A critical review on the recent progress in application of electro-Fenton process for decontamination of wastewater at near-neutral pH.pdf

Chemical Engineering Journal 474 (2023) 145741
Available online 3 September 2023
1385-8947/© 2023 Elsevier B.V. All rights reserved.
Review
A critical review on the recent progress in application of electro-Fenton
process for decontamination of wastewater at near-neutral pH
Zahra Heidari a,b,c
, Rasool Pelalak a,b,c
, Minghua Zhou a,b,c,*
a
Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin,
300350, China
b
Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media Pollution, Nankai University, Tianjin, 300350, China
c
Tianjin Advanced Water Treatment Technology International Joint Research Center, College of Environmental Science and Engineering, Nankai University, Tianjin,
300350, China
A R T I C L E I N F O
Keywords:
Neutral pH
Electro-Fenton process
Wastewater treatment
Heterogeneous catalysts
Cathode modification
Chelating agents
A B S T R A C T
One of the greatest engineering challenges of this century is development of new technologies for removing
emerging hazardous contaminants from water sources. Electro-Fenton (EF) as one of the most promising ap
proaches of advanced oxidation processes (AOPs) has received widespread attention for its excellent performance
in the removal of recalcitrant pollutants due to generation of strong oxidizing species like hydroxyl radicals
(•
OH). However, the practical application of classical homogeneous EF process is hampered by a narrow pH
range (2–4) and production of sludge at higher pH values. The information obtained so far highlights the need to
enhance the removal efficiency and reduce the reaction time of EF process under neutral pH conditions. Herein,
this review summarizes efficient approaches in recent years (2017–2023) applied in EF system with the aim of
overcoming the low pH implementation barrier, such as application of heterogeneous catalysts in EF process
(HEF), cathode modifications, using chelating agents, and hybridizing the EF with other treatment methods,
including adsorption, membrane, and photo/catalysis. These strategies can accelerate the iron cycle and Fe3+
reduction; therefore, more H2O2 and •
OH can be generated. Promoting the H2O2 utilization efficiency can
provide more oxidative species, which leads to higher degradation and mineralization of contaminant. On the
other hand, some strategies by preventing the Fe3+
precipitation at higher pH values help the system work in a
wider pH range. In the final section, the challenges of these plans are discussed and perspectives for future
research are proposed to improve the practicability and feasibility of the EF for wastewater treatment.
1. Introduction
Many persistent contaminants have been detected in different sur
face and groundwater bodies at very low concentrations which can in
crease bacterial resistance in the environment [1,2]. Advanced
oxidation processes (AOPs), including sonocatalysis, photocatalysis,
electrochemical-based AOPs (EAOPs), and ozone-based processes have
attracted widespread interest for the effective removal of different
classes of persistent pollutants from water [3–7]. Among different AOP,
EAOPs and more specifically, electro-Fenton (EF) systems have been
extensively applied in recalcitrant contaminant remediation due to the
in situ and continuous electrogeneration of strong oxidizing agents,
excellent energy and cost efficiency, ease of operation as well as fast
degradation and mineralization [8–10]. In a typical EF process,
hydrogen peroxide (H2O2) is continuously produced from the two-
electron reduction of injected O2 at the carbonaceous cathode in
acidic solution, according to Eq. (1). The generation of •
OH occurs
through Eq. (2), which is the classical Fenton reaction, from the reaction
of ferrous iron (Fe2+
) with H2O2 [11,12].
O2 + 2H+
+ 2e−
→H2O2 (1)
H2O2 + Fe2+
→Fe3+
+ •
OH + OH−
(2)
The oxygen electro reduction occurs in the presence of protons; thus,
the production of H2O2 and other oxidizing agents is more achievable
under acidic conditions. Moreover, the instability of Fe2+
and the low
* Corresponding author at: Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education, College of Environmental Science and Engi
neering, Nankai University, Tianjin, 300350, China.
E-mail address: zhoumh@nankai.edu.cn (M. Zhou).
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
https://doi.org/10.1016/j.cej.2023.145741
Received 6 June 2023; Received in revised form 10 August 2023; Accepted 28 August 2023

2
activity of H2O2 and •OH under neutral and alkaline conditions neces
sitate working at low pH values (2.0–4.0) [13,14]. This constrains large-
scale EF application by medium acidification and subsequent neutrali
zation after treatment, which cause extra treatment costs and increases
the medium salinity [15]. Furthermore, in acidification step some gases
can be released in the presence of cyanides or sulfides, while in the
neutralization step, iron sludge (ferric hydroxide) can be generated
which causes secondary pollution and decreases the activity of iron
[16,17]. Also, the addition of high concentration of ionic iron to the
solution and the difficulty of reusability and recyclability are other
drawbacks restrict the EF practical applicability. It has been reported
that the amount of iron remaining in the treated effluent exceeds the
standards of the European Union and should be removed from the final
treated water, which also imposes extra process costs [18,19]. These
weaknesses created a gap between the results obtained from research
works in the laboratory and real implementation of EF process. There
fore, from an economic and environmental point of view, the application
of conventional EF treatment process is not efficacious. A similar trend
was also observed in the application of other treatment processes in
neutral condition. For example, UV/H2O2 process successfully used for
the decontamination of surface water samples at pH 7.0 [20]. Gon
charuk et al. [21] confirmed the superior mineralization ability of O3/
TiO2/UV process (90%) in fulvic acids removal at initial pH of 6.0
compared to H2O2/TiO2/UV (73%) and O2/TiO2/UV (41%) processes.
Complete ibuprofen degradation in 20 min of reaction at neutral pH was
reported in the O3/Fe2+
/H2O2 and UV/Fe2+
processes by Lara-Romas
et al. [22]. In photochemical treatment of wastewater application of
TiO2 confirmed its significant performance at neutral pH [23–25].The
successful application of ultrasound assisted processes at neutral pH
have been proven in elimination of wide range of organic pollutants
from wastewater in different studies [26–28].
According to the literature review, some approaches have been
proposed to perform EF process at neutral pH as shown in Fig. 1(a),
which not only reduces the treatment costs but also makes the process
more compatible with urban wastewater effluents and large-scale ap
plications. The application of heterogeneous catalysts in EF (HEF) hin
ders sludge production and iron leaching at higher pH values, and also
the heterogeneous catalysts can be recovered and reused for continuous
cycles [29]. H2O2 and subsequent oxidizing agents can be produced
through surface and homogeneously catalyzed reactions which are both
influenced by catalytic ability. In homogeneous reactions, Fe3+
/Fe2+
cycle promotes the H2O2 generation while diffusion of gaseous and/or
dissolved O2 to the solid–liquid interface promotes H2O2 generation in
surface-catalyzed reactions [30]. An alternative technique to accomplish
the EF process at neutral and near-neutral pH values is modification of
the cathode. There is a burning desire to provide stable, eco-friendly,
and highly tunable bifunctional cathodes using metals, bimetals, and
non-metals to improve cathode characteristics and H2O2 electro
generation [31,32]. Another technique that successfully prolongs the EF
working pH range is the use of chelating agents or ligands. These ma
terials form soluble species through coordination with Fe2+
/Fe3+
and
prevent iron precipitation in the form of iron hydroxide at neutral and
near-neutral pH values [17].
Although the Fenton process has been known for more than a cen
tury, its application at neutral pH is still a topic of discussion. Recent
advances in homogeneous photo-Fenton processes working at near-
neutral or neutral pH values by chelating agent addition have been re
ported by Clarizia et al. [33]. Different cathode modifications, including
modifications based on materials, (electro)chemical and thermal treat
ments in the EF processes, were reviewed by Lin et al. [32]. The appli
cation of chelating agents at high pH levels in Fenton and Fenton-like
systems was reviewed by Zhang and Zhou [17]. However, to the best of
our knowledge, a specific review related to recent trends in overcoming
the pH adjustment drawback in EF processes is lacking. Therefore, the
present review specifically provides an insight to highlight recent ad
vances in extending the working pH in EF processes to the near-neutral
and neutral pH values that appear in the literature so far. Fig. 1(b) shows
the co-occurrence network of the top keywords with the most frequent
employed in articles (2017–2023) on the EF system to broaden the
working pH. Based on the type of procedure, modifications can be
classified into four categories including the application of (1) novel or
modified heterogeneous catalysts, (2) cathode modification, (3)
chelating agents, and (4) combination of EF process with other treat
ment methods, such as photo-EF (PEF). In the following sections, eco
nomic aspects of applying these approaches are highlighted. Finally,
challenges and proposals are presented for future investigations that can
make significant contributions to this specific field.
2. Application of heterogeneous catalysts in EF process
The leached iron in homogeneous EF process at higher pH values
forms ferric hydroxide complexes such as Fe (OH)−
4 , Fe(OH)2+
, and Fe
(OH)3, which coat the catalyst surface can reduce the catalytic activity
[19]. Owing to the occurrence of catalytic reactions on the catalyst
(a) (b)
Fig. 1. (a) Different strategies used in EF system to broaden the working pH, and (b) co-occurrence network of the top keywords with the most frequent employed in
articles on the EF system to broaden the working pH (2017–2023).
Z. Heidari et al.

3
active site, the surface characteristics of catalysts, as well as cost, sta
bility, selectivity, and catalytic activity play key roles in the HEF effi
ciency [34]. Heterogeneous catalysts in HEF can be classified into
different categories: based on the nature of the active sites in the catalyst
(catalysts with only iron or catalysts with iron and other metals), the
synthesis method (minerals or synthetic catalysts), and the form of
catalysts applied to the reaction media (supported or suspended cata
lysts). Here, HEF systems are reviewed according to the application of
suspended and supported catalysts.
2.1. Suspended catalysts
2.1.1. Metal nanoparticles
Application of nanoparticles (NPs) as EF catalysts due to the small
particle size, can improve the available active surface for interaction
with other molecules and atoms; so increase the process efficiency.
Special attention has been given to the ferromagnetic NPs because of
ease of separation from the solution with an external magnetic field,
high surface areas, and no obvious iron sludge production at neutral pH.
By implanting different ions like Co2+
, Mn2+
, Fe2+
, and Fe3+
in the cubic
spinel structure of magnetic catalysts the generation of •
OH from H2O2
can be increased owing to the enhancement of the cyclic electron
transfer process by the embedded ions. Campos et al. [35] reported 30%
and 60% improvement in Nafcillin (36 mg/L) removal at neutral pH
using Fe/Cu bimetallic NPs compared to Fe and Cu NPs, respectively.
The generated •OH at the anode and the Fenton reaction with Cu and Fe
NPs were mainly responsible for contaminant degradation. Combination
of Co and Fe in CoFe2O4/natural organic matter (NOM) catalysts led to
complete removal of Acid Black 210 dye at pH 6.0 [36]. The satisfactory
performance of catalyst at near-neutral pH was attributed to the high
number of active sites, large surface area, and also, the cyclic electronic
transfer capacity of the ≡Fe3+
/≡Fe2+
and ≡Co3+
/≡Co2+
ion pairs,
which promoted the H2O2 transformation to •
OH. CoFe2O4 also
confirmed its potential application in HEF process by O2 activation at
cathode surface and also another chemical activation route from
disilicate-Fe(II) complexes. The higher reduction ability of the disilicate-
Fe(II) complexes at near-neutral pH led to chemical activation of O2 and
higher production of H2O2 in the system [37].
The metal encapsulation in a carbonaceous shell can prevent NPs
dissolution and agglomeration [38,39]. The coordinated interaction
between the metal particles in the core and carbonaceous materials in
the shell can improve the NPs catalytic ability [40]. For example, iron
sulfide encapsulated in a boron-doped graphene nanocomposite
(FeS2@BrGO) completely degraded bisphenol A (50 mg/L) at neutral pH
in 20 min due to the direct electron transferring from reductive sulfur
and Fe3+
free radicals and also the coordination of the oxygen-
containing functional groups on catalyst surface with Fe(III) that
reduced Fe3+
/Fe2+
reduction potential (Fig. 2(a)) [41].
Metal organic framework (MOF)-derived metal incorporated carbons
demonstrated higher conductivity compared to metal oxides thus widely
used in HEF processes. Ye et al. [42] obtained complete fluoxetine (10
mg/L) removal at near neutral pH using FeS2/C nanocomposite. The
simultaneous sulfidation and carbonization of iron-based MOF provided
unique structure of FeS2 in the core and acted as Fe2+
shuttles for ho
mogeneous and heterogeneous Fenton process, whereas the porous
carbon in the shell enhanced mass transport. The excellent electron
donor characteristic of FeS2 led to conversion of S2
2−
to sulfate so that
released H+
and Fe2+
(Fig. 2(b)). A similar mechanism was observed by
Fig. 2. Schematic illustration of the proposed reaction mechanism for HEF process catalyzed by: (a) FeS2@BrGO catalysts for bisphenol A degradation at pH 7.2 [41],
(b) FeS2/C catalysts for fluoxetine degradation at mild pH [42], (c) MIL-53(Fe)/S(1:2)-350 catalysts for sulfamethazine degradation at pH 7.0 [43], and (d) Fe2Co/
NPC catalysts for tetracycline degradation at pH 7.0 [44].
Z. Heidari et al.

