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The nature of Schwertmannite and Jarosite mediated by two strains of Acidithiobacillus ferrooxidans with different ferrous oxidation ability
Materials Science and Engineering C 33 (2013) 2679–2685
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The nature of Schwertmannite and Jarosite mediated by two strains of Acidithiobacillus ferrooxidans with different ferrous oxidation ability Jianyu Zhu a, b, c, Min Gan a, b, Dan Zhang a, b, Yuehua Hu a, b, Liyuan Chai b, c,⁎ a b c
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Key Laboratory of Biohydrometallurgy of Ministry of Education, Changsha 410083, China School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 10 June 2012 Received in revised form 31 December 2012 Accepted 17 February 2013 Available online 24 February 2013 Keywords: Jarosite Schwertmannite Acidithiobacillus ferrooxidans Ferrous oxidation ability Cr(VI) adsorption
a b s t r a c t Jarosite and Schwertmannite are iron-oxyhydroxysulfate materials. These materials gain increasing interest in geological and metallurgical fields. Especially, for it can effectively scavenge heavy metals, less toxic ions and better biocompatibility, the application potential in environment becomes more and more intriguing. In this study, the nature of Jarosite and Schwertmannite mediates synthesized by two strains of Acidithiobacillus ferrooxidans with different ferrous oxidation ability is investigated. The precipitates are characterized by SEM, XRD, FTIR, and TG/DSC analysis. The materials are varied in color, shape, surface area, elemental composition and crystallinity. The crystallinity of precipitate produced by A. ferrooxidans 23270 with lower oxidation ability in optimized medium is significantly better than the precipitate produced by A. ferrooxidans Gf. A. ferrooxidans Gf will tend to mediate the formation of Schwertmannite with the decreasing of monovalent cation concentration in optimized medium. Cr(VI) adsorption capacity difference exists among the four materials. The adsorption efficiency of Schwertmannite is higher than Jarosite. Adsorption capacity of the materials formed by A. ferrooxidans Gf is higher than that of A. ferrooxidans 23270. Adsorption capacity decreases with the increasing of crystallinity. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Jarosite and Schwertmannite are iron-oxyhydroxysulfate materials [1,2], which are commonly found in hyperacid, sulfate rich environments associated with acid mine drainage [3]. The chemical composition of naturally occurring Jarosite and Schwertmannite is variable [1]. Jarosite is a member of the isostructural jarosite-alunite group. It has also been found on the surface of Mars based on the spectroscopy finding by the Opportunity rover [4]. Schwertmarmite has a tunnel structure akin to akaganéite (β-FeOOH) with high specific surface area, surface reactivity and poor crystallinity [5,6]. Jarosite and Schwertmannite have gained increasing interest in geological, environmental, metallurgical fields in recent years, because it can scavenge heavy metals and some toxic ion effectively by adsorption, coprecipitation or structural incorporation/substitution [7–10]. Researches show that the maximum adsorption capacity of Schwertmannite to arsenate can reach upto 113.9 mg/g [8,11]. Both of trivalent and hexavalent chromium can be removed from the aqueous solution by these ironoxyhydroxide materials [12]. Cadmium and lead would be incorporated into Jarosite during its precipitation process [13,14]. The adsorption behavior of copper to Schwertmannite in ⁎ Corresponding author at: Key Laboratory of Biohydrometallurgy of Ministry of Education, School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China. Tel.: +86 731 88836944. E-mail address: [email protected] (L. Chai). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.02.026
presence of dissolved organic matter has also been studied [7]. Schwertmannite and Jarosite possess the adsorption ability to almost all the heavy metals. The scavenging capacity of these materials is strongly correlated with their morphology, size, and crystallinity [8,12]. The study aimed at the nature of the material and its adsorption capacity has important theoretical and practical significance. Acidithiobacillus ferrooxidans has been assured to be one of the most important mesophiles for extracting metals from sulfuric ores in biohydrometallurgy industry [15,16]. It will produce acid condition with high concentration of Fe 3+ and SO42−. A. ferrooxidans will mediate the formation of iron oxyhydrosulfate in such environment [17]. In the process of forming such iron oxyhydrosulfate, the A. ferrooxidans not only oxidizes ferrous ion but also plays an important role in biomineralization [18–20]. The inorganic material will migrate, enrich, transform and form the secondary minerals under precise control or induced by the groups on bacteria surface or metabolites, so the property of the formed precipitate would be varied with the bacteria [21]. The existing research indicates that the formed iron oxyhydroxides like Schwertmannite and Jarosite would be varied with the nature change of bacterium [20,21]. For ferrous oxidation ability of A. ferrooxidans determines the Fe 3+ supplement rate, so it is a very important influence factor to the nature of the formed material. These two strains of A. ferrooxidans applied in this experiment have identical properties except the oxidation ability, so they are the appropriate choice for us to investigate the influence of oxidation ability to the material. The results of this research can provide some
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theoretical basis for us to biosynthesis better heavy metal adsorption materials. Hexavalent chromium ions widespread exist in industry environment and are more hazardous to living organisms than the trivalent form [22]. The most classical way to separate Cr(VI) is carried out by reduction method, which mainly included the reduction of hexavalent chromium to trivalent form [23]. Other harness methods including activated carbon adsorption, liquid–liquid extraction, membrane separation, ion exchange and reactive polymer methods, but the effect is unsatisfactory. Cr(VI) removal from wastewaters has been highlighted recently by the use of various sorbent materials [24]. Schwertmannite and Jarosite have become a potential adsorbent. 2. Materials and methods 2.1. Strain domestication and sample preparation Experiments were conducted with two different A. ferrooxidans, A. ferrooxidans 23270 and A. ferrooxidans Gf. A. ferrooxidans 23270 (ATCC 23270) was obtained from the American Type Culture Collection while A. ferrooxidans Gf was separated and preserved by the Key Laboratory of Biohydrometallurgy of Ministry of Education. The latter possesses higher ferrous oxidation ability than that of A. ferrooxidans 23270. First, two strains of iron-oxidizing A. ferrooxidans were enriched in optimized 9K medium which was comprised of 0.5 g/L (NH4)2SO4, 0.1 g/L KCl, 0.5 g/L K2HPO4 · 3H2O, 0.5 g/L MgS04 · 7H2O and 44.7 g/L FeSO4 · 7H2O at pH 2.5,30 °C and 180 rpm. The composition of the optimized 9K medium was determined on the basis of a series of experiments. It reduced the content of (NH4)2SO4 and removed Ca(NO3)2 from common 9 K medium. The formed ammoniojarosite would be less in this kind of medium. Its effect has been confirmed by our previous experiments. The medium used in the experiment was autoclaved for 20 min at 121 °C. Then, increasing the content of Fe2+ gradually to domesticate the bacteria until the content of FeSO4 · 7H2O reached 210 g/L. The bacteria were cultivated repeatedly until Fe 2+ oxidation rate was basically stable. In four 500 mL conical flasks, each was added with 250 mL optimized medium, two of them were inoculated with A. ferrooxidans 23270 and the other two were inoculated with A. ferrooxidans Gf. The initial pH of the medium was adjusted to 2.5 and inoculum concentration was 10% (v/v). The culture was incubated at 30 °C and shook at 180 rpm. In the third day, a bottle of A. ferrooxidans 23270 and A. ferrooxidans Gf were filtered through a 0.45 μm filter paper to remove the precipitate when the proliferation entered into the stationary phase. Then the filtrate was centrifuged at 12,000 rpm for 20 min to get the cells. Finally, the obtained cells were added into 250 mL double distilled water which only contained 52.5 g FeSO4 · 7H2O. The reaction system was cultured at initial pH 2.5, 30 °C and 180 rpm. The bacterium cannot proliferate but still possesses the ferrous oxidation ability under the condition with only FeSO4 ·7H2O. pH and Fe(II) concentration of reaction solutions were monitored every 12 h and 24 h. The precipitate was collected with 0.45 μm filter paper through filtration in the sixth day, washed twice with distilled water at pH 2.0, dried at 60 °C for 10 h. The precipitates were characterized by SEM, XRD, FTIR, and TG/DSC analyses. 2.2. Analytical methods pH values of reaction solutions were determined by using a pHS-3C model digital pH-meter. The concentration of Fe(II) in solutions was determined by using 1, 10-phenanthroline method. Scanning electron microscope (SEM) analyses were performed with a JSM-6360LV instrument operated at 15 kV accelerating voltage. Samples were mounted on Al-stubs with double-sided carbon tabs and coated with a thin layer of gold in a Pelco Model 3 Sputter 91000 coater.
