Gamma-ferric oxide nanoparticles decoration onto porous layered double oxide belts for efficient removal of uranyl

Accepted Manuscript Gamma-ferric oxide nanoparticles decoration onto porous layered double oxide belts for efficient removal of uranyl Kairuo Zhu, Changlun Chen, Haiyan Wang, Yi Xie, Muhammad Wakeel, Abdul Wahid, Xiaodong Zhang PII: DOI: Reference:

S0021-9797(18)31194-9 https://doi.org/10.1016/j.jcis.2018.10.005 YJCIS 24160

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

29 June 2018 29 September 2018 3 October 2018

Please cite this article as: K. Zhu, C. Chen, H. Wang, Y. Xie, M. Wakeel, A. Wahid, X. Zhang, Gamma-ferric oxide nanoparticles decoration onto porous layered double oxide belts for efficient removal of uranyl, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.10.005

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Gamma-ferric oxide nanoparticles decoration onto porous layered double oxide belts for efficient removal of uranyl Kairuo Zhua,b, Changlun Chen*a,c, Haiyan Wang,c Yi Xiea, Muhammad Wakeeld, Abdul Wahidd, Xiaodong Zhang*a a

CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of

Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR China b

University of Science and Technology of China, Hefei 230000, PR China

c

Institute of Physical Science and Information Technology, Anhui University, Hefei

230601, P.R. China d

Department of Environment Sciences, Bahauddin Zakariya University, Multan 60800,

Pakistan *Corresponding author. Tel: 86-551-65592788, Fax: 86-551-65591310 E-mail address: [email protected] (C.L. Chen)；[email protected] (X.D. Zhang)

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ABSTRACT Layered double oxides (LDO) and γ-Fe2O3 have been demonstrated to be promising adsorbents to remove radioactive elements from aqueous media. Herein, magnetic γ-Fe2O3 nanoparticles decoration onto porous layered double oxides belts (γ-Fe2O3/LDO) were fabricated by in situ solid-state thermolysis technique combined with Fe(III)-loaded layered double hydroxides as a precursor. The microstructure, chemical composition, and magnetic properties of γ-Fe2O3/LDO were characterized in detail. The as-obtained γ-Fe2O3/LDO was employed as an adsorbent for the elimination of U(VI) from water. The adsorption process followed the Langmuir model with the maximal adsorption capacity of U(VI) onto γ-Fe2O3/LDO being 526.32 mg·g-1 at 303 K and pH 5, which surpassed pristine LDO and many other materials. The Fourier transformed infrared spectra and the X-ray photoelectron spectra analysis suggested that the interaction mechanism was mainly controlled by the surface complexation and electrostatic interactions. All in all, the γ-Fe2O3/LDO with remarkable adsorption capacity, excellent regeneration, and easy magnetic separation opens a new expectation as a suitable material for the cleanup of U(VI) from contaminated water.

Keywords: Layered double oxides belts; Magnetic γ-Fe2O3 nanoparticles; U(VI); Adsorption

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1. Introduction Uranium, one of the most commonly detected radioactive element in water and wastewater, has attracted wide concern [1-11]. The existence of highly mobile hexavalent uranium (U(VI)) in the aquatic environment would exceedingly hazard human being and natural ecosystems [12-16]. In order to avoid serious harm to mankind, it is highly desirable to find a promising efficient approach for U(VI) removal from water. Many water purification technologies including photocatalysis, ion exchange, and adsorption, etc [17-26]. have so far been implemented to treat U(VI) contamination. Adsorption is regarded as one of the simple and most cost-effective methods among these technologies, because of its simple operation, environmental friendliness and independence on facilities. To date, a plenty of adsorbents, i.e., bio-absorbents, metal (hydr)oxides [27], clay minerals [28], and active carbon [29], have been utilized to treat U(VI)-contaminated groundwater. Nevertheless, low adsorption capacities and efficiencies of these above adsorbents restrain their actual applicability. Therefore, it’s still urgent need to synthesize advanced adsorbents with simple recovery and highly efficient enrichment of U(VI). As functional material, γ-Fe2O3 have great potential applications in analysis and separation of pollutants because of their low toxicity, great biocompatibility and magnetic characteristic [30, 31]. In particular, the nanosized γ-Fe2O3 shows unique properties that differ from their corresponding bulk material counterparts due to their small particle size, high surface area and reactive activity. Nevertheless, bare γ-Fe2O3 nanoparticles are prone to aggregate [32]. To address this issue, various natural and synthetic materials have been proposed to immobilize γ-Fe2O3 as a supporter to reduce particle agglomeration and improve their reaction performance [33-38]. Layered double oxides (LDO), the post-calcination product of layered double 3

hydroxides (LDHs) with great surface area and high thermal stability, have been recognized as a promising adsorbent to purify the contaminated wastewater [39, 40]. A survey of study on the removal of U(VI) shows the efficacy of LDO as an effective material for uptake of U(VI) from water [18, 41, 42]. Meanwhile, it should be noted that LDO can serve as supports which can induce or/and restrict the formation of metal nanoparticles with specific morphology and high dispersion by means of an exterior confinement effect from the LDO surface. Such an exterior confinement effect is advantage of avoiding the sintering/aggregation of metal nanoparticles, thus affording metal nanoparticles with high stability [43]. With respect to LDO-supported materials, the previous studies have successfully verified that the combination of LDO with metal nanoparticles such as Au [44], Pt [45], and CoMo alloy [46], not only solves the metal nanoparticles agglomeration problem, but also shows higher reaction activity. Incorporating the magnetic γ-Fe2O3 to the porous LDO to obtain the γ-Fe2O3/LDO composites may preserve or even improve the major features of each phase in the composites, and furthermore, new properties may come from the synergy of both components, the construction of the γ-Fe2O3/LDO composites should be very anticipated. Against this background, a facile and scalable approach was developed to synthesize γ-Fe2O3/LDO composites via in-situ thermal pyrolysis of Fe(III)-loaded LDHs under Ar conditions. The as-prepared LDO and γ-Fe2O3/LDO samples were used and compared as versatile adsorbents for U(VI) immobilization from aqueous solutions. The objectives of the present investigation are (i) to study the characteristics of the γ-Fe2O3/LDO systemically; (ii) to elucidate the U(VI) removal efficiency at different environmental factors like equilibrium time, pH, CO32- concentration, and temperature; and (iii) to identify the possible interaction mechanisms of U(VI) onto 4

