In the name of God
University of Patras
Department of Chemical Engineering
Section of Process and Environmental Engineering
M.Sc. Thesis in Chemical Engineering
(Environment & Energy)
Novel Photo-Catalytic Materials for Wastewater Treatment
Photo-catalytic degradation of endocrine disruptor chemicals by solar driven
Ag3PO4/TiO2 composite catalyst
Mir Edris Taheri
Supervisor Professor
Prof. Dionissios Mantzavinos
Thesis Committee
Prof. Dionissios Mantzavinos
Assoc. Prof. Dimitris Kondarides
Assoc. Prof. Alexandros Katsaounis
Autumn 2015
I
ACKNOWLEDGEMENT
First of all, thanks God, for everything, especially this moment! I am so proud and thankful to
have the opportunity to study in the University of Patras, during three years of my studies in the
programme of postgraduate studies "M.Sc. in Chemical Engineering", I’ve been supported and
encouraged by many people. I want to express my deepest thanks and sincere gratitude to my
thesis supervisor, dear Professor Dionissios Mantzavinos for his caring, supports, aspiring
guidance, valuable advices and patience throughout my master studies, it was a great honor for
me to study under his guidance.
I would also like to thank Professor Dimitris Kondarides and Professor Alexandros Katsaounis
for helping me in different ways. My special thanks go to Dr. Zacharias Frontistis for his time,
constant helps, supports, useful suggestions and advices during my studies and researches,
especially during preparation of my master’s thesis.
I have to acknowledge all of the chemical engineering programme professors who changed my
world for better with offering me their time, wisdom, expertise and I greatly appreciated their
endless kindness and supports during my time at the university. Moreover, I would like to thank
all my dear classmates for their friendship, encouragement and also for helping me to carry out
my studies.
I am also grateful to my family, my mother, father and brothers for their unconditional and never
ending helps and supports. Finally, my heartfelt thanks to my companion in the life’s journey,
my darling wife, without her love, help and understanding I would not be able to pass this part
of my life.
It is worth mentioning that, part of this work was supported by Grant Ε056 from the Research
Committee of the University of Patras (Program C. Caratheodory), also part of this research has
been co-financed by the European Union (European Social Fund ESF) and Greek national funds
through the Operational Program ‘Education and Lifelong Learning’ of the National Strategic
Reference Framework (NSRF) - Research Funding Program: Thales; and investing in knowledge
society through the European Social Fund (PhotoFuelCell project).
Mir Edris Taheri
Patras, Greece.
October 2015.
II
Summary
Over the last three decades there has been increasing global concern over the public health
impacts attributed to environmental pollution. It is estimated that about a quarter of the diseases
facing mankind today occur due to prolonged exposure to environmental pollution, which in
many ways, water pollution is as serious - if not more so - than air contamination. Thousands of
industrial and natural chemicals are daily discharged into the aquatic environment that some of
them are not only toxic but also partly biodegradable; therefore they are not easily and completely
removed in conventional wastewater treatment plants. Although most of them are present at low
concentrations, but many of these micro-pollutants raise considerable toxicological concerns,
particularly when present as components of complex mixtures in the texture of living organisms.
Advanced oxidation processes are set of the emerging treatment technologies for this purpose, in
which developing photo-chemical processes, especially photo-catalysis via solar driven
semiconductor photo-catalysts are more desirable because of using solar energy as a free, clean
and renewable energy that will be present to last us indefinitely. Although relevant inorganic
semiconductors are often non-toxic, cheap and abundant, but not all of them good enough in
absorbing solar irradiation, quantum efficiency and chemical stability together. Designing heterostructure semiconductor catalysts is one of the strategies in order to overcome mentioned
problem, in which at least two semiconductors with different characteristics are joined to each
other to enhance strengths and/or to improve weaknesses of them.
By the same token, in this research Degussa-P25 TiO2 that is famously known as an efficient
catalyst in UV range with a high chemical stability and reusability, was added to the structure of
Silver Orthophosphate (Ag3PO4) that is an efficient catalyst in visible range with a high quantum
yield (90%) and high photo-catalytic activity, in order to boost both photo-catalytic activity and
stability of them, as a novel synthesized solar driven composite photo-catalyst for degradation of
Bisphenol A as a serious micro-pollutant in water.
BPA as one of the most famous endocrine disruptor chemical that has possible hazards to fetuses,
infants, young children, and has been associated with congenital defects, reduced fertility, and
several diseases such as neural circuit’s distress, obesity, diabetes mellitus, and cancer in humans,
is widely known for its tendency to leach from polycarbonate plastics and epoxy resins,
particularly those found in food packaging industries, drinking water bottles, foods and beverages
tin/steel/aluminum cans, food and beverage containers, baby bottles, etc., which has made it a
subject of public health and environmental concern.
III
In this project, photo-catalytic degradation of BPA under full spectrum simulated solar light
irradiation has been studied in different conditions, including different types of catalyst, dark
adsorption, different types of radiation, different concentration of the composite catalyst,
different concentration of BPA, different pH and different water matrices. Also photo-catalytic
activity and stability of the composite catalyst were checked during sequential runs.
The optimum ratio of Ag3PO4/TiO2 for the composite catalyst was defined 3:1 that showed higher
photo-catalytic activities in comparison to its two components, under all three types of irradiation
including simulated solar and its UV and visible parts, also it showed much higher efficiency
under real solar irradiation. The optimum required time for appropriate adsorption of BPA at the
surface of the composite catalyst under dark condition was defined 15 minutes. Full conversion
of BPA under simulated solar irradiation was observed in 7.5 minutes that follows a pseudo-first
order reaction kinetics with respect to initial concentration of BPA, whereas the kinetic constant
decreases as the initial concentration of BPA increases. Also with increasing the catalyst
concentration, BPA degradation was increased considerably. Photo-catalytic degradation of BPA
in acidic pH was noticeably more efficient than in alkaline pH, whereas high pH had detrimental
effects on the composite catalyst. In presence of Humic acid as a ubiquitous troublesome oxidant
consumer in water matrices, efficiency reduction was not observed in photo-catalytic degradation
of BPA, and also the same happened in presence of Sodium bicarbonate as a radical scavenger,
which was beyond expectation for both. Full degradation of BPA in drinking bottled water
containing different inorganic ions that act as radical scavengers and catalyst deactivators, was
observed in 10 minutes, which looks acceptable; but degradation of BPA in secondary
wastewater treatment effluent was not efficient at all, whereas only 66.5% conversion was
observed after 8 hours. Furthermore, photo-catalytic stability/activity of the composite photocatalyst was enhanced in comparison to pure silver phosphate, after 5 consecutive runs of BPA
photo-catalytic degradation, also it was found that such composite photo-catalyst appeared to be
a somewhat more desirable in repeated and/or long-term application under visible part of
simulate solar irradiation.
IV
CONTENT
Title
Page
Chapter 1: INTRODUCTION ……………………………………………………………… 1
1.1. Environmental Pollution ………………………………………………….. 2
1.1.1. Environmental Pollutants ………………………………………….. 3
1.1.2. Micro-pollutants ……………………………………………………. 4
1.1.3. Persistent Organic Pollutants (POPs) ……………………………... 6
1.1.4. Endocrine Disrupting Chemicals (EDCs) …………………………. 7
1.2. Bisphenol A (BPA) ………………………………………………………. 8
1.2.1. Applications ……………………………………………………….. 10
1.2.2. Health and Environmental Issues …………………………………. 12
1.3. Water/Wastewater Treatment ……………………………………………. 15
1.3.1. Fourth Treatment Stage ……………………………………………. 15
1.3.2. Advanced Oxidation Processes (AOPs) …………………………… 16
1.4. Photochemical Processes ………………………………………………... 21
1.5. Heterogeneous Photo-Catalysis …………………………………………. 22
1.5.1. Titanium Dioxide Photo-Catalyst …………………………………. 24
1.5.2. Silver Phosphate Photo-Catalyst …………………………………. 27
1.5.3. Silver Phosphate Based Composite Photo-Catalysts ……………… 32
Chapter 2: EXPERIMENTAL ……………………………………………………………. 35
2.1. Equipment ……………………………………………………………….. 36
2.1.1. Reactor ……………………………………………………………. 37
2.1.2. Irradiation Source …………………………………………………. 38
2.1.3. Analytical Techniques …………………………………………….. 40
2.2. Materials …………………………………………………………………. 41
2.3. Catalyst …………………………………………………………………... 42
2.3.1. Measurements of specific surface areas …………………………… 42
2.3.2. X-ray diffraction (XRD) measurements …………………………... 42
2.3.3. Diffuse Reflectance Spectroscopy (DRS) …………………………. 43
2.4. Procedure of experiments ………………………………………………... 47
Chapter 3: RESULTS & DISCUSSION ………………………………………………….. 48
3.1. Catalyst Screening ……………………………………………………….. 50
3.2. Dark Adsorption ………………………………………………………….. 53
3.3. The Effect of Radiation ………………………………………………….. 55
3.4. The Effect of Catalyst Concentration ……………………………………. 59
3.5. The Effect of BPA Concentration ……………………………………….. 62
3.6. The Effect of pH ………………………………………………………….. 66
3.7. The Effect of Water Matrix ……………………………………………… 69
3.8. Catalyst Stability ………………………………………………………… 74
Chapter 4: CONCLUSION ……………………………………………………………….. 80
REFERENCES …………………………………………………………………………….. 84
V
Chapter 1
INTRODUCTION
1
1.1. Environmental Pollution
Environmental pollution had been a fact of life for many centuries, but it became a real issue
since the start of the industrial revolution, when the environment could not process and neutralize
harmful by-products and impacts of human activities at appropriate time without any structural
or functional damage to its ecosystem. [1]
Pollution is the introduction of contaminants into the natural environment that cause adverse
change (i.e. harm or discomfort to humans or other living organisms or damage the environmental
ecosystem) which can take in the form of chemical substances or energy or detrimental action.
Air pollution, water pollution, soil pollution, radioactive pollution, litters, plastic pollution,
thermal pollution, noise pollution, light pollution, visual pollution and personal pollution are the
major forms of environmental pollution. [2]
In general, air, water and soil pollution are the most important types of environmental pollution,
because they are directly linked to the basics of life. Among these, the role of water pollution
should be more highlighted due to its impairments in vital and essential usages of water such as
drinking, sanitation, irrigation, aquaculture, fishing, ecosystem maintenance, and also its high
ability in absorbing and transferring of air pollution to water cycle during rainfall process, as well
its significant permeability in soil and food chains.
Figure 1.1 - Overview of main health effects on humans from some common types of pollution. [2]
2
Water pollution is the contamination of natural water bodies (e.g. ponds, lakes, rivers, oceans,
aquifers and groundwater) by physical, chemical and microbial extrinsic agents that affects the
entire biosphere, plants and organisms living in these bodies of water. It occurs when pollutants
are directly or indirectly discharged into water bodies without adequate treatment to remove
harmful compounds that makes water unfit for its intended purpose.
By the same token, water pollution is often classified as; [3]
A) Point-Source Pollution originates from a single and identifiable source and usually
occurs in surface water and groundwater, in which a plume that has the highest
concentrations of the pollutant nearest the source (such as the end of a pipe or an
underground injection system) and diminishing concentrations farther away from the
source. The various types of point-source pollutants found in waters are as varied as the
types of business, industry, agricultural, and urban sources that produce them.
B) Nonpoint-Source Pollution does not originate from a single source or point and occurs
as water moves across the land or through the ground and solves and picks up natural
and human-made pollutants, which can then be deposited in ponds, lakes, rivers,
wetlands, coastal waters, and even groundwater. The water that carries nonpoint-source
pollution may originate from natural processes such as rainfall or snowmelt (airborne
pollutants), or from human activities such as crop irrigation or lawn maintenance
(fertilizers and pesticides).
1.1.1. Environmental Pollutants
A pollutant is the constituent part of any pollution process, which is a waste material or energy
introduced into the environment (i.e. air, water or soil) that has undesired effects, or adversely
affects the usefulness of a resource. A pollutant may cause short- or long-term damage by
changing the growth rate of plant or animal species, or by interfering with human amenities,
comfort, health, or property values. As the actual executing agent of environmental pollution they
can be foreign or naturally occurring contaminants, and 3 factors determine the severity of a
pollutant: its chemical nature, the concentration and the persistence. [4]
From the environmental point of view, chemical substances as the major part of environmental
pollutants are divided into; [1], [4]
A) Biodegradable Pollutants that can be broken down and processed by living
organisms/micro-organisms
(biologically),
including
organic
waste
products,
phosphates, inorganic salts, etc. Therefore, they will not persist in the environment in the
long term, and can be neutralized and converted into harmless or even useful compounds.
3
B) Non-biodegradable Pollutants that cannot be decomposed by living organisms/microorganisms and therefore persist in the ecosphere for extremely long periods of time,
including heavy metals, persistent organic pollutants (POP), environmental persistent
pharmaceutical pollutants (EPPP), polyclinic aromatic hydrocarbons (PAHs),
environmental xenobiotics/Pharmaceutically active compounds (PhACs), and some
volatile organic compounds (VOCs). Physicochemical processes can decompose and
convert some of them into the harmless compounds.
At the moment, trace compounds become the biggest challenge for environmental ecosystems,
especially they leak into bodies of water from wastewater treatment plants and through diffuse
inputs from sources such as agriculture. They could be organic or mineral substances that have
toxic, persistent and bio-accumulative properties, which affect negatively the environment and/or
organisms. [5], [6]
1.1.2. Micro-Pollutants
Micro-pollutants are organic and also inorganic compounds that are found in a few ng/L to
several μg/L concentration range in the air, water and soil, which are considered to be potential
threats to environmental ecosystems. The way that these compounds enter the environment
depends on their uses and the mode of application. The major routes seem to be agricultural and
urban runoff, municipal and industrial wastewater discharge, sludge disposal, hospital activity,
transport and machinery, atmospheric fallout, direct emissions and accidental spills. Accordingly,
different groups of compounds are included in this category, which are present in many products
that we consume daily at home, industry and agriculture such as insecticides, pesticides and
phytosanitary products, flame retardants, PCBs, PAHs and perfluorinated compounds,
pharmaceuticals, surfactants, personal care products and cosmetics. In most cases these
compounds are liable to have potentially chronic direct or indirect effects on ecosystems, and
even on human and animal health. [6], [7], [8]
By the same token, it is clear that micro-pollutants can be found ubiquitously in the aquatic
environment, in which their occurrence have been frequently associated with a number of
negative effects, including short-term and long-term toxicity, endocrine disruption, hormonal
balance disorders, cancers, birth defects, bio-accumulation, bio-magnifications and antibiotic
resistance of microorganisms. [9]
Micro-pollutants are usually organic and non-biodegradable, but may also degrade sometimes
into even more toxic chemicals. They can find their way into water supplies and food chains that
finally affect environmental health.
4
Category
Major Sources
Important Subclasses
Distinct
Domestic wastewater Sources that are not
NSAIDs, lipid regulator,
anticonvulsants, antibiotics,
Pharmaceuticals
antidepressants, analgesics,
stimulants
Products
(from excretion)
exclusive to
Hospital effluents
individual categories
Run-off from CAFOs include:
β-blockers, bronchodilators,
chemotherapy products and
Personal Care
Nonexclusive
(concentrated animal
Industrial wastewater
feeding operations)
(from product
and aquaculture
manufacturing
Domestic wastewater discharges)
Fragrances, disinfectants,
UV filters, cosmetics, and
insect repellents
(from bathing,
Landfill leachate
shaving, spraying,
(from improper
swimming and etc.)
disposal of used,
Domestic wastewater defective or expired
Steroid
Hormones
(from excretion)
Estrogens
Run-off from CAFOs
and aquaculture
Domestic wastewater
(from bathing,
Surfactants
laundry, dishwashing
Non-ionic surfactants, and
and etc.)
detergents
Industrial wastewater
(from industrial
cleaning discharges)
Industrial
Plasticizers, fire retardants,
Chemicals
solvents, and preservatives
Domestic wastewater
(by leaching out of
the material)
Domestic wastewater
(from improper
Pesticides
Insecticides, insecticides,
cleaning, run-off from
herbicides and fungicides
gardens, lawns and
roadways and etc.)
Agricultural runoff
Metals
Metalloids, heavy metals,
Wastewater from labs
and radioactive elements
and hospitals
Table 1.1 - Sources of micro-pollutants in the aquatic environment.
5
items)
1.1.3. Persistent Organic Pollutants (POPs)
Persistent organic pollutants are resistant organic compounds to environmental degradation
through chemical, biological, and photolytic processes, which can bio-accumulate and pass from
one species to the next through the food chains (more) and water supplies (less), and pose a risk
of causing significant adverse effects on environmental and human health around the world. [10]
Although some POPs arise naturally (from volcanoes and various biosynthetic pathways), most
are man-made and since the boom in industrial production after World War II, thousands of
synthetic POPs have used as pesticides, solvents, pharmaceuticals, and industrial chemicals. [11]
They negatively affect environmental ecosystem through two processes, "long-range transport"
by wind and water, which allows them to travel far from their source (regions where they have
never been used or produced them), and "bio-accumulation", which reconcentrates these
chemical compounds to potentially dangerous levels. In people and animals, reproductive,
developmental, behavioral, neurologic, endocrine, and immunologic adverse health effects have
been linked to POPs, especially endocrine disruption, destruction of reproductive system, various
kinds of cancers, cardiovascular disease, obesity, diabetes and threatening the health of fetus and
next generation in human. [12]
Aldrin, Chlordane, Dieldrin, Endrin, Heptachlor, Hegzachlorobenzene (HBC), Mirex,
Toxaphene, Polychlorinated biphenyls (PCBs), Dichlorodiphenyltrichloroethane (DDT),
Dioxins and Polychlorinated dibenzofurans are the most famous POPs that are introduced by
Stockholm Convention list in 1995. Moreover, Polycyclic aromatic hydrocarbons (PAHs),
Chlordecone, Hexachlorocyclohexanes (α/β-HCH), Lindane, Pentachlorobenzene (PeCB),
Bromodiphenyl Ethers (tetra/penta/hexa/hepta/octa-BDE), Perfluorooctanesulfonic acid (PFOS),
Endosulfans, Brominated Flame Retardants (i.e. Hexabromocyclododecane or HBCD) and
Tributyltin (TBT) have added to the Stockholm Convention list since 2001. [13]
Figure 1.2 - Background on Persistent Organic Pollutants
6
1.1.4. Endocrine Disrupting Chemicals (EDCs)
Endocrine disruptors are chemicals that, at certain doses, can interfere with the body’s endocrine
system (also named hormone systems) in human, mammals, birds, fish, and many other types of
living organisms. EDCs with low dose matters, because of ubiquitous exposure and persistence
of biological effects, have wide range of adverse health effects. [14]
A wide range of compounds (both natural and man-made) can cause endocrine disruption,
including Dioxin and Dioxin-like compounds, Pharmaceuticals (i.e. DES), many of Pesticides
(Insecticides such as Endosulfan, Kepone, Amitraz and DDT, Herbicides such as Atrazine, and
Fungicides such as Vinclozolin), Polychlorinated Dibenzo group (PCDBDs and PCDBFs),
Polycyclic Aromatic Hydrocarbons (PAHs), Polybrominated Diphenyl Ethers (PBDE's),
Plasticizers (i.e. Bisphenols), Polychlorinated Biphenyls (PCBs), Phthalate Esters (i.e. DEHP),
Perfluorooctanoic Acid (PFOA), Tributyltin (TBT), some Phenol derivatives and Oxybenzone
that are synthetic, and also Phytoestrogens (i.e. Genistein) and Mycoestrogens (i.e. Zearalenone)
that are natural xenoestrogens. They are found in many everyday products such as plastic bottles,
food packaging materials, foods, detergents, flame retardants, toys, cosmetics, drugs, etc. [15]
When an EDC absorbed into the body (Fig. 1.3), it can disrupts the normal functions of different
organs through; a) Mimic or partly mimic naturally occurring hormones in the body like
estrogens, androgens, and thyroid hormones, potentially producing overstimulation; b) Bind to a
receptor within a cell and block the endogenous hormone from binding, which the normal signal
then fails to occur and the body fails to respond properly; or c) Interfere or block the way natural
hormones or their receptors are made or controlled, by altering their metabolisms. Subsequently,
it can produce adverse developmental, reproductive, neurological, and immune effects and also
cause forming of cancerous tumors in humans, domesticated and non-domesticated species. [14]
One of the most common EDCs that is very important because of its large quantity production
and also its wide utilization in many things, is Bisphenol group.
Figure 1.3 - When an endocrine disruptor absorbed in the body; a) it can decrease or increase normal
hormone levels, b) mimic the body's natural hormones, or c) alter the natural production of hormones. [14]
7
1.2. Bisphenol A (BPA)
BPA with the chemical formula (CH3)2C(C6H4OH)2 is a carbon-based synthetic compound
derived from styrene, acrylonitrile and butadiene, which is the most popular representative of the
