Kinetics of adsorption of the cobalt ions on the "electrolytic solution/γ-alumina" interface

In the present work we studied, for the first time, the kinetics of adsorption of the Co(H2O)(6)(2+) species on the "electrolytic solution/gamma-Al2O3" interface at pH = 7 and 25 degrees C for a very broad range of Co(II) surface concentrations ranged from 0.03 to 6 theoretical Co(H2O)(6)(2+) surface layers. Moreover, we studied the surface dissolution of gamma-alumina in the presence of the Co(H2O)(6)(2+) ions in the impregnating solution, the contribution of the Co(II) desorption on the whole deposition process and the deposition isotherm. It was found that under the conditions where the deposition has taken place, the dissolution of the gamma-alumina surface is negligible even in the presence of the Co(H2O)(6)(2+) species in the impregnating solution. It was, moreover, inferred that the Co(II) desorption does not participate significantly to the whole deposition process. It was found that the deposition kinetics may be described by the following kinetic expression r(Co.bulk) = k'C-Co,bulk(2), which relates the rate of disappearance of the Co(H2O)(6)(2+) ions from the impregnating solution, r(Co,bulk), with their concentration C-Co,C-bulk. This kinetic expression may be derived assuming the following deposition scheme: nS + 2[Co-(H2O)(6)(2+)] -> S-n - [Co(H2O)(x,x < 6)(2+)](2), where S represents the surface reception sites. The above expressions indicated that two Co(H2O)(6)(2+) ions are involved, from the side of the interface, in the reaction with the reception sites. It seems probable that the deposition step involves the simultaneous adsorption and dimerization of the two interfacial Co(H2O)(6)(2+) ions through (hydr)oxobridges. On the other hand, the sigmoidal form of the deposition isotherm and the dependence of the apparent rate constant, k', on the interfacial Co(II) concentration suggested that the already deposited Co(II) species may be involved in the reception sites, S, promoting the adsorption and resulting to the formation of multinuclear complexes and Co(II) surface precipitates. Finally, reasonable interface potential values for oxides were determined for the first time using kinetic results.