3.1 Characterization
SEM was carried out to observe the morphological and structural changes of the Fe3O4@ZIF-8@ZIF-67 formation, adsorption, and Fenton-like oxidation. SEM micrographs (Fig. 1a and b) show that Fe3O4@ZIF-8 exhibited uniform rhombic dodecahedral shape crystals and was not undermined by Fe3O4 NP loading. Figure 1c and d shows that the material nanostructure changed from a granular to a needle-shaped morphology with a coating of ZIF-67. Fe3O4 NPs were distributed on the surface and inside the ZIF-8@ZIF-67. After adsorption, the particle surface became significantly visually rougher, which is attributed to TC adsorption on the Fe3O4@ZIF-8@ZIF-67 surface. After Fenton-like oxidation, the appearance coarsened more than for the Fe3O4@ZIF-8@ZIF-67 after adsorption and showed several pinholes and folds. This appearance most likely resulted because the generated •OH acted on the structure.
To clarify the elemental distribution, EDS was used in an individual Fe3O4@ZIF-8@ZIF-67 particle. Figure 2 shows that elemental Co gathered in the core section of the nanocomposite and the Fe, Zn, O, C, and N were dispersed evenly in the shell layer. The mapping of Co, Zn, C, and N can be considered as ZIFs, and the presence of Fe and O can be ascribed to Fe3O4. The abundant distribution of Co provided valid active sites for Fenton-like oxidation. Based on these results, it can be concluded that the ZIF-67 as an outer layer coated the inner ZIF-8 and that Fe3O4 NPs distributed homogeneously on the ZIF-8@ZIF-67 structure.
To determine the chemical structure, Fe3O4, Fe3O4@ZIF-8, ZIF-8@ZIF-67, and Fe3O4@ZIF-8@ZIF-67 were analyzed by XRD. The diffraction pattern for Fe3O4@ZIF-8@ZIF-67 had eight main peaks of ZIF-8@ZIF-67 (Fig. 3b) at ~ 10.1°, 13.2°, 14.5°, 16.4°, 18.1°, 19.6°, 26.1°, 27.9°, and 29.2°, which agrees with the results in the literature (Wu et al., 2020). Characteristic peaks were visible at 30.0°, 35.6°, 43.4°, 53.7°, 57.2°, and 62.8°, which indicates the successful loading of Fe3O4. The introduction of Fe3O4 NPs did not destroy the ZIF-8 or ZIF-67 structure, which confirms that Fe3O4@ZIF-8@ZIF-67 was synthesized. To verify the crystalline structure stability, the XRD patterns of Fe3O4@ZIF-8@ZIF-67 after adsorption and Fenton-like oxidation (Fig. 3b) were analyzed. In Fig. 3, the position and intensity of the Fe3O4 diffraction peaks remained consistent before and after adsorption and Fenton-like oxidation, which shows that the Fe3O4 structure was stable. After batch reactions and Fenton-like oxidation, the magnetic property of Fe3O4 remained unchanged and met the practical application of repeated and strong separation. However, after adsorption and Fenton-like oxidation, the diffraction peak intensities of ZIF-8@ZIF-67 decreased compared with that observed prior to reaction, which may be attributed to TC adsorption, which resulted in clogged pores of Fe3O4@ZIF-8@ZIF-67 and changes in the nanoparticle surface (Li et al., 2019). The crystal form and structure of the Fe3O4@ZIF-8@ZIF-67 did not show changes because of the existence of characteristic peaks. These results agree with the SEM observations (Fig. 1) and element mapping (Fig. 2).
