Journal Pre-proof Anodic oxidation of organic pollutants: anode fabrication, process hybrid and environmental applications Zhongzheng Hu, Jingju Cai, Ge Song, Yusi Tian, Minghua Zhou PII:
To appear in:
Current Opinion in Electrochemistry
Received Date: 14 October 2020 Revised Date:
16 November 2020
Accepted Date: 21 November 2020
Please cite this article as: Hu Z, Cai J, Song G, Tian Y, Zhou M, Anodic oxidation of organic pollutants: anode fabrication, process hybrid and environmental applications, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2020.100659. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Anodic oxidation of organic pollutants: anode fabrication, process
hybrid and environmental applications
Zhongzheng Hu 1&, Jingju Cai1& , Ge Song 1, Yusi Tian 1, Minghua Zhou 1∗
Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media Pollution,
College of Environmental Science and Engineering, Nankai University, Tianjin 300350, P. R.
Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education,
Manuscript submit to special issue of
Current Opinion in Electrochemistry
These authors contributed equally to this work and should be considered as co-first authors Corresponding author. E-mail address: [email protected]
ABSTRACT This review summarizes the recent progress in anodic oxidation of organic
pollutant for water and wastewater treatment. It supplies the advances in anodes
fabrication to improve the anodic performance by different modifications and
preparation strategies, focusing on non-active anodes including boron-doped diamond
(BDD), PbO2, SnO2 and Ti-based anode (e.g., Ti4O7, blue titanium oxide). Meanwhile,
the tendency of anodic oxidation coupled or combined with other processes
(adsorption, membrane separation, biological treatment and advanced oxidation
process) for pretreatment or advanced treatment of organic pollutant is presented.
Finally, anodic oxidation for environmental application are briefly described, several
challenges need to be overcome and perspectives for future study are critically
Keywords: Anodic oxidation; Electrode preparation; Process hybrid; Organic pollutant
degradation; Advanced oxidation processes
Introduction Recently electrochemical advanced oxidation processes (EAOPs) have attracted
great interests for treatment of biorefractory organic pollutants due to their advantages
including high efficiency, cost-effectiveness and environmental compatibility [1-3].
Many processes have been developed, e.g., anodic oxidation, electro-Fenton (EF),
photo-electro-Fenton (PEF), and have been attempted for the removal of emerging
contaminants, and different industrial wastewaters containing phenols, dyes,
pharmaceuticals or membrane concentrates, as well as municipal wastewaters [4-6].
Among these processes, anodic oxidation (AO) may be the simplest and most
effective alternative owned to the direct or indirect generation of active species on the
anode, hence the nature of anode material plays an essential role on both treatment
efficiency and selectivity. Many works have proved that some non-active anodes, such
as BDD, SnO2, and PbO2, are ideal anodes for the mineralization of organic pollutants
to final products of CO2 and water . It has been one of the research hot spots to
fabricate and modify or develop new electrodes to enhance the anodic performance
and electrode stability.
Till now, AO has been successfully applied to the treatment of various organic
pollutants and real wastewaters as a pretreatment or advanced treatment process.
Specifically, to suit different treatment objective and achieve cost-effectiveness of the
method, many other processes (adsorption, membrane separation, biological treatment
and advanced oxidation process) are combined with anodic oxidation, which further
facilitate environmental application of AO . 3
In this review, it highlights recent advance in the anodic oxidation of organic
pollutants, illuminating the approaches for sound anodes preparation, summarizing
possible hybrid process to improve treatment efficiency, and critically presenting the
existing problems or limitations in this research area.
