Facile preparation and highly efficient sorption of magnetic composite graphene oxide/Fe3O4/GC for uranium removal

In this work, we reported for the first time a novel magnetic composite graphene oxide/Fe3O4/glucose-COOH (GO/Fe3O4/GC) that was facilely prepared from glucose through the hydrothermal carbonization and further combination with graphene oxide (GO). The chemical and structural properties of the samples were investigated. By the batch uranium adsorption experiments, the magnetic composite GO/Fe3O4/GC exhibits an excellent adsorption performance and fast solid–liquid separation for uranium from aqueous solution. GO/Fe3O4/GC (the maximum adsorption capacity (Qm) was 390.70 mg g−1) exhibited excellent adsorption capacity and higher removal rate (> 99%) for U(VI) than those of glucose-COOH (GC) and magnetic GC (MGC). The effect of the coexisting ions, such as Na+, K+, Mg2+, Ca2+, and Al3+, on the U(VI) removal efficiency of GO/Fe3O4/GC was examined. The equilibrium sorption and sorption rate for the as-prepared adsorbents well fit the Langmuir model and pseudo second-order kinetic model, respectively. The thermodynamic parameters (ΔH0 = 11.57 kJ mol−1 and ΔG0 < 0) for GO/Fe3O4/GC indicate that the sorption process of U(VI) was exothermic and spontaneous. Thus, this research provides a facile strategy for the preparation of the magnetic composite with low cost, high efficiency and fast separation for the U(VI) removal from aqueous solution.


Results and discussion
Characterization. The Fourier transform infrared (FTIR) spectra of glucose, Fe 3 O 4 , GC, MGC and GO/ Fe 3 O 4 /GC were shown in Fig. 2A. In the FTIR spectrum of GC, most of the characteristic peaks disappeared compared to glucose, but the intensity of the peak at 1714 cm −1 attributed to the group -COOH was higher than that of glucose, which indicated that GC was successfully obtained after hydrothermal and calcination treatment, and the number of carboxyl, carbonyl and ester groups significantly increased on the surface. These characteristic peaks of GC were similar to those of HTC-COOH reported in the reference 32 . In the FTIR spectrum of GO/Fe 3 O 4 /GC, the characteristic peaks at ~ 567 cm −1 and ~ 352 cm −1 belonged to Fe-O stretching vibration 42 appeared, suggesting that the magnetic composite GO/Fe 3 O 4 /GC was successfully prepared.The crystal phases of the samples were presented in Fig. 2B. In the X-ray diffraction (XRD) pattern of glucose, the strong diffraction peaks reflected great crystallization of glucose. However, the crystal phase of GC is amorphous owing to the calcination process in 300 ℃, which accords with the reference 43 44 , were clearly shown in Fig. 2B. The XRD patterns of GO/Fe 3 O 4 /GC and MGC are basically consistent with those of Fe 3 O 4 , but the intensity of the peaks has significantly reduced because of the addition of GO and GC, revealing that the decoration process of Fe 3 O 4 did not change the crystal phase of magnetite composite.
To reveal the thermal stability of the samples, the thermogravimetric analysis (TGA) curves of glucose, GC, MGC and GO/Fe 3 O 4 /GC in the range of temperatures from 30 to 900 °C are shown in Fig. 2C. Glucose is a kind of organic compound, and starts to decompose at 200 °C by TGA. GC is obtained from glucose through hydrothermal and carbonization at high temperature. While in the preparation process of GO/Fe 3 O 4 /GC and MGC inorganic substance Fe 3 O 4 was introduced into their molecular. In general, thermal stability of organic substances is poorer than that of inorganic substances. In the structure of GO/Fe 3 O 4 /GC there are more interaction including electrostatic interaction, ionic interaction, and π-π stacking interaction than MGC. Therefore, the thermal stability of GO/Fe 3 O 4 /GC is higher than those of glucose, GC and MGC. The composite GO/Fe 3 O 4 /GC presented the smallest weight loss (4.28%) when the temperature was up to 900 °C, which showed that GO/Fe 3 O 4 /GC had the excellent thermal stability and almost no thermal decomposition took place. While the thermal stability of glucose, GC and MGC was poor, and the weight losses at 900 °C were 76.79%, 49.67% and 16.48%, respectively. Based on the above characterization results, a possible formation mechanism for GO/Fe 3 O 4 /GC is illustrated in Fig. 4. Firstly, GO nanosheet is physically mixed with GC in an ultrasonic bath to form the complex GO · GC through hydrogen bond interaction as shown in Eq. (1). Then, Fe n+ (n = 2, 3) ions were formed in the above suspension by adding Fe 2+ ions because a part of Fe 2+ ions were oxidized to Fe 3+ in air by the redox reaction as shown in Eq. (2). With the hydrolysis of Fe 3+ and the addition of 30% ammonia solution the magnetic composite GO/Fe 3 O 4 /GC was obtained as expressed in Eqs.    