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Cite this: Nanoscale, 2014, 6, 722
Received 6th July 2013 Accepted 7th November 2013
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Novel fungus–titanate bio-nanocomposites as high performance adsorbents for the efficient removal of radioactive ions from wastewater† Mingze Xu,a Guodong Wei,*b Na Liu,a Liang Zhou,a Chengwei Fu,a M. Chubik,c A. Gromova and Wei Han*a
DOI: 10.1039/c3nr03467d www.rsc.org/nanoscale
Reclaimable adsorbents have a critical application in the adsorption of radioactive materials. In this study, the novel bio-nanocomposites comprising fungi and titanate nanotubes are successfully synthesized by a simple and low-cost method. Morphological characterizations and composite mechanism analysis confirm that the composites are sufficiently stable to avoid dust pollution resulting from the titanate nanomaterials. Adsorption experiments demonstrate that the bionanocomposites are efficient adsorbents with a saturated sorption capacity as high as 120 mg g1 (1.75 meq. g1) for Ba2+ ions. The results suggest that the bio-nanocomposites can be used as promising radioactive adsorbents for removing radioactive ions from water caused by nuclear leakage.
The rapid development of nuclear energy has led to the establishment of more than 400 nuclear power plants in operation in 30 countries. These plants produce about 12 000 metric tons of nuclear waste every year.1 Thus, emergency measures in the event of a nuclear leakage must be considered for safety. However, no effective way has been established to avoid or reduce damage once such a leakage occurs. For example, 11.5 thousand tons of nuclear wastewater was dumped into the sea from the Fukushima Daiichi nuclear power plant, leading to serious radioactive contamination (over 100 MBq m3) of the nearby seashore.2–6 To address this serious issue, techniques must be developed for removing radioactive ions from the environment (mainly from wastewater). Natural inorganic cation exchange materials, such as clays and zeolites, have been extensively studied and used to remove radioactive ions from water via ion exchange.7,8
a
College of Physics, Jilin University, Changchun 130012, P.R. China. E-mail: whan@ jlu.edu.cn; Fax: +86 (0)43185167869
b
College of Science, Ningbo University of Technology Country, Ningbo 315016, P.R. China. E-mail: [email protected]
c
Tomsk Polytechnic University, Tomsk 634050, Russia
† Electronic supplementary information (ESI) available: The experimental section and supplementary gures are shown in supplementary information. See DOI: 10.1039/c3nr03467d
722 | Nanoscale, 2014, 6, 722–725
Synthetic advanced materials such as polymer materials,9 macrocyclic compounds,10–12 porous materials,13,14 inorganic nonmetal nanomaterials15–17 and composites18–21 also are of great interest for this purpose. Inorganic adsorbents, especially titanate nanomaterials, exhibit excellent performance (high adsorption capacity and high stability) for removing radioactive ions from wastewater because of their specic crystal structures.22–28 However, the efficient reclaim of these adsorbents remains one outstanding unresolved problem for titanate nanomaterial application. Two major disadvantages are observed: (1) titanate nanomaterials of extremely small size are difficult to reclaim when dispersed in water and thus can lead to dust pollution. (2) Over time, most of these materials precipitate to the bottom of the water and thus signicantly increase the difficulty of reclaim. Therefore, the solution to the problems lies in nding ways to overcome these limitations of titanate nanomaterials' drawbacks and to effectively reclaim used adsorbents from wastewater. As early as 1991, Lovely et al. found that microorganisms can reduce uranium(VI) to uranium(IV) with high efficiency and low cost.29 Aside from their ability to remove radioactive ions from water, microorganisms as living templates can be used to extend the length scale of inorganic material patterning.30,31 Given this capability, designing a material with the merits of both microorganisms and titanate nanomaterials will meet the strict requirements for adsorbing radioactive ions. In our study, novel fungus–titanate composites are found to possess the following merits: (1) titanate nanotubes as the main adsorbents in bio-nanocomposites can evenly grow on the whole surface of microorganisms with no agglomeration, signicant enrichment and high exposure of the surface area of nanotubes; (2) fungus as the template can be used to direct and control the structure of the composite at the micro-scale with low price and high compatibility with titanate nanotubes. Most importantly, the fungus can always make titanate nanomaterials oat in water instead of sinking to the bottom. Thus, fungus–titanate composites can easily be reclaimed from water aer use.
