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An Energy-efficient Electrochemical Strategy for the Oxidative Sequestration of As(III) in the Synthesized Anoxic Groundwater Bo Jiang, Shuaishuai Xin, Yijie Liu, Congcong Nin, Xuejun Bi, and jianliang xue Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01013 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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Industrial & Engineering Chemistry Research

An Energy-efficient Electrochemical Strategy for the Oxidative Sequestration of As(III) in the Synthesized Anoxic Groundwater Bo Jiang†,‡,*, Shuaishuai Xin†, Yijie Liu†, Congcong Nin†, Xuejun Bi†, Jianliang Xue§,

†

School of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266033, PR China;

‡

State Key Laboratory of Petroleum Pollution Control, CNPC Research Institute of Safety and Environmental Technology, Beijing, 102206, China

§

College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, PR China

*Corresponding author: [email protected] (B. Jiang)

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In this study, a wave rectified alternating current (RAC) electrocoagulation process was developed for the oxidative sequestration of As(III) via simultaneous generating Fe(II) and O2 in the synthesized anoxic groundwater. The optimal TFe-anode/TMMO-anode:Tpower-off ratio (T is the time-taken in corresponding stage) and reaction period for As(III) sequestration were 1:2:1 and 24 s, respectively. Elevating electric current applied on the alternate iron/MMO anodes (30-50 mA) and solution pH (6.0-9.0) promoted As(III) sequestration, whereas the presence of HCO3− and PO43- deteriorated As(III) sequestration. The intermediate Fe(IV) primarily accounted for the oxidation of As(III) to As(V), which can be readily sequestrated by the freshly generated Fe(III) (oxyhydr)oxides. The energy required for As(III) sequestration in the RAC electrocoagulation process was only 0.11 kW·h m-3, which is much less than those in the traditional DC/AC electrocoagulation processes. Generally, this study offers an energy-efficient strategy to improve access to safe groundwater for millions of people.

Keywords: Arsenic sequestration; Electrocoagulation; Anoxic groundwater; Alternate anode; Oxidation


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1. Introduction Groundwater is extensively utilized as the drinking water resource all over the world in consideration of its relatively stable biochemical properties 1. Unfortunately, more than 100 million people worldwide are at risk of exposure to the elevated levels of hypertoxic arsenic (As), a natural groundwater contaminant, most of them in India, Bangladesh and Vietnam, etc.

2, 3

. Specifically, As

concentrations in millions of water wells in Bangladesh and West Bengal were estimated up to 1,000 mg L-1, which are significantly above the World Health Organization (WHO) maximum contaminant level (10 µg L-1) 4, 5. In the anoxic groundwater, As exists primarily in the form of trivalent arsenite (As(III)) as a result of chemically reducing aquifer conditions 6. As compared with As(V), As(III) exhibits more toxic and owns higher mobility, which can be easily desorbed from its minerals 7. Thus, removing As(III) from the contaminated groundwater is a challenging but urgent task to avoid the risk of As(III) in certain regions. Till now, to mitigate this problem, a variety of in-situ technologies for As(III) removal from groundwater are developed and some have been implemented over the last few years. In alkaline Fe(II)-deficient groundwater, aerated water and iron ions can be simultaneously injected to the subsurface through the tube-wells to achieve the remediation of As(III) 8, 9. For instance, Welch et al. 8 studied subsurface arsenic (30-36 µg L-1) removal with a combination of aerated water and the injection of Fe(III) ion (0.15-6.35 mg L-1) in two wells with depths of >30 m in Carson Valley, NV, USA. By setting the flow rate about 120 L h-1, the injected water volume was 790-2880 L and the extracted volume was 1340-3900 L, which finally reduced the As levels below 6 µg L-1 after conducting several trials. But the operation of co-injecting aerated water and iron ions is difficult to control and regulate during the in-situ remediation of As in the groundwater 10.



Recently, continuous introduction of Fe(II) via iron-based electrocoagulation process offers significant potential for remedying inorganic/organic-contaminated groundwater owning to its several unique benefits, such as easy operation, low production of solid waste and sustainable operation in rural areas

11-14

. During this process, a small direct current (DC) voltage is normally

applied between two iron electrodes, resulting in the anodic dissolution of iron electrode into dissolved Fe(II). When combining with the aeration process, the produced Fe(II) could be oxidized to Fe(III), which then hydrolyzes into the poorly soluble Fe(III) (oxyhydr)oxides with an appreciable As adsorption affinity at circumneutral pH. For instance, in field trials for the treatment of 50 L of water, As concentration (449−667 µg L−1) could be successfully decreased to 10 µg L−1 within 1.5 h with a energy consumption of 0.72−0.78 kW·h m−3 for the DC electrocoagulation process 15. In spite of this, the utilization of O2 in the air, externally aerated into the subsurface, is considerably low due to the relatively low solubility of O2 (0.3 mM, 25 ºC), which therefore adds energy costs to the electrocoagulation process. To address this limitation, a dual anode system was originally developed by Yuan et al. 10, 16 for the remediation of the synthesized anoxic As(III)-contaminated groundwater. In this reaction system, electric current was proportionately applied on iron anode and another inert anode, which could give rise to the simultaneous productions of Fe(II) and O2, respectively, in the anoxic and iron-deficient groundwater. The experimental results show that 500 µg L-1 As(III) can be almost completely oxidized to As(V) and then precipitated from the solution within 30 min when 60 mA was equally applied to the two anodes. However, this DC-based treatment strategy is complicated and inevitably suffers from the limited diffusion of oxygen. Thus, it is essential to develop an energy-efficient electrochemical strategies to create a simply and well-controlled artificial environment for the in-situ


