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Immobilization of U(VI) on hierarchical NiSiO@MgAl and NiSiO@NiAl nanocomposites from wastewater Shuang Song, Qiang Huang, Gong Cheng, Weixue Wang, Zhanhui Lu, Rui Zhang, Tao Wen, Yihan Zhang, Jian Wang, and Xiangke Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05698 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Sustainable Chemistry & Engineering

Immobilization

of

U(VI)

on

hierarchical

NiSiO@MgAl and NiSiO@NiAl nanocomposites from wastewater Shuang Song, Qiang Huang, Gong Cheng, Weixue Wang, Zhanhui Lu*, Rui Zhang*, Tao Wen*, Yihan Zhang, Jian Wang, Xiangke Wang* MOE Key Lab of Resources and Environmental System optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, P.R. China * corresponding authors. E-mail addresses: [email protected] (R. Zhang), [email protected] (T. Wen), [email protected] (X. Wang), [email protected] (Z. Lu).

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ABSTRACT: The emerging radioactive pollution caused by human activity has attracted increasing attention in the recent years. Uranium (U(VI)) is one of the key radionuclides in the environment, but its migration behavior is firmly controlled by the sorption properties on the environmental matrix. Herein, three hollow spherical materials (NiSiO, NiSiO@MgAl and NiSiO@NiAl) were rationally designed and constructed by the template method, which showed great potential in the immobilization of U(VI). The engineered functional nanomaterials exhibited fast sorption kinetics, which could be illustrated by the pseudo-second-order model, and favorable thermodynamics, which showed spontaneous and endothermic process. Based on the batch experimental investigations under different conditions, the immobilization of U(VI) on the as-synthesized nanomaterials followed the mono-layer mechanism in the form of inner-sphere surface complexes. The maximum removal capacity (Qmax) of U(VI) on NiSiO@NiAl (136.94 mg∙g-1) was much higher than those of NiSiO@MgAl (41.82 mg∙g-1) and NiSiO (14.77 mg∙g-1). The XPS investigations revealed that the excellent sorption property of NiSiO@NiAl originated from its surface Ni-OH group, which had higher coordination ability towards U(VI) than the others. This work paves an avenue to rational design and synthesize novel materials in the application of radionuclide pollution treatment. KEYWORDS: Hierarchical materials; LDH; Sorption; Spectroscopy; Uranium


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INTRODUCTION With the rapid growth of nuclear industrialization, a large amount of radioactive wastes are being released into the environment, which has caused serious pollution on a global scale.1-3 Uranium is the most important element in the nuclear industry, which is not only used as fuel for the nuclear reactor but also a key component in the nuclear waste.4 However, it has been pointed out that the hexavalent uranium (U(VI)) is a hazardous radioactive substance owing to its long half-life and high toxicity, which might directly or indirectly lead to health problems, such as renal injury, brain damage and even death.5-6 For the sake of human health and ecological stability, it is crucial to evaluate the fate and transport behavior of U(VI), and then extract U(VI) ions from polluted areas by proper methods before they are discharged into the environment.7,8 In the past decades, various technologies have been applied for eliminating

radionuclides

from

aquatic

systems,

including

photocatalytic

degradation/oxidation, sorption, electrocoagulation, reverse osmosis and membrane filtration.9-13 Nevertheless, most of these methods suffer from several limitations, such as narrow range of pH operation and limited tolerance towards high salt concentrations, which severely impede their applications.14,15 Of all these methods, sorption has been applied as one of the most efficient ways to remove radionuclides due to its simple design, easy manipulation and versatility.16-20 Recently, a great number of nano-adsorbents, including graphene oxides, polymers, minerals, metal and metal-oxides, have been applied for the elimination of U(VI) from aqueous solutions, but their applications are still handicapped either by the low sorption capacities or by the high running costs.21-29 Hollow-structured adsorbents have recently attracted great attention owing to their structural flexibility and physicochemical properties,30 however, limited contribution has been devoted to the treatment of radioactive wastewater. Towards this end, fabricating hybrid



nanocomposites and constructing hierarchical structures by low-cost materials have been proved to be the effective method.31-35 In terms of materials, silicate is green and highly abundant in natural environment, and nickel silicate, which has a typical layered structure, has received research interests in the removal of pollutants.36,37 However, its low sorption capacity for U(VI) still cannot satisfy the demand of practice. Layered double hydroxides (LDHs) is a family of two-dimensional anionic clays with unique physical and chemical properties, including rapid dispersion, high surface areas, excellent anion exchange property, which make them promising adsorbents for many organic and inorganic pollutants, but the easy-to-aggregate feature is still the fatal shortcoming.38-41 Therefore, the rational construction of hybrid composite by nickel silicate and LDHs can be a promising way to increase the number of surface functional groups of the adsorbent and overcome the aforementioned shortcoming in the application of LDHs. Inspired by this idea, we have prepared hierarchical nickel silicate (NiSiO) hollow spheres wrapped with MgAl (NiSiO@MgAl) and NiAl (NiSiO@NiAl) LDHs. The structures and morphologies of the resulting materials are carefully characterized by XRD, SEM and TEM, and the sorption performances towards U(VI) are systemically studied by batch sorption experiments under various environmental conditions (e.g., solid contents, contact time, pH value, ionic strength, temperature and initial concentration). The interaction mechanisms are further investigated by XPS techniques.

