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Front. Environ. Sci. Eng. 2020, 14(5): 85 https://doi.org/10.1007/s11783-020-1264-7 REVIEW ARTICLE Recent advances in the electrochemical oxidation water treatment: Spotlight on byproduct control Yang Yang (✉) Department of Civil and Environmental Engineering, Clarkson University, Potsdam, NY 13699, USA HIGH LIGHTS GRAPHIC A BSTRA CT � Byproduct formation mechanisms during elec- trochemical oxidation water treatment. � Control byproduct formation by quenchers. � Process optimization to suppress byproduct formation. AR TICL E IN F O Article history: Received 16 March 2020 Revised 3 May 2020 Accepted 19 May 2020 Available online 30 June 2020 Keywords: ABSTRA CT Electrochemical water treatment Electrochemical oxidation (EO) is a promising technique for decentralized wastewater treatment, Byproducts owing to its modular design, high efﬁciency, and ease of automation and transportation. The catalytic Perchlorate destruction of recalcitrant, non-biodegradable pollutants (per- and poly-ﬂuoroalkyl substances (PFAS), pharmaceuticals, and personal care products (PPCPs), pesticides, etc.) is an appropriate niche for EO. EO can be more effective than homogeneous advanced oxidation processes for the degradation of recalcitrant chemicals inert to radical-mediated oxidation, because the potential of the anode can be made much higher than that of hydroxyl radicals (E = 2.7 V vs. NHE), forcing the direct transfer of OH electrons from pollutants to electrodes. Unfortunately, at such high anodic potential, chloride ions, which are ubiquitous in natural water systems, will be readily oxidized to chlorine and perchlorate. Perchlorate is a to-be-regulated byproduct, and chlorine can react with matrix organics to produce organic halogen compounds. In the past ten years, novel electrode materials and processes have been developed. However, spotlights were rarely focused on the control of byproduct formation during EO processes in a real-world context. When we use EO techniques to eliminate target contaminants with concentrations at μg/L-levels, byproducts at mg/L-levels might be produced. Is it a good trade-off? Is it possible to inhibit byproduct formation without compromising the performance of EO? In this mini- review, we will summarize the recent advances and provide perspectives to address the above questions. © The Author(s) 2020. This article is published with open access at link.springer.com and journal.hep. com.cn 2020 1 Introduction ✉ Corresponding author E-mail: firstname.lastname@example.org Electrochemical oxidation (EO) is a promising technology for the fast treatment of recalcitrant chemicals. It features a Special Issue—Accounts of Aquatic Chemistry and Technology compact and modular design. A typical EO reactor only Research (Responsible Editors: Jinyong Liu, Haoran Wei & Yin Wang) 2 Front. Environ. Sci. Eng. 2020, 14(5): 85 requires DC power supply and electrode array, and it is Agency (EPA) proposed a maximum contaminant level at usually operated in open air at room temperature. The 56 μg/L. Some US states have already regulated ClO in treatment capacity of EO processes is scalable, depending drinking water at lower levels (2 μg/L for Massachusetts on the numbers and areas of electrode arrays used. EO and 6 μg/L for California). Current experimental results exhibited excellent performance on the destruction of non- indicate that, once it is formed, the elimination of ClO is biodegradable pollutants (PFAS, PPCPs, pesticides, etc.) very challenging (Schaefer et al., 2007; Liu et al., 2016). (Carter and Farrell, 2008; Zhuo et al., 2012; Jasper et al., In most of the cases, free chlorine generated in EO 2016). For example, recent advances show that one-log treatment facilitates the removal of organic and enhances removal of typical PFAS (perﬂuorooctanesulfonic acid the inactivation of pathogens. However, it also reacts with (PFOS) and perﬂuorooctanoic acid (PFOA)) and PPCPs organics to form disinfection byproducts (DBPs). USEPA (carbamazepine, metoprolol, and ciproﬂoxacin) can be regulated ﬁve haloacetic acids (HAAs), including mono- achieved at energy consumptions less than 10 kWh/m chloroacetic acid (MCAA), dichloroacetic acid (DCAA), (Jasper et al., 2016; Le et al., 2019; Yang et al., 2019a). trichloroacetic acid (TCAA), monobromoacetic acid By tuning the anodic potential, various oxidants (OCl , (MBAA), and dibromoacetic acid (DBAA), with a O ,Cl$ , $OH, etc.) can be produced at the anodes (Fig. 1). combined concentration of < 60 mg/L in drinking water, EO is more efﬁcient than the homogeneous advanced and four trihalomethanes (THMs: chloroform, bromodi- oxidation processes (AOPs) on the degradation of chloromethane, dibromochloromethane, and bromoform) pollutants that barely react with radicals, because the with a maximum combined concentration of 80 mg/L potential of anodes can be made much higher than that of (USEPA, 2010). Nowadays, more than 600 DBPs were hydroxyl radicals (E = 2.7 V vs. NHE), forcing the identiﬁed (Richardson et al., 2007; Li and Mitch, 