Glycerol-Modified Binary Layered Double Hydroxide

Research Article pubs.acs.org/journal/ascecg

Glycerol-Modified Binary Layered Double Hydroxide Nanocomposites for Uranium Immobilization via Extended X‑ray Absorption Fine Structure Technique and Density Functional Theory Calculation Yidong Zou,†,‡ Yang Liu,† Xiangxue Wang,† Guodong Sheng,† Suhua Wang,† Yuejie Ai,*,† Yongfei Ji,§ Yunhai Liu,*,‡ Tasawar Hayat,∥ and Xiangke Wang*,†,∥ Downloaded via UNIV OF TOLEDO on June 29, 2018 at 20:13:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

School of Environment and Chemical Engineering, North China Electric Power University, Beijing 102206, P. R. China School of Chemistry, Biological and Materials Sciences, East China Institute of Technology, Nanchang, 330013, P. R. China § Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, Roslagstullsbacken 15, 10691 Stockholm, Sweden ∥ NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: Novel, efficient, glycerol-modified nanoscale layered double hydroxides (rods Ca/Al LDH-Gl and flocculent Ni/Al LDH-Gl) were successfully synthesized by a simple onestep hydrothermal synthesis route and showed excellent adsorption capacities for U(VI) from aqueous solutions under various environmental conditions. The advanced spectroscopy analysis confirmed the existence of abundant oxygen-containing functional groups (e.g., C−O, O−CO, and CO) on the surfaces of Ca/Al LDH-Gl and Ni/Al LDH-Gl, which could provide enough free active sites for the binding of U(VI). The maximum adsorption capacities of U(VI) calculated from the Sips model were 266.5 mg·g−1 for Ca/Al LDH-Gl and 142.3 mg·g−1 for Ni/Al LDH-Gl at 298.15 K, and the higher adsorption capacity of Ca/Al LDH-Gl might be due to more functional groups and abundant high-activity “Ca−O” groups. Macroscopic experiments proved that the interaction of U(VI) on Ca/Al LDH-Gl and Ni/Al LDH-Gl was due to surface complexation and electrostatic interactions. The extended Xray absorption fine structure analysis confirmed that U(IV) did not transformation to U(VI) on solid particles, and stable innersphere complexes were not formed by reduction interaction but by chemical adsorption. The density functional theory (DFT) calculations further evidenced that the higher adsorption energies (i.e., Ead = 4.00 eV for Ca/Al LDH-Gl-UO22+ and Ead = 2.43 eV for Ca/Al LDH-Gl-UO2CO3) were mainly attributed to stronger hydrogen bonds and electrostatic interactions. The superior immobilization performance of Ca/Al LDH-Gl supports a potential strategy for decontamination of UO22+ from wastewater, and it may provide new insights for the efficient removal of radionuclides in environmental pollution cleanup. KEYWORDS: Nanocomposites, Layered double hydroxides, Immobilization, U(VI), EXAFS

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INTRODUCTION Recently, erosion-environmental effect and global environmental deterioration issue caused by the application of fossil fuels have attracted intense interests due to their severe hazard to human beings and environmental toxicity.1,2 It leads more and more countries to search for new energy to satisfy their basic energy demand and reduce environmental pressure. Nuclear power has become the potential energy and essential method for solving this energy crisis.3,4 With the development of nuclear technology and extensive utilization of nuclear energy, exhaustion of nuclear resources has been the current urgent problem for maintaining sustainable development of nuclear energy.5,6 In addition, radioactive pollution, which was caused by unreasonable utilization and exploitation of nuclear © 2017 American Chemical Society

sources, has already become one of the forefront environmental and energy issues because of its potential life-threatening and environmental effects.7−9 Nevertheless, uranium, the main material of nuclear power, a toxic radionuclide, has been excessively disposed into the natural environment in hexavalent form (U(VI)).2,10,11 It can cause serious comprehensive environmental radiation and potential toxicological effects, which results in different levels of water pollution or soil pollution,12,13 so it is essential and urgent to recycle and aggregate uranium from natural environment. Received: February 11, 2017 Revised: March 1, 2017 Published: March 13, 2017 3583

DOI: 10.1021/acssuschemeng.7b00439 ACS Sustainable Chem. Eng. 2017, 5, 3583−3595

Research Article

ACS Sustainable Chemistry & Engineering

transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and theoretical calculations. This paper provides new insights and scientific understanding to adsorption or aggregation of U(VI) on LDHs by advanced EXAFS and density functional theory (DFT) calculations, which is beneficial for reasonable application of LDHs in radionuclide pollution cleanup during nuclear industry processes.

