Nitrate Ion Adsorption Characteristics of Activated Carbon Fibers with and without Quaternary Nitrogen Effective for Anion Adsorption.

By comparing the two types of activated carbon fibers (ACFs), characteristics of adsorption sites for nitrate ion other than quaternary nitrogen (N-Q) were investigated. Using phenol resin as precursors, activation with ZnCl2 was performed, and then heat treatment at 950 °C was carried out to prepare ACFs without N-Q, while ACFs with N-Q was prepared in the same method using polyacrylonitrile-based carbon fiber as precursors. We assessed the amount of functional groups, elemental composition, porous properties, and model of unit crystal size of graphene. For both ACFs with N-Q and without N-Q, equilibrium adsorption amount was not always simply proportional to surface area, but to the average number of benzene rings (Bz-rings) of graphene universally. PhR-5.0Z4 had only 20 benzene rings per graphene unit, but after heat treatment at 950 °C, the number drastically increased to 1088 (PhR-5.0Z4-9.5HT30). However, when the ACFs contained a large amount of oxygen, the number of Bz-rings was limited to 792 (PhR-5.0Z4-Ox-9.5HT30) even after heat treatment at 950 °C, and did not increase sufficiently. Cπ sites are more susceptible to oxygen inhibition than N-Q in adsorbing nitrate ions. For ACFs having Cπ sites as main adsorption sites, the heat treatment at 950 °C without oxidation can enhance the nitrate ion adsorption capacity.