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Catalytic effect of Ag+ on arsenic bioleaching from orpiment (As2S3) in batch tests with Acidithiobacillus ferrooxidans and Sulfobacillus sibiricus
Journal of Hazardous Materials 283 (2015) 117–122
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Catalytic effect of Ag+ on arsenic bioleaching from orpiment (As2 S3 ) in batch tests with Acidithiobacillus ferrooxidans and Sulfobacillus sibiricus Guangji Zhang a , Xingwu Chao a,b , Pei Guo a , Junya Cao a , Chao Yang a,∗ a National Key Laboratory of Biochemical Engineering and Key Laboratory of Green Process and Engineering, Institute of Processing Engineering, Chinese Academy of Sciences, Beijing 100190, China b China University of Petroleum-Beijing, Beijing 102249, China
h i g h l i g h t s • • • •
Orpiment was leached by Fe3+ under acidic condition. The leaching rate was significantly improved in the presence of Ag+ . Both At. ferrooxidans and S. sibiricus survived in the pulp containing orpiment. At. ferrooxidans and S. sibiricus showed different performance in the bioleaching.
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Article history: Received 7 July 2014 Received in revised form 26 August 2014 Accepted 14 September 2014 Available online 22 September 2014 Keywords: Orpiment Bioleaching Ag+ Acidithiobacillus ferrooxidans Sulfobacillus sibiricus
a b s t r a c t Orpiment is one of the major arsenic sulfide minerals which commonly occurs in the gold mine environment and the weathering of this mineral can lead to the contamination of arsenic. In this study, chemical leaching experiments using 10 g/L Fe3+ at 35 ◦ C and 50 ◦ C were carried out and the results show that orpiment can be leached by Fe3+ and the leaching rate of orpiment was significantly enhanced in the presence of Ag+ . The bioleaching experiments with mesophilic bacteria Acidithiobacillus ferrooxidans and moderate thermophilic bacteria Sulfobacillus sibiricus were carried out, showing that these two strains can survive in the mineral pulp and oxidize Fe2+ to regenerate Fe3+ . Based on above results, it is believed that the leaching action of the acidic mining drainage by some bacteria can lead to the release of arsenic from orpiment. Different performances of At. ferrooxidans and S. sibiricus in the tests suggest they follow two different mechanisms and this point of view is further confirmed based on analyses of the composition and morphology of the mineral residue by SEM and EDS. © 2014 Elsevier B.V. All rights reserved.
1. Introduction As arsenic is a very dangerous carcinogen, environmental regulators are adopting a more stringent attitude to arsenic exposure, and release of arsenic causes worldwide concerns increasingly. Arsenic ranks the 20th among the most abundant elements in the earth crust and is associated with a variety of ores including those of copper, gold, nickel, lead and zinc [1–3]. Arsenic containing minerals are commonly found in the mine tailings and exist primarily as sulfide. The weathering and oxidation of these arsenic sulfide minerals in ambient conditions can lead to the release of arsenic and other heavy metals into the environment. Orpiment (As2 S3 ) is one of the major arsenic sulfide minerals, which is associated
∗ Corresponding author. Tel.: +86 10 62554558; fax: +86 10 82544928. E-mail address: [email protected] (C. Yang). http://dx.doi.org/10.1016/j.jhazmat.2014.09.022 0304-3894/© 2014 Elsevier B.V. All rights reserved.
with some gold bearing minerals and can be found in many gold mine sites worldwide. Long and Dixon [4] studied the pressure oxidation kinetics of orpiment and found that the oxidation rates were significantly affected by changes in temperature and particle size, but were relatively insensitive to oxygen partial pressure. Lengke and Tempel [5] investigated the oxidation of orpiment by dissolved oxygen at 25–40 ◦ C with varying pH from 6.8 to 8.2, and found that the oxidation rate increased with the increase of pH. Therefore, they suggested that the addition of limestone into AMD (acid mine drainage) may not retard the release of arsenic because the limestone increased the solution pH [6]. Despite these pioneering works, more studies have to be carried out for understanding the weathering of orpiment. Actually, in the mining environment the weathering of sulfide minerals often occurs at lower pH condition and AMD is often found to have elevated concentrations of arsenic. Microorganism plays an important role in this process [2,7]. Almost all the sulfide can be bioleached with the presence of iron and sulfur oxidizing bacteria, and these
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Fig. 1. XRD pattern of natural orpiment.
processes have been studied intensively in past decades [8–12]. But few studies are involved with the bioleaching of orpiment probably due to its lack of industrial significance. However, the bioleaching behavior of this mineral as an arsenic sulfide is critical for assessment of its environmental impact. Sand and collaborators [9] proposed that orpiment can be dissolved by ferric ion and proton produced by the bacteria, and the main intermediates would be polysulfides and elemental sulfur, but they did not show experimental evidence. Our previous work showed that orpiment can be leached by Fe3+ but the leaching rate was very low [13]. However, our recent work showed that the presence of some ions can increase the bioleaching rate of realgar (As2 S2 ) [14]. The accelerating effect of some heavy metal ions on the bioleaching of sulfide minerals has been investigated by many researchers and silver is of special interest for its effectiveness [15–18]. In the nature, silver minerals are commonly associated with gold, and therefore, they often occur together with orpiment as well. These minerals co-existing with orpiment in the tailings piles can be oxidized by some microorganisms in the presence of dissolved oxygen and release Fe3+ and Ag+ into the water. These ions can accelerate greatly the bioleaching of orpiment. In this study the bioleaching behavior of orpiment is investigated with addition of ferric or ferrous ions with two bacterial strains, and the effect of Ag+ on the dissolution of orpiment is studied. The results will help hopefully in understanding the process of orpiment bioleaching, which in turn provide a sound basis for further development of technology for suppressing the release of arsenic from the arsenic-containing solid waste.
