Degradation of microplastics during wet air oxidation treatment

Microplastics (MPs) are emerging contaminants in ecology and environment. Most of MPs in sewage are finally enriched in sludge during biological wastewater treatment, and they could be released to soil or natural water bodies along with land use of sludge. It is a key step to cut off the transfer of microplastics from human activities to nature, especially via sewage treatment and discharge. Wet air oxidation (WAO), a promising sludge treatment technology, should have a potential to degrade MPs during sludge treatment since it can destroy complex and resistant organic substances. In this study, this guess was tested, and the degradation of three common MPs, i.e., polyethylene (PE), polystyrene (PS), and polyethylene terephthalate (PET) in WAO was investigated through a series of WAO experiments. The results showed that cracks, holes, and oxygen-containing groups occurred in the surface of MPs, and the particles were rapidly dissolved within 15 min at 240 degrees C and oxygen pressure of 2 MPa. Hydroxyl radicals ((OH)-O-center dot) generated in WAO played a key role, which was contributed to depolymerize MPs into alkyl compounds (alkane, aceton, etc.), and alkyl radicals and further degrade into low molecular aldehydes and ketones, finally into short chain carboxylic acids, even CO2 and H2O. After 120 min WAO treatment, 43.6% of PEMPs, 68.2% of PSMPs, and 0.9% of PETMPs were mineralized to CO2, respectively, and only 23.8% of PEMPs, 5.8% of PSMPs, and 77.3% of PETMPs remained in residues. These findings confirmed the advantage and potential of WAO in controlling the release of MPs sludge to environment.