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What drives contaminant adsorption in photoaged microplastics? Polymer identity outweighs contaminant class and environmental conditions
Summary
When sunlight breaks down plastic pollution in water, this can make microplastics better at soaking up harmful chemicals—but this new analysis of 30 studies found that the *type* of plastic (like PE, PS, or PVC) matters more than how sunny, salty, or warm the environment is. This means some plastics may pose a bigger risk than others as chemical "sponges" in our water and food chain, and future safety research should focus more on which plastics we're exposed to rather than just where they end up.
Microplastic (MP) pollution is a growing global concern due to its ubiquity in aquatic ecosystems and its potential to act as a vector for environmental contaminants. Although laboratory studies have examined how photoaging alters the adsorptive behavior of MPs, their findings remain fragmented and often contradictory. Here, we conducted a systematic review and a three-level random-effects meta-analysis to quantitatively assess the impact of photoaging on the adsorption capacity of MPs. A total of 30 studies met the inclusion criteria, yielding 256 control-treatment comparisons. Extracted data included adsorption capacity (mean ± SD), polymer type, crystallinity, contaminant class, and experimental conditions, which were evaluated as potential moderators. Overall, photoaging exhibited a marginally positive effect on adsorption capacity, although results varied substantially across studies. Polymer-related properties, particularly type and hydrophobicity, emerged as stronger predictors of adsorption outcomes than environmental or operational variables, whereas crystallinity showed no significant effect. Among environmental moderators, only pH significantly influenced adsorption, with higher effects under acidic conditions. In contrast, salinity, temperature, and contaminant class showed no consistent moderating role. Notably, the type of microplastic was a stronger determinant than either contaminant concentration or chemical class, underscoring the central role of polymer-specific features in determining adsorption behavior after photoaging. More hydrophobic polymers, especially PE, PS, and PVC displayed greater increases in adsorption capacity after photoaging than hydrophilic ones. These findings suggest that the physicochemical nature of MPs, especially polymer type and hydrophobicity, plays a more decisive role in shaping adsorption patterns after photoaging than external conditions or contaminant characteristics. By identifying the key sources of variability across studies, our findings provide a robust evidence base to refine environmental risk assessments and to guide future research priorities.