Selective Immobilization of Highly Valent Radionuclides by Carboxyl

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Selectively Immobilization of High-Valent Radionuclides by Carboxyl Functionalized Mesoporous Silica Microspheres: Batch, XPS and EXAFS Analyses Chenlu Zhang, Xing Li, Zhihao Jiang, Yihan Zhang, Tao Wen, Ming Fang, Xiaoli Tan, Ahmed Alsaedi, Tasawar Hayat, and Xiangke Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04146 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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ACS Sustainable Chemistry & Engineering

Selectively Immobilization of High-Valent Radionuclides by Carboxyl Functionalized Mesoporous Silica Microspheres: Batch, XPS and EXAFS Analyses Chenlu Zhang,a Xing Li,a Zhihao Jiang,a Yihan Zhang,a Tao Wen,a,* Ming Fang,a Xiaoli Tan,a,* Ahmed Alsaedi,b Tasawar Hayat,b,c Xiangke Wanga,b,* a

College of Environment and Chemical Engineering, North China Electric Power

University, Beijing 102206, P.R. China b

NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia c

Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan

* ：

Corresponding

authors.

Email:

[email protected]

(T.

Wen);

[email protected] (X.L. Tan); [email protected] (X.K. Wang). Phone(Fax): 86-10-61772890.

1

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ABSTRACT The removal of radionuclides on natural inorganic materials is pursuing issue for many nuclear-related processes requiring remediation of radioactive wastewater. However, the current natural adsorbents with low-cost and great biocompatibility are suffered from limitations in removal efficiency, regeneration capability, or/and operation conditions. Herein, silicon dioxide (SiO2) as the major ingredient of clay minerals was modified by carboxyl groups to improve its adsorption affinity for high-valent radionuclides. The batch experiments for U(VI) capture showed that the carboxyl-modified mesoporous silica (SiO2-COOH) microspheres had fast sorption velocity and were exceptionally capable in efficiently sequestering U(VI) under various relevant interferences. The resulting SiO2-COOH possessed the great potential for controlled loading of high-valent radionuclides in a selective adsorption order of U(VI) > Th(IV) > trivalent ions > divalent ions > Cs(I). The XPS and EXAFS analyses further confirmed that the interaction mechanism between SiO2-COOH and U(VI) was mainly attributed to the inner-sphere complexation with one or two oxygen atoms shared between the UO22+ and the carboxyl ligand. Carboxyl group modification of natural inorganic materials provides a general and powerful approach to eliminate high-valent radionuclides in various wastewater systems. KEYWORDS: High-valent radionuclides; Carboxyl group; Mesoporous silica microspheres; Adsorption; Interaction mechanism.

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INTRODUCTION Nuclear power, a clean and highly efficient technology, has become an alternative to the traditional energy. However, the consequent radioactive waste (~2300 t/year) from nuclear-related processes is one of the serious bottlenecks constraining nuclear energy development.1,

2

High-valent radionuclides and their

fission products could emit alpha, beta and gamma particles during the decay process which may disturb the metabolism in body organs and even directly induce DNA damage or cancerization.3, 4 As the current major fuel in commercial reactors, uranium with high internal radiation and extremely long half-life might threat the environment and living organisms with destruction, even at low concentrations.4, 5 To date, natural inorganic materials have been widely adopted for radioactive wastewater treatment.6-10 Among these natural existing adsorbents, clay minerals (such as kaolinite, montmorillonite, sepiolite and illite, etc.9,

11-13

) are generally

composed of stacked sheets of Al-substituted octahedra, Si-substituted tetrahedra, and water,14,

15

frequently along with moderate-to-minor amounts of magnesium, iron,

alkalis, and alkaline earths, showing great biocompatibility. Unfortunately, most of these materials inevitably suffered from slow adsorption kinetics, low adsorption capacities, limited selectivity, and poor reusability under harsh conditions of inhomogeneous contaminants, extreme pH and high salt concentrations, making them unsuitable for practical applications.16 As the major ingredient of clay minerals, silica is considered as an ideal adsorbent in the immobilization of radionuclides (U(VI), Th(IV), Eu(III), La(III), Co(II), Cs(II)).17-21 In particular, substantial progresses of 3



