Comprehensive Analysis of Oxy-Naphthenic Acids in Produced Water via Liquid–Liquid Extraction and Orbitrap Mass Spectrometry

• Published in: Energy & fuels 2023-11, Vol.37 (21), p.16378-16387
• Main Authors: Rodrigues, Marcella F., Simas, Rosineide C., Lima, Nerilson M., Pereira, Rosana C. L., Dufrayer, Gabriel H. M., Vieira, Michel M. F., de Carvalho, Rogério M., Medeiros Júnior, Iris, dos Santos, Gabriel F., Vaz, Boniek G.
• Format: Article
• Language: English
• Subjects: Traditional Fossil Fuels
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/acs.energyfuels.3c02381

Produced water, a complex saline concoction, houses naphthenic acids (NAs), some of the most potent organic toxins that significantly threaten aquatic life. Deciphering the molecular intricacy of NAs is a formidable task as a result of their composite nature comprising numerous isomers that are hard to distinguish and characterize. While substantial research has examined NAs in oil sand process-affected water, there is a dearth of studies regarding their characterization in produced water from offshore oil platforms. Consequently, for an efficient disposal process, it is critical to understand the organic composition of the produced water comprehensively. This study endeavors to delineate the application of liquid–liquid extraction using various solvents, n-hexane, toluene, ethyl ether, dichloromethane (DCM), and ethyl acetate, at pH 2. An electrospray ionization (−) Q-Orbitrap mass spectrometry analysis is conducted to examine the efficiency of these solvents in isolating NAs from produced water. Our observations indicate that DCM and ethyl acetate extract a wider variety of oxy-naphthenic acid (oxy-NA) compounds and were, hence, selected for sequential and non-sequential liquid–liquid extraction. These extractions were executed at pH values of 2, 7, and 10 to assess the influence of pH and the efficiency of the extraction methodologies in isolating oxy-NAs. With regard the extraction methodology using these two solvents, a more significant number of molecular formulas were witnessed in non-sequential extraction. Moreover, the analysis of the extraction capacity for NAs at pH 2, 7, and 10 for the O2 class revealed higher molecular formula species and NAs with increased carbon numbers and a broader double bond equivalent range at pH 10.