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 × 10 3 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 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-oxidative embrittlement. 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 × 10 2 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. 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 ( R 2 = 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 and, consequently, smaller sizes than pristine particles. 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. • UV aging modified plastic surface susceptible to mechanical fragmentation. • Shear-mixing alone released nanoparticles while pre-UV-aging amplified the counts. • NTA found one-magnitude higher concentrations in UV-aged vs pristine samples. • The nanoparticle counts increased while their size decreased exponentially.
Kabir et al. (Wed,) studied this question.