This article was downloaded by: [University of Haifa Library] On: 18 September 2013, At: 09:56 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20 BIODESULPHURIZATION OF COAL: MECHANISM AND RATE LIMITING FACTORS Anushree Malik a b , Manisha Ghosh Dastidar a & Pradip Kumar Roychoudhury c a Indian Institute of Technology, Centre for Energy Studies, New Delhi, 110016, India b Department of Energy and Environmental Science, Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi, 321-8585, Japan c Department of Biotechnology and Biochemical Engineering, Indian Institute of Technology, New Delhi, 110016, India Published online: 06 Feb 2007. To cite this article: Anushree Malik , Manisha Ghosh Dastidar & Pradip Kumar Roychoudhury (2001) BIODESULPHURIZATION OF COAL: MECHANISM AND RATE LIMITING FACTORS, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 36:6, 1113-1128, DOI: 10.1081/ESE-100104135 To link to this article: http://dx.doi.org/10.1081/ESE-100104135 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.
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This article was downloaded by: [University of Haifa Library]On: 18 September 2013, At: 09:56Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Environmental Scienceand Health, Part A: Toxic/HazardousSubstances and EnvironmentalEngineeringPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lesa20
BIODESULPHURIZATION OF COAL:MECHANISM AND RATE LIMITING FACTORSAnushree Malik a b , Manisha Ghosh Dastidar a & Pradip KumarRoychoudhury ca Indian Institute of Technology, Centre for Energy Studies, NewDelhi, 110016, Indiab Department of Energy and Environmental Science, Graduate Schoolof Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya,Tochigi, 321-8585, Japanc Department of Biotechnology and Biochemical Engineering, IndianInstitute of Technology, New Delhi, 110016, IndiaPublished online: 06 Feb 2007.
To cite this article: Anushree Malik , Manisha Ghosh Dastidar & Pradip Kumar Roychoudhury (2001)BIODESULPHURIZATION OF COAL: MECHANISM AND RATE LIMITING FACTORS, Journal of EnvironmentalScience and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 36:6,1113-1128, DOI: 10.1081/ESE-100104135
To link to this article: http://dx.doi.org/10.1081/ESE-100104135
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.
This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
1Centre for Energy Studies and2Department of Biotechnology and BiochemicalEngineering, Indian Institute of Technology,
New Delhi 110016, India
ABSTRACT
The pyrite sulphur removal from coal by Thiobacillus ferrooxidans wasstudied in batch reactor. A combination of SEM, IR and XRD was usedto study the presence of superficial phases and the changes in solid sur-face during biodesulphurization. Biodesulphurization was found to be athree-step process. In the first step (0–4 days), direct oxidation of pyriteby bacteria brought about 28% pyritic sulphur removal. Both direct andindirect oxidation contributed to the second step (4–10 days) resulting in51% pyrite removal. The deposition of elemental sulphur, jarosite andferric sulphate precipitates in the third step reduced the pyrite availabilityand ferric iron concentration in the leachate and brought the process ofbiodesulphurization to an end.
The severe pollution caused by sulphur dioxide emission during com-bustion of high sulphur coal is considered to be responsible for acid rain,ozone layer depletion and various health hazards such as respiratory tractcancer and cardio-respiratory diseases. Microbial desulphurization is emer-ging as an attractive coal cleaning technique in view of the environmentallysafe utilization of high sulphur coal. The key organism used for removal ofpyritic sulphur from coal and oxidation of other sulphide ores is Thiobacillusferrooxidans [1, 2]. The effect of particle size, pulp density, cell density andvarious other process parameters on biodesulphurization process has beenstudied extensively [3, 4]. In the last two decades, various reactors have beendeveloped to attain higher cell densities and high rates of oxidation of pyriteand other sulphidic ores [5–7].
Bioleaching is a complex phenomenon governed by a chain of reactionsrepresenting direct (bacterial) and indirect attack on sulphide [8, 9]. Duringthe process, various oxidized and precipitated phases appear on pyrite sur-face, governed by physico-chemical and biological changes occurring in thesolution phase [10]. These surface alterations cause the leaching to becomeincreasingly controlled by solid-state diffusion. To understand the evolutionof ‘‘Thiobacillus ferrooxidans-leachate-sulphide system’’, parallel studies onchanges in solid and solution phase composition are needed. In the lastdecade, some studies on sulphidic ores (pyrite, arsenopyrite and sphalerite)have provided the knowledge about bacterial activity and evolution of sur-face-oxidized phases [11–14]. However, similar studies on coal are very muchrequired.