mesoporous silica materials in U(VI) remediation have been achieved due to their chemical stability, large surface area, uniform particle size and well-defined pore diameter. Wang et al.22 found that mesoporous SBA-15 with surface area ~800 m2/g showed high binding ability for uranium (80.68 mg/g) at pH 4.0. Although the large specific surface area of the material is a beneficial factor, the relative abundant active sites play a crucial role in effective utilization of the surface area to afford high adsorption capability.23 Hence, it is urgent to select a highly effective modified method of natural inorganic materials to overcome pre-existing limitations. Various chemical modification strategies have been applied to improve the active sites of these materials, such as element doping, polymerization, esterification, oxidation, chemical grafting and so on.24-32 The carboxylation is a simple method to introduce carboxyl groups (-COOH) onto the surface of substrates which can improve the affinity and selectivity of adsorbents towards radionuclides.33 After being functionalized with carboxyl groups, two terminal oxygen atoms can present highly electron-rich sites for electrostatic attraction of target ions.34 Singh et al.35 opened up the facile soft-chemical functionalization of Fe3O4 nanoparticles with carboxyl which showed obvious superiority in the adsorption of toxic metal ions (Cr(III), As(III), Cd(II), Co(II), Pb(II), Ni(II) and Cu(II)). For U(VI) adsorption, Wang et al.33 reported that the carboxyl-mesoporous carbon using sulfuric acid solution of ammonium persulfate (APS) had higher U(VI) adsorption capacity than the unmodified one. However, the adsorption behaviors between raw and carboxyl-modified adsorbents have not been systematically investigated, and the proposed interaction mechanism 4


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between the carboxyl groups and radionuclides under different conditions still lacks experimental verification and spectrum analyses as well. As a result of the above considerations, we aimed to graft carboxyl groups onto the surface of mesoporous silica microspheres (SiO2-COOH), and applied to enrich radionuclides from wastewater. Different characterization approaches (SEM, TEM, FT-IR, XPS, BET, TG and zeta-potential techniques) were applied to compare the physicochemical properties of SiO2 and SiO2-COOH microspheres. Batch experiments were conducted to investigate selectivity and recyclability of SiO2 and SiO2-COOH towards U(VI) under various conditions. Moreover, the luminous point of this work was to discuss the interaction mechanism by using spectroscopic analyses (XPS and EXAFS). This paper highlighted the wide applicability of carboxyl functionalized adsorbents in the purification of high-valent radionuclides under diverse conditions from aqueous solutions, which is significant for the real application of nanomaterials in nuclear waste management. EXPERIMENTAL SECTION Materials The

U(VI) stock solution (200 mg/L) was obtained by dissolving

UO2(NO3)2·6H2O (Sigma Aldrich) into 0.01M HNO3 solution. Tetraethyl orthosilicate (TEOS, 99%), n-pentanol (≥98%), cyclohexane (≥99.5%), cetylpyridinium bromide hydrate (CBH, 96%), (3-aminopropyl) triethoxysilane (APTES, ≥99%), succinic anhydride and other reagents used in this work were purchased from Sinopharm Chemical Reagent Co. 5



Synthesis of SiO2 microspheres SiO2 microspheres were prepared by a modified hydrothermal method.23 Firstly, urea (1.0 g) and CBH (2.0 g) were dissolved in 60 mL deionized water. Secondly, a stirred solution of TEOS (5.4 mL) in 50 mL of cyclohexane and 3.0 mL of n-pentanol was then added in the above solution. After continuely stirring for 2 h at 40°C, the resulting solution was poured into a Teflon lined autoclave and maintained at 120 °C for 5 h. The synthesized material was centrifuged, washed by acetone and deionized water thrice, and dried at 60 °C for 12 h. Finally, the as-prepared solid was calcined at 550 °C for 6 h to remove the residual surfactant. Synthesis of SiO2-COOH microspheres SiO2 microspheres (0.5 g) were dispersed into ethanol (50 mL), and then APTES (0.4 mL) was added into the solution under vigorous stirring conditions. After 3 h stirring at 80 °C, the product was separated by centrifugation and redispersed into N, N-dimethylformamide (DMF) solvent. Succinic anhydride (1 g dissolved in DMF) was mixed with the above solution by stirring for 12 h to get the optimal loading of carboxyl groups. The resulting SiO2-COOH microspheres were puriﬁed by ethanol and deionized water and dried by freeze-dryer. Batch adsorption experiments The removal of radioactive ions on SiO2 and SiO2-COOH were investigated using batch experiments. Typically, the stock solution of target ions, the background electrolyte (NaNO3) and the certain amount of adsorbent were added to 50 mL 6


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container. The pH value was adjusted by adding negligible volume of 0.1 or 1 mol/L HNO3 or NaOH. The containers were stirred in the oscillator for an appropriate time, and then centrifuged at 8000 rpm for 20 min. The concentration of target ion in the supernatant was detected by the inductively coupled plasma-optical emission spectrometer (ICP-OES). Removal efficiency (%) and adsorption capability of target ions on adsorbents (qe (mg/g)) were calculated as followed:

Removal (%) =

qe =

C0 − Ce ×100% C0

(C0 − Ce ) × V m

(1)

