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One-pot preparation of NaA zeolite microspheres for highly selective and continuous removal of Sr(II) from aqueous solution Kaituo Wang, Fan Wang, Fan Chen, Xuemin Cui, Yuezhou Wei, and Lin Shao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05349 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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ACS Sustainable Chemistry & Engineering

One-pot preparation of NaA zeolite microspheres for highly selective and continuous removal of Sr(II) from aqueous solution Kaituo Wang*†, Fan Wang†, Fan Chen†, Xuemin Cui ‡, Yuezhou Wei†, Lin Shao†, Meihua Yu† † School of Resources, Environment and Materials, Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University, Daxue Road 100, Nanning, Guangxi 530004, P.R.China

‡ School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Daxue Road 100, Nanning, Guangxi 530004, P.R.China *Corresponding author E-mail: [email protected]; Tel and Fax: +86-0771-3810975

Abstract: This paper describes synthesized method and adsorption properties of geopolymer microspheres

toward

strontium

ions;

firstly,

it

was

prepared

by

dispersion-pelletizing-solidification (DPS) method followed transformed into NaA zeolite microspheres (about 100 μm) through an in-situ heat curing process. The adsorption experiments were investigated and the experimental data were fitted well by the pseudo-second-order kinetic model and Freundlich isotherm model. Loading experiments were performed by bath and column process techniques. The maximum adsorption capacity in batch process assigned to be 106.28 mg/g, the zeolite microspheres adsorption strontium ions reached adsorption equilibrium approximately in 15 min. In the dynamic column, the most suitable flow rate (4 mL/min), this found to be higher compared with other sorbents with the same particle size. Moreover, the zeolite microspheres has a good dynamic separation effect, the concentration of the outlet Sr(II) ions from the column began to rise after 18 h with bed height 1.5 cm. The competitive adsorption capabilities are investigated and have the following order Na+ ＜Mg2+ ＜K+ ＜Ca2+, indicating that this adsorbent has a good adsorption effect in the real seawater. Through the analysis of the solution after adsorption, the process is not only chemical adsorption but also ion exchange. The used adsorbents could be easily regenerated using 0.05 mol/L EDTA-2Na solution. This results showed that NaA zeolite microspheres are convenient and low-cost adsorbents for the removal of Sr(Ⅱ) from liquid wastes.

Keywords: metakaolin; geopolymers; zeolite; microspheres; strontium Introduction Nuclear radiation pollution is more and more serious with the rapid development of the world

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science and technology. With the energy shortage in developing countries, many of them have started to build nuclear power plants [1-5]. However, these plants also produce large amounts of liquid radioactive wastes (LRW), which seriously affect the environment and human body. In nuclear power plant accidents, it produce more serious damage as in 2011 of Fukushima-Daiichi nuclear power plant. Strontium is a common radionclide of LRW; it has a long half-life of 29 years and sustainable release beta ray [6]. Meanwhile, since the strontium and calcium ion radius are very close, strontium can partially replace calcium in the human body, leading to teratogenic, cancerigenic, genetic mutations and threat to human health [7]. Therefore, so many research focused on the removal of radioactive Sr(II) from LRW. So far, a lot of methods and suitable processes have been developed to efficiently remove radioactive ions from LRW. These methods included chemical precipitation and flocculation [8], ion exchange [9], membrane process [10-12], solvent extraction [13], and adsorption [14]. Considering the simplicity of the operation, effectiveness and low cost, adsorption was found to be the more common way used between the methods mentioned above. Moreover, various natural and synthetic adsorbents have been used for adsorption of strontium in LRW. These adsorbent materials included natural aluminosilicate minerals [15-17], zeolites [18,19], metal oxides [20-23], MOFs [24] and titanate-based materials [25,26]. Since most of these materials are powder, it is difficult to separate them in practical application process. Many researchers have already prepared the adsorption materials into different shapes or load on other materials, but this increased the cost and operation difficulty. The one-pot preparation method only need to mix the raw materials in a certain proportion, without separation or load processes in the preparation process and directly get the final product. Compared with the traditional preparation method, it avoids the tedious operation process, which is economical and environmentally friendly. Geopolymers are a kind of gel material with three-dimensional network consisting of structural aluminum silicate inorganic polymer [27]; their excellent mechanical properties include high temperature stability [28], acid resistance [29], low temperature and vacuum stability [30,31]. Recently, there are many researches used such geopolymers as adsorbent of heavy metals; this is due to have a suitable porosity and surface area [32-35]. However, very few studies were reported on the removal of radioactive ions. NaA zeolite microspheres can be directly packed in column beds, and easily retrieved. In this


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work, the one-pot preparation of highly porosity NaA zeolite microspheres was used in a continuous column process for LRW, the filtering process is highly efficient and continuous. The process of preparation is simple, environment-friendly and low cost. Adsorption properties have been evaluated through batch and column experiments.

