Fast ATR-FTIR method for quantifying silicates presence on PE plastic fragments from soil

The interaction of plastic debris with the soil environment remains insufficiently studied. In particular, we have recently reported the incorporation of a mechanically stable clay phase—mainly composed of silicates—onto polyethylene (PE) macro-, meso-, and microplastic surfaces. This incorporation transforms plastic fragments into a composite material, potentially leading to significant changes in properties such as density, hydrophobicity, and contaminant sorption capacity. Therefore, quantifying the siliceous fraction is essential to better understand plastic–environment interactions. Determination of silicon by EDX is a conventional method, but is time-consuming, technically demanding, and not widely accessible. Moreover, the presence of clay onto the PE matrix complicates the identification of oxygen-containing functional groups due to spectral overlap between C–O and Si–O stretching vibrations in the sample's FTIR spectra. In this study, a rapid and straightforward ATR-FTIR-based methodology for the quantitative determination of silicon on weathered PE mulch fragments was developed. Furthermore, a reliable approach for the identification of Si–O and C–O functional groups in PE samples with high silicon content was established. The peak area of the Si–O stretching band showed a strong linear correlation with silicon concentration in PE–sand standards (R²=0.9878). The proposed method was validated against EDX measurements of PE samples extracted from agricultural soils, showing good agreement. Additionally, sodium citrate treatment effectively removed the siliceous fraction without the use of hazardous hydrofluoric acid, allowing for accurate determination of oxidation indices. The developed method is simple, rapid, and requires minimal sample preparation, offering a practical alternative for laboratories lacking access to advanced analytical techniques.