Photochemical formation of perchlorate from aqueous oxychlorine anions

Evidence of atmospherically produced perchlorate is being accumulated, yet information regarding its formation process is largely unknown. For the first time, the present study demonstrates that perchlorate can be generated as an end-product of photochemical transformation reactions of chlorine precursors such as aqueous salt solutions of hypochlorite, chlorite, and chlorate upon exposure to ultraviolet (UV) radiation. For example, under exposure to UV light from photochemical reactor lamps at a peak wavelength of 253.7 nm for 7 days, the observed perchlorate concentrations were 5, 25, and 626 microg/L at initial chlorite concentrations of 100, 1000, and 10,000 mg/L, respectively. In addition, perchlorate was generated within 7 days from aqueous chlorite solutions at mid-latitude (33 degrees 59'N, 101 degrees 89'W) spring and summer solar radiation. Via UV radiation from the artificial lamps and sunlight, chlorite was converted to chloride (68%) and chlorate (32%) as end-products on the basis of molar percentage. However, perchlorate was not detected from aqueous chloride solutions at initial concentrations up to 10,000 mg/L under the experimental conditions. Relevant mechanistic pathways were proposed based on the fact that chlorine dioxide (as a primary intermediate) may play a significant role in phototransformation of the precursors leading to perchlorate.