Monovalent cation concentrations determine the types of Fe(III) hydroxysulfate precipitates formed in bioleach solutions

Hydrometallurgy 94 (2008) 29–33

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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Monovalent cation concentrations determine the types of Fe(III) hydroxysulfate precipitates formed in bioleach solutions Jonathan P. Gramp a, F. Sandy Jones b, Jerry M. Bigham b, Olli H. Tuovinen a,c,⁎ a b c

Department of Microbiology, Ohio State University, 484 West 12th Avenue, Columbus, Ohio 43210, USA School of Environment and Natural Resources, Ohio State University, 2021 Coffey Road, Columbus, Ohio 43210, USA Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101 Tampere, Finland

A R T I C L E

I N F O

Available online 9 July 2008 Keywords: Jarosite Acidithiobacillus ferrooxidans Iron oxidizing bacteria Schwertmannite

A B S T R A C T Iron oxidation in bioleaching systems commonly produces mixtures of hydroxysulfate minerals. The purpose of this study was to characterize iron precipitates formed in liquid media inoculated with iron-oxidizing acidophiles (Acidithiobacillus ferrooxidans). Air-dried precipitates were characterized by X-ray diffraction (XRD), elemental analysis, specific surface area, color, and scanning electron microscopy. The Fe(III)hydroxysulfates produced in this study included schwertmannite (Fe8O8(OH)6SO4) and various solid solution jarosites (K,Na,NH4,H3O)Fe3(SO4)2(OH)6). The precipitates were composed entirely of schwertmannite in the presence of insufficient concentrations of monovalent ions. The levels of NH+4, K+, and Na+ were determined for the formation of the corresponding jarosites. The levels varied depending on the jarosite type and were the lowest for K+ and highest for Na+. With natrojarosite especially, when synthesized with 500 mM Na+, the highest level tested in this study, the precipitate still contained poorly ordered schwertmannite as a major phase. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Fe(III)-hydroxysulfate minerals are commonly found in sediments of acid mine drainage impacted environments. Among the most common of these are schwertmannite, Fe8O8(OH)6SO4, goethite, FeOOH, and jarosites, XFe3(SO4)2(OH)6 where X is K+, NH+4, Na+, or H3O+ or their mixtures. Schwertmannite is poorly crystalline and metastable, and its transformation to both goethite (Bigham et al., 1996; Bigham and Nordstrom, 2000; Acero et al., 2006) and jarosite (Wang et al., 2006) has been documented. The latter transformation requires monovalent ions for jarosite formation and is facilitated at elevated temperatures (Wang et al., 2006). Schwertmannite has a relatively high surface area and has been shown to adsorb metals and metalloids such as arsenate (Carlson et al., 2002; Fukushi et al., 2004, Regenspurg and Peiffer, 2005). By comparison to schwertmannite, jarosites contain twice as much S but only about one third of the Fe on a molar basis. Therefore, their formation serves different purposes in controlling iron solubility and the removal of sulfate from leach solutions. The purpose of this work was to synthesize and characterize jarosites in liquid media inoculated with iron-oxidizing acidophiles and to determine the practical concentrations of monovalent cations ⁎ Corresponding author. Department of Microbiology, Ohio State University, 484 West 12th Avenue, Columbus, Ohio 43210, USA. Tel.: +1 614 292 3379; fax: +1 614 292 8120. E-mail address: [email protected] (O.H. Tuovinen). 0304-386X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2008.05.019

required for jarosite precipitation. While such concentrations may be predicted from geochemical speciation models using appropriate thermodynamic parameters, the paragenetic pathways of mineral formation are often driven by kinetics. 2. Materials and methods 2.1. Bacterial cultures and media Enrichment cultures of iron-oxidizing acidophiles (Acidithiobacillus ferrooxidans) (Wang et al., 2007) were maintained at 22±2 °C by transferring 5 mL of inoculum to 100 mL of fresh K- and Na-free media weekly. Culture flasks were incubated on a shaker at 120 rev/min. The medium was composed of two components: (A) an autoclaved mineral salts solution (0.5 g (NH4)2SO4, 0.5 g MgSO4·7H2O, and 0.36 g H3PO4 per L of 10 mM H2SO4) and (B) a filter-sterilized ferrous sulfate solution (167 g FeSO4·7H2O per L of 10 mM H2SO4). The two components were combined in a volume ratio of 4 parts A and 1 part B. The medium had an initial pH of 2.2±0.1. Liquid media for jarosite synthesis experiments were prepared with a range of K+, Na+, and NH+4 concentrations except that all media contained 6.1 mM NH+4 to provide a nitrogen source for the bacteria. This concentration was insufficient to initiate ammoniojarosite formation. The concentration of K+ was adjusted with KH2PO4 to 1, 4, 6, 12, and 31 mM; the concentration of Na+ was adjusted with Na2SO4 to 50, 100, 250, and 500 mM; and the concentrations of NH+4 were adjusted with NH4(SO4)2 to 6.1, 80, 160, and 320 mM NH+4. Media

