Preparation of mesoporous carbon from sodium lignosulfonate by

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Preparation of mesoporous carbon from sodium lignosulfonate by hydrothermal and template method and its adsorption of uranium (VI) Wenhui Zhao, Xiaoyan Lin, Huaming Cai, Tao Mu, and Xuegang Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02854 • Publication Date (Web): 14 Oct 2017 Downloaded from http://pubs.acs.org on October 15, 2017

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Industrial & Engineering Chemistry Research

Preparation of mesoporous carbon from sodium lignosulfonate by hydrothermal and template method and its adsorption of uranium (VI) Wenhui Zhao†,‡, Xiaoyan Lin*,†,‡, Huaming Cai†,‡, Tao Mu§ and Xuegang Luo‡ †

School of Materials Science and Engineering, ‡Engineering Research Center of

Biomass Materials, Ministry of Education, Southwest University of Science and Technology, Mianyang, 621010 Sichuan, China and §China Academy of Engineering Physics, Mianyang, 621900 Sichuan, China Abstract: A novel adsorbent of mesoporous carbon with high specific surface area was successfully prepared by hydrothermal and template method, using sodium lignosulfonate (LSs) as a raw material and cetyltrimethylammonium bromide (CTAB) as a template agent. The mesoporous carbon was characterized by SEM, TEM, BET, FTIR and XPS. The formation mechanism of the mesoporous carbon was analyzed. The adsorption of U (VI) on the mesoporous carbon from the simulated aqueous solution and actual radioactive wastewater was respectively investigated. The optimum conditions for U (VI) adsorption were determined by studying experimental variables including pH, contact time, sorbent dose, initial concentration and temperature. The results indicated that the maximum adsorption capacity of the mesoporous carbon for the U (VI) in the simulated aqueous solution and actual radioactive wastewater was respectively 109.46 mg/g at pH 5.5 and 328.15 K, and 195.6 mg/g at pH 5.5 and the initial U (VI) concentration of 189.75mg/L. The adsorption data could be well described by the pseudo-second-order model and Freundlich isotherm model. The adsorption of U (VI) on the mesoporous carbon was an endothermic and spontaneous process. The adsorption mechanism may be a

Corresponding author at: School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China. E-mail address: [email protected] (X. Lin).

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complex chemical reaction between uranium and the oxygen-containing functional groups on the mesoporous carbon. Keywords: Hydrothermal; Template; Uranium (VI); Adsorption. 1. Introduction Uranium is one of the most widely used radioactive elements. However, uranium is highly toxic in all of its oxidation states, which has caused a potential environment and health risk to the biosphere through water pollution.1-3 Therefore, the removal and enrichment of uranium from the waste water are necessary in order to avoid environmental contamination and save uranium resources. At present, a variety of methods have been used in U (VI) removal and enrichment process, including ion exchange,4-5 coagulation sedimentation6 and adsorption.7-8 Among those methods, adsorption has attracted a great deal of attention from scientists because of its process simplicity, wide application range and high removal efficiency without releasing harmful by-products.9 In recent years, carbonaceous materials in adsorbent have attracted attention due to their superior performance in adsorption.10 In addition, the raw materials for carbonaceous materials are extensive. Currently, people prefer to use various biomass materials and waste as carbon sources. For example, Akshay Jain et al. reported that hydrothermal carbon with mesoporous areas of up to 94% has been prepared from coconut shell and ZnCl2, which is used in electrode materials. Huaming Cai et al. reported one-step hydrothermal synthesis of carbonaceous spheres from glucose with an aluminum chloride catalyst and its adsorption characteristic for uranium (VI).11-12 Besides, there are corn cobs, sawdust, fruit shell, pollen grains, LSs and other natural substances as carbon sources.13-16 LSs is the third most abundant natural polymer, next to cellulose and chitin, and ranks as one of the most abundant phenolic natural polymers, where LSs has been used as a carbon source to prepare various carbonaceous materials due to its high carbon content and solubility in water.17 Moreover, LSs can be obtained from a variety of low-cost precursors such as woody


