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Synthesis of magnetite nanoparticles from mineral waste
Accepted Manuscript Synthesis of magnetite nanoparticles from mineral waste Rohit Kumar, R. Sakthivel, Reshma Behura, B.K. Mishra, D. Das PII: DOI: Reference:
S0925-8388(15)01382-1 http://dx.doi.org/10.1016/j.jallcom.2015.05.089 JALCOM 34205
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
24 November 2014 28 April 2015 11 May 2015
Please cite this article as: R. Kumar, R. Sakthivel, R. Behura, B.K. Mishra, D. Das, Synthesis of magnetite nanoparticles from mineral waste, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/ j.jallcom.2015.05.089
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Synthesis of magnetite nanoparticles from mineral waste Rohit Kumar1, R. Sakthivel*1, Reshma Behura1, B. K. Mishra1, D. Das2 1
CSIR- Institute of Minerals and Materials Technology, Bhubaneswar-751 013, INDIA 2
UGC-DAE Consortium, Kolkata, India
Abstract Magnetite nanoparticles were synthesized from iron ore tailings; - a mineral waste collected from the iron ore processing plant. Mechanical milling followed by chemical route is employed to obtain the magnetite nanoparticles from the waste. The magnetite nanoparticles were characterized by X-ray diffractometer, Field Emission Scanning Electron Microscope, Fourier Transform Infrared Spectrometer and Vibrating Sample Magnetometer. X-ray diffraction pattern confirms the existence of a magnetite phase. Field Emission Scanning Electron Microscopic (FE-SEM) pictures reveal that the particle size is below 100 nm. Fourier Transform Infrared (FTIR) spectrum shows a band at 570 cm-1 for the Fe-O bond vibration. Vibrating Sample Magnetometric (VSM) study shows high saturation magnetization value of 60 emu/g at low applied magnetic field. Silver coated magnetite nanoparticles exhibits antibacterial property whereas bare magnetite does not. Keywords: Mineral waste, iron ore tailings, magnetite nanoparticles, saturation magnetization, antibacterial property.
*
Corresponding author Email: [email protected], [email protected]
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1.
Introduction Enormous quantity of iron ore tailings is disposed off as a mineral waste from the iron ore
processing industries due to rapidly increasing demand and growth of iron and steel industries in India. It has been reported that ~18-20 million tons of tailings per year in India [1]. Since it does not have direct or indirect commercial use, it gradually keeps on accumulating. Safe disposal of this waste is most essential as it occupies a larger area that results in land pollution. Due to the fine nature of its particles, it easily contaminates the ground water as well as various water bodies through rain and by wind; it ultimately affects the human beings and aquatic animals [2]. Therefore, more efforts are needed to utilize this waste for its possible applications. Indian iron ore tailings generally contain 26.8% Fe2O3, 72.4% SiO2 and 0.7% Al2O3. As it contains high amount of SiO2 (>80%), it is suggested as a raw material for construction, ceramics, glass, and to make a new class of silicate [3, 4]. Effort has been made to prepare försterite from iron ore tailings due to its wider applications as it has good creep stability, high refractory temperature and also it provides better thermal insulation [5]. This observation has given the scope for effective utilization of tailings for the preparation of value added materials. Extraction of iron from tailings and converting that into products that are more valuable can find wider applications. If the hematite content of iron ore tailings is converted into magnetite nanoparticles, it can find its suitability in various applications. Magnetite nanoparticles find applications in the field of catalysis [6], information storage [7], adsorption [8], medicine [9], nano-tagging [10], ferrofluids [11], removal of heavy ions like arsenic from water [12] and even in the field of biomedical applications like magnetic separation [13], drug delivery [14], hyperthermia treatments [15] and magnetic resonance imaging (MRI) contrast enhancements [16]. In the literature, various methods such as sol-gel [17], thermal decomposition [18], co-precipitation [19, 20], reverse micelles [21, 22], hydrothermal 2
synthesis [23] etc. are reported for the synthesis of magnetite nanoparticles. However, the main challenges lie in obtaining desired shape and narrow size distribution of particles as they play a critical role in their practical applications [24]. Some control on these parameters could be achieved by optimizing the parameters [25-26] such as reaction temperature, pH, reaction time and the environment maintained for synthesis. Other than chemical methods, mechanical milling approach is also suggested by Carvalho et al. [27] and Can et al. [28], in which they reported that the milling of metallic iron powder with distilled water at variable milling times results in formation of a variable size range (12-20 nm) of magnetite nanoparticles; reaction involved in this method is given below. 3Fe + 4H2O à Fe3O4 + 4H2
(1)
Alcala et al. [29] demonstrated that hematite could be transformed into magnetite by milling it with iron according to the following reaction scheme: 4Fe2O3 + Fe à 3Fe3O4
(2)
Mechanical milling is a very simple technique unlike chemical methods that involve optimizing parameters like pH, reaction temperature, filtration, washing, etc. In that direction, in our previous study, we attempted to prepare magnetite out of iron ore tailings through mechanical milling with iron powder as a reductant. Unfortunately, we failed to obtain the desired magnetite product; instead ended up obtaining undesired fayalite (Fe 2SiO4) phase due to the interference of the high amount of silica (SiO2) in the tailings during mechanochemical reaction [30]. Inhibition of this side reaction of silica becomes very difficult since the mechanical milling is not under thermodynamic equilibrium condition. To overcome this issue, in this investigation, we adopted a suitable method involving both physical and chemical approaches for transforming hematite portion of tailings into 3
magnetite nanoparticles. It has been reported that magnetite is used in cancer therapy due to its good biocompatible nature and also used as a support for metal nanoparticles [31]. Silver nanoparticles exhibit excellent antimicrobial and antibacterial activity without light. Its antibacterial activity increases with the size reduction, but there is a great risk in using these finer nanoparticles because of removal difficulties after their use. Its persistence in the mammiferous cells causes argyrosis and argyia [32-33]. It can also cause clinical toxicity in human even at lower concentration of 100 ppb [34]. Hence, in the water purification process, use of silver nanoparticles to remove bacteria is restricted. So there is a need to develop suitable method to eradicate this practical difficulty. Here, we demonstrate the synthesis of magnetite nanoparticles from mineral waste and using it as a support for silver nanoparticles for its application in removal of bacteria from water. 2. Materials and methods Iron ore tailings collected from the iron ore processing plant contain about 26.8% Fe 2O3, 72.4% SiO2 and 0.7% Al2O3. In order to convert hematite (Fe2O3) portion of tailings into magnetite, an appropriate amount of iron was added. The molar concentration ratio of added Fe to Fe2O3 content of tailings was maintained at 1:4. Then this mixed sample was subjected to ball milling for 2 hr for uniform mixing/distribution of iron in the tailings. The sample obtained after milling was transferred into a round bottom flask, subjected to acid digestion at 95 °C for 4-5 hr with addition of required amount of hydrochloric acid solution and cooled down to room temperature. During acid digestion, iron (III) leached out from the tailings as iron (III) chloride and a portion of it reacted with metallic Fe mixed in the tailings to form iron (II) chloride solution. These two processes happened simultaneously and resulted in the formation of 2:1 molar ratio of iron (III) and iron (II) chloride solution. Then, this solution was separated out from the acid 4
insoluble residue by filtration. After separation, the obtained solution was quantified for both Fe (III) and Fe (II) by standard wet chemical analysis and was found that they are in the 2:1 molar ratio. This solution was transferred into a round bottom flask and sufficient amount of urea was added to it. It was refluxed at 95 °C until black colored fine particles of magnetite was obtained. During this process, color of the solution was changed from brown to black due to changes in the
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(
2
3
+
2
+
2
+
3)
ℎ
)
(
2 ℎ
(
)
. 2
+
.
95°
6
pH by slow release of NH3 by the decomposition of urea. Then, it was cooled down to room temperature and fine magnetite particles were separated out from aqueous solution by magnetic separation and washed with de-oxygenated distilled water to remove chloride followed by alcohol to remove moisture and dried in a vacuum desiccator at room temperature. A schematic representation for preparation of magnetite nanoparticles from tailings is given above. A portion of magnetite nanoparticles was coated with silver nanoparticles by incipient wet impregnation of aqueous silver acetate followed by sonication, washing and drying. Since the obtained Ag-functionalized magnetite (Ag/Fe3O4) is hydrophobic in nature, it was converted into hydrophilic by mixing it with 10 wt.% aqueous tetra methyl ammonium hydroxide up on stirring, sonication and washed with a mixed solution containing 8:1:1 of acetone, ethanol and distilled water respectively[35]. Amount of silver on magnetite nanoparticles was maintained at 1 wt.% and the same was used for antibacterial activity.
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Antibacterial activity of Fe3O4, hydrophobic and hydrophilic Ag-Fe3O4 nanomaterials was tested against Escherichia coli (reference bacterial strain MTCC-739). Qualitative antibacterial investigation was carried out by using spot inoculation method in which sample was placed as a spot on the petri dish containing E. coli cultured in agar media. After its incubation period of 24 h at 37 °C, the zone of inhibition area indicating E. coli destruction was measured. For quantitative estimation of antibacterial property, a test solution was prepared by dispersing 1 mg/ml of sample in 25 ml sterilized tap water containing initial E. coli concentration of 107 CFU/ml. 1 ml of this test solution was drawn at various time intervals and diluted serially up to 10 -4 dilution. Then 0.1 ml of this diluted solution was spread evenly over an agar plate with the help of a sterile L-shaped glass rod. These agar plates were allowed to undergo incubation at 37 °C for 24 h. Then the number of bacterial colonies formed was counted. All antibacterial tests were carried out under dark condition at ambient atmosphere. X-ray diffraction (XRD) studies were performed using X-ray powder diffractometer (Panalytical Instrument-Model: X’ pert Pro, Netherland) using Co K α radiation (λ =1.78901 Å) having programmable divergence slit. The diffractometer was operated at 40kV and 30 mA. Diffractograms were recorded from 10 to 80 ° 2q with a scan speed of 2.15s/step and step size of 0.05° (2q). The diffraction lines were analyzed for different phases present in the sample using Highscore™ software available with the instrument.
For identification of functional groups
present in the sample, Fourier Transform Infrared (FTIR) spectrometer (spectrum GX Perkin Elmer, resolution 4cm−1) was used to record the spectrum from 400-4000 cm−1. Before recording the spectrum, KBr spectrum was taken as a reference and then sample mixed with KBr was measured. Thermogravimetry analysis of sample was carried out with Mettler Toledo (TGS/SDTA851e) instrument at a heating rate of 20°C/min. About 15 mg of sample was used for 8
TGA measurement. Further confirmation of phases present in the sample, room temperature Mössbauer spectrum was recorded with Mössbauer spectrometer using a 57Co source in a constantacceleration transmission. The spectrometer was calibrated with a standard a-Fe foil. The morphology and elemental characterization of various samples were carried out through Field emission scanning electron microscope (Zeiss Supra55). The samples for FE-SEM studies were prepared by dispersing the sample in ethanol by sonication and this homogenous solution was dispersed on an aluminum foil support. Magnetic property of the sample was evaluated through a vibrating sample magnetometer (PAR 155) at room temperature.
3. Results and Discussion X-ray diffraction pattern of iron ore tailings shown in Fig. 1 reveals that the sample is crystalline and shows diffraction peaks corresponding to hematite (Fe 2O3) and quartz (SiO2) phases. Presence of these phases is confirmed by matching the observed diffraction peaks with standard powder diffraction patterns of hematite (JCPDS F. No.85-0599) and quartz (JCPDS F. No. 86-1630). It is noticed that the intensity of diffraction peaks of quartz is more prominent than hematite indicating that quartz exists as major phase and hematite as minor. Acid leached residue obtained after acid digestion of mixed iron ore tailings and iron powder followed by solid-liquid separation shows diffraction peaks only for quartz phase (Fig. 1). It reveals that both mixed iron and hematite phase in the tailings are leached out completely during acid digestion. X-ray diffraction pattern of magnetite nanoparticles synthesized from acid leached solution is shown in Fig. 2. It shows very prominent diffraction peaks, which are well matched with the standard powder diffraction pattern of magnetite (JCPDS F. No.19-0629) as a single phase. Broadness of diffraction peaks indicates the nano-crystalline nature of the sample. The average crystallite size 9
calculated from the diffraction peaks using the following Debye-Scherrer formula is found to be 14 nm. =
l b∗
q
Where K = 0.94, l is the wavelength of X-ray, b is full width half maximum (FWHM) of the peak, q is the diffraction angle. Lattice parameter calculated from the X-ray powder diffraction data using the following equation shows the value 8.394 Å and this is in good agreement with the values reported in the standard JCPDS F.No.19-0629. =
+
+
FE-SEM images of uncoated and Ag coated magnetite nanoparticles are shown in Fig. 3. It shows that particles are well distributed and has nearly spherical morphology with narrow particle size range from 50 to 60 nm [36]. Since capping reagent is not used during the synthesis, the particles show a high tendency for agglomeration due to its fineness. The size of the particles seen from FESEM corresponds to the overall polycrystalline material, while that determined from XRD is related to the crystallite size. Evidence for coating of Ag on magnetite nanoparticles are well noticed in Fig. 3a. Fig. 3b shows the EDS spectra of Fe3O4 and Ag-Fe3O4 nanoparticles. It reveals peaks for Fe, Al, and O in both the samples whereas an additional peak for Ag confirms the presence of Ag in Ag-Fe3O4 composite. Peak observed for Al is due to aluminum foil support used in the sample preparation before FE-SEM study. The FTIR spectra (Fig. 4) show a distinct absorption band at 608 cm-1 corresponding to intrinsic stretching vibrations of Fe-O bond at the tetrahedral site (Fetetra « O) of magnetite. Similar observation has been reported for magnetite nanoparticles [37-38]. Whereas bands observed at around 3650 cm-1 and 1660 cm-1 corresponds to 10
OH stretching and OH bending of adsorbed water molecule in the lattice respectively. Whereas other bands observed at 3750 and 1023 cm-1 are for OH stretching and bending mode of hydroxyl group in akaganite. The characteristic bands of akaganeite are observed at around 670 and 880 cm 1
which are correspond to OH…Cl hydrogen bond and OH respectively [39,40]. The translational
stretching of Fe-O bond vibration in akaganite is noticed at 464 cm-1[41]. The bands observed in the region 1430 cm−1 are attributed to adsorbed anionic species like carbonates on the surface of magnetite. The adsorbed carbonate species could have come from the decomposition of urea during hydrolysis. It is observed that the band at 1023 cm-1 in case of magnetite sample gets disappeared in case of Ag/Fe3O4 samples and it might be due to Ag coating and conversion of hydrophobic to hydrophilic nature brought by tetra methyl ammonium hydroxide treatment. TGA curve of magnetite sample is shown in Fig. 5. It indicates the two stages of weight loss up to 375°C, one up to 150°C and another between 200 and 375°C. The first stage of weight loss accounts for removal of physically adsorbed water molecules whereas second stage for dehydration of b-FeOOH phase present as an impurity phase in the sample. The percentage of weight loss found to be 0.62% at second stage and it accounts for 6.12% quantity of b-FeOOH in the magnetite sample.
4. Magnetic property To find out qualitative determination of its magnetic property, the synthesized magnetite nanoparticles dispersed in hexane and placed near the magnet. Fig. 6a shows that the dispersion of magnetite nanoparticles in the absence and presence of magnetic field. It clearly demonstrates that the well dispersed particles in hexane in the absence of magnetic field get attracted after application of magnetic field. Quantitative magnetic property of magnetite nanoparticles is shown
11
as M¾H curve in Fig. 6b. It shows the saturation magnetization value of 60 emu/g. This saturation magnetization is very much sensitive to the particle size. Usually, when the size of particles reduced to nanometer range, its magnetic properties reduce drastically and they exhibit super paramagnetic behavior [42]. When we deal with fine particles, magnetization is generally characterized by two parameters viz., remanence and coercivity. Between them, coercivity is more important as it is strongly dependent on the particle size. Initially, when the particle size reduces to a critical value from larger size, coercivity increases up to a certain maximum value, after that it starts decreasing rapidly as it becomes a single domain system and it completely vanishes to exhibit super-paramagnetic behavior [43]. The coercivity and remanence of the magnetite nanoparticles studied in this investigation are 23 Oe and 4 emu/g respectively and these observations indicate that the sample approaches towards superparamagnetic behavior. Magnetic property and particle morphology of magnetite nanoparticles obtained from the tailings is very much similar to those synthesized from hydrothermal method [44]. The saturation magnetization value of 65 emu/g is reported for the magnetite having an average particle size of 70 nm with spherical shape, whereas magnetite particles studied in this investigation show 60 emu/g for the particle size range of 50-60 nm with the same morphology. A comparative VSM curve of uncoated and silver coated magnetite is shown in Fig. 6c. It shows that silver coating of magnetite reduces the saturation magnetization value from 65 to 55emu/g but this trend becomes reverse in case of coercivity values. Bare magnetite has coercivity value of 23 Oe, whereas Ag/Fe 3O4 has 74.3Oe. This reduction in the saturation magnetization and increased coercivity values brought by silver coating could be due to the reduction in overall magnetic phase concentration and also by the interaction of nonmagnetic silver with magnetic phase. These observations are in good agreement with literature reported for Ag/Fe3O4 [45,46]. Hence, the synthetic procedure adopted in this
12
investigation is comparatively more facile in nature and requires less temperature compared to hydrothermal method reported in the literature [44] to generate good quality magnetite nanoparticles from the mineral waste, which follows the concept of waste to wealth. Mössbauer spectrum of magnetite nanoparticles obtained at room temperature is shown in Fig. 7. It shows mainly two sextets corresponding to the various oxidation states of Fe in magnetite. These observed peaks are in good accordance with the reported literature [47]. Magnetite (Fe3O4) has inverse spinel structure with fcc close packed oxygen anions and Fe cations occupying at interstitial tetrahedral and octahedral sites [48]. The magnetite is represented with the formula [Fe3+]A [Fe2+1-3x Fe3+1+2x ¨x]B O4 with x=0, ¨ for vacancies. ,where A and B designate tetrahedral and octahedral sites respectively. Two sextets observed in the spectrum can be resolved into one for high spin Fe3+ ion on the tetrahedral sites (Bhf =487.66 kOe) and the other one for Fe2+ ion on the octahedral site (Bhf =455.67 kOe). The rapid electron hopping between Fe 2+ and Fe3+ ions in the octahedral site at room temperature is accountable for good thermal conductivity which makes magnetite an important class of semi-metallic materials [49]. The isomer shift (IS) value of 0.37 mm s-1 observed from the Mössbauer spectrum confirms the presence of Fe 3+ ions in A and B-sites, whereas other IS value, 0.56 mms -1 is the characteristic for Fe2+ ions in B-site. These IS values (Table 1) are very much close to the reported value for magnetite [50]. However, weak features observed in the middle of the spectrum are the indications for the presence of trace amounts of other phase which might be akaganéite (b-FeOOH) as its IS value, 0.35 mm s-1 matches for the Fe3+ of b-FeOOH [51]. Formation of akaganéite/goethite is quite possible due to magnetically induced oxidation of Fe2+ on the octahedral site (from a low spin state Fe2+ to a high spin state Fe3+) during magnetic separation of magnetite nanoparticles from aqueous solution [52]. Existence of b-FeOOH phase confirmed from TGA and FTIR studies is well supporting the 13
Mossbauer results. Further, it can be stated that the concentration of b-FeOOH phase determined from TGA and Mossbauer spectrum are almost identical. Escherichia Coli, a gram-negative bacterium, is used to investigate the relative antibacterial activity of Fe3O4, hydrophobic Ag-Fe3O4 and hydrophilic Ag-Fe3O4 nanomaterials. Qualitative insight on the relative antibacterial property of these samples made with spot inoculation method is shown in Fig. 8. It reveals that Fe3O4 does not show any zone of inhibition area, whereas hydrophobic Ag-Fe3O4 and hydrophilic Ag-Fe3 O4 exhibit significant zone of inhibition area confirming the role of Ag on Fe3O4. However, hydrophobic Ag-Fe3O4 exhibits relatively less activity than hydrophilic Ag-Fe3O4. The quantitative antibacterial activity of these samples examined with respect to various time intervals up to 120 min (Fig. 9) shows that bare Fe 3O4 does not show any antibacterial activity up to 120 min. However, hydrophobic Ag-Fe 3O4 and hydrophilic Ag-Fe3O4 show significantly improved antibacterial activity, particularly from 60 to 120 min. Among hydrophobic and hydrophilic Ag-Fe3O4 nano-composites, hydrophilic Ag-Fe3O4 shows superior activity indicating its preferred affinity towards E.coli. These observations support the fact that the increase in the hydrophobicity of the nanoparticle surface leads to a decrease in nanoparticle dispersibility in aqueous solutions whereas the hydrophilic nanoparticles overcome this barrier due to the high dispersibility in aqueous solutions [53]. Therefore, conversion of nanoparticle surface from hydrophobic to hydrophilic by a suitable method widens their possible potential applications [54, 55]. It is reported that silver and its compounds are shown to be effective against both aerobic and anaerobic bacteria by precipitating bacterial cellular proteins and by blocking the microbial respiratory chain system. There are some other possible mechanisms for antibacterial property of silver nanoparticles reported are: (i) attachment of silver nanoparticles to the cell membrane and its penetration inside the bacteria, (ii) its attack on the respiratory chain in 14
bacterial mitochondria and leading to cell death and (iii) its sustained release of Ag+ inside the bacterial cells inducing oxidative stress by generating free radicals [56 and references therein]. Although Ag nanoparticles exhibit excellent antibacterial activity, it has severe limitation due to difficulty in removing them from the system causing nanotoxicity to human. However, this can be overcome by anchoring it on magnetite nanoparticles that brings the change for easy separation by a suitable magnet in order to reuse them for a number of times. Hydrophilic Ag-Fe 3O4 nanocomposite studied in this investigation could be used in the removal of bacteria from water. Hence, magnetite nanoparticles derived from iron ore tailings can act as a good support for silver nanoparticles to use it effectively in the water disinfection.
Conclusions Iron ore tailings considered as waste from a mineral industry is used effectively as a prime raw material for the synthesis of magnetite nanoparticles. This study demonstrates the origin of a nanomaterial from mineral waste. Mechanical milling followed by chemical route is employed for the facile synthesis of magnetite nanoparticles. Magnetite nanoparticles derived from waste exhibit very prominent magnetism by showing saturation magnetization value of 60 emu/g. Its surface modification with silver (Ag-Fe3O4) induces the antibacterial property, which provides a scope of its utilization in water treatment. Therefore, Ag-Fe3O4 can become a better option in water treatment process compared to bare silver. Overall, this paper shows the path to convert waste into wealth and also provides a possible application of the end product in water treatment. Acknowledgement
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Science and Engineering Research Board, New Delhi is acknowledged for financial support and Council of scientific and Industrial Research, New Delhi is acknowledged for providing infrastructural facilities. Diptipriya Sethi, RJNRF is acknowledged for antibacterial study.
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[51] V. K. Sharma, G. Klingelhöfer, T. Nishida “Mossbauer Spectroscopy- Applications in Chemistry, Biology, and Nanotechnology” John Wiley & Sons (2013) pp-419-421. [52] J. Wang, Q. Chen, X. Li, L. Shi, Z. Peng, C. Zeng, Chem. Phys. Lett. 390 (2004) 55-58. [53] S. Sekiguchi, K. Niikura, Y. Matsuo, K. Ijiro, Langumir 28 (2012) 5503−5507. [54] B. Chudasama, A. K. Vala, Nidhi Andhariya, R. V. Upadhyay, R. V. Mehta, Nano Res. 2 (2009) 955-965. [55] C. M. Johns, E. M. V. Hoek, J. Nanopart Res 12 (2010) 1531-1551. [56] E. Abbasi, M. Milani ,S. F. Aval, M. Kouhi, A. Akbarzadeh, H. T. Nasrabadi, P. Nikasa, S. W. Joo, Y. Hanifepour ,K. N. Koshki, M. Samiei “Silver nanoparticles: Synthesis methods, bioapplications and properties” Crit. Rev. Microbiol. DOI:10.3109/1040841X.2014.912200
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Fig. 1. X-ray powder diffraction pattern of iron ore tailings mixed with iron powder and its acid insoluble residue after acid digestion.
21
Fig. 2. X-ray powder diffraction pattern of magnetite nanoparticles.
22
23
Fig. 3a. FE-SEM micrographs of (a) uncoated Fe 3O4 and (b). Ag coated magnetite nanoparticles. 24
Fig. 3b. EDS spectra of Fe3O4 (left) Ag-Fe3O4 (right) nanoparticles
25
Fig. 4. FTIR spectra of (a) Bare magnetite (b) Hydrophobic Ag coated magnetite and (c) Hydrophilic Ag coated magnetite nanoparticles.
Fig. 5 TGA curve of uncoated magnetite nanoparticles
26
Fig. 6a. Magnetite nanoparticles dispersed in hexane before (left) and after (right) application of magnetic field.
Fig. 6b. VSM curve of magnetite nanoparticles.
27
Fig. 6c. Comparative VSM curve of magnetite and Ag coated magnetite nanoparticles.
28
Fig. 7. Mossbauer spectrum of magnetite nanoparticles.
Fig. 8. Relative antibacterial activity of uncoated magnetite, Hydrophobic Ag coated magnetite and Hydrophilic Ag coated magnetite nanoparticles observed through spot inoculation method.
29
Fig. 9. Quantitative antibacterial activity of various samples.
30
Table 1. Mössbauer parameters of magnetite nanoparticles measured at room temperatures. (Isomer shift, IS given in mm/s, quadrupole splitting De given in mm/s, Hhf is the hyperfine magnetic field given in kOe, and RAA is relative absorption area of various components of the spectrum given as percentages).
Sample
Magnetite (Fe3O4)
Akaganéite (β- FeOOH)
IS (mm/s)
De(mm/s)
Components
RAA (%)
Hhf (kOe)
Fe3+(Tetrahedral site)
46.08
0.37
0.05
487.66
Fe2+ , Fe3+(Octahedral site)
45.25
0.56
-0.05
455.67
Fe3+
5.66
0.35
0.66
31
Highlights ·
Mineral waste becomes a valuable source for the synthesis of magnetite
·
Milling helps uniform mixing of reductant with iron ore tailings
·
Magnetite nanoparticles exhibit saturation magnetization of 60 emu/g
·
Ag coating induces antibacterial activity of magnetite
32
S0925-8388(15)01382-1 http://dx.doi.org/10.1016/j.jallcom.2015.05.089 JALCOM 34205
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
24 November 2014 28 April 2015 11 May 2015
Please cite this article as: R. Kumar, R. Sakthivel, R. Behura, B.K. Mishra, D. Das, Synthesis of magnetite nanoparticles from mineral waste, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/ j.jallcom.2015.05.089
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Synthesis of magnetite nanoparticles from mineral waste Rohit Kumar1, R. Sakthivel*1, Reshma Behura1, B. K. Mishra1, D. Das2 1
CSIR- Institute of Minerals and Materials Technology, Bhubaneswar-751 013, INDIA 2
UGC-DAE Consortium, Kolkata, India
Abstract Magnetite nanoparticles were synthesized from iron ore tailings; - a mineral waste collected from the iron ore processing plant. Mechanical milling followed by chemical route is employed to obtain the magnetite nanoparticles from the waste. The magnetite nanoparticles were characterized by X-ray diffractometer, Field Emission Scanning Electron Microscope, Fourier Transform Infrared Spectrometer and Vibrating Sample Magnetometer. X-ray diffraction pattern confirms the existence of a magnetite phase. Field Emission Scanning Electron Microscopic (FE-SEM) pictures reveal that the particle size is below 100 nm. Fourier Transform Infrared (FTIR) spectrum shows a band at 570 cm-1 for the Fe-O bond vibration. Vibrating Sample Magnetometric (VSM) study shows high saturation magnetization value of 60 emu/g at low applied magnetic field. Silver coated magnetite nanoparticles exhibits antibacterial property whereas bare magnetite does not. Keywords: Mineral waste, iron ore tailings, magnetite nanoparticles, saturation magnetization, antibacterial property.
*
Corresponding author Email: [email protected], [email protected]
1
1.
Introduction Enormous quantity of iron ore tailings is disposed off as a mineral waste from the iron ore
processing industries due to rapidly increasing demand and growth of iron and steel industries in India. It has been reported that ~18-20 million tons of tailings per year in India [1]. Since it does not have direct or indirect commercial use, it gradually keeps on accumulating. Safe disposal of this waste is most essential as it occupies a larger area that results in land pollution. Due to the fine nature of its particles, it easily contaminates the ground water as well as various water bodies through rain and by wind; it ultimately affects the human beings and aquatic animals [2]. Therefore, more efforts are needed to utilize this waste for its possible applications. Indian iron ore tailings generally contain 26.8% Fe2O3, 72.4% SiO2 and 0.7% Al2O3. As it contains high amount of SiO2 (>80%), it is suggested as a raw material for construction, ceramics, glass, and to make a new class of silicate [3, 4]. Effort has been made to prepare försterite from iron ore tailings due to its wider applications as it has good creep stability, high refractory temperature and also it provides better thermal insulation [5]. This observation has given the scope for effective utilization of tailings for the preparation of value added materials. Extraction of iron from tailings and converting that into products that are more valuable can find wider applications. If the hematite content of iron ore tailings is converted into magnetite nanoparticles, it can find its suitability in various applications. Magnetite nanoparticles find applications in the field of catalysis [6], information storage [7], adsorption [8], medicine [9], nano-tagging [10], ferrofluids [11], removal of heavy ions like arsenic from water [12] and even in the field of biomedical applications like magnetic separation [13], drug delivery [14], hyperthermia treatments [15] and magnetic resonance imaging (MRI) contrast enhancements [16]. In the literature, various methods such as sol-gel [17], thermal decomposition [18], co-precipitation [19, 20], reverse micelles [21, 22], hydrothermal 2
synthesis [23] etc. are reported for the synthesis of magnetite nanoparticles. However, the main challenges lie in obtaining desired shape and narrow size distribution of particles as they play a critical role in their practical applications [24]. Some control on these parameters could be achieved by optimizing the parameters [25-26] such as reaction temperature, pH, reaction time and the environment maintained for synthesis. Other than chemical methods, mechanical milling approach is also suggested by Carvalho et al. [27] and Can et al. [28], in which they reported that the milling of metallic iron powder with distilled water at variable milling times results in formation of a variable size range (12-20 nm) of magnetite nanoparticles; reaction involved in this method is given below. 3Fe + 4H2O à Fe3O4 + 4H2
(1)
Alcala et al. [29] demonstrated that hematite could be transformed into magnetite by milling it with iron according to the following reaction scheme: 4Fe2O3 + Fe à 3Fe3O4
(2)
Mechanical milling is a very simple technique unlike chemical methods that involve optimizing parameters like pH, reaction temperature, filtration, washing, etc. In that direction, in our previous study, we attempted to prepare magnetite out of iron ore tailings through mechanical milling with iron powder as a reductant. Unfortunately, we failed to obtain the desired magnetite product; instead ended up obtaining undesired fayalite (Fe 2SiO4) phase due to the interference of the high amount of silica (SiO2) in the tailings during mechanochemical reaction [30]. Inhibition of this side reaction of silica becomes very difficult since the mechanical milling is not under thermodynamic equilibrium condition. To overcome this issue, in this investigation, we adopted a suitable method involving both physical and chemical approaches for transforming hematite portion of tailings into 3
magnetite nanoparticles. It has been reported that magnetite is used in cancer therapy due to its good biocompatible nature and also used as a support for metal nanoparticles [31]. Silver nanoparticles exhibit excellent antimicrobial and antibacterial activity without light. Its antibacterial activity increases with the size reduction, but there is a great risk in using these finer nanoparticles because of removal difficulties after their use. Its persistence in the mammiferous cells causes argyrosis and argyia [32-33]. It can also cause clinical toxicity in human even at lower concentration of 100 ppb [34]. Hence, in the water purification process, use of silver nanoparticles to remove bacteria is restricted. So there is a need to develop suitable method to eradicate this practical difficulty. Here, we demonstrate the synthesis of magnetite nanoparticles from mineral waste and using it as a support for silver nanoparticles for its application in removal of bacteria from water. 2. Materials and methods Iron ore tailings collected from the iron ore processing plant contain about 26.8% Fe 2O3, 72.4% SiO2 and 0.7% Al2O3. In order to convert hematite (Fe2O3) portion of tailings into magnetite, an appropriate amount of iron was added. The molar concentration ratio of added Fe to Fe2O3 content of tailings was maintained at 1:4. Then this mixed sample was subjected to ball milling for 2 hr for uniform mixing/distribution of iron in the tailings. The sample obtained after milling was transferred into a round bottom flask, subjected to acid digestion at 95 °C for 4-5 hr with addition of required amount of hydrochloric acid solution and cooled down to room temperature. During acid digestion, iron (III) leached out from the tailings as iron (III) chloride and a portion of it reacted with metallic Fe mixed in the tailings to form iron (II) chloride solution. These two processes happened simultaneously and resulted in the formation of 2:1 molar ratio of iron (III) and iron (II) chloride solution. Then, this solution was separated out from the acid 4
insoluble residue by filtration. After separation, the obtained solution was quantified for both Fe (III) and Fe (II) by standard wet chemical analysis and was found that they are in the 2:1 molar ratio. This solution was transferred into a round bottom flask and sufficient amount of urea was added to it. It was refluxed at 95 °C until black colored fine particles of magnetite was obtained. During this process, color of the solution was changed from brown to black due to changes in the
5
(
2
3
+
2
+
2
+
3)
ℎ
)
(
2 ℎ
(
)
. 2
+
.
95°
6
pH by slow release of NH3 by the decomposition of urea. Then, it was cooled down to room temperature and fine magnetite particles were separated out from aqueous solution by magnetic separation and washed with de-oxygenated distilled water to remove chloride followed by alcohol to remove moisture and dried in a vacuum desiccator at room temperature. A schematic representation for preparation of magnetite nanoparticles from tailings is given above. A portion of magnetite nanoparticles was coated with silver nanoparticles by incipient wet impregnation of aqueous silver acetate followed by sonication, washing and drying. Since the obtained Ag-functionalized magnetite (Ag/Fe3O4) is hydrophobic in nature, it was converted into hydrophilic by mixing it with 10 wt.% aqueous tetra methyl ammonium hydroxide up on stirring, sonication and washed with a mixed solution containing 8:1:1 of acetone, ethanol and distilled water respectively[35]. Amount of silver on magnetite nanoparticles was maintained at 1 wt.% and the same was used for antibacterial activity.
7
Antibacterial activity of Fe3O4, hydrophobic and hydrophilic Ag-Fe3O4 nanomaterials was tested against Escherichia coli (reference bacterial strain MTCC-739). Qualitative antibacterial investigation was carried out by using spot inoculation method in which sample was placed as a spot on the petri dish containing E. coli cultured in agar media. After its incubation period of 24 h at 37 °C, the zone of inhibition area indicating E. coli destruction was measured. For quantitative estimation of antibacterial property, a test solution was prepared by dispersing 1 mg/ml of sample in 25 ml sterilized tap water containing initial E. coli concentration of 107 CFU/ml. 1 ml of this test solution was drawn at various time intervals and diluted serially up to 10 -4 dilution. Then 0.1 ml of this diluted solution was spread evenly over an agar plate with the help of a sterile L-shaped glass rod. These agar plates were allowed to undergo incubation at 37 °C for 24 h. Then the number of bacterial colonies formed was counted. All antibacterial tests were carried out under dark condition at ambient atmosphere. X-ray diffraction (XRD) studies were performed using X-ray powder diffractometer (Panalytical Instrument-Model: X’ pert Pro, Netherland) using Co K α radiation (λ =1.78901 Å) having programmable divergence slit. The diffractometer was operated at 40kV and 30 mA. Diffractograms were recorded from 10 to 80 ° 2q with a scan speed of 2.15s/step and step size of 0.05° (2q). The diffraction lines were analyzed for different phases present in the sample using Highscore™ software available with the instrument.
For identification of functional groups
present in the sample, Fourier Transform Infrared (FTIR) spectrometer (spectrum GX Perkin Elmer, resolution 4cm−1) was used to record the spectrum from 400-4000 cm−1. Before recording the spectrum, KBr spectrum was taken as a reference and then sample mixed with KBr was measured. Thermogravimetry analysis of sample was carried out with Mettler Toledo (TGS/SDTA851e) instrument at a heating rate of 20°C/min. About 15 mg of sample was used for 8
TGA measurement. Further confirmation of phases present in the sample, room temperature Mössbauer spectrum was recorded with Mössbauer spectrometer using a 57Co source in a constantacceleration transmission. The spectrometer was calibrated with a standard a-Fe foil. The morphology and elemental characterization of various samples were carried out through Field emission scanning electron microscope (Zeiss Supra55). The samples for FE-SEM studies were prepared by dispersing the sample in ethanol by sonication and this homogenous solution was dispersed on an aluminum foil support. Magnetic property of the sample was evaluated through a vibrating sample magnetometer (PAR 155) at room temperature.
3. Results and Discussion X-ray diffraction pattern of iron ore tailings shown in Fig. 1 reveals that the sample is crystalline and shows diffraction peaks corresponding to hematite (Fe 2O3) and quartz (SiO2) phases. Presence of these phases is confirmed by matching the observed diffraction peaks with standard powder diffraction patterns of hematite (JCPDS F. No.85-0599) and quartz (JCPDS F. No. 86-1630). It is noticed that the intensity of diffraction peaks of quartz is more prominent than hematite indicating that quartz exists as major phase and hematite as minor. Acid leached residue obtained after acid digestion of mixed iron ore tailings and iron powder followed by solid-liquid separation shows diffraction peaks only for quartz phase (Fig. 1). It reveals that both mixed iron and hematite phase in the tailings are leached out completely during acid digestion. X-ray diffraction pattern of magnetite nanoparticles synthesized from acid leached solution is shown in Fig. 2. It shows very prominent diffraction peaks, which are well matched with the standard powder diffraction pattern of magnetite (JCPDS F. No.19-0629) as a single phase. Broadness of diffraction peaks indicates the nano-crystalline nature of the sample. The average crystallite size 9
calculated from the diffraction peaks using the following Debye-Scherrer formula is found to be 14 nm. =
l b∗
q
Where K = 0.94, l is the wavelength of X-ray, b is full width half maximum (FWHM) of the peak, q is the diffraction angle. Lattice parameter calculated from the X-ray powder diffraction data using the following equation shows the value 8.394 Å and this is in good agreement with the values reported in the standard JCPDS F.No.19-0629. =
+
+
FE-SEM images of uncoated and Ag coated magnetite nanoparticles are shown in Fig. 3. It shows that particles are well distributed and has nearly spherical morphology with narrow particle size range from 50 to 60 nm [36]. Since capping reagent is not used during the synthesis, the particles show a high tendency for agglomeration due to its fineness. The size of the particles seen from FESEM corresponds to the overall polycrystalline material, while that determined from XRD is related to the crystallite size. Evidence for coating of Ag on magnetite nanoparticles are well noticed in Fig. 3a. Fig. 3b shows the EDS spectra of Fe3O4 and Ag-Fe3O4 nanoparticles. It reveals peaks for Fe, Al, and O in both the samples whereas an additional peak for Ag confirms the presence of Ag in Ag-Fe3O4 composite. Peak observed for Al is due to aluminum foil support used in the sample preparation before FE-SEM study. The FTIR spectra (Fig. 4) show a distinct absorption band at 608 cm-1 corresponding to intrinsic stretching vibrations of Fe-O bond at the tetrahedral site (Fetetra « O) of magnetite. Similar observation has been reported for magnetite nanoparticles [37-38]. Whereas bands observed at around 3650 cm-1 and 1660 cm-1 corresponds to 10
OH stretching and OH bending of adsorbed water molecule in the lattice respectively. Whereas other bands observed at 3750 and 1023 cm-1 are for OH stretching and bending mode of hydroxyl group in akaganite. The characteristic bands of akaganeite are observed at around 670 and 880 cm 1
which are correspond to OH…Cl hydrogen bond and OH respectively [39,40]. The translational
stretching of Fe-O bond vibration in akaganite is noticed at 464 cm-1[41]. The bands observed in the region 1430 cm−1 are attributed to adsorbed anionic species like carbonates on the surface of magnetite. The adsorbed carbonate species could have come from the decomposition of urea during hydrolysis. It is observed that the band at 1023 cm-1 in case of magnetite sample gets disappeared in case of Ag/Fe3O4 samples and it might be due to Ag coating and conversion of hydrophobic to hydrophilic nature brought by tetra methyl ammonium hydroxide treatment. TGA curve of magnetite sample is shown in Fig. 5. It indicates the two stages of weight loss up to 375°C, one up to 150°C and another between 200 and 375°C. The first stage of weight loss accounts for removal of physically adsorbed water molecules whereas second stage for dehydration of b-FeOOH phase present as an impurity phase in the sample. The percentage of weight loss found to be 0.62% at second stage and it accounts for 6.12% quantity of b-FeOOH in the magnetite sample.
4. Magnetic property To find out qualitative determination of its magnetic property, the synthesized magnetite nanoparticles dispersed in hexane and placed near the magnet. Fig. 6a shows that the dispersion of magnetite nanoparticles in the absence and presence of magnetic field. It clearly demonstrates that the well dispersed particles in hexane in the absence of magnetic field get attracted after application of magnetic field. Quantitative magnetic property of magnetite nanoparticles is shown
11
as M¾H curve in Fig. 6b. It shows the saturation magnetization value of 60 emu/g. This saturation magnetization is very much sensitive to the particle size. Usually, when the size of particles reduced to nanometer range, its magnetic properties reduce drastically and they exhibit super paramagnetic behavior [42]. When we deal with fine particles, magnetization is generally characterized by two parameters viz., remanence and coercivity. Between them, coercivity is more important as it is strongly dependent on the particle size. Initially, when the particle size reduces to a critical value from larger size, coercivity increases up to a certain maximum value, after that it starts decreasing rapidly as it becomes a single domain system and it completely vanishes to exhibit super-paramagnetic behavior [43]. The coercivity and remanence of the magnetite nanoparticles studied in this investigation are 23 Oe and 4 emu/g respectively and these observations indicate that the sample approaches towards superparamagnetic behavior. Magnetic property and particle morphology of magnetite nanoparticles obtained from the tailings is very much similar to those synthesized from hydrothermal method [44]. The saturation magnetization value of 65 emu/g is reported for the magnetite having an average particle size of 70 nm with spherical shape, whereas magnetite particles studied in this investigation show 60 emu/g for the particle size range of 50-60 nm with the same morphology. A comparative VSM curve of uncoated and silver coated magnetite is shown in Fig. 6c. It shows that silver coating of magnetite reduces the saturation magnetization value from 65 to 55emu/g but this trend becomes reverse in case of coercivity values. Bare magnetite has coercivity value of 23 Oe, whereas Ag/Fe 3O4 has 74.3Oe. This reduction in the saturation magnetization and increased coercivity values brought by silver coating could be due to the reduction in overall magnetic phase concentration and also by the interaction of nonmagnetic silver with magnetic phase. These observations are in good agreement with literature reported for Ag/Fe3O4 [45,46]. Hence, the synthetic procedure adopted in this
12
investigation is comparatively more facile in nature and requires less temperature compared to hydrothermal method reported in the literature [44] to generate good quality magnetite nanoparticles from the mineral waste, which follows the concept of waste to wealth. Mössbauer spectrum of magnetite nanoparticles obtained at room temperature is shown in Fig. 7. It shows mainly two sextets corresponding to the various oxidation states of Fe in magnetite. These observed peaks are in good accordance with the reported literature [47]. Magnetite (Fe3O4) has inverse spinel structure with fcc close packed oxygen anions and Fe cations occupying at interstitial tetrahedral and octahedral sites [48]. The magnetite is represented with the formula [Fe3+]A [Fe2+1-3x Fe3+1+2x ¨x]B O4 with x=0, ¨ for vacancies. ,where A and B designate tetrahedral and octahedral sites respectively. Two sextets observed in the spectrum can be resolved into one for high spin Fe3+ ion on the tetrahedral sites (Bhf =487.66 kOe) and the other one for Fe2+ ion on the octahedral site (Bhf =455.67 kOe). The rapid electron hopping between Fe 2+ and Fe3+ ions in the octahedral site at room temperature is accountable for good thermal conductivity which makes magnetite an important class of semi-metallic materials [49]. The isomer shift (IS) value of 0.37 mm s-1 observed from the Mössbauer spectrum confirms the presence of Fe 3+ ions in A and B-sites, whereas other IS value, 0.56 mms -1 is the characteristic for Fe2+ ions in B-site. These IS values (Table 1) are very much close to the reported value for magnetite [50]. However, weak features observed in the middle of the spectrum are the indications for the presence of trace amounts of other phase which might be akaganéite (b-FeOOH) as its IS value, 0.35 mm s-1 matches for the Fe3+ of b-FeOOH [51]. Formation of akaganéite/goethite is quite possible due to magnetically induced oxidation of Fe2+ on the octahedral site (from a low spin state Fe2+ to a high spin state Fe3+) during magnetic separation of magnetite nanoparticles from aqueous solution [52]. Existence of b-FeOOH phase confirmed from TGA and FTIR studies is well supporting the 13
Mossbauer results. Further, it can be stated that the concentration of b-FeOOH phase determined from TGA and Mossbauer spectrum are almost identical. Escherichia Coli, a gram-negative bacterium, is used to investigate the relative antibacterial activity of Fe3O4, hydrophobic Ag-Fe3O4 and hydrophilic Ag-Fe3O4 nanomaterials. Qualitative insight on the relative antibacterial property of these samples made with spot inoculation method is shown in Fig. 8. It reveals that Fe3O4 does not show any zone of inhibition area, whereas hydrophobic Ag-Fe3O4 and hydrophilic Ag-Fe3 O4 exhibit significant zone of inhibition area confirming the role of Ag on Fe3O4. However, hydrophobic Ag-Fe3O4 exhibits relatively less activity than hydrophilic Ag-Fe3O4. The quantitative antibacterial activity of these samples examined with respect to various time intervals up to 120 min (Fig. 9) shows that bare Fe 3O4 does not show any antibacterial activity up to 120 min. However, hydrophobic Ag-Fe 3O4 and hydrophilic Ag-Fe3O4 show significantly improved antibacterial activity, particularly from 60 to 120 min. Among hydrophobic and hydrophilic Ag-Fe3O4 nano-composites, hydrophilic Ag-Fe3O4 shows superior activity indicating its preferred affinity towards E.coli. These observations support the fact that the increase in the hydrophobicity of the nanoparticle surface leads to a decrease in nanoparticle dispersibility in aqueous solutions whereas the hydrophilic nanoparticles overcome this barrier due to the high dispersibility in aqueous solutions [53]. Therefore, conversion of nanoparticle surface from hydrophobic to hydrophilic by a suitable method widens their possible potential applications [54, 55]. It is reported that silver and its compounds are shown to be effective against both aerobic and anaerobic bacteria by precipitating bacterial cellular proteins and by blocking the microbial respiratory chain system. There are some other possible mechanisms for antibacterial property of silver nanoparticles reported are: (i) attachment of silver nanoparticles to the cell membrane and its penetration inside the bacteria, (ii) its attack on the respiratory chain in 14
bacterial mitochondria and leading to cell death and (iii) its sustained release of Ag+ inside the bacterial cells inducing oxidative stress by generating free radicals [56 and references therein]. Although Ag nanoparticles exhibit excellent antibacterial activity, it has severe limitation due to difficulty in removing them from the system causing nanotoxicity to human. However, this can be overcome by anchoring it on magnetite nanoparticles that brings the change for easy separation by a suitable magnet in order to reuse them for a number of times. Hydrophilic Ag-Fe 3O4 nanocomposite studied in this investigation could be used in the removal of bacteria from water. Hence, magnetite nanoparticles derived from iron ore tailings can act as a good support for silver nanoparticles to use it effectively in the water disinfection.
Conclusions Iron ore tailings considered as waste from a mineral industry is used effectively as a prime raw material for the synthesis of magnetite nanoparticles. This study demonstrates the origin of a nanomaterial from mineral waste. Mechanical milling followed by chemical route is employed for the facile synthesis of magnetite nanoparticles. Magnetite nanoparticles derived from waste exhibit very prominent magnetism by showing saturation magnetization value of 60 emu/g. Its surface modification with silver (Ag-Fe3O4) induces the antibacterial property, which provides a scope of its utilization in water treatment. Therefore, Ag-Fe3O4 can become a better option in water treatment process compared to bare silver. Overall, this paper shows the path to convert waste into wealth and also provides a possible application of the end product in water treatment. Acknowledgement
15
Science and Engineering Research Board, New Delhi is acknowledged for financial support and Council of scientific and Industrial Research, New Delhi is acknowledged for providing infrastructural facilities. Diptipriya Sethi, RJNRF is acknowledged for antibacterial study.
16
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Fig. 1. X-ray powder diffraction pattern of iron ore tailings mixed with iron powder and its acid insoluble residue after acid digestion.
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Fig. 2. X-ray powder diffraction pattern of magnetite nanoparticles.
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Fig. 3a. FE-SEM micrographs of (a) uncoated Fe 3O4 and (b). Ag coated magnetite nanoparticles. 24
Fig. 3b. EDS spectra of Fe3O4 (left) Ag-Fe3O4 (right) nanoparticles
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Fig. 4. FTIR spectra of (a) Bare magnetite (b) Hydrophobic Ag coated magnetite and (c) Hydrophilic Ag coated magnetite nanoparticles.
Fig. 5 TGA curve of uncoated magnetite nanoparticles
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Fig. 6a. Magnetite nanoparticles dispersed in hexane before (left) and after (right) application of magnetic field.
Fig. 6b. VSM curve of magnetite nanoparticles.
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Fig. 6c. Comparative VSM curve of magnetite and Ag coated magnetite nanoparticles.
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Fig. 7. Mossbauer spectrum of magnetite nanoparticles.
Fig. 8. Relative antibacterial activity of uncoated magnetite, Hydrophobic Ag coated magnetite and Hydrophilic Ag coated magnetite nanoparticles observed through spot inoculation method.
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Fig. 9. Quantitative antibacterial activity of various samples.
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Table 1. Mössbauer parameters of magnetite nanoparticles measured at room temperatures. (Isomer shift, IS given in mm/s, quadrupole splitting De given in mm/s, Hhf is the hyperfine magnetic field given in kOe, and RAA is relative absorption area of various components of the spectrum given as percentages).
Sample
Magnetite (Fe3O4)
Akaganéite (β- FeOOH)
IS (mm/s)
De(mm/s)
Components
RAA (%)
Hhf (kOe)
Fe3+(Tetrahedral site)
46.08
0.37
0.05
487.66
Fe2+ , Fe3+(Octahedral site)
45.25
0.56
-0.05
455.67
Fe3+
5.66
0.35
0.66
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Highlights ·
Mineral waste becomes a valuable source for the synthesis of magnetite
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Milling helps uniform mixing of reductant with iron ore tailings
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Magnetite nanoparticles exhibit saturation magnetization of 60 emu/g
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Ag coating induces antibacterial activity of magnetite
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