Introduction

Uranium is a primary and valuable constituent of nuclear fuel of pressurized water reactor [1]. Additionally, U(VI) is a highly toxic and radioactive metal ion [2], even in trace amounts, can harm organs such as the kidneys, liver, and bones [3], etc. Uranium-containing wastewater may have generated during nuclear fuel fabrication including mining, hydrometallurgy, and conversion. Therefore, there is an urgent need to efficiently eliminate and reuse uranium from contaminated water [4].

Among numerous techniques for U(VI) capture, adsorption is considered the most favorable owing to its high effectiveness, simple operation, and lack of secondary pollution. Moreover, fostering a high efficient adsorbent is the key for this certain objective. Mesoporous silicas (MSs) has attracted increasing attentions for their excellent properties, such as remarkable stability, controllable pore diameter, high specific area [2], rich in porosity [5]. Many MSs adsorbents have been applied into recovery of U(VI) from wastewater, including MSs functionalized with phosphonate [6], amidoxime [7], and amino groups [8].

As a kind of nano-adsorbent, the MMs are found to be difficult to recovery from aqueous solution [9]. With strong magnetic responsivity [10], magnetic mesoporous silicas (MMSs) have tremendous potential in the fields of U(VI) adsorption [10,11,12], which can easily separation from water by an external magnetic field. But, the MMSs matrix applied in removal of U(VI) were different, such as magnetic MCM-41 [2], and magnetic SBA-15 [13].

Herein, the adsorptive performances for U(VI) at MMSs were compared. Firstly, magnetic MCM-41, MCM-48 and SBA-15 were synthesized using a hydrothermal method. The structure and physicochemical properties of three kinds of MMSs were characterized by small angle XRD, VSM, ζ potential analyzer, contact angle measurer, and XPS, etc. The adsorption mechanism was clarified by using the relevant adsorption models, FT-IR and, XPS.

Experimental

Reagents and instruments

Ferric chloride hexahydrate (AR), chloroacetic acid (AR), sodium fluoride (AR), and anhydrous sodium acetate (AR) were obtained from Shanghai Macklin Biochemical Technology Co., Ltd. P123 (MW 5800) was acquired from Sigma Aldrich (Shanghai) Trading Co., Ltd. Cetyltrimethylammonium bromide (CTAB, AR), Arsenazo III (AR), and TEOS (AR) were sourced from Shanghai Aladdin Biochemical Technology Co., Ltd.

The equipment used in the study included centrifuge (TG12-WS, Shanghai Spectral, China), XRD instrument (D8 ADVANCE, Bruke, Germany), FT-IR spectrometer (Nicolet iS5, Thermo Fisher, USA), FEI-TEM (Tecnai F20, FEI, USA), XPS (K-alpha, Thermo Fisher, USA), Vibrating sample magnetometer (VSM) (7404 type, Lakeshore, China), Particle potential titration analyzer (Stabino, PMX, Germany), and an automatic gas adsorption system (ASAP2020, Micromeritics, USA).

Preparation of magnetic mesoporous silica

Preparation of Fe3O4@ SiO2 (MNPS)

The Fe3O4@SiO2 composite was synthesized using a sol–gel method. 0.30 g of Fe3O4 was dispersed in a 70.0 mL ethanol–water solution (Vethanol/Vwater = 6/1). After ultrasonic treatment for 10 min, 2.0 mL of TEOS and 1.0 mL of ammonia solution were introduced. The solution was mechanically stirred at 25 °C for 10 h. Then, wash it with water and ethanol for three times, separate the product with external magnetic field, and dry it at 60 °C.

Preparation of MMSs

The preparation route for the MMSs is illustrated in Fig. 1. Initially, a mixture comprising 1.50 g of MNPS, 50.0 mL of H2O, and 5.0 mL of NH3·H2O was subjected to oscillation using an ultrasonic cleaner for 30 min. After adding 10 mL of TEOS, 0.90 g of NaOH, and 0.19 g of NaF, the resulting mixture was then mechanically blended at 25 °C for 2 h. Subsequently, 3.00 g of CTAB was introduced, and the stirring process was continued for an additional 2 h. After thorough mixing, the resulting solution was transferred to a reaction kettle and aged at 100 °C for 48 h. The obtained product was subjected to washing and dried at 60 °C for 12 h. Finally, the product underwent calcination at 300 °C for 3 h to yield the ultimate product MMCM-41. The preparation of MMCM-48 followed the same procedure, with the only difference being the use of 7.0 g of CTAB.