γ-Fe2O3/LDO by the Fourier transformed infrared (FT-IR) spectra and the X-ray photoelectron spectra (XPS) analysis. 2. Experimental 2.1. Materials synthesis All solvents and chemicals were obtained from Sinoreagent without further purification. 120 mg·L-1 U(VI) stock solutions were prepared by dissolving UO2(NO3)2·6H2O in Milli-Q water. The γ-Fe2O3/LDO was prepared via three step routes. The Ca-Mg-Al-LDHs were prepared by a simple hydrothermal reaction [27]. Generally, 24 g urea, 5.625 g Al(NO3)3·9H2O, 5.125 g Mg(NO3)2·6H2O and 2.375 g Ca(NO3)2·4H2O were slowly added into the 50 mL Milli-Q water solution. The mixture was stirred for 6 h and then sealed in a 100 mL Teflon-lined stainless-steel autoclave and the autoclave was heated to 140 ℃ and kept for 36 h. The white products were washed with Milli-Q water several times, centrifuged, and dried overnight. Then, 6 g of LDHs powder was dispersed in 400 mL of FeCl3·6H2O solution (2.5 g·L-1), subsequently, the pH of above mixed suspensions was adjusted to 5.0. This pH value guarantees the good survivability of LDHs. After shaking for 4 h, the yellow Fe(III)-loaded LDHs powder was obtained by centrifugal separation at 7500 rpm for 5 min and dried at 60 ℃ under vacuum for one day. Finally, to get black γ-Fe2O3/LDO sample, Fe(III)-loaded LDHs powder was loaded into a tube furnace and heated at 500 ℃ for 2 h under Ar atmosphere, with a heating rate of 5 ℃·min-1. As a comparison, LDO, γ-Fe2O3 and γ-Fe2O3/LDO with LDHs/FeCl3·6H2O mass ratios of 6:0.5 and 6:1.5 were fabricated through the same pyrolysis procedure. 2.2. Batch adsorption experiments A series of adsorption tests were conducted to evaluate the effect of different process parameters. Typically, the suspensions of adsorbents were mixed with 0.01 5

mol·L-1 Na2CO3 solutions in 10 mL polyethylene centrifuge tubes, and then the stock solution of U(VI) (120 mg·L-1) was added to the tubes with the desired concentrations, and 0.1 or 1 mol·L-1 HCl or NaOH solutions were employed to control the initial pH of the suspensions. The effect of CO32- ion was investigated with various concentration gradients (0.01-0.20 M) at pH = 5.0 ± 0.1.The above suspensions were shaken for 240 min to achieve desired removal equilibrium, and then centrifugal separation at 9000 rpm for 10 min. The concentration of U(VI) was determined by an Arsenazo-III spectrophotometer method at a wavelength of 650 nm [47]. The removal efficiency of U(VI) were calculated from the difference between the initial concentration (Co) and the equilibrium one (Ce) (adsorption capacity Qe = (Co - Ce) / m × V and adsorption percentage (%) = 100 % × (Co - Ce) / Co), where V is the volume of mixed suspension (L), and m is the mass of LDO or γ-Fe2O3/LDO (g). Characterization methods and data analysis model are presented in the Supplementary data. 3. Results and discussion 3.1. Synthesis of γ-Fe2O3/LDO A schematic illustration of the pathway for the synthesis of the γ-Fe2O3/LDO is shown in Scheme 1. Firstly, urea would be gradually hydrolyzed and generated NH4+, CO2- and OH- under the hydrothermal conditions (CO(NH2)2 + 3H2O → 2NH4+ + CO2↑+ 2OH-), thus raising the pH value of solution. Consequently, Mg 2+, Ca2+ and Al3+ start to aggregation and growth to form LDHs crystals. Then, Fe3+ is loaded by the LDHs at pH = 5 and converted into Fe-hydroxide (FeO(OH)) composites in the drying process [48]. The subsequent pyrolysis step of Fe(III)-loaded LDHs at 500 ℃ under Ar atmosphere leads to nucleation and growth of magnetic γ-Fe2O3 and the release of porosity of LDO, resulting in the magnetic γ-Fe2 O3 anchored onto porous LDO belts. 6