Fig. 2. Evolutions of orpiment leaching rate and Eh in the chemical leaching: (a) leaching rate; (b) Eh.
temperature is between 28 and 37 ◦ C. The moderately thermophilic Sulfobacillus sibiricus is from the German Collection of Microorganisms and Cell Cultures (DSMZ No.: 17363) and its optimal growth temperature is 50 ◦ C. All strains were cultured on the Leathen medium containing 0.15 g/L (NH4 )2 SO4 , 0.05 g/L KCl, 0.05 g/L K2 HPO4 , 0.5 g/L MgSO4 ·7H2 O and 0.01 g/L Ca(NO3 )2 [19]. Before the experiments, the strains were grown on a ferrous sulfate containing medium. Yeast extract (0.2 g/L) was particularly added to support the growth of S. sibiricus. The initial pH of At. ferrooxidans culture is 2.0 and the value for S. sibiricus is 1.5 in all experiments.
2. Materials and methods 2.3. Experimental procedure 2.1. Mineral The mineral used in this study is natural orpiment, which is originally yellow blocks from Hunan province, China and provided by the National Museum of Geology, Beijing, China. Chemical analysis of the sample reveals that this mineral contains 58.24% As. X-ray diffraction analysis shows that the most important mineral phase is orpiment (Fig. 1). The mineral was finely grounded and sieved to obtain the size fraction of 49–74 m. 2.2. Strains and media Two stains are used in this study. Acidithiobacillus ferrooxidans is a mesophilic bacterium, provided by Institute of Microbiology, Chinese Academy of Sciences, Beijing. The optimal growth
2.3.1. Chemical leaching experiments The experiments were carried out in a rotary shaker at 140 rpm using 250 mL Erlenmeyer flasks containing 100 mL iron-free Leathen medium and 0.5 g orpiment, namely, the pulp density is 0.5% (w/v). 3.57 g Fe2 (SO4 )3 corresponding to 10 g/L Fe3+ was added as the oxidizing agent. Silver was added in the form of Ag2 SO4 . The initial pulp pH value is 1.6 after Fe2 (SO4 )3 was dissolved in the medium and was not adjusted during the leaching experiments. 1.0 mL supernatant was taken from each flask as sample every two days for analyzing the concentration of As. pH and the redox potential (Eh) were periodically measured. The water loss by evaporation was compensated by adding deionized water. In the control experiments free of Ag, all the conditions and procedure were kept the same as the chemical leaching, but no Ag2 SO4
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Fig. 3. XRD patterns of the residue after the chemical leaching in presence of 0.05 g/L Ag+ : (a) chemical leaching at 35 ◦ C; (b) chemical leaching at 50 ◦ C.
Fig. 4. Evolutions of orpiment bioleaching fraction and Eh in the tests with At. ferrooxidans: (a) leaching rate; (b) Eh.
was added. In the iron free experiments, the initial pH of the pulp was adjusted to 1.6, but no Fe2 (SO4 )3 and Ag2 SO4 were added.
by an inductively coupled plasma atomic emission spectroscope (ICP-OES, Optima 5300DV). At the end of the experiment, the solution was filtered and the solids were rinsed several times and dried at ambient conditions. The solid samples were analyzed by X-ray diffraction (XRD, X’Pert PRO), scanning electron microscopy (SEM, JSM-7001F) and energy dispersive X-ray spectroscopy (EDS, X-MAX150).
2.3.2. Bioleaching experiments The strains of At. ferrooxidans and S.sibiricus were grown in the respective ferrous sulfate containing medium until the cell density reached 108 cells/mL when harvested by filtration. The cells were resuspended in 100 mL fresh Leathen medium as the inoculum. 10 mL (10% in volume ratio) of this inoculum was added into the 250 mL Erlenmeyer flask containing 90 mL medium and 5 g FeSO4 ·7H2 O (corresponding to 10 g/L Fe2+ ), and the yeast extract (0.2 g/L) was particularly added for the growth of S. sibiricus. The initial pH of culture was adjusted with H2 SO4 of 1 mol/L. The flasks were inoculated in the rotary shaker at 140 rpm at the temperature of 35 ◦ C for At. ferrooxidans and 50 ◦ C for S. sibiricus. After 2 days, in the exponential growth phase of the bacteria, 0.5 g orpiment was added into each flask and the initial pH was adjusted again to 2.0 for At. ferrooxidans and 1.5 for S. sibiricus. 0.0016 g Ag2 SO4 (0.01 g/L) was added into the flasks for testing the catalytic effects of the added ions. The bioleaching experiments were concurrently started to ensure identical cell counts and solution chemistry. In the control experiments, no Ag2 SO4 was added. The bioleaching experiments were duplicated. 2.3.3. Analysis methods The pH value was measured with a pH meter being calibrated using standard pH 4.01 and 6.86 buffers. The redox potential (Eh) was determined using a Pt electrode with reference to the Ag/AgCl electrode (the same reference for all potential measurements in this work). The concentration of dissolved arsenic was analyzed
3. Results and discussion 3.1. Fe3+ chemical leaching of orpiment As shown in Fig. 2, the leaching fractions of arsenic in the presence of Fe3+ reached 15% and 21% after 7 days at 35 ◦ C and 50 ◦ C, respectively. Meanwhile, the Eh decreased monotonously. On the other hand, both at 35 ◦ C and 50 ◦ C, no more than 4% arsenic were leached and Eh increased very moderately in the control tests without Fe3+ added. The results show that orpiment can be dissolved by Fe3+ , suggesting that the orpiment in the mining tailings can be leached by Fe3+ which was produced in the biooxidation of iron containing mineral. But in comparison with other arsenic containing sulfide minerals such as arsenopyrite and enargite [20], the leaching rate is rather low. It should be noted that at an elevated temperature (50 ◦ C) a higher leaching rate was achieved. Silver has been proven to be effective in increasing the bioleaching rate of chalcopyrite in many researches [15,17,21], but few studies are involved with the catalytic effect of Ag+ on the bioleaching of other sulfide minerals [16,18]. Ag+ can react with sulfide MeS through a cationic interchange reaction as follows: MeS + Ag+ → Ag2 S + Me2+
(1)
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Fig. 6. Evolutions of orpiment bioleaching fraction and Eh in the tests with S. sibiricus: (a) leaching rate; (b) Eh.
