Energy-Efficient Oxidation and Removal of Arsenite from Groundwater

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Letter

Energy-Efficient Oxidation and Removal of Arsenite from Groundwater Using Air-Cathode Iron Electrocoagulation Yanxiao Si, GUANGHE LI, and Fang Zhang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.6b00430 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016

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Environmental Science & Technology Letters

1

Energy-Efficient

Oxidation

and

Removal

of

Arsenite

2

Groundwater Using Air-Cathode Iron Electrocoagulation

3 4 5

Yanxiao Si, Guanghe Li*, Fang Zhang*

6

School of Environment and State Key Joint Laboratory of Environment Simulation

7

and Pollution Control, Tsinghua University, Beijing, 100084, China

8

Key Laboratory of Solid Waste Management and Environment Safety (Tsinghua

9

University), Ministry of Education, Beijing 100084, China

10 11

*Corresponding author.

12

Guanghe Li: Email: [email protected], phone: 86-10-62792236

13

Fang Zhang: Email: [email protected], phone: 86-10-62789655

14

1

ACS Paragon Plus Environment

from


15

+

e-

e-

Fe0 Fe(II)

+

Air-cathode

H2O2 As (III)

Fe (IV) ·OH

O2

As (V)

O2 Fe (III)

16 17

2


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Abstract: Arsenic contamination in groundwater has affected many countries

19

especially in Southeast Asia. Although aerated electrocoagulation (EC) provides an

20

effective way for arsenite removal, this process is energy-intensive due to aeration and

21

relatively poor cathode performance. To overcome these disadvantages, a novel EC

22

system was proposed using an air-cathode to generate H2O2 in situ for improved

23

energy-efficiency of As(III) removal. With the air-cathode, the H2O2 production rate

24

was 3.7 ± 0.1 mg L–1 h–1, which indirectly promoted the As(III) oxidation by

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interaction with Fe(II). At a current density of 4 A m–2, the average cell voltage in the

26

air-cathode electrocoagulation (ACEC) was 1.0 V, compared to 1.9 V in the EC and

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aerated EC systems. The energy consumption in the ACEC system was 17.0 ± 0.7

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Wh log–1 m–3, much lower than those in the EC (67.8 ± 0.9 Wh log–1 m–3) and

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aerated EC systems (65.1 ± 0.8 Wh log–1 m–3), which can be attributed to the

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effective As(III) oxidation, no need for aeration, and increased cathode performance

31

with the air-cathode. These results showed that the ACEC system is a promising

32

technology for energy-efficient arsenite removal from contaminated groundwater.

33 34

Introduction

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Arsenic contamination in groundwater occurs naturally throughout the world,

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especially in Southeast Asia, including Bangladesh, India, China and Vietnam.1-3 Tens

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of millions of people worldwide are exposed to arsenic due to the use of contaminated

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groundwater as their drinking water supplies.2, 3 Arsenic primarily exists as inorganic

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form in natural waters, i.e., negatively charged arsenate [As(V), HAsO42-] and

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uncharged arsenite [As(III), H3AsO3]. Arsenite can account for as much as 50-90% of

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the total arsenic, due to highly reducing conditions in the subsurface.4, 5 Because of

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chronic effects including skin, cardiovascular and respiratory diseases caused by 3



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long-term arsenic exposure,6-8 many countries including the US and China have

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adopted stricter arsenic standards for drinking water, lowing the maximum arsenic

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contaminant level from 50 to 10 µg L–1.9

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Due to the much smaller affinity of arsenite to absorbent,10 arsenite is usually

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oxidized to negatively charged arsenate for better removal efficiency. Various

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synergistic oxidation and removal techniques have been employed for arsenite

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removal, including ZVI/O2,11 synthetic siderite,12 magnetic iron oxide nanoparticles,13

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iron and manganese binary oxide.14,

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Fenton-like reactions or Mn(IV) to oxidize As(III) and remove As(V) by the generated

