| Fluorine is a kind of essential trace elements, which can maintain the normal physiological function of human body. A small amount of intake can prevent dental caries, however, excessive intake will lead to fluorosis, such as dental fluorosis, periosteum hyperplasia, bone sclerosis and cancer. The main way of intake of fluorine is drinking water containing excessive fluoride. In China, the groundwater containing excessive fluoride is widely distributed, and industrial production process contains fluorine pollution. So poisoning incidents caused by fluorine appear easily. Therefore, it is an urgent task to ensure the safety of drinking water.At present, the commonly used methods of defluorination are adsorption, coagulating sedimentation, electrocoagulation, electrodialysis, reverse osmosis, etc. But all of these methods have defects, and are not suitable for practical application. It is very important to search for a material and process of defluorination, which is easily operated, inexpensive and efficient.In this study, based on the skeleton material, polyacrylamide gel, through immobilizing chelation groups(diethylenetriamine, that is DETA; triethylenetetramine, that is TETA) by grafting, and then chelating zirconium to form a coordination center, the defluorinating agent was gained. According to the fact that zirconium and fluorine could form a complex, ligand exchange occured between the zirconium of chelation-loading-zirconium gel and the fluorine in water, so that fluorine ions could be fixed. They could be removed through solid-liquid separation. So the deep defluorination was realized. Fluorine ions of the chelation-loading-zirconium gel after removing fluoride, could be eluted. Then it was regenerated and reused, after simple treatment.Main contents and conclusions of this study were as follows:1.The preparation of defluorinating agent: By means of Mannich reaction, based on the skeleton material, the multihole polyacrylamide gel which was homemade, through the connection of methanol, and grafting DETA and TETA onto polyacrylamide gel respectively, DETA grafted gel and TETA grafted gel were gained. The effects of material ratial, p H, temperature and time were explored. The optimum grafting conditions of preparation of DETA grafted gel were as follows: Material ratial was 1:3:4, p H was 9, temperature was 50℃, and two periods of time were both 2 h. For TETA grafted gel, material ratial was 1:2.1:2.3, p H was 11, temperature was 50℃, and two periods of time were both 2 h. Under the optimal grafting conditions after enlargement, the amination degree of DETA grafted gel was 84.79%, and that of TETA grafted gel was 90.86%. Putting the two kinds of chelation group grafted gels in the solution of zirconium to load zirconium by chelation, DETA-loading-zirconium gel and TETA-loading-zirconium gel were obtained. The effects of p H, time, temperature, concentration of zirconium and solid-liquid ratio were investigated. The optimum loading-zirconium conditions of preparation of DETA-loading-zirconium gel were as follows: p H was 5, time was 3 h, temperature was 45℃, concentration of zirconium was 4 g/L, solid-liquid ratio was 0.02 g/m L. For TETA-loading-zirconium gel, p H was 5, time was 3 h, temperature was 45℃, concentration of zirconium was 8 g/L, solid-liquid ratio was 0.02 g/m L. Under the optimal loading-zirconium condition after enlargement, the zirconium content of DETA-loading-zirconium gel was 57.41 mg/g, and that of TETA-loading-zirconium gel was 80.62 mg/g.2. Experiments and conclusions of removing fluoride by ligand exchange adsorption: The two kinds of loading-zirconium gels were used to remove fluoride, testing their properties of defluorination. At room temperature, p H 6, shaking rate of 120 times/min, adsorption time for 180 min, solid-liquid ratio of 0.003 g/m L, using DETA-loading-zirconium gel, fluorine ions under the concentration of 10 mg/L were removed to 90.48%. And using TETA-loading-zirconium gel, at room temperature, p H 6, shaking rate of 120 times/min, adsorption time for 120 min, solid-liquid ratio of 0.006 g/m L, they were removed to 98.30%. The two kinds of loading-zirconium gels were suitable for dealing with weakly acidic water containing fluoride. SO42-, H2PO4-, and CO32- had influences on defluorination of DETA-loading-zirconium gel; for TETA-loading-zirconium gel, CO32- had influences. The chelation-loading-zirconium gels after removing fluoride, could be regenerated and reused.3. Conclusions of mechanism research: The adsorption for fluoride by chelation-loading-zirconium gels could be described by the pseudo-second-order kinetic model and Langmuir isotherm model. The adsorption for fluoride by DETA-loading-zirconium gel released heat, however, for TETA-loading-zirconium gel, it absorbed heat. The values of maximum adsorption capacity of DETA-loading-zirconium gel and TETA-loading-zirconium gel were 10.31 and 9.62 mg/g, respectively. The values of apparent activation energy were individually 55.84(DETA-loading-zirconium gel) and 42.98(TETA-loading-zirconium gel) k J/mol. And the enthalpy change were-10.63(DETA-loading-zirconium gel) and 20.24(TETA-loading-zirconium gel) k J/mol, respectively.The material adopts adsorption to remove fluoride. It is easily operated, inexpensive and efficient, having great prospect in practical defluorination. |
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