Biomass-derived composite aerogels with novel structure for removal/recovery of uranium from simulated radioactive wastewater

KGM (Mw∼980 000) solution (10 g l⁻¹) was obtained by dissolving 1 g of KGM powders in the 100 ml of deionized water (DIW) by constantly stirring for 0.5 h. The homogeneous KGM–GO suspension was obtained by dispersing different amounts of GO (0, 0.1, 0.2 g) powders in 100 ml of as-prepared KGM solution by constantly stirring for 3 h .

KGM/GO composites were successfully prepared by bidirectional freezing technology in liquid nitrogen. After complete freezing, the as-prepared KGM/GO composites were dried in a vacuum for 7 days. The KGM carbon aerogels and KGM/GO composite aerogels were obtained by further pyrolysis in a tube furnace at 1200 °C under N₂ atmosphere. The resulting final product was referred to as KGCA-x, where x (0.1, 0.2) is the percentage weight content of GO to KGM. When the GO was omitted, the synthesized sample was referred to as KCA.

The structure of the KGCAs was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM. Crystalline structure and functional groups of KGCAs were characterized by Fourier-transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS) and x-ray diffraction. The degree of graphitization of KGCAs was detected by Raman spectroscopy. The specific surface area and the porous features of the KGCAs were investigated by nitrogen isothermal adsorption/desorption measurements on a QuadraSorb SI system (QuantachromeInstruments). The potential change of KGCAs was detected by a potentiometric titrator. The compressive tests of the KGCAs were executed on the material testing machine. The viscosity of KGM solution with mass concentration (2, 4, 6, 8, 10 g l⁻¹) was measured by an NDJ-1 rotary viscometer

Batch adsorption experiments were performed in a glass bottle including 5 mg of adsorbent and 20 ml of U(VI) solution. The experimental system studied the influence of conventional environmental factors, for example, the effect of pH value, contact time, temperature, and ionic strength on the KGCAs enrichment capability.

Simulated seawater: 17 mg of uranyl nitrate (UO₂(NO₃)₂.6H₂O), 25.6 g of sodium chloride (NaCl) and 193 mg of sodium bicarbonate (NaHCO₃) were dissolved in 1 L of deionized water to obtain the simulated seawater. The pH of the solution was adjusted to 8.3. Batch adsorption experiments were conducted according to the procedure in section 2.4. High salt concentration uranium solution was prepared by adding 4 g NaCl to 100 ml of a 100 mg l⁻¹ uranium solution.

The U(VI) adsorption with coexisting ions was conducted by adding a different proportion of cations or anions in the above glass bottle. For the effects of radiation, the KGCAs were exposed to different doses of γ irradiation for 24 h. The irradiated materials were then used for batch adsorption experiments. The effects of radiation on the adsorption properties of the materials were examined at different temperatures and pH values.

The adsorbents loaded with U(VI) were further treated by excess HCl (0.1 mol l⁻¹) for 4 h under an ultrasonication condition. Then, the adsorbents were further washed by DIW three times before next cycle enrichment experiment.

The synthesis of KCGAs is schematically shown in figure 1(a). To obtain an ideal lamellar structure, firstly we developed an optimized facile bidirectional freeze-casting technique to prepare lamellar KGM/GO nanocomposites with a homogeneous mixture of GO suspension and KGM as raw material (Details of the bidirectional technology are available in the supporting material, and figure S1 is available online at stacks.iop.org/NANO/30/455602/mmedia) [31]. Through this procedure, KGM/GO nanocomposites composed of parallel, aligned, and thin lamellas were readily obtained. The frozen 3D layered KGM/GOs were then freeze-dried to form the KGM/GO composite aerogels, which were finally carbonized in flowing N₂ to obtain a black KGCA (figures 1(a), S2, and S2). The as-prepared KCA and KGCAs had an ultralow density of 3.4–10.9 mg cm⁻³ which were measured using Archimedes' principle. Thus, it was so light it could stand on a blade of Asparagus setaceus (figure 1(f)). Besides, KGCA could be made into arbitrary shapes by existing silicone molds, for example the pyramids, horses, Eiffel Tower, heart-shaped, drop-shaped, and so on (figure 1(b)). This excellent feature of KGCA could provide unlimited possibilities in future applications.

