Adsorption of cobalt, chromium and barium on ripidolite and kaolinite as examined by x-ray photoelectron spectroscopy

Abstract

X-ray photoelectron spectroscopy (XPS) has been used to study the bonding of adsorbed metal cations to clay minerals. Binding energy differences of the adsorbed metal cations can be related to changes in the electron density or charge on the atom of interest.

Adsorption experiments were carried out in aqueous solution at controlled pH's of 2, 4, 6, 8, and 10 for Ba²⁺, Co²⁺, Cr³⁺, adsroption on the clays kaolinite and ripidolite.

Solution processes were monitored by measuring the solution concentrations of dissolved silica and the metal ions Fe³⁺, Mg²⁺, K⁺, Al³⁺, Cr³⁺, and Co²⁺ at the beginning and the end of the experiment. Atomic absorption spectroscopy was used to determine the metal ion concentrations and dissolved silica was determined spectrophotometrically as a molybdate complex.

Examination of the adsorbed cation species on the clay surface by XPS indicated that the clays behaved as nucleation centers at or near the pH of precipitation of the cations. Further it was found that high spin Co²⁺ in solution became low spin Co²⁺ or formed a highly covalent bond when adsorbed on kaolinite at pH's 4, 6, and 7 and on ripidolite at pH 2 and 4. Finally, if the clay has a negatively charged surface which donates some of its charge to the positive cation, then the barium XPS data indicated that ripidolite has a greater negative surface charge than kaolinite. This idea was supported by calculations of the surface charge density from CEC and surface area data.