Selective removal of uranyl from aqueous solutions containing mix of

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Selective removal of uranyl from aqueous solutions containing mix of toxic metal ions using core/shell MFe2O4-TiO2 nanoparticles of montmorillonite edge sites Liang Bian, Jianan Nie, Xiaoqiang Jiang, Mianxin Song, Faqin Dong, Weimin Li, Liping Shang, Hu Deng, Huichao He, Bing Xu, Bin Wang, and Xiaobin Gu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03129 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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ACS Sustainable Chemistry & Engineering

Selective removal of uranyl from aqueous solutions containing mix of toxic metal ions using core/shell MFe2O4-TiO2 nanoparticles of montmorillonite edge sites

Liang Bian1,3,*, Jianan Nie1, Xiaoqiang Jiang1, Mianxin Song1,*, Faqin Dong1, Weimin Li1, Liping Shang1, Hu Deng1, Huichao He1, Bing Xu2, Bin Wang1, Xiaobin Gu3

1 Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education, State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, South West University of Science and Technology, Mianyang 621010, Sichuan, China 2 Sichuan Civil-military Integration Institute, Mianyang 621010, Sichuan, China 3 Institute of Gem and Material Technology, Hebei GEO University, Shijiazhuang 050000, Hebei, China

*

Corresponding Authors: Liang Bian ([email protected]), Mianxin Song

([email protected]) Mailing address: 1 and 2: 59 Qinglong Road, Mianyang,Sichuan, P.R.China 621010; 3: Huai An Road No. 136, Shijiazhuang, Hebei, P.R.China 050031

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Abstract Usually, uranyl (UO22+) is competitively adsorbed by coexisting potentially toxic metal ions (Xn+: Rb+, Sr2+, Cr3+, Mn2+, Ni2+, Zn2+, Cd2+) that limit the adsorbent application. Here, the core-shell MFe2O4-TiO2 (M=Mn, Fe, Zn, Co, or Ni) nanoparticles were synthesised on K-montmorillonite (MMT) edge sites and assessed as new selective adsorbents. The results revealed that UO22+ and Xn+ were simultaneously adsorbed on the TiO2(101) surfaces, MFe2O4(111)-TiO2(101)/MMT(100)-MFe2O4(111)

interfaces

and

MMT

inner

layers.

Specifically, the Xn+ were mainly adsorbed on the TiO2(101) surfaces. We note that according to Freundlich models, UO22+ and Cr3+ were selectively adsorbed on the MFe2O4(111)-TiO2(101) interface.

High

adsorption

capacity

of

UO22+

was

109.11

mg·g-1

in

the

MMT-Fe3O4(111)-TiO2(101) interface. The interface electron gases transferred from MMT(100)-MFe2O4(111) to MFe2O4(111)-TiO2(101) prevent the Cr3+ oxidation-reduction reaction and further adsorption. Our results suggested that MMT-MFe2O4-TiO2 is a suitable candidate of highly selective uranyl removal. Keywords: uranyl, toxic metal, ferrite, titanium oxide, montmorillonite

Introduction In the oxidising environments of uranium contaminated soils and sediments, the most stable uranium (U6+) state usually occurs as linear uranyl (UO22+) (1), which originates from mining, sea-water extraction, the nuclear industry, etc. A remarkable coexistence phenomenon can be found between low concentration (ppm level) cations and uranyl. These cations include the solid 2 ACS Paragon Plus Environment

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solution within the fuel matrix: 90Sr (89Sr), 50Cr, etc. (2); and natural aqueous cations: Sr2+, Cr3+ (3), Cd2+, Zn2+, Ni2+ (4), etc. The competitive adsorption reaction will occur between UO22+ and coexisting toxic metal (Xn+) cations (e.g., Cr3+) (5). It blocks the uranyl adsorption pathway that the mixture solution will carry the uranyl ions into the natural environment. This presents a problem for the disposal of classified materials; therefore, the investigation of the selective reaction of UO22+ and Xn+ is necessary. Currently, various technologies have been developed for the direct removal of ppm levels of uranyl in aqueous solutions (6, 7). Recently, adsorption method was considered as highly efficient approach for highly selective removal of ppm levels of UO22+ from aqueous Xn+ solution (8, 9). Among the various adsorbents, montmorillonite (MMT), a common component of soil and sediment, is a potential backfill/barrier material for future advanced uranyl processing facilities. Kowalfouchard et al (10) reported that UO22+ was preferentially adsorbed on the (110) edge sites of montmorillonite. However, UO22+ ions were blocked on MMT edge sites by a Cr-UO2 competitive reaction, as shown by the Freundlich isotherm results (Cr: 2.35 mg·g-1, U: 2.69 mg·g-1) at normal pH (5~7) (11). This suggests that Al2O3 surfaces were covered with H3O+ ions, which formed ≡Al(OH)2UO22+. However, the largest adsorption capacities of Cr ions were higher than that of UO22+, on the surface (Langmuir isotherm model: Cr: 104.82 mg·g-1, U: 54.79 mg·g-1) and gap binding sites (Dubinin-Radushkevich: Cr: 75.23 mg·g-1, U: 36.55 mg·g-1). Thus, the adsorption reaction of UO22+ was weakened in the aqueous Xn+ solution (12). The subsequent investigation was focused on the modification of montmorillonite edge sites to improve UO22+ selective adsorption. 3 ACS Paragon Plus Environment


