Density-Based Multi-Stage Flotation Sorting of Microplastics in Beach Sand

Microplastics (MPs) are ubiquitous in the environment and pose an increasing concern for the world’s terrestrial and marine ecosystems due to their persistence and potential toxicity. Density sorting of MPs in beach sand, combined with heat treatment to remove impurities such as wood fragments, enhances the analysis of MP contamination. While density sorting does not identify the composition of MPs, it provides insight into their sources and potential for re-drift into the ocean. In this study, we evaluated the accuracy of a multi-stage Flotation sorting technique in separating MPs based on their density in beach sand. A major challenge in density sorting is interference from impurities such as wood fragments. To address this, heat treatment is performed to remove wood fragments. We also evaluated the effects of heat treatment on the density and weight of MPs. The findings indicate that most MPs experienced a density change and a weight loss of less than 4% and 1%, respectively, suggesting the minimal effects of the heat treatment. However, certain types of MPs, such as those containing voids (e.g., PVC-NS), showed significant density changes, which impacted their sorting behavior, resulting in some misclassification during the flotation sorting. Unless the heat treatment caused a density change, the multi-stage Flotation sorting method, including water and saturated calcium chloride (SCC) solutions, achieved high recovery rates (90%-110%) for light MPs, heavy MPs, and wood and sand mixtures. In other words, light and heavy MPs and the wood and sand mixture were separated without misjudgment and loss. Overall, this study confirms the feasibility and efficiency of multi-stage flotation sorting for MP analysis in beach sand and highlights the need to carefully consider heat treatment effects in future environmental studies on MPs.