2018), OH direct transfer of electrons from target compounds to and some unregulated nitrogenous DBPs (nitrosamine, electrodes. For instance, perﬂuorooctanoate (C F COO ) haloacetonitriles, etc.) have orders of magnitude higher 7 15 is resistant to $OH oxidation. However, it can be readily toxic potencies than THMs and HAAs (Wagner and Plewa, destroyed by EO via direct electron transfer (DET) 2017). oxidation (Niu et al., 2013). In the area of electrochemical disinfection, balancing the The generation of oxidants requires high anodic acute risk caused by pathogens against the chronic potential. Unfortunately, at such high anodic potential, carcinogenic risk associated with DBPs is an endless chloride ions, which are ubiquitous in natural water debate. As for the treatment of recalcitrant chemical systems, will be readily oxidized to free chlorine (HClO/ contaminants and pathogen inactivation, it is clear that OCl ;pK 7.5) and then further oxidized to perchlorate DBPs (μg/L-level) and perchlorate (mg/L-level) are (ClO ). Perchlorate is an endocrine disruptor posing an produced during the removal of μg/L-level target com- adverse impact on thyroid gland function (Urbansky and pounds (or cells/L-level pathogens) in chloride bearing surface water and waste stream (Anglada et al., 2009, Schock, 1999). In 2019, the US Environmental Protection Fig. 1 Oxidation power of oxidants and direct electron transfer reactions characterized in the scale of redox potentials. The bottom frame shows the mechanism of indirect oxidation, in which target compounds react with reactive chlorine species (RCS) and reactive oxygen species (ROS) generated electrochemically. The top frame demonstrates the oxidation reactions based on the direct electron transfer mechanism. Yang Yang. Recent advances in byproduct control during electrochemical water treatment 3 2010; Bagastyo et al., 2013; Schaefer et al., 2015; Yang surface water, which could potentially be utilized down- et al., 2019a). These facts keep us wondering whether stream as a potable water supply. adopting EO treatment is a good trade-off. 3) The author has received review comments from Despite that breakthroughs were made on the develop- almost every article and proposal, requiring the evaluation ment of novel electrode materials, it is equally important to of DBPs formation in the EO processes, even though the work on the control of byproducts. This review primarily technique was proposed for wastewater treatment. These focuses on the control of THMs, HAAs, and ClO as they comments were raised due to the high expectations from are facing regulatory pressure. It is also important to note broad scientiﬁc and industrial communities on the that THMs and HAAs could serve as indicators of emerging EO technology. “The control of byproducts in exposure to the complex mixture of DBPs. The concentra- the EO reactions in all matrices.” The author believes this is a question of when, not if. tions of THMs and HAAs are usually proportional to the total organic halogen concentrations in treated efﬂuents (Pourmoghaddas and Stevens, 1995). Although this correlation has not been proved for the EO systems, we 2 Byproduct formation during EO treatment speculate that through the control of THMs, HAAs, and ClO , the overall reduction of other unknown halogen 2.1 Electrode materials byproducts can be achieved. It is also important to address that health concerns of The performance of EO and the byproduct generation rates DBPs and ClO were initially raised by the drinking water are largely dependent on the properties of electrodes. As treatment sector. EO is less likely to be applied to treat shown in Fig. 2(a), depending on the activity of water drinking water with low conductivity. In wastewater oxidation, electrodes can be categorized as active and treatment, the control of byproducts seems to be nonactive electrodes (Chaplin, 2014; Martínez-Huitle et al. unnecessary as the efﬂuents are not required to be treated 2015; Chaplin, 2019). The property of electrodes can be to a drinkable degree. However, the author believes it is identiﬁed by cyclic voltammetry (CV) analysis (Fig. 2(b)). still important to investigate the byproduct suppression In these tests, electrodes are immersed into the inert strategies in the EO treatment of wastewater, and in some electrolyte (e.g., Na SO or NaClO ). The anodic potential 2 4 4 case, drinking water due to the following reasons: is increased at a ﬁxed rate, and the response of current is recorded. Assume that all the electrons withdrawn from the 1) Some studies demonstrated that EO could be used to anode are only contributed by water oxidation (2H O ! treat the emerging contaminants and inactivate pathogens + – O + 4H + 4e ), the larger current recorded implies faster in drinking water and surface water at acceptable energy reaction rates. In the CV analysis of the active electrodes, consumption levels (less than 10 kWh/m ) (Martínez- the sharp increase of current usually occurs at 1.5 V . Huitle and Brillas, 2008; Schaefer et al., 2015; Yang et al., RHE These onset potentials are slightly higher than the 2019a). theoretical potential for water oxidation (1.23 V ). 