Rates of Reduction of N-Chlorinated Peptides by Sulfite:  Relevance to Incomplete Dechlorination of Wastewaters

Biologically induced fragmentation of proteins during wastewater treatment produces peptides, which form long-lasting organic chloramines when the water is disinfected with Cl2. To protect aquatic wildlife from residual chlorine, including chloramines, wastewaters are often treated with sulfur dioxide or sulfite salts. This strategy incompletely eliminates residual chlorine species. Here we report that dechlorination rate constants of N-chloropeptides are 1−2 orders of magnitude smaller than those for NH2Cl and some aliphatic organic chloramines. Slow rates explain the prevalence of N-chloropeptides in dechlorinated wastewaters after faster reacting chlorine species have been eliminated. Dechlorination is subject to general acid catalysis. For N-chlorinated leucylalanine, the rate law above pH 6 in phosphate buffer at 25 °C and I ≈ 0.1 M is as follows: rate = (9.92 ± 0.41 × 103[H2PO4 -] + 5.70 ± 0.52 × 108[H3O+] + 5.3 ± 0.2)[SO3 2-][Cl-Leu-Ala] (concentrations in M, time in s). Rate constants for other peptides appear to be of similar magnitude; variations in the acid-catalyzed terms among different hydrophobic peptides correlate with solvation energies of side chains. The kinetic data suggest that reducing N-chloropeptides in wastewaters by 75% or more will require reaction times generally >0.5 h at environmentally acceptable SIV doses and pH values.