Hazard reduction of heavy metals by co-pyrolysis of modified vermiculite with paper mill sludge/municipal solid waste: Characterization, risk and reaction mechanism study in pyrolytic environment

The increasing volume of paper mill sludge (PM) and municipal solid waste (MW) poses environmental problems associated with heavy metals. Co-pyrolysis of PM/MW with additives effectively reduces the risk of heavy metal contamination. In this study, co-modified vermiculite (LMV) was prepared using the modification methods of intercalation-exfoliation, Mg2+ impregnation, and thermal activation. PM and MW were mixed at different doping ratios, and a non-doping experiment was performed to compare the pyrolysis processes and the retention rates for different heavy metals (Zn, Cr, Cu, Cd, and Pb), as well as the potential ecological risks, and the form distribution of the heavy metals. The addition of LMV reduced the risk posed by heavy metals. In the absence of doping, a higher pyrolysis temperature is conducive to an increase in the oxidizable and residual fractions of heavy metals without higher retention rates. Among these heavy metals, Zn, Cr, and Cu had higher retention rates and low risks, whereas Cd had a relatively high risk with low retention rates. Simulations showed that there was a transfer of the Mulliken charge after the reaction, indicating that all the heavy metals can interact with O, Al, and Si in the LMV; this reaction mainly occurs around the No. 17 O atom of the LMV. The monomers, oxides, and chlorides of Zn/Cr/Cu exhibited better adsorption on vermiculite compared to that of Pb/Cd.