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.