BDD Own to its wide potential window, good stability and strong corrosion resistance,
BDD electrode is regarded as the most efficient anode material, and has been
extensively studied for anodic oxidation of organic pollutants. Several recent reviews
have summarized the preparation and electrochemical properties of BDD electrode,
the electrocatalytic process and degradation mechanisms of the electrochemical
oxidation of refractory pollutants . Though BDD electrode is commercially
available, it is still expensive and possess the largest application bottleneck. Hence,
this work will focus on BDD preparation to highlight some recent efforts made for
improving the cost-effectiveness, which indicated four trends:
1) The improvement of substrate material. BDD film can be deposited on
substrates such as Si, Nb and Ta , and recent works have shown that Ti-BDD
composite prepared by spark plasma sintering demonstrate a bright prospect for
application due to the extended service life and good electrochemical oxidation ability
2) Regulation of surface properties. It is reported that surface boron doping level,
graphite ratio (sp3/sp2), crystal size and morphology roughness of diamond affect the 4
anodic oxidation performance. A recent research indicated that low doping level is
more efficient for urine removal by anodic oxidation, in which the best results were
found for the BDD with a boron content of 200 ppm, capable of removing 100% of
the antibiotic, reducing toxicity by 90%, and eradicating the antibiotic effect . It
had been established that a higher sp3/sp2 ratio brought about a more efficient
degradation process since the more C-sp3 loaded BDD favors strong oxidation instead
of forming ineffective secondary compounds . A high-temperature oxidation
etching technology was employed for decreasing the content of sp2 phase on the
surface of BDD to prepare an Si/BDD electrode with an irregular cone structure (Fig.
1a), whose removal rate of tetracycline hydrochloride was increased by 1.57 times
compared with BDD .
boron-doped vertically aligned graphene walls (BCNWs) was grown on a BDD
interfacial layer (Fig. 1b), which resulted in a higher current exchange density and an
enhanced COD removal . Similarly, the fabricated BDD nanowire electrode (Fig.
1c) enhanced effective surface area several times compared to the conventional planar
BDD electrode, and also significantly improved COD and TOC removal and current
efficiency . To overcome shortcomings of two dimensional BDD electrodes, a
novel three dimensional macroporous BDD (3D-BDD) foam electrode with a
structure of evenly distributed pores and interconnected networks was prepared (Fig.
1d), whose electroactive surface area and electrochemical oxidation reaction rate
constant of RB-19 were increased by 20 times and 350 times respectively .
PbO2 and SnO2 PbO2 and SnO2 are two of the most common non-active metal oxide anodes for
degrading pollutants due to the advantages of high oxygen evolution potential, strong
oxidation ability, excellent electrical conductivity, and low cost [18-20]. As known, Ti
is usually used as the substrate of PbO2 electrode [21, 22]. Nevertheless, it had been
proved that the produced active oxygen during electrolysis would diffuse to the
surface of the matrix to form TiO2 insulator, reducing the conductivity and
electrocatalytic activity of the electrode . For pristine SnO2 anode, it has the
problems of short service life and poor conductivity, hindering its industrial
application. To overcome the defects of pure PbO2 and SnO2 electrodes, various
methods had been attempted that mainly involve elements doping, introducing
intermediate layers and nano-architecture.
1) The electrocatalytic performance of PbO2 and SnO2 electrodes can be improved
by introducing functional elements into their coating. The commonly used doping
elements are F [24, 25], Bi , Al [27, 28], Fe [29, 30], Cu , Pd , Pt  and
many rare earth elements, including Ce , La  and Yb . For example, Xia et
al.  found that In-doped PbO2 anode had a higher oxygen over-potential (2.08 V)
than that of the undoped one, and the removal efficiency of aspirin reached up to
76.45% within 2 h. After Sb was proved to increase the electrocatalytic activity of
SnO2 anode, Zhang et al.  further constructed a new type of aluminum-doped
SnO2-Sb (SnO2-Sb-Al) electrode, and observed that the mineralization of phenol by
SnO2-Sb-Al was 1.5 times higher than SnO2-Sb electrode. In addition, multi-elements
co-doping was found effective to enhance the electrocatalytic activity of SnO2 anode,
e.g., Co/Pr co-doped Ti/PbO2 . 2) The intermediate layer can strengthen the bonding between the electrode active
layer and the matrix, avoid the shedding of the active layer, improve the
electrocatalytic activity and prolong the electrode service life. At present the
commonly used intermediate coatings are Pt , IrO2 , TiO2 nanotubes ,
graphene nanosheet , metal oxide (SnO2-Sb, SnO2-Sb2O3)  and so on. Tang's
group  prepared the novel Ti/MnO2-WC/β-PbO2 electrode by electrodeposition,
showing that the MnO2-WC composite intermediate layer increased the surface active
sites and enhanced the electrocatalytic activity as well as accelerated anode lifetime.
Mameda et al.  demonstrated inserting the mixed C and N interlayers between the
Ti substrate and Sb-SnO2 catalyst could increase the lifetime 25 times longer than that
of the Ti/Sb-SnO2 electrode.
