Hybrid porous magnetic bentonite-chitosan beads for selective removal of radioactive cesium in water

Accepted Manuscript Title: Hybrid Porous Magnetic Bentonite-Chitosan Beads for Selective Removal of Radioactive Cesium in Water Authors: Wang Kexin, Ma Hui, Pu Shengyan, Yan Chun, Wang Miaoting, Yu Jing, Wang Xiaoke, Chu Wei, Zinchenko Anatoly PII: DOI: Reference:

S0304-3894(18)30754-4 https://doi.org/10.1016/j.jhazmat.2018.08.067 HAZMAT 19692

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

17-3-2018 19-8-2018 20-8-2018

Please cite this article as: Wang K, Ma H, Pu S, Yan C, Wang M, Yu J, Wang X, Chu W, Zinchenko A, Hybrid Porous Magnetic Bentonite-Chitosan Beads for Selective Removal of Radioactive Cesium in Water, Journal of Hazardous Materials (2018), https://doi.org/10.1016/j.jhazmat.2018.08.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hybrid Porous Magnetic Bentonite-Chitosan Beads for Selective Removal of Radioactive Cesium in Water WANG Kexina,1, MA Huia,d,1, PU Shengyana,b,, YAN Chuna, WANG Miaotinga, YU Jinga, WANG Xiaokea, CHU Weib, ZINCHENKO Anatolya,c a

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State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu

University of Technology), 1#, Dongsanlu, Erxianqiao, Chengdu 610059, Sichuan, P.R.China

b

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p

Faculty of Construction and Environment, The Hong Kong Polytechnic University, Hong

Kong 999077, P.R.China

Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan

d

Department of Plant and Environmental Sciences, University of Copenhagen,

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Thorvaldsensvej 401871 Frederiksberg, Denmark

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c

Corresponding

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author at: State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology), Chengdu 610059, PR China. Tel./fax: +86 (0) 28 8407 3253; E-mail addresses: [email protected] (S. Pu); [email protected] (A. Zinchenko) 1

Notes: The authors declare that there is no conflict of interests regarding the publication of

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this paper. K. Wang and H. Ma contributed equally to this work.

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Graphical Abstract

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•The magnetic bentonite-chitosan beads were synthesized as adsorbent for Cs+

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

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Highlights

removal.

•The porous adsorbent functioned by the synergistic effect of bentonite and

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

•Bentonite-chitosan beads capture Cs+ selectively in presence of the co-existing cations.



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

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chitosan.

•Adsorbent was recycled by 0.1 mol L-1 of Mg2+ for quantitative desorption of

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Cs+ from beads.

Abstract: Easy-to-obtain magnetic bentonite-chitosan hybrid beads (Bn-CTS) were

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prepared by immobilizing bentonite within a porous structure of chitosan beads to achieve a hybrid adsorption effect for the removal of cesium ion (Cs+) from water. The hybrid adsorbent, which had a porous structure and abundant binding sites contributed by both chitosan and bentonite, ensured superb adsorption characteristics. The paramagnetic character of the beads enabled their facile separation for recycling. The chitosan/bentonite ratio, pH and contact time were optimized to achieve the ideal 2

Cs+ efficiency, and the adsorption kinetics and isotherms were thoroughly discussed. The adsorption kinetics obeyed the pseudo-second-order model, and the best fitted equation for equilibrium data was the Langmuir isotherm model. The maximum adsorption capacity of the bentonite-chitosan beads was 57.1 mg g-1. The adsorbent had excellent selectivity towards Cs+ adsorption in the presence of abundant cations (Li+, Na+, K+ and Mg2+). The adsorbent was able to be recycled by treating the beads

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with 0.1 mol L-1 of MgCl2 to quantitatively desorb Cs+ from the beads. Overall, the magnetic bentonite-chitosan beads can be used as a highly efficient adsorbent for

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radioactive waste disposal and management.

Keywords: Cesium; bentonite; hydrogel beads; magnetic adsorbent; radioactive

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wastewater

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1. Introduction

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In March 2011, the Kanto and Northeastern regions of northern Japan were

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widely contaminated by leaching wastewater containing radioactive elements during the Fukushima Daiichi accident [1]. Wastewater containing radioactive elements

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introduced a permanent threat to the environment since all of the waste washed and taken up by contaminated water became a high-risk radiation pollution source [2].

