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Contaminant Removal from Source Waters Using Cathodic Electrochemical Membrane Filtration: Mechanisms and Implications Junjian Zheng, Jinxing Ma, Zhiwei Wang, Shaoping Xu, T. David Waite, and Zhichao Wu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05625 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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Environmental Science & Technology

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Contaminant Removal from Source Waters Using Cathodic

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Electrochemical Membrane Filtration: Mechanisms and Implications

3 4

Junjian Zheng1, Jinxing Ma2, Zhiwei Wang*,1, Shaoping Xu1, T. David Waite2, Zhichao Wu1

5 6

1

7

Science and Engineering, Tongji University, Shanghai 200092, China

8

2

9

NSW 2052, Australia

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

School of Civil and Environmental Engineering, University of New South Wales, Sydney,

10 11 12

Revised Manuscript for Environmental Science & Technology (clean version)

13

January 20, 2017

14 15 16 17 18 19 20 21

1

ACS Paragon Plus Environment


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ABSTRACT: Removal of recalcitrant anthropogenic contaminants from water calls for the

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development of cost-effective treatment technologies. In this work, a novel electrochemical

24

membrane filtration (EMF) process using a conducting microfiltration membrane as the

25

cathode has been developed and the degradation of sulphanilic acid (SA) examined. The

26

electrochemical degradation of SA in flow-by mode followed pseudo-first-order kinetics with

27

the degradation rate enhanced with increase in charging voltage. Hydrogen peroxide as well

28

as oxidants such as HO• and Fe(IV)O2+ were generated electrochemically with HO• found to

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be the dominant oxidant responsible for SA degradation. In addition to the anodic splitting of

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water, HO• was formed via a heterogeneous Fenton process with surface-bound Fe(II)

31

resulting from aerobic corrosion of the steel mesh. In flow-through mode, the removal rate of

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SA was 13.0% greater than obtained in flow-by mode, presumably due to the better contact of

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the contaminant with the oxidants generated in the vicinity of the membrane surface. A

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variety of oxidized products including hydroquinone, p-benzoquinone, oxamic acid, maleic

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acid, fumaric acid, acetic acid, formic acid and oxalic acid were identified and an

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electrochemical degradation pathway proposed. These findings highlight the potential of the

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cathodic EMF process as an effective technology for water purification.

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2


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INTRODUCTION

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The world is facing increasingly severe water scarcity with many utilities conscripted to

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use source water containing elevated levels of contaminants arising from, variously, the

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disposal of hazardous waste, discharge of wastewater and surface runoff containing

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agricultural fertilizers and pesticides.1, 2 In China, more than half of the lakes and accessed

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ground waters do not meet the quality criteria for source water supply.3 As a consequence, a

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critical need exists for innovative and robust technologies for removal of these contaminants

48

with those based on low-pressure membrane filtration (such as microfiltration and

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ultrafiltration) of particular attraction in view of their small footprint, high efficacy in

50

removing bacteria and colloidal matter, and, if designed and operated appropriately, ease of

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maintenance.4-6

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While a considerable number of water/wastewater treatment plants using low-pressure

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membrane filtration are in operation, concerns remain with regard to the inability of the

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membranes to remove anthropogenic contaminants. In addition to contributing to significant

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loss of membrane permeability,7 many of these pollutants are refractory to conventional

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biological treatment (e.g., sulfonated aromatic amines are known to suppress biodegradation8),

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and, once discharged, will become secondary point sources of contamination.9 These facts

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have, in recent years, motivated numerous studies into the possibility of incorporating

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advanced oxidation processes into membrane filtration with preliminary results suggesting

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that hybrid membrane filtration-electrochemcial advanced oxidation processes offer real

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potential as a cost-effective means of treating contaminated source waters.6, 10, 11

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In an electrochemical membrane filtration (EMF) system, the functionalized membrane 3



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operates as a physical filter as well as an anode (or cathode) for electrochemical oxidation of

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organic pollutants when subject to pressure and current. Under anodic polarization of the

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membranes, degradation of recalcitrant organic compounds has been reported to occur as a

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result of both hydroxyl radical (HO•) mediated oxidation and direct electron transfer (DET)

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reactions at the anode surface.6, 12 A recent study using density functional theory calculations

68

indicated that the mechanisms involved in contaminant degradation in EMF might be a

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function of potential with an increase in anodic potential resulitng in a shift in

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p-methoxyphenol removal from a one-electron DET process to a two-electron DET process

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that generated p-benzoquinone.13

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While oxidative degradation of contaminants at a positively charged anode has been

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emphasized above, the possibility of degradation of contaminants at the negatively charged

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cathode also exists. In particular, hydrogen peroxide (H2O2) may be produced via the

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two-electron reduction of oxygen at the cathode with the possibility of producing HO• and/or

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higher valent iron species (such as Fe(IV)O2+) via Fenton processes if Fe(II) species are

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present.14, 15 Although benefits from cathodic polarization of electrochemical membranes such

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as fouling control have previously been suggested,11,16-20 there is surprisingly little

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information on the nature of these cathodic processes or on the nature and reactivity of the

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oxidants produced. For example, do homogeneous Fenton reactions in solution or

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heterogeneous reactions at the membrane surface dominate the oxidation process? Further

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understanding of these processes is required if cathodic EMF is to be used effectively and

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reliably for contaminant removal.

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In this study, we systemically investigated the oxidation of sulphanilic acid (SA) using 4


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an innovative, cost-effective electrochemical microfiltration membrane.19 SA was chosen

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because of its wide use as a precursor in synthesizing sulfonamide pharmaceuticals,

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sulfonated azo dyes, optical brighteners, pesticides and concrete plasticisers,8, 21 as well as its

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ubiquitous occurrence in industrial wastewaters, rivers and surface waters.22 Key questions

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addressed in the present work include: (i) what kind of oxidant species are generated and

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account for the oxidation of SA during the cathodic polarization of the electrochemical

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membrane? (ii) are homogenous or heterogeneous Fenton reactions in cathodic EMF of most

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importance in degrading SA? and (iii) what is the detailed degradation pathway of SA in the

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cathodic EMF process?

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MATERIALS AND METHODS

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Materials. All chemicals were analytical reagent grade and were used as received except

97

for 2,4-dinitrophenyl hydrazine (DNPH) that was recrystallized three times by acetonitrile.