To compensate for the lack of UO22+ selective adsorption, the edge modification of MMT was explored based on covalent binding or physical coating using iron-based semiconductors (13). Therein, the excellent segregative adsorption capacity of magnetite (Fe3O4) adsorbent was considerable attention (12.33 mg·g-1) of uranyl (14, 15). However, the edge adsorption sites of MMT surfaces were occupied by Fe3O4, which resulted in the lower U6+ maximum adsorption capacity at pH=5.6±0.1 of 28.8 mg·g-1 for Fe3O4/MMT compared to that of pure MMT (35.24 mg·g-1) (16). And there was a competitive adsorption between UO22+ and Xn+ (e.g. Cu2+: 70.92 mg·g-1; Ni2+: 65.78 mg·g-1 with less than 2 min) on Fe3O4/MMT surfaces (17). This reaction blocked additional UO22+ adsorption. Thus, to separate the electron-hole pairs for enhancing the complex strength with UO22+, a novel design being proposed combined a wide band gap (e.g., TiO2: 3.0~3.2 eV) (18) shell-type semiconductor around a narrow band gap (MFe2O4: 1.9~2.7 eV) core-type semiconductor (19). Here, the TiO2(101) facet in thermodynamic equilibrium could preferentially grow on the two predominant natural cleavage MFe2O4(311) and (111) facets (20). A low concentration of uranyl with a bidentate coordination appears on the TiO2(101) surface, which is attributed to either i) the bridging-bridging site (relative energy: 795 meV) or ii) the bridging-top site (relative energy: 903 meV) (21). For example, the maximum uranium adsorption capacity was 118.8 mg·g-1 via a core-shell Fe3O4-TiO2 at a pH of 6.0 (22). Simultaneously, a competitive adsorption reaction inevitably occurs on the TiO2 surface, due to the highly efficient cation adsorption, such as 59.3 mg of Cr·g-1 using 50% TiO2-Fe3O4 (23); Cd2+: 4.0 ng·L-1, Cr3+: 2.6 ng·L-1, Mn2+: 1.6 ng·L-1 from 20 ng·L-1 stock solutions via 40 mg Fe3O4-SiO2-TiO2 (24), etc. These main adsorption models were still evaluated as Langmuir 4 ACS Paragon Plus Environment

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models. (25-27). Therefore, motivated by these findings, we used these transition metals (M=Mn2+, Fe2+, Co2+, Zn2+, or Ni2+) as mediators to modify the Fe-O tetrahedral structures of MMT-MFe2O4-TiO2 interfaces to enhance the selective adsorption strength of UO22+ from coexisting Xn+ solutions.

Experimental details Synthesis The MMT-MFe2O4-TiO2 was synthesized utilizing a two-step hydrothermal refluxing technique. Firstly, MFe2O4 was composited on the montmorillonite edge sites: 10 mmol MCl2∙nH2O (M=Fe, Mn, Cu, Zn, Ni), 20 mmol FeCl3∙6H2O, 10 mmol K-montmorillonite and 100 mL of water were heated, refluxed, and mixed at 368 K at a pH of 11 for 1 h. The MMT-MFe2O4 was obtained after being cooled, filtrated, and washed (three times) and then dried (343 K and 18 h) (28). The second step was to coat the TiO2 shell on the MMT-MFe2O4: 10 mmol MMT-MFe2O4, 5.5 mL tetra-n-butyl orthotitanate and 150 mL mixture solution (HNO3, water and ethanol) were heated and refluxed at 358 K at a pH of 3 for 3 h (29). Finally, the TiO2 was successfully wrapped on the MFe2O4 surfaces on the MMT edge sites after the process of cooling, filtration, washing, and drying. Characterisation Scanning electron microscopy (SEM) was carried out to determine the MMT-MFe2O4-TiO2 morphologies. The element distributions were analysed using X-ray photoelectron spectroscopy (XPS) (30). Lattice distances were imaged with high-resolution transmission electron 5 ACS Paragon Plus Environment


