Title

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

Document Type

Article

Journal/Book Title/Conference

Journal of Environmental Quality

Volume

29

Publication Date

1-1-2000

First Page

1826

Last Page

1834

Abstract

Phytoremediation of metal-contaminated soils requires an understanding of the interactions between metal-tolerant plant species and soil chemical properties controlling the bioavailability of metals. We conducted controlled laboratory studies to investigate the effects that river birch (Betula occidentalis Hook.) and smallwing sedge (Carex microptera Mack.) had on the fate and behavior of Pb in a contaminated soil (3000 mg Pb/kg) and railings (13 000 mg Pb/kg) collected from an abandoned mining site in Utah. Significant Pb accumulation in aboveground tissue was observed in smallwing sedge (≥1000 mg/kg dry wt.) in both the soil and railings, but Pb was primarily excluded by birch (≤300 mg/kg dry wt.). Lead exclusion in birch resulted in elevated concentrations of Pb in the rooting zone in both the soil and railings. In the soil, the exchangeable Pb concentration of the unplanted control was not significantly different than the birch rhizosphere but was higher than the birch bulk (nonrhizosphere) soil fraction. This suggested that plants using exclusionary mechanisms of metal resistance may promote soil Pb stabilization by sequestering normally mobile fractions of Pb in the rhizosphere. However, both birch and smallwing sedge increased the leachate Pb concentration by 2 mg/L and decreased the pH by one unit in the railings compared with unplanted controls. Leachate Pb concentrations and pH were not significantly affected by plants in the soil. This indicated that the ability of metal-resistant plants to promote soil Pb stabilization is soil specific and depends on the level of Pb contamination and soil characteristics controlling the solubility and mobility of Pb. Phytoremediation of metal-contaminated soils requires an understanding of the interactions between metal-tolerant plant species and soil chemical properties controlling the bioavailability of metals. We conducted controlled laboratory studies to investigate the effects that river birch (Betula occidentalis Hook.) and smallwing sedge (Carex microptera Mack.) had on the fate 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. Significant Pb accumulation in aboveground tissue was observed in smallwing sedge (≥1000 mg/kg dry wt.) in both the soil and tailings, but Pb was primarily excluded by birch (≤300 mg/kg dry wt.). Lead exclusion in birch resulted in elevated concentrations of Pb in the rooting zone in both the soil and tailings. In the soil, the exchangeable Pb concentration of the unplanted control was not significantly different than the birch rhizosphere but was higher than the birch bulk (nonrhizosphere) soil fraction. This suggested that plants using exclusionary mechanisms of metal resistance may promote soil Pb stabilization by sequestering normally mobile fractions of Pb in the rhizosphere. However, both birch and smallwing sedge increased the leachate Pb concentration by 2 mg/L and decreased the pH by one unit in the tailings compared with unplanted controls. Leachate Pb concentrations and pH were not significantly affected by plants in the soil. This indicated that the ability of metal-resistant plants to promote soil Pb stabilization is soil specific and depends on the level of Pb contamination and soil characteristics controlling the solubility and mobility of Pb. In pots containing lead-contaminated soil, birch seedlings and smallwing sedge seedlings were planted, and uptake of Pb, survival, and biomass production were assessed and compared. Three different soils were used, which differed in metal content, pH, and nutrient content. The effect of the plants on soil Pb fate was also considered. Both species were found to be Pb-resistant, but the resistance was attributed to different mechanisms. Under birch, the rhizosphere became enriched in Pb over time, suggesting that the plant promoted soil Pb stabilization. Smallwing sedge, in contrast, accumulated Pb, which appeared to promote soil Pb stabilization and decontamination by reducing the relative mobility of the metal. However, both species promoted Pb leaching from highly contaminated tailings, which was associated with a decrease in pH due to root exudation of organic acids. The immobilization and mobilization of Pb observed in the treatments could be explained possibly by plant processes affecting the levels of sulfate or phosphate in the soil.

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