Anodic oxidation of organic pollutants: Anode fabrication, process hybrid and environmental applications

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:

S2451-9103(20)30212-X

DOI:

https://doi.org/10.1016/j.coelec.2020.100659

Reference:

COELEC 100659

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.

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Anodic oxidation of organic pollutants: anode fabrication, process

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hybrid and environmental applications

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Zhongzheng Hu 1&, Jingju Cai1& , Ge Song 1, Yusi Tian 1, Minghua Zhou 1∗

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Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media Pollution,

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College of Environmental Science and Engineering, Nankai University, Tianjin 300350, P. R.

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China

Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education,

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Manuscript submit to special issue of

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Current Opinion in Electrochemistry

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These authors contributed equally to this work and should be considered as co-first authors Corresponding author. E-mail address: [email protected] (M. Zhou)

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ABSTRACT This review summarizes the recent progress in anodic oxidation of organic

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pollutant for water and wastewater treatment. It supplies the advances in anodes

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fabrication to improve the anodic performance by different modifications and

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preparation strategies, focusing on non-active anodes including boron-doped diamond

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(BDD), PbO2, SnO2 and Ti-based anode (e.g., Ti4O7, blue titanium oxide). Meanwhile,

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the tendency of anodic oxidation coupled or combined with other processes

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(adsorption, membrane separation, biological treatment and advanced oxidation

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process) for pretreatment or advanced treatment of organic pollutant is presented.

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Finally, anodic oxidation for environmental application are briefly described, several

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challenges need to be overcome and perspectives for future study are critically

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proposed.

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Keywords: Anodic oxidation; Electrode preparation; Process hybrid; Organic pollutant

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degradation; Advanced oxidation processes

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Introduction Recently electrochemical advanced oxidation processes (EAOPs) have attracted

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great interests for treatment of biorefractory organic pollutants due to their advantages

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including high efficiency, cost-effectiveness and environmental compatibility [1-3].

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Many processes have been developed, e.g., anodic oxidation, electro-Fenton (EF),

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photo-electro-Fenton (PEF), and have been attempted for the removal of emerging

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contaminants, and different industrial wastewaters containing phenols, dyes,

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pharmaceuticals or membrane concentrates, as well as municipal wastewaters [4-6].

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Among these processes, anodic oxidation (AO) may be the simplest and most

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effective alternative owned to the direct or indirect generation of active species on the

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anode, hence the nature of anode material plays an essential role on both treatment

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efficiency and selectivity. Many works have proved that some non-active anodes, such

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as BDD, SnO2, and PbO2, are ideal anodes for the mineralization of organic pollutants

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to final products of CO2 and water [7]. It has been one of the research hot spots to

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fabricate and modify or develop new electrodes to enhance the anodic performance

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and electrode stability.

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Till now, AO has been successfully applied to the treatment of various organic

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pollutants and real wastewaters as a pretreatment or advanced treatment process.

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Specifically, to suit different treatment objective and achieve cost-effectiveness of the

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method, many other processes (adsorption, membrane separation, biological treatment

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and advanced oxidation process) are combined with anodic oxidation, which further

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facilitate environmental application of AO [8]. 3

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In this review, it highlights recent advance in the anodic oxidation of organic

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pollutants, illuminating the approaches for sound anodes preparation, summarizing

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possible hybrid process to improve treatment efficiency, and critically presenting the

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existing problems or limitations in this research area.

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Anode fabrication

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BDD Own to its wide potential window, good stability and strong corrosion resistance,

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BDD electrode is regarded as the most efficient anode material, and has been

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extensively studied for anodic oxidation of organic pollutants. Several recent reviews

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have summarized the preparation and electrochemical properties of BDD electrode,

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the electrocatalytic process and degradation mechanisms of the electrochemical

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oxidation of refractory pollutants [9]. Though BDD electrode is commercially

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available, it is still expensive and possess the largest application bottleneck. Hence,

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this work will focus on BDD preparation to highlight some recent efforts made for

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improving the cost-effectiveness, which indicated four trends:

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1) The improvement of substrate material. BDD film can be deposited on

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substrates such as Si, Nb and Ta [10], and recent works have shown that Ti-BDD

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composite prepared by spark plasma sintering demonstrate a bright prospect for

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application due to the extended service life and good electrochemical oxidation ability

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[11].

