Highly Efficient Removal of Uranium from an Aqueous Solution by a Novel Phosphonic Acid-Functionalized Magnetic Microsphere Adsorbent

Abstract

The development of adsorption materials which can efficiently isolate and enrich uranium is of great scientific significance to sustainable development and environmental protection. In this work, a novel phosphonic acid-functionalized magnetic microsphere adsorbent Fe₃O₄/P (GMA-MBA)-PO₄ was developed by functionalized Fe₃O₄/P (GMA-MBA) prepared by distill-precipitation polymerization with O-phosphoethanolamine. The adsorption process was endothermic, spontaneous and kinetically followed the pseudo second-order model. The maximum uranium adsorption capacity obtained from the Langmuir model was 333.33 mg g⁻¹ at 298 K. In addition, the adsorbent also had good acid resistance and superparamagnetic properties, which could be quickly separated by a magnetic field. XPS analysis showed that the adsorption of adsorbent mainly depended on the complexation of phosphonic acid group with uranium. This work offers a promising candidate for the application of magnetic adsorbents in the field of uranium separation and enrichment.

1. Introduction

With the intensification of the energy crisis, nuclear energy as a kind of clean energy has been widely studied [1,2,3]. The demand for uranium as an important nuclear fuel is increasing. However, uranium is characterized by radioactivity and heavy metal toxicity. If it accumulates in large quantities in the environment, it will do harm to the environment. Therefore, it is important to develop the separation technology for the efficient separation and enrichment of uranium for the exploitation of uranium resources and environmental protection [4,5,6]. At present, many methods including ion exchange, solvent extraction, chemical precipitation, membrane separation, photocatalysis and adsorption have been used to recover uranium from an aqueous solution [7,8,9,10,11,12]. Among these methods, adsorption has proved to be a promising separation technique due to its wide source of materials, low cost, high adsorption selectivity and high volumetric value [13,14,15]. The key to the adsorption method is the choice of adsorption material. At present, many materials such as carbon materials, silicon materials, polymer materials and metal–organic frames have been used in the field of uranium adsorption. However, these materials often suffer from complicated separation steps such as filtration and centrifugation, which hinder their application in the field of uranium adsorption to a great extent. Therefore, the development of a uranium adsorption medium with a high adsorption efficiency and easy separation is one of the current research key points in the field of environmental radiochemistry [16,17].

Recently, magnetic polymer nanomaterials have received much attention [18,19,20,21,22]. On the one hand, the nano size can make this material have a larger specific surface area and more surface atoms than ordinary materials, thus showing strong adsorption and performance. On the other hand, magnetic polymer nanomaterials can be separated quickly by magnetic force, which solves the problem of the separation difficulty of traditional adsorption materials [23,24,25,26,27,28,29,30,31]. In addition, the surface of the material can also be rich in organic functional groups by means of copolymerization or post-modification, which can improve the adsorption capacity for uranium. Therefore, magnetic polymer nanomaterials have obvious advantages in the separation and enrichment of uranium, which cannot be replaced by conventional materials in many aspects. Based on these advantages, the development of magnetic polymer nanomaterials has important scientific significance and practical value for the healthy and rapid development of the nuclear industry.

Up to now, as an efficient ligand for uranium, the phosphonic acid group has been widely used for the separation and enrichment of uranium due to its strong complexing ability with uranyl ion. For example, in our preliminary work, many phosphonic acid-functionalized adsorbents were prepared by solvothermal polymerization, distillation precipitation polymerization and other methods, and all of them had good adsorption properties for uranium [1,16]. In addition, Broda et al. [32] reported a nanocomposite hydroxyapatite/white clay with excellent adsorption properties for uranium. Therefore, considering these factors, this work hoped to develop a phosphonic acid-functionalized magnetic polymer microsphere adsorbent for the efficient separation and enrichment of uranium from aqueous solutions [33,34].

