Performance and Mechanism of Uranium Adsorption from Seawater to

Article pubs.acs.org/est

Performance and Mechanism of Uranium Adsorption from Seawater to Poly(dopamine)-Inspired Sorbents Fengcheng Wu,† Ning Pu,† Gang Ye,*,†,‡ Taoxiang Sun,† Zhe Wang,† Yang Song,† Wenqing Wang,† Xiaomei Huo,† Yuexiang Lu,† and Jing Chen*,†,‡ †

Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology, and Beijing Key Lab of Radioactive Waste Treatment, Tsinghua University, Beijing 100084, China

‡

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S Supporting Information *

ABSTRACT: Developing facile and robust technologies for effective enrichment of uranium from seawater is of great significance for resource sustainability and environmental safety. By exploiting mussel-inspired polydopamine (PDA) chemistry, diverse types of PDA-functionalized sorbents including magnetic nanoparticle (MNP), ordered mesoporous carbon (OMC), and glass fiber carpet (GFC) were synthesized. The PDA functional layers with abundant catechol and amine/imine groups provided an excellent platform for binding to uranium. Due to the distinctive structure of PDA, the sorbents exhibited multistage kinetics which was simultaneously controlled by chemisorption and intralayer diffusion. Applying the diverse PDA-modified sorbents for enrichment of low concentration (parts per billion) uranium in laboratory-prepared solutions and unpurified seawater was fully evaluated under different scenarios: that is, by batch adsorption for MNP and OMC and by selective filtration for GFC. Moreover, high-resolution X-ray photoelectron spectroscopic and extended X-ray absorption fine structure studies were performed for probing the underlying coordination mechanism between PDA and U(VI). The catechol hydroxyls of PDA were identified as the main bidentate ligands to coordinate U(VI) at the equatorial plane. This study assessed the potential of versatile PDA chemistry for development of efficient uranium sorbents and provided new insights into the interaction mechanism between PDA and uranium.

1. INTRODUCTION With increasing worldwide concern about the shortage of fossil fuels and global warming, nuclear power, as a clean energy without greenhouse gas emission, has been perceived as a critical component of energy systems.1 Uranium is well-known as a strategic resource in the nuclear industry that is crucial for sustainable nuclear energy production. Meanwhile, uraniumcontaining wastes generated in the nuclear fuel cycle, or released during nuclear accidents, can be environmental hazards with radioactive and chemical toxicity. In recent years, great efforts have been made to develop effective technologies for enrichment of uranium from different media such as seawater, groundwater, and radioactive wastes.2−5 Lately, inspired by the adhesive properties of mussel foot proteins, polydopamine (PDA) chemistry has emerged as a popular strategy for surface modification since the pioneering work by Messersmith and co-workers.6,7 This strategy, due to the mild and environmentally benign synthesis and availability for virtually any type of substrate, has been attracting huge interest in fields like energy storage,8 catalysis,9 biosensors,10,11 drug delivery,12 and environmental remediation.13 Based on PDA chemistry, a uniform PDA layer with tunable thickness can be readily deposited, by manipulating the spontaneous polymerization of dopamine in weak alkaline solutions, onto various bulk surfaces14,15 and nanoparticles.16 © 2017 American Chemical Society

Up to now, structure of the PDA aggregates, and their formation mechanism involving covalent polymerization and noncovalent self-assembly, have not been fully understood.17,18 However, the presence of abundant catechol and imine/amine groups in PDA structure can well improve the substrate’s hydrophilicity while providing active sites for the binding to heavy metal ions and organic contaminants.19,20 For instance, researchers have attempted to employ PDA chemistry for surface modification of graphene oxides21 and mesoporous silicas.13 The resultant products were found to show strong affinity for uranium in aqueous solutions. In these studies, however, the coordination mechanism between PDA and U(VI) was not elucidated, and the adsorption ability of PDAmodified sorbents for low-concentration uranium was not demonstrated. Overall, PDA chemistry has brought about inspiration for the synthesis of effective sorbents for uranium, but in practical terms, how much would the promising strategy benefit this area? Extensive research is still required to expand the family of PDA composites for uranium enrichment and to Received: Revised: Accepted: Published: 4606

January 26, 2017 March 22, 2017 March 23, 2017 March 23, 2017 DOI: 10.1021/acs.est.7b00470 Environ. Sci. Technol. 2017, 51, 4606−4614