2) Efﬂuents from the EO wastewater treatment units RHE containing (DBPs) will eventually be discharged to the Therefore, it is concluded that active electrodes have low Fig. 2 (a) Classiﬁcation and properties of active and non-active anodes. (b) Cyclic voltammetry analysis of electrodes in 30 mmol/L N SO electrolyte. IrO and blue TiO nanotube array (NTA) represent active and non-active electrodes, respectively. Data was collected 2 4 2 2‒x from reference (Yang and Hoffmann, 2016). 4 Front. Environ. Sci. Eng. 2020, 14(5): 85 overpotentials for oxygen evolution. Typical Active produce Cl (rxn. 2 and 3), which subsequently hydrolyze electrodes include RuO and IrO . As for nonactive to HOCl or OCl depending on pH. 2 2 electrodes, such as PbO , Sb-SnO , boron-doped diamond With longer electrolysis duration, free chlorine will 2 2 (BDD), and sub-stoichiometric TiO electrodes, their undergo a combination of DET oxidation (rxn. 4, 6, and 8) 2– x ‒ ‒ onset potentials usually are higher than 2 V . and $OH mediated oxidation to form ClO and ClO RHE 3 4 The different activities toward oxygen evolution stems (Bergmann et al., 2014; Hubler et al., 2014). from the surface chemistry of electrode materials. – – OCl ↕ ↓OCl⋅ þ e (4) Mechanisms are well explained in previous studies (Trasatti, 1984; P. Chaplin, 2014). During electrolysis, – þ OCl⋅ þ ⋅OH↕ ↓ClO þ H (5) $OH is produced from water as a surﬁcial intermediate. 2 The $OH strongly binds with the active electrode (M O), – – and is rapidly incorporated into its lattice structure to form ClO ↕ ↓ClO ⋅ þ e (6) 2 2 n+1 higher oxide (M O), then the reaction proceeds to produce O . As for non-active electrode, $OH loosely – þ ClO ⋅ þ ⋅OH↕ ↓ClO þ H (7) 2 3 binds with the electrode surface. The oxygen production is hence disfavored. – – ClO ↕ ↓ClO ⋅ þ e (8) 3 3 Assume that both active and non-active electrodes are operated at a constant current density of 10 mA/cm – þ ClO ⋅ þ ⋅OH↕ ↓ClO þ H (9) (Fig. 2(b)), the active electrode at 2 V can only produce 3 4 RHE chlorine (Cl /Cl 1.36 V ). The non-active electrode has 2 RHE The homogeneous reaction of chlorite (ClO ) and free a potential of 3.2 V , which is high enough to satisfy the RHE chlorine could produce ClO (Gordon and Tachiyashiki, thermodynamic criteria for the production of multiple 1991). However, the contribution of this step to ClO oxidants such as $OH, O , and H O (Fig. 1). 3 2 2 formation in EO treatment might be minimal, as the Since oxygen evolution shares similar mechanisms with chlorine balance analyses showed that the concentrations chlorine evolution (Krishtalik, 1981), active electrodes of ClO in the bulk electrolyte are negligible (Yang et al., have high efﬁciency for chlorine production. The free 2016, 2019a; Mostafa et al., 2018). Our studies on the EO chorine produced can efﬁciently inactivate pathogens and treatment of latrine wastewater (a mixture of feces and remove ammonium via breakpoint chlorination. As for urine) showed that at moderate current density (10 mA/ nonactive electrodes, they demonstrate superior perfor- cm ), IrO electrode was inert to, while BDD electrode was mance on the degradation of recalcitrant organic com- quite reactive to ClO generation (Yang et al., 2016; Yang pounds that are inert to free chlorine but reactive toward and Hoffmann, 2016; Jasper et al., 2017). Interestingly, a radicals. study focusing on EO treatment of urine reported that at a Electrolysis using active electrodes tends to produce 2 higher current density (20 mA/cm ), the IrO electrode was DBPs due to the buildup of free chlorine to react with able to produce ClO . When active electrode Pt/Ti was matrix organics in bulk solution (vide infra). In contrast, 2 ‒ operated at 270 mA/cm , the production of ClO was nonactive electrodes are oxidative enough to mineralize observed as well (Jung et al., 2010). These recent studies organic contaminants and destroy the as-formed DBPs break the stereotype that active anode cannot produce (Jasper et al., 2017), but their strong oxidation capability ClO . In fact, both active and nonactive electrodes can leads to the rapid oxidation of free chlorine to ClO . produce ClO when the thermodynamic criteria (e.g., anodic potential and current density) are met. The kinetics 2.2 Formation of perchlorate of the multi-step oxidation of chlorine oxyanions (rxn 4‒9) ‒ ‒ at the electrode/electrolyte interface determines how fast In the EO reaction, Cl is oxidized to free chlorine, ClO , ‒ the ClO can be produced. then to ClO . The electrochemical chlorine evolution follows the Volmer-Heyrovsky mechanism (Trasatti, 1987; ‒ 2.3 Formation of DBPs Consonni et al., 1987): Cl is ﬁrst adsorbed onto the anode surface to discharge one electron (rxn. 1). The DBP formation mechanisms in EO treatment were – – MnO þ Cl ↕ ↓MO ðCl⋅Þþ e (1) rarely investigated speciﬁcally. Therefore, mechanisms x x discovered in disinfection are revealing. THMs and HAAs – – result from reactions between free