3.1 Characterization
The micro-morphologies of the PS and PS-Fe3O4 were characterized by the analytical technologies of SEM and TEM (Fig. 2). As shown from Fig. 2, the surface morphology of PS was smooth, and the surface morphology of PS-Fe3O4 was a craggy and irregular structure. It might be the reason that the PS-Fe3O4 composites were covered by porous nanoscale Fe3O4 particles. The EDS images also could confirm that porous nanoscale Fe3O4 particles were appeared and well distributed on the surface of PS (Fig. 3).
Figure 3a showed that C, O and Si elements were distributed in the structure of PS. Their contents were 68.47, 31.35 and 0.17%, respectively. It depicted that the elements of C and O were the main component in the material of PS. In addition to the appearance of C, O and Si elements, the element of Fe also was observed on the structure of PS-Fe3O4 composites (Fig. 3b). The content of Fe reached 20.67%. The contents of C, O and Si were 42.76, 36.29 and 0.25%, respectively. Compared with PS, the content of C was decreased, and the contents of O and Fe were increased. It could be confirmed that the functionalized porous nanoscale Fe3O4 particles supported by PS (PS-Fe3O4 composites) were successfully prepared.
The surface area and pore size of PS and PS-Fe3O4 composites were calculated by N2 adsorption-desorption isotherms. Results showed that porous structure was present for PS-Fe3O4 composites. BET specific surface areas of PS and PS-Fe3O4 composites were 11.35 and 46.13 m2/g, respectively. Due to nanoscale Fe3O4 particles supporting, the BET surface area of nanoscale was increased obviously compared with PS. It also depicted that PS could effectively decrease the aggregation of nanoscale Fe3O4 particles. The adsorption average pore width of nanoscale Fe3O4 particles were 6.94 and 6.31 nm, respectively. The results of FT-IR spectra about PS and PS-Fe3O4 composites were shown in Fig. 4a. The four strong characteristic peaks at 3398 cm−1, 2330 cm−1, 1610 cm−1 and 1377 cm−1 were observed. Some related researches indicated that the characteristic peak at 3398 cm−1 and 2330 cm−1 should be related to the stretching vibration of –OH and –C≡C, respectively (Yuan et al. 2015). The stretching vibration peak at 1610 cm−1 contributed to the C=C, and the characteristic peak at 1377 cm−1 represented the stretching vibration of C-O-C (Li et al. 2020). Compared with PS, the characteristic peak at 582 cm−1 was present on the surface of PS-Fe3O4 composites. The peak at 582 cm−1 corresponded to the stretching vibration of Fe-O. It also indicated that PS was coated with nanoscale Fe3O4 particles successfully. The crystal structures of PS and PS-Fe3O4 composites were determined by XRD patterns (Fig. 4b).
For PS, the peak at 26.58 º should be the characteristic peak of biochar. In addition to the peak at 26.58, the characteristic peak at 34.84 º also was observed on the surface of PS-Fe3O4 composites. According the related studies (Fatehi et al. 2017), it should be the appearance of Fe3O4 crystal structure on the surface of PS-Fe3O4 composites.
Formation of Fe3O4 particles could be depicted as following (Badi et al. 2018):
Fe3+ + 3OH- ↔ Fe(OH)3 (1)
Fe(OH)3 ↔ FeOOH + H2O (2)
Fe2+ + 2OH- ↔ Fe(OH)2 (3)
2FeOOH + Fe(OH)2 ↔ Fe3O4 + 2H2O (4)
According to the results of SEM, TEM, EDS, FT-IR, XRD and N2-BET, it could be concluded that the functionalized porous nanoscale Fe3O4 particles supported PS from peanut shell was obtained. PS was successfully coated with the nanoscale PS-Fe3O4 particles. It was a craggy and irregular structure with BET specific surface area of 46.13 m2/g and the adsorption average pore width of 6.31 nm.
3.2 Effect of operation parameters
The influence of operation parameters on the adsorption of Pb(II) ions in solution were very important for investigating the adsorption capacity of PS-Fe3O4 composites. In this research, the influences of contact time, initial concentration of Pb(II) ions in solution, initial pH in solution and temperature on the adsorption of Pb(II) ions in solution were tested. The details experiments could be found in Supporting Information. Effect of contact time on adsorption of Pb(II) ions in solution by PS-Fe3O4 composites was shown in Fig. 5a. The adsorption capacity of Pb(II) ions in solution by PS-Fe3O4 composites increased significantly with the increasing of contact time. At the first 100 min, the adsorption capacity of Pb(II) ions in solution increased rapidly. Then, it begun to increase slowly, and reached adsorption equilibrium. In the initial stage of adsorption process, there are a lot of adsorption sites on the surface of PS-Fe3O4 composites. They would facilitate adsorption process, and the adsorption capacity of Pb(II) ions in solution increased quickly. Then, as the contact time increased, the adsorption sites on the surface of PS-Fe3O4 composites begun to decrease. Therefore, the adsorption capacity of Pb(II) ions in solution increased slowly. The adsorption of Pb(II) ions in solution by PS-Fe3O4 composites reached adsorption equilibrium about 360 min.