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This study aims to develop reclaimable adsorbents with efficient reclaiming and high adsorption capacity for removing radioactive ions from water contaminated by the nuclear leakage. To achieve this goal, bio-nanocomposites are successfully synthesized by coupling fungi and titanate nanotubes. Experimental results conrm that the resulting bio-nanocomposite is a potential candidate for efficiently removing radioactive ions from contaminated water. Titanate nanotubes (0.5 g) (Fig. S3†) with a BET specic surface area of 260 m2 g1 were dispersed into a solution containing fungi (0.6 g) prepared by the same method as reported in our previous paper.22 Subsequently, the mixed solution was placed in a constant temperature shaking incubator for 3 days to grow composites. Finally, bio-nanocomposites can be obtained by washing several times with ethanol. The detailed preparation procedure can be found in the experimental section in the ESI.† XRD was employed to characterize the crystal structures of the bio-nanosamples before and aer calcination at 500 C. As shown in Fig. 1, the XRD pattern (red) with a broad band has a high background, indicating that it contains a large number of organic materials. Sharp peaks can be indexed as a crystal phase of H2Ti2O4(OH)2 with an orthorhombic structure, which belongs to a typical member of the layered titanate family.22 Aer calcination, no more broad diffraction patterns were observed, and all the sharp peaks (black) could be indexed to the anatase phase of TiO2 because organic hyphae in the composites were removed by the calcination of 500 degrees Celsius. The H2Ti2O4(OH)2 nanotubes were dehydrated by the release of H2O located between the TiO6 octahedral layers. The nanotubes were then transformed to anatase TiO2. Additionally, the mass proportion of the nanotubes in the composites can be calculated as 36% by comparing the quality changes of the bio-nanocomposites before and aer calcination. Structural and morphological characterizations of the black aspergillus–titanate nanotube composites by TEM are displayed in Fig. 2. Fig. 2(a) presents a typical composite structure with numerous tiny materials uniformly decorated on the whole surface of fungus bers similar to a high quality coating. Such a composite structure can be stable, and can maintain the
XRD patterns of bio-nanocomposites before and after calcination at 500 C.
Fig. 1
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(a–c) Typical TEM images at different magnifications of black aspergillus–titanate nanotube composites. (d and e) HRTEM images of a single titanate nanotube adhered to the fungus, showing the lattice fringes separated by 0.185 and 0.350 nm, respectively. (f) The SAED pattern of the nanotubes taken from the area marked in (c).
Fig. 2
complete structure under a harsh environment for a long time. These composites can withstand an hour of the ultrasonic process in our TEM characterization experiments (Fig. S5†). With a close look, as observed in Fig. 2(b) and (c), the dark tiny materials possess open ends in accordance with the titanate nanotube features. The nanotubes have an estimated length of 200 nm to 500 nm and a diameter of 5 nm to 20 nm. These sizes are identical to those of the original ones. As shown in Fig. 2(d) and (e), the nanotubes have lattice spacing values of 0.185 and 0.350 nm, respectively, comparable with the (020) and (110) facets of H2Ti2O4(OH)2. In addition, a corresponding selected area electron diffraction (SAED) pattern (Fig. 2(f)) taken from the area marked in Fig. 2(c) further conrms that the nanotubes are titanate nanotubes, whose diffraction rings can be indexed to the (020) and (110) planes of anatase. The titanate nanotubes can be rmly attached to the surface of the whole fungus, and the fungus is fully covered by the titanate nanotubes. It is worth noticing that the composites can still maintain their complete structure even when they are subjected to a harsh environment. Thus, some chemical bonds must have formed between the nanotubes and fungus instead of weak physical adsorption. FTIR spectroscopy was used to test whether chemical bonds existed between them. To obtain reliable results, a contrast sample was produced by a simple mechanism of mixing the titanate nanotubes and black aspergillus in a certain proportion (the details are shown in ESI 1.4†). We see that the nanotube content in the mixture is the same as that in the composites. As shown in Fig. 3(A) and (B), the relative wavenumber positions of the hydroxyl group stretching mode are not identical and undergo a blue shi to a higher wavenumber
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Fig. 3 FTIR spectroscopy of titanate–fungus composites (A) and the mixture produced by simple mechanical mixing of titanate nanotubes and fungus in a certain proportion (titanate–fungus ¼ 0.36 (g) : 0.64 (g)) (B).