group named Bisphenols that most of them are based on Diphenylmethane (except M, P and S)
with two Hydroxyphenyl functionalities. [16]
Bisphenol A
Bisphenol F
Bisphenol AF
Bisphenol G
Bisphenol AP
Bisphenol M
Bisphenol B
Bisphenol P
Bisphenol BP
Bisphenol PH
Bisphenol C
Bisphenol S
Bisphenol D
Bisphenol TMC
Bisphenol E
Bisphenol Z
Table 1.2 – Bisphenols group members and their structural formula. [17]
8
Transparent colorless crystals
Appearance
White solid powder
White to light brown flakes
Structure
IUPAC Name
4,4'-(propane-2,2-diyl)diphenol
Other Names
p,p'-isopropylidenebisphenol
2,2-bis(4-hydroxyphenyl)propane
Bis(4-hydroxyphenyl) dimethylmethane
Chemical Formula (CH3)2C(C6H4OH)2 or C15H16O2
Molar Mass
228.29 g/mole
Density
1.195 g/cm3
Melting Point
158 °C
Boiling Point
220 °C
Flash Point
227 °C
Kindling Point
600 °C
Solubility in water
120-300 ppm
Vapour Pressure
5×10-6 Pa at 25 °C
Odour
Weak medicine (Mild phenolic)
Half-life time
0.2 day in air, 38 days in water, 75 days in soil, and 340 days in sediment
Risk Phrases
R36: Irritating to eyes.
R37: Irritating to respiratory system.
R38: Irritating to skin.
R43: May cause sensitization by skin contact.
Safety Phrases
S24: Avoid contact with skin.
S26: In case of contact with eyes, rinse immediately with plenty of water and seek
medical advice.
S37: Wear suitable gloves.
NFPA 704
Flammability: 0
Materials that will not burn under typical fire conditions, or intrinsically
noncombustible materials (Materials that will not burn in air when exposed to a
temperature of [820 °C] for a period of 5 minutes).
Health: 3
Short exposure could cause serious temporary or moderate residual injury.
Instability/Reactivity: 0
Normally stable, even under fire exposure conditions, and is not reactive with water.
Special Notice:
Its powder is a significant dust explosion hazard (dust cloud by static electricity).
It is incompatible with strong oxidizers, bases, acid chlorides and acid anhydrides.
Table 1.3 - Chemical infobox of Bisphenol A. [16]
9
It is a colorless crystalline solid (white flakes and powder) with mild phenolic odour that is
soluble in organic solvents, but poorly soluble in water (less than 1 g/L at 21.5 ºC). It was
discovered by Russian chemist Aleksandr Dianin in 1891, and after discovering its reaction with
Phosgene (COCl2) that produced a clear hard resin known as polycarbonate, it has been widely
in commercial use since 1957. [16]
Figure 1.4 - Bisphenol A synthesis through condensation of Acetone with Phenol. (Catalyzed by HCl)
It is a high production volume chemical that its world production exceeded 7 million tonnes in
2013, and it is predicted to surpass the 9.6 million tonnes mark by 2020. [18]
1.2.1. Applications
Bisphenol A is predominantly used as an intermediate for producing of other products in nearly
every industry, such as binding, plasticizing and hardening functions in making various plastics,
paints, lacquers, binding and filling-in materials. It is widely used in producing polycarbonate
plastics and one of the key monomer in producing epoxy resins. It also plays an important role
in manufacturing of unsaturated polyester resin, polyacrylate, polyetherimide, polyether ketones,
polysulfone resins and flame retardants.
Plastics made of BPA (i.e. polycarbonates) have superior properties such as physical stability,
heat and chemical resistant, durability and strength at low temperature, resiliency and flexibility,
hardness, high-impact collision sustainability, light weight, slick surface, high transparency and
optical clarity, and easy processing that make them appropriate to manufacture variety of
consumer goods such as food and beverage packages and containers, constructional materials,
playground and sports equipment, toys, electronic equipment, water bottles, infant bottles and
Sippy cups, compact disks (i.e. CD, DVD and Blue-ray disk), impact-resistant safety equipment
(e.g. safety glasses, face guards, motorcycle helmets, and bullet-resistant windows), medical and
dental devices (e.g. heart-lung machines, incubators, artificial kidneys, syringes, contact lenses,
and dental sealants, composites and fillers,), lenses (e.g. sunglass/eyeglass, automotive headlamp
and lighting lenses), and smart phones. [18], [19]
Also, BPA based epoxy resins because of their high quality performance in thermal stability,
chemical resistivity, moisture resistivity, corrosion protection, mechanical strength and adhesion,
and thermal and electrical resistivity/conductivity, are used in lining the inner surfaces of almost
all food and beverage cans and vats and containers, bottle tops and water supply pipes as
10
protective coatings, in lining of carbonless thermal receipt papers, in producing of fiber glasses,
laminates, thermoplastics, thermosetting polymers/resins, paints, coatings, lacquers, anticorrosion sprays, structural adhesives (polyurethane, acrylic, cyanoacrylate, etc.), and
electronic/electrical systems (i.e. integrated circuits, transistors and hybrid circuits, printed circuit
boards, insulators, etc.) as the base/raw material.
In addition BPA has other applications in production of different materials such as in adhesives,
sealant chemicals and flame retardants (Tetrabromo Bisphenol A) as a base material, in thermal
receipt papers as additive and colour development component, in PVC as a stabilizer, in rubber
tires, brake fluid, high-temperature cables and some plastics as an antioxidant, and also in paints,
thermal printing inks, toners, carbonless copy paper and some other colorant products.
Figure 1.5 - BPA in some products.
11
1.2.2. Health and Environmental Issues
In the early 1930s, for the first time Edward Charles Dodds (British biochemist) discovered that
BPA acts as an artificial estrogen, but it was never used as a drug. BPA is categorized as
xenoestrogens that are a kind of xenohormone (hormone-like), which imitates estrogen and has
estrogenic effects on a living organism. From 1997, many researches have determined that both
high-dose and low-dose exposure to BPA can adversely affect human and animals health as an
endocrine disruptor. Adverse effects of BPA came from its ability to mimic the effects of natural
estrogen derive from the similarity of phenol groups on both BPA and estradiol, which enable
this synthetic molecule to bind and activate the same estrogen receptor as the natural hormone
and trigger estrogenic pathways in the body.
Figure 1.6 - Because of phenol group’s similarity, BPA binds to proteins meant to interact with estradiol.
BPA can cross the placental barrier in mammals and detects in maternal and fetal serum and their
wombs and placental tissues. It can adversely affect reproduction and development in mammals
by interfering with their endocrine systems. It has also been discovered to exert very subtle effects
on sexual behaviour in animals. [19]
Bisphenol A as an endocrine disruptor chemical has been associated with congenital defects,
reduced fertility, and several diseases such as obesity, diabetes mellitus, and cancer in humans.
It can distress neural circuits that regulate feeding behavior, which has been proposed to increase
the risk of obesity. Also it has negative effects on fetal and infant brain development and
behavior, in which interference with brain cell connections vital to memory, learning, and mood.
Furthermore, it affects various dopaminergic processes to enhance mesolimbic dopamine activity
resulting in hyperactivity, attention deficits, and a heightened sensitivity to drugs of abuse. In
addition, it binds to thyroid hormone receptor and has adverse effects on its functions.
BPA suppresses DNA methylation, which is involved in epigenetic changes that increases cancer
risk, for instance alters breast development and increases breast cancer risk, and also promotes
the growth, invasiveness and metastasis of neuroblastoma cancer cells. As well as, it increases
risk of coronary heart disease and high blood pressure significantly, and may cause diabetes and
abnormally high levels of certain liver enzymes.
12
Also exposure to BPA during childhood increases risk of asthma about 10 times. Moreover, it
causes childhood externalizing behaviors, recurrent miscarriage and postmenopausal
inflammation in women, altering hormone levels and declining male sexual function in men, and
oxidative stress and immune deficiency in both.
On the other hand, BPA also has negative effects on environment. Although released BPA to soil
has low mobility because of its low water solubility, but in plants it causes an increase in
micronuclei in root-tip cells, for instance it was shown to interfere with nitrogen fixation at the
roots of leguminous plants (bean family) associated with the bacterial symbiont Sinorhizobium
meliloti. The tendency for leached BPA to accumulate in the tissues of marine and fresh water
species is quite low because of its very low concentration, but prolonged exposure to even low
concentrations of BPA (1 μg/L to 1 mg/L) can cause DNA damage and abnormalities in aquatic
invertebrates, amphibians, and reptiles, in which fishes are the most sensitive species. Also in
certain species of reptiles whose sex determination normally is influenced by temperature, BPA
exposure causes sex ratios to become biased toward females. [19]
Dose
(μg/kg.day)
0.025
0.025
1
2
2
2.4
2.5
10
10
30
50
50
Adverse Effects
Permanent changes to genital tract.
Changes in breast tissue that predispose cells to hormones and carcinogens.
Long-term adverse reproductive and carcinogenic effects.
Increased prostate weight 30%.
Lower bodyweight, increase of anogenital distance in both genders, signs of
early puberty and longer estrus.
Decline in testicular testosterone.
Breast cells predisposed to cancer.
Prostate cells more sensitive to hormones and cancer.
Decreased maternal behaviors.
Reversed the normal sex differences in brain structure and behavior.
Adverse neurological effects occur in non-human primates.
Disrupts ovarian development.
Table 1.4 - Examples of some adverse effects due to BPA low dose exposure in animals. [16]
In 2015, European Food Safety Authority (EFSA) for the new Tolerable Daily Intake (TDI)
considerably reduced the safe dose of BPA from 50 µg/kg.day (micrograms per kilogram of body
weight per day) to 4 µg/kg.day, for exposure from a combination of sources, including dietary,
dust, cosmetics and thermal paper, whereas in 2010, the U.S. Environmental Protection Agency
reported that about 500 tonne of BPA are released into the environment annually, including soil,
rivers, lakes, and oceans. [16], [20]
13
In macro scale, the primary sources of environmental release of BPA are expected to be
effluents and emissions from its manufacturing facilities, facilities which manufacture
epoxy, polycarbonate, and polysulfone resins, coat and staining manufacturers, BPA based
materials recycling (i.e. cans, vats and paper mills), and foundries who use BPA in casting sand.
But in micro scale, BPA is widely known for its tendency to leach from the BPA based products,
especially infant and water bottles, food and beverage canes and vats and containers, tableware
(e.g. plates and mugs), storage containers, thermal receipt papers, and epoxy lined water supply
pipes. It can also enter the environment through leaching from plastic, paper and metal wastes in
landfills, ocean-borne plastic trashes, and wastewater treatment plants. [19]
Researches show BPA existence in surface water and sediment, frequently came from municipal
wastewater. Although 75% to 90% of BPA usually can/may be removed from water during
treatment at municipal wastewater/water treatment plants, but BPA even with very low
concentration (less than 1 μg/L) can progressively bio-accumulate and bio-magnificate in
different aquatic and terrestrial organisms.
Samples
Concentration
Water bottle (at 25 °C)
0.5 – 8.82
μg/L
Infant bottle (at 80 °C)
5.0 – 8.0
μg/L
Canned beverages (soft drinks)
0.032 – 4.5
μg/L
Canned foods
2.6 – 790
μg/kg
Municipal wastewater
0.01 – 10
mg/kg
Effluent of wastewater treatment plants
nondetect – 370 μg/L
(most cases < 5 μg/L)
Surface water
nondetect – 56
(avg. 0.014 – 1.3 mg/L)
Hazardous waste landfill leachates
nondetect – 17.2 mg/L
Municipal sewage sludge
0.01 – 100
μg/L
mg/kg
(most cases ~1 μg/L)
(dry weight)
(dry weight)
Table 1.5 - Average BPA concentration in some samples. [21], [22], [23], [24], [25], [26]
Since BPA is a non-biodegradable persistent organic pollutant, what we shall do for its removal
from water/wastewater in order to protect human, animals and aquatic environment?!
14
1.3. Water/Wastewater Treatment
Generally treatment means removing impurities from water being treated, in which water
treatment is set of processes that makes water more acceptable for an end-use (i.e. drinking,
agriculture, industry, or medicine) by removing water contaminants or so reduce their
concentration to its standards; and wastewater treatment is the same but for converting
wastewater (water that is no longer needed or suitable for its most recent use) into an effluent
that can be either returned to the water cycle with minimal environmental issues or reused for
various purposes (e.g. agriculture or industry).
Treatment plants frequently consist several stages, including Pre-treatment (bar screening, grit
removing, flow equalizing basin, and fat and grease removing), Primary treatment (primary
sedimentation tanks), Secondary treatment (biological treatment and secondary sedimentation),
Tertiary treatment (filtration, lagoon/pond settling, biological nutrient removal, and disinfection),
and recently Fourth treatment stage. [27]
1.3.1. Fourth Treatment Stage
Micro-pollutants such as pharmaceuticals, ingredients of household chemicals, chemicals used
in small businesses or industries, environmental persistent organic pollutant or pesticides may
not be eliminated in conventional treatment processes (primary, secondary and tertiary treatment)
and therefore lead to water pollution. Although concentrations of those substances and their
decomposition products are quite low, but there is still a chance to harm terrestrial and aquatic
organisms. Treatment of toxicological compounds such as endocrine disruptors, genotoxics, and
bacterial resistant development enhancer that mainly belong to the group of environmental
persistent organic pollutants, are the intended subject in this step that could be also as a
supplementary process in combination with secondary or tertiary treatment processes.
Accordingly, the fourth treatment stage is also named post-treatment or water purification.
However, since related techniques for eliminating of micro-pollutants via a fourth treatment stage
during water/wastewater treatment are still costly, they are not yet applied on a regular basis. [27]
Related technologies to the treatment of micro-pollutants in water and wastewater are typically
̵
categorized in 5 groups, including; [28]
Membrane separation: Micro-filtration (MF) for removing bacteria and suspended solids;
Ultra-filtration (UF) for removing colloids, viruses and certain proteins; Nano-filtration (NF)
for softening (polyvalent cation removal) and removing of disinfection by-product
precursors; and Hyper-filtration or Reverse Osmosis (RO) for desalination.
15
̵
Adsorption & Coagulation: Activated Carbone (Charcoal) for effective removing of different
molecules and ions, but not effective in minerals, salts, and dissolved inorganic compounds,
Zeolites (Clays) in which due to the variety of their base materials and structures, adsorptive
selectivity towards different molecules and particles is also wide; Ion Exchange Resins or
Polymers that have been used in water softening and purification, because of their high
mechanical and chemical stabilities in combination with high adsorptive capacity for ion
exchange, complexation, reduction-oxidation, and precipitation of different macromolecular
materials, cataions and anions; Inorganic Ion Exchangers with higher thermal, chemical and
radioactive stability, and more efficient selectivity. Enhanced coagulation and oxidationcoagulation in which use organic (polymeric) and inorganic coagulants to enhance removal
of micro-pollutants such as natural organic matters and disinfection by-products.
̵
Advanced Biological Treatment: removal of micro-pollutants by Algae and specific plant
species (Phytoremediation), and Fungi (Mycoremediation), and also using specific microorganisms and Genetically Engineered Micro-organisms (GEMs) for Bioaugmentation in
order to enhance bioremediation/biodegradation of hard biodegradable and nonbiodegradable micro-pollutants.
̵
Advanced Oxidation Processes (AOPs): are set of different technologies for chemical
treatment of organic and sometimes inorganic environmental contaminants, almost based on
̵
oxidation through reactions with hydroxyl radicals.
Hybrid processes: actually are combination of minimum two different processes from
mentioned former groups, even combination of different technologies from the same group.
1.3.2. Advanced Oxidation Processes
Advanced Oxidation Processes (AOPs) refer to a set of oxidative soil, water, and air treatment
methods that can be used to treat/transform persistent organic pollutants and toxic organic
compounds (i.e. dyes, pesticides, pharmaceutical compounds, endocrine disruptors, etc.) into
safe/low-risk biodegradable substances. It has been precisely defined by Dr. William H. Glaze
and his research team in 1987, as several water treatment processes performed at room
temperature and normal pressure and based on the in situ generation of powerful oxidizing agents,
at a sufficient concentration to effectively decontaminate waters. Although Fenton reagent
oxidation process has been in use since late 19th century, but it was an analytical reagent at that
time and it was not utilized as oxidative process in water treatment. Nowadays AOPs are
considered environmental-friendly and high efficiency physicochemical processes due to their
thermodynamic viability and capable to produce deep changes in the chemical structure of wide
range of contaminants in groundwater, surface water, swimming pools, water recycling,
disinfection, ultrapure water, municipal and industrial wastewater and sludge, leachates, odour
and volatile organic compounds via the participation of free radicals. [29], [30], [31]
16
Oxidation process is defined as the transfer of one or more electrons from an electron donor to
an electron acceptor (reductant to oxidant), which has a higher affinity for electrons. These
electron transfers result in the chemical transformation of both the oxidant and the reductant, and
also in some cases producing chemical species with an odd number of valence electrons. These
species, known as radicals, tend to be highly unstable and, therefore, highly reactive because one
of their electrons is unpaired. Oxidation reactions that produce radicals tend to be followed by
additional oxidation reactions between the radical oxidants and other reactants (both organic and
inorganic) until thermodynamically stable oxidation products are formed. [32]
The ability of an oxidant to initiate chemical reactions is measured in terms of its oxidation
potential. Except fluorine as the most powerful oxidant, the hydroxyl radical (∙OH) that is neutral
form of the hydroxide ion (OH−), is highly reactive and consequently short-lived, which plays a
key role in oxidative destruction of organic pollutants using advanced oxidation processes. Since
oxidizing a wide range of organic compounds by hydroxyl radicals is frequently billion times
faster than with Ozone, the aim of most AOPs is to produce the hydroxyl radicals in water.
Besides hydroxyl radicals, AOPs can also generate other oxidizing species that can boost the
process, such as perhydroxyl radical (∙O2H), superoxide radical (∙O2−), alkoxyl radical (∙OR),
peroxyl radical (∙OOR), and singlet oxygen (1O2). However, the term advanced oxidation
processes refers specifically to processes in which oxidation of organic contaminants occurs
primarily through reactions with hydroxyl radicals, and generally the effectiveness of an AOP is
proportional to its ability to generate hydroxyl radicals. [32], [33]
Oxidizing Agent
Fluorine
Half-reaction
⇌
Oxidant
F2(g) + 2 H+ + 2 e−
⇌
Hydroxyl radical
∙OH + H+ + e−
Oxygen (Atomic)
∙O∙ + 2 H + 2 e
Ozone
O3(g) + 2 H+ + 2 e−
Hydrogen peroxide
H2O2(aq) + 2 H+ + 2 e−
+
−
Perhydroxyl radical ∙O2H(aq) + 3 H + 3 e
+
−
Permanganate
MnO4−(aq) + 4 H+ + 3 e−
Hypobromous acid
HBrO(aq) + H + 2 e
Chlorine dioxide
Hypochlorous acid
−
Electrochemical
Oxidation
Reductant
Potential (V)
2 HF(aq)
3.05
⇌
H2O(aq)
2.80
⇌
H2O(aq)
2.42
⇌
H2O + O2(g)
2.08
⇌
2H2O
1.78
⇌
2H2O
1.70
⇌
H2O + MnO2(s)
1.70
−
⇌
H2O + Br (aq)
1.59
ClO2(g) + 4 H+ + 5 e−
⇌
2H2O + Cl−(aq)
1.57
HClO(aq) + H+ + 2 e−
⇌
H2O + Cl−(aq)
1.49
⇌
−
1.36
2H2O(aq)
1.23
+
−
Chlorine
Cl2(g) + 2 e
Oxygen (O2)
O2(g) + 4 H+ + 4 e−
⇌
2Cl
Table 1.6 - Comparison of oxidizing potential of several famous oxidants. [34]
17
In general AOP mechanism involve two major stages, first formation of strong oxidants, and
second generated oxidizing species oxidize organic contaminants mainly by hydrogen abstraction
or by electrophilic addition to double bonds to generate organic free radicals (∙R), which can react
with oxygen molecules forming perhydroxyl radicals (∙O2H) and initiate oxidative degradation
chain reactions that may lead to the complete/partial oxidation of the organics, in which the end
products will be mainly water, carbon dioxide and perhaps salts (mineralization) and may also
some amounts of biodegradable compounds (partial oxidation). [32]
(1)
∙RH
(2)
∙OH
(3)
∙R
(4)
∙R
(5)
∙OOR
+
+
+
+
+
∙OH
∙OH
∙ H2O2
∙ O2
∙ RH
→
→
→
→
→
∙R
+
∙ H2O
+
∙OH
+
∙R
∙ H2O2
∙ ROH
∙∙OOR
∙ ROOH
Table 1.7 - A common reaction is the abstraction of hydrogen atom to initiate a radical chain oxidation.
More than anything else destruction of pollutants in AOPs is based on formation of hydroxyl
radicals, because of their high oxidation capability, which enable them to attack the most organic
molecules rapidly and non-selectively. Basically formation of hydroxyl radicals in AOPs needs
one or more primary oxidant chemicals as source of ∙OH (i.e. O3, H2O2 and O2), or energy sources
for producing ∙OH (e.g. UV), or using a combination of both of them as a hybrid process, in
which they may involve some catalysts (e.g. TiO2). Accordingly there are several types of
advanced oxidation processes that each one may use similar or different mechanism and
technologies for producing hydroxyl radicals, other radicals and different oxidant species which
can be applied in water and can virtually oxidize any compound present in the water matrix. [35]
Figure 1.7 - Some characteristic features of hydroxyl radical. [36]
18
In water treatment applications, classic AOPs usually refer to a specific subset of processes that
involve O3, H2O2, and/or UV light. Also Fenton oxidation method that is a mixture of H2O2 and
Iron catalyst (a soluble Fe2+ salt), known as the oldest and widely used chemical AOP for
industrial wastewater treatment. However, nowadays AOPs are used to refer to a general group
of processes as bellow that using several oxidant chemicals and energy sources through different
single and hybrid processes; [32], [37]

