Retracted Article: Reductive immobilization of uranium by PAAM–FeS/Fe₃O₄ magnetic composites

Abstract

Ternary magnetic composites of poly(acrylamide)-stabilized FeS/Fe₃O₄ (denoted as PAAM–FeS/Fe₃O₄) were in situ synthesized by chemical coprecipitation of FeS/Fe₃O₄ in the presence of acrylamide (AAM), followed by dielectric barrier discharge (DBD) plasma-induced polymerization of AAM on the FeS/Fe₃O₄ surface. The modified PAAM improved greatly the stabilization and dispersion properties of PAAM–FeS/Fe₃O₄ in aqueous solutions. This as-prepared composite was applied as an adsorbent for the preconcentration of U(VI) cations from aqueous solutions. The maximum enrichment capacity of U(VI) on this adsorbent at pH 5.0 and 20 °C was calculated to be 311 mg g⁻¹. The presence of other cations had no obvious influence on the sorption of U(VI) on PAAM–FeS/Fe₃O₄, suggesting that this absorbent can remove U(VI) with high selectivity. According to the results of XPS spectra analysis, both FeS and amide groups on PAAM–FeS/Fe₃O₄ surfaces have strong chemical affinities to U(VI), and FeS can also reduce U(VI) to U(IV) effectively. The coagulation of magnetic U-laden PAAM–FeS/Fe₃O₄ can be easily separated and recovered from aqueous solution by simple magnet separation. This paper highlights the potential application of PAAM–FeS/Fe₃O₄ composites as suitable materials for the preconcentration and extraction of trace U(VI) ions from aqueous solutions in environmental radioactive pollution control and peaceful utilization of nuclear energy, such as the extraction of trace U(VI) ions from seawater.

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Water impact

In this work, PAAM–FeS/Fe₃O₄ composites were in situ synthesized by the chemical coprecipitation–dielectric barrier discharge plasma technique and used as adsorbents for the preconcentration of U(VI) from aqueous solutions. PAAM improved greatly the stabilization properties and adsorption capability of PAAM–FeS/Fe₃O₄ in aqueous solutions. The coagulation of PAAM–FeS/Fe₃O₄ after adsorption of U(VI) can be easily separated and recovered from aqueous solution by simple magnet separation. The results show that PAAM–FeS/Fe₃O₄ is a suitable material for the preconcentration and extraction of trace U(VI) ions from aqueous solutions in environmental radioactive pollution control and peaceful utilization of nuclear energy.

Introduction

With the fast development of nuclear techniques, large quantities of radioactive contaminants were released into the environment under certain circumstances during the past decades,^1,2 which resulted in serious environmental radioactive pollution. Uranium, one fundamental material of nuclear energy, is one common and important radioactive contaminant in soils, sediments, and groundwater throughout the world.³ The immobilization and enrichment of uranium from radioactive wastewater is an important issue in the environmental friendly utilization of nuclear energy.

The environmental risk and diffusion of uranium is strongly affected by its redox state.^1,4–6 The reduction of soluble U(VI) to insoluble U(IV) solid phases is widely considered as an effective method for the in situ immobilization/stabilization of uranium contaminants.^6,7 The reductive immobilization and enrichment of U(VI) with a variety of minerals and synthesized materials, such as sulfate-reducing bacteria,^3,7 FeS,^1,4,5,8 and FeS₂,^2,6,9 have been extensively studied. Among them, FeS can effectively reduce U(VI) to U(IV) and inhibit U(IV) re-oxidation under varied environmental conditions and is recognized as an important material for uranium contamination remediation. However, the easy aggregation of FeS particles in aqueous solution greatly restricts their application.^10–12 It is desirable to modify the surface of FeS to enhance its stabilization and dispersion properties in aqueous solutions. Surface modification of FeS/Fe₃O₄ by hydrophilic materials, such as itaconic acid,¹⁰ agarose,¹¹ carboxymethyl cellulose (CMC),^12,13etc., is a feasible approach to enhance its stabilization and dispersion properties in aqueous solutions. Due to its excellent hydrophilic properties and complexation ability, poly(acrylamide) (PAAM) has been widely introduced to modify various materials for the enrichment and separation of radionuclides (such as U(VI) and Th(IV)) and metal ions (such as Pb(II), Cu(II), Cd(II), and Hg(II)) from aqueous solutions.^14–16 The modified PAAM could provide more available sites for the selective extraction of U(VI) from aqueous solutions.

The effective separation technique for U-laden FeS-based composites from aqueous solution is also critical for the application of FeS in uranium contamination cleanup. Magnetic materials, such as nano- or micro-sized Fe₃O₄, have attracted great interest in various fields because they can be easily separated from aqueous solution by utilizing a simple magnetic field.¹⁷ Based on the reasons mentioned above, one can see that PAAM–FeS/Fe₃O₄ composites would be attractive materials in uranium pollution management because of the excellent enrichment capacity of PAAM, the reduction of U(VI) to U(IV) by FeS, and the magnetic properties of Fe₃O₄ for magnetic separation on a large scale.