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remediation of As-contaminated groundwater. To this end, this study developed a wave rectified alternating current (RAC) electrocoagulation process and evaluated its performance for As(III) removal from the anoxic groundwater. The RAC electrocoagulation process can in-situ produce Fe(II) and O2 simultaneously with utilizing an iron rod and a Ti plate coated mixed IrO2/Ta2O5 (MMO) as the alternate anode. Here, the effects of electric current and TFe-anode/TMMO-anode:Tpower-off ratio (T is the time-taken in corresponding stage) on As(III) sequestration were evaluated. In addition, the influences of solution chemistry (e.g, solution pH, inorganic ions and temperature) were also investigated during the sequestration of As(III). Moreover, scavenging experiments were conducted to qualitatively detect the intermediate oxidants in the RAC electrocoagulation process to clarify the underlying mechanisms of As(III) sequestration. In general, present study sought to examine the feasibility of RAC electrocoagulation process for As(III) sequestration from the anoxic groundwater. 2. Experimental methods 2.1. Chemicals and reagents Antimony potassium tartrate (K(SbO)C4H4O6·0.5H2O, >99%), ammonium molybdatetetrahydrate ((NH4)6Mo7O24·4H2O, >99%), L-ascorbic acid (C6H8O6, >99.7%), isopropyl alcohol (99.5%), Na2SO4 (99%), H2SO4 (73.0%~75.0%), NaHCO3 (99.5%), NaOH (>96%), CaCl2 (99.5%), HCl (36%~38%), MgCl2 (98%), NaH2PO4 (99%), and Na2SiO3 (98%) were supplied by Sinopharm Chemical Reagent Co. Ltd., China. Sodium arsenite (NaAsO2, 97%) and sodium arsenate (Na2HAsO4·7H2O, 99%) were purchased from Xiya Reagent and Jiangxi Qianhua Industry Co., Ltd., respectively. Ultrapure water (resistivity 18.2 MΩ·cm) was utilized in all experiments. 2.2. Experimental procedure



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The stock As(III) solution (1.0 mM) was diluted to prepare all working solutions with 0.02 M Na2SO4 as supporting electrolyte. Concentrated H2SO4 or NaOH solution was used to adjust the solution pH to the desired values. A 9 cm2 MMO (IrO2 and Ta2O5 coated on Ti plate, Shanxi Kaida Chemical Ltd., China) and a stainless steel rod (φ6 mm × 50 mm) were used as the alternate anodes. The electrodes gap was fixed to be 20 mm. Galvanostatic electrolyses were performed by a DC regulated power supply (voltage 0-30.0 V, electric current 0-5.0 A). A programmable time relay (JSPG–120–B, China) was applied to output the wave rectified alternating current and regulate the working electric parameters of RAC electrocoagulation process. The schematics of the RAC electrocoagulation process were provided in Fig. 1. Compressed N2 (99.99%) was bubbled through the solution at a flow rate of 300 mL min-1 using a fritted glass diffuser to maintain an anoxic condition for the electrocoagulation process. Then, the reactor was sealed under N2 atmosphere and stirred with a magnetic stir bar. And the typical aqueous reaction of 250 mL working solution ([As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8.0, I = 40 mA, temperature 25 °C) was initiated after 30 min bubbling of compressed N2. Aqueous samples were taken from the cell at various intervals for chemical analysis. 2.3. Analysis After sampling, 3 mM 2, 2'-bipyridine was immediately introduced to quench the oxidation of Fe(II) by O2 in the aqueous samples 17. One group of aqueous samples were filtered through a 0.45 µm pore-sized nylon filters for determining the concentrations of the dissolved As and Fe. Adding 1 M HCl into the other group to dissolve the precipitant for the analysis of total As and Fe. As and Fe species in solid phase were calculated from the difference of the above two measurements. The concentration of As(V) were quantified using the molybdate-based method