(2)

where Ce (mg/L) and C0 (mg/L) are the equilibrium and initial concentrations of U(VI), respectively. The parameters of m (g) and V (L) are the dosage of powders and the volume of testing solution. The adsorption experimental data were the average values of triplicate determinations. EXAFS analysis Uranium LIII-edge EXAFS spectra were recorded at room temperature at Shanghai Synchrotron Radiation Facility (SSRF, China) using a fixed-exit double-crystal Si (111) monochromator. U(VI) adsorption samples were collected and were mounted in a Teflon sample cell for EXAFS measurements. The electron storage ring energy was 3.5 GeV and the maximum storage beam current was 220 mA. The spectrum of the uranyl acetate was recorded in transmission mode while the spectra of the adsorption samples and UO22+(aq) were collected in fluorescence mode using a high-throughput 30-element purity Ge solid-state detector. EXAFS data reduction and 7



modeling were performed using IFEFFIT software and their graphical interfaces ATHENA and ARTEMIS.36 Radial structure functions (RSFs) were obtained by Fourier transformed k3-weighted χ(k) functions between 3.6-12.8 Å-1 for all the samples using a Kaizer-Bessel window function. Theoretical scattering paths were calculated using FEFF7.37, 38 The relevant paths included U-Oax, U-Oeq, U-Si and U-C were calculated based on the crystal structures of soddyite and uranyl acetate. All the fits were carried out for Fourier transform (FT) spectra in R-space from 0.9 to 4.0 Å. Accuracies of the radial distance (R) and coordination number (N) were estimated to be ±0.03% and ±30%. RESULTS AND DISCUSSION Characterization SEM images of SiO2 and SiO2-COOH microspheres showed that the materials consisted of uniform size colloidal spheres with an average diameter of 280 nm (Figure 1a, 1b, 1d, 1e and Figure S1). The pore channels were radially extended outwards from the center and distributed uniformly in all directions. More structural characterizations of the synthesized SiO2 and SiO2-COOH were examined by TEM (Figure 1c and 1f). Evidently, the two samples possessed well-defined and symmetric fibers arranged in three dimensions to form spheres, which could induce facile method for the higher surface area. After grafting with carboxyl groups, SiO2-COOH inherited the original morphology, however, the surface layer of SiO2-COOH became more nebulous than that of SiO2. In addition, elemental mappings of the individual SiO2 and SiO2-COOH microspheres showed that Si, O and C elements were 8


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homogeneously distributed in the whole nanostructure (Figure 1g and 1h). Figure S2a showed the FT-IR spectra of SiO2 and SiO2-COOH microspheres. A broad band located at around 3300 cm-1 was associated with hydrogen-bonded silanol group, and the strong peaks at 1090, 808, and 467 cm-1 were relevant to the asymmetrical stretching vibration, symmetrical stretching vibration and bending vibration modes of Si-O-Si, respectively.39,

40

For SiO2-COOH, several new

characteristic bands in the range of 1800-1400 cm-1 were originated from the joint contribution of carboxyl groups. A new peak appeared at 2938 cm-1 was relevant to the stretching vibration of C-H. While, the peaks at about 1724, 1645, and 1409 cm-1 corresponded to the C=O stretching vibration, the carboxy C-O deformation vibration and the O-H bands in-plane deformation vibration, implying the existence of grafted carboxyl groups.41 Specifically, the characteristic peak at 1558 cm-1 was the hybrid of C-N of secondary amide stretching vibration and N-H deformation vibration, which could be assigned to the infusion of C–NH2 groups during the synthetic process.42 In the XPS survey spectrum of SiO2, the characteristic peaks of Si and O elements could be observed in the two samples, while C 1s peak at 284.80 eV and N 1s at 400.08 eV appeared in the XPS spectrum of SiO2-COOH (Figure S2b). The binding energy shifts of O, C, and Si elements could be interpreted from the chemical interactions in SiO2-COOH (Figures 3b, 3c and S8c). The amount of carboxyl groups on the surface of SiO2-COOH microspheres as determined by Boehm titration was shown in Figure S3.43 The highest concentration of -COOH on SiO2-COOH surface was approximately 0.70 mmol/g. Considering the removal performance and production 9



cost, carboxyl groups with concentration of 0.66 mmol/g were efficient enough for adsorption experiments. As shown in Figure S2c, the type IV with H3 hysteresis loops have been exhibited in the N2 adsorption–desorption isotherms of SiO2 and SiO2-COOH, typical for the well-defined mesoporous material. The BET surface areas were greatly reduced from 411.09 m2/g for SiO2 to 80.72 m2/g for SiO2-COOH. The pore size distribution of SiO2-COOH microspheres was centered in the range of 40-60 nm, which was smaller than that of SiO2 (50-80 nm). The gifted carboxyl groups occupied the narrow pore channel of microspheres. Thus, the coverage of carboxyl groups on the surface of SiO2-COOH could be regarded as the result of the decrease in BET surface areas. These phenomena revealed that the pore structures of microspheres were slit pores piled up by plate-like particles, which were agreement with TEM images. As shown in Figure S2d, the TG curve of SiO2 had no significant weight loss, which suggested that the prepared SiO2 microspheres have good thermal stability. With respect to SiO2-COOH, a great mass loss (~27.63%) occurred with an increase of temperature, which could be interpreted from the decomposition of organic carbon on the surface of microspheres. The aforementioned characterization results indicated the successful modification of carboxyl groups on microspheres. Effect of solution pH and coexisted ions Figure 2a showed the results of U(VI) adsorption onto SiO2 and SiO2-COOH in a wide pH range of 2.0-11.0. The removal efficiencies of U(VI) onto SiO2 and SiO2-COOH increased dramatically from 1.0% to 82.0% and 3.2% to 90.5% at pH 10


