BURAPHA UNIVERSITY
DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL ENGINEERING
RESEARCH PROPOSAL
APPLICATIONS OF FENTON AND FENTON-LIKE REACTIONS FOR
DERUSTING WASTEWATER TREATMENT
COMMITTEE:
PRINCIPAL ADVISER: DR. TONGCHAI SRIWIRIYARAT
SUBMITTED BY
MR. PISETH SOM
55910117
DATE: JULY 20, 2013
ii
TABLE OF CONTENT
TABLE OF CONTENT .................................................................................................ii
LIST OF TABLES ........................................................................................................ iv
LIST OF FIGURES ....................................................................................................... v
ABBREVIATION......................................................................................................... vi
CHAPTER 1 INTRODUCTION ................................................................................... 1
Background .............................................................................................................. 1
Problem Statements ................................................................................................. 3
Research Hypothesis ................................................................................................ 5
Objectives of the Study ............................................................................................ 6
Scope of the Study ................................................................................................... 6
Significance of the Study ......................................................................................... 7
CHAPTER 2 LITERATURE REVIEWS ..................................................................... 9
Advanced Oxidation Processes (AOPs)................................................................... 9
Fenton Reagent Mechanism ................................................................................... 11
Basic Principle .............................................................................................. 11
Conventional Fenton Process ........................................................................ 12
Fenton-like Process ....................................................................................... 14
Hydroxyl Radical Reaction with Organic Compounds ......................................... 15
General Mechanisms ..................................................................................... 16
Iron Ligand, Chelators and Coordination ..................................................... 18
Factors Affecting Fenton and Fenton-like Process ................................................ 21
Effect of pH................................................................................................... 21
Effect of Temperature ................................................................................... 22
Effect of Iron (ferrous or ferric) Concentration ............................................ 23
iii
Effect of H2O2 dose....................................................................................... 24
Effect of Iron/ H2O2 ratio .............................................................................. 25
Effect of Reaction Time ................................................................................ 27
Effect of Post-treatment ................................................................................ 27
EDTA Degradation by Various Fenton Processes ................................................. 29
CHAPTER 3 RESEARCH METHODOLOGY .......................................................... 37
De-rusting Wastewater........................................................................................... 37
Experimental Design .............................................................................................. 37
Materials and Chemical Reagents .......................................................................... 39
Experimental Procedure ......................................................................................... 40
Analytical Method ................................................................................................. 42
Kinetic Modeling Methods .................................................................................... 43
REFERENCES ............................................................................................................ 44
APPENDICES ............................................................................................................. 50
Appendix A Activities plan ................................................................................. 50
iv
LIST OF TABLES
Table 1
Oxidizing potential for conventional oxidizing agents.............................. 10
Table 2
Fenton reaction mechanisms and reaction rate constants .......................... 13
Table 3
Mechanisms for hydroxyl radical reaction with organic compounds ........ 16
Table 4
Hydrogen atom abstraction reaction .......................................................... 17
Table 5
Further oxidation of organic free radicals.................................................. 17
v
LIST OF FIGURES
Figure 1
advanced oxidation processes (AOPs) classification. ............................... 11
Figure 2
schematic experimental procedure of Fenton and Fenton-like process ..... 41
vi
ABBREVIATION
AOPs
: Advanced Oxidation Processes
EDTA
: ethylenediamine tetraacetic acid
COD
: chemical oxygen demand
BOD5
: biological oxygen demand in 5 day
NTU
: Nephelometric Turbidity Unit
1
CHAPTER 1
INTRODUCTION
This chapter covers the fundamental background of research relating
chemical cleaning or de-rusting wastewater and the determining factors in
consideration of Advanced Oxidation Processes (AOPs) based on Fenton and Fentonlike processes. The research motivation and justification are identified in the problem
statement. Then, the research objectives and research hypothesis are formulated
accordingly. Finally, scope and limitation of the study are also provided.
Background
Development and industrialization of mining, pulp and paper manufacturing,
plating facilities, mental bleaching and finishing, detergent manufacturing and
chemical cleaning processes have recently posted a great concern for environment
because of the presence of heavy metals and other toxic or recalcitrant organic
compounds. The above industries have applied various organic compounds in the
production facilities, which result in very complicated toxic wastewater. Chelating or
complexing agents are among the most common organic compounds that have been
used. Indeed, chemical cleaning of the secondary side of pipes, tanks, boilers, and
power plant cleaning services have threaten natural environment because of the
generation of highly chelated wastewater particularly ethylenediamine tetraacetic acid
(EDTA) and citric acid waste (Lan et al., 2012; Fu et al., 2012).
EDTA and Citric acid are strong chelating agents. They are commonly used
in numerous applications based on its ability to control the action of different metal
ions by complexation and coordination. They are used as an important
decontiminating agent in nuclear industry and removing the rust or scale metal
industrial equipment (Chitra et al., 2011). Juang & Lin and references therein (2000)
mentioned that strong chelating agents including EDTA, citric acid are applied in
such industries as metal plating, passivation of the steel, textile and paper processing,
and industrial cleaning. In addition, migration behavior of metals in aqueous
environment is significantly altered by complexation. Particularly, EDTA does not
biodegradable rapidly and show its effects of magnified persistence. The presence of
2
EDTA and citric complexation with other metals could result in inhibitory and toxic
condition, which is resistant to biological treatment (Chitra et al., 2011).
In addition, the presence of chelating agents in the wastewater could have
constraints and ineffective application of conventional treatment processes such as
chemical precipitation, coagulation, ion exchange, interior microelectrolysis and
adsorption due to metal complexation and stabilization of the complexes (Citra et al.,
2011; Lan et al., 2012; Fu et al, 2009, 2012). To degrade or mineralize the metalEDTA complexes, there is an urgent need to search for such an approach, which may
be applicable, efficient, economical, and eco-friendly (Bautista et al., 2008; Bianco et
al., 2011).
More recently, advanced oxidation processes (AOPs) have been applied
successfully for the removal or degradation of wide range of organic pollutants
including toxic and recalcitrant compounds based on the high oxidative power of the
HO· radical. Among AOPs, Fenton process is the most cost effective and has already
been widely studied and applied in full-scale wastewater treatment operations. Fenton
process employed catalytic method based on the generation of hydroxyl radicals
(HO·) from hydrogen peroxide with iron ions acting as homogeneous catalyst at
acidic pH and ambient conditions. The process may be applied to wastewaters
treatment in term of organic pollutant destruction, toxicity reduction, biodegradability
improvement, COD removal, and odor and color removal (Matthew Tarr, 2003).
Furthermore, this Fenton process has been modified to increase its efficiency
and solve operational problems. Common modified Fenton technologies include
electro-Fenton process, photo-Fenton process, Fenton-like process, and the photo
electro-Fenton process (Ameta et al., 2012; Lou & Huang, 2009; Poyatos et al., 2010).
Although electro-Fenton, photo-Fenton, and the photo electro-Fenton
processes are remarkably known for their better performances in a wide range of
industrial wastewater comparing to conventional Fenton process as reported in
literature, it should be justified in term cost effective. An important drawback to
industrial application of photo-electro catalytic processes required large consumption
of electrical energy, which was accounted for more than 60% of total cost (Chitra et
al., 2011). Therefore, Fenton and Fenton-like oxidation processes are considerably
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selected because they may allow feasible and applicable treatment performance for
de-rusting wastewater.
Problem Statements
Advanced oxidation processes (AOPs) are better treatment options than the
conventional treatment methods commonly adopted in wastewater treatment plant.
Advanced oxidation processes are known for their capability to mineralize,
decompose, and degrade non-biodegradable (inhibitory /recalcitrant / refectory)
organic compounds (Bautista et al., 2008; Lucas & Peres, 2009; Neyens & Baeyens,
2003; Sekaran et al., 2012). In particular, advanced oxidation processes (AOPs)
based on Fenton reaction are the most common process has been applied for many
different industrial wastewaters due to the economic advantages, ease of application,
and effectiveness in the contaminant reduction. Fenton oxidation was considered and
presented one of the best methods for clean and safe processes for the degradation of
organics even at higher initial organic content (Bianco et al., 2011; Lucas & Peres,
2009; Bautista et al., 2008). In addition, Bautista et al.,(2007) added that there is no
energy input to activate the hydrogen peroxide therefore, it may offers feasible and
cost-effective method for remediation of chelated organic complexes in the cleaning
wastewater in term of high COD reduction and metals could be conveniently removed
by subsequent chemical precipitation.
Recently, Lan et al.(2012) applied interior microelectrolysis (IM) and
Fenton reaction followed by chemical coagulation to remove EDTA-Cu(II) complex.
It was found that Fenton reaction was successfully applied to degrade EDTA complex
with utilization of existing Fe(III)-rich IM effluent with additional iron. Free Cu (II)
was subjected to be removed completely by coagulation. In addition, Fu et al.(2009)
successfully incorporated Fenton and Fenton-like reactions with hydroxide
precipitation to removal synthetic NiEDTA complex. Total iron and Ni were removed
by hydroxide precipitation which was conveniently achieved by increase Fenton’s
effluent pH to 11.5. However, Fe-EDTA complex has not been conducted yet.
Therefore, to our knowledge Fe-EDTA removal by Fenton and Fenton-like is
important since their applications are not limited.
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Kim et al.,(2010) mentioned that de-rusting operation processes generate
wastewater containing highly concentrated citric acid and EDTA. Both citric acid and
EDTA are strong and persistent chelating agents, so they are specialized in chelating
with predominant Fe (III) species in the wastewater. This situation causes some
metal-laden wastewater and violet the COD requirement effluent discharge because
the toxicity of heavy metal complexes prevents the reduction of COD by most
biological treatment processes. More importantly, EDTA are recently suspected of
eutrophication driven substance because it is in group aminopolycarbocylic acids that
contains nitrogen atom in its chemical structure (Ghiselli et al., 2004). In this case,
pretreatment by Fenton oxidation can adequately reduce the COD prior to biological
treatment as recommended by USPeroxide.
The uses of chelating agents have become widespread over the past decade.
Wastewaters containing strong chelating agents such as citric acid and EDTA have
become environmental concerns. Citric acid and EDTA themselves are relatively
harmless to human, but they form very stable chelates with metals and can extend
mobilization of toxic heavy metal. Chelating heavy mental must be treated not only
for the toxic heavy mental, but also the chelating agent. Indeed, the presence of the
EDTA and citric acid in contaminated wastewater cause complexation of cation
resulting in interferences in their removal by various treatments such as chemical
precipitation, iron exchange resin and membrane separation as the examples, and can
negatively influence on the quality of final effluent of wastewater owing to the
stabilization and mobilization of metal in chelating effect (Chitra et al., 2011; Fu et
al., 2012; Kim et al., 2010). Chitral et al., (2011) and references therein indicated that
EDTA is not easily biodegradable, scarcely degradable by chlorine and hardly
retained by activated carbon filters. In the light of increasing concern over
contamination of the environment by hazardous chemicals, there is a great effort to
develop innovative techniques for safe destruction of toxic pollutants.
