Removal of cesium ions using nickel hexacyanoferrates-loaded bacterial cellulose membrane as an effective adsorbent

Journal of Molecular Liquids 294 (2019) 111682

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Removal of cesium ions using nickel hexacyanoferrates-loaded bacterial cellulose membrane as an effective adsorbent Shuting Zhuang a, Jianlong Wang a,b,⁎ a b

Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR China Beijing Key Laboratory of Radioactive Waste Treatment, INET, Tsinghua University, Beijing 100084, PR China

a r t i c l e

i n f o

Article history: Received 28 May 2019 Received in revised form 17 July 2019 Accepted 3 September 2019 Available online 04 September 2019 Keywords: Biosorbent Hexacyanoferrates Bacterial cellulose membrane Cesium

a b s t r a c t Radiocesium is a major fission product that has negative effects on ecosystems and human health due to its radioactivity and chemical toxicity. Metal hexacyanoferrates (MHCF), as efficient adsorbent for cesium removal has aroused great interest because of their high selectivity and adsorption capacity for cesium ions. In this study, a novel biosorbent, sodium nickel hexacyanoferrates-loaded bacterial cellulose membrane (BC/NiHCF), was prepared by loading NiHCF in porous and shape controllable bacterial cellulose (BC). The as-prepared biosorbent presented porosity, flexibility, and good stability in aqueous solutions, and showed a good adsorptive removal capacity for cesium ions. The effect of experimental conditions (e.g. contact time and initial cesium ion concentration) on the uptake of Cs+ was examined. For adsorption kinetics, the data was best described by the pseudo-second-order model with R2 = 0.998; for adsorption isotherms, the data could be efficiently modelled by the Langmuir model (R2 = 0.9715). Meanwhile, its maximum adsorption capacity was estimated to be 175.44 mg g−1, indicating the showcase adsorbent was efficient for Cs+ removal. Additionally, the adsorption mechanism was tentatively proposed. In a word, BC/NiHCF exhibited great adsorption performance towards Cs+. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Radioactive wastewater is generated in nuclear power plants during their daily operations, maintenance and decommissioning. Compared to municipal sewage and industrial wastewaters, radioactive wastewater is much more dangerous because of the presence of radionuclides with radioactivity. These radionuclides (e.g. 60Co, 137Cs, and 90Sr) will undergo radioactive decay with energy emissions, which may cause illness, gene mutation, and even death for organisms. The separation of these radionuclides by adsorption is a popular method owing to its low cost, high efficiency, and feasibility in large scale application [1,2]. Compared with other radionuclides, the separation of 137Cs is more important and challenging. On the one hand, with a half-life of N30 years and the chemical similarity of K+, 137Cs can be easily taken into body, resulting in more than elemental poison, but also long time internal exposure of radiation; on the other hand, most adsorbents presented low adsorption capacity or poor adsorption selectivity towards this monovalent group IA element [3]. Various technologies have been studied for the removal of radionuclides from aqueous solution [4–10]. For the separation of cesium ions,

⁎ Corresponding author at: Energy Science Building, Tsinghua University, Beijing 100084, PR China. E-mail address: [email protected] (J. Wang).

https://doi.org/10.1016/j.molliq.2019.111682 0167-7322/© 2019 Elsevier B.V. All rights reserved.

mainly the adsorption and membrane technologies have been applied [11–15]. Among them, hexacyanoferrates have been widely studied due to their high selectivity and adsorption capacity to cesium [3]. Metal hexacyanoferrates (MHCF) are coordination polymers containing transition metal ions, such as Co2+, Fe3+, Cu2+, and Ni2+, and bridging cyano-groups [4,16]. Sodium nickel hexacyanoferrates falls into this category. Attributing to the low cost and strong affinity towards cesium ions, MHCF have been utilized for the treatment of Cs+containing solutions, such as the radioactive wastewater of Fukushima nuclear power plant accident. It has been widely accepted that the high selectivity of MHCF towards cesium ions can be attributed to the guest-host structure, where the micro-pore of the face-centered cubic structure of MHCF that is usually occupied by alkaline cations (e.g. Na+ and K+) and water molecules can instead be replaced by cesium ion of a similar size. However, the MHCF usually obtained via chemical co-precipitation comes in an ultra-fine powder form. This is bad for separation and column operation. Previously, we reviewed MHCF-based adsorbents for cesium ions [3]. Immobilization and magnetic modification are the widely adopted methods for functional application. Several types of organic polymers (e.g. polyacrylic acid [17], polyvinyl alcohol [18], chitosan [19,20], and alginate [21]) and inorganic solids (e.g. SiO2 [22], clinoptilolite [23] and hydroxyapatite [24]) have been reported for the loading of MHCF. This creates a range of adsorbents that have ideal forms and stronger mechanical strength for practical uses, as well as the inherent benefits of the organic/inorganic carriers.

