Sorption of Np(V) on kaolinite from solutions of MgCl₂ and CaCl₂

Abstract

The Np(V) distribution coefficients between kaolinite and solutions of MgCl₂ and CaCl₂ were determined experimentally at various concentrations of the electrolytes and Np. The Np sorption decreases with increasing concentration of the supporting electrolyte. The sorption is completely reversible. The sorption equilibrium is attained in approximately one week after the start of the sorption-desorption experiments. The constants of NpO₂ ⁺-Mg²⁺ and NpO₂ ⁺-Ca²⁺ binary ion exchange on kaolinite were determined by fitting the experimental results with an ion-exchange equation for the restricted sorption capacity: $\log K_{NpO_2^ + - Mg^{2 + } } = 1.26 \pm 0.08$ and $\log K_{NpO_2^ + - Ca^{2 + } } = 0.96 \pm 0.10$. These constants describe well the experimental data at low Np concentrations (≤1×10⁻⁶ M). The ion-exchange capacity of kaolinite, calculated from the experimental data on Np sorption from solutions (3.03×10⁻⁴ g-equiv kg⁻¹ MgCl₂), somewhat differs from that in CaCl₂ solutions (2.15×10⁻⁴ g-equiv kg⁻¹).