Adsorption of Uranium(VI) from a Simulated Saline Solution by Alkali

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Adsorption of Uranium(VI) from a Simulated Saline Solution by Alkali-Activated Leather Waste Tingsong Yan,†,‡ Xuegang Luo,*,†,‡ Zhongqiang Zou,†,‡ Xiaoyan Lin,†,‡ and Yu He†,‡ †

Department of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China ‡ Engineering Research Center of Biomass Materials, Ministry of Education, Mianyang, Sichuan 621010, China S Supporting Information *

ABSTRACT: A porous adsorbent was prepared from leather waste by activation with alkali. The adsorbent, alkali-activated leather waste (AALW), was applied to adsorb uranium(VI) and characterized by scanning electron microscopy, energy-dispersive X-ray detection, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The influence of the pH, initial uranium(VI) concentration, temperature, and contact time on the adsorption of uranium(VI) was systematically investigated. The adsorption of uranium(VI) on AALW obeyed the Langmuir isotherm model and was attributed to ion exchange and complexation coordination. Thermodynamic and kinetic studies showed that the adsorption process was spontaneous and endothermic, and it reached adsorption equilibrium in 360 min. Moreover, the selective adsorption of uranium(VI) from an aqueous solution containing coexisting ions and adsorption of trace uranium(VI) from a simulated high-salinity environment showed that AALW had not only a strong affinity but a high selectivity for uranium(VI).

1. INTRODUCTION Uranium, a radioactive metal, is one of the heaviest elements found in nature. It was widely used in nuclear fuel after the discovery of nuclear fission. In the background of a global lowcarbon economy, nuclear fuel was proven to be a better choice for supplying energy because of its clean energy, large power generation, and high efficiency. In 2011, 14% of the world’s electricity was supplied by nuclear power.1 With the development of nuclear technologies, the demand for uranium is growing rapidly and uranium resources in minerals are expected to decrease over time, leading to an uncertain future. Hence, the heavy demand requires that researchers refine uranium from other resources such as seawater.2 Nevertheless, the extremely low concentration of uranium (3.3 μg L−1) in seawater increases the difficulty of extraction.3 In addition to uranium in ores and seawater, salt lake is another important uranium resource. The concentration of uranium in a salt lake is about 1000 times higher than that in seawater.4 Therefore, it is necessary to research uranium extraction from salt lake brines, which is expected to expand the scope of uranium resources. Up to now, many methods have © XXXX American Chemical Society

been proposed for uranium extraction from aqueous solutions.5,6 Among them, adsorption is one of the most promising methods. As we know, the adsorbent is a key factor in adsorption. In recent years, porous materials such as porous silica7 and metal oxide8 have been widely used for adsorption because of their structural flexibility. It should be noted that these porous materials have some shortcomings such as high cost, low yield,9 and easily-caused secondary pollution because they are difficult to degrade.10 Alternative adsorbents from industrial wastes and natural materials with low-cost and easy degradation properties have been widely investigated recently.11,12 In China, the utilization of pelts is only about 35%, and the solid leather waste generated is enormous.13 So, it is of significant importance to handle these leather wastes reasonably. Leather waste is abundant in collagen fibers, Received: Revised: Accepted: Published: A

November 14, 2016 March 2, 2017 March 5, 2017 March 5, 2017 DOI: 10.1021/acs.iecr.6b04425 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Technology Corp., Beijing, China). AALWs before and after uranium(VI) adsorption were measured by a FTIR spectrometer (Nicolet-5700, PerkinElmer Instruments Corp., Waltham, MA) in the wavenumber range of 400−4000 cm−1. The binding energies of the elements before and after uranium(VI) adsorption were obtained by XPS (Thermo Scientific Escalab 250, Thermo Fisher Corp., Waltham, MA). 2.4. Adsorption Experiments. A total of 0.05 g of AALW was added to a conical flask that contained 50 mL of a uranium(VI) solution or multiion solution at a given pH. The pH of the solution was adjusted by 0.1 mol L−1 HNO3 or 0.1 mol L−1 NaOH solutions and determined by a pH meter (PHS3BW PHscan, Bante Instruments, Shanghai, China). Then, the mixtures were agitated in a shaking apparatus at the investigated temperature for a predetermined time. After that, solid−liquid separation was achieved by filtration, and the residual uranium(VI) concentrations of the filtrates were analyzed by a UV−vis spectrophotometer (UV-3900, Hitachi Corp., Tokyo, Japan; the limit of analytical detection of uranium is 0.1 mg L−1) with arsenazo(III) as the complex agent.21 The uranium(VI) concentrations were measured at 650 nm using a calibration curve (linear equation: Y = 0.24071X − 0.00255, where X is the concentration of uranium(VI) in mg L−1 and Y is the absorbency; R2 = 0.999), which was established from the known concentration of standard uranium(VI) solutions (0.4, 0.8, 1.6, 2.4, 3.2, and 4 mg L−1, respectively). During a study of the selectivity of AALW, the concentrations of uranium(VI) and other metal ions were determined by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7700X, Santa Clara, CA; the limit of analytical detection of uranium is 0.3 × 10−3 mg L−1). The equilibrium uranium adsorption capacity was calculated according to the following equation:22

