Comparative aquatic toxicity of a commercial naphthenic acid and processes for mitigating risks

Comparative toxicity data can inform predictions of relative risk, and can be used to implement strategies for altering exposures to mitigate risk. Naphthenic acids (NAs) are a complex group of carboxylic acids that naturally occur in petroleum sources and energy-derived process waters (e.g. refinery effluents and oil sands process affected waters). These compounds are relatively persistent in water and can be a source of toxicity to aquatic organisms. In the first experiment of this thesis, responses of sentinel aquatic organisms to 7-d exposures of commercial (Fluka) NAs were measured (in terms of acute toxicity) to discern relative sensitivities. In terms of sensitivities, fish\u3einvertebrates\u3eplant for exposures to Fluka NAs. Once toxicity was determined, two potential processes for altering exposures were investigated. In the second experiment, photocatalytic degradation of Fluka NAs was measured using fixed-film titanium dioxide (TiO2) irradiated with sunlight for 8 hours. Confirmation of changes in NA concentrations by photocatalytic degradation was accomplished analytically and with toxicity tests using sentinel vertebrate and invertebrate species. The half-life for Fluka NAs achieved by photocatalytic degradation was approximately 1 hour, with toxicity eliminated to both test species (Pimephales promelas and Daphnia magna) by the 5th hour of the sunlight exposure. In the third experiment, environmental conditions that can influence aerobic degradation for altering exposures of NAs were evaluated. Effects of nitrogen and phosphorus concentrations, dissolved oxygen concentrations, pH, and temperature on rates and extents of aerobic degradation of Fluka NAs were measured. Environmental conditions that positively influenced aerobic degradation rates of Fluka NAs included nutrients (C:N 10:1-500:1, C:P 100:1-5000:1), DO (4.76-8.43 mg/L), pH (6-8), and temperature (5-25ºC). At an initial Fluka NA concentration of 61 mg/L (±8), a removal rate of 11.7 mg/L day-1 was achieved (half-life approximately 2.5 days) in treatments with C:N and C:P molar ratios of 10:1 and 100:1, respectively (with other macro- and micronutrients supplied), DO \u3e8 mg/L, pH ≈8, and temperatures \u3e23ºC. Commercial NAs differ structurally from energy-derived NAs (e.g oil sands process affected waters), but environmental conditions systematically evaluated in this study are also expected to affect rates and extents of aerobic degradation of compositionally complex NAs. Ultimately, experiments conducted in this thesis can serve as a model approach for evaluating comparative toxicity of NAs, in terms of relative sensitivities of a taxonomic range of sentinel species, and using that information to implement effective strategies for mitigating risks in aquatic systems