J. Phys. Chem. C 2008, 112, 16275–16280
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Layered Titanate Nanofibers as Efficient Adsorbents for Removal of Toxic Radioactive and Heavy Metal Ions from Water Dongjiang Yang,† Zhanfeng Zheng,† Hongwei Liu,† Huaiyong Zhu,*,† Xuebin Ke,† Yao Xu,‡ D. Wu,‡ and Y. Sun‡ School of Physical and Chemical Sciences, Queensland UniVersity of Technology, Brisbane, Qld 4001, Australia, State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: June 24, 2008
Titanate nanofibers with two formulas, Na2Ti3O7 and Na1.5H0.5Ti3O7, respectively, exhibit ideal properties for removal of radioactive and heavy metal ions in wastewater, such as Sr2+, Ba2+ (as substitute of 226Ra2+), and Pb2+ ions. These nanofibers can be fabricated readily by a reaction between titania and caustic soda and have structures in which TiO6 octahedra join each other to form layers with negative charges; the sodium cations exist within the interlayer regions and are exchangeable. They can selectively adsorb the bivalent radioactive ions and heavy metal ions from water through ion exchange process. More importantly, such sorption finally induces considerable deformation of the layer structure, resulting in permanent entrapment of the toxic bivalent cations in the fibers so that the toxic ions can be safely deposited. This study highlights that nanoparticles of inorganic ion exchangers with layered structure are potential materials for efficient removal of the toxic ions from contaminated water. Introduction Radioactive contaminants, such as 226Ra2+ ions from the tailings and heap-leach residues of uranium mining industry and 90Sr2+ ions from the byproduct of nuclear fission reaction and the leakage of the nuclear reactor, can cause long-term issues that seriously threaten the health of a large population.1-6 Treatment of the radioactive wastes is needed to produce a waste product suitable for long-term storage and disposal. Advanced materials for this purpose are of great interest as more nuclear power stations are in operation or construction and the quantity of nuclear waste is increasing.1-8 Natural inorganic cation exchangers, such as clays and zeolites, have been extensively used for removal of the radioactive ions from water by ionexchange process and subsequent safe disposal9,10 because of their stability against radiation, chemicals, and thermal changes. Synthetic inorganic cation exchangers, such as synthetic micas2,4 and γ-zirconium phosphate,1 niobate molecular sieves,5,6 and titanate8,11 have also been studied for this purpose. The synthetic exchangers are far superior to the natural materials for selective removal of the radioactive cations from water.1-6,8 The radioactive cations can be preferably exchanged with sodium ions or protons in the synthetic exchangers. More importantly, structural collapse of the exchangers occurred after the ion exchange proceeds to a certain fraction,2-4 forming a stable solid with the radioactive cations trapped inside permanently, and the immobilized radioactive cations could then be disposed safely. The property that the uptake of large radioactive cations eventually triggers the trapping of the cations themselves is a desirable intelligent property for decontaminating water of radioactive cations. * To whom correspondence should be addressed. E-mail: (H.Y.Z.)
[email protected]. † Queensland University of Technology. ‡ Chinese Academy of Sciences.
