Theoretical investigation of adsorptive nitrate ion removal by pure graphene, Mo-decorated graphene and reduced graphene oxide based adsorbents: a DFT study

Density functional theory (DFT) calculations were used to investigate the efficacy of pure graphene (G), Mo-decorated graphene and Mo-decorated reduced graphene oxide (rGO) in removing nitrate anion (NO₃⁻) pollutants. Initially, the adsorption mechanism was analyzed to identify the most probable position of nitrate adsorption through optimized geometries, adsorption energy, bond length and electronic structures. Subsequently, a comprehensive analysis was executed to examine the adsorption properties of the NO₃⁻ anion. Analyses of the adsorption energy, charge density difference and density of states indicated that defect sites, functional groups and Mo-atom decorations in graphene could significantly enhance the nitrate adsorption energy. The results indicated that the adsorption mechanisms of the NO₃⁻ anion on pure G, Mo-decorated G and Mo-decorated rGO were different. NO₃–Mo-decorated rGO demonstrated the highest adsorption energy. Conversely, NO₃–pure G exhibited the lowest adsorption energy, while the NO₃–Mo-decorated G showed the highest Fermi energy. Bader and projected density of states analyses suggest that the orbitals in the NO₃–Mo-decorated G structure occupy the largest share in the valence band compared with the NO₃–Mo-decorated rGO structure, which led to high electron accumulation. Consequently, the NO₃–Mo-decorated rGO structure allows the complete absorption of nitrate, resulting in the breaking of chemical bonds. These results indicate that the NO₃–Mo-decorated rGO structure has the highest nitrate absorption energy among the studied structures.