The ECHA guidance on persistence (P) assessment considers non-extractable residues (NER) as non-degraded parent compound. In degradation tests with radiolabeled substances, total NER consist of the fractions sorbed/sequestered (NER I), covalently bound (NER II), and bioNER (incorporated into the biomass, NER III). Mainly NER I pose a risk due to potential release.
Degradation half-lives DegT50 of 24 substances were determined for four scenarios: i) extractable parent compound, ii) parent plus total NER, iii) parent plus xenoNER, and iv) parent plus NER I (few data). Using the Microbial Turnover to Biomass (MTB) method, xenoNER (the sum of NER I and II) were calculated as total NER minus bioNER.
The inclusion of NER in P assessment had significant effects on the DegT50, with shortest half-lives in scenario i) extractable parent only followed by iv) parent+NER I, then iii) parent+xenoNER, while ii) parent+total NER had the longest half-lives. One third of the substances had DegT50 above the P criteria with scenario i, but two thirds with scenario ii. Experimental quantification of NER I (scenario iv) significantly reduced half-lives, compared to total NER. For two compounds, the subtraction of calculated bioNER led to a change to `not persistent´.
For this comparison of half-lives, the SFO (Single First-Order) kinetics was chosen. Double-First-Order in Parallel (DFOP) gave often better fits, but the half-lives found for the slow phase of the DFOP gave puzzling results and were above the P criterion for all tested substances with scenario ii) total NER.
Non-Extractable Residues and the Change in Persistence Assessment
Pesticide Research No. 228, November 2024