In order to remove and recycle radioactive U(VI) from water system, various treatment technologies (e.g., ion-exchange,14 chemical (co)precipitation,15 solvent extraction,14,16 coagulation,17 filtration,18 permeation,19 and membrane separation20) have been applied to separate or aggregate radionuclides from wastewater or groundwater system. However, a series of defects, such as secondary pollution, high investment, and complex operation, significantly limited these methods’ application in environmental remediation.21 Compared with these methods, adsorption, an efficient and environmentalfriendly technology, is widely applied with great potential application in the removal of U(VI) from aqueous solutions.3,5,22,23 Various adsorbents, such as graphene-based materials,1,5,23,24 carbon-based materials,3,16 clay minerals-based materials,6,12,25 polymer-based materials,26,27 or metal-based materials,10 have been applied for the removal and enrichment of U(VI) from wastewater or groundwater. Nonetheless, the effectiveness and practicability of these materials depend on the adsorption efficiency, production cost, and complexity.21 Thus, novel and multifunctional adsorbents should be developed and applied for the efficient removal of U(VI) from the natural environment. Layered double hydroxides (LDHs), a typical 2D-structured anionic clay of mineral brucite, have aroused increasing attention in adsorption, photocatalytic, coprecipitation, or other fields based on their novel structures and easily exchanged intercalated layer anions.28,29 In general, LDHs are described by the chemical formula [M1−x2+Mx3+(OH)2]x+(An−)x/n·yH2O, where M2+ and M3+ represent the divalent or trivalent metal cations, respectively (e.g., Mg2+, Zn2+, Co2+, Mn2+, Ni2+, Ca2+, Fe3+, Cr3+, and Al3+), and An− represents the interlayer anions with high activity (e.g., Cl−, ClO4−, NO3−, CO32−, and SO42−), x is regarded as the molar ratio of M2+/(M2+ + M3+).30−35 LDHs and their composites have been considered as superior adsorbents for the removal and enrichment of U(VI) from aqueous solutions, which may be attributed to the special structure and physicochemical properties of LDHs, such as high stability, excellent anion exchange capacities, high specific surface area and effective active sites.28,36,37 In order to obtain novel LDHs or their derivatives with higher performance, lamellar, flakes, spheres, or porous LDHs have been investigated in environmental pollution cleanup and displayed excellent removal efficiency for various pollutants.28,30,33 For example, Zou et al.28 applied lamellar LDH-CO3 and LDH-Cl to coagulate graphene oxides (GO) and exhibited high coagulation capacities for (GO) in aqueous solutions. To further evaluate the adsorption behavior and interaction mechanism of LDHs with U(VI), herein, rods glycerol-modified nanocrystallined Ca/Al LDHs (Ca/Al LDH-Gl) and flocculent glycerol-modified nanocrystallined Ni/Al layered double hydroxide (Ni/Al LDH-Gl) were synthesized and applied as superior adsorbents for the efficient removal of U(VI) from aqueous solutions. The objectives of this paper are (1) to fabricate rods Ca/Al LDH-Gl and flocculent Ni/Al LDH-Gl with a facile-simple hydrothermal process; (2) to investigate the adsorption behavior of U(VI) on the as-prepared Ca/Al LDH-Gl or Ni/ Al LDH-Gl under various conditions (e.g., solution pH, ionic strength, solid contents, contact time, temperature and concentration of CO32−); (3) to evaluate the interaction mechanism between U(VI) and Ca/Al LDH-Gl or Ni/Al LDHGl by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier

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MATERIAL AND METHODS

The synthesis and characterization of Ca/Al LDH-Gl and Ni/Al LDHGl composites and batch adsorption experiments are described in the Supporting Information (SI). Ca/Al LDH-Gl and Ni/Al LDH-Gl were fabricated by a typical facile-cheap hydrothermal method (Figure 1).

Figure 1. One-step hydrothermal synthesis of Ca/Al LDH-Gl and Ni/ Al LDH-Gl.

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RESULTS AND DISCUSSION Morphology Analysis and Structure Characterization. The microstructures and morphologies of as-prepared Ca/Al LDH-Gl and Ni/Al LDH-Gl were characterized by various spectroscopy techniques. According to SEM (Figure 2) and TEM images (Figure S1), significant differences existed in SEM and TEM images between Ca/Al LDH-Gl and Ni/Al LDH-Gl. It showed that Ca/Al LDH-Gl possessed a typical rodlike 3D structure, and the average diameter could be adjusted in the range of 0.247−0.524 μm. The rodlike structure is beneficial for the surface adsorption and particle diffusion.35,38 Interestingly,

Figure 2. SEM images of Ca/Al LDH-Gl (a and b) and Ni/Al LDHGl (c and d). 3584


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Figure 3. Characterization of as-prepared Ca/Al LDH-Gl and Ni/Al LDH-Gl samples: (a) FT-IR spectra, (b) XRD patterns, (inset) partial XRD lattice distance, (c) XPS spectra of survey scan, (d) O 1s spectra, (e) C 1s spectrum of Ca/Al LDH-Gl, and (f) C 1s spectrum of Ni/Al LDH-Gl.

Ni/Al LDH-Gl samples exhibited flocculentlike structures, and the flocculent composites were aggregated with smooth layerby-layer nanoplates. Specifically, the TEM image of Ni/Al LDH-Gl (Figure S1b) showed that flocculent composites with a ternary network were formed, and the pileup pellets were beneficial for the removal of U(VI) on the surface of flocculent, which was attributed to its high specific surface area. The bonding types of Ca/Al LDH-Gl and Ni/Al LDH-Gl were evaluated by FT-IR technique. As shown in Figure 3a, the FT-IR spectra exhibited no significant changes of Ca/Al LDHGl and Ni/Al LDH-Gl in the main peaks. The peak at 3443 cm−1 is assigned to the stretching mode of hydroxyl (ν(OH)),28 and the wide bonds between ∼3000 and ∼3180 cm−1 are defined as the water molecules, which are formed with

the hybridization of hydrogen-bonded and CO32− in the interlayer (H2O−CO32−).35 Compared with Ni/Al LDH-Gl, a special peak at 2515 cm−1 in the FT-IR spectrum of Ca/Al LDH-Gl exhibits the typical C−H stretching vibration,39 which is due to the dehydration of glycerol molecule.40 Similarly, the special peak at 2185 cm−1 in the FT-IR of Ni/Al LDH-Gl demonstrates the ionic structures associated with CN conjugation formed by the cross-linking among glycerol, CO32−, and NO3− in aqueous solution.41 The FT-IR spectra indicated that glycerol was successfully introduced into the LDHs, and the grafted of glycerol molecule could beneficial for the improvement of stability, dispersity, and decreasing of chemical resistance. The platform from ∼1375 to ∼1560 cm−1 may be attributed to the ν3 vibration of CO32−,42 and the strong 3585