EXPERIMENTAL SECTION Materials. All chemical reagents, including Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Ni(NO3)2·6H2O, CO(NH2)2, HNO3, NH4Cl, NaOH, NH3·H2O, NaNO3and tetraethyl orthosilicate (TEOS) were of analytical grade, and used as received without


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purification. Fabrication of three hierarchical materials. The total preparation processes are illustrated in Scheme 1. Fabrication of NiSiO hollow spheres. The silicon dioxide template was firstly fabricated and the NiSiO hollow sphere was prepared according to the previous reports.33,42Briefly, 0.2 g SiO2 was dispersed into 40 mL water to form solution A. 2.7 mmol Ni(NO3)2·6H2O and 10 mmol NH4Cl was dissolved into 40 mL distilled water and stirred for about 15 min. Subsequently, 2 mL NH3·H2O was added into the solution slowly, which was remarked as solution B. The solution B was added dropwise to the suspension A. Ultimately, the mixed suspension was under vigorous stirring for 10 min, moved into a 100 mL autoclave and then remained at 120 oC for 24 h. The precipitate was rinsed with water and ethanol several times and then dried at 60 oC overnight. Preparation of NiSiO@MgAl and NiSiO@NiAl LDHs. In a typical procedure, the NiSiO@MgAl and NiSiO@NiAl LDHs were prepared by the hydrothermal method. Specifically, 1 g NiSiO hollow sphere, 6 mmol Mg(NO3)2·6H2O, 2 mmol Al(NO3)3·9H2O and 40 mmol urea were dispersed in 60 mL water. Subsequently, the mixed suspension was moved into the autoclave and remained at 120 °C for 12 h. The sediment was rinsed with ethanol and Milli-Q water three times. Then the obtained samples were dried in vacuum at 60 °C for about 12 h. The NiSiO@NiAl LDH was prepared through a similar method using 1 g NiSiO hollow sphere, 6 mmol Ni(NO3)2·6H2O, 2 mmol Al(NO3)3·9H2O and 40 mmol urea as raw materials. Characterizations. The powder X-ray diffraction (XRD) patterns were



conducted using a Rigaku D/max 2500 diffractometer. The N2 adsorption/desorption isotherms were recorded from a Micromeritics ASAP2010 instrument. The surface morphologies of materials were characterized by Hitachi S-4800 scanning electron microscopy (SEM). The transmission electron microscopy (TEM) images were conducted on the JEM-1011 transmission electron microscopy. Energy-dispersive X-ray (EDX) spectroscopy was obtained on Hitachi S-4800. The Zeta-potentials were measured on a Nanoscale and Zeta potential analyzer (Malvern Zetasizer Nano ZS). The X-ray photoelectron spectroscopy (XPS) spectra were obtained using the ESCALab220i-XL electron spectrometer. The Fourier transformed infrared (FTIR) spectra were characterized on a Nicolet Magana-IR 750 spectrometer using the KBr pellet method. Batch sorption experiments. All sorption experiments were carried out in polyethylene test tubes under various experimental conditions, including reaction time (0-360 min), solid-to-liquid ratio (0.1-0.6 g∙L-1), pH value (pH 4-10), ionic strength (0.001-0.5 M NaNO3), temperature (25-55 ℃) and initial concentration (5.0 -70.0 mg·L−1). Firstly, the adsorbents were pretreated under the desired pH conditions at room temperature for 24 h to reach equilibrium. Next, the adsorbent, NaNO3 solution, U(VI) stock solution (200 mg∙L-1) were added to the desired contents with the total volume of 6 mL. The pH was adjusted and controlled by adding trace amount of 0.1 or 0.01 M HNO3 or NaOH and the temperature was controlled by a


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vapor-bathing constant temperature shaker (140 rpm). After continuous shaking for 24 h, the solid and liquid phases were separated by centrifuging at 8000 rpm for 5 min. The residual U(VI) concentration in the supernatant was analyzed by an UV-Vis spectrophotometer. Briefly, 1 mL supernatant and 1 mL arsenazo(III) (1 g∙L-1) were added in volumetric flask and diluted with 0.1 M HNO3 to a final volume of 10 mL. After incubating for 30 min at room temperature, the absorbance was measured at the wavelength of 650 nm. Desorption experiments were conducted after sorption equilibrium. In details, 2 mL of the centrifuged supernatant was removed out, and then an equal volume of NaNO3 solution with the same ionic strength was added and the same pH was adjusted to the same value. Next, the following processes were kept in accordance with those of the sorption experiments. The effect of different background electrolytes on the desorption of U(VI) from NiSiO@NiAl were conducted under similar conditions with the total volume of 40 mL. Briefly, after reaching sorption equilibrium at pH 5.0, all the centrifuged supernatant was poured out and the solid was rinsed by small amount of Milli-Q water. Then, NaCl, NaNO3 or Na2SO4 solution with different concentrations (0.01 or 0.1 M) was added into the tube and the pH was readjusted to 5.0. Next, the same operations were performed as those of the sorption experiments. Reusability tests of NiSiO@NiAl and NiSiO@MgAl were conducted as follows. After reaching sorption equilibrium, the tube, with the total volume of 40 mL, was centrifuged at 8000 rpm for 5 min. Then, the supernatant was poured out and the adsorbent was re-dispersed into 0.5 M Na2CO3 solution. After shaking for 6 h, the centrifuged solid was washed by large amount of Milli-Q water and absolute alcohol



