Highly Efficient Removal of Organic Dyes from Waste Water Using

Article pubs.acs.org/JPCC

Highly Efficient Removal of Organic Dyes from Waste Water Using Hierarchical NiO Spheres with High Surface Area Ting Zhu,† Jun Song Chen,†,‡ and Xiong Wen (David) Lou*,†,‡ †

School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637457, Singapore ‡ Energy Research Institute @ NTU, Nanyang Technological University, 50 Nanyang Drive, Singapore 637553, Singapore S Supporting Information *

ABSTRACT: A facile solvothermal method has been developed for large-scale preparation of uniform spheres of a nickel−ethylene glycol complex (Ni-EG complex) with a hierarchical nanostructure. The dispersibility and hierarchical structure of the Ni-EG particles can be tuned by varying the concentration of additives added. On the basis of experimental observations, a plausible mechanism has been proposed to understand the formation process of the Ni-EG complex spheres. Calcining these as-prepared Ni-EG complex spheres at 300 °C in air results in uniform porous NiO spheres with a high specific surface area of 222 m2 g−1. When served as the adsorbent for Congo red in water, the colloidal suspension of the as-prepared NiO hierarchical spheres exhibits a high adsorption capacity for the dye removal, suggesting their potential use in water treatment.

1. INTRODUCTION The efficient removal of dyestuffs in the effluents has raised great concerns in the past few years, because the dyes in wastewater can inhibit sunlight penetration into the stream and, therefore, reduce the photosynthetic reaction, and some synthetic dyes are quite toxic and even carcinogenic, which is a severe health threat to human beings.1 Hence, it is essential to remove the dyes (such as Congo red) in wastewater to alleviate the water problems faced by society. As an effective approach, adsorption processes are extensively employed in water treatment, in which a variety of materials have been studied as adsorbents, such as clay materials, bottom ash, activated carbon, natural zeolite, and soya.2−6 However, there are still some challenges that restrict the adsorption approach, such as low selectivity and adsorption capacity, for the current materials. Therefore, it is of great importance to explore new adsorbents with high adsorption capacity and selectivity.7,8 Nickel oxide (NiO) is an important transition-metal oxide that is employed in various fields, such as catalysis,9 lithium-ion batteries,10,11and supercapacitors,12−14 because of its high chemical and thermal stability, and environmental benignity.15,16 As an adsorbent in water treatment, NiO nanosheets were reported previously for Congo red removal with relatively low adsorption capacities ranging from 30 to 150 mg g−1.17,18 Many other types of NiO have also been prepared via various approaches,19,20 in which the NiO products are commonly prepared from their hydroxide precursors. Alkaline precipitants, such as sodium hydroxide, ammonia, and urea, are usually used in the synthesis, which may cause environmental concerns. Therefore, it is practically significant to develop facile and © 2012 American Chemical Society

environment-friendly routes to prepare NiO materials with novel structures. Ethylene glycol (EG) is an automotive antifreeze with a relatively high boiling point (ca. 197 °C), and it has been widely employed to synthesize nanomaterials of metal oxides (TiO2, SnO2, In2O3, and PbO) in large scale by forming glycolate intermediates,21,22 because of its strong chelating ability with the transition-metal cations.23,24 In one of our previous works, cobalt-based intermediates with various structures have been prepared using a solvothermal method in polyethylene glycol (PEG).25 Herein, we report a glycolassisted solvothermal method to synthesize a Ni-EG intermediate complex in the presence of additives, sodium chloride (NaCl) and sodium acetate (NaAc). By controlling the concentrations of nickel ions and additives, uniform Ni-EG nanospheres can be easily obtained. Control experiments indicate that NaCl and NaAc play important roles in the formation of these uniform nanospheres. The corresponding hierarchical NiO nanospheres assembled from nanoflakes can be acquired by calcining the as-prepared spherical Ni-EG complex intermediate at 300 °C, and the porous structure after calcination is confirmed by the N2 sorption measurement. In virtue of the unique assembled hierarchical nanostructures and high surface area, the NiO hierarchical nanospheres exhibit excellent Congo red adsorption capability. The Congo red adsorption is performed without adjusting the pH value, Received: January 8, 2012 Revised: February 29, 2012 Published: March 1, 2012 6873

