The effects of some common inorganic soil components on the pyrolytic analysis of plastics

Plastics and microplastics are major hazards for the environment and human health. Plastic materials in the sedimentary record are a marker for modern anthropogenic activity. One environmental reservoir of plastics and microplastics is soil. One method to detect the presence of plastics in soil, and to thereby recognise the presence of its hazards, is pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). Yet soils are multicomponent mixtures and any analytical artefacts generated will reduce our ability to monitor the presence of plastics. We have studied the Py-GC-MS responses for the polymers polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC), in the presence of individual phyllosilicate (kaolinite, illite and montmorillonite) and iron oxyhydroxide/oxide (goethite and magnetite) minerals that may be present in soil. Our data show the distinctive polymer fingerprint becomes obscured when analysed with minerals. We have isolated the effects of some potential inorganic soil components so that responses from complex matrices can be deconvolved. The mineral-assisted aromatisation of compounds affects the diagnostic signals of the original polymer, particularly for those polymers (PE and PP) which have a large aliphatic hydrocarbon signature when analysed alone. Iron oxyhydroxide/oxide are shown to have a greater effect on the data than phyllosilicates. Our results will help guide future interpretations of Py-GC-MS data obtained when monitoring soils for plastic contamination. Py-GC-MS is most commonly applied as a qualitative method, yet detailed quantitation of the organic-inorganic reactions we have described, albeit analytically complex, would represent a useful future study.