Sensitive quantification and morphological analysis of microfibers in laundry wastewater: Standardization and validation of a fluorescence-based method

Microplastics and microfibers are widespread environmental pollutants, with synthetic microfibers comprising 40–90 % of microplastics in aquatic systems. Accurate quantification is essential for assessing and mitigating their impact, but conventional methods face several challenges, including high costs, labor-intensive procedures, and limited accessibility. This study standardizes and validates a cost- and time-effective fluorescence-based method for the sensitive detection and automated quantification of polyester-based microfibers. Exploiting the fluorescence properties of polyester fibers under UV light, the method enables direct visualization and automated quantification of the fibers from images obtained using a simple camera setup through open-source software. The method was developed using synthetic, fluorescent microfiber suspensions with concentrations ranging from 1 µg/L to 125 µg/L and processed through glass-fiber filters. This procedure yielded a linear calibration curve (R² = 0.987) between the automatically computed fluorescent area on the filter surface and the mass of filtered microfibers weighted on an analytical balance. The limit of detection (LOD) and quantification (LOQ) were estimated to be 1 µg and 2.5 µg, respectively, demonstrating high sensitivity. Validation with washing machine wastewater samples showed excellent accuracy, with computed concentrations through fluorescence aligning closely with known suspension values. Additionally, the method quantified total fiber length and number, showing a strong correlation between fiber length and area. Morphological analysis revealed shorter fiber lengths in wash water samples compared to synthetic fibers with an average of 1.13 mm and 2.31 mm, respectively, likely due to fragmentation during laundering. Furthermore, washing machine wastewater samples consistently had higher fiber counts, indicating increased fragmentation. These findings highlight the method's robustness, adaptability, and potential for broad applications, allowing for both quantitative and morphological microfiber analysis in environmental and industrial settings, while also contributing to the development of future reference standards.