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Solutions were prepared using 18.2 MΩ cm−1 Milli-Q water (Millipore) with glass- and

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plastic-ware soaked in 5% v/v HCl for at least three days followed by rinsing with Milli-Q

100

water prior to use. All experiments were carried out in 1 mM piperazine-N,

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N′-bis(ethanesulfonic acid) (PIPES) buffer at pH 7 unless otherwise stated. The circumneutral

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pH was chosen because it applies to most water/wastewater treatment in regards to the certain

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buffer capacity of waters. Solution pH was adjusted to 7 using 1 M H2SO4 or 1 M NaOH. The

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electrochemical microfiltration membrane was prepared by a phase inversion method as

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described previously.19 In brief, upon drying at 80 °C for 24 h to eliminate moisture, a

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predetermined amount of 8 wt% polyvinylidene fluoride (PVDF) and 3 wt% of polyvinyl 5



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pyrrolidone (PVP, pore-forming agent) was dissolved in dimethyl sulphoxide (DMSO). The

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solution was then agitated at 80 °C for 48 h to form a homogeneous casting solution. Finally,

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the solution was coated uniformly on a steel mesh (pore size = 96 µm, thickness = 43 µm)

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assembled on a nonwoven support (Shanghai Tianlu Advanced Textile Co., Ltd) at a casting

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knife gap of 300 µm. The cast film was exposed to ambient air (20 ± 1 °C, 30 ± 5% relative

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humidity) for 30 s to allow partial evaporation of the solvents and then immersed in a

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deionized water bath for phase-inversion at room temperature, inducing the formation of

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membrane pores.

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A scanning electron microscope with energy dispersive spectrometer (SEM/EDS)

116

(Model XL-30, Philips, Netherlands) was used to determine the surface morphology of

117

membrane and composition of the steel mesh. SEM images of membrane surface and section

118

were provided in the Supporting Information (SI) Figure S1. Linear sweep voltammetry (LSV)

119

measurements were carried out to characterize the electrochemical properties of both the

120

pristine membrane and conductive membrane in a conventional three-electrode cell driven by

121

an electrochemical workstation (CHI 660D, Shanghai Chenhua Instrument Co. Ltd., China).

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The species of the iron oxides formed on the steel mesh surface were determined using X-ray

123

photoelectron spectroscopy (XPS) (AXIS UltraDLD130, Kratos Analytical Ltd., U.K.) with

124

the binding energy of C 1s calibrated to 284.6 eV as an internal reference.

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Experimental Setup. All experiments were carried out in the dark in an electrochemical

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membrane reactor (for details, see SI Figure S2) made of plexiglass in which two graphite

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plates (dimension = 5 × 5 cm; thickness = 0.5 cm) were used as the anodes whilst the

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composite membrane module (dimension = 5 × 5 cm; effective area = 18 cm2) was used as the 6


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physical filter and cathode. The distance between the anodes and the cathode was set at 1

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cm.23 A diffuser was installed at the bottom of the reactor to (i) provide air or nitrogen gas at a

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flow rate of 300 mL min−1 and (ii) scour the membrane surface for fouling control.5 Each

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experiment was initiated by adding 10 µM of SA (or, in some tests, probe compounds or

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scavengers) to 250 mL of air-saturated water (or, in some cases, water which had been

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deoxygenated by sparging with nitrogen) containing 1 mM PIPES buffer and 50 mM Na2SO4.

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Recent evidence suggests that the application of relatively low voltages (0.2 ~ 2.0 V) could be

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effective in improving the anti-biofouling behavior of the EMF process.19, 24, 25 In order to

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investigate the process of SA removal under low electric field strength and to avoid the

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consequences of the increase in the internal resistance during the experiments that could result

139

in electrode potential excursion and activation of side reactions, constant voltage mode with

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charging voltages of 0, 0.5, 1.0, 1.5 or 2.0 V was employed during the experiment using a DC

141

power supply (CHI1030C, Jiecheng Co. Shanghai, China). Samples were then collected at

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predetermined time intervals using 5 mL glass syringes and filtered immediately through 0.45

143

µm nylon syringe filters. Experiments were first conducted under flow-by mode, in which the

144

influent and effluent valves were turned off, with quantification of the concentrations of

145

oxidants (H2O2 and HO•) and SA degradation products generated over a 60-minute period.

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Subsequently, performance in flow-through mode was assessed with solution passing through

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the membrane at a membrane flux of 140 L m-2 h−1, resulting in a hydraulic retention time of

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60 min (an equivalent time period to that used in the flow-by mode).

149

Analytical Methods. The concentrations of SA and any aromatic intermediates

150

produced as a result of oxidation of the parent compound were measured using 7



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reversed-phase high-performance liquid chromatography (HPLC, Agilent 1200) while the

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concentrations of any carboxylic acids generated were determined using ion-exclusion

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high-performance

154

chromatography (IC, Agilent 1000) and gas chromatography (GC, Agilent6890N) (detailed

155

procedures can be found in SI Section S1). Concentrations of ammonium (NH4+), nitrite

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(NO2−) and nitrate (NO3−) ions in aqueous solutions were determined using an AQ2 Discrete

157

Analyzer (SEAL Analytical).

liquid

chromatography

(HPLC,

Agilent

1200),

ion

exchange

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The concentration of aqueous H2O2 generated during cathodic polarization was

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determined using the metavanadate spectrophotometric method.26 Specifically, ammonium

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metavanadate (NH4VO3) is oxidized by H2O2 in acidic medium resulting in the formation of a

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red-orange color peroxovanadium cation demonstrating a maximum absorbance at 450 nm.

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Following filtration, samples withdrawn from the reactor were immediately mixed with

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NH4VO3 and H2SO4 stock solution in a quartz cuvette at final NH4VO3 and H2SO4

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concentrations of 6.2 mM and 58 mM respectively. The absorbance of the samples was then

165

measured using a TU-1810DPC spectrophotometer (PERSEE, Beijing, China).

