Redox Conversion of Chromium(VI) and Arsenic(III) with the

(22) H2O2 was analyzed colorimetrically at 400 nm by using potassium ...... Shukra Raj Paudel , Sushant Prasad Banjara , Amrita Wagle , Friedemann T. ...
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Redox Conversion of Chromium(VI) and Arsenic(III) with the Intermediates of Chromium(V) and Arsenic(IV) via AuPd/CNTs Electrocatalysis in Acid Aqueous Solution Meng Sun,†,§ Gong Zhang,†,§ Yinghua Qin,†,§ Meijuan Cao,∥ Yang Liu,‡ Jinghong Li,*,‡ Jiuhui Qu,† and Huijuan Liu*,† †

Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡ Department of Chemistry, Beijing Key Laboratory for Microanalytical Methods and Instrumentation, Tsinghua University, Beijing 100084, China § University of Chinese Academy of Sciences, Beijing 100039, China ∥ Beijing Engineering Research Center of Printed Electronics, School of Printing and Packaging Engineering, Beijing Institute of Graphic Communication, Beijing 102600, China S Supporting Information *

ABSTRACT: Simultaneous reduction of Cr(VI) to Cr(III) and oxidation of As(III) to As(V) is a promising pretreatment process for the removal of chromium and arsenic from acid aqueous solution. In this work, the synergistic redox conversion of Cr(VI) and As(III) was efficiently achieved in a three-dimensional electrocatalytic reactor with synthesized AuPd/ CNTs particles as electrocatalysts. The AuPd/CNTs facilitated the exposure of active Pd{111} facets and possessed an approximate two-electrontransfer pathway of oxygen reduction with the highly efficient formation of H2O2 as end product, resulting in the electrocatalytic reduction of 97.2 ± 2.4% of Cr(VI) and oxidation of 95.7 ± 4% of As(III). The electrocatalytic reduction of Cr(VI) was significantly accelerated prior to the electrocatalytic oxidation of As(III), and the effectiveness of Cr(VI)/As(III) conversion was favored at increased currents from 20 to 150 mA, decreased initial pH from 7 to 1 and concentrations of Cr(VI) and As(III) ranging from 50 to 1 mg/L. The crucial intermediates of Cr(V) and As(IV) and active free radicals HO• and O2•− were found for the first time, whose roles in the control of Cr(VI)/As(III) redox conversion were proposed. Finally, the potential applicability of AuPd/CNTs was revealed by their stability in electrocatalytic conversion over 10 cycles.

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INTRODUCTION

oxidative weathering or reductive dissolution of As-containing minerals, suggesting the immobilization of As(III) in acid solution more strictly depends on the redox conditions.7 Therefore, it is advisible to preoxidize As(III) to As(V), coupled with a following adsorption or coprecipitation of the As(V) by using adsorbents or coagulants, to finally enhance the removal of arsenic species in acid aqueous solution.8,9 Similarly, Cr(VI) is quite soluble and mobile in acid wastewater,10 while Cr(III) is less toxic and insoluble, and can be effectively immobilized as the hydroxide in a wide pH range.11,12 Thus, the prereduction of Cr(VI) to Cr(III) is conducive to the complete removal of chromium species in acid condition.

The combined pollution of heavy metals in acid wastewater has become a formidable problem to environmental scientists and engineers for decades. As a consequence of improper treatment, effluents containing abundant heavy metal ions such as lead, nickel, copper, arsenic, or chromium may inflow into surface water or seep into groundwater, which pose great threats to aquatic life and drinking water sources.1,2 Thus, it is urgent to remove the heavy metals prior to their discharge into the environment. Arsenic and chromium, as priority pollutants, are highly toxic and frequently coexist with highly acidic wastewater (e.g., acid mine drainage).3 The valence states of arsenic or chromium have decisive impacts on their environmental fate and toxic effect.4,5 For example, As(III) is the most prevalent form of arsenic in the natural environment, which is more toxic than As(V) to mammals.6 Additionally, As(III) is also more mobile than As(V), because As(III) is quite easier originated from the © XXXX American Chemical Society

Received: April 8, 2015 Revised: June 28, 2015 Accepted: July 8, 2015

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and active radicals, as well as the catalytic stability of the electrocatalysts. Finally, mechanisms for the Cr(V)/As(III) redox conversion in the presence of the vital intermediates and radicals were proposed. We demonstrate that this approach holds great promise in practical environmental application.