4
Du et al. [43] who provided reduced S-modified MOF by sulfurizing
MIL-53 (Fe), and removed 95.8% of sulfamethazine at initial pH 7.0
(Fig. 2(c)). The radical scavenging and Electron Paramagnetic Reso
nance (EPR) tests showed the main role of •
OHads and auxiliary role of
•OHfree for sulfamethazine degradation. The turnover frequency value of
the synthesized catalysts was about 6.8-fold of that for commercial FeS2
and the rate constant in HEF process was 16 times higher than that in
homogenous EF process.
Studies have been shown that the N doping can adjust the charge
distribution in electronic structure of carbon and provide partially
negatively and positively charged groups, which improves the interfa
cial electron transfer and H2O2 production [9,44]. The bimetallic MOF-
based N-doped porous carbon (Fe2Co/NPC) catalysts showed effective
tetracycline degradation and mineralization at pH 7.0 [44]. The simul
taneous presence of low-valence Co and N-doped carbon support not
only enhanced the regeneration of ≡FeII
/≡CoII
but also provide a proper
surface for high dispersion of active sites, thus ensuring the continuous
formation of •
OH (Fig. 2(d)).
2.1.2. Natural catalysts
The reuse of abundant waste resources and the synthesis of new
materials through the principles of green chemistry can overcome the
costs of catalyst synthesis. A good example of the application of natural
catalysts is the production of a magnetic hybrid (HbLM) catalyst from an
eco-friendly and simple approach by reusing iron mine tailings waste
from the Mariana disaster (Brazil). dos Santos et al. [45] demonstrated
the high catalytic efficiency of synthesized catalyst in HEF and complete
degradation of Acid blue 29 diazo dye (25 mg/L) under neutral pH
condition. Another natural source of iron for the HEF process are min
eral iron oxides such as hematite (α-Fe2O3), pyrite (FeS2), goethite
(α-FeOOH), and magnetite (Fe3O4) [46,47]. Although these materials
produce H+
after dissolution in water and naturally reduce the solution
pH to near 3, a neutralization step is still needed at the end of the pro
cess. Labiadh et al. [46] analyzed the catalytic ability of pyrite for the
removal of textile dye (175 mg/L) in HEF system. Complete dye
degradation (in 30 min), 20% higher mineralization (90 %in 300 min),
and significantly lower energy consumption compared to the conven
tional EF process were obtained, which can be attributed to the self-
regulation of iron ions in the presence of oxygen. A similar perfor
mance was reported by Barhoumi et al. [48] using pyrite as Fe2+
source
for degradation and mineralization of sulfamethazine (0.2 mM).
Apart from Fe2+
, which is the main promoter of Fenton reaction,
other transition metals like Cu2+
/Cu+
, Co3+
/Co2+
, Mn4+
/Mn3+
can
promote Fenton-like reaction. Application of chalcopyrite (CuFeS2) in
HEF process completely mineralized tetracycline (0.2 mM) by •
OH
produced from both Fenton’s reaction (Fe2+
and) and Fenton’s-like re
action (Cu+
and H2O2), as well as the •
OH generated at the surface of
anode [47]. Notably, like pyrite, chalcopyrite acidifies the reaction so
lution by releasing H+
and the synergistic effect between Fe2+
and Cu2+
accelerated the Fe2+
regeneration, promoting the Fenton’s reaction and
H2O2 electrogeneration. This can be attributed to the faster reaction rate
constant of Cu(II)-carboxylate complexes and •OH compared with Fe
(III)-carboxylate complexes.
The characteristic properties of natural minerals are significantly
influenced by their origin and many reports confirmed the presence of
impurities in mineral ores. The properties of natural martite (Fe2O3) can
be improved by applying non-thermal plasma treatment, so that about
52.2% improvement in paraquat herbicide (20 mg/L) degradation,
lower electrical energy consumption, excellent stability and reusability
were achieved at near neutral pH [49]. This can be explained by the
higher surface area and further active sites on the plasma-treated martite
nanocatalyst, confirmed through BET analysis, which led to higher
electrogeneration of H2O2. In another study, Yu et al. [50] observed
complete degradation and 82% mineralization of diclofenac sodium at
neutral pH using hydrothermally synthesized pyrite. The higher effi
ciency of the HEF process can be explained by the higher activation of
oxygen molecules due to extra surface bound ferrous ions on pyrite,
which can produce more superoxide anions, prompting the Fe(II)/Fe(III)
cycle and consequently producing more •OH for contaminant
degradation.
2.1.3. Zero valent iron
The low cost and high reduction capacity of zero-valent iron (Fe0
)
make it a promising substitute for homogeneous Fe2+
, reducing iron
leaching in solution and extending the efficient working pH range. In
addition, the efficiency of the Fenton reaction can be improved in the
presence of Fe0
since Fe0
readily produces Fe2+
via Eq. (3) by losing two
electrons and hampering the formation of ferric hydroxides by recycling
Fe3+
according Eq. (4):
2Fe0
+ O2 + 2H2O→2Fe2+
+ 4OH−
(3)
2Fe3+
+ Fe0
→3Fe2+
(4)
Nevertheless, Fe0
corrosion reduces catalytic activity, which can be
overcome by the pre-magnetization of Fe0
. The catalytic activity and
feasibility of Fe0
and pre-magnetized Fe0
for the removal of persistent
and highly toxic p-nitrophenol (10 mg/L) were compared by Tian et al.
[10], who confirmed the higher catalytic activity of pre-magnetized Fe0
at neutral pH. At a catalyst concentration of 0.5 mM, current value of 25
mA, and pH 7.0, complete contaminant and 60% TOC removal were
obtained using pre-magnetized Fe0
(90 and 480 min, respectively),
while only 54% contaminant and 48% TOC removal were obtained with
Fe0
. The measurement of Fetot and •
OH concentration showed a decrease
after 60 min of reaction in both processes; however, the presence of Fe0
in the HEF system led to a twice as much decrease as in the HEF system
with pre-magnetized Fe0
, which can be due to Fe0
corrosion. In another
work, Tian et al. [51] modified micron-Fe0
to pre-magnetized-sulfidized
Fe0
(pre-S/Fe0
) particles for EF degradation of carbamazepine (CBZ)
over a wide range of pH (3.0–10.0). Comparison of different HEF pro
cesses using Fe0
, pre/Fe0
, pre-S/Fe0
, and S/Fe0
confirmed the superior
catalytic activity and stability of pre-S/Fe0
catalysts due to the syner
gistic effect of pre-magnetization and sulfidation in the catalyst syn
thesis process. Sulfidation of Fe0
not only accelerated the electron
transfer between catalyst and contaminant but also increased the reac
tivity of the S/Fe0
catalyst through the formation of sulfides, which
hindered the passivation of Fe0
by replacing iron oxide on the surface of
Fe0
[52,53]. Also, pre-magnetization by preventing Fe0
corrosion
resulted in complete CBZ removal over a wide pH range, whereas
complete CBZ removal was achieved only at pH values of 3.0 and 4.0 in
the other aforementioned processes. XPS analysis confirmed the reaction
between Fe and S and formation of species like FeS and FeS2, which can
lower pH of the solution via Eq. (5) and (6), and facilitated the trans
formation of electrons from the core of iron to the surface, as shown in
Fig. 3(a). The fast release of Fe2+
in the solution phase provide more
H2O2, •OH, •O2
–
, and also, 1
O2 which can participate in rapid CBZ
mineralization.
2FeS2 + 15H2O2→2Fe3+
+ 14H2O + 4SO2−
4 + 2H+
(5)
FeS + 8Fe3+
+ 4H2O→9Fe2+
+ SO2−
4 + 8H+
(6)
Ye et al. [54] used nano ZVI encapsulated in porous carbon rods,
obtained from high-temperature (800 ◦
C) pyrolysis of pristine and NH2-
doped Fe-based MOF (MIL(Fe)-88B), in EF degradation of gemfibrozil
(10 mg/L). The highest gemfibrozil degradation at near-neutral pH was
obtained in EF process using super paramagnetic core–shell nano-
ZVI@C-N which was about 40% greater than that of EF process using
the pristine MOF and nano-ZVI@C catalyst. Relatively low iron leach
ing, high catalytic activity and good stability were obtained mainly due
to carbon doping with N, which could upgrade the electron transfer and
enhance the FeIII
/FeII
cycle (Fig. 3(b)).
Z. Heidari et al.