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Figure 1. Field emission scanning electron microscopy (FESEM) images (a and b), X-ray diffraction (XRD) pattern (c), and thermogravimetric analysis (TGA) curve (d) of the typical nickel−ethylene glycol (Ni-EG) complex spheres (scale bar: 500 nm).

rpm at room temperature for 3 h, the NiO was separated by centrifugation (12 000 rpm) and the supernatant solutions were analyzed with UV−vis spectroscopy (Shimadzu UV-2450) to obtain the concentrations of Congo red in solution. The concentrations of Congo red were determined using a linear calibration curve over 5−80 mg L−1 based on the absorbance value at 495 nm. To evaluate the adsorption capacity, the initial concentrations of Congo red solutions were scaled in the range of 100−500 mg L−1, and the dosage of NiO was kept at 0.5 g L−1. The mixtures were stirred at 300 rpm at room temperature for 3 h, and the concentrations of final Congo red solutions were measured using UV−vis spectroscopy after centrifugation at 12 000 rpm. Adsorption Kinetic. The adsorption kinetic is studied with a NiO dosage of 0.5 g L−1 and an initial Congo red concentration of 100 mg L−1. The concentration of Congo red solution is determined using UV−vis spectroscopy at different time intervals. After the adsorption experiment, the NiO adsorbent is collected by centrifugation, and redispersed into DI water. The mixture is stirred at room temperature for 48 h, and the supernatant is collected by centrifugation and examined by UV−vis spectroscopy to study the reversibility of the adsorption process. Recycling Experiment. After the first time it is used, the NiO adsorbent is washed thoroughly using ethanol by ultrasonication and centrifugation several times and then dried in vacuum at 80 °C for 10 h. The dried NiO sample is reused for adsorption with a Congo red concentration of 300 mg L−1. After each run, the concentration of Congo red is determined using UV−vis spectroscopy.

revealing their superiority over many other adsorbents that operate effectively only at optimal pH values.

2. EXPERIMENTAL SECTION Material Preparation. All the chemicals are of analytical grade and used as received without further purification. In a typical synthesis of a Ni-EG complex intermediate, 2 mmol of nickel nitrate (Ni(NO3)2·6H2O, Aldrich) and 0.234 g of sodium chloride (NaCl, Aldrich) are sequentially added into 20 mL of ethylene glycol (EG, Aldrich) to form a clear light green solution, to which then 2.624 g of sodium acetate (NaAc, Aldrich) is added to give a concentration of 2.0 M. After vigorous stirring for 20 min, the solution is transferred into a Teflon-lined stainless steel autoclave and heated at 190 °C for 8 h. After the heat treatment, the autoclave is allowed to cool naturally to room temperature, and the green products are collected and rinsed with distilled water and ethanol several times. The final products are then dried in an oven at 60 °C for 24 h. To obtain the nickel oxide product, the dried green powder is calcined in air at 300 °C for 2 h with a ramping rate of 1 °C min−1. Material Characterization. Crystallographic phases of the products were investigated by powder X-ray diffraction (XRD, Shimadzu XRD-6000, Cu Kα, λ = 1.5406 Å). Morphologies of samples were examined by field emission scanning electron microscopy (FESEM, JEOL, JSM-6700F), and transmission electron microscopy (TEM, JEM-2010, 200 kV). Thermal behavior of the Ni-EG complex was characterized by thermogravimetric analysis (TGA, Shimadzu TGA-60) at a heating rate of 5 °C min−1 from room temperature to 600 °C in a dynamic atmosphere of air (200 mL min−1) using α-alumina crucibles. Measurement of the specific surface area and analysis of the porosity for the NiO product were carried out through measuring N2 adsorption−desorption isotherms at 77 K with a Quantachrome NOVA-3000 system. Adsorption of Congo Red. In a typical experiment, 5 mg of the as-prepared NiO nanospheres powder was mixed with 10 mL of aqueous solutions of Congo red. After stirring at 300