In the present study, detailed investigations were made on thechanges occurring in physico-chemical properties and bacterial distribu-tion of the leachate during coal biodesulphurization. The change in coal/pyrite surface characteristics was simultaneously monitored using ScanningElectron Microscopy (SEM), Infra Red Spectroscopy (IR) and X-RayDiffraction (XRD) analyses. Based on these results, the mechanism ofbiodesulphurization was elucidated and limitations of the process wereidentified.
MATERIALS AND METHODS
Coal and Pyrite
Coal and pyrite samples collected from Assam coal fields (North-easternIndia) and Amjhore mines (Eastern India), respectively, were ground andsieved to obtain below 250 mm size fraction. The characteristics of Assamcoal are shown in Table 1. Pyrite contains 40.1% total iron and 47.0%sulphur.
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Organism and Inoculum Preparation
Thiobacillus ferrooxidans (ATCC 13984) was grown in 500ml Erlen-meyer flask in a shaking incubator at 30�C and 250 rpm using modified9K medium of the following composition (g/l): (NH4)2SO4, 1.0;MgSO4.7H2O, 0.5; K2HPO4, 0.5; KCl, 0.1; FeSO4, 30.0. The pH of themedium was adjusted at 1.5. A 10% inoculum of two day old culture wasused for desulphurization studies.
Biodesulphurization Studies
The study on coal biodesulphurization was conducted in a 2L bioreac-tor (Bioengineering AG) with 1.6L working volume which was equippedwith various controls. The aeration rate was maintained at 1 vvm and theair was enriched with 1% CO2. Coal suspension (10% pulp density) wasprepared in 9K mineral medium, sterilized and inoculated with 7.5� 1010
iron grown cells. Periodically leachate samples were drawn and analyzed fornumber of free cells, total iron, ferrous and ferric iron and redox potential.The coal samples were also drawn at regular intervals, washed with distilledwater, dried overnight at 105�C and analyzed for pyrite sulphur content andsolid phase protein. Since coal is very complex and heterogeneous material, itwas assumed that the identification of corrosion patterns and surface specieswill be easier with pure pyrite. Bioleaching studies on pure pyrite were simi-larly conducted at 2% pulp density (data not shown). Both coal and pyritesamples were analyzed by SEM, IR and XRD at different stages of theprocess.
Pyritic sulphur 1.72Organic sulphur 1.84Gross calorific value 27,421.29 kJ/kg
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Analytical Methodology
Moisture, ash and volatile matter of a sample of coal were estimatedaccording to the standard procedures [15]. The elements carbon, hydrogenand nitrogen were determined with the help of Perkin Elmer 240-C elementalanalyzer. The total, pyritic and sulphate sulphur contents of coal before andafter biodesulphurization were estimated [16]. The organic sulphur was esti-mated by subtracting pyritic and sulphate sulphur from total sulphur. Oxygenwas determined by subtracting the sum of C, H, N and total S from the total as100. Calorific value of coal was determined using a Bomb Calorimeter. TheSEM analysis of solid samples was done using a Scanning ElectronMicroscope (Stereoscan 360, Cambridge Scientific Instruments Limited,England) at an accelerating voltage of 20 kV. IR spectra of solid sampleswere recorded on Nicolet Protege 460 ESP model. Powder diffraction X-rayanalysis of the solid samples was conducted using CuKa radiation with aRigaku Rotaflex (model RV 2008) wide-angle Goniometer equipped with adiffracted beam monochromator. Soluble iron concentration in the leachateduring coal/pyrite desulphurization was estimated using o-Phenanthrolinemethod [17]. For determination of attached protein concentration, the coal/pyrite samples were boiledwith 0.1NNaOH, centrifuged and the supernatantswere analyzed by Lowry’s method [18]. The concentration (number of freecells/ml) of Thiobacillus ferrooxidans in liquid phase was estimated by directmicroscopic count using a Haemocytometer. Redox potential and pH weremeasured by using pH meter.
RESULTS
The bacterial oxidation of pyrite in coal is characterized by three phasesof bacterial growth, which form the three main steps of the process.
