Proton binding by hydrous ferric oxide and aluminum oxide surfaces interpreted using fully optimized continuous pKa...

Area of Science:

• Surface Chemistry
• Geochemistry
• Environmental Science

Background:

• Understanding proton binding on metal oxide surfaces is crucial for contaminant transport and water quality.
• Existing methods for determining surface complexation parameters can be limited in accuracy and scope.
• Hydrous ferric oxide (HFO) and aluminum oxide are common soil and mineral components influencing environmental processes.

Purpose of the Study:

• To develop and validate a modified regularized least squares pKa spectrum approach for determining proton stability constants and concentrations.
• To apply this method to analyze binding sites on hydrous ferric oxide (HFO) and aluminum oxide surfaces.
• To compare the obtained pKa distributions with theoretical models and experimental data for HFO.

Main Methods:

• Utilized a modified regularized least squares pKa spectrum approach to analyze acid-base titration data.
• Employed a continuous binding site model represented as a pKa spectrum.
• Optimized simultaneously for pKa spectrum smoothness and goodness-of-fit, improving upon traditional methods.

Main Results:

• The modified method accurately recovered theoretical values when tested on aluminum oxide.
• Applied to HFO samples, the method yielded pKa distributions consistent with theoretical binding site stability constants for crystalline iron oxides.
• The binding constants for the HFO samples aligned closely with those of lepidocrocite and goethite.

Conclusions:

• The modified regularized least squares pKa spectrum approach is a robust method for characterizing proton binding on metal oxide surfaces.
• The study provides valuable insights into the nature and stability of binding sites on HFO.
• Findings support the application of this method in environmental geochemistry and soil science research.