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J.P. Gramp et al. / Hydrometallurgy 94 (2008) 29–33

Fig. 1. X-ray diffractogram of schwertmannite collected after 2 weeks from medium containing 6.1 mM NH+4, 0 mM K+, 0 mM Na+ and 160 mM Fe2+. The peak positions are identified by the corresponding Å-values. The vertical bar shows the scale of relative counts.

with 0 mM K+ and Na+ were also prepared. All media contained 160 mM Fe2+ added as FeSO4. The media (pH 2.2 ± 0.1) were inoculated (5 ml inoculum + 150 ml medium) with cultures that had been maintained in K- and Na-free media. All cultures were incubated at 22 ± 2 °C on a shaker at 120 rev/min. The K+ and NH+4 series were incubated for 2 weeks and the Na+ series was incubated for 2 and 6 weeks. Precipitates were collected by centrifugation at 15,300 ×g for 10 min followed by washing with 0.01 M H2SO4 and double distilled H2O. The samples were air dried and stored in a desiccator at 22 ± 2 °C. 2.2. Analytical methods Powder X-ray diffraction (XRD) was performed using a Philips PW 1316/90 diffractometer (Philips Electronics, New York, NY) with CuKα radiation. All samples were packed into a zero background quartz sample holder and step-scanned from 3 to 80°2Θ using a step interval

Fig. 2. XRD pattern of precipitates collected after 2 weeks from media containing 6.1, 80, 160, and 320 mM NH+4. The peak positions are identified by the corresponding Å-values. The vertical bar shows the scale of relative counts.

Fig. 3. XRD pattern of precipitates collected after 2 weeks from media containing 0, 1, 4, 6, 12 and 31 mM K+. The peak positions are identified by the corresponding Å-values. The vertical bar shows the scale of relative counts.

of 0.05°2Θ and a counting time of 4 s. The instrument was calibrated with a quartz reference material several times in the course of the study. XRD patterns were processed using the computer programs Jade (Materials Data Incorporated, Livermore, CA) and Microsoft Excel (Microsoft Corporation, Redmond, WA). Water content was determined gravimetrically after drying at 105 °C for 24–48 h. Total sulfur was determined by iodometric titration of SO2 evolved following sample combustion at 800 °C in a LECO model 521 induction furnace (LECO Corporation, St. Joseph, MI).

Fig. 4. XRD pattern of precipitates collected from liquid media containing 0, 50, 100, 250, and 500 mM Na+. All precipitates were held in solution for 6 weeks except for 0 mM Na+, which was analyzed after 2 weeks. The peak positions are identified by the corresponding Å-values. The vertical bar shows the scale of relative counts.

J.P. Gramp et al. / Hydrometallurgy 94 (2008) 29–33 Table 1 Comparison of color and d-values for end-member jarosites Cation

Concentration (mM)

NH+4

160 320

K+

12 31

Na+

500

ICDD PDF2 card no.

Munsell color (H V/C)a

8.2 YR 5.0/10.0 9.3 YR 6.2/9.8 26-1014 9.6 YR 6.6/8.3 3.0 Y 7.9/6.3 22-0827 9.6 YR 5.9/10.1 36-0425

d-Value (Å) (006) Peak 3

(024) Peak 4

(122) Peak 5

2.8777 2.8914 2.9090 2.8301 2.8260 2.8610 –b 2.7930

2.5610 2.5622 2.5660 2.5365 2.5342 2.5420 – 2.5310

2.3066 2.3098 2.3180 2.2663 2.2633 2.2870 – 2.3070

a Munsell notation: H = hue, V = value (lightness: 0 = black, 10 = white), C = chroma (intensity or saturation: 0 = neutral). b Diagnostic peaks obscured by the presence of schwertmannite.