5
2.2. Supported catalysts
According to the literature most of suspended catalysts suffer from
iron leaching, lack of stability, and catalyst activity reduction during
long-term usage [55]. Loading iron ions or iron oxides on a substrate
seems to be an effective by providing a larger surface area and catalytic
efficiency, sufficient contact between the catalyst, electrolyte, and sub
strate and also, preventing iron from leaching [34]. Different porous
solid supports including activated carbon, zeolites, and biochar as well
as various synthesis methods, such as precipitation, sol–gel, wet
impregnation, and adsorption have been used for the synthesis of
effective heterogeneous catalysts in EF systems [56–59]. Biodegradable
polymers beside the advantages of low toxicity and cost, have many
natural functional groups that can easily attach to the iron catalyst and
use as support. This section reviews the applications of natural and
synthetic supports in HEF process at neutral and near-neutral pH.
2.2.1. Natural based supports
Iron or iron oxide loaded on clay minerals can be a promising
candidate for heterogeneous catalysts in the HEF process. Kaolin is a
cheap and environmentally friendly clay with a high adsorption capacity
which has been used as a support of many catalysts. For example, Ozcan
et al. [60] reported complete degradation and mineralization of enox
acin antibiotic (0.25 mM) at four different pH values: 2.0, 3.0, 5.1, and
7.1 using Fe2O3 modified kaolin as a heterogeneous catalyst of HEF
system. Similarly, Zhang et al. [61] used kaolin as a support of hematite
and cuprous oxide (Fe-Cu/kaolin) and degrade 94.8% of Rhodamine B
dye at pH 6.7. The result of this study showed that the calcination
temperature had a key role in the system performance followed by
kaolin to pore-making agent ratio, calcination time, and catalyst dosage.
Application of two metals with a potential difference to accelerate
electron transfer inside catalysts can affect the efficiency of EF systems.
Qi et al. [62] studied this effect on preparation of Cu-Fe-FeC3 catalyst
immobilized on nitrogen-doped biochar (Cu-Fe-Fe3C@NDB) using chi
tosan as carrier material and evaluated its performance in amoxicillin
(100 mg/L) removal at near-neutral pH by HEF process. The obtained
results showed 99.3% contaminant degradation in 35 min of reaction
using Cu-Fe-Fe3C@NDB, whereas only 57.0% of amoxicillin was
degraded under the same condition without using Cu in the catalyst.
Along with the superior catalytic performance, the synthesized catalysts
exhibited low metal ion leaching and excellent stability at neutral pH
which could be due to the synergistic effect of Fe and Cu, as well as metal
particles wrapping in the carbon structure. In addition, nitrogen doping
improved the electron transfer rate and adjusted the electronic
structure. The role of various free radicals and the catalytic mechanism
in amoxicillin degradation were evaluated by radical scavenger exper
iments and XPS before and after the reaction, and the amoxicillin
removal mechanism was suggested as depicted in Fig. 4(a). Wang et al.
[63] used activated carbon (AC) supported Fe3O4 for the abatement of
diuron (10 mg/L) from aqueous solution by HEF process at pH 6.7.
Porous AC from the coconut shell was used as the substrate, and poly
tetrafluoroethylene (PTFE) was utilized to modify the catalyst, delay
inactivation, and extend the useful life of the catalyst. More than 95% of
contaminant was removed after 120 min and less than 0.1 mg/L ferric
ions leaching was observed even after 10 consecutive degradation runs.
Among natural polymeric matrices, chitosan due to the presence of
chelating agents like hydroxyl and amine groups presents good prop
erties for the immobilization of metals. TaheriAshtiani and Ayati [64]
utilized glutaraldehyde cross-linked magnetic chitosan (Fe3O4/CS/GA)
NPs as heterogeneous catalysts in EF process for dye degradation in a
cylindrical reactor with rotating cathodes, as shown in Fig. 4(b). The
authors reported more than 94% degradation of Acid Blue 25 dye (150
mg/L) in 90 min at neutral pH (6.8) and after 120 min, the TOC and COD
removal efficiencies reached 61.53% and 74.19%, respectively. The
superior degradation ability of HEF system in a wide pH range can be
attributed to the presence of iron ions in solid support which hindered
the iron ion precipitation at neutral and higher pH values. Rosales et al.
[31] prepared three novel iron chitosan-epichlorohydrin spheres to
degrade high concentrations of diclofenac (140 mg/L) through HEF
process at near-neutral pH. The reaction between epichlorohydrin
(crosslinking agent) and chitosan forms a polymeric network, improving
the stability, contact area, and catalytic activity. Iron fixation on the
chitosan-epichlorohydrin support was performed through three
different methods: impregnation, entrapment, and co-precipitation. The
obtained results demonstrate superior TOC decay (74.39%) in the HEF
system using entrapped iron catalysts without noticeable iron release,
whereas the other catalysts led to lower TOC decay.
2.2.2. Synthetic nanocomposite supports
To overcome problems related to practical applications of EF such as
sludge production and catalyst loss due to recovery and filtration pro
cedures, there is an interest in introducing bulky solid materials like
metal foams, as synthetic support of catalysts. Three-dimensional (3D)
nanostructured array frameworks of metal foams offer higher surface
area, and enhance electron transport pathways and ion diffusion. In this
regard, Chen et al. [65] synthesized novel Fe3O4 core–shell catalysts on
nickel foam (NF) support for degradation of salicylic acid (100 mg/L) by
HEF. NF@Fe3O4, NF@Fe3O4@SiO2, NF@Fe3O4@MgAl-LDH, and NF@
Fig. 3. The possible enhancement mechanism in HEF process: (a) carbamazepine degradation using pre-S/Fe0
catalysts [51], (b) gemfibrozil degradation using nano-
ZVI@C-N catalysts [54].
Z. Heidari et al.

6
Fe3O4@SiO2@MgAl-LDH were prepared using solvothermal, sol–gel,
hydrothermal, and co-precipitation methods, respectively. The catalysts
were layered two times by SiO2 and MgAl-LDH, which significantly
enhanced the loading ability and uniform dispersion of Fe3O4 on the NF
substrate, providing a large surface area and increased pore volume
(Fig. 5(a)). These characteristics provided excellent stability and great
performance of Fe3O4@SiO2@MgAl-LDH catalyst in removal of salicylic
acid (72.8%) and COD (52.12%) from solution at pH 7.0 which was due
to the uniform dispersion of Fe3O4 on the NF substrate. Graphitic carbon
nitride (g-C3N4) has a two-dimensional graphite-like configuration that
gathered significant attention due to its low cost, environmentally
friendly structure, excellent chemical stability, and affinity for entrap
ping transition metal ions. Doping Cu+
on g-C3N4 through the coordi
nation of Cu+
with pyridinic N results in a stable catalyst with high
catalytic activity for degradation of contaminants in Fenton-like catal
ysis at neutral pH [66]. In a recent study, the application of Cu-doped g-
C3N4 substrate using one-pot pyrolysis method (Fig. 5(b)) was explored
as a heterogeneous catalyst in EF process by Pan et al. [67] for degra
dation of amoxicillin (100 mg/L) at neutral pH. Amoxicillin degradation
was more favorable in neutral pH than at acidic pH due to higher •OH
scavenging by H+
(Eq. (7)) and higher Cu2+
production at pH 3.0, which
inhibits amoxicillin oxygenolysis according to Eq. (8). The remarkable
stability (after five cycles) and catalytic performance of samples at
neutral pH could be due to the Cu0
and Cu+
entrapment in the g-C3N4
structure obtained through one-step pyrolysis synthesis method.
H+
+ •
OH + e−
→H2O (7)
4Cu+
+ O2 + 4H+
→4Cu2+
+ 2H2O (8)
Recently, metal single-atom catalysts have been widely applied for
the removal of organic contaminants from water bodies due to their
superior inherent characteristics and potential for both oxygen evolu
tion reaction (OER) and ORR [68,69]. Zhang et al. [70] successfully
prepared a hollow sea-urchin-shaped carbon-anchored single-atom iron
(SAFe@HSC) from MOF of Zn and Fe for the degradation of thiamphe
nicol (20 mg/L) contaminant by HEF in a wide range of pH. A schematic
of synthesis procedure is depicted in Fig. 5(c). Almost total removal of
contaminant and 85% of TOC were obtained at neutral pH, which can be
due to the uniform dispersion of Fe and the unique hollow sea-urchin
structure of catalyst led to full access to the active sites, accelerating
the reduction of O2 to H2O2 and •OH production.
3. Cathode modification methods
Due to some limitations such as low electronic conductivity, high
synthesis costs, aggregation, and difficulty in gathering suspended par
ticles, the application of heterogeneous catalysts may not be easy in
practical engineering applications. In the past few years, the attachment
of catalyst particles to highly porous electrodes has enhanced the elec
trode surface to electrolyte volume ratio, solved the mass transport
limitations, and improved the EF performance in contaminant removal
at neutral and near-neutral pH values [71].
Fig. 4. (a) The effect of initial pH and reaction schematic of amoxicillin degradation by EF process using Cu–Fe–FeC3@NDB microsphere catalyst synthesized from
CuFe2O4@chitosan. Reproduced from Ref. [62], and (b) The effect of initial pH and reaction schematic of Acid Blue dye degradation by EF process using Fe3O4/CS/
GA NPs. Reproduced from Ref.[64]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Z. Heidari et al.

7
3.1. Metal based cathode modifications
Modification of the cathode with metals not only improves the
conductivity of cathode and reduces iron sludge formation, but also
leads to in-situ generation of simultaneous H2O2 and •OH by oxygen
reduction process at the cathode and thus broaden the working pH. Iron
and iron-based materials supported on carbonaceous composites are the
most studied electrodes in EF systems in neutral condition [32,55].
Accelerating the Fe(II)/Fe(III) redox cycle is an important step in
decomposition of H2O2 and EF process efficiency, which can be achieved
by introducing other metals such as Co, Cu, and Mn, that form redox
couples for Fe(III) reduction [15,40,72]. For example, Huang et al. [15]
confirmed no obvious differences were observed in terms of ciproflox
acin degradation (95.40% in 30 min), mineralization (93.77%, in 2 h),
and MCE (20.26%) using binary oxide of Fe-Mn on the carbon cathode at
pH 7.0, compared with those at pH 3.0. A similar catalysis mechanism of
homogeneous EF (the Haber-Weiss mechanism) can happen in this HEF
system according to Eqs. (9)–(13) [55,73,74]:
≡ FeIII
− OH + e−
→ ≡ FeII
− OH (9)
≡ FeIII
− OH + H2O2→ ≡ FeIII
− OH(H2O2)s (10)
FeIII
− OH(H2O2)s→ ≡ FeII
− OH
(
HO•
2
)
s
+ H+
(11)
FeII
− OH
(
HO•
2
)
s
→ ≡ FeII
− OH + HO•
2 + H+
(12)
FeII
− OH + H2O2→ ≡ FeIII
− OH + •
OH + OH−
(13)
On the other hand, Mn oxides can also involve the regularity like
Haber-Weiss mechanism [75,76] to produce active radicals such as O2
•−
and •OH according to Eqs. (14)–(16):
≡ MnIV
+ H2O2→ ≡ MnIII
+ HO•
2 + H+
(14)
HO2→ ≡ H+
+ O•−
2 (15)
≡ MnIII
+ H2O2→ ≡ MnIV
+ •
OH + OH−
(16)
Some reports in the literature have shown a decrease in the 2e−
ORR
as a consequence of the metal support on carbonaceous materials [9]. To
avoid this problem, encapsulation of metals in carbon-based materials
can be a good strategy, so that the reduction activity will remain intact.
Enhanced H2O2 yield, removal rate constant, and extremely low iron
leaching were observed by Su et al. [77] when iron was encapsulated in
the interior structure of CNT, as shown in Fig. 6(a). After 120 min of
Fig. 5. (a) Schematic illustration of synthesis steps for NF@ Fe3O4@SiO2@MgAl-LDH catalysts. Reproduced from Ref. [65]. (b) Effect of pH on contaminant
degradation and reaction mechanism for pyridinic N in EF system catalyzed by Cu-doped on g-C3N4 substrate. Reproduced from Ref. [67], and (c) Schematic
illustration of synthesis steps for SAFe@HSC catalysts, Reproduced from Ref. [70].
Z. Heidari et al.

8
Fig. 6. Schematic diagram in electrocatalytic process for: (a) Fe0
-in/out-CNTs cathode. Reproduced from Ref. [77], (b) Fe@BN-C cathode. Reproduced from
Ref. [78], and (c) FeAl/SUS cathode. Reproduced from Ref. [80].
Z. Heidari et al.

