Oxalic Acid Adsorption on Rutile: Experiments and Surface Complexation Modeling to 150 °C

Area of Science:

• Geochemistry
• Surface Science
• Environmental Chemistry

Background:

• Understanding anion adsorption on metal oxides is crucial for environmental remediation and geochemical processes.
• Rutile (TiO2) is a common mineral, and its interaction with organic anions like oxalate influences contaminant transport and mineral weathering.

Purpose of the Study:

• To characterize oxalate adsorption on rutile across varying pH, ionic strength, and temperature conditions.
• To reconcile conflicting interpretations of oxalate binding mechanisms (inner-sphere vs. outer-sphere) using surface complexation modeling and molecular simulations.

Main Methods:

• Experimental adsorption measurements in NaCl media (0.03 and 0.30 m) from pH 3 to 10 and 10-150 °C.
• Surface complexation modeling (SCM) using the CD-MUSIC formalism.
• Molecular modeling including molecular dynamics, free-energy, and ab initio calculations.
• Literature data from IR spectroscopy was also incorporated.

Main Results:

• Oxalate adsorption increased with decreasing pH.
• No systematic trends were observed with ionic strength or temperature.
• Molecular simulations indicated outer-sphere oxalate binding via hydrogen bonds.
• IR spectroscopy suggested predominant inner-sphere complex formation.
• Both SCM approaches (constrained by molecular modeling or IR data) equally fit the adsorption data.

Conclusions:

• The discrepancy in binding mechanisms likely arises from differences in rutile surface characteristics (perfect (110) faces vs. defect-rich powders).
• Surface defects on rutile powders may be responsible for inner-sphere oxalate adsorption.
• Further research is needed to confirm the role of surface defects in oxalate adsorption.