Total Fe, K, and Na were determined by atomic absorption spectroscopy on a SpectrAA-5 instrument (Varian, Palo Alto, CA) following complete dissolution of 30 mg of sample with 5 mL of 6 M HCl over a 24 h period. Total N was determined using a Thermoquest NC 2100 soil analyzer (CE Instruments, Milan, Italy) with atropine as the standard. Surface area measurements were determined using a Micromeritics Flowsorb II 2300 surface area analyzer (Micromeritics, Norcross, GA). Samples were dried over P2O5 for 24 h before analysis using the singlepoint Brunauer-Emmett-Teller (BET) method with N2 as the adsorbate. Color measurements were performed using a Minolta CR-300 Chroma Meter (Konica Minolta Sensing Americas, Mahwah, NJ). Color data were recorded in Munsell color notation using Hue, Value, and Chroma (HVC). For morphological characterization, precipitates were examined under a Hitachi S-3500N scanning electron microscope (Hitachi High Technologies America, Schaumburg, IL). Powder samples were applied as a dusting to carbon tape on aluminum specimen holders and examined without coating. Samples were examined with an accelerating voltage of 20 kV. 3. Results and discussion

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holding in solution at 22 ± 2 °C. In contrast to schwertmannite, ammoniojarosite yields sharp XRD peaks indicative of better crystallinity (Fig. 2). XRD analyses of this series showed that jarosite peaks were present at 160 mM NH+4 (Fig. 2). However, schwertmannite was still present even at 320 mM NH+4, as was apparent from the broad background peaks forming part of the baseline. Potassium jarosite was synthesized in liquid media with 0, 1, 4, 6, 12 and 31 mM K+ and the precipitates were collected after holding in solution for 2 weeks. XRD analyses of this series showed that jarosite peaks appeared at 4 mM K+ (Fig. 3), and schwertmannite was absent in the samples prepared at ≥12 mM K+. Natrojarosite was synthesized in liquid media with 0, 50, 100, 250, and 500 mM Na+ and the precipitates were collected after holding in solution for 2 weeks and 6 weeks. Jarosite peaks were present at N100 mM Na+ (Fig. 4), but pure, end-member natrojarosite was not formed. Schwertmannite was still present even after 6 weeks of holding in solution at 500 mM Na+ (Fig. 4). The end-member jarosite XRD data for the samples produced in this study were compared to the data in the International Centre for Diffraction Data (ICDD) powder diffraction file (PDF2) database (rev. 2001). The XRD data indicated slight differences in peak positions depending on the monovalent cation and its concentration. Table 1 lists the d-values for three of the most sensitive peaks for endmember jarosites. Natrojarosite is also included in Table 1 even though the sample formed at 500 mM Na+ still had a substantial admixture of schwertmannite that obscured some diagnostic peaks. There was an average difference of 0.01 Å in matching peaks between the XRD data of jarosites produced in this study and the ICDD data, and some differences were as high as 0.04 to 0.07 Å. We have previously investigated ammoniojarosite formation as a function of temperature and NH+4 concentration (Wang et al., 2007). Ammoniojarosite in the present work was slightly less ordered because of a shorter holding time (2 weeks vs. 8 weeks) and a lower synthesis temperature (22 °C vs. 36 °C). Even under the most ideal synthesis conditions, ammoniojarosite included H3O+ substitutions for NH+4 (Wang et al., 2007). Such solid solutions could also be expected in this work, especially when low levels of monovalent cations were used to precipitate jarosites.

3.1. Iron hydroxysulfate minerals 3.2. Color The Fe(III)-hydroxysulfate minerals synthesized in this study were schwertmannite (Fe8O8(OH)6SO4), ammoniojarosite (NH4Fe3(SO4)2 (OH)6), potassium jarosite (KFe3(SO4)2(OH)6), and natrojarosite (NaFe3(SO4)2(OH)6). With 6.1 mM NH+4 and the absence of K+, or Na+, schwertmannite precipitated concurrently with ferrous iron oxidation in liquid media. The characteristic schwertmannite XRD pattern had 8 broad peaks (Fig. 1) with high background, indicating poor crystallinity. In the presence of sufficient levels of NH+4, K+, or Na+, jarosite was formed, but schwertmannite was also present at low cation concentrations. Ammoniojarosite was synthesized in liquid media with 6.1, 80, 160, and 320 mM NH+4 and the precipitates were collected after 2 weeks of

