| Fluoride contamination in drinking water has become a matter of great concern around the world due to its serious threat to human health. Among the different treatment technologies, adsorption process for fluoride removal is considered cost-effective, flexible, and easy to design and operate. However, several problems of the conventional adsorbents, such as narrow available pH range, poor selectivity and low defluoridation capacity, are still crucial limiting factors as they will affect the efficiency of fluoride uptake. Nanoadsorbents usually showed an excellent fluoride removal performance due to their high surface areas and high density of adsorptive sites. However, the nanopowder form of the adsorbent makes it difficult to be immobilized, separated and recycled in the practical defluoridation process. To maintain the excellent fluoride removal performance and address the limitations of nanoadsorbents, it is an ideal way to make use of a micro-/nanostructured materials for fluoride removal or load the nanoadsorbents onto support materials to facilitate engineering applications. Besides, recent studies showed that the surface sulfate anions have been proved to be effective active groups, which can participate in the ion-exchange process as well as surface hydroxyl groups. Thus, doping sulfate would also be an alternative way to improve fluoride removal performance for adsorbents.This dissertation focus on the preparation of nanostructured hierarchical adsorbents and their adsorption performance for the removal of fluoride from drinking water. The details are described as follows:(1) Sulfate-doped hydroxyapatite (HAP) hierarchical hollow microspheres, was prepared and developed for fluoride removal from water. Batch of adsorption experiments were investigated to study the fluoride removal performance. The sulfate-doped HAP showed enhanced fluoride removal performance with a defluoridation capacity of 28.3 mg g-1 with initial fluoride concentration of 100 mg L-1 at 25 ℃ under neutral conditions, which was higher than other previously reported HAP-related adsorbents. The adsorption isotherm could be better defined by Ferundlich model than Langmuir model. The adsorption kinetic followed the pseudo-second-order model. And the sulfate-doped HAP had a relatively high defluoridation capacity in a wide pH range of 3.0-10.0. The study of adsorption mechanism suggested that the hydroxyl groups and the sulfate groups on the surface were all involved in the fluoride adsorption, and these active sites cooperatively enhanced the fluoride removal.(2) Hierarchically porous Ce-Zr oxide nanospheres encapsulated calcium alginate millimeter-sized beads (CZ-CABs) were synthesized by using a sol-gel templating technique. Their defluoridation performance, including static and dynamic adsorption, was systematically evaluated. The adsorption kinetic followed the pseudo-second-order model. The adsorption isotherm could be divided into two distinct regions depending on the fluoride concentrations, and the CZ-CABs exhibited a Langmuir-Freundlich maximum fluoride adsorption capacity of 137.6 mg g-1 under neutral conditions. Such a specific adsorption isotherm indicated that various mechanisms were involved in the fluoride adsorption depending on fluoride concentrations, which were further demonstrated by FTIR and XPS analyses. The effect of pH and co-existing anions on fluoride adsorption was studied. Furthermore, column adsorption experiments were conducted, and the results showed a high efficiency of the CZ-CABs for the removal of fluoride from water on a continuous flow basis. |
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