Optimization of heavy metal removal and salt recovery from MSWI fly ash through pre-washing combined with hydrothermal treatment

Abstract

The environmental risks posed by heavy metals in municipal solid waste incineration fly ash are a critical concern, requiring effective safety training and disposal strategies. This study examined the combination of pre-washing with hydrothermal treatment to remove heavy metals and recover salts from fly ash. The samples were analyzed using XRF, XRD, SEM, BET, and ICP-MS to assess the effect of combined treatment on heavy-metal stabilization. Hydrothermal conditions were varied in terms of temperature, liquid-to-solid (L/S) ratio, duration, and additives (H₂O₂ and HCl) to enhance metal immobilization and reduce the leachability of As, Pb, Cr, Cd, Ni, and Zn. Pre-washing removed up to 85 % of salts, reducing dissolved solids. After hydrothermal treatment, inorganic reagents Na₂S and Na₃PO₄ were used to separate heavy metals from the highly toxic, high-salt wastewater phase. The experimental results demonstrate that pre-washing combined with hydrothermal treatment (210 °C, 15min, 5:1 L/S ratio, and 2 mol/L HCl) is suitable for leaching heavy metals from fly ash by forming zeolites that stabilize them. Na₂S and HCl treatments optimized salt recovery, indicated by increased sodium salt extraction. Multivariate analyses, including correlation heatmaps, PCA, and cluster analysis, were performed after hydrothermal treatment to explore relationships among heavy metals. Overall, this combined process offers a promising approach for removing heavy metals, enhancing resource recovery, and reducing environmental risks associated with fly ash disposal.