Standard Operating Procedures for Analysis of Naphthenic Acids from Oil Sands Process-Affected Water

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  • Naphthenic acids (NAs) are considered the main source of chronic and acute toxicity in oil sands process-affected water (OSPW). The purpose of this investigation is to establish elements of a standard procedure to determine and minimize the amount of NA loss during storage, sample preparation and pretreatment, and radiation emission such as sunlight, microwave and ultraviolet. In addition, efficiency of solid phase extraction (SPE) and industrial resins for NA separation from aqueous phase were studied. For quantification of NAs, fluorescence spectroscopy was used, which requires minimum sample manipulation. Two different types of NAs, OSPW-associated (O-NAs) in oil sands process- affected water as well as Merichem NAs (M-NAs) dissolved in phosphate buffer (0.05M, pH = 8.5), were tested and compared in this research. The lowest NA loss among the tested filter membranes was observed in the PVDF filter membrane (Durapore 0.45 µm, Millipore), and among the tested filter syringes was observed in the Target GL microfiber (0.7 µm, National Scientific). Pre-wetting PTFE membranes with methanol may increase the NA loss. According to our observations, no significant difference was detected between the PVDF filter membrane (Durapore 0.45 µm, Millipore) and centrifugation in glass tubes (30 min, 3500 ×g) for solid separation (P-value>0.05). For storage containers, the best performance (smallest NA loss) was observed in lime-soda and borosilicate glass; however, glass silanization may increase the adsorption of M-NAs on the glass surface. Significant M-NA loss was observed in all three plastic bottles (HDPE, LDPE, and PP). Despite that no significant reduction in concentration of O-NAs was observed, long term storage in plastic bottles is not suggested. Three types of centrifuge tubes (polypropylene ultra-high-performance (PP-UHP), polypropylene high-performance (PP-HP), and polystyrene) were tested and high M-NA loss was observed, especially in polystyrene centrifuge tubes. Similar to the plastic bottles, no considerable O-NA loss was detected, but long term storage in centrifuge tubes is not suggested because it is expected that those NA molecules with similar structure to M-NAs are suspected to be adsorbed on the surface of any plastic made containers. The best storage condition was storage in the fridge (4oC). Addition of methanol (50% v/v), pH increase to 11.1, or pH reduction to 2.1 caused false-positive and false-negative errors in NA concentration measured by fluorescent instrument. Freezing did not influence the NA concentration, however, possible NA loss due to storage in a plastic bottle or centrifuge tube should be taken into consideration. In the cap liner material test, the best performance was seen for PTFE and Tinfoil cap liners. The leakage of contaminants, interfering with NA measurement, from white rubber and polyethylene cap liners was seen. For long term storage of water samples, the PTFE cap liner is suggested. Small reduction in M-NA concentration was observed after UV exposure, but the microwave did not influence either M-NAs or O-NAs. In rotavapor experiment with O-NAs, it was found that O-NA loss increases at low pH (2); however for M-NAs, no considerable difference in NA loss was seen at high (9) or low pH (2). The highest NA loss was observed in DCM solvent. The C18 SPE laboratory cartridge and L493 industrial resins displayed the best performance (in terms of NA adsorption from aqueous solution and subsequent NA release into the eluent solution) among the tested SPE cartridges and industrial resins. The results from this investigation elucidated the unknown aspects of sampling, storage conditions, and processing of NA containing water samples. Still, more investigations are required to optimize the performance of SPE laboratory cartridges and industrial resins. For future research, the amount of recovered NAs from industrial resins can be optimized by using various eluent solutions. The type of eluent solution is crucially important for further treatment of NAs on an industrial scale. In addition, in this research only one type of OSPW sample was tested. It is hypothesized that if the OSPW sample is fresh or derived from a refinery, there is a chance that such a sample may behave in a similar fashion to M-NAs. Long-term storage and consecutive use of storage containers may also result in serious losses. Measurement of NA using fluorescence instrument requires a minimum sample preparation and manipulation which reduce the error from NA loss. However, this method comes with some inherent issues in terms of accuracy. For future research we suggest that a high resolution instrument is used for quantification, fingerprinting and characterizing of NA molecules.

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    Attribution 3.0 International