Fig. 5A. The results show that the adsorption process for U(VI) obviously depends on the pH value of the solution. The pH value of the solutions greatly affects the surface charge of the samples. At pH < 4.0, the surface of the sorbents were protonated to form the positively charged surface, and then the electrostatic repulsion between these positive charge (including H 3 O + ) and UO 2 2+ led to the poor adsorption capability for U(VI) 14 . With the appearance of the positive species (UO 2 (OH) + , (UO 2 ) 3 (OH) 5 + , and (UO 2 ) 4 (OH) 7 + ) the removal efficiency of U(VI) significantly increased at pH 4.0-7.0 due to the electrostatic interaction between these complex uranium ions with positive charges and the negatively charged adsorbents. When pH is above 7.0, the negatively charged U(VI) species ((UO 2 ) 3 (OH) 7 − and UO 2 (OH) 3 − ) are the dominant U(VI) species in solution which result in the reduction of the U(VI) removal efficiency 45 . The maximum removal rate for GC, MGC and GO/Fe 3 O 4 /GC was 66.30% (pH 6.0), 73.30% (pH 5.0), and 98.70% (pH 5.0), respectively. The sorption efficiency of U(VI) by GO/Fe 3 O 4 /GC was much higher than that of GC indicating that the addition of GO in the GC molecular enhanced greatly the adsorption property for U(VI). As a consequence, the optimal pH for GC, MGC and GO/Fe 3 O 4 /GC was selected as 6.0, 5.0 and 5.0 in the next U(VI) adsorption tests, respectively.
The influence of some important co-existing cations (e.g., Na + , K + , Ca 2+ , Mg 2+ , and Al 3+ ) on U(VI) sorption by GO/Fe 3 O 4 /GC at 25 °C and pH 5.0 was shown in Fig. 5B. When no coexisting ions were added into the uranium solution, the removal rate of U reached 99.10%. It was clearly seen that Na + , K + , Mg 2+ and Ca 2+ had no significant competition effects on the sorption of U(VI). In contrast, the presence of Al 3+ had a suppressive effect on U(VI) sorption. The results showed that the binding ability of cations to U(VI) followed the priority sequence: + 3 valence cations (e.g., Al 3+ ) < + 2 valence cations (e.g., Mg 2+ and Ca 2+ ) < + 1 valence cations (e.g., Na + and K + ) which indicated that the better electrostatic interaction between high valence cations and the adsorbent GO/Fe 3 O 4 /GC result in the decrease of the adsorption efficiency for U(VI).
Adsorption isotherm. The investigation of the adsorption isotherm reveals that how the adsorbate distribute between the liquid and the solid phase when the solution reach the adsorption equilibrium. The fit results of Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models for the U(VI) adsorption on GC and GO/Fe 3 O 4 /GC are presented in Fig. 6. The Langmuir, Freundlich and D-R isotherm parameters are calculated  where c 0 is the initial adsorbate concentration (mg L −1 ). The R L value is related to the strength of the adsorption. The values of R L > 1, R L = 1, 0 < R L < 1, and R L = 0 indicate that weak, linear, strong or irreversible adsorptions,   www.nature.com/scientificreports/ respectively. According to Table 1 it was seen that the k L value of GO/Fe 3 O 4 /GC was 0.3420 and the calculated R L value was 0.2262, indicating that strong adsorption between the adsorbent GO/Fe 3 O 4 /GC and U(VI). According to Langmuir isotherm fit result, the maximum sorption capacity (Q m ) of U(VI) on GC and GO/ Fe 3 O 4 /GC was determined to be 396.85 mg g −1 and 390.70 mg g −1 , respectively, higher than those of the previously reported glucose-based materials (see Table 2), which indicated that GO/Fe 3 O 4 /GC was a promising adsorbent for the treatment of the uranium-bearing wastewater. The fit for the data for the lowest uranyl concentrations is poor, which might result from the poor adsorption efficiency of the as-prepared adsorbent for the lower concentration uranium solutions. In this study, the as-prepared GO/Fe 3 O 4 /GC is a more promising adsorbent compared to other GO-based adsorbent due to the use of glucose with low-cost, environmental friendliness and anti-bacterial property as an initial material. The loading-U(VI) GO/Fe 3 O 4 /GC can be rapidly separated from the liquid phase through external magnetic fields due to the presence of magnetic Fe 3 O 4 .