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Figures 1(c)–(e) show that KGCA had a unique layered structure parallel to the growth direction, which minimizes capillary forces. This unique layered structure is very akin to the structure of mother-of-pearl shell, which greatly improves the mechanical properties of the KGCA. In addition, according to TEM characterization results, KGCA exhibited a lamellar microstructure corresponding to the results of the SEM. High-resolution transmission electron microscopy (HRTEM) exhibited that KGCA had a continuous porous structure, which could increase the specific surface area of the material (figure 1(h)). Furthermore, figure 1(i) shows that KGCA had some lattice stripes (aligned graphite layers) that corresponded to the graphite (002) plane, suggesting partial graphitization of the KGCA. The results of SEM and TEM showed that KGCA had a unique ordered layered structure, which was beneficial to enhancing the mechanical properties, specific surface area, and adsorption performance of the material.

Figure 2(a) displays the x-ray powder diffraction (XRD) patterns for the as-prepared KCA and KGCAs, which all contained a weak peak and a broad peak representing the reflections of the (101) faces and the graphitic stacking of (002), respectively. Compared with KCA (002), the characteristic peak clearly shifted to a lower angle, which indicated an increase in the interlayer distance of carbon [32]. The chemical structures of KCA and KGCAs were further investigated using an FTIR spectrometer within 500–4000 cm⁻¹ (figure 2(b)). The FTIR spectra of the KGCAs were very similar to those of the KCA. The characteristic broad and strong band appearing at around 3426.2 cm⁻¹ might correspond to O-H vibrations. The absorption peak near 2925.6 cm⁻¹ originated from the anti-symmetric stretching vibration of C-H in -CH₃ and -CH₂. And the N-H bending vibration in amide I was found at the absorption band at 1380.2 cm⁻¹ [33]. Interestingly, a stronger amine band appeared on the surface of KGCA after the addition of GO. Therefore, the above analysis results indicated that KGCA possessed a large number of active groups, e.g., carboxyl groups, hydroxyl groups, and amides, which were favorable for the capture of U(VI).

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The intensities of the D band and G band indicate the degree of disordered carbon and graphitized carbon, respectively [32, 34, 35]. Generally, the D/G intensity ratio (I_D/I_G) can be reflected by the defect density of carbon material. Figure 2(c) presents that the intensity ratios of the G to D band (I_D/I_G) of KCA, KGCA-0.1, and KGCA-0.2 were 0.819, 0.893, and 0.936, respectively, demonstrating that the chemical cross-linking of GO nanosheets and KGM molecules led to a decrease of orderliness with the increase of GO. Figure 2(d) shows that the Zeta potential of KGCAs were significantly lower than KCA at different pH values, which means that the surface of KGCAs were covered with more negative charges to facilitate the adsorption of U(VI) by electrostatic interaction.

The pore sizes of KCA and KGCAs were analyzed more accurately by nitrogen adsorption/desorption isotherms. The specific surface area of KCA was 605.8 m² g⁻¹, however, that of the KGCAs rose to 896.9 m² g⁻¹ and 1119.7 m² g⁻¹, respectively, with increasing amount of GO (figure 2(e)). Figure 2(f) shows that KCA and KGCAs were mainly made up of microspores and mesopores ranging from 1.6–10 nm. And the high specific surface area and appropriate pore structure of KGCAs have could greatly improve the adsorption properties of the material.

Compared to the brittleness of common biomass-derived carbonaceous aerogels, the unique layered structure of KGCA-0.2 exhibited strong mechanical properties, and its layered structure allowed for large deformation without cracks. Similar to the layered structure material, two distinct compression states were observed in the stress–strain (σ–ε) curve (figure S4(a)): at 0% < ε < 40%, a gently compressed state corresponded to the elastic buckling of the cell wall, and a densification compressed state for ε > 40% with σ rising sharply [36]. Surprisingly, there were no major changes in the modulus of layered KGCA-0.2 after 1000 loading–unloading cycles (figure S4(b)).

