Spectroscopic Investigation of Enhanced Adsorption of U (VI) and Eu

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Materials and Interfaces

Spectroscopic investigation of enhanced adsorption of U(VI) and Eu(III) on magnetic attapulgite in binary system Yi Xie, Dadong Shao, Xirui Lu, Tasawar Hayat, Njud S. Alharbi, Changlun Chen, Gang Song, Diyun Chen, and Yubing Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Industrial & Engineering Chemistry Research

Spectroscopic investigation of enhanced adsorption of U(VI) and Eu(III) on magnetic attapulgite in binary system Yi Xiea, Dadong Shaoa, Xirui Luc, Tasawar Hayatd, Njud S. Alharbie, Changlun Chena,d,e, Gang Songf, Diyun Chenf, Yubing Sunb* a

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

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

School of Environment and Chemical Engineering, North China Electric Power

University, Beijing 102206, P.R. China c

Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory,

Southwest University of Science Technology, Mianyang 621010, China d

NAAM Research Group, Department of Mathematics, Faculty of Science, King

Abdulaziz University, Jeddah 21589, Saudi Arabia e

Department of Biological Sciences, Faculty of Science, King Abdulaziz University,

Jeddah, Saudi Arabia f

Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and

Resources, Guangzhou 510006, China ABSTRACT: The co-adsorption of uranium and europium on magnetic attapulgite (M-ATT) was explored through batch experiments and spectroscopic tests. The adsorption processes of U(VI) and Eu(III) onto M-ATT were highly affected by the solution pH but not by the ionic strength, indicating that the adsorption of the two metals was predominated through inner-sphere surface complexation. Adsorption 1

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isotherms demonstrated that the maximum adsorption capacities of U(VI) on M-ATT in the single-solute system (e.g., 60.48 mg g-1 at pH 6.0) were lower than that in the binary-solute system (e.g., 63.03 mg g-1 at pH 6.0), the same trend was noticed for Eu(III) adsorption due to their synergistic effect. The existence of U(VI) and U(IV) species evidenced the partial reduction of adsorbed U(VI) to U(IV) by M-ATT based on XPS tests. Hence, the enhanced adsorption of U(VI) in the existence of Eu(III) is ascribed to the primary co-adsorption and then the redox of adsorbed U(VI) to U(IV) by M-ATT, as well as the formation of new sorbent active sites derived from reductive co-precipitation (e.g., UO2+X(s)), which also increased the Eu(III) adsorption on M-ATT. The findings are significant for co-adsorption of radionuclides by magnetic adsorbents in environmental pollution management. Keywords: Magnetic attapulgite, U(VI), Eu(III), Synergistic effect

1. Introduction The safe and effective disposal of radioactive waste has become a main challenge to make peaceful use of nuclear power [1, 2]. The release of radionuclides (e.g.,

234

U,

239

Pu,

241

Am,

242

Cm,

154

235

Eu,

Np, etc.) poses a potential threat to

ecological environment, humans, animals and other species [3]. Therefore, there is an urgent need to dispose these radionuclides in the aqueous environments. Among various approaches, adsorption has attracted the most attention owing to its cost-effective, simple regeneration and easily implementation. In this respect, diverse materials such as clays [4-7], alumina [8-10], activated carbon [11-13] have been investgated for radionuclides removal. In these researches, the adsorption behaviour 2


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of radionuclides in the adsorption process was thoroughly elucidated via batch experiments and spectroscopic technologies. However, many radionuclides were co-existed in the real aquatic environment such as a deep or shallow underground radioactive waste repository [14]. Although the co-adsorption of different radionuclides had been widely elucidated, few studies about co-adsorption mechanism of various radionuclides were available nowadays [15, 16]. Hence, more attention should be focus on the simultaneous adsorption mechanism of various radionuclides from aqueous environments. Attapulgite (ATT) as a phyllosilicate clay mineral consists of two silicon-oxygen tetrahedron linked with magnesium of magnesium-oxygen octahedron coordination [17, 18]. Owing to those superior physicochemical performances such as a large amount of favorable ion-exchange capacity, negatively charged reaction sites, large specific surface area [19-21], ATT has been considered as a potential adsorbent for radionuclides adsorption from aquatic environment [22-24]. Nevertheless, the properties that are hard to separate ATT from aqueous solutions hindered its actural application. In recent years, magnetic separation technology has aroused due to its fast separation and redox performance [25-27]. Reaserches on the adsorption of radionuclide by magnetic ATT (M-ATT) from water solutions have been reported [18, 28]. Fan et al. explored the interaction mechanisms between M-ATT and U(VI), and they found that U(VI) adsorbed on M-ATT through the inner-sphere surface complexation [18]. Lu et al. reported the Eu(III) adsorption on M-ATT with a saturated adsorption of 75.07 mg g-1 at pH 5.0 [28]. Although the adsorption of 3



