Y₂O₃ functionalized natural palygorskite as an adsorbent for methyl blue removal

Abstract

Y₂O₃ functioned palygorskite (Pal) composite as a novel adsorbent has been successfully synthesized and characterized, which shows stable and rapid decolorization performance for methyl blue (MB). HRTEM images showed that Y₂O₃ nanoparticles with size about 2–5 nm evenly dispersed on palygorskite, and the increase of the binding energy of Y₂O₃ (Y3d_5/2) confirmed that several bonds such as Y–OH and Y–O–Si were existed in Y₂O₃/Pal adsorbent. Y₂O₃ modification greatly increased the number of negatively charged groups as Y₂O₃/Pal showed lower negative zeta potential than that of Pal. Therefore, the electrostatic interaction between Y₂O₃/Pal and MB is impossible to be the adsorption mechanism. What's more, it is found that the adsorption isotherm obeys the Langmuir model, with the maximum adsorption capacity greatly enhanced to 1579.06 mg g⁻¹, exhibiting potential applications in wastewater treatment.

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1. Introduction

The dyes are generally used in leather, textile, plastics, food and paper industries. They are difficult to decompose because of their complex components, dark color and high levels of organic pollutants. The methods commonly adopted to decompose organic dye wastewater include chemical oxidation,^1–3 biological treatment,^4–6 and photocatalytic degradation.^7–9 However, the processes mentioned above are limited to a certain extent. Chemical oxidation shows high efficiency and high cost. Biological process is not applicable because organic dye wastewater contains toxic or non-biodegradable substances. Photocatalytic degradation needs UV irradiation. Although the emerging ozonation method is quite effective in decolorizing dye wastewater, it would lead to additional problems such as high pH values.¹⁰

Adsorption has attracted great attention because it can operate effectively without UV irradiation and oxidant. The latest finds suggest that the adsorption performance of a adsorbent could be substantially enhanced by forming hydrogen bond with dye molecule,^11–15 in which the rules of design of composites that could decolorize dye wastewater in an efficient and rapid way are suggested. Palygorskite is a natural product with large specific surface area and adsorption capacity.^16–18 Palygoskite is very cheap and offers an option for the removal of organic and inorganic contaminants.^19–24 Palygorskite is readily available materials functioning as excellent cation exchangers, which have often been used to adsorb metallic contaminants.^25–27

Y₂O₃ is a promising host matrix for luminescence due to its good chemical durability, thermal stability.²⁸ Y₂O₃ can host optically active dopants, stabilize metal oxides for using in catalytic and ceramic materials.^29–31 The potential application of Y₂O₃ in dye wastewater purification is suggested since it could react with water to produce YOOH,³² while YOOH might greatly accelerate the decolorization by forming bond with dye molecule. However, to the best of our knowledge, Y₂O₃ has not been applied in the purification of dye wastewater.

Our group has done a lot of research on surface functionalization^33–46 and structure modification^47–52 of clay minerals, in which the surface or structure properties of clay minerals can be adjust according to the demand. Y₂O₃/Pal might be an option for the rapid purification of dye wastewater as both of them have special surface properties. Meanwhile, the safety assessment for Y₂O₃/Pal composite is also very important. According to Yttrium oxide (Y₂O₃) Material Safety Data Sheet (MSDS),⁵³ Y₂O₃ is a white and odourless powder. Its water solubility is insoluble. Toxicity is LD₅₀ intraperitoneal – rat – 230 mg kg⁻¹. Palygorskite is chemical inert and non polluting. It is used widely in medicine.⁵⁴ It can inhibit the growth of microorganisms and absorb toxic volatile components. Palygorksite is used as the raw material of cosmetics, and is particularly common in foundation, catapasm and lipstick.⁵⁵ No information on health risks for this material has been entered into the database.⁵⁵ It could be included that Y₂O₃/Pal is environmental friendly when used under appropriate conditions. To the best of our knowledge, the chemical, physical, and toxicological properties of Y₂O₃/Pal have not been thoroughly investigated. It is very important that investigation on Y₂O₃ functionalized natural palygorskite as an adsorbent for methyl blue removal in the environmental application. Therefore, Y₂O₃/Pal was designed, synthesized and characterized. The effects of Y₂O₃ functionalization on the structure and surface properties of Pal were studied. Also, the mechanism of decolorization was proposed, in which chemical adsorption between Y₂O₃/Pal and methyl blue was revealed. Results obtained from this work suggested that Y₂O₃ functionalized Pal might enlarge the application of Pal in the fields of dye wastewater purification.

