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.

Biodegradation is one of the most important processes influencing the fate of organic contaminants in the environment. Quantitative understanding of the spatial variability in environmental biodegradation is still largely uncharted territory. Here, we conducted modified OECD 309 tests to determine first-order biodegradation rate constants for 97 compounds in 18 freshwater river segments in five European countries: Sweden, Germany, Switzerland, Spain, and Greece. All but two of the compounds showed significant spatial variability in rate constants across European rivers (ANOVA, P < 0.05). The median standard deviation of the biodegradation rate constant between rivers was a factor of 3. The spatial variability was similar between pristine and contaminated river segments. The longitude, total organic carbon, and clay content of sediment were the three most significant explanatory variables for the spatial variability (redundancy analysis, P < 0.05). Similarities in the spatial pattern of biodegradation rates were observed for some groups of compounds sharing a given functional group. The pronounced spatial variability presents challenges for the use of biodegradation simulation tests to assess chemical persistence. To reflect the variability in the biodegradation rate, the modified OECD 309 test would have to be repeated with water and sediment from multiple sites.

Surfactants are a class of chemicals released in large quantities to water, and therefore bioconcentration in fish is an important component of their safety assessment. Their structural diversity, which encompasses nonionic, anionic, cationic and zwitterionic molecules with a broad range of lipophilicity, makes their evaluation challenging. A strong influence of environmental pH adds a further layer of complexity to their bioconcentration assessment. Here we present a framework that penetrates this complexity. Using simple equations derived from current understanding of the relevant underlying processes, we plot the key bioconcentration parameters (uptake rate constant, elimination rate constant and bioconcentration factor) as a function of its membrane lipid/water distribution ratio and the neutral fraction of the chemical in water at pH 8.1 and at pH 6.1. On this chemical space plot, we indicate boundaries at which four resistance terms (perfusion with water, transcellular, paracellular, and perfusion with blood) limit transport of surfactants across the gills. We then show that the bioconcentration parameters predicted by this framework align well with in vivo measurements of anionic, cationic and nonionic surfactants in fish. In doing so, we demonstrate how the framework can be used to explore expected differences in bioconcentration behavior within a given sub-class of surfactants, to assess how pH will influence bioconcentration, to identify the underlying processes governing bioconcentration of a particular surfactant, and to discover knowledge gaps that require further research. This framework for amphiphilic chemicals may function as a template for improved understanding of the accumulation potential of other ionizable chemicals of environmental concern, such as pharmaceuticals or dyes.

It is of considerable interest to identify chemicals which may represent a hazard and risk to environmental and human health in remote areas. The OECD P_OV and LRTP Screening Tool (“The Tool”) for assessing chemicals for persistence (P) and long-range transport potential (LRTP) has been extensively used for combined P and LRTP assessments in various regulatory contexts, including the Stockholm Convention (SC) on Persistent Organic Pollutants (POPs). The approach in The Tool plots either the Characteristic Travel Distance (CTD, in km), a transport-oriented metric, or the Transfer Efficiency (TE, in %), which calculates the transfer from the atmosphere to surface compartments in a remote region, against overall persistence (P_OV). For a chemical to elicit adverse effects in remote areas, it not only needs to be transported and transferred to remote environmental surface media, it also needs to accumulate in these media. The current version of The Tool does not have a metric to quantify this process. We screened a list of >12 000 high production volume chemicals (HPVs) for the potential to be dispersed, transferred, and accumulate in surface media in remote regions using the three corresponding LRTP metrics of the emission fractions approach (EFA; ϕ₁, ϕ₂, ϕ₃), as implemented in a modified version of The Tool. Comparing the outcome of an assessment based on CTD/TE and P_OV with the EFA, we find that the latter classifies a larger number of HPVs as having the potential for accumulation in remote regions than is classified as POP-like by the existing approach. In particular, the EFA identifies chemicals capable of accumulating in remote regions without fulfilling the criterion for P_OV. The remote accumulation fraction of the EFA is the LRTP assessment metric most suited for the risk assessment stage in Annex E of the SC. Using simpler metrics (such as half-life criteria, P_OV, and LRTP–P_OV combinations) in a hazard-based assessment according to Annex D is problematic as it may prematurely screen out many of the chemicals with potential for adverse effects as a result of long-range transport.

The environmental relevance of standard biodegradation tests such as OECD 309 has been questioned. Challenges include the interpretation of changing degradation kinetics over the 60–90 incubation days and the effects of chemical spiking on the microbial community. To ameliorate these weaknesses, we evaluated a modified OECD 309 test using water and sediment from three Swedish rivers. For each river, we had three treatments (no spiking, 0.5 μg L^–1 spiking, and 5 μg L^–1 spiking). The dissipation of a mixture of 56–80 spiked chemicals was followed over 14 days. Changes in dissipation kinetics during the incubation were interpreted as a departure of the microbial community from its initial (natural) state. The biodegradation kinetics were first-order throughout the incubation in the no spiking and 0.5 μg L^–1 spiking treatments for almost all chemicals, but for the 5 μg L^–1 treatment, more chemicals showed changes in kinetics. The rate constants in the no spiking and 0.5 μg L^–1 treatments agreed within a factor of 2 for 35 of 37 cases. We conclude that the environmental relevance of OECD 309 is improved by considering only the initial biodegradation phase and that it is not compromised by spiking multiple chemicals at 0.5 μg L^–1.