Sorption behavior of Zn(II) ions on synthetic apatitic calcium phosphates

Abstract

The synthesis, characterization and the reactivity of apatitic calcium phosphates (Ca-HA, chemical formula Ca10(PO4)6(OH)2) is reported. Calcium carbonate (CaCO3) and potassium dihydrogen orthophosphate (KH2PO4) were selected as economical starting materials for the synthesis of Ca-HA under atmospheric conditions. Monocalcium phosphate monohydrate (MCPM), dicalcium phosphate dihydrate (DCPD), and octacalcium phosphate pentahydrate (OCP) were identified as the main intermediates of the synthesis reaction. The product obtained after 48h of reaction contains mainly low-crystalline Ca-HA and small amounts of other calcium phosphates such as octacalcium phosphate (OCP), B-type carbonate apatite (CAP), as well as unreacted calcium carbonate. This Ca-HA was found to be active for the removal of Zn2+ from an aqueous solution. Its sorption capacity reached up to 120mg of Zn2+ per g of Ca-HA powder after 24h of reaction. The monitoring of soluble Zn, Ca and P during the sorption experiment allowed characterizing the mechanism of Zn uptake. Dissolution–precipitation, ionic exchange and surface complexation are the three main mechanisms involved in the sorption processes. The contribution of these mechanisms is discussed in detail.