2. Experimental

2.1 Materials preparation

Y(NO₃)₃·6H₂O (5.11 g) was dissolved in deionized water (200 mL), the ultrasonically dispersed suspension (10.02 g Pal in 100 mL deionized water) was slowly added later. The mixture was stirred at 80 °C for 8 h, dried in an oven at 80 °C overnight. The solid product was put into a crucible and heated at 600 °C for 2 h (at a heating rate of 5 °C min⁻¹) to produce the Y₂O₃/Pal composite. Pure Y₂O₃ powders were obtained after calcination of Y(NO₃)₃·6H₂O under the same thermal conditions. Other Pal supported rare earth oxides were synthesized from their nitrates in the same way.

2.2 Characterization

X-ray diffraction (XRD) patterns of the samples were recorded by a DX-2700 diffractometer with Cu Kα radiation. Fourier transform infrared (FTIR) spectra were recorded in a Shimadzu FTIR 8120 spectrometer by using the dried KBr disk technique. X-ray photoelectron spectroscopy (XPS) measurements were performed on a spectrometer (K-Alpha 1063, Thermo Fisher Scientific) equipped with an Al Kα X-ray radiation source. The XPS binding energy (BE) was internally referenced to the C1s peak (BE = 284.68 eV). Transmission electron microscopy (TEM) was conducted using a JEM 2100F microscope operated at 200 keV. The zeta-potential was determined by a Zeta Meter System (Backmen-Coulter Delsa 440SX, USA).

2.3 Decolorization property

Decolorization of methyl blue (MB) was performed in a glass reactor equipped with a digital thermometer controller. 30 mg sample was added into 300 mL MB solution with magnetic stirring at 400 rpm. The pH value was maintained at 7.1 during the whole process. 5 mL mixture was taken at regular intervals without addition of fresh liquid. The mixture was treated by centrifugation at 10 000 rpm for 5 min before UV-vis measurement. Used samples were collected. The maximum absorbance wavelength (610 nm) was detected and used as concentration index. A 2600 UV/VIS spectrophotometer was used to measure the concentration. The decolorization of MB was calculated by the following equation: Decolorization = 1 − C_e/C₀, where C₀ and C_e were the initial and final concentrations (mg L⁻¹) of dye, respectively.

3. Results and discussion

Fig. 1 shows the effect of Y₂O₃ on the structure of Pal. The characteristic reflection of Pal at 2θ = 8.3° shrunk after calcination at 600 °C. Our previous work indicated that the structure of Pal changed into amorphous SiO₂ upon calcination due to the removal of structural water.^16,17 Therefore, the XRD spectrum of the Y₂O₃/Pal showed very low intensity compared with the other spectra.

Fig. 1 XRD patterns of different samples.

Some other reflections belonging to Pal shrunk or disappeared after modification with Y₂O₃, which might be caused by the replacement of Si–O by Y–O. Y(NO₃)₃ was completely decomposed to Y₂O₃ after calcination at 600 °C for 2 h with a particle size of 18.6 nm calculated from Scherrer's equation, and the Y₂O₃ content (with a mass ratio of 15% to Pal) was far more than the minimum detection limit of XRD. Therefore, yttrium was supposed to connect with Pal by a Y–O–Si structure or in the form of small particles as reflections belonging to Y₂O₃ was not obvious.

