3.1. Aerogel characterization
SEM
As shown in Fig. 2, the internal microscopic morphologies of the four aerogels were captured by scanning electron microscopy (SEM). Figure 2(a) showed that the pure BC aerogel had amazing 3D filamentous fibers with many pores were formed between the fibers. These pores could provide a certain guarantee for the mechanical properties of the composite aerogel, and they also provide a good basis for adsorbing Dy(III) in water. Compared with the pure BC aerogel, the fibers of the OBC aerogel in Fig. 2(b) were slightly broken, but the original fiber network was basically maintained, which indicated that the carboxylation process introduced hydroxyl groups on the surface of cellulose, and the micromorphology of BC was slightly damaged. The microscopic morphologies of APT-GT-OBC aerogel and I-APT-GT-OBC aerogel were shown in Fig. 2(c) and Fig. 2(d), and it could be seen that there were a large number of slender rod APT adhered to OBC and formed many tiny pores. Compared with the non-imprinted aerogel, the APT on the surface of the eluted imprinted aerogel was more uniform, but the structure did not change significantly, indicating the strong stability of the BC based aerogel.
BET
In order to study the pore structure of the aerogels, the N2 adsorption-desorption experiments were carried out on four aerogels. The adsorption-desorption isotherms and pore performance of the aerogels were shown in Fig. 3 and Table 1. It could be seen that the SBET of the pure BC aerogel was 37.12 cm2 g− 1. The pore diameter was enlarged, which was due to the partial fracture of the BC fibers caused by carboxylation process, which made the pores of the aerogel collapse to a certain extent. APT adhered to many tiny pores formed on OBC by GT, resulting in an increase in SBET and a decrease in average pore size of two composite aerogels. The average pore size of I-APT-GT-OBC was 12.77 nm, and the SBET was 28.01 cm2 g− 1. Significantly smaller than SBET of non-imprinted aerogels, which was believed to be due to the loss of APT in the material and a certain degree of fiber breakage in the BC caused by imprinting eluting and subsequent drying process. These results were consistent with SEM. Therefore, the introduction of APT could effectively increase the SBET of the composite aerogel, which was beneficial to the subsequent adsorption of Dy(III), while the carboxylation and elution process would have a certain impact on the structural properties of the material.
Table 1
The pore structure parameters of four aerogels
Sample | Surface Area (cm2/g) | Pore Size (nm) |
BC | 37.12 | 13.81 |
OBC | 13.6 | 15.65 |
APT-GT-OBC | 41.55 | 16.98 |
I-APT-GT-OBC | 28.01 | 12.77 |
FT-IR: The group in the aerogel range of 4000-500 cm-1 was analyzed by FT-IR, as shown in Fig. 4. It could be seen that the four aerogels all showed the characteristic peaks of BC at 3342 cm-1, 2883 cm-1 and 1054 cm-1, including -OH stretching vibration related to intramolecular and intermolecular hydrogen bonds, as well as stretching vibration of -CH2 and C-O-C in cellulose glucose rings, respectively. Compared with the pure BC, the infrared spectra of OBC, APT-GT-OBC and I-APT-GT-OBC all showed new a characteristic peak at 1654 cm-1, this is mainly due to the anti-symmetrical stretching of -COOH in the carboxylate, indicating that TEMPO oxidation was capable of converting the hydroxyl groups on the surface of cellulose into carboxyl groups. Besides, the carboxyl characteristic peaks of the APT-GT-OBC and I-APT-GT-OBC aerogels were enhanced compared with OBC, which was due to the addition of GT. Against the spectrum of OBC, APT-GT-OBC and I-APT-GT-OBC appeared at 975 cm-1 with a new characteristic peak due to the presence of Si-O-Si (Song et al. 2020; Zhu et al. 2020) in APT. That is, TEMPO can effectively modify BC, and APT and GT successfully introduced into the composite aerogel.
TGA
Thermal stability tests were carried out on four aerogels under N2. The mass loss of the aerogels at 30–800°C was shown in Fig. 5. It could be seen that the mass loss of the four aerogels was relatively small below 200°C, indicating that the residual water in the materials was very small. The BC and OBC had obvious mass loss at 200–350°C, which decreased by about 80% and 55% respectively, which may be due to the thermal decomposition of the BC fiber structure, while APT-GT-OBC and I-APT-GT-OBC had only lost about 20% at this stage, which was due to the reduction of BC content in composites. The material entered the carbonization stage, and the loss of the material tended to stop after 450°C. At this time, the main components of BC and OBC residues were carbon, and the residual mass was 3.76% and 26.06%, respectively, while the main components of APT-GT-OBC and I-APT-GT-OBC residues were silicon and carbon. Therefore, the residual mass of these two kinds of composite aerogel were greatly improved compared with the former two. In other words, the carboxylation of BC and the addition of APT and GT can significantly enhance the thermal stability of the composite aerogel. Compared with APT-GT-OBC, I-APT-GT-OBC had a low total mass loss rate, which indicated that the thermal stability of the imprinted aerogel is better.