Chemical Processes:
-
Ozonation: O3 , O3/UV||US , O3/Catalyst , O3/UV/Catalyst
-
Peroxidation: H2O2 , H2O2/UV||MW||US , H2O2/UV/MW , H2O2/Catalyst ,
H2O2/UV/Catalyst , H2O2/Catalyst/UV/MW
-
Peroxonation: O3/H2O2 , O3/H2O2/UV , O3/H2O2/Catalyst , O3/H2O2/Catalyst/UV ,
O3/H2O2/Catalyst/UV/MW
-
−
2−
2−
Per-Sulfate Oxidation: (HSO−
5 ||HS2 O8 ||SO5 ||S2 O8 )/Heat||UV||MW||US||Catalyst
-
Fenton’s Reagent: Dark Fenton (H2O2/Fe2+) , Electro-Fenton (Anodic & Cathodic) ,
Photo-Fenton (H2O2/Fe2+/UV||Visible||Solar) , Sono-Fenton (H2O2/Fe2+/US||HC) ,
Photo-Electro-Fenton , Sono-Electro-Fenton , Sono-Photo-Electro-Fenton (SPEF) ,
Microwave-Fenton , Fenton-Microbial , and Heterogeneous Fenton-like processes

Electrochemical processes: Direct anodic oxidation (∙OH generation by electron transfer) ,
Indirect oxidation (oxidant species generation via electrochemical oxidation of liquid bulk) ,
Sono-Electrochemical oxidation (US/EO) , and Electro-Fenton oxidation (EF)

Photochemical processes: Photolysis , Homogeneous||Heterogeneous Photo-Catalysis and
Photo-Electro-catalysis , and Photo-degradation (Photo-Oxidation)

Sonochemical processes: Ultrasound Sonolysis (US) , Hydraulic Cavitation (HC) ,
Homogeneous||Heterogeneous Sono-Catalysis (US/Catalyst) , Electro-Hydraulic cavitation ,
Sono-Fenton-like reactions, Microwave assisted Sonolysis, Sono-Photolysis (US/UV),
US/H2O2 , US/O2 , US/H2O2/ Fe||Fe2+||Fe3+

Radiolysis (Ionizing Radiation): Gamma ray, X-ray, and Electron Beam

Thermal processes: Wet Air Oxidation, and Supercritical Water Oxidation (SCWO)

