The evolving interface of aged microplastics and heavy metals: implications for environmental fate and toxicity

Microplastics (MPs) ubiquitously contaminate ecosystems and serve as efficient vectors for heavy metals (HMs), amplifying their environmental mobility and bioavailability. Although the individual toxicological impacts of MPs and HMs are well-documented, their combined effects, driven by complex adsorption dynamics and synergistic toxicity, remain poorly understood. This review systematically synthesizes recent advances in MP-HM interactions, with a focus on adsorption mechanisms such as electrostatic attraction, biofilm facilitation, and co-precipitation. Key factors governing adsorption efficiency, including polymer crystallinity, environmental aging, biofilm formation, and water chemistry, are critically examined. Furthermore, we elucidate the compounded toxicity of MP-HM complexes across aquatic and terrestrial organisms, manifesting as oxidative stress, multi-organ damage, and endocrine disruption, with bioaccumulation risks that propagate through food chains to humans. By identifying critical knowledge gaps, particularly regarding long-term ecotoxicological outcomes and transgenerational effects, this review provides a mechanistic framework to guide future research and evidence-based policy for mitigating composite pollution in a rapidly changing environment.