from 3369 cm1 (bio-nanocomposites) to 3377 cm1 (the mixture).32,33 At the same time, the adsorption band of Ti–O–Ti (ref. 34) centered at 500 cm1 is also shied. These results, together with Fig. S4,† indicate that the hydroxyl groups of black aspergillus have strong interactions with Ti–O bonds of the titanate nanotubes,35 and thus, certain chemical bonds between black aspergillus and the nanotubes can be inferred. Obviously, this property can benet our product applications and avoid dust pollution from titanate nanotubes. Considering the high toxicity of radioactive isotopes and the similar ionic diameter of Ba2+ ions to those of radioactive ions and thus similar ion exchange behavior,36 aqueous solutions of nonradioactive ions (Ba2+ ions) instead of radioactive ions were used in our adsorption experiments. The adsorption equilibrium isotherms for the adsorption of Ba2+ ions are shown in Fig. 4. The adsorption data of the fungus–titanate bio-nanocomposites can be tted according to the linear form of the Langmuir adsorption equation. From the slope and intercept, the saturated adsorption capacity can be estimated to be 120 mg g1 (1.75 meq. g1). This value is larger than those for clays,7 zeolites8 and several other inorganic nanostructures (magnesia
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and sulphide materials),37,38 and even larger than the capacity of the titanate nanotubes (1.6 meq. g1).22 The high adsorption capacity can be mainly attributed to the signicantly exposed surface area of the titanate nanotubes in the adsorbent compared with that in the pure titanate nanotubes which easily agglomerate. In our composite case, the titanate nanotubes can be uniformly attached to the fungus surface without agglomeration and can thus increase the effective surface area of the adsorbent and enhance the probability of adsorption of contaminative ions. In addition, according to our experiment, pure fungus can slightly contribute to adsorption capacity. Compared with pure titanate nanotubes which are very difficult to reclaim, these bio-nanocomposites are in the macroscopic scale and can even be observed by the naked eye. Thus, these composites can readily be reclaimed aer use and can be regarded as promising adsorbents with excellent performance in Ba2+ ion uptake. The adsorption capacity of radioactive ions must be similar to that of the adsorption capacity of Ba2+ ions, and the detailed properties of radioactive ion removal will be further examined in our future work. In summary, the novel fungus–titanate bio-nanocomposites as green, reclaimable and highly efficient radioactive adsorbents are successfully prepared in our laboratory. The adsorption experiments demonstrate that the bio-nanocomposites are highly efficient adsorbents with a saturated adsorption capacity as high as 120 mg g1 for Ba2+ ions. As efficient adsorbents, these composites can be used to remove radioactive ions from water because of their similar adsorption behavior to Ba2+ ions. The bio-nanocomposites with special structural characteristics and chemical properties not only have high adsorption capacity, but can also avoid dust pollution resulting from titanate nanotubes, which is a great challenge in this eld. It is believed that the bio-nanocomposites can be regarded as a new type of radioactive water purication materials. In particular, as emergency adsorbents, these composites can be applied to nuclear leakage with their high efficiency and reclaimable properties. Our results also suggest that the bio-nanocomposites may facilitate the creation of a new and general method for designing promising adsorbents.