Based on the previous study [27], LDO calcined at 500 ℃ showed the highest adsorption performance due to the more active surface sites, therefore, this heat treatment temperature was chosen in the study. In the X-Ray Fluorescence (XRF) result analysis of γ-Fe2O3/LDO (Table S1), one can find that the major compositions of γ-Fe2O3/LDO were Al2O3 (45.07 wt %), Fe2O3 (34.83 wt %), MgO (11.26 wt %) and CaO (9.42 wt %). 3.2. Structural characterization and material properties Scanning electron and transmission electron microscopy (SEM and TEM) were employed to examine the morphologies and structures of the pristine LDO and γ-Fe2O3/LDO and displayed in Fig. 1. From Fig. 1A, a typical belt-like structure was exhibited and the orderliness is clear. One can find that the width of LDO was not uniform and the maximal thickness is ~480 nm. Interestingly, plenty tiny pores could be observed on the belt-like LDO after calcinations treatment (Fig. 1B). From Fig. 1C, more discrete γ-Fe2O3 nanoparticles were evenly supported on the surface of LDO belts, and it can be observed from Fig. 1D that the size of γ-Fe2O3 nanoparticles was about 30-80 nm without obvious aggregations. Notably, the well-dispersed γ-Fe2O3 nanoparticles were in situ generated during the calcinations process, which are further investigated and evidenced by HRTEM images. Fig. 1E depicted a randomly selected magnified particle, which further indicated γ-Fe2O3 nanoparticles grown on porous LDO belts. Moreover, the HRTEM image of the particle displayed a regular lattice and the lattice distance of 0.25 nm, which is well-assigned to the (311) plane of γ-Fe2O3 [49]. As the results from the energy dispersive spectroscopy (EDS) analysis of the γ-Fe2O3/LDO shown in Fig. 1F, Mg, Al, Ca, O and Fe were detected. Elemental mapping of the γ-Fe2O3/LDO suggested the distribution of the five elements (Mg, Al, Ca, O and Fe) within the belt structures (Fig. S2). It can be obviously seen that Fe is 7

widely distributed in the LDO belts. To further confirm the formation of γ-Fe2O3/LDO, X-ray diffraction (XRD) patterns of LDO, γ-Fe2O3, and γ-Fe2O3/LDO were displayed in Fig. 2A and S3a. In the XRD pattern of LDO, the peak at 2θ = 62.1° displayed the typical (113) crystal planes, which implied that mixed oxides of magnesium, aluminum, and calcium were produced by the pyrolysis process [39]. In the XRD patterns of γ-Fe2O3/LDO and γ-Fe2O3, typical peaks at 30.1°, 35.5°, 43.1°, 53.5°, 57.0°, and 62.5° were assigned to the (220), (311), (222), (400), (422), (511), and (440) facets of γ-Fe2O3 (JCPDS No. 39-1346), respectively [48], which indicated that the γ-Fe2O3 of γ-Fe2O3/LDO had the high crystallinity and purity. The FT-IR spectra were used to study the structural information and chemical components of the LDO and γ-Fe2O3/LDO and shown in Fig. 2B. The wide peak of two samples at 3446 cm-1 corresponded to the stretching mode of -OH group [50], and three other peaks at 1051, 876, and 710 cm-1 corresponded to M-O (M = Ca, Mg, and Al) [39, 40]. Furthermore, the particular characteristic peaks at 1629 and 1420 cm-1 were assigned to the water bending vibration and CO32- vibrations, respectively [50]. Notably, the band at 592 cm-1 on γ-Fe2O3/LDO was involved in the stretching vibration of the Fe-O bond [51]. From the above results, one can be concluded that γ-Fe2O3/LDO composites were successfully developed. Zeta potential values of LDO and γ-Fe2O3/LDO were depicted in Fig. 2C, and the point of zero charge (pHzpc) values of LDO and γ-Fe2O3/LDO were ~10.02 and ~6.78, respectively. The γ-Fe2O3/LDO surface was positively charged ranging at pH < 6.78 due to the γ-Fe2O3 grown on the LDO surface, which implied the Fe-O groups on the surface of γ-Fe2O3/LDO were responsible for the surface negative charge. The magnetization curve of the γ-Fe2O3/LDO was presented in Fig. 2D. The saturation 8

magnetization value was measured to be ~7.02 emu·g -1, the inset digital picture revealed that γ-Fe2O3/LDO composites can be easily separated from aqueous solution by a permanent magnet. The Brunner-Emmet-Teller (BET) surface area and pore-size distribution were well-known to be vital for adsorption performances. The isotherm of the γ-Fe2O3/LDO in Fig. 2E clearly demonstrated a typical isotherm of type-IV with the hysteresis loop of type H3 in the P/P0 range from 0.50 to 1.00. The appearance of hysteresis loop presented the existence of mesopores [52]. The BET surface area of the LDO and γ-Fe2O3/LDO were calculated to be ~105.79 and 127.41 m2·g-1, respectively. According to the pore-size distribution from Fig. 2F, the γ-Fe2O3/LDO exhibited porous structures with micropores (< 2 nm) and mainly mesopores (2-50 nm) [53]. It's really the opposite to the LDO. Therefore, we concluded that the growth of γ-Fe2O3 crystals on the LDO surface would block the LDO micropores, resulting in the decrease of micropores. Meanwhile, new mesopores would be generated between γ-Fe2O3 particles growing on the surface of LDO. The high surface area and porous property of the γ-Fe2O3/LDO can provide abundant active sites and they are conducive to the efficient transportation of metal ions to interconnected pore structure systems, thus leading to enhancing adsorption properties. haracteristic

ssbauer spectra of γ-Fe2O3/LDO collected at 300 K are shown in

Fig S3b. The spectra exhibited both quadrupole and magnetically split contributions; however, the former contribution dominated the absorption area in all spectra. At 300 K, the resonant absorption lines of the magnetically split contributions were rather broad, whereas a set of two magnetically (M1 and M2) and one quadrupole (SP) split components were used to fitted these spectra. The

