Cation Exchange Selectivity versus concentration of competing heavy metal cations (Pb2+,Zn2+) : case of Na-montmorillonite

Cation Exchange Selectivity (CES) for Wyoming montmorillonite was determined by equilibration of the clay with a mixed equinormal solution containing two competing cations (i.e. Pb2+, Zn2+). This paper aims at characterizing the structural change and selectivity of a Na-dioctahedral smectite (Wy-Na). The quantitative analysis of XRD patterns is achieved using an indirect method based on the comparison of XRD experimental patterns to calculated ones. Two reference samples were prepared by saturation with Pb2+ or Zn2+ (i.e. two heavy metal cations occurring in hold house trash). The resulting complexes were respectively labelled Wy-Pb and Wy-Zn. After that, the Wy-Na sample was dispersed in solutions containing 0.5Pb2+ and 0.5Zn2+ with different concentrations (from 10−2 N to 10−4 N) in order to understand the concentration effect on the selectivity process of the Na-montmorillonite. The XRD quantitative analysis shows that for low concentrations the d001 spacing value corresponds to Wy-Na complex, whereas for high concentrations the d001 spacing value can be attributed to the Wy-Zn and/or Wy-Pb. At low concentrations, the sample presents a homogeneous state and the cation exchange capacity is saturated with Na+ cation which is characterized by one water layer hydration state (1W). For high concentrations, interstratified hydration behavior appears and the clay has a tendency to exchange in minor contribution the Zn2+ cation and in major contribution Pb2+ cation characterized by a mixed hydration state between one (1W) and two water layers (2W).