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Rapid degradation of azo dye methyl orange using hollow cobalt nanoparticles
Chemosphere 144 (2016) 1530e1535
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Short communication
Rapid degradation of azo dye methyl orange using hollow cobalt nanoparticles Yingying Sha a, Iswarya Mathew a, Qingzhou Cui a, Molly Clay a, Fan Gao a, Xiaoqi Jackie Zhang b, Zhiyong Gu a, * a b
Department of Chemical Engineering, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, USA Department of Civil and Environmental Engineering, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Novel hollow Co nanoparticles were synthesized for azo dye degradation. The hollow Co nanoparticles exhibited rapid degradation toward methyl orange. pH had a significant effect on methyl orange degradation by hollow Co nanoparticles. The hollow Co nanoparticles are easily recycled through magnetic separation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2015 Received in revised form 3 October 2015 Accepted 10 October 2015 Available online xxx
A rapid and efficient method for methyl orange degradation using hollow cobalt (Co) nanoparticles is reported. Hollow Co nanoparticles were fabricated by a galvanic replacement reaction using aluminum (Al) nanoparticles as the template material. The methyl orange degradation characteristics were investigated by measuring the time dependent UVeVis absorption of the dye solution, which showed a very fast degradation rate under acidic conditions. At an initial methyl orange concentration of 100 mg/L (pH ¼ 2.5) and Co nanoparticle dosage of 0.5 g/L, the azo dye degradation efficiency reached up to 99% within 4 min, and the degradation constant rate was up to 2.444 min1, which is the highest value among other studies. A comparison of the decolorization rates at similar conditions with several other azo dyes, including Congo red, Amaranth, and Orange G, showed that the dye with a simpler structure and lower molecular mass decolorized considerably faster than the ones having a more complicated structure (higher molecular mass). The methyl orange degradation was also conducted using hollow nickel (Ni) nanoparticles and commercially available solid spherical Co and Ni nanoparticles. The results showed that Co-based nanoparticles outperformed Ni-based nanoparticles, with the hollow Co nanoparticles exhibiting the fastest degradation rate. Using the hollow Co nanoparticles is a very promising approach for the remediation of methyl orange dye containing wastewater due to the fast degradation rate and high degradation efficiency. In addition, these hollow Co nanoparticles are easily recycled because of their magnetic property. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Azo dye Methyl orange Reductive degradation Cobalt Hollow particles Magnetic separation
* Corresponding author. E-mail address: [email protected] (Z. Gu). http://dx.doi.org/10.1016/j.chemosphere.2015.10.040 0045-6535/© 2015 Elsevier Ltd. All rights reserved.
Y. Sha et al. / Chemosphere 144 (2016) 1530e1535
1. Introduction Synthetic dyes have been widely used in various fields, especially in textile industries. Azo dyes constitute a significant proportion of synthetic dyes, and have been identified as potential genotoxic and carcinogenic agent (Brown and De Vito, 1993). Dye removal or degradation from wastewater has been extensively studied to reduce the impact on the environment. A wide range of physicoechemical and biological methods, such as adsorption (Abramian and El-Rassy, 2009; Roy et al., 2013; Goudarzi et al., 2014), ozonation (Yıldırım et al., 2011; Liakou et al., 1997; Manivel et al., 2015), Fenton (Nidheesh et al., 2013; Lucas and Peres, 2006; n et al., 2006; Peternel et al., 2007; Gomathi Devi et al., Chaco 2009), photocatalysis (Konstantinou and Albanis, 2004; Chen et al., 2011; Singla et al., 2014), reductive degradation (Bigg and Judd, 2001; Bokare et al., 2008; Cao et al., 1999), microbial process (Nigam et al., 1996; McMullan et al., 2001; Martins et al., 1999), and two or more combination methods have been successfully applied to the treatment of various dyes (Peternel et al., 2007). Among these methods, reductive degradation is a fast and low cost method, which is easy to implement for the industrial level dye removal. Up to date, the following particles (including nanoparticles) have been utilized for azo dye removal, including zerovalent iron (Fe0) (Bigg and Judd, 2001; Cao et al., 1999; Zhang, 2003; Shu et al., 2007; Fan et al., 2009; Shih et al., 2010) and iron-based bimetallic nanoparticle, such as Fe/Ni (Bokare et al., 2008). Recently, Cobalt (Co) nanoparticle has emerged as a promising agent to treat azo dyes (Liang and Zhao, 2012; Naz et al., 2014).
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However, some report indicated adsorption as the main decolorization mechanism (Liang and Zhao, 2012), and some indicated catalytic reduction degradation (Naz et al., 2014). In this paper, we report a very fast reductive degradation method for methyl orange dye based on novel hollow Co nanoparticles which were synthesized by a galvanic replacement reaction. The hollow Co nanoparticles were applied on the methyl orange degradation study by monitoring the change of the dye concentration over time. The reductive degradation effectiveness of the hollow Co nanoparticles on methyl orange was illustrated by comparing the results with other azo dyes including Orange G, Congo red and Amaranth. In addition, the kinetic study of dye degradation was conducted by comparing the hollow Co nanoparticles with the hollow Ni nanoparticles and commercial solid Ni and Co nanoparticles. 2. Materials and methods 2.1. Materials and chemicals Aluminum (Al) nanoparticles (80 nm and 120 nm) were purchased from Novacentrix (Austin, Texas). The Al nanoparticles were sealed in an inert gas during shipping and handling and were immediately stored in an argon-filled glove box upon arrival. For the hollow metal nanoparticle fabrication and dye degradation test, chemicals used and purchased from Arcos Organics include ammonium chloride (NH4Cl, 99.6%), cobalt (II) chloride hexahydrate (CoCl2$6H2O, 98%), nickel sulfate hexahydrate (NiSO4$6H2O,
Fig. 1. (A) TEM image of Al nanoparticles as purchased with the nominal diameter of 120 nm; (B) SEM images of as synthesized hollow Co nanoparticles through the galvanic replacement reaction; (C) EDS measurement of hollow Co nanoparticles; (D) SAED pattern of a single hollow Co nanoparticle.
