Density separation with calcium chloride solution

Calcium chloride (CaCl₂) was used for the density separation in order to separate synthetic fibers and fragments from the mineral fraction of the sediment. The salt is inexpensive and environmentally friendly. We produced the salt solution by dissolving the salt in deionized water under permanent stirring at 20 °C. The density of the solution was measured with an aerometer and its initial density was 1.42 g/cm³. However, during sample processing, the density of the solution occasionally dropped to a minimum of 1.34 g/cm³, depending on the water content in the sediment samples and on sample mass. Solution density was then re-adjusted to 1.4–1.42 g/cm³ by adding CaCl₂ after sample processing. Sediment samples from the different sites as well as the different size fractions from each of the sites were processed separately. For each density separation, a maximum of 1 kg of sediment was mixed with 1 l of CaCl₂ solution in an Erlenmeyer flask. This was the maximum ratio of sediment to CaCl₂ solution that still allowed an effective ventilation of the mix during the separation. On average, 629 (± 290) g of sediment were filled into one Erlenmeyer flask and mixed with 1 l of CaCl₂ solution. Sediment size fractions that exceeded 1 kg in wet weight were distributed to more than one Erlenmeyer flask and mixed with 1 l of CaCl₂ solution per flask. The resulting suspensions were ventilated for a total of 40 min, and the flasks were rotated every 10 min by 90°. This was done to ensure that all the sediment in the flasks was whirled up by the air stream. Thereby, organic particles or microplastics were separated from the heavier sediment and were brought into suspension. After 40 min, we let the suspensions rest for 15 to 18 h. After this period, the supernatants were carefully sucked into a washing bottle by the use of a vacuum pump. Subsequently, they were then filtered with a 0.2-mm polyester round filter, which was placed in a Büchner funnel that was put on top of a 5-l glass bottle. When the glass bottle was full, the filtered CaCl₂ solution was used again for further separations. The extraction step was repeated three times according to the recommendation of Besley et al. (2016), who observed an increase in microplastic recovery with an increasing number of extractions. Finally, the filter was rinsed with 35% hydrogen peroxide and stored in a closed glass Petri dish to dissolve the organic matter that was present in the sample. After 24 h, each filter was carefully rinsed with deionized water to remove remaining organic residues and stored again in a separate glass Petri dish until inspection. The applied density separation with air-venting and rotation of the Erlenmeyer flasks as well as the treatment with hydrogen peroxide was done following the approach by Stolte et al. (2015).