Non-Thermal Processes: Non-Thermal Plasmas, and Pulsed Corona Discharge
19
Advanced oxidation processes hold several advantages that are specially unparalleled in the field
of water/wastewater treatment, such as ability to destroy wide range of organic pollutants and
removal of some inorganics (i.e. heavy metals), and also disinfection ability through inactivating
of bacteria, amoebas, viruses, etc. However beside water decontamination, they are good in air
purification, odour elimination, and soil remediation, but it should be realized that AOPs are not
perfect and they also have several drawbacks.
Advantages
Able to eliminate large variety of organic compounds and some inorganics effectively.
Able to full mineralization of pollutants.
Able to destroy of refractory compounds resistant to other treatment processes.
Able to remove some heavy metals from aquatic environment in forms of precipitated M(OH)x .
Able to use in high toxicity wastewaters that adversely affect biological treatment systems.
Generated ∙OH virtually react with almost every aqueous pollutants non-selectively & immediately.
Accepted to use for in situ treatment of soil and water matrix.
Develop byproducts reaction intermediates that submitted to a post treatment may be mineralized.
Able to improve organoleptic properties of treated water (i.e. drinking water).
Disinfection process could also be achieved as an integrated part for air and water treatment.
Present high power with high oxidizing reaction kinetics.
Theoretically do not introduce any new hazardous substances into the water.
Relatively cheap to install.
Disadvantages
High costs because of high energy consumption and expensive chemical reagents.
Not all of them are available at appropriate scales.
Not all of them apply to wastewater with high organic capability, turbidity, optical or color.
They may not individually handle a large amount of wastewater.
They usually require a pre-treatment step of wastewater to ensure reliable performance.
Free radical scavengers (i.e. HCO̅3) can inhibit contaminant destruction efficiency.
Excessive dosages of chemical oxidizers may act as a scavenger.
Formation of oxidation intermediates that potentially are toxic.
Engineers are required for their design and often also for operation.
Handling, safety, protection, repair and maintenance service of their related equipment are costly and
difficult due to presence of strong oxidant species.
Table 1.8 - General advantages and disadvantages of advanced oxidation processes.
20
1.4. Photochemical Processes
Photochemical process is a chemical reaction initiated by the absorption of energy in the form of
light. Generally, this term is used to describe a chemical reaction caused by absorption of specific
range of electromagnetic waves, including ultraviolet (wavelength from 100 to 400 nm), visible
light (400 - 750 nm) or infrared radiation (750 - 2500 nm). The photo-chemical technologies
present the advantages to be simple, clean, relatively inexpensive, and generally more efficient
than chemical AOPs. Beside pollutants destruction capability, they also have acceptable abilities
in water disinfection. [38]
Figure 1.8 - Spectrum of electromagnetic waves.
During a photo-chemical reaction, the energy of absorbed photon is transferred to electron in the
absorptive molecule and briefly changes its configuration via promoting the molecule from a
ground state to an excited state. The excited state will represent what is essentially a new
molecule, because frequently an excited state molecule is not kinetically stable in the presence
of air or water, even sometimes by itself, therefore spontaneously will decompose through
oxidation, hydrolysis or dissociation. Excited state molecules can also decompose to produce
high energy species such as radicals, and naturally unstable fragments will react with other
molecules around them. These processes are collectively referred to as direct or indirect
photolytic destruction, and both mechanisms contribute to the removal of pollutants. [39]
Depend on reagents that are present in a photo-chemical process, absorbed photon can interact
in the process through three chemical mechanisms, including photolysis, photo-catalysis, and
photo-degradation, which enables them to degrade and/or destroy micro-pollutants. [29]
Photolysis (photo-decomposition or photo-dissociation) is a chemical reaction in which a
chemical compound is directly broken down by absorption of one or more photons with sufficient
energy (specific wavelength) that can affect the chemical bonds of the target molecule. [40]
𝐌 𝐨 (ground state) + 𝒉𝝂 (specific wavelength (𝜆)) → 𝐌 ∗ (exited state)
𝐀𝐁 + 𝒉𝝂 → 𝐀 + 𝐁
21
Photo-degradation (or photo-oxidation) is an oxidizing chemical reaction in which a chemical
compound is oxidized by generated radicals from other present chemicals such as ozone, oxygen,
hydrogen peroxide, and per-sulfates that are photolytically excited via photon absorption. Also
radicals and other oxidant species can generate through photolysis of the target molecule. [41]
𝐎𝟑
+
𝒉𝝂
→
∙𝐎∙
+ 𝐎𝟐
𝐎𝟐
+
𝒉𝝂
→
∙𝐎−𝐎∙
𝐇𝟐 𝐎𝟐 +
𝒉𝝂
→
∙ 𝐎𝐇 + ∙ 𝐎𝐇
𝐒𝟐 𝐎−
𝟖
+
𝒉𝝂
→
−
∙ 𝐒𝐎−
𝟒 + ∙ 𝐒𝐎𝟒
𝐇𝐒𝐎−
𝟓 +
𝒉𝝂
→
∙ 𝐒𝐎−
𝟒 + ∙ 𝐎𝐇
Photo-catalysis is the acceleration of a photo-reaction in the presence of a catalyst, which its
broad definition involves two distinct categories, photo-generated catalysis and catalyzed
photolysis. The former process is production of a catalyst by specific photo-reaction that also
named photo-production of a catalyst. But catalyzed photolysis is a photo-catalytic oxidation
mechanism, in which a catalyst molecule is electronically excited via photon absorption that
enables the catalyst to create electron-hole pairs (e−–h+) for generating oxidant species such as
hydroxyl radicals and superoxide anions, which are able to undergo secondary reactions leading
to degradation and mineralization of pollutants. During a photo-catalytic reaction either the
catalyst molecule or the substrate molecule, or both, are in an electronically excited state during
the catalytic step. Depend on the nature and the phase of catalysts, photo-catalytic reactions can
take place homogeneously as an enhanced photo-chemical process (i.e. photo-Fenton reaction)
or heterogeneously (i.e. semiconductor photo-catalysis). [36], [42]
1.5. Heterogeneous Photo-Catalysis
The heterogeneous photo-catalysis that also named semiconductor photo-catalysis, is a process
based on the absorption of usually UV to visible photons by a solid semiconductor, in which the
degradation reaction or transformation of pollutants may occur in the interface area between the
solution and the electrically excited solid, but without changing the chemical structure of the
semiconductor. [43]
The electronic structure of such semiconductor materials (usually metal oxides) comprises a
highest occupied band full of electrons (valance band), and a lowest unoccupied band without
electrons (conductance band), which are separated by a region that is largely devoid of energy
levels, and the difference in energy between the two bands is called the band-gap energy. [33]
Theoretically, when the absorbed photons have an energy equal to or greater than the band-gap
energy (ultra-bandgap illumination) of the semiconductor, electron-hole pairs are formed, which
can either recombine to liberate heat, or migrate to the surface of the semiconductor material,
22
where they have the possibility of reacting with adsorbed target molecules on the surface of the
semiconductor catalyst, which can directly react with adsorbed target molecules on the surface
of the catalyst, or they can generate radicals from the bulk (air/water) that can indirectly oxidize
the target molecules. Accordingly, generated valence band holes can react with adsorbed electron
donors (directly or indirectly) to generate oxidized products, and similarly, generated conduction
band electrons can react with electron acceptors (directly or indirectly) to generate reduced
products. Electrons may reduce absorbed oxygen, ultimately to water, and holes may oxidize any
adsorbed pollutant, ultimately to its mineral form, that is usually carbon dioxide and water. On
the other hand, recombination, either at the surface, or in the bulk of the semiconductor catalyst,
is the usual fate of photo-generated electron-hole pairs. [29], [33]
Figure 1.9 - Schematic illustration of the major
processes that occur on a semiconductor photocatalyst particle upon ultra-bandgap excitation in
an aqueous solution containing dissolved oxygen
and a pollutant. The processes include:
.
(a) electron-hole recombination in the bulk,
(b) electron–hole recombination at the surface,
(c) direct or indirect (through trap sites) reduction
of oxygen, or oxidizing intermediates by the
photo-generated electron at the surface of the
semiconductor and (d) direct or indirect (through
trap sites) oxidation of the pollutant, or an oxidized
intermediate by the photo-generated hole at the
surface of the semiconductor, leading eventually
to the mineralization of the pollutant. [33]
In the case of water purification as an AOP, photo-degradation in the presence of suspended
semiconductor particles may be either by a direct process, by organic molecules adsorbed on the
surface of the particles which interact with holes and hydroxyl radicals on the surface, or they
may be indirect, by interaction of the organic molecules with hydroxyl radicals in the bulk of the
solution. Generally, the electron acceptor is invariably dissolved oxygen, and the electron donor
is the pollutant. Under these circumstances, the overall process is the semiconductor photocatalyzed oxidative mineralization of the pollutant by dissolved oxygen in the water. [33], [44]
Figure 1.10 - Scheme of semiconductor photo-catalysis mechanism when hν≥Ebg.
23
1.5.1. Titanium Dioxide Photo-Catalyst
Titanium dioxide, also known as Titanium peroxide or Titania, is the naturally occurring oxide
of titanium, with chemical formula TiO2, which exists in nature as three well-known crystalline
form, including rutile, anatase and brookite. It has a wide range of applications that the most
important areas are paints and varnishes as well as paper and plastics, which account for about
80% of the world's titanium dioxide consumption. Other pigment applications such as printing
inks, fibers, rubber, cosmetic products and foodstuffs account for another 8%. The rest is used in
other applications, for instance the production of technical pure titanium, glass, glass and
electrical ceramics, catalysts, electric conductors and chemical intermediates. [45]
Appearance
White solid powder
Structure
IUPAC Name
Other Names
Chemical Formula
Molar Mass
Density
Melting Point
Boiling Point
Flash Point
Solubility in water
Vapour Pressure
Odour
Bandgap
Risk Phrases
Safety Phrases
Titanium dioxide and Titanium(IV) oxide
Titania, Rutile, Anatase, Brookite, and Titanium peroxide
TiO2
79.866 g/mole
4.23 g/cm3 (Rutile)
&
3.78 g/cm3 (Anatase)
1843 °C
2972 °C
Non-flammable
Insoluble
~0 mmHg
Odourless
3.05 eV (Rutile)
&
3.2 eV (Anatase)
R20: Harmful by inhalation.
R20/21/22: Harmful by inhalation, in contact with skin and if swallowed.
R36/37: Dust may irritate eyes and respiratory system.
S26: In case of contact with eyes, rinse immediately with plenty of water and seek
medical advice.
S25: Avoid contact with eyes.
S36/37: Wear suitable protective clothing and gloves.
Flammability: 0
Intrinsically noncombustible material.
Health: 1
Exposure would cause irritation with only minor residual injury.
Instability/Reactivity: 0
Normally stable, even under fire exposure conditions, and is not reactive with water.
Special Notice:
Its dust particles deposition in lungs causes impaired lung clearance, cell injury,
fibrosis, mutations and ultimately respiratory tract cancer.
Table 1.9 - Chemical infobox of Titanium Dioxide. [45]
NFPA 704
24
In 1972, for the first time Fujishima and Honda showed the possibility of using the photo-excited
semiconductor TiO2 to split water molecule into H2 and O2 in a photo-electrochemical solar cell.
Subsequently, fundamental works of Mills & Lee Hunte, Fujishima et al., and Pelaez et al. led to
the development of a new AOP technology, based on semiconductor photo-catalysis, for
numerous environmental and energy applications. This so-called heterogeneous photo-catalysis
involves irradiation of TiO2 (preferably in the form of rutile in front of anatase: Degussa-P25) as
a semiconducting catalyst, by near-UV light, which is easily photo-excited to form electrondonating and electron-accepting sites, permitting to induce redox reactions. [46], [47], [48], [49]
𝐓𝐢𝐎𝟐 + 𝒉𝝂 (λ < 380 nm) ►►►
e−
cb
+
h+
vb
=> {
→ ∙ O−
2
−
+ 𝐇𝟐 𝐎 → ∙ 𝐎𝐇 + 𝐇 +
e−
cb + 𝐎𝟐
h+
vb
Titania has been mostly chosen for the application of heterogeneous photo-catalysis processes to
water treatment because it is a material close to being a practically ideal photo-catalyst in several
important aspects. It is highly stable chemically and biologically inert, very easy to produce,
inexpensive, active from the photo-catalysis standpoint, and it has an energy gap comparable to
that of solar photons. Moreover, the photo-generated holes are enough strong oxidants to oxidize
water to create hydroxyl radicals, and the photo-generated electrons are reducing enough to yield
superoxide from oxygen molecule. Also its photo-generated holes are able to either directly
oxidize the absorbed pollutants or as the most favored degradation pathway, oxidize the hydroxyl
groups located at its surface to form hydroxyl radicals that interact with pollutants and oxidize
them indirectly. In addition, it is possible to enhance this process by increasing the number of
hydroxyl radicals via adding H2O2 or O3 into the solution of photo-reactor, which can be
photolyzed by UV irradiation. Furthermore, it can be utilized either under dispersed form
(powder, aqueous suspension) or in thin film form. [29], [48], [50]
Figure 1.11 - Scheme of TiO2 photo-catalysis of organic pollutants in water.
25
Accordingly, the heterogeneous TiO2 photo-catalysis has been widely applied in recent years,
particularly in the case of degradation and destruction of refractory and persistent organic
pollutants (i.e. pesticides, pharmaceuticals, surfactants, endocrine disruptors, dyes, etc.) by other
conventional AOPs as hybrid processes. Also, this technology is generally efficient for treating
a substantial range of inorganic as well as organic pollutants, for instance toxic inorganic ions
such as cyanide, bromate, nitrite, and sulfite, are easily oxidized by this process into non-toxic or
weakly toxic compounds, respectively carbon dioxide, bromide, nitrate, and sulfate. Also,
disinfection is another ability of this technology as a side application that can completely destroy
pathogenic biologic pollutants, including viruses, bacteria, and mold. [29], [47]
Despite of outstanding characteristics of TiO2 as an ideal semiconductor photo-catalyst with a
large bandgap energy for water purification, it is hardly surprising that it does not absorb photons
with the wavelength more than 380 nm, which means it is not absorb visible light. By the same
token, TiO2 is only able to absorb UV photons, which unfortunately represents a small fraction
of the solar spectrum (maximum 6%), whereas an ideal semiconductor photo-catalyst should be
activated by sunlight.
Figure 1.12 - Spectrum of solar irradiation at three levels.
However, the very positive features of TiO2 as a semiconductor photo-catalyst far outweigh the
limitations of its spectral profile and thus, it has become the semiconducting material for research
in the field of semiconductor photo-catalysis for water purification. Accordingly, various
strategies have developed to modify titanium dioxide for converting it to a visible light active
TiO2 photo-catalytic material, including non-metal/metal doping, dye sensitization, and coupling
semiconductors. [33], [49]
26
1.5.2. Silver Phosphate Photo-Catalyst
Silver phosphate, also known as Silver orthophosphate, is a light sensitive yellow (darkens when
heated or exposed to light), and almost water-insoluble chemical compound, with chemical
formula Ag3PO4, which is formed as a precipitate by the reaction between a soluble silver
compound (i.e. silver nitrate), with a soluble orthophosphate compound (i.e. sodium phosphate).
It has several applications, including as a magnifying agent for phosphate in silver staining of
biological materials that is important in analytical chemistry, as a light sensitive agent in early
photography, as an incorporating antibacterial agent in bacteria killer coatings, and as a photocatalyst for the visible light photo-chemical splitting of water. [51]
Appearance
Translucent yellow
Structure
IUPAC Name
Silver(I) Phosphate
Other Names
Silver Orthophosphate, Argentous Phosphate, Phosphoric acid trisilver(I) salt
Chemical Formula Ag 3 PO4
Molar Mass
418.57 g/mole
Density
6.37 g/cm3 at 20 °C
Melting Point
849 °C
Flash Point
Non-flammable
Solubility in water
6.5 mg/L (Micro-soluble in water)
Vapour Pressure
~0 mmHg
Odour
Odourless
Bandgap
2.45 eV
Risk Phrases
R36/37/38: Irritating to eyes, respiratory system, and skin.
Safety Phrases
S26: In case of contact with eyes, rinse immediately with plenty of water and seek
medical advice.
S36: Wear suitable protective clothing.
Flammability: 0
Intrinsically noncombustible material.
Health: 2
Intense or continued but not chronic exposure could cause temporary incapacitation
or possible residual injury.
Instability/Reactivity: 0
Normally stable, even under fire exposure conditions, and is not reactive with water.
Special Notice:
It is an irritant material, which is light sensitive.
Table 1.10 - Chemical infobox of Silver Phosphate. [51]
NFPA 704
27
In 2010, Yi and his coworkers presented the pioneering work on exploring the photo-catalytic
properties of silver phosphate that exhibit extremely high photo-oxidative capabilities for the
oxygen evolution from water and the decomposition of organic pollutants under visible light
irradiation. Analysis of the absorption spectrum revealed that Ag3PO4 has an indirect bandgap of
2.43 eV and a direct bandgap of 2.61 eV, which can absorb part of solar energy with a wavelength
shorter than ~530 nm. The most interesting thing about this novel photo-catalyst is its quantum
efficiency that achieves up to 90% at wavelengths greater than 420 nm, which is significantly
higher than the best previous reported values. Also at wavelengths less than 480 nm, the
recombination of photo-excited electrons and holes within the catalyst is very weak. Accordingly,
the actual photo-degradation rate of organic pollutants over Ag3PO4 gets dozens of times faster
than the rate over the Degussa-P25 TiO2, BiVO4, and commercial N-doped TiO2. [52]
The morphology of silver phosphate’s crystals directly affects the properties of the catalyst, and
researches show that all various synthesized Ag3PO4 nanostructures including spherical grains,
cubes, rhombic dodecahedrons, concave trisoctahedrons, and tetrapods exhibit much more
excellent photo-catalytic activity than mentioned catalysts, under both UV and visible light
irradiation. Among them, the rhombic dodecahedrons exhibits much higher photo-catalytic
activity in comparison to other shapes in degradation of organic contaminants. [53]
The experimental energy bandgap value of silver phosphate is equal to 2.45 eV, in which the
bottom of conduction band is well dispersive in comparison to the top of valence band, which
indicates that the photo-generated electrons possess smaller effective mass and, therefore, higher
migration ability. The redox ability of Ag3PO4 is evaluated by determining the energy positions
of valence and conduction bands, in which its valence band maximum potential is equal to 2.67 V,