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99%), sodium hydroxide (NaOH, extra pure), methyl orange (C14H14N3NaO3S, pure, conform to ACS), Orange G (C16H10N2Na2O7S2, pure, certified), Congo red (C32H22N6Na2O6S2, indicator grade) and Amaranth (C20H11N2Na3O10S3, 85%). Additionally, sodium citrate dihydrate (Na3C6H5O7$2H2O, 99.0%) and anhydrous ethyl alcohol (denatured) were used as purchased from Fisher Scientific. Hydrochloric acid (HCl, 36.5%) was purchased from VWR Scientific. Commercial Ni and Co solid nanoparticles were purchased from American Elements (Los Angeles, CA) and were used without further treatment.
suspension in ethanol on a piece of TEM grid and dried under ambient conditions. Selected area electron diffraction (SAED) was taken by Zeiss Libra 120 TEM at Oak Ridge National Laboratory. A UV/Vis spectrophotometer (Lambda 25, Perkin Elmer) was utilized to collect the absorption spectra of azo dye degradation. The pH value was measured by a benchtop pH meter (model pH 510, Fisher Scientific).
2.2. Synthesis of hollow metallic nanoparticles
3.1. Hollow Co nanoparticles
The hollow Co and Ni nanoparticles were synthesized by the galvanic replacement method (Cui et al., 2011; Clay et al., 2012; Guo et al., 2005, 2007). The reaction was carried out in the glove box (Plas-Labs Inc., Model 818-GB) to minimize the nanoparticle oxidation. Briefly, the replacement precursor solution was prepared with 0.23 M CoCl2 or NiSO4, 1.87 M NH4Cl, and 0.44 M Na3C6H5O7, in an aqueous solution. Then, 1 g of aluminum nanoparticles was mixed with 30 mL of the precursor solution to initiate the reaction. Periodic agitation of the solution continued throughout the reaction. At the end of the reaction, the nanoparticles were rinsed with 2 M NaOH to remove any residual Al template material followed by washing with DI water and ethanol. Finally, the nanoparticles were dried under vacuum and collected as dry powders which were then weighed and applied quantitatively to the following degradation tests.
Fig. 1 shows the nanoparticle characterization results of the template Al nanoparticles and the hollow Co nanoparticles that have been made. Fig. 1(A) is the TEM image of Al nanoparticles as purchased with a nominal diameter of 120 nm; Fig. 1(B) shows the SEM image of as-synthesized hollow Co nanoparticles through the galvanic replacement reaction; either single hole or multiple holes were observed on each single Co nanoparticle. Fig. 1(C) shows the EDS results of Co hollow nanoparticles, which indicates that a complete removal of the Al template is achieved and the Co nanoparticles contain pure composition of Co with slight surface oxidation; Fig. 1(D) shows the selected area electron diffraction (SAED) ring pattern of a single Co hollow nanoparticle under TEM. The procedure for indexing the ring pattern typically follows the steps as below: (1) measuring the diameter of the ring by Image J;
3. Results and discussion
2.3. Degradation of azo dyes The as synthesized hollow metal nanoparticles were used for the methyl orange degradation at different conditions. A 10 mL centrifuge tube was used as a reaction vessel for the dye degradation studies. The tube was fixed to a rotator, which was rotating at 10 rpm to help facilitate the dispersion of metal nanoparticles and maintain solution uniformity during the degradation process. The initial concentration of the dye solution was kept at 100 mgL1 and the pH was adjusted to the desired value by the addition of 2 M HCl or 2 M NaOH solution. The methyl orange solution was added to the tube containing a weighed amount of hollow metal nanoparticles to initiate the degradation. The reaction was monitored by measuring the UVevis absorption spectra of the sample solution taken out at regular intervals (e.g., 1, 3, 5, 10, 20, 30, 60, and 90 min). For the UVevis spectrophotometric analysis, 150 mL of sample solution was taken out of the reaction vessel and transferred to a 1.5 mL centrifuge tube and centrifuged immediately to stop the degradation reaction by precipitating the metal nanoparticles. It took about 20 s for the centrifugation to separate the metal nanoparticles from the reaction mixture. A 100 mL of above clear solution was diluted to 1 mL with DI water and absorbance spectra was recorded over a wavelength range of 200e900 nm. The sampling time interval was adjusted to obtain smooth kinetics curve depending on the initial concentration and pH of the dye solution, the type of metal nanoparticle, and its dosage. 2.4. Instruments and characterization The nanoparticle imaging was performed using a JEOL JSM7401F field emission scanning electron microscope (FESEM). The SEM was equipped with an energy-dispersive x-ray spectrometer (EDS) for elemental analysis of the nanoparticles. Transmission electron microscopy (TEM) imaging was performed using a Philips EM400 transmission electron microscope operated at 100 kV. TEM samples were prepared by placing a drop of the nanoparticle
Fig. 2. (A) Methyl orange dye solution (pH ¼ 2.5) before and after hollow Co nanoparticle treatments; (B) Time-dependent UV/Vis absorption measurement of methyl orange degradation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Y. Sha et al. / Chemosphere 144 (2016) 1530e1535
(2) converting the distances into interplanar values via the equation d ¼ lL/r, where d is the lattice spacing, l is the wavelength of the electrons, L is the camera focal length and r is the distance between the central spot and the ring; (3) comparing the measured inteplanar values with standard (Andrews et al., 1971). The index (100) (002) (102) (110) indicates the hexagonal polycrystalline structure of the Co nanoparticles. Similarly, hollow Co nanoparticles can also be synthesized by 80 nm Al nanoparticle template.