In this work, a novel phosphonic acid-functionalized magnetic polymer microsphere adsorbent Fe₃O₄/P (GMA-MBA)-PO₄ was developed by functionalizing Fe₃O₄/P (GMA-MBA) prepared by distill-precipitation polymerization with O-phosphoethanolamine. Surprisingly, the Fe₃O₄/P (GMA-MBA)-PO₄ showed a good adsorption capacity, with the theoretical maximum adsorption capacity of uranium reaching 333.33 mg g⁻¹ at pH 4.5. In addition, the adsorbent also had good structural stability and superparamagnetic character, resulting in the quick separation and recovery by magnetic force of the adsorbent. XPS analysis showed that the adsorption of the adsorbent mainly depended on the complexation of the phosphonic acid group with uranium. This work offers a promising candidate for the application of magnetic adsorbents in the field of uranium separation and enrichment.

2. Results and Discussion

2.1. Structural Analyses

To verify that Fe₃O₄/P (GMA-MBA)-PO₄ was successfully synthesized by Scheme 1, the physical and chemical properties of the adsorbent were characterized by TEM, FT-IR, XRD, VSM and TGA, respectively.

The morphology of Fe₃O₄, activated Fe₃O₄, Fe₃O₄/P(GMA-MBA) and the magnetic adsorbent Fe₃O₄/P (GMA-MBA)-PO₄ were investigated using TEM. As shown in Figure 1A, the prepared Fe₃O₄ had a uniform particle size with an average diameter of about 200 nm. The magnetic sphere surface had a fluffy structure [35]. The mean particle size and morphology of Fe₃O₄ did not change significantly after the modification of γ-MPS (Figure 1B), suggesting that the activation process did not have much effect on the morphology of Fe₃O₄. Figure 1C shows that there was a thick polymer shell layer around the Fe₃O₄, and the thickness of the polymer shell was around 35 nm, indicating that the copolymerization of GMA and MBA had occurred smoothly on the surface of Fe₃O₄ to obtain the matrix material Fe₃O₄/P (GMA-MBA). Figure 1D lists the TEM image of Fe₃O₄/P (GMA-MBA)-PO₄. From Figure 1D, the magnetic adsorbent still maintained the core–shell structure, indicating the matrix material Fe₃O₄/P (GMA-MBA) had an excellent medium resistance.

Figure 2 shows the FTIR spectra of Fe₃O₄, Fe₃O₄ activated by KH570, Fe₃O₄/P(GMA-MBA) and Fe₃O₄/P (GMA-MBA)-PO₄. As shown in Figure 2a, the characteristic peaks located around 585 cm⁻¹ belonged to the characteristic absorption of Fe-O in Fe₃O₄. Compared to Fe₃O₄, a series of new peaks ascribed to γ-MPS can be observed in Figure 2b. For example, tow new peaks located at approximately, 1636 cm⁻¹ and 1720 cm⁻¹ were attributed to the -C=C and -C=O groups of γ-MPS, respectively, indicating that Fe₃O₄ had been functionalized by KH570 successfully [36]. In the spectrum of Fe₃O₄/P (GMA-MBA) (Figure 2c), many new characteristic absorption bands ascribed to P (GMA-MBA) could be observed. For instance, the two obvious peaks located at 1528 cm⁻¹ and 1650 cm⁻¹ belonged to the stretching vibration of N-H and C=O from MBA. In addition, the three obvious peaks located at 842 cm⁻¹, 906 cm⁻¹ and 1721 cm⁻¹ belonged to the stretching vibration of epoxy group and C=O from GMA. Thus, all these results indicated that the co-polymerization of GMA and MBA had occurred on the surface of activated Fe₃O₄. Figure 2d shows the infrared spectrum of Fe₃O₄/P(GMA-MBA)-PO₄. The two obvious peaks located at 940 cm⁻¹ and 1232 cm⁻¹ belonged to the stretching vibration of P-OH and P=O [37]. Besides, the epoxy group characteristic absorption peak (842 cm⁻¹ and 906 cm⁻¹) appearing in Figure 2c almost disappeared. Thus, all these results suggest that Fe₃O₄/P(GMA-MBA) was successfully modified with O-phosphoethanolamine.