FTIR analysis was carried out to prove the synthesis of Fe3O4@ZIF-8@ZIF-67 and to obtain an improved understanding of the possible reaction mechanisms (Fig. 4). As shown in Fig. 4b, the appearance of a peak at 581 cm− 1 could be related to Fe–O–Fe vibration, which suggests that Fe3O4 NPs loaded the nanocomposite. The peaks at 1136–1306 cm− 1 are ascribed to imidazole ring vibration, whereas those at 2917–3107 cm− 1 were assigned to the stretching vibration of the saturated hydrocarbon C–H (CH3) and unsaturated hydrocarbon C–H of the 2-methylimidazolium. Therefore, the 2-methylimidazolium served as an organic ligand in the nanocomposite. The peak at 425 cm− 1 was caused by the Zn–N stretching vibration, and the peaks at 994–1106 cm− 1 resulted from C = N in ZIF-67, which confirms the presence of a ZIF-8 and ZIF-67 structure. These characteristic peaks confirm the synthesis of nanocomposite Fe3O4@ZIF-8@ZIF-67 in accordance with the XRD results (Fig. 3). In Fig. 5, after adsorption and Fenton-like oxidation, the 1618 cm− 1 peak that was associated with the benzene rings broadened, which indicates that TC was adsorbed on the Fe3O4@ZIF-8@ZIF-67. The appearance of the 1458 cm− 1 peak established the TC adsorption. Compared with the original Fe3O4@ZIF-8@ZIF-67, the main characteristic peaks exhibited no obvious alterations after adsorption or Fenton-like oxidation, thus it was inferred that the magnetic nanocomposite structure had not been destroyed.
The hysteresis loop from VSM was used to obtain the magnetic properties of Fe3O4, Fe3O4@ZIF-8, and Fe3O4@ZIF-8@ZIF-67. According the VSM results in Fig. 5, the saturation magnetization (Ms) of Fe3O4, Fe3O4@ZIF-8, and Fe3O4@ZIF-8@ZIF-67 was 70.21, 37.16, and 18.38 emu/g, respectively. The phenomenon that the magnetic intensity decreased shows that ZIF-8 and ZIF-67 were coated successively. The as-prepared nanocomposite still exhibited sufficient magnetic responsibility and could be separated with an external magnetic field to facilitate recycling and reuse. Figure 6 shows that Fe3O4@ZIF-8@ZIF-67 started to decompose strongly at 500℃, which indicates its good thermal stability.
3.2 Adsorption performance
Different factors affect the adsorption process, and the Fe3O4@ZIF-8@ZIF-67 dosage, temperature, pH, primary concentration of TC, and HA were investigated by a variable-controlling strategy.
To determine the optimum adsorbent dosage, we explored the adsorbent effect on the TC removal efficiency. As shown in Fig. 7a, with an increase of adsorbent to 20 mg, the TC removal efficiency improved, and reached a maximum removal efficiency of more than 88%. The adsorption quantity dropped gradually from 352.88 to 181.00 mg L− 1 when the amount of the adsorbent continued to increase. This behavior results from the limited adsorption sites on the adsorbent that are occupied by TC and reach adsorption saturation. Under the action of many adsorbents, the effective collision quantity per unit mass and the concentration gradient decrease, which leads to a decrease in adsorption capacity.
Temperature determines the velocity of the molecular motion and the energy of the molecular surface, which affects the mass transfer rate. Therefore, it is important to study the effect of temperature on the adsorption process. Figure 7b shows that, with an increase in temperature, the adsorption quantity of Fe3O4@ZIF-8@ZIF-67 on TC increases slightly, which indicates that the adsorption is an endothermic process. This phenomenon may be attributed to the improved dispersion rate of TC molecules as the temperature increases, and as a result, TC molecules can pass through the external boundary faster.
We investigated the adsorption quantity of double-layer MOF on TC at different pHs. As indicated in Fig. 7c, the removal efficiency of TC exceeded 80% and remained stable for a pH of 3–11, below or above which, that is, under strongly acidic (pH 3) or basic (pH 11) conditions, respectively, the adsorption efficiency decreases, which probably occurs because strong acid or strong base destroys the molecular structure of the MOFs.
As the primary concentration of TC increases, the amount of adsorbent on the TC improves, which shows that the primary concentration of contaminants may affect the mass-transfer rate. A higher primary concentration increases the effective collision probability between the adsorbate and the adsorbent, which causes the adsorption to move in a positive direction. When the primary concentration reaches 120 mg L− 1, the adsorption rate exceeds 90%. However, there is no further significant change in adsorption quantity when the primary concentration continues to increase. This behavior may be related to the saturation of occupied active sites in Fe3O4@ZIF-8@ZIF-67, which cannot adsorb excess TC at higher concentrations.
