Waste medical mask-derived carbon quantum dots enhance the photocatalytic degradation of polyethylene terephthalate (PET) over BiOBr/g-C3N4 S-scheme heterojunction

The coronavirus disease 2019 (COVID-19) pandemic has increased the necessity of medical masks, and to date, many waste masks have been discarded without being reprocessed, causing environmental harm. PET, a commonly used plastic product, presents certain hurdles to its natural degradation. In this work, waste medical masks were converted into carbon quantum dots (MCQDs) with blue fluorescence emissions using a simple solvothermal process and then doped into BiOBr/g-C 3 N 4 composite material to construct S-scheme heterojunctions for PET degradation. Density functional theory (DFT) calculations revealed that an interfacial electric field (IEF) was formed between g-C 3 N 4 and BiOBr. The findings demonstrate that the MCQDs, as a cocatalyst for electron transmission and storage, encourage S-scheme heterojunctions to further separate photogenerated electrons and holes. Levofloxacin (LEV) was used as a molecular probe to visually compare the catalytic activities of various catalysts. These catalysts with different photocatalytic activity were then used to degrade PET. The findings demonstrate that the degradation efficiency of PET over the BiOBr/g-C 3 N 4 /3MCQDs in seawater is 39.88 ± 1.04 % (weight loss), which is 1.37 times higher than that of BiOBr/g-C 3 N 4 , and also better than those reported in most of the literature. Free radical capture tests, electrostatic field orbital trap high-resolution gas chromatography-mass spectrometry (HRGC-MS), and ultra-performance liquid chromatography-mass spectrometry (UPLC-MS) experiments uncovered and briefly revealed the key products in the photocatalytic degradation of PET, as well as the relevant mechanism of photocatalytic degradation of PET. The degradation products are expected to become precursors for the further production of polymers and medicines, etc . This study offers fresh perspectives for the creation of innovative photocatalysts for the ecologically benign breakdown of PET, which helps to further lessen environmental damage caused by microplastics (MPs) and enhance resource sustainability. Carbon quantum dots (MCQDs) derived from the waste medical masks were used to modify BiOBr/g-C 3 N 4 to improve the photocatalytic efficiency of the resulted samples (BiOBr/g-C 3 N 4 /MCQDs), and the optimized BiOBr/g-C 3 N 4 /3MCQDs achieved the photocatalytic degradation of PET MPs with a weight loss of 39.88 ± 1.04 %, outperforming most reported photocatalysts for PET degradation. • Waste medical mask-derived carbon quantum dots (MCQDs) were prepared by a simple, non-toxic and low-cost process. • MCQDs act as an electron sink and cocatalyst, promoting BiOBr/g-C 3 N 4 /3MCQDs S-scheme heterojunctions light-harvesting and enhancing e − /h + separation. • The spectral absorption of MCQDs compensates for the absorption edge and intensity of catalyst and enhances the photocatalytic degradation of PET. • The BiOBr/g-C 3 N 4 /3MCQDs photocatalytic system achieved a PET degradation of 39.88 ± 1.04 % within 6 h in seawater circumstance.