A MECHANISTIC DESCRIPTION OF NI AND ZN SORPTION ON NA-MONTMORILLONITE.1. TITRATION AND SORPTION MEASUREMENTS

In this paper experimental investigations into the acid/base titration characteristics of Na-montmorillonite and the sorption behaviour of N i 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 sorpti on data are analysed to determine the parameters in cation exchange an d surface complexation based models which together provide a quantitat ive description of the titration and sorption data. A conditioning pro cedure was applied to the SWy-1 Na-montmorillonite starting material i n order to remove background metal impurities, soluble salts and spari ngly soluble minerals which could influence titration and sorption mea surements. The purified clay, in the homo-ionic Na form, was thoroughl y 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 ex change processes on the form of the titration curves is discussed. Tit ration data can be analysed to yield site capacities and protonation/d eprotonation constants for the amphoteric surface hydroxyl groups (=SO H). The acid endpoint in the titration data was used to estimate an =S OH site capacity of 0.08 mol kg(-1). The sorption of Ni and Zn on cond itioned Na-montmorillonite was studied at trace concentrations as a fu nction of pH over a range from similar to 3 to similar to 10 to produc e so-called ''sorption edges''. In the case of Ni, such measurements w ere carried out as a function of the NaClO4 background electrolyte con centration. In addition, sorption isotherms were determined for both n uclides at several fixed pH values in 0.1 M NaClO4. From the form of t he ''edges'' it was deduced that two main sorption mechanisms were con trolling the uptake of Ni and Zn onto the clay mineral; a pa-independe nt component, identified as cation exchange on the permanent charge si tes, and a pH-dependent one, interpreted as surface complexation on th e amphoteric surface hydroxyl groups. The non-linearity of the sorptio n isotherms indicated that at least two different =SOH type sites were contributing to the overall sorption on Na-montmorillonite. (C) 1997 Elsevier Science B.V.