XRD analysis was conducted with CuKα radiation (40 kV/250 mA) in a RINT2000 vertical goniometer which is equipped with a fixed monochrometer and a theta-compensating slit. Samples were scanned from 5° to 80° with a step increment of 0.02° and 4 s counting time. The FT-IR spectra were taken on a Nicolet Nexus670 Fourier Transform spectrometer at a resolution of 4 cm−1 using a KBr beamsplitter, a DTGS detector with KBr window, and a sample shuttle for the transmittance measurements. The background was taken on a disk made from 400 mg KBr. Thermal analyses (TG/DSC) were performed on a Netzsch STA449C apparatus. The carrier gas argon 20 was flowed at 110 mL/min. The heating rate was 15 °C/min from 35 °C to 1010 °C. 2.3. Cr(VI) adsorption experiment The Cr(VI) adsorption experiment was completed in 50 mL centrifuge tubes. 20 centrifuge tubes were divided into four groups and added with 0.1 g adsorbent respectively. Then, each group was added with 20 mL of potassium chromate solution which has Cr(VI) concentrations of 50, 100, 150, 200, and 250 mg/L. The solution pH was adjusted to 7.5. The centrifuge tubes were oscillated for 5 h on shaker at 180 rpm and 30 °C. Finally, the Cr(VI) concentration in supernatant was detected by spectrophotometry after resting for 5 min following the 1,5-diphenyl-carbazide method [25]. Each experimental treatment was conducted in duplicate. 3. Results and discussion 3.1. Changes of pH and Fe(II) concentrations It is noted in Fig. 1(a) that the pH of all the four systems increases first and then decreases continuously followed by a constant value. The rising trend of pH value observed in the early stage can attribute to the acid-consuming Fe(II) oxidation process. The subsequent remarkable dropping trend is due to the Fe(III) hydrolysis and precipitation. The pH dropping speed in the medium which was inoculated with A. ferrooxidans Gf is faster than that of A. ferrooxidans 23270. The pH in optimized 9K medium remains lower than corresponding pH in pure FeSO4 system. Fig. 1(b) shows that the Fe(II) concentration in solution decreases rapidly in the first 24 h, but the Fe(II) oxidation rate varied due to different ferrous oxidation of the A. ferrooxidans and medium composition. Ferrous oxidation rate in the system inoculated with A. ferrooxidans Gf is faster than the one inoculated with A. ferrooxidans 23270 which is consistent with the oxidation ability. The Fe(II) concentration in optimized 9K medium is lower than in pure FeSO4 system at last. This is because A. ferrooxidans cannot proliferate in this system and the lack of monovalent. 3.2. Characteristics of iron precipitates 3.2.1. SEM analysis Apparent difference exists in the surface morphology of the precipitation formed in optimized 9K medium by two strains of A. ferrooxidans with different ferrous oxidation ability. The precipitation formed by A. ferrooxidans 23270 is light yellow and dense while precipitation formed by A. ferrooxidans Gf is dark red, uniform and loose. As shown in SEM images Fig. 2a, the crystal formed by A. ferrooxidans 23270 in optimized medium is polyhedron-shaped and coarse, while the other one (Fig. 2b) exists with villous structure on the surface which is derived from polyhedral core. This kind of villous structure formed on the polyhedral core has not been reported before. As the villous structure is attached to the polyhedral core, thus it can be speculated that the formation time should be late. Studies show that the factors which affect Jarosite crystal morphology are the supply rate of Fe3+ and the composition of monovalent cation [26]. Because the monovalent cation composition of the optimized 9K medium is identical, A. ferrooxidans
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Fig. 1. Dynamic changes of pH (a) and Fe(II) concentration(a): (a) In optimized 9K medium with A. ferrooxidans 23270, (b) In optimized 9K medium with A. ferrooxidans Gf, (c) In pure FeSO4 system with A. ferrooxidans 23270, (d) In pure FeSO4 system with A. ferrooxidans Gf.
