Adsorption of pollutant cations from their aqueous solutions on graphitic carbon nitride explored by density functional theory

In this study, adsorption of important pollutant cations on the surface of graphitic carbon nitride (g-C3N4) was investigated by density functional theory. The calculations indicated that N-6 cavity surrounded by triazine units is the most probable adsorption site on this surface. The structural optimizations also predicted a planar surface for Cr3+, and Ni2+/g-C3N4 systems while the structure of the surface for other systems indicated a considerable distortion with strong dependency on the cation size. Also, g-C3N4 surface exhibited the high adsorption energies for Cr3+, As3+, and Sb3+ ions in the gas phase. However, formation energies of the metal-aquo complexes of these cations indicated that only adsorption of Sb3+, As3+, Pb2+, Hg2+ and Cd2+ cations from the aqueous solution is favorable in a thermodynamic point of view, in such a way that efficiency of adsorption obeys a Sb3+ > As3+ > Pb2+ > He2+ > Cd2+ trend. Moreover, time-dependent density functional calculations indicated metal to ligand charge transfer vertical excitations for Cr3+/g-C3N4 structure, and ligand to metal charge transfer excitations for Hg2+/g-C3N4, Cd2+/g-C3N4, As3+/g-C3N4, and Sb3+/g-C3N4 systems, which indicates the potential of these systems for future use in variety fields of nanotechnology and catalysis. (C)2018 Elsevier B.V. All rights reserved.