Comparative study on intercalation-exfoliation and thermal activation modified kaolin for heavy metals immobilization during high-organic solid waste pyrolysis

With the new municipal solid waste classification policy implemented in China, attention on achieving the waste-to-energy disposal of dry waste has been growing. Pyrolysis conversion of organic waste into value-added chemicals is a promising method to treat solid waste. However, after removing the non-combustible components of dry waste, the obtained high-organic solid waste (HSW) contains various heavy metals, which requires urgent attention during thermochemical conversion. To mitigate heavy metals risk, kaolin was employed as additive during HSW pyrolysis, and intercalation-exfoliation and thermal activation modifications were performed on the kaolin to further immobilize and stabilize heavy metals in the derived chars. The characterization results illustrated that the interlayer spacing, pore volume and diameter of kaolin were expanded after intercalation-exfoliation modification, providing more opportunities for the adsorption of metals. The thermal activation method favored the transformation of kaolin into metakaolin via dehydroxylation to enhance its nonhexacoordinated Al proportion and chemisorption. During 450-650 degrees C, kaolin exhibited an effective solid enrichment performance for targeting heavy metals, and the intercalation-exfoliation and thermal activation modification further enhanced the adsorption capacity of the kaolin for Cd, Cr, Pb and Cr, Cu, Pb, Zn, respectively. Compared with Cu and Zn, additives demonstrated better stabilization effects for Cd, Pb, and Cr, transforming more bioavailable fractions to the residual speciation. Overall, a higher pyrolytic temperature (650 degrees C) and the addition of effective additives could simultaneously increase the residual fraction and decrease the bioavailable fraction of heavy metals in HSW-derived chars, reducing the potential ecological risk.

Automated summary

With the implementation of China's new municipal solid waste classification policy, attention has been focused on achieving waste-to-energy disposal of dry waste. Pyrolysis conversion of high-organic solid waste is a promising method for treating solid waste. The addition of kaolin as an additive during pyrolysis can effectively immobilize and stabilize heavy metals, with effective solid enrichment performance observed between 450-650 degrees Celsius.