Consideration of non-extractable residues (NER) in PBT-assessment

The formation of NER in simulation studies on degradation in the environment is an issue considered in all regulations of chemical substances in Europe. Based on efforts in the last years, to differentiate NER into different categories of concern, in September 2018 this project was initiated by UBA in order to harmonise the results with the procedures described in the ECHA discussion paper on NER (Kästner et al., 2018). In the project, the applicability of a harmonised approach was evaluated experimentally. A set of three different substances was tested in a standard OECD 307 approach. Different procedures for exhaustive extraction were applied in order to compare NER determination approaches. We conducted these experiments using test substances labelled with either 13C or 14C in parallel. Results indicate that data from literature using either label can be reconciled. We showed that NER characterization methods like EDTA extraction and silylation are reproducible and applicable for routine analyses. Experiments were accompanied by the modelling tool Microbial Turnover to Biomass (MTB) (Trapp et al., 2018). This tool relies on the CO2 measured from the mineralization of test substance as indication of microbial degradation in order to estimate the formation of biogenic NER. Experimental data determined in the current project were used to verify the outcome of this approach. The results of the project were discussed at an international workshop at the German Environment Agency in February 2021. The aim of the workshop was to present and discuss a practicable approach for a harmonised test procedure for NER characterisation. We hereby present two approaches for discussion to determine the relevant fractions of non-extractable residues and to consider them for the derivation of half-lives in the framework of persistency assessment (PBT/ vPvB) of these substances. The approaches are in line with the requirements of ECHA for the handling of non-extractable residues in the assessment of persistence.