KIT-6 - ACS Publications - American Chemical Society

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Large-Pore 3D Cubic Mesoporous (KIT-6) Hybrid Bearing a Hard−Soft Donor Combined Ligand for Enhancing U(VI) Capture: An Experimental and Theoretical Investigation Li-Yong Yuan,†,# Lin Zhu,†,‡,# Cheng-Liang Xiao,*,‡ Qun-Yan Wu,† Nan Zhang,† Ji-Pan Yu,† Zhi-Fang Chai,†,‡ and Wei-Qun Shi*,† †

Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China

‡

S Supporting Information *

ABSTRACT: A preorganized tetradentate phenanthrolineamide (DAPhen) ligand with hard and soft donors combined in the same molecule has been found to possess high extraction ability toward actinides over lanthanides from acidic aqueous solution in our previous work. Herein we grafted phenanthrolineamide groups onto a large-pore three-dimensional cubic silica support by the reaction of DAPhen siloxane with KIT-6 substrate to prepare a novel uranium-selective sorbent, KIT-6-DAPhen. The assynthesized sorbent was well-characterized by scanning electron microscopy, high-resolution transmission electron microscopy, N2 adsorption/desorption, X-ray diffraction, FT-IR, 13C crosspolarization magic-angle spinning NMR, and TGA techniques, which confirmed the consummation of the functionalization. Subsequently, the effects of contact time, solution pH, initial U(VI) concentration, and the presence of competing metal ions on the U(VI) sorption onto KIT-6-DAPhen sorbent were investigated in detail. It was found that KIT-6-DAPhen showed largely enhanced sorption capacity and excellent selectivity toward U(VI). The maximum sorption capacity of KIT-6-DAPhen at pH 5.0 reaches 328 mg of U/g of sorbent, which is superior to most of functionalized mesoporous silica materials. Density functional theory coupled with quasi-relativistic small-core pseudopotentials was used to explore the sorption interaction between U(VI) and KIT-6-DAPhen, which gives a sorption reaction of KIT-6-DAPhen + [UO2(H2O)5]2+ + NO3− ⇄ [UO2(KIT-6-DAPhen)(NO3)]+ + 5H2O. The findings of the present work provide new clues for developing new actinide sorbents by combining new ligands with various mesoporous matrixes. KEYWORDS: phenanthroline, mesoporous silica, KIT-6, uranium, DFT

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contributed by Florek et al.4 on the topic of mesoporous materials for actinide extraction has been published in which a series of examples of the successful application of such materials in actinide separation are summarized. Fryxell and coworkers,5−9 for example, pioneered the synthesis of functional ordered mesoporous silica adsorbents, called self-assembled monolayers on mesoporous supports (SAMMS), for actinide sequestration. After grafting of glycinyl−urea, salicylamide, acetamide−phosphonate, and hydroxypyridinone ligands (Figure 1) onto mesoporous MCM-41 supports, these organic− inorganic hybrid materials showed fast and selective capture of actinides. In addition, Yousefi et al.10 grafted a 5-nitro-2furaldehyde (fural) group (Figure 1) onto the MCM-41 substrate for U(VI) sorption. Chen and co-workers11,12

INTRODUCTION Although the Fukushima nuclear accident in 2011 had a negative effect on the development of nuclear power, after three years the situation shows signs of a turnaround. The consequent nuclear wastes with long-term radiotoxicity, especially resulting from actinide ions, still cause major social and environmental problems, which would restrict further expansion of nuclear power. From the viewpoint of environmental protection and a sustainable nuclear fuel cycle, it is highly desired to develop efficient and selective sorbents for the removal of actinides from wastewater. Mesoporous silica materials are defined in terms of their pore diameters of 2−50 nm, such as MCM-41, SBA-15, and KIT-6, which exhibit distinct advantages of large surface area, welldefined pore size, chemical stability, and easy modification.1 Such excellent characteristics make mesoporous silica an ideal support to synthesize functional sorbents for the separation of radiotoxic actinides from wastewater.2,3 A recent review © 2017 American Chemical Society

Received: December 6, 2016 Accepted: January 9, 2017 Published: January 9, 2017 3774

DOI: 10.1021/acsami.6b15642 ACS Appl. Mater. Interfaces 2017, 9, 3774−3784

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ACS Applied Materials & Interfaces

Figure 1. Functional organic ligands used in the mesoporous silica sorbents for sequestering actinides.

requires the lowest energy to complex a target guest ion with less structural change. We recently reported a novel preorganized tetradentate 2,9diamide-1,10-phenanthroline (DAPhen) ligand, Et-Tol-DAPhen, which exhibited high extraction ability and excellent selectivity toward actinides.21−23 The “ligand preorganization” here takes place in the two nitrogen atoms of the phenanthroline moiety in view of its rigid coplanar structure. Unlike dipyridyl derivatives, in which a certain flexibility is provided by the single bond between the two pyridines, the rigid coplanar structure of phenanthroline forces the two nitrogen donors be positioned on the same side, thus making phenanthroline preorganized for forming complexes with U(VI) ions. Following our previous success with actinide extraction, in the present work we proposed to graft DAPhen groups onto the surface of the large-pore 3D cubic mesoporous KIT-6 support to obtain the sorbent KIT-6-DAPhen, aiming to selectively recover actinides from wastewater. The synthesis, characterization, and U(VI) sorption properties of this mesoporous KIT-6-DAPhen sorbent were investigated in detail, and the sorption mechanism was explored by density functional theory (DFT) calculations.

