Retention of U(VI) by the Formation of Fe Precipitates from Oxidation

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Retention of U(VI) by the Formation of Fe Precipitates from Oxidation of Fe(II) Huiyang Mei, Xiaoli Tan, Liqiang Tan, Yuedong Meng, Changlun Chen, Ming Fang, and Xiangke Wang ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00055 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Earth and Space Chemistry

Retention of U(VI) by the Formation of Fe Precipitates from Oxidation of Fe(II) Huiyang Mei,†,‡ Xiaoli Tan,†,‡,* Liqiang Tan,† Yuedong Meng,‡ Changlun Chen,‡ Ming Fang,†,* Xiangke Wang†,§ †

School of Environment and Chemical Engineering, North China Electric Power

University, Beijing 102206, P. R. China ‡

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 §

Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education

Institutions, Suzhou, Jiangsu, P. R. China * Corresponding author Email: [email protected] or [email protected] (X. Tan); [email protected] (M. Fang)

ABSTRACT: The fate of radionuclide contaminants can be strongly affected by Fe(II) oxidation process at natural redox boundaries as the formative Fe precipitates may serve as scavengers for radionuclides. In this regard, we investigated the retention efficiency of U(VI) by the formative Fe precipitates in dilute FeCl2 solutions under mild oxidative conditions. The formed precipitates were characterized by using spectroscopic methods. The results identified that the presence of U(VI) facilitated the formation of Fe precipitates as lepidocrocite. The high retention efficiency of U(VI) through co-precipitated reaction is ascribed to the synergetic effect of incorporation 1

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and adsorption process. The successive acid washing experiments, energy dispersive X-ray spectra and excitation-emission matrix spectra revealed that the adsorbed U was washed off while the incorporated one was still in the precipitates. As a redox-active trace element, U incorporated into the solids could be in the state of U(V) or U(VI) as analyzed from X-ray photoelectron spectroscopy. These results may be of value in evaluating the potential of permanent retention of trace element through redox and precipitation of iron oxides. KEYWORDS: U(VI); Fe(II); Fe precipitate; oxidation; incorporation 1. INTRODUCTION The release of radionuclides through processes like refining of nuclear fuel and nuclear reactor operation raises serious public concerns due to their inherent toxicity and radioactivity.1-6 It is imperative to gain a thorough understanding of the migration and transformation behaviors of radionuclides and to immobilize the radionuclide contamination permanently. However, the usually used method of adsorption may cause slow re-release of the radionuclides, especially under acidic conditions.7,8 Uranium (U), a most common radionuclide contaminant, is very redox sensitive due to the instability of the 4f electrons in its structure.9 The oxidation state exerts great influence on the mobility of U.10 The hexavalent U(VI), existing primarily as the uranyl cation (UO22+), is the most soluble species and can easily migrate under oxidizing conditions.11 In contrast, tetravalent U(IV) is sparingly soluble and immobile, which may form precipitates such as uraninite (UO2(s)) under common environmental conditions.7 Pentavalent U(V) is very rare and the relevant natural 2


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mineral is seldom documented.12 Both theoretical and experimental approaches confirmed the incorporation of U(V) in the structure of the minerals, which is significant for the understanding of the redox transformation and the fate of U in the environments.13-19 Reducing U(VI) to the less mobile oxidation state of U(IV/V) is an efficient way to immobilize U. The retention of U(VI) has been investigated intensively by utilizing various techniques (such as adsorption, precipitation, incorporation, reduction, etc.) over the past decade.4,20-24 Reduction of U(VI) leads to the formation of highly insoluble U-containing phase, which could seemingly be used for long-term in-situ immobilization.25 The fluctuation in the geochemical conditions may cause reoxidation of U(IV) phases (even sparingly soluble in water) and therefore lead to significant remobilization of U(VI) in the environment. The geochemical cycling of the trace radionuclides is closely associated to the cycle of Fe element in natural systems.9,26-29 At environmental redox boundaries, for example, in aquifers and sediments with spatial redox gradients, the redox reaction between Fe(II) and U(VI) and the transformation of precipitate phase can critically influence the fate of trace radionuclides.30,31 The Fe precipitates formed by oxidation of Fe(II) with nanometer-range size may serve as “scavengers” of U.32 Retention of radionuclides by embedding them into Fe host minerals is advantageous over reduction process in redox-stratified environments as the resultants are recalcitrant to re-mobilization and extraction.33 Scavenging U(VI) by interacting with Fe(II) has been studied previously, most

