Cationic substances released in the environment can sorb to negatively charged particulate or dissolved matter, e.g. humic acids, microorganisms, algae, clay and silica. Those sorption properties can significantly affect the bioavailability and thus apparent toxicity of cationic substances to aquatic organisms. Part of the adverse effects observed can be attributed to indirect (physical) effects resulting, for example, from the clogging of respiratory organs such as gills. This is especially true for cationic polymers which are not expected to pass biological membranes and are rather expected to affect the outer membranes of aquatic organisms, or their near environment (Nabholz & Zeeman, 1991; Cumming, 2007, 2008).
Dissolved Organic Carbon (DOC) species such as humic acids can reduce the apparent toxicity of cationic polymers by one or two orders of magnitude (Cary et al., 1987, 1989). This mitigation effect has been known for decades and is considered since the 90’s in US testing guidelines (e.g. OPPTS 850.1085 Fish Acute Toxicity Mitigated by Humic Acid – EPA, 1996) and regulatory assessment procedures (e.g. Environmental Assessment of Polymers under the U.S. Toxic Substances Control Act – Boethling & Nabholz, 1997). The situation is quite different in Europe as there is no guidance at the moment on the way cationic substances should be tested and assessed. Shall ecotoxicity tests be performed with tap water, tap water supplemented with humic acids, or river water? Shall toxicity measurements be based on dissolved or freely dissolved concentrations? The situation is all the more complex as there is a lack of consistency between OECD testing guidelines regarding the characteristics of acceptable dilution waters (e.g. the concentration in Total Organic Carbon).
If truly bioavailable concentrations of cationic polymers are not determined in an appropriate way, this can lead not only to an erroneous estimation of aquatic hazards but also to an erroneous estimation of exposures and risks. For example, if experimental toxicity data obtained with tap water are directly compared with exposure concentrations measured in the field, this might lead to an overestimation of risks if no distinction is made between freely dissolved and DOC-sorbed polymer concentrations. To better assess bioavailable concentrations, Solid-Phase MicroExtraction (SPME) method could be used to monitor the freely dissolved concentrations of polymers (Andersson, 2012). This method has already been demonstrated to allow reliable measurements of the freely dissolved concentrations of cationic surfactants in various environmental samples (Chen et al., 2012, 2013). Alternatively, ECETOC (2003) proposes to follow a bulk approach relying on modified ecotoxicity tests with humic acids on one side, and on field measurements addressing total concentrations (bulk = dissolved and sorbed) on the other side. However, ECETOC provides only general orientations and gives no methodological scheme. In addition, while performing risk assessments, exposure concentrations are most often derived from in silico predicted data provided by models (e.g. EUSES) rather than from in situ measurements. But are those models applicable to cationic polymers? Besides, if the sorbing behavior of cationic polymers cannot be appropriately predicted, how can we expect the exposure and risk assessments to be accurate?
The project aims at assessing in a realistic and consistent way the environmental threat posed by cationic polymers. A first objective is to provide a review of the various methods and tools currently available to measure or predict the toxic and fate properties of cationic polymers (e.g. standard guidelines, sampling methods, SAR equations, fate models, etc.). A second objective is to clarify the methodological scheme that should (or shall) be followed when assessing cationic polymers:
- Environmental hazards: definition of the characteristics of appropriate dilution waters (i.e. tap water, tap water supplemented with humic acids, or river water) for standards and modified ecotoxicity tests; recommendation of methods for the appropriate monitoring of bioavailable concentrations; proposition of mitigation factors to be used when deriving classification from tests run with river water;
- Environmental risks: thorough description of the bulk approach; review of exposure models’ applicability to cationic polymers; proposition of mitigation factors to be used when deriving PNECaqua from tests run with tap water.
The project aims at providing a common methodological approach to assess the environmental hazards and risks associated with the water release of cationic polymers, e.g. polyquaterniums (PQ-6, PQ-10, etc.), dimethylamine-epichlorohydrin copolymer or guar hydroxypropylrimonium chloride. The recommended methodological approach will be established from a state-of-art review of the toxic and fate properties of cationic polymers, and of the ways to assess these properties in a regulatory context. The definition of “hazard” and “toxicity” (i.e. “means the intrinsic property of a substance to cause adverse effects to aquatic species” – CLP Regulation), as well as the concept of bioavailability, will be at the core of the discussions when questioning the relevancy of toxicity tests performed with humic acids. The project could include an experimental phase dedicated to SPME sampling methods in order to compare toxic effects expressed either based on total or freely dissolved concentrations of polymers. The project could build on the work of Y. Chen, S. T. Droge and J. L. Hermens towards a better understanding of the partition behavior of cationic surfactants (Chen et al., 2012, 2013) as part of the ERASM project on the bioavailability of cationic surfactants.
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