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Preliminary experiments showed that H2O2 alone was unable to oxidize SA effectively

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(SI Figure S3). As a result, several probe compounds were introduced to quantify the rate and

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extent of production of reactive oxidants and to identify the nature of the oxidants. Methanol,

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which is known to react with both HO• and Fe(IV) species,27 was used in this study with its

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oxidation product formaldehyde (HCHO) quantified after DNPH derivatization (detailed

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procedures can be found in SI Section S2).28 While it has been reported that both 2-propanol

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and benzoic acid exhibit high selectivity for HO•, 2-propanol was used here to determine the 8


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relative role of HO• in view of the low solubility of benzoic acid and its tendency to associate

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with membrane surfaces at high concentrations.27, 29 The primary product (i.e., acetone) from

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the reaction of 2-propanol with HO• was determined using a protocol outlined previously.30

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The concentrations of Fe species present in solution including aqueous Fe(II), Fe(III) and

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colloidal Fe(III) (oxyhydro)oxide were quantified using the ferrozine spectrometric method.31

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In brief, a 0.4 mL aliquot of 0.5% ferrozine was added to a 5 mL sample to bind Fe(II) with

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the absorbance of the resultant Fe(II)-ferrozine complex determined at 510 nm using a

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UV-Vis spectrophotometer (PERSEE, Beijing, China). Analyses of dissolved Fe(II) and

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Fe(III) species were undertaken after samples had been passed through 0.45 µm nylon syringe

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filters with Fe(III) reduced to Fe(II) with the addition of hydroxylamine hydrochloride. For

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quantification of the total Fe species concentrations, acidification to pH 1 using concentrated

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HCl was conducted prior to sample filtration, with the fraction of colloidal Fe(III) calculated

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according to the differences between the total and dissolved Fe concentrations. In order to

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determine the specific contribution of anodic reaction(s) to SA degradation, sufficient

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ferrozine (1 mM) was added in the reactor under anoxic conditions in order to “lock up” Fe(II)

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via formation of the Fe(II)-ferrozine complex thereby preventing Fe(II) generated at the

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cathode from reaction with H2O2 and, as a result, excluding degradation of SA via the Fenton

190

reaction.

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RESULTS AND DISCUSSION

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Characterization of the Conductive Membrane. It can be observed from Figure S1

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that the composite membrane exhibited the typical structure of microfiltration membranes 9



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with an average pore size of 0.397 ± 0.074 µm. While the polymer coating film (PVDF and

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PVP) of the composite membrane is less conductive, the formation of water channels through

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the bulk material during the phase-inversion process due to the presence of PVP (a

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pore-forming agent) enabled oxygen and dissolved organic matter to readily reach the steel

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mesh surface with the redox reactions occurring consequently. That the membrane prepared in

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this manner was indeed conducting is apparent from the potential polarization curve shown in

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SI Figure S4. The pristine steel mesh (pore size of 96 µm) is (as expected) quite inefficient in

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retaining suspended solids and/or colloids. Following coating with PVDF, the composite

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conductive membrane is expected to achieve simultaneous membrane separation and

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dissolved organic pollutant removal. The filtration behavior of the conducting membrane is

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shown in Figure S5 with test procedures documented in SI Section S3. It is evident that the

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conducting membrane could achieve efficient separation of colloidal materials and provide

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enhanced performance for filtration of sodium alginate and bovine serum albumin.

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Considering that benefits from cathodic polarization of electrochemical membranes including

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fouling control and enhanced filtration behavior have been confirmed as above and also

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previously suggested11,16-20, we focus in this study on investigation of the mechanisms relating

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to oxidant generation and organic pollutant degradation.

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Electrochemical Degradation of SA. Figure 1a demonstrates the performance of the

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electrochemical membrane in flow-by aerobic degradation of SA when different voltages are

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applied. Overall, rapid removal of aqueous SA (>60% following one hour of operation)

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occurred at charging voltages over 1.5 V while the efficacy was low when the voltage was

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decreased to 0.5 V. For any specific condition involving the cathodic polarization of the 10


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membrane, SA remaining in the solution exhibited pseudo-first-order decay during the

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experiment. Apparent rate constants for SA removal (kapp) as a function of voltage applied are

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shown in Figure 1b; for example, kapp at 2.0 V was 3.3 × 10−4 s−1 with this value about two

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orders of magnitude higher than that (7.5 × 10−6 s−1) obtained at 0.5 V. While it has been

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reported that capacitive electrosorption associated with non-Faradaic reactions is a possible

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approach to pollutant removal, especially in the case where a relatively low voltage is used,32,

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33

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electrosorption is not likely to be responsible for SA removal during the cathodic polarization

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of the electrochemical membrane used in this study. This finding is, however, unsurprising

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given (i) the low specific surface areas of the electrodes (2.4×10−4 m2 g−1 for the anode and

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0.03 m2 g−1 for the cathode) and (ii) the fact that deprotonation of SA will occur at pH 7.0 (Eq.

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1) leading to electrostatic repulsion between the anionic form of SA and the

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negatively-charged membrane. In view of the high rate of SA removal at 2.0 V and the

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insignificant inhibition on microbial activity observed when integrating the conductive

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membrane with biological treatment,19 subsequent experiments were carried out using this

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voltage unless otherwise stated.

the negative results at 0.5 V in the present work (Figure 1a and SI Figure S6) indicate that

Ka = 10−3.25 M

233 234

11


(1)


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Figure 1. (a) Time course results of the proportion of aqueous sulphanilic acid (SA)

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remaining in the reactor following the application of different voltages under oxic conditions

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and (b) pseudo-first-order rate constants (kapp) of SA degradation. In Figure 1a, dashed lines

239

represent pseudo-first-order descriptions of SA degradation with the apparent rate constants

240

provided in Figure 1b. Experimental conditions: pH = 7.0 and [SA]0 = 10 µM. Error bars are

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the standard deviation of duplicate measurements.

242 243

Oxidant Species Responsible for SA Degradation. While the results of this study show

244

that cathodic polarization of the membrane under oxic conditions could lead to the generation

245

of H2O2 (SI Figure S7) through the two-electron reduction of oxygen (Eq. 2), this oxidant,

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while putatively contributing to fouling control during cathodic EMF,19, 20 was unable to

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induce the direct oxidation of SA (SI Figure S3). In comparison, SA degradation was evident

248

in the H2O2 solution following the addition of aqueous Fe(II) (SI Figure S3) with this effect

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presumably associated with the generation of more powerful oxidants such as HO• and/or

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Fe(IV)O2+ species via Fenton processes (Eqs. 3 and 4).

251

O 2 + 2H + + 2e − → H 2 O 2 12


(2)

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Fe II + H 2 O 2 → Fe III + HO• + OH −

(3)

253

Fe II + H 2 O 2 → Fe IV O 2 + + H 2 O

(4)

254

To assess the nature of the oxidants contributing to SA degradation, control experiments

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were first performed at 2.0 V with the use of probe compounds (methanol and 2-propanol

256

respectively) in the absence of SA. As can be seen from Figure 2a, the concentration of

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acetone, the oxidation product of 2-propanol, follows a similar trend to that of formaldehyde

258

(the oxidation product of methanol) with the yield increasing as a function of the elapsed time.

