| Fluoride is one of the essential elements to maintain the normal physiological function of human health. The proper level of fluoride is beneficial for the formation of healthy bones and teeth, but excessive intake of fluoride causes fluorosis. Fluoride mainly in human body form drinking water. Drinking water of high fluoride concentration for a long time may result in dental fluorosis. The serious consequence may be distortion of skeleton, paralysis or even death. Thus, the economical and effective adsorption method is widely appreciated enough. Adsorption method by activated alumina (AA)is an cost-effective and mature method for removing fluoride, but the adsorption capacity and service life of AA is not enough, it is limited development. So this paper studied modified activated alumina (MAA) preparation, static and dynamic adsorption performance,regeneration method and defluorination mechanism.This paper used activated carbon, zeolite and activated alumina as materials. Three kinds of materials were prepared by impregnation-calcination method, loading lanthanum, magnesium, and lanthanum-magnesium respectively. The results show that La-Mg-loaded AA has the best adsorption performance in the evaluation. On this condition, the static and dynamic adsorption experiments were studied in order to research the fluoride removal performance of MAA. The different influence factors of static experiment such as react time,dosage, initial pH, oscillation frequency and coexisted negative ions on defluoridation in groundwater have been studied. The different influence factors of dynamic experiment such as initial concentration, flow rate and the bed length on defluoridation in groundwater have been studied.The results of static adsorption show that The optimal adsorption conditions is contact time120min,dosage2.0g/L, initial pH7.0±0.2, oscillation frequency180r/min, the removal rate of10mg/L fluoride solution is97.7%. The max adsorption capacity of MAA is8.56mg/g. Static adsorption capacity of MAA is2.0times of that of AA. The influence of different anions on the adsorption is CO32-> SO42-> Cl-> NO3-.The adsorption follows pseudo-second order kinetics. The adsorption isotherm could be described by the Langmuir equation. The adsorption is a spontaneous process, the higher the temperature is, the more beneficial to adsorption is. Adsorption rate of F-on MAA is mainly controled by liquid film diffusion and intra-particle diffusion, liquid film diffusion holds leading position. The results of dynamic adsorption by MAA show that column kinetics fit well with the Thomas model and the dynamic adsorption capacity is2.99mg/g. Dynamic adsorption capacity of MAA is1.24times of that of AA. The results of dynamic adsorption experiment show that breakthrough and adsorption saturation time are influenced by influent concentration, flow rate, and filter material height. The lower influent concentration, flow rate, bed length, the shorter breakthrough and adsorption saturation time is.This paper researched the regeneration conditions by using NaOH solution and H2SO4solution for desorption-activated regeneration of the saturated MAA.And the influence conditions include desorption solution concentration,desorption time and recycle numbers. The results show that0.25mol/LNaOH solution desorption for6.0h and1%H2SO4solution activation for1.0h.The adsorption performance has no decline after continuous running four static adsorption cycles or three dynamic adsorption cycles respectively.The properties of AA and MAA were characterized by Scanning Electron Microscopy (SEM),X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FT-IR) et al,in order to reveal adsorption mechanism. The results show that defluoridation performance of MAA is increased. It is concluded that lanthanum and magnesium play an important role in the process of defluoridation. Adsorption mechanism of MAA is electrostatic adsorption and ion exchangeLa-Mg-loaded AA has excellent adsorption performance for large capacity absorption, long service life and better regenerating effect. It has a good application prospect in defluoridation in groundwater. |
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