Analysis of the effect and mechanism of heavy metals in stabilized landfill humus soil using fly ash-based materials☆

With the increasing excavation of landfills, the safe disposal of humus soil contaminated with heavy metals has become an urgent environmental challenge. This study developed a novel fly ash-based material (PHB-FA), modified with potassium dihydrogen phosphate, humic acid, and biochar, to enhance the stabilization of heavy metals in landfill humus soil. Compared to unmodified fly ash, PHB-FA significantly improved stabilization efficiency due to its optimized composition and enlarged surface area. At the optimized mass ratio of KH2PO4:HA: BC (4.7:1.0:3.0) and a dosage of 5 %, PHB-FA reduced the leaching toxicity of Cd, Zn, and Pb in prepared humus soil by 87.60 %, 92.36 %, and 99.76 %, respectively. The transformation of heavy metals from exchangeable to more stable residual forms markedly decreased ecological risks. Mechanism analysis revealed that pozzolanic reactions produced stable hydration products (AFt and C-(A)-S-H), while metal phosphate precipitation further minimized heavy metal leaching. Finally, PHB-FA was applied to actual landfill humus soil, effectively reducing leachable heavy metal concentrations to within regulatory limits. These findings demonstrate the superior performance and long-term stability of PHB-FA in heavy metal stabilization. The material shows great potential for large-scale landfill applications, offering environmental benefits and engineering feasibility for the resource utilization and safe disposal of contaminated humus soil.