Acknowledgements We would like to thank Prof. Jin Wang at the State University of New York at Stony Brook and Guangshan Zhu at the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University for polishing the manuscript and the experimental support. This work was supported by National Natural Science Foundation of China (NSFC, Grant no. 51202115 and 61106066).
Notes and references Adsorption isotherm for Ba2+ ion uptake by the bionanocomposites. The inset shows the linearized Langmuir isotherm, indicating that the adsorption of Ba2+ ions follows the Langmuir adsorption equation. Fig. 4
724 | Nanoscale, 2014, 6, 722–725
1 A. Adamantiades and I. Kessides, Energy Policy, 2009, 37, 5149–5166. 2 A. Hosseini, J. E. Brown, J. P. Gwynn and M. Dowdall, Sci. Total Environ., 2012, 438, 325–333.
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3 H. J. Jeong, W. T. Hwang, E. H. Kim and M. H. Han, Ann. Nucl. Energy, 2009, 36, 1313–1318. 4 P. Kurttio, A. Auvinen, L. Salonen, H. Saha, J. Pekkanen, I. M¨ akel¨ ainen, S. B. V¨ ais¨ anen, I. M. Penttil¨ a and H. Komulainen, Environ. Health Perspect., 2002, 110, 337– 342. 5 R. Nair, F. Sunny, M. Chopra, L. Sharma, V. Puranik and A. Ghosh, Environ. Earth Sci., 2013, DOI: 10.1007/s12665013-2501-1. 6 E. Saad, R. Mansour, A. El-Asmy and M. El-Shahawi, Talanta, 2008, 76, 1041–1046. 7 S. Komarneni, N. Kozai and W. J. Paulus, Nature, 2001, 410, 771. 8 C. Oliveira and J. Rubio, Miner. Eng., 2007, 20, 552–558. 9 S. Yan, M. Zhao, G. Lei and Y. Wei, J. Appl. Polym. Sci., 2012, 125, 382–389. 10 V. Suresh, N. Ahmed, I. S. Youn and K. S. Kim, Chem.–Asian J., 2012, 7, 658–663. 11 N. Ahmed, B. Shirinfar, I. Geronimo and K. S. Kim, Org. Lett., 2011, 13, 5476–5479. 12 N. Ahmed, I. Geronimo, I. C. Hwang, N. Singh and K. S. Kim, Chem.–Eur. J., 2011, 17, 8542–8548. 13 L. Hu, J. y. Mei, Q. w. Chen, P. Zhang and N. Yan, Nanoscale, 2011, 3, 4270–4274. 14 V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza, R. Zboril and K. S. Kim, Chem. Rev., 2012, 112, 6156–6214. 15 B. Li, J. Liao, J. Wu, D. Zhang, J. Zhao, Y. Yang, Q. Cheng, Y. Feng and N. Liu, Nucl. Sci. Tech., 2008, 19, 88–92. 16 Y. Yang, Y. Zhuang, Y. He, B. Bai and X. Wang, Nano Res., 2010, 3, 581–593. 17 X. Yu, T. Luo, Y. Jia, R. Xu, C. Gao, Y. Zhang, J. Liu and X. Huang, Nanoscale, 2012, 4, 3466–3474. 18 K. C. Kemp, H. Seema, M. Saleh, K. Mahesh, V. Chandra and K. S. Kim, Nanoscale, 2013, 5, 3149–3171. 19 V. Chandra and K. S. Kim, Chem. Commun., 2011, 47, 3942– 3944. 20 V. Chandra, J. Park, Y. Chun, J. W. Lee, I.-C. Hwang and K. S. Kim, ACS Nano, 2010, 4, 3979–3986.
