Mechanistic insight into suppression of bromate formation by dissolved organic matters in sulfate radical-based advanced oxidation processes

Abstract

Serious concerns have been raised on the formation of bromate in sulfate radical (SO4−)-based advanced oxidation process. Previous studies have shown that dissolved organic matters (DOMs) can efficiently suppress bromate formation, but the involved mechanisms are not clearly understood. In this work, we found that bromate evolution in UV/peroxydisulfate (PDS) in the presence of oxalate (a DOM model compound) exhibited biphasic kinetics, i.e., bromate was undetectable in the lag phase and rapid increased in the secondary rapid phase. Increasing oxalate concentrations resulted in the increase of duration time of lag phase and the decrease of rate of bromate formation in secondary rapid phase. Kinetic simulation was conducted by including elementary reactions of oxalate with various reactive radicals involved in UV/PDS/Br− system, where CO2 was the final product (i.e., no interference by organic intermediates) Simulation results indicated that scavenging SO4− by oxalate contributed little to the suppression of bromate formation, while scavenging reactive bromine atoms (Br) by oxalate played an important role. In addition, the superoxide formed during the reaction of oxalate with Br could also contribute to bromate suppression via reducing the reactive bromine species. Similarly, other DOM model compounds were also observed to inhibit bromate formation, and their effects decreased in the order of l-tyrosine>phenol>tert-butanol>critic acid>methanol>oxalate under identical conditions. This order was consistent with the amounts of oxidant needed for their complete mineralization, suggesting that their oxidation intermediates continuously scavenge reactive radicals to suppress bromate formation. In parallel, the inorganic bromine during the kinetic runs almost kept constant. These results obtained in this work suggest that DOMs react with reactive bromine species yielding Br− and thus prevent their further oxidation to bromate.