The Fenton oxidation process has been employed successfully to treat
different types of industrial wastewater. They were included textile wastewater
(Karthikeyan et al., 2011; Kang et al., 2002), cork cooking wastewater (Guedes et al.,
2003),chemical laboratory wastewater (Benatti et al., 2006), cosmetic wastewater
(Bautista et al., 2007), livestock wastewater (Lee & Shoda, 2008), chemical industrial
5
wastewater (Bautista et al., 2008a) , olive mill wastewater (Kallel et al., 2009;
Karthikeyan et al., 2011; Lucas & Peres, 2009; Nieto et al., 2011), coking wastewater
(Güçlü et al., 2011), bleaching wastewater (Wang et al., 2011), complex industrial
wastewater (Bianco et al., 2011), chelated heavy metal containing wastewater (Fu et
al., 2012). However, de-rusting and cleaning wastewater was not extensively
documented. Thus, advanced oxidation process based on Fenton’s reagent is primarily
considered for treating this type of wastewater.
Compared to other AOPs, the Fenton process is a widely studied and applied
catalytic method based on the generation of hydroxyl radicals (HO·) from hydrogen
peroxide with iron ions acting as homogeneous catalyst at acidic pH and ambient
conditions. As reported, Kim et al., (2010) and Lan et al. (2012) indicated the
cleaning (de-rusting) industrial wastewater have already been predominated by iron
concentration (Fe2+/Fe3+) which can reach up to hundreds of mg/L. Apparently,
Fenton reactions should take place to generate highly oxidative hydroxyl radicals
when H2O2 is added to the iron-rich wastewater. It was also supported by Kitis &
Kaplan (2007) that iron oxide (Fe2O3) particle already presented in wastewater can be
an effective catalyst in the generation of strong oxidant (HO·). If it works, this is
another economic advantage of Fenton or Fenton-like process to initiate reaction by
utilizing the abundant iron in wastestream.
Final rational for this study also concerns with the effluent standard as
stipulated in effluent regulation. According to the characteristics of de-rusting
wastewater, the concentration of total Fe (1500 mg/L) and COD (32232 mg/L) which
are much higher than the effluent standard which are limited by total iron (5 mg/) and
COD (400 mg/L). Thus, this type of wastewater is not allowed to discharge central
wastewater treatment facilities or environment without proper treatment. More
importantly, the de-rusting wastewater itself contains low biodegradability which
biological processes are not directly used.
Research Hypothesis
1.
The presence of chelating agent, EDTA, can inhibit the Fenton and
Fenton-like reactions.
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2.
Utilization of existing Fe (III)/ Fe (II) and additional iron can be beneficial
for Fenton and Fenton-like oxidation for degradation of EDTA complex in
term of COD reduction and total Fe removal.
3.
Kinetic degradation and removal efficiency of COD by Fenton process
can allow faster and better performance than Fenton-like oxidations.
Objectives of the Study
The overall objective of this study is to evaluate the feasibility and
efficiency of Fenton and Fenton-like oxidation for removal of organic and inorganic
pollutant measured in COD and total Fe as the main parameters in de-rusting
wastewater. Specific objectives are:
1.
To determine and compare the optimum condition between Fenton and
Fenton-like oxidation for COD, total iron, turbidity, TDS and TSS
removal efficiency
2.
To investigate the impact of operating parameters ( pH, ferrous/ferric
dosage, peroxide dosage, reaction time) on COD, total iron, turbidity,
TDS and TSS removal efficiency
3.
To monitor the kinetic of degradation in order to determine the kinetic
constant with respect to COD reduction
Scope of the Study
This study is limited in following conditions.
- Treatment performance evaluation is conducted using Jar test apparatus
under normal laboratory room temperature at Department of Chemical
Engineering, Faculty of Engineering, Burapha University.
- Real de-rusting wastewater from Kation Power Ltd (Thailand).is used for
Fenton and Fenton-like reactions.
- Chemical oxygen demand (COD) and total iron (ferrous and ferric) are main
objective parameters. Other wastewater parameters including total
suspended solid (TSS), total dissolved solid (TDS), turbidity, BOD are also
in-line monitored in influent and effluent of Fenton processes.
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- Kinetic degradation and reaction rate and EDTA will NOT be conducted
directly because both organic acids are measured in COD value. Therefore,
kinetic degradation and kinetic constant is conducted in respected to COD
- Oxidation products are not monitored in this study
- Independent variables are concluded reaction time, Fe2+/ Fe3+ concentration,
H2O2 concentration, pH, molar ratio of Fe2+/ Fe3+ :H2O2 whereas COD and
total iron are dependent variables. Control variables are included
temperature, mixing speed, and characteristic of wastewater.
Significance of the Study
The overal motivation of the present study is to investigate the applicability
of Fenton and Feton-like processes in the treatment of high concentrated derusting
wastewater from rust cleaning processes. Better water and wastewater management is
of great importance to economic advantages, regulation compliances, and ecological
considerations. The results of this study can contribute and illustrate several
beneficial considerations.
Firstly, this study demonstrates the fessible applicability of Fenton and
Fenton-like process as a means to solve the encountered derusting wastewater
treatment problem as practiced in accordance with standard effluent stipulated in
national regulation.
Secondly, even though Fenton oxidation have been applied extensively and
enormously in many differrent types of wastewater, its application for derusting
wastewater was not well documented in literatures. Thus, this present study will
contribute to comprehensive and extensive knowlegde and discussion on de-rusting
wastewater treatment which is known be be contiminated with chelating organic
compounds and high in metal concentration.
Thirdly, it is probably advantageous for Fenton and Fenton-like process to
utilize iron metal (ferric and ferrous ion) that already existed in de-rusting wastewater.
If they do, there will be economical and cost-effective for reagents uses for the
treatment of this wastewater.
Finally, it was supported in general that Fenton oxidation has its own
specification and commonness for many different types of industrial wastewater due
8
to the economic advantages, ease of application, effectiveness and non-selectivity in
the contaminant reduction and removal. Therefore, this study could bring the new
insight for the better treatment option for industry.
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CHAPTER 2
LITERATURE REVIEWS
This chapter provides the comprehensive reviews on advanced oxidation
technologies (AOTs). Indeed, Theoretical and empirical reviews on Fenton and
Fenton-like reaction mechanism, kinetics, influencing factors and their applications
are critiqued consecutively. Finally, the application of EDTA and citric acid and the
treatment method by Fenton oxidation are also reviewed.
Advanced Oxidation Processes (AOPs)
Industrial activities generate wastewaters with a wide variety of
contaminants, such as phenol and derivatives, hydrocarbons, halogenated sulphur and
nitrogen-containing organic compounds, heavy metals and other organic complexes.
Frequently these wastewaters contain a mixed pool of pollutants in a wide range of
concentrations. The development of cost-effective technical solutions is needed to
successfully deal with the increasingly complex problems arising in the field of
industrial wastewater. In recent decades, chemical treatment methods involving the
generation of hydroxyl radicals, known as advanced oxidation processes (AOPs),
have been applied successfully for the removal or degradation of recalcitrant
pollutants based on the high oxidative power of the HO· radical with electrochemical
oxidation potential (EOP) 2.8 V which is comparatively be second to fluorine as
shown in table 1 (Poyatos et al., 2010).
A chemical wastewater treatment using AOPs can produce the complete
mineralization of pollutants to CO2, water, and inorganic compounds, or at least their
transformation into more harmless products. Furthermore, the partial decomposition
of non-biodegradable organic pollutants can lead to biodegradable intermediates. For
this reason, combined AOPs as pre-treatments, followed by biological or chemical
processes, are both cost efficient and extremely viable from an economic perspective
(Poyatos et al., 2010).
AOPs represent the newest methods in H2O2 technology which include
photochemical degradation processes (UV/O3, UV/ H2O2), photocatalysis (TiO2/UV,
photo-Fenton reaction), and chemical oxidation processes (O3, O3/ H2O2, H2O2/Fe2+).
10
Although advanced oxidation processes (AOPs) have employed different reagent
systems, they all produce hydroxyl radicals. These radicals are very reactive, attack
most organic molecules, and nonselective (Kalra et al., 2011; Lucas & Peres, 2009;
Poyatos et al., 2010).
Table 1 Oxidizing potential for conventional oxidizing agents
Oxidizing agent
Oxidation Potential (EOP),
EOP relative to
V
Chlorine (V)
Fluorine
3.06
2.25
Hydroxyl radical (HO·)
2.80
2.05
Oxygen (atomic)
2.42
1.78
Ozone
2.08
1.52
Hydrogen peroxide
1.78
1.30
Hypochlorite
1.49
1.10
Chlorine
1.36
1.00
Chlorine dioxide
1.27
0.93
Oxygen (molecular)
1.23
0.90
Poyatos et al., (2010) have described in detail that these advanced oxidation
processes can be classified either as homogeneous or heterogeneous. Homogeneous
processes can be further subdivided into processes that use energy and processes that
do not use energy (Figure. 1). The following sections describe a wide range of
advanced oxidation systems that are currently being studied for their possible use in
wastewater treatment.
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Advanced Oxidation Processes
Homogeneuos
process
Using Energy
Ultraviolet
Radiation
- O3/UV
- H2O2/UV
- H2O2/O3/UV
- PhotoFenton(Fe2+/
H2O2/UV)
Without Energy
Ultrasound
Energy
- O3/US
- O3/US
Heterogeneuos
process
Electrical
Energy
- Electrochemical
oxidation
- Anodic
Oxidation
- Electro-Fenton
- O3 in alkaline
Medium
- O3/ H2O2
- Fenton Process
Fe2+/ H2O2
- Fentton-like
Fe3+/H2O2
- Advanced
Fenton
Fe0/H2O2
- Catalytic
Ozonization
- Photocatalytic
Ozonization
-Heterogeneous
Photo-catalysis
Figure 1 advanced oxidation processes (AOPs) classification.
Among the above advanced oxidation technologies, Fenton oxidation is the
most common method which has been used for many different industrial wastewaters
due to economic advantages, ease of application, and effectiveness in the contaminant
reduction and mineralization. It was also considered that Fenton oxidation presents
one of the best methods for clean and safe processes for the degradation of organics
even at higher initial organic content (Bianco et al., 2011; Lucas & Peres, 2009).
Fenton Reagent Mechanism
Basic Principle
The term Fenton’s reagent refers to the aqueous mixture of Fe (II) and
hydrogen peroxide. The Fenton’s reagent was first discovered and used by H. J. H.
Fenton in 1894 when he observed that the rate of oxidation of tartaric acid increased
dramatically when dilute hydrogen peroxide with the solution containing dissolved
Fe2+ ions. Forty years later, after a controversial history about the reaction mechanism
12
of Fenton’s reaction, its reaction mechanism was interpreted by Haber and Weiss in
1934 that Fenton’s chemistry is a reaction between hydrogen peroxide (H2O2) and
Fe2+ ions forming hydroxyl radicals, which is the main oxidizing agent. However the
hydroxyl radical mechanism of the Fenton’s reaction for destroying toxic organics
was not applied until the late 1960s (Ciambelli et al., 2008; Matthew Tarr, 2003;
Neyens & Baeyens, 2003).
