Controllable Synthesis of Ca-Mg-Al Layered Double Hydroxides and

Research Article pubs.acs.org/journal/ascecg

Controllable Synthesis of Ca-Mg-Al Layered Double Hydroxides and Calcined Layered Double Oxides for the Efficient Removal of U(VI) from Wastewater Solutions Yidong Zou,†,‡ Xiangxue Wang,‡ Fen Wu,§ Shujun Yu,‡ Yezi Hu,‡ Wencheng Song,‡ Yunhai Liu,*,† Hongqing Wang,∥ Tasawar Hayat,⊥ and Xiangke Wang*,‡,∥,⊥ †

School of Chemistry, Biological and Materials Sciences, East China Institute of Technology, Nanchang 330013, P. R. China School of Environment and Chemical Engineering, North China Electric Power University, Beijing102206, P. R. China § School of Materials Engineering, Shanghai University of Engineering Science, P.O. Shanghai 201620, P.R. China ∥ School of Chemistry and Chemical Engineering, University of South China, 28 Changsheng West Road, Henyang, Hunan421001, P.R. China ⊥ NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah21589, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: Novel rod-like ternary nanoscale layered double hydroxides (Ca-Mg-Al-LDH) and their bimetal derivatives (Ca-Mg-Al-LDOx, x: 200, 300, 400, 500, and 600 °C) were fabricated with a simple-green hydrothermal and calicination process. The interaction mechanism and adsorption property of U(VI) on Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx were investigated by a batch technique and spectroscopy analysis, and the results indicated that U(VI) could form strong and stable surface complexes on Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx surfaces. The adsorption capacity of U(VI) on various adsorbents could be controlled and adjusted through changing the calcination temperature, which was attributed to the different contents of various metal−oxide bonds (e.g., Ca−O, Mg−O, and Al−O). The adsorption capacities of U(VI) on these adsorbents were in the order of Ca-Mg-Al-LDO500 (486.8 mg/g) > Ca-Mg-Al-LDO600 (373.4 mg/g) > Ca-Mg-Al-LDO400 (292.5 mg/g) > Ca-Mg-Al-LDO300 (260.0 mg/g) > Ca-Mg-Al-LDO200 (223.5 mg/g) > Ca-Mg-Al-LDH (132.5 mg/g), which might be attributed to more active surface sites and abundant “Ca−O and Al−O” with the increase of calcination temperature. The results of kinetic and thermodynamic studies demonstrated that the adsorption was a spontaneous and endothermic chemical process, and the better fitted Sips model revealed that the adsorption reaction was multilayer adsorption at low concentration of U(VI) and monolayer adsorption at high concentration of U(VI). This study provided highlights on the interaction mechanism of U(VI) with various metal−oxide bonds, and it could play an important role for the controllable adsorption capacity and effcient application in environmental remediation. KEYWORDS: Controllable synthesis, Layered double hydroxides, Nanomaterials, U(VI)

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(e.g., 2.6 mg/kg soil7) enlarged the complexity of enrichment and utilization from the natural environment. Therefore, in order to solve the sources of U(VI) for nuclear energy and reduce the environmental pollution, it is necessary to remove and aggregate U(VI) from contaminated wastewater or groundwater. Various efficient separation technologies, such as chemical (co)precipitation,8 solid-phase extraction,9,10 membrane separation,11 and ion-exchange,12 have been applied as traditional treatment methods for the removal and recycle of U(VI) from

INTRODUCTION The serious energy crisis and the environmental pollution thus induced have been one of the most important scientific problems to be urgently solved in the social development process.1,2 Uranium, one essential nuclear material, mainly exists in soils, minerals, and seawater,2 and it has been regarded as the basic material for the nuclear industry and nuclear power.1,3 However, uranium is a highly soluble radioactive element with long half-life of radiation, and uranium containing wastewater is inevitably released into the natural environment, which can lead to high toxicity and radiation to human beings and other organisms (such as the Fukushima accident).2,4−6 In addition, the uneven distribution and low concentration of U(VI) © 2016 American Chemical Society

Received: October 22, 2016 Published: November 3, 2016 1173

DOI: 10.1021/acssuschemeng.6b02550 ACS Sustainable Chem. Eng. 2017, 5, 1173−1185

Research Article

ACS Sustainable Chemistry & Engineering

is favorable for fine-tuning active sites and adsorption capacity.34,38 However, it is also facing a great challenge to synthesize LDO with adjustable adsorption capacity, and the interaction mechanism of U(VI) on ternary LDO composites is still unclear. In order to explore the interaction mechanism of U(VI) on ternary LDH or LDO composites, herein the novel rod-like Ca-Mg-Al-LDH, rod-like Ca-Mg-Al-LDO300, and Ca/Mg/ Al-LDO600 were fabricated by a one-step hydrothermal process and calcination treatment. The effects of various environmental conditions, such as solid contents, pH, contact time, ionic strength, temperature, and concentration of CO32−, were investigated by a batch technique. In addition, the interation mechanism and adsorption driving forces of U(VI) on Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, or Ca-Mg-Al-LDO600 were evaluated and compared through SEM, FT-IR, XRD, and XPS techniques. This paper exhibited new insights for the interaction of U(VI) on LDHs and LDO composites, which were valuable for solving the lack of nuclear sources and alleviating the environmental pollution pressure.