microscopy (HRTEM; JEOL-2010, Japan). X-ray diffraction (XRD) patterns were analysed to investigate the crystal plane characterisation in the range 2θ=15~70° (31). Fourier transform-infrared spectroscopy (FT-IR) was used to study the bond structures. Raman scattering measurements were conducted under 514.5 nm excitation light at 30 mW (32). The measurement of Zeta potential and diffuse reflectance spectroscopy (DRS) were implemented to reflect the surface charge and electron transfer information, respectively (33). Terahertz time-domain spectroscopy (THz-TDS) was carried out to investigate the triggered photon spectrum in the interfaces (34). Selective adsorption In a typical experiment, 1.0 g of UO2Cl2, RbCl, SrCl2, CrCl3, MnCl2, NiCi2, ZnCl2 and CdCl2 were dissolved and mechanically stirred in 1000 mL of deionised water. These precursor solutions (0.5, 1, 1.5, 2, 2.5, 3 mL) were diluted for the 10~60 mg·L-1 of UO22+-Xn+ (X=Rb+, Sr2+, Cr3+, Mn2+, Ni2+, Zn2+ and Cd2+; n=1, 2 and 3) chlorate aqueous solutions. Then, 0.03 g MMT-MFe2O4-TiO2 was fixed in 50 mL mixture solutions at a pH of 5.0 with 24 h of mechanical shaking (35-37). The relative atom ratios on the surfaces were reflected by high-resolution transmission electron microscopy-energy dispersive X-ray spectroscopy (HRTEM-EDS) (38). The UO22+-Xn+ ion concentrations were characterised via an inductively coupled plasma-optical emission spectrometer (ICP-Oes) (39). The adsorption behaviours and the relative maximum adsorption capacities were fitted and described using Langmuir, Freundlich and Dubinin-Radushkevich (DR) isotherm equations. The charge transfer information


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in the interfaces was analyzed by an electrochemistry workstation (Parstat4000, USA) at room temperature (40).

Results and discussion Structural characterisation of MMT-MFe2O4-TiO2 In both the MFe2O4-TiO2 and MMT-MFe2O4 interfaces, the M occupies in the tetrahedron that provides the oxygen bridge to connect with the Ti-O and MMT edge bonds. It had two advantages including the MMT surface agglomeration reduction and the enhanced electronic transition (41). Firstly, the inevitable agglomeration occurred as magnetic Fe3O4 nanoparticles surround MMT layer surfaces and edge sites in the growth process, as displayed in Fig. 1 a. Since the crystal energies of MFe2O4 (MnFe2O4: -33000.22 eV; ZnFe2O4: -35432.89 eV; CoFe2O4: -36106.85 eV; NiFe2O4: -38601.21 eV) were lower than that of Fe3O4 (Fe3O4: -34690.42 eV) (42), the average nanoparticle diameters increased (e.g., 80 nm for MnFe2O4; 73 nm for ZnFe2O4; 72 nm for CoFe2O4; and 79 nm for NiFe2O4) compared to that (33 nm) of Fe3O4, under the same synthesis conditions. Additionally, the MFe2O4 nanoparticles were wrapped up at MMT edge sites, due to the Fe and M ion precursors were preferentially absorbed on the MMT edge sites. And its reunion phenomenon on the MMT surface was effectively avoided. They were uniformly surrounded by small TiO2 nanoparticles (2~3 nm in length). Fig. 1 b reflected the clear division (two sides of orange dotted lines) between the MMT edge and MFe2O4-TiO2. It indicated that the MFe2O4-TiO2 nanoparticles were mainly distributed at MMT edge sites. 7 ACS Paragon Plus Environment


