Principal Investigator
Dr. Boris Meisterjahn
Fraunhofer Institute for Molecular Biology and Applied Ecology (IME)
Auf dem Aberg 1
57392 Schmallenberg
Germany
Email: boris.meisterjahn@ime.fraunhofer.de
Phone no: +49 2972 302 435
Collaborators
Megan Griffiths, Ricardo, Megan.Griffiths@ricardo.com
Dr. Glauco Battagliarin, BASF, glauco.battagliarin@basf.com
Christopher Hughes, Embark Chemical Consulting, chris@embarkchemical.com
Description
Biodegradation screening testing and more complex biodegradation simulation studies are requested under regulatory frameworks such as REACH for persistency assessment of low molecular weight (“small”) molecules. While polymers and polymeric substances were initially exempted from REACH, this will change in the near future, and implementation of polymer-specific modifications of the respective ordinances and guidance are necessary.
In this context, the question arises whether standard test systems used for the determination of monomeric substances degradability, are appropriate for polymers and polymeric compounds. These tests might need modifications to obtain meaningful data on the biodegradation of polymers in environmental compartments, as previously reported in the literature (Menzies et al., 2023). Possible modifications of the tests are not limited to the test setup and technical aspects (e.g., test duration, etc.), but possibly require alternative endpoints for test evaluation, which might be different when compared to small molecules (e.g. changes of the molecular weight distribution, definition of primary degradation).
Within biodegradation screening tests (OECD 301 or 310 guidelines) mineralization are tested using relatively high-test substance concentrations (2-100 mg/L). In simulation tests, usually performed in complex matrices such as surface water, soil, sediment or sewage sludge, typically radiolabelled (14C) materials are used. Using radiolabelled materials allow testing at much lower and more realistic concentrations (OECD 309 ca. 10 to 100 µg/L), which might prove benefits in representing the actual biodegradation behaviour in the environment. In addition, while not changing the substance in its chemical structure, the radiolabel offers the possibility to obtain data on distribution between compartments, differentiate metabolites and degradation products from natural backgrounds and enables the establishment of mass balances. Especially the possibility to close the mass balance across the different compartments of the respective test matrices enables the assessment of otherwise not accessible fractions such as non-extractable residues (NER), which can be of different types, representing either a hidden hazard (e.g. remobilisable, physically entrapped NER) or a safe sink (e.g. NER material incorporated in the biomass or covalently bound to the environmental matrix). The radiolabel allows for a highly specific analysis of the test substance in complex matrices and thus can be applied for the evaluation of new or adapted analytical methodologies used within these studies.
While investigations on polymers using biodegradation screening tests (OECD 301 or 310 guidelines or ISO standard methods) have been previously reported, only few data similar to OECD degradation simulation studies (OECD TGs 307, 308, 309) are available on polymers so far (Hahn and Hennecke, 2022, Duis et al., 2021). This is mainly a consequence of the low availability of 14C labelled polymeric materials because of the high costs of the starting materials and the limited infrastructures for the synthesis and characterization of radiolabelled polymers. However, using 14C materials is considered as one approach to overcome some of the challenges related to polymer testing. An evaluation of the standard test systems for biodegradability testing of polymers is already available in recent reviews and within the CEFIC LRI Eco52 reports (2022).
In recent years the environmental fate of polymers of plastics and microplastics has been receiving increasing attention. Water soluble polymers have remained by far less investigated, despite their importance in many applications, such as home and personal care or agrochemical formulations. Among the most used biodegradable water-soluble polymers are e.g. PEGs or PVOHs (Vandermeulen et al., 2022). Water soluble polymers exhibit high molecular weights that hinder direct microbial uptake, as in the case of plastics and microplastics. However, their solubility in water impacts their partitioning and interaction with surfaces and microorganisms. Considering the current regulatory discussion, detailed investigations targeting water soluble polymers are needed.
For the investigation of polymer biodegradation, gaps remain also in the identification of suitable reference materials. Reference substances are of key importance during the development of test methods, to better understand the impact of the polymer concentration on biodegradation profile, which might have significant influence. Typically, in standard test systems reference substances are used to demonstrate that the inoculum used is sufficiently active to biodegrade the chemicals under investigation. The most commonly used reference substances in OECD testing guidelines are sodium benzoate and aniline. However, these substances have previously been criticised as too easily biodegraded. Due to their low molecular weights, they are readily available for metabolization and might not be a good proxy to indicate the presence of suitable exo-enzymes needed for polymer degradation. This was recently criticised in screening test setups where the inoculum is treated in a way that exo enzymes will be mostly eliminated before the test. As part of the ECO55 project, various other candidate reference substances have been investigated for use in the OECD 309 test guideline, with a ring trial currently taking place using some of the candidates. It is also worth noting that reference substances are not currently used in either OECD TG 307 or OECD TG 308 tests, leaving a gap in understanding degradation processes of polymers in those matrices.