prepared mesoporous SBA-V-based materials bearing phosphine oxide ligands for binding of uranium in HNO3 media. We recently developed several efficient mesoporous silica materials for solid-phase extraction of actinides from aqueous solution. Phosphonate, dihydroimidazole, and amino groups (Figure 1) were successfully grafted onto mesoporous MCM-41 or SBA15 supports, and the resulting materials exhibited extremely high sorption capacity toward U(VI) ions.13−17 Compared with mesoporous SBA-15 and MCM-41 supports, the KIT-6 support has an interconnected large-pore threedimensional (3D) cubic mesoporous nature, which is beneficial for reducing the risk of pore blocking during functionalization with large ligands. In addition, the 3D mesopores are highly accessible for the solution and metal ions during the sorption process. Larivière, Kleitz, and co-workers18,19 opened up the postsynthetic modification of mesoporous KIT-6 silicas with (2-diethylphosphatoethyl)triethoxysilane (DTPS) and found this sorbent to show obvious superiority over its 2D hexagonal analogue (SBA-15-DTPS) in the sorption of U(VI) ions. Furthermore, the same group reported mesoporous KIT-6 hybrid materials functionalized with diglycolylamide (DGA) groups (Figure 1) for efficient recovery and enrichment of rareearth elements and some actinides.20 In these novel sorbents, there may be an adverse effect on the sorption performance because of the rotation of σ−σ bonds in the DGA groups. If one could reduce the flexibility of the diglycolyamide groups, the sorption properties would improve. From a chemical point of view, further modifications in the design and synthesis of new preorganized ligands for selective capture of actinides should be made. A so-called preorganized ligand is one that

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EXPERIMENTAL SECTION

Chemicals and Materials. Tetraethoxysilane (TEOS, 98%) and Pluronic P123 ethylene oxide−propylene oxide−ethylene oxide triblock copolymer were purchased from Sigma-Aldrich. 3(Aminopropyl)trimethoxysilane (APTS) (97%) was purchased from Meryer (Shanghai, China). Uranyl nitrate hexahydrate (UO2(NO3)2· 6H2O, ACS grade) was purchased from Merck. All of the other chemical materials (thionyl chloride, HCl, butanol, etc.) were of 3775


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Figure 2. Schematic representation of the synthesis of phenanthrolineamide-functionalized KIT-6. analytical grade and used without further purification. The U(VI) stock solution was prepared by dissolving the appropriate amount of UO2(NO3)2·6H2O in deionized water. The type and purity of the metal ions used in the selectivity experiments are listed in Table S4 in the Supporting Information. The concentration of each metal ion was around 0.5 mmol L−1. The deionized water used in all of the experiments was obtained from a Milli-Q water purification system. Synthesis of the KIT-6-DAPhen Sorbent. The mesoporous KIT-6 support was prepared according to the method reported by Kleitz and co-workers20 and used after thermal activation at 150 °C overnight. 1,10-Phenanthroline-2,9-dicarboxylic acid was synthesized using our previous procedures.21 The KIT-6-DAPhen sorbent was synthesized as depicted in Figure 2. DAPhen Siloxane. First, 1,10-phenanthroline-2,9-dicarboxylic acid (0.509 g, 1.9 mmol) was refluxed with thionyl chloride (20 mL) under an inert atmosphere. After 3 h, the mixture was cooled, and thionyl chloride was removed by reduced-pressure distillation. When the solvent was completely evaporated, the gray solid was dissolved in dry toluene (25 mL) and cooled in ice water. Then 1 mL (4.0 mmol) of (3-aminopropyl)triethoxysilane and 0.6 mL of triethylamine were added to the above solution, and the resulting mixture was stirred for 24 h under reflux and N2 protection. This afforded the DAPhen siloxane solution. To confirm that the reactant, i.e., 1,10-phenanthroline-2,9-dicarboxylic acid, was fully converted into its silanized version, thin-layer chromatography analysis was performed using petroleum ether/ethyl acetate (1:1) as the eluent. Only two components besides toluene and triethylamine were obtained. One component was confirmed to be unreacted APTS since a 5% excess of APTS was used, and the other component was characterized by 1H NMR spectroscopy (see Figure S1 in the Supporting Information). All of the resonance signals in the NMR spectrum could be assigned to appropriate H atoms of N2,N9-bis(3-(triethoxysilyl)propyl)-1,10phenanthroline-2,9-dicarboxamide, providing evidence that the DAPhen siloxane had been successfully prepared. KIT-6-DAPhen. Activated KIT-6 support (0.5 g) and 0.5 mL of triethylamine were added to 25 mL of dry toluene. The DAPhen siloxane solution was transferred into the above suspension, and the resulting mixture was left to stir for 24 h under reflux and N2 protection. After cooling to room temperature, the mixture was filtered and washed with large amounts of toluene, ethanol, and dichloromethane six times. The white solid product, named KIT-6DAPhen, was obtained after drying at 70 °C overnight under vacuum. Characterization Methods. The morphologies and microstructures of the samples were characterized by field-emission scanning electron microscopy (SEM) on a HITACHI S-4800 microscope. Highresolution transmission electron microscopy (HRTEM) was performed with a JEOL JEM-2100 microscope operating at 200 kV.

Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8-Advance X-ray diffractometer with Cu Kα radiation. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TA Instruments, Q500) from 20 to 900 °C at a heating rate of 5 °C min−1 under an air flow. The N2 sorption experiments were performed on a Micromeritics ASAP 2020 HD88 instrument at liquid nitrogen temperature (−196 °C). The samples were degassed under vacuum at 120 °C before the measurements. The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) method. The pore size was obtained from the maximum of the pore size distribution curve calculated by the Barrett−Joyner−Halenda (BJH) method using the sorption branch of the isotherm. The total pore volume was evaluated by the single-point method. Fourier transform infrared (FTIR) spectra were recorded on a Bruker Tensor 27 spectrometer with a potassium bromide pellet method. 13C cross-polarization magic-angle spinning (CP/MAS) NMR spectra were measured on a Mercury Plus 400 spectrometer at 100 MHz at a sample spinning frequency of 3 kHz. Inductively coupled plasma optical emission spectrometry (ICPOES, Horiba JY2000-2) was used to determine the residual concentrations of tested ions in supernatants in the selectivity test experiment. The UV absorbance of the arsenazo III−U(VI) complex was recorded in photometry mode on a Hitachi UV-3900 spectrophotometer with a quartz cuvette of 1 cm path length. Sorption Experiments. All of the sorption experiments were carried out using the batch method in air at room temperature with initial concentrations of U(VI) ranging from 5 to 200 mg L−1. The solution pH was adjusted by adding negligible volumes of dilute nitric acid or sodium hydroxide. In a typical experiment, 4 mg of sorbent was added to 10 mL of U(VI) solution or multi-ion test solution in a flask (the solid−liquid ratio was 0.4 g/L). The control experiment was performed simultaneously using the identical U(VI) solution in the absence of the sorbent. After the solution was stirred for the desired time, the solid phase was separated from the aqueous solution using a 0.22 μm nylon membrane filter, and then the concentration of U(VI) and other metal ions in the aqueous phase was determined by the arsenazo III spectrophotometric method or ICP-OES. For the arsenazo III spectrophotometric method (the detection limit is below 0.1 ppm), the supernatant was diluted 2.5−100 times to make sure that the U(VI) concentration in the dilution was 0.1−5 μg/ mL, corresponding to a UV absorbance of 0.05−1.0 at 656 nm. For the ICP-OES method (the detection limit is below 0.01 ppm), the supernatant was diluted 25−100 times to make sure that the concentration of metal ions in the dilution was 1−5 μg/mL. The sorption capacity (q) of metal ions is defined as q = (C0 − Ce) × Vsolution/msorbent, where C0 and Ce represent the concentrations of metal ions in the aqueous phase after 2 h of stirring in the control experiment and the sorption experiment, respectively, and msorbent and 3776


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ACS Applied Materials & Interfaces Vsolution are the weight of the sorbent and the solution volume used in the sorption experiment, respectively. All of the values were measured in duplicate with an uncertainty within 5%. Theoretical Methods. The geometries were optimized using the B3LYP hybrid functional in Gaussian 09.24 The two-component smallcore quasi-relativistic effective core potentials (RECPs),25,26 which replace 60 core electrons for the uranium atom, were adopted here in combination with the corresponding basis set with a segmented contraction scheme. For light atoms (H, C, N, O, and Si), the polarized all-electron 6-31G(d) basis set was used for geometry optimizations and energy calculations. The Mayer bond order and electron density of the bond critical point were calculated using the Amsterdam Density Functional (ADF) 2012 package.27 The B3LYP functional and Slater-type orbital (STO) basis set with the triple-ζ plus polarization (TZP)-quality basis set were used without the frozen core.28 The scalar relativistic effects were taken into account using the zeroth-order regular approximation (ZORA) approach.29

KIT-6, two new characteristic bands in the FT-IR spectrum of KIT-6-DAPhen at 1551 and 1501 cm−1 can be assigned to the stretching vibration of CC groups of the aromatic ring from the grafted phenanthrolineamide groups, while the new bands at 3051 and 2943 cm−1 represent the stretching vibrations of C−H groups. When the spectra were enlarged (shown in the inset in Figure 4d), another new peak for KIT-6-DAPhen was revealed at 1655 cm−1, which overlapped with the band assigned to H−O−H deformation vibrations of absorbed H2O molecules at 1630 cm−1. These peaks clearly indicate the successful introduction of phenanthrolineamide groups into the mesoporous silica. The solid-state 13C CP/MAS NMR spectrum of KIT-6-DAPhen is shown in Figure 4e. Three intense resonance peaks can be observed at chemical shifts δ = 12, 24, and 45 ppm, which can be definitely assigned in turn to methylene carbon atoms from the silicon end to the nitrogen end in (−Si−CH2−CH2−CH2−N−) (no. 1−3, respectively). A small peak corresponding to the amide carbon atoms was revealed at δ = 167 ppm (no. 4). Several peaks in the δ range of 120−160 ppm overlap each other but can be reasonably assigned to aromatic carbon atoms of the phenanthroline moiety (no. 5−10). Besides, it is noted that a small peak assigned to ethoxy groups in (−Si−O−CH2CH3) appears at δ = 60 ppm, and the wide peak at δ = 24 ppm is believed to mask the signal of the remaining methyl carbon species, which is an indication that a little part of the ethoxysilane remained unreacted during the functionalization. From the NMR data, however, it is possible to conclude that covalent attachment of the proper extraction agent analogue functionality (i.e., phenanthrolineamide groups) on the KIT-6 pore surface was successful. From the TGA curve of KIT-6-DAPhen (Figure 4f), we can see that four distinct stages of weight loss occur with increasing temperature. The first stage arises from the volatilization of physically adsorbed water and organic solvent, while stages II−IV can be mainly attributed to the pyrolysis of the grafted phenanthrolineamide on the surface of KIT-6 since the pyrolysis temperature of DAPhen ligand is between 200 and 600 °C, as shown in Figure S5. The total weight loss is 22.9%, excluding the volatilization of water and toluene, from which the grafting ratio of the targeted functional groups can be calculated to be ∼0.66 mmol g−1 on the basis of the assumption that all of the weight losses are assigned to phenanthrolineamide degradation (the molecular mass of phenanthrolineamide functional groups is 348 u). The grafting rate, however, is slightly overestimated because the unreacted amino groups also contribute the weight loss in this temperature range. U(VI) Sorption onto KIT-6-DAPhen Sorbent. To further explore the sorption behavior of U(VI) on the KIT-6 and KIT6-DAPhen sorbents, we conducted batch sorption of U(VI) from aqueous solution under various conditions of pH, contact time, and initial U(VI) concentration. The selectivity toward U(VI) and reusability of the sorbent were also assessed for the aim of practical applications. Sorption Kinetics. To determine the sorption kinetics of U(VI) ions onto KIT-6 and KIT-6-DAPhen, the sorption over a time range of 1−360 min was performed with an initial U(VI) concentration of 100 mg/L. As shown in Figure 5, the sorption of U(VI) ions by both KIT-6 and KIT-6-DAPhen is ultrafast in the initial 10 min, indicating a high affinity between U(VI) ions and the surface of the sorbents. With the decrease in the number of active sites, the uptake rate of U(VI) ions slowly declines and then reaches an equilibrium. The final sorption equilibrium times are about 60 and 120 min for the KIT-6 and

