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 N6 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+>Hg2+>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.