Experimental Materials The metakaolin used in this experiments was produced by the calcination of kaolin (obtained from Beihai in Guangxi Province, P.R. China) after treatment at 800 ℃ for 2 h. The chemical composition of metakaolin was measured by X-ray Fluorescence to be found (mass%):SiO2=52.89, Al2O3=43.50, K2O=1.80, Fe2O3=1.38, MgO=0.43. The sodium hydroxide and HCl were produced by Xilong Chemical Company. Sr(II) ion solution was prepared by dissolving SrCl2 • 6H2O (AR/Sinopharm Chemical Reagent Co., Ltd) in distilled water. NaCl, KCl, MgCl2 and CaCl2 were obtained from Guangdong Guanghua Sci-Tech Co., Ltd. Ethyenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na) was obtained from Beijing J ＆ K Sci-Tech Co., Ltd. Real seawater (Beihai, Guangxi, China) and natural surface water (Nanning, Guangxi, China) were applied to study the Sr(II) elimination behaviors from contaminated water.

Preparation of NaA zeolite microspheres Figure 1 shows the NaA zeolite microspheres preparation. This process performed by DPS method and in-situ heat curing technology. The slurry was dispersed into particles through high-speed rotating impeller, and then formed spherical droplets under the surface tension of silicone oil. Finally, the spherical droplets solidified into microspheres in 80 ℃ of silicone oil and converted into NaA zeolite microspheres over a period of time at 90 ℃. Because the metakaolin-based geopolymer was found to have a fast solidification characteristic under certain temperature [36].



Figure 1. Schematic diagram of the preparation porous zeolite microspheres and Fixed-bed column adsorption device.

The preparation process of NaA zeolite microspheres was mainly divided into three steps. First step (the preparation of slurry): on the basis of preliminary experimentation, mix proportioning of the specimen number 1-3 was made with a Na2O/Al2O3 molar ratio=0.7-0.9 (H2O/Na2O molar ratio=16.5), and mix proportioning of the specimen number 4-6 was made with H2O/Na2O molar ratio=15.5-17.5 (Na2O/Al2O3 molar ratio=0.8). Details of the compositions are presented in Table 1S. Figure 1S and 2S, shows that the metakaolin-based geopolymer slurry optimal experimental parameters were as follow Na2O/Al2O3 molar ratio=0.8, H2O/Na2O molar ratio=16.5; the slurry was mixed intensively for 1 min using an electrical mixer at 1000 rpm under room temperature. Second step: slurry was quickly loaded into syringes and continuously injected into silicone oil with mechanical dispersion using an electrical mixer at 600 rpm under an 80 ℃ water bath; prepared slurry microspheres immediately solidified in the subsidence process of silicone oil under high temperature, and then geopolymer microspheres were cured for 12 h at 90 ℃, NaA zeolite conversion mechanism was shown in Figure 2. This process mainly includes


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depolymerization of the solid aluminosilicate source, reorganization of the aluminate and silicate species, and through polymerization transformed for NaA zeolite. Third step: NaA zeolite microspheres were collected through vacuum filtration, then cleaned, dried and sieved (size distribution of NaA zeolite microspheres as shown in Figure 1), and the microspheres of particle size in 75-150 μm were obtained.

Figure 2. The NaA zeolite conversion mechanism.

Characterizations X-ray diffraction analysis (XRD) was performed on powdered samples using a Rigaku MiniFlex 600 instrument with Ni-filtered and Cu (Kα) radiation (λ = 1.392 Å), operated at 40 kV and 15 mA with a step size of 0.02°, a dwell time 0.5 s and a scanning rate 5°/min from 10 to 70° (2θ). Fourier transform infrared (FT-IR) spectroscopy of NaA zeolite microspheres were tested by Shimadzu IRTracer-100 FT-IR spectrometer, Japan, all the samples were dried at 100 ℃ before analyzed and data was recorded in the range of 400-4000 cm-1 using a KBr disk containing 1% finely ground samples. NaA zeolite microspheres surface morphological characteristica were observed using scanning electron microscopy (SEM, Phenom ProX, Holland, 15.0 kV accelerating voltage), transmission electron microscopy (TEM, Tecnai G2 F30 S-TWIN/X-MAX 80, USA, 300.0 kV accelerating voltage) and chemical composition was analyzed by an attached energy