Article

Environmental Science & Technology

Figure 1. (a) Illustrative synthesis of polydopamine (PDA)-modified magnetic nanoparticles (MNP), ordered mesoporous carbon (OMC), and glass fiber carpet (GFC). (b−g) TEM images of (b) Fe3O4, (c) Fe3O4@PDA, (d) CMK-3, (e) CMK-3@PDA, (f) GFC, and (g) GFC@PDA. (h, i) Digital photos of (h) GFC and (i) GFC@PDA.

clarify the underlying coordination mechanism between PDA and U(VI). Herein, we report the facile development of diverse types of uranium sorbents via PDA chemistry, including magnetic nanoparticles (MNPs), ordered mesoporous carbons (OMCs), and glass fiber carpet (GFC), and we systematically evaluate their potential for uranium enrichment from laboratoryprepared low-concentration (parts per billion, ppb) solutions and from real seawater. The candidate sorbents were selected by taking into account their physical−chemical properties and structural features, leading to uranium separation under different scenarios. MNPs and OMCs are two classes of popular nanomaterials holding great promise for environmental applications.22−27 MNPs, with favorable magnetic properties, allow facile magnet-assisted separation and reclamation of the sorbents.28−33 OMCs, due to the large specific surface area, mesoscale regularity, and tunable pore size, can promote the adsorption process in terms of both kinetics and capacity.34−36 On the other side, GFC represents a more practical candidate, because of its commercial availability with low cost and accessibility for surface modification,37,38 which provides an alternative choice for separation of metal ions by selective filtration instead of batch adsorption. Surface modification of MNP, OMC, and GFC via PDA chemistry is illustrated in Figure 1a. By controlling the selfpolymerization of PDA in Tris buffer solutions, well-defined PDA layers with tunable thickness were deposited on the substrates. In this study, with the PDA-encapsulated MNP as a model, the structural tuning, morphology characterization, magnetic properties, and ζ potential measurement are detailed. The adsorption behavior toward uranium was investigated, showing interesting adsorption kinetics different from previous reports. On this basis, uranium enrichment from lowconcentration (ppb) solutions and unpurified seawater by use of diverse PDA-modified sorbents was evaluated under different scenarios. Regeneration of PDA-modified sorbents for cycle use was demonstrated. Moreover, the goal of this study also lies in probing the coordination mechanism between PDA and U(VI)

by using spectroscopic tools, such as X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS). This work is expected to pave the way for developing efficient sorbents via PDA chemistry for uranium enrichment and other environmental applications, while providing new insights into the interaction mechanism between PDA and uranium.

2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. First, uniform Fe3O4 MNPs were obtained by a solvothermal method.39 For synthesis of PDA-encapsulated MNPs, 0.1 g of Fe3O4 was dispersed in a mixture of 45 mL of ethanol and 45 mL of Tris buffer solution (20 mM) by ultrasonication treatment for 30 min. Then, 0.2 g of dopamine hydrochloride was dissolved in 10 mL of ethanol aqueous solution (1:1) which was added dropwise to the dispersion. The resulting mixture was mechanically stirred at 25 °C. By control of the reaction time, PDA-encapsulated MNPs (named as Fe3O4@PDA-x, where x represents the reaction time in hours) with different PDA thicknesses were obtained. The products were washed with ethanol several times and dried at 35 °C under vacuum for 15 h. Surface modification of OMC was performed according to a previously reported method.35 CMK-3 type ordered mesoporous carbon (100 mg) was dispersed in 70 mL of mixed ethanol and water solution with volume ratio 4:3. After ultrasonic treatment for 10 min, 400 mg of dopamine hydrochloride was added under magnetic stirring. Five minutes later, 20 mL of 25 mM Tris buffer solution was added dropwise, followed by adjustment of the pH value to 8.5 by use of HCl solution. The coating process was maintained at 25 °C for 10 h. The obtained CMK-3@PDA was separated by centrifugation and washed with ethanol several times. Then the product was placed in a vacuum oven at 60 °C for 24 h. For synthesis of GFC@PDA, 25 mg of glass fiber carpet (GFC), after being rinsed by ethanol and deionized water, was immersed in Tris buffer solution (25 mM) containing 20 mM dopamine. After contact for 24 h in a constant-temperature 4607