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RESULTS AND DISCUSSION Characterization of KIT-6-DAPhen Sorbent. Figures 3 and 4 show the characterization results for the KIT-6 and KIT-

Figure 3. SEM/TEM images of (a, b) KIT-6 and (c, d) KIT-6DAPhen.

6-DAPhen sorbents. The irregular morphology of KIT-6 and KIT-6-DAPhen can be seen from SEM images, and the 3D cubic pore structures were determined by TEM (Figure 3). The small-angle XRD pattern (Figure 4a) indicates that the mesoporous structure is commensurate with Ia3d symmetry. In addition, the diffraction peak of KIT-6-DAPhen is not largely shifted compared with that of KIT-6, which may be due to the large pore diameter and pore volume of KIT-6. The N2 sorption/desorption isotherms of KIT-6 and KIT-6-DAPhen are shown in Figure 4b. Both isotherms are typical type IV curves with the H1 hysteresis loop according to the IUPAC classification. The surface areas of KIT-6 and KIT-6-DAPhen were calculated to be 746.5 and 395.6 m2/g, respectively, using the BET method. The total pore volumes were estimated to be 0.95 and 0.46 cm 3 /g for KIT-6 and KIT-6-DAPhen, respectively. After the grafting with the phenanthrolineamide groups, the average pore size was reduced from 8.9 to 5.6 nm (Figure 4c). To confirm the covalent attachment and the chemical stability of the functional groups in the prepared hybrid materials, FT-IR, solid-state NMR, and TGA measurements were performed. As shown in Figure 4d, compared with that of 3777


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Figure 4. Characterizations of KIT-6 and KIT-6-DAPhen sorbents. (a) XRD patterns, (b) N2 sorption/desorption isotherms, (c) Pore size distribution, (d) FT-IR spectra, (e) 13C CP/MAS NMR spectra, and (f) TGA profiles.

Besides, to further assess the effect of intraparticle diffusion on the entire sorption process, the sorption kinetics data were also analyzed using the intraparticle diffusion model given by Weber and Morris (Table S2 and Figure S3). It is clear that before the sorption reaches equilibrium, the experimental data reasonably fit with the intraparticle diffusion model, suggesting that intraparticle diffusion is one of the key points for the rate determination in the sorption of U(IV) by both KIT-6 and KIT-6-DAPhen. The better correlation coefficient (R2 = 0.98) and lower sorption rate constant (kid) for KIT-6-DAPhen imply that intraparticle diffusion plays more an important role for U(VI) sorption by KIT-6-DAPhen than by KIT-6, corresponding to the slower sorption kinetics for KIT-6-DAPhen than for KIT-6. This is actually understandable from the porous structure of the sorbents. That is, the introduction of the bulky organic ligand (i.e., the phenanthrolineamide group) into KIT-6 causes an obvious decrease in the surface area, pore volume, and pore size of the sorbent as denoted by the BET measurements, and the connectivity of pores is also partly reduced. In such a case, the intraparticle diffusion of U(VI) in KIT-6-DAPhen is limited, thus leading to the slower sorption kinetics. Effect of pH. The pH of the aqueous solution is one of the important factors in the sorption process, since H+ ions not only can affect the surface charge of the sorbent but also can change the speciation of metal ions in the solution. Additionally, H+ ions can compete with metal ions in the complexation with functional groups. Herein we investigated the effect of pH ranging from 2.0 to 6.0 on the U(VI) sorption onto the KIT-6 and KIT-6-DAPhen sorbents. As shown in Figure 6, the U(VI) sorption by the two sorbents rapidly increases with augmentation of the solution pH. As reported previously,30 the U(VI) speciation in aqueous solution largely depends on the solution pH. We thus here calculated the aqueous and solid speciation of U(VI) as a function of pH in the absence of sorbent according to the thermodynamic data reported previously,31 as shown in section SI-2. It is clear that higher pH leads to the formation of multinuclear hydroxide complexes,32 and the multinuclear

Figure 5. Effect of the contact time on the U(VI) sorption by KIT-6 and KIT-6-DAPhen (pH 5.5, msorbent/Vsolution = 0.4 mg mL−1, [U]initial = 100 mg/L). The solid lines show pseudo-second-order model fitting results.

