Investigation on the thermal activation of montmorillonite and its application for the removal of U(VI) in aqueous solution

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Investigation on the thermal activation of montmorillonite and its application for the removal of U(VI) in aqueous solution Qianru Zuo a, Xiaoqing Gao a,b, Junqiang Yang a, Peng Zhang a, Geng Chen a, Yaming Li a, Keliang Shi a,c,∗, Wangsuo Wu a,c a b c

Radiochemistry Lab, School of Nuclear Science and Technology, Lanzhou University, 730000 Lanzhou, PR China Environmental Monitor Center of Gansu Province, 730000 Lanzhou, PR China Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, 730000 Lanzhou, PR China

a r t i c l e

i n f o

Article history: Received 8 June 2017 Revised 11 September 2017 Accepted 13 September 2017 Available online xxx Keywords: Montmorillonite Thermal activation Uranium(VI) Sorption Wastewater

a b s t r a c t The application for montmorillonite to deal with toxic metals (including radionuclides) becomes interesting based on its excellent physicochemical properties. In this work, the thermal activation method was utilized to pre-treat montmorillonite before use. The sample was characterized by FTIR, XRD, zeta potential, BET and potentiometric titration to clarify the variation of montmorillonite before and after thermal activation. Batch techniques were used to investigate the sorption ability of montmorillonite to U(VI) under different environmental conditions. The electrical double layer model has been introduced to describe the variation of pre-treated sample as well as the principal mechanism for U(VI) uptake. In addition, the irradiation effects of samples for U(VI) sorption was also investigated. Based on the optimum condition for U(VI) uptake, it can be deduced that the thermally activated montmorillonite has potential application for the removal of U(VI) in wastewater. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Uranium is a naturally occurring radioactive element that is a primary raw material for nuclear energy. However, the resources of uranium in nature are limited comparing to the large consumption of uranium based on the fast development of nuclear energy [1–3]. Meanwhile, excessive amounts of uranium have been released into the environment through the activities of nuclear industry, which would cause a public health problem because of the toxic nature of uranium [4]. Therefore, it is necessary to remove, concentrate and recovery uranium to meet the energy demand for the future and prevent radioactive contamination of the environment. Numerous treatment methods have been applied for the removal or recovery of uranium from non-conventional resources such as seawater, industrial wastewater, and other waste sources [5–7]. Among these methods, the sorption techniques have been extensively studied due to the advantages of lower cost, easy operation, high efficient, and friendly to the environment [3]. Montmorillonite is often used to decontaminate metal ions from water bodies based on the various outstanding physicochemical properties such as large specific surface area, strong ad∗ Corresponding author at: Radiochemistry Lab, School of Nuclear Science and Technology, Lanzhou University, 730 0 0 0 Lanzhou, PR China. E-mail address: [email protected] (K. Shi).

sorptive affinity, low permeability, high cation exchange capacity, great swelling property, stability, and low cost [8–10]. It is a 2:1 type of mineral, and its unit layer structure consists of one octahedral alumina layer placed between two tetrahedral silica layers. The lattice damage and isomorphic substitution of Si4+ by Al3+ in the tetrahedral layer and Al3+ by Mg2+ or Zn2+ in the octahedral layer results in a net negative surface charge on the clay [11]. Accordingly, montmorillonite often shows advantages for dealing with metal cations such as Cs+ , Sr2+ , Eu3+ , Th4+ and UO2 2+ in wastewater [12,13]. However, as the limited surface-active sites, the sorption capability of raw montmorillonite to metal ions is relatively small. To enhance the sorption abilities of montmorillonite/bentonite, different techniques such as organic modification [2,14], acid activation [15] and thermal calcination [16–20] have been used to pre-treat samples. Among these, thermal calcination is a straightforward and efficient method. In the thermal calcination process, the samples would undergo four reactions: (1) dehydration, (2) dehydroxylation, (3) decomposition and (4) recrystallization. The H2 O in interlayers would lose at temperatures higher than 220 °C, and dehydroxylation takes place with the temperature range of 350–10 0 0 °C [21–23]. Although some work for the thermal treatment of bentonite have been proposed [1], the activation parameters of sample seem not to be well optimized because of the insufficient experimental data. In addition, the interpretation of variation of

https://doi.org/10.1016/j.jtice.2017.09.016 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Q. Zuo et al., Investigation on the thermal activation of montmorillonite and its application for the removal of U(VI) in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.016