The X-ray powder diffraction (XRD) pattern of Fe₃O₄ and Fe₃O₄/P(GMA-MBA)-PO₄ was shown in Figure 3A. The XRD pattern of Fe₃O₄/P(GMA-MBA)-PO₄ showed many characteristic peaks of Fe₃O₄. Moreover, a peak of dispersion at a 2θ of about 12.5° representing amorphous P(GMA-MBA) was also seen in addition to the characteristic peaks of Fe₃O₄. The VSM pattern of Fe₃O₄/P (GMA-MBA)-PO₄ is shown in Figure 3B. As shown in Figure 3B, the hysteresis loop passed through the origin with a coercivity force of zero, confirming the magnetic polymer microsphere adsorbent was superparamagnetic with a specific saturation magnetization of about 10.0 emu g⁻¹. Thus, Fe₃O₄/P (GMA-MBA)-PO₄ could be quickly separated and recovered from the aqueous solution by applying an external magnetic field within 20 s (Figure 3B inset). The thermal stability of the material was tested, and the results are shown in Figure 3C. From Figure 3C, the lost weight of the sample was mainly caused by the volatilization of the small molecule including water, solvent and monomers remaining on the surface of Fe₃O₄/P (GMA-MBA)-PO₄ within 50–293 °C. However, when the temperature was above 293 °C, the mass of the magnetic adsorbent dropped sharply, which might have been due to the oxidative degradation of the polymer chain. Thus, according to Figure 3C, the prepared magnetic adsorbent had a high thermal stability. As shown in Figure 3D, the elemental composition of Fe₃O₄/P (GMA-MBA)-PO₄ was also analyzed by XPS, and the magnetic adsorbent contained C, N, O and P. Combined with the results of infrared spectroscopy analysis, it is clear that the phosphonic acid group was successfully introduced to the surface of the magnetic sphere. Based on these results, the phosphonic acid functionalized core–shell magnetic sorbent Fe₃O₄/P(GMA-MBA)-PO₄ was successfully prepared.

2.2. Effect of pH

In general, the adsorption capacity of uranium is often significantly affected by pH, due to which the speciation of uranium and the surface charge of the adsorbent are affected to a large extent. Hence, the uranium adsorption capacity on Fe₃O₄/P (GMA-MBA) and Fe₃O₄/P (GMA-MBA)-PO₄ was investigated with an initial pH ranging from 1.0 to 4.5, and the experimental results are shown in Figure 4A. From Figure 4A, the uranium adsorption capacity of the two magnetic adsorbents increased with the value of pH. In aqueous solutions of pH 4.5, the adsorption capacity of Fe₃O₄/P (GMA-MBA) and Fe₃O₄/P (GMA-MBA)-PO₄ could reach up to 52.21 and 98.73 mg g⁻¹, respectively. From Figure 4A, the uranium adsorption capacity of Fe₃O₄/P (GMA-MBA)-PO₄ was much higher than that of Fe₃O₄/P (GMA-MBA). This result confirms that the prepared magnetic polymer microsphere adsorbent had the ability to efficiently separate the enriched uranyl ions in the aqueous solution, which was mainly due to the complexation of phosphoric acid functional groups on the surface of the adsorbent with uranyl ions.

As shown in Figure 4A, under highly acidic conditions (at pH < 2), the U (VI) adsorption capacity of Fe₃O₄/P(GMA-MBA)-PO₄ was very low. This was mainly because the material surface group was protonated when the solution pH value was very low, and the adsorbent surface was positively charged. Since uranium (VI) mainly exists in the form of UO₂²⁺ when the pH value was less than 4.5 (Figure S1), electrostatic repulsion between the adsorbed material and UO₂²⁺ reduced the adsorption performance of uranium. When the acidity of the system decreased, the protonation degree of the adsorbent surface reduced. In such a case, the electrostatic repulsion between the adsorption material and UO₂²⁺ was constantly reduced, and the complexation ability of the functional group of the adsorbent surface with uranyl ion was enhanced, resulting in the increase in the uranium adsorption capacity of the adsorbent. In addition, when the value of pH was greater than 4.5, uranium became unstable and easy to precipitate. Therefore, pH of 4.5 was applied as the best operation condition for further experiments.

2.3. Sorption Static Kinetics, Isotherms and Thermodynamic Analysis

To acquire sorption kinetics, isotherms and thermodynamic data, the effect of time, uranium concentration and temperature on the adsorption capacity of the adsorbent for uranium was evaluated, and the results are listed in Figure 4B–D.

