Testing the use of sodium polytungstate for an efficient extraction of microplastic particles from river sediments

Rivers are the primary pathways for microplastic (MP) particles from terrestrial sources to the sea, but they can also be temporary reservoirs of MPs, that can be easily stored in alluvial sediments. Knowing the MPs content in sediments is critical for i) understanding where and how they accumulate over time, ii) assessing their toxic effects, and iii) developing mass balance models based on their fate and fluxes. Given the rapid growth of the research field and the lack of standardized methods, extraction strategies for MP particles from sediments have become inconsistent, and new techniques are constantly being developed. Several studies highlight that density separation using high-density concentrated saline solutions is one of the most reliable and efficient separation methods. In this study, we tested the efficiency of sodium polytungstate (ρ = 3.1 g · cm^-3), diluted with distilled water to a density of 1.6 g · cm^-3, as a density separation agent. Sodium polytungstate has an intermediate density between that of sediments and plastic and has also been used successfully for the gravity separation of minerals and rocks. Furthermore, it is non-toxic, it is stable in the pH range of 2-14 and can be easily recovered and reused. In this study, artificial sediment samples were created by adding 50 MP items into 25 grams of plastic-free sand. MPs-free sediments were sampled from Pleistocene alluvial deposits of the Upper Valdarno Basin (Italy) after removing the surficial layer to avoid contamination. Sediments were divided into three different particle size ranges (i.e., 250-63 μm, <63 μm, <250 μm). Different shapes of MPs were used in the experiments: i) fibres obtained by cutting 500 μm long segments of a nylon drawstring, ii) fragments created by cutting PVC pipes, and iii) PET glitters used as films. A total of 45 experiments were performed. Before applying the procedure for density separation, samples were prepared using Wet Peroxide Oxidation (WPO) to remove the organic matter. Successively, a sodium polytungstate saline solution was used for the extraction technique and the suspended solids were collected and transferred to filters. The MP particles were counted using a stereomicroscope. Results highlight the very high efficiency of the system, although some variance is present and related to the different shapes of MPs and the sediment particle size.