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carbon nanotube composites
Author’s Accepted Manuscript Synthesis, Processing, Characterization, and Applications of Red Mud/Carbon Nanotube Composites Saloumeh Mesgari Abbasi, Alimorad Rashidi, Azam Ghorbani, Gholamreza Khalaj www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)31219-6 http://dx.doi.org/10.1016/j.ceramint.2016.07.146 CERI13371
To appear in: Ceramics International Received date: 13 June 2016 Revised date: 14 July 2016 Accepted date: 21 July 2016 Cite this article as: Saloumeh Mesgari Abbasi, Alimorad Rashidi, Azam Ghorbani and Gholamreza Khalaj, Synthesis, Processing, Characterization, and Applications of Red Mud/Carbon Nanotube Composites, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.07.146 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, Processing, Characterization, and Applications of Red Mud/Carbon Nanotube Composites Saloumeh Mesgari Abbasia*, Alimorad Rashidib, Azam Ghorbanic, Gholamreza Khalaja a
Department of Materials Engineering, Saveh Branch, Islamic Azad University, Saveh, Iran. Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. c Department of Chemistry, Saveh Branch, Islamic Azad University, Saveh, Iran.
b
[email protected] [email protected] * Corresponding author. Tel.: +98 8642433342; fax: +98 8642433049
Abstract In the present study, red mud/carbon nanotube (RM/CNT) composites were synthesized by decomposition of hydrocarbon gas using the chemical vapor deposition (CVD) method. Red mud, a by-product of the Bayer process of alumina production, was used as a catalyst in this process. Synthesis was done at different growth temperatures (600, 700, 750 and 850 °C). The microstructure and morphology of the synthesized nanocomposite was characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Brunauer– Emmett–Teller (BET), Simultaneous Thermal Analysis (STA), and Raman spectra analysis. The prepared nanocomposite, which was prepared at 750 °C, was used for Pb (II) adsorption. The adsorption characteristics of the nanocomposite for Pb (II) removal were investigated as a function of pH, adsorbent dose, and contact time. The best result was obtained with 0.05 g of adsorbent at a pH = 5with 40 min of contact time. The results proved that RM/CNT composites are a good adsorbent for lead ions in comparison with unprocessed RM or pristine CNTs. Keywords: Red mud; Carbon nanotubes; Nanocomposite; Adsorption; Lead.
1- Introduction 1
Red mud is a solid waste residue formed after the caustic digestion of bauxite ores during the production of alumina [1]. Red mud is a highly alkaline waste material with a pH of 10-13, because of the sodium hydroxide solution used in the refining process. Red mud is primarily composed of fine particles containing aluminum, iron, silicon, titanium oxides, and hydroxides [1]. Each year, about 90 million tons of red mud is produced globally. For every ton of alumina produced, the process can leave behind a third of a ton to more than two tons of red mud [1, 2]. Because of its alkaline nature, and the chemical and mineralogical species present in red mud, this solid waste causes a significant impact on the environment, and proper disposal of waste red mud presents a huge challenge where alumina industries are installed [1,2]. Many attempts have been made in past years to find some practical applications for red mud. Some of the successful applications have included construction materials and ceramics [3], surface treatment for carbon steel [4], and a low-cost adsorbent for removal of pollutants from aqueous solutions or the gas phase [5,6]. In addition, red mud can also be employed as a catalyst for hydrogenation, hydro dechlorination, and hydrocarbon oxidation [7, 8]. Of these processes, the most promising efficient technique has been identified as adsorption with a suitable adsorbent. Carbon nanotubes (CNTs) are unique and versatile adsorbents because of the availability of extensive surface area, its micro porous structure, and high adsorption capacity; but the high cost restricts their use [9]. Hence, the production of carbon nanotubes from cheaper materials is a need, and has gained significant attention for waste water treatment in developing countries. Various researchers have developed carbon nanotubes from red mud by fluidized bed chemical vapor deposition (CVD) [10]. Red mud has thermally stable oxides, e.g., Al2O3, SiO2, and TiO2, which can play two important roles: to disperse the Fe phases for the CVD reaction, and to
2
contribute to relating to the amphiphilic character of the product with their hydrophilic surfaces [11]. CVD is a relatively simple and economical technique for the synthesis of CNTs as compared with arc-discharge, and laser ablation methods [12]. It can be done at low temperatures and ambient pressures. This method offers higher purity, a larger yield, and better control of the growth parameters and structure. To date, CVD is considered a cost-effective method for the production of good-quality CNTs. It has the potential to scale up the production of CNTs to the commercial level [13]. In recent years, metal oxide nanoparticles supported on carbon nanotubes have been extensively studied and found to be excellent catalysts, as well as an effective adsorbent for the removal of heavy metal ions and hazardous organic chemicals from water [14]. G. E. J. Poinern et al. [15] synthesized a CNT-Ferrihydrite nanocomposite and reported that CNT-Ferrihydrite proved to be a good adsorbent for arsenic. V. K. Gupta et al. [16] synthesized alumina-coated multiwall CNTs. They showed that this material was found to be an effective adsorbent for the removal of lead ions from aqueous solutions. However, RM /CNT composites have not been previously investigated for the remediation of lead ions from water. The objective of this study is to investigate the ability of RM/CNT composites to remove lead ions from contaminated water by adsorption. The potential of CNT/RM composites for lead removal was determined by carrying out batch adsorption studies. In this work, we report the synthesis of RM/CNT composites by catalytic decomposition of methane (CH4), as a hydrocarbon gas, using the CVD method and red mud as a catalyst.