chlorine and natural MO ðCl⋅Þþ Cl ↕ ↓MO þ Cl þ e (2) x x 2 organic matters (NOMs) in the drinking water disinfection process (Bond et al., 2012). NOMs could be considered as 2MO ðCl⋅Þ↕ ↓2MO þ Cl (3) x x 2 surrogates for most of the organic contaminants of The adsorbed chlorine radical then combines with Cl in concern, as NOMs have multiple functional groups: the bulk solution or with neighboring surﬁcial Cl$ to carboxylic acid, enolic hydrogen, phenolic hydrogen, Yang Yang. Recent advances in byproduct control during electrochemical water treatment 5 quinine, alcoholic hydroxyl, ether, ketone, aldehyde, ester, because the former produces more free chlorine. However, lactone, amide and amine (Thurman, 2012). Free chlorine a few conﬂicting results were reported. Bagastyo et al. reacts with NOMs through oxidation, addition, and (2012, 2013) studied the formation of THMs and HAAs electrophilic substitution. In general, aromatic precursors during the electrolysis of RO concentrate using IrO and have THM and HAA formation potentials (2‒ BDD electrodes. In this study, the BDD electrode produces 1892 μg/mg C) two to three orders of magnitude higher more HAAs and THMs than IrO . The authors speculated than aliphatic precursors (Bond et al., 2012). High pH that BDD oxidized chloride to chlorine radicals (Cl$) and favors the formation of DBPs by promoting the hydrolysis dichloride radical anion (Cl $ ). The chlorine radicals of halogenated leaving group (Fig. 3). Long contact time might be able to react with organics to form DBPs. and high chlorine/organic ratio promote the formation of THMs and HAAs (Sérodes et al., 2003; Sun et al., 2009). Learning from the above, we know that both aliphatic 3 Control strategies and aromatic compounds, no matter they are target pollutants or matrix organics, can react with chlorine to 3.1 Fabricate anode materials produce DBPs. But the formation of DBPs is not that easy. It requires multiple steps (long contact time), appropriate Pralay et al. use perﬂuoro-decyl trichlorosilane to fabricate pH, and high chlorine dose. Therefore, it is possible to the surface of BDD with the goal of inhibiting DET steps control DBPs by adjusting the operational parameters and (rxn. 5, 7, and 9) via steric hindrance and hydrophobic reaction conditions of EO treatment (vide infra). effects (Gayen and Chaplin, 2017). The ﬂuorinated In homogeneous chlorination disinfection operation, polymeric coating prevents the direct contact between free chlorine at high concentration is spiked in water ClO and BDD surface but allows $OH to diffuse into the initially. As for EO treatment, chlorine is gradually bulk electrolyte. Another study reported that covering the generated. DBP formation is usually insigniﬁcant at the BDD surface with perﬂuorooctanoic acid (PFOA) could early-stage of electrolysis because free chlorine will be inhibit the formation of ClO (Jawando et al., 2015). This instantly consumed by excessive organics and ammonium. result implies that the EO process could produce less ClO Organics are not yet broken down to small molecule during the treatment of per- and poly-ﬂuorinated chemical precursors with abundant halogen leaving groups. Sharp waste at high concentrations. It is important to note that the above studies operated BDD at low current densities (ca. 1 increase of DBPs was reported when the majority of mA/cm ). At higher current densities, the stability of the organics was removed (>90% removal of COD) accom- surface coating will be a critical issue, as BDD electrodes panied by the rapid increase of free chlorine concentration can oxidize the poly-and perﬂuoroalkyl coatings. Our (Zöllig et al., 2015; Jasper et al., 2017). These results indicate that the DBP formation during electrolysis is still study found that signiﬁcant formation of ClO was still dominated by the homogeneous reactions in the bulk observed when BDD was operated at 10 mA/cm to treat solution. Most of the studies reported that active electrodes 10 mg/L PFOA in the presence of 3 mmol/L Cl (Yang tend to produce more DBPs than non-active electrodes et al., 2019a). Forming less ClO during treatment of high Fig. 3 Formation of trichloromethane and trichloroacetic acid in the chlorination of a) aliphatic carbohydrates and b) aromatic compounds (based on ref’s (Rook, 1977; Boyce and Hornig, 1983; Navalon et al., 2008; Bond et al., 2012)). Cleavage at sites A and B leads to the formation of trichloromethane (TCM) and dichloroacetic acid (DCAA), respectively. 