Chelate-assisted phytoextraction of lead from contaminated soils

Phytoextraction, a remediation strategy for lead (Pb)-contaminated soils that removes soil Pb through plant uptake and harvest, may be enhanced by use of synthetic chelates. We evaluated Pb desorption from four contaminated soils (total Pb 1278-14 349 mg kg(-1)) by seven chelates (CDTA, DTPA, EDDHA, EGTA, HEDTA, HEIDA, and NTA) at three rates (0.2, 2.0, and 20 mmol chelate kg soil(-1)). The three most effective chelates (CDTA, DTPA, and HEDTA) were used in greenhouse studies with an uncontaminated soil and a Pb-contaminated soil (total Pb = 3212 mg kg(-1)) to determine the effect of chelate type and rate on growth, Pb uptake, and plant elemental composition. Lead desorption varied with chelate and soil and increased with chelate rate, averaging 948 mg Pb kg(-1) at the 20 mmol kg(-1) rate vs. 28 mg Pb kg(-1) by the control. The general ranking of chelate effectiveness, based on total Pb desorbed, was HEDTA > CDTA > DTPA > EGTA > HEIDA > EDDHA approximately NTA. Plant uptake of Pb from the contaminated soil was enhanced by CDTA, DTPA, and HEDTA, but with even the most effective treatment (corn, high CDTA rate), the amount of Pb extracted by plants was rather low (0.4 kg Pb ha(-1)). Lead extractable by the Toxicity Characteristic Leaching Procedure) (TCLP) was increased from 9 mg L(-1) in the control to from 47 to 174 mg L(-1) in soils treated with 20 mmol kg(-1) CDTA or DTPA and chelates generally caused a shift in Pb from resistant to more soluble chemical fractions.