Determination of monochloramine dissipation kinetics in various surface water qualities under relevant environmental conditions - Consequences regarding environmental risk assessment.

A total 190 experiments were performed to study the dissipation kinetics of monochloramine (NHCl, CAS no 10599-90-3) in surface water samples from six rivers (Loire, Rhône, Meuse, Garonne, Seine and Moselle) and an artificial reservoir (Mirgenbach), all located in France. Experiments were conducted in an open reactor, under relevant controlled environmental conditions. The impact of various parameters such as initial NHCl concentration, temperature, pH, presence of sediments, sampling site and collection period was investigated. It was found that NHCl dissipated rapidly without any lag phase, and that decay follows an apparent first-order kinetics (r > 0.99). Presence of sediment greatly accelerated decay. Half-lives were generally <1 h in river water in presence of natural sediment, but of several hours without sediment. The impact of pH was low for the normal river water pH range. However, increase in temperature significantly accelerated decay. The combination of high initial NHCl concentrations and elevated temperatures generally gives half-lives similar to those obtained at lower temperatures and lower concentrations. Short half-lives (0.06 to 1.50 h) were found in all the surface waters examined, regardless of geographic location of sampling site or collection period, indicating no temporal or site-specific effects on NHCl dissipation. Decay was slightly faster at lower initial concentrations, which supports extrapolation of half-lives measured in this study to a wide range of environmental concentrations. It can thus be assumed that NHCl degradation in river and reservoir waters is mainly determined by presence of sediments and temperature.