Rare earth element sorption onto hydrous manganese oxide: a modeling study

Area of Science:

• Geochemistry
• Environmental Science
• Mineralogy

Background:

• Manganese oxides are key sorbents for rare earth elements (REE) in aquatic environments.
• Modeling REE sorption on manganese oxides is challenging due to incomplete surface complexation models and data inconsistencies.

Purpose of the Study:

• To develop a comprehensive surface complexation model for REE sorption onto hydrous manganese oxide.
• To investigate the influence of pH, ionic strength, and metal loading on REE sorption.
• To elucidate the heterogeneity of Mn oxide binding sites for different REE.

Main Methods:

• Utilized a two-site surface complexation model (SCM) with diffuse double layer theory.
• Fixed key surface parameters: specific surface area (746 m²/g), site density (2.1 mmol/g), and point of zero charge (pH(zpc) = 2.2).
• Determined equilibrium surface complexation constants using published REE sorption data.

Main Results:

• Modeled REE sorption constants (LogK) increase from light to heavy REE, exhibiting a convex tetrad effect.
• Identified distinct REE binding preferences: ≡YOH sites dominate at low metal loading, while ≡XOH sites dominate at high metal loading.
• Demonstrated heterogeneity in Mn oxide binding site distribution across the REE series.

Conclusions:

• The developed SCM accurately describes REE sorption on manganese oxides, accounting for site heterogeneity.
• The findings enhance the understanding of REE behavior and transport in hydrosystems.
• This work provides a robust framework for predicting REE interactions with manganese oxides.