Does As(III) interact with Fe(II), Fe(III) and organic matter through ternary complexes?

Area of Science:

• Environmental Chemistry
• Geochemistry
• Biogeochemistry

Background:

• Limited research exists on arsenic binding with organic matter via iron bridges, particularly concerning Fe(II).
• Understanding these interactions is crucial for predicting arsenic behavior in iron and organic matter-rich aquatic environments.

Purpose of the Study:

• To investigate the complexation mechanisms of As(III) with Leonardite humic acid (HA) in the presence of Fe(II) and Fe(III).
• To develop and refine geochemical models that accurately describe As(III)-Fe-OM interactions.

Main Methods:

• Complexation isotherms were performed using As(III), Fe(II)/Fe(III), and Leonardite humic acid.
• Geochemical modeling with PHREEQC/Model VI was employed and subsequently modified to account for ternary complex formation.

Main Results:

• Fe(III) binding of As(III) to HA primarily involved thiol groups, consistent with existing models.
• Fe(II) binding showed a different mechanism, with a proposed mononuclear bidentate complexation of As(III) to a bidentate Fe(II)-HA complex, though model refinement was needed.
• A general scheme for As(III)-Fe-OM complex formation in relevant aquatic systems was proposed.

Conclusions:

• Arsenic binding mechanisms with organic matter differ significantly between Fe(II) and Fe(III).
• The study provides a refined model for As(III)-Fe(II)-OM interactions and a comprehensive overview of As(III)-Fe-OM complexation.
• These findings enhance our understanding of arsenic fate and transport in natural waters.