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montmorillonite before and after thermal activation is not clear although the enhancement of U(VI) sorption was observed. Furthermore, for the treatment of radionuclides, the irradiation stability of sorbent materials should be considered, however, there is no relative work reported for thermal activated montmorillonite. Accordingly, a comprehensive investigation on montmorillonite activation and its application for U(VI) sorption is still necessary. The objectives of this study are: (i) to optimize the conditions for montmorillonite thermal pre-treatment; (ii) to clarify the variation of montmorillonite before and after thermal activation; (iii) to study the irradiation stabilities of thermal activated montmorillonite and (iv) to investigate the potential application for the removal of U(VI) from wastewater by activated montmorillonite. 2. Materials and methods 2.1. Materials and reagents The raw montmorillonite (regards as RM) sample purchased from Beijing Boyu New Materials Co., Ltd. Shijiazhuang, China and used in the sorption experiment after thermal activation (regards as TAM). All reagents used for experiments were of analytical grade. A stock solution of U(VI) was prepared by dissolving an appropriate amount of UO2 (NO3 )2 •6H2 O salt (A.R.) in high purity water (18.2 M cm). The stock solution was then diluted to prepare for working solutions. The high purity water was used during the tests, and reagent blank was run for every sample solution. 2.2. Sorption experiment Sorption experiment was carried out using batch techniques in the air conditions. The general procedure is as following: suitable amount of montmorillonite sample is dispersed in NaCl solution contained in a polyethylene tube with a solid-to-liquid ratio of 2 g/L. The pH of suspensions was adjusted to desired values with a small amount of NaOH/HCl solution. The suspensions with U(VI) were then shaken for 48 h to make sure that the steady state is reached (the pH was monitored during these periods). After that, the pH of the suspensions was measured and the suspension was separated by centrifugation (18,0 0 0 g × 30 min) to get the concentration of U(VI) in aqueous phase. The sorption percentage (%), sorption amount (q, mol/g) and distribution coefficient (Kd , mL/g) of U(VI) onto montmorillonite can be calculated by Eqs. (1)–(3), respectively:

%=

(C0 − Caq ) C0

q = (C0 − Caq ) ·

Kd =

· 100%

(1)

V m

(2)

(C0 − Caq ) V Caq

·

m

(3)

Where V(L) is the volume of aqueous solution, m (g) is the mass of sorbent, Caq (mol/L) is the concentration of U(VI) in aqueous phase, C0 (mol/L) is the initial concentration of U(VI) added. For each sorption experiment, three replicates were performed. The results (e.g., Kd , q or %) are presented as the mean values of three replicates ±10% uncertainties estimated in the whole sorption experiment. To investigate the irradiation stability of sorbent, the TAM sample was irradiated with a γ dose rate of 2.5 kGy/h using a 60 Co energy source (2.22 × 1015 Bq) at room temperature. After irradiation, U(VI) sorption experiment was carried out with TAM sample using batch technique mentioned above.

Fig. 1. The effect of activation temperature (activation time 5 h, conditions for sorption: m/V = 2 g/L; t = 24 h; I = 0.1 mol/L NaCl; T = 25 ± 1 °C; [U(VI)]0 = 9.96 × 10−5 mol/L and pH = 5.0 ± 0.2). The error bars represent 10% uncertainties estimated in the whole experiment.

