A mechanistic description of Ni and Zn sorption on Na-montmorillonite Part I: Titration and sorption measurements

In this paper experimental investigations into the acid/base titration characteristics of Na-montmorillonite and the sorption behaviour of Ni and Zn under a wide variety of conditions are presented. From these measurements the dominant sorption mechanisms could be deduced. In the following paper (Bradbury and Baeyens, 1997) the titration and sorption data are analysed to determine the parameters in cation exchange and surface complexation based models which together provide a quantitative description of the titration and sorption data. A conditioning procedure was applied to the SWy-1 Na-montmorillonite starting material in order to remove background metal impurities, soluble salts and sparingly soluble minerals which could influence titration and sorption measurements. The purified clay, in the homo-ionic Na form, was thoroughly physico-chemically characterised before carrying out batch titration measurements on suspensions in 0.1 and 0.5 M NaClO4. The influence of background impurities, not removed by the conditioning, and cation exchange processes on the form of the titration curves is discussed. Titration data can be analysed to yield site capacities and protonation/deprotonation constants for the amphoteric surface hydroxyl groups (≡SOH). The acid endpoint in the titration data was used to estimate an ≡SOH site capacity of 0.08 mol kg−1. The sorption of Ni and Zn on conditioned Na-montmorillonite was studied at trace concentrations as a function of pH over a range from 3 to 10 to produce so-called “sorption edges”. In the case of Ni, such measurements were carried out as a function of the NaClO4 background electrolyte concentration. In addition, sorption isotherms were determined for both nuclides at several fixed pH values in 0.1 M NaClO4. From the form of the “edges” it was deduced that two main sorption mechanisms were controlling the uptake of Ni and Zn onto the clay mineral; a pH-independent component, identified as cation exchange on the permanent charge sites, and a pH-dependent one, interpreted as surface complexation on the amphoteric surface hydroxyl groups. The non-linearity of the sorption isotherms indicated that at least two different ≡SOH type sites were contributing to the overall sorption on Na-montmorillonite.