23270 and A. ferrooxidans Gf as two different strains of the same species, the latter's ferrous oxidation rate is faster than the former, so the apparent difference between the precipitations can attribute to the different oxidation ability. The groups on bacterial surface maybe varied with the ferrous oxidation ability. And these groups play an important role on the formation of precipitate nucleation, grading and the growth process, but there is no direct evidence to prove it. For there is no difference in monovalent cation composition in the system with only pure FeSO4, so the difference is not obvious. Both of the precipitates are spherical aggregates and villous. The diameter of the former (Fig. 2c) is 2 μm approximately which is bigger than the latter (Fig. 2d). EDAX spectra show that elemental content of Fe and S (SO42−) of the material formed by A. ferrooxidans Gf (Fig. 2b1, d1) is higher than that of A. ferrooxidans 23270 (Fig. 2a1, c1). This phenomenon is consistent in both Jarosite and Schwertmannite. 3.2.2. X-ray diffraction analysis The X-ray diffraction analysis is used as the most effective and common means to identify the crystal and amorphous body. The specific substance can be determined according to the characteristic peak. Analysis results demonstrate that the main component of the precipitate formed in optimized 9K medium by A. ferrooxidans 23270 and A. ferrooxidans Gf are Jarosite as well as a small amount of other phases such as ammoniojarosite and Schwertmannite through the comparative analysis using Jade-5 software (Fig. 3a, b). There are some differences on the location, number and intensity of characteristic peaks between the patterns Fig. 3a and 3b. There are also significant differences existing in the peak intensity and area, the maximum peak height of Fig 3a is close to 10,000, sharp and symmetrical, it indicates high symmetry crystal structure and good crystallinity. But the maximum peak height is only 5500 for the precipitate formed in optimized 9K medium by A. ferrooxidans Gf which demonstrates that the crystallinity of the latter is much worse. These results are consistent with SEM images. The reason leading to this phenomenon may be due to A. ferrooxidans Gf's selective utilization of H3O + and H2O instead of K + and OH− in the later of the reaction, so the villous Schwertmannite formed on the surface of the core. The XRD patterns of the precipitate which formed in the pure FeSO4 system (Fig. 3c, d) is quite alike, no single, sharp, and intense and well recognizable peaks can be searched. It shows that these two samples are amorphous body-Schwertmannite which is similar with the result reported before [27]. 3.2.3. FTIR analysis The FTIR spectra of Fig. 4a and b are basically in accordance with the standard Jarosite [28]. Main vibration bands are marked. The peak positions in the FTIR spectra of the two precipitates are nearly the same, but both have its own characteristics in peak shape and
intensity. This shows the differences in its composition. The intense absorption observed in the region 2900 to 3700 cm −1 can be attributed to O\H stretching (vOH). There are obvious differences existing in the peak shape and intensity about the O\H stretching peak, the peak shape of the former is sharp and transmissivity is only 16.7% while the latter 's peak shape is smooth and the transmissivity reaches 62.6%. This is caused by the diversity of monovalent cations which will increase hydrogen bonds energy within the structure [29]. The band observed at 1629.4 to 1630.8 cm −1 is attributing to H\O\H deformation and both of them are fairly consistent in peak position and transmissivity. It indicates that the moisture content in the sample is basically identical [30]. Three intense absorption bands are observed near 1204.5, 1086.9, 1006.2 cm−1 in sample a and 1201.8, 1084.7, 1003.6 cm−1 in sample b, the first two of these three absorption bands are due to the υ3 (doublet) vibrations of SO42− [31,32]. Powers et al. attribute the band observed at 1006.2 and 1003.6 cm−1 to O\H deformation [28]. The intensity of these three peaks between the two samples are different, the absorption intensity of the precipitate formed by A. ferrooxidans Gf is significantly greater than the sample formed by the A. ferrooxidans 23270. Several absorptions are also observed in the 400 to 1000 cm−1 region. The peak around 632.7, 633.5 cm−1 and a weak shoulder observed near 682.2 cm−1 can attribute to the v4 vibration mode of the sulfate [31]. The v4 (SO42−) doublet is distinguishable due to the decrease in symmetry of the sulfate species in the Jarosite structure. Note that the vibrations υ1 (SO42−) and υ2 (SO42−) could not be seen here, most probably due to the overlap by other nearby intense absorption [32,33]. The bands observed near 509.0, 475.2 cm−1 in sample a and 506.0, 461.1 cm−1 in sample b are vibrations of FeO6 coordination octahedral [34]. The band near 1428.1 and 1429.1 cm−1 is the adsorption band of NH4+. The presence of NH4+ absorption further confirms that the precipitate mixture contain ammoniojarosite [35]. The corresponding FTIR spectra of the Schwertmannite (Fig. 4c, d) are simpler than Jarosite (Fig. 4a, b). The difference between them is also smaller. The peak positions are almost identical and only subtle difference in intensity. The overall transmissivity of sample c is smaller than sample d. The most obvious diversity with the mixture above is that we cannot find the adsorption band of NH4+ in the spectrum for there is no ammoniojarosite. 3.2.4. Thermal analysis The detailed peaks of the thermal decomposition of the Jarosite and Schwertmannite are marked. The Jarosite mixture TG/DSC profiles Fig. 5a and b are similar, although some subtle variations in the position and intensity are observed. Both of the DSC curves show three obvious endothermic peaks, a series of weaker endothermic peaks and an exothermic peak during Jarosite thermal decomposition [36–39]. The first
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Fig. 2. SEM and EDAX of the mediated precipitates: (a, a1) Jarosite by A. ferrooxidans 23270, and (b, b1) Jarosite by A. ferrooxidans Gf in optimized 9K medium; (c, c1) Schwertmannite by A. ferrooxidans 23270, and (d, d1) Schwertmannite by A. ferrooxidans Gf in pure FeSO4 system.
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Fig. 3. XRD patterns of the mediated precipitates: (a) Jarosite by A. ferrooxidans 23270, and (b) Jarosite by A. ferrooxidans Gf in optimized 9K medium; (c) Schwertmannite by A. ferrooxidans 23270, and (d) Schwertmannite by A. ferrooxidans Gf in pure FeSO4 system.
endothermic peak is emerged at 72 °C and 82 °C respectively that can be attributing to the loss of adsorbed water [34]. The dehydration process temperature ranges from 30 °C–229 °C to 30 °C–317 °C for the two samples respectively. The second smaller endothermic peak, sample a at 392.35 and sample b at 415.69, can be assigned to dehydroxylation and ammoniojarosite's ammonium desorption, which will lead to the formation of yavapaiite-like compounds [37–39]. Both of the samples emerged an endothermic peak at almost the same position, it can be assigned to the crystallization of hematite (a-Fe2O3). This process can be summarized by equation as: KFe3 ðSO4 Þ2 ðOHÞ6 →3KFeðSO4 Þ2 þ Fe2 O3 þ 3H2 O The position and intensity of the maximum endothermic peak for the two samples are quite consistent. The maximum of peak is observed at 680.81 °C and 675.16 °C and accompanied by a dramatic weight loss process and the weight loss reaches 22.52% and 19.44% respectively. It is caused by thermal decomposition of yavapaiite-type structures: KFeðSO4 Þ2 →1=2Fe2 O3 þ 1=2 K2 SO4 þ SO3 ðg Þ There are limited thermal analyses about Schwertmannite reported in the literature so there are little references about the concrete thermal decomposition process. The peak of DSC curve of Schwertmannite (Fig. 5c, d) decomposition process is not obvious as that of Jarosite which is due to the Schwertmannite belonging to an amorphous substance and its element composition is simpler. The DSC curves differences of the two Schwertmannite samples are more obvious than the previous Jarosite but the TG curves are basically the same. This
shows that although the endothermic situation is different during thermal decomposition process, decomposition process is basically identical. Absorbed water desorps first and then followed by the dehydroxylation process. Finally, it decomposed as Fe2O3 and SO3. The quantity of heat demanded for Schwertmannite decomposition is much less than the corresponding heat of Jarosite, which indicates that Schwertmannite's thermal stability is poorer than Jarosite. 3.2.5. Cr(VI) adsorption ability Ligand exchange and anionic adsorption would be occurring between the Cr(VI) and sorbent [40]. Fig. 6 shows that the trend of Cr(VI) removal efficiency of the four adsorbent materials is identical, the removal efficiency became smaller with the increasing of Cr(VI) concentration for surface exchange sites on adsorbent saturated with the increasing of Cr(VI) concentration. The removal efficiency among the four absorbents ranked as sample d, c, b, a from high to low. The highest adsorption efficiency absorbent, Schwertmannite formed in pure FeSO4 system by A. ferrooxidans Gf, reaches 83.68% when the Cr(VI) concentration at 50 mg/L and the lowest efficiency reaches 59.745% at 250 mg/L. The adsorption efficiency of absorbent Jarosite composite material formed in optimized 9K medium by A. ferrooxidans 23270 is relatively poor, 69.7% at 50 mg/L and 30.92% at 250 mg/L. The adsorption efficiency of Schwertmannite is higher than Jarosite from the perspective of material form; the adsorbent mediated by A. ferrooxidans Gf is more efficient than the adsorbent mediated by A. ferrooxidans 23270 from the view of the strain. The heavy metal adsorption ability becomes lower with the increasing of crystallinity. SEM picture and XRD pattern show that Schwertmannite presents an amorphous state while Jarosite is well crystalline. Different crystallinity caused the differences on heavy metal
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Fig. 4. FTIR spectra of the mediated precipitates: (a), and (b); (c), and (d).
