Phytoextraction of copper from contaminated soil by Elsholtzia splendens as affected by EDTA, citric acid, and compost.

Phytoextraction of copper (Cu) from contaminated soils greatly depends on the metal bioavailability in the soils and metal uptake ability of the plant. In this study, the effects of chelators [ethylenediamine tetraacetic acid (EDTA), citric acid (CA)] and compost amendments on Cu phytoextraction potential by a tolerant and accumulating plant species (E. splendens) were examined in two types of contaminated soils, ie., the mined soil from Cu-mined area (MS) and a paddy soil polluted by Cu refining (PS). The results showed that EDTA application at 2.5-5.0 mmol kg(-1) increased phytoextraction of Cu by four- and eight-fold from both MS and PS, respectively, which is mainly attributed to increased H2O extractable Cu in the soil. The Cu amount extracted by the shoots of E. splendens reached 800-1000 microg Cu plant(-1) from the MS and 400-700 microg Cu plant(-1) from the PS at EDTA application rates of 2.5-5.0 mmol kg(-1). The application of CA at 5.0 mmol kg(-1) had minimal effects on Cu extractability in both soils and slightly decreased Cu extraction efficiency by E. splendens. Plant biomass production was enhanced by CA at 0.25 mmol L(-1) in nutrient solution, but inhibited by CA at 5.0 mmol kg(-1) in both MS and PS. Increasing the compost rate significantly decreased H2O extractable Cu in the MS, but raised H2O-extractable Cu in the PS, which resulted mainly front the reduced exchangeable Cu in the MS and the increased exchangeable and organic fractions of Cu in the PS by compost. At high compost rate (5%), the shoots of E. splendens extracted 3.6-fold higher Cu from the PS than from the MS. These results indicate that, among the soil amendments, efficiency of Cu phytoextraction is enhanced mostly by 2.5-5.0 mmol kg(-1) EDTA, followed by 5% (w:w) compost, whereas < 5.0 mmol kg(-1) CA has minimal effects on Cu phytoextraction by E. splendens in the PS. As for the MS, only 2.5-5.0 mmol kg(-1) EDTA can elevate the efficiency of Cu, while 5% compost amendment and < 5.0 mmol kg(-1) CA application have no marked effects on Cu phytoextraction by E. splendens.