Diffusion of neptunium(V) in clays

The diffusion coefficients (D) and distribution ratios (RD) of neptunium (237Np) have been studied in three clays; Wyoming montmorillonite, St Austell kaolinite and Fithian illite. The clays were homoionic in either sodium or calcium and the background electrolyte was a perchlorate solution of the appropriate cation. Montmorillonite and kaolinite were investigated with a range of electrolyte concentrations and with two concentrations of EDTA, citrate and Aldrich humic acid. The diffusion coefficients attributable to the liquid phase were calculated from the distribution ratio and the tracer diffusion coefficient of chloride for each clay preparation. The measured values of RD are relatively low and those of D correspondingly high. No time dependent adsorption was observed and the adsorption isotherm was linear in the concentration range covered. Values of diffusion coefficients were also calculated assuming diffusion in the liquid phase only. The measured and calculated diffusion coefficients agree well in most cases. There is no evidence of the mobility of adsorbed species. Only in the calcium-montmorillonite was an effect of ionic strength on RD and D observed. For a given ligand and clay, the concentration of the ligand (10-5 or 10-3 M of EDTA and citrate, and 1 or 100 mg dm-3 of humic acid) had no effect on any measured parameter. The presence of organic ligand had no significant effect on the distribution ratio or on the tracer diffusion coefficient of chloride, nor hence on the calculated liquid phase diffusion coefficient. The effect of organic ligands was most clear for humic acid which caused a clear decrease in D. This is attributed to the presence of complexes with a lower value of the limiting diffusion coefficient in ideal solution and with a liquid phase impedance factor lower than that of chloride. (orig.)