Preparation And Application Of Activated Carbon Based On Rice-Straw

Rice straw,a low-cost kind of agricultural residues,was selected as a precursor for the production of activated carbon(AC) in this research.It 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:4C+2(NH₄)₂HPO_→4CO₂↑+3H₂↑+4NH₃↑+2PWe selected three pretreatment methods(without pre-oxidation RN-2;air pre-oxidation before and after impregnation RN-1 RN-3,respectively) and different activation temperatures to produce activated carbon.The optimal preparation conditions were chosen to be activated in 700℃for an hour with air pre-oxidation after impregnation.The characteristics of the three samples RN-1,RN-2 and RN-3,which were activated at 700℃,were determined with N₂ adsorption- desorption isotherms, Scanning Electron Microscope(SEM),FTIR analysis,X-ray diffraction(XRD), Boehm titration and estimation of pH_pzc.It was found that pre-oxidation after impregnation RN-3 could greatly increase AC's BET surface area(1078 m²/g) compared to RN-2 without pre-oxidation(603.8 m²/g).And pre-oxidation before impregnation(RN-1) would obviously decrease BET surface area(358.5 m²/g).It is proved by FTIR analysis that there are some kinds of oxygen-containing groups in existence on the surface of AC.According to Boehm titrations,both two air pre-oxidation methods lead to higher concentrations of surface acidic groups.The three ACs were employed in wastewater treatment containing ammonia nitrogen and other two common pollutants,phenol and methylene blue(MB). Compared to phenol and MB,the adsorption capacity of ammonia nitrogen was very low.This suggests that ammonia nitrogen molecule be dissociated into ions in water,however,the adsorption of AC in liquid phase is physical process.The results showed that the data of adsorption kinetics fitted the pseudo second-order model better.And the data of phenol adsorption isotherms fitted the Langmuir model better. The data of MB adsorption isotherms fitted the Freundlich model better except RN-1. Specifically,RN-3 had the biggest adsorption capacity for phenol(up to 187 mg/g), and MB(up to 166 mg/g).Hence,the surface acidic groups were found to decrease the adsorption of MB in aqueous solutions.Moreover,from the estimation ofâ–³G, there maybe the exisistence chemical adsoprion in the adsorption of RN-3 for MB. However it proved to be a slight effect during adsorption according to estimation of E_a(surface activation energy).