Adsorption of Organic Compounds onto Multiwall and Nitrogen-Doped Carbon Nanotubes: Insights into the Adsorption Mechanisms

Abstract

In this work, the single species and competitive (multispecies) adsorption of pyridine, phenol, and p-nitrophenol present in aqueous solution on multiwalled carbon nanotubes (MWCNTs), and nitrogen-doped carbon nanotubes (CNx) were studied. The physicochemical properties of MWCNTs and CNx were related to their capability for the adsorption of the organic molecules. Adsorption isotherms were developed at 25 °C, at pH 7 and 10. All compounds were favorably adsorbed on both materials, with differences in the adsorption capacities. The CNx phenol adsorption capacity outperformed that of MWCNTs; however, CNx demonstrated a lower adsorption capacity for pyridine and p-nitrophenol than MWCNTs. The adsorption capacities for each material could be associated with the particular adsorption mechanisms that control the adsorption of the organic molecules. Based on the results, it is proposed that three mechanisms might be responsible for the adsorption of the organic molecules: hydrogen bonding, π-π interactions, and electron-donor reactions. The prevalence of any of the specific mechanism depends on the geometry of the carbon nanotubes, the size and shape of the organic target molecules to adsorb, and the presence of other organic molecules in solution. The aqueous chemistry of the adsorbates at the solution pH during adsorption, played a relevant role during adsorption as well. The differences in nanotube selectivity were attributed to the presence of oxygen and nitrogen in the adsorbent structure.