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

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 (Kd) 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(Kd) 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, CaCl2, and MgCl2), additional batch experiments were performed at room temperature to investigate the influence of 0.1 and 0.4 M NaClO4, NaCl, CaCl2, and MgCl2 electrolytes on the sorption of Np(V) on OPA at pH 7.6. The results showed significant differences between monovalent (Na+) and divalent (Mg2 +, Ca2 +) electrolytes. While the sorption isotherms in sodium electrolytes (I = 0.4 M) provided Kd values of 231 ± 13 (NaClO4) and 224 ± 17 L/kg (NaCl), low distribution coefficients of 54 ± 6 L/kg (MgCl2, 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 NaClO4 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.

► Np(V) sorption strongly depends on temperature and use of background electrolyte.
► Np(V) sorption on Opalinus Clay (OPA) is an endothermic process.
► Sorption experiments in NaClO4 overestimate Np(V) uptake by OPA in pore water.
► Divalent cations determine the sorption behavior of Np(V) on OPA in pore water.