Preparation of ZnNiAl-LDHs microspheres and their adsorption behavior and mechanism on U(VI)

Ternary zinc-nickel-aluminum hydrotalcites (ZnNiAl-LDHs) were prepared by hydrothermal synthesis. The structure and morphology of the materials were characterized using X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), nitrogen adsorption–desorption (BET) and other test techniques. ZnNiAl-LDHs was applied in the treatment of uranium-containing wastewater, the effects of initial pH of the solution, adsorption temperature and contact time on its adsorption performance were systematically investigated, and the adsorption performance of ZnNiAl-LDHs and ZnAl-LDHs on uranyl ions were compared. The result showed that ZnNiAl-LDHs were 3D microspheres self-assembled from flakes, with a specific surface area of 102.02 m2/g, which was much larger than that of flake ZnAl-LDHs (18.49 m2/g), and the saturation adsorption capacity of ZnNiAl-LDHs for uranyl ions (278.26 mg/g) was much higher than that of ZnAl-LDHs for uranyl ions (189.16 mg/g), so the ternary ZnNiAl-LDHs had a more excellent adsorption capacity. In addition, kinetic and thermodynamic studies showed that the adsorption process of ZnNiAl-LDHs on uranyl ions conformed to the pseudo-second-order kinetic model and Langmuir isotherm model. The positive value of ΔH and the negative value of ΔG indicated that the adsorption process was endothermic and spontaneous. The adsorption mechanism was analyzed by X-ray energy spectroscopy (EDS), fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The results showed that the adsorption of uranyl ions by ZnNiAl-LDHs mainly consisted of complexation and ion substitution. The research results prove that ZnNiAl-LDHs is an adsorbent with low cost and excellent performance, and it has a good application prospect in the field of uranium-containing wastewater treatment.

www.nature.com/scientificreports/ examined the selectivity of adsorption. Zhu et al. 20 synthesized amino-trimethylene phosphonic acid (ATMP) intercalated ZnA1-LDHs by anion-exchange method and used it to absorb Cu 2+ and Pb 2+ from wastewater. The adsorption capacity of the modified LDHs for pollutants was significantly improved. Besides, there have been researches related to the use of hydrotalcite as adsorbent for uranium-containing wastewater treatment by researchers. For example, Xie et al. 21 prepared binary flake FeAl-LDHs as adsorbent by ultrasonic-assisted precipitation method and used it for the removal of U(VI) in wastewater solutions. The experimental result showed that the maximum adsorption capacity of FeAl-LDHs was 113.64 mg/g at 308 K through the synergistic effect of adsorption-reduction. Li et al. 22 synthesized binary flake CaAl-LDHs by modified coprecipitation method under ultrasonic technology. The result showed that the maximum adsorption capacity of CaAl-LDHs for U(VI) was 54.79 mg/g, and the maximum adsorption rate was 90.28%. However, the binary hydrotalcites prepared by the traditional method are mostly in the form of hexagonal flakes with low specific surface area, which limits the application of hydrotalcites in the field of adsorption. In recent years, in order to expand the application of layered hydrotalcite compounds, the preparation of ternary and multivariate hydrotalcites with special properties by introducing other divalent or trivalent metal ions on the basis of Mg-Al hydrotalcite is the focus of research in this field at home and abroad. In this paper, ZnNiAl-LDHs with 3D microspherical structure were prepared by a simple hydrothermal synthesis method and applied in the treatment of uranium-containing wastewater. XRD, FT-IR, SEM, BET, EDS and XPS characterization methods were used to analyze the material and explain the adsorption mechanism. The effects of various factors on the adsorption performance of ZnNiAl-LDHs for uranyl ions were discussed. In order to understand the adsorption process deeply, the adsorption kinetic, thermodynamic and isotherm were analyzed. The results show that ternary hydrotalcites synthesized in this study has a good adsorption performance for uranyl ions.