Furthermore, the element identification and chemical bonding were confirmed by the XPS spectra (see Fig. 1 c). The 460 eV, 466 eV, 712 eV, 725.5 eV, 99.8 eV, 150.5 eV and 72.7 eV characteristic peaks indicated the presence of Ti-2p3/2, Ti-2p1/2 (Ti4+), Fe-2p1/2 (Fe2+), Fe-2p3/2 (Fe3+), Si-2p3, Si-2s (Si4+) and Al-2p3 (Al3+) spin-orbit doublet components in the TiO2 shell, MFe2O4 core and MMT carrier, respectively. The small M2+ characteristic peaks occurred at 660, 1022, 809, and 888 eV, respectively. Therefore, the elements were uniformly distributed in MMT-MFe2O4-TiO2. To analyse the crystalline structures of MMT-MFe2O4-TiO2 nanoparticles, we characterised the typical XRD patterns (see Fig. 2 a). Therein, the 2θ angles at approximately 3° and 19.8° corresponded to the (001) and (100) characteristic reflections of K-Montmorillonite (JCPDS 29-1498) (39), the layer surface and edge construction, respectively. The (111), (220), (311), (400), (422), (511), and (440) characteristic reflections demonstrated the cubic structures of MnFe2O4 (JCPDS 10-0319), Fe3O4 (JCPDS 19-0629) (43), ZnFe2O4 (JCPDS 89-1010), CoFe2O4 (JCPDS 22-1086) and NiFe2O4 (JCPDS 87-2338) (44) core structures. The clear diffraction peak could be assigned to the shell-type anatase TiO2(101) characteristic reflection (JCPDS 21-1272) (45). As a result, the highly crystalline core-shell MFe2O4-TiO2 nanoparticles wrapped up at MMT edge sites had been successfully obtained. Secondly, it was previously reported that MFe2O4 has two predominant natural cleavage facets of (111) and (311). The surface energy (1.056~1.184 J∙m-2) (46) of the (111) facet was lower than that of the high-index (311) facet (18). Thus, the MMT-MFe2O4 interfacial interaction was stable for the MFe2O4 preferential growth on the exposed Al2O3(100) facet of the MMT edge site (47). On the preferential MFe2O4(111) facet, the TiO2(101) facet preferentially 8 ACS Paragon Plus Environment

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complexed to the exposed tetrahedral and octahedral coordinated Fe-O bond. This was due to the negative surface free energy (-0.015 eV∙A-2) in thermodynamic equilibrium, in comparison to the TiO2(001) facet with 0.026 eV∙A-2 (48), a positive surface free energy. Therefore, similar lattice distances (e.g., TiO2(101): 24.7°; MFe2O4(111): 19°; MMT(100): 19.8°) will constitute two interfaces, which was close to those of the XRD results. More detailed characterisations of the MMT-MFe2O4-TiO2 interfaces were determined from HRTEM images, as shown in Fig. 2 b. The relative lattice distances (d) and crystal facets were similar to the reported data (49, 45, 50-52) (see Table 1). The lattice distance difference between MFe2O4(111) and MMT(100) was only 0.03 nm, which revealed the high lattice matching of the resulting edge-localised MFe2O4(111) facets with the exposed Al2O3(100) of MMT edge sites. However, the lattice matching degrees between MFe2O4(111) and TiO2(101) facets were reduced based on the lattice distance difference (0.08~0.12 nm). This interfacial bonding distorted the surface atomic structure of the TiO2(101) facets, inducing the gap formation among TiO2 nanowires. Accounting for the effect of M2+-Fe2+ double charge transfer in the tetrahedron of the Fe3O4(111) facet, we noted that M2+ acting as an electron acceptor created a built-in oxygen vacancy to preferentially complex to the TiO2(101) facet. It was thought that Fe3+-O-Fe2+(M2+) charge compensation greatly suppresses the formation of oxygen vacancies, which results in the interesting small O-auger peaks (980 eV) in XPS lines. The partial O-2p4 electrons in the oxygen vacancy sites were sensitive to the transfer between long-range O-O p-p orbitals of the MFe2O4(111)-TiO2(101) interface. However, it is difficult to determine the interface bonding information of the MMT-MFe2O4-TiO2 based on XRD and HRTEM results. The bonding patterns can be 9 ACS Paragon Plus Environment


distinguished using the FT-IR spectrum. Two broad bands at 570 and 1090 cm-1 reflected the stretching vibration frequencies of the Fe-O and Si-O bonds (see Fig. 3 a). On the MFe2O4 surface, a partial Fe3+ and Ti4+ forms a Ti-O-Fe stretching vibration at 451 cm-1. The Raman modes were depicted by the characteristic peaks in Fig. 3 b. Therein, the Si-O tetrahedron and Al-O octahedron with high symmetries revealed extremely weak peaks. The characteristic peaks at 303 and 667 cm-1 were assigned to the two-fold degenerate (Eg) and non-degenerate (A1g) modes of MFe2O4(111), respectively. The 409 and 637 cm-1 characteristic peaks appeared as weak overtone scattering (B1g) and the Eg vibration modes of TiO2(101). In particular, the long-range O-O bonds in the MFe2O4(111)-TiO2(101) interfaces were observable at strong peaks (1370 cm-1). The weak peaks near 1617 cm-1 reflected the long-range electronic transition of AlO-OFe(M) bonds. These active electrons transferred and complexed to the -OH groups of H2O (O-H stretching, and bending vibration modes at 3370, 1650, 1405 cm-1). There was no consensus agreement among researchers on why the water molecules dissociated to compensate for surface oxygen vacancies (53). The surface hydroxyl radicals seen at 3630, 1018 and 916 cm-1 in FT-IR patterns were associated with the SiO-OH-OAl, HO-OTi and AlO-OH-OFe stretching vibrations, respectively. Electron transfer mechanism of MMT-MFe2O4-TiO2 Before studying the selective adsorption of UO22+-Xn+, we tested the surface potentials of MMT-MFe2O4-TiO2, to explain the electron transfer behaviour on the interfaces. Although the reported surface potential (22.5~25 mV) (54) of TiO2 was higher than that of MFe2O4 (MnFe2O4: 17.5 mV (55); Fe3O4: 17.5 mV (56); ZnFe2O4: 20 mV (57); CoFe2O4: -15 mV (58); NiFe2O4: -30 10 ACS Paragon Plus Environment