In the last decades, advanced oxidation processes (AOPs) based on Fenton
reaction has been frequently involved in many different industrial wastewater
treatment processes for degrading and remediating of a wide range of contaminants,
predominately toxic, recalcitrant , and persistent organic pollutants (POPs) (Matthew
Tarr, 2003). The primary benefits of Fenton reagent are the ability to convert broad
range of pollutants to harmless or biodegradable products, its benign nature (residual
reagent do not pose environmental threat), the availability of relatively low cost
reagent (Bautista et al., 2007; Matthew Tarr, 2003), ease of implementation, no
energy input , short reaction time, and considerably clean and safe process the
degradation of organic compounds (Bianco et al., 2011; Lucas & Peres, 2009).
However, Matthew Tarr, (2003) stated that the major drawback to utilize Fenton’s
reagent are the interferences from non-pollutant species, difficulty in application to
the subsurface, generation of excessive or explosive heat under aggressive condition
and waste reagent cost due to inefficient application or inefficient pollutant
degradation in the subsurface, as well as the sludge generation, but these problems
can be justified in term of its advantages.
Conventional Fenton Process
The oxidation mechanism in the Fenton process, a cost-effective method,
easy to apply, involves the reactive hydroxyl radical generated under acidic conditions
by the catalytic decomposition of hydrogen peroxide initiated with ferrous ions (Fe2+).
The generated hydroxyl radical reacts unselectively within few seconds to minutes
with organic substances (RH), which are based on carbon chains or rings and also
contain hydrogen, oxygen, nitrogen, or other elements. The oxidation mechanism of
Fenton’s reagent is very complex, and the widely accepted major chemical reactions
are summarized as shown below (Ameta et al., 2012; Bianco et al., 2011; Jiang et al.,
13
2010; Lee & Shoda, 2008; Lucas & Peres, 2009; Matthew Tarr, 2003; Neyens &
Baeyens, 2003; Munter, 2001).
Table 2 Fenton reaction mechanisms and reaction rate constants
No.
Reactions
Rate constants (k)
(1)
Fe2+ + H2 O2 → Fe3+ + OH• + OH− ( chain initiation) 70 M-1s-1
(2)
Fe2+ + •OH → Fe3+ + OH− (chain termination)
3.2 108 M-1s-1
(3)
Fe2+ + HO2• → Fe3+ + HO2−
1.3 106 M-1s-1
(4)
•
OH + H2 O2 → HO2• + H2O (scavenging effects)
3.3 107 M-1s-1
(5)
Fe3+ + H2O2 → Fe2+ + H+ + HO2•
0.001-0.01 M-1s-1
(6)
Fe3+ + HO2• → Fe2+ + H+ + O2
1.2 106 M-1s-1
(7)
RH + •OH → R• + H2O → further oxidation
107 -1010 M-1s-1
(8)
R• + Fe3+ → product + Fe2+
NA
As shown in equation (1), the ferrous iron (Fe2+ ) initiates and catalyses the
decomposition of hydrogen peroxide (H2O2) to generate the hydroxyl radicals (OH• ).
The reaction (1) is commonly known as the main reaction of Fenton process. The
generation of the radicals involves the complex reaction sequence in an aqueous
solution. In addition to the main reaction, various additional competitive reactions are
also possible involving ferrous ions (Fe2+) , ferric ions (Fe3+ ), superoxide (HO2• ) ,
and hydroxyl radicals (OH• ) as shown through reaction (2)-(6) in table 2.
Moreover, the newly formed ferric ions (Fe3+) may catalyze hydrogen peroxide,
causing it to be decomposed into water and oxygen. Ferrous ions and radicals are also
formed in the reaction as shown in reaction (5)-(6) in table 2. The reaction of
hydrogen peroxide with ferric ions is referred to Fenton-like reaction (Ameta et al.,
2012; Bianco et al., 2011; Matthew Tarr, 2003; Neyens & Baeyens, 2003).
In the presence of organic substrates (RH), highly reactive hydroxyl radical
which are species with a relatively short life-span (rate constants in the range 107 -1010
M-1s-1), undergoes oxidation generating a new radical as provided in reaction (7). The
possible organic compounds present in reaction mixture can suffer an abstraction of a
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hydrogen atom (proton abstraction) or addition of hydroxyl radical (OH•) with the
production of organic radicals (R•) which can subsequently be oxidized by ferric ions
(Fe3+) as indicated in reaction (8). Indeed, the reaction (8) regenerates ferrous ions
(Fe2+) which ensure the continuity of the chain reaction. As long as the concentration
of reactants are not limited or available in the system, the iron species continually
cycle between Fe2+ and Fe3+ unless additional reaction result in formation of
insoluble iron oxides and hydroxides. This can lead ultimately to the decomposition
of organic substrate in carbon dioxide (CO2) and water in case of substituted organic
compounds and inorganic salts (Lucas & Peres, 2009; Matthew Tarr, 2003; Neyens &
Baeyens, 2003).
Fenton-like Process
The conventional Fenton has been modified to improve treatment efficiency
with the reduced inorganic sludge production and prevention of inhibition reaction of
some ions. Among the emerging Fenton technologies, photo-Fenton, electro-Fenton,
electro-photo Fenton and Fenton-like reaction are commonly investigated and applied.
Fenton-like process uses other transition metal catalyst other than Fe2+.
Some studied investigated the use of Fe-containing zeolites, soluble
manganese (II) and amorphous and crystalline manganese (IV) oxide, soluble iron
(III), mixture of Fe2+/ Cu2+ and Fe3+/ Cu2+ , suspended iron power, clay-based Fe
nanocomposite . However, conventional and lower cost FeCl3 were recommended for
a common practice of Fenton-like reaction initiation. Since the Fenton-like reaction
can be applied interchangeably or comparatively with Fenton reaction, It was recently
selected for wastewater treatment application in term of cost-effectiveness, efficiency,
and easy of application as reported in literatures (Fu, Wang, & Tang, 2009; Hodaifa et
al., 2013; Jiang et al., 2010, 2013; Kim et al., 2010; Kiril Mert et al., 2010; Li et al.,
2013).
Although Fenton did not observe hydroxyl radical-mediated reaction for
mixtures of Fe3+ and hydrogen peroxide, recent studies have illustrated that such
system can produce hydroxyl radical. The classic Fenton’s reaction was interpreted
by Haber and Weiss (1934) and reaction consists of a combination of H2O2 and
ferrous ions (Fe2+) in aqueous acidic medium, which leads to the decomposition of
H2O2 into a hydroxyl ion and a hydroxyl radical and the oxidation of Fe2+ to Fe3+ and
15
the oxidized Fe3+ starts further reaction with excessive H2O2 to produce hydroxyl
radical (Ameta et al., 2012). Additionally, Haber and Weiss originally proposed a free
radical mechanism for the Fe3+ -catalyzed decomposition of hydrogen peroxide. These
reactions are illustrated in reaction (1)-(6) in table 2. The treatment of pollutants with
Fe3+/ H2O2 mixture has been referred to the Baber-Weiss process. It is clear that the
same set of reaction is involved in both the Baber-Weiss process and Fenton process.
The hydroperoxyl radical (HO2•) formed in reaction (4) and (5) is a good reducing
agent and under some circumstances, it may reduce pollutant species.
More specifically, Jaing et al. (2013) has indicated the interconversion of
Fe(III)/Fe(II) in Fenton and Fenton-like reaction that they are co-occurring or
coexisting. A Fenton-like reaction involves a classical Fenton reaction, and Fenton
reaction may also involve a Fenton-like reaction step. However, Jaing et al. (2010)
and Neyens & Baeyens (2003) demonstrated the major differences in their system.
Conventional Fenton reaction is referred to Fe2+/H2O2 system, whereas Fenton-like
reaction is include in Fe3+/H2O2 system. Therefore, the reaction mechanisms are
similar in both systems; they differentiate according to the typical Fe3+/ Fe2+ are
utilized to initiate the reaction.
Hydroxyl Radical Reaction with Organic Compounds
A hydroxyl radical is applicable and capable for oxidizing a wide range of
organic compounds or molecules because of its interesting characteristics, which are
important in Fenton processes. These characteristics are short-lived, easily produced,
powerful oxidant, electrophilic in behavior, ubiquitous in nature, highly reactive, and
nonselective. Thus, hydroxyl radicals have emerged not only as an effective but also
as an economic and eco-friendly species (Ameta et al., 2012).
The contaminants are degraded to smaller or less harmful fragments and, in
the majority of cases, complete mineralization of the pollutants has been achieved.
Even persistent organic pollutants (POPs) can be degraded to the desirable extent
using Fenton process involving hydroxyl radicals as an active oxidizing agent. The
complete mineralization of an organic pollutant leads to the formation of carbon
dioxide (CO2), water, and/or some inorganic ions, depending upon the molecular
16
composition of that pollutant. Once generated, the hydroxyl radicals aggressively
attack virtually all organic compounds (Munter, 2001).
General Mechanisms
For the reaction of hydroxyl radical with organic species, there are three
common reaction pathways: (a) hydroxyl radical addition to an unsaturated compound
(Aromatic or aliphatic) to form the free radical products, (b) hydrogen abstraction
where an organic free radical and water are formed (c) electron transfer, where ions of
higher valence state are formed reducing hydroxyl radical to hydroxide ions (Matthew
Tarr, 2003; Munter, 2001; Neyens & Baeyens, 2003). These generic mechanisms are
illustrated in detail in Table 4 below.
Table 3 Mechanisms for hydroxyl radical reaction with organic compounds
No.
(9)
Reaction Mechanism Pathways
RH + •OH
→ (OH)RH•
(Hydroxyl Radical Addition)
C6H6 + •OH → (OH) C6H6•
(10)
RH + •OH
→ R• + H2O
(Hydrogen Abstraction)
CH3OH + •OH → CH2OH• + H2O
(11)
RH + •OH
→ (RH)• + + OH−
(Direct Electron Transfer)
[Fe(CH)6]4− + •OH → [Fe(CH)6]3− + OH−
Matthew Tarr, (2003) has emphasized that non-radical reactants, all three
mechanisms result in initial products that are radicals. Subsequent reactions follow to
yield non-radical products. Hence, additional reactants including Fe2+, Fe3+, H2O, O2,
H+ , •OH , other metals, other organics, and other radicals present in the system are
necessary to complete these subsequent reactions. This leads to the facts that the
further oxidation processes are continuously occur and dimerizeation can also occur if
the initially formed radical species reacts with another identical radical. Even though
the complete oxidation and mineralization of organic compounds are complicated and
various in reaction mechanisms, Munter, (2001) and Neyens & Baeyens, (2003) have
explained that in addition to the common abstraction of hydrogen atom to initiate a
radical chain oxidation in reaction (9), other possible reactions including radical
17
interaction where the hydroxyl radical reacts with other hydroxyl radical to combine
or to disproportionate to form the stable products are shown as following:
Table 4 Hydrogen atom abstraction reaction
No.
Reactions
(12)
•
OH + •OH
→ H2O2
(13)
R•
+ H2O2
→ ROH + •OH
(14)
R•
+ O2
→ ROO•
(15)
ROO• + RH
(dimerization of •OH)
→ ROOH + R•
More importantly, the organic free radical produced in the above reactions
may then be oxidized by Fe3+ reduced by Fe2+, or dimerized according to the
following reactions.