The uranium adsorption capacity of Fe₃O₄/P(GMA-MBA) and Fe₃O₄/P(GMA-MBA)-PO₄ at different adsorption times is shown in Figure 4B. From Figure 4B, the adsorption of uranium at pH 4.5 could be roughly divided into three stages. In the first 120 min, the adsorption rate was very fast and the adsorption capacity increased rapidly. At 120–180 min, the adsorption rate was reduced. The adsorption was gradually balanced after 180 min. Since the Fe₃O₄/P(GMA-MBA)-PO₄ was rich in the phosphate acid group, the saturation adsorption capacity of Fe₃O₄/P(GMA-MBA)-PO₄ was greater than that of Fe₃O₄/P(GMA-MBA).

Table S1 lists the kinetic parameters obtained by pseudo-first-order, pseudo-second-order and intra-particle diffusion models [38,39,40]. According to Table S1 and Figures S2–S4, the adsorption process was better correlated with the pseudo-second-order kinetic models. Calculated from the linear equation of the pseudo-second-order, the theoretical adsorption volume q_e values of Fe₃O₄/P(GMA-MBA) and Fe₃O₄/P(GMA-MBA)-PO₄ at pH 4.5 were 52.91 mg g⁻¹ and 101.21 mg g⁻¹, very close to the actual experimental results (52.21 mg g⁻¹ and 98.73 mg g⁻¹). This showed that the pseudo-second-order kinetic model was better suited for describing the adsorption process of uranium by Fe₃O₄/P(GMA-MBA)-PO₄, since the pseudo-second-order model is based on the assumption that the adsorption rate is controlled by chemical adsorption. Thus, the adsorption mechanism of uranium by Fe₃O₄/P (GMA-MBA)-PO₄ and Fe₃O₄/P(GMA-MBA) in pH 4.5 solution was mainly dominated by chemical adsorption.

The effect of the initial concentration of uranyl ion on the uranium adsorption capacity of Fe₃O₄/P(GMA-MBA) and Fe₃O₄/P(GMA-MBA)-PO₄ in aqueous solutions at pH 4.5 was investigated, and the results are shown in Figure 4C. From Figure 4C, the adsorption capacity of the two magnetic adsorbents increased as the concentration of the uranium solution increased from 25 mg L⁻¹ to 200 mg L⁻¹. When the concentration of the solution reached above 250 mg L⁻¹, the adsorption capacity of the two magnetic adsorbents increased. When the concentration was 300 mg L⁻¹, the equilibrium adsorption capacity of Fe₃O₄/P (GMA-MBA) reached 148.59 mg g⁻¹. When the adsorption capacity of Fe₃O₄/P (GMA-MBA)-PO₄ increased to 303.59 mg g⁻¹, the adsorption capacity remained unchanged with the continued increase in concentration. The main reason for this was that when the concentration of uranium ions in solution increased, the adsorption sites on the surface of the adsorbent were occupied by uranyl ions. When the concentration increased to a certain extent, the adsorption sites were occupied completely. In such case, there were no more phosphate groups to coordinate with the uranyl ion, meaning that the adsorption balance was reached. Moreover, the equilibrium adsorption capacity of Fe₃O₄/P (GMA-MBA)-PO₄ was much higher than that of Fe₃O₄/P (GMA-MBA), due to the fact that Fe₃O₄/P (GMA-MBA)-PO₄ had a high concentration of phosphonic acid groups.

The uranium adsorption performance of Fe₃O₄/P (GMA-MBA) and Fe₃O₄/P (GMA-MBA)-PO₄ at different temperatures is shown in Figure 4D. With the increase in contact temperature, the uranium adsorption capacity of the two adsorbents to uranium gradually increased, indicating that the uranium adsorption process was endothermic, and the increase in temperature was beneficial to the adsorption. To further investigate the effect of temperature on uranium adsorption performance, the three parameters including enthalpy changes ΔH° (KJ mol⁻¹), ΔS° (J mol⁻¹ K⁻¹) and Gibbs free energy changes ΔG° (KJ mol⁻¹) were studied. ΔH° and ΔS° were calculated using the thermodynamic formulas Equations (S1) and (S2). The ΔH° and ΔS° were calculated from the linear plots of lnK_d and 1/T (Figures S7 and S8). The ΔG° of Fe₃O₄/P (GMA-MBA) and Fe₃O₄/P (GMA-MBA)-PO₄ were calculated, respectively, by using the Van’t Hoff equation. According to Table S3, since ΔH° was positive, the adsorption of Fe₃O₄/P (GMA-MBA)-PO₄ to uranium was endothermic, suggesting the increase in the temperature was beneficial to the adsorption. The value of ΔS° was positive, indicating that the surface confusion and randomness of Fe₃O₄/P (GMA-MBA)-PO₄ increased during the adsorption process. The value of ΔG° was negative, indicating that the uranium adsorption process was spontaneous.

