Mechanism of adsorption of selected monovalent and divalent inorganic ionsat the alumina/electrolyte interface

The mechanism of adsorption of selected mono- and divalent ions (Na+, Ca2+, Cl-, SO42-) at the metal oxide (Al2O3)/electrolyte interface has been stud ied using different experimental techniques: potentiometric titration, zeta potential and adsorption density measurements. Radiotracers and ion-select ive electrodes were used to measure the adsorption densities of ions. It wa s found that the accuracy of both methods was almost the same. Ion-selectiv e electrodes allowed to acquire adsorption data for shorter times of contac t between the oxide and electrolyte solution. Based on the potentiometric t itration data, ionization and complexation constants of surface hydroxyl gr oups were calculated using the site binding theory of the electrical interf acial layer. The adsorption density of Ca2+, at the same pH and ion concent ration, was significantly higher than that observed for sodium ions and rev ealed strong pH dependence (adsorption edge). A shift of the pzc and iep of alumina was observed in the systems containing chloride, sulphate or calci um ions, due to the specific adsorption of those ions at the oxide/electrol yte interface. The calculated pK(A) values for different anions showed that the stability of surface complexes increased in the following sequence: Cl O4- < Cl- < SO42-.