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21 J. Woo´ aLee and S. Bin´ aKim, Nanoscale, 2011, 3, 3583–3585. 22 M. Xu, G. Wei, S. Li, X. Niu, H. Chen, H. Zhang, M. Chubik, A. Gromov and W. Han, J. Nanosci. Nanotechnol., 2012, 12, 6374–6379. 23 D. Yang, H. Liu, Z. Zheng, S. Sarina and H. Zhu, Nanoscale, 2013, 5, 2232–2242. 24 D. Yang, S. Sarina, H. Zhu, H. Liu, Z. Zheng, M. Xie, S. V. Smith and S. Komarneni, Angew. Chem., Int. Ed., 2011, 50, 10594–10598. 25 D. Yang, Z. Zheng, H. Liu, H. Zhu, X. Ke, Y. Xu, D. Wu and Y. Sun, J. Phys. Chem. C, 2008, 112, 16275–16280. 26 D. Yang, Z. Zheng, Y. Yuan, H. Liu, E. R. Waclawik, X. Ke, M. Xie and H. Zhu, Phys. Chem. Chem. Phys., 2010, 12, 1271–1277. 27 D. Yang, Z. Zheng, H. Zhu, H. Liu and X. Gao, Adv. Mater., 2008, 20, 2777–2781. 28 Y. Masuda, in Nanofabrication of Metal Oxide Nanostructures in Aqueous Solutions, ed. Y. Masuda, InTech, 1st edn, 2011, ch. 4, pp. 118–126, ISBN: 978-953-307-912-7. 29 D. R. Lovley, E. J. Phillips, Y. A. Gorby and E. R. Landa, Nature, 1991, 350, 413–416. 30 S. A. Davis, S. L. Burkett, N. H. Mendelson and S. Mann, Nature, 1997, 385, 420–423. 31 Z. Li, S. W. Chung, J. M. Nam, D. S. Ginger and C. A. Mirkin, Angew. Chem., Int. Ed., 2003, 115, 2408–2411. 32 J. Schmitt and H.-C. Flemming, Int. Biodeterior. Biodegrad., 1998, 41, 1–11. 33 T. Bezrodna, G. Puchkovska, V. Shymanovska, J. Baran and H. Ratajczak, J. Mol. Struct., 2004, 700, 175–181. 34 X. Li, S. Chen, W. Hu, S. Shi, W. Shen, X. Zhang and H. Wang, Carbohydr. Polym., 2009, 76, 509–512. 35 J. Xu, L. Li, Y. Yan, H. Wang, X. Wang, X. Fu and G. Li, J. Colloid Interface Sci., 2008, 318, 29–34. ˜ ez-Regil and M. Mim´ 36 M. J. Vargas, M. Oligu´ın, E. Erd´ on enezReyes, J. Radioanal. Nucl. Chem., 1997, 218, 153–156. 37 Q. Li, X. Xu, H. Cui, J. Pang, Z. Wei, Z. Sun and J. Zhai, J. Environ. Manage., 2012, 98, 98–106. 38 J. Watson, I. Croudace, P. Warwick, P. James, J. Charnock and D. Ellwood, Sep. Sci. Technol., 2001, 36, 2571–2607.
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Cite this: Nanoscale, 2014, 6, 722
Received 6th July 2013 Accepted 7th November 2013
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Novel fungus–titanate bio-nanocomposites as high performance adsorbents for the efficient removal of radioactive ions from wastewater† Mingze Xu,a Guodong Wei,*b Na Liu,a Liang Zhou,a Chengwei Fu,a M. Chubik,c A. Gromova and Wei Han*a
DOI: 10.1039/c3nr03467d www.rsc.org/nanoscale
Reclaimable adsorbents have a critical application in the adsorption of radioactive materials. In this study, the novel bio-nanocomposites comprising fungi and titanate nanotubes are successfully synthesized by a simple and low-cost method. Morphological characterizations and composite mechanism analysis confirm that the composites are sufficiently stable to avoid dust pollution resulting from the titanate nanomaterials. Adsorption experiments demonstrate that the bionanocomposites are efficient adsorbents with a saturated sorption capacity as high as 120 mg g1 (1.75 meq. g1) for Ba2+ ions. The results suggest that the bio-nanocomposites can be used as promising radioactive adsorbents for removing radioactive ions from water caused by nuclear leakage.