the natural environment. Compared with these methods, adsorption, a simple and mild reaction process, has been considered as the most effective and potential treatment method for environmental pollutants based on its rapid adsorption and high selectivity.13−22 Unique nanomaterials and inorganic materials, such as carbon-family materials,17−20 graphene-family materials,5,6 clay-family materials,15,23,24 and metal-family materials,4,5,13,25 have exhibited excellent adsorption capacity to U(VI) in aqueous solutions. However, in practical applications, the superior adsorbents are dependent on the actual costs and adsorption efficiency.26 Thus, novel and naturally existing adsorbents have attracted tremendous interests, and the interaction mechanism of U(VI) on natural adsorbents deserves research and anticipation. Recently, layered double hydroxides (LDHs), a class of 2D-structured anionic inorganic material,27,28 have been widely studied and applied as predominant adsorbents,29−31 catalysts,27,32 flocculants,33 or other matrix materials based on their exchangeable interlayer anions and the “memory effect” of calcined derivatives.34,35 Most LDHs could be found in the natural environment and exist in authigenic minerals, and the general formula of LDHs could be described as [M1−x2+Mx3+(OH)2]x+(An−)x/n·yH2O, where M2+ and M3+ are di- and trivalent metal cations (e.g., Ca2+, Mg2+, Zn2+, Ni2+, Fe2+, Cr3+, Mn3+, Ga3+, and Al3+),36 respectively, and An‑ represents the incorporated anion with high activity (e.g., Cl−, ClO4−, NO3−, CO32−, and SO42−) for charge neutrality and structure stability;37 x is considered as the molar ratio of M2+/(M2+ + M3+).33 The binary and ternary LDHs have similar microstructure, while ternary LDHs possess superior physicochemical properties due to their complex element components. Calcination treatment as an efficient modification method is favorable to enhance the surface defect of derivatives and is beneficial for inner reaction. Calcinated LDHs (layered double oxide, defined as LDO) under a certain temperature can maintain a “memory effect” in aqueous solutions, and the treatment process can be divided into four parts based on the various temperatures, such as dehydration (process I), dehydroxylation (process II), decomposition of anions (process III), and oxide reformation (process IV), which can be expressed as32

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Figure 1. Proposed hydrothermal synthesis and calcination route of Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx (x: 200, 300, 400, 500, and 600).

[M1 − x 2 +Mx 3 +(OH)2 ]x + (An −)x / n ·y H2 O → [M1 − x 2 +Mx 3 +(OH)2 ]x + (An −)x / n

Characterization. The scanning electron microscopic (SEM) images of all samples were obtained with a JEOL JSM-JSM-6330F analyzer. The specific surface area and pore diameter were determined with N2 adsorption−desorption on an ASAP 2020 analyzer. The Fourier transformed infrared (FT-IR) spectra were mounted in a Nicolet Magana-IR 750 spectrometer in the range of 400 cm−1 to 4000 cm−1, and the X-ray diffraction (XRD) patterns were recorded with a Philips X’ Pert Pro Super X-ray diffractometer (Cu Kα source, λ = 1.5418 Å). The X-ray photoelectron spectroscopy (XPS) spectra were achieved with a Thermo Escalab 250 electron spectrometer. Batch Adsorption Experiments. The whole adsorption experiments of U(VI) on LDH and LDOx were carried out in 10 mL polyethylene test tubes at pH 5.0 ± 0.1 under various influence factors. A mixed suspension was formed by 0.1 g/L Ca-Mg-Al-LDH or Ca-MgAl-LDOx, 30 mg/L U(VI) solution, and 0.01 M NaNO3 solutions, and the desired pH was adjusted with negligible amounts of 0.01 or 0.05 M HNO3 or NaOH solution. The effect of carbonate ions was investigated with various concentration gradients (0.01−0.20 M) at fixed pH (5.0 ± 0.1), and the adsorption capacity was compared at 0.10 M CO32− in the range of pH 2.0−11.0. The reaction system achieved equilibrium under oscillation for 24 h, and then was centrifuged at 10000 rpm for 10 min. The concentration of U(VI) was determined with the Arsenazo-III method at 650 nm in UV-2550 spectrophotometry, and the adsorption efficiency of U(VI) on Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx was obtained from the difference between the

(process I)

[M1 − x 2 +Mx 3 +(OH)2 ]x + (An −)x / n → [M1 − x 2 +Mx 3 +O]x + (An −)x / n

(process II)

[M1 − x 2 +Mx 3 +O]x + (An −)x / n → M1 − x 2 +Mx 3 +O1 + x /2 (BOy ) (process 2+

3+

2+

2+

MATERIAL AND METHODS

Materials Synthesis. The Ca-Mg-Al layered double hydroxides (defined as Ca-Mg-Al-LDH) or calcined LDHs (donated as Ca-Mg-AlLDOx, “x” represented various calcination temperature) were synthesized by modification methods (see SI). The possible synthesis route of these matrials was shown in Figure 1.