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for the first time, (2) study the adsorption performance of cesium ions by BC/NiHCF, and (3) identify the possible adsorption mechanism.

Nomenclature C0 (mg g−1) initial concentrations of cesium ions in the solution Ce (mg g−1) equilibrium concentrations of cesium ions in the solution Ct (mg g−1) the concentrations of cesium ions in the solution at time t V (L) solution volume m (mg) adsorbents' mass k1 (min−1) kinetic rate constant of the pseudo-first order model qe1 (mg g−1) the calculated equilibrium adsorption capacity of the pseudo-first order model k2 (g mg−1 min−1) the kinetic rate constant of the pseudo-second order model qe2 (mg g−1) the calculated adsorption capacity of the pseudo-second order model ki(mg g−1 min-1/2) the intra-particle diffusion rate constant C (mg g−1) a constant of the intra-particle diffusion model relating to the thickness of the boundary layer α (mg g−1 min−1) the initial metal sorption rate β (g mg−1) the desorption constant

Cellulose derived from bacterial, denoted as bacterial cellulose (BC), is a type of renewable biomaterial with the merits of benign tensile strength, malleability, and water holding capacity, as well as an ultrafine network architecture [25,26]. This green material has been widely utilized in medical, food and paper industries, as well as cosmetic (mask). These unique characteristics of BC also inspired us to fabricate its composite with MHCF. Previous studies proved that BC with ultrafine network architecture can be utilized as a good carrier for catalysts, cells, enzymes, and others [27]. In addition, with abundant –OH groups, it could coordinate with transition metal ions, which is an important step of the successive synthesis of MHCF in supporter. This implies the feasibility of using it as a carrier for MHCF, which has not been tried before. What is more, as a vital component of adsorbent, its outstanding ultrafine network architecture is a promising structure for mass transfer. This study was intended to (1) synthesize and characterize the bacterial cellulose supported sodium nickel hexacyanoferrates (BC/NiHCF)

Soaked in the NiCl2 solution

(a) BC membrane

2. Experimental section 2.1. Chemical and reagents Bacterial cellulose membranes (thickness is about 3 mm) were purchased from Hainan Yide Food Industry Co., Ltd. (China) and were cut into small round pieces (diameter: 3 mm). Other chemicals of chemical pure were obtained from Aladdin. 2.2. Preparation of adsorbent (BC/NiHCF) NiCl2·6H2O (23.8 g) was dissolved in 200 mL deionized water, where BC membranes were immersed for 2 h. Then the Ni2+-loaded BC membranes were transferred into sodium ferrocyanide solution (48.4 g Na4Fe(CN)6·10H2O dissolved in 23.8 g H2O) and shaken for another 2 h. After that, the obtained BC/NiHCF membranes was washed and stored in deionized water for later use. The shape of adsorbents can be tailored by the malleable BC. 2.3. Characterization The characterization of BC/NiHCF, including FTIR, SEM, EDX and XRD. FTIR spectra of BC/NiHCF was recorded using Bruker FT-IR device (VERTEX 70). The spectra were collected after 16 scans with a resolution of 2 cm−1. Additionally, SEM and EDX spectra of BC/NiHCF before and after adsorption of cesium ions were recorded on JSM-7001F field emission scanning electron microscopy equipped with an Oxford INCA energy- dispersive X-ray spectrometer. The XRD spectra were obtained on a Rigaku TTRAX III diffractometer. 2.4. Adsorption experiments The adsorption experiments were performed with contact time or initial concentrations as the respective controlled factors. To prepare Cs+-containing solution, CsCl was dissolved by deionized water without pH adjusting (about 6). All samples were contained in 50 mL sealable plastic tubes and shaken at 160 rpm on a vortex shaker at a set temperature (25 °C). Experiments were conducted in duplicate. Cesium ion concentration in the solution was quantified using a ZA3000 Polarized Zeeman Atomic Absorption Spectrophotometer with flame atomic emission (HITACHI, Japan). The samples were prepared with 2% (w/v) nitric acid and 5 g L−1 K+ as an ionization interference inhibitor. Other adsorption parameters, e.g. the adsorption capacities qt (at time t/min, mg g−1) and qe (at equilibrium, mg g−1), and removal ratio (r), were calculated by Eqs. (1)–(3): qt ¼ ðC0 –Ct Þ V=m

ð1Þ

Soaked in the Na4Fe(CN)6 solution

(b) BC membrane after adsorption of Ni2+ Fig. 1. Schematic diagram of the formation of BC/NiHCF.

