Improvement of flotation-sieving method for extracting soil microplastics and delineating measurable particle-size fractions

This study developed a mass-based quantification approach combined with a modified flotation-sieving method to determine microplastics (MPs) of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) in soils. MPs extraction was improved at three levels: (1) the standard procedure to collect floating MPs from the surface of the soil suspension, (2) a optimized procedure to collect suspended MPs from the top 10 cm of the soil suspension, and (3) determination of the measurable MPs limits. Three particle-size ranges (0.053-0.149 mm, 0.149-2 mm, and 2-5 mm) and three farmland soils were used to prepare MPs-spiked samples. Results showed that for 1% MPs-spiked soil of LDPE, HDPE, and PP with particle sizes larger than 0.15 mm, the standard extraction achieved recoveries exceeding 90%. For particle sizes of 0.053-0.149 mm, the standard extraction yielded recoveries below 60%. Using the optimized extraction, recoveries of small particles (0.053-0.149 mm) increased to over 80% for HDPE and PP but not for LDPE. This indicates that the measurable limits for the former two but not for the latter could approach 1% in soil. Further testing for LDPE in soils spiked with 2%, 5%, and 10% MPs showed recoveries of small particles above 80%, indicating a measurable limit of approximately 2%.