Ion exchange properties of hydrous ceria—III thermodynamics of anion exchange

Abstract

The thermodynamics of the $\text{NO}{}_3{}^{\mathrm{-}}\text{Cl}{}^{\mathrm{-}}\text{,}\text{NO}{}_3{}^{\mathrm{-}}\text{Br}{}^{\mathrm{-}}$ and $\text{NO}{}_3{}^{\mathrm{-}}\text{SCN}{}^{\mathrm{-}}$ exchanges in hydrous ceria was studied. The $\text{NO}{}_3{}^{\mathrm{-}}\text{Cl}{}^{\mathrm{-}}$ and $\text{NO}{}_3{}^{\mathrm{-}}\text{SCN}{}^{\mathrm{-}}$ exchanges in this exchanger are enthalpy directed while the $\text{NO}{}_3{}^{\mathrm{-}}\text{Br}{}^{\mathrm{-}}$ exchange is entropy directed. Consideration of these results and other results on hydrous zirconia shows that although selectivity reversals of the “normal” selectivity sequence of monovalent anions do occur in the weak-base hydrous oxides, these reversals may be due to changes in the oxide entropy, which are ignored in the Eisenman model. Noncoulombic electrostatic interactions, which are also ignored in the model, are probably dominant in certain cases and seem to contribute considerably to the enthalpy and solid entropy changes. The Eisenman model seems to throw some light on the phenomena involved in anion exchanges on hydrous oxides.