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Phytoremediation - Greenhouse TestingAthens, GASource: Valentine A. Nzengung, Ph.D., Associate Professor Department of Geology, University of Georgia, Athens, Georgia 30605, Phone: 706-542-2699, Fax: 706-542-2425, Email: vnzengun@uga.edu Project Summary: The following text was excerpted from project summary information provided by University of Georgia personnel, September 2000: Green plants (aquatic and terrestrial) were screened at the request of the United States Air Force and species capable of degrading perchlorate to chloride were identified (Table 1). Subsequent investigations focused on long-term studies and possible factors that may influence phytoremediation of perchlorate-contaminated water. The specific objectives were to: (1) show that woody, edible, and wetland plants effectively remove perchlorate from water, (2) identify factors that enhance or inhibit uptake, phytodegradation and rhizodegradation of perchlorate, (3) isolate rhizosphere bacteria and verify their contribution to the degradation of perchlorate, (4) determine the efficacy of phytoremediation in the clean up of a mixed-contaminants of perchlorate and trichloroethylene (TCE). Bench scale studies show that selected woody, edible, and aquatic plants, microbial mats and algae can be used to detoxify water, sediment and soil contaminated by perchlorate. The initial slow uptake and phytodegradation of perchlorate by plants exposed to perchlorate changes to very rapid removal by rhizodegradation after several days depending on the plant physiology and environmental conditions. Initial or prolonged exposure of rooted green plants to perchlorate-dosed media biostimulates the rapid growth of perchlorate-degrading microorganisms in the rhizosphere. Although rhizodegradation accounts for the removal of most of the perchlorate in solution by plants with roots, uptake of perchlorate into the plants may become significant in environments with high nitrate concentrations (a competing terminal electron acceptor). The results of these studies demonstrate that rhizotransformation predominates if nitrate is replaced by ammonium/urea as the nitrogen source and the trees have a higher fraction root mass. Exudates secreted by the plant roots supply nutrients (carbon and electron sources) that sustain the growth of rich and diverse consortia of the perchlorate-degrading microorganisms. Rhizodegradation is a very rapid process that minimizes the uptake of perchlorate into the tree leaves and branches. Any perchlorate taken up into the green plants is not simply accumulated, but slowly transformed. Degradation of perchlorate by pureed and crude extracts of edible plants (spinach and French tarragon) provided direct evidence of phytodegradation of perchlorate. Willow trees were used to successfully treat water contaminated by both perchlorate and TCE in bench-scale tests. The rate of perchlorate removal from solution in willow bioreactors with and without TCE was the same. The simultaneous phytotransformation of perchlorate and TCE observed in our greenhouse tests was verified and confirmed in experiments conducted with the radiolabeled forms of these compounds. No evidence of long-term phytoaccumulation of either contaminant was observed. Possible perchlorate phytoremediation schemes may include (1) an intensively cultivated plantation of trees with phreatophytic characteristics at field sites with shallow groundwater, (2) irrigation of planted trees with perchlorate-contaminated groundwater pumped from deep aquifers; and (3) use of constructed anaerobic treatment wetlands. The costs of phytoremediation of perchlorate are similar to those of phytoremediation system currently used to treat organic contaminants and nutrients in soils and groundwater. A potentially effective algae and microbial mat-based bioreactor treatment system for the rapid removal of perchlorate from water (to non-toxic levels - <4 ppb, or <2 ppb if background noise is insignificant) at perchlorate concentrations within the range of 0 to 300 mg/L (ppm) was also designed and tested. The essential components of the design are a reaction vessel, microbial mats or algae, light, and the bioactive solids. The microbial mat bioreactors performed better and rapidly degraded perchlorate in solution at a faster rate (half-life = 1.2 days). Additionally, the microbial mats survived harsh environmental conditions better than the algae. Possible application of the microbial mats and algae systems could be in treatment ponds exposed to sunlight. Table 1. Different plants, algae, microbial mats, nutrient amendments and bioreactor systems used in perchlorate degradation studies
Additional Info Source: Nzengung, Valentine A., Wang, Chuhua, Harvey, Greg. 1999. Plant-Mediated Transformation of Perchlorate into Chloride, Environmental Science & Technology, vol. 33, pp. 1470-1478. Nzengung, Valentine A., Chuhua Wang, Harvey, G., McCutcheon, S.C., and Wolfe, N.L. 1999. Phytoremediation of Perchlorate Contaminated Water: Laboratory Studies. Symposium Series: Fifth International Symposium on In Situ and On-Site Bioremediation: Phytoremediation. Editors; Leeson Andrea and B. C. Alleman. Battelle Press, pp 239-244. Nzengung, Valentine A., Wang Chuhua. 2000. Influences on Phytoremediation of Perchlorate Contaminated Water. American Chemical Society (ACS) Special Symposium Series: Perchlorate in the Environment. Editor: Urbansky. Kluwer Academic/Plenum Publishers, New York. Chapter 21, pp 219 - 229. OšNiell, L.W., Nzengung, Valentine A., Adesida Adebola. 2000. Treatment of Perchlorate Contaminated Water in Microbial Mat, Algae, and Ebb-and-Flow Hydroponic Bioreactors. Symposium Series: Case Studies in the Remediation of Chlorinated and Recalcitrant Compounds. Editors: Godage B. Wickramanayake, Arun R. Gavaskar, James T. Gibbs, and Jeffrey L. Means. Battelle Press, Columbus, Ohio. 2(7), Pp 101-106 Susarla, Sridhar, and Nzengung, Valentine A. Chapter 25: Degradation of Perchlorate by Plants. in Phytoremediation: Scientific Advances to Manage Contamination by Organic Compounds. Editors: Steven C. McCutcheon and Jerald L. Schnoor (In Preparation 2001.) |
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