Figure 1. Schematic diagram illustrating (A) transformation of nanoparticles (NPs) in environmental systems and (B) key mechanisms underlying the toxicity of transformed NPs relative to pristine particles.

NPs may encounter physical-, chemical-, or biological-based transformations in different environmental media, i.e., air, soil, and water. The physicochemical traits of NPs in combination with environmental features determines the transformation pathway. During the transformation process, NPs may interact with other co-pollutants and biomacromolecules, thereby resulting in the alteration of physicochemical traits, bioavailability, and differential toxic responses mediated by transformed versus pristine NPs. In an environmental viewpoint, the potential toxicity of transformed NPs not only relies on physicochemical traits (e.g., size and size distribution, charge, coating, aggregation/agglomeration state) but also on the presence of co-pollutants (e.g., heavy metals, organic pollutants etc.) as well as interaction with biomolecules in living organisms (e.g., proteins). MNPs, Magnetic nanoparticles; IS, Ionic strength; OM, organic matter; NOM, natural organic matter.