Effects of chemical pretreatment on natural fibers removal and microplastics integrity for wastewater characterization

Nine digestion protocols were tested to quantify microplastics in wastewater using nine polymeric and three natural fiber controls representative of common microplastics in wastewater. Protocols were also evaluated for their impact on natural fibers, which can interfere with microplastic quantification. Control size change and visual integrity were assessed, revealing that a sequential 24-h treatment with 6% NaClO at room temperature (RT) followed by 24 h with 30% H₂O₂ at 40 °C preserved polymer integrity while fully oxidizing natural fibers, even when preincubated in real wastewater samples. A Fourier-transform infrared spectroscopy (FTIR) validation using the carbonyl index (CI) and carbon–oxygen index (COI) showed significant changes in poly(ethylene terephthalate) (PET) and polyvinyl chloride (PVC) after digestion but did not compromise FTIR spectrum recognition. The protocol applied to raw wastewater samples showed optimal performance at 300 mg Cl₂/L, achieving up to 95% Chemical Oxygen Demand (COD) and 92% turbidity reduction. No further improvements in COD or turbidity removal were observed beyond this dose, regardless of initial COD levels. The present approach affords greater comparability with existing studies thanks to a large range of polymeric, natural controls, and oxidant dose investigations regarding common water quality parameters.