Capture of radioactive cations from water using niobate nanomaterials with layered and tunnel structures

Abstract

Niobates with one-dimensional (1D) morphology, layered KNb₃O₈ nanorods and tunnel structured Na₂Nb₂O₆·H₂O nanofibers, are fabricated readily by a reaction between niobium oxide and alkali hydroxides under hydrothermal conditions. They exhibit ideal properties for removal of radioactive cations such as Sr²⁺, Ba²⁺ (as simulant for ²²⁶Ra²⁺) and Cs⁺ ions from wastewater through an ion exchange process. Compared with the Na₂Nb₂O₆·H₂O nanofibers, the KNb₃O₈ nanorods displayed better performance for the irreversible entrapment of the toxic cations, particularly Ba²⁺ cations, due to the deformation of the layer structure. Besides, these 1D niobates are acid resistant with selective uptake of the radioactive cations in the presence of a large excess of K⁺ or Na⁺ ions.

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Introduction

With the wide popularity of fission-based nuclear power, soil and water pollution resulting from accidents or the disposal of radioactive contaminants continues to be a great concern worldwide. Particularly, environmental pollution with radioactive cations, such as ¹³⁷Cs⁺, ⁹⁰Sr²⁺, and ²²⁶Ra²⁺, that stems from the processing of uranium or the leakage of nuclear reactors is a serious threat to the health of a large percentage of the population, since the radioactive cations can leach into groundwater and contaminate drinking-water supplies for large population areas. Therefore, these highly toxic cations must be removed completely from the contaminated water to prevent nearby populations from developing cancer and other serious illnesses.^1,2

The key issue in developing technologies for purification of the radioactive wastewater and their subsequent safe disposal is to devise highly efficient adsorbents, which are able to capture radioactive cations from contaminated water selectively, irreversibly, and promptly. Inorganic cation exchangers, such as zeolites, clay minerals, synthetic γ-zirconium phosphate,³ micas,^4,5 niobate molecular sieves^6,7 and titanate nanomaterials,^8–15 have been developed for the removal of radioactive cations because of their high ion exchange capacity as well as their stability to withstand intense radiation and elevated temperatures.¹⁶ Zeolites are only used in neutral or alkaline media for Cs adsorption, and the adsorption efficiency decreases seriously with the introduction of competitive cations. Compared with most of natural or modified inorganic cation exchangers, synthetic ones are far superior in selective and irreversible removal of the radioactive cations from water. Generally, layered or tunnel structure is desirable for the subsequent safe disposal, because such a metastable structure may deform and even collapse in the ion exchange process to entrap the radioactive cations into the adsorbents irreversibly.^17–26 For instance, layered sodium tri-titanate (Na₂Ti₃O₇) and tunnel structured sodium hexa-titanate (Na₂Ti₆O₁₃) nanofibers have been developed to selectively and irreversibly adsorb radioactive ¹³⁷Cs⁺, ⁹⁰Sr²⁺, and ²²⁶Ra²⁺, respectively.^11,12

In the present study, layered potassium niobate (KNb₃O₈) nanorods and tunnel structured sodium niobate (Na₂Nb₂O₆·H₂O) nanofibers were easily synthesized at low cost through a wet chemical reaction in the presence of niobium oxide and alkali hydroxides under hydrothermal conditions. The KNb₃O₈ nanorods possess a typical layered structure that is composed of Nb₃O₈-layers with the exchangeable K⁺ ions at the interlayers.^27–29 The Na₂Nb₂O₆·H₂O nanofibers comprise a framework of NbO₆ and NaO₆ octahedra with the exchangeable Na⁺ ions located in the tunnels.³⁰ Both phase compositions and their crystallographic parameters are shown in Table S1.† The niobate nanofibers and nanorods were used to adsorb Sr²⁺, Cs⁺ and Ba²⁺cations from water via ion-exchange process in this study. In addition to their equilibrium capacities, the adsorption selectivity and kinetics, and the structure evolution were investigated. Besides, niobates are inorganic oxides and hence radiation damage is minimal to them unlike polymeric materials, making them ideal candidates for entrapment of radioactive cations.

