Effect of Polymer Aging on Uptake/Release Kinetics of Metal Ions and Organic Molecules by Micro- and Nanoplastics: Implications for the Bioavailability of the Associated Compounds

Abstract

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.

Keywords: aquatic contamination; intraparticulate diffusion; polymers; transient flux.

Figure 1

Schematic illustration of the temporal evolution of concentration gradients in the aqueous and polymer phases during (A) steady-state and (B) transient regimes. The nomenclature adopted for the relevant concentrations of X considered in the formalism, i.e., c_X,w^*, c_X,w^int, c_X,p^int, and c_X,p^*, is specified. See the text for details. Adapted from Bayen et al.

Figure 2

Absorption of Cd(II) on (A) pristine PS, with the inset showing data at short times, and (B) PS aged for 7 days in a mixture of H₂O₂ and Fe²⁺. Experimental data (black solid circles) from ref (40) for c_Cd,w^* = 1.8 × 10^–5 mol dm^–3 (2 ppm). Computed curves correspond to the involved integral fit, eq 18 (including the initial 0-absorption point: green solid curve), the monoexponential, eq 21 (including the initial 0-absorption point: blue dashed line; excluding the initial 0-absorption point: blue dotted line), and the monoexponential, eq 20 (including the initial 0-absorption point: red dot-dashed line). See text for details of the fitting procedures.

Figure 3

Absorption of tetracycline on (A) pristine PS and (B) aged PS, each with the inset showing data at short times. Experimental data (black solid circles) from ref (48). Computed curves correspond to the involved integral fit, eq 18 (including the initial 0-absorption point: green solid curve), the monoexponential, eq 21 (including the initial 0-absorption point: blue dashed line; excluding the initial 0-absorption point: blue dotted line) and the monoexponential, eq 20 (including the initial 0-absorption point: red dot-dashed line). See text for details of the fitting procedures.

Figure 4

Absorption of Cd(II) on (A) pristine PS and (B) aged PS. Experimental data (black solid circles) are from ref (44). Computed curves correspond to the involved integral fit, eq 18 (including the initial 0-absorption point: green solid curve), the monoexponential, eq 21 (including the initial 0-absorption point: blue dashed line; excluding the initial 0-absorption point: blue dotted line), and the monoexponential, eq 20 (including the initial 0-absorption point: red dot-dashed line). See the text for details of the fitting procedures.

Figure 5

Absorption of atrazine by (A) pristine PS and (B) aged PS, each with an inset showing data at short times. Experimental data (black solid circles) from ref (42). Computed curves correspond to the involved integral fit, eq 18 (including the initial 0-absorption point: green solid curve), the monoexponential, eq 21 (including the initial 0-absorption point: blue dashed line; excluding the initial 0-absorption point: blue dotted line), and the monoexponential, eq 20 (including the initial 0-absorption point: red dot-dashed line). See the text for details of the fitting procedures.

Figure 6

Absorption of sulfamethoxazole by (A) pristine PP and (B) aged PP, with the inset showing data at a short time. Experimental data (black solid circles) from ref (47). Computed curves correspond to the involved integral fit, eq 18 (including the initial 0-absorption point: green solid curve), the monoexponential, eq 21 (including the initial 0-absorption point: blue dashed line; excluding the initial 0-absorption point: blue dotted line), and the monoexponential, eq 20 (including the initial 0-absorption point: red dot-dashed line). See the text for details of the fitting procedures.

Figure 7

(A) Relative change in K_w,p and D_X,p upon the aging of plastic particles. (B) Plot of the dimensionless transient flux term, J₀⁺/(c_X,w^*σ), versus the magnitude of the intraparticulate diffusion layer thickness relative to the particle radius, δ_p/a. Dashed lines are guides to the eye. In both (A,B), parameters are obtained by involved integral fitting (eq 18, including the 0-absorption point at t = 0) of published experimental sorption curves.⁻

Figure 8

Relationship between log of the dimensionless transient flux term, J₀⁺/(c_X,w^*σ), and log D_X,p for Cd(II), tetracycline (“tet”), and oxytetracycline (“oxytet”) absorption by plastic particles of various polymer types. Dashed lines are guides to the eye. Parameters obtained by involved integral fitting (eq 18, including the 0-absorption point at t = 0) of published experimental sorption curves.^,,,−