which is more positive than electrode potential of O2/H2O (1.23 V), indicating that Ag3PO4 has
the ability to oxidize H2O to produce O2 or oxidizing the pollutants, whereas the conduction band
minimum potential of Ag3PO4 is equal to 0.24 V, which is lesser than H+/H2 (0 V) and
unfortunately it cannot reduce H+ to produce H2. [54]
Figure 1.13 - Calculated VBM and CBM potentials of Ag3PO4. [53]
28
Figure 1.14 - XRD patterns of the Ag3PO4 powders. Inset: Schematic drawing of the crystal structure. [52]
Figure 1.15 - SEM images of different crystal shapes of Ag3PO4 and their ultraviolet-visible diffusive
absorption spectrums. [52], [53]
29
Table 1.11 - Preparation and photo-catalytic properties of Ag3PO4 with different morphologies. [55]
30
Researches show that silver phosphate is the highest performing light-sensitive semiconductor
photo-catalyst for oxygen evolution in comparison to mentioned commercial photo-catalysts,
which produces vigorous bubbles of O2, and also ∙OH and ∙O2− radicals, as soon as light
irradiation commenced. It should be the most important and desirable advantage of silver
phosphate for its environmental applications, especially treatment of micro-pollutants. [52]
As mentioned before, since the electrode potential of Ag/Ag3PO4 is lower than that of the
hydrogen electrode, unfortunately the Ag3PO4 semiconductor would decompose during the water
oxidization if no sacrificial reagent was involved. Aside the large amount of silver as an
expensive noble metal, this insufficiency perhaps is the most important drawback of this catalyst.
It is worth mentioning that even without sacrificial reagent, the silver phosphate still shows strong
photo-oxidative ability. [52]
However, finding the silver orthophosphate as the novel semiconductor photo-catalyst,
potentially opens an avenue for solving current energy crisis and environment problems with
abundant solar light. Despite the fact that Ag3PO4 is a promising candidate for environmental
remediation and renewable energy, its mentioned drawbacks strongly limit its practical
environmental applications. Accordingly, many efforts have been devoted to further improving
the photo-catalytic stability of Ag3PO4 while maintaining its high photo-catalytic activity or even
improving and optimizing its photo-electric and photo-catalytic properties. [53]
Figure 1.16 - Schematic drawing of redox potentials of Ag3PO4.
31
1.5.3. Silver Phosphate Based Composite Photo-Catalysts
Among the various photo-catalysts developed, titanium dioxide is undoubtedly the most popular
and widely used photo-catalyst since it is of low cost, high photo-catalytic activity, chemical and
photo-chemical stability. However, TiO2 is not ideal for all purposes and performs rather poorly
in processes associated with solar photo-catalysis due to its wide bandgap (3-3.2 eV), thus
making impractical overall technological process based on TiO2. Generally, two strategies have
been proposed to design visible-light driven photo-catalysts, including modification of wide
bandgap photo-catalysts such as TiO2 by doping or by producing hetero-junctions between them
and other materials, and exploration of novel semiconductor materials capable of absorbing
visible light, such as Ag3PO4. Accordingly, to harvest photons in visible region, many narrow
bandgap metal oxides or chalcogenides have been coupled with TiO2 to fabricate visible-light
photo-catalysts, which exhibit visible-light photo-catalytic activity to a certain extent. Such a
strategy is also applied to modify silver phosphate photo-catalyst to enhance its photo-catalytic
activity and/or improve its photo-catalytic stability. [53]
Figure 1.17 - Relationship between energy bandgap structures of various semiconductor photo-catalysts.
Wang et al. showed that the photo-catalytic activity of silver phosphate can be enhanced by
deposition of silver nanoparticles on the surface of Ag3PO4 catalyst particles, because part of the
photo-generated electrons are captured during Ag3PO4 decomposition reaction, which prevent
the recombination of electron-hole pairs within the Ag3PO4 catalyst particles at the initial stage
of photo-catalytic reactions. Improvement of Ag3PO4 stability by covering Ag o nanoparticles on
the surface of the catalyst particles is attributed to the localized surface plasmon resonance effects
of silver nanoparticles and a large negative charge of phosphate ions (PO3−
4 ), which effectively
inhibit the reducibility of silver ions (Ag + ) in the Ag3PO4 lattice. Also Ag3PO4 can be rejuvenated
from weak photo-catalytically active silver as a recyclable highly efficient photo-catalyst by
oxidizing Ag with produced H2O2 during the photo-catalysis, under a PO3−
4 ion atmosphere. [56]
32
However, this strategy is not ideal from the practical application perspective, because with
increasing the silver nanoparticle contents in order to increasing the photo-catalytic stability, the
photo-activity decreases due to the formation of silver layers on the surface of the catalyst particle
that first shield photon absorption, second inhibit the transfer of holes from the valance band of
silver phosphate to the interface between photo-catalyst surface and solution, and third hinder the
contact of oxidizing molecules (pollutants or water) with catalyst surface, and accordingly, the
photo-catalytic activity deteriorates gradually. Thus, fabrication of silver phosphate based
composite photo-catalysts with high photo-catalytic activity and excellent stability as well as
lower silver usage for their large scale applications is desirable. [56], [57]
Yao et al. synthesized Ag3PO4/TiO2 visible-light driven photo-catalyst by depositing of Ag3PO4
nanoparticles onto the surface of TiO2 (Degussa-P25), which the hetero-structured photo-catalyst
shows enhanced activity and much more stability in comparison with unsupported Ag3PO4. The
enhanced activity is because of effective electron-hole separation and larger surface area of the
composite photo-catalyst, while the enhanced stability is due to the chemical adsorption of silver
cations (Ag + ) in Ag3PO4 and oxygen anions (O− ) in TiO2. Besides that, the silver weight
percentage of the synthesized photo-catalyst in comparison to pure silver phosphate decreases
from 77% to 47%, which means significantly reducing the cost of Ag3PO4 based photo-catalysts
for the Ag3PO4/TiO2 composite. [58]
Liu et al. synthesized Ag3PO4/TiO2 composite hetero-structures photo-catalyst by depositing of
Ag3PO4 nanoparticles onto the surface of TiO2 nanobelts, which UV photo-catalytic activity,
stability and reusability of the designed photo-catalyst was substantially enhanced in comparison
to silver phosphate nanoparticles or titanium dioxide nanobelts alone. These results were
attributed to the improved charge separation of the photo-generated holes and electrons under
UV irradiation at the hetero-structures photo-catalyst interface and/or surfactant-like, and
function of the nanobelts in stabilizing the silver phosphate nanoparticles, which cause
Ag3PO4/TiO2 composite hetero-structures be more desirable in long-term applications with
enhanced chemical stability and photo-catalytic activity. [59]
Lee et al. fabricated a novel hetero-junction structure of Ag3PO4-core/TiO2-shell by covering the
silver phosphate nanoparticles with polycrystalline titanium dioxide by sol-gel method, which
notably enhanced photo-catalytic activity of the prepared composites photo-catalyst in
decomposing of gaseous 2-propanol and evolving CO2 compared to bare Ag3PO4 and TiO2. They
inferred that, since the valence band level of silver phosphate with +2.67 V (versus NHE) is lower
than that of titanium dioxide with +3 V (versus NHE), Ag3PO4 can be severed as an appropriate
sensitizer for TiO2, accordingly unique relative band positions of the two semiconductors cause
unusually high visible-light photo-catalytic activity of composite catalyst that shows. [60]
33
Yang et al. used chemically derived graphene oxide with oxygen-containing functional groups
in construction of Ag3PO4/TiO2 composite photo-catalysts, which improved photo-catalytic
performance in efficient removal of organic contaminants and enhanced bacterial inactivation
under visible-light irradiation. In this bifunctional nanocomposite catalyst, negatively-charged
active sites of graphene oxide on its high-surface-area sheets enhanced visible-light absorption
and obviously leads to an improved visible-light photo-catalytic performance. Also geraphene
oxide sheets could facilitate charge transfer and suppress the recombination of photo-generated
holes and electrons in the photo-catalytic system. Besides comparative low-cost fabrication of
the TiO2/Ag3PO4/GR nanocomposite photo-catalyst, better structural stability and recyclability
under visible-light irradiation were also observed. [61]
Bi and coworkers have reported that AgX/Ag3PO4 (X = Cl, Br, I) hetero-crystals prepared by in
situ ion-exchange method show more promising and fascinating advantages in comparison to the
pure silver phosphate. Among them, AgBr/Ag3PO4 hybrid synthesized catalyst displayed much
higher stability and photo-catalytic activity than single silver bromide or silver phosphate under
visible-light irradiation. The high stability was attributed to the formed Ag@AgBr/Ag3PO4@Ag
plasmonic system, which effectively retains the activity of hybrid photo-catalyst due to the
efficient transfer of photo-induced electrons. [62], [63]
To sum up, fabricating of hetero-structure composites is a routine strategy to enhance the activity
of photo-catalysts as well as to overcome some application barriers. Accordingly, Ag3PO4 based
composite photo-catalysts such as TiO2/Ag3PO4 (decreasing the silver content to reduce cost),
Ag3PO4/Carbon Nanotube-stabilized Pickering emulsion (enhancing activity by surfacechemical design of novel micro-reaction system), Fe3O4/Ag3PO4 (magnetic separable),
Ag@(Ag2S/Ag3PO4) (facilitating migration of charge carriers and enhancing activity via
synergistic effect of Ag and Ag2S), AgX/Ag3PO4 (improving stability via core-shell structure),
In(OH)3/Ag3PO4 (enhancing absorption by tuning surface electric property), Ag3PO4/Graphene
and Graphene Oxide (increasing the photon absorption) and so forth have been successfully
synthesized and studied. Despite many of these hetero-structure composites photo-catalysts being
effective for the degradation of organic pollutants, water splitting, oxygen evolution, and
disinfection up to date, the present achievements are still far from the ideal goal. [55]
34
Chapter 2
EXPERIMENTAL
35
2.1. Equipment
For all experiments, operating temperature and pressure were adjusted at an almost constant
temperature (25-27 °C) and atmospheric pressure respectively. The related pH and temperature
were measured by METTLER TOLEDO - 7 Compact pH Meter (S230/Conductivity). Weighing
the catalysts was done by OHAUS - Adventurer Analytical Balance (AR3130). Sampling for
measurement was done by DRAGONMED Mechanical Micropipette (100-1000 μl). For sample
holding 1.5 ml Eppendorf vials and 2 ml HPLC glass vials were used as the primary and the
secondary sample reservoir respectively. Also normal 5 ml medical syringes and WHATMAN 0.2 μm PVDF syringe filters were used for filtering the samples.
Figure 2.1 - Set of related used equipment in all experiments.
36
2.1.1. Reactor
All experiments were conducted in a 500 ml cylindrical pyrex glass reactor with 8 cm diameter
that was fully covered by an aluminium foil jacket in order to avoid the effect of ambient light.
Also a prefect performance heating magnetic stirrer with a 3 cm teflon coted magnet stir bar was
used for mixing the batch reactor at 380 rpm for all the experiments.
Figure 2.2 - Set of related things that were used for reactor and stirring system.
37
2.1.2. Irradiation Source
Required solar irradiation was performed using an Oriel LCS - 100 (ABB class) Solar Simulator
equipped with a 100 W Xenon Ozone-free lamp and an Air Mass 1.5 Global Filter simulating
solar radiation reaching the surface of the earth at a zenith angle of 48.2°. According to the related
spectral irradiance data given by the manufacturer, the simulated solar radiation contains about
5% UVA radiation, and 0.1% UVB radiation, while the filter cuts radiations with wavelengths
below 280 nm. In addition, two optic filters were used for providing visible part & UV part of
solar irradiation that cut solar photons with the wavelengths less than and more than 420 nm
respectively. The incident radiation intensity on the photochemical reactor in the UV region of
the electromagnetic spectrum was measured using Ferrioxalate as the chemical actinometer and
it was found to be 1.3×10-4 E/(m2.s). For those runs performed under UV and visible light, two
filters with UV Bandpass 420 nm and Cut-on 420 nm ability were employed (Newport FSQ-UG5
and FSQ-GG420, 50.8 mm×50.8 mm); in this case, the intensity was measured at 7×10-5 E/(m2.s).
Related specifications to the solar simulator and its two visible and ultra-violet optical filters are
presented in figures 2.4 to 2.6. [64], [65], [66]
Figure 2.3 - Solar simulator system and its related visible and UV optic filters. [64]
38
Figure 2.4 - Spectral output of LCS-100 Solar Simulator with standard AM1.5G filter. [64]
Figure 2.5 - Specification related to Newport FSQ-UG5 (UV Bandpass 420 nm) optic filter. [65]
Figure 2.6 - Specification related to Newport FSQ-GG420 (Cut-on 420 nm) optic filter. [66]
39
2.1.3. Analytical Techniques
The Waters Alliance 2695 - High Performance Liquid Chromatography (HPLC) was employed
to monitor the concentration of BPA. Separation was achieved on a Kinetex XB-C18 100A
column (2.6 μm, 2.1 mm×50 mm) and a 0.5 μm inline filter (KrudKatcher Ultra). The mobile
phase consisting of 68:32 UPW:Acetonitrile eluted isocratically at 0.32 mL/min and 45 °C, while
the injection volume was 20 μL. A Waters 474 - Scanning Fluorescence Detector (FLD) for
measuring in microgram range and a Waters 2996 - Photo Diode Array Detector (PDA) for
measuring in milligram range were connected to the Waters Alliance 2695 HPLC. For our
mesurment the detection was achieved through the Waters 474 FLD, in which the related
excitation wavelength was 280 nm and the emission wavelength was 305 nm. Under these
conditions, the related retention time was 1.56 min, also the limit of detection was 4.7 μg/L and
the limit of quantitation was 12.4 μg/L. [67], [68], [69]
Figure 2.7 - The HPLC system, FLD and PDA detectors. [70], [71], [72]
40
2.2. Materials
Most of the experiments were carried out in ultrapure water with pH equal to ~6, taken from the
Barnstead Thermolyne Easypure RF Water Filtration Purification System (Model: D7031).
Bisphenol A (C15H16O2, CAS # 80-05-7) was purchased from Sigma-Aldrich that 220 μg/L added
to ultrapure water as the target pollutant.
Commercially available drinking bottled water with pH equal to 7.5 and 400 μS/cm conductivity
that containing 211 mg/L bicarbonate, 10 mg/L chloride, 15 mg/L sulphate, 5 mg/L nitrate and
78 mg/L of various metal ions, which is used instead of ultrapure water and was employed for
the water matrix experiments. Sodium bicarbonate (NaHCO3, CAS # 144-55-8) was purchased
from Sigma-Aldrich that 500 mg/L added to ultrapure water as a radical scavenger for the water
matrix experiments. Humic acid (C187H186O89N9S1, CAS # 1415-93-6) was purchased from
Sigma-Aldrich that 3 mg/L added to ultrapure water for the water matrix experiments. Secondary
treated wastewater effluent with pH equal to 8 and COD equal to 21 mg/L, was taken from the
campus wastewater treatment plant of the University of Patras, which is used instead of ultrapure
water for the water matrix experiments.
Methanol (CH3OH, CAS # 67-56-1) was purchased from Sigma-Aldrich that 0.4 ml was preinjected in all primary sample reservoirs in order to stop the probable reactions as a radical
scavenger, to solve the absorbed BPA on the surface of catalyst particles and also to solve the
trapped BPA in the texture of the micro-filter.
Phosphoric acid (H3PO4, CAS # 7664-38-2) and Sodium hydroxide (NaOH, CAS # 1310-73-2)
were purchased from Sigma-Aldrich that added to aqueous solution for pH adjustment to acidic
and alkaline conditions.
Degussa P25 Titania (TiO2, CAS # 13463-67-7), Silver nitrate (AgNO3, CAS # 7761-88-8) and
Monosodium phosphate (NaH2PO4, CAS # 7558-80-7) were purchased from Sigma-Aldrich that
were used in catalyst synthesizing, in which silver nitrate and monosodium phosphate were used
for producing silver phosphate as a component of composite catalyst, and titanium dioxide was
used as a component of composite catalyst, and also in some experiments as a photo-catalyst.
41
2.3. Catalyst
Silver orthophosphate - Titanium dioxide (Ag3PO4/TiO2) composite photo-catalysts in powder
form were synthesized employing a simple method. For this, an appropriate amount of
commercial TiO2 (Degussa P25) particles was dispersed in 50 mL distilled water and sonicated
for 15 minutes. After sonication, 3.05766 g of AgNO3 was added to this solution and left under
magnetically stirring for 15 minutes. After that, 1.91 g of NaH2PO4 dissolved in 50 mL distilled
water, was dropwise added into the solution and kept stirring for 240 min at room temperature.
The Ag3PO4/TiO2 composites formed after that procedure were filtered off, washed with water
and ethanol and dried in an oven at 60 °C for 12 hours. Pure Ag3PO4 was prepared using the same
method but in the absence of TiO2. [73]
A partial aim of this study was to investigate the effect of TiO2 content on the activity and stability
of the composite Ag3PO4/TiO2 photo-catalyst. For this reason the content of Ag3PO4 was
unchanged whereas by changing the concentration of TiO2 powders in the starting solution a
series of composite photo-catalysts with variable molar ratios of Ag3PO4 to TiO2 were prepared.
The molar ratio of Ag3PO4 to TiO2 was 6:1, 3:1, 1:1, and 3:10 named as Ag3PO4/TiO2 [6:1],
Ag3PO4/TiO2 [3:1], Ag3PO4/TiO2 [1:1] and Ag3PO4/TiO2 [3:10] respectively.
2.3.1 Measurements of specific surface areas
The specific surface areas (SSA) of the synthesized materials were determined with the BrunauerEmmett-Teler (BET) method with the use of a Micromeritics (Gemini III 2375) instrument,
employing Nitrogen physisorption at the temperature of liquid nitrogen (77 K). Prior to each
measurement, the sample was outgassed under dynamic vacuum at 250 °C for 2 hours.
2.3.2. X-ray diffraction (XRD) measurements
The X-ray diffraction (XRD) patterns of the catalyst powders were obtained using a Brucker D8
Advance instrument employing Cu Ka source (λ= 1.5496 Å) operated at 40 kV and 40 mA. Data
were collected in the 2θ range of 2° to 85° at a scan rate of 0.05° s-1 and a step size of 0.015°.
Phase identification was based on JCPDS cards. The primary crystallite size of nanocrystals was
estimated by means of the Debye-Scherrer’s formula; [74]
d
0.9
B cos 
Equation (I)
where λ is the X-ray wavelength corresponding to Cu Ka radiation (0.15406 nm), θ is the
diffraction angle and B is the line broadening (in radians) at half of its maximum. Diffraction
peaks used for the estimation of d were located at angle (2θ) of 25.6° and 33.5°.
42
2.3.3. Diffuse Reflectance Spectroscopy (DRS)
The diffuse reflectance spectra were recorded on a UV-Vis spectrophotometer (Varian Cary 3)
equipped with an integration sphere, using BaSO4 as a reference. The catalyst powders were
loaded into a quartz cell and spectra were obtained at room temperature in the wavelength range
of 200-800 nm. The DR measurements were converted into the equivalent absorption coefficient
by applying the transformation based on the Kubelka-Munk function;
K ( ) 1  R 
Equation (II)