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plants and even industrial waste such as paper mill effluents.18 Therefore, the use of LSs as a carbon source can not only protect the environment but also save resources. It is well known that there are many methods for preparing carbonaceous materials wherein the hydrothermal method is presently more popular. However, the structural properties of hydrothermal carbon are not very good, such as a low specific surface and pore size.19 Thus, in order to improve the structural properties of hydrothermal carbon, the hydrothermal method and the template method are combined in this paper to prepare the mesoporous carbon to greatly improve the specific surface area and pore volume of the new carbon material. The template method has been widely used in the preparation of carbon materials because of its simple method, high repetition rate, good predictability and stable performance. Simultaneously, the template method includes a hard template and a soft template method.20 Yet, for hard templates, they require prefabrication and subsequent removal, resulting in the multiple-step processes and high cost. In contrast, the soft templates, which include anionic and cationic surfactants, block copolymers, polymers and macromolecules, do not need to be removed because they may be incorporated into the framework of carbons by carbonization at high temperatures, rendering high cost-effectiveness and environmental benignity for scalable production.21-23 Therefore, the soft templates are chosen in this paper as template agents. The CTAB is a cationic surfactant, which is suitable for dispersing and pore-making. Accordingly, we can choose CTAB as a template agent. 2. Experiment Section 2.1. Materials and Methods 2.1.1. Materials All reagents were of AR grade without further purification in this experiment. LSs (the molecular weight is 5000-10000, from a Sigma-Aldrich) was used as a carbon precursor. CTAB was used as a template and purchased from Sigma-Aldrich.



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Hydrochloric acid and ultrapure water were used as the liquid phase. Uranium nitrate hexahydrate (UO2(NO3)2·6H2O) was used as the preparation of simulated uranium wastewater and purchased from Kelong chemicals, China. Arsenazo-III solution was used as the preparation of uranium indicator by dissolving 1.0 g of the reagent in 1000 mL ultrapure water. 2.1.2. Preparation of mesoporous carbon The mesoporous carbon was prepared as follows. Firstly, LSs (3 g) and HCl (30 ml) were mixed with ultrapure water (30 ml) and stirred for 2 h at room temperature to form a LSs hydrolysis solution. Then CTAB (3 g) was added into the hydrolysis solution and stirred for 60 min. After that, a 100 mL Teflon-lined stainless steel autoclave was 70% filled with this solution, which was placed in a preheated oven and heated at 180 oC overnight. After cooling to room temperature, the black solid powder was separated by centrifugation and washing with ultrapure water and ethanol until the filtrate was colorless and pH neutral. The black solid powder was dried at 60 oC and then calcined at 800 oC for 2 h with a heating rate of 5 oC/min in a tube furnace under N2 atmosphere. Finally the black solid powder was washed with ultrapure water and dried in an oven at 60 oC for 12 h. The formation process of the mesoporous carbon is displayed in Scheme 1. As a control sample, the carbon material was also prepared via the above method from the lignin sulfonate hydrolysis solution without CTAB. The carbon material, the templates agent of CTAB was added or not during the hydrothermal carbonization process of LSs, is respectively denoted as Carbon-LSs+CTAB and Carbon-LSs.

Scheme 1. Schematic diagram of the formation process of the Carbon-LSs+CTAB.


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2.2. Characterization The mesoporous carbon was imaged by transmission electron microscopy (TEM; Carl zeiss irts and instrument operated at 200 kV). A field-emission scanning electron microscopy (SEM; Zeiss Ultra 55) was employed to characterize the surface morphology. Nitrogen adsorption-desorption measurement was used to analyze the mesoporous pore size of the carbon using a JW-BK112 mesoporous pore-size analyzer. The carbon powder was dispersed in the ultrasonic disperser for 10 min, and then a laser particle size analyzer (90PLUS, Brookhaven company, USA) was used to analyze the particle size of the carbon. Fourier transform infrared spectroscopy (FTIR) of mesoporous carbon was recorded on Perkin Elmer Nicolet-5700 FTIR Spectrophotometer. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a spectrometer (XPS; ESCALAB250, Thermo Fisher Corportation, USA) to obtain the elements binding energies of C, O, and U. 2.3. Batch adsorption experiment of mesoporous carbon for U (VI) Triplicate batch experiments were conducted using 200 mL Erlenmeyer flask at room temperature (298.15 K) on an electrically thermostatic reciprocating shaker at 180 r/min. In this adsorption experiment, we considered the effect of pH, time, sorbent dose, concentration and temperature on the adsorption of uranium from the aqueous solution. After the adsorption experiment, solid-liquid separation was achieved by filtration and the residual U (VI) concentrations of filtrates were analyzed by a U-3900 ultraviolet spectrophotometer. The adsorption capacity qe (mg/g) and removal efficiency RE (%) of mesoporous carbon were calculated by: qe =

(C 0 − C e ) V m

RE =

(1)

C0 − Ce × 100% C0

(2)



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where C0 (mg/L) and Ce (mg/L) stands for the initial and final equilibrium concentration of U (VI), respectively. V (L) is the volume of solution and m (g) is the mass of adsorbent.