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Among all of the radioactive contaminants, the leaked radioactive 137Cs, which has a half-life of 30.28 years [3], is the major radioactive source and has a toxic effect of

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emitting beta-particles and strong gamma rays [4]. Cs contamination remediation is urgent but challenging due to the high solubility and mobility of Cs ions in the environment their bioavailability to terrestrial and aquatic organisms [5]. In the

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environment, radioactive substance emitted direct and inevitable adverse radiation on all species, and persistent cesium could also be ingested and accumulated by organisms and bioconcentrated through the food chain, eventually posing a threat to human beings [6]. The permanent damage caused by radioactive cesium at the cellular level leads to irreparable destruction of the biological structure and functions of organisms, which may lead to cancer, genetic mutation, genetic disorders, and other 3

diseases[7, 8]. Therefore, from the perspective of environmental protection and the common interest of all life on earth, remediation of Cs+ in the environment with minimal interference to the original environment has become a particularly important issue. To date, Cs removal techniques, including adsorption/ion exchange [9, 10],

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precipitation [11], solvent extraction [12], electrochemical treatment [13] and membrane separation processes [14], have been developed. The primary wastewater treatment system at Fukushima is mainly based on Cs+ adsorption and separation

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through a reverse osmosis (RO) unit to reduce salinity before cycling back to the

reactors [15]. Specifically, the adsorption/ion exchange property possessed by clay mineral adsorbents, such as vermiculite [1], phlogopite [16], and smectite [17], is

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acknowledged to be an effective treatment technique due to its high processing

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capacity and selectivity for Cs+. Among all clay minerals, bentonite is widely used for

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Cs+ adsorption [25, 26] and adsorption of other radioactive nuclides (Ra [18], Pb [19],

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Sr [20]), due to its superior characteristics, such as low permeability, large specific surface area, high swelling ability and ion-exchange capacity, as well as colloidal

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properties [23]. In the field of adsorption techniques, natural biomass materials, such as tea waste [21], sawdust [22], marine algae [23], bacterial exopolymer [24], and

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chitosan [25], have also attracted great interest from researchers due to their low cost and biocompatibility. Their favorable adsorption capacity also makes it feasible to

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utilize biomass materials as adsorbent or adsorbent supporting materials [26]. Chitosan is regarded as one of the most effective biopolymer adsorbents due to the abundant active functional groups on its macromolecule chain [27]. However,

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previous experience and research showed that direct utilization of either bentonite or chitosan as an adsorbent was restricted in the application for Cs+ removal. The adsorption performance of chitosan showed a limited adsorption capacity, and it was found that for bentonite, it was difficult to thoroughly separate the adsorbent composites from the environment after the removal process because its submicron size is not suitable for filtration [28]. However, utilization of powdered adsorbents for 4

the treatment of contaminated water in an actual aquatic environment is also unrealistic because there is no easy way to collect adsorbents after contaminant collection [29]. Chitosan-bentonite nanoparticles have been fabricated to solve the problem of separation by introducing magnetism [30]. In spite of their high removal efficiency due to the nanometer size effect, their application was restricted due to problems such as their high price and adsorbent loss during adsorption. In this case,

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fixing bentonite on a supporting carrier with both a suitable size and favorable

adsorption capacity seems to be a feasible solution to overcome these disadvantages

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[31]. In our previous research, porous magnetic chitosan hydrogel (PMCH) beads

were developed as an adsorbent for heavy metal ion removal [27]. In addition to its outstanding adsorption capacity, the highly ordered porous structure of this size-

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controlled adsorbent also presented great potential as a framework to immobilize

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other powdery adsorbents [32].

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In the present study, powdery bentonite was immobilized onto a supporting

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matrix of porous chitosan-based beads. The preparation of the adsorbent was optimized by adjusting the weight fraction of bentonite in the beads, and the effect of

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the parameters during the removal process, including contact time, pH, initial concentration of Cs+, and the presence of co-existing cations, were systematically

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studied.

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2. Materials and methods 2.1 Materials

Chitosan (80-95% deacetylation, viscosity average molecular weight 3.0 × 105 g

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mol-1) and 133CsCl provided by Aladdin Reagent Factory (Shanghai, China). Bentonite (Na-Bentonite) was obtained from Borun Foundry Material Co., Ltd. (Henan, China). All reagents were purchased of analytical grade and used as received without further purification. Ultra–pure water prepared with a Milli-Q water purification system (U pure Corporation) was used throughout this work.

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2.2 Preparation of porous magnetic Bn-CTS beads A series of bentonite suspensions with different mass percentages was obtained by mixing 0, 0.75, 1.5, 3, 7.5 and 10.5 g of bentonite (in 40 mL of 2.0% (v/v) acetic acid) with stirring for 30 min at 1000 r min-1. A 1.5 g sample of chitosan was added to the prepared suspension with stirring for another 30 min. Then, 5 mL of a Fe3+/Fe2+

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solution (Fe3+: 0.875 mol L-1, Fe2+: 0.438 mol L-1) was added to the chitosan solution during stirring, and a homogeneous bentonite/chitosan/Fe suspension was obtained

after 30 min. Subsequently, the dark brown suspension was slowly dropped into an

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alkaline crosslinking solution (sodium citrate: 0.1 mol L-1, NaOH: 1.25 mol L-1) using

a peristaltic pump with soaking for 12 h. The obtained beads were extensively washed several times with a diluted hydrochloric acid solution and deionized water to remove

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the residual alkali and cross-linker. Eventually, gel beads with different mass ratios

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(bentonite: chitosan) of 0.5:1, 1:1, 2:1, 5:1, 7:1 and chitosan gel beads without

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bentonite (as a comparison expressed as ratio 0:1) were freeze-dried at -70℃ to 25℃

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2.3 Characterizations

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for 30 h.

