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 mixe d solvents (aqueous methanol and dioxane up to 30%) on silica and alum ina at given pH and ionic strength increases with the temperature. How ever, at given surface charge density and ionic strength, sodium adsor ption in the above systems is practically independent of temperature o ver 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 s tudied systems is purely electrostatic in character. In contrast, adso rption of Cl- ions on alumina at constant pH and ionic strength is ind ependent of the temperature. This observation is in line with adsorpti on 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 ads orption systems has been confirmed.