Electroactive Filter Technology for Water Treatment

CHAPTER 3

Electroactive Filter Technology for Water Treatment Yanbiao Liu1, 2, Shengnan Yang1, Fang Li1, 2, Chensi Shen1, 2, Jianshe Liu1, 2, Wolfgang Sand1, 3 1

Textile Pollution Controlling Engineering Center of Ministry of Environmental Protection, College of Environmental Science and Engineering, Donghua University, Shanghai, China; 2Shanghai Institute of Pollution Control and Ecological Security, Shanghai, China; 3Institute of Biosciences, Freiberg University of Mining and Technology, Freiberg Germany

1. Introduction Water purification is a critical security, environmental, and economic issue that requires immediate attention and innovative solutions. In particular, with the increasing global population and rapid industrialization, huge amounts of organic wastewater are discharged into water bodies, causing serious environmental pollution. Therefore, it is highly desirable to develop effective and affordable technologies to remove these pollutants from aqueous solutions. Among currently available technologies, electrochemical oxidation has emerged as a rapid, environmentally friendly, and promising solution to address this challenging issue. Compared with the widely applied biological processes that require significant concentrations of organic material, a relatively large working space, and a moderate or high working temperature, and still tend to be quite slow, the electrochemistry process usually takes only seconds or minutes to complete the reaction (Liu, 2015). Moreover, many industrial wastewaters have certain toxic and hazardous contents that render the biological processes ineffective (Ben et al., 2018). The electrochemical oxidation process also demonstrates enhanced oxidation power to degrade these toxic and refractory chemicals or microbes (Moreno-Andre´s et al., 2018). Numerous studies have demonstrated the ability of electrochemical processes to degrade refractory organic compounds by anodic oxidation via reactive radicals (e.g., hydroxyl radical, OH•), direct electron-transfer (DET) reactions, or both (Chaplin, 2014). Various advanced electrode materials have been developed and applied to electrochemical oxidation and the most common materials include doped-SnO2, PbO2 and doped-PbO2, Microbial Wastewater Treatment. https://doi.org/10.1016/B978-0-12-816809-7.00003-8 Copyright © 2019 Elsevier Inc. All rights reserved.

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44 Chapter 3 carbon-based materials, boron-doped diamond, and substoichiometric- and doped-TiO2 (Chaplin, 2014; Zhang et al., 2014). However, mass-transport-limited kinetics and high energy consumption are two key factors restricting the wide application of electrochemical technologies (Liu, 2015; Feng et al., 2016; Radjenovic and Sedlak, 2015). A typical electrochemical process employs two electrode plates and is operated in a flow-by mode. This flow configuration results in a relatively large hydrodynamic diffusive boundary layer (>100 mm), which is not favorable for the rapid reaction of organics with OH• due to diffusion limitations. Theoretical modeling and experimental studies suggest that this oxidation reaction only occurs within 1 mm of the anode surface (Donaghue and Chaplin, 2013). To address this limitation, efforts have been devoted to combining membrane filtration with an electrooxidation process that can be operated in an alternative flowthrough mode. In this case, the thickness of the bounder layer could be significantly minimized, resulting in advection-enhanced mass transport rates of organic molecules toward the active sites at the anode surface (e.g., compared with the flow-by mode or conventional batch mode). Another limiting factor of the high energy requirements result from the competitive oxygen evolution reaction, which is shown in Eq. (3.1):
2H2 O/O2 þ 4e þ 4Hþ E0 ¼ 1:03 V vs: Ag=AgCl

(3.1)