It is meaningful to explore the effect of HA on the adsorption of TC because HA exists extensively in natural water and wastewater. Figure 7d shows the variation tendency in adsorption capacity of Fe3O4@ZIF-8@ZIF-67 with the addition of different amounts of HA when the TC concentration ranges from 0 to 8 mg L− 1. The adsorption capability of Fe3O4@ZIF-8@ZIF-67 hardly improves as the HA concentration increases, which occurs because the adsorption quantity of HA on the adsorbent is too low to affect the TC adsorption. This phenomenon also shows that Fe3O4@ZIF-8@ZIF-67 is an efficient adsorbent in HA-enriched water.
3.3 Oxidation performance
An evaluation of TC removal from aqueous solution in different systems is shown in Fig. 8. The H2O2 system yielded a 1.31% TC removal in 180 min, whereas for the Fe3O4@ZIF-8@ZIF-67/H2O2 system, 95.76% TC removal exceeded the sum of the H2O2 and Fe3O4@ZIF-8@ZIF-67 systems (89.03%). These findings led us to conclude that H2O2 showed limited degradation of TC and Fe3O4@ZIF-8@ZIF-67/H2O2 could be a favorable catalyst for H2O2 in the TC removal process. The addition of H2O2 compensated for the deficiency that Fe3O4@ZIF-8@ZIF-67 alone had a weak adsorption efficiency for a low concentration of TC, and the TC removal from aqueous solution rose to a new high. Figure 9 shows that the TC removal efficiency of the Fe3O4@ZIF-8@ZIF-67/H2O2 system minus the Fe3O4@ZIF-8@ZIF-67 system decreased progressively, which indicates that Fenton-like oxidation acted mainly on the TC molecules that were adsorbed on the Fe3O4@ZIF-8@ZIF-67 instead of the free TC molecules in aqueous solution. Therefore, in synergetic adsorption and Fenton-like oxidation, TC molecules were adsorbed preferentially on the Fe3O4@ZIF-8@ZIF-67 and afterwards some were oxidized by •OH radicals that were generated by H2O2.
In Fenton-like reactions, the concentration of H2O2, the primary pH of the solution, and the adsorbent dosage may influence the oxidation performance, and thus, the effect of the abovementioned conditions on the catalytic oxidation of TC was investigated.
The Fe3O4@ZIF-8@ZIF-67 dosage affects TC removal. As shown in Fig. 10, when the usage of Fe3O4@ZIF-8@ZIF-67 was low, the removal efficiency increased as the dosage of Fe3O4@ZIF-8@ZIF-67 increased, and peaked at 25 mg, with a maximum removal efficiency of 95.75%. A possible reason for this alteration is that the catalysis was stimulated when the number of active centers in Fe3O4@ZIF-8@ZIF-67 increased, which accelerated H2O2 degradation. At a high level of Fe3O4@ZIF-8@ZIF-67 usage, even if the dosage continued to increase, the removal efficiency could not be improved significantly, which confirms that the oxidation is a rate-limiting reaction. Under this circumstance, the adsorbent may eliminate hydroxyl radicals, which adversely affects TC removal. The low concentration of TC may limit further improvements in removal efficiency.
H2O2 can oxidize a variety of organic compounds, such as carboxylic acids, alcohols, and esters into inorganic states, and thus, it is important in Fenton-like reactions. Therefore, it is necessary to explore the influence of H2O2 concentration. As shown in Fig. 11, the removal efficiency of TC improves with an increase in H2O2 concentration, from the initial 90.40% (H2O2-free solution) to a maximum of 98.97% when the concentration of H2O2 reaches 35 mM. During this process, many hydroxyl radicals are generated, which leads to an increase in oxidation efficiency. With a further increase in the amount of H2O2, the removal efficiency decreased gradually. A reason for the behavior may be the decrease in oxidizing radicals because a high concentration of H2O2 will induce the elimination of hydroxyl radicals instead.