of

which

were

performed

in

anaerobic

3


environments.34-36

Though


thermodynamically favorable, homogeneous reduction of U(VI) by Fe(II) has not been observed at neutral pH in an anaerobic system.36 Once the hematite nanoparticles acting as intermediates were added in the system, U(VI) reduction started and reached equilibrium rapidly. However, there are limited studies on the interaction between U(VI) and Fe(II) at mild oxidizing condition that generally appears at environmental redox boundaries, which is of great importance in modeling the geochemical behavior of Fe(II) and U(VI) in the real environment. During the formation of Fe precipitates from oxidation and hydrolysis of Fe(II), U(VI) can be immobilized on the meta-stable Fe intermediate species (e.g. ferrihydrite). As the intermediate precipitates transform into more stable phases (iron oxyhydroxides), the binding U(VI) may be stabilized and incorporated into the structure of the precipitates.33 It has been found that the formative species of Fe precipitates is closely related to the aqueous Fe(II) concentration (Table S1), which has influence on the retention efficiency of U(VI).37,38 Many researchers focused on the interactions of U(VI) and goethite, the formation of which is favored in relatively high Fe(II) concentration.7,33 But the knowledge about the reactivity and retention ability of Fe precipitates forming in dilute aqueous Fe(II) solutions, which are typically relevant to reduced soil pore and ground waters, is still lacking. During the formation of Fe precipitates, the retention of U(VI) involves incorporation as well as adsorption. The comparison of these two scarcely investigated processes is necessary and helpful for us to understand the full mechanism. In this work, we studied the retention of U(VI) during the formation of Fe 4


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precipitates at low concentrations of Fe(II) under mild oxidative conditions. Different initial U:Fe molar ratios were adopted to investigate the retention efficiency of U(VI) through incorporation in the crystal structure or adsorption on the surface of Fe precipitates. Experiments of U(VI) adsorbed on the preformed Fe precipitates were also conducted. The objectives of the research were (i) to identify whether U was actually incorporated into the crystal structure of the Fe precipitates; (ii) to evaluate the effects of incorporation and adsorption on the retention process and (iii) to explore the redox process of U(VI) during the oxidation of Fe(II). These combined results are essential for understanding the retention process of U(VI) through oxidative hydrolysis of dilute aqueous Fe(II) solutions under mild oxidative conditions, which commonly occurs at environmental redox boundaries. 2. MATERIALS AND METHODS 2.1 Materials All chemicals, including FeCl2·4H2O, NaHCO3, concentrated HCl, and UO2(NO3)2·6H2O were analytical grade from Sigma Aldrich or Sinopharm Chemical Reagent Co., Ltd and used without further treatment. All stock solutions were prepared by dissolving the chemicals in doubly deionized water (DDI-H2O) (18 MΩ, Millipore system). The Fe(II) stock solution (50 mM FeCl2·4H2O, 1.0 mM HCl) was prepared freshly before experiments and placed in N2 atmosphere to avoid oxidation. 2.2 Co-precipitation Experiments Co-precipitation experiments of U(VI) with Fe(II) solutions under aerated conditions were conducted as follows. NaHCO3 solution was added into the 5



Erlenmeyer flask to achieve a buffer concentration of 8 mM. The pH was lowered to 7.0 (±0.1) by adding 1.0 M HCl slowly with vigorous mechanical stirring. The U(VI) stock solution was then pipetted in to obtain variable initial concentrations (0.025-0.10 mM). The oxidation experiment was triggered by adding Fe(II) stock solution to achieve the initial concentration of 1.0 mM. Afterwards, the Erlenmeyer flask was transferred to the oscillator for continuous shaking. The flask was not sealed to ensure that the solution remained exposed to air, which is closer to the natural environmental conditions. The reaction was terminated after 24 h. The co-precipitation samples are abbreviated as Fe-U0.10ppt (0.10 denotes that the initial concentrations of U(VI) is 0.10 mM), Fe-U0.05ppt and Fe-U0.025ppt (Table 1). The solid phases were obtained by centrifugation at 9000 rpm for 20 min. The concentration of Fe(III) in the final supernatants was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and U(VI) concentration was analyzed by a kinetic phosphorescence analyzer (KPA-11, Richland, USA). The precipitates were then washed with DDI water for several times and dried in air for the following experiments and characterization. The control sample was prepared as described above except for the addition of U(VI) solution. 2.3 Adsorption Experiments The Fe precipitate suspension was prepared as described above (Section 2.2) except for the addition of U(VI). After the termination of the Fe precipitate formation (24 h), the needed U(VI) was introduced into the suspension and reacted for another 24 h. The adsorption samples are abbreviated as Fe-U0.10ads, Fe-U0.05ads and 6


