Transformation of Iron in a Waterlogged Soil as Influenced by Redox Potential and pH

The distribution of different forms of iron in a waterlogged soil was studied over a wide range of closely controlled redox potential and pH conditions. Increases in water soluble and exchangeable iron were favored by a decrease in both redox potential and pH. The critical redox potentials for iron reduction and consequent dissolution was between +300 mV and +100 mV at pH 6 and 7, and −100 mV at pH 8, while at pH 5 appreciable reduction occurred at +300 mV. The distribution between water soluble and exchangeable iron fractions was highly pH dependent with a decrease in pH at a given redox potential increasing the relative amount of ferrous iron in the soil solution at the expense of that on the exchange complex. A thermodynamic approach to the equilibria between solid phase ferric oxyhydroxide and a water soluble species of iron (Fe 2+ ) indicated that it was largely governed by the Fe 2+ − Fe(OH) 3 system in which the ferric oxyhydroxide was a mixture of goethite and amorphous material.