Understanding the attenuation rate of organic chemicals in aquatic environments is essential for assessing their long-term ecological risks and informing chemical management. Biodegradation is a key attenuation pathway for many chemicals, however, measuring biodegradation in situ is inherently complex, resource-intensive, and subject to high environmental variability. Consequently, regulatory frameworks and research efforts often rely on laboratory tests, intended to provide controlled and reproducible conditions. Despite their widespread application, few studies have evaluated how well these laboratory results represent biodegradation in real aquatic environments. In this study, we compared biodegradation rate constants (k) from a modified OECD 309 laboratory test with those derived from a field experiment in a subtropical river (Queensland, Australia). Biodegradation was measurable in both laboratory and field experiments for 23 chemicals spanning diverse structures. Rank-order agreement between laboratory k and field k was strong (Spearman ρ = 0.7, p < 0.001), and linear association was high (Pearson r = 0.8, p < 0.001) for the 16 chemicals that did not sorb strongly. The relative magnitudes of laboratory and field log k also aligned well (slope of the linear regression = 0.8), however, uncertainty remained (average root-mean-square-error equivalent to a factor of 2). These results demonstrate that the modified OECD 309 can capture much of the rank-order and relative magnitude of biodegradation, supporting its role as a higher-tier laboratory simulation test for persistence assessment.

Describing and dealing with the large temporal and spatial variability in biodegradation rate constants is a requirement for robust persistence and exposure assessment. Chemical benchmarking uses a well-characterized reference chemical in a manner analogous to an internal standard in analytical chemistry; its measured biodegradation rate constant captures environmental-specific information that is used to predict the variability of the rate constants of other chemicals. We compiled 1656 biodegradation rate constants for 97 chemicals in European and Australian aquatic ecosystems, all measured with the same modified OECD 309 test protocol. Two benchmarking approaches were assessed for their ability to reduce the spatiotemporal variability in the data: (i) normalizing all chemicals to a single benchmark chemical (universal benchmarking); and (ii) grouping chemicals and normalizing the chemicals within each group to a benchmark chemical chosen from within that group (group-specific benchmarking). Universal benchmarking did not reduce the measured variability, while group-specific benchmarking did when the grouping of chemicals was optimized using the data. However, when chemical grouping was predicted based on the molecular fingerprint MACCS (Molecular ACCess System) or initial biotransformation rules, there was no reduction in variability for most chemicals. Group-specific benchmarking has promise as a tool to predict spatiotemporal variability in biodegradation rate constants when an appropriate chemical grouping is possible, but our current understanding of the key chemical features associated with biodegradability is insufficient to reliably group chemicals a priori.

The rate of biodegradation in aquatic environments is a large source of uncertainty in chemical exposure assessments. Currently, there are insufficient biodegradation data and limited understanding of spatial variation in biodegradation, particularly across diverse aquatic environments. This study investigated the spatial variation in chemical biodegradation rate constants (k) across seven distinct aquatic environments in Australia including marine, mangrove, estuarine, and freshwater ecosystems, using a modified OECD 309 test. Significant spatial variation in k was found for 36 of 38 chemicals (ANOVA, p < 0.05). In addition, the average log k for the chemicals was strongly correlated with the average log k of the same chemicals in 18 European rivers measured using the same method (Pearson r = 0.94, p < 0.001), illustrating intercontinental consistency in the relative biodegradation behavior of the chemicals. However, the standard deviation in log k (which is a relative metric equivalent to a fold difference) was larger across the Australian sites than across the European sites for many chemicals. These findings provide two key insights: first, that diverse aquatic environments demonstrate greater spatial variability in k than rivers; and second, that despite this spatial variability, the modified OECD 309 method can reliably rank chemicals according to their biodegradability and thus be useful in persistence assessments.

Biodegradation makes major contributions to the removal of micropollutants from contaminated environments, but the factors that influence the removal rate in specific environments remain unclear. This study examined environmental biochemical oxygen demand (BOD) as a potential indicator of site-specific attenuation rate constants for 47 micropollutants. A modified BOD protocol for water-sediment systems was developed and applied in parallel with a modified OECD 309 biodegradation test on samples collected from 10 sites along an urban wastewater-impacted river. The biodegradation test provided attenuation rate constants for 34 compounds that met quality criteria for at least three sites. After normalizing by sediment dry weight, 23 compounds showed a significant positive correlation (p < 0.05) between BOD and attenuation rate constant, with an average linear regression R² of 0.71. Normalization by BOD significantly (p < 0.05) decreased the observed variability in attenuation rates. If biodegradation mainly occurs through co-metabolism, BOD may act as an integrative indicator of microbial metabolic activity relevant to micropollutant transformation. Our results suggest that a water–sediment BOD test may provide a practical indicator of relative biodegradation rate across sites for a subset of compounds.

