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The main driver of the potential toxicity of micro- and nanoplastics toward biota is often the release of compounds initially present in the plastic, i.e., polymer additives, as well as environmentally acquired metals and/or organic contaminants. Plastic particles degrade in the environment via various mechanisms and at different rates depending on the particle size/geometry, polymer type, and the prevailing physical and chemical conditions. The rate and extent of polymer degradation have obvious consequences for the uptake/release kinetics and, thus, the bioavailability of compounds associated with plastic particles. Herein, we develop a theoretical framework to describe the uptake and release kinetics of metal ions and organic compounds by plastic particles and apply it to the analysis of experimental data for pristine and aged micro- and nanoplastics. In particular, we elucidate the contribution of transient processes to the overall kinetics of plastic reactivity toward aquatic contaminants and demonstrate the paramount importance of intraparticulate contaminant diffusion.

A robust description of the bioavailability of Ni(II) in freshwaters is fundamental for the setting of environmental quality standards. Current approaches assume that bioavailability is governed by the equilibrium concentration of the free metal ion in the bulk aqueous medium. Such strategies generally have limited predictive value: a suite of empirical fitting parameters is required to deal with variations in water chemistry. Herein we compile data on Ni(II) speciation under typical freshwater conditions and compute the lability of Ni(II) complexes with typical molecular and nanoparticulate components of dissolved organic carbon. In combination with an analysis of the kinetic setting of Ni(II) biouptake by freshwater organisms, we assess the potential contribution from dissociation of Ni(II) complexes to the diffusive supply flux of free Ni 2+ . The strategy takes into account the absolute and relative magnitudes of the Michaelis-Menten bioaffinity and bioconversion parameters for a range of freshwater organisms, together with dynamic chemical speciation descriptors under environmentally relevant conditions. The results show that the dissociation kinetics of Ni(II) complexes play a crucial role in buffering the free metal ion concentration at the biointerface. Our results highlight the need to couple the timescales of chemical reactivity with those of biouptake to properly identify the bioavailable fraction of Ni(II) in freshwaters., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)