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Given that both methanol and 2-propanol react with HO•, it can be concluded that activation

260

of H2O2 occurs in the EMF reactor. This speculation is further confirmed by the quenching of

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the aerobic oxidation of SA under cathodic polarization of the electrochemical membrane on

262

addition of methanol or 2-propanol. As can be seen from Figure 2b, both methanol (which is

263

recognized to scavenge both HO• and Fe(IV)O2+ species) and 2-propanol (which scavenges

264

HO• only) resulted in very similar (p>0.05, two-tailed) inhibition of SA removal with this

265

result suggesting that HO• is most likely responsible for SA removal in the EMF reactor with

266

minimal influence of Fe(IV)O2+. This is perhaps not particularly surprising as high valence

267

metal species such as Fe(IV)O2+ are recognized to be more important under alkaline

268

conditions (with HO• expected to dominate at the near neutral conditions used here). 34

252

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13



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Figure 2. (a) Formaldehyde and acetone production under oxic and anoxic conditions.

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Nitrogen gas was used for deoxygenation under the anoxic condition. Experimental

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conditions: pH = 7.0, voltage = 2.0 V and [methanol]0 = [2-propanol]0 = 100 mM. (b) Change

274

of reaction kinetics of SA degradation following the addition of 2-propanol and methanol

275

scavenging of HO• and/or Fe(IV)O2+ species. In the blank assay, no scavengers were used.

276

Experimental conditions: pH = 7.0, voltage = 2.0 V, [SA]0 = 10 µM and [2-propanol]0 =

277

[methanol]0 = 100 mM.

278 279

While cathodic reduction of oxygen to H2O2 with subsequent production of HO• as a

280

result of reaction of H2O2 with Fe(II) is likely in the system being used here, HO• could also

281

be generated at the anode by direct electrolysis of water (Eq. 5)14, 15, 21 since the setup was

282

configured as an undivided electrochemical cell.

283

H 2 O → HO • + H + + e −

284

In order to examine the significance of this degradation pathway, the possibility of the

285

Fenton reaction occurring was removed by both adding excess ferrozine to remove reactive

286

Fe(II) and undertaking the study under anoxic conditions thereby preventing H2O2 formation. 14


(5)

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With the Fenton reaction excluded, the degradation of contaminants can be mainly attributed

288

to (i) anodic HO• mediated oxidation and/or (ii) direct anode oxidation. Under this

289

circumstance, the apparent rate constant for SA removal (kapp) was 1.2 × 10−4 s−1 (SI Figure

290

S8), which is much lower than that obtained under the oxic condition (kapp = 3.3 × 10−4 s−1)

291

with the difference between the kapp values exemplifying the significant contribution of

292

aerobic production of HO• via iron activation of the electro-generated H2O2.

293

Iron-Mediated Fenton Reactions. Compared to composite membranes prepared using

294

conductive polymers,10,23,24 the incorporation of steel mesh might contribute to the production

295

of more powerful oxidants (e.g., HO•) that may induce the oxidative degradation of refractory

296

organic compounds such as SA. It is generally expected that Fenton reactions are initiated by

297

the reaction between Fe(II) (or other metallic species) and H2O2 through an inner-sphere

298

mechanism.35 EDS analysis of the pristine steel mesh shows that the atomic concentration of

299

Fe is on the order of 81.10% and, given the dominant presence of Fe, we have focused on its

300

role in producing powerful oxidants. Other redox active elements (such as Cr) are present but,

301

given their low concentrations relative to Fe and limited ability to drive Fenton-like processes,

302

their role in activating O2 or H2O2 is expected to be minor (SI Figure S9). Although Fe(II)

303

was not added to the reactor, 2.6 µM dissolved Fe(II) and 1.9 µM colloidal Fe(III)

304

(oxyhydro)oxide were detected in the bulk solution after one-hour reaction at 2.0 V (Figure 3a)

305

with these Fe species presumably resulting from the aerobic corrosion of the steel mesh by

306

oxygen and/or H2O2. These processes are represented by Eqs. 6-8 where Fe0n-1-FeII in these

307

reactions represents surface-bound Fe(II) which is subsequently released to solution. After

308

~1000 h testing, the used membrane was retrieved from the reactor with SEM analysis 15



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revealing the presence of rust spots on the mesh surface. While Fe 2p XPS analysis of the

310

pristine and used membranes (after 1000-h operation) indicated that these spots contained a

311

variety of ferrous and ferric oxides (SI Figure S10), the TMP variation of the pristine and used

312

membranes operating in flow-through mode were quite similar over period of 7 days (SI

313

Figure S11) with this result suggesting that any corrosion of the steel mesh that might have

314

occurred over this time period (1000 h) had minimal adverse impact on the filtration

315

performance of the conductive membrane. Moreover, no morphological or structural changes

316

of the pores on the used membrane surface were evident (SI Figure S12) while further study

317

on a long-term operation of a continuous-flow reactor is needed to clarify the effect of

318

corrosion on the lifespan of the conductive membrane. +

319

2H Fe0n + O2  → Fe0n −1 -FeII + H 2 O2 +

320 321

H Fe 0n + H 2 O 2  2 → Fe n0 −1 -Fe II + 2H 2 O

Fe 0n −1 - Fe II ⇔ Fe n0 −1 + Fe II ( aq )

(6) (7) (8)

322

To elucidate the role of the homogenous Fenton reaction (i.e., reaction of aqueous Fe(II)

323

with H2O2) in SA degradation, 2.6 µM Fe(II) was added to a buffer solution containing 47 µM

324

H2O2 and 10 µM SA with an initial pH of 7.0 with these dosages of Fe(II) and H2O2 similar to

325

the steady-state concentrations of Fe(II) and H2O2 in the aqueous phase following one-hour of

326

charging of the electrochemical membrane (Scenario: Homo I in Figure 3b). It can be seen

327

from Figure 3b that only 4.6% of SA was oxidized following 1 h reaction suggesting that

328

homogeneous Fenton reactions play a minor role in SA degradation in the electrochemical

329

membrane reactor. A similar conclusion was obtained in the alternative scenario (Homo II in

330

Figure 3b) in which the aqueous oxidants generated during one-hour charging of the 16


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membrane caused only 3.2% of SA (added subsequent to charging) to be degraded in the

332

ensuing 1 h. In comparison, 69% of SA was removed over the course of 1 h reaction within

333

the electrochemical cell (Figure 3b). Moreover, only 0% and 1.3% of SA removal were

334

obtained after the addition of 2.6 µM Fe(II) and 1.8 µM Fe(III) respectively to neutral bulk

335

solutions, suggesting that the contribution of Fe(II) and Fe(III) hydroxides to SA removal via

336

adsorption was negligible (SI Section S4 and Figure S13). A schematic representation of the

337

processes leading to generation of oxidants is presented in Figure 4. In view of the results

338

obtained, we can conclude that the effectiveness of the heterogeneous reactions arises from

339

the interaction of surface-associated Fe(II) with cathodically generated H2O2 resulting in

340

extensive generation of oxidants at the membrane surface (Eq. 9) with the Fe(III) species

341

produced in the Fenton reaction being subsequently reduced to Fe(II) at the cathode (Eq. 10)

342

thereby enabling the ongoing production of HO•.15, 36 The mechanism disclosed in this study

343

extends our knowledge of the process occurring in electrochemical membranes10-13,23-25 with

344

important new features including (i) quantitative understanding of the rate and extent of

345

oxidant production during cathodic polarization of the conductive membrane and (ii)

346

clarification of the roles of homogeneous and heterogeneous Fenton reactions in pollutant

347

degradation.