individual uranium or europium onto M-ATT have been fully investigated, no reports have been performed concerning the simultaneous adsorption behaviors and mechanisms of the two radionucldes onto M-ATT. In this study, uranium(VI) and europium(III) were selected as representative radionuclides. The aims of this research include: (1) to prepare M-ATT by co-coprecipitation method; (2) to characterize M-ATT via X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption-desorption, magnetization analysis and zeta-potential techniques; (3) to explore the individual and simultaneous adsorption of the two metals onto M-ATT in the condition of varying pH and initial metal concentration; (4) to research the co-adsorption mechanisms by Fourier transformed infrared (FT-IR) spectra and X-ray photoelectron spectroscopy (XPS). These investigations are of great significance for the disposal of radionuclides using magnetic composites in the environmental pollution improvement.

2. Experimental section 2.1. Chemicals and Materials ATT employed in the research was derived from Guanshan Mine, Anhui Province, China. The purification procedure of ATT was as follows: firstly, the natural ATT powder was crushed and sieved to 0.45-0.90 mm. Then, the ATT powder was dispersed into a HCl solution with concentration of 0.1 mol/L under magnetic stirring condition for 24 h to eliminate the carbonate. Afterwards, ATT was separated via filtration, washed several times with deionized water, and then vacuum freeze-dried

4


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overnight at room temperature. Finally, the purified ATT was obtained by screening with 200 mesh standard sieve. The chemicals of FeCl3·6H2O, FeSO4·7H2O and NH4OH were purchased of reagent grades from Sinopharm Chemical Reagent Co., Ltd. (China). The stock solution of U(VI) (0.6 mmol/L) was prepared through the dissolution of UO2(NO3)2·6H2O (purity 99.99%, Sigma-Aldrich) in deionized water. The stock solution of Eu(III) (0.6 mmol/L) was prepared through dissolution and evaporation of Eu2O3 (purity 99.99%) with 0.1 mol/L HClO4 and then dilution. 2.2. Preparation of M-ATT The M-ATT was synthesized using co-precipitation of FeCl3·6H2O and FeSO4·7H2O on the surface of ATT at the alkaline conditions. In detail, 4.0 g of purified ATT were dissolved into the 250 mL flask under ultrasonic bath for 30 min, then, 3.1 g FeCl3·6H2O and 2.4 g FeSO4·7H2O were provided under the stirring (200 rpm at 60 ºC) and N2 conditions. After reaction of 0.5 h, 10 mL 25 % NH4OH solution was added dropwise, subsequently the suspensions were stirred for 1 h followed by aging for 2 h. Finally, the solid phase was separated through magnetic force and then rinsed repeatedly with DI water, and then the as-prepared M-ATT was obtained at vacuum freeze-dried overnight. 2.3. Characterization The structure and micro-morphology of ATT/M-ATT were characterized by XRD and SEM, respectively. The XRD spectrums with a 2θ ranging from 5° to 70° were 5



acquired by a PANalytical X’Pert PRO MPD diffractometer using Cu Kα radiation (λ = 1.5406 Å). SEM patterns were collected using a scanning electron microscope (SU8020, Japan). The BET surface area was determined by Quantachrome NOVA 4200e instrument and the Zeta potential was recorded on a Malvern Zeta sizer Nano ZS instrument, respectively. A vibrating sample magnetometer (VSM, Lakeshore 7404) device was utilized for magnetization measurement at 25 ºC. The chemical composition of ATT and M-ATT were characterized by X-ray fluorescence (XRF, Shimadzu-1800, Japan). FT-IR test was performed on a Fourier transform spectrophotometer (Nexus 670, Thermo Nicolet) employing the KBr pellet approach. The data of XPS test were acquired with a VG Scientific ESCALAB 250Xi spectrometer, and were fitted by the software of XPSPEAK (version 4.1). 2.4. Batch Adsorption and Desorption Tests Batch adsorption test was conducted with 1 g/L M-ATT and 0.1 mmol/L radionuclide solution in 0.01 or 0.1 mol/L NaNO3 at room temperature. In brief, the suspensions of M-ATT and NaNO3 were mixed for 24 h firstly, then 0.1 mmol/L uranium or europium solution was spiked to the bulk suspensions. Subsequently, these suspensions were agitated for 24 h, which was enough to get fully kinetic equilibrium according to our kinetic data. The pH of the solution was adjusted by adding neglected amounts of NaOH or HNO3 (1.0-0.05 mol/L). The adsorption isotherms were explored in the existence of NaNO3 solution (0.01 mol/L) with initial U(VI) or Eu(III) concentrations in the range of 0.01-0.35 mmol/L at pH 4.0, 6.0 and 9.0, respectively. As to the binary-solute systems, the equivalent concentrations of U(VI) 6