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Fe-U0.025ads. The difference of concentrations between the final and the initial one could be taken as the adsorption amount of U(VI) on Fe precipitates, which were then compared with those of co-precipitation samples. 2.4 Successive Washing and Digestion Experiments To track where the initially dosed U(VI) are going, the collected co-precipitation and adsorption samples were washed by successive process, i.e., they were first washed by DDI water, followed by HCl solution starting from 10-3 M H+ (pH = 3.0), then 10-2 M H+ (pH = 2.0) and 10-1 M H+ (pH = 1.0) for 30 min, respectively, and finally dried in vacuum. To obtain the U/Fe molar ratios in bulk structure, the Fe precipitate samples before and after the successive washing process were digested in 6 M HCl. The concentrations of Fe and U in the digestion solution were determined. Due to the inevitable loss of solid mass during the successive washing processes, it is inaccurate to compare the absolute concentrations of Fe and U for different samples after digestion. Thus, the relative concentration ratio of U/Fe was employed for comparison. 2.5 Characterization of Fe Precipitates The morphology images, selected-area electron diffraction (SAED) and energy dispersive X-ray spectra (EDS) of the Fe precipitates before and after successive washing process were investigated by transmission electron microscope (TEM, JEM-2010). The crystal phases of the solids were identified using X-ray diffraction (XRD, Rigaku MiniFlex 600) equipped with a Cu Kα radiation source (λ = 1.5406 Å) of 10-70° by step size of 0.02°. Fourier transformed infrared (FT-IR) spectra of the Fe 7



precipitates were conducted on a Nicolet IR spectrometer from a KBr pellet over a range of 400-4000 cm-1. The fluorescence spectra were measured on a spectrofluorimeter (HITACHI F-7000). The detailed procedure was described in the Supporting Information. X-ray photoelectron spectra (XPS) were accomplished on a Thermo Escalab 250 with an Al Kα X-ray source. The C1s peak (284.8 eV) was employed as a reference to calibrate the binding energies. The XPSPEAK software was used to process the obtained XPS data. 3. RESULTS AND DISCUSSION 3.1 Retention Efficiency of U(VI) The retention of U(VI) during the formation of Fe precipitates was carried out under aerobic conditions. The reaction conditions and final concentrations of U(VI) and Fe(III) in solutions are shown in Table 1. In all experiments, only a small amount of Fe (< 10%) was left in solution after 24 h of oxidation. Maintaining the same U(VI) initial concentration, the U(VI) left in solution after adsorption was higher than that after co-precipitation, demonstrating the high removal efficiency of U(VI) by the co-precipitation method. The retention efficiency was calculated from the difference between the initial and final U(VI) concentration in solution (Table 1). For all the experiments, the retention efficiency of U(VI) decreased with increasing initial U(VI) concentration. In addition, the retention of U(VI) was also evaluated by the molar concentration ratio of U and Fe in the precipitates. The U/Fe molar ratios in the solids increased with increasing initial U(VI) concentration, which is ascribed to the fact that more amount of U(VI) was immobilized by the precipitates. By comparison, U/Fe 8


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molar ratio of the co-precipitation sample was higher than that of the adsorption one, which is consistent with the result of retention efficiency. The retention percentages of U(VI) were quite small for the adsorption experiments (Table 1), elucidating the weak affinity of U(VI) to the surface of Fe precipitates at near-neutral conditions. The addition of bicarbonate might suppress the hydrolysis of U(VI) and facilitate the formation of aqueous UO2(CO3)22- and UO2(CO3)34- complexes at near-neutral conditions.39 The adsorption of U(VI) on iron oxyhydroxides was mainly due to inner sphere surface complexation at near-neutral pH.40 It is difficult for the anionic U(VI) species to form complexes with the negatively charged surfaces of precipitates and this may inhibit adsorption of U(VI), leading to the low retention efficiency. The co-precipitation process is different from adsorption as some soluble U(VI) might first be adsorbed by the preformed meta-stable Fe intermediates. Then the binding U(VI) was likely to be locked in the matrix through the crystallization of the Fe precipitates. The incorporation effect should be responsible for the improved retention efficiency of the co-precipitation process. At near-neutral conditions, the retention of U(VI) was not attributed to the precipitation of schoepite because of the low precipitation constant (logKsp = -5.39).39 Thus, both adsorption and incorporation mechanisms are responsible for the retention of U. In order to evaluate the stability of U immobilized by the precipitates, the successive washing process was employed. The U/Fe molar ratios in bulk precipitates after the successive washing process are shown in Table 1. As for the Fe-U0.10ads, 9