2.3.1 Scanning electron microscope (SEM)

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SEM images were collected by SEM (Sigma S-3000N, Germany) at 20 kV. Samples for SEM imaging were mounted using conductive tape on specimen stub.

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The surface morphology of the Bn-CTS beads of whole sized, vertical cross section and horizontal cross section were analyzed.

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2.3.2 X-ray diffraction (XRD) XRD patterns of the pure bentonite, magnetic chitosan beads and Bn-CTS beads

(before and after adsorption) samples were collected on a X-ray diffractometer (Rigaku Corp, Japan) with Cu sealed tube radiation source (λ = 0.154 nm, 40 kV) in the range of 3° to 80°.

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2.3.3 X-ray photoelectron spectroscopy (XPS) XPS measurements were recorded on a spectrometer (Thermo Scientific Escalate 250X, USA), using an analyser pass energy of 100.0 eV. The surface elemental composition of Bn-CTS beads before and after adsorption was analyzed. All binding energies were referenced to C1s peak at 284.8 eV of the surface adventitious carbon

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to correct the shift caused by charge effect. Spectra were analyzed using XPS peak41 software.

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2.3.4 Vibrating sample magnetometry (VSM)

All samples before adsorption were dried in an oven at 70C for 6 h. Then the magnetic properties of Bn-CTS beads were measured by VSM (Quantum, USA) at

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room temperature in the magnetic field range of -7 to 7 T. The saturation

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2.3.5 Fourier transform infrared (FTIR)

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magnetization was evaluated.

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FTIR (Nicolet iS 10, USA) was used to analyze the functional groups in the pure chitosan, Bn-CTS beads and magnetic bentonite-chitosan before and after adsorption.

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The test powders were prepared by KBr disk method. The sample was ground and mixed thoroughly. Then the composite was compressed to form a thin sheet of the

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cm-1.

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sample. Finally, the adopted samples were scanned in a wavelength range 4000 to 400

2.4 Adsorption experiments In this study, 133Cs, an isotope of 137Cs [33], was chosen as the representative but

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safer targeted contaminant ion, reflecting the adsorption behavior of the radioactive ions. Batch experiments were conducted in 50 mL glass bottles with stirring (120 r min-1) at room temperature (25±1ºC) for 12 h, with a Bn-CTS bead dosage of 5 g L-1. In pH effect experiments, the initial solution pH was adjusted to 3.5, 4.5, 5.3, 7, 8.5 and 9.8 using an HCl (0.1 or 1 mol L-1) / NaOH (0.1 or 1 mol L-1) solution. Batch adsorption experiments with different contact times ranging from 0 to 24 h were 7

conducted to investigate the Cs+ adsorption kinetics and evaluate the optimum adsorption contact time. The adsorption isotherm study for Cs+ removal with an initial concentration of Cs+ ranging from 50 to 500 mg L-1 was designed to describe the adsorption interaction behavior between the adsorbent and Cs+. The interference due to coexisting ions was studied by conducting a Cs+ adsorption experiment with 0.001 mol L-1 of Mg2+ to 0.001 mol L-1 K+, 0.001 mol L-1 Li+. The interference due to Na+

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was evaluated at concentrations of 0.001, 0.01, 0.1 and 0.4 mol L-1. The concentration of Cs+ was measured by a flame atomic absorption spectrometer (Ggx-9, Haiguang,

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China). The calibration curve equation was y = 0.0347x + 0.0035, R2 = 0.9999 (Fig. S2, Supporting Information); operating current: 10 mA, sensitive line: 852.1 nm, LOD: 0.001 μg mL-1, LOQ: 0.01 μg mL-1.

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2.5 Cycling experiment

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After adsorption of Cs+ from a 0.001 mol L-1 solution for 12 h, Cs+ loaded Bn-

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CTS beads were transferred to 20 mL of a 0.1 mol L-1 Mg2+ solution stirring at 120 r

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min-1 for 24 h at room temperature. The Mg2+ solution continued to cycle with a fresh

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Mg2+ solution until Cs+ stopped releasing from the beads. 2.6 Experimental calculation

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The removal efficiency (R, %) and corresponding equilibrium adsorption capacities (Qe, mg g-1) of the hydrogels were calculated using equations (1) and (2).