In this paper, the magnetic composites of PAAM-stabilized FeS/Fe₃O₄ (denoted as PAAM–FeS/Fe₃O₄) were in situ synthesized by chemical coprecipitation of FeS/Fe₃O₄ in the presence of AAM, followed by DBD plasma-induced polymerization of AAM on the FeS/Fe₃O₄ surface. The as-synthesized PAAM–FeS/Fe₃O₄ was applied as an adsorbent to selectively enrich and separate U(VI) from aqueous solutions under a variety of operation conditions. The results showed that PAAM–FeS/Fe₃O₄ presented exceptional performance in the selective enrichment and separation of trace U(VI) ions from aqueous solutions, and the surface-adsorbed U(VI) was also partly reduced to insoluble U(IV). This paper highlighted the applicability of PAAM–FeS/Fe₃O₄ composites for the enrichment and reduction of U(VI) ions from aqueous solutions in uranium contamination cleanup.

Experimental

Synthesis of PAAM–FeS/Fe₃O₄ composites

PAAM–FeS/Fe₃O₄ composites were in situ synthesized by chemical coprecipitation of FeS/Fe₃O₄ in the presence of AAM, followed by DBD plasma-induced polymerization of AAM on the FeS/Fe₃O₄ surface. NaOH and Na₂S were used as precipitants for Fe₃O₄ and FeS, respectively. Briefly, under argon and continuous mechanical stirring conditions, the solution containing NaOH (20.0 g L⁻¹) and Na₂S (0–12.5 g L⁻¹) was added dropwise into a suspension containing FeCl₂·4H₂O (1.7 g L⁻¹), FeCl₃·6H₂O (4.6 g L⁻¹), and AAM (0–2.0 g L⁻¹) to achieve pH ~7.0. After aging for 1 h, the argon DBD plasma technique was used to induce the polymerization of AAM on the FeS/Fe₃O₄ surface. The DBD plasma discharging power, time, and argon flow rate were 50 W, 1 h, and 10 mL min⁻¹, respectively. The product was centrifuged and rinsed repeatedly with degassed Milli-Q water and dried at 50 °C for 24 h in a vacuum drier to obtain PAAM–FeS/Fe₃O₄ composites. In order to compare the sorption capacities of PAAM–FeS/Fe₃O₄ composites, Fe₃O₄ and PAAM/Fe₃O₄ were synthesized by the same method.

Characterization

The physiochemical properties of PAAM–FeS/Fe₃O₄ were measured and evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) – element distribution mapping, a vibrating sample magnetometer (VSM), Fourier transform infrared (FT-IR) spectroscopy, UV-vis spectroscopy, powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) in detail. SEM images were obtained with a JSM 6320F FE-SEM (JEOL). TEM image and element distribution mapping investigations were carried out using a JEOL JEM 2010 microscope. VSM curves were obtained using a Model 155 VSM at room temperature in the measurement range of 0 to ±25 kOe. FT-IR spectroscopy measurements were performed using a Perkin-Elmer 100 spectrometer in KBr pellets under environmental conditions. UV-vis spectroscopy analysis was performed using a Shimadzu 2550 UV-vis spectrophotometer. The XRD pattern of PAAM–FeS/Fe₃O₄ was obtained using a Rigaku D/max 2550 X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) at room temperature. XPS spectroscopy measurements were performed with an ESCALab220i XL surface microanalysis system (VG Scientific) equipped with an Al Kα (hν = 1486.6 eV) source at a chamber pressure of 3 × 10⁻⁹ mbar. The surface charging effects were corrected with the C 1s peak at 284.4 eV as a reference.

Batch enrichment

The enrichment properties of radionuclides (U(VI), 90-Sr(II), and 137-Cs(I)) and metal ions (Pb(II), Cu(II), and Ni(II)) on Fe₃O₄, PAAM/Fe₃O₄, and PAAM–FeS/Fe₃O₄ were studied at 20 ± 1 °C by using the batch technique. The bulk suspensions of adsorbent and NaCl were pre-equilibrated for 24 h, then the solution containing radionuclides or metal ions was added, and the pH of the suspensions was adjusted to the desired values. The experiments were carried out under argon protection by shaking for 48 h to achieve sorption equilibration. The solid and liquid phases were separated by centrifugation at 18 000 rpm for 30 min (Beckman Coulter 64R). The concentration of U(VI) in the supernatant was analyzed by the arsenazo III spectrophotometric method at 650 nm. The concentrations of 90-Sr(II) and 137-Cs(I) were analyzed by liquid scintillation counting (Packard 3100 TR/AB liquid scintillation analyzer, Perkin-Elmer) with the scintillation cocktail (Ultima Gold AB™, Packard). The concentrations of Pb(II), Cu(II), and Ni(II) were determined by atomic absorption spectrometry. All the experimental data were the average of triplicate determinations, and the data relative errors were <5%.