Fig. 1
figure 1

Schematic synthesis diagram of magnetic mesoporous silicas

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In a separate process, 4.00 g of P123 and 0.05 g of NH4F were dissolved in 138.0 mL H2O and 1.2 mL concentrated hydrochloric acid within a three-neck flask. The solution underwent ultrasonication for 10 min after the addition of 0.60 g of Fe3O4@SiO2. Afterthat, 9.0 mL of TEOS was introduced into the flask, and the solution was stirred at 40 °C and then at 100 °C for 24 h each. The resulting filter residue was obtained after washing with H2O and subsequent dehydration at 60 °C for 24 h. The final product, MSBA-15, was achieved through calcination at 550 °C for 6 h.

U(VI) adsorption experiment

The adsorption properties of the MMSs for U(VI) were investigated using a batch experiment. In a conventional adsorption procedure, 50 mL of U(VI) solution was introduced into a 150 mL conical flask and adjusted to a fixed pH value with NaOH and HNO3 solutions. The conical flask with 50 mL solution and 10.0 mg of adsorbent was shaken for a certain time. Then, the U(VI) concentration of supernatant was determined by an Arsenazo III spectrophotometry. Finally, the amount of U(VI) adsorbed [14, 15] (q, mg·g−1) and the distribution coefficient [16] (Kd, mg·mL−1) were calculated by the Eq. S1 and S2, respectively.

Selective experiment

Weighed amounts of Zn(NO3)2·6H2O, Co(NO3)2·6H2O, Sr(NO3)2, Pb(NO3)2, Ni(NO3)2·6H2O, Ce(NO3)3·6H2O, Gd(NO3)3·6H2O, and La(NO3)3·6H2O were ultrasonically dissolved in a beaker. And, the mixture with 10 mL of 5 g·L−1 U(VI) solution was diluted to 1 L volume by deionized water. The concentration of these ions in the prepared solution was 50 mg·L−1. After shaking for 240 min, 10 mL of supernatant was taken and added with 2.5 mL of HNO3 solution (2 mol·L−1). Finally, the residual concentrations of the aforementioned ions were measured using an ICP-AES (ICAP-6300, Thermo Fisher, USA).

Results and discussion

Characterization

The micro morphology and ordered structure of the MMSs were examined using TEM and small angle XRD, respectively. The TEM images of MSBA-15 is shown in Fig. 2a and b. Fe3O4 is covered with an ordered SiO2 layer, whose pore wall is approximately 5.13 nm. The small-angle XRD patterns of three types of MMSs are presented in Fig. 2c. Concerning MSBA-15, the peaks at 0.9°, 1.7° and 2.1° correspond to the (100), (110), and (200) crystal planes, revealing that it has hexagonal structure [17]. The peak of MMCM-48 at 2.4° corresponds to the (100) crystal plane, and the characteristic peak of MMCM-41 and MMCM-48 at 2.1° corresponds to the (200) crystal plane, has a good mesoporous structure [18]. The small-angle XRD results indicate that the structure of MMSs is both mesoporous and ordered.

Fig. 2
figure 2

TEM images of MSBA-15 (a) × 8 × 105, (b) × 1.6 × 10.6, (c) small-angle XRD patterns of MSBA-15, MMCM-41 and MMCM-48, (d) FT-IR spectra of MSBA-15, MMCM-41 and MMCM-48

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The surface functional groups of MMSs were analyzed using FT-IR, and the findings are illustrated in Fig. 2d. In the infrared spectra of MMSs, there are typical characteristic peaks of mesoporous silica. The bands at 1041 and 804 cm−1are Si–O–Si stretching vibration peaks [19, 20].The stretching and bending vibration of the hydroxyl group connected to the silica atom appears at 1647 and 3750 cm−1 [21, 22].The weak absorption peak at about 593 cm−1 can be attributed to the vibration of Fe–O bond [23]. The FT-IR results indicate the successful synthesis of three distinct MMSs.

