Phytoremediation of Perchlorate from Fireworks

Ammonium and potassium perchlorate are strong oxidants widely employed in the pyrotechnic compositions in fireworks. Although its reduction is thermodynamically highly favorable, the perchlorate anion’s reactivity is kinetically inhibited, primarily attributable to steric hindrance of access to the central chlorine atom within the tetrahedral ClO4 structure. This relative inactivity has historically made perchlorate a spectator anion in aqueous solution chemistry and explains its classification as a persistent environmental contaminant. The perchlorate anion is a potent competitive inhibitor of iodide (IG) transport by the sodium iodide symporter (NIS) expressed in the basolateral membranes of human thyroid follicular cells. This inhibition can result in the reduction of thyroid hormone production critical for the normal growth and development of fetuses, infants and young children. Perchlorate has no other adverse physiological effects and is normally excreted quantitatively. However, public concern over the suppression of thyroid function has engendered government regulation and the research, development and use of technologies for the detection, measurement and removal of perchlorate contamination in drinking water supplies. In 2005, the U.S. Environmental Protection Agency adopted a Reference Dose (RfD) for ClO4 of 0.7 :g/kg/day and recently established an Interim Drinking Water Health Advisory level of 15 :g/l. In the last ten years, researchers have investigated the transformation of perchlorate to chloride (ClG), called mineralization, in which plants and plantmicrobe systems serve as the mediators. This plant-assisted degradation of perchlorate is an example of phytoremediation. Perchlorate can be decomposed in the plant’s leaves (phytodegradation) and in the root environment or rhizosphere (rhizodegradation). Phytodegradation is a slow process and is normally accompanied by phytoaccumulation, in which perchlorate is stored inside plant tissues. This accumulated perchlorate will likely either return to the plant’s local environment or enter the food chain. Rhizodegradation is a much faster process involving bacteria in anaerobic conditions in which perchlorate is sequentially converted to chlorate (ClO3G), chlorite (ClO2G) and chloride by reaction with many organic compounds (e.g., acetate). This rhizodegradation is hampered by the presence of nitrate, which is preferentially reduced by the bacteria in the root zone. Many species of terrestrial and aquatic plants are capable of remediating perchlorate, provided adequate levels of oxidizable (electron donating) organic compounds are available. A recently reported study of the contamination of