Effects of mixture of chelating agents with different pH values on phytoextraction and heavy metals removal

The mixture of chelators (MC) can remove heavy metals from contaminated soils, but the appropriate pH value and its impacts on the following phytoextraction need to be elucidated. Pot experiments were carried out to evaluate the efficiency of heavy metals removal by MC at different pH values (2.75, 5, 7 and 9). The ptytoextraction by Sedum alfredii in the soil after washing with MC were also assessed. Result showed that soil washing with MC at pH 5 and 7 significantly increased the leached Cd, Pb and Cu. Heavy metals removal enhanced by addition of Ca2+ into MC instead of Na+ or K+. The biomass of S. alfredii enhanced after soil washing with MC at pH value 7 and 9, but the concentrations of Cd and Zn in S. alfredii and phytoextraction rates reduced compared with the initial MC at pH 2.75. In this combined technology, Zn and Cd removal mainly depended on phytoextraction, its removal rates were from 30% to 40% of the total soil Cd and from 6.5% to 6.9% for Zn. Pb and Cu removal relied on soil washing, the removal rates were from 2.3% to 2.6% of the total soil Pb and from 1.6% to 2.0% for Cu. Overall, the combination of soil washing with pH value 9 MC and phytoextraction was preferred to remove simultaneously Cd, Zn, Pb and Cu from the acidic contaminated soil and to decrease its available pools in the soil.