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Pyrolysis behavior of sewage sludge coexisted with microplastics: Kinetics, mechanism, and product characteristics
Summary
Researchers investigated the co-pyrolysis behavior of sewage sludge mixed with polyethylene and polylactic acid microplastics. They found that the presence of microplastics improved the overall pyrolysis performance and altered the composition of the resulting bio-oil and gas products. The study suggests that understanding how microplastics in sewage sludge affect thermal treatment could help optimize waste processing at wastewater treatment plants.
Microplastics can accumulate in the excess sludge from wastewater treatment plants through domestic wastewater. This study investigated the co-pyrolysis behavior of sewage sludge coexisting with two types of microplastics (polyethylene (PE) and polylactic acid (PLA)) and found a superior comprehensive pyrolysis performance. By calculating the difference between theoretical and experimental weight loss during the pyrolysis process, it was found that the incorporation of microplastics PE and PLA created a synergistic effect at 270°C-450 °C, which was confirmed through the Malek method analysis from a pyrolysis mechanism perspective that it could increase the random nuclei on each particle, that is, enhance the heterogeneous diffusion of volatiles. The average activation energy was reduced by 84.99 kJ/mol, as determined using three isoconversional methods: Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), and Starink. Regarding the products, the aforementioned synergistic effect led to a reduction in char retention and larger specific surface area of the biochar, while the quantities of gaseous products and bio-oil escalated. Through a thermogravimetric analyzer and Fourier transform infrared spectroscopy (TG-FTIR), an increase in aromatic hydrocarbons, alkanes, aldehydes, ethers, and esters in the gaseous products were detected. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) revealed an increase in hydrocarbons, esters, and alcohols in the bio-oil, and acids and aldehydes decreased, overall enhancing the quality of the bio-oil. This study elucidated that pyrolysis completely transformed microplastics in sludge, thus eliminating environmental risks and provided a theoretical reference for understanding the pyrolysis behavior of sludge containing microplastics.