3.2. Analysis of adsorption results
Effect of pH on adsorption
We found the optimal adsorption pH by testing the adsorption capacity of four aerogels to Dy(III) at different pH values. Figure 6 showed the adsorption properties of these aerogels. It could be seen that the adsorption capacity of the pure BC, OBC, APT-GT-OBC and I-APT-GT-OBC aerogel for Dy(III) were gradually enhanced. Among them, the maximum adsorption amount of imprinted aerogel reached 47.74 mg g− 1. This demonstrated that carboxylation and the introduction of APT and GT further increased the active sites provided from the carboxyl groups, thus improving the ability of composite aerogel selection and adsorption of Dy(III). With the increase of pH value, the adsorption effect of aerogel changed significantly. The adsorption effect of all four aerogels increased slowly when the pH value was below 5.0, while when the pH value was 5.0, the adsorption capacity increased significantly. However, the increased trend of the adsorption capacity of aerogel slowed down again when the pH value was above 5.0, which might be due to the decrease of the carboxyl dissociation degree, resulting in the decrease of the affinity of materials for Dy(III). The separation ability of the imprinted aerogel was measured by imprinting factor (IF). When the value of pH was 5.0, I-APT-GT-OBC showed a maximum IF value of 1.12. This showed that the imprinted aerogel has the best adsorption performance when pH = 5.0, so we set the pH to 5.0 in subsequent study of optimal conditions.
Adsorption dynamics
We tested the adsorption effects of four aerogels at different contact times and fitted them with PFOKM and PSOKM. Figure 7 and Table 2 were the adsorption data, the fitting curves and the related parameters, respectively. It is not difficult to see that the conclusion was the same as the pH adsorption experiment, the adsorption effect of APT-GT-OBC and I-APT-GT-OBC composite aerogel on Dy(III) was far superior to BC and OBC, which further reflected the necessity of subsequent material modification. In addition, the adsorption of the four aerogels occurred rapidly and began to reach equilibrium in about 100 minutes, suggesting that BC itself was an excellent adsorbent for Dy(III). Combined with the fitting curves in the Fig. and the correlation coefficient (R2) in the table, it could be seen that the fitting curves of the four aerogels were more consistent with PSOKM, which meant that the adsorption process of the prepared aerogels were mainly caused by chemical action, while physical action played an auxiliary role, and the adsorption occured quickly in a very short period of time.
Table 2
Kinetic parameters for fitting curves of PFOKM and PSOKM
Sorbents | PFOKM | PSOKM |
Qe (mg g− 1) | k1 (min− 1) | R2 | Qe (mg g− 1) | k2 (g mg− 1 min− 1) | R2 |
BC | 13.834 | 0.041 | 0.940 | 14.511 | 0.004 | 0.994 |
OBC | 21.509 | 0.062 | 0.915 | 22.463 | 0.004 | 0.993 |
APT-GT-OBC | 36.656 | 0.072 | 0.908 | 38.251 | 0.003 | 0.996 |
I-APT-GT-OBC | 38.772 | 0.087 | 0.905 | 40.279 | 0.004 | 0.994 |
Adsorption isotherm: We studied the influence of different initial concentrations on the adsorption effect of the aerogels, and obtained the maximum theoretical adsorption capacity and adsorption principle of aerogels by fitting adsorption data through Frundlich isothermal model and Langmuir isothermal model. The fitting curves and adsorption parameters were shown in Fig. 8 and Table 3, respectively. It could be seen that a higher initial concentration contributed to the adsorption of the aerogels. The closer the correlation parameter R2 is to 1, the better the model fits. It could be seen that the isotherm data of the four aerogels and the Langmuir isotherm model had a better fitting effect, and the R2 was 0.993-0.999, which meant that the adsorption for Dy(III) of the BC aerogel was mainly due to the monolecolar adsorption. In addition, through fitting analysis, it could be concluded that the maximum theoretical amount for Dy(III) of I-APT-GT-OBC was 48.762 mg g-1, which was greatly improved compared with the OBC, pure BC and APT-GT-OBC aerogel.
