Environmental Research 237 (2023) 117005
Contents lists available at ScienceDirect
Environmental Research
journal homepage: www.elsevier.com/locate/envres
Review article
A review on recent advancements in extraction, removal and recovery of
phenols from phenolic wastewater: Challenges and future outlook
Kirubanandam Grace Pavithra a, Panneerselvam Sundar Rajan b, Jayaseelan Arun c,
Kathirvel Brindhadevi d, e, Quynh Hoang Le f, g, Arivalagan Pugazhendhi f, g, *
a
Department of Infrastructural Engineering, Saveetha School of Engineering, Chennai, 602105, India
Department of Chemical Engineering, Saveetha Engineering College, Thandalam, Chennai, Tamil Nadu, 602105, India
Centre for Waste Management - ‘International Research Centre’, Sathyabama Institute of Science and Technology, Jeppiaar Nagar, Chennai - 600119, Tamil Nadu,
India
d
School of Engineering, Lebanese American University, Byblos, Lebanon
e
University Centre for Research & Development, Department of Civil Engineering, Chandigarh University, Mohali-140103, India
f
School of Medicine and Pharmacy, Duy Tan University, Da Nang, Viet Nam
g
Institute of Research and Development, Duy Tan University, Da Nang, Viet Nam
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Phenol
Environment
Treatment
Adsorption
Toxic pollutant
Water pollution is the major problem seen in today’s scenario and even pollutants at low concentration harms
our environment. In industrial sector usage of phenol is seen even at low concentrations. The interaction of
phenol in the environment provides adverse effects to living beings. This review focuses on the toxicity of phenol
and its impact towards environment and human health. The treatment techniques such as distillation, extraction,
wet air oxidation, membrane process, electrochemical oxidation, biological treatment and finally adsorption
techniques were discussed. Among many treatment techniques so far utilized in the treatment of phenol,
adsorption was considered as one of the best technique due to its advantages such as reusability, ease in oper­
ation, large availability etc., This review also highlights the adsorption technique for the cleaner removal of
phenol from aqueous solution with novel as well as low-cost adsorbents in the removal of phenolic compounds.
This review also discusses about the drawbacks and issues related with adsorption of phenolic compounds.
1. Introduction
Commercial development in the industries apart from invasion of
new services and products creates contaminations to our environment.
In turn animals, plants and human health are affected due to the pres­
ence of organic contaminants in the surface as well as ground water. This
is the main reason for the limitation guidelines followed by many
developed as well as developing countries with legal commitment
(GracePavithra et al., 2019). Phenol is considered as organic colourless
pollutant found in abundance as phenolic molecules presence were seen
in wastewater which exist as an effluent mainly from ceramic, leather
and petrochemical industries and also seen in textile, pharmaceutical,
paints, plastics, oil refineries, resin plants, wood plants manufacturing
and utilizing industries. Due to phenolic discharge to our environment
water, soil and air gets affected. In ocean environment, chemical spills
also have a possibility in the release of phenol. In aquatic environment,
phenol release may impact in ecological damage due to its toxicity as it
is highly soluble. According to a research done by Duan et al. (2018)
Cirrhinus mrigala organism seen in fresh waters shows phenol (96 h-LC50
mg/L) presence. At low concentrations, phenol was found to be toxic
pollutant as it causes deleterious effect to human kind and to the envi­
ronment. It is prioritized as 11th among 126 chemical pollutants under
priority pollutants according to USEPA (US Environmental Protection
Agency) (Caturla et al., 1988).
As per annual estimation, 10 million tons of phenolic waste were
exposed to our environment without any kind of treatment (Li et al.,
2017). According to Unite States Environmental Protection Agency,
phenol discharge level in drinking water should not be greater than 1
parts per billion and in water bodies the permissible level is 1 ppm
(Kazemi et al., 2014; Adegoke and Bello, 2015). Long-term exposure to
phenol in lethal dosage may result in muscle weakness, respiratory ar­
rest, breathing difficulties, tremor and coma. Chronic effect of phenol
* Corresponding author. School of Medicine and Pharmacy, Duy Tan University, Da Nang, Viet Nam.
E-mail address: arivalaganpugazhendhi@duytan.edu.vn (A. Pugazhendhi).
https://doi.org/10.1016/j.envres.2023.117005
Received 7 June 2023; Received in revised form 23 August 2023; Accepted 25 August 2023
Available online 3 September 2023
0013-9351/© 2023 Elsevier Inc. All rights reserved.
K. Grace Pavithra et al.
Environmental Research 237 (2023) 117005
may result in sudden weight loss, vertigo, anorexia, diarrhoea
(Mohammadi et al., 2015). On the whole, due to anthropogenic activity
and industrial production, phenol has become a risk factor to our
ecosystem and to human health. Proper treatment before the disposal of
phenol from industries is necessary. Phenolic compounds are generally
classified as bisphenol, chlorophenol, alkylphenol, triclosan’s, cresols,
nitrophenols, aminophenol’s and endocrine disrupting compounds.
material. Some of the industrial activities like production of phenol,
caprolactam, cokes, insulation material and in processing phenol emis­
sions were seen.
1.2. Phenol and industries
Phenols often produced as a by-product from the industries like
plastics, dyes, paper and pulp, petroleum, antioxidants and drugs.
Phenolic compounds are utilized in various industries, household
products and as intermediates on industrial synthesis. In the production
of dye and dye intermediates, drugs, explosives and plastics phenol play
major role. In smoked food presence of phenol were seen. In coal gasi­
fication, petrochemical and refinery plants, coke-oven batteries and in
other industries like pesticides, antioxidants, pulp and paper, herbicides,
insecticides, synthetic chemicals. Wastewater contains phenolic com­
pounds (Chakraborty et al., 2010).
Phenolic resins and bisphenol A were the primary compound ob­
tained from phenol. Other major products of phenol include aniline,
caprolactam, alkylphenols and adipic acid. Catalytic reduction of phenol
results in cyclohexanol which produces polyamides and polycarbonates
were obtained from bisphenol A. Thermosetting polymers are also
produced using phenol and phenol in the presence of methanol gener­
ates phenol-methanol resins using catalyst. In medical sector, the role of
1.1. Phenol-classification
1.1.1. Natural source
The organic waste in presence of benzene produces phenol. In major,
the metabolization of benzene produces phenol which is abundant in our
environment. From the volatile components of liquid manure, phenols
were detected. Wood, decaying vegetation also produces phenol as it
contains hydroxyl groups with benzene which is produced in woody
substance (Míguez et al., 2021).
1.1.2. Anthropogenic sources
Phenol forms a base for all the commercial phenolic resins like alkyl
phenols, chlorophenols, caprolactam. In chipboards, insulation mate­
rials, paints, casting sand foundries and triplex uses phenol resins. The
proportion of phenolic resin content changes for each above-mentioned
Fig. 1. Chemical structure of priority phenol pollutants.
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Environmental Research 237 (2023) 117005
phenol in the form of disinfectant, antiseptic, lotions, salves, mouth­
washes, ointments and slimicide. In paint, lacquers, rubber, ink, tanning
dyes, perfumes, toys, soaps and varnish removers use phenols. Some of
the phenol structures are represented in Fig. 1 and their toxic effects are
described in Table 1.
para-aminophenols were used. In paracetomol, the main component is
aminophenol as it has analgesic and anti-inflammatory capacities. In
colouring of hair aminophenol isomers were used.
1.2.6. Alkyl phenols
The presence of alkyl phenols is seen in shale oils and in rock-oil.
Alkyl phenols are used in the process like alkylation, buthylation and
in methylation. Alkyl phenols were used as dispersing agents, emulsi­
fiers and wetting agents. In the manufacturing of steel, resins, paints and
in processing of oil, textile, paper and pulp nonylphenol polyethoxylated
were used.
