Adsorption of phosphate on vaterite

Abstract

The adsorption of orthophosphate on the surface of vaterite has been investigated at 25.0 °C. The adsorption obeys the Henry adsorption isotherm at low phosphate concentrations and the Freundlich adsorption isotherm at high concentrations. The unprotonated phosphate ion is adsorbed on vaterite, forming a negatively charged ion pair with the accompanying calcium ion, Ca²⁺·PO^3–₄. The adsorption of a negatively charged species is attributed to the positive charge of the vaterite surface. A fraction of the adsorbed PO^3–₄ ion is irreversibly incorporated into the vaterite lattice. From the temperature dependence of distribution ratio (15–45 °C) the enthalpy change of the adsorption of the ion pair was evaluated to be almost zero.