3. Results and Discussion 3.1. Morphology and Microstructure of mesoporous carbon The surface morphologies of Carbon-LSs and Carbon-LSs+CTAB are demonstrated by SEM and TEM images (Figure 1). It was depicted in Figure 1a, b that Carbon-LSs+CTAB exhibited spherical and cotton-like shape, whereas Carbon-LSs appeared single spherical shape. As is shown in the TEM images of Carbon-LSs+CTAB and Carbon-LSs (Figure 1c, d), a part of the Carbon-LSs+CTAB appear round and another part of the Carbon-LSs+CTAB appear irregular shape, whereas almost all of the Carbon-LSs are round. The round and irregular structures of Carbon-LSs+CTAB in the TEM images correspond to the spherical and cotton-like texture of Carbon-LSs+CTAB in the SEM images, respectively. A part of the Carbon-LSs+CTAB with cotton-like texture and the other with spherical shape existing in the carbon powder in this study may be due to that partial LSs is hydrolyzed and the other not.24 The reaction mechanism was speculated and illustrated in Figure 2. As is shown in Figure 2, firstly, the ammonium ion in CTAB reacts with the sulfonate group in LSs in the presence of H+ (coming from HCl) and CTAB. Then, one part of LSs is hydrolyzed into monomer and dimer,25 which is polymerized to form the mesoporous carbon with spherical shape by dehydration reaction between monomer and dimer. Meanwhile, the other part of the LSs, which is not hydrolyzed, forms a mesoporous carbon with cotton-like texture by solid-solid reaction under hydrothermal conditions.26 Finally, the CTAB is removed by the calcination of the mesoporous carbon at 800 oC.


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Figure 1. SEM images of (a) Carbon-LSs+CTAB, (b) Carbon-LSs; TEM images of (c) Carbon-LSs+CTAB, (d) Carbon-LSs

Figure 2. Possible formation mechanism of Carbon-LSs+CTAB 3.2. Surface area, porosity and particle size analysis of mesoporous carbon The nitrogen adsorption-desorption isotherm of mesoporous carbon is presented in Figure 3. And the specific surface, pore volume and average pore size of mesoporous carbon are listed in Table 1. It can be seen from Figure 3, whether the templates agent is added or not during the hydrothermal carbonization process of LSs, the nitrogen adsorption-desorption isotherm of all of the carbon material exhibit similar pattern, which is composite of type III and type V adsorption isotherms



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according to IUPAC classification. The result reveals that the carbon material has mainly mesoporous structure with wide pore size distribution.27 From Table 1, we can see that the Brunauer-Emmett-Teller (BET) surface area of Carbon-LSs+CTAB, which the template agent is added in the hydrothermal carbonization process, is nine times more than that of Carbon-LSs, without addition of template agent in the hydrothermal carbonization process. Meanwhile, the Barrett-Joyner-Halenda (BJH) desorption cumulative volume and Barrett-Joyner-Halenda (BJH) desorption average pore width of Carbon-LSs are slightly high than that of Carbon-LSs+CTAB (Table 1). However, the median diameter and mean diameter of Carbon-LSs+CTAB is twice smaller than that of Carbon-LSs, as seen from Table 1. It is supposed that the high specific surface area of Carbon-LSs+CTAB may be caused by small diameter and cotton-like texture of the carbon material. Simultaneously, the results indicate that the addition of template can improve the structural properties, especially, the specific surface area of carbonaceous material, which is beneficial for the adsorption of U (VI). Based on the above results, the Carbon-LSs+CTAB is selected as the adsorbent for the following U (VI) adsorption test.

