Far-UVC-driven Cr(VI) reduction in alkaline solution: Synergistic effects of iodide and sulfite

Hexavalent chromium (Cr(VI)) is an extremely toxic pollutant in aqueous environment. Chemical reduction is the most employed method in decontamination of Cr(VI). However, the chemical reduction was usually conducted in acidic media, resulting in considerable waste of acid reagents and the following neutralizing agents. In this study, kinetics and mechanisms of Cr(VI) reduction by sulfite in alkaline conditions (pH: 7-10) were investigated. It reveals that Cr(VI) reduction follows pseudo-zero-order kinetics, where the rate constants increased markedly with an in situ irradiation of Far-UVC (UV₂₂₂). Decreasing pH levels slightly favored the reduction. Iodide ion displayed a notable accelerating effect, which not only save the energy input but also minimize the reductant usage. Chloride, sulfate, and carbonate ions exhibit little effect on the reduction, whereas nitrate and nitrite ions, dissolved oxygen as well as Cu(II) suppressed the reduction significantly, implying that hydrated electrons produced by UV₂₂₂ played the most important role in the reaction. Compared to the UV₂₅₄/sulfite/iodide process, UV₂₂₂/sulfite/iodide demonstrates clear advantages in the reduction kinetics and the sulfite utilization efficiency, underscoring its potential for effective Cr(VI) remediation in various environmental settings.