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3 times respectively, and then dried at 60 oC. Finally, the regenerated adsorbent was used for the next cycle of sorption experiments. The amount of U(VI) remained in the supernatant (sorption (%)) and the equilibrium sorption capacity (Qe) were obtained according to following equations:

Sorption(%) 

Qe 

C0  Ce 100% C0

(1)

(C0  Ce )  V m

(2)

In which V (L) and m (mg) denoted the material dosage and suspension volume, and C0 and Ce (mg∙L-1) represented the initial and equilibrium U(VI) concentrations, respectively. All the data in this study was evaluated by averages of duplicate experiments, and the relative errors of those data were ~5%.

RESULTS AND DISCUSSION Characterization

of

NiSiO,

NiSiO@MgAl

and

NiSiO@NiAl.

The

crystallographic structures of the as-prepared adsorbents are firstly characterized by XRD. As depicted in Figure 1a, the sharp characteristic peaks of the NiSiO pattern at 2θ = 19.2°, 33.2°, 38.6°, 52.1°, 59.3° and 62.8° can be assigned to the (001), (100), (011), (012), (110) and (111) planes of Ni(OH)2 (PDF#73-1520), while the peaks at 12.0° and 24.3° are well coincident with the (002) and (004) planes of Ni3Si2O5(OH)4 (PDF#49-1859). After coated by MgAl LDHs, the characteristic peaks of NiSiO are still distinguishable, and the strong peaks at 2θ = 11.7°, 23.5°, 35.0°, 39.6°, 47.1°, and 60.8° are originated from the (003), (006), (222), (225), (228) and (600) planes of Mg4Al2(OH)12CO3∙3H2O (PDF#51-1525). In contrast, the characteristic peaks of NiSiO are quite indistinct in the XRD pattern of NiSiO@NiAl, and the emerging broad peaks at 2θ = 11.4°, 23.3°, 35.0°, and 60.8° can be assigned to the (003), (006),


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(009) and (112) planes of Ni2Al(CO3)2(OH)3 (PDF#48-0594). The well preserved NiSiO peaks indicate that the growth of the MgAl LDH is a non-destructive process, while the formation of the NiAl LDH is due to the isomorphic substitution effect accompanied by the epitaxial growth process. N2 adsorption-desorption isotherms are recorded to obtain the specific surface area and pore size distribution. As shown in Figure 1b, all the three curves are the typical IV shape in the IUPAC classification,43 suggesting that the as-prepared adsorbents are mesoporous structured. In the p/p0 region of 0.4~1, three hysteresis loops are clearly observed, which can be assigned to the H3-type, suggesting that the mesopores are generated by the stacking and accumulation of the nanosheets. While in the low-pressure region, the relationship can be well described by the BET (Brunauer-Emmett-Teller) model (Figure S1), the linear fitting of which yields the specific area (SBET) of 59.09, 136.99 and 169.20 m2∙g-1 for NiSiO, NiSiO@MgAl and NiSiO@NiAl, respectively. According to the BJH (Barrett-Joiner-Halenda) model, the pore sizes of the hollow spheres are in the mesoporous region (Figure 1c), which coincide with the above conclusions. SEM and TEM are applied to uncover the morphologies and microstructures of three samples. As shown in Figure 2a, the as-prepared NiSiO particles are spherical with satisfactory monodispersity, which are well inherited from their precursors (Figure S2). A close observation shows that the diameter of the sphere is about 700 nm and the surface of which is covered by vertically arranged sheet array. The TEM image shows that the NiSiO spheres are hollow structure, which is in good agreement with the expectation, and the height of the sheet array is estimated to be about 50 nm (Figure 2b). From the high-resolution TEM (HR-TEM) image (Figure 2c), the nanosheets show well-resolved crystal lattice fringes with a distance of 0.23 nm, which is very consistent with the (011) plane of Ni(OH)2, indicating that the



