The Henry's constant of monochloramine

Abstract

Monochloramine is a secondary disinfectant used in drinking water and is also formed in chlorinated wastewater. While known to hydrolyze over time and react with dissolved organic matter, its partitioning between the aqueous and gas phase has not been extensively studied. Preliminary experiments demonstrated that monochloramine concentrations in solutions open to the atmosphere or actively aerated decreased more rapidly than in sealed solutions, indicating significant losses to the atmosphere. For example, a monochloramine solution open to the atmosphere yielded a loss rate constant of 0.08 d−1, a value twice that for sealed samples without headspace (0.04 d−1) where loss occurs exclusively as a result of hydrolysis. A solution aerated at 10 mL s−1 had a loss rate constant nearly 10× greater than that for hydrolysis alone (0.35 d−1). To better understand partitioning of monochloramine to the gas phase and potential for volatilization, the dimensionless Henry's law constants of monochloramine (KH) were determined using an equilibrium headspace technique at five different temperatures (11, 16, 21, 27, and 32 °C). The resulting values ranged from 8 × 10−3 to 4 × 10−2, indicating a semi-volatile compound, and were found to be consistent with quantitative structure activity relationship predictions. At 20 °C, monochloramine exhibits a dimensionless Henry's constant of about 1.7 × 10−2 which is 35 times greater than ammonia but comparable to the Henry's constant of inorganic semi-volatile compounds such sulfur dioxide. The Henry's constant values for monochloramine suggests that volatilization could be a relevant loss process in open systems such as rivers receiving chlorinated wastewater effluent, swimming pools and cooling towers.