3. RESULTS AND DISCUSSION The morphology and hierarchical structure of a typical Ni-EG complex product are presented in Figure 1. Clearly, the assynthesized Ni-EG particles are all spherical with a mean diameter of about 600 nm (Figure 1a). The high-magnification FESEM image (Figure 1b) reveals that the hierarchical nanospheres are composed of many densely packed well6874

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(Ti4+, Sn2+, In3+, and Pb2+), and the as-formed chainlike complexes would congregate and self-assemble into bundles when such chains become sufficiently long.22 As illustrated in Figure 2, initially, the nickel ions (Ni2+) would diffuse in the EG solution and then attach onto the EG molecules to form glycolates by the strong chelating interaction. The polymerization of nickel glycolates then occurs (Figure 2, step a), which is an important process that has been discussed in many other reports for the formation of nanostructured metal oxides.29−31 Nickel glycolates would subsequently grow into a bigger size, and the thermally stable Ni-EG complex can finally be formed (Figure 2, step b). It should also be mentioned that the formation of typical well-defined Ni-EG spheres has been assisted by the additives (NaCl and NaAc) present in the system. That is to say, the presence of NaCl and NaAc may have strong effects on the chelating interaction between nickel ions and EG molecules and the interactions among the formed Ni-EG chains, which should be responsible for the morphological transformation, as discussed above. Uniform NiO nanospheres can be readily obtained by calcining the Ni-EG complex precursor, with the morphology shown in Figure 3a. The high-magnification FESEM image

defined flakelike structures. This suggests that the Ni-EG nanospheres are self-assembled from these fine flakes, which may further give rise to a porous structure of the Ni-EG spheres due to the congregation of the building blocks. The crystal phase of the Ni-EG complex spheres is revealed by the XRD pattern (Figure 1c). As can be seen, a strong diffraction peak appears at around 10.3°, which is the characteristic of coordination polymers from metal ions and ethylene glycol.23,24 In some similar systems, α-Ni(OH)2 was synthesized as the precursor for the preparation of NiO.9,26 The TGA curve in Figure 1d shows a total weight loss of about 51.4% up to 600 °C in air, which is comparable to a previously reported value (58%) for the Ni-EG complex.27 For the present hierarchical Ni-EG complex spheres, it might be relatively more difficult to completely combust the deeply trapped organic molecules. To study the roles of NaCl and NaAc in the formation of uniform Ni-EG nanospheres, control experiments have been carried out. Ni-EG particles were prepared in the absence of NaCl while keeping other conditions unchanged. Most Ni-EG particles are found to be composed of flower-like structures with random shapes, which indicates that the presence of NaCl is critical for the formation of well-defined flake-constructed nanospheres (Figure S1a, Supporting Information). In the absence of NaAc, a largely interconnected Ni-EG complex with spherical subunits is obtained (Figure S1b, Supporting Information). Furthermore, the sample prepared at a lower NaAc concentration of 1.09 M consists of relatively uniform spherical particles with different structures (Figure S1c, Supporting Information). When the concentration of NaAc is increased to 4.0 M, aggregates of the Ni-EG complex with thickened flakelike building blocks are obtained (Figure S1d, Supporting Information). The crystal phases of all the samples are confirmed, as shown in Figure S2 (Supporting Information). Hence, it is apparent that the formation of well-defined spheres is very sensitive to the concentration of NaAc. It seems that the NaAc has favored the formation of uniform Ni-EG particles at a lower concentration. On the basis of the above observations, a plausible formation mechanism has been proposed in Figure 2. It has been reported

Figure 3. FESEM image (a), TEM images (b and c), and XRD pattern (d) of the NiO nanospheres obtained after calcining Ni-EG nanospheres at 300 °C for 2 h (scale bar: 500 nm).

