The effect of the mineral matrix during thermal analysis of polymers: Implications for microplastics characterization

The exponential growth of global plastic production complicates waste management, leading to the accumulation of microplastics in the environment. Currently, the identification and quantification of microplastics rely on various analytical methods that often require sample pretreatments, which can introduce errors. The Rock-Eval® thermal method, through the total quantity of hydrocarbon ( Total HC parameter) and the temperature of their maximum release ( T peak ), identifies and quantifies polymers when individually analyzed. However, mineral matrices in natural sediments can influence these parameters. In this study, synthetic mixtures of various mineral matrices and polymers were analyzed using the thermal Rock-Eval® method. Three effects of mineral matrices on the thermal degradation of polymers have been identified: (1) catalysis, which accelerates the polymer degradation;(2) multimodal inhibition, characterized by a fractionated release of hydrocarbons, first at expected pyrolysis temperatures and then at higher temperatures; and (3) retention, which reduces total HC released. The same mineral matrix can produce different effects depending on the type of polymer with which it is mixed. For instance, illite catalyzes the degradation of polyethylene (PE), but inhibits that of polyethylene terephthalate (PET). Despite these effects, linear regressions between the Total HC and polymer content in synthetic mixtures provide a coefficient of determination (r²) of 0.99, confirming that matrix effects do not hinder polymer quantification. Furthermore, mixtures with natural matrices show effects comparable to those observed with synthetic matrices. These findings represent a first step towards understanding the effects of the matrix and confirm the possibility of using the thermal Rock-Eval® method as a simple tool to identify and quantify microplastic pollution in natural sediments. Conceptual model of the mineral matrix effects illustrating catalysis, retention, and thermal desorptionIntroduction • Strong mineral matrix effects are shown when analyzing polymers using Rock-Eval • Clay minerals affect polymer degradation through catalytic or thermal desorption effects • Opposite effects appear when mixing different polymer with clays • Goethite alters hydrocarbon yields via a marked pyrolysis product retention effect • Despite mineral matrix effects, each polymer may be characterized using Rock-Eval