It would be of great significance if the simultaneous reduction of Cr(VI) to Cr(III) and oxidization of As(III) to As(V) could be achieved in acid solution by feasible methods. However, the direct redox between Cr(VI) and As(III) under standard conditions is not possible due to the fact that the theoretical Gibbs free energy (ΔrGm) of the reaction is 468 J· mol−1·K−1.13 Although some works have devised strategies to overcome the thermodynamic restriction of Cr(VI)/As(III) redox conversion,14,15 these attempts are still far from meeting the demands of practical application. Recently, Liu et al. have emphasized that H2O2 can be utilized to achieve the simultaneous redox conversion of Cr(VI) and As(III). They stated that the oxidation of As(III) to As(V) by hydroxyl radicals (HO·) was kinetically feasible in acid solution by virtue of the Cr(V)/H2O2 Fenton-like reaction, and the vital intermediate of Cr(V) originating from the reduction of Cr(VI) by H2O2 played an important role.16 This advanced method for the H2O2-based aqueous redox conversion of Cr(VI) and As(III) possessed outstanding advantages, such as rapid reaction, no secondary pollution, and convenient operation, exhibiting favorable applicability in removing chromium and arsenic from acid aqueous solution. Nevertheless, some weaknesses of this promising strategy may exist and need further improvement. For instance, the use of H2O2 involves several shortcomings including the uneconomical production, dangerous transportation and storage; and those organic matter, particularly Cr(V) ligands, can inhibit the serial reactions of Cr(V) decomposition, which significantly weakens its ability to practical use. More importantly, the significance and evidence for the intermediates and radicals in redox conversion of Cr(VI) and As(III) are still lacking. It is necessary to clarify the effects of the Cr(V)/H2O2 reaction and its resulting free radicals on the conversion of As species in detail. To overcome these barriers, developing novel H2O2-based routes for the efficient redox conversion of Cr(VI) and As(III) is imperative. Based on our previous works,17,18 we have efficiently removed Cr(VI) and As(III) by electrochemical processes, respectively. Furthermore, our recent progress in the low load of noble metal based (Au and Pd) electrocatalytic synthesis of H2O2 has shown great potential in the remediation of organic contaminants.19 Sufficient H2O2 was continuously generated via the AuPd nanoparticles supporting on the carbon nanotubes, which served as the stable and promising substrates, preventing the agglomeration of active particles and contributing to the excellent electrocatalytic degradation of organic pollutants. Therefore, based on these cases, we propose that the synergetic redox conversion of Cr(VI) and As(III) can be effectively facilitated by the in situ synthesis of H2O2 via an electrocatalytic process, and the roles of the intermediates may be revealed in depth. Herein, to meet the demands for optimizing the generation of H2O2 in the electrocatalytic system, a three-dimensional electrocatalysis reactor was designed with a pair of dimensionally stable electrodes (DSE, ruthenium-coated titanium electrode) to provide H2 and O2 from water electrolysis, and assembled with dispersed AuPd alloys/carbon nanotubes (AuPd/CNTs) as the electrocatalysts to improve the contact efficiency among catalysts, H2 and O2. The AuPd/CNTs for highly efficient production of H2O2 were synthesized and characterized. Thereafter, the redox conversion of Cr(VI) and As(III) via the AuPd/CNTs electrocatalysis process was systemically investigated in terms of the effects of electrocatalytic parameters, and identification of crucial intermediates

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EXPERIMENTAL SECTION Chemicals and Materials. All the used chemical reagents were at least of analytical grade and are listed in the Supporting Information. The AuPd/CNTs electrocatalysts were prepared according to our previous reports.19 Au/CNTs and Pd/CNTs particle electrodes with specific loads of Au or Pd were also prepared for comparison using a similar procedure. Experimental Apparatus and Procedure. The experiments were performed in a cylindrical glass reactor of 150 mL at 20 °C. The working anode and cathode consisted of DSE (2.5 cm × 5.0 cm), which were set vertically with a horizontal distance of 3 cm. A power adapter with frequently switched polarity could provide constant current from 0 to 1000 mA and voltage from 0 to 10 V. For each experiment, As(III), Cr(VI), 1.42 g Na2SO4 and 0.1 g electrocatalysts were mixed in 100 mL solution, and constantly stirred at 600 rpm after ultrasonic treatment for 10 min. Subsequently, 1.0 M H2SO4 was utilized to adjust the initial pH. Three mL samples were collected periodically and filtered (0.45 μm, PTFE) for chemical analysis. The electrocatalytic durability of AuPd/CNTs was tested for 10 cycles, in which electrocatalysts were recycled by filtration and washing with ultrapure water after each cycle. Characterization and Analytical Methods. All experiments were performed in duplicate, and the analysis of each parameter was carried out in triplicate for each run. Powder Xray diffraction (XRD) analysis was recorded on a Bruker D8 advance X-ray diffractometer. High resolution transmission electron microscopy coupled with energy-dispersive X-ray spectrometry (HRTEM/EDS) was obtained with a TEM H800 (Hitachi, Japan) at an accelerating voltage at 200 kV. The X-ray photoelectron spectroscopy (XPS) data were taken on an AXIS-Ultra instrument (Kratos Analytical, UK) using monochromatic Al Kα radiation. Linear sweep voltammetry (LSV) measurements were performed by using a CHI 830 electrochemical analyzer coupled with a rotating ring-disk electrode (RRDE, Princeton Applied Research, model 636).20 The Cr(VI) and total chromium (after oxidation by potassium permanganate) were measured at 540 nm by the 1,5diphenylcarbazid analytical method.21 The speciation of As(III) and As(V) was separated and quantified using the UPLC-ICPMS method.22 H2O2 was analyzed colorimetrically at 400 nm by using potassium titanium oxalate solution.23 Electron spin resonance (ESR) analysis was obtained using a Bruker electron paramagnetic resonance spectrometer (ESP 300E). The HO• and O2•− were captured by using DMPO as scavenger, and detected by ESR according to the reported parameters.24 The ESR parameters of Cr(V) and OEC(As(IV)) were operated according to the literatures, respectively.25,26 The computational model and descriptions were provided in the Supporting Information.