DOI: 10.1021/acs.est.7b00470 Environ. Sci. Technol. 2017, 51, 4606−4614

Article


Figure 2. (a) Influence of pH value on U(VI) adsorption efficiency at a contact time of 200 h and (b) adsorption capacity of U(VI) varying with contact time at pH 5.0. [U(VI)] = 100 mg/L, INaClO4 = 0.01 mol/L, T = 298 K, m/v = 1.5 g/L. (Inset) Kinetic fitting of adsorption process based on intraparticle diffusion model.

centered cubic structure of Fe3O4 (JCPDS card 19-629) was well-preserved after deposition of PDA encapsulation layers, suggesting the mild nature of PDA chemistry for surface modification. Magnetic properties of Fe3O4@PDA were studied with a vibrating magnetometer. The magnetization curves in Figure S4A indicate a high saturation magnetization value of 58.4 emu/g for pristine Fe3O4. With PDA deposition, the saturation magnetization of Fe3O4@PDA shows a declining trend due to the decreased magnetic fraction in total mass. Nevertheless, fast responses were still observed when the Fe3O4@PDA MNPs were placed in a magnetic field, where a magnetic separation could be accomplished in 15 s (Figure S4B). The amount of PDA deposited onto the MNPs was determined by elemental analysis of C, H, and N elements. on the basis of mass percentage of nitrogen, it can be calculated that ∼1.22 mmol/g dopamine was deposited onto Fe3O4 within 5 h (Table S1). Regarding the N species, previous research about PDA structure showed that there mainly existed -NHspecies in the indole units, together with fewer -NH2 species in open-chain oligomers and a small part of cyclized −N species.44 Here, a colorimetric method (see details in Supporting Information) was used to determine the number of primary amines in [email protected] The results in Table S1 show that uncyclized -NH2 groups make up less than 30% of the total N species. Interestingly, the density of -NH2 in Fe3O4@PDA remained almost constant when the self-polymerization time was longer than 5 h. Accordingly, the ratio of -NH2 in total N species shows a decrease with increasing PDA thickness. Thus, it can be inferred that, the majority of -NH2 groups in dopamine monomers are engaged in cyclization to generate indole units during self-polymerization. With prolonged reaction time, the residual-free -NH2 in PDA would be continuously transformed to cyclized -NH- species.20 The structure of CMK-3 OMC was examined by TEM (Figure 1d). Ordered pore channels were maintained after the surface modification by PDA deposition (Figure 1e). The absence of large PDA aggregates by self-nucleation implies that the dopamine molecules underwent a heterogeneous nucleation at the surface of CMK-3 OMC due to π−π stacking interactions.46−48 According to our previous work,35 the amount of PDA deposited on CMK-3 OMC under the experimental conditions was about 1.86 mmol/g. The SEM images in Figure 1f shows the morphology of GFC with smooth surface for each glass fiber (GF) (average diameter ∼7 μm). After contact with dopamine in Tris buffer, large amounts

oscillator, the PDA-modified GFC was taken out, washed repeatedly with ethanol, and placed in a vacuum oven at 60 °C for 24 h before use. 2.2. Extended X-ray Absorption Fine Structure Measurements and Analysis. Samples for EXAFS measurements were placed in 2 mm thick holders with a round solid film. Uranium L3-edge EXAFS spectra were recorded in fluorescence mode by use of a 30-element Ge array solid detector on beamline 1W1B at Beijing Synchrotron Radiation Facility (BSRF), China. The ring storage energy of the synchrotron radiation accelerator during data collection was 2.5 GeV with current intensity of 60−110 mA. EXAFS data reduction was performed based on the standard operation with pre-edge background subtraction, edge normalization to unit step height, conversion to momentum space, and extraction of the EXAFS with a spline function.40 The k2-weighted EXAFS data were analyzed by use of the Athena and Artemis interfaces to the IFEFFIT program package.41

