Competitive encapsulation of multiple heavy metals by magnesium potassium phosphate cement: Hydration characteristics...

Magnesium potassium phosphate cement (MKPC) is increasingly used in the solidification/stabilization (SS) of heavy metal (HM) pollutants. However, research on composite HM pollutants remains limited. In this study, four heavy metals (Pb/Zn/Cu/Cd) were individually and simultaneously introduced into MKPC systems with different magnesium/phosphorus (M/P) molar ratios. The introduction of HMs altered the extent of hydration and morphology of MgKPO₄·6H₂O. Among the MKPC pastes, those with M/P = 2 and 3 had the highest HM solidification efficiency and strength, respectively. The HM solidification efficiency of all specimens exceeded 99 %. In samples with M/P = 3, the codoping of four HMs slightly increased the M/P ratio, thereby increasing MgKPO₄·6H₂O content and enhancing strength. Pb could generate additional low-solubility precipitates, such as PbHPO₄, Pb₃ (PO₄)₂, Pb₅ (OH) (PO₄)₃, and Pb (OH)₂, which easily accumulated in pores and were encapsulated by MgKPO₄·6H₂O, leading to the highest solidification efficiency of Pb by MKPC. Pb and Cu could also form the composite phosphate products Pb₂Cu (PO₄)₃ (OH)·4H₂O, thus promoting the S/S effect of Cu. Therefore, the use of MKPC with M/P ratio of 2-3 for the S/S of complex pollutants containing Pb and Cu is a promising approach.