Step I
This phase observed between 0–4 days corresponds to solubilization ofpyrite. Till 4 days, the number of free bacteria in the leachate remainsconstant while the number of bacteria attached to pyrite surface increasesconsiderably (Figure 1). The presence of hexagonal pyrite crystals and adhe-sion of bacteria to pyrite surface is also evident from the SEM pictures(Figures 4A and B, later). The selective adhesion of Thiobacillus ferrooxidansto pyrite surface has recently been reported to involve specific interactions viaorganic capsule or cell surface proteins rather than the physical (electrostatic/hydrophobic) interactions [19]. Redox potential decreased slightly and thenbecame stable till 4 days (Figure 2). This is contrary to earlier studies on
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arsenopyrite bioleaching in which adhesion of bacteria is accompanied withsubstantial rise in the redox potential [13]. The soluble iron concentrationwas found to increase with most of the iron existing in ferrous form (Figure3). In this step, 28% pyritic sulphur reduction was observed (Figure 2).
As compared to coal (Figure 4C), the irregular and weak corrosionpatterns are more clear in the SEM photograph of pure pyrite sample leachedover a 2 day period. Such patterns which seem to be controlled by fragilityzones and crystallinity of the particles is a feature of bacterial attack onthe surface [20].
BIODESULPHURIZATION OF COAL 1117
0 5 10 15 200
2
4
6
8
10
Pro
tein
(µg
/ml)
Time (days)
Figure 1. Change in free and attached cell protein concentration during biodesulphurizationof coal. Symbols: e, free cell protein: f, attached cell protein.
0 5 10 150
2
4
6
Cel
l (1
0 8 c
ells
/ml)
Time (day)
400
500
600
Re
dox po
tential (mV
)
0.0
0.4
0.8
1.2
1.6
Pyr
itic
sulp
hur
(%
)
Figure 2. Change in cell number, pyritic sulphur content and redox potential duringbiodesulphurization of coal. Symbols: g, cell number; +, redox potential; e, pyritic sulphur.
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Step II
During the second step (4–10 days), number of free bacteria increasedgreatly while the number of attached bacteria remained constant (Figure 1).Pyrite solubilization occurred at high rates with 79% pyrite reduction till 10day period. Iron solubilization was also enhanced and all the ferrous iron wasquickly oxidized to ferric form (Figure 3). Redox potential increased sharplyfrom 438mV to 544mV (Figure 2).
The uniformly distributed ovoid pits were clearly observed on purepyrite surface (Figure 4D). Apart from this, the pyrite surface was alsopartially covered by some deposition.
Step III
Beyond 10 days, the number of both free and adhered bacteria becameconstant. The redox potential also reached a plateau during this step. Thepyrite solubilization was reduced to a great extent and finally stopped beyond15 day period. The ferric iron concentration in the leachate decreasedwhereas the ferrous iron concentration increased simultaneously. The accu-mulation of ferrous iron in the third step indicates that most of the bacteriawere inactive or dead.
The corrosion patterns, especially the uniformly distributed pitsincreased considerably and the breaking of particles was also observedwith pure pyrite (35 day) crystal (Figure 4E). Presence of coarse precipitateswas also observed on biodesulphurized coal (50 day) sample (Figure 4F).
1118 MALIK, DASTIDAR, AND ROYCHOUDHURY
0 5 10 15 200
500
1000
1500
2000
2500
So
lubl
e ir
on
( µg/
ml)
Time (days)
Figure 3. Change in ferrous and ferric iron concentration during biodesulphurization ofcoal. Symbols: f, ferrous iron; e, ferric iron.
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BIODESULPHURIZATION OF COAL 1119
A
B
C
Figure 4. SEM during different stages of biodesulphurization. Coal, 0 day: presence of cubicpyrite crystals (A), coal, 4 day: adhesion of bacteria to pyrite surface (B), pyrite, 4 day:appearance of irregular and weak corrosion patterns (C), pyrite, 10 day: appearance of
ovoid pits and deposition on the surface (D), pyrite, 35 day: uniform distribution of pits,breaking of particles (E), coal, 50 day: presence of precipitates on the surface (F).