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2.0-5.5, respectively, and maintained at the high level (∼85.7% for SiO2 and ∼88.4% for SiO2-COOH) at pH 5.5–8.0. Finally, U(VI) adsorption onto both samples decayed at pH 8.0−11.0. To explore the tendency of pH-dependent U(VI) adsorption on SiO2 and SiO2-COOH, the surface charge of adsorbents and the distribution of U(VI) species in solution were collectively investigated (Figure S4a and S4b). The zero-point charge of SiO2-COOH (pHzpc 3.3) was slight lower than that of SiO2 (pHzpc 3.8), and then the surface electronegativity of two samples strengthen with the increase of pH. In Figure S4b, UO22+ and positive hydrolysed ions ((UO2)3(OH)5+ and UO2(OH)+) were the main species at pH < 5.5. At pH < pHzpc, it was difficult for positively charged species to adsorb on the positively charged surface of microspheres because of electrostatic repulsion, whereas the dramatic increase of U(VI) adsorption on two materials from pHzpc to pH 5.5 could be ascribed to the strong electrostatic attractions between positively charged species and negatively charged surface of adsorbents. The better performance of U(VI) adsorption on SiO2-COOH was resulted from the modification of carboxyl groups, which made the increase in negative charge of the SiO2-COOH surface and directly bonded to the positively charged species by electrostatic attractions. Notably, the surface coprecipitate (i.e., schoepite) formed at pH 5.5−8.0 resulted in the high U(VI) removal onto both adsorbents. However, the successive deprotonation of carboxyl groups increased the surface electronegativity and thereby restrained the adsorption of negative carbonato−uranyl complexes (UO2(CO3)34-) under alkaline condition due to electrostatic repulsion. Thus, the U(VI)

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adsorption onto SiO2 was slightly higher than that on SiO2-COOH at pH 8.0−11.0 due to the high electronegativity of SiO2-COOH surface property. In consideration of the complexity and heterogeneity of water systems, the influence of ionic strength and coexisted ions on adsorption process should be evaluated. As shown in Figure S4c and S4d, the concentration of NaNO3 (0.001 M, 0.01 M, and 0.1 M) had negligible effect on the adsorption of U(VI) onto SiO2 and SiO2-COOH, reflecting the remarkable tolerance of two adsorbents to ionic strength variation. The ionic strength-independent characteristics revealed that the adsorption process was predominated by inner-sphere surface complexation with the formation of chemical bonds between U(VI) and electron-donating sites of oxygen-containing materials.44 To estimate the influence of coexisted ions on electrostatic interaction between materials and uranyl ions, Na+, K+, Mg2+ and Ca2+ cations, as well as NO3-, Cl-, CO32- SO42- and PO43- anions were regarded as the coexisted ions on the adsorption process. From Figure 2b, SiO2-COOH exhibited excellent adsorption performance in the presence of partially coexisted ions. Nevertheless, the coexistence of Ca2+ resulted in the decrease of U(VI) removal (~35.13% for SiO2 and ~67.08% for SiO2-COOH) due to the formation of ternary Ca-uranyl-carbonate complexes (CaUO2(CO3)3 and Ca2UO2(CO3)3(aq)), which were the main species in calcium rich solution at pH < 8.4.45 The superiority of as-synthesized SiO2-COOH on U(VI) coexisted with various ions implied the strong surface complexation between grafted carboxyl groups and target ions. Adsorption kinetics 12


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Adsorption kinetics of U(VI) on SiO2 and SiO2-COOH were investigated to provide insight on the adsorption rate and the rate limiting steps (Figure 2c). The U(VI) adsorption on two adsorbents were significantly enhanced within the initial stage, and then the rate decreased until reached the equilibrium (less than 15 min). Notably, SiO2-COOH exhibited a higher U(VI) capture efficiency of 85% than SiO2 sample (65%) at pH 5.0, which was mainly due to the abundant binding sites on SiO2-COOH surface, further confirming the important role of the carboxyl groups for U(VI) capture. The short time to achieve equilibration is critical for the application of the materials in real applications. The kinetic curves of U(VI) adsorption were further simulated by two kinds of typical models (pseudo-first-order and pseudo-second-order models), and the fitting procedure was described in SI. In Figure S5a, S5b and Table S2, the kinetic curves of U(VI) on two adsorbents were highly matched with the pseudo-second-order model (R2 > 0.999), suggesting that the chemical interaction of surface functional groups with U(VI) governed the adsorption processes on SiO2 and SiO2-COOH. Adsorption isotherms Considering the various wastewater systems, the isotherms of U(VI) adsorption on SiO2 and SiO2-COOH were investigated under the conditions of three different pH values (pH 3.0, 5.0, and 8.0) and 298 K, which could appraise the adsorption capacities of materials at saturation. From Figure 2d, the U(VI) immobilization on adsorbents were constantly accumulated with the increasing Ce and then reached equilibrium state. The phenomenon indicated that concentration-driven target ions 13



