| Hexavalent chromium(Cr(Ⅵ))was listed as the priority pollutants in the aquatic en-vironment by U.S.EPA and the high risk heavy metal pollutant in the Chinese "125"programming for its seriously toxicity including teratogenicity,carcinogenicity,and mutagenecity.China is the biggest nonferrous metals producer in the world,and discharge a great deal of chromium-containing water and residue during the industry production,seriously affecting the drinking water and industrial water.With the serious lack of the water and heavy metal pollution crisis,the governance of Cr(Ⅵ)in water is urgently needed.Adsorption method is efficient,simple,well selectivity and recyclable for Cr(Ⅵ)removal in aqueous media,whose key lies in the selection of the adsorbent.Mesoporous silica materials SBA-15 and MCM-41 had synthesized with different silicon source,different template,different aging and modified method.These mesoporous silica materials were used as adsorbents for Cr(Ⅵ)removal in aqueous solution by static experiment,and the optimal adsorbent were selected from these adsorbents by comparing their adsorption capacity.The result is that the MCM-41 synthesized by alkali-melting process-microwave-assisted and fly ash as silicon source while CTAB as template,which maned FMC has the best adsorption capacity.The mesoporous silica materials synthesized by fly ash were characterized by XRD,and N2 adsorption-desorption.The characterization show that the materials is mesoporous silica but without well order degree and high specific surface area,for the impurity of fly ash going against the formation and distribution of pore in the mesoporous silica materials.The effect factors including initial pH values,adsorption time,initial Cr(Ⅵ)concentration,coexisting ion,light condition and adsorption temperature on the adsorption properties of Cr(Ⅵ)onto FMC were investigated.The adsorption kinetics,thermodynamics and adsorption-desorption regeneration were also studied.FMC were characterized by XRD,N2 adsorption-desorption,and FT-IR.The experiment result show that the FMC removal effectively and high selectively for Cr(Ⅵ),which the coexisting ions have no effected on the adsorption.The acidic conditions is beneficial to the adsorption of Cr(VI),and FMC has the largest capacity for Cr(VI)when pH 1.5～2.5.The light condition is significantly beneficial to the removing of Cr(VI)in higher concentration.The high adsorption temperature is beneficial to adsorption but not obviously.The adsorption fits the Langmuir isotherm model,the adsorption capacity inferred from the Langmuir model is 256.41 mg/g under light condition,while 153.85 mg/g under no light condition.The adsorption follows pseudo-second order kinetic model,and it is an exothermic process.We infer the adsorption mechanism according to the adsorption performance of adsorption and the analysis of XRD,N2 adsorption-desorption and FT-IR.The adsorption happens in acid solution mainly,and the Cr(VI)anion is the adsorption object while the positively charged silicon species on the surface and the ammonium cation in the inner surface of FMC are the adsorption sites.The two adsorption sites combine with Cr(VI)anion by electrostatic adsorption in acid solution,bonding by N-H.The electrostatic adsorption between silicon surface and Cr(VI)anion becomes weaken or disappears for the silicon surface becoming negative charge in alkaline solution,and the ammonium cation in the inner surface of FMC combines with the OH-by electrostatic adsorption,which competing with Cr(VI)anion,resulting the removal of Cr(VI)decreasing as the pH increasing.The desorption is the inverse process of adsorption and the FMC can be regenerated by alkaline solution.The FMC show negative charge in alkaline solution and is competitive with the Cr(VI)anion,which is already adsorbed in the FMC,resulting Cr(VI)anion releasing into the solution.These study suggest that mesoporous silica materials FMC has great potential applications for Cr(VI)removal in aqueous solution. |
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