Table 5 Further oxidation of organic free radicals
No.
Reactions
(16)
R•
+ Fe3+ -oxidation → R+ + Fe2+
(17)
R•
+ Fe2+ -reduction → R− + Fe3+
(18)
R•
+ R• -dimerization → R−R
Neyens & Baeyens,( 2003) concluded that the sequences of reaction (1), (2),
(9), and (16) constitute the present accepted scheme for the Fenton’s reaction chain.
Reaction (9) is typical of aliphatic and alcohols, whereas reaction (10) is
common for double bonds, especially in conjugated and aromatic systems. To form
the final products, the radicals R• and ROH• undergo additional reactions. Free radical
scavengers are a very important component of Fenton systems (Matthew Tarr, 2003).
Typical reaction products include oxygenated products (alcohols, aldehydes,
oxyacids, ketones, etc.), ring opening products, and dimers. In some cases, complete
mineralization yields carbon dioxide as a final product.
18
If hydroxyl radical availability is high enough, the pollutants present will be
degraded to CO2 and other mineralized products. However, if a species of low
reactivity is produced, this species may become a major degradation product, and
complete mineralization will not be observed. For example, oxalic acid has a
relatively low rate constant for reaction with hydroxyl radical (1.4  106 M-1sec-1,
about three to four orders of magnitude lower than aromatics). Such highly
oxygenated compounds, however, are likely to be biodegradable. Consequently, such
species can be acceptable end points for remediation or waste stream treatment.
Matthew Tarr (2003) and Neyens & Baeyens (2003) mentioned that the
presence of hydroxyl radical scavengers also plays a key role in the overall fate of the
peroxide. Perhaps the biggest limitation of Fenton reagent for subsurface remediation
is the difficulty of delivering the reagent to the pollutant. In addition to physical
limitations in reagent injection efficiency, there are also limitations on a molecular
scale. Non-pollutant species act as hydroxyl radical scavengers, consuming the radical
before it can reach the pollutant. Scavenging of hydroxyl radical minimizes the
occurrence of reactions (2), and (4).
In applying Fenton’s Reagent for industrial waste treatment, the conditions
of the reaction are adjusted so that first two mechanisms (hydrogen abstraction and
oxygen addition) predominate. Typical rates of reaction between the hydroxyl radical
and organic materials are 109 – 1010 k (M-1 s-1).
Iron Ligand, Chelators and Coordination
Rate constants, hydroxyl radical concentration, pollutant concentration, and
the presence of other species could have significant impacts on the kinetic and
mechanism of Fenton-base degradation. The presence of both in organic and organic
iron ligands and coordination of Fe2+ and Fe3+ in natural systems or waste streams can
have a dramatic influence on the Fenton reaction. Their presence not only influences
on reaction rate, but also alters the lifetime of hydroxyl radical. This will result in
changes of Fe2+ concentration as well as further influence hydroxyl radical formation
rate. The common ligands may include OH−, Cl−, ClO4−, CO3−, PO43−, NO2−, NO3−,
and SO42−. Coordination by anionic ligands makes oxidation of Fe2+ easier which
yield an in increased rate constant for reaction. Conversely, Fe3+-coordinated by
anions may be difficult to reduce to Fe2+. Such changes in oxidation/reduction
19
potentials for Fe2+ and Fe3+ not only alter the rate constant for reaction (1), but also
change the relative concentration of the two ion species. Consequently, it leads to the
alteration of several reaction rates in iron/peroxide system (Fenton process). Some
anions form insoluble salts with iron. In these cases, the precipitation of iron may
severely decrease or eliminate its participation in Fenton reaction. Iron hydroxide and
oxyhydroxides are likely to precipitate under higher pH condition. As a result, Fenton
processes generally must be carried out at pH value well below 7, unless ironsolubilizing agents (chelators) are used.
Matthew Tarr (2003) mentioned that iron solubility has played an obvious
role in Fenton oxidation because the rate of hydroxyl radical formation is directly
proportional to [Fe2+]. The dramatic decline or decrease in hydroxyl radical formation
rate is the consequence of formation and precipitation of iron hydroxide and oxide
under elevated pH value. However, this issue can be overcome by using iron chelator
or lowering the pH value in operating condition. Another associated problem is that
the insufficient rate of Fe3+ reduction to Fe2+ can result in Fe2+ concentration that are
too low to produce hydroxyl radical at sufficient rates.
In addition to the inorganic ligands, The presence of organic ligands has
several important implications for Fenton chemistry: (a) the chelated iron species will
have a kinetic behavior different from that of pure aqueous iron; (b) the distribution
and cycling between Fe2+ and Fe3+ states will vary with different ligands; (c) the
solubility of iron, especially at high pH values, can be dramatically increased through
chelation; (d) oxidizable ligands bound directly to iron (the site of hydroxyl radical
formation) may be more likely to react with the radicals than other species, including
pollutants; and (e) some iron complexes form alternate oxidants other than hydroxyl
radical (Matthew Tarr, 2003). A number of reports have evaluated the Fenton reaction
as a function of iron ligands. Fenton reactions are substantially or completely
inhibited by some ligands including phosphate, desferal, diethylenetriamine
pentaacetic acid (DTPA) and phytate. These ligands are all strong coordinating
ligands or chelators.
It was contradicted that some ligands inhibit the Fenton process whereas
others may enhance it. The addition or presence of resolubilizing chelators or
chelating agents could cause an increase in the occurrence of reaction (1) and other
20
reaction necessary in the catalytic Fenton process. In contrast, complicating factor is
the hydroxyl radical –scavenging ability of the chelator. A good scavenger may
appear to have a lower production rat of hyrdoxyl radical due to rapid trapping of the
radical by the chelator. Matthew Tarr (2003) concluded that the inability of hydroxyl
radical to reach sorbed or sequestered pollutants is one of the major drawback to the
application of Fenton degradation method. However, it was suggested that aggressive
conditions including high H2O2 concentration could make possibility for direct
degradation of sorbed species.
Several studies have been investigated the effect of chelators on Fenton
reaction. Addition of chelators to Fe(III)- H2O2 systems (Fenton-like reaction) allows
for effective degradation at near neutral pH values. The influence of the iron chelators
form increased solubility of iron species at higher pH value. Iron chelators improved
the Fenton oxidation of pollutant by increasing iron solubility, and probably by
increasing the rate constant for hydroxyl radical formation from peroxide. The
chelators also act as hydroxyl radical from potential interaction with pollutants. Over
time, loss or absence of chelator results in poor iron solubility and slow rates of
hydroxyl radical formation. Earlier studies indicated that at pH 7.3, each EDTA-Fe
complex was able to produce more than 50 hydroxyl radical before being degraded
(Eckenfelder, 2000). The relative efficiencies of the chelators for hydroxyl radical
formation will determine whether the added chelators will have a positive or negative
effect on radical formation. Furthermore, increased radical formation is not always
correlated with improved pollutant degradation.
Matthew Tarr, (2003) reported that significant increase in k1 when iron was
chelated by DTPA, EDTA, nitrilotriacetic acid (NTA), and several aminophosphonic
acids. In comparison to aqueous Fe2+, increases in k1 from 1000-fold to 5000-fold
were identified. A study of 50 different iron chelators assessed the affect of each
chelator on the Fenton process initiated with Fe3+ and hydrogen peroxide. Among the
nine classes of chelators tested, the results indicated that the chelator ranged from
inactive to highly active in term of hydroxyl radical formation. It reaches to a
conclusion that the presence of iron ligands and coordination could bring both
positive and negative influences on Fenton process depending on specific property of
iron-coordinating complex.
21
Iron speciation is a major factor in Fenton chemistry. As discussed above,
iron solubility, redox potentials, and concentration of Fe2+ and Fe3+ are all dependent
on iron ligands and coordination. Very strong iron chelators inhibit the formation of
hydroxyl radical. Iron ligands can also act as hydroxyl radical scavengers. Ligands are
more likely to react with hydrxyl radical than pollutants that are not in close proximity
to the iron because radical is always formed in close proximity to these ligands. Such
coordination will alter the kinetics of hydroxyl radical formation as well as the
dynamics of hydroxyl radical interaction with pollutants.
Factors Affecting Fenton and Fenton-like Process
Operating condition plays a vital role in determining the efficiency of
Fenton and Fenton-like oxidation. The most significant factors affecting the both
processes are included reagent concentration (H2O2 and Iron Concentration), pH,
reaction time, temperature, and organic content. They are discussed in detail as
following:
Effect of pH
The optimal pH rang for the application of the Fenton process was also
determined to be at pH 3 to pH 6. The application of the Fenton process at high pH
value will result into inhibition of Fenton reaction since the Fe2+ ions will form the
colloidal Fe3+ ions. Likewise, the application of Fenton at very low pH value would
result into the decomposition of hydrogen peroxide into water and oxygen by iron
without forming hydroxyl radical (Neyens & Baeyens, 2003). Furthermore, Bautista
et al., (2007, 2008) reported that Fenton oxidation presented the maximum catalytic
activity at pH 2.8-3.0. At very low pH, H2O2 is stabilized as H3O2+. The reaction
between •OH and H+ also occurs. More importantly, Fe2+ regeneration by the reaction
of Fe3+ with H2O2 is inhibited at more acidic pH value. On the other hand, at the pH
higher than 3, Fe3+ can precipitate as Fe(OH)3 and decompose of H2O2 into O2 and
H2O without •OH production. Lou & Huang (2009) conducted a study on EDTA
degradation by Fenton process with pH ranged from 2 to 7. It was found that the
degradation of EDTA decreased from 80.3% to 27.5% over the reaction time of 10
min. This result indicated that the pH value significantly influenced the removal of
EDTA by directly affecting the generation of •OH and was similar to other studies,
22
which found that the optimum range for Fenton oxidation was 2-4 (San Sebastián
Martinez et al., 2003; Bautista et al., 2007; Z. Wang et al., 2011; Jiang et al., 2013).
At high pH, oxidation yield of the process decrease due to the precipitation of Fe3+ as
Fe(OH)3 which hindered the reaction between Fe3+ and H2O2 and thus influenced the
regeneration of Fe2+. Moreover, Fe(OH)3 functionally catalyzed the decomposition of
H2O2 into O2 and H2O which decrease the production of •OH. Therefore, pH of 2 was
the optimum condition for Fenton method in removal of EDTA. Similarly, Fu et
al.(2009, 2012) and Lan et al.(2012) found the optimal pH of 3 and 2-5, accordingly
for metal-EDTA complex wastewater treatment.
There have been some recent developments using non-radical scavenging
sequestering or chelating agents (e.g., gallic acid and EDTA) to extend the useful pH
range to pH 8-9, but no commercial applications are known (www.USPeroxide.com).
