Preparation Of Expanded Graphite-based Carbon/Carbon Composites And Its Surface Modification

Owing to its toxicity and good solubility, phenol has already been listed as one of the top priority contaminants and the most important substructure of potentially carcinogenic pollutants discharged from fine chemical plants. In the past two decades, many methods were applied to deal with phenolic wastewater, and adsorption by activated carbon is shown to be effective due to its large adsorption capacity and high BET surface area. However, fragile mechanical performance and high wear rates of activated carbon as well as the mineral ashes have been the main hindrance in actual application.In previous studies, expanded graphite has been suggested to be a new carbonaceous absorbent material. Expanded graphite-based carbon/carbon composites (EGCs) were prepared by impregnation of expanded graphite (EG) in H₃PO₄/sucrose solution followed by carbonization, activation in this work. The textural, structural characteristics of the composites were investigated by scanning electron microscopy (SEM) and nitrogen adsorption method. The results indicated that the worm-like particles were partly covered by pyrolytic carbons while the large open spaces among entangled worm-like particles were scarcely any, most of the activated carbon (AC) was coated the surface of interior pores of EGCs in the form of thin carbon layer with the thickness of 100nm. The obtained EGCs had the highest surface area of 1948 m²/g with Xp=0.9, 350℃activation temperature and 120min activation time. And the phenol removal capability of EGCs obtained was 173.1mg/g.The results showed that the thickness of carbon layer could be controlled by changing the concentration of sucrose solution. The dynamic adsorption properties of phenol were studied. The results indicated that a pseudo-second-order equation provided the best correlation with the adsorption data. The initial adsorption rates derived from the model increased with the rising of phenol concentration and decreasing of the thickness of the activated carbon layer.The results of Boehm titration and phenol static adsorption data showed that the EGCs, which were activated by H₃PO₄) had lots of acid functional groups on the surface. And the acid functional groups were harmful for phenol adsorption. So its phenol removal capacity was only 173.1mg/g. After the material was modified by urea, the acid functional groups decomposed and the nitrogen-containing functional groups appeared. Moreover in the meantime the adsorption capacity reached up to 212.1mg/g.The XPS studies indicated that there were pyridinic and pyrrolic structures on the surface of EGCs after modified by urea. They were slightly alkaline and good for phenol removing. The research showed that nitric acid modification could bring more oxygen-containing functional groups and the nitrogen-containing functional groups can be more easily grafted. So EGCs which was modified by urea after nitric acid modification, acquired a higher phenol adsorption capacity of 242.1mg/g, 37% higher than the unmodified ones.