Kinetic and mechanistic study of the effect of Cu(II) on monochloramine stability in bromide-containing waters

Abstract

Naturally occurring bromide can reduce the stability of monochloramine (NH2Cl) by forming brominated halamines, the most abundant being bromochloramine (NHBrCl). Cu(II) is found in drinking water distribution systems from copper pipes leaching or added as a biocide. This study showed that Cu(II) can catalyse the decay of NHBrCl (proved by Membrane Introduction Mass Spectrometry) during the chloramination process. 11 % of the total oxidant (chloramines, bromamines and NHBrCl) was consumed during chloramination in bromide-containing waters at pH 6.5 after 6 h, while the presence of Cu(II) remarkably accelerated the total oxidant decay with 65 % consumption. Higher bromide concentrations and lower pH led to an increasing total oxidant decomposition. This was primarily ascribed to the higher formation of NHBrCl and its further catalytic degradation from Cu(II) under these conditions. The nitrogen by-products in the presence of bromide and Cu(II) were identified. Nitrogen gas and ammonia were shown to be the main degradation products and a mechanism involving NHBrCl decomposition is proposed. It was also verified that Cu(II) enhanced the disinfectant decay in the absence/presence of bromide when natural organic matter was present. These findings can provide useful information to water utilities to better manage and maintain the stability of disinfectant residual allowing safe distribution of drinking water over a wide geographical area.