The oxygen evolution reaction significantly reduces the energy efficiency of the organic oxidation reaction. To address this issue, these electrode materials can be modified or designed to achieve a high overpotential toward oxygen evolution to suppress this competing reaction. Besides anodic oxidation of pollutants, an equally important but often overlooked aspect is the cathodic reduction reaction. Within an electrochemical filtration system, organic pollutants are oxidized at the anode and titanium or platinum plates/wires serve as countercathodes to provide the required electrical potential. However, the role of a cathode in electrochemical systems beyond a counterelectrode has not been thoroughly investigated. A cathode provides electrons and only supports reduction reaction, instead of oxidation, and, therefore, it cannot be directly used to oxidize organic compounds in wastewater. However, some cathodic materials are excellent catalysts of the oxygen reduction reaction (ORR). For example, A cathode composed of carbon nanotubes (CNT) can be used to reduce oxygen to generate hydrogen peroxide with the counter-CNT electrode serving as a functional cathode as in Eq. (3.2) (Liu et al., 2015): O2 þ 2Hþ þ 2e /H2 O2

(3.2)

As a relatively strong oxidant (E0 ¼ 1.76 V vs. SHE), H2O2 can directly oxidize various organic pollutants or react with transitional metal ions (e.g., [Cu2þ], [Fe2þ], and [Ce3þ]) to

Electroactive Filter Technology for Water Treatment 45 initiate the Fenton reaction to achieve degradation. The latter process belongs to electroFenton technology, which offers significant advantages over conventional Fenton processes (Nidheesh and Gandhimathi, 2012). For example, such process could produce H2O2 in situ via ORR and avoid the addition of hazardous and expensive H2O2 (He and Zhou, 2017). The electro-Fenton process is a recently developed Fenton process and has raised much interest for the treatment of refractory organic contaminants. This chapter focuses on the recent advances in wastewater treatment using continuous-flow electrochemical systems. Of the electrode materials reported, a specific focus is dedicated to carbon electrodes, as they are the most promising and most extensively studied electrodes. This chapter considers processes involving reactive radicals or DET at the anode surface and electro-Fenton process that occurs at the cathode surface. The working principle and performance of each system are briefly introduced.

2. Electroactive Filter Based on Anodic Oxidation Electrochemical oxidation technology has emerged as a promising process for the degradation of refractory organic compounds in aqueous solution. A high-performance electrode material is essential to this technology. To date, researchers have built several conductive anodic reactive electrochemical filter systems by integrating membrane separation processes with an electrochemical oxidation process. Several state-of-the-art designs based on the anodic oxidation reaction are introduced here.

2.1 Anodication Oxidation Based on Carbon Electrodes CNTs can be easily formed into porous 3D networks for contaminant sorption (SSA 30e500 m2/g) and electrochemical degradation (104e106 S/m). A hybrid electrochemical reactor and membrane filter using CNT as the anode was first proposed by Vecitis et al. in 2010 (Vecitis et al., 2011). The unique porous structure of CNT allows liquid to flow through the micrometer scale pores. The forced convective flow significantly enhances mass transport for adsorption and electrooxidation by bringing target molecules toward the active sites at the anode surface. This process combines separation and oxidation technology into one single unit and allows for in situ degradation of pollutants without producing concentrate that requires additional treatment. A schematic illustration of the electrochemical filtration apparatus is provided in Fig. 3.1. A commercial 47-mm Whatman polycarbonate filtration casing was modified to allow for simultaneous electrochemistry. Preliminary studies have demonstrated that electrochemical CNT filters are effective for the adsorption and electrooxidation of selected dyes, pharmaceutical and organic compounds, and inorganic ions. For example, electrochemical filtration of 0.2 mM tetracycline at a total cell potential of 2.5 V and a flow rate of 1.5 mL/min (hydraulic residence time <2 s) resulted in an oxidative flux of 0.03 mol/h/m/2 and >95%

46 Chapter 3

Figure 3.1 Depiction and images of the electrochemical filtration apparatus. (A) Design of the modified commercial polycarbonate filtration casing consisting of (1) a perforated stainless-steel cathode, (2) an insulating silicone rubber separator and seal, (3) a titanium anodic ring that is pressed into the carbon nanotube anodic filter, and (4) the CNT anodic filter supported by a PTFE membrane. (B, C) Images of the modified filtration casing. (D, E) Images of the CNT network before and after electrochemical filtration, respectively. Reproduced with permission from Vecitis et al. (2011a). Copyright 2011 American Chemical Chemistry.