(continued)
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1120 MALIK, DASTIDAR, AND ROYCHOUDHURY
D
E
F
Figure 4. Continued
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Infrared (IR) and X-ray Diffraction (XRD) Measurements:
Identification of Surface Species
The IR spectra of original and bioleached (35 days) pyrite samples areshown in Figure 5. Appearance of new peaks at 777 cm�1 and 691 cm�1 wasobserved in bioleached sample. These peaks correspond to thiosulphate andvarious surface oxidation products such as goethite, ferric-ferrous sulphate,iron oxide and ferrous sulphide species [21]. The increased intensity and shiftin 1148 cm�1 peak to 1087 cm�1 are due to solubilization of pyrite to ferroussulphate and oxidation to ferric form. Appearance of new peak at 470 cm�1
indicates the formation of elemental sulphur on pyrite surface. The formationof elemental sulphur during bioleaching is well documented [9, 22].
As compared to pyrite, the IR spectra of coal exhibited several addi-tional peaks (Figure 6). The absorption bands at 3619 cm�1 and 1008 cm�1 inthe original coal are assigned to kaolinite (3600–3700 cm�1) and silicate(800–1100 cm�1), respectively [23, 24]. Disappearance of kaolinite peak and
BIODESULPHURIZATION OF COAL 1121
Figure 5. Infrared (IR) spectra of pyrite during biodesulphurization. 0 day (A), 35 day (B).
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decrease in silicate band indicate dissolution and removal of these mineralsduring biodesulphurization. Increase in peaks at 1092 cm�1, 1035 cm�1 and537 cm�1 in the bioleached sample are assigned to various intermediates ofpyrite oxidation such as sulphate, thiosulphate, pyrosulphite etc. Increasedabsorption at 470 cm�1 peak indicates the appearance of elemental sulphuron coal surface. Increase in 1617 cm�1 peak suggests the deposition of ferricsalts on the surface [13].
The X-ray diffractograms of original and desulphurized pyrite (10 and35 day) shows gradual reduction in pyrite peaks over the course of bioleach-ing (Figure 7). Few peaks for elemental sulphur appear after 10 days andtheir number is considerably increased in sample leached for 35 days, sug-gesting gradual accumulation of sulphur over time. Jarosite and ferricsulphate peaks are also present in 35 day sample. Jarosite and sulphur pre-cipitation on the surface of bioleached pyrite and arsenopyrite has beenreported earlier [14, 25].
1122 MALIK, DASTIDAR, AND ROYCHOUDHURY
Figure 6. Infrared (IR) spectra of Assam coal. 0 day (A), 50 day (B).
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As compared to pyrite, several peaks corresponding to other mineralswere detected on coal surface (Figure 8). Besides pyrite, the peaks for silicates(Si), gypsum (G), kaolinite (K) illite-montomor (I-M), hemalite (H), zircon(Z) and magnetite (M) also disappeared during desulphurization process. Asobserved with pure pyrite, new peaks corresponding to elemental sulphur (S),jarosite (J) and ferric sulphate [Fe2(SO4)3] were detected on the desulphurizedcoal sample (50 day).
DISCUSSION
Based on the simultaneous observations at solid and solution phases,the process of biodesulphurization can be demarcated into three main steps.
BIODESULPHURIZATION OF COAL 1123
Figure 7. XRD spectra of pyrite during biodesulphurization. 0 day (A), 10 day (B), 35 day(C). Symbols: P, pyrite; Q, quartz; S, sulphur; J, jarosite.
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In the first step, bacteria get attached to pyrite surface and initiate the solu-bilization of pyrite to ferrous sulphate. As the number of attached bacterialcells increases, more and more ferrous iron is released into the leachate. Thiscauses a decline in redox potential of the leachate. The weak and irregularcorrosion patterns on the pyrite surface can also be assigned to the directbacterial attack during this step. Therefore, direct bacterial oxidationdominates during the first step.
The increase in soluble ferrous iron concentration leads to multiplica-tion of free bacteria during second step. The growth of free cells causesoxidation of ferrous iron to ferric iron and subsequently redox potential ofthe leachate increases. The bacterially generated ferric iron can now cause
1124 MALIK, DASTIDAR, AND ROYCHOUDHURY
Figure 8. XRD spectra of coal during biodesulphurization. 0 day (A), 20 day (B), 50 day
(C). Symbols: K, kaolinite; G, gypsum; H, hematite; F, ferrite; C, calcite; M, magnetite; Z,zircon; E, epidote; I-M, illite-montomer; P, pyrite; Q, quartz; S, sulphur; J, jarosite.7.
