Introduction

Uranium is a naturally-occurring actinides element, attracting huge attention since the utilization of nuclear energy. Due to extreme energy release via the fission reaction, uranium has been well known as a superior nuclear fuel [1]. Uranium of the hexavalent state is mainly in the formation of uranyl, which is highly mobile in aqueous environment [2]. Once enterring into human body, it would result in seriously irreversible injury because of heavy metal and radioactive toxicity. To avoid the potential risk, the US Environmental Protection Agency require the safe uranium concentration less than 20 µg/L [3]. To meet the requirement, treating uranium-containing wastwater is thus of significance before the discharge. Nowdays, the techniques solving heavy metal contamination mainly involve adsorption, solvent extraction, precipitation and membrane. Adsorption highlights feasibility, low secondary pollution and abundant adsorbents. Commonly used adsorbents include clay [4], metal oxide [5], polymer [6], and active char [7].

Layered double hydroxides (briefly as LDHs) is promising in the field of removal of heavy metal due to superior surface area, thermal stability and adjustable structure. Its chemical formula can be described as [M2 + 1–xM3+x(OH)2]x+(An–)x/n⋅mH2O (M2+, M3+ and An– mean divalent, trivalent cations and interlamination anions). In the past, most studies paid large attention to the removal performance for various anions [8] and organic molecules [9] of LDHs by taking advantage of hydrophilicity and exchangeable anion of LDHs. For example, Zubair et al. utilized biochar modified MgAl-LDH to adsorb an organic compound, methylene blue. The results proved the adsorption amount reached 302.75 mg/g [10]. Jung et al. found that Mg-Al layered double hydroxides-functionalized hydrochar composite possessed certain adsorption capacity toward anion, 16.222 mg/g for arsenate and 20.265 mg/g for phosphate [11]. Shi et al. used glycine-modified Fe/Cu-layered double hydroxides as a specific As(V) adsorbent. The adsorbent could uptake As(V) in chemical way with a maximal adsorption capacity of 808.71 mg/g [12]. The compounds composed of M-O bonds have been used as various cations adsorbents. For example, MnO2 is able to complex with lead (II), copper (II), zinc (II), uranium and Cd(II) [13,14,15]. It is noteworthy that there are plenty of M-O bonds in the LDHs structure. The bond has extreme potential to complex uranium.

In addition, LDH with unique layered characteristic is a useful raw material to prepare layered double oxides (LDO). LDO yielded from calcining LDH achieves stronger adsorption performance due to (1) sufficient expose of metal atom sites; (2) high specific surface area; and (3) superior scatter than original LDH. LDO thus have gained ascending attention in the fields of catalysis and adsorption. Hong et al. CoMgAl-LDO and CoMgFe-LDO had excellent catalyst performance toward atrazine and carbamazepine because of the expose of three metal atoms [16, 17].

To promote the adsorption capacity of LDH and LDO, much attention has been paid on modification with various chemically foreign objects. For example, Deng et al. increased the congo red adsorption amount of layered double-oxide from 166 mg/g to 344.83 mg/g by coating biochar [18]. Hou et al. calcinated the humic acid loading MgAl-layered double hydroxide into magnetic layered double oxide/carbon. The results proved that the product showed excellent adsorption efficiencies, able to adsorb 386.1 mg/g Cd(II), 359.7 mg/g Pb(II), and 192.7 mg/g Cu(II), respectively [19]. Wu et al. used rare-earth oxides such as La2O3, Sm2O3, and Er2O3 to modify Mg-Al layered double oxides. The consequent material showed higher adsorption-photocatalytic activity toward tetracycline hydrochloride. The removal efficiency reached 99.87% within 110 min under initial TCH concentration of 100 mg/L [20]. Although the post-modification is effective for improving adsorption ability, it introduces other chemical additives and meanwhile increases preparation process step, bringing more pollution and cost.