2. Materials and methods
3
The red mud waste was obtained from the Jajarm Alumina Plant (Iranian Alumina Co.). The red mud was washed with distilled water several times until it reached a neutral pH, and was then dried to a constant weight before use. Then, it was sieved through 250 mesh steel and divided for chemical and mineralogical analysis.RM/CNT composites were synthesized by the CVD method at different temperatures (600, 700, 750 and 850 °C). For each run, 10 g of as-prepared red mud (as a catalyst) was put into a quartz boat and then placed in a horizontal tubular quartz reactor under atmospheric pressure. Subsequently, a mixture of methane and a carrier gas (CH4/H2: 1/10) was introduced into the quartz tube, which was maintained at the reaction temperature for 45 min before the furnace was cooled down to room temperature under N2 protection. The total flow rate was 500 sccm, and from room temperature to the desired temperature, the heating rate was 10 °C min-1.
2-1. Characterization methods To identify the chemical composition of the red mud, X-ray fluorescence (XRF) was employed. The morphologies and particle sizes of the as-received and RM/CNT composites were characterized by transmission electron microscopy (TEM) (FEG, Philips CM200) at an accelerating voltage of 120 kV, and by field emission scanning electron microscopy (FESEM) (Hitachi, S-4160, Vacc 15kV), with electron beam energy in the 10-30 kV range. Energydispersive X-ray spectroscopy (EDS) was used to identify the catalyst present in the methanetreated red mud using an INCA, from Oxford Instruments. X-ray diffraction (XRD) and Raman spectra analysis were used to confirm the catalyst responsible for CNT formation. Raman spectra analysis and XRD measurements were carried out with the Thermonicolet (Almega, USA), and the PAN analytical X’pert XRD System (Philips, Netherlands), with Cu kα = 1.5406 Å, 40kV,
4
and 30 mA, respectively. The thermal stability of the RM/CNT composite was determined by STA 409 PC Luxx (Netzsch, Germany).
2-2. Adsorption experiments The adsorption capacity of the RM/CNT composite was investigated. For this, a Pb solution was prepared by diluting the appropriate volume of a stock solution. The pH of the solutions was adjusted by adding 0.1 M HNO3 or 0.1M NaOH as required. The adsorption studies were carried out by placing known volumes of the Pb solutions in contact with predetermined amounts of the adsorbents in tightly closed bottles. The bottles were subjected to shaking for different periods of time by placing them in a Perth Scientific shaking water bath at 75 rpm and 25°C. After shaking, the suspension was filtered twice using a 0.22 μm Millipore syringe filter unit; then the filtrate was analyzed for residual lead by flame atomic absorption spectroscopy.
3 - Results and discussion 3-1. Characterization of red mud (RM) The chemical composition of the red mud that was determined by the X-ray Fluorescence (XRF) analyzer is given in Table 1.
Table 1- The primary chemical constituents of red mud (%). Al2O3
SiO2
Fe2O3
TiO2
CaO
Na2O
MgO
K2O
L.O.I
17.25
19.29
28.78
7.36
21.35
1.79
1.75
0.63
14.1
It can be seen from Table 1 that the primary chemical compositions of red mud are Fe2O3, Al2O3, SiO2, CaO, Na2O, TiO2, K2O, and MgO. Red mud, a by-product of the aluminum industry,
5
consists of a mixture of metal oxides. The X-ray diffraction pattern (XRD) of the red mud is shown in Figure 1.
Figure 1 - XRD pattern of red mud.
Some of the combinations identified in the sample are hematite (Fe2O3), calcite (CaCO3), sodium calcium
silicon
oxide
(Na1.8Ca1.1(Si6O14)),
kaolinite
(Al2Si2O5(OH)4),
katoite
(Ca3Al2(SiO4)(OH)8) , anatase and rutile (TiO2), and cancrinite (Na6Ca2Al6Si6O24(CO3)2·2H2O). The particle size distribution of the red mud is shown in Figure 2.
6
Figure 2- The particle diameter distribution of red mud.
It can be seen that the red mud particles are mostly in the range of 2–80 μm with a mean value of 30 μm.
3-2. RM/CNT composite characterization XRD analysis is an important tool used for clarifying the as-synthesized CNTs in the catalysts [13]. The XRD patterns of the RM/CNT composite synthesized at different temperatures are shown in Figure 3. As can be seen in the pattern in Figure 3(a), no diffraction peaks attributed to the CNTs were observed at 600 °C. In Figure 3(a), the peaks at 2θ = 52° are relevant to iron (Fe), the diffraction peaks at 2θ = 38°, 42°, and 50° are reflections of magnetite )Fe3O4( and the cancrinite [Na6Ca2Al6Si6O24(CO3)2.2H2O] has many diffraction peaks at 2θ = 32°, 38°, and 62°. There are no peaks characteristics of carbon.
7
It is obvious from the XRD pattern in Figure 3(b) that the diffraction peaks at 2θ = 30° and 50° correspond to the (002) and (100) planes of the graphite, which is attributed to the graphitic structure of CNTs [12-13], while other peaks are attributed to the red mud support. For instance, in pattern in figure 3(b), the diffraction peaks at 2θ = 28°, 33°, 36°, 41°, and 61° are related to gehlenite (Ca2Al2SiO7), while the peaks at 2θ = 52° and 57° are relevant to cohenite (Fe3C) and those at 2θ = 38° and 46° are related to tricalcium aluminate (3CaO·Al₂O₃).
Figure 3- XRD pattern of RM/CNT composite synthesized at: a) 600 °C and b) 750 °C.
8
The XRD confirmed the conversion of hematite (Fe2O3) to magnetite (Fe3O4) after reaction with methane at 600 °C; after an increase in temperature to 750 °C, magnetite was converted to iron carbide (Fe3C), and then carbon was formed from the decomposition of iron carbide [17]. Based on these data and the observed microstructure, a synthesized RM/CNT composite is possible at 750 °C. The EDS spectrum of the as- received RM and RM/CNT composite is shown in Figure 4(a, b).
Figure 4- EDS of: a) as- received RM and b) RM/CNT composite synthesized at 750 °C.
9
It can be seen that after the growth of the CNTs, it shows a distinct C peak, indicating the higher content of deposited carbon on this catalyst (Fig. 4(b)).
