Role of hydroxyl radicals and hypobromous acid reactions on bromate formation during ozonation.

摘要

Ozonation of waters containing bromide ions (Br− results in Br− oxidation by ozone and its decomposition by-product (e.g., hydroxyl radical (HO•)) to form different intermediate brominated species (e.g., hypobromous acid (HOBr), hypobromite ions (OBr−), bromite (BrO₂−), bromide radicals (Br• ), and hypobromite radicals (BrO−)) and eventually to form bromate (BrO₃−), a suspected carcinogen. In this study, bench- and pilot-scale experiments were conducted with Colorado River Water (CRW) to investigate the kinetics and control of bromate formation.; Bromate formation is strongly influenced by water quality and treatment variables. Some water quality parameters (e.g., bromide level, pH, temperature and (bi)carbonate alkalinity) have positive effects on bromate formation, while the presence of natural organic matter (NOM) and ammonia reduce the amount of bromate formed. Increasing ozone dose and/or contact time increases bromate formation. Quantification of bromate formation is expressed by the R_CT value, the ratio of HO• exposure (or concentration) to ozone exposure (or concentration). Two phases of R _CT are performed and the values of R_CT remain unchanged throughout the ozonation process with a set of water quality. The R_CT values for ozonation of CRW water range between 10−7 and 10 −9, resulting in HO• concentrations of on the order of 10−2 to 10−14 M.; Bromate formation can be controlled by adding acid or ammonia to decrease intermediate OBr−. Acid addition to lower water pH from ambient (8.2) to 7.5 and 6.5 reduces bromate formation by 30% and 80%, respectively. In comparison, adding ammonia reduces bromate formation up to 60% and 85% at pH 7.5 and 8.5, respectively. Ammonia's efficiency to mitigate bromate formation is lessened with lowered water pH. Bromate formation by HO • radical pathways cannot be controlled unless radicals are being scavenged. Inorganic carbonate species can scavenge HO• and form carbonate radicals that react with OBr− to form bromate.; The reaction rate of chlorine or bromine with NOM is rapid and dependent upon NOM characteristics. Bromination is approximately one order of magnitude faster than chlorination. The slower NOM reaction sites have a rate constant of approximately 50 M−1s−1, which is 3 to 4 orders of magnitude less than the fast NOM reaction sites. Pre-ozonation reduces the NOM reactivity by approximately 50%. Kinetically, the impact of bromate reduction by the reaction of intermediate HOBr and NOM is only important during the fast ozonation stage (t 2 minutes). A negligible impact on bromate reduction by HOBr and NOM reactions at slow ozonation stage (t > 2 minutes) can be implied based upon the rate constants and reactivity of NOM.