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

Abstract

Under the European REACH regulation, chemicals are assessed for persistence as part of weight-of-evidence determinations of persistence, bioaccumulation and toxicity (PBT), as required under Annex XIII and supported by an Integrated Assessment and Testing Strategy (ITS). This study describes the persistence assessment of phenanthrene, a data-rich polycyclic aromatic hydrocarbon (PAH), in accordance with this framework. All available data from screening and simulation tests, for water, soil and sediment compartments, plus other relevant information, have been compiled. These have been evaluated for reliability and relevance, and a weight-of-evidence determination of persistence has been carried out. Aspects relevant to the assessment, such as degradation metabolites, non-extractable residues (NER), test temperature and bioavailability, have also been considered. The resulting assessment considered a wide range of evidence, including 101 experimental data points. Phenanthrene was demonstrated to be readily biodegradable, a first-tier screen for non-persistence in the ITS. Furthermore, weight-of-evidence assessment of data for water, soil and sediment compartments supported a conclusion of “not persistent” (not P). In non-standard soil studies with sludge-amended soils, longer half-lives were observed. This was attributable to pyrogenic sources of and significantly reduced bioavailability of phenanthrene, highlighting the importance of bioavailability as a major source of variability in persistence data. Available simulation test data for the sediment compartment were found to be unreliable due to the anoxic impact of the use of a biodegradable solvent in a closed system, and were inconsistent with the broader weight of evidence. Estimation of photodegradation using AOPWIN and the APEX model demonstrated this to be an important fate process not currently considered in persistence assessments under REACH. The assessment is not in agreement with a recent regulatory decision in which phenanthrene was determined to be very persistent (vP). This assessment provides a case study for persistence assessment using the REACH ITS and highlights the need for improved guidance to improve consistency and predictability of assessments. This is particularly important for complex cases with data-rich chemicals, such as phenanthrene.

Predicting Primary Biodegradation of Petroleum Hydrocarbons in Aquatic Systems: Integrating System and Molecular Structure Parameters using a Novel Machine‐Learning Framework

“…In addition, guidance on ranges of values for environmental and test conditions that are most relevant for persistence assessment could reduce uncertainty and provide clear direction for persistence assessment of petroleum hydrocarbons in complex environmental systems. These considerations would provide a quantitative framework that could be used to supplement the existing “weight of evidence” approaches (Hughes et al, 2020) that are currently used to evaluate the persistence properties of petroleum hydrocarbons.…”

Moving persistence assessments into the 21st century: A role for weight‐of‐evidence and overall persistence

“…The methods used to measure half‐lives are often not standard, defined, or transparent (Hughes et al, 2020 ; Webster et al, 1998 ), which can reduce the certainty associated with persistence classification. Currently, guidance is lacking to assess the suitability of nonstandard experimental methods for use in persistence assessments.…”

“…Before the publication of the OECD WoE guiding principles (OECD, 2019 ), many of the key elements outlined by the OECD were already applied in chemical safety assessments (Brandt et al, 2016 ; Bridges & Solomon, 2016 ; Giesy et al, 2014 ; Hughes et al, 2020 ; Wassenaar & Verbruggen, 2021 ). Giesy et al ( 2014 ) used a WoE approach to assess the organophosphorus pesticide chlorpyrifos.…”

“…Hughes et al ( 2020 ) also used the same R.11 guidance process to assess the persistence of phenanthrene. The studies were assessed and qualitatively weighted by relevance (similarity to standard test guidelines).…”

“…In this framework, a unit‐world approach is used to focus attention on relevant physicochemical properties driving the fate of the molecule (e.g., solubility, adsorption, volatility), and focus the evaluation on key degradation and fate processes that influence the persistence. Such an approach was applied by Hughes et al ( 2020 ) to evaluate the persistence of phenanthrene, with a P ov ‐like calculation being performed using a Level III model (see also the example given in Supporting Information).…”