Proton adsorption and electrokinetics of an Argentinean montmorillonite

Suspensions of an argentinean Na+-exchanged montmorillonite were studied by acid-base potentiometric titrations, mass titrations and electrophoresis, Potentiometric titrations were employed to obtain proton adsorption vs pH curves at different NaCl concentrations whereas mass titrations were used to determine the dependence of the point of zero net proton charge (PZNPC) with the ionic strength. The studied Na+ montmorillonite presented proton adsorption vs pH curves typical of materials carrying mainly structural negative charges and few hydroxyl surface groups: the PZNPC decreased with the ionic strength, and the different titration curves ran almost parallel to each other. No crossing point could be observed for several titration curves performed at different NaCl concentrations. The EM was also typical of particles that carry mainly negative charges: there was only a slight decrease in the negative electrophoretic mobility when the pH was decreased from 11 to 3. The kinetics of proton adsorption-desorption processes was fast in alkaline media and slower at pH lower than ca. 8.5. This behavior appears to be a consequence of edge-to-face interactions and suggests that the edges become positively charged at pH <8.5. A model of the 2:1 type clay water interface is proposed to fit proton adsorption and electrophoretic data of the studied sample and those previously published for an illite, The model assumes the presence of structural negative charges in the clay particle whereas variable charge sites and cation exchange sites are assumed to be located at the particle surface. The model predicts that Hi adsorption at high pH takes place mainly on variable charge sites. They become positively charged at pHs lower than the PZNPC, which is in agreement with kinetics measurements. Hf adsorption is only important at pH < 6 and it reaches values close to the cation exchange capacity of the sample at pHs 2-3, Although this proton adsorption is high at low pHs, it takes place mainly by a H+-Na+ exchange mechanism and does not modify appreciably the net negative charge of the particle. This nearly constant charge induces a negative surface potential that is responsible for negative zeta potential of the particles. The negative surface potential also modifies the normal behavior of the variable charge sites so that the PZNPC is predicted to change by changing the electrolyte concentration. The model also predicts that the affinity of surface sites for cations is higher in illites than in montmorillonites. This is probably due to the fact that isomorphic substitutions take place beneath the plane of basal oxigens in illites, modifying their electronic population and increasing their cation binding capabilities. (C) 1998 Academic Press.