Synergistic Physical–Chemical Immobilization of Mn in Electrolytic Manganese Residue Co-Solidified with Sludge Biochar: Interfacial Speciation, Pore Evolution, and LCA

The long-term immobilization of highly mobile Mn2+ in electrolytic manganese residue (EMR) remains a critical challenge for sustainable waste management, as conventional cement-based solidification often suffers from carbon-intensive clinker demand and insufficient interfacial stabilization. Herein, we propose a co-solidification strategy integrating municipal sewage sludge-derived biochar (SBC) into cementitious matrices to enhance Mn immobilization. Multi-scale characterization revealed that SBC functioned as a micro-aggregate to increase pore network tortuosity by 124% and further modified the hydration/interfacial environment of the cementitious matrix. BCR sequential extraction showed that the acid-soluble/exchangeable Mn fraction decreased from 63% in raw EMR to 41% in ECB without SBC and further to 28% after the incorporation of 5 wt% SBC, indicating a progressive redistribution of Mn toward less mobile fractions. These coupled chemical and physical effects reduced Mn leaching from 14.5 mg/L to 0.2 mg/L after 28 days of curing, corresponding to a 98.6% reduction and meeting the relevant leaching criterion for non-hazardous classification. Mechanistic analysis indicated that Mn immobilization was governed by the combined effects of hydration-product association, interfacial interactions, matrix densification, and pore-network refinement. Life-cycle assessment further showed that the climate benefit of the SBC route was dependent on the sludge-management scenario, particularly the energy demand for sludge pretreatment and drying, auxiliary fuel use, and energy recovery. This study elucidates the interfacial engineering principles of SBC-amended cementitious systems and offers a technically feasible pathway for the co-valorization of industrial and municipal solid wastes.