Development of an Electrochemical Ceramic Membrane Filtration

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Environmental Processes

Development of an Electrochemical Ceramic Membrane Filtration System for Efficient Contaminant Removal from Waters Junjian Zheng, Zhiwei Wang, Jinxing Ma, Shaoping Xu, and Zhichao Wu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06407 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Environmental Science & Technology

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Development of an Electrochemical Ceramic Membrane Filtration System

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for Efficient Contaminant Removal from Waters

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Junjian Zheng,† Zhiwei Wang,*, †, § Jinxing Ma, ‡ Shaoping Xu,† Zhichao Wu†

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†

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Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China

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‡

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of New South Wales, Sydney, NSW 2052, Australia

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§

State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental

UNSW Water Research Centre, School of Civil and Environmental Engineering, University

Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China

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Revised Manuscript for Environmental Science & Technology (Clean version)

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February 23, 2018

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ABSTRACT: Inability to remove low molecular weight anthropogenic contaminants is a

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critical issue in low-pressure membrane filtration processes for water treatment. In this work,

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a novel electrochemical ceramic membrane filtration (ECMF) system using TiO2@SnO2-Sb

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anode was developed for removing persistent p-chloroaniline (PCA). Results showed that the

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ECMF system achieved efficient removal of PCA from contaminated waters. At a charging

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voltage of 3 V, the PCA removal rate of TiO2@SnO2-Sb ECMF system under flow-through

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mode was 2.4 times that of flow-by mode. The energy consumption for 50% of PCA removal

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for TiO2@SnO2-Sb ECMF at 3 V under flow-through mode was 0.38 Wh/L, much lower than

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that of flow-by operation (1.5 Wh/L), which was attributed to the improved utilization of the

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surface adsorbed HO• and dissociated HO• driven by the enhanced mass transfer of PCA

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towards the anode surface. Benefited from the increased production of reactive oxygen

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species such as O2•−, H2O2 and HO• arising from excitation of anatase TiO2, TiO2@SnO2-Sb

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ECMF exhibited a superior electrocatalytic activity to the SnO2-Sb ECMF system. The

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degradation pathways of PCA initiated by OH• attack were further proposed, with the

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biodegradable short-chain carboxylic acids (mainly formic, acetic and oxalic acids) identified

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as the dominant oxidized products. These results highlight the potential of the ECMF system

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for cost-effective water purification.

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INTRODUCTION

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Low-pressure membrane filtration (e.g., microfiltration (MF) and ultrafiltration (UF)) has

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become one of the dominant technologies for water/wastewater treatment in last decades.1-4

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Membranes made of porous water-permeable polymeric and ceramic matrices are commonly

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used in these processes,5,6 allowing continuous separation of aquatic contaminants through

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size-exclusion effects. The advantages for low-pressure membrane filtration include

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high-quality effluent, sound pathogen removal, small footprint, and relatively low life-cycle

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costs.1,2,5,6 Nevertheless, critical challenges remain with regard to conventional MF/UF

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filtration incapable of eliminating low molecular weight anthropogenic contaminants (e.g.,

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toxic and/or refractory organic micropollutants),2,7 which, if present, can either deposit

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on/inside membrane matrices causing the loss of membrane flux2,4 and/or pass through the

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membranes leading to their occurrence in receiving waters.8 Therefore, integration with other

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robust technologies is of great importance for low-pressure membrane filtration in order to

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mitigate the associated environmental concerns due to the inefficient removal of low

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molecular weight contaminants.

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Of the alternatives, electrochemical advanced oxidation process (EAOP) has gained

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popularity for its amenability to automation, low cost and no secondary pollution,9,10

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prompting the integration of EAOP into membrane filtration in recent studies; for instance,

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electrochemical membranes have been developed to serve as both electrode and filter media

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when subject to external electric field.7,11-17 It has been reported that the use of

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electrochemical membranes under cathodic polarization can degrade aquatic contaminants,

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which is largely attributed to the interaction between H2O2 generated at cathode surface and 3 ACS Paragon Plus Environment


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the interfacial Fe(II) species producing strong oxidants (e.g., HO•).7,14 However, these

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Fenton-based processes might produce large amount of chemical sludge and block the

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membrane matrices, thus limiting their full-scale applications.18

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In contrast, anodic oxidation techniques are becoming more preferable to be integrated

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into membrane filtration processes because this clean approach allows, in many cases, a full

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mineralization of contaminants with no need of chemical additives.16,19 It is worth noting that

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the efficiency of anodic oxidation strongly depends on the properties of electrodes,9,20

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highlighting the importance of selecting proper electrocatalytic materials. According to a few

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previous studies, anodic filters including Ti4O7 membranes,11,12 carbon membranes,15,16 and

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carbon nanotubes17,21 are capable of degrading recalcitrant compounds via direct oxidation

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and mediated production of HO•. However, these filters suffer from drawbacks such as

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rigorous preparation conditions and/or poor electrochemical stability (i.e., electrode corrosion

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by oxidants). Dimensionally stable anode (DSA) is a potential alternative material for

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electrochemical membrane filtration for its cost-effectiveness, high oxygen evolution

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potential and good electrocatalytic activity.22,23 In particular, titanium oxide could be

