Repeatable separation of microplastics integrating mineral oil extraction and a PDMS-Ni foam adsorbent in real soil

The accumulation of microplastics (MPs) in the environment has accelerated. It is very challenging to separate MPs from complicated solid substrate matrices such as soil and sediments not only because solid substrates usually adhere to or surround MPs but also because there is no standardized method for extracting MPs. Here, we report a novel adsorption-based repeatable process for separating and efficiently recovering MPs from environmental matrices using mineral oil and a polydimethylsiloxane (PDMS)-coated nickel (Ni) foam adsorbent. The hydrophobic/magnetic adsorbent was produced by chemical vapor deposition of PDMS on Ni foam. This process created a hydrophobic surface throughout three-dimensional (3D) porous Ni network, providing high porosity, high surface area, and robustness. Compared to tested vegetable oils, mineral oil was superior at extracting seven types of MPs (LDPE, PP, PS, PC, PET, PVC, and PTFE) with different densities of 0.94 - 2.2 g/cm3 from real soil samples. After the mineral oil extraction, by stamping the PDMS-Ni foam on the floating oil layer of the extracted materials, oil-captured MPs of various types and sizes (70 - 1200 mu m) were successfully recovered and identified in seconds without accompanying unwanted particles. In addition, due to the reusability of the PDMS-Ni foam, the MPs were isolated several times simply by supplementing the sample with mineral oil, and this procedure results in over 99% of MPs becoming isolated from complex solid matrices. Thus, our adsorption-based repeatable process for separating MPs can allow for an accurate quantitative analysis of microplastic accumulation in solid matrices.