Microwave-assisted Modification Of Coal-based Activated Carbon And Studies On PAHs Adsorption Properties

The persistent organic pollutants, polycyclic aromatic hydrocarbons(PAHs) are widespread in the environment. Because of its persistent organic pollutants with high toxicity, hard degradation, and long distance migration characteristic had caused serious pollution to environment. It is very important siginificant for choosing appropriate approach and method for removal of PAHs solution. Coal-baed activated carbon is used as an absorbent, which can removal of PAHs solution with disadvantage of poor performance and poor selective adsorption. In order to improve its adsorption properties, choosing the rich coal in xinjiang as raw material, on the basis of the preparation of coal-based activated carbon, and then studying microwave-assisted modification of modified coal-based activated carbon, and thus regulating the surface structure and chemical properties of CAC, and adsorption of the typical PAHs(naphthalene,phenanthrene and pyrene), the results as following:Taking advantage of the microwave heating using microwave treatment modification, the effect of the different time of microwave and microwave power were study. The optimum modification technology conditions werw microwave time of 8 min and microwave power of 500 W. The characterization result in different microwave power as follows: the surface of pore increased of the modified M-CAC, the content of carbon is increased from 74.67 of CAC to 80.67 of M-CAC-500 W, the corresponding containing oxygen content is reduced from 23.04 to 23.04; and the corresponding specific surface area increased from 671.88m2/g to 1061.95 m2/g, total volume increased from 0.38 cm3/g to 0.61 cm3/g. Morever, the content of C=O enhanced, C-O, R-O*-C=O and O*=C-OH decreased; as well as the surface acid reduced and pHPZC values decreased alkaline increased. The M-CACs sample adsorb naphthalene, which is spontaneous and exothermic reaction, adsorption in 40 min to reach adsorption equilibrium; and adsorption capacity decreased with the increase of temperature, adsorption belongs to the multilayer adsorption; M-CAC-500 W sample has good reproducibility and stability.Through single factor experiment, before and after modified samples adsorption of iodine and methylene blue were determined. The optimum conditions were nitrate concentration of 20%, nitrate modification time of 16 h, microwave time of 8 min, microwave power of 500 W. The surface of the of MN-CAC-16 h distributed more pores, having high specific surface area, surface oxygen-containing functional groups from 1.19 mmol/g down to 0.087 mmol/g, but alkaline properties increased from 0.14mmol/g to 0.45 mmol/g, leading to pHPZC values increase from 7.98 to 8.20. The adsorption capacity increased with the increase of equilibrium concentration of naphthalene and phenanthrene, which belongs to multilayer adsorption and were fitted well by pseudo-second-order model.Microwave-assisted modification with different metal salt modifier, which obtained the best modifier for nitric acid iron, the best modification condition is nitric acid iron concentration of 0.05 M, microwave time of 5 min and microwave power of 300 W. The pores of the modified sample become more developed,because the iron has higher activity and loaded the surface of 0.05 Fe-MCAC, the pore is mainly mespore,the average pore diameter increased from CAC of 3.111 nm to 0.05Fe-MCAC of 3.816 nm; the Fe-Ostretching vibration were found by FTIR analysis, and the content of loaded iron is 0.135 mmol/g. The adsorption capacity for naphthalene, phenanthrene, pyrene were pyrene > phenanthrene > naphthalene and low temperature is beneficial to the adsorption PAHs, which belongs to the physical adsorption.The optimal modified coal-based activated carbon conditions were obtained as ammonia concentration of 10%, modification temperature of 35°C, treatment time 18 h, microwave time of 8 min, and microwave power of 500 W. The content of C increased significantly, the content of O increased, leading C/O increased, and the content of N increased, the oxygen-containing groups were removed, and the basicity enhanced. And the average pore diameter were enhanced from 3.111 nm to A-CAC of 3.825 nm, M-CAC of 3.830 nm, MA-CAC of 3.831 nm, the total volume enhaced by 5.70%, 45.0% and 102.04%, respectively.The adsorption capacity of pyrene enhanced significantly, especially the MA-CAC samples.The modified samples made pores structure become more develpoed, the BET surface area and average pore size increased. In addition, the surface acidity reduced and the surface basicity increased. The adsorption capacity of naphthalene, phenanthrene, and pyrene enhanced with ringe number increased. The Langmuir model can describe the phenomenon for adsorption naphthalene, phenanthrene, and pyrene using coal-based activated carbon, which belongs to monolayer adsorption. And the adsorption isotherm experimental data were best described using the Freundlich isotherm model for the modified samples,which is multilayer adsorption. The pseudo-second-order model could be fitted well for before and after modification coal-based activated carbon, which was caused by chemical adsorption. And PAHs adsorption was found to a spontaneous and exothermal adsorption process. The lower temperature is benefit for adsorption of PAHs solutions, and the process is physical adsorption. Microwave-assisted modified CAC is an effective modification technology, and improved its performance for PAHs adsorption. In addition, it has a broad market prospect and application value.