Sorption of neptunium(V) on silicate gels

Aqueous corrosion of nuclear waste glasses induces the formation of a gel layer enriched in hydrolyzable elements. To assess the role of such layers on the migration of Np(V) from repositories to environment, and determine the retention mechanisms, experiments have been undertaken on synthetic pure silica and on ferrisilicate gels as model solids. No detailed and systematic studies on silicate gels as scavengers for Np(V) appear to have been devoted to the influence of solution parameters and gel composition on the sorption of Np(V) on silicate gels. In this work, the fractional uptake of Np(V) on a synthetic pure silica gel and on synthetic ferrisilicate gels of controlled Si/Fe proportions (3 and 10) was measured at different ionic strengths (0.5 to 0.01 M) and Np(V) concentration levels ≅10-10 M and 10-6 M with pH as the main variable (3 to 10). The pH dependence of Np(V) sorption on the alternation gel was also investigated for different initial concentrations of Np(V) in the aqueous phase ≅10-10 M, 10-6 M and 10-5 M. Batch experiments were carried out at 298 K under CO2-free conditions to avoid the formation of Np(V) carbonate complexes in the solution. As a part of this work, the charge characteristics of the gel surfaces were determined either by studying the retention of the electrolyte ions as counterions to the surface charges, as a function of pH and ionic strength of the solution contacted with gels, or by performing acid/base titrations of the alteration gel suspensions. (J.P.N.)