Influence of temperature and background electrolyte on the sorption of neptunium(V) on Opalinus Clay

Abstract The temperature dependence of the sorption of 8 μM neptunium(V) on Opalinus Clay (OPA, Mont Terri, Switzerland) was investigated between 40 and 80 °C and compared to previous results for 20 °C. The batch experiments were performed under ambient air conditions in synthetic pore water (pH = 7.6, I = 0.4 M) under variation of the solid-to-liquid ratio between 2 and 20 g/L. Under these conditions the sorption of Np(V) on OPA was found to be an endothermic process as the average distribution coefficient (K d ) increases continuously with increasing temperature from 23 ± 4 L/kg at 40 °C to 678 ± 254 L/kg at 80 °C. In the temperature range of 40-80 °C, the plot of log(K d ) vs. 1/T could be fitted to the Van't Hoff equation delivering an apparent sorption enthalpy of 80 ± 16 kJ/mol and an apparent sorption entropy of 278 ± 48 J/K·mol. Since pore water consists of several constituents (mainly NaCl, CaCl 2 , and MgCl 2 ), additional batch experiments were performed at room temperature to investigate the influence of 0.1 and 0.4 M NaClO 4 , NaCl, CaCl 2 , and MgCl 2 electrolytes on the sorption of Np(V) on OPA at pH 7.6. The results showed significant differences between monovalent (Na + ) and divalent (Mg 2 + , Ca 2 + ) electrolytes. While the sorption isotherms in sodium electrolytes (I = 0.4 M) provided K d values of 231 ± 13 (NaClO 4 ) and 224 ± 17 L/kg (NaCl), low distribution coefficients of 54 ± 6 L/kg (MgCl 2 , I = 0.1 M) and less were determined in magnesium and calcium electrolytes. A weak dependence on ionic strength was observed only in the batch experiments using NaClO 4 as electrolyte. In all other electrolytes the influence of ionic strength on the sorption of Np(V) was negligible between I = 0.1 and 0.4 M.