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co-doped onto the anode surface as catalysts due to its special energy band structure.24

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In this work, a novel electrochemical ceramic membrane filtration (ECMF) system was

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developed using TiO2@SnO2-Sb DSA, and the performance of the ECMF in degradation of

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p-chloroaniline (PCA) was evaluated. PCA was chosen as the model pollutant because of its

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wide-spread use in the production of plastics, pigments, cosmetics, agricultural chemicals and

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pharmaceuticals.25,26 Due to its low biodegradability and high persistence in environment,27


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PCA has been listed as a priority pollutant by US EPA and EU legislation.25,26 Key questions

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addressed in this study include: (i) how does the performance (e.g., PCA removal) correlate

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with the physicochemical properties of anodes and operating conditions of the system? (ii)

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which scenario is likely prevailing in the generation of oxidants responsible for PCA

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degradation under anodic polarization? and (iii) what is the detailed oxidation pathway of

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PCA?

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MATERIALS AND METHODS

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Reagents. All chemicals used were of reagent grade unless stated otherwise.

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p-chloroaniline (PCA), piperazine-N,N′-bis(ethanesulfonic acid) (PIPES) and sodium sulfite

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were purchased from Sigma-Aldrich (U.S.). Citric acid, ethylene glycol, SnCl4·5H2O, SbCl3,

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ethanol and tetrabutyl titanate were supplied by Aladdin (China). HPLC-grade methanol,

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acetonitrile, pentanol and phosphate acid were obtained from Sinopharm (China). The porous

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ceramic membrane was purchased from ItN-Nanovation AG (Germany). Milli-Q water (18.2

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MΩ/cm) was used for preparing all solutions. Solution pH was adjusted using either 0.1 M

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H2SO4 or 0.1 M NaOH when necessary.

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Fabrication of the Electrochemical Membrane Module. Rectangular titanium meshes

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(Hebei-Anheng, China) with a mean pore size of 170 µm and dimension of 5 cm×8 cm were

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used as the substrates. The raw titanium meshes were first mechanically polished with

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abrasive papers, degreased in 5 wt% NaOH solution at 90°C for 1 h, etched in boiling oxalic

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acid (10 wt%) for 0.5 h, rinsed with twice-distilled water and dried. SnO2-Sb coated Ti-mesh



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was prepared according to the sol-gel method.23 TiO2@SnO2-Sb electrode was then

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fabricated by dip-coating TiO2 onto the modified Ti-mesh (Scheme 1a). The details for the

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preparation of SnO2-Sb and TiO2@SnO2-Sb/TiO2 coated Ti-meshes are documented in SI

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Section S1.

108 109

Scheme 1. Schematic representation of (a) the fabrication of SnO2-Sb and TiO2@SnO2-Sb

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coated titanium meshes, assembly of (b) the composite anodic membrane and (c) built-in

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electrochemical ceramic membrane filtration (ECMF) module. The surface morphology of

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ceramic membrane and Ti-mesh is shown in Figure S1.

113 114

The ECMF membrane was prepared using epoxy resin adhesive to assemble the edges

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of ceramic membrane (pore size=0.4 µm; dimension=5×8 cm) onto the TiO2@SnO2-Sb

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coated Ti-mesh (termed TiO2@SnO2-Sb ECMF) (Scheme 1b). The SnO2-Sb ECMF

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membrane was also prepared for comparison. The ECMF membranes could be used as not

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only anode but also separation filter. For fabricating the ECMF module, two composite 6 ACS Paragon Plus Environment

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anodic membranes were placed at both sides of a PVC-bracket whilst a pristine titanium

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mesh was installed in between to serve as the cathode (Scheme 1c). The anodes and cathode

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were connected to a DC-power supply (CHI1030C-Jiecheng, China) via titanium wires with

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the distances between them set at 1 cm.28 Surface morphology and/or physicochemical

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properties of the raw ceramic membrane, pristine titanium mesh, used titanium mesh (after

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~500-h testing) and prepared electrodes were characterized according to the protocols

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documented in Section S2. Testing procedure for separation performance and anti-fouling

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behaviors of the ECMF membranes is provided in Section S3.

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Experimental Setup. Performance of the ECMF module was investigated in an aeration

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reactor made of plexiglass (for details, see Figure S2) with a diffuser installed at the bottom

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for air or nitrogen supply (flow rate=300 mL/min). The ECMF module was placed in the

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middle of the reactor. In all conditions, the reactor was operated via thermostatic control (to

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maintain the solution temperature at 25±1°C) by a thermostatic water bath. Electrochemical

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degradation of 10 µM PCA was performed in a 250-mL solution containing 50 mM Na2SO429

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(and, in some cases, reactive oxygen species (ROS) scavengers were added). In view of the

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circumneutral pH and buffer capacity of real water/wastewater, solution pH was adjusted to 7

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and controlled by adding 1 mM PIPES buffer. Prior to all experiments, the solution was

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purged with the selected gas for 30 min to reach air or nitrogen saturation. Recent studies

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have shown that the use of constant voltages (0.2~5.0 V) on electrochemical membranes

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enables their successful integration into biological processes for membrane fouling

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control,30-32 implying a luminous practical application prospect. Therefore, constant voltages



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(1.0~5.0 V) were herein applied to the ECMF module using the power supply, for the

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clarification of PCA removal behaviors. All experiments were performed at least twice in the

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dark.

