Hexavalent Chromium Generation within Naturally Structured Soils and Sediments

Area of Science:

• Environmental Chemistry
• Geochemistry
• Microbiology

Background:

• Chromium(VI) poses a significant threat to groundwater quality, originating from the oxidation of chromium(III) minerals.
• The generation of chromium(VI) is critically dependent on the interaction between chromium(III) and manganese oxides within soil and sediment matrices.

Purpose of the Study:

• To investigate the oxidation of chromium(III) by manganese oxides within structured solid matrices representative of soils and sediments.
• To understand the influence of mineral solubility and environmental conditions (organic carbon, redox) on chromium(VI) generation.

Main Methods:

• Artificial soil aggregates were created using chromium(III) hydroxide or iron-substituted chromium(III) hydroxide coated quartz grains.
• Aggregates were combined with synthetic birnessite or inoculated with the manganese-oxidizing bacterium Leptothrix cholodnii.
• Chromium(VI) concentrations in advecting solutes were measured under varying conditions.

Main Results:

• Chromium(VI) concentrations exceeded drinking water standards in low-organic carbon environments.
• Increasing iron substitution in chromium minerals reduced chromium(VI) generation by 37% due to decreased solubility.
• High organic carbon environments created reducing conditions that suppressed chromium(VI) efflux.

Conclusions:

• Chromium(VI) is generated through reactions with manganese oxides in structured soil and sediment media.
• Hydrologic and redox conditions significantly influence coupled chromium and manganese cycling, impacting chromium(VI) formation and transport.