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

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, D _eff, 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.

Keywords: bioavailability; dynamic metal speciation; kinetics; microplastic; nanoplastic.

Figure 1

Typical time, τ_rel, required for complete release of M from plastic particles as a function of the effective diffusion coefficient D_eff (y-axis) and the particle radius, r_p (indicated above each line). Typical values of D_eff reported for M in polymers are indicated by horizontal gray dashed lines (see Table 1).

Figure 2

Fraction of total M released from a plastic particle in 1 h as a function of the particle radius, r_p, and the effective diffusion coefficient, D_eff, values indicated on the figure. The inset is an amplification of the result for D_eff = 10⁻²⁰ m² s⁻¹.

Figure 3

Comparison of experimentally measured (points) (Turner and Lau, ; Turner, 2018) and computed (dashed curves) release of Pb (blue symbols) and Cd (green and red symbols) as a function of time from fragments of beach-collected plastics. The dashed curves were computed using Equation (4) with particle radius = 3 × 10⁻³ m and the D_eff values indicated on the figure. The plastic samples correspond to ca. 0.1 g “shavings” of polyurethane (blue symbols), polypropylene (red symbols), and polyethylene (green symbol). See text for details.