6 Front. Environ. Sci. Eng. 2020, 14(5): 85 PFAS concentration may be important in the PFAS efﬁciency, the sluggish reduction kinetics could largely be remediation process. However, it cannot be applied as a assigned to the fact that DBPs and perchlorate are neutral general strategy to mitigate ClO formation. or negatively charged at circumneutral pH, making their Another option is to use alternative electrodes for BDD. adsorption on cathode difﬁcult. Recently Chaplin et al. Active electrodes such as Pt/Ti, IrO /Ti, and RuO /Ti have developed a carbon-Ti O reactive electrochemical mem- 2 2 4 7 lower ClO formation potentials, but they are also less brane (REM) cathode (Almassi et al., 2020). The REM was efﬁcient for organic oxidation. Recently, sub-stoichio- operated in a ﬂow-through mode, in which water was metric TiO NTA and Ti O phase anode were developed forced to pass through the microporous cathode. The ﬁrst- 2‒x 4 7 with comparable performance with BDD (Zaky and order rate constant for dibromoacetic acid reduction is 9.16 –1 Chaplin, 2013; Zaky and Chaplin, 2014; Yang and min , which is 57 to 1110 times higher than those obtained Hoffmann,2016;Yangetal.,2018).These TiO in batch mode (Mao et al., 2016, 2018). Changing the 2‒x electrodes produced less ClO than BDD (Yang and operational mode from ﬂow-by to ﬂow-through dramati- Hoffmann, 2016; Wang et al., 2020). Though the cally reduces the thickness of the diffusional boundary molecular-scale mechanism is still unclear, we suspected layer from ~100 μm for plate-type electrodes to ~1 μm for that $OH-mediated oxidation, rather than DET, is the major REM electrodes (Chaplin, 2019), leading to a more contaminant removal mechanism on TiO electrodes efﬁcient contact between target compounds and the 2‒x with abundant surﬁcial titanol groups (Kesselman et al., electrode surface. Given the above, it is promising to 1997; Bejan et al., 2012), while BDD has higher reactivity develop tandem REM anode + REM cathode modules to for DET mediated oxidation, which favors the ClO remove contaminants and eliminate byproducts simulta- formation. neously. 3.2 Enhance cathodic dehalogenation 3.3 Quench byproduct precursors Typical EO cells use stainless steel or Ti metal as cathodes, Introducing free chlorine quencher during electrolysis on which hydrogen evolution reaction (HER) occurs via could suppress the formation of DBPs and ClO . proton reduction. It is viable to replace the HER cathodes Hydrogen peroxide (H O ) is an ideal quencher because 2 2 with catalytic electrodes that can reductively remove it is effective, inexpensive, and widely used in site perchlorate and DBPs. Perchlorate can be electrochemi- remediation projects. H O readily reacts with free 2 2 cally reduced by Rh, Pt, Sn, Cu, and Ni (Horányi and chlorine to form Cl and H O (Connick, 1947). Our recent Bakos, 1992; Wasberg and Horányi, 1995; Wang et al., study (Yang et al., 2019a) found that adding 50 mmol/L 2007). Reactions can be described by the Langmuir- H O can effectively inhibit ClO formation in EO 2 2 4 Hinshelwood model: ClO is adsorbed on to cathode treatment of PFOS using BDD anode (Fig. 4). According surface, transfer one oxygen atom to the cathode, and then to rxn. 4‒9, ClO formation could be suppressed by react with neighboring reactive atomic hydrogen ($H), scavenging free chlorine and $OH. Through computational which is an intermediate of HER. Alloy could have a kinetic modeling, it is found that although H O reacts 2 2 – 4 higher ClO reduction activity. Ni-Pt and Co-Pt out- with free chlorine more slowly than with $OH (~10 vs. 7 –1 –1 performed pure Pt due to the enhanced production of $H ~10 M s ), the former reaction contributes the most to (Rusanova et al., 2006; Mahmudov et al., 2008). However, ClO inhibition. long electrolysis duration (t =4‒5 h), acidic media, and H O can be in situ generated by cathode via oxygen 1/2 2 2 + – elevated temperature (>50 °C) are required for electro- reduction reaction (O + 2H + 2e ! H O ). For 2 2 2 – ‒ chemical ClO reduction. The sluggish cathodic ClO example, in the electro-Fenton process, H O is generated 4 4 2 2 2+ reduction kinetics is incomparable to the fast formation at carbonaceous cathodes and then react with Fe to rate on the anode. produce $OH as an oxidant (Brillas et al., 2009). – – Cathode materials like Fe, Pd-Fe, Pd, Cu, graphene, and Interestingly, in the electro-Fenton process, ClO , ClO 3 4 graphite exhibit high activity for THM and HAA reduction and THMs were not detected (Cotillas et al., 2015). Note (Li and Farrell, 2000; Radjenović et al., 2012; Mao et al., that this study used low current densities (0.12- 2016; Mao et al., 2018). The electrochemical reductive 2.5 mA/cm ). Thus, the intrinsic formation rates of DBPs ‒ – removal of THMs and HAAs is faster than ClO reduction and ClO might be small. Electro-peroxone process is 4 4 as THMs and HAAs are directly reduced by $H, and no another EO technique that involves cathodic H O 2 2 oxygen transfer step is involved. Complete removal HAAs production (Yuan et al., 2013). In this process, O and could be achieved within an hour. The removal of THMs is O are purged to the electrolyte simultaneously. Oxygen even faster due to the volatilization effect. will be reduced to H O and then react with O to produce 2 2 3 To the best of our knowledge, there is no study showing radicals (rxn. 10). that cathodic dehalogenation can completely eliminate the DBPs and perchlorate produced by anode to achieve the – þ H O þ O ↕ ↓⋅OH þ ⋅O þ H þ O (10) 2 2 3 2 2 zero-DBP discharge. Aside from the low $H production Yang Yang. Recent advances in byproduct control during