Fig. 5. The TG/DSC curve of the mediated precipitates: (a), and (b); (c), and (d).
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The adsorption efficiency of Schwertmannite is higher than Jarosite. Adsorption capacity of the materials varies inversely with its crystallinity. Adsorption capacity of the material formed by A. ferrooxidans Gf is higher than that of A. ferrooxidans 23270. Acknowledgments This research was supported by the National Natural Science Foundation of China (51174239 and 30700008), China Postdoctoral Science Foundation funded project (20090461028 and 201003526), Hunan Provincial Natural Science Foundation of China (10JJ3001) and Xiaolan Foundation (11XL07). References Fig. 6. The removal efficiency of Cr(VI) by the mediated precipitates: (a), and (b); (c), and (d).
adsorption ability. Adsorption capacity may also relate to the elemental composition of the material. The materials synthesized by A. ferrooxidans Gf have a higher elemental content of Fe and S (SO42−) possibly leading to the differences. Table 1 shows that the adsorption ability of samples d, c and b are basically consistent in Cr(VI) concentration that ranges from 50 to 150 mg/L. However, the adsorption ability of sample a is lower than the others due to its well polyhedral shape and high crystallinity. The differences become more obvious when the Cr(VI) concentration increased. Schwertmannite formed in pure FeSO4 system by A. ferrooxidans Gf has a greater adsorption capacity in relative high Cr(VI) concentration compared with others. Most of the studies existed now focused on the influence of factors to the adsorption of heavy metals such as Cu, As(III) [10,32]. But they didn't put emphasis on its adsorption ability. The study conducted by S. Regenspurg [41] revealed that the Cr concentration incorporated in Schwertmannite which was in fluenced by acid mine drainage in natural environment could reach 0.812 mg/g. Their study also showed that Cr content in Schwertmannite could reach 15.3 wt.%. The result of our study revealed the nature and Cr adsorption ability difference of the materials produced by the bacteria with the different ferrous oxidation ability. 4. Conclusions The materials formed by two strains A. ferrooxidans with different oxidation activity, A. ferrooxidans 23270 and A. ferrooxidans Gf, are varied in color, shape, surface area, elemental composition and crystallinity. Materials biosynthesized by the bacteria with higher oxidation ability also have a higher elemental content of Fe and S (SO42−). The crystallinity of the precipitate produced by A. ferrooxidans 23270 with lower oxidation ability in optimized medium is significantly better than the precipitate crystallinity which produced by A. ferrooxidans Gf. A. ferrooxidans Gf will tend to mediate the formation of Schwertmannite with the decreasing of monovalent cations concentration.
Table 1 The adsorption capacity in different Cr(VI) concentration (mg/g). Adsorbent
a b c d
The Cr(VI) concentration (mg/L) 50
100
150
200
250
6.97 8.11 8.07 8.37
11.81 13.86 13.33 14.42
13.12 19.60 20.10 19.66
13.09 22.67 22.78 25.43
15.46 25.63 27.79 29.87
(a) Jarosite by A. ferrooxidans 23270, and (b) Jarosite by A. ferrooxidans Gf in optimized 9K medium; (c) Schwertmannite by A. f 23270, and (d) Schwertmannite by A. ferrooxidans Gf in pure FeSO4 system.
[1] J.M. Bigham, U. Schwertmann, S.J. Traina, R.L. Winland, M. Wolf, Geochim. Cosmochim. Acta 60 (1996) 2111–2121. [2] J. Daoud, D. Karamanev, Miner. Eng. 19 (2006) 960–967. [3] J.M. Bigham, D.K. Nordstrom, Mineral. Geochem. 40 (2000) 351–403. [4] A. Navrotsky, F. Forray, C. Drouet, Icarus 176 (2005) 250–253. [5] J.M. Bigham, U. Schwertmann, L. Carlson, E. Murad, Geochim. Cosmochim. Acta 54 (1990) 2743–2758. [6] J.G. Hockridge, F. Jones, M. Loan, W.R. Richmond, J. Cryst. Growth 311 (2009) 3876–3882. [7] J. Jönsson, S. Sjöberg, L. Lövgren, Water Res. 40 (2006) 969–974. [8] Y. Liao, J. Liang, L.X. Zhou, Chemosphere 83 (2011) 295–301. [9] M. Egal, C. Casiot, G. Morin, Chem. Geol. 265 (2009) 432–441. [10] S. Paikaray, J. Göttlicher, S. Peiffer, Chem. Geol. 283 (2011) 134–142. [11] J.E. Dutrizac, D.J. Hardy, T.T. Chen, Hydrometallurgy 41 (1996) 269–285. [12] J.E. Dutrizac, Hydrometallurgy 73 (2004) 11–30. [13] R. Gieré, N.V. Sidenko, E.V. Lazarev, Appl. Geochem. 18 (2003) 1347–1359. [14] N.K. Lazaridis, C. Charalambous, Water Res. 39 (2005) 4385–4396. [15] A.B. Jensen, C. Webb, Process Biochem. (Oxford) 30 (1995) 225–236. [16] M.E. Hoque, O.J. Philip, Mater. Sci. Eng. C 31 (2011) 57–66. [17] H.M. Wang, J.M. Bigham, O.H. Tuovinen, Mater. Sci. Eng. C 26 (2006) 588–592. [18] J.F. Banfield, S.A. Welch, H. Zhang, T.T. Ebert, Science 289 (2000) 751–754. [19] S. Weinera, L. Addadib, Trends Biochem. Sci. 16 (1991) 252–256. [20] Y. Che, W. Huang, D. Liu, J. Chen, Z. Sun, Mater. Sci. Eng. C 26 (2006) 606–609. [21] S. Regenspurg, A. Brand, S. Peiffer, Geochim. Cosmochim. Acta 68 (2004) 1185–1197. [22] V.M. Boddu, K. Abburi, J.L. Talbot, E.D. Smith, Environ. Sci. Technol. 37 (2003) 4449–4456. [23] Z. Modrzejewska, W. Kaminski, Ind. Eng. Chem. Res. 38 (1999) 4946–4950. [24] M. Owlad, M.K. Aroua, W.A.W. Daud, S. Baroutian, Water. Air. Soil. Pollut. 200 (2009) 59–77. [25] Standard Methods for the Examination of Water and Wastewater, 19, APHA, AWWA, WPCF, Washington, DC 20005, 1995. [26] K. Sasaki, Can. Mineral. 38 (2000) 45–56. [27] Y.H. Liao, L.X. Zhou, J.R. Liang, H.X. Xiong, Mater. Sci. Eng. C 29 (2009) 211–215. [28] D.A. Powers, G.R. Rossman, H.J. Schugar, J. Solid State Chem. 13 (1975) 1–13. [29] R.J. Eagan, C.G. Bergeron, J. Am. Ceram. Soc. 55 (1972) 53–54. [30] Z.Y. Yuan, B.L. Su, Chem. Phys. Lett. 381 (2003) 710–714. [31] H. Xiong, Y. Liao, L. Zhou, Environ. Sci. Technol. 42 (2008) 8681–8686. [32] J. Jönsson, P. Persson, S. Sjöberg, L. Lövgren, Appl. Geochem. 20 (2005) 179–191. [33] C. Drouet, Geochim. Cosmochim. Acta 67 (2003) 2063–2076. [34] D. Baron, C.D. Palmer, Geochim. Cosmochim. Acta 60 (1996) 185–195. [35] P. Víctor, M.P. Mercedes, Res. Microbiol. 155 (2004) 703–709. [36] J.L. Kulp, H.H. Adler, Am. J. Sci. 248 (1950) 475–487. [37] J. Kubisz, Mineral. Pol. 2 (1971) 51–59. [38] C. Drouet, K.L. Pass, D. Baron, S. Draucker, Geochim. Cosmochim. Acta 68 (2004) 2197–2205. [39] R.L. Frost, M.L. Weier, W. Martens, J. Therm. Anal. Calorim. 82 (2005) 115–118. [40] J. Bajpai, R. Shrivastava, A.K. Bajpai, Colloids Surf. A 236 (2004) 81–90. [41] S. Regenspurg, S. Peiffer, Appl. Geochem. 20 (2005) 1226–1239.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
