ABSTRACT
The effect of soil reduction on trace metal solubility has important implications to both plant availability and toxicity, and chemical transport The release of metals associated with Mn and Fe oxides following reductive dissolution is an important mechanism that can potentially increase the soluble concentrations of metals.1,2 The potential for the release of trace metals following soil reduction appears to be the greatest for slightly to moderately reduced soils, with redox potentials between 100 and 400 mV. Under these conditions, redox potentials are sufficiently low to dissolve Mn or Fe oxides, but not low enough to precipitate metal sulfides. In highly reduced soils with redox potentials less than approximately 0 mV, the precipitation of metal sulfides limits the soluble concentration of trace metals.1,3 Dissolution of Mn oxides precedes Fe oxide dissolution because of the lower redox potential required to dissolve Fe oxides.4 Manganese (IV) oxides become unstable at a redox potential (EH) of approximately 300 mV, whereas Fe (III) oxides are stable until EH decreases to less than 100 mV, with the exact values of EH required to initiate reductive dissolution dependent on pH. Consequently, the dissolution of Mn oxides may play a more important role in metal solubilization in the early stages of soil reduction when redox potential is low enough to dissolve Mn oxides, but Fe oxides may still be stable.