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. Surface morphology and microstructure of the PDA-modified sorbents were characterized. The TEM image (Figure 1b) shows that pristine Fe3O4 MNPs have a nearly spherical shape, with a rough surface and diameter of ∼200 nm. In addition, the inset shows that these MNPs are formed by the aggregation of many small primary clusters with a size of ∼7 nm.42 After contact with dopamine in Tris buffer solution for 5 h, a well-defined PDA encapsulation layer with thickness of ∼20 nm was deposited on the Fe3O4 MNPs (Fe3O4@PDA-5). The core−shell structured Fe3O4@PDA MNPs show smoother surfaces than the pristine Fe3O4 (Figure 1c). During the autoxidative polymerization of dopamine, ethanol was introduced as a radical-trapping agent in the Tris buffer solution to slow down the nucleation rate.43 This would promote heterogeneous nucleation of dopamine on the surfaces of the MNPs, thereby forming uniform PDA coatings rather than random-sized PDA agglomerates. By adjusting the deposition time within 24 h, a tunable PDA encapsulation layer with a linear increase of thickness from 10 to 80 nm could be achieved (Figure S1). The presence of PDA on the MNPs was evidenced by FT-IR spectra (Figure S2), where characteristic bands corresponding to the stretching vibration of aromatic rings (1602 cm−1) and the shearing vibration of -NH units (1506 cm−1) were observed.16 X-ray diffraction (XRD) patterns (Figure S3) showed that the face4608


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the substrates allow the penetration of ions and small molecules.50 Therefore, the adsorption kinetics, except for being influenced by chemisorption reactions between U(VI) and binding sites of PDA, would be also affected by the static resistance and steric hindrance when the uranyl ions diffuse into the inner layer of PDA. This is why the Fe3O4@PDA-5 MNPs, with a well-established 20 nm PDA shell, showed an evidently different adsorption process compared with PDAmodified graphene oxide21 or silica,13 on which only scattered PDA aggregates were deposited with deficient coverage of the substrates. Such an understanding would help explain the adsorption kinetics found in our study. Upon contact with U(VI), a sharp increase of the adsorption capacity was observed (stage I) due to the instantaneous adsorption or external surface adsorption.51 With occupation of binding sites on the outer surfaces of PDA layers, uranyl ions continuously diffused into the PDA layer (stage II). Intraparticle diffusion was the main ratelimiting factor of this stage. Then, the adsorption capacity of U(VI) was approaching a constant when intraparticle diffusion slowed down to an equilibrium state (stage III). This adsorption process could be described by the intraparticle diffusion kinetic model:52

of PDA aggregates with uniform coverage onto the surfaces of GFs were observed (Figure 1g). By comparing the TGA curves of GFC- and PDA-modified products (Figure S5), it could be estimated that the amount of PDA deposited on the surface of the GFC was about 1.83 mmol/g. The GFC preserved the physical appearance, with the color turned from white to black recorded by a digital camera (Figure 1h,i). 3.2. Uranium Adsorption Behavior. Due to the existence of abundant oxygen- and nitrogen-containing functionalities, PDA has potential for capturing uranyl ions from aqueous solutions. Before evaluation of the ability of PDA-modified sorbents for uranium enrichment in low-concentration solutions and in seawater, taking advantage of the PDAencapsulated MNPs mediated by facile magnetic separation, the adsorption behaviors toward U(VI) were investigated. Influence of pH value on the adsorption efficiency of U(VI) by Fe3O4@PDA-5 is shown in Figure 2a. The adsorption efficiency increased sharply with the pH value rising from 3.0 to 6.0, followed by a decline at pH values higher than 6.5. This variation is associated with the distribution of uranium species and the surface charge of PDA at different pH values. The ζpotential curve shows that surface charge of Fe3O4@PDA-5 turns negative when the pH value is higher than ∼3.7 (Figure S6). Hence, at lower pH values (6.0), negatively charged uranyl hydroxide and carbonate complexes were formed with decreased affinity for the negatively charged surfaces of Fe3O4@PDA-5, leading to declining adsorption efficiency of U(VI). It is should be pointed out that the PDA coatings deposited on the Fe3O4 MNPs were stable under the test conditions (pH 3.0−8.0). After contact with pH 3.0 HNO3 solution for 24 h, the core−shell structure of the Fe3O4@PDA MNPs was well-preserved (Figure S7). The Fe3O4@PDA MNPs prepared in this study have welldefined PDA encapsulation layers with considerable thickness. Thus, except for the binding sites, the self-assembling structure of PDA aggregates would affect the adsorption behavior toward U(VI). Figure S8 shows a nonlinear increase of U(VI) adsorption capacity with the thickness of PDA deposited onto the MNPs. This implies there might be diffusion resistance and/or steric hindrance for U(VI) to access the binding sites of PDA. In this respect, the adsorption kinetics of Fe3O4@PDA in U(VI) solution at pH 5.0 was investigated. Figure 2b shows the adsorption capacity variation of Fe3O4@ PDA-5 upon contact with U(VI). An interesting kinetics was observed, which could be roughly divided into three stages. In the first stage (I, 0−60 h), the uptake of U(VI) increased rapidly in the initial 20 h. Thereafter, the adsorption rate slowed down, but an equilibrium was not approached. The second stage (II, 60−150 h) featured a constant adsorption rate with a linear increase of U(VI) adsorption capacity. In the third stage (III, 150−288 h), a further decrease in adsorption rate was observed, and the adsorption process gradually reached equilibrium with qe ≈ 44.5 mg·g−1. Basically, the self-assembling structure of PDA aggregates consists of various oligomers linked by covalent and noncovalent interactions.18 PDA layers deposited on the surfaces of