Experimental

Synthesis of KNb₃O₈ nanorods

The KNb₃O₈ nanorods were fabricated by the reaction between concentrated KOH solution and niobium pentoxide (Nb₂O₅) under hydrothermal conditions.³¹ Specifically, 1 g of Nb₂O₅ powder was dispersed in 80 mL of 2.5 M KOH aqueous solution under vigorously stirring for 1 h until an approximate transparent liquid was observed. Then the liquid was sealed in a Teflon-lined stainless steel autoclave and kept at 180 °C for 48 h. After the hydrothermal treatment, the suspension was collected and the pH value of the suspension was adjusted to ∼5.5 by drop-wise addition of diluted hydrochloric acid (HCl) aqueous solution. Then it was put back into 100 mL autoclaves and kept at 180 °C for 96 h to yield KNb₃O₈ nanorods. The solid in the autoclaved mixture was recovered and washed with deionized water to remove any soluble salts. Finally, the collected white powder was dried at 80 °C under vacuum for 12 h.

Preparation of Na₂Nb₂O₆·H₂O nanofibers

The Na₂Nb₂O₆·H₂O nanofibers were synthesized by the reaction between concentrated NaOH solution and Nb₂O₅ under hydrothermal conditions.³² Specifically, 4 g of Nb₂O₅ was dissolved in 80 mL of 12 M NaOH solution under vigorously stirring for 1 h to achieve an approximate transparent liquid, and then the obtained liquid was sealed in a Teflon-lined stainless steel autoclave and kept at 150 °C for 4 h. Then the resulting white powder in the autoclave was recovered and washed with deionized water. Finally, the washed powder was dried at 80 °C under vacuum for 12 h.

Adsorption test

Given the high toxicity of radioactive cations, the nonradioactive isotopes were used to substitute the radioactive ones. In addition, Ba²⁺ ion was used to substitute for Ra²⁺, since both of them possess similar ionic diameter and ion exchange behavior.³³ The sorption isotherms of the Sr²⁺, Ba²⁺, and Cs⁺ cations were obtained by equilibrating 100 mL Sr(NO₃)₂, Ba(NO₃)₂ or CsCl solution with different concentrations containing 100 mg KNb₃O₈ or Na₂Nb₂O₆·H₂O for 48 h at room temperature. The saturated adsorption capacities were determined under the pH value of 6 to 7, to avoid the formation of SrCO₃ or BaCO₃ on the adsorbent. The sorption tests (sorption isotherms, uptake kinetics, selective uptake and leaching test) were conducted at 25 °C. After the sorption experiments, the used adsorbents were collected by centrifugation for further characterization.

Characterization

The surface morphology and composition of the KNb₃O₈ nanorods and Na₂Nb₂O₆·H₂O nanofibers were observed on a scanning electron microscope (SEM; Hitachi, S-4800 with an accelerating voltage of 5 kV) and energy-dispersive X-ray spectroscopy (EDX) attached to the used SEM, respectively. High-resolution transmission electron microscopy (HRTEM) images were collected on a JEOL JEM-2100F field emission electron microscope under an accelerating voltage of 200 kV. The crystallized phases were detected by powder X-ray diffraction (XRD) analysis using an X-ray diffractometer (DX-2700, China) with Ni-filtered Cu Kα radiation (λ = 1.5406 Å)at 40 kV and 30 mA with a fixed slit. The concentrations of various cations were determined by using atomic absorption spectroscopy (AAS, TAS 990, China).

