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Carbothermal synthesis of ordered mesoporous carbon-supported nano zero-valent iron with enhanced stability and activity for hexavalent chromium reduction
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Carbothermal synthesis of ordered mesoporous carbon-supported nano zero-valent iron with enhanced stability and activity for hexavalent chromium reduction Ying Dai a,b,1 , Yuchen Hu a,1 , Baojiang Jiang a , Jinlong Zou a,c,∗ , Guohui Tian a , Honggang Fu a,∗ a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China b School of Civil Engineering, Heilongjiang Institute of Technology, Harbin 150050, China c Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion, College of Heilongjiang Province, Heilongjiang University, Harbin 150080, China
g r a p h i c a l
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 22 January 2015 Received in revised form 4 April 2015 Accepted 6 April 2015 Available online xxx Keywords: Nano zero-valent iron Ordered mesoporous carbon Carbothermal reduction
a b s t r a c t Composites of nano zero-valent iron (nZVI) and ordered mesoporous carbon (OMC) are prepared by using simultaneous carbothermal reduction methods. The reactivity and stability of nZVI are expected to be enhanced by embedding it in the ordered pore channels. The structure characteristics of nZVI/OMC and the removal pathway for hexavalent chromium (Cr(VI)) by nZVI/OMC are investigated. Results show that nZVI/OMC with a surface area of 715.16 m2 g−1 is obtained at 900 ◦ C. nZVI with particle sizes of 20–30 nm is uniformly embedded in the OMC skeleton. The stability of nZVI is enhanced by surrounding it with a broad carbon layer and a little ␥-Fe is derived from the passivation of ␣-Fe. Detection of ferric state (Fe 2p3/2 , around 711.2 eV) species confirms that part of the nZVI on the outer surface is inevitably
∗ Corresponding authors at: Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China. Tel.: +86 451 8660 8549; fax: +86 451 8660 8549. E-mail addresses: [email protected] (J. Zou), [email protected] (H. Fu). 1 These authors contributed equally to this work and should be considered as co-first authors. http://dx.doi.org/10.1016/j.jhazmat.2015.04.013 0304-3894/© 2015 Elsevier B.V. All rights reserved.
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2 Hexavalent chromium Reduction
oxidized by O2 , even when unused. The removal efficiency of Cr(VI) (50 mg L−1 ) by nZVI/OMC is near 99% within 10 min through reduction (dominant mechanism) and adsorption. nZVI/OMC has the advantage in removal efficiency and reusability in comparison to nZVI/C, OMC and nZVI. This study suggests that nZVI/OMC has the potential for remediation of heavy metal pollution in water. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nano zero-valent iron (nZVI) is an environmentally friendly and useful material that has the characteristics of small particle size, high surface area, strong reducing ability, fast reaction, etc. [1]. Therefore, in recent years, nZVI has been widely used in the field of remediation of bodies of water. nZVI has been proven to be highly effective in the removal/degradation of a wide range of pollutants, including PCBs [2], bromides [3], dyes [4], nitrates [5], and heavy metals [6]. The rank for removal of heavy metals by nZVI has been shown to be as follows: As > Cr > Cu > Hg > Pb > Zn > Cd > Ni [7]. This shows that nZVI has a universal capability for reducing of heavy metals in water, and the pathway (mechanism) for the removal of the heavy metals includes reduction, adsorption, and co-precipitation [8,9]. Therefore, the use of nZVI as a reducing agent is a simple and convenient method for eliminating the heavy metal pollution from water. However, nZVI has some shortcomings areas where it has been applied, such as poor air stability, difficulty of preservation, and easy reunion [1,6,8–10]. In recent years, many studies have been conducted to find methods for preparing stable and efficient nZVI. Initially, a small amount of surfactant, which can play a protective role for nZVI, was added during the preparation process to improve the dispersibility of nZVI [11]. However, this method does not completely solve the problem of poor stability. Subsequently, it has been found that the use of a carrier (supporter) is a very effective way to improve the stability of nZVI. The carrier materials can lessen the oxidization of nZVI (to some extent) and improve its performance, even after use or storage [12]. The dispersibility and surface area of nZVI can be improved by using the carrier materials. The carriers usually used include inorganic compounds, organic compounds, silica, polymers, clays, carbon materials, etc. Petala et al. have successfully synthesized a stable nZVI-composite by using MCM-41 as a carrier [13]. By using diatom mud as the carrier, nZVI has been successfully loaded onto its surface and pore channels [14]. This carrier can greatly improve the dispersion of nZVI and enhance its capacity of remediation of bodies of water polluted by chlorinated organics. The cation exchange resin-supported nZVI-composite has been used for the decoloration of azo dye acid blue 113, and it shows a strong decolorization capacity (100%, within 30 min) [15]. Currently, the most common method for supporting nZVI is to immerse the carrier in a solution of an iron salt and then reduce the Fe(II)/Fe(III) to Fe0 with a reducing agent, such as NaHB4 . The ordered mesoporous carbon (OMC) impregnated with FeSO4 ·7H2 O was pyrolyzed at 300 ◦ C for 4 h and then treated with NaBH4 (2.1 M) to reduce the Fe(II) to metallic Fe0 [16]. The peak at 44.6◦ for nZVI in the nZVI/OMC (452 m2 g−1 ) is negligible, which implies that the crystallinity and stability of nZVI are relatively poor [16]. Although, this method is fast and effective, there are also some disadvantages: NaHB4 is expensive, and the conditions for reduction are rigorous. Therefore, some researchers choose the carbothermal reduction method to synthesize the nZVI/C composites [17]. By comparison with the conventional liquid-phase reduction method, it has the following advantages: the carbon is capable of supporting a large amount of nZVI and the stability of the nZVI in air is improved [18]. By pyrolyzing the mixture of carbon black and iron nitrate
Fig. 1. XRD patterns of nZVI/OMC carbonized at different temperatures (a) and with different dosage of ferric nitrate (b).
under argon [19], Fe3 O4 /C and Fe0 /C composites can be obtained at 300–500 ◦ C and 600–800 ◦ C, respectively. By pyrolyzing a mixture of the modified activated carbon colloid and iron salt, an Fe0 /C composite is obtained at 700 ◦ C [20] that still retains the colloidal characteristics and high reactivity. Using sucrose as the carbon source, the spherical Fe0 /C composites are obtained by using a twostep carbothermal reduction process [21]. The aerosol of Fe3 O4 /C composites is first synthesized at 1000 ◦ C, and then the Fe3 O4 is converted to Fe0 at 720 ◦ C [21]. The morphology of this Fe0 /C is regular (spherical) and the loading content of Fe0 is higher than in other methods [21]. The carbon carriers used in the reported literature usually have the problems of small SBET and pore volumes [16] and non-regular pore structure [19–21], which limits the
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Fig. 2. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of nZVI/OMC-x (x = 1–5).
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Fig. 4. (a) Effect of reaction time on removal efficiency of Cr(VI); (b) recycling tests for removal of Cr(VI) by using nZVI/OMC-3, nZVI/C, nZVI and OMC.
In this study, OMC with high surface area is selected as a carrier to support nZVI. OMC can play a protective role for nZVI and provide a certain adsorption capacity because of its uniform nanopore structure and high SBET . The nZVI/OMC composites, which are expected to have high stability and reactivity, are prepared using the carbothermal reduction method. These composites will gain the synergistic effects of the high reactivity of nano-particles and the high efficiency of the mass transfer of pollutants through the porous structure of the OMC, while the adsorption and reduction efficiency of nZVI/OMC will consequently be improved. Cr(VI) is selected as the target pollutant, and the efficiency and stability of nZVI/OMC for removing Cr(VI) is thoroughly investigated. Moreover, based on the analyses of the reduction efficiency of Cr(VI), the relationship between the structure and properties of nZVI/OMC is also discussed. 2. Materials and methods 2.1. Synthesis of nZVI/OMC composite Fig. 3. TG curves of the nZVI/OMC composites with different dosage of ferric nitrate.
performance of the adsorption and mass transfer of pollutants [22]. If these problems are properly solved, the stability and pollutantsremoval capacity of the nZVI-composite can be further improved.