The inorganic cation exchangers can also be used to remove heavy metal ions in wastewaters released from chemical manufacturing, painting and coating, mining, extractive metallurgy, nuclear, and other industries.12,13 These ions are toxic or carcinogenic and hence present a serious threat to human health and the environment.14,15 Various adsorbents, including activated carbon, oxide minerals, resins, polymer fibers, and biosorbents, have been used to remove heavy metal ions from water. The research in this area has focused on the development of materials with increased affinity, capacity, and selectivity for the heavy metal ions.13,16 For example, quaternary ammonium compounds and/or thiols have been used to modify clays such as montmorillonite and other smectite clays to produce materials with high sorption capacity of the heavy metal ions.17-21 Recently, mesoporous silicas have become more attractive to be employed as inorganic support due to their large surface area. MCM-41,22-25 HMS,26,27 SBA-15,28 and SBA-125 have been modified with various groups, such as thiol and amido groups, to strongly entrap heavy metal cations. Titanates have been considered as a possible nuclear waste host for decades.8,11 These materials are usually fabricated by pyrochemical process in commercial scale and available for various uses. In the present work, we synthesized two types of titanate nanofibers by a reaction between concentrated NaOH solution and a titanium compound under hydrothermal conditions.11,29 The nanofibers have a structure in which TiO6 octahedra join each other to form layers carrying negative electrical charges, and the sodium cations exist between the layers are exchangeable. Generally, the ion exchangers with layered structure are less stable than the exchangers of three-dimensional crystal structures; serious deformation of the layers can take place under moderated conditions. It has also been found that nanostructures of an inorganic solid are ready to react with other species or to be converted to another crystal phase under moderate conditions30 and thus are substantially less stable than the
10.1021/jp803826g CCC: $40.75 2008 American Chemical Society Published on Web 10/01/2008
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Figure 1. The XRD patterns of the fibers before and after sorption of M2+ ions: (a) T3 and (b) T3(H).
corresponding bulk particles. With this knowledge, we choose two types of titanate nanofibers as adsorbents to remove radioactive ions (Sr2+ and Ra2+) and heavy metal ions (Pb2+) and expect that the sorption of bivalent cations may induce structural change and trap the cations in the adsorbents. The experimental results confirm that the fibril titanate sorbents not only adsorb Sr2+, Ra2+, and Pb2+ from water efficiently but also entrap these toxic ions permanently in the fibers. This irreversible sorption ability made the fibers superior to conventional adsorbents for practical applications in removal of hazardous metal ions. On the basis of the sorption data and structure change measurement, a model was advanced to explain the sorption process and the trapping mechanism. Experimental Section Preparation of Titanate Nanofibers. Trititanate fibers (Na2Ti3O7) were synthesized by a reaction between concentrated NaOH solution and a titanium compound under hydrothermal conditions.11,29 For example, a solution of 10.7 g of TiOSO4 · XH2O (98%, from Fluka) in 80 mL of water was mixed with 200 mL of 15 M NaOH solution. The mixture was then autoclaved at 473 K for 2 days (hydrothermal reaction) to yield sodium titanate fibers. The solid in the autoclaved mixture was recovered and washed with deionized water. After drying at 353 K for 6 h, the product white powder was labeled as T3 in the present study. Upon heating at 483 K, the trititanate is converted to a new phase Na1.5H0.5Ti3O7 [labeled as T3(H)]. The chemical equation for this transition is 483K