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The distribution coefficient Kd (mL g-1), a mass-weighted partition coefficient between the solid phase and liquid supernatant that reflects the selectivity for target

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ions [34], was calculated according to the equation (3): 𝑄𝑒 = 𝑅=

𝐶0 −𝐶𝑒 1000

𝐶0 −𝐶𝑡 𝐶0

𝐾𝑑 =

𝑉

×𝑀

(1)

× 100%

(2)

𝐶0 −𝐶𝑡 𝐶𝑡

𝑉

×𝑀

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(3)

where C0 (mg L-1) and Ce (mg L-1) are the initial and final concentration of Cs+, respectively; V (mL) is the volume of the solution containing metal ions; and M (g) is the weight of the adsorbent. 3. Results and discussion

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3.1. Formation mechanism and characterization of Bn-CTS beads The schematic diagram of the synthesis of the Bn-CTS composite beads is shown in Fig. 1. The bentonite/chitosan/Fe solution was added drop-wise to an alkaline

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sodium citrate solution that was aged for 12 h to form Bn-CTS beads. The formation of bentonite-chitosan hydrogel beads is based on a combination of the breath figure phenomenon with a co-precipitation reaction [32]. When hydroxide ions of NaOH

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diffused in a bentonite/chitosan/Fe mixture and were neutralized with acetic acid, the

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interaction between chitosan/bentonite and iron provided sufficient interfacial tension,

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which allowed hexagonal water droplet matrices to be ordered at the interface. When

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water droplets were away from the interface, a multi-layered honeycomb structure formed [32] and bentonite powder was immobilized within the chitosan. Chitosan

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intercalates into the interlayers rather than the surface of bentonite [35], and both the chelating reaction of chitosan and ion exchange affected the function of bentonite

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during the adsorption process.

Scanning electron microscopy (SEM) was used to visualize the morphology and

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internal structure of the Bn-CTS beads containing bentonite-to-chitosan. As shown in Fig. 2a, the spherical beads were uniformly shaped with a diameter of approximately 1.8 mm. The SEM image of the cross section of Bn-CTS beads showed they had a

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multi-layer structure (Fig. 2b). The SEM image in Fig. 2c shows a well-ordered honeycomb structure and that each layer contained pores with a diameter of approximately of approximately 7.3 μm. Notably, the average diameter of the BnCTS beads was smaller than that of PMCH beads and the pore size of the PMCH beads was approximately 11.62 μm (Fig. S1 in Supporting Information). The large

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surface area provided by the porous structure of Bn-CTS beads exposed more available active sites during the adsorption process for the capture of Cs+. Bentonite powder was immobilized into chitosan beads, and the binding of bentonite to the chitosan matrix prevented the loss of bentonite powder during application. Furthermore, to achieve separability, magnetism was introduced into the

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adsorbent during co-precipitation. The magnetism of Bn-CTS beads at different ratios of bentonite was measured by VSM. As shown in Fig. 3, the saturation magnetization of beads with bentonite-to-chitosan ratios of 0:1, 0.5:1, 1:1, 2:1, 5:1 and 7:1 were

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15.15, 10.05, 4.57, 1.24, 1.06 and 0.40 emu g-1, respectively. Apparently, the

magnetic properties of the beads gradually decreased with the increasing bentonite fraction. The superparamagnetic behavior (zero coercive field and zero remnant

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magnetization) was observed for all of the studied samples. Therefore, well-formed

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magnetic Bn-CTS could be collected and easily separated by an external magnetic

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field, providing a potential advantage for bead separation, recovery and reuse during

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practical treatment by manipulating an external magnetic field. To further investigate the mechanism of Cs+ adsorption, Fourier-transform

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infrared (FTIR) spectra of (a) pure chitosan, (b) non-magnetic Bn-CTS, (c) Bn-CTS beads and (d) Bn-CTS beads after equilibrium with a 200 mg L-1 Cs+ ion solution

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were analyzed to identify the active site of the adsorbent during the adsorption procedure. The strong characteristic absorption peak of bentonite at 1035 cm-1 among

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samples (b), (c) and (d) is due to the Si-O-Si stretching vibration [36], and the vibration peaks of Si-O-Si bending at 792 cm-1 [37] and 464 cm-1 [36], indicate that the bentonite functioned profoundly within the hydride adsorbent. The stretching peak

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at 3627 cm-1 represents the free -OH of the pristine bentonite surface [38]. The strong stretching vibration at 3444 cm-1 represents the typical –OH [37] and –NH2 [39] of the adsorbent. The symmetric stretching vibration peak at 2872 cm-1 is caused by CH3 stretching [40], while the absorption band at 1645 cm-1 represents the typical bending vibration band of water and the stretching vibration of carbonyl (C=O). A comparison of the spectra between the different samples indicates that most of the functional 10

groups of chitosan and bentonite remained in the adsorbent before and after Cs+ adsorption, which indicated that the chemical composite of the adsorbent was stable. As shown in Fig. 4e for magnetic Bn-CTS beads before and after equilibrium with Cs+ in the range of 3750-3250 cm-1, the -NH2 peak became weaker after Cs+ adsorption, which demonstrated the critical role of the functional group.

