Effect of Soil Washing and Liming on Bioavailability of Heavy Metals in Acid Contaminated Soil

Soil washing with acidic chelators followed by liming is under consideration for remediating multi-metal contaminated soils. This technique was tested by pot experiments with a mixture of chelators (MC) as a washing agent. After washing and liming treatments, corn (Zea mays L.) was planted to assess bioavailability of heavy metals. A phytoextraction with Sedum alfredii was additionally performed after corn harvest to further decrease heavy metals in the soil. The results showed that the concentrations of Cd, Pb, and Zn in the corn grain with the washing treatment were still higher than the food standard limits in China, and liming the washed soil did not effectively decrease Cd and Pb concentrations in the corn grain. Liming the washed soil significantly increased heavy metals concentrations in leachate. The concentrations of Cd, Pb, Zn, and Cu in S. alfredii after washing were increased by 42.8, 51.2, 7.3, and 25.8%, respectively, compared to nonwashed soils. Phytoextraction rates of Cd and Zn by one crop of S. alfredii after soil washing were 47.8 and 9.82%, respectively, of the soil total metal. The phytoextraction of soils with S. alfredii, and also the liming treatment alone without phytoextraction or washing, were able to reduce exchangeable heavy metal concentrations in soils. This study suggests that the bioavailability of heavy metals could still be high after washing, liming the washed soil may release the adsorbed metal-chelates, and the phytoextraction by S. alfredii after soil washing is an effective approach to deplete the mobile Cd activated by soil washing.