A mechanistic description of Ni and Zn sorption on Na-montmorillonite Part II: modelling

Titration and sorption edge/isotherm measurements, carried out under a wide variety of conditions on Na-montmorillonite, were reported in Part I (Baeyens and Bradbury, 1997). These data are modelled here in terms of cation exchange and surface complexation mechanisms using a computer code called MINSORB. This code allowed the uptake of radionuclides by both mechanisms to be calculated simultaneously; also taking into account competitive reactions from other cations present. A stepwise iterative fitting/modelling procedure is described. For the case of Na-montmorillonite it is demonstrated that an electrostatic term in the surface complexation model is not required. From the modelling of the titration results, values for site capacities and protonation/deprotonation constants were deduced. These values were then fixed and used in all further surface complexation modelling of the sorption measurements. The main study was carried out with Ni, but impurity cations present in the system, particularly Zn, had to be examined in addition due to their competitive effects on Ni sorption. The surface complexation behaviour of Ni and Zn was investigated in detail to give intrinsic surface complexation constants on two of the ≡SOH type sites included in the model. The sorption of Mn is also considered, though in less detail, and estimated surface complexation constants for this element are presented. Cation exchange was included in all of the calculations. Measured selectivity coefficients for Ni---Na and Zn---Na exchange reactions are given. The model, with the derived parameters, allowed all the experimental data from titration measurements through sorption edges to sorption isotherms to be quantitatively described.