348

349

>Fe II + H 2 O 2 → >Fe III + HO• + OH −

>FeIII + e− → >FeII

17


(9) (10)


350 351

Figure 3. (a) Time course of dissolved Fe(II) and colloidal Fe(III) concentrations in the

352

solution at a charging voltage of 2.0 V. (b) SA remaining in the solution following 1 h

353

reaction under different scenarios. Oxic: aerobic degradation of 10 µM SA at 2.0 V; Anoxic:

354

anoxic degradation of 10 µM SA at 2.0 V with nitrogen gas used for deoxygenation and

355

ferrozine for locking up Fe(II); Fe(II) + H2O2 (Homo I): degradation of 10 µM SA with the

356

dosages of 2.6 µM Fe(II) and 47 µM H2O2; Fe(II) + H2O2 (Homo II): after 1 h polarization at

357

2.0 V in a 1 mM PIPES buffer solution containing 50 mM Na2SO4, the electrochemical

358

membrane and graphite anodes were withdrawn from the system followed by addition of 10

359

µM SA. In Figure 3a, dissolved Fe(III) was, unsurprisingly, undetectable given the low

360

solubility of the iron oxide phases formed.

361

18


Page 18 of 33

Page 19 of 33


362 363

Figure 4. A schematic representation of the proposed mechanistic model for oxidant

364

generation during cathodic electrochemical membrane filtration (EMF). Numbers of relevant

365

reactions (Eqs. 2-10) are also listed in the scheme.

366 367

Degradation Pathways. Determination of the products resulting from the degradation of

368

SA at pH 7.0 was initially attempted for a SA dosage of 10 µM but the concentrations of the

369

resultant

370

chromatographically. As a result, a higher SA concentration (150 µM) was used to elucidate

371

the degradation pathways of SA in the electrochemical reactor (Figure 5).

aromatic

and

aliphatic

compounds

were

too

low

to

be

detectable

372

In this study, the concentration of the primary aromatic products, i.e., hydroquinone and

373

p-benzoquinone, generated at different voltages (0, 0.5, 1.0, 1.5 and 2.0 V) were determined

374

using reverse-phase HPLC with the chromatograms revealing the formation of hydroquinone

375

at a retention time of tR = 7.2 min via dihydroxylation of the initial compound with direct loss 19



376

of NH4+ and SO42− from the C(1)- and C(4)- positions (Figure 5).21 In comparison, a retention

377

time of 9.7 min was evident for the oxidation derivative p-benzoquinone. Temporal variation

378

of the solution concentrations of SA, hydroquinone and p-benzoquinone is shown in Figure

379

S14. In accordance with the previous findings when low concentrations of SA (10 µM) were

380

used (Figure 1), kapp for SA removal at higher initial concentrations are also dependent on the

381

voltages applied with a much higher kapp obtained at 2.0 V compared to 0.5 V (SI Table S1).

382

As can be seen from Figures S14b and c, the steady-state concentrations of the aromatic

383

products increased as a function of voltage applied, suggesting that using a higher voltage (or

384

higher current density) benefits the dihydroxylation of SA. This process leads to the

385

concomitant production of NH4+ followed by further oxidation of NH4+ to NO3− with

386

insignificant accumulation of NO2− (SI Figure S15).37 It can be observed from Figures S14

387

and S15 that the steady-state concentrations of the aromatic products are much lower than that

388

of the concomitant products (i.e., NH4+), indicating that further degradation of

389

p-benzoquinone to secondary products occurred in the cathodic EMF reactor. This is

390

confirmed by the observation that the kapp values for SA removal (2.5 × 10−6 to 3.6 × 10−4 s−1)

391

are one order of magnitude higher than those for hydroquinone generation (0 to 3.1 × 10−5 s−1)

392

as shown in Table S1.

393

Short-chain carboxylic acids are the expected secondary products of the degradation of

394

aromatic compounds by advanced oxidation processes (Figure 5).14, 21, 37 In this study, the ion

395

exclusion chromatograms of samples exhibited peaks38, 39 corresponding to acetic acid (tR =

396

15.2 min), fumaric acid (tR = 14.2 min), maleic acid (tR = 8.3 min), oxalic acid (tR = 6.7 min)

397

and formic acid (tR =13.3 min) with a correction of tR for acetic acid (4.7 min) and formic acid 20


Page 20 of 33

Page 21 of 33


398

(5.8 min) by ion exchange chromatography. It has been established that the attack of HO• on

399

p-benzoquinone could yield either (i) intermediate aliphatic compounds that evolve into

400

carboxylic acids such as fumaric and maleic that are eventually oxidized into oxalic acid, or

401

(ii) ultimate acids such as acetic and formic (Figure 5).40 As can be seen from Figures S16 a-c,

402

at charging voltages of 1.5 and 2.0 V, the concentrations of fumaric and maleic acids are

403

substantially lower than that of their oxidized product (i.e., oxalic acid) which clearly shows

404

that the reaction of intermediate carboxylic acids with HO• should be faster than the rate of

405

reaction with oxalic acid as this end product is relatively recalcitrant to hydroxyl radical

406

attack.14 A similar accumulation was observed for other highly oxidised acids (Figures S16d

407

and e); for example, the concentrations of acetic and formic acids reached 18.0 and 62.2 µM

408

following charging of the system at 2.0 V for 1 h. Moreover, Figure S16f indicates that there

409

is another pathway associated with the direct ring opening involving SA degradation under

410

cathodic EMF (Figure 5). The oxamic acid containing the −NH2 group was formed at

411

charging voltages of 1.0, 1.5 and 2.0 V though the steady-state concentrations were lower

412

than other end-product acids (Figures S16 c-f).