transferred from aqueous phase to solid surface until the binding groups of microspheres reached saturation. Further studies of isotherms could be acquired using Langmuir and Freundlich models, and the fitting results were shown in SI. The higher value of correlation coefficients (R2 > 0.97) derived from Langmuir model suggested that U(VI) adsorption onto SiO2 and SiO2-COOH under different conditions were mainly dominated by monolayer adsorption (Table S3). Specifically, the resulting microspheres have homogeneous surface with equivalent binding sites, which independently interacted with uranium ions. The maximum U(VI) adsorption capacities of SiO2 were 3.76 mg/g (pH 3.0), 76.57 mg/g (pH 5.0), and 82.05 mg/g (pH 8.0), whereas the maximum adsorption capacities of SiO2-COOH were 46.53 mg/g (pH 3.0), 167.79 mg/g (pH 5.0), and 328.95 mg/g (pH 8.0). The phenomena further supported the proposition that the introduction of carboxyl groups facilitated U(VI) loading on the microspheres by increasing the amounts of active sites. And the remarkable performance of SiO2-COOH at pH 8.0 could be interpreted from the formation of surface coprecipitate (i.e., schoepite), which coincided with the pH-dependent adsorption of U(VI) on SiO2-COOH. From Table S4, the adsorption ability of SiO2-COOH was dramatically higher than those of other Si-based adsorbents, demonstrating that SiO2-COOH microspheres were promising materials for efficient decontamination of U(VI) containing wastewater. XPS analysis In Figure S8b, the double peaks of U 4f at around 392 and 381 eV were detected in the XPS survey scans after U(VI) adsorption, which were identified to the 14


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characteristic peaks of U 4f5/2 and U 4f7/2.44 Compared to SiO2_U pH 5, the higher intensity of U 4f signal for SiO2-COOH_U pH 5 suggested that the presence of carboxyl groups onto SiO2 enhanced their loading at pH 5.0. Moreover, U 4f peaks of SiO2-COOH_U shifted to the lower binding energy with the increase of solution pH. From Figure S8c, Si 2p spectra indicated the appearance of Si-O (103.4 eV) and Si-C (102.7 eV). The binding energy of Si-O bond for SiO2 was red shifted after U(VI) adsorption, revealing the bonding interaction between U(VI) and hydroxyl groups on the surface of SiO2 microspheres. However, the negligible differences in Si 2p spectrum of SiO2-COOH_U confirmed that Si was not involved in U(VI) adsorption onto SiO2-COOH. The specific functions of grafted carboxyl groups on U(VI) uptake could be deduced from the fitting of C 1s and O 1s spectra, and the corresponding binding energies were listed in Table S5. The C 1s spectra were decomposed into three sub-bands with binding energies of ~284.6 eV (Si-C/C-C), ~285.5 eV (C-O) and ~288.3 eV (O-C=O) (Figure 3b). While the O 1s spectra could be deconvoluted into two peaks at 531.1 eV and 532.5 eV, which were relevant to O-C=O and Si-O/C-O (Figure 3c).40 High resolution C 1s spectra (O-C=O and C-O) and O 1s spectra (O-C=O and Si-O/C-O) of SiO2-COOH_U shifted to higher binding energy after adsorption, demonstrating that these oxygen-containing groups were responsible for the interaction between SiO2-COOH and U(VI). When the new chemical bond between O and U was formed, the extranuclear electron density of O element decreased because of the coordinating role, in which electron transferred from the electron-rich orbit of O to the unoccupied orbit of U. The shielding effect on O 15



nucleus became weaker and the attractive force exerted by the nucleus on its 1s electron was strengthened. As a result, the electronic excitation from the innermost orbit needed X-ray with higher energy, and the photoelectron binding energy of O 1s became higher after adsorption. The decrease in electron density of O would force the shared electrons of Si-O (SiO2)/C-O (SiO2-COOH) shifting to O, resulting in the higher binding energy of Si 2p (SiO2)/C 1s (SiO2-COOH). Notably, the relative content of O-C=O in O 1s spectra decreased from 11.59% (SiO2-COOH) to 10.69% (SiO2-COOH_U pH 3), 10.31% (SiO2-COOH_U pH 5) and 10.23% (SiO2-COOH_U pH 8) after adsorption. The order of the decline degree was consistent with the result of adsorption isotherms, further demonstrating that carboxyl groups acted as the mainly active sites for the adsorption of U(VI) onto SiO2-COOH. EXAFS analysis Figure 4 showed the raw k3-weighted fluorescence U LIII-edge EXAFS spectra and the corresponding Fourier transforms (FT) for the samples and references. The parameters obtained from the fitting were listed in Table 1. For all the samples, the first peak with a bond distance of 1.77-1.79 Å corresponded to the two axial oxygen atoms (Oax), and the equatorial oxygen atoms (Oeq) with a number range from 4-6 corresponded to the second peak.46 Compared with UO22+ ions, the fitting of uranyl acetate data showed the structural changes in the equatorial region, and a split equatorial oxygen shell coincided with the asymmetry of the local structure environmental of U(VI) produced upon the inner-sphere complex. The spectrum of the uranyl acetate showed an obvious feature with a new peak at R value around 3 Å, 16


