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STUDY OF THE LIGANDS INVOLVED IN METAL BINDING TO ALFALFA BIOMASS

Laboratory experiments were performed to determine the pH profiles, time dependency, capacity for metal binding, as well as the recovery of the metals bound. For most of the metal ions studied, the biomass showed a high affinity for metal binding around pH 5.0 within a short time period.

Binding capacity experiments revealed the following amounts of metal ions bound per gram of biomass: 19.7 mg Cu(II), 4.11 mg Ni(II), 7.1 mg Cd(II), 7.7 mg Cr(III), 43 mg Pb(II), and 4.9 mg Zn(II). Most of these metals were recovered from the biomass by treatment with O. l M HCl, with the exception of chromium(III). Since no chromium(VI) binding occurred, none was recovered.

Currently we are studying the possible mechanisms involved in metal binding by the alfalfa biomass by direct and indirect approaches. The direct approach involves investigations of the alfalfa shoot biomass by X-ray absorption spectroscopic analysis (XANES and EXAFS), which are being performed at Brokhaven National Laboratories.

Preliminary results from these studies suggest that Ni(II) binding may occur through coordination with oxygen or nitrogen ligands, and Cr(III) binding may be occurring at nitrogen ligands. Indirect approaches consist of chemical modification of some chemical functional groups such as carboxylate groups which have shown to play a role in metal binding.

These studies are important for determining the ligands that may be involved in the binding of metal ions to the alfalfa biomass, thus aiding in the innovative removal and recovery of metal ions from contaminated waters through phytofiltration. Results from these experiments will be presented.

Key words: phyto-filtration, alfalfa, Medicago sativa, metal ligands, heavy metal binding