which offer many functional groups (such as amino, carboxyl, and hydroxyl groups) for binding metal ions. Many researchers extracted the collagen fibers from leather waste as adsorbents to adsorb organic dyes.14 However, the extraction of collagen fibers is a complicated process, which prevents its wide use in industrial application and environmental fields. Because of its ease of operation and high yield, alkali activation is extensively used to promote the adsorption property of the adsorbent.15,16 Meanwhile, alkali activation is known to be effective for the pore formation of biomaterials used for heavy-metal ion adsorption.17 Some biomaterials have been modified with NaOH to adsorb heavy-metal ions, such as Aleppo pine sawdust18 and pummel peel.19 Alkali activation can remove the impurities on the surface of biomaterials and expose more metal-ion binding sites. Buema et al.20 found that the adsorption capacity of power-plant ash for uranium increased from 126 to 206 mg g−1 after alkali activation with 5 mol L−1 NaOH. In this work, chrome-tanned cowhide waste particles were activated by NaOH and a porous leather adsorbent was prepared. The leather adsorbent was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, energy-dispersive X-ray (EDX) detection, and Xray photoelectron spectroscopy (XPS) to analyze the adsorbent and adsorption mechanism. Additionally, the influence of the solution pH, initial uranium(VI) concentration, temperature, and contact time on the adsorption of uranium(VI) was discussed in detail. Meanwhile, different adsorption isotherms and kinetic models were investigated to understand the adsorption process. Furthermore, the adsorption of uranium(VI) from an aqueous solution containing coexisting ions and adsorption of trace uranium(VI) from a simulated high-salinity environment were conducted.

qe = (C0 − Ce)V /m

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. A uranium(VI) stock solution was prepared by dissolving uranyl nitrate [UO2(NO3)2·6H2O] in deionized water. NaOH, HNO3, KNO3, Na2SO4, NaHCO3, Na2CO3, and other metal nitrates were purchased from Chengdu Kelong Chemical Reagent Co., Ltd., China. All reagents were of analytical grade. The chrometanned cowhide waste was provided by the National Engineering Laboratory for Clean Technology of Leather Manufacture of Sichuan University, China, and characterized including the ash content, moisture content, total chromium, pH of soluble matter, and specific surface area. The results are listed in Table S1. 2.2. Preparation of AALW. Chrome-tanned cowhide leather waste was soaked for 48 h in deionized water and cut into uniform square particles of 0.25 × 0.25 cm2 area, and then these particles were dried at 313.15 K for 12 h in an oven. The dried particles (1 g) were immersed in 100 mL of a 0.075 mol L−1 NaOH solution and shaken for 4 h at 323.15 K. After shaking, the leather particles were filtered and washed with deionized water until neutral pH was reached. After being frozen for 12 h and dried for 12 h in vacuum, the adsorbent, alkali-activated leather waste (AALW), was obtained. 2.3. Characterization of the Adsorbent. The surface morphology and element composition of the raw leather waste and AALW were observed by SEM complimented with EDX (Ultra 55, Carl Zeiss, Oberkochen, Germany). The nitrogen adsorption isotherm was performed in an automated volumetric system (JW-BK112, Beijing Subtle Triglobal Science and

(1)

