Abstract

Alicyclic bicyclic acids have been reported to be the major naphthenic acids (NA) in oil sands process-affected water (OSPW) but due to the extreme complexity of OSPW none have yet been identified. A previous study using the Microtox™ assay indicated that some synthetic alicyclic bicyclics were the most acutely toxic acids tested so it is important that these compounds are identified and their toxicities determined.

Using comprehensive multidimensional gas chromatography-mass spectrometry (GCxGC-MS) we show that >100 C_8-15 bicyclic acids are typically present in OSPW. Synthesis or purchase facilitated GCxGC retention times of methyl esters of these acids to be established and their mass spectra compared with unknowns. Having identified numerous alicyclic bicyclic acids by comparison with authentic standards, we suggest that many of the remaining unknowns are simply analogues of the identified acids with longer alkanoate chains and/or alkyl substituents.

Previously we reported the predicted toxicities of a range of NA, many of which had been identified in OSPW, but the alicyclic bicyclic acid structures modelled had only been found in commercial NA mixtures or crude oil. Now, having identified numerous novel acids, we report their toxicities to Pimephales promelas (fathead minnow), the protozoan Tetrahymena pyriformis and the water flea Daphnia magna as predicted by Admet Predictor™ (Simulations Plus). Also, neural network ensemble models were used to assess a compound's likelihood of binding to the estrogen or androgen receptors. Furthermore, the persistence of NA is a huge problem for the oil sands extraction industries so the likelihood of biodegradation was also modelled.

Predicted L_C50s for fathead minnows and water fleas were in the range 0.01 – 3.54 mmol/L and EC50s for T. pyriformis 0.01 – 0.7 mmol/L. As expected for this type of structure, no binding affinities for either estrogen or androgen receptors were predicted. Of the structures tested, about 30% were considered not readily biodegradable as defined by the model based on biological oxygen demand and theoretical oxygen demand. Structures with terminal branches were least likely to readily biodegrade.

Several of the novel alicyclic bicyclic acids identified were predicted to be more toxic than the equivalent molecular weight decalin acids which were previously modelled (and experimentally tested with Microtox™) to be the most toxic of this group of compounds.

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