9
process at neutral conditions, the H2O2 concentration reached 61.20
mg/L, while only 0.078 mg/L iron was found in the medium. Fe0
was
confined in the interior cavities of CNTs instead of iron oxide, which
promoted the catalytic activity of encapsulated iron compared to
external loading of iron on the CNTs walls. Recently, the same research
group showed that by encapsulating iron in the B, N-codoped CNT
structure, H2O2 with high selectively (94–98%) can be converted to
•
OOH/O2
•−
, and then through the reaction with •
OH, dominant 1
O2 can
be generated [78]. The confined nanosized Fe0
particles significantly
enhanced the removal of sulfamethazine (10 mg/L) over a wide pH
range (3.0–11.0) with low electrical energy consumption through the
1
O2 degradation mechanism (Fig. 6(b)). Apart from iron, other transi
tion metals (M = Cu, Co, and Ni) encapsulated in N-doped CNTs (M@N-
C) can also provide high potential as bifunctional cathodes for degra
dation of sulfamethazine in a wide pH range [79]. Surprisingly, Co@N-C
showed the highest sulfamethazine degradation rate at neutral condi
tion, followed by Fe@N-C, Ni@N-C, and Cu@N-C. At acidic condition,
Co was more conductive to activate H2O2 via atomic H*, while at a wider
pH range (up to 9.0) Fe produced atomic H* through pyridinic N, which
led to higher stability and reusability with lower leaching Fe after 10
cycles at pH 7.0. Recently, a study conducted by Choe et al. [80] showed
magnetite coated stainless steel (SUS) meshes as an effective cathode for
removal of methylene blue (10 mg/L) dye at neutral pH. Synthesized
bimetallic cathode showed higher H2O2 generation and lower Fe2+
leaching, not only in acidic pH but also in neutral conditions (Fig. 6(c)).
The complete degradation of dye at neutral pH was approximately 20%
higher than that of homogeneous EF process, which benefited from the
enhanced catalytic effect of iron oxide layer.
MOF are well known materials for synthesis of functional carbo
naceous cathodes due to large surface area, tunable functionalities, and
highly ordered structure that can provide more active sites [32]. Liu
et al. [71] reported the use of multiphase porous catalysts derived from
carbonized Fe-based MOF (CMOF) on the porous carbon monoliths
(PCM) cathode, achieving high degradation of herbicides at circum
neutral pH. The electrochemically produced H2O2 on the PCM surface,
and the generated •OH by the CMOF together with the narrow pore size
distribution of CMOF on the PCMs enhanced the oxidation of Fe(II) to Fe
(III) with negligible leaching at pH value of 7. The application of
bimetallic particles of iron and other metals instead of iron alone can
improve catalytic activity, FeII
/FeIII
recycling, and H2O2 production in
the EF process. In this regard, Fu et al. [81] prepared a bimetallic MOFs
modified graphite felt cathode (Cu0.33Fe0.67NBDC-300/GF), which led
to complete removal of sulfamethoxazole (10 mg/L) at pH 7.0. More
over, the authors confirmed the production of high-valent iron-oxo
species at near-neutral pH condition, which is in agreement with pre
vious researches [82,83]. The Ce-based MOF loaded on carbon encap
sulated Fe3O4 particles showed high H2O2 generation efficiency and
relatively low electric energy consumption (EEC) in sulfamethazine
degradation in a wide pH range (3.0–11.0) [84]. Yu et al. [85] synthe
sized a trimetallic MOF-derived catalysts (Fe, Al, and Ni on the PCM)
and achieved 96% napropamide pesticide (10 mg/L) degradation at
neutral pH, 39% higher than that of monometallic catalyst.
Layered double hydroxides (LDHs) are two-dimensional materials
containing interlayer anions and positively charged host layers.
Recently, it has been reported that transition metal LDHs by enhancing
the electron transfer rate can accelerate the catalytic activity of Fe-based
LDH in EF process [86]. For example, a hierarchical CoFe-LDH grown on
a CF cathode provided 3.5 times higher mineralization of Acid Orange II
(0.1 mM) than homogeneous system at pH 7.1 [87]. This can be due to
the enhanced electroactive surface of cathode and the synergistic effect
of Co and Fe catalyst. Yang et al. [88] investigated CF modification using
FeII
FeIII
-LDH at neutral pH, noting complete ofloxacin (0.1 mM)
mineralization in 480 min and a three-time higher degradation rate at
pH 7.0 when compared to that of homogeneous EF process at pH 3.0.
The significant performance as well as negligible amounts of iron
leaching and released iron at neutral pH could be attributed to the
occurrence of surface-catalyzed process for ofloxacin mineralization at
the solid–liquid interface which hinders the iron precipitation in the
form of Fe(OH)3. Similar observation was reported by Yao et al. [89]. It
is important to mention that working in a pH condition close to the
natural pH can change the nature of EF process to electro-coagulation,
and samples should be analyzed to confirm which process is domi
nant. Some synthesis methods, such as the production of iron based
catalysts in the form of core–shell structures and immobilization of them
on the cathode, could prevent the coagulation of iron ions.
3.2. Iron-free and metal-free cathode modifications
Transition metal materials such as copper, cerium, cobalt, and va
nadium confirmed their applicability in reaction with H2O2, performing
Fenton-like reactions, generating different ROSs, and promote the nar
row working pH values [90]. The low valence states of vanadium oxide
(VO2, V6O13, and V2O3) can be utilized to generate •
OH from H2O2
decomposition, which is beneficial for contaminant degradation.
Moreover, the rich oxygen vacancies in the VO structures can improve
the electrocatalytic efficiency and promote the OER activity [91].
Recently, an increasing attention has been paid to the copper ions due to
its higher reaction rate with H2O2 compare to Fe2+
which can adapt to a
wider pH range in Fenton-like and EF-like systems [37,92,93]. In this
regard Zou et al. [94] produced CuCo–O@CNTs/nickel foam (NF)
electrode by electrodeposition method, which led to significantly higher
sulfamethoxazole (10 mg/L) degradation and lower metal ion at neutral
and near-neutral pH condition compared to acidic pH (Fig. 7(a)). The
unique spherical structure of Cu oxides accelerated the interaction be
tween oxygen and transition metal oxides, producing H2O2, which
further reacted with transition metal redox pairs (Co3+
/Co2+
and Cu2+
/
Cu+
) to generate different ROSs (including •
OH and •
O2
–
) for contami
nant degradation.
Similar to HEF catalysts, N-doping on cathode can activate the
delocalization of electrons in the neighboring atoms. Xiao et al. [95]
confirmed that the incorporated nanocrystals of iron nitride with the
pyridinic and graphitic-N species can improve the catalytic activity of N-
doped carbon-coated Fe3N composite for production and activation
H2O2 in HEF process. The synthesized cathode showed a high stability
and catalytic activity in a wide pH range (3–9) with low iron leaching
(Fig. 7(b)). The N-doped modified CF cathode (Co9S8-xNx@CF) with high
electronegativity and strong spin polarization was synthesized by Wang
et al. [96] which provided more H2O2 production and led to higher
amitriptyline degradation, electrical conductivity, and redox capacity
compared to the pristine Co9S8. This study endowed a deeper under
standing about the mechanism of H2O2 production after N-doping. The
electronic structure analysis revealed a redistribution of electrons sur
rounding the Co atoms, causing asymmetrical occupation of the π* or
bitals of *O2. As a result, *O2 exhibited radical nature and became active
for hydrogenation, which led to a significant reduction in the absorption
energy required during the rate-determining step.
Due to the unavoidable metal leaching of metal-based catalysts,
recently different research groups studied the application of carbon-
based materials for in situ production of •OH from H2O2 in the absence
of metal catalysts due to their inherent characteristics such as good
conductivity, chemical stability, and electronic conductivity [18]. Qin
et al. [97] designed O-doped CNT as a bifunctional metal-free EF cath
ode for the removal of phenol contaminant (20 mg/L) at near-neutral pH
(6.5). The excellent performance of OCNTs (complete phenol removal in
60 min and 76.6% TOC removal in 300 min) could be attributed to the
promoted H2O2 production by catalyzing the 2e−
ORR so that a large
number of •OH was generated. The high content of sp3
-C bond can
accelerate the oxygen adsorption during the ORR and increase the H2O2
production. In the same way, the higher fraction of –C = O had a weak
binding affinity to H2O2, thus reduced the activation energy and
increased the H2O2 decomposition. In addition to O, doping other het
eroatoms such as N and S, on carbon materials has been confirmed to
Z. Heidari et al.

10
provide an outstanding bifunctional metal-free cathode with high cat
alytic activity [98]. This improvement can be attributed to the regula
tion of chemical properties and construction of active sites on the surface
or inner structure of carbonaceous materials. Su et al. [99] observed a
high H2O2 selectivity (78.02%) and production rate (8.6 mg h− 1
cm− 2
)
with N-doped graphene bifunctional cathode for phenol degradation
(25 mg/L) in neutral pH solution. Graphite N by promoting the 2e−
ORR
reaction increased H2O2 production, and pyridine N activated H2O2 to
produce more •
OH. Although the main ROS in phenol degradation was
•OH, the obtained results confirmed the contribution of •O2
–
as well.
Similar results were obtained in O doping of granular bamboo-based
biochar (GB) deposited on SS mesh cathode, which resulted in a
61.2% improvement in H2O2 production compared to GBSS cathode at
neutral pH [18]. Also, the coexisting role of radical (•OH and •O2
–
) and
non-radical (1
O2) oxidants using a N-self-doped biochar bifunctional
cathode was confirmed in degradation of tetracycline (100 mg/L) at
neutral pH [98]. The N vacancies and graphitic N, as electroactive sites
in the electrode structure, had a strong capability for oxygen adsorption
and electron trapping; therefore, these sites contributed in production of
non-radical oxidants. Moreover, graphitic N acted as an important active
site for production of H2O2 and generation of •
OH from H2O2. Yang et al.
[100] used the advantages of synergistic effects from binary
heteroatoms (sulfur and nitrogen) co-doped graphene cathode for
degradation of phenol, antibiotics, and dyes at pH 7.0. The simultaneous
generation and activation of H2O2 via (2 + 1) e−
consecutive reduction
reactions led to 10 times increase in phenol removal rate constant
compared to conventional EF.
3.3. Application of aerogels
Aerogel forms of materials compared with their traditional materials
demonstrate significant properties like larger porosity, environmental
compatibility, extremely lower density, higher specific surface area, and
electrical conductivity. Therefore, these materials were subject of great
interest in different water treatment applications [73]. The unique
properties of aerogels are due to the high portion of air in the structure
which can provide excellent mass transfer and ease of functionalization.
Recently, some researchers used iron-based aerogel composites as
cathodic material in EF system at neutral pH. Distributing the iron oxide
into the aerogel framework not only improve the contact efficiency and
a great interior reaction spaces for in situ production of H2O2 but also,
prevent the agglomeration of catalyst particles [16]. Graphene aerogel
with low density, high porosity, and corrosion resistance can be
considered an appropriate candidate for catalytic application. A novel
Fig. 7. (a) Effect of pH in sulfamethoxazole degradation and metal ions release in EF process using CuCo–O@CNTs/NF electrode. Reproduced from Ref. [94]. (b)
Experimental setup and possible degradation mechanism of rhodamine B in EF system using Fe3N@NG/NC/GF cathode. Reproduced from Ref. [95].
Z. Heidari et al.

