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Doctoral Degrees (Chemistry)

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Browsing Doctoral Degrees (Chemistry) by Subject "Adsorption studies."

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    Assessment of antiretroviral drugs uptake by vegetables from contaminated soil and their adsorption by exfoliated graphite in river and wastewater.
    (2022) Kunene, Philisiwe Nganaki.; Mahlambi, Precious Nokwethemba.
    This study was directed toward vegetable uptake of the commonly used antiretroviral drugs (ARVDs), abacavir, nevirapine, and efavirenz. Antiretroviral drugs are used to treat the human immune-deficiency virus (HIV). South Africa (SA) is one of the countries with a high number of infected people on ARV therapy, therefore, the ARVDs are anticipated to be existing at high concentrations in the South African environment than in other countries worldwide. In recent years, the presence of ARVDs in the environment has drawn attention; hence studies have reported their presence in aquatic environments while very few studies have been conducted on their uptake using vegetables. This work was therefore based on the optimization and application of sensitive, simple, cost-effective, and robust techniques for quantifying ARVDs in vegetables. Based on this information, ultrasonic extraction (UE) and microwave-assisted extraction (MAE) were used to isolate target compounds from vegetable samples to the aqueous phase. Dispersive liquid-liquid microextraction (DLLME) and solid-phase extraction (SPE) were utilized to preconcentration and clean up the extracts from UE and MAE, respectively. A liquid chromatography photodiode array detector (LC-PDA) was utilized to detect and quantify the extracted compounds. The UE with and without DLLME cleanup were compared with each other, also, MAE with and without SPE cleanup were compared with each other. The methods comparison was done in terms of their detection (LOD) and quantification limits (LOQ), extraction efficiencies (%Recovery), relative standard deviations (%RSD), and concentrations of ARVDs found in vegetable samples. In comparison of UE and ultrasonic-assisted dispersive liquid-liquid microextraction (UADLLME), the LOD and LOQ obtained ranged between 0.0081 - 0.015 μg/kg and 0.027 - 0.049 μg/kg for UE and 0.0028 -0.0051 μg/kg and 0.0094 - 0.017 μg/kg for UADLLME respectively. High recoveries ranging from 93 to 113% in UE and 85 to 103% in UADLLME with less than 10% RSD in both procedures were obtained. These results indicated that UADLLME is more sensitive than the UE method, although they are both accurate and precise. The UE can be recommended for routine analysis as UADLLME showed the inability to extract analytes from root vegetables. The optimized UE and UADLLME methods were applied to extract ARVDs from vegetables bought from local fruit and veggie supermarket. Vegetables were categorized as root (carrot, potato, and sweet potatoes), leaf (cabbage and lettuce), and fruit (green paper, butternut, and tomato). The target ARVDs were quantified in most samples with concentrations up to 8.18 μg/kg. The concentrations obtained were slightly high in UADLLME than in UE as a result of its high sensitivity. Efavirenz was the most dominant drug, while the potato was the most contaminated vegetable. In the comparison of MAE and MAESPE, the obtained LOD and LOQ ranged from 0.020 to 0.032 μg/kg and 0.068 to 0.109 μg/kg for MAE and 0.019 to 0.066 μg/L and 0.065 to 0.22 μg/L for MAE-SPE. The obtained recoveries ranged from 85 to 103% for MAE and from 82 to 98 % for MAE-SPE, respectively, and the RSDs were all less than 6%. These results showed that both methods have comparable sensitivity; however, the recoveries values for MAE were slightly higher than those obtained in MAE-SPE, which signals MAE’s high accuracy. The optimized MAE and MAE-SPE methods were applied to remove ARVDs in the root (potatoes, onions, and beetroot), leaf (lettuce, and spinach), and fruit (green paper, cucumber, and eggplant) vegetables bought from local fruit and veggie supermarket. The obtained ARVDs concentration range was 1.48 ± 0.5 - 27.9 ± 1.2 μg/kg. The MAE-SPE resulted in low concentration compared to MAE without cleanup. Beetroot exhibited high concentrations of the target ARVDs, while nevirapine was found to have high concentration and as a dominant compound. The results obtained revealed that