18

. Briefly, 0.5 mL of

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methanol, 0.5 mL of 2% HCl and 0.3 mL of coloring reagent were sequentially added into each 5.0 mL of aqueous sample. After 30 min's color development, the absorbance of the mixture was detected at 880 nm on an UV−vis spectrophotometer (F97PRO, Lengguang Tech.). To determine total As (As(tot)), 2% HCl containing 2 mmol L-1 KIO3 was used instead of 2% HCl solution. As(III) concentration was obtained by calculating the concentration difference between As(tot) and As(V). When phosphate co-existed with As, As concentration was analyzed by inductively coupled plasma spectrometer (ICP). Fe(II) concentration was spectrophotometrically determined at the wavelength of 510 nm using a modified phenanthroline method 19. The premix was prepared by orderly adding 2 mL of ultrapure water, 6 mL of chromogenic agent, 0.2 mL of aqueous solution and 0.2 mL of sample solution. For total Fe (Fe(tot)) analysis, 2 mL of 10% hydroxylamine was used instead of 2 mL deionized water. Fe(III) concentration was obtained by calculating the concentration difference between Fe(tot) and Fe(II). The precipitates formed in RAC electrocoagulation process were washed with ultrapure water and dried at 50 °C for 24 h. The abrasive powder was characterized by XPS (PHI5000 Versa Probe, ULVAC-PHI, Japan). All the operations including filtration, washing and drying were conducted under an N2 atmosphere. 3. Results and discussions 3.1. Oxidative sequestration of As(III) in the RAC electrocoagulation system In consideration of high efficiency, environmental friendly and easy to attain, iron is usually used as the active anode in the DC electrocoagulation process. During this process, Fe(III) (oxyhydr)oxides can be generated from the dissolution of iron anode, which are capable to immobilize toxic As(III)/As(V) species via coprecipitation or adsorption. However, Fig. 2 (a) shows that, in the synthesized anoxic groundwater, only 7% of As(tot) was removed after 30 min's DC



electrocoagulation process with Fe as the anode. Although approximately 1.35 mM Fe species can be electrochemically produced in the electrolyte, which primarily exists as Fe(II) (data not shown). The derived Fe(II) (oxyhydr)oxides in pale green owns inferior As adsorption affinity to that of Fe(III) (oxyhydr)oxides

15, 20

. Besides, electrolysis by non-corrosive MMO anode is often applied for the

production of O2 because it is efficient for sustaining the O2 evolution reaction. In addition, MMO electrode has been used for electrocatalytic oxidation of refractory organic compounds in wastewaters

21

. Consequently, it is rational in Fig. 2(a) that approximately 100 µg L-1 As(III) was

oxidized to As(V) within 30 min in the DC electrolysis system when employing MMO and iron as the anode and cathode, respectively. However, As(tot) was negligibly removed from the electrolyte due to no production of flocculant in present reaction process. Based on the above experimental results, productions of O2 and iron flocculant should be indispensable for As(tot) sequestration in anoxic environment. As shown in Fig. 1, in the RAC electrocoagulation process, driven by positive potential applied to iron (Stage I), the iron anode can electrochemically dissolve into Fe(II) ion, while in Stage III, the positive potential applied to the MMO anode results in the production of O2. The RAC electrocoagulation process is power off in Stage II and IV. In these two stages, Fe(II) produced in Stage I and O2 generated in Stage III can diffuse into the bulk solution for the oxidative formation of Fe(III) (oxyhydr)oxides. Fig. 2(a) shows that, in the RAC electrocoagulation system, approximately 92% of As(tot) was precipitated out from the electrolyte within 30 min, which mainly existed as As(V) in the precipitant, when applying an electric current of 40 mA on the alternate MMO and Fe anodes. The concentration of As(III) gradually decreased to approximately 5 µg L-1 within 30 min; while the concentration of As(V) in solution increased to 78 µg L-1 after 5 min's reaction but decreased afterward. These experimental


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results indicate that As(V) formation cannot be only ascribed to the heterogeneous oxidation of As(III) on MMO anode. Besides, as shown in Fig. 2(b), approximately 0.275 mM Fe(tot) was produced within 30 min and then almost entire Fe species transformed into the precipitant and existed as Fe(III). This suggests that the produced O2 was sufficient for the oxidative conversion of Fe(II) to Fe(III)

15, 20

. It should be noted that in present RAC electrocoagulation process, although

reaction time of 30 min was required for the removal of 500 µg L-1 As(III), actual working time of iron as the anode was 7.5 min, which was much less than that required in the DC electrocoagulation process. For example, approximately 1.94 mM Fe species was produced within 2 h at a current of 22 mA, which was required for the removal of 100 μg L-1 As(III) in the DC electrocoagulation process [12]. This suggests that the utilization of Fe(II) species dissolved from iron anode was high in the RAC electrocoagulation process, which therefore produced minimal amounts of sludge. Based on the above experimental results, As(III) removal mechanism in the RAC electrocoagulation process was proposed to be the oxidation of As(III) to As(V) and the subsequent As(V) sequestration by Fe(III) (oxyhydr)oxides. The findings in previous literatures concerned with As removal in the DC electrocoagulation process can support this deduction

22, 23

. It has been

previously demonstrated that the oxidation of As(III) to As(V) occurs as a result of the oxidative intermediates in Fe(II) oxidation by the dissolved O2 11. In addition, the co-existence of Fe(II) and Fe(III) (oxyhydr)oxides can also jointly induce the generation of intermediate oxidants, which leads to the oxidation of As(III)

