Modified oil extraction of pristine and weathered synthetic polyurea microcapsules and polyethylene microplastics from soil

Microplastics (MPs) in soil are an emerging environmental concern due to their widespread distribution, persistence, potential toxicity to soil organisms, possible transfer into crops and groundwater as well as their potential ability to alter soil functions. MP polymers, such as linear low-density polyethylene (LLDPE) and polyurea (PUA) are commonly applied to soils for mulching and pesticide application and their fate needs to be better understood. For qualitative and quantitative analysis, MPs need to be efficiently isolated from soils using suitable extraction methods. Therefore, we developed a modified oil extraction method using n-octanol and compared it with the widely used density extraction method, both preceded by a potassium hydroxide (KOH) extraction step. Pristine and artificially weathered (light-irradiated) small-sized LLDPE mulch film particles (cryo-milled to a median size of 147 µm) and PUA microcapsules used for pesticide applications (median diameter of 2 µm) were spiked into 25 g samples of two standard agricultural soils. Both MP types differed in size, shape, density and especially their composition, as the PUA microcapsules predominantly consisted of oil. After MP isolation, precise MP quantification was facilitated by 14C-radiolabelling of the polymers, which enabled accurate mass balancing while eliminating potential interferences from background polymers. The modified oil extraction method, achieved extraction efficiencies of up to 75% of the applied radioactivity (AR), compared to a maximum efficiency of 62%AR using the conventional density extraction method combined with KOH.