Effect of mineral additives on pyrolytic characteristics and heavy metal behavior during co-pyrolysis of industrial sludge and hyperaccumulator plant

In this study, the pyrolysis of industrial sludge (IS), Sedum plumbizincicola L. (SP) and their mixtures were carried out in a thermogravimetric analyzer and fixed bed reactor. The effects of blending ratio, temperature and mineral additives (metakaolin, diatomite, and bentonite) on the pyrolysis characteristics and heavy metal immobilization were investigated. The results showed a positive synergistic effect during the co-pyrolysis process of IS and SP. The addition of mineral additives reduced the activation energy and reaction termination temperature and shortened the reaction process. The changes in Gibbs free energy, enthalpy and entropy of them were evaluated by thermodynamics, and the results showed that pyrolysis is a non-spontaneous process and the mineral additives enhance the pyrolysis reactivity. Besides, the mineral additives facilitated the conversion of heavy metals (Pb, Zn and Cd) from unstable states (F1 and F2) to more stable states (F3 and F4) in biochar, which effectively reduced their leaching toxicity. In conclusion, the best synergy can be obtained by pyrolysis of IS and SP at 550 °C with a mass ratio of 3:1, and diatomite is the best heavy metal adsorbent.