Unveiling the synergic mechanism in ultraviolet/iodide for redox conversion of toxic oxysalts

Abstract

The concurrent presence of heavy metal ions such as chromium and arsenic at acid mine drainage locations significantly complicates the treatment of wastewater. This research developed a synergistic redox strategy for Cr(VI) and As(III) using iodide(\({\mathrm{I}}^{-}\)) as an activator under UV light irradiation to enhance the removal of these metal ions from acidic solutions. Experimental findings indicate that \({\mathrm{I}}^{-}\) notably boosts the redox process for both Cr(VI) and As(III) when present together in the solution. During UV irradiation, \({\mathrm{I}}^{-}\) converts into iodine radicals (I•) and triiodide ions (\({\mathrm{I}}_{3}^{-}\)), facilitating the oxidation of As(III) to As(V). Furthermore, the simultaneous presence of Cr(VI) significantly enhances the formation of I• and \({\mathrm{I}}_{3}^{-}\) while also contributing to the reduction of Cr(VI) to Cr(III). \({\mathrm{I}}_{3}^{-}\) was identified as the main oxidant in the conversion of As(III) to As(V), with hydroxyl radicals (•OH) playing a minimal role in the reaction. The study introduces a “clock reaction” mechanism, based on the cycling of \({\mathrm{I}}^{-}\) and \({\mathrm{I}}_{3}^{-}\), to explain the redox interactions in the Cr(VI)/KI/As(III) system.