In the study, two layered double hydroxides (O-MgAl-LDHs and F-MgAl-LDHs were prepared, where “O” and “F” denotes oven and freezing processing methods, respectively) and two bimetallic oxides (O-MgAl-LDO and F-MgAl-LDO) through two processing technique. To our knowledge, there is no detailed data about the adsorption performance relevant with the dry route. The results in the study proved that the freezing process produced larger specific surface area and pore size than the oven process. The difference led to different adsorption performance. Effects of pH, shaking time, ionic strength, initial uranium concentration and temperature on the adsorption of uranium by O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO were illustrated. The adsorption mechanism was explained by isotherms, kinetics, thermodynamic models and instrument characteristics.

Materials and methods

Reagents and instruments

Magnesium nitrate (Mg(NO3)2·6H2O), aluminium nitrate (Al(NO3)3·9H2O), sodium hydroxide (NaOH), arsenazo III, chloroacetic acid (ClCH2COOH) and anhydrous sodium acetate (CH3COONa ) of analytical grade were received from Aladdin Reagent (Shanghai) Co., Ltd, China and directly used without further purification. The sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3) and nitric acid (68%) of analytical grade were provided by Sinopharm Chemical Reagent Co., Ltd. Triuranium octoxide (U3O8) was provided by Quanxin Chemical Industry Co., Ltd.

Scanning electron microscope (SEM) analyses was conducted with Nova Nano SEM 450. The internal structure and crystallinity of layered bimetallic (hydrogen) oxides was recorded using by X-ray diffractometer (D8 ADVANCE). Fourier transform-infrared spectra was obtained using Nicolet-IS10. Surface area and porosity distribution were analyzed using N2 adsorption-desorption isotherms (ASAP2460). Thermal stability was analyzed by thermogravimetric (TG, Nestal) with a heating rate of 10 oC/min from room temperature up to 750 oC. XPS (Thermo ESCALAB 250XI) was used to analyzed the adsorption mechanism.

Preparation of layered bimetallic (hydrogen) oxides

The layered double hydroxide, MgAl-LDHs, was prepared using a modified co-precipitation method [21], which is highly feasible and able to produce a resultant with a uniform internal lattice. In detail, Mg(NO3)2·6H2O of 12.82 g and Al(NO3)3·9H2O of 3.75 g was dissolved in ultrapure water of 200 mL, followed by addition of the Na2CO3-NaHCO3 buffer. The consequent mixture adjusted to pH 7.0 was stirred for 12 h to make sufficient precipitation reaction. By filtration, the solid product was collected. A part of the solid product was oven-dried at 60 oC for 10 h produce O-MgAl-LDHs. The other was freeze-dried to fabricat F-MgAl-LDHs. The parameters of the freezing-dry were set as the temperature − 78 oC, the pressure 7 Pa and the duration time 10 h. Further calcination at 550 oC transferred O-MgAl-LDHs and F-MgAl-LDHs into the corresponding products, O-MgAl-LDO and F-MgAl-LDO.

Adsorption experiments

The adsorption experiments were conducted with a static batch strategy. In addition, the duplicate parallel experiments were executed to ensure the data no more than 5% relative error. A typcial adsorption operation was as the following. 5.0 mg adsorbent substance (O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs or F-MgAl-LDO) and 50 mL uranium-containing solution of 50 mg/L was mixed in a 250 mL conical flask. Subsequently, the flask was fixed in a thermostatic culture oscillator and oscillated at a constant temperature for a given period. After the adsorption contact, 2.0 mL suspension was taken out and treated by high-speed centrifuge of 9000 r/min. The supernatant was measured to determine the concentration of uranium by a UV-vis spectrophotometry at 660 nm with the standard arsenazo III method. The adsorption amount (mg/g), i.e. the weight of uranium adsorbed onto the adsorbents, was calculated out using Eq. (1)

$${q}_{\text{e}}=\frac{({C}_{0}-{C}_{\text{e}})V}{m}$$
(1)

where C0 (mg/L) and Ce (mg/L) are the initial and final uranium concentration, respectively; V (L) is the volume of uranium-containing solution; m (g) is the mass of O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs or F-MgAl-LDO.