The adsorption–desorption plots were obtained under a N2 atmosphere. MSBA-15, MMCM-41, and MMCM-48 were degassed at 373.15 K for more than 15 h before the determination. The specific surface of the sample (SBET, m2·g−1) is calculated from the BET equation [24] at a relative pressure (P/P0) between 0.05 and 0.25. Pore distribution is calculated by BJH model [25]. In Fig. 3a, the N2 adsorption–desorption isotherms of MMSs at 77 K. It can be seen from the Fig. that their adsorption isotherms all show the characteristics of type IV curves, indicating that they all conform to the characteristics of mesopores [26]. MSBA-15's type hysteresis ring is a typical H2 type [27], suggesting that it has a narrow mesoporous structure. The N2 desorption adsorption curves of MMCM-41 and MMCM-48 are H4 type hysteresis loops, showing micro-mesoporous structure. Figure 3b illustrates the BJH pore size distribution of MMSs. The pore size distribution of MMSs is relatively narrow. The most probable pore diameters of MSBA-15, MMCM-41, and MMCM-48 is 8.36, 3.79, and 2.75 nm, respectively. The pore structure data are itemized in Table 1. The SBET and average pore diameter values of MSBA-15, MMCM-41 and MMCM-48 was 675.50, 605.38 and 638.70 m2·g−1; 6.69 nm, 3.60 nm and 3.56 nm, respectively.

Fig. 3
figure 3

MSBA-15, MMCM-41 and MMCM-48, (a) N2 adsorption and desorption curves, (b) BJH pore size distribution, (c) VSM plots, (d) ζ potential

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Table 1 Pore structure parameters of MSBA-15, MMCM-41 and MMCM-48
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Magnetic properties of MMSs are measured by VSM (Fig. 3c). The magnetic symmetrical hysteresis curves of MSBA-15, MMCM-41 and MMCM-48 all show an S-shape. And, the saturation magnetization is 6.29, 13.66 and 16.17 emu·g−1, respectively. Upon the removal of the template from the as-synthesized MSBA-15 at high temperatures, there is a consequential partial oxidation of ferric oxide. It's noteworthy that MSBA-15 can swiftly aggregate using a magnet, as depicted in the inset of Fig. 2c. This property proves beneficial for the recovery of U(VI) through magnetic means.

The ζ potential at different pH values is illustrated in Fig. 3d. The surface potential of the three materials declined with the increasing pH values. Specifically, the surface potential of MSBA-15, MMCM-41, and MMCM-48 decreases from -1.32, 1.53, and 19.83 mV to −48.59, −38.49, and −31.45 mV, respectively, when the pH value is within the range of 2.0–8.0. The pH values of the zero potential points (pHIZE) of MMCM-41 and MMCM-48 is 2.6 and 5.6, respectively. When the pH value of the external system exceeds the pHIZE of the material, the surface potential of the material becomes negative. This negative surface potential contribute to the adsorption of U(VI) through electrostatic attraction.

The hydrophilicity of materials was explored through contact angle. Contact angle of MMSs after dropping liquid for 5 s is illustrated in Fig. 4. The contact angles of MSBA-15, MMCM-41, and MMCM-48 was 22.5°, 24.5°, and 31.5°, indicating that MSBA-15 has best hydrophilicity [28]. Excellent hydrophilicity of MSBA-15 facilitates the contact with aqueous solution and improves the separation ability of U(VI) [29].

Fig. 4
figure 4

Contact angle of MSBA-15, MMCM-41 and MMCM-48

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The XPS was employed to characterize the element composition and chemical form of the prepared MSBA-15 material. The corresponding spectra are shown in Fig. 5. The peaks of Fe, O, C and Si appear in the full spectrum, indicating Fe3O4 and mesoporous silica combined together closely. The spectrum peak of Fe 2p shows that Fe 2p1/2 and 2P3/2 is located at 708.9 and 714.1 eV, respectively [30]. It signposted that Fe3O4 exists in MSBA-15, showing a good match with the results of TEM and FT-IR. The C 1 s peak separation that the synthesized magnetic mesoporous silica material contains two peaks of C–C and C–Si, locating at 284.1 and 282.6 eV, respectively. It noted that the synthesized MSBA-15 also contained a small amount of template that has not been removed completely. Figure 5d is the spectrum of Si 2p, with Si–OH at 102.1 eV and Si–C at 101.4 eV [31]. In the O 1 s spectrum, the peak at 531.3 and 530.5 eV corresponds to Si–OH and O–H. In summary, the XPS analysis corroborates the presence of Fe3O4 in MSBA-15. The characterization indicates a substantial presence of silica hydroxide groups on the material's surface.

