Plant-mediated transformation of perchlorate into chloride

The decontamination of perchlorate-contaminated water by woody plants was i nvestigated in sand and hydroponic bioreactors. Willow trees were found to be the most favorable woody plants with phraetophytic characteristics in co mparative screen tests with eastern cottonwoods and Eucalyptus cineria. Wil lows decontaminated aqueous solutions dosed with 10-100 mg/L of perchlorate to below the method detection limit of 2 mu g/L. Two phytoprocesses were i dentified as important in the remediation of perchlorate-contaminated water : (a) uptake and phytodegradation of perchlorate in the tree branches and l eaves and (b) rhizodegradation. Exposure of rooted willow trees to perchlor ate-dosed media stimulated rhizodegradation. Homogeneous degradation studie s using media from the root zone of dosed willow trees confirmed that rhizo sphere-associated microorganisms mediated the degradation of perchlorate to chloride. Experiments conducted with varying ranges of nitrate concentrati ons clearly indicated that high nitrate concentrations interfered with rhiz odegradation of perchlorate. This study provides evidence that the efficacy of phytoremediation of perchlorate-contaminated environments may depend on the concentration of competing terminal electron accepters, such as nitrat e, and the nitrogen source of the nutrient solution. Since perchlorate does not volatilize from water readily, a perchlorate remediation scheme may in volve an intensively cultivated plantation of trees with phraetophytic char acteristics and irrigation with the contaminated water.