ssbauer hyperfine parameter

values resulting from these fits are listed in Table S2, which showed that all 9

components characterize Fe3+ high spin (S = 5/2) ions in oxygen coordinated environment. Combining the information available from XRD and TEM measurements, one can conclude that the iron bearing phase in our samples is that of γ-Fe2O3. Due to the particle size distribution, γ-Fe2O3 nanoparticles hold both superparamagnetic and ferromagnetic

ssbauer signals at room temperature. In Raman spectra of

γ-Fe2O3/LDO (Fig. S3c), as clearly illustrated in the region 800-400 cm-1 where a broad band appeared for high iron oxide concentration, which was recorded from a reference sample containing small nanoparticles of γ-Fe2O3 [54]. Thermo gravimetric analysis (TGA) curve of γ-Fe2O3/LDO was depicted in Fig. S3d. It was evidenced that one step of weight loss only occurred during the calcination process, which can be due to the evaporation of absorbed water. The XPS technique was further employed to study the surface chemical properties of LDO and γ-Fe2O3/LDO, as shown in Fig. 3. Both LDO and γ-Fe2O3/LDO exhibited similar elemental compositions as indicated from the main peaks at 24.4, 50.3, 74.2, 119.4, 347.2, and 531.6 eV in the survey spectra, corresponding to the elemental species of O 2s, Mg 2p, Al 2p, Al 2s, Ca 2p, and O 1s, respectively (Fig. 3A). For γ-Fe2O3/LDO, the presence of the additive peaks at 705~735 eV was attributed to Fe 2p. The fitted high-resolution spectrum of Fe 2p (Fig. 3B) revealed that the peaks located at ~724.8 and 711.1 eV were assigned to Fe 2p1/2 and Fe 2p3/2 of γ-Fe2O3 rather than Fe3O4, which were consistent with the XRD results [55]. Both peaks were accompanied by satellite structures with higher binding energy, which was characteristic of the Fe3+ species in Fe2O3. As shown in Fig. 3C, the peaks at ~530.10, 531.39 and 532.60 eV in O 1s should be assigned to Ca-O, Al-O and Mg-O bonds of LDO, respectively [27]. Similar peak deconvolution results were also observed in the XPS curves of the γ-Fe2O3/LDO (Fig. 3D), while the new peak at ~529.78 eV for Fe-O 10

bonds was an evidence of iron species [48]. The addition of the Fe-O bonds could improve the stability of the formation of U(VI)-oxides, which was beneficial for U(VI) uptake from aqueous solutions based on the high activity of Fe-O [56]. 3.3. Kinetics of uranium adsorption U(VI) adsorption onto γ-Fe2O3/LDO as a function of contact time was shown in Fig. 4A. The adsorption amount of the U(VI) onto γ-Fe2O3/LDO increased quickly at first; afterwards, it kept stable with the rising time. At pH 5.0, the U(VI) removal could obtain equilibrium states within 40 min, whilst at pH 7.0, longer adsorption time about 60 min was required. In the pH 7.0 solution, the γ-Fe2O3/LDO had a negative surface (pHzpc = 6.78 as shown in Fig. 2C), whereas uranium was mainly exist in the form of very stable uranium carbonate complexes (i.e., UO2CO3 (aq), UO2(CO3)22- and UO2(CO3)34-) [57]. In the consequence, the electrostatic repulsion between the γ-Fe2O3/LDO and negative uranium species may be the cause for the slow uptake kinetics [58]. In addition, the correlation coefficient of the pseudo-second-order rate equation (Eq. S1) for the linear plot is 0.99 (Fig. S4a), indicating that the U(VI) removal process was primarily involved in chemical reaction [59]. 3.4. Effect of pH, CO32- concentration and foreign ions Effect of pH on the U(VI) adsorption was investigated in detail from pH 2.5 to 11.0. The results were depicted in Fig. 4B, the removal efficiency of U(VI) increased sharply when solution pH increases from 2.5 to 4.5, and maintains almost 90 % at pH 4.5-6.0, followed a gradual decrease at pH > 6.0. Such a tendency is in good accordance with the literature reports about the U(VI) removal on the pure LDH, LDO, and relevant composites [60]. Moreover, the optimum pH value was 5.1 (Fig. S4b), showing the highest removal percentage. And as displayed in Fig. 4C and Table S3, U(VI) mainly existed as positively charged species (i.e., UO22+ and UO2OH+) at pH < 11

6.0 and negatively charged species (i.e., UO2(CO3)22- and UO2(CO3)34-) at pH > 7.0, the result was calculated by Visual MINTEQ ver. 3.0 [61]. Consequently, the increased removal amount of U(VI) onto γ-Fe2O3/LDO at pH 2.0-6.0 could be attributed to the strong electrostatic interaction between positive charged U(VI) species and negatively charged surface of the γ-Fe2O3/LDO. At pH > 6.4, the negative surface charge of γ-Fe2O3/LDO was suppressive for electrostatic interaction with positive cationic U(VI) species. The selectivity of γ-Fe2O3/LDO for U(VI) (Fig. S4c) was evaluated by measuring the efficient removal of UO22+ in the presence of competing cations likes Na+ (50 mg·L-1), K+ (50 mg·L-1), and Mg2+ (50 mg·L-1) at pH = 4 and 6. The experiment was conducted for 24 h. It is worth noting that the solution didn’t add the

