Abstract
The increasing desire for better life in terms of health and food in developing countries has led to the increasing demand for both better health and animal production. This, in turn, has resulted in the extensive use of antibiotics in order to meet this demand. Antibiotics such as tetracyclines (TCs) find their way into the environment through human and animal excretion since they are partially metabolized in the body of the consumer. Other major sources of TCs in the environment are domestic sewage works, hospital waste disposal, expired and unused antibiotics disposal, and agricultural and aquatic farming activities. The majority of conventional wastewater treatment plants (WWTPs) fail to completely remove antibiotic residues from wastewater, and their effluents are discharged into the environment containing trace amounts of TCs which exacerbates the environmental pollution problem. Furthermore, the lack of stringent guidelines to control the discharge and presence of TCs in the environment also adds to the growing concern. The occurrence of these antibiotics in the water systems has become an increasing concern due to the threat they pose to human health and aquatic species when there is prolonged exposure. In order to address this issue, it is, therefore, crucial to determine and monitor the concentration of TCs in the environment. With this in mind, the principal aim of this study was to prepare magnetic chitosan-based adsorbents using a co-precipitation method and develop a simple and fast sample preparation method for the removal and pre-concentration of TCs in water samples. This was achieved by synthesizing magnetic chitosan-kaolin nanocomposite, magnetic chitosan-zeolite nanocomposite, and magnetic chitosan-polypyrrole nanocomposite. The synthesized chitosan-based nanocomposites were characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET). Magnetic solid phase extraction (MSPE) was employed as the sample preparation method. Following the sample clean-up, removal, or pre-concentration step, the TCs were quantified by high performance liquid chromatography coupled with a diode array detector (HPLC-DAD).
Overall, one of the key objectives involved the synthesis, characterization, and application of magnetic chitosan-kaolin nanocomposites (freeze-dried and oven-dried) in the removal and pre-concentration of the tetracycline antibiotic (TC) in water. The BET results indicated that the freeze-dried nanocomposite had a higher surface area of 37.4 m2 g-1 compared to the 19.3 m2 g-1 of the oven-dried nanocomposite. During the adsorption process,
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the freeze-dried and oven-dried nanocomposites followed the Langmuir isotherm model with adsorption capacity of 28 and 27 mg g-1, respectively, and were both described by the pseudo-first order kinetic model. Due to the higher surface area for the freeze-dried magnetic chitosan-kaolin nanocomposite, it was applied for the extraction and pre-concentration of TC from wastewater and river water samples. The central composite design (CCD) was used to optimize the magnetic solid phase extraction (MSPE) method. Under optimum conditions, the method yielded good linearity with a limit of detection (LOD) and a limit of quantification (LOQ) of 0.21 and 0.63 μg L-1, respectively. The method showed satisfactory accuracy of 89-103% and was specific for the pre-concentration of TC. Of the three real samples tested (influent, effluent, and river water), TC was detected in the influent with a concentration of 2.92 μg L-1, while the effluent had a concentration which was below the quantification limit. Secondly, magnetic chitosan-zeolite nanocomposite was synthesized, characterized, and applied in the ultrasound-assisted magnetic solid phase extraction (UA-MSPE) for the extraction and pre-concentration of oxytetracycline (OT), tetracycline (TC) and doxycycline (DC) in wastewater and river water prior to analysis with high performance liquid chromatography equipped with a photodiode array detector. The maximum adsorption capacities were 103, 104, and 97 mg g-1 for OT, TC, and DC, respectively. The proposed method was sensitive with LOD and LOQ values that ranged from 0.05-0.67 μg L-1 and 0.14-2.03 μg L-1, respectively. An acceptable linear range of LOQ-400 μg L-1 and percentage recoveries of 90.3-100.7% were obtained, suggesting that the complex matrix did not affect the analytical method.
An adsorbent that consisted of chitosan mixed with polypyrrole and iron oxide nanoparticles was prepared, characterized, and used in magnetic solid-phase extraction. The prepared adsorbent was useful for the removal of (OT), (TC), chlortetracycline (CT), and DC in wastewater and river water. Their removal allowed for the pre-concentration and analysis of these antibiotics, which mostly enter the environment in trace amounts. The adsorption of these TCs was described best by the Langmuir isotherm model and pseudo-first order kinetics. Satisfactory removal efficiencies were obtained, with the nanocomposite being able to be reused five times before a considerable reduction (of 50% or below) in the removal efficiency was observed.