The details of the chemicals used and the synthesis methods for SBA-15 [23] and OMC [24] are introduced in Supporting information (SI). X (x = 1–5) g of Fe(NO3 )3 ·9H2 O was dissolved in 15 mL of a mixture of ethanol and distilled water (1:1). 1 g OMC was wet-impregnated with the above-mixed solution, which was then stirred for over 2 h. After drying at 60 ◦ C, the resultant samples were
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Fig. 5. TEM (a) and HRTEM (b) image of nZVI/C; TEM (c) and HRTEM (d) image of nZVI/OMC-3.
carbonized at different temperatures (500–1000 ◦ C, at an interval of 100 ◦ C) in N2 flow (50–60 mL min−1 ) for 3 h (with a heating rate of 5 ◦ C min−1 ). After being naturally cooled to room temperature (25 ◦ C), the obtained samples were marked as nZVI/OMCs-x (x = 1–5). The number (1–5) for x was defined as the initial mass ratio of Fe(NO3 )3 ·9H2 O/OMC. The synthesis route for the nZVI/OMC composites are shown in Fig. S1 (SI). 2.2. Synthesis of comparison samples To investigate the influence of OMC on the reduction properties of nZVI/OMC composites, the ordinary carbon-supported nZVI composite (nZVI/C), nZVI and OMC were prepared as the contrast samples. 1 g of sucrose and 1 g of Fe(NO3 )3 ·9H2 O was dissolved in 10 mL of the mixture of ethanol and distilled water (1:1) and then stirred for over 2 h [25]. The resultant mixture was dried in an oven at 100 ◦ C for 6 h, and the oven temperature was raised to 160 ◦ C for 6 h. The resultant samples were carbonized in N2 flow at 900 ◦ C for 3 h (5 ◦ C min−1 ). The synthesis of nZVI was carried out as follows [26]: 100 mL of 20 mM NaBH4 aqueous solution was added drop-by-drop to a three-necked flask containing 100 mL of 4 mM FeSO4 ·7H2 O aqueous solution, stirring vigorously under N2 protection at ambient temperature. After stirring for 1.5 h, the ferrous iron was completely reduced by the borohydrate to form the nZVI particles. The synthesized nZVI particles were separated and washed twice with deionized water. Finally, the nZVI was dried at room temperature (25 ◦ C) under vacuum conditions, ready for instant usage.
2.3. Characterization methods X-ray diffraction (XRD) patterns were recorded with a Rigaku D/max-IIIB diffractometer using Cu K␣ ( = 1.5406 Å) radiation at a step scan of 0.02◦ from 10◦ to 80◦ . The accelerating voltage and the applied current were 40 kV and 20 mA, respectively. The resulting powder diffraction patterns were analyzed according to the Joint Committee on Powder Diffraction Standard (JCPDS) data. All of the thermogravimetric (TG) experiments were carried out in a TA instruments (NEJSCH STA 449C) TG apparatus. Transmission electron microscopy (TEM) images were taken by using a JEM2100 electron microscope (JEOL) with an acceleration voltage of 200 kV. The nitrogen adsorption/ desorption isotherms were measured at 77 K using a Micromeritics Tristar II. The specific surface area of materials was calculated by the Brunauer–Emmett–Teller (BET) theory. Pore size distribution was computed by using the Barrett–Joyner–Halenda (BJH) method from the adsorption branch of the isotherms. The surface components were determined by using X-ray photoelectron spectroscopy (XPS, Kratos-AXIS UL TRA DLD, Al K␣ X-ray source). The XPS data for each atom were fitted with the ‘XPS peak’ software. The used full width at half maximum (FWHM) was not more than 2.7 eV and allowed to float for all atoms. The % Lorentzian–Gaussian was set as 20% for all fittings. 2.4. Batch experiments Y g nZVI/OMC (6 mg nZVI), Z g nZVI/C (6 mg nZVI), 6 mg nZVI and 0.014 g OMC, respectively, were added to a 20 mL solution
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containing 50 mg L−1 Cr(VI) (K2 Cr2 O7 ) for adsorption. By using the thermogravimetric method, the iron (Fe2 O3 ) content of nZVI/OMC and nZVI/C could be obtained, and then the mass ratio (i.e., Y and Z) of the nZVI/composite could be approximately calculated. The adsorption tests were carried out at 25 ◦ C (air bath) and 110 rpm. The effects of the Cr(VI) concentration, the absorbents’ dosage, and pH on adsorption performance of the four samples were investigated. The pH of the suspensions was adjusted by adding HCl (0.1 M) or NaOH (0.1 M) to the solution. After filtration with a 0.45 m membrane, the Cr(VI) concentration of the filtrates was determined by using the 1, 5-diphenylcarbazide method (spectrophotometry).
3. Results and discussion 3.1. Structure characteristics of nZVI/OMC composites Fig. S2a and b (SI) shows the N2 adsorption–desorption isotherms and pore size distributions of SBA-15 and OMC, respectively. Fig. S2c and d (SI) shows the pore structure of OMC observed by TEM. The XRD patterns in Fig. 1a shows that the main crystalline phases of the samples carbonized at 700–1000 ◦ C are different from each other. At temperatures of 500–700 ◦ C, the curves are almost the same and show typical XRD patterns of the Fe3 O4 nanoparticles (JCPDS Card No. 75-1609) with a face-centered cubic structure, which indicates that the obtained composites are Fe3 O4 /OMC. At 800 ◦ C, the peaks correspond to the typical diffraction peaks of ␣Fe (44.5◦ and 65◦ ) with a body-centered cubic structure and FeO (36◦ , 42◦ , 60◦ , 72◦ , and 76◦ ) [16,27], which means that the Fe3 O4 in Fe3 O4 /OMC is gradually reduced by the carbothermal reduction. At 900 ◦ C and 1000 ◦ C, the diffraction peaks correspond to ␣-Fe (44.5◦ and 65◦ ) and ␥-Fe (51◦ and 78◦ ) with a face-centered cubic structure [28], which implies that the Fe3 O4 nanoparticles is completely converted to Fe0 . To save energy, 900 ◦ C is selected as the optimum temperature for preparation of the nZVI/OMC. XRD patterns of the composites (900 ◦ C) with different dosage of Fe(NO3 )3 are shown in Fig. 1b. It can be seen that the typical diffraction peaks of nZVI/OMC-1 and nZVI/OMC-2 are Fe3 O4 . At x = 3, 4 and 5, only the peaks of ␣-Fe and ␥-Fe are observed in the composites. At lower dosages of Fe(NO3 )3 , the carbothermal reduction reaction is not complete, which results in the formation of the highly-crystallized Fe3 O4 particles [29]. Based on the above analyses, x = 3 is selected as the optimal ratio for preparation of nZVI/OMC-x. N2 adsorption–desorption isotherms and pore size distributions of nZVI/OMC–x (x = 1–5) are shown in Fig. 2a and b, respectively. It can be seen that the adsorption–desorption hysteresis loops are postponed in the relative pressure range of 0.45–0.8 as iron content increases. Because nZVI is firmly loaded onto the surface and within the channels of OMC, the porous structure of OMC is inevitably destroyed to a certain extent [30]. Therefore, the SBET of nZVI/OMC–x (x = 1–5) decreases to 965.51, 841.37, 715.16, 596.12 and 518.81 m2 g−1 , respectively, as x increases. As shown in Fig. 2b, the sharp peaks centered at approximately 3.6 nm in the pore size distributions gradually become broader as x increases. This indicates that part of the pore channels of OMC are filled with the loaded nZVI, which inevitably leads to the decrease of SBET . The N2 adsorption–desorption isotherms and SBET (10.81 m2 g−1 ) of the pure nZVI are shown in Fig. S3a (SI). The TG curves of nZVI/OMC–x (x = 1–5) with different dosages of Fe(NO3 )3 are shown in Fig. 3 (heated in an air atmosphere). It can be seen that an obvious weight loss occurred at 450–560 ◦ C (approximately), which can be attributed to the combustion of carbon (OMC). After 560 ◦ C, there was no weight loss and the stable residuals (reddish brown) are iron oxide (Fe2 O3 , as shown in Fig. S3b, (SI)), which originated from the oxidation of nZVI in the
Fig. 6. Effects of pH (a), Cr(VI) concentration (b) and nZVI/OMC-3 dosage (c) on the removal of Cr(VI) from water.
