Technical Report NTB 95-04

A Quantitative Mechanistic Description of Ni, Zn and Ca Sorption onNa-Montmorillonite, Part I: Physico-Chemical Characterisation and Titration Measurements

The work presented in this report is primarily concerned with characterisation studies and titration measurements on conditioned Na-montmorillonite.

The starting material, SWy-1 Na-montmorillonite (Crook County, Wyoming, USA), contains background metal impurities, soluble salts and sparingly soluble minerals which can influence titration and sorption measurements and thereby lead to difficulties in the interpretation of the data. A conditioning procedure is described which yielded a purified clay material in the homo­ionic sodium form with a particle size < 0.5 μm. This so-called conditioned Na-montmorillonite was thoroughly physico-chemically characterised before carrying out batch titration measurements on suspensions in 0.1 and 0.5 M NaCIO4. The reasons for choosing a batch method, coupled with back titration of the supernatant solutions, instead of a continuous procedure, are fully discussed. Chemical analyses data for the supernatant solutions are presented in detail. The influence of background impurities, not removed by the conditioning, and cation exchange processes on the form of the titration curves were investigated. Net Na-montmorillonite titration curves at two different ionic strengths were derived which are considered to reflect only the acid/base behaviour of ≡SOH type surface sites. The acid end point in the titration data was used to estimate an ≡SOH site capacity of 0.08 moles per kg conditioned Na-montmorillonite.