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mV) (59) and MMT (-36 mV) (60) at pH=5.4~6, the interface effect was mainly attributed to the synergistic oxidation-reduction reaction between M2+ and Fe3+ in MFe2O4. As shown in Fig. 4 a, the electrons were annihilated in the oxygen bridges of short-range Mn2+-Mn3+-O-Fe3+-Fe2+ bonds and two long-range Fe3+-O-O-Ti4+ and Fe3+-O-O-Al3+ interfaces, which indicated a synergistic charge equilibrium to 0 mV surface potential at a pH of 5.5. However, the active electrons were mainly exchanged in the interface of NiFe2O4 (-30 mV) and MMT (-36 mV). Therefore, the origin of the positive surface potential (13 mV) of MMT-MFe2O4-TiO2 was from the weakened TiO2 surface potential. By the effect of stable charge M2+ mediators, the active electrons transferred from the core-type MFe2O4 to shell-type TiO2 surfaces, and the active holes were annihilated in the negative potential surfaces of MMT edge sites. This was why the surface potentials of MMT-MFe2O4-TiO2 (M: Co, Zn) were below that (-5 mV) of MMT-Fe3O4-TiO2 (61). To explain the details of surface potential weakening and interface electron/hole transfer, we calculated the band gaps based on the abscissa of the tangent line, as seen in Fig. 4 b. Therein, the MMT was nearly transparent above the 300-nm wavelength, which was identical to the result reported by Fatimah et al (62). The electron transferred from valance bands to conduction bands mainly appeared at MFe2O4(111) and TiO2(101). The absorption edges of MMT(100)-MFe2O4(111) were blue-shifted (~2.16 eV) compared to those of MFe2O4 nanoparticles (e.g., CoFe2O4: 1.1 eV) (63). Interestingly, M2+ acting as a mediator in the MFe2O4 acceptor stored holes from the electron donor TiO2 (2.48~3.1 eV), which formed small band



gaps (0.79~0.87 eV) in the conductor bands, where the pair electrons will directly complex with the -OH groups of H2O molecules, forming Ti-O-OH hydroxyl layers. To investigate the electron transfer mechanism in both the MMT(100)-MFe2O4(111) and MFe2O4(111)-TiO2(101) interfaces, we used the THz spectra to characterise the sensitive interface information. The light velocities were almost unchanged through the MFe2O4-TiO2 nanoparticles at the MMT edge sites, and they could penetrate through MMT structures (64). As seen in Fig. 5a, the peak widths of the equipment-emitted pulses were less than 0.75 ps, and the corresponding amplitudes were around 70~125 V. Refraction index information could be gathered from low active photons triggered in the interfaces. Two refraction index peaks at approximately 1.65 and 2~2.2 THz in Fig. 5b represented the MMT(100)-MFe2O4(111) and MFe2O4(111)-TiO2(101) interface structures, respectively (65). Therein, two adsorption peaks near 1.65 THz were consistent with the photon adsorption of AlO-OFe(M) and SiO-OFe(M) at the MMT(100)-MFe2O4(111) structures, respectively (see Fig. 5c). The absorption response at 2~2.2 THz reflected the characteristic peaks of the MFe2O4(111)-TiO2(101) interfaces. These characteristic peaks indicated the effect of synergistic hybridised orbitals (e.g., O-O sp2, TiO-OM sp2d2, TiO-OFe sp2d2, etc.) that originate from the electronic transition in the TiO-OFe(M) interface multipaths. These were identical to the AlO-OFe(M) and TiO-OFe(M) bonds as two traps that capture the active electron gas in the interfaces. This result was in agreement with the previous observation of high frequency Raman peaks (1617 and 1370 cm-1). To better represent the THz reflection, we calculated the tangent data according to the frequency difference in Fig. 5d. Therein, high tangent data implied that the paired electrons increase the 12 ACS Paragon Plus Environment