A second aspect of pH deals with its shift as the reaction progresses. Provided an
initial wastewater pH of 6.0, the following profile is typical of Fenton reactions result
in degrease of pH. The first inflection is caused by the addition of FeSO4 catalyst
which typically contains residual H2SO4. A second, more pronounced drop in pH
occurs as the H2O2 is added, and continues gradually at a rate which is largely
dependent on catalyst concentration. This drop in pH is attributed to the fragmenting
of organic material into organic acids. This pH change is often monitored to ensure
that the reaction is progressing as planned--the absence of such a pH decrease may
mean that the reaction is inhibited and that a potentially hazardous build-up of H2O2 is
occurring within the reaction mixture. In highly concentrated waste streams (>10 g/L
COD), it may be necessary to perform the oxidation in steps, readjusting the pH
upwards to pH 4-5 after each step so as to prevent low pH from inhibiting the reaction
(www.USPeroxide.com)
Effect of Temperature
The reaction temperature is another crucial parameter in the Fenton process.
The effect of temperature on the rate of reaction of the Fenton process was also
studied and was found to increase as the solution temperature increase. However, the
effect of temperature was only obvious at temperature lower than 20 °C. In addition to
this application of temperature greater than 40 °C, the treatment efficiency declined
due to the decomposition of H2O2 into oxygen and water. Application of the Fenton
23
process has been normally conducted at temperature of 20 to 40 °C (Bautista et al.,
and references therein, 2008). However, San Sebastián Martinez et al., (2003) found
that temperature showed only a mild positive effect on COD removal. Wang (2008)
conducted a comparative study of Fenton and Fenton-like reaction kinetics in
decolorization of wastewater. The result has been indicated that temperature had little
influence on overall dye degradation in the range 15-45 °C. Dye degradation rate
decreased when the temperature greater than 30 °C due to decomposition of H2O2 at
higher temperature. Fu et al. (2009) gave the significance of temperature influencing
the Fenton and Fenton-like oxidation. It was clear that the increase of temperature
could increase the removal efficiency in the system because higher temperature
increases the reaction between hydrogen peroxide and Fe2+/Fe3+, and improve the
generation rate of hydroxyl radicals. The increase temperature from 25 to 50 °C, In
addition, the removal efficiency of Ni increased from 72.1 to 97.2% for Fenton and
from 74.3 to 96.7% for Fenton-like after 20 min. This is because that the reaction of
Fenton and Fenton-like could be accelerated by increasing the temperature. However,
Ni removal efficiency is marginal for both systems after 60 min of reaction time.
Nieto et al., (2011) gave fact allows working on an industrial level in the treatment of
OMW without temperature control, whereas the Fenton reaction is exothermic
(optimal temperature varied from 20 to 30 °C). Consequently, temperature was not
considered in the optimization of Fenton’s reaction in highly polluted industrial
wastewater. In short, this leads to a conclusion that temperature is important but not
necessary for Fenton reactions because of exothermic effects of reaction leading to
increase of temperature in a suitable of range as found in the work of Bautista et al
(2008); Wang (2008); Fe et al. (2009, 2012);and Lan et al.(2012).
Effect of Iron (ferrous or ferric) Concentration
Iron concentration plays a vital role in determining the treatment efficiency
of Fenton and Fenton-like reactions because production rate of •OH is proportional to
the concentration of iron and hydrogen peroxide. However, iron content is the
determining factors in sludge production as a challenge for Fenton reaction (Z. Wang
et al., 2011). In the absence of iron, there is no evidence that •OH is produced in
wastewater. In case of inadequate concentration of iron in the operating condition will
lead to insufficient production of •OH, whereas overdosing of iron can favor the
24
scavenging reaction which prevents the reaction of •OH with contaminants resulted in
poor treatment efficiency (Matthew Tarr, 2003; Neyens & Baeyens, 2003). Lou &
Huang (2009) also investigated the influence of ferrous concentration on EDTA
degradation by Fenton oxidation. It was found that the increase of ferrous
concentration from 10-4 M to 10-2 M resulted in degradation of EDTA from 29.8% to
98.5% at a reaction time of 10 min respectively. Thus, the efficiency increased
progressively with the Fe2+ concentration increased because large amounts of the OH·
radical were generated by the reaction. However, increasing Fe2+ concentration from
10-2 M to 10-1 M could decrease EDTA degradation from 98.5% to 44.9%
accordingly. A higher of Fe2+ dose provided the scavenging reaction between Fe2+ and
OH·. As reported in the work of Fu et al. (2009) that Ni removal efficiency were
influenced by the concentration of Fe2+ and Fe3+ for Fenton and Fenton-like,
respectively. The increase of initial Fe2+ or Fe3+ from 0 to 1.0 mM resulted in
increasing the removal efficiency remarkably. When Fe2+ or Fe3+ concentration was
1.0 mM, Fenton and Fenton-like systems achieved 92.8% and 94.7% of Ni removal
efficiency at after 60 min, accordingly. However, further increase of Fe2+ and Fe3+
concentration did not achieve the improvement in Ni removal. This has been revealed
that the use of much Fe2+concentration could lead to the self-scavenging of •OH by
Fe2+ as explained literatures (Matthew Tarr, 2003; Neyens & Baeyens, 2003; Lou &
Huang (2009). A minimal threshold concentration of 3-15 mg/L Fe which allows the
reaction to proceed within a reasonable period of time regardless of the concentration
of organic material. A constant ratio of Fe: substrate above the minimal threshold,
typically 1 part Fe per 10-50 parts substrate, which produces the desired end products.
It should be noted that the ratio of Fe: substrate may affect the distribution of reaction
products. A supplemental aliquot of Fe which saturates the chelating properties in the
wastewater, thereby availing unsequestered iron to catalyze the formation of hydroxyl
radicals. Iron dose may also be expressed as a ratio to H2O2 dose. Typical ranges are 1
part Fe per 5-25 parts H2O2 (wt/wt) (www. USPeroxide.com).
Effect of H2O2 dose
The amount of H2O2 is considered one of the most important factors in
Fenton and Fenton-like reaction owing to economic cost, sources of •OH generation ,
improvement of treatment efficiency and side effects in overdosing. The H2O2 dose
25
has to be fixed according to the initial pollutant concentration. It is frequent to use an
amount of H2O2 corresponding to the theoretical stoichiometric H2O2 to chemical
oxygen demand (COD) ratio, although it depends on the response of the specific
contaminants to oxidation and on the objective pursued in term of reduction of the
contaminant load (Neyens & Baeyens, 2003; Bautista et al., 2007; Lan et al.,2012).
Indeed, Lan et al. (2012) signified the effect of H2O2 on the removal of COD with the
initial value of [H2O2]/[COD] from 0.5 to 6.0. The results indicated that the increase
in [H2O2]/[COD] from 0.5 to 2.0, the COD removal by Fenton oxidation was
increased remarkably from 73.6% to 89.4%. However, the further increase in
[H2O2]/[COD] from 2.0 to 6.0, the removal of COD was negligible or unchanged.
Similarly, W. Wang et al. (2011) also investigated the [H2O2]/[COD] ratio at 0.6, 2, 4,
and 8 for the reduction of initial COD of 300 mg/L. The results were shown that the
removal of COD was enhanced from 70.3 % to 82.2% within 120 min of reaction
time by increasing [H2O2]/[COD] from 0.6 to 4 (37.5 mg/L). However, when
[H2O2]/[COD] was increased to 8, the COD removal was improved slightly.
Similarly, Bautista et al. (2007) specified the H2O2 dose for cosmetic wastewater
treatment (initial COD= 2395 mg/L). It was found that at the condition of the initial
Fe2+ of 200 mg/L, [H2O2]/[COD] ratio at 2.12 and [Fe2+]/ [H2O2] at 1:10 resulted in
about 60 % of COD removal. Nevertheless, insignificant effect was observed as H2O2
dose increased. The marginal change or improvement of COD removal in the reported
works may be explained by the scavenging effect of excessive H2O2 to •OH and
recombination of •OH as shown previously in reaction (4) which were supported in
literatures (Neyens & Baeyens, 2003; Matthew Tarr, 2003; Bautista et al., 2007, 2008;
Wang, 2008; Wang et al., 2011; Lucas & Peres, 2009). Therefore, stoichiometric
relation between COD and H2O2 are significant for Fenton reaction and acceptable
[H2O2]/[COD] weight ratio should in the range of 2-4.
Effect of Iron/ H2O2 ratio
The dose of H2O2 and the concentration of Fe2+ are the two relevant factors
affecting the Fenton process and performance. For most applications, it is important to
optimize the molar ratio of [Fe2+/3+]/ [H2O2] because it does not only directly affect the
economic cost and hydroxyl radical generation in Fenton reaction, but also relate to
the amount of sludge generated from Fenton oxidation process. It does not matter
26
whether Fe2+ or Fe3+ salts are used to catalyze the reaction (Matthew Tarr, 2003;
Neyens & Baeyens, 2003).
Kim et al. (2010) drawn a conclusion that the presence of Fe2+ or Fe3+ salts
not only functions as catalytic reagents to decompose H2O2 for •OH generation but
also reduce the scavenging effect of •OH radical from H2O2. The role of Fe3+ plays an
important role in oxidizing the target organic compound and producing •OH radical
through Fe2+ reaction as shown previously in reaction (1), (5) and (8). [Fe2+/3+]/
[H2O2] is difficult to specify and varied according to the degradation of different
pollutants covering the range from 1:1 to 1:400 for a complete oxidation as reported
in De Souza et al. (2006).
As a reflection, Wang et al. (2011) monitored the effect of [Fe2+]/ [H2O2]
molar ratio of 1:50, 1:20, 1:10: , 3:4 while [H2O2] was kept constant at [H2O2]= 4
COD (mM/L), pH 3, temperature at 30 °C , and reaction time for 120 min for COD
removal ([COD]0 = 300 mg/L). The results were indicated that higher reaction rate of
COD removal (>55%) were achieved in the first 10 min at higher [Fe2+]/ [H2O2]
molar ratio which resulted from higher generation of generation of •OH radical
according to reaction (1) as shown previously. However, It was noted that the 120 min
COD removal tended to decline in case of molar ratio of [Fe2+]/ [H2O2] greater than
1:20 which can be explained by the quenching or scavenging effects of OH radical by
excessive Fe2+ according to reaction (2). It can be concluded that [Fe2+]/ [H2O2] ratio
of 1:20 attain highest performance for greater than 85% of COD removal. It was also
supported by San Sebastián Martinez et al (2003) who conducting pre-oxidation of an
extremely polluted industrial wastewater (effluent COD value of 362000 mg/L). To
achieve 90% of COD removal, it was required to maintain the optimal [Fe2+]/ [H2O2]
molar ratio of 1:10, while [H2O2] was 3M. This molar ratio was comparatively found
to be lower than that of [Fe2+]/ [H2O2] molar ratio at 1:15 resulted in the study of
Lucas and Peres (2009). It is clear that [Fe2+]/ [H2O2] molar ratio is varied
corresponding to type and concentration of organic pollutant existing in wastewater
and the typical range of Fe2+]/ [H2O2] ratio is 1:5-25 as reported in Bautista et al.
(2008) and www.USperoxide.com.
27
Effect of Reaction Time
The time needed to complete a Fenton reaction will depend on the many
variables discussed above, most notably catalyst dose and wastewater strength.