Recovery of uranium

The recovery of uranium from U-laden PAAM–FeS/Fe₃O₄ was investigated by dispersing 20 mg of U-laden PAAM–FeS/Fe₃O₄ in a beaker containing 50 mL of HNO₃ solution with different HNO₃ concentrations. The reduced U(IV) can be re-oxidized to U(VI) effectively under oxidizing conditions,^3,10 which would dissolve in acidic solutions. After being exposed to air and mechanical stirring for 3 h, the suspension was centrifuged and analyzed under the same conditions as in the enrichment experiments. The residual materials in the 0.10 mol L⁻¹ HNO₃ solution were washed repeatedly with Milli-Q water, dried at 50 °C for 24 h under argon protection, and denoted as recovered PAAM–FeS/Fe₃O₄ for XPS and FT-IR analysis.

Results and discussion

PAAM–FeS/Fe₃O₄ characterization

SEM, TEM and element distribution mapping were employed to investigate the morphology, microstructure and element distribution of PAAM–FeS/Fe₃O₄. As depicted in Fig. 1A and B, PAAM–FeS/Fe₃O₄ nanoparticles are highly dispersed and spherical-like in shape with a diameter of 10–20 nm, which is much smaller than that of bare and CMC-stabilized FeS particles (38.5 ± 5.4 nm) measured by TEM.^12,13 It reveals that AAM can effectively prevent the aggregation of FeS/Fe₃O₄ particles in aqueous solution. Compared with aggregated bare FeS, stabilized FeS particles present higher specific surface area, which is due to their sorption capacity and affinity toward contaminants.

Fig. 1 Characterization of PAAM–FeS/Fe₃O₄. SEM image (A), TEM image and the related element distribution mapping (B), XPS survey spectrum (C), and XRD pattern (D) of PAAM–FeS/Fe₃O₄.

The Fe, O, N, and S element mapping (Fig. 1B) demonstrates that nitrogen and sulfur are uniformly dispersed. This indicates the homogeneous distribution of PAAM and FeS on the surfaces of PAAM–FeS/Fe₃O₄.

The successful formation of PAAM–FeS/Fe₃O₄ was also confirmed by XPS analysis. Fig. 1C shows the XPS survey spectrum of PAAM–FeS/Fe₃O₄. The peaks at ~168, ~285 and ~400 eV are related to S 2p, C 1s, and N 1s, which confirm the successful introduction of FeS and PAAM onto Fe₃O₄ surfaces.

Fig. 1D presents the XRD patterns of Fe₃O₄, PAAM/Fe₃O₄, and PAAM–FeS/Fe₃O₄. The XRD patterns of Fe₃O₄ and PAAM/Fe₃O₄ were similar to each other, and the characteristic XRD peaks at 2θ values of 30.3°, 35.6°, 37.3°, 43.3°, 53.7°, 57.2°, and 62.8° can be assigned to the (220), (311), (222), (400), (422), (511) and (440) planes of Fe₃O₄, respectively (JCPDS card no. 19-0629). Due to the fact that FeS shows the characteristic XRD peaks at similar 2θ values of 30.0°, 33.7°, 43.2°, 53.2°, and 56.9°,^18,19 the XRD pattern of PAAM–FeS/Fe₃O₄ presents a new characteristic XRD peak at the 2θ value of 33.7° and has a much broader peak width at other positions. The average crystallite size of PAAM–FeS/Fe₃O₄ was calculated to be ~15.8 nm by using the Debye–Scherrer formula (D = 0.9λ/(β cos θ), where D is the crystallite size, λ is the wavelength of X-ray, β is the value of full width at half maximum and θ is the Bragg angle).

