A modeling assessment of the physicochemical properties and environmental fate of emerging and novel per- and polyfluoroalkyl substances
Introduction
In the last decade, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkane sulfonic acids (PFSAs) were recognized as persistent (Remde and Debus, 1996, Key et al., 1998) and those with “long” perfluoroalkyl chains were shown to be bioaccumulative (Houde et al., 2006) and toxic (Kennedy et al., 2004, Borg et al., 2013). Our definition of “long” chain refers to PFCAs with 7 or more fluorinated carbons (including PFOA, which is designated as bioaccumulative under REACH; ECHA, 2013) and their precursors as well as PFSAs with 6 or more fluorinated carbons alnd their precursors (Buck et al. 2011).
Long-chain PFCAs and PFSAs are globally present, including in the abiotic environment (Yamashita et al., 2005, Young et al., 2007) and wildlife (Kannan et al., 2002) in remote regions, indicating the long-range transport potential of these substances. Furthermore, humans in industrialized countries contain relatively high levels of long-chain PFCAs and PFSAs in their serum (Kannan et al., 2004, Olsen et al., 2003), suggested to be due to the historical presence of these substances and their precursors in a wide range of consumer products (Vestergren and Cousins, 2009). Due to concern regarding their hazardous properties, there have been a number of actions by industry and regulatory authorities to reduce the environmental release of long-chain PFCAs, PFSAs and their precursors. In the period 2000–2002, 3M phased out its global production of perfluorooctane sulfonic acid (PFOS) and related chemicals derived from perfluorooctane sulfonyl fluoride (POSF, C8; i.e., POSF-based chemicals) and replaced their use in certain key products with perfluorobutane sulfonyl fluoride (PBSF, C4)-based chemicals. In 2009, PFOS and related POSF-based chemicals were added to Annex B (restriction of production and use) of the Stockholm Convention on Persistent Organic Pollutants (UNEP, 2009). Similar actions have also taken place for perfluorooctanoic acid (PFOA) and other long-chain PFCA homologues. For example, the US Environmental Protection Agency (US EPA) and eight major global fluoropolymer and fluorotelomer manufacturers have agreed to work toward the elimination of long-chain PFCAs and their precursors from point-source emissions and products by 2015 (US EPA, 2006). In addition, PFOA and its ammonium salt (APFO) as well as C11–C14 PFCAs have been listed in the Candidate List of Substances of Very High Concern under the European chemicals regulation, REACH (ECHA, 2013).
A common feature of all the above actions is an on-going industrial transition to replace long-chain PFCAs, PFSAs and their precursors with alternatives, particularly other poly- and perfluoroalkyl substances (PFASs) such as shorter-chain homologues and functionalized perfluoropolyethers (PFPEs) in applications where extremely low surface tension and/or durable oil- and water-repellency is needed (Holt, 2011). Although the identity of fluorinated substances used in industrial processes and consumer products is often claimed as “confidential business information” (CBI) by the manufacturers, a number of fluorinated alternatives used in different industrial branches and consumer products were identified by Wang et al. (2013). A key question is: are these fluorinated alternatives less hazardous for humans and the environment than their predecessors? There have been other historical examples showing the problems associated with removing a chemical from the market and replacing it with other structurally similar chemicals from the same class of substances (Strempel et al., 2012, Goldstein et al., 2013). Wang et al. (2013) reviewed available knowledge on the identified fluorinated alternatives and highlighted the scarcity of information on their production volumes, emissions, (bio)degradability, bioaccumulative potential and (eco)toxicity. Conducting experiments to generate missing data for all these fluorinated alternatives is expensive and time-consuming. However, a preliminary assessment using in silico methods including quantitative structure–property/activity relationships (QSPRs/QSARs) can provide valuable insights and help to prioritize future research needs (Strempel et al., 2012, Gawor and Wania, 2013, Howard and Muir, 2010).