Results and discussion

Characterization

Microstructure of O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO are displayed in Fig. 1. One can see that layered double hydroxide and bimetallic oxide were heaps of many thin slices. In comparison, F-MgAl-LDHs and F-MgAl-LDO showed petal-shaped structure and more uneven surface than O-MgAl-LDHs and O-MgAl-LDO. The thermogravimetric data proved O-MgAl-LDH included more moisture content than F-MgAl-LDH. Thus crystalline grains of O-MgAl-LDH tended to form a monoblock, which exposed few active adsorption sites for uranium. On the contrary, F-MgAl-LDH with smaller particle size possessed a larger specific surface area, providing more effective adsorption sites.

Fig. 1
figure 1

SEM images of O-MgAl-LDHs (a), O-MgAl-LDO (b), F-MgAl-LDHs (c) and F-MgAl-LDO (d)

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Figure 2(a) shows four XRD patterns of layered double hydroxide and bimetallic oxide. Most important of all, the characteristic (003) and (006) peaks of the prepared O-MgAl-LDHs and F-MgAl-LDHs highly fitting with the standard card ((Mg4Al2)(OH)12CO3·3(H2O))0.5 (PDF:70-2151) as well as O-MgAl-LDO and F-MgAl-LDHs and F-MgAl-LDO perfectly fitting the standard card MgO (PDF:45–0946) indicated that four bimetallic product were the design compound. In addition, O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO were single crystal structure of high purity in view of the narrow and sharp peak.

It can be seen the crystal plane spaces of O-MgAl-LDHs and F-MgAl-LDHs were determined as 8.07 Å and 8.16 Å calculated from D003. The larger plane space of the latter was attributed to the freeze-drying way, in which water molecule left off the solid structure by ice sublimation. Moreover, the average particle size (7.14 nm) of the latter, less than 7.63 nm of the former, proved that the freeze-drying technique resulted in finer particle and larger specific surface area, which was profitable in the adsorption process.

Figure 2(b) shows four FT-IR patterns of layered double hydroxide and bimetallic oxide. The peaks in the wavelength range of 450–735 cm-1 observed in O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO patters were assigned as M-OH and M-O (M = Al or Mg) lattice vibrations. However, the strong and broad peak at 3550–3735 cm-1 well known as hydroxyl groups and the characteristic peak at 1640 cm-1 assigned as the bending vibration of interlayer water molecules [22] appeared in O-MgAl-LDHs and F-MgAl-LDHs patters and disappeared in O-MgAl-LDO and F-MgAl-LDO patters. The difference was resulted from the the calcination.

The N2 adsorption-desorption isotherms (Fig. 2c) was used to characterize O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO. The type IV isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification indicated existance of micropores and mesopores in the prepared substances. Besides, the specific surface area of F-MgAl-LDHs (117.46 m2/g) is 4.3 times larger than that of O-MgAl-LDHs (27.41 m2/g). And F-MgAl-LDO has the largest specific surface area (163.15 m2/g), also bigger than that of O-MgAl-LDHs (88.97 m2/g). The reason why the freeze-drying process resulted in bigger specific surface area may be that the oven-drying treatment caused structural collapse due to the fierce escape of water molecules while the freeze-drying process had no such adverse influence.

The embedded figure in Fig. 2c showed the average pore sizes of O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO were 23.62, 22.47, 25.61 and 23.93 nm, respectively, indicating mesopores pores were dominant.

The mass loss of O-MgAl-LDHs in the first stage (Fig. 2d) was 14.9%, 5.9% higher than that of F-MgAl-LDHs (9.0%), indicating a higher dry extent made by freeze dry. In the second stage, two mass losses were similar, indicating the identical chemistry component. The TG results indicated that the drying method would not vary the main components except moisture content.

Fig. 2
figure 2

XRD (a), FT-IR patterns (b), N2 adsorption-desorption isotherms (c) and TG curves (d)

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Effect of pH and ionic strength

Effect of pH and ionic strength on the adsorption of uranium onto O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs or F-MgAl-LDO were studied (Fig. 3). qe for O-MgAl-LDHs and O-MgAl-LDO increased as pH increased from 3.0 to 5.0. Further increase in pH to 7.0 lead to decrease in qe. Thus the optimal pH for the uranium adsorption by O-MgAl-LDHs or O-MgAl-LDO were determined as 5.0. The similar tendency were observed for the adsorption by F-MgAl-LDHs or F-MgAl-LDO except the the optimal pH of 5.5.