H2O2+·OH→H2O + HO2· (3)
HO2·་·OH→H2O་O2 (4)
pH is an important factor to control the generation of ions and free radicals in Fenton-like reactions. When the pH ranges from 3 to 9, the removal efficiency remains above 90%, which shows a favorable TC removal performance. In the Fe3O4@ZIF-8@ZIF-67 system with pH 3, the removal ratio increases by 12.24% compared with the independent Fe3O4@ZIF-8@ZIF-67 system, which corresponds with the fact that the most appropriate pH is 3 in Fenton-like reactions. For pH 7, the removal efficiency peaks at 95.52%, which shows that the adsorbent has a good TC removal under acid and neutral conditions because of the electrostatic attractions and hydrophobic interactions between TC and Fe3O4@ZIF-8@ZIF-67, and the hydroxyl radicals that are produced by H2O2 in an acidic environment. Fe3O4@ZIF-8@ZIF-67 has a wider applied pH range than normal Fenton-like reagents, with a better environmental adaptability. In the Fe3O4@ZIF-8@ZIF-67 system at pH > 9, the removal efficiency drops to 78.69%, possibly because H2O2 tends to decompose rapidly to water and oxygen in a basic environment, instead of producing massive free radicals. A high pH may lead to an expansion in TC anion species or change the status of the electropositive TC. Therefore, in an alkali environment, the adsorbent surface reaches a charge balance and prevents TC diffusion into the catalytic reaction zone.
3.4 Kinetics analysis
TC removal consists of two sections, i.e., adsorption and Fenton-like oxidation. The following explains the two sections:
3.4.1 Adsorption kinetics
Adsorption kinetics were described by the pseudo-first-order (Eq. (5)) and pseudo-second-order (Eq. (6)) kinetic models:
where qe and qt (mg g− 1) are the amount of TC adsorption at equilibrium and time t (min), respectively; and k1 (min− 1) and k2 [g (mg min− 1)] are the rate constants for the pseudo-first-order and pseudo-second-order kinetic models, respectively. The best-fit kinetic parameters of TC adsorption are presented in Table 1 at 25℃. The resulting best linear correlation coefficients for the pseudo-second-order kinetic model (R2 = 0.99788) were greater than those for the pseudo-first-order kinetic model (R2 = 0.97004) (Table 1), which shows that the adsorption process followed the pseudo-second-order kinetic model at 25℃ and chemisorption was dominant in the speed limit.
Table 1
Best-fit kinetics parameters for TC adsorption by Fe3O4@ZIF-8@ZIF-67.
Reaction
temperature (℃)
|
Pseudo-first-order kinetic model
|
Pseudo-second-order kinetic model
|
k1 (min− 1)
|
R2
|
k2 (gmg− 1min− 1)
|
R2
|
25
|
65.9845
|
0.97004
|
6.11182E-4
|
0.99788
|
3.4.2 Oxidation kinetics
To explore the TC removal process, data of the Fenton-like oxidation fitted the pseudo-first-order (Eq. (7)) and pseudo-second-order (Eq. (8)) kinetic models:
where C0 is the initial concentration of TC and Ct (mg L− 1) is the concentration of TC at t min, and kobs (min− 1) and k [g (mg min− 1)] are the rate constants for the pseudo-first-order and pseudo-second-order kinetic models, respectively. As shown in Table 2, the correlation coefficients for the pseudo-first-order kinetic model (R2 = 0.94738) were greater than those for the pseudo-second-order kinetic model (R2 = 0.93004), which means that the pseudo-first-order kinetic model was more appropriate for oxidation.
Table 2
Best-fit kinetics parameters for TC oxidation.
Reaction
temperature (℃)
|
Pseudo-first-order kinetic model
|
Pseudo-second-order kinetic model
|
k1 (min− 1)
|
R2
|
k2 (gmg− 1min− 1)
|
R2
|
25
|
0.08357
|
0.94738
|
0.17699
|
0.92126
|
3.5 Comparison of various materials
The double-layer magnetic MOF (Fe3O4@ZIF-8@ZIF-67) was compared with the single-layer magnetic MOF (Fe3O4@ZIF-8) and other common antibiotics adsorbents (AC and nZVI), to reflect the adsorption performance. Figure 13 shows that the adsorption efficiency of Fe3O4@ZIF-8@ZIF-67 (90.01%) was two or more times that of Fe3O4@ZIF-8 (38.47%), which is attributed primarily to the high porosity and larger specific surface area of the former, which results from the double-layer structure. In contrast with AC (68.53%) and nZVI (18.01%), Fe3O4@ZIF-8@ZIF-67 showed an exceedingly good adsorption efficiency. The above results indicate that Fe3O4@ZIF-8@ZIF-67 was an outstanding TC adsorbent.