nanosheet array is mainly made up by crystallized Ni(OH)2, while the Ni3Si2O5(OH)4 may be mainly present in the inner-shell layer. The selected area electron diffraction (SAED) pattern shows two distinct diffraction rings (Figure 2d), which can be indexed to the (011) and (111) planes of Ni(OH)2, further confirming that Ni3Si2O5(OH)4 does not exist in the nanosheet layer. After grafting with LDHs, the spherical morphologies are well preserved, but the NiSiO@MgAl spheres are surrounded by plate-like clusters (Figure 2e) while the NiSiO@NiAl spheres are covered by foam-like ones (Figure 2i). The TEM image shows that the height of sheet array is more than 100 nm for NiSiO@MgAl (Figure 2f), while it nearly keeps unchanged for NiSiO@NiAl (Figure 2j). According to the HR-TEM images, the crystal lattice fringes can be indexed to the MgAl LDHs (Figure 2g) and NiAl LDHs (Figure 2k), respectively, but the SAED patterns show diffraction rings for both Ni(OH)2 (white arrows) and LDHs (red arrows), demonstrating that the LDHs are successfully grafted onto the Ni(OH)2 nanosheet arrays (Figure 2h and 2l). The EDX spectra and the relative elemental mappings show that the component elements are uniformly presented on the surface, indicating that the host spheres and the LDHs are chemically hybridized instead of physically mixed (Figure S3-S5). Effect of solid content. In order to make a reasonable choice of adsorbent dosage for the following batch experiments, the influence of the solid content is carefully investigated. As demonstrated in Figure 3a, NiSiO hollow spheres exhibit weak sorption ability towards U(VI) at the whole tested solid contents. On the contrary, the amounts of U(VI) adsorbed on NiSiO@MgAl and NiSiO@NiAl increase significantly with the increase adsorbent dosage from 0.1 to 0.3 g∙L-1, and then keep at a high level (nearly 100%), suggesting that both hold great potential in the cleanup of uranium-containing industrial wastewater (Figure 3b and 3c). Furthermore, the value of distribution coefficient (Kd) is almost independent of the


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solid content, suggesting that a physicochemical sorption process occurs on the surface of these materials.44,45 Since the removal percentage can achieve 70% and 90% at 0.3 g∙L-1 of NiSiO@MgAl and NiSiO@NiAl, respectively, the following studies are all conducted under this appropriate solid-to-liquid ratio. Sorption kinetics. The removal rate is a crucial parameter to assess the efficiency of an adsorbent in applications. As illustrated in Figure 4a, the sorption reactions can quickly reach equilibrium in 2 h, showing the considerably fast removal rates. However, the maximum sorption percentages of U(VI) on NiSiO@MgAl LDH and NiSiO@NiAl are ∼70% and ~90%, respectively, which are much higher than that of U(VI) on pure NiSiO hollow spheres (∼21%), indicating that the modification with LDHs is an efficient way to enhance the sorption capacities. To go further into the kinetic process, two typical kinetic models (i.e., pseudo-first-order and pseudo-second-order kinetic models) are applied to describe the kinetic data,46 the equations of which are shown as following, and the fitting results are listed in Table 1.

ln(Qe  Qt )  ln Qe  k1t t 1 t   2 Qt k 2Qe Qe

(3) (4)

In which, Qe and Qt denote the amount of U(VI) adsorbed on solid phases after reaching equilibrium and at time t, respectively. k1 and k2 are the pseudo-first-order and pseudo-second-order kinetic rate constants, respectively. From Figure 4b, the pseudo-first-order model can roughly describe the data in 2 h, but with low correlation coefficients (Table 1), demonstrating that the ≡S-U(VI) is not the favorable surface complex (≡S represents the surface site). While from Figure 4c and Table 1, the kinetic data can be well fitted by the pseudo-second-order model, and correlation coefficients of which are higher than 0.99, indicating that (≡S)2-U(VI)



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may be the dominant surface complex. Besides, the higher k2 value of NiSiO may be due to its much lower sorption capacity, while the faster sorption kinetics of U(VI) on NiSiO@NiAl than on NiSiO@MgAl may be caused by the thinner LDHs shell, which can shorten the transport path of target ion from suspension to the interface. Effect of pH and ionic strength. Generally, pH could affect the sorption process because both U(VI) species and electric properties, i.e., surface charges and surface potentials of the adsorbents are greatly influenced by solution pH. As illustrated in Figure 5a, sorption percentages of U(VI) increase visibly from pH 4 to 6, indicating that H+ is a key influence factor for surface reactions. It’s found that the polynuclear complexes, such as (UO2)3(OH)5+, gradually replace uranyl (UO22+) to become the dominate U(VI) species with pH increasing from 4 to 6 (Figure 5b), indicating that uranyl could coordinate with oxygen-rich groups at lower concentration of H+. Meanwhile, the zeta potentials of the three adsorbents decrease significantly from pH 4 to 6 (Figure 5c), indicating that the surface charges may originate from the sorption or coordination of H+. Consequently, the sorption reactions are enhanced due to the reduced electrostatic repulsion between the solid surface and the aqueous species, the reduced competition between H+ and positive charged

U(VI)

species,

and

the

enhanced

coordination

of

the

surface

oxygen-containing functional groups. In the neutral pH region, i.e., pH 6 to 8, H+ has little effect on sorption due to its low concentration, however, the amounts of U(VI) adsorbed on NiSiO@MgAl and NiSiO@NiAl are independent of pH while it shows decreasing tendency on NiSiO with pH increasing. It’s widely accepted that both precipitation and the dissolved CO2 play considerable roles in this region,47,48 the former will lead to apparent high sorption performances while the coordination effect of the latter, i.e. UO2(CO3)34-, could enhance the mobility of the adsorbed U(VI), leading to the decreased sorption.


