Electrolyte ion binding at iron oxyhydroxide mineral surfaces.

Area of Science:

• Environmental science
• Surface chemistry
• Mineralogy

Background:

• Iron oxyhydroxides (FeOOH), including goethite and lepidocrocite, are prevalent in natural environments and industrial processes.
• Understanding electrolyte ion interactions at mineral surfaces is crucial for predicting environmental fate and optimizing technological applications.

Purpose of the Study:

• To investigate electrolyte ion (Cl-, ClO4-, Na+) loadings on synthetic goethite and lepidocrocite surfaces.
• To correlate electrolyte ion adsorption with potential determining ion (pdi; H+, OH-) loadings.
• To compare the adsorption behavior of chloride and perchlorate ions.

Main Methods:

• Cryogenic X-ray photoelectron spectroscopy (XPS) was used to determine atomic concentrations of adsorbed ions.
• Potentiometric titrations were employed to quantify potential determining ion (pdi) loadings.
• Synthetic goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) particles were pre-equilibrated in NaCl and NaClO4 solutions.

Main Results:

• Chloride (Cl-) showed higher pdi adsorption than perchlorate (ClO4-) due to its charge-to-size ratio, but both followed similar loading dependencies.
• Lepidocrocite exhibited stronger electrolyte adsorption responses to pdi loadings, particularly on the (010) plane, and higher sodium loadings than goethite.
• Perchlorate adsorption enhanced sodium loadings on lepidocrocite, potentially by thickening the interfacial region.
• Rougher goethite surfaces showed greater pdi and ion loadings compared to smoother surfaces.

Conclusions:

• The study elucidated the relationship between pdi and electrolyte ion loadings at FeOOH mineral surfaces.
• Differences in adsorption behavior were observed between goethite and lepidocrocite, influenced by surface properties and specific ion characteristics.
• Findings provide critical insights into the surface chemistry of iron oxyhydroxides relevant to environmental and technological contexts.