Phytoremediation of lead contaminated soils

Despite considerable national public health efforts to reduce lead (Pb) exposure, Pb poisoning remains the most common environmental health problem affecting the children in the U.S. Phytoremediation is emerging as an attractive option for cleanup of Pb paint-contaminated soils in housing facilities built prior to 1978. The objective of this study was to investigate the use of a high biomass, metal tolerant grass, Vetiveria zizanioides (vetiver grass), to remediate Pb paint-contaminated residential soils of variable physico-chemical properties. Developing a successful phytoremediation model requires a good understanding of the role of soil properties governing Pb availability for plant uptake, biochemical mechanisms involved in Pb tolerance, and changes in Pb bioaccessibility. Therefore, a comprehensive greenhouse-based study was conducted to evaluate the effectiveness of vetiver in Pb uptake and reducing soil Pb concentrations. Chelation of Pb is an important factor in enhancing its solubility and therefore availability to vetiver to promote phytoremediation. We compared the effects of two chelating agents---namely ethylenediaminetetracetic acid (EDTA) and ethylenediaminedisuccinic acid (EDDS)---at 4 different concentrations such as 0, 5, 10 and 15 mmol kg-1 soils. Our studies indicate that (1) soil properties determine the extent of soluble Pb; (2) EDTA is more effective than EDDS in mobilizing bound soil Pb; (3) unlike EDTA, EDDS reduced the leachable Pb fraction in soils; (4) activities of antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase were elevated in vetiver under Pb stress; (5) lead-binding phytochelatins were induced as a Pb tolerance mechanism; and (6) Pb bioaccessibility in soils decreased with an increase in Pb uptake by vetiver. This study demonstrated that EDDS could be regarded as a good chelant candidate for the environmentally safe phytoextraction of Pb using vetiver grass in Pb paint-contaminated residential soils.