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In this region, the existent forms of U(VI) are both positive ((UO2)3(OH)5+ and (UO2)4(OH)7+) and negative ((UO2)2CO3(OH)3-). Because there is no exchangeable ions in the NiSiO layers, the sorption behavior is mainly controlled by the electrostatic attraction, which could facilitate the interaction between surface functional groups and aqueous ions. As the zeta potentials of NiSiO are negative, only the positive U(VI) species can be adsorbed, as a result, the sorption percentage decreases drastically with the increasing of (UO2)2CO3(OH)3-. In sharp contrast, both MgAl and NiAl LDHs are layer-structured materials, the CO32- in the interlayer space could act either as coordinating sites for the central uranyl species or as ion exchange sites for the negative species. Thus, the sorption of U(VI) on NiSiO@MgAl and NiSiO@NiAl are governed by the repulsion between the surface and the U(VI) species with the same charge, and the aforementioned effect of lattice CO32-. As pH further increases to 10, UO2(CO3)34- becomes the dominant species, the electrostatic repulsion will hamper it to contact with NiSiO, which results in the continues decrease in the sorption percentages of U(VI). However, the sorption percentages on NiSiO@MgAl and NiSiO@NiAl are nearly unaffected, indicating that the exchangeable CO32- in the interlayer space plays an important role in the sorption process under high pH conditions. Furthermore, ionic strength (I) has great impact on the thickness of the double electrical layer,49 which plays an essential role in surface reactions. Thus, the sorption performances of U(VI) on these three adsorbents under various ionic strengths are systemically investigated. The sorption percentages are nearly independent of the concentration of the background electrolyte over a wide region (Figure 5d), but the sorption edges are pushed backward under high ionic strength (Figure S6). Fundamentally, the double electrical layer will be compacted at high I conditions, so that it’s hard for U(VI) to enter the surface region due to the strong competition effect



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of the background ions, and as a result, the sorption reactions will be suppressed to a certain extent. These results indicate that U(VI) may be adsorbed through the formation of inner-sphere surface complexes.50 Sorption isotherms. Sorption isotherms are performed to investigate the sorption mechanism, meanwhile, Langmuir (Equation 5), Freundlich (Equation 6), Dubinin-Radushkevitch (Equation S1) and Temkin (Equation S3) models51-53 models are applied to depict the data: Qe 

K LQmax Ce 1  K LCe

Qe=KFCe1/n

(5) (6)

In which, Ce (mg∙L-1) denotes the residual U(VI) concentration, Qe (mg∙g-1) is the mass of U(VI) adsorbed on solid surface per weight, Qmax (mg∙g-1) denotes the saturation sorption capacity and KL (L∙g-1) is regarded as the equilibrium constant of sorption, KF (mg1−n∙Ln∙g-1) represents Freundlich constant and 1/n is the sorption intensity. Figure 6a shows that the introduction of LDHs is an effective way to promote the sorption capability and complexation ability of adsorbents towards U(VI) because of the synergistic effects between the spherical morphology and the vertical nano-arrays that exposing a great number of various functional groups. With the increasing of the initial concentration of U(VI), a sorption platform can be observed for each of the three isotherms, indicating that the sorption of U(VI) on these adsorbents is a finite process, which conflicts with the hypothesis of the Freundlich model. Although the data can be fitted by the Dubinin-Radushkevitch and Temkin models, the significant deviations in some regions suggest that they are not the optimal models (Figure S8 and Table S1). By contrast, the isotherms can be better described by the Langmuir model (Table 2), indicating that the sorption of U(VI) on


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the surface of the as-synthesized adsorbents is a monolayer chemical sorption process. Accordingly, the maximum sorption capacity of NiSiO@NiAl is calculated to be 136.94 mg·g−1, which is much higher than those of NiSiO@MgAl (41.82 mg·g−1) and NiSiO (14.77 mg·g−1) at 298 K, and also comparable or better to the other reported materials (see Table 3), indicating that NiSiO@NiAl is a quite promising adsorbent in the removal of U(VI). Furthermore, when normalized by the specific area, NiSiO@NiAl also shows the best sorption performance (Table S2), indicating that the sorption site density of NiSiO@NiAl is higher than those of NiSiO and NiSiO@MgAl. To reveal the effect of the Ni:Al ratio on the sorption of U(VI), NiSiO@NiAl(1:1) and NiSiO@NiAl(2:1) are also synthesized (Figure S9). It’s found that NiSiO@NiAl(3:1) shows the best maximum sorption capability among the three candidates (Figure S10). Besides, statistical significance test shows that NiSiO@NiAl has significant advantages over NiSiO@MgAl in the removal of U(VI) (Table S3 and S4).60 The separation factor RL (see equation 7, in which KL is the Langmuir constant, C0,max is the maximum initial concentration (mg∙L-1)) is applied to assess the essential characteristics of Langmuir isotherms.61

RL 

1 1  K LC0,max

(7)

As shown in Table 2, all the RL values are in the region of (0-1) in this study, indicating that the sorption is a favorable and reversible process under the experimental conditions. To provide more convinced evidences for the reversibility, desorption experiments are further performed. The distribution coefficients of the desorption process match well with those of the sorption ones (Figure 6b), and the desorption isotherms are nearly overlapped with the sorption ones (Figure S7),