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Two operation modes were tested in this study; i.e., for flow-by mode, experiments were

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conducted by turning off the influent and effluent peristaltic pumps (Figure S2) while in

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flow-through mode the wastewater was continuously fed into the reactor and pumped out

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through the membrane module at a constant flow rate (i.e., membrane fluxes at 11.6~138.9

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L/(m2 h) resulting in a hydraulic retention time (HRT) of 360~30 min (calculated according

148

to Eq. S1 in Section S4), respectively, comparable to the operating time used in flow-by

149

mode). The electro-generated ROS was quantified without adding PCA. Scavenging

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experiments were conducted in flow-through mode by dosing excess scavengers (i.e., 200

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mM isopropanol (i-PrOH)33 and 20 mM 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy

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(TEMPOL)34) to investigate the roles of different ROS such as hydroxyl radicals (HO•),

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hydrogen peroxides (H2O2) and superoxide anions (O2•−) in PCA degradation. i-PrOH can

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readily react with HO• (1.9×109 M−1 s−1)35 and has a proven capacity for dissociated HO•

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quenching due to its low affinity for semiconductor surfaces in the aqueous media34-37 while

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TEMPOL was used to quench O2•−.34 In this study, nitrogen sparging was applied to inhibit

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the generation of O2•− and H2O2 on the electrode surface.7 Samples collected from the reactor

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(flow-by mode), and inlet and outlet (flow-through mode) were determined immediately after

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filtration using 0.45-µm nylon syringe-filters.

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Analytical Methods. Identification and/or quantification of PCA and its aromatic


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intermediates were performed by reversed-phase high-performance liquid chromatography

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(HPLC, Agilent-1200), and the generated carboxylic acids were measured by ion-exclusion

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chromatography (IEC) and gas chromatography (GC, Agilent-6890N, U.S.), with their

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chromatograms and retention time compared with those of pure standards (for detailed

165

procedures, see Section S5). Concentrations of inorganic nitrogen species (i.e., NH4+, NO2−

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and NO3−) were determined on an automated discrete analyzer (AQ2-SEAL, U.K.).38

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ROS generated in the system was measured by ROS-detection kit39 (H2DCF-DA, Life

168

Technologies, U.S.). Concentrations of H2O2 in the reactor and near membrane surface were

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determined using hydrogen peroxide assay kit40 (Beyotime, China). More details regarding

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ROS and H2O2 determination are shown in Section S6. The total organic carbon (TOC)

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concentrations in aqueous solutions were analyzed by using a TOC analyzer

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(TOC-5000A-Shimadzu, Japan). The concentrations of Ti, Sn and Sb in effluents were

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quantified on an inductive coupled plasma atomic emission spectrometer (ICP-AES-720ES,

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Agilent, U.S.), with the detailed pretreatment procedure of effluent samples provided in

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Section S7. In all cases, the errors of metal ion determination were less than 2%.

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The energy demand for PCA degradation (degradation rate 50%) in flow-by and flow-through modes can be calculated by Eq. 1.41

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E=

U × I × t1/2 V

(1)

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where U is the applied voltage (V), I is the average current (A), t1/2 is the half-life (h) and V is

180

the electrolysis solution volume (L).

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RESULTS AND DISCUSSION

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Characterization of the SnO2-Sb and TiO2@SnO2-Sb ECMF Membrane. The

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ECMF modules with ceramic membrane assembled onto the SnO2-Sb or TiO2@SnO2-Sb

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electrode surface exhibited sound separation performance for inorganic colloidal materials

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with ~98% of turbidity removal (Figure S3). While this was also observed for the bare

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SnO2-Sb or bare TiO2@SnO2-Sb electrode following the deposition of SiO2 particles on the

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electrode surface, it significantly interfered the interaction of the generated oxidants with the

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pollutants, and thus resulted in deterioration of current efficiency (Figure S4). After coating

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TiO2 layer onto the SnO2-Sb surface, the charge-transfer resistance (RCT) of the composite

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ECMF anode was decreased from 40 Ω to 18 Ω (See Figure S5a), with the current magnitude

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of linear sweep voltammetry for TiO2@SnO2-Sb was obviously higher than SnO2-Sb

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membrane (Figure S5b). The enhanced performance should be ascribed to (i) the triggered

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redox reactions resulting in the generation of reactive species (e.g., conduction electron (ecb-),

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holes (hvb+) and ROS) at the electrode/electrolyte interface when the electric field higher than

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the band gap of TiO2 was applied16,42 that accelerated the electron transfer and/or (ii) the

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formed “staggered” type II heterojunction at the interface of SnO2/TiO2 improving the charge

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separation of electron-hole pairs.43-46

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As can be seen from Figures S6a~c, nano-sized SnO2-Sb particles were distributed on

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the SnO2-Sb composite anode, providing electroactive sites for the oxidation of pollutants.41

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After coating TiO2, new clusters of particles were present on the electrode surface (Figures

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S6d~f). Elemental analysis on an energy dispersive spectrometer indicated the TiO2 layer had


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a good coverage of the Sn and Sb matrix of the composite electrode (Table S1) with XRD

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and XPS analysis (Figure S7 and Figure S8) demonstrating the abundance of pure rutile SnO2

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and anatase TiO2 on SnO2-Sb and TiO2@SnO2-Sb surface, respectively. The results of BET

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measurements further verified that the specific surface area of SnO2-Sb and TiO2@SnO2-Sb

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electrode reached 22.4±2.5 m2/g and 22.7±2.2 m2/g respectively, yielding a large total surface

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area of the electrodes (~35.5 m2 for SnO2-Sb and ~39.1 m2 for TiO2@SnO2-Sb).

