Effects-Directed Analysis of Dissolved Organic Compounds in Oil Sands Process-Affected Water

Acute toxicity of oil sands process-affected water (OSPW) is caused by its complex mixture of bitumen-derived organics, but the specific chemical classes that are most toxic have not been demonstrated. Here, effects-directed analysis was used to determine the most acutely toxic chemical classes in OSPW collected from the world’s first oil sands end-pit lake. Three sequential rounds of fractionation, chemical analysis (ultrahigh resolution mass spectrometry), and acute toxicity testing (96 h fathead minnow embryo lethality and 15 min Microtox bioassay) were conducted. Following primary fractionation, toxicity was primarily attributable to the neutral extractable fraction (F1-NE), containing 27% of original organics mass. In secondary fractionation, F1-NE was subfractionated by alkaline water washing, and toxicity was primarily isolated to the ionizable fraction (F2-NE2), containing 18.5% of the original organic mass. In the final round, chromatographic subfractionation of F2-NE2 resulted in two toxic fractions, with the most potent (F3-NE2a, 11% of original organic mass) containing predominantly naphthenic acids (O₂^–). The less-toxic fraction (F3-NE2b, 8% of original organic mass) contained predominantly nonacid species (O⁺, O₂⁺, SO⁺, NO⁺). Evidence supports naphthenic acids as among the most acutely toxic chemical classes in OSPW, but nonacidic species also contribute to acute toxicity of OSPW