oxidizing rate of Ag2 S speeded up at higher temperature, but part of dissolved arsenic can co-precipitate with jarosite and resulted in the decline of the leached arsenic in the solution. Fig. 5. SEM images of mineral residues after bioleaching with At. ferrooxidans: (a) control test; (b) test with 0.01 g/L Ag+ .
The formed Ag2 S is oxidized in the presence of an oxidizing agent such as Fe3+ and Ag+ is then regenerated: Ag2 S + 2Fe3+ → 2Ag+ + 2Fe2+ + S0
(2)
Reaction (1) is independent of bacteria actions. The role of bacteria in this process is oxidizing Fe2+ to Fe3+ and then generating the oxidizing agent by Reaction (2). In the bioleaching of chalcopyrite, the presence of Ag2 S could affect the conductivity of the sulfur produced or its morphology, and then eliminate its passivation [22]. The cationic interchange reaction between Ag+ and orpiment can occur theoretically, but this is seldom addressed in relevant researches. In our study, 0.01 g/L Ag+ was added in the chemical leaching media with 10 g/L Fe3+ . It can be found that in comparison with the control test without Ag+ , the leaching rate was significantly enhanced. XRD patterns (Fig. 3) show that elemental sulfur and trace of Ag2 S were produced after leaching at 35 ◦ C. This fact suggests that the leaching of orpiment complied with the mechanism described as Reactions (1) and (2). But due to the hydrolysis of dissolved arsenic, it was found that the pH value decreased in all the tests. Thus, the overall reaction can be described as Reaction (3): 6Fe3+ + As2 S3 + 6H2 O → 2H3 AsO3 + 6Fe2+ + 6H+ + 3S0
(3)
At 50 ◦ C, the XRD pattern shows that the peaks of Ag2 S disappear and jarosite can be detected instead. This result implies that the
3.2. Bioleaching with At. ferrooxidans in presence of Ag+ A, ferrooxidans is the most common microorganism which is related to the weathering of sulfide minerals. It is almost ubiquitous in mine environment and is thought to be responsible for the formation of acid mine drainage and the release of a variety of heavy metals including arsenic [7]. This bacterium can oxidize Fe2+ to Fe3+ and the present chemical leaching results indicate that orpiment can be leached by Fe3+ . Therefore, it is very possible that the bioleaching of orpiment occurs commonly in the tailings piles. In this study, the bioleaching experiments were carried out to test the leaching behavior with At. ferrooxidans. Fig. 4 shows that similar to the chemical leaching, the bioleaching rate was significantly improved in the presence of 0.01 g/L Ag+ , while the Eh variation shows some different features from chemical leaching. Eh of the test medium with 0.01 g/L Ag+ added decreased rapidly and then stabilized gradually. In the control test without Ag+ , Eh fluctuated slightly in the first stage and increased evidently after 5 days. Since the value of Eh in the bioleaching culture was determined by the couple of Fe3+ /Fe2+ , these results show that in the later stage of bioleaching, the production of Fe3+ was faster than the consumption, indicating that the bacteria worked well. It is worth being noted that the cell density decreased rapidly from around 8.0 × 106 cells/mL to 2.5 × 104 cells/mL after the addition of the mineral. It is very possible that the free bacteria were absorbed on the surface of mineral particles. The attachment of At. ferrooxidans to ore particles in the bioleaching process was confirmed in many researches [9,10,23,24]. Some researchers
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ferrooxidans, but the evolution of Eh in these tests is very different: the initial Eh in the tests with S. sibiricus was much higher and decreased slowly or increased slightly at the beginning of the tests; moreover during the last period, Eh decreased rapidly and it never rebounded like in the test with At. ferrooxidans. On the other hand, it was observed under microscope that in all the tests with S. sibiricus the density of the free cells in the solution did not decrease evidently after the addition of orpiment as occurred in the tests with At. ferrooxidans. These results show that S. sibiricus was not adsorbed on the mineral surface as At. ferrooxidans did. In the last 3 days of the test the cell density decreased to 6.0 × 104 cells/mL accompanied with the decline of Eh. Some researchers supposed that it was due to the consumption of yeast extract rather than the toxicity of metal ions in the solution [27]. The SEM images in Fig. 7 show that no layer covers the surface of the mineral residue, but the mineral surface bioleached with Ag+ added was severely etched compared to that from the control test. The different bioleaching behaviors of S. sibiricus and At. ferrooxidans suggest that the two strains acted differently in the bioleaching of orpiment. The similar results were reported in our early work [28] and by other researchers [29]. Unlike in the chemical leaching with Fe3+ , in the bioleaching the microorganisms can oxidize Fe2+ to Fe3+ continuously and sustain the leaching process of orpiment for a long time. Our experimental results show that the bacteria can grow in the pulp containing orpiment and leach the mineral in the presence of Fe2+ or Fe3+ . Therefore, it is possible that the bioleaching of orpiment can occur in the tailings pool and the released arsenic can form the continual contamination.
