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Metal-Free Electro-Activated Sulfite Process for As(III) Oxidation in Water Using Graphite Electrodes

Cite this: Environ. Sci. Technol. 2020, 54, 16, 10261–10269
Publication Date (Web):July 27, 2020
Copyright © 2020 American Chemical Society
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Transition-metal-activated sulfite [S(IV)] processes for water decontamination have recently received intense attention in the field of decontamination by advanced oxidation processes (AOPs). However, the drawback with respect to the secondary metal sludge contamination involved in various AOPs has been argued often. In this work, we developed a novel electro-sulfite (ES) process using stable and low-cost graphite electrodes to address that concern. Arsenite [As(III)] was used as the target compound for removal by the ES process because of its wide presence and high toxicity. Parameters, including cell voltage, S(IV) concentration, solution pH, and water matrix, and the mechanisms for reactions on anode and cathode were investigated in electrolytic cells containing one or two compartments, respectively. The results show that the ES process using 1 mM S(IV) and 2 V cell voltage oxidizes 5 μM As(III) at a rate of 0.127 min–1, which is 15-fold higher than mere electrolysis without S(IV) addition (0.008 min–1) at pH 7. Further studies using radical scavengers and electron spin resonance assays demonstrated that oxysulfur radicals (i.e., SO5•– and SO4•–) and HO are responsible for As(III) oxidation in the ES process. However, HO2 produced via the oxygen reduction reaction in the EO process plays a major role in As(III) oxidation, which explains the lower reaction rate in the absence of S(IV). The effectiveness of the ES process was moreover evidenced by 60–82% As(III) oxidation in field water within 40 min. Overall, this work realizes the metal-free activation of S(IV) and significantly leverages the S(IV)-based water treatment technologies.

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  • Reaction between 1O2 and As(III); determination of second-order rate constants; determination of S(IV) by ion chromatography; determination of FFA and p-benzenediol by HPLC; speciation of As(III) and As(V); SEM of the electrode; economic analysis of different systems for As(III) oxidation; calculation of CE; experimental setup with the one-compartment and two-compartment electrolysis cells used in the ES and EO processes; photochemical reactor setup for 1O2 generation; As(III) oxidation in the ES process with different types of electrodes; comparison of the anodic electrode before and after use and SEM images of the new electrode and anodic electrode used for long time; As(III) oxidation at various initial As(III) concentrations in the ES process; effects of different buffers for the electro-sulfite system; effect of pH on As(III) oxidation in the EO process or with sulfite alone; effect of potential for As(III) oxidation in the EO process; change of As(III) concentration in the solutions containing H2O2 without electrolysis; changes of As(III) concentration in the EO process and the ES process in the presence of different scavengers; As(III) oxidation in the presence of H2O2 in the anodic compartment in the ES process; ESR spectra with DMPO as spin trap for HO radicals in the EO process; changes of As(III) and FFA concentrations in the visible-light MB+ system; ESR spectra with TEMP as spin trap; changes of As(III) and p-benzenediol concentrations; changes of S(IV) and DO concentrations in the ES process; effect of matrix components on As(III) oxidation in the ES system; As(III) oxidation in real field waters using the ES process; changes of pH and DO concentration in the real field waters; changes of current in the ES process; characterizations of the field water samples; reactions among reactive species, As(III), and scavengers; and economic analysis based on the EE/O index of different systems (PDF)

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This article is cited by 12 publications.

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