Adsorption of Carbamazepine from Water by Hydrothermally Treated and Steam Activated Flax Shives and Oat Hulls

Abstract

Recently water pollution is one of the many threats that the world population is confronting, which requires immediate consideration. Numerous emerging pharmaceutical pollutants are commonly found in aquatic environments, wastewater, and ground surface because of their extensive uses for human beings and animals. Carbamazepine (CBZ), an antiepileptic drug, is one of the emergent pollutants. The existence of the pollutants turns into a serious health risk to people. In order to control these pollutants in the environment, it is necessary to develop efficient removal methods. In this thesis, flax shives and oat hulls representing cellulosic materials of agricultural byproducts were used to develop hydrochars and steam activated hydrochar adsorbents. They were used to adsorb CBZ as a model pharmaceutical pollutant from artificially contaminated water. The adsorbents were characterized by surface properties and composition. Hydrothermal carbonization increased the porosity and surface area of raw flax shives and raw oat hulls, which were further significantly enhanced by steam activation. In addition, both hydrothermal carbonization and steam activation increased the carbon content and decreased the oxygen content and ratio of H/C of the flax shive and oat hull adsorbents. These thermal treatments generated aromatic structure and functional groups such as hydroxyls and carbonyls or carboxyls which were important for CBZ adsorption. CBZ equilibrium adsorption was investigated at various temperatures (20-40oC) and solution pH (2-10). Steam activation significantly enhanced the adsorption capacity of the hydrochars made from either flax shives or oat hulls. However, both the hydrochars with or without steam activation demonstrated effective CBZ adsorption in comparison with numerous reported adsorbents. The achieved adsorption capacities were not significantly affected by solution pH, indicating that the adsorbents developed in this work are applicable to remove CBZ and alike pharmaceutical pollutants from water/wastewater in a wide range of pH values. The results implied that electrostatic interaction between the charged adsorbents and CBZ molecules, and hydrogen bonding may not be the major adsorption mechanisms. The π-π Electron-Donor-Acceptor (EDA) interactions and hydrophobic interaction could play important roles in the adsorption. The CBZ adsorption was effective at room temperature, which was enhanced at a higher temperature, showing the adsorption was endothermic in nature. In addition, the isotherms of CBZ adsorption by the steam activated hydrochars were experimentally generated at various temperatures (20-40oC), which were successfully simulated by the Sips model. The site energy distribution of CBZ adsorption based on the Sips modeling results was further determined. The higher weighed mean of site energy obtained at 40°C indicated higher adsorption affinity, thus more favorable for adsorption. Furthermore, the thermodynamic parameters of CBZ adsorption were determined. The results again indicated the CBZ adsorption was endothermic and spontaneous. The experimental CBZ adsorption kinetic data of both hydrochars and steam activated hydrochars were generated and successfully simulated by the pseudo-second-order kinetic model. Because of higher adsorption rate constants and lower activation energy, steam activated hydrochars adsorbed CBZ faster than the hydrochars. Desorption of CBZ from hydrochars and steam activated hydrochars was investigated with different solvents and at various pH levels. The highest desorption efficiency was obtained with organic solvent ethanol. Overall, the results demonstrated that flax shives and oat hulls are promising feedstock’s to be made into adsorbents for treatment of wastewater contaminated by CBZ and additional similar pharmaceuticals.