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Figure 4. Effect of environmental conditions on U(VI) adsorption from aqueous solutions on Ca/Al LDH-Gl and Ni/Al LDH-Gl. (a) Effect of solid content for Ca/Al LDH-Gl at pH = 5.0 ± 0.1, I = 0.01 NaNO3. (b) Effect of solid content for Ni/Al LDH-Gl at pH = 5.0 ± 0.1, I = 0.01 NaNO3. (c) Effect of pH on U(VI) adsorption with various NaNO3 concentrations at m/v = 0.1 g/L. (d) Species distribution of U(VI) as a function of pH (precipitation is not considered). (f) Species distribution of U(VI) as a function of pH (precipitation is considered). C[U(VI)initial] = 30 mg·L−1, T = 298.15 K.

peaks at ∼1102 and ∼859 cm−1 represented the ν1(CO32−) and ν2(CO32−) of various LDHs, respectively.37 Typically, a series of characteristic peaks at 980, 765, 635, and 485 cm−1 are exhibited the M−O lattice vibrations and M−O−H bending (M: Ca, Al, or Ni).29,43 The phase distribution and purity were characterized by XRD (Figure 3b), the strong diffraction peak at 2θ = 11.55° was assigned to the (002) planes of Ni/Al LDH-Gl,29,31 with the basal spacing d(002) of 7.61 nm. The diffraction peaks at 2θ = 15.11°, 26.80°, 30.58°, 34.77°, 39.36°, 44.30°, and 52.67° corresponded to the (003), (006), (009), (012), (015), (018), and (1010) planes of Ca/Al LDH-Gl and Ni/Al LDH-Gl, which indicated that the LDHs had a special hydrotalcite structure with relatively well-formed crystalline.28,44 In addition, the weak peak at 2θ = 29.31° was attributed to the (100) plane of Ca/Al LDH-Gl (d(100) = 3.03 nm), and the similar weak peak at 2θ = 62.43° may correspond to the (110) plane of Ni/Al

LDH-Gl, which was due to the synergistic effects between various divalent cations and Al3+ in the composites.33,29 Furthermore, according to Bragg’s law,45 it can be seen that the d(003) and d(006) space are independent of the types of divalent cations (d(003) = 5.83 nm, d(006) = 3.32 nm for Ca/Al LDH-Gl and d(003) = 5.81 nm, d(006) = 3.31 nm for Ni/Al LDHGl). The XPS spectra of Ca/Al LDH-Gl and Ni/Al LDH-Gl were displayed in Figure 3c, various peaks (e.g., Al 2s, Al 2p, Ca 2p, Ni 2p, C 1s, O 1s, and O 2s) indicated that Al, O, C, and Ni were the predominant elements in Ni/Al LDH-Gl sample, and Al, O, C, and Ca were the main elements in Ca/Al LDH-Gl sample.46 In order to further explain the distribution of functional groups, the high XPS O 1s spectra resolution of Ni/ Al LDH-Gl and Ca/Al LDH-Gl were shown in Figure 3d, and the relative contents of different groups were listed in Tables S1 and S2. The O 1s spectrum of Ca/Al LDH-Gl can be 3586


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Figure 5. (a) Adsorption isotherms of U(VI) on Ca/Al LDH-Gl. (b) Adsorption isotherms of U(VI) on Ni/Al LDH-Gl. (c) Adsorption kinetics of U(VI). (inset) Pseudo-second-order kinetic plots of U(VI). (d) UV−vis spectra of U(VI) in Ca/Al LDH-Gl or Ni/Al LDH-Gl aqueous suspensions as a function of reaction time. m/v = 0.1 g/L, pH = 5.0 ± 0.1, I = 0.01 NaNO3.

active sites on the surface of Ca/Al LDH-Gl,9,36 and oxygencontaining functional groups (e.g., −OH, C−O, CO, and O−CO) could improve adsorption energy and specific binding between guest molecules and targets; moreover, these highly active groups could provide electrons for the adsorption process through participating in protonation/deprotonation reaction, which was consistent with the results of XPS spectra analysis. Moreover, it can be clearly seen that the Kd values were independent of solid contents, which was accordance with the properties of Kd.47 In general, the distribution coefficient (Kd) value was independent of solid contents when the competition adsorption among various solid particles was negligible and in-apparent at low solid contents.39 Once increasing the solid content from 0.10 to 0.60 g/L, the adsorption efficiency increased slowly, thus 0.10 g/L was the satisfied content for the adsorption process. Effect of pH and Ionic Strength. During the adsorption process, solution pH plays an important role because it can change the surface charge, the protonation/deprotonation process of solid particles, and U(VI) ions.2,47 Figure 4c and d exhibited the effect of solution pH on U(VI) adsorption on Ca/ Al LDH-Gl and Ni/Al LDH-Gl (I = 0.01 M NaNO3). Dependent on the high-activity of Ca−O and charge-effect of Ca/Al LDH-Gl, the adsorption process of UO22+ could be divided into three various stages, which was the result of synergistic effect including surface complexation, electrostatic interaction, and chemical precipitation. In addition, according to the high adsorption capacities of U(VI) on both adsorbents under low pH, this indicated that Ca/Al LDH-Gl and Ni/Al LDH-Gl were stable and highly active. One can see that the removal percentage of U(VI) from aqueous solutions on Ca/Al