electrochemical water treatment 7 Fig. 4 Degradation of (a) PFOS (10 mg/L) and (b) benzoic acid (1 mmol/L), and (c) the formation of ClO during electrolysis in 15 mmol/L Na SO + 3 mmol/L NaCl electrolytes in the presence or absence of H O (50 mmol/L). BDD anode was coupled with stainless 2 4 2 2 steel cathode and operated at 10 mA/cm . Data was collected from reference (Yang et al., 2019a). þ – The combination of BDD based EO and cathodic H O 2 2 Cl⋅ þ H O ↕ ↓HO ⋅ þ H þ Cl 2 2 2 production produced 50% less [ClO ] than EO alone. However, when O was introduced, efﬂuent [ClO ] raised 3 4 9 – 1 – 1 k ¼ 2 10 M s (12) up due to the depletion of H O (Lin et al., 2016). A recent 2 2 study of (Yao et al., 2019) shows that H O could suppress Ammonium is another common chlorine scavenger. It 2 2 the formation of TCM and HAAs during the electro- reacts with free chlorine to form chloramines. In the peroxone treatment of chloride-containing water. These chlorination disinfection process, ammonium/ammonia is results imply that H O can inhibit the reactions between intentionally introduced to produce chloramine as a 2 2 chlorine and organic precursors in the bulk solution. disinfectant with less formation potentials of THMs and It is important to note that the quenching effect of H O HAAs (Qi et al., 2004; Hong et al., 2013). The same 2 2 may be valid within a current density window. Studies strategy can be adopted in EO treatment. In the EO + – showed that H O could suppress ClO formation when treatment of wastewater containing NH and Cl (e.g., 2 2 4 the BDD anode was operated at current densities equal or latrine wastewater), chloramines are the dominant reactive below 10 mA/cm (Lin et al., 2016; Yang et al., 2019a). chlorine species before breakpoint chlorination (Yang However, ClO cannot be suppressed by H O when et al., 2016; Jasper et al., 2017). Negligible ClO , THM, 4 2 2 current densities are above 16 mA/cm (Lin et al., 2016). It and HAA formation was observed when chloramines seems that 10 mA/cm is a critical point beyond which the prevailed (Jasper et al., 2017; Zhang et al., 2018). The high beneﬁtofH O addition could be attenuated. concentration of NH (ca. 30 mmol/L) in the latrine 2 2 The addition of H O could incur different impacts on 2 2 wastewater enables the efﬁcient quenching of free the EO treatment efﬁciency, depending on the properties of chlorine. For the EO treatment of wastewater with less + + target pollutants. As shown in Fig. 4, the degradation of abundant NH , whether NH should be added inten- 4 4 PFOS is primarily contributed by DET oxidation. It is tionally should be evaluated on a case-by-case basis. found that H O barely affects PFOS removal due to its 2 2 weak afﬁnity to the BDD surface. However, the inhibitory 3.4 Optimize operational parameters effect of H O is observed in the EO treatment of benzoic 2 2 acid- a probe readily reacts with $OH. These results imply Acidic pH disfavors the formation of THMs and HAAs in that if the target compounds are primarily removed via EO treatment (Bagastyo et al., 2012; Yang et al., 2019b), radical-mediated oxidation, then the addition of H O likely through the inhibition of hydrolytic conversion of 2 2 could be detrimental due to the radical quenching effect precursors to THMs and HAAs (Fig. 3) (Chen, 2011). The (rxn. 11 and 12) (Buxton et al., 1988; Yu, 2004). acidiﬁcation of water can be readily realized without acid addition in a membrane electrolysis cell (Yang et al., ⋅OH þ H O ↕ ↓HO ⋅ þ H O 2 2 2 2 2019b). In this cell conﬁguration, the anode is separated from cathode by an ion-exchange membrane. Wastewater 7 – 1 – 1 k ¼ 2:7 10 M s (11) in the anodic chamber will be rapidly acidiﬁed to pH < 2, 8 Front. Environ. Sci. Eng. 2020, 14(5): 85 while electrolyte in the cathodic chamber will be alkalized However, electrodialysis reduces the overall conductivity to pH>13. Membrane assisted electrolysis was found to of water, leading to higher energy consumption of EO accelerate COD and ammonium removal and produce less treatment. Instead of removing chloride, transferring target THMs and HAAs than membrane-free electrolysis (Yang compounds from wastewater to electrolytes with con- et al., 2019b). The acidic efﬂuent can be neutralized by trolled composition might be more practical. For instance, passing through the cathodic chamber for safe discharge. PFAS in water can be adsorbed by anion exchange resins. Attention should be paid to the control of reaction It is a typical operation to regenerate the spent resins by endpoints. That means overtreatment should be avoided. NaCl solutions (Yu et al., 2009; Deng et al., 2010). The Take latrine wastewater treatment for example. The eluent is composed of high concentration Cl and PFAS. complete removal of COD and NH requires an One can envision that the EO treatment of PFAS in the electrolysis duration of 2-4 h (Yang et al., 2016; Yang resin regenerant could produce considerable amounts of and Hoffmann, 2016). Under the same condition, the byproducts. Alternatively, if resins are regenerated by pathogen