Effects of activated carbon on the reactions of combined chlorine with phenols

In the presence of ammonia, prechlorination in drinking water treatment results in contact of combined chlorine (monochloramine) with activated carbon, which is used to remove organic compounds from water. Monochloramine reacts very slowly with phenolic compounds in aqueous solution, giving low yields of chlorinated phenols. When monochloramine reacts with phenols adsorbed on granular activated carbon (GAC), however, several oxidized products, principally hydroxylated biphenyls, are formed. Some of the hydroxylated biphenyls are chlorinated (hydroxylated PCBs). Their formation is particularly important because of their potential toxicity. Such compounds are major reaction products from chlorophenol, but they are also formed in small amounts from nonchlorinated phenols. Most of the monochloramine-GAC-phenolic compound reaction products are also produced in similar reactions with free chlorine, indicating that similar reaction mechanisms (free radical mechanisms) take place on carbon's surface. No organic compounds are produced from the reaction of monochloramine with GAC alone.