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dispersive X-ray spectroscopy detector (EDX, the Elemental Identification software available on the Phenom ProX, Holland). Prior to imaging of samples in SEM and TEM, dry samples were placed onto aluminum studs with carbon tape and sputter coated with gold using an ion sputtering instrument (SCB-12 Zhongke, China). The BET specific surface area test and pore size distributions were performed on a Micromeritics TriStar II (USA) system at 77 K, and using the BET equation with N2 gas and desorption branches of isotherms based on BJH methods, respectively. BET samples were swept for 4 h at 120 ℃ with N2 gas before testing. The concentrations of Sr(II), Ca2+ and Mg2+ ion were tested by inductively coupled plasma atomic emission spectroscopy (ICPS-7510, Shimadzu, Japan) under a plasma gas flow of 15 L/min and a nebulizer gas flow of 0.6 L/min. The concentrations of K+ and Na+ ion were investigated by atomic absorption spectrophotometer (AA-7000, Shimadzu, Japan) under a C2H2 gas flow of 1.8 L/min and an air gas flow of 15 L/min. The particle size distribution of NaA zeolite microspheres was determined using a laser diffraction particle size analyzer (OMEC LS603, China), and taken the average of 5 times. 0.5 g NaA zeolite microspheres with 100 mL distilled water into the beaker and ultrasound assisted dispersion 5 minutes for preparation suspensions. The zeta potential of NaA zeolite microspheres at different pH was measured using a Zetasizer (Malvern NANO ZS90, UK). 0.1 g NaA zeolite microspheres with distilled water in 100 mL volumetric flasks for preparation suspensions (0.1 wt.%). The suspensions of pH values in the range 2-8 were prepared by adjusting with 0.1 M HCl and 0.1 M NaOH.

Batch adsorption studies Effect of contact time: In this procedure, 0.1 g of NaA zeolite microspheres was added to 100 mL prepared synthetic solutions of SrCl2 with initial concentrations of 50 mg/L, the pH of solution was 6 at 25 ℃. Adsorption capacity (Qe) and removal efficiency (R) were calculated by the following mass balance equations.

R（%）

Qe 

Ci  C e  100 Ci

V Ci  Ce  m

Desorption efficiency 

Ce  V  100% 1000  Qe  m


(1)

(2) (3)

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where Ci and Ce are the initial and equilibrium concentrations of strontium ions in solution (mg/L), respectively. V is solution volume (L) and m is adsorbent mass (g). Effect of pH: Different solutions of Sr(II) were prepared at pH range (2-8) using 0.1 M HCl and 0.1 M NaOH, 100 mL of Sr(II) solutions (50 mg/L) was mixed with 0.1 g of NaA zeolite microspheres for 0.5 h at 25 ℃. Effect of adsorbent dosage: For the determination of the optimal adsorbent dosage, a series of adsorption tests were performed by varying the amount of adsorbent dosage (0.03 g, 0.05g, 0.065 g, 0.1 g, 0.135 g, 0.17 g, 0.205 g, 0.24 g, 0.275 g, 0.31 g per 100 mL of Sr(II) ions solutions) and mixed for 0.5 h at pH 6. Before testing, the solutions were filtered using NYLONM (diameter: 13 mm, pore size: 0.45 μm). Effect of initial concentration of Sr(II) solutions: The effect of the initial concentration of Sr(II) was assessed through changing the initial concentration (10-130 mg/L, interval 20 mg/L); 0.05 g of adsorbents was added to 100 mL solutions and the contact time was 0.5 h at 25 ℃. Column separation experiments: Column separation experiments were carried out using a glass column (1 cm in diameter and 5 cm in length) as a simulation of industrial adsorption procedures. Column was packed with adsorbent as shown in Figure 3S. The initial Sr(II) concentration of 50 mg/L, pH 6 and 25 ℃ were used as experimental conditions. Different bed heights (i.e. 0.5 cm, 1 cm and 1.5 cm) related to 0.22 g, 0.50 g and 0.68 g, respectively of the masses NaA zeolite microspheres were investigated. Flow rates (1 mL/min and 4 mL/min ) were applied and the effluent solutions were collected by a fractional collector. Selectivity adsorption: In order to investigate the influence of the coexisted cations, the Sr(II) concentration of 50 mg/L was prepared then adjustment the concentration of Na+, K+, Mg2+ and Ca2+ to 0.01 mol/L, 0.1 mol/L, 0.2 mol/L and 0.5 mol/L, respectively. 0.1 g of adsorbents were separately added into 100 mL of the above solutions at pH 6. These samples were placed into constant temperature oscillation device at 25 ℃ with a shaking speed of 160 rpm, as shown in Figure 4S. Samples were taken after 0.5 h and analyzed for Sr(II) concentration using ICP. Desorption and recyclability experiments: 0.1 g of adsorbent was added to 100 mL solutions of Sr(II) (50 mg/L) at pH 6, the solution undergoes shaking with velocity 160 rpm for 0.5 h at 25 ℃, after filtered of the sorbent, 100 mL of different desorbing reagents were added (i.e. distilled water, HCl, NaCl and EDTA-2Na solutions) and shacked with shaking speed 160 rpm at



25 ℃ for 1 h. After the desorption process, the concentration of Sr(II) in eluate was analyzed by ICP and the adsorbent filtered off, washed with water for other cycle.