S ( )
2 R
where K and S are the absorption and scattering coefficients, respectively, and R∞ = R / Rref is the
2
F ( R ) 
reflectance. The optical band gaps of the semiconductors were evaluated based on the following
expression; [75], [76]
 h 
1/ n
 B  h  Ebg 
Equation (III)
where a is the absorption coefficient, hν is the incident photon energy, Ebg is the band gap energy,
B is a constant related to the effective masses of charge carriers associated with valance and
conduction bands, and n is a factor that depends on the kind of optical transition induced by
photon absorption. Band gap energies (absorption thresholds) were estimated assuming that F(R)
values are proportional to the optical absorption coefficients and that the synthesized materials
are indirect semiconductors, for which n is equal to 2. Thus, the values of Ebg were obtained from
the plot of [F(R)hν]1/2 versus hν (Tauc plot) in the region of high absorption and the extrapolation
of the linear region to the horizontal axis, at zero F(R). [77], [78]
The XRD patterns obtained for Ag3PO4 [Pure] and the Ag3PO4/TiO2 composites are presented in
Fig. 2.8. For comparison the XRD pattern of commercial TiO2 [P25] is also included in the
mentioned figure. It is clear that all the diffraction peaks of the as prepared Ag3PO4 [Pure],
Ag3PO4/TiO2 [6:1] and Ag3PO4/TiO2 [3:1] catalysts can be indexed to the cubic structure of
standard Ag3PO4 (JCPDS 01-1058). However, when TiO2 content increases to 3Ag:10Ti molar
ratio, an additional peak located at 2θ = 25.6° can be clearly discerned in the XRD spectra of
Ag3PO4/TiO2 [3:1] catalyst together with less distinguishable peaks located at 38.1°, 54.2° and
63.01°. These peaks are attributed to the Anatase phase of TiO2 (JCPDS 02-406). Meanwhile,
because the characteristic structure of Ag3PO4 kept in all composite samples, it is considered that
TiO2 should be merely placed on the surface of Ag3PO4 crystal without doped into the crystal
lattice. This is in accordance with previous studies. [79]
The XRD patterns of Fig. 2.8 were used to estimate the mean crystallite size of the phase detected
according to Equation (I). According to the Table 2.1, the calculated crystallite sizes of Ag3PO4
were approximately 44-52 nm, while for the TiO2 phase the primary crystallite size was found to
be approximately 20 nm in all cases.
43
According to the BET method, it was found that the as prepared Ag3PO4 exhibits very low BET
surface value equal to 0.1 m2/g, in agreement with previous reports. Moreover, it is observed that
the specific surface area converts towards higher values with the addition of TiO2 in the
synthesized composite samples. This behavior was expected since commercial Degussa P25 has
a much higher specific surface area (~50 m2/g) than pure Silver phosphate. [80]
Figure 2.9 shows the absorption spectra of the Ag3PO4, TiO2 and the synthesized composite
samples. It is observed that Ag3PO4 [Pure] absorbs sunlight with a wavelength less than 515 nm
approximately in agreement with previous reports. The composite catalysts are characterized by
the presence of two absorption edges at 508-516 nm and at 398-440 nm. These edges correspond
to the absorption of Ag3PO4 [Pure] and commercial TiO2 [P25] respectively. As a general trend
absorption at the visible light region decreases with increase of the amount of TiO 2 in the
composite samples. However, it is shown that the hetero-structured photo-catalysts seems to be
a promising candidate as a visible light photo-catalyst. [81]
The optical band gaps of the synthesized composite catalysts were determined according to the
Tauc method (Eq. III), and representative results are shown in Fig. 2.10. Regarding the
commercial TiO2 [P25] sample, it is observed that extrapolation of the linear part of the Tauc
plot at zero F(R) yields an absorption threshold of 3.12 eV. Similarly, the band gap energy of
Ag3PO4 [Pure] catalyst was found to be equal to 2.34 eV. The optical band gaps of synthesized
composite samples are consisted of two absorption thresholds located at Ca. 2.3 eV and 2.7 eV.
Also, all the calculated band gaps listed in Table 2.1.
Sample
Calcination temperature
(oC) - gas atmosphere
TiO2 [Degussa P25]
Ag3PO4
[Pure]
Ag3PO4/TiO2 [6:1]
Ag3PO4/TiO2 [3:1]
Ag3PO4/TiO2 [3:10]
70
70
70
70
SSA (a)
(m2.g-1)
50
0.1
3
5
27
Primary crystallite size (b)
(nm)
21
44
52
47
52
(a) Specific surface area, determined with the B.E.T. method.
(b) Primary crystallite size estimated from XRD line broadening.
(c) Energy band gap estimated from Tauc plot.
Table 2.1 - Physicochemical characteristics of the synthesized catalysts.
44
Ebg(c)
(eV)
3.12
2.33
2.33
2.34
2.33
Figure 2.8 - X-ray diffraction patterns of TiO2, Ag3PO4 and synthesized Ag3PO4/TiO2 composites.
45
Figure 2.9 - UV-Vis diffuse reflectance spectra of TiO2, Ag3PO4 and synthesized Ag3PO4/TiO2 composites.
Figure 2.10 - Tauc plots obtained for the TiO2, Ag3PO4 and synthesized Ag3PO4/TiO2 composites.
46
2.4. Procedure of experiments
In a typical photo-catalytic run, 120 mL of the aqueous solution containing the desired
concentration of Bisphenol A and desired amount of catalyst, were loaded in an open-glass,
cylindrical reaction vessel at ambient temperature and pressure, with inherent pH, under
continuous stirring. In general, the base concentration of BPA and the base amount of the
catalysts were 220 μg/L and 250 mg/L, respectively. The solution was suspended with the
appropriate amount of catalyst and magnetically stirred (380 rpm) for 15 minutes in the absolute
dark condition to ensure complete equilibration of adsorption/desorption of BPA onto the catalyst
surface. After that period, the solar simulator was turned on and the slurry mixture was exposed
to simulated solar irradiation. Samples of exact 1 mL were periodically taken from the reactor
and directly injected to 1.5 ml Eppendorf vials that had prefilled with 0.4 ml pure Methanol in
order to stop the probable chain reactions and also solving the absorbed BPA on the surface of
catalyst particles, then taken samples were filtered by 0.2 μm PVDF syringe filters in order to
separate catalyst particles from the solution and injected to 2 ml HPLC glass vials for analyzing
via HPLC.
It should be noted that in general, solution pH was adjusted but not buffered to its initial value;
however, it remained relatively constant, as changes were ±0.5 pH units, depending on the
experimental conditions throughout the course of the reaction.
47
Chapter 3
RESULTS
&
DISCUSSION
48
In this part, Bisphenol A (BPA) was chosen as a representative model compound of the endocrine
disrupting chemicals family that are a kind of persistent organic micro-pollutants, to investigate
its degradation by a novel kind of synthesized Ag3PO4/TiO2 composite photo-catalyst under
simulated solar irradiation. In continue, performance of mentioned photo-catalytic process in
presence of the best composite photo-catalyst defined via catalyst screening, was tested by
affecting several factors, including irradiation time, irradiation type, adsorption time, catalyst
concentration, BPA concentration, different pH and water matrix. Also photo-catalytic activity
and stability of the composite photo-catalyst was checked during 5 sequential runs, under
simulated solar irradiation.
The aim of each photo-catalytic reaction is full degradation, destruction or mineralization of BPA
in aqueous solution, through hydroxylation, oxidation and subsequent dealkylation reactions by
photo-generated oxidant species such as hydroxyl radicals, super oxide radicals and oxygen.
Figure 3.1 - The supposed reaction pathway for the photo-catalytic degradation of BPA. [82]
49
3.1. Catalyst Screening
At the first step, 250 mg/L of TiO2 [P25], Ag3PO4 [Pure], and also four chosen synthesized
composite catalysts including Ag3PO4/TiO2 [3:10], Ag3PO4/TiO2 [1:1], Ag3PO4/TiO2 [3:1] and
Ag3PO4/TiO2 [6:1] were individually participated in the photo-catalytic degradation of 220 μg/L
BPA in 120 mL aqueous solution under the simulated solar irradiation and ambient operating
conditions in order to check their performances and finding the best photo-catalyst.
According to the achieved results from HPLC, Figure 3.2 shows that all synthesized composite
photo-catalysts have better efficiencies in comparison with their two base components, where
pure silver phosphate shows faster BPA degradation in comparison to another base component
Degussa P25 titanium dioxide, under the same condition. Also according to the Figure 3.2, Figure
3.3 and related bar graph to K value, the optimum ratio of Ag3PO4/TiO2 composite photo-catalyst
was found to be 3:1. The synthesized Ag3PO4/TiO2 [3:1] composite catalyst with the K value
equal to 0.9065 min-1, completely degraded BPA in less than 7.5 minutes under simulated solar
irradiation and ambient conditions, which shows higher efficiency in comparison with the
Ag3PO4 [Pure] and TiO2 [Pure] that need 15 and 25 minutes for the same purpose, respectively.
Meanwhile, according to the relevant rate constant, photo-catalytic degradation of BPA via
Ag3PO4/TiO2 [3:1] composite catalyst seems to have compliance with a psudo-first order kinetic.
The improved photo-catalytic performance of the Ag3PO4/TiO2 [3:1] hetero-structure as
compared to that of the commercial TiO2 [P25], may be accounted for by the electronic band
structures of the composite catalyst. Because, the conduction and valence bands of anatase TiO2,
occur at ca. -4.34 and -7.44 eV, respectively; and for Ag3PO4, the valence band maximum with
-7.34 eV is very close to that of anatase TiO2 as the main part of TiO2 [P25], whereas the
conduction band lies at about -4.9 eV. Under simulated solar irradiation, because of presence of
both UV and visible photons, electrons in both Ag3PO4 and TiO2 would be excited from the
valence band to the conduction band. On silver orthophosphate, this would most likely lead to
the reduction of silver ions into metallic silver particles that because of their large Helmhotz
double-layer capacitance, metal silver nanoparticles are generally good electron sinks that may
facilitate the interfacial transfer of the TiO2 photo-electrons to the Ag3PO4 conduction band. The
holes that remained on both TiO2 and Ag3PO4 then served as powerful oxidizing reagents for the
degradation of BPA.
50
Figure 3.2 - Comparing the efficiencies of selected catalysts in photo-catalytic degradation of BPA under
simulated solar irradiation.
51
Figure 3.3 - Degradation of BPA as a function of Ln(C/C0) vs. Time of exposure to simulated solar
irradiation in the presence of selected catalysts, and their related rate constants.
52
3.2. Dark Adsorption
At the second step, 250 mg/L of Ag3PO4/TiO2 [3:1] was participated in a simple mixing in
presence of 220 μg/L BPA in 120 mL aqueous solution under absolute dark condition and
ambient operating conditions, in order to check the catalyst ability and capacity in adsorption of
BPA and its behavior during the mixing time, and also finding the optimum time for adsorption
of BPA in the dark condition.
Figure 3.4 shows that the adsorption of BPA at the surface of catalyst in the first 15 minutes is
somewhat fast but only about 11% of the base concentration of BPA. After that time, interaction
between the catalyst and BPA tends to a semi-equilibrium phase that after passing 2 hours the
adsorbed BPA at the surface of the catalyst slowly reached to ~13%. Accordingly, it seems that
15 min be an adequate time for mixing the slurry aqueous solution under the dark condition.
It should be due to the cubic shape of the produced silver phosphate crystals via the mentioned
process in the section 2.3., since the silver phosphate crystals are not porous and the content of
titanium dioxide in Ag3PO4/TiO2 [3:1] is only 25%, the synthesized composite catalyst grains
most probably are not porous, which means low surface area of the composite catalyst has
occupied by the BPA molecules in a short period of time.
According to Table 2.1, the specific surface areas of Ag3PO4 [Pure], Ag3PO4/TiO2 [3:1] and TiO2
[P25] that determined with the B.E.T. method are 0.1, 5 and 50 m2/g, respectively. It conforms
to previous discussion about the low porosity and accordingly low surface are of the produced
synthesized composite catalyst crystals, which causes fast and low initial adsorption of BPA.
53
Figure 3.4 - The effect of time on adsorption of BPA at the surface of the Ag3PO4/TiO2 [3:1] catalyst.
54
3.3. The Effect of Radiation
At the fourth step, 250 mg/L of Ag3PO4 [Pure], TiO2 [P25] and Ag3PO4/TiO2 [3:1] were
individually participated in the photo-catalytic degradation of 220 μg/L BPA in 120 mL aqueous
solution under three types of radiation, including simulated solar irradiation, UV part of
simulated solar irradiation and visible part of simulated solar irradiation, in order to check the
performance and photo-catalytic activity of each catalyst under different types of radiation in
ambient operating conditions. The UV and visible parts of solar irradiation were supplied by
using mentioned 2 optic filters that cut wavelengths more than and less than 420 nm respectively.
Figure 3.5 shows that Ag3PO4 [Pure], TiO2 [P25] and Ag3PO4/TiO2 [3:1] naturally have better
photo-catalytic activities under simulated solar irradiation in comparison to its UV and visible
parts. Only pure silver phosphate shows better response to the visible part of solar irradiation in
comparison to the UV part, while the response of commercial Degussa P25 is very weak to the
visible part of solar irradiation. It is interesting to know that the selected synthesized composite
catalyst shows higher efficiency and K value under three types of irradiation in compare with the
best efficiencies of its two components.
In addition according to the Figure 3.7, in photo-catalytic degradation of BPA by the best selected
composite catalyst under real solar irradiation (at 26/7/2015, 1:15 PM, approximately clear sky)
as a real case experiment, full conversion of BPA was observed in 3 minutes with the rate
constant equal to 1.6662 min-1.
55
Figure 3.5 - Comparing the efficiencies of Ag3PO4/TiO2 [3:1], Ag3PO4 [Pure] and TiO2 [P25] catalysts in
photo-catalytic degradation of BPA under simulated solar and its UV part and visible part irradiations.
56
Figure 3.6 - Comparing the photo-catalytic activities and related rate constants of Ag3PO4/TiO2 [3:1],
Ag3PO4 [Pure] and TiO2 [P25] in degradation of BPA under simulated solar irradiation.
57
Figure 3.7 - Comparing the photo-catalytic activity and related rate constants of Ag3PO4/TiO2 [3:1] in
degradation of BPA under simulated solar, and its UV part and visible part irradiations.
58
3.4. The Effect of Catalyst Concentration
At the fifth step, different amount of Ag3PO4/TiO2 [3:1] including 50 mg/L, 100 mg/L, 250 mg/L
and 500 mg/L were individually participated in the photo-catalytic degradation of 220 μg/L BPA
in 120 mL aqueous solution under simulated solar irradiation and ambient operating conditions,
in order to check the effect of the catalyst concentration on the photo-catalytic reaction.
Figure 3.8 and Figure 3.9 show that the reaction rate is increased by increasing the amount of
catalyst, and inversely. Also the K value is increased/decreased approximately proportional to
the ratio of catalyst concentration to its base concentration. The full conversion of BPA in its
photo-catalytic degradation process related to 50 mg/L Ag3PO4/TiO2 [3:1] was observed shortly
after 30 minutes, while related reaction time to 500 mg/L Ag3PO4/TiO2 [3:1] for full conversion
of BPA was less than 4 minutes. This is presumably due to firstly higher production rate of the
photo-generated oxidant species (especially ∙OH) through the reaction, and secondly larger
surface area of the catalyst that directly causes increasing number of active sites available for the
reactions, which both are pertaining to the higher amount of catalyst.
Since the reaction is rapid, probable photolysis and/or photo-oxidation only marginally
contributes to the process, which implies that degradation of BPA is almost due to the interactions
between photonic energy and the composite catalyst. The apparent reaction rate appears to
increase proportionately with the catalyst concentration in the range 50-500 mg/L, above which
it reaches a plateau. This is a typical behavior in slurry photo-catalytic processes, indicating a
heterogeneous catalytic regime with the rate depending on the catalyst active sites available for
reaction; this occurs up to a point where all photons are fully absorbed. Excessive catalyst
concentrations may also lead to decreased reaction rates due to light penetration being impeded
by the catalyst particles, however at the conditions employed in this set of experiments, such
behavior was not observed. [82], [83]
59
Figure 3.8 - The effect of catalyst concentration on photo-catalytic degradation of BPA.
60
Figure 3.9 - Degradation of BPA as a function of Ln(C/C0) vs. Time of exposure to simulated solar
irradiation in the presence of different amount of catalyst, and relevant rate constants.
61
3.5. The Effect of BPA Concentration
At the sixth step, 250 mg/L of Ag3PO4/TiO2 [3:1] was individually participated in photo-catalytic
degradation of different BPA concentration including 110 μg/L (0.5 time), 220 μg/L (1 time),
440 μg/L (2 times), 880 μg/L (4 times) and 2200 μg/L (10 times) in 120 mL aqueous solution
under simulated solar irradiation and similar ambient operating conditions, in order to check the
ability and photo-catalytic activity of the selected catalyst in degradation of different
concentration of target micro-pollutant.
Figure 3.10 and Figure 3.11 show that the reaction rate constant is normally decreased by
increasing the initial concentration of BPA, and inversely. Also, the full degradation of BPA with
0.5, 1, 2, 4 and 10 times of its base concentration, was observed at 4.5, 7.5, 10, 20 and 40 minutes,
respectively. According to the K values in Figure 3.11, at the base concentration of BPA, the
reaction appears to be a first order rate expression as follow;
−
𝐂𝐨
𝐝𝐂
= 𝐤 𝐚𝐩𝐩 ∙ 𝐂 ↔ 𝐋𝐧 ( ) = 𝐤 𝐚𝐩𝐩 ∙ 𝐭 ↔ −𝐋𝐧(𝟏 − 𝐗) = 𝐤 𝐚𝐩𝐩 ∙ 𝐭
𝐂
𝐝𝐭
(I)
where C and Co are BPA concentrations at time t and 0, respectively, X is the conversion and kapp
is an apparent rate constant. However, as initial concentration increases/decreases, there appears
to be a deviation to lower/higher order kinetics, although data fitting to the mentioned pseudofirst order rate expression is still good. But since the K value changes with varying BPA
concentration, the reaction is not true first order. This presumably is explicable by according to
set of graphs in Figure 3.12, where BPA conversion in the process at second minute is different
in different BPA initial concentration that decreases with increasing BPA concentration, in the
event that for real first order reaction, the conversion with different initial concentrations must
be the same at the same time. Also apparent rate constant at the second minute of the reaction
decreases with increasing the initial concentration of BPA that again shows the reaction is not
true first order, even tends to zeroth order at higher initial concentration of BPA. Initial rate of
the first order reaction in different initial concentrations must increase/decrease exactly by the
factor of the initial concentration, while the initial rate of the reaction with 2200 μg/L (10 times)
BPA at the second minute, is less than 4 times initial rate of the reaction with the base
concentration, which shows noticeable difference in comparison to the real first order reaction.
On the other hand, if for degradation of 10 times BPA concentration, the reaction is assumed
zeroth order with the rate expression as follow;
−
𝐝𝐂
𝐭
= 𝐤 𝐚𝐩𝐩 ↔ 𝐂𝐨 − 𝐂 = 𝐤 𝐚𝐩𝐩 ∙ 𝐭 ↔ 𝐗 = 𝐤 𝐚𝐩𝐩 ∙
𝐝𝐭
𝐂𝐨
(II)
conversion would be inversely proportional to the BPA concentration, while it would be
independent of concentration in the case of first order. The fact that conversion and the rate