Typical reaction times are 30 - 60 minutes for simple wastewater. For more complex
or more concentrated wastes, the reaction may take several hours. Determining the
completion of the reaction may prove troublesome. However, effects of reaction time
for Fenton oxidation have been reported in literatures. Fu et al. (2009, 2012) tested the
reaction time from 20 – 120 min for Fenton and Fenton-like reactions. It was found
that at 60 min both processes achieved about 98.4% Ni removal with the reduction of
Ni concentration from 50 mg/L to 1 mg/L and COD decreased from 252 mg/L to 53.3
mg/L, indicating about 78.8% COD removal. After 60 min of reaction, the removal
efficiency was marginal or almost unchanged. Similarly, Lan et al. (2012) also
emphasized COD and Cu(II) removal as a function of time for Fenton oxidationcoagulation process. After 20 min of interior microelectolysis (IM), the proper
reaction time of 60-80 min were required for a complete Cu(II) and 87.0% removal
efficiency for Fenton oxidation-coagulation. However, Wang et al. (2011)
investigated the required reaction of Fenton oxidation for bleaching effluent. With
heterogeneous and complicated characteristics of wastewater, it required 120 min to
yield 88.7% of COD removal, indicating the reduction of COD from 300 mg/L to 40
mg/L. Unlike S. Wang et al. (2011), San Sebastián Martinez et al. (2003) and Jiang et
al. (2013) achieved optimal removal efficiency in the first 10 min of Fenton reaction
due to the fast reaction in the first stage of Fenton oxidation, while prolonging the
reaction time remained efficiency insignificantly changed. However, it was required
longer than 1 hour reaction time for metal-complex wastewater treatment due to their
persistency in waste stream Pirkanniemi et al. (2003). It was interesting to note that
the application of Fenton oxidation to industrial wastewater treatment typical ranged
from 1 to 4 hours for optimal reaction time as reviewed in Bautista et al.(2008).
Effect of Post-treatment
As reported in literatures, Fenton reaction functions as the pre-treatment
process for organic destruction, COD removal, biodegradability improvement, and
detoxification of industrial wastewater, which can be subsequently followed by
conventional treatment methods such as chemical precipitation, coagulation and
28
flocculation, and biological processes (Neyens & Baeyens, 2003; Bautista et al., 2007;
Matthew Tarr, 2003; Fu, Wang, & Tang, 2009; Fu et al., 2012; Lan et al., 2012).
Neyens & Baeyens (2003) and Kang et al.(2002) also reported that Fenton’s
reagent could serve both oxidation and coagulation functions depending on the
[H2O2]/ [Fe2+] ratio. When amount of Fe2+ required exceeds the amount of H2O2 tends
to have oxidation effects. However, when both reagents are reversed, the treatment
tends to have chemical oxidation effects. A combination of process was studied and
reported in literatures. Ma & Xia (2009) applied Fenton process combined with
coagulation for water-based printing ink wastewater treatment. It was found that
92.4% of COD and 86.4% of color removal were achieved after 30 min of settling
time at following condition: pH=4, [H2O2]= 50 mg/L, [Fe2+]=25 mg/L. The Fenton
effluent was treated continuously by coagulation using polyaluminium chloride and
FeSO4 to improve Fenton treated effluent in reducing flocs settling time, enhancing
color, COD removal. Overall removal of color, COD and SS were obtained 100%,
93.4% and 87.2%, respectively.
Fu et al. (2009, 2012) incorporated Fenton reactions with chemical
precipitation for NiEDTA wastewater treatment. Treatment mechanisms were simply
explained that Fenton reaction was responsible for organic complex degradation
measure in term of COD, while hydroxide precipitation was employed to precipitate
heavy mentals by conveniently adding alkali (increase pH to 11.5). As a result, greater
than 92.8% of Ni and about 78.8% of COD were obtained. In addition, the recent
work of Lan et al. (2012) employed interior microelectrolysis and Fenton oxidationcoagulation for EDTA-Cu(II) containing wastewater treatment. The results were
shown that interior microelectrolysis (IM) primarily contributed for Cu(II) removal
from EDTA-Cu(II) complex; nevertheless, IM effluent found to be rich in Fe(II).
Therefore, Fenton oxidation proceeded automatically when adding H2O2. As a result,
for Cu (II) removal, the contribution of IM, Fenton oxidation, and coagulation were
97.5%, 0%, and 25%, respectively. However, IM, Fenton oxidation, and coagulation
could achieve 22.3%, 47.8% and 10.9%, accordingly, for COD removal. This study
had a consistency with Ma & Xia (2009).
As above mention, Fenton oxidation plays a vital role in biodegradability
improvement for highly polluted industrial wastewater. after treatment, Lan et al
29
(2011) also monitored the biodegradability in term of BOD5/COD ratio and was found
to be enhanced from 0 to 0.42, which indicating that EDTA was effectively oxidized
in the combine system. This biodegradability improvement was consistency to the
work of S. Wang et al. (2011). It was found the increase of BOD5/COD ratio from
0.22 to 0.59, which could reach to an extent compatible with biological treatment
(BOD5/COD > 0.5) as reported in Bautista et al. (2008).
EDTA Degradation by Various Fenton Processes
As reported in literatures, there were a number of studies of advanced
oxidation processes based on Fenton oxidation in order to degrade or mineralize the
chelating agents particularly EDTA. Due to the decontaminating or mineralizing
ability of H2O2 for organic pollutants, it is considered as eco-friendly and safe reagent
(Bautista et al., 2008). It was reflected by Sillanpää and Rämö (2001) in the result of
degradation of EDTA, DTPA, and ß-ADA by H2O2 oxidation in alkaline environment
at the same pollutant concentration of 0.04 mM at pH 10 with oxidation time of 30
min. The result indicated that degradation of ß-ADA (29%) by H2O2 was highest if
compared to the degradation of EDTA and DTPA (6%). EDTA degradation was
applied higher dose of H2O2 (147 mM) than DTPA (35.29 mM). This can be inferred
that EDTA and DTPA are more recalcitrant to the oxidation’s degradation than ßADA as similarly reported in other literatures (Ghiselli et al., 2004; Chitra et al.,
2011; Fu et al., 2009). In the meanwhile, the authors studied the degradation of EDTA
by hydrogen peroxide in alkaline conditions (pH 10-11) found that to minimize the
use of EDTA wherever possible, until an effective means for its removal has been
developed. It was recommended that the use of an effective catalyst might appreciably
increase the conversion rate into more biodegradable decomposition products (Rämö
& Sillanpää, 2001).
To improve degree of mineralization or degradation of targeted chelating
agents, H2O2 have to be activated or catalyzed by homogenous catalyst such as Fe2+.
Addition of iron catalyst can produce very potential hydroxyl radicals which plays a
vital role in attack and destroy other pollutants (Matthew Tarr, 2003). This concept
could be reflection of a study on treatment of waste containing EDTA by chemical
oxidation (Tucker et al., 1999). It was indicated that only 90% of EDTA was
30
degraded at the initial concentration of 70 mM in 45 min. This could be explained that
EDTA degradation by Fenton oxidation was suppressed by presence of mentalchelating complex which was counterproductive to degradation. It was also found in
the study that the presence of dissolved cations such as Mn (II) and Cr(III) inhibited
the degradation rate of EDTA if their concentrations are significantly higher than
chelating agents.
In another study, Ghiselli et al.,( 2004) applied the Fenton and Fenton-like
reactions under UV-A irradiation to degrade the EDTA. It was reported that with the
initial EDTA concentration of 5 mM, dark Fenton and Fenton-like reactions achieved
about 80% of EDTA removal with EDTA:Fe2+ and EDTA:Fe3+ ratio of 1:1 with the
initial peroxide concentration of 100 mM. in 4 hours. However, in both cases the
reaction rates were increased after 4 hours irradiation with the total EDTA
mineralization of 92 % (Fe2+, Fe3+, Fe3++Cu2+ system). The study concluded that the
rate of EDTA degradation was highly depended on the iron concentration and amount
of peroxide, as well as solution pH. The photolysis of Fe(III)-EDTA complex in
EDTA destruction can make use of high peroxide concentration unnecessary. The
authors have stressed that photo-Fenton reaction was suitable for the treatment of
wastewater from cleaning and decontamination of nuclear power plant because this
wastewater contained small amount of Fe2+ and Fe3+ coming from corrosion process.
Like other chelating agents, EDTA is prone to auto-oxidation. When Fe (II)
is added into the system, the initiation of chain reactions by the hydroxyl radical lead
EDTA to be degraded. Pirkanniemi et al.(2007) tested Fenton’s oxidation to degrade
EDTA from bleaching wastewater. It was reported that an almost complete removal
of EDTA was achieved at its concentration of 76 mM. This result was comparatively
higher than whose previously accomplished by Tucker et al.(1999), who only
degraded 90% of EDTA at an initial concentration of 70 mM.
In addition, Fenton-like system has been developed in recent year using
iron-supported catalyst like Fe(III) and zero-valent iron to improve efficiency and
sludge associated problem caused by conventional Fenton process (Neyens &
Baeyens, 2003; Bautista et al., 2008a; Jiang et al., 2013; Zhou et al.,2009and 2010).
Noradoun and Cheang (2005) attained 95% of EDTA degradation at an initial
concentration of 1 mM at pH 6.5 within 2.5 h. The researchers applied an oxygen
31
activation scheme to mineralize EDTA in zero-valent iron system. In another study,
Zhou et al. (2009) has applied an oxidative treatment by using heterogeneous ZVI and
ultrasound to facilitate reduction of O2 to H2O2. While being oxidized to Fe2+, ZVI
induced series of Fenton-like oxidation and degraded EDTA itself. In the system,
EDTA acts as a complexing agent with the dissolved Fe2+ and generates H2O2. The
result indicated that a lower EDTA degradation (81%) at its concentration of 0.32 mM
at pH 7.5 due to excessive iron catalyst added in solution that prevented the formation
of O-2-FeII/III EDTA, slowing down EDTA degradation by Fenton-like oxidation.
Indeed, it also was found that a non-radical degradation pathway was proposed for the
neutral US/Fenton like system. Ferryl-EDTA complex ([FeIVO] EDTA) instead of
OH• was identified as the dominant oxidant in the system.
Due to low cost and thermal stability, Pirkanniemi et al.(2003) applied
Fenton-like oxidation using metallophthalocyanine to degrade five different chelating
agents including EDTA from bleaching effluent. The rate of EDTA degradation was
found to be dependent on the concentration of Fe2+, H2O2, its molar ratio to the
Fenton’s reagent, pH, and temperature. It was found that almost complete degradation
of iron complexes of chelating agents studied was remarkably observed between 60%
to 100% under pH 1.5 and initial chelants concentration of 0.1M within a reaction
time of 1 h. In addition, the most relevant iron, manganese, sodium, copper and
calcium EDTA complexes can be successfully eliminated, the conversions being 93,
76, 68, 62 and 49%, respectively, after 3h of reaction.
A few years later, Pirkanniemi et al. (2007) further investigated the
application of Fenton’s reagent to degrade EDTA and novel complexing agents in
pulp and paper mill wastewater and elementary chlorine free bleaching effluent. The
result indicated that over 90% of EDTA was degraded within 3 min when temperature
was 60 °C, pH 4, and molar ratio of H2O2:Fe2+:EDTA was 70:2:1 (0.26 mM EDTA).
