Salt crust-assisted thermal decomposition method for direct and simultaneous quantification of polypropylene microplastics and organic contaminants in high organic matter soils

Direct quantitative analysis of soil polypropylene microplastics (MPs) via thermal method is still a challenge due to its sensitivity to the soil matrix during the thermal decomposition. In this work, the impact of soil organic matter (SOM) on MPs decomposition in real soil was estimated, and high SOM contents was found have significantly negative effect on the qualitative and quantitative analysis of PP. To solve this problem, a salt crust-assisted thermal decomposition method was developed to reduce the soil matrix effect. By adding salt solution and heating in appropriate temperature program, salt crust can be formed between MPs and soil matrix to isolate PP MPs and soils during the heating process, and thereby to avoid the influence of SOM. Thermal desorption/decomposition coupling with headspace solid phase microextraction (HS-SPME) was used extraction of MPs decomposition products. All the thermal decomposition products of PP were identified, several alkenes and dialkenes were selected as candidates for quantitative analysis, and 2,4,6,8,10-pentamethyltridec-1-ene with optimal linearity was finally established for quantification. Besides, this method was also established for simultaneous determination of PP MPs and organic contaminants (OCs) including PCBs and PAHs, since it is capable of providing a new approach for investigation of their interaction in real soil environment. A theoretical LOD of 0.002 wt% was obtained for PP MPs, and the LOD of OCs ranges from 0.05 to 1.87 ng/g. Determination of aged soil samples reveal that adsorption of OCs is not a predominating effect of MPs in soil environment, and MPs contaminants may reduce the soil retention capacity. This method provides a new approach for direct quantification of soil PP MPs, and applicable for investigation of the interactions between MPs and OCs in real soils.