Two factors are the main reason for this adsorption phenomenon. (1) One factor is the H+ concentration in solution, affecting the uranium species distribution (Fig. 3d) and the protonation degree of the adsorbents; (2) The other is surface charge of the adsorbents (Fig. 3c

Fig. 3
figure 3

(a, b) The effect of pH on adsorption under different ionic strengths; (c) Zeta potential; (d) uranium ion form distribution

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O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs or F-MgAl-LDO were positively charged as pH < 5.5 but the charge degree decreased as pH increased. On the other hand, the protonation degree of the adsorbents also declined as pH increased. Both explained qe plots as pH < 5.0 or 5.5. As pH beyond 5.0, the dominant uranium species was complex state of uranyl, which was not favorable for the adsorption.

The effects of ionic strengths controlled by 0–0.1 M NaClO4 on the uranium adsorption were also studied. One can see that adsorption amount of uranium by MgAl-LDHs and MgAl-LDO were almostly not influenced by the NaClO4 concentration. It can be concluded that uranium uptake onto O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs or F-MgAl-LDO were regarded as the inner spherical surface complexation.

Adsorption kinetics

Adsorption kinetics was usually used to judge the adsorption rate and to evaluate the practical application potential of adsorbents. In order to further understand the adsorption process and mechanism of uranyl ions on O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO. The adsorption of uranyl on O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO varying with shaking time were fitted using the pseudo-first-order, pseudo-second-order and intra-particle diffusion model [23], which were expressed as Eq. 2, Eq. 3 and Eq. 4.

$${q}_{t}={q}_{e}(1-{e}^{-{k}_{1}t})$$
(2)
$${q}_{t}=\frac{{q}_{e}^{2}{k}_{2}t}{1+{q}_{e}{k}_{2}t}$$
(3)
$${q}_{t}={k}_{i}{t}^{1/2}+C$$
(4)

where qt (mg/g) and qe (mg/g) are the uranium adsorption amount of materials at time t and equilibrium, respectively; k1(min-1), k2(g/(mg·min)) and ki(mg/ g·min1/2) are the adsorption rate parameters, respectively; C is constant.

The curved fit are conducted in Fig. 4a,b. The calculated parameters are listed in Table 1. One can see that the values of correlation coefficients of the pseudo-second-order model are greater than the values of the Pseudo-first-order and Intra-particle diffusion models. Thus the pseudo-second-order model was appropriative for the reactions between uranyl and O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs or F-MgAl-LDO, verified by the highest relevant coefficients, 0.991, 0.997, 0.993 and 0.987. Furthermore, the adsorption amount of uranium on O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO, 347.98 mg/g, 610.90 mg/g, 588.98 mg/g and 968.11 mg/g, respectively determined from pseudo-second-order model were approximate to the experiment values, 338.23 mg/g, 586.97 mg/g, 565.80 mg/g and 882.60 mg/g. Therefore, the adsorption of O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO toward uranyl were identical with the assumption of the pseudo-second-order kinetic model, indicating the adsorption were controlled via chemisorption

Fig. 4
figure 4

(a) Effect of time on adsorption and fitting with pseudo-first-order and pseudo-second-order models; (b) Fitting with intra-particle diffusion model

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Table 1 Model parameters of pseudo-first-order, pseudo-second-order and intra-particle diffusion
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Adsorption isotherms

The adsorption isotherm is often used to judge the adsorption nature and to calculate the adsorption capacity. In the study, Langmuir, Freundlich and Dubinin-Radushkevich isotherm models were utilized to fit the isotherm data (Fig. 5).

