EFFECTS OF CHELATING AGENTS ON PHYTOEXTRACTION OF LEAD BY VETIVER GRASS FROM LEAD-CONTAMINATED SOILS

The effects of two chelating agents, ethylenediaminetetraacetic acid (EDTA) and ethylenediaminedisuccinic acid (EDDS) on lead (Pb) extractability and its uptake and accumulation by vetiver grass (Vetiveria zizanioides) were investigated in a greenhouse setup. Four soil types with varying physico-chemical properties (Immokalee, Pahokee Muck, Tobosa and Millhopper) were spiked with lead nitrate at 3 different concentrations (400, 800 and 1200 mg Pb/ kg dry soil). All treatments (3 replicates) were compared with controls (same soil type, no added lead). Chelates were applied at two different rates (10 and 15 mM/kg EDTA or EDDS), and compared with unamended treatments. Vetiver was harvested after two weeks of chelate addition. Soils, leachates, plant root and shoot samples were collected appropriately and analyzed for Pb. Sequential extraction of the soil samples show a large proportion of Pb being mobilized into ‘soluble pool' by EDTA compared to EDDS treatments. Lead accumulation in vetiver shoots increased significantly with increasing amount of chelate added to the soils. Step wise multiple linear regression analysis and correlation studies show that uptake of Pb in vetiver is influenced by soil properties, and it was in the order of: Millhopper > Immokalee > Tobosa > Pahokee Muck. Vetiver grown in Millhopper soil series, showed a maximum accumulation of Pb up to 9670 and 629 mg / kg root and shoot tissue, respectively. Organic matter rich Pahokee Muck and clay rich Tobosa showed higher retention of Pb in soils resulting in relatively lower levels of exchangeable Pb for plant uptake. The decrease in the total soil Pb was well explained by the amount of Pb found in the vetiver tissue and loss in leachates as calculated using mass balance approach. Significant amount of Pb was leached in EDTA treatments compared to EDDS amended soils. Based on the results from this investigation we recommend using EDDS-aided Pb phytoextraction with vetiver grass in contaminated residential sites.