2Na2Ti3O7 + H2O 98 2Na1.5H0.5Ti3O7 + NaOH Sorption Experiments. In consideration of the high toxicity of the radioactive isotopes, we used the aqueous solutions of nonradioactive ions in the sorption experiments such as Sr2+ and Ba2+. Ba2+ ions have similar ionic diameters as the radioactive 226Ra2+ ions and similar ion exchange behavior.31 The isotherm of M2+ ions (M2+ ) Sr2+ and Ba2+) sorption were determined by equilibrating 30 mg of the nanofibers in 30 mL SrCl2 or BaCl2 solutions having normality between 6 × 10-6 to 2 × 10-3 M, respectively, for 48 h at room temperature. To avoid Sr2+ or Ba2+ depositing on the fiber surface in the form of SrCO3 or BaCO3, the pH value of the solutions was adjusted to the range between 6 and 7 using dilute HCl solution during the sorption process. After the equilibration the liquid and solid phases were separated by centrifugation. The experiment of heavy metal ions sorption was carried out using the aqueous solutions of Pb2+ ions. We determined a Pb2+ exchange isotherm by equilibrating 20 mg of the nanofibers in 50 mL of Pb(NO3)2 having normality between 5 × 10-5 M to
1.5 × 10-3 M. The pH value of the solutions was also adjusted to the range between 6 and 7 by HNO3 during the sorption process to avoid the formation of PbCO3 or Pb(OH)2 on the fiber surface. The sorption kinetics was also investigated. We measured the amounts (Mt) of M2+ ions adsorbed by the fibril adsorbents after equilibrating an adsorbent with the solution for 1, 6, 12, 24, 36, and 48 h, respectively. Characterization. The titanate fibers were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), Raman spectra, and diffuse reflectance UV-visible (DR-UV-vis) spectroscopy techniques prior to and after sorption experiments. SEM images were taken with an FEI Quanta 200 scanning electron microscope. The composition of some samples was determined by energy-dispersive X-ray spectroscopy (EDS) attached on the same microscope. The specimens were deposited onto a copper microgrid coated with a holey carbon film. XRD patterns of the sample powder were recorded on a Philips PANalytical X’pert pro diffractometer equipped with a graphite monochromator. Cu KR radiation and a fixed power source (40 kV and 40 mA) were used. The data were collected over a 2θ range between 4 and 75°, at a scanning rate of 2.5°/min. The concentration of M2+ in the aqueous solution was analyzed by inductively coupled plasma (ICP) technique using a Varian Liberty 200 ICP-OES. Results and Discussion The crystal structure of the titanate nanofibers was analyzed by XRD and is shown in Figure 1. The XRD pattern of T3 fibers is in good agreement with that of primitive monoclinic Na2Ti3O7 phase (PDF Number: 72-0148, a ) 0.8571 nm, b ) 0.3804 nm, c ) 0.9135 nm). Na2Ti3O7 phase can be converted into Na2Ti6O13 phase17,18 by a heating at temperatures higher than 573 K. Considering that T3(H) was obtained by heating T3 at 483 K, it should be an intermediate product between Na2Ti3O7 and Na2Ti6O13 phase. According to the XRD pattern, it is a new titanate phase. To determine the structure of T3(H), it was dissolved into 15 M HNO3 solution, and its molar ratio of Na to Ti was measured by ICP, being about 1:2. Thus the chemical formula of the T3(H) fibers is Na1.5H0.5Ti3O7. On the basis of the knowledge of literature32-37 and the current study, the diagrams of the structures of the two nanofibers are illustrated in Figure 2. As a product of ion exchange, (Na,H)2Ti3O7 has the similar crystal structure with the precursor of Na2Ti3O7. Both of them have a layered structure. The major difference is the distance of interlayer according to their XRD patterns. If all the cations of Na+ at corners of slabs of TiO6 octhedra are replaced by H+, that is, one-fourth of Na+ ions is exchanged into H+, then the variation of diffraction angles is
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Figure 2. Schematic diagrams of the structures of T3 and T3(H) nanofibers.