qt = kt 1/2 + C

(1)

where k (mg·g−1·h−1/2) is the rate constant, qt (mg·g−1) is the adsorption capacity of U(VI) at time t (hours), and the intercept C (mg·g−1) reflects the thickness of the boundary layer. The inset in Figure 2b shows the plot of qt versus t1/2. Based on a linear regression, the value of the rate constant k was determined to be ∼3.4 ± 0.2 mg·g−1·h−1/2. In addition, it is worth mentioning that similar kinetics was observed when PDA-encapsulated Fe3O4 MNPs were used for the adsorption of lead ions (data not given), showing a multistage kinetic process that was basically controlled by intraparticle diffusion. 3.3. Uranium Enrichment from Seawater. Concern about uranium enrichment from seawater is of significance for resource sustainability and environmental safety. But it remains quite challenging to effectively capture trace uranium, in the main form of [UO2(CO3)3]4− anions, from seawater, where a large amount of competing metal ions and various microorganisms coexist.53−55 Here, we first evaluated the adsorption ability of PDA-modified MNP and OMC by performing a series of batch experiments in the seawater from Bohai Sea in China. Except for the natural seawater, extra uranium (50 to 2000 ppb) was added to simulate seawater contaminated by radioactive wastes. Figure 3a shows that both Fe3O4@PDA-5 and CMK-3@PDA exhibited remarkable adsorption ability. The uranium in the unpurified seawater and the simulated solutions was almost completely removed. In comparison, pristine Fe3O4 with inert surface showed a low adsorption efficiency, E (%). It is noted that CMK-3 precursors before the PDA modification, due to the presence of oxygen-containing impurities on the surfaces, could adsorb >80% of the uranium from natural seawater and the “seawater + 50 ppb” system. But the limited functional groups on the CMK-3 surfaces could not guarantee satisfactory adsorption in seawater solutions with uranium addition more than 100 ppb. Influence of the coexisting metal ions in seawater was examined. Table 1 shows the concentrations of the relevant metal ions in seawater used in this study.56 After contact with the PDA-modified MNPs, the adsorption efficiency of each 4609


