Addition of water hyacinth and attapulgite during hydrothermal carbonization of sewage sludge: Migration behavior of heavy metals and fuel characteristics of hydrochar

The treatment of sewage sludge (SS) presents challenges in terms of the effective simultaneous immobilization of heavy metals (HMs). This study analyzed the heavy metal migration behavior and fuel characteristics of hydrochar generated by co-hydrothermal carbonization (co-HTC) of SS and water hyacinth (WH) under the addition of attapulgite. The results showed that the co-HTC treatment facilitated the conversion of Cu, Cr, and Ni from the bioavailable fraction to the relatively stable fraction, with little effect on Mn and Pb. At 20% attapulgite of co-HTC, not only did the total concentration of HMs decreased significantly but the bioavailability of all measured metals decreased, reducing the potential ecological risk of the hydrochars. Moreover, the fuel ratio of co-HTC hydrochars was found to be improved (0.03─0.29), and an increase in the higher heating values (3.16─12.44 MJ/kg) compared to hydrochar from SS. The results of TG, FTIR spectra, and kinetic analysis further indicate that co-HTC promotes the aromaticity of the hydrochars and improves the fuel grade. From the perspectives of HMs immobilization and clean fuel production, co-HTC with SS and WH and the addition of attapulgite is an effective SS treatment method.