The Langmuir model (Eq. 5) supposes that the adsorbates adsorbed in the monolayer way on the homogeneous surface of adsorbent [24]. The Freundlich model (Eq. 7) is an empirical relationship describing the quantity of adsorbate on an adsorbent [25]. The Dubinin-Radushkevich isotherm model (Eq. 8) is able to estimate the mean free energy of adsorption [26].

$${q}_{e}=\frac{{K}_{L}{q}_{m}{C}_{e}}{1+{K}_{L}{C}_{e}}$$
(5)
$${q}_{e}={K}_{F}{C}_{e}^{1/n}$$
(7)
$${q}_{e}={q}_{m}{e}^{-{K}_{DR}{\epsilon }^{2}}$$
(8)
$$\epsilon =RT{ln}(1+\frac{1}{{C}_{e}})$$
(9)
$$E=\frac{1}{\sqrt{-2{K}_{DR}}}$$
(10)

where qe (mg/g) and qm (mg/g) are the adsorption capacity at equilibrium and the maximum adsorption capacity, respectively; C0 (mg/L) and Ce (mg/L) are the initial and equilibrium uranium concentrations, respectively; KL (L/mg), KF[(mg/g) (L/mg)1/n] and KDR((mol/kJ)2) are constants of Langmuir, Freundlich and Dubinin-Radushkevich models, respectively; n a characteristic constant related to the adsorption strength; ε (kJ/mol) a constant of the Dubinin-Radushkevich isotherm model. E (kJ/mol) is the mean free energy. If E value locates between 1.0 and 8.0 kJ/mol, the adsorption is physical; If E value belongs to the range of 8.0–16 kJ/mol, the adsorption is chemical in nature [27].

Table 2 listed the parameters’ value of three isotherm models above. Higher correlation coefficient R2 of 0.997, 0.992, 0.996 and 0.997 were achieved via fit the adsorption of O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO with the Langmuir model than that of Freundlich and Dubinin-Radushkevich models. In addition, the theoretical maximum adsorption capacities of O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO calculated by Langmuir model were 476.94 mg/g, 777.91 mg/g, 764.16 mg/g and 1099.93 mg/g, respectively, close to the actual measured results, 406.79 mg/g, 696.04 mg/g, 666.04 mg/g and 1004.55 mg/g. The above fit results proved that the adsorption behavior conformed to the Langmuir assumption. The isotherm fit results demonstrated that the adsorption of uranium onto four adsorbents adopted monolayer way. The uranium adsorption amount of 1099.93 mg/g of F-MgAl-LDO (see the comparation in Table 3) ranked a very competitive position in the uranium specific adsorbents. The outstanding performance verified benefit of the freeze-dry processing for the adsorption.

Table 2 Isotherms parameters of Langmuir, Freundlich and Dubinin-Radushkevich models
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Table 3 The maximal adsorption amount of F-MgAl-LDHs, O-MgAl-LDHs, F-MgAl-LDO, O-MgAl-LDO and some other typical adsorbents
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$${R}_{L}=\frac{1}{1+{K}_{L}{C}_{0}}$$
(6)

The no-dimensional factor of the Langmuir isotherm model, RL, can determine the nature of the adsorption process. The adsorption was determined as irreversible adsorption when RL = 0, as favorable adsorption when 0 < RL < 1, as linear adsorption when RL = 1 and as adverse adsorption when RL >1 [39]. RL values of the adsorption of O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO are listed in Table 4. All RL values located between 0 and 1, indicating favorable adsorption.

Table 4 Langmuir isotherm model parameters, RL (T = 298 K)
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Fig. 5
figure 5

(a) Effect of initial uranium concentration on adsorption and fitting with Langmuir and Freundlich models; (b) Fitting with Duninin-Radushkevich isotherm model

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Effect of temperature

Effect of temperature on the adsorption of uranium on to O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO were investigated. The adsorption capacity were obviously influenced by the temperature (Fig. 6). As the temperature rose from 282.5 to 312.5 K, qe of O-MgAl-LDHs, O-MgAl-LDO, F-MgAl-LDHs and F-MgAl-LDO increased from 274.16 to 400.59 mg/g, 522.98 to 649.71 mg/g, 493.72 to 617.05 mg/g, 827.78 to 949.78 mg/g, indicating a higher temperature were beneficial to the adsorption.

The feasibility and mechanism of the adsorption reactions were evaluated by free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) [40]. The thermodynamic equations are given as the following.

$${K}_{d}=\frac{{q}_{e}}{{C}_{e}}$$
(11)
$${ln}{K}_{d}=\frac{\varDelta S}{R}-\frac{\varDelta H}{RT}$$
(12)
$$\varDelta G=\varDelta H-T\varDelta S$$
(13)

where Kd is the distribution coefficient (L/g), T and R represent the absolute temperature (K) and ideal gas constant (8.314 J/(mol·K)), respectively.

