How aging microplastics influence heavy metal environmental fate and bioavailability: A systematic review

Microplastics (MPs) are now pervasive in the environment, with annual emissions estimated to range from 10 to 40 million metric tons. Aging (weathering) processes induced by environmental changes, gradually degrade MPs into smaller particles with higher surface reactivity. These particles readily adsorb surrounding heavy metals (HMs), forming complex pollutants. Such composite contaminants can bioaccumulate through the food chain, ultimately posing significant threats to ecosystems and human health. At present, this type of combined pollution has emerged as a pressing global challenge requiring urgent attention. Although research on the impact of MPs aging processes on the environmental behavior of HMs has increased in recent years, there remains a lack of systematic reviews. Therefore, there is an urgent need to collate relevant studies to better assess and mitigate the risks of composite pollution by MPs and HMs. This paper provides a comprehensive review of the effects of aging processes on the physicochemical properties of MPs and explores the mechanisms of adsorption, mobility, and bioavailability of HMs by aged MPs, systematically summarizing the key environmental factors influencing the interactions between aged MPs and HMs. Finally, the prospects for research on the co-occurrence of MPs and HMs in the environment were discussed. This review provides a scientific basis for the environmental risk assessment of such combined pollution and holds substantial practical significance for advancing ecological conservation.