Results and discussions Characterizations of NaA zeolite microspheres XRD data of the metakaolin and zeolite microspheres are shown in Figure 3(A). The XRD patterns of zeolite microspheres compared with JCPDS showed that the major phase was Zeolite A,(Na) (JCPDS#39-0223 Na96Al96Si96O384). XRD patterns of metakaolin compared with JCPDS indicated that the major phases were kaolinite and quartz. The peaks of metakaolin disappear in zeolite microspheres, which means metakaolin could be dissolved in alkaline condition and produce geopolymer. Figure 3(B) presents FTIR spectra of metakaolin and zeolite microspheres. As seen from Figure 3(B), there are remarkable differences between metakaolin and zeolite microspheres. Peaks at 466 and 467 cm-1 were attributed to Si-O bending and Al-O linkages [31,37]. A broad band was observed around 550 cm-1, i.e. close to the value of the Si-O-Al in octahedral coordination [38]. Band at 664 cm-1 was due to Al-O-Si vibrations [39]. The broad band of metakaolin located at 800 cm-1 is assigned to Al-O bonds in Al2O3, but this band was not observed in NaA zeolite microspheres [40]. The vibration band at 1090 cm-1 for metakaolin is attributed to stretching band of Si-O bonds in SiO2. This band shifted towards 1000 cm-1 for zeolite microspheres likely due to a T-O-Si (T: Si or Al) antisymmetric stretching vibration in aluminosilicates [40]. NaA zeolite microspheres were also studied by the N2 adsorption-desorption test which indicated the pore size distribution of zeolite microspheres (Figure 3(C)) to be found in the range of 1.7-254.6 nm, with average pore diameter of 26.93 nm, the most probable pore size was about 124.39 nm and the pore volume was 0.23 mL/g and the porosity provided more active positions for the adsorption of heavy metal ions. Figure 1 and Figure 3(D) respectively present the optical microscope photos and SEM images of the zeolite microspheres. According to Figure 1, zeolite microspheres size distribution was uniform due to the use of the DPS method. From SEM images (Figure 3(D)), zeolite microspheres presented a good sphericity and zeolite A,(Na) crystallites were formed in the surface of the zeolite microspheres. In order to obtain a detailed


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characterization of the morphologies and crystal structures of the zeolite A,(Na), TEM and HRTEM analysis were performed and the images are shown in Figure 3(E) and (F). Through the measurements and calculations of nano-scale ordered structures in HRTEM image (Figure 3(F)), the lattice spacings of 0.30 nm corresponded to the peak (800) crystal faces of Zeolite A,(Na) (JCPDS#39-0223 Na96Al96Si96O384). This result is highly consistent with the XRD patterns of zeolite microspheres.

Figure 3. XRD analysis (A) and FTIR spectra (B) of the zeolite microspheres and metakaolin, SEM images of the surface (C) and BJH pore-size distribution (D) of the zeolite microspheres, TEM image (E) and HRTEM image (F) of the zeolite microspheres.

Strontium ion adsorption kinetics Because Sr(OH)2 has a certain solubility in water at 25 ℃, the Sr(II) concentration used in this



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study does not produce precipitation at pH 2-8, and the precipitation of Sr(II) was ignored. In order to obtain the equilibrium time between the zeolite microspheres and Sr(II), adsorption capacities of Sr(II) changed over time as shown in Figure 4(A). 0.1 g NaA zeolite microspheres were added to 100 mL test solutions of SrCl2 with initial Sr concentrations of approximately 50 mg/L at pH 6 and 25 ℃. From Figure 4(A), it was shown that the amount of Sr(II) adsorbed increased with time and reached adsorption equilibrium after 0.5 h from this we can concluded that 0.5 h is the maximum adsorption time and fixed for other experiments.

Figure 4. (A) Adsorption kinetics of the zeolite microspheres for Sr(II); (B) pseudo-first-order plots for adsorption of Sr(II) by the zeolite microspheres; (C) pseudo-second-order plots for adsorption of Sr(II) by the zeolite microspheres.; (D) fitting by intra-particle diffusion model.