3-3. FESEM and TEM analysis The microstructure and morphology of both the as- prepared and the purified nanocomposite were characterized by TEM and FESEM. Figure 5(a) shows FESEM micrographs of the asprepared composite. According to these results, at 600 °C no growth of CNTs was observed, but uniform circular particles of red mud or metal oxides were grown instead. On increasing the growth temperature to 700 °C, the growth of the CNTs is shown in Figure 5(b), along with circular carbon particles. These particles are larger than those observed at the previous growth temperature, but still they maintain the circular shape. These results are, as the results of other researchers [14, 15].
10
Figure 5- Comparison of FESEM images of synthesized RM/CNT composite growth at different reaction temperatures: a) at 600 °C, b) at 700 °C, and c1), c2) at 750 °C before HNO3 purification.
Further increase in growth temperature up to 750 °C results in better growth of CNTs (see Figure 5(c1, c2)). Figure 6(a, b) shows the TEM images of RM/CNT composite samples. The carbon nanostructures formed over red mud were observed both by FESEM and by TEM.
Figure 6- The TEM images of synthesized RM/CNT composite after HNO3 purification (a, b).
The TEM images showed RM particles encapsulated by carbon.
3-3-1. Carbon yield 11
In order to investigate the effect of red mud and temperature on the growth of CNTs, carbon yield calculated using the following equation [18]: Carbon yield = [MTotal _ MCatalyst] / M Catalyst × 100% Where MTotal is the total mass of the final catalyst and carbon products and MCatalyst is the initial mass of the catalyst. Carbon yield as a function of growth temperature is plotted in Figure 7.
Figure 7- Carbon yield versus growth temperature.
As shown in Figure 7, the maximum yield of carbon from methane on red mud obtained at 750 °C. Carbon formation via methane decomposition is based on the following reaction [12]: CH4
2 H2 + C
Graphitic carbons are generally formed at reaction temperatures greater than 500 °C, while at low reaction temperatures, both carbidic and graphitic carbons may be observed [12,18]. The atomic carbon may give rise to amorphous carbon and graphitic carbon by undergoing polymerization, or it may lead to the formation of carbide by dissolution into the metal lattice [12, 18]. According to the two widely accepted ‘tip-growth’ and ‘base-growth’ mechanisms, the hydrocarbon gas decomposes on the surface of the metal particles to release carbon. The carbon atoms will dissolve in these metal particles and diffuse through the particle, resulting in the 12
precipitation of the filament. Moreover, the metal-support interactions are found to play a determinant role for the growth mechanism [18, 19]. 3-4. Raman spectra analysis The Raman spectra are used for understanding the quality of the carbon nanotubes [18]. Figures 8(a) and 8(b) show the Raman spectra of RM and the RM/CNT composite, respectively. Figure 8(a) shows the Raman spectra of the as-received red mud. A series of bands related to the different oxides, e.g., oxides of Fe, Si, Al, and Ti, that are present in the RM, can also be observed in the Raman spectra (range 100-1500 cm-1).
Figure 8- Raman spectra of: a) RM and b) RM/CNT composite.
As expected, Raman spectra, Figure 8(b), showed D and G bands at 1356 and 1588 cm−1, respectively, after the reaction with methane. It should be noted that the presence of an intense D band at about 1340 cm−1 suggests the formation of more defective carbonaceous structures, such as amorphous carbon, while the G band at about the 1580 cm-1 wave length shows the formation of more organized carbon, such as graphite and carbon nanotubes (CNTs) [19, 20].
13
As seen, this sample has a ratio of D band intensity to G band intensity (ID/IG) of 1.437 (to the highest value observed in these studies). This ratio indicates the quality of the products. The higher the ratio is, the higher the quality of the products is [19, 20].
3-5. TGA studies Thermogravimetric analysis of the RM, depicted in Figure 9(a), shows a weight loss around 200– 300 °C primarily related to the dehydration of small amounts of amorphous FeOOH and AlOOH [6, 7].
Figure 9 - Thermogravimetric analysis of: a) RM and b) RM/CNT composite.
Figure 9(b) shows the presence of enlarged peaks with shoulders from about 400 up to 500 °C, indicating the formation of different forms of carbon, such as amorphous, as well as more organized carbon, e.g., graphitic structures. From these weight losses, the carbon content for the composites was estimated.
3-6. Surface area studies
14
Table 2, presents the specific surface area of the as-received red mud before and after exposure to methane. A significant increase in the surface area is observed in case of the synthesized RM/CNT composite at 750 °C.
Table 2- Surface area analysis of red mud samples before and after exposure to methane.
Sample code
BET surface area, m2/g
RM
11
RM/CNT composite (650˚C)
36
RM/CNT composite (750˚C)
78
The surface area of the RM (11 m2·g−1) showed a small increase (to 36 m2·g−1) after CVD at 650 °C, but the synthesized RM/CNT composite at 750 °C has a higher specific surface area than the red mud. On the other hand, a strong increase was observed after CVD at 750 °C (to 78 m2·g−1). The TEM images suggest the presence of irregular carbon nanotubes which are likely responsible for the increase on the surface area of the composites. This result is one of the reasons that led to the increase in the RM/CNT composite’s adsorption capacity.
3-7. Adsorption properties To find the optimum adsorption condition of the RM/CNT composite, the parameters that affected the adsorption properties, such as adsorption dosage, pH, and contact time, were tested. RM/CNT composites with different dosages of 0.01-0.1 g were tested. The results show that 0.05 g of prepared composite have good capacity for Pb ion removal .As the pH is one of the important parameter in adsorption properties, it was tested in the range of 2-11. The best results were obtained with pH = 5. Contact time is another parameter, which was tested in the range of 20-70 min. As the result shows, by increasing contact time from 20 to 50, the adsorption capacity 15
increases, so the highest results were obtained in 50 min of contact time. Beyond that, the removal capacity of lead ions decreased.
