Interactions of Microplastics and Heavy Metals: Mechanisms, Bioavailability, and Potential Impacts on Ecological Health

The widespread presence of microplastics (MPs) is an emerging health and environmental issue, posing significant threats to ecosystems and living organisms. Even remote regions, such as mountain catchments and polar environments, are not exempt from MP pollution. Once deposited, any plastic items can undergo extensive, yet not fully characterized, ageing processes, breaking down into much smaller and numerous MPs. Besides their presence, MPs act as vectors for transporting various environmental pollutants, including heavy metals (e.g., mercury (Hg), lead (Pb), cadmium (Cd), and arsenic (As)). The interactions between MPs and heavy metals have drawn much attention recently because MPs can significantly influence the chemical forms, bioavailability, bioaccumulation, and toxicity. Our current understanding of their interactions is primarily based on studies conducted under controlled conditions, using pristine or lab-induced degraded MPs, while naturally degraded MPs, which can be more abundant in the environment, remain understudied. Degradation alters many MP characteristics, including surface roughness and the abundance of oxygen-containing functional groups, potentially enabling degraded MPs to have increased affinities with metal pollutants. This review discusses the interactions between MPs and four common heavy metals (i.e., Hg, Pb, Cd, and As) in terrestrial environments and the toxicological effects of MP–heavy metal on plants and organisms. The environmental factors influencing their interactions are also included. By considering the associated risks and the factors affecting the interactions and exposure, further studies are needed to determine the current exposure to MP–metal contamination in the terrestrial environment. It is essential to address how the interactions between MPs and various pollutants change after they enter the natural environment with prolonged ageing under real-world conditions. © 2027 Ming Hung Wong.