The Preparation Of Magnesium-based Composite Materials And Its Adsorption Performance For Fluoride

Fluoride is one of the essential elements for human health,with symbol F and atomic number 9.Human beings mainly ingest fluoride from drinking water,the intake of appropriate fluoride is beneficial for the dental and skeletal growth.But the ingestion of excessive fluoride will cause diseases of multiple organs and tissues,osteoporosis,fluorosis,thyroid disorder and other health problems.In China,high-fluorine water is widely distributed in 27 provinces,municipalities and autonomous regions,especially in Northwest and Inner Mongolia.Qinghai salt lakes are rich in magnesium resources,but the explorations of magnesium resources in salt lakes delay.Therefore,the full utilization of magnesium resources in salt lakes and the effect elimination of fluoride from contaminated water are of great significance for protecting the environment and human health.In this paper,layered Mg-Fe-Ce composite oxides?Mg-Fe-Ce-O?,magnetic Mg-Al composite oxides?Fe₃O₄@Mg-Al-O?and flowerlike Mg-Al-CO₃-LDH@MIL-88A were successfully prepared.The materials were optimized and characterized by powder X-ray diffraction?XRD?,scanning electron microscopy?SEM?,Fourier transform infrared spectroscopy?FTIR?,X-ray photoelectron spectroscopy?XPS?and so on.The adsorption performances of materials for fluoride in aqueous solution were studied by various parameters,such as contact time,temperature,initial solution pH,and coexisting anions.The adsorption kinetics,adsorption isotherms and removal mechanisms were also discussed.1.Mg-Fe-Ce composite oxides?Mg-Fe-Ce-O?were obtained by calcining CeO₂/Mg-Fe-Ce-CO₃-HTlcs precursor at 400^oC,which was successfully synthesized by a co-precipitation method.The method is simple.Adsorption properties and mechanisms of fluoride by Mg-Fe-Ce-O were investigated.The results showed that the precursor had better crystal structure of hydrotalcite and the calcined product possessed the best adsorption effect with the Fe/Ce molar ratio of 1:3.There was no obvious influence of pH on fluoride adsorption and the optimal pH was 4.0-9.0.The fluoride removal was best fitted with pseudo-second-order kinetic and Freundlich isotherm,revealing the adsorption process was multilayer adsorption and chemisorption was the rate controlling step.Based on the results of XRD,FTIR and XPS,the removal mechanism was mainly ligand exchange.2.The magnetic Fe₃O₄ microspheres with uniform particle size were prepared via a solvothermal method,which were loaded onto the surface of Mg-Al-LDH and then calcined at 400^oC to get magnetic Mg-Al composite oxides?Fe₃O₄@Mg-Al-O?.The as-prepared adsorbent was characterized by XRD,FTIR,SEM and BET.The results indicated that the spherical Fe₃O₄ nanoparticles were adhered to the surface of lamellar Mg-Al-O and the surface of adsorbent was rough.Fe₃O₄@Mg-Al-O displayed a larger BET surface area(S_BET=196.238 m²·g^-1)compared with the pure Mg-Al-O(S_BET=129.439 m²·g^-1).Single factor experiments demonstrated that as the Fe₃O₄/Mg?NO₃?₂·6H₂O mass ratio was 0.02:1,the adsorption capacity of fluoride on Fe₃O₄@Mg-Al-O was the largest.The optimal pH was 4.0-9.0 and the material had high selectivity for fluoride.The adsorption process was spontaneous and exothermic,and conformed pseudo-second-order kinetic and Freundlich isotherm.The theoretical maximum adsorption capacity reached 170.02 mg·g^-1,and the removal process was mainly carried out by the exchange of surface hydroxyl groups?M-OH?with the fluoride.3.Mg-Al-CO₃-LDH@MIL-88A was prepared by water bath stirring with the supporting material of flowerlike Mg-Al-CO₃-LDH,which was synthesized via solvothermal method.The characterization methods such as XRD,SEM and FTIR confirmed that MIL-88A was successfully immobilized on flowerlike Mg-Al-CO₃-LDH,the original flowerlike morphology of Mg-Al-CO₃-LDH was well-maintained.The adsorption activities were evaluated by single factors,adsorption kinetics and isotherms experiments.The experimental results revealed the adsorption reached equilibrium in approximately 420 min and the equilibrium adsorption capacity was larger than Mg-Al-CO₃-LDH.The adsorption process was multimolecular layer adsorption and chemical adsorption was the rate controlling step.Thermodynamic study suggested that the fluoride adsorption was a spontaneous endothermic process,and accompanied with physisorption on the basis of the low value of enthalpy change.