High-efficiency and low-carbon remediation of zinc contaminated sludge by magnesium oxysulfate cement

Highlights: • High dosage of ZnO addition insignificantly hindered the reaction process of MOSC. • MgO-to-MgSO₄ molar ratio determined the formation of different MOSC products. • ZnO incorporated the structure of 5Mg(OH)₂·MgSO₄.7H₂O phase in MOSC matrices. • MOSC exhibited excellent performance on S/S of Zn-rich electroplating sludge. Electroplating sludge is classified as a hazardous waste due to its extremely high leachability of potentially toxic elements. This study concerns the use of magnesium oxysulfate cement (MOSC) for the stabilisation/solidification (S/S) of Zn-rich electroplating sludge. According to X-ray diffraction and thermogravimetric analyses, Zn was mainly immobilised through both chemical interaction and physical encapsulation in the MOSC hydrates of 5Mg(OH)₂·MgSO₄.7H₂O (5−1−7) phase. The crystal size analysis, elemental mapping, and extended X-ray absorption fine structure (EXAFS) analysis proved that the Zn²⁺ was also incorporated in the structure of 5–1–7 phase. Unlike Portland cement system, hydration kinetics, setting time, and compressive strength of the MOSC system were only negligibly modified by the presence of Zn, indicating its superior compatibility. Subsequent S/S experiments demonstrated that the MOSC binder exhibited an excellent performance on immobilisation efficiency of Zn (up to 99.9%), as well as satisfying the requirements of setting time and mechanical strength of sludge S/S products. Therefore, MOSC could be an effective and sustainable binder for the treatment of the Zn-rich industrial wastes.