4. Conclusions
Fig. 7. SEM images of mineral residues after bioleaching with S. sibiricus: (a) control test; (b) test with 0.01 g/L Ag+ .
proposed that the attached cells can make mineral components solubilized and these components would be further oxidized by the suspended bacteria in the solution [25,26]. The SEM images (Fig. 5) show that in the test with added Ag+ , the residual mineral is covered by a porous layer and the EDS analysis shows that the composition of the layer is probably elemental sulfur. On the other hand, the SEM images show that there are some crystalline particles adhered on the mineral surface in the control test. EDS results (not included here) show that the iron appears in high levels in these crystalline particles, so it is presumed that these crystals are jarosite. 3.3. Bioleaching with S. sibiricus in presence of Ag+ The temperature inside the tailings pool and bioleaching heap can reach up to 50–60 ◦ C due to the heat from oxidation of sulfide minerals and the mesophilic bacteria such as At. ferrooxidans cannot survive under these conditions [11]. The moderate thermophilic microorganisms can work at higher temperature and have a kinetic advantage over the mesophilic bacteria [21]. Fig. 6 shows that the leached arsenic in the test with Ag+ added achieved around 60% in 9 days, while in the bioleaching test without Ag+ the rate was only 10% when the moderate thermopile S. sibiricus was used. This result is similar to that of the test with At.
In this study the bioleaching of orpiment was investigated. The results of chemical leaching experiments using Fe3+ show that orpiment can be leached by Fe3+ and the leaching can be significantly enhanced in presence of Ag+ . Bioleaching experiments with mesophilic bacteria At. ferrooxidans and moderate thermophilic bacteria S. sibiricus were carried out to test the effect of the microorganisms on the leaching of orpiment. It was found that both At. ferrooxidans and S. sibiricus can survive in the pulp containing orpiment and oxidize Fe2+ to Fe3+ continually, suggesting that the microbial activities will lead to the dissolution of orpiment in the mine environment. The microscopic examinations show that At. ferrooxidans can adhere on the surface of orpiment particles resulting in the decrease of cell density in the solution. In contrast, in the tests with S. sibiricus no microbial adsorption was observed. The SEM analysis shows that the residuals from the tests with At. ferrooxidans in presence of Ag+ were covered by elemental sulfur, while no elemental sulfur was detected on the residue surface from the test with S. sibiricus. The performance difference between At. ferrooxidans and S. sibiricus implies that different bioleaching mechanisms were involved in the bioleaching of orpiment. In the mining environment, AMD is the main source of Fe3+ and heavy metal ions, so avoiding the contact of orpiment containing mineral with AMD is necessary for suppressing the release of arsenic. But some researchers found that the oxidation rate of orpiment increased with the increase of pH. Therefore, the addition of limestone for prevention of AMD may not retard the release of arsenic because the limestone increased the solution pH [6]. So it might be a better alternative that covers the orpiment bearing solid waste with waterproof material to stop the contact between AMD and the orpiment, and if possible we do not mix the mineral and iron containing minerals in the tailing pool. Although the possibility of the release of arsenic due to the activity of bacteria and heavy metals were confirmed, the further
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study on the bioleaching mechanisms of orpiment is needed. On the other hand, in this study all the experiments were carried out in the shake flasks and the conditions are very different from the real mine environment. In the real environment, the interactions between Fe3+ and arsenic can be more complicated. For example, the arsenic in the solution can be adsorbed and co-precipitated with ferrihydrite. So the investigation involved the action of Fe3+ should also be carried out under the conditions closer to the real environment. Acknowledgements The authors acknowledge 973 Program (2010CB630904), the National Natural Science Foundation of China (51174185, 21427814), 863 Key Project (2012AA061503), the Major National Scientific Instrument Development Project (21427814) and the National Science Fund for Distinguished Young Scholars (21025627). References [1] M. Leist, R.J. Casey, D. Caridi, The management of arsenic wastes: problems and prospects, J. Hazard. Mater. B 76 (2000) 125–138. [2] A.B.M.R. Islam, J.P. Maity, J. Bundschuh, C. Chen, B.K. Bhowmik, K. Tazaki, Arsenic mineral dissolution and possible mobilization in mineral– microbe–groundwater environment, J. Hazard. Mater. 262 (2013) 989–996. [3] B.K. Mandal, K.T. Suzuki, Arsenic round the world: a review, Talanta 58 (2002) 201–235. [4] H. Long, D.G. Dixon, Pressure oxidation kinetics of orpiment (As2 S3 ) in sulfuric acid, Hydrometallurgy 85 (2007) 95–102. [5] M.F. Lengke, R.N. Tempel, Reaction rates of natural orpiment oxidation at 25 to 40 ◦ C and pH 6.8 to 8.2 and comparison with amorphous As2 S3 oxidation, Geochim. Cosmochim. Acta 66 (18) (2002) 3281–3291. [6] M.F. Lengke, R.N. Tempel, Geochemical modeling of arsenic sulfide oxidation kinetics in a mining environment, Geochim. Cosmochim. Acta 69 (2) (2005) 341–356. [7] H. Cheng, Y. Hu, J. Luo, B. Xu, J. Zhao, Geochemical processes controlling fate and transport of arsenic in acid mine drainage (AMD) and natural systems, J. Hazard. Mater. 165 (2009) 13–26. [8] K. Sasaki, M. Tsunekawa, T. Ohtsuka, H. Konno, The role of sulfur-oxidizing bacteria Thiobacillus thiooxidans in pyrite weathering, Colloids Surf. A: Physicochem. Eng. Aspects 133 (1998) 269–278. [9] W. Sand, T. Gehrke, P. Jozsa, A. Schippers, (Bio)chemistry of bacterial leaching – direct vs. indirect bioleaching, Hydrometallurgy 59 (2001) 159–175.