Experimental sections
Materials and reagents. Zinc 2 were dissolved in 100 mL deionized water, stirred for 1 h at room temperature, and then transferred to 100 mL in a reaction kettle lined with polytetrafluoroethylene, put it in an oven at 100 °C to react for 24 h. After the reaction, the reactor was cooled to room temperature, and the precipitate was collected by centrifugation. The precipitate was washed with deionized water to neutrality, and then dried at 60 °C for 12 h to obtain ZnNiA1-LDHs. 0.015 mol Zn(NO 3 ) 2 ·6H 2 O, 0.005 mol A1(NO 3 ) 3 ·9H 2 O and 0.066 mol CO(NH 2 ) 2 were used as raw materials, ZnA1-LDHs were prepared using the same steps as ZnNiA1-LDHs.
Characterization. Fourier transform infrared spectroscopy (Nicolet-iS10 type, Thermo Fisher Scientific, USA) was used for IR analysis of samples. The samples were characterized by XRD with an X-ray diffractometer (Bruker D8, Bruker, Germany), Cu target, Kα radiation, Ni filter, tube voltage 40 kV, tube current 40 mA, scanning range 2θ = 5° ~ 90°. The morphology of samples was characterized by scanning electron microscope (QUANTA FEG 450 type, FEI company, USA) and transmission electron microscope (Talos F200X type, Thermo Scientific company, USA). The specific surface area was analyzed and determined by Brunauer-Emmett-Teller (Micro for TriStar II Plus 2.02, Micro Company, USA). Energy dispersive spectroscopy was used to analyze the element content of samples. X-ray photoelectron spectroscopy (Escalab 250Xi, Thermo Fisher Scientific, USA) was used for XPS analysis to explore the adsorption mechanism.
Adsorption experiments. Batch adsorption experiments under different experimental conditions such as pH, time and temperature were carried out in a 100 mL conical flask. Add 5.00 mg of adsorbent to a conical flask containing 40 mL of uranium solution and adjust pH with HNO 3 or NaOH solution. The adsorption experiment was carried out on a reciprocating water bath shaking table, and the supernatant was taken for detection after the reaction. The concentration of U(VI) was determined by an UV spectrophotometer at a wavelength of 652 nm using arsenazo III spectrophotometry. In order to reduce the experimental error, all experimental data were the average of 3 repeated experiments. The equilibrium adsorption capacity (q e , mg/g) and adsorption rate (R, %) of LDHs for U(VI) were calculated by the following two equations: where C 0 and C e were the initial concentration and equilibrium concentration (mg/L) of the uranium solution, V was the solution volume (L), and m was the mass of adsorbent (g).