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iron oxide.14, 16 However, As(III) removals were less than 80% within 1 hour in these

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studies due to a relatively slow reaction kinetics.11-15 Electrocoagulation (EC), with

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iron as the anode, has shown faster and higher arsenite removal when combined with

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aeration.17,

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oxidation of Fe(II) by dissolved oxygen, which could effectively oxidize As(III).19 For

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example, a 99% removal of As(III) was achieved at 37~46 A m–2 in 90 s using a

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relatively low initial arsenite concentration of 25~50 µg L–1 in Mexico.20 With a

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higher initial concentration of ~133 µg L–1, the final concentration fell below 10 µg L–

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1

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µg L–1) could be decreased to 10 µg L–1 in 1.5 h, with a total energy consumption of

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0.72~0.78 kWh m–3.22 This process is energy-intensive, partially due to aeration,

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which requires quite amount of energy (0.04~0.5 kWh m–3). 23, 24

18

15

These methods utilize heterogeneous

Some oxidant species [Fe(IV) or ·OH] were generated during the

within 20 s (30 A m–2).21 In field trials for treatment of 50 L water, arsenic (449~667

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The air-cathode, which contains a hydrophobic gas diffusion layer to allow direct

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exposure of that side to air and thus passive oxygen diffusion, has been widely studied

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in electrochemical systems due to the advantage of energy-saving without the need for

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aeration.

25, 26

The oxygen transfer coefficients by air-cathodes in previous studies 4


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were approximately 7.7~16.1 ×10–9 m2 s–1, higher than those in aeration systems

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(1.98~2.41 ×10–9 m2 s–1).25,

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carbon black or graphite as a catalyst showed effective two-electron oxygen reduction

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to produce H2O2,28, 29 which could contribute to As(III) oxidation directly or indirectly

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through the interaction with Fe(II). In this study, a novel energy-efficient EC system,

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air-cathode iron electrocoagulation (ACEC), was proposed for synergistic oxidation

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and removal of As(III), using an air-cathode instead of the Fe cathode. The As(III)

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removal rate and energy consumption of the new ACEC and the conventional EC

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systems were compared.

27

In addition to the four-electron reduction pathway,

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Materials and methods

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Reactor construction

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Bottle-shaped ACEC and EC reactors were constructed using armed media

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bottles (100 mL capacity).31 The air-cathode was prepared using a phase inversion

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method as previously described,26 with a stainless steel mesh (mesh size of 60 per

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inch, wire diameter of 0.15 mm) as the current collector, activated carbon and carbon

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black mixture (activated carbon loading of 26.5 mg cm–2 and carbon black loading of

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2.65 mg cm–2, ratio of 10:1) as the catalysts, and PVDF (loading of 8.8 mg cm-2) as

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the binder and diffusion layer. The cathode used in the EC reactor was bare stainless

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steel mesh, which is commonly used in EC processes.21,

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projected surface area of 11.2 cm2. Anodes in the ACEC and EC reactors were

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stainless steel mesh with a projected surface area of 5 cm2. The spacing between the

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two electrodes was 3 cm.

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Batch experiments

5


22

Both cathodes had a


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Experiments were carried out using synthetic Bangladesh groundwater (SBGW)

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as described in British Geological Survey.1, 32 SBGW contained 8.2 mM NaHCO3, 2.5

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mM CaCl2, 1.6 mM MgCl2, 0.025 mM NaH2PO4 and 0.246 mM Na2SiO3

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(conductivity of 1500 µs cm–1). NaAsO2 (99.8%, Shanghai Sinopharm, China) was

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added to prepare As(III) solutions of 500 µg L–1.