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Catalytic effect of Ag+ on arsenic bioleaching from orpiment (As2 S3 ) in batch tests with Acidithiobacillus ferrooxidans and Sulfobacillus sibiricus Guangji Zhang a , Xingwu Chao a,b , Pei Guo a , Junya Cao a , Chao Yang a,∗ a National Key Laboratory of Biochemical Engineering and Key Laboratory of Green Process and Engineering, Institute of Processing Engineering, Chinese Academy of Sciences, Beijing 100190, China b China University of Petroleum-Beijing, Beijing 102249, China
h i g h l i g h t s • • • •
Orpiment was leached by Fe3+ under acidic condition. The leaching rate was significantly improved in the presence of Ag+ . Both At. ferrooxidans and S. sibiricus survived in the pulp containing orpiment. At. ferrooxidans and S. sibiricus showed different performance in the bioleaching.
a r t i c l e
i n f o
Article history: Received 7 July 2014 Received in revised form 26 August 2014 Accepted 14 September 2014 Available online 22 September 2014 Keywords: Orpiment Bioleaching Ag+ Acidithiobacillus ferrooxidans Sulfobacillus sibiricus
a b s t r a c t Orpiment is one of the major arsenic sulfide minerals which commonly occurs in the gold mine environment and the weathering of this mineral can lead to the contamination of arsenic. In this study, chemical leaching experiments using 10 g/L Fe3+ at 35 ◦ C and 50 ◦ C were carried out and the results show that orpiment can be leached by Fe3+ and the leaching rate of orpiment was significantly enhanced in the presence of Ag+ . The bioleaching experiments with mesophilic bacteria Acidithiobacillus ferrooxidans and moderate thermophilic bacteria Sulfobacillus sibiricus were carried out, showing that these two strains can survive in the mineral pulp and oxidize Fe2+ to regenerate Fe3+ . Based on above results, it is believed that the leaching action of the acidic mining drainage by some bacteria can lead to the release of arsenic from orpiment. Different performances of At. ferrooxidans and S. sibiricus in the tests suggest they follow two different mechanisms and this point of view is further confirmed based on analyses of the composition and morphology of the mineral residue by SEM and EDS. © 2014 Elsevier B.V. All rights reserved.
1. Introduction As arsenic is a very dangerous carcinogen, environmental regulators are adopting a more stringent attitude to arsenic exposure, and release of arsenic causes worldwide concerns increasingly. Arsenic ranks the 20th among the most abundant elements in the earth crust and is associated with a variety of ores including those of copper, gold, nickel, lead and zinc [1–3]. Arsenic containing minerals are commonly found in the mine tailings and exist primarily as sulfide. The weathering and oxidation of these arsenic sulfide minerals in ambient conditions can lead to the release of arsenic and other heavy metals into the environment. Orpiment (As2 S3 ) is one of the major arsenic sulfide minerals, which is associated
∗ Corresponding author. Tel.: +86 10 62554558; fax: +86 10 82544928. E-mail address: [email protected] (C. Yang). http://dx.doi.org/10.1016/j.jhazmat.2014.09.022 0304-3894/© 2014 Elsevier B.V. All rights reserved.
with some gold bearing minerals and can be found in many gold mine sites worldwide. Long and Dixon [4] studied the pressure oxidation kinetics of orpiment and found that the oxidation rates were significantly affected by changes in temperature and particle size, but were relatively insensitive to oxygen partial pressure. Lengke and Tempel [5] investigated the oxidation of orpiment by dissolved oxygen at 25–40 ◦ C with varying pH from 6.8 to 8.2, and found that the oxidation rate increased with the increase of pH. Therefore, they suggested that the addition of limestone into AMD (acid mine drainage) may not retard the release of arsenic because the limestone increased the solution pH [6]. Despite these pioneering works, more studies have to be carried out for understanding the weathering of orpiment. Actually, in the mining environment the weathering of sulfide minerals often occurs at lower pH condition and AMD is often found to have elevated concentrations of arsenic. Microorganism plays an important role in this process [2,7]. Almost all the sulfide can be bioleached with the presence of iron and sulfur oxidizing bacteria, and these
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Fig. 1. XRD pattern of natural orpiment.
processes have been studied intensively in past decades [8–12]. But few studies are involved with the bioleaching of orpiment probably due to its lack of industrial significance. However, the bioleaching behavior of this mineral as an arsenic sulfide is critical for assessment of its environmental impact. Sand and collaborators [9] proposed that orpiment can be dissolved by ferric ion and proton produced by the bacteria, and the main intermediates would be polysulfides and elemental sulfur, but they did not show experimental evidence. Our previous work showed that orpiment can be leached by Fe3+ but the leaching rate was very low [13]. However, our recent work showed that the presence of some ions can increase the bioleaching rate of realgar (As2 S2 ) [14]. The accelerating effect of some heavy metal ions on the bioleaching of sulfide minerals has been investigated by many researchers and silver is of special interest for its effectiveness [15–18]. In the nature, silver minerals are commonly associated with gold, and therefore, they often occur together with orpiment as well. These minerals co-existing with orpiment in the tailings piles can be oxidized by some microorganisms in the presence of dissolved oxygen and release Fe3+ and Ag+ into the water. These ions can accelerate greatly the bioleaching of orpiment. In this study the bioleaching behavior of orpiment is investigated with addition of ferric or ferrous ions with two bacterial strains, and the effect of Ag+ on the dissolution of orpiment is studied. The results will help hopefully in understanding the process of orpiment bioleaching, which in turn provide a sound basis for further development of technology for suppressing the release of arsenic from the arsenic-containing solid waste.