2.3. U(VI) measurement using spectrophotometry The concentration of U(VI) in solution was measured using spectrophotometric techniques [24]. The main steps for U(VI) analysis is described as follows: 2.0 mL of Arsenazo-III solution (1%) was added in a 25 mL volumetric flask contained target sample, the acidity was maintained with 1.0 mL 0.5 mol/L HCl. After diluted and mixed with deionized water, the solution was then stayed for 30 min to ensure that the color reaction has completed. After that, the absorbance of solution was measured at 652 nm using spectrophotometer (7230G-N) and the concentration of U(VI) in solution was calculated according to the standard carve of U(VI) obtained. The uncertainties for U(VI) measurement is less than 3%. 2.4. Sorbent characterization The RW and TAM samples were characterized using Fourier transform infrared spectroscopy (FT-IR) over spectral range from 40 0 0 to 40 0 cm−1 at a resolution of 2 cm−1 . The phase and texture characterizes of samples were conducted by X-ray powder diffraction (XRD) analyzer using Cu target radiation source with 2θ degree of 5–80. The specific surface area of samples was obtained through BET measurement. By using potentiometric acidbase titration techniques [25], the surface character of RM and TAM samples were investigated. The particle sizes (in the range of 60 0–150 0 nm) as well as the zeta potential of samples were measured using Malvern Nano ZS at room temperature. 3. Results and discussion 3.1. Optimization of calcination temperature for montmorillonite on U(VI) sorption The thermal modification of materials changes their composition, structure and sorption abilities [26,27]. In order to optimize the conditions for thermal treatment of montmorillonite sample, the influence of temperature (20 0–80 0 °C) on the variation of uptake ability of TAM to U(VI) were investigated. The results are shown in Fig. 1. It is evident that the temperature significantly affects the sorption ability of sample. The sorption capacity of TAM for U(VI) enhances with the increase of calcination temperature at first and then decreases when the temperature is above 600 °C (see Fig. 1). It can be explained that the rise of temperature breaks the crystal structure through the dehydration and dehydroxylation processes, which causes the variation of absorbability [1]. Based on

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Table 1 The effect of calcination temperature on the specific surface area of montmorillonite sample. Temperature (°C)

25

500

550

600

650

700

BET (m2 /g)

63.2

38.9

37.8

34.1

31.4

30.1

Fig. 2. The FT-IR spectra of raw and thermally activated montmorillonite samples.

Fig. 4. Schematic diagram of electrical double layer model.

Fig. 3. XRD patterns of raw and thermally activated montmorillonite samples.

the experimental data, 600 °C was chosen as the optimum temperature for the sample pre-treatment in practice. The further information for montmorillonite sample activation has been show in support information (SI: Figs. S-1 and S-2). 3.2. FT-IR, XRD and BET characterization of sorbents The FT-IR spectrums of RM and TAM sample have been shown in Fig. 2. In the spectra of RM sample, the peak at 1034.9 cm−1 represents for stretching vibration of Si–O–Si, 795.9 cm−1 and 468 cm−1 for stretching and bending vibration of Si–O–Fe, and 520.3 cm−1 for bending vibration of Si–O–Mg, respectively. The peak near 3626.1 cm−1 represents stretching vibration of –OH in Si–OH–Al and 914.5 cm−1 represents Al–Al–OH hydroxyl vibration. Peaks at 3447.5 cm−1 and 1637.2 cm−1 meet with stretching and bending vibration adsorption bands of O–H bond of water [28]. Compared with RM, the TAM sample maintains most of the absorption bands of the basic structure of montmorillonite. However, variations occur in the bands related to the group of -OH and absorbed water. The transition intensity in the regions that represent H2 O (3447.5 cm−1 and 1637.2 cm−1 for RM sample) is obviously decreased, indicating that adsorbed water has been lost. The shrink of peaks at 3626.1 cm−1 and 914.5 cm−1 suggests that the structural –OH groups were lost while calcination at high temperature [29,30]. XRD is a useful technique to study the layer structure and the basal spacing of crystal material. The XRD patterns for RM and TAM samples observed by wide-angle range are presented in Fig. 3. It is clear that there are some differences between the patterns for RM and TAM. The peak at 7° is the diffraction angle of d001