Fig. 5
figure 5

XPS spectra of MSBA-15: (a) survey, (b) C 1 s, (c) Fe 2p, (d) Si 2p, (e) O 1 s

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pH value

The impact of pH value on the adsorption of U(VI) by MSBA-15, MMCM-41 and MMCM-48 was studied in the range of pH 2.0–7.0 (C0 = 50 mg·L−1, m = 10 mg, V = 50 mL, T = 298.15 K, t = 240 min). The results are displayed in Fig. 6. Under acidic conditions, U(VI) predominantly exists in the form of UO22+. And, the competition of H+ with U(VI) for the active sites on the MMSs resulted in lower adsorption [32]. With the increasing of pH values, the ζ potential of MMSs decreased according to the Fig. 3d. Consequently, the values of q increased accordingly. At a pH value of 6.0, the adsorption performance of the three types of MMSs for U(VI) reached its peak, with adsorption capacities of 158.18, 126.74, and 125.38 mg·g−1. The best adsorption ability of MSBA-15 may be due to highest pore volume, biggest pore diameter (Table 1) as well as smallest contact angle (Fig. 4). When pH was over 6.0, the anions of UO2CO3(OH)33− and UO2(CO3)34− were major components [33]. Thus, the electrostatic repulsion weakened the removal capacity for U(VI). As a result, during the subsequent batch experiments, the solution's pH was adjusted to 6.0.

Fig. 6
figure 6

Effect of pH on the adsorption of U(VI) by MSBA-15, MMCM-41, and MMCM-48 (C0 = 50 mg·L−1, m = 10 mg, V = 50 mL, T = 298.15 K, t = 240 min)

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Adsorption kinetics

The effect of contact time on U(VI) adsorption by MMSs (C0 = 50 mg·L−1, m = 10 mg, V = 50 mL, T = 298.15 K, pH = 6.0) was explored. In Fig. 7a, U(VI) rapidly enriched on the surface of MMSs in the initial 90 min. Notably, MSBA-15, MMCM-41, and MMCM-48 achieved adsorption equilibrium at 120, 120, and 240 min, respectively.

Fig. 7
figure 7

Fitted curves of (a) quasi-primary and quasi-secondary kinetic models for MSBA-15, MMCM-41 and MMCM-48 adsorbed U(VI), (b) fitted curves of internal diffusion model (C0 = 50 mg·L.−1, m = 10 mg, V = 50 mL, T = 298.15 K, pH = 6.0)

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Quasi-first order [34, 35], quasi-second order [34] and intra-particle diffusion model [36, 37] were employed to analyze the experimental data and the process of adsorption of U(VI) by MMSs. The formulas of them are shown in form of S3, S4, and S5. The sorption kinetics of U(VI) by MSBA-15, MMCM-41, and MMCM-48 are depicted in Fig. 7a. The fitting parameters are detailed in Table 2. The correlation coefficient (R2) of the quasi-second order kinetic model for three kinds of MMSs is closer to 1.0. Furthermore, the theoretical adsorption capacity (176.23, 154.86, and 144.33 mg·g−1) is closer to the experimental value (172.40, 152.86, and 140.85 mg·g−1). Consequently, the U(VI) adsorption processes of on MMSs align more closely with the quasi-second-order equation, suggesting that chemical action predominantly governs the adsorption process.

Table 2 Kinetic parameters of U(VI) adsorption by MSBA-15, MMCM-41 and MMCM-48
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The intra-particle diffusion model [38] of MMSs for U(VI) sorption by MMSs is illustrated in Fig. 7b. The fitting results can be found in Table 3. The internal diffusion rate constants (kid) for the three kinds of MMSs consistently follow the order kid,1 > kid,2 > kid,3, indicating that the adsorption process occurs in three distinct steps.

Table 3 Internal diffusion model parameters for MSBA-15, MMCM-41 and MMCM-48 adsorbed U(VI)
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During the initial phase of rapid adsorption, U(VI) in the solution comes into contact with the surface of MMSs and swiftly diffuses towards the material surface. Subsequently, in the slower adsorption process, U(VI) penetrates the inner surface and binds to active sites within. Upon reaching adsorption equilibrium, the diffusion rate approaches 0.

Adsorption isotherm

Figure 8 illustrates the impact of varying equilibrium concentrations of U(VI) (Ce, mg·L−1) on the adsorption efficiency of MMS for U(VI). In the lower concentration range, the adsorption capacity of U(VI) shows a rapid increase. Conversely, at higher concentrations, MSBA-15 demonstrates a gradual attainment of adsorption satiation, reaching a capacity of 319.55 mg·g−1. The saturated adsorption capacities for MMCM-41 and MMCM-48 are 217.88 and 190.54 mg·g−1, respectively.