FTIR was employed to characterize the band change of Pal before and after modification. Bands at 800, 880, 1180 and 1030 cm⁻¹ were the different vibrations of Si–O (Fig. 2).¹⁷ Bands at 1620, 3589 and 3650 cm⁻¹ were the different OH vibrations of Pal.^16,56 Some Si–O vibrations almost disappeared after modification with Y₂O₃. Band at 536 cm⁻¹ was the Y–O vibration of Y–O–Y, and band at 576 cm⁻¹ was Y–O vibration originated from Y–O–Si. The strength of surface OH groups of Pal reduced in Y₂O₃/Pal because Y₂O₃ were bonded on the surface of Pal.

Fig. 2 FTIR spectra of Pal and Y₂O₃/Pal. Pal was calcined at the same thermal conditions.

TEM was applied to reveal the distribution of Y₂O₃ nanoparticles. Y₂O₃ were uniformly dispersed on Pal with size of 2–5 nm (Fig. 3). The small image inserted in Fig. 3b shows a interplanar spacing of 2.08 Å, coinciding with the (134) plane of Y₂O₃ (PDF#43-1036). Y₂O₃ nanoparticles supported by Pal showed regular shape and size compared with the reported results.^57,58 These regular particles produced a large number of active sites, which would enhance the dye adsorption performance.

Fig. 3 (a) TEM and (b) HRTEM images of Y₂O₃/Pal composite (inset is the interplanar spacing), c TEM and d HRTEM images of Pal.

XPS was employed to characterize the surface chemical composition and the oxidation state of the sample. The fitting of O1s region with three peaks indicated three kinds of oxygen species on Y₂O₃/Pal (Fig. 4). Binding energy at 534.2 eV was assigned to adsorbed water oxygen, 532.2 eV was attributed to hydroxyl groups on the nanoparticle surface, 531.0 eV was caused by lattice oxygen.⁵⁹ The intensity of the –OH peak was considerably higher than the others. This clearly indicates that the Y₂O₃/Pal sample has more hydroxyl groups on the surface, which would be beneficial to enhancing the hydrophilicity of Y₂O₃/Pal.

Fig. 4 XPS spectra of O1s, Si2p and Y3d regions of Y₂O₃/Pal composite.

Binding energy of Y₂O₃ (Y3d_5/2) was reported at 156.8,²⁹ 156.4 (ref. 59) or 156.2 eV.⁶⁰ However, none of them was observed in this work. The peaks of the four yttrium contributions showed higher chemical shift. Water molecules would be strongly adsorbed on Y₂O₃ and react with bulk oxide layers to form a strongly hydrated species (in the form of YOOH or Y(OH)₃) even at room temperature. The Y3d_5/2 binding energy of vanadium–yttrium hydrates (158.6 eV) was higher than that of Y₂O₃ (156.8 eV) because of the presence of hydrogen.⁶¹ Therefore, the binding energies at 158.8 (3d_5/2) and 160.5 (3d_3/2) eV with a spin–orbit splitting of 1.7 eV were assigned to the yttrium-hydroxyl.

The binding energies at 157.9 (3d_5/2) and 159.7 (3d_3/2) eV with a spin–orbit splitting of 1.8 eV indicated that the chemical bond of YOOH groups on the nanoparticle surface was more than that of YOH. In addition, binding energies at 102.9 and 103.5 eV belonged to Si–O–Si. Si was linked to surface Y by a Si–O–Y chemical bond, suggested by the lower binding energies at 101.9 and 102.5 eV.^62,63

The decolorization of MB (Fig. 5) was used to evaluate the adsorption performance of samples. Y₂O₃/Pal showed better performance than that of Pal. The T₅₀ (time that used to reach the decolorization of 50%) of Y₂O₃/Pal was in 10 min while that of Pal was about 40 min. In addition, the decolorization of Y₂O₃/Pal was much more stable than that of Pal. The decolorization performance and its rate were both enhanced by Y₂O₃ modification.

Fig. 5 Decolorization of MB by different samples.