However, in bleaching effluent up to 42% of EDTA was degraded in similar reaction
condition due to complex mixture of chemical components and higher concentration
of organic matter apparently competed with EDTA for Fenton’s catalyst in bleaching
effluent. EDTA degradation increased remarkably with higher peroxide concentration
and increased reaction time in pulp and paper wastewater as well as bleaching
effluent. It was also stressed that nearly complete degradation of EDTA were achieve
32
when 74 mM peroxide, pH 4, and Fe2+:EDTA molar ration 2:1 were used. The
findings suggest that Fenton’s process could be applied as pretreatment step for
EDTA-containing effluents prior to biological process. The authors also
recommended the further study to characterize the oxidation products and their fate in
the environment; to check the applicability of this method for the treatment of real
wastewater; and to develop heterogeneous catalysts for this process.
Since the appropriate dose of iron added into system may facilitate the
formation of reactive intermediates such as superoxide ions (O-2) to enhance a higher
EDTA degradation. Englehardt et al. (2007) addressed the drawback in the work of
Zhou et al. (2009) by aerating of mental-EDTA solution for 25 h at ambient
temperature. The result was reported that an iron oxyhydroxide layer was developed
and almost complete removal of EDTA at its concentration of 3.24 mM was achieved,
while the EDTA was degraded into glyoxyleic acid and formaldehyde as oxidation
by-products.
To enhance EDTA degradation rate, Chitra et al. (2004) integrated the
Fenton’s oxidation with ultrasound for treatment of liquid waste containing EDTA. In
this study, ultrasound + Fenton’s reagent and ultrasound + H2O2 at lower and higher
frequencies of 40 and 130 KHz were investigated in comparison with the chemical
degradation using H2O2 and Fenton’s reagent. It was found that sonochemical and
chemical Fenton’s oxidation achieved almost complete degradation of EDTA at
different rates. After 6 h of ultrasound Fenton’s oxidation, complete removal of
EDTA was attained at its initial concentration of 2000 mg/l at pH 3 in the frequency
of 130 kHz whereas, ultrasound + H2O2 (130 kHz ), H2O2 and Fenton’s reagent
required 30, 120, 150 h respectively for almost complete degradation of EDTA. It was
concluded that pH changes during the chemical degradation and sono-Fenton
processes, there is loss of chelating ability of EDTA. Formation of amides in all the
US + Fe(II) + H2O2 processes was confirmed and formation of acidic intermediates is
suspected for the processes using Fenton’s reagent and US + H2O2. This result is
comparatively higher than that achieved by Zhou et al. (2009), who only removed
81% of EDTA at initial concentration of 0.32 mM at neutral environment. This could
be due to the fact that the main oxidant in ultrasound Fenton’s oxidation was hydroxyl
radical, whereas ultrasound Fenton-like oxidation attained by FeIV-EDTA as an
33
oxidant. it was also recommended that Studies using transition metals like Cu2+ in
combination with H2O2 can be carried out for further improvement in the kinetics of
degradation. The synergistic effect of both photochemical and sonochemical
processes can also be explored for the improvement of kinetics of degradation and
also for the complete mineralization of EDTA.
Theoretically, an optimum dose ratio of H2O2 to Fe2+ is required to optimize
the degradation of EDTA to a certain degree of mineralization, while Fe2+
concentration is important to accelerated reaction kinetics. It is important that either
H2O2 or Fe2+ is proportional so that a maximum amount of OH radical can be
generated not only to minimize scavenging effects. The addition of an excessive
amount of H2O2 can act as a scavenger for the OH radicals by reducing the oxidation
rate of Fenton’s process. In general, the performances of iron-facilitated AOP vary,
depending on pH, reaction time, and molar ratio between oxidant (oxidant dose) and
chelates. As reported, Fenton’s system gave satisfactory degradation of chelating
agents with over 76% of removal at initial concentrations ranging from 0.03 to 76
mM in acidic pH range. In spite of its advantages, there are drawbacks that may
restrict its industrial applications. The complexation of EDTA with iron minimized
free ions for Fenton’s oxidation, resulting in a slow generation of OH radical
(Sillanpää et al., 2011). However, the chelating agent may activate H2O2 oxidation at
a neutral pH range. This pH ranges might affect the Fenton’s process due to iron
precipitation (Ghiselli et al., 2004).
Fu et al. (2009) conducted a comparative study for Fenton and Fenton-like
reaction followed by hydroxide precipitation in removal of Ni from NiEDTA
wastewater. The results indicated that Fenton-like process representing higher Ni(II)
removal efficiency than Fenton process can be attributed to the mechanism of ligand
exchange. However, both processes considerably achieved above 92% Ni removal
efficiency at optimal condition with [Ni]0 =25 mg/L. The removal of Ni depends on
initial concentration of Fe ions, H2O2 and initial and precipitation pH. Optimal
operation conditions were found in [H2O2]0= 141 mM, [Fe2+]0=1 mM, [Fe3+]0=1 mM,
initial pH=3, precipitation pH=11, and temperature in the range of 40-50 °C after
reaction time 60 min. It was quantitatively observed that the complete disappearance
of NiEDTA due to the fragmentation of EDTA by oxidation, indicating Fenton and
34
Fenton-like processes were effective to degrade EDTA. A few years later, Fu et al.
(2012) applied advanced Fenton-chemical precipitation to the treatment of strong
stability chelated heavy metal (NiEDTA) containing wastewater. The process used
zero-valent iron (ZVI) and hydrogen peroxide to initiate the advanced Fenton reaction
for chelated compound, followed by alkali precipitation of heavy metals (Ni). The
results indicated that at condition of [H2O2]0= 35 mM, [Fe0]0=2 g/L, initial pH=2.5,
precipitation pH=11.5, and reaction time for 60 min was the optimal condition which
resulted in 98.2% Ni removal efficiency and the COD decreased from 252 mg/L to
53.3 mg/L; indicating about 78.8% COD reduction. Lower percentage of COD
removal may be attributed to the formation of intermediates of acetate and formate. In
comparison, it is clearly shown that advanced Fenton process shows higher removal
efficiency of Ni and requires lower H2O2 amount than Fenton or Fenton-like process.
However, Fenton and Fenton-like requires lower iron concentration. More
importantly, less than 0.03 mg/L of residue iron concentration was identified after
Fenton type processes, which require no further treatment options. This leads to a
conclusion that Fenton type processes seems to be an economically and
environmentally friendly process to removal for remediation of strong stability
chalated heavy metal wastewater.
Kim et al. (2010) employed UV photo-Fenton-like oxidation to treat naval
de-rusting wastewater containing high concentration iron and organic acid (about 8 %
synthetic citric acid solution). Since the de-rusting wastewater was high in Fe3+,
Fenton oxidation was preferred because this process could occur automatically when
hydrogen peroxide was added. It was significantly found that UV/H2O2/Fe3+ could
decomposed citric acid better than UV/H2O2 and Fe2+/H2O2. This is apparently due to
the important role of UV in allowing Fe3+ and H2O2 to function as strong oxidant in
producing radical chain reaction. In Fe2+/H2O2 system without UV, only 10% of
complex removed due chelating effects and precipitation. Lower decomposition of
citric acid in UV/H2O2 basically results from precipitation of Fe(OH)3 which further
interferes in UV radiation as well as the scavenging effects. In addition, 93% COD
reduction was achieved from de-rusting wastewater for UV/H2O2/Fe3+. As reported,
destruction mechanism occurring in de-rusting wastewater include the UV induced
destruction, OH radical induced destruction, and ferric ion induced destruction. It was
35
also concluded that the presence of iron salt can reduce the scavenging effect of OH
radical from H2O2 as reported in literatures (Matthew Tarr, 2003; Neyens & Baeyens,
2003).
Similarly, Chitra et al. (2011) conducted a pilot-scale study on photodegradation of EDTA using Fenton’s reagent. It was reported that photo-Fenton
reaction could retain complete degradation of 20000 mg/L EDTA. Photo-Fenton
oxidation with the application of visible radiation, UV radiation, and sunlight resulted
a complete degradation within 31, 6 and 3 hours, respectively. It is clear that the
kinetics of photodegradation using solar-Fenton reaction follow the order of solarFenton > UV(254 nm)- Fenton > Visible-Fenton. In addition, it was also observed that
the pH changes from acidic to alkaline range during the photo-Fenton process,
indicated that there is a loss of chelating ability of EDTA and formation of amide was
confirmed. Therefore, the design and treatment of large volume of decontamination
waste containing EDTA using a solar Fenton process is easy, cost effective, and safe
to operate.
Lan et al. (2012) applied the combined process of interior microelectrolysis
and Fenton oxidation-coagulation (IM-FOC) to treat EDTA-Cu(II) containing
wastewater. In this study, chemical oxygen demand (COD) was used to indirectly
determine the concentration level of EDTA species in the wastewater. The results
demonstrated that IM process higher treatment efficiencies for Cu(II) and yielded
336.1 mg/L Fe(II) concentration at very low pH (pH=1.39) in accordance with IM
reaction mechanism as reported in literature reviews (Ju et al., 2011; Ju & Hu, 2011).
The poor treatment performance of COD by IM and low BOD5/COD ratio of IM
effluent indicated that EDTA species cannot be effectively decomposed into small
biodegradable organic molecules by IM process. The Fe(II)-rich effluent of IM was
suitable for direct treatment in a subsequent Fenton oxidation without Fe(II) addition
or pH adjustment. The optimal conditions for Fenton oxidation-coagulation were
found as follow: [H2O2]: [COD] of 2.0, [Fe (II)]/ [H2O2] of 0.2-0.3 , initial pH of 2.05.0, and reaction time of 60-80 min. Under the optimal operating condition, Cu(II)
and COD decrease from 225.3 mg/L and 1096.6 mg/L to 0 mg/L and 142.6 mg/L with
overall removal efficiency of 100% and 87%, respectively by IM-FOC process. Of
100% Cu(II) removed, 97.5% was removed by IM process, while only 2.5% was
36
removed by coagulation. This indicated that IM process was responsible for the
removal the majority of Cu(II) from wastewater. However, the COD removed by IM
process was relatively low which accounted for only 22.3%. Most of COD (47%) was
degraded by the Fenton oxidation, and approximately 10.9% of COD was removed by
coagulation and hydroxide precipitation. From this research findings, IM-FOC is a
highly efficiency and low-cost method for detoxification and mineralization of nonbiodegradable wastewater containing EDTA-Cu(II) which is relatively consistent with
other novel finding (Fu et al., 2009, 2012).
37
CHAPTER 3
RESEARCH METHODOLOGY
This chapter also provides materials and reagents required for research
process. Experimental variables are also determined. Then, precise experimental
procedure, analytical method, and kinetic modeling are described accordingly.
De-rusting Wastewater
The de-rusting wastewater used in this study was obtained from the Kation
Power, a cleaning service company, located in Rayong Province, Thailand. This
cleaning service company produces varying amount of wastewater according to the
numbers and types of cleaning processes. The approximate amount of wastewater is
ranged from 15 to 600 m3/week. The wastewater is originally produced from cleaning
processes of boilers. It is collected and stored in temporary storage tank for treatment.