Fe-U0.05ads and Fe-U0.025ads samples after the successive washing process, the concentration of U in the digestion solution was below the detection limit, indicating that the adsorbed phase of U(VI) was on the surface of Fe precipitates and could be removed by the acid washing process. For the Fe-U0.10ppt, Fe-U0.05ppt and Fe-U0.025ppt samples after the successive washing process, there is still a small amount of U in the digestion solution, suggesting that a proportion of retained U is stable and recalcitrant to the acid washing process. The U/Fe molar ratios in the solids increased with increasing initial U(VI) concentration. This is presumably because more U was incorporated in the crystal structure of precipitates with higher initial U(VI) concentration. 3.2 Crystal Structures of Fe Precipitates The XRD patterns of the Fe-U0.10ppt, Fe-U0.10ads and control sample before and after the successive washing process are shown in Figure 1. The dominant broad peaks of the control sample agreed well with the reference mineral, lepidocrocite. In addition, some typical peaks of goethite could also be observed, demonstrating that the control sample contains some goethite phase. The observation is in line with previous research that a minor fraction of goethite formed in Na electrolyte by the oxidation of 1.0 mM Fe(II).37 As for the adsorption sample (Fe-U0.10ads), apart from the dominant phase of lepidocrocite, a typical peak of goethite phase was detected. However, the peaks assigned to goethite disappeared after the successive washing process. For the unwashed co-precipitation samples (Fe-U0.10ppt), the peaks of goethite were also not observed. The co-precipitation samples (Fe-U0.10ppt) after the 10


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successive washing process exhibited sharper peaks compared with the unwashed ones, which indicated that the amorphous phase was washed off. The absence of goethite peaks implied that the presence of U(VI) inhibited the formation of goethite phase in co-precipitation experiments. Carbonate anions have been demonstrated to facilitate the formation of goethite,41 but the strong complexation of U(VI) with carbonate may weaken the effect of carbonate and then inhibit the formation of goethite. Figure 2A shows the FT-IR spectra of the Fe-U0.10ppt, Fe-U0.10ads and control sample before and after the successive washing process. Some characteristic bands of lepidocrocite appeared at ~ 3420, 3180, 1632 and 472 cm-1.42,43 The band at ~ 3420 cm-1 belonged to the stretching vibration of O–H. The band at ~ 3180 and 1632 cm-1 were assigned to the bending vibrations of the adsorbed water molecules.43 The bands at ~ 1020 and 740 cm-1 were ascribed to the in-plane and out of plane bend modes of OH group, respectively. The typical peak at 472 cm-1 attributed to the symmetric Fe-O stretch mode became sharpened after the successive washing process (Figure 2B). In addition, the shoulder band around 1157 cm-1 also belonged to lepidocrocite phase.42 The bands of goethite at 796 and 885cm-1 were shown in the spectra of control and adsorption samples, indicating the presence of goethite phase (Figure 2C).44 These bands can not be observed in the spectra of co-precipitation samples, implying that no goethite phase was formed. The result is consistent with that of XRD analysis. 3.3 Morphology and Composition of Fe Precipitates To identify the morphology and the composition of the Fe precipitates, 11



Fe-U0.10ppt, Fe-U0.10ads and Fe-U0.10ppt after successive washing were chosen and characterized through TEM and SAED patterns (Figure 3). The Fe-U0.10ppt sample exhibited spherical and loose morphology with a diameter of several hundred nanometers, which was mainly formed through the precipitation or aggregation of the Fe flocs (Figure 3A-1).38 The Fe-U0.10ppt sample showed polycrystallinity as indicated by the well-defined rings of SAED pattern (Figure 3A-2). Different from the co-precipitation sample, tighter packing morphology could be observed for the Fe-U0.10ads sample (Figure 3B-1). The corresponding SAED pattern with scattered points indicated the poor crystallinity of the adsorption sample compared to that of the co-precipitation sample (Figure 3B-2). For the Fe-U0.10ppt sample, after successive washing process the agglomerates formed by the aggregation of short rods emerged and many thin films on the edge disappeared due to corrosion by the acid (Figure 3C). The relevant EDS spectra of those samples showed the existence of dominant elements (Fe, O and U) (Figure 4). No signal of U could be detected for the Fe-U0.10ads sample after successive washing process (Figure S1), implying the complete removal of the adsorbed U. The Fe-U0.10ppt sample after successive washing process displayed the signal of U (Figure 4C), which could be regarded as the evidence of incorporation of U in the crystal structure of Fe precipitates. The U elemental intensity of the sample decreased a lot compared with the Fe-U0.10ppt sample, indicating the loss of the adsorbed U after successive washing. The U/Fe molar ratios in the bulk structure calculated from EDS measurements could provide an insight into the relative proportion of U and Fe in the samples. The U/Fe molar 12


