Mechanism of chloroform formation by chlorine and its inhibition by chlorine dioxide.

Chlorination of drinking waters leads to the formation of trihalomethanes arising from the reaction of chlorine and organic substances. Therefore, chlorine dioxide (ClO2) which does not produce trihalomethanes is being considered as an alternative disinfectant. It has been reported that rat blood chloroform levels were significantly decreased after treatment with ClO2. Studies were conducted to investigate the mechanisms of chloroform formation by chlorine (HOCl) and its inhibition by ClO2 (5 mg/liter) in the presence of HOCl (5, 10, 20 mg/liter) using sodium citrate (1 mM) as an organic substance. When citrate was reacted with HOCl, beta-ketoglutaric acid, monochloroacetone, dichloroacetone, and trichloroacetone were produced as reaction intermediates and chloroform as a final product. There was a linear relationship between the concentrations of HOCl and the formation of chloroform. When ClO2 was substituted for HOCl, neither chloroform was formed nor citrate concentration was changed. Further, chloroform formation was inhibited by ClO2 in the presence of HOCl and citrate and the degree of inhibition depends on the ratio of ClO2/HOCl. Gas chromatograph/mass spectrometer analysis indicates that this inhibition is related to the reaction of ClO2 with beta-ketoglutaric acid to form malonic acid. Chlorine dioxide also oxidizes other intermediates such as monochloroacetone and dichloroacetone to acetic acid. These studies indicate that ClO2 inhibits chloroform formation from citrate and HOCl by the oxidation of the intermediates which were involved in the reaction of chloroform formation.