Efficient reductive recovery of arsenic from acidic wastewater by a UV/dithionite process.

The removal of arsenic (As(III)) from acidic wastewater using neutralization or sulfide precipitation generates substantial arsenic-containing hazardous solid waste, posing significant environmental challenges. This study proposed an advanced ultraviolet (UV)/dithionite reduction method to recover As(III) in the form of valuable elemental arsenic (As(0)) from acidic wastewater, thereby avoiding hazardous waste production. The results showed that more than 99.9 % of As(III) was reduced to As(0) with the residual concentration of arsenic below 25.0 μg L within several minutes when the dithionite/As(III) molar ratio exceeded 1.5:1 and the pH was below 4.0. The content of As(0) in precipitate reached 99.70 wt%, achieving the purity requirements for commercial As(0) products. Mechanistic investigations revealed that SO and H· radicals generated by dithionite photolysis under UV irradiation are responsible for reducing As(III) to As(0). Dissolved O, Fe(III), Fe(II), Mn(II), dissolved organic matter (DOM), and turbidity slightly inhibited As(III) reduction via free radicals scavenging or light blocking effect, whereas other coexisting ions, such as Mg(II), Zn(II), Cd(II), Ni(II), F(-I), and Cl(-I), had limited influence on As(III) reduction. Moreover, the cost of treating real arsenic-containing (250.3 mg L) acidic wastewater was estimated to be as low as $0.668 m, demonstrating the practical applicability of this method. This work provides a novel method for the reductive recovery of As(III) from acidic wastewater.