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, NH4, H3O)-jarosites produced with various combinations of NH4+ + 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 NH4+ 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 Fe8O8(OH)6(SO4).nH2O) and/or as solid solution jarosites [(Na, NH4, H3O)-Fe3(SO4)2(OH)6)] depending on the relative and absolute concentrations of NH4+ 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 NH4+ (≤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 NH4+ 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 <1.0 m2/g. Unit cell edge lengths and cell volume calculations from powder XRD data indicated that the jarositic phases produced were ternary (Na, NH4, H3O)-solid solutions. Most products also appeared to be deficient in structural Fe, especially at low NH4 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 NH4+ in the synthesis media. No phase separations involving discrete, end-member Na-, NH4-, or H3O-jarosites were detected.