Capture of toxic radioactive and heavy metal ions from water by using titanate nanofibers

Journal of Alloys and Compounds 614 (2014) 389–393

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Capture of toxic radioactive and heavy metal ions from water by using titanate nanofibers Jiasheng Xu a,⇑, He Zhang a, Jie Zhang a, Eui Jung Kim b a Liaoning Province Key Laboratory for Synthesis and Application of Functional Compounds, College of Chemistry, Chemical Engineering and Food Safety, Center of Science and Technology Experiment, Bohai University, 19 Sci-tech Road, Jinzhou 121013, PR China b School of Chemical Engineering and Bioengineering, University of Ulsan, Ulsan 680-749, Republic of Korea

a r t i c l e

i n f o

Article history: Received 22 October 2013 Received in revised form 13 June 2014 Accepted 18 June 2014 Available online 27 June 2014 Keywords: Titanate nanofibers Inductively coupled plasma Langmuir mode Ion exchangers Toxic ions

a b s t r a c t Three types of titanate nanofibers (sodium titanate nanofibers (TNF-A), potassium/sodium titanate nanofibers (TNF-B), potassium titanate nanofibers (TNF-C)) were prepared via a hydrothermal treatment of anatase powders in different alkali solutions at 170 °C for 96 h, respectively. The as-prepared nanofibers have large specific surface area and show availability for the removal of radioactive and heavy metal ions from water system, such as Ba2+ (as substitute of 226Ra2+) and Pb2+ ions. The TNF-A shows a better capacity in the removal of Ba2+ and Pb2+ than TNF-B and TNF-C. Structural characterization of the materials was performed with powder X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS) and with inductively coupled plasma optical emission spectrometry (ICP-OES). It is found that the equilibrium data fit well with the Langmuir model. 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. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Radioactive contaminants, such as 226Ra2+ ions from the tailings and heap-leach residues of uranium mining industry can cause the long-term issues that seriously threaten the health of a large population [1–3]. They must be removed completely from radioactive waste water to prevent nearby populations from developing cancer and other serious illnesses. Sorption is an effective approach for the decontamination of highly toxic radioactive ions if the sorbents can adsorb the ions irreversibly. Irreversible sorption assures that the adsorbed radioactive ions will not be released from the sorbent to cause secondary pollution [1,4–6]. Compared with natural inorganic ion exchangers, synthetic ones are far superior in selective removal of the radioactive ions from water [1–3,7]. The radioactive ions could be preferably exchanged with sodium/potassium ions or protons in the synthetic exchangers. More importantly, a structural collapse of the exchangers occurs after the ion exchange proceeds to a certain fraction, thereby forming a stable solid with the radioactive ions being permanently trapped inside [1]. Thus the immobilized radioactive ions can then be disposed safely. The inorganic exchangers can also be used to remove heavy metal ions in waste water ⇑ Corresponding author. Tel.: +86 15941604563; fax: +86 416 3400240. E-mail address: [email protected] (J. Xu). http://dx.doi.org/10.1016/j.jallcom.2014.06.128 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

[8–10]. Many types of adsorbents have been used to remove heavy metal ions or sequester trace amount of metals from waste water [7,10]. The research in this area has concentrated on the development of materials with increased affinity, capacity, and selectivity for the heavy metal ions [11,12–16]. In this work, we synthesized three types of titanate nanofibers by a reaction between concentrated NaOH/KOH solution and anatase under hydrothermal conditions. We choose three types of titanate nanofibers as adsorbents to remove radioactive ions (using Ba2+ as substitute of 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 exhibit larger sorption capacities of these radioactive and heavy metal ions in the contaminated water [2–4].