Effect of initial concentration of Pb(II) ions in solution was depicted in Fig. 5b. Along with the increase of initial concentration of Pb(II) ions, the adsorption capacity of Pb(II) ions by PS-Fe3O4 composites increased. The high concentration gradient would accelerate the diffusion of Pb(II) ions to the PS-Fe3O4 composites (Song et al. 2016; Javaheri et al. 2019). The adsorption capacity of Pb(II) ions under different initial pH value conditions could be found in Fig. 5c. It could be seen that the adsorption capacity of Pb(II) ions in solution was increased with the increase of initial pH value at pH<6.0. It might be the reason that the large amount of H+ ions at lower pH could generate electrostatic repulsion against Pb(II) ions. When the initial pH>6.0, the adsorption capacity of Pb(II) ions in solution begun to decrease. The increase of OH− ions in solution could promote the precipitation formation of Pb(OH)2 (Li et al. 2020). Effect of temperature on the adsorption capacity of Pb(II) ions in solution by PS-Fe3O4 composites was shown in Fig. 5d. It could be concluded that temperature was benefit for the adsorption capacity of Pb(II) ions in solution.
3.3 Adsorption kinetic, adsorption isotherm and thermodynamic
To evaluate the adsorption process of Pb(II) ions in solution by PS-Fe3O4 composites, adsorption kinetic, adsorption isotherm and thermodynamic were investigated according to the experimental data of Fig. 5. In this research, pseudo first-order kinetic model, pseudo second-order kinetic model, Langmuir isotherm model and Freundlich isotherm model were chosen for describing the adsorption process. The details of equations were provided in Supporting Information. The adsorption kinetics for adsorption of Pb(II) ions in solution by PS-Fe3O4 composites were described in Fig. 6a-b. It could be concluded that the pseudo second-order kinetic model was more fitted for describing the adsorption process of Pb(II) ions in solution (0.9989>0.9641). Therefore, it also could be confirmed that the adsorption process of Pb(II) ions in solution by PS-Fe3O4 composites was mainly controlled by the chemical adsorption process (Chen et al. 2019). Fig. 6c-d was the adsorption isotherms for adsorption of Pb(II) ions in solution by PS-Fe3O4 composites.
It could be observed that the R2 of Langmuir model and Freundlich model were 0.9983 and 0.9374, respectively. Therefore, the adsorption process of Pb(II) ions in solution by PS-Fe3O4 composites could be depicted by the Langmuir model. It also could be implied that the adsorption processes were the homogeneous and monolayer adsorption (Zama et al. 2017). The maximum adsorption capacity of Pb(II) ions removal in solution by PS-Fe3O4 composites could reach 188.68 mg/g. Compared with the adsorption capacity of Pb(II) ions by other magnetic adsorbents, PS-Fe3O4 composites exhibited well performance of Pb(II) removal (The details of the adsorption capacity of Pb(II) ions in solution removal by other magnetic adsorbents could be found in Supporting Information). Therefore, PS-Fe3O4 composites were thought as a low-cost and high efficiency adsorption material for removal of Pb(II) ions in solution. According to the experimental data of Fig. 5d, thermodynamic parameters of Pb2+ ions in solution removal by PS-Fe3O4 composites could be calculated (They could be found in Supporting Information). The negative value of ΔG0 could be concluded that adsorption process of Pb(II) ions removal by PS-Fe3O4 composites was a spontaneous process. When reaction temperature increased, the value of ΔG0 decreased. It indicated that temperature was benefit for enhancing adsorption capacity of Pb(II) ions removal by PS-Fe3O4 composites. Furthermore, ΔH0 and ΔS0 both were positive value. It also depicted that adsorption process of Pb(II) ions removal by PS-Fe3O4 composites also was an endothermic process.
3.4 Adsorption mechanism
In order to elaborate the adsorption mechanism of Pb(II) ions removal in solution by PS-Fe3O4 composites, the samples of PS-Fe3O4 composites before and after adsorption of Pb(II) ions also were characterized by XPS spectroscopy. The XPS spectra of PS-Fe3O4 composites before and after adsorption of Pb(II) ions were shown in Fig. 7.