tetracycline oxidation within a single pass (Liu et al., 2015). Another interesting finding from the same study indicated that the electrochemical filtration process can degrade the tetracycline molecular structure and significantly decrease its antimicrobial activity simultaneously, as confirmed by liquid chromatography-mass spectrometry and disk agar biocidal diffusion tests. The general electrochemical reaction mechanism includes three steps: (1) mass transfer to the electrode; (2) adsorption to and desorption from the electrode; (3) DET at the electrode surface (Vecitis et al., 2011a). However, the CNT anode has limited oxygen evolution potential (OEP), which may lead to low current efficiencies at higher anode potentials due to competition with anode corrosion and water oxidation. To address this problem, Vecitis et al. developed an electrosorption-hydrothermal method to load nanoscale SnO2 and doped-SnO2 particles onto the sidewalls of CNT (Liu et al., 2013). Electrochemical characterization of the anode materials confirmed that the stability of CNT filters at higher anode potential was improved with the OEP increasing from 1.25 V (vs. Ag/AgCl) of CNT to 1.85 V (vs. Ag/AgCl) of SnO2-CNT. The rapid oxidation kinetics and characteristics of CNT filters have shown that this technology could potentially serve as an effective wastewater treatment system to remove organic pollutants or as a point-of-use wastewater treatment system.

Electroactive Filter Technology for Water Treatment 47

2.2 Anodication Oxidation Based on TiO2 and Substoichiometric TiO2 Electrodes As one of the most well-known photocatalysts, TiO2 has been extensively studied for photocatalytic degradation of organic pollutants via direct oxidation and mediated production of OH• (Arif et al., 2017). Besides photoactivation, TiO2 can also be electrically activated once the applied electrical energy exceeds the band gap of TiO2. Yang et al. developed a nano-TiO2 functionalized electrocatalytic membrane reactor for oily wastewater treatment (Yang et al., 2012). The performance test indicate that the oil removal rate increases with decrease in the liquid hourly space velocity (LHSV) through the electrocatalytic reactor. During the treatment of 200 mg/L oily water, a complete chemical oxygen demand (COD) removal efficiency could be obtained at an LHSV of 7.2 h1. A similar flow-through design was recently reported by Zheng et al., who coated nano-TiO2 onto a SnO2eSb-loaded Ti mesh support (Fig. 3.2) (Zheng et al., 2018). At an applied total cell potential of 3.0 V, the removal rate of a selected refractory compound, pchloroaniline, was 2.4 times higher than that in conventional flow-by mode. The electrically activated reactive oxygen species, like O2  , H2O2, and OH•, were responsible for the superior electrocatalytic activity. Moreover, a synergistic effect was noted between the membrane filtration and the electrocatalytic oxidation.

Figure 3.2 Schematic representation of the reactive oxygen species-mediated mechanisms for oxidant generation on TiO2@SnO2eSb electrocatalytic filtration system. Reproduced with permission from Zheng, J., Wang, Z., Ma, J., Xu, S., Wu, Z., 2018. Development of an electrochemical ceramic membrane filtration system for efficient contaminant removal from waters. Environmental Science & Technology 52(7), 4117e4126. Copyright 2018 American Chemical Chemistry.