3.2. Catalytic degradation efficiency The eN]N- double bond in the azo dyes is the chromophoric group for color (Aljamali, 2015). Typically, the absorption band of methyl orange shows red shift with a decrease in pH, which results in the color change from yellow to red, as illustrated in S1. The increase in wavelength suggests an increase in delocalization in the methyl orange molecules (Del Nero et al., 2005). Fig. 2(A) shows the photograph of methyl orange dye solution (pH ¼ 2.5) before and after hollow Co nanoparticle treatments (2 g/L). Fig. 2(B) shows the time-dependent UV/Vis absorption measurement of methyl orange degradation. It is observed that the dye degradation occurred immediately upon the addition of the hollow Co nanoparticles into the dye solution, and the decolorization was completed in about 4 min. At t ¼ 0 min, the curve represents the absorption spectra of methyl orange solution without Co nanoparticle treatment. After the addition of the hollow Co nanoparticles at pH ¼ 2.5, a decrease in the intensity of the absorption band at ~500 nm is observed. This indicates the adsorption of methyl orange molecules by hollow Co nanostructure and possible cleavage of the azo bond, which is the chromophoric group and thereby decolorizing the solution. The
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formation of aromatic products is indicated by the appearance of a new peak ~248 nm and the intensity of this absorption peak increases over reaction time. This suggests successful decomposition of methyl orange dye in the solution by the reduction of azo bond to two or more possible chemical structures with amines (NH2). The emerging of a new peak indicates that the decolorization of the methyl orange dye was due to dye degradation from the solution, instead of only physical adsorption. The proposed two products are sulfanilic acid and dimethyl-4-phenylenediamine, which were analyzed and compared with the standard pure species under UV/ Vis analysis, as shown in S2.
3.3. Degradation mechanism The decolorization experiments were performed on several other azo dyes with different molecular structures as presented in Fig. 3, including Congo red, Amaranth and Orange G. The chemical structures of each dye are inserted in the figure. For the four dyes studied, only the methyl orange showed very fast decolorization rate (Fig. 3(D)), and the other three dyes showed slower decolorization rates. In addition, there were no new peaks formed in the spectra of the other three dyes, indicating that the dye decolorization process was mostly through surface adsorption, instead of degradation. The remarkably faster decolorization of methyl orange compared to the other three azo dyes can be attributed to its relatively simpler chemical structure and lower molecular weight. It is possible that the larger molecular structures of dyes will: (1) hurdle the access to the eN]N- double bond, so the active Co atoms on the surface of the hollow Co nanoparticles cannot easily access the dye molecule, except methyl orange which has a relatively small size with only one benzene ring on either side of the
Fig. 3. Time-dependent UV/Vis absorption measurement of different azo dyes at a pH value of 3.0. (A) Congo red, (B) Amaranth, (C) Orange G, (D) Methyl orange. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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eN]N- double bond; (2) block the sensitive surface of the Co catalyst therefore reducing the availability of active sites. On the basis of the results discussed above, the mechanism for methyl orange degradation using hollow Co nanoparticles can be proposed as below: When the catalyst is introduced, the methyl orange molecule was first adsorbed on the surface of the hollow Co nanoparticle, then the Co atoms performed as a catalyst and reacted with the eN]N- double bond and broke it to single phenyl ring compounds with amine group as possible products, thereby decolorizing the methyl orange (see Figure S3). The proposed reaction mechanism for methyl orange degradation by hollow Co nanoparticles is similar to the mechanisms observed for the azo dye treatment by zero valent iron nanoparticles, which have been reported for the reductive degradation of methyl orange (Fan et al., 2009) and acid orange II (Cao et al., 1999). 3.4. Kinetic study The dye decolorization kinetics by the hollow Co nanoparticles were studied for the four azo dyes. The concentration of each dye was quantified at their absorbance peak maxima (Regression analysis wavelength: Methyl orange ¼ 470 nm, Congo red ¼ 500 nm, Orange G ¼ 475 nm and Amaranth ¼ 520 nm). Based on the discussions above, although the reactive chemical degradation only took place for methyl orange, other azo dyes also followed a pseudo first order kinetics (except for Congo red, which had a very low decolorization rate and showed some instability at the early stage), as shown by the fitting curves in Fig. 4(A). With the same amount of the hollow Co nanoparticles used, the fastest dye decolorization rate was obtained from the methyl orange dye degradation; while for the other three dyes, where surface adsorption dominated the dye removal process, the decolorization rate followed the order of Amaranth > Orange G > Congo Red. Besides the hollow Co nanoparticles, the galvanic replacement method could also be used to synthesize hollow Ni nanoparticles (Cui et al., 2011; Clay et al., 2012). In order to understand the hollow structure effect on the methyl orange dye degradation, commercially purchased solid spherical Co and Ni nanoparticles have been compared with the hollow Co and Ni nanoparticles. Fig. 4(B) shows the kinetics of the methyl orange degradation by using different nanoparticles. The hollow Co nanoparticle performed better degradation efficiency than the solid Co nanoparticle, because the hollow Co structure has much more surface area than solid Co particles at the same mass loading, which provides more adsorption binding site of Co to react with methyl orange, hence improved the catalytic activity and degradation rate. Both hollow and commercial Co nanoparticles performed better than