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which is attributed to the backscatter from the nearest C neighbor and the fitting result suggested the existence of U-C bond. The peak of U-C shell could be caused by a uranyl-acetate mononuclear and/or bidentate U-C interaction. Two adsorption samples showed apparent evidence for the existence of U(VI) species. For SiO2_U, the EXAFS data could be satisfactorily fitted by involving two uranium coordination shells with two axial (Oax) at 1.79 Å and five equatorial (Oeq) oxygen atoms at 2.28 Å and 2.45 Å, and a coordination shell with Si atom at 3.15 Å, respectively. The procedure gave U-Si bond distance of ~ 3.15 Å and coordination number of 0.7. The FT feature of U-Si shell verified that the formation of inner-sphere U(VI) complexes was the main adsorption mechanism.46 As in the case of the binary system SiO2_U, uranium formed bidentate uranyl-silica complexes by edge sharing with silicon tetrahedra. Carboxyl functional groups were also supposed to affect the speciation of U(VI) on the adsorbent surface, since COO- ligands were known to form complex with UO22+.47 The effect of the surface coated -COOH could be verified by comparing the k3-weighted and the corresponding Fourier transforms EXAFS spectra of SiO2_U and SiO2-COOH_U (Figure 4). For SiO2-COOH_U, additional FT peaks were observed at ~2.2, 2.8, and 3.2 Å, R+∆R, which could be ascribed to C, Si, and multiple scattering paths.48, 49 The FT feature can be fitted by the shell with C and Si together with the U-Oax MS contribution. The results gave bond distance of U-C around 3.01 Å and the coordination number of 1.3. UO22+ bound to SiO2-COOH with the surface carboxyl groups, with a mean distance between U atom and C atom of 3.01 Å. This U-C bond 17



distance indicating an inner-sphere complex with one or two oxygen atoms shared between the UO22+ and the carboxyl ligand. Reusability, selectivity tests and application of SiO2 and SiO2-COOH in simulated wastewater Reusability Tests: To assess the stability of SiO2 and SiO2-COOH as adsorbents in long-period application, the reusability experiments were also carried out. As a result of pH-dependent experiment, the low pH value of solution was inefficient for U(VI) adsorption on solid. Combined with high chemical stability of SiO2 under acidic condition, acid picking (0.01 M HNO3) is a feasible approach to regenerate the samples after adsorption. After elution, the collected materials were rinsed with deionized water until the pH rose to 6.0 and U(VI) was not detected in the supernatant. From Figure 5a, the regeneration of SiO2 and SiO2-COOH through five consecutive adsorption/desorption cycles were carried out. The removal efficiencies decreased from 61.76% to 53.24% for SiO2 and from 90.52% to 83.06% for SiO2-COOH, respectively. The slight decreases throughout the cycles might be ascribed to the incompletely desorption and the mass loss during the regeneration procedure. In Figure S9, SiO2-COOH after regeneration mainly inherited the original morphology. However, several bands in the range of 1800-1400 cm-1 were weakened, which corresponded to the decorated carboxyl groups (Figure S10). The new peak at 964 cm-1 was ascribed to the antisymmetric vibration of O=U=O, suggesting the incompletely desorption of U(VI).50 In conclusion, SiO2 and SiO2-COOH had remarkable reusability for U(VI) adsorption in long-period environmental 18


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remediation. Selectivity Tests: The adsorption performances of other radioactive ions on SiO2 and SiO2-COOH were investigated to evaluate the selectivity towards U(VI) and evaluated in terms of the removal efficiency. As shown in Figure 5b, both two adsorbents showed a higher removal efficiency for U(VI), Th(IV), Ce(III), Eu(III) and La(III) than for other cations (Cd(II), Sr(II), Co(II), Zn(II), Ni(II) and Cs(I)) from multi-component systems, suggesting the excellent selectivity of SiO2 and SiO2-COOH towards high-valent ions. On the basis of removal efficiency, the order of the selective adsorption was U(VI) > Th(IV) > trivalent lanthanides > divalent metal ions > monovalent metal ion. The results could be attributed to the intrinsic properties of cations and their specific coordination affinities to the active sites of SiO2 and SiO2-COOH, as high-valent ions held the higher electronegativity and charge density than other cations.51-55 Obviously, SiO2-COOH also presented enhanced binding capacities for high-valent radioactive ions compared to SiO2. Application in Simulated Wastewater: Simulated adsorption experiments were carried out in deionized water, tap water, sea water, ground water and surface water with an initial U(VI) concentration of 10 ppm (Figure 5c). The major chemical components of different water systems were listed in Table S1. It was found that the SiO2-COOH sample still exhibited superior performance (89.5% for deionized water, 41.5% for tap water, 67.1% for sea water, 50.0% for ground water, and 52.2% for surface water) compared with pure SiO2 (59.6% for deionized water, 23.4% for tap water, 25.5% for sea water, 27.7% for ground water, and 38.2% for surface water). 19



