Reduction and Reoxidation Cycles of Manganese and Iron in Flooded Soil and in Water Solution

Abstract Cycles of manganese (Mn) and iron (Fe) reduction and reoxidation were examined in suspensions of Crowley silt loam (a thermic Typic Albaqualf) and in water solutions. In stirred soil suspensions at a constant pH of 7.0, the redox potential was varied stepwise from reducing conditions to oxidizing conditions and then stepwise back to reducing conditions. For 50‐mV incremental changes in redox potential, similar redox potential‐pH curves were obtained for the oxidizing half of the cycle and for the reducing half of the cycle when a 2‐week incubation period at each controlled redox potential was used. A 1‐week incubation period did not result in completion of the oxidation and/or reduction reactions, and the curves obtained during oxidation did not coincide well with those obtained during reduction. For soil‐free water solutions, the solubility of Mn was governed almost completely by pH with little effect of redox potential being evident. Iron reduction and reoxidation in water solutions were more sensitive to redox potential, especially at pH 5.0 and at pH 6.0. At pH 4.0, Fe remained in solution at all redox potentials. At pH 7.0 and 8.0, little Fe was present as soluble Fe 2+ when the redox potential was decreased.