3) Nano-structure construction is also an effective way to enhance the
electrocatalytic activity, basically including two approaches, one is to build a
nano-structured matrix to improve the active catalyst loading, and the other is to
fabricate nano-sized or nanocrystalline PbO2 or SnO2. Many literatures indicated that
a well-aligned TiO2 nanotubes prepared by anodic oxidation could greatly improve
oxygen evolution potential, effective area, and electrocatalytic performance [45-48].
Based on the enhanced nanotube array (ENTA) as internal structure, Chen et al. 
coated two kinds of anodes, Ti-ENTA/SnO2-Sb and Ti-ENTA/SnO2-Sb/PbO2. They 7
found that the ENTA improved not only the electrochemical properties (e.g., oxygen
evolution potential, •OH production), but also the service lifetime, and concluded that
anodic oxidation on these novel electrodes was cost-effective and promising for the
treatment of reverse osmosis concentrates. On the other hand, Xu et al.  prepared
a hydroxyl multi-wall carbon nanotube-modified nanocrystalline PbO2 anode, whose
oxygen evolution potential and effective area were 1.5 and 3.7-fold higher than the
traditional PbO2 electrode, boosting the decay rate of pyridine (93.8%).
Ti based oxide
Titanium (Ti) is cheap and abundant in nature, but it is easy to be oxidized to
form a TiO2 layer, which leads to poor conductivity unsuitable and is not suitable for
anodic oxidation. To solve this problem, there are two ways to improve the
conductivity of TiO2 by reduction in H2 atmosphere [50, 51] or electrochemical
reduction [52, 53].
TiO2 reduction in H2 atmosphere would form the sub-stoichiometric titanium
oxide (TinO2n-1, n≥3), i.e., Ti4O7, Ti5O9 and Ti6O11. Previous studies have shown that
Ti4O7 electrode is a good non-active anode , exhibiting a better performance than
the classical anodes Pt and dimensionally stable anode (DSA), and can constitute an
alternative to BDD anode for a cost-effective anodic oxidation . Oturan’s group
 extended Ti4O7 as anode for electro-Fenton oxidation, concluding that Ti4O7
provided similar oxidation rate and mineralization current efficiency as BDD, while
remarkably superior to DSA and Pt anodes. Le et al.  prepared Ti4O7 reactive
electrochemical membrane (REM) for oxidation of perfluorooctanesulfonic acid
(PFOS) (Fig. 2a) with the lowest energy consumption of 6.7 kWh/m3, which was
much lower than that obtained on Ti4O7 anode (32 kWh/m3). Electrochemical reduction of TiO2 always generates the Blue color TiO2 named
Blue-TiO2 [57, 58,59]. Chang et al.  investigated the performance of Blue-TiO2
for degradation of salicylic acid, obtaining the degradation rate was 6.3 times higher
than that on Pt anode. Cai et al.  prepared Blue-TiO2 by electrochemical reduction
in formic acid solution, obtaining a higher •OH production activity (1.7 × 10-14 M)
than BDD (9.8 ×10-15 M) and inducing a higher TOC removal with a lower
accumulation of phenol degradation intermediates. Both •OH and SO4•- were
responsible for the phenol degradation, and the contribution of radicals was
influenced by current density, pH and Na2SO4 concentration (Fig. 2b). Gan et al. 
employed self-doped TiO2 nanotubes arrays (DNTA) as the anode for degradation of
phenol, and concluded DNTA had a higher TOC removal than BDD and Pt due to the
surface-bound •OH oxidation mechanism (Fig. 2c). Nevertheless, the disadvantage of
this anode is the short lifetime, which can be enhanced by elements doping, e.g., Fe,
Ni, Co and B doping [62, 63]. Yang et al.  employed cobalt-doped Black TiO2
array as an anode, the lifetime significantly increased from 2.3 h of the unmodified
one to 100 h.