3+

M1 − x Mx O1 + x /2 (BOy ) → M O + M M 2 O4 + BOy (process IV)

where BOy represents the species of decomposed anion. Interestingly, the “memory effect” of LDO can be obtained under the intermediate calcination temperature (600 °C).29,32 LDO showed higher adsorption capacity than LDHs due to its synergistic effect of “memory effect” and “size effect”.32,33 Therefore, controllable synthesis of LDO is beneficial for improving the surface property and chemical reactivity, which 1174


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ACS Sustainable Chemistry & Engineering initial concentration (C0, mg·L−1) and the equilibrium one (Ce, mg·L−1). The removal percentage (%) was recorded as adsorption (%) = 100% × (1 − Ce/C0), and the distribution coefficient (Kd) was described as Kd = (C0 − Ce)/Ce × V/m. The amount of adsorbed U(VI) was achieved by Cs = (C0 − Ce)/m × V, where m was the mass of Ca-Mg-Al-LDH or Ca-Mg-Al-LDOx, and V was the volume of mixed suspension. All the adsorption experiment data were the average of duplicate determinations, and the relative error was ∼5%.

Interestingly, after calcination treatment, the regular crystal and rod-like structure have been destroyed and transformed to an amorphous crystal, and the disorder degree could be improved with the increasing of temperature.39,40 Compared with the SEM images of the samples before and after calcination, the hardness of Ca-Mg-Al-LDH was higher than those of Ca-Mg-Al-LDO300 and Ca-Mg-Al-LDO600. In addition, the average diameter could be adjusted and controlled in the range ∼1.7 μm to ∼1.3 μm, which was beneficial for the adjusting of the adsorption capacity of U(VI). Elemental mapping of asprepared nanomaterials (Figure 3(a)−3(c)) showed the homogeneous distribution of four elements (Ca, Mg, Al, and O) within the hierarchical structures, which indicated that the four main elements were uniformly deposited on the surface of these three adsorbents. In addition, it showed the increase of Ca and Al elements with the improving of calcination temperature. It can be seen from Figure 4(a) and 4(b), the Brunauer− Emmett−Teller (BET) surface areas were 72.8 m2·g−1 for Ca-Mg-Al-LDH with an average pore diameter (Barrett− Joyner−Halenda (BJH)) of ∼12.7 nm, and 43.0 m2·g−1 for Ca-Mg-Al-LDO300 with an average pore diameter of ∼11.5 nm,

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RESULTS AND DISCUSSION Microstructure and Characterization. The morphologies of as-prepared Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600 were characterized by SEM. As shown in Figure 2, a typical rod-like structure was exhibited and the orderliness was clear. In Figure 2(a), one can see that the width of Ca-Mg-Al-LDH is not uniform and the maximum width is near 1.73 μm, and the surface defect or surface roughness is not lower than that of calcined Ca-Mg-Al-LDHs (according to Figure 2(b), 2(d), and 2(f)). The maximum width of Ca-Mg-Al-LDO300 (Figure 2(c), d = 1.32 μm) is similar to that of Ca-Mg-Al-LDO600 (Figure 2(e), d = 1.34 μm), indicating that the dimension is independent of calcination temperature.

Figure 2. SEM images of as-prepared Ca-Mg-Al-LDH (a and b), Ca-Mg-Al-LDO300 (c and d), and Ca-Mg-Al-LDO600 (e and f). 1175


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Figure 3. Elemental mapping of the homogeneous dispersion of Ca, Mg, Al, and O elements in the as-prepared Ca-Mg-Al-LDH (a), Ca-Mg-AlLDO300 (b), and Ca-Mg-Al-LDO600 (c).

Figure 4. (a) N2 adsorption−desorption isotherms of as-prepared Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300 samples (inset: N2 adsorption−desorption isotherms of Ca-Mg-Al-LDO600). (b) Pore size distribution of Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600.

and 157.8 m2·g−1 for Ca-Mg-Al-LDO600 with an average pore diameter of ∼2.1 nm. The higher surface area could provide more active sites and improved sufficient interaction between adsorbents and U(VI), and the narrow pore-size distribution indicated that the mesopores structure was determined. In Figure 4(b), a typical IV isotherm with a H3 hysteresis loop was exhibited in the curve of Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300, while a H4 hysteresis loop was observed in the isotherm of

Ca-Mg-Al-LDO600. It also confirmed the presence of a mesoporous in Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300, and the presence of a micropore in Ca-Mg-Al-LDO600. As shown in Figure S1 and Table S1, the results of energy-dispersive spectroscopy (EDS) demonstrated that Ca, Mg, Al, and O were confirmed and the chemical composition was quantitative, which exhibited the increase of the relative content of Ca and Mg with the increase of calcination temperature. 1176


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Figure 5. Characterization of as-prepared Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx samples: (a) FT-IR spectra, (b) XRD patterns, (c) XPS survey spectra, (d) Ca 2p spectra, (e) O 1s spectrum of Ca-Mg-Al-LDH, and (f) O 1s spectra of Ca-Mg-Al-LDO300 and Ca-Mg-Al-LDO600.