During the cleaning processes, chelating agent (EDTA) is applied for removal of rust
or corrosion tank. Furthermore, the de-rusting wastewater is in the dark red color due
to high iron content, which will form a complex with the EDTA. Therefore, this high
content of iron can play an essential role in the Fenton oxidation The specific
characteristic of cleaning wastewater will be attached in chapter 4.
Experimental Design
The proposed experimental design is conducted and divided into 2 sets of
Fenton reactions. First set of experiment is referred to the Fenton and Fenton-like
reactions initiated with additional [Fe2+] and [Fe3+]. However, for second set of
experiment, [H2O2] is solely added without additional [Fe2+] and [Fe3+] because it is
intended to investigate the extent of efficiency by Fenton reaction when utilizing the
existed iron in original wastewater.
Treatment efficiency of Fenton and Fenton-like reactions are the function of
the operating parameters including dosage of hydrogen peroxide [H2O2], [Fe2+] and
[Fe3+], initial pH , reaction time, temperature, agitation and weight ratio of
[COD]/[H2O2] on removal of COD and total iron as the objective functions..
Therefore, variables of the experiment are classified and described as following:
38
a.
b.
Independent Variables
-
Reaction time at 0, 30, 60, 90, 120, 150, and 180 min
-
Initial pH value of 2, 3, 4, 5, 6, 8
-
[Fe2+] and [Fe3+] vary at 0.0005, 0.001, 0.005, 0.01, 0.05, and 0.1M
-
[Fe2+/3+]/[H2O2] molar ratio of 1:5, 1:10, 1:15, 1:20, 1:25
Dependent Variables
-
c.
COD and total iron as objective parameters
Control Variables
-
Room Temperature (28°C) corresponding to the wastewater
temperature during Fenton process
-
Rapid mixing at 150 rpm for 2 min followed by slow mixing at 50
min for 180 min
-
Homogenous wastewater characteristic in all experiments
The range of values for the operating conditions (pH, [Fe2+] and [Fe3+],
[Fe2+/3+]/[H2O2], agitation and reaction time) above are chosen according to
previously values reported in literature. In this study, reaction time is allowed to be 3
hours and since treatment efficiency of Fenton and Fenton-like reactions are generally
optimal less than 3 hour for highly polluted wastewater and chelated wastewater.
[Fe2+] and [Fe3+] vary from 0.0005, to 0.1M (Jiang et al., 2010, 20113; Fu et el., 2009,
2012; San Sebastián Martinez et al., 2003; Kiril Mert et al., 2010). Indeed, pH is
varied from 2 to 8 due to the presence of Fe-EDTA in the waste. Since chelating agent
may extend initial pH range to be feasibly operated in basic condition because
chelating agents improve resolubilization of iron (Matthew Tarr, 2003). In addition,
[Fe2+/3+]/[H2O2] molar ratio will be varied at 1:5-25 because greater than this range
result in scavenging effects and insignificant treatment efficiency (Wang et al., 2011;
Bautista et al. (2008) and this range is also recommended for industrial wastewater
treatment regardless organic concentration (www.USperoxide.com). The temperature
will be constant according to the room and Fenton operating temperature at 28°C
owing to difficulty in temperature control and mild effect of temperature for industrial
wastewater treatment as reported in literature (Nieto et al., 2011; San Sebastián
Martinez et al., 2003).
39
Materials and Chemical Reagents
The entire reagents used in this study are the analytical reagent grade and are
used without any further purification. Deionized or distilled water will be used in all
experiments. The chemical reagent are included the chemical reagents for Fenton and
Fenton-like processes and chemical reagents for wastewater parameters analysis.
1.
Chemicals for Fenton and Fenton-like Processes
1.1 Hydrogen peroxide (H2O2 -35% w/w)
1.2 Sodium Hydroxide (NaOH, 10N)
1.3 Sulfuric acid (H2SO4, 5N)
1.4 Ferrous sulfate (FeSO4 -7H2O) for Fenton reaction
1.5 Ferric chloride (FeCl3) for Fenton-like reaction
1.6 Manganese dioxide (MnO2)
2.
Chemicals for Parameters Analysis
2.1 COD
2.1.1 Standard potassium dichromate digestion solution
2.1.2 Sulfuric acid reagent
2.1.3 Ferroin indicator
2.1.4 Stand ferrous ammonium sulfate titrant (FAS)
2.2 Total Iron (ferric and ferrous)
2.2.1 Hydrochloric acid (HCl) conc,
2.2.2 Hydroxylamine solution
2.2.3 Ammonium acetate buffer solution
2.2.4 Sodium acetate solution
2.2.5 Phenanthroline solution
2.2.6 Potassium permanganate (KMnO4)
2.2.7 Stock iron solution
3.
Equipment and Materials
3.1
Jar test apparatus (six paddles and six beakers with volume of 1L)
3.2
pH meter (EUTECH)
3.3
Analytical balance (OHAUS)
3.4
UV-Vis Spectrophotometer
3.5
Turbidity meter (EUTECH)
40
3.6
Drying oven
3.7
Evaporating dishes
3.8
Suction flask
3.9
Desiccator
3.10 0.45m filter paper (GF/C )
3.11 Burette stand
3.12 Separatory funnel
3.13 Centrifugal machine (Harmonic Series)
3.14 Other glass wares (pipettes, burette, measuring cylinder, volumetric
flash, small beakers...)
Experimental Procedure
The Fenton process experiments are going to be conducted by using jar test
method under normal laboratory room temperature (28) 0C. The jar test apparatus
equipments (JR-6A model, Metrology Technical, Thailand) comprise six paddles that
have adjustable rotating speeds of 20–350 r/min and is equipped with six beakers of
1L each. To have a homogenous characteristic of wastewater, all de-rusting
wastewater is transferred from small storage tanks into big mixing tank which is
prepared for whole experimental process. For each batch of experiments, de-rusting
wastewater is initially taken from the a big mixing tank in amount of 3L and then
every beaker is first filled with 500 ml of the wastewater sample. The wastewater in
each beaker will mixed homogenously before conducting any experiments to adjust
the wastewater temperature to the room temperature. In the meanwhile, the pH
adjustment is carried out with diluted sulfuric acid (5N) and sodium hydroxide (10N)
solution. The pH of the sample will adjusted to 3.0 by sulfuric acid. Then ferric and
ferrous ions will be injected into the wastewater sample under stirring. Finally, a
known amount of H2O2 will be added under vigorous stirring to initiate the Fenton
and Fenton-like reaction as presented in figure 2. The reaction time is taken in
account immediately when H2O2 is injected. After the introduction of hydrogen
peroxide and ferrous sulfate into the wastewater sample, the processes will proceed
with rapid mixing of wastewater sample at 150 rpm for 2 min, and slow mixing at 50
rpm will be maintained until the reactions is completed and stopped at 180 min. The
41
reactions are stopped by increasing the pH to 8 by NaOH 10 N and adding amount of
manganese dioxide in order to consume the remaining hydrogen peroxide
instantaneously in order to inhibit the reactions. Then, solid precipitate will be
allowed to settle down for 30 min. Samples for COD, and total iron measurement
will taken out from each batch reactor through pipette at reaction time of 30, 60, 120,
150, and 180 min. The supernatant is obtained by centrifugal separation at 2000 rpm
then the analysis of parameters can be started immediately.
Sample (de-rusting
wastewater)
Homogenueous Mixing
Without adding/adding the
amount of Fe2+ and Fe3+
pH adjustment to 3
Addition of H2O2
pH adjustment to >8
Additon of MnO2
Parameters analysis at
determined time interval
Figure 2 schematic experimental procedure of Fenton and Fenton-like process
The optimum condition is determined for each ferrous/ferric and hydrogen
peroxide concentration by computing the removal efficiencies of pollutants at
different varying concentration of reagent used in Fenton and Fenton-like oxidation.
The removal efficiency (R) is calculated by the following equation:
Removal Efficiency (R) =
A- B
100
A
42
where, A represents the initial characteristic of the objective parameters; B represents
the final characteristics of the objective parameters. The objective parameters include
the TSS, TDS, turbidity, total iron, chemical oxygen demand (COD).
Analytical Method
The analytical methods for each parameters will be analyzed according to
the Standard Method for the Examination of Water and Wastewater (APHA, 1998).
They are briefly described as following:
-
The COD of treated water is determined by the close reflux titrimetric
method (Method 5520) with dichromate solution as the oxidant in strong
acid media.
-
The pH of solution was adjusted using sulfuric acid or sodium hydroxide
and then the pH will be measured with a EUTECH pH meter.
-
Total iron, ferric and ferrous concentration will be analyzed by UV-Vis
spectrophotometer according to Phenanthroline Method (Standard Method
3500). Samples will be filtered through a 0.45 m membrane filter. Then the
filtrate will be analyzed for total Fe concentration. The concentration of
Fe(III) is determined by subtracting the Fe(II) concentration from total Fe
concentration. The total Fe concentration is determined by measuring the
Fe(II) concentration after Fe(III) was reduced with 1 N hydroxylamine.
-
Total suspended solid (TSS) and total dissolved solid (TDS) are also
determined by standard method (Method 2540). The total solids include total
suspended solids and total dissolved solids. Total solid and total dissolved
solids (TDS) dried at 103-105°C are determined according to Standard
Method (Method 2540) and quantified by
mg total solids/L 
where:
(A  B) 1000
sample volume, mL
A = weight of dried residue + dish, mg
B = weight of dish, mg
Total suspended solid is measured by subtracting total dissolved solid from
total solids.
-
Turbidity is measured by turbidity meter
43
The detail description of each analytical method is referred to Appendix B.
Kinetic Modeling Methods
The kinetics of Fenton and Fenton-like oxidations of COD removal can be
represented by the flollowing nth-order reaction kinetics as described in Skoog and
West (2004)
dC
 kC n
dt
where C represents the COD concentration, n is the order of the reaction, k is
the reaction rate coefficient and t is the time.
For a first-order reaction, the above equation after integration becomes
C  C 0 exp( kt)
For a second-order reaction the integrated equation becomes
1
1

 kt
C C0
In addition, by assuming that •OH concentration is constant during the
reaction, a pseudo-first order reaction can be simplified and the equation after
integration is given as below
C  C 0 exp( k app t )
where C0 is the initial COD concentration and kapp is the pseudo-first order
apparent rate constant. The kapp constants are obtained from the slope of the straight
lines by plotting –ln (Ct /C0) as a function of time t.
Kinetic modeling is conducted by calculating the rate constant using the
COD degradation data with respect to time. The data is fitted using the first order and
second order rate equation. The best fit is chosen when the coefficient of linearity is
nearly equal to the value of 1.
44
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overview of the application of Fenton oxidation to industrial wastewaters
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Bautista, P., Mohedano, A. F., Gilarranz, M. A., Casas, J. A., & Rodriguez, J. J.
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APPENDICES
Appendix A
Activities plan
2013
Activities
Topic Selection
Literature Review
Pre-test Method
Proposal Defense
Proposal Revision
Laboratory Testing
Data Analysis
Thesis Reporting
Thesis submission
and Revision
Thesis Defense
Publication
Jan
Feb
Mar
Apr
May
Jun
Jul
2014
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
51