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3.2. Cr(VI) removal efficiency and reusability comparison
Fig. 7. Removal pathway of Cr(VI) from water by using nZVI/OMC.
composite. Based on the mass of the residual Fe2 O3 , the nZVI content in the composite can be calculated. For example, the mass ratio of residual Fe2 O3 is approximately 41% after combustion; therefore, the total mass ratio of nZVI in nZVI/OMC-3 is approximately 30%. The TG curve of the nZVI/C composite is shown in Fig. S3c (SI).
Fig. 4a shows the removal efficiency of Cr(VI) in aqueous solution with an initial concentration of 50 mg L−1 (20 mL solution, a pH of approximately 5.6 and a sample dosage of 20 mg mL−1 ) using 20 mg nZVI/OMC-3, 23.1 mg nZVI/C (each of the two samples contains 6 mg nZVI according to the TG analyses), 6 mg nZVI and 14 mg OMC. It can be seen that the removal efficiencies of Cr(VI) by nZVI and OMC are approximately 39% and 57%, respectively, within 10 min, which indicates that OMC (1277.91 m2 g−1 , SI) has a stronger adsorption capacity for Cr(VI). nZVI/OMC-3 (715.16 m2 g−1 ) is the best of the four materials with respect to the removal efficiency or reaction rates, and complete removal (near 99%) is achieved within 10 min. This implies that at least approximately 40% of the Cr(VI) is reduced to the less toxic Cr(III) by nZVI/OMC-3 within 10 min. The removal efficiency of Cr(VI) by nZVI/C (approximately 94%) is nearly equivalent to that of nZVI/OMC-3 after the equilibrium (60 min), but its reaction rate is far slower than that of nZVI/OMC-3. The reusability (the experimental conditions are the same as that of Fig. 4a) of nZVI/OMC-3, nZVI/C, nZVI and OMC is compared by recycling seven times (Fig. 4b). The removal efficiency of Cr(VI) by nZVI/OMC-3 gradually decreases with increasing number of cycles, but no less than 60% efficiency is still maintained after the 7th cycle. It can be determined that the adsorption capacity of OMC for removal of the Cr(VI) is nearly exhausted after the 3rd cycle. From the beginning of the 4th cycle, Cr(VI) is only being reduced by the nZVI of nZVI/OMC-3, and the reaction activity of nZVI can be quickly recovered after each cycle. The removal efficiency of Cr(VI) by nZVI/C dramatically decreases after the first use and remains at only 19% after the 7th cycle. The adsorption capacity of nZVI/C is completely exhausted before recycling again and the removal relies only on the reduction activity of nZVI. The reusability of nZVI/OMC3 is better than those of nZVI/C and nZVI, which implies that the stability of nZVI is more important than the reaction activity for removal of Cr(VI) from water. These results once again prove that the pore channels of OMC can not only provide protective layers for inhibiting the oxidation of nZVI but can also enhance the stability and reaction activity of nZVI in nZVI/OMC-3. To distinguish the role of carbon carriers, the TEM morphologies of nZVI/C and nZVI/OMC-3 are compared. As shown in Fig. 5a and b, some nZVI with irregular shapes and particle sizes are dispersed on the surface of the nZVI/C, and the obvious agglomeration phenomenon can be observed. As shown in Fig. 5c, nZVI with particles sizes of 20–30 nm is uniformly distributed in the structure of nZVI/OMC-3 and no agglomeration is observed. In Fig. 5d, the ordered structure of the mesopores for nZVI/OMC-3 can be clearly observed, but part of the channels of nZVI/OMC-3 are plugged and bent after iron loading, which implies that the ordered mesoporous structure is slightly deformed. It can be seen that the morphologies of nZVI/C and nZVI/OMC-3 are dramatically different from each other, which results in their differences in reductive activity and stability. At temperatures ≤700 ◦ C, Fe3 O4 is formed in the nZVI/OMC-3, which is further reduced by carbon to form nZVI particles at higher temperatures (700–900 ◦ C). During the reduction process, the walls of the pore channels of OMC are first softened by the growth of Fe-species nano-particles, and, subsequently, some carbon atoms in the walls are removed [27]. Therefore, a little deterioration of the ordered mesoporous structure occurs, which is accompanied by in situ formation of nZVI in OMC. 3.3. Effect of reaction conditions on removal efficiency of Cr(VI) by nZVI/OMC-3
Fig. 8. (a) XRD patterns of nZVI/OMC-3 and (b) panoramic XPS diagrams of nZVI/OMC-3 before use, after the 7th use, and after storage in air for 15 days.
As shown in Fig. 6a, the removal efficiency of Cr(VI) (50 mg L−1 ) by nZVI/OMC-3 (solid dosage of 20 mg mL−1 ) is dramatically affected by pH. The Cr(VI) removal efficiency in acidic pH is higher
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Fig. 9. High resolution XPS of Fe 2p and O 1s spectra for nZVI/OMC-3 before use (a and b), after the 7th use (c and d), and after storage in air for 15 days (e and f); (1) Fe 2p3/2 → Fe0 (␣-Fe), (2) Fe 2p3/2 → Fe3+ , (3) Fe 2p3/2 → Fe3+ (satellite peaks), (4) Fe 2p1/2 → Fe3+ (satellite peaks), (5) O 1s → O-physically absorbed or carbonates, (6) O 1s → Fe O, and (7) O 1s → O C = O.
than that in alkaline pH, and it can reach approximately 99% at pH 4. The pH dependence of Cr(VI) removal is mainly related to the ion (Cr(VI)) chemistry in the solution [31]. In aqueous solution, Cr(VI) has two forms, including Cr2 O7 2− under acidic conditions and CrO4 2− under alkaline conditions. Cr2 O7 2– has the characteristic of strong oxidation resistance, which can be easily reduced [31]. As the pH decreases, the surface potential of nZVI/OMC-3 is increased, while the capacity of the surface complexation of nZVI with anions is correspondingly enhanced [32]. Therefore, precipitation of the Cr-Fe complex occurs is formed during the reaction, which further enhances the Cr(VI) removal efficiency. It can be seen from Eqs. (1)–(3) below that the reaction of Fe0 and Cr2 O7 2– needs H+ , therefore, the Cr(VI) reduction rate increases as the pH of the solution decreases [33]. Cr2 O7 2– (aq) + Fe0 (s)-- + 14H+ (aq) → 2Cr3+ (aq) + 3Fe2+ (aq) + 7H2 O
(1)
Cr2 O7 2– (aq) + 6Fe2+ (aq)-- + 14H+ (aq) → 2Cr3+ (aq) + 6Fe3+ (aq) + 7H2 O
(2)
xCr3+ (aq) + (1−x)Fe3+ (aq)-- + H2 O → Crx Fe(1−x) (OH)3 ↓ + 3H+ (3) The effects of Cr(VI) concentration and nZVI/OMC-3 dosage on removal efficiency of Cr(VI) are shown in Fig. 6b and c, respectively. Fig. 6b shows that Cr(VI) removal efficiency decreases as the initial concentration of Cr(VI) increases (pH 4 and solid dosage of 20 mg mL−1 ) and the reaction equilibrium can be reached within
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3.4. Stability analysis of the nature of nZVI/OMC-3 XRD patterns of the nZVI/OMC-3 samples before use, after the 7th use, and after storage in air for 15 days are shown in Fig. 8a. The results show that the main crystalline phases of the three samples do not have substantial differences. The diffraction peaks of ␥-Fe originating from the passivation of nZVI (␣-Fe) are found at approximately 44◦ for the samples after use and storage for 15 days [28]. However, no diffraction peaks of the hematite (␣-Fe2 O3 , JCPDS, No. 87-1165) can be observed, indicating the excellent stability of these samples. The carrier OMC can greatly enhance the stability of nZVI, making it difficult to be deeply oxidized even in an air atmosphere. There is no doubt that the stability of nZVI/OMC-3 is far greater than that of pure nZVI. Panoramic XPS tests are conducted to detect the chemical compositions of nZVI/OMC-3 before use, after the 7th use, and after storage in air for 15 days (Fig. 8b). From the survey spectra, the predominant C 1s peaks (narrow) are located at around 284.6 eV, along with O 1s peaks at around 530 eV and Fe 2p peaks at around 710 eV for the three samples. It should be noted that although the used nZVI/OMC-3 is washed with deionized water three times before the XPS test, the Cr 2p peak at around 575 eV is still observed, which confirms that Cr-species (i.e., Cr3+ ) are firmly absorbed into the porous structure of OMC and the reduction reaction (Cr6+ → 2Cr3+ ) is indeed present [34]. These results clearly indicate that the differences in the surface (or skeleton) elements constituting the three samples are minor, implying good stability and reusability of nZVI/OMC-3. Fig. 9 shows the high resolution XPS of the Fe 2p and O 1s spectra for the three samples. As shown in Fig. 9a, c and e, two obvious bands are observed in the curves, which correspond to the low energy (Fe 2p3/2 ) and high energy (Fe 2p1/2 ) asymmetric bands originating from the spin orbital splitting, respectively [35,36]. The Fe 2p peaks can be decomposed into four components, originating from the metallic iron (␣-Fe, around 707.2 eV), Fe 2p3/2 ferric state (around 711.2 eV), Fe 2p3/2 ferric state (satellite peaks, around 713.0 eV), and Fe 2p1/2 ferric state (satellite peaks, around 725.1 eV). The satellite peaks are caused by the process of shake-off and shakeup, and the surface plasmons [35]. The presence of the ferric state species (such as Fe2 O3 ) confirms that part of the nZVI on the composite surface is inevitably oxidized by O2 , even when unused. It should be noted that the Fe2 O3 is not detected in the XRD tests. It may be that only a small amount of nZVI on the surface layer is oxidized, which is not sufficient to be detected by XRD. The Fe 2p3/2 XPS spectra do not show any Fe3 O4 (708.2 eV) peaks, which are already detected by XRD. The deep inlay of a little Fe3 O4 in the skeleton of OMC results in the formation of a protective (carbon) layer, which restricts the XPS detection.
