Selectivity of Na–montmorillonite in relation with the concentration of bivalent cation (Cu2+, Ca2+, Ni2+) by quantitative analysis of XRD patterns

This paper aims at characterizing the structural evolution and the selectivity of a dioctahedral smectite (i.e. Wyoming montmorillonite) saturated with two different couples of cations. A reference sample was prepared by saturation with Ca2+, Cu2+ or Ni2+. The clay was dispersed in solutions of (Ca2+ and Cu2+) or (Cu2+ and Ni2+). The quantitative analysis of XRD patterns is achieved using an indirect method based on the comparison of XRD experimental patterns to calculated ones. The XRD quantitative analysis shows that for low concentrations of cations the basal spacing value corresponds to Wy–Cu, whereas for high concentration the d001 spacing value can be attributed to Wy–Ca or Wy–Ni. At low concentrations, the exchangeable sites were saturated with low hydration-state cations (i.e.Cu2+) which is characterized by one water layer, whereas at high concentrations, the clay fixes the cations with high hydration state (i.e. Ca2+or Ni2+) characterised by a two water layers.