(c) BC/NiHCF

S. Zhuang, J. Wang / Journal of Molecular Liquids 294 (2019) 111682

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Fig. 2. SEM of BC/NiHCF.

qe ¼ ðC0 –Ce Þ V=m

ð2Þ

r ¼ ðC0 –Ct Þ=C0  100%

ð3Þ

3. Results and discussion 3.1. Preparation and characterization of adsorbents

Fig. 3. XRD of BC/NiHCF.

The preparation of BC/NiHCF was obtained via a two-step reaction, as shown in Fig. 1. Firstly, Ni2+ of high concentration was adsorbed into BC due to the abundant –OH groups on the surface via dipoledipole interactions [28]. After that, Ni2+ was concentrated in BC, resulting a color change (Fig. 1a and b). Secondly, after washed several times, it was transferred into a sodium ferrocyanide solution and the deposition in BC could be observed in Fig. 1(c), indicating that some kind of reactions happened in the membranes, which was likely to be the formation of NiHCF [29]. To confirm the presence and the composition of NiHCF in BC, SEMEDX and XRD were applied. With a magnification of 20.31 k, SEM

Fig. 4. The effect of contact time on the adsorption capacity and removal ratio of cesium ions (V = 50 mL, C0 (Cs+) = 175 mg L−1, dosage = 147 mg/50 mL).

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Fig. 5. Plot of kinetic models for sorption of cesium ions on BC/NiHCF (a) the pseudo first-order model; (b) the pseudo second-order model; (c) the intra-particle diffusion model; and (d) Elovich model.

image of BC/NiHCF showed the crossed nanofiber structure of BC and the precipitation of crystalline with a diameter b200 nm inside the porous structure (Fig. 2). The nanoporous BC enriched the composites with high surface area and porous structure, which was good for the growth of NiHCF in it and the penetration of Cs+. To identify the chemical composition of crystalline in the BC, a semiqualitative analysis, EDX data and XRD pattern was applied on synthesized composites. According to the EDX data, the atomic molar ratio of Ni/Fe was close to 1.55. Therefore, it could be calculated that the theoretical stoichiometry of BC/NiHCF was close to Na0.9Ni1.55Fe(CN)6. The XRD pattern, as shown in Fig. 3, was used to evaluate the composition and structure of the material. The intensive peaks in the curve implied a terrific crystallization of the sample. Compared with the standard card, the characteristic peaks at 17.4°, 24.7°, 35.3°, 39.6°, 43.7°, 50.7°, 54.1° and 57.3° confirmed the presence of NiHCF in the composites [30–32]. According to the XRD spectrum, the NiHCF supported by BC proved to be a phase-pure face-centered cubic structure [32]. The

other characteristic peaks at 15.1°, 21.4° and 22.6° in the front end of the XRD spectrum showed the structure of Iα for BC [25,33]. The obtained results indicated that NiHCF could be supported by BC membrane successfully without changing its crystal structure, which was good for the Cs decontamination. From macro-morphological changes to micro-morphology picture, as well as the elements and structural analyses, the synthesized materials could be well believed to be the combination of BC and NiHCF with good structures. NiHCF was immobilized into BC porous structure and the as-prepared adsorbents inherited the merits of both components. 3.2. Adsorption performance Adsorption kinetics and isothermals of cesium ions adsorbed by the showcase adsorbent were explored to identify its adsorption equilibrium time and saturated adsorption capacity towards cesium ions.

Table 1 Values of the parameters of different kinetic models for the adsorption of cesium ions by BC/NiHCF. Kinetic models Pseudo first-order model

Linearized forms logðqe1 −qt Þ ¼ logqe1 −

Pseudo second-order model

t 1 t ¼ þ q k2 q2e2 qe2

Intraparticle diffusion model

qt = kit0.5 + C

Elovich model

1 1 qt ¼ 1nðαβÞ þ 1nt β β

Parameters k1 t 2:303

−1

k1 (min ) qe1 (mg g−1) R2 k2 (g mg−1 min−1) qe2 (mg g−1) R2 ki,1 (mg g−1 min−1/2) ki,2 (mg g−1 min−1/2) ki,3 (mg g−1 min−1/2) α (mg g−1 min−1) β (g mg−1) R2

Values 0.032 65.599 0.996 0.001 82.645 0.998 9.4170 2.0183 0.1772 0.0591 10.7829 0.9553

S. Zhuang, J. Wang / Journal of Molecular Liquids 294 (2019) 111682

Fig. 6. The effect of initial Cs+ concentrations on qm and removal ratio (t = 3 h, V = 20 mL, dosage = 30 mg/20 mL).