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surface oxygen vacancy concentrations of MFe2O4(111) and TiO2(101) (66). Therefore, the exposed electron gases in both the MMT(100)-MFe2O4(111) and MFe2O4(111)-TiO2(101) interfaces will be decisive for the selective adsorption of UO22+ from aqueous Xn+ solutions. Selective removal of uranyl from aqueous toxic metal ion solution using the MFe2O4(111)-TiO2(101) interfaces at the montmorillonite edge sites To distinguish the selective adsorption behaviours of UO22+ and Xn+, we described the adsorption sites and maximum adsorption capacities (see Fig. 6) (67). Therein, the linear correlation coefficients of Xn+ were in approximate agreement with each other: Langmuir (0.912~0.998), Freundlich (0.889~0.997) and Dubinin–Radushkevich (0.829~0.942). It turned out that Xn+ are synchronously absorbed at the TiO2(101) surfaces, interfaces (e.g., MMT(100)-MFe2O4(111), MFe2O4(111)-TiO2(101)), and MMT inner layers. The Langmuir adsorption pathway was a single-electron transfer from the valence bands of TiO2(101) electron acceptors to superoxide anion (radical O2-), which creates hydroxyl radicals (·OH) to complex with positively charged Xn+ as a Xn+-·OH-TiO2(101) sp2d2 hybridised orbital, which formed an ion monolayer to block the adsorption of UO22+ (68). The O end adsorption energy (2.68 eV) was lower than that of horizontal adsorption (4.14 eV), and as a result, the adsorption mechanism was the oriented surface adsorption geometry at the bidentate site. Although this blocking behaviour was exactly the same as inner layer (Dubinin-Radushkevich) adsorption, the maximum adsorption capacities (qmax: 79.42~782.33 mg·g-1) of Xn+ (e.g., Sr2+, Mn2+, or Ni2+) ions in the first-order Langmuir equation were higher than those (Dubinin-Radushkevich: 71.47~150.08 mg·g-1), comparing to the adsorption capacities (Langmuir: 0.86~13.47 mg·g-1; 13 ACS Paragon Plus Environment


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Dubinin–Radushkevich: 0.99~13.17 mg·g-1) of UO22+. Therefore, the Xn+ ions were mainly absorbed on the TiO2(101) surfaces with different adsorption mechanisms. Therein, the surface ·OH strongly complexed with the 4s1/5s1 orbitals of Rb+, Cr3+, or Zn2+ to block the neighbouring Xn+ adsorption. This demonstrated that the selective adsorption is unlike that of reported collaborative adsorption (Zn2+: 137 mg·g-1, Cd2+: 148 mg·g-1 using Fe3O4@SiO2@TiO2) (40). Intuitively, the HRTEM-EDS images in Fig. 7 verified that the UO22+ and Xn+ were uniformly distributed on the MMT-MFe2O4-TiO2 surfaces. Ignoring the dissociated and inner-layer Xn+ ions, we found that UO22+ and Cr3+ are collaboratively and competitively absorbed, based on the redox reaction of U-5f36d1-Cr3+-4s1 (14). An interesting adsorption pathway

was

when

UO22+-Cr3+

was

highly

selectively

adsorbed

in

the

MFe2O4(111)-TiO2(101)/MMT(100)-MFe2O4(111) interfaces (see Fig. 6 b). The adsorption mechanisms were considered as the static second-order electronic transition of two-dimensional electron gases in the long-range TiO-OM(Fe) and AlO-OFe(M) bonds, where the electrons transferred to surface water molecules, creating surface hydroxyl radical layers in aqueous solution. The UO22+-Xn+ adsorption mechanism in the interfaces involved successive monoelectronic transfer steps that can be postulated as i) the direct electron gas transfer for increasing ·OH; and ii) the complexation of ·OH-UO22+/Xn+. The stable adsorption capacity of UO22+ was 109.11 mg·g-1 in the MMT(100)-Fe3O4(111)-TiO2(101) interface, which was similar to that reported for single UO22+ data on both the surface and interface of Fe3O4@TiO2 (118 mg·g-1) (20) and Fe3O4@SiO2 (111.13 mg·g-1) (69); TiO2(101)/RGO/Fe3O4(311) (88.2 mg·g-1) (70), etc. Furthermore, the enhanced collaborative active holes in both the Fe3O4(111)-TiO2(101) 14 ACS Paragon Plus Environment