Results and discussion

As shown in Fig. 1a, the KNb₃O₈ nanorods reach a adsorption plateau with the increase of concentration of Sr²⁺, Cs⁺ and Ba²⁺ cations. The plateau value is the experimental saturated adsorption capacity. For the divalent Sr²⁺ and Ba²⁺ ions, the saturated adsorption capacities by KNb₃O₈ nanorods are ∼0.52 and ∼0.18 mmol g⁻¹, respectively. The cation exchange capacity (CEC) of the niobates were calculated from its formulas (CEC = mol_K/M_w(niobate)). The CEC of KNb₃O₈ nanorods for divalent and univalent cations is 1.12 and 2.24 mmol g⁻¹, respectively. This means about 46% and 16% of the K⁺ ions could be exchanged by Sr²⁺ and Ba²⁺ cations, respectively. For univalent Cs⁺ ions, the saturated adsorption capacity is ∼0.38 mmol g⁻¹, indicating that only 17% of the K⁺ ions are exchanged by the toxic Cs⁺ ions. These data reveal that a large fraction of the K⁺ ions are still retained at the interlayers of KNb₃O₈ nanorods. Obviously, the KNb₃O₈ nanorods displayed much better adsorption performance for Sr²⁺ cations than Ba²⁺ and Cs⁺ ions. Similarly, the Na₂Nb₂O₆·H₂O nanofibers also exhibited very high adsorption capacity for Sr²⁺ cations. As shown in Fig. 2a, the saturated capacities of divalent Sr²⁺ and Ba²⁺ are ∼2.53 and ∼1.05 mmol g⁻¹, respectively. Given that the CEC value of Na₂Nb₂O₆·H₂O for divalent cations is 2.89 mmol g⁻¹, the saturation capacity reaches up to ∼87% of the CEC value for Sr²⁺ sorption, much higher than that for Ba²⁺ ions (∼36%). But unfortunately, the Na₂Nb₂O₆·H₂O nanofibers did not show any sorption of Cs⁺ ions, probably because the Na⁺ ions in the tunnel structure could not be displaced by the much larger Cs⁺ ions.

Fig. 1 Removal of Sr²⁺, Cs⁺ and Ba²⁺cations by the KNb₃O₈ nanorods. (a) The adsorption isotherms of Sr²⁺, Ba²⁺, and Cs⁺ ions. The horizontal axis represents the concentration of the cations (mmol L⁻¹) remained in solution, while the vertical axis shows the saturation capacity (mmol g⁻¹) after reaching the sorption equilibrium. (b) The dynamic adsorption curves of Sr²⁺, Ba²⁺, and Cs⁺ ions (the initial adsorbate cations is 5 mmol L⁻¹). The horizontal axis represents the sorption time, while the vertical axis shows the amount of the cations taken up by the adsorbent.

Fig. 2 Removal of Sr²⁺ and Ba²⁺ cations by the Na₂Nb₂O₆·H₂O nanofibers. (a) The adsorption isotherms of Sr²⁺ and Ba²⁺. (b) The dynamic adsorption curves of Sr²⁺ and Ba²⁺ (the initial adsorbate cations is 5 mmol L⁻¹).

As shown in Fig. 1b, the KNb₃O₈ nanorods could remove all three toxic cations within the first 5 h, and more than 50% of the toxic Sr²⁺ and Ba²⁺ ions were taken up within the first 1 h. Similarly, Sr²⁺ and Ba²⁺ ions could reach the sorption equilibrium within 2 and 5 h by the Na₂Nb₂O₆·H₂O nanofibers, respectively (Fig. 2b). In addition, the kinetic features in Fig. S1† suggest that the replacement of K⁺ (or Na⁺) ions in the interlayers or tunnels of niobate nanomaterials with Cs⁺ (Sr²⁺, Ba²⁺) occurred in a solution. Apparently, the nanostructured adsorbents can provide sufficient surface contact and distribution between adsorbate and adsorbent, and thus can reach adsorption equilibrium rapidly.

