Study on the effects of catalysts on the immobilization efficiency and mechanism of heavy metals during the microwave pyrolysis of sludge

In order to enhance the immobilization of heavy metals in the bio-char during microwave pyrolysis, the immobilization efficiency and mechanism of heavy metals in the microwave pyrolysis of sludge with different alkaline catalysts were explored. Results showed that the leaching concentrations of heavy metals reduced greatly after pyrolysis, which were lower when catalyzed by CaO than those of Fe2O3. CaO was more favorable for the immobilization of Cr, Cu, Zn, Pb and Ni while Fe2O3 was more favorable for Cd. Different species distributions of heavy metals in the bio-char affected the leaching concentrations. Adding catalyst could significantly reduce the ecological risks of heavy metals in the bio-char, and CaO (RI = 15.17-20.43) had a better performance than Fe2O3 (RI = 16.88-21.79). When catalyzed by CaO, the formation of pores and co-crystal compounds in the bio-char determined the immobilization efficiencies of heavy metals.