24

. In general, it is feasible to apply RAC electrocoagulation process for

the appreciable sequestration of As(III) via supplying the sufficient amount of O2 and Fe(II) under anoxic condition. 3.2. Effect of electric current



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Electric current applied to the anode is a determinant for the yields of Fe(tot) and O2, which essentially impacts the performance of RAC electrocoagulation system for the oxidative sequestration of As(III). Fig. 3(b) shows that as the current was elevated from 30 to 50 mA, Fe(tot) amount increased from 0.22 to 0.27 mM within 30 min in the RAC electrocoagulation system. In addition, the behavior of Fe(tot) accumulation fitted pseudo zero-order kinetic reaction and its rate constant increased from 7.6×10-3 to 1.03 ×10-2 mM min-1. According to the Faraday's laws, the theoretical molar ratio of [O2]/[Fe(II)] was 1:1 when TFe-anode/TMMO-anode ratio was 1:2, suggesting that the produced O2 sufficiently oxidized Fe(II) ion to the poorly soluble Fe(III) (oxyhydr)oxides. As a consequence, with increasing the current from 30 to 50 mA, the residual As concentration in electrolyte decreased from 138 to 41 μg L-1 (Fig. 3(a)). Accordingly, the pseudo first-order rate constant of As(tot) removal enhanced from 0.041 to 0.091 min−1 (R2 = 0.98) with the increase of current in the range of 30-50 mA. There are two explanations for the positive relationship between As(tot) removal and the electric current. For one hand, increasing the current improved the oxidative transformation of As(III) to As(V). At pH 8.0, the negatively charged As(V) was more readily sequestrated by the positively charged Fe(III) (oxyhydr)oxide than As(III) with the neutral charge

11

. For the other hand, the

formation of Fe(III) (oxyhydr)oxides was facilitated by enhancing the current because of the enhancement in productions of Fe(II) and O2. Thus, the enhancements in As(III) oxidation and Fe(III) (oxyhydr)oxides formation induced by increasing electric current collectively benefited the removal of As(tot). In spite of this, it should be noted that elevating the current from 40 to 50 mA induced the slight increase of As(III) sequestration. Thus, applying the electric current above 40 mA might be not an economical strategy to enhance As(III) sequestration.


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3.3. Working electric parameters An appreciable electrocoagulation performance for As sequestration cannot be achieved with a relatively low yield of Fe(tot), while a high yield of Fe(tot) inevitably leads to the excessive production of precipitant. Moreover, a low molar ratio of [O2]/[Fe(II)] would result in an inferior process efficiency of As(III) sequestration because of the competition between the excess Fe(II) and As(III) for the intermediate oxidants. Therefore, an appropriate molar ratio of [O2]/[Fe(II)] should be attained to achieve the effective removal of As(III). Here, different TFe-anode/TMMO-anode ratio, such as 1:4, 1:2, 1:1 and 2:1, theoretically equal to the stoichiometric [O2]/[Fe(II)] ratio of 2:1, 1:1, 1:2 and 1:4, respectively, was examined to explore the best stoichiometric [O2]/[Fe(II)] ratio on As(III) sequestration. As shown in Fig. 4, when elevating TFe-anode/TMMO-anode ratio from 1:4 to 1:2, Fe(tot) in precipitant increased from 0.20 to 0.26 mM after 30 min's reaction, almost all of which was Fe(III). This facilitated the oxidative sequestration of As(III) with decreasing residual As concentration in electrolyte from 95 to 41 μ g L-1 in TFe-anode/TMMO-anode ratio range from 1:4 to 1:2. Although, with the further increase of TFe-anode/TMMO-anode ratio from 1:2 to 2:1, the yield of Fe(tot) increased from 0.26 to 0.33 mM, residual As(tot) concentration in electrolyte increased from 41 to 252 μg L-1. The experimental results can be ascribed to the insufficient yield of O2 for the complete oxidation of the freshly generated mixed Fe(II)/Fe(III) (oxyhydr)oxides, which can be validated by the significant appearance of Fe(II) in precipitant at TFe-anode/TMMO-anode ratio of ＞1:2. For example, when TFe-anode/TMMO-anode ratio was 1:2, Fe(II) content in solid was 0.02 mM, while it increased up to 0.27 mM at TFe-anode/TMMO-anode ratio of 2:1. Consequently, the removal of As(tot) from the electrolyte decreased from 459 to 247 µg L-1 in TFe-anode/TMMO-anode ratio range of 1:2-2:1. This suggests that the