48 Chapter 3 Recently, the conductive Magneli phase suboxides or substoichiometric of titanium oxide (TinO2n-1, 4  n  10) have gained attention as a promising electrode material for water treatment (Arif et al., 2017). The electronic properties of substoichiometric titanium oxide are dramatically different from that of stoichiometric TiO2. The most conductive compound of the series is Ti4O7, which has an electrical conductivity of 166 U1 cm1, several orders of magnitude higher than TiO2 (w109 U1 cm1) (Walsh and Wills, 2010). Zaky and Chaplin designed a reactive electrochemical membrane using porous Ti4O7 as a functional anode and a stainless-steel rod as the cathode (Zaky & Chaplin, 2013, 2014). Oxidation experiments with model organic compounds showed that the as-developed reactive membrane was active for both direct oxidation reactions and the formation of OH•. Advection-enhanced mass transfer rates could also be observed. These results provide conclusive experimental and DFT modeling evidence that the dominant mechanism for p-substituted phenolic compound removal at a Ti4O7 anode is a function of both the electrode potential and the substituent type. At an anode potential of 1.7e1.8 V/SHE, p-nitrophenol and p-methoxyphenol were removed primarily by an electrochemical adsorption/polymerization mechanism on the reactive membrane surface. However, at an increased anode potential of 1.9e3.2 V/SHE, the electroassisted adsorption mechanism contributed far less to p-methoxyphenol removal than to removal of p-nitrophenol.

2.3 Anodication Oxidation Based on Ti/SnO2eSb Electrodes Due to the high cost of CNTs and the complexity of fabrication of the Ti4O7 membrane, researchers are still seeking alternative inexpensive and porous electrode materials. As a typical “non-active” electrode, the Sb-doped SnO2 electrode is low cost, easy to prepare, and has excellent catalytic activity as well as stability. In a recent study, Yang et al. developed a porous Ti/SnO2eSb filter and applied it for water treatment (Fig. 3.3; Yang et al., 2018). An improved convection-enhanced rate constant of 4.35  104 m/s could be obtained for FeðCNÞ6 4 oxidation. When challenging 50 mg/L rhodamine B solution, the reactive electrochemical filter system in flow-through mode resulted in an 8.6-fold enhancement in rhodamine B oxidation as compared to those in a flow-by mode under the same conditions. Moreover, the energy consumption was comparative to the state-of-theart electrochemical oxidation processes with energy consumptions in the range of 0.1e40 kW h/m3.

3. Electroactive Filter Based on Cathodic Reduction While oxidative degradation of organic pollutants at a positively charged anode has been extensively studied, there is also the possibility of degradation of pollutants at the negatively charged cathode. Although a cathode cannot support an oxidation reaction and, thus, cannot be applied to oxidize organic pollutants directly, some cathodic

Electroactive Filter Technology for Water Treatment 49

Figure 3.3 A schematic diagram of the experimental setup. (1) The design of the reactive electrochemical filter system consisting of two perforated stainless-steel pipes acting as water distributors, one of which is also used as the inner cathode, and a stainless-steel pipe acting as the outer cathode and a tubular porous Ti/SnO2eSb filter acting as the anode; (2) a photo of the reactive electrochemical filter system. Original drawing was done by us.

materials are excellent ORR catalysts. Under appropriate applied potentials, H2O2 (E0 ¼ 1.76 V vs. SHE) can be produced via a two-electron-transfer process that can directly oxidize certain organics or initiate an electro-Fenton reaction to generate the nonselective OH• (E0 ¼ 2.38 V vs. SHE). Although attempts to employ electricityassisted membrane fouling alleviation have been reported (Zhang et al., 2017), there is still very limited information on a flow-through system to oxidize organic pollutants. Recent advances of the electrochemical filter using carbon as functional cathodes have attracted attention for pollutants remediation. Liu et al. developed a novel wastewater treatment system by combining both adsorption and oxidation at the CNT anode and additional oxidation with in situegenerated hydrogen peroxide at the CNT cathode (Liu et al., 2015). As shown in Fig. 3.4, the CNT was used as both the anode and cathode materials and phenol was used as a model pollutant in this design. One interesting finding was that the H2O2 production rate was maximized at neutral pH. It is of note that acidic medium is preferred (e.g., pH w3.0) in most reported electro-Fenton systems as a basic pH adversely affects the overall efficiency by producing iron sludge precipitates (Radwan