3.2.1. Adsorption kinetics

Figure 3(a) depicts the removal of U(VI) as a function of time in the initial U(VI) concentration (90 mg l⁻¹) at pH ∼ 5.5, T = 313 K. The results showed that adsorption saturation could be achieved within 40 min. Compared with other konjac or graphene-based adsorbent materials [37, 38], even pure GO [30] reached the adsorption equilibrium in 60 min, and this adsorbent still had obvious advantages in adsorption rate. In order to clarify the adsorption mechanism of U(VI) on KCA and KGCAs, the pseudo-first-order (equation (S1)) and pseudo-second-order (equation (S2)) kinetics models were further utilized to study the sorption kinetics of U(VI) on KCA and KGCAs (figure S4). The kinetic constants of U(VI) adsorption indicated that the pseudo-second-order models of KCA and KGCAs are better fitting than the pseudo-first-order model (table S1). Thus the adsorption capacity of KCA and KGCAs might rely on a lot of surface active sites of the KCA and KGCAs, and the U(VI) adsorption on KCA and KGCAs could be through the physicochemical adsorption process, including the electron sharing and ion exchange, etc occurring between the KCA and KGCAs and U(VI)-hydroxo complexes [28].

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3.2.2. Adsorption isotherms

3.2.3. Effects of temperature and pH on U(VI) adsorption

3.2.4. Effects of coexisting ions and irradiation

Results in figure 4(a) showed that the U(VI) removal efficiency of KGCA-0.2 maintained a high level with the presence of various competing anions, indicating that anions in aqueous solutions had negligible effects on the U(VI) adsorption on KGCA-0.2. Furthermore, U(VI) removal by KGCA-0.2 showed a slight decrease with the presence of cations, such as Cs⁺, Cu²⁺, Ca²⁺, Mn²⁺, and Sr²⁺, indicating that the competition between the ions has a negligible effect on the removal of U(VI) (figure 4(b)). The above results indicate that the KGCAs have good selectivity for U(VI).

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In addition, the actual radioactive wastewater contains a large amount of radiation, and whether the KGCAs have a good enrichment capability after irradiation has attracted our attention. Considering this, we evaluated the irradiation stability of the material at a total dose of 50, 100, and 200 kGy under ⁶⁰Co γ-ray irradiation. From figure S5, no significant changes were found in the morphology and functional groups of KGCA-0.2. At different pH values and temperatures, the adsorption of U(VI) by KGCA-0.2 after irradiation was almost insignificant, indicating that the modified material could be applied to the actual radioactive environment from laboratory simulation conditions (figures 4(c) and (d)).

3.2.5. Adsorption under simulated seawater and high salt conditions

The adsorption ability of the material under the conditions of simulated seawater or high salt concentration is also an important factor for its practical application, especially for U(VI) enrichment. Thus, we simulated these two extreme conditions in the laboratory and examined the KGCA's enrichment capability under these conditions. Surprisingly, KGCA-0.2 demonstrated a fascinating enrichment ability of 55 mg g⁻¹ at simulated seawater, which was much higher than that of KCA (figure 5(a)). In addition, under the high salt concentration condition, the KGCA's enrichment capacity showed little decrease compared to that in the normal environment, and it still maintained a relatively high enrichment capacity (340.2 mg g⁻¹) (figure 5(b)). Therefore, KGCA can be used as an effective uranium adsorbent under simulated seawater or high salt conditions.

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3.2.6. Recyclability of KGCA

Recyclability of an ideal adsorbent is an important parameter for practical application, which facilitates the cost reduction. Most traditional adsorption materials are difficult to be recovered because of their powdered form which limits their practical application. The 3D KGCAs avoid the problem of the conventional adsorbent being difficult to recycle. Here, repetitive adsorption experiments were performed six times. The desorption of U(VI) could be achieved by NaOH or HCl. As can be seen from figure 6, the material maintained a high enrichment capability for U(VI), with a slight decrease from 457.4–410.4 mg g⁻¹ after six adsorption–desorption cycles. Thus, these results suggested KGCA-0.2 had a good recyclability for U(VI) removal/recovery compared to the conventional adsorption material (cellulose) after the second cycle of adsorption performance retention rate of 12%–80% [42].