Conclusions In this work, RM/CNT composite was synthesized by the decomposition of methane over RM as a catalyst. It was observed that certain parameters, like growth temperature, have a strong influence on both the synthesis and the carbon yield. Under experimental setup, the optimum conditions were obtained at a temperature of 750 °C, with a 1 hour reaction time, and at a total flow rate of gas of 500 sccm. The composite obtained by CVD with methane led to 48% carbon deposition. The surface area of the composite compared with the RM used in the preparation is very high. The TEM images suggest the presence of irregular carbon nanotubes, which are likely responsible for the increase in the surface area of the composites. While RM itself is a poor adsorbent for lead, the RM/CNT composite is an excellent adsorbent, and like RM, the adsorption of the RM/CNT composite is pH-dependent. The adsorption increased as the pH increased from 2 to 5, then the adsorption capacity decreased as pH was increased up to 11. This property of the RM/CNT composite adsorbent would make it a valuable adsorbent for lead remediation from acidic hydrometallurgical waste liquors. The results show that the fabricated RM/CNT composite had an effective adsorbent for the removal of lead aqueous solution.
References [1] Rashad A. M. Alkali-activated metakaolin. A short guide for civil Engineer– An overview. Construction and Building Materials, 41, 751–765, (2013) [2] Autef A, Joussein E, Gasgnier G, Rossignol S. Role of the silica source on the geopolymerization rate. Journal of Non-Crystalline Solids, 366:2886–2893, (2012) 16
[3] Aly Z, Vance E.R, Perera D.S. Aqueous dissolution of sodium aluminosilicate geopolymers derived from metakaolin. Journal of Nuclear Materials, 424, 164–170, (2012) [4] Mesgari Abbasi, S., Ahmadi, H., Khalaj, G., Ghasemi, B., 2016, Microstructure and mechanical properties of a Metakaolinite-based geopolymer nanocomposite reinforced with carbon nanotubes, Ceramics International. Online [5] Nazari, A., Khalaj, G., 2012, Prediction compressive strength of lightweight geopolymers by ANFIS, Ceramics International, 38, 4501- 4510 [6] Jin F, Park S. A review of the preparation and properties of carbon nanotubesreinforced polymer composites. Carbon Letters, 12, 57-69, (2011) [7] Baco S, Arshad S. E, Salleh S, Alias A, Yassin F. MD. Preparation and characterization of carbon nano fiber Carbon nanofibers/ Metakaolin geopolymer based nanocomposite. Int. J. Environ. Sci. Techno, 4,474 – 481(2015) [8] Saafi M, Andrew K, Tang P.L, Ghon D.M, Taylor S, Rahman M, Yang S, Zhou X. Multifunctional properties of carbon nanotube/fly ash geopolymeric Nanocomposites, Construction and Building Materials, 49 , 46–55, (2013) [9] Tyson B, Al-Rub A, Yazdanbakhsh A, Grasley Z. Carbon Nanotubes and Carbon Nanofibers for Enhancing the Mechanical Properties of Nanocomposite Cementitious Materials. J. Mater. Civ. Eng. 23, 1028-1035, (2011) [10] Khatera H.M, Gawaad H.A. Characterization of alkali activated geopolymer mortar doped with MWCNT. Adv. Mater. Res, 4, 45-61, (2015) [11] Hui L, Baomin W, Yu H, Jun Q, Zhiqiang G. The influence of carbon nanotubes/ silica fume on the mechanical properties of cement– based composites, Romanian Journal of Materials, 45, 29 –34,(2015) [12] Hunashyal A. M, Tippa S. V, Quadri S. S, Banapurmath N. R, Experimental Investigation on Effect of Carbon Nanotubes and Carbon Fibres on the Behavior of Plain Cement Mortar Composite Round Bars under Direct Tension. International Scholarly Research Notices nanotechnology, 20, 1-6,(2011) [13] Slaty F, Khoury H, Wastiels J, Rahier H. Characterization of alkali activated kaolinitic clay. Appl. Clay Sci., 75–76, 120–125, (2013) [14] Liew Y.M, Kamarudin H, Mustafa Al Bakri A.M, Luqman M, Khairul Nizar I, Ruzaidi C.M, Heah C.Y. Processing and characterization of calcined kaolin cement powder, Construction and Building Materials, 30, 794–802,(2012) [15] G. E. J. Poinern, D. Parsonage, T. B. Issa, M.K. Ghosh, E. Paling and P. Singh, Preparation, characterization and As(V) adsorption behaviour of CNT-ferrihydrite composites, International Journal of Engineering, Science and Technology, Vol. 2, No. 8, 2010, pp. 13-24. [16] V.K. Gupta, S. Agarwal, T.A.Saleh, Synthesis and characterization of aluminacoated carbon nanotubes and their application for lead removal, J Hazard Mater, 2011 Jan 15;185(1):17-23. [17] Kuenzel C, Neville T.P, Donatello S, Vandeperre L, Boccaccini A.R, Cheeseman C.R. Influence of metakaolin characteristics on the mechanical properties of geopolymers. Appl. Clay Sci., 83– 84, 308-314, (2013) [18] Ferone C, Colangelo F, Roviello G, Asprone D, Menna C, Balsamo A, Prota A, Cioffi R, Manfredi G. Application-Oriented Chemical Optimization of a Metakaolin Based Geopolymer. Materials, 6, 1920-1939, (2013)
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[19] Khalaj, G., Hassani, S.E.S., Khezrloo, A., Haratifar, E.-A.-D., 2014, Split tensile strength of OPC-based geopolymers: Application of DOE method in evaluating the effect of production parameters and their optimum condition, Ceramics International, 40, 10945- 10952. [20] I. F. Teixeira, T. P.V. Medeiros, P. E. Freitas, M. G. Rosmaninho, J. D. Ardisson, R. M. Lago, Carbon deposition and oxidation using the waste red mud: A route to store, transport and use offshore gas lost in petroleum exploration, Fuel, 124 (2014) 7–13.