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3.2. Removal of Cs+ by Bn-CTS beads 3.2.1. Cs+ adsorption kinetics and mechanism

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Cs+ adsorption kinetics was studied by measuring the time dependence of the

amount of Cs+ in a solution containing 5 g L-1 of Bn-CTS beads with a bentonite-tochitosan ratio of 5:1. Fig. 5a shows that the adsorbed amount of Cs+ ions significantly

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increased within the first 3 hours after Bn-CTS bead addition. A further incubation for

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3 hours resulted in an additional slight increase of Cs+ adsorption after reaching

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adsorption saturation. The equations for kinetic models are listed in Supporting

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Information (Text S4).

Comparison of the Qe values calculated from the models, and the experimental Qe values are

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also given.

The parameters K1, K2 and Qe calculated from the slopes and intercepts of the

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fitting lines (Fig. S2 in Supporting Information) are listed in Table 1. Cs+ adsorption onto the Bn-CTS beads is better fitted by pseudo-second-order kinetics, and the

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experimental value of Qe (36.5 mg g-1) is similar to the calculated value for this model. This result indicates that Cs+ adsorption on porous Bn-CTS beads occurs via chemical adsorption, suggesting that the large number of functional groups exposed

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by the large specific area of the porous composite play a critical role in adsorption. 3.2.2 Effect of pH Bn-CTS presented excellent stability at pH values ranging from 3 to 10. The bentonite powder was immobilized within the magnetic chitosan network, and there were no significant physical or chemical changes in the adsorbent with pH flocculation due to the crosslinking effect. The effect of pH on Cs+ adsorption by Bn11

CTS beads is shown in Fig. 5b. The adsorption efficiency changed slightly from 90% to 93% over a wide range of pH values from 3.5 to 9.8. The highest removal was achieved at a certain pH, but was hardly distinguishable from the other conditions, and the distribution coefficients (Kd) were induced to illustrate the effect of pH on the adsorption [34] because Eq. (3) shows that a small change in Ct can lead to a large difference in Kd. Similar to the results of Dahu Ding et al.[34], the highest distribution

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coefficient was measured at pH 8.5, and the higher distribution coefficients obtained under alkaline conditions compared to a low pH were consistent with the pH results.

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The lower distribution coefficients under acidic conditions may be due to competitive adsorption of the abundant H+ in solution. Despite a difference in the distribution coefficients, the Bn-CTS beads showed a sufficient removal performance (over 90%)

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the treatment of wastewater with varying acidity.

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over the whole studied pH range, indicating that this adsorbent would be suitable for

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3.2.3 Hybrid effect of the bentonite–chitosan composition on Cs+ adsorption

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To clarify the effect of the hybrid bead composition on the adsorbtion properties of the beads, the weight ratio between bentonite and chitosan in the beads was varied

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between 0:1 to 7:1, corresponding to weight percentages of bentonite between 0 and 87.5%. The weight percentage of the pure bentonite sample was taken as 100%. As

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shown in Fig. 5c, the Cs+ removal rate and Kd increased as the bentonite concentration increased. Without the influence of chitosan, the graph shows a linear dependence.

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Otherwise, the effect of chitosan can be understood from the shape of the graph. As observed from Fig. 5c, Cs+ removal does not show a simple linear dependence, as an inflection point exists at a 7:1 ratio and the adsorption of Cs+ with pure bentonite is

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less effective than that of Bn-CTS beads. It was observed that bentonite and chitosan beads both increased the adsorption rate of Cs+ and that bentonite was the major contributor to the improved adsorption efficiency. As shown in the X-ray diffraction data, obvious peaks of montmorillonite and quartz were detected in the bentonite and Bn-CTS bead (before and after adsorption) samples, indicating the main composition of the Bn-CTS beads is montmorillonite 12

and quartz. Fig. 6 shows peaks at 5.48° for bentonite and 5.08° for Bn-CTS beads, indicating that the d (001) spacing in natural bentonite was 1.61 nm. However, after incorporation of bentonite into the porous chitosan beads to form Bn-CTS beads, chitosan was inserted between the interlayers of bentonite to increase the d (001) spacing to 1.74 nm. In addition, diffraction of the (311) crystal planes of Fe3O4 can be observed in PMCH beads and Bn-CTS beads (before and after adsorption) [30]. We

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hence conclude that the Bn-CTS bead composite was successfully formed. After the

adsorption of Cs+, the layer spacing d (001) of the Bn-CTS beads changed from 1.74

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to 1.88 nm. In addition, the adsorption of Cs+ induced an important reduction in the peak intensity. These changes in the position and intensity may be due to the

exchange of the interlayer hydration cations of the bentonite [41, 42]. From the

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results, one can conclude that ion exchange is the main behavior of Cs+ adsorption

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onto Bn-CTS beads.

