| Three kinds of magnesium hexasilicate gels (with the same chemical formula ofMgO·6SiO2, wMgO<10%) were synthesized by hydrothermal method, using sodium metasilicate,magnesium chloride and sulfuric acid as raw materials. As raw materials were added in differentorder, products were recorded as H-SM-6, SM-H-6and MS-H-6respectively. Different factorsaffecting the preparation, i.e. pH, reaction temperature and reaction time were studied.Experiments found that synthetic time and synthetic temperature had little influence on thecontent of silicon and magnesium, pH was the decisive factor for Si/Mg mole ratios of themagnesium silicate gels.The resulting samples were characterized by BET method, Fourier transform infraredspectra (FT-IR), Scanning electron microscope (SEM) and potential titration analysis. Theresults showed that the sample particles were round or irregular in shape. Agglomerationhappened in different degree among particles. The surface area of H-SM-6, SM-H-6andMS-H-6were369m2·g-1、274m2·g-1and285m2·g-1, separately. Porous structures of the threemagnesium hexasilicate gels were mainly mesoporous and the pore size distribution was around2~5nm. The pHZPCof H-SM-6, SM-H-6and MS-H-6occurred around pH of8.3,8.0and8.4respectively.The water uptake behaviors of magnesium hexasilicate gels were evaluated under differentrelative humidity. The effect of roasting temperature on the adsorption performance was studied.Results showed that under low relative humidity (RH=40%), there was little difference amongthe water absorbencies of H-SM-6, SM-H-6and MS-H-6. The water vapor adsorptioncapacities of the three kinds of gels were about100mg·g-1, but the values decreased after thesamples were roasted. Under high relative humidity (RH=90%), calcination of the magnesiumsilicate gels at550～600℃facilitates their adsorption of water. H-SM-6roasted at550℃hadthe best adsorption performance and the adsorption capacity was421.6mg·g-1. The dye adsorption performance of the three magnesium hexasilicate gels was conductedin Methylene Blue. The effects of particle size and roasting temperature were studied. Resultsshowed that calcination did little to the adsorption of Methylene Blue. Compared with SM-H-6and MS-H-6, H-SM-6had the best adsorption capabilities and the best thermal stability. TakeH-SM-6as sample, the dye adsorption performance of magnesium hexasilicate gels wasevaluated. The experiments showed that H-SM-6had good adsorption performance for cationicdyes (Methylene Blue and Basic Brown) but poor adsorption capabilities for anionic dyes(Congo Red and Acid Fuchsin). Effects of the functions of adsorbent dosage, initialconcentration of solution, contact time and initial pH on the adsorption performance as well asthe adsorption isotherm and kinetics were studied. The results showed that at room temperature,the saturated adsorption capacities for Methylene Blue and Basic Brown on H-SM-6were182.8mg·g-1and77.1mg·g-1, respectively. The adsorption process of Methylene Blue was fitted toLangmuir isotherm and the adsorption process of Basic Brown was fitted to Freundlich isotherm.Their adsorption mechanisms could be better described by pseudo-second-order kinetic model.The adsorption performance of the three magnesium hexasilicate gels for Co2+wasevaluated and the effect of roasting temperature was studied. Results showed that calcinationhad obvious effect on the adsorption of Co2+. SM-H-6roasted under550℃had the bestadsorption performance for Co2+compared with H-SM-6and MS-H-6. Take SM-H-6as sample,the adsorption performance of magnesium hexasilicate gels for metal ions (Co2+、Ni2+、Fe2+) wasstudied. The experiments showed that SM-H-6had good adsorption capabilities for Co2+andNi2+but poor adsorption capabilities for Fe2+. At room temperature, the saturated adsorptioncapacities for Co2+and Ni2+on SM-H-6were46.0mg·g-1and18.1mg·g-1, respectively. Theadsorption process of Co2+and Ni2+on SM-H-6could be both described by Langmuir isothermand pseudo-second-order kinetic model. |
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