title: Perchlorate production by photodecomposition of aqueous chlorine solutions.
abstract: Aqueous chlorine solutions (defined as chlorine solutions (Cl(2,T)) containing solely or a combination of molecular chlorine (Cl(2)), hypochlorous acid (HOCl), and hypochlorite (OCl(-))) are known to produce toxic inorganic disinfection byproduct (e.g., chlorate and chlorite) through photoactivated transformations. Recent reports of perchlorate (ClO(4)(-)) production-a well-known thyroid hormone disruptor- from stored bleach solutions indicates the presence of unexplored transformation pathway(s). The evaluation of this potential ClO(4)(-) source is important given the widespread use of aqueous chlorine as a disinfectant. In this study, we perform detailed rate analysis of ClO(4)(-) generation from aqueous chlorine under varying environmental conditions including ultraviolet (UV) light sources, intensity, solution pH, and Cl(2,T) concentrations. Our results show that ClO(4)(-) 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 ClO(4)(-) produced depends on the starting concentrations of Cl(2,T) and ClO(3)(-), UV source wavelength, and solution pH, but it is independent of light intensity. We hypothesize a mechanistic pathway derived from known reactions of Cl(2,T) photodecomposition that involves the reaction of Cl radicals with ClO(3)(-) to produce ClO(4)(-) with calculated rate coefficient (k(ClO4-)) 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 ClO(4)(-) 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, phototransformation of aqueous chlorine solutions at concentrations relevant to drinking water treatment would result in ClO(4)(-) 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 ClO(4)(-) formation by atmospheric transformations.
authors.affiliation: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.
authors.forename: Balaji
authors.initials: B
authors.lastname: Rao
authors.forename: Nubia
authors.initials: N
authors.lastname: Estrada
authors.forename: Shelly
authors.initials: S
authors.lastname: McGee
authors.forename: Jerry
authors.initials: J
authors.lastname: Mangold
authors.forename: Baohua
authors.initials: B
authors.lastname: Gu
authors.forename: W Andrew
authors.initials: WA
authors.lastname: Jackson
mesh_terms: D002713:Chlorine / Q000737:chemistry* / Q000528:radiation effects; D004202:Disinfectants / Q000737:chemistry* / Q000528:radiation effects; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D010472:Perchlorates / Q000737:chemistry* / Q000528:radiation effects; D010782:Photolysis; D012996:Solutions; D014466:Ultraviolet Rays
chemical_list: D004202:Disinfectants; D010472:Perchlorates; D012996:Solutions; D002713:Chlorine; C494474:perchlorate