Study On Activated Carbon Fiber Based On Kapok

A new kind of activated carbon fiber(ACF)based on kapok,an abundant and inexpensive natural resource in South China and Southeast Asia,was prepared by activation with diammonium hydrogen phosphate [(NH₄)₂HPO₄].Based on thermogravimetric analysis(TG),the mechanism of activation of[(NH₄)₂HPO₄]maybe the following reaction;6C+2(NH₄)₂HPO₄→4CO+2CO₂+3H₂+4NH₃+2PThree pretreatment methods(without pre-oxidation,air pre-oxidation after and before impregnation)and different activation temperatures were adopted to produce activated carbon fibers.The optium preparation conditions for kapok-based ACF were chosen to be activated temperature at 650℃with air pre-oxidation after impregnation.The characteristics of the three samples from different pretreatment methods were determined with N₂ adsorption-desorption isotherms, scanning electron microscope(SEM),FTIR analysis,Boehm titration and estimation of pH_pzc.It was found that pre-oxidation after impregnation (AK-2)could greatly increase ACF's BET surface area(1518m²/g) compared to AK-1 without pre-oxidation(825.1m²/g).And pre-oxidation before impregnation(AK-3)would slightly decrease BET surface area (799.2m²/g).FTIR analysis proved the exisistence of different oxygen-containing groups.According to Boehm titrations,both two air pre-oxidation methods led to higher concentrations of surface acidic groups.However,whether activated or not,pre-oxidation or not,all the carbon fiber from kapok have almost the same average size of cavities, about 2nm.The three ACFs(AK-1,AK-2,and AK-3)were employed in wastewater treatment containing two common pollutants,phenol and methylene blue(MB).The results showed that the data of adsorption kinetics fitted the pseudo second-order model better and the data of adsorption isotherms fitted the Freundlich model better except AK-1 in adsorption of MB.Specifically,AK-1 had the biggest adsorption capacity for phenol(up to 65.8mg/g),and AK-3 for MB(up to 156.7mg/g). Although AK-2 had the largest surface aeras,its adsorption capacity for phenol or MB was not high.This phenomenum could be explained that AK-2 owned the highest concentrations of surface acidic groups and the surface acidic groups on the ACF were not favourable to the adsorption of both phenol and MB in aqueous solutions.The experiments displayed that the adsorption behaviors of the three samples in phenol and methylene blue exhibited physical adsorption and spontaneous process. Moreover,the rate of adsorption was controlled by liquid film diffusion in initial adsorption stages and internal diffusion of particle subsequently. On the other hand,from the fitting of Elovich equation,there maybe some chemical adsoprion in the adsorption process of phenol,the reason was conferred that the exisistence of ester bonds were generated between -OH of phenol and-COOH of carbon fiber surface.However,it proved to be a slight effect during adsorption according to estimation of AG.