2. Experimental 2.1. Preparation of titanate nanofibers The titanate nanofibers are synthesized by a relatively simple chemical approach in the different alkaline solutions, which is similar to the method reported in our previous work [17,18]. Potassium and sodium titanate nanofibers were prepared successfully using TiO2 as the starting material, KOH and NaOH solution as the solvents (the detail excremental conditions are shown in Table 1). Then the mixed solution was slowly transferred into a Teflon-lined stainless steel autoclave,

J. Xu et al. / Journal of Alloys and Compounds 614 (2014) 389–393 exchange behavior [19]. The isotherm of Ba2+ and Pb2+ ions sorption were determined by equilibrating 30 mg of the adsorbent in 30 mL of BaCl2 and Pb(Ac)2 solutions having normality ranging from 4  104 to 4  103 M for 48 h at room temperature. To avoid Ba2+ and Pb2+ depositing on the fiber surface in the form of BaCO3 and PbCO3 (or Pb(OH)2), the pH value of the solutions was adjusted to 6–7 using dilute HCl and HAc solution during the sorption process. After the equilibration, the liquid and solid phases were separated by centrifugation. The concentration of Ba2+ and Pb2+ in the aqueous solution was analyzed by inductively coupled plasma (ICP) technique using a Varian Liberty 200 ICP-OES.

Table 1 Summary of titanate nanofibers in different synthesis conditions. No.

KOH (mol)

NaOH (mol)

T (°C)

TNF-A TNF-B TNF-C

0 0.15 0.3

0.3 0.15 0

170 170 170

filled up to 80% of the total volume. The autoclave was sealed and maintained at 170 °C for 96 h. The precipitates were recovered by centrifugation, and washed with deionized water for dried at 50 °C for 5 h.

3. Result and discussion

2.2. Sorption experiments

The crystal structure of the titanate nanofibers was analyzed by SEM and XRD is shown in Fig. 1. From the SEM observation, it can be seen that the titanate have nearly fiber-like morphology. These TNF have diameters ranging from 15 to 50 nm and the length

In consideration of the high toxicity of the radioactive isotopes, we used the aqueous solutions of nonradioactive ions-Ba2+ in the sorption experiments. Ba2+ ions have ionic diameters similar to the radioactive 226Ra2+ ions and similar ion

310

200

A2

Intensity / a.u.

A1

10

20

301

390

30

JCPDS card 47-0124 40

50

60

70

80

90

2 Theta / degree

JCPDS card 47-0124

110

-

-311 203

310

301

200

B2

Intensity / a.u.

B1

JCPDS card 40-0403 10

20

30

40

50

60

70

80

90

2 Theta / degree

C1

110

-

203 311

Intensity / a.u.

C2

JCPDS card 40-0403

10

20

30

40

50

60

70

80

90

2 Theta / degree Fig. 1. SEM images and XRD spectra of nanofibers before sorption of Ba2+ and Pb2+: (A1 and A2) TNF-A, (B1 and B2) TNF-B and (C1 and C2) TNF-C. The big image is Lowmagnification, scale bar = 1 lm; the images that have been inserted is High-magnification, scale bar = 200 nm.

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J. Xu et al. / Journal of Alloys and Compounds 614 (2014) 389–393

Fig. 2. SEM images and EDS spectra of titanate powders after sorption of Ba2+ and Pb2+: (A and B) TNF-ABa and (C and D) TNF-APb. Different experimental conditions is shown in Table 1. The scale bar of SEM image is 100 nm.

2.0

1.5

1.0

TNF-A TNF-B TNF-C Langmuir model Freundlich model

0.5

1

2

Pb 2+ uptake (mmol g-1)

Ba 2+ uptake (mmol g -1)

2.0

1.5 TNF-A TNF-B TNF-C Langmuir model Freundlich model

1.0

0.5

1

2

0.0 0.0

0.0 0.0

0.5

1.0

1.5

2.0

2.5

Ba 2+ in equilibrium solution (mmol L-1) Fig. 3. Langmuir and Freundlich adsorption isotherms of Ba2+ on different titanate adsorbents. The TNF-A in an aqueous suspension before (1) and after sedimentation in 2 h (2).