Fig. 2. Evolutions of orpiment leaching rate and Eh in the chemical leaching: (a) leaching rate; (b) Eh.
temperature is between 28 and 37 ◦ C. The moderately thermophilic Sulfobacillus sibiricus is from the German Collection of Microorganisms and Cell Cultures (DSMZ No.: 17363) and its optimal growth temperature is 50 ◦ C. All strains were cultured on the Leathen medium containing 0.15 g/L (NH4 )2 SO4 , 0.05 g/L KCl, 0.05 g/L K2 HPO4 , 0.5 g/L MgSO4 ·7H2 O and 0.01 g/L Ca(NO3 )2 [19]. Before the experiments, the strains were grown on a ferrous sulfate containing medium. Yeast extract (0.2 g/L) was particularly added to support the growth of S. sibiricus. The initial pH of At. ferrooxidans culture is 2.0 and the value for S. sibiricus is 1.5 in all experiments.
2. Materials and methods 2.3. Experimental procedure 2.1. Mineral The mineral used in this study is natural orpiment, which is originally yellow blocks from Hunan province, China and provided by the National Museum of Geology, Beijing, China. Chemical analysis of the sample reveals that this mineral contains 58.24% As. X-ray diffraction analysis shows that the most important mineral phase is orpiment (Fig. 1). The mineral was finely grounded and sieved to obtain the size fraction of 49–74 m. 2.2. Strains and media Two stains are used in this study. Acidithiobacillus ferrooxidans is a mesophilic bacterium, provided by Institute of Microbiology, Chinese Academy of Sciences, Beijing. The optimal growth
2.3.1. Chemical leaching experiments The experiments were carried out in a rotary shaker at 140 rpm using 250 mL Erlenmeyer flasks containing 100 mL iron-free Leathen medium and 0.5 g orpiment, namely, the pulp density is 0.5% (w/v). 3.57 g Fe2 (SO4 )3 corresponding to 10 g/L Fe3+ was added as the oxidizing agent. Silver was added in the form of Ag2 SO4 . The initial pulp pH value is 1.6 after Fe2 (SO4 )3 was dissolved in the medium and was not adjusted during the leaching experiments. 1.0 mL supernatant was taken from each flask as sample every two days for analyzing the concentration of As. pH and the redox potential (Eh) were periodically measured. The water loss by evaporation was compensated by adding deionized water. In the control experiments free of Ag, all the conditions and procedure were kept the same as the chemical leaching, but no Ag2 SO4
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Fig. 3. XRD patterns of the residue after the chemical leaching in presence of 0.05 g/L Ag+ : (a) chemical leaching at 35 ◦ C; (b) chemical leaching at 50 ◦ C.
Fig. 4. Evolutions of orpiment bioleaching fraction and Eh in the tests with At. ferrooxidans: (a) leaching rate; (b) Eh.
was added. In the iron free experiments, the initial pH of the pulp was adjusted to 1.6, but no Fe2 (SO4 )3 and Ag2 SO4 were added.
by an inductively coupled plasma atomic emission spectroscope (ICP-OES, Optima 5300DV). At the end of the experiment, the solution was filtered and the solids were rinsed several times and dried at ambient conditions. The solid samples were analyzed by X-ray diffraction (XRD, X’Pert PRO), scanning electron microscopy (SEM, JSM-7001F) and energy dispersive X-ray spectroscopy (EDS, X-MAX150).
2.3.2. Bioleaching experiments The strains of At. ferrooxidans and S.sibiricus were grown in the respective ferrous sulfate containing medium until the cell density reached 108 cells/mL when harvested by filtration. The cells were resuspended in 100 mL fresh Leathen medium as the inoculum. 10 mL (10% in volume ratio) of this inoculum was added into the 250 mL Erlenmeyer flask containing 90 mL medium and 5 g FeSO4 ·7H2 O (corresponding to 10 g/L Fe2+ ), and the yeast extract (0.2 g/L) was particularly added for the growth of S. sibiricus. The initial pH of culture was adjusted with H2 SO4 of 1 mol/L. The flasks were inoculated in the rotary shaker at 140 rpm at the temperature of 35 ◦ C for At. ferrooxidans and 50 ◦ C for S. sibiricus. After 2 days, in the exponential growth phase of the bacteria, 0.5 g orpiment was added into each flask and the initial pH was adjusted again to 2.0 for At. ferrooxidans and 1.5 for S. sibiricus. 0.0016 g Ag2 SO4 (0.01 g/L) was added into the flasks for testing the catalytic effects of the added ions. The bioleaching experiments were concurrently started to ensure identical cell counts and solution chemistry. In the control experiments, no Ag2 SO4 was added. The bioleaching experiments were duplicated. 2.3.3. Analysis methods The pH value was measured with a pH meter being calibrated using standard pH 4.01 and 6.86 buffers. The redox potential (Eh) was determined using a Pt electrode with reference to the Ag/AgCl electrode (the same reference for all potential measurements in this work). The concentration of dissolved arsenic was analyzed
3. Results and discussion 3.1. Fe3+ chemical leaching of orpiment As shown in Fig. 2, the leaching fractions of arsenic in the presence of Fe3+ reached 15% and 21% after 7 days at 35 ◦ C and 50 ◦ C, respectively. Meanwhile, the Eh decreased monotonously. On the other hand, both at 35 ◦ C and 50 ◦ C, no more than 4% arsenic were leached and Eh increased very moderately in the control tests without Fe3+ added. The results show that orpiment can be dissolved by Fe3+ , suggesting that the orpiment in the mining tailings can be leached by Fe3+ which was produced in the biooxidation of iron containing mineral. But in comparison with other arsenic containing sulfide minerals such as arsenopyrite and enargite [20], the leaching rate is rather low. It should be noted that at an elevated temperature (50 ◦ C) a higher leaching rate was achieved. Silver has been proven to be effective in increasing the bioleaching rate of chalcopyrite in many researches [15,17,21], but few studies are involved with the catalytic effect of Ag+ on the bioleaching of other sulfide minerals [16,18]. Ag+ can react with sulfide MeS through a cationic interchange reaction as follows: MeS + Ag+ → Ag2 S + Me2+
(1)
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Fig. 6. Evolutions of orpiment bioleaching fraction and Eh in the tests with S. sibiricus: (a) leaching rate; (b) Eh.
