Environmental Carriers for Metal Nanoparticles: Transport, Fate, and Eco-risks

The occurrence of metal nanoparticles (MNPs) is an established environmental menace. After MNPs are released into the environment, they can interact with surrounding components and further be embedded within environmental substances, forming MNPs-carrier composites. Many studies have documented MNPs as individual particles. Yet, the specific impacts of environmental substances on the transport, fate, and eco-risks of MNPs-carrier composites have not been systematically analyzed. Here, we conducted a comprehensive review to illustrate the environmental carrier role for MNPs by different environmental substances and components, influencing their occurrence, transport route, transformation, and eco-effect, which calls for a reconsideration of the MNPs’ eco-/health-connectivity. Studies on MNPs’ environmental carrier in primary environmental domains reveal that in the atmosphere, MNPs-carrier composites are from nature-originated particulates followed by anthropogenic sources (e.g., traffic and industry exhausts); in the hydrosphere, various components (e.g., natural organic matter, extracellular polymeric substances, and proteins) can act as major carriers for MNPs; in the biosphere, microorganism and plant tissues can carry MNP-composites, which can be bioaccumulated and biomagnified through trophic transfer. Moreover, MNPs-carrier composites undergo distinct biogeochemical transformations in different environmental components, including aggregation, sedimentation, chemical transformation and dissolution, and the consequential biotransformation. Carriers’ impacts on MNPs’ environmental behavior manifested in changed bioavailability with widespread ecotoxicity via cellular uptake, oxidative stress and metal ions release, as well as in human health with adverse effects. Overall, this review can facilitate the understanding of MNPs-carrier composites’ origin, transformation, fate, and eco-/health risks in the environment, proposing future research needs.