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demonstrating that the sorption and desorption processes follow the same mechanism, i.e. the reactions between U(VI) and the surface functional groups are reversible. The effect of background electrolytes are further studied under different concentrations. It shows that the desorption percentage of U(VI) from NiSiO@NiAl follows the sequence of Na2SO4 > NaNO3 > NaCl under the same condtions (Figure S11). To investigate the sorption thermodynamics, sorption isotherms are further measured under different temperatures (298 K, 313 K and 328 K, see Figure S12). It is obvious that the sorption capacity increased remarkably with the increase of temperature (Figure 6c), meanwhile, the isotherms still follow the Langmuir model, demonstrating that the promoted uptake of U(VI) is due to the chemical equilibrium shift of surface complexation instead of the changing of sorption mechanism. Furthermore, thermodynamic functions, such as standard free energy (ΔG0), standard enthalpy change (ΔH0) and standard entropy change (ΔS0), are also extracted to quantitatively describe the temperature effect on sorption. The free energy change (ΔG0) is expressed by the following equation: G 0  H 0  T S 0

(8)

while ΔH0 and ΔS0 are achieved from Eq.(9): ln K 0 

S 0 H 0  R RT

(9)

In which K0 is regarded as the constant of thermodynamic equilibrium, R represents the ideal gas constant (8.3145 J∙mol-1∙K-1), T is considered as the absolute temperature (K). As shown in Figure 6d and Table S5, ΔH0 values are positive, implying that the U(VI) sorption process on these three adsorbents are endothermic, which is in agreement with the observed temperature effect. It’s believed that the measured ΔH0 is a net result of the endothermic dehydration of U(VI) species, the deprotonation of


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surface functional groups and the exothermic surface complexation reaction. The experimental results indicate that the former two reactions play more important roles in the total sorption processes, which leads to the fact that high temperature could enhance the sorption process. Moreover, the positive values of ΔS0 indicate that the randomness will be increased in U(VI) sorption process, while the negative values of ΔG0 suggest that the sorption process of U(VI) on the three samples are spontaneous in thermodynamics. Effect of the co-existing ions on NiSiO@NiAl LDH. The adsorption performances of NiSiO@NiAl LDH towards U(VI) at the presence of co-existing ions, such as Ca2+, K+, Mg2+, Na+, are further investigated to evaluate the selectivity. As shown in Figure 7, the sorption percentage of U(VI) was 97.3%, which is much higher than that of Ca2+(18.76%), K+(38.43%), Mg2+ (66.79%) and Na+(22.78%), indicating that NiSiO@NiAl LDH has better biding ability for U(VI). One possible reason is that the CO32- in the interlayer space could coordinate to UO22+ forming anionic complexes retaining in the LDH gallery, while K+ and Na+ may be adsorbed in the electric double layer and Ca2+ and Mg2+ may precipitate on the surface.38 It’s worth mentioning that these competing ions are the major background cations in the sea water, therefore, the high selectivity endows NiSiO@NiAl LDH to be a promising candidate in the separation and enrichment of uranium from sea water. XPS analysis. XPS technique is performed to investigate the change of surface elements in the chemical environment. A pair of peaks appear at binding energy of around 400 eV in the post-sorption spectra (Figure S13a, S14a and S15a), which can be assigned to the characteristic peaks of U 4f,29 indicating that uranium is loaded on the surface. Generally, oxygen-containing functional groups are believed to be the surface sites for complexing U(VI) species, and the corresponding change of oxygen is firstly investigated. As shown in Figure 8a, the high-resolution O 1s spectra can be



deconvolved into two peaks. The first one at 531.5 ± 0.2 eV can be assigned to the M-OH groups (M = Ni, Mg or Al),62,63 while the second one at 532.5 ± 0.2 eV can be ascribed to the Si-OH groups64. After sorption, all the peaks show clear blue shifts, indicating that all the surface groups may take part in the complexation reactions. To further reveal the interaction, the high-resolution Ni 2p3/2 spectra are subsequently investigated. The peak at around 856.4eV is related to Ni3+,65 which can be assigned to the Ni-OH group, and the peak at around 862.0 eV is the typical shake up satellite peak. After sorption, a blue shift of 0.12 eV can be found in NiSiO, indicating that Ni-OH is involved in the surface complexation. In comparison, the shifts are 0.24 eV and 0.25 eV for NiSiO@MgAl and NiSiO@NiAl, respectively, indicating that the interactions are strong on the two adsorbents. From the high-resolution Si 2p and Al 2p spectra, all the Si-OH and Al-OH groups show blue shifts (Figure S13-S15), while the content of the Mg-OH groups significantly increases after U(VI) sorption (Figure S14d), indicating that these groups also take part in the surface complexation, which is in good agreement with the conclusions obtained from O 1s spectra. As illustrated in Figure 9, Ni-OH and Si-OH are major functional groups on the surface of NiSiO, while (a) and (b) may be major reactions, through which the coordination effect of O with U(VI) will result in the photoelectrons of O, Ni and Si shifting to higher binding energy. The large atomic radius and the abundant electron of Ni could weaken its impact on Si, resulting in a small shift. However, on the surface of NiSiO@MgAl, (d) and (b) may be the major and side reactions, respectively, due to the greatly decreased content of Ni (Figure S4). Furthermore, synergistic effect may also occur among Si-OH, Al-OH and Mg-OH. Since Mg has strong binding ability to its 1s electrons, it will show weak "buffering effect" when the synergistic reaction occurs between Mg-OH and Si-OH, leading to the obvious change in the binding energy of Si. As for NiSiO@NiAl, the content of Ni is higher