disinfection (5 log removal of E. coli) and the Na SO solution, then the byproduct formation can be 2 4 removal of trace pharmaceuticals can be readily achieved minimized. (Liang et al., 2018) within 1 h (Huang et al., 2016; Jasper et al., 2016). If disinfection is the primary design goal, then the EO 3.6 Feasibility analysis treatment should be terminated at 1 h to avoid reaching break-point chlorination. Jasper et al. (2017) found that The section aims to address the advantages and limitations when breakpoint chlorination occurs, the concentrations of of the above control strategies in views of cost-effective- free chlorine rapidly increased, and such transition is ness and system complexity. From high to low, the reﬂected as the sharp increase of oxidation-reduction feasibility of strategies (by section number) is ranked in potential (ORP). From an engineering point of view, the order of 3.3>3.4>3.2>3.5>3.1. The addition of optimum reaction endpoints can be controlled by sensors quenchers (3.3) and the control of reaction endpoints (ORP probe, NH selective electrode, free chlorine online (3.4) do not require the modiﬁcation of EO units. detector, etc.). Byproduct suppression can be obtained instantly. The addition of H O will not affect the performance of DET 2 2 3.5 Process integration oxidation of some compounds. Thus, the energy consump- tion of such EO system will not be increased. However, it Perchlorate is resistant to homogenous reduction reactions. will be increased for the removal of compounds relying on At neutral pH, it even barely reacts with the hydrated radical-mediated oxidation, as extended treatment duration electron, the strongest reductant known (Vellanki et al., will be required to offset the radical-scavenging effect of 2013). Fortunately, perchlorate can be reduced by H O . Attention should also be paid to the elimination of 2 2 biological reactions. Microorganisms collected from residual H O before distribution (Barazesh et al., 2015). 2 2 saturated aquifer could gain the ability of ClO reduction Anodic oxidation reactions can readily convert H O to 4 2 2 after 20 days of inoculation using emulsify oil as an oxygen (E = -0.682 V ). Additional EO post-treatment NHE electron donor (Schaefer et al., 2007). Schaefer et al. can be deployed, in which active electrodes should be used (2017) used sand columns bioaugmented with perchlorate- and operated at a low anodic potential (e.g., E < E – : Cl =Cl degrading bacterium Azospira suillum to treat the efﬂuent 1.36 V ) to avoid the re-formation of chlorine and NHE of the EO process. It was found that after 150 days of byproducts. Signiﬁcant system modiﬁcation is required to inoculation, sand columns achieved three order-of-magni- implement the strategy of cathodic dehalogenation (3.2) tude removal of ClO at a 9-day residence time. Although and process integration (3.5). Cathodic dehalogenation Biological processes have low capital and operational requires the change of HER active cathode to dehalogena- costs, the long residence time limits its treatment capacity. tion catalysts. This modiﬁcation will not affect energy Recently, a series of bio-inspired Rhenium (Re) complex consumption as the water treatment efﬁciency is still catalysts with high ClO reduction efﬁciency were determined by the anodic reactions. Though both treatment developed. (Liu et al., 2015, 2016) The structures of Re efﬁciency and efﬂuent concentration can be well controlled complex catalysts are designed to mimic the structure of by process integration, this strategy deﬁnitely increases the molybdopterin in bacterial reductase. Using H gas as an complexity of the treatment train, which will increase electron donor, a complete reduction of perchlorate can be capital and operational costs. More studies are required to achieved within 2 h. These breakthroughs enable the investigate the optimum technology combination and the catalytic hydrogenation techniques to serve as post- seamless connection between units. The modiﬁcation of treatment processes after EO treatment. anode material is identiﬁed as a strategy with low Electrodialysis was used to removed chloride from feasibility. Because DBPs are primarily derived from wastewater and consequently reduced byproduct formation redox reactions in the bulk solution. The surface coatings in the downstream EO treatment (Bagastyo et al., 2013). for the suppression of DET are not stable. Yang Yang. Recent advances in byproduct control during electrochemical water treatment 9 2015). The GDE cell conﬁguration enables the energy- efﬁcient production of H O at low costs because 2 2 pressurized gas sources (pump, cylinder, etc.) are no longer required. Recent progress demonstrated that the air- GDE pair could produce H O ranges from 2 to 2 2 300 mmol/L (0.0068 wt.%–1 wt.