Immobilization of Aqueous Heavy Metal Cations with Phosphates and Sulfates

The dissolution-precipitation mechanism has been suggested for the immobilization of aqueous lead cation with calcium hydroxyapatite (Ca10(PO4)6(OH)2; CaHAp). Here Na2HPO4, K2HPO4, MgHPO4 and CaHPO4 were employed for CaHAp. These hydrogenphosphates could favorably remove Pb^2+, Cd^2+, Co^2+ and Cu^2+ from aqueous solution for 0.5 h at 293 K. The removability of those hydrogenphosphates was evidently greater than that of CaHAp and seemed to be dependent on solubility of those hydrogenphosphates and the hydration enthalpy of those heavy metal cations. The structures of phosphates recovered after immobilization of Pb^2+, Cd^2+, and Co^2+ were strongly influenced by the pH in the aqueous solution, while the structures after immobilization of Cu^2+ for 0.5 and 12 h were amorphous compound and Cu2(PO4)(OH), respectively. The employment of solid state 31P MAS NMR and XRD revealed that the amorphous compound was also Cu2(PO4) (OH). The removability found with those hydrogenphosphates was also observed in the immobilization of Pb^2+ with MgSO4 and CaSO4.