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than that of Al (Figure S5), indicating that (a) and (c) may be the major reactions, and the side reaction of (b) may lead to a relative weak shift of Si 2p. From the changes in the blue shift of Ni 2p3/2 and red shift of U 4f (Figure 8c), it can derive at least two conclusions: first, the coordination ability of the functional groups follows the sequence of Ni-OH (in NiAl LDH) >Al-OH & Mg-OH> Ni-OH (in NiSiO), which is in agreement with the results of Qmax; second, the sorption energy of U(VI) on LDHs is higher than that of U(VI) on NiSiO, which are also confirmed by the thermodynamic data (Figure 6d). FT-IR analysis. FT-IR technique is further employed to investigate the change of surface functional groups. As shown in Figure S16a, a new peak at 1115 cm-1 appears after U(VI) sorption which is distinct from the one at 1015 cm-1. The latter is the characteristic peak of the surface hydroxyl group, so that the former can be assigned to the surface complex. From Figure S16b, the emerging peak at 1521 cm-1can be attributed to the uranyl-carbonate species66, indicating that the CO32- ions in the interlayer space of MgAl LDH play important role in the sorption of U(VI). In the Post-NiAl spectrum, the two new peaks can be found together (Figure S16c), indicating that the sorption mechanism of U(VI) on NiSiO@NiAl follows both the surface complexation reaction and the ion exchange reaction. Furthermore, the morphologies of the post-sorption samples are also investigated by SEM. The spheres and nanosheet arrays are well preserved (Figures S17-S19), confirming that NiSiO, NiSiO@MgAl and NiSiO@NiAl have excellent stability in the elimination process of U(VI). Besides, the adsorbed U(VI) are homogeneously presented on the solid surface (Figure S20-S22), demonstrating that the sorption energy of the binding sites is uniform, which matches well with the hypothesis of the Langmuir model. Reusability test. Since the reusability of adsorbent is an important parameter in



adsorption experiment, cycle experiments of NiSiO@NiAl and NiSiO@MgAl in the sorption of U(VI) are further conducted. After five cycles of regeneration experiments, both of the two adsorbents remain high sorption efficiency towards U(VI) (Figure S23), indicating their great potential in the treatment of radioactive wastewater with excellent reusability and stability.

CONCLUSION In conclusion, we have synthesized three hollow spherical adsorbents, NiSiO, NiSiO@MgAl and NiSiO@NiAl, by the template method and conducted detailed investigations on their removal performances towards U(VI). Batch experiments show that the sorption edges are nearly free from dependence on ionic strength, implying the interaction of U(VI) on the solid surface may be the inner-sphere surface complexation, and the sorption isotherms conform to the Langmuir model, indicating the mono-layer sorption mechanism. The sorption kinetics results suggest a pseudo-second-order process, and the sorption thermodynamics data reveal the spontaneous and endothermic behavior. NiSiO@NiAl shows the best Qmax of 136.94 mg∙g-1, which can be reasonably due to its large specific surface area and double layer capacitance. XPS investigation reveals that the Ni-OH group on NiAl LDH has better coordination ability towards U(VI) than that on NiSiO and the Al-OH and Mg-OH groups on MgAl LDH, which can be an important fundamental reason for its high sorption capacity. This work shows that NiSiO@NiAl is an efficient adsorbent, which hold great potential in environmental radioactive pollution remediation. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications


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website at DOI: Additional supporting characterizations, effect of pH, ionic strength and temperature on sorption, fitting results of adsorption isotherm data with Dubinin-Radushkevitch and Temkin models, effect of background electrolyte on desorption, stability and recyclability of materials. AUTHOR INFORMATION Corresponding Authors *Phone: 86-10-61772890. Fax: 86-10-61772890. E-mail: [email protected] (X. Wang),

[email protected]

(Z.

Lu),

[email protected]

(R.

Zhang),

[email protected] (T. Wen).

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is financially supported by NSFC (201836001) and Science Challenge Project (TZ2016004).

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Scheme1. The preparation illustrations of hierarchical hollow spheres.


DOI


Figure 1. Structural characterizations of NiSiO, NiSiO@MgAl and NiSiO@NiAl. (a) XRD patterns, (b) N2 adsorption/desorption isotherms, and (c) pore size distributions.


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Figure 2. Morphological characterizations of NiSiO (top panel), NiSiO@MgAl (middle panel) and NiSiO@NiAl (bottom panel). (a), (e) and (i) SEM, the scale bars of the insert images are 500 nm. (b), (f) and (j) TEM. (c), (g) and (k) HR-TEM. (d), (h) and (l) SAED.



Figure 3. The effect of solid content on the sorption of U(VI) by (a) NiSiO, (b) NiSiO@MgAl, and (c) NiSiO@NiAl. C0,U(VI) =10.0 mg∙L-1, pH = 5.0 ± 0.1, I = 0.01 M NaNO3, T = 298 K and t = 24 h.

Figure 4. (a) Effect of contact time for the sorption of U(VI) on NiSiO, NiSiO@MgAl and NiSiO@NiAl. (b) Pseudo-first-order model simulation and (c) Pseudo-second-order kinetic model simulation. C0,U(VI) =10.0 mg∙L-1, m/V =0.3 g∙L-1, pH = 5.0 ± 0.1, I = 0.01 M NaNO3 and T = 298 K.