ranging from 800 nm to 5 lm. Based on the analysis of previous work [17,18,20], all peaks cannot be clearly indexed as a pure monoclinic phase of titanate. The diffraction peaks of 2h  9.78°, 27.85° and 33.38° correspond with the (2 0 0) plane, (3 1 0) plane and (3 0 1) plane of H2Ti2O5H2O (JCPDS No. 47-0124). The layer lattice structure of Na2Ti2O5H2O is similar to that of H2Ti2O5H2O. The diffraction peaks (9.78°, 27.85° and 33.38°) may be supposed to correspond with (2 0 0), (3 1 0) and (3 0 1) plane of the Na2Ti2O5H2O. The diffraction peaks (24.11°, 29.94° and 30.13°) may correspond  and (2 0 3)  plane of the K2Ti6O13 (JCPDS with the (1 1 0), (3 1 1) No. 40-0403). The as-prepared sample may be a mixed phase of

0.5

1.0

1.5

2.0

2.5

Pb 2+ in equilibrium solution (mmol L-1) Fig. 4. Langmuir and Freundlich adsorption isotherms of Pb2+ on different titanate adsorbents. The TNF-A in an aqueous suspension before (1) and after sedimentation in 2 h (2).

Na2Ti2O5H2O, Na2Ti4O9H2O, K2Ti6O13 and K2Ti8O17, as observed in our previous work [17,18]. The SEM images and EDS spectra of the TNF-A nanofibers after sorption of Ba2+ and Pb2+ ions according to SEM images are shown in Fig. 2. Obviously, the titanate nanofibers maintain almost the same the fiber-like morphology after sorption. The fibril adsorbents can readily be dispersed into a solution because their large surface/volume ratio and weak aggregation of the nanofibers may significantly reduce diffusion distances [21]. The EDS spectra indicate that the presence of barium and lead in the used adsorbents (after sorption experiment). As shown in Figs. 3 and 4, the fibers in aqueous suspension precipitate in 2 h, while it may take over 10 h for layered clays to

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sediment. The difficulty in separation could be reduced if large particles of clay or zeolite are used as adsorbents. But the sorption process 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 [22]. In order to obtain insight into the possible mechanisms for removing Ba2+ and Pb2+ ions from aqueous solution onto different titanate adsorbents, the adsorption equilibrium data were analyzed using Langmuir and Freundlich models [23,24]:

Q e ¼ C e Q m b=ð1 þ C e bÞ

ð1Þ

Q e ¼ K f C 1=n e

ð2Þ

where Ce is the equilibrium metal ion concentration in solution (mol/L), Qe is the amount of adsorbed ions (mol/g), Qm (mol/g) and b (L/mol) are Langmuir constants related to the maximum adsorption capacity and energy of adsorption, respectively, and Kf (mol/g(L/mol)1/n) and n are the Freundlich constants. The Langmuir isotherm is based on the assumption that uptake occurs on homogeneous surface by monolayer sorption and there is no migration of absorbed molecules/ions on the surface, while the Freundlich isotherm, which is derived to model the multilayer adsorption, assumes that the adsorption energy of a metal ion binding to a site on an adsorbent depends on whether the adjacent sites are occupied [25]. The values of Qm, b, Kf, 1/n and the correlation coefficients (R) for Langmuir and Freundlich models were given in Table 2. The Langmuir model is found to fit the experimental data for the adsorption of Ba2+ and Pb2+ onto these titanate adsorbents better than the Freundlich model. This indicates the monolayer adsorption of Ba2+ and Pb2+ ions on the surface of titanate, and the active sites on titanate can be considered to be homogenously distributed. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless separation factor or equilibrium parameter, RL, which is defined as [26,27]:

RL ¼ 1=ð1 þ bC 0 Þ

ð3Þ

where C0 is the initial metal ion concentration (mol/L). The value of RL implies the adsorption to be unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). Values of RL for all the titanate adsorbents were found to be far lower than 1, and the value of RL for TNF-A is far lower than those for TNF-B and TNF-C, showing favorable adsorption of Ba2+ and Pb2+ by these titanate adsorbents, especially TNF-A. The fitting results of both Langmuir and Freundlich models are shown in Figs. 3 and 4. The saturation capacities of TNF-A, TNF-B

Table 2 Langmuir and Freundlich isotherm constants for the adsorption of metal ions onto different titanate adsorbents. Adsorbate

Ba

2+

Pb2+

Adsorbent

TNF-A TNF-B TNF-C TNF-A TNF-B TNF-C

Langmuir Qm (mmol/g)

b (L/mmol)