the vegetables from the studied area are contaminted with ARVDs, which could indicate their possible irrigation with wastewater effluent or the use of sludge as biosolids in the agricultural areas. This is a concern as it leads to unintentional consumption by consumers which could lead to drug resistance by the human body or have human health effects. The study was then expanded by conducting the phytoremediation approach to investigate the uptake of abacavir, nevirapine, and efavirenz by beetroot, spinach, and tomato from the contaminated soil. The three selected vegetable plants were planted and irrigated with ARVDs spiked (at 2000 and 5000 μg/L) water over a period of three months. The optimized UE and LC-PDA methods were used to extract and quantify the selected ARVDs from the target vegetables and soil. The obtained results showed that the studied vegetables have the potential to take up abacavir, nevirapine, and efavirenz from contaminated soil, be absorbed by the root, and translocate to the aerial part of the plants. Abacavir was found at high concentrations to a maximum of 40.21 μg/kg in the root, 18.43 μg/kg in the stem, and 6.77 μg/kg in the soil, while efavirenz was the highest concentrations, up to 35.44 μg/kg in leaves and 8.86 μg/kg in fruits. Spinach root accumulated more ARVDs than beetroot and tomato. The bio-accumulation factor ranged from 2.0-14 μg/kg in beetroot, 3.6 - 15 μg/kg in spinach, and 6 – 10 μg/kg in tomato. The root concentration factor range was 0.047 – 17.6 μg/kg; 0.34-5.9 μg/kg, and 0.14-2.82 μg/kg in beetroot, spinach, and tomato, respectively. The translocation factor range obtained was 0.40 – 38 μg/kg, 0.08 – 19 μg/kg, and 0.14 – 49 μg/kg in beetroot, spinach, and tomato, respectively. However, the accumulation of ARVDs in all studied plants showed that they could be used in phytoremediation. The results obtained in the phytoremediation approach revealed that the utilization of the contaminated water has an influence on the presence ARVDs in vegetables; hence this work also focused on evaluating the exfoliated graphite adsorption of ARVDs in water. Natural graphite was intercalated with acids and exfoliated with thermal shock to obtain the exfoliated graphite. The scanning electron microscopy images showed that the exfoliated graphite had increased c-axis distance between the layers with accordion-like structure which were confirmed by the lower density of exfoliated graphite material (0.0068 gmL-1) compared to the natural graphite (0.54 g mL-1). Fourier Transformed Infrared Spectroscopy results showed the C=C in natural and exfoliated graphite at 1635 cm-1 stretching. The phenolic, alcoholic, and carboxylic groups were observed from 1000 to 1700 cm-1 for the intercalated and exfoliated graphite. The Energy-dispersive X-ray results further confirmed these results, which showed carbon and oxygen peaks in the intercalated and exfoliated graphite spectrum, whereas natural graphite showed only a carbon peak. Raman spectroscopy results showed that the material’s crystallinity was not affected by the intercalation and exfoliation processes as observed from the ratios of the G and D peaks and the G' and D'. Natural, intercalated and exfoliated graphite contained the D, G, D', and G' peaks at about 1350 cm-1, 1570 cm-1, 2440 cm-1, and 2720 cm-1, respectively. The exfoliated graphite material showed the characteristic of a hexagonal phase graphitic structure by (002) and (110) reflections in the X-ray diffraction results. The exfoliated graphite adsorption method was optimized based on the pH of a solution, adsorbent dosage, and adsorption time prior to application to water samples. The optimum pH solution, adsorbent dosage, and adsorption time were 7, 30 mg, 0.01 μg/L, and 30 minutes respectively. The kinetics and isotherm studies were conducted to assess the model that best fit and explain the experimental data obtained. The kinetic model and adsorption isotherm studies showed that the experimental data fit well pseudo-second-order kinetics and is well explained by Freundlich’s adsorption isotherm. The maximum adsorption capacity of the exfoliated graphite (EG) for ARVDs ranges between 1.660-197.0, 1.660-232.5, and 1.650-237.7 mg/g for abacavir, nevirapine, and efavirenz, respectively. These results showed that under proper operating conditions, the EG adsorbent could potentially be applied as a water purifying tool for the removal of ARVDs pollutants.