1.2.1. Catechol and chlorocatechols
In photographic reagents, plastic and rubber production, insecticide,
dyes, cosmetics dyeing and drug synthesis chlorocatechols are utilized.
With the help of microorganisms, catechol’s are produced as interme­
diate products in the aromatic compound degradation. Catechol chlo­
rinated derivatives. Due to environmental transformations.
1.3. Environmental release and distribution
1.2.2. Nitrophenols
In the process of degradation and production of pesticides like 4,6dinitro-2-methylphenol (DNMP) and 2-buthyl-4,6-dinitrophenol (Dino­
seb) nitrophenols were formed. The DNMP and Dinoseb compounds are
used as precursors and components in drug and polymer production and
also as preservatives and photographic developers. 4-Nitro-3-phenyl­
phenol and 3-methyl-4-nitrophenol are released in the environment
due to vehicular emissions.
Due to the usage and manufacturing, phenol is released in water and
air. It as high solubility and low vapour pressure as a result, phenols are
seen in water phase more than in air phase. During transportation
loading and storage tank, phenols are released in atmosphere. Other
sources are automobile exhaust and burning of residential wood. The
product such nitrophenols. Dihydroxy-benzenes are formed due to the
reaction of phenols with hydroxyl radical which are produced photo­
chemically. Due to emissions in traffic, phenol is seen in high concen­
trations in urban areas. The water from manufacturing industries like
steel, leather, resins, fibers, plastics, iron and adhesives, phenol is
released. Apart from this, other phenol containing products such as
disinfectants, slimicides, in medical preparations, gargles, antiseptic
lotions and mouthwashes. Chlorophenols are normally seen in industrial
as well as domestic wastewaters. In rainwater, drinking water, industrial
effluents, sediments and in hazardous waste areas some amount of
phenol are seen. In marine environment, phenol percentage are seen
naturally due to at the time of manufacturing, phenol is released into the
soil via spilling at the time of loading and transport. Due to leaching and
biodegradation, phenol concentrations are seen in minimal amount but
the number of phenols can be identified from a point source or from a
continuous source. At hazardous sites phenol are seen in groundwater.
Due to landslide leaching and continuous discharge from spills, phenols
are seen in abundant and they are biodegraded rapidly. Microbial
degradation, oxidation and photo-oxidation are the transformation
process seen in water with respect to phenol and may be found in the
dissociate form. Reduction of heart rate, liver, blood vessels, intestines
loss of gills function were seen in fresh water species due to intervention
of phenols in water. In cold sore lotions, nose and ear drops, ointments,
1.2.3. Methyl phenols
In asphalt production and wood impregnation, methylphenols are
seen in high concentrations. In the production of drug, dyes and epoxy
resins methyl phenols are utilized. During gasoline and coal combustion,
the substance like di-methylphenol, cresols and 2,4,6-trimethylphenol
are formed. Due to fragrance, explosive, pesticide and disinfectant
production, m-cresol are seen in our environment. Lisole which is the
combination of potassium soap and cresols solution used in medicinal
field as disinfectant (Michałowicz and Duda, 2007).
1.2.4. Chlorophenols
Due to the usage of phenolic biocides, growth regulators and phe­
noxy herbicides, chlorophenols are formed. The pesticide named
pentachlorophenol can also permeate in skin, textile and wood products
as it has fungicide properties. The chlorophenol compounds can be
formed from pesticides such as pentachoronitrobenzene, hexa­
chlorobenzene, pentachlorobenzene and hexachlorocyclohexane.
1.2.5. Aminophenols
In photographic developers, lubricants and in oil some amount of
Table 1
Literature note on different phenolic compounds and their toxic effects.
Compounds
Uses
Chemical Characteristics
Toxic effects
Phenol
Used as disinfectant
Renal damage, eye irritation, blindness, respiratory
arrest, central nervous system disorders, headache
Nitrophenols
In manufacturing of dyes,
fungicides, insecticides, drugs
Aminophenol
Catechols
- Final intermediate in
paracetamol synthesis
- Used in oxidative dyes
- Used in photographic developers
- On combustion and heating formation of toxic
fumes are seen
- Causes fire when reacts with oxidants
- Does not ignite but burns by emitting nitrogen
oxides.
- Toxic in all type of intakes
- Act as reducing agent
Methylphenols
- Used as analytical reference
Bisphenols
- Used in hardening of plastics.
- Dental sealants, compact discs,
medical devices, water bottles
Used as antiseptics and
antibacterial agent
Chlorophenols
Buthylhydroxyanisole/
buthylhydroxytoluene
Used as synthetic oxidants
When reacts with oxidants irritating fumes were
formed
–
- Due to temperature increase, the molecules in the
container may break and interacts with the
substance
- On heating corrosive fumes and toxic fumes were
produced.
- Reacts with anhydrides, acid chlorides and
oxidants
–
3
Eyes irritation, abdominal pain, unconsciousness,
confusion
Eye irritation, asthma, anoxia and eczemas
DNA damage or DNA arylation, protein damage
Pain in throat and mouth, headache, hypotension,
paralysis, death and coma
Diabetes, heart disease, abnormalities in heart and
liver, hormone imbalance
Vomiting, irregular pulse, muscle weakness, death,
convulsions, pain in throat and mouth
Adrenal gland damage, liver and brain weight
increase
K. Grace Pavithra et al.
Environmental Research 237 (2023) 117005
gargles, analgesic rubs, antiseptic lotions and in throat lozenges phenol
presence were seen. Apart from workplace exposures due to inhalation
of contaminated air, usage of tobacco, usage of lotions which consist of
phenol are the other unknown discharge of phenol. From lungs and skin
phenol inhalation to our body occurs. The toxicity of phenol in human
body are represented in Fig. 2.
Many technologies were available in the removal of phenol from
aqueous solution and they are classified as chemical, physical and bio­
logical. The physical methods include flocculation, filtration, adsorp­
tion, membrane separation, the chemical treatment includes
electrocoagulation,
electro-fenton process, oxidation, photodegradation and biological treatment includes, microbial degradation,
enzymatic degradation (Villegas et al., 2016). The schematic represen­
tation of the treatment regimes are mentioned in Fig. 3. In this review,
some of the removal techniques practised in the phenol removal have
been highlighted and their advantages and disadvantages are explained
in detail. This manuscript equally shares the concept of all the treat­
ments related to phenol removal and the adsorption section has been
highlighted among all the treatment in the phenol removal due to the
merits available in the treatment.
2. In-depth analysis
On the whole this review provides wide knowledge regarding the
source and the form of phenol in the aqueous solution and the discussion
regarding the impact of phenol to our environment are also discussed in
detail. The methodologies which are discussed in this review regarding
phenol pollution impact to aqueous solution are explained by the
techniques such as distillation, extraction methods, wet air oxidation,
membrane process, chemical oxidation, electrochemical treatment,
biological treatment, photocatalytic degradation, enzymatic process,
ozonation and finally adsorption and treatment techniques summarized
in this review are listed in a diagrammatic representation Fig. 4. Their
merits as well as demerits are discussed in detail. Based upon the out­
comes and the clarifications of the researchers from this field which are
gathered from well-known journals and scientific books are gathered
and provided in this review. The cited literature references are mostly
from Elsevier, Scopus, Springer and Google scholar. The schematic
writing format was framed from various published article and the key­
words such as, “Phenol”, “Environment”, “Treatment”, “Adsorption”,
“Toxic pollutant” were utilized in order to screen the reference content.
The content of the abstract was framed in a manner to find the content
such as Phenol/treatment techniques/adsorption.
Overall, this review proposes the idea regarding the techniques
Fig. 2. Health effects caused upon phenolic exposure on individual human organs.
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Environmental Research 237 (2023) 117005
Fig. 3. Schematic representation of treatment regimens upon sustainable processing of phenolic wastewater.
