(623c) Adsorption of Ammonium From Water Solution Onto Carbon Nanotubes and Activated Carbon Fibers

The ammonium adsorption from water solution onto multi-walled carbon nanotubes (MWCNTs) and different types of activated carbon fibers (ACF) were investigated in this work. The MWCNTs and the ACFs were used as pristine carbon materials or oxidized with a nitric acid solution. ACFs were used in the forms of cloth and felt and the precursors of the ACFs were polyacrylonitrile and phenolic resin. The MWCNTs and ACFs were characterized by several techniques such as scanning electron microscopy, FTIR and N2 physisorption. Furthermore, the acid and basic sites and surface charge distribution of all the carbon materials were determined by acid-base titration. The adsorption equilibrium data were determined using a batch adsorber and the isotherms of Langmuir and Freundlich were fitted to the data. The Langmuir isotherm adjusted the experimental data better, presenting smaller average absolute percentage deviations than those of the Freundlich isotherm. The dependence of the adsorption capacity on the type and nature of the ACF, temperature and pH of the solution was studied, and it was found that the capacities of the MWCNTs and ACFs for adsorbing ammonium were dependent upon these factors. The adsorption capacity of both types of adsorbents is highly dependent on the nature of the adsorbent and the solution pH. The nature of the surface of both ACF and MWCNTs was modified by oxidation with a HNO3 solution. In the oxidation, the adsorption capacity of both types of adsorbents was enhanced due to that the oxygenated groups were introduced in the surface of the adsorbent. The capacity of the ACFs and MWCNTs for adsorbing ammonium was considerably increased by raising the pH from 3 to 7 and this behavior was attributed to the electrostatic interactions between the surface of the adsorbents and the ammonium cations present in the water solution.