%) (Luo et al., 2015; Barazesh et al., 2015, 2018), which are sufﬁciently higher than the effective 50 mmol/L concentration to inhibit perchlorate and DBP formation. Therefore, coupling air- Fig. 5 Transformation of Cl to chlorine radicals and free GDE with novel anode materials is an exciting direction to chlorine during EO treatment. The ﬁgure was modiﬁed from ref (Yang et al. 2016). advance EO technology. In this article, we place the spotlight on THMs and HAAs, which are only the tip of the DBP iceberg. We introduced that NH could suppress the formation of 4 Future outlook THMs and HAAs by converting free chlorine to Previously, a considerable amount of studies focusing on chloramine. However, chloramine might react with matrix electrode material development preferred to evaluate the organics to form N-nitrosodimethylamine (NDMA), a electrode performance in inert electrolytes (Na SO , carcinogenic chemical (McCurry et al., 2017; Selbes et al., 2 4 NaClO Na HPO , etc.). In future studies, it is critical to 2018). Only a handful of studies reported the electro- 4, 2 4 include the tests performed in chloride-bearing solutions, chemical treatment of NDMA (Chaplin et al., 2009; which is more environmentally relevant. In this case, the Almassi et al., 2019). So far, no study reported the radical chemistry needs to be revisited. According to the formation of NDMA in the electrochemical treatment of + – reaction chains in Fig. 5, Electrodes that demonstrated high wastewater containing both NH and Cl . The report on activity for $OH production in inert electrolytes might end the formation and transformation of other nitrogen-based – – up producing Cl $ and Cl$ in real wastewater and surface and iodinated DBPs is scarce as well. More importantly, water containing chloride (Park et al., 2009; Yang et al., the removal of detectable byproducts does not indicate all – – 2016). Because Cl $ and Cl$ have lower redox potentials toxic substances are removed. Given above, the author than $OH, the EO treatment efﬁciencies of compounds believes that challenges and opportunities down the road only react with $OH (e.g., nitrobenzene) could be include: 1) develop advanced electrocatalysts and identify compromised. One must also note that chloride will be best practice to balance treatment efﬁciency and byproduct converted to free chlorine at concentrations serval orders of formation; 2) close the mass balance of total organic magnitude higher than all of the radicals. (Yang et al., halogen formed in EO treatment by advanced mass- 2016) Though it is even less oxidative than chlorine spectrometric techniques; 3) evaluate the toxicity of the EO treated efﬂuents. radicals, free chlorine can signiﬁcantly enhance the removal of some organics (e.g., phenol, salicylic acid) Acknowledgements This work is supported by the Bill and Melinda Gates that are vulnerable to chlorination (Park et al., 2009; Cho Foundation (BMGF INV-003227). et al., 2014). We have demonstrated that H O at a concentration 2 2 Open Access This article is licensed under a Creative Commons Attribution higher than 50 mmol/L can effectively control byproduct 4.0 International License, which permits use, sharing, adaptation, distribution formation during the EO treatment of PFAS contaminated and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative groundwater (Yang et al., 2019a). However, the addition of Commons licence, and indicate if changes were made. The images or other H O obviously deviate from the original design principle 2 2 third party material in this article are included in the article’s Creative of EO as “chemical-free” processes. The addition of 50 Commons licence, unless indicated otherwise in a credit line to the material. ‒3 mmol/L H O could incur $ 0.6‒1.2 m extra costs to the 2 2 If material is not included in the article’s Creative Commons licence and your EO treatment (Yang et al., 2019a). Thus, the in situ intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To generation of H O is desired. With this in mind, 2 2 view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. improvement of EO can be made by combining anodes with carbonaceous cathodes to in situ generate H O . 2 2 Using pure oxygen, up to 20 wt.% H O can be generated 2 2 References electrochemically (Xia et al., 2019). The electro-Fenton or electro-peroxone reactions introduced above all used pure Almassi S, Li Z, Xu W, Pu C, Zeng T, Chaplin B P (2019). 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Before joining Yang Y, Hoffmann M R (2016). Synthesis and stabilization of blue-black Clarkson University in 2019, he was a TiO nanotube arrays for electrochemical oxidant generation and postdoc and a senior research scientist at wastewater treatment. Environmental Science & Technology, 50(21): Caltech. His research interests have 11888–11894 spanned the subject areas of electrochem- Yang Y, Kao L C, Liu Y, Sun K, Yu H, Guo J, Liou S Y H, Hoffmann M istry, water chemistry, and heterogeneous R (2018). Cobalt-doped black TiO nanotube array as a stable anode catalysis. for oxygen evolution and electrochemical wastewater treatment. ACS
Frontiers of Environmental Science & Engineering – Springer Journals
Published: Oct 1, 2020
Keywords: Electrochemical water treatment; Byproducts; Perchlorate
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