Micro/nanoplastics as environmental mediators: A systematic review of sources and interfacial processes driving cross-media transport and impacts

Micro- and nanoplastics (MNPs) are pervasive across terrestrial, aquatic, and atmospheric environments, originating from both primary emissions and the secondary fragmentation of macro-plastics. However, current understanding remains fragmented across compartments, with cross-media pathways poorly integrated and terrestrial and atmospheric systems underrepresented in existing syntheses. This review systematically synthesizes field, laboratory, and modeling evidence and advances a process-based framework that links sources, transformation, and impacts to address this gap. Rather than behaving as inert particles, MNPs undergo continuous aging, weathering, and biofilm colonization that reshape size, morphology, surface chemistry, and reactivity. These transformations regulate mobility, aggregation, and interactions with co-contaminants including metals, organic pollutants, and pathogens. Across compartments, MNPs act as reactive carriers that facilitate long-range transport and time-dependent release of sorbed substances, while biofilms further modify surface properties, deposition, sedimentation, and contaminant exchange. System-specific drivers-pH, ionic strength, organic matter, salinity, temperature, humidity, and irradiation-modulate these pathways. Collectively, the synthesis shows that conditioned MNPs function as environmental mediators that redirect contaminant pathways across interconnected environmental media, providing a mechanistic basis for predicting fate, exposure, and risk.