O32- ions. The

composite showed excellent selectivity towards UO22+ because the concentration of UO22+ in any environment is significantly lower than the concentrations of coexisting cations, such as Na+, K+, and Mg2+. The selectivity preference of γ-Fe2O3/LDO for U(VI) is also reflected by its Kd value, which is as high as 15.11 and 221.48 mL·g-1 at pH = 4.0 and 6.0, as well as 10~40 times higher than those for other ions (Table S4). In addition, in the acidic medium, the H+ ions compete with Na+ and K+ for adsorption site, and therefore, the adsorption of Na+ and K+ was less as compared to UO22+ [62]. In a water system, CO32- has been the essential component due to the high solubility of CO2 in aqueous solutions, and the effect of CO32- concentration in the U(VI) removal from aqueous solutions should be considered. As shown in Fig. 4D, with the increase of CO32- concentration from 0.01 to 0.20 M, the removal capacity of U(VI) on γ-Fe2O3/LDO decreased slightly, which might be attributed to the abundant CO32- that changed the distribution of surface charge of solid adsorbents (Fig. S4d), and it caused strong electrostatic repulsion between UO22+ and adsorbents [63]. In the 12

meantime, the impact of various electrolyte anions for U(VI) removal onto γ-Fe2O3/LDO was shown in Fig. S4e. The results depicted that HCO3-, NO3-, SO32-, SO42- and PO43- had a positive impact on U(VI) adsorption onto γ-Fe2O3/LDO. These anions and U(VI) could form stable soluble complexes, leading to the increase of U(VI) adsorption. Moreover, it can be observed that PO43- had the significant impact on U(VI) removal. The presence of PO43- increased U(VI) removal on γ-Fe2O3/LDO, which can be due to the formation of the coordination between PO43- and UO22+. 3.5. Adsorption isotherms and thermodynamics U(VI) adsorption isotherms for the LDO, γ-Fe2O3, and γ-Fe2O3/LDO with vary mass ratio of LDHs to FeCl3·6H2O (6: 0.6, 6:1 and 6:1.5) at pH 5.0 were displayed in Fig. 5A and Fig. S4f. One can be observed that the adsorption amount of U(VI) onto absorbents increased as the U(VI) concentration increased because of stronger driving forces at higher U(VI) concentrations. It is worth noting that stronger driving forces are helpful for the transportation of U(VI) species from the solution to the surface of adsorbents, leading to a promotion in collisions between U(VI) and the active sites on adsorbents. γ-Fe2O3/LDO exhibited higher U(VI) removal capability than pure LDO and γ-Fe2O3, indicative of additional functional γ-Fe2O3 nanoparticles enhancing U(VI) removal performance. To measure the adsorption capacity of the γ-Fe2O3/LDO, the experimental data were fitted by the Langmuir, Freundlich and D-R models (Eqs. S2-S4). The relative parameters of three isotherm models were calculated and listed in Table 1. From the correlation coefficients (R2), the experimental data clearly showed better fit the Langmuir model than the Freundlich and D-R model, indicating that the binding energy on the entire surface of adsorbents was uniform, and chemisorption was related to the adsorption process [64]. Furthermore, we can also note that the γ-Fe2O3/LDO synthesized with LDHs/FeCl3·6H2O mass ratio = 6:1 possessed the 13

maximal U(VI) removal capacity (526.32 mg·g -1), which was much higher than those of many reported adsorbents (Table S5). The maximum removal capacity of U (VI) by γ-Fe2O3 and LDO was 27.31 and 432.21 mg/g at 303 K and pH 5.0, respectively, which suggested that LDO dominated U(VI) adsorption and γ-Fe2O3 synergism enhanced U(VI) removal. In addition, the U(VI) adsorption performance of the γ-Fe2O3/LDO under three different temperatures was depicted in Fig. 5B. The adsorption capacity significantly increased as adsorption temperatures rise from 303 to 333 K, suggesting that high temperature promotes U(VI) adsorption performance. The values of thermodynamic parameters were computed according to Eqs. S8 and S9. The related thermodynamic parameters were calculated by line regression (Fig. 5F) and listed in Table 2, which presented positive ΔH0, positive ΔS0 and negative ΔG0 values, indicative of spontaneous and endothermic reaction under the conditions applied and improved disorderliness of the solid-solution system during the adsorption process [65]. 3.6. Desorption and regeneration The influence of background electrolyte on desorption was performed. Typically, after adsorption, the adsorbent was dispersed in 5 mL 0.2 mol·L-1 NaCl, 0.2 mol·L-1 NaNO3, 0.2 mol·L-1 NaClO4, 0.2 mol·L-1 Na2CO3, 0.2 mol·L-1 Na2SO4, 0.2 mol·L-1 HCl, and 0.2 mol·L-1 HNO3 solution, respectively. 24 h later, the concentrations of uranium were detected in the same method. As displayed in Fig. 5G, Na2CO3 revealed much higher desorption efficiency than that of NaNO3, NaClO4, NaCl, Na2SO4, HCl, and HNO3. The results suggested that high level of carbonate could hinder the surface complexation while general inorganic salts had a little effect on adsorption and high concentration of strong acid destroyed the structure of γ-Fe2O3/LDO. Therefore, it’s 14