Intensity (cps)
a C in graphite (284.6 eV)
C-O (286.4 eV)
275
280
285
290
295
300
Binding Energy (eV)
b C in graphite (284.6 eV)
Intensity (cps)
30 min. The removal efficiency can reach above 93% within 5 min at the initial concentration of 75 mg L−1 . As shown in Fig. 6c, the removal efficiency of Cr(VI) increases as the sample dosage increases. Under the conditions of pH 4 and initial Cr(VI) concentration of 50 mg L−1 , the removal efficiency of Cr(VI) by nZVI/OMC-3 with dosage of 10 mg mL−1 is comparable to that with 20 mg mL−1 with pH 5.6. The removal process is fast and efficient, which indicates that the dominant pathway for Cr(VI) removal must be the reduction reaction. The deduced pathway (mechanism) for removal of Cr(VI) by nZVI/OMC is shown in Fig. 7. During the reduction process, Cr(VI) can be quickly attracted to the area surrounding the nZVI by adsorbing on the surface of OMC, which provides greater chances for the contact (reaction) of Cr(VI) and nZVI/OMC-3 (Fig. 7). Furthermore, the mass transfer of Cr(VI) through the body of nZVI/OMC-3 is enhanced by the abundant pore channels of OMC, which correspondingly enhances the reduction rate and efficiency (Fig. 7).
C-O (286.0 eV)
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Binding Energy (eV)
c C in graphite (284.7 eV)
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Binding Energy (eV) Fig. 10. High resolution XPS of C 1s spectra for nZVI/OMC-3 before use (a), after the 7th use (b), and after storage in air for 15 days (c).
As shown in Fig. 9b, d and f, the O 1 s peaks can be decomposed into three components originating from the physically-absorbed O or carbonates (around 530.7 eV), Fe O (around 531.2 eV) and O C = O (around 533.0 eV). The presence of the Fe O (Fe2 O3 ) bond in the O 1s spectra confirms the oxidation of nZVI on the surface of OMC, which is consistent with the Fe 2p analysis. The binding energy (peak) of the Fe O bond in the unused nZVI/OMC-3 is slightly lower than those of other two samples, which can be attributed to the further oxidation of nZVI after use or storage in air. As shown in Fig. 10, the C 1s peaks can be decomposed into two components originating from the C in graphite (around
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284.6 eV) and C O (around 286.0 eV) [36,37]. The C 1s band at around 286.0 eV may correspond to the C O in carboxyl, epoxy, or phenol groups in the OMC skeleton [36,37]. However, the C 1s peaks for the three samples remain almost unchanged, implying that the characteristics of the OMC carrier are very stable. These XPS results provide solid evidence that the nZVI/OMC-3 is stable and suitable to be used for eliminating pollutants from water. 4. Conclusions In summary, the load-type composites of nZVI/OMC are prepared and applied to remediate the Cr(VI) pollution in water. Stable nano-magnetic nZVI/OMC is prepared under the conditions of 900 ◦ C and an iron/carbon ratio of 3: 1. Cr(VI) can be rapidly removed by nZVI/OMC-3, which has high SBET (715.16 m2 g−1 ) and small nZVI particle sizes (20–30 nm), which are conducive to the reduction and mass transfer of Cr(VI). Furthermore, the stability and activity of the nZVI in nZVI/OMC are dramatically enhanced by the protective effect of the OMC skeleton, which prevents it from being easily oxidized under O2 exposure. Although, slight damage to the ordered mesoporous structure and the nZVI oxidization inevitably exists, the performance of nZVI/OMC was far better than those of nZVI/C, nZVI and OMC. Therefore, nZVI/OMC prepared by this method can be regarded as an effective and feasible method for removal of heavy metals from bodies of water, being associated with the advantages of proven technology and relatively low reactant toxicity. Acknowledgements We acknowledge the support by Key Program Projects of the National Natural Science Foundation of China (21031001), National Natural Science Foundation of China (51108162, 20971040), Natural Science Foundation of Heilongjiang Province (B201411), Postdoctoral Science (Start–up) Foundation of Heilongjiang Province (LBH-Q14137), Excellent Young Researchers Fund and Hundred Young Talents Fund of Heilongjiang University, and Young Teachers Fund of Heilongjiang Institute of Technology (QJ201202). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2015.04.013. References [1] D.L. Wu, S.S. Zheng, A.Q. Ding, G.D. Sun, M.Q. Yang, Performance of a zero valent iron-based anaerobic system in swine wastewater treatment, J. Hazard. Mater. 286 (2015) 1–6. [2] E.J. Petersen, R.A. Pinto, X.Y. Shi, Q.G. Huang, Impact of size and sorption on degradation of trichloroethylene and polychlorinated biphenyls by nano-scale zero valent iron, J. Hazard. Mater. 243 (2012) 73–79. [3] H.M. Zhou, Y.Y. Shen, P. Lv, J.J. Wang, P. Li, Degradation pathway and kinetics of 1-alkyl-3-methylimidazolium bromides oxidation in an ultrasonic nanoscale zero-valent iron/hydrogen peroxide system, J. Hazard. Mater. 284 (2015) 241–252. [4] Y. He, J.F. Gao, F.Q. Feng, C. Liu, Y.Z. Peng, S.Y. Wang, The comparative study on the rapid decolorization of azo, anthraquinone and triphenylmethane dyes by zero-valent iron, Chem. Eng. J. 179 (2012) 8–18. [5] C.L. Tang, Z.Q. Zhang, X.N. Sun, Effect of common ions on nitrate removal by zero-valent iron from alkaline soil, J. Hazard. Mater. 231 (2012) 114–119. [6] S.A. Kim, S. Kamala-Kannan, K.J. Lee, Y.J. Park, P.J. Shea, W.H. Lee, B.T. Oh, Removal of Pb(II) from aqueous solution by a zeolite–nanoscale zero-valent iron composite, Chem. Eng. J. 217 (2013) 54–60. [7] R.A. Crane, T.B. Scott, Nanoscale zero-valent iron: future prospects for an emerging water treatment technology, J. Hazard. Mater. 211 (2012) 112–125. [8] E. Petala, K. Dimos, A. Douvalis, T. Bakas, J. Tucek, R. Zboˇril, M.A. Karakassides, Nanoscale zero-valent iron supported on mesoporous silica: characterization and reactivity for Cr(VI) removal from aqueous solution, J. Hazard. Mater. 261 (2013) 295–306.
