Multi-scale interfacial mechanisms of Pb immobilization in contaminated soil using phosphoric acid-activated coal gangue-electrolytic manganese residue geopolymers

The accumulation of contaminated soils and solid waste stockpiles has led to significant resource wastage and environmental pollution. This study investigates a novel solidification/stabilization (S/S) curing agent, named ECPG, synthesized from electrolytic manganese residue (EMR), coal gangue (CG), and phosphoric acid, for the treatment of Pb-contaminated soils. The influences of ECPG dosages, CG to EMR ratios, Pb 2+ and phosphoric acid concentrations, curing age, and durability on the strength development and leaching characteristics of the solidified soils were systematically investigated. Durability tests under different freeze-thaw and wet-dry cycles were also conducted to assess long-term performance. The mechanisms of geopolymerization, S/S, and strength enhancement were elucidated through tests and molecular simulations. The results demonstrated that the optimal CG to EMR ratio was 8:2 with 7 mol/L phosphoric acid, resulting in a peak strength of 5.9 MPa and immobilization rates of 99.7%, 99.3%, and 81.5% for Pb, Mn, and after curing for 28 d. Freeze-thaw and wet-dry cycles demonstrated excellent durability, with unconfined compressive strength (UCS) above 85% and Pb leaching value of 0.42 mg/L. Molecular simulations revealed that Pb and Mn were primarily immobilized by adsorption, with ion exchange serving as a secondary mechanism, and a minor fraction resulted in the formation of phosphate precipitates. Conversely, was predominantly immobilized via ion exchange, with a portion forming MgNH 4 PO 4 ·6H 2 O and MnNH 4 PO 4 ·H 2 O. This study can not only contribute to the advancement of safe disposal and resource reuse of contaminated soil and solid waste, but also offers a theoretical foundation for S/S technology from a multi-scale perspective.