Color measurements are listed in Table 1. Schwertmannite is a yellowish-red mineral with a typical Munsell hue of 8–10 YR, whereas jarosites usually have yellow Munsell hues (Bigham, 1994; Bigham et al., 1994). All except the end-member potassium jarosite had yellowish-red colors due to the schwertmannite remaining in the samples. 3.3. Elemental and surface area analyses Total Fe, S, N, and K and specific surface area were determined for precipitates produced in the K+ series at 6.1 mM and 320 mM NH+4 (Table 2). Reported specific surface areas for poorly crystalline

Table 2 Elemental analyses of jarosite samples and their idealized formulations NH+4 (mM)

K+ (mM)

Fe (wt.%)

Fe (mol%)

S (wt.%)

S (mol%)

N (wt.%)

N (mol%)

K (wt.%)

K (mol%)

0 12 31

26.4 38.4 31.3

2.26 3.44 2.80

9.9 12.3 13.6

1.48 1.91 2.12

1.3 b0.3 b0.3

0.44 b0.1 b0.1

0 4.9 6.5

0 0.63 0.83

35.0 33.5 34.6 34.9 57.8

10.0 12.5 12.5 11.1 24.2

13.4 12.9 13.3 13.4 4.15

6.67 8.33 8.33 7.41 3.03

2.93 0 0 0 0

3.33 0 0 0 0

0 7.82 0 0 0

0 4.17 0 0 0

320 6.1 6.1 Idealized formulas: Ammoniojarosite Potassium jarosite Natrojarosite Hydronium jarosite Schwertmannite

Na (wt.%)

Na (mol%)

Mol ratio Fe/S 1.5 1.8 1.3

0 0 4.74 0 0

0 0 4.17 0 0

1.5 1.5 1.5 1.5 8

Specific surface area (m2/g) 8–10 0.8

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Fig. 5. SEM micrographs of Fe(III) precipitates collected from A. ferrooxidans cultures. A. Precipitates were collected after 2 weeks of incubation. The medium contained 6.1 mM NH+4, 0 mM K+, and 0 mM Na+. Note the fuzziness of schwertmannite surface. Scale bar = 5 μm. B. Precipitates were collected after 2 weeks of incubation. The medium contained 320 mM NH+4, 0 mM K+, and 0 mM Na+. Smooth crystals represent ammoniojarosite and the fine-sized aggregates are poorly crystalline schwertmannite. Scale bar = 10 μm. C. Precipitates were collected after 2 weeks of incubation. The medium contained 6.1 mM NH+4, 31 mM K+, and 0 mM Na+. Smooth crystals represent K-jarosite. Some schwertmannite may still be present. Scale bar = 10 μm. D. Precipitates were collected after 11 weeks of incubation. The medium contained 6.1 mM NH+4, 0 mM K+ and 500 mM Na+. Some smooth crystals represent Najarosite and are indicated with arrows, and the rest are poorly crystalline schwertmannite. Scale bar = 10 μm.

schwertmannite are from 100 to 200 m2g− 1 (Bigham et al., 1994), while well-crystalline jarosites have specific surface areas of less than 1 m2g− 1 (Sasaki and Konno, 2000; Wang et al., 2006). The purest jarosite precipitates in this study showed typical specific surface areas of 0.6 to 0.8 m2g− 1 (Table 2). The ideal chemical formulas and elemental compositions of jarosites and schwertmannite are also presented for comparison to data collected in this study. While the ideal Fe/S ratio for schwertmannite is 8, a lower Fe/S ratio of around 4.5 is commonly observed in nature, especially at low pH. Schwertmannite has a very high surface area which develops positive charge at low pH and adsorbs excess sulfate, and that in turn raises the sulfur content in the analysis (Bigham et al., 1994). The Fe/S molar ratios of the purest ammonio- and potassium jarosites prepared in this study are near the ideal Fe/S molar ratio of 1.5 for jarosites. 3.4. Scanning electron microscopy The schwertmannite precipitates were highly aggregated (Fig. 5A) and possessed a characteristic “pin-cushion” surface morphology that contributes to the large surface area of the mineral. Ammoniojarosite and potassium jarosite crystals had smooth surfaces, but some schwertmannite was also present in the ammoniojarosite sample (Fig. 5B and C). The natrojarosite samples also contained large amounts of schwertmannite (Fig. 5D). Only a few smooth crystals of natrojarosite were present in the precipitate produced from the A. ferrooxidans culture solution with 500 mM Na+ even after 11 weeks of incubation. 4. Conclusions The concentrations of NH+4, K+ and Na+ required for jarosite formation varied depending on the type of jarosite. The formation of potassium jarosite required the lowest level of monovalent cation as contrasted with high levels of NH+4 and Na+ needed to produce ammoniojarosite and natrojarosite. Even at the highest NH+4 and Na+

