Examination of Np(V) sorption to montmorillonite as a function of temperature (25–55 °C) and ionic strength

Abstract

Because of the prevalence of clays in rocks, soils, and sediments, and their proposed use in nuclear waste storage sites, clay materials are an important consideration in radionuclide interactions in the environment. This study examined Np(V) sorption to Na-montmorillonite as a function of temperature and ionic strength. Batch sorption experiments were conducted at 25 °C, 35 °C, 45 °C, 55 °C for 0.01, 0.1, and 1.0 M NaCl ionic strengths. The data indicate that Np(V) sorption to montmorillonite at pH values above 6 increases slightly with increasing temperature. The displacement of hydrating waters from the neptunium cation and clay surface may be the driving force behind the increased sorption, which is postulated based on the entropically driven removal of water molecules from the primary hydration sphere to bulk water. A double-layer (DLM) surface complexation model was applied to describe the Np(V) sorption data with activities of the dissolved aqueous species and estimate their associated log K values, allowing for an estimation of the adsorption enthalpy. X-ray absorption spectroscopy (XAS) measurements confirmed one of the essential assumptions for modeling at high ionic strengths: no variation in surface speciation appears between low and high ionic strength samples. XAS analysis confirmed Np(V) remained in the pentavalent oxidation state and indicated similar surface speciation for both low and high ionic strength conditions, with no evidence of Np precipitation. These results provide insights into Np(V) sorption behavior under conditions relevant to nuclear waste repositories, including elevated temperatures and high ionic strengths. The slight increase in sorption with temperature suggests Np(V) mobility may be somewhat reduced in heated repository environments. However, the overall weak sorption indicates Np(V) remains relatively mobile under most conditions studied.