Surface Charge and Electrophoretic Properties of Colloids Obtained From Homoionic and Natural Bentonite

Abstract

Suspensions of colloids obtained from a Spanish bentonite were studied by potentiometric acid-base titrations and electrophoresis in order to analyse their surface chemical properties, which are responsible of their stability behaviour and that are very important to consider in radionuclides sorption modelling. "Fast" titrations and "batch -back" titrations techniques were used to determine the contribution of the pH -dependent charge and the difference in the results obtained are discussed. Experimental data obtained by acid/base titrations were interpreted according the EDL theory. The model prediction agreed satisfactorily with the experimental data in the alkaline pH range. Protonation / deprotonation reactions of surface functional groups (SOH) appeared to be the main surface charge -determining mechanism in the alkaline pH range whereas ion -exchange type reactions, had to be taken into account over the acidic pH range. Surface potentials were calculated for different salt concentrations, from experimental data and taking into account both layer and edge sites charge contributions. INTRODUCTIONIt has been recently shown that bentonite colloids can be generated at the near/field far field interface of a radioactive waste repository [1] and can therefore affect the transport of contaminants to the biosphere. The study of the surface charge and electrophoretic properties of such colloids is necessary to improve the understanding of the mechanisms that lead to the formation of pseudo-colloids at the repository/far-field interface, as well as of their stability or sorption properties.

On the Applicability of DLVO Theory to the Prediction of Clay Colloids Stability

“…Different methods for determining the surface charge from titration experiments have been proposed (13), some of them being more reliable if the material is appreciably soluble. In a separate work, titration experiments onto FEBEX bentonite and the comparison of the results obtained with different methods are discussed (14). In this work, the results of a "fast" titration method were used.…”

Effect of colloids on transfer of radionuclides by subsurface water

“…The buffer layer also may be a source of colloidal particles. The properties of bentonite colloids were investigated by Missana et al (2000).…”