Photooxidation of chloride to perchlorate in the presence of desert soils and titanium dioxide


Glenn C. Miller,, Environmental Sciences and Health, University of Nevada, Mail Stop 199, Reno, NV 89557, Viktoriya Lepak,, Department of Natural Resources and Environmental Science, University of Nevada, Reno, Mail Stop 199, Reno, NV 89557, Reya Kempley, Department of Chemistry, University of Nevada, Reno, NV 89557, and Jalal Awadh, University of Yemen, Sanaa.
Naturally occurring perchlorate has been observed in playa 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 sunlight and laboratory ultraviolet light in the presence of desert soils and titanium dioxide. Several desert soils were surveyed to determine which soils contained the highest amounts of naturally occurring perchlorate. A soil from Death Valley (29/μg/kg) and a soil from the Black Rock Desert (16 μg/kg) were selected, along with titanium dioxide for further studies. The soils were washed to remove the existing perchlorate; additional chloride was added and exposed to sunlight and ultraviolet light for 1-4 months. Perchlorate was generated in both sunlight and ultraviolet light on the soils (<4-29 μg/kg). Higher amounts were generated on titanium dioxide. The mechanism proposed for formation is a step-wise oxidation of chloride. The presence of both chlorate and perchlorate was confirmed by ion chromatography-mass spectrometry. These data suggest a potential mechanism for natural generation perchlorate on soils in the desert southwest.