Chlorine-ozone interactions: Formation of chlorate

The chlorate ion is one of the contaminants being considered for regulation in drinking water under the disinfection by-product rule. In addition to its presence in water as a result of chlorine dioxide use, chlorate may be formed during intermediate ozonation if residual chlorine is present. The majority of chlorate forms through a free radical pathway in natural source waters as opposed to by both molecular ozone and free radical pathways in low-DOC waters. The monitoring of OH radicals using p-chlorobenzoic acid indicated that the formation of chlorate is proportional to OH radical generation. The increase in pH favored chlorate formation and the presence of alkalinity enhanced chlorate formation due to secondary reactions between carbonate radicals and chlorine species. The addition of ammonia and peroxide before ozonation reduced chlorate formation whereas the simultaneous addition of peroxide during ozonation enhanced chlorate formation.