Adsorption of Antibiotics and Nickel from Aqueous Solutions on Heterogeneous Adsorbents Based on Barley Straw
Date
2018-01-05
Authors
Journal Title
Journal ISSN
Volume Title
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ORCID
0000-0002-3541-222X
Type
Thesis
Degree Level
Doctoral
Abstract
Water security is essential in order to achieve sustainability. Concerns of antibiotic-contaminated water bodies have prompted research of effective water treatment technologies. Researchers are particularly concerned about two commonly prescribed fluoroquinolones antibiotics with broad-spectrum activity and good oral absorption: levofloxacin (LEV) and norfloxacin (NOR). However, these drugs cannot be completely metabolized in humans or animals, nor can they be effectively removed using current wastewater treatment technologies such as activated sludge. As a result, they are discharged into the environment and become emerging environmental contaminants, which coexist with metals such as nickel ions in diverse aquatic systems. Consequently, they may threaten human health.
Adsorption, the partitioning of a target compound between the fluid and adsorbent phase, is an alternative technology for pollutant removal. In this work, raw barley straw (RBS)--an abundantly generated agricultural byproduct mainly composed of cellulose, hemicellulose, and lignin--was pretreated by H3PO4 impregnation and microwave heating. This pretreated barley straw (PBS) was used as an adsorbent for the removal of LEV, NOR, and nickel ions, which are representatives of antibiotics and heavy metals, from artificial wastewater. The research included the following phases:
1. Phase I: Pretreatment and characterization of adsorbents based on barley straw. PBS that had a high surface area (1314 ± 10 m2/g) was obtained at conditions of 5% (w/v) H3PO4 impregnation concentration and 9 min 700 W microwave heating. The total organic carbon (TOC) released into suspensions from the adsorbents significantly reduced from 34.4 ± 0.9 mg/g (RBS) to 0.9 ± 0.2 mg/g (PBS) indicating the enhanced stability of PBS. Barley straw adsorbents were characterized by particle size distribution, elemental compositions, scanning electron microscopy, and thermogravimetric analysis.
2. Phase II: Adsorption of LEV on PBS. PBS demonstrated high LEV adsorption capacities in a wide range of solution pH (2.47-9.60). The experimental maximum LEV adsorption capacity of PBS (408 ± 5 mg LEV/g at pH 6.88 and 298.15 K) was much higher than that of RBS and the reported adsorbents. The adsorption kinetics and equilibrium at different temperatures were investigated, and the kinetic and isotherm data were well-fitted by the pseudo-second-order kinetic model and Langmuir-Freundlich model, revealing that the adsorption of LEV on PBS was an endothermic process. The activation energy was determined to be 45.9 kJ/mol. The desorption of LEV loaded on PBS was examined, and the site energy and its distribution of PBS for LEV adsorption were estimated. The weighted mean and standard deviation of the distribution were employed to depict the interaction strength between the adsorbent and adsorbate, and adsorption heterogeneity. The π-π electron-donor-acceptor (EDA) interaction between the π* aromatic C=C of PBS and the π* carbon atom in the benzene ring attached to fluorine of LEV was proposed as one of the major adsorption mechanisms. Such interaction was investigated by C K-edge X-ray absorption near-edge structure (XANES) spectroscopy.
3. Phase III: Adsorption of NOR on PBS. High NOR adsorption capacities of PBS were also obtained in a wide pH range (2.67-10.50). The experimental maximum NOR adsorption capacity of PBS (396 ± 14 mg NOR/g at pH 6.96 and 298.15 K) was much higher than that of RBS and the reported adsorbents. The adsorption kinetics and equilibrium with respect to temperature were evaluated using the pseudo-second-order kinetic model and Langmuir-Freundlich model, respectively. The adsorption of NOR on PBS was an endothermic processes with an activation energy of 22.2 kJ/mol. The desorption of NOR loaded on PBS was also examined. The adsorption site energy and its distribution were also determined to exhibit the adsorption mechanism. At the tested temperatures of 298.15-318.15 K, the higher the temperature, the higher the weighted mean (therefore the stronger the adsorption affinity), and the slightly weaker the heterogeneity. The percentage of adsorption sites, whose site energy is greater than or equal to a specific E* value, was estimated. The proposed dominant forces, EDA interactions (n-π and π-π), were investigated by the C and O K-edge XANES spectroscopy.
4. Phase IV: Adsorption of nickel on PBS and impact of nickel on LEV adsorption. The adsorption isotherms of nickel on PBS and RBS were well-fitted using the Langmuir-Freundlich model. Solution pH played an important role during the nickel adsorption process. Results of XANES and extended X-ray absorption fine structure spectroscopy indicated that the adsorbed Ni remained the same oxidation state (II) as NiSO4 and was associated with 6 oxygen atoms from water or the functional groups of PBS (e.g., carboxyl group) at an atomic distance of 2.043 ± 0.013 Å. The site energy and its distributions of PBS and RBS for nickel adsorption were determined and demonstrated that the adsorption affinity between PBS and nickel was stronger than that of RBS. Ni(II) slightly suppressed the LEV adsorption on PBS at the tested pH values (4.0, 7.0, and 9.0) since the cationic Ni(II) competed with LEV for the negatively charged adsorption sites of PBS.
5. Phase V: The extended application of PBS in selective water removal from water-ethanol mixture was investigated to produce fuel grade ethanol. PBS demonstrated a higher water adsorption capacity (0.63 ± 0.02 mol/g) and a close water to ethanol adsorption ratio (4.85) compared with RBS (0.50 ± 0.01 mol/g and 5.00). Utilization of PBS in ethanol concentration can be optimized by a dynamic system control in order to achieve a higher concentration of ethanol.
Description
Keywords
pretreated barley straw, antibiotics and heavy metal adsorption, electron-donor-acceptor interactions, X-ray absorption near edge structure, site energy distribution.
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
School of Environment and Sustainability
Program
Environment and Sustainability