Results and discussion
Characterization. Figure 1a shows the XRD patterns of ZnA1-LDHs and ZnNiA1-LDHs. As shown,  24 . Figure 1b is the FT-IR spectrum of ZnA1-LDHs and ZnNiA1-LDHs, the bands at about 3400 cm −1 and 1650 cm −1 are the stretching and bending vibration peaks of -OH, respectively, and the peak near 1400 cm −1 is related to the asymmetric stretching vibration of interlayer CO 3 2− . In the low-frequency region, the peaks of 500-900 cm −1 can be regarded as metal-oxygen-metal and oxygen-metal-oxygen bending vibration peaks 25 . The above conclusions indicate that two LDHs materials were successfully prepared. Figure 2a is the SEM picture of ZnAl-LDHs. It can be seen from the picture that ZnAl-LDHs present a regular hexagonal sheet structure with the size of 0.3-0.5 μm, and the sheet layer is relatively thin, which is similar to the general LDHs structure 26 . Figure 2b,c are SEM pictures of ZnNiAl-LDHs. The pictures show that ZnNiAl-LDHs are composed of a large number of uniform, well-dispersed microspheres with an average diameter of 1-2 μm. Figure 2d,e are TEM images of a single microsphere of ZnNiAl-LDHs, which are consistent with the SEM image observed in Fig. 2c. The microsphere is formed by nanosheets growing from the center to the spherical surface. At the same time, the lattice fringes of the material can be clearly observed through the high magnification transmission electron microscope image of Fig. 2f. The above characterization results indicate that the prepared ZnNiAl-LDHs crystal structure is complete and the crystallinity is excellent, which is consistent with the XRD characterization. Figure 3a is the nitrogen adsorption-desorption isotherms of ZnAl-LDHs and ZnNiAl-LDHs. The structure of two materials are similar, and the adsorption and desorption isotherms are typical type IV isotherms, which belong to mesoporous adsorption materials 27 . The hysteresis loops of two materials both appear in the higher relative pressure range, which are of type H 3 , indicating that there are slit holes formed by the accumulation of flake particles, so the two materials may be flake or layered materials similar to clay 28 . As can be seen from the pore size distributions diagram in Fig. 3b, the pore size distributions of ZnAl-LDHs and ZnNiAl-LDHs are relatively wide, ranging from 5 to 45 nm and 5 to 25 nm, respectively. Therefore, ZnNiAl-LDHs has a more uniform pore structure. In addition, the BET characterization parameters are shown in Table 1, the specific surface area, pore volume and pore diameter of ZnNiAl-LDHs are 102.02 m 2 /g, 0.346 cm 3 /g and 13.573 nm, respectively. While the specific surface area, pore volume and pore size of ZnAl-LDHs are 18.49 m 2 /g, 0.075 cm 3 /g and 16.115 nm, respectively, suggesting that the microsphere structure of ZnNiAl-LDHs has richer spatial structure and can provide more binding sites than the flake structure, which is beneficial to show its excellent adsorption performance.
Effect of initial pH. In the adsorption of U(VI) by LDHs, the initial pH in the solution is an important influencing factor, so the adsorption of U(VI) on LDHs under different pH conditions were carefully investigated in this experiment. Figure 4a shows that ZnAl-LDHs and ZnNiAl-LDHs have similar adsorption trends in the pH range of 3 to 9. Therefore, ZnNiAl-LDHs were selected for Zeta potential analysis to investigate the relationship between the species distribution of uranium in solution at different pH and the surface charge properties of the adsorbent, and the Zeta potential of ZnNiAl-LDHs and U(VI) species distribution with pH are shown in Fig. 4b,c.
When the pH is 3 ~ 4, due to the high buffering capacity of LDHs, adsorbent will partially dissolve and release OH − to increase the pH. At the same time, H + will also compete with U(VI) for adsorption 29 , so the adsorption www.nature.com/scientificreports/ capacity is reduced. With the increase of pH, the removal efficiency of U(VI) increases rapidly. The adsorption capacities of ZnAl-LDHs and ZnNiAl-LDHs both reach the peak value at pH 6, and then gradually decrease.
The main reason for this phenomenon may be that at pH < 6, the uranium in solution exists mainly as UO 2 2+ , UO 2 OH + and (UO 2 ) 3 (OH) 5 + . As the pH increases, the deprotonation of LDHs increases, and the positive charge on the surface is relatively less, and the repulsion of the adsorbent from U(VI) decreases, so the adsorption capacity gradually increases 30 . When the pH is greater than 6, U(VI) mainly exists in the form of negative complexes such as (UO 2 ) 3 (OH) 7 − and UO 2 (OH) 3 − and carbonate complexes such as UO 2 (CO 3 ) 2 2− and UO 2 (CO 3 ) 3

4−31
. These compounds can exist stably in the solution and are difficult to be adsorbed by adsorbents, resulting in the decrease of adsorption capacities, suggesting that the adsorption process is mainly controlled by the complexation reaction of U(VI) with surface functional groups rather than electrostatic forces. Therefore, in subsequent www.nature.com/scientificreports/ experiments, ZnAl-LDHs and ZnNiAl-LDHs were adsorbed at pH 6. On the whole, the adsorption performance of ZnNiAl-LDHs is better than that of ZnAl-LDHs. The reason is that ZnNiAl-LDHs has a larger specific surface area, so it has better pH buffering capacity and adsorption capacity.

