Abstract
Perfluorooctanoic acid (PFOA) is a fluoro- organic compound with water, heat, grease, and dust repellent properties widely used in the manufacturing industry. Due to the various health effects posed by PFOA in humans such as cancer and infertility, many studies around the world have attempted to quantify PFOA in water systems. The challenge in the quantification of PFOA is that it is present at low concentration and associated with complex matrices. Therefore, because of sample complexity, PFOA in environmental samples direct detection and quantification using sensitive analytical technique ultra-high performance liquid chromatography-electrospray-tandem mass spectrometry (UHPLC-ESI-MS/MS) is impossible. Therefore, extraction, preconcentration and cleanup is required before PFOA quantitative determination using UHPLC-ESI-MS/MS. The literature showed that traditional solid phase extraction (SPE) is the most used clean-up method. However, the operation tradition SPE procedures is tedious and require large adsorbent amounts, thus making the sample preparation method more expensive.
Owing to the need for improved sample cleanup methods, cost-effective, easy to separate, and environmentally friendly adsorbents for PFOA extraction are need. This study prepared tea waste activated carbon modified with Fe3O4 nanoparticles (TWAC-Fe3O4) and magnetic β-cyclodextrin molecularly imprinted polymer (TWAC-Fe3O4@-β-CD-MIP) as adsorbents in dispersive magnetic solid phase extraction (DSPEME) of PFOA in water bodies prior UHPLC-ESI-MS/MS quantification. The TWAC-Fe3O4@-β-CD-MIP was selected because of its capabilities in selectively binding PFOA to mitigate the problem of matrix effects, thus enhancing the PFOA quantification at trace concentration levels in real water samples. The project aimed to keep the cost of synthesizing the adsorbent material minimal by using tea waste.
Evidently, through the identification of magnetic nanoparticles on the surface of TWAC structure through Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) spectroscopy transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis, the TWAC-Fe3O4 was successfully synthesized. Batch adsorption studies were conducted and the mechanism of adsorbing PFOA on TWAC-Fe3O4 followed the Freundlich model (R2 = 0.9331), which suggested a multilayer adsorption and that the adsorbent was heterogenous. Validation experiments towards PFOA determination in
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model water and real water samples showed that the developed TWAC-Fe3O4 DMSPE-UHPLC-ESI-MS/MS technique was sensitive and precise. The method had an acceptable precision (%RSD = 2.4-3.1%) and percentage recoveries (%R) were from 88.2-98.4% in spiked real water samples. The TWAC-Fe3O4 adsorbent was stable and re-usable up to four adsorption-desorption cycles. Compared to other studies conducted for PFOA around the world, the developed TWAC-Fe3O4 DMSPE-UHPLC-ESI-MS/MS method achieved acceptable LOD and LOQ values of 11.8 ng/Land 39 ng/L, respectively.
Selective extraction and preconcentration of PFOA was also investigated using the TWAC-Fe3O4@-β-CD-MIP DMSPE. The successful synthesis of the adsorbent was confirmed by FTIR, XRD, SEM-EDS, BET and TEM. Unlike the TWAC-Fe3O4, the TWAC-Fe3O4@-β-CD-MIP interaction with PFOA during adsorption followed the Sips isotherm, which suggested a multilayer coverage (heterogenous surface). Comparative investigation with the TWAC-Fe3O4@-β-CD-NIP indicated that using the TWAC-Fe3O4@-β-CD-MIP adsorbent improved adsorption by a factor of 7.6. Moreover, the MIP adsorbent was highly selective towards PFOA with a selectivity coefficient (α’) of 6.1 compared to the TWAC-Fe3O4@-β-CD-MIP. Validation experiments showed that the developed TWAC-Fe3O4@-β-CD-MIP DMSPE method was selective, sensitive and repeatable. Compared to the TWAC-Fe3O4-MSPE method, the TWAC-Fe3O4@-β-CD-MIP-MSPE technique had higher spike recoveries (98.8-105%) at an acceptable precision (<5 %). The adsorbent material was stable and re-usable up to 2 adsorption-desorption cycles. Compared to similar studies in the world, the developed TWAC-Fe3O4 DMSPE-UHPLC-ESI-MS/MS technique obtained comparable LOD and LOQ values of 5.0 ng/L and 16.7 ng/L.
The applicability of both developed methods (TWAC-Fe3O4 and TWAC-Fe3O4@-β-CD-MIP as clean-up techniques for PFOA determination in real water was tested and PFOA was successfully quantified in 3 river water matrices namely Umgeni River, Hennops River, and Sand River at ng/L levels. The adsorbent materials were easily separated after application using a magnet. Lastly, using the TWAC-Fe3O4@-β-CD-MIP adsorbent enhanced PFOA detection in the presence of other contaminants in real water samples. According to the reviewed literature, there is currently no evidence of PFOA based TWAC-Fe3O4@-β-CD-MIP adsorbents for DMSPE procedures towards PFOA enrichment in water matrices. Therefore, this study the first to prepare the TWAC-Fe3O4@-β-CD-MIP adsorbent for application towards extracting and preconcentrating PFOA to enhance its detection in river water systems, including validation of the approach.