Photo-mechanical synergy in plastic weathering: UV aging accelerates shear-driven nanoparticle release in water

This study examined the shear-driven fragmentation of the UV-aged (UV-A: 340 nm; 0.91 W·m^-2 over 16 weeks; dose = 8.81 × 10³ kJ·m^-2) and unaged polyethylene microspheres (180-212 μm) in water. Scanning electron microscopy revealed that UV aging and shear mixing altered the particle surface morphology, whereas the unaged particles without shear mixing remained intact. The continuous increase in carbonyl (0.15-0.36), vinyl (0.14-0.3), and ester (0.14-0.34) values indicated a progressive photo-oxidation in the microspheres. After shear-mixing of both the UV-aged and pristine groups in the jar-tester (240 rpm; energy density 2.84 KJ/L; shear rate, G_bulk = 6.3 × 10² s^-1) over different time points (30, 60, 90, and 120 min), nanoparticle tracking analysis revealed a one-magnitude higher nanoparticle concentration in UV-aged suspensions than in the pristine ones by 120 min, with mean concentrations of 2.7 × 10⁷ and 4.5 × 10⁶ particles mL^-1, respectively. The nanoparticle release rate was six times higher in UV-aged (3556 particles·s^-1) than in the pristine particles (576 particles·s^-1). Nanoparticle mean size decreased to 62.7 nm from 128 nm, and to 72.3 nm from 122.6 nm for UV-aged and pristine particles, respectively. First-order exponential fits (R² = 0.956-0.985) for particle count growth and size decay revealed that UV-aged particles fragmented 1.2 times faster, resulting in shorter doubling times and half-lives. The nanoparticle counts increased while their size decreased exponentially. UV-aging reduced the negative surface charge, while shear mixing exhibited a neutralizing effect on the particles. Shear mixing alone released nanoparticles, while pre-UV-treatment amplified the effect. This study provided a shear-kinetic framework that serves as a baseline for establishing photo-mechanical synergy in plastic degradation.