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and Eu(III) (0.1 mmol/L) were pre-mixed to guarantee a simultaneous reaction with M-ATT. The above suspension was separated using a centrifuge at the rotate speed of 8000 rpm for 15 min after equilibrium. Besides, the blank test was carried out to eliminate the adsorption of radionuclides on the wall of polycarbonate tube. The residual U(VI) and Eu(III) concentration were determined through inductively coupled plasma-atomic emission spectrometer (ICP-AES, CAP6300, Thermo Scientific). It is confirmed that the interference effect between U(VI) and Eu(III) measurement can be neglected because of different wavelengths of the two metals used during the ICP-AES test. The percentages of adsorption (%) and capacities (Qe) were determined according to Eqns. (1) and (2) as follows: Adsorption (%) = Qe ሺmmo/gሻ =

(C0 -Ce ) C0

×100%

(1)

V×(C0 -Ce )

(2)

m

where C0 and Ce represent, respectively, the initial and final equilibrium concentration of adsorbate (mmol/L); V(ml) represents the volume of the suspension; m(g) is the mass of the absorbents. The desorption tests of the two radionuclides were carried out in single-solute [29]. After reaching equilibrium of the adsorption process, half of the supernatant was substituted with the equal quantity of NaNO3 solution to maintain the same experimental conditions during the adsorption-desorption. When equilibrium was reached, the concentration of supernatants was determined to calculate the residual metal ions concentrations by the principle of mass conservation.

7



2.5. Preparation and Analysis of FT-IR and XPS FT-IR and XPS characterization were conducted to explore the interaction mechanisms between U(VI)/Eu(III) and M-ATT. The samples for FT-IR and XPS measurements were prepared as following procedures: 0.1 g M-ATT and 10 mL 0.1 mol/L NaNO3 were added to 100 mL U(VI) or Eu(III) solution under stirring condition. The solution pH was adjusted using neglected volume of NaOH or HNO3 (1.0-0.05 mol/L). After 24 h of reaction, the suspensions were centrifuged, and then the solid phase was acquired by vacuum freeze-dried overnight.

3. Results and discussion 3.1. Characterization Fig. 1A and 1B displayed the micromorphologies of ATT and M-ATT, respectively. It can be seen that ATT presented a fibrous structure with sizes ca. 1 µm length × 0.2 µm width, while M-ATT maintained the fibrous structure with the precipitation of the iron oxides on the surfaces of the ATT. In addition, the much coarser surface of M-ATT was observed, which was attributed to the introduction of iron oxides. In additions, the colors of ATT and M-ATT were grey (inset in Fig. 1A) and brown (inset in Fig. 1B), respectively, revealing iron oxides was successfully formed on the surface of ATT. Fig. 1C demonstrates the XRD patterns of the ATT and M-ATT. For the ATT, the typical peaks at 2θ = 8.44, 13.81, 16.38, 19.84, 27.58, 35.34º were in good agreement with diffraction of the (110), (200), (130), (040), (400), (161) planes of the ATT, respectively [30, 31]. For the M-ATT, the other peaks at 2θ = 30.27, 8


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35.74, 43.47, 53.89, 57.56, 62.73º were very close to the (220), (311), (400), (422), (511), (440) planes of Fe3O4 (JCPD standards 89-3854), respectively [18, 32]. The related chemical reactions could be indicated as Eqn. (3): ATT+Fe2++2Fe3++8OH- → ATT/Fe3O4+4H2O

(3)

