Effects of pre-treatment on characterization of microplastics in biosolids via optical photothermal infrared spectroscopy

Microplastics (MPs) in biosolids used as soil amendments are of growing concern. The purpose of this study was to improve the characterization of MPs in complex biosolid matrices by optimizing sample preparation for morphological and chemical analyses with different spectroscopic techniques. We compared extraction procedures involving Fenton oxidation (F), Fenton plus sodium dodecyl sulfate (SDS), and Fenton plus cellulase (FE). We performed partial particle sample counting along with a helical shape, corresponding to 56 % of sample area, and total particle counting. Chemical characterization was performed using sub-micron optical-photothermal infrared (O-PTIR) spectroscopy, and the results were compared with those obtained via commonly employed Raman and Fourier transform infrared absorption microspectroscopy technique (µ-FTIR). Our FE protocol yielded a slightly higher total sample mass removal (97 %±0.3 %) compared to other pre-treatment methods. No significant difference was observed in the total MPs count between the two approaches, indicating a homogeneous distribution across the filter and supporting reliable quantification using only half the filter in the helical method. O-PTIR's high spatial resolution (down to 0.5 µm) and absence of spectral artefacts compared to Raman and µ-FTIR enabled accurate identification of fine fibers (2 µm wide) and small particles (∼5 µm). Single-frequency O-PTIR imaging revealed well-defined particles clearly separated from their surroundings, highlighting the technique's potential for particle identification. The findings highlight the need to combine effective sample pre-treatment with high-resolution chemical analysis to improve understanding of plastic fate in the environment and supporting future policy development or regulatory updates on plastic content in biosolids.