Trihalomethanes formation in water treated with chlorine dioxide

Formation of trihalomethanes (THMs) was investigated in water treated with chlorine dioxide (ClO2) and/or chlorine (Cl2) where humic acid (HA) was used as THMs precursors. When ClO2 was used as the only disinfectant, no THMs were detected in bromide-free water; while only CHBr3 was formed in water containing bromide ion because ClO2 could oxide bromide to form hydrobromous acid which subsequently reacted with HA, and the yield CHBr3 increased with bromide concentration and ClO2 dosing. When water was treated with ClO2 combined with Cl2, only CHCl3 was formed in the absence of bromide, however, all four species of THMs were formed in the presence of bromide; the THMs formation potential decreased gradually with an increase in the ratio of ClO2 to Cl2 because ClO2 reacted with HA to render them unreactive or unavailable for THMs production. When water (with or without bromide ion) was irradiated by light, the yield of THMs was increased as a function of irradiation time to a maximum, and thereafter decreased markedly; the possible mechanism is that irradiation could activate the THMs precursors in HA, and at the same time destroy the reactivity of ClO2 or Cl2. The same results could be collected from natural water treated by ClO2 with or without irradiation.