Table 4 Thermodynamic parameters
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ΔH and ΔS were calculated out by linear fit of ln Kd and 1/T. The corresponding ΔG at different temperatures were deduced using Eq. 13. Table 3 lists the values of ΔH, ΔS and ΔG. Negative values of ΔH, -13.947 kJ·mol-1 for O-MgAl-LDHs, -12.610 kJ·mol-1 for O-MgAl-LDO, -11.823 kJ·mol-1 for F-MgAl-LDHs, -31.566 kJ·mol-1 for F-MgAl-LDO meant the adsorption were exothermic. Positive values of ΔS, 60.360 (J·mol-1) for O-MgAl-LDHs, 64.411 (J·mol-1) for O-MgAl-LDO, 60.779 (J·mol-1) for F-MgAl-LDHs, and 142.798 (J·mol-1) for F-MgAl-LDO indicated increase in randomness during the adsorption processes. Negative values of ΔG illustrated that the influence of enthalpy changes were less than that of entropy change, revealing adsorption processes were spontaneous at each studied temperature. In addition, the value of ΔG decreasing with the increase in temperature indicated a higher temperature was favorable to the adsorption. The ΔG value of F-MgAl-LDO was significantly less than that of O-MgAl-LDHs, O-MgAl-LDO and F-MgAl-LDHs at the same temperature, highlighting superior uranium adsorption performance.

Fig. 6
figure 6

(a) Effect of temperature on adsorption; (b) Fitting with lnKd vs. 1/T

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Selectivity, elution efficiency and reusability

The freeze-dried F-MgAl-LDHs and F-MgAl-LDO show high adsorption capacity for uranium, which indicates that F-MgAl-LDHs and F-MgAl-LDO have the potential of selective adsorption of uranium. Therefore, a mixed solution containing Ni2+, Co2+, Sr2+, Zn2+, La3+, Gd3+, Ce3+, Sn4+ and UO22+ was treated with F-MgAl-LDHs and F-MgAl-LDO.

Uranium adsorption rates of F-MgAl-LDHs and F-MgAl-LDO in the mixed solution are as high as 84.67% and 96.15%, respectively (Fig. 7). In addition, F-MgAl-LDHs and F-MgAl-LDO have almost no adsorption for Ni, Co, Sr and Zn (less than 10%), suggesting that F-MgAl-LDO has a better selectivity for U(VI) than F-MgAl-LDHs. F-MgAl-LDO has a good selectivity for uranium, probably because its layer spacing matches the diameter of the uranyl ion, keeping the uranyl ion in interlamination.

The elution and reusability of F-MgAl-LDHs and F-MgAl-LDO was conducted by using alkaline solution as the eluent. 0.1 mol/L Na2CO3, NaOH and EDTA were able to eluate uranium from the used F-MgAl-LDHs and F-MgAl-LDO (Fig. 7(a)). Of the three eluents, Na2CO3 showed the best effectivity, with desorption efficiency of 97.84% and 98.52%, respectively. The possible reason may be that CO32- had the most affinity toward uranyl in the three eluents.

Figure 7(b) presents the reusability of F-MgAl-LDHs and F-MgAl-LDO by using Na2CO3 as the eluent. The five adsorption-desorption cycles made qe of F-MgAl-LDO decreases from 882.6 to 534.53 mg/g, a drop about 39.44% and qe of F-MgAl- LDHs from 565.8 to 285.13 mg/g, a drop about 49.60%. One interesting result should be found that although evident drop were found for both adsorbent, the adsorption capacity were still competitive.

Fig. 7
figure 7

The effect of coexisting cations on the adsorption on F-MgAl-LDHs (a) and F-MgAl-LDO (b) ; F-MgAl-LDHs and F-MgAl-LDO elution performance (c) and 5 times adsorption-desorption experiments (d)

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Adsorption mechanism

The FT-IR spectrum of the uranyl loaded F-MgAl-LDHs and F-MgAl-LDO (Fig. S1) exhibited the presence of O = U = O special characteristic peaks at 910 and 911 cm-1, indicating the complexation of the uranyl.

