Biochemodynamic Features of Metal Ions Bound by Micro- and Nano-Plastics in Aquatic Media.

Raewyn M. Town,Ronny Blust,Herman P. van Leeuwen

doi:10.3389/fchem.2018.00627

Raewyn M. Town, Ronny Blust + Show 1 more

Open Access

https://doi.org/10.3389/fchem.2018.00627

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Abstract

A simple model, based on spherical geometry, is applied to the description of release kinetics of metal species from nano- and micro-plastic particles. Compiled literature data show that the effective diffusion coefficients, Deff, for metal species within plastic polymer bodies are many orders of magnitude lower than those applicable for metal ions in bulk aqueous media. Consequently, diffusion of metal ions in the aqueous medium is much faster than that within the body of the plastic particle. So long as the rate of dissociation of any inner-sphere metal complexes is greater than the rate of diffusion within the particle body, the latter process is the limiting step in the overall release kinetics of metal species that are sorbed within the body of the plastic particle. Metal ions that are sorbed at the very particle/medium interface and/or associated with surface-sorbed ligands do not need to traverse the particle body and thus in the diffusion-limiting case, their rate of release will correspond to the rate of diffusion in the aqueous medium. Irrespective of the intraparticulate metal speciation, for a given diffusion coefficient, the proportion of metal species released from plastic particles within a given time frame increases dramatically as the size of the particle decreases. The ensuing consequences for the chemodynamics and bioavailability of metal species associated with plastic micro- and nano-particles in aquatic systems are discussed and illustrated with practical examples.

Highlights

The typical timescale, τrel, required for the complete release of M from nano- and micro-plastic particles is shown in Figure 1 as a function of particle size and intraparticulate mobility (Deff) of the species of M
The results show that metal species with a Deff of 5 × 10−13 m2 s−1 would be practically completely released within 1 h from particles with rp up to order 100 μm
The association of metal species with plastic particles alters the spatial scale and timescale of the external and internal exposure conditions in a manner that depends on the physicochemical features of the particles, notably rp, as well as the physiology of the organisms, e.g., feeding behavior, gastric digestion, and gut retention times

Summary

Introduction

Microplastics (MPs) are ubiquitous worldwide in the water column of freshwater and marine systems (Barnes et al, 2009; Baldwin et al, 2016; Leslie et al, 2017; Schmidt et al, 2017), in sediments (Blumenröder et al, 2017; Graca et al, 2017; Leslie et al, 2017; Wang et al, 2017), in soils (Scheurer and Bigalke, 2018; Zhang and Liu, 2018; Zhou et al, 2018), and within biota (Foekma et al, 2013; Goldstein and Goodwin, 2013; Lusher et al, 2015; Leslie et al, 2017; Digka et al, 2018; Piccardo et al, 2018; van der Hal et al, 2018). Equation (4) is used to predict the extent to which inherent and sorbed M is released from the body of MPs and NPs. For the case of metal ions that are present at the particle surface, i.e., sorbed at the particle/water interface or sorbed by surface-sorbed ligands such as natural organic matter, the release process does not involve diffusion through the polymer phase.

Results

Conclusion