Figure 3. Nitrogen adsorption-desorption isotherm of Carbon-LSs+CTAB (a) and Carbon-LSs (b), respectively. Table 1. Specific surface, pore volume desorption cumulative, desorption average width and particle size of Carbon-LSs+CTAB and Carbon-LSs Sample

S

V

pore size

median diametereter dean diameter


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(m2/g) Carbon-LSs+CTAB 633.14 Carbon-LSs

66.734

(cm3/g)

(nm)

(nm)

(nm)

0.407

4.589

720.3

722.1

0.414

5.171

1453.2

1456.8

3.3. Effect of pH on adsorption The pH of the solution determines the surface charge and the ionization degree of the adsorbent, and adsorbent characteristics.28 Therefore, the pH is one of the most important parameters affecting the adsorption process. The effect of the solution pH on the adsorption capacity of Carbon-LSs+CTAB toward U (VI) is given in Figure 4. The adsorption capacity of Carbon-LSs+CTAB for U (VI) increased in the range of pH 1.5-5.5, and reached a maximum of 223.49 mg/g at pH 5.5. When the pH >5.5, the adsorption capacity of Carbon-LSs+CTAB tended to decline gradually. The effect of pH on the adsorption of U (VI) is interpreted as follows. Firstly, at low pH, there is a competitive relationship between H+ and U (VI), and a large amount of H+ occupies a large number of adsorption sites, resulting in the slow adsorption of U (VI). Secondly, with the increase of the solution pH, in the range of 3 to 5.5, the amount of H+ decreases, the competition from U (VI) becomes strong and the uranium exists in the form of UO2OH+, (UO2)3(OH)5+ and (UO2)2(OH)22+, which are beneficial for being adsorbed.29 Finally, at the pH more than 5.5, the adsorption capacity of Carbon-LSs+CTAB for U (VI) declines due to the precipitation of uranium (UO2(CO3)2−,UO2(CO3)24−). Other research groups also received similar results.28,30 The pH value of 5.5 is selected in following adsorption experiment.



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Figure 4. Effect of the pH on the adsorption of U (VI) onto the Carbon-LSs+CTAB. (C0=140 mg/L, t=24 h, V=50 ml, T=298.15 K and m=0.02 g) 3.4. Effect of sorbent dose on adsorption Sorbent dose is an important factor affecting on adsorption capacity of the sorbent at a certain initial concentration. The effects of sorbent dose varied from 0.01 to 0.06 g on the adsorption capacity and removal efficiency of Carbon-LSs+CTAB for U (VI) were given in Figure 5. In the range of sorbent dose between 0.01-0.04 g, the removal efficiency of Carbon-LSs+CTAB increased with the increasing quality of sorbent dose, which was attributed to the increase of surface area and active adsorption sites.31 Subsequently, when the sorbent dose was higher than 0.04 g, the adsorption of uranium was close to equilibrium. Conversely, the adsorption capacity of Carbon-LSs+CTAB decreased with increasing sorbent dose, which was probably due to that the adsorption active sites on the adsorbent were unsaturated when the amount of the adsorbent increased under the condition of the total amount of U (VI) in the solution was constant.32 Therefore, 0.04 g of sorbent was used in following batch experiments.


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Figure 5. Effect of the sorbent dose on the adsorption of U (VI) onto the Carbon-LSs+CTAB. (C0=140 mg/L, t=24 h, V=50 ml, T=298.15 K and pH=5.5) 3.5. Effect of contact time and adsorption kinetics on adsorption The effect of contact time on the adsorption capacity of Carbon-LSs+CTAB for U (VI) is shown in Figure 6. It was clear from Figure 6 that the adsorption capacity of Carbon-LSs+CTAB increased rapidly within 9 h. And the adsorption capacity of Carbon-LSs+CTAB was slowly increased between 9-19 h, with a maximum of 223.49 mg/g at the 19 h. Consequently, subsequent adsorption experiments were performed for 24 h in order to ensure complete equilibrium. The kinetic data were analyzed using the following pseudo-first-order and pseudo-second-order equations, respectively:33 In(q e − q t ) = Inq e − k1t

(3)

t 1 t = + 2 q t k 2q e qe

(4)

where qe (mg/g) and qt (mg/g) are the adsorption capacity of Carbon-LSs+CTAB for U (VI) at equilibrium and at any time, t (min), respectively. The k1 (min-1) and k2 (g/(mg·min)) are the pseudo-first-order rate constant and the pseudo-second-order rate constant of adsorption, respectively.



