6.3.2. Removal of organic pollutants

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To remove organic material bound to microplastics, 30 m/m% H2O2, HCl solution, Fenton reagent (H2O2 +Fe(II)), NaOH solution, or enzymatic treatment are most commonly used, but the applied material also depends on the sample matrix. For living organisms, tissue is usually removed with 10 m/m% KOH solution, 65 m/m% HNO3, or NaOCl [202]. When choosing the chemical to use, it is important that it does not damage the material structure of the microplastic particle itself or that no further fragmentation occurs during treatment. Although hydrogen peroxide proved to be more effective than hydrochloric acid or sodium hydroxide in removing biological residues, when using an H2O2 solution with a concentration higher than 30 m/m%, discoloration and foaming of the plastic particles were observed [178]. NaOH has proven to be effective for cleaning microplastics at higher temperatures but at concentrations of 10 mM at 60 °C it can already damage and discolor microplastic particles [191]. Strong mineral acids, such as sulfuric or nitric acid, can also damage plastics (e.g. PE, polyamide, polystyrene) [49], [54], [175]. The Fenton reagent can also oxidize other organic compounds bound to microplastics, so it is not recommended if one want to measure these as well [175]. Enzyme treatment offers a gentler solution for sample cleaning, but at the same time it takes much more time than chemical treatments. Their efficiency is often not sufficient for complete purification, and the most used proteinase K enzyme is much more expensive than the traditional chemicals mentioned above. A multistep procedure, such as enzyme treatment + H2O2, is usually required for efficient but gentle sample purification [171]. Ultrasonic treatment has also been described for removing organic material, but the disadvantage of this method is that it can cause further fragmentation of microplastics [49], [196].
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