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Carbothermal synthesis of ordered mesoporous carbon-supported nano zero-valent iron with enhanced stability and activity for hexavalent chromium reduction Ying Dai a,b,1 , Yuchen Hu a,1 , Baojiang Jiang a , Jinlong Zou a,c,∗ , Guohui Tian a , Honggang Fu a,∗ a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China b School of Civil Engineering, Heilongjiang Institute of Technology, Harbin 150050, China c Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion, College of Heilongjiang Province, Heilongjiang University, Harbin 150080, China
g r a p h i c a l
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 22 January 2015 Received in revised form 4 April 2015 Accepted 6 April 2015 Available online xxx Keywords: Nano zero-valent iron Ordered mesoporous carbon Carbothermal reduction
a b s t r a c t Composites of nano zero-valent iron (nZVI) and ordered mesoporous carbon (OMC) are prepared by using simultaneous carbothermal reduction methods. The reactivity and stability of nZVI are expected to be enhanced by embedding it in the ordered pore channels. The structure characteristics of nZVI/OMC and the removal pathway for hexavalent chromium (Cr(VI)) by nZVI/OMC are investigated. Results show that nZVI/OMC with a surface area of 715.16 m2 g−1 is obtained at 900 ◦ C. nZVI with particle sizes of 20–30 nm is uniformly embedded in the OMC skeleton. The stability of nZVI is enhanced by surrounding it with a broad carbon layer and a little ␥-Fe is derived from the passivation of ␣-Fe. Detection of ferric state (Fe 2p3/2 , around 711.2 eV) species confirms that part of the nZVI on the outer surface is inevitably
∗ Corresponding authors at: Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China. Tel.: +86 451 8660 8549; fax: +86 451 8660 8549. E-mail addresses: [email protected] (J. Zou), [email protected] (H. Fu). 1 These authors contributed equally to this work and should be considered as co-first authors. http://dx.doi.org/10.1016/j.jhazmat.2015.04.013 0304-3894/© 2015 Elsevier B.V. All rights reserved.
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2 Hexavalent chromium Reduction
oxidized by O2 , even when unused. The removal efficiency of Cr(VI) (50 mg L−1 ) by nZVI/OMC is near 99% within 10 min through reduction (dominant mechanism) and adsorption. nZVI/OMC has the advantage in removal efficiency and reusability in comparison to nZVI/C, OMC and nZVI. This study suggests that nZVI/OMC has the potential for remediation of heavy metal pollution in water. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nano zero-valent iron (nZVI) is an environmentally friendly and useful material that has the characteristics of small particle size, high surface area, strong reducing ability, fast reaction, etc. [1]. Therefore, in recent years, nZVI has been widely used in the field of remediation of bodies of water. nZVI has been proven to be highly effective in the removal/degradation of a wide range of pollutants, including PCBs [2], bromides [3], dyes [4], nitrates [5], and heavy metals [6]. The rank for removal of heavy metals by nZVI has been shown to be as follows: As > Cr > Cu > Hg > Pb > Zn > Cd > Ni [7]. This shows that nZVI has a universal capability for reducing of heavy metals in water, and the pathway (mechanism) for the removal of the heavy metals includes reduction, adsorption, and co-precipitation [8,9]. Therefore, the use of nZVI as a reducing agent is a simple and convenient method for eliminating the heavy metal pollution from water. However, nZVI has some shortcomings areas where it has been applied, such as poor air stability, difficulty of preservation, and easy reunion [1,6,8–10]. In recent years, many studies have been conducted to find methods for preparing stable and efficient nZVI. Initially, a small amount of surfactant, which can play a protective role for nZVI, was added during the preparation process to improve the dispersibility of nZVI [11]. However, this method does not completely solve the problem of poor stability. Subsequently, it has been found that the use of a carrier (supporter) is a very effective way to improve the stability of nZVI. The carrier materials can lessen the oxidization of nZVI (to some extent) and improve its performance, even after use or storage [12]. The dispersibility and surface area of nZVI can be improved by using the carrier materials. The carriers usually used include inorganic compounds, organic compounds, silica, polymers, clays, carbon materials, etc. Petala et al. have successfully synthesized a stable nZVI-composite by using MCM-41 as a carrier [13]. By using diatom mud as the carrier, nZVI has been successfully loaded onto its surface and pore channels [14]. This carrier can greatly improve the dispersion of nZVI and enhance its capacity of remediation of bodies of water polluted by chlorinated organics. The cation exchange resin-supported nZVI-composite has been used for the decoloration of azo dye acid blue 113, and it shows a strong decolorization capacity (100%, within 30 min) [15]. Currently, the most common method for supporting nZVI is to immerse the carrier in a solution of an iron salt and then reduce the Fe(II)/Fe(III) to Fe0 with a reducing agent, such as NaHB4 . The ordered mesoporous carbon (OMC) impregnated with FeSO4 ·7H2 O was pyrolyzed at 300 ◦ C for 4 h and then treated with NaBH4 (2.1 M) to reduce the Fe(II) to metallic Fe0 [16]. The peak at 44.6◦ for nZVI in the nZVI/OMC (452 m2 g−1 ) is negligible, which implies that the crystallinity and stability of nZVI are relatively poor [16]. Although, this method is fast and effective, there are also some disadvantages: NaHB4 is expensive, and the conditions for reduction are rigorous. Therefore, some researchers choose the carbothermal reduction method to synthesize the nZVI/C composites [17]. By comparison with the conventional liquid-phase reduction method, it has the following advantages: the carbon is capable of supporting a large amount of nZVI and the stability of the nZVI in air is improved [18]. By pyrolyzing the mixture of carbon black and iron nitrate
Fig. 1. XRD patterns of nZVI/OMC carbonized at different temperatures (a) and with different dosage of ferric nitrate (b).
under argon [19], Fe3 O4 /C and Fe0 /C composites can be obtained at 300–500 ◦ C and 600–800 ◦ C, respectively. By pyrolyzing a mixture of the modified activated carbon colloid and iron salt, an Fe0 /C composite is obtained at 700 ◦ C [20] that still retains the colloidal characteristics and high reactivity. Using sucrose as the carbon source, the spherical Fe0 /C composites are obtained by using a twostep carbothermal reduction process [21]. The aerosol of Fe3 O4 /C composites is first synthesized at 1000 ◦ C, and then the Fe3 O4 is converted to Fe0 at 720 ◦ C [21]. The morphology of this Fe0 /C is regular (spherical) and the loading content of Fe0 is higher than in other methods [21]. The carbon carriers used in the reported literature usually have the problems of small SBET and pore volumes [16] and non-regular pore structure [19–21], which limits the
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Fig. 2. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of nZVI/OMC-x (x = 1–5).
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Fig. 4. (a) Effect of reaction time on removal efficiency of Cr(VI); (b) recycling tests for removal of Cr(VI) by using nZVI/OMC-3, nZVI/C, nZVI and OMC.
In this study, OMC with high surface area is selected as a carrier to support nZVI. OMC can play a protective role for nZVI and provide a certain adsorption capacity because of its uniform nanopore structure and high SBET . The nZVI/OMC composites, which are expected to have high stability and reactivity, are prepared using the carbothermal reduction method. These composites will gain the synergistic effects of the high reactivity of nano-particles and the high efficiency of the mass transfer of pollutants through the porous structure of the OMC, while the adsorption and reduction efficiency of nZVI/OMC will consequently be improved. Cr(VI) is selected as the target pollutant, and the efficiency and stability of nZVI/OMC for removing Cr(VI) is thoroughly investigated. Moreover, based on the analyses of the reduction efficiency of Cr(VI), the relationship between the structure and properties of nZVI/OMC is also discussed. 2. Materials and methods 2.1. Synthesis of nZVI/OMC composite Fig. 3. TG curves of the nZVI/OMC composites with different dosage of ferric nitrate.
performance of the adsorption and mass transfer of pollutants [22]. If these problems are properly solved, the stability and pollutantsremoval capacity of the nZVI-composite can be further improved.