The Dubinin-Radushkevich (D-R) model is adopted to better explain the U(VI) adsorption behaviour (chemical adsorption or physical adsorption) onto the adsorbents. According to the D-R isotherm parameters, the obtained E values reveal the physical or chemical sorption mechanism. According to the literature 47 , if E lies between 8 and 16 kJ mol −1 , the sorption process takes place chemically whereas E < 8 kJ mol −1 follows the physical sorption. For GC and GO/Fe 3 O 4 /GC, low E value (< 8 kJ mol −1 ) obtained in this study suggested that the adsorption process was mainly physical adsorption, which was in accordance with the reference 47 .
Adsorption kinetics. The adsorption kinetic mechanism is controlled by a mass transfer process involving equilibsrium time as well as physical and chemical adsorption characteristics. Figure 7 presents the timedependent U(VI) adsorption rate over contact time ranging from 5 min to 24 h at initial U(VI) concentration of 10 mg L −1 by GC, MGC and GO/Fe 3 O 4 /GC. From Fig. 7 it is clear that the adsorption amount of U(VI) increases  www.nature.com/scientificreports/ significantly with the extension of time until it reaches an equilibrium. As shown in Fig. 7, the adsorption kinetic of GO/Fe 3 O 4 /GC toward U(VI) indicated a fast adsorption process, and the remove of U(VI) could reach above 98% within 30 min. But the remove of U(VI) by GC could reach 97% after 24 h. Moreover, as presented in Fig. 7  (insert A and B), the correlation coefficients of pseudo second-order model were superior compared to pseudo first-order model which showed that the adsorption of UO 2 2+ ions onto GO/Fe 3 O 4 /GC was well fitted by the pseudo-second-order model. Adsorption kinetic parameters of the pseudo first-order and pseudo second-order model for GC, MGC and GO/Fe 3 O 4 /GC were given in Table 3. The results suggested that chemisorption is the rate-controlling step, implying the strong complexation between U(VI) ions and organic functional groups on the structures of GC, MGC and GO/Fe 3 O 4 /GC 50 . Fig. 8. The thermodynamic parameters such as enthalpy (ΔH 0 ), entropy (ΔS 0 ) and standard free energy (ΔG 0 ) from 303 to 333 K in the adsorption processes were calculated according to Eqs. (6) and (7) and given in Table 4. The negative value of ΔH 0 for GC reflected that the adsorption reaction was endothermic. While the positive value of ΔH 0 for GO/Fe 3 O 4 /GC reflected that the adsorption reaction was endothermic. The positive ΔS 0 and negative ΔG 0 suggested that the spontaneity of the adsorption process.   Adsorption experiments. The influence of pH, co-existing cations, contact time, initial U(VI) concentration, and temperature on the U(VI) removal was investigated. The U(VI) solution pH was adjusted to the desired value using HCl and NaOH. The as-prepared adsorbent was added to 20 mL solution and shaken in a shaker (Kangshi, China). After filtration, the U(VI) concentrations in solutions were determined by an MUA microquantity uranium analyzer (Beijing Yulun, China). The removal rate (R, %) and adsorption capacity (Q, mg g −1 ) were calculated according to Eqs. (8) and (9), respectively.

Adsorption thermodynamics. The plots of lnK d versus 1/T onto GC and GO/Fe 3 O 4 /GC were shown in
where c 0 (mg L −1 ) is the initial U(VI) concentration; c t (mg L −1 ) is the U(VI) concentration at time t; V is the volume of the solution (L); W is the dosage of the adsorbent (g).

Conclusions
In summary, three adsorbents GC, MGC and GO/Fe 3 O 4 /GC were facilely prepared using the inexpensive and environmentally benign glucose as a raw material for U(VI) capture via the simple hydrothermal carbonization and magnetization reaction. The optimum adsorption conditions for U(VI) with the initial concentration of 10 mg L −1 was at a pH of 5.0, a dosage of 0.15 g L −1 , and contact time of within 30 min when using GO/Fe 3 O 4 / (7) G 0 = H 0 − T S 0 ,