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functionalized sorbents was vanadium.53 But here, no evident uptake of vanadium was found by the PDA-modified MNPs, indicating there was no coordination interaction between vanadium and PDA. It is interesting to note that PDA-modified MNPs showed obviously enhanced adsorption efficiency for molybdenum (E ≈ 72.4%), as compared to pristine Fe3O4 MNPs (E ≈ 4.6%). Further work is being scheduled to investigate the underlying interaction between molybdenum and PDA.∼ Employing PDA-modified GFC for uranium separation by filtration represents a more practical application scenario. In this respect, we comparatively studied the adsorption ability of GFC and the PDA-modified product toward uranium in unpurified seawater and laboratory-prepared low-concentration (ppb) solutions. The experimental setup for filtration separation is illustrated in Supporting Information. Untreated GFC showed moderate adsorption of uranium, especially in the natural seawater (E ≈ 53.1%) and the seawater + 50 ppb system (E ≈ 67.7%) (Figure 3b, columns). After surface modification, GFC@PDA showed enhanced adsorption efficiency in each of the test solutions. However, the increments were not as significant as those obtained when Fe3O4@PDA-5 and CMK-3@PDA were used in batch experiments. This could be attributed to the slow kinetics discussed above, leading to unsaturated adsorption of uranium during dynamic filtration of the seawater. Similar results were obtained when GFC and GFC@PDA were used for filtration separation of uranium in laboratory-prepared solutions with concentrations varying from 50 to 2000 ppb (Figure 3b, curves). However, because of the absence of competing metal ions and various microorganisms, both GFC and GFC@PDA exhibited evidently increased adsorption efficiency over that obtained in unpurified seawater systems. Overall, the above results demonstrated that PDA-modified sorbents were effective for uranium adsorption from lowconcentration (ppb) solutions and unpurified seawater. From a practical point of view, the PDA-modified sorbents, due to the slow kinetics toward U(VI), should be more appropriately applied by the manner of batch adsorption rather than some other dynamic processes, such as filtration and column chromatography. Besides, the uranium adsorbed by PDAmodified MNPs could be effectively stripped by use of 5% (NH4)2CO3 aqueous solution, compared to the other elution solutions listed in Table S2. This resulted in regeneration of the sorbents without compromising the structure of the PDA coatings (Figure S9). The regenerated Fe3O4@PDA MNPs were also demonstrated to have the potential for cycle use (Figure S10). 3.4. Coordination Mechanism between Polydopamine and Uranium(VI). 3.4.1. X-ray Photoelectron Spectroscopic Analysis. Before discussion of the coordination mechanism between PDA and U(VI), a basic understanding about the structure of PDA should be established. So far, the mechanism regarding the self-polymerization of dopamine and PDA’s exact structure have not been fully elucidated. Previous studies suggested that PDA existed as an oligomer mixture containing indole units formed through oxidative cyclization with different degrees of saturation, together with some openchain dopamine units.16,18 Figure S11 shows some building blocks proposed in the literature, which were stacked by covalent polymerization and noncovalent self-assembly to form PDA aggregates.18,19 The presence of abundant nitrogen- and oxygen-containing functionalities make PDA a favorable

Figure 3. (a) Removal of uranium by PDA-modified Fe3O4 MNPs and CMK-3 OMCs from unpurified seawater in batch adsorption. (b) Removal of uranium by filtration using GFC@PDA and GFC from both laboratory-prepared solutions and unpurified seawater. Seawater was obtained from the Bohai Sea in China. Contact time = 168 h, T = 298 K, m/v = 1.5 g/L.

Table 1. Concentrations of Metal Ions in Natural Seawatera and Adsorption Efficiency of Each Metal Ionb adsorption efficiency E (%) element K Na Mg Sr Ca Ni V Zn Mo Al U

initial concn (ppb) 3.5 × 1.1 × 8.5 × 5.8 × 3.6 × 8.4 1.8 3.5 7.8 6.9 2.8

5

10 107 105 103 105

Fe3O4

Fe3O4@PDA-5

2.0 6.9 2.1 0.8 2.9 6.7 1.6 23.8 4.6 8.7 21.3

2.9 1.8 2.2 1.7 4.1 8.1 3.3 37.9 72.4 4.5 99.8

a

From Bohai Sea in China. bBy use of Fe3O4@PDA-5 and pristine Fe3O4 MNPs. Contact time = 168 h, T = 298 K, m/v = 1.5 g/L.

metal ion was measured. The pristine Fe3O4 MNPs were used for blank controls. The results suggested that the PDAmodified MNPs exhibited the highest affinity for uranium in the seawater. Alkali and alkaline earth metal ions were not adsorbed by the PDA-modified MNPs despite their high concentration. According to a previous study, the main interference to uranium enrichment from seawater by use of amidoxime4610


