Sorption Characteristics of Neptunium by Sodium-Smectite

The sorption characteristics of 237Np by sodium-smectite have been studied by batch type sorption-desorption experiments over a pH range of 2 to 8. The association of 237Np with smectite was examined by the sequential extraction technique in which IM KCl and IM HCl solutions were used as the extraction agents. Two kinds of neptunium-smectite association were identified by their characteristic desorption properties. In the first association, neptunium is desorbed from smectite by 1 M KCl. In the second association, neptunium is stable in the basic solution but is desorbed by 1 M HCl. The latter association was observed when neptunium was sorbed at low pH. Powder X-ray diffraction showed an expansion of the basal spacing of smectite from 1.25 to 1.48 nm in low pH solutions, consistent with specific sorption of neptunium at a H+ expanded interlayer of smectite.