Approaches to surface complexation modeling of Ni(II) on Callovo-Oxfordian clayrock

Callovo-Oxfordian formation (COX) is as potential host formation for emplacement of long-term nuclear waste repositories in France. The objective of this work is to assess whether a simplified “bottom-up” approach may explain the retention of Ni(II) by the COX considering two levels of ‘upscaling’: (i) from clay surfaces to rock clay fraction and (ii) from clay fraction to whole rock samples. To this end, Ni(II) adsorption was investigated by batch equilibrium, XPS, and EXAFS techniques on a representative sample extracted at the location where the storage is supposed to be built (clay content of about 50%) and on the corresponding carbonate-free < 2 μm fractions. The results showed that a simplified “bottom-up” approach based on published models available for illite and montmorillonite cannot explain Ni(II) adsorption on the < 2 μm fraction when the retention is controlled only by surface complexation on the reactive clay edge sites. An operational model based on the generalized composite modeling approach was used instead. The developed model considers an interaction between two Ni(II) species with one type of clay edge sites. The model developed for the clay fraction gives a satisfactory estimation of Ni(II) adsorption data for the representative clayrock sample. Complementary experiments were performed by X-ray photoelectron (XPS) and X-ray absorption (EXAFS) spectroscopies for both clay fraction and raw COX sample at high Ni(II) loadings. Spectroscopic data were characterized by similar fitting parameters considering formation of Ni phyllosilicate. This indicates that the clay fraction governs the retention of Ni(II), as it was concluded from the batch experiments. Complementary adsorption experiments preformed with COX samples having clay contents representative of the variability occurring at the formation scale ( i.e. 1.5–47% in weight) show that one cannot neglect the retention properties of the non-clay phases, mainly dominated by calcite, when the clay content becomes the minority. Retention values in the range of 60–300 L/kg can finally be given for describing adsorption properties of trace concentrations of Ni(II) for the clay contents representative of the majority of the Callovo-Oxfordian formation.