(Figure 3a, inset) shows that the overall structure is completely intact after the thermal treatment, indicating good thermal stability. The TEM image shown in Figure 3b clearly shows the flake-constructed nanospheres of this calcined product. Furthermore, the TEM image of a single nanosphere presented in Figure 3c reveals that the structure is composed of radially oriented ultrathin flakelike nanosheets, consistent with the above FESEM observation. The crystallographic phase of NiO is confirmed by XRD analysis (Figure 3d), and all the diffraction peaks agree well with the standard pattern of NiO (PDF card no. 71-1179), which reveals the complete conversion from the Ni-EG complex to nickel oxide. The textural characteristic of as-prepared NiO nanospheres is investigated by the gas sorption measurement at 77 K. The nitrogen adsorption−desorption isotherm of the NiO sample is

Figure 2. Schematic illustration of the formation of the Ni-EG complex: chelation between nickel ions and ethylene glycol chainlike molecules, followed by the formation of the Ni-EG complex (a), and the growing process of the Ni-EG complex (b).

that there is strong chelating interaction between ethylene glycol and metal ions. For example, Yamamoto et al. employed the glycol (primarily ethylene glycol) chelates to control the hydrolysis and polycondensation rates of tin alkoxides.28 It was also found by Xia et al. that glycols could serve as a ligand to form a chainlike coordination complex with many metal ions 6875

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shown in Figure 4, and the inset shows the Barrett−Joyner− Halenda (BJH) pore size distributions obtained from the

Figure 5. UV−vis absorption spectra of initial Congo red with a concentration of 100 mg L−1 (I), and solutions (10 mL) after treated with the as-prepared NiO nanospheres with various dosages: 0.1 (II), 0.2 (III), and 0.5 g L−1 (IV). The inset shows photos of initial Congo red (100 mg L−1) aqueous solution and treated with dosage IV.

Figure 4. N2 adsorption−desorption isotherm of the prepared NiO nanospheres, and the inset is the pore size distribution.

of this NiO adsorbent. After the adsorption, the NiO is collected for the reversible adsorption test, and the UV−vis spectroscopy only detects a Congo red concentration of 2.04 mg L−1, which shows the good adsorption affinity of this NiO adsorbent to Congo red. The equilibrium study presents the capacity of an adsorbent and describes the adsorption isotherm that reveals the surface property and affinity to the absorbate.17 To investigate the adsorption capacity of these porous NiO hierarchical nanospheres, a series of Congo red solutions with higher concentrations are employed to study the equilibrium isotherm. The absorbed amount of Congo red, q (mg g−1), at equilibrium can be calculated from the following equation

desorption branch. This isotherm profile can be categorized as type IV with a small hysteresis loop observed at a relative pressure of 0.5−1.0, indicating the presence of a mesoporous structure in the sample. As a result, the sample has a high specific surface area of 222 m2 g−1. This large surface area is believed to be brought about by the combustion of the organic molecules trapped in the particles and recrystallization of NiO, both of which would have created considerable porosity in the structure. It can be seen from the pore size distribution that the porous NiO nanospheres have pores with diameters in the range of 4−10 nm, which are formed by organization of nanoflakes and also crystal reconstruction during the calcination.32 As widely reported, a high surface area usually gives rise to high adsorption capacity for an adsorbent in water treatment because of more active adsorption sites available.18 The hierarchical and mesoporous structure is beneficial to improving the adsorption capacity and efficiency of absorbate molecules.33,34 The isoelectric point of NiO is 10.3.9,35 Hence, in a neutral solution, the NiO nanocolloids will be positively charged, which is apt for the adsorption of anionic dyes, such as Congo red. The as-prepared NiO nanospheres are composed of ultrathin nanoflakes, which can provide sufficient active sites for the adsorption of Congo red molecules by electrostatic force and hydrogen bonds. In our adsorption experiment, a certain amount of NiO powder is dispersed into 10 mL of Congo red solution with an initial concentration of 100 mg L−1 by mild stirring without any other additives. After a contact time of 3 h, the supernatant was collected by centrifugation and then examined by UV−vis spectroscopy. Figure 5 shows the UV−vis spectra of Congo red solutions before and after adsorption in the colloidal suspension of the NiO hierarchical nanospheres with different dosages. At a NiO dosage of 0.5 g L−1, the colloidal suspension is able to adsorb almost 100% of Congo red (Figure 5, spectrum IV), and the photos before and after adsorption also confirm the complete removal of the Congo red at this dosage of NiO (Figure 5, inset). Figure 6a shows the adsorption kinetic of these NiO nanospheres with an initial Congo red concentration of 100 mg L−1. As expected, the adsorption is very fast during the first 30 min, and the adsorption process is nearly finished after a 90 min contact time, exhibiting the effective and efficient property

