Neptunium(V) sorption on kaolinite

Abstract

The sorption behavior of neptunium(V) onto the clay mineral kaolinite was studied in batch experiments under different experimental conditions: [Np(V)]=7 × 10⁻¹²–8 × 10⁻⁶ M, solid-to-liquid ratio 2–20 g L⁻¹, I=0.1 and 0.01 M NaClO₄, pH=6–10, ambient air and Ar atmosphere. The short-lived isotope ²³⁹Np (T_1/2=2.36 d) was used instead of ²³⁷Np (T_1/2=2.14 × 10⁶ a) to study the sorption behavior of Np(V) at environmentally-relevant concentrations, i.e., 7 × 10¹² M Np. In addition, ²³⁹Np(V) served as tracer to measure sorption isotherms over six orders of magnitude in Np concentration (4.8 × 10⁻¹²–1.0 × 10⁻⁴ M). The results show that Np(V) sorption on kaolinite is strongly influenced by pH, CO₂, and ionic strength. The sorption of 8 × 10⁻⁶ M Np(V) at pH 9.0, and ionic strength of 0.1 M NaClO₄ was proportional to the solid-to-liquid ratio of kaolinite in the range of 2–10 g L⁻¹. In the absence of CO₂, the Np(V) uptake increased continuously with increasing pH value up to 97% at pH 10. Under ambient CO₂, the sorption of Np decreased above pH 8 up to zero at pH 10. An increase of Np(V) concentration from 7 × 10⁻¹² to 8 × 10⁻⁶ M resulted in a shift of the sorption pH edge by up to one pH unit to higher pH values. The ionic strength influenced the Np(V) sorption onto kaolinite only in the presence of ambient CO₂. Under Ar atmosphere the sorption of Np(V) was independent from ionic strength, indicating the formation of inner-sphere complexes of Np(V) with kaolinite. Time-dependent batch experiments at pH 9.0 under ambient CO₂ showed that the sorption of Np(V) on kaolinite is fast and fully reversible over six orders in Np(V) concentration.