anticipated, just like the experimental results indicated. So, the molecular formula of T3(H) was deduced as Na1.5H0.5Ti3O7. The cation exchange capacity (CEC) for the Na2Ti3O7 and Na1.5H0.5Ti3O7 nanofibers were calculated from their formulas (CEC ) molNa/Mw(titanate)). The ion-exchange capacity for bivalent ions are 3.31 and 2.57 mmol/g, respectively. Such CECs of the titanates are relatively high, compared with those of other inorganic ion exchangers, such as layered clays, zeolites, and γ-ZrP, which have CEC in a range between 0.4 and 1.6 mmol/g.1-4 As shown in Figure 3, the isotherms approach a plateau that is the experimental saturate sorption capacity. For the sorption of Ba2+ ions, the saturate capacities are 1.17 mmol/g for T3 and 0.95 mmol/g for T3(H) fibers. The capacities of Sr2+ ion sorption are 0.63 mmol/g for T3 and 0.57 mmol/g for T3(H) nanofibers; the capacities of Pb2+ ion sorption are 1.35 mmol/g for T3 and 1.18 mmol/g for T3(H) nanofibers. These values are larger than the corresponding capacities for clays, zeolites, synthetic mica, and niobate.1-6 The sorption isotherm by the whiskers of potassium titanate, which was synthesized by pyrosynthesis at high temperature (>1000 °C), is also given in Figure 3 for comparison. The sorption ability of the whiskers is far below those of the nanofibers. The whiskers also possess layered structure with exchangeable potassium ions, but they are much larger than the nanofibers in size. They are about 100 µm long and 3 µm thick according to SEM measurement. Therefore, the large dimension of the whiskers should be the reason for their poor sorption performance. The selective uptake of Sr2+, Ba2+, and Pb2+ ions in the presence of a large excess of Na+ ions was also determined by equilibrating the nanofibers for 2 days within the solution containing 6 × 10-4 M SrCl2 (or 1.2 × 10-3 M BaCl2, 5 × 10-4 M Pb(NO3)2) and Na+ ions (NaCl or NaNO3) of a concentration from 0.01 to 0.1 M at room temperature. The distribution coefficient Kd, which is the ratio of the amount of M2+ ions adsorbed by one gram of the sorbent to the amount of M2+ remaining in solution (per milliliter),2,5,8 was derived for the systems containing sodium ions of various concentrations and is listed in Table 1. The presence of sodium ions reduces the sorption capacity of Ba2+ and Sr2+ ions by the fibers and results in a lower Kd. Nonetheless, further increases in the concentration of Na+ ions cause only slight decreases in the capacity of the M2+ ions and Kd value after the initial capacity reduction. It is also noted that the in the presence of Na+ ions, the Kd values for T3(H) fibers are higher than the corresponding values for T3 fibers while the capacities of Ba2+ and Sr2+ ions sorption by T3(H) fibers are similar to those by T3 fibers. This means that T3(H) fibers possess a better ability to selectively adsorb the bivalent radioactive ions than the T3 fibers. Interestingly, the presence
Figure 3. The isotherms of M2+ sorption by the T3 and T3(H) nanofibers: (a) Ba2+, (b) Sr2+, and (c) Pb2+.
of sodium ions does not reduce the sorption capacity of Pb2+ by the fibers and the Kd values are higher than 105, displaying a super selective sorption ability of the fibers for removal of Pb2+ ions from water. Using ratio of Mt/M48 as the vertical axis, we plot the kinetic curves of M2+ sorption in Figure 4. For the three adsorbate ions, the adsorbed amounts in the first 1 and 24 h are about 60 and 80% of the final equilibrium capacity (48 h), respectively. Compared with the traditional adsorbents such as γ-ZrP1 and synthetic clays,3 the titanate fibril adsorbents are more prompt and thus more efficient in terms of sorption kinetics for removal of the radioactive and heavy metal ions from contaminated water. Generally safe deposal of the adsorbed radioactive and heavy metal cations is required after sorption process. For safe deposal it is desired that the radioactive and heavy metal cations are permanently trapped in the adsorbent. If the adsorbed cations release from the adsorbents by ion exchange process, they will cause secondary contamination. Experiments were conducted
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TABLE 1: Kd Values for M2+ Ion Sorption by Titanate Nanofibers
a
CNaa
Ba2+-T3
Sr2+-T3
Pb2+-T3
Ba2+-T3(H)
Sr2+-T3(H)
Pb2+-T3(H)
0 0.01 M 0.05 M 0.1 M
16000 3000 2100 1900
11000 1300 1050 620
>100000 >100000 >100000 >100000
14000 4000 3300 2700
10000 1700 1000 960
>100000 >100000 >100000 >100000
CNa is the concentration of NaCl or NaNO3 in the solution of M2+ ions.
Figure 4. The dynamic isotherms of M2+ sorption by the (a) T3 and (b) T3(H) nanofibers.