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Electrochemical Degradation of PCA. Degradation of PCA using the ECMF module

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was initially assessed in flow-by mode. As can be seen from Figure S9, the degradation

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efficiencies of PCA on SnO2-Sb and TiO2@SnO2-Sb ECMF systems are dependent on the

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operating voltages with the removal rates following 240-min reaction gradually increasing

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from 12.2%~14.6% to 46.8%~52.3% with charging voltage increasing from 1.0 to 5.0 V. In

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all cases, the electrochemical degradation of PCA follows pseudo-first-order kinetics (Table

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S2). It is evident from Figure 1a that kapp of TiO2@SnO2-Sb ECMF is higher than SnO2-Sb

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ECMF system, indicating that TiO2@SnO2-Sb has a better electrocatalytic activity.

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Figure 1. (a) Pseudo-first-order rate constant (kapp) of the electrochemical degradation of

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PCA in flow-by mode, (b) comparison of PCA removal under flow-by and flow-through 11 ACS Paragon Plus Environment


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modes as a function of voltages at an equivalent HRT of 240 min and (c) effects of operating

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time (or HRT) on PCA removal at 3.0 V. Experimental conditions: pH=7.0, [PCA]0=10 µM

222

and T=25±1°C. Error bars are the standard deviations of duplicate measurements.

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Figure 1b compares the PCA removal performance of the ECMF system under flow-by

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and flow-through modes at different charging voltages. A constant membrane flux of 17.4

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L/(m2 h) was used under flow-through mode, resulting in an HRT of 240 min, equivalent to

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an operating period of 240 min in flow-by mode. Obviously, PCA degradation rates under

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flow-through mode are significantly higher compared to flow-by mode. For example, PCA

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removal efficiencies were 36.7% and 97.9% using TiO2@SnO2-Sb ECMF at 1.0 and 5.0 V

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respectively, which were 2.5 and 1.9 times of those obtained in flow-by operation. This

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phenomenon should be attributed to the improvement of mass transfer and increase in ROS

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generation at membrane surface under flow-through mode.7,11,14 Further verification was

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carried out by examining the mass transfer rate constant (km) of the ECMF system (with the

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testing procedure and calculation equation (Eq. S2) documented in Section S8), and the

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results are shown in Figure S10. At a charging voltage ranging from 1.0 to 5.0 V, compared to

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the km of flow-by mode in TiO2@SnO2-Sb ECMF (9.7×10-6~9.6×10-5 cm/s), an

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approximately 3.5~5.4 times of increase in km was observed in its flow-through operation

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(5.1×10-5~3.4×10-4 cm/s), with these results clearly indicating an advection-enhanced

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transport of PCA molecules towards the anode surface. In addition, PCA removal rates of

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TiO2@SnO2-Sb ECMF at 1.0~5.0 V were higher than those of SnO2-Sb under the same

240

conditions (Figure 1b), confirming the superior performance of TiO2@SnO2-Sb.


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Energy consumption is an important parameter for the application of ECMF to

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water/wastewater treatment. As shown in Figure S11a, the energy demand of the ECMF in

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flow-by mode increases with an increase in the applied voltage, and TiO2@SnO2-Sb

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exhibited slightly lower energy consumption than SnO2-Sb under the same conditions.

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Notably, the energy demand of TiO2@SnO2-Sb ECMF at 4.0 and 5.0 V reached 8.6 and 23.1

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Wh/L respectively, 5.9 and 15.9 times that of 3.0 V (1.5 Wh/L). The low charge efficiency is

247

largely ascribed to the side reactions such as water splitting at the voltage higher than the

248

oxygen evolution potential,7 which results in intensified formation of gas bubbles (O2 and H2

249

at the anode and cathode respectively) that may block the electroactive reaction sites of the

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electrodes and clog the membrane pores, thus leading to a decrease in current efficiency.11,47

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In contrast, energy consumption under flow-through mode was significantly lower compared

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to flow-by mode, and also TiO2@SnO2-Sb ECMF consumed less energy than SnO2-Sb under

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flow-through mode (Figure S11b). In view of the appreciable PCA removal rate (85.5%) on

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TiO2@SnO2-Sb at 3.0 V under flow-through mode (Figure 1b), as well as the probably lower

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energy demand in this scenario compared to flow-by operation, 3.0 V was therefore chosen

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for the subsequent flow-through experiments.

