Efficient removal of toxic organics and reduction of Cr(VI) to Cr(III) from tannery sludge: A comparative study of microwave pyrolysis and conventional pyrolysis

•Microwave pyrolysis is superior to conventional methods in treating tannery sludge.•Microwave pyrolysis facilitates the reduction of Cr(VI) to Cr(III).•Microwave pyrolysis efficiently removes toxic substances from tannery sludge.•Microwave pyrolysis effectively fixes heavy metals in the residues. The efficient and safe treatment of tannery sludge (TS) are causing significant concern. Pyrolysis stands as a pivotal technology in the realm of sludge treatment, offering the benefits of degrading organic matters within TS while simultaneously transforming the sludge into energy and other valuable substances. In order to develop an effective pyrolysis method with good resource recovery performance, this study employs microwave pyrolysis (MP) and conventional pyrolysis (CP) at temperatures ranging from 300 to 900 °C to investigate the transformation of chromium morphology. The results indicate that TS experienced a weight loss of 43 % during the pyrolysis process, with a combined average activation energy measured at 169.58 kJ/mol. MP, due to its more uniform and efficient heating, resulted in pronounced solid degradation, leading to increased pyrolysis gases. This enhanced the removal of organics at lower temperatures and contributed to the ring opening of aromatic hydrocarbons in the high-temperature segment. MP effectively reduced Cr(VI) to Cr(III) in TS, whereas CP showed relatively ineffective reduction of Cr(VI). Furthermore, metal compounds in TS treated by both MP and CP predominantly existed in the residue as FeCr2O4 and Cr2O3 at high temperatures. This suggests that more Cr(III) was immobilized in the pyrolysis residue. This study revalues the significance of MP as a promising treatment for TS, holding the potential for resource utilization.