Experiment on washing procedures to extract positively buoyant microplastics (> 500 µm) from beach wrack

The "beach wrack - plastic separator" is the prototype of a simple construction that we used to assess two methods for washing beach wrack. Its main component is a polypropylene container that is 40 x 30 x 22 cm in size and has a volume of 20.8 l. Furthermore, we used two grids (39.5 cm x 25.5 cm) that could be inserted horizontally into the container and between which a beach wrack sample of 500 g could be placed. The grids were made of aluminium and had a mesh size of 1 cm x 1 cm. Two handles were attached to the lower of the two grids that allowed to move the grids vertically inside the container. As soon as the container was filled with 10 litres of tap water, the upper grid prevented the beach wrack from floating to the surface, while the lower kept it from settling on the bottom of the container. The separation of the plastic particles from the sample material was induced by moving the grids up and down manually or by bubbling pressured air, which was supplied through air inlets, through the beach wrack material.For assessing the extraction efficiency of the two washing procedures, 48 kg of beach wrack were collected at the beach of Falckenstein, which is located at the western shore of the outer Kiel Fjord, Germany, (N 54.391250, E 10.190728) from May to August 2018. After collection, we divided the 48 kg into 96 batches of 500 g each, which were then spiked with microplastic particles. Half of the batches consisted of Zostera leaves, while the other half consisted of thalli of Fucus spp. We did not dry the material prior to spiking and we used two types of polymers of three size classes to be able to assess the influence of polymer type and particle size on the recovery rate. We have used polypropylene (PP) fragments with a density of 0.88 to 0.91 g/cm3 (Herrera et al., 2018) and spheres of expanded polystyrene (EPS), with a density of 0.01 to 0.05 g/cm3 (Herrera et al., 2018). The densities of both polymer types are lower than seawater. The different particle size classes were either manually created or directly purchased. We tested the following three size classes: 500 to 1000 µm, 1000 to 2000 µm and 2000 to 5000 µm. We have produced PP fragments from plastic cups (wall thickness: 0.5 mm) that we collected at the driftline of the beach of Falckenstein, and which had the polymer type indicated on their bottom. The cups, which were free of epibionts, were cut into quadratic fragments that fell into the three size classes using a scissor. The EPS spheres were purchased in the same size classes. A defined number of particles from each of the three size classes was weighed on a laboratory scale. Then the particles were carefully mixed into the beach wrack at a weight ratio of 1 : 1x104 (2000 to 5000 µm) or 1 : 1x105 (the two remaining size classes). To achieve the weight ratios mentioned above, we either added a) 19 particles of PP or 15 particles of EPS in the size range of 1000 to 2000 µm, b) 30 particles of PP or 20 particles of EPS in the size range of 500 to 1000 µm or c), 12 particles of PP or 26 particles of EPS in the size range of 2000 to 5000 µm to one individual batch. For each replicate, the PP and EPS particles were counted and weighed individually. After spiking, we let the beach wrack rest for a maximum of 20 minutes and then placed the material between the two grids in the beach wrack - plastic separator. The lower grid was at a distance of 1 to 2 cm to the bottom, what would allow negatively buoyant microplastic particles (not tested in this study) to sink to the bottom of the container and to accumulate underneath the lower grid. The separation efficiency of both procedures was then analysed regarding the following four factors: 1. type of beach wrack, 2. polymer type 3. polymer size and 4. duration of washing. The principle component of the separation process was the induction of a water flow, which detached the plastic particles from the surface of the macrophyes and also released them from hollows between their leaves or thalli. In the manual washing process, this was achieved by moving the grids up and down ten times in quick succession with an amplitude of 15 cm. The amplitude as well as the speed and number of repetitions then resulted in an up- and downward flow of water through the sample material that was strong enough to separate the microplastic particles from the macrophytes. The released particles floated up and were picked manually from the water surface. The particles were identified as either PP or EPS particles belonging to one of the three size classes. This was done after each single movement of the grids (up and down), so that particle extraction success could be assessed for each polymer type/size class as a function of the washing effort. This procedure was repeated with both types of beach wrack (Zostera marina L. and Fucus spp.), for both particle types (PP fragments and EPS spheres) and for all size classes within each particle type. For each of these 12 treatment combinations (beach wrack type with two levels x particle type with two levels x size class with three levels), we had four replicates and we used new beach wrack and new plastic material for each of them. The air-facilitated washing of the beach wrack was also done with tap water. For this, three cylindrical diffusor stones (diameter: 50 mm) were connected to an electric air compressor (Pontec PondoAir Set 200) via tubes (diameter: 4 mm, total length: 100 cm) and placed underneath the lower grid. Again, individual batches of 500 g of beach wrack were placed in between the two grids and were then bubbled with air for four hours at an overall discharge rate of 200 l pressured air/h. This rate generated a water flow through the sample material with a velocity that was sufficient to separate the microplastic particles from the beach wrack material. In addition to this, the air bubbles themselves presumably released particles from macrophyte surfaces or from hollows between their thalli or leaves. This was done by the shear stress they exerted when getting in direct contact with a particle or by transferring a momentum that set the particle in motion.