q=

C0 − C V W

(1) −1

where C0 and C (mg L ) are concentrations of Congo red solutions at the initial and equilibrium states, respectively, V (L) is the volume of the solution, and W (g) is the mass of the adsorbent. Figure 6b shows the adsorption isotherm and Congo red removal percent with a NiO dosage of 0.5 g L−1. A high adsorption capacity of 440 mg g−1 can be calculated with 73.4% of the dye removed for these NiO nanospheres when the initial concentration of Congo red is 300 mg L−1, indicating their good adsorption capability to Congo red. The recycle study of these NiO nanospheres also shows that 93% of the adsorption capacity can be retained for the first recycling and 77% of the capacity still remained after use for four runs, which evidences the practical significance of this NiO adsorbent. As a comparison, NiO nanorods are also evaluated for Congo red adsorption (Figure S3a, Supporting Information). A maximum adsorption capacity of only 237 mg g−1 can be obtained with the concentrations of Congo red in the range of 100−500 mg L−1 (Figure S3b, Supporting Information), which is much lower than that of the hierarchical NiO nanospheres. For other metal oxides studied in previous works, maximum Congo red adsorption capacities of only 239 and 66 mg g−1 are reported for α-FeOOH and α-Fe2O3, respectively.36 A recent work of MnO2 hollow nanostructures done by Li et al. demonstrates an adsorption capacity of 60 mg g−1.37 It should also be noted that many adsorption experiments are conducted at an optimal pH value; thus their high capacities may not be realized at a natural pH value,38,39 while the obtained adsorption capacity in our work is achieved without any pH 6876