deconvoluted into two components at 531.0 eV (bridging −OH, 13.81%) and 531.9 eV (CO 86.19%), and the O 1s spectrum of Ni/Al LDH-Gl can be also deconvoluted into two components at 530.8 eV (bridging −OH, 18.74%) and 531.8 eV (CO, 81.26%).26 Similarly, as shown in Figure 3e and f, the C 1s peak of Ca/Al LDH-Gl can be deconvoluted into four components at 284.6 eV (CC, 51.75%), 285.5 eV (C−C, 15.41%), 286.8 eV (C−O, 8.47%), and 289.3 eV (O−CO, 24.37%), and the C 1s peak of Ni/Al LDH-Gl can also be deconvoluted into four components at 284.6 eV (CC, 56.80%), 285.9 eV (C−C, 16.27%), 287.3 eV (C−O, 9.23%), and 288.7 eV (O−CO, 17.70%), respectively.32,28,47 The content of total oxygen-containing functional groups calculated from C 1s spectrum was 32.84% (C−O and O−CO) for Ca/ Al LDH-Gl, which was higher than that for Ni/Al LDH-Gl (26.93%). More oxygen-containing functional groups can provide more active sites for the binding of U(VI) in adsorption process. Effect of Solid Content. In natural application of environmental remediation, solid content can affect the adsorption efficiency of U(VI) and economic benefit.48,49 In order to achieve excellent removal efficiency and obtain considerable economic value, the effect of solid content on the adsorption process was investigated with the range of 0.0− 0.6 g/L. As shown in Figure 4a and b, the removal percentage of U(VI) from aqueous solutions on Ca/Al LDH-Gl increased from ∼8% to ∼57% with the solid content increasing from 0.01 to 0.10 g/L, while the removal percentage of U(VI) on Ni/Al LDH-Gl increased from ∼11% to ∼43% at same conditions. The higher adsorption efficiency of Ca/Al LDH-Gl might be attributed to more oxygen-containing functional groups and 3587


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ACS Sustainable Chemistry & Engineering LDH-Gl increased from ∼1% to ∼56% with the increase of pH from 2.3 to 5.0 (process I). In process I, the positive charge of U(VI) species (e.g., UO22+, UO2OH+, (UO2)2(OH)22+, and (UO2)3(OH)5+) are observed from Figure 4e and f, and the positive charged adsorbents can cause electrostatic repulsion with U(VI).28,50 With the increase of C[H+], the protonation process was promoted and produced more complexes. In addition, changing the concentration of NaNO3 from 0.001 to 0.1 M, the removal percentage was also changed obviously, which indicated that the adsorption process was dependent on ionic strength, and demonstrated that the adsorption behavior in process I was mainly dominated by outer-sphere surface complexation and electrostatic repulsion.50,51 While continues to increase the pH from 5.0 to 7.2, the removal percentage decreased from 56% to 37%, which was caused by the deprotonation process and the strong electrostatic repulsion interaction in process II.12,13 At alkaline conditions (7.2 < pH < 10.9) (process III), the adsorption efficiency was improved again and achieved at 85%, which may be due to the formation of precipitates (schoepite) (Figure 4f), and process III was dominated by surface precipitation and outer-sphere surface complexation. Furthermore, the negative derivatives of U(VI) (e.g., UO2(OH)3− and UO2(OH)42−) can improve the electrostatic attraction to positively charged Ca/Al LDHGl.9,10 Compared to Ca/Al LDH-Gl, the adsorption behavior of U(VI) on Ni/Al LDH-Gl was more simple. At low pH values (2.3 < pH < 6.0), the removal percentage increased from ∼2% to ∼70%, and the process was little independent of ionic strength, which demonstrated that the adsorption reaction was controlled by inner-sphere surface complexation and electrostatic attraction.50 In addition, based on the stabilizer effect of glycerol molecule, both adsorbents decreased the proteolytic phenomenon and exhibit high activity and reactivity even in acidic solution. At high pH values (6.0 < pH < 10.9), the process was strongly dependent on ionic strength, indicating that the main interaction was also dominated by surface precipitation and outer-sphere surface complexation.52 Interestingly, with the increasing of background electrolyte, the adsorption capacity of U(VI) on Ca/Al LDH-Gl was decreased while that on Ni/Al LDH-Gl was increased, and it was attributed to the following reasons: (1) stronger competitive adsorption among NaNO3, UO22+ and Ca/Al LDH-Gl based on the higher active of M−O (e.g., Ca−O) on the surface of Ca/Al LDH-Gl; (2) higher electrostatic repulsion between UO22+ and Ca/Al LDH-Gl with the increasing of Na+.39 The different adsorption behavior between Ca/Al LDH-Gl and Ni/ Al LDH-Gl may be attributed to the easily changeable charge of Ca2+ and its derivatives. Adsorption Isotherms and Thermodynamic Behavior. In order to explore the adsorption mechanism and thermodynamic behaviors, adsorption isotherms of U(VI) on Ca/Al LDH-Gl and Ni/Al LDH-Gl were investigated at three temperatures (298.15, 313.15, and 333.15 K). As shown in Figure 5a and b, the adsorption efficiencies of U(VI) on Ca/Al LDH-Gl and Ni/Al LDH-Gl were promoted obviously at higher temperature, especially in high U(VI) concentration. In addition, three traditional isotherms, i.e., Langmuir, Freundlich, and Sips models, were applied to simulate the experimental data. Typically, a Langmuir isotherm represents monolayer adsorption, and the adsorption reaction is carried on the surfaces and bulk phase of homogeneous adsorbents, which is described as47

Ce Ce 1 = + Cs Cs,maxKL Cs,max

(1)

The Freundlich isotherm stands for multilayer adsorption, and the adsorption process is considered to produce on the surface of heterogeneous adsorbents, which can be described as3,26 1 log Cs = log KF + log Ce (2) n The Sips isotherm is regarded as a combined form of Langmuir and Freundlich equations, which can predict the heterogeneous adsorption process, and it is expressed as51 Cs =