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PII: DOI: Reference:
S0272-8842(16)31219-6 http://dx.doi.org/10.1016/j.ceramint.2016.07.146 CERI13371
To appear in: Ceramics International Received date: 13 June 2016 Revised date: 14 July 2016 Accepted date: 21 July 2016 Cite this article as: Saloumeh Mesgari Abbasi, Alimorad Rashidi, Azam Ghorbani and Gholamreza Khalaj, Synthesis, Processing, Characterization, and Applications of Red Mud/Carbon Nanotube Composites, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.07.146 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, Processing, Characterization, and Applications of Red Mud/Carbon Nanotube Composites Saloumeh Mesgari Abbasia*, Alimorad Rashidib, Azam Ghorbanic, Gholamreza Khalaja a
Department of Materials Engineering, Saveh Branch, Islamic Azad University, Saveh, Iran. Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. c Department of Chemistry, Saveh Branch, Islamic Azad University, Saveh, Iran.
b
[email protected] [email protected] * Corresponding author. Tel.: +98 8642433342; fax: +98 8642433049
Abstract In the present study, red mud/carbon nanotube (RM/CNT) composites were synthesized by decomposition of hydrocarbon gas using the chemical vapor deposition (CVD) method. Red mud, a by-product of the Bayer process of alumina production, was used as a catalyst in this process. Synthesis was done at different growth temperatures (600, 700, 750 and 850 °C). The microstructure and morphology of the synthesized nanocomposite was characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Brunauer– Emmett–Teller (BET), Simultaneous Thermal Analysis (STA), and Raman spectra analysis. The prepared nanocomposite, which was prepared at 750 °C, was used for Pb (II) adsorption. The adsorption characteristics of the nanocomposite for Pb (II) removal were investigated as a function of pH, adsorbent dose, and contact time. The best result was obtained with 0.05 g of adsorbent at a pH = 5with 40 min of contact time. The results proved that RM/CNT composites are a good adsorbent for lead ions in comparison with unprocessed RM or pristine CNTs. Keywords: Red mud; Carbon nanotubes; Nanocomposite; Adsorption; Lead.
1- Introduction 1
Red mud is a solid waste residue formed after the caustic digestion of bauxite ores during the production of alumina [1]. Red mud is a highly alkaline waste material with a pH of 10-13, because of the sodium hydroxide solution used in the refining process. Red mud is primarily composed of fine particles containing aluminum, iron, silicon, titanium oxides, and hydroxides [1]. Each year, about 90 million tons of red mud is produced globally. For every ton of alumina produced, the process can leave behind a third of a ton to more than two tons of red mud [1, 2]. Because of its alkaline nature, and the chemical and mineralogical species present in red mud, this solid waste causes a significant impact on the environment, and proper disposal of waste red mud presents a huge challenge where alumina industries are installed [1,2]. Many attempts have been made in past years to find some practical applications for red mud. Some of the successful applications have included construction materials and ceramics [3], surface treatment for carbon steel [4], and a low-cost adsorbent for removal of pollutants from aqueous solutions or the gas phase [5,6]. In addition, red mud can also be employed as a catalyst for hydrogenation, hydro dechlorination, and hydrocarbon oxidation [7, 8]. Of these processes, the most promising efficient technique has been identified as adsorption with a suitable adsorbent. Carbon nanotubes (CNTs) are unique and versatile adsorbents because of the availability of extensive surface area, its micro porous structure, and high adsorption capacity; but the high cost restricts their use [9]. Hence, the production of carbon nanotubes from cheaper materials is a need, and has gained significant attention for waste water treatment in developing countries. Various researchers have developed carbon nanotubes from red mud by fluidized bed chemical vapor deposition (CVD) [10]. Red mud has thermally stable oxides, e.g., Al2O3, SiO2, and TiO2, which can play two important roles: to disperse the Fe phases for the CVD reaction, and to
2
contribute to relating to the amphiphilic character of the product with their hydrophilic surfaces [11]. CVD is a relatively simple and economical technique for the synthesis of CNTs as compared with arc-discharge, and laser ablation methods [12]. It can be done at low temperatures and ambient pressures. This method offers higher purity, a larger yield, and better control of the growth parameters and structure. To date, CVD is considered a cost-effective method for the production of good-quality CNTs. It has the potential to scale up the production of CNTs to the commercial level [13]. In recent years, metal oxide nanoparticles supported on carbon nanotubes have been extensively studied and found to be excellent catalysts, as well as an effective adsorbent for the removal of heavy metal ions and hazardous organic chemicals from water [14]. G. E. J. Poinern et al. [15] synthesized a CNT-Ferrihydrite nanocomposite and reported that CNT-Ferrihydrite proved to be a good adsorbent for arsenic. V. K. Gupta et al. [16] synthesized alumina-coated multiwall CNTs. They showed that this material was found to be an effective adsorbent for the removal of lead ions from aqueous solutions. However, RM /CNT composites have not been previously investigated for the remediation of lead ions from water. The objective of this study is to investigate the ability of RM/CNT composites to remove lead ions from contaminated water by adsorption. The potential of CNT/RM composites for lead removal was determined by carrying out batch adsorption studies. In this work, we report the synthesis of RM/CNT composites by catalytic decomposition of methane (CH4), as a hydrocarbon gas, using the CVD method and red mud as a catalyst.
2. Materials and methods
3
The red mud waste was obtained from the Jajarm Alumina Plant (Iranian Alumina Co.). The red mud was washed with distilled water several times until it reached a neutral pH, and was then dried to a constant weight before use. Then, it was sieved through 250 mesh steel and divided for chemical and mineralogical analysis.RM/CNT composites were synthesized by the CVD method at different temperatures (600, 700, 750 and 850 °C). For each run, 10 g of as-prepared red mud (as a catalyst) was put into a quartz boat and then placed in a horizontal tubular quartz reactor under atmospheric pressure. Subsequently, a mixture of methane and a carrier gas (CH4/H2: 1/10) was introduced into the quartz tube, which was maintained at the reaction temperature for 45 min before the furnace was cooled down to room temperature under N2 protection. The total flow rate was 500 sccm, and from room temperature to the desired temperature, the heating rate was 10 °C min-1.
