From Electronic Structure to Environmental Remediation: Adsorption of Ionized Glyphosate on COOH-Modified Carbon Nanotube

RESUMO

Glyphosate is a widely used herbicide whose persistence and toxicity in aquatic and terrestrial environments demand efficient removal strategies. Here we employ GFN2 xTB calculations with implicit ALPB aqueous solvation and automated docking to investigate the adsorption of all five pH dependent ionized forms of glyphosate (G1-G5) on (10,0) single walled carbon nanotubes covalently functionalized with carboxyl groups at 0-25% coverage. Adsorption energies reveal a clear charge dependent trend: the dianionic (G4) and trianionic (G5) species exhibit the most negative binding energies over the entire functionalization range, while the protonated and neutral forms (G1, G2) bind weakly, approaching reversible adsorption at high COOH contents. The deprotonated form G5 strengthens from -2.17 eV on pristine CNT to about -5.7 eV at 10% and 25% functionalization, with a local minimum near 15-20% COOH due to steric and electrostatic crowding of adjacent groups. Decomposition of the solvation free energy shows dominant electrostatic stabilization complemented by increasingly favorable hydrogen bond contributions as carboxyl density grows. All complexes display very small HOMO-LUMO gaps (0.02-0.22 eV), indicating high electronic sensitivity to adsorption and functionalization. Overall, CNT+COOH systems can operate in both strong capture and regenerable regimes depending on glyphosate ionization state, offering a tunable platform for pH responsive remediation.