| Alumina is a kind of oxide with several polymorphisms,having common crystal forms of?-Al2O3 and?-Al2O3.?-Al2O3 is usually prepared by thermal decomposition of aluminum salts or by phase transformation of?-Al OOH at high temperature.However,the main disadvantage of this traditional method is that the preparation temperature is too high,which requires a high temperature calcination process above1200oC;and thus it is difficult to prepare small particles of nano?-Al2O3by the method.Therefore,the synthesis of small size?-Al2O3nanoparticles has become an important research topic in the field of nano alumina research.In this paper,?-Al2O3 nanoparticles with uniform distribution and small size(less than 20nm nanoparticles)were prepared by one-step thermal decomposition at 550oC?600oC using the self-made urea aluminum complex as a precursor.Furthermore,small size?-Al2O3nanoparticles(less than 10 nm nanoparticles),?/?-Al2O3 and high-temperature?-Al2O3 nanoparticles were also prepared,and their adsorption properties for Congo red and weak acid dark blue were investigated.The results are summarized as follows:1.Aluminum-urea complex was synthesized by a solid phase method using Al(NO3)3·9H2O and CON2H4 as reactants;and Al2O3 nanoparticles with different crystal forms were prepared by the calcination of aluminum-urea complex at different temperatures.The products were characterized by TG-DTA,FTIR,XRD,BET,SEM and TEM analyses.The products obtained at different temperature are as follows:(1)550oC?600oC:low-temperature type of?-Al2O3(LT?-Al2O3,LT=Low Temperature)with an average crystallite size of less than 20 nm nanoparticles;(2)at 700oC:?-Al2O3,7.6 nm;(3)750oC?850oC:?/?-Al2O3(mixed type),25 nm;(4)at 1000oC:high-temperature type F of?-Al2O3(HT?-Al2O3,HT=High Temperature),35.2 nm.The BET characterization results show that LT-?-Al2O3,?-Al2O3,?/?-Al2O3 and HT?-Al2O3 possess mesoporous structures with specific surface areas of3.7 m2/g,12.5 m2/g,29.8 m2/g and 18.4 m2/g,respectively.The SEM images indicated that all samples have sponge morphologies with irregular pore diameter.2.LT-?-Al2O3,?-Al2O3,?/?-Al2O3 and HT?-Al2O3 were used as adsorbents to study the adsorption performance for anionic dye of Congo red(CR),and the optimal adsorption conditions were investigated.The results show that the all adsorbents have the the best adsorption performances for CR under acidic condition.The adsorption of CR on LT?-Al2O3,?-Al2O3 and?/?-Al2O3 are in accordance with Freundlich isothermal adsorption model,indicating that multi-molecular layer adsorption is dominant.While the adsorption of CR on HT?-Al2O3 is consistent with the Langmuir and Freundlich isotherm models,which indicates that the adsorption process is a complex adsorption process of monolayer and multi-layer.The maximum adsorption capacities of LT?-Al2O3,?-Al2O3,?/?-Al2O3 and HT?-Al2O3 are 139.28 mg/g,118.91mg/g,140.25 mg/g and 131.41 mg/g,respectively.3.The above-mentioned four different adsorbents were used for adsorption of Weakly Acid Dark Blue(5R),and the optimal adsorption conditions were investigated.The results show that the adsorption of 5R is favorable at acidic conditions.The adsorption of 5R on LT?-Al2O3,?-Al2O3 and HT?-Al2O3 fit Langmuir adsorption isotherm model better,indicating that the adsorption is monolayer adsorption.The adsorption of5R on?/?-Al2O3,however,consistent with both Langmuir and Freundlich adsorption isotherm model,indicating that the adsorption process is complex monolayer and multi-layer adsorption.The maximum adsorption capacities of LT?-Al2O3,?-Al2O3,?/?-Al2O3 and HT?-Al2O3 for 5R are estimated to be 107.07 mg/g,269.54 mg/g,332.23 mg/g and 168.63 mg/g,respectively. |
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