Waste Control by Waste: Red Mud-Based Porous Carbothermal Composite for Efficient Remediation of Manganese and Ammonia Nitrogen in Contaminated Soil

The co-contamination of manganese ions (Mn2+) and ammonia nitrogen (NH4+) caused by the stockpiling of manganese residue poses a serious threat to the ecological environment. In this study, a series of the composite was prepared from red mud, bentonite, and corn straw via oxygen-limited pyrolysis. The effects of pyrolysis temperature and raw material ratio on the material properties were investigated, and the synergistic remediation performance of the composites for Mn2+ and NH4+ in manganese residue-contaminated soil was evaluated through a 180-day soil column experiment. The results showed that the composite prepared with a raw material ratio of 1:1:1 at a pyrolysis temperature of 700 °C exhibited the largest specific surface area and the most developed pore structure, achieving a Mn2+ removal rate of 92.72% ± 0.85% in aqueous solution. In the soil column experiment, the material prepared at 700 °C gave the highest immobilization rate for soil Mn2+ (96.22% ± 0.5%), whereas the combined addition of materials prepared at 700 °C and 500 °C achieved the best removal efficiency for NH4+ (99.33% ± 0.23%). Mechanistic studies revealed that the stabilization of Mn2+ is primarily attributable to alkaline precipitation and mineral lattice solid solution induced by the composite, leading to the formation of stable spinel phases (e.g., (Fe,Mn)3O4) and insoluble manganese phosphate-carbonate salts. The removal of NH4+ is proposed to proceed via adsorptive enrichment by the porous structure and Fe0-mediated Fenton-like catalytic oxidation, ultimately converting NH4+ to N2 gas. The 180-day monitoring results demonstrated that the remediation effect continuously increased over time, indicating good long-term stability of the composite. This study provides an efficient, low-cost functional material derived from solid waste for the remediation of manganese residue-contaminated soil and offers a theoretical basis for the synergistic resource utilization of multiple solid wastes.