The specific experimental conditions were given after Fig. 4. As presented in Fig. 4, Cs+ in the solution was quickly adsorbed into the adsorbent of abundant unoccupied adsorption sites at the initial stage, resulting in the fast increase in removal ratio and adsorption capacity (qt). In addition, the porous fiber structure (as shown in SEM imagine) was also good for the fast diffusion of pollutants into the inner adsorption sites. With the gradual saturation of adsorption sites, the uptake of Cs+ slowed down until equilibrium. Almost 90% of the pollutants was separated from the solution at the first 80 min. It took about 150 min to achieve adsorption equilibrium, with 96.3% removal ratio and qe of 74.4 mg g−1. Therefore, 4 h was fixed in the later experiments

5

to ensure the adsorption equilibrium. Results indicated that the showcase BC/NiHCF was an efficient adsorbent for cesium ions. Four kinds of classic kinetic models were applied to simulate the kinetic data (Fig. 5), and their related parameters and mathematic expression were given in Table 1. In terms of R2, both of the pseudo-first [34] and pseudo-second [35] order models were up to 0.99, whilst the Elovich model [36], which was suitable for chemical adsorption [37], was the last one in fitness to the data. Additionally, experimental qe (74.4 mg g−1) was close to the calculated qe1 (65.599 mg g−1) and qe2 (82.645 mg g−1) obtained from above two models. To further explore the diffusion step of adsorption, intra-particle diffusion [38] model was utilized, as shown in Fig. 5(c) and Table 1. The adsorption process could be categorized into three stages, and the diffusion rate constants followed the order of ki,1 N ki,2 N ki,3. During the steep slope at first stage (0 min to 40 min), the linear plot almost passed through the origin, implying intra-particle diffusion-dominated type rate-controlling step at this stage [39]. As the surface adsorption sites been occupied, as well as the lower concentration gradient of Cs+, the rate of adsorption was likely to be limited by pore size distribution of BC/NiHCF and the concentration of cesium ions, together with its affinity to adsorbent at the second stage (60 min–120 min) [40]. In the third stage, the value of ki,3 was close to zero, which was probably caused by the equilibrium. The removal ratio and adsorption capacity were showed in Fig. 6 when initial concentrations of cesium ions varied (50 mg L−1– 1100 mg L−1). The results were further analyzed via several isothermal models, as shown in Fig. 7 and Table 2. As shown in Fig. 6, the adsorption capacity of showcase adsorbent towards cesium ions increased with the increasing of initial Cs+ owing to the stronger driving force of the mass transfer at higher concentration. Whilst, its increasing rate gradually slowed down at higher concentration until reaching its maximum adsorption capacity.

Fig. 7. Plot of isotherm models for sorption of cesium ions onto BC/NiHCF.

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Table 2 Different adsorption isotherm models and its fitting results. Adsorption isotherm models

Forms 1 ln qe ¼ ln C e þ ln K F n

Freundlich isotherm model

Langmuir isotherm model

Ce 1 Ce ¼ þ qe qm K L qm

Temkin model

qe ¼

Dubinin-Radushkevich equation

lnqe = ln qm − KDε2 E = (2KD)−0.5

RT ln ðaC e Þ b

Parameters

Values

KF (L mg−1) n R2 qm (mg g−1) KL (L mg−1) R2 B (J mol−1) a (L g−1) R2 qm (mg g−1) KD (mol2·kJ−2) E (kJ mol−1) R2

6.5013 2.0040 0.8033 175.44 0.0081 0.9715 66.346 0.0940 0.9325 160.58 0.0101 7.0360 0.9269

Table 3 Adsorption performance of cesium ions by different sorbents (qm was obtained from the Langmuir model). Adsorbent

Dosage (g L−1)

T (°C)

pH

Cs+ concentration (mg L−1)

qm (mg g−1)

References

SBA-15@FC FC-Cu-EDA-SAMMS NaCuHCF-PEI-MNCs PB/Fe3O4/GO P-MSC composite NiHCF-sericite poly(AAc-co-B18C6Am) hydrogels BC/NiHCF