The rapid development of nuclear energy has led to the establishment of more than 400 nuclear power plants in operation in 30 countries. These plants produce about 12 000 metric tons of nuclear waste every year.1 Thus, emergency measures in the event of a nuclear leakage must be considered for safety. However, no effective way has been established to avoid or reduce damage once such a leakage occurs. For example, 11.5 thousand tons of nuclear wastewater was dumped into the sea from the Fukushima Daiichi nuclear power plant, leading to serious radioactive contamination (over 100 MBq m3) of the nearby seashore.2–6 To address this serious issue, techniques must be developed for removing radioactive ions from the environment (mainly from wastewater). Natural inorganic cation exchange materials, such as clays and zeolites, have been extensively studied and used to remove radioactive ions from water via ion exchange.7,8
a
College of Physics, Jilin University, Changchun 130012, P.R. China. E-mail: whan@ jlu.edu.cn; Fax: +86 (0)43185167869
b
College of Science, Ningbo University of Technology Country, Ningbo 315016, P.R. China. E-mail: [email protected]
c
Tomsk Polytechnic University, Tomsk 634050, Russia
† Electronic supplementary information (ESI) available: The experimental section and supplementary gures are shown in supplementary information. See DOI: 10.1039/c3nr03467d
722 | Nanoscale, 2014, 6, 722–725
Synthetic advanced materials such as polymer materials,9 macrocyclic compounds,10–12 porous materials,13,14 inorganic nonmetal nanomaterials15–17 and composites18–21 also are of great interest for this purpose. Inorganic adsorbents, especially titanate nanomaterials, exhibit excellent performance (high adsorption capacity and high stability) for removing radioactive ions from wastewater because of their specic crystal structures.22–28 However, the efficient reclaim of these adsorbents remains one outstanding unresolved problem for titanate nanomaterial application. Two major disadvantages are observed: (1) titanate nanomaterials of extremely small size are difficult to reclaim when dispersed in water and thus can lead to dust pollution. (2) Over time, most of these materials precipitate to the bottom of the water and thus signicantly increase the difficulty of reclaim. Therefore, the solution to the problems lies in nding ways to overcome these limitations of titanate nanomaterials' drawbacks and to effectively reclaim used adsorbents from wastewater. As early as 1991, Lovely et al. found that microorganisms can reduce uranium(VI) to uranium(IV) with high efficiency and low cost.29 Aside from their ability to remove radioactive ions from water, microorganisms as living templates can be used to extend the length scale of inorganic material patterning.30,31 Given this capability, designing a material with the merits of both microorganisms and titanate nanomaterials will meet the strict requirements for adsorbing radioactive ions. In our study, novel fungus–titanate composites are found to possess the following merits: (1) titanate nanotubes as the main adsorbents in bio-nanocomposites can evenly grow on the whole surface of microorganisms with no agglomeration, signicant enrichment and high exposure of the surface area of nanotubes; (2) fungus as the template can be used to direct and control the structure of the composite at the micro-scale with low price and high compatibility with titanate nanotubes. Most importantly, the fungus can always make titanate nanomaterials oat in water instead of sinking to the bottom. Thus, fungus–titanate composites can easily be reclaimed from water aer use.