which represents the layer space of RM [31,32]. From the results, we can find that the high temperature caused the degree of d001 increasing and the intensity is also reduced. In other words, the space distance of montmorillonite interlayer is declined after calcination. Combining XRD and FT-IR results, it can be deduced that the reason for degree of d001 increasing is due to the loss of interlayer water, similar research exploration was also proposed by Andrini et al. for montmorillonite sample pre-treated at 800 °C [33]. The BET specific surface area of samples calcined at different temperature was investigated. Results (Table 1) show that the surface area of TAM declines with the increase of calcination temperature. This can be explained that the rise of temperature would cause the collapse of the montmorillonite interlayer spaces which inhibited the entrance of the probe molecule, and further decrease the specific surface area of sample [34]. Although the specific surface area of sample decreased slightly with the increase of calcination temperature, the sorption ability of TAM to U(IV) enhanced evidently with the temperature increased up to 600 °C (see in Fig. 1), suggesting that the specific surface area is not the only parameter which affects the sorption ability of sorbent. 3.3. Transformation of surface properties of montmorillonite after calcination Montmorillonite is constituted with two silicon-oxygen tetrahedron layer and a central aluminum-oxygen octahedron layer. Because of the lattice damage and isomorphic substitution, an apparent electronegative surface of montmorillonite is usually present. The negative charges are compensated by cations adsorbed in the interlayer and basal space such as Na+ , K+ or Ca2+ , Mg2+ [35]. To balance its surface charges when it is immersed in water, an electrical double layer would be formed in montmorillonite surface. The electrical double layer contains a Stern layer and a diffusive layer (see in Fig. 4). Compacted counter ions connected to interfacial charges directly composes Stern layer. Diffusive layer is formed by ions attracted by electric field around the surface in solution [36], it is far from the surface and relatively instability compared with Stern layer. Because the ions between Stern layer and mineral surface are very tight, it’s hard to measure the surface potential, so

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Fig. 7. The irradiation stability of TAM with different γ dose. The error bars represent 10% uncertainties estimated in the whole experiment.

Fig. 5. Zeta potential of thermally activated montmorillonite samples.

that the dehydroxylation at excessively high temperature may undergo another process. Firstly, proton in surface hydroxyl transferred to another adjacent hydroxyl:

≡ SOH + ≡ SOH → ≡ SHOH+ + ≡ SO−

(6)

Then, the dehydration reaction took placed in the hydroxyl sites with two protons. At last, the two active hydroxyl sites become inactivation by forming a stable structure with each other through silicon–oxygen bond:

Fig. 6. Potentiometric titration result of raw and thermally activated montmorillonite samples.

zeta potential is used to characterize the surface electrical property of TAM in the present work. According to Fig. 5, the zeta potential of TAM kept stable at beginning and had a sudden rise at 500 °C but descended after 600 °C with the increasing of calcination temperature. The growing potential can ascribe to the absence of interlayer H2 O in basal spacing, which may be connected to the negatively charged sites on montmorillonite surface by hydrogen bonding. The loss of H2 O leads to exposure of these sites directly at high temperature. The surface potential would not change because H2 O molecular is neutral, but the sites can combine with proton or other cations easily in solution because of the decrease of resistance. Thus, the zeta potential of TAM in solution is increased. In aqueous solution, interfacial hydroxyl (≡SOH) undergoes deprotonation and protonation reactions:

≡ SOH ↔ ≡ SO− + H+

(4)

≡ SOH + H+ ↔ ≡ SOH+ 2

(5)

