Colloid Mobilization During Soil Iron Redox Oscillations

Abstract

In redox-dynamic soils, iron reduction-oxidation events may initiate wide shifts in the concentration of colloidal and dissolved material because of either Fe mineral dissolution or pH shifts associated with Fe oxidation state changes. This can have profound effects on the mobilization of organic and metal constituents. We conducted laboratory studies of colloid dynamics in a Hawaiian soil subjected to four consecutive 14-day reduction-oxidation cycles across the "soil-Fe" (Fe(OH)3)/(Fe2+(aq)) equilibrium. Size fractionated samples were isolated by differential centrifugation and characterized based on analysis of the framework and trace elements (Si, C, Fe, Ti, Al, Zr, Nb, La, and U). Intracycle oscillations in all colloidal (3 kDa to 160 nm) elements peaked during the reduction half-cycles, mobilizing 10% of total soil Ti and from 1-5% of total soil Zr, Nb, La, and U at peak dispersion. Colloid dynamics were dependent on pH shifts accompanying the redox oscillations rather than the fluctuating solubility of Fe oxides. TEM/EDS and mass-balance calculations suggest a carbon-based colloid matrix with zones of metal enrichment. The cumulative effects of four redox cycles included an apparent increase in colloid stability. Proton production/consumption associated with Fe-redox cycling has important implications for mobilization colloid-borne trace elements and sorbed contaminants.