The nature of Schwertmannite and Jarosite mediated by two strains of Acidithiobacillus ferrooxidans with different ferrous oxidation ability Jianyu Zhu a, b, c, Min Gan a, b, Dan Zhang a, b, Yuehua Hu a, b, Liyuan Chai b, c,⁎ a b c
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Key Laboratory of Biohydrometallurgy of Ministry of Education, Changsha 410083, China School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 10 June 2012 Received in revised form 31 December 2012 Accepted 17 February 2013 Available online 24 February 2013 Keywords: Jarosite Schwertmannite Acidithiobacillus ferrooxidans Ferrous oxidation ability Cr(VI) adsorption
a b s t r a c t Jarosite and Schwertmannite are iron-oxyhydroxysulfate materials. These materials gain increasing interest in geological and metallurgical fields. Especially, for it can effectively scavenge heavy metals, less toxic ions and better biocompatibility, the application potential in environment becomes more and more intriguing. In this study, the nature of Jarosite and Schwertmannite mediates synthesized by two strains of Acidithiobacillus ferrooxidans with different ferrous oxidation ability is investigated. The precipitates are characterized by SEM, XRD, FTIR, and TG/DSC analysis. The materials are varied in color, shape, surface area, elemental composition and crystallinity. The crystallinity of precipitate produced by A. ferrooxidans 23270 with lower oxidation ability in optimized medium is significantly better than the precipitate produced by A. ferrooxidans Gf. A. ferrooxidans Gf will tend to mediate the formation of Schwertmannite with the decreasing of monovalent cation concentration in optimized medium. Cr(VI) adsorption capacity difference exists among the four materials. The adsorption efficiency of Schwertmannite is higher than Jarosite. Adsorption capacity of the materials formed by A. ferrooxidans Gf is higher than that of A. ferrooxidans 23270. Adsorption capacity decreases with the increasing of crystallinity. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Jarosite and Schwertmannite are iron-oxyhydroxysulfate materials [1,2], which are commonly found in hyperacid, sulfate rich environments associated with acid mine drainage [3]. The chemical composition of naturally occurring Jarosite and Schwertmannite is variable [1]. Jarosite is a member of the isostructural jarosite-alunite group. It has also been found on the surface of Mars based on the spectroscopy finding by the Opportunity rover [4]. Schwertmarmite has a tunnel structure akin to akaganéite (β-FeOOH) with high specific surface area, surface reactivity and poor crystallinity [5,6]. Jarosite and Schwertmannite have gained increasing interest in geological, environmental, metallurgical fields in recent years, because it can scavenge heavy metals and some toxic ion effectively by adsorption, coprecipitation or structural incorporation/substitution [7–10]. Researches show that the maximum adsorption capacity of Schwertmannite to arsenate can reach upto 113.9 mg/g [8,11]. Both of trivalent and hexavalent chromium can be removed from the aqueous solution by these ironoxyhydroxide materials [12]. Cadmium and lead would be incorporated into Jarosite during its precipitation process [13,14]. The adsorption behavior of copper to Schwertmannite in ⁎ Corresponding author at: Key Laboratory of Biohydrometallurgy of Ministry of Education, School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China. Tel.: +86 731 88836944. E-mail address: [email protected] (L. Chai). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.02.026
presence of dissolved organic matter has also been studied [7]. Schwertmannite and Jarosite possess the adsorption ability to almost all the heavy metals. The scavenging capacity of these materials is strongly correlated with their morphology, size, and crystallinity [8,12]. The study aimed at the nature of the material and its adsorption capacity has important theoretical and practical significance. Acidithiobacillus ferrooxidans has been assured to be one of the most important mesophiles for extracting metals from sulfuric ores in biohydrometallurgy industry [15,16]. It will produce acid condition with high concentration of Fe 3+ and SO42−. A. ferrooxidans will mediate the formation of iron oxyhydrosulfate in such environment [17]. In the process of forming such iron oxyhydrosulfate, the A. ferrooxidans not only oxidizes ferrous ion but also plays an important role in biomineralization [18–20]. The inorganic material will migrate, enrich, transform and form the secondary minerals under precise control or induced by the groups on bacteria surface or metabolites, so the property of the formed precipitate would be varied with the bacteria [21]. The existing research indicates that the formed iron oxyhydroxides like Schwertmannite and Jarosite would be varied with the nature change of bacterium [20,21]. For ferrous oxidation ability of A. ferrooxidans determines the Fe 3+ supplement rate, so it is a very important influence factor to the nature of the formed material. These two strains of A. ferrooxidans applied in this experiment have identical properties except the oxidation ability, so they are the appropriate choice for us to investigate the influence of oxidation ability to the material. The results of this research can provide some
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theoretical basis for us to biosynthesis better heavy metal adsorption materials. Hexavalent chromium ions widespread exist in industry environment and are more hazardous to living organisms than the trivalent form [22]. The most classical way to separate Cr(VI) is carried out by reduction method, which mainly included the reduction of hexavalent chromium to trivalent form [23]. Other harness methods including activated carbon adsorption, liquid–liquid extraction, membrane separation, ion exchange and reactive polymer methods, but the effect is unsatisfactory. Cr(VI) removal from wastewaters has been highlighted recently by the use of various sorbent materials [24]. Schwertmannite and Jarosite have become a potential adsorbent. 