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RESULTS AND DISCUSSION Characterization of AuPd/CNTs. The preparation of the AuPd/CNTs electrocatalysts was designed with the goal of obtaining excellent electrocatalytic properties and long durability for the synthesis of H2O2. The exposure of

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Figure 1. (a) High resolution XPS spectra of AuPd/CNTs. (b) XRD patterns of Pd/CNTs, Au/CNTs, and AuPd/CNTs. (c) TEM image of AuPd/ CNTs. The inset shows the corresponding HRTEM images of the red zone with measured lattice spacing. d1 = 0.224 nm, corresponds to the Pd{111}; d2 = 0.194 nm, corresponds to the Pd{200}. (d) RRDE curves of CNTs, Pd/CNTs, Au/CNTs, and Au/Pd/CNTs in O2-saturated H2SO4 solution (pH 3) at a rotation rate of 1600 rpm. Sweep rate: 5 mV·s−1. The inset shows the corresponding electron transfer number of AuPd/CNTs from −0.3 to −0.6 V.

Pd{200}, and Pd{220} planes on AuPd/CNTs were also strengthened significantly compared with Pd/CNTs. It was found that Au not only promoted the exposure of active Pd{111} but also Pd{200} and Pd{220}, suggesting the high crystallinity of the AuPd alloys.30 This high crystalline structure with specific exposed polar facet has been proved for the contribution to electrocatalytic superiority of nanocatalysts.31 Moreover, the HRTEM image of the AuPd alloy provides further evidence that Au facilitated the exposure of active Pd{111} facets. As shown in Figure 1c, AuPd nanospheres with a diameter of 2−10 nm are dispersed on the surface of CNTs. The inset of Figure 1c indicates the crystal lattice planes with well-defined spacing of 0.224 and 0.194 nm, corresponding to Pd{111} and Pd{200}, respectively. More morphological and crystal insights of the AuPd alloy illustrated the promotional effects of Au on the AuPd alloy nanostructure (Figure S2). The interlaced three-dimensional network of CNTs takes the advantages of their high electron transfer efficiency, good structural stability, and micromesoporous pore size distribution (Supporting Information Figure S3), not only ensuring sufficient contact between the gas and solid phases, but also avoiding the leaching and agglomeration of AuPd alloys, potentially contributing to electrocatalytic synthesis of H2O2 and long-term stability. LSV curves of CNTs, Pd/CNTs, Au/CNTs, and AuPd/ CNTs were measured using a rotating ring-disk electrode (RRDE) in O2-saturated acidic solution at a rotation rate of 1600 rpm to investigate the electrocatalytic performance in the synthesis of H2O2 via the oxygen reduction processes (Figure

Pd{111} and Pd{200} facets on AuPd alloy particles, which has been proved to be beneficial for the production of H2O2 in theory,27 is required. Moreover, the promotional effects of Au on the exposure of Pd active facets and the stabilization of the AuPd alloy nanostructure have been demonstrated.28 Additionally, a substrate with good effectiveness in electron transfer and varying pore size distribution can trigger sufficient homogenization of H2, O2, reactants, and electrocatalysts, and prevent the agglomeration of AuPd alloy particles in harsh conditions. Thus, in view of the above-mentioned aspects, the AuPd/CNTs electrocatalysts were elaborately designed, and a series of characterizations indicated that the as-prepared AuPd/ CNTs showed superior performance in the synthesis of H2O2. The XPS results show that only C 1s, Pd 3d, and Au 4f can be found on AuPd/CNTs, without any impurities (Figure 1a). The binding energies of Pd 3d and Au 4f were both found to correspond to the signals of Pd0 and Au0, suggesting the presence of pure metallic Pd and Au supported on the CNTs (see the inset in Figure 1a). Compared with Pd/CNTs and Au/ CNTs (Supporting Information Figure S1), the slight shifts of binding energies of Pd 3d and Au 4f on AuPd/CNTs implied that the nature of AuPd alloys may affect their internal atom arrangement and crystallinity, resulting in the exposure of active crystal faces.29 As shown in Figure 1b, the AuPd particles exhibited typical XRD peaks of alloy crystals. In particular, the AuPd alloy peak at 39.4° was located between the Au{111} and Pd{111} peak locations, closer to Pd{111}, suggesting better growth of Pd-enriched AuPd alloy and more exposure of active facets of Pd{111} on the AuPd surface. Besides Pd{111}, C