11
nanocomposite of α-Fe2O3 wrapped in graphene aerogel (α- Fe2O3/GA)
completely removed rhodamine B dye (10 mg/L) under neutral condi
tion with low iron leaching (<2.3 mg/L) and excellent reusability and
stability after six consecutive runs [101]. Wang et al. [16] synthesized a
novel aerogel cathode from carbonized graphene and γ-FeOOH com
posite to integrate the high conductivity of graphene and the high sur
face area and uniform dispersion of the iron oxide in aerogel, which
successfully total sulfamethoxazole (0.1 mM) and 89% of TOC were
removed under neutral pH condition. According to the SEM images the
aerogel cathode showed an open porous structure that can place both
catalyst particles and pollutant into its micropores which not only can
reduce the distance between catalyst, pollutant, and ROSs but also both
Fe2+
regeneration and Fenton reaction can happen rapidly in each pore
(Fig. 8(a)). Another interesting study was the application of reduced
graphene oxide (rGO) aerogel cathode, sacrificial iron anode, and tri
polyphosphate (TPP) electrolyte leading to more than 90% of sulfame
thazine (5 mg/L) removal in 90 min of treatment at pH 7.7 [102]. The
authors concluded that the porous configuration and high conductivity
of cathode together with abundant oxygen functional groups on the rGO
aerogel could greatly accelerate the mass and electron transfer. More
over, the TPP formed a film on the iron anode surface preventing iron
sludge production and excessive corrosion of the iron anode through
phosphorization under near-neutral condition. Also, more H2O2 (about
two times) and •OH could be generated from the molecular oxygen
activation pathway provided by Fe-TPP complexes. Further discussion
on the application of chelating agents in EF process can be find in Sec
tion 4.
Vanadium is a transition metal that proved its ability in •OH
production from H2O2 decomposition and it has shown an ideal over
potential for vanadium reduction reaction close to the 2e−
oxygen
reduction overpotential [91]. Vanadium doped chitosan carbon aerogel
cathode effectively improved the ciprofloxacin degradation (17.0%) and
mineralization (41.7%) at neutral pH compared to iron doped chitosan
carbon aerogel cathode [103]. According to the DFT calculation vana
dium was more favorable in H2O2 decomposition and •OH electro
generation compared to conventional Fe3O4. The enhanced performance
of the novel EF system at neutral pH can be due to the strong porous
structure and wide pore size distribution of aerogel cathode that facili
tate the mass transport, accessibility and application of surface and in
ternal active sites. The authors also predicted the ecotoxicity of the
produced intermediates using ECOSAR software. According to these
results, some intermediates presented higher toxicity than ciprofloxacin
against organisms. However, the inhibition zone test confirmed toxicity
attenuation with increasing the treatment time.
The introduction of metal reagents into carbon aerogels can plug the
open pores, increase the mass transport resistance, and decrease the
yield of H2O2 production [73]. To overcome this, Zhao et al. [104]
synthesized an iron-copper bimetallic carbon aerogel electrode and
activated it with CO2 to improve the pores accessibility and N2 to
develop the porosity and regeneration of ultra-dispersed Fe0
with
reductive carbon. The reductive reactivity of ZVI and the presence of a
Cu promoter accelerated Fe reduction and complete degradation and a
high mineralization efficiency of methylene blue dye (50 mg/L) were
obtained over a wide pH ranges (3.0–9.0, Fig. 8(b)) which was higher
than conventional EF with carbon aerogel and Fe2+
at acidic pH. This
can be due to both the high dispersion of metallic NPs implanted in the
Fig. 8. (a) Schematic of reaction occurred on the γ-FeOOH loaded on aerogel cathode surface. Reproduced from Ref. [16]. (b) Schematic of reaction occurred on the
aerogel FeCuC cathode surface and the effect of pH on TOC removal of homogeneous EF and EF with activated aerogel electrode. Reproduced from Ref. [104].
Z. Heidari et al.

12
3D structure of carbon aerogel and gas activation treatment with N2 and
CO2. Other studies related to the application of modified cathodes in EF
systems to broaden working pH reported in the recent years are sum
marized in Table 1.
4. Application of chelating agent
The addition of chelating agents or ligands to the EF system is a good
plan to overcome the drawbacks of the classical EF process. Appropriate
iron chelating agents with at least two functional groups can form stable
complexes with iron ions and form a ring structure named chelate ring
which prevents iron precipitation and widen the narrow EF working pH
[17,122], since the stability constants of these coordination materials
are higher than those of Fe-hydroxide compounds [123]. Moreover, Fe-
complexes due to the presence of the ligand have a lower standard
reduction potential which can facilitate electron transfer from Fe2+
and
accelerate the reaction with H2O2 [124]. Different organic (such as
ethylenediamine-N, N’-disuccinic acid (EDDS), nitrilotriacetic acid, and
ethylenediaminetetraacetic acid (EDTA)) and inorganic (such as poly
phosphates) chelating agents can be utilized to expand the working pH
to neutral pH [122,123]. Varindani et al. [125] obtained 67% COD
removal of mixed industrial wastewater at near-neutral pH using 80 mg/
L of both Fe2+
and EDTA, which was comparable to the COD removal at
pH 2.5 (66%). EDTA by forming a complex with Fe ions provided a
higher rate of Fe2+
/Fe3+
cyclic reaction, which led to higher generation
of H2O2 and •OH. When EDTA did not used in the medium, Fe3+
precipitated as ferric hydroxide at neutral pH, which led to coagulation.
Application of humic acid ligand in EF with Ni doped CF (Ni-CF) cathode
showed 10 times higher ciprofloxacin degradation rate and two times
higher mineralization compared to the CF cathode at natural pH [126].
The experimental (electron spin resonance spectrometer, ESR) and
computational (density functional theory, DFT) results revealed that the
atomic H* introduced via Ni-CF system can improve Fe(II)-complex
regeneration by reducing chemisorbed Fe(III) and hence broaden the
working pH. The reduction of Fe(III)-ligand complexes and the regen
eration of Fe(II)-complexes are important steps and evaluating the
mechanism of these reactions by experimental and theoretical studies
would be of great interest. The mechanism can be determined through
the different properties of chelating agents, including the available
functional groups, spin state of the metal ions, and orbital hybridization.
Electrochemical reduction can occur in the bulk of solution or at the
cathode surface through the introduced atomic H* or direct electron
transfer, respectively [123]. Liu et al. [127] conducted a comprehensive
study on the ibuprofen degradation mechanisms using Ni-CF-EF (H*-
dominated) and CF-EF (electron-dominated) systems in the presence of
four common Fe(III)-ligand complexes (ethylenediamine-N,N’-dis
uccinic acid (EDDS), EDTA, phthalocyanine (Pc), and HA ligands) and
studied the reduction priority of electrons and atomic H* using ESR and
Raman spectroscopy analyses as well as DFT calculations. According to
the outcomes addition of EDDS and EDTA ligands to the electron-
dominated EF system lead to 8.31 and 16.23 times higher degradation
rates at neutral pH which can be attributed to the higher Fe(III)
chelating efficiency of these ligands compared to that of H2O. These two
chelating-EF systems had lower activation energy gaps and the electron
distribution was deposited on the Fe atom, which led to more rapid
electron reduction. On the other hand, in the atomic H*-dominated EF
system the addition of HA ligand lead to 10 times higher degradation
rate than the electron-dominated EF system, which can be attributed to
the higher chelation ability and lower energy barrier of [Fe(III)-HA]3+
(Fig. 9(a)). Among the studied ligands, addition of Pc to the EF system
had no significant effect on the ibuprofen removal which could be
related to its lower ability to make a complex with Fe(III) compared to
H2O under neutral pH condition. Zhang et al. [128] also used EDTA in a
UV assisted flow-through EF system for rhodamine B (0.08 mM)
abatement leading to 90% degradation at pH 7.0. According to the
radical scavenging experiments •OH played a dominant role in
contaminant removal, whereas UV photolysis of the Fe(III)-EDTA com
plex led to EDTA degradation.
Recently, tetrapolyphosphate, a derivative of polyphosphate, was
introduced as a promising inorganic chelating agent capable of •
OH
production without ROSs scavenging at different pH values [124]. Deng
and coworkers used a more stable and cheaper form of this ligand,
namely tripolyphosphate (TPP), in EF degradation of phenol
[124,129,130]. A novel Ni-Fe-Foam composite cathode with TPP sup
porting electrolyte, instead of conventional Na2SO4 electrolyte, were
designed to hinder iron sludge production at near-neutral pH conditions,
which effectively removed total phenol (1.4 mM) in 40 min and 75%
TOC in 4 h, with 18 times higher rate constant than EF system using
sulfate electrolyte at pH 3.0 [124]. In another work of this group, Ni-Fe-
Foam cathode was replaced with a Ni-Foam cathode with lower cost,
higher lifetime, and controlled Fe2+
release [130]. Under the optimal
operating condition calculated by RSM, total phenol degradation was
attained in 25 min at pH 5.8, with a 3.2-fold higher rate constant than
that of EF using sulfate electrolyte at pH 3. The plausible cathode and
bulk Fe(III) reduction mechanisms are shown in Fig. 9(b). By replacing
Fe-Foam catalyst in the EF system 8.55-fold improvement in the phenol
degradation rate, more than 59% increment in mineralization (in 8 h),
and lower energy consumption (0.10 kWhg− 1
vs. 0.98 kWhg− 1
) were
obtained compared to conventional EF process (Fe2+
/k2SO4) at cir
cumneutral pH [129]. 8.55-fold improvement in degradation rate, more
than 59% increment in mineralization yield, and lower energy con
sumption (0.10 kWhg− 1
vs. 0.98 kWhg− 1
) were observed compared to
conventional EF process (Fe2+
/k2SO4) [131]. The application of
chelating agents make concerns about secondary pollution to the envi
ronment. Some researchers introduced nitrilotriacetic acid (NTA),
which is a biodegradable agent and the produced complex with iron
could be degraded very fast by photodegradation. Zhang et al. [29]
obtained complete degradation of phenol (10 mg/L) at pH 7.0 in a flow-
through EF system using NTA. The obtained results confirmed the lower
redox potential of Fe(III)-NTA complexes, facilitating the Fe(II) oxida
tion by H2O2 leading to a 40% improvement in rate constant compared
to that in the absence of NTA.
It is worth mentioning that the generated •
OH in the system can
attack chelating agents, decreasing their capacity. For example, it has
been shown that chelating agents like tetrasodium-iminodisuccinate
completely inhibited •OH performance, however, EDTA had no effect
on •
OH formation [132]. Therefore, the performance of chelate-
modified EF processes and the reactivity of chelating agents with •OH
should be carefully considered.
5. Combination of EF processes with other treatment methods
In recent years, the utilization of hybrid EF processes has gained
significant attention, which combine the advantages of two or more
conventional treatment methods such as adsorption, biological treat
ment, and filtration resulting in the degradation of organic pollutants
and inorganic contaminants. At neutral pH, the use of an integrated EF
process can mitigate the limitations of the traditional EF process, such as
its low stability and low reaction rate. The idea of integrating the great
EF oxidation power and low cost advantages of biological treatment
systems treatment (BEF system) might be an effective alternative to
overcome the EF shortcomings, such as narrow pH range, high energy
consumption, and improved removal efficiency [133]. Hybridization of
EF and microbial fuel cell (BEF-MFC) in a single setup can remove the
requirements of external chemicals for Fenton oxidation, thereby
reducing the operational costs. The BEF-MFC system comprises a
divided cell in which microorganisms in the anodic chamber produce
electrons via microbial metabolism of a substrate under anaerobic
conditions. The bio-generated electrons can be conveyed to the cathode
chamber, where used by different terminal electron acceptors, including
oxygen. In the cathodic compartment application of iron-based cathode
or external addition of iron can electrogenerate the Fenton’s reagent and
Z. Heidari et al.

