Synthesis and solubility of hopeite Zn₃(PO₄)₂·4H₂O

Minerals from the phosphate groups are used in environmental engineering as thermodynamically stable vehicles for heavy metals such as zinc. The hopeite Zn₃(PO₄)₂·4H₂O was synthesized and characterized by X-ray diffraction and scanning electron microscopy. The solubility of the hopeite was measured at 25°C. The average solubility product, log K_sp, for the reaction Zn₃(PO₄)₂·4H₂O ⇔ 3Zn²⁺ + 2PO₄^3- + 4H₂O at 25°C is –35.72 ± 0.03. The free energy of formation, ΔG°_f,298, calculated from this measured solubility product is –3597.4 ± 1.0 kJ mol^-1. The immobilization of zinc in the hopeite structure offers the possibility of developing an effective method for removing Zn from wastewater, water and soils.