The stabilization and dispersion of Fe₃O₄, FeS/Fe₃O₄, and PAAM–FeS/Fe₃O₄ in aqueous solution is straightly demonstrated in Fig. 2A. After allowing to stand for 4 min, most Fe₃O₄ and FeS/Fe₃O₄ particles aggregated and precipitated at the bottom of the bottle. Moreover, the visible color change in the FeS/Fe₃O₄ suspension from black to black yellow indicated that part of FeS/Fe₃O₄ could be oxidized by dissolved oxygen in solution.^20,21 On the other hand, due to the abundant hydrophilic amide groups in the molecular chains of PAAM, PAAM–FeS/Fe₃O₄ composites dispersed well in aqueous solution and no precipitate was formed even after allowing to stand for 6 months, which was further confirmed by UV-vis analysis (Fig. 2B). Meanwhile, the color of PAAM–FeS/Fe₃O₄ in aqueous solution was kept as gray black during the measurements, which demonstrated the excellent physical and chemical stability of PAAM–FeS/Fe₃O₄ in aqueous solution. Black yellow coagulation of U-laden PAAM–FeS/Fe₃O₄ could be observed when U(VI) solutions were added (Fig. 2A), which can be explained by the deposition of insoluble U(IV) on the oxidized PAAM–FeS/Fe₃O₄ surface. The coagulation and precipitation of U-laden FeS in aqueous solutions was consistent with the previous reports.^1,5,6,22 Bi et al.¹ found loose and porous U(IV) precipitates on FeS surfaces by using TEM images. The coagulation can be easily separated from the supernatant, which results in the separation and recovery of U(VI) from aqueous solutions by PAAM–FeS/Fe₃O₄ composites.

Fig. 2 Photographs of dispersion (A) after allowing to stand for 20 minutes and UV-vis absorbance at 380 nm as a function of standing time (B) of Fe₃O₄, FeS/Fe₃O₄, raw and U-laden PAAM–FeS/Fe₃O₄.

The magnetization properties of PAAM–FeS/Fe₃O₄ were also studied (Fig. SI-1†). The saturation magnetization of PAAM–FeS/Fe₃O₄ was detected to be 41.9 emu g⁻¹ at room temperature and at a magnetic field of ±25 kOe. As observed from Fig. SI-1,† the PAAM–FeS/Fe₃O₄ composites could be attracted quickly by using a simple permanent magnet, and the clear solution can be easily removed by using a pipet or decantation. This result proved the excellent magnetic properties of PAAM–FeS/Fe₃O₄ composites, which can be used as magnetic adsorbents to enrich and preconcentrate U(VI) ions from aqueous solutions, followed by easy separation from aqueous solutions.

Enrichment capability of PAAM–FeS/Fe₃O₄ composites

The enrichment capability of PAAM–FeS/Fe₃O₄ is critical for its potential application. The effect of AAM and Na₂S concentration used in the experiments on the enrichment capability of PAAM–FeS/Fe₃O₄ for U(VI) was investigated. Firstly, we studied the effect of AAM concentration on PAAM/Fe₃O₄ enrichment capability for U(VI). As illustrated in Fig. 3A and C, the enrichment of U(VI) on PAAM/Fe₃O₄ was much higher than that of U(VI) on Fe₃O₄. The PAAM/Fe₃O₄ composites prepared with 1.0 g L⁻¹ AAM exhibited the highest enrichment ability for aqueous U(VI) under the experimental conditions. Therefore, 1.0 g L⁻¹ AAM was selected to prepare the PAAM–FeS/Fe₃O₄ composites in this study. The effect of Na₂S concentration on the sorption capability of PAAM–FeS/Fe₃O₄ for U(VI) was also investigated with the highest enrichment capability at 10.0 g L⁻¹ Na₂S (Fig. 3B and D). Together, 1.0 g L⁻¹ AAM and 10.0 g L⁻¹ Na₂S solutions were used to prepare PAAM–FeS/Fe₃O₄ in this study.

Fig. 3 Effect of AAM (A, C) and Na₂S (B, D) concentrations used in the experiment on the enrichment capability of PAAM–FeS/Fe₃O₄ for U(VI), pH-dependent enrichment of U(VI) on PAAM–FeS/Fe₃O₄ and the distribution of 50.0 mg L⁻¹ U(VI) species in aqueous solution at different pH values (E), and the effect of contact time on U(VI) enrichment on PAAM–FeS/Fe₃O₄ (F). T = 20 ± 1 °C, m/V = 0.10 g L⁻¹, C[NaCl] = 0.10 mol L⁻¹. Fig. 3A–D: contact time: 48 h, pH = 5.0 ±0.1. Fig. 3E: contact time: 48 h, C[U(VI)]_(initial) = 50.0 mg L⁻¹. Fig. 3F: C[U(VI)]_(initial) = 50.0 mg L⁻¹, pH = 5.0 ±0.1.