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ratios followed the order of Fe-U0.10ppt (2.27%) > Fe-U0.10ads (1.34%) > Fe-U0.10ppt after successive washing process (0.710%), which is similar to that of the experiment results (Table 1). 3.4 Mechanism of U(VI) Retention As U is a fluorescent actinide, some useful information could be obtained from the fluorescence contour maps. The excitation-emission matrix (EEM) spectrum of aqueous uranyl is shown as a contour map in Figure S2. Two dominant peaks centered at 290EX/496EM and 420EX/497EM nm could be observed. To explore the species of retained U, the EEM spectra of Fe-U0.10ppt and Fe-U0.10ads before and after successive washing were recorded and shown in Figure 5. There is a dominant peak centered at 201EX/467EM nm for the Fe-U0.10ppt (Figure 5A), which is different from that of aqueous uranyl. This indicated the variation of chemical environments between free aqueous uranyl and the immobile U. In case of the spectrum of Fe-U0.10ads (Figure 5C), no obvious difference can be observed compared with that of Fe-U0.10ppt, in spite of the fact that the U chemical environments in adsorption and co-precipitation phases are different. The characteristic peak of the Fe-U0.10ppt after successive washing (205EX/467EM nm) shifted up slightly (Figure 5B). It meant that the different U speciation existed in the Fe-U0.10ppt sample, and the remaining U immobilized by co-precipitation phase still remained unchanged after successive washing. The co-precipitation phase is different from the adsorption phase. As for the EEM spectrum of the Fe-U0.10ads after successive washing (Figure 5D), no signal of U was detected, which agrees with the above analysis. 13



The valence state of trace elements might change during the oxidation of Fe(II). U may be incorporated into the structure of iron oxides or oxyhydroxides in the state of U(V).45 In order to evaluate the change of species and the valence state of U in the bulk structure, XPS was employed to characterize the co-precipitation sample (Fe-U0.10ppt) after successive washing process. Figure 6A shows the XPS survey spectra of the sample, where the peaks of dominant elements (Fe, O, U) appeared clearly. The high-resolution Fe 2p spectrum of the sample is present in Figure 6B. Peaks at 711.46 eV, 719.64 eV and 725.07 eV were assigned to the binding energies of Fe 2p3/2, satellite 2p3/2 and 2p1/2, respectively.46 The satellite peak is associated with the charge transfer screening, which is solely due to the presence of the Fe3+ ions of lepidocrocite.47 For the crystal structure of the lepidocrocite, some Fe3+ cations are octahedrally coordinated while the others are tetrahedrally coordinated.48 Thus, the Fe 2p peaks were decomposed into two peaks, one for Fe(III) octahedra and the other for Fe(III) tetrahedra. The high-resolution O 1s spectrum of the sample is shown in Figure 6C and could be decomposed into three components. The peak at 529.8 eV corresponded to lattice O2- from lepidocrocite. The peak located at 531.1 eV was assigned to the lattice OH-. The peak centered at 531.9 eV was attributed to the adsorbed OH- located on the iron oxides surface.46 As a redox-active element, the valence state of U might change during the oxidation of Fe(II). Fe(II) can complex with the U(VI) species on the surface of the formed lepidocrocite, which can thus lead to an electron transfer from Fe(II) to U(VI) and create U(V) and Fe(III). The formed U(V) would then be stabilized by 14