K sCens 1 + asCens

(3)

where Ce (mg·L−1) and Cs (mg·g−1) are the equilibrium concentration and the amount of U(VI) adsorbed on adsorbents, Cs,max (mg·g−1) is the maximum amount under per unit weight of adsorbents, and KL (L·mg−1) is a constant related to Langmuir isotherm, with increase of KL, the affinity and bonding of adsorbent can be promoted. KF (mg1−n·Ln·g−1) is a constant related to the adsorption capacity of Freundlich model, and 1/n corresponds to the adsorption intensity. ns is the Sips’ heterogeneity parameter, and Ks = Cs,max·ns (L·g−1) is a Sips constant. The corresponding parameters calculated from the three models were tabulated in Tables S3. One can see that the adsorption isotherms of U(VI) on Ca/Al LDH-Gl were well fitted by Sips mode (R2 > 0.95), and the maximum adsorption capacities of U(VI) calculated from Sips model was 266.5 mg· g−1 at 298.15 K (Table S3). It demonstrated that the adsorption process was multilayer adsorption at low concentration of U(VI) and monolayer adsorption at high concentration.51 However, the adsorption curves of U(VI) on Ni/Al LDH-Gl were well fitted by Langmuir model (R2 > 0.91) and Sips model (R2 > 0.90), and the maximum adsorption capacity calculated from Sips model was 142.3 mg·g−1 at 298.15 K, indicating that the adsorption reaction was monolayer coverage.3 The higher adsorption capacity of U(VI) on Ca/Al LDH-Gl might be attributed to more oxygen-containing functional groups (e.g., C−O, O−CO, and CO), which provided more active sites, promoted the deprotonation reactions, and thereby, enhanced the binding of U(VI) ions.10,12,52 To further study the feasibility and stability of the adsorption reaction, various thermodynamic parameters (e.g., standard entropy change (ΔS0, J·K−1·mol−1), standard free energy change (ΔG0, kJ·mol−1), and standard enthalpy change (ΔH0, kJ·mol−1)) were calculated according to the following equations:12 ln K 0 =

ΔS 0 ΔH 0 − R RT

(4)

(5) ΔG 0 = −RT ·ln K 0 0 where K is the adsorption equilibrium constant, and the values of ΔH0 and ΔS0 are obtained from the slope and intercept of linear regression of ln K0 versus T−1 (Figure S2). The values of ΔG0 at different temperatures were calculated by eq 5, T is the absolute temperature in Kelvin, and R is the gas constant (8.314 J·mol−1·K−1). The relative values of thermodynamic parameters for the adsorption of U(VI) at different temperatures (i.e., 298.15, 313.15, and 333.15 K) were shown in Table 1. 3588


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Table 1. Thermodynamic Parameters of U(VI) Adsorption on Ca/Al LDH-Gl and Ni/Al LDH-Gl at Various Temperatures (298.15, 313.15, and 333.15 K) ΔG0 (kJ·mol−1) −1

−1

−1

adsorbents

ΔH (kJ·mol )

ΔS (J·mol ·K )

298.15 K

313.15 K

333.15 K

Ca/Al LDH-Gl Ni/Al LDH-Gl

28.32 30.03

116.83 121.73

−6.51 −7.97

−8.27 −8.09

−10.60 −10.52

0

0

As shown in Table 1, the positive values of ΔH0 suggested that the adsorption process of U(VI) on Ca/Al LDH-Gl and Ni/Al LDH-Gl was an endothermic reaction process.3 In addition, the positive ΔS0 values demonstrated that the reaction was a spontaneous adsorption process with high affinity, which indicated that more ligands incorporated with UO22+ and more H+ or OH− participated the adsorption process.56 The negative ΔG0 values further proved the spontaneous adsorption process,47,56 and the decrease of ΔG0 with the increase of temperature showed that better adsorption efficiency could be obtained under higher temperature, which was due to the easy dehydration of U(VI) adsorbed on Ca/Al LDH-Gl or Ni/Al LDH-Gl.56 The adsorption capacities of U(VI) on different materials were summarized in Table 2, and it was clear that Ca/

7 h for Ca/Al LDH-Gl), abundant free binding sites and active sites were available for the removal of U(VI) from aqueous solutions, and the adsorption was quick at the initial contact time. Especially, in the initial 4 h, the adsorption rate of U(VI) on Ni/Al LDH-Gl was higher than that on Ca/Al LDH-Gl, which was controlled by the higher stability of Ni/Al LDH-Gl in weak-acid solution and lower diffusion resistance of solid particles. However, at the later stage (t > 4 h for Ni/Al LDH-Gl or t > 7 h for Ca/Al LDH-Gl), most of the active sites were occupied by U(VI) ions and the adsorption of U(VI) would proceed in the inside of adsorbents, which needed longer diffusion ranges to bonding with inner-free binding sites, and the adsorption increased slowly.9,47 As shown in Figure 5d, the highest peak position of U(VI) UV−vis spectra in both adsorbent suspensions was found at the wavelength of 650 nm, and it did not change with the increase of contact time, suggesting that the basic characteristic of U(VI) (i.e., species, microstructures etc.) was maintained and did not transfer to other new components.47 To evaluate the mass transfer process in adsorption system, two typical adsorption kinetic models (pseudo-first-order and pseudo-second-order kinetic) were applied to simulate the kinetic adsorption process, which can be described as follows:3,47

Table 2. Comparison of the Adsorption Capacities of U(VI) on Ca/Al LDH-Gl and Ni/Al LDH-Gl with Other Adsorbents adsorbents mesoporous carbon magnetic chitosan resins graphene oxide nanosheets hematite hydrothermal carbon spheres carbonaceous nanofiber NKF-6 zeolite cyclodextrinmodified graphene oxide nanoporous alumina Ca/Al LDH-Gl Ni/Al LDH-Gl

experimental conditions −1

Cs,max (mg·g−1)

ref

C0 = 50 mg·L , pH = 6.0, T = 298.15 K C0 = 80 mg·L−1, pH = 5.0, T = 298 K C0 = 110 mg·L−1, pH = 5.0, T = 293 K C0 = 5.4 mg·L−1, pH = 5.5, T = 293 K C0 = 50 mg·L−1, pH = 6.0, T = 298.15 K C0 = 10 mg·L−1, pH = 4.5, T = 298 K C0 = 5.4 mg·L−1, pH = 5.5, T = 293 K C0 = 10 mg·L−1, pH = 5.0, T = 288 K