50 Chapter 3 Influent

+ Silicon R ubber

Anode C NT Adsorption/Oxidation

PTFE Support

Cathode CNT O 2 Reduction

Current Collector

Reference Electrode

Effluent

Figure 3.4 Schematic of the electrochemical carbon nanotube filter coupled with in situ generated H2O2. Original drawing was done by us.

et al., 2018). It is highly desirable to achieve in situ generation of Fenton reagents at neutral pH, which avoids the drawback of having to regularly add acids to keep the iron species dissolved. This inconsistency of the optimal pH may be due to the variability of operational conditions and electrode materials, as well as the physicochemical characteristics of the target pollutants. The H2O2 will likely react with a phenol species that was anodically activated to a radical form (i.e., phenol electropolymerization), since the H2O2 alone cannot remove phenol efficiently. A stable phenol removal efficiency of 87.0  1.8% within 4 h of continuous operation could be achieved with an average oxidation rate of 0.06 mol/hr/m2. Scavenger tests showed that phenol oxidation was mainly due to H2O2. However, this is different from a previous report on phenolic compounds oxidation at a Ti4O7 reactive electrochemical membrane in which the produced OH• was the main contributor (Zaky and Chaplin, 2014). Another follow-up study by Gao et al. used the same device to achieve sequential electro-Fenton reactions (Gao et al., 2015). As shown in Fig. 3.5, the employed CNT membrane stack (with a total thickness of w200 mm) consisted of (1) a CNT network cathode for O2 reduction to H2O2; (2) a CNT-COOFe2þ cathode to chemically reduce H2O2 to OH• and HO and to regenerate ferrous ions in situ; (3) a porous PTFE insulating separator; and (4) a CNT filter anode for remaining intermediate oxidation. In this system, H2O2 can be produced in situ and the ferric ions can be easily reduced back to ferrous ions by the electrons at the cathode and so the electroregeneration of Fenton reagents can be accomplished. Moreover, the ferrous ions are chemically bonded onto the oxygen-containing functional groups of the CNT cathodes, thus preventing the loss of ferrous ions and ensuring long-term oxidative ability. Using oxalate as a model compound, a synergistic effect between electrochemistry and the conventional Fenton reaction was observed. The corresponding oxidation rate of oxalate by the sequential electro-Fenton process was 207 mgC/m2/h, which is fourfold greater than the sum of

Electroactive Filter Technology for Water Treatment 51

Figure 3.5 Sandwiched electro-Fenton system based on carbon nanotube membrane stacks. (A) Images of the unfolded sandwich membrane stacks including four layers, and (B) schematic of main roles of every layer in membrane stacks, and [P] and [P]m are pollutants and their oxidation intermediates, respectively. Reproduced with permission from Gao, G., Zhang, Q., Hao, Z., Vecitis, C. D., 2015. Carbon nanotube membrane stack for flow-through sequential regenerative electro-Fenton. Environmental Science & Technology 49(4), 2375e2383. Copyright 2015 American Chemical Chemistry.

the individual electrochemistry (16 mgC/m2/h) and Fenton (33 mgC/m2/h) reaction fluxes.


A CNT cathode could also activate peroxydisulfate to produce sulfate radicals SO4  . Compared to OH• produced in photocatalytic and Fenton-like processes, SO4  has a comparable or even higher oxidative potential (2.5e3.1 V for SO4  vs. 2.7 V for OH•) and can adapt to a wider pH range (Anipsitakis and Dionysiou, 2003; Liu et al., 2016). Nie et al. designed a flow-through electrochemical cell and used CNT as the cathode (Nie et al., 2018). Results show that a 98% phenol could be achieved over 3 h of continuous operation at an applied voltage of 0.6 V. A synergistic effect between aniline electrosorption and electrochemical activation of peroxydisulfate was confirmed in this system. This leads to a fast electron-transfer process between the organic compounds and the generated reactive species on the CNT cathode surface. Based on these observations, the authors proposed a nonradical oxidation mechanism for aniline removal. The overall results suggest that the removal of organic pollutants by SO4  produced from electrochemical activation is a promising, effective, and energy-saving process.

