Nitrogen-Enhanced Charge Transfer Efficacy on the Carbon Sheet: A Theoretical Insight Into the Adsorption of Anionic Dyes

Dye removal over the surface of activated carbon is a well-established approach for the treatment of wastewater effluents from pharmaceutical, food, and textile industries. Tuning the surface chemistry of the adsorbent plays a vital role in improving the dye adsorption performance. Herein, we report the mechanistic insights from first-principles calculations on the removal of common anionic dyes using nitrogen-enriched carbon sheets. The adsorption characteristics and chemical reactivity patterns of methyl blue, methyl orange, and methyl red were thoroughly accounted for based on their computed reactivity descriptors, adsorption energies, and structural and electronic properties. Nitrogen enrichment of the carbon sheet resulted in the significant decrease in the energy gap and global hardness values, suggesting an improvement in charge transfer efficacy and dye adsorption capacity. Among the three dyes investigated, methyl blue exhibited the lowest energy gap of 0.960 and 1.064 eV and the global hardness of 0.480 and 0.532 eV at the B3LYP/6-31G(d) and the B3LYP/6-311G(d,p) levels of theory, respectively, displaying a higher electrostatic attraction toward nitrogen-enriched carbon sheets in accordance with the hard-soft acid–base principle. Predicted adsorption energies of 28.69, 13.18, and 16.90 kcal/mol at 6-31G(d); and 28.91, 12.08, and 16.88 kcal/mol at 6-311G(d,p) basis sets were obtained for methyl blue, methyl orange, and methyl red, respectively, suggesting the selectivity of the nitrogen-enriched carbon sheet to the methyl blue dye. Further, the protonation of the nitrogen-enriched carbon sheets would tune the selectivity of the adsorbent resulting in a significant enhancement of the electrostatic attraction, in line with the experimental findings.