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incorporation into the structure upon the crystal growth of the host mineral phase.7,33 The incorporation of trace elements is usually accompanied by lattice deformation of lepidocrocite.18 The greater the radius disparity between dopant and the cation in the matrix crystal structure, the more difficult it is to incorporate the dopant.49,50 U(V) can be incorporated in the Fe precipitations because of the similar ionic radius of U(V) (0.76 Å) with Fe(III) (0.645 Å).51 It could be observed that both the primary U 4f7/2 (~ 381 eV) and U 4f5/2 spectra (~ 392 eV) were obviously split into two components (Figure 6D), indicating the presence of two distinct chemical states of U. The 4f7/2 spectrum was decomposed into two peaks, located at 381.80 and 380.40 eV, which were assigned to the contribution of U(VI) and U(V), respectively.52 The 4f5/2 spectrum was also decomposed into two peaks of U(VI) (~392.57 eV) and U(V) (~391.12 eV). However, the result of fitting the primary peak could only be taken as supportive information for identifying U(V). It has been suggested that the valence state of U can be confirmed explicitly by analyzing the satellite peak structures.15 As for the U 4f7/2 primary peak, the separation between the U(IV), U(V) and U(VI) primary peaks and their associated satellite ones were ~ 6.0-7.0 eV, ~ 7.8-8.5 eV and ~ 4.0 eV, respectively.53 Two satellite peaks corresponding to U(VI) (~ 385.64 eV) and U(V) (~ 388.56 eV), but no evidence for U(IV), were observed (Figure 6D). The fitting result indicated the presence of a fraction of U(V), which was regarded as the partial species incorporated into the crystal structure of iron oxyhydroxides,33 while the main amount of U belonged to the U(VI) species. It is unlikely for U(VI) to be incorporated in the structure of Fe precipitates in the form of UO22+, due to the 15



significant gap between the size of UO22+ (1.80 Å) and Fe(III) (0.645 Å).51 However, previous reports indicated that U6+ (with ionic radii of 0.73 to 0.81 Å) could be incorporated into the structure of Fe oxides.20 The incorporated U(VI) might be in the state of U6+ without axial O atoms. Thus, the incorporated U in the precipitates in this study might be in the state of U(V) as well as U(VI). In addition, the XPS survey spectrum of the adsorption sample Fe-U0.10ads and corresponding U 4f spectrum (Figure S3) were also provided for comparison. It could be clearly observed that only U(VI) existed in the adsorption sample, which is quite different from that of the coprecipitation one. Based on the experimental results, characterization and analysis, the retention mechanism of U by the Fe(II) oxidation process can be depicted in Figure 7. Under the oxidizing conditions, U could be incorporated in the bulk structure in the state of U(V) through the electron transfer from Fe(II) ions during the crystallization of Fe precipitates. The incorporated U was recalcitrant to acid washing process, which provides a new insight into the immobilization of U. The results indicated that U(VI) was partially reduced to U(V) with no evidence of U(IV). 4. CONCLUSION The higher retention efficiency of co-precipitation experiments than adsorption experiments demonstrated the complex mechanism of incorporation for the retention process. The incorporation effect could increase the retention efficiency of U by Fe precipitates and stabilize U better than adsorption, which was revealed by the successive washing experiments. The XRD and FT-IR spectra indicated that the dominant phase for the Fe precipitates is lepidocrocite. The fact that only a small 16


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fraction of U was incorporated into the crystal structure was confirmed by the digestion experiments and EDS spectra. As a redox-active trace element, U incorporated into the solids might be in the state of U(V) as well as U(VI) as indicated by XPS analysis. These results shed light on the retention ability and mechanisms of U by the Fe precipitates formed from dilute Fe(II) solutions in natural redox boundaries. Due to the short-term nature of this study, long term experiments are needed in the future to assess the stability and phase transformation of the resultant precipitates. ACKNOWLEDGMENTS Financial supports from National Natural Science Foundation of China (U1607102, 11675210), Science Challenge Project (TZ2016004), the Fundamental Research Funds for the Central Universities (2018ZD11), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions are acknowledged. REFERENCES (1) Tan, X.; Fang, M.; Wang, X. Sorption speciation of lanthanides/actinides on minerals by TRLFS, EXAFS and DFT studies: a review. Molecules 2010, 15, 8431-8468. (2) Tan, X.; Liu, G.; Mei, H.; Fang, M.; Ren, X.; Chen, C. The influence of dissolved Si on Ni precipitate formation at the kaolinite water interface: Kinetics, DRS and EXAFS analysis. Chemosphere 2017, 173, 135-142. 17



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Ding,

C.;

Cheng,

W.;

Sun,

Y.;

Wang,

X.

