Effects of oxidation state on metal ion binding by Medicago sativa (alfalfa): Atomic and X-ray absorption spectroscopic studies with Fe(II) and Fe(III)

We present here experimental results that investigate the effects of metal-ion binding on iron-ion sorption to and recovery from alfalfa biomass. Fe(II)- and Fe(III)-ion binding were measured in order to ascertain the differences in binding strengths due to changes in oxidation state. Stronger binding was found for iron(III)-biomass as compared to iron(II)-biomass. The optimal pH for iron uptake was determined to be 5. The results of pH binding profile, iron desorption, and temperature-dependent binding experiments as well as X-ray spectroscopic (XAS) measurements all suggest that binding of iron by alfalfa biomass may be occurring through carboxyl ligands. The XAS experiments further demonstrate that the metal binding proceeds without an oxidation state change, and both iron(II) and iron(III) have similar coordination environments. The information presented will assist in understanding the binding of other metals to alfalfa biomass and in developing methods for their recovery.