Sorption of Am(III) on natural sediment: experiment and modeling

Release of long-lived radioactivity to the aquatic bodies from various nuclear fuel cycle related operations is of great environmental concern in view of their possible migration into biosphere. This migration is significantly influenced by various factors such as pH, complexing ions present in aquatic environment and sorption of species involving radionuclides on the sediments around the water bodies. Am-241/243 are two major radionuclides which can contribute a great deal to radioactivity for several thousand years. In the present study, Am-241 sorption on natural sediment collected from site near a nuclear installation in India, has been investigated under the varying conditions of pH (3-10) and ionic strength [I = 0.01-1 M (NaClO4)]. The sorption of Am increased with pH of the aqueous medium [10% (pH 2) to similar to 100% (pH 10)], which was explained in terms of the increased negative surface charge on the sediment particles. There was marginal variation in Am(III) sorption with increased ionic strength (within error limits) of the aqueous medium suggesting inner-sphere complexation/sorption process. Sediment was characterized for its elemental composition and structural phases using Energy Dispersive X-Ray (SEM-EDX) and X-Ray Diffraction (XRD) techniques. Zeta-potential measurement at I = 0.1 M (NaClO4) suggested that Point of Zero Charge (pH(PZC)) was similar to 2, indicating the presence of silica as major component in the sediment. Kurabtov plot using sorption data as a function of pH at fixed I = 0.1 M (NaClO4) indicated the presence of multiple Am(III) species present on the surface. Potentiometric titration of the suspension indicated the presence of mineral oxide like behavior and assuming a generic nature (a parts per thousand XOH) for all types of surface sites, protonation-deprotonation constants and total number of sites have been obtained. The sorption data has been modeled using 2-pK Diffuse Double Layer Surface Complexation Model (DDL-SCM). a parts per thousand XOAm2+ has been identified as the main species responsible for the sorption profile.