Microplastic fragmentation into nanoplastics by water shear forces during wastewater treatment: Mechanical insights and theoretical analysis

Nanoplastics (NPs) are generated from the fragmentation of microplastics (MPs) through mechanical forces such as mixing, sonication and homogenization in wastewater treatment plants (WWTPs). Despite their environmental significance, the formation mechanisms and size distribution of NPs in WWTPs are not well understood. This study presents an in-depth investigation into the fragmentation mechanisms of polyethylene (PE) and polystyrene (PS) MPs, sized 250 μm and 106 μm, under simulated WWTP conditions. Our findings demonstrate that under water shear forces ranging from 32 to 100 kJ/L weathered PS and PE particles were further disintegrated into nano-sized particles. Nanoparticle tracking analysis results revealed a significant increase in NP numbers from 8.34 × 10⁸ to 1.54 × 101⁰ NPs/mL as the water shear force increased from 32 to 100 kJ/L. Notably, the smallest NP, measuring 54.2 nm, was produced from 106 μm PS particles at 100 kJ/L. Scanning electron microscope images confirmed micro-cracks on the particle surfaces as the dominant fragmentation mechanism. A robust correlation between experimental NP sizes and theoretical predictions underscores the continuous production of NPs during water treatment processes. These results offer groundbreaking insights into the transformation of MPs within WWTPs and underscore the urgent need for effective strategies to mitigate NP pollution.