the Ni nanoparticles in the degradation of methyl orange. At pH ¼ 3, the hollow Ni nanoparticles exhibited faster degradation rate than the commercial solid Ni nanoparticles. The initial pH has a significant effect on the methyl orange degradation. When the pH was increased to 10 (basic solution), the dye degradation process was significantly slowed down; thus, acidic conditions are preferred for methyl orange degradation. The additional measurement of pH effect can be found from Figure S4. It is believed that the hollow cobalt catalyst particles act as an electron donor and the methyl orange dye molecules acts as electron acceptor, and when combined with Hþ ions form a transitional product as shown in the scheme (see Figure S3). The acidity of the solution and the presences of surface active sites are the influencing factors for the dye degradation reaction. At a pH value of 2.5, the hollow Co nanoparticles exhibited the highest degradation rate among all the nanoparticles studied, showing a reaction rate constant of 2.444 min1 (R2 ¼ 0.9999). Table 1 compares the reaction rate constant for methyl orange degradation from our result to the ones obtained from the
Fig. 4. Azo dyes decolorization curves. (A) Comparison of different azo dye decolorization; (B) Comparison of synthesized hollow nanoparticles and commercial nanoparticles toward methyl orange degradation efficiency. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
literature. Clearly, the hollow Co nanoparticles showed a much faster reaction rate toward methyl orange degradation. For practical applications, there are concerns for the hazardous and environmental effect of nanoparticles in industrial manufacturing and usage. In this work, the hollow Co nanoparticles synthesized in this article can be easily recycled due to their magnetic property. In Figure S5, a simple magnetic separation experiment showed that the hollow Co nanoparticles in the solution could quickly move and stick on the side of the reaction container upon applying a magnet, which facilitates easy separation of the nanoparticles from the reaction medium and would maximally eliminate the effect of nanoparticle toxicity to the environment in the practical applications. Table 1 Reaction rate constants of several catalysts for methyl orange degradation. Catalyst
Rate constant (min1)
Reference
Fe0 Nanoparticles Nanoporous Au TiO2 Nanotube Arrays Fe0eUVeH2O2 Hollow Co nanoparticles
0.559 2.1 103 0.0515 0.1025 2.444
(Fan et al., 2009) (Hakamada et al., 2012) (Zhang et al., 2007) (Gomathi Devi et al., 2009) This work
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4. Conclusions Methyl orange is one of the most stable azo dyes which is extensively used in the textile industry and resistant to biodegradation. This study demonstrated the effectiveness of the hollow Co nanoparticles in methyl orange azo dye degradation. The hollow Co nanoparticles synthesized by the galvanic replacement method exhibited fast catalytic property during methyl orange degradation and reduced methyl orange to amine compounds in a matter of few minutes. The hollow Co nanoparticles showed rapid degradation with a rate constant of 2.444 min1, the fastest degradation rate towards methyl orange to date, to the best of our knowledge. The dye degradation rates were very sensitive to pH, and worked best in the acidic conditions. Compared to the solid nanoparticles, the nanoparticles with hollow structures possess much higher surface area for high concentration organic compound adsorption and surface reaction, hence increase the methyl orange degradation rate. Further study is needed to analyze the degradation products to better understand the different decolorization mechanisms (adsorption vs. degradation) for different azo dyes. Acknowledgments Financial support from the UMass CVIP Technology Development Fund is acknowledged. Partial funding support from the NSF (Award # CMMI-1031532) is also acknowledged. We thank Dr. Jihua Chen at the Oak Ridge National Laboratory for his help in the selected area electron diffraction measurements. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.10.040. References Abramian, L., El-Rassy, H., 2009. Adsorption kinetics and thermodynamics of azodye Orange II onto highly porous titania aerogel. Chem. Eng. J. 150 (2e3), 403e410. Aljamali, N., 2015. Review in azo compounds and its biological activity. Biochem. Anal. Biochem. 4 (2), 1000169. Andrews, K.W., Dyson, D.J., Keown, S.R., 1971. Interpretation of Electron Diffraction Patterns. Plenum Press, New York. Bigg, T., Judd, S., 2001. Kinetics of reductive degradation of azo dye by zero-valent iron. Process Saf. Environ. Prot. 79 (5), 297e303. Bokare, A.D., Chikate, R.C., Rode, C.V., Paknikar, K.M., 2008. Iron-nickel bimetallic nanoparticles for reductive degradation of azo dye Orange G in aqueous solution. Appl. Catal. B Environ. 79 (3), 270e278. Brown, M.A., De Vito, S.C., 1993. Predicting azo dye toxicity. Crit. Rev. Environ. Sci. Technol. 23 (3), 249e324. Cao, J., Wei, L., Huang, Q., Wang, L., Han, S., 1999. Reducing degradation of azo dye by zero-valent iron in aqueous solution. Chemosphere 38 (3), 565e571. n, J.M., Leal, M.T., Sa nchez, M., Bandala, E.R., 2006. Solar photocatalytic Chaco degradation of azo-dyes by photo-Fenton process. Dyes Pigment. 69 (3), 144e150. Chen, C., Liu, J., Liu, P., Yu, B., 2011. Investigation of photocatalytic degradation of methyl orange by using nano-sized ZnO catalysts. Adv. Chem. Eng. Sci. 1 (01), 9. Clay, M., Cui, Q., Sha, Y., Chen, J., Rondinone, A.J., Wu, Z., Chen, J., Gu, Z., 2012. Galvanic synthesis of bi-modal porous metal nanostructures using aluminum nanoparticle templates. Mater. Lett. 88, 143e147. Cui, Q., Sha, Y., Chen, J., Gu, Z., 2011. Galvanic synthesis of hollow non-precious metal nanoparticles using aluminum nanoparticle template and their
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Short communication
Rapid degradation of azo dye methyl orange using hollow cobalt nanoparticles Yingying Sha a, Iswarya Mathew a, Qingzhou Cui a, Molly Clay a, Fan Gao a, Xiaoqi Jackie Zhang b, Zhiyong Gu a, * a b