Novel

fungus-Fe3O4

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Ca

on

Fe(III)-precipitates

formed

in

aerated

Fe(II)-

and

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(42) Lewis, D.; Farmer, V. Infrared absorption of surface hydroxyl groups and lattice vibrations in lepidocrocite (γ-FeOOH) and boehmite (γ-AlOOH). Clay Miner. 1986, 21, 93-100. (43) Supattarasakda, K.; Petcharoen, K.; Permpool, T.; Sirivat, A.; Lerdwijitjarud, W. Control of hematite nanoparticle size and shape by the chemical precipitation method. Powder Technol. 2013, 249, 353-359. (44) Cumplido, J.; Barrón, V.; Torrent, J. Effect of phosphate on the formation of nanophase lepidocrocite from Fe(II) sulfate. Clay. Clay Miner. 2000, 48, 503-510. (45) Roberts, H. E.; Morris, K.; Law, G. T. W.; Mosselmans, J. F. W.; Bots, P.; Kvashnina, K.; Shaw, S. Uranium(V) incorporation mechanisms and stability in Fe(II)/Fe(III) (oxyhydr)oxides. Environ. Sci. Technol. Lett. 2017, 4, 421-426. (46) Liu, A.; Liu, J.; Pan, B.; Zhang, W. Formation of lepidocrocite (γ-FeOOH) from oxidation of nanoscale zero-valent iron (nZVI) in oxygenated water. RSC Adv. 2014, 4, 57377-57382. (47) Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254, 2441-2449. (48) Grosvenor, A.; Kobe, B.; Biesinger, M.; McIntyre, N. Investigation of multiplet splitting of Fe2p XPS spectra and bonding in iron compounds. Surf. Interface Anal. 2004, 36, 1564-1574. (49) Lee, G.; Bigham, J.; Faure, G. Removal of trace metals by coprecipitation with Fe, Al and Mn from natural waters contaminated with acid mine drainage in the Ducktown Mining District, Tennessee. Appl. Geochem. 2002, 17, 569-581. 23



(50) Curti, E. Coprecipitation of radionuclides with calcite: estimation of partition coefficients based on a review of laboratory investigations and geochemical data. Appl. Geochem. 1999, 14, 433-445. (51) Shannon, R. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. A 1976, 32, 751-767. (52) Schindler, M.; Hawthorne, F.; Freund, M.; Burns, P. XPS spectra of uranyl minerals and synthetic uranyl compounds. I: The U 4f spectrum. Geochim. Cosmochim. Acta 2009, 73, 2471-2487. (53) Ilton, E.; Bagus, P. XPS determination of uranium oxidation states. Surf. Interface Anal. 2011, 43, 1549-1560. (54) Cambier, P. Infrared study of goethites of varying crystallinity and particle size: I. Interpretation of OH and lattice vibration frequencies. Clay Miner. 1986, 21, 191-200. (55) Nasrazadani, S. The application of infrared spectroscopy to a study of phosphoric and tannic acids interactions with magnetite (Fe3O4), goethite (α-FeOOH) and lepidocrocite (γ-FeOOH). Corros. Sci. 1997, 39, 1845-1859.

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Table 1. Synthesis conditions and final concentrations of U and Fe in solutions and solids. sample

[U(VI)]i (mM)

[Fe(III)]f (mM)

[U(VI)]f (mM)

([U(VI)]i-[U(VI)]f)/ [U(VI)]i (%)

[U/Fe]b (%)

[U/Fe]a (%)

control

0

0.0514

0

0

0

0

Fe-U0.10ppt

0.10

0.0553

0.0839

16.1

1.96

1.04

Fe-U0.05ppt

0.050

0.0675

0.0401

19.8

1.05

0.61

Fe-U0.025ppt

0.025

0.0634

0.0181

27.6

0.70

0.51

Fe-U0.10ads

0.10

0.0431

0.0854

14.6

1.59

0*

Fe-U0.05ads

0.050

0.0166

0.0419

16.2

0.82

0*

Fe-U0.025ads

0.025

0.0144

0.0197

21.2

0.54

0*

[U(VI)]i is the initial concentration of U(VI) added in solution. [Fe(III)]f and [U(VI)]f are final concentrations of Fe(III) and U(VI) in solution, respectively, after 24 h reaction period. ([U(VI)]i-[U(VI)]f)/[U(VI)]i (%) refers to the removal percentage of U(VI) from solution if only initial and final U(VI) concentrations are taken into account. [U/Fe]b (%) is the molar ratio of U and Fe in the precipitates before the successive washing process. [U/Fe]a (%) is the molar ratio of U and Fe in the precipitates after the successive washing process. * The values below a threshold of 0.01 are reported as 0 in the table.

