Quantification of microplastics in agricultural soils by total organic carbon -solid sample combustion analysis

Accurate quantification of microplastics (MPs) in soils is a significant challenge due to the complex nature of the organo-mineral matrix. Fine mineral particles and organic matter often interfere with the efficiency of extraction, identification and quantification of MPs from soils. Here, an optimized MP extraction and quantification method is proposed, using total organic carbon analyser-solid sample combustion unit (TOC-SSM) analysis. The approach entails a field survey, digestion of organic matter by Piranha solution, density separation, and quantification. This method achieves a high total recovery rate of 97.39 ± 14.25 (SE) % for particles sized between 300 and 600 µm, and 94.80 ± 13.48 (SE) % for particles less than 300 µm with spiked soil as samples. The optimised method is then applied to strawberry farm soils that use plastic mulch films to quantify MP contamination levels. Our results indicate MP concentrations of 12.24 ± 3.65 (SE) mg kg^-1 (for particles of 300-2000 µm in size) and 2.62 ± 0.66 (SE) mg kg^-1 (for particles smaller than 300 µm). With improved simplicity and the ability to provide the actual weight of plastics for the extraction and quantification of MPs, this work offers a potential approach for assessing low-density plastics in the northeastern Australian agricultural soils with a dominant MP contamination, specifically polyethylene (PE).