Department of Chemical Engineering, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, USA Department of Civil and Environmental Engineering, University of Massachusetts Lowell, One University Ave, Lowell, MA 01854, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Novel hollow Co nanoparticles were synthesized for azo dye degradation. The hollow Co nanoparticles exhibited rapid degradation toward methyl orange. pH had a significant effect on methyl orange degradation by hollow Co nanoparticles. The hollow Co nanoparticles are easily recycled through magnetic separation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2015 Received in revised form 3 October 2015 Accepted 10 October 2015 Available online xxx
A rapid and efficient method for methyl orange degradation using hollow cobalt (Co) nanoparticles is reported. Hollow Co nanoparticles were fabricated by a galvanic replacement reaction using aluminum (Al) nanoparticles as the template material. The methyl orange degradation characteristics were investigated by measuring the time dependent UVeVis absorption of the dye solution, which showed a very fast degradation rate under acidic conditions. At an initial methyl orange concentration of 100 mg/L (pH ¼ 2.5) and Co nanoparticle dosage of 0.5 g/L, the azo dye degradation efficiency reached up to 99% within 4 min, and the degradation constant rate was up to 2.444 min1, which is the highest value among other studies. A comparison of the decolorization rates at similar conditions with several other azo dyes, including Congo red, Amaranth, and Orange G, showed that the dye with a simpler structure and lower molecular mass decolorized considerably faster than the ones having a more complicated structure (higher molecular mass). The methyl orange degradation was also conducted using hollow nickel (Ni) nanoparticles and commercially available solid spherical Co and Ni nanoparticles. The results showed that Co-based nanoparticles outperformed Ni-based nanoparticles, with the hollow Co nanoparticles exhibiting the fastest degradation rate. Using the hollow Co nanoparticles is a very promising approach for the remediation of methyl orange dye containing wastewater due to the fast degradation rate and high degradation efficiency. In addition, these hollow Co nanoparticles are easily recycled because of their magnetic property. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Azo dye Methyl orange Reductive degradation Cobalt Hollow particles Magnetic separation
* Corresponding author. E-mail address: [email protected] (Z. Gu). http://dx.doi.org/10.1016/j.chemosphere.2015.10.040 0045-6535/© 2015 Elsevier Ltd. All rights reserved.
Y. Sha et al. / Chemosphere 144 (2016) 1530e1535
1. Introduction Synthetic dyes have been widely used in various fields, especially in textile industries. Azo dyes constitute a significant proportion of synthetic dyes, and have been identified as potential genotoxic and carcinogenic agent (Brown and De Vito, 1993). Dye removal or degradation from wastewater has been extensively studied to reduce the impact on the environment. A wide range of physicoechemical and biological methods, such as adsorption (Abramian and El-Rassy, 2009; Roy et al., 2013; Goudarzi et al., 2014), ozonation (Yıldırım et al., 2011; Liakou et al., 1997; Manivel et al., 2015), Fenton (Nidheesh et al., 2013; Lucas and Peres, 2006; n et al., 2006; Peternel et al., 2007; Gomathi Devi et al., Chaco 2009), photocatalysis (Konstantinou and Albanis, 2004; Chen et al., 2011; Singla et al., 2014), reductive degradation (Bigg and Judd, 2001; Bokare et al., 2008; Cao et al., 1999), microbial process (Nigam et al., 1996; McMullan et al., 2001; Martins et al., 1999), and two or more combination methods have been successfully applied to the treatment of various dyes (Peternel et al., 2007). Among these methods, reductive degradation is a fast and low cost method, which is easy to implement for the industrial level dye removal. Up to date, the following particles (including nanoparticles) have been utilized for azo dye removal, including zerovalent iron (Fe0) (Bigg and Judd, 2001; Cao et al., 1999; Zhang, 2003; Shu et al., 2007; Fan et al., 2009; Shih et al., 2010) and iron-based bimetallic nanoparticle, such as Fe/Ni (Bokare et al., 2008). Recently, Cobalt (Co) nanoparticle has emerged as a promising agent to treat azo dyes (Liang and Zhao, 2012; Naz et al., 2014).
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However, some report indicated adsorption as the main decolorization mechanism (Liang and Zhao, 2012), and some indicated catalytic reduction degradation (Naz et al., 2014). In this paper, we report a very fast reductive degradation method for methyl orange dye based on novel hollow Co nanoparticles which were synthesized by a galvanic replacement reaction. The hollow Co nanoparticles were applied on the methyl orange degradation study by monitoring the change of the dye concentration over time. The reductive degradation effectiveness of the hollow Co nanoparticles on methyl orange was illustrated by comparing the results with other azo dyes including Orange G, Congo red and Amaranth. In addition, the kinetic study of dye degradation was conducted by comparing the hollow Co nanoparticles with the hollow Ni nanoparticles and commercial solid Ni and Co nanoparticles. 2. Materials and methods 2.1. Materials and chemicals Aluminum (Al) nanoparticles (80 nm and 120 nm) were purchased from Novacentrix (Austin, Texas). The Al nanoparticles were sealed in an inert gas during shipping and handling and were immediately stored in an argon-filled glove box upon arrival. For the hollow metal nanoparticle fabrication and dye degradation test, chemicals used and purchased from Arcos Organics include ammonium chloride (NH4Cl, 99.6%), cobalt (II) chloride hexahydrate (CoCl2$6H2O, 98%), nickel sulfate hexahydrate (NiSO4$6H2O,
Fig. 1. (A) TEM image of Al nanoparticles as purchased with the nominal diameter of 120 nm; (B) SEM images of as synthesized hollow Co nanoparticles through the galvanic replacement reaction; (C) EDS measurement of hollow Co nanoparticles; (D) SAED pattern of a single hollow Co nanoparticle.