13
Table 1
Application of cathode modifications reported in the recent years to broaden working pH in EF systems.
Cathode/
Anode
Catalyst Pollutant pH Operational condition Efficiency (removal) Ref
CuFe nanoLDH-CNTs on
graphite/
Pt sheet
CuFeNLDH-CNTs
nanocomposite by
hydrothermal method
Pharmaceutical: cefazolin 3.0–12.0 pH = 6.0
[pollutant]0 = 20 mg/L
[Na2SO4] = 50 mM
I = 300 mA
AFR = 10 L/h
100 min: 95.8% CFZ (pH
3.0)90.9% CFZ
(pH 6.0)
[105]
CoFe@NC-CNTs-CNTs-NF /
Pt sheet
N-doped carbon CoFe alloy
anchored on
CNTs
Herbicides: Atrazine 3.0–9.0 pH = 5.9
[Na2SO4] = 50 mM
J = 4.5 mA/cm2
AFR = 0.6 L/min
105 min: 100% ATZ
420 min: 80% TOC
ECTOC: 2.88 kWh g− 1
[106]
FeOCl/carbon cloth/AC
fiber/
Ti/RuO2 mesh
FeOCl Pharmaceutical:
trimethoprim
3.0–9.0 pH = 6.8
[Na2SO4] = 12.5 mM
J = 9 mA/cm2
Flux rate = 210 mL/min
60 min: 100% TMP
180 min: 62.6% TOC
the average current
efficiency = 69.9%
EC = 0.28 kWh kg− 1
vs. 2.96
kWh kg− 1
in traditional EF
[107]
Cu–B–F-modified graphite/
BDD
Cu–B–F Pharmaceutical:
propranolol, atenolol
3.0, 7.0, &
8.0
pH = 7.0
[pollutant]0 = 200 ng
L− 1
[Na2SO4] = 0.02 M
I = 100 mA
10 min: 100% degradation [108]
Cit-Fe–ACFs/
Ti–RuO2
Ferric citrate Pharmaceutical: ibuprofen 3.0 & 6.8 pH = 6.8
[Na2SO4] = 0.05 mM
J = 5 mA/cm2
AFR = 100 mL/min
120 min: 97% IBU
45% TOC
MCE = 46% (2 h)
EEO = 1.46 kWh log− 1
m− 3
[109]
Fe3O4-NP@CNF/
Pt
Fe3O4-NP Pharmaceutical:
carbamazepine
4.0, 7.0, &
10.0
pH = 7.0
[Na2SO4] = 0.1 M
40 min: 100% CBZ
180 min: 100% TOC
[110]
N-doped 3D CF FeOOH Organic compound:
Bisphenol A Herbicide:
Atrazine
6.0–9.0 pH = 7.0
[FeOOH] = 0.5 g/L
[Na2SO4] = 0.1 M
AFR = 300 L/min
150 min: 97% Bis A
91% ATZ
300 min: 66% TOC
current efficiency = 70% in
15 min
[111]
Fe3O4–CaO2 cathode
promoted by PMS/
Graphite
electrode
Fe3O4–CaO2 composite by co-
precipitation
Pharmaceutical:
levofloxacin
3.0–9.0 pH = 5.6
[PMS] = 5 mM
[Na2SO4] = 0.1 M
J = 10 mA/cm2
60 min: 92.1% LVO
74.5% TOC
EE/O: 0.76 kWh m− 3
order− 1
[112]
AC-SS composite/
Ti-mixed metal oxide
mesh
Iron-free Dye:
reactive blue 19
2.0–12.0 pH = 7.0
[pollutant]0 = 100 mg/
L
[Na2SO4] = 50 mM
90 min: 61.5% RB19
720 min: 74.3% TOC
[113]
3D Co/CoOx-N-CF
nanoparticle/
Pt mesh
with/without FeSO4⋅7H2O as
homogeneous EF catalyst
Dye: acid orange 7 3.0 & 6.0 pH = 6.0
[Iron] = 0.2 mM
[pollutant]0 = 20 mM
[Na2SO4] = 0.05 M
J = 10 mA/cm2
40 min: 93.3% TOC (pH 6.0)
92.4% TOC
(pH 3.0)
[114]
Iron–anchored graphite
cloth/
SS plate
Loaded FeSO4 Dye: methyl orange 3.0 – 11.0 pH = 7.0
[Na2SO4] = 50 mM
J = 100 mA/m2
AFR = 5 L/min
60 min: 98% degradation
Max produced H2O2:
28.67 mg/L (pH 3.0)
19.88 mg/L (pH 7.0)
[115]
N-doped
CNTs and CNT-COOFe2+
membrane cathode/Pt
CNT-COOFe2+
as the catalyst
layer on the cathode electrode
Organic compound:
p-nitrophenol
3.0, 5.0, &
7.0
pH = 7.0
[Iron] = 3.33 mM
[Na2SO4] = 0.1 M
120 min: 96.04% pollutant
180 min: 80.26% TOC
MCE: 13.13%
CE: 0.26 kWh/g TOC
[116]
graphene/Fe3O4/
Pt
Fe3O4 Organic compound:
bisphenol A
3.0 – 8.5 pH = 7.0
[Na2SO4] = 0.05 M
AFR = 1 L/min
40% TOC
[117]
Fc-ErGO on GF/
Pt
Fc-ErGO Pharmaceutical:
ciprofloxacin
3.0, 7.0, &
9.0
pH = 7.0
[Na2SO4] = 0.05 M
rate constant: 0.222 min− 1
•
OH production: 247 mM
[118]
Indium tin oxide/
Pt
Structural Fe(III) in single sheet
iron oxide
Dye:
orange II
7.0 – 10.0 pH = 7.0
[Iron] = 0.03 mM
[pollutant]0 = 0.23 mM
[Na2SO4] = 0.05 M
250 min: 92% dye
rate constant: 0.0048 min− 1
[119]
FeS2 on CF/
DSA
Synthesized pyrite (FeS2) Pharmaceutical:
carbamazepine
7.0 [Iron] = 1.875 mg/cm2
[pollutant]0 = 0.02 mM
[Na2SO4] = 0.05 M
AFR = 100 L/min
240 min: 26.94% TOC
[120]
(continued on next page)
Z. Heidari et al.

14
finally produce •
OH to oxidize recalcitrant contaminant [134]. Sathe
et al. [135] observed more than 85% degradation of three different
surfactants (20 mg/L) in a BEF-MFC system at neutral pH with an iron
catalyst from spent toner cartridge ink. In addition, treatment of real
wastewater confirmed high TOC removal efficiency of BEF-MFC system
even after 11 cycles. A novel composite of iron and activated carbon on a
titanium mesh (Fe/AC/Ti) was used as an air cathode in EF process in
combination with anodic catalytic oxidation to treat the nanofiltration
concentrate of landfill leachate in an undivided chamber [136]. More
than 97% COD removal at 280 min and improvement in BOD5/COD
ratio and color removal were achieved at neutral pH. In spite of the
synergistic effect of cathodic EF and anodic electrocatalysis the treated
effluent was at weakly alkaline or neutral pH, highlighting the reduction
in operational cost and environmentally friendly manner of the
combined system.
Coupling of cation exchange resin and EF was also explored in a
novel reactor arrangement, evidencing high efficiency and low cost
wastewater treatment technology under neutral condition [137]. The
designed setup consisted of three series of compartments including two
packed chambers with Fe-loaded resin and a polarized AC column
placed between them for Fenton reaction. In the first chamber the
contaminant molecules (Orange II dye) were adsorbed by the AC packed
bed, then in the middle chamber polarization of AC was applied to
produce the Fenton reagent. The obtained results suggested a combi
nation of reasons for the polarization effect including electro-oxidation
and electro-sorption as well as EF discoloration which enhances the in
situ electrochemically generated H2O2 and •
OH. A dual-compartment
EF-membrane reactor was successfully explored for the treatment of
Table 1 (continued)
Cathode/
Anode
Catalyst Pollutant pH Operational condition Efficiency (removal) Ref
Boron-doped graphene
aerogel (BGA)
modified GDE/
Pt
BGA
(metal free process)
Organic compound:
bisphenol A
3.0 – 9.0 pH = 7.0
BGA doping = 2.4%
[Na2SO4] = 50 mM
60 min: 95.5% degradation [121]
AFR: Air flow rate; TOC: Total organic carbon, COD: Chemical oxygen demand; BOD: Biochemical oxygen demand, AFC: Activated carbon fibers; NP: Nanoparticles;
Fc-ErGO: Ferrocene functionalized electrochemically reduced graphene oxide, GDE: Gas diffusion electrode.
Fig. 9. Possible cathode and bulk reduction mechanism of Fe(III) in EF system at neutral pH using (a) EDTA. Reproduced from Ref. [127], and (b) tripolyphosphate.
Reproduced from Ref. [130].
Z. Heidari et al.