between ∼1372 and ∼1555 cm−1 were assigned to the ν3 vibration of CO32−,45 and the vibration of ν1(CO32−) and ν2(CO32−) of Ca-Mg-Al-LDH could be observed at ∼1105 cm−1 and ∼852 cm−1, respectively.46 In addition, a series of characteristic peaks at 979 cm−1, 747 cm−1, 633 cm−1, and 480 cm−1 also represented the M−O lattice vibrations and M−O-H bending (M: Ca or Al or Mg).47−49 Compared to the M-O of Ca-Mg-Al-LDOx, the positions of lattice vibrations for Ca-Mg-Al-LDH were variable and the purity of bonding types for Ca-Mg-Al-LDOx was higher, which was consistent with the results of SEM. The XRD patterns (Figure 5(b)) of Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx demonstrated that the dominant crystalline phases were similar among these Ca-Mg-Al-LDOx. In the XRD pattern of Ca-Mg-Al-LDH, the main diffraction peaks at 2θ = 15.22°, 26.87°, 30.77°, 31.93°, 34.81°, 42.18°, 45.30°, and 52.67° exhibited the (003), (006), (100), (009), (012), (015), (018), and (1010) crystal planes of Ca-Mg-Al-LDH, respectively, which showed that Ca-Mg-Al-LDH had a typical hydrotalcite-like structure with high crystallinity and purity.28,48,49

In order to compare the difference of bonding types about Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx (x: 200, 300, 400, 500, and 600 °C), the FT-IR spectra were shown in Figure 5(a). It demonstrated that the main peaks of Ca-Mg-Al-LDOx were similar and that the same characteristic peaks disappeared after calcination. The wide peak of all samples at 3446 cm−1 corresponded to the stretching mode of −OH (ν(OH)),18,33 and the weak peak of Ca-Mg-Al-LDH near 3000 cm−1 was assigned to the water molecules (ν(H2O)), which was attributed to the hybridization of CO32− and hydrogen-bonded (CO32−-H2O) in the interlayer.41 The absence of this weak peak in Ca-Mg-Al-LDOx indicated that the interlayer anions were destroyed during the calcination process. The common peaks of Ca-Mg-Al-LDOx at 1428 cm−1, 873 cm−1, and 726 cm−1 corresponded to M−O and M−OH (M = Ca, Mg, and Al),33,34,42−44 and the same peaks were also observed in the FT-IR spectrum of Ca-Mg-Al-LDH, which indicated that the basic bonding types of all samples were consistent and similar structure was maintained after calcination treatment. Furthermore, the particular characteristic peaks of Ca-Mg-Al-LDH 1177


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of solid content should be considered. As shown in Figure 6(a), the adsorption percentages of U(VI) on Ca-Mg-Al-LDH, CaMg-Al-LDO300, and Ca-Mg-Al-LDO600 increased rapidly with the increasing amount of adsorbents at low contents (m/V < 0.2 g·L−1). It could be found that the removal percentage of U(VI) increased from ∼10% to ∼47% for Ca-Mg-Al-LDH, from ∼26% to ∼70% for Ca-Mg-Al-LDO300, and from ∼53% to ∼81% for Ca-Mg-Al-LDO600 with the solid contents increasing from 0.025 g·L−1 to 0.1 g·L−1, which might be attributed to more active sites for the formation of U(VI) surface complexes on Ca-Mg-Al-LDH or Ca-Mg-Al-LDO300 or Ca-Mg-AlLDO600.33,49 In addition, the higher adsorption capacity of Ca-Mg-Al-LDO600 might be due to the abundant metal−oxide bonds, higher surface area, and “size effect”, which could provide more active sites for the binding of U(VI) on Ca-MgAl-LDO600.34,43,57 Interestingly, the surface area of Ca-Mg-AlLDO300 is lower than that of Ca-Mg-Al-LDH (Figure 4(a)), while the higher adsorption capacity of Ca-Mg-Al-LDO300 can be due to the abundant metal−oxide bonds (Ca−O and Al−O, Table S1) and “memory effect”. However, at higher solid contents, the relative available surface area of adsorbents was decreased and most energy sites were occupied by a “solid effect”, which could lead to the slight increase of adsorption capacity.58 The distribution coefficients (Kd) of U(VI) on various adsorbents were exhibited in Figure S2, and it can be seen that the Kd values were weakly dependent on solid contents under the experimental uncertainties, which was consistent with the physicochemical properties of the Kd values; that is, the Kd value was independent of solid contents if the competition among the solid paritcles was negligible at low solid contents. Effect of pH and Ionic Strength. The adsorption of U(VI) on Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600 as a function of pH in 0.001 M, 0.01 M, and 0.1 M NaNO3 solutions was studied. As displayed in Figure 6(b)−6(d), one can see that the adsorption behavior of U(VI) on Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300 was similar, and the adsorption process can be divided into three parts. The adsorption of U(VI) on CaMg-Al-LDH and Ca-Mg-Al-LDO300 increased with pH increasing at pH < 5.0 (process I), decreased with pH increasing in the pH range of 5.0−8.0 (process II), and then increased again at pH > 8.0 (process III) (Figure 6(b) and 6(c)). At pH < 5.0, there were a small dissolution of Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600, and the amount of dissolution was very low due to their high stability, which did not interfere significantly in the adsorption process. The free UO22+ and (UO2)3(OH)5+ were the dominant species (Figure 6(e)), which can form strong surface complexes on solid particle surfaces.4,5 In process II, the decrease of adsorption with the increase of pH might be due to the fact that more OH− could make the surface of adsorbents negatively charged, greatly weakening the electrostatic attraction between Ca-Mg-Al-LDH or Ca-Mg-Al-LDO300 and the negatively charged U(VI).5 But the main reason is that the U(VI) does not easily form complexes with protonated carboxylic acid groups on the interlayer of Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300. However, in process III (alkaline solutions, 8.0 < pH < 11.0), the removal percentages increased again with the increase of pH, which was due to the formation of chemical precipitate (schoepite) (Figure 6(f)).4,5,59 It was very interesting to note that the sorption of U(VI) on Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300 was enhanced at higher NaNO3 concentrations (Figure 6(b) and 6(c)), and this phenomenon might be due to the following

