# Notes on Common Misconceptions in Microplastics Removal from Water
## 3 Microplastics Removal from Water
### Separation  
There are also methods that do not attempt to eliminate microplastic pollutants but intend to concentrate them and separate them from the effluent. Separated particles can be collected and transported to a municipal waste incineration plant. Separation methods comprise density separation, vacuum filtration through a porous filter preceded by a sieve and sieving through a series of sieves with decreasing mesh sizes down towards 50 \(\upmu\)m [100]. Due to its relatively simple experimental arrangement, it can be considered a dominating method. However, nanoparticles would require very fine sieves, which can exhibit high pressure drop, quick clogging and requirement of frequent replacement and cleaning. Density separation might thus provide a more meaningful way. Density separation was further developed (various extraction liquids, leaching in H\({}_{2}\)O\({}_{2}\), flowing liquid, stirred-shaken sediment in gravitational or centrifugal field, in counter-current gas = foam flotation or elutriation) and high separation efficiency was achieved using a drum separator [101], or by electromagnetic density separation in a ferroliquid, which can change its density according to EM radiation ([PERSON] and Franchetti [102]). Some of the plastics may have very similar density to water and that could be a major drawback of the practical application of such method. The most often used separation technique, flotation, can be divided into three subgroups: selective flotation, froth flotation and hydrodynamic flotation [103]. Selective flotation in a standing liquid practically corresponds to density separation (the particles are divided according to different densities; sink-float method). The method is based on the separation of one kind of microplastic from a mixture of two or more. Separation is most often performed from sediments [104; 105]. However, these methods reveal poor efficiency with particles smaller than 200 \(\upmu\)m [106]. HDPE particles (100-850 \(\upmu\)m) were tested for flotation in salty solutions in yields above 90% for high concentration samples and about 80% for low particle concentration samples [107]. The content of salt (NaCl, NaI) enhances the flotation of microparticles, because salt increases the difference between the density of water and microplastic particles. That provides an indication that the method would be feasible for salt water; however, not all types of microplastics would be possible to separate in practice--it would have to be carefully chosen materials with appropriate density, which would be different enough from water density. High flotation yields were obtained using potassium ferrite; almost 100% yield was obtained for PC and PS particles of 2-4 mm [108]. Treatment with K\({}_{2}\)FeO\({}_{4}\) changes surface properties and makes particles more floatable, due to increasing surface wettability. The question is, why particles of such size were not just filtered. Moreover, another aspect is that the water must be treated from K\({}_{2}\)FeO\({}_{4}\). Selective flotation might be a very useful tool in the area of microplastic removal. The method is able to remove particles larger than 0.2 mm and, therefore, it can serve as the first significant method of microplastic reduction in the environment. These particles are easy to remove because they can still be visible to the human eye and, therefore, do not require a complicated membrane separation (membrane surface can be easily clogged and fouled by particles of smaller dimensions). Froth flotation, where the particles are divided according to densities and air bubbles, uses the different hydrophobicity of the particle surfaces for their separation. Very high efficiency was achieved for removal of PC, PVC and PMMA particles in a solution containing potassium permanganate [109]. However, removal of potassium permanganate must be consequently dealt with. Hydrodynamic flotation (elutriation) is a method where the distribution of particles according to density is supported by hydrodynamic flow.