Photocatalytic conversion of polylactic acid microplastics using CoO/g-C3N4 composite with improved formation of low-molecular-weight oxygenates

CoO/g-C 3 N 4 shows marginal improvement in PLA microplastic degradation. • CoO modification narrows the distribution of low-molecular-weight oxygenated products. • A lactate/lactic-acid-type product is tentatively identified as a key intermediate. • O 2 • ˉ is confirmed as the dominant reactive species during photocatalysis. Polylactic acid (PLA) microplastic endures in the environment despite the biodegradable nature of PLA, requiring effective methods for conversion under mild conditions. In this work, a CoO/g-C 3 N 4 composite was synthesized by a wet impregnation-calcination method and evaluated for UV-driven photocatalytic conversion of PLA microplastics. A loading-screening study identified the nominal 0.53 wt% CoO/g-C 3 N 4 composite as the best-performing composition among the tested loadings, and ICP analysis confirmed an elemental Co content of 0.4170 wt%, close to the theoretical value of 0.4168 wt%. Replicated photocatalytic tests showed apparent conversions of 14.50 ± 0.06%, 56.07 ± 0.72%, 65.46 ± 0.27%, and 69.68 ± 0.65%, after 100 min for UV only, CoO, g-C 3 N 4 , and CoO/g-C 3 N 4 , respectively. The corresponding apparent pseudo-first-order rate constant for CoO/g-C 3 N 4 was 0.01153 ± 0.00021 min −1 . The recyclability experiment showed that the catalyst retained 92.02% of its initial apparent conversion after four cycles, and ICP-MS analysis of the final-cycle filtrate showed a trace Co concentration of 0.083 μg/L. Radical-trapping and electron paramagnetic resonance analysis suggested that O 2 • ˉ was the dominant reactive species, whereas • OH and h + provided secondary contributions. Product analysis by total organic carbon, high-performance liquid chromatography, liquid chromatography-mass spectrometry, and pyrolysis-gas chromatography/mass spectrometry suggested that CoO/g-C 3 N 4 composite promoted a narrower distribution of low-molecular-weight oxygenated products compared with pristine g-C 3 N 4 . Additional LC/MS analysis using an authentic lactic acid standard and a lactic acid-spiked PLA sample supported the assignment of the m / z 89.02 product signal to lactic-acid/lactate-type species. These results indicate that CoO modification influences both the apparent conversion and the product distribution during UV-driven photocatalytic PLA transformation. Download: Download high-res image (156KB) Download: Download full-size image