Quantum chemical study on surface complex structures of phosphate on gibbsite

Area of Science:

• Geochemistry
• Surface Chemistry
• Computational Chemistry

Background:

• Phosphate adsorption on mineral surfaces is crucial for nutrient cycling and environmental processes.
• Gibbsite (Al(OH)₃) is a common aluminum hydroxide mineral influencing phosphate mobility.
• Understanding surface complexation at varying pH is key to predicting phosphate behavior.

Purpose of the Study:

• To computationally identify and characterize phosphate surface complexes on gibbsite across a range of pH.
• To investigate the thermodynamic favorability and reaction mechanisms of phosphate adsorption.
• To correlate theoretical vibrational frequencies with experimental spectroscopic data.

Main Methods:

• Density functional theory (DFT) calculations using the Al₆(OH)₁₈(H₂O)₆ cluster model.
• Modeling adsorption via ligand exchange of phosphate species with surface hydroxyls.
• Calculation of stretching and bending vibrational frequencies for different surface structures.
• Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy.

Main Results:

• DFT predicts phosphate surface complexes are thermodynamically more favorable at acidic pH.
• Ligand exchange involving release of aquo groups facilitates aluminum vacant site generation.
• Calculated vibrational frequencies shift to higher values at low pH compared to high pH.
• ATR-FTIR spectra show a dominant band (800-840 cm⁻¹) with frequency shifts consistent with DFT predictions.

Conclusions:

• Phosphate adsorption on gibbsite is pH-dependent, favoring acidic conditions.
• DFT calculations accurately predict the thermodynamic stability and vibrational characteristics of phosphate surface complexes.
• The study provides atomic-level insights into phosphate-mineral interactions, supporting experimental observations.