Technical Report NTB 95-05

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

The main focus of the experimental work presented here is the sorption behaviour of Ni on conditioned Na-montmorillonite. Ca and Zn were also studied as important background impurity cations which can, depending on the circumstances, be competitive for the available sorption sites. Distribution ratios were measured for all three radionuclides at trace concentrations as a function of pH over a range from 2 to 10.5 to produce so-called "sorption edges". In the cases of Ca and Ni such measurements were carried out as a function of the NaCIO4 background electrolyte concentration.

From the form of the sorption edges it was deduced that two main mechanisms were controlling the uptake of radionuclides onto the solid; a pH independent component, identified as cation exchange, and a pH dependent pocess interpreted as surface complexation on ≡SOH type sites.

In addition to sorption edges, sorption isotherms were determined for Ni and Zn in solutions of 0.1 M NaCIO4 at several pH values. The non-linear sorption isotherms indicated that in addition to cation exchange, at least 2 different ≡SOH type sites were present in Na-montmorillonite contributing to the overall sorption.