Fig. 6 shows the result of potentiometric titration. The curve displays proton excess of RM and TAM at pH ranging from 5.0–9.5. It can be obtained from the result that there is much more proton on TAM surface than that on RM at same pH values, which indicates TAM gains stronger ability to combine proton. The result is consistent with zeta potential data. Accordingly, it can be deduced that the enhancement of sorption ability of TAM is mainly caused by the variation of surface properties. It should be mentioned that the enhancement of activation temperature does not make voltage continuous increase (see Fig. 5). When the activation temperature higher than 600 °C, zeta potential of TAM in solution was declined. The reason for this is

≡ SHOH+ → ≡ S+ + H2 O ↑

(7)

≡ SO− + ≡ S+ → ≡ SS > O

(8)

Therefore, when the calcination temperature is excessive, some of the sites with electronegativity are disappeared and the sorption abilities of TAM to U(VI) becomes lower. 3.4. Irradiation stability of TAM sample The irradiation would destroy the clay mineral structure by breaking the chemical bonding of Al–O and Si–O [37, 38]. To clarify the irradiation stability of TAM sample, γ irradiation experiment was conducted under different doses (50, 100 and 200 kGy) at room temperature. From the results (Fig. 7) we can see that the irradiated sample matches well with the originated one according to the sorption abilities towards U(VI) (the uptake ratio of U(VI) maintains around 60% in all case), suggesting that the stability of TAM sample is well when the irradiation doses is lower than 200 kGy. Similar behaviour was also observed on gammairradiation of natural and synthetic inorganic sorbents [39]. 3.5. Application for the removal of U(VI) by TAM 3.5.1. Effect of contact time The effect of contact time was studied using an initial U(VI) concentration of 9.96 × 10−5 mol/L at room temperature (see in Fig. 8 (A)). It can be found that the sorption increased quickly within the early 1 h and increased gradually from 1 to 6 h, afterwards it trended to be stable, suggesting that the state of equilibrium was reached. The initial faster rate may be due to surface sorption and in the initial stage, the surface is free and reaction proceeds at a faster rate. Once the available free surface is clogged, then the adsorbate molecules penetrate as intra-particle diffusion resulted in the gradual increase [1]. When the active sites on the surface of the particles and on the generated fractures, pores and channels within the particles had gradually been taken up by U(VI) ions, the sorption reached the dynamic equilibrium.

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Fig. 8. The parameters which affect the uptake of U(VI) by TAM with conditions of T = 25 ± 1 °C and m/V = 2 g/L. (A) contact time; (B) initial concentration of U(VI); (C) solution pH and (D) ionic strengths. The error bars represent 10% uncertainties estimated in the whole experiment.

To better interpret the behaviour of the sorbents and passible controlling mechanism, the kinetics experimental data were fitted with the pseudo second-order model, the linear form of the equation was expressed as follows:

equations respectively:

t 1 t = + qt qe k2qe 2

log qe = log KF +

(9)

Where qe (mol/g) is the equilibrium sorption capacity, and qt (mol/g) the amount of U(VI) sorbed at time t; k2 (g/(mol h)) is the rate constant of pseudo-second-order equation. The parameters in the model were determined from the straight-line plots of t/qt versus t and were shown in Table S-1. The fitting correlation coefficient shows that pseudo-second-order kinetic model is suitable for describing the sorption process (R2 > 0.99), suggesting that the chemical sorption is the predominant step for the uptake process of U(VI) [24]. 3.5.2. Effect of U(VI) initial concentration The effects of the initial U(VI) ions concentration on the sorption were investigated and the results have been shown in Fig. 8(B). The amount of U(VI) adsorbed onto TAM enhanced when the initial U(VI) concentration increases. However, the sorption efficiency of U(VI) ions increased at first and then decreased gradually with the enhancement of U(VI) initial concentration (the results were not presented here). This can be explained that there were a limited number of silanols and unsaturated sites on TAM surface, and they can adsorb and exchange U(VI) ions rapidly. With the increasing of U(VI) concentration, U(VI) ions become excessive in the solution. Afterwards, the sorption reached the equilibrium and then the surface free energy decreased gradually, leading to the decline of sorption efficiency [1]. The Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R) isotherm models have been applied to model the experimental data [40,41]. These models can be described in the following