The

experimental

data

were

evaluated

with

the

pseudo-first-order

(Eqs.(4))

and

pseudo-second-order (Eqs.(5)) kinetic model, respectively [39,41].

ln Qe  Qt   ln Qe  k1t

(4)

t 1 t   2 Qt k 2Qe Qe

(5)

Where Qt (mg/g) is the amount absorbed at time t (min) and Qe (mg/g) is the amount absorbed at equilibrium time t (min). Thus, k1 (min-1) and k2 (g mg-1 min-1) represent the pseudo-first-order and pseudo-second-order rate constant of adsorption process respectively.


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The intra-particle diffusion model (Eqs.(6)) was used to study the adsorption mechanism and rate-controlling steps of Sr(II) in the adsorbent [40].

Qt  k id t 1 / 2  Ci

(6)

Where kid (mg/(g min1/2)) is the intra-particle diffusion constant, and Ci represents the thickness of the liquid film. The fitting results are presented in Figure 4(B) and (C), and the parameters of the three kinetic models are shown in Table 1. As can be seen from Table 1, the calculated correlation coefficient value (R2) for the pseudo-second-order (0.999) is greater than that for the pseudo-first-order (0.987). Besides, the theoretical values (Qe,cal=58.45 mg/L) of pseudo-second-order equation was close to the experimental equilibrium adsorption amounts (Qe,exp=53.53 mg/L). These results demonstrated that the adsorption mechanism followed the pseudo-second-order kinetic models and indicated the adsorption process mainly for the chemical adsorption. As shown in Figure 4(D), the diffusion process of Sr(II) mainly contained three stages on the NaA zeolite microspheres, and rate-controlling process consisted of one or more steps. The first portion was the adsorbate transfer across the external boundary layer film of solution surrounding the outside of the adsorbent (external adsorption), the second part was attributed to the diffusion of the adsorbate molecules to an adsorption site (intra-particle diffusion), and the third part described the adsorption is approaching equilibrium. The parameters of intra-particle diffusion model are listed in Table 1. The external adsorption was predominant in 5 minutes before, the intra-particle diffusion was predominant in 5-15 minutes, and the adsorption was approaching equilibrium in 15 minutes. But all lines did not passed through the origin (Figure 4(D)), which indicated that the rate-controlling step consisted of external adsorption and intra-particle diffusion [42]. Table.1 Kinetics parameters for adsorption of Sr(II) onto the zeolite microspheres.

Experimental Qe

Pseudo-first-order kinetic model

Pseudo-second-order kinetic model

Intra-particle diffusion model

mg/L

Qe.cal

k1(×10-4)

R2

Qe.cal

k2(×10-4)

R2

Kid

C

R2

53.53

44.75

1983

0.987

58.48

62.61

0.999

8.95

17.36

1

In order to further explore the adsorption mechanism of the zeolite microspheres adsorption to strontium ions. At the same time, the concentration of sodium ions was tested by ICP in the adsorption process. From Fig.5 it is observed that the concentration of sodium ions increased and



strontium ions decreased in the solution with the time extension. Therefore, there is ion exchange effect in the process of adsorption of Sr(II) by the zeolite microspheres. When equilibrium was achieved the molar ratio △C(Sr2+):△C(Na+)=1:2 only contains the ion exchange, but actually the molar ratio △C(Sr2+):△C(Na+)≈1:2.29. It was indicated that there are chemical adsorption and ion exchange in the process of removing Sr(II) by the zeolite microspheres.

Figure 5. The changes of concentration of Sr2+ and Na+ in the process of adsorption.

Effect of dosage and pH As could be seen in Figure 6(A), addition of 0.3-3.1 g/L NaA zeolite microspheres to 100 mL SrCl2 solutions with initial concentration (50 mg/L) and pH 6 found the removal efficiency of Sr(II) increased from 55.25% to 100%. It can be concluded that the removal efficiency increased with increasing of the adsorbent dosage; and reached to 100% with dosage 1.0 g/L. However, the adsorption amount decreased when the NaA zeolite microspheres dosage increased; this could be explained by the fact that the active sites were excessive with increasing adsorbent dosage, leading to a decrease of the adsorption amount [43]. Therefore, it is very important to choose the optimal adsorbent dosage to determine the maximum adsorption capacity. Through the analysis of Figure 6(A), the experiments of adsorption isotherm were adopted for 0.5 g/L for Sr(II) solution of various initial concentrations. The pH value of the solution has a great influence on the adsorption process, playing on the change of the surface charge of the adsorbent and the existence of acid affect and hydrolysis of metal ions [44]. In order to study the effect of pH on adsorption, the pH range of 2-8 is shown in Figure 6(B). The pH values of solution have a marked effect on the removal efficiency of Sr(II).


