PROBING THE ORIGINS OF NATURAL PERCHLORATE: EXPOSURE OF CHLORIDE TO OZONE IN THE PRESENCE OF SAND AND AQUEOUS PHOTOCHEMICAL REACTIONS OF OXYCHORINE ANIONS

Although current forensic investigations of perchlorate occurrence have demonstrated that some of the contamination is from a non-anthropogenic source, natural perchlorate production pathways are still not clearly understood and many of the proposed routes of production have not been supported with experimental evidence. We therefore attempted to probe atmospheric pathways other than electrochemical reactions previously reported.

The first feasibility study involved catalytic ozone reactions with chloride on sand particles. Our experimental setup employed a chloride solution, ozone, silica sand as a surrogate for dust or mineral oxides, and ozone-inert Teflon^® tubings and glassware. The experiments were conducted with no sand, hydrated or dehydrated sand for 7 days. Control experiments were also performed using filtered, compressed air in place of ozone. Aqueous species and gaseous species produced were captured in alkaline trapping solutions.

The possibility of perchlorate generation via photochemical pathways was tested in a subsequent study. Current understanding indicates that ultraviolet (UV) radiation is an important variable in rapid transformation of less oxidized oxychlorine anions (ClO^-, ClO₂^-) into more oxidized species (ClO₃^-) in aqueous solutions. All photochemical experiments were conducted in a Rayonet^® Chamber Reactor housing UV lamps. Chlorite and chlorate were tested initially. Irradiated samples and dark controls were prepared in quartz reaction vessels and subject to UV radiation for 7 days. Samples at Time 0 as an additional set of controls were also prepared. Additionally, hydrogen peroxide, pure oxygen, and ozone/oxygen mixtures were added or sparged into the system to examine which oxygen donor(s) are potentially responsible for photo-production of perchlorate in aqueous solutions.

For both studies, the concentrations of oxychlorine anions including perchlorate were measured using various ion chromatographic methods. Based on the mass balance of Cl atom, the potential routes of perchlorate generation were established. The results from the simulation experiments may reveal the existence of important pathways, depending upon the relative abundance of the reaction variables, and provide the clues for the ubiquitous detection of trace-level perchlorate in the environment.