Ce 1 Ce = + qe qmax KL qmax log Ce n

(10) (11)

qe = BT log KT + BT log Ce

(12)

ln qe = ln qmax + βε 2

(13)

Where qmax (mol/g) is the theoretical sorption capacity of U(VI); Ce (mol/L) is the equilibration concentration of U(VI) in aqueous phase; KL is the Langmuir equilibrium constant; KF and n are the Freundlich constants indicating the capacity and intensity of the sorption, respectively; BT is related to the heat of sorption and KT is equilibrium binding constant. β is the activity coefficient related to mean sorption energy and ε is the Polanyi potential, which is equal to:

  1 ε = RT ln 1 + Ce

(14)

The mean free energy of sorption E (kJ/mol), can be calculated from D-R parameter β as follows:

E=

1



2β

(15)

The modelling parameters have been shown in Table S-2. As can be seen from the calculated results, the Freundlich isotherm model can well delineate the U(VI) sorption process by TAM compared to the Langmuir and Temkin isotherm models because of the higher correlation coefficient (R2 ˃0.95), and that the sorption of U(VI) ions onto TAM surface is probably a multilayer sorption process [24]. The sorption data have been applied to D-R model (R2 = 0.97)

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based on the heterogeneous surface of the sorbate as in Freundlich isotherm in order to distinguish between physical and chemical sorption. The mean free energy of sorption E was calculated according to the Eq. (15) and the value is 8.05 kJ/mol, which is in the range of 8–16 kJ/mol, revealing that the type of U(VI) sorption onto TAM is chemical process which corresponds to the kinetic analysis above [24]. 3.5.3. Effect of solution pH and ionic strength The pH of the solution is an important parameter for the sorption of metals on the sorbents, through which the possible sorption reactions can be deduced. In the present work, the effect of pH on U(VI) sorption onto montmorillonite samples were studied in the pH region of 3–9 at room temperature. The results show that the amount of U(VI) sorbed increased with the increase of solution pH from 3–7, then reached a peak soon afterwards decreased (Fig. 8(C)). The inflection of sorption curves of U(VI) against pH results from variation sorption mechanisms. U(VI) sorption was usually caused by cation exchange or formation of surface complexes according to the characterizes of sorbent [42,43]. For U(VI) sorption onto TAM, based on the investigation on the effect of ionic strength (see Fig. 8(D)), both cation exchange and surface complexes are the control processes because the sorption was significantly influenced by the variation of ionic strength from 0.001 to 0.1 mol/L, but the Kd values keep stable when it > 0.1 mol/L. The variation of U(VI) sorption with the increasing of solution pH is usually caused by the change of U(VI) species in solutions [44]. As shown in Fig. S-3, the species of U(VI) in the aqueous phase strongly depend on environmental conditions such as solution pH and the presence of ligands like CO3 2− (the distribution of U(VI) speciation with pH was calculated using the Medusa freeware (google: Medusa kth)). The reactions of U(VI) species and chemical equilibrium constants have been shown in Table S-3. At pH < 5, UO2 2+ is the dominant species (more than 85%), and (UO2 )3 (OH)5 + is the main specie in the pH ranges of 5–6. The carbonate specie of (UO2 )2 CO3 (OH)3 − becomes dominant at the higher pH (pH in the range of 6–10). According to the structure of montmorillonite, two kinds of surface sites named the layer site (permanently negatively charged site, X− ), and edge site (≡SOH) were often considered during the sorption processes [25,43]. Based on the evaluation on the effect of ionic strength, solution pH and species of U(VI) in solution, the possible surface reactions of U(VI) onto TAM can be proposed and described as follows: + 2XNa + UO2+ 2  X2 UO2 +2Na

