3.1 SEM Analysis
The morphology and microstructure of materials were characterized using SEM and mapping analyses. Figure 2a shows the microstructure of the PC and it can be seen that the PC indicates the presence of distinct pore structures that provide a rich carrier for ZIF-67 loads. After a facile in-situ growth process, a large numbers of ZIF-67 nanosheet are successfully covered evenly on the surface of PC and it has a sheet structure about 400 nm in length (Fig. 2b). The ZIF-67/PC is finally transformed into ZIF-67/PC@C via carbonization in a N2 atmosphere. After carbonization, the morphology of ZIF-67/PC@C shows that small particles with irregular shape are covered on the surface of PC (Fig. 2c). In addition, EDS mapping of Fig. 2(d-h) shows that C, N, O, and Co elements are evenly distributed in ZIF-67/PC@C. The inefficiency of separating the powdered formulation from the treated water was overcome by adding metal cobalt to make ZIF-67/PC@C magnetic.
The N2 adsorption-desorption isotherms for PC, ZIF-67/PC and ZIF-67/PC@C are presented in Fig. 3, and the specific surface area and pore volume are shown in Table 1. According to the IUPAC classification, the isotherms of PC, ZIF-67/PC and ZIF-67/PC@C can be classified as type I, indicating that there are abundant micropores and mesopores in PC, corresponding to the reversible adsorption process of langmuir monolayer, indicating that the combination of PC and MOFs does not change the adsorption type of PC. The specific surface area of PC is 1859.08 m²/g. The specific surface area of ZIF-67/PC after impregnation with ZIF-67 is 928.59 m²/g, and that of ZIF-67 /PC@C after pyrolysis at 800℃ is 1091.14 m²/g. The micropore volume and average pore size of PC, ZIF-67/PC and ZIF-67/PC@C shows the same trend, and the micropore volume of PC, ZIF-67/PC and ZIF-67/PC@C are 0.47, 0.21 and 0.30 cm3/g respectively and the average pore sizes of PC, ZIF-67/PC and ZIF-67/PC@C are 2.13, 2.17 and 2.25 nm, respectively. The decrease in specific surface area, micropore volume and average pore size of PC after impregnation with ZIF-67 may be resulted from the blocking of some pores by ZIF-67. The surface area of ZIF-67/PC@C increases to 1091.14 m2/g after calcination at 800 ℃, indicating that the pyrolysis of ZIF-67 at 800 ℃ causes some pores to be cleared and the specific surface area to increase.
Table 1
The porosity characteristics of BLPC synthesized at different parameters.
Parameters | PC | ZIF-67/PC | ZIF-67/PC @C |
Specific surface area (m2/g) | 1,876.88 m²/g | 928.59 m²/g | 1,091.14 m²/g |
Micropore volume (cm3/g) | 0.47cm3/g | 0.21cm3/g | 0.30 cm3/g |
Average pore diameter (nm) | 2.13 nm | 2.17 nm | 2.25 nm |
3.2 XRD, FTIR and XPS Analyses
The XRD patterns of the PC, ZIF-67/PC and ZIF-67/PC@C samples are shown in Fig. 4a. Two broad peaks at 23° and 43° are the typical diffraction peaks for PC, which is consistent with amorphous and disordered carbon material[26]. After ZIF-67 impregnation, several new peaks emerge at 2θ < 30°, which is in accordance with the peaks of ZIF-67, indicating that ZIF-67 is loaded onto PC successfully. Most of the peaks of ZIF-67 disappear after further pyrolysis, and only two broad peaks of PC could be observed due to the fact that the crystal structure of ZIF-67 is decomposed after calcination at high temperature.
FTIR spectra of PC, ZIF-67/PC and ZIF-67/PC@C are illustrated in Fig. 4b. An obvious peak near 3410 cm− 1 is attributed to stretching vibration of hydroxyl and carboxyl. The strong absorption peak around 2300 cm− 1 is assigned to characteristics peak of carbon dioxide trapped on the surface of PC. The peaks located around 1500 and 1600 cm− 1 are attributed to C = N stretching vibrations groups. Sharp peak around 1100 cm− 1 is assigned to C-O-C linkages.
The chemical valence of Co in ZIF-67/PC@C was further analyzed by XPS measurement. According to Fig. 4c, XPS spectra further confirmed that Co and N were successfully loaded onto the ZIF-67/PC@C surface. Four distinguishable peaks of Co2p are shown in the high-resolution narrow scan due to spin-orbit splitting (Fig. 4e). In Co2p narrow high resolution scanning (Fig. 4b) of 781.2, 786.2, 797.81 and 804.1 eV in the binding energy of correspond to the Co3+ 2 p3/2, Co2+2p3/2, Co3+2p1/2 and Co2+2p1/2, This indicates that cobalt mainly coexists as Co2+ and Co3+.
