Abstract
Metformin (MET) is an antidiabetic drug developed from natural compounds found in plants to treat type 2 diabetes. Metformin is one of the most prescribed and widely used drugs worldwide since a large population suffers from diabetes. Due to overuse, it is detected in water systems. Furthermore, it is not completely metabolised by the human body and can be excreted through urine and stools and enters the water cycle through the sewage system. Research shows that this drug has been found in concentrations ranging from 10 ng/L–100 ng/L in surface, ground, and drinking water. Toxicological studies show that this drug is a potential endocrine disruptor. Environmental risk assessment of MET in water bodies reveals that it may pose risks to aquatic organisms, such as fish. This project has reported the success of synthesising the magnetic metal-organic frameworks (UiO-66@Fe3O4), UiO-66-molecularly imprinted polymers (MOF-MIP) and UiO-66-non-imprinted polymers (MOF-NIP) using the solvothermal method and co-precipitation. These materials were characterised to determine their structural, elemental, and morphological properties using various analytical techniques.
Results from Fourier-transform infrared spectroscopy (FTIR) analysis showed the different functional groups present in each synthesised nanomaterial. The scanning electron microscope (SEM) analysis for the Fe3O4 showed a spherical shape with an average size of 13 nm, while UiO-66 showed cubic shapes with less aggregation due to the high viscosity of the Deep Eutectic Solvents (DES) with a particle size of 76 nm. The nanocomposite showed that the functionalised Fe3O4 did not change the morphology of UiO-66, and the shape of UiO-66@Fe3O4 nanoparticles remained unchanged, but their particle size decreased. The Energy-dispersive X-ray spectroscopy (EDS) showed the different elements present in each material. The Fe3O4 confirmed the presence of iron (Fe) and oxygen (O), and UiO-66 showed the presence of zirconium (Zr) and oxygen (O). Lastly, the EDS of the nanocomposite confirmed that the zirconium-based MOF was successfully incorporated into the Fe-O of the magnetite. Zeta potential measurements showed the point of zero charge and helped understand the interaction between MET and the adsorbent. The nanocomposite had a net charge of pH 5.6. The Nitrogen adsorption-desorption analysis showed the surface area, and the pore volume of the materials, and all the prepared materials were mesoporous and followed a type IV hysteresis isotherm. The Brunauer-Emmett-Teller (BET) surface area of Fe3O4, UiO-66, and UiO-66@Fe3O4 were equal to 89.4 m2/g, 649 m2/g, and 567 m2/g,
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respectively. The MOF-MIP and MOF-NIP FTIR results were similar, indicating the successful removal of MET. However, the SEM image of MOF-MIP, when compared with the MOF-NIP was different, which could mean that solvents used to remove the template changed the structure of the MOF-MIP, and the pores were observed for the analyte to bind.
Dispersive solid phase extraction (dSPE) was developed to isolate the analyte of interest from the sample matrix before the determination using high performance liquid chromatography-diode array detector (HPLC-DAD). The method effectively extracted and preconcentrated MET in ultrapure water, surface water, and wastewater. The procedure was optimised using a central composite design, and parameters such as pH, mass of adsorbent, extraction time, eluent volume, and elution time were optimised. The magnetic UiO-66 material was used to extract and preconcentrate MET at optimum conditions. The extraction efficiency of the dSPE method using UiO-66@Fe3O4 ranged between 84% - 112% of MET. The limits of detection (LOD) and quantification (LOQ) for MET were 0.16 μg/L and 0.53 μg/L, respectively. The dSPE/HPLC-DAD method showed acceptable linearity in the concentration ranging from 0.5 - 100 μg/L with a correlation of determination of 0.9987. The intraday (n=10) and interday (n = 5 working days) precision expressed as relative standard deviations (RSD %) were less than 5%. The results indicate that the HPLC-DAD is a reliable and sensitive alternative for the detection of MET in ultrapure water, wastewater, and surface water. The magnetic UiO-66@Fe3O4 material can be a promising adsorbent for the analysis of MET in aqueous solutions.
The results obtained using UiO-66@Fe3O4 revealed the application of this adsorbent in surface and wastewater extract and preconcentrate MET and other analytes, therefore, UiO-66@MIP (MOF-MIP) material was synthesised to improve the selectivity of the sorbent material. The adsorption behavior of MOF-MIP and MOF-NIP to MET in aqueous solution was investigated. This was achieved by performing a sequence of adsorption experiments. The results obtained demonstrated that the MOF-MIP had high transfer mass rates and high sensitivity and selectivity affinity for MET as compared to MOF-NIP. The Scatchard analysis model suggested that the MOF-MIP has two binding affinities towards MET with the qmax of 26.7 mg/g and 60.9 mg/g, respectively. Whereas the Langmuir isotherm model displayed that the MOF-MIP adsorbent had high binding capacity suggesting the adsorbent large number of specific binding sites. The Freundlich model using aristolochic acid I (AAI) as a template molecule indicated that the AAI could be readily absorbed by MIP. The
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pseudo-second-order kinetic model well described the adsorption of MET. The adsorption capacities of MOF-MIP and MOF-NIP for adsorption of MET were 61 mg/g and 20.8 mg/g, respectively.