Fig. 8
figure 8

Langmuir, Freundlich and Sips models for MSBA-15, MMCM-41 and MMCM-48 adsorption U(VI) (m = 10 mg, V = 50 mL, T = 298.15 K, pH = 6.0, t = 240 min)

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To further analyze the adsorption processes, Langmuir [39, 40], Freundlich [41], and Sips [42, 43] isotherm models were used. The formula is presented as Eqs. S6, S7, and S8. The fitting results including the values of the R2 and relevant parameters are revealed in Fig. 8 and Table 4. As shown from Table 7, MSBA-15 exhibit a greater consistency with the Sips adsorption isotherm model with a higher R2values (0.99) than Freundlich (R2 = 0.87) and Langmuir models (R2 = 0.98). Furthermore, the saturated adsorption capacity (341.94 mg·g−1) obtained by fitting the Sips adsorption isotherm model of MSBA-15 is near to the actual value (319.55 mg·g−1). However, the saturated adsorption capacities of Langmuir adsorption isotherm models for MMCM-41 and MMCM-48 (250.60 mg·g−1, 232.88 mg·g−1) are closer to the actual values (217.88 mg·g−1, 190.54 mg·g−1), indicating a single molecular layer adsorption dominated by a chemical process.

Table 4 Adsorption isotherm model parameters for U(VI) adsorption by MSBA-15, MMCM-41 and MMCM-48
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To investigate the impact of the initial U(VI) concentration on MSBA-15, MMCM-41, and MMCM-48, the Dubin-Radushkevich (D-R) [44], and the Temkin [45] adsorption isotherm models were employed to compute the binding energy (E, kJ·mol−1) and the constant (B, J·mol−1). The calculation formula for the D-R adsorption isotherm model is represented by Eqs. S9, S10, and S11.

The D-R and Temkin adsorption isotherm models are illustrated in Fig. 9a and b, respectively. The relevant parameters are displayed in Table 4. The values of D-R adsorption isotherm models’ R2 (0.98, 0.97, 0.96) of MSBA-15, MMCM-41, and MMCM-48 are closer to 1. The results are higher than the Temkin adsorption isotherm models (0.90, 0.96, 0.93), presenting that the enrichment of U(VI) by MMSs is more consistent with the D-R adsorption isotherm model. Furthermore, the binding energies of the three kinds of MMSs (11.18, 11.78, 11.95 kJ·mol−1) are all over 8.0 kJ·mol−1. And again, it indicated that adsorption is dominated by chemical reaction [46].

Fig. 9
figure 9

MSBA-15, MMCM-41 and MMCM-48 adsorption U(VI) isothermal models: (a) D-R model; (b) Temkin model

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The adsorption capacity of MSBA-15 for U(VI) was compared with that of other adsorbents, and the relevant results are presented in Table 5. Observing the table, it becomes apparent that of MSBA-15 shows a greater saturated adsorption capacity for U(VI) than that of the studied functionalized Fe3O4, SBA/SA, MCM-41, and MCM-48, etc.

Table 5 Comparison of the adsorption performance of different mesoporous material adsorbents on U(VI) at 298.15 K
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Adsorption thermodynamics

The effect of temperature on the adsorption of U(VI) by MMSs is depicted in Fig. 10a. The equilibrium adsorption capacity of U(VI) increases with rising temperature, suggesting that adsorption of U(VI) is an endothermic process.

Fig. 10
figure 10

(a) Effect of temperature on the adsorption of U(VI) by MSBA-15, MMCM-41 and MMCM-48, (b) thermodynamic curves (C0 = 50 mg·L.−1, m = 10 mg, V = 50 mL, t = 240 min, pH = 6.0)

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To further explore the influence of temperature, the corresponding thermodynamic functions [52] of ΔH, ΔS, and ΔG are calculated by using formulas S13 and S14. Thermodynamic fitting plots are presented in Fig. 10b. According to the results presented in Table 6, ΔH > 0 and ΔG < 0 indicate that the adsorption processes are endothermic and spontaneous.

Table 6 Thermodynamic parameters of adsorbed U(VI) for MSBA-15, MMCM-41 and MMCM-48
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Adsorption selectivity

The adsorption selectivity of MMSs for U(VI) was studied in the solution with coexisting ions. In Fig. 11, the adsorption capacity of MSBA-15, MMCM-41 and MMCM-48 for U(VI) in the presence of eight ions is 49.03, 44.82 and 52.48 mg·g−1, respectively, accounting for 50.81, 42.89 and 44.47% of the total adsorption capacity. Among them, U(VI) accounts for the largest proportion of MSBA-15 adsorption, thanks to the excellent mesoporous structure and abundant Si–OH of MSBA-15.