The details of the chemicals used and the synthesis methods for SBA-15 [23] and OMC [24] are introduced in Supporting information (SI). X (x = 1–5) g of Fe(NO3 )3 ·9H2 O was dissolved in 15 mL of a mixture of ethanol and distilled water (1:1). 1 g OMC was wet-impregnated with the above-mixed solution, which was then stirred for over 2 h. After drying at 60 ◦ C, the resultant samples were
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Fig. 5. TEM (a) and HRTEM (b) image of nZVI/C; TEM (c) and HRTEM (d) image of nZVI/OMC-3.
carbonized at different temperatures (500–1000 ◦ C, at an interval of 100 ◦ C) in N2 flow (50–60 mL min−1 ) for 3 h (with a heating rate of 5 ◦ C min−1 ). After being naturally cooled to room temperature (25 ◦ C), the obtained samples were marked as nZVI/OMCs-x (x = 1–5). The number (1–5) for x was defined as the initial mass ratio of Fe(NO3 )3 ·9H2 O/OMC. The synthesis route for the nZVI/OMC composites are shown in Fig. S1 (SI). 2.2. Synthesis of comparison samples To investigate the influence of OMC on the reduction properties of nZVI/OMC composites, the ordinary carbon-supported nZVI composite (nZVI/C), nZVI and OMC were prepared as the contrast samples. 1 g of sucrose and 1 g of Fe(NO3 )3 ·9H2 O was dissolved in 10 mL of the mixture of ethanol and distilled water (1:1) and then stirred for over 2 h [25]. The resultant mixture was dried in an oven at 100 ◦ C for 6 h, and the oven temperature was raised to 160 ◦ C for 6 h. The resultant samples were carbonized in N2 flow at 900 ◦ C for 3 h (5 ◦ C min−1 ). The synthesis of nZVI was carried out as follows [26]: 100 mL of 20 mM NaBH4 aqueous solution was added drop-by-drop to a three-necked flask containing 100 mL of 4 mM FeSO4 ·7H2 O aqueous solution, stirring vigorously under N2 protection at ambient temperature. After stirring for 1.5 h, the ferrous iron was completely reduced by the borohydrate to form the nZVI particles. The synthesized nZVI particles were separated and washed twice with deionized water. Finally, the nZVI was dried at room temperature (25 ◦ C) under vacuum conditions, ready for instant usage.
2.3. Characterization methods X-ray diffraction (XRD) patterns were recorded with a Rigaku D/max-IIIB diffractometer using Cu K␣ ( = 1.5406 Å) radiation at a step scan of 0.02◦ from 10◦ to 80◦ . The accelerating voltage and the applied current were 40 kV and 20 mA, respectively. The resulting powder diffraction patterns were analyzed according to the Joint Committee on Powder Diffraction Standard (JCPDS) data. All of the thermogravimetric (TG) experiments were carried out in a TA instruments (NEJSCH STA 449C) TG apparatus. Transmission electron microscopy (TEM) images were taken by using a JEM2100 electron microscope (JEOL) with an acceleration voltage of 200 kV. The nitrogen adsorption/ desorption isotherms were measured at 77 K using a Micromeritics Tristar II. The specific surface area of materials was calculated by the Brunauer–Emmett–Teller (BET) theory. Pore size distribution was computed by using the Barrett–Joyner–Halenda (BJH) method from the adsorption branch of the isotherms. The surface components were determined by using X-ray photoelectron spectroscopy (XPS, Kratos-AXIS UL TRA DLD, Al K␣ X-ray source). The XPS data for each atom were fitted with the ‘XPS peak’ software. The used full width at half maximum (FWHM) was not more than 2.7 eV and allowed to float for all atoms. The % Lorentzian–Gaussian was set as 20% for all fittings. 2.4. Batch experiments Y g nZVI/OMC (6 mg nZVI), Z g nZVI/C (6 mg nZVI), 6 mg nZVI and 0.014 g OMC, respectively, were added to a 20 mL solution
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containing 50 mg L−1 Cr(VI) (K2 Cr2 O7 ) for adsorption. By using the thermogravimetric method, the iron (Fe2 O3 ) content of nZVI/OMC and nZVI/C could be obtained, and then the mass ratio (i.e., Y and Z) of the nZVI/composite could be approximately calculated. The adsorption tests were carried out at 25 ◦ C (air bath) and 110 rpm. The effects of the Cr(VI) concentration, the absorbents’ dosage, and pH on adsorption performance of the four samples were investigated. The pH of the suspensions was adjusted by adding HCl (0.1 M) or NaOH (0.1 M) to the solution. After filtration with a 0.45 m membrane, the Cr(VI) concentration of the filtrates was determined by using the 1, 5-diphenylcarbazide method (spectrophotometry).
3. Results and discussion 3.1. Structure characteristics of nZVI/OMC composites Fig. S2a and b (SI) shows the N2 adsorption–desorption isotherms and pore size distributions of SBA-15 and OMC, respectively. Fig. S2c and d (SI) shows the pore structure of OMC observed by TEM. The XRD patterns in Fig. 1a shows that the main crystalline phases of the samples carbonized at 700–1000 ◦ C are different from each other. At temperatures of 500–700 ◦ C, the curves are almost the same and show typical XRD patterns of the Fe3 O4 nanoparticles (JCPDS Card No. 75-1609) with a face-centered cubic structure, which indicates that the obtained composites are Fe3 O4 /OMC. At 800 ◦ C, the peaks correspond to the typical diffraction peaks of ␣Fe (44.5◦ and 65◦ ) with a body-centered cubic structure and FeO (36◦ , 42◦ , 60◦ , 72◦ , and 76◦ ) [16,27], which means that the Fe3 O4 in Fe3 O4 /OMC is gradually reduced by the carbothermal reduction. At 900 ◦ C and 1000 ◦ C, the diffraction peaks correspond to ␣-Fe (44.5◦ and 65◦ ) and ␥-Fe (51◦ and 78◦ ) with a face-centered cubic structure [28], which implies that the Fe3 O4 nanoparticles is completely converted to Fe0 . To save energy, 900 ◦ C is selected as the optimum temperature for preparation of the nZVI/OMC. XRD patterns of the composites (900 ◦ C) with different dosage of Fe(NO3 )3 are shown in Fig. 1b. It can be seen that the typical diffraction peaks of nZVI/OMC-1 and nZVI/OMC-2 are Fe3 O4 . At x = 3, 4 and 5, only the peaks of ␣-Fe and ␥-Fe are observed in the composites. At lower dosages of Fe(NO3 )3 , the carbothermal reduction reaction is not complete, which results in the formation of the highly-crystallized Fe3 O4 particles [29]. Based on the above analyses, x = 3 is selected as the optimal ratio for preparation of nZVI/OMC-x. N2 adsorption–desorption isotherms and pore size distributions of nZVI/OMC–x (x = 1–5) are shown in Fig. 2a and b, respectively. It can be seen that the adsorption–desorption hysteresis loops are postponed in the relative pressure range of 0.45–0.8 as iron content increases. Because nZVI is firmly loaded onto the surface and within the channels of OMC, the porous structure of OMC is inevitably destroyed to a certain extent [30]. Therefore, the SBET of nZVI/OMC–x (x = 1–5) decreases to 965.51, 841.37, 715.16, 596.12 and 518.81 m2 g−1 , respectively, as x increases. As shown in Fig. 2b, the sharp peaks centered at approximately 3.6 nm in the pore size distributions gradually become broader as x increases. This indicates that part of the pore channels of OMC are filled with the loaded nZVI, which inevitably leads to the decrease of SBET . The N2 adsorption–desorption isotherms and SBET (10.81 m2 g−1 ) of the pure nZVI are shown in Fig. S3a (SI). The TG curves of nZVI/OMC–x (x = 1–5) with different dosages of Fe(NO3 )3 are shown in Fig. 3 (heated in an air atmosphere). It can be seen that an obvious weight loss occurred at 450–560 ◦ C (approximately), which can be attributed to the combustion of carbon (OMC). After 560 ◦ C, there was no weight loss and the stable residuals (reddish brown) are iron oxide (Fe2 O3 , as shown in Fig. S3b, (SI)), which originated from the oxidation of nZVI in the
Fig. 6. Effects of pH (a), Cr(VI) concentration (b) and nZVI/OMC-3 dosage (c) on the removal of Cr(VI) from water.
