Effect of Ca–Si–Al Element Proportion on the Formation of Aluminosilicate Minerals and Detoxification of PAHs in Fly Ash from MSW Incineration during the Hydrothermal Process

Abstract

Fly ash (FA) produced during municipal solid waste incineration contains hazardous polycyclic aromatic hydrocarbons (PAHs). Hydrothermal treatment (HT), a promising technology for the degradation of organic compounds, was applied in this study for hydrothermal detoxification of PAHs in FA. Due to different levels of Ca/(Si + Al) and Al/(Si + Al) molar ratios achieved by adjusting the SiO2 and Al2O3 added to FA, different kinds of aluminosilicate minerals were formed and different detoxification efficiencies of PAHs were observed in HT products. For aluminosilicate minerals formed at lower (0.17) and moderate (0.51) Al/(Si + Al) ratio levels, tobermorite was obtained and it enhanced with the decrease of Ca/(Si + Al) ratio levels. At higher Al/(Al + Si) ratio levels of 0.71, tobermorite cannot be synthesized completely even when the Ca/(Si + Al) ratio for FA-30-3 (1.01) was low and in the suitable range for tobermorite formation (0.67–1.20). Compared with the Ca/(Si + Al) ratio, the Al/(Si + Al) ratio not only affected the synthesis of tobermorite but also had much more significant influence on both the removal percentage of the concentration of PAHs and the removal percentage of the toxicity equivalent quality (TEQ) of PAHs. Attributing to the fact that tobermorite cannot be synthesized at higher Al/(Al + Si) ratio levels of 0.71, the weakest detoxification efficiency of PAHs was observed. When tobermorite was effectively synthesized in FA-30-1, for which both the Ca/(Al + Si) and Al/(Al + Si) ratios were in the suitable range for tobermorite formation [Ca/(Al + Si) ratio = 0.67–1.20 and Al/(Al + Si) ratio = 0–0.22], the best detoxification efficiency of PAHs was achieved. The formation of tobermorite affected by the Ca–Si–Al element proportion during the HT process has a positive effect on the detoxification of PAHs in FA. The results of regression analysis performed for investigating the relationship between the content of minerals formed and the detoxification efficiency of PAHs showed that the formation of tobermorite is helpful to promote the detoxification of PAHs but the existence of hydrocalumite reduced the detoxification. Compared with the Ca/(Si + Al) ratio, the Al/(Si + Al) ratio has more significant influence on the formation of tobermorite and hydrocalumite.

Road Pavement Thickness and Construction Depth Optimization Using Treated and Untreated Artificially-Synthesized Expansive Road Subgrade Materials with Varying Plasticity Index

“…and it is responsible for the detoxification and strength gain in a mix. According to investigations conducted by [63], the relationship between the content of minerals formed in a mix and their detoxification efficiency shows that the formation of tobermorite helps to promote detoxification in a mix. This means the presence of toxic elements found in a mix due to the addition of binders (especially waste materials or industrial by-products, which can be very toxic due to leaching) can be detoxified due to the formation of a high amount of tobermorite (C-S-H and C-A-H gel) in the mix.…”

Advances and Outlook of Heavy Metal Treatment Technology from Municipal Solid Waste Incineration Fly Ash: A Review

“…Therefore, they are collectively referred to as zeolite minerals or zeolite media. 68,203,205,206 Conversely, the traditional hydrothermal treatment method may have drawbacks, such as a sluggish reaction rate and low energy conversion efficiency, when producing zeolite-like crystals from fly ash. 203,207,208 Jin et al 209 determined that after conventional hydrothermal treatment, the zeolite-like minerals generated stabilized heavy metals by precipitation, physical encapsulation, ionic adsorption, and ion exchange, leading to a stabilization efficiency of more than 95% for heavy metals.…”

“…Recently, hydrothermal treatment (HT) has become an effective technology because of its ability to utilize aluminum or silicon sources in fly ash at lower temperatures (150–200 °C) or by adding external aluminum and silicon; fly ash is converted to zeolites, katazite, solids, and other materials while immobilizing hazardous heavy metals in the final product. − These silicate minerals, which are like zeolite, have remarkable adsorption capacity and cation exchange capacity (CEC), are thermally stable, and can effectively stabilize heavy metals in hydrothermal processes. Therefore, they are collectively referred to as zeolite minerals or zeolite media. ,,, Conversely, the traditional hydrothermal treatment method may have drawbacks, such as a sluggish reaction rate and low energy conversion efficiency, when producing zeolite-like crystals from fly ash. ,, Jin et al determined that after conventional hydrothermal treatment, the zeolite-like minerals generated stabilized heavy metals by precipitation, physical encapsulation, ionic adsorption, and ion exchange, leading to a stabilization efficiency of more than 95% for heavy metals. Shi et al indicated that the leaching toxicity of Cr, Zn, Cd and Cu was greatly reduced by the hydrothermal application of MSWI FA, which was below the values stipulated in the Toxicity Characteristic Leaching Procedure (TCLP), which indicated that the hydrothermal application of MSWI FA had a beneficial effect on the toxicity reduction of MSWI FA.…”