EFFECT OF CYSTEINE AND MANGANESE ON THE CRYSTALLIZATION OF NONCRYSTALLINE IRON(III) HYDROXIDE AT PH-8

To provide a greater understanding of the crystallization of iron oxides under natural aqueous conditions, the combined effect of an inorganic ion (Mn2+) and a reducing organic ligand (L-cysteine) on the conversion of noncrystalline ferric hydroxide to goethite and/or hematite was investigated at pH 8. At cysteine: Fe ratios ≥ 0.2, L-cysteine caused noncrystalline iron(III) hydroxide to transform rapidly into geothite at pH 8; in the absence of the organic ligand, hematite was the predominant reaction product. The adsorption of cysteine on noncrystalline iron(III) hydroxide was immediately followed by the oxidation of cysteine to the disulfide with simultaneous reduction of a proportion of the interfacial ferric ions. The partly reduced noncrystalline iron(III) hydroxide dissolved at pH 8 more rapidly than the original material, thus facilitating the formation of goethite. In Mn(II)-noncrystalline iron(III) hydroxide coprecipitates, the interfacial oxidation/reduction reaction with cysteine was reduced. Additional aspects of the study are discussed.