Organo-mineral interaction between plastic film and sedimentary rock induced by UV irradiation

Organo-mineral interaction is an important mechanism affecting the fate of natural organic matter in the environment. Field discoveries since 2014 report a novel type of organo-mineral interaction, where "recalcitrant" plastic polymers either physically or chemically attach onto or fuse into host rocks or sediments. During our previous field observation in 2022, it was reported that weathered low-density polyethylene (LDPE) plastic films were chemically attached onto sedimentary rocks. It was hypothesized that sunlight irradiation may have triggered chemical interactions following physical attachment of plastic film onto rock surfaces. In this study, we tested this hypothesis through accelerated aging experiments. Specifically, we first explored the interaction mechanisms between low-density polyethylene film and crushed sedimentary rock powders, including sandstone, shale, limestone, and ironstone, through accelerated UV irradiation ranging from 0 to 168 h. Subsequently, we conducted experiments using the plastic film and consolidated shale rocks. We found that chemical binding took place after 48 h of UV irradiation equivalent to 35 days of exposure of plastic film to sunlight on the land surface. The organo-mineral interaction process started with the surface oxidation of plastic film leading to the generation of carbonyl and ether groups. After that, irreversible mineral binding occurred. Visualization of the plastic films following the aging experiments showed that mineral particles were incorporated within the polymer instead of surface attachment. Organo-mineral interaction mechanisms between LDPE film and sedimentary rocks were further explored via multiple approaches including XPS, STEM-EELS, TOF-SIMS, and theoretical calculations. Multiple lines of evidence suggested binding mechanisms for this process, including the formation of chemical bonds such as Si-O-C, corroborating well with our previously reported field findings. The findings of this study offer robust evidence that natural weathering processes, such as sunlight irradiation, can initiate chemical reactions between plastic and rock materials, deepening our understanding of plastic pollution dynamics within the geological context.