Our previous work has indicated that the zeta potential of Pal decreased from +3.2 to −14.8 mV with increasing pH. Y₂O₃ modification greatly increased the number of negatively charged groups as Y₂O₃/Pal showed lower negative zeta potential than that of Pal (Fig. 6). Therefore, the good decolorization performance was impossible to benefit from the electrostatic adsorption between Y₂O₃/Pal and MB molecule as Y₂O₃/Pal was negatively charged at pH 7.1 and MB molecules also negatively charged in one of the –SO₃ groups.

Fig. 6 Zeta potential of Y₂O₃/Pal as a function of pH (inset is the molecular structure of MB).

The performance of sample at different temperatures was showed in Fig. 7. Increasing temperature benefited the adsorption, which means the adsorption process was endothermic. The dye adsorption curves of sample at 60 °C were simulated with Langmuir, Freundlich and Dubinin–Radushkevich equations⁶⁴ (Fig. 8 and Table 1). The curve was best simulated with Langmuir equation, suggesting the monolayer chemical adsorption of MB. The maximum adsorption capacity is up to 1579.06 mg g⁻¹, exhibiting potential applications in wastewater treatment.

Fig. 7 Adsorption performance of sample at different temperatures.

Fig. 8 Langmuir, Freundlich and Dubinin–Radushkevich fitting of sample at 60 °C.

Table 1 Isotherm parameters for adsorption of methyl blue on Y₂O₃/Pal^a

Adsorbent	T/°C	Langmuir adsorption isotherm qe = qmKLCe/(1 + KLCe)			Freundlich adsorption isotherm qe = KFCe^1/n			Dubinin–Radushkevich adsorption isotherm ln qe = ln qmax − βε^2; ε = RT(1 + 1/Ce); E = 1/(2β)^1/2
		KL (L mg^−1)	qm (mg g^−1)	R^2	n	Kf (mg g^−1)	R^2	β (mol^2 kJ^−1)	E (kJ mol^−1)	R^2
a Note: qe: adsorbing capacity (mg g^−1), qm: saturated extent of adsorption (mg g^−1), KL: Langmuir adsorption equilibrium constant (L mg^−1); KF: Freundlich adsorption coefficient, n: Freundlich constant; β: adsorption coefficient, ε: the adsorption potential energy, E: characteristic energy.
Y2O3/Pal	60	0.0077	1579.06	0.9879	1.054	12.18	0.9840	0.5788	0.9295	0.8183

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As the Y₂O₃ modification can increase the number of negatively charged groups of Pal, Y₂O₃/Pal can be used in removing other toxicants from the environment. The capacity of adsorbing heavy metal cations of Y₂O₃/Pal is obviously higher than that of Pal. The increase in the surface negatively charged groups could greatly enhance the removal efficiency of heavy metal cations. Also, Y₂O₃ modification have effect on capturing cationic surface-active agent from water.

Molecular sieve and activated carbon are kind of common adsorbent that used in wastewater treatment besides clay minerals. Y₂O₃/Pal showed faster and more stable performance than that of Pal in this work. The treatment of molecular sieve and activated carbon in wastewater is mainly a process of physical adsorption, the rate of which is usually slower than that of chemical adsorption. Also, the desorption can be hardly avoid because of dynamic equilibrium of physical adsorption.