−1

where qe (mg g ) is the adsorption capacity of uranium(VI) on the adsorbent and C0 (mg L−1) and Ce (mg L−1) are the initial and equilibrium uranium(VI) concentrations, respectively. V is the volume of the solution (L), and m is the weight of the adsorbent (g). 2.5. Adsorption of Trace Uranium(VI) from a Simulated High-Salinity Environment. The concentrations of uranium in salt lake brine were several hundreds of micrograms per liter, and some are a few milligrams per liter.4,23,24 Thus, the adsorption of uranium(VI) onto AALW was investigated in low-uranium(VI)-concentration solutions ranging from 0.1 to 10 mg L−1 (0.1, 0.5, 0.8, 5, and 10 mg L−1) at pH 5. Furthermore, salt lake brines contain many kinds of disturbing ions with high concentrations. Na+, K+, Mg2+, Ca2+, Cl−, SO42−, and HCO3− generally existed in salt lake brines, so these common ions were selected to study the effect of the ion strength on the adsorption of trace uranium(VI). The adsorption of uranium(VI) from the binary solutions of UO22+-Na+, UO22+-K+, UO22+-Mg2+, UO22+-Ca2+, UO22+-Cl−, UO22+-SO42−, UO22+-CO32−, and UO22+-HCO3− were implemented at pH 5. The initial concentrations of Na+, K+, Mg2+, Ca2+, Cl−, SO42−, and HCO3− were varied from 0.5 to 8 g L−1. The initial concentration of uranium(VI) was 800 μg L−1. Then, 0.05 g of AALW was mixed with 50 mL of a binary solution, and the mixtures were shaken for 4 h at 298.15 K. After adsorption, solid−liquid separation was achieved by filtration, and the residual uranium(VI) concentrations of the filtrates were analyzed by a uranium microanalyzer (the limit of B

DOI: 10.1021/acs.iecr.6b04425 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research analytical detection of uranium is 0.3 × 10−4 mg L−1) with the standard addition method. The adsorption ratio of uranium(VI) was calculated by the following equation:25 adsorption ratio = −1

Csi − Csf × 100% Csi

(2)

−1

where Csi (μg L ) and Csf (μg L ) are the initial and final uranium concentrations in the binary solutions. All experiments mentioned above were carried out in triplicate tests, and the reported results were the average value of three data sets.

3. RESULTS AND DISCUSSION 3.1. Mechanism Analysis. On the basis of characterization analysis in the Supporting Information, a likely mechanism for uranium(VI) adsorption onto AALW contains ion exchange and complexation. The preparation of AALW and the adsorption mechanism of uranium(VI) on AALW are described in Figure 1. Leather waste is rich in collagen fibers, which are

Figure 2. Effect of the solution pH on the adsorption of uranium(VI) onto an adsorbent (adsorbent dose = 0.05 g; initial uranium concentration = 20 mg L−1; contact time = 24 h; temperature = 298.15 K).

reaching a maximum adsorption capacity at pH 5. At pH values below 2, UO22+ is the predominant ion form and excess H+ competes with UO22+ for active sites on AALW. Moreover, the adsorption sites on the surface of the adsorbent are easily protonated at low pH, causing electrostatic repulsion between the positively charged UO22+ and the protonated adsorbent.27 Hence, the adsorption of uranyl ions is limited, leading to poor adsorption amounts. The content of H+ decreased with increasing pH. Meanwhile, the competition for the adsorption sites and the electrostatic repulsion weakened, leading augmentation of the adsorption amount.28 A further increase of the pH value will generate negatively charged species, including UO2(OH)53− and UO2(OH)75−,29 which resulted in a decrease of the adsorption amount. Therefore, all of the following experiments were conducted at pH 5. 3.3. Effect of the Initial Uranium(VI) Concentration on the Adsorption. In order to study the effect of the initial uranium(VI) concentration, AALW was used for adsorption in different concentrations of uranium(VI) ranging from 20 to 200 mg L−1. It was found that the adsorption amount of uranium on AALW ascended with increasing uranium(VI) concentration and finally reached a plateau of 92.62 mg g−1 at a concentration of 170 mg L−1. The rise of the adsorption amount might result from the fact that the higher initial uranium concentration increased the driving force to overcome the mass-transfer resistance of uranyl ions between the aqueous and solid phases.30 The plateau occurs because of saturation of the adsorption sites on AALW. 3.4. Effect of the Temperature on the Adsorption. It is recognized that high temperature has an effect on the adsorption process because it can promote the movement of molecules. So, the effect of the temperature on adsorption of uranium(VI) onto AALW, ranging from 298.15 to 328.15 K, at different initial uranium(VI) concentrations (20, 50, 80, 110, and 140 mg L−1), was studied. As can be seen from Figure 3, the adsorption capacity of uranium(VI) increased gradually with a rise of the temperature, which suggested an endothermic nature of uranium(VI) adsorption on AALW. This may be ascribed to the diffusion process of uranium(VI) and exchange of UO22+ with Na+ needing the bond breaking of groups.31 3.5. Isotherm Model. To quantify the adsorption data and to gain a better understanding of the adsorption mechanism,