52 Chapter 3 Besides CNT, steel mesh is another promising conducting support for membranes. Zheng et al. employed a phase-inversion process to fabricate a conductive PVDF membrane onto a steel mesh support and applied this membrane as a functional cathode (Zheng et al., 2017). The as-developed electrochemical membrane reactor demonstrated an enhanced oxidation rate for sulphanilic acid in flow-through mode compared with that in flow-by mode. This can be explained by the better contact of contaminant molecules with oxidants generated at the membrane surface compared with that in flow-by mode. The sulphanilic acid degradation originated from the iron-mediated Fenton reactions, which were themselves initiated by cathodic reduction of oxygen and the formation of membrane surface-bound ferrous ions due to corrosion of the steel mesh under acidic conditions. Further experiments suggested that HO• is most likely responsible for sulphanilic acid removal in this system with minimal influence of higher valence iron species. The limited energy consumption and excellent degradation performance highlight the potential of cathodic electrochemical membrane systems to address the current water crisis.

4. Challenges and Future Perspectives Although the electroactive filter technology is a promising process for water purification, there are still several challenges that need to be addressed before they can be used in practical engineering applications. First, novel porous conductive electrode materials that are inexpensive to fabricate and are chemically stable during operation are required. Although the price of CNTs has decreased significantly compared with that of a decade ago due to advanced mass production technologies, their price is still much higher than activated carbon and other widely applied carbon materials. Also, in some cases, the target compounds cannot be directly mineralized to CO2 but produce byproducts or intermediates after the electrochemical treatment and the toxicity of some pharmaceutical byproducts was even higher than their mother compounds (Du et al., 2017). This should be taken into consideration when applying the technology, especially when dealing with wastewater containing halogen ions. To date, only limited information is available regarding the environmental impacts of the technology although this information is crucial. Second, most reports only employ laboratory- or bench-scale devices and only deal with synthetic wastewater containing one or few components. Their treatment performance under such conditions does not ensure a similar performance when dealing with industrial wastewater with a complex composition. For example, the ubiquitous presence of natural organic matter may decrease the performance dramatically. Membrane fouling is another key issue that needs to be solved before its large-scale application, especially when dealing with aromatic compounds (e.g., phenol), which have strong p-p interaction with the sp2-conjugated CNT sidewalls. These aromatic compounds cannot be effectively

Electroactive Filter Technology for Water Treatment 53 removed even by chemical washing. Therefore, alternative methods need to be developed to replenish the active sites on the membrane and to regenerate the membrane. The filtration performance in actual wastewater treatment plant effluent and the surface water matrix also requires study. Moreover, to ensure high flux and long durability for pilot-scale applications, other robust membranes (e.g., ceramic) could be used as mechanical support or directly used as flow-through membrane materials after certain chemical modifications. Third, these organic pollutants can also be seen as resources. In parallel with the anodic oxidation reaction, the electrons can be further utilized at the cathode for H2 production and/or CO2 reduction. Using smart filtration device design together with suitable porous electrode materials, a flow-through filtration system enabling anodic oxidation of pollutants together with cathodic energy production could be achieved. This is an interesting topic to explore in the future from economic and environmental perspectives. Further, the development of electroactive membranes with multiple functions also deserves more study. Besides the removal of organic compounds, the electrical field could also accelerate the adsorptive kinetics of compounds and other charged ions (Li et al., 2011), and reduce toxic heavy metal ions (e.g., Cr6þ) to a less-toxic form (Choi et al., 2017). By integrating a flow-through design, it can be envisaged that the performance could be further improved.

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