Mechanisms of Perchlorate Formation with UV and Ozone in Relation to the Isotopic Composition of Natural Perchlorate

摘要

Recent studies have demonstrated that there is a ubiquitous occurrence of natural per-chlorate (ClO_4~-) throughout the United States and the world. Natural ClO_4~appears to be atmospherically generated and deposited in precipitation (wet and dry). A number of studies have demonstrated possible ClO_4~generation mechanisms involving oxidation of chlorine species by ozone (O3) or by photo-oxidation. Past stable isotope studies of natu-ralClO_4~from the Atacama Desert support an atmospheric oxidation mechanism related to a substantial 17O anomaly (Δ~(17)O = +4 to +11 ‰) caused by reactions with O3. However, more recent work indicates that some natural ClO_4~from groundwater and the unsaturated zone in the Southern High Plains of Texas and Rio Grande Basin of New Mexico have relatively small 17O anomalies (~(17)O = 0 to +1 ‰). The lower ~(17)O values in these samples could indicate that the natural ClO_4~originated with higher ~(17)O, but later this characteristic was partly erased by exchange with other oxygen-bearing species having ~(17)O = 0; alternatively, the ClO_4~may not have been produced from reactions with O3. We have examined production mechanisms and stable isotopic compositions of ClO_4~-produced in the laboratoryby O3 oxidation and photo-oxidation of aqueous oxy-chlorine anions (eg. Cl~-, OCl~, ClO_2~-, ClO2). In addition to evaluating the mechanisms of production we have also evaluated the impact of major environmental parameters such as pH, reactant concentration, and irradiation wavelength. The major final products for all experiments were Cl~and ClO_3~with ClO_4~constituting a minor end species (2–3 orders of magnitude lower).