Phosphonic acid-functionalized g-C₃N₄ nanosheets for chromium (Cr⁶⁺) adsorption from aqueous solutions: Experimental insights and density functional theory calculations

The surface of graphitic carbon nitride (g-C₃N₄) was functionalized with phosphonic acid (PSA) to improve its capability to adsorb Cr⁶⁺, as investigated using experimental and density functional theory (DFT) calculations. The optimal initial concentration of Cr⁶⁺ (200 mg/L), pH (6), and adsorbent mass (0.04 g) were determined using the central composite design methodology. The experimental data followed the second-order kinetic equation and Freundlich isotherm for Cr⁶⁺ adsorption on g-C₃N₄ and phosphonic acid-functionalized g-C₃N₄ (PSA/g-C₃N₄) nanosheets. The maximum adsorption capacities for g-C₃N₄ and PSA/g-C₃N₄ were 98.22 and 319.21 mg/g, respectively. According to the DFT calculations, the adsorption energies for Cr⁶⁺/g-C₃N₄ and Cr⁶⁺/PSA/g-C₃N₄ complexes were −39.63 and −48.83 kcal/mol, respectively. The band gap of g-C₃N₄ decreased from 2.51 eV to 0.39 eV when PSA was added through surface functionalization. The experimental and theoretical results underscore the remarkable adsorption capability of the PSA/g-C₃N₄ nanosheet for water treatment.