important to make sure Na2CO3 is the most effective in the desorption process. The relationship between regeneration cycle number and adsorption amounts of U(VI) was also studied. Typically, after each adsorption, the recycled adsorbent was immersed in 0.2 mol·L-1 Na2CO3 solution and shaken for 12 h, then isolated by a magnet and washed by Milli-Q water repeatedly several times for the adsorbent regeneration. The collected solid was then freeze-dried at -60 oC for 24 h using FD-1A-50 freeze dryer. As depicted in Fig. 5H, the U(VI) removal percentage slightly decreases from ~89 % to ~81 % after six cycles. The slight decrease in the maximal removal efficiency may be due to mass loss during the adsorption-desorption process. The stable property, the excellent regeneration as well as reusability capacity suggests that γ-Fe2O3/LDO can be used as a renewable material for U(VI) removal from various water systems [66]. 3.7. Stability The blank test for the survivability of γ-Fe2O3/LDO and LDHs was shaken at pH 2.5 and 5 in the 10 mL polyethylene centrifuge tube, respectively. The experimental data were the average of triplicate experiments. As depicted in Fig. S4g and Fig. S4h, the blank test displayed that the mass loss of γ-Fe2 O3/LDO and LDHs were 4.41 % and 2.83 %, respectively, which suggested that high survivability and good stability of γ-Fe2O3/LDO at pH 2.5 and LDHs at pH 5. The magnetization curve of the sample of γ-Fe2O3/LDO after U(VI) removal is presented in Fig. S5. The saturation magnetization value was measured to be 3.72 emu·g-1, the digital picture suggested the easy separation of the adsorbent in the reaction system with a magnet. The structure and textural property of the sample after six reaction-recovery cycles were studied by 15

the characterization of XRD and N2 adsorption-desorption. As shown in Fig. S6a, there is the obvious existence of the crystalline of γ-Fe2O3 in the sample, suggesting that the good stability of γ-Fe2O3/LDO in the experiments. From Fig. S6b, one can see that large amounts of macropores and few mesopores in the structures of the sample after five reaction-recovery cycles, indicated that the UO22+ loaded would block the γ-Fe2O3/LDO mesopores, resulting in a large amount of mesopores decreasing. 3.8. Interaction mechanism In this section, FT-IR and XPS investigations were ascertained to gain insight into the possible interaction mechanism. The FT-IR results of the γ-Fe2O3/LDO and γ-Fe2O3/LDO after U(VI) removal were depicted in Fig. S7. After uptake, the sharp peak at ~1513 cm-1 was probably due to the adsorbed CO3 - asymmetric stretching on γ-Fe2O3/LDO which originated from uranium carbonate complexes [56]. Specifically, the stretching vibration of the Mg-O groups led to a slightly shift from 876 to 879 cm-1, and the relative intensity of the Fe-O absorption band decreased after U(VI) adsorption. These changes may be attributed to the surface complexation of U(VI) onto γ-Fe2O3/LDO [67]. The full survey and high resolution XPS spectra of the γ-Fe2O3/LDO and γ-Fe2O3/LDO after U(VI) removal are depicted in Fig. 6A-D. As shown in Fig. 6A, U species successfully were loaded on γ-Fe2O3/LDO. According to the above U 4f spectrum analysis (Fig. 6B), the presence of U 4f 7/2 and U 4f 5/2 peaks located at 381.78 and 392.68 eV, respectively, were due to the multiple splitting of electrons with unpaired spins in the atomic shells [68]. Afterward, the Fe 2p and O 1s peaks were further analyzed. It can be detected that the representative spectrum of Fe 2p for γ-Fe2O3/LDO after U(VI) adsorption exhibited two broad peaks at ~711.38 and 16

~725.08 eV. Considering the fitted high-resolution spectrum of Fe 2p for the γ-Fe2O3/LDO after U(VI) adsorption, the U(VI) loaded can lead to an obvious peak shifts with the value of ~0.28 and ~0.19 eV (Fig. 6C). The evident energy shifts demonstrated that the Fe-O group would exert strong adsorption complex with highly mobile U(VI) ions [69]. From Fig. 6D, it was clearly observed that the O 1s peak strength reduced considerably after U(VI) removal, indicating that the O species were responsible for the highly effective removal of U(VI) onto γ-Fe2O3/LDO. The complex O 1s spectrum of γ-Fe2O3/LDO after U(VI) adsorption can also be deconvoluted into four different oxygen containing functional groups: (a) Ca-O at ~530.23 eV, (b) Al-O at ~531.45 eV, (c) Mg-O at ~532.78 eV, and (d) Fe-O at ~529.77 eV [27, 48]. After U(VI) loaded, the content of Al-O obviously increased, whereas the contents of Mg-O, Fe-O reduced and the content of Ca-O groups mainly kept unchangeable (Table 3). The consequence revealed that surface complex between Mg-O, Fe-O and Ca-O and UO22+ may participate in the adsorption processes, in which Mg-O and Fe-O group dominate the surface complexation.

4. Conclusions The magnetic γ-Fe2O3 nanoparticle immobilization onto LDO belts was successfully performed by loading Fe3+ onto LDHs belts, and following in-situ calcination process under Ar atmosphere. According to characterization results, the γ-Fe2O3/LDO presented the porous structure and magnetic property. The macroscopic adsorption experiments displayed the U(VI) adsorption kinetics and isotherms onto γ-Fe2O3/LDO fitted the pseudo-second-order kinetic model and the Langmuir model well, respectively, and U(VI) removal on the γ-Fe2O3/LDO was highly dependent on pH and CO32concentration. Thermodynamic parameters displayed that U(VI) removal on the γ-Fe2O3/LDO was an endothermic and spontaneous process. The γ-Fe2O3/LDO 17

