PHOTOOXIDATION OF CHLORIDE TO PERCHLORATE IN THE PRESENCE OF TITANIUM DIOXIDE AND DESERT SOILS

Perchlorate has been observed in playa soils in areas of the western United States, in nitrate deposits from northern Chile, and has also been observed at low concentrations in surface and groundwater in the southwestern United States, distant from industrial sources. Studies were conducted to determine if perchlorate could be formed when dry chloride salts were exposed to ultraviolet light (sunlight or ultraviolet lamps) in the presence of titanium dioxide and soils. Production of perchlorate was observed, and varied with the conditions used; for example, 59 days of UV-B irradiation of 8 mg of chloride in the presence of 200 mg of titanium dioxide on a petri dish resulted in the production of 260 ng/plate of perchlorate. In the presence of thin layers (ca. 3 mm) of desert soil collected from Death Valley, Calif. and exposed to sunlight for 3 months, perchlorate was observed at concentrations up to 45 ng/gm. Additionally, a limited number of desert soils were analyzed by ion chromatography to determine if perchlorate was present. Concentrations varied from non-detectable to 63 ng/gm of soil. Because titanium dioxide, and to a lesser extent, desert soils, have previously been shown to generate hydroxyl radical under irradiation, these results suggest a natural source of perchlorate in the arid regions of the southwestern United States.