KIT-6-DAPhen sorbents, respectively. That is, U(VI) sorption onto KIT-6-DAPhen shows slower kinetics. In subsequent experiments, the mixture of the sorbent and the solution were all stirred for at least 240 min to ensure equilibrium. In order to clarify the sorption process of U(VI) onto KIT-6 and KIT-6-DAPhen sorbents, the pseudo-first-order and pseudo-second-order kinetic models were applied to analyze the sorption kinetic data (Figure S3). The detailed description of the two models and the fitting results are presented in section SI-3 and Table S2 in the Supporting Information. It was found that the pseudo-first-order model poorly matches the experimental kinetic data (R2 = 0.8), whereas the pseudosecond order model gives a much better correlation coefficient (R2 > 0.99). In addition, the qe values calculated from the pseudo-second-order kinetic model for KIT-6 and KIT-6DAPhen are in good agreement with the experimental data. Obviously, the pseudo-second order model is more appropriate for explaining the kinetics of U(VI) sorption by both KIT-6 and KIT-6-DAPhen, implying that the sorption process is mainly dominated by chemical sorption. 3778


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Figure 6. Effect of pH on the U(VI) sorption by KIT-6 (▲) and KIT6-DAPhen (■) (msorbent/Vsolution = 0.4 mg/mL, [U]initial = 100 mg/L). The blank control data are also shown (●); the two dash-dot lines represent the zeta potentials of KIT-6-DAPhen (top) and KIT-6 (bottom) as functions of pH.

Figure 7. Sorption isotherms of U(VI) for the KIT-6 and KIT-6DAPhen sorbents (pH 5.0 ± 0.1, msorbent/Vsolution = 0.4 mg mL−1). The solid and dashed lines are fits to the Langmuir and Freundlich models, respectively.

hydroxide complexes are believed to be more favored by the sorbents, thus leading to an enhancement of the U(VI) uptake with increasing solution pH. On the other hand, the pHdependent surface charge of the sorbent may also be responsible for the pH-dependent U(VI) sorption. Also shown in Figure 6 are the zeta potentials of KIT-6-DAPhen and KIT-6 as functions of pH. It can be seen that at a lower pH (here pH 2), the surface of both KIT-6 and KIT-6-DAPhen are positively charged, and almost no U(VI) sorption occurred for KIT-6, whereas for KIT-6-DAPhen a U(VI) sorption of ca. 28 mg/g was accomplished, which is of practical value for most applications. This result clearly reveals the important role of the grafted ligand in KIT-6-DAPhen for binding U(VI).21 As the solution pH increases, the surface charge for KIT-6 becomes negative as a result of deprotonation of the surface −OH groups, and thus, the U(VI) sorption is enhanced because of the electrostatic interaction.33,34 For KIT-6-DAPhen, however, the surface charge remains positive over the whole test pH range, and the U(VI) sorption enhancement with increasing pH is probably due to the improved complexation ability between phenanthrolineamide and U(VI) ions resulting from weak protonation of the phenanthrolineamide groups. Such a result further confirms the binding effect of the grafted ligand in KIT-6-DAPhen.21 Besides, both the blank control data as shown in Figure 6 and the calculations (section SI-2) show that a precipitate such as schoepite occurs at pH ≥6. To avoid the interference of the U(VI) precipitation and achieve higher sorption capacity, a pH range of 4−5 was selected for further sorption experiments. Sorption Isotherms. The sorption isotherms of the KIT-6 and KIT-6-DAPhen sorbents toward U(VI) were determined at a constant pH of 5.0 ± 0.1 by varying the initial U(VI) concentration over the range from 5 to 200 mg/L. As shown in Figure 7, the U(VI) sorption onto KIT-6-DAPhen is significantly higher than that onto unmodified KIT-6. The maximum sorption capacity of KIT-6-DAPhen for U(VI), for example, is larger than 300 mg/g at Ce > 60 mg L−1, while that of KIT-6 is only ca. 125 mg/g at Ce = 30 mg/L. The result clearly reveals the important role of the grafted phenanthrolineamide groups in binding U(VI) ions. Besides, it is noted that the U(VI) sorption onto KIT-6-DAPhen has a sharp peak at Ce < 10 ppm, corresponding to ca. 180 mg/g of sorption, and beyond this value, C e greatly exceeds the maximum

concentration limit set by the World Health Organization for U(VI) in drinking water (15 ppb). That is, the effective sorption capacity of KIT-6-DAPhen for U(VI) during the purification of water was deemed as less than 180 mg/g, while the saturation sorption capacity is larger than 300 mg/g. In order to understand the mode of U(VI) sorption onto KIT-6 and KIT-6-DAPhen, the sorption data were fitted to the Langmuir, Freundlich, and Dubinin−Radusckevich (D−R) models. The detailed descriptions of the three models are presented in section SI-4, and the fitting plots as well as the parameters are given in Figure S4 and Table S3. It can be seen that the Langmuir model gives a better correlation coefficient (>0.99) and saturated capacities (143 mg/g for KIT-6, 328 mg/ g for KIT-6-DAPhen) much closer to the experimental values (125 mg/g for KIT-6, 310 mg/g for KIT-6-DAPhen), suggesting that Langmuir model (i.e. a monolayer and uniform sorption mode35) is more appropriate to explain the sorption of U(VI) in both KIT-6 and KIT-6-DAPhen. Besides, from the D−R fitting, mean free energies (E) of 11.3 kJ mol−1 for KIT-6 and 10.7 kJ mol−1 for KIT-6-DAPhen were obtained. Since numerical values of E in the ranges of 1−8 and 9−16 kJ/mol forecast physical sorption and chemical sorption, respectively, the E values obtained in this work clearly suggest that the sorption of U(VI) in both KIT-6 and KIT-6-DAPhen is chemical sorption (i.e., chemisorption). The chemical nature of the sorption is actually in good agreement with uniform monolayer sorption indicated by the Langmuir model. Table 1 compares the maximum sorption capacity of KIT-6DAPhen toward U(VI) with those of other functionalized mesoporous silica materials reported previously in the literature. From the sorption capacity viewpoint, KIT-6DAPhen is superior to most of the mesoporous silica materials with respect to U(VI) sorption. This may be due to the interconnected large-pore 3D cubic mesopore structure and the strong complexation ability of phenanthrolineamide functional groups toward U(VI). Thus, it is believed that the KIT-6DAPhen sorbent is quite promising in uranium removal and recovery from aqueous solution. Thermodynamic Study. To obtain thermodynamic data for U(VI) sorption onto KIT-6-DAPhen and further understand the nature of the sorption process, the sorption of U(VI) by KIT-6-DAPhen sorbent at different temperatures was performed, and the results are shown in Figure 8. It can be seen 3779


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Sr(II), La(III), Nd(III), Sm(III), Gd(III), and Yb(III) was investigated at pH 4.0 and 5.0, respectively. The results are shown in Figure 9. The U(VI) uptake by KIT-6-DAPhen is

Table 1. Comparison of U(VI) Sorption onto Various Functionalized Mesoporous Silica Sorbents sorbent

experimental conditions

MSU-H MSA-III MSPh-III DIMS NP10 PA-SBA-15 SBA-15-DT4A KIT-6-DAPhen

ambient temperature, pH = 8.3 ± 0.1 ambient temperature, pH = 8.3 ± 0.1 ambient temperature, pH = 8.3 ± 0.1 ambient temperature, pH = 5.0 ± 0.1 T = 298 K, pH = 6.9 ± 0.2 T = 284 K, pH = 5.5 T = 298 ± 1 K, pH = 4.00 ± 0.02 T = 298 K, pH = 5

qmax (mg/g)

ref

81

36

68

36

185

36

268

16

303 373 122 310

14 15 37 this work

Figure 9. Competitive sorption of coexisting ions by KIT-6-DAPhen (Cmetal ions = 0.5 mmol/L).

∼40 mg/g at pH 4.0, while the value at pH 5.0 reaches ∼120 mg/g. In addition, the U(VI) uptake decreases compared with that in the absence of competing ions, which means there is a negative effect of the competing ions on the U(VI) sorption. However, the KIT-6-DAPhen sorbent exhibits superior sorption of U(VI) over other metal ions. The selectivity coefficient is commonly used to assess the selectivity of a sorbent for a certain metal ion. Herein, the selectivity coefficient (SU/M) for U(VI) relative to competing ions is defined as38

Figure 8. U(VI) sorption onto KIT-6-DAPhen at different temperatures (pH 5.0, msorbent/Vsolution = 0.4 mg mL−1, [U]initial = 150 mg L−1). The inset shows the plot of ln Kd vs 1/T.

SU/M =

that the U(VI) uptake onto KIT-6-DAPhen increases with increasing temperature, indicating that a higher temperature would be more favorable for the U(VI) sorption. It is wellknown that the temperature dependence of a sorption process is associated with changes in the thermodynamic parameters. Herein the three basic parameters, the standard free energy (ΔG°), standard enthalpy (ΔH°), and standard entropy (ΔS°) were derived from the equations ΔG° = ΔH° − TΔS° and ln Kd = −ΔH°/RT + ΔS°/R, where Kd is the distribution coefficient (in mL g−1), R is the gas constant (8.314 J mol−1 K−1), and T is the absolute temperature (in K). The results are shown in the Figure 8 inset and Table 2. The positive value of

ΔH° (kJ mol−1)

ΔS° (J mol−1 K−1)

ΔG° (kJ mol−1)

298 308 328

11.2

108.7

−21.2 −22.3 −24.5

KdM

where KUd and KM d are the distribution coefficients of U(VI) and competing ions, respectively. It is found that the SU/M value for U(VI) relative to all the competing ions is larger than 11 at pH 5.0, revealing that the KIT-6-DAPhen sorbent shows a desirable selectivity for U(VI) over a range of competing metal ions at this pH. This result is clearly understandable since there are several fundamental differences between uranyl ion and the other tested metal ions, e.g., different sizes, different effective charges, and different ionic potentials. In future work we will compare the selectivity of other actinides over lanthanides by KIT-6-DAPhen to confirm our previous conclusion that the preorganized tetradentate phenanthrolineamide (DAPhen) ligand with combined hard and soft donors shows selective extraction ability toward actinides over lanthanides.21 Stability Assessment, Desorption, and Reusability of the Sorbent. XRD and BET techniques were employed to gain knowledge about the structural and textural evolution of the sorbent upon its interaction with U(VI), as shown in Figure S6. It was observed that the small-angle XRD pattern of KIT-6DAPhen showed no large changes following U(VI) sorption, suggesting that the ordered mesostructure of the sorbent is maintained during the sorption. The decrease in intensity is reasonably attributed to disturbance of the sorbed U(VI) ions. The N2 adsorption−desorption measurements show that after U(VI) sorption, both the surface area and the pore volume of KIT-6-DAPhen were slightly reduced, and these reductions are a certain indication that the sorbed U(VI) ions enter into the sorbent pores.

Table 2. Thermodynamic Parameters for U(VI) Sorption onto KIT-6-DAPhen T (K)

KdU

ΔH° (11.2 kJ mol−1) indicates the endothermic nature of U(VI) sorption onto the KIT-6-DAPhen sorbent, while the negative values of ΔG° suggest the spontaneous nature of the sorption process. Besides, the more negative values of ΔG° at higher temperatures further confirm that a higher temperature is more favorable for the sorption process. Selectivity Tests. The sorption of U(VI) by KIT-6-DAPhen from aqueous solutions containing Co(II), Ni(II), Zn(II), 3780


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ACS Applied Materials & Interfaces To desorb the U(VI) ions from KIT-6-DAPhen, the U(VI)loaded sorbent was treated using HNO3 solution. Quantitative desorption of U(VI) using various concentrations of HNO3 is shown in Table S5. It was found that most of the U(VI) ions (>88%) were desorbed from KIT-6-DAPhen using 0.05 mol L−1 HNO3 solution, while complete (∼100%) desorption of U(VI) was achieved by using 0.5 mol L−1 or more concentrated HNO3 solution. To further evaluate the stability of the sorbent following U(VI) desorption, the reusability of the KIT-6-DAPhen sorbent was tested by equilibrating 4 mg of reclaimed KIT-6DAPhen with 10 mL of U(VI) solution (100 mg L−1) at pH 5. After 4 h of sorption and determination of the solution U(VI) concentration, the desorption of U(VI) was performed as described above, thus forming a sorption−desorption cycle. For comparison, a reference sorption experiment was also conducted using fresh KIT-6-DAPhen under the same conditions during each sorption−desorption cycle. The results are listed in Figure 10. It can be seen that the U(VI) sorption

Table 3. U(VI) Extraction from Simulated Seawater by KIT6-DAPhen (4 mg/10 mL) U concentration (μg/L) pH

C0

Ce

U extraction (%)

8.30 8.27 8.33

3.8 16.5 42.0

0.0055 0.026 0.042

>99 98.3 98.0

Interaction between Uranyl and KIT-6-DAPhen. To elucidate the binding modes of the U(VI) ion onto KIT-6DAPhen, we simulated uranyl complexes theoretically. To simplify the computation, DAPhen siloxane (L) was selected as a molecular model, but the Si atoms in the model ligand were fixed during the modeling process to simulate the case that the DAPhen ligand is anchored to the silica surface via the silanol groups. Three structures of the uranyl complexes with DAPhen siloxane ligand were optimized at the B3LYP/6-31G(d) level of theory (Figure 11). In these structures, the ligand is coordinated in a tetradentate chelating mode through two nitrogen donors of the phenanthroline moiety and two oxygen donors of the amide moieties. Unlike the [UO2L]2+ complex, in the [UO2L(H2O)2]2+ and [UO2L(NO3)]+ complexes one uranyl ion is also coordinated with two water molecules and one nitrate anion, respectively. The selected bond lengths are listed in Table 4. It can be seen that the UO bond length in the [UO2L(NO3)]+ complex is about 1.766 Å, the longest bond length in the three complexes. The distance between the uranium atom and the nitrogen atom of the phenanthroline moiety (U−NL) is longer than that between the uranium atom and the oxygen of the amide moiety (U−OL) for each complex. In [UO2L(NO3)]+ complex, for example, the length of the U− NL bond is about 2.74 Å, which is 0.29 Å longer than that of the U−OL bond. The U−NL and U−OL bond distances increase when the water molecules and nitrate anion are considered. In addition, the distance between the uranium atom and the oxygen atom of the nitrate anion (U−Onitrate) is shorter than that between the uranium atom and the oxygen atom of the water molecule (U−Owater), which indicates that U(VI) ion binds more strongly with the oxygen atom of the nitrate anion than that of the water molecules. To determine the nature of the bonding between the uranyl ion and the DAPhen siloxane ligand, we investigated the Mayer bond order (MBO) and the electron density (ρ) at the bond critical point (BCP) (Table 4). It is important to point out that the MBO for the UO bond is about 2.00, an indication that the double-bond character is maintained. For the U−OL bonds, the largest value is only about 0.44 in the [UO2L]2+ complex. For the U−Onitrate bond, the MBO is about 0.40, which is higher than that for the U−Owater bond, which suggests that the affinity of U(VI) ion for the nitrate anion is stronger than that for water molecules. These results are in accordance with the analysis of the corresponding bond lengths. The nature of the bonding between the U(VI) ion and the DAPhen siloxane ligand was also revealed by the topological analysis of the electron density. As we know, the bonding interactions can be characterized and classified according to the properties of the electron density. In general, when the electron density at the BCP is ρ(r) > 0.20 a.u., the bond can be described as a covalent bond, while ρ(r) < 0.10 au indicates an ionic bond. The electron densities of the UO bonds are >0.30, indicating that they are covalent bonds. For the other bonds connected to the uranium atom, their electron densities are 98% extraction was observed. At C0 = 3.8 ppb (close to the U(VI) concentration in real seawater), the U(VI) extraction was almost complete (>99%). Such a result clearly reveals the vast opportunity to apply this kind of sorbent in U(VI) uptake from seawater. Further detail work, however, should be done before its real application. 3781


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Figure 11. Optimized structures of the uranyl complexes with the ligand. H, C, N, O, and U atoms are represented by gray, green, blue, red, and pink spheres, respectively.