R2

1.93 1.67 1.74 1.91 1.88 1.79

42.65 13.95 4.09 96.79 53.79 38.73

0.999 0.995 0.995 0.999 0.999 0.998

Kf (mmol/g(L/mmol)1/n)

R2

Freundlich 1/n Ba

2+

Pb2+

TNF-A TNF-B TNF-C TNF-A TNF-B TNF-C

0.18 0.21 0.31 0.14 0.16 0.17

1.912 1.47 1.29 1.87 1.81 1.79

0.863 0.908 0.925 0.833 0.851 0.878

and TNF-C for Ba2+ (Pb2+) were 1.86 (1.89) mmol/g, 1.55 (1.84) mmol/g and 1.42 (1.75) mmol/g, respectively. These values are larger than those for clays, zeolites, synthetic micas, and niobates. For example, the capacities for Ba2+ ion sorption by Na-4-mica, zeolite Y, and niobate are 0.2–1.39 [2,3,19]. The capacities for Pb2+ ion sorption by sepiolite and zeolite are 0.08–0.45 mmol/g, respectively [23]. Furthermore, it is obvious that the adsorption capacity of TNF-A is larger than those of TNF-B and TNF-C. The above results indicate that TNF-A might be a promising adsorbent for the removal of radioactive and heavy metal ions. The diversity of adsorption capacity for these titanate adsorbents may be related with their structures. For TNF-A, the nanometer-size overall structure is composed of many ultra-thin nanosheets. Thus the diffusion of Ba2+ and Pb2+ ions from the surface active sites into the interlayer region of titanate is very easy and fast, and most of the Na+ or K+ originally present in the interlayer region can be replaced by Ba2+ and Pb2+ ions. These titanate nanofibers with exchangeable Na+ and K+ ions located in the tunnels. The interlayer spacing of the sodium titanate is bigger than potassium titanate. As a result, TNFA possesses high adsorption capacity. Generally a safe deposal of the adsorbed radioactive and heavy metal cations is required after sorption process. Experiments were conducted to investigate the release of the adsorbed Ba2+ and Pb2+ ions from the nanofibers in water. The titanate fibers adsorbing the saturated amount of Ba2+ and Pb2+ ions were separated by centrifugation, rinsed with a small amount of water to remove the Ba2+ and Pb2+ ions on the fiber surface, and then dispersed into pure water. The suspension was shaken for 48 h, and the Ba2+ and Pb2+ ion concentration in the solution was determined by ICP. It was found that about 1–2% of the adsorbed Ba2+ and Pb2+ ions were released from the sorbent to pure water, which are probably the ions physically adsorbed on the outer surface of the fibers. Obviously, these ions have been immobilized in the sorbents without further treatment to the sorbents after the sorption. These nanofibers can be deposited safely without the risk of causing secondary contamination.

4. Conclusions In this paper, three types of titanate nanofibers (TNF-A, TNF-B, TNF-C) were synthesized by a conventional hydrothermal method. The application of titanate nanofibers as adsorbent for the removal of radioactive and heavy metal ions, such as barium and lead, was investigated. The equilibrium data are fitted well with the Langmuir model. It is found that the adsorption capacity of TNF-A is far larger than those of TNF-B and TNF-C. Additionally, the nanofibers possess a number of advantages for the practical applications. First, they can be synthesized easily and the fabrication cost is relatively low. Second, these sorbents can be readily dispersed into solution and have large surface/volume ratio; these features significantly reduce diffusion distances. Third, the nanofibers can be readily separated from a liquid after the sorption and the discharged sodium/potassium ions in water are innocuous to the environment. The ability to tailor these structural features to enhance uptake and trapping of ions can be exploited for further development of new, selective and environment-friendly adsorbents for the removal of other toxic metal ions that may be found in the other wastewater.

Acknowledgement The authors gratefully acknowledge the financial support of the Liaoning Province Department of Education General Scientific and Technological Research projects (L2012403).

J. Xu et al. / Journal of Alloys and Compounds 614 (2014) 389–393

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