Y₂O₃/Pal showed nearly 90% decolorization in the first run of recycle test (Fig. 9). The 30 min decolorization in all recycling runs was higher than 80%. Therefore, Y₂O₃/Pal was stable and could be reused in the decolorization of MB. Additionally, increasing Y₂O₃ content benefited the performance of Y₂O₃/Pal (Fig. 10a) and caused more replacement of Si–O structure by Y–O structure (Fig. 10b). Meanwhile, the adsorption performance of Y₂O₃/Pal was enhanced when the pH was increased (Fig. 10c). Palygorskite modified by other rare earth oxides such as CeO₂ and Eu₂O₃ with ratio of rare earth oxides to Pal of 15 wt% also enhanced the performance of Pal (Fig. 10d). These results demonstrate that palygorskite attaches rare earth oxides such as Y₂O₃, CeO₂ and Eu₂O₃ could provide a kind of novel adsorbents for enhanced dye adsorption. However, as all we know, rare earth compounds may cause delayed blood clotting leading to hemorrhages. Inhalation of rare earths may cause sensitivity to heat, itching, and increased awareness of odor and taste, coagulation abnormalities, gastrointestinal disturbance. Inhalation may result in asthma, cough, muscles, damage to the lungs.⁵³ In addition, although Y₂O₃ is not radioactive, palygorskite has the possibility as radioactive elements might integrated into its structure because of geologic action. Therefore, we had to think carefully about whether this material is suitable for environmental pollutant removal. Considering that clay minerals are plastic substances, Pal can be made of honeycomb or other shapes after sintering,^17,18 therefore, Y₂O₃/Pal sintered is stable and can be easily collected from wastewater and recycled. In addition, although Y₂O₃ is stable in neutral water or alkali solution, it may be soluble in acidic solution to cause environmental pollution in some extent. Therefore, the solubility of Y₂O₃/Pal in acidic solution need to be measured, and radiation detection should be employed to ensure the safety of Pal in the future work.

Fig. 9 Life span of Y₂O₃/Pal in the decolorization of MB.

Fig. 10 Effects of (a) Y₂O₃ mass ratio, (c) pH value and (d) rare earth oxides at Y₂O₃, CeO₂, Eu₂O₃ to Pal of 15 wt% on the decolorization of MB, (b) XRD patterns of Y₂O₃/Pal at different mass ratio.

4. Conclusions

A novel adsorbent was designed and synthesized, which showed stable and rapid decolorization performance for methyl blue. Y₂O₃ nanoparticles played an important role in enhancing the performance because they increased the number of surface bonds of Pal and could form chemical bond with dye molecule. Therefore, methyl blue was decolorized in a stable and rapid way by chemical adsorption. Besides Y₂O₃/Pal, Pal supported CeO₂ and Eu₂O₃ also have good performance for enhanced dye adsorption. Furthermore, the Y₂O₃/Pal composite could be easily separated by centrifugation, and could be reused in the purification of dye wastewater.

Acknowledgements

This work was supported by the National Science Fund for Distinguished Young Scholars (51225403), the National Natural Science Foundation of China (51374250), the Specialized Research Fund for the Doctoral Program of Higher Education (20120162110079) and Hunan Provincial Innovation Foundation for Postgraduates (CX2012B122).