EDS-mapping images of various element distributions are given in Fig. S2. One can see that the distribution of main elements (O, Mg, Al, U) through the F-MgAl-LDHs and F-MgAl-LDO surface was homogeneous.

The atom composition and its surrounding environment of the uranium loading F-MgAl-LDHs and F-MgAl-LDO were studied by XPS. The mechanism of uranium adsorption was therefore deduced. The spectrum in Fig. 8(a) shows that the main elements in F-MgAl-LDHs and F-MgAl-LDO including C, O, Mg and Al, and an obvious U peak, indicating that uranium was adsorbed. The double peaks at 381.3 eV and 392.1 eV with difference of 10.8 eV in Fig. 8(b) was characteristic spectrum of uranium, assigned as U 4f7/2 and U 4f5/2, respectively [41].

Overall, the binding energies of C1s, O1s, Mg1s and Al2p increased after interacting F-MgAl-LDHs and F-MgAl-LDO with uranium. This may be because uranium combined with the interlayer anion (MOCO2-) and metal oxide (M-O) of the material to form (MOCO2)2UO2+ and (MO)2UO2+. In detail, O1s binding energy of F-MgAl-LDHs shifted 521.93 eV denoted as O-H bond to 532.13 eV after adsorption, while O1s binding energy of F-MgAl-LDO varied slightly, proving a stronger complex ability of O-H toward uranium [42]. In addition, binding energies of C1s, Mg1s and Al2p (Fig. 8(d,e,f)) increased slightly, illustrating the three atoms indirectly interacted with uranium [43]. Based on the results, the adsorption mechanism could be proposed as Eq. 14 and Eq. 15.

The bond energy of C-O bond in C1s after adsorption also increases (Fig. 8(d)), forming a stable C-O-U bond. The charge density on the C-O and O-H bonds is transferred to the C-C and M-O bonds, resulting in lower bond energies. The adsorption mechanism of uranium by F-MgAl-LDHs and F-MgAl-LDO includes co-precipitation and complexation on the inner surface of the sphere, and the possible complexation reaction is as the following.

$${2\text{M}\text{O}\text{C}\text{O}}_{2}^{-}+{\text{U}\text{O}}_{2}^{2+}\to {\left({\text{M}\text{O}\text{C}\text{O}}_{2}\right)}_{2}{\text{U}\text{O}}_{2}$$
(14)
$$2\text{M}\text{O}+{\text{U}\text{O}}_{2}^{2+}\to {\left(\text{M}\text{O}\right)}_{2}{\text{U}\text{O}}_{2}$$
(15)
Fig. 8
figure 8

XPS spectra of F-MgAl-LDHs and F-MgAl-LDO before and after U (VI) adsorption: (a) total spectrum; (b) U 4f; (c) C 1s; (d) O 1s; (e) Mg 1s; (f) Al 2p

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Conclusion

Mg-Al layered double hydroxides (LDH) (F-MgAl-LDHs and O-MgAl-LDHs) and Mg-Al layered double oxides (LDO) (F-MgAl-LDO and O-MgAl-LDO) nanosheets have been fabricated for the uranium adsorption. The adsorption as functions of pH, initial uranium concentration, contact time, temperature and coexistence ions were illustrated. The main results were as the following: (1) The freeze dry could significant promote the adsorption ability; (2) The optimal pH for the adsorption were at 5.0 ~ 5.5; (3) Of the four materials, the maximum adsorption capacity of F-MgAl-LDO reached 1099.93 mg/g, highlighted the freeze dry as a powerful craft of material process; (4) The adsorption conformed to the assumption of Langmuir and pseudo-second-order model, proving monolayer and chemical adsorption; (5) The adsorption process was spontaneous and endothermic; (6) M-O and C-O bonds participated the complex process in the uranium adsorption. The results proved qualification of F-MgAl-LDHs, O-MgAl-LDHs, F-MgAl-LDO and O-MgAl-LDO as candidate adsorbents for removal of uranium from wastewater and proposed the freeze dry as an efficient method to promote adsorbent performance.