RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Batch experiments were performed in order to investigate the influence of pH, neptunium concentration, and concentration of carbonate in solution on the sorption of Np(V) on a synthetic hydrargilite. The concentration of Np(V) at hydrargilite surface was found to be proportional to the concentration of dissolved Np(V) at equilibrium, within the Np(V) concentration range ≈ 10⁻¹³ to 10⁻⁷ M, in CO₂-free 0.1 M NaClO₄ solutions at pH = 7.45±0.05. The sorption of tracer concentration of Np(V) on hydrargilite was strongly pH-dependent. Increasing the total concentration of carbonate from about 0 to 10⁻² M in the aqueous phase resulted in a net decrease of the extent of Np(V) uptake in the alkaline pH region. Surface complexation modeling suggested that Np(V) sorption behavior in carbonate solutions resulted mainly from a competition between surface complexation and aqueous carbonate complexation of NpO₂⁺. The surface complexation reactions and conditional constants proposed in this study for the non-electrostatic model allowed an accurate description of the sorption behavior of Np(V) on hydrargilite over a wide range of pH and carbonate concentration of the aqueous phase.