Effect of synthesized and neoformed ettringite on immobilization of toxic metals in municipal solid waste incineration fly ash = Effect of synthesized and neoformed ettringite on immobilization of toxic metals in municipal solid waste incineration fly ash

다국어 초록 (Multilingual Abstract)

Municipal solid waste incineration (MSWI) fly ash is usually classified as hazardous waste owing to high contents of toxic metals such as Pb. In Japan, chelating treatment is mainly used to immobilize toxic metals. However, unreacted and/or remained chelating reagent in leachate at landfill causes long-tern leachate treatment owing to COD component and nitrification inhibition. Thus, stabilization of landfill site is delayed. Therefore, new metal immobilization method is desirable instead of chelating treatment or in order to reduce the amount of chelating reagent. In this context, this study investigated the effect of mineralogical immobilization, in particular synthesized and neoformed ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O), on toxic metals in MSWI fly ash.
MSWI fly ash sample used in this study was collected from bag filter of a stoker-type MSW incinerator in Japan. The incineration capacity is 80 tons/day. Ettringite was synthesized by mixing 0.02 mol/L Ca(OH)₂ solution and 0.7 mol/L Al₂(SO₄)₃·14-18H₂O solution in sealed plastic bottle in order to avoid CO₂ via atomosphere. The mixing volume of these solutions were theoretically decided according to molar ratio of Ca/Al (= 6) in ettringite. After mixing the solutions by magnetic stirrer for 24 hours, synthesized ettringite was collected by vacum filtration and dried at room temperature. Synthesized ettringite was added to MSWI fly ash sample at different dosage (0%, 5%, 25%, 50%, and 100% of fly ash weight). In the case of neoformed ettringite, powdary Ca(OH)₂ and Al₂(SO₄)₃·14-18H₂O were added to MSWI fly ash. The amount of powdery additives were stoichiometrically decided in order to form the same amount as synthesized ettringite added to MSWI fly ash. Then, these mixtures were subjected to leaching experiments. The mixture added distilled water were shaken at 200 rpm for 6 hours in leaching test bottle. The liquid to solid (fly ash) ratio was 10. After shaking the bottle, the suspension was filtrated through 0.45 μm membrane filter in order to obtain leachate. Leaching concentrations of regulated heavy metals (As, Cd, Cr, Pb, and Se) in leachate were measured by ICP-MS and -OES.
The results showed that only Pb concentration exeeded regulated limit ( > 0.3 mg/L). Concentration of As and Cd were lower than regulated limits (0.3 mg/L and 1.5 mg/L, respectively). Concentrations of Cd and Se were below detection limit in this analysis (0.4 μg/L and 3.0 μg/L, respectively). Lead concentration in leachate is shown in Figure 1. Leaching concentration of Pb slightly decreased until at 25 % dosage (see Figure 1-A). Lead leaching from MSWI fly ash strongly depends on pH value. However, leachate pH at all conditions added synthesized ettringite as well as neoformed ettringite was almost equal to 11.9. Therefore, this result suggests that Pb was immobilized by synthesized ettringite through interaction with negatively charged ettringite surface and/or substitution of Ca²⁺ ion in ettringite structure during leaching experiment. In contrast, leaching concentration of Pb under neoformed ettringite was significantly lower than that under synthesized ettringite (see Figure 1-B). It is considered that Pb was efficiently incorporated into ettringite structure and immobilized by ettringite neoformation during leaching experiment. This means that selectively forming ettringite into MSWI fly ash might have possibility of mineralogical metal immobilization.