Figure 1. Simple schematic of the process of alkali−alkali and the adsorption of uranium(VI).

closely intertwined. After activation of NaOH, the collagen fibers are hydrolyzed, leading to the porous structure of AALW. Meanwhile, sodium was loaded onto the surface of AALW. In the process of uranium(VI) adsorption, Na+ ions are replaced by some UO22+ ions. Besides, there is coordination complexation between UO22+ and functional groups (amino and carboxyl). 3.2. Effect of the pH on the Adsorption. It is well-known that the pH value of the solution is one of the most important factors affecting the adsorption capacity of the adsorbent. So, the effect of the pH on the uranium(VI) adsorption was first investigated using a series of experiments. At every run, 0.05 g of the adsorbent was added to 20 mg L−1 uranium solutions containing different pH values. The pH values ranged from 1 to 6 because uranyl ions in the designed system would precipitate at higher pH.26 The adsorption amounts of raw uranium(VI) at different pH values are shown in Figure 2, for which it could be found that the adsorption amount of raw leather waste is very low. This is mainly because there are no functional groups that can interact with uranium(VI) because of the presence of large amounts of impurities on the surface of raw leather waste. As illustrated in Figure 2, the adsorption amounts of uranium(VI) on AALW increase dramatically with increasing pH from 1.0 to 5.0. Subsequently, the adsorption amount of uranium(VI) declines, C

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the parameters calculated are given in Table 1. From Table 1, the values of the correlation coefficient (R2) of the Langmuir model are 0.945, 0.931, 0.990, and 0.946 at 298.15−328.15 K, respectively, which are all higher than those of the Freundlich model. Thus, the experimental adsorption data are more satisfactorily fitted by using the Langmuir adsorption isotherm, indicating that the adsorption process is monolayer adsorption on a homogeneous surface. The fitting maximum adsorption capacity of uranium(VI) on AALW is 133.15 mg g−1 at 328.15 K. Generally, the separation factor RL can be used to predict the affinity between the adsorbent and adsorbate. The value of RL can be estimated according to the following equation:34 Figure 3. Effect of the temperature on the adsorption of uranium(VI) onto AALW (pH 5; contact time = 24 h; adsorbent dose = 0.05 g).

RL =

(3)

where qe and Ce are the equilibrium adsorption amount (mg g−1) and the equilibrium concentration of uranium (mg L−1), respectively, qmax is the maximum uranium adsorption capacity (mg g−1), and b is the Langmuir constant related to the energy of adsorption. The Freundlich adsorption isotherm assumes that the adsorption site distributions are not consistent and the adsorbent has different surface properties. It is based on nonideal multilayer adsorption processes. The Freundlich model is given by33 ln qe = ln K f +

⎛1⎞ ⎜ ⎟ ln C e ⎝n⎠

(5)

where b is the Langmuir equilibrium constant and C0 is the initial concentration of uranium(VI). The value of RL indicated the nature of isotherm like unfavorable (RL > 1), linear (RL = 1), and favorable (0 < RL < 1). The calculated values of RL varied from 0.072 to 0.504, which confirmed the suitability of AALW as the adsorbent for uranium(VI) from aqueous solutions. 3.6. Thermodynamic Studies. In order to evaluate the thermodynamic feasibility, the thermodynamic parameters for the adsorption process have been calculated by the following equations:35

the widely used Langmuir and Freundlich models were used to simulate the experimental data. The Langmuir isotherm, the most widespread adsorption model, assumes ion adsorption to the ligand site in a single layer on the surface of the adsorbent without any interaction between the adsorbed molecules. The linear form of the Langmuir isotherm equation can be expressed as follows:32 Ce C 1 = + e qe bqmax qmax