performance of RAC electrocoagulation process for As(tot) removal was closely related with the extent of Fe(II) oxidized by O2. And the optimal TFe-anode/TMMO-anode ratio for As(tot) removal was 1:2. In the RAC electrocoagulation process, a reaction period consists of four stages, i.e., anodic dissolution of iron electrode (Stage I), power off stage (Stage II and IV) and oxygen evolution reaction on MMO (Stage III). Fig. 5(a) illustrates the effect of RAC reaction period (i.e., 8 s, 16 s, 24 s and 48 s) on As(tot) removal. Short time scale such as “6 s ON+2 s OFF” and “12 s ON + 4 s OFF” showed less As(tot) removal efficiency than that in the case of “18 s ON + 6 s OFF”. In spite of generating higher concentration of Fe(tot) (Fig. 5(b)), long time scale such like “36 s ON + 12 s OFF” exhibited the inferior performance for As(tot) removal to the case of “18 s ON + 6 s OFF”. Two reasons may account for the above experimental results. For short time scale such like “6 s ON+2 s OFF”, O2 and Fe(II) cannot be efficiently generated on the alternate MMO/iron anode within 4 s and 2 s, respectively. Moreover, the power-off time (1 s Stage II and 1 s Stage IV) is so short that the generated reactants on the electrodes did not have sufficient time to diffuse into the bulk solution. This inevitably deteriorated the reaction between O2 and Fe(II) and thereby As(tot) sequestration. Long time scale such like “36 s ON + 12 s OFF”, 36 s ON (12 s Fe + 24 s MMO) and 12 s OFF (6 s Stage II and 6 s Stage IV) were sufficient for the generations of O2 and Fe(II) and their diffusion to bulk electrolyte, respectively. However, too long time scale of anodic reactions on MMO (Stage III) probably resulted in excess O2 generation, which may diffuse into gas phase owning to its relatively low solubility (0.3 mM, 25°C). Meanwhile, during this process, excess Fe(II) dissolved from iron anode may be sequestrated by Fe(III) (oxyhydr)oxide rather than fully transforming to Fe(III) because of the insufficient presence of O2. Generally, “18 s ON + 6 s OFF” and TFe-anode/TMMO-anode


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ratio of 1:2 achieved the best combination of O2 generation and Fe(II) dissolution for the sequestration of As(III). 3.4. Solution pH The extent of As(III) oxidation and the affinity of As to Fe(III) (oxyhydr)oxides depend highly on solution pH

25

, therefore solution pH is likely to affect As(tot) sequestration in the RAC

electrocoagulation system. Fig. 6(a) shows that As(tot) removal increased from approximately 363 µg L-1 at pH 6.0 to 455 µg L-1 at pH 9.0. The large difference in As(tot) removal at various solution pH can be explained as below. At higher solution pH, Fe(II) can be rapidly transformed to Fe(III) (oxyhydr)oxides mediated by O2 upon being produced from the anodic iron electrode

26

.

Consequently, the competition of dissolved Fe(II) with As(III) for the intermediate oxidants was minor, which facilitated the oxidation of As(III) and thereby the sequestration of As(tot). In contrast, Fe(II) oxidation rate is slow at lower solution pH

26

. Therefore, dissolved Fe(II) could gradually

accumulate in the electrolyte (Fig. 6(b)) and inevitably compete with As(III) for the intermediate oxidants. Consequently, more As(tot) was present in the form of As(III) and less As(tot) was sequestrated owning to less formation of Fe(III) (oxyhydr)oxides. After 30 min's reaction, solution pH slightly increased from 8.0 to 8.36 (data not shown), which was a combined effects of the hydroxyl ion generated on the alternate MMO/Fe cathode, the hydrogen ion generated on the MMO anode and the consumption of hydroxyl ion for the oxidative formation of Fe(III) (oxyhydr)oxides. 3.5. Coexisting inorganic ions In addition to As, many other cations and anions are also prevalent in groundwater

16

. For

example, HCO3- (8.2 mM), Ca2+ (2.5 mM), Mg2+ (1.6 mM), PO43- (0.025 mM), and SiO32- (0.246 mM) were detected in the Bangladesh groundwater by the British Geological Survey


27

. In this


section, the individual and combined effects of these inorganic ions on the oxidative sequestration of As(III) was investigated in the RAC electrocoagulation system. According to previous study 28, Ca2+ and Mg2+ can be removed in traditional DC electrocoagulation process. The electrostatic attraction may induce the uptake of Ca2+ and Mg2+ by Fe(III) (oxyhydr)oxides, which could provide additional adsorption sites or increasing the accessibility of oxyanions to the precipitant via relieving the electrostatic barrier

25, 29

. Thus, it can be expected that the sequestration of As(V), existing as

HAsO42− and H2AsO4−, could be facilitated in the presence of bound bivalent cations 29. However, in the RAC electrocoagulation system, as shown in Fig. 7, Ca2+ and Mg2+ exerted negligible effects on the removal of As(tot). Besides, we find that the presence of 0.246 mM SiO32- also negligibly influenced the removal efficiency of As(tot). This can be explained by the inferior adsorption infinity of SiO32- with Fe(III) (oxyhydr)oxides to that of As(V) species 25. Many previous literatures demonstrated that Cl- addition posed enhanced effect on As(tot) removal 30. In the electrolysis process, Cl2 (E(Cl-/Cl2)=1.36V) can be produced from the electrolysis of Cl- on the inert anode and can successfully oxidize As(III) to As(V), facilitating As(tot) sequestration by the positively charged Fe(III) (oxyhydr)oxides. However, 8.2 mM Cl- exerted negligible effect on As(tot) removal in the RAC electrocoagulation system. This result can be ascribed to the fact that when MMO served as the anode, Cl- probably did not transform to Cl2 due to the lower potential of O2 evolution (E0(H2O/O2) = 1.23 VNHE) than Cl2 evolution (E0(Cl-/Cl2) = 1.36 VNHE)

