Evaluating the Ion Adsorption Properties of CB3 and CN3 Nanoflakes for Nitrate and Nitrite Removal in Water Purification: A DFT Approach

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Abstract

In this study, the adsorption and sensing performance of nitrate (NO₃⁻) and nitrite (NO₂⁻) ions on CB₃ and CN₃ nanoflakes were systematically investigated in both gas and aqueous environments. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed at the ωB97XD/6-31+G (d, p) level to optimize the geometries and evaluate the electronic, thermodynamic, and sensing-related properties of the NO₃⁻/NO₂⁻@CB₃ and NO₃⁻/NO₂⁻@CN₃ complexes. The calculated adsorption energies reveal that the CB₃–NO₂⁻ (model 2) complex exhibits the strongest interaction among all studied systems, indicating that CB₃ nanoflakes are highly effective adsorbents for nitrate and nitrite ions. The dipole moments and polarizabilities of the NO₃⁻@CB₃ and NO₃⁻@CN₃ complexes are significantly higher than those of their NO₂⁻ counterparts, suggesting greater sensitivity toward nitrate ions. Thermodynamic analyses confirm that the adsorption processes are exothermic and thermodynamically spontaneous, with enthalpy changes (ΔH) showing a slight decrease as the temperature increases from 298.15 to 328.15 K. The reduced energy gap (E_gap) and chemical hardness (η) values for the adsorbed complexes indicate enhanced electrical conductivity and reactivity, supporting their suitability as sensitive sensors and efficient adsorbents. Further insights from atoms-in-molecules (AIM), electron localization function (ELF), and reduced density gradient (RDG) analyses demonstrate that the dominant interactions between NO₃⁻/NO₂⁻ ions and CB₃/CN₃ nanoflakes are governed by electrostatic forces and van der Waals interactions, with the NO₃⁻@CN₃ complex exhibiting the strongest interaction characteristics. Overall, the findings highlight CB₃ and CN₃ nanoflakes as promising candidates for the development of high-performance sensors and adsorbents for nitrate and nitrite ion removal from environmental systems.