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Environmental Science & Technology 1d). The results indicated that AuPd/CNTs enjoyed a more positive onset potential and higher peak current density than the other samples, implying that AuPd/CNTs possessed the most kinetically favorable oxygen reduction process. Furthermore, the fact that AuPd/CNTs showed the highest ring current demonstrated that the oxygen reduction process on AuPd/CNTs was more likely to follow a two-electron pathway to produce H2O2 eq 1), rather than a four-electron process to generate H2O (eq 2. O2(g) + 2H+ + 2e− → H 2O2 E0 = 0.69V

(1)

O2(g) + 4H+ + 4e− → 2H 2OE0 = 1.23V

(2)

The accurate electron transfer number (n) per oxygen molecule in oxygen reduction and the corresponding generation rate of H2O2 can be calculated derived from the RRDE measurements according to eqs 3, 4: H 2O2 % = 200 × n=4×

iR /N iD + iR /N

iD iD + iR /N

(3)

(4)

where iD is the disk current, iR is the ring current, and N is the current collection efficiency of the Pt ring. N was calculated to be 0.36 from the reduction of K3Fe(CN)6, which is consistent with the manufactureŕs value of 0.37.32 As shown in the inset in Figure 1d, the calculated n values of AuPd/CNTs were 2.25− 2.54 from −0.3 to −0.5 V, further indicating that AuPd/CNTs promoted an approximate two-electron oxygen reduction pathway, preferable for the generation of H2O2. Meanwhile, the calculated generation rate of H2O2 over the AuPd/CNTs at −0.4 V was 84.32% (Supporting Information Figure S4). In contrast, the corresponding yields of H2O2 over Au/CNTs, Pd/ CNTs, and CNTs were 65.08, 76.63, and 31.57%, respectively, much lower than that of AuPd/CNTs. In summary, the synthesized AuPd/CNTs exhibited superior electrocatalytic activity in the generation of H2O2 via an approximate twoelectron oxygen reduction process. Electrocatalytic Redox Conversion of Cr(VI) and As(III). As shown in Figure 2a, the redox conversion of Cr(VI) and As(III) under different conditions was examined. The results indicated that the conversion in the presence of AuPd/ CNTs alone was unachievable. The reasons can be attributed to the poor adsorption of Cr(VI) on AuPd/CNTs electrocatalysts, and the inefficient conversion of As(III) to As(V) by AuPd/ CNTs (Supporting Informaiton Figure S5). In comparison, for electrolysis alone, the Cr(VI)/As(III) redox conversion was elevated via electrolysis of only 75 mA, indicating that the removal of Cr(VI) and the accumulation of As(V) profited from the cathodic reduction of Cr(VI) and anodic oxidation of As(III). The simultaneous electro-reduction of Cr(VI) and electro-oxidation of As(III) under different electrolysis conditions were further investigated (Supporting Information Figure S6). The results show that the Cr(VI)/As(III) redox conversion was accelerated with increasing current from 0 to 600 mA. Although the Cr(VI)/As(III) conversion was simultaneously accomplished under electrolysis of 300 mA within 90 min, such a large current and long electrolysis time would result in great loss of current efficiency (Supporting Information Figure S7 and Table S1), predicting that the direct electro-conversion of Cr(VI) and As(III) has bleak prospects

Figure 2. (a) Redox conversion of Cr(VI) and As(III) in the presence of AuPd/CNTs, electrolysis and AuPd/CNTs electrocatalysis, respectively. (b) Contributions of electrocatalytic removal of Cr(VI) and generation of As(V) based on the electrocatalytic conditions of 75 mA, pH 3, 0.1 M Na2SO4, and 1 g/L 1% AuPd/CNTs.

for practical application. In sharp contrast, the Cr(VI)/As(III) redox conversion was notably promoted by AuPd/CNTs electrocatalysis of 75 mA, in which the Cr(VI) was entirely removed only in 15 min, and the concentration of As(V) increased from 0 to 10 ± 0.3 mg/L within 20 min. This acceleration process greatly shortened the treatment time compared with electrolysis alone and the reported chemical conversion of Cr(VI)/As(III).13,16 Clearly, this enhancement of Cr(VI)/As(III) redox conversion could be attributed to the contribution of the AuPd/CNTs electrocatalysts, which had been proved to provide sufficient H2O2 to accelerate the simultaneous reduction of Cr(VI) and oxidation of As(III). The contributions of Cr(VI) removal and As(V) generation by AuPd/CNTs electrocatalysis and electrolysis were analyzed separately. As shown in Figure 2b, the contribution of electrocatalytic oxidation of As(V) gradually increased from 12.5 to 77.4 (±2)% as a function of time, whereas electrolysis only accounted for 5−21 (±1)%. As for Cr(VI), the contribution of electrocatalysis increased to 72.1 (±2.3)% rapidly at 5 min and remained at ∼80% until 15 min. The results suggest that the kinetic conversion processes of Cr and As were different. Because it was achieved 5 min prior to the conversion of As(III), the rapid conversion of Cr(VI) implies its higher activity in AuPd/CNTs electrolysis. These findings are different from the previous results,13,16 and make AuPd@ D