Effect of contact time and adsorption kinetics.
Contact time is a key factor affecting the mass transfer rate of U(VI) during adsorption. In order to determine the adsorption equilibrium time and perform adsorption kinetic analysis, an investigation of the effect of contact time on adsorption performance was conducted. It can be seen from Fig. 5 that in the first 2 h, the adsorption of uranyl ions by ZnAl-LDHs and ZnNiAl-LDHs increase rapidly. This phenomenon is due to the fact that in the initial stage of adsorption, the surface of LDHs provides a large number of active sites to combine with U(VI) 32 . As the reaction time increases, the available binding sites gradually decrease. U(VI) needs to slowly diffuse to the inner surface, causing the adsorption rate to gradually slow down. And finally the adsorption capacities of ZnAl-LDHs and ZnNiAl-LDHs for uranyl ions reach the adsorption equilibrium at 8 h and 6 h, respectively. Since the adsorption capacity of ZnNiAl-LDHs was much larger than that of ZnAl-LDHs, ZnNiAl-LDHs was chosen as the object of study in the subsequent experiments to investigate the effects of various factors on the adsorption performance of LDHs for uranyl ions and to probe the mechanism of the interaction between LDHs and U(VI). In order to describe the relationship between adsorption capacity and contact time and to analyze the adsorption mechanism, the pseudo-first-order kinetic model, the pseudo-second-order kinetic model and Elovich model are used to fit the adsorption process of LDHs on U(VI), and the three models are expressed as follows: pseudo-first-order kinetic model: pseudo-second-order kinetic model: Elovich model:  www.nature.com/scientificreports/  www.nature.com/scientificreports/ where k 1 (min −1 ) and k 2 (g/(mg·min)) are the rate constants of two models, α (mg/g × min) and β (mg/g) are the Elovich model constants, q t (mg/g) is the adsorption capacity at time t and q e (mg/g) is the adsorption capacity at equilibrium. Figure 6 show the ftting curves of the pseudo-first-order kinetic model (a), pseudo-second-order kinetic model (b) and Elovich model (c), the related parameters are listed in Table 2. The R 2 value of the pseudo-secondorder kinetic model is higher than those of the pseudo-first-order kinetic model and Elovich model, moreover, the theoretical equilibrium adsorption capacity (303.95 mg/g) of ZnNiAl-LDHs calculated by the pseudo-secondorder kinetic model is closer to the real experimental result (278.26 mg/g), implying that the pseudo-secondorder kinetic model can better describe the kinetic process of adsorption of uranium ions by ZnNiAl-LDHs, indicating the adsorption process of the adsorbent is dominated by chemisorption on a homogeneous surface 33,34 . Adsorption isotherms. The adsorption isotherm is a curve describing the change of adsorption capacity with equilibrium concentration at a certain temperature. In order to further evaluate the adsorption capacity of  Table 2. The adsorption kinetic parameters of the adsorption of uranyl ions by ZnNiAl-LDHs. The units of q e (mg/g), k 1 (min −1 ), k 2 (g/(mg·min)), α (mg/g × min), β (mg/g).

