Lipid-based passive dosing to study the effects of volatile hydrophobic chemicals and their mixtures

Ngoc Lam Trac*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

More than 100,000 industrial chemicals are estimated to be produced today and potentially emitted to the environment. The assessment of the associated risks requires both experimental testing and a solid scientific basis. However, for chemicals that are hydrophobic and volatile, both the toxicity testing and the subsequent assessment are associated with technical and conceptual challenges.
Hydrophobicity and volatility are the physicochemical properties that make the testing difficult when it comes to aquatic toxicity tests. Chemicals with these properties can be difficult to dissolve in aqueous medium and are prone to losses due to sorption and evaporation. As a result, they become almost untestable with currently available methods. The technical aim of this thesis is therefore to develop and optimize novel dosing methods that facilitate toxicity testing at well-defined and constant exposure for volatile hydrophobic organic chemicals (VHOCs) and their complex mixtures. Along with the method development and application in standard toxicity tests, new concepts and exposure parameters such as “concentration in the membrane at equilibrium with the donor” and chemical activity are employed. By linking toxicological responses on a thermodynamic exposure basis, the ultimate goal of the PhD project is to improve the experimental as well as scientific basis of environmental toxicological research and testing of these difficult-to-test chemicals and their mixtures.
The novel passive dosing method developed in this thesis employs a liquid as partitioning donor: The liquid test substance is used to control exposures at the saturation level, whereas purified plant oil containing the test substance is used at lower concentrations. The working principle is simple: The donor solution is kept in a glass insert placed in the test vial to separate it from the test medium/organisms, test chemicals are then passively dosed to the test medium/organisms by equilibrium partitioning between the donor and the target phases via the headspace, i.e., headspace passive dosing (HS-PD).
The HS-PD method was applied to various toxicity tests with a number of selected single VHOCs and complex mixtures in individual studies: (1) pure liquid chemicals were tested exactly at their saturation levels in standard algal growth inhibition tests with the freshwater algae Raphidocelis subcapitata. (2) Dose-response testing with R. subcapitata and the terrestrial springtail Folsomia candida to test the baseline toxicity hypothesis and explore toxic effects towards the two test organisms. (3) Aquatic toxicity testing of six complex mixtures, three petroleum products and three essential oils, with both algae and the water flea Daphnia magna. These complex test mixtures are categorized as substances of Unknown or Variable composition, Complex reaction products and Biological materials (UVCBs) under the European chemical regulatory framework.
By the use of the HS-PD method, it was possible to test and determine inherent toxic effects of difficult-to-test chemicals such as alkanes, terpenes, and cyclic volatile methylsiloxanes, as well as UVCBs, at better controlled and better defined exposure conditions. Furthermore, the application of this equilibrium partitioning-based method allowed exposure to be expressed on a basis of internal exposure in the lipid membranes of the test organisms, as well as chemical activity, which are used to categorize the observed effects as baseline or excess toxicity. The research of this thesis explores new possibilities and concepts that contribute to an improved experimental and scientific basis for environmental risk assessment of chemicals.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark
Number of pages39
Publication statusPublished - 2019

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