Fig. 4. Some of the treatment strategies in the removal of phenol from aqueous solution are listed.
involved in the removal of phenol from aqueous solution and the tech­
niques economic viability. The merits and demerits of using adsorption
in the removal of phenol were elaborated and the utilization of various
adsorbent for the removal of phenol were discussed.
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Environmental Research 237 (2023) 117005
3. Different treatment strategies
which phenolic compounds will be gathered on coacervate and sepa­
rated from utilized surfactants using pH variations. In a research, phenol
was removed from olive mill wastewater using Triton X-100 as surfac­
tant. The removal efficiency of phenol majorly depends upon the con­
centration of surfactant and temperature and 67 ◦ C. When the
temperature was raised to 70–90 ◦ C, the concentration of removal in­
creases to 20% with Triton X-100 concentration (El-Abbassi et al.,
2014).
3.1. Distillation
Distillation using relative volatility for removal of phenol in aqueous
phase was performed as one of the treatment methods. This technology
was found to be destructive or non-destructive and the type of distilla­
tion was found to be azeotropic or steam (Mohammadi et al., 2015). Two
types of output liquid and gaseous state were seen in distillation. In
liquid state the intermediates were found to be hydroquinone, pyro­
catechol, butanedioic acid, maleic acid and muconic acid and in gaseous
state the products like carbon-di-oxide, carbon monoxide and hydrogen
were seen. In a research, olive mill wastewater was noted with the
presence of phenolic compounds and treated using solar distillation. In
olive mill wastewater two types of phenolic compounds such as tyrosol
and hydroxytyrosol and more than 50 phenolic compounds were found
in olive mill wastewater (Sklavos et al., 2015). In a research, for a
concentrated solution of phenol (about 3000 mg/L) extractive distilla­
tion were applied in which, catechol was separated from phenol with
greater efficiency (Zhang et al., 2009). The drawbacks such as more
energy consumption, lack of treatment efficiency for phenol at low
concentration were seen in this treatment (Villegas et al., 2016).
3.3. Wet air oxidation
When the pollutant is found to be more toxic for bio-treatment and
diluent for incineration, wet air oxidation play role. This treatment uses
less cost for operation with low air discharge and considered as clean
process as it doesn’t utilize chemical as reagents which are harmful to
our environment and the final products obtained where in the form of
water and carbon-di-oxide. However, high pressure and temperature
with residence time for the oxidation of organic compounds was
considered as main drawback for WAO. For some pollutants containing
propionic and acetic acids, ethanol, methanol and acetaldehyde WAO
was considered as pretreatment technique as WAO cannot completely
degrade these type of oxygenated compounds (Luck, 1999; Bhargava
et al., 2006). To overcome the drawbacks seen in normal WAO treat­
ment, researchers were invented CWAO (Catalytic Wet Air Oxidation)
treatment, in which high pressure and temperature usage can be low­
ered with the usage of heterogenous/homogenous catalyst. The mostly
used homogenous catalyst so far are transition metals such as Fe2+ and
Cu+. Researchers were also used heterogenous catalyst such as metal
oxides, noble metals and mixed oxides for phenolic model compounds
(Kim and Ihm, 2011).
3.2. Extraction methods
3.2.1. Liquid-liquid extraction (LLE)
This treatment is considered as one of the conventional treatments in
treating the phenol wastewater by relative solubilities. The solvents such
as hexane, tributyl phosphate (TBP), kerosene, 2-octanol, oxygenated
compounds and hydrocarbons were utilized (Abbassian et al., 2015).
Jiang et al. (2003) investigated with phenol containing concentration of
6000 mg/L and salt 5% using extractants such as organic acids, amines
and alcohol and 995 of phenol removal was obtained. In another
research, phenol from 100 to 500 mg/L was removed with 70% removal
efficiency using kerosene as solvent with pH 6, temperature 50 ◦ C and
100 mg/L of phenol solution as optimal condition. For the concentra­
tions of phenol higher than 50 mg/L, LLE was found to be an efficient
technique with environmental friendly manner (Abbassian et al., 2011).
The solvents utilized during the treatment can harm our environment
and selectivity of phenolic compounds were found to be low are some of
the disadvantages seen in LLE treatment (Bhatnagar and Anastopoulos,
2017; Zhou et al., 2018).
3.4. Membrane process
Membrane technologies are found to be economically feasible and
reliable for its change in concentration gradient for pollutants, low
consumption and operating cost while the limitations in membrane are
due to fouling of membrane due to colloids and particles in the feed
steam. The major systems such as membrane-based solvent extraction
(MBSE), membrane distillation and pervaporation were seen in the
removal of organic pollutants under membrane process (Shirazi et al.,
2013).
Membrane process were majorly differentiated by their driving
forces like concentration difference (Liquid membrane), pressure dif­
ference (Nanofiltration, pervaporation and reverse osmosis), thermal
difference (Membrane distillation). MBSE process combines the process
of solvent extraction and the interfacial exchange area (Mei et al., 2020).
Fig. 5 describes various membrane and their sizes in the diagrammatic
representation.
3.2.2. Solid-phase extraction (SPE)
Some of advantages such as low cost, simplicity and usage of ecofriendly agents are seen in this treatment. In a research done by Ferri
et al. (2011). Purchased five resins namely IRA96, Isolute ENV+, XAD4,
XAD7 and XAD16 were utilized in the removal of polyphenols from olive
mill wastewater (OMW) inorder to obtain high value added products
from OMW and in same experiment desorption were performed using
water, ethanol, methanol, basified ethanol and acidified ethanol
desorbing agents. The phenol adsorption removal efficiency was seen
high for ENV + resin with desorbing phase as ethanol. The resin was
found to be better in absorbing target compounds with high adsorption
ratios and high desorption ratios in the presence of ethanol. In a research
it is concluded that, instead of activated carbon as adsorptive material,
poly methyl methacrylate (PMMA) can be utilized for low concentration
of phenol solutions with increase in temperature causes increase in
adsorption capacity (Ala’a et al., 2011). The efficiency of this treatment
purely depends on the selectivity of adsorbing and desorbing material.
3.4.1.
This type of membrane acts as physical barrier between organic
solvent and aqueous feed. The organic pollutants were separated by
transmembrane pressure difference (Kislik, 2009; Nabavinia et al.,
2012). In research, for the phenol concentration of 5 g/L polypropylene
(PP) membrane were used for the removal of phenol using methyl ter­
butyl ether (MTBE). The removal concentration of phenol was found to
be higher, as MTBE increases the H-bons possibilities and polarity to­
wards phenol (Kujawski et al., 2004b).
3.4.2. Emulsion liquid membrane
ELM was found to be widely utilized technique in the removal of
phenol from aqueous phase due to its advantages over other methods. In
ELM two types of operations such as extraction and stripping occur at a
time and except hydroquinone other phenolic compounds can be treated
in ELM. Reis et al. (2011) removed phenolic compounds such as tyrosol
and p-coumaric acid using cyanex 923 (2%) as extractant with 99%
3.2.3. Cloud-point extraction
This treatment is based upon the two phases (bulk solution and
coacervate) separation using surfactants. This treatment utilizes tem­
perature and pH as main factor through-out the treatment, where
particular temperature of the solution was considered as cloud-point in
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Environmental Research 237 (2023) 117005
Fig. 5. Membrane treatment regimens for removal and recovery of phenolics from phenolic wastewater.
phenol recovery with contact time of 5–6 min. In some research sup­
ported liquid membrane (SLM) were utilized in the removal of phenol.
SLM is nothing but a membrane liquid supported by inorganic or poly­
meric films. This type of films was seen from feed to strip area and also
disintegrate with pores of membrane. In a research, vegetable oil was
utilized
as
membrane
liquid
with
polypropylene
and
poly-tetrafluoroethylene with 95% of phenol removal (Venkateswaran
and Palanivelu, 2006).
