Effect of Carboxylic Acid Content on the Acute Toxicity of Oil Sands Naphthenic Acids

Fractions of methylated naphthenic acids (NAs) isolated from oil sands process-affected water were collected utilizing Kugelrohr distillation and analyzed by proton nuclear magnetic resonance (1H NMR) spectroscopy. 1H NMR analysis revealed that the ratio of methyl ester hydrogen atoms to remaining aliphatic hydrogen atoms increased from 0.130 to 0.214, from the lowest to the greatest molecular weight (MW) fractions, respectively, indicating that the carboxylic acid content increased with greater MW. Acute toxicity assays with exposure to monocarboxyl NA-like surrogates demonstrated that toxicity increased with increasing MW (D. magna LC50 values of 10 ± 1.3 mM and 0.59 ± 0.20 mM for the respective lowest and highest MW NA-like surrogates); however, with the addition of a second carboxylic acid moiety, the toxicity was significantly reduced (D. magna LC50 values of 10 ± 1.3 mM and 27 ± 2.2 mM for the respective monocarboxyl and dicarboxyl NA-like surrogates of similar MW). Increased carboxylic acid content...