As observed from Fig. 7a, the spectrum of C 1s, O 1s and Fe 2p at binding energies of 284.91, 530.69 and 710.52 eV appeared in the survey XPS spectrum before adsorption of Pb(II) ions. The elemental atomic compositions of C, O and Fe were 76.27, 20.28 and 3.45%, respectively. It was implied that the elements on the surface of PS-Fe3O4 composites were C, O and Fe. After adsorption of Pb(II) ions, the elemental atomic compositions of C, O and Fe on the surface of PS-Fe3O4 composites were changed. They were 64.69, 24.34 and 3.97%, respectively. Additionally, the new Pb 4f at binding energies of 143.26 eV appeared in the survey XPS spectrum. Therefore, it could be concluded that Pb(II) ions in solution could be successfully adsorbed by PS-Fe3O4 composites.
The spectrum of high resolved Pb 4f had two peaks at the binding energies of 138.41 and 143.41 eV (Fig. 7b). They were corresponded to Pb 4f7/2 and Pb 4f5/2, respectively. Additionally, they were in accordance with Pb(II) (Zhu et al. 2017). It also indicated that Pb(II) ions in solution should be adsorbed into the active sites on the surface of PS-Fe3O4 composites without being oxidized. This result was consistent with the result of the Fe 2p spectra (Fig. 7c). As from Fig. 7c, it could be observed that two characteristic peaks at the binding energies of 710.35 and 724.38 eV were presented on the surface of PS-Fe3O4 composites. They were corresponded to Fe(II) and Fe(III), respectively. The intensity of Fe 2p decreased after adsorption of Pb(II) ions in solution. It was indicated that PS-Fe3O4 composites could adsorb Pb(II) ions with Fe-O coordination reaction (Liu et al. 2016). Additionally, this result could also be confirmed from the O 1s spectra (Fig. 7d). The peak at binding energy of 531.35 eV was corresponded to Fe-O before adsorption of Pb(II) ions. After adsorption of Pb(II) ions, the intensity of O 1s increased. It was implied that the interaction of PS-Fe3O4 composites and Pb(II) ions was happened through Fe-O coordination reaction. According to the result of influence of initial pH, it could be concluded that pH had an important effect on the adsorption capacity of Pb(II) ions in solution. The initial pH in solution not only affected the charges of functional groups on the surface of PS-Fe3O4 composites, but also affected the species of Pb(II) ions in solution. Therefore, PS-Fe3O4 composites could adsorb the Pb(II) ions in solution through electrostatic interaction. Additionally, Pb(II) ions in solution could be co-precipitated on the surface of PS-Fe3O4 composites. The related equations were following, such as, Fe3O4 + 8H+ → Fe2+ + 2Fe3+ + 4H2O and Pb2+ + 2OH− → Pb(OH)2↓. As shown from the results of FT-IR, the large number of functional groups (such as –OH, –C≡C, C=C, C-O-C and Fe-O functional groups) were appeared on the surface of PS-Fe3O4 composites. They could interact with Pb(II) ions in solution through complexation reaction, ion exchange and Fe-O coordination reaction.
In a word, the adsorption process of Pb(II) ions in solution by PS-Fe3O4 composites was mainly controlled by the chemical adsorption process. The possible reaction mechanisms of Pb(II) ions removal by PS-Fe3O4 composites were depicted in Fig. 8. The possible mechanism of Pb(II) ions by PS-Fe3O4 composites included Fe-O coordination reaction, co-precipitation, complexation reaction and ion exchange.
3.5 Regeneration experiment
In order to investigate the regeneration of Pb(II) ions removal by PS-Fe3O4 composites, the adsorption or desorption experiment of Pb(II) ions removal by PS-Fe3O4 composites were carried out. After the adsorption experiment of Pb(II) ions removal by PS-Fe3O4 composites reached equilibrium, the solution was centrifuged at 4000 rpm for 20 min and the adsorbent of PS-Fe3O4 composites was obtained. Next, they were washed for three times with 0.1 M H2SO4, and dried at 60℃ for 24 h. Then, the dried PS-Fe3O4 composites were used for adsorption experiment again. The experimental results were depicted in Fig. 9. After four regeneration experiments, the adsorption capacity of Pb(II) ions removal by PS-Fe3O4 composites only decreased slightly, and it still retained 71.86%. It also could be implied that PS-Fe3O4 composites were reused for Pb(II) ions in solution removal, the regeneration performance of PS-Fe3O4 composites was good.