# Microplastics in Water Resources: Threats and Challenges
## 4 Methods for Separation and Extraction of MPs from Environmental Matrices
### Density Separation  
Currently, the most common and widely developed method for separating MPs from environmental samples is density separation. High-density solutions are prepared from filtered water in which large amounts of highly soluble salts are dissolved [102]. As a result, plastic fragments with a density lower than that of the separating solution can float to the surface, while heavier particles remain at the bottom. MPs can then be extracted by filtering the separation medium, followed by analysis and characterization. Since the density of most plastics ranges from 0.8 g/cm\({}^{3}\) (silicone) to around 1.5 g/cm\({}^{3}\) (polyethyleneterephthalate (PET), polyvinyl chloride (PVC), and polyoxymethylene), the denser the solution, the more types of polymers can be extracted from sediments [103]. However, it is important to note that the density of polymers like PVC can vary significantly due to the presence of plasticizers, which are commonly added to increase flexibility. For example, pure PVC has a density of \(\sim\)1.4 g/cm\({}^{3}\), making it difficult to separate using standard density solutions, as it often sinks even in high-density media. On the contrary, plasticized PVC can have a reduced density of \(\sim\)1.2 g/cm\({}^{3}\), making it easier to float. This variability complicates the separation process and highlights a key limitation of density-based methods for certain polymers [104]. In reality, polymers such as PVC and polytetrafluoroethylene (PTFE) have densities higher than those of others; hence, they are more challenging to extract via density separation. This method is most effective for samples containing a high proportion of mineral components [105]. Soil and sediment samples contain a significant amount of organic matter, which has a density similar to polymers, approximately in the range of 1 to 1.4 g/cm\({}^{3}\)[106]. Consequently, not only polymer fragments but also all organic particles with similar densities float to the surface of the separating liquid [107]. Current devices used for density separation, including beakers and separators shaped as inverted funnels with valves, have drawbacks in terms of recovery speed and operational complexity [108]. There is still a need to refine this method and objectively consider it as just one of the sample preparation processes [109]. Figure 2 presents the most widely used separating salts along with the density of their saturated solutions. All plastics with a density higher than the given salt density can be separated using it. The densities of the plastics are approximate, as they can vary depending on the filler and additive content. As seen in Figure 3, the most effective salts for microplastic density separation are K\({}_{2}\)CO\({}_{3}\), ZnCl\({}_{2}\), NaI and ZnBr\({}_{2}\). These salts differ in not only density but also other characteristics. NaBr, NaI, ZnCl\({}_{2}\), and ZnBr\({}_{2}\) are toxic salts with a high potential environmental impact and are also costly [103]. Therefore, the most optimal salts in terms of cost and safety are K\({}_{2}\)CO\({}_{3}\) and CaCl\({}_{2}\). These salts are characterized by low toxicity and have even been approved as food additives, making them safer alternatives for laboratory use. Figure 3: The applicability of saturated salt solutions in the process of density separation of microparticles of individual plastics. Plastics with a density lower than the density of the salt flow to its surface [103]. PP—polypropylene, LDPE—low-density polypropylene, HDPE—high-density polyethylene, PS—polystyrene, PA—polyamide, PUR—polyurethane, PC—polycarbonate, PVC—polyvinyl chloride, PET—polyethylene terephthalate, POM—polyoxymethylene. To minimize contamination risks, it is crucial to implement blank controls and filter all salt solutions before use. This step is particularly important because chemical reagents are often packaged in plastic containers and may come into contact with plastics during production, potentially introducing additional MPs into the samples.