1 1 + bC0

ln K 0 = −

ΔH ° 1 ΔS° + R T R

ΔG° = −RT ln K 0

(6) (7)

where K0 is the distribution coefficient equal to qmaxb of the Langmuir isotherm, R is the universal gas constant (8.314 J mol−1 K−1), T is the temperature (K), ΔH° is the enthalpy change (kJ mol−1), ΔS° is the entropy change (J mol−1 K−1), and ΔG° is the Gibbs free energy change (kJ mol−1) in the given process. The plot of ln K0 versus 1/T is shown in Figure 5, and the thermodynamic parameters for the adsorption process of uranium(VI) on AALW are listed in Table 2. It can be seen that all of the ΔG° values are negative in the temperature range of 298.15−328.15 K, indicating that the uranium(VI) adsorption on AALW is feasible and spontaneous. The positive values of ΔH° confirm that the adsorption of uranium(VI) on AALW is endothermic under the experimental conditions. The positive values of ΔS° suggest increasing

(4)

where Kf and n are the adsorption constants that are related to the adsorption capacity and adsorptive intensity, respectively. Values of Kf and n can be calculated from the intercept and slope of the linear plot of ln qe versus ln Ce. The curves of Langmuir and Freundlich constants obtained by the linear regression method are depicted in Figure 4, and

Figure 4. Linearization of the (a) Langmuir and (b) Freundlich adsorption isotherm models. D

DOI: 10.1021/acs.iecr.6b04425 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Langmuir and Freundlich Models Langmuir

Freundlich

T (K)

experimental qmax(mg g−1)

qmax (mg g−1)

b (L mg−1)

R2

Kf [(mg g−1)(L mg−1)1/n]

n

R2

298.15 308.15 318.15 328.15

83.99 91.93 110.99 118.94

92.93 101.42 120.63 133.15

0.049 0.071 0.077 0.091

0.945 0.931 0.990 0.946

7.585 11.974 13.169 14.581

1.739 1.980 1.790 1.698

0.941 0.884 0.808 0.822

equilibrium was achieved within 360 min. The initial fast adsorption might be attributed to the fact that the adsorption sites were unoccupied at the initial stage and uranium(VI) could interact with them easily. As time went on, the unoccupied adsorption sites reduced gradually and the ion concentration gradient at the solid−liquid interface decreased,36 which inhibited the adsorption rate correspondingly. 3.8. Adsorption Kinetics. To investigate the mechanism of the adsorption process further, the two main types of adsorption kinetic models, namely, pseudo-first-order and pseudo-second-order, were applied to fit the experimental data. The linear forms of the pseudo-first-order and pseudosecond-order models can be respectively expressed as

Figure 5. Plot of ln K0 versus 1/T for the adsorption of uranium(VI) on AALW.

Table 2. Thermodynamics Parameters for the Adsorption of Uranium by AALW T (K)

ΔH° (kJ mol−1)

ΔS° (J mol−1 K−1)

ΔG° (kJ mol−1)

298.15 308.15 318.15 328.15

22.378 22.378 22.378 22.378

88.321 88.321 88.321 88.321

−3.787 −4.997 −5.953 −6.380

ln(qe − qt) = ln qe − k1t

(8)

t 1 t = + qt qe k 2qe 2

(9)

where qe (mg g−1) and qt (mg g−1) are the amount of uranium(VI) adsorbed on the adsorbent at equilibrium and at any contact time t, respectively. k1 (min−1) and k2 (g mg−1 min−1) are the rate constants whose units depend upon the order of the reaction. The parameters of the pseudo-first-order and pseudo-secondorder kinetic models can be obtained directly from the intercept and slope in Figure 7, and the values are listed in Table 3. It could be clearly seen that the correlation coefficient of the pseudo-second-order kinetic model (R2) is higher. Moreover, the calculated value of the adsorption capacity (95.51 mg g−1) of the pseudo-second-order kinetic model is closer to the experimental data (92.26 mg g−1) than that of the pseudo-first-order kinetic model. As a result, the pseudosecond-order kinetic model is more representative than the pseudo-first-order kinetic model for fitting the kinetic data. It is signified that the rate-controlling step is chemical adsorption involving electron sharing or electron transfer between uranium(VI) and AALW.37 3.9. Selective Adsorption of AALW. Generally, efficient adsorption of uranium(VI) often requires high selectivity because there are many competitive metal ions in aqueous solutions. Therefore, in order to evaluate the selectivity of AALW, the adsorption of uranium(VI) from an aqueous solution that contains seven coexisting metal ions was studied at pH 5. The concentrations of these seven coexisting metal ions were 20 mg L−1. As shown in Figure 8, the adsorption capacity of uranium(VI) onto AALW is much higher than those of other metal ions, indicating the excellent selectivity of AALW for uranium(VI). The highly selective adsorption property mainly resulted from the fact that the collagen fibers contain a lot of amino groups. The average U−N bond distance is remarkably shorter than the corresponding distances in other metal−nitrogen cations (i.e., Ni2+, Cd2+, Co2+, and Zn2+),

