Plastic Beyond the Surface: Multi-Scale Alteration Mechanisms of Polypropylene in Soils

Plastics have become pervasive contaminants in terrestrial environments, notably through compost amendments that introduce large quantities of fragments into agricultural soils. Once in the soil, plastics undergo weathering and degradation, leading to their fragmentation into microplastics (1 µm–5 mm) and nanoplastics (< 1 µm), which can be transported through the soil profile (1). Mobility of nanoplastics is particularly concerning because they can transport adsorbed metals such as lead, titanium and emerging contaminants such as rare earth elements (2,3). While thermal, photo- and mechanical degradation pathways are documented (4,5), the structural transformations induced by soil weathering and their role in generating nanoplastics remain poorly understood.

Here, we investigated polypropylene (PP) macrofragments aged ~30 years in agricultural soils at Meung-sur-Loire (Loiret, France). Using multi-scale synchrotron imaging and diffraction techniques, we characterized the surface alteration layers and assessed their implications for nanoplastic formation.

Synchrotron X-ray fluorescence (s-XRF) shows the heterogeneous distribution of metallic additives containing Ca, Ti, Cr, Mn and Fe, together with surface parallel cracks, trapping soil minerals. Micro-computed tomography (micro-CT) evidences that these surface cracks propagate down to ~150 µm, demonstrating that degradation extends into the interior of the polymer. Rietveld refinement of synchrotron grazing-incidence X-ray diffraction (s-GIXD) reveals that these cracks reflect strong vertical gradients in crystallinity and atomic positions between the altered surface nanolayers and the underlying interior, consistent with surface recrystallization and the development of a deep alteration front. These surface modifications coincide with the formation of large subsurface voids (up to ~300 µm) linked to surface roughening, recording the break-up of the polymer into smaller micro- and nanoplastic fragments. At the nanoscale, synchrotron nano-CT highlights heterogeneous nanoporosity (up to ~2.5 %) in regions enriched in nano-additives, whereas additive-poor regions show < 0.5 % porosity. This spatial correlation demonstrates that metallic additives act as preferential sites that promote localized degradation.

Altogether, this multi-scale structural analysis evidences that soil weathering induces deep structural degradation that is controlled by the distribution of metallic additives. These structural features shed light on the processes controlling the formation and release of potentially harmful nanoplastics in soils.

References

1. Wahl et al., (2024). Journal of Hazardous Materials, 476, 135153.

2. Davranche et al., (2019). Environmental pollution, 249, 940-948.

3. Blancho et al., (2022). Environmental Science: Nano, 9(6), 2094-2103.

4. Cai et al., (2018). Science of the Total Environment, 628, 740-747.

5. He et al., (2018). TrAC Trends in Analytical Chemistry, 109, 163-172.