Table 3
Adsorption equilibrium constants for isotherm models
Sorbents | Langmuir | Freundlich |
Qm (mg g− 1) | KL (L mg− 1) | R2 | KF (mg g− 1) | 1/n | R2 |
BC | 30.597 | 0.026 | 0.995 | 3.648 | 0.381 | 0.932 |
OBC | 34.298 | 0.074 | 0.999 | 9.587 | 0.243 | 0.933 |
APT-GT-OBC | 43.650 | 0.246 | 0.993 | 20.388 | 0.155 | 0.805 |
I-APT-GT-OBC | 48.762 | 0.455 | 0.994 | 25.778 | 0.134 | 0.773 |
Adsorption thermodynamics: We measured the ability of aerogels to adsorb Dy(III) at different temperatures to study the thermodynamic properties of these aerogels. Fig. 9 showed the linear relationship between Cs and ln(Cs/Ce), and the thermodynamic equilibrium constant K° is the vertical intercept value of the curve. Fig. 10 showed the linear relationship between 1/T and the thermodynamic equilibrium constant K°. Table 4 showed the thermodynamic adsorption parameters such as ΔH°, ΔS° and ΔG°. The ΔH° value was less than zero, which meant that the adsorption process of Dy(III) by aerogel was an endothermic process, and a moderate increase in temperature could promote the occurrence of the reaction, mainly because the increase in temperature leaded to thermal diffusion of molecules, thus increasing the contact area of the adsorption reaction. The ΔS° value was greater than zero, which meant that the process of adsorption was a chaotic process. ΔG° values ranged from -7.25 to -4.75, all of which were less than zero, it is explained that the adsorption process of Dy(III) by aerogel was spontaneous. The value of ΔG° of the imprinted aerogel was the minimum, which meant that the adsorption of I-APT-GT-OBC aerogel was more spontaneous. In summary, the adsorption process of imprinted aerogel was an endothermic, entropy-enhanced and spontaneous process.
Table 4
Thermodynamic parameters of four aerogels
Sorbents | ΔH° (kJ mol− 1) | ΔS° (J mol− 1) | T (K) | K° | ΔG° (kJ mol− 1) | R2 |
BC | 4.30 | 36.33 | 298.15 | 6.80 | -4.75 | 0.999 |
308.15 | 8.00 | -5.33 |
318.15 | 9.19 | -5.87 |
OBC | 8.61 | 48.89 | 298.15 | 8.82 | -5.40 | 0.998 |
308.15 | 10.98 | -6.14 |
318.15 | 13.16 | -6.82 |
APT-GT-OBC | 15.77 | 71.00 | 298.15 | 11.08 | -5.96 | 0.991 |
308.15 | 12.56 | -6.48 |
318.15 | 13.78 | -6.94 |
I-APT-GT-OBC | 11.90 | 55.87 | 298.15 | 13.90 | -6.52 | 0.999 |
308.15 | 14.69 | -6.88 |
318.15 | 15.50 | -7.25 |
Selective test: To demonstrate the adsorption selectivity of the imprinted aerogel, we tested and compared the maximum adsorption effects of four aerogels when coexisting with Pr(III), Nd(III) and Dy(III) in a mixed solution, and the separation constant (Kd) was used to analyze whether it is selective adsorption. The results were shown in Fig. 11 and Table 5. It could be seen that the ion imprinting technology played an advantage in competitive adsorption. Compared with Nd(III) and Pr(III), I-APT-GT-OBC had a higher separation constant for Dy(III), which was 4084.40 mL g-1. In other words, the Dy(III) was the unique adsorption target for the I-APT-GT-OBC aerogel. There were pores in the imprinted aerogel that were specific for adsorption of Dy(III) and were unable to absorb other rare earth ions, which the other non-imprinted aerogels were unable to do. Through the competitive adsorption experiments, it was determined that I-APT-GT-OBC aerogel had a preference for Dy(III) during the adsorption process, and Dy(III) could be preferentially separated from multi-rare earth ionic solutions.
Table 5
The Kd value of Dy(III) adsorbed by four aerogels in mixed solution
Cation | BC | OBC | APT-GT-OBC | I-APT-GT-OBC |
Cf (mg L− 1) | Kd (mL g− 1) | Cf (mg L− 1) | Kd (mL g− 1) | Cf (mg L− 1) | Kd (mL g− 1) | Cf (mg L− 1) | Kd (mL g− 1) |
Pr(III) | 35.911 | 392.33 | 28.292 | 767.28 | 12.993 | 2848.23 | 12.958 | 2858.62 |
Nd(III) | 35.873 | 393.81 | 28.253 | 769.72 | 12.898 | 2876.57 | 12.81 | 2903.20 |
Dy(III) | 35.977 | 389.78 | 28.314 | 765.91 | 13.027 | 2838.18 | 9.834 | 4084.40 |
Repetitive experiment: In addition to the price of raw materials, the reusability of adsorbents also greatly affects the cost of the adsorption process. Recyclable adsorbents can greatly reduce the cost of separation and recovery, thus expanding the application range of adsorbents. As shown in Fig. 12, in order to verify the reusability of I-APT-GT-OBC aerogels, five adsorption-desorption experiments were performed on the aerogels, and the adsorption capacity of each cycle was tested respectively. It could be seen that the adsorption capacities after five cycles of the pure BC, OBC, APT-GT-OBC and I-APT-GT-OBC were 71.9%, 74.3%, 78.5% and 84.5% of the first adsorption, respectively. The decrease of adsorption capacities was mainly due to the destruction of pore structure during the elution, but all showed good stability. In other words, compared with non-imprinted aerogel, the imprinted aerogel showed stronger stability and reusability after multiple adsorption cycles.