Micro plastic analytics - mass content approach
Recently, quite many studies and projects investigated the presence of micro plastic in the enivornment. Basically, two analytical prinicples resulting in two different parameters are being used:
● Particle number, either with reference to the area (surface water bodies, for example particles / m², particles / km²) or reference to the volume (particles / m³), determined by spectroscopic processes, mainly used for ecotoxicological evaluation
● Mass content or concentration with reference to volume (mg / m³, μg / m³), determined by thermoanalytical methods for the quantitative description of the wastewater composition.
These two principles reflect the traditions in the disciplines ecology and urban water management. Mass concentrations can be converted into loads, so material flow balances can be created. Mass balances help to justify regulatory frameworks based on the occurence of micro plastic. It is not possible to convert results gained with either method into the respective other.
The department of Urban Water Management established routines for a thermoanalytical method at Technische Universität Berlin.
Results and next steps
Depending on the analytical method, the compartment and the matrix effects, samples have to be processed further. The procedures for removing inorganic and organic components are necessary but time-consuming. In order to ensure a loss-free and contamination-free processing without affecting the analyte, the laboratory samples for the measurement with the thermal extraction desorption-gas chromatography- mass spectrometry (TED-GC-MS) were freeze-dried, mass balanced and then grinded for homogenization. As a result of the grinding, the average particle diameter is reduced, which is intended to prevent small particles that occur much more frequently in terms of quantity from becoming less relevant than larger particles that are more relevant to mass. The TED-GC-MS includes the following steps:
- The sample is heated up to 600 ° C under a nitrogen atmosphere. The sample and the polymers present in it decompose and the degradation products are collected on an adsorber material.
- This then goes into the gas chromatograph, where the degradation products are desorbed again and loaded onto the column of the gas chromatograph. There the substances are separated.
- The final step is the detection in a mass spectrometer.
In a recent study, this method was used to determine the five different polymers that represent a total of 63% of the production volume in 2018. In our investigations, detection limits in the range of 1.1 - 4.4 µg could be achieved for the various polymers. The detection of the polymer PVC turned out to be difficult due to the large number of degradation products that also occur in other wastewater components and was therefore postponed for the time being. Appropriate validation methods must be used to generate scientifically sound results. One parameter here is reproducibility, which also provides information about the homogeneity of the sample. This is a particular challenge for samples that are rich in solids. For the TED-GC- MS in the present study, a reproducibility of 24% was achieved from ten measurements of the same sample. To check the accuracy of the results, recoveriy tests were also carried out with synthetic samples made from rock flour and various pure polymers and with spiked matrix samples. First preliminary results show a recovery rate of 79-119% for the synthetic samples in the inorganic matrix and 70-83% for the polymers PE and PP in the spiked matrix samples. In order to be able to classify the measurements correctly, round robin tests to compare the analysis with other laboratories are still pending.
More information about the Project is available in the article, which is also linked:
T. Fuhrmann, I. Urban, H. Scheer (Essen), P. Lau, L. Reinhold, M. Barjenbruch (Berlin), K. Bauerfeld, S. Meyer (Braunschweig) (2021): Mikroplastik-Emissionen aus Kläranlagen – Welche Rolle spielt die Abwasserbehandlung? In: KA Korrespondenz Abwasser, Abfall, Jg. 69, Nr. 9, 2021, S 730-741
Potential applications in the BSR
All measures, that help to reduce the amount of micro plastic released or circulating in the environment, have to be evaluated. Two parallel approaches, one particle based, one mass based are pursued in science, both showing promissing results. The load based approach suggested by the Department of Urban Water Management allows to directly quantify the efficiency of treatment methods. In combination with the sampling method ROSI, a suggestion for efficient and reliable micro plastic has been developed and is waiting for further adaptation and standardization.