31

. In contrast, the presence of 8.2 mM HCO3- in electrolyte evidently decreased As(tot)

removal from 458.9 to 408.8 µg L-1. As reported by King et al. 32, in near neutral solution containing 2 mM HCO3−, approximately 70% of total Fe(II) existed as FeCO30 and was oxidatively transformed to soluble Fe(III)-carbonate complex by both dissolved O2 and H2O2. Consequently, it is likely that


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in the RAC electrocoagulation system, Fe(II) primarily remained as FeCO30, resulting in the decreased production of insoluble Fe(III) (oxyhydr)oxides. Besides, 0.025 mM PO43- also played an inhibitory role in As(tot) sequestration, because PO43- competed with As for the adsorption sites of Fe(III) (oxyhydr)oxides and hindered the oxidization of Fe(II) to Fe(III) via forming complex with Fe(II)

23, 33

. Consequently, the negative effects of HCO3− and PO43- on the removal of As(tot) were

jointly responsible for the declined performance of RAC electrocoagulation system for As(tot) removal in the synthesized Bangladesh groundwater (Fig. 7). 3.6. Temperature The temperture of groundwater is probably influenced by the heat transfer from a warm ground surface or infiltration of warm water from the ground surface34. In view of this, we focused on the effect of solution temperature on oxidative sequestration of As(III) in the temperature range from 25 °C to 40 °C. As shown in Fig. 8, decreasing temperature led to the promoted performance of the RAC electrocoagulation process in As(tot) removal, but the temperature dependency for As(tot) removal was observed to be relatively small. For example, the removal of As(tot) in solution and its rate constant reduced from 459 to 431 μg L-1 and from 0.086 to 0.068 min-1, respectively, with the temperature increasing from 25 °C to 40 °C (R2 = 0.98). The plot of log k versus 1/T gave a linear plot (R2 = 0.99) and the activation energy (Ea) was calculated to be 6.5 kJ mol-1 from the slope of the linear plot. Accordingly, we may conclude that solution temperature may be not a critical parameter in the RAC electrocoagulation process for As(tot) sequestration. 3.7. Mechanisms of As(tot) removal in the RAC electrocoagulation system In the RAC electrocoagulation system, As(III) oxidation possibly proceeded via the following mechanisms: heterogeneous oxidation on the MMO anode

35

, and homogeneous oxidation by the



intermediate oxidants produced during the process of Fe(II) oxidation by O2

Page 16 of 34

36, 37

. Here, several

additional experiments were conducted to evaluate the contributions of the above two mechanisms for As(III) oxidation via selectively scavenging the intermediate oxidants. As shown in Fig. 9, approximately 100 µg L-1 was oxidized on the MMO anode in the DC electrolysis system. However, in the RAC electrocoagulation process, the working time of MMO anode was half of that in the DC electrolysis system when reaction period was 24 s and TFe-anode/TMMO-anode:Tpower-off ratio was 1:2:1. Thus, actually, much less As(III) was oxidized on MMO anode in the RAC electrocoagulation process, and some other intermediates primarily accounted for As(III) oxidation . The rate constant of 2-propanol reacting with •OH is 8×109 s-1, higher than that of As(III) reacting with •OH (2×109 s-1), while the reactivity of 2-propanol toward high-valent oxoiron complexes, e.g., Fe(IV), was poor 38, 39. Therefore, to inspect the predominant oxidant for As(III) oxidation, 100 mM 2-propanol as the specific •OH scavenger was added into the As(III)-containing solution in the RAC electrocoagulation system. As shown in Fig. 9, the decrease in the oxidation of As(III) was negligible in the presence of 2-propanol, implying the insignificant contribution of •OH to As(III) oxidation. The similar effect of 2-propanol on As(III) oxidation at neutral pH was also observed in the reaction system of iron-catalyzed oxidation of As(III) by O2

36

. Unlike 2-propanol, 2,2′-bipyridine can

preferentially coordinate with Fe(II), which thus hinders the oxidation of Fe(II) to Fe(III)

40

. As a

result, the addition of 2,2′-bipyridine (10 mM) remarkably depressed the oxidation of As(III) from 495 to 61 µg L-1. It has been previously confirmed that an intermediate oxidant, Fe(IV) species, can be produced in the process of Fe(II) reacting with O2, and can selectively react with As(III) via a one-electron transfer process at a rate consant of ＞104 M-1 s-1 as depicted in Eqs. (1-3) 36, 40, 41. Thus, the oxidation of As(III) was mainly ascribed to Fe(IV) rather than