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This study aims to develop reclaimable adsorbents with efficient reclaiming and high adsorption capacity for removing radioactive ions from water contaminated by the nuclear leakage. To achieve this goal, bio-nanocomposites are successfully synthesized by coupling fungi and titanate nanotubes. Experimental results conrm that the resulting bio-nanocomposite is a potential candidate for efficiently removing radioactive ions from contaminated water. Titanate nanotubes (0.5 g) (Fig. S3†) with a BET specic surface area of 260 m2 g1 were dispersed into a solution containing fungi (0.6 g) prepared by the same method as reported in our previous paper.22 Subsequently, the mixed solution was placed in a constant temperature shaking incubator for 3 days to grow composites. Finally, bio-nanocomposites can be obtained by washing several times with ethanol. The detailed preparation procedure can be found in the experimental section in the ESI.† XRD was employed to characterize the crystal structures of the bio-nanosamples before and aer calcination at 500 C. As shown in Fig. 1, the XRD pattern (red) with a broad band has a high background, indicating that it contains a large number of organic materials. Sharp peaks can be indexed as a crystal phase of H2Ti2O4(OH)2 with an orthorhombic structure, which belongs to a typical member of the layered titanate family.22 Aer calcination, no more broad diffraction patterns were observed, and all the sharp peaks (black) could be indexed to the anatase phase of TiO2 because organic hyphae in the composites were removed by the calcination of 500 degrees Celsius. The H2Ti2O4(OH)2 nanotubes were dehydrated by the release of H2O located between the TiO6 octahedral layers. The nanotubes were then transformed to anatase TiO2. Additionally, the mass proportion of the nanotubes in the composites can be calculated as 36% by comparing the quality changes of the bio-nanocomposites before and aer calcination. Structural and morphological characterizations of the black aspergillus–titanate nanotube composites by TEM are displayed in Fig. 2. Fig. 2(a) presents a typical composite structure with numerous tiny materials uniformly decorated on the whole surface of fungus bers similar to a high quality coating. Such a composite structure can be stable, and can maintain the
XRD patterns of bio-nanocomposites before and after calcination at 500 C.
Fig. 1
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Nanoscale
(a–c) Typical TEM images at different magnifications of black aspergillus–titanate nanotube composites. (d and e) HRTEM images of a single titanate nanotube adhered to the fungus, showing the lattice fringes separated by 0.185 and 0.350 nm, respectively. (f) The SAED pattern of the nanotubes taken from the area marked in (c).
Fig. 2
complete structure under a harsh environment for a long time. These composites can withstand an hour of the ultrasonic process in our TEM characterization experiments (Fig. S5†). With a close look, as observed in Fig. 2(b) and (c), the dark tiny materials possess open ends in accordance with the titanate nanotube features. The nanotubes have an estimated length of 200 nm to 500 nm and a diameter of 5 nm to 20 nm. These sizes are identical to those of the original ones. As shown in Fig. 2(d) and (e), the nanotubes have lattice spacing values of 0.185 and 0.350 nm, respectively, comparable with the (020) and (110) facets of H2Ti2O4(OH)2. In addition, a corresponding selected area electron diffraction (SAED) pattern (Fig. 2(f)) taken from the area marked in Fig. 2(c) further conrms that the nanotubes are titanate nanotubes, whose diffraction rings can be indexed to the (020) and (110) planes of anatase. The titanate nanotubes can be rmly attached to the surface of the whole fungus, and the fungus is fully covered by the titanate nanotubes. It is worth noticing that the composites can still maintain their complete structure even when they are subjected to a harsh environment. Thus, some chemical bonds must have formed between the nanotubes and fungus instead of weak physical adsorption. FTIR spectroscopy was used to test whether chemical bonds existed between them. To obtain reliable results, a contrast sample was produced by a simple mechanism of mixing the titanate nanotubes and black aspergillus in a certain proportion (the details are shown in ESI 1.4†). We see that the nanotube content in the mixture is the same as that in the composites. As shown in Fig. 3(A) and (B), the relative wavenumber positions of the hydroxyl group stretching mode are not identical and undergo a blue shi to a higher wavenumber
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Fig. 3 FTIR spectroscopy of titanate–fungus composites (A) and the mixture produced by simple mechanical mixing of titanate nanotubes and fungus in a certain proportion (titanate–fungus ¼ 0.36 (g) : 0.64 (g)) (B).
