Phytoavailability Of Copper In A Heavy Metal-Polluted Calcareous Soil
The presence of carbonates in soils polluted by heavy metals affects their solubility in soil and their removal by plants. Acidic amendments are often used to remediate these soils by soil washing and phytoremediation techniques.The phytoavailability of copper (Cu) in heavy metal-polluted soils increases under acidic conditions. The objective of this study was to measure the effect of acidification on Cu accumulation in shoots of four metal-tolerant plant species (brown mustard, perennial ryegrass, red fescue and alfalfa) grown in a heavy metal-polluted soil containing carbonates. Several portions (600 g) of soil were mixed with 100 ml of H2SO4 solution (0.005 M and 0.05 M ) and 2.95 g of elemental sulfur (S0) as follows: soil without amendment (T0); 0.005 M H2SO4 (T1), 0.05 M H2SO4 (T2) and S0 (T3). The cultivated soil was then subjected to the following sequential extraction procedure: 0.05 MCa(NO3)2 (CuCa), 0.1 M Mg(NO3)2(CuMg), 0.005 MDTPA-0.1M TEA-0.01 M CaCl2 (pH 7.3) (CuDTPA), and 0.1 MHCl (CuHCl). The labile Cu pools, as percentage of total Cu in soil samples increased in the following order: CuHCl (18.3 to 21.8%) ˃ CuDTPA (7.6 to 19.7%) ˃ CuMg (1.4 to 3.0%) ˃ CuCa (0.8 to 1.7%). Although the application of acidic amendments increased the total labile Cu pool (CuCa+ CuMg + CuDTPA + CuHCl) by 14.2% (T1), 22.1% (T2) and 38.1% (T3) compared to control, a large percentage (53.9 to 71.9%) of total Cu was is tightly bound to soil constituents (i.e. non-labile or immobile). This result indicates that Cu is not easily phytoavailable in heavy metal contaminated calcareous soils. The mean Cu concentrations in shoots (CuShoots) ranged from 8.3 to 22.1 mg/kg (dw) across treatments. Bioaccumulation ratio for Cu in shoot biomass ([Cu]shoot biomass/[Cu]soil) ranged from 0.07 in soil without amendment to 0.22 in soil treated with S0. H2SO4 and S0 amendments converted a portion of the solid Cu pool into the phytoavailable pool. The results suggest that sulfuric acid or elemental S added to soil before plant seedling increased the total labile Cu pools in calcareous soil as well as the rate of Cu accumulation in the shoot biomass of the plants.
Keywords - Heavy metal, Phytoremediation, Trace element, Pollution, Decontamination.