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Investigation on the effects of HRT on PCA removal showed that at 3.0 V 58.0% of PCA

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could be removed with TiO2@SnO2-Sb ECMF at an HRT of 120 min while a prolonged HRT

259

(e.g., 180 min) was required for SnO2-Sb to achieve the same treatment efficiency (Figure 1c).

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In flow-through operation, the reaction kinetics on SnO2-Sb and TiO2@SnO2-Sb also follows

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pseudo-first-order kinetics, respectively with HRT in range of 30~360 min and 30~240 min



262

(Table S3). As shown in Figure S11b, the energy consumption of SnO2-Sb and

263

TiO2@SnO2-Sb ECMF in flow-through mode was merely 0.54 and 0.38 Wh/L, respectively,

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much lower than 1.7 and 1.5 Wh/L needed for their flow-by operation, and relatively lower

265

than typical energy consumption of 0.6 Wh/L for wastewater treatment.48 In addition, the

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PCA removal of TiO2@SnO2-Sb ECMF in flow-through mode was only increased by 4.6%

267

by further prolonging HRT from 240 to 360 min (Figure 1c), possibly as a result of mass

268

transfer limit. Hence, HRT was chosen to be 240 min in subsequent experiments based on

269

economic consideration. The deterioration in PCA removal efficiency of flow-by mode in

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ECMF system (operating time >240 min) might be induced by the occurrence of pH

271

excursion (caused by hydrogen ions accumulation in the vicinity of anode surface) that led to

272

formation of protonated PCA49 (Scheme 2), and thereby resulted in electrostatic repulsion of

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the anode for cationic form of PCA. This was further corroborated by observation from

274

Figure S12a and Figure S12b that reducing pH excursion (due to improved mass transfer by

275

proper operation) could lead to an increase in degradation rates of PCA under flow-by mode.

276 277 278

Scheme 2. Schematic representation of the protonation reaction of PCA.

279 280

ROS-Mediated Mechanisms for PCA degradation. The electrochemical oxidation of

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organic compounds at the anode surface involves direct and indirect oxidation.22,50 Cyclic

282

voltammetry (CV) analyses of SnO2-Sb and TiO2@SnO2-Sb were performed in a 50 mM


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Na2SO4 solution with/without adding 1 mM PCA. As shown in Figure S13, no additional

284

anodic peaks were found in CV curves lower than the oxygen evolution potential, and the

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addition of PCA resulted in negligible change of the CV patterns. This finding indicated that

286

direct oxidation of PCA by the anode played a minor role and that ROS-mediated redox

287

reactions should account for the electrochemical degradation of PCA.

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ROS production in the absence of PCA was first assessed under aerobic conditions using

289

H2DCF-DA as the probe. It can be seen from Figure 2a that the total ROS production of

290

TiO2@SnO2-Sb is 40.1% and 63.2% higher than that of SnO2-Sb in flow-by and flow-through

291

modes, respectively. This phenomenon might be ascribed to the excitation of TiO2,16,42

292

enhancing electron-transfer rate of the electrode when subject to external electric field. It is

293

expected that conduction electrons (ecb−) and holes (hvb+) could be generated from TiO2

294

excitation (Eq. 2),15 with the produced ecb− and hvb+ subsequently reacting with the

295

surface-absorbed oxygen, water and/or hydroxyl groups to generate ROS (O2•−, H2O2 and

296

HO•) for PCA oxidation (Eqs. 3~7).15,16,30 It should be noticed that ROS production by

297

TiO2@SnO2-Sb in flow-through mode was 5.0 times of that in flow-by mode, consistent with

298

the results of PCA removal (Figure 1b and Figure 1c).

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Figure 2. (a) ROS production in flow-by and flow-through modes under aerobic conditions.

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Experimental conditions: [PCA]0=0 µM, pH=7.0, voltage=3.0 V, operating time (HRT)=240

302

min and T=25±1°C. (b) Effects of scavengers (i.e., TEMPOL for O2•−, N2 gas for O2•− and

303

H2O2, i-PrOH for dissociated HO•, and N2+i-PrOH for O2•−, H2O2 and dissociated HO•) on

304

PCA removal in flow-through mode. In the control test, no scavengers were used.

305

Experimental conditions: [PCA]0=10 µM, pH=7.0, voltage=3.0 V, operating time (HRT)=240

306

min, [TEMPOL]0=20 mM, [i-PrOH]0=200 mM and T=25±1°C.

307 308

TiO 2 → TiO 2 ( hvb + + ecb − )

(2)

309

hvb + + ≡ TiIV OH → H+ + ≡ TiIVO•

(3)

310

hvb+ + H2O → H+ + HO• ( or > HO• )

(4)

311

ecb− + O2 ( or > O2 ) → O2•− ( or > O2•− )

(5)

312

ecb− + O2•− ( or > O2•− ) + 2H+ → H2O2 ( or > H2O2 )

(6)

313

ecb− + H2O2 ( or > H2O2 ) → OH− + HO• ( or > HO• )

(7)

314

Further studies were carried out with consideration given to the ROS species in

315

flow-through operation of the ECMF system. H2O2, a relatively stable oxidant compared to

316

O2•− and HO•,34 could be generated (i) near the anode surface via the combination of ecb− and

317

O2•− (Eq. 6) and disproportionation of O2•− (Eq. 8) and/or (ii) on the cathode via two-electron

318

reduction of oxygen (Eq. 9).7,15,29,51,52 Steady-state concentrations of H2O2 in the bulk

319

solution and near the membrane module were quantified with the results clearly showing that

320

H2O2 was more abundant near the membrane surface (Figure S14a). While the following


Page 17 of 36


321

experiments demonstrated that H2O2 itself was incapable of degrading PCA (Figure S14b),

322

H2O2 might mediate the production of other oxidants (Eq. 7) accounting for the oxidation.