The results of XRD patterns demonstrated that the magnetic Fe3O4 were successfully formed on the surfaces of the ATT. As depicted by zeta potential in Fig. 1D, the pHPZC (pH at zero charge point) values of ATT and M-ATT are 2.8 and 6.1, respectively. The significant increase of pHPZC for M-ATT was mainly attributed to the introduction of iron oxides. Fig. 1E shows the N2-adsorption-desorption isotherm of the M-ATT. Generally, pores are classified into three types on the basis of the diameter of macropores (d > 50 nm), mesopores (2 nm < d < 50 nm) and micropores (d < 2 nm)[33]. The pore size distribution curves was inserted in Fig. 1E, it can be seen that the pore size of M-ATT exhibited a multipeaks distribution, including primarily mesopores, as well as a small number of macropores and micropores. In the light of the Barrett-Joiner- Halenda (BJH) calculation, the average pore diameter and the total pore volume were 17.08 nm and 4.86 × 10−1 cc/g, respectively. On the basis of the N2 adsorption analysis, the BET of M-ATT was calculated to be 113.821 m2/g. Fig. 1F describes the magnetization curves of M-ATT. The weak hysteresis phenomenon suggested that the as-prepared M-ATT was nearly superparamagnetic, which could avoid the self-agglomeration of the carrier material caused by residual magnetism [18, 34]. The specific saturation magnetization (Ms) of the M-ATT was 23.21 emu/g. The insert image in Fig. 1F 9



shows that the as-prepared M-ATT could be efficiently separated by a magnet. The chemical compositions of the ATT and M-ATT were determined by XRF analysis. As given in Table 1, it can be seen that the major constituents of ATT were SiO2 (71.72 wt %), MgO (14.11 wt %), and Al2O3 (7.77 wt %), whereas the content of iron oxides for M-ATT (37.36 wt %) was observably higher than that of ATT (4.67 wt %). The characterization results indicated that Fe3O4 was successfully modified on ATT surface by chemical co-precipitation method. 3.2. Effect of the Reaction Time The kinetic data describes the solute uptake rate that determines the required equilibration time and the optimal contact time for an adsorption process. [35]. Fig. 2 shows the individual and simultaneous uptake of uranium and europium on M-ATT against contact time at pH 4. As can be seen from Fig. 2A, the uptake of U(VI) on the M-ATT was rapid within the initial 1 h reaction time, and then gradually reached equilibrium within the next 4 h. Besides, the U(VI) adsorption on M-ATT in binary-solute were obviously enhanced compared with the individual U(VI) adsorption. Similar trends were found for Eu(III) adsorption on M-ATT in individual and competitive systems (Fig. 2B). To acquire further insight into the adsorption behaviours of M-ATT towards the two radionuclides, the obtained kinetic data were simulated by the pseudo-first-order and pseudo-second-order models. More detailed information referring to this simulation are offered in the Supporting Information (SI). According to the fitting, the adsorption of the two metals on M-ATT in all systems could be favorable described through the pseudo-second-order kinetic model (R2 > 10


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0.999) (Fig. S1 and Table S1). 3.3. Effect of pH Undoubtedly, the pH of solution plays a vital role in the uranium and europium adsorption on M-ATT. As we know, the changes of solution pH would influence the species of radionuclides and the surface characteristics of adsorbents in aquatic environments [36-38]. Fig. 3A and 3B depicted the impact of pH on the adsorption of uranium and europium on M-ATT in single or binary-solute in 0.01 mol/L NaNO3 solutions, respectively. As displayed in Fig. 3A, the uptake of U(VI) increased with the increase of solution pH over the range of 2.0-6.0, and a plateau was noted when pH > 6.0. The enhanced Eu(III) adsorption on M-ATT in single-solute was also observed (as shown in Fig. 3B) at pH 3.0-7.0, subsequently, the adsorption kept a high plateau in alkaline conditions. The species distribution of the two metal ions in water solution over pH 2.0-10.0 was calculated using Visual MINTEQ version 3.0 [39]. According to the calculation results (Fig. 4A and 4B), U(VI) mainly existed as positive species (e.g., UO22+, (UO2)3(OH)5+, (UO2)2(OH)22+ species) when pH < 6.0, while Eu(III) existed mainly in the form of Eu3+ when pH < 7.0. As exhibited in Fig. 1D, the positive charged M-ATT surface were noticed when pH < 6.1. Thus, the increased uptake of uranium and europium on M-ATT in pH range of 2.0-6.0 are not on account of electrostatic forces, which may be ascribed to the surface complexation or reduction [40]. It was found that the U(VI)-bearing precipitates (e.g., schoepite) formed at pH > 6.0 (Fig. 4A), the Eu(OH)3 precipitates were also noticed at pH > 7.0 (Fig. 4B). Hence, the high adsorbance of the two radionuclides might be partly owing 11



