Plastic materials of various forms have become ubiquitous in the environment and are thus of significant concern. Microplastics, in particular, are notorious for passing through filters in water treatment plants and actively contaminate various aquatic bodies. Plastics are relatively buoyant and typically persist in surficial water layers where sunlight can irradiate and degrade organic materials. As a result, microplastics are photochemically altered and partially solubilized as dissolved organic matter (DOM). A particular area of interest is the structural characterization of the photochemically produced particulate materials as well as of their DOM counterpart. However, this presents an analytical challenge as analytical instrumentation rarely allows for both liquid- and solid-state analysis of samples, a necessity for studying the photochemistry of plastic particles. Here, we show an application of gel-state NMR spectroscopy for assessing both the water-insoluble photo-aged microplastics as well as well as for the water-soluble DOM. We photo-irradiated four polymers of varying photochemical reactivity: polyethylene (C-C bonds, medium photo-reactivity), polyethylene terephthalate (hydrolyzable ethylene bonds, medium photo-reactivity), polyvinyl chloride (C-C bonds, low photo-reactivity), and polystyrene (C-C bonds, high photo-reactivity). Both the insoluble plastic remains as well as the soluble DOM were obtained by lyophilization. Gel-state NMR analyses allowed for liquid-state-like two-dimensional NMR spectra using 1H-1H TOCSY, 1H-13C HSQC, and 1H-13C HMBC pulse sequences. The obtained structural information allowed us to elucidate the photochemical mechanisms leading to the degradation of plastic materials. This provides for a better understanding of the production of biologically labile molecules that become entrained in DOM and lead to the disruption of global biogeochemical cycles.

Leveraging gel-state NMR spectroscopy for the structural elucidation of plastics photochemistry

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