Influence of UV lamp, sulfur(IV) concentration, and pH on bromate degradation in UV/sulfite systems: Mechanisms and applications.

Bromate (BrO) is a possible human carcinogen regulated worldwide at a strict standard of 10 μg/L in drinking water. Removal of BrO by advanced reduction processes (ARPs) has attracted much attention due to its high reduction efficiency and easier combination with ultraviolet (UV) disinfection. In this study, we employed a UV/sulfite process to degrade BrO and studied the effects of UV lamp, sulfur(IV) concentration, and pH on effectiveness of the system in degrading BrO. Low-pressure UV lamps (UV-L) instead of medium-pressure UV lamps (UV-M) were selected because of the high ultraviolet-C (UV-C) efficiency of UV-L. The increased sulfur(IV) concentration is proportionally correlated with enhanced degradation kinetics. BrO reduction was improved by increasing pH when pH is within 6.0-9.0, and principal component analysis demonstrated that pH is the most influential factor over sulfur(IV) concentration and type of UV lamp. Degradation mechanisms at different pH levels were subsequently investigated. Results showed that the reduction reactions are induced by hydrated electron (e) at pH > 9.0, by H at pH 4.0, and by both e and H at pH 7.0. Effective quantum efficiency for the formation of e and H in the photocatalytic systems was determined to be 0.109 ± 0.001 and 0.034 ± 0.001 mol E, respectively. Furthermore, mass balance calculation of bromine and sulfur at pH 7 showed that bromide, sulfate and possibly dithionate ions were the major products, and a degradation pathway was proposed accordingly. Moreover, UV/sulfite processes could reduce the initial bromate concentration of 0.1 mM by 82% and 95% in the presence and absence of O in tap water respectively, and 99% in the absence of O in deionized water within 20 min at pH 9.0 and 2.0 mM sulfur (IV).