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Figure 6. Adsorption kinetics (a) and adsorption isotherm with the corresponding percentage removal of Congo red at various initial concentrations with a contact time of 3 h (b). NiO dosage: 0.5 g L−1. (11) Varghese, B.; Reddy, M. V.; Yanwu, Z.; Lit, C. S.; Hoong, T. C.; Rao, G. V. S.; Chowdari, B. V. R.; Wee, A. T. S.; Lim, C. T.; Sow, C. H. Chem. Mater. 2008, 20, 3360. (12) Lang, J. W.; Kong, L. B.; Wu, W. J.; Luo, Y. C.; Kang, L. Chem. Commun. 2008, 4213. (13) Yuan, C. Z.; Zhang, X. G.; Su, L. H.; Gao, B.; Shen, L. F. J. Mater. Chem. 2009, 19, 5772. (14) Ding, S. J.; Zhu, T.; Chen, J. S.; Wang, Z. Y.; Yuan, C. L.; Lou, X. W. J. Mater. Chem. 2011, 21, 6602. (15) Chang, K. H.; Hu, C. C.; Chou, C. Y. Chem. Mater. 2007, 19, 2112. (16) Hu, C. C.; Chang, K. H.; Lin, M. C.; Wu, Y. T. Nano Lett. 2006, 6, 2690. (17) Song, Z.; Chen, L. F.; Hu, J. C.; Richards, R. Nanotechnology 2009, 20, 275707. (18) Cheng, B.; Le, Y.; Cai, W. Q.; Yu, J. G. J. Hazard. Mater. 2011, 185, 889. (19) Dong, L. H.; Chu, Y.; Sun, W. D. Chem.Eur. J. 2008, 14, 5064. (20) Kuang, D. B.; Lei, B. X.; Pan, Y. P.; Yu, X. Y.; Su, C. Y. J. Phys. Chem. C 2009, 113, 5508. (21) Wang, Y. L.; Jiang, X. C.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 16176. (22) Jiang, X. C.; Wang, Y. L.; Herricks, T.; Xia, Y. N. J. Mater. Chem. 2004, 14, 695. (23) Scott, R. W. J.; Coombs, N.; Ozin, G. A. J. Mater. Chem. 2003, 13, 969. (24) Khushalani, D.; Dag, O.; Ozin, G. A.; Kuperman, A. J. Mater. Chem. 1999, 9, 1491. (25) Zhu, T.; Chen, J. S.; Lou, X. W. J. Mater. Chem. 2010, 20, 7015. (26) Song, X. F.; Gao, L. J. Am. Ceram. Soc. 2008, 91, 4105. (27) Tao, F. F.; Guan, M. Y.; Zhou, Y. M.; Zhang, L.; Xu, Z.; Chen, J. Cryst. Growth Des. 2008, 8, 2157. (28) Yamamoto, O.; Sasamoto, T.; Inagaki, M. J. Mater. Res. 1992, 7, 2488. (29) Murakami, Y.; Matsumoto, T.; Yahikozawa, K.; Takasu, Y. Catal. Today 1995, 23, 383. (30) Korb, G.; Levy, G.; Brini, M.; Deluzarc., A. J. Organomet. Chem. 1970, 23, 437. (31) Pommier, J. C.; Mendes, E.; Valade, J.; Housty, J. J. Organomet. Chem. 1973, 55, C19. (32) Jiao, F.; Hill, A. H.; Harrison, A.; Berko, A.; Chadwick, A. V.; Bruce, P. G. J. Am. Chem. Soc. 2008, 130, 5262. (33) Yu, J. G.; Su, Y. R.; Cheng, B. Adv. Funct. Mater. 2007, 17, 1984. (34) Yu, X. X.; Yu, J. G.; Cheng, B.; Jaroniec, M. J. Phys. Chem. C 2009, 113, 17527. (35) Lewis, J. A. J. Am. Ceram. Soc. 2000, 83, 2341. (36) Fei, J. B.; Cui, Y.; Zhao, J.; Gao, L.; Yang, Y.; Li, J. B. J. Mater. Chem. 2011, 21, 11742. (37) Fei, J. B.; Cui, Y.; Yan, X. H.; Qi, W.; Yang, Y.; Wang, K. W.; He, Q.; Li, J. B. Adv. Mater. 2008, 20, 452.

value adjustment. It is believed that the unique hierarchical nanostructure and high surface area of the NiO spheres are responsible for the excellent performance in Congo red adsorption. From this point of view, the as-prepared porous hierarchical NiO nanospheres might be promising in water treatment.

4. CONCLUSIONS In summary, a facile solvothermal method has been developed to synthesize uniform Ni-EG complex nanospheres by forming the glycolate. Uniform NiO nanospheres with a porous hierarchical structure can be obtained after calcination of the Ni-EG nanospheres. Such obtained NiO hierarchical spheres are shown to exhibit excellent adsorption capability for Congo red with a maximum capacity of 440 mg g−1. This improved adsorption performance could be attributed to the unique hierarchical structure and high surface area of the assynthesized porous NiO nanospheres.

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ASSOCIATED CONTENT

S Supporting Information *

More data of FESEM, XRD, and UV−vis absorption spectra about control experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +65-63168879. Notes

The authors declare no competing financial interest.

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REFERENCES

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