Chelate-Assisted Pb Phytoextraction: Pb Availability, Uptake, and Translocation Constraints

Chelates have been shown to enhance phytoextraction of Pb from contaminated soil. Mechanisms behind this phenomenon, however, remain largely unexplored. In this paper we examine chelate effect on Pb dissolution, plant Pb uptake, and internal plant Pb translocation. EDTA was found to be the most efficient in increasing watersoluble Pb concentration in our test soil. Unfortunately, PbEDTA is highly water-soluble and poses potential risks to groundwater in its application. In addition, it would not appear to be ideally suited for plant uptake and translocation based upon the relative water solubility of Pb-EDTA. We demonstrated that N,N′-di(2-hydroxybenzyl)ethylenediamine N,N′-diacetic acid (HBED) resulted in Zea mays root Pb content significantly higher than did EDTA, indicating that a chelate better than EDTA might be designed. Fortuitously, EDTA appears to increase overall plant transpiration, the driving force in phytoextraction of the Pb-chelate complex from soil. We also found that there was a significant increase in Pb uptake and translocation for corn transplanted into soil, then treated with EDTA, as compared to plants germinated and grown in Pb-contaminated soil to which EDTA was subsequently applied. These results demonstrate that significant improvement over current chelateassisted phytoextraction of Pb may be possible.