Synthesis and properties of ammoniojarosites prepared with iron-oxidizing acidophilic microorganisms at 22–65 °C

Abstract

Ammoniojarosite [(NH 4,H 3O)Fe 3(OH) 6(SO 4) 2], a poorly soluble basic ferric sulfate, was produced by microbiological oxidation of ferrous sulfate at pH 2.0–3.0 over a range of NH 4 + concentrations (5.4–805 mM) and temperatures (22–65 °C). Ammoniojarosites were also produced by chemical (abiotic) procedures in parallel thermal (36–95 °C) experiments. At 36 °C, schwertmannite [ideally Fe 8O 8(OH) 6(SO 4)] was the only solid product formed at <10 mM NH 4 + concentrations. Between 11.5 and 85.4 mM NH 4 + , a mixed product of ammoniojarosite and schwertmannite precipitated, as identified by X-ray diffraction. In excess of 165 mM NH 4 + , ammoniojarosite was the only solid phase produced. An increase in the incubation temperature using thermoacidophiles at 45 and 65 °C accelerated the formation of ammoniojarosite in culture solutions containing 165 mM NH 4 + . Both the biogenic and chemical ammoniojarosites were yellow (2Y–4Y in Munsell hue), low surface area (<1 m 2/g), well crystalline materials with average c o and a o unit cell parameters of 17.467 ± 0.048 Å and 7.330 ± 0.006 Å, respectively. Strong positive correlations were observed between unit cell axial ratios ( c o/ a o) and increasing synthesis temperature in both biotic and abiotic systems. All samples were N deficient compared to stoichiometric ammoniojarosite, and both chemical and X-ray data indicated partial replacement of NH 4 + by H 3O + to form solid solutions with 0.14–0.24 mole H 3O + per formula unit. The morphology of the biogenic jarosites included aggregated discs, pseudo-cubic crystals and botryoidal particles, whereas the chemical specimens prepared at 36–95 °C were composed of irregular crystals with angular edges. Morphological information may thus be useful to evaluate environmental parameters and mode of formation. The data may also have application in predicting phase boundary conditions for Fe(III) precipitation in biogeochemical processes and treatment systems involving acid sulfate waters.