Oxidation of V(IV) by Birnessite: Kinetics and Surface Complexation

Area of Science:

• Environmental Chemistry
• Geochemistry
• Environmental Science

Background:

• Vanadium (V) is a contaminant of concern due to potential health risks.
• Groundwater vanadium levels often exceed EPA notification levels.
• Geochemical factors controlling vanadium mobility remain unclear.

Purpose of the Study:

• Investigate the redox interaction between vanadium IV (V^IV) and birnessite (MnO₂).
• Understand the impact of this interaction on vanadium mobility and toxicity in groundwater.

Main Methods:

• Utilized continuously stirred batch reactors under neutral pH conditions.
• Employed Synchrotron X-ray absorption spectroscopy (XAS) for in situ and ex situ analysis.
• Characterized V^IV oxidation, MnO₂ reduction, and resulting mineral phases.

Main Results:

• Birnessite rapidly oxidized sparingly soluble V^IV to mobile, toxic vanadate (HVO₄^3-).
• V^IV oxidation occurred in two rapid stages, faster than V^IV solid dissolution.
• Reduction of birnessite produced soluble Mn^II, forming Mn^III oxyhydroxide (β-MnOOH).
• XAS confirmed a V^V-birnessite complex, but V^V retention was minimal.

Conclusions:

• Manganese oxides are potent oxidants of V^IV in environmental settings.
• Redox cycling involving Mn oxides can elevate dissolved vanadium concentrations in aquifers.
• This process has significant implications for understanding vanadium's environmental fate and transport.