Surface analysis and the adsorption of Co(II) on goethite

The adsorption of aqueous Co(II) on goethite (α-FeOOH) was studied as a function of pH. Adsorption reactions were carried out in solutions with cobalt concentrations in the range 1 × 10−2 to 1 × 10−1M and at pH between 4 and 10. The quantity of cobalt adsorbed increases with increasing pH. The chemical nature of adsorbed cobalt was probed using X-ray photoelectron spectroscopy (XPS). The general features of the cobalt XPS spectra indicate that cobalt(II) is adsorbed at pH 6–7.5. At pH 8 and 10 cobalt is present as a mixture of Co(II) and Co(III). Analysis of X-ray diffraction patterns and interpretation of electrophoretic mobility results lead to the suggestion that cobalt-containing surface species are best represented as a mixture of Co(OH)2 and CoOOH at pH 8 and 10.