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The results were shown in Table S1 and Figure 7. Comparing the two models, the calculated equilibrium adsorption capacity qe (134.1 mg/g) in the pseudo-second-order model was closer to the experimental value (119.97 mg/g) and the R2 value (0.9976) was also higher than that of pseudo-first-order kinetic model. The results indicated that the adsorption of U (VI) onto Carbon-LSs+CTAB fitted the pseudo-second-order kinetics model well, which implied that the rate limiting step in adsorption of U (VI) process was chemisorption.34

Figure 6. Effect of contact time on the adsorption of U (VI) onto the Carbon-LSs+CTAB. (C0=140 mg/L, pH=5.5, V=50 ml, T=298.15 K and m=0.04 g)


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Figure 7. (a) and (b) Pseudo-first-order kinetics and pseudo-second-order kinetics plots for U (VI) adsorption on Carbon-LSs+CTAB, respectively. 3.6. Effect of initial concentrations and isotherm models on adsorption The adsorption capacity of Carbon-LSs+CTAB for U (VI) was investigated in the range of 20-340 mg/L. As is shown in Figure 8, the adsorption capacity of Carbon-LSs+CTAB increases along with the increasing of initial U (VI) concentration and reaches equilibrium at a concentration of 300 mg/L.

Figure 8. Effect of initial U (VI) concentration on the adsorption of U (VI) onto the Carbon-LSs+CTAB. (t=24 h, pH=5.5, V=50 ml and m=0.04 g) The adsorption isotherm can not only investigate the adsorption capacity of adsorbent, but also illustrate the adsorption mechanism. In this study, Langmuir and Freundlich isotherms are applied to evaluate the adsorption equilibrium data.35 The Langmuir model, a simple theoretical adsorption isotherm, has been derived based on the assumption of adsorption homogeneity, monolayer surface coverage and interaction between adsorbed species.36 The linear and non-linear forms of Langmuir isotherm equation can be written as:37



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Ce C 1 = + e (linear) q e K Lq m q m qe =

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(5)

K Lq m Ce (non-linear) 1 + K L Ce

(6)

Where Ce (mg/L) is the equilibrium U (VI) concentration. KL (L/mg) is the Langmuir equilibrium adsorption constant, which is related to the adsorption capacity. The qm (mg/g) is the maximum adsorption capacity of Carbon-LSs+CTAB for U (VI). As an empirical equation, Freundlich sorption presumes a multilayer sorption on a heterogeneous surface and supposes that molecules binding on the surface site will affect the adjacent sites.38 The linear and non-linear forms of Freundich isotherm equation are described as follows:37

Inq e = InK F + 1/ n

q e = K FC e

1 InCe (linear ) n

(non-linear)

(7)

(8)

where KF ((mg/g)·(L/mg)1/n) and n are the Freundlich equilibrium adsorption constant, which can be determined from the intercept and slope of the linear plot of lnqe against lnCe in Figure 9b. Figure 9 shows the linear and non-linear of Langmuir and Freundlich. The parameters of Langmuir and Freundlich equations are shown in Table S2. As we can see from Table S2, the non-linear Freundlich model is better fitted than linear Freundlich model and Langmuir model (based on the highest regression coefficient value i.e. R2 value), indicating that the adsorption process of U (VI) is a chemical adsorption, and the adsorption belongs to multilayer adsorption.31


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Figure 9 (a) and (c) The linear and non-linear fitting of Langmuir model for U (VI) adsorption on Carbon-LSs+CTAB; (b) and (d) The linear and non-linear fitting of Freundich model for U (VI) adsorption on Carbon-LSs+CTAB.

3.7. Effect of temperature on adsorption The effect of temperature on the sorption of U (VI) onto the Carbon-LSs+CTAB was investigated at different temperatures ranging from 288.15 to 328.15 K. As is shown in Figure 10, with the rising in solution temperature at the constant uranium concentration, the adsorption capacity of Carbon-LSs+CTAB for U (VI) increased. According to the influence of temperature on adsorption of uranium, the thermodynamic equilibrium constant and the reaction Gibbs free energy, enthalpy change and entropy change can be used to estimate the corresponding thermodynamic data. The values of △H0 and△S0 could be calculated from the slope and intercept of the line of 1nK0 versus 1/T (Figure 11). The Gibbs free energy change was determined by equation (11). The effect of temperature on the adsorption of U (VI) onto Carbon-LSs+CTAB was given from calculated thermodynamic parameters (Table S3), which were used to evaluate the thermodynamic feasibility and investigate the



