RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The sorption of tracer concentrations of neptunium(V) on hydrargilite was studied both under CO₂-free conditions and with aqueous solutions with a low total concentration of carbonate (<10⁻⁴ mol · L⁻¹). The fractional uptake of Np(V) by hydrargilite was measured as a function of pH (4-9), using various conditions of ionic strength (0.1, 0.01 and 0.001 mol · L⁻¹ NaClO₄) and two solid concentrations (10 and 50 g · L⁻¹). At nearly neutral and high pH values, hydrargilite acts as an efficient trap for Np(V) at trace levels. The pH is the main solution parameter that governs the Np(V) sorption, whereas major changes in ionic strength induce no changes in Np(V) uptake. The presence of small amounts of carbonate in solution has no measurable effect on Np(V) sorption. The experimental results were modeled using the non-electrostatic surface complexation model (NEM), assuming that only monodentate surface species (SOH) and the neptunyl ion NpO₂⁺ participate in the adsorption reaction. To account for effects of the acid/base behaviour of aluminol groups at the hydrargilite surface, the protonation and dissociation constants of the surface groups were determined for the NEM from CO₂-free potentiometric titrations. Application of the non-electrostatic model provided an accurate description of Np(V) sorption at trace levels on hydrargilite.