Effect of nano silica on solidification/stabilization of heavy metal in alkali-activated MSWI fly ash solidified body

Abstract

Municipal solid waste incineration fly ash (MSWIFA) containing considerable heavy metals is classified as a hazardous waste. Alkali-activated technology can effectively improve the environmental safety of MSWIFA. Due to the low silica content of MSWIFA, the addition of nano silica (NS) solves the problem to enhance the ability of solidification/stabilization (S/S) and maximize the utilization of MSWIFA. NS/MSWIFA solidified body was prepared by alkali-activated technology in this work. The mechanical and environmental properties of NS/MSWIFA solidified body with various dosages (0–3 wt%). NS were characterized by compressive strength, leaching concentration, and chemical speciation distribution of heavy metals. A comparative study of MSWIFA and solidified bodies for exploring S/S mechanisms was investigated using XRD, FTIR, TG/DTG, NMR, and SEM/EDS tests. The results showed that the leaching concentration of Pb in MSWIFA based on different leaching methods was 23.54 mg/L and 31.7 mg/L, 19.6 times higher than the standard of hazardous waste landfill and 126.8 times higher than the standard of landfill site of municipal solid waste, respectively. The total amount of Zn and Pb was 4739 mg/kg and 2451 mg/kg, respectively, exceeding other heavy metals by 1–2 orders of magnitude. When the addition of NS was 2 wt% and 3 wt%, the compressive strength and the leaching concentration of heavy metals in solidified bodies reached the highest and the lowest, respectively. Comparative analysis indicated that adding NS promoted heavy metals in MSWIFA to be converted into a stable state, and increased hydration product C-(A)-S-H gels formation, making the structure denser. On a broader perspective, NS/MSWIFA solidified body was developed as a low-carbon and environment-friendly binder.