Adsorption of Chromium (VI) on Calcium Phosphate: Mechanisms and Stability Constants of Surface Complexes

The adsorption of chromate on octacalcium phosphate (OCP) was investigated as a function of contact time, surface coverage, and solution pH. The ion exchange method was adapted to establish the interaction mechanism. Stoichiometry exchange of H+/OH- was evaluated at a pH range of 3-10, and obtained values ranged between 0.0 and 1.0. The surface complexes formed between chromate and OCP were found to be <mml:semantics> > S ( HCrO 4 ) <overbar> </mml:mover> </mml:semantics> and <mml:semantics> > S ( CrO 4 ) <overbar> </mml:mover> </mml:semantics> . The logarithmic stability constant logK(1-1), and the logK(10) values of the complexes, were 6.0 in acidic medium and 0.1 in alkaline medium, respectively. At low pH and low surface coverage, the bidentate species <mml:semantics> > S <mml:mo>( HCrO 4 <mml:mo>) <mml:mo stretchy="true"><overbar> </mml:mover> 2 </mml:semantics> with logK(10.5) of about 2.9, was favored at a hydration time of less than 150 min. The contribution of an electrostatic effect to the chromium uptake by the OCP sorbent, was also evaluated. The results indicate that the adsorption of chromate on OCP is of an electrostatic nature at a pH 5.6, and of a chemical nature at a pH > 5.6.