Sorption and Colloidal Behavior of Np (IV) in a Bentonite-Carbonate Solution System

Abstract

Sorption of Np(IV) on bentonite in the presence of carbonate was studied by a batch method. The sorption was considered mainly due to Np(OH)₂(CO₃)₂^2- and Np(OH)₄(CO₃)₂^4-. The distribution coefficient (K_d) of Np(OH)₂(CO₃)₂^2- and Np(OH)₄(CO₃)₂^4- was determined to be 10^4.01±0.24ml/g and 10^3.93±0.37 ml/g, respectively. The increasing Kd at high pH was attributed to the sorption of NpO₂(OH)₂^- with the K_d of 10^5.82±0.39 ml/g.
The formation of colloidal species was observed under a reducing condition. By assuming Np(OH)₄ as the colloidal species, the formation constant was evaluated to be β_c ?? 10^53.85. Under an oxidizing condition, the formation of the colloidal species was less observed possibly due to the transformation of Np(OH)₄ to NpO₂OH.