from 3369 cm1 (bio-nanocomposites) to 3377 cm1 (the mixture).32,33 At the same time, the adsorption band of Ti–O–Ti (ref. 34) centered at 500 cm1 is also shied. These results, together with Fig. S4,† indicate that the hydroxyl groups of black aspergillus have strong interactions with Ti–O bonds of the titanate nanotubes,35 and thus, certain chemical bonds between black aspergillus and the nanotubes can be inferred. Obviously, this property can benet our product applications and avoid dust pollution from titanate nanotubes. Considering the high toxicity of radioactive isotopes and the similar ionic diameter of Ba2+ ions to those of radioactive ions and thus similar ion exchange behavior,36 aqueous solutions of nonradioactive ions (Ba2+ ions) instead of radioactive ions were used in our adsorption experiments. The adsorption equilibrium isotherms for the adsorption of Ba2+ ions are shown in Fig. 4. The adsorption data of the fungus–titanate bio-nanocomposites can be tted according to the linear form of the Langmuir adsorption equation. From the slope and intercept, the saturated adsorption capacity can be estimated to be 120 mg g1 (1.75 meq. g1). This value is larger than those for clays,7 zeolites8 and several other inorganic nanostructures (magnesia
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and sulphide materials),37,38 and even larger than the capacity of the titanate nanotubes (1.6 meq. g1).22 The high adsorption capacity can be mainly attributed to the signicantly exposed surface area of the titanate nanotubes in the adsorbent compared with that in the pure titanate nanotubes which easily agglomerate. In our composite case, the titanate nanotubes can be uniformly attached to the fungus surface without agglomeration and can thus increase the effective surface area of the adsorbent and enhance the probability of adsorption of contaminative ions. In addition, according to our experiment, pure fungus can slightly contribute to adsorption capacity. Compared with pure titanate nanotubes which are very difficult to reclaim, these bio-nanocomposites are in the macroscopic scale and can even be observed by the naked eye. Thus, these composites can readily be reclaimed aer use and can be regarded as promising adsorbents with excellent performance in Ba2+ ion uptake. The adsorption capacity of radioactive ions must be similar to that of the adsorption capacity of Ba2+ ions, and the detailed properties of radioactive ion removal will be further examined in our future work. In summary, the novel fungus–titanate bio-nanocomposites as green, reclaimable and highly efficient radioactive adsorbents are successfully prepared in our laboratory. The adsorption experiments demonstrate that the bio-nanocomposites are highly efficient adsorbents with a saturated adsorption capacity as high as 120 mg g1 for Ba2+ ions. As efficient adsorbents, these composites can be used to remove radioactive ions from water because of their similar adsorption behavior to Ba2+ ions. The bio-nanocomposites with special structural characteristics and chemical properties not only have high adsorption capacity, but can also avoid dust pollution resulting from titanate nanotubes, which is a great challenge in this eld. It is believed that the bio-nanocomposites can be regarded as a new type of radioactive water purication materials. In particular, as emergency adsorbents, these composites can be applied to nuclear leakage with their high efficiency and reclaimable properties. Our results also suggest that the bio-nanocomposites may facilitate the creation of a new and general method for designing promising adsorbents.
Acknowledgements We would like to thank Prof. Jin Wang at the State University of New York at Stony Brook and Guangshan Zhu at the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University for polishing the manuscript and the experimental support. This work was supported by National Natural Science Foundation of China (NSFC, Grant no. 51202115 and 61106066).
Notes and references Adsorption isotherm for Ba2+ ion uptake by the bionanocomposites. The inset shows the linearized Langmuir isotherm, indicating that the adsorption of Ba2+ ions follows the Langmuir adsorption equation. Fig. 4
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