The results demonstrated that SiO2-COOH was a versatile uranium trap possessing the great potential for practical applications. Given the above, it is reasonable that the pre-treatment of real wastewater is a vital step to guarantee the capture performance of U(VI) onto SiO2-COOH. Hence, the carboxylation treatment could improve the practical ability of adsorbents, indicating the great potential of SiO2-COOH as adsorbent for controlled loading of high-valent radionuclides in nuclear waste management. Conclusion This article reported on the modification of mesoporous silica microspheres with carboxyl groups to enhance the adsorption performance for high-valent radionuclides. Highly porous structure and abundant active sites made SiO2-COOH an excellent adsorbent for removing U(VI) under various conditions as compared to SiO2 without modification. More fundamentally, the mechanism for improving adsorption capacity with carboxyl groups was demonstrated using spectroscopic analyses, where carboxyl groups on the surface of SiO2-COOH played the combinatorial roles as (1) hydrophilic groups, (2) electron withdrawing groups, and (3) binding sites in promoting U(VI) adsorption. This work indicated that the engineering adsorbent with carboxyl groups is a plausible solution to immobilize high-valent radionuclides in real environment cleanup. Supporting Information

20


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More detailed on the characterization technologies and results. The effect of adsorbents dosage and the corresponding models and parameters of isotherms and kinetics were provided in SI Acknowledgements Financial supports from the National Key Research and Development Program of China (2017YFA0207002), the Fundamental Research Funds for the Central Universities (2018ZD11, 2017YQ001, 2017MS045) and NSFC (21836001, 21607042, 21707033, 21777039, 21577032) are acknowledged. References (1) Liu, H.; Li, M.; Chen, T.; Chen, C.; Alharbi, N. S.; Hayat, T.; Chen, D.; Zhang, Q.; Sun, Y. New Synthesis of nZVI/C Composites as an Efficient Adsorbent for the Uptake of U(VI) from Aqueous Solutions. Environ. Sci. Technol. 2017, 51 (16), 9227-9234. DOI: 10.1021/acs.est.7b02431. (2) Li, Z.; Huang, Z.; Guo, W.; Wang, L.; Zheng, L.; Chai, Z.; Shi, W. Enhanced Photocatalytic Removal of Uranium(VI) from Aqueous Solution by Magnetic TiO2/Fe3O4 and Its Graphene Composite. Environ. Sci. Technol. 2017, 51 (10), 5666-5674, DOI: 10.1021/acs.est.6b05313. (3) Hu, Y.; Zhao, C.; Yin, L.; Wen, T.; Yang, Y.; Ai, Y.; Wang, X. Combining batch technique with theoretical calculation studies to analyze the highly efficient enrichment of U(VI) and Eu(III) on magnetic MnFe2O4 nanocubes. Chem. Eng. J. 2018, 349 (1), 347-357, DOI: 10.1016/j.cej.2018.05.070. (4) Bleise, A.; Danesi, P. R.; Burkart, W. Properties, use and health effects of 21



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doi.org/10.1016/j.cej.2018.03.107.

Table 1. EXAFS structural parameters of uranium complexes in references and samples Sample

Shell

R (Å)

N

σ2 (Å2)

UO22+ (aq)

U-Oax

1.79

1.8

0.0002

U-Oeq

2.41

5.4

0.0075

U-Oax

1.78

2.1

0.0032

U-Oeq1

2.28

3.2

0.0085

U-Oeq2

2.44

1.6

0.0024

U-C

3.01

1.9

0.0047

U-Oax

1.77

2

0.0035

U-Oeq1

2.28

3.1

0.0045

U-Oeq2

2.45

1.7

0.0036

U-Si

3.15

0.7

0.0065

U-Oax

1.77

2

0.005

U-Oeq1

2.29

3.7

0.0022

U-Oeq2

2.45

1.2

0.0048

U-C

3.01

1.3

0.0101

U-Si

3.15

0.3

0.0057

R-factor 0.0079

Uranyl-citrate

0.0025

SiO2_U

0.0031

SiO2-COOH_U

0.0035

UO22+ and Uranyl-citrate are reference and the others are adsorption samples. N, coordination number; R, radial distance (Å); σ2, Debye–Waller factor (Å2).

30


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a)

b)

c)

d)

e)

f)

g)

h)

Figure 1. SEM images of (a, b) SiO2 and (d, e) SiO2-COOH microspheres; TEM images of (c) SiO2 and (f) SiO2-COOH microspheres; and the corresponding elemental mapping images of Si, O and C for (g) SiO2 and (h) SiO2-COOH, respectively.