As mentioned above, metastable structures such as layered or tunnel structure are desirable for the irreversible adsorption of radioactive cations, because the structural deformation or collapse occurs during the ion-exchange process. Herein, we compared the XRD patterns of the KNb₃O₈ nanorods before and after ion exchange to detect the structural evolution. As shown in Fig. 3a–c, the interlayer spacing of the KNb₃O₈ nanorods (d₀₂₀ spacing) does not decrease evidently when the K⁺ exchange sites in the nanorod are partially occupied by Ba²⁺, Cs⁺ and Sr²⁺ ions. This is different from the layered Na₂Ti₃O₇ nanofibers, in which the dehydration of the adsorbed cations resulted in serious collapse of the interlayers, because of the extremely high charge density of the layers in this phase coupled with the relatively low hydration energy of the adsorbed cations.³⁴ The interlayer spacing of the KNb₃O₈ nanorods is ∼1.05 nm, larger than that of the Na₂Ti₃O₇ nanofibers (0.86 nm). It is large enough to resist the interaction change between the cations and the negatively charged layers. Furthermore, the random distribution of the adsorbed toxic cations (see Fig. 3b) results in a significant loss of intensity for the d₀₂₀ diffraction. It is well-known that the X-ray diffraction intensity of a plane is dominated by (i) the structural factor (F_hkl), (ii) atomic scattering factors (f_Z) of all the atoms in the unit cell of the investigated crystal and (iii) the crystallographic equivalent coordinate (XYZ) of every atoms. The introduction of radioactive cations in (020) can change their atomic number, mass and occupational ratio, which will contribute to structural factor F_hkl and the XRD peak intensity variation.¹²

Fig. 3 The structural evolution of the KNb₃O₈ nanorods before and after sorption of Ba⁺, Cs²⁺ and Sr²⁺cations. (a) The XRD patterns of the KNb₃O₈ nanorods before and after ion exchange. (b) The local amplification of the XRD patterns in planes (020). (c) Schematic illustration of the ion exchange between K⁺ ions and the target cations.

Compared with the layered KNb₃O₈ nanorods, the structural variation is more significant for the tunnel structured Na₂Nb₂O₆·H₂O nanofibers. As shown in Fig. 4a and b, the diffraction intensity of (200) and (002) planes decreases substantially when the divalent Sr²⁺ and Ba²⁺ ions were adsorbed into the fibers. In addition, the diffraction peaks of (200) and (002) planes shift to higher angle with the introduction of Ba²⁺ and Sr²⁺ ions. As depicted in Fig. 4c, the d₂₀₀ and d₀₀₂ spacings are corresponding to the width and height of the tunnel. Apparently, the dehydration of the adsorbed Ba²⁺ and Sr²⁺ ions leads to the deformation of the narrow tunnel.

Fig. 4 The Na₂Nb₂O₆·H₂O structure change before and after sorption. (a) The XRD patterns of the Na₂Nb₂O₆·H₂O nanofibers before and after ion exchange. (b) The local amplification of the XRD patterns in planes (200) and (002). (c) Schematic illustration of the ion exchange between Na⁺ ions and the target cations.

The morphology of the KNb₃O₈ nanorods and the Na₂Nb₂O₆·H₂O nanofibers before and after adsorption of toxic cations was observed by FESEM and shown in Fig. 5. The signals of the adsorbed Ba⁺, Cs²⁺ and Sr²⁺ ions could be detected from the EDS spectra (insets in Fig. 5). The 1D morphology of the nanorod and nanofiber possesses very important advantages for practical application in water purification.¹¹ For instance, rod or fiber adsorbents can readily be dispersed into a solution and are easily accessed by the toxic cations, which can efficiently enhance the kinetics of the sorption process. Furthermore, the adsorbents with 1D morphology are able to be readily separated from a liquid after the sorption by filtration, sedimentation, or centrifugation, and this will significantly reduce the cost of the separation of the adsorbents from a liquid.^11,12

Fig. 5 FESEM images and EDS spectra (insets) of (a) the pristine KNb₃O₈ nanorods and the sample after adsorption of (b) Ba²⁺, (c) Cs⁺ and (d) Sr²⁺ ions; (e) the pristine Na₂Nb₂O₆·H₂O nanofibers and the sample after adsorption of (f) Ba²⁺and (g) Sr²⁺ ions. The Cu signal originates from the copper foil used for SEM observation.