133.5

3

160.8

4

97.5

5

6.3

6

80.0

8

125.0

2

C0 = 8 mg·L−1, pH = 4.5, T = 298 K C0 = 30 mg·L−1, pH = 5.0, T = 298.15 K C0 = 30 mg·L−1, pH = 5.0, T = 298.15 K

4.1

53

97.3

54

3.1

55

266.5

this study

142.3

this study

ln(qe − qt ) = ln qe − k1t

(6)

t 1 t = + 2 qt qe k 2qe

(7)

where qe (mg·g−1) and qt (mg·g−1) are the adsorption capacity at equilibrium and time t (h), respectively. k1 (min−1) and k2 (g· mg−1·min−1) represent the constant of pseudo-first order and pseudo-second order rate, respectively. The values of k1, k2, and qe can be calculated from the linear plot of ln (qe − qt) versus (t) or t/qt versus (t), and the basic parameters are listed in Table S4. As inset in Figure 5c and Figure S3, the higher R2 values (R21 = 0.996 and R22 = 0.999) demonstrated that the adsorption kinetics of U(VI) on Ca/Al LDH-Gl and Ni/Al LDH-Gl followed the pseudo-second order model, and the adsorption process was mainly attributed to chemical reaction.3,47 Effect of Carbonate Ions. In the natural environment, various anions and cations are widespread existed in aqueous solution, and many researchers have studied the effect of these ions in the removal of environmental pollutants.7,11 However, the results demonstrated that CO32− ions could produce significant influence on the removal of U(VI), which was due to the abundant carbon dioxide in the natural condition and its high water solubility,56 thus the effect of CO32− ions should be considered and discussed. As shown in Figure 6a and b, the observed increase in the enrichment of U(VI) at low concentration of CO32− (0.001 < C[CO32−] < 0.02 for Ca/Al LDH-Gl and 0.001 < C[CO32−] < 0.01 for Ni/Al LDH-Gl) could be explained that CO32− promoted

Al LDH-Gl and Ni/Al LDH-Gl could be the promising potential adsorbents for the efficient removal of U(VI) from aqueous solutions in environmental pollution remediation and cleanup. Adsorption Kinetics. In the practical application, adsorption rate is also an significant factor for the evaluating various adsorbents. The effects of contact time on U(VI) adsorption on Ca/Al LDH-Gl and Ni/Al LDH-Gl were compared in Figure 5c; it is demonstrated that the adsorption of U(VI) on Ni/Al LDH-Gl increased rapidly and reached ∼40% in the first 4 h of contact time and then increased slowly. While the adsorption on Ca/Al LDH-Gl increased to ∼60% in the first 7 h of contact time and maintained a high level with further increase of contact time. At the first stage (t < 4 h for Ni/Al LDH-Gl or t < 3589


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Figure 6. (a) Effect of C[CO32−] on U(VI) adsorption on Ca/Al LDH-Gl and Ni/Al LDH-Gl. (b) Effect of pH on U(VI) removal at C[CO32−] = 0.01 mol·L−1. (c) Species distribution of U(VI) as a function of pH at C[CO32−] = 0.01 mol·L−1. (d) UV−vis spectra of U(VI) in a Ca/Al LDH-Gl or Ni/Al LDH-Gl aqueous suspensions as a function of C[CO32−]. m/v = 0.1 g/L, pH = 5.0 ± 0.1, I = 0.01 NaNO3, C[U(VI)initial] = 30 mg·L−1.

Figure 7. U LIII-edge background subtracted, k2-weighted χ(k) data (a) and corresponding Fourier transformed EXAFS spectra (b) of standards (UO22+ or UO2) and adsorption samples.

the formation of UO2CO3(aq) (Figure 6c), which could reduce the electrostatic repulsion from various cation complexes at pH = 5.0. However, increasing the concentration of CO32− continuously, the adsorption efficiency of U(VI) was decreased gradually, and it might be attributed to the strong surface complexation between U(VI) and CO32−. Furthermore, the

effect of C[CO32−] for Ca/Al LDH-Gl was more evident than that for Ni/Al LDH-Gl, and it might be due to the easily binding of Ca2+ and CO32−. From the UV−vis spectra of U(VI) (Figure 6d), it could be found that the characteristic adsorption peak of U(VI) was maintained at 650 nm under different CO32− concentration, indicating that the adsorption process was a 3590


