Abstract:

Polycyclic aromatic hydrocarbons (PAHs) are among the most harmful persistent organic pollutants that possess high carcinogenicity and teratogenicity; hence, establishing a highly sensitive analytical method for monitoring PAHs in environmental samples is an urgent need. However, due to the low PAHs content in environmental samples and the complex matrix of the samples, it is difficult to directly determine the amount of PAHs using the existing analytical instruments. Therefore, an essential pretreatment of environmental samples should be carried out before instrumental analysis. In most pretreatment techniques, the extraction efficiency depends on the characteristics of the extraction materials. Currently, metal-organic framework materials (MOFs), which are porous materials self-assembled by metal ions and organic ligands, are used as solid-phase microextraction (SPME) coating materials for the extraction of PAHs. However, the following problems limit the application of MOFs in the SPME field: (1) MOF coating materials often require a long equilibration time for extraction because the it is difficult for the target to reach the deep adsorption sites; (2) In addition, most MOFs are formed by the coordination of single metal ions with organic monomers. The single type of open metal active sites is not conducive for realizing high extraction performance. In this study, a hollow bimetal-organic framework (H-BiMOF) was synthesized by the solvothermal method and characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption-desorption analysis, thermogravimetric analysis, etc. The TEM images and XRD patterns demonstrated the successful synthesis of H-BiMOF with a hollow structure, which was formed through the competitive coordination between benzoic acid and water. The H-BiMOF material showed type-Ⅳ isotherms with a surface area of 1437 m²/g and excellent thermal stability. Subsequently, a H-BiMOF-coated SPME fiber was prepared by the physical adhesion method and used to extract trace PAHs from environmental samples. Due to the hollow structure of H-BiMOF, the prepared fibers offer the advantages of high utilization of specific surface area as well as short mass transfer distance, so that the extraction process quickly reaches equilibrium. At the same time, the introduction of bimetals provides a variety of metal active sites, which improves the extraction efficiency of the fiber against electron-rich cloud targets such as PAHs. The prepared fiber also had good service life, with at least 150 cycles. Combined with gas chromatography-tandem mass spectrometry (GC-MS/MS), a new method for the determination of PAHs in environmental water samples was established. Single factor experiments were performed to investigate the effects of the SPME conditions on the analytical performance. Under the optimal conditions, the established method showed low limits of detection (0.01-0.08 ng/L), wide linear range (0.03-500.0 ng/L), good linearity (correlation coefficients≥0.9986), and acceptable reproducibility (relative standard deviations≤9.8%, n=5). Finally, typical water samples were analyzed by the established method. Four environmental water samples were collected from Dianchi Lake, Poyang Lake, Taihu Lake, and Xihu Lake in China. No benzo(a)anthracene (BaA) and chrysene (CHR) were detected in any of the water samples. However, 17.9 ng/L of fluorene (FLU) and 5.3 ng/L of phenanthrene (PHE) were found in the Poyang Lake sample; 11.3 ng/L of fluoranthene (FLA) and 24.2 ng/L of pyrene (PYR) were found in the Taihu Lake sample; 50.0 ng/L of FLU, 19.5 ng/L of PHE, 14.9 ng/L of anthracene (ANT), 34.2 ng/L of FLA, and 44.5 ng/L of PYR were found in the Xihu Lake sample. The contents of the PAHs detected in all the lake water samples were lower than the Chinese National Standard GB 5749-2006 (2000.0 ng/L). The results of this study indicate that the developed method is suitable for the sensitive detection of trace levels of PAHs in real environmental water samples.

Key words: bimetal, metal-organic frameworks (MOFs), solid-phase microextraction (SPME), gas chromatography-tandem mass spectrometry (GC-MS/MS), polycyclic aromatic hydrocarbons (PAHs)