Figure 5. The Raman spectra of the fibers before and after sorption of M2+ ions: (a) T3 and (b) T3(H).
to investigate the release of the adsorbed M2+ ions from the nanofibers in water. The titanate fibers that adsorbed the saturated amount of M2+ ions were separated by centrifugation, rinsed with small amount of water to remove the M2+ ions on the fiber surface, and then dispersed into water and 0.1 M Na+ aqueous solution, respectively. The suspension was shaken for 48 h and the M2+ ion concentration of the solution was determined by ICP. There is no release of the M2+ ions from the sorbent to pure water. About 5-8% of the Ba2+ and Sr2+ ions adsorbed by the fibers in saturated amount was released to the Na+ solution. However, the release of Pb2+ ions was also not detected in the solution containing Na+. Obviously, these toxic bivalent ions, especially Pb2+ ions, have been immobilized in the nanofiber sorbents without further treatment to the sorbents after the sorption. The used fibers can be deposited safely without the risk of causing secondary contamination. We noted that the saturate sorption capacities of M2+ ions by the fibers are substantially below the theoretical exchange capacities calculated from their formulas: the saturate capacities of Ba2+ and Sr2+ ions sorption are 35-37% and 19-22% of the theoretical ones, respectively; they are 40-45% of the theoretical ones for Pb2+ ions sorption. A large fraction of the sodium ions in the interlayer region of the titanate nanofibers had not been replaced by the M2+ ions as the sorption reached saturation. The XRD patterns of the fibers after the sorption reached the saturate capacity were measured (Figure 1), and
we find that the sorption of the M2+ ions induced substantial change in the crystal structures of the fibers. For T3 fibers, the interlayer spacing (d100 spacing) decreases from 0.859 to 0.750 nm when 19.3 and 40% of the interlayer Na+ ions of the nanofibers were exchanged with Sr2+ and Pb2+ ions, respectively, and decreases to 0.798 nm when 35.2% of the Na+ ions are replaced by Ba2+ ions (Figure 1a). For T3(H) fibers, the interlayer spacing (d100 spacing) decreases from 0.748 to 0.714 nm when about 22.3% of interlayer Na+ ions in the nanofibers were replaced by Sr2+ ions and decreases to 0.703 and 0.736 nm when about 36.7 and 45% of interlayer Na+ ions had been exchanged with Ba2+ ions and Pb2+ ions, respectively (Figure 1b). The considerable contraction of the interlayer spacing indicates the considerable deformation of the layer structure, which prevents further uptake of the ions, and is responsible for the incomplete exchange. The interaction between the M2+ ions and the negatively charged layers is greater than that between Na+ ions and the layers because of the higher charges and can cause the structural deformation of the negatively charged layers in the nanofibers. The structural deformation also brings about the desired outcome: trapping the toxic cations in the fibers permanently. For safe disposal of the radioactive cations after their sorption, trapping the cations permanently inside the sorbent is essential. In addition to the shift of the peak position of (100) diffraction, the cation sorption also results in substantial decrease
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Figure 6. SEM images and EDS spectra of the T3 and T3(H) nanofibers after sorption of M2+ ions: (a,d) T3_Sr, (b,e) T3(H)_Ba, and (c,f) T3_Pb.
in the diffraction intensity of the (003) planes in the fibers (Figure 1). This unveils the positions of the adsorbed M2+ ions in the fibers, because the (003) diffraction is affected when the adsorbed M2+ ions exist on the (003) planes. As illustrated in the Figure 2, the adsorbate M2+ ions replace the Na+ ions on the (003) planes and stay at the corner of the step-shaped interlayer space. Because only part of the Na+ ions on (003) planes were exchanged by M2+ions, the periodicity on (003) planes deteriorates, resulting in the serious loss of the intensity of d003 diffraction. The Raman spectra of T3 and T3(H) fibers before and after sorption of M2+ ions are presented in Figure 5. The Raman spectra shown in Figure 5a are essentially similar to spectrum reported for sodium trititanate. The set of observed bands and spectrum features are in good agreement with the reported Raman data for Na2Ti3O7.34-37 The band at 230 cm-1, which corresponds to Na-O-Ti bond vibration, disappears after M2+ ion sorption. Meanwhile the band at 883 cm-1 also disappears. This band is assigned to the stretching vibration of the shortest Ti-O bond, which is the bond between the terminal oxygen atom and the center titanium atom of a distorted TiO6 octahedron.34-37 When the M2+ ions existed on the (003) planes, as shown in Figure 5, they affected the vibrations of this shortest Ti-O bonds. The spectrum obtained from the sample T3(H) is basically similar to that obtained by Papp and co-workers for H2Ti3O7.34 The spectra of T3(H) after structural deformation caused by the sorption of M2+ions is similar to those of the T3 fibers after structural deformation (Figure 5b). The morphology and EDX spectra of the T3 and T3(H) nanofibers after sorption of M2+ ions according to SEM images
Figure 7. The fibers in an aqueous suspension before (bottle a) and after sedimentation in 30 min (bottle b).