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characterizations are in accordance with the standard structure of UO22+ and UO2 reported before.13,52,57,58 For the sample of Ni/Al LDH-Gl-U(VI) at pH 5.0, the spectral fits divided into ∼2.0 Oax at ∼1.82 Å and ∼6.0 Oeq at ∼2.41 Å, which was a typical composite of U(VI) rather than U(IV), and the U−Oax shell at ∼1.82 Å with the U−Oeq shell at ∼2.41 Å displayed the distance of UO22+. The similar FT spectra of Ca/Al LDH-Gl-U(VI) at pH 5.0 indicated that the adsorption process on both adsorbents without the changes of element valence state.57,58 The FT features at ∼2.6 Å, 3.0 and 3.1 Å for two various adsorbents could be fitted well by the U− Al (∼3.3 Å), U−Ni (∼3.6 Å), and U−Ca (∼3.7 Å), respectively, which demonstrated that the robust inner-sphere U(VI) complexes was formed on the oxide shells of LDHs. DFT Calculations. To explain the detailed interaction mechanism between U(VI) and Ca/Al LDH-Gl or Ni/Al LDHGl, the adsorption structure and energy of U(VI) on various adsorbents have been explored by plane-wave-based DFT calculations using the Vienna Ab-initio Simulation Package (VASP) code.28,47 The reaction system with various metal− metal ratio, such as 1:1, 1:2, and 3:1, have been calculated to evaluate and compare the adsorption mechanism, and computational details were presented in the SI. The optimized structures of LDHs-UO22+ complexes (Ca:Al = 3:1, Ni:Al = 3:1 as a typical sample) are displayed in Figure 8, and other possible coordination structures under different metal−metal ratio are shown in Figures S6 and S7. As shown in Figure 8, the U−O distance between UO22+ and LDHs for Ni/Al LDHsUO22+ complexes is ∼2.345 Å, which is longer than that for Ca/ Al LDHs-UO22+ (∼2.181 Å). It indicated that there is stronger electrostatic attraction existed in Ca/Al LDHs-UO22+ system. Furthermore, extra electrostatic interactions between Ca2+ and O atom of UO22+ (d = 2.361 Å) further improved the chelating ability of Ca/Al LDH-Gl, which was consistent with the results of batch experiments. Abundant metal−oxygen funcational groups could form stronger surface complexes with U(VI) in aqueous solutions, indicating that the adsorption process was chemisorption rather than physical sorption or ion exchange.1,11 These driving forces might be attributed to stronger electrostatic interactions. In order to consider the effect of CO32− anion, the UO2− CO3 model has been built for various LDHs. Taking Ca:Al = 3:1, Ni:Al = 3:1 as a typical sample, the optimized structures of LDHs-UO2CO3 complexes are shown in Figure 9, and the calculated results of adsorption energy (Ead) of possible complexes are summarized in Table 4. For example, as shown in Figure 9, abundant hydrogen bonds between CO32− or UO22+ and LDHs existed in Ca/Al LDH-Gl-UO2CO3 and Ni/Al LDH-Gl-UO2CO3 systems. However, in the adsorption system of Ca/Al LDH-Gl-UO2CO3, extra and typical Ca−O (UO22+) and Ca−O (CO32−) bonds existed, which could improve the removal of U(VI) on the surface of Ca/Al LDHGl. It is accordance with the results of Ead, and the Ead of Ca/Al LDH-Gl-UO2CO3 system is ∼2.43 eV (Ca:Al = 3:1), which is also higher and more positive than Ni/Al LDH-Gl-UO2CO3 system (∼2.39 eV, Ni:Al = 3:1). It demonstrated that stable derivative and metal−oxygen bonds have been producted and existed in aqueous solutions.11,28 Furthermore, comparing the adsorption reaction of U(VI) on same adsorbent with the existence of CO32− or not, it could be found that the adsorption energy (Ead) of LDH-UO22+ was significantly higher than that of LDH-UO2CO3 system, and it indicated that CO32− could inhibit the adsorption reaction of U(VI) through competitive

mild and rapid reaction process. According to Figure 6b, at low pH (2.3 < pH < 5.0), the adsorption behavior of U(VI) was similar to the state without CO32−, and the reaction was mainly controlled by deprotonation process and electrostatic attraction. The deprotonation reaction could inhibit the formation of stable uranyl carbonate chelate-complexes instead of unstable and active uranyl-containing hydroxide, such as UO2OH+ and UO22+, which was beneficial for the adsorption process of U(VI) on both adsorbents. While in the pH range of 5.0−11.0, the adsorption efficiency of U(VI) decreased rapidly. The situation was due to the formation of more stable U(VI)− carbonate complexes (e.g., UO2CO3, UO2(CO3)22−, and UO2(CO3)34−) in aqueous solutions (Figure 6c), various charge variation of carbonate complexes endowed more positve charges of uranyl-chelates, which resulted strong adsorption resistance for the uptake of U(VI) in aqueous solutions. Especially, stronger competitive adsorption and ion trapping was existed between negative CO32− and positive adsorbents. Compared with the adsorption system without CO32−, the adsorption process would be inhibited by the competitive adsorption of CO32− with U(VI). Thus, the existence of CO32− should be avoided in natural applications, and more stable uranyl-chelate formed between LDHs and U(VI) without CO32− or soluble CO2. EXAFS Analysis. In order to explore the adsorption mechanism from element specific or short-range structure between LDHs and U(VI), X-ray absorption fine structure (XAFS) spectroscopy, an advanced qualitative and quantitative technique, was applied to study the local atomic structure and compositional environment for adsorbing atoms.13,58 Herein, the oxidation state and local atomic structure of U(VI) sequestered onto Ca/Al LDH-Gl and Ni/Al LDH-Gl were evaluated by EXAFS spectra, and the k2-weighted χ(k) data, U LIII-edge background subtracted and corresponding Fourier transformed EXAFS spectra of Ca/Al LDH-Gl-U(VI) and Ni/ Al LDH-Gl-U(VI) at pH 5.0 are exhibited in Figure 7a and b, respectively. It can be seen that the FT features of UO22+ at ∼1.38 and 1.88 Å can be fitted well by ∼2.0 axial oxygen atom (Oax) at ∼1.86 Å and ∼6.0 equatorial oxygen atom (Oeq) at ∼2.42 Å, respectively (Table 3). In addition, for the reference of UO2, two typical peaks located at ∼1.49 and ∼3.12 Å are for U−O and U−U interaction, respectively, which can be fitted by ∼8.1 O at 2.36 Å and ∼10.6 U at 3.85 Å. These Table 3. EXAFS Analysis for Standards and the Adsorption of U(VI) samples UO2

2+

UO2 Ca/Al LDH-Gl (pH = 5.0)

Ni/Al LDH-Gl (pH = 5.0)

shell

R (Å)a

CNb

σ2 (Å2)c

U−Oax U−Oeq U−O U−U U−Oax U−Oeq U−Al U−Ca U−Oax U−Oeq U−Al U−Ni

1.86(2) 2.42(1) 2.36(3) 3.85(2) 1.85(1) 2.39(3) 3.34(3) 3.71(2) 1.82(3) 2.41(2) 3.37(2) 3.63(3)

