Kinetics of chlorination of phenolchlorophenolic tastes and odors.

ONE of the major problems associated with the disinfection of water supplies by chlorination is that the organoleptic properties of the chlorinated water may be increased. This malodorous water is often produced by a reaction between the added chlorine and trace concentrations of organic compounds present in the water. By far the most notorious of the organic compounds found in water which increase the taste and odors in the water upon chlorination are phenol and some of its homologs. The nature of the compound or compounds which give rise to "chlorophenolic" tastes and odors resulting from the reaction between aqueous chlorine and phenolic compounds, has been repeatedly investigated (ADAMS, 1931; ETTINGER, 1951; ETTINGER and RUCHOFT, 1951; INGOLS and RIDENOUR, 1948; TODD, 1947). However, very little was known with certainty about the chemical reactions involved in the production or destruction of chlorophenolic tastes and odor until recent appearances of a significant paper by BURTTSCHELL, ROSEN, MIDDLETON and ETTINGER (1959). In their important study, the chlorophenols that are formed from the chlorination of phenol in dilute aqueous solutions were for the time isolated and identified. From a knowledge of the chlorophenols formed, they were able to propose a reaction scheme, FIG. 1, to account for the production and subsequent elimination of "chlorophenolic" tastes and odors in water supplies arising from the chlorination of phenol. According to BURTTSCHELL et al., the chlorination of phenol proceeds by the stepwise substitution of the 2, 4 and 6 positions of the aromatic ring. Initially, phenol is chlorinated to form either 2– or 4–chlorophenol. Then 2–chlorophenol is chlorinated to form either 2,4– or 2,6–dichlorophenol, while 4–chlorophenol to form 2,4–dichlorophenol. Both 2,4– and 2,6–dichlorophenol are chlorinated to form 2,4,6–trichlorophenol. The 2,4,6–trichlorophenol reacts with aqueous chlorine to form a mixture of non-phenolic oxidation products. BURTTSCHELL et al. also determined the threshold odor concentration, i.e., the maximum dilution at which odor can be detected by any individual with an average olfactory sensitivity, for the chlorophenols formed from the chlorination of phenol. These threshold odor concentrations are listed below each of the compounds in BURTTSCHELL'S et al. reaction scheme presented in FIG. 1. The compounds found to have the strongest organoleptic properties were 2-chlorophenol, 2,4–dichlorophenol, and 2,6–dichlorophenol; these were detectable at concentration of 2 to 3 μg/L. In contrast, phenol, 4–chlorophenol, 2,4,6–trichlorophenol were detectable only at much higher concentrations. Consequently, it appears from their study that 2–chlorophenol, 2,4– and 2,6–dichlorophenol are the compounds primarily responsible for the "chlorophenolic" tastes and odors in water supplies arising from the chlorination of phenol.