Cr(OH)3(s) oxidation induced by surface catalyzed Mn(II) oxidation

Area of Science:

• Environmental Chemistry
• Geochemistry
• Environmental Science

Background:

• Chromium (VI) is a toxic contaminant often found in groundwater.
• Chromium (III) hydroxide [Cr(OH)3(s)] is considered a stable remediation product of Cr(VI).
• Natural processes that generate Cr(VI) from Cr(III) are not fully understood.

Purpose of the Study:

• To investigate the potential for Cr(OH)3(s) oxidation mediated by Mn(II) oxidation under typical groundwater conditions.
• To determine if Cr(VI) can be naturally produced from Cr(OH)3(s) via surface-catalyzed Mn(II) oxidation.

Main Methods:

• Reacting dissolved Mn(II) with synthesized Cr(OH)3(s) at varying pH (7.0-9.0) and oxygen conditions (oxic/anoxic).
• Monitoring Mn(II) removal and Cr(VI) formation.
• Utilizing X-ray absorption near-edge structure (XANES) to characterize the manganese oxide phase.

Main Results:

• Homogeneous Mn(II) oxidation was negligible at pH ≤ 8.0.
• At pH 9.0, Mn(II) was completely removed and precipitated as hausmannite.
• Cr(OH)3(s) was oxidized, releasing Cr(VI) when Mn(II) was added at pH ≥ 8.0 under oxic conditions.
• Surface-catalyzed Mn(II) oxidation on Cr(OH)3(s) formed a mixed valence Mn(III/IV) oxide phase.

Conclusions:

• Cr(OH)3(s) oxidation coupled with Mn(II) oxidation can naturally produce toxic Cr(VI).
• Sparingly soluble Cr(OH)3(s) poses a potential environmental hazard, contrary to its previous classification as a stable remediation product.
• This pathway highlights a previously unrecognized source of Cr(VI) contamination in groundwater.