Crystallisation of CaCO3 polymorphs induced by layered PET-based microplastic particles

Background

Polyethylene terephthalate (PET)-based microplastics are prevalent marine pollutants, yet their impact on calcifying organisms remains understudied. This study investigates PET glitter microplastics as nucleation sites for Ca–Mg carbonates, assessing their role in biomineralisation. Laboratory experiments simulated seawater conditions (21–60 °C, 2–50 mM Ca and CO₃, varied Mg/Ca ratios) to induce specific carbonate polymorphs (calcite, Mg-calcite, aragonite, vaterite, monohydrocalcite) on six PET glitter variants. Mineral phases and PET surface interactions were characterised using scanning electron microscopy with energy dispersive spectroscopy, infrared spectroscopy, and powder X-ray diffraction.

Results

PET glitter actively promoted Ca–Mg carbonate crystallisation, with nucleation preferentially occurring at surface irregularities. Polymorph selection and morphology remained consistent with control experiments. Calcite formed rhombohedral crystals (1–20 µm), vaterite and monohydrocalcite appeared as spherical aggregates (5–10 µm, 100–200 nm nanocrystals), Mg-calcite exhibited a granular texture (< 50 nm), and aragonite displayed branching morphologies, with secondary aragonite forming reduced branching and columnar structures (< 10 µm). Crystallisation was rapid: vaterite and ACC-derived calcite formed within 2–3 min, solution-derived calcite within 5–10 min, Mg-calcite within 2–3 h, and monohydrocalcite within 6 h. Secondary transformations of vaterite and aragonite, as well as monohydrocalcite-derived aragonite, completed after 6 h. All CaCO₃ phases strongly adhered to PET, except primary aragonite, which displayed weaker attachment. PET degradation was observed during crystallisation, with cracks and surface peeling releasing microplastic fragments.

Conclusions

PET uniquely influences surface CaCO₃ nucleation compared to other microplastics. Unlike polystyrene or polyethylene, which require organic coatings for encapsulation, PET actively promotes crystallisation via ester (–COO–) and hydroxyl (–OH) groups that facilitate Ca²⁺ adsorption, creating local supersaturation zones. Surface defects further concentrate ions, accelerating mineral growth. Crystallisation in confined PET features enhances fragmentation, increasing micro- and nanoplastic release. The strong attachment of CaCO₃ phases to PET may affect biomineralisation in marine organisms, impacting shell formation and skeletal integrity. Additionally, PET degradation through crystallisation-driven fragmentation raises concerns about increased microplastic bioavailability and long-term environmental pollution.

Graphical Abstract