•

OH in the RAC

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrocoagulation system. The oxidation of As(III) by Fe(IV) gives rise to the formations of Fe(III) and As(IV), which then reacts almost diffusion controlled mainly with dissolved O2 to As(V) and O2•(Eqs. (4,5)) 36. As shown in Fig. 10(a), the wide scan XPS spectrum of arsenic-bearing precipitate indicates the presences of Fe and As in the precipitant. The XPS spectrum of Fe2p, near 711.1 eV, suggests that γ-FeOOH dominated the oxidation state of iron oxides in the arsenic-bearing precipitates

42

. The

binding energy of the As3d at 44.3–44.5 eV and 45.2–45.6 eV were assigned to As(III) and As(V), respectively 43. Thus, the As3d could be fitted with two components of binding energies at 44.35 and 45.22 eV, corresponding to As(III) and As(V), respectively (Fig. 10(b)). The quantitative analysis based on the area ratio of two overlapped peaks shown that 96 % of As in the solid existed as As(V), indicating that As predominantly existed as As(V) in the precipitant. It is rational because H3AsO3 remains the dominant As(III) species at pH = 8.0 (see Fig. S1) and cannot ionically bind to Fe(III) (oxyhydr)oxides. As for As(V), the dominant ions in the solution are ionized HAsO42- at pH = 8.0. As reported in previous literatures

15, 28

, the sequestration of As(V) proceeded via substitution of

HAsO42- for hydroxyl group on insoluble Fe(III) (oxyhydr)oxides as expressed in Eq. (6) in the electrocoagulation process. In many electrocoagulation process, a small quantity of symplesite was detected in the solution with high As(V) concentration up to 1000 mg L-1

44

. In spite of this,

adsorption onto Fe(III) (oxyhydr)oxides is the primary sequestration mechanism of As(V) in the RAC electrocoagulation process because of no peak of symplesite in the XRD results (see Fig. S2). Based on these discussions, it can be determined that the most of As(III) were oxidized to As(V) and then sequestrated by the produced Fe(III) (oxyhydr)oxides during the RAC electrocoagulation process.



Fe(II) + O → Fe(III) + O•

Page 18 of 34

(1)

Fe(II) + O• + 2H → Fe(III) + H O

(2)

Fe(II) + H O → intermediate → Fe(IV)

(3)

Fe(IV) + As(III) → Fe(III) + As(IV)

(4)

O + As(VI) → O• + As(V)

(5)

3FeOOH + HAsO → (FeO) HAsO + H O + 2OH

(6)

3.8. Environmental application When applying electric current of 40 mA in the RAC electrocoagulation process, the corresponding voltage was 2.5 V and 0.5 V on alternate MMO and Fe anodes, respectively. And the calculated energy for the oxidative sequestration of As(III) in anoxic environment was 0.11 kW·h m-3, which is much less than those in the previous literatures

15, 45

. For example, Vasudevan et al.

45

reported that the removal efficiency of As(V) (500 µg L-1) can reach up to 98.3% and 97.9% at a current density of 0.2 A dm-2 in AC and DC electrocoagulation process, respectively. The calculated energy consumption in the above AC and DC electrocoagulation process was 0.724 kW·h m-3 and 1.035 kW·h m-3, respectively. In addition, 449−667 µg L−1 As(tot) could be effectively removed to approximately 10 µg L−1 within 1.5 h in field trials for the treatment of As-contaminated groundwater, with an energy consumption of 0.72−0.78 kW·h m-3

15

. Notably, in above processes, aeration is an

indispensable process to provide the sufficient amount of O2 for the oxidation of Fe(II). However, the RAC electrocoagulation process does not only consume less electric energy but also in-situ produces O2 on MMO anode instead of aeration for the oxidative formation of Fe(III) (oxyhydr)oxides. The electric parameters, such as TFe-anode/TMMO-anode ratio, electric current and reaction period can be regulated to achieve the appreciable performance for As(III)-contaminated groundwater. Generally,


Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the RAC electrocoagulation is a promising strategy for remedying the anoxic As(III)-contaminated groundwater owning to its cost-effective performance and the minimal production of sludge. 4. Conclusions In this study, the RAC electrocoagulation system with MMO and iron as the alternate anodes was evaluated for the oxidative sequestration of As(III) from the synthesized anoxic groundwater. As(III) removal efficiency greatly depended on the electric parameters, such as TFe-anode/TMMO-anode ratio, electric current and reaction period. A slightly alkaline pH was favorable for the oxidative sequestration of As(III) in the RAC electrocoagulation process. The presence of HCO3− and PO43posed negative influences on the process efficiency of As(tot) removal. Decreasing the temperature of groundwater from 40