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mechanism involved in the adsorption of U (VI), were obtained by employing the following equations from the experimental data:39 K0 = qm × KL

InK0 =

(9)

− ∆H 0 1 ∆S0 × + R T R

(10)

∆G 0 = −RTInK0

(11)

where △H0,△S0, and △G0 are enthalpy change (kJ/mol), entropy change (J/(mol·K)) and Gibbs free energy change in a given process (kJ/mol), respectively. K0 is the distribution coefficient and R is the ideal gas constant (8.314 J/(mol·K)). It can be seen from Table S3 that the value of △H0 is positive, indicating that the adsorption of uranium on Carbon-LSs+CTAB is an endothermic reaction. △S0 value is positive, indicating that the process of uranium removal by Carbon-LSs+CTAB will increase the degree of confusion. The negative values of △G0 indicate that the adsorption of uranium ions on Carbon-LSs+CTAB is a spontaneous process. In summary, in a temperature ranging from 288.15 to 328.15 K, the adsorption of uranium ions on Carbon-LSs+CTAB is a spontaneous and endothermic process.

Figure 10. Effect of temperature on the adsorption of U (VI) onto the Carbon-LSs+CTAB.


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(t=24 h, pH=5.5, V=50 ml and m=0.04 g)

Figure 11. The plot of ln K0 versus 1/T for the adsorption of U (VI) onto the Carbon-LSs+CTAB.

3.8. Chemical Structure of Carbon-LSs+CTAB before and after adsorption The FTIR spectra of Carbon-LSs+CTAB before and after adsorption of U (VI) are represented in Figure 12. The band at 3433 cm-1 belongs to characteristic hydrogen bonded phenolic O-H vibration bands. The peaks at 2923 and 2854 cm-1 are attributed to C-H stretching vibrations.10 The band at 1634 cm-1 in the spectrum belongs to C=C stretching of aromatic.40 Meanwhile, some bands appeared in the 1452 and 1383 cm-1 are attributed to C-H bending vibrations.41 The band at 1105 cm-1 corresponded to the alcohol and phenol C-O vibration. Furthermore, comparing the spectrum of mesoporous carbon (Figure 12a), there appeared a distinct band at 924 cm-1 showed in Figure 12b, which strongly affirmed that U (VI) had been successfully adsorbed on the Carbon-LSs+CTAB.34



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Figure 12. (a) and (b) The FTIR spectra of Carbon-LSs+CTAB and Carbon-LSs+CTAB-U (VI). The XPS wide scan data of Carbon-LSs+CTAB before and after adsorption are presented in Figure 13e. It can be seen that the U 4f peaks are found after adsorption of U (VI), which corroborates the existence of U (VI) on Carbon-LSs+CTAB after adsorption. Moreover, the C 1s peaks at 284.6 and 285.9 eV is related to the carbon group (C=C, CHx, C-C), and -C-OR.42 Figure 13c,d is the XPS O 1s spectra of Carbon-LSs+CTAB, which contain two different atomic states of oxygen. The peak at 532.0 eV is assigned to the oxygen in C=O groups. The peak at 534.6 eV is related to the -COOH groups.43 Compared the O 1s XPS peaks in Figure 13c with the corresponding peaks in Figure 13d, it was founded that the binding energy significantly increases and the normalized intensity decreases comparing to that for Figure 13d, which could be attributed to the overlap of bond energy both uranyl and carbonyl.44 This result showed that the sorption of uranyl ions was mainly attributed to

the

complexation

of

oxygen-containing

functional

Carbon-LSs+CTAB.


groups

on

the

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Figure 13. XPS spectra of (a-b) C 1s for Carbon-LSs+CTAB before and after U (VI) adsorption. (c-d) O 1s for Carbon-LSs+CTAB before and after U (VI) adsorption. (e) XPS spectra of Carbon-LSs+CTAB before and after U (VI) adsorption, (f) High resolution spectra of U 4f after U (VI) adsorption. 3.9. Mechanism analysis of uranium adsorption In this study, Carbon-LSs+CTAB was synthesized from LSs using CTAB as template by hydrothermal and template method. The analysis of FTIR and XPS reveals the Carbon-LSs+CTAB possesses a large amount of oxygen-containing



