Studies have shown that biomaterials have the capacity to adsorb heavy metals and metal oxo-cations from aqueous solution. In addition, previous studies have shown that biomaterials have the ability to bind uranyl cations from solution with capacities that are comparable to or greater than some commercially available synthetic ion-exchange resins. By using chemical modification, inductively coupled plasma optical emission spectroscopy (ICP-OES), and X-ray absorption spectroscopy (XAS), we have found that the primary functional group on alfalfa biomass responsible for the binding of uranyl cations from aqueous solution is the carboxyl functionality. Batch pH dependency experiments show a direct relationship between the increase in binding and the increase in pH (up to pH 4.5). XAS experiments showed that the major ligand involved in the binding of uranyl cations from aqueous solution was either a nitrogen or oxygen ligand with coordination numbers ranging from 6 to 10 +/- 1.