Removal of Se(IV) by the Dithionite/Ultraviolet Advanced Reduction Process: Effects of Process Variables

This study investigates removal of selenite [Se(IV)] by reductive precipitation during treatment with an Advanced Reduction Process (ARP) that uses dithionite activated by ultraviolet (UV) irradiation. Our screening experiments evaluated a number of ARP and found that the dithionite/UV ARP was most effective in removing soluble selenite. Furthermore, this work considers effects of operating conditions such as dithionite dose, solution pH, initial selenite concentration, and light intensity on reduction of Se(IV). Selenite [Se(IV)] was completely removed in 120 min when initial Se(IV) concentration was 0.023 mM, dithionite dose was 1 mM, and the initial pH was ∼4.5. Higher dithionite doses, lower pH, and higher incident UV irradiance increased soluble Se(IV) removal. Selenium in the solids was effectively removed from solution by conversion to solids that were removed by filtration. Scanning emission microscopy/energy dispersive X-ray spectroscopy (EDS), X-ray diffraction, and X-ray Photoelectron Spectroscopy results showed that Se(IV) was reduced by the dithionite/UV ARP to form solids identified as elemental Se or as a compound composed of both Se and S (e.g., SemSn). Elemental Se was the primary solid, especially at higher initial Se(IV) concentrations. In the dithionite/UV ARP, rapid removal of soluble Se(IV) at low pH is attributed to photolysis of dithionite or a dithionite decomposition product (e.g., bisulfite, metabisulfite, thiosulfate, and trithionate) that is initially present in the dithionite solution.