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2) Regulation of surface properties. It is reported that surface boron doping level,

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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

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more efficient for urine removal by anodic oxidation, in which the best results were

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found for the BDD with a boron content of 200 ppm, capable of removing 100% of

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the antibiotic, reducing toxicity by 90%, and eradicating the antibiotic effect [12]. It

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had been established that a higher sp3/sp2 ratio brought about a more efficient

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degradation process since the more C-sp3 loaded BDD favors strong oxidation instead

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of forming ineffective secondary compounds [13]. A high-temperature oxidation

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etching technology was employed for decreasing the content of sp2 phase on the

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surface of BDD to prepare an Si/BDD electrode with an irregular cone structure (Fig.

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1a), whose removal rate of tetracycline hydrochloride was increased by 1.57 times

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compared with BDD [14].

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boron-doped vertically aligned graphene walls (BCNWs) was grown on a BDD

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interfacial layer (Fig. 1b), which resulted in a higher current exchange density and an

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enhanced COD removal [15]. Similarly, the fabricated BDD nanowire electrode (Fig.

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1c) enhanced effective surface area several times compared to the conventional planar

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BDD electrode, and also significantly improved COD and TOC removal and current

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efficiency [16]. To overcome shortcomings of two dimensional BDD electrodes, a

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novel three dimensional macroporous BDD (3D-BDD) foam electrode with a

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structure of evenly distributed pores and interconnected networks was prepared (Fig.

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1d), whose electroactive surface area and electrochemical oxidation reaction rate

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constant of RB-19 were increased by 20 times and 350 times respectively [17].

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PbO2 and SnO2 PbO2 and SnO2 are two of the most common non-active metal oxide anodes for

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degrading pollutants due to the advantages of high oxygen evolution potential, strong

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oxidation ability, excellent electrical conductivity, and low cost [18-20]. As known, Ti

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is usually used as the substrate of PbO2 electrode [21, 22]. Nevertheless, it had been

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proved that the produced active oxygen during electrolysis would diffuse to the

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surface of the matrix to form TiO2 insulator, reducing the conductivity and

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electrocatalytic activity of the electrode [23]. For pristine SnO2 anode, it has the

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problems of short service life and poor conductivity, hindering its industrial

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application. To overcome the defects of pure PbO2 and SnO2 electrodes, various

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methods had been attempted that mainly involve elements doping, introducing

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intermediate layers and nano-architecture.

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1) The electrocatalytic performance of PbO2 and SnO2 electrodes can be improved

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by introducing functional elements into their coating. The commonly used doping

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elements are F [24, 25], Bi [26], Al [27, 28], Fe [29, 30], Cu [31], Pd [23], Pt [32] and

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many rare earth elements, including Ce [33], La [34] and Yb [35]. For example, Xia et

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al. [36] found that In-doped PbO2 anode had a higher oxygen over-potential (2.08 V)

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than that of the undoped one, and the removal efficiency of aspirin reached up to

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76.45% within 2 h. After Sb was proved to increase the electrocatalytic activity of

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SnO2 anode, Zhang et al. [27] further constructed a new type of aluminum-doped

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SnO2-Sb (SnO2-Sb-Al) electrode, and observed that the mineralization of phenol by

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SnO2-Sb-Al was 1.5 times higher than SnO2-Sb electrode. In addition, multi-elements

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co-doping was found effective to enhance the electrocatalytic activity of SnO2 anode,

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e.g., Co/Pr co-doped Ti/PbO2 [37]. 2) The intermediate layer can strengthen the bonding between the electrode active

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layer and the matrix, avoid the shedding of the active layer, improve the

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electrocatalytic activity and prolong the electrode service life. At present the

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commonly used intermediate coatings are Pt [38], IrO2 [39], TiO2 nanotubes [40],

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graphene nanosheet [41], metal oxide (SnO2-Sb, SnO2-Sb2O3) [42] and so on. Tang's

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group [43] prepared the novel Ti/MnO2-WC/β-PbO2 electrode by electrodeposition,

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showing that the MnO2-WC composite intermediate layer increased the surface active

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sites and enhanced the electrocatalytic activity as well as accelerated anode lifetime.

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Mameda et al. [44] demonstrated inserting the mixed C and N interlayers between the

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Ti substrate and Sb-SnO2 catalyst could increase the lifetime 25 times longer than that

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of the Ti/Sb-SnO2 electrode.