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functional groups, which can take part in a complex chemical reaction with uranium. The adsorption mechanism was speculated and illustrated in Figure 14 (a) The uranium ions can participate in ion-exchange reaction with the hydrogen ions on the oxygen-containing functional groups to achieve uranium removal. (b) The uranium ions can participate in coordination reaction with two oxygen atoms in hydroxyl of Carbon-LSs+CTAB.31,45

(a)

(b)

Figure 14. Supposed mechanism of the adsorption of U (VI) onto the Carbon-LSs+CTAB. 3.10. The effectiveness of the adsorbent applied in actual radioactive wastewater In order to demonstrate the effectiveness of the adsorbent of Carbon-LSs+CTAB applied in actual radioactive wastewater, the sample of Carbon-LSs+CTAB was supplied by us to China Academy of Engineering Physics, the adsorption experiment was conducted and the results were supplied by the researchers in the institute. The adsorption experiment was conducted at the optimum experimental conditions obtained from the above batch experiment, such as pH=5.5, t=24 h, V=50 ml, T=298.15 K and m=0.04 g, using 200 mL Erlenmeyer flasks on an electrically thermostatic reciprocating shaker. The concentrations of uranium in actual radioactive wastewater before and after adsorption was measured with a trace uranium analyzer (WGJ-III, Daji Photoelectric Instrument Co Ltd, Hangzhou), and the results were presented in the Table 2. We can see from Table 2 that the adsorption capacity of Carbon-LSs+CTAB increases along with the increasing of uranium concentration of


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the actual radioactive wastewater. The maximum adsorption capacity of the mesoporous carbon for the U (VI) in the actual radioactive wastewater was 195.6 mg/g at pH 5.5 and the initial U (VI) concentration of 189.75 mg/L. The results have demonstrated clearly that the Carbon-LSs+CTAB adsorbent is very promising for the removal of U(VI) from the actual radioactive wastewater. Table 2 The adsorption capacity of Carbon-LSs+CTAB applied in actual radioactive wastewater with different initial U (VI) concentration Initial concentration 13.22

76.80

189.75

7.067

45.47

195.60

(mg/L) Adsorption capacity (mg/g)

4. Conclusion In this study, Carbon-LSs+CTAB with a high specific surface area was successfully prepared for adsorption of U (VI) from aqueous solutions. The BET indicates that the specific surface area of the Carbon-LSs+CTAB is nine times more than that of the Carbon-LSs. The formation of mesoporous carbon with spherical shape and cotton-like texture may be through the process of hydrolysis, polymerization and solid-solid reaction of LSs under hydrothermal conditions. The results of batch experiments of Carbon-LSs+CTAB show that the maximum adsorption capacity of 109.46 mg/g is obtained when the pH value and temperature of the solution are 5.5 and 328.15 K, respectively. The result of Carbon-LSs+CTAB applied in actual radioactive wastewater indicates the adsorption capacity of Carbon-LSs+CTAB increases along with the increasing of uranium concentration of the actual wastewater. And the maximum adsorption capacity of the absorbent for the U (VI) in the actual radioactive wastewater is 195.6 mg/g at pH 5.5 and the initial U (VI) concentration of 189.75 mg/L. In addition, the adsorption kinetic and isotherm were well fitted by the pseudo-second-order kinetic model and Freundlich isotherm model, respectively. The thermodynamic parameters for the sorption process indicated



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that the adsorption of U (VI) onto Carbon-LSs+CTAB was endothermic and spontaneous. The adsorption mechanism of U (VI) by Carbon-LSs+CTAB could be proposed of the ion-exchange and coordination reaction.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Parameters of pseudo-first-order kinetics and pseudo-second-order kinetics for U (VI) removal (Table S1), parameters of Langmuir and Freundlich equations for U (VI) removal (Table S2) and three basic thermodynamic parameters (△H0, △S0, △G0) (Table S3).

Author Information Corresponding Author *E-mail: [email protected]. Phone: +86 08166089372.

Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by the National Key Scientific Projects for Decommissioning of Nuclear Facilities and Radioactive Waste Management (14zg6101) and Longshan academic talent research support plan of Southwest University of Science and Technology (17LZX302). The authors gratefully thank the technology support of Engineering Research Center of Biomass Materials, Ministry of Education, Southwest University of Science and Technology.

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Abstract Graphic


Preparation of mesoporous carbon from sodium lignosulfonate by

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