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99%), sodium hydroxide (NaOH, extra pure), methyl orange (C14H14N3NaO3S, pure, conform to ACS), Orange G (C16H10N2Na2O7S2, pure, certified), Congo red (C32H22N6Na2O6S2, indicator grade) and Amaranth (C20H11N2Na3O10S3, 85%). Additionally, sodium citrate dihydrate (Na3C6H5O7$2H2O, 99.0%) and anhydrous ethyl alcohol (denatured) were used as purchased from Fisher Scientific. Hydrochloric acid (HCl, 36.5%) was purchased from VWR Scientific. Commercial Ni and Co solid nanoparticles were purchased from American Elements (Los Angeles, CA) and were used without further treatment.
suspension in ethanol on a piece of TEM grid and dried under ambient conditions. Selected area electron diffraction (SAED) was taken by Zeiss Libra 120 TEM at Oak Ridge National Laboratory. A UV/Vis spectrophotometer (Lambda 25, Perkin Elmer) was utilized to collect the absorption spectra of azo dye degradation. The pH value was measured by a benchtop pH meter (model pH 510, Fisher Scientific).
2.2. Synthesis of hollow metallic nanoparticles
3.1. Hollow Co nanoparticles
The hollow Co and Ni nanoparticles were synthesized by the galvanic replacement method (Cui et al., 2011; Clay et al., 2012; Guo et al., 2005, 2007). The reaction was carried out in the glove box (Plas-Labs Inc., Model 818-GB) to minimize the nanoparticle oxidation. Briefly, the replacement precursor solution was prepared with 0.23 M CoCl2 or NiSO4, 1.87 M NH4Cl, and 0.44 M Na3C6H5O7, in an aqueous solution. Then, 1 g of aluminum nanoparticles was mixed with 30 mL of the precursor solution to initiate the reaction. Periodic agitation of the solution continued throughout the reaction. At the end of the reaction, the nanoparticles were rinsed with 2 M NaOH to remove any residual Al template material followed by washing with DI water and ethanol. Finally, the nanoparticles were dried under vacuum and collected as dry powders which were then weighed and applied quantitatively to the following degradation tests.
Fig. 1 shows the nanoparticle characterization results of the template Al nanoparticles and the hollow Co nanoparticles that have been made. Fig. 1(A) is the TEM image of Al nanoparticles as purchased with a nominal diameter of 120 nm; Fig. 1(B) shows the SEM image of as-synthesized hollow Co nanoparticles through the galvanic replacement reaction; either single hole or multiple holes were observed on each single Co nanoparticle. Fig. 1(C) shows the EDS results of Co hollow nanoparticles, which indicates that a complete removal of the Al template is achieved and the Co nanoparticles contain pure composition of Co with slight surface oxidation; Fig. 1(D) shows the selected area electron diffraction (SAED) ring pattern of a single Co hollow nanoparticle under TEM. The procedure for indexing the ring pattern typically follows the steps as below: (1) measuring the diameter of the ring by Image J;
3. Results and discussion
2.3. Degradation of azo dyes The as synthesized hollow metal nanoparticles were used for the methyl orange degradation at different conditions. A 10 mL centrifuge tube was used as a reaction vessel for the dye degradation studies. The tube was fixed to a rotator, which was rotating at 10 rpm to help facilitate the dispersion of metal nanoparticles and maintain solution uniformity during the degradation process. The initial concentration of the dye solution was kept at 100 mgL1 and the pH was adjusted to the desired value by the addition of 2 M HCl or 2 M NaOH solution. The methyl orange solution was added to the tube containing a weighed amount of hollow metal nanoparticles to initiate the degradation. The reaction was monitored by measuring the UVevis absorption spectra of the sample solution taken out at regular intervals (e.g., 1, 3, 5, 10, 20, 30, 60, and 90 min). For the UVevis spectrophotometric analysis, 150 mL of sample solution was taken out of the reaction vessel and transferred to a 1.5 mL centrifuge tube and centrifuged immediately to stop the degradation reaction by precipitating the metal nanoparticles. It took about 20 s for the centrifugation to separate the metal nanoparticles from the reaction mixture. A 100 mL of above clear solution was diluted to 1 mL with DI water and absorbance spectra was recorded over a wavelength range of 200e900 nm. The sampling time interval was adjusted to obtain smooth kinetics curve depending on the initial concentration and pH of the dye solution, the type of metal nanoparticle, and its dosage. 2.4. Instruments and characterization The nanoparticle imaging was performed using a JEOL JSM7401F field emission scanning electron microscope (FESEM). The SEM was equipped with an energy-dispersive x-ray spectrometer (EDS) for elemental analysis of the nanoparticles. Transmission electron microscopy (TEM) imaging was performed using a Philips EM400 transmission electron microscope operated at 100 kV. TEM samples were prepared by placing a drop of the nanoparticle
Fig. 2. (A) Methyl orange dye solution (pH ¼ 2.5) before and after hollow Co nanoparticle treatments; (B) Time-dependent UV/Vis absorption measurement of methyl orange degradation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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(2) converting the distances into interplanar values via the equation d ¼ lL/r, where d is the lattice spacing, l is the wavelength of the electrons, L is the camera focal length and r is the distance between the central spot and the ring; (3) comparing the measured inteplanar values with standard (Andrews et al., 1971). The index (100) (002) (102) (110) indicates the hexagonal polycrystalline structure of the Co nanoparticles. Similarly, hollow Co nanoparticles can also be synthesized by 80 nm Al nanoparticle template.