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As one of the important parameters, the effect of the breakthrough time and the effluent concentration represent the adsorption equilibrium relationship between the stationary phase (absorbent) and the mobile phase (aqueous solution of U(VI)). Figure 7(a) shows a simplified schematic of a fixed-bed column experiment, which displays the process of uranium-containing wastewater passing through the fixed-bed column to become clean water. The critical parameters of breakthrough curves for the removal of U(VI) by KCA and KGCA-0.2 are summarized in table S3. The C/C₀ value on KGCA-0.2 still maintained below 5% at t < 915 min, while the C/C₀ value for KCA was only achieved at t = 475 min (figure 7(b)). It can be intuitively indicated that KGCA-0.2 needs more U(VI) than KCA to achieve adsorption equilibrium, which means that KGCA-0.2 exhibits superior adsorption due to the breakthrough point of KGCA-0.2 (defined as C/C₀ ≤  5%), far more than KCA. The high column efficiency further demonstrated that KGCA-0.2 (η = 0.85) can be a promising adsorbent for the treatment of nuclear wastewater. This may be due to the unique layered structure of KGCA-0.2. In detail, the U(VI) solution flows in the layered structure as the spring water flows between the mineral rock layers, which can greatly increase the contact time and area of the KGCA-0.2 with the U(VI) solution and thereby improve the adsorption performance of the material.

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The Thomas and Yan model constants both revealed good agreement between the experimental results and the above models (the correlation coefficients (R²) >0.998) (table S4)). In addition, the maximum adsorption capacities fitted by the Thomas and Yan model were also very close to the actual values of the batch experiments. The high fixed-bed column experiment also proved that KGCA can be used as one of the promising adsorbent for the solidification and preconcentration of U(VI) in nuclear waste management and geological repositories.

3.3.1. Mechanisms for adsorption

XPS and FTIR spectra were employed for the characterization of the changes of the surface functional groups and composition of KGCA after U(VI) adsorption (figure 8). Figure 8(a) shows the FTIR spectra of KGCA-0.2 after adsorption of U(VI) at initial concentrations of 50, 100, and 150 mg l⁻¹. The C-O vibration peak gradually decreased at ∼1037.9 cm⁻¹ with increasing U(VI) concentration, which might be due to the chemical interactions of U(VI) with alkoxy groups from GO or C-O groups in the epoxy by comparing KGCA-0.2 before and after U(VI) adsorption [41, 43]. Furthermore, the adsorbed -OH peak (3440.1 cm⁻¹) also became broad and weak, which may be due to the coordination of KGCA-0.2 and U(VI).

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As an important characterization, XPS is able to evaluate the effect of various organic functional groups on the U(VI) adsorption of the KGCA-0.2 surface. As shown in figure 8(b), the U(VI) peaks appeared in KGCA-0.2 after adsorption of U(VI) in the high-resolution U1s spectrum. In addition, the XPS spectra of U4f5/2 and U4f7/2 are shown in figure 8(c). The two peaks at 393.2 and 382.5 eV were found to fit the experimental data well, with two corresponding satellite peaks, indicating that the adsorbed U(VI) was in the form of an oxidized state. In addition, the high-resolution C1s spectrum of the KGCA-0.2 (figures S7(a) and (b)) was divided into four peaks at 287.4 eV (O-C-O and O-C=O), 286.7 eV (C-OH, C-NH₂, and C-O), 285.1 eV(C-OH), and 284.8 eV (C-C), respectively. The various oxygen-containing functional groups and amino groups of KGCA-0.2 through U(VI) adsorption showed obvious changes, demonstrating that the oxygen-containing functional group (hydroxyl and carboxyl) chelated with U(VI) [44].