90% (Ozaki and Li, 2002). NF are mainly utilized on the front of RO in
order to achieve better removal efficiency and a research done by Sun
et al. (2015) proved that, before RO treatment ultrafiltration or micro­
filtration’s can be done with organic pollutants to avoid fouling of
membrane. Praveen and Loh (2016) utilized osmatic membrane biore­
actor in continuous operation for the degradation of phenol. The
removal of phenol was achieved using membrane rejection and
biodegradation. 600–2500 mg/L of phenol concentration were treated
for 54 days and complete removal was achieved. The treatment with
biofouling and salt accumulation and to regain the performance of
filtration washing was performed. From FTIR analysis, the fouling
causing agents were found to be proteins. In a research, hydrophilic
ultrafiltration membrane was utilized in the removal of phenol. The bed
used was polymide matrix with modified silica-based nanofilters. The
modifications of silica-based nanoparticles were performed using
post-impregnation method.
3.4.3. Pervaporation
Especially for low volatile organic compounds PV technique was
found to be useful with no addition of entertainer and with energy
consumption (Hao et al., 2009). In this technique, volatile organic
compounds and water were separated in feed and permeate area of the
membrane with evaporation of permeate compound simultaneously.
Kujawski et al. (2004a) utilized membranes such as zeolite-filled poly­
dimethylsiloxane (PDMS), PDMS and polyether-block-amide (PEBA).
Among three membranes, PEBA were found to be best but PEBA is not
commercially available. Studies have concluded that combination of
pervaporation and evaporation provides higher efficiency in phenol
removal.
3.5. Chemical oxidation
In oxidative treatment chemicals such as ozone, permanganate, fer­
rate, chlorine oxide, chlorine, ozone and chloramines were utilized
under different operating conditions (pH and temperature). Due to the
high reduction potentials of ferrate and permanganate (Ẻ(K2 FeO4 ) = 2.2 V,
3.4.4. RO, NF and UF
It is membrane-based technique in which ions are separated from
aqueous based solutions. In RO, membranes behaves like selective bar­
riers in which solids are separated from water solution (Kaghazchi et al.,
2010). In research, three different RO membranes (DSS-HR98PP,
DESAL-3B and SEPA-MS05 were utilized for phenol removal with low
percentage of rejection. From the research it is concluded that, the value
of rejection decreases with pressure increase. For the same phenol
concentration, nanofiltration (NF-97, DSS-HR98PP and NF-99) were
performed with same RO concentration and the rejection value was
found to be 80% (Bódalo et al., 2009). Ozaki and Li (2002) used different
experimental conditions such as initial phenol concentration, pH and
pressure using ultra-low pressure RO (ULPRO) for the phenolic com­
pounds like nitro and chlorophenols. It was reported that under pH 9,
the removal percentage of chlorophenol and nitrophenol were 60% and
Ẻ(KMnO4 ) = 1.68 V). Permanganate was found to be low in cost and easy
to handle and not found to be efficient for brominated or chlorinated byproducts. Ferrate has an ability to oxidize organic pollutamt which
contains wide range of pH (Yates et al., 2014; Jiang et al., 2015). Peings
et al. (2015) performed mechanism of oxidation for phenol containing
concentration of 30 mg/L using sulfarrate and the same concentration of
phenol was performed using hypochlorite and permanganate and the
results concluded that, phenol removal using potassium sulfatoferrate
was found to be efficient. In a research, p-nitrophenol was treated using
ozonation without pH control and under pH 2, 7 and 10 with a scavenger
(t-butanol). The intermediates such as catechol, 4-nitrocatechol,
hydroxyquinone, maleic acid, fumaric acid and oxalic acid were detec­
ted along the treatment. The rate of decomposition in ozonation was
computed at each experimental run. At pH 7 and 10, the decomposition
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Environmental Research 237 (2023) 117005
of p-nitrophenol was found to be same. At pH 2, the scavenger decreases
the intermediates concentration and the results conclude that pH de­
termines the production of intermediates along the path (Kuosa et al.,
2015). However, ozone treatment was considered to be expensive and
less solubility in water. The chemicals utilized during the statement
were found to be harmful and there is formation of intractable chemicals
during the usage of metal oxides.
temperature and 9pressure (Anglada et al., 2009). In a research,
phenolic wastewater was treated using electrochemical method with
ultrasound assistance with fly ash loaded TiO2–Fe3+.
The removal efficiency was found high when the ultrasound fre­
quency is 45 kHz, 1.2 A current, 200 W power, 5 pH and 3 g of particles.
It is concluded that the microwave has the efficiency to speed up the
reaction rate and reduces the metal ions number with reduction in cost
(Zhang et al., 2020b). In a research, leachate containing phenol com­
pounds were treated using Pt/Ti anode electrode in a single-chambered
electrochemical reactor with current densities of 50, 100 ND 150
Ma/cm2. The COD removal rate was found to be 1.73 h− 1 at 150
mA/cm2 (Zambrano and Min, 2020). In a research, shape-tailored TiO2
was utilized as anode and the removal efficiency was high. On the
whole, TiO2 based anodic material showed higher degradation of
phenolic removal from aqueous solution (Liu et al., 2019a). Fig. 6 il­
lustrates the basic design of electrochemical treatment for the removal
of phenol.
3.6. Electrochemical treatment
Two types of electrochemical oxidation were seen in phenol removal
(i.e) direct oxidation and indirect oxidation as per, Tasic et al. (2014)
and Martinez-Huitle and Ferro (2006) research. In direct oxidation,
phenols were adsorbed on the surface of anode and in indirect oxidation,
chemicals like metal ions, chlorine, hydrogen peroxide were utilized as
intermediates in the electrochemical process. In electrochemical treat­
ment, phenol degradation follows pseudo first-order kinetics and the
efficiencies of removal depends upon electrochemical oxidation index
and current efficiency. Two-way pf possibilities in direct anodic oxida­
tion were seen mainly and they are a) electrochemical conversion
(partial oxidation of organic compounds as per the reaction) and b)
electrochemical combustion (decomposition of organic compounds to
CO2, H2O and other inorganic compounds). The problem seen in this
type of system is electrode fouling due to catalytic activity decreases and
formation of polymer layer. Anode deactivation is also possible in direct
oxidation and its mainly depends upon the surface adsorption proper­
ties, the concentration and nature of organic compounds. The fouling of
electrodes was prevented in indirect electrochemical oxidation by
omitting the transfer of electron on the anode surface and organic
compound directly. The oxidation agents utilized may have high
oxidation potential (such as cobalt, silver, celenium) or it may be of
stronger type of oxidizing agent (such as ozone, hydrogen peroxide,
chlorine, perphosphate, persulphate and percarbonate). In the presence
of chloride species wasterwater performs different behaviour and this is
due to the active chlorine species such as ClO− , HClO, Cl2 and ClO−2 . The
usage of electrode materials plays major role as it should have high
chemical and physical stability, corrosion and erosion resistance,
passivation layers formation, higher electrical conductivity with cata­
lytic selectivity and activity. Some of the advantages such as simple
handling of equipment, ease operation with retention time, robustness,
automation, versatility and comparing to supercritical oxidation,
incineration electrochemical oxidation requires less amount of
3.7. Biological treatment
This type of treatment was considered as inexpensive method with
simple design and maintenance in the degradation of phenol to carbondioxide and water. Phenol can be converted to simple end products with
aerobic or anaerobic environment with the help of microorganisms. As
far now low concentration of phenol was treated with biodegradation
but from the industries phenol with high concentrations were released
which is difficult for microorganisms to degrade and this statement was
evident from the research done by Busca et al. (2008) in which for 300
and 500 mg/L of phenol, 100% degradation was achieved but for
800–900 mg/L the degradation rate decreased to 94%. The degradation
was performed using activated sludge which contains microbial con­
sortium. In a research, mixed culture activated sludge were utilized in
sequential batch reactor for the biodegradation of phenol was investi­
gated with varying initial phenol concentration and temperature.
