Ubiquitous photoaging of microplastics: Potential challenges to air flotation

The effect of photoaging on microplastic removal by air flotation and its potential mechanisms are unclear and deserve further investigation. Five common microplastics in nature (PP, PE, PET, PVC, and PS) were subjected to simulated photoaging, and the experimental data revealed that the air flotation efficiency of microplastics gradually decreased with the increase of the degree of light aging. Although photoaging has adverse effects, the densities of PP and PE are lower than water. Their air flotation efficiencies decreased only marginally by 2.3 % and 5.3 % respectively compared to their non-photoaged counterparts. In contrast, the air flotation removal rates of PS, PVC, and PET decreased to 25.7 %, 13.7 %, and 7.3 %, respectively, after 72 h of light exposure. Mechanistic analysis via SEM, FTIR, zeta potential, and contact angle measurements revealed increased fragmentation and disintegration of the five microplastics with increased photoaging severity, alongside significant physicochemical changes: reduced zeta potential, enhanced electronegativity, increased oxygen-containing functional groups, and lowered surface contact angle. Overall, the surface hydrophilicity of the five microplastics increased significantly, which led to the deterioration of the flotation performance of microplastics and the reduction of air flotation removal rate. The effects of ultraviolet light on the aging of the five microplastics were in the following order: PE < PP < PS < PET < PVC. This study reveals the adverse effects of light aging in nature on the removal of microplastics by air flotation, and provides theoretical guidance for the evaluation and optimisation of microplastic air flotation removal processes.