Perchlorate production by ozone oxidation of chloride in aqueous and dry systems

Overwhelming evidence now exists that perchlorate is produced through natural processes and can be ubiquitously found at environmentally relevant concentrations in arid and semi-arid locations. A number of potential production mechanisms have been hypothesized and ClO₄^- production by ozone oxidation of surface bound Cl^- was demonstrated. However, no information concerning the impact of concentration, final reaction products distribution, impact of reaction phase, or oxidation of important oxychlorine intermediates has been reported. Using IC-MS-MS analysis and replicate oxidation experiments, we show that exposing aqueous solutions or Cl^- coated sand or glass surfaces to O₃ (0.96%) generated ClO₄^- with molar yields of 0.007 and 0.01% for aqueous Cl^- solutions and 0.025 and 0.42% for Cl^- coated sand and glass, respectively. Aqueous solutions of Cl^- produced less ClO₄^- than Cl^- coated sand or glass as well as a higher ratio of ClO₃^- to ClO₄^-. Reduction of the initial Cl^- mass resulted in substantially higher molar yields of ClO₄^- and ClO₃^-. In addition, alkaline absorbers that captured gaseous products contained substantial quantities of Cl^-, ClO₃^-, and ClO₄^-. Solutions of possible oxychlorine intermediates (OCl^- and ClO₃^-) exposed to O₃ produced only scant amounts of ClO₄^- while a ClO₂^- solution exposed to O₃ produced substantial molar yields of ClO₄^- (4% molar yield). Scanning electron microscopy coupled with energy energy-dispersive X-ray analysis demonstrated a significant loss of Cl^- and an increase in oxygen on the Cl^- coated silica sand exposed to O₃. While the experimental conditions are not reflective of natural conditions this work clearly demonstrates the relative potential of Cl^- precursors in perchlorate production and the likely importance of dry aerosol oxidation over solution phase reactions. It also suggests that ClO₂^- may be a key intermediate while ClO₃^- and OCl^- are unlikely to play a significant role.