Pseudo-first-order model
Pseudo-second-order model Elovich model www.nature.com/scientificreports/ ZnNiAl-LDHs for U(VI), the adsorption isotherms of the initial concentration of uranium solution in the range of 10 ~ 80 mg/L were studied at 293 K, 298 K and 303 K, respectively. It can be seen from the Fig. 7 that in the initial stage, as the concentration increases, the adsorption capacity of ZnNiAl-LDHs on U(VI) increases rapidly. This is because under the force of the concentration gradient, the adsorbent can more fully contact U(VI), the adsorption sites on the surface are easier to bind to U(VI). However, when the initial concentration continues to increase, the adsorption capacity of ZnNiAl-LDHs on U(VI) tends to balance, the reason is that adsorption site of the materials is almost completely occupied, and the adsorption reaches saturation 35 . Langmuir model, Freundlich model and Temkin model were used to fit the adsorption isotherms, the three models are expressed as: Langmuir model: Freundlich model: Temkin model: where q e (mg/g) is the adsorption capacity of ZnNiAl-LDHs in adsorption equilibrium, q m (mg/g) is the theoretical maximum adsorption capacity, k L (L/mg) is the adsorption constant of Langmuir model, k F ((mg/g) (L·mg) 1/n ) and n are the adsorption constants related to the adsorption capacity and adsorption strength of Freundlich model, respectively. A T (L/mg) and b T (J/mol) are Temkin model constants.
The ftting results of Langmuir model, Freundlich model and Temkin model are shown in Fig. 8, and the parameters fitted by three models are shown in Table 3, the Langmuir model (R 2 > 0.99) can better describe the adsorption data, indicating that adsorption process is a homogeneous surface adsorption process dominated by monolayer adsorption 36 . In addition, under the same other conditions, the adsorption capacity of ZnNiAl-LDHs increases significantly with the increase of temperature, which illustrates that the adsorption of U(VI) by ZnNiAl-LDHs is an endothermic reaction. Therefore, a proper temperature rise is conducive to the progress of adsorption.
Adsorption mechanism. In order to explore the adsorption mechanism of ZnNiAl-LDHs on U(VI), EDS, FT-IR and XPS were analyzed. Figure 9 shows the EDS patterns of ZnNiA1-LDHs before and after the adsorption of U(VI). Compared with Fig. 9a, the content of U appears in Fig. 9b, confirming that ZnNiA1-LDHs has an adsorption effect on U(VI). Figure 10a is XPS full scan spectrum before and after ZnNiA1-LDHs adsorption of U(VI), the results show that material surface contains C, O, Zn, Ni and Al elements, and the adsorbed material shows the characteristic peaks of U(VI), indicating that the material has adsorbed U(VI). The U 4f spectrum in Fig. 10b shows two characteristic peaks at 381.5 eV and 392.3 eV, which correspond to U 4f 7/2 and U 4f 5/2 , respectively 37 . In addition, the O 1s and C 1s of ZnNiA1-LDHs were subjected to peak separation processing, and the results were shown in Fig. 10c,d. The O 1s spectrum before adsorption can be divided into two parts, M-O (metal containing oxygen bond) and -OH at 531.21 eV and 531.80 eV, respectively 38 . Compared with the O 1s spectrum before adsorption, the peaks after adsorption move to higher binding energy and the occupied peak area is reduced, confirming that M-O and -OH in ZnNiA1-LDHs have reacted with U(VI). In addition, the Similarly, the C 1 s spectrum shows HCO 3 − and CO 3 2− related groups at 284.65 eV and 289.06 eV, and about 285.0 eV corresponds to the C elemental calibration energy peak 40 . The changes in the positions of above two peaks and the weakening of the peak intensities indicate that HCO 3 − and CO 3 2− between the ZnNiA1-LDHs layers are chelated with U(VI) 41 . In addition, according to Fig. 10e, it is found that the structural element of adsorbed ZnNiA1-LDHs may have changed. Ni 2p peaks are observed at 855.91 eV (Ni 2p 3/2 ) and 873.59 eV (Ni 2p 1/2 ), and the small peaks are related satellite peaks 42,43 . The positions and peak intensities of Ni 2p peaks have changed after adsorption, indicating the changes in the local bonding environment and chemical structure. The reason may be that Ni 2+ has the same valence state and similar ionic radius as UO 2

2+
, so UO 2 2+ may have ion exchange with the nickel oxygen bond on the surface of ZnNiA1-LDHs 44,45 .   www.nature.com/scientificreports/ Figure 10f shows the FT-IR spectra of ZnNiAl-LDHs before and after adsorption. The spectrum shows that the adsorbed ZnNiA1-LDHs still have the characteristic peaks of LDHs, and the characteristic peak of UO 2 2+ appears at about 830 cm −146 , indicating that the material successfully adsorbs U(VI). In addition, the peak intensities of the characteristic peaks corresponding to -OH, -CO 3 2− and M-O are all weakened and the peak positions are shifted, which may be due to the reaction with U(VI). The SEM images of ZnNiAl-LDHs after adsorption are shown in Fig. 11a,b. The material still maintains a good microsphere structure, but a few irregular nanosheets appear on the surface, probably due to the reaction of the constituents of LDHs with U(VI), which causes some structural damage. Combined with FT-IR and SEM analysis, it can be seen that ZnNiAl-LDHs still maintain their original structure after adsorption, meaning that the material has great stability.
Based on the above analysis, this study proposes that the potential mechanism for removing U(VI) may be the complexation of U(VI) with surface hydroxyl groups, chelation with interlayer functional ligands and ion substitution.
Effect of temperature and thermodynamic data. In order to discuss the adsorption performance of ZnNiAl-LDHs for U(VI) from the perspective of thermodynamics, different adsorption temperatures were selected for research to determine the type of adsorption reaction (endothermic or exothermic). As shown in Fig. 12, when T < 303 K, with the increase of temperature, the adsorption capacity shows an upward trend. The main reason is that the increase in temperature aggravates the movement of molecules and promotes the contact of U(VI) with active sites on the surface of ZnNiAl-LDHs 47 . When T = 303 K, the adsorption capacity reaches the maximum, and the U(VI) in original solution is almost completely adsorbed. When T > 303 K, as the temperature increases, adsorption capacity basically keeps the peak value, so 303 K is selected as the optimal adsorption temperature. Thermodynamic parameters such as enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG) are used to describe the process. These parameters can be obtained according to the following equations: where R (8.314 J/(mol·K)) is the ideal gas constant, T (K) is the reaction temperature, and K d is the solid-liquid partition coefficient; the calculated slope and intercept of the curve of lnK d versus 1/T correspond to ∆H 0 /R and ∆S 0 /R, respectively. The ΔG 0 value is calculated by the Eq. (10), and the thermodynamic parameters are shown in the Table 4: According to the data in Table 4, the positive value of ∆H 0 indicates that the adsorption process is endothermic, and this result is also consistent with the analysis of adsorption isotherms. The negative value of ΔG 0 means that the adsorption of U(VI) on ZnNiA1-LDHs is spontaneous 48 . In addition, the positive value of ∆S 0 reveals that the adsorption process is proceeding in the direction of increasing disorder 49 . In this study, the potential adsorption mechanism of ZnNiA1-LDHs to remove U(VI) in the solution is ion substitution, the material structure changes, which leads to more chaos in the entire system, this analysis is also in line with the previous analysis results.

