Use of X-ray absorption spectroscopy and esterification to investigate Cr(III) and Ni(II) ligands in alfalfa biomass

Previously performed studies have shown that alfalfa shoot biomass can bind an appreciable amount of nickel(II) and chromium(III) ions from aqueous so lution. Direct and indirect approaches were applied to study the possible m echanisms involved in metal binding by the alfalfa biomass. The direct appr oach involves investigations of the metal-bound alfalfa shoot biomass by X- ray absorption spectroscopic analysis (XANES and EXAFS). Results from these studies suggest that nickel(II) and chromium(III) binding mostly occurs th rough coordination with oxygen ligands. Indirect approaches consist of chem ical modification of carboxylate groups that have been shown to play an imp ortant role in metal binding to the alfalfa biomass. An appreciable decreas e in metal binding resulted after acidic methanol esterification of the bio mass, indicating that carboxyl groups are entailed in the metal binding by the alfalfa biomass. In addition, base hydrolysis of the alfalfa biomass in creased the binding of these metals, which further indicates that carboxyl groups play an important role in the binding of these metal ions from solut ion. Therefore, by combining two different techniques, our results indicate that carboxylate groups are the major ligands responsible for the binding of nickel(II) and chromium(III) by alfalfa biomass.