TEM and HRTEM were used to determine the detailed structure of KNb₃O₈ nanorods after sorption of toxic cations. As presented in Fig. 6a–d, the 1D morphology of the KNb₃O₈ nanorods with average size of 500–600 nm is maintained after the adsorption of Ba⁺, Cs²⁺ and Sr²⁺ ions. The HRTEM images (Fig. 6e–h) and the fast Fourier transform (FFT) images (insets) reveal different lattice planes of the nanofibers. For instance, one set of the fringe spacing of 0.287 nm is observed from the image of the pristine KNb₃O₈ nanorods, which is corresponding to the {211} plane. The {160}, {311}, and {111} planes were observed from the images of KNb₃O₈ nanorods after adsorption of Ba⁺, Cs²⁺ and Sr²⁺ ions, respectively. Unfortunately, unlike the KNb₃O₈ nanorods, the TEM and HRTEM images of the Na₂Nb₂O₆·H₂O nanofibers were not collected since the nanofibers are unstable under beam during TEM observation.

Fig. 6 TEM and HRTEM images of the KNb₃O₈ nanorod: (a and e) the pristine KNb₃O₈ and the sample after adsorption of (b and f) Ba²⁺, (c and g) Cs⁺, and (d and h) Sr²⁺ ions. Insets: FFT images of the samples.

To prove the immobilization of the toxic cations in KNb₃O₈ nanorods and Na₂Nb₂O₆·H₂O nanofibers, the adsorbents after adsorption of equilibrium amount of Cs⁺, Sr²⁺ and Ba²⁺ ions were collected by centrifugation and washed with a small amount of water. Then the collected powder was dispersed in water again and stirred for 48 h. We measured concentration of the various ions in the solution by AAS. For the KNb₃O₈ nanorods, the quantities of Ba²⁺ ions released from the used samples into pure water are very low or below the detection limits. However, about 11% of Sr²⁺ and 20% Cs⁺ ions were released from the KNb₃O₈ nanorods. The leaching or desorption behaviour stems from the ion-exchange between toxic ions and protons in deionized water. Therefore, the pH value of solution is slightly increased with the extension of the desorption time, as shown in Fig. S2.† Apparently, the KNb₃O₈ nanorods displayed a superior irreversible adsorption for Ba²⁺ ions. This should be ascribed to the larger layer contraction (d₀₂₀ spacing) of KNb₃O₈ nanorods after adsorption of Ba²⁺ ions (Fig. 3b). However, about 22% of Sr²⁺ and 31% of Ba²⁺ were released from the Na₂Nb₂O₆·H₂O nanofibers, much higher than those of the KNb₃O₈ nanorods and hence the former material is not good for fixation compared to the latter. The fixation behaviour of Na₂Nb₂O₆·H₂O nanofibers is similar to the tunnel structured Na₂Ti₆O₁₃.^11,12

The adsorption selectivity of KNb₃O₈ nanorods and Na₂Nb₂O₆·H₂O nanofibers to adsorb Cs⁺, Sr²⁺, and Ba²⁺ ions in the presence of a large excess of K⁺ or Na⁺ ions was also investigated. The distribution coefficient K_d (ref. 35) of KNb₃O₈ nanorods and Na₂Nb₂O₆·H₂O nanofibers, which is the ratio of the metal ions adsorbed into the adsorbent (per gram) to the adsorbate remaining in solution (per milliliter), is listed in Tables 1 and 2, respectively. Clearly, the existence of a small amount of K⁺ or Na⁺ ions can result in large decrease in adsorption of all the toxic cations. However, further increase of K⁺ or Na⁺ ions concentration only causes a slight decrease of the adsorption capacities and the K_d values. This means the KNb₃O₈ nanorods and the Na₂Nb₂O₆·H₂O nanofibers possess good adsorption selectivity. Compared with the previous reports,^3–7 the K_d values in this work are much lower, because the initial concentrations of the toxic cations (0.8–3.6 mmol L⁻¹) are much higher than those of the previous work.

