Heavy metal distribution between contaminated soil and Paulownia tomentosa, in a pilot-scale assisted phytoremediation study: influence of different complexing agents

The distribution of Cd, Cu, Pb and Zn between a contaminated soil and the tree species Paulownia tomentosa was investigated in a pilot-scale assisted phytoremediation study. The influence of the addition of EDTA, tartrate and glutamate at 1, 5 and 10mM concentrations on metal accumulation by the plant and on metal mobilization in soil was evaluated. Root/shoot metal concentration ratios were in the range of 3-5 for Zn, 7-17 for Cu, 9-18 for Cd and 11-39 for Pb, depending on the type and concentration of complexing agent. A significant enhancement of metal uptake in response to complexing agent application was mainly obtained in roots for Pb (i.e. 359 mg kg(-1) for EDTA 10mM and 128 mg kg(-1) for the control), Cu (i.e. 594 mg kg(-1) for glutamate 10mM and 146 mg kg(-1) for the control) and, with the exception of glutamate, also for Zn (i.e. 670 mg kg(-1) for tartrate 10mM and 237 mg kg(-1) for the control). Despite its higher metal mobilization capacity, EDTA produced a metal accumulation in plants quite similar to those obtained with tartrate and glutamate. Consequently the concentration gradient between soil pore water and plant tissues does not seem to be the predominant mechanism for metal accumulation in Paulownia tomentosa and a role of the plant should be invoked in the selection of the chemical species taken up. Metal bioavailability in soil at the end of the experiment was higher in the trials treated with EDTA than in those treated with tartrate and glutamate, the latter not being significantly different from the control. These findings indicated the persistence of a leaching risk associated to the use of this chelator, while an increase of the environmental impact is not expected when glutamate and tartrate are applied.