to the formation of precipitates on the M-ATT surface at pH > 6.0. Besides, compared to the previous studies, the high removal effciency of U(VI) in alkaline condition may be ascribed to the contribution of redox of U(VI) to U(IV) by Fe3O4 nanoparticles [41-43]. Ding et al. based on XPS and EXAFS techniques demonstrated that the adsorbed U(VI) was reduced to U(IV) by Fe3O4 nanoparticles at circumneutral conditions [44]. It can be deduced that both adsorption and reduction made a contribution to the removal of U(VI) by M-ATT. The effects of pH on the simultaneous adsorption of the two radionuclides by M-ATT were displayed in Fig. 3A and 3B. Interested enough, the removal percentages of uranium on M-ATT in the existence of europium were obviously higher than that in the absence of europium, indicating that the addition of europium promoted the uranium adsorption onto M-ATT. The same trends were observed for Eu(III) adsorption in the existence of U(VI), revealing that the two radionuclides exerted a synergistic effect on each other’s adsorption in our systems. The generation of synergistic effects may be explained by following three reasons: i) the simultaneous adsorption of uranium and europium can facilitated the co-precipitation of the two metal ions at high concentration; ii) the reduction of adsorbed U(VI) to non-stoichiometrical UO2+x co-precipitate provided the reactive sites for Eu(III) adsorption; iii) the corrosion products of Fe3O4 nanoparticles in M-ATT generated the additional active sites available to adsorb U(VI) and Eu(III) simultaneously [45]. 3.4. Effect of CO32- and ionic strength In the aquatic environment, CO32- is an important component since CO2 is highly 12


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soluble in water solution. The existence of carbonate plays an important part in the migration, transport, and fate of radionuclides in groudwater [46]. Herein, the impact of CO32- on the two radionuclides adsorption by M-ATT was probed. Fig. 3A and 3B show the effects of pH on the two radionuclides adsorption by M-ATT in the absence or presence of 1 mmol/L Na2CO3. It was found that the U(VI) uptake was inhibited by the existence of carbonates when pH > 7.0, whereas this inhibition effect for Eu(III) adsorption was not clearly observed. The decreased U(VI) adsorption may because of the formation of uranyl-carbonate chelate species (e.g. UO2(CO3)22- and UO2(CO3)34complexes) at high pH (Fig. 4C). The adsorption process was inhibited by repulsion between negative uranyl-carbonate complexes and negative charged M-ATT at high pH [5]. However, the adsorption capacities of the two metals in the binary-solute were also higher than that in the single-solute, which indicated that the synergistic effects still work even in the presence of Na2CO3. The ionic strength could influence the potential of M-ATT interfaces, and have effects on the properties of the binding sites [41, 47]. The impacts of ionic strength on the individual U(VI) or Eu(III) uptake by M-ATT are depicted in Fig. 3C and 3D, respectively. The adsorption of the two metals on M-ATT did not depended on ionic strength over the entire pH range. Generally, inner-sphere surface complexation is irrelative to ionic strength, while ion exchange or outer-sphere surface complexation is sensitive to ionic strength [48]. Therefore, it was demonstrated that the inner-sphere surface complexation dominated the uptake of U(VI) and Eu(III) on M-ATT under the applied experimental conditions [16]. 13



