Neptunium(V) sorption onto gibbsite

Abstract

The sorption behavior of neptunium(V) on gibbsite (γ-Al(OH)3) was studied by batch experiments in the presence/absence of ambient CO2 as a function of solid-to-liquid ratio, pH, ionic strength, and Np(V) concentration. The sorption of 8×10−6 M Np(V) at ionic strength of 0.1 M NaClO4 was proportional to the solid-to-liquid ratio of gibbsite with a Kd value of ∼45±6 mL/g at pH 9.0. The sorption behavior of Np(V) on gibbsite was strongly affected by ambient CO2 and pH in the pH range of 6.0–10.0. In the presence of CO2 at ionic strengths of 0.01 and 0.1 M NaClO4, the maximum Np(V) uptake was at pH 8.5 for 8×10−6 and 7×10−12 M Np(V). In the absence of CO2 at the same ionic strengths, the Np(V) uptake increased with increasing pH up to ∼95% at pH 10.0. The sorption edges of Np(V) shifted by approximately one unit toward higher pH while increasing the concentration from 7×10−12 to 8×10-−6 M Np(V) and the amount of Np(V) sorbed at pH 8.5 decreased by ca. 40% under ambient air conditions. Ionic strength had no effect on the sorption of Np(V) onto gibbsite whether CO2 was present or not, indicating the formation of inner-sphere surface complexes between Np(V) and γ-Al(OH)3.