Table 1. Crystallite Sizes and d-Spacing of Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx from XRD Analysis d-spacing (Å)

Crystallite sizes (nm)

Samples

(100)

(009)

(015)

(100)

(009)

(015)

Ca-Mg-Al-LDH Ca-Mg-Al-LDO200 Ca-Mg-Al-LDO300 Ca-Mg-Al-LDO400 Ca-Mg-Al-LDO500 Ca-Mg-Al-LDO600

2.91 2.92 2.93 2.92 3.02 3.02

2.81 2.81 2.81 2.81 2.92 2.91

2.14 2.15 2.14 2.14 2.11 2.11

24.4 24.5 22.9 15.7 21.7 20.6

17.7 19.3 20.9 24.0 14.7 21.2

18.8 15.5 18.2 20.1 12.4 16.1

The d-spacing and crystallite sizes of all samples obtained from XRD patterns were listed in Table 1, and the similar d-spacing or crystallite sizes indicated that similar structure characteristics (rod-like) of all samples were exhibited.50 The XRD patterns of Ca-Mg-Al-LDOx demonstrated that the regular LDHs phases disappeared and were completely replaced by the mixed phase of double metal-oxides after calcination. Moreover, the absence of the (003), (006), and (012) reflection peaks suggested that the uniform structure of LDH has been destructed through calcination at high temperatures.28 Interestingly, the stronger reflection peaks of (018) and (1010) for Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx at low calcined temperature (T < 500 °C) demonstrated that the short-range ordered structure was contained,34,49 while the new peak at 2θ = 62.11° for Ca-Mg-AlLDO500 and Ca-Mg-Al-LDO600 showed the typical (113) crystal planes,51 which implied that mixed oxides of magnesium, aluminum, and calcium were produced via the decomposition process. From the aforementioned results, it can be found that the differences of basic structures and crystalline phase were more obvious among Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx with the increase of calcination temperature. To unravel the element distribution and surface state of Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx, the XPS spectra of Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600 were exhibited in Figure 5(c). The XPS spectra revealed that calcium, magnesium, aluminum, and oxygen were the predominant elements in all samples, and the photoelectron lines at binding energies of ∼24.4 eV, 51.7 eV, 74.5 eV, 119.4 eV, 351.5 eV, and 531.9 eV corresponded to O 2s, Mg 2p, Al 2p, Al 2s, Ca 2p, and O 1s, respectively.33,49 Interestingly, the relative contents of Ca increased after the calcination process (Figure 5(d)), and the positions of Ca 2p3/2 and Ca 2p1/2 peaks for Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300 were centered at ∼347.5 and 351.3 eV, respectively, while the peak positions for Ca-Mg-Al-LDO600 were shifted to low binding energies (347.3 or 350.9 eV), which indicated that the relative contents of Ca were increased.28,52 The relative contents of other elements were listed in Table S2. The high XPS O 1s spectral resolutions of Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600 were displayed in Figure 5(e) and 5(f), and the O 1s spectra of all samples could be deconvoluted into three components, such as “Ca−O”, “Al−O”, and “Mg−O” bonds.53−56 The relative contents of various metal-oxide bonds were summarized in Table S3, and the contents of “Ca−O” increased with the increase of calcinated temperature, which was attributed to the reduction of interlayered CO32−. In addition, “Ca−O” could improve the stability of the formation of U(VI)-oxides, which was beneficial for U(VI) removal from aqueous solutions based on the high activity of “Ca−O”. Effect of Solid Content. In order to reduce the cost under the effective removal percentage in real applications, the effect 1178


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Figure 6. (a) Effect of solid content for Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600 at pH = 5.0 ± 0.1 and I = 0.01 NaNO3. (b) Effect of pH on U(VI) adsorption on Ca-Mg-Al-LDH with various NaNO3 concentrations at m/V = 0.1 g/L. (c) Effect of pH on U(VI) adsorption on Ca-Mg-Al-LDO300 with various NaNO3 concentrations at m/V = 0.1 g/L. (d) Effect of pH on U(VI) adsorption on Ca-Mg-Al-LDO600 with various NaNO3 concentrations at m/V = 0.1 g/L. (e) 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, T = 298.15 K.