Biomass in activated sludge was acclimatized to increase the population.
The phenol concentration ranging from 250 mg/L to 1000 mg/L stability
was achieved with temperature variation (Kamali et al., 2019). In a
research, Pseudomonas putida was utilized for the removal of phenol
from aqueous solution. The mechanism involves both meta and ortho
pathways, the steps involved are conversion of phenol to catechol
through cleavage (ortho or meta) and catechol is further metabolized
into tricarboxylic acid cycle intermediates which leads to complete
Fig. 6. Electrochemical treatment a simultaneous mode of phenolic wastewater treatment and electricity generation.
8
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Environmental Research 237 (2023) 117005
mineralization of phenol. The research shows that, Pseudomonas putida
has a promising candidate in the removal of phenol from aqueous so­
lution (He et al., 2022). In another research, Acinetobacter sp. Was uti­
lized for the removal of phenol and the mechanism involves,
hydroxylation of phenol to catechol leading to the generation of in­
termediates by ortho cleavage pathway and finally to the complete
mineralization of phenol (Van Dexter and Boopathy, 2019).
phosphate was used and in stripping phase, sodium hydroxide was used.
Membrane treatment and adsorption were performed simultaneously in
the same system in order to remove phenol. The high concentrations of
phenol (1000–3000 mg/L) was reduced within 2–4 h. In a research, Li
et al. (2018) removed phenol using the combination of activated carbon
in rotating packed bed. The removal amount was found to be higher
when compared to traditional packed bed. The kinetics of adsorption
was well explained by intraparticle diffusion model. The adsorption
capacity was found to be 51.3 mgg− 1 at 2 h for the phenol concentration
of 1800 mg/L.
3.7.1. Biological reactors
Dionisi and Etteh (2019) performed the degradation of phenol and
paracetamol in a open mixed cultures as a energy sources with phenol
concentration increase in the range of 0.2–0.5 g/L. In SBR, 1.0 g/L
phenol concentration was feed as carbo source and 90% of phenol
removal was seen. In research, thermophilic anaerobic upflow packed
bed reactor (UPER) were utilized in the treatment of untreated olive mill
wastewater and the treated effluents toxicity were tested using micro­
biotest Thamnotoxkit™. Six phenolic compounds such as caffeic acid
(CA), vanillic acid (VA), syringic acid (SA), 4-ethylphenol (4-EP),
oleuropein (OLEU) was biodegraded with removal of tyrosol (93.92%)
and hydroxytyrosol (94.87%) and existence of recalcitrant pollutant
such as 4-hydroxybenzoic acid (PHBA), 3.4-dihydroxybenzoic acid
(DBA), were p-coumaric acid (p-CA) were seen (Genethliou et al., 2020;
Zheng et al., 2021). Basak et al. (2019) performed biodegradation of
phenol using packed bed reactor with immobilized Candida tropicalis
and sugarcane bagasse were utilized. The microorganism was isolated
from steel plant’s coke oven effluent. 97% of phenol was removed using
PBR with initial phenol concentration of 2400 mgL− 1 at 54 h with 4 mL
min− 1. The utilized SCB was considered as low-cost packing material in
this study. In a study, Packed Bed Reactor (PBR) was filled with Ralstonia
eutropha for biodegradation of phenol. Different initial concentrations
with various circulation flow rates were performed. The efficiency of
recycling was seen when the phenol concentration was found to be less
in last cycles when compared to first cycles (Amani et al., 2018). In a
research, Bacillus cereus MTCC9817 in an immobilized calcium-alginate
was utilized in re-circulated up-flow packed bed reactor in phenol
biodegradation. The phenol removal rate was identified using various
initial phenol concentration and it is concluded that the phenol
biodegradation rate depends upon the flowrate (Banerjee and Ghoshal,
2016). Spouted bed bioreactor was designed by El-Naas et al. (2010) for
the degradation of phenol. The bed was designed using Pseudomonas
putida which is immobilized using polyvinyl alcohol. The effects of air
and liquid flow rate, PVA particle size, initial phenol concentration for
1.1 L volume of reactor were performed. The continued biodegradation
rate was increased when there is increase in phenol concentration. Dey
and Mukherjee (2016) biodegraded phenol and m-cresol obtained from
coking industry using aerobic batch reactor system. The biodegradation
was performed using mixed microbial culture isolated from effluent
treatment plant. Brink et al. (2017) designed aerobic moving bed biofilm
reactor (MBBR) for treating the phenol concentration in paper and pulp
mill effluents using activated sludge. Four types of phenol concentra­
tions were taken for the experiment and monitored individually. At
Hydraulic Retention Time of 16 h the efficiency of removal was found to
be 86%, 65%, 60% and 46%.
3.8. Photocatalytic degradation
Photocatalyst utilizes sunlight as clean energy in the wastewater
treatment particularly in industrial wastewater discharges. This type of
treatment comes under the category of green environmental protection
and it has a capacity to degrade the pollutants into non-toxic substances.
In recent times, photocatalytic material with excellent performances
were invented and among them TiO2 was found to be excellent with high
band gap energy, TiO2 found to be active under ultraviolet light which
presence is seen only 305 in solar energy. To overcome were these types
of difficulties the response range of the photocatalytic materials were
widened to visible light for the effective removal of pollutants. The
limitations were overwhelmed by coupling TiO2 with other materials
which are of semiconductors like TiO2/Co-g-C3N4, N-doped TiO2-carbon
composite fibres (Deng et al., 2017), carbon nitride loaded into titanium
incorporated SBA-15 mesoporous silica (Ti-SBA15-CN) (Liu et al.,
2017). Other than TiO2, many semiconductors like Au–Pd nanoparticles
loaded on g-C3N4/MCM-41 (Patnaik et al., 2020), PANI-sensitized
g-C3N4/ZnFe2O4 heterostructure (Patnaik et al., 2018), bismuth oxoio­
dide/reduced graphene oxide/bismuth sulphide (BiOI/Rgo/Bi2S3)
(Chen et al., 2017). Graphene based materials were uiltized in more
amount due to its recombination ability of electron-hole pair. The ma­
terials conduction band is reduced due to the electronic charge in­
teractions between graphene sheets and semiconductor materials. Fig. 7
represents the photocatalytic treatment setup for the removal of phenol.
In a research, photocatalysis using hybrid nanocomposite such as
Graphene Oxide coated TiO2 and graphene-Iron. The performance was
tested with pH, adsorbent dosage and phenol concentration. 97.78%
removal efficiency was achieved at 0.210 g/L of adsorbent dosage, 5.5 P
h and with phenol concentration of 10 mg/L with graphene-TiO2
(Ganguly et al., 2020). 16 phenolic compounds which are identified in
refinery wastewater. Bojuru sand was used as the catalyst with 95 W UV
lamp at 7 pH with 5 g of bojuru sand was utilized for 1 litter of refinery
3.7.2. Hybrid technologies
In a research, Photocatalytic Hybrid Sequencing Batch Reactor
(PHSBR) for the removal of phenol was performed. In this process, single
system was designed in which photocatalysis ad SBR occurs simulta­
neously. There was 72% removal of phenol with photocatalytic reaction
for 100 mg/L of phenol and 50–70% of removal efficiency were seen for
the same amount of phenol in biodegradation. In the hybrid technique
up to 90% of removal efficiency was achieved. This is due to higher
mineralization of phenol with stable biodegradation performance
(Yusoff et al., 2018). Praveen and Loh (2015) utilized two-phase parti­
tioning membrane with extraction and stripping method for high con­
centration phenol removal. In organic phase, kerosene dissolved tributyl
Fig. 7. Photocatalytic treatment of phenolic contaminated wastewater.
