Formation and characterization of ternary (Na, NH4, H3O)-jarosites produced from Acidithiobacillus ferrooxidans cultures
Abstract The purpose of this study was to characterize a series of (Na, NH 4 , H 3 O)-jarosites produced with various combinations of NH 4 + + Na + in cultures of Acidithiobacillus ferrooxidans that simulated acid solutions from bioleaching systems. The solution concentrations utilized were 6.1, 80, 160 and 320 mM for NH 4 + and 0, 50, 100, 250 and 500 mM for Na + as their respective sulfates. Media at pH 2.2 were inoculated with iron-oxidizing A. ferrooxidans and incubated in shake flasks at 22 ± 2 °C. As the bacteria oxidized ferrous sulfate, ferric iron hydrolyzed and precipitated as schwertmannite (idealized formula Fe 8 O 8 (OH) 6 (SO 4 ).nH 2 O) and/or as solid solution jarosites [(Na, NH 4 , H 3 O)-Fe 3 (SO 4 ) 2 (OH) 6 )] depending on the relative and absolute concentrations of NH 4 + and Na + . The precipitates were characterized by elemental analysis, X-ray diffraction, specific surface area, and Munsell color. Schwertmannite was the dominant mineral product at low combinations of Na + (≤50 mM) and NH 4 + (≤80 mM) in the media after 2 weeks of aging. At higher single or combined concentrations and with aging for 6 and 11 weeks, the formation of yellowish, solid solution jarosites was enhanced. Precipitation of jarosite-group minerals was favored by NH 4 + relative to Na + . Color (Munsell hue) was a useful tool for assessing sample mineralogy after extended aging, but the presence of abundant, poorly crystalline schwertmannite tended to mask the color of admixed jarosite-group minerals after only 2 weeks of contact with the culture media. The purest samples of jarosite-type minerals had specific surface areas 2 /g. Unit cell edge lengths and cell volume calculations from powder XRD data indicated that the jarositic phases produced were ternary (Na, NH 4 , H 3 O)-solid solutions. Most products also appeared to be deficient in structural Fe, especially at low NH 4 contents. Thus, ferric iron precipitation from the simulated bioleaching systems yielded solid solutions of jarosites with chemical compositions that were dependent on the relative concentrations of Na + and NH 4 + in the synthesis media. No phase separations involving discrete, end-member Na-, NH 4 -, or H 3 O-jarosites were detected. Highlights Ternary (Na, NH 4 , H 3 O) jarosites were precipitated in cultures of A. ferrooxidans . H 3 O substitution for structural Na/NH 4 occurred in all samples. Crystallization of jarosites was enhanced by NH 4 + relative to Na + and by aging. Schwertmannite was formed at low NH 4 + (≤80 mM) and all Na + concentrations. Graphical abstract [DISPLAY OMISSION]
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