THE ROLE OF P_CO2, PH AND GROUNDWATER COMPOSITION ON NEPTUNIUM(V) ADSORPTION TO NATURITA ALLUVIAL AQUIFER SEDIMENT: COMPARISON TO URANIUM(VI)

Np(V) adsorption to untreated and treated alluvial aquifer sediment was studied in batch experiments under pH, P_CO2 and groundwater conditions similar to those observed at the former uranium mill site at Naturita, Colorado (USA). Treated sediment underwent carbonate removal using sodium acetate buffered at pH 5 with acetic acid. Untreated sediment was largely composed of quartz, feldspars and calcite, with lesser amounts of magnetite and clay minerals [1]. Sediment grains exhibited illite/smectite clay coatings with occluded ferrihydrate and goethite [2].

Steady-state concentrations of added Np(V) were reached within 24 hours. Np(V) adsorption to untreated and treated sediment decreased considerably as P_CO2 increased while the percent adsorbed decreased as Np(V) concentrations increased. Np(V) adsorption to treated sediment was reduced compared to adsorption to untreated sediment, possibly a result of partial dissolution of the clay coatings due to the extraction procedure. Np(V) adsorption to untreated sediment was appreciably lower in artificial groundwater than 0.1 NaClO₄. This reduction was likely a consequence of competition for surface sites between Ca²⁺ and Mg²⁺ and aqueous neptunyl species. Np(V) adsorption edges for the treated sediment over a range of pH values and variable P_CO2 were obtained. Consistent with the formation of aqueous neptunyl carbonato complexes, adsorption edges showed a decrease in adsorption with increasing pH and P_CO2. A comparison of Np(V) adsorption to U(VI) adsorption studies using Naturita alluvial aquifer sediment will be presented [1, 3].

[1] Davis, J. A. et al. (2004) Approaches to Surface Complexation Modeling of Uranium(VI) Adsorption on Aquifer Sediments. Geochimica et Cosmochimica Acta. 68:3621-3641.

[2] Jove-Colon C. F. et al. (2004) A Combined Analytical Study to Characterize Uranium Soil Contamination: The Case of the Naturita UMTRA Site and the Role of Grain Coatings. U.S. Nuclear Regulatory Commission, in press.

[3] Sanpawanitchakit, C. (2002) The Application of Surface Complexation Modeling to the Adsorption of Uranium (VI) on Natural Composite Materials. PhD Thesis, Colorado School of Mines, Environmental Science and Engineering, Golden, CO, USA.