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To gather further evidence regarding the observed hybrid effect of bentonite and

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chitosan on Cs+ adsorption, the elemental composition of the beads was compared by XPS before and after Cs+ adsorption. The spectrum shown in Fig. 7a shows the

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presence of Al, Si, C, N, O, Fe and Na in both samples and the appearance of Cs after Cs+ adsorption by the beads. The characteristic peaks Cs 3d at Cs 3d3/2 = 738.9 eV

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and Cs 3d5/2 = 724.9 eV indicate that Cs is present on the surfaces of the Bn-CTS beads or in the interlayers [43, 44]. Slight changes were found in the spectra of N1s

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(Fig. 7c): the increase in the peak after adsorption at 401.8 eV indicates the production of NH-metal complexes because the increase in binding energy may be due to the lone pair of electrons provided by N, resulting in a decrease in the electron

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cloud density [45]. The typical Na1s peak with bonding energy at 1071.33 eV for magnetic porous beads was obviously reduced after Cs+ adsorption, indicating that Na+ ions in the bentonite were exchanged with Cs+. 3.2.4 Adsorption isotherms To investigate the effects of bentonite on the Cs+ sorption process, we compared Bn-CTS beads and PMCH beads regarding their Cs+ adsorption capacity (Fig. 5d) 13

with the initial concentrations of CsCl ranging from 50 to 500 mg·L-1. To quantify the adsorption capacity of the Bn-CTS beads, Langmuir (Fig. 5d), Freundlich (Fig. 5d), Tempkin (Figure S5a, Supporting Information) and Dubinin-Radushkevich (Figure S5b, Supporting Information) methods were used to interpret the adsorption process. The equations for these four isotherm models are listed in Text S6 of the Supporting

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Information. For the four isotherm models that we studied, the data accurately obeyed the

Langmuir model with the highest R2 value, which can be related to the homogeneity

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of the Bn-CTS bead surface and monolayer coverage of Cs+ onto the adsorbent.

Langmuir isotherm fitting gave the maximum adsorption capacity of Bn-CTS beads, i.e., 57.084 mg g-1, which was 1.16 times higher than that of pure bentonite (49.358

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mg g-1) and 8.75-fold greater than that of PMCH beads (6.524 mg g-1). The values of

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n in the Freundlich equation over the range of 1 to 10 showed that adsorption was

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favorable [46]. As shown in Table 2, the n values of all three samples were in this

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range and the n value of Bn-CTS beads was higher than that of bentonite above PMCH beads, illustrating that Bn-CTS beads had good adsorption characteristics

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towards Cs+. Moreover, the Tempkin model had better applicability than the Freundlish model, implying that the binding energy sites of the adsorbent were

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uniform. In the Dubinin-Radushkevich model, the maximum adsorption capacities, 49.552 mg/g (Bn-CTS beads), 41.997 (bentonite), and 4.993 (PMCH beads), were

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lower than the Langmuir adsorption capacities. All of the E values were found to be greater than 16 kJ mol-1, indicating that the adsorption process for Cs+ was predominately chemisorption, which is in agreement with the adsorption kinetics.

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Combining the characterization results and the adsorption isotherm analysis, the

possible reactions on Bn-CTS beads could be concluded to be as follows: ̅̅̅̅ Na + Cs+ ⇋ ̅̅̅ Cs + Na+

(4)

R − NH2 + Cs+ → R − NH2 Cs +

(5)

R − NH2 + H + → R − NH3+

(6)

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R − NH3+ + Cs + → R − NH2 Cs+ + H +

(7)

Equation (4) shows that bentonite adsorbs Cs+ by ion exchange, where ̅̅̅̅ Na and ̅̅̅ Cs refer to sodium and cesium, respectively, on the solid and M and Cs refer to solution species. In addition, the chitosan composites adsorb Cs+ prior to H+ due to electrostatic interactions between N atoms and Cs+ [25]. The functional groups in the adsorbent play an important role in Cs+ adsorption by affecting the ion exchange

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process, and the Cs+ removal procedure was dominated by the strong ion exchange of the bentonite interlayer.

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3.2.5 Interference of coexisting cations

The interference effect of the coexisting cations on Cs+ adsorption to Bn-CTS

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beads is shown in Fig. 8. Cs+ adsorption based on the mechanism of cation exchange contributed by bentonite and the chelating reaction by chitosan, as well as the

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adsorption capacity of Cs+, decreased with the increase in ionic strength, as adjusted

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by the addition of NaCl. The competition between Na+ ions with Cs+ ions for binding

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to the adsorbent showed adverse effects during the removal process. The effect of other cations, including Mg2+, Na+, and K+, is also compared in Fig. 8a, indicating

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that the sorption of Cs+ on bentonite was slightly dependent on background cations. The interference of the cation affects the adsorption capacity in the sequence Li+ >

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Na+ > K+ > Mg2+. Generally, ions with a higher charge density can bind to larger water clusters [47], and the order of hydrated ionic radii follows the sequence

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Mg2+>Li+>Na+>K+>Cs+. Unlike monovalent cations, Mg2+ can be adsorbed more easily due to its stronger positive charge. For ions with the same charge, adsorption can be achieved more easily with ions with smaller hydration radii [48, 49].