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Treatment of synthetic groundwater with different types of reactor configurations

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(ACEC, conventional EC, and EC with aeration) were examined at a set current

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density of 4 A m–2 using a potentiostat (VMP3; BioLogic, Claix, France) in a

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two-electrode setup. For each test, the concentration of dissolved oxygen (DO) in

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solution was reduced to 1 mg L–1 by purging with N2 (99.99%). In the aerated EC

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system, the reactor configuration and electrode materials were the same as the

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conventional EC system, and pure oxygen was sparged into the reactor with a flow

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rate of 40 mL min-1. All the reactors were operated in duplicate at room temperature

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and stirred at 200 rpm. The initial pH after N2 sparging was 8.9, without adjustment

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during the experiments. The variations of pH in all systems were less than 1.

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To explore the role of H2O2 in ACEC system, a cation exchange membrane

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(CEM, Membranes International Inc.) was added to separate the cathode and anode

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chamber so that the anode solution had little H2O2. The As(III) removal rates in the

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anode chamber with or without 3 mg L–1 H2O2 addition were measured. Both

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chambers were stirred at 200 rpm.

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Measurements and calculations

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The samples were first filtered through a 0.45-µm membrane, and then the As(III)

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and As(Tot) concentrations were measured using a hydride generation atomic

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fluorescence spectrophotometer (HG-AFS; Jitian Co., China). To selectively detect

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As(III), we followed the procedures outlined by Roberts et al.33 H2O2 was measured 6


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with a UV-spectrophotometer with the titanium (IV) sulfate method at 405 nm.34

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Linear sweep voltammetry (LSV) was used to measure the electrochemical

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performance of different cathodes in a 50 mM phosphate buffer (pH = 7). Both the Fe

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cathode and the air-cathode were tested in the electrochemical cell with and without

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aeration (40 mL min–1), using the three-electrode setup with the Ag/AgCl reference

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electrode (potential of 0.198 V vs standard hydrogen electrode, SHE) and Pt counter

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electrode.

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Energy consumption W (Wh m–3) for the electrode reaction in the three systems

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was calculated as WE=UIt/V where U was the voltage (V), I the current (A), t the

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electrolysis time (h), and V the volume (m3). To provide a reference for the

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comparison between EC with aeration and ACEC, the aeration (WO) energy was

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assumed to be 0.04 kWh m–3 when an aeration system was applied with a proper and

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efficient blower.35 EEO is the electrical energy (kWh) required to reduce the As(Tot)

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concentration per order of magnitude per m3 of SBGW.29

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EEO =

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where c0 and c are the initial and final As(Tot) concentrations.

(

)

133 134

Results and discussion

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Arsenic removal and energy consumption in ACEC and EC systems

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The As(III) and As(Tot) removal in the air-cathode electrocoagulation (ACEC)

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was investigated and compared with the conventional EC and EC with aeration

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(EC/O2). With an initial As(III) concentration of 500 µg L–1 and a current density of 4

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A m–2, As(III) removal was 91.5 ± 3.2% (± standard deviation for duplicate reactors)

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in 30 min and further increased to 98.8 ± 0.02% after 60 min in the ACEC reactor (Fig.

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1). The As(Tot) removal was slightly lower and reached 96.3 ± 0.4% after 60 min in 7



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the ACEC system. Conventional EC had a much smaller As(III) removal of 80.9 ± 4.2%

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and As(Tot) removal of 78.7 ± 4.8% after 60 min, likely due to the ineffective

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oxidation of As(III) without aeration and the slower removal kinetics of As(III)

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compared to As(V) (Fig. S1). In the EC/O2 system, although the initial removals were

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higher than those in the ACEC reactor, they became lower after ~30 min, with 95.9 ±

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1.5% for As(III) and 95.4 ± 3.6% for As(Tot) at 60 min (Fig. 1A). The ACEC system

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outcompeted the EC/O2 system after 30 min, likely due to the different As(III) and

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Fe(II) oxidation mechanisms with the direct production of H2O2 in the ACEC system

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compared to that with O2 in the EC/O2 system.36

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The energy per order of magnitude (EEO) was estimated to compare the energy

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cost of ACEC with the EC and EC/O2 systems. For the ACEC and EC systems, we