9
K. Grace Pavithra et al.
Environmental Research 237 (2023) 117005
wastewater. The efficiency of the phenol removal was found to be effi­
cient at t1/2 value is 0.01–3.6 h (Salcedo et al., 2020).
metals (Li et al., 2012; Dai et al., 2017; Mei et al., 2020). In catalytic
ozonation process, lower valence active component represents high
removal of organic pollutant (He et al., 2012). In a research, biochar
synthesized from sludge was utilized for the removal of phenol from
coking wastewater. At 30 min of reaction, 95% of phenol was removed
using ozonation. When compared to non-catalytic ozonation biochar
assisted ozonation is superior in the removal of phenol. After the pro­
cess, deactivation of biochar occurs and it may be due to ozone
adsorption on the biochar surface (Bagreev et al., 2001). In another
research,
cerium-doped
alumina
was
prepared
using
impregnation-calcination method and utilized as catalyst in the ozona­
tion of phenol. By ozonation 39.7% and by catalytic ozonation 46.3%
removal of phenol was achieved at 50 min of reaction (Li et al., 2020). In
a research, magnesium oxide was impregnated with activated carbon
(MgO/AC). At the surface of activated carbon MgO was inserted in the
form of nano-crystal particles. The catalytic performance of the acti­
vated carbon was increased by MgO incorporation. The phenol and COD
concentration removal was found to be 88.5% and 83.5% (Zhou et al.,
2020). For the ozonation treatment, Magnetic NiFe2O4 was utilized as
catalyst. When comparing with ozonation alone, the involvement of
magnetic NiFe2O4 as catalyst showed high removal efficiency of phenol
and additionally the catalyst can be easily separated from the mixture
using a magnet (Zhao et al., 2013).
3.9. Enzymatic process
Enzymatic remediation has become most attractive technology as it
doesn’t produce sludge. This type of system found to be efficient for low
concentration pollutants. DNA recombination at low cost makes the
enzyme utilization in wastewater remediation. The main disadvantage
of the enzyme system is the stability of the enzyme as it depends upon
the pH and temperature. Two types of enzymatic reactions namely ho­
mogeneous and heterogeneous are available. The most widely used
homogeneous enzymes are laccases and peroxidases. Peroxidases have
either redox-active selenocystine/cysteine residues or heme factor at
their active sites which make easy access to pollutants removal espe­
cially phenol and its derivatives (Husain and Qayyum, 2013). The main
drawback in the usage of peroxidases is the addition of H2O2 at the time
of the reaction. With high concentration of H2O2 the peroxidases
become inactive (Chiong et al., 2016). Laccases can be obtained from
fungi. Due to its non-specificity (Beck et al., 2018), wide range of sub­
strates and multicopper oxidases (Asadgol et al., 2014), laccases are
utilized in wastewater remediation. The limitation of homogenous
enzymatic system is biocatalyst recovery and this can be rectified by
heterogenous enzymatic system and in heterogenous system enzyme
stability can be seen (Ba et al., 2013; Ji et al., 2017).
3.11. Adsorption
Due to its low operational cost, efficiency, ease in handling,
simplicity, low energy consumption, reduced sludge generation,
adsorption are used in the removal of organic pollutants (Yin et al.,
2016; Capra et al., 2018). Table 2 represents some of the adsorbents
utilized in the removal of phenol.
3.10. Ozonation
In the treatment of organic pollutants, catalytic ozonation plays
major role by radicals generation at higher oxidation-reduction poten­
tials. In the promotion of free radical production noble metals plays
major role and this step is found to be critical step in catalytic ozonation
but in economical aspect the noble metals were replaced with transition
Table 2
Some of the adsorbents commonly utilized in the removal of phenol.
Origin of sorbents
From algae
Biomass
Resin
GO-Fe3O4
Graphite
Pyrolytic tire
char
Semi coke
Moroccan clay
Adsorbent preparation
Mode of
operation
Type of treatment
Adsorption
capacity
Reference
Spirulina
HyCNT
Spirulina & Chitosan
Chitosan hydrogel/CNT
Batch
447.6 mg g− 1
404.2 mg g− 1
Peres et al. (2018)
Alves et al. (2019)
CS/MWCNT
Batch
86.96 mg g− 1
Guo et al. (2019)
Rice husk
Rice husk
Toona sinensis leaves
Peanut shell
Chitosan & Multi wall carbon
nanotube
KOH & EDTA-4Na
KOH
NaOH & N2
Quartz reactor
Batch
Batch
Batch
Batch
Thermal treatment at 80 ◦ C
Thermal treatment with
constant vacuum
Thermal treatment with
constant vacuum
Carbonization at 750 ◦ C
Pyrolysis AT 750 ◦ C
Carbonization at 800 ◦ C
Heated at 600 ◦ C
Black wattle bark
Peanut shell
ZnCl2
–
Batch
Batch
Carbonized at 700 ◦ C
Carbonized at 600 ◦ C
Ziziphus leaves
Heated at 105 ◦ C
Acacia mangium
Azadirachta indica leaves
Black wattle bark waste
Tithonia diversifolia
activated carbon
Oil palm frond
Low rank coal
Pomelo peel
Sugarcane bagasse
Rosin
Phosphoric acid 50%
Heated at 80 ◦ C
Heated at 700 ◦ C
2,4-dinitrophenol
Batch
Batch
Batch
Batch
Carbonization
Thermal treatment
Thermal treatment
KOH
Exfoliated graphite
Steam pyrolysis at 500 ◦ C
Carbonization
Pyrolyzed with ferric chloride
Dried at 100 ◦ C
Rosin/ethylenediamine
Graphene oxide/Fe3O4
–
Batch
Batch
Batch
Batch
Batch
Batch
Batch
–
KOH
Thermal
Thermal
Refluxing for 4 h
Sonication
Vaporization
62.89 mg g− 1
549.6 mg g− 1
16.37 mg g− 1
57.7 mg g− 1
82 mg g− 1
79 mg g− 1
–
Tire char
Acid treatment
Batch
Pyrolysis at 500 ◦ C
65.8 mg g− 1
–
–
KOH solution
Sulphuric acid treated
Batch
Batch
Calcinated at 600 ◦ C
Heated at 110 ◦ C
42.75 mg g− 1
Thermal treatment
10
− 1
215 mg g
201 mg g− 1
325 mg g− 1
55.43 mg g− 1
98.57 mg g− 1
21 ± 2.11 mg
g− 1
–
96.92 mgg− 1
98.57 mg g− 1
42.607 mg g− 1
Lv et al. (2020)
Fu et al. (2019)
Kong et al. (2020)
da Gama et al.
(2018)
Lütke et al. (2019)
da Gama et al.
(2018)
Al Bsoul et al.
(2020)
Zhang et al. (2021)
Mandal et al. (2020)
Lütke et al. (2019)
Supong et al. (2020)
Lawal et al. (2021)
Gokce et al. (2020)
Dong et al. (2021)
Elayadi et al. (2021)
Liu et al. (2019b)
Tshemese et al.
(2021)
Makrigianni et al.
(2015)
Gao et al. (2017)
Elayadi et al. (2021)
K. Grace Pavithra et al.
Environmental Research 237 (2023) 117005
3.11.1. Biosorption
Biosorption utilizes renewable and natural materials for the removal
of pollutants from aqueous phase. This type of treatment is considered as
fast, economic, clean and efficient treatment.
utilized for the modification of carbon nanotubes. The adsorption ca­
pacity of crystal violet dye and phenol was found to be 394 mg g− 1 and
298 mg g− 1 (Lawal et al., 2019).
