Validation of a density separation technique for the recovery of microplastics and its use on marine & freshwater sediments

Currently there is no standardised method for the collection or separation of microplastics from sediment samples, hindering the generation of data on their presence and potential environmental impact. Density separation using various brine solutions has been a popular method for extracting microplastics from sediment. The aims of this work were (i) to validate a density separation method using a new brine solution of zinc bromide in comparison to other solutions for separating microplastics from sediments and (ii) to apply this method to marine and freshwater sediment samples to isolate and identify environmental microplastics. The efficiency of four brine solutions (sodium chloride, sodium bromide, sodium iodide and zinc bromide) and water to separate out microplastics from a marine sediment (200-400 µm) spiked with different plastics was tested. The plastics included polyethylene (PE), high density polyethylene (HDPE), nylon, polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC). From the validation test it is evident that the best recoveries (>90%) were obtained with both the sodium iodide or the zinc bromide solution. Although sodium iodide gave higher recoveries for PVC (97%) and HDPE (98%), overall the zinc bromide gave better recoveries for the majority of the plastics tested including the polyethylene face wash (99%) and the plastic mixture (99%). The zinc bromide brine solution was used to test for microplastics in marine and freshwater sediments taken along the firth of Clyde, Scotland. Density separation using zinc bromide brine solution is an effective method for the separation of microplastics from sediments. Although expensive, zinc bromide can be sucessfully reused indefinatily (once re-saturated to 25%) and is a cost effective and efficient method as high recovery rates are achieved after one sample run, as opposed to the three runs needed when using sodium chloride.