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The removal efficiency and adsorption amount increased with the increase of the pH values. In the initial pH 2, the removal efficiency of Sr(II) was only 2.23%. In the pH values range of 4-8, the removal efficiency of Sr(II) reached 100%. In order to further study the adsorption performance at different pH values, the surface charge of NaA zeolite microspheres at different pH were investigated. As can be seen from Figure 6(C), it showed that a great relationship between zeta potential and concentration of H+ in the solution. NaA zeolite microspheres was positively charged attributed by protonation at low pH, and not favored the adsorption of Sr(II). The zero charge (pHPZC) of NaA zeolite microspheres emerged at pH=3.9. The negative charge on the surface of the NaA zeolite microspheres promoted the adsorption of Sr(II) when the pH value exceeded pHPZC. This shows that NaA zeolite microspheres dosage can be used under condition of a wide range of pH. The pH value of the preparation of SrCl2 solutions is about 6, therefore the following experiments used the pH 6 of the Sr(II) solution.

Figure 6. (A) Influence of the zeolite microspheres dosage on Sr2+ removal (pH 6, initial concentration 50 mg/L, equilibrium contact time 0.5 h); (B) Effect of pH variation on the adsorption efficiency of Sr2+ by the zeolite microspheres (adsorbent dosage 1.0 g/L, initial concentration 50 mg/L, equilibrium contact time 0.5 h); (C) Zeta potential of zeolite microspheres as a function of pH (adsorbent dosage 1.0 g/L)

Adsorption isotherms Figure 7(A) shows the maximum adsorption capacity of the zeolite microspheres toward



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strontium ions. The equilibrium adsorption amount increased with increasing equilibrium concentration of Sr(II). When the adsorbent dosage was fixed, the available active sites of the zeolite microspheres gradually saturated with the increase of the equilibrium concentration of strontium ions. Through changing the initial concentration of Sr(II), the maximum adsorption capacity of Sr(II) by the zeolite microspheres was 106.28 mg/g. The experimental data were fitted with Langmuir and Freundlich isotherm model, as shown in Figure 7(B) and (C) respectively according to the following Eqs. (7) and (8) [45,46]:

1 1 1   Qe Qmax k L Ce Qmax

(7)

1 log Qe  log k F  log Ce n

(8)

Where Qmax (mg/g) is the maximum equilibrium adsorption capacity, kL (L/mg) and n are model constants; kF ((mg/g)(L/mg)1/n) is the adsorption equilibrium constants of Freundlich model. The fitting results of Langmuir and Freundlich for adsorption of Sr(II) on the zeolite microspheres were listed in Table 2.

Figure 7. (A) Experimental isotherms of Sr(II) adsorption by the zeolite microspheres; (B) Langmuir isotherm plots for adsorption of Sr(II) by the zeolite microspheres; (C) Freundlich isotherm plots for adsorption of Sr(II) by the zeolite microspheres.


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From the two adsorption model it was shown that, the experimental plots agree well with the Freundlich adsorption isotherm model, which presented a more significant correlation coefficient (0.995) than that of Langmuir isotherm model (0.931). Therefore, the adsorption process belonged to multilayer adsorption due to heterogeneous surface of the zeolite microspheres and existing interactions between adsorbed molecules. Thus the magnitude of exponent 1/n is indication to the favorability and capacity of the adsorption; values where n＞1 represent favorable adsorption. In this experiment the exponent n=4.93 showed beneficial adsorption. From Table 3, it was shown the advantage of NaA zeolite microspheres. The adsorption capacity of NaA zeolite microspheres is much greater (about 16 times) than similar commercial spherical 4A molecular sieve, and compared with potassium tetratitanate whisker the adsorption capacity is very close under the same conditions of temperature and pH. NaA zeolite microspheres also showed advantageous adsorption capacity when compared to other adsorbents, such as sodium trititanate whisker, FeOOH-BT1, modified hydroxyl apatite and alginate/Fe3O4 composite. Moreover, the NaA zeolite microspheres adsorbent which can be used under pH 4-8, and it could be directly packed with column beds for a continuous dealing with LRW, and easily retrieved. Table.2 Langmuir and Freundlich parameters for adsorption of Sr(II) onto the zeolite microspheres.

Langmuir isotherm

Freundlich isotherm

Qmax

kL

R

109.89

0.1572

0.931

2

n

kF

R2

4.93

43.70

0.995

Table.3 Comparisons of adsorption capacities of the present adsorbent with other adsorbents designed for Sr(II).