constant decrease as concentration increases, denotes kinetic between zeroth and first order.
62
Figure 3.10 - The effect of BPA concentration on photo-catalytic activity of Ag3PO4/TiO2 [3:1] in
degradation of BPA.
63
Figure 3.11 - Degradation of BPA in different concentration as a function of Ln(C/C0) vs. Time of exposure
to simulated solar irradiation in the presence of 250 mg/L Ag3PO4/TiO2 [3:1], and relevant rate constants.
64
Figure 3.12 - Comparing BPA conversion, initial rate and its apparent rate constant in different BPA
concentration at the second minute of the reaction.
65
3.6. The Effect of pH
At the last step, 250 mg/L of Ag3PO4/TiO2 [3:1] was individually participated in photo-catalytic
degradation of 220 μg/L BPA in 120 mL aqueous solution under simulated solar irradiation and
similar ambient operating conditions with three different pH including 3, 9 and inherent pH, in
order to evaluate the effect of alkaline and acidic pH that usually found in domestic and industrial
wastewaters, on BPA degradation. In this experiment the initial pH of ultrapure water was
measured 6.1±0.1 that after injecting of 220 μg/L BPA, the pH decreased to 5.9±0.1. For alkaline
pH, specific volume of NaOH added to aqueous solution that increased the pH to 10±0.1, and
then was declined by adding the catalyst to 9±0.1. For acidic pH, specific volume of H3PO4 added
to aqueous solution that decreased the pH to 2.9±0.1, and then was increased by adding the
catalyst to 3.2±0.1. For inherent pH, after adding the catalyst into the aqueous solution, the pH
was dropped from 5.9±0.1 to 5±0.1. [84]
Figure 3.13 and Figure 3.14 show the effect of changing the initial pH in the range 3-9 on BPA
degradation in ultrapure water, in which full degradation of BPA in acidic pH was observed
before 7.5 minutes that does not show significant change in comparison to that in inherent pH,
but full degradation of BPA in alkaline pH was observed at 20 minutes with the 69% decreased
relevant K value in comparison to that in inherent pH. Moreover, the extent of dark adsorption
in acidic pH was approximately the same with initial pH, while in alkaline pH it was diminished
about 45%, which implies that the discrepancy between alkaline and inherent/acidic conditions
could be explained by the possibly different extent of electrostatic forces developed between the
catalyst surface and the organic pollutant molecules, because BPA exists mainly in molecular
form at acidic zone and becomes negatively charged. In general, it seems that unlike acidic
condition, alkaline condition has a considerable negative effect on kinetic. [82], [85]
It should be mentioned that, silver(I) cations in aqueous solution can give protons and behave
like a weak Bronsted acid and convert to silver hydroxide (AgOH) at relatively alkaline pH that
easily and spontaneously yields the silver oxide (Ag 2 O) due to noble character of silver, which
is thermodynamically more stable. Phosphate anions in aqueous solution also behave as a weak
acid but they are famous enough as buffering agents. Aqueous phosphate exists in four forms
depend on the pH, in which the phosphate ion (PO3−
4 ) predominates in strongly basic conditions,
whereas the hydrogen phosphate ion (HPO2−
4 ) is prevalent in weakly basic conditions, also the
dihydrogen phosphate ion (H2 PO−
4 ) is most common in weakly acid conditions, and trihydrogen
phosphate (H3 PO4 ) is the main form in strongly acidic conditions. [86], [87]
66
Figure 3.13 - The effect of acidic and alkaline pH on photo-catalytic activity of Ag3PO4/TiO2 [3:1] in
degradation of BPA under simulated solar irradiation.
67
Figure 3.14 - Degradation of BPA in different pH as a function of Ln(C/C0) vs. Time of exposure to
simulated solar irradiation in the presence of 250 mg/L Ag3PO4/TiO2 [3:1], and relevant rate constants.
68
3.7. The Effect of Water Matrix
At the seventh step, 250 mg/L of Ag3PO4/TiO2 [3:1] was individually participated in photocatalytic degradation of 220 μg/L BPA in 120 mL different aqueous solution under simulated
solar irradiation and similar ambient operating conditions, to evaluate the effect of various
organic and inorganic constituents typically found in environmentally relevant samples on BPA
degradation. Experiments were conducted in 5 different water matrices, including ultrapure
water, 500 mg/L NaHCO3 in ultrapure water, 3 mg/L Humic acid in ultrapure water, bottled
drinking water, and 21 mg/L COD wastewater.
Figure 3.15 and Figure 3.16 show that in the presence of 3mg/L Humic acid in ultrapure water
and also 500 mg/L NaHCO3 in ultrapure water, the related reaction rates to the photo-catalytic
degradation of BPA were abnormally increased, but no significant changes. The addition of
Humic acid, an analog of the recalcitrant natural organic matter typically found in waters and
wastewaters, normally has a detrimental effect on the rate, which is consistent with the
nonselective behavior of the photo-catalytically generated oxidizing species that can directly
decreases conversion of the target pollutant. Retardation effects of Humic acid could be ascribed
to the three reasons; firstly Humic acid competes with the target pollutant molecules for the active
sites of the catalyst, secondly Humic acid can attenuate the incident light in the suspension that
causes decreasing the photo-catalyst activation, and thirdly Humic acid substances contain many
reactive functional groups that could effectively scavenge the photo-generated hydroxyl radicals
and holes. However, in this experiment full degradation of BPA was observed before 7.5 minutes
and relevant K value was amplified about 22% in comparison to the same experiment with
ultrapure water, which means presence of Humic acid in aqueous solution unusually does not
have detrimental effect on degradation of BPA. [82], [88]
The addition of Sodium bicarbonate to aqueous solution is in order to check the photo-catalytic
degradation of BPA in presence of a radical scavenger that naturally found in wastewater and
water as a typical inorganic compound. In general bicarbonates are known as radical scavengers,
which reduce reaction rate of micro-pollutants degradation via scavenging hydroxyl radicals to
form carbonate radicals that eventually recombine, as follows;
+ 𝐇𝟐 𝐎
𝐇𝐂𝐎−
+ ∙ 𝐎𝐇
→
∙ 𝐂𝐎−
𝟑
𝟑
−
−
∙ 𝐂𝐎𝟑
+ ∙ 𝐂𝐎𝟑
→
𝐂𝐎𝟐 + 𝐂𝐎𝟐−
𝟒
∙ 𝐎𝐇
. + ∙ 𝐎𝐇
→
𝐇𝟐 𝐎𝟐
But in this experiment, the addition of 500 mg/L NaHCO3 in ultrapure water was decreased the
(I)
(II)
(III)
required reaction time for full degradation of BPA to 5 minutes and the relevant K value was
amplified about 32% in comparison to the referent experiment in ultrapure water. It shows that
unlike usual, presence of bicarbonates in aqueous solution as a radical scavenger does not have
negative effect on photo-catalytic degradation of BPA. This could be explained that although the
69
carbonate radicals with E°=1.78 V at pH=7 are less oxidizing in comparison to hydroxyl radicals
with E°=2.23 V at pH=7, but they are very strong one-electron oxidants that act by both electron
transfer and hydrogen abstraction mechanisms to produce radicals from the oxidized target
molecules. The carbonate radical as a selective electrophilic reagent shows a wide range of
reactivity towards aromatic compounds with the rate constants depending on the substituents of
the aromatic ring system. In this respect, for instance ∙CO−
3 reacts with benzene very slowly with
k < 104 1/M.s, while reacts with each of two phenol moieties, appearing in BPA molecule, more
rapidly by a factor of ca 103. The replacement of ∙OH with ∙CO−
3 may be offset by the fact that
reaction II is two orders of magnitude slower than the recombination of hydroxyl radicals in
reaction III. In other words, carbonate radicals have a greater chance than hydroxyl radicals to
react with the substrate rather than recombine. [82], [89], [90], [91], [92], [93]
In another water matrix experiment, bottled drinking water was used instead of ultrapure water
in order to check the photo-catalytic degradation of BPA in presence of 211 mg/L bicarbonate,
10 mg/L chloride, 15 mg/L sulphate, 5 mg/L nitrate and 78 mg/L of various metal ions such as
Ca, Mg, Na, Zn, Cu, Fe, etc. According to the related figures, the full conversion of BPA was
observed before 10 minutes and the relevant K value was reduced about 55% in comparison to
the referent experiment. In this respect, the presence of bicarbonates in aqueous solution cannot
justify the reduced degradation rate in comparison to the basic run in ultrapure water, this may
be due to the presence of firstly sulphates and chlorides that also behave as radical scavengers,
and secondly mentioned metal ions that may either scavenge hydroxyl radicals and/or block
active sites on the surface of the catalyst, also some anions and cations can react with catalyst
component molecules. [82], [94], [95]
In the last water matrix experiment, secondary treated wastewater with 21 mg/L COD was used
instead of ultrapure water in order to check the photo-catalytic degradation of BPA in complex
matrices with detrimental effect of wide range of organic and inorganic constituents that is close
to real case, unlike ideal case with ultrapure water. According to the Figure 3.17, only 66.5%
conversion of BPA was observed after 8 hours unceasing photo-catalytic reaction that in
comparison to the same conversion of BPA in ultrapure water that was observed before 85
second, the reaction rate in wastewater is 340 times slower. Also, in a supplementary experiment,
4 times of the catalyst (1 g/L) was used to check the difference with the previous test, which full
degradation of BPA was observed at 4 hours that is 32 times slower in comparison to ideal case
with the base amount of catalyst. Besides different detrimental effects of wide range of organic
and inorganic constituents on the catalyst and reaction, since the secondary treated wastewater
has inherent alkaline pH (~8), as mentioned before in the section 3.6 it should be noted that poor
performance of the composite catalyst in this case reminds the noticeable negative effect of
alkaline pH on photo-catalytic degradation of BPA in ideal case.
70
Figure 3.15 - The effect of water matrix on photo-catalytic activity of Ag3PO4/TiO2 [3:1] in degradation
of BPA under simulated solar irradiation.
71
Figure 3.16 - Degradation of BPA in different water matrix as a function of Ln(C/C0) vs. Time of exposure
to simulated solar irradiation in the presence of 250 mg/L Ag3PO4/TiO2 [3:1], and relevant rate constants.
72
Figure 3.17 - Comparing the effect of different amount of Ag3PO4/TiO2 [3:1] on photo-catalytic
degradation of 220 μg/L BPA in wastewater, and relevant rate constants.
73
3.8. Catalyst Stability
At the third step, 250 mg/L of Ag3PO4 [Pure] and Ag3PO4/TiO2 [3:1] were individually
participated in five consecutive runs of photo-catalytic degradation of 220 μg/L BPA in 120 mL
aqueous solution under simulated solar irradiation and ambient operating conditions, to evaluate
their photo-catalytic activity upon repeated use. After the first cycle, at the initial moment of each
cycle, specific volumes of BPA and ultrapure water were injected to the slurry aqueous solution
in order to keep the BPA concentration and also total volume of the solution constant, exactly at
220 μg/L BPA in 120 mL aqueous solution for all cycles. Also, except the last cycle, after the
last sampling of each photo-catalytic cycle, the slurry aqueous solution was exposed to excess 15
minutes solar irradiation in order to full degradation and/or destruction of probable leftover BPA
and/or intermediates and byproducts. In addition, after the injection of BPA in the start point of
each cycle, the slutty aqueous solution was left to equilibrate for 15 min in the absolute dark
condition, then irradiated for the next period of time that was 15 min for Ag3PO4 [Pure] and 10
min for Ag3PO4/TiO2 [3:1]. Moreover, each sampling took place after complete pause of the
reaction (stopped radiation and stopped stirring) and participating of suspended catalyst particles,
in order to avoid decreasing the amount of catalyst during sampling, and keep the concentration
of the catalysts in the aqueous solution approximately constant, for all cycles.
According to the achieved results from HPLC, both procedures of five cycles of consecutive
photo-catalytic reactions related to Ag3PO4 [Pure] and Ag3PO4/TiO2 [3:1] and their relevant
results are shown in the Figure 3.18, in which it appears that both photo-catalytic activities are
partly lost upon repeated use during periods of time. The 15 min full conversion of BPA by
Ag3PO4 [Pure] in the first cycle decreased to ~70% in the fifth cycle, while the 10 min full
conversion of BPA by Ag3PO4/TiO2 [3:1] in the first cycle decreased to ~91% in the fifth cycle.
In both case, this could be attributed to the formation and accumulation of transformation
byproducts that cause increasing the residual organic content of the reaction mixture
progressively, which would compete with BPA for the photo-generated oxidizing species. Also,
some of these intermediates may strongly be adsorbed on the catalyst surface, thus reducing the
number of active sites available for the reactions. In addition, related results to K value of each
cycle for both catalysts are presented in the Figure 3.19 and the Figure 3.20, which show the K
value related to Ag3PO4 [Pure] at fifth cycle shows ~80% decreasing in comparison to its first
cycle, while this K value drop for Ag3PO4/TiO2 [3:1] is about ~66%. It should be noted that, even
though all sampling had done in stagnant and motionless state of aqueous solution, but small
amount of catalysts were observed in micro-filters after filtration of each cycle.
As mentioned before in the third paragraph of the section 3.1. about the band structures of the
composite catalyst components, such a band structure may also account for the enhanced stability
74
of the Ag3PO4/TiO2 [3:1] hetero-structure, in comparison to Ag3PO4 [Pure], which compromised
the repeated uses of both catalysts in consecutive photo-catalytic reactions that also exhibited
enhanced recoverability of the photo-catalytic activity. Since the TiO2 valence band is situated
slightly lower than that of Ag3PO4, effective transport of holes from TiO2 to Ag3PO4 might also
occur, which leading to the re-oxidation of metallic silver to silver ions in silver phosphate and/or
silver oxide. It is likely that such synergistic interfacial charge transport was facilitated by the
intimate contact between the two semiconductor catalysts. Further contributions might arise from
the surfactant-like function of the TiO2 for the structural stabilization of the Ag3PO4.
Accordingly, the Ag3PO4/TiO2 [3:1] hetero-structure appeared to be a somewhat more desirable
in repeated and/or long-term applications because of its enhanced stability. [52], [53], [59]
Furthermore about the stability of synthesized composite photo-catalyst, the silver leaching from
the synthesized composite catalyst structure to aqueous solution as determined by flame atomic
absorption spectrometry was below 10% after five sequential runs. Meanwhile, according to the
Figure 3.21, from the visual point of view it was observed that the synthesized composite catalyst
shows better stability under visible part of simulated solar irradiation in comparison with normal
simulated solar irradiation, wherein UV photons have the ability to change the catalyst
composition and cause photo-corrosion. The color of the Ag3PO4/TiO2 [3:1] catalyst powder is a
mixture of dark yellow and beige that the initial color of its slurry aqueous solution seems dilute
pale yellow. Also the color of metallic silver, AgOH and Ag2O powders seem metallic
grey/silver, light beige and dark brown, respectively. According to the picture of final state under
UV irradiation, the color of slurry aqueous solution seems to be dilute pale brown to dark orange,
which shows most probably presence of insoluble Ag2O that means photo-corrosion/destruction
of the synthesized composite catalyst. It is interesting to know that, even with this changing the
color of slurry aqueous solution, the next photo-catalytic runs were efficient in degradation of
BPA! According to the picture of final state under Visible irradiation, the color of slurry aqueous
solution seems to be dilute pale beige to light grey, which shows most probably presence of
insoluble AgOH and metallic silver nanoparticles that means again photo-corrosion/destruction
of the synthesized composite catalyst, but as mentioned before, presence of silver nanoparticles
at the surface of catalyst particles can improve photo-catalytic activity of both Ag3PO4 and TiO2,
and also they could be re-oxidize to silver ions during photo-catalytic reaction and react with
exist phosphate ions in aqueous solution. Metallic silver and AgOH in presence of UV photons
with high amount of energy that can produce stronger oxidant species are oxidized and tend to
be convert to AgO2 that is more stable thermodynamically, but in presence of visible photons
with lower energy, this process probably needs much more time.
It is worth mentioning that, in this visual analyzing we ignore the effect of the color of TiO2 and
metallic titanium because of the low content of TiO2 in the composite catalyst.
75
Figure 3.18 - Comparing the activities of Ag3PO4 [Pure] and Ag3PO4/TiO2 [3:1] in 5 consecutive cycles.
76
Figure 3.19 - Degradation of BPA as a function of Ln(C/C0) vs. Time of exposure to simulated solar
irradiation during 5 consecutive cycles for Ag3PO4 [Pure] and Ag3PO4/TiO2 [3:1].
77
Figure 3.20 - Comparing the rate constants of BPA degradation by Ag3PO4 [Pure] and Ag3PO4/TiO2 [3:1].
78
Figure 3.21 - Visual comparing of the pictures related to the initial and final states of slurry aqueous
solution under UV and Visible parts of simulated solar irradiations.
79
Chapter 4
CONCLUSION
80
Silver orthophosphate as a photo-catalyst has excellent photo-catalytic activity and yields a high
quantum efficiency of nearly 90% under visible light that found for the evolution of oxygen in
water photolysis that enables it to oxidize persistent organic micro-pollutants such endocrine
disruptor chemicals (i.e. Bisphenol A) efficiently, which leads to complete photo-catalytic
degradation/destruction of target molecules.
Nevertheless this is not the only requirement, a semiconductor photo-catalyst must also be robust
and stable enough to last for an extended period of time, and furthermore be cheap enough to be
cost effective. Unfortunately, silver orthophosphate has not shown appropriate stability during
the photo-catalytic processes, and also silver as a component of this semiconductor catalyst
(~77%) is an expensive noble metal.
Accordingly, in this study we tried to design and synthesize a new hetero-structure semiconductor
catalyst by addition of another semiconductor catalyst to the structure of silver phosphate in order
to enhance its stability and photo-catalytic activity, which commercial Titanium dioxide
(Degussa P25) as one of the most famous catalysts being used in this field with thoroughly known
properties and abilities, was selected for this purpose.
Bisphenol A (BPA) was chosen as a representative model compound of the endocrine disrupting
chemicals family that are a kind of persistent organic micro-pollutants in water, to investigate its
photo-catalytic degradation by the synthesized Ag3PO4/TiO2 composite photo-catalysts under
simulated solar irradiation. Performance of the best selected composite catalyst was tested by
affecting several factors, including irradiation time, irradiation type, adsorption time, catalyst
concentration, BPA concentration, different pH and water matrix. Also photo-catalytic activity
and stability of the composite catalyst was tested during 5 sequential runs, under simulated solar
irradiation.
According to the achieved results, the outcomes of the issues that have been studied in this
project, are listed below;