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Figure 5. (a) Sorption edges of U(VI) on NiSiO, NiSiO@MgAl and NiSiO@NiAl. C0 =10.0 mg∙L-1, m/V =0.3 g∙L-1, I = 0.01 M NaNO3, T = 298 K and t = 24 h. (b) Relative distribution of U(VI) species as a function of pH. C0,U(VI) =10.0 mg∙L-1, p(CO2) = 3.8×10-4 atm, I = 0.01 M NaNO3 and T = 298 K. (c) Zeta potentials at different pH values. (d) Effect of ionic strength on the sorption of U(VI). C0 =10.0 mg∙L-1, m/V =0.3 g∙L-1, pH = 5.0 ± 0.1, T = 298 K and t = 24 h.



Figure 6. (a) Sorption isotherms of U(VI) on NiSiO, NiSiO@MgAl and NiSiO@NiAl. The solid line represents the Langmuir model and the dashed line represents the Freundlich one. (b) The sorption/desorption behavior of U(VI) on NiSiO, NiSiO@MgAl and NiSiO@NiAl. I = 0.01 M NaNO3, m/V = 0.3 g∙L-1, pH = 5.0 ± 0.1 T = 298 K and t = 24 h. (c) The maximum sorption capacities (Qmax) at different temperatures. (d) The enthalpy (ΔH0) and entropy (ΔS0) of U(VI) sorption on NiSiO, NiSiO@MgAl and NiSiO@NiAl.


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Figure 7. The effect of co-existing ions on the adsorption of U(VI) by the NiSiO@NiAl. The initial concentration of all the ions were 10 mg∙L-1, the solution volume was 40 mL, m/V =0.3 g∙L-1, t =24 h, T = 298 K, and pH = 5.0 ± 0.1.

Figure 8. High-resolution XPS spectra of (a) Ni 2p3/2, (b) O 1s and (c) U 4f. Post-NiSiO, Post-MgAl, Post-NiAl refer to the post-sorption samples.



Figure 9. Illustration of surface complexation reactions.


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Table 1. Fitting parameters of sorption kinetics. Sorbent

Pseudo-first order

Pseudo-second order

k1 (min-1)

Qe (mg∙g-1)

R2

k2 (g∙mg-1∙min-1)

Qe (mg∙g-1)

R2

NiSiO

3.01×10-3

7.71

0.804

2.46×10-2

7.60

0.998

NiSiO@MgAl

5.83×10-3

25.25

0.836

4.59×10-3

24.88

0.999

NiSiO@NiAl

2.13×10-3

31.76

0.862

9.56×10-3

29.96

0.999

Table 2. Fitting results of the sorption isotherms by Langmuir and Freundlich models. Langmuir Sorbent

NiSiO

NiSiO@MgAl

NiSiO@NiAl

Freundlich

T(K) KL (L∙mg-1)

Qmax (mg∙g-1)

R2

RL

KF (mg1-n∙Ln∙g-1)

n

R2

298

0.09

14.77

0.980

0.133

3.04

2.88

0.907

313

0.11

32.36

0.989

0.108

7.71

3.11

0.944

328

0.17

49.55

0.978

0.072

14.35

3.44

0.96

298

0.37

41.82

0.997

0.037

44.02

3.05

0.951

313

0.52

81.80

0.988

0.027

69.31

3.38

0.91

328

0.84

128.91

0.988

0.017

101.79

3.45

0.836

298

0.31

136.94

0.966

0.047

19.21

5.18

0.845

313

0.46

184.23

0.973

0.033

33.25

4.31

0.831

328

0.78

254.58

0.984

0.019

56.19

4.29

0.840



Page 40 of 41

Table 3. Comparison of maximum sorption capacities towards U(VI) Materials

Experimental Conditions

Qmax (mg∙g-1)

Ref.

NiSiO

T = 298 K, pH = 5.0

14.8

This work

NiSiO@MgAl

T = 298 K, pH = 5.0

41.8

This work

NiSiO@NiAl

T = 298 K, pH = 5.0

136.9

This work

NiFeAl LDH

T = 298 K, pH = 5.0

51.6

40

MMT@C

T = 298 K, pH = 4.9

66.2

47

SBA-AO-0.0

T = 298 K, pH = 5.0

119.9

54

G/AgCS

T = 298 K, pH = 5~6

13.3

55

Oxime-CMK-5

T = 283 K, pH = 4.5

65.1

56

Fe3O4@TiO2

T = 298 K, pH = 6.0

91.1

57

TiO2-x

T = 298 K, pH =5.0

40.02

26

Fe3O4@MnOx

T = 298 K, pH = 5.0

107

58

palygorskite

T = 333 K, pH = 4.0

46.8

59


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For Table of Contents Use Only Synopsis The NiSiO@NiAl LDH hierarchical hollow spheres showed high sorption capacity towards uranium in the presence of many co-existing ions, which holds great potential in the separation and enrichment of uranium from sea water.


Immobilization of U(VI) on Hierarchical NiSiO ... - ACS Publications

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