Ion-Exchange Aspects of Toxic Metal Uptake by Indian Mustard

ABSTRACT

Uptake of lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) as +2 ions by excised roots of Indian mustard was demonstrated to be an ion-exchange process with existing Ca or protons released to the solution. This initial reaction at the root-aqueous interface is important in the uptake of these toxic metals from contaminated soil. Ethylene diamine tetraacetic acid (EDTA)-amended soil for phytoremediation has Pb in anionic form as [Pb-EDTA]^{2 −}, which was not taken up by excised roots. In nonliving B. juncea, Pb²⁺ was translocated from a solution through a cut stem to petiole and leaves much more quickly than anionic [Pb-EDTA]^{2 −}. However, in living plants [Pb-EDTA]^{2 −} was more quickly translocated from a solution through roots and petiole to leaves than Pb²⁺. The final amount of uptake on roots of the living plants was the same for both forms of Pb. The present results are important toward understanding the mechanism of phytoremediation of toxic metal-contaminated soil for two reasons: 1) the initial process, uptake of metal ions by roots, was shown to occur by cation exchange and 2) since [Pb-EDTA]^{2 −} was not sorbed by excised roots, other factors such as transpiration and active transport are important in applications using EDTA-amended soils contaminated by Pb.

Keywords:

• hyperaccumulation
• metal uptake
• EDTA
• phytoremediation
• Brassica juncea

ACKNOWLEDGMENTS

The authors wish to express their gratitude to Drs. Rufus Chaney and Michael Blaylock for helpful discussions, and also to Michael Blaylock for his generous donation of B. juncea plants and seeds.