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3) Nano-structure construction is also an effective way to enhance the

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electrocatalytic activity, basically including two approaches, one is to build a

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nano-structured matrix to improve the active catalyst loading, and the other is to

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fabricate nano-sized or nanocrystalline PbO2 or SnO2. Many literatures indicated that

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a well-aligned TiO2 nanotubes prepared by anodic oxidation could greatly improve

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oxygen evolution potential, effective area, and electrocatalytic performance [45-48].

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Based on the enhanced nanotube array (ENTA) as internal structure, Chen et al. [48]

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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

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evolution potential, •OH production), but also the service lifetime, and concluded that

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anodic oxidation on these novel electrodes was cost-effective and promising for the

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treatment of reverse osmosis concentrates. On the other hand, Xu et al. [49] prepared

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a hydroxyl multi-wall carbon nanotube-modified nanocrystalline PbO2 anode, whose

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oxygen evolution potential and effective area were 1.5 and 3.7-fold higher than the

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traditional PbO2 electrode, boosting the decay rate of pyridine (93.8%).

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Ti based oxide

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Titanium (Ti) is cheap and abundant in nature, but it is easy to be oxidized to

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form a TiO2 layer, which leads to poor conductivity unsuitable and is not suitable for

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anodic oxidation. To solve this problem, there are two ways to improve the

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conductivity of TiO2 by reduction in H2 atmosphere [50, 51] or electrochemical

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reduction [52, 53].

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TiO2 reduction in H2 atmosphere would form the sub-stoichiometric titanium

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oxide (TinO2n-1, n≥3), i.e., Ti4O7, Ti5O9 and Ti6O11. Previous studies have shown that

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Ti4O7 electrode is a good non-active anode [54], exhibiting a better performance than

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the classical anodes Pt and dimensionally stable anode (DSA), and can constitute an

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alternative to BDD anode for a cost-effective anodic oxidation [55]. Oturan’s group

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[56] extended Ti4O7 as anode for electro-Fenton oxidation, concluding that Ti4O7

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provided similar oxidation rate and mineralization current efficiency as BDD, while

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remarkably superior to DSA and Pt anodes. Le et al. [51] prepared Ti4O7 reactive

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electrochemical membrane (REM) for oxidation of perfluorooctanesulfonic acid

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(PFOS) (Fig. 2a) with the lowest energy consumption of 6.7 kWh/m3, which was

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much lower than that obtained on Ti4O7 anode (32 kWh/m3). Electrochemical reduction of TiO2 always generates the Blue color TiO2 named

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Blue-TiO2 [57, 58,59]. Chang et al. [60] investigated the performance of Blue-TiO2

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for degradation of salicylic acid, obtaining the degradation rate was 6.3 times higher

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than that on Pt anode. Cai et al. [61] prepared Blue-TiO2 by electrochemical reduction

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in formic acid solution, obtaining a higher •OH production activity (1.7 × 10-14 M)

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than BDD (9.8 ×10-15 M) and inducing a higher TOC removal with a lower

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accumulation of phenol degradation intermediates. Both •OH and SO4•- were

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responsible for the phenol degradation, and the contribution of radicals was

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influenced by current density, pH and Na2SO4 concentration (Fig. 2b). Gan et al. [58]

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employed self-doped TiO2 nanotubes arrays (DNTA) as the anode for degradation of

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phenol, and concluded DNTA had a higher TOC removal than BDD and Pt due to the

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surface-bound •OH oxidation mechanism (Fig. 2c). Nevertheless, the disadvantage of

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this anode is the short lifetime, which can be enhanced by elements doping, e.g., Fe,

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Ni, Co and B doping [62, 63]. Yang et al. [63] employed cobalt-doped Black TiO2

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array as an anode, the lifetime significantly increased from 2.3 h of the unmodified

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one to 100 h.

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Hybrid process

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AO can be as a single process for pollutants abatement, and also as a hybrid process

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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

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two types, one is the coupling process in one cell, and the other is combined process

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with physical, chemical or biological process [64-76]. AO process can be coupled

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with electro-Fenton (EF), photo-electro-Fenton (PEF), or ultrasound (US) oxidation

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for obtaining higher decontamination efficiencies. Moreover, AO process can be the

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pretreatment process for biologic process, such as wetland and membrane bioreactor

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(MBR), and for adsorption and ultrafiltration process, as well as some oxidation

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process including ozonation or UV irradiation. AO process can also act as advanced

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treatment for electrocoagulation (ECO). However, the hybrid process with AO would

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increase the capital investment and treatment cost due to the introduction of external

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fields (e.g. UV and US), thus a synergistic effect in the processes would help to

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expand the removal performance (e.g., TOC removal) and increase the benefits. In

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addition, the coordination between AO and biological processes should also be taken

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into consideration, for example, pH adjustment before biological process.