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Therefore, the competitive adsorption capacity of coexisting ions decreases in the order Mg2+ > K+ > Na+ > Li+, consistent with the experimental results from the interference of coexisting cations. The divalent cations (Mg2+) desorbed more Cs+ than monovalent Li+, Na+, and K+ cations from bentonite clay fractions. The Mg2+ solution was selected as the ideal 15

desorption reagent in the cycling experiment due to its high competitive effect. Cs+ was quantitatively desorbed after 5 cycles of desorption treatment with 0.1 mol L-1 Mg2+ (Fig. 8b). In addition, with increasing numbers of treatment repetitions, a decrease in the desorption yield in subsequent sequential operations is shown in Fig. 8b, indicating a gradually increased resistance of Cs desorption due to the steric effect. The Cs+ ions near the central core area of the bentonite obstructed Cs+

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interlayer diffusion for desorption in subsequent sequential extractions. Although the desorption efficiency presented a slight decrease due to interlayer collapse, Bn-CTS

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still showed a high adsorption capacity after 5 cycles of regeneration. After 5 cycles of treatment, a 95–100% removal of saturated Cs+ was achieved.

The comparison between Bn-CTS beads and other similar adsorbents used for

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Cs+ extraction is shown in Table 3. Bn-CTS beads showed a good adsorption

N

capacity, which might be contributed by their large specific surface area. Chitosan-

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grafted magnetic bentonite showed higher adsorption characteristics, likely due to the

M

higher purity of the raw material bentonite used in this research, but the fabrication methods and experimental conditions investigated in our research were simpler,

ED

cheaper and more environmentally friendly than those used in other studies. The comparison provides context on the use of the potential adsorbent in real radioactive

PT

liquid waste treatment. However, the maximum adsorption amount also depends on the Cs concentration, solution acidity, adsorbent functionality and nature of coexisting

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ions.

“——”Not mentioned in the literature.

A

4. Conclusions

In this study, magnetic Bn-CTS beads that serve as an effective adsorbent for Cs+

removal were prepared by immobilizing bentonite powder onto a chitosan matrix during in situ co-precipitation. The selective adsorption capacity of chitosan and bentonite were strengthened by the hybrid effect. The removal of Cs+ by the Bn-CTS 16

beads was based on both amine chelation and cation exchange, which were contributed by chitosan and bentonite, respectively. The adsorption behavior and mechanism were analyzed by fitting isotherms and kinetic data. The best fitted Langmuir isotherm model and pseudo-second-order kinetic equation indicated that Cs+ removal obeyed the monolayer chemical adsorption process and that the

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maximum Cs+ adsorption capacity of beads with bentonite and chitosan with a mass ratio of 5:1 was 57.084 mg g-1. The adsorbent retained a good adsorption ability for

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Cs+ within a wide pH range, from 3.5 to 9.8, in the presence of Li+, Na+, K+ and Mg2+ cations. Bn-CTS showed excellent reusability after 5 cycles, suggesting that the immobilized bentonite porous material has important significance and prospects for

N

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the removal of Cs+.

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Acknowledgments

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This work was supported by the National Natural Science Foundation of China

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(41772264) and the Research Fund of State Key Laboratory of Geohazard Prevention

References

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IP T SC R U N

A

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M

A

Fig. 1. Schematic diagram for the fabrication of Bn-CTS beads and the removal of Cs+ from aqueous solution.

21

Fig. 2. SEM images of Bn-CTS beads with a bentonite-to-chitosan ratio of 5:1. (a) SEM image of a single bead, inset: photograph of hybrid Bn-CTS beads. (b-c) SEM image of the

M

A

N

U

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Bn-CTS bead cross-section at a different magnification.

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Fig. 3. Saturation magnetization of beads with bentonite-to-chitosan ratios of 0:1, 0.5:1, 1:1, 2:1, 5:1 and 7:1. The insets show the hysteresis curve of Bn-CTS beads with different mass ratios of bentonite to chitosan and schematic images of the separation of Bn-CTS beads with

A

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bentonite-to-chitosan ratios of 1:1 under a magnetic field.

22

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Fig. 4. FTIR spectra of (a) pure chitosan, (b) non-magnetic Bn-CTS beads, (c) Bn-CTS beads

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(d) Bn-CTS beads after equilibrium with Cs+ and (e) magnified fragments of Bn-CTS beads

A

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ED

M

A

N

spectra in the range of 3750-3250 cm-1 recorded before and after equilibrium with Cs+.

