Technical Report NTB 95-06

A Quantitative Mechanistic Description of Ni, Zn and Ca Sorption onNa-Montmorillonite, Part III: Modelling

Titration and sorption measurements, carried out under a wide variety of conditions on Na-montmorillonite, were examined in terms of cation exchange and surface complexation mechanisms. A computer code called MINSORB was developed and used throughout this work. 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. A basic data set comprising of site capacities and protonation/deprotonation constants was defined, which was valid for all surface complexation sorption reactions. 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 Mg, Ca and Mn is also considered, though in less detail, and estimated surface complexation constants for these nuclides are presented. Cation exchange was included in all of the calculations. Measured selectivity coefficients for Ni-Na, Zn-Na and Ca-Na exchange reactions are given. The model, with the derived parameters, allowed all the data from titration measurements through sorption edges to sorption isotherms to be calculated.