Redox Conversion of Chromium(VI) and Arsenic(III) with the Intermediates of Chromium(V) and Arsenic(IV) via AuPd/CNTs Electrocatalysis in Acid Aqueous Solution

Simultaneous reduction of Cr­(VI) to Cr­(III) and oxidation of As­(III) to As­(V) is a promising pretreatment process for the removal of chromium and arsenic from acid aqueous solution. In this work, the synergistic redox conversion of Cr­(VI) and As­(III) was efficiently achieved in a three-dimensional electrocatalytic reactor with synthesized AuPd/CNTs particles as electrocatalysts. The AuPd/CNTs facilitated the exposure of active Pd{111} facets and possessed an approximate two-electron-transfer pathway of oxygen reduction with the highly efficient formation of H₂O₂ as end product, resulting in the electrocatalytic reduction of 97.2 ± 2.4% of Cr­(VI) and oxidation of 95.7 ± 4% of As­(III). The electrocatalytic reduction of Cr­(VI) was significantly accelerated prior to the electrocatalytic oxidation of As­(III), and the effectiveness of Cr­(VI)/As­(III) conversion was favored at increased currents from 20 to 150 mA, decreased initial pH from 7 to 1 and concentrations of Cr­(VI) and As­(III) ranging from 50 to 1 mg/L. The crucial intermediates of Cr­(V) and As­(IV) and active free radicals HO^• and O₂^•– were found for the first time, whose roles in the control of Cr­(VI)/As­(III) redox conversion were proposed. Finally, the potential applicability of AuPd/CNTs was revealed by their stability in electrocatalytic conversion over 10 cycles.