23

Fig. 5. (a) Kinetic curve of Cs+ adsorption by Bn-CTS beads with a bentonite-to-chitosan ratio 5:1 at T = 25±1ºC, msorbent/Vsolvent = 5 g L-1, [Cs+]initial = 200 mg L-1, contact time 20 h. (b) Variation of the Cs+ removal efficiency and distribution coefficient of Cs+ adsorption by BnCTS beads as a function of the solution pH at T = 25±1ºC, msorbent/Vsolvent = 5 g L-1, [Cs+]initial = 200 mg L-1, contact time 12 h. (c) Effect of the weight fraction of bentonite (constant total weight of adsorbent) on removal efficiency of Cs+ at T = 25±1ºC, msorbent/Vsolvent = 5 g L-1, and [Cs+]initial = 200 mg L-1, contact time 12 h. (d) Adsorption isotherms of Cs+ uptake by Bn-CTS

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ED

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A

N

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beads, bentonite, and PMCH beads at T = 25±1ºC, msorbent/Vsolvent = 5 g L-1, [Cs+]initial = 50, 100, 200, 300, 400, 500 mg L-1, contact time 12 h.

A

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Fig. 6. XRD curves of bentonite, PMCH beads and Bn-CTS beads (before and after adsorption).

24

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A

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M

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Fig. 7. XPS spectra of beads before and after Cs+ adsorption. (a) Survey scan, (b) Cs 3d spectrum, (c) N 1s spectrum, and (d) Na 1s spectrum.

25

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Fig. 8. (a) Effect of coexisting Mg2+, K+, Li+, and Na+ cations on the adsorption of Cs+ by Bn-CTS beads with bentonite-to-chitosan ratio 5:1; (b) Desorption of Cs+ by treatment with

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0.1 mol L-1 solution of Mg2+ over five cycles. T = 25±1ºC, msorbent/Vsolvent = 5 g L-1, [Cs+]initial

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= 0.001 mol L-1, [Mg2+]initial = 0.1 mol L-1.

Table 1

+

A

Pseudo-first-order and pseudo-second-order constants and R2 values for Cs adsorption

Qexp(mg g-1) 36.65

Pseudo-first-order

Pseudo-second-order

K1 (h-1)

Qcal (mg g-1)

R2

3.20

35.36

0.9736 11.49

SD

26

K2 (h-1)

Qcal (mg g-1)

R2

0.24

36.50

0.9998 0.15

SD

Table 2 Isotherm parameters for the adsorption of Cs+ ions

Freundlich

Tempkin

Bentonite 49.358 0.052 0.972 2.281 10.157 3.513 0.936 2.161 8.196 1.226 0.966 1.833 22.191 41.997 0.150 0.974 0.372

PMCH beads 6.524 0.012 0.993 0.135 0.697 2.891 0.954 0.157 1.406 0.124 0.990 0.943 223.667 4.993 0.047 0.882 0.633

N

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Dubinin- Radushkevich

Bn-CTS beads 57.084 0.090 0.991 1.096 16.689 4.303 0.877 4.875 9.741 1.671 0.947 1.440 7.566 49.552 0.257 0.920 0.41

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Langmuir

Isotherm constants Qm(mg g-1) KL R2 SD KF n R2 SD β Kt R2 SD β Qm (mg g-1) E (kJ mol-1) R2 SD

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Isotherms

A

Table 3

+

M

Comparison of the maximum adsorption capacities of Cs onto same class of adsorbents.

Composite methods

Magnetic chitosan

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Adsorbent

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Chitosan-grafted magnetic bentonite(CS-g-MB)

A

Phosphate-modified montmorillonite clay

Cross-linked with glutaraldehyde Hydrothermal process for MB； Plasma-induced method for CS-g-MB

Phosphate-modified

Montmorillonite-iron oxide composite

Montmorillonite coated by iron oxides

Ammonium-pillared montmorillonite/Fe3O4 composite

Co-precipitation and situ aggregation

Ammonium-pillared montmorillonite-CoFe3O4

Co-precipitation method for 27

Experimental condition Stirred for 3 h At 180°C for 10 h； low-temperature plasma-induced grafting procedure in Ar atmosphere Washed with distilled water at 60°C; at room temperature for 24 h Heated solution at 70°C; in a muffle furnace at 500°C for 4 h Stirred for 1.5 h at 80°C under nitrogen atmosphere Stirred for 2.5 h at 80°C under a

Qmax (mg g1) 3.9

Reference [25]

160.93

[30]

57.0

[50]

52.6

[51]

27.5

[52]

86.5

[53]

Ca-Mt Ethyl-Mt

—— Ethylamine-modified

Magnetic bentonitechitosan beads

In situ coprecipitation and cross-linking

nitrogen atmosphere for CoFe3O4; stirred for 12 h at room temperature for NH4-MMT —— Shook the mixed suspension in a water bath shaker at 30°C for 24 h Stirred for 1.5h at room temperature and soaked for 12 h

60.0 80.3

[54]

57.1

This study

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CoFe3O4；Ion exchange method for NH4-MMT；Crosslinking method for MCCA

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M

A

N

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composite caged in calcium alginate beads (MCCA)

28