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

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.