°

C to 25 °C slightly enhanced the removal of As(tot). In the RAC

electrocoagulation process, the intermediate oxidant, i.e., Fe(IV), generated during the reaction between Fe(II) and O2, induced the oxidation of As(III) to As(V), and the produced As(V) can be effectively sequestrated by the freshly generated amorphous Fe(III) (oxyhydr)oxides. Supporting Information The speciation distribution of As(III) and As(V), and XRD patterns obtained for the precipitants. Acknowledgments This work is ﬁnancially supported by the National Natural Science Foundation of China (No. 51608284), National Major Science and Technology Program for Water Pollution Control and Treatment (No. 2017ZX07101-006), China Postdoctoral Science Foundation (No. 2017M610413), Shandong Province Postdoctoral Science Foundation (No. 201702041), the State Key Laboratory of Petroleum Pollution Control (No. PPC2016009), the Stated Key Laboratory of Heavy Oil Processing. Yijie Liu and Shuaishuai Xin contributed equally to this work.



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Page 22 of 34

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Fig. 1. Schematics of RAC electrocoagulation process. (a) electric wave pattern of RAC process; (b) anodic dissolution of the Fe electrode; (c) diffusion of Fe(II) and OH- ions to bulk; (d) oxygen evolution reaction on the MMO; (e) diffusion of O2 and OH- ions to bulk and the generation of Fe(III) (oxyhydr)oxides; (f) anodic dissolution of the Fe electrode. Fig. 2. The variation of As concentration (a) and Fe production (b) in the RAC and DC electrocoagulation

systems

(RAC

system:

[As(tot)]0

=

[As(III)]0

=

500

µg

L-1,

TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, I = 40 mA, pH = 8; DC system: I = 40 mA, [As(III)]0 = 500 µg L-1, pH = 8.0). Fig. 3. The effect of electric current on As(III) removal (a), total Fe production and Fe production rate (b) in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8). Fig. 4. Effect of TFe-anode/TMMO-anode ratio on As removal (a) and Fe content in solid (b) in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, Tperiod = 24 s, I = 40 mA, pH = 8). Fig. 5. Effect of electrochemical reaction period on As removal (a) and Fe content in solid (b) in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, I = 40 mA, pH = 8). Fig. 6. Effect of solution pH on As removal (a) and Fe(II) concentration in solution (b) in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, I = 40 mA, Tperiod = 24 s). Fig. 7. Effect of various inorganic ions on As removal in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8.0, I = 40 mA). Fig. 8. Effect of solution temperature on As removal in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8.0, I = 40 mA). Fig. 9. Effect of various chemical additions on As removal in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8.0, I =



40 mA). Fig. 10. XPS wide scan spectrum, Fe2p spectrum (the inset) and As3d spectrum of arsenic-bearing precipitant ( pH = 8.0, As(III) = 500 µg L-1). Fig. 1.

Fig. 1. Schematics of RAC electrocoagulation process. (a) electric wave pattern of RAC process; (b) anodic dissolution of the Fe electrode; (c) diffusion of Fe(II) and OH- ions to bulk; (d) oxygen evolution reaction on the MMO; (e) diffusion of O2 and OH- ions to bulk and the generation of Fe(III) (oxyhydr)oxides; (f) anodic dissolution of the Fe electrode.


Page 24 of 34

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Fig. 2

Fig. 2 The variation of As concentration (a) and Fe production (b) in the RAC and DC electrocoagulation

systems

(RAC

system:

[As(tot)]0

=

[As(III)]0

=

500

µg

L-1,

TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, I = 40 mA, pH = 8; DC system: I = 40 mA, [As(III)]0 = 500 µg L-1, pH = 8.0).



Fig. 3.

Fig. 3. The effect of electric current on As(III) removal (a), total Fe production and Fe production rate (b) in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8).


Page 26 of 34

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Fig. 4.

Fig. 4. Effect of TFe-anode/TMMO-anode ratio on As removal (a) and Fe content in solid (b) in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, Tperiod = 24 s, I = 40 mA, pH = 8).



Fig. 5.

Fig. 5. Effect of electrochemical reaction period on As removal (a) and Fe content in solid (b) in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, I = 40 mA, pH = 8).


Page 28 of 34

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Fig. 6.

Fig. 6. Effect of solution pH on As removal (a) and Fe(II) concentration in solution (b) in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, I = 40 mA, Tperiod = 24 s).



Fig. 7.

Fig. 7. Effect of various inorganic ions on As removal in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8.0, I = 40 mA).


Page 30 of 34

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Fig. 8.

Fig. 8. Effect of solution temperature on As removal in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8.0, I = 40 mA).



Fig. 9.

Fig. 9. Effect of various chemical additions on As removal in the RAC electrocoagulation system ([As(tot)]0 = [As(III)]0 = 500 µg L-1, TFe-anode/TMMO-anode:Tpower-off = 1:2:1, Tperiod = 24 s, pH = 8.0, I = 40 mA).


Page 32 of 34

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Fig. 10.

Fig. 10. XPS wide scan spectrum, Fe2p spectrum (the inset) and As3d spectrum of arsenic-bearing precipitant ( pH = 8.0, As(III) = 500 µg L-1).



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