randomness at the solid−liquid interface during the process of uranium(VI) adsorption. 3.7. Effect of the Contact Time on the Adsorption. The effect of the contact time for AALW on the adsorption of uranium(VI) at pH 5.0 is plotted in Figure 6. It was observed that the adsorption amount of uranium increased linearly with time in the initial 180 min and then slowed gradually until

Figure 6. Effect of the contact time on the adsorption of uranium(VI) onto AALW (pH 5; initial uranium(VI) concentration = 170 mg L−1; adsorbent dose = 0.05 g). E

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Figure 7. (a) Pseudo-first-order and (b) pseudo-second-order models.

concentration, and it reaches as high as 80%, suggesting that AALW can be a promising adsorbent for uranium recovery from an aqueous solution containing a low concentration of uranium. The influence of the ion strength (Na+, K+, Mg2+, Ca2+, Cl−, SO42−, and HCO3−) is exhibited in Figure 10. From Figure 10a, it can be clearly seen that there is no suppression in the uranium(VI) adsorption when the ion strength increases to 8 g L−1 (the molar ratio of ions to uranium was 10000). As shown in Figure 10b, the adsorption ratio of uranium(VI) was limited by the rise of HCO3−. This may be attributed to hydrolysis of UO22+ with HCO3−, causing the existing form of uranium to change into UO2(CO3−)22−. AALW has no affinity with the uranium charged negatively, so the adsorption ratio decreases with increasing HCO3−. 3.11. Desorption of AALW. The HNO3 solution was the universal reagent for the desorption of uranium(VI) from the adsorbent.38 According to the result that that adsorption of uranium(VI) by AALW was poor at low pH (Figure 2), the desorption of uranium(VI) adsorbed by AALW was carried out in HNO3 solutions with different pH values ranging from 0.5 to 3. The result is shown in Figure 11. The desorption efficiency of AALW was poor when pH > 1.5, and it increased with decreasing pH. The highest desorption efficiency was achieved at pH 0.5. However, the overly acidic environment could destroy the collagen fibers on AALW, which has an influence on the adsorption property of the adsorbent. The desorption efficiency at pH 1.5 is much closer to the value at pH 1 and could reach up to 90%. Furthermore, AALW is stable in the solution of pH 1.5. Therefore, pH 1.5 was selected as the optimum desorption pH in this study. 3.12. Comparison with Other Adsorbents. A comparison of the adsorption capacity of different adsorbents for uranium(VI) has been made among AALW and previously reported adsorbents. The results are shown in Table 4. All of them are low-cost adsorbents that come from industrial and agricultural waste. It can be found that the adsorbents activated by NaOH are superior to unmodified adsorbents. Besides, the adsorption capacity of AALW is higher than those of most of the adsorbents listed in Table 4. Therefore, AALW can be used as an efficient adsorbent with low cost for uranium(VI) ions.