The experimental data were fitted with the classical Langmuir model (C_s = b × C_smax× C_e/(1 + b × C_e), C_smax is the maximum enrichment capacity, b is the Langmuir constant) and the Freundlich model (C_s = K × C_e^1/n, K and 1/n are the constants indicative of enrichment capacity and enrichment intensity, respectively), which are the two most popular models applied in the reductive enrichment of multi-valent metal ions and radionuclides, such as U(VI),⁶ Hg(II),¹² Se(IV) and Se(VI).²³ Herein, the correlation coefficients (R²) (Table 1) indicated that the experimental data could be better described by the Langmuir model than by the Freundlich model. The applicability of the Langmuir model suggested that PAAM/Fe₃O₄ and PAAM–FeS/Fe₃O₄ surfaces were uniform and homogeneous for U(VI) sorption. The maximum enrichment capacity of U(VI) on PAAM–FeS/Fe₃O₄ at pH 5.0 and 20 °C was calculated to be 311 mg g⁻¹ by the Langmuir model, which was much higher than that of U(VI) on Fe₃O₄ (90 mg g⁻¹) and on PAAM/Fe₃O₄ (121 mg g⁻¹) and comparable to today's nanomaterials under similar experimental conditions (see Table SI-1†). This observation highlights the potential application of PAAM–FeS/Fe₃O₄ as a suitable material for the high sorption of U(VI) from aqueous solutions in radioactive pollution cleanup.

Table 1 Parameters calculated from Langmuir and Freundlich models for U(VI) sorption on Fe₃O₄, PAAM/Fe₃O₄, and PAAM–FeS/Fe₃O₄ at pH = 5.0 ± 0.1 and T = 20 ± 1 °C

AAM (g L^−1)	Na2S (g L^−1)	Langmuir model			Freundlich model
		C smax (mg g^−1)	b (L mg^−1)	R ^2	K (mg g^−1)	1/n	R ^2
Fe3O4
—	—	89.5	0.152	0.995	27.7	0.272	0.919
PAAM/Fe3O4
0.25	—	100	0.184	0.998	35.0	0.245	0.905
0.50		113	0.162	0.995	36.2	0.265	0.910
1.0		121	0.157	0.996	37.1	0.279	0.913
2.0		112	0.118	0.996	29.4	0.302	0.923
PAAM–FeS/Fe3O4
1.0	2.5	231	0.0358	0.993	22.9	0.469	0.955
	5.0	249	0.0461	0.996	31.5	0.431	0.965
	7.5	290	0.0778	0.995	55.5	0.361	0.950
	10.0	311	0.0251	0.997	83.0	0.301	0.927
	12.5	251	0.0576	0.996	39.7	0.391	0.973

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Solution pH is an important factor that affects U(VI) sorption. The effect of pH on U(VI) sorption on PAAM–FeS/Fe₃O₄ was investigated by the batch technique in 0.10 mol L⁻¹ NaCl solutions. As illustrated in Fig. 3E, the sorption of U(VI) on the PAAM–FeS/Fe₃O₄ surface increased with increasing solution pH. At pH ~7.0, PAAM–FeS/Fe₃O₄ can effectively remove ~95% U(VI) from aqueous solutions under experimental conditions. It is widely accepted that the protonation–deprotonation reactions of functional groups on the adsorbent surface and the aqueous U(VI) species are fairly dependent on solution pH.^14,24 The relative distribution of U(VI) species (50.0 mg L⁻¹) in aqueous solution was calculated from the hydrolysis constants of U(VI)²⁴ and is also shown in Fig. 3E. The distribution of U(VI) species in aqueous solution was dependent on pH values, and the dominating U(VI) species were positively charged under the experimental conditions. U(VI) mainly existed as free uranyl ions (UO₂²⁺) at pH <4.5 and mainly existed as U(VI) hydrolysis complexes (such as UO₂OH⁺) and U(VI) multinuclear hydroxide complexes (such as (UO₂)₂(OH)₂²⁺, (UO₂)₃(OH)₅⁺, and (UO₂)₄(OH)₇⁺) at pH >4.5 in aqueous solutions. Moreover, the enrichment capacity of FeS was also affected by solution pH because the hydroxyl and bisulfide functional groups on FeS surfaces would undergo different protonation–deprotonation reactions as solution pH changes.²⁵ Similar phenomena were confirmed by other studies, such as those on U(VI),^5,9,26 Se(IV) and Se(VI),²³ and Hg(II),¹² and organic contaminants.²⁵