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3.2. Cr(VI) removal efficiency and reusability comparison
Fig. 7. Removal pathway of Cr(VI) from water by using nZVI/OMC.
composite. Based on the mass of the residual Fe2 O3 , the nZVI content in the composite can be calculated. For example, the mass ratio of residual Fe2 O3 is approximately 41% after combustion; therefore, the total mass ratio of nZVI in nZVI/OMC-3 is approximately 30%. The TG curve of the nZVI/C composite is shown in Fig. S3c (SI).
Fig. 4a shows the removal efficiency of Cr(VI) in aqueous solution with an initial concentration of 50 mg L−1 (20 mL solution, a pH of approximately 5.6 and a sample dosage of 20 mg mL−1 ) using 20 mg nZVI/OMC-3, 23.1 mg nZVI/C (each of the two samples contains 6 mg nZVI according to the TG analyses), 6 mg nZVI and 14 mg OMC. It can be seen that the removal efficiencies of Cr(VI) by nZVI and OMC are approximately 39% and 57%, respectively, within 10 min, which indicates that OMC (1277.91 m2 g−1 , SI) has a stronger adsorption capacity for Cr(VI). nZVI/OMC-3 (715.16 m2 g−1 ) is the best of the four materials with respect to the removal efficiency or reaction rates, and complete removal (near 99%) is achieved within 10 min. This implies that at least approximately 40% of the Cr(VI) is reduced to the less toxic Cr(III) by nZVI/OMC-3 within 10 min. The removal efficiency of Cr(VI) by nZVI/C (approximately 94%) is nearly equivalent to that of nZVI/OMC-3 after the equilibrium (60 min), but its reaction rate is far slower than that of nZVI/OMC-3. The reusability (the experimental conditions are the same as that of Fig. 4a) of nZVI/OMC-3, nZVI/C, nZVI and OMC is compared by recycling seven times (Fig. 4b). The removal efficiency of Cr(VI) by nZVI/OMC-3 gradually decreases with increasing number of cycles, but no less than 60% efficiency is still maintained after the 7th cycle. It can be determined that the adsorption capacity of OMC for removal of the Cr(VI) is nearly exhausted after the 3rd cycle. From the beginning of the 4th cycle, Cr(VI) is only being reduced by the nZVI of nZVI/OMC-3, and the reaction activity of nZVI can be quickly recovered after each cycle. The removal efficiency of Cr(VI) by nZVI/C dramatically decreases after the first use and remains at only 19% after the 7th cycle. The adsorption capacity of nZVI/C is completely exhausted before recycling again and the removal relies only on the reduction activity of nZVI. The reusability of nZVI/OMC3 is better than those of nZVI/C and nZVI, which implies that the stability of nZVI is more important than the reaction activity for removal of Cr(VI) from water. These results once again prove that the pore channels of OMC can not only provide protective layers for inhibiting the oxidation of nZVI but can also enhance the stability and reaction activity of nZVI in nZVI/OMC-3. To distinguish the role of carbon carriers, the TEM morphologies of nZVI/C and nZVI/OMC-3 are compared. As shown in Fig. 5a and b, some nZVI with irregular shapes and particle sizes are dispersed on the surface of the nZVI/C, and the obvious agglomeration phenomenon can be observed. As shown in Fig. 5c, nZVI with particles sizes of 20–30 nm is uniformly distributed in the structure of nZVI/OMC-3 and no agglomeration is observed. In Fig. 5d, the ordered structure of the mesopores for nZVI/OMC-3 can be clearly observed, but part of the channels of nZVI/OMC-3 are plugged and bent after iron loading, which implies that the ordered mesoporous structure is slightly deformed. It can be seen that the morphologies of nZVI/C and nZVI/OMC-3 are dramatically different from each other, which results in their differences in reductive activity and stability. At temperatures ≤700 ◦ C, Fe3 O4 is formed in the nZVI/OMC-3, which is further reduced by carbon to form nZVI particles at higher temperatures (700–900 ◦ C). During the reduction process, the walls of the pore channels of OMC are first softened by the growth of Fe-species nano-particles, and, subsequently, some carbon atoms in the walls are removed [27]. Therefore, a little deterioration of the ordered mesoporous structure occurs, which is accompanied by in situ formation of nZVI in OMC. 3.3. Effect of reaction conditions on removal efficiency of Cr(VI) by nZVI/OMC-3
Fig. 8. (a) XRD patterns of nZVI/OMC-3 and (b) panoramic XPS diagrams of nZVI/OMC-3 before use, after the 7th use, and after storage in air for 15 days.
As shown in Fig. 6a, the removal efficiency of Cr(VI) (50 mg L−1 ) by nZVI/OMC-3 (solid dosage of 20 mg mL−1 ) is dramatically affected by pH. The Cr(VI) removal efficiency in acidic pH is higher
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Fig. 9. High resolution XPS of Fe 2p and O 1s spectra for nZVI/OMC-3 before use (a and b), after the 7th use (c and d), and after storage in air for 15 days (e and f); (1) Fe 2p3/2 → Fe0 (␣-Fe), (2) Fe 2p3/2 → Fe3+ , (3) Fe 2p3/2 → Fe3+ (satellite peaks), (4) Fe 2p1/2 → Fe3+ (satellite peaks), (5) O 1s → O-physically absorbed or carbonates, (6) O 1s → Fe O, and (7) O 1s → O C = O.
than that in alkaline pH, and it can reach approximately 99% at pH 4. The pH dependence of Cr(VI) removal is mainly related to the ion (Cr(VI)) chemistry in the solution [31]. In aqueous solution, Cr(VI) has two forms, including Cr2 O7 2− under acidic conditions and CrO4 2− under alkaline conditions. Cr2 O7 2– has the characteristic of strong oxidation resistance, which can be easily reduced [31]. As the pH decreases, the surface potential of nZVI/OMC-3 is increased, while the capacity of the surface complexation of nZVI with anions is correspondingly enhanced [32]. Therefore, precipitation of the Cr-Fe complex occurs is formed during the reaction, which further enhances the Cr(VI) removal efficiency. It can be seen from Eqs. (1)–(3) below that the reaction of Fe0 and Cr2 O7 2– needs H+ , therefore, the Cr(VI) reduction rate increases as the pH of the solution decreases [33]. Cr2 O7 2– (aq) + Fe0 (s)-- + 14H+ (aq) → 2Cr3+ (aq) + 3Fe2+ (aq) + 7H2 O
(1)
Cr2 O7 2– (aq) + 6Fe2+ (aq)-- + 14H+ (aq) → 2Cr3+ (aq) + 6Fe3+ (aq) + 7H2 O
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xCr3+ (aq) + (1−x)Fe3+ (aq)-- + H2 O → Crx Fe(1−x) (OH)3 ↓ + 3H+ (3) The effects of Cr(VI) concentration and nZVI/OMC-3 dosage on removal efficiency of Cr(VI) are shown in Fig. 6b and c, respectively. Fig. 6b shows that Cr(VI) removal efficiency decreases as the initial concentration of Cr(VI) increases (pH 4 and solid dosage of 20 mg mL−1 ) and the reaction equilibrium can be reached within
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3.4. Stability analysis of the nature of nZVI/OMC-3 XRD patterns of the nZVI/OMC-3 samples before use, after the 7th use, and after storage in air for 15 days are shown in Fig. 8a. The results show that the main crystalline phases of the three samples do not have substantial differences. The diffraction peaks of ␥-Fe originating from the passivation of nZVI (␣-Fe) are found at approximately 44◦ for the samples after use and storage for 15 days [28]. However, no diffraction peaks of the hematite (␣-Fe2 O3 , JCPDS, No. 87-1165) can be observed, indicating the excellent stability of these samples. The carrier OMC can greatly enhance the stability of nZVI, making it difficult to be deeply oxidized even in an air atmosphere. There is no doubt that the stability of nZVI/OMC-3 is far greater than that of pure nZVI. Panoramic XPS tests are conducted to detect the chemical compositions of nZVI/OMC-3 before use, after the 7th use, and after storage in air for 15 days (Fig. 8b). From the survey spectra, the predominant C 1s peaks (narrow) are located at around 284.6 eV, along with O 1s peaks at around 530 eV and Fe 2p peaks at around 710 eV for the three samples. It should be noted that although the used nZVI/OMC-3 is washed with deionized water three times before the XPS test, the Cr 2p peak at around 575 eV is still observed, which confirms that Cr-species (i.e., Cr3+ ) are firmly absorbed into the porous structure of OMC and the reduction reaction (Cr6+ → 2Cr3+ ) is indeed present [34]. These results clearly indicate that the differences in the surface (or skeleton) elements constituting the three samples are minor, implying good stability and reusability of nZVI/OMC-3. Fig. 9 shows the high resolution XPS of the Fe 2p and O 1s spectra for the three samples. As shown in Fig. 9a, c and e, two obvious bands are observed in the curves, which correspond to the low energy (Fe 2p3/2 ) and high energy (Fe 2p1/2 ) asymmetric bands originating from the spin orbital splitting, respectively [35,36]. The Fe 2p peaks can be decomposed into four components, originating from the metallic iron (␣-Fe, around 707.2 eV), Fe 2p3/2 ferric state (around 711.2 eV), Fe 2p3/2 ferric state (satellite peaks, around 713.0 eV), and Fe 2p1/2 ferric state (satellite peaks, around 725.1 eV). The satellite peaks are caused by the process of shake-off and shakeup, and the surface plasmons [35]. The presence of the ferric state species (such as Fe2 O3 ) confirms that part of the nZVI on the composite surface is inevitably oxidized by O2 , even when unused. It should be noted that the Fe2 O3 is not detected in the XRD tests. It may be that only a small amount of nZVI on the surface layer is oxidized, which is not sufficient to be detected by XRD. The Fe 2p3/2 XPS spectra do not show any Fe3 O4 (708.2 eV) peaks, which are already detected by XRD. The deep inlay of a little Fe3 O4 in the skeleton of OMC results in the formation of a protective (carbon) layer, which restricts the XPS detection.