AO can be as a single process for pollutants abatement, and also as a hybrid process
to achieve a more efficient or deeper treatment. Table 1 summarizes the typical hybrid 9
processes with the main results and highlights, which can be basically divided into
two types, one is the coupling process in one cell, and the other is combined process
with physical, chemical or biological process [64-76]. AO process can be coupled
with electro-Fenton (EF), photo-electro-Fenton (PEF), or ultrasound (US) oxidation
for obtaining higher decontamination efficiencies. Moreover, AO process can be the
pretreatment process for biologic process, such as wetland and membrane bioreactor
(MBR), and for adsorption and ultrafiltration process, as well as some oxidation
process including ozonation or UV irradiation. AO process can also act as advanced
treatment for electrocoagulation (ECO). However, the hybrid process with AO would
increase the capital investment and treatment cost due to the introduction of external
fields (e.g. UV and US), thus a synergistic effect in the processes would help to
expand the removal performance (e.g., TOC removal) and increase the benefits. In
addition, the coordination between AO and biological processes should also be taken
into consideration, for example, pH adjustment before biological process.
At present, various types of pollutants (e.g. disinfection by-products (DBPs),
endocrine disruptors (EDCs), pharmaceutical and personal care products (PPCPs)
from synthesized wastewater, domestic wastewater, greywater, and many real
wastewaters including dyes, pesticides, phenols, RO concentrates and landfill leachate)
have been verified effective removal by AO . AO has been extended from organic
pollutant removal in aqueous solution to soil remediation and gas gaseous effluents 10
purification . However, there are still some issues needed to be paid attention or
solved. 1) Many current studies focus on ideal solutions (e.g., single target contaminant),
but do not consider the water matrix constituents in real water , which would
render omission the great impact from other important co-existed inorganic/organic
pollutants. Also owing to the difference in contraction ranges, it will make the
diffusion mass transport rates dramatically different between laboratory and field
applications. For example, various salts, such as sulfates, chlorides, carbonates, or
phosphates are present in wastewater and natural water, which could generated strong
oxidants to react with organics . However, the existed in water of chlorides will
participate in the AO, and lead to the formation of chlorinated by-products, i.e.,
organochlorinated compounds, chlorates, or perchlorates . Such by-products may
be more toxic than parent contaminants and should be avoided as much as possible. A
special polymer exchange membrane (PEM)-electrolyzer equipped with BDD would
be an alternative to prevent the formation of chlorates and perchlorates . On the
other hand, in the case of very low concentration of pollutant and electrolytes, the
high resistivity would be an important limitation to reduce the cell voltage and the
ohmic loses. Many efforts on electrochemical engineering towards specific
geometrical designs that could minimize the interelectrodic gap, maximize the
turbulence with promoters to be made. Presently, microfluidic reactor and
flow-through reactor showed the cost-effectiveness .
2) Scale-up has not always been faced in the right way, the full applications for 11
industrial use, demonstration or even pilot scale are still very limited, probably due to
the economic difficulties and immature electrode fabrication or reactor design .
Therefore, the application of electrochemical technologies driven by renewable
energy sources (e.g. solar photovoltaics, wind turbines) for treating hazardous
pollutants in wastewater would help to reduce the treatment cost issues . 3) The comprehensive treatment (PPCPs removal, ammonia removal, disinfection)
other than single organic pollutant removal would be a research trend to improve the
process economics and integrated performance .
Challenges and perspectives
Although anodic oxidation has been successfully applied in many research areas
in environmental engineering, such as water reclamation and wastewater treatment,
gas purification and soil remediation. Besides the environmental application problems
stated before, there are still many challenges to be tacked to advance AO research and
1) Electrode activity, stability, and its degradation mechanism. Till now, many
efforts on electrode fabrication and modifications have already been made to prepare
high activity anodes, which are used to construct small electrochemical devices to
treat wastewater. Liang et al.  used an Fe plate anode, a stainless-steel plate
cathode, and a mixed metal oxide (MMO) anode to construct single electrochemical
reactor for treatment of phosphite-laden wastewater, achieving a phosphite removal
efficiency of 74.25% that significantly higher than that in the control experiments in
the absence of an MMO anode (< 23.41%). Despite of these progresses, excellent 12
anodes with low cost, strong stability, long service lifetime and enhanced
electrocatalytic activity are urgently required to develop industrial scale-up to meet
the practical demand . Though there are some materials or electrochemical
methods to characterize the prepared electrode, the AO performance is usually
evaluated or analyzed by the organic pollutant degradation/mineralization
performance or detection of degradation intermediates or identification of possible
radicals in the solution. However, as an anodic interface process, it is urgently needed
to develop the characterization or identification method for surface reaction especially
for the real-time or in-situ detection of possible radicals involved in the pollutant
degradation, so that an intrinsic mechanism would be well disclosed to benefit anode
preparation and selection.