Adsorbent

pH

Temperature/℃

Qmax(mg/g)

References

6

25

13.44

[47]

10.5

30

38.5

[48]

This work

6

25

106.28

Commercial spherical 4A molecular sieve

7

25

6.66

Potassium tetratitanate whisker

6

25

111.78

[49]

Sodium trititanate whisker

6

25

85.26

[49]

Alginate/Fe3O4 composite

6

25

12.5

[50]

Modified hydroxyl apatite FeOOH-BT1

The diffusion of Sr2+ in the zeolite microspheres, distribution in the internal cross section of post-adsorption was investigated using EDS. As showed in Figure 8(D), Sr2+ with a weight



percentage of 5.57% spread to the interior of the zeolite microspheres, and this is consistent with the adsorption capacities of Sr(II) as shown in Figure 4(A). Because of the zeolite microspheres exposing highly porous and a large surface area, Sr2+ in the internal diffusion resistance is small, so that it quickly achieved the adsorption equilibrium. By comparing Figure 8(B) and (D) found that the Na+ weight percentage decreased slightly after adsorption, which could be due to ion exchange between Na+ and Sr2+. This result is consistent with the changes of concentration of Na+ in the process of adsorption (Figure 5).

Figure 8. SEM images/EDS maps of the zeolite microspheres (A) and (B) before adsorption; (C) and (D) after adsorption; (E) Sr2+ maps in image c.

Column separation experiments For further studying of the dynamic adsorption of the zeolite microspheres, using of column process technique to evaluate the removal effeciency of Sr(II) as shown in Figure S2 of column


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used on this study after filled with adsorbent. which photo of fixed-bed column adsorption device is shown in. Effect of bed height of the column on the adsorption efficiency were studied, the used bed height and corresponding weight are 0.5 cm/0.22 g, 1.0 cm/0.5 g and 1.5 cm/0.68 g on the other hand the corresponding breakthrough curves were showed in Figure 9; it expressed the ratio between outlet and inlet concentration of Sr(II) as a function of time. The experimental data of Sr(II) breakthrough curves have been fitted with Thomas model as the following Eqs. (9) [51].

Ct  C0

1 1 e

 q m   KT  0 C0t     Q  

(9)

Where Q (mL/min) is the flow speed of Sr(II) solution, q0 (mg/g) is the amount absorbed at equilibrium time t (min), m (g) is the mass of the adsorbent. Thus, KT (mL mg-1 min-1) represent the rate constant of adsorption process respectively.

Figure 9. (A), (B) Effect of column heights and flow rates on breakthrough curves of Sr(II) from the column packed with the zeolite microspheres; (C), (D) The breakthrough curves of different flow rates fitted by the Thomas model.

From Figure 9(A), it was shown that the time required for attaining the saturation (breakthrough time) increased with column heights at a flow rate of 1 mL/min. When the column height was 0.5 cm, the concentration of Sr(II) (column outlet) began to rise in 3 h and reached saturation in 16.5 h, meanwhile with 1.0 cm bed height, the concentration of Sr(II) (column outlet) began to rise in 9.5



h and reached saturation in 27 h. And for 1.5 cm bed height, the concentration of Sr(II) (column outlet) began to rise in 18 h and reached saturation after 29 h. As shown in Figure 9(B), even if the flow rate reached 4 mL/min, the concentration of Sr(II) (column outlet) began to rise in 3 h and reached saturation in 10 h, still had a good separation effect. Furtherly, the Thomas model was used to fit the breakthrough curves of different flow rates as shown in Figure 9(C) and (D). The maximum dynamic adsorption capacity found to be 105.83 mg/g (R2=0.967) and 123.46 mg/g (R2=0.940), for of flow rates 1 mL/min and 4 mL/min respectively. 105.83 mg/g was close to the value observed in batch experiment (106.28 mg/g). In addition, the actual dynamic adsorption amount of flow rate of 1 mL/min and 4 mL/min were 105.65 mg/g and 119.43 mg/g according to the mass balance. The actual value was very consistent with the estimated value. Hence, the zeolite microspheres has a high removal efficiency of Sr(II) through the dynamic adsorption experiment. Moreover, the zeolite microspheres were not only low-cost and environmental friendly adsorbents, but also could be easily handled in packed column beds for the removal of radioactive Sr(II) from liquid radioactive wastes in industry.

Effect of cations

Figure 10. A: Effect of different cations on the adsorption amount of Sr(II) (pH=6, adsorbent dosage 1.0 g/L, Sr(II) concentration 50 mg/L, equilibrium contact time 30 min). B: The removal percentages of Sr(II) on zeolite microspheres from different water systems and adsorbent dosage(pH=6, Sr(II) concentration 50 mg/L, equilibrium contact time 30 min).