References

1. J. M. Aquino, R. C. Rocha-Filho, P. C. C. Sáez and M. A. Rodrigo, Environ. Sci. Pollut. Res., 2014, 21, 8442–8448.
2. H. Olvera-Vargas, N. Oturan, C. T. Aravindakumar, M. M. S. Paul, V. K. Sharma and M. A. Oturan, Environ. Sci. Pollut. Res., 2014, 21, 8379–8386.
3. Z. Wang, C. Fang and M. Megharaj, ACS Sustainable Chem. Eng., 2014, 2, 1022–1025.
4. M. Hakimelahi, M. R. A. Moghaddam and S. H. Hashemi, Biotechnol. Bioprocess Eng., 2012, 17, 875–880.
5. H.-J. Wang, C.-F. Wang, Y.-Y. Sun and Y. Cao, Chem. Eng. J., 2012, 209, 442–450.
6. R. M. Jain, K. H. Mody, J. Keshri and B. Jha, Mar. Pollut. Bull., 2014, 84, 83–89.
7. L. Liu, J. Ding, C. Huang, M. Li, H. Hou and Y. Fan, Cryst. Growth Des., 2014, 14, 3035–3043.
8. A. A. P. Mansur, H. S. Mansur, F. P. Ramanery, L. C. Oliveira and P. P. Souza, Appl. Catal., B, 2014, 158, 269–279.
9. C. Tan, G. Zhu, M. Hojamberdiev, K. Okada, J. Liang, X. Luo, P. Liu and Y. Liu, Appl. Catal., B, 2014, 152, 425–436.
10. M. Tokumura, T. Katoh, H. Ohata and Y. Kawase, Ind. Eng. Chem. Res., 2009, 48, 7965–7975.
11. M. A. Al-Ghouti, Y. S. J. Li, N. Al-Laqtah, G. Walker and M. N. M. Ahmad, J. Hazard. Mater., 2010, 176, 510–520.
12. M. Greluk and Z. Hubicki, Chem. Eng. J., 2013, 215–216, 731–739.
13. Z. Jiang, J. Xie, D. Jiang, Z. Yan, J. Jing and D. Liu, Appl. Surf. Sci., 2014, 292, 301–310.
14. E. Kharlampieva and S. A. Sukhishvili, Langmuir, 2004, 20, 9677–9685.
15. N. Rajendiran and R. K. Sankaranarayanan, Carbohydr. Polym., 2014, 106, 422–431.
16. C. Huo and H. Yang, Appl. Clay Sci., 2010, 50, 362–366.
17. X. He, A. Tang, H. Yang and J. Ouyang, Appl. Clay Sci., 2011, 53, 80–84.
18. C. Huo and H. Yang, J. Colloid Interface Sci., 2012, 384, 55–60.
19. D. N. Taha and I. S. Samaka, J. Oleo Sci., 2012, 61, 729–736.
20. Y. G. Peng, D. J. Chen, J. L. Ji, Y. Kong, H. X. Wan and C. Yao, Environ. Prog. Sustainable Energy, 2013, 32, 1090–1095.
21. W. T. Wu, Z. F. Nie, F. I. Tan, F. Xu, L. Xu and K. I. Gao, Polym. Polym. Compos., 2013, 21, 565–572.
22. X. G. Wang, S. Y. Lu, C. M. Gao, X. B. Xu, X. J. Zhang, X. Bai, M. Z. Liu and L. Wu, Chem. Eng. J., 2014, 252, 404–414.
23. Y. Guan, H. Qian, J. Q. Guo, S. G. Yang, X. Wang, S. M. Wang and Y. S. Fu, Appl. Clay Sci., 2015, 114, 124–132.
24. H. Zhao, F. X. Qiu, J. Yan, J. Wang, X. Li and D. Y. Yang, Appl. Clay Sci., 2016, 121, 137–145.
25. E. Alvarez-Ayuso and A. Gardia-Sanchez, J. Hazard. Mater., 2007, 147, 594–600.
26. Y. Kong, J. X. Wei, Z. L. Wang, T. Sun, C. Yao and Z. D. Chen, J. Appl. Polym. Sci., 2011, 122, 2054–2059.
27. C. Yao, Y. Y. Xu, Y. Kong, W. J. Liu, W. J. Wang, Z. L. Wang, Y. Wang and J. L. Ji, Appl. Clay Sci., 2012, 67–68, 32–35.
28. G. Jia, H. You, Y. Song, Y. Huang, M. Yang and H. Zhang, Inorg. Chem., 2010, 49, 7721–7725.
29. W. O. Gordon, B. M. Tissue and J. R. Morris, J. Phys. Chem. C, 2007, 111, 3233–3240.
30. J. Yang, Z. Quan, D. Kong, X. Liu and J. Lin, Cryst. Growth Des., 2007, 7, 730–735.
