Phytoavailability of Lead And Aluminum in A Soil Amended With Sulfuric Acid and Elemental Sulfur
The excessive concentration of lead (Pb) in contaminated soils poses a serious threat to the environment. Phytoextraction is a biological method to remove metallic trace elements from contaminated soils. The phytoavailability of Pb in heavy-metal polluted soils increases under acidic conditions while aluminum (Al) accumulates in the plant as pH decreases. The objective of this study was to measure the effect of acidification on Pb and Al accumulation in shoots of four metal-tolerant plant species grown in a heavy metal-polluted soil containing carbonates. Soil samples were treated with 100 ml of H2SO4 solution (0.005 M and 0.05 M) and 2.95 g of elemental sulfur (S). The Pb and Al in soils were fractionated into "exchangeable" (EXC), "carbonate" (CAR), "oxides" (OX), "organic" (ORG), and "residual" (RES) pools. The amounts of Pb, expressed as % of total Pb in soil samples increased in the following order: PbRES (89.5 to 94.4%) ˃ PbOX (3.68 to 7.50%) ˃ PbCAR (0.44 to 1.02%) ˃ PbEXC (0.18 to 0.34%). Most of Pb occurred in the residual (immobile) fraction, assumed to be incorporated into the crystal lattice of minerals. Compared to control, elemental S increased 1.6-fold the labile Pb pool (EXC+CAR). The Pb concentrations in shoots ranged from 19.1 to 41.0 mg/kg (dw) across treatments. Bioaccumulation ratio for Pb in shoot biomass ([Pb]shoot biomass/[Pb]soil) ranged between 0.04 without amendment to 0.09 in soil treated with elemental S. H2SO4 and S amendments converted a portion of the solid Pb pool into the soluble or phytoavailable pool. Cultivated soil samples contained very small amount of labile Al (5-12 mg/kg). Most of Al (97.0-97.7% of total Al) was present in the residual fraction. Under the experimental conditions, plants accumulated more Al (1.61-5.64 mg) than Pb (0.19- 0.64 mg) in their shoot biomass. Aluminum in the cultivated heavy-metal polluted soil samples (pH 6.70-7.62) does not constitute a potential source of soil and groundwater pollution. The results suggest that sulfuric acid or elemental S added to soil before plant seedling increased the phytoextraction efficiency of metallic trace elements.
Indexterms- Heavy metal, Phytoremediation, Trace element, Pollution, Decontamination.