Abstract
The prevalence of pharmaceuticals and their metabolites in the environment is causing great concern. These pharmaceuticals are widely and steadily used in veterinary and human medicine and are continuously released into the environment. The direct release of treated and untreated wastewater from wastewater treatment plants is the primary cause of the widespread presence of pharmaceutically active chemicals, which are identified as emerging contaminants in aquatic environments. Among these emerging contaminants, antibiotics such as tetracyclines (TCs) have been frequently detected in wastewater, natural and drinking water systems at concentrations ranging from ng/L to μg/L. Therefore, there is a need for the development of rapid and sensitive analytical methods for the extraction, preconcentration and monitoring of these pollutants in the environmental matrices.
The combination of environmental analytical chemistry and nanotechnology presents attractive prospects for the development of rapid, cost-effective, miniaturized, selective and sensitive sample preparation methods for environmental monitoring. The application of nanomaterials in sample pretreatment has gained popularity over the last decade. Among the wide variety of nanoadsorbents, metal-organic frameworks and molecularly imprinted polymer (MOF-MIP) based materials have shown promising performance in environmental applications. The hybrid material possesses great characteristic properties such as thermal stability, greater surface area, chemical ability and tuneable pores.
Therefore, this study aimed to develop sample preparation methods for extraction and preconcentration of TCs prior to their chromatographic quantification using High performance liquid chromatography coupled with a diode array detector (HPLC-DAD). To achieve the aim of the study, magnetic nanocomposites were synthesized and explored as adsorbents for ultrasound-assisted-dispersive magnetic solid-phase extraction (UA-DMSPE) of different TCs in wastewater samples. These adsorbents included magnetic chromium-based metal-organic framework (Fe3O4@MIL-101(Cr)) and magnetic chromium-based metal-organic framework-molecularly imprinted polymer (Fe3O4@MIL-101(Cr)@MIP) nanocomposites. The adsorbents were characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Brunauer–Emmett–Teller (BET). Parameters affecting the developed sample preparation methods were optimised using the design of experiment tools such as central composite design. The performance of each adsorbent in the development of analytical methods for the analysis of TCs is discussed in the subsequent paragraphs.
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Firstly, the suitability and practicability of Fe3O4@MIL-101(Cr) adsorbent in UA-DMSPE were investigated. Under optimum conditions, the UA-DMSP.HPLC-DAD method had linear ranges between 0.16-2000 and 0.11-2500 μg/L for Oxytetracycline (OTC) and doxycycline (DXC), respectively. The limits of detection (LOD) and quantification (LOQ) of the method ranged from 0.03-0.049 μg/L and 0.16 μg/L and 0.11 μg/L, respectively. The detection limits ranged from 0.049 to 0.03 μg/L. The recoveries obtained for OTC and DXC, after spiking wastewater samples, ranged from 98 to 107%. The adsorbent showed good precision in terms of repeatability and reproducibility.
Secondly, the Fe3O4@MIL-101(Cr)@MIP nanocomposite was used for selective preconcentration of chlorotetracycline (CTC) from water. The analytical performance of the synthesized Fe3O4@MIL-101(Cr)@MIP was investigated. The linearity and detection limits of the developed method ranged from 0.11-1000 μg/L and 0.033-0.39 μg/L, and the quantification limits of the UA-DMSPE/HPLC-DAD procedure were 0.11 μg/L and 0.13 μg/L for OTC and CTC. The recoveries obtained for OTC and CTC, after spiking wastewater samples,, ranged from 98-107%, with relative standard deviations not exceeding 4%. The molecularly imprinted polymer showed high selectivity and high adsorption capacity compared to the non-imprinted polymer. These findings suggested the developed method is an acceptable analytical method for the determination of these TCs in wastewater samples.