Design And Preparation Of Nano- Adsorbents And Adsorption Mechanism For Fluoride In Water

The pollution of fluoride in groundwater has seriously threatened the environment, human health, and social development. Thus, it becomes extremely important and necessary to remove micropollutant fluoride from groundwater. And developing a highly-efficient, stabile and low cost removal technologies for fluoride is of important scientific significance for removing fluoride from groundwater. However, fluoride is adsorbed on adsorbents through electrostatic attraction and/or ion exchange. Surface hydroxyl groups are mainly reported to be involved in the ion exchange step. These adsorbents possess narrow available pH range, low selectivity and bad adsorption capacity for fluoride removal. The presence of anions significantly affects fluoride removal from complex groundwater. So, designing and developing new adsorbent based on new defluoride mechanism is of important for adsorbent application.Based on the above problems, the dissertation aims to design and prepare metal oxide nano-material, control the morphology and apply these materials into removing fluoride from complicated groundwater. And the defluoridation properties of metal oxide nano-material in groundwater were investigated. In addition, the adsorption mechanism of nanomaterials has been further demonstrated. The details are described as follows:(1) Nanostructured Fe-Al-Mn trimetal oxide adsorbent was synthesized by simple coprecipitation method and used for removing fluoride from simulated groundwater. We characterized the morphology and composition of the adsorbent, and studied its defluoridation properties and mechanism. The effects of pH and adsorption temperature on kinetic and thermodynamic adsorption of fluoride by the Nanostructured Fe-Al-Mn trimetal oxide adsorbent were investigated. The results show that the adsorbent possesses excellent adsorption capacity for fluoride removal. The fluoride removal mechanism was studied by Fourier transform infrared spectroscopy (FT-IR), indicating that the fluoride adsorption mainly resulted from the exchange of the hydroxyl groups on the surface of the Nanostructured Fe-Al-Mn trimetal oxide adsorbent with fluoride anions. In addition, the adsorption of F-by the Nanostructured Fe-Al-Mn trimetal oxide adsorbent was a spontaneously endothermic process.(2) A novel friendly environmental adsorbent, micro-nano hierarchical structured flower-like MgO/MgCO3, was developed for fluoride removal from simulated groundwater. We characterized the morphology and composition of the adsorbent, and investigated its defluoridation properties and mechanism. The effects of pH and various co-existing ions on kinetic and thermodynamic adsorption of fluoride by the micro-nano hierarchical structured flower-like MgO/MgCO3 adsorbent were studied. The results show that this adsorbent possessed excellent adsorption capacity for fluoride removal, and the fluoride adsorption capacities even reached up to 149 mg/g at pH= 11 for equilibrium concentrations of 50 mg/L. The adsorbent possessed a very wide available pH range of 5-11. The effects of co-existing anions indicated that the anions had less effect on fluoride removal except phosphate. Moreover, the fluoride removal mechanism analysis revealed that the wide available pH range and less effects of co-existing anions toward fluoride were mainly resulted from the exchange of the carbonate and hydroxyl groups on the surface of the MHS-MgO/MgCO3 with fluoride anions.(3) A new uniform-sized CeCO3OH nanosphere adsorbent was prepared, and developed to establish its efficiency for fluoride removal. The results demonstrated that the CeCO3OH nanospheres showed much high adsorption capacities for fluoride anions due to electrostatic interactions and exchange of the carbonate and hydroxyl groups on the adsorbent surface with fluoride anions. Adsorption kinetics was described well by the pseudo-second-order model, and adsorption isotherm data were fitted well by Langmuir model with the max adsorption capacity of 45 mg/g at pH 7.0±0.2. Thermodynamic examination demonstrated that fluoride removal on the CeCO3OH nanospheres was reasonably spontaneous and endothermic. Moreover, the CeCO3OH nanospheres have less influence on adsorption of fluoride by pH and co-exiting ions, and the adsorption efficiency is very high at the low initial fluoride concentrations in the basis of the equilibrium adsorption capacities. This study indicated that the CeCO3OH nanospheres could be developed into a very viable technology for highly effective removal of fluoride from simulated groundwater.