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Figure 3. (a) Hyperfine ESR spectra of the Cr(V) and (b) UV−visible absorption spectra of Cr(VI) and Cr(V) during the AuPd/CNTs electrocatalytic reduction of Cr(VI) as a function of time. (c) ESR spectra of the [(OEC)As(IV)]+ and (d) UPLC-ICP-MS analysis of dissolved As(III) and As(V) during the AuPd/CNTs electrocatalytic oxidation of As species. The hyperfine coupling signals of Cr(V) (giso = 1.9798) and As(IV) (giso = 2.01) are marked by asterisks.

increased. It is possible that the intermediates resulting from the reduction of Cr(VI) gave high priority to the decomposition of H2O2, and soon after, boosted the oxidation of As(III).16 However, the measured half-life for the reaction of Cr(VI) with H2O2 is about 0.02 s,35 thus, the previous results cannot make a convincing explanation for the delay of the conversion of As. Hence, it can be presumed that more intermediates and active species are involved in the Cr(VI)/ As(III) redox conversion via AuPd/CNTs electrocatalysis, which significantly control the conversion pathways of Cr(VI) and As(III). The Crucial Intermediates: Cr(V) and As(IV). As mentioned above, to reveal the intermediate-dependent redox conversion pathways of Cr(VI) and As(III), the typical hyperfine peaks of the Cr(V) and (octaethylcorrole)As(IV) spin adducts ([(OEC)As(IV)]+) were simultaneously detected for the first time by Electron Spin Resonance (ESR) in AuPd/ CNTs electrocatalysis. As seen from Figure 3a, the distinct hyperfine coupling signals of Cr(V) at giso = 1.9798 reached their highest level at 5 min, and after that, began to decrease until 15 min, suggesting that the accumulation of Cr(V) occurred within 5 min. The corresponding changes in the UV absorption spectra during the AuPd/CNTs electrocatalysis process were investigated. The distinct UV absorption peak at 352 nm and spectrum >500 nm corresponded to the [Cr(VI)O4]2− and tetraperoxochromate-(V) anion complex, respectively.16,36 The peak at 352 nm declined, while the absorbance >500 nm increased at 5 min, confirming the abovementioned hypothesis that Cr(V) was initially accumulated (Figure 3b). Moreover, both the decrease of the peak at 352 nm and absorbance >500 nm up to 15 min suggested that the

CNT electrocatalytic process hold great potential for rapid Cr(VI)/As(III) redox conversion. Effects of Current, Initial pH, and Initial Concentration of Cr(VI) and As(III). The Cr(VI)/As(III) redox conversion was investigated with various electrocatalytic parameters such as current, initial pH, and initial concentration of Cr(VI) and As(III) with the same dosage of 1 g/L AuPd/CNTs (Supporting Information Figure S8). To keep the paper reasonably concise, some important results are summarized as following. Although the efficiency of AuPd/CNTs electrocatalytic Cr(VI)/As(III) redox conversion increased as current increased from 75 to 300 mA, the overlarge current of 300 mA resulted in a significant loss of H2O2 of nearly 50% within 60 min compared to that of 75 mA, and also a decline of the efficiency of Cr(VI) conversion, suggesting the strong scavenging of H2O2 by the anode cannot be ignored.33 On the contrary, this negative effect did not affect the oxidation of As(III), implying that the oxidation of As(III) in this electrocatalytic system possibly depended on the resulting free radicals (e.g., HO·) rather than H2O2.34 As for the effects of initial pH, the results show that both the reduction of Cr(VI) and the oxidation of As(III) were favored in a low pH condition. In particular, the removal of Cr(VI) followed pseudo-first order kinetics at pH 5−1 (Supporting Information Figure S9), with a rate constant increasing from 0.14 to 0.49 min−1, verifying a typical H2O2based reduction of Cr(VI) in acid condition.35 The oxidation of As(III) greatly slowed down when the Cr(VI) increased from 10 to 50 mg/L. In other words, the electrocatalytic reduction of Cr(VI) was completed prior to the electrocatalytic oxidation of As(III) as the Cr(VI) and As(III) E

