Biodegradation Kinetics Testing of Hydrophobic Organic Chemicals in Mixtures at Low Concentrations

Rikke Høst Hammershøj*

*Corresponding author for this work

    Research output: Book/ReportPh.D. thesis

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    Biodegradation is the most important pathway for removal of organic chemicals from the environment. If a chemical is degraded at a slow rate it can accumulate in the environment, where it can potentially cause harmful effects. Yet, standard biodegradation data are lacking for more than half of the chemicals on the European market and there is a need for new methods to generate biodegradation data faster. Today, regulatory biodegradability tests are conducted almost solely on single chemicals though many chemical substances are mixtures, and ultimately all chemicals that enter the environment will end up in mixtures at low concentrations. Conducting biodegradation testing of chemicals in mixtures could accelerate the generation of environmentally relevant biodegradation data.
    Chemical mixtures pose new challenges for biodegradation testing as the physicochemical properties and biodegradability can vary widely between chemicals. Mixtures of hydrophobic organic chemicals (HOCs) pose additional challenges due to their low water solubility and potential volatility. The primary aim of this PhD study is to explore, discuss and further develop the application of a novel experimental and analytical platform for aquatic biodegradation testing of HOCs in mixtures. This research includes well-defined mixtures of a limited number of chemicals as well as complex mixtures containing hundreds or even thousands of chemicals.
    Biodegradation is a highly variable process that depends not only on substance specific properties but also on the microbial communities and the specific experimental conditions. Currently, there is a knowledge gap concerning how the presence of multiple substrates affect the biodegradation kinetics of individual chemicals when testing at environmentally relevant concentrations. A second aim of this PhD thesis is to study the effect of multiple substrates and chemical concentration on biodegradation kinetics of chemicals in mixtures.
    Testing of mixtures was found to be a practical way to generate primary biodegradation kinetic data for a large number of chemicals in one test. Biodegradation data obtained from chemicals tested in mixtures are generated with the same inoculum and at the same test conditions and are thus directly comparable. This is a clear advantage when studying how different variables affect biodegradation of chemicals, as it reduces the number of confounding factors, and can for instance also be used to generate training set data for biodegradation models. Testing of complex mixtures may be the most environmentally relevant choice but it also increases the complexity of a study and thus the analytical challenges. The presented biodegradation platform can readily be used for mixtures with a limited number of chemicals while testing of complex mixtures requires additional studies.
    Many previous studies have observed distinct effects of multiple substrates on the biodegradation kinetics of individual chemicals when testing with pure or simple mixture bacterial cultures and at concentrations near the water solubility of the chemicals. However, in this thesis it was observed that the number of mixture components had only limited effects on the biodegradation half times (DT50, lag phase + one half-life) for hydrocarbons when tested at environmentally relevant concentrations. The biodegradation half times were affected more by the substrate concentrations than by the number of mixture constituents, at least for mixtures of up to sixteen hydrocarbons. Further research is needed to address the difference in biodegradation kinetics for chemicals tested alone and as part of a complex mixture at environmentally relevant concentrations.
    When conducting biodegradation tests with complex microbial communities and a complex mixture of substrates, many different interactions can take place, and it is not straightforward to predict the effect of changing the substrate concentration. Two complex mixtures, diesel oil and lavender oil, were tested at concentrations near water solubility and two orders of magnitude lower. Diesel oil was observed to stimulate the microbial activity at both test concentrations and no marked concentration effect was observed on biodegradation kinetics for the diesel oil constituents. Lavender oil on the other hand stimulated the microbial activity at low concentration but inhibited the microbial activity at higher concentration, which coincided with a delayed and limited biodegradation that was best explained by toxicity near the solubility limit. This underlines the importance of conduction biodegradation kinetics testing of HOCs, tested alone or in mixtures, at low concentrations.
    The findings of this PhD study are a step towards improved biodegradation kinetics testing of HOCs at low concentrations and emphasize the importance of controlling the exposure in aquatic biodegradation testing of HOC mixtures.
    Original languageEnglish
    Place of PublicationKgs. Lyngby
    PublisherTechnical University of Denmark
    Number of pages41
    Publication statusPublished - 2020


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