oxidizing rate of Ag2 S speeded up at higher temperature, but part of dissolved arsenic can co-precipitate with jarosite and resulted in the decline of the leached arsenic in the solution. Fig. 5. SEM images of mineral residues after bioleaching with At. ferrooxidans: (a) control test; (b) test with 0.01 g/L Ag+ .
The formed Ag2 S is oxidized in the presence of an oxidizing agent such as Fe3+ and Ag+ is then regenerated: Ag2 S + 2Fe3+ → 2Ag+ + 2Fe2+ + S0
(2)
Reaction (1) is independent of bacteria actions. The role of bacteria in this process is oxidizing Fe2+ to Fe3+ and then generating the oxidizing agent by Reaction (2). In the bioleaching of chalcopyrite, the presence of Ag2 S could affect the conductivity of the sulfur produced or its morphology, and then eliminate its passivation [22]. The cationic interchange reaction between Ag+ and orpiment can occur theoretically, but this is seldom addressed in relevant researches. In our study, 0.01 g/L Ag+ was added in the chemical leaching media with 10 g/L Fe3+ . It can be found that in comparison with the control test without Ag+ , the leaching rate was significantly enhanced. XRD patterns (Fig. 3) show that elemental sulfur and trace of Ag2 S were produced after leaching at 35 ◦ C. This fact suggests that the leaching of orpiment complied with the mechanism described as Reactions (1) and (2). But due to the hydrolysis of dissolved arsenic, it was found that the pH value decreased in all the tests. Thus, the overall reaction can be described as Reaction (3): 6Fe3+ + As2 S3 + 6H2 O → 2H3 AsO3 + 6Fe2+ + 6H+ + 3S0
(3)
At 50 ◦ C, the XRD pattern shows that the peaks of Ag2 S disappear and jarosite can be detected instead. This result implies that the
3.2. Bioleaching with At. ferrooxidans in presence of Ag+ A, ferrooxidans is the most common microorganism which is related to the weathering of sulfide minerals. It is almost ubiquitous in mine environment and is thought to be responsible for the formation of acid mine drainage and the release of a variety of heavy metals including arsenic [7]. This bacterium can oxidize Fe2+ to Fe3+ and the present chemical leaching results indicate that orpiment can be leached by Fe3+ . Therefore, it is very possible that the bioleaching of orpiment occurs commonly in the tailings piles. In this study, the bioleaching experiments were carried out to test the leaching behavior with At. ferrooxidans. Fig. 4 shows that similar to the chemical leaching, the bioleaching rate was significantly improved in the presence of 0.01 g/L Ag+ , while the Eh variation shows some different features from chemical leaching. Eh of the test medium with 0.01 g/L Ag+ added decreased rapidly and then stabilized gradually. In the control test without Ag+ , Eh fluctuated slightly in the first stage and increased evidently after 5 days. Since the value of Eh in the bioleaching culture was determined by the couple of Fe3+ /Fe2+ , these results show that in the later stage of bioleaching, the production of Fe3+ was faster than the consumption, indicating that the bacteria worked well. It is worth being noted that the cell density decreased rapidly from around 8.0 × 106 cells/mL to 2.5 × 104 cells/mL after the addition of the mineral. It is very possible that the free bacteria were absorbed on the surface of mineral particles. The attachment of At. ferrooxidans to ore particles in the bioleaching process was confirmed in many researches [9,10,23,24]. Some researchers
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ferrooxidans, but the evolution of Eh in these tests is very different: the initial Eh in the tests with S. sibiricus was much higher and decreased slowly or increased slightly at the beginning of the tests; moreover during the last period, Eh decreased rapidly and it never rebounded like in the test with At. ferrooxidans. On the other hand, it was observed under microscope that in all the tests with S. sibiricus the density of the free cells in the solution did not decrease evidently after the addition of orpiment as occurred in the tests with At. ferrooxidans. These results show that S. sibiricus was not adsorbed on the mineral surface as At. ferrooxidans did. In the last 3 days of the test the cell density decreased to 6.0 × 104 cells/mL accompanied with the decline of Eh. Some researchers supposed that it was due to the consumption of yeast extract rather than the toxicity of metal ions in the solution [27]. The SEM images in Fig. 7 show that no layer covers the surface of the mineral residue, but the mineral surface bioleached with Ag+ added was severely etched compared to that from the control test. The different bioleaching behaviors of S. sibiricus and At. ferrooxidans suggest that the two strains acted differently in the bioleaching of orpiment. The similar results were reported in our early work [28] and by other researchers [29]. Unlike in the chemical leaching with Fe3+ , in the bioleaching the microorganisms can oxidize Fe2+ to Fe3+ continuously and sustain the leaching process of orpiment for a long time. Our experimental results show that the bacteria can grow in the pulp containing orpiment and leach the mineral in the presence of Fe2+ or Fe3+ . Therefore, it is possible that the bioleaching of orpiment can occur in the tailings pool and the released arsenic can form the continual contamination.