2. Materials and methods 2.1. Strain domestication and sample preparation Experiments were conducted with two different A. ferrooxidans, A. ferrooxidans 23270 and A. ferrooxidans Gf. A. ferrooxidans 23270 (ATCC 23270) was obtained from the American Type Culture Collection while A. ferrooxidans Gf was separated and preserved by the Key Laboratory of Biohydrometallurgy of Ministry of Education. The latter possesses higher ferrous oxidation ability than that of A. ferrooxidans 23270. First, two strains of iron-oxidizing A. ferrooxidans were enriched in optimized 9K medium which was comprised of 0.5 g/L (NH4)2SO4, 0.1 g/L KCl, 0.5 g/L K2HPO4 · 3H2O, 0.5 g/L MgS04 · 7H2O and 44.7 g/L FeSO4 · 7H2O at pH 2.5,30 °C and 180 rpm. The composition of the optimized 9K medium was determined on the basis of a series of experiments. It reduced the content of (NH4)2SO4 and removed Ca(NO3)2 from common 9 K medium. The formed ammoniojarosite would be less in this kind of medium. Its effect has been confirmed by our previous experiments. The medium used in the experiment was autoclaved for 20 min at 121 °C. Then, increasing the content of Fe2+ gradually to domesticate the bacteria until the content of FeSO4 · 7H2O reached 210 g/L. The bacteria were cultivated repeatedly until Fe 2+ oxidation rate was basically stable. In four 500 mL conical flasks, each was added with 250 mL optimized medium, two of them were inoculated with A. ferrooxidans 23270 and the other two were inoculated with A. ferrooxidans Gf. The initial pH of the medium was adjusted to 2.5 and inoculum concentration was 10% (v/v). The culture was incubated at 30 °C and shook at 180 rpm. In the third day, a bottle of A. ferrooxidans 23270 and A. ferrooxidans Gf were filtered through a 0.45 μm filter paper to remove the precipitate when the proliferation entered into the stationary phase. Then the filtrate was centrifuged at 12,000 rpm for 20 min to get the cells. Finally, the obtained cells were added into 250 mL double distilled water which only contained 52.5 g FeSO4 · 7H2O. The reaction system was cultured at initial pH 2.5, 30 °C and 180 rpm. The bacterium cannot proliferate but still possesses the ferrous oxidation ability under the condition with only FeSO4 ·7H2O. pH and Fe(II) concentration of reaction solutions were monitored every 12 h and 24 h. The precipitate was collected with 0.45 μm filter paper through filtration in the sixth day, washed twice with distilled water at pH 2.0, dried at 60 °C for 10 h. The precipitates were characterized by SEM, XRD, FTIR, and TG/DSC analyses. 2.2. Analytical methods pH values of reaction solutions were determined by using a pHS-3C model digital pH-meter. The concentration of Fe(II) in solutions was determined by using 1, 10-phenanthroline method. Scanning electron microscope (SEM) analyses were performed with a JSM-6360LV instrument operated at 15 kV accelerating voltage. Samples were mounted on Al-stubs with double-sided carbon tabs and coated with a thin layer of gold in a Pelco Model 3 Sputter 91000 coater.
XRD analysis was conducted with CuKα radiation (40 kV/250 mA) in a RINT2000 vertical goniometer which is equipped with a fixed monochrometer and a theta-compensating slit. Samples were scanned from 5° to 80° with a step increment of 0.02° and 4 s counting time. The FT-IR spectra were taken on a Nicolet Nexus670 Fourier Transform spectrometer at a resolution of 4 cm−1 using a KBr beamsplitter, a DTGS detector with KBr window, and a sample shuttle for the transmittance measurements. The background was taken on a disk made from 400 mg KBr. Thermal analyses (TG/DSC) were performed on a Netzsch STA449C apparatus. The carrier gas argon 20 was flowed at 110 mL/min. The heating rate was 15 °C/min from 35 °C to 1010 °C. 2.3. Cr(VI) adsorption experiment The Cr(VI) adsorption experiment was completed in 50 mL centrifuge tubes. 20 centrifuge tubes were divided into four groups and added with 0.1 g adsorbent respectively. Then, each group was added with 20 mL of potassium chromate solution which has Cr(VI) concentrations of 50, 100, 150, 200, and 250 mg/L. The solution pH was adjusted to 7.5. The centrifuge tubes were oscillated for 5 h on shaker at 180 rpm and 30 °C. Finally, the Cr(VI) concentration in supernatant was detected by spectrophotometry after resting for 5 min following the 1,5-diphenyl-carbazide method [25]. Each experimental treatment was conducted in duplicate. 3. Results and discussion 3.1. Changes of pH and Fe(II) concentrations It is noted in Fig. 1(a) that the pH of all the four systems increases first and then decreases continuously followed by a constant value. The rising trend of pH value observed in the early stage can attribute to the acid-consuming Fe(II) oxidation process. The subsequent remarkable dropping trend is due to the Fe(III) hydrolysis and precipitation. The pH dropping speed in the medium which was inoculated with A. ferrooxidans Gf is faster than that of A. ferrooxidans 23270. The pH in optimized 9K medium remains lower than corresponding pH in pure FeSO4 system. Fig. 1(b) shows that the Fe(II) concentration in solution decreases rapidly in the first 24 h, but the Fe(II) oxidation rate varied due to different ferrous oxidation of the A. ferrooxidans and medium composition. Ferrous oxidation rate in the system inoculated with A. ferrooxidans Gf is faster than the one inoculated with A. ferrooxidans 23270 which is consistent with the oxidation ability. The Fe(II) concentration in optimized 9K medium is lower than in pure FeSO4 system at last. This is because A. ferrooxidans cannot proliferate in this system and the lack of monovalent. 3.2. Characteristics of iron precipitates 3.2.1. SEM analysis Apparent difference exists in the surface morphology of the precipitation formed in optimized 9K medium by two strains of A. ferrooxidans with different ferrous oxidation ability. The precipitation formed by A. ferrooxidans 23270 is light yellow and dense while precipitation formed by A. ferrooxidans Gf is dark red, uniform and loose. As shown in SEM images Fig. 2a, the crystal formed by A. ferrooxidans 23270 in optimized medium is polyhedron-shaped and coarse, while the other one (Fig. 2b) exists with villous structure on the surface which is derived from polyhedral core. This kind of villous structure formed on the polyhedral core has not been reported before. As the villous structure is attached to the polyhedral core, thus it can be speculated that the formation time should be late. Studies show that the factors which affect Jarosite crystal morphology are the supply rate of Fe3+ and the composition of monovalent cation [26]. Because the monovalent cation composition of the optimized 9K medium is identical, A. ferrooxidans
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Fig. 1. Dynamic changes of pH (a) and Fe(II) concentration(a): (a) In optimized 9K medium with A. ferrooxidans 23270, (b) In optimized 9K medium with A. ferrooxidans Gf, (c) In pure FeSO4 system with A. ferrooxidans 23270, (d) In pure FeSO4 system with A. ferrooxidans Gf.
