These protocols are likely to be effective to destroy adhering biofilms that also predominately are composed of polysaccharides, proteins and lipids. (2017) found highest efficiencies in applying KOH with NaClO (1:1) to fish stomach samples. Alkaline solutions, such as potassium hydroxide (KOH) also provide good results concerning the digestion of biotic tissue ( Foekema et al., 2013 Nuelle et al., 2014 Collard et al., 2015 Dehaut et al., 2016 Karami et al., 2017 Prata et al., 2019). (2017) confirmed this for HNO 3 and hydrochloric acid (HCl) digestion of fish tissue. For example, applying strong acids, such as nitric acid (HNO 3) led to very good digestion results regarding biota tissue, mainly consisting of proteins, carbohydrates and fats ( Nuelle et al., 2014 Lusher et al., 2017 Naidoo et al., 2017). The efficiency of biogenic organic matter destruction is matrix-dependent. For this purpose chemical digestion with acids, bases and oxidizing agents and enzymes are applied.Ī number of studies address a comparison of different digestion methods to optimize the efficiency of biogenic organic matter destruction ( Nuelle et al., 2014 Cole et al., 2015 Dehaut et al., 2016 Enders et al., 2017 Karami et al., 2017 Herrera et al., 2018 Hurley et al., 2018 Munno et al., 2018 Prata et al., 2019 Duan et al., 2020). The optimal digestion method for the respective samples eliminates the biogenic organic matter as much as possible while preserving the target particles of synthetic polymers. The isolation of plastic particles requires the removal of the natural sample matrix consisting of mineral and biogenic organic substances. However, especially considering the detection of small particles, the separation of plastic particles from the sample matrix still poses a major challenge. The investigation of environmental pollution by synthetic polymers and its effects is a rapidly developing research discipline. As a concluding recommendation the selection of the appropriate digestion method should be specifically tested within initial pre-tests to account for the specific composition of the sample matrix and the project objectives. This effect is most pronounced for polyamide in the presence of acids and for polyethylene terephthalate digested with bases.
This is most evident for polyamide and polyethylene terephthalate, which show losses in weight between 15 and 100% when the digestion temperature is increased. Increasing the concentrations and temperatures, however, results in accelerated degradation of all polymers. The resistance of tested polymer types can be rated as good except for polyamide and polyethylene terephthalate. The additional temperature increase to 60–70☌ showed a further but less effective improvement, compared to the initial temperature increase. However, the other reagents can also have good effects, especially by increasing the temperature to 40–50☌. Soft and hard tissue were most efficiently destroyed by sodium hypochlorite. Calcareous shells showed no to very low reaction to oxidizing agents and bases, but were efficiently dissolved with both tested acids at all concentrations and at all temperatures. The efficiency of organic matter destruction is highly variable. Before and after the application of different digestion protocols, the material was weighed in order to determine the degree of digestion efficiency and polymer resistance, respectively. Tests were carried out in three parallels on organic material (soft tissue-leaves, hard tissue-branches, and calcareous material-shells) and six polymers (low-density polyethylene, high-density polyethylene, polypropylene, polyamide, polystyrene, and polyethylene terephthalate) in two size categories. Within this study we tested acids (nitric acid and hydrochloric acid), bases (sodium hydroxide and potassium hydroxide), and oxidizing agents at different concentrations, temperature levels, and reaction times on their efficiency of biogenic organic matter destruction and the resistance of different synthetic polymers against the applied digestion protocols. The selected protocols comprise different solutions, concentrations, and reaction temperatures. Therefore, a concise literature survey has been undertaken to identify the most commonly applied digestion protocols with a special focus on water and sediments samples.
Organic residues hamper the separation of polymer particles especially within density separation or polymer identification via spectroscopic and staining methods. The digestion of biogenic organic matter is an essential step of sample preparation within microplastic analyses. Department of Earth Sciences, Center for Earth System Research and Sustainability, Universität Hamburg, Hamburg, Germany.Felix Pfeiffer and Elke Kerstin Fischer *
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