The effect of contact time on the removal of U(VI) ions from aqueous solutions on PAAM–FeS/Fe₃O₄ was studied to evaluate the required operation time of PAAM–FeS/Fe₃O₄ in real possible applications. As can be seen from Fig. 3F, the sorption of U(VI) on PAAM–FeS/Fe₃O₄ increased quickly within 1 h and then maintained the high level with further increasing contact time. Hua and Deng⁵ also found that the uptake of U(VI) from aqueous solution by amorphous FeS was quick and the sorption equilibrium was achieved within 1 h. The experimental data were analyzed by using pseudo-first-order kinetic models (ln(q_e − q_t) = ln q_e − k₁t, where q_e and q_t are the maximum and the experimental enrichment capacity, respectively; K₁ (h⁻¹) is the rate constant of enrichment) and pseudo-second-order kinetic models (t/C_s = 1/(2K′·C^′_s) + t/C^′_s, where C^′_s (mg g⁻¹) is the enrichment capacity; K′ (g mg⁻¹ h⁻¹) is the rate constant of enrichment). The parameters for pseudo-first-order were calculated to be q_e = 224.7 mg g⁻¹, K₁ = 9.07 h⁻¹, and R² = 0.725, and the parameters for pseudo-second-order were calculated to be C^′_s = 225.1 mg g⁻¹, K′ = 0.199 g mg⁻¹ h⁻¹, and R² = 0.999. The pseudo-second-order kinetic model fitted the experimental data better than the pseudo-first-order model. In the reaction of U(VI) with iron sulfide materials, the enrichment behavior was widely considered as a complex process of surface sorption of U(VI), partial reduction of U(VI) to U(IV), and precipitation of U(IV)/U(VI).^6,9 The principle mechanism of U(VI) sorption on PAAM–FeS/Fe₃O₄ was further evidenced by XPS characterization.

Under the effective immobilization efficiency uncertainties, the lesser the amount of PAAM–FeS/Fe₃O₄ used, the lower the cost. Thus, the effect of PAAM–FeS/Fe₃O₄ content on the enrichment of U(VI) was studied and the results are shown in Fig. SI-2.† It is worth noting that a very small amount of PAAM–FeS/Fe₃O₄ (such as 0.10 g L⁻¹) was needed to extract U(VI) from aqueous solutions, and the distribution coefficient (K_d) values reached the 10⁴ mL g⁻¹ level. Thus, PAAM–FeS/Fe₃O₄ presents high enrichment capacity in the immobilization and separation of U(VI) from large volumes of aqueous solutions.

Selectivity is a critical factor that affects the extraction of U(VI) from aqueous solutions due to the possible competitive sorption of other radionuclides. Herein 137-Cs(I), 90-Sr(II), Pb(II), Cu(II), and Ni(II) ions were selected as coexisting radionuclides and metal ions. According to the results of selective experiments (Fig. 4), it is observed that PAAM–FeS/Fe₃O₄ presents an excellent selectivity for U(VI) under the experimental conditions. The experimental data were also simulated by Langmuir and Freundlich models, and the relative fitting results are listed in Table 2. The Langmuir model matched the experimental data much better than the Freundlich model. This observation further confirmed that PAAM–FeS/Fe₃O₄ is a promising adsorbent for the selective removal and extraction of U(VI) ions from aqueous solutions.

Fig. 4 Comparison of the enrichment performance of PAAM–FeS/Fe₃O₄ for radionuclides and metal ions (A) and the related C_smax values calculated from the Langmuir model (B). Contact time: 48 h, T = 20 ±1 °C, m/V = 0.10 g L⁻¹, pH = 5.0 ±0.1, C[NaCl] = 0.10 mol L⁻¹.

Table 2 Parameters calculated from Langmuir and Freundlich models for radionuclide sorption on PAAM–FeS/Fe₃O₄ at pH = 5.0 and T = 20 ± 1 °C

Ions	Langmuir model			Freundlich model
	C smax (mg g^−1)	b (L mg^−1)	R ^2	K (mg g^−1)	1/n	R ^2
U(VI)	311	0.0251	0.997	83.0	0.301	0.927
Pb(II)	89.7	0.226	0.999	32.1	0.243	0.901
Cu(II)	85.0	0.170	0.998	26.5	0.270	0.902
Ni(II)	78.2	0.132	0.999	21.0	0.299	0.934
Sr(II)	74.5	0.0710	0.990	14.9	0.347	0.916
Cs(I)	64.5	0.0836	0.983	15.0	0.314	0.892

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Enrichment mechanism

In order to get a better understanding of the interaction mechanism between U(VI) and PAAM–FeS/Fe₃O₄, the U-laden and recovered PAAM–FeS/Fe₃O₄ samples were characterized by the XPS technique. The XPS spectra of raw, U-laden, and recovered PAAM–FeS/Fe₃O₄ are shown in Fig. 5. In the U-laden PAAM–FeS/Fe₃O₄ sample, the occurrence of U 4f in U-laden PAAM–FeS/Fe₃O₄ is in accordance with the apparent changes in the N 1s, O 1s, and S 2p spectra, which suggested that the enrichment of U(VI) on PAAM–FeS/Fe₃O₄ was relevant to FeS and amide groups.

Fig. 5 XPS S 2p spectra (A), O 1s spectra (B), N 1s spectra (C), and U 4f (D) spectra of raw, U-laden, and recovered PAAM–FeS/Fe₃O₄.