3.5. Adsorption Isotherms The adsorption isotherms of U(VI) and Eu(III) on M-ATT at pH 4.0 are exhibited in Fig. 5A and 5B, respectively. One can see that the adsorption efficiencies of the two radionuclides increased dramatically with the increase of initial metal ion concentration in both single- and binary-solute. The adsorption capacities of the two radionuclides on M-ATT in binary-solute was higher than that in single-solute, suggesting the promotion role of one metal on the other. The same trend could be observed at pH 6.0 and 9.0 from Fig. 6. The maximum adsorption capacities of U(VI) in the single-solute system (60.48 and 58.93 mg g-1, at pH 6.0 and 9.0, respectively) were slightly lower than that in the binary-solute system (63.03 and 60.79 mg g-1, at pH 6.0 and 9.0, respectively). As to Eu(III), the maximum adsorption capacities in single-solute system (29.78 and 43.47 mg g-1, at pH 6.0 and 9.0, respectively) were also slightly lower than that in binary-solute system (33.20 and 44.69 mg g-1, at pH 6.0 and 9.0, respectively). Besides, the adsorbance of uranium and europium at pH 6.0 and 9.0 were all higher than those at pH 4.0, which were accordance with the consequences of pH-dependent adsorption. These observations suggested that the synergetic effect worked in all investigated pH values, which was accordant with the findings of pH adsorption and kinetics tests. The Langmuir equation and Freundlich equation were further utilized to fit the experimental data [1, 49]. More detailed information of the two equations is offered in the SI. It was found that the uptake of both U(VI) and Eu(III) followed the Langmuir model (R2 > 0.99) (Fig. S2 and Table S2), implying the adsorption of the two radionuclides on M-ATT was monolayer 14


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coverage [50, 51]. Besides, the maximal adsorption capacities of uranium in single-solute and binary-solute calculated based on Langmuir model at the three pH and 25 ºC were higher than those of europium. These findings suggested that uranium and europium exerted a synergistic effect on each other’s adsorption, which indicated that M-ATT could be employed as potential adsorbent for removing U(VI) and Eu(III) from water solution in environmental management. Fig. 5C and 5D displayed the adsorption and desorption isotherms of uranium and europium in single-solute at pH 4.0, respectively. It was found that the adsorption isotherm of U(VI) or Eu(III) are higher than their desorption isotherm, revealing that the uptake of the two metals on M-ATT was an irreversible procedure. The regeneration tests indicated that the two radionuclides adsorbed onto M-ATT could not be easily desorbed to rebuild the adsorption equilibrium under applied conditions. The irreversible adsorption also implyed that U(VI) and Eu(III) were mainly adsorbed on M-ATT through the strong inner-sphere surface complexation. 3.6. Adsorption Mechanism The co-adsorption mechanisms of the two metal on M-ATT were testified through FT-IR and XPS analysis. As displayed in FT-IR spectra (Fig. 7A), the peaks of original M-ATT at 3614 and 3440 cm−1 were associated with the stretching mode of OH- group and zeolitic water, respectively [52]. The band at 1630 cm−1 represented the stretching mode of the coordination water molecules and absorbed water [53]. The stretching vibration of Si-O-Si band could be noted at 1036 and 480 cm-1 [54, 55]. The band at 580 cm-1 belonged to vibration of Fe-O groups, further manifesting that 15



iron oxides was modified on the surface of ATT [56]. These oxygenic functional groups could provide the abundant reactive sites for the metal adsorption. Note that the OH- groups (at 3614 cm-1) of M-ATT shifted to a lower frequency for M-ATT+Eu and M-ATT+U (at 3613 cm-1), while almost disappeared for M-ATT+U+Eu, indicating involvement of OH groups in the adsorption process. It is extremely hard to acquire precise and reasonable surface species only by FT-IR spectra. XPS can provide underlying skills to appraise the elemental identification on solid surfaces because of its high sensitivity [18, 57]. Fig. 7B-F show XPS scans of the M-ATT before and after metal ions adsorption at pH 4.0 and 8.0. As exhibited by total scans in Fig. 7B, the main Si 2p, O 1s and Fe 2p peaks of M-ATT were noticed. The occurrence of U 4f and Eu 3d bands after uranium and europium adsorption revealed that the two radionuclides were adsorbed on M-ATT [58]. Besides, the relative intensities of U 4f and Eu 3d peaks at pH 8.0 were visibly higher than those of relative intensities at pH 4.0, demonstrating that the adsorption amounts of uranium and europium were observably enhanced with increasing solution pH. Compared with the M-ATT before adsorption, the relative intensity of the O 1s band of M-ATT after metal adsorption decreased, and the binding energy (BE) moved to the side of higher energy. In additions, the binding energy of O 1s for M-ATT-U+Eu was higher than that for M-ATT, demonstrating that oxygen-bearing functional groups contributed to the metal ions adsorption on M-ATT [59, 60]. Fig. 7C displays the U 4f spectra after fitting for MATT-U+Eu. Previous researches have reported that the BE of the spin orbit split U4f peaks was depended 16


