Effect of temperature on the retention of Cs+ by Na-montmorillonite: microcalorimetric investigation

The microcalorimetric investigation of the system composed of a homoionic Na-montmorillonite extracted from ANDRA MX80 bentonite and CsCl in aqueous solution at neutrality has allowed us to determine directly the enthalpy for the Na+ → Cs+ exchange reaction as a function of the degree of exchange. To our knowledge, this provides the first experimental evidence that the enthalpy of ion exchange varies greatly with the degree of exchange: ΔH′e goes from −15.0 ± 0.5 kJ mol−1 when only traces of Cs+ are sorbed to −4.1 ± 0.2 kJ mol−1 when almost all the Na+ ions of the clay are exchanged for Cs+. It also shows that the Na+ → Cs+ exchange process is clearly driven by the favourable enthalpy of reaction. The enthalpy values are used to predict the effect of temperature on the retention of traces of Cs+ by the Na-montmorillonite in pure water but also in aqueous solutions containing Na+ ions in order to mimic natural waters. Our microcalorimetric results show that when the concentration of Na+ initially present in the aqueous solution is relatively low, as expected in natural environments, the retention of traces of 137Cs by montmorillonite will be only slightly modified by a temperature rise of the underground nuclear waste repository due to radioactivity. Interestingly, for the first time a directly measured enthalpy value for the exchange of traces of Cs+ by a Na-montmorillonite can be compared with enthalpy values indirectly obtained from Kd values at different temperatures: our microcalorimetric investigation demonstrates that good results can be deduced from van't Hoff plots.