As can be seen from the S 2p spectra in Fig. 5A, the raw PAAM–FeS/Fe₃O₄ showed the typical S 2p XPS peaks related to FeS at 161.90 and 163.33 eV.^5,27 After the sorption of U(VI), U-laden PAAM–FeS/Fe₃O₄ also showed a new peak centered at ~161.30 eV, which was related to the formation of polysulfides (S_2+x²⁻).^1,5,9,28 Researchers^1,5,6,9,26 have reported this observation that the reaction of U(VI) with FeS_x resulted in the formation of U(IV) species and a variety of various sulfur species, including S_2+x²⁻, dissolved S₂O₃²⁻, dissolved S₂O₄²⁻, etc. The formed S_2+x²⁻ could further react with U(VI) and showed a typical peak centered at ~164.39 eV.²⁸ The conversion of insoluble FeS to dissolved sulfur species also significantly decreased the relative peak intensities and signal-to-noise ratios of S 2p in the XPS spectrum of U-laden PAAM–FeS/Fe₃O₄. Bargar et al.⁴ found that the distribution of uranium on the FeS surface was just associated with the sulfur-containing materials on the scale of the whole sediment due to the reaction of U(VI) with FeS. After rinsing with 0.1 mol L⁻¹ HNO₃, the peak centered at 164.39 eV related to S_2+x²⁻ shifted to 164.08 eV,²⁹ which was due to the removal of U(IV)/U(VI) from the surface of PAAM–FeS/Fe₃O₄. The regenerated PAAM–FeS/Fe₃O₄ also showed a weak S 2p XPS peak related to FeS at 161.90 eV, indicating that the quantitative recovery of uranium from the surfaces of PAAM–FeS/Fe₃O₄ can be achieved by elution with 0.1 mol L⁻¹ HNO₃.

The O 1s spectra of raw, U-laden, and recovered PAAM–FeS/Fe₃O₄ samples (Fig. 5B) can be deconvoluted into the O²⁻ anions in the bulk structure of Fe₃O₄ and the surface bond oxygen^29,30 on the PAAM–FeS/Fe₃O₄ surface. The significantly increasing content of chemical bond oxygen in the U-laden PAAM–FeS/Fe₃O₄ sample confirmed the enrichment of U(IV)/U(VI)³⁰ on the PAAM–FeS/Fe₃O₄ surface (Table 3). Moreover, after enrichment of U(VI), the peak related to the O²⁻ anions in the bulk structure of Fe₃O₄ increased from ~529.9 eV^29,30 to ~530.4 eV, which is similar to the report by Scott et al.³⁰ that the O 1s spectrum of Fe₃O₄ showed a ~0.4 eV shift to a higher binding energy after enriching with U(VI). They explained the shift by the electron transfer between U(VI) and structural Fe(II) on the Fe₃O₄ surface, which resulted in the oxidation of Fe(II) to Fe(III) and the reduction of U(VI) to U(IV).

Table 3 Curve fitting results of XPS O 1s and N 1s spectra

	Peak	BE^a (eV)	FWHM^b (eV)	%
a Binding energy. b Full width at half maximum.
Raw PAAM–FeS/Fe3O4
O 1s	Fe3O4	529.87	1.19	52.2
	Bridging OH	531.32	1.76	47.8
N 1s	–NH2	399.90	1.70	87.1
	–N–O	401.64	0.52	8.49
	–NH2^+	403.30	0.90	4.45
U-laden PAAM–FeS/Fe3O4
O 1s	Fe3O4	530.39	1.08	40.7
	Bridging OH	531.38	2.22	59.3
N 1s	–NH2	400.10	1.66	33.1
	–N–O	401.60	2.17	12.7
	–NH2^+	403.50	2.61	54.2
Recovered PAAM–FeS/Fe3O4
O 1s	Fe3O4	530.31	1.08	54.0
	Bridging OH	531.41	2.33	46.0
N 1s	–NH2	400.00	1.55	87.7
	–N–O	401.76	0.95	9.44
	–NH2^+	403.50	1.00	2.90

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The high-resolution scans for the N 1s spectra (Fig. 5C and Table 3) of raw, U-laden, and recovered PAAM–FeS/Fe₃O₄ can be deconvoluted into two peaks at 400.0 ± 0.1 and 401.7 ± 0.1 eV, corresponding to –NH₂ and –N–O, respectively.^31,32 The U-laden PAAM–FeS/Fe₃O₄ presents a new peak at 403.4 ± 0.1 eV corresponding to the complex of protonated amide groups and U(VI), which was ascribed to one of the uranyl oxo-oxygen atoms by Sather et al.³³ from the protonation of amide groups (denoted as –NH₂⁺). After the recovery of U(IV)/U(VI) from the surface of PAAM–FeS/Fe₃O₄, the relative content of –NH₂⁺ decreased significantly and can be neglected. The results of XPS spectrum analysis indicated that amide groups and FeS on the surface of PAAM–FeS/Fe₃O₄ were attributed to the enrichment of U(VI).