Table S2 lists the sorption isotherm parameters of the Fe₃O₄/P (GMA-MBA) and Fe₃O₄/P (GMA-MBA)-PO₄ calculated from the Langmuir and Freundlich models [41,42]. As shown in Figures S5 and S6 and Figures S5 and S6, the correlation coefficients R² of Fe₃O₄/P (GMA-MBA) and Fe₃O₄/P (GMA-MBA)-PO₄ were 0.995 and 0.9982 according to the Langmuir Model, which was much higher than that of Freundlich Model. The maximum adsorption capacity of Fe₃O₄/P (GMA-MBA)-PO₄ for U (VI) (333.33 mg g⁻¹) calculated by the Langmuir adsorption equation was closer to the experimental data (303.59 mg g⁻¹). Compared with the Freundlich model, the Langmuir model was more compatible with the experimental data. Thus, it could be argued that the adsorption of Fe₃O₄/P (GMA-MBA)-PO₄ for uranium was dominated by monolayer adsorption.

2.4. Structural Stability Analysis

To investigate the acid resistance of Fe₃O₄/P (GMA-MBA)-PO₄, the adsorbent was soaked at a pH of 4.5 of HNO₃ for 24 h, and then separated by an external magnet and freeze-dried. As shown in Figure 5, the soaked magnetic absorbent was characterized by FTIR, VSM and TEM. Figure 5A confirms that its structure did not significantly change after immersion and the surface functional group remained unchanged. The VSM test results are shown in Figure 5B, where the adsorbent magnetic response decreased slightly, but it still showed superparamagnetic character. The TEM images of the adsorbent before and after soaked are shown in Figure 5C,D. From Figure 5C,D, the material remained intact with a spherical structure after 24 h of soaking with no significant changes. The above results show that Fe₃O₄/P (GMA-MBA)-PO₄ had a good structural stability at a pH of 4.5 of HNO₃ for 24 h.

2.5. Comparison of the U(VI) Sorption Capacity of Fe₃O₄/P (GMA-MBA)-PO₄ with That of Other Adsorbents in Aqueous Solutions

As shown in

Table 1. Comparison of uranium adsorption performance between Fe₃O₄/P(GMA-MBA)-PO₄ and other adsorbents.

Absorbent	qmax (mg g−1)	pH	Ref.
Fe-DETA	236.0	6.0	[43]
Amidoxime-modified Fe3O4@SiO2	105.0	5.0	[44]
Fe3O4/GO	69.5	5.5	[45]
Fe3O4@C-KO	38.7	6.0	[46]
Quercetin-modified Fe3O4 nanoparticles	12.3	3.7	[47]
Fe3O4@TiO2	118.8	6.0	[48]
Fe3O4@PAM	220.9	5.0	[49]
Fe3O4@SiO2	52.0	6.0	[50]
Phosphoramide-functionalized Fe3O4	95.2	6.0	[51]
Fe3O4	<50	7.0	[52]
MCFN	91.1	5.5	[53]
MNHA	310.0	5.0	[54]
Fe3O4/P(GMA-MBA)-PO4	333.3	4.5	This work

, the Fe₃O₄/P (GMA-MBA)-PO₄ of adsorption properties were compared with other adsorbents. From Table 1, the q_max of Fe₃O₄/P (GMA-MBA)-PO₄ was up to 333.33 mg g⁻¹, which was better than that of the other magnetic adsorbents listed in Table. For example, unmodified Fe₃O₄ had a uranium adsorption capacity of less than 50 mg g⁻¹ at pH 7.0. After modifying the Fe₃O₄ (such as Fe₃O₄/GO, Fe₃O₄@C-KO, Fe₃O₄@TiO₂, MCFN, et al.) uranium adsorption capacity increased, but its saturation adsorption capacity was still lower than that of Fe₃O₄/P (GMA-MBA)-PO₄. Most magnetic adsorbents show a good adsorption capacity at pH 5.5–6.0, but the adsorption capacity decreased with the increase in acidity. Thus, Fe₃O₄/P (GMA-MBA)-PO₄ could be regarded as a promising candidate for the separation and preconcentration of uranium.