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Environmental application

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At present, various types of pollutants (e.g. disinfection by-products (DBPs),

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endocrine disruptors (EDCs), pharmaceutical and personal care products (PPCPs)

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from synthesized wastewater, domestic wastewater, greywater, and many real

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wastewaters including dyes, pesticides, phenols, RO concentrates and landfill leachate)

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have been verified effective removal by AO [77]. AO has been extended from organic

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pollutant removal in aqueous solution to soil remediation and gas gaseous effluents 10

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purification [78]. However, there are still some issues needed to be paid attention or

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solved. 1) Many current studies focus on ideal solutions (e.g., single target contaminant),

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but do not consider the water matrix constituents in real water [79], which would

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render omission the great impact from other important co-existed inorganic/organic

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pollutants. Also owing to the difference in contraction ranges, it will make the

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diffusion mass transport rates dramatically different between laboratory and field

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applications. For example, various salts, such as sulfates, chlorides, carbonates, or

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phosphates are present in wastewater and natural water, which could generated strong

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oxidants to react with organics [80]. However, the existed in water of chlorides will

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participate in the AO, and lead to the formation of chlorinated by-products, i.e.,

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organochlorinated compounds, chlorates, or perchlorates [77]. Such by-products may

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be more toxic than parent contaminants and should be avoided as much as possible. A

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special polymer exchange membrane (PEM)-electrolyzer equipped with BDD would

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be an alternative to prevent the formation of chlorates and perchlorates [81]. On the

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other hand, in the case of very low concentration of pollutant and electrolytes, the

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high resistivity would be an important limitation to reduce the cell voltage and the

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ohmic loses. Many efforts on electrochemical engineering towards specific

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geometrical designs that could minimize the interelectrodic gap, maximize the

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turbulence with promoters to be made. Presently, microfluidic reactor and

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flow-through reactor showed the cost-effectiveness [82].

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2) Scale-up has not always been faced in the right way, the full applications for 11

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industrial use, demonstration or even pilot scale are still very limited, probably due to

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the economic difficulties and immature electrode fabrication or reactor design [7].

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Therefore, the application of electrochemical technologies driven by renewable

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energy sources (e.g. solar photovoltaics, wind turbines) for treating hazardous

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pollutants in wastewater would help to reduce the treatment cost issues [83]. 3) The comprehensive treatment (PPCPs removal, ammonia removal, disinfection)

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other than single organic pollutant removal would be a research trend to improve the

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process economics and integrated performance [84].

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Challenges and perspectives

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Although anodic oxidation has been successfully applied in many research areas

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in environmental engineering, such as water reclamation and wastewater treatment,

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gas purification and soil remediation. Besides the environmental application problems

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stated before, there are still many challenges to be tacked to advance AO research and

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application.

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1) Electrode activity, stability, and its degradation mechanism. Till now, many

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efforts on electrode fabrication and modifications have already been made to prepare

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high activity anodes, which are used to construct small electrochemical devices to

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treat wastewater. Liang et al. [85] used an Fe plate anode, a stainless-steel plate

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cathode, and a mixed metal oxide (MMO) anode to construct single electrochemical

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reactor for treatment of phosphite-laden wastewater, achieving a phosphite removal

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efficiency of 74.25% that significantly higher than that in the control experiments in

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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

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electrocatalytic activity are urgently required to develop industrial scale-up to meet

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the practical demand [86]. Though there are some materials or electrochemical

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methods to characterize the prepared electrode, the AO performance is usually

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evaluated or analyzed by the organic pollutant degradation/mineralization

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performance or detection of degradation intermediates or identification of possible

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radicals in the solution. However, as an anodic interface process, it is urgently needed

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to develop the characterization or identification method for surface reaction especially

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for the real-time or in-situ detection of possible radicals involved in the pollutant

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degradation, so that an intrinsic mechanism would be well disclosed to benefit anode

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preparation and selection.

