Thermodynamics of Sr-Mg vermiculite exchange and the effect of PVA on Mg release

The aim of this work is to study the thermodynamics of Sr-Mg vermiculite ex change and the effect of polyvinyl alcohol (PVA) on the interlayer magnesiu m release from the clay surface. The results indicated that the affinity of adsorption decreases as the unhydrated ionic radius increases. Also it was found that the surface of vermiculite exhibits more selectivity for Sr by treating Mg-vermiculite with PVA than in its absence. With increasing tempe rature the exchange of Mg by Sr increases. The enthalpy value (Delta H) was positive, i.e. the exchange reaction is endothermic. X-ray diffraction of the clay indicates that the c-spacing of vermiculite increases gradually as the adsorbed amount of Sr ion increases on the vermiculite surface. The ac tivity coefficient ratio of f(Sr)/f(Mg) varies at 20 degrees C from 0.042 a t 10% Sr saturation to 0.364 at 90% Sr saturation, i.e. the ratio increased with increasing Sr saturation. (C) 1999 Elsevier Science B.V. All rights r eserved.