Adsorption of heavy metals by microplastics in aquatic environments: mechanism, multi-factor regulation and ecological risks

Microplastics (MPs) can adsorb heavy metals well in water. When these toxic substances interact, the ecological risks will be greatly aggravated, constituting compound pollution. In this review, we summarized how microplastics adsorb heavy metals - such as Pb, Cd and Cu - and found the following three points: (1) Polar polymers, such as polyamide (PA), can adsorb Cu 2 to 3 times more than non-polar polymers (such as polyethylene (PE)). This is because their surface and amide groups combine with each other to form a complex. This highlights the necessity of distinguishing polymer types in risk assessment protocols. (2) Plastics such as polystyrene (PS) will absorb 40 % to 60 % more Cd during aging (such as ultraviolet irradiation). This is mainly because new oxygen-rich groups (such as carboxyl or hydroxyl groups) begin to form on their surfaces. This indicates that current regulations based on virgin plastics may significantly underestimate the transport capacity of MPs in real environments. (3) When microplastics and heavy metals are mixed, their combined toxicity makes aquatic organisms experience oxidative stress twice to twice as much as when exposed to a single pollutant alone. This quantifies the synergistic amplification effect that must be accounted for in ecological safety standards. We also built a framework called "Polymer Type-Environmental Factors-Adsorption Model" and found some gaps we know, such as the differences in adsorption in the mixed state of multiple pollutants and the impact of aging on adsorption. Overall, this review connects mechanistic insights to ecological risks, providing a theoretical basis to formulate more precise plans to control pollution.