2-1. Characterization methods To identify the chemical composition of the red mud, X-ray fluorescence (XRF) was employed. The morphologies and particle sizes of the as-received and RM/CNT composites were characterized by transmission electron microscopy (TEM) (FEG, Philips CM200) at an accelerating voltage of 120 kV, and by field emission scanning electron microscopy (FESEM) (Hitachi, S-4160, Vacc 15kV), with electron beam energy in the 10-30 kV range. Energydispersive X-ray spectroscopy (EDS) was used to identify the catalyst present in the methanetreated red mud using an INCA, from Oxford Instruments. X-ray diffraction (XRD) and Raman spectra analysis were used to confirm the catalyst responsible for CNT formation. Raman spectra analysis and XRD measurements were carried out with the Thermonicolet (Almega, USA), and the PAN analytical X’pert XRD System (Philips, Netherlands), with Cu kα = 1.5406 Å, 40kV,
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and 30 mA, respectively. The thermal stability of the RM/CNT composite was determined by STA 409 PC Luxx (Netzsch, Germany).
2-2. Adsorption experiments The adsorption capacity of the RM/CNT composite was investigated. For this, a Pb solution was prepared by diluting the appropriate volume of a stock solution. The pH of the solutions was adjusted by adding 0.1 M HNO3 or 0.1M NaOH as required. The adsorption studies were carried out by placing known volumes of the Pb solutions in contact with predetermined amounts of the adsorbents in tightly closed bottles. The bottles were subjected to shaking for different periods of time by placing them in a Perth Scientific shaking water bath at 75 rpm and 25°C. After shaking, the suspension was filtered twice using a 0.22 μm Millipore syringe filter unit; then the filtrate was analyzed for residual lead by flame atomic absorption spectroscopy.
3 - Results and discussion 3-1. Characterization of red mud (RM) The chemical composition of the red mud that was determined by the X-ray Fluorescence (XRF) analyzer is given in Table 1.
Table 1- The primary chemical constituents of red mud (%). Al2O3
SiO2
Fe2O3
TiO2
CaO
Na2O
MgO
K2O
L.O.I
17.25
19.29
28.78
7.36
21.35
1.79
1.75
0.63
14.1
It can be seen from Table 1 that the primary chemical compositions of red mud are Fe2O3, Al2O3, SiO2, CaO, Na2O, TiO2, K2O, and MgO. Red mud, a by-product of the aluminum industry,
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consists of a mixture of metal oxides. The X-ray diffraction pattern (XRD) of the red mud is shown in Figure 1.
Figure 1 - XRD pattern of red mud.
Some of the combinations identified in the sample are hematite (Fe2O3), calcite (CaCO3), sodium calcium
silicon
oxide
(Na1.8Ca1.1(Si6O14)),
kaolinite
(Al2Si2O5(OH)4),
katoite
(Ca3Al2(SiO4)(OH)8) , anatase and rutile (TiO2), and cancrinite (Na6Ca2Al6Si6O24(CO3)2·2H2O). The particle size distribution of the red mud is shown in Figure 2.
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Figure 2- The particle diameter distribution of red mud.
It can be seen that the red mud particles are mostly in the range of 2–80 μm with a mean value of 30 μm.
3-2. RM/CNT composite characterization XRD analysis is an important tool used for clarifying the as-synthesized CNTs in the catalysts [13]. The XRD patterns of the RM/CNT composite synthesized at different temperatures are shown in Figure 3. As can be seen in the pattern in Figure 3(a), no diffraction peaks attributed to the CNTs were observed at 600 °C. In Figure 3(a), the peaks at 2θ = 52° are relevant to iron (Fe), the diffraction peaks at 2θ = 38°, 42°, and 50° are reflections of magnetite )Fe3O4( and the cancrinite [Na6Ca2Al6Si6O24(CO3)2.2H2O] has many diffraction peaks at 2θ = 32°, 38°, and 62°. There are no peaks characteristics of carbon.
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It is obvious from the XRD pattern in Figure 3(b) that the diffraction peaks at 2θ = 30° and 50° correspond to the (002) and (100) planes of the graphite, which is attributed to the graphitic structure of CNTs [12-13], while other peaks are attributed to the red mud support. For instance, in pattern in figure 3(b), the diffraction peaks at 2θ = 28°, 33°, 36°, 41°, and 61° are related to gehlenite (Ca2Al2SiO7), while the peaks at 2θ = 52° and 57° are relevant to cohenite (Fe3C) and those at 2θ = 38° and 46° are related to tricalcium aluminate (3CaO·Al₂O₃).
Figure 3- XRD pattern of RM/CNT composite synthesized at: a) 600 °C and b) 750 °C.
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The XRD confirmed the conversion of hematite (Fe2O3) to magnetite (Fe3O4) after reaction with methane at 600 °C; after an increase in temperature to 750 °C, magnetite was converted to iron carbide (Fe3C), and then carbon was formed from the decomposition of iron carbide [17]. Based on these data and the observed microstructure, a synthesized RM/CNT composite is possible at 750 °C. The EDS spectrum of the as- received RM and RM/CNT composite is shown in Figure 4(a, b).
Figure 4- EDS of: a) as- received RM and b) RM/CNT composite synthesized at 750 °C.
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It can be seen that after the growth of the CNTs, it shows a distinct C peak, indicating the higher content of deposited carbon on this catalyst (Fig. 4(b)).
