Impact of cellulose digestion on the accuracy and reproducibility of microplastic and synthetic microfiber quantification

Microfibers are widespread microplastics, but their quantification is hindered by their similarity to natural and regenerated cellulose fibers. The ASTM D8333 protocol incorporates a cellulose digestion step using Schweizer's reagent, though its efficiency and selectivity remain uncertain. The objectives of this study were to assess cellulose removal efficiency, polymer integrity (mass change and spectral preservation), and classification accuracy following cellulose digestion when performed after peroxide oxidation. Ten representative materials, including plastics, polymer-cellulose textile blends, municipal wastewater influent samples, and cellulose-based controls, were treated with peroxide oxidation followed by digestion in Schweizer's reagent for up to 48 h at room temperature. Mass loss, fiber counts normalized to mass removed, and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra were compared with manufacturer-reported cellulose content to assess removal and over-digestion. Regenerated cellulose fabrics (e.g., viscose) exhibited near-complete removal within 1 h, cotton-rich blends required up to 24 h, and some polymer–cellulose blends showed non-target polymer loss after 48 h. Synthetic textiles remained stable, though polyester and polyurethane shed fibers. Spectral classification via OpenSpecy remained robust (>80% correct) for most polymers. The polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT) bioplastic blend was frequently misclassified as polyester terephthalate (PET), with implications for source and fate assessments. Prolonged reagent exposure occasionally produced copper-rich residues, potentially interfering with analysis; filtration and light-protected storage are recommended. In wastewater influent, 47–57% mass loss occurred after 24 h, yet FTIR spectra still indicated cellulose-based materials, underscoring the resistance of non-plastic organics. These findings provide guidance on optimizing cellulose digestion conditions to balance removal efficiency with polymer preservation, enhancing microfiber quantification in environmental studies.