4. Conclusions
Fig. 7. SEM images of mineral residues after bioleaching with S. sibiricus: (a) control test; (b) test with 0.01 g/L Ag+ .
proposed that the attached cells can make mineral components solubilized and these components would be further oxidized by the suspended bacteria in the solution [25,26]. The SEM images (Fig. 5) show that in the test with added Ag+ , the residual mineral is covered by a porous layer and the EDS analysis shows that the composition of the layer is probably elemental sulfur. On the other hand, the SEM images show that there are some crystalline particles adhered on the mineral surface in the control test. EDS results (not included here) show that the iron appears in high levels in these crystalline particles, so it is presumed that these crystals are jarosite. 3.3. Bioleaching with S. sibiricus in presence of Ag+ The temperature inside the tailings pool and bioleaching heap can reach up to 50–60 ◦ C due to the heat from oxidation of sulfide minerals and the mesophilic bacteria such as At. ferrooxidans cannot survive under these conditions [11]. The moderate thermophilic microorganisms can work at higher temperature and have a kinetic advantage over the mesophilic bacteria [21]. Fig. 6 shows that the leached arsenic in the test with Ag+ added achieved around 60% in 9 days, while in the bioleaching test without Ag+ the rate was only 10% when the moderate thermopile S. sibiricus was used. This result is similar to that of the test with At.
In this study the bioleaching of orpiment was investigated. The results of chemical leaching experiments using Fe3+ show that orpiment can be leached by Fe3+ and the leaching can be significantly enhanced in presence of Ag+ . Bioleaching experiments with mesophilic bacteria At. ferrooxidans and moderate thermophilic bacteria S. sibiricus were carried out to test the effect of the microorganisms on the leaching of orpiment. It was found that both At. ferrooxidans and S. sibiricus can survive in the pulp containing orpiment and oxidize Fe2+ to Fe3+ continually, suggesting that the microbial activities will lead to the dissolution of orpiment in the mine environment. The microscopic examinations show that At. ferrooxidans can adhere on the surface of orpiment particles resulting in the decrease of cell density in the solution. In contrast, in the tests with S. sibiricus no microbial adsorption was observed. The SEM analysis shows that the residuals from the tests with At. ferrooxidans in presence of Ag+ were covered by elemental sulfur, while no elemental sulfur was detected on the residue surface from the test with S. sibiricus. The performance difference between At. ferrooxidans and S. sibiricus implies that different bioleaching mechanisms were involved in the bioleaching of orpiment. In the mining environment, AMD is the main source of Fe3+ and heavy metal ions, so avoiding the contact of orpiment containing mineral with AMD is necessary for suppressing the release of arsenic. But some researchers found that the oxidation rate of orpiment increased with the increase of pH. Therefore, the addition of limestone for prevention of AMD may not retard the release of arsenic because the limestone increased the solution pH [6]. So it might be a better alternative that covers the orpiment bearing solid waste with waterproof material to stop the contact between AMD and the orpiment, and if possible we do not mix the mineral and iron containing minerals in the tailing pool. Although the possibility of the release of arsenic due to the activity of bacteria and heavy metals were confirmed, the further
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study on the bioleaching mechanisms of orpiment is needed. On the other hand, in this study all the experiments were carried out in the shake flasks and the conditions are very different from the real mine environment. In the real environment, the interactions between Fe3+ and arsenic can be more complicated. For example, the arsenic in the solution can be adsorbed and co-precipitated with ferrihydrite. So the investigation involved the action of Fe3+ should also be carried out under the conditions closer to the real environment. Acknowledgements The authors acknowledge 973 Program (2010CB630904), the National Natural Science Foundation of China (51174185, 21427814), 863 Key Project (2012AA061503), the Major National Scientific Instrument Development Project (21427814) and the National Science Fund for Distinguished Young Scholars (21025627). References [1] M. Leist, R.J. Casey, D. Caridi, The management of arsenic wastes: problems and prospects, J. Hazard. Mater. B 76 (2000) 125–138. [2] A.B.M.R. Islam, J.P. Maity, J. Bundschuh, C. Chen, B.K. Bhowmik, K. Tazaki, Arsenic mineral dissolution and possible mobilization in mineral– microbe–groundwater environment, J. Hazard. Mater. 262 (2013) 989–996. [3] B.K. Mandal, K.T. Suzuki, Arsenic round the world: a review, Talanta 58 (2002) 201–235. [4] H. Long, D.G. Dixon, Pressure oxidation kinetics of orpiment (As2 S3 ) in sulfuric acid, Hydrometallurgy 85 (2007) 95–102. [5] M.F. Lengke, R.N. Tempel, Reaction rates of natural orpiment oxidation at 25 to 40 ◦ C and pH 6.8 to 8.2 and comparison with amorphous As2 S3 oxidation, Geochim. Cosmochim. Acta 66 (18) (2002) 3281–3291. [6] M.F. Lengke, R.N. Tempel, Geochemical modeling of arsenic sulfide oxidation kinetics in a mining environment, Geochim. Cosmochim. Acta 69 (2) (2005) 341–356. [7] H. Cheng, Y. Hu, J. Luo, B. Xu, J. Zhao, Geochemical processes controlling fate and transport of arsenic in acid mine drainage (AMD) and natural systems, J. Hazard. Mater. 165 (2009) 13–26. [8] K. Sasaki, M. Tsunekawa, T. Ohtsuka, H. Konno, The role of sulfur-oxidizing bacteria Thiobacillus thiooxidans in pyrite weathering, Colloids Surf. A: Physicochem. Eng. Aspects 133 (1998) 269–278. [9] W. Sand, T. Gehrke, P. Jozsa, A. Schippers, (Bio)chemistry of bacterial leaching – direct vs. indirect bioleaching, Hydrometallurgy 59 (2001) 159–175.
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