Table 1 K_d values of Sr²⁺, Ba²⁺ and Cs⁺ sorption by KNb₃O₈ nanorods

Ck^a (mol L^−1)	Sr^2+ (2.3 mmol L^−1)	Ba^2+ (3.6 mmol L^−1)	Cs^+ (0.8 mmol L^−1)
a Ck is the concentration of KCl in a solution.
0	500	53	1010
0.01	430	23	680
0.05	410	20	590
0.1	380	16	600

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Table 2 K_d values of Sr²⁺ and Ba²⁺ sorption by Na₂Nb₂O₆·H₂O nanofibers

CNa^a (mol L^−1)	Sr^2+ (2.3 mmol L^−1)	Ba^2+ (3.6 mmol L^−1)
a CNa is the concentration of NaCl in a solution.
0	1310	410
0.01	410	270
0.05	350	230
0.1	310	210

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Given that the radioactive liquid waste is usually an acidic solution, for example, ¹³⁷Cs⁺ and ⁹⁰Sr²⁺ acidic aqueous solution,^16,34 it is very important for the adsorbents to have acid resistance. To investigate the influence of the pH value on the sorption of Cs⁺, Sr²⁺, and Ba²⁺ ions, we equilibrated KNb₃O₈ nanorods or Na₂Nb₂O₆·H₂O nanofibers for 2 days in a solution with determined concentration of target cations and different pH values (1 to 14). As shown in Fig. S3,† the target ions sorption amount on the KNb₃O₈ nanorods can reach the maximum when the pH value is greater than 4. Even when the pH value equals to 2, the KNb₃O₈ nanorods can still keep 88, 91, and 46% of its equilibrium capacity to adsorb Sr²⁺, Cs⁺ and Ba²⁺ ions. For the Na₂Nb₂O₆·H₂O nanofibers, the Sr²⁺ and Ba²⁺ sorption amounts reach to the maximum when the pH is 4 and 6, respectively (Fig. S4†). The results indicate that these 1D niobates are suitable for removing radioactive ions from slightly acidic solutions.

Conclusions

The layered KNb₃O₈ nanorods and tunnel structured Na₂Nb₂O₆·H₂O nanofibers, which are potential adsorbents for radioactive ions can be easily synthesized in large scale through a hydrothermal reaction between KOH (NaOH) and niobium sources. They possess a flexible layered or tunnel structure, which guarantees a relatively high and stable cation exchange capacity for radioactive cations such as Sr²⁺, Ba²⁺, and Cs⁺. Compared with traditional ion exchangers, KNb₃O₈ nanorods and Na₂Nb₂O₆·H₂O nanofibers are more efficient and fast to remove these toxic cations from contaminated water. With the presence of competitive K⁺ or Na⁺ ions in a solution, 1D KNb₃O₈ or Na₂Nb₂O₆·H₂O nanomaterials can selectively adsorb the toxic cations even when the concentration of K⁺ or Na⁺ ions is higher than that of the target cations. More importantly, there was a structure deformation of KNb₃O₈ and Na₂Nb₂O₆·H₂O after ion exchange procedure, resulting in a permanent trapping of the radioactive ions between the interlayers or tunnels. They also can stay active in a lightly acidic environment and can be readily separated from water by cost-effective filtration, sedimentation or centrifugation. In summary, the KNb₃O₈ nanorods with layered structure and Na₂Nb₂O₆·H₂O nanofibers with tunnel structure are promising candidates as adsorbents for the removal of radioactive cations from waste water.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 21207073 and No. 50673046), ARC Discovery Project (No. 130104759), and the 47^th Scientific Research Foundation for the Returned Overseas Chinese Scholars.

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Footnote

† Electronic supplementary information (ESI) available: The phase compositions and crystallographic parameters of the KNb₃O₈ nanorods and Na₂Nb₂O₆·H₂O nanofibers (Table S1), the effect of pH value on equilibrium capacity for Sr²⁺, Ba²⁺ and Cs⁺ adsorption by the KNb₃O₈ nanorods (Fig. S1), and the effect of pH value on equilibrium capacity for Sr²⁺and Ba²⁺ adsorption by the Na₂Nb₂O₆·H₂O nanofibers (Fig. S2). See DOI: 10.1039/c5ra10907h

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