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2) Hybrid process with AO has extended the application scale of the process and

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improves the effectiveness for the goal of deep or advanced treatment. However, a

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combined process would increase the energy input, or increase the treatment

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investment and operation cost, how to coordinate process compatibility (e.g., no need

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for extra pH adjustment, pretreatment) and improve the hybrid process integration

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(e.g., multifunction, synergic effect) will be great challenges. Besides the AO, the

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simultaneous electrochemical generation/activation of other oxidants or active species

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(e.g., SO4•-, • OH and active chlorine) will increase benefit [80]. The recent

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development of REM as a flow-through electrode has proven to be a breakthrough

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innovation, leading to both high electrochemically active surface area and

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convection-enhanced mass transport of pollutants, and thus deserves to further study

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[87].

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Acknowledgments

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This work was financially supported by Natural Science Foundation of China

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(nos. 21773129, 21976096, 21811530274 and 21273120), Tianjin Science and

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Technology Program (19PTZWHZ00050), Tianjin Development Program for

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Innovation and Entrepreneurship, Tianjin Post-graduate Students Research and

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Innovation Project (2019YJSB075), National Key Research and Development

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Program (2016YFC0400706), and Fundamental Research Funds for the Central

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Universities, Nankai University.

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Papers of particular interest, published within the period of review, have been

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highlighted as:

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1.

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2●●.Ganiyu SO, Zhou M, Martinez-Huitle CA: Heterogeneous electro-Fenton and

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photoelectro-Fenton processes: A critical review of fundamental principles and

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application for water/wastewater treatment. Appl Catal B: Environ 2018, 235:

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103-129.

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This review focus on heterogeneous electro-Fenton and photoelectro-Fenton 14

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Feier B, Florea A, Cristea C, Săndulescu R: Electrochemical detection and

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Yu X, Zhou M, Hu Y, Groenen Serrano K, Yu F: Recent updates on

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29

a-2

a-1

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Fig. 1 SEM before and after heat treatment (a) [14], BDD/Boron-doped carbon nanowall (BCNW) (b) [15], Boron-doped diamond nanowire electrode (c) [16] and 3D BDD (d) [17].

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618 619 620 621 622 623 624 625

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a

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628 629 630 631 632 633 634 635

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Fig. 2 (a) The intermediates of PFOA and PFOS in REM system [51], (b) The mechanism on Blue-TiO2 in sulfate electrolyte [61], (c) Comparison between BDD and DNTA for phenol degradation [58].

636

1

Table 1 Hybrid processes with anodic oxidation Main results

Remarks

Ref

AO: 40 % TOC AO/EF: 55.1% TOC

TOC removal can be increased by Fenton.

[64]

f

Hybrid process

oo

Type

EF-AO-DSA: 64.3% TOC EF-AO-BDD: 97.1% TOC

The versatility of EF mainly depend upon the anode materials.

[65]

AO/PEF process can efficiently absorb photons and generate (h+)/(e–) pairs.

[66]

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.

[67]

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.

[68]

AO + Wetland

AO: 28% TOC AO + Wetland: 61% TOC

The toxic AO by-products were removed substantially in the vertical flow constructed wetland.

[69]

AO + MBR

AO: 55% of ibuprofen AO-MBR: 99% of ibuprofen

The high COD and nitrogen removal by MBR and pharmaceuticals removal by AO.

[70]

PEF: 53% of methyl orange AO/PEF: 71% of methyl orange

Pr

AO/PEF

al

Coupling process

epr

AO/EF

AO/US

Combined process—AO as pretreatment

Jo u

rn

AO: 60.6% ofloxacin AO/US: 95% ofloxacin

1

continued Hybrid process

Main results

Remarks

Ref

AO + Adsorption

Adsorption: 20% mecoprop AO + Adsorption: ~ 100% mecoprop

The AO would overcome some drawbacks as poor adsorption affinity of some compounds.

[71]

AO + UF

AO: 13%DCO AO + UF: 53% DOC

AO + Ozonation

AO + UV Combined process—AO as advanced treatment

ECO + AO

[72]

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.

[73]

AO: 84.71% TOC AO + UV : 96.25% TOC

The UV would decompose of electrogenerated oxidants into radical species, such as peroxide and peroxydisulfate.

[74]

ECO: 17.1% TOC ECO + AO: 84.6% TOC

AO would promote the mineralization of organic substances in wastewater.

[75]

ECO: 56% COD ECO + AO: 72% COD

ECO enhances the chemical coagulation process.

[76]

rn

al

Pr

epr

AO as a pretreatment stage can reduce membrane fouling and operation cost, and improve the water quality for water reuse applications.

Jo u

Combined process—AO as pretreatment

oo

f

Type

2

Declaration of interest statement

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There is no competing financial interest among the authors.