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Abstract
Electro-activated sulfite or bisulfite [E-S (IV)] system is a green technology in environmental remediation. This study developed a comprehensive kinetic model to simulate S (IV) oxidation and contaminants degradation. The results revealed that the mechanism and kinetics of S (IV) oxidation varied across different regions, leading to the dominance of different radicals. In kinetics-controlled and mix-controlled regions at low current density, the rate-determining step (RDS) for S (IV) electro-oxidation was the first electron transfer with the generation of SO3•-/HSO3• contributing to BrO3- reduction. In diffusion-controlled region at high current density, oxygen evolution accelerated the rate of quenching SO3•-/HSO3•, being the RDS for S (IV) oxidation. Consequently, SO4•- dominated benzoic acid (BA) oxidation. However, due to the SO3•-/HSO3• oxidation by oxygen and SO4•- reduction by S (IV), the utilization of specific radicals was limited. To maximize the reduction and oxidation efficiency, the influence of current density and S (IV) concentration on the electrode reaction rates (i.e., electro-generation rates of SO3•-/HSO3• (r1) and oxygen (rO2)) and radical conversion efficiencies were simulated. Enhancing r1 but inhibiting rO2 in mix-controlled region improved the reduction rate. Meanwhile, balancing r1 and rO2 in diffusion-controlled region maximized the oxidation rate. Specifically, the E-HSO3- system was more effective for reduction due to the lower electro-oxidation rate and oxygen oxidation rate of HSO3• compared to SO3•-. Conversely, the E-SO32- system was more suitable for oxidation with higher rates of oxysulfur radical conversions. Based on the kinetic model, the most cost-effective current density and S (IV) concentration for reduction, oxidation and simultaneous reduction and oxidation were determined. This study significantly guided the utilization of the E-S (IV) system for various contaminants degradation.
Published Version
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