are shown in Figure 6. Obviously, the titanate fibers maintain fibril morphology after sorption. Fibril morphology possesses very important advantages for application in removal of toxic ions from water. Fibril adsorbents can readily be dispersed into a solution because the fibers do not aggregate seriously as clay and zeolites. This property enhances the kinetic of the sorption process. Furthermore, the adsorbents can be readily separated from a liquid after the sorption by filtration, sedimentation, or centrifugation because of their fibril morphology, and this will significantly reduce the cost of the separation of the adsorbent from the liquid. As shown in Figure 7, the fibers in aqueous suspension sediment down in 30 min, while it may take over
16280 J. Phys. Chem. C, Vol. 112, No. 42, 2008 10 h for layered clays to sediment. The difficulty in separation could be reduced if large particles of clay or zeolite are used as adsorbents. But the sorption will be slow owing to the diffusion in the particles. This is similar to the situation observed for the titanate nanofibers and large titanate whiskers. Pure titanate nanofibers can be manufactured in large quantity by the hydrothermal reaction of NaOH with abundant raw materials, such as TiO2 minerals.35 Therefore, the fabrication cost of the nanofibers is much lower than that of titanate whiskers prepared by pyro-synthesis at high temperatures. This is an important advantage of the nanofibers when used as adsorbents. Conclusion Titanate nanofibers with two formulas, Na2Ti3O7 and Na1.5H0.5Ti3O7, and layered structure were used to remove the bivalent radioactive and heavy metal ions from wastewater, such as 90Sr2+, 226Ra2+, and Pb2+ ions. They exhibit larger sorption capacities of these toxic ions than the traditional adsorbents, such as layered clays and zeolites. The sorption by the titanate fibers is also much faster than those by the traditional adsorbents. The fibers can selectively adsorb M2+ ions in the presence of Na+ ions whose concentrations are much larger than that of the target M2+ ions. More importantly, the sorption can cause considerable deformation of the layered structure in nanofibers when approaches a certain extent. Most of the adsorbed toxic M2+ ions were trapped permanently in the nanofibers because of the structure deformation and could not be released from the fibers to water. So the toxic ions can be safely deposed after sorption without further treatment. In addition to these superior properties, the nanofibers possess a number of advantages for practical applications in removal of the radioactive cations. The titanate nanofiber can be prepared readily by hydrothermal reactions between caustic soda and a titanium compounds (even industrial grade rutile minerals)30 and the manufacturing cost is relatively low. The fibril sorbents can be readily dispersed into a solution because the fibers do not aggregate seriously as clay and zeolites. Moreover, the adsorbents can be readily separated from a liquid after the sorption by filtration, sedimentation, or centrifugation because of their fibril morphology. This study demonstrates that nanoparticles of inorganic ion exchangers with layered structure are potential candidates of the intelligent sorbents. This knowledge may guide us for searching new sorbent materials for this purpose. Acknowledgment. This research is supported by the Australian Research Council (ARC). References and Notes (1) Komarneni, S.; Roy, R. Nature 1982, 299, 707. (2) Komarneni, S.; Roy, R. Science 1988, 239, 1286.
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JP803826G