15
methyl orange dye (50 mgL− 1
) at pH 7.0 [19]. A novel magnetite/multi-
walled carbon nanotubes (Fe3O4/MWCNTs) composite on gas diffusion
electrode (GDE) was coupled with Al2O3 ceramic membrane flow
reactor to enhance the mass transfer, oxygen utilization, and H2O2
electrogeneration. The high-speed charge channel of MWCNTs and
accumulation of H2O2 by ceramic membrane improved contaminant
degradation by more than 23% compared to that of homogeneous EF
system.
The application of carbonaceous adsorbents in EF processes are
interesting due to their high H2 evolution overpotential and low H2O2
reduction electrocatalytic activity which can act as both contaminant
adsorbent and H2O2 electro production. García-Espinoza et al. [138]
designed a novel three phase EF reactor for simultaneous application of
electrochemical reactions and adsorption under neutral pH for removal
of Orange II dye (10 mgL− 1
). In the absence of AC adsorbent, dye
degradation was slightly lower (10% in 90 min) in real effluent than in
the synthetic dye solution, which can be attributed to the presence of
•
OH scavengers in the real effluent. In the presence of AC, a fast dye
removal was observed in the 10 min of treatment process because of the
increased electrical conductivity between the polarized surface of elec
trode and the AC particles. In another study, the synergetic advantages
of the adsorption process and heterogeneous EF process without any
oxidants addition, was confirmed in continuous operation at neutral pH
in pilot-scale [139]. The novel EF ‘filter’- type system consisted of three
cathode/anode pairs and cathodes were impregnated with γ-Fe2O3/F3O4
NPs leading to more than 80% diclofenac (1 mgL− 1
) and 36 %TOC
removal.
It is well known that photo (UV or solar) irradiation can increase the
Fe(II) regeneration rate by the accelerating the Fe(III)/Fe(II) cycle,
which enhances •OH generation. Moreover, irradiation is helpful to
improve the H2O2 decomposition and production of •
OH. The combi
nation of photo oxidation process and EF process is known as photo-
assisted EF (PEF) process, which lead to faster and higher removal of
contaminant [140,141]. However, a major challenge in PEF process is
scaling up to the industrial level because UV irradiation is energy-
intensive and the cost of UV lamps is high. On the other hand, visible
light-assisted EF (vis-EF) or solar-PEF (SPEF) process uses free sunlight
to reduce the costs of UV irradiation, which is economically feasible and
highly desired. In this regard, Martínez-Pachón et al. [142] compared
valsartan removal by EF and PEF processes using CF cathode and Ti/IrO2
doped with SnO2 as anode at near neutral pH (5.0) and NaCl as sup
porting electrolyte. A fluorescent black-light blue tube of max 368 nm
with input photoionization energy of 1.4 W/m2
was used as light source
(UVA) for irradiation of solution. Complete valsartan removal was ob
tained after 2 h of PEF process at natural pH in presence of oxalic acid,
while under the same conditions EF process just removed 55% of val
sartan. This can be related to the photolysis of iron complexes by light,
providing more ROSs at near neutral pH in the PEF system. Different
routes have been proposed for contaminant degradation, including •
OH
produced from photo-Fenton and Fenton reactions, and H2O2 generated
at the cathode. Moreover, chlorine species produced from NaCl elec
trolyte led to production of various chlorinated oxidizing species, such
as Cl2 and HOCl, at the anode. Similarly, the synergistic effect between
photolysis and EF process was confirmed by the effective removal of
losartan and valsartan in PEF process [143]. The breakdown of iron
complexes formed with organic acids under irradiation, H2O2 genera
tion at the cathode (GDE), and anodic oxidation at the anode (DSA)
provided high mineralization and degradation of contaminants at near-
neutral pH. The same research group studied the decontamination of
real wastewater treatment plant effluent containing 16 different phar
maceutical contaminants in EF, PEF, and PEF in the presence of added
oxalic acid (PEFox) [144]. At near-neutral pH values PEFox, as the
tertiary treatment process, showed the highest degradation of contam
inants, which can be related to the formation of Fe(III)-oxalate com
plexes and increased availability of soluble iron ions. As stated by the
authors, the addition of oxalic acid to the PEF system not only reduced
the energy consumption but also lowered the environmental hazard of
the wastewater by more than ten times. A study conducted by Manrique-
Losada et al. [145] indicated effective removal of four representative
pharmaceuticals in SPEF process at near neutral pH (7.7), using a nat
ural SPEF enhancer namely autochthonous Amazonian fruit copoazu
(COPE). Actual sunlight was used as light source of the two SPEF sys
tems, including BDD anode in Na2SO4 electrolyte and IrO2 anode with
NaCl electrolyte. The results showed the superior performance of IrO2-
NaCl system (almost complete degradation) compared to BDD- Na2SO4
system (49% degradation), which could be due important role of elec
trogenerated reactive chlorine species in decontamination of waste
water. Also, COPE favored the iron catalytic cycle in the system and
improved the ferrous ions regeneration because it served as a natural
complexing agent. Chen et al. [146] stated that the use of CuxO nano
arrays on the surface of copper mesh anode (CuxO@Cu) showed excel
lent methylene blue degradation rate and H2O2 utilization efficiency at
pH of 7.0 under visible light. The comparison of PEF, photo-Fenton (PF),
and EF processes confirmed the synergistic effect of PF and photo-
electro-catalysis under simulated sunlight irradiation led to higher
methylene blue removal. According to their founding, CuxO was excited
under visible light and photogenerated electron-hole pairs, which can
produce a large number of •OH or other free radicals from the reaction
with H2O2. A comparative study was conducted by Fenton, EF, and PEF
processes (UVC lamp, 32 W) to evaluate their performance in treatment
of real desalination unit effluent at different pH values (3.0, 5.0, and 7.0)
[147]. The obtained results indicated that compared to other processes
PEF process showed higher integrated efficiency, leading to 98.72%
dissolved iron removal in 90 min at neutral pH. Under the same
experimental conditions, the Fenton, EF, and PF processes yielded
89.04%, 97.39%, and 95.04%, respectively.
Recently, special attention has been paid to the application of 3D
electrochemical systems to improve the efficiency of EF process. 3D
electrodes overcome the mass transfer problems and improves the
conductivity by decreasing the distance between electrodes and
contaminant. However, limitations such as a narrow working pH, iron
sludge production, and low current efficiency still exist under neutral
pH. Some researchers have successfully applied novel catalytic particle
electrodes at near-neutral condition. The details of these studies,
together with other attempts to widen the EF working pH over the recent
years, are summarized in Table 2.
6. Economic aspects
The application of EF process at neutral or near-neutral pH can be a
promising technology for treatment of wastewater containing organic
contaminants, which eliminates the need for pH adjustment and reduces
the energy consumption and costs associated with the treatment process.
However, working in a neutral pH environment requires a higher con
centration of catalyst due to leaching of catalysts. Therefore, calculating
the energy consumption is a beneficial approach to evaluate the EF
practical application at near neutral pH from the economic point of
view. Generally different parameters involved the operational cost of
wastewater treatment using EF process, including sludge dewatering
and discarding, labor cost, system maintenance, costs of electrodes,
chemicals, and energy consumption. However, the highest portion of
operating costs is related to the electrical energy consumption ($
kWh− 1
). Energy consumption can be shown with respect to the energy
consumed per TOC or COD elimination (kWh per TOC or COD removal)
or amount of treated wastewater (kWh per m3
removal). Mineralization
current efficiency (MCE) and energy consumption (EC) are two impor
tant parameters for evaluating the performance and cost effectiveness of
EF process. Yang et al. [88] performed a comparative study on MCE and
EC for ofloxacin removal (0.1 mM) during the EF process using FeII
FeIII
-
LDH modified CF cathode at neutral pH and homogeneous EF using raw
carbon felt cathode at acidic pH. The obtained results indicated that the
MCE and EC evolution of novel EF system were superior to those of the
Z. Heidari et al.

16
Table 2
Different strategies reported in the recent years to broaden working pH in EF systems.
Strategy Cathode/
Anode
Catalyst Pollutant pH Operational
condition
Efficiency (removal) Ref
Novel Fe-based HEF Graphite electrode
plates
Co nanowires and NP doped
on CoAl-LDHs@Fe3O4
Organic
phosphonates:
HEDP and PMG
3.0–9.0 pH = 7.0
[Iron] = 1.0 g/L
[pollutant]0 = 100
mg/L
[Na2SO4] = 0.05 M
J = 0.3 A/cm2
AFR = 150 mL/
min
120 min: 71% HEDP
42% CODHEDP
95.3% PMG
37% CODPMG
[148]
Novel Fe-based HEF CF/
Pt
Rhombohedral FeCO3 Organic compound:
p-benzoquinone
3.0 – 9.0 pH = 5.6
[Iron] = 0.75 mM
[pollutant]0 = 0.5
mM
[Na2SO4] = 50 mM
550 min: 95% TOC [149]
Novel Fe-based HEF Graphite plate/ Pt
sheet
Cu–Fe–NLDH Pharmaceutical:
gentamicin
3.0–12.0 pH = 6.0 (natural
pH)
[Iron] = 1.25 g/L
[pollutant]0 = 20
mg/L
[Na2SO4] = 0.05 M
I = 400 A
AFR = 10 L/min
100 min: 91.3% GTM
300 min: 77% COD
[150]
Cathode modification
and application of
sulfite
CF modified with
CB/
DSA
Homogeneous Fe3+
(Fe2(SO4)3)
Pharmaceutical:
carbamazepine
3.0–9.0 pH = 7.0
[Fe3+
] = [sulfite]
= 0.2 mM
[pollutant] = 10
mg/L
[Na2SO3] = 1 mM
60 min: 93.9% CBZ
32.6% TOC
ECTOC: Fe3+
-EF/sulfite
=
0.081 kWh g− 1
TOC
Fe3+
-EF =
1.08 kWh g− 1
TOC
[151]
New reactor design:
Tubular U-shaped
GDE
GDE/
Anode sleeve
coating with IrO2
Homogenous Iron Pharmaceutical:
ibuprofen
3.0–11.0 pH = 7.0
[Fe3+
] = 0.7 mM
[pollutant]0 = 20
mg/L
[Na2SO4] = 0.05 M
J = 10 mA/cm2
AFR = 0.6 L/min
2 h: >92% IBO
50% TOC
H2O2 = 769.82 mg/L
EEC = 370 kWh kg− 1
[152]
New reactor design:
Floating sandwich EF
system
Natural air
diffusion electrode
and AC interlayer/
DSA
Fe2(SO4)3 Pharmaceutical:
carbamazepine
3.0–9.0 pH = 7.0
[pollutant]0 = 20
mg/L
[Fe3+
] = 8 mg/L
[Na2SO4] = 10.0
mM
I = 30 mA
60 min: 95% CBZ
48% TOC
[153]
New reactor design:
droplet
flow-assisted EF
Cu–B–F-modified
graphite/BDD
Cu–B–F Pharmaceutical:
propranolol, atenolol
3.0, 7.0,
& 8.0
pH = 7.0
[pollutant]0 = 200
ng/L
[Na2SO4] = 0.02 M
I = 100 mA
10 min: 100%
degradation
[108]
New reactor design:
rotating-disc
electrode
Fc-ErGO on CF/
plain carbon felt
disc
Fc-ErGO Pharmaceutical:
ciprofloxacin and
carbamazepine
3.0, 7.0,
& 9.0
pH = 7.0
[pollutant]0 = 10
mg/L
[Na2SO4] = 0.05 M
30 min: 100%
degradation
60 min: 100% TOC
[154]
Dual cathode without
aeration
natural air
diffusion electrode
and modified CF/
DSA
Fe2(SO4)3 Pharmaceutical:
bronopol
3.0––9.0 pH = 7.0
[Fe2(SO4)3] =
0.25 mM
[pollutant]0 = 100
mg/L
[Na2SO4] = 50 mM
current = 200 &
30 mA
60 min: 93%
degradation
72% TOC
[155]
EF without O2 or H2O2
addition in the
presence of Na2MoO4
Graphite rod/
Annular iron plate
– Pharmaceutical:
carbamazepine
3.0–11.0 pH = 6.3
[pollutant]0 = 5
mg/L
[Na2SO4] = 1 M
[Na2MoO4] = 1 M
J = 0.435 mA/cm2
AFR = 0.6 L/min
60 min: 100% CBZ
kobs = 0.0417 min− 1
[iron]tot = 4.93 mg/L
current efficiency =
76.4% in 120 min
[156]
3D EF CPE Graphite plate bentonite-based various iron
species: Fe3O4-Bent, Fe2O3-
Bent and Fe0
-Bent
Organic compound:
phenol
6.24 [Fe-Bent] = 2 g
[pollutant]0 = 100
mg/L
J = 125 mA/cm2
180 min (max by Fe0
-
Bent):
92.48% phenol
67.48% COD
Fe2O3-Bent: 51%
phenol
[157]
(continued on next page)
Z. Heidari et al.

