Rao, B., Estrada, N., McGee, S. Mangold, J., Gu, B, Jackson, W.A. Perchlorate production by photo-decomposition of aqueous chlorine solutions. Environ Sci Technol. 2012 Sep 10.
Aqueous chlorine solutions (defined as chlorine solutions (Cl2,T) containing solely or a combination of molecular chlorine (Cl2) ), hypochlorous acid (HOCl) and hypochlorite (OCl-)) are known to produce toxic inorganic disinfection by-products (e.g., chlorate and chlorite) through photo-activated transformations. Recent reports of perchlorate (ClO4-) production-a well-known thyroid hormone disruptor- from stored bleach solutions indicates the presence of unexplored transformation pathway(s). The evaluation of this potential ClO4- source is important given the widespread use of aqueous chlorine as a disinfectant. In this study, we perform detailed rate analysis of ClO4- generation from aqueous chlorine under varying environmental conditions including ultra-violet (UV) light sources, intensity, solution pH, and Cl2,T) concentrations. Our results show that ClO4- is produced upon UV exposure of aqueous chlorine solutions with yields ranging from 0.09 × 10-3 to 9.2 × 10-3 % for all experimental conditions. The amount of ClO4- produced depends on the starting concentrations of Cl2,T) , ClO3-, UV source wavelength, and solution pH but is independent of light intensity. We hypothesize a mechanistic pathway derived from known reactions of Cl2,T) photo-decomposition that involves the reaction of Cl radicals with ClO3- to produce ClO4- with calculated rate coefficient (kClO4-) of (4-40) × 10^5 M^-1 s^-1 and (3‒250) × 10^5 M^-1 s^-1 for UV-B/C and UV-A, respectively. The measured ClO4- concentrations for both UV-B and UV-C experiments agreed well with our model (R^2= 0.88 ‒ 0.99), except under UV-A light exposure (R^2 = 0.52 ‒ 0.93), suggesting the possible involvement of additional pathways at higher wavelengths. Based on our results photo-transformation of aqueous chlorine solutions at concentrations relevant to drinking water treatment would result in ClO4- concentrations (~ 0.1 µg L-1) much below the proposed drinking water limits. The importance of the hypothesized mechanism is discussed in relation to natural ClO4- formation by atmospheric transformations.