Insight into the stabilization/solidification of Cd, Pb and Zn in MSWI FA by three stages: Vulcanization, precipitation and bio-oxidation

Abstract

Bio-oxidation has been perceived as a promising treatment for the solidification/stabilization (S/S) of heavy metals (HMs) in mining soils. Nonetheless, no research exists on the integration of the iron-sulfur oxidizing bacterium Acidithiobacillus ferrooxidans as a microbial agent into the treatment of municipal solid waste incineration-fly ash (MSWI-FA). The effect of vulcanization (Stage I), precipitation (Stage II) and bio-oxidation (Stage III) on the stabilization of HMs in MSWI-FA was unexplored. This study investigated the stabilization effects of Stage I and Stage II by utilizing Na2S and FeSO4 to adjust the Fe/S ratio. The findings contributed to identifying suitable methods to enhance the stabilization effect of the microbial agent on MSWI-FA (Stage III). The results indicated that the vulcanization (Stage I) increased the compressive strength of MSWI-FA to 1.87 MPa when the concentration of Na2S reached 0.03 mM. The leaching toxicity of Cd, Pb, and Zn at 28 days after Stage II was 0.12 mg/L, 0.99 mg/L, and 356 mg/L, respectively, which improved by 96.3%, 94.8%, and 26.9% compared to the untreated samples. The compressive strength of the samples peaked at 1.97 MPa when the Fe/S ratio was 0.25 (precipitation (Stage II)). Despite the inhibitory effect of MSWI-FA on microbial activity, the leaching toxicity of Cd, Pb, and Zn in the presence of Acidithiobacillus ferrooxidans decreased to 0.07 mg/L, 0.17 mg/L and 97.3 mg/L at 28 days after Stage III, with no significant further increase in compressive strength. This study suggested that bio-oxidation utilizing iron sources may change the formation of iron oxides, thereby impacting the mechanical and chemical properties of MSWI-FA-based materials. The treatment sequence of vulcanization, precipitation and bio-oxidation proved effective in immobilizing HMs, particularly Pb, Cd, and Zn in MSWI FA. This research highlighted the potential of employing this technique to treat MSWI FA, yet it is essential to identify suitable reaction systems with appropriate additives to further enhance the long-term effectiveness and mechanical properties of MSWI-FA-based materials.