exhibited extremely high efficiency for U(VI) adsorption, considerably higher than that of pristine LDO. On the base of FT-IR and XPS analysis, the adsorption mechanisms were ascribed to the electrostatic interaction and surface complexation. These consequences determined that the γ-Fe2O3/LDO could be promising candidates for the enrichment of U(VI) in practical applications. Acknowledgments Financial supports from the National Natural Science Foundation of China (21477133) and CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences are acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. References [1] F. Yuan, C. F. Wu, Y. W. Cai, L. J. Zhang, J. Q. Wang, L. H. Chen, X. K. Wang, S. T. Yang, S. A. Wang, Synthesis of phytic acid-decorated titanate nanotubes for high efficient and high selective removal of U(VI), Chem. Eng. J. 322 (2017) 353–365. [2] Y. W. Cai, C. F. Wu, Z.Y. Liu, L. J . Zhang, L. H. Chen, J. Q. Wang, X. K. Wang, S. T. Yang, S. A. Wang, Fabrication of a phosphorylated graphene oxide-chitosan composite for highly effective and selective capture of U(VI), Environ. Sci. Nano 4 (2017)1876–1886. [3] R. Zhang, C. L. Chen, J. Li, X. K. Wang, Investigation of interaction between U(VI) and carbonaceous nanofibers by batch experiments and modeling study, J. Colloid Interface Sci. 460 (2015) 237–246. 18

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Highly

efficient

lead(II)

sequestration

using

size-controllable

polydopamine microspheres with superior application capability and rapid capture, ACS Sustain. Chem. Eng. 5 (2017) 4161–4170.

28

Scheme 1 Schematic preparation diagram of the γ-Fe2O3/LDO. Fig. 1. SEM, TEM images of LDO (A and B), γ-Fe2O3/LDO (C and D); HRTEM (E)and EDS (F) images of the γ-Fe2O3/LDO. Fig. 2. (A) XRD patterns, (B) FT-IR spectra, (C) zeta potentials of the LDO and γ-Fe2O3/LDO; (D) room-temperature magnetization curve (the inset is a digital photograph of the magnetic separation), nitrogen adsorption-desorption isotherms (E), and the pore-size distributions (F) of the γ-Fe2O3/LDO. Fig. 3. XPS spectra of the LDO and γ-Fe2O3/LDO: (A) survey spectra, (B) Fe 2p, (C and D) O 1s. Fig. 4. (A) Adsorption kinetics of U(VI) on γ-Fe2O3/LDO (C0 = 25 mg·L-1, m/V = 0.1 g/L, I = 0.01 mol·L-1 Na2CO3, and T = 303 K); (B) effect of pH on U(VI) adsorption onto γ-Fe2O3/LDO (C0 = 25 mg·L-1, m/V = 0.1 g·L-1, I = 0.01 mol·L-1 Na2CO3, and T = 303 K); (C) species distribution of U(VI) as a function of pH (C(Na2CO3) = 0.01 mol·L-1); (D) impact of CO32- concentration on U(VI) adsorption onto γ-Fe2O3/LDO (C0 = 25 mg·L-1, pH 5.0 ± 0.1, m/V = 0.1 g·L-1, and T = 303 K). Fig. 5. (A) Adsorption isotherms of U(VI) on LDO and γ-Fe2O3/LDO with LDHs/FeCl6·H2O mass ratios of 6:0.5, 6:1.0, and 6:1.5 (pH 5.0 ± 0.1, m/V = 0.1 g·L-1, I = 0.01 mol·L-1 Na2CO3, and T = 303 K), (B) adsorption isotherms of U(VI) onto γ-Fe2O3/LDO at 303, 318 and 333 K (pH 5.0 ± 0.1, m/V = 0.1 g·L-1, and I = 0.01 mol·L-1 Na2CO3), line plots of the Langmuir (B), Freundlich (C) and D-R (D) models, plot of △G0 versus T (F), desorption experiments at different background electrolyte (G), recycle property of the γ-Fe2O3/LDO (H). 29

Fig. 6. XPS spectra of the γ-Fe2O3/LDO and γ-Fe2O3/LDO after U(VI) adsorption: (A) survey spectra, (B) U 4f, (C) Fe 2p and (D) O 1s.

load Fe3+

500℃/2h

Ar LDHs

Fe(III)-loaded LDHs Scheme 1

30

γ-Fe2O3/LDO

Fig. 1.

31

♦ γ-Fe2O3 ♥ LDO

Intensity (a.u.)

♦ ♦

♦

♥ ♥

γ-Fe2O3/LDO ♦

♦

♥

LDO

B

γ-Fe2O3/LDO

Intensity (a.u.)

A

♦

♥

LDO 592

1629 1051 1420 876 716

3446

20

40

4000

60

LDO

20 0

γ-Fe2O3/LDO

-20 5

6

7

8

9

10 11 12

2000

1000

-1

D

3 0 -3 -6 -9

-1

E

6

0.025

-1

pH

9

3000

Wavenumber (cm )

0.020

-20000 -10000

0

10000 20000

Magnetic field (Oe)

F

3

150

4

Adsorption dV/dD (cm ·g ·nm )

-40

Quantity adsorbed (cm ·g STP)

-1

50

C

180

3

40

2 Theta (degree)

Magnetization (emu·g-1)

Zeta potential (mV)

60

30

120

LDO γ-Fe2O3/LDO

90 60 30 0 0.0

0.2

0.4

0.6

0.8

0

LDO γ-Fe2O3/LDO

0.015 0.010 0.005 0.000 1 2

1.0

5 10 20 30 40 50 60 70 80 90

Pore diameter (nm)

Relative pressure (P/P )

Fig. 2.

32

200

400

600

800

Binding energy (eV)

C

Fe 2p3/2 Fe 2p1/2 Fe 2p3/2 Satellite

Fe 2p1/2 Satellite

705 710 715 720 725 730 735

Binding energy (eV)

Fig. 3.

