Solidification/stabilization of Pb2+ and Zn2+ in the sludge incineration residue-based magnesium potassium phosphate cement: Physical and chemical mechanisms and competition between coexisting ions.

In order to exhaustively investigate the physical and chemical mechanisms of heavy metal immobilization in sludge incineration residue (SIR)-based magnesium potassium phosphate cement (MKPC), this work investigated the influence of Pb2+ and Zn2+ on the compressive strength and microstructure of SIR-based MKPC, and the efficiency of Pb and Zn immobilization. Taking the difference of Ksp (solubility product) of different heavy metal compounds as the entry point, the physical and chemical mechanisms of Pb and Zn immobilization, and the competitive mechanism between coexisting ions, were comprehensively analyzed. It was discovered that Pb2+ is in the form Pb3(PO4)2, and Zn2+ is immobilized in the form Zn2(OH)PO4 [Zn3(PO4)2 is preferentially formed, when the pH > 7, Zn3(PO4)2 is converted to Zn2(OH)PO4]. The low solubility of heavy metal phosphates is the main reason that Pb2+ and Zn2+ are well immobilized. The preferential formation of Pb3(PO4)2 (Ksp = 8 × 10-43) and Zn3(PO4)2 (Ksp = 9.0 × 10-33) reduced the amount of MgKPO4·6H2O (Ksp = 2.4 × 10-11), resulting in a decrease in compressive strength. Besides, coexisting Pb2+ and Zn2+ has a competitive effect: Pb2+ will weaken the immobilization efficiency of Zn2+. The new exploration of these mechanisms provide a theoretical basis for rationally adjusting the Magnesia/Phosphate ratio to enhance the compressive strength and improve the efficiency of heavy metals immobilization.