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chemical or indirect oxidation of pyrite under existing favourable redoxconditions. The biological leaching of sulphides is reported to be primarilyan indirect process with ferric iron (Feþ3) acting as the chemical oxidant andredox carrier [8, 26]. During the indirect oxidation, ferric iron acts on pyriteto yield ferrous iron and elemental sulphur. The ferrous iron is oxidized backto ferric form by actively growing free cells. Hence, it becomes a cyclicprocess until the growth of free bacteria is not hindered. Therefore, in thesecond step, both the direct oxidation by attached cells and indirect chemicaloxidation by bacterially generated ferric iron contribute to pyrite oxidation.The SEM photograph taken during this step shows some deposition on pyritesurface and the corresponding sample (XRD) reveals the appearance of ele-mental sulphur peaks. The elemental sulphur formed as a result of chemicaloxidation is not oxidized by Thiobacillus ferrooxidans and gets accumulatedon pyrite surface. The attached bacteria are coated by sulphur layer andhence their multiplication and the direct oxidation get retarded. Insteadsulphur grown Thiobacillus ferrooxidans cells have been recently reportedto oxidize sulphur layer and enhance the biodesulphurization rate [27]. Theuniform distribution of ovoid pits on pyrite surface also indicates that thechemical oxidation is dominant during the second step.
The last step is characterized by the decline in desulphurization rate, freecell growth and attainment of stable redox potential value. The IR and XRDanalyses of the desulphurized coal samples very clearly show that apart frompyrite, several other minerals such as kaolinite and silicates also get solubi-lized into the leachate. It was recently observed that fraction solubilized fromsolid phase is responsible for toxicity of black-shale leachate [28]. Further,the activity of Thiobacillus ferrooxidans, which is the key to the biodesulphur-ization process, gets inhibited in presence of various metals, cations andanions [29]. The present authors observed that toxicity of coal leachatecould be attributed to presence of Si and Al [30]. It appears that the con-centration of dissolved inorganic/organic components build up to toxic levelduring this step and inhibit the growth of free cells. Therefore, the cyclicphenomenon observed during step II is disrupted. Consequently, concentra-tion of ferric iron is reduced and ferrous iron concentration builds up in theleachate. The rate of desulphurization is drastically reduced as only the indir-ect oxidation occurs during this step. Finally, the precipitation of jarositesand hydrated ferric sulphate observed on 35 day leached pyrite and 50 dayleached coal sample coat the surface of crystal and reduce the pyrite avail-ability. This precipitation can also result in the decrease of ferric iron in theleachate, thereby stopping the indirect chemical oxidation. In this way, boththe oxidation mechanisms cease towards the end of third step. Availability offresh pyrite, ferric/ferrous ratio and level of toxic minerals in the leachateneed to be simultaneously controlled by using suitable operational strategy toenhance the biodesulphurization rate. This was successfully attempted andreported by the group of present authors using three different operational
BIODESULPHURIZATION OF COAL 1125
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strategies viz. Constant Volume Pulse Feeding (CVPF), Increasing VolumePulse Feeding (IVPF) and Leachate Recycle Operation [31]. The IVPF wasfound to be the best strategy for operation of biodesulphurization process forlongest duration at enhanced rate due to sufficient substrate availability andminimization of the toxicity of the leachate.
CONCLUDING REMARKS
In the present study, the bacterial distribution and physico-chemicalcomposition of leachate was correlated with the evolution of corrosion pat-terns and nature of surface oxidized phases on solid surface to propose themechanism of biodesulphurization as a three step process. The first step wascharacterized by direct oxidation of pyrite through attached bacteria. Thesecond step which began with oxidation of soluble ferrous iron to ferric formby fast growing free cells was soon taken over by indirect oxidation of pyriteby ferric iron. A bioleaching cycle involving both direct and indirect oxida-tion occurred during this step and towards the end of the step, attachedbacteria were coated by sulphur layer and the direct oxidation got retarded.Solubilization of toxic components from the coal caused inhibition of freecell growth and consequently the bioleaching cycle was disrupted in the thirdstep. Finally, the precipitation of jarosite and ferric sulphate coated pyritesurface and reduced the ferric iron concentration in the leachate to effectcessation of indirect oxidation. Then the process of desulphurization cameto a standstill as both the direct and indirect oxidation got stopped.
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