UV exposure to PET microplastics increases their downward mobility in stormwater biofilters undergoing freeze–thaw cycles

Abstract

Stormwater conveys microplastics and accumulates them in stormwater treatment systems such as biofilters, where they are exposed to sunlight and natural weather conditions such as drying, freezing, and wetting cycles. Thus, the mobility of microplastics through biofilters could depend on the interactive effects of the weather conditions. Yet, how weathering of microplastics under UV light affects their mobility during freeze–thaw cycles has not been evaluated. This study estimates to what extent UV weathering could affect the rate of microplastic transport through sand filters during freezing and thawing cycles. To compare the mobility of unweathered and weathered microplastics based on their concentration at different depths, PET microplastic particles weathered to different degrees under a UV light were deposited on sand-packed columns and subjected the columns to freeze–thaw cycles. Our results confirm that an increase in exposure to UV light alters the surface properties of microplastics, particularly contact angle, leading to increased surface hydrophilicity. Additionally, the depth distribution of microplastics varies with weathering of microplastics, with most weathered microplastics moving farthest into the subsurface. We attribute these results to a combination of changes in surface properties due to weathering and changes in the interaction of weathered plastics with either ice or water interfaces, resulting in a net increase in the downward mobility of microplastics over time. The results imply that UV weathering, in conjunction with the freeze–thaw cycle process, could substantially increase the mobility of microplastics in subsurface soil, and they should be included in models to predict the transport of microplastics in subsurface systems.