1 0.1–1 0.1 2.5 0.5–2.5 1–10 3–15 1.5

25 20 25 1–50 20 15–45 25 25

7 0.1–7.3 4–10 4–10 – 2–7 2–8 6

10–800 0.25–50 5–75 25–150 0.5–50 1–20 0–1980. 50–1100

13.90 21.7 166.67 43.52 108.06 16.58 74.63 175.44

[46] [22] [47] [17] [49] [50] [51] This study

However, the curve of removal ratio presented a different trend. At low concentrations (b100 mg L−1), the removal ratio increased with the increasing initial concentration. As the initial concentration continually increased, the value of removal ratio decreased. Its optimal removal ratio should be obtained when the initial concentrations was within the range of 100 mg g−1 to 300 mg g−1. Four kinds of classis adsorption isotherms (Table 2) were applied to simulate the isothermal data. The Freundlich [41] and Langmuir

[42] models are the most popular ones in simulating adsorption experiments. The former is an experimental one; the latter was derived from the assumption that the adsorbent of homogeneous surface adsorbed adsorbates via monolayer adsorption without further interaction between adsorbates. Besides, more isothermal models have been applied here, including Temkin model [43] and DubininRadushkevich equation. The former was come up with taking into account the sorbate/sorbent interactions; the latter was applied for

Fig. 8. FTIR spectra of BC/NiHCF before and after adsorption of cesium ions.

S. Zhuang, J. Wang / Journal of Molecular Liquids 294 (2019) 111682

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characteristic bands at around 2095 cm−1 were assigned to be Fe-CNNi tensions [48]. That proved the presence of both ingredients in the composites. At the same time, these bands could still be seen in the spectra of BC/NiHCF after adsorption of cesium ions. No obvious band changes were observed in Fig. 8, indicating that the functional groups may not dominate the process via chelating. Instead, the cesium ions adsorbed by BC/NiHCF can be well explained by the ion exchange between monovalent cations presented in the lattice of crystal NiHCF [52]. During the adsorption process, Cs+ with the similar size of the crystal lattice of NiHCF could penetrate the composites and replace the Na+ from the lattice (Fig. 9). The good fitting results of the monolayer adsorption model (Langmuir model) has proved it to some degree [53]. Besides, the pseudo-first order kinetic and Dubinin-Radushkevich isothermal models both indicated that diffusion and physisorption dominated the adsorption process. 4. Conclusion A novel BC/NiHCF was synthesized, characterized, and used as a biosorbent for cesium ions removal. The biosorbent was characterized by SEM, EDX, XRD and FTIR. NiHCF was successfully loaded in the porous BC with a good crystal structure. The adsorption of Cs+ onto BC/ NiHCF could achieve equilibrium within 150 min and followed the pseudo-second order model. 175.44 mg g−1 of the maximum adsorption capacity was obtained according to the Langmuir model (R2 = 0.9715). The FTIR spectra showed no functional groups involving in the adsorption process, and an ion exchange mechanism between the monovalent cations presented in the lattice of crystal NiHCF was responsible for the uptake of Cs+. Acknowledgments The research was supported by the National Key Research and Development Program(2016YFC1402507), National Natural Science Foundation of China (51578307) and the Program for Changjiang Scholars and Innovative Research Team in College (IRT-13026). Fig. 9. Possible mechanism of Cs+ adsorption by BC/NiHCF.

the determination on the nature of adsorption process as a physical or chemical one. The fitting results of these models were presented in Fig. 7 and Table 2. The Langmuir model (R2 = 0.9715) showed best fitness, orderly followed by the Temkin (R2 = 0.9325), Dubinin-Radushkevich (R2 = 0.9269), and Freundlich (R2 = 0.8033) isotherm models. That implicated the adsorption process may involve monolayer adsorption. Additionally, the qm derived from the Langmuir model was 175.44 mg g−1. It was higher than other reported ones as presented in Table 3. Furthermore, its mean energy (E) and KD derived from the DubininRadushkevich model was 7.0360 kJ mol−1 and 0.0101(mol2·kJ−2), respectively, indicated the adsorption process may be owed to the physical adsorption [44,45]. 3.3. Adsorption mechanism The FTIR spectra of BC/NiHCF with or without loaded cesium ions were observed to identify which functional groups were responsible for adsorbing cesium ions. As presented in Fig. 8, the characteristic bands of BC could be seen in both pictures at around 3348 cm−1 (–OH stretching vibration), 1618 cm−1 (in-plane deformation vibration of –OH), and 2920 cm−1 (-CH2 stretching vibration), as well as 1163 cm−1 (C-O-C vibration), 1036 cm−1 (the vibration of the pyridine ring skeleton of the saccharide) and 1059 cm−1 (C\\O vibration) [24,33]. As for NiHCF, the

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