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considered only the electrical energy consumption (WE=UIt/V), while aeration energy

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was also included for the EC/O2 system. At a current density of 4 A m–2, the average

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cell voltage in the ACEC reactor was 1.0 V, compared to the average voltage of 1.9 V

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in the EC and EC/O2 systems (Fig. S2). The EEO in the ACEC reactor was only 17.0

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± 0.7 Wh log–1 m–3, which was only ~26% of those in the EC/O2 (65.1 ± 0.8 Wh

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log–1 m–3) and EC (67.8 ± 0.9 Wh log–1 m–3) systems (Fig. 1 B). This much lower

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EEO in the ACEC reactor was due to the more favorable oxygen reduction reaction

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(ORR) that occurred at the air cathode than the energy-intensive hydrogen evolution

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reaction (HER) for the conventional EC cathode, as well as the high arsenic removal

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efficiency in the ACEC system. Considering only the electrical energy without the

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contribution of aeration, the electrical energy per order of magnitude in the EC/O2

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was 34.4 ± 0.8 Wh log–1 m–3, lower than that in the EC reactors using the same

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electrodes, which can be attributed to the enhanced arsenic oxidation and removal

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with the aeration. 8


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The ACEC system showed comparable arsenic removals, but much lower values

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of EEO (17.0 ± 0.7 Wh log–1 m–3) than those reported in the literature (Table S1). In

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the previous aerated EC systems, the energy per order of magnitude was 51.1~56.5

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Wh log–1 m–3, with a lower initial As(III) concentration of 131 µg L–1.21 With a higher

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As(III) initial concentration of 1000 µg L–1, the value increased to 187.5 Wh log–1 m–

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3 22

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for As(III) removal in ACEC was also less than that for As(V) removal in the EC

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system (150 µg L–1, 25.7 Wh log–1 m–3).37 These results suggest that the ACEC system

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is a promising energy-efficient approach for effective arsenic removal.

, one order of magnitude higher than that obtained in our study. The energy required

176 177

Cathode performance in electrochemical tests

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LSV tests were conducted using the air-cathode and the Fe cathode to evaluate

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the electrochemical performance of cathodes. Compared with the Fe cathode, the

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air-cathode showed much higher current densities at the same applied potentials (Fig.

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2), suggesting greatly improved the cathode performance. Based on the

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voltammograms, the reaction at the air-cathode had a more favorable ORR (onset

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potential of 0.65 V) compared to the HER at the Fe cathode (onset potential of –0.70

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V).

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Aeration showed little effect on voltammograms for both cathodes. For the

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air-cathode, changing the operation mode from passive oxygen diffusion to aeration

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did not affect the cathode performance, suggesting that passive oxygen diffusion

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provided a similar oxygen flux as aeration. The small change of voltammogram

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values for the Fe cathode indicated that the reaction at the cathode remained HER

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under the aeration condition. This electrochemical result was consistent with the

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previous EC operation result that similar cell voltages were obtained with and without 9



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aeration.

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The role of cathodically produced H2O2 in As(III) oxidization

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The CEM was used to separate the anode and cathode chambers in the ACEC

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reactor to examine the role of cathodically produced H2O2 in As(III) oxidation. The

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measured H2O2 production rate was 3.7 ± 0.1 mg L–1 h–1 in the cathode chamber (Fig.

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3). The addition of the CEM greatly reduced the H2O2 intrusion to the anode chamber,

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resulting in dramatically decreased As(III) removals in the anode chamber from 98.8

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± 0.02% to 48.9 ± 4.5% within 60 min, at a set current of 4 A m–2 (Fig. 3). The As(III)

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removal increased back to 93.6 ± 0.5% when 3 mg L–1 H2O2 was added in the anode

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chamber. These results suggested that cathodically produced H2O2 was crucial for

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As(III) oxidation and removal. However, when 3 mg L–1 H2O2 was added in the anode

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chamber under open circuit condition, the As(III) removal was only 7.8 ± 0.02% after