Effect of cations.
In the presence of cations such as Na + , K + , Mg 2+ and Al 3+ , the adsorption capacity of ZnNiA1-LDHs for U(VI) in the solution is shown in Fig. 13. The result shows that in most cases, ZnNiA1-LDHs has excellent selectivity to UO 2 2+ , which is attributed to the fact that Na + , K + and Al 3+ have different valence states and ion radii compared with UO 2 2+ , but Mg 2+ has the same valence state and similar ionic radius as Ni 2+ and UO 2

2+
. According to the literature, the main laminate of LDHs is formed by the co-edge of octahedra of divalent and trivalent metal ions, so the structural stability of the minimum structural motif MO 6 octahedra may determine the stability of the main laminate, and the introduction of Mg 2+ into the LDHs materials can obtain a stable laminate structure. Therefore, Mg 2+ may partly exchange with the nickel-oxygen bonds on the surface of ZnNiA1-LDHs, thereby being introduced into the hydrotalcite structure, occupying adsorption sites, affecting the adsorption of UO 2 2+ in the solution by the adsorbent, resulting in a decrease in uranium adsorption 50,51 . www.nature.com/scientificreports/ Reusability of adsorbent. In practical applications, the regenerability of adsorbents is an important basis for evaluating materials. 0.1 mol/L Na 2 CO 3 solution was selected as the desorption agent, mixed with the adsorbed ZnNiA1-LDHs and shaken for 12 h, washed with deionized water to neutrality, and the dried material was subjected to repeated adsorption experiments. As shown in Fig. 14, after 4 cycles, the removal rate of U(VI) by ZnNiA1-LDHs still maintained more than 80%, proving that the material has good regeneration performance and recyclability. www.nature.com/scientificreports/

Conclusion
ZnNiAl-LDHs with microspherical structure were prepared by a simple hydrothermal synthesis method, and the microspheres had uniform particle size and good dispersibility. The adsorption properties of ZnNiAl-LDHs and ZnAl-LDHs for U(VI) were studied, experimental results showed that ZnNiAl-LDHs exhibited better adsorption capacity for U(VI) due to the larger specific surface area and more active sites in the microsphere structure of ZnNiAl-LDHs. The adsorption of ZnNiAl-LDHs on U(VI) reached equilibrium within 6 h, and the fit of experimental data was consistent with the pseudo-second-order kinetic model, while the Langmuir isotherm model fitted the adsorption isotherm well, indicating that the adsorption process was dominated by monolayer coverage and chemosorption. The thermodynamic parameters indicated that the adsorption of U(VI) by ZnNiAl-LDHs was an endothermic and spontaneous process. In addition, the presence of common co-existing cations in the solution had no obvious effect on the removal of U(VI) in most cases, indicating that the adsorbent had good selectivity for U(VI). Moreover, ZnNiAl-LDHs could still exhibit good adsorption performance after several cycles. Finally, the adsorption mechanism was explained by FT-IR and XPS characterization. The result showed