2.6. Analysis of the Interactions between Uranium and Fe₃O₄/P(GMA-MBA)-PO₄

To study the interaction between magnetic adsorbent and uranium ions, XPS analysis of Fe₃O₄/P (GMA-MBA)-PO₄ before and after uranium adsorption is shown in Figure 6A–D. As shown in Figure 6A, the two peaks located at 133.9 eV and 133.0 eV were the characteristic peaks of P 2p1/2 and P 2p3/2. Compared with Figure 6A, the binding energy strength and position of Fe₃O₄/P (GMA-MBA)-PO₄ of P 2p1/2 changed significantly (Figure 6B), indicating a new complexation between uranyl with P. As shown in Figure 6C, the O 1s spectra could be decomposed into three peaks, and the characteristic peaks at 530.3 eV, 531.2 eV and 532.5 eV belonged to P=O, C=O, and P-OH. After adsorption of Fe₃O₄/P (GMA-MBA)-PO₄ for uranium, the position of the binding energy and strength of O 1s of P=O and P-OH bond changed significantly, while the position of O 1s of C=O bond remained almost unchanged (Figure 6D). Thus, it could be concluded that the adsorption process mainly depended on the interaction between the phosphonic acid group with uranium.

3. Materials and Methods

3.1. Materials

Bisacrylamide (MBA), O-phospethanolamine, γ-(Methacryloxypropyl) trimethoxy silane (KH570), glycidyl methacrylate (GMA) and azobisisobutyronitrile (AIBN) were bought from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China) Acetonitrile, nitrate acid (HNO₃), sodium hyfroxide (NaOH) and FeCl₃·6H₂O were supplied from Xilong Chemical Co., Ltd. (Shantou, China) O-phospethanolamine was purchased from Shanghai Perlingway Chemical Technology Co., Ltd. (Shanghai, China) UO₂ (NO₃)₂·6H₂O (ACS grade) was purchased from Shanghai Reagents (Shanghai, China). All other chemicals used in the experiments were of analytical grade. Deionized water used for all experiments was supplied from a Milli-Q (Milli-pore Corporation, Burlington, MA, USA) water purification system.

3.2. Preparation of Fe₃O₄

Fe₃O₄ microspheres were prepared by the solvothermal method. The synthetic procedure was as follows: 2.5 g of FeCl₃·6H₂O was dissolved in 80 g of ethylene glycol (EG), and then 7.2 g of sodium acetate (NaAc) and 2.0 g of polyethylene glycol acid (PEG) were added to the solution. The mixture was stirred at 50 °C for 1 h and transferred to a 100 mL autoclave lined with polytetrafluoroethylene. The autoclave was heated to 200 °C for 6 h, and then left to cool naturally to room temperature. The Fe₃O₄ microspheres were washed with ethanol several times to remove the impurities under ultrasonic condition. Finally, Fe₃O₄ microspheres were kept at room temperature.

3.3. Preparation of Activated Fe₃O₄

In order to improve the coating rate of Fe₃O₄ in the polymer shell, the Fe₃O₄ hollow pellets were activated by γ-(Methacryloxypropyl) trimethoxy silane (KH570) [55]. The synthetic procedure was as follows: 0.3 g of Fe₃O₄, 128.0 mL of anhydrous ethanol, 4.0 mL γ-(Methacryloxypropyl) trimethoxy silane (KH570), 4.0 mL NH₃·H₂O and 36.0 mL of distilled water were added to a three-neck flask with a capacity of 250 mL. The mixture was stirred for 12 h at 40 °C. After the reaction, the product was washed several times to remove impurities. Finally, activated Fe₃O₄ microspheres by KH570 were kept at room temperature.

3.4. Preparation of Fe₃O₄/P (GMA-MBA)

Fe₃O₄/P(GMA-MBA) was prepared by the distillation and precipitation method. In a typical run, 0.10 g of surface-activated Fe₃O₄ microsphere and 80.0 mL acetonitrile were moved into 250 mL three-neck round bottom flask under ultrasonic oscillations. Then, 0.08 g of AIBN, 0.30 g of GMA and 0.30 g of MBA were added to the mixture. The mixture was heated to 90 °C for the reaction for 2 h. At the end of the reaction, the obtained magnetic microspheres were separated by an external magnetic field, washed several times and dried by lyophilization.

