Fate and behavior of lead in soils planted with metal-resistant species (river birch and smallwing sedge)

Phytoremediation of metal-contaminated soils requires an understanding of t he interactions between metal-tolerant plant species and soil chemical prop erties controlling the bioavailability of metals. We conduct ed controlled lab oratory studies to investigate the effects that river birch (Betula occ identalis Hook.) and smallwing sedge (Carex microptera Mack.) had on the fa te and behavior of Pb in a contaminated soil (3000 mg Pb/kg) and tailings ( 13 000 mg Pb/kg) collected from an abandoned mining site in Utah. Significa nt Pb accumulation in aboveground tissue nas observed in smallwing sedge (g reater than or equal to 1000 mg/kg dry wt.) in both the soil and tailings, but Pb was primarily excluded by birch (less than or equal to 300 mg/kg dry wet). Lead exclusion in birch resulted in elevated concentrations of Pb in the rooting zone in both the soil and tailings. In the soil, the exchangea ble Pb concentration of tbe unplanted control was not significantly differe nt than the birch rhizosphere but was higher than the birch bulk (nonrhizos phere) soil fraction. This suggested that plants using exclusionary mechani sms of metal resistance may promote soil Pb stabilization by sequestering n ormally mobile fractions of Pb in the rhizosphere, However, both birch and smallwing sedge increased the leachate Pb concentration by 2 mg/L and decre ased the pH by one unit in the tailings compared with unplanted controls. L eachate Pb concentrations and pH were not significantly affected by plants in tbe soil. This indicated that the ability of metal-resistant plants to p romote soil Pb stabilization is soil specific and depends on the level of P b contamination and soil characteristics controlling the solubility and mob ility of Pb.