levels evaluated, the presence of poorly crystalline schwertmannite was still evident from the XRD patterns and SEM micrographs. Based on our previous study of biogenic jarosites (Wang et al., 2007) some hydronium solid solution likely occurred in all jarosite samples. Published methods of hydronium synthesis (e.g., Brophy and Sheridan, 1965; Dutrizac and Kaiman, 1976; Basciano and Peterson, 2007) require high temperatures that are prohibitive to all known iron-oxidizing microorganisms. The results presented in this study may serve as a baseline for further studies of monovalent cations in the schwertmannite–jarosite system, which directly impacts ferric iron solubility in acid solutions in bioleaching processes. Acknowledgments We thank D.E. Fulton and A. Kaszas, Molecular and Cellular Imaging Center of the Ohio Agricultural Research and Development Center, for their advice on SEM. Partial salary and research support were provided to J.M. Bigham by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, Ohio State University and to O.H. Tuovinen by the Finnish Funding Agency for Technology and Innovation (Finland Distinguished Professor Program, 402/06). References Acero, P., Ayora, C., Torrentó, C., Nieto, J.-M., 2006. The behavior of trace elements during schwertmannite precipitation and subsequent transformation into goethite and jarosite. Geochim. Cosmochim. Acta 70, 4130–4139. Basciano, L.C., Peterson, R.C., 2007. Jarosite–hydronium jarosite solid-solution series with full iron site occupancy: mineralogy and crystal chemistry. Am. Mineral. 92, 1464–1473. Bigham, J.M., 1994. Mineralogy of ochre deposits formed by sulfide oxidation. In: Blowes, D.W., Lambor, J.L. (Eds.), The Environmental Geochemistry of Sulfide Mine-wastes. Mineralogical Association of Canada, Waterloo, Ontario, Canada, pp. 103–132. Bigham, J.M., Nordstrom, D.K., 2000. Iron and aluminum hydroxysulfates from acid sulfate waters. Rev. Mineral. Geochem. 40, 351–403.

J.P. Gramp et al. / Hydrometallurgy 94 (2008) 29–33 Bigham, J.M., Carlson, L., Murad, E., 1994. Schwertmannite, a new iron oxyhydroxysulphate from Pyhäsalmi, Finland, and other localities. Mineral. Mag. 58, 641–648. Bigham, J.M., Schwertmann, U., Traina, S.J., Winland, R.L., Wolf, M., 1996. Schwertmannite and the chemical modelling of iron in acid sulfate waters. Geochim. Cosmochim. Acta 60, 2111–2121. Brophy, G.P., Sheridan, M.F., 1965. Sulfate studies IV: the jarosite–natrojarosite– hydronium jarosite solid solution series. Am. Mineral. 50, 1595–1607. Carlson, L., Bigham, J.M., Schwertmann, U., Kyek, A., Wagner, F., 2002. Scavenging of As from acid mine drainage by schwertmannite and ferrihydrite: a comparison with synthetic analogues. Environ. Sci. Technol. 36, 1712–1719. Dutrizac, J.E., Kaiman, S., 1976. Synthesis and properties of jarosite-type compounds. Can. Mineral. 14, 151–158. Fukushi, K., Sato, T., Yanase, N., Minati, J., Yamada, H., 2004. Arsenate sorption on schwertmannite. Am. Mineral. 89, 1728–1734.

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Regenspurg, S., Peiffer, S., 2005. Arsenate and chromate incorporation in schwertmannite. Appl. Geochem. 20, 1226–1239. Sasaki, K., Konno, H., 2000. Morphology of jarosite-group compounds precipitated from biologically and chemically oxidized Fe ions. Can. Mineral. 38, 45–66. Wang, H., Bigham, J.M., Tuovinen, O.H., 2006. Formation of schwertmannite and its transformation to jarosite in the presence of acidophilic iron-oxidizing microorganisms. Mater. Sci. Eng. C26, 588–592. Wang, H., Bigham, J.M., Tuovinen, O.H., 2007. Synthesis and properties of ammoniojarosites prepared with iron-oxidizing acidophilic microorganisms at 22 to 65 °C. Geochim. Cosmochim. Acta 71, 155–164.