25



Figure captions Figure 1. The XRD patterns of the Fe-U0.10ppt, Fe-U0.10ads and control sample before and after the successive washing process. Figure 2. FT-IR spectra for the Fe-U0.10ppt, Fe-U0.10ads and control sample before and after the successive washing process (A), corresponding enlarged regions of 430-520 cm-1 (B) and 670-850 cm-1 (C). The spectra of pure goethite54 and lepidocrocite55 are provided for comparison. Figure 3. TEM and corresponding SAED images of Fe-U0.10ppt (A-1, A-2) and Fe-U0.10ads (B-1, B-2) before successive washing process and Fe-U0.10ppt after successive washing process (C-1, C-2). Figure 4. EDS spectra of Fe-U0.10ppt (A) and Fe-U0.10ads (B) before successive washing process and Fe-U0.10ppt after successive washing process (C). Figure 5. Fluorescence excitation-emission matrix spectra of Fe-U0.10ppt before and after successive washing process (A, B) and Fe-U0.10ads before and after successive washing process (C, D). Figure 6. XPS survey spectrum of the co-precipitation sample Fe-U0.10ppt after successive washing process (A), corresponding Fe 2p (B), O1s (C) and U 4f spectra (D). Figure 7. The schematic of the retention mechanism of U by the Fe(II) oxidation process.

26


Page 26 of 35

Page 27 of 35

successive washing Fe-U0.10ads Fe-U0.10ads

successive washing Fe-U0.10ppt

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe-U0.10ppt

control successive washing control PDF#44-1415, lepidocrocite PDF#29-0713, goethite

10

20

30

40

50

2 Theta (degree)

Figure 1

27


60

70


(A)

successive washing Fe-U0.10ads successive washing Fe-U0.10ppt successive washing

Transmittance (%)

control Fe-U0.10ads Fe-U0.10ppt

control lepidocrocite

goethite

1020 740 796 472 885 0

500

3180 3420

1632

1000

1500

2000

2500

3000

3500

4000

4500

5000

-1

Wavenumber (cm ) (B) successive washing Fe-U0.10ads successive washing Fe-U0.10ppt successive washing control Fe-U0.10ads Fe-U0.10ppt

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

control lepidocrocite

-1

472 cm

430

440

450

460

Fe-O stretch mode

470

480

490

500

510 -1

Wavenumber (cm )

28


520

530

540

550

Page 29 of 35

successive washing

(C)

Fe-U0.10ads successive washing Fe-U0.10ppt successive washing control Fe-U0.10ads Fe-U0.10ppt

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


control goethite lepidocrocite

740

680

720

796

760

800

840 -1

Wavenumber (cm )

Figure 2

29


880


Figure 3

30


Page 30 of 35

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Figure 4

31



EX (nm)

250

460.0 250

(A)

1200

(B)

240

368.0 240

960.0

230

276.0

720.0

184.0

220

230

480.0

220

92.00

210

240.0

210 0

200 300

350

400

450

500

550

0

200 300

600

250

350

400

450

500

550

600

450.0 250

(C)

EX (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

200.0

(D)

240

360.0 240

160.0

230

270.0 230

120.0

180.0

80.00

220

220

90.00

210

40.00

210 0

200 300

350

400

450

500

550

600

0

200 300

350

EM (nm)

Figure 5

32


400

450 500 EM (nm)

550

600

Page 33 of 35

(B)

(A) O-1s

Fe(III)-2p3/2 Fe(III)-2p1/2

Fe-2p Intensity (a.u.)

Intensity (a.u.)

Fe(III) Otc.

C-1s

Fe(III) Tet. Fe(III)-sat

U-4f

200

400 600 800 Binding Energy (eV)

1000

705

710 (D)

(C)

-

adsorbed OH 2-

-

lattice OH

8.5 eV U(VI) satellite

3.8 eV 528

529


533

534

730

data fit envelope U-4f 5/2 U(V) U(VI)

U-4f7/2

O1s

lattice O


Intensity (a.u.)

0

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


376

380

Figure 6

33


U(V) satellite


396


Figure 7

34


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Graphical abstract

Retention of U(VI) by the formation of Fe precipitates from oxidation of Fe(II).

35


Retention of U(VI) by the Formation of Fe Precipitates from Oxidation

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