DISSOLUTION OF HYDROUS IRON(III) OXIDES BY REDUCTIVE MECHANISMS

Reductive mechanisms play an important role in the dissolution of iron(III) (hydr)oxides in nature. For iron(III) (hydr)oxides, reductive mechanisms are usually faster than nonreductive mechanisms, because release of iron(II) from the surface of the crystal lattice is energetically more favorable than release of iron(III). We have studied two different types of dissolution, which both involve a reductive step. The first reaction is the reductive dissolution of hematite (alpha-Fe2O3) by ascorbate and the second is the dissolution of goethite (alpha-FeOOH) by iron(II) complexes, where a reduction step leads to a fast overall dissolution rate. In both cases, the dissolution rate is proportional to the concentration of the reducing species on the surface of the dissolving solid. Dissolution rates for different pH values have been determined. The effect of the pH value is pronounced and explanations are given for the trend observed. The presence of aluminum is shown to influence the dissolution of hematite by ascorbate at pH 4, but not at pH 3. This observation is explained by the blocking of the surface by adsorption of aluminum at pH 4.