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Environmental Science & Technology simultaneous reduction of Cr(VI) and Cr(V) by H2O2 was dominant. Compared with the conversion of Cr species, the conversion of As behaved differently. As(IV), as another crucial intermediate in the oxidation of As(III), was captured by OEC acid and detected by ESR. In Figure 3c, the changes of typical As(IV) signals at giso = 2.01 show trends opposite to those of Cr(V), that is, the signal intensities of [(OEC)As(IV)]+ were gradually enhanced as electrolysis time increased from 5 to 15 min, implying that the oxidation of As(III) to As(IV) was slow compared with the accumulation of Cr(V). Figure 3d shows the specific oxidation process of As(III) to As(V) as a function of electrocatalysis time as analyzed by HPLC-ICP-MS. Although the As(IV) cannot be well separated by the chromatographic conditions, the results still provided some insight, namely that the generation of As(V) highly depended on the accumulation of As(IV). It can be inferred that the conversion of As species mainly followed two steps: initial oxidation of As(III) to As(IV) within 10 min; and subsequent oxidation of As(IV) to As(V). In comparison, the direct conversion of As(III) to As(V) was kinetically sluggish, because, if it could take place, the As(V) would be largely oxidized by HO· when Cr(V)/H2O2 Fentonlike reactions dominated in the initial time period. However, as shown in Figure 3d, the concentration of As(V) at 5 min was too low to support this hypothesis. Therefore, it can be concluded that the Cr(VI) to Cr(V) conversion was initiated first, and thereafter, the conversion of As species was activated by Cr(V)/H2O2. As(IV) controlled the conversion of As(III) to As(V), that is, the majority of As(V) was finally produced via the oxidation of As(IV) to As(V) rather than the direct oxidation of As(III) to As(V). The Involved Free Radicals: HO• and O2•−. To better reveal the conversion pathway of As(III) to As(V), the roles of the different free radicals generated from the electrocatalytic Cr(VI)/As(III) redox conversion were carefully examined by ESR (Figure 4). Tertiary butanol (TBA) and benzoquinone (BQ) which are known as HO• and O2•− radical scavengers, were used to separately test the contributions of HO• and O2•− radicals on the oxidation of As(III). As shown in Figure 4a, the ESR signals in the presence of AuPd/CNTs were clearly observed, which was in sharp contrast with that without AuPd/ CNTs. With the addition of TBA and BQ to scavenge HO• and O2•− respectively, typical characteristic peaks of the DMPOHO• and DMPO-O2•− spin adducts gave hard evidence that HO• and O2•− coexisted in AuPd/CNTs electrocatalytic Cr(VI)/As(III) redox conversion. In addition to HO•, O2•− can be generated both from the reduction of Cr(VI)37 and over the surface of AuPd alloys via interactions between electrons and O2.38 O2•− has also been proved to affect the oxidation of As(III). Coincidentally, it was found that 21(±4)% of As(III) was oxidized to As(V) in the presence of O2•− only, while 96(±4)% of Cr(VI) was efficiently removed (Figure 4b). In contrast, HO• alone contributed 83.3(±3.4)% to As(III) oxidation and 92(±1)% to Cr(VI) reduction. When HO• and O2•− were both scavenged, there was almost no effect on Cr(VI) conversion, but this had a great influence upon the conversion of As(III), suggesting a preferable oxidation pathway of As(III) by HO•. It can be concluded that the role of O2•− in the oxidation of As(III) cannot be ignored, although the dominant oxidant was HO•; and the effects of active free radicals on the conversion of Cr species were slight, implying that the H2O2-based conversion of Cr(VI) was quite independent and little affected by the conversion of As, but not vice versa.

Figure 4. (a) Hyperfine EPR spectra of the different active free radicals detected in the typical electrocatalysis conditions of 15 min, 75 mA, pH 3, 0.1 M Na2SO4, and 10 mg/L dosages of Cr(VI) and As(III). TBA or BQ were used as radical scavengers. (b) Effects of different free radicals on redox conversion of Cr(VI) and As(III) derived from the corresponding electrocatalysis condition.

The Electrocatalytic Cr(VI)/As(III) Redox Conversion Mechanisms. From the above results, it has been well demonstrated that both the intermediates of Cr(V) and As(IV) and active radicals of HO• and O2•− play vital roles in the highly efficient Cr(VI)/As(III) redox conversion via the AuPd/ CNTs electrocatalysis. Herein, the mechanisms in the presence of these components are deduced in Figure 5. Moreover, density functional theory (DFT) calculations were also used to probe the mechanism in depth from the angle of thermodynamics (Figure 6). First, based on the above results, H2O2 was verified to be in situ synthesized efficiently over the surface of AuPd/CNTs electrocatalysts by using O2 and H2 during the electrocatalytic process eqs 5−7. In this process, the active {111} facet on AuPd alloys weaken the interaction of the surface with H2O2, hence decrease the hydrogenation of the OH group to H2O and thus suppress the dissociation of H2O2, and, finally, facilitate the release of H2O2.39 Subsequently, H2O2 initiated the Cr(VI)-H2O2 reaction. To be specific, [Cr(VI)O4]2− was reduced to [Cr(V)(O2)4]3− by virtue of H2O2 via the electrocatalysis, resulting in the production of superoxide (O2•−) eq 8.36 As shown in Figure 6, the calculated ΔGH of the conversion of Cr(VI) to Cr(V) is 0.22 eV, suggesting that F

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Figure 5. Proposed mechanism of simultaneous transformation of Cr(VI) and As(III) under AuPd/CNTs electrocatalysis.

