Synthesis Of Mesoporous Oxides And Their Use In Efficient Removal Of Fluoride From Aqueous Solution

Today drinking water in more than20countries and regions worldwide has excessive fluoride concentration. The problem can easily cause fluoride toxicity in human, thus World Health Organization (WHO) sets the maximum concentration limit of fluoride ions in drinking water to1.0mg/l. In China, more than70million people are using drinking water with excessive fluoride concentration. Regional endemic fluoride toxicity has been reported in almost every province except Shanghai, Hainan and Taiwan. Therefore, removal of excessive fluoride ion in drinking water is critical to people's health. However, conventional water purification technology, that is,"Flocculation—Precipitation—Filter—Disinfection", does not effectively remove fluoride ion in drinking water. So there is immediate need to develop new fluoride removal technology that is highly efficient, low-cost and pollution-free.This paper is specifically designed to meet the demand of deep purification treatment on drinking water with excessive fluoride concentration. A few nano-sized adsorption materials were synthesized to specific structure and surface topography, including mesoporous silica, mesoporous alumina Lanthanum oxide mesoporous compound and Porous layered double hydroxides. Different approaches to regulate micro-pore structure of those materials, such as pore size, pore distribution and surface site density, were explored. Interaction between mesoporous material and fluoride in aqueous solution was studied. Adsorption thermodynamics and kinetics process were analyzed. The mechanism of high efficient fluoride adsorption was discussed, and the relationship between micro-structure and fluoride adsorption property was investigated. The main contents of this paper are as follows:1. Regular six-edge columnar and cylindrical La-silicon composite mesoporous materials were synthesized using MCM-41as hard template. Pore size of the La-silicon composite was4nm and surface area was800.40m2/g. With the increase of lanthanum amount in the material, though always kept below10%, ordered mesoporous structure gradually evolved into worm-like mesoporous structure. Thus it was possible to adjust Zeta potential on the surface of the material significantly, which then improved fluoride adsorption capability of the material. So this work resolved the issue that silica mesoporous materials could not be effectively applied in inorganic anions adsorption. Sorption experiments indicated that La-silicon composite mesoporous materials had excellent adsorption property at low fluoride concentration (≤10mg/l). When initial fluoride concentration was5.72mg/l, fluoride adsorption capacity went up to25.36mg/g within60minutes. Kinetic data were well described by pseudo second-order model.2. Regulated synthesis of mesoporous alumina:Highly ordered mesoporous alumina was prepared using Aluminium isopropoxide as precursor and P123as template agent. Its surface area, pore size, and pore volume were338m/g,9nm and0.88cm3/g respectively. Mesoporous wall was crystallized in high temperature. When the temperature was raised to850℃, highly ordered crystalline mesoporous alumina was synthesized successfully, its surface area and pore volume were224.4m2/g and0.664cm3/g respectively. Two kinds of worm-like spherical mesoporous alumina were prepared by sol-gel method, using aluminum nitrate and aluminum chloride as precursor separately. The resulting surface area was127.1m/g and85.68m/g, pore size was10nm and14nm, pore volume was0.33cm/g and0.35cm/g respectively. In addition, study was done regarding the impact of synthesis conditions, such as precursor and temperature, on the morphology, pore size, pore distribution, and pore structure of mesoporous alumina. 3. Adsorption properties of mesoporous alumina:The synthesized mesoporous alumina demonstrated excellent adsorption capacity. The adsorption capacity of highly-ordered mesoporous alumina and worm-like mesoporous alumina was115.4mg/g and96.18mg/g respectively, about tens of times of traditional γ-Alumina adsorbent under the same condition. In addition, both types of mesoporous alumina had high adsorption rate (10min to reach equilibrium). Apparently highly-ordered mesoporous alumina had better adsorption capacity and adsorption rate than worm-like mesoporous alumina. Crystalline mesoporous alumina showed better co-existing anions resistance and regeneration property because of the stability of its crystal structure. The experimental data in high fluoride concentration were better described by Freundlich model, but Langmuir model fit better in low fluoride concentration (≤10ppm)4. Nano-sized Mg/Al-C03hydrotalcite-like compound (LDHs) was synthesized by hydrothermal synthesis method. The sample had uniform and thin hexagonal platelets with a mean lateral size as large as200～300nm and a thickness of about20nm. The XRD analysis indicated that LDHs had the so-called "memory effect" The adsorption experiments were performed with low initial fluoride ion concentration (≤10mg/L), The effect of adsorption time, adsorbent amount, solution's pH value on adsorption capacity and fluoride ion removal rate were studied. Adsorption thermodynamics and kinetics property were also discussed. The regeneration results indicate that nano hydrotalcite-like compounds have good reusable nature.5. Analysis on fluoride adsorption mechanism of mesoporous material:After comprehensive analysis, three processes could used to describe the fluoride adsorption on nano materials:diffusion process, adsorption process, and ion exchange process. These processes can be explained by ion transport model, adsorption thermodynamic model and ion exchange mechanism respectively. According to the three models, the fluoride adsorption mechanism of La-silicon composite mesoporous material, mesoporous alumina, and Mg/Al hydrotalcite-like compound were discussed using material analysis method.