3.2. Catalytic degradation efficiency The eN]N- double bond in the azo dyes is the chromophoric group for color (Aljamali, 2015). Typically, the absorption band of methyl orange shows red shift with a decrease in pH, which results in the color change from yellow to red, as illustrated in S1. The increase in wavelength suggests an increase in delocalization in the methyl orange molecules (Del Nero et al., 2005). Fig. 2(A) shows the photograph of methyl orange dye solution (pH ¼ 2.5) before and after hollow Co nanoparticle treatments (2 g/L). Fig. 2(B) shows the time-dependent UV/Vis absorption measurement of methyl orange degradation. It is observed that the dye degradation occurred immediately upon the addition of the hollow Co nanoparticles into the dye solution, and the decolorization was completed in about 4 min. At t ¼ 0 min, the curve represents the absorption spectra of methyl orange solution without Co nanoparticle treatment. After the addition of the hollow Co nanoparticles at pH ¼ 2.5, a decrease in the intensity of the absorption band at ~500 nm is observed. This indicates the adsorption of methyl orange molecules by hollow Co nanostructure and possible cleavage of the azo bond, which is the chromophoric group and thereby decolorizing the solution. The
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formation of aromatic products is indicated by the appearance of a new peak ~248 nm and the intensity of this absorption peak increases over reaction time. This suggests successful decomposition of methyl orange dye in the solution by the reduction of azo bond to two or more possible chemical structures with amines (NH2). The emerging of a new peak indicates that the decolorization of the methyl orange dye was due to dye degradation from the solution, instead of only physical adsorption. The proposed two products are sulfanilic acid and dimethyl-4-phenylenediamine, which were analyzed and compared with the standard pure species under UV/ Vis analysis, as shown in S2.
3.3. Degradation mechanism The decolorization experiments were performed on several other azo dyes with different molecular structures as presented in Fig. 3, including Congo red, Amaranth and Orange G. The chemical structures of each dye are inserted in the figure. For the four dyes studied, only the methyl orange showed very fast decolorization rate (Fig. 3(D)), and the other three dyes showed slower decolorization rates. In addition, there were no new peaks formed in the spectra of the other three dyes, indicating that the dye decolorization process was mostly through surface adsorption, instead of degradation. The remarkably faster decolorization of methyl orange compared to the other three azo dyes can be attributed to its relatively simpler chemical structure and lower molecular weight. It is possible that the larger molecular structures of dyes will: (1) hurdle the access to the eN]N- double bond, so the active Co atoms on the surface of the hollow Co nanoparticles cannot easily access the dye molecule, except methyl orange which has a relatively small size with only one benzene ring on either side of the
Fig. 3. Time-dependent UV/Vis absorption measurement of different azo dyes at a pH value of 3.0. (A) Congo red, (B) Amaranth, (C) Orange G, (D) Methyl orange. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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eN]N- double bond; (2) block the sensitive surface of the Co catalyst therefore reducing the availability of active sites. On the basis of the results discussed above, the mechanism for methyl orange degradation using hollow Co nanoparticles can be proposed as below: When the catalyst is introduced, the methyl orange molecule was first adsorbed on the surface of the hollow Co nanoparticle, then the Co atoms performed as a catalyst and reacted with the eN]N- double bond and broke it to single phenyl ring compounds with amine group as possible products, thereby decolorizing the methyl orange (see Figure S3). The proposed reaction mechanism for methyl orange degradation by hollow Co nanoparticles is similar to the mechanisms observed for the azo dye treatment by zero valent iron nanoparticles, which have been reported for the reductive degradation of methyl orange (Fan et al., 2009) and acid orange II (Cao et al., 1999). 3.4. Kinetic study The dye decolorization kinetics by the hollow Co nanoparticles were studied for the four azo dyes. The concentration of each dye was quantified at their absorbance peak maxima (Regression analysis wavelength: Methyl orange ¼ 470 nm, Congo red ¼ 500 nm, Orange G ¼ 475 nm and Amaranth ¼ 520 nm). Based on the discussions above, although the reactive chemical degradation only took place for methyl orange, other azo dyes also followed a pseudo first order kinetics (except for Congo red, which had a very low decolorization rate and showed some instability at the early stage), as shown by the fitting curves in Fig. 4(A). With the same amount of the hollow Co nanoparticles used, the fastest dye decolorization rate was obtained from the methyl orange dye degradation; while for the other three dyes, where surface adsorption dominated the dye removal process, the decolorization rate followed the order of Amaranth > Orange G > Congo Red. Besides the hollow Co nanoparticles, the galvanic replacement method could also be used to synthesize hollow Ni nanoparticles (Cui et al., 2011; Clay et al., 2012). In order to understand the hollow structure effect on the methyl orange dye degradation, commercially purchased solid spherical Co and Ni nanoparticles have been compared with the hollow Co and Ni nanoparticles. Fig. 4(B) shows the kinetics of the methyl orange degradation by using different nanoparticles. The hollow Co nanoparticle performed better degradation efficiency than the solid Co nanoparticle, because the hollow Co structure has much more surface area than solid Co particles at the same mass loading, which provides more adsorption binding site of Co to react with methyl orange, hence