2H+ + 2e− = H 2E θ = 0V

(5)

O2(g) + 2H+ + 2e− = H 2O2 E θ = 0.695V

(6)

AuPd/CNTs

O2(g) + H 2(g) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H 2O2

(7)

[Cr VIO4 ]2 − + nH 2O2 → [Cr VI(O)3 (O)2 ]2 − → ... + → [Cr V(O2 )4 ]3 − + O•− 2 + nH

IV [Cr V(O2 )4 ]3 − + O•− + O2 2 → Cr + Cr IV + H 2O2 → Cr III + O•− 2 + 2H Pd(111)

+ • O•− 2 + H ⎯⎯⎯⎯⎯⎯⎯→ OOH

(8) (9) (10) (11)

Pd(111)

OOH• + H• ⎯⎯⎯⎯⎯⎯⎯→ H 2O2

(12)

Cr V + H 2O2 → Cr VI + HO− + HO•

(13)

According to the above results, the accumulation of Cr(V) occurred prior to that of As(IV), thus, the conversion of As(III) to As(IV) is proposed as the first step. It has been stated that the HO• was more kinetically active in this conversion (k12 = 9 × 109 M−1 s−1) than O2•− (k13 = 3.6 × 106 M−1 s−1).40 HO• and O2 can both facilitate the subsequent conversion of As(IV) to As(V) easily (Figure 6, ΔGH < 0), improving the conversion of As. It is worth noting that the HO• chemically interacts with As(IV) by seizing O from O−H bond to form HO2•, resulting in a slight decrease of the distance of the As−H bond (from 1.833 to 1.655 Å). The activation barrier of −0.02 eV of the transition state (TS) by HO• pathway is lower than that of 0.31 eV by O2, indicating the HO• is more able to break the thermodynamic barrier of this conversion of As compared with O2. These results agreed well with our experimentally observed contribution of HO• on As(III) oxidation in Figure 4b. Given all this, the corresponding reactions are presented as following eqs 14−17.

Figure 6. Density functional theory calculations of the intermediate reactions with Cr(V), As(IV), HO• and O2•−. The calculated distance of two atoms and energies are displayed in Å and eV. White, red, purple, and gray indicate O, H, As, and Cr, respectively.

the reduction of Cr(VI) to Cr(V) is a nonspontaneous reaction. This conversion by receiving O atom from H2O2 to Cr(VI)-H band edges was also benefited from the energy derived from electrocatalysis. That is to say, the conversion of Cr(VI) to Cr(V) will be existed dependently in the AuPd/CNTs electrocatalytic process. As a result of the accumulation of Cr(V), two subsequent steps could proceed. On one hand, [Cr(V)(O2)4]3− could be further converted to Cr(III) via H2O2 and O2•− eqs 9−10.37 In this period, O2•− can effectively interact with two hydrogen atoms on Pd{111} active facets, and reassemble to H2O2 for recycling eqs 11 and 12. 38 On the other hand, the decomposition of H2O2 by Cr(V) induced the Cr(V)/H2O2 Fenton-like reaction, providing mounts of active HO• eqs 13.

HO• + AsO−2 + 2H 2O → As IV + OH− G

(14)

DOI: 10.1021/acs.est.5b01759 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology IV As III + O•− + O22 − 2 → As

(15)

As IV + O2 → As V + O•− 2

(16)

HO• + As IV → As V + OH−

(17)

