Influence of mineral and organic matrices on the thermal characterization of microplastics

Qualifying and quantifying microplastic (MP) pollution in sediments represent a methodological challenge. Most used methods often require sample pretreatments to separate the MP particles of interest from the rest of the sediment. It has been demonstrated that the sampling preparation can influence the analytical results. To overcome these difficulties, Romero-Sarmiento et al. (2022) proposed the use of the pyrolysis and oxidation based-thermal method (a Rock-Eval® device), originally developed to characterize sedimentary organic matter. According to these authors, several parameters calculated from Rock-Eval® analyses, such as Total HC (quantity of hydrocarbons released during pyrolysis) and Tpeak (cracking temperature of plastic polymers), could be used to qualify and quantify microplastics, without sediment pretreatments. However, unpublished preliminary studies have shown that the Tpeak parameter, can vary with catalytic and thermal desorption effects, depending on the nature of the associated mineral and organic matrices. To understand the origin of these effects, we studied matrix effects during the thermal analysis of composite samples. In this study, synthetic mixtures of various mineral matrices, organic materials and pure polymers at different concentrations were analyzed using a Rock-Eval®. As expected, the results show that Total HC varies with the amount of polymer present in the samples. However, Total HC also varies according to the type of the mineral matrix. Indeed, some samples, such as clays and particularly goethite, show retention effects when the mineral matrices are characterized by a high adsorption capacity. Furthermore, for a given polymer, the Tpeak parameter can vary according to the mineral matrix. For example, it seems that some mixtures of polymers and mineral matrices enhance the catalytic cracking of hydrocarbons. While highest expected Tpeak values were obtained for some synthetic blends indicating a delayed release of hydrocarbons. The same mineral matrix can also induce different effects depending on the organic compounds present in the sample. In addition, analyses using natural versus artificial matrices were performed to compare these effects. Obtained results were complemented by scanning electron microscope observations and X-ray diffraction measurements at different temperatures during pyrolysis, to visualize the possible organo-mineral interactions and analyze morphological changes respectively. Understanding these effects of the mineral and organic matrices on the determination of MP concentrations will enable us to refine a more accurate method to quantify the impact of plastic pollution in sediments. Romero-Sarmiento, M.-F., Ravelojaona, H., Pillot, D., Rohais, S., 2022. Polymer quantification using the Rock-Eval® device for identification of plastics in sediments. Science of The Total Environment 807, 151068. https://doi.org/10.1016/j.scitotenv.2021.151068