| Due to its excellent performance,alumina powder is widely used in the fields of catalysts,adsorbents,ceramics and abrasives.In various crystalline alumina,α-Al2O3 andγ-Al2O3 are the two most widely used oxides.Among them,γ-Al2O3 has a high specific surface area and is widely used in the adsorption and separation of macromolecular dyes and ions.γ-Al2O3 prepared by traditional methods has relatively low specific surface area(<250 m2?g-1),wide pore size distribution and is easy to change intoα-Al2O3 stable phase,which limits its catalytic and adsorption applications.Compared withγ-Al2O3,the traditional synthesis method ofα-Al2O3 required higher temperature（>1200℃）.However,high temperature could easily lead to agglomeration ofα-Al2O3particles.The reduction of the crystal transformation temperature ofα-Al2O3 and particle contact opportunities are useful ways to overcome particle agglomeration.Therefore,low-temperature synthesis is one of the effective means to solve above problems.γ-Al O（OH）,as the precursor ofγ-Al2O3,usually has a high specific surface area,and thus it could also be used in the adsorption field.In this paper,α-Al2O3 powder,high specific surface areaγ-Al2O3nanoparticles andγ-Al O（OH）powder were successfully prepard by one-step thermal decomposition method,solvothermal method and hydrothermal method,respectively,using aluminum-urea as the precursor.The obtained samples were characterized by X-ray diffraction analysis（XRD）,differential thermal analysis and gas chromatography mass spectrometry（TG-GCMS）,N2 adsorption-desorption measurements（BET,BJH）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,and isoelectric point measurements.The results showed that,the hexagonal single crystal ofα-Al2O3 with grain size of 47.3 nm was prepared at 550℃.The specific surface area,pore volume and most probable pore size of theα-Al2O3were 0.91 m2?g-1,0.023 cm3?g-1 and 3.822 nm,respectively.Theγ-Al2O3 with grain size of 2.6 nm was prepared at 300℃,its specific surface area,pore volume and most probable pore size were 601.0 m2?g-1,0.936 cm3?g-1and 3.4nm,respectively.Fibrousγ-Al O（OH）with grain size of 9.80 nm were prepared at 150℃,its specific surface area,pore volume and most probable pore size were 83.0 m2?g-1,1.193 cm3?g-1and 3.8 nm,respectively.The three samples were used to adsorb the simulated fluorine-containing groundwater solution in the laboratory.The effects of time,amount of adsorbent,p H of solution,initial concentration of solution,temperature and coexisting ions on the adsorption of fluoride were investigated to obtain the best adsorption conditions.At the same time,the adsorption kinetics and adsorption isotherm model for fluoride adsorption were also investigated.The results,are summarized as follows:（1）α-Al2O3 showed a low adsorption rate for fluoride.After 60 min of adsorption with 0.2 g of adsorbent,the adsorption rate of fluoride solution with concentration of 10 mg·L-1 had not reached 20%.The p H of solution influenced adsorption of fluoride onα-Al2O3 greatly,with the highest adsorption rate under acidic condition.The adsorption of fluoride onα-Al2O3 conformed to the pseudo-first-order kinetic model and Langmuir isotherm model indicating monolayer adsorption.The maximum adsorption capacity for fluoride was estimated to be 1.9 mg·g-1.（2）The optimal condition for adsorption of fluoride onγ-Al2O3 was as follows:adsorbent dosage=0.05 g,contact time=30 min,solution p H?7.Under the optimum condition,the adsorption rate reached 93.6%,and the concentration of the simulated fluoride wastewater with the concentration of 10mg·L-1 was reduced to the allowable value of the national standard(<1.0 mg·L-1).The adsorption of fluoride ions byγ-Al2O3 was an endothermic reaction.The adsorption of fluoride onγ-Al2O3 conformed to the pseudo-second-order kinetic model,indicating that the fluoride adsorption onγ-Al2O3 belongs to chemical adsorption.The adsorption isotherm conformed to the Freundlich isothermal adsorption model,implying that adsorbed layer is multi-molecular layer coverage.The maximum adsorption capacity was 10.9 mg·g-1.（3）The optimal adsorption condition ofγ-Al O（OH）for fluoride was as follows:adsorbent dosage=0.1 g,contact time=30 min,solution p H?7.At this condition,the adsorption rate of fluoride solution with concentration of 10mg·L-1 reached 94.2%.The adsorption of fluoride onγ-Al O（OH）conformed to the pseudo-second-order kinetic model and Langmuir isothermal adsorption model,indicating that the fluoride adsorption onγ-Al O（OH）is monolayer adsorption.The maximum adsorption capacity was 5.2 mg·g-1.（4）The adsorption performance of three adsorbents for fluoride were compared,and the conclusion was drawn:The order of adsorption amount of fluoride on three adsorbents was as follows:α-Al2O3<γ-Al O（OH）<γ-Al2O3,which indicates that the adsorption performance was determined by specific surface area.The larger the specific surface area,the better the adsorption performance. |