413

21



Page 22 of 33

Sulfanilic acid

Pathway I

HO

Pathway II

Dihydroxylation

SO42NO2-

HO

NH4+

Oxidative ring opening

SO42-

fast HO

NO3Hydroquinone Oxamic acid

HO

HO Maleic acid

HO

NH4+

fast p-benzoquinone Oxalic acid

HO Oxidative ring opening

HO

NO2-

slow Fumaric acid

HO

CO2

fast

NO3Acetic acid

HO slow

414

Formic acid

415

Figure 5. Proposed reaction sequence for sulphanilic acid (SA) degradation under the neutral

416

condition by using cathodic electrochemical membrane filtration (EMF).

417 418

Enhanced oxidation of SA in flow-through mode. Figure S17 depicts the effluent

419

concentration of SA (initial concentration of 10 µM) under flow-through mode in which the

420

reactor was operated with the solution passing through the membrane at a membrane flux of

421

140 L m-2 h−1 yielding a hydraulic retention time (60 min) equivalent to that in the flow-by

422

mode. The removal rate of SA was found to be 13.0% higher in flow-through membrane

423

filtration compared to flow-by mode, presumably as a result of the better contact of the

424

contaminant with the oxidants generated in the vicinity of the membrane surface when

425

operated in flow-through mode (Figure 4). These findings are consistent with the results of

426

previous literature in which the use of appropriate operating modes can enhance mass transfer 22


Page 23 of 33


427

and accelerate (i) the generation of ROS and (ii) oxidation of pollutants on the conductive

428

membrane surface.10, 11 Oxidation efficiency of SA during cathodic EMF was then evaluated

429

by increasing the initial SA concentration to 150 µM with mass balance on the basis of carbon

430

stoichiometry conducted (i) following 1 h reaction under flow-by cathodic polarization, and

431

(ii) at steady state for the flow-through mode, with the residual proportion calculated by

432

subtracting the defined proportions (SA remaining in the solution, aromatic compounds,

433

intermediate products, ultimate acids and oxamic acid) from the total carbon. In this study, the

434

residual proportion is associated with the production of CO2 and/or other poorly characterized

435

products (such as the products of oxamic acid oxidation). It can be observed from Figure 6

436

that SA is significantly oxidized under neutral conditions at high applied voltages, with the

437

dominant end-products including oxalic, acetic and formic and/or CO2. Considering the molar

438

ratio of end-product acids to oxamic acid, we conclude that pathway I in Figure 5 is the

439

primary route for SA degradation in spite of the possibility that the generation of oxamic acid

440

is underestimated due to its further mineralization to CO2. Moreover, a higher oxidation

441

efficiency of SA has been achieved under flow-through mode than flow-by mode. For

442

example, ultimate acids (i.e., acetic, formic and oxalic acids) in the solution contributed 31.9%

443

of the total carbon remaining following 1 h of flow-by SA oxidation at 2.0 V. In comparison,

444

the proportion increased to 65.0% when the solution underwent cathodic filtration. Another

445

advantage is that the cathodic EMF process inhibits the accumulation of aromatic compounds

446

with hydroquinone and p-benzoquinone accounting for 8.6% and 1.2% of the total carbon for

447

the flow-by and flow-through modes, respectively. While the detailed mechanism requires

448

further study, its environmental importance is profound with regard to the critical concern 23



449

arising from the release of quinones and/or their derivatives with halobenzoquinones possibly

450

formed during the chlorination of effluents.

451 452

Figure 6. Mass balance on the basis of carbon stoichiometry. SA: SA (C6) remaining in the

453

solution; Aromatic compounds: the hydroquinone (C6) and p-benzoquinone (C6);

454

Intermediate: the fumaric (C4) and maleic acids (C4); Ultimate acids: the acetic (C2), formic

455

(C1) and oxalic acid (C2). The proportion of CO2 + Unknown was calculated by subtracting

456

the defined proportions (SA remaining in the solution, aromatic compounds, intermediate

457

products, ultimate acids and oxamic acid) from the total carbon. Experimental conditions: pH

458

= 7.0 and [SA]0 = 150 µM. For experiments conducted under the flow-by mode, calculation

459

was carried out following 1 h reaction.

460

461

IMPLICATIONS

462

Integration of electrochemical oxidation technology into the membrane filtration process

463

is an attractive technology to treat wastewater containing recalcitrant organic contaminants. In

24


Page 24 of 33

Page 25 of 33


464

this study, a cathodic EMF system, comprised of a composite conductive microfiltration

465

membrane cathode and cost-effective graphite plate anodes, exhibited effective SA removal.

466

Cathodic heterogeneous oxidation reactions played a dominant role in SA degradation

467

compared to either homogeneous reactions or oxidation by HO• generated from the anodic

468

splitting of water. More importantly, SA was removed more effectively in flow-through mode

469

than in flow-by mode with the higher removal efficacy related to the better contact of the

470

contaminant with the oxidants generated in the vicinity of membrane surface compared with

471

the case in flow-by mode.

472

Moreover, a higher oxidation efficiency of SA was achieved in flow-through mode with

473

the residual quantity of aromatic compounds (i.e., hydroquinone and p-benzoquinone) in this

474

mode much lower (1.2% of the total carbon) than was the case in flow-by mode (8.6%). SA

475

degradation was induced primarily by iron-mediated Fenton reactions with these reactions

476

initiated by cathodic reduction of oxygen and formation of membrane surface-bound Fe(II)

477

(Fe0n-1-FeII). These interfacial Fenton reactions overcome one of the major drawbacks of

478

conventional microfiltration/ultrafiltration, that is, the inability to remove low molecular

479

weight (< 1000 Da) organic contaminant41 and, as such, broaden the scope of application of

480

low-pressure membrane treatment technology. While there was no obvious decrease in the

481

extent of SA oxidation over the seven day studies described here, a variety of processes

482

including (i) ion deposition (or adsorption) to the electrodes, (ii) local pH excursion and/or (iii)

483

membrane fouling might lead to changes in the efficiency of contaminant degradation over

484

extended times.

485

The energy consumption associated with application of an external electric field is an 25



486

important parameter for evaluating the applicability of the EMF system to water treatment.

487

The values of electrical efficiency per log order reduction (EE/O)42 for the electrochemical

488

oxidation process are calculated to be 0.01, 0.09 and 0.16 kWh m−3 at charging voltages of 1

489

V, 1.5 V and 2 V respectively, indicating that the energy consumption of the EMF system is

490

relatively low compared to a typical energy consumption of 0.6 kWh m−3 for wastewater

491

treatment43 though there is a concern that the high voltage used (2.0 V) in this study is beyond

492

the voltage at which water electrolysis would be expected to occur (typically 1.23 V),

493

resulting in adverse consequences such as low charging efficiency. In the future,

494

consideration should be given to the optimization of the operating conditions and the

495

minimization of unwanted side-reactions through the preparation of electrodes that have high

496

overpotentials for oxygen and hydrogen evolution.