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30 min. The removal efficiency can reach above 93% within 5 min at the initial concentration of 75 mg L−1 . As shown in Fig. 6c, the removal efficiency of Cr(VI) increases as the sample dosage increases. Under the conditions of pH 4 and initial Cr(VI) concentration of 50 mg L−1 , the removal efficiency of Cr(VI) by nZVI/OMC-3 with dosage of 10 mg mL−1 is comparable to that with 20 mg mL−1 with pH 5.6. The removal process is fast and efficient, which indicates that the dominant pathway for Cr(VI) removal must be the reduction reaction. The deduced pathway (mechanism) for removal of Cr(VI) by nZVI/OMC is shown in Fig. 7. During the reduction process, Cr(VI) can be quickly attracted to the area surrounding the nZVI by adsorbing on the surface of OMC, which provides greater chances for the contact (reaction) of Cr(VI) and nZVI/OMC-3 (Fig. 7). Furthermore, the mass transfer of Cr(VI) through the body of nZVI/OMC-3 is enhanced by the abundant pore channels of OMC, which correspondingly enhances the reduction rate and efficiency (Fig. 7).
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Binding Energy (eV) Fig. 10. High resolution XPS of C 1s spectra for nZVI/OMC-3 before use (a), after the 7th use (b), and after storage in air for 15 days (c).
As shown in Fig. 9b, d and f, the O 1 s peaks can be decomposed into three components originating from the physically-absorbed O or carbonates (around 530.7 eV), Fe O (around 531.2 eV) and O C = O (around 533.0 eV). The presence of the Fe O (Fe2 O3 ) bond in the O 1s spectra confirms the oxidation of nZVI on the surface of OMC, which is consistent with the Fe 2p analysis. The binding energy (peak) of the Fe O bond in the unused nZVI/OMC-3 is slightly lower than those of other two samples, which can be attributed to the further oxidation of nZVI after use or storage in air. As shown in Fig. 10, the C 1s peaks can be decomposed into two components originating from the C in graphite (around
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284.6 eV) and C O (around 286.0 eV) [36,37]. The C 1s band at around 286.0 eV may correspond to the C O in carboxyl, epoxy, or phenol groups in the OMC skeleton [36,37]. However, the C 1s peaks for the three samples remain almost unchanged, implying that the characteristics of the OMC carrier are very stable. These XPS results provide solid evidence that the nZVI/OMC-3 is stable and suitable to be used for eliminating pollutants from water. 4. Conclusions In summary, the load-type composites of nZVI/OMC are prepared and applied to remediate the Cr(VI) pollution in water. Stable nano-magnetic nZVI/OMC is prepared under the conditions of 900 ◦ C and an iron/carbon ratio of 3: 1. Cr(VI) can be rapidly removed by nZVI/OMC-3, which has high SBET (715.16 m2 g−1 ) and small nZVI particle sizes (20–30 nm), which are conducive to the reduction and mass transfer of Cr(VI). Furthermore, the stability and activity of the nZVI in nZVI/OMC are dramatically enhanced by the protective effect of the OMC skeleton, which prevents it from being easily oxidized under O2 exposure. Although, slight damage to the ordered mesoporous structure and the nZVI oxidization inevitably exists, the performance of nZVI/OMC was far better than those of nZVI/C, nZVI and OMC. Therefore, nZVI/OMC prepared by this method can be regarded as an effective and feasible method for removal of heavy metals from bodies of water, being associated with the advantages of proven technology and relatively low reactant toxicity. Acknowledgements We acknowledge the support by Key Program Projects of the National Natural Science Foundation of China (21031001), National Natural Science Foundation of China (51108162, 20971040), Natural Science Foundation of Heilongjiang Province (B201411), Postdoctoral Science (Start–up) Foundation of Heilongjiang Province (LBH-Q14137), Excellent Young Researchers Fund and Hundred Young Talents Fund of Heilongjiang University, and Young Teachers Fund of Heilongjiang Institute of Technology (QJ201202). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2015.04.013. References [1] D.L. Wu, S.S. Zheng, A.Q. Ding, G.D. Sun, M.Q. Yang, Performance of a zero valent iron-based anaerobic system in swine wastewater treatment, J. Hazard. Mater. 286 (2015) 1–6. [2] E.J. Petersen, R.A. Pinto, X.Y. Shi, Q.G. Huang, Impact of size and sorption on degradation of trichloroethylene and polychlorinated biphenyls by nano-scale zero valent iron, J. Hazard. Mater. 243 (2012) 73–79. [3] H.M. Zhou, Y.Y. Shen, P. Lv, J.J. Wang, P. Li, Degradation pathway and kinetics of 1-alkyl-3-methylimidazolium bromides oxidation in an ultrasonic nanoscale zero-valent iron/hydrogen peroxide system, J. Hazard. Mater. 284 (2015) 241–252. [4] Y. He, J.F. Gao, F.Q. Feng, C. Liu, Y.Z. Peng, S.Y. Wang, The comparative study on the rapid decolorization of azo, anthraquinone and triphenylmethane dyes by zero-valent iron, Chem. Eng. J. 179 (2012) 8–18. [5] C.L. Tang, Z.Q. Zhang, X.N. Sun, Effect of common ions on nitrate removal by zero-valent iron from alkaline soil, J. Hazard. Mater. 231 (2012) 114–119. [6] S.A. Kim, S. Kamala-Kannan, K.J. Lee, Y.J. Park, P.J. Shea, W.H. Lee, B.T. Oh, Removal of Pb(II) from aqueous solution by a zeolite–nanoscale zero-valent iron composite, Chem. Eng. J. 217 (2013) 54–60. [7] R.A. Crane, T.B. Scott, Nanoscale zero-valent iron: future prospects for an emerging water treatment technology, J. Hazard. Mater. 211 (2012) 112–125. [8] E. Petala, K. Dimos, A. Douvalis, T. Bakas, J. Tucek, R. Zboˇril, M.A. Karakassides, Nanoscale zero-valent iron supported on mesoporous silica: characterization and reactivity for Cr(VI) removal from aqueous solution, J. Hazard. Mater. 261 (2013) 295–306.
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Please cite this article in press as: Y. Dai, et al., Carbothermal synthesis of ordered mesoporous carbon-supported nano zero-valent iron with enhanced stability and activity for hexavalent chromium reduction, J. Hazard. Mater. (2015), http://dx.doi.org/10.1016/j.jhazmat.2015.04.013