31. T. Ishihara and K. Goto, Catal. Today, 2011, 164, 484–488.
32. Y. Kuroda, H. Hamano, T. Mori, Y. Yoshikawa and M. Nagao, Langmuir, 2000, 16, 6937–6947.
33. X. He and H. M. Yang, Dalton Trans., 2015, 44, 1673–1679.
34. X. He, Q. Yang, L. J. Fu and H. M. Yang, Funct. Mater. Lett., 2015, 8, 1550056.
35. J. Jin, L. J. Fu, J. Ouyang and H. M. Yang, Sci. Rep., 2015, 5, 12429.
36. X. Y. Li, J. Ouyang, Y. H. Zhou and H. M. Yang, Sci. Rep., 2015, 5, 13736.
37. S. Y. Liu and H. M. Yang, Energy Technol., 2015, 3, 77–83.
38. H. L. Lun, J. Ouyang, A. D. Tang and H. M. Yang, Nano, 2015, 10, 1550078.
39. K. Peng, J. Y. Zhang, H. M. Yang and J. Ouyang, RSC Adv., 2015, 5, 66134–66140.
40. X. Li and A. D. Tang, RSC Adv., 2016, 15585–15591.
41. K. Peng, L. J. Fu, J. Ouyang and H. M. Yang, Adv. Funct. Mater., 2016 DOI:10.1002/adfm.201504942.
42. K. Peng, L. J. Fu, H. M. Yang and J. Ouyang, Sci. Rep., 2016, 6, 19723.
43. Y. Zhang, A. D. Tang, H. M. Yang and J. Ouyang, Appl. Clay Sci., 2016, 119, 8–17.
44. Z. Zhou, J. Ouyang, H. M. Yang and A. D. Tang, Appl. Clay Sci., 2016, 121, 63–70.
45. Q. Yang, M. Long, L. Tan, Y. Zhang, J. Ouyang, P. Liu and A. D. Tang, ACS Appl. Mater. Interfaces, 2015, 7, 12719–12730.
46. M. Long, L. Tan, H. T. Liu, Z. He and A. D. Tang, Biosens. Bioelectron., 2014, 59, 243–250.
47. W. J. Ding, H. M. Yang and J. Ouyang, RSC Adv., 2015, 5, 67184–67194.
48. L. J. Fu, C. L. Huo, X. He and H. M. Yang, RSC Adv., 2015, 5, 20414–20423.
49. C. C. Li, L. J. Fu, J. Ouyang, A. D. Tang and H. M. Yang, Appl. Clay Sci., 2015, 115, 212–220.
50. K. Peng, H. M. Yang and J. Ouyang, Powder Technol., 2015, 286, 678–683.
51. Y. Y. Xie, A. D. Tang and H. M. Yang, Nano, 2015, 10, 155005.
52. W. J. Ding, J. Ouyang and H. M. Yang, Powder Technol., 2016, 292, 169–175.
53. http://www.guidechem.com/msds/1314-36-9.html.
54. https://en.wikipedia.org/wiki/Palygorskite.
55. http://www.mindat.org/min-3072.html#.
56. A. L. Valant, N. Bion, F. Can, D. Duprez and F. Epron, Appl. Catal., B, 2010, 97, 72–81.
57. B. Guo, M. Mukundan and H. Yim, Powder Technol., 2009, 191, 231–234.
58. S. A. Song, K. Y. Jung and S. B. Park, Langmuir, 2009, 25, 3402–3406.
59. X. C. Zhang, H. M. Yang and A. D. Tang, J. Phys. Chem. B, 2008, 112, 16271–16279.
60. G. B. Sun, K. Hidajat, X. S. Wu and S. Kawi, Appl. Catal., B, 2008, 81, 303–312.
61. V. Bondarenka, S. Grebinskij, S. Kaciulis, G. Mattogno, S. Mickevicius, H. Tvardauskas, V. Volkov and G. Zakharova, J. Electron Spectrosc. Relat. Phenom., 2001, 120, 131–135.
62. G. Liu, G. Hong and D. Sun, J. Colloid Interface Sci., 2004, 278, 133–138.
63. C. M. Hessel, A. T. Heitsch and B. A. Korgel, Nano Lett., 2010, 10, 176–180.
64. Y. Shao, Q. Sun, Q. Zhang, L. Li and P. Liu, Acta Sci. Circumstantiae, 2014, 34, 3011–3016.

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Footnote

† These authors contributed equally to the research.

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