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and MMT(100)-Fe3O4(111) interface electron gases preferentially hybridised to the U-5f36d1 orbital in comparison to the Cr3+-4s1 orbital, which weakens the UO22+-Cr3+ redox reaction (71). Hence, UO22+ was mainly absorbed in the interface area. In particular, M2+ ions could competitively complex to the same toxic metal ions (e.g., Mn2+, Zn2+ and Ni2+), which displayed a low Langmuir adsorption capacity. The formation of a half-full 3d5 hybridised orbital as the Zn-3d10-Fe-3d5 hybridised orbital could complex with ·OH in a pentagonal geometry (72). Compared to the hexagonal bipyramid orbitals in the other MFe2O4-TiO2 interfaces, there was a lack of surface adsorption sites for Xn+ due to the missing electron in the pentagonal geometry. Thus, the maximum Langmuir adsorption capacities of Xn+ were lower than that of the other MMT(100)-MFe2O4(111)-TiO2(101); however, the maximum adsorption capacity was only 51.15 mg·g-1 in the MMT(100)-ZnFe2O4(111)-TiO2(101) interface. Consequently, the high adsorption of UO22+ was realised in the MMT(100)-Fe3O4(111)-TiO2(101) interface, and its selective enhancement was controlled through the active electron/hole regulation by M-site modification. To explain the charge transition of UO22+-Xn+ as an intermediate reaction, we used cyclic voltammograms to test the oxidation-reduction reaction. As seen in Fig. 8 a, the oxidised peaks in the anodic scan indicated that the UO22+ (U6+) could undergo a one-electron reduction to UO2+ (U5+). However, the integral area comparison of the reduction-oxidation cyclic voltammogram reflected that only partial UO2+ was oxidised back to UO22+, in the -3.75 V cathodic scan. This confirmed that UO22+ was heterogeneously reduced to UO2+, and there was no further two-electron reduction process of U6+→U4+ that was inhibited by sp2d2 hexagonal geometry in 15 ACS Paragon Plus Environment


the MFe2O4(111)-TiO2(101) interface (73). Simultaneously, the interface electron gas facilitated the exchange of active electrons and holes and prevents the single Xn+ redox reaction, e.g., that of Sr2+ and Cr3+. Their electric signals will overlay into the total peaks (see Fig. 8 b), which showed that the data are weakened. This origin was the active holes in the electron gases transferred from UO2+ to the neighbouring Cr3+, as a result of the formation of the UO22+-UO2+-Cr3+-Cr4+ redox reaction. However, some Cr4+ ions were only reduced to Cr3+, due to other Cr4+ ions transferring to the oxygen vacancy sites of the TiO2(101) surfaces. The relative oxidation reaction was inhibited, showing a weakened total oxidation signal. In summary, we found that the MMT(100)-MFe2O4(111)-TiO2(101) interfaces were the main reaction area of the UO22+→UO2+ charge transition.

Conclusion In this paper, we investigated the selective removal of uranyl from aqueous toxic metal ion solution via core/shell MFe2O4-TiO2 nanoparticles of montmorillonite edge sites. Therein, Xn+ ions were mainly absorbed on the TiO2(101) surfaces, which was in agreement with monolayer adsorption behaviour. We payed particular attention to the fact that the UO22+→UO2+ charge transition appeared at the MFe2O4(111)-TiO2(101) interface, and this highly selective removal was improved by the enhanced active holes of the MMT(100)-MFe2O4(111) interfaces. These different UO22+-Xn+ adsorption behaviors are important from a technical perspective for highly selective and efficient removal of UO22+. Further research will pay close attention to the effect of the environmental factors (e.g. pH, anion, humic substances, etc) on the UO22+ extraction process 16 ACS Paragon Plus Environment

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from real water system (e.g. mine/industrial wastewater, sea water, etc), in order to optimize highly efficient and selective removal technology.

Acknowledgements The authors acknowledge the financial supports by the National Natural Science Foundation of China (41872039 and 41831285), the One-Thousand-Talents Scheme in Sichuan Province, and Sichuan Science and Technology Foundation (2018JY0462).

