Morphology of jarosite-group compounds precipitated from biologically and chemically oxidized Fe ions

Jarosite-group compounds [MFe3(SO4)(2)(OH)(6): argentojarosite (M+ = Ag+),jarosite (M+ = K+), ammoniojarosite (M+ = NH4+)] were synthesized by supplying Fe3+ ions in three different ways: biological oxidation of Fe2+ ions by T. ferrooxidans (biological products), chemical oxidation of Fe2+ ions by slow addition of H2O2 (chemical products 1), and chemical oxidation by rapid addition of H2O2 (chemical products 2). These were characterized by XRD, FTIR chemical analysis and SEM; as well, the morphological features were compared with those formed by the hydrothermal method (standard substances). The jarosite-group compounds so synthesized do not contain crystalline by-products, as revealed by XRD, but the order of purity inferred from IR spectra, which is determined by the intensity of specific peaks, was found to be dependent on the method of preparation and is independent of the jarosite species; the order was found to be standard substances > chemical products 2 > chemical products 1 > biological products. Two main factors were found to affect the morphology, the method and rate of supply of Fe3+ ions to the system and the nature of the monovalent cations, which determine the intrinsic rate of formation under given conditions. Where Fe3+ ions are present in the system from the beginning, the order of rate of formation is confirmed to be argentojarosite > jarosite > ammoniojarosite at 30 degrees C. Morphological features of jarosite-group phases formed by the biological method were rendered distinguishable by the effect of extracellular substances. Morphological information is useful to distinguish the mode of occurrence of jarosite-group phases in natural samples, since it may be difficult to do so by other analytical techniques, such as XRD, FTIR Raman spectroscopy and chemical analysis.