Tracing Plastic Degradation in Soils Using Copper Isotopes: Novel Insights from Laboratory Experiments

Global plastic production now exceeds 400 million tons per year and continues to increase, despite plastics being ubiquitous and pervasive in every environmental compartment. Soils are increasingly recognized as major sinks for macro-, micro- and nanoplastics. The presence of plastic in soils can affect their quality and impair ecosystem services. Yet, degradation pathways in soils remain insufficiently constrained, limiting our ability to predict plastic persistence and associated risks. Manufactured plastics contain complex organic and inorganic additive packages (e.g. pigments, fillers, stabilizers, flame retardants, and catalyst residues), which may leach, transform or fragment during aging. Copper (Cu)-bearing pigments, widely used in green and blue plastics, offer an opportunity to couple elemental and isotopic information to track additive mobilization during degradation.

Here, we investigate temporal changes in elemental composition and copper isotope ratios (δ⁶⁵Cu) during a controlled laboratory degradation experiment in plastic-contaminated soil systems. Three common polymers - polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC) - were cut into 1 x 1 cm pieces. For each polymer, 400 mg were exposed to two conditions over 1 to 24 weeks (1, 2, 4, 8 and 24 weeks): (i) agitation in a soil suspension prepared with ultrapure water (soil:water 1:1) and (ii) incubation in water-saturated soil under static conditions. Triplicates and controls were prepared for each condition and duration.

Following microwave digestion, major and trace element concentrations were determined by ICP-MS. Copper was purified by chromatographic separation, and δ⁶⁵Cu was measured by MC-ICP-MS on plastics recovered at each time point. Under static conditions, δ⁶⁵Cu remained unchanged for all polymers during the 24 weeks. Under agitation, PE showed no significant δ⁶⁵Cu shift, whereas PVC and PP displayed a slight decrease during the first two weeks, followed by an increase at 4, 8 and 24 weeks. In contrast, most major and trace element concentrations remained stable over the 24-week experiment in both conditions.

Overall, our results show that Cu isotopes can capture subtle, time-dependent processes during plastic aging in soils that are not apparent from bulk elemental concentrations alone, providing an additional tracer dimension for assessing the fate and behaviour of plastics in terrestrial environments. Ongoing experiments extending to 40 weeks and future comparisons with field-collected plastics from contaminated sites will further test and scale up this approach.