Various Digestion Protocols Within Microplastic Sample Processing—Evaluating the Resistance of Different Synthetic Polymers and the Efficiency of Biogenic Organic Matter Destruction

Abstract

The digestion of biogenic organic matter is an essential step of sample preparation within microplastic analyses. Organic residues hamper the separation of polymer particles especially within density separation or polymer identification via spectroscopic and staining methods. 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. The selected protocols comprise different solutions, concentrations, and reaction temperatures. Within this study we tested acids (nitric acid and hydrochloric acid), bases (sodium hydroxide and potassium hydroxide), and oxidizing agents [hydrogen peroxide, sodium hypochlorite and Fenton's reagent (hydrogen peroxide 30% in combination with iron(II)sulfate 0.27%)] 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. 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. 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. The efficiency of organic matter destruction is highly variable. 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. Soft and hard tissue were most efficiently destroyed by sodium hypochlorite. However, the other reagents can also have good effects, especially by increasing the temperature to 40–50°C. The additional temperature increase to 60–70°C showed a further but less effective improvement, compared to the initial temperature increase. The resistance of tested polymer types can be rated as good except for polyamide and polyethylene terephthalate. Increasing the concentrations and temperatures, however, results in accelerated degradation of all polymers. This is most evident for polyamide and polyethylene terephthalate, which show losses in weight between 15 and 100% when the digestion temperature is increased. This effect is most pronounced for polyamide in the presence of acids and for polyethylene terephthalate digested with bases. 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.

Development of screening criteria for microplastic particles in air and atmospheric deposition: critical review and applicability towards assessing human exposure

“…For studies where a digestion step was necessary, a score of '2' is assigned where the impact of the digestion method on different polymers has been assessed and where incubation temperatures are < 50°C [34,63], identified as being important for ensuring that damage to MPs or changes in glass transition temperature have not occurred. Most studies, however, used temperatures > 50°C, and were assigned a score of '1' or provided insufficient information to fully evaluate how the samples were treated, receiving an evaluation score of '0'.…”

Measuring impacts of microplastic treatments via image recognition on immobilised particles below 100 μm

“…In general, negative impacts of elevated temperatures during treatments have also been demonstrated by various studies. For instance, increased mass loss for PA in H 2 O 2 (among other treatments), was reported for temperatures between 60 and 70°C [14] and for > 70°C [25]. In our qualitative analysis PA did not show signs of degradation, however, we cannot infer whether particle losses or area changes may have resulted from these treatments, as no treatments involving elevated temperatures were part of our quantitative analysis.…”

“…We defined the categories with the goal to differentiate the spectra into groups of similar chances for successful spectroscopic polymer recognition. Similar categorisation approaches have been taken by other MP treatment validation studies [1,14]. Category "x" was assigned when an evaluation could not be conducted, because, independent of treatment, no exploitable spectral bands spawned from the respective polymer with the respective spectroscopic technique.…”

“…For this purpose, as a positive control, we used one additional wafer with PA6 particles. This wafer was submerged in hydrochloric acid (37%) for 20 min at room temperature (which was shown to completely dissolve PA6, [14]). Pre-and post-imaging, as well as any rinsing steps were conducted as described above.…”

“…Microplastic (MP) purification techniques were developed by a number of studies in order to facilitate measurements of abundance of this inhomogeneous group of environmental contaminants [1][2][3][4][5][6][7][8][9][10][11][12] and reviews of recent developments are available [13][14][15][16][17][18]. Within MP, practically any synthetic polymeric material can be of interest, among others classical thermoplastics, varieties of elastomers as well as paint resins.…”