3-3. FESEM and TEM analysis The microstructure and morphology of both the as- prepared and the purified nanocomposite were characterized by TEM and FESEM. Figure 5(a) shows FESEM micrographs of the asprepared composite. According to these results, at 600 °C no growth of CNTs was observed, but uniform circular particles of red mud or metal oxides were grown instead. On increasing the growth temperature to 700 °C, the growth of the CNTs is shown in Figure 5(b), along with circular carbon particles. These particles are larger than those observed at the previous growth temperature, but still they maintain the circular shape. These results are, as the results of other researchers [14, 15].
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Figure 5- Comparison of FESEM images of synthesized RM/CNT composite growth at different reaction temperatures: a) at 600 °C, b) at 700 °C, and c1), c2) at 750 °C before HNO3 purification.
Further increase in growth temperature up to 750 °C results in better growth of CNTs (see Figure 5(c1, c2)). Figure 6(a, b) shows the TEM images of RM/CNT composite samples. The carbon nanostructures formed over red mud were observed both by FESEM and by TEM.
Figure 6- The TEM images of synthesized RM/CNT composite after HNO3 purification (a, b).
The TEM images showed RM particles encapsulated by carbon.
3-3-1. Carbon yield 11
In order to investigate the effect of red mud and temperature on the growth of CNTs, carbon yield calculated using the following equation [18]: Carbon yield = [MTotal _ MCatalyst] / M Catalyst × 100% Where MTotal is the total mass of the final catalyst and carbon products and MCatalyst is the initial mass of the catalyst. Carbon yield as a function of growth temperature is plotted in Figure 7.
Figure 7- Carbon yield versus growth temperature.
As shown in Figure 7, the maximum yield of carbon from methane on red mud obtained at 750 °C. Carbon formation via methane decomposition is based on the following reaction [12]: CH4
2 H2 + C
Graphitic carbons are generally formed at reaction temperatures greater than 500 °C, while at low reaction temperatures, both carbidic and graphitic carbons may be observed [12,18]. The atomic carbon may give rise to amorphous carbon and graphitic carbon by undergoing polymerization, or it may lead to the formation of carbide by dissolution into the metal lattice [12, 18]. According to the two widely accepted ‘tip-growth’ and ‘base-growth’ mechanisms, the hydrocarbon gas decomposes on the surface of the metal particles to release carbon. The carbon atoms will dissolve in these metal particles and diffuse through the particle, resulting in the 12
precipitation of the filament. Moreover, the metal-support interactions are found to play a determinant role for the growth mechanism [18, 19]. 3-4. Raman spectra analysis The Raman spectra are used for understanding the quality of the carbon nanotubes [18]. Figures 8(a) and 8(b) show the Raman spectra of RM and the RM/CNT composite, respectively. Figure 8(a) shows the Raman spectra of the as-received red mud. A series of bands related to the different oxides, e.g., oxides of Fe, Si, Al, and Ti, that are present in the RM, can also be observed in the Raman spectra (range 100-1500 cm-1).
Figure 8- Raman spectra of: a) RM and b) RM/CNT composite.
As expected, Raman spectra, Figure 8(b), showed D and G bands at 1356 and 1588 cm−1, respectively, after the reaction with methane. It should be noted that the presence of an intense D band at about 1340 cm−1 suggests the formation of more defective carbonaceous structures, such as amorphous carbon, while the G band at about the 1580 cm-1 wave length shows the formation of more organized carbon, such as graphite and carbon nanotubes (CNTs) [19, 20].
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As seen, this sample has a ratio of D band intensity to G band intensity (ID/IG) of 1.437 (to the highest value observed in these studies). This ratio indicates the quality of the products. The higher the ratio is, the higher the quality of the products is [19, 20].
3-5. TGA studies Thermogravimetric analysis of the RM, depicted in Figure 9(a), shows a weight loss around 200– 300 °C primarily related to the dehydration of small amounts of amorphous FeOOH and AlOOH [6, 7].
Figure 9 - Thermogravimetric analysis of: a) RM and b) RM/CNT composite.
Figure 9(b) shows the presence of enlarged peaks with shoulders from about 400 up to 500 °C, indicating the formation of different forms of carbon, such as amorphous, as well as more organized carbon, e.g., graphitic structures. From these weight losses, the carbon content for the composites was estimated.
3-6. Surface area studies
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Table 2, presents the specific surface area of the as-received red mud before and after exposure to methane. A significant increase in the surface area is observed in case of the synthesized RM/CNT composite at 750 °C.
Table 2- Surface area analysis of red mud samples before and after exposure to methane.
Sample code
BET surface area, m2/g
RM
11
RM/CNT composite (650˚C)
36
RM/CNT composite (750˚C)
78
The surface area of the RM (11 m2·g−1) showed a small increase (to 36 m2·g−1) after CVD at 650 °C, but the synthesized RM/CNT composite at 750 °C has a higher specific surface area than the red mud. On the other hand, a strong increase was observed after CVD at 750 °C (to 78 m2·g−1). The TEM images suggest the presence of irregular carbon nanotubes which are likely responsible for the increase on the surface area of the composites. This result is one of the reasons that led to the increase in the RM/CNT composite’s adsorption capacity.
3-7. Adsorption properties To find the optimum adsorption condition of the RM/CNT composite, the parameters that affected the adsorption properties, such as adsorption dosage, pH, and contact time, were tested. RM/CNT composites with different dosages of 0.01-0.1 g were tested. The results show that 0.05 g of prepared composite have good capacity for Pb ion removal .As the pH is one of the important parameter in adsorption properties, it was tested in the range of 2-11. The best results were obtained with pH = 5. Contact time is another parameter, which was tested in the range of 20-70 min. As the result shows, by increasing contact time from 20 to 50, the adsorption capacity 15
increases, so the highest results were obtained in 50 min of contact time. Beyond that, the removal capacity of lead ions decreased.