Table 3. Parameters of Pseudo-First-Order and PseudoSecond-Order Kinetics pseudo-first-order model qe(mg g−1) 80.27

k1(min−1) 0.010

R2 0.960

pseudo-second-order model qe(mg g−1) 95.51

k2(g mg−1 min−1) 0.4 × 10

−3

R2 0.998

Figure 8. Competitive adsorption capacities of coexisting ions onto AALW (initial concentration of the metal ions = 20 mg L−1; adsorbent dose = 0.05 g; contact time = 4 h; temperature = 298.15 K).

leading to a strong trend of nitrogen-containing functional groups toward UO22+.34 3.10. Adsorption of Trace Uranium(VI) from a Simulated High-Salinity Environment. As shown in Figure 9, although at low concentration, the adsorption ratio of uranium increases with a rise of the initial uranium(VI)

4. CONCLUSIONS In this work, a porous adsorbent was prepared from leather waste by a simple method of activation with alkali (AALW). AALW was applied to uranium(VI) adsorption and characterized by SEM, FTIR, EDX, and XPS analysis. Meanwhile, the influence of the pH, initial uranium(VI) concentration, temperature, and contact time on the adsorption of uranium-

Figure 9. Adsorption ratio of uranium(VI) onto AALW at low initial concentration (pH 5; adsorbent dose = 0.05 g; temperature = 298.15 K; contact time = 4 h). F

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Figure 10. Adsorption of trace uranium(VI) onto AALW from a high-salinity environment: (a) cation strength; (b) anion strength (pH 5; initial uranium concentration = 800 μg L−1; adsorbent dose = 0.05 g; contact time = 4 h; temperature = 298.15 K).

133.15 mg L−1 at 328.15 K. The adsorption kinetic data fitted well with the pseudo-second-order model. (3) Thermodynamic implied that the adsorption process was spontaneous and endothermic. (4) The selective adsorption of uranium(VI) from an aqueous solution containing coexisting ions and adsorption of trace uranium(VI) from a simulated high-salinity environment indicated that AALW had not only a strong affinity but also a high selectivity for uranium(VI). Therefore, AALW has great potential to be a low-cost and high-efficiency adsorbent for uranium recovery from a highsalinity environment.

■

* Supporting Information

Figure 11. Desorption of uranium(VI) by AALW under different pH values (adsorbent dose = 0.05 g; initial uranium concentration = 20 mg L−1; contact time = 4 h; temperature = 298.15 K).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04425. Experimental details, Table S1, and analyses (PDF)

■

Table 4. Comparison of the Adsorption Capacities for Uranium(VI) of Different Adsorbents adsorption capacity (mg g−1)

ASSOCIATED CONTENT

S

AUTHOR INFORMATION

adsorbent

pretreatment

pH

concn of uranium(VI) (mg L−1)

wheat straw dried tea wastes poplar leaves sawdust sawdust coir pith power plant ash power plant ash leather waste AALW

NaOH

3.0

100

11.29

16

unmodified

4.0

1000

59.5

39

The authors declare no competing financial interest.

unmodified

4.0

10

2.3

40

unmodified NaOH unmodified unmodified

5 5 4−6 3

250 350 200 1400

19.95 48.66 97.43 126

18 18 41 20

NaOH

3

1000

206

20

ACKNOWLEDGMENTS This work was financially supported by the Nuclear Energy Special Project (Grant 13zg610301) and a Postgraduate Innovation Fund Project by Southwest University of Science and Technology (Grant 16ycx017). Technology support of Engineering Research Center of Biomass Materials, Ministry of Education, Southwest University of Science and Technology, is greatly appreciated.

unmodified

5

20

3.41

NaOH

5

140

133.15

Corresponding Author

*E-mail: [email protected] or [email protected]. Phone: +86 816 6089009.

ref

Notes

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this study this study

REFERENCES

(1) Karakosta, C.; Pappas, C.; Marinakis, V.; Psarras, J. Renewable energy and nuclear power towards sustainable development: Characteristics and prospects. Renewable Sustainable Energy Rev. 2013, 22, 187. (2) Anderson, R. F. Concentration, vertical flux, and remineralization of particulate uranium in seawater. Geochim. Cosmochim. Acta 1982, 46, 1293. (3) Kim, J.; Tsouris, C.; Mayes, R. T.; Oyola, Y.; Saito, T.; Janke, C. J.; Dai, S.; Schneider, E.; Sachde, D. Recovery of uranium from seawater: A review of current status and future research needs. Sep. Sci. Technol. 2013, 48, 367.

(VI) by AALW was systematically investigated. The results are as follows: (1) The adsorption mechanism for uranium(VI) by AALW was mainly attributed to coordination complexation and ion exchange. (2) The adsorption isotherm can be well described by the Langmuir model, and the maximum adsorption capacity was G

DOI: 10.1021/acs.iecr.6b04425 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b04425 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX