- Email: [email protected]
Influence of carbonate on sequestration of U(VI) on perovskite
Accepted Manuscript Title: Influence of carbonate on sequestration of U(VI) on perovskite Authors: Songhua Lu, Kairuo Zhu, Tasawar Hayat, Njud S. Alharbi, Changlun Chen, Gang Song, Diyun Chen, Yubing Sun PII: DOI: Reference:
S0304-3894(18)30944-0 https://doi.org/10.1016/j.jhazmat.2018.10.035 HAZMAT 19855
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
21-6-2018 11-10-2018 12-10-2018
Please cite this article as: Lu S, Zhu K, Hayat T, Alharbi NS, Chen C, Song G, Chen D, Sun Y, Influence of carbonate on sequestration of U(VI) on perovskite, Journal of Hazardous Materials (2018), https://doi.org/10.1016/j.jhazmat.2018.10.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of carbonate on sequestration of U(VI) on perovskite Songhua Lua,b, Kairuo Zhub, Tasawar Hayatc, Njud S. Alharbid, Changlun Chenb,c,d, Gang Songe, Diyun Chene, Yubing Suna,* a
College of Environmental Science and Engineering, North China Electric Power
IP T
University, Beijing 102206, PR China b
Key Laboratory of Photovoltaic and Energy Conversation, Institute of Plasma
c
SC R
Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, 230031, PR China
NAAM Research Group, Department of Mathematics, Faculty of Science, King
Department of Biological Sciences, Faculty of Science, King Abdulaziz University,
N
d
U
Abdulaziz University, Jeddah 21589, Saudi Arabia
Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and
M
e
A
Jeddah, Saudi Arabia
ED
Resources, Guangzhou 510006, PR China
PT
* Corresponding author: E-mail: [email protected] (Y. Sun).
A
CC E
Graphical Abstract
1
IP T SC R U
Highlights
Cubic perovskite was successfully synthesized by a facile solvothermal method
Inner-sphere complexation dominated U(VI) sequestration under neutral
M
A
N
condition
Abstract
ED
Carbonate inhibited the sequestration and photocatalytic reduction of U(VI)
PT
CC E
Cubic perovskite (CaTiO3) was successfully synthesized by a facile solvothermal method and was utilized to sequestrate U(VI) from aqueous solutions. The batch
A
experiments revealed that carbonate inhibited U(VI) sequestration at pH > 6.0 due to the formation of uranyl-carbonate complexes. The maximum sequestration capacity of U(VI) on perovskite was 119.3 mg/g (pH 5.5). The sequestration mechanism of U(VI) on perovskite were investigated by XPS and EXAFS techniques. According to XPS 2
analysis, the presence of U(IV) and U(VI) oxidation states revealed the photocatalytic reduction of U(VI) by perovskite under UV-vis irradiation. In addition, photocatalytic reduction performance significantly decreased in the presence of carbonate. Based on EXAFS analysis, the occurrence of U-Ti and U-U shells revealed the inner-sphere
IP T
surface complexation and reductive precipitation of U(VI) on perovskite. These findings herein are crucial for the application of perovskite-based composites in the
SC R
decontamination of U(VI) in aquatic environmental cleanup.
Keywords: Perovskite; EXAFS; U(VI); Sequestration; Carbonate.
U
1. Introduction
N
Uranium could discharge into sub-environments during mining of uranium and
A
reprocessing of spent nuclear fuel, which pose serious threats to aquatic life and
M
human health due to its radioactivity and chemical toxicity [1-3]. Consequently, the
ED
sequestration of uranium from the contaminated sites, including adsorption, redox and surface co-precipitation, is a topic of major concern nowadays [4-7]. In addition, the
PT
sequestration of uranium was strongly influenced by various environmental factors
CC E
such as pH and foreign ligands. For example, U(VI) and carbonate can generate stable uranyl-carbonate complexes (UO2(CO3)22-, UO2(CO3)34-) under circumneutral and alkaline conditions [8-11], which strongly suppressed the sequestration of U(VI) on
A
solid adsorbents due to the electrostatic repulsion [8,12]. Owing to the high adsorption performance, titanate-based materials have been extensively applied in sequestration uranium in recent years [13, 14]. García-Rosales et al., investigated the removal of U(VI) onto SrTiO3 was relative to temperature [15]. 3
Perovskite-structure titanate as semiconductor material presents the strong light absorption in the UV-light range (< 390 nm) due to the relative large band gap [16, 17]. Recently, it is demonstrated that perovskite displayed excellent adsorption and/or photocatalytic activity for heavy metals or organic pollutants [18-21]. In these studies,
IP T
it is found that the high adsorption of Pb2+, Cd2+ and Zn2+ on perovskite was attributed to adsorption, whereas the As(III) removal and dye degradation was due to the
SC R
favorable photocatalytic activity under UV-light conditions. However, the limited
reports regarding the effect of carbonate on the adsorption and photocatalytic activity
U
of uranium towards perovskite were available nowadays.
N
The objectives of this study are to (1) synthesize perovskite by a facile solvothermal
A
process and characterize it by XRD, FT-IR, SEM and XPS techniques; (2) investigate
M
the influence of carbonate on U(VI) sequestration by perovskite under different pH
ED
condition; (3) determine the sequestration mechanism of perovskite towards U(VI) using UV-Vis, XPS and EXAFS techniques. These findings are expected to provide a
PT
new understanding of the perovskite-type materials for the sequestration of U(VI) in
CC E
actual environmental cleanup.
2. Experimental details 2.1 Materials
A
U(VI) stock solutions (1.0 mmol/L) were obtained by dissolving UO2(NO3)2·6H2O (analytical reagent, Hubei Chushengwei Chemical Co. Ltd.) into ultrapure water. Other chemicals of analytical grades (i.e., Na2CO3, Ca(NO3)2, Ti(OC4H9)4, NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. 4
2.2 Synthesis and characterization of perovskite Perovskite was synthesized through a facile solvothermal method [24, 25]. Typically, 1.0 mmol Ca(NO3)2 was dissolved into 20 mL polyethylene glycol (PEG-200) under ultrasonic conditions. Afterwards, 0.33 mL of Ti(OC4H9)4 solution was added
IP T
drop-wise into the above mixture, then 0.88 g NaOH was added under vigorous magnetic stirring. Subsequently, the suspension was heated at 180 °C for 15 h. The
SC R
perovskite were obtained by washing it with acetone, diluted acetic acid, distilled water several times and then centrifugation it at 6000 rpm for 30 min.
U
The mineralogy of as-prepared perovskite was characterized by X-ray diffractometer
N
(XRD, Rigaku Mini Flex 600, Japan). The morphology of the perovskite was
A
performed using SEM (FEI-JSM 6320F). The FT-IR spectra were obtained by using a
M
FT-IR spectrophotometer (JASCO FTIR 410). Thermo Escalab 250 X-ray
ED
photoelectron spectrometer with Al Kα radiation at 150 W was used to detect the surface groups and fitting of O 1s, Ti 2p and U 4f peaks was analyzed using the
PT
XPSPEAK v. 4.1 software. Zeta potentials at different pH were recorded by Zetasizer
CC E
Nano ZS (Malvern Instruments). UV-Vis absorption spectrum was performed using Shimadzu UV-2500 UV-Vis spectrophotometer. 2.3 Batch sequestration experiments
A
The triplicate sequestration of U(VI) (20 mg/L) onto perovskite (0.2 g/L) were performed in polycarbonate tubes at glovebox condition (293 K). The experiments were conducted with the daylight (indoor) in day, and in the night were irradiated with a 300 W xenon lamp equipped with a cut-off UV filter (λ ≥ 400 nm). The irradiation 5
of UV-light experiment was conducted using a 300 W Xenon lamp equipped with a 250-380 nm UVREF filter. Typically, perovskite and NaClO4 were pre-equilibrated for 24 h, and then U(VI) stock solutions were spiked into the bulk suspension gradually to avoid the generation of uranium precipitate at high pH. The pH was
IP T
adjusted by using dilute HClO4 or NaOH solutions (0.1-1.0 mol/L). After sequestration equilibrium (24 h), the solid phases were separated from liquid phases
SC R
by centrifugation at 9500 rpm for 10 min, and then the supernatant was filtered
through a 0.22 μm nylon membrane. The concentration of U(VI) was analyzed using
U
spectrophotometric method with arsenazo-III as the chromogenic agent at wavelength
N
of 652 nm. The desorption of U(VI) from perovskite was conducted for XPS analysis.
A
Briefly, the uranium-bearing perovskite was obtained by centrifuging it after
M
adsorption equilibrium. Then 0.05 mol/L Na2CO3 was added under vigorous stirring
ED
conditions. After desorption 12 h, perovskite was obtained by centrifuging it and drying it for XPS analysis. The sequestration percentage (Sequestration %) and
ion
(%)
CC E
Sequestrat
PT
capacity (Qe (mg/g) were calculated according to Eqns. (1) and (2), respectively:
Qe
C0 Ce
100 %
(1)
C0
( C 0 C e ) V
(2)
m
A
where C0 (mg/L) and Ce (mg/L) are the initial U(VI) concentration and the aqueous U(VI) concentration after sequestration equilibrium, respectively. The m (g) and V (mL) are the mass of perovskite and the volume of the suspension, respectively. 2.4 Preparation and analysis of XANES and EXAFS spectra 6
The perovskite-U(VI) sample was prepared at pH 7.5 at glovebox conditions. The detailed preparation process as the following protocols: the perovskite, NaClO4 (0.01 mol/L) and Milli-Q water were weighted into 250 mL flask bottle, and then UO2(NO3)2 solution was gradually dropwise added to the suspension under vigorously
IP T
stirring condition to avoid the formation of U(VI) precipitate (m/V = 0.2 g/L). Afterwards, the solution was adjusted to pH 7.5 by adding negligible volume HClO4
SC R
or NaOH dilute solution. After equilibration, the experimental system was centrifuged at 9500 rpm for 10 min. The wet pastes were placed in a sealed chamber under
U
vacuum condition for the XANES and EXAFS measurement. The solid U(IV)O2(s)
N
and 1 mmol/L UO2(NO3)2 solution were selected as U(IV) and U(VI) standards,
A
respectively. The uranium LIII-edge (17179 eV) XANES and EXAFS spectrum was
M
conducted at Shanghai Synchrotron Radiation Facility (SSRF, BL14W beamlines)
ED
using Si(111) double-crystal monochromator with 30-element solid-state Ge detector. The spectra of U(IV)O2 crystalline and U(VI)-containing samples were measured in
PT
transmission and fluorescence mode, respectively. The data of all samples were
CC E
recorded at a relatively small grazing-incidence angle to avoid or minimize selfabsorption effects. The optimized parameters (i.e., path, coordination number (CN), interatomic length (R), Debye-Waller factor (σ2)) were obtained by setting them as
A
default values during the fitting process. The κ3-weighted EXAFS data were processed by Athena and Artemis interfaces to IFFEFIT 7.0 software [26, 27].
3. Results and discussion 7
3.1 Characterization The mineralogy of perovskite before and after U(VI) sequestration were identified by XRD patterns. As shown in Fig. 1A, the diffraction peaks at 2θ = 23.23, 32.89, 47.50 and 59.04° were well consistent with the (110), (020), (220) and (204) planes of
IP T
orthorhombic CaTiO3 phase (JCPDS No.82-0228), respectively [24, 25]. Further, no obvious changes of XRD patterns for perovskite after U(VI) sequestration under
SC R
daylight and UV-light condition indicated no distinct destruction of crystal structure during the sequestration process. The FT-IR spectra of perovskite before and after
U
U(VI) sequestration were shown in Fig. 1B. Major absorption bands at 3432 and 1630
N
cm−1 could be belonged to O-H (hydroxyl groups) stretching vibration and H-O-H
A
(bond water molecule) binding vibration, respectively [28]. The absorption band at
M
1400 cm−1 was attributed to the coexisted NO3− groups [29]. The absorption band at
ED
620 cm−1 can be attributed to the Ti-O-Ti stretching vibration [30]. Similar with the XRD patterns, the FT-IR spectra of perovskite after sequestration of U(VI) were
PT
identical to that of perovskite, implying no change of the perovskite structure after
CC E
sequestration process. The SEM image was used to characterize the morphology of perovskite. As shown by SEM image in Fig. 1C, the cubic structure with heterogeneous sizes was observed. As shown by zeta potentials in Fig. 1D, the
A
isoelectric points of perovskite in 0 and 10.0 mmo/L Na2CO3 solutions were calculated to be ~3.9 and 3.4, respectively, indicating that negative charge was observed at pH > 4.0. N2-BET surface area, pore volume and average pore diameter of perovskite was 126.055 m²/g, 0.103 cm³/g and 2.853 nm, respectively (Fig. S1A). 8
Such high surface area and suitable pore size could provide abundant sequestration sites for the sequestration process. 3.2 pH and carbonate effect The sequestration of U(VI) on perovskite affected by carbonate under different pH
IP T
condition was shown in Fig. 2A. Clearly, The sequestration performance of perovskite towards U(VI) increased dramatically with increasing pH from 3.0 to 6.0, then
SC R
maintained the high-level sequestration at pH 6.0-7.0, whereas the decreased
sequestration trend was observed at pH > 7.0. The pH-dependant sequestration
U
performance of perovskite towards U(VI) could be attributed to the electrostatic
N
interaction. Fig. 1D shows that the perovskite revealed a negatively charged surface at
A
pH > 4.0. Additionally, the mainly U(VI) species were positive charged species (e.g.,
M
UO2OH+, UO22+, (UO2)3OH5+ and (UO2)4OH7+ species) at pH < 6.0 in aqueous
ED
solutions (Fig. S1B), whereas the uranyl-carbonate species (UO2(CO3)22- and
(Fig. 2B).
PT
UO2(CO3)34-) were predominately formed under circumneutral and alkaline condition
CC E
Therefore, the increased sequestration performance of perovskite toward U(VI) at pH < 6.0 could be attribute to the strong electrostatic attraction generated by negatively charged surface of the perovskite and cationic U(VI) species [31]. The high
A
sequestration of U(VI) on perovskite at neutral pH could be ascribed to the surface complexation of uranyl with hydroxyl groups of perovskite and/or surface co-precipitation such as schoepite [32]. However, the significant decrease of U(VI) sequestration at pH > 7.0 was attribute to the electrostatic repulsion between 9
negatively charged surface of the perovskite and anionic uranyl species. After addition of Na2CO3, the slight enhance and great decrease of U(VI) sequestration was exhibited at pH < 6.0 and > 6.0, respectively. At pH < 6.0, the sequestration performance of perovskite towards U(VI) at 1.0 mmol/L Na2CO3 was slightly lower
IP T
than that of sequestration performance at 10.0 mmol/L Na2CO3, whereas the lower sequestration performance at 10.0 mmol/L Na2CO3 was observed at pH > 6.0 (Fig.
SC R
2A). As shown in Fig. 1D, the addition of carbonate significantly decreased the zeta potential of perovskite. Besides, the uranyl-carbonate species were observed at pH >
U
4.0 (Fig. 2B). Therefore, the increased sequestration of U(VI) on perovskite at pH <
N
6.0 with increasing carbonate concentration was attributed to the surface
A
complexation and/or electrostatic attraction of positive U(VI) species and negative
M
charged of perovskite, whereas the significant decrease of U(VI) sequestration at
ED
pH > 6.0 could be due to the electrostatic repulsion between negative charged perovskite and negative U(VI) species such as UO2(CO3)34- species. Therefore, the
PT
influence of carbonate on U(VI) sequestration at low and high pH was also ascribed to
CC E
the electrostatic attraction and repulsion, respectively. 3.3 Sequestration kinetics and isotherms The sequestration kinetics and isotherms of U(VI) onto perovskite (pH 5.5, 7.5) are
A
shown in Fig. 2C and 2D, respectively. The comparison kinetic sequestration of U(VI) on perovskite under daylight and xenon lamp light (λ ≥ 400 nm) was showed in Fig. S2 of SI. No obvious difference of sequestration result indicated the feasibility of the xenon lamp light (λ ≥ 400 nm) as an alternative light source in the night for the 10
kinetics experiments. The sequestration of perovskite toward U(VI) significantly enhanced with increasing sequestration time from 0 to 4 h, then slight enhancement of U(VI) sequestration was observed at long-term reaction time (4~30 h). After addition of Na2CO3 (10.0 mmol/L), the significant decreased sequestration of U(VI) was
IP T
observed at pH 7.5. The decrease of U(VI) sequestration at high pH could be due to the formation of uranyl-carbonate complexation, which were not favorable for U(VI)
SC R
sequestration in aqueous system [33, 34]. In the similar way, the slightly enhanced
sequestration of U(VI) under acidic condition with Na2CO3 was assigned to the
U
electrostatic attraction generated by negatively charged surface of perovskite and
N
cationic U(VI) species (e.g., UO22+). The pseudo-first-order and pseudo-second-order
A
kinetic models were applied to fit the sequestration kinetic data of U(VI). The more
M
detail information of the fitting result was provided in SI. Fig. S3 and Table S2 show
ED
the fitted correlation coefficients were R2 < 0.935 and R2 > 0.999 for pseudo-first-order model and pseudo-second-order model, respectively, indicating that
PT
sequestration kinetics of U(VI) was well fitted with the latter model.
CC E
The sequestration isotherms of U(VI) on perovskite at pH 5.5 and 7.5 were shown in Fig. 2D. The sequestration of U(VI) at 5.5 was obviously lower than that of U(VI) sequestration at pH 7.5. The data of sequestration isotherms were fitted by Langmuir
A
and Freundlich models. More details regarding Langmuir and Freundlich models were provided in SI. As shown in Fig. S4 and Table S3, the sequestration isotherms of U(VI) on perovskite at pH 5.5 and 7.5 can be satisfactorily fitted by Langmuir model with high correlation coefficient (R2 ≥ 0.99) compared to Freundlich model (R2 ≤ 0.97). 11
The maximum sequestration capacities of U(VI) at pH 5.5 and 7.5 were 119.3 and 126.8 mg/g, respectively. To demonstrate the excellent sequestration performance of perovskite, the comparison of maximum sequestration capacities of different titanate-based adsorbents towards U(VI) were summarized in Table 1 [35-39]. The
IP T
sequestration performance of perovskite is higher than yolk-shell magnetic titanate nanosheets (Fe3O4@TNS) (82.85 mg/g at pH 5.0 and 298 K) [35], defective TiO2−x
SC R
(65.41 mg/g at pH 5.0 and 298 K) [36], Fe3O4@TiO2 (91.1 mg/g at pH 6.0 and 298 K)
[37], graphene oxide nanosheets (GONS) (97.5 mg/g at pH 5.0 and 293 K) [38],
U
oxidized multiwalled carbon nanotubes (MWCNTs) (33.32 mg/g at pH 5.0 and 293 K)
N
[39]. These results suggested that perovskite can be used as a promising adsorbent for
A
the immobilization of U(VI) from aqueous solutions in actual environmental cleanup.
M
3.4 Comparison sequestration experiments
ED
Fig. 3A shows the effect of carbonate on U(VI) sequestration towards perovskite at pH 7.5 under different light irradiation conditions. Typical, the comparison of
PT
sequestration experiments were firstly conducted under dark condition for 2 h, and
CC E
then samples were irradiated with daylight or UV-light for 3 h. It was found that the significant enhanced sequestration capacity at pH 7.5 under UV-light condition compared to that under daylight condition in the absence of carbonate. Nevertheless,
A
no obvious changes of U(VI) sequestration was observed in the presence of carbonate under daylight and UV-light conditions. These results indicated that the presence of carbonate significantly inhibited the U(VI) sequestration. Moreover, the photocatalytic reduction of U(VI) was also hindered in the presence of carbonate. 12
The photocatalytic activity of U(VI) on perovskite was determined by analysis of UV-Vis absorption spectra. As shown in Fig. 3B, an optical steep and strong absorption peak was observed at ~380 nm. The band gap energy (Eg) of perovskite was calculated to be about 3.18 eV. The conduction band potential (ECB) and valence
IP T
band potential (EVB) were calculated to be -0.69 and 2.49 V, respectively (SI). Further, the negative conduction band potential (ECB = -0.69 V) of perovskite was
SC R
lower than that of positive redox potential of U(VI)/U(IV) (∼ + 0.411 V) at pH 6.9 [40]. Generally, the photocatalytic reduction includes two ways: (i) the
U
photogenerated electrons directly react with adsorbate; (ii) the photo-generated holes
N
derived from the oxidizing of organic matter can produce intermediate to reduce
A
adsorbate [41]. In this study, the photocatalytic activity of perovskite was excited by
M
UV-light irradiation. As a consequence, the peroskite generated the photo-generated
ED
electrons, which directly participated in the reduction of U(VI) to U(IV) [40]: (perovskite) + hv (< 390 nm) → perovskite (e- +h+)
(6)
PT
U(VI)+2 e- → U(IV)
(5)
CC E
Therefore, the significant enhanced sequestration capacity under UV-light condition (pH 7.5) was attributed to the photocatalytic reduction of U(VI). 3.5 XPS analysis
A
Fig. 4 shows the XPS spectra of perovskite before and after U(VI) sequestration (noted as P and P-U(VI)). As shown by survey scans in Fig. S5, the relative intensities of U 4f peak for P-U(VI)-Na2CO3 sample was lower than that of P-U(VI), suggesting that carbonate inhibited the U(VI) sequestration. Additionally, the higher relative 13
intensities of U 4f peak under UV-light irradiation indicated the increased photocatalytic activity of U(VI) on perovskite. As shown in Fig. 4A, O 1s peaks can be deconvoluted into three sub-peaks at ~ 529.5, 531.0 and 532.0 eV, which were ascribed to crystal lattice oxygen of TiO6, -OH of adsorbed H2O/acidic Ti–OH(a) and
IP T
-OH of basic Ti–OH(b), respectively [42-44]. As shown in Fig. 4A-B, the binding energy of O 1s and Ti 2p peaks after U(VI) sequestration were slightly shifted to the
SC R
lower binding energy, implying coordination of U(VI) with the oxygen-containing groups of the perovskite [45, 46].
U
As shown in Fig. 4C, two doublet peaks of U 4f peaks at ∼382 and 392 eV were
N
assigned to U 4f 7/2 and U 4f 5/2, respectively [47]. Moreover, U 4f 7/2 and U 4f 5/2
A
peaks can be fitted with the components at 380.0/381.6 eV and at 390.8/392.5 eV,
M
respectively. The peaks at 380.0/390.8 and 381.6/392.5 eV were corresponded to
ED
U(IV) and U(VI) phase, respectively [33, 48]. Moreover, the relative intensity of U(IV) phase for P-U(VI)-Na2CO3 was lower than that of U(IV) phase for P-U(VI),
PT
indicating that the existence of carbonate influenced the photocatalytic reduction of
CC E
U(VI) due to the formation of uranyl-carbonate complexes [22, 23]. Hua et al. also found that the U(VI)-carbonate species could inhibit the U(VI) reduction by hydrogen sulfide at high carbonate concentration [22]. Yan et al. concluded that carbonate
A
inhibited U(VI) reduction by nanoscale zero valent iron due to the formation of U(VI)-carbonate complexes, which decreased U(VI) adsorption and suppressed the U(VI) reduction [23]. Furthermore, the relative intensities of U(IV) phase in P-U(VI) and P-U(VI)-Na2CO3 samples under UV-light condition was higher than that of U(IV) 14
phase under daylight condition, suggesting that the UV-light irradiation was benefited to the photocatalytic reduction of U(VI) phase. As shown in Fig. 4D, the low relative intensities of U 4f peaks after desorption indicated that most of uranium was desorbed from perovskite by 0.05 mol/L Na2CO3. However, the relative intensities of U(IV)
IP T
component was significantly higher than that of U(VI), indicating U(IV) species generated from photocatalytic reduction were stabilized and hardly desorbed by
SC R
carbonate [49]. Besides, the relative intensities of Na 1s and Na KLL were apparent
increased, which indicated that the Na+ ions adsorbed on the perovskite surface in the
U
desorption process due to the Na2CO3 as the desorption solution.
N
3.6 XANES and EXAFS analysis
A
The uranium LIII-edge XANES and Fourier transformed (FT) EXAFS spectra of
M
U(IV)O2, U(VI)O22+ and U(VI)-loaded samples were shown in Fig. 5A-B. Table 2 listed
ED
the optimized parameters of EXAFS analysis, and the fitted data of U(IV)O2 crystalline were derived from the previous study [50]. Fig. 5A shows the energies of absorption
PT
edge of U(IV)O2 and U(VI)O22+ were ~ 17176 and 17179 eV, respectively [33], and that
CC E
of U-loaded perovskite at pH 7.5 was ~ 17178 eV located between U(IV)O2 and U(VI)O22+ standards. The above result implied that a fraction of sequestrated U(VI) phase was photocatalytic reduced to U(IV) phase due to the photocatalytic activity of
A
perovskite. As shown in Fig. 5B, the FT features at ~ 1.4 Å can be fitted by two axial oxygen shell (U-Oax) at 1.78 Å, whereas the second FT features at ~ 1.9 Å can be fitted by 5-6 equatorial oxygen shells at approximately 2.35 Å (~two U-Oeq1) and 2.58 Å (three U-Oeq2) [51]. The great uncertainties of CN of U-Oeq1 and U-Oeq2 15
could be attributed to the strong correlation with high Debye-Waller factor. The values of Debye-Waller factor significantly increased with increasing bonding distance. As shown in Figure S6, the EXAFS spectra of U(VI)-loaded perovskite in k-range was similar to that of U(IV)O2, indicating the U(VI) was photo-reduced to U(IV) by
IP T
perovskite under UV-vis irradiation condition. Compared to UO22+, the splitting of distance for U-Oeq1 and U-Oeq2 shells indicated the formation of inner-sphere
SC R
surface complexation [52]. The FT features at ~ 2.50 Å could be attributed to the U-Ti shell at 3.15 Å [53, 54], further indicating the inner-sphere complexation of U(VI) on
U
perovskite. It is noted that the FT feature at ~ 3.02 Å can be fitted by U-U shell of
N
uraninite at 3.88 Å [54], which revealed that U(VI) was reduced to U(IV) by
A
perovskite. The results of XANES and EXAFS analysis suggested that inner-sphere
M
complexation dominated the U(VI) sequestration, moreover a fraction of sequestrated
4. Conclusions
ED
U(VI) was photocatalytic reduced to U(IV) on perovskite at daylight irradiation.
PT
In this study, cubic perovskite was successfully synthesized by a facile solvothermal
CC E
method. The introduction of carbonate inhibited the sequestration of U(VI) and further influenced the photocatalytic reduction of U(VI) on perovskite due to the formation of uranyl-carbonate complexes. The high sequestration of U(VI) on
A
perovskite was attributed to the coordination of U(VI) with the oxygenated groups of perovskite. In addition, a fraction of U(VI) can be photocatalytically reduced to U(IV) by the analysis of UV-vis, XPS and EXAFS techniques. These observations are crucial for the application of perovskite-based materials for the immobilization of 16
U(VI) in nuclear waste management, specially carbonate-rich environment.
Acknowledgements Financial support from the Natural Science Foundation of China for Outstanding Young Foundation (21822602) and Research Fund Program of Guangdong Provincial
IP T
Key Laboratory of Radionuclides Pollution Control and Resources (GZDX2017K002)
SC R
is acknowledged.
References
U
[1] C.C. Ding, W.C. Cheng, Y.B. Sun, X.K. Wang, Determination of chemical affinity
N
of graphene oxide nanosheets with radionuclides investigated by macroscopic,
A
spectroscopic and modeling techniques, Dalton. Trans., 43 (2014) 3888-3896.
M
[2] Y.B. Sun, S.H. Lu, X.X. Wang, C. Xu, J.X. Li, C.L. Chen, J. Chen, T. Hayat, A.
ED
Alsaedi, N.S. Alharbi, X.K. Wang, Plasma-facilitated synthesis of amidoxime/carbon nanofiber hybrids for effective enrichment of U-238(VI) and Am-241(III), Environ.
PT
Sci. Technol., 51 (2017) 12274-12282.
CC E
[3] J. Li, X.X. Wang, G. X. Zhaom C. L. Chen, Z. F. Chai, A. Alsaedi, T. Hayat, X. K. Wang, Metal-organic framwork-based materials: superior adsorbents for the capture of toxic and radioactive metal ions, Chem. Soc. Rev., 47(2018) 2322-2356.
A
[4] M.O. Barnett, P.M. Jardine, S.C. Brooks, U(VI) adsorption to heterogeneous subsurface media: Application of a surface complexation model, Environ. Sci. Technol., 36 (2002) 937-942. [5] D.S. Alessi, B. Uster, H. Veeramani, E.I. Suvorova, J.S. Lezama-Pacheco, J.E. 17
Stubbs, J.R. Bargar, R. Bernier-Latmani, Quantitative separation of monomeric U(IV) from UO2 in products of U(VI) reduction, Environ. Sci. Technol., 46 (2012) 6150-6157. [6] S.W. Zhang, H.H. Gao, J.X. Li, Y.S. Huang, A. Alsaedi, T. Hayat, X.J. Xu, X.K.
IP T
Wang, Rice husks as a sustainable silica source for hierarchical flower-like metal silicate architectures assembled into ultrathin nanosheets for adsorption and catalysis, J.
SC R
Hazard. Mater., 321 (2017) 92-102.
[7] X. X. Wang, S. B. Yang, W. Q. Shi, J. X. Li, T. Hayat, X. K. Wang, Different
U
interaction mechanisms of Eu(III) and Am-243(III) with carbon nanotubes studies by
N
batch, spectroscopy technique and theoretical calcualtion, Environ. Sci. Technol.,
A
49(2015) 11721-11728.
M
[8] Y.B. Sun, J.X. Li, X.K. Wang, The retention of uranium and europium onto
ED
sepiolite investigated by macroscopic, spectroscopic and modeling techniques, Geochim. Cosmochim. Acta, 140 (2014) 621-643.
PT
[9] M. Marques Fernandes, B. Baeyens, R. Daehn, A.C. Scheinost, M.H. Bradbury,
CC E
U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study, Geochim. Cosmochim. Acta, 93 (2012) 262-277. [10] S. Kerisit, C.X. Liu, Diffusion and adsorption of uranyl carbonate species in
A
nanosized mineral fractures, Environ. Sci. Technol., 46 (2012) 1632-1640. [11] Y. Arai, P.B. Moran, B.D. Honeyman, J.A. Davis, In situ spectroscopic evidence for neptunium(V)-carbonate inner-sphere and outer-sphere ternary surface complexes on hematite surfaces, Environ. Sci. Technol., 41 (2007) 3940-3944. 18
[12] Z.B. Zhang, J. Liu, X.H. Cao, X.P. Luo, R. Hua, Y. Liu, X.F. Yu, L.K. He, Y.H. Liu, Comparison of U(VI) adsorption onto nanoscale zero-valent iron and red soil in the presence of U(VI)-CO3/Ca-U(VI)-CO3 complexes, J. Hazard. Mater., 300 (2015) 633-642.
IP T
[13] L. Yin, P. Wang, T. Wen, S. Yu, X. Wang, T. Hayat, A. Alsaedi, X. Wang, Synthesis of layered titanate nanowires at low temperature and their application in
SC R
efficient removal of U(VI), Environ. Pollut., 226 (2017) 125-134.
[14] F. Yuan, C. Wu, Y. Cai, L. Zhang, J. Wang, L. Chen, X. Wang, S. Yang, S. Wang,
U
Synthesis of phytic acid-decorated titanate nanotubes for high efficient and high
N
selective removal of U(VI), Chem. Eng. J., 322 (2017) 353-365.
A
[15] G. Garcia-Rosales, R. Drot, F. Mercier-Bion, G. Lagarde, E. Simoni, Interaction
M
between U(VI) and SrTiO3 surfaces versus temperature, J. Colloid Interface Sci., 333
ED
(2009) 104-113.
[16] O. Ruzimuradov, G. Hasegawa, K. Kanamori, K. Nakanishi, Preparation of
PT
hierarchically porous nanocrystalline CaTiO3, SrTiO3 and BaTiO3 perovskite
CC E
monoliths, J. Am. Ceram. Soc., 94 (2011) 3335-3339. [17] W.X. Dong, B. Song, W.J. Meng, G.L. Zhao, G.R. Han, A simple solvothermal process to synthesize CaTiO3 microspheres and its photocatalytic properties, Appl.
A
Surf. Sci., 349 (2015) 272-278. [18] D. Zhang, C.-l. Zhang, P. Zhou, Preparation of porous nano-calcium titanate microspheres and its adsorption behavior for heavy metal ion in water, J. Hazard. Mater., 186 (2011) 971-977. 19
[19] J.D. Zhuang, Q.F. Tian, S. Lin, W.B. Yang, L.H. Chen, P. Liu, Precursor morphology-controlled formation of perovskites CaTiO3 and their photo-activity for As(III) removal, Appl. Catal. B-environ., 156 (2014) 108-115. [20] T. Xian, H. Yang, Y.S. Huo, Enhanced photocatalytic activity of CaTiO3-graphene
IP T
nanocomposites for dye degradation, Physica Scripta, 89 (2014). [21] Y.H. Chen, Y.D. Chen, Kinetic study of Cu(II) adsorption on nanosized BaTiO3
SC R
and SrTiO3 photocatalysts, J. Hazard. Mater., 185 (2011) 168-173.
[22] B. Hua, H.F. Xu, J. Terry, B.L. Deng, Kinetics of uranium(VI) reduction by
U
hydrogen sulfide in anoxic aqueous systems, Environ. Sci. Technol., 40 (2006)
N
4666-4671.
A
[23] S. Yan, B. Hua, Z.Y. Bao, J. Yang, C.X. Liu, B.L. Deng, Uranium(VI) removal by
M
nanoscale zerovalent iron in anoxic batch systems, Environ. Sci. Technol., 44 (2010)
ED
7783-7789.
[24] X. Yang, I.D. Williams, J. Chen, J. Wang, H. Xu, H. Konishi, Y. Pan, C. Liang, M.
PT
Wu, Perovskite hollow cubes: morphological control, three-dimensional twinning and
CC E
intensely enhanced photoluminescence, J. Mater. Chem., 18 (2008) 3543-3546. [25] X. Yang, J. Fu, C. Jin, J. Chen, C. Liang, M. Wu, W. Zhou, Formation mechanism of CaTiO3 hollow crystals with different microstructures, J. Am. Chem. Soc., 132
A
(2010) 14279-14287. [26] M. Newville, EXAFS analysis using FEFF and FEFFIT, J. Synchrotron. Radiat., 8 (2001) 96-100. [27] B. Ravel, M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: data analysis for 20
X-ray absorption spectroscopy using IFEFFIT, J. Synchrotron. Radiat., 12 (2005) 537-541. [28] W. Liu, X. Zhao, T. Wang, D. Zhao, J. Ni, Adsorption of U(VI) by multilayer titanate nanotubes: Effects of inorganic cations, carbonate and natural organic matter,
IP T
Chem. Eng. J., 286 (2016) 427-435. [29] Y.B. Xie, C.W. Yuan, Visible-light responsive cerium ion modified titania sol and
SC R
nanocrystallites for X-3B dye photodegradation, Appl. Catal. B-environ., 46 (2003) 251-259.
U
[30] H. Liu, X. Dong, X. Wang, C. Sun, J. Li, Z. Zhu, A green and direct synthesis of
A
Chem. Eng. J., 230 (2013) 279-285.
N
graphene oxide encapsulated TiO2 core/shell structures with enhanced photoactivity,
M
[31] X.L. Tan, Q.H. Fan, X.K. Wang, B. Grambow, Eu(III) Sorption to TiO2 (Anatase
ED
and Rutile): Batch, XPS, and EXAFS Studies, Environ. Sci. Technol., 43 (2009) 3115-3121.
PT
[32] M. Konstantinou, L. Pashalidis, Competitive sorption of Cu(II), Eu(III) and U(VI)
CC E
ions on TiO2 in aqueous solutions - A potentiometric study, Colloids Surf. A, 324 (2008) 217-221.
[33] C.C. Ding, W.C. Cheng, Y.B. Sun, X.K. Wang, Effects of Bacillus subtilis on the
A
reduction of U(VI) by nano-Fe0, Geochim. Cosmochim. Acta, 165 (2015) 86-107. [34] R. Zhang, C.L. Chen, J. Li, X.K. Wang, Preparation of montmorillonite@carbon composite and its application for U(VI) removal from aqueous solution, Appl. Surf. Sci., 349 (2015) 129-137. 21
[35] L. Yin, S. Song, X.X. Wang, F.L. Niu, R. Ma, S.J. Yu, T. Wen, Y.T. Chen, T. Hayat, A. Alsaedi, X.K. Wang, Rationally designed core-shell and yolk-shell magnetic titanate nanosheets for efficient U(VI) adsorption performance, Environ. Pollut., 238 (2018) 725-738.
IP T
[36] S. Song, S.Y. Huang, R. Zhang, Z.S. Chen, T. Wen, S.H. Wang, T. Hayat, A. Alsaedi, X.K. Wang, Simultaneous removal of U(VI) and humic acid on defective
SC R
TiO2-x investigated by batch and spectroscopy techniques, Chem. Eng. J., 325 (2017) 576-587.
U
[37] L.C. Tan, X.F. Zhang, Q. Liu, X.Y. Jing, J.Y. Liu, D.L. Song, S.X. Hu, L.H. Liu, J.
N
Wang, Synthesis of Fe3O4@TiO2 core-shell magnetic composites for highly efficient
A
sorption of uranium(VI), Colloids Surf. A., 469 (2015) 279-286.
M
[38] G.X. Zhao, T. Wen, X. Yang, S.B. Yang, J.L. Liao, J. Hu, D.D. Shao, X.K. Wang,
ED
Preconcentration of U(VI) ions on few-layered graphene oxide nanosheets from aqueous solutions, Dalton Trans, 41 (2012) 6182-6188.
PT
[39] Y.B. Sun, S.T. Yang, G.D. Sheng, Z.Q. Guo, X.K. Wang, The removal of U(VI)
CC E
from aqueous solution by oxidized multiwalled carbon nanotubes, J. Environ. Radioactiv., 105 (2012) 40-47. [40] S.C. Brooks, J.K. Fredrickson, S.L. Carroll, D.W. Kennedy, J.M. Zachara, A.E.
A
Plymale, S.D. Kelly, K.M. Kemner, S. Fendorf, Inhihition of bacterial U(VI) reduction by calcium, Environ. Sci. Technol., 37 (2003) 1850-1858. [41] S.W. Zhang, J.X. Li, X.K. Wang, Y.S. Huang, M.Y. Zeng, J.Z. Xu, Rationally designed
1D
Ag@AgVO3
nanowire/graphene/protonated 22
g-C3N4
nanosheet
heterojunctions for enhanced photocatalysis via electrostatic self-assembly and photochemical reduction methods, J. Mater. Chem. A, 3 (2015) 10119-10126. [42] H.H. Ou, C.H. Liao, Y.H. Liou, J.H. Hong, S.L. Lo, Photocatalytic oxidation of aqueous ammonia over microwave-induced titanate nanotubes, Environ. Sci. Technol.,
IP T
42 (2008) 4507-4512. [43] D.E. MacDonald, N. Deo, B. Markovic, M. Stranick, P. Somasundaran,
SC R
Adsorption and dissolution behavior of human plasma fibronectin on thermally and chemically modified titanium dioxide particles, Biomaterials, 23 (2002) 1269-1279.
U
[44] W. Liu, J.R. Ni, X.C. Yin, Synergy of photocatalysis and adsorption for
N
simultaneous removal of Cr(VI) and Cr(III) with TiO2 and titanate nanotubes, Water
A
Res., 53 (2014) 12-25.
M
[45] C.C. Ding, W.C. Cheng, X.X. Wang, Z.Y. Wu, Y.B. Sun, C.L. Chen, X.K. Wang,
ED
S.H. Yu, Competitive sorption of Pb(II), Cu(II) and Ni(II) on carbonaceous nanofibers: A spectroscopic and modeling approach, J. Hazard. Mater., 313 (2016) 253-261.
PT
[46] Y.B. Sun, Z.Y. Wu, X.X. Wang, C.C. Ding, W.C. Cheng, S.H. Yu, X.K. Wang,
CC E
Macroscopic and microscopic investigation of U(VI) and Eu(III) adsorption on carbonaceous nanofibers, Environ. Sci. Technol., 50 (2016) 4459-4467. [47] Y.B. Sun, C.C. Ding, W.C. Cheng, X.K. Wang, Simultaneous adsorption and
A
reduction of U(VI) on reduced graphene oxide-supported nanoscale zerovalent iron, J. Hazard. Mater., 280 (2014) 399-408. [48] O. Riba, T.B. Scott, K.V. Ragnarsdottir, G.C. Allen, Reaction mechanism of uranyl in the presence of zero-valent iron nanoparticles, Geochim. Cosmochim. Acta, 23
72 (2008) 4047-4057. [49] H.B. Jung, M.I. Boyanov, H. Konishi, Y. Sun, B. Mishra, K.M. Kemner, E.E. Roden, H. Xu, Redox behavior of uranium at the nanoporous aluminum oxide-water interface: Implications for uranium remediation, Environ. Sci. Technol., 46 (2012)
IP T
7301-7309. [50] E.J. Schofield, H. Veeramani, J.O. Sharp, E. Suvorova, R. Bernier-Latmani, A.
SC R
Mehta, J. Stahlman, S.M. Webb, D.L. Clark, S.D. Conradson, E.S. Ilton, J.R. Bargar,
Structure of biogenic uraninite produced by shewanella oneidensis strain MR-1,
U
Environ. Sci. Technol., 42 (2008) 7898-7904.
N
[51] E.R. Sylwester, E.A. Hudson, P.G. Allen, The structure of uranium (VI) sorption
A
complexes on silica, alumina, and montmorillonite, Geochim. Cosmochim. Acta, 64
M
(2000) 2431-2438.
ED
[52] Y. B. Sun, R. Zhang, C. C. Ding, X. X. Wang, W. C. Cheng, C. L. Chen, X. K. Wang, Adsorption of U(VI) on sericite in the presence of Bacillus subtilis: A
PT
combined batch, EXAFS and modeling techniques. Geochim. Cosmochim. Acta,
CC E
180(2016) 51-65.
[53] C. Den Auwer, R. Drot, E. Simoni, S.D. Conradson, M. Gailhanou, J.M. de Leon, Grazing incidence XAFS spectroscopy of uranyl sorbed onto TiO2 rutile surfaces, New
A
J. Chem., 27 (2003) 648-655. [54] M.C. Duff, D.B. Hunter, D.T. Hobbs, S.D. Fink, Z. Dai, J.P. Bradley, Mechanisms of strontium and uranium removal from high-level radioactive waste simulant solutions by the sorbent monosodium titanate, Environ. Sci. Technol., 38 (2004) 5201-5207. 24
Figure captions Fig. 1 Characterization of perovskite, A-B: XRD patterns and FT-IR spectra before and after sequestration of U(VI); C: SEM image; D: zeta potentials. Fig. 2 A: Effect of carbonate on U(VI) sequestration by perovskite under different pH
IP T
conditions; B: Distribution of U(VI) speciation at 1.0 mmol/L Na2CO3 and C0(U(VI)) = 20 mg/L; C and D: Sequestration kinetics and isotherms of U(VI) on
SC R
perovskite, m/V = 0.2 g/L; I = 0.01 mol/L NaClO4 and T = 293 K.
Fig. 3 A：Comparison sequestration experiments under different irradiation condition
U
m/V = 0.2 g/L, V = 100 mL, C0(U(VI)) = 20 mg/L and pH = 7.5; B: UV-Vis absorption
N
spectra (Inset: (Ahv)1/2 versus E (eV)).
A
Fig. 4 XPS analysis of U-loaded perovskite, A-C: the high resolution of O 1s, Ti 2p
M
and U 4f, respectively; D: the high resolution of U 4f and total survey scans after
ED
U(VI) desorption, C0(U(VI)) = 20 mg/L, m/V = 0.2 g/L, I = 0.01 mol/L NaClO4 and T = 293 K.
PT
Fig. 5 A: U LIII-edge XANES spectra, and B: Fourier transform of the EXAFS spectra
CC E
for the reference samples and perovskite-U(VI) sample at pH 7.5, C0(U(VI))= 20
A
mg/L, m/V = 0.2 g/L, I = 0.01 mol/L NaClO4 and T = 293 K.
25
B
(a) P-Perovskite (b) P+U(VI) Daylight (c) P+U(VI) UV-light
Relative transmittance (%)
P+U(VI) UV-light
(c)
(400)
(204)
(a)
(220)
(200)
(b) (110)
P+U(VI) Daylight
Perovskite
3432
Perovskite PDF#82-0228
10
1400 1630
620
20
30
40
50
3500
60
3000
2500
2000
1500
-1 Wavenumber(cm )
2 Theta (degree)
D 20
SC R
Zeta potential (mV)
0 -10 -20
U
-30
N
-40 3.0
A
2.5
M ED PT CC E A
26
500
0 mM Na2CO3 10.0 mM Na2CO3
Perovskite
10
Fig. 1
1000
IP T
Relative intensity (a.u.)
A
3.5
4.0
4.5
5.0
pH
5.5
6.0
6.5
7.0
7.5
100
100
A
B
0 mM Na2CO3 1 mM Na2CO3
U(VI) species (%)
Sequestration (%)
80
10 mM Na2CO3
80
UO22+
60
40
UO2CO3(aq)
UO2(CO3)3
4-
60
40
UO2(CO3)2
(UO2)2(OH)22+
20
2-
UO2(OH)+
20 0
4
5
6
7
8
9
3
4
5
6
D
C
100
80
90
Qe (mg/g)
60
Qe (mg/g)
pH 5.5 pH 7.5
40 0 mM Na2CO3
pH 7.5
20
80 70 60 50
10.0 mM Na2CO3
U
40
0 30
5
10
15
20
25
0
30
Time (hours)
9
2
3
4
5
Ce (mg/L)
A
Daylight 0 mM Na2CO3
B
ED
UV-light 10.0 mM Na2CO3
pH 7.5
0.6
PT
0.4
CC E
0.0
-50
0
50
0.8 0.6
2.0 1.5 1.0
Eg=3.18 eV
0.5
0.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0
0.4
E (eV)
0.2 0.0
100
300
150
400
500
600
Wavelength (nm)
Time (min)
Fig. 3
A
-100
2.5
(Ahv)1/2
0.8
0.2
3.0
1.0
Daylight 10.0 mM Na2CO3
Dark
Ce/C0
1.2
UV-light 0 mM Na2CO3
Absorbance (a.u.)
1.0
M
A
Fig. 2
1
N
0
8
SC R
100
7
pH
pH
IP T
3
27
700
Ti-OH(b) Ti-OH(a) [TiO6]
B
O 1s
(a) P-Perovskite (b) P-U(VI) Daylight (c) P-U(VI) UV-light
Relative intensity (a.u.)
P+U(VI) Daylight
P+U(VI) UV-light
(d) P-U(VI)-Na2CO3 Daylight
(a) (b)
P+U(VI)+Na2CO3 Daylight
533
(d)
532
531
530
529
468
528
466
464
462
Binding Energy (eV) C
U 4f 7/2 U(VI)
D
386
394
392
390
N 384
382
380
378
U(VI)
SC R
396
A
388
Binding Energy (eV)
450
Desorption
UV-light P-U(VI)-Na2CO3
390
452
U(IV)
U(IV)
C 1s Ca 2p
Daylight P-U(VI)-Na2CO3
454
388
386
384
382
U
Relative intensity (a.u.)
Relative intensity (a.u.)
P-U(VI) UV-light
392
456
U 4f 7/2
U(VI)
P-U(VI) Daylight
394
458
U 4f 5/2
U(IV)
Ti 2p Na KLL
U 4f 5/2 U(IV)
U(VI)
460
Binding Energy (eV)
IP T
534
(c)
0
200
380
378
Na 1s
Relative intensity (a.u.)
P-Perovskite
O 1s
A
Desorption
400
600
800
1000
1200
Binding Energy (eV)
ED
M
Fig. 4
(IV)
U
O2(s)
ax -O U
FT
PT
CC E
U-U
eq
A
B
-O U
Adsorption
A
U(IV) U(VI) pH 7.5
U
(VI)
U-Ti pH 7.5
17172 17178 17184 17190 17196
0
Energy (eV)
Fig. 5
28
1
2
R(Å)
3
4
2+
O2
5
Table 1. Comparison of U(VI) sequestration capacity of perovskite with other adsorbents. Experimental condition
qmax(mg/g)
References
Yolk-shell Fe3O4@TNS
pH 5.0, T=298 K
82.85
[35]
Defective TiO2−x
pH 5.0, T=298 K
65.41
[36]
Fe3O4@TiO2
pH 6.0, T=298 K
91.1
GONS
pH 5.0, T=293 K
97.5
Oxidized MWCNTs
pH 5.0, T=298 K
Perovskite
pH 5.5, T = 293 K
IP T
Materials
[37]
SC R
[38] [39]
119.3
This study
N
U
33.32
Shells
M
Samples
PT
CC E
Perovskite-U(VI) pH 7.5
A
a
U-Oax U-Oeq1 U-Oeq2 U-Oax U-U U-Oax U-Oeq1 U-Oeq2 U-Ti U-U
ED
U(VI)O22+
U(VI)O2(s)d
A
Table 2. Uranium LIII-edge EXAFS spectra for standards and U(VI)-loaded sample R(Å)a
CNb
σ2(Å 2)c
1.78e 2.35(2)f 2.53(1) 2.35e 3.86e 1.78(0) 2.38(2) 2.56(2) 3.15(3) 3.88(2)
2.0e 2.0(5) 3.1(4) 8.0e 12.0e 2.0(1) 2.2(3) 3.3(1) 1.4(0) 7.9(2)
0.0053(2) 0.0027(1) 0.0075(1) 0.0052(6) 0.0034(4) 0.0039(3) 0.0059(1) 0.0087(3) 0.0123(5) 0.0263(7)
R: bond distance; b CN: coordination number; cσ2: Debye-Waller factor; d data from
Schofield et al. (2008) [50]; e fixed number; f digit in bracket: uncertainties.
29
S0304-3894(18)30944-0 https://doi.org/10.1016/j.jhazmat.2018.10.035 HAZMAT 19855
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
21-6-2018 11-10-2018 12-10-2018
Please cite this article as: Lu S, Zhu K, Hayat T, Alharbi NS, Chen C, Song G, Chen D, Sun Y, Influence of carbonate on sequestration of U(VI) on perovskite, Journal of Hazardous Materials (2018), https://doi.org/10.1016/j.jhazmat.2018.10.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of carbonate on sequestration of U(VI) on perovskite Songhua Lua,b, Kairuo Zhub, Tasawar Hayatc, Njud S. Alharbid, Changlun Chenb,c,d, Gang Songe, Diyun Chene, Yubing Suna,* a
College of Environmental Science and Engineering, North China Electric Power
IP T
University, Beijing 102206, PR China b
Key Laboratory of Photovoltaic and Energy Conversation, Institute of Plasma
c
SC R
Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, 230031, PR China
NAAM Research Group, Department of Mathematics, Faculty of Science, King
Department of Biological Sciences, Faculty of Science, King Abdulaziz University,
N
d
U
Abdulaziz University, Jeddah 21589, Saudi Arabia
Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and
M
e
A
Jeddah, Saudi Arabia
ED
Resources, Guangzhou 510006, PR China
PT
* Corresponding author: E-mail: [email protected] (Y. Sun).
A
CC E
Graphical Abstract
1
IP T SC R U
Highlights
Cubic perovskite was successfully synthesized by a facile solvothermal method
Inner-sphere complexation dominated U(VI) sequestration under neutral
M
A
N
condition
Abstract
ED
Carbonate inhibited the sequestration and photocatalytic reduction of U(VI)
PT
CC E
Cubic perovskite (CaTiO3) was successfully synthesized by a facile solvothermal method and was utilized to sequestrate U(VI) from aqueous solutions. The batch
A
experiments revealed that carbonate inhibited U(VI) sequestration at pH > 6.0 due to the formation of uranyl-carbonate complexes. The maximum sequestration capacity of U(VI) on perovskite was 119.3 mg/g (pH 5.5). The sequestration mechanism of U(VI) on perovskite were investigated by XPS and EXAFS techniques. According to XPS 2
analysis, the presence of U(IV) and U(VI) oxidation states revealed the photocatalytic reduction of U(VI) by perovskite under UV-vis irradiation. In addition, photocatalytic reduction performance significantly decreased in the presence of carbonate. Based on EXAFS analysis, the occurrence of U-Ti and U-U shells revealed the inner-sphere
IP T
surface complexation and reductive precipitation of U(VI) on perovskite. These findings herein are crucial for the application of perovskite-based composites in the
SC R
decontamination of U(VI) in aquatic environmental cleanup.
Keywords: Perovskite; EXAFS; U(VI); Sequestration; Carbonate.
U
1. Introduction
N
Uranium could discharge into sub-environments during mining of uranium and
A
reprocessing of spent nuclear fuel, which pose serious threats to aquatic life and
M
human health due to its radioactivity and chemical toxicity [1-3]. Consequently, the
ED
sequestration of uranium from the contaminated sites, including adsorption, redox and surface co-precipitation, is a topic of major concern nowadays [4-7]. In addition, the
PT
sequestration of uranium was strongly influenced by various environmental factors
CC E
such as pH and foreign ligands. For example, U(VI) and carbonate can generate stable uranyl-carbonate complexes (UO2(CO3)22-, UO2(CO3)34-) under circumneutral and alkaline conditions [8-11], which strongly suppressed the sequestration of U(VI) on
A
solid adsorbents due to the electrostatic repulsion [8,12]. Owing to the high adsorption performance, titanate-based materials have been extensively applied in sequestration uranium in recent years [13, 14]. García-Rosales et al., investigated the removal of U(VI) onto SrTiO3 was relative to temperature [15]. 3
Perovskite-structure titanate as semiconductor material presents the strong light absorption in the UV-light range (< 390 nm) due to the relative large band gap [16, 17]. Recently, it is demonstrated that perovskite displayed excellent adsorption and/or photocatalytic activity for heavy metals or organic pollutants [18-21]. In these studies,
IP T
it is found that the high adsorption of Pb2+, Cd2+ and Zn2+ on perovskite was attributed to adsorption, whereas the As(III) removal and dye degradation was due to the
SC R
favorable photocatalytic activity under UV-light conditions. However, the limited
reports regarding the effect of carbonate on the adsorption and photocatalytic activity
U
of uranium towards perovskite were available nowadays.
N
The objectives of this study are to (1) synthesize perovskite by a facile solvothermal
A
process and characterize it by XRD, FT-IR, SEM and XPS techniques; (2) investigate
M
the influence of carbonate on U(VI) sequestration by perovskite under different pH
ED
condition; (3) determine the sequestration mechanism of perovskite towards U(VI) using UV-Vis, XPS and EXAFS techniques. These findings are expected to provide a
PT
new understanding of the perovskite-type materials for the sequestration of U(VI) in
CC E
actual environmental cleanup.
2. Experimental details 2.1 Materials
A
U(VI) stock solutions (1.0 mmol/L) were obtained by dissolving UO2(NO3)2·6H2O (analytical reagent, Hubei Chushengwei Chemical Co. Ltd.) into ultrapure water. Other chemicals of analytical grades (i.e., Na2CO3, Ca(NO3)2, Ti(OC4H9)4, NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. 4
2.2 Synthesis and characterization of perovskite Perovskite was synthesized through a facile solvothermal method [24, 25]. Typically, 1.0 mmol Ca(NO3)2 was dissolved into 20 mL polyethylene glycol (PEG-200) under ultrasonic conditions. Afterwards, 0.33 mL of Ti(OC4H9)4 solution was added
IP T
drop-wise into the above mixture, then 0.88 g NaOH was added under vigorous magnetic stirring. Subsequently, the suspension was heated at 180 °C for 15 h. The
SC R
perovskite were obtained by washing it with acetone, diluted acetic acid, distilled water several times and then centrifugation it at 6000 rpm for 30 min.
U
The mineralogy of as-prepared perovskite was characterized by X-ray diffractometer
N
(XRD, Rigaku Mini Flex 600, Japan). The morphology of the perovskite was
A
performed using SEM (FEI-JSM 6320F). The FT-IR spectra were obtained by using a
M
FT-IR spectrophotometer (JASCO FTIR 410). Thermo Escalab 250 X-ray
ED
photoelectron spectrometer with Al Kα radiation at 150 W was used to detect the surface groups and fitting of O 1s, Ti 2p and U 4f peaks was analyzed using the
PT
XPSPEAK v. 4.1 software. Zeta potentials at different pH were recorded by Zetasizer
CC E
Nano ZS (Malvern Instruments). UV-Vis absorption spectrum was performed using Shimadzu UV-2500 UV-Vis spectrophotometer. 2.3 Batch sequestration experiments
A
The triplicate sequestration of U(VI) (20 mg/L) onto perovskite (0.2 g/L) were performed in polycarbonate tubes at glovebox condition (293 K). The experiments were conducted with the daylight (indoor) in day, and in the night were irradiated with a 300 W xenon lamp equipped with a cut-off UV filter (λ ≥ 400 nm). The irradiation 5
of UV-light experiment was conducted using a 300 W Xenon lamp equipped with a 250-380 nm UVREF filter. Typically, perovskite and NaClO4 were pre-equilibrated for 24 h, and then U(VI) stock solutions were spiked into the bulk suspension gradually to avoid the generation of uranium precipitate at high pH. The pH was
IP T
adjusted by using dilute HClO4 or NaOH solutions (0.1-1.0 mol/L). After sequestration equilibrium (24 h), the solid phases were separated from liquid phases
SC R
by centrifugation at 9500 rpm for 10 min, and then the supernatant was filtered
through a 0.22 μm nylon membrane. The concentration of U(VI) was analyzed using
U
spectrophotometric method with arsenazo-III as the chromogenic agent at wavelength
N
of 652 nm. The desorption of U(VI) from perovskite was conducted for XPS analysis.
A
Briefly, the uranium-bearing perovskite was obtained by centrifuging it after
M
adsorption equilibrium. Then 0.05 mol/L Na2CO3 was added under vigorous stirring
ED
conditions. After desorption 12 h, perovskite was obtained by centrifuging it and drying it for XPS analysis. The sequestration percentage (Sequestration %) and
ion
(%)
CC E
Sequestrat
PT
capacity (Qe (mg/g) were calculated according to Eqns. (1) and (2), respectively:
Qe
C0 Ce
100 %
(1)
C0
( C 0 C e ) V
(2)
m
A
where C0 (mg/L) and Ce (mg/L) are the initial U(VI) concentration and the aqueous U(VI) concentration after sequestration equilibrium, respectively. The m (g) and V (mL) are the mass of perovskite and the volume of the suspension, respectively. 2.4 Preparation and analysis of XANES and EXAFS spectra 6
The perovskite-U(VI) sample was prepared at pH 7.5 at glovebox conditions. The detailed preparation process as the following protocols: the perovskite, NaClO4 (0.01 mol/L) and Milli-Q water were weighted into 250 mL flask bottle, and then UO2(NO3)2 solution was gradually dropwise added to the suspension under vigorously
IP T
stirring condition to avoid the formation of U(VI) precipitate (m/V = 0.2 g/L). Afterwards, the solution was adjusted to pH 7.5 by adding negligible volume HClO4
SC R
or NaOH dilute solution. After equilibration, the experimental system was centrifuged at 9500 rpm for 10 min. The wet pastes were placed in a sealed chamber under
U
vacuum condition for the XANES and EXAFS measurement. The solid U(IV)O2(s)
N
and 1 mmol/L UO2(NO3)2 solution were selected as U(IV) and U(VI) standards,
A
respectively. The uranium LIII-edge (17179 eV) XANES and EXAFS spectrum was
M
conducted at Shanghai Synchrotron Radiation Facility (SSRF, BL14W beamlines)
ED
using Si(111) double-crystal monochromator with 30-element solid-state Ge detector. The spectra of U(IV)O2 crystalline and U(VI)-containing samples were measured in
PT
transmission and fluorescence mode, respectively. The data of all samples were
CC E
recorded at a relatively small grazing-incidence angle to avoid or minimize selfabsorption effects. The optimized parameters (i.e., path, coordination number (CN), interatomic length (R), Debye-Waller factor (σ2)) were obtained by setting them as
A
default values during the fitting process. The κ3-weighted EXAFS data were processed by Athena and Artemis interfaces to IFFEFIT 7.0 software [26, 27].
3. Results and discussion 7
3.1 Characterization The mineralogy of perovskite before and after U(VI) sequestration were identified by XRD patterns. As shown in Fig. 1A, the diffraction peaks at 2θ = 23.23, 32.89, 47.50 and 59.04° were well consistent with the (110), (020), (220) and (204) planes of
IP T
orthorhombic CaTiO3 phase (JCPDS No.82-0228), respectively [24, 25]. Further, no obvious changes of XRD patterns for perovskite after U(VI) sequestration under
SC R
daylight and UV-light condition indicated no distinct destruction of crystal structure during the sequestration process. The FT-IR spectra of perovskite before and after
U
U(VI) sequestration were shown in Fig. 1B. Major absorption bands at 3432 and 1630
N
cm−1 could be belonged to O-H (hydroxyl groups) stretching vibration and H-O-H
A
(bond water molecule) binding vibration, respectively [28]. The absorption band at
M
1400 cm−1 was attributed to the coexisted NO3− groups [29]. The absorption band at
ED
620 cm−1 can be attributed to the Ti-O-Ti stretching vibration [30]. Similar with the XRD patterns, the FT-IR spectra of perovskite after sequestration of U(VI) were
PT
identical to that of perovskite, implying no change of the perovskite structure after
CC E
sequestration process. The SEM image was used to characterize the morphology of perovskite. As shown by SEM image in Fig. 1C, the cubic structure with heterogeneous sizes was observed. As shown by zeta potentials in Fig. 1D, the
A
isoelectric points of perovskite in 0 and 10.0 mmo/L Na2CO3 solutions were calculated to be ~3.9 and 3.4, respectively, indicating that negative charge was observed at pH > 4.0. N2-BET surface area, pore volume and average pore diameter of perovskite was 126.055 m²/g, 0.103 cm³/g and 2.853 nm, respectively (Fig. S1A). 8
Such high surface area and suitable pore size could provide abundant sequestration sites for the sequestration process. 3.2 pH and carbonate effect The sequestration of U(VI) on perovskite affected by carbonate under different pH
IP T
condition was shown in Fig. 2A. Clearly, The sequestration performance of perovskite towards U(VI) increased dramatically with increasing pH from 3.0 to 6.0, then
SC R
maintained the high-level sequestration at pH 6.0-7.0, whereas the decreased
sequestration trend was observed at pH > 7.0. The pH-dependant sequestration
U
performance of perovskite towards U(VI) could be attributed to the electrostatic
N
interaction. Fig. 1D shows that the perovskite revealed a negatively charged surface at
A
pH > 4.0. Additionally, the mainly U(VI) species were positive charged species (e.g.,
M
UO2OH+, UO22+, (UO2)3OH5+ and (UO2)4OH7+ species) at pH < 6.0 in aqueous
ED
solutions (Fig. S1B), whereas the uranyl-carbonate species (UO2(CO3)22- and
(Fig. 2B).
PT
UO2(CO3)34-) were predominately formed under circumneutral and alkaline condition
CC E
Therefore, the increased sequestration performance of perovskite toward U(VI) at pH < 6.0 could be attribute to the strong electrostatic attraction generated by negatively charged surface of the perovskite and cationic U(VI) species [31]. The high
A
sequestration of U(VI) on perovskite at neutral pH could be ascribed to the surface complexation of uranyl with hydroxyl groups of perovskite and/or surface co-precipitation such as schoepite [32]. However, the significant decrease of U(VI) sequestration at pH > 7.0 was attribute to the electrostatic repulsion between 9
negatively charged surface of the perovskite and anionic uranyl species. After addition of Na2CO3, the slight enhance and great decrease of U(VI) sequestration was exhibited at pH < 6.0 and > 6.0, respectively. At pH < 6.0, the sequestration performance of perovskite towards U(VI) at 1.0 mmol/L Na2CO3 was slightly lower
IP T
than that of sequestration performance at 10.0 mmol/L Na2CO3, whereas the lower sequestration performance at 10.0 mmol/L Na2CO3 was observed at pH > 6.0 (Fig.
SC R
2A). As shown in Fig. 1D, the addition of carbonate significantly decreased the zeta potential of perovskite. Besides, the uranyl-carbonate species were observed at pH >
U
4.0 (Fig. 2B). Therefore, the increased sequestration of U(VI) on perovskite at pH <
N
6.0 with increasing carbonate concentration was attributed to the surface
A
complexation and/or electrostatic attraction of positive U(VI) species and negative
M
charged of perovskite, whereas the significant decrease of U(VI) sequestration at
ED
pH > 6.0 could be due to the electrostatic repulsion between negative charged perovskite and negative U(VI) species such as UO2(CO3)34- species. Therefore, the
PT
influence of carbonate on U(VI) sequestration at low and high pH was also ascribed to
CC E
the electrostatic attraction and repulsion, respectively. 3.3 Sequestration kinetics and isotherms The sequestration kinetics and isotherms of U(VI) onto perovskite (pH 5.5, 7.5) are
A
shown in Fig. 2C and 2D, respectively. The comparison kinetic sequestration of U(VI) on perovskite under daylight and xenon lamp light (λ ≥ 400 nm) was showed in Fig. S2 of SI. No obvious difference of sequestration result indicated the feasibility of the xenon lamp light (λ ≥ 400 nm) as an alternative light source in the night for the 10
kinetics experiments. The sequestration of perovskite toward U(VI) significantly enhanced with increasing sequestration time from 0 to 4 h, then slight enhancement of U(VI) sequestration was observed at long-term reaction time (4~30 h). After addition of Na2CO3 (10.0 mmol/L), the significant decreased sequestration of U(VI) was
IP T
observed at pH 7.5. The decrease of U(VI) sequestration at high pH could be due to the formation of uranyl-carbonate complexation, which were not favorable for U(VI)
SC R
sequestration in aqueous system [33, 34]. In the similar way, the slightly enhanced
sequestration of U(VI) under acidic condition with Na2CO3 was assigned to the
U
electrostatic attraction generated by negatively charged surface of perovskite and
N
cationic U(VI) species (e.g., UO22+). The pseudo-first-order and pseudo-second-order
A
kinetic models were applied to fit the sequestration kinetic data of U(VI). The more
M
detail information of the fitting result was provided in SI. Fig. S3 and Table S2 show
ED
the fitted correlation coefficients were R2 < 0.935 and R2 > 0.999 for pseudo-first-order model and pseudo-second-order model, respectively, indicating that
PT
sequestration kinetics of U(VI) was well fitted with the latter model.
CC E
The sequestration isotherms of U(VI) on perovskite at pH 5.5 and 7.5 were shown in Fig. 2D. The sequestration of U(VI) at 5.5 was obviously lower than that of U(VI) sequestration at pH 7.5. The data of sequestration isotherms were fitted by Langmuir
A
and Freundlich models. More details regarding Langmuir and Freundlich models were provided in SI. As shown in Fig. S4 and Table S3, the sequestration isotherms of U(VI) on perovskite at pH 5.5 and 7.5 can be satisfactorily fitted by Langmuir model with high correlation coefficient (R2 ≥ 0.99) compared to Freundlich model (R2 ≤ 0.97). 11
The maximum sequestration capacities of U(VI) at pH 5.5 and 7.5 were 119.3 and 126.8 mg/g, respectively. To demonstrate the excellent sequestration performance of perovskite, the comparison of maximum sequestration capacities of different titanate-based adsorbents towards U(VI) were summarized in Table 1 [35-39]. The
IP T
sequestration performance of perovskite is higher than yolk-shell magnetic titanate nanosheets (Fe3O4@TNS) (82.85 mg/g at pH 5.0 and 298 K) [35], defective TiO2−x
SC R
(65.41 mg/g at pH 5.0 and 298 K) [36], Fe3O4@TiO2 (91.1 mg/g at pH 6.0 and 298 K)
[37], graphene oxide nanosheets (GONS) (97.5 mg/g at pH 5.0 and 293 K) [38],
U
oxidized multiwalled carbon nanotubes (MWCNTs) (33.32 mg/g at pH 5.0 and 293 K)
N
[39]. These results suggested that perovskite can be used as a promising adsorbent for
A
the immobilization of U(VI) from aqueous solutions in actual environmental cleanup.
M
3.4 Comparison sequestration experiments
ED
Fig. 3A shows the effect of carbonate on U(VI) sequestration towards perovskite at pH 7.5 under different light irradiation conditions. Typical, the comparison of
PT
sequestration experiments were firstly conducted under dark condition for 2 h, and
CC E
then samples were irradiated with daylight or UV-light for 3 h. It was found that the significant enhanced sequestration capacity at pH 7.5 under UV-light condition compared to that under daylight condition in the absence of carbonate. Nevertheless,
A
no obvious changes of U(VI) sequestration was observed in the presence of carbonate under daylight and UV-light conditions. These results indicated that the presence of carbonate significantly inhibited the U(VI) sequestration. Moreover, the photocatalytic reduction of U(VI) was also hindered in the presence of carbonate. 12
The photocatalytic activity of U(VI) on perovskite was determined by analysis of UV-Vis absorption spectra. As shown in Fig. 3B, an optical steep and strong absorption peak was observed at ~380 nm. The band gap energy (Eg) of perovskite was calculated to be about 3.18 eV. The conduction band potential (ECB) and valence
IP T
band potential (EVB) were calculated to be -0.69 and 2.49 V, respectively (SI). Further, the negative conduction band potential (ECB = -0.69 V) of perovskite was
SC R
lower than that of positive redox potential of U(VI)/U(IV) (∼ + 0.411 V) at pH 6.9 [40]. Generally, the photocatalytic reduction includes two ways: (i) the
U
photogenerated electrons directly react with adsorbate; (ii) the photo-generated holes
N
derived from the oxidizing of organic matter can produce intermediate to reduce
A
adsorbate [41]. In this study, the photocatalytic activity of perovskite was excited by
M
UV-light irradiation. As a consequence, the peroskite generated the photo-generated
ED
electrons, which directly participated in the reduction of U(VI) to U(IV) [40]: (perovskite) + hv (< 390 nm) → perovskite (e- +h+)
(6)
PT
U(VI)+2 e- → U(IV)
(5)
CC E
Therefore, the significant enhanced sequestration capacity under UV-light condition (pH 7.5) was attributed to the photocatalytic reduction of U(VI). 3.5 XPS analysis
A
Fig. 4 shows the XPS spectra of perovskite before and after U(VI) sequestration (noted as P and P-U(VI)). As shown by survey scans in Fig. S5, the relative intensities of U 4f peak for P-U(VI)-Na2CO3 sample was lower than that of P-U(VI), suggesting that carbonate inhibited the U(VI) sequestration. Additionally, the higher relative 13
intensities of U 4f peak under UV-light irradiation indicated the increased photocatalytic activity of U(VI) on perovskite. As shown in Fig. 4A, O 1s peaks can be deconvoluted into three sub-peaks at ~ 529.5, 531.0 and 532.0 eV, which were ascribed to crystal lattice oxygen of TiO6, -OH of adsorbed H2O/acidic Ti–OH(a) and
IP T
-OH of basic Ti–OH(b), respectively [42-44]. As shown in Fig. 4A-B, the binding energy of O 1s and Ti 2p peaks after U(VI) sequestration were slightly shifted to the
SC R
lower binding energy, implying coordination of U(VI) with the oxygen-containing groups of the perovskite [45, 46].
U
As shown in Fig. 4C, two doublet peaks of U 4f peaks at ∼382 and 392 eV were
N
assigned to U 4f 7/2 and U 4f 5/2, respectively [47]. Moreover, U 4f 7/2 and U 4f 5/2
A
peaks can be fitted with the components at 380.0/381.6 eV and at 390.8/392.5 eV,
M
respectively. The peaks at 380.0/390.8 and 381.6/392.5 eV were corresponded to
ED
U(IV) and U(VI) phase, respectively [33, 48]. Moreover, the relative intensity of U(IV) phase for P-U(VI)-Na2CO3 was lower than that of U(IV) phase for P-U(VI),
PT
indicating that the existence of carbonate influenced the photocatalytic reduction of
CC E
U(VI) due to the formation of uranyl-carbonate complexes [22, 23]. Hua et al. also found that the U(VI)-carbonate species could inhibit the U(VI) reduction by hydrogen sulfide at high carbonate concentration [22]. Yan et al. concluded that carbonate
A
inhibited U(VI) reduction by nanoscale zero valent iron due to the formation of U(VI)-carbonate complexes, which decreased U(VI) adsorption and suppressed the U(VI) reduction [23]. Furthermore, the relative intensities of U(IV) phase in P-U(VI) and P-U(VI)-Na2CO3 samples under UV-light condition was higher than that of U(IV) 14
phase under daylight condition, suggesting that the UV-light irradiation was benefited to the photocatalytic reduction of U(VI) phase. As shown in Fig. 4D, the low relative intensities of U 4f peaks after desorption indicated that most of uranium was desorbed from perovskite by 0.05 mol/L Na2CO3. However, the relative intensities of U(IV)
IP T
component was significantly higher than that of U(VI), indicating U(IV) species generated from photocatalytic reduction were stabilized and hardly desorbed by
SC R
carbonate [49]. Besides, the relative intensities of Na 1s and Na KLL were apparent
increased, which indicated that the Na+ ions adsorbed on the perovskite surface in the
U
desorption process due to the Na2CO3 as the desorption solution.
N
3.6 XANES and EXAFS analysis
A
The uranium LIII-edge XANES and Fourier transformed (FT) EXAFS spectra of
M
U(IV)O2, U(VI)O22+ and U(VI)-loaded samples were shown in Fig. 5A-B. Table 2 listed
ED
the optimized parameters of EXAFS analysis, and the fitted data of U(IV)O2 crystalline were derived from the previous study [50]. Fig. 5A shows the energies of absorption
PT
edge of U(IV)O2 and U(VI)O22+ were ~ 17176 and 17179 eV, respectively [33], and that
CC E
of U-loaded perovskite at pH 7.5 was ~ 17178 eV located between U(IV)O2 and U(VI)O22+ standards. The above result implied that a fraction of sequestrated U(VI) phase was photocatalytic reduced to U(IV) phase due to the photocatalytic activity of
A
perovskite. As shown in Fig. 5B, the FT features at ~ 1.4 Å can be fitted by two axial oxygen shell (U-Oax) at 1.78 Å, whereas the second FT features at ~ 1.9 Å can be fitted by 5-6 equatorial oxygen shells at approximately 2.35 Å (~two U-Oeq1) and 2.58 Å (three U-Oeq2) [51]. The great uncertainties of CN of U-Oeq1 and U-Oeq2 15
could be attributed to the strong correlation with high Debye-Waller factor. The values of Debye-Waller factor significantly increased with increasing bonding distance. As shown in Figure S6, the EXAFS spectra of U(VI)-loaded perovskite in k-range was similar to that of U(IV)O2, indicating the U(VI) was photo-reduced to U(IV) by
IP T
perovskite under UV-vis irradiation condition. Compared to UO22+, the splitting of distance for U-Oeq1 and U-Oeq2 shells indicated the formation of inner-sphere
SC R
surface complexation [52]. The FT features at ~ 2.50 Å could be attributed to the U-Ti shell at 3.15 Å [53, 54], further indicating the inner-sphere complexation of U(VI) on
U
perovskite. It is noted that the FT feature at ~ 3.02 Å can be fitted by U-U shell of
N
uraninite at 3.88 Å [54], which revealed that U(VI) was reduced to U(IV) by
A
perovskite. The results of XANES and EXAFS analysis suggested that inner-sphere
M
complexation dominated the U(VI) sequestration, moreover a fraction of sequestrated
4. Conclusions
ED
U(VI) was photocatalytic reduced to U(IV) on perovskite at daylight irradiation.
PT
In this study, cubic perovskite was successfully synthesized by a facile solvothermal
CC E
method. The introduction of carbonate inhibited the sequestration of U(VI) and further influenced the photocatalytic reduction of U(VI) on perovskite due to the formation of uranyl-carbonate complexes. The high sequestration of U(VI) on
A
perovskite was attributed to the coordination of U(VI) with the oxygenated groups of perovskite. In addition, a fraction of U(VI) can be photocatalytically reduced to U(IV) by the analysis of UV-vis, XPS and EXAFS techniques. These observations are crucial for the application of perovskite-based materials for the immobilization of 16
U(VI) in nuclear waste management, specially carbonate-rich environment.
Acknowledgements Financial support from the Natural Science Foundation of China for Outstanding Young Foundation (21822602) and Research Fund Program of Guangdong Provincial
IP T
Key Laboratory of Radionuclides Pollution Control and Resources (GZDX2017K002)
SC R
is acknowledged.
References
U
[1] C.C. Ding, W.C. Cheng, Y.B. Sun, X.K. Wang, Determination of chemical affinity
N
of graphene oxide nanosheets with radionuclides investigated by macroscopic,
A
spectroscopic and modeling techniques, Dalton. Trans., 43 (2014) 3888-3896.
M
[2] Y.B. Sun, S.H. Lu, X.X. Wang, C. Xu, J.X. Li, C.L. Chen, J. Chen, T. Hayat, A.
ED
Alsaedi, N.S. Alharbi, X.K. Wang, Plasma-facilitated synthesis of amidoxime/carbon nanofiber hybrids for effective enrichment of U-238(VI) and Am-241(III), Environ.
PT
Sci. Technol., 51 (2017) 12274-12282.
CC E
[3] J. Li, X.X. Wang, G. X. Zhaom C. L. Chen, Z. F. Chai, A. Alsaedi, T. Hayat, X. K. Wang, Metal-organic framwork-based materials: superior adsorbents for the capture of toxic and radioactive metal ions, Chem. Soc. Rev., 47(2018) 2322-2356.
A
[4] M.O. Barnett, P.M. Jardine, S.C. Brooks, U(VI) adsorption to heterogeneous subsurface media: Application of a surface complexation model, Environ. Sci. Technol., 36 (2002) 937-942. [5] D.S. Alessi, B. Uster, H. Veeramani, E.I. Suvorova, J.S. Lezama-Pacheco, J.E. 17
Stubbs, J.R. Bargar, R. Bernier-Latmani, Quantitative separation of monomeric U(IV) from UO2 in products of U(VI) reduction, Environ. Sci. Technol., 46 (2012) 6150-6157. [6] S.W. Zhang, H.H. Gao, J.X. Li, Y.S. Huang, A. Alsaedi, T. Hayat, X.J. Xu, X.K.
IP T
Wang, Rice husks as a sustainable silica source for hierarchical flower-like metal silicate architectures assembled into ultrathin nanosheets for adsorption and catalysis, J.
SC R
Hazard. Mater., 321 (2017) 92-102.
[7] X. X. Wang, S. B. Yang, W. Q. Shi, J. X. Li, T. Hayat, X. K. Wang, Different
U
interaction mechanisms of Eu(III) and Am-243(III) with carbon nanotubes studies by
N
batch, spectroscopy technique and theoretical calcualtion, Environ. Sci. Technol.,
A
49(2015) 11721-11728.
M
[8] Y.B. Sun, J.X. Li, X.K. Wang, The retention of uranium and europium onto
ED
sepiolite investigated by macroscopic, spectroscopic and modeling techniques, Geochim. Cosmochim. Acta, 140 (2014) 621-643.
PT
[9] M. Marques Fernandes, B. Baeyens, R. Daehn, A.C. Scheinost, M.H. Bradbury,
CC E
U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study, Geochim. Cosmochim. Acta, 93 (2012) 262-277. [10] S. Kerisit, C.X. Liu, Diffusion and adsorption of uranyl carbonate species in
A
nanosized mineral fractures, Environ. Sci. Technol., 46 (2012) 1632-1640. [11] Y. Arai, P.B. Moran, B.D. Honeyman, J.A. Davis, In situ spectroscopic evidence for neptunium(V)-carbonate inner-sphere and outer-sphere ternary surface complexes on hematite surfaces, Environ. Sci. Technol., 41 (2007) 3940-3944. 18
[12] Z.B. Zhang, J. Liu, X.H. Cao, X.P. Luo, R. Hua, Y. Liu, X.F. Yu, L.K. He, Y.H. Liu, Comparison of U(VI) adsorption onto nanoscale zero-valent iron and red soil in the presence of U(VI)-CO3/Ca-U(VI)-CO3 complexes, J. Hazard. Mater., 300 (2015) 633-642.
IP T
[13] L. Yin, P. Wang, T. Wen, S. Yu, X. Wang, T. Hayat, A. Alsaedi, X. Wang, Synthesis of layered titanate nanowires at low temperature and their application in
SC R
efficient removal of U(VI), Environ. Pollut., 226 (2017) 125-134.
[14] F. Yuan, C. Wu, Y. Cai, L. Zhang, J. Wang, L. Chen, X. Wang, S. Yang, S. Wang,
U
Synthesis of phytic acid-decorated titanate nanotubes for high efficient and high
N
selective removal of U(VI), Chem. Eng. J., 322 (2017) 353-365.
A
[15] G. Garcia-Rosales, R. Drot, F. Mercier-Bion, G. Lagarde, E. Simoni, Interaction
M
between U(VI) and SrTiO3 surfaces versus temperature, J. Colloid Interface Sci., 333
ED
(2009) 104-113.
[16] O. Ruzimuradov, G. Hasegawa, K. Kanamori, K. Nakanishi, Preparation of
PT
hierarchically porous nanocrystalline CaTiO3, SrTiO3 and BaTiO3 perovskite
CC E
monoliths, J. Am. Ceram. Soc., 94 (2011) 3335-3339. [17] W.X. Dong, B. Song, W.J. Meng, G.L. Zhao, G.R. Han, A simple solvothermal process to synthesize CaTiO3 microspheres and its photocatalytic properties, Appl.
A
Surf. Sci., 349 (2015) 272-278. [18] D. Zhang, C.-l. Zhang, P. Zhou, Preparation of porous nano-calcium titanate microspheres and its adsorption behavior for heavy metal ion in water, J. Hazard. Mater., 186 (2011) 971-977. 19
[19] J.D. Zhuang, Q.F. Tian, S. Lin, W.B. Yang, L.H. Chen, P. Liu, Precursor morphology-controlled formation of perovskites CaTiO3 and their photo-activity for As(III) removal, Appl. Catal. B-environ., 156 (2014) 108-115. [20] T. Xian, H. Yang, Y.S. Huo, Enhanced photocatalytic activity of CaTiO3-graphene
IP T
nanocomposites for dye degradation, Physica Scripta, 89 (2014). [21] Y.H. Chen, Y.D. Chen, Kinetic study of Cu(II) adsorption on nanosized BaTiO3
SC R
and SrTiO3 photocatalysts, J. Hazard. Mater., 185 (2011) 168-173.
[22] B. Hua, H.F. Xu, J. Terry, B.L. Deng, Kinetics of uranium(VI) reduction by
U
hydrogen sulfide in anoxic aqueous systems, Environ. Sci. Technol., 40 (2006)
N
4666-4671.
A
[23] S. Yan, B. Hua, Z.Y. Bao, J. Yang, C.X. Liu, B.L. Deng, Uranium(VI) removal by
M
nanoscale zerovalent iron in anoxic batch systems, Environ. Sci. Technol., 44 (2010)
ED
7783-7789.
[24] X. Yang, I.D. Williams, J. Chen, J. Wang, H. Xu, H. Konishi, Y. Pan, C. Liang, M.
PT
Wu, Perovskite hollow cubes: morphological control, three-dimensional twinning and
CC E
intensely enhanced photoluminescence, J. Mater. Chem., 18 (2008) 3543-3546. [25] X. Yang, J. Fu, C. Jin, J. Chen, C. Liang, M. Wu, W. Zhou, Formation mechanism of CaTiO3 hollow crystals with different microstructures, J. Am. Chem. Soc., 132
A
(2010) 14279-14287. [26] M. Newville, EXAFS analysis using FEFF and FEFFIT, J. Synchrotron. Radiat., 8 (2001) 96-100. [27] B. Ravel, M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: data analysis for 20
X-ray absorption spectroscopy using IFEFFIT, J. Synchrotron. Radiat., 12 (2005) 537-541. [28] W. Liu, X. Zhao, T. Wang, D. Zhao, J. Ni, Adsorption of U(VI) by multilayer titanate nanotubes: Effects of inorganic cations, carbonate and natural organic matter,
IP T
Chem. Eng. J., 286 (2016) 427-435. [29] Y.B. Xie, C.W. Yuan, Visible-light responsive cerium ion modified titania sol and
SC R
nanocrystallites for X-3B dye photodegradation, Appl. Catal. B-environ., 46 (2003) 251-259.
U
[30] H. Liu, X. Dong, X. Wang, C. Sun, J. Li, Z. Zhu, A green and direct synthesis of
A
Chem. Eng. J., 230 (2013) 279-285.
N
graphene oxide encapsulated TiO2 core/shell structures with enhanced photoactivity,
M
[31] X.L. Tan, Q.H. Fan, X.K. Wang, B. Grambow, Eu(III) Sorption to TiO2 (Anatase
ED
and Rutile): Batch, XPS, and EXAFS Studies, Environ. Sci. Technol., 43 (2009) 3115-3121.
PT
[32] M. Konstantinou, L. Pashalidis, Competitive sorption of Cu(II), Eu(III) and U(VI)
CC E
ions on TiO2 in aqueous solutions - A potentiometric study, Colloids Surf. A, 324 (2008) 217-221.
[33] C.C. Ding, W.C. Cheng, Y.B. Sun, X.K. Wang, Effects of Bacillus subtilis on the
A
reduction of U(VI) by nano-Fe0, Geochim. Cosmochim. Acta, 165 (2015) 86-107. [34] R. Zhang, C.L. Chen, J. Li, X.K. Wang, Preparation of montmorillonite@carbon composite and its application for U(VI) removal from aqueous solution, Appl. Surf. Sci., 349 (2015) 129-137. 21
[35] L. Yin, S. Song, X.X. Wang, F.L. Niu, R. Ma, S.J. Yu, T. Wen, Y.T. Chen, T. Hayat, A. Alsaedi, X.K. Wang, Rationally designed core-shell and yolk-shell magnetic titanate nanosheets for efficient U(VI) adsorption performance, Environ. Pollut., 238 (2018) 725-738.
IP T
[36] S. Song, S.Y. Huang, R. Zhang, Z.S. Chen, T. Wen, S.H. Wang, T. Hayat, A. Alsaedi, X.K. Wang, Simultaneous removal of U(VI) and humic acid on defective
SC R
TiO2-x investigated by batch and spectroscopy techniques, Chem. Eng. J., 325 (2017) 576-587.
U
[37] L.C. Tan, X.F. Zhang, Q. Liu, X.Y. Jing, J.Y. Liu, D.L. Song, S.X. Hu, L.H. Liu, J.
N
Wang, Synthesis of Fe3O4@TiO2 core-shell magnetic composites for highly efficient
A
sorption of uranium(VI), Colloids Surf. A., 469 (2015) 279-286.
M
[38] G.X. Zhao, T. Wen, X. Yang, S.B. Yang, J.L. Liao, J. Hu, D.D. Shao, X.K. Wang,
ED
Preconcentration of U(VI) ions on few-layered graphene oxide nanosheets from aqueous solutions, Dalton Trans, 41 (2012) 6182-6188.
PT
[39] Y.B. Sun, S.T. Yang, G.D. Sheng, Z.Q. Guo, X.K. Wang, The removal of U(VI)
CC E
from aqueous solution by oxidized multiwalled carbon nanotubes, J. Environ. Radioactiv., 105 (2012) 40-47. [40] S.C. Brooks, J.K. Fredrickson, S.L. Carroll, D.W. Kennedy, J.M. Zachara, A.E.
A
Plymale, S.D. Kelly, K.M. Kemner, S. Fendorf, Inhihition of bacterial U(VI) reduction by calcium, Environ. Sci. Technol., 37 (2003) 1850-1858. [41] S.W. Zhang, J.X. Li, X.K. Wang, Y.S. Huang, M.Y. Zeng, J.Z. Xu, Rationally designed
1D
Ag@AgVO3
nanowire/graphene/protonated 22
g-C3N4
nanosheet
heterojunctions for enhanced photocatalysis via electrostatic self-assembly and photochemical reduction methods, J. Mater. Chem. A, 3 (2015) 10119-10126. [42] H.H. Ou, C.H. Liao, Y.H. Liou, J.H. Hong, S.L. Lo, Photocatalytic oxidation of aqueous ammonia over microwave-induced titanate nanotubes, Environ. Sci. Technol.,
IP T
42 (2008) 4507-4512. [43] D.E. MacDonald, N. Deo, B. Markovic, M. Stranick, P. Somasundaran,
SC R
Adsorption and dissolution behavior of human plasma fibronectin on thermally and chemically modified titanium dioxide particles, Biomaterials, 23 (2002) 1269-1279.
U
[44] W. Liu, J.R. Ni, X.C. Yin, Synergy of photocatalysis and adsorption for
N
simultaneous removal of Cr(VI) and Cr(III) with TiO2 and titanate nanotubes, Water
A
Res., 53 (2014) 12-25.
M
[45] C.C. Ding, W.C. Cheng, X.X. Wang, Z.Y. Wu, Y.B. Sun, C.L. Chen, X.K. Wang,
ED
S.H. Yu, Competitive sorption of Pb(II), Cu(II) and Ni(II) on carbonaceous nanofibers: A spectroscopic and modeling approach, J. Hazard. Mater., 313 (2016) 253-261.
PT
[46] Y.B. Sun, Z.Y. Wu, X.X. Wang, C.C. Ding, W.C. Cheng, S.H. Yu, X.K. Wang,
CC E
Macroscopic and microscopic investigation of U(VI) and Eu(III) adsorption on carbonaceous nanofibers, Environ. Sci. Technol., 50 (2016) 4459-4467. [47] Y.B. Sun, C.C. Ding, W.C. Cheng, X.K. Wang, Simultaneous adsorption and
A
reduction of U(VI) on reduced graphene oxide-supported nanoscale zerovalent iron, J. Hazard. Mater., 280 (2014) 399-408. [48] O. Riba, T.B. Scott, K.V. Ragnarsdottir, G.C. Allen, Reaction mechanism of uranyl in the presence of zero-valent iron nanoparticles, Geochim. Cosmochim. Acta, 23
72 (2008) 4047-4057. [49] H.B. Jung, M.I. Boyanov, H. Konishi, Y. Sun, B. Mishra, K.M. Kemner, E.E. Roden, H. Xu, Redox behavior of uranium at the nanoporous aluminum oxide-water interface: Implications for uranium remediation, Environ. Sci. Technol., 46 (2012)
IP T
7301-7309. [50] E.J. Schofield, H. Veeramani, J.O. Sharp, E. Suvorova, R. Bernier-Latmani, A.
SC R
Mehta, J. Stahlman, S.M. Webb, D.L. Clark, S.D. Conradson, E.S. Ilton, J.R. Bargar,
Structure of biogenic uraninite produced by shewanella oneidensis strain MR-1,
U
Environ. Sci. Technol., 42 (2008) 7898-7904.
N
[51] E.R. Sylwester, E.A. Hudson, P.G. Allen, The structure of uranium (VI) sorption
A
complexes on silica, alumina, and montmorillonite, Geochim. Cosmochim. Acta, 64
M
(2000) 2431-2438.
ED
[52] Y. B. Sun, R. Zhang, C. C. Ding, X. X. Wang, W. C. Cheng, C. L. Chen, X. K. Wang, Adsorption of U(VI) on sericite in the presence of Bacillus subtilis: A
PT
combined batch, EXAFS and modeling techniques. Geochim. Cosmochim. Acta,
CC E
180(2016) 51-65.
[53] C. Den Auwer, R. Drot, E. Simoni, S.D. Conradson, M. Gailhanou, J.M. de Leon, Grazing incidence XAFS spectroscopy of uranyl sorbed onto TiO2 rutile surfaces, New
A
J. Chem., 27 (2003) 648-655. [54] M.C. Duff, D.B. Hunter, D.T. Hobbs, S.D. Fink, Z. Dai, J.P. Bradley, Mechanisms of strontium and uranium removal from high-level radioactive waste simulant solutions by the sorbent monosodium titanate, Environ. Sci. Technol., 38 (2004) 5201-5207. 24
Figure captions Fig. 1 Characterization of perovskite, A-B: XRD patterns and FT-IR spectra before and after sequestration of U(VI); C: SEM image; D: zeta potentials. Fig. 2 A: Effect of carbonate on U(VI) sequestration by perovskite under different pH
IP T
conditions; B: Distribution of U(VI) speciation at 1.0 mmol/L Na2CO3 and C0(U(VI)) = 20 mg/L; C and D: Sequestration kinetics and isotherms of U(VI) on
SC R
perovskite, m/V = 0.2 g/L; I = 0.01 mol/L NaClO4 and T = 293 K.
Fig. 3 A：Comparison sequestration experiments under different irradiation condition
U
m/V = 0.2 g/L, V = 100 mL, C0(U(VI)) = 20 mg/L and pH = 7.5; B: UV-Vis absorption
N
spectra (Inset: (Ahv)1/2 versus E (eV)).
A
Fig. 4 XPS analysis of U-loaded perovskite, A-C: the high resolution of O 1s, Ti 2p
M
and U 4f, respectively; D: the high resolution of U 4f and total survey scans after
ED
U(VI) desorption, C0(U(VI)) = 20 mg/L, m/V = 0.2 g/L, I = 0.01 mol/L NaClO4 and T = 293 K.
PT
Fig. 5 A: U LIII-edge XANES spectra, and B: Fourier transform of the EXAFS spectra
CC E
for the reference samples and perovskite-U(VI) sample at pH 7.5, C0(U(VI))= 20
A
mg/L, m/V = 0.2 g/L, I = 0.01 mol/L NaClO4 and T = 293 K.
25
B
(a) P-Perovskite (b) P+U(VI) Daylight (c) P+U(VI) UV-light
Relative transmittance (%)
P+U(VI) UV-light
(c)
(400)
(204)
(a)
(220)
(200)
(b) (110)
P+U(VI) Daylight
Perovskite
3432
Perovskite PDF#82-0228
10
1400 1630
620
20
30
40
50
3500
60
3000
2500
2000
1500
-1 Wavenumber(cm )
2 Theta (degree)
D 20
SC R
Zeta potential (mV)
0 -10 -20
U
-30
N
-40 3.0
A
2.5
M ED PT CC E A
26
500
0 mM Na2CO3 10.0 mM Na2CO3
Perovskite
10
Fig. 1
1000
IP T
Relative intensity (a.u.)
A
3.5
4.0
4.5
5.0
pH
5.5
6.0
6.5
7.0
7.5
100
100
A
B
0 mM Na2CO3 1 mM Na2CO3
U(VI) species (%)
Sequestration (%)
80
10 mM Na2CO3
80
UO22+
60
40
UO2CO3(aq)
UO2(CO3)3
4-
60
40
UO2(CO3)2
(UO2)2(OH)22+
20
2-
UO2(OH)+
20 0
4
5
6
7
8
9
3
4
5
6
D
C
100
80
90
Qe (mg/g)
60
Qe (mg/g)
pH 5.5 pH 7.5
40 0 mM Na2CO3
pH 7.5
20
80 70 60 50
10.0 mM Na2CO3
U
40
0 30
5
10
15
20
25
0
30
Time (hours)
9
2
3
4
5
Ce (mg/L)
A
Daylight 0 mM Na2CO3
B
ED
UV-light 10.0 mM Na2CO3
pH 7.5
0.6
PT
0.4
CC E
0.0
-50
0
50
0.8 0.6
2.0 1.5 1.0
Eg=3.18 eV
0.5
0.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0
0.4
E (eV)
0.2 0.0
100
300
150
400
500
600
Wavelength (nm)
Time (min)
Fig. 3
A
-100
2.5
(Ahv)1/2
0.8
0.2
3.0
1.0
Daylight 10.0 mM Na2CO3
Dark
Ce/C0
1.2
UV-light 0 mM Na2CO3
Absorbance (a.u.)
1.0
M
A
Fig. 2
1
N
0
8
SC R
100
7
pH
pH
IP T
3
27
700
Ti-OH(b) Ti-OH(a) [TiO6]
B
O 1s
(a) P-Perovskite (b) P-U(VI) Daylight (c) P-U(VI) UV-light
Relative intensity (a.u.)
P+U(VI) Daylight
P+U(VI) UV-light
(d) P-U(VI)-Na2CO3 Daylight
(a) (b)
P+U(VI)+Na2CO3 Daylight
533
(d)
532
531
530
529
468
528
466
464
462
Binding Energy (eV) C
U 4f 7/2 U(VI)
D
386
394
392
390
N 384
382
380
378
U(VI)
SC R
396
A
388
Binding Energy (eV)
450
Desorption
UV-light P-U(VI)-Na2CO3
390
452
U(IV)
U(IV)
C 1s Ca 2p
Daylight P-U(VI)-Na2CO3
454
388
386
384
382
U
Relative intensity (a.u.)
Relative intensity (a.u.)
P-U(VI) UV-light
392
456
U 4f 7/2
U(VI)
P-U(VI) Daylight
394
458
U 4f 5/2
U(IV)
Ti 2p Na KLL
U 4f 5/2 U(IV)
U(VI)
460
Binding Energy (eV)
IP T
534
(c)
0
200
380
378
Na 1s
Relative intensity (a.u.)
P-Perovskite
O 1s
A
Desorption
400
600
800
1000
1200
Binding Energy (eV)
ED
M
Fig. 4
(IV)
U
O2(s)
ax -O U
FT
PT
CC E
U-U
eq
A
B
-O U
Adsorption
A
U(IV) U(VI) pH 7.5
U
(VI)
U-Ti pH 7.5
17172 17178 17184 17190 17196
0
Energy (eV)
Fig. 5
28
1
2
R(Å)
3
4
2+
O2
5
Table 1. Comparison of U(VI) sequestration capacity of perovskite with other adsorbents. Experimental condition
qmax(mg/g)
References
Yolk-shell Fe3O4@TNS
pH 5.0, T=298 K
82.85
[35]
Defective TiO2−x
pH 5.0, T=298 K
65.41
[36]
Fe3O4@TiO2
pH 6.0, T=298 K
91.1
GONS
pH 5.0, T=293 K
97.5
Oxidized MWCNTs
pH 5.0, T=298 K
Perovskite
pH 5.5, T = 293 K
IP T
Materials
[37]
SC R
[38] [39]
119.3
This study
N
U
33.32
Shells
M
Samples
PT
CC E
Perovskite-U(VI) pH 7.5
A
a
U-Oax U-Oeq1 U-Oeq2 U-Oax U-U U-Oax U-Oeq1 U-Oeq2 U-Ti U-U
ED
U(VI)O22+
U(VI)O2(s)d
A
Table 2. Uranium LIII-edge EXAFS spectra for standards and U(VI)-loaded sample R(Å)a
CNb
σ2(Å 2)c
1.78e 2.35(2)f 2.53(1) 2.35e 3.86e 1.78(0) 2.38(2) 2.56(2) 3.15(3) 3.88(2)
2.0e 2.0(5) 3.1(4) 8.0e 12.0e 2.0(1) 2.2(3) 3.3(1) 1.4(0) 7.9(2)
0.0053(2) 0.0027(1) 0.0075(1) 0.0052(6) 0.0034(4) 0.0039(3) 0.0059(1) 0.0087(3) 0.0123(5) 0.0263(7)
R: bond distance; b CN: coordination number; cσ2: Debye-Waller factor; d data from
Schofield et al. (2008) [50]; e fixed number; f digit in bracket: uncertainties.
29