Adsorption of phosphate on calcite

Abstract

The distribution of orthophosphate on the calcite surface has been investigated at 25.0 °C. The distribution obeys the Freundlich adsorption isotherm. The unprotonated species, PO^3–₄, is adsorbed on the calcite surface at low phosphate concentration and by increasing its concentration the adsorbed specie changes to HP^2–₄, where the phosphates are adsorbed as neutral species by an accompanying calcium ion. At very low phosphate concentration, the PO^3–₄ ion exchanges with the carbonate ion in the calcite surface and is irreversibly incorporated into the calcite lattice. The enthalpy changes of the adsorption of phosphates are comparable to those of ion-pair formation of calcium-phosphate.