Co-pyrolysis of industrial sludge with phytoremediation residue: Improving immobilization of heavy metals at high temperature

Abstract

Phytoremediation as an attractive technology for the degradation or removal of pollutants from the environment inevitably produces large amounts of heavy metal-accumulated plants. Although pyrolysis is effective in treating the generated biomass, the problem of considerable emissions of heavy metals into the atmosphere has not received deserved attention. In this study, industrial sludge (IS) was attempted as an additive to improve the immobilization performance of pyrolysis for heavy metals at high temperature. Based on the separate pyrolysis and co-pyrolysis experiments of IS and the phytoremediation residue (PR) rich in Cd, Pb and Zn at 350–750 °C, the results showed that, compare to separate pyrolysis, introducing IS to PR pyrolysis system could reduce the emission of heavy metals while enhancing their stability in resultant biochar. It was also found that the complexation of heavy metals with functional groups, the combination of heavy metals with π electrons and the formation of heavy metal crystalline compounds could be the main reasons improving the immobilization of heavy metals during co-pyrolysis. Correspondingly, the heavy metal ecological risk evaluation indicated that the biochar obtained from feedstocks containing IS was at no risk or low risk while other biochar possessed a higher risk. This study demonstrated that IS additive was able to optimize the migration and transformation behavior of heavy metals in hyperaccumulator biomass during pyrolysis, which provides new insights into the environmentally sound treatment of heavy metal-rich biomass.