Mechanisms of UV-Light Promoted Removal of As(V) by Sulfide from Strongly Acidic Wastewater.

Strongly acidic wastewater with a high arsenic concentration is produced by a number of industries. The removal of As(V) (HAsO) by sulfide from strongly acidic wastewater remains a difficult issue. This study proposed a UV-assisted method to efficiently remove As(V) by sulfide, and the involved mechanisms were systematically investigated. In the dark, the low removal efficiency of As(V) by sulfide was attributed to the slow formation and transformation of an intermediate species, i.e., monothioarsenate (HAsOS), in the As(V) sulfuration reaction, which were the rate-controlling steps in this process. However, UV irradiation significantly promoted the removal efficiency of As(V) not only by promoting the formation of HAsOS through light-induced HS and •H radicals but also by enhancing the transformation of HAsOS through a charge-transfer process between S(-II) and As(V) in the HAsOS complex, leading to the reduction of As(V) to As(III) and the oxidation of S(-II) to S(0). The formed As(III) species immediately precipitated as AsS under excess S(-II). Kinetic modeling offered a quantitative explanation of the results and verified the proposed mechanisms. This study provides a theoretical foundation for the application of light-promoted As(V) sulfuration removal, which may facilitate the recycling and reuse of arsenic and acid in strongly acidic wastewater.