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Table 1. The lattice distances (d) and crystal facets of MMT-MFe2O4-TiO2, where “MM” indicates MMT, “F” indicates MFe2O4, and “T” indicates TiO2. Samples

d (nm)

Crystal facets

MFe2O4 (M = Ni, Co, Mn, Mg, Zn)a

F: 0.3, 0.47~0.49

F: (220), (111)

TiO2b

T: 0.33

T: (101)

CoFe2O4-TiO2c

F: 0.25, T: 0.35

F: (311), T: (101)

MFe2O4-TiO2 (M = Ni, Co, Zn and

F: 0.48; 0.25, T: 0.32

F: (111); (311), T: (101)

MM: 0.45, F: 0.25

MM: (100), F: (311)

MMT-MnFe2O4-TiO2

MM: 0.45, F:0.48, T: 0.28

MM: (100), F: (111), T: (101)

MMT-Fe3O4-TiO2

MM: 0.45, F: 0.48, T: 0.28

MM: (100), F: (111), T: (101)

MMT-ZnFe2O4-TiO2

MM: 0.45, F: 0.48, T: 0.3

MM: (100), F: (111), T: (101)

MMT-CoFe2O4-TiO2

MM: 0.45, F: 0.48, T: 0.28

MM: (100), F: (111), T: (101)

MMT-NiFe2O4-TiO2

MM: 0.45, F: 0.48, T: 0.28

MM: (100), F: (111), T: (101)

References

Sr)d MMT-Fe3O4e Present work

Expermenta-e: Reported d data and crystal planes by HRTEM (49, 45, 50-52) . Present work: HRTEM results.


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Figure captions

Figure 1. a) HRTEM, b) SEM images and c) XPS spectra of MMT-MFe2O4-TiO2.

Figure 2. a) XRD spectra and b) HRTEM images of MMT-MFe2O4-TiO2.

Figure 3. a) FT-IR patterns and b) Raman spectra of MMT-MFe2O4-TiO2. Residual organics mean the two -CH2 groups present in the spectrum of residual organics, which are present in the inner layer of MMT. However, they do not further complex with surface ·OH of TiO2(101).

Figure 4. a) Zeta and b) DRS curves of MMT-MFe2O4-TiO2.

Figure 5. a) Time dependence, b) Refraction index, c) Absorption and d) Frequency difference (FD) power of the THz signal spectra of MMT-MFe2O4-TiO2.

Figure 6. Maximum adsorption capacities of uranyl-toxic metal ions at MMT-MFe2O4-TiO2. a) Langmuir and Dubinin-Radushkevich results. b) Illustrates the competitive adsorption mechanism and Freundlich results in the interfaces.

Figure 7. HRTEM-EDS of uranyl-toxic metal ions at MMT-MFe2O4-TiO2, where “AR” indicates the atomic ratio. 31 ACS Paragon Plus Environment


Figure 8. Cyclic voltammograms of FTO glass electrode in 60 ppm a) UO22+-Cr3+-Sr2+ and b) Mixed aqueous solution (pH 5.5). Therein, “O” and “R” represent the oxidation and reduction peaks, respectively.


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Figure 1. a) HRTEM, b) SEM images and c) XPS spectra of MMT-MFe2O4-TiO2. 279x215mm (300 x 300 DPI)



Figure 2. a) XRD spectra and b) HRTEM images of MMT-MFe2O4-TiO2. 279x215mm (300 x 300 DPI)


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Figure 3. a) FT-IR patterns and b) Raman spectra of MMT-MFe2O4-TiO2. Residual organics mean the two CH2 groups present in the spectrum of residual organics, which are present in the inner layer of MMT. However, they do not further complex with surface ·OH of TiO2(101). 289x203mm (300 x 300 DPI)



Figure 4. a) Zeta and b) DRS curves of MMT-MFe2O4-TiO2. 279x215mm (300 x 300 DPI)


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Figure 5. a) Time dependence, b) Refraction index, c) Absorption and d) Frequency difference (FD) power of the THz signal spectra of MMT-MFe2O4-TiO2. 279x215mm (300 x 300 DPI)



Figure 6. Maximum adsorption capacities of uranyl-toxic metal ions at MMT-MFe2O4-TiO2. a) Langmuir and Dubinin-Radushkevich results. b) Illustrates the competitive adsorption mechanism and Freundlich results in the interfaces. 279x215mm (300 x 300 DPI)


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Figure 7. HRTEM-EDS of uranyl-toxic metal ions at MMT-MFe2O4-TiO2, where “AR” indicates the atomic ratio. 289x203mm (300 x 300 DPI)



Figure 8. Cyclic voltammograms of FTO glass electrode in 60 ppm a) UO22+-Cr3+-Sr2+ and b) Mixed aqueous solution (pH 5.5). Therein, “O” and “R” represent the oxidation and reduction peaks, respectively. 279x215mm (300 x 300 DPI)


Page 40 of 41

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TABLE OF CONTENTS (TOC) GRAPHIC: An interesting selective adsorption pathway is UO22+ being selective removal in the MMT(100)-MFe2O4(111)-TiO2(101) interface. 83x47mm (300 x 300 DPI)


Selective removal of uranyl from aqueous solutions containing mix of

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