Experimental study on the stabilization effect of phosphorus-modified oyster shell on fly ash

Safe disposal of municipal solid waste incineration (MSWI) fly ash, a hazardous waste rich in leachable heavy metals (e.g., Pb, Cd, and Zn), poses severe environmental challenges.

In this study, we fabricated K₂HPO₄-modified waste oyster shell (MOS) as a novel, cost-effective stabilization material and investigated its effectiveness and mechanism for immobilizing heavy metals in MSWI fly ash. The modification process was systematically optimized through single-factor experiments. The optimal conditions were determined to be a K₂HPO₄-to-oyster shell mass ratio of 0.2, a calcination temperature of 400 ℃, and a calcination time of 30 min. Characterization analyses, including scanning electron microscopy, X-ray diffractometry, and Fourier transform infrared spectroscopy, revealed that the modification process successfully transformed the smooth surface of raw oyster shells into a porous “coral-like” structure, substantially increasing the specific surface area. Further, the modification introduced active components such as calcium hydrogen phosphate (CaHPO₄) and portlandite(Ca(OH)₂), thereby enhancing the material’s reactivity.

Stabilization experiments conducted on real MSWI fly ash demonstrated the high efficiency of MOS. With an addition of only 5% MOS, the leaching concentrations of key hazardous metals, Pb and Cd, were drastically reduced to 0.03 and 0.05 mg/L, respectively. These values are well below the stringent regulatory limits for hazardous waste landfill disposal stipulated by the Chinese standard (GB 16889-2008). To elucidate the stabilization mechanism, sequential extraction procedures (BCR) were employed to analyze changes in the chemical speciation of heavy metals before and after treatment. The results indicated that MOS effectively promoted the conversion of heavy metals from unstable, acid-extractable fractions (F4) to the highly stable residual fraction (F5). Notably, the proportion of Pb in the residual fraction increased to more than 85%, significantly reducing its potential mobility and bioavailability. The immobilization mechanisms were identified as a combination of several synergistic processes. (1) Precipitation: Phosphate ions (PO₄^3–) released from MOS reacted with heavy metal ions (e.g., Pb²⁺ and Cd²⁺) to form highly insoluble phosphate precipitates (e.g., Pb₃(PO₄)₂ and Cd₃(PO₄)₂), which are extremely stable in the environment. (2) Ion exchange: Calcium ions (Ca²⁺) from the MOS matrix could be exchanged with heavy metal ions on the surface of fly ash particles or formed compounds. (3) Adsorption and incorporation: The formed hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), a major component after modification, provided ample adsorption sites and could incorporate heavy metal ions into its crystal lattice through isomorphic substitution. Ecological risk assessment, evaluated using the risk assessment code and individual contamination factor, confirmed that MOS treatment substantially reduced the mobility and potential ecological risk of heavy metals in the fly ash.

In conclusion, this study successfully transforms waste oyster shells into a highly efficient stabilization agent for MSWI fly ash. MOS offers a promising, low-cost, and “waste-treating-waste” strategy for the safe disposal of MSWI fly ash, contributing to both waste resource utilization and environmental remediation. The findings provide a theoretical foundation and technical support for developing sustainable and economically stable materials for heavy metal contamination management.