improved the catalytic activity and degradation rate. Both hollow and commercial Co nanoparticles performed better than the Ni nanoparticles in the degradation of methyl orange. At pH ¼ 3, the hollow Ni nanoparticles exhibited faster degradation rate than the commercial solid Ni nanoparticles. The initial pH has a significant effect on the methyl orange degradation. When the pH was increased to 10 (basic solution), the dye degradation process was significantly slowed down; thus, acidic conditions are preferred for methyl orange degradation. The additional measurement of pH effect can be found from Figure S4. It is believed that the hollow cobalt catalyst particles act as an electron donor and the methyl orange dye molecules acts as electron acceptor, and when combined with Hþ ions form a transitional product as shown in the scheme (see Figure S3). The acidity of the solution and the presences of surface active sites are the influencing factors for the dye degradation reaction. At a pH value of 2.5, the hollow Co nanoparticles exhibited the highest degradation rate among all the nanoparticles studied, showing a reaction rate constant of 2.444 min1 (R2 ¼ 0.9999). Table 1 compares the reaction rate constant for methyl orange degradation from our result to the ones obtained from the
Fig. 4. Azo dyes decolorization curves. (A) Comparison of different azo dye decolorization; (B) Comparison of synthesized hollow nanoparticles and commercial nanoparticles toward methyl orange degradation efficiency. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
literature. Clearly, the hollow Co nanoparticles showed a much faster reaction rate toward methyl orange degradation. For practical applications, there are concerns for the hazardous and environmental effect of nanoparticles in industrial manufacturing and usage. In this work, the hollow Co nanoparticles synthesized in this article can be easily recycled due to their magnetic property. In Figure S5, a simple magnetic separation experiment showed that the hollow Co nanoparticles in the solution could quickly move and stick on the side of the reaction container upon applying a magnet, which facilitates easy separation of the nanoparticles from the reaction medium and would maximally eliminate the effect of nanoparticle toxicity to the environment in the practical applications. Table 1 Reaction rate constants of several catalysts for methyl orange degradation. Catalyst
Rate constant (min1)
Reference
Fe0 Nanoparticles Nanoporous Au TiO2 Nanotube Arrays Fe0eUVeH2O2 Hollow Co nanoparticles
0.559 2.1 103 0.0515 0.1025 2.444
(Fan et al., 2009) (Hakamada et al., 2012) (Zhang et al., 2007) (Gomathi Devi et al., 2009) This work
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4. Conclusions Methyl orange is one of the most stable azo dyes which is extensively used in the textile industry and resistant to biodegradation. This study demonstrated the effectiveness of the hollow Co nanoparticles in methyl orange azo dye degradation. The hollow Co nanoparticles synthesized by the galvanic replacement method exhibited fast catalytic property during methyl orange degradation and reduced methyl orange to amine compounds in a matter of few minutes. The hollow Co nanoparticles showed rapid degradation with a rate constant of 2.444 min1, the fastest degradation rate towards methyl orange to date, to the best of our knowledge. The dye degradation rates were very sensitive to pH, and worked best in the acidic conditions. Compared to the solid nanoparticles, the nanoparticles with hollow structures possess much higher surface area for high concentration organic compound adsorption and surface reaction, hence increase the methyl orange degradation rate. Further study is needed to analyze the degradation products to better understand the different decolorization mechanisms (adsorption vs. degradation) for different azo dyes. Acknowledgments Financial support from the UMass CVIP Technology Development Fund is acknowledged. Partial funding support from the NSF (Award # CMMI-1031532) is also acknowledged. We thank Dr. Jihua Chen at the Oak Ridge National Laboratory for his help in the selected area electron diffraction measurements. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.10.040. References Abramian, L., El-Rassy, H., 2009. Adsorption kinetics and thermodynamics of azodye Orange II onto highly porous titania aerogel. Chem. Eng. J. 150 (2e3), 403e410. Aljamali, N., 2015. Review in azo compounds and its biological activity. Biochem. Anal. Biochem. 4 (2), 1000169. Andrews, K.W., Dyson, D.J., Keown, S.R., 1971. Interpretation of Electron Diffraction Patterns. Plenum Press, New York. Bigg, T., Judd, S., 2001. Kinetics of reductive degradation of azo dye by zero-valent iron. Process Saf. Environ. Prot. 79 (5), 297e303. Bokare, A.D., Chikate, R.C., Rode, C.V., Paknikar, K.M., 2008. Iron-nickel bimetallic nanoparticles for reductive degradation of azo dye Orange G in aqueous solution. Appl. Catal. B Environ. 79 (3), 270e278. Brown, M.A., De Vito, S.C., 1993. Predicting azo dye toxicity. Crit. Rev. Environ. Sci. Technol. 23 (3), 249e324. Cao, J., Wei, L., Huang, Q., Wang, L., Han, S., 1999. Reducing degradation of azo dye by zero-valent iron in aqueous solution. Chemosphere 38 (3), 565e571. n, J.M., Leal, M.T., Sa nchez, M., Bandala, E.R., 2006. Solar photocatalytic Chaco degradation of azo-dyes by photo-Fenton process. Dyes Pigment. 69 (3), 144e150. Chen, C., Liu, J., Liu, P., Yu, B., 2011. Investigation of photocatalytic degradation of methyl orange by using nano-sized ZnO catalysts. Adv. Chem. Eng. Sci. 1 (01), 9. Clay, M., Cui, Q., Sha, Y., Chen, J., Rondinone, A.J., Wu, Z., Chen, J., Gu, Z., 2012. Galvanic synthesis of bi-modal porous metal nanostructures using aluminum nanoparticle templates. Mater. Lett. 88, 143e147. Cui, Q., Sha, Y., Chen, J., Gu, Z., 2011. Galvanic synthesis of hollow non-precious metal nanoparticles using aluminum nanoparticle template and their
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