17
conventional EF system, which confirmed the economic characteristics
of prepared electrode. Zhou et al. [163] introduced a novel cost-effective
and high-efficiency EF system to reduce the electrical energy con
sumption without aeration and pH adjustment, using commercial mo
lybdenum powder as co-catalyst in a cathode prepared by granular AC
wrapped with stainless steel mesh. The electrical energy consumption
calculations proved the new intermittent system as an energy saving
system (9.39 kWh/kg dye) at near-neutral pH (6.8) compared to classic
EF (24.41 kWh/kg dye), electrochemical oxidation (16.97 kWh/kg dye),
and electrocoagulation (27.02 kWh/kg dye). The performance of iron
free HEF system using vanadium oxide deposited on carbon paper anode
(V3O7@CP) and cathode (VOx@CP) under neutral pH condition was
compared with that of different iron-based EF systems previously re
ported in the literature [91]. The energy consumption of these processes
showed the priority of VO-based EF system as low energy consumption
and H2O2 residual even in high contaminant concentration (30 mg/L).
Varindani et al. [125] compared the energy consumption based on COD
removal for the chelate-modified EF (using EDTA) and EF processes in
industrial wastewater treatment. Similar energy consumption was
observed in the chelate-modified EF at near-neutral pH and the EF
process at pH 2.5. Moreover, the MCE according the changes in the TOC
concentrations were compared for these processes at optimum condi
tions. Although EF process showed a higher MCE (32.5%) in the first
stages of treatment process compared to chelate-modified EF (17%), for
both processes the MCE remained almost the same when the process
progressed (5% and 6%, respectively, at 60 min of treatment). In a study
conducted by Song et al. [151], the energy consumption per TOC
removal (ETOC), MCE, and EEC were compared for EF process coupled
with sulfite (using CF modified with CB) and EF process in carbamaze
pine removal. The lowest EEC of 34.63 kWh kg− 1
was obtained for EF/
sulfite process at 25 mA and pH = 7.0. Also, the results showed signif
icantly higher amounts of MCE and lower amounts of ETOC in EF/sulfite
process at different contaminant concentrations. For example, at an
initial carbamazepine concentration of 10 mgL− 1
, 0.081 kWh kg− 1
TOC
was calculated for EF/sulfite process, while this amount was 1.08 kWh
kg− 1
TOC in EF process. The research conducted by Su et al. [84] indi
cated that at neutral pH, the EEC for sulfamethazine removal was
significantly reduced by the incorporation of Ce-based MOF on carbon
encapsulated Fe3O4 particles as cathode (21.52 before and 7.65 kWh
kg− 1
sulfamethazine after Ce-based MOF deposition). By application of
the novel bifunctional cathode, both CE and EEC for H2O2 generation
were significantly exceeded the values reported in previous studies
(3.52 kWh kg− 1
H2O2 and 80.7%, respectively). Also, in the potential
range of − 0.15 V to 0.45 V, the EEC for sulfamethazine removal was
between 24.23 and 3.13 kWh kg− 1
SMT, whereas EEC values of 16–4 ×
106 kWh kg− 1
pollutants have been reported for contaminant removal
Table 2 (continued)
Strategy Cathode/
Anode
Catalyst Pollutant pH Operational
condition
Efficiency (removal) Ref
Fe3O4-Bent: 25%
phenol
3D EF Granular AC as
CPE
Graphite bipolar
plates
Ferrous sulfate powder Stabilizer:
Ni-EDTA
3.0–9.0 pH = 7.0
[Iron] = 1 mM
[pollutant]0 = 50
mg/L
[Na2SO4] = 0.047
M
[Granular AC] = 4
g/L
J = 10 mA/cm2
120 min: 84.89% Ni-
EDTA
[158]
3D EF CPE GF/
Ti–PbO2
Iron-coated nickel foam
particles
Pharmaceutical:
ibuprofen
naproxen
3.0–9.0 pH = 6.3
[Iron] = 6 g/L
[pollutant]0 = 10
mg/L
J = 15 mA/cm2
38 min: 100%
degradation
100 min: 83.2% COD
60.1% TOC
40 min:
EC = 77.9 kWh kg
COD− 1
[159]
3D EF CPE AC fiber/BDD Waste rice straw and coal fly
ash composite as CPE
Actual azo dyes
containing
wastewater
2.0–8.0 pH = 6.5
COD0 = 240 mg/L
[CPE] = 2 g/L
J = 10 mA/cm2
AFR = 5 L/min
240 min: 90.5%
degradation
120 min: 73.5% COD
BOD5/COD = 0.46%
[160]
Using chelating
agent
(TPP)
CF brush/
DSA mesh, Ti-IrO2
RuO2 or BDD
Ferrous sulfate powder A mixture of
pharmaceutical
pollutants
6 [Iron] = 3.33 mM
[Na5TPP] = 0.25
M or [K2SO4] =
0.05 M
TPP = 10 mM
J = 4.67 mA/cm2
AFR = 0.2 L/min
BDD anode had the
highest mineralization
efficiency.
180 min: >90% TOC
[161]
Using chelating
agent
(citrate)
Citrate-Fe–ACFs/
Ti–RuO2
Ferric citrate Pharmaceutical:
ibuprofen
3.0 & 6.8 pH = 6.8
[pollutant]0 = 10
mg/L
[Na2SO4] = 0.05
mM
J = 5 mA/cm2
AFR = 100 mL/
min
120 min: 97% IBU
45% TOC
MCE = 46% (2 h)
EEO = 1.46 kWh log− 1
m− 3
[109]
Graphite/
Aluminum
Ferrous sulfate powder Pharmaceutical:
amoxicillin
3.0–11.0 pH = 7.6
[Iron] = 0.1 mM
[pollutant]0 = 100
mg/L
[Na2SO4] = 0.02 M
J = 5.5 mA/cm2
120 min: 95% AMX
EEC: 155 kWh kg− 1
[162]
CPE: catalytic particle electrode.
Z. Heidari et al.

18
using a bifunctional cathode in the literature. Interestingly, when the
potential was less than 0.05 V EEC of sulfamethazine removal was about
7.65 kWh kg− 1
at neutral pH. The authors corresponded this to the
multilayered reaction centers and interface confinement that meaning
fully enhanced the simultaneous production and degradation of H2O2.
Yu et al. [164] calculated the current efficiency and EEC for H2O2 pro
duction at different pH values in the HEF process using nitrogen-doped
porous carbon (NPC) and carbon black (CB) cathode and FeS2 catalysts.
The highest CE value of 45.49% was obtained at neutral pH, followed by
42.05%, 40.39%, and 30.99% at pH values of 5.0, 3.0, and 9.0,
respectively. The lowest amount of EEC was also found at pH 7.0, which
was about 47.82 kWh kg− 1
. The absence of H+
and the presence of •
HO2
in the alkaline solution decomposed H2O2, while in acidic pH values
large amount of H+
led to H2O2 decomposition. Therefore, the highest
production of H2O2 was achieved at pH 7 in this work. Roales et al. [31]
reported an economical treatment of diclofenac using novel iron
chitosan-epichlorohydrin catalysts in HEF process at near-neutral pH.
The calculations showed, 428.89 kWh kg− 1
diclofenac, and 44.14 €kg− 1
diclofenac
economical cost were obtained at optimized condition. It is important to
noted that some indirect costs, including aeration (2.19 kWh) and gas
bubbling (0.22 € h− 1
) should also be considered in energy consumption
calculations. The obtained EC of 6.331 kWh m− 3
was more than five-fold
lower than that reported previously for treatment of diclofenac.
Recently, Conde et al. [165] comprehensively compared the opera
tional and chemical costs of different Fenton-based systems under the
same operating condition through the Life Cycle Inventory of the 100 L
sequential batch reactor. In the classic EF process the acidification
(usually using H2SO4) and neutralization of medium (usually using
NaOH) are unavoidable steps. Therefore, the chemical costs in EF pro
cess at neutral pH will be much lower than EF process at acidic pH. The
cost of chemicals for EF at neutral pH was about 0.22 € m− 3
for
providing Na2SO4 electrolyte, which was much lower than the cost of
chemicals for EF at acidic pH including 1.07 € m− 3
for providing H2SO4
(0.17 € m− 3
), NaOH (0.68 € m− 3
), and Na2SO4 (0.22 € m− 3
). The
additional details are provided in Table 3. Considering Eurostat bian
nual in 2021 the electricity price for non-household consumers was
about 0.1053 € kWh− 1
for the European Union. According to the pre
sented data, the conventional EF at acidic pH and photo-Fenton at
neutral pH had the highest electrical costs which was mainly due to
stirring, while both EF and photo-EF processes at neutral pH were the
most cost-effective processes. As the authors concluded, the slow
degradation kinetics in the photo-Fenton process at neutral pH lead to
higher requirement of stirring and illumination resulting in high con
sumption of electricity.
Comparing the cost and energy consumption of EF process at acidic
and neutral condition can be very complex and depends on the type and
concentration of contaminants, type and scale of reactor, characteriza
tion of influent, required composition of produced solution, and the
specific treatment requirements. For example, using chelating agents in
the EF process lead to significant increase in the reagent cost. Further
research is needed to determine the most cost-effective process and fully
understand the economic benefits and limitations of this technology.
7. Challenges and future consideration
The application of strategies mentioned in this review for performing
EF process at neutral pH presents a number of challenges that need to be
addressed to make it more practical and economically viable. Some
major challenges include:
I. Low reaction rate: Further research should aim to improve the
reaction kinetics by minimizing the mass transfer limitations
through designing catalysts with abundant nano/micropores, to
enhance accessibility to catalytic sites and improve the ORR ki
netics [166], improving oxygen mass transfer through designing
cathodes like natural air diffusion electrodes (NADE) [167], or
using flow-through system [168,169], application of anodes with
high stability and catalytic activity to improve •
OH production
[170], chemical functionalization of catalyst and cathode with
metal and non-metal atoms which can provide more active sites
for ORR improvement [78], and hybrid EF process with different
treatment methods to synergistically enhance the degradation
kinetics [9].
II. Insufficient studies on mechanism: A deeper understanding of
different aspects of mechanism at neutral pH, not only the radical
mechanism (mainly by •OH) but also the nonradical mechanisms
(mainly by singlet oxygen (1
O2)) in the EF process, should be
considered to improve understanding of the overall process by
means of analytical tests (such as FTIR and EPR) or theoretical
calculations (such as DFT to improve the understanding of ROS
production and find the potential attacked sites, produced in
termediates, and possible O2 adsorption sites [51,55]), quenching
experiments [171], and micro electro sensors [172].
III. Limited availability of appropriate catalysts: One of the key
challenges in the EF process at neutral pH is the limited avail
ability of stable catalysts that are effective for the oxidation of
contaminants at this pH range. This has been a major hindrance
to the commercialization of the EF process in neutral pH [34].
IV. Electrode fouling and scaling: The accumulation of reaction by-
products and contaminants on electrodes can lead to fouling
and scaling, which can cause significant performance degrada
tion and increase maintenance costs [173]. These potential
problems, along with the corrosion during long-term use, should
be discussed in detail in future investigations.
V. Chelating agent toxicity: Different parameters, such as chemical
species of chelating agents and their interaction with metal spe
cies can affect the transformation and environmental fate of
chelating agents. Therefore, the ecotoxicity and biodegradability
of the components formed after addition of chelating agents
should be investigated [17,33].
VI. Side effects of EF combinations with other treatment methods:
Although the combined systems provide higher degradation ef
ficiency, increasing the rate of side reactions as well as process
cost and non-radical pathways (in a few cases) remain challenges,
and further studies are needed before their implementation in the
real field [13].
VII. Developing EF process for continuous flow or scaled-up systems:
Large-scale EF explorations are needed to further assess the
possibility of practical application to identify scale-up parameters
and optimize industrial applications.
VIII. Evaluating performance of EF system in real wastewater: The
complexity of real water matrix due to presence of different
scavenging radical species, inions, bacteria, and organisms may
lead to different results in activity and stability of catalysts.
Table 3
Detailed operational cost for EF process at acidic and neutral pH for a 100 L
sequential batch reactor. Reproduced from Ref. [165].
EF process pH 3.0 neutral pH
Chemicals / € m¡3
(total) 1.07 0.22
H2SO4 0.17 –
NaOH 0.68 –
Na2SO4 0.22 0.22
Electricity consumption / € m¡3
(total) 0.22 0.44
Stirring 0.09 0.18
H2O2 generation 0.11 0.11
Air supply 0.02 0.02
Total costs/ € m¡3
1.27 0.66
Z. Heidari et al.

A critical review on the recent progress in application of electro-Fenton process for decontamination of wastewater at near-neutral pH.pdf

A critical review on the recent progress in application of electro-Fenton process for decontamination of wastewater at near-neutral pH.pdf

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