Chlorate Reduction in Water using Advanced Reduction Processes

Abstract

Chlorate is one of the disinfection byproducts when chlorine/chlorine dioxide is used as the primary disinfectant. Excess amounts of chlorate in drinking water above permissible limits are toxic. This study investigated chlorate removal from water by photochemical degradation using advanced reduction processes (ARP). This new class of treatment processes combines a reducing agent and an activation method to generate reducing radicals that can efficiently destroy persistent oxidized contaminants. The effectiveness of combinations of four reducing agents (sulfite, dithionite, sulfide, and ferrous iron) and three UV light sources (UV-L, UV-M, and UV-B) were evaluated for chlorate removal. Dithionite irradiated by broad-band UVB lamp having the output between 280 nm and 320 nm with peak energy at 312 nm (UV-B) showed the highest chlorate removal. In pursuit of finding the optimum ARP conditions, the process variables including pH, reducing agent dose, initial chlorate concentration and light intensity were investigated. Dithionite/UV-B ARP was effective at weakly acidic conditions less than pH 5, and chlorate removal occurred in two steps. The first was an initial rapid decrease in chlorate concentration that occurred before initiating UV irradiation and was attributed to reaction with dithionite decomposition products. Bisulfite or metabisulfite in equilibrium with bisulfite are believed to be the decomposition products responsible for chlorate reduction. The second step was a slow decrease during UV irradiation that is caused by radicals produced by photolysis of the products of dithionite decomposition. The major product of chlorate destruction was chloride, with negligible amounts of chlorite produced.