Microplastics as a Modulator of Mercury Biogeochemistry: Transport, Transformation, and Environmental Impacts

Microplastics (MPs) are ubiquitous in global ecosystems, yet their influence on the biogeochemical cycling of mercury (Hg), a highly toxic metal capable of global-scale and cross-medium transport, remains poorly understood. This Review synthesizes the multifaceted role of MPs in Hg transport and transformation across aquatic, soil, and atmospheric systems. We analyze how MPs act as novel vectors and reaction interfaces, modulating Hg dynamics through adsorption-desorption processes, redox reactions, and methylation via mechanisms including the release of dissolved organic matter (DOM) and modulation of microbial activity. This study for the first time quantifies the Hg adsorption potential of MPs in global freshwater and marine environments alongside Hg reduction and biological desorption potentials mediated by MPs. These MP-Hg interactions are governed by the physicochemical properties of MPs, Hg species, and ambient conditions. Despite these findings, critical knowledge gaps persist regarding MP-Hg interactions and the underlying mechanisms under environmentally realistic conditions. Particularly, research on atmospheric MPs is still in its early stages, although they may influence Hg transport and deposition as airborne carriers. Filling these gaps is essential for refining risk assessments and developing integrated pollution control strategies for both Hg and MPs.