33

Intensity (a.u.)

LDO

B Intensity (a.u.)

Fe 2p

O 1s

C 1s Ca 2p

O 2s Al 2p Al 2s

0

γ-Fe2O3/LDO

Mg 2p

Intensity (a.u.)

A

γ-Fe2O3/LDO

Al-O 531.48

Mg-O 532.60

Ca-O 530.40 Fe-O 529.78 Al-O 531.39

LDO Mg-O 532.60

Ca-O 530.10

528

530

532

534

Binding energy (eV)

536

B) 100 Removal percentage (%)

A)250 150

pH 5.0 pH 7.0

100 50

60 40 20

100 120

UO2(CO3)3

4-

60 +

UO2OH

40

UO2(CO3)2

2-

20 0

2

3

4

5

6

7

5

8

9

pH

7

8

9

10 11

80 60 40 20 0

10

6

pH

D)100

UO2CO3(aq)

2+

4

2-

-1

CO3 concentration (mol·L )

Fig. 4.

34

0.20

UO2

80

3

0.18

C) 100

2

0.15

80

0.12

60

t (min)

0.06

40

0.03

20

Removal percentage (%)

0

0.09

0

Species (%)

80

0.01

Qt (mg·g

-1

)

200

A LDO LDHs:Fe = 6:0.5 LDHs:Fe = 6:1.0 LDHs:Fe = 6:1.5

200

450 300

303K 318K 333K

150

Ce /Qe

Qe (mg·g

-1

-1

300

2.8

12 )

16

-1

0

4

12

γ-Fe2O3/LDO 318 K

6.0

γ-Fe2O3/LDO 333 K

lnQe

LDO 303 K

γ-Fe2O3/LDO 333 K LDO 333K 0.8

3

6

9

0.0

0.2

0.4

2

0.6

ln (1+1/Ce)

G

100

-13.5 -14.0 -14.5

0.8

300 305 310 315 320 325 330 335

T (K)

-1

Qe (mg·g )

60 40

H

60 40

3

O N H

4

Cl H

SO 2

a

CO 2

a N

aC N

l

4

0 3

0

lO

20

O

20

aN

18

-13.0

80

aC

15

F

-12.5

100

80

N

12

Ce

-15.0

1.2

lgCe Desorption (%)

0.4

N

0.0

4.5

N

2.0 1.8

5.0

γ-Fe2O3/LDO 318 K

3

lgQe

γ-Fe2O3/LDO 303 K

2.2

0

5.5

2.4

0

-12.0

γ-Fe2O3/LDO 303 K

E

2.6

0.00

16

-1

Ce (mg·L )

6.5

D

8

0.02

-1

8

Ce (mg·L

ΔG (KJ·mol )

4

0.03

0.01

100 0

C

γ-Fe2O3/LDO 303 K γ-Fe2O3/LDO 318 K γ-Fe2O3/LDO 333 K LDO 303 K

0.04

)

Qe (mg·g )

400

0.05

B

600

1

2

3

4

5

Cycle number

Fig. 5.

35

6

B U 4f

after U(VI) loaded

γ-Fe2O3/LDO

0

200

400

600

U 4f7/2

800

378 381 384 387 390 393 396

Binding energy (eV)

Binding energy (eV)

C 711.38

D

725.08

Intensity (a.u.)

Intensity (a.u.)

after U(VI) loaded 0.28 eV

705

0.19 eV

711.10

724.89

γ-Fe2O3/LDO

710

715

720

U 4f5/2

Intensity (a.u.)

Intensity (a.u.)

A

725

730

after U(VI) loaded

Al-O Ca-O

Mg-O

Fe-O

γ-Fe2O3/LDO

Al-O

Mg-O

Ca-O Fe-O

528

530

532

534

Binding energy (eV)

Binding energy (eV) Fig. 6.

36

536

Table 1 Summary of the Langmuir, Freundlich and D-R model for U(VI) adsorption.

samples

T (K)

Langmuir Qm

KL

(mg·g-1)

(L·mg-1)

Freundlich 2

R

KF

n

D-R 2

R

(mg1-n·Ln·g-1)

Qm

β

(mg·g-1)

(mg2·kJ-2)

R2

LDO

303

432.21

0.344

0.985

115.63

0.465

0.931

313.09

2.14

0.897

6:0.5

303

464.62

0.356

0.983

129.51

0.456

0.927

337.98

2.26

0.892

6:1.0

303

522.08

0.362

0.983

150.28

0.442

0.939

366.98

1.68

0.824

318

625.00

0.516

0.991

222.27

0.378

0.926

472.10

1.49

0.910

333

666.67

0.833

0.989

331.29

0.278

0.893

585.23

1.51

0.925

303

505.99

0.362

0.993

139.48

0.469

0.938

373.23

2.29

0.935

6:1.5

Table 2 Thermodynamic parameters for U(VI) adsorption onto γ-Fe2O3/LDO. T (K) 303 318 333

△G0 (kJ·mol-1) -12.22 -13.93 -14.75

△S0 (J·mol-1·k-1)

△H0 (kJ·mol-1)

84.20

13.15

Table 3 XPS results of the oxygen-containing functional groups of γ-Fe2O3/LDO before and after U(VI) adsorption. sample γ-Fe2O3/LDO after U(VI) adsorption

Mg-O 21.76% 13.11%

Al-O 59.39% 69.35%

37

Ca-O 10.38% 10.96%

Fe-O 8.47% 6.58%

Graphic abstract

Electrostatic Attraction Surface Complexation

UO22+

38

γ-Fe2O3/LDO