reasons: (1) the activity of Ca-Mg-Al-LDH and Ca-Mg-AlLDO300 increased with the increase of Na+; (2) the interlayer anions (CO32−) increased particle dispersion with the increase of NO3− by electrostatic replusion.60 Compared with the above two adsorbents, the adsorption behavior of U(VI) on Ca-MgAl-LDO600 was different, and the removal percentage of U(VI) increased rapidly at pH < 5.0 in 0.001 and 0.01 M NaNO3 solutions, and then maintained the high level at pH > 5.0. However, it is necessary to note that the sorption of U(VI) on Ca-Mg-Al-LDO600 increased with pH increasing at whole experimental pH conditions in 0.1 M NaNO3 solution, which was different in 0.001 and 0.01 M NaNO3 solutions. The difference might be attributed to the fact that U(VI) could form an electrical double layer complex in 0.001 and 0.01 M NaNO3 solutions, while foreign cations could compete with U(VI) and thereby decreased the adsorption of U(VI) on Ca-Mg-Al-LDO600, which demonstrated that the interaction mechanism between metal−oxide bonds and U(VI) was partly an ionic interaction in nature.29,31,43,61

According to the difference of ionic strength, it can be found that the adsorption processes of U(VI) on Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300 were dependent on ionic strength at all pH values, indicating that the adsorption process was dominated by outer-sphere surface complexation,5,6 which was attributed to abundant interlayer anions (CO32−). During the adsorption process, interlayer anions (CO32−) could participate protonation/deprotonation reaction, and it could also ion-exchange with U(VI). However, the adsorption process of U(VI) on Ca-Mg-Al-LDO600 was strongly dependent on ionic strength in the pH range of 2.0−8.0, and the interlayer anions (CO32−) was disappeared and transferred to metal-oxide composites, which demonstrated that the adsorption reaction was also controlled by outer-sphere surface complexation or ion-exchange.21 The weakly ionic strength-dependent adsorption at 8.0 < pH < 11.0 suggested that the process was domianted by inner-sphere surface complexation.20,42 Adsorption Isotherms and Thermodynamic Studies. To analyze the adsorption mechanism of U(VI), the adsorption 1179


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Figure 7. (a) Adsorption isotherms of U(VI) on Ca-Mg-Al-LDH. (b) Adsorption isotherms of U(VI) on Ca-Mg-Al-LDO300. (c) Adsorption isotherms of U(VI) on Ca-Mg-Al-LDO600. (d) Comparison of adsorption isotherms of Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx at 298.15 K (Dots represent the experimental data, and fitting lines represent the Sips model). m/V = 0.1 g/L, pH = 5.0 ± 0.1, I = 0.01 NaNO3.

Table 2. Adsorption Isotherm Parameters of U(VI) Adsorption on Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx (x: 200, 300, 400, 500, and 600) at 298.15 K Adsorbents Sips model

Parameters −1

KS (L·g ) as (L·mg−1) 1/ns Cs,max (mg·g−1) R2

Ca-Mg-Al-LDH

Ca-Mg-Al-LDO200

Ca-Mg-Al-LDO300

Ca-Mg-Al-LDO400

Ca-Mg-Al-LDO500

Ca-Mg-Al-LDO600

0.204 0.002 0.349 132.5 0.971

0.171 7.6 × 10−4 0.266 223.5 0.946

2.231 0.009 0.382 260.0 0.944

5.520 0.019 0.454 292.5 0.971

0.779 0.002 0.248 486.8 0.953

0.001 3.2 × 10−6 0.148 373.4 0.993

From the relative parameters (Table S4), it can be seen that the adsorpton isotherms of U(VI) on all adsorbents were well fitted by the Sips model (R2 > 0.94), which implied that the adsorption reaction was multilayer adsorption at low concentration of U(VI) and monolayer adsorption at high concentration of U(VI).63 This was attributed to the adsorption rate of U(VI) on adsorbents sustaining increases at low concentration of U(VI), while the adsorption rate of U(VI) was not increased at high concentration of U(VI).64 In order to explore the controlled adsorption capacity of Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx, the adsorption isotherms of U(VI) on Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx (x: 200, 300, 400, 500, and 600) were compared and fitted by the Sips model at 298.15 K, and the relative parameters were tabulated in Table 2. This demonstrated that the adsorption capacities of U(VI) on various adsorbents followed the order Ca-Mg-AlLDO500 (486.8 mg·g−1) > Ca-Mg-Al-LDO600 (373.4 mg·g−1) > Ca-Mg-Al-LDO 400 (292.5 mg·g −1 ) > Ca-Mg-Al-LDO 300 (260.0 mg·g−1) > Ca-Mg-Al-LDO200 (223.5 mg·g−1) > Ca-Mg-Al-LDH (132.5 mg·g−1), which might be attributed to more active surface sites and a “memory effect” provided by metal-oxide bonds (e.g., Ca−O or Al−O) with the increase of calcination temperature under T < 600 °C, while the reduction of the U(VI) adsorption capacity at T = 600 °C could be due to

isotherms were studied under three different reaction temperatures (298.15 K, 313.15 K, and 333.15 K). As displayed in Figure 7, the adsorption capacities of various adsorbents were improved rapidly with the increase of temperature. Three typical isotherm models, i.e., the Langmuir, Freundlich, and Sips models, were applied to evaluate the maximum adsorption capapcity. These models can be expressed as eqs 1−3:62,63 Ce Ce 1 = + Cs Cs ,maxKL Cs ,max

log Cs = log KF +

Cs =

KsCens 1 + asCens

1 log Ce n

(1) (2)