Models to predict the environmental fate of micropollutants are needed for alternatives assessment and safe-by-design efforts. Wastewater treatment plants (WWTPs) are the main barrier to prevent micropollutants from entering receiving water bodies, and WWTP breakthrough is an important indicator of chemical persistence. State-of-the-art models to predict breakthrough are limited by their need for first-order degradation rate constants, a metric that is often unavailable. Here, we build models that predict removal in conventional treatment directly from the chemical structure using data from field-scale monitoring for over 1000 chemicals. The best predictions were achieved using substructure-based fingerprints (i.e., MACCS) and random forests, and identified influential substructures agree with structural moieties relevant for biotransformation. We show that our models are more reliable than existing process-based models used in EU and US regulatory contexts, making them important contributions to the in silico toolbox for alternatives assessment, the design of more benign chemicals in industrial research and development, and even exposure modeling in a risk assessment context. Moreover, our data sets along with our extensive systematic evaluation of different curation criteria and the scripts to reproduce it are key for future model advancement. Our model is publicly available (pepper-app) along with the training data and the scripts to reproduce the data curation process (github.com/FennerLabs/pepper).

We appreciate the interest that our Perspective on the Stockholm Convention (Convention) on Persistent Organic Pollutants (POPs) (1) has elicited. A vigorous exchange of ideas by those with a shared interest in assuring that the Convention lives up to its full potential can only be valuable. Clearly, there is much common ground with Scheringer et al. (2) Most importantly, we share the desire to safeguard the effectiveness of the Convention and to maintain or even strengthen the ability to globally regulate chemicals that are likely as a result of their long-range environmental transport (LRET) to lead to significant adverse human health or environmental effects. Another point of agreement is that the number of chemicals that meet the four screening criteria of persistence, bioaccumulation, toxicity, and LRET potential and therefore are eligible for listing in the annexes of the Convention is potentially large, although there may not be consensus as to how large.

Scheringer et al. (2) insist that a high number of nominations is not a problem and the logical response is to strengthen the implementation process. This would entail not only a greatly expanded capacity of the Persistent Organic Pollutant Review Committee (POPRC) to assess nominations and of the Conference of the Parties (CoP) to adopt amendments. Equally importantly, it would require an increased capacity of the more than 180 Parties to the Convention (Parties) to develop National Implementation Plans (NIPs) and enshrine the decisions of the COPs in national policy and legislation. We do not share the optimism that the latter is realistically achievable. We note that some of the authors of the correspondence stated in a policy analysis in 2022 that it is already “increasingly challenging for all Parties, particularly low- and middle-income ones, to compile an overview of POPs present within their national borders and transport via goods and waste, and to develop and implement effective control measures”. 

The removal efficiency (RE) of organic contaminants in wastewater treatment plants (WWTPs) is a major determinant of the environmental impact of chemicals which are discharged to wastewater. In a recent study, non-target screening analysis was applied to quantify the percentage removal efficiency (RE%) of more than 300 polar contaminants, by analyzing influent and effluent samples from a Swedish WWTP with direct injection UHPLC-Orbitrap-MS/MS. Based on subsets extracted from these data, we developed quantitative structure–property relationships (QSPRs) for the prediction of WWTP breakthrough (BT) to the effluent water. QSPRs were developed by means of multiple linear regression (MLR) and were selected after checking for overfitting and chance relationships by means of bootstrap and randomization procedures. A first model provided good fitting performance, showing that the proposed approach for the development of QSPRs for the prediction of BT is reasonable. By further populating the dataset with similar chemicals using a Tanimoto index approach based on substructure count fingerprints, a second QSPR indicated that the prediction of BT is also applicable to new chemicals sufficiently similar to the training set. Finally, a class-specific QSPR for PEGs and PPGs showed BT prediction trends consistent with known degradation pathways.

Biodegradation plays a key role in the fate of chemicals in the environment. The variability of biodegradation in time can cause uncertainty in evaluating the environmental persistence and risk of chemicals. However, the seasonality of biodegradation in rivers has not yet been the subject of environmentally relevant testing and systematic investigation for large numbers of chemicals. In this work, we studied the biodegradation of 96 compounds during four seasons at four locations (up- and downstream of WWTPs located on two Swedish rivers). Significant seasonality (ANOVA, p < 0.05) of the first-order rate constant for primary biodegradation was observed for most compounds. Variations in pH and total bacterial cell count were not the major factors explaining the seasonality of biodegradation. Deviation from the classical Arrhenius-type behavior was observed for most of the studied compounds, which calls into question the application of this relationship to correct biodegradation rate constants for differences in environmental temperature. Similarities in magnitude and seasonality of biodegradation rate constants were observed for some groups of chemicals possessing the same functional groups. Moreover, reduced seasonality of biodegradation was observed downstream of WWTPs, while biodegradation rates of most compounds were not significantly different between up- and downstream.

Twenty years since coming into force, the Stockholm Convention has become a “living” global agreement that has allowed for the addition of substances that are likely, as a result of their long-range environmental transport (LRET), to lead to significant adverse effects. The recent listing of the phenolic benzotriazole UV-328 in Annex A and a draft nomination of three cyclic volatile methylsiloxanes (cVMS) for Annex B draw attention to the fact that many chemicals are subject to LRET and that this can lead to questionable nominations. The nomination of UV-328 and the draft nomination of cVMS also raise the spectre of regrettable substitutions. At the same time, atmospheric monitoring across the globe reveals that environmental releases of several unintentionally produced POPs listed in Annex C, such as hexachlorobenzene and hexachlorobutadiene, are continuing unabated, highlighting shortcomings in the enforcement of the minimum measures required under Article 5. There is also no evidence of efforts to substitute a chemical whose use has been known for three decades to unintentionally produce polychlorinated biphenyls. These developments need to be rectified to safeguard the long-term viability and acceptance of a global treaty of undeniable importance.