3.5. Preparation of Fe₃O₄/P (GMA-MBA)-PO₄

In a typical run, Fe₃O₄/P (GMA-MBA) microspheres (0.1 g), O-phospethanolamine (1 g), DMF (50 mL) and H₂O (50 mL) were added to a round-bottom flask. The mixture was stirred at 80 °C for 8 h. Then, the obtained Fe₃O₄/P (GMA-MBA)-PO₄ was separated by an external magnetic field, washed several times and dried by lyophilization. The synthesis process is described in Scheme 1.

3.6. Characterization

The XRD pattern was measured with a D8 ADVANCE Da Vinci. FT-IR spectroscopy analysis were acquired on a Nicolet 6700 (Thermo Fisher, Waltham, MA, USA). Thermogravimetric analysis (TGA) was measured with an STA 449 F3 (Nichi, Bavaria, Germany). TEM images were performed on a TALOS F200X (Thermo Fisher, Waltham, MA, USA). X-ray photoelectron spectra (XPS) were acquired on an AXIS UItraDLD (Shimadzu, Kyoto, Japan). Magnetic properties (VSM) were measured on a MPMS3 (Quantum Design, San Diego, CA, USA).

3.7. Adsorption Tests

The adsorption experiments were carried out using the batch method to measure the adsorption property of uranium of the magnetic adsorbent. In a typical run, 10 mg of Fe₃O₄/P (GMA-MBA)-PO₄ was placed into 25 mL of uranium solution at different pH values that could be adjusted by adding negligible volumes of diluted HNO₃ or NaOH. After stirring for an appropriate time at a given temperature, the Fe₃O₄/P (GMA-MBA)-PO₄ was collected by magnetic separation. The concentrations of UO₂²⁺ in the aqueous solution, before and after adsorption, were determined by UV-vis spectrophotometer with arsenazo (III) as the complex agent.

The adsorption capacity q_e (mg g⁻¹) and the distribution coefficient, K_d (mL g⁻¹), were calculated by Equations (1) and (2):

$${\mathrm{q}}_e=\frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)\times \mathrm{V}}{\mathrm{M}}$$

(1)

$${\mathrm{K}}_d=\frac{{\mathrm{C}}_0-{\mathrm{C}}_e}{{\mathrm{C}}_0}\times 100\%$$

(2)

where C₀ and C_e are the initial concentration and equilibrium concentration of metal cations (mg L⁻¹), K_d is the distribution coefficient (mL g⁻¹), V is the volume of testing solution (L) and M is the amount of adsorbent (g).

3.8. Stability Tests

To investigate the structural stability of the magnetic adsorbent in an acidic solution, the prepared adsorbent was immersed at a pH of 4.5 of HNO₃ for 24 h, and then was magnetically separated and freeze-dried with an external magnet. The adsorbent was subsequently characterized using TEM, FTIR, XRD and VSM to determine whether the structure had changed.

4. Conclusions

In this work, a novel phosphonic acid-functionalized core–shell magnetic polymer microsphere adsorbent Fe₃O₄/P (GMA-MBA)-PO₄ was prepared by functionalized Fe₃O₄/P (GMA-MBA) with O-phosphoethanolamine. The physical and chemical properties and microstructure of the material were characterized by TEM, FT-IR, TG, XRD, XP and, VSM. The effect of pH, time, concentration and temperature on the adsorption behavior of the adsorbent was studied. The results showed that the adsorbent had a good adsorption performance for uranium, and its theoretical saturated adsorption capacity was up to 333.3 mg g⁻¹. The study of adsorption kinetics and thermodynamics showed that the adsorption process was a fast, spontaneous and endothermic process, which accorded with the pseudo-second-order model and the Langmuir model. In addition, the adsorbent also had a good acid resistance and superparamagnetic character, which could be quickly separated by an external magnet. The XPS analysis showed that the adsorption process was mainly dependent on the interaction between the phosphonic acid group with uranium. Based on these, Fe₃O₄/P(GMA-MBA)-PO₄ could be a promising candidate for the separation and preconcentration of uranium in actual water treatment.