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 Pb 2+ and Zn 2+ 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 Pb 2+ is in the form Pb 3 (PO 4 ) 2 , and Zn 2+ is immobilized in the form Zn 2 (OH)PO 4 [Zn 3 (PO 4 ) 2 is preferentially formed, when the pH > 7, Zn 3 (PO 4 ) 2 is converted to Zn 2 (OH)PO 4 ]. The low solubility of heavy metal phosphates is the main reason that Pb 2+ and Zn 2+ are well immobilized. The preferential formation of Pb 3 (PO 4 ) 2 (K sp  = 8 × 10 −43 ) and Zn 3 (PO 4 ) 2 (K sp  = 9.0 × 10 −33 ) reduced the amount of MgKPO 4 ·6H 2 O (K sp  = 2.4 × 10 −11 ), resulting in a decrease in compressive strength. Besides, coexisting Pb 2+ and Zn 2+ has a competitive effect: Pb 2+ will weaken the immobilization efficiency of Zn 2+ . 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.