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on the oxidation state of U [44, 61]. As revealed in Fig. 7C, the U 4f7/2 and U 4f5/2 bands can be fitted the U(VI) centered at 393.0 ± 0.1 and 382.1 ± 0.1 eV, and non-stoichiometric U(IV) situated at 391.9 ± 0.3 and 380.9 ± 0.3 eV (Table 2), respectively [62]. The emergence of U(IV) demonstrated that U(VI) was partially reduced by magnetic particles of M-ATT [43]. Furthermore, the relative abundance of U(IV) at pH 8.0 was higher than the relative abundance at pH 4.0, demonstrating the increased reduction at high pH. These observations indicated that both adsorption and reduction made a contribution to U(VI) uptake on M-ATT. As depicted in Fig. 7D, the high resolution scan of Eu 3d was fitted with two doublets peaks, namely Eu 3d3/2 and Eu 3d5/2 peaks at ~1165 and ~1135 eV, respectively, which was accordance with the previous research [63]. The fitting results of Fe 2p spectra were displayed in Fig. 7E and the corresponding peak parameters were listed in Table 3. One can see from Fig. 7E that the Fe 2p spectra can be deconvoluted into three peaks at 713.5, 711.5 and 709.9 eV belonging to the Fe(III)tet, Fe(III)oct and Fe(II)oct, respectively [64]. The decreased relative intensity of Fe(II)oct for M-ATT-U+Eu evidenced the occurrence of U(VI) reduction by structural Fe(II) in the surface reaction [44]. Moreover, the relative abundance of Fe(II) at pH 8.0 was lower than that at pH 4.0, suggesting the oxidation of Fe(II) to Fe(III) at high pH. As shown in Fig. 7F, the O1s peaks of initial M-ATT could be deconvolved to three peaks at 530.6, 531.9 and 532.8 eV, which was attributed to the anionic oxygen in Fe3O4 of the adsorbent, and OH- groups and adsorbed water, respectively [65, 66]. However, compared with M-ATT, the BE for O 17



1s of M-ATT-U+Eu were slightly moved to higher BE, demonstrating that U(VI) and Eu(III) were bonded with oxygen-bearing functional groups. Besides, the relative abundance of OH- group at pH 4.0 was distinctly lower than that at pH 8.0, revealing that the OH- group played a significant part for the radionuclides adsorption at pH 4.0. These results of XPS test manifested that the effective adsorption of U(VI) and Eu(III) were probably ascribed to the oxygen-bearing functional groups of M-ATT.

4. Conclusions The M-ATT composites were successfully prepared through co-precipitation of Fe3+ and Fe2+ on the ATT surface by adding NH4OH under N2 conditions. The simultaneous adsorption mechanisms of uranium and europium on M-ATT were explored through batch experiments and spectral analysis techniques. The results of batch tests exhibited that adsorption capacities of the two radionuclides in binary-solute systems were obviously higher than that in single-solute systems. The adsorption isotherms of both metals adsorbed onto M-ATT could be well simulated by Langmuir models. The spectroscopic data demonstrated that U(VI) can be reduced to non-stoichiometric U(IV), meanwhile the abundant new secondary phase (e.g., UO2+X precipitate and corrosion product of Fe3O4) were indicated by FT-IR and XPS analysis. Therefore, both adsorption and partially reduction contributed to U(VI) adsorption on M-ATT in binary-solute system, whereas the enhanced Eu(III) adsorption in binary-solute was ascribed to the re-adsorption of Eu(III) on these new reactive sites. These findings in present work are crucial for removing radionuclides from water solution in environmental remediation applications. 18


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Associated content Supporting information Detailed figures for the fitting results of kinetic and isotherm data, and tables for the calculated parameters of adsorption kinetic and isotherm models.

Author information Corresponding Authors *[email protected] (Y. Sun); Tel: 86-551-65591368; Fax: 86-551-65591310. ORCID: Yubing Sun: 0000-0003-4931-8039 Notes The authors declare no competing financial interest.

Acknowledgements Financial supports from the National Natural Science Foundation of China (21477133, 11675210), Research Fund Program of Guangdong Provincial Key Laboratory of Radionuclides

Pollution

Control

and

Resources

(GZDX2017K002),

NASF

(U1530131) and The Key Lab of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences are acknowledged.

