Synthesis Of Composite Materials Of Zr-Based MOFs And Graphene Oxide And Its Application For Adsorption Of Fluoride Ions

The fluorine is one of the essential trace elements for humans. However, long-term drinking water containing fluoride will lead to "fluorosis". Therefore, the technology for removing fluorine of high content in drinking water is always one of the focuses of attention and research in the field of water treatment. Metal-organic frameworks (MOFs) materials have characteristics of large surface area, tunable pore size, saturated or unsaturated metal sites and controllable surface properties. Herein, the MOFs materials have become good potential adsorbents. Graphene oxide (GO) is an excellent adsorption material because of its huge large specific surface area, and lots of hydrophilic functional groups (-OH,-COOH, C-O-C) on its surface, which can provide many binding sites for the growth of MOFs and guide the growth of MOFs when GO is applied to form composites with MOFs. Meanwhile, these hydrophilic functional groups may be helpful to improve the dispersion of the material in water. Therefore, MOFs/GO composite is expected to be an excellent adsorbent for fluoride ions.In this thesis, adsorbent of UiO-66(Zr) with mesoporous structure was prepared via improved solvothermal method using Zr(NO3)4·6H2O as metal source and terephthalic acid (H2BDC) as organic ligands. and then, UiO-66(Zr)/GO composite materials named as ZrUG3 was prepared through the similar solvothermal method by adding graphene oxide (GO). Static adsorption experiments wereperformed to investigate the adsorption behavior of the two kinds of the adsorbent material for removing fluoride ions in aqueous solution.The physicochemical properties of UiO-66(Zr) and ZrUG3 were characterized by XRD, SEM, TEM, TG-DTA, BET, FTIR and XPS. Effects of several parameters, including the dosage of adsorbent, pH value of solution, time, temperature, the initial concentration of fluoride ions solution and co-existed ions on the absorption process were investigated. Pseudo-second-order kinetics equation, Langumir adsorption isotherm model, Freundlich adsorption isotherm model and thermodynamic equation were used to fit and analyze the experiment results. Also the mechanism of the adsorption of fluoride ions was discussed.By optimizing the synthesis conditions, UiO-66(Zr) with higher specific surface area than that prepared in literature were prepared and the obtained UiO-66(Zr) has mesopore structure with BET specific surface area of 1018.63m2·g-1. With its high specific surface area as well as the strong interaction of the hard acid and hard base between the central metal Zr(IV) and fluoride ions, UiO-66(Zr) had a good adsorption performance for fluoride ions. In order to improve the particle dispersion of UiO-66(Zr) and dispersion of UiO-66(Zr) in water, GO was introduced to prepare UiO-66(Zr)/GO composite to further improve the adsorption properties of the adsorbent. When the addition amount of GO was 4%, the obtained ZrUG3 composites also had a mesoporous structure with BET specific surface area increasing up to 1234.34m2·g-1, the dsorption performance for fluoride ions of ZrUG3 was greatly improved. The morphologies of UiO-66(Zr) and ZrUG3 were spherical particles with the diameter of 200-300nm and 100-200nm, respectively. However, spherical particles of ZrUG3 were made of original nano-crystallites in size of of 30-80 nm, which was lower than that in UiO-66(Zr) with an average diameter of 100-200 nm.The pH of the solution had a great influence on the adsorption capacity of fluoride ions for UiO-66(Zr) and ZrUG3. In the range of pH 2.0-7.0, adsorption capacity could maintain a high level. And the maximum adsorption capacity was achieved at pH3.0 and pH4.0, respectively.The adsorption/desorption equilibrium of fluoride ions on UiO-66(Zr) and ZrUG3 could be reached in 150 minutes and 120 minutes, respectively. The adsorption processes could be well described by the pseudo-second-order equation with correlation coefficient R2≈1, accompanied by a process of intra-particle diffusion. The adsorption of fluoride ions on UiO-66(Zr) and ZrUG3 were fitted with both Langmuir and Freundlich model, but fitted with Langmuir better than Freundlich. The theoretical maximum adsorption capacity was 52.80mg·g-1 and 65.19mg·g-1, respectively. The study of thermodynamics showed that in the experimental temperature range from 15℃ to 55℃, the adsorption of fluoride by UiO-66(Zr) and ZrUG3 was an exothermic, spontaneous process with reducing entropy.The co-existed ions of Cl-, SO42-, and NO3- in water had no influence on removal ratio of fluoride ionson UiO-66(Zr) and ZrUG3. However, the removal ratio of fluoride ions was negatively correlated with the concentration of CO32- and PO43-, which indicated that there were adsorption competition among carbonates, phosphates and fluoride ions. The adsorption of fluoride on UiO-66(Zr) and ZrUG3 were a surface absorption process, and electrostatic interactions between the two kinds of the adsorbent material and Fluoride ions was the main absorption mechanism.