It is well known that the complex composition of wastewater containing strontium, and some coexisted cations may produce certain influence on the adsorption. Hence, the influence of coexisted cations in adsorption process should be considered. The removal percentages of Sr(II) on the zeolite microspheres in the presence of different cations were illustrated in Figure 10(A). In this paper, Na+, K+, Mg2+ and Ca2+ were chosen as representative anions due to their widely distributed in wastewater. The types and concentrations of the coexisted cations have an effect on


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the adsorption properties of Sr(II), the adsorption amount and removal percentages of Sr(II) decreased with the increase of the concentration of the coexisted cations in Figure 10(A).When the concentration of Na+, K+, Mg2+ and Ca2+ were 0.2 mol/L, the adsorption capacity of Sr(II) was reduced by 39.6%, 68.7%, 47.9% and 88.6% compared with pure water, respectively. Therefore, the Ca2+ and K+ have a significant effect on the adsorption process, this result indicates that the competitive adsorption capabilities are as follows: Na+＜Mg2+＜K+＜Ca2+. As shown in Figure 5S and Table2S, the NaA zeolite microspheres has the highly selective adsorption of Sr(II). In order to further verify the possible practicability of adsorbnets, Figure 10(B) showed the adsorption amount and removal percentages of Sr(II) (pH=6.0) on zeolite microspheres from different water systems and adsorbent dosage. The Sr(II) adsorption amount and removal percentages in different waters were in the order of pure water ＞ natural surface water ＞ real seawater, which was consistent with above-mentioned results of competitive adsorption. The Sr(II) removal efficiency in pure water, natural surface water and real seawater reached maximum 100%, 100% and 78.5% with dosage 1.0 g/L, 1.5 g/L and 15 g/L, respectively. Although the adsorption amount decrease in real seawater, this make zeolite microspheres promising adsorbent from various wastewater.

Desorption and recyclability study

Figure.11. A: Dsorption efficiency of Sr(II) from zeolite microspheres with different desorption concentrations (adsorbent dosage 1.0 g/L). B: Effect of cycle number on the adsorption-desorption of zeolite microspheres (EDTA-2Na concentration 0.05 mol/L, adsorbent dosage 1.0 g/L, equilibrium contact time 1.0 h).

Regeneration and recycling performance are important parameters for adsorbents. In this study using of distilled water, HCl, NaCl and EDTA-2Na solutions as desorption agent. As shown in Figure 11(A), the desorption efficiency was gradually increased with the increase of HCl



concentration. When the concentration of HCl was 0.015 mol/L, the desorption efficiency reached 83.88%, but the zeolite microspheres appeared the dissolution. Therefore, using NaCl and EDTA-2Na to further explore the desorption effect of the zeolite microspheres. By comparison, it was found that the change of NaCl concentration did not obtain a better desorption effect, and when the concentration of EDTA-2Na was 0.05mol/L, the desorption efficiency reached 83.93%, this reflects excellent desorption property. Through the adsorption-desorption cycles of zeolite microspheres adsorbent. Figure 11(B) present regeneration cycles using 0.05 mol/L EDTA-2Na solution for 3 cycles performance test found that adsorption efficiency of zeolite microspheres decreased to 34% in the first cycle and then adsorption efficiency constant stage. Moreover, the desorption efficiency remained almost unchanged after recycling 3 times. These results shown that the zeolite microspheres have excellent regeneration and recycling properties. By comparing the adsorption-desorption experiment, it can be found that the desorption efficiency of the second and third cycles was better than that of the first cycle, so the rebinding mechanism of NaA zeolite microspheres was based on ion exchange.

Conclusions This work describes one-pot process to fabricate NaA zeolite microspheres (about 100 μm) from metakaolin-based geopolymers. SEM and XRD characterization clearly displayed that the surface of geopolymer microspheres generated a large number of NaA zeolites. From adsorption studies, the NaA zeolite microspheres exhibited excellent removal efficiency for Sr(II), and fitted by pseudo-second-order kinetic model and Freundlich isotherm model. The process included both chemical adsorption and ion exchange. The zeolite microspheres were found to have a high removal efficiency of Sr(II) through the dynamic adsorption experiment. Desorption and recyclability experiment shown that the product have excellent regeneration and recycling properties. Although the adsorption amount decrease in real seawater, and the zeolite microspheres was a promising adsorbent from various wastewater.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:xxxxxxxxx.


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The slurry optimal experimental parameters, the photo of relevant fixed-bed column adsorption device and extracted Sr(II) from different water systems in oscillation device, competitive adsorption results.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by the Chinese Natural Science Fund (Grants: 51772055, 21566006 and 11675102). We thank Dr Mohammed F. HAMZA for helping me.

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For Table of Contents Use Only:

Brief synopsis: NaA zeolite microspheres prepared using suspension dispersion solidification method and in-situ heat curing process with highly efficient and continuous removal of strontium.


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One-Pot Preparation of NaA Zeolite Microspheres ... - ACS Publications

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