(3) −1

where Cs,max (mg·g ) is the maximum adsorption capacity per unit weight of adsorbents and KL (L·mg−1) is the Langmuir constant related to the affinity and bonding of adsorbent. KF (mg1−n·Ln·g−1) is the Freundlich constant related to the adsorption capacity, 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 concerned with adsorption behavior. 1180


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Figure 8. (a) Adsorption kinetics of U(VI) on various adsorbents. (b) Effect of C[CO32−] on U(VI) adsorption on Ca-Mg-Al-LDH, Ca-Mg-AlLDO300, and Ca-Mg-Al-LDO600. (c) Effect of pH on U(VI) removal at C[CO32−] = 0.1 mol·L−1. (d) Species distribution of U(VI) as a function of pH at C[CO32−] = 0.1 mol·L−1.

the lack of a “memory effect” and an enhanced “size effect” of Ca-Mg-Al-LDO600, which could provide the possibility for selective control the adsorption capacity of U(VI) through changing calcination temperature. Traditional thermodynamic parameters, such as standard entropy change (ΔS°, J·K−1·mol−1), standard free energy change (ΔG°, kJ·mol−1), and standard enthalpy change (ΔH°, kJ·mol−1), were obtained from the temperature-dependent isotherms and applied to evaluate the interaction mechanism. ΔG° and ΔH° could be calculated by eqs 4 and 5:17,20 ln K ° =

ΔS° ΔH ° − R RT

ΔG° = −RT ln K °

removal percentage of U(VI) on Ca-Mg-Al-LDO300 and CaMg-Al-LDO600 increased rapidly at the first contact time of 2 h, and the removal percentage reached over 50% at the first 10 min, which might be attributed to abundant free binding sites and active sites based on more metal-oxide bonds.34,38 While the adsorption rate and adsorption capacity were increased slightly for Ca-Mg-Al-LDH, this was due to its more smooth surface and less surface defects. The adsorption processes were simulated by pseudo-firstorder and pseudo-second-order kinetic models, which could be expressed as eqs 6 and 7:17 ln(qe − qt ) = ln qe − k1t

(4)

(6)

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

(5)

where R = 8.314 J·mol−1·K−1 represents the gas constant and K° represents the thermodynamic equilibrium constant, and the values of In K° are calculated by plotting In Kd vs Ce and extrapolating to zero (Figure S3(a)−S3(d)). The thermodynamic parameters were listed in Table S5, and the positive ΔH° values implied that U(VI) adsorption on Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx were endothermic processes.15 The negative ΔG° values and the positive ΔS° values revealed that the process was spontaneous with high affinity, which demonstrated that more ligands chelated with uranyl and more water molecules (H2O) participated in the reaction at higher adsorption temperature.58 Kinetic Studies. In order to explore the adsorption ratecontrolling steps (e.g., mass transport or chemical reaction), the effects of contact time on U(VI) adsorption to Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600 were investigated and analyzed. As shown in Figure 8(a), it can be seen that the

−1

(7) −1

where qt (mg·g ) and qe (mg·g ) represented the adsorption capacity at time t (h) and equilibrium, respectively. k1 (min−1) and k2 (g·mg−1·min−1) were the constants of the pseudo-firstorder and pseudo-second-order rates, respectively. The values of k1, k2, and qe obtained from the linear plot of ln(qe - qt) vs (t) or t/qt vs (t), and the related parameters are listed in Table S6. The kinetic curves (Figure S4 and S5) indicated that the adsorption process could be better fitted by a pseudo-secondorder model, suggesting a chemical reaction process.3,4,17 The results were in accordance with the rapid adsorption process of U(VI) on Ca-Mg-Al-LDO300 and Ca-Mg-Al-LDO600. Effect of CO32‑. In a water system, CO32− has been the essential component because of the high solubility of CO2 in aqueous solutions, and the effect of CO32− in the removal of U(VI) from aqueous solutions should be considered. As shown in Figure 8(b), with the increase of C[CO32−] from 0.01 to 0.20 M, 1181


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and necessary for the fate and stability of environmental management.65 Natural clay minerals will play an important role in the environmental in situ remediation, which can reduce secondary pollution through traditional carbon-family materials or other synthetic materials. Ternary rod-like Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx could be controlled and adjusted by changing the calcination temperatures, and they could be promising adsorbents for the removal of U(VI), especially for hierarchical processing environmental pollutants. As shown in Figure 9, the interaction mechanism between U(VI) and Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx could be attributed to strong surface complexation and electrostatic interactions, and with the increase of calcination temperature, the adsorption capacity could be adjusted and controlled, which was beneficial for hierarchical processing of various pollutants. The adsorption capacities and thermodynamic parameters on different materials were summarized in Table 3, and it demonstrated that the adsorption capacity of Ca-Mg-Al-LDOx was higher than those of most synthetic materials and clay minerals. It was clear that Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx could be controllable materials for the adsorption of U(VI) from wastewater or groundwater in environmental pollution cleanup.