3.3 Adsorption kinetics
The adsorption capacity of PC, ZIF-67/PC and ZIF-67/PC @C as a function of adsorption time was investigated and the results are shown in Fig. 5. Adsorption capacity significantly increases at the initial stage and then slightly increase with rise of contact time. The adsorption capacity of PC, ZIF-67 /PC and ZIF-67 /PC@C for MG for 24 h of adsorption time are 1277,1786 and 1810 mg/g, respectively .Obviously, the improvement of adsorption performance of carbon is related to the ZIF-67. The models of pseudo-first-order and pseudo-second-order were used to analyze the adsorption kinetics of PC, ZIF-67/PC和ZIF-67/PC@C for MG according to Equations (3) and (4), respectively.
\({Q}_{t}={q}_{e}\times (1-{e}^{-{K}_{1}t})\) (3)
\({Q}_{t}=\frac{{q}_{e}^{2}{K}_{2}t}{1+{q}_{e}{K}_{2}t}\) (4)
Kinetic parameters such as rate constants (k1, k2), equilibrium adsorption capacity (qe) and correlation coefficient (R2) obtained by equation fitting are shown in Table 2. Compared with the quasi-first-order model, the correlation coefficients between the quasi-second-order model and PC, ZIF-67/PC and ZIF-67/PC@C are 0.98, 0.97 and 0.99, respectively, which are much higher than those of the quasi-first-order model, indicating that the quasi-second-order model has better applicability to the adsorption process.
Table 2
Parameters of kinetics models for the adsorption on PC, ZIF-67/PC and ZIF-67/PC@C
sample | qe,exp (mg/g) | Pseudo-first-order kinetic model | Pseudo-second-order kinetic model |
| | k1(10− 2 min− 1) | qe (mg/g) | R2 | k2 (g/mg min) | qe (mg/g) | R2 |
PC | 1254 | 2.26 | 1232 | 0.89 | 0.044 | 1308 | 0.98 |
ZIF-67/PC | 1765 | 2.28 | 1737 | 0.92 | 0.054 | 1821 | 0.97 |
ZIF-67/PC@C | 1772 | 2.60 | 1738 | 0.86 | 0.037 | 1822 | 0.99 |
3.4 Adsorption isotherm
The adsorption mechanism of MG on PC, ZIF-67/PC and ZIF-67/PC@C was analyzed by Langmuir and Freundlich adsorption isotherms. The Langmuir isotherm assumes that the adsorbent is covered by a single layer on the surface of a uniform adsorbent. Langmuir equation refer to Eq. (5).
$${Q}_{e}=\frac{{Q}_{\text{o}}{K}_{t}{C}_{e}}{1+{K}_{t}{C}_{e}}$$
5
Qe (mg/g) is the adsorption capacity of the adsorbent at the Ce concentration. Qo (mg/g) is the maximum adsorption capacity of the adsorbent. Kt (L/mg) is Langmuir adsorption constant, and Ce (mg/L) is the equilibrium concentration of MG. The Freundlich isotherm indicates the presence of multiple layers on the surface of the adsorbent. Freundlich equation refer to Eq. (6).
.\({Q}_{e}={K}_{F}{C}_{e}^{\frac{1}{n}}\) (6)
Qe (mg/g) is the adsorption capacity of the adsorbent at the Ce concentration. Ce (mg/L) is the equilibrium concentration of MG. KF and n are Freundlich constants, which represent adsorption capacity and adsorption intensity of the adsorbent, respectively. The adsorption isotherm fitted with Langmuir and Freundlich models are illustrated in Fig. 6a and the calculated parameters are listed in Table 3. It can be seen from Table 3 that R2 values calculated from Langmuir model is higher than that from Freundlich model. This suggests that the data fitted Langmuir equation well and furthermore the MG adsorption onto three samples is probably in a monomolecular-layer adsorption. It could be seen that the amount of adsorbed MG gradually increases with the rise of MG concentration for PC, ZIF-67/PC and ZIF-67/PC@C samples. Obviously, ZIF-67/PC and ZIF-67/PC@C adsorb much more MG than PC at the same condition. The maximum adsorption capacity of ZIF-67/PC is 1783 mg/g, and that of ZIF-67/PC@C is 1808 mg/g, indicating the superior performance of PC after ZIF-67 modification. This high capacity is resulted from contributed by the surface imidazole ring in 2-MIM or Co oxide nanoparticles, which could act as active sites for MG adsorption[27]. The comparison of adsorption capacity of various adsorbents for MG is listed in Fig. 6b[28–33]. It can be seen that the high adsorption capacity make ZIF-67/PC@C for great potential in practical application.
Table 3
Parameters of Langmuir and Freundlich isotherms using PC, ZIF-67/PC and ZIF-67/PC@C.
sample | