Synthesis Of Nitrogen-containing Carbon Materials For Hexavalent Chromium Removal From Aqueous Solution

CrⅥ-containing wastewater is a major source of water pollution because of its great toxicity,strong acidic property and complex water quality.Many adsorbents(i.e.,magnesium-based,aluminum-based,and iron-based materials)failed in practical acidic CrⅥ-containing wastewater,especially in strong acidic solution.In recent years,nitrogen-containing carbon materials have been successfully applied in gas adsorption and water treatment due to their good chemical and acid and alkali resistance.It was reported that nitrogen functionality on the carbon surface not only improved adsorption ability as well as build a reductive environment for Cr(VI)detoxification.In addition,from the point of view of practical application,spherical adsorbent can improve fluid permeability and phase separation in the real fluidized bed.Thus,a new type of carbon spheres possessing a high nitrogen content should combine the two advantages described above and prove desirable for Cr(VI)removal.(1)Nitrogen-containing ordered mesoporous carbons(NOMC)were synthesized by the organic-organic self-assembly water solution method using m-aminophenol and hexamethylenetetramine as raw materials.The morphology and pore structure of NOMC were characterized by SEM,TEM,BET and element analysis.TEM and BET show that NOMC have highly ordered pore structure,and pore size was distributed in 4～5 nm.Elemental analysis indicates that the nitrogen content of NOMC is 4.25 wt%.NOMC have a larger Cr(VI)adsorption capacity with pH value 1-2,and adsorption capacity is as high as 76 mg/g.The pseudo-second-order and Freundlich model fitted the Cr(VI)adsorption behavior well.(2)Nitrogen-enriched carbon spheres(NECS)were obtained after ZnCl2 activation of high nitrogen content polymer spheres prepared by one-pot hydrothermal synthesis using m-phenylenediamine and hexamethylenetetramine as raw materials.The NECS were characterized by SEM,BET,FT-IR,and XPS.It was found that the NECS possessed homogeneously spherical morphology with diameters of about 0.6～1.2 μm and a high specific surface area(1237 m2/g).Elemental analysis indicated that the nitrogen content of NECS were up to 10.21 wt%.When the Cr(Ⅵ)in acidic solution was used to evaluate the adsorption performance,the maximum Cr(Ⅵ)removal capacity was as high as 279 mg/g at pH 2.0.The pseudo-second-order and Freundlich model fitted the Cr(Ⅵ)adsorption behavior well.Based on zeta potential and XPS analysis,the Cr(VI)removal mechanism was proposed to be an electrostatic adsorption of Cr(Ⅵ)anions to the NECS surface at low pH,followed by a reduction reaction of Cr(Ⅵ)to Cr(Ⅲ)via the nitrogen functionality of carbon spheres and possible coordination reaction of the resulting Cr(Ⅲ)with the lone pair electrons of nitrogen.The NECS adsorbent was shown to maintained over 90%Cr(Ⅵ)removal efficiency after six cycles.Finally,the Cr(Ⅵ)removal efficiency of NECS reached 99.9%with a dosage of 12 g/L for real acidic electroplating wastewater(the initial concentration 937 mg/g and pH 1.5),clearly showing the promises of this novel adsorbent for treating CrⅥ-containing wastewater.