The aim of this work is to provide a preliminary assessment of emerging and novel fluorinated alternatives with state-of-the-art in silico tools. We use the terminology of “emerging” and “novel” that has previously been applied to brominated flame retardants (Bergman et al., 2012). Emerging fluorinated alternatives are defined as alternatives have been recently identified in the environment, wildlife, food or humans (e.g. Adona). Most of the alternatives included in this study are novel alternatives, i.e. those are known to be present in manufacturing processes, materials and products but have not yet been identified in environmental samples, wildlife, food or humans. First, COSMOtherm and SPARC are used to predict physicochemical properties and EPISuite is used to predict degradation half-lives in air, water and soil. COSMOtherm and SPARC were previously used to estimate the physicochemical properties of long-chain PFASs, including partition coefficients (Arp et al., 2006, Wang et al., 2011) and acid dissociation constants (pKas) (Goss, 2008). The US EPA EPISuite software package is a well-established QSPR/QSAR tool used to estimate physicochemical properties and degradation half-lives in hazard assessments (Strempel et al., 2012, Zarfl et al., 2012). It has, however, been shown to be inaccurate for estimating the physicochemical properties of PFASs (Arp et al., 2006) and is therefore only used here for estimating the environmental degradation half-lives. The structural differences of the fluorinated alternatives and their estimated physicochemical properties are analyzed to provide insights into the impact of structural changes on physicochemical properties. Second, based on the estimated physicochemical properties and degradation half-lives, the environmental fate of the fluorinated alternatives, more specifically the overall persistence (POV) and long-range transport potential (LRTP), is assessed by using the OECD Overall Persistence and Long-Range Transport Potential Screening Tool (hereafter “the OECD Tool”). The OECD Tool was developed as a “consensus model” combining the essential aspects of nine multimedia fate and transport models (Wegmann et al., 2009). It should be noted that this study focuses on the physicochemical properties and possible environmental fate (POV and LRTP) of the selected fluorinated alternatives. The prediction of bioaccumulation potential (B) and (eco)toxicity (T) is not included in this study because there is a lack of mechanistic understanding about the possible oleo- and proteinophilic bioaccumulation behavior as well as the toxic mode-of-action of these fluorinated alternatives. However, a discussion of possible strategies for assessing B is included in the Discussion section.
Section snippets
Selected fluorinated alternatives
A total of 16 emerging and novel fluorinated alternatives were investigated, including five perfluoroether carboxylic acids (PFECAs) and two perfluoroether sulfonic acids (PFESAs) identified by Wang et al. (2013) and for which the chemical structures were known (see Table 1). The selection of the fluorinated alternatives for this study was limited by the large amount of unknown fluorinated chemical structures in the products identified by Wang et al. (2013). Furthermore, some fluorinated
Physicochemical properties of fluorinated alternatives
Fig. 1A shows estimated log KAW and log KOW for PFOS, PFOA and 8:2 FTOH together with PFECAs, PFESAs, 6:2 FTCA and n:1 FTOHs that are replacing these substances. Estimated values of log KAW and log KOW for the fluorinated alternatives replacing certain POSF- and/or fluorotelomer-based substances (i.e., Forafac, PFBSaPA, EF-N, Novec, PFOTSi, RM720) and the six degradation products are presented in Table A2 of the Supplementary data.
Log KOW and log KAW of the two PFESAs (i.e., F-53 and F-53B) are
Influence of structural variability on physicochemical properties
This study provides a preliminary assessment of emerging and novel fluorinated alternatives that replace long-chain perfluoroalkyl acids and their precursors. Even though PFECAs and PFESAs, which are alternatives to PFOA and PFOS, respectively, contain some structural differences such as ether linkage(s) between perfluorinated carbon chains and replacement of fluorine atom with chlorine atom at the end of perfluoroether chain, their physicochemical properties are not significantly changed
Outlook
To date, the absence of experimental data on degradation half-lives and physicochemical properties is a major drawback for carrying out hazard and risk assessments of emerging chemicals such as fluorinated alternatives. At this stage, the use of in silico methods remains the only time- and resource-saving tool for the identification of potential hazardous chemicals. Despite the inherent uncertainties, our study provides qualitative evidence that most of the alternatives do not differ
Acknowledgments
European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no. 316665 (A-TEAM project) and the Swiss Federal Office for the Environment (FOEN) for their financial support.
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