Table 4. Selected Bond Lengths (B, in Å), Mayer Bond Orders (MBO), and Electron Densities (ρ, in a.u.) at the Bond Critical Point for the Uranyl Complexes B

MBO

ρ

species

UO

U−NL

U−OL

U−Onitrate

[UO2L(NO3)]+ [UO2L(H2O)2]2+ [UO2L]2+ [UO2L(NO3)]+ [UO2L(H2O)2]2+ [UO2L]2+ [UO2L(NO3)]+ [UO2L(H2O)2]2+ [UO2L]2+

1.767 1.758 1.755 2.002 2.027 2.060 0.317 0.324 0.326

2.740 2.669 2.541 0.205 0.269 0.326 0.036 0.043 0.060

2.451 2.454 2.324 0.298 0.331 0.440 0.056 0.057 0.078

2.445

U−Owater 2.582

0.402 0.157 0.064 0.043

Figure 12. Diagrams of canonical valence MOs and the corresponding energies (eV) of the complex [UO2L(NO3)]+. H, C, N, O, and U atoms are represented by gray, green, blue, red, and pink spheres, respectively. The negative and positive portions of the MOs are shown in yellow and blue, respectively.

that the interactions between the U(VI) ion and the DAPhen siloxane ligand are predominately ionic. In order to further understand the interaction between the uranyl cation and the ligands, the canonical valence molecular orbitals (MOs) relevant to the uranyl cation and ligands are provided. Because the MOs of three complexes are similar, here we show only the MOs of the complex [UO2L(NO3)]+ (Figure 12); the atomic orbital compositions of these MOs are listed in Table S6. It is worth mentioning the MOs possess σ character and are mainly contributed by the p atomic orbital of the oxygen/nitrogen atom of the ligand and the f and d orbitals of the uranium atom. In addition, the percentage of the f orbital is predominant in the MOs, indicating that the f orbitals make significant contributions to all of the MOs. In order to explore the complexation mechanism of the uranyl cation and the KIT-6-DAPhen ligand, three thermodynamic reactions were considered. As shown in Table 5, the changes in Gibbs free energy are negative, which indicates that these three reactions seem to be thermodynamically favorable. However, the change in Gibbs free energy for the reaction L + [UO2(H2O)5]2+ + NO3− ⇄ [UO2L(NO3)]+ + 5H2O (−390.11 kcal/mol) is the most negative among the three reactions, revealing that the most likely mechanism of sorption of U(VI)

Table 5. Calculated Changes in Gibbs Free Energy (ΔGg, in kcal/mol) for Reactions of Uranyl Cation with L in the Gas Phase reaction

ΔGg

L + [UO2(H2O)5]2+ + NO3− ⇄ [UO2L(NO3)]+ + 5H2O L + [UO2(H2O)5]2+ ⇄ [UO2L(H2O)2]2+ + 3H2O L + [UO2(H2O)5]2+ ⇄ [UO2L]2+ + 5H2O

−390.11 −245.85 −219.71

by KIT-6-DAPhen is described by this reaction. This result is consistent with the reported DAPhen−U(VI) complex in our previous works.21 Four of the six coordination sites in the equatorial plane of UO22+ are occupied by the tetradentate DAPhen ligand, and the remaining two are insufficient to bind with another tetradentate ligand. Moreover, the steric hindrance of the bulky DAPhen also limits the approach of water molecules and nitrate ions to the first coordination sphere of UO22+, and thus, one of the two nitrate ions acts as both a counterion and a chelating agent in the inner sphere, while the other one is only a counterion in the outer sphere. 3782


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CONCLUSION We have reported here a new, efficient, and feasible U(VI) sorbent, phenanthrolineamide-functionalized mesoporous KIT6 (KIT-6-DAPhen). The efficiency is clearly demonstrated by the fast sorption kinetics of ca. 10 min, the large sorption capacity of more than 300 mg/g at a relatively low pH, and the desirable selectivity for U(VI) over a range of competing metal ions. The practicability arises from the fact that the sorbent shows good potential for reusability, which would reduce the cost of its practical application. With regard to the detailed sorption mode, the preorganized phenanthroline moiety of KIT-6-DAPhen provides rigidity, while the lateral carbon chains provide a certain flexibility. When U(VI) ions are incorporated, the flexibility of the lateral carbon chains allow the oxygen donors of the amide moieties to rotate toward the nitrogen side of phenanthroline, finally forming most favorable complex geometry as denoted by the DFT calculations. In addition, the thermodynamic analysis further indicated that the sorption might be described by the reaction KIT-6-DAPhen + [UO2(H2O)5]2+ + NO3− ⇄ [UO2(KIT-6-DAPhen)(NO3)]+ + 5H2O. This work provides new clues for assessing the feasibility of mesoporous silica as a support material for recovery of uranium from wastewater and/or preconcentration of uranium from seawater. Further work is in progress to test the adsorption ability of KIT-6-DAPhen toward americium, thorium, and europium and to determine the radiation stability of this new sorbent in order to assess its feasibility for application in group separation of actinides over lanthanides.

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work were obtained on the ScGrid of the Supercomputing Center, Computer Network Information Center of the Chinese Academy of Sciences.

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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/acsami.6b15642. 1 H NMR spectra of DAPhen siloxane; calculated aqueous speciation of U(VI); U(VI) sorption data obtained from fits to kinetic and isotherm models; TGA profile of the DAPhen ligand; compositions of the coexisting ions solution in the selectivity test; XRD patterns and N2 sorption/desorption isotherms of KIT-6DAPhen before and after U(VI) sorption; desorption of U(VI) from KIT-6-DAPhen; percent contributions of U, the ligand NL and OL atoms, and the nitrate ON atom to the delocalized canonical MOs for the [UO2L(NO3)]+ complex (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel: 86-10-88233968. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei-Qun Shi: 0000-0001-9929-9732 Author Contributions #

L.-Y.Y. and L.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21471153, U1432103, 21577144, and 11675192), and the Science Challenge Project (JCKY2016212A504). Some of the results described in this 3783


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