A comparison of the synergetic adsorption and Fenton-like oxidation of TC by various materials is shown in Fig. 14. For the two classical Fenton reagents: Fe (nZVI) and Fe2+ (FeSO4•7H2O), as high as 82.70% and 84.50% TC, respectively, could be removed in 100 min, which depended on •OH radical generation. By comparison, Fe3O4@ZIF-8@ZIF-67 (95.47%) exhibited a superior removal performance compared with the conventional Fenton reagents. After the reaction, nZVI decreased because of oxidative degradation and transformed into Fe2+/Fe3+, such as FeSO4•7H2O, which remained in solution and yielded difficulties in separation and recycling(Guo et al., 2020).
To investigate the synergetic removal mechanism of the structure and composition of Fe3O4@ZIF-8@ZIF-67, Fe3O4, Fe3O4@ZIF-8, and ZIF-8@ZIF-67 were applied to this catalytic system. With the existence of H2O2, the TC removal efficiency in the Fe3O4@ZIF-8 system increased by 7.99%, whereas the ZIF-8@ZIF-67 system (93.81%) showed an increase of 34.85% (Figs. 13 and 14). Fe3O4@ZIF-8 or ZIF-8@ZIF-67 has a similar structure to Zn-ZIFs, which indicates that Co-ZIFs had a high catalysis and was the primary active substance. Zn-ZIFs could not catalyze H2O2 but the adsorption was dominated because of the stable valence states of Zn (Wu et al., 2020). The promotion of removal efficiency in the Fe3O4@ZIF-8 system resulted mainly from the slight catalysis of Fe3O4 NPs that were attached to the ZIF-8 surface (Fig. 14). Only 5.46% TC removal resulted, given that the loaded Fe3O4 NPs probably occupied part of the active sites of Co-ZIFs, which resulted in the Co-ZIFs failing to provide a high catalytic performance. Nevertheless, Fe3O4@ZIF-8@ZIF-67 exhibited a significantly higher removal than other materials in this work.
3.6 Removal mechanism
The Fenton-like oxidation reaction mainly catalyzes the H2O2 to produce potent oxidative hydroxyl radicals (•OH) to degrade organic pollutants. To determine the type of free radicals that were produced during the reaction, tert-butyl alcohol was introduced as a quencher for •OH. As shown in Fig. 15, with the increase in tert-butyl alcohol concentrations, the TC removal rate decreased from 94.48–84.99%, which indicates that tert-butyl alcohol scavenged and trapped the generated •OH. TC removal at a high tert-butyl alcohol concentration depended mainly on the adsorption reaction.
Based on these results, a proposed pathway and mechanism for Fenton-like oxidation of TC using Fe3O4@ZIF-8@ZIF-67 was proposed. This possible mechanism was supported by the obtained characterization (SEM, EDS, XRD, FTIR, VSM, and TG), and kinetics (adsorption and oxidation) data suggested that Fe3O4@ZIF-8@ZIF-67 functioned as an adsorbent and a catalyst in the Fenton-like oxidation system. TC molecules were adsorbed rapidly on to the Fe3O4@ZIF-8@ZIF-67. Co2+ contacted the H2O2 to generate numerous •OH radicals. TC molecules were oxidized and mineralized to H2O and CO2 with adsorption proceeding until reaction equilibrium.
3.7 Fe3O4@ZIF-8@ZIF-67 reusability
Figure 14 shows the reusability of the adsorption and Fenton-like oxidation experiments of Fe3O4@ZIF-8@ZIF-67 over five cycles. The removal efficiencies of TC in the adsorption and oxidation experiments decreased to different extents; the removal efficiency in the oxidation experiment was always better than that in the adsorption. With a high removal performance maintained, the removal rate in the adsorption experiment decreased from 88.96–79.27%, and that in the oxidation experiment decreased from 93.38–82.94%, which was sufficiently high for practical application. The decrease in removal performance after repeated use may result because of the drop in adsorption performance that results from the change in porous structure that is caused by clearing and washing between the repeated experiments, or the leaching of the loaded Fe3O4 as Fe2+/Fe3+, which cannot transfer Co3+ to Co2+, and results in a low Fenton-like oxidation efficiency.