Surface complexation modeling of Cr(VI) adsorption at the goethite-water interface

Area of Science:

• Environmental Chemistry
• Geochemistry
• Surface Chemistry

Background:

• Goethite is a common iron oxyhydroxide mineral in soils and sediments.
• Chromate (Cr(VI)) is a toxic oxyanion pollutant of environmental concern.
• Understanding Cr(VI) adsorption onto mineral surfaces is vital for remediation strategies.

Purpose of the Study:

• To develop and validate a charge distribution multisite surface complexation model (CD-MUSIC) for chromate adsorption onto goethite.
• To elucidate the adsorption mechanism and identify dominant surface species.
• To assess the model's applicability under various environmental conditions.

Main Methods:

• Investigated Cr(VI) adsorption as a function of pH, ionic strength, and concentration.
• Utilized a CD-MUSIC approach to model macroscopic adsorption data.
• Proposed and constrained surface complexation species based on spectroscopic evidence.
• Verified the model through competitive adsorption experiments with phosphate.

Main Results:

• Cr(VI) adsorption on goethite is primarily governed by inner-sphere complexation, with minimal influence from ionic strength.
• A bidentate complex (≡Fe2O2CrOOH) dominates at low pH, while a monodentate complex (≡FeOCrO3(-3/2)) becomes prevalent at higher pH.
• The developed CD-MUSIC model successfully predicted competitive adsorption, indicating phosphate has a higher affinity for the goethite surface than chromate.
• The model demonstrated applicability across various conditions, enhancing the database for oxyanion adsorption onto goethite.

Conclusions:

• The CD-MUSIC model provides a robust framework for understanding chromate adsorption on goethite.
• The findings highlight the importance of pH in determining surface complexation mechanisms and species distribution.
• This study offers a valuable tool for predicting and managing Cr(VI) contamination in environmental settings.