STANDARD ENTHALPIES OF ION ADSORPTION ONTO OXIDES FROM AQUEOUS-SOLUTIONS AND MIXED-SOLVENTS

Adsorption of sodium from 0.001-0.1 M NaCl solutions in water and mixed solvents (aqueous methanol and dioxane up to 30%) on silica and alumina at given pH and ionic strength increases with the temperature. However, at given surface charge density and ionic strength, sodium adsorption in the above systems is practically independent of temperature over the range 15-35-degrees-C; thus the apparent positive (endothermic) enthalpy, calculated from iso-pH data, reflects the increase in the negative surface charge. This suggests that cation adsorption in the studied systems is purely electrostatic in character. In contrast, adsorption of Cl- ions on alumina at constant pH and ionic strength is independent of the temperature. This observation is in line with adsorption of various anions on goethite at high fractional surface coverage. Cobalt adsorption on silica and alumina in the presence of 0.1 M NaCl at constant surface charge density also increases with the temperature. Thus the tendency found for other (hydr)oxide-multivalent cation adsorption systems has been confirmed.