323

The enrichment of H2O2 on the membrane (and electrode) surface correlated with the

324

superior performance of TiO2@SnO2-Sb ECMF in flow-through mode in which the

325

interaction between PCA and H2O2 (and other oxidants) was enhanced.

326

O2•− + O2•− + 2H+ → O2 + H2O2

(8)

327

O2 + 2e− + 2H+ → H2O2

(9)

328

SnO2 -Sb H2O   →H+ + e− + HO• ( or > HO• )

(10)

329

ROS scavengers were then introduced into the system to further illustrate the roles of

330

O2•−, HO• and H2O2 in the electrochemical degradation of PCA. In the control test (no

331

scavengers), 67.9% and 85.5% of PCA were removed for SnO2-Sb and TiO2@SnO2-Sb

332

ECMF under flow-through mode, respectively (Figure 2b). In contrast, with the addition of

333

TEMPOL (O2•− scavenger) under oxic condition, PCA removal rate of TiO2@SnO2-Sb was

334

decreased to 73.6%. Further experiment in N2-saturated solution to exclude O2•- and H2O2

335

resulted in the decrease of PCA removal rate to 63.9%, almost the same as that of SnO2-Sb,

336

(two-tailed, p>0.05). These results confirmed that TiO2@SnO2-Sb enabled the activation of

337

oxygen (Eqs. 5 and 6) that largely accounted for the improvement of PCA removal compared

338

to SnO2-Sb though it had been found that direct oxidation of PCA by H2O2 was inefficient

339

(Figure S14b) and the oxidation potential of O2•− (1.3 V) is lower than E0 (H2O2/H2O). In

340

addition to the surface oxidation of H2O (Eqs. 4 and 10),11 O2•− and H2O2 are also involved in

341

the production of powerful oxidizing agent, i.e., HO• (Eq. 7). Following experiments using



Page 18 of 36

i-PrOH to scavenge dissociated HO• resulted in the decrease of PCA removal rate by 22.9%

343

and 31.4% for SnO2-Sb and TiO2@SnO2-Sb systems, respectively, suggesting that the

344

dissociated HO• in the diffuse layer due to the diffusion of HO• from the surface (Eqs. 4 and

345

10) contributed to PCA removal. The remaining removal efficiency (44.9% and 54.1% for

346

SnO2-Sb and TiO2@SnO2-Sb, respectively), however, should be ascribed to the

347

surface-bound >HO• (and ≡TiO•). As a result, it is possible to conclude that >HO• (and

348

≡TiO•) produced by reaction of anode and hvb+ with absorbed hydroxyl groups and/or water

349

(Eqs. 3, 4 and 10) played an important role in PCA removal. Moreover, the use of dualistic

350

scavengers (N2 and i-PrOH) led to a further decrease in PCA removal rate for TiO2@SnO2-Sb

351

while this strategy had little effect on SnO2-Sb (Figure 2b). This phenomenon indicated that

352

the heterogeneous activation of oxygen (Eqs. 5~7) also generated >HO• with the elimination

353

of O2 and O2•− inhibiting the oxidizing agent production and consequently PCA degradation.

354

The schematic representation of the ROS-mediated mechanisms on TiO2@SnO2-Sb is

355

provided in Figure 3.

+

342

Diffuse layer

·

O2

O2

Electrolyte

Reduction site H2O2

-

ecb

-

CB

H2O2

TiO2

hγ

hvb+

VB

Oxidation site SnO2-Sb H2O


·OH

Page 19 of 36


357

Figure 3. Schematic representation of the ROS-mediated mechanisms for oxidant generation

358

on TiO2@SnO2-Sb ECMF system.

359 360

Degradation Pathways of PCA. Our preliminary experimental results indicated that the

361

aromatic and aliphatic products of PCA with an initial concentration of 10 µM were hard to

362

detect chromatographically. Therefore, a higher PCA dosage (150 µM) was utilized to

363

elucidate the degradation pathways of PCA in the ECMF system at 3.0 V. Figure S15 reveals

364

the time-course results of aromatic compounds concentrations during oxidation with a

365

reaction scheme including five routes for PCA proposed in Figure 4. These oxidation

366

processes were assumed to be initiated by HO• (and >HO• and ≡TiO•) attack to different

367

reaction sites (i.e., A~E). In route A, hydrogen abstraction on amino group occurred, resulting

368

in

369

4,4’-dichloroazobenzene via dimerization.55 The subsequent attack of HO• to its diazo group

370

led to the generation of 4-chloronitrobenzene54 followed by the transformation to PCA

371

through reduction reactions (e.g., ecb−)56,57 or to 4-chlorophenol due to further oxidation

372

(Figure 4).54 Routs B and C described the cleavage of Cl-C bond,49,58 leading to the

373

production of aniline and 4-aminophenol, respectively, with the concomitant release of Cl−.