3.12.1. Silica gel
Calixarenes which is considers as supermolecular compounds and
synthetic receptor in removal of many contaminants due to the presence
of cyclodextrins and crown ethers. Calix [4]arene-bonded silica gels (C
[4]BS and DMC [4]BS) were synthesized for the removal of phenol, mnitrophenol, p-nitrophenol, p-chlorophenol from aqueous phase and
among them C [4]BS showed better performance in the removal under
optimum conditions such as 25 mg of adsorbent dosage, 25 ◦ C, 1 h
contact time, pH 6 and 5.0 × 10− 5 M of p-nitrophenol concentration
obtains 0.65 mgg− 1 of adsorption capacity (Dolaksiz et al., 2018). In a
research, Carbon xerogels surface was functionalized with acidity, ba­
sicity and combination of basic/acidic active centres by hydrothermal
method, gas treatments and thermal modifications and it was concluded
carbon xerogel surface chemical structure alters the catalytic and
adsorption properties (Malaika et al., 2020).
3.11.2. Microbes
In a research, Candida tropicalis ATCC 750 was immobilized using
cashew apple bagasse and utilized as biocatalyst in the removal of
phenol. The synthesized biocatalyst shows high stability towards oper­
ation and storage. The removal efficiency was found high under 30 ◦ C,
150 RPM, pH 6.0 and 1100 mg L− 1 (E Silva et al., 2019).
3.11.2.1. Algae. In a research, algae like Spirulina were utilized for the
removal of phenol in combination with chitosan obtained from shrimp
wastes and the specific surface area was found to be 1112 m2 g-1 (Peres
et al., 2018). A new type of foam with the combination of Spirulina and
chitosan for the removal of phenol from aqueous solution. From the
characterization the surface of the foam was found to be irregular and
rough surface. The adsorption capacity was found to be 447.6 mg g− 1
(Alves et al., 2020).
3.12.2. Sludge
In a research, sludge from oxygen furnace in steel plant was taken as
adsorbent in the removal of phenol. Studies are made with different
phenol concentration, adsorbent dosage, time and temperature. At op­
timum conditions such as adsorbent dose of 20 g/L, pH of 7, phenol
concentration of 10 mg/L, adsorption time of 240 min and at the tem­
perature of 35 ◦ C 63 5 of removal efficiency was achieved (Mandal and
Das, 2019).
3.11.2.2. From plants. In a research, M-AC was prepared by modifying
rice husk with KOH-activation and EDTA-4Na-modification. The
adsorption capacity for phenol was found to be 215 mg g− 1 (Lv et al.,
2020). From black wattle bark waste, micro/mesoporous activated
carbon has been prepared and utilized for phenol adsorption which has
98.57 mg g− 1 of adsorption capacity. The black wattle bark was utilized
in tannin extraction industries. The efficiency of phenolic removal was
seen to be 95.89% (Lütke et al., 2019). The bark from Acacia mangium
was activated using phosphoric acid and utilized for phenol removal.
The adsorption capacity of the phenol increases with increased phenol
concentration. The maximum adsorption capacity was found to be
96.92 mg g− 1 (Zhang et al., 2020a). The biochar from oil palm fronds
was synthesized using pyrolysis method and used for the phenol
removal. The adsorption capacity obtained is 62.89 mgg− 1 using 3 g/L
dosage of biochar at optimum conditions such as pH 6.5, 45 ◦ C of
temperature and 8 h of time (Lawal et al., 2021). The biochar synthe­
sized from rice husk using two-step KOH catalysed pyrolysis. Two-step
and one-step KOH catalysed pyrolysis was compared and among them
two-step KOH catalysed pyrolysis provides higher amount of biochar wit
maximum adsorption capacity of 201 mg g− 1 (Fu et al., 2019). In a
research, peanut shell was utilized as activated carbon for the removal of
phenol as well as cadmium ions. From the results, the carbonaceous
material was found to be mesoporous and amorphous predominantly
and the removal of phenol was found to be 55.43 mgg− 1 (da Gama et al.,
2018; Niknam et al., 2021). In another research, activated carbon was
produced from fox nutshell with the help of zinc chloride and from the
BET analysis, the pore size was found to be 2869 m2/g with micropore
and macropore volume. The material was prepared using impregnation
technique and showed good removal efficiencies (Kumar and Jena,
2016; Míguez et al., 2021).
3.12.3. Hybrid system
In a research, Bacillus sp. SAS19 was combined with porous carbo­
naceous gels. The bacterium was isolated from activated sludge by
addition of resuscitation-promoting factor. The biodegradation was
performed using free as well as immobilized bacterium for various
phenol concentrations. For long time utilization and for removal of high
concentrations of phenol, immobilized bacteria play role when
compared to free bacteria. At third recycling, 87.2% of phenol degra­
dation efficiency was achieved (Ke et al., 2018). Ma et al. (2013)
immobilized Pseudomonas putida with activated carbon fiber in order to
treat phenol (10,000 mg/L) by acclimatization and cells-immobilized
technique. The phenol removal followed adsorption with decrease of
concentration from 10,000 mg/L to 2000 mg/L in 120 h.
4. Phenol recovery from wastewater
4.1. Adsorption
Kalogerakis et al. recovered phenol using Wettability switchable
surface molecularly imprinted polymer based on TiO2 loaded hollow
carbon sphere (WSMIP@HC-Ti) by washing the adsorbent with meth­
anol at room temperature. Up to 98% of phenol was recovered by uti­
lizing 120 mL of methanol per 5 mL of adsorbent (Kalogerakis et al.,
2013). Darvishi et al. utilized ceria and manganese oxide modified
rice-straw activated carbon which is calcined at different temperatures.
After removal of phenol using the prepared activated carbon-metal
composites. In 10th cycle of adsorption-oxidation experiments Ac–Mn
showed best performance among composites of AC metal (Cañadas
et al., 2021). Chaari et al. utilized Tunisian natural clay for the phenolic
compound’s removal. The adsorbent was prepared by thermal activation
method that is at 600 ◦ C and the efficiency was found to be 84.21% at
acidic pH. Desorption studies shows that the raw clay negative sites
favours way for desorption of phenol (Chaghaganooj et al., 2021). The
regeneration of adsorbent material involves addition of chemicals and
the interaction with the targeted material occurs.
3.12. Carbon nanotubes
Nanotubes were considered as nanoscale adsorbent and provides
promising applications in nanotechnology field. Nanotubes showed
good performance in the removal of hydrophobic pollutants present in
water. The phenyl compound presence in phenol makes phenol a hy­
drophobic nature at room temperature (Han et al., 2020). In research,
MWCNTs/TiO2 was utilized as nano adsorbent in the removal of phenol.
By impregnation method, the adsorbent was developed with 400 ◦ C of
calcination. Phenol and cyanide removal was seen at high temperature
(Kariim et al., 2020). In a research, modified carbon nanotubes was
utilized for the removal of phenol and crystal violet dye. The composi­
tion of glycerol and methyltriphenylphosphonium bromide were
11
K. Grace Pavithra et al.
Environmental Research 237 (2023) 117005
4.2. Membrane technologies
IRA-67 weak-base anion exchange and Amberlyst A26 strong-base anion
exchange resin. The phenol was washed using distilled water with 0.042
L min− 1 for the first 10 min and the flow rate was increased to 0.33 L
min− 1. The recovery efficiency was found to be 100% for both the resins
(Víctor-Ortega et al., 2016). Zhang et al. researched with electrochem­
ically researched switched ion exchange (ESIX), ferricyanide (FeHCF)
coated electrode and the phenol was extracted with sodium (Na+) ions
by FeHCF redox state. The pH was maintained at 12.84 with a energy
consumption of 9.677 × 10− 3 kWh mol− 1. For the regeneration the pH
maintained was 12.84 with the energy consumption of 2.083 × 10− 3
(Zhang et al., 2020c). Ion exchange provides high flow rate and
concentrate the sample and the disadvantages include, the sample uti­
lized for ion exchange should have low ionic strength and the positively
charged residues causes negatively charged protein to bind cation
exchanger and likewise for negatively charged residues.