Bearing Solid Wastes Generated during Arsenic Removal from Drinking Water. Environ. Sci. Technol. 2013, 47, 10799−10812. (2) Adra, A.; Morin, G.; Ona-Nguema, G.; Menguy, N.; Maillot, F.; Casiot, C.; Bruneel, O.; Lebrun, S.; Juillot, F.; Brest, J. Arsenic Scavenging by Aluminum-Substituted Ferrihydrites in a Circumneutral pH River Impacted by Acid Mine Drainage. Environ. Sci. Technol. 2013, 47, 12784−12792. (3) Shokes, T. E.; Möller, G. Removal of Dissolved Heavy Metals from Acid Rock Drainage Using Iron Metal. Environ. Sci. Technol. 1999, 33, 282−287. (4) Cullen, W. R.; Reimer, K. J. Arsenic speciation in the environment. Chem. Rev. 1989, 89, 713−764. (5) Rifkin, E.; Gwinn, P.; Bouwer, E. Peer Reviewed: Chromium and Sediment Toxicity. Environ. Sci. Technol. 2004, 38, 267A−271A. (6) Kim, D.-h.; Bokare, A. D.; Koo, M. s.; Choi, W. Heterogeneous Catalytic Oxidation of As(III) on Nonferrous Metal Oxides in the Presence of H2O2. Environ. Sci. Technol. 2015, 49, 3506−3513. (7) O’Day, P. A.; Vlassopoulos, D.; Root, R.; Rivera, N. The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions. Proc. Natl. Acad. Sci. U. S. A. 2004, 101 (38), 13703−13708. (8) Liu, R.; Sun, L.; Qu, J.; Li, G. Arsenic removal through adsorption, sand filtration and ultrafiltration: In situ precipitated ferric and manganese binary oxides as adsorbents. Desalination 2009, 249, 1233−1237. (9) Lee, Y.; Um, I.-h.; Yoon, J. Arsenic(III) Oxidation by Iron(VI) (Ferrate) and Subsequent Removal of Arsenic(V) by Iron(III) Coagulation. Environ. Sci. Technol. 2003, 37 (24), 5750−5756. (10) Edward, L. K.; John, A. N. The Interaction of Chromium(III) and Chromium(VI) in Acidic Solution. J. Am. Chem. Soc. 1955, 77, 3186−3189. (11) Fruchter, J. Peer Reviewed: In-Situ Treatment of ChromiumContaminated Groundwater. Environ. Sci. Technol. 2002, 36, 464A− 472A. (12) Døssing, A. Recent advances in the coordination chemistry of hydroxo-bridged complexes of chromium(III). Coord. Chem. Rev. 2014, 280, 38−53. (13) Kim, K.; Choi, W. Enhanced Redox Conversion of Chromate and Arsenite in Ice. Environ. Sci. Technol. 2011, 45, 2202−2208. (14) Bachate, S. P.; Nandre, V. S.; Ghatpande, N. S.; Kodam, K. M. Simultaneous reduction of Cr(VI) and oxidation of As(III) by Bacillus Firmus TE7 isolated from tannery effluent. Chemosphere 2013, 90, 2273−2278. (15) Jiang, B.; Guo, J. B.; Wang, Z. H.; Zheng, X.; Zheng, J. T.; Wu, W. T.; Wu, M. B.; Xue, Q. Z. A green approach towards simultaneous remediations of chromium(VI) and arsenic(III) in aqueous solution. Chem. Eng. J. 2015, 262, 1144−1151. (16) Wang, Z.; Bush, R. T.; Sullivan, L. A.; Liu, J. Simultaneous Redox Conversion of Chromium(VI) and Arsenic(III) under Acidic Conditions. Environ. Sci. Technol. 2013, 47, 6486−6492. (17) Zhao, X.; Zhang, B.; Liu, H.; Qu, J. Removal of arsenite by simultaneous electro-oxidation and electro-coagulation process. J. Hazard. Mater. 2010, 184, 472−476. (18) Hou, Y.; Liu, H.; Zhao, X.; Qu, J.; Chen, J. P. Combination of electroreduction with biosorption for enhancement for removal of hexavalent chromium. J. Colloid Interface Sci. 2012, 385, 147−153. (19) Sun, M.; Zhang, G.; Liu, Y.; Liu, H.; Qu, J.; Li, J. Highly Efficient AuPd/Carbon Nanotube Nanocatalysts for the Electro-Fenton Process. Chem. - Eur. J. 2015, 21, 7611−7620. (20) Sun, M.; Dong, Y.; Zhang, G.; Qu, J.; Li, J. α-Fe2O3 spherical nanocrystals supported on CNTs as efficient non-noble electrocatalysts for the oxygen reduction reaction. J. Mater. Chem. A 2014, 2, 13635−13640. (21) Demoisson, F.; Mullet, M.; Humbert, B. Pyrite Oxidation by Hexavalent Chromium: Investigation of the Chemical Processes by Monitoring of Aqueous Metal Species. Environ. Sci. Technol. 2005, 39, 8747−8752. (22) Zheng, J.; Iijima, A.; Furuta, N. Complexation effect of antimony compounds with citric acid and its application to the speciation of

Implications and Perspective. In this work, the AuPd/ CNTs electrocatalysis system was first introduced as a preprocessing method for the synergetic redox conversion of Cr(VI) and As(III) with crucial intermediates of Cr(V) and As(IV) and active free radicals HO• and O2•−. The as-prepared AuPd/CNTs electrocatalysts facilitated the exposure of active Pd{111} facets and possessed an approximate two-electrontransfer pathway of oxygen reduction with highly efficient formation of H2O2, which greatly contributed to the Cr(VI)/ As(III) coconversion efficiency. Moreover, the structured AuPd/CNTs electrocatalysts exhibited promising and outstanding applicability. On one hand, the high removal rates of Cr(VI) and As(III) via AuPd/CNTs electrocatalysis over 10 cycles were easily obtained (Supporting Information Figure S11a), demonstrating that AuPd/CNTs could be effective in enabling the Cr(VI)/As(III) coconversion for long time. Further insights on the binding energy of Au and Pd after 10 cycles revealed that the AuPd alloy excellently maintained compositional and crystalline stability (Supporting Information Figure S12). In the real environment and actual wastewater, this superiority of AuPd/CNTs holds great feasibility for solving some practical problems, such as the pH fluctuation, the presence of other competitive metal ions (Fe, Cu, Pb, or Sb),41 and the negative effects of ligands.42 On the other hand, the AuPd/CNTs also show desirable settleability, indicating the ease of reuse and promise of economic benefits (Supporting Information Figure S11b). For the better improvement, we propose that the structural decoration of electrocatalysts for efficient recycle (e.g., magnetic fabrication or surface wettability adjustment) is still challenging and need to scale up efforts by environmental scientists and engineers in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b01759.

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AUTHOR INFORMATION

Corresponding Authors

*(H.L.) Phone/fax: +86-10-62849160; e-mail: [email protected]. cn. *(J.L.) Phone/fax: +86-10-62795290; e-mail: jhli@mail. tsinghua.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the Major Program of National Natural Science Foundation of China (No. 51290282), National Science Fund for Distinguished Young Scholars of China (Grant No. 51225805), National Basic Research Program of China (No. 2011CB935704, No. 2013CB934004).

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DOI: 10.1021/acs.est.5b01759 Environ. Sci. Technol. XXXX, XXX, XXX−XXX