374

Note that in addition to route C, 4-aminophenol could be also generated from further

375

hydroxylation of aniline produced via route B, resulting in the accumulation of

376

4-aminophenol in bulk solution to a higher level than other aromatic compounds (Figure S15).

377

4-chlorophenol, arising from either (i) conversion of 4-chloronitrobenzene via route A or (ii)

the

formation

of

anilinyl-radical49,53,54

that


was

rapidly

evolved

into


Page 20 of 36

378

HO• attack to N position of PCA49,54 via route D, was expected to have the same fate as

379

4-aminophenol generated via routes B and C since these aromatic compounds were further

380

oxidized to hydroquinone and benzoquinone as a result of the successive oxidation by HO•

381

(Figure 4).59,60 The transformation of PCA towards hydroquinone via routes A~D occurred

382

along with the generation of NH4+, NO2− and NO3− (Figure S16). In all cases, NH4+ was

383

preferentially accumulated compared to NO2− and NO3− despite the oxidation of NH4+ by

384

HO• remaining.7 Moreover, Figures S15d~f show that the total concentrations of aromatic

385

products of PCA including 4-chlorophenol and quinones were much lower than the products

386

of amine group, indicating that benzoquinone could be further degraded into secondary

387

products in the ECMF system. A NH2

B Cl

A

C

C Cl

D E

E Cl-

-

NH4+

NO3-

NH4+

388 389

Figure 4. Schematic diagram illustrating the electrochemical degradation pathways of PCA.


Page 21 of 36


391

It has been reported that the cleavage of the benzene ring of aromatic compounds leads

392

to the formation of carboxylic acids during electrochemical oxidation.7,59-61 Chromatograms

393

of the treated solutions demonstrated characteristic peaks related to (i) intermediate acids

394

including 2-ketoglutaric, maleic, fumaric, succinic and malonic acids,7,59,62 and (ii) ultimate

395

acids such as oxamic, formic, acetic and oxalic acids that could be directly oxidized to

396

(bi)carbonate.59,60,62 Evolution of the intermediate and ultimate acids is shown in Figure S17

397

and Figure S18, respectively. The cleavage of benzoquinone led to the generation of

398

2-ketoglutaric and formic acids with the further oxidation of 2-ketoglutaric resulting in the

399

production of formic, maleic and its trans-isomer fumaric acid (Figure 4).62 Figure S18a

400

shows that a small amount of oxamic acid was detected, suggesting that the direct ring

401

opening of PCA (i.e., route E) might play a minor role in PCA degradation. As shown in

402

Figures S18b~d, after operation in flow-through mode, the ultimate acids including formic,

403

acetic and oxalic acids were accumulated in the effluent. This is, however, not surprising as

404

they are more persistent to HO• attack compared to their parent compounds.7,63

405

Stoichiometry carbon balance analysis of PCA and its products was then performed to

406

evaluate the treatment efficiency using SnO2-Sb and TiO2@SnO2-Sb. It can be seen from

407

Figure 5a that flow-through operation requires a much lower equivalent operating time (i.e.,

408

HRT) than flow-by operation to achieve similar PCA removal rates (30 min vs. 240 min) with

409

the former mode being capable of inhibiting the accumulation of intermediate products (e.g.,

410

aromatic products). This should be attributed to the enhanced mass transfer and interaction

411

between pollutants and HO• leading to more efficient oxidation of quinones to ultimate acids.



412

Moreover, Figure 5b shows that an increase in HRT from 30 min to 240 min facilitates the

413

degradation of PCA to formic, acetic and oxalic acids during flow-through operation of

414

TiO2@SnO2-Sb ECMF whilst the recovery rates of carbon decrease from 89.4% to 70.7%,

415

likely associated with the production of CO2 and/or unidentified byproducts. It is of high

416

interest to notice that TiO2@SnO2-Sb demonstrates a higher PCA oxidation efficiency than

417

SnO2-Sb ECMF. For example, the concentration of accumulated ultimate acids under

418

flow-through operation of TiO2@SnO2-Sb was increased by 11.4% compared to SnO2-Sb at

419

an HRT of 240 min (Figure 5a). This can be explained by the increased ROS production in

420

the TiO2@SnO2-Sb ECMF system benefiting the oxidation of PCA and its intermediate

421

byproducts.

422 423

Figure 5. Mass balance on the basis of carbon stoichiometry during electrochemical

424

degradation of PCA using SnO2-Sb and TiO2@SnO2-Sb ECMF under (a) flow-through mode

425

with different HRT and (b) flow-by (operating time=240 min) and flow-through mode

426

(HRT=240 min). Aromatic compounds (C6): 4-chloronitrobenzene, aniline, 4-aminophenol,

427

4-chlorophenol, hydroquinone and benzoquinone. Intermediate acids: 2-ketoglutaric (C5),


Page 22 of 36

Page 23 of 36


428

fumaric (C4), maleic (C4), succinic (C4), malonic acids (C3). Ultimate acids: oxamic (C2),

429

formic (C1), acetic (C2) and oxalic acids (C2). For mass balance of carbon, the proportion of

430

CO2+unknown was calculated according to the differences between defined proportions

431

(undegraded PCA, aromatic compounds, intermediate acids and ultimate acids) and total

432

carbon. Experimental conditions: pH=7.0, voltage=3.0 V, 50 mM Na2SO4 supporting

433

electrolyte, [PCA]0=150 µM and T=25±1°C.