2.1(2) 5.9(3) 8.1(2) 10.6(1) 1.8(2) 5.7(4) 1.6(3) 0.8(4) 1.9(2) 5.8(4) 1.4(3) 1.1(1)

0.0023(4) 0.0041(5) 0.0048(6) 0.0057(5) 0.0025(5) 0.0034(6) 0.0057(5) 0.0078(4) 0.0046(5) 0.0051(4) 0.0062(5) 0.0069(6)

a

R is the bond distance. bCN is coordination numbers of neighbors. σ is the Debye−Waller factor.

c 2

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Figure 8. Optimized complexes for Ni/Al LDH-Gl (Ni:Al = 3:1) interaction with UO22+ (upper-layer) and Ca/Al LDH-Gl (Ca:Al = 3:1) interaction with UO22+ (sublayer): green Ca atom, ice blue Ni atom, red oxygen atom, blue U atom, mauve Al atom, white H atom.

Figure 9. Optimized complexes for Ni/Al LDH-Gl (Ni:Al = 3:1) interaction with UO2CO3 (upper-layer) and Ca/Al LDH-Gl (Ca:Al = 3:1) interaction with UO2CO3 (sublayer): green Ca atom, ice blue Ni atom, red oxygen atom, blue U atom, mauve Al atom, white H atom.

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adsorption. Thus, it further evidenced that carbanion should be avoided in the natural environmental remediation of U(VI).39 Interestingly, through the DFT calculation, it can be found that the increase of the relative content of Ca or Ni, the adsorption process could be promoted and the Ead increased. Moreover, high-activity of “Ca” is beneficial for the formation of Ca−O, which can improve the surface active and interfacial energy of Ca/Al LDH-Gl, and it is favorable for the adsorption and complexing of UO22+ or UO2CO3.

CONCLUSIONS

In summary, rods of Ca/Al LDH-Gl and flocculent Ni/Al LDH-Gl were prepared by a simple and green hydrothermal synthesis technology. The chemical affinities of U(VI) on Ca/ Al LDH-Gl and Ni/Al LDH-Gl were determined by batch adsorption experiments and advanced spectroscopy techniques (e.g., SEM, TEM, XRD, FT-IR, XPS, and EXAFS). The maximum adsorption capacity of U(VI) on Ca/Al LDH-Gl (266.5 mg·g−1) was higher than that on U(VI) on Ni/Al LDHGl (142.3 mg·g−1), which was attributed to the various content of oxygen-containing functional groups. In addition, the 3592


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ACS Sustainable Chemistry & Engineering Table 4. Calculated Adsorption Energies Ead (eV) of Ca/Al LDH-Gl or Ni/Al LDH-Gl with UO22+ or UO2CO3 Complexes Under Various Metal Ratios various system LDH-Gl-UO22+ LDH-Gl+UO22+ LDH-Gl-UO22+ LDH-Gl-UO22+

Ca/Al Ni/Al Ca/Al Ni/Al Ca/Al LDH-Gl-UO2CO3 Ni/Al LDH-Gl-UO2CO3 Ca/Al LDH-Gl-UO2CO3 Ni/Al LDH-Gl-UO2CO3

metal ratios

ETotal (eV)

Ead (eV)

(Ca:Al = 3:1) (Ni:Al = 3:1) (Ca:Al = 1:2) (Ni:Al = 1:1) (Ca:Al = 3:1) (Ni:Al = 3:1) (Ca:Al = 1:2) (Ni:Al = 1:1)

−564.231 −547.440 −404.559 −618.219 −570.413 −554.476 −412.060 −624.211

4.00 3.11 2.38 2.21 2.43 2.39 2.13 0.45

Institute of Technology Graduate Student Innovation Fund (YC2015-S273) are acknowledged. X.W. acknowledges the CAS Interdisciplinary Innovation Team of the Chinese Academy of Sciences. The Swedish National Infrastructure for Computing (SNIC) is acknowledged for computer time.

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adsorption process for Ni/Al LDH-Gl was weakly dependent on the concentration of CO32− under low C[CO32−] (C[CO32−] < 0.04 mol·L−1), while the adsorption reaction for Ca/Al LDH-Gl was strongly dependent on the concentration of CO32− under low C[CO32−] (C[CO32−] < 0.04 mol·L−1). The results indicated that the adsorption process for Ca/Al LDH-Gl was dominated by outer-sphere surface complexation and electrostatic interactions, and the interaction of U(VI) on Ni/Al LDH-Gl was controlled by inner-sphere surface complexation and electrostatic interactions. The results of EXAFS and DFT calculations further indicated that the adsorption process on both adsorbents without the changes of element valence state, and the main driving force of adsorption process was hydrogen bonds and electrostatic interactions, which confirmed the important role of “Ca−O” in chemisorption reaction. It can provide new highly efficient adsorbents for the enrichment and uptake of U(VI) from natural environment and is beneficial for the environmental remediation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00439. Preparation and characterization of Ca/Al LDH-Gl with Ni/Al LDH-Gl nanocomposites; batch adsorption experiments, additional figures and tables for TEM images, XPS spectra, kinetic parameters, and adsorption isotherm models. More detailed information on EXAFS measurements and DFT calculation (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: 86-10-61772890. Fax: 86-10-61772890. E-mail: [email protected] or [email protected] (X.W.). *E-mail: [email protected] (Y.A.). *E-mail: [email protected] (Y.L.). ORCID

Xiangke Wang: 0000-0002-3352-1617 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Science Challenge Project (JCKY2016212A04), NSFC (91326202, 21577032, 21403064), the Fundamental Research Funds for the Central Universities (JB2015001), and the Project of East China 3593


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Glycerol-Modified Binary Layered Double Hydroxide

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