Conclusions In this work, RM/CNT composite was synthesized by the decomposition of methane over RM as a catalyst. It was observed that certain parameters, like growth temperature, have a strong influence on both the synthesis and the carbon yield. Under experimental setup, the optimum conditions were obtained at a temperature of 750 °C, with a 1 hour reaction time, and at a total flow rate of gas of 500 sccm. The composite obtained by CVD with methane led to 48% carbon deposition. The surface area of the composite compared with the RM used in the preparation is very high. The TEM images suggest the presence of irregular carbon nanotubes, which are likely responsible for the increase in the surface area of the composites. While RM itself is a poor adsorbent for lead, the RM/CNT composite is an excellent adsorbent, and like RM, the adsorption of the RM/CNT composite is pH-dependent. The adsorption increased as the pH increased from 2 to 5, then the adsorption capacity decreased as pH was increased up to 11. This property of the RM/CNT composite adsorbent would make it a valuable adsorbent for lead remediation from acidic hydrometallurgical waste liquors. The results show that the fabricated RM/CNT composite had an effective adsorbent for the removal of lead aqueous solution.
References [1] Rashad A. M. Alkali-activated metakaolin. A short guide for civil Engineer– An overview. Construction and Building Materials, 41, 751–765, (2013) [2] Autef A, Joussein E, Gasgnier G, Rossignol S. Role of the silica source on the geopolymerization rate. Journal of Non-Crystalline Solids, 366:2886–2893, (2012) 16
[3] Aly Z, Vance E.R, Perera D.S. Aqueous dissolution of sodium aluminosilicate geopolymers derived from metakaolin. Journal of Nuclear Materials, 424, 164–170, (2012) [4] Mesgari Abbasi, S., Ahmadi, H., Khalaj, G., Ghasemi, B., 2016, Microstructure and mechanical properties of a Metakaolinite-based geopolymer nanocomposite reinforced with carbon nanotubes, Ceramics International. Online [5] Nazari, A., Khalaj, G., 2012, Prediction compressive strength of lightweight geopolymers by ANFIS, Ceramics International, 38, 4501- 4510 [6] Jin F, Park S. A review of the preparation and properties of carbon nanotubesreinforced polymer composites. Carbon Letters, 12, 57-69, (2011) [7] Baco S, Arshad S. E, Salleh S, Alias A, Yassin F. MD. Preparation and characterization of carbon nano fiber Carbon nanofibers/ Metakaolin geopolymer based nanocomposite. Int. J. Environ. Sci. Techno, 4,474 – 481(2015) [8] Saafi M, Andrew K, Tang P.L, Ghon D.M, Taylor S, Rahman M, Yang S, Zhou X. Multifunctional properties of carbon nanotube/fly ash geopolymeric Nanocomposites, Construction and Building Materials, 49 , 46–55, (2013) [9] Tyson B, Al-Rub A, Yazdanbakhsh A, Grasley Z. Carbon Nanotubes and Carbon Nanofibers for Enhancing the Mechanical Properties of Nanocomposite Cementitious Materials. J. Mater. Civ. Eng. 23, 1028-1035, (2011) [10] Khatera H.M, Gawaad H.A. Characterization of alkali activated geopolymer mortar doped with MWCNT. Adv. Mater. Res, 4, 45-61, (2015) [11] Hui L, Baomin W, Yu H, Jun Q, Zhiqiang G. The influence of carbon nanotubes/ silica fume on the mechanical properties of cement– based composites, Romanian Journal of Materials, 45, 29 –34,(2015) [12] Hunashyal A. M, Tippa S. V, Quadri S. S, Banapurmath N. R, Experimental Investigation on Effect of Carbon Nanotubes and Carbon Fibres on the Behavior of Plain Cement Mortar Composite Round Bars under Direct Tension. International Scholarly Research Notices nanotechnology, 20, 1-6,(2011) [13] Slaty F, Khoury H, Wastiels J, Rahier H. Characterization of alkali activated kaolinitic clay. Appl. Clay Sci., 75–76, 120–125, (2013) [14] Liew Y.M, Kamarudin H, Mustafa Al Bakri A.M, Luqman M, Khairul Nizar I, Ruzaidi C.M, Heah C.Y. Processing and characterization of calcined kaolin cement powder, Construction and Building Materials, 30, 794–802,(2012) [15] G. E. J. Poinern, D. Parsonage, T. B. Issa, M.K. Ghosh, E. Paling and P. Singh, Preparation, characterization and As(V) adsorption behaviour of CNT-ferrihydrite composites, International Journal of Engineering, Science and Technology, Vol. 2, No. 8, 2010, pp. 13-24. [16] V.K. Gupta, S. Agarwal, T.A.Saleh, Synthesis and characterization of aluminacoated carbon nanotubes and their application for lead removal, J Hazard Mater, 2011 Jan 15;185(1):17-23. [17] Kuenzel C, Neville T.P, Donatello S, Vandeperre L, Boccaccini A.R, Cheeseman C.R. Influence of metakaolin characteristics on the mechanical properties of geopolymers. Appl. Clay Sci., 83– 84, 308-314, (2013) [18] Ferone C, Colangelo F, Roviello G, Asprone D, Menna C, Balsamo A, Prota A, Cioffi R, Manfredi G. Application-Oriented Chemical Optimization of a Metakaolin Based Geopolymer. Materials, 6, 1920-1939, (2013)
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[19] Khalaj, G., Hassani, S.E.S., Khezrloo, A., Haratifar, E.-A.-D., 2014, Split tensile strength of OPC-based geopolymers: Application of DOE method in evaluating the effect of production parameters and their optimum condition, Ceramics International, 40, 10945- 10952. [20] I. F. Teixeira, T. P.V. Medeiros, P. E. Freitas, M. G. Rosmaninho, J. D. Ardisson, R. M. Lago, Carbon deposition and oxidation using the waste red mud: A route to store, transport and use offshore gas lost in petroleum exploration, Fuel, 124 (2014) 7–13.
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