American Chemical Society
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Determination and Characterization of Oxy-Naphthenic Acids in Oilfield Wastewater

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journal contribution
posted on 2013-08-20, 00:00 authored by Beili Wang, Yi Wan, Yingxin Gao, Min Yang, Jianying Hu
Oxy-naphthenic acids (oxy-NAs) are one of the major components of NA mixtures in wastewaters from petroleum industries. The limited available data indicated that oxy-NAs were considered as a potential marker for the degradation of NAs, and some oxy-NAs exhibited endocrine disrupting activities. However, the lack of information on the structures and occurrences of oxy-NAs in oilfield wastewaters limited the interpretations of the biotransformation pathways of NAs and structure-specific toxicity. A sensitive method for simultaneous determination of oxy-NAs together with NAs was developed by combining MAX extraction column and UPLC-ESI-QTOF-MS. The 2000-fold SPE preconcentration step was highly specific for acids and the prewash solvent greatly reduced matrix effects in the UPLC-ESI-QTOF-MS analysis, resulting in an increase in sensitivity down to detection limits in the ng/L range. To provide structural information within each oxy-NA isomer class, a new method was developed by derivatizing oxy-NAs with dansyl chloride by UPLC-ESI+-QTOF-MS. The molecular ion dansyl derivatives from the corresponding oxy-NAs and characteristic fragmentation ions, not detected before derivatization, were observed in the extracts of oilfield wastewater, providing evidence that O3–NAs and O4–NAs were mainly composed of OH–NAs and (OH)2–NAs, respectively. Semiquantification of oxy-NAs and NAs in various oilfield wastewaters revealed NAs, O3–NAs, and O4–NAs present at concentrations of 187–397, 44–146, and 40–108 μg/L, respectively. Significantly different profiles of NA mixtures were observed in petroleum refinery wastewater and oil sands extraction water, but the profile of oxy-NAs was similar to NAs in different wastewaters suggesting the existence of biotransformation between NAs and oxy-NAs in the environment, and hydroxylation could be one of the major biotransformation pathways of NAs.