434 435

IMPLICATIONS

436

With the design of a novel electrochemical ceramic membrane module, anodic oxidation is

437

successfully integrated into low-pressure membrane filtration process, exhibiting an efficient

438

removal of PCA. Flow-through operation showed a higher removal rate compared to flow-by

439

mode, attributed to enhanced mass transfer of PCA towards membrane surface leading to the

440

improved utilization efficiency of surface adsorbed HO• and dissociated HO•. The main

441

degradation products were identified as the nontoxic short-chain carboxylic acids (mainly

442

formic, acetic and oxalic acids), implying that this technology is feasible for source water

443

purification. Stable PCA removal was observed under flow-through operation (Figure S19),

444

and the possibility of the electrode deactivation, if this system is applied for real water

445

purification, could be reduced since deposition of existing inorganic particles (e.g., SiO2)64 on

446

electrode surface would be prevented due to ceramic membrane filtration. Therefore, despite

447

dual use of ceramic membrane and electrodes may increase the invest cost, it is of great

448

importance that ceramic membrane filtration process enables the enhanced reliability and



449

effectiveness of anodic oxidation technology.

450

Regarding anode material, despite plate-like SnO2-Sb is a widely-used electrode,23

451

TiO2@SnO2-Sb developed in this work demonstrated its higher efficiency because of its

452

higher ROS production allowing for more effective oxidation of PCA. Despite this, in the

453

future, consideration should be given to comparison with other classical DSA anodes (e.g.,

454

PbO210), to further improve the oxidation power of the ECMF system. Possible release of

455

toxic ions from metal oxide electrodes is one of the concerns for their application to water

456

treatment.65 In this ECMF system, the concentration of Ti, Sn and Sb ions in the effluents was

457

quite low (see Table S4) for SnO2-Sb and TiO2@SnO2-Sb. For instance, the maximum

458

content of Sb ions was merely 0.005 mg/L, lower than the drinking water ordinance limits of

459

US EPA (0.006 mg/L). Despite the existence of Ti and Sn ions in some effluents, Ti and Sn

460

ions at low concentration is considered to be nontoxic.10,65 Therefore, both SnO2-Sb and

461

TiO2@SnO2-Sb can be used for water decontamination. Nevertheless, the detailed release

462

kinetics/mechanism of metal ions, and the potential toxicity of SnO2-Sb and TiO2

463

nanoparticles requires further study.66,67 Additionally, compared to pristine titanium mesh

464

cathode, no apparent decrease in electrical conductivity was observed on the used titanium

465

mesh after ~500-h testing (Figure S20). However, further investigation on long-term

466

operation of ECMF system is still needed to clarify the effect of corrosion on the service life

467

of the electrodes.

468

Current wastewater treatment plants are not specifically designed for micropollutants

469

elimination. Consequently, many of these micropollutants can pass through the treatment


Page 24 of 36

Page 25 of 36


470

processes, accumulating in the aquatic environment due to their continuous release and/or

471

persistency.8 Given the satisfactory removal performance for micropollutants (e.g., PCA) and

472

the produced biodegradable carboxylic acids, the ECMF system can be integrated into

473

biological processes, which can either be utilized as a pre- or post-treatment step of

474

bioreactors, or be directly incorporated into bioreactors since microbial viability is not

475

affected by exerting external electric field.31,32 Although PCA could not induce membrane

476

fouling (Figure S21a), the enhanced antifouling performance of the ECMF system compared

477

to conventional membrane filtration was demonstrated by using sodium alginate as model

478

pollutant of polysaccharides, attributed to the in-situ cleaning of membranes by ROS (e.g.,

479

H2O2)31 when the external electric field was applied (Figure S21b). These benefits highlight

480

that the ECMF can be used as an effective, safe, and promising technology for water

481

purification.

482 483

SUPPORTING INFORMATION

484

Figures S1~S21, Tables S1~S4; Text sections S1~S8. This information is available free of

485

charge via the Internet at http://pubs.acs.org.

486

AUTHOR INFORMATION

487

Corresponding Author

488

*Phone: +86-21-65975669, Fax: +86-21-65980400. E-mail: [email protected]

489

Notes

490

The authors declare no competing financial interest.



491

ACKNOWLEDGMENTS

492

We thank Key Special Program on the S&T for the Pollution Control and Treatment of Water

493

Bodies (2017ZX07201005) for supporting this work. Dr. Jinxing Ma acknowledges the

494

receipt of a UNSW Vice-Chancellor’s Postdoctoral Research Fellowship (RG152482).

495 496

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497

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TOC figure

+

e-


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Development of an Electrochemical Ceramic Membrane Filtration

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