Certainties and uncertainties in accessing toxicity of non-extractable residues (NER) in soil

Abstract

Background: Discussion concerning the bioavailability and ecotoxicological relevance of non-extractable residues (NER) in soil is still ongoing. Is NER formation a detoxification process or a hidden hazard? The use of radiolabelled chemicals enables detection of NER, but the identity of NER is usually unknown. Regulations require clear measurable parameters and the approach of Ortega-Calvo et al. (Environ Sci Technol 49:10255-10264, 2015) defines these. Results: Following that approach, we studied the fate of three ecotoxic, NER-forming chemicals over a period of 6 months after application to three different soils. Initial 14 C experiments showed formation of NER for all chemicals. For the chemical 2,4,6-trinitrotoluene (TNT), NER-formation was reproducible in all soils. We applied a recently standardized method using Tenax ® to remove the bioavailable fraction of the chemical at test start and test end. Removing the bioavailable fractions also removed toxicity. Further experiments without radiolabelled TNT clearly showed that the toxicity measured in applied soils was caused by the bioavailable chemical and not by NER. Conclusions: The tool developed can be used if the fate of the chemical including NER formation is well known and reproducible. The other selected chemicals, cypermethrin and carbendazim, showed unexpected behaviour in 14 C-fate experiments. The degree of biodegradation was not reproducible for cypermethrin and unexpected losses occurred with carbendazim. This indicated a very large uncertainty when using non-radiolabelled compounds in NER experiments and thus the tool is not suitable in non-radiolabelled experiments.

Can a chemical be both readily biodegradable AND very persistent (vP)? Weight-of-evidence determination demonstrates that phenanthrene is not persistent in the environment

“…The relevance of NER, which are only detectable in radiolabelled studies, to environmental hazard and risk assessment continues to be debated, i.e. : are they a "safe sink" or a "hidden hazard" [154][155][156][157][158]? Current ECHA guidance applies the latter interpretation [10,159].…”

Improved Assessment of Soil Nonextractable Residues of the Pyrethroid Insecticide Cyphenothrin

“…In contrast, bio-NER are generated through microbial assimilation of xenobiotics into biomolecular components ( e.g ., fatty acids and amino acids [AAs]), which can be further metabolized and incorporated into soil organic matter (SOM) after microbial cell death. , The bioaccessibility and bioavailability of xenobiotics may explain the impact of NER on the surrounding environment . Irrespective of the NER type, conversion into NER may generally reduce both of the abilities . However, while type II xeno-NER and bio-NER can be considered to be of minimum environmental concern due to their irreversible binding strength and equivalency to natural compounds, respectively, some researchers warn that type I xeno-NER could still be unaltered active substances with the potential to be released back into the environment and cause environmental damage .…”

“…10 Irrespective of the NER type, conversion into NER may generally reduce both of the abilities. 11 However, while type II xeno-NER and bio-NER can be considered to be of minimum environmental concern due to their irreversible binding strength and equivalency to natural compounds, respectively, some researchers warn that type I xeno-NER could still be unaltered active substances with the potential to be released back into the environment and cause environmental damage. 12 Despite numerous studies on NER, xenobiotics-derived chemical constituents have not hitherto been fully clarified yet.…”

Root-mediated bacterial accessibility and cometabolism of pyrene in soil

“…A pragmatic and justifiable approach for use in retrospective risk assessment was recently reported, which might be especially useful when decisions on soil remediation and reuse must be made 1 . Soil bioremediation can often initially increase chemical pollution risks as a result of biological processing of the most bioavailable pollutant fractions, leading to the formation of byproducts that are more toxic and mobile than the parent chemicals 1,2 or to nonextractable residues with unknown risks 3 . In this context, and considering PAHs in particular, cometabolism is relevant; cometabolism is a major PAH removal mechanism and often results in incomplete transformation of the PAH molecular structure, leading to the formation of products that may have increased risks.…”