Unearthing nanoplastics in soil: optimising extraction and purification while preserving particle integrity

Nanoplastics (NPs) are considered to be widespread environmental pollutants but little is known about their occurrence and properties in soils. Here, we evaluate and optimise an extraction and purification method for NPs in soil, aiming to preserve particle integrity and assess the potential for a single extraction workflow to support characterisation by both advanced microscopy techniques and mass-based techniques such as Py-GC-MS. This targets comprehensive characterisation of NPs, including size, shape, polymer chemistry, and mass concentration data. Individual extraction and purification steps were optimised, including density separation by centrifugation using a sucrose solution, filtration to < 1 μm by vacuum filtration and concentration combined with purification using direct flow ultrafiltration. Recovery tests using Pd-doped NPs aided baseline performance quantification. Recoveries were 38% for extraction (7% SD), 74% for density separation (18% SD), 92% for filtration (15% SD), and 74% for ultrafiltration (7% SD). The final combined method comprising these steps in sequence had a low recovery of 1.4% (0.4% SD), demonstrating the challenge of particle-preserving NPs extraction from soil. Next, we assessed the suitability of our final combined NP extraction method for analysis of NPs by Py-GC-MS and SEM. As part of this feasibility assessment, we tested a step to transfer the extracted NPs for Py-GC-MS analysis by dissolving them in a mixture of 1,2,4-trichlorobenzene and p-xylene and drying aliquots in pyrolysis cups (87% recovery, 41% SD). However, high sample dilution during the extraction resulted in a high method detection limit, which was unsuitable for quantitative Py-GC-MS analysis of NPs extracted from soil. In contrast, the method was well suited to qualitative analysis of NPs using advanced microscopy techniques, with SEM images revealing highly purified samples and minimal contamination from soil. As the first study to both evaluate recovery for a particle-preserving NP extraction method in soil and demonstrate its application to analytical techniques, this work provides a foundation for future improvements in NP extraction and analysis methods.