Abstract: With extensive use of plastic products, lots of fibrous microplastics have been discharged and accumulated in sewage sludge, posing a great challenge for sludge pyrolysis treatment. Moreover, high concentration of fibrous microplastics may alter the sludge pyrolysis behavior and sludge biochar structure, impacting the subsequent reuse of sludge. Therefore, it is urgent to understand the influence of fibrous microplastics on sludge pyrolysis behavior as well as sludge biochar structure. In this study, polyethylene terephthalate (PET, ≤0.15 mm), the typical fibrous microplastic, had been added into sludge with various mass ratios (0, 15% and 30%), and then the mixture was pyrolyzed at different temperatures (300-700 ℃). Results showed that the synergistic interaction of PET and sludge reduced yield of biochar at lower pyrolysis temperatures, while promoted yield of sludge at higher pyrolysis temperatures. Meanwhile, high abundance of PET in sludge could enhance the fraction of larger fused aromatic ring systems and increase degree of structural arrangement in biochar. In addition, for the biochar obtained at lower pyrolysis temperatures, the carbon retention ratio and long-term stability were improved by PET. However, for the biochar pyrolyzed at higher temperatures, high abundance of PET could reduce the carbon retention ratio and long-term stability.

Key words: sludge pyrolysis, sludge biochar, fibrous microplastics, polyethylene terephthalate (PET), carbon structure