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Environmental Science & Technology platform for the capture of uranyl ions.13,21 In this work, using the PDA-modified MNPs as a model, spectroscopic studies based on XPS and EXAFS were performed, aiming to provide more insights into PDA’s structure and the coordination interactions to U(VI). First, overall XPS surveys of Fe3O4@PDA-5 and the counterpart after binding to U(VI), labeled as Fe3O4@PDA5-U, were obtained (Figure S12a). The typical N 1s signal at 399.8 eV corresponds to nitrogen species in the PDA layers on the magnetic cores. For Fe3O4@PDA-5-U, adsorbed U(VI) is evidenced by emergence of the U 4f signal, which shows two asymmetrical peaks assigned to U 4f5/2 (392.5 eV) and U 4f7/2 (382.0 eV) with a spin−orbit splitting of ∼10.5 eV (Figure S12b).57 Besides, the characteristic U 4f7/2 peak shows a binding energy of 382.0 eV, suggesting that the valence state of U(VI) remains unchanged after binding to PDA.58 Deconvolution analysis of high-resolution XPS spectra affords an effective way to distinguish the specific chemical species or functional groups on solid surfaces, while revealing their interactions with metal ions.59,60 In this respect, deconvolution of the C 1s spectrum of Fe3O4@PDA-5 resulted in four components: C−C/CC (284.6 eV), C−N/CN (285.5 eV), C−O (286.3 eV), and CO (287.8 eV) (Figure 4a).61 After U(VI) adsorption, the C−O peak (286.5 eV) and

To further identify the binding sites of PDA, deconvolution analysis of the N 1s and O 1s spectra was implemented. In the N 1s spectrum of Fe3O4@PDA-5 (Figure 4c), the peaks of −N (399.1 eV) and -NH- (400.0 eV) correspond to the cyclized nitrogen in the indole units.62 Meanwhile, the -NH2 component was obtained at 400.7 eV, confirming the existence of open-chain oligomers in PDA aggregates. After binding to U(VI), the N 1s peak of the −N species remained unchanged, whereas the peaks of -NH- and -NH2 showed a slight shift to higher binding energy (Figure 4d), which was consistent with the change of the C−N/CN species found by deconvolution analysis of C 1s spectra. Such a tiny shift of binding energy implied that the -NH- and -NH2 species might participate, but not principally, in the complexation to U(VI) with a relatively weak binding energy. In addition, it is noted that, after binding to U(VI), no N 1s peak at 406.0 eV or Cl 2p peak at 200.2 eV was observed, indicative of the absence of nitrate ions and perchlorate ions in the PDA−U(VI) complex structure as counterions.63 Likewise, deconvolution analysis of the O 1s spectrum of Fe3O4@PDA-5 gave three component signals assigned to Fe− O (530.8 eV), CO (530.8 eV), and C−O (533.2 eV) species (Figure 4e).64 After binding to U(VI), only the C−O signal evidently shifted to higher binding energy of ∼533.5 eV. Meanwhile, the peak area of C−O species in O 1s spectra was intensively decreased (Figure 4f). This suggests that it was the O atoms in catechol groups, rather than the carbonyl O atoms in dopamine-quinone, that played a main role in PDA’s complexation to U(VI) by sharing electrons for the formation of U−O bonds. 3.4.2. Extended X-ray Absorption Fine Structure Study. To determine the structure of PDA−U(VI) complex, EXAFS was utilized to examine the changes in coordination environment of the uranium inner shell after binding to Fe3O4@PDA-5. Figure 5 shows the k3-weighted L3-edge EXAFS spectra of the U(VI)-

Figure 5. (a) L3-edge EXAFS spectra of U(VI)-adsorbed Fe3O4@PDA and (b) corresponding Fourier transformations. Figure 4. Deconvolution analysis of (a) C 1s, (c) N 1s, and (e) O 1s XPS spectra of Fe3O4@PDA-5 and (b) C 1s, (d) N 1s, and (f) O 1s XPS spectra of Fe3O4@PDA-5-U.

adsorbed Fe3O4@PDA and the corresponding Fourier transformations (FT). The EXAFS spectrum of U(VI) solution (pH = 5.0, 100 ppm) is also provided as a reference (Figure S13). It can be seen that Fe3O4@PDA-5-U and Fe3O4@PDA-8-U exhibit almost the same oscillation modes and FT peaks in the range 0−6 Å, suggesting that the binding of U(VI) to PDA occurred in the same manner regardless of the thickness of PDA layers. Besides, due to the relatively low amounts of U(VI) in total mass, all the spectra show a poor signal-to-noise ratio when k > 7 Å−1.