497

Previous studies have reported that the conductive membrane cathode is also effective in

498

mitigating membrane fouling.11,

18-20

499

membrane bioreactors (MBRs) has no negative impacts on microbial viability.19 As such, if

500

the EMF system is combined with biological processes (e.g., MBRs), a synergistic microbial

501

mineralization process of the electrochemically degraded products of organic contaminants,

502

e.g., short-chain carboxylic acids, is anticipated. These benefits highlight the potential of

503

cathodic EMF to be used as an effective technology for water purification.

Importantly, the use of external electric field in

504 505

SUPPORTING INFORMATION

506

Text Sections S1-S4, Figures S1-S17, and Table S1 are included. This information is available

507

free of charge via the Internet at http://pubs.acs.org. 26


Page 26 of 33

Page 27 of 33


508

AUTHOR INFORMATION

509

Corresponding Author

510

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

511

Notes

512

The authors declare no competing financial interest.

513 514

ACKNOWLEDGMENTS

515

We thank the National Natural Science Foundation of China (Grant 51422811) for the

516

financial support of this work. Dr. Jinxing Ma acknowledges the receipt of a UNSW

517

Vice-Chancellor’s Postdoctoral Research Fellowship (RG152482).

518 519

REFERENCES

520

1. Karnik, B. S.; Davies, S. H.; Baumann, M. J.; Masten, S. J., Fabrication of catalytic

521

membranes for the treatment of drinking water using combined ozonation and ultrafiltration.

522

Environ. Sci. Technol. 2005, 39 (19), 7656-7661.

523

2. Chakraborti, D.; Das, B.; Murrill, M. T., Examining India’s groundwater quality

524

management. Environ. Sci. Technol. 2011, 45 (1), 27-33.

525

3. Cheng, H. F.; Hu, Y. A.; Hao, J. F., Meeting China's water shortage crisis: Current

526

practices and challenges. Environ. Sci. Technol. 2009, 43 (2), 240-244.

527

4. Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A.

528

M., Science and technology for water purification in the coming decades. Nature 2008, 452

529

(7185), 301-310. 27



530

5. Wang, Z. W.; Ma, J. X.; Tang, C. Y. Y.; Kimura, K.; Wang, Q. Y.; Han, X. M., Membrane

531

cleaning in membrane bioreactors: A review. J. Membr. Sci. 2014, 468, 276-307.

532

6. Ganiyu, S. O.; van Hullebusch, E. D.; Cretin, M.; Esposito, G.; Oturan, M. A., Coupling

533

of membrane filtration and advanced oxidation processes for removal of pharmaceutical

534

residues: A critical review. Sep. Purif. Technol. 2015, 156, 891-914.

535

7. Yamamura, H.; Okimoto, K.; Kimura, K.; Watanabe, Y., Hydrophilic fraction of natural

536

organic matter causing irreversible fouling of microfiltration and ultrafiltration membranes.

537

Water Res. 2014, 54, 123-136.

538

8. Chen, G.; Cheng, K. Y.; Ginige, M. P.; Kaksonen, A. H., Aerobic degradation of sulfanilic

539

acid using activated sludge. Water Res. 2012, 46 (1), 145-151.

540

9. Audenaert, W. T. M.; Vandierendonck, D.; Van Hulle, S. W. H.; Nopens, I., Comparison

541

of ozone and HO induced conversion of effluent organic matter (EfOM) using ozonation and

542

UV/H2O2 treatment. Water Res. 2013, 47 (7), 2387-2398.

543

10. Zhao, F.; Liu, L.; Yang, F.; Ren, N., E-Fenton degradation of MB during filtration with

544

Gr/PPy modified membrane cathode. Chem. Eng. J. 2013, 230, 491-498.

545

11. Li, Y.; Liu, L.; Yang, F., High flux carbon fiber cloth membrane with thin catalyst coating

546

integrates bio-electricity generation in wastewater treatment. J. Membr. Sci. 2016, 505,

547

130-137.

548

12. Zaky, A. M.; Chaplin, B. P., Porous substoichiometric TiO2 anodes as reactive

549

electrochemical membranes for water treatment. Environ. Sci. Technol. 2013, 47 (12),

550

6554-6563.

551

13. Zaky, A. M.; Chaplin, B. P., Mechanism of p-substituted phenol oxidation at a Ti4O7 28


Page 28 of 33

Page 29 of 33


552

reactive electrochemical membrane. Environ. Sci. Technol. 2014, 48 (10), 5857-5867.

553

14. Brillas, E.; Sires, I.; Oturan, M. A., Electro-Fenton process and related electrochemical

554

technologies based on Fenton's reaction chemistry. Chem. Review 2009, 109 (12), 6570-6631.

555

15. Qiu, S.; He, D.; Ma, J. X.; Liu, T. X.; Waite, T. D., Kinetic modeling of the electro-Fenton

556

process: Quantification of reactive oxygen species generation. Electrochim. Acta 2015, 176,

557

51-58.

558

16. Bani-Melhem, K.; Elektorowicz, M., Development of a novel submerged membrane

559

electro-bioreactor (SMEBR): Performance for fouling reduction. Environ. Sci. Technol. 2010,

560

44 (9), 3298-3304.

561

17. Liu, J. D.; Liu, L. F.; Gao, B.; Yang, F. L., Integration of bio-electrochemical cell in

562

membrane bioreactor for membrane cathode fouling reduction through electricity generation.

563

J. Membr. Sci. 2013, 430, 196-202.

564

18. Ma, J.; Wang, Z.; Mao, B.; Zhang, J.; Wu, Z., Electrochemical membrane bioreactors for

565

sustainable wastewater treatment: Principles and challenges. Curr. Environ. Eng. 2015, 2,

566

38-49.

567

19. Huang, J.; Wang, Z.; Zhang, J.; Zhang, X.; Ma, J.; Wu, Z., A novel composite conductive

568

microfiltration membrane and its anti-fouling performance with an external electric field in

569

membrane bioreactors. Sci. Rep. 2015, 5, 9268.

570

20. Wang, Y. K.; Li, W. W.; Sheng, G. P.; Shi, B. J.; Yu, H. Q., In-situ utilization of generated

571

electricity in an electrochemical membrane bioreactor to mitigate membrane fouling. Water

572

Res. 2013, 47 (15), 5794-5800.