23270 and A. ferrooxidans Gf as two different strains of the same species, the latter's ferrous oxidation rate is faster than the former, so the apparent difference between the precipitations can attribute to the different oxidation ability. The groups on bacterial surface maybe varied with the ferrous oxidation ability. And these groups play an important role on the formation of precipitate nucleation, grading and the growth process, but there is no direct evidence to prove it. For there is no difference in monovalent cation composition in the system with only pure FeSO4, so the difference is not obvious. Both of the precipitates are spherical aggregates and villous. The diameter of the former (Fig. 2c) is 2 μm approximately which is bigger than the latter (Fig. 2d). EDAX spectra show that elemental content of Fe and S (SO42−) of the material formed by A. ferrooxidans Gf (Fig. 2b1, d1) is higher than that of A. ferrooxidans 23270 (Fig. 2a1, c1). This phenomenon is consistent in both Jarosite and Schwertmannite. 3.2.2. X-ray diffraction analysis The X-ray diffraction analysis is used as the most effective and common means to identify the crystal and amorphous body. The specific substance can be determined according to the characteristic peak. Analysis results demonstrate that the main component of the precipitate formed in optimized 9K medium by A. ferrooxidans 23270 and A. ferrooxidans Gf are Jarosite as well as a small amount of other phases such as ammoniojarosite and Schwertmannite through the comparative analysis using Jade-5 software (Fig. 3a, b). There are some differences on the location, number and intensity of characteristic peaks between the patterns Fig. 3a and 3b. There are also significant differences existing in the peak intensity and area, the maximum peak height of Fig 3a is close to 10,000, sharp and symmetrical, it indicates high symmetry crystal structure and good crystallinity. But the maximum peak height is only 5500 for the precipitate formed in optimized 9K medium by A. ferrooxidans Gf which demonstrates that the crystallinity of the latter is much worse. These results are consistent with SEM images. The reason leading to this phenomenon may be due to A. ferrooxidans Gf's selective utilization of H3O + and H2O instead of K + and OH− in the later of the reaction, so the villous Schwertmannite formed on the surface of the core. The XRD patterns of the precipitate which formed in the pure FeSO4 system (Fig. 3c, d) is quite alike, no single, sharp, and intense and well recognizable peaks can be searched. It shows that these two samples are amorphous body-Schwertmannite which is similar with the result reported before [27]. 3.2.3. FTIR analysis The FTIR spectra of Fig. 4a and b are basically in accordance with the standard Jarosite [28]. Main vibration bands are marked. The peak positions in the FTIR spectra of the two precipitates are nearly the same, but both have its own characteristics in peak shape and
intensity. This shows the differences in its composition. The intense absorption observed in the region 2900 to 3700 cm −1 can be attributed to O\H stretching (vOH). There are obvious differences existing in the peak shape and intensity about the O\H stretching peak, the peak shape of the former is sharp and transmissivity is only 16.7% while the latter 's peak shape is smooth and the transmissivity reaches 62.6%. This is caused by the diversity of monovalent cations which will increase hydrogen bonds energy within the structure [29]. The band observed at 1629.4 to 1630.8 cm −1 is attributing to H\O\H deformation and both of them are fairly consistent in peak position and transmissivity. It indicates that the moisture content in the sample is basically identical [30]. Three intense absorption bands are observed near 1204.5, 1086.9, 1006.2 cm−1 in sample a and 1201.8, 1084.7, 1003.6 cm−1 in sample b, the first two of these three absorption bands are due to the υ3 (doublet) vibrations of SO42− [31,32]. Powers et al. attribute the band observed at 1006.2 and 1003.6 cm−1 to O\H deformation [28]. The intensity of these three peaks between the two samples are different, the absorption intensity of the precipitate formed by A. ferrooxidans Gf is significantly greater than the sample formed by the A. ferrooxidans 23270. Several absorptions are also observed in the 400 to 1000 cm−1 region. The peak around 632.7, 633.5 cm−1 and a weak shoulder observed near 682.2 cm−1 can attribute to the v4 vibration mode of the sulfate [31]. The v4 (SO42−) doublet is distinguishable due to the decrease in symmetry of the sulfate species in the Jarosite structure. Note that the vibrations υ1 (SO42−) and υ2 (SO42−) could not be seen here, most probably due to the overlap by other nearby intense absorption [32,33]. The bands observed near 509.0, 475.2 cm−1 in sample a and 506.0, 461.1 cm−1 in sample b are vibrations of FeO6 coordination octahedral [34]. The band near 1428.1 and 1429.1 cm−1 is the adsorption band of NH4+. The presence of NH4+ absorption further confirms that the precipitate mixture contain ammoniojarosite [35]. The corresponding FTIR spectra of the Schwertmannite (Fig. 4c, d) are simpler than Jarosite (Fig. 4a, b). The difference between them is also smaller. The peak positions are almost identical and only subtle difference in intensity. The overall transmissivity of sample c is smaller than sample d. The most obvious diversity with the mixture above is that we cannot find the adsorption band of NH4+ in the spectrum for there is no ammoniojarosite. 3.2.4. Thermal analysis The detailed peaks of the thermal decomposition of the Jarosite and Schwertmannite are marked. The Jarosite mixture TG/DSC profiles Fig. 5a and b are similar, although some subtle variations in the position and intensity are observed. Both of the DSC curves show three obvious endothermic peaks, a series of weaker endothermic peaks and an exothermic peak during Jarosite thermal decomposition [36–39]. The first
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Fig. 2. SEM and EDAX of the mediated precipitates: (a, a1) Jarosite by A. ferrooxidans 23270, and (b, b1) Jarosite by A. ferrooxidans Gf in optimized 9K medium; (c, c1) Schwertmannite by A. ferrooxidans 23270, and (d, d1) Schwertmannite by A. ferrooxidans Gf in pure FeSO4 system.
