Transient Plastic Spin Labeling with Chlorine Dioxide

Abstract

Plastic waste, being one of the most important problems for humankind, poses severe threats to ecosystems, wildlife, and human health. Tracing, quantifying, and identifying types of plastic waste is of crucial importance to understand its environmental pathways and develop targeted strategies for reduction, recycling, and remediation. To contribute to addressing this global issue, we investigated the spin-labeling capabilities of aqueous chlorine dioxide (ClO$_2$) radicals upon introduction into poly(ethylene terephthalate) and utilized electron spin resonance spectroscopy for detection. The technique is capable of identifying plastic species as the unpaired electron of the radical molecule is strongly sensitive to its local environment through its coupling parameters. Temperature-dependent measurements revealed that the molecules are immobilized at low temperatures and exhibit well-resolved anisotropic and hyperfine spectra that are quantitatively described by a model spin Hamiltonian. Even above the melting point of water, some degrees of freedom remain blocked as a result of the polymer matrix. Furthermore, employing a time-series measurement at room temperature enabled us to determine the diffusion coefficient of the molecule in the polymer.

Figures (3)

• Figure 1: ESR spectra of ClO$_2$ in aqueous solution with three different concentrations, $30$, $300$, and $3000$ ppm at $298$ K. Multipliers indicate the relative intensity increase, determined by double integration, compared to the $30$ ppm solution. While a tenfold increase in concentration from $30$ to $300$ ppm increases the signal almost tenfold, above this threshold, the solution becomes too concentrated and broadening effects occur due to stronger interactions between the molecules. Solid curves are simulations from the spin-Hamiltonian with parameters fitted to match the experimental data. A detailed analysis is found in the text.
• Figure 2: a) Detected derivative cw ESR spectra of ClO$_2$ in a $3000$ ppm aqueous solution at $298$, $223$, and at $173$ K (top) and the same for $100~\mu$m thick PET sample soaked in the solution for days. Please note the drastic change in the lineshape depending on the solid matrix. Solid lines are simulations with parameters given in panels b to g. The details of the spin model can be found in the main text. b-g) Parameters obtained from the spin-model: $g$-factors, hyperfine couplings ($A$), nuclear quadrupole coupling constant ($e^2qQ/h$) and its asymmetry parameter ($\eta$), linewidths ($\Delta B$) and rotational correlation time ($\tau_{\text{correlation}}$), respectively. In the solid phase, all parameters show little variation with temperature. On the other hand, around the phase transition of water, they do change drastically. Moreover, please note the significant difference between the aqueous solution and when the molecules are embedded in the PET matrix. Vertical dashed lines denote the melting point of water under ambient pressure. The semi-transparent dashed blue line in panel d suggests that the quadrupolar coupling averages to zero at $273$ K in the aqueous ClO$_2$.
• Figure 3: a) A time series of ESR measurements performed on a $12~\mu$m thick PET film, initially soaked in a $2900$ ppm aqueous solution of ClO$_2$. All measurements were performed at room temperature. The feature at $357$ mT arises from the sample holder and was excluded from later evaluation steps. The time difference between two adjacent spectra is $5$ minutes. Please note the systematic decrease in the spectral intensity. b) Time dependence of the ESR intensity from double integration of the spectrum for three separate runs. The curves follow an exponential decay in time as dictated by the out-diffusion and removal of the ClO$_2$ molecules from the PET film. The orange curve is a fit based on the theoretical discussion presented in the main text. Please note that the decay characteristic is independent of the initial settings. The obtained diffusion coefficient is $D=(3.91 \pm 0.74)\times 10^{-15}$ m$^2$/s at room temperature.