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Figure captions Fig. 1. Characterization of ATT and M-ATT, A and B: SEM images of ATT and M-ATT, respectively; C: XRD patterns; D: Zeta-potentials; E: the N2- adsorption/ desorption isotherm; F: Magnetic hysteresis loop. Fig. 2. Effect of reaction time on the adsorption of U(VI) (A) and Eu(III) (B) on M-ATT in single- and binary-solute systems, pH = 4 ± 0.1, C0 = 0.1 mmol/L, m/V = 1 g/L, I = 0.01 mol/L NaNO3, T = 25 ºC. Fig. 3. Effect of pH on the adsorption of U(VI) (A) and Eu(III) (B) on M-ATT in the adsence and presence of 1 mmol/L Na2CO3 in the single- and binary-solute systems; Effect of ionic strength on adsorption of U(VI) (C) and Eu(III) (D) in single-solute; C0 = 0.1 mmol/L, m/V = 1 g/L, T = 25 ºC. Fig. 4. The distribution of aquoes species of radionuclides, (A) U(VI) and (B) Eu(III), (C) U(VI) with 1 mmol/L CO32-, and (D) Eu(III) with 1 mmol/L CO32-, C0 = 0.1 mmol/L, I = 0.01 mol/L NaNO3, T = 25 ºC. Fig. 5. Adorption isotherms of U(VI) (A) and Eu(III) (B) on M-ATT in single- and binary-solute systems; adsorption-desorption of U(VI) (C) and Eu(III) (D) from M-ATT in single-solute, pH = 4 ± 0.1, m/V = 1 g/L, I = 0.01 mol/L NaNO3, T = 25 ºC. Fig. 6. Adsorption isotherms of U(VI) and Eu(III) on M-ATT in single- and binary-solute systems; (A) and (B) at pH = 6 ± 0.1, (C) and (D) at pH = 9 ± 0.1, m/V = 1 g/L, I = 0.01 mol/L NaNO3, T = 25 ºC. Fig. 7. (A) FT-IR spectra of M-ATT before and after radionuclides adsorption; (B−F) Survey and high resolution scans of XPS spectra of U(VI) and Eu(III) adsorption on 27



M-ATT: (B) total survey, (C) U 4f, (D) Eu 3d, (E) Fe 2p, (F) O 1s.

28


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Table 1 Chemical compositions of ATT and M-ATT Compositions

ATT (wt %)

M-ATT (wt %)

SiO2

71.72

47.10

MgO

14.11

8.31

Al2O3

7.77

5.24

Fe2O3

4.67

37.36

K2O

1.06

0.62

TiO2

0.47

0.34

Others

0.21

1.02

Table 2 Peak parameters for U 4f XPS spectra for M-ATT-U+Eu at pH 4.0 and 8.0 U 4f 7/2

U 4f 7/2

U 4f 5/2

U 4f 5/2

U(IV)

U(VI)

U(IV)

U(VI)

pH 4.0

380.74

382.10

391.94

393.00

pH 8.0

381.11

382.17

391.94

393.08

Binding energy (eV)

29



Page 30 of 38

Table 3 Peak parameters for Fe 2p and O 1s XPS spectra for M-ATT (1), after U(VI) and Eu(III) adsorption at pH 4.0 (2) and pH 8.0 (3) Samples

(1)

(2)

(3)

Element

Fe 2p3/2

Fe 2p3/2

Fe 2p3/2

Assignment

BE(eV)

Fe(II)oct

709.91

Fe(III)oct

711.50

Fe(III)tet

Element

Assignment

BE(eV)

Fe-O

530.60

-OH

531.91

713.50

sorbed H2O

532.80

Fe(II)oct

709.91

Fe-O

531.10

Fe(III)oct

711.50

-OH

532.24

Fe(III)tet

713.50

sorbed H2O

533.15

Fe(II)oct

709.91

Fe-O

530.79

Fe(III)oct

711.50

-OH

532.08

Fe(III)tet

713.50

sorbed H2O

533.13

O 1s

O 1s

O 1s

30


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Fig. 1.

31



Fig. 2.

32


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Fig. 3. .

33



Fig. 4.

34


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Fig. 5.

35



Fig. 6.

36


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Fig. 7.

37



GRAPHICAL ABSTRACT

38


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Spectroscopic Investigation of Enhanced Adsorption of U (VI) and Eu

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