573

21. El-Ghenymy, A.; Garrido, J. A.; Centellas, F.; Arias, C.; Cabot, P. L.; Rodríguez, R. M.; 29



574

Brillas, E., Electro-Fenton and photoelectro-Fenton degradation of sulfanilic acid using a

575

boron-doped diamond anode and an air diffusion cathode. J. Phys. Chem. A 2012, 116 (116),

576

3404-3412.

577

22. Zerbinati, O.; Vincenti, M.; Pittavino, S.; Gennaro, M. C., Fate of aromatic sulfonates in

578

fluvial environment. Chemosphere 1997, 35 (10), 2295-2305.

579

23. Liu, L.; Zhao, F.; Liu, J.; Yang, F., Preparation of highly conductive cathodic membrane

580

with graphene (oxide)/PPy and the membrane antifouling property in filtrating yeast

581

suspensions in EMBR. J. Membr. Sci. 2013, 437, 99-107.

582

24. de Lannoy, C. F.; Jassby, D.; Gloe, K.; Gordon, A. D.; Wiesner, M. R., Aquatic biofouling

583

prevention by electrically charged nanocomposite polymer thin film membranes. Environ. Sci.

584

Technol. 2013, 47 (6), 2760-2768.

585

25. Liu, L.; Liu, J.; Gao, B.; Yang, F., Minute electric field reduced membrane fouling and

586

improved performance of membrane bioreactor. Sep. Purif. Technol. 2012, 86, 106-112.

587

26. Nogueira, R. F. P.; Oliveira, M. C.; Paterlini, W. C., Simple and fast spectrophotometric

588

determination of H2O2 in photo-Fenton reactions using metavanadate. Talanta 2005, 66 (1),

589

86-91.

590

27. Keenan, C. R.; Sedlak, D. L., Factors affecting the yield of oxidants from the reaction of

591

nanoparticulate zero-valent iron and oxygen. Environ. Sci. Technol. 2008, 42 (4), 1262-1267.

592

28. Zhou, X.; Mopper, K., Determination of photochemically produced hydroxyl radicals in

593

seawater and freshwater. Mar. Chem. 1990, 30 (90), 71-88.

594

29. Moon, G.; Kim, W.; Bokare, A. D.; Sung, N.; Choi, W., Solar production of H2O2 on

595

reduced graphene oxide–TiO2 hybrid photocatalysts consisting of earth-abundant elements 30


Page 30 of 33

Page 31 of 33


596

only. Energy Environ. Sci. 2014, 7 (12), 4023-4028.

597

30. Asmus, K. D.; Moeckel, H.; Henglein, A., Pulse radiolytic study of the site of hydroxyl

598

radical attack on aliphatic alcohols in aqueous solution. J. Phys. Chem. 2002, 77 (10),

599

1218-1221.

600

31. Wang, X. M.; Waite, T. D., Iron speciation and iron species transformation in activated

601

sludge membrane bioreactors. Water Res. 2010, 44 (11), 3511-3521.

602

32. Foo, K. Y.; Hameed, B. H., A short review of activated carbon assisted electrosorption

603

process: An overview, current stage and future prospects. J. Hazard. Mater. 2009, 170 (2-3),

604

552-559.

605

33. Porada, S.; Zhao, R.; Wal, A. V. D.; Presser, V.; Biesheuvel, P. M., Review on the science

606

and technology of water desalination by capacitive deionization. Prog. Mater. Sci. 2013, 58

607

(8), 1388-1442.

608

34. Bataineh, H.; Pestovsky, O.; Bakac, A., pH-induced mechanistic changeover from

609

hydroxyl radicals to iron(IV) in the Fenton reaction. Chem. Sci. 2012, 3 (5), 1594-1599.

610

35. Ensing, B.; Buda, F.; Gribnau, M. C. M.; Baerends, E. J., Methane-to-methanol oxidation

611

by the hydrated iron(IV) oxo species in aqueous solution: a combined DFT and car-parrinello

612

molecular dynamics study. J. Am. Chem. Soc. 2004, 126 (13), 4355-4365.

613

36. Lair, V.; Antony, H.; Legrand, L.; Chaussé, A., Electrochemical reduction of ferric

614

corrosion products and evaluation of galvanic coupling with iron. Corros. Sci. 2006, 48 (8),

615

2050-2063.

616

37. Özcan, A.; Oturan, M. A.; Oturan, N.; Şahin, Y., Removal of acid orange 7 from water by

617

electrochemically generated Fenton's reagent. J. Hazard. Mater. 2009, 163 (2-3), 1213-1220. 31



618

38. Sirés, I.; Centellas, F.; Garrido, J. A.; Rodríguez, R. M.; Arias, C.; Cabot, P. L.; Brillas, E.,

619

Mineralization of clofibric acid by electrochemical advanced oxidation processes using a

620

boron-doped diamond anode and Fe2+ and UVA light as catalysts. Appl. Catal. B Environ.

621

2007, 72 (3-4), 373-381.

622

39. El-Ghenymy, A.; Oturan, N.; Oturan, M. A.; Garrido, J. A.; Cabot, P. L.; Centellas, F.;

623

Rodríguez, R. M.; Brillas, E., Comparative electro-Fenton and UVA photoelectro-Fenton

624

degradation of the antibiotic sulfanilamide using a stirred BDD/air-diffusion tank reactor.

625

Chem. Eng. J. 2013, 234 (234), 115-123.

626

40. Hammami, S.; Bellakhal, N.; Oturan, N.; Oturan, M. A.; Dachraoui, M., Degradation of

627

acid orange 7 by electrochemically generated • OH radicals in acidic aqueous medium using a

628

boron-doped diamond or platinum anode: A mechanistic study. Chemosphere 2008, 73 (5),

629

678-684.

630

41. Cartwright, P. S., The role of membrane technologies in water reuse applications. Desalin.

631

Water Treat. 2013, 51 (25-27), 4806-4816.

632

42. Lin, H.; Niu, J.; Xu, J.; Huang, H.; Li, D.; Yue, Z.; Feng, C., Highly efficient and mild

633

electrochemical mineralization of long-chain perfluorocarboxylic acids (C9-C10) by

634

Ti/SnO2-Sb-Ce, Ti/SnO2-Sb/Ce-PbO2, and Ti/BDD electrodes. Environ. Sci. Technol. 2013, 47

635

(22), 13039-13046.

636

43. Mccarty, P. L.; Bae, J.; Kim, J., Domestic wastewater treatment as a net energy

637

producer--Can this be achieved? Environ. Sci. Technol. 2011, 45 (17), 7100-7106.

638 639 32


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TOC Figure

641 642

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