The optimum ratio of the Ag3PO4/TiO2 composite catalyst was found to be 3:1.

The performance of the Ag3PO4/TiO2 [3:1] appears to be the best among the catalysts tested,
with completely 100% full degradation of BPA in 7.5 minutes.

The maximum adsorption of BPA at the surface of Ag3PO4/TiO2 [3:1] was observed less than
13% after 2 hours, also the optimum required time for appropriate adsorption of BPA under
dark condition was defined 15 minutes that ~11% BPA adsorption was observed.

The photo-catalyst Ag3PO4/TiO2 [3:1] is substantially more active than its both basic
components Ag3PO4 [Pure] and TiO2 [P25], under each three types of irradiation, including
simulated solar irradiation, and its UV and visible parts.
81

Ag3PO4/TiO2 [3:1] showed better photo-catalytic activity and faster BPA degradation under
simulated solar irradiation in comparison to UV/visible parts of simulated solar irradiation.

As a real case, photo-catalytic degradation of BPA in presence of Ag3PO4/TiO2 [3:1] under
real solar irradiation (at late July, noon, normal sky) took place very fast, in 3 minutes.

Photo-catalytic degradation of BPA by Ag3PO4/TiO2 [3:1] follows a pseudo-first order
reaction kinetic with respect to initial concentration of BPA, whereas in the range of 110 to
2200 μg/L the kinetic constant decreases as the initial concentration increases, and inversely.

Also degradation of BPA was increased considerably with increasing the catalyst
concentration from 50 to 500 mg/L, in which decreasing the reaction rate due to impeded
light penetration phenomena by excessive catalyst concentrations was not observed.

Photo-catalytic degradation of BPA in acidic and inherent pH was considerably more
efficient than in alkaline pH, in which acidic, inherent and alkaline pH were 3, 5 and 9
respectively.

In water matrix, the composite catalyst showed good performances in photo-catalytic
degradation of BPA in presence of Humic acid (as troublesome oxidant consumer) and also
in presence of Sodium bicarbonate (as radical scavenger), with slightly positive difference in
comparison to in ultrapure water.

Full degradation of BPA in drinking bottled water was observed in less than 10 minutes that
in comparison to in ultrapure water, it was good in presence of different inorganic cataions
and anions that act as radical scavengers and catalyst deactivators.

Unfortunately, degradation of BPA in secondary wastewater treatment effluent was not
efficient (66.5% conversion after 8 hours), and even by using 1 g/L catalyst concentration
full degradation was observed in near 4 hours.

Photo-catalytic stability/activity of Ag3PO4/TiO2 [3:1] to some extent was enhanced in
comparison with Ag3PO4 [Pure] after 5 consecutive runs of BPA photo-catalytic degradation.

Full conversion of BPA in 15 min by the pure silver phosphate, ~30% decreased after 5 runs;
while full conversion of BPA in 7.5 minutes by the composite catalyst, 9% decreased after 5
photo-catalytic runs.

The silver leaching from Ag3PO4/TiO2 [3:1] structure to aqueous solution as determined by
flame atomic absorption spectrometry was below 10% after five sequential runs.

Also from the visual point of view, Ag3PO4/TiO2 [3:1] showed better stability under visible
part of simulated solar irradiation in comparison with the full spectrum simulated solar
irradiation. It was observed that UV photons that naturally present in the normal solar
irradiation have the ability to change the catalyst composition and cause photo-corrosion.

The Ag3PO4/TiO2 [3:1] hetero-structure appeared to be somewhat more desirable in repeated
and/or long-term applications under visible part of simulated solar irradiation.
82
The research some extent achieved the main goals set out, by adding Degussa P25 TiO2 to the
structure of pure Ag3PO4 as one of the newest photo-catalysts. Despite these successes, and taking
into account the originality of the composite photo-catalyst investigated, the research should also
be viewed as a simple basic for future works, and therefore there are many possible follow up
investigations that could be pursued in order to meet the ideal solar driven photo-catalyst that be
able to degrade wide range of micro-pollutants in real environment with acceptable efficiency,
stability and reusability.
The End …
83
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