(16)

+ + ≡ SOH + UO2+ 2 ≡ SOUO2 +H

(17)

+ ≡ SOH + 3UO2+ 2 + 5H2 O ≡ SO (UO2 )3 (OH )5 + 6H

(18)

2− 2− + ≡ SOH + 2UO2+ 2 + CO3 +3H2 O ≡ SO (UO2 )2 CO3 (OH )3 + 4H

(19) Although the possible surface complexes of U(VI) onto TAM were proposed in the present paper, the modelling exercises based on the surface properties of TAM as well as the spectroscopic evidences are still needed to confirm the assumption because it is difficult to get a reliable conclusion at molecular level through macroscopic data analyses. The relative work will be carried out in our future studies. It should be pointed out that the TAM sample shows stronger sorption capacity toward U(VI) at all pH scope compared to RM (see Fig. 8(C)), which is consistent with the results of Fig. 1. The

Fig. 9. Desorption of U(VI) from TAM with different desorbing reagents. The error bars represent 10% uncertainties estimated in the whole experiment.

optimum condition for U(VI) sorption by TAM is at neutral medium with Kd value in the order of 103 , suggesting that the TAM have potential application for the removal of U(VI) in wastewater or underground water.

3.5.4. Desorption of adsorbed U(VI) from TAM Desorption is an important process from which the spent adsorbent can be regenerated and the target element can be recovered. In this work, different reagents (H2 O, 0.5 mol/L NaOH, 1.0 mol/L NaCl, 1.0 mol/L Na2 CO3 , 1.0 mol/L CH3 COONH4 , 1.0 mol/L HCl and 1.0 mol/L HNO3 ) were tested for the desorption of U(VI) from TAM under the contact time of 60 min at room temperature, the results have been presented in Fig. 9. TAM shows the lower desorption yield (< 10%) using NaCl or H2 O compared to the other desorptive solutions such as Na2 CO3 , HCl or HNO3 . Similar observation was also reported for U(VI) desorption from bentonite [1]. By optimizing the concentration and kinds of desorptive reagents, it is found that 1 mol/L HNO3 has a maximum desorption yield (more than 90%) for U(VI). The possible explanation for U(VI) desorption by HNO3 is the exchange reaction between UO2 (CO3 )3 4− and NO3 − in acid solution [45]. Accordingly, the U(VI) in wastewater can be concentrated with TAM and recovered using 1 mol/L HNO3 at room temperature at one stage.

4. Conclusion The montmorillonite sample was pre-treated by thermal activation method and applied for the removal of U(VI) in aqueous. On the basis of work presented, the following conclusions can be drown out: (1) The sorption ability of montmorillonite toward U(VI) is enhanced dramatically when the sample was pre-treated at 600 °C, the enhancement is mainly caused by the loss of hydroxyl and structural water of montmorillonite sample, which can reveal the sites with negative potential and enhance its activity; (2) Using electrical double layer model, the sorption mechanism of U(VI) can be deduced, that is, U(VI) is supposed to adsorb in both Stern layer and diffusive layer. U(VI) in Stern layer can form relatively stable bonds with montmorillonite surface (inner-sphere surface complex) while U(VI) in diffusive layer is easily exchanged; (3) The irradiation effect of TAM is not obvious, suggesting that the irradiation stability of TAM is well; (4) The optimum condition for U(VI) sorption by TAM is at neutral medium, indicating the potential application for U(VI) removal from wastewater. The adsorbed U(VI) on TAM can be easily and quantitatively recovered by 1 mol/L HNO3 .

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Please cite this article as: Q. Zuo et al., Investigation on the thermal activation of montmorillonite and its application for the removal of U(VI) in aqueous solution, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.016