Fig. 11
figure 11

Selectivity of MSBA-15, MMCM-41 and MMCM-48 for different ions (pH = 6.0, C0 = 50 mg·L.−1, m = 10 mg, V = 50 mL, T = 298.15 K, t = 240 min)

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To further theoretically evaluate the adsorption selectivity of MMSs for U(VI), the selectivity coefficient (S) [53] is calculated. In Table 7, comparing with the other eight ions, the three MMSs have higher partition coefficients for U(VI), and the selectivity coefficient S is greater than 1, indicating that they can selectively adsorb U(VI) in an aqueous solution.

Table 7 Distribution ratios and selectivity coefficients of MSBA-15、MMCM-41and MMCM-48
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Repetitive experiment

HCl solutions (0–0.50 mol·L−1) are chosen as the desorption solution. Then the reusability experiments of MSBA-15 were carried out (Fig. 12a). The elution rate (D, %) is determined by the formula S16 [54]. The U(VI) adsorbed on MSBA-15 can be eluted with 0.30 mol·L−1HCl with the best elution rate of 99.4%. Thus, 0.30 mol·L−1 HCl solution was selected as the eluent for MSBA-15 in subsequent repeated experiments.

Fig. 12
figure 12

(a) Elution effect of different hydrochloric acid concentrations on MSBA-15, (b) reproducibility of MSBA-15

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Adsorption–desorption-regeneration cycles were repeated for 5 times, as shown in Fig. 12b. After repeating the experiment for 4 times, the adsorption capacity of MSBA-15 for U(VI) decreased to 80.7% of the initial value. It may be related to the destruction of the adsorption active site on MSBA-15. The above results show that MSBA-15 can be reused in recovery of U(VI) from an aqueous solution.

Adsorption mechanism

The potential mechanism of U(VI) adsorption on MSBA-15 was assessed through FT-IR and XPS analyses. In the FT-IR spectrum following the adsorption of U(VI) (Fig. 13a), an asymmetric vibration absorption peak of UO22+ emerged at 911 cm−1. Furthermore, the stretching and bending vibration peaks (3750 and 1647 cm−1) of the hydroxyl group were intensified.

Fig. 13
figure 13

(a) FT-IR spectrum of MSBA-15 after adsorption, XPS spectra of MSBA-15 before and after adsorption: (a) total spectrum, (b) U 4f, (c) Si 2p, (d) O 1 s, (e) C 1 s

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The XPS spectra of MSBA-15 before and after U(VI) uptake are shown in Fig. 13b. Apart from the Fe 2p, O 1 s, C 1 s and Si 2p peaks, the U 4f peak (Fig. 13c) also manifested in the spectrum after adsorption. It indicated that U(VI) was successfully adsorbed by MSBA-15. Additionally, in the Si 2p spectrum (Fig. 13d), the Si–OH at 102.1 eV shifted to 102.5 eV after adsorption, indicating that there was an interaction between the Si–OH groups and U(VI). In the spectrum of O 1 s (Fig. 13 (e)), the Si–OH peak shifted to 531.3 eV, and the O–H peak at 530.5 eV did not change significantly. In addition, the spectral diagram at C 1 s (Fig. 13f, the C–C peak at 284.1 eV exhibited no change, whereas the C-Si peak at 282.6 eV has shifted. These findings indicate that U(VI) coordinates with the silica hydroxide group on the surface of MSBA-15.

Conclusion

Magnetic mesoporous silicas, MMCM-41, MMCM-48, and MSBA-15, were successfully synthesized. The ordered-mesoporous structure of MSBA-15 was identified by TEM, small angle XRD and N2 adsorption–desorption curves. The BET specific surface areas of MSBA-15, MMCM-41 and MMCM-48 is 675.50, 605.38 and 638.70 m2·g−1, respectively. The optimal adsorption pH values of MMSs were all 6.0, and reached adsorption equilibrium within 240 min. MSBA-15 has a higher saturated adsorption capacity (341.94 mg·g−1) and superior selectivity for U(VI) than MMCM-41 and MMCM-48. The adsorption capacity of MSBA-15 for U(VI) decreased to 80.7% of the initial value after 4 recycles. The adsorption of U(VI) on MSBA-15 is mainly the coordination of Si–O–H group with U(VI). In conclusion, MSBA-15 has outstanding performance for U(VI) and is expected to be used in treatment and recovery of U(VI) from radioactive waste water.