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60 min (Fig. S3), suggesting that H2O2 could not directly oxidize the As(III) at a

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considerable rate. Thus in the ACEC system, the As(III) was more likely oxidized by

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the oxidant species generated during the reaction between cathodically produced H2O2

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and anodically generated Fe(II). This result showed that the interaction between H2O2

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and the iron species was important for enhanced As(III) removal in our ACEC system,

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which was consistent with previous studies that addition of H2O2 to the ZVI/O2

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corrosion system accelerated the As(III) oxidization and removal.38

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Outlook

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ACEC has the advantage of easy control and operation, providing a simple and

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efficient approach for synergistic arsenite oxidation and removal. The air-cathode was

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~18% more expensive than the stainless steel cathode (Table S2), but it could lead to

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an energy savings of 74% compared to the conventional EC system. The materials 10


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needed for making the air-cathode are readily available, and the fabrication process

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(phase-inversion method) is scalable. Based on previous long-term evaluations of

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air-cathodes in bioelectrochemical systems,39 it is promising that our air-cathode iron

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electrocoagulation can be applied either for water treatment plants or for decentralized

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water supply systems with good long-term stability in the regions affected by arsenic

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contaminated groundwater. 40

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ASSOCIATED CONTENT

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Supporting Information Available: Figures S1-S3 and Table S1-S2. This material is

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

228 229

AUTHOR INFORMATION

230

Corresponding Author

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*Phone: 86-10-62792236 ; Email: [email protected]

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*Phone: 86-10-62789655 ; Email: [email protected]

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Notes

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The authors declare no competing financial interest

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ACKOWLEDGEMENT

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This work was supported by the National Natural Science Foundation of China

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(No. 41672236), National High Technology Research and Development Program of

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China (2013AA06A207), State Key Joint Laboratory of Environment Simulation and

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Pollution Control (15Y03ESPCT), and Tsinghua University Initiative Scientific

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Research Program (20151080353). Dr. Fang Zhang would also like to acknowledge

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the support from the Thousand Talents Plan for Young Professionals, and Yong Elite 11



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Scientist Sponsorship Program by CAST (2015QNRC001).

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Figure captions:

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Fig. 1 (A) Arsenic removal, and (B) Energy per order of magnitude of total arsenic

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removed (EEO) by the ACEC, EC and EC/O2 systems with an applied current density

354

of 4 A m–2 and initial As(Tot) concentration of 500 µg L–1. (Error bars ± SD based on

355

measurements of two duplicate reactors.)

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Fig. 2 LSV of air-cathode and closed air-cathode with aeration; Fe cathode and Fe

357

cathode with aeration in 50 mM phosphate buffer (pH = 7).

358

Fig. 3 As(III) removals in the ACEC reactor, and in the anode chamber separated by

359

the CEM with and without addition of 3 mg L–1 of H2O2. The y-axis on the right is for

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H2O2 production in the cathode chamber.

15



Table of Contents 49x32mm (300 x 300 DPI)


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Fig. 1 (A) Arsenic removal, and (B) Energy per order of magnitude of total arsenic removed (EEO) by the ACEC, EC and EC/O2 systems with an applied current density of 4 A m–2 and initial As(Tot) concentration of 500 µg L–1. (Error bars ± SD based on measurements of two duplicate reactors.) manuscript 146x157mm (300 x 300 DPI)



Fig. 2 LSV of air-cathode and closed air-cathode with aeration; Fe cathode and Fe cathode with aeration in 50 mM phosphate buffer (pH = 7). manuscript 203x143mm (300 x 300 DPI)


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Fig. 3 As(III) removals in the ACEC reactor, and in the anode chamber separated by the CEM with and without addition of 3 mg L–1 of H2O2. The y-axis on the right is for H2O2 production in the cathode chamber. manuscript 77x47mm (300 x 300 DPI)


Energy-Efficient Oxidation and Removal of Arsenite from Groundwater

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