Colloid mobilization during soil iron redox oscillations.

Area of Science:

• Environmental Science
• Soil Science
• Geochemistry

Background:

• Redox-dynamic soils experience iron reduction-oxidation cycles.
• These cycles can alter concentrations of dissolved and colloidal materials.
• Mobilization of organic and metal constituents is influenced by Fe mineral dissolution and pH shifts.

Purpose of the Study:

• To investigate colloid dynamics in a Hawaiian soil under redox cycling.
• To determine the impact of iron reduction-oxidation on trace element mobilization.
• To understand the role of pH shifts versus Fe oxide solubility in colloid dynamics.

Main Methods:

• Laboratory studies simulating four consecutive 14-day reduction-oxidation cycles.
• Isolation of size-fractionated samples via differential centrifugation.
• Characterization of elemental composition (Si, C, Fe, Ti, Al, Zr, Nb, La, U) using TEM/EDS and mass-balance calculations.

Main Results:

• Colloidal elements peaked during reduction half-cycles, mobilizing significant portions of Ti, Zr, Nb, La, and U.
• Colloid dynamics were primarily driven by pH shifts associated with redox oscillations.
• A carbon-based colloid matrix with metal enrichment was identified.
• Four redox cycles led to increased colloid stability.

Conclusions:

• pH fluctuations accompanying iron redox cycling are key drivers of trace element and contaminant mobilization.
• Understanding these dynamics is crucial for predicting element transport in soils.
• Proton production/consumption during Fe-redox cycling influences colloid-borne contaminant mobility.