pH-Responsive Glyphosate Adsorption on Hydroxylated Carbon Nanotubes: From Electronic Structure to Molecular Dynamics

TL;DR

Glyphosate pollution is addressed by exploring pH-responsive adsorption on hydroxyl-functionalized carbon nanotubes using a hybrid computational workflow. The study combines $xTB$ geometry optimization, $QTAIM$ topological analysis, and molecular dynamics to examine glyphosate adsorption across five ionization states ($G1$–$G5$) and OH coverages from 5% to 25% on (10,0) CNTs. Key findings show that OH functionalization enhances adsorption, with more negative adsorption energies for higher OH content and for deprotonated forms ($G4$/$G5$), and that electronic coupling is optimized at 20–25% OH in the presence of $G4$/$G5$, supported by a high density of bond critical points and RDF-based evidence of stronger surface organization. Collectively, the results indicate covalent-like donor–acceptor interactions and environmentally viable adsorption on functionalized CNTs, suggesting potential for glyphosate detection and remediation across varying pH conditions; moderate interactions may enable regenerable adsorbents, offering practical routes for environmental applications.

Abstract

This computational study investigates glyphosate adsorption mechanisms on hydroxyl-functionalized carbon nanotubes (CNTs) as an alternative approach for environmental remediation. Single-walled CNTs with (10,0) zigzag chirality were functionalized with hydroxyl groups at concentrations of 5-25% and evaluated for interactions with glyphosate in five different ionization states (G1-G5) corresponding to pH-dependent protonation. Using semi-empirical tight-binding methods implemented in xTB software, molecular geometry optimization, electronic property calculations, topological analyses via Quantum Theory of Atoms in Molecules (QTAIM), and molecular dynamics simulations at 300K were performed. Results demonstrate that functionalization significantly enhances adsorption capacity, with binding energies becoming increasingly negative at higher OH concentrations and with more deprotonated glyphosate forms (G4 and G5). Electronic coupling analyses reveal optimized charge reactivity and transport in systems with 20-25% OH functionalization. Topological characterization identified 477 bond critical points, confirming donor-acceptor interactions with strong covalent contributions, particularly in highly functionalized systems. Radial distribution function profiles from molecular dynamics simulations demonstrate that functionalization promotes spatial organization on nanotube surfaces, increasing contact regions and reducing molecular mobility. Systems with moderate interactions (CNT+OHx+G1 and CNT+OHx+G3) present environmentally and economically viable solutions, enabling adsorbent regeneration and reuse. The findings indicate that OH-functionalized carbon nanotubes show significant promise for glyphosate detection and capture applications in environmental monitoring and remediation, regardless of the pesticide's ionization state.

Figures (9)

• Figure 1: 2D and 3D illustration of the ionic glyphosate forms. Representations based on pKa values associated with each protonation state.
• Figure 2: Computational procedure flowchart.
• Figure 3: Optimized structures for pristine CNT, functionalized nanotubes, glyphosate and complexes.
• Figure 4: Structure of the frontiers orbitals of all systems. (A) HOMO and (B) LUMO.
• Figure 5: 2D representation of the electronic density, $\rho$.