Organic chloramines in disinfected wastewaters: Rates of reduction by sulfite and toxicity

Peptides and protein fragments that have been monochlorinated at terminal amino-N sites are known to react sluggishly with S(IV) dechlorinating agents in wastewater treatment plants. Persistence of these chloramines may imperil downstream aquatic organisms. Reactivity of sidechain amino-N is explored in this paper, and previous kinetic information on chloramine reduction by S(IV) is summarized. Much like other chloramines, side-chain chlorinated N-chloroacetyllysine reacts with SO1/2- in borate buffer at 25°C according to the law rate = [SO2−3 [chloramine](520[H3BO3] + 5.7 × 1010[H3O+] + 4.1) (time in s, concentration in M). Apparently because of their greater basicity, aliphatic monochloramines such as Nchloroacetyllysine are reduced rapidly at near-neutral pH, in contrast to terminally chlorinated peptides. At pH > 10, all monochloramines will be reduced at nearly the same, quite slow rate (t1/2 ≈ 0.1/[SO2−3]; time in s, SO2−3 in M). Microtox™ (Azur Environmental, Carlsbad, CA, USA) bioassays indicate that two terminally chlorinated N-chloropeptides as well as side-chain chlorinated N-chloroacetyllysine are much less toxic to Vibrio fisheri than NH2Cl, N-chloroglycine, and N-chloropiperidine. An astonishingly simple relationship describes the bioassay results at 5 min exposure as In(EC50) = −3.684 + 0.0221 (MW) (EC50 in mg/L as C12; molecular weight [MW] in Daltons). These exploratory data offer a basis for optimism that S(IV)-surviving N-chloropeptides will prove less toxic in natural ecosystems than rapidly reduced chloramines, but further investigations are needed.