The immobilized uranium on PAAM–FeS/Fe₃O₄ is most likely to exist as U(IV)/U(VI) mixture species because the enrichment of U(VI) on the surfaces of PAAM–FeS/Fe₃O₄ is a combined process of sorption and reductive immobilization. The XPS U 4f of U-laden PAAM–FeS/Fe₃O₄ was soundly determined and quantitatively resolved into U(IV) (centered at 380.66 and 391.53 eV) and U(VI) (centered at 382.09 and 392.94 eV).^5,9,28 Although the formed U(IV) might be partly re-oxidized^1,5,6,8 when it was exposed to air, the relative ratio of U(IV) to U(VI) on the PAAM–FeS/Fe₃O₄ surface (Fig. 5D) was still found to be ~0.63 : 1. It further confirmed that PAAM–FeS/Fe₃O₄ could reduce U(VI) to U(IV) on PAAM–FeS/Fe₃O₄ surfaces. Similarly, Hua and Deng⁵ reported that U(VI) was firstly adsorbed on the FeS surface and then reduced to U(IV), which resulted in U(IV)/U(VI) mixture species.

The reductive precipitation of U(VI) to U(IV) was also reported as the major reaction mechanism for the enrichment of U(VI) by reductive materials, such as sulfate-reducing biofilms,³ iron sulfide materials,⁹ and zero-valent iron.³⁴ Compared with that of U-laden PAAM–FeS/Fe₃O₄, the XPS U 4f of recovered PAAM–FeS/Fe₃O₄ was undetectable, which indicated that U(VI) and U(VI) were completely rinsed from the surfaces of PAAM–FeS/Fe₃O₄.

The recovery of uranium from PAAM–FeS/Fe₃O₄ was also investigated. The adsorbed U(IV) can effectively be re-oxidized to soluble U(VI) under oxidizing conditions.^3,10,34,35 Cerrato et al.³⁵ reported that the oxidization and dissolution rates of U(IV) increased with increasing dissolved oxygen and decreasing pH values. The recovery efficiency of U(IV)/U(VI) versus HNO₃ concentration is shown in Fig. 6A. It can be seen that 0.1 mol L⁻¹ HNO₃was enough for quantitative elution of adsorbed uranium, indicating the promising recovery efficiency of uranium from PAAM–FeS/Fe₃O₄. The recovery of U(IV)/U(VI) from PAAM–FeS/Fe₃O₄ was also confirmed by FT-IR spectroscopy analysis.

Fig. 6 Effect of HNO₃ concentration on the recovery of U(VI) from PAAM–FeS/Fe₃O₄ (A) and the related FT-IR spectra of raw, U-laden, and recovered PAAM–FeS/Fe₃O₄ (B).

As depicted in the FT-IR spectrum of U-laden PAAM–FeS/Fe₃O₄ (Fig. 6B), the immobilized uranium on the PAAM–FeS/Fe₃O₄ surface was evidenced by the broad peak at ~910 cm⁻¹ related to O=U=O stretching vibration.^33,36,37 Compared with the peak of free aqueous U(VI) ions at ~963 cm⁻¹, the significant red shift indicated the strong affinity of uranium to PAAM–FeS/Fe₃O₄.³⁸ After rinsing with 0.1 mol L⁻¹ HNO₃ solution, the peak related to O=U=O stretching vibration disappeared, which confirmed the quantitative recovery of uranium from the surfaces of PAAM–FeS/Fe₃O₄ by elution with 0.1 mol L⁻¹ HNO₃.

Conclusions

In conclusion, PAAM–FeS/Fe₃O₄ composites were successfully in situ synthesized by chemical coprecipitation of FeS/Fe₃O₄ in the presence of AAM, followed by DBD plasma-induced polymerization of AAM on the FeS/Fe₃O₄ surface. The enrichment of U(VI) on PAAM–FeS/Fe₃O₄ was mainly attributed to the complexation of U(VI) with amide groups and the reductive reaction of U(VI) with FeS to U(IV). Enrichment results indicated that PAAM–FeS/Fe₃O₄ presented a highly efficient enrichment for U(VI) and can be easily separated and recovered from aqueous solution by a simple magnetic separation method. The results highlighted the potential application of PAAM–FeS/Fe₃O₄ in the reductive immobilization of U(VI) in nuclear waste management and environmental pollution cleanup.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (21272236, 91326202 and 21225730), the Science Foundation of Institute of Plasma Physics, Chinese Academy of Sciences (DSJJ–13–YY01), and Hefei Center for Physical Science and Technology (2012FXZY005).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ew00014e

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