Stabilization Effect of Combined Stabilizing Agent on Heavy Metals in Hazardous Waste Incineration Fly Ash and Effect on Solidification Volume

Based on the need for safe disposal of hazardous waste incineration fly ash (HFA), this study evaluated the solidification/stabilization (S/S) performance of silicate cement, sodium dihydrogen phosphate (NaH2PO4), and sodium dimethyl dithiocarbamate (SDD) used individually and in combination. The raw HFA failed the leaching test for Pb, Zn, Cd, and Ni, with their concentrations exceeding the GB16889-2024 limits by factors of 3.1, 2.45, 1.67, and 1.1, respectively. While cement (150% dosage) effectively immobilized Pb, and Cd with >90% leaching reduction, it resulted in significant volume expansion (2.7-fold). NaH2PO4 excelled in Pb stabilization (100% efficiency at 20% dosage) via insoluble phosphate formation but required high doses. SDD effectively chelated Zn (63.4% efficiency at 5% dosage) but was less effective for Pb and costly. A synergistic combination of 5% cement, 15% SDD, and 10% NaH2PO4 was identified as the possible optimal formulation, successfully immobilizing all heavy metals within regulatory limits. This combined approach minimized dosage, controlled volume expansion ratio (RVE) (~1.31), and reduced cost. The low initial dioxin content (7.6 ng TEQ/kg) was unaffected by S/S treatments and remained compliant. Mechanistic analyses (XRD, FTIR, SEM-EDS) confirmed the formation of C-S-H gels from cement, insoluble phosphates from NaH2PO4, and metal chelates from SDD, collectively transforming the HFA into a compact, low-porosity matrix conducive to safe disposal. This stabilization and solidification strategy not only achieves the safe disposal of hazardous waste incineration fly ash but also contributes to the goals of sustainable waste management by reducing the environmental footprint of treatment processes and minimizing the final disposal volume.