Pressure driven membrane process are utilized in recent times for the
separation, fractionation and concentration of phenolic compounds as
they provide many advantages over conventional methods in separa­
tion. Some of the advantages of membrane process are no additive re­
quirements, no phase change and low energy consumption. The NF and
UF membrane separate targeted compounds based on molecular weight
cut off (MWCO) and apart from MWCO the other parameters such as
concentration polarization, membrane fouling, hydrophobic and
coulombic interaction shas to be noted. The combination of different
membrane process is recently utilized in the recovery of phenolic
compounds. Cassano et al. utilized MF, UF and NF membranes in a order
where MF removes suspended solids, UF removes macromolecules by
conserving the phenolic compounds in the retentate stream and NF
utilized to recover the concentration of solute (Cassano et al., 2011). It
has been proved that using this sequential design, the permeate can be
utilized directly for the industrial process. Russo investigated the re­
covery of polyphenols from olive wastewater by the sequential design of
MF, UF and RO membranes. The wastewater was fed into MF with
centrifugation as primary step and this MF membrane process accounts
severe fouling. The permeate from this MF and UF consist of hydroxy
tyrosol, caffeic acid, oleuropein, protocatechuic acid and tyrosol. The
permeate was sent to RO which results in low yield of polyphenols
(Russo, 2007). The disadvantages of membrane process is the membrane
fouling and high investments.
5. Future perspective
The removal techniques such as distillation, extraction, wet air
oxidation, membrane process, chemical oxidation, electrochemical
oxidation, biological treatment and adsorption were discussed in the
review and their merits as well as demerits are described in Table 3.
When compared to all the treatment techniques, adsorption was found
to be economic, easy operation and efficient treatment with wide range
of pH. The waste from plants such as barks, seeds, leaves and sludges
from industrial waste are in turn used as adsorbent with some modifi­
cation in physical as well as chemical aspect. The adsorbent utilized for
single treatment can be repeated for n number of times and the
repeatability depends upon the efficiency of the adsorbent. Although,
the final utilized adsorbent was seen as major problem in adsorption
treatment, regeneration technique has rectified the problem. In future
acceptive, for the treatment of phenol persistent water, the utilization of
cost-effective adsorbent with multiple regeneration aspect is needed for
the minimal utilization of resources for larger treatment of wastewater.
The researchers are focussing on the utilization of low-cost adsorbent
and spent adsorbent and this manuscript will give gist about the utili­
zation of various adsorbent in the removal of phenol.
4.3. Extraction
Duan et al. utilized micro pipeline system for the extraction of
phenol, tributyl phosphate was prepared by dissolving in kerosene and
the striping phase was NaOH aqueous solution of 0.5 mol/L. The pipe­
line system consists of main micro pipe, loop pipeline by two upright
expanded pipes, T-type joints and a buffer pipe. The recovery of phenol
was found to be 94% (Duan et al., 2021). Some of the disadvantages of
extraction are the time consumption and target compound degradation
in the presence of high temperatures. Kiai et al. extracted phenol from
olive processing wastewater using cloud pint extraction (CPE) method.
This method is utilized as a preconcentration step for phenol from olive
processing wastewater using three different surfactants such as Genapol
X-080, Tween 80 and Triton X100. The experimental conditions such as
pH, concentration, incubation time and equilibrium conditions. The
optimum conditions were found to be 2 pH, 70 ◦ C with the equilibrium
time of 30 min. The recovery factor for the phenol from olive processing
wastewater was found to be 65%, 62% and 68% while using 10% of
Triton X100, Genapol X-080 and Tween 80 (Kiai et al., 2018). Canadas
et al. recovered wine-derived phenolic compounds from liquid effluents
by hydrophobic eutectic solvents (HESs). The eutectic solvents consist of
fatty acids, DL-menthol and ammonium salts and utilized as
liquid-liquid extraction solvents. The extraction time, dilution factor,
solvent to feed volume ratio and the HESs types. The extraction of
phenol from winery wastewater and from gallic acid solutions using
extraction method and the efficiencies are found to be 74.865 and
92.31%. N8881Cl-Octanoic acid and N8881Cl-DL-Menthol solvents
showed high potential for recovery of phenolic compounds (Cañadas
et al., 2021).
6. Conclusion
Phenol, as a priority environmental pollutant, poses significant risk
to terrestrial and aquatic health. Adsorption is found to be a promising
and efficient strategy in the removal of phenol from wastewater. The
low-cost adsorbents show great potential towards the removal of phenol
due to its cost-effectiveness, simple implementation and the ability to
target specific pollutants. However, the factors such as temperature, pH,
initial phenol concentration, contact time are to be optimized to achieve
the adsorption efficiency. While adsorption offers benefits, it is impor­
tant to note that it is not a complete solution. The adsorbed phenol has to
be further processed before releasing back to our environment. Several
biological treatment techniques such as anaerobic digester, bioremedi­
ation and hydrothermal liquefaction can be employed to process the
adsorbed phenol. Overall, the combination of adsorption with biological
treatment methods offers environmental friendly and sustainable
approach to tackle phenol pollution. Continuous research in future is
much needed in this field to ensure the applicability and efficiency of the
proposed method.
4.4. Ion exchange process
Declaration of competing interest
Ortega et al. utilized continuous adsorption in fixed-bed column
which is simple in process and in future it can be scaled-up. This research
highlights the separation of phenol from olive wastewater using strongbased Amberlyst A26 and weak-base Amberlite IRA-67 anion exchange
resins. The pH value maintained was 7 throughout the experiment and
the removal efficiency of phenol attained was 57%. The elution was
performed using NaOH 4% and aqueous solution of 2% for Amberlite
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
12
K. Grace Pavithra et al.
Environmental Research 237 (2023) 117005
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Table 3
The advantages and disadvantages of the treatments involved in phenolic
removal.
Treatment
techniques
Merits
Demerits
Distillation
In distillation of phenol, the
compounds such as muconic
acid, maleic acid,
pyrocatechol, butanedioic
acid and hydroquinone in its
liquid phase and in gaseous
phase carbon monoxide,
carbon dioxide and hydrogen
were seen.
Suitable for high
concentrations.
Solvents recovery are possible
Utilized at low concentrations
High consumption in terms
of energy and cost
Extraction
Wet air oxidation
Membrane process
The pollutant which is dilute
for incineration and toxic for
biological treatment can
adopt this method
Phenol destruction occurs
Low power consumption,
quality effluent
Chemical
oxidation
Low energy cost and low
consumption of reagent
Electrochemical
treatment
Low cost and no reagent
requirement
Biological
treatment
Inexpensive method, simple in
design, easy maintenance
Enviroenmnetal friendly
No or less chemical
consumption
Can be used as primary or in
combination of other
treatments
Photocatalytic
degradation
Enzymatic process
Ozonation
Adsorption
Less treatment time, easy
operation and installation, no
handling of chemicals
Simple technique
Cost efficiency in low
concentrations
Recovery of adsorbents from
the adsorbates are possible
• Disposal of used solvents
are tedious
• High investments
• Use of toxic solvents
Works under high pressur
and energy utilization
conditions
Membrane fouling,
concentration polarization,
Low discharge cannot be
handled by membrane
separation method
Chemicals are required
High consumption of energy
Secondary treatment are
necessary
Installation of equipment
requires initial cost and
energy
Creation of favourable
environment for biological
treatment is tedioud process
Economically not suitable
for wide range of usage
Less availability,
Low stability,
Isolation of enzymes from
reaction media
Before ozonation
pretreatment is mandatory
The efficiency of removal
depends upon the type of the
adsorbent being utilized and
the recycling parameters
Data availability
Data will be made available on request.
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