31



a)

b)

100

SiO2 SiO2-COOH

100 80

Removal (%)

Removal (%)

80 60 40

60 40

20

100

4 3-

4 2-

PO

3 2-

O

SO

C

2+

3 -

-

C l

N

d)

a

0

11

N O

10

2+

9

g

8

C

7 pH

+

6

M

5

+

4

a

3

K

2

c)

20

SiO2 SiO2-COOH

0

250 SiO2-COOH (pH 8)

80 200 60 Qe (mg/g)

Removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

40 20

150 SiO2-COOH (pH 5) 100 50

SiO2 (pH 5) SiO2-COOH (pH 5)

0 0

40

80 120 160 Reaction Time (min)

200

SiO2-COOH (pH 3) 0

240

0

10

20 30 Ce (mg/L)

40

50

60

Figure 2. Effect of (a) pH and (b) coexisted ions (C0 = 10.0 mg/L) on U(VI) adsorption onto SiO2 and SiO2-COOH; (c) Adsorption kinetics (C0 = 10.0 mg/L and pH = 5.0 ± 0.1) and (d) adsorption isotherms (C0 = 5.0-80.0 mg/L and pH = 3.0 ± 0.1, 5.0 ± 0.1, and 8.0 ± 0.1) of U(VI) adsorption on SiO2-COOH. Same condition: T = 298 K, m/V = 0.2 g/L and I = 0.01 M.

32


Page 33 of 36

a)

U 4f7/2

SiO2-COOH_U pH 8

b)

SiO2-COOH_U pH 8 C-O O-C=O

Si-C/C-C

c)

Si-O/C-O

SiO2-COOH_U pH 8

O-C=O

U 4f5/2 SiO2-COOH_U pH 5

SiO2-COOH_U pH 5

SiO2-COOH_U pH 3

SiO2-COOH_U pH 3

SiO2-COOH

Intensity (a.u.)

SiO2-COOH_U pH 3

Intensity (a.u.)

SiO2-COOH_U pH 5

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SiO2-COOH

SiO2_U pH 5

SiO2_U pH 5 SiO2_U pH 5 SiO2

370

375 380 385 390 395 Binding Energy (eV)

280 282 284 286 288 290 292 Binding Energy (eV)

Si-O

SiO2

526 528 530 532 534 536 538 Binding Energy (eV)

Figure 3. XPS analyses: high resolution (a) U 4f, (b) C 1s and (c) O 1s of SiO2, SiO2-COOH before and after U(VI) adsorption at given pH (SiO2_U pH 5, SiO2-COOH_U pH 3, SiO2-COOH_U pH 5 and SiO2-COOH_U pH 8), respectively.

33



a)

UO22+(aq)

b)

U-Oax U-Oeq

uranyl acetate

SiO2_U

SiO2-COOH_U

Fourier Transform Magnitude

UO22+(aq)

k3χ (k)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

U-C uranyl acetate

U-Si

SiO2_U

SiO2-COOH_U

3

4

5

6

7

8 o

9 10 11 12

0

1

2

3

4

5

6

o

-1

R+∆R(A)

k(A )

Figure 4. (a) k3-weighted EXAFS spectra and (b) the corresponding Fourier transforms of the reference and adsorption samples.

34


Page 35 of 36

a) 100

SiO2

SiO2-COOH

b) 100

SiO2 SiO2-COOH

c) 100

40

60 40

20

20

0

0

1

2

3 4 Cycle Number

5

SiO2 SiO2-COOH

80

80

60

Removal (%)

80 Removal (%)

Reuseability (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20

U Th Ce Eu La Cd Sr Co Zn Ni Cs

0 Deionized Tap Sea Ground Surface water water water water water

Figure 5. (a) Adsorption recyclability of SiO2 and SiO2-COOH for U(VI) over five cycles, pH = 5.0 ± 0.1 and I = 0.01 M; (b) Adsorption selectivity of SiO2 and SiO2-COOH towards high-valent ions (U(VI), Th(IV), Ce(III), Eu(III), La(III), Cd(II), Sr(II), Co(II), Zn(II), Ni(II) and Cs(I)), and (c) Treatment capacity of SiO2 and SiO2-COOH for simulative uranium-contaminated water (deionized water (pH = 6.98), tap water (pH = 6.98), real sea water (pH = 6.86), synthetic ground water (pH = 7.31) and synthetic surface water (pH = 7.14)). Same condition: C0 = 10.0 mg/L, T = 298 K and m/V = 0.2 g/L.

35



TOC Synopsis: The application and mechanism of SiO2-COOH in radionuclides’ removal were investigated to lay the foundation for carboxylation of natural inorganic materials in real nuclear waste management.

inner-sphere complexation

SiO2-COOH

100

SiO2 SiO2-COOH

80 Removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60 40 20 0

U Th Ce Eu La Cd Sr Co Zn Ni Cs

36


Page 36 of 36

Selective Immobilization of Highly Valent Radionuclides by Carboxyl

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