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Fig. 3. XRD patterns of the mediated precipitates: (a) Jarosite by A. ferrooxidans 23270, and (b) Jarosite by A. ferrooxidans Gf in optimized 9K medium; (c) Schwertmannite by A. ferrooxidans 23270, and (d) Schwertmannite by A. ferrooxidans Gf in pure FeSO4 system.
endothermic peak is emerged at 72 °C and 82 °C respectively that can be attributing to the loss of adsorbed water [34]. The dehydration process temperature ranges from 30 °C–229 °C to 30 °C–317 °C for the two samples respectively. The second smaller endothermic peak, sample a at 392.35 and sample b at 415.69, can be assigned to dehydroxylation and ammoniojarosite's ammonium desorption, which will lead to the formation of yavapaiite-like compounds [37–39]. Both of the samples emerged an endothermic peak at almost the same position, it can be assigned to the crystallization of hematite (a-Fe2O3). This process can be summarized by equation as: KFe3 ðSO4 Þ2 ðOHÞ6 →3KFeðSO4 Þ2 þ Fe2 O3 þ 3H2 O The position and intensity of the maximum endothermic peak for the two samples are quite consistent. The maximum of peak is observed at 680.81 °C and 675.16 °C and accompanied by a dramatic weight loss process and the weight loss reaches 22.52% and 19.44% respectively. It is caused by thermal decomposition of yavapaiite-type structures: KFeðSO4 Þ2 →1=2Fe2 O3 þ 1=2 K2 SO4 þ SO3 ðg Þ There are limited thermal analyses about Schwertmannite reported in the literature so there are little references about the concrete thermal decomposition process. The peak of DSC curve of Schwertmannite (Fig. 5c, d) decomposition process is not obvious as that of Jarosite which is due to the Schwertmannite belonging to an amorphous substance and its element composition is simpler. The DSC curves differences of the two Schwertmannite samples are more obvious than the previous Jarosite but the TG curves are basically the same. This
shows that although the endothermic situation is different during thermal decomposition process, decomposition process is basically identical. Absorbed water desorps first and then followed by the dehydroxylation process. Finally, it decomposed as Fe2O3 and SO3. The quantity of heat demanded for Schwertmannite decomposition is much less than the corresponding heat of Jarosite, which indicates that Schwertmannite's thermal stability is poorer than Jarosite. 3.2.5. Cr(VI) adsorption ability Ligand exchange and anionic adsorption would be occurring between the Cr(VI) and sorbent [40]. Fig. 6 shows that the trend of Cr(VI) removal efficiency of the four adsorbent materials is identical, the removal efficiency became smaller with the increasing of Cr(VI) concentration for surface exchange sites on adsorbent saturated with the increasing of Cr(VI) concentration. The removal efficiency among the four absorbents ranked as sample d, c, b, a from high to low. The highest adsorption efficiency absorbent, Schwertmannite formed in pure FeSO4 system by A. ferrooxidans Gf, reaches 83.68% when the Cr(VI) concentration at 50 mg/L and the lowest efficiency reaches 59.745% at 250 mg/L. The adsorption efficiency of absorbent Jarosite composite material formed in optimized 9K medium by A. ferrooxidans 23270 is relatively poor, 69.7% at 50 mg/L and 30.92% at 250 mg/L. The adsorption efficiency of Schwertmannite is higher than Jarosite from the perspective of material form; the adsorbent mediated by A. ferrooxidans Gf is more efficient than the adsorbent mediated by A. ferrooxidans 23270 from the view of the strain. The heavy metal adsorption ability becomes lower with the increasing of crystallinity. SEM picture and XRD pattern show that Schwertmannite presents an amorphous state while Jarosite is well crystalline. Different crystallinity caused the differences on heavy metal
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Fig. 4. FTIR spectra of the mediated precipitates: (a), and (b); (c), and (d).
Fig. 5. The TG/DSC curve of the mediated precipitates: (a), and (b); (c), and (d).
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The adsorption efficiency of Schwertmannite is higher than Jarosite. Adsorption capacity of the materials varies inversely with its crystallinity. Adsorption capacity of the material formed by A. ferrooxidans Gf is higher than that of A. ferrooxidans 23270. Acknowledgments This research was supported by the National Natural Science Foundation of China (51174239 and 30700008), China Postdoctoral Science Foundation funded project (20090461028 and 201003526), Hunan Provincial Natural Science Foundation of China (10JJ3001) and Xiaolan Foundation (11XL07). References Fig. 6. The removal efficiency of Cr(VI) by the mediated precipitates: (a), and (b); (c), and (d).
adsorption ability. Adsorption capacity may also relate to the elemental composition of the material. The materials synthesized by A. ferrooxidans Gf have a higher elemental content of Fe and S (SO42−) possibly leading to the differences. Table 1 shows that the adsorption ability of samples d, c and b are basically consistent in Cr(VI) concentration that ranges from 50 to 150 mg/L. However, the adsorption ability of sample a is lower than the others due to its well polyhedral shape and high crystallinity. The differences become more obvious when the Cr(VI) concentration increased. Schwertmannite formed in pure FeSO4 system by A. ferrooxidans Gf has a greater adsorption capacity in relative high Cr(VI) concentration compared with others. Most of the studies existed now focused on the influence of factors to the adsorption of heavy metals such as Cu, As(III) [10,32]. But they didn't put emphasis on its adsorption ability. The study conducted by S. Regenspurg [41] revealed that the Cr concentration incorporated in Schwertmannite which was in fluenced by acid mine drainage in natural environment could reach 0.812 mg/g. Their study also showed that Cr content in Schwertmannite could reach 15.3 wt.%. The result of our study revealed the nature and Cr adsorption ability difference of the materials produced by the bacteria with the different ferrous oxidation ability. 4. Conclusions The materials formed by two strains A. ferrooxidans with different oxidation activity, A. ferrooxidans 23270 and A. ferrooxidans Gf, are varied in color, shape, surface area, elemental composition and crystallinity. Materials biosynthesized by the bacteria with higher oxidation ability also have a higher elemental content of Fe and S (SO42−). The crystallinity of the precipitate produced by A. ferrooxidans 23270 with lower oxidation ability in optimized medium is significantly better than the precipitate crystallinity which produced by A. ferrooxidans Gf. A. ferrooxidans Gf will tend to mediate the formation of Schwertmannite with the decreasing of monovalent cations concentration.
Table 1 The adsorption capacity in different Cr(VI) concentration (mg/g). Adsorbent
a b c d
The Cr(VI) concentration (mg/L) 50
100
150
200
250
6.97 8.11 8.07 8.37
11.81 13.86 13.33 14.42
13.12 19.60 20.10 19.66
13.09 22.67 22.78 25.43
15.46 25.63 27.79 29.87
(a) Jarosite by A. ferrooxidans 23270, and (b) Jarosite by A. ferrooxidans Gf in optimized 9K medium; (c) Schwertmannite by A. f 23270, and (d) Schwertmannite by A. ferrooxidans Gf in pure FeSO4 system.
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