C−N/CN peak (285.7 eV) were shifted to relatively higher binding energy region (Figure 4b). No obvious change was found for the peaks of C−C/CC and CO. This implies that the C−O species in the catechol groups and the C−N/ CN species in the indole building blocks might be involved in the complexation to U(VI). 4611



■

The EXAFS data were fitted to the single scattering theoretical phase and amplitude functions (Figure 5b). The corresponding parameters are summarized in Table 2. For both

Fe3O4@PDA-5 Fe3O4@PDA-5 Fe3O4@PDA-8 Fe3O4@PDA-8

shell

CNa

Rb(Å)

σ2c(Å2)

E0d(eV)

Rfactore

U−Oax U−Oeq U−Oax U−Oeq

f

1.79 2.39 1.79 2.39

0.003 0.009 0.003 0.009

−1.08 −1.08 −0.71 −0.71

0.0173

2 4.7 2f 4.6

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b00470. Additional text with experimental details, setup for filtration separation of uranium, and reusability evaluation; 13 figures showing SEM images, FT-IR spectra, XRD patterns, magnetization and TGA curves, ζpotential measurements, TEM images, influence of PDA thickness on uranium adsorption, building blocks of PDA aggregates, and XPS and EXAFS spectra; two tables listing elemental analysis and determination of free amines on Fe3O4@PDA and elution of adsorbed U(VI) (PDF)

Table 2. EXAFS Parameters of U(VI)-Adsorbed Fe3O4@ PDA-5 at k3-Weighted L3 Edge sample

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0.0139

Coordination number CN ± ∼20%. bBond distance R ± ∼0.03 Å. Debye−Waller factor. dEnergy shift linked for all paths. eGoodnessof-fit parameter. fFixed parameter. a c

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samples, evident differences were observed in the FT spectra as compared to that of the initial U(VI) solution, implying a change in coordination environment after binding to PDA. The axial U−O distance of ∼1.79 Å was the typical structure of uranyl (OUO), and the U−Oax σ2 value (0.003 Å2) was in accordance with the reported results.65 Only a single equatorial U−Oeq shell with a distance of 2.39 ± 0.03 Å was observed. The average number of O atoms equatorially coordinated to uranium was approximately 4.7 ± 0.9, suggesting that U(VI) should be surrounded by water molecules and/or hydroxyl groups. This coordination manner in the inner shell of uranium is similar to the binding of U(VI) to graphene oxide (GO), carboxylated GO, and reduced GO.2,66,67 In addition, the absence of U−Neq shell in the spectra suggests that the free amines in the PDA layers have negligible contribution to the coordination with U(VI) at equatorial plane, and neither do the cyclized N atoms in the indole rings, due to steric hindrance. This is consistent with the preeding XPS analysis. Moreover, when the XPS evidence that no nitrate ions and perchlorate ions as counterions were adsorbed onto Fe3O4@PDA-5 is considered, to keep charge balance, two hydroxyl groups would act as both binding sites and counterions for the coordination with uranium, while another three O atoms involved in the coordination were provided by the catechol groups in PDA and water molecules. 3.4.3. Environmental Implications. Developing viable methods for uranium extraction from seawater is of strategic value for the sustainability of nuclear energy worldwide. Meanwhile, effective enrichment of uranium from other systems, such as groundwater and radioactive wastes, is also an environmental concern. This work presents a new strategy to prepare diverse types of uranium sorbents by taking advantage of the bioinspired PDA chemistry. Due to abundant catechol and amine/imine groups, the PDA functional layers provide an excellent platform for uranium binding. The findings in this work highlight the adsorption behavior of PDA-modified sorbents and the performance for enrichment of low concentration uranium in natural seawater. Moreover, insights into the interaction mechanism between PDA and uranium are obtained from spectroscopic studies. To conclude, this work presents a comprehensive assessment of PDA chemistry for developing effective uranium sorbents, showing promise for uranium enrichment from natural seawater. It is envisioned that the facile and non-surface-specific PDA chemistry would provide a platform technique for surface modification of either nano- or bulk-sized materials for more environmental applications.

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (G.Y.). *E-mail [email protected] (J.C.). ORCID

Gang Ye: 0000-0002-7066-940X Taoxiang Sun: 0000-0003-4690-3566 Yuexiang Lu: 0000-0003-2755-7733 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Changjiang Scholars and Innovative Research Team in University (IRT13026), the National Science Fund for Distinguished Young Scholars (51425403), and National Natural Science Foundation of China under Projects 51673109 and 51473087.

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Performance and Mechanism of Uranium Adsorption from Seawater to

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