2) Hybrid process with AO has extended the application scale of the process and
improves the effectiveness for the goal of deep or advanced treatment. However, a
combined process would increase the energy input, or increase the treatment
investment and operation cost, how to coordinate process compatibility (e.g., no need
for extra pH adjustment, pretreatment) and improve the hybrid process integration
(e.g., multifunction, synergic effect) will be great challenges. Besides the AO, the
simultaneous electrochemical generation/activation of other oxidants or active species
(e.g., SO4•-, • OH and active chlorine) will increase benefit . The recent
development of REM as a flow-through electrode has proven to be a breakthrough
innovation, leading to both high electrochemically active surface area and
convection-enhanced mass transport of pollutants, and thus deserves to further study
This work was financially supported by Natural Science Foundation of China
(nos. 21773129, 21976096, 21811530274 and 21273120), Tianjin Science and
Technology Program (19PTZWHZ00050), Tianjin Development Program for
Innovation and Entrepreneurship, Tianjin Post-graduate Students Research and
Innovation Project (2019YJSB075), National Key Research and Development
Program (2016YFC0400706), and Fundamental Research Funds for the Central
Universities, Nankai University.
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Fig. 1 SEM before and after heat treatment (a) , BDD/Boron-doped carbon nanowall (BCNW) (b) , Boron-doped diamond nanowire electrode (c)  and 3D BDD (d) .
618 619 620 621 622 623 624 625
628 629 630 631 632 633 634 635
Fig. 2 (a) The intermediates of PFOA and PFOS in REM system , (b) The mechanism on Blue-TiO2 in sulfate electrolyte , (c) Comparison between BDD and DNTA for phenol degradation .
Table 1 Hybrid processes with anodic oxidation Main results
AO: 40 % TOC AO/EF: 55.1% TOC
TOC removal can be increased by Fenton.
EF-AO-DSA: 64.3% TOC EF-AO-BDD: 97.1% TOC
The versatility of EF mainly depend upon the anode materials.
AO/PEF process can efficiently absorb photons and generate (h+)/(e–) pairs.
Ultrasound can clean the electrode, increase the mass transport across the electrode surface, prevent the electrode passivation, and activate the persulfate to generate free sulfate radicals.
AO: 40% of methylene blue AO/US: 91.41% of methylene blue
Ultrasound can improve the charge transfer ability and wettability to promote mass transport.
AO + Wetland
AO: 28% TOC AO + Wetland: 61% TOC
The toxic AO by-products were removed substantially in the vertical flow constructed wetland.
AO + MBR
AO: 55% of ibuprofen AO-MBR: 99% of ibuprofen
The high COD and nitrogen removal by MBR and pharmaceuticals removal by AO.
PEF: 53% of methyl orange AO/PEF: 71% of methyl orange
Combined process—AO as pretreatment
AO: 60.6% ofloxacin AO/US: 95% ofloxacin
continued Hybrid process
AO + Adsorption
Adsorption: 20% mecoprop AO + Adsorption: ~ 100% mecoprop
The AO would overcome some drawbacks as poor adsorption affinity of some compounds.
AO + UF
AO: 13%DCO AO + UF: 53% DOC
AO + Ozonation
AO + UV Combined process—AO as advanced treatment
ECO + AO
AO: 92.1% TOC at 120 min AO + Ozonation: 98.5% TOC after 60 min
The reluctance of oxalic acid to be oxidized by either AO or ozonation alone can be overcome by the combination of both.
AO: 84.71% TOC AO + UV : 96.25% TOC
The UV would decompose of electrogenerated oxidants into radical species, such as peroxide and peroxydisulfate.
ECO: 17.1% TOC ECO + AO: 84.6% TOC
AO would promote the mineralization of organic substances in wastewater.
ECO: 56% COD ECO + AO: 72% COD
ECO enhances the chemical coagulation process.
AO as a pretreatment stage can reduce membrane fouling and operation cost, and improve the water quality for water reuse applications.
Combined process—AO as pretreatment
Declaration of interest statement
There is no competing financial interest among the authors.