the adsorption capacity of U(VI) on Ca-Mg-Al-LDH and Ca-Mg-Al-LDO300 decreased slightly, which might be attributed to the abundant CO32− that changed the distribution of surface charge of solid adsorbents, and it caused strong electrostatic repulsion between UO22+ and adsorbents.2,13 However, the removal percentage of U(VI) on Ca-Mg-Al-LDO600 was weakly dependent on C[CO32−], which could be due to the fact that the main components of Ca-Mg-Al-LDO600 were metal oxides and lacking a “memory effect”. Figure 8(c) displayed that the removal percentages of U(VI) on Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600 increased with pH increasing at 2.0 < pH < 5.0 in C[CO32−] = 0.1 mol·L−1. In this pH range, the main components of U(VI) were UO22+, UO2OH+, and UO2CO3 (aq), and it might be controlled by a protonation process and electrostatic attraction.16 While the adsorption decreased rapidly with the increase of pH at 5.0 < pH < 11.0, the main components in aqueous solutions were UO2(CO3)22− and UO2(CO3)34−, which indicated that the adsorption process was inhibited by more stable carboanion and free radical middle substance, and it produced steric hindrance to an adsorption reaction.13,16,24 This revealed that the effect of CO32− should be avoided and reduced in the environmental remediation.

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■

CONCLUSIONS A series of ternary rod-like Ca-Mg-Al-LDH and Ca-Mg-AlLDOx were synthesized via hydrothermal and calicination methods, and the interaction mechanism of U(VI) with Ca-MgAl-LDH and Ca-Mg-Al-LDOx was investigated from SEM, FT-IR, XRD, and XPS techniques. The results indicated that the adsorption capacity could be adjusted and controlled by changing the calination temperature, and followed the order Ca-Mg-Al-LDO 500 (486.8 mg·g −1 ) > Ca-Mg-Al-LDO 600 (373.4 mg·g−1) > Ca-Mg-Al-LDO400 (292.5 mg·g−1) > Ca-Mg-Al-LDO 300 (260.0 mg·g −1 ) > Ca-Mg-Al-LDO 200 (223.5 mg·g−1) > Ca-Mg-Al-LDH (132.5 mg·g−1). The adsorption process was inhibited by more stable carboanion and free radical middle substances, which indicated that the effect of CO32− should be avoided and reduced in the environmental remediation. Compared with other nanomaterials, this revealed that Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx are promising potential nanomaterials for the efficient removal of U(VI) from aqueous solutions in natural environmental pollution cleanup.

ADSORPTION MECHANISM AND ENVIRONMENTAL APPLICATION With the development of nuclear energy, the removal and recycle of U(VI) from wastewater or groundwater is essential

Figure 9. Schematic representation mechanism and environmental behavior of U(VI) adsorption onto Ca-Mg-Al-LDH and Ca-Mg-AlLDOx.

Table 3. Comparison of the Adsorption Capacities of U(VI) on Ca-Mg-Al-LDH, Ca-Mg-Al-LDO300, and Ca-Mg-Al-LDO600 with Other Adsorbents at 298.15 Ka Thermodynamic parameters

a

−1

Adsorbents

ΔH° (kJ·mol )

ΔS° (J·mol−1·K−1)

ΔG° (kJ·mol−1)

Sorption Capacity (mg·g−1)

ref

Phosphate-functionalized graphene oxide Amidoxime-functionalized magnetic mesoporous silica Bentonite Hydrothermal carbon spheres Hydrothermal carbon Graphene oxide nanosheets Amidoximated magnetite/graphene oxide composites Amidoxime modified Fe3O4@SiO2 Magnetic chitosan resins Mesoporous carbon Ca-Mg-Al-LDH Ca-Mg-Al-LDO300 Ca-Mg-Al-LDO600

93.00 n.a. 17.94 n.a. 23.53 16.95 16.71 14.85 −33.87 14.60 26.70 26.08 17.24

348.00 n.a. 120.80 n.a. 165.13 115.92 130.80 114.40 −29.59 115.10 109.38 114.27 87.49

−12.50 n.a. −18.07 n.a. −25.71 −17.08 −22.34 −18.43 −25.05 −18.50 −5.91 −7.99 −8.84

251.7 277.3 23.6 179.9 273.0 97.5 284.9 105.0 160.7 133.5 132.5 260.0 373.4

58 59 23 18 19 6 5 4 2 17 This study This study This study

n.a.: not applicable. 1182


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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.6b02550. Preparation of Ca-Mg-Al-LDH and Ca-Mg-Al-LDOx composites; additional figures and tables for XPS analysis, kinetic spectra, kinetic parameters, and adsorption isotherm models (PDF)

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AUTHOR INFORMATION

Corresponding Authors

* Phone: 86-10-61772890. Fax: 86-10-61772890. E-mail: [email protected] or [email protected] (X. Wang), [email protected] (Y. Liu). * Phone: 86-10-61772890. Fax: 86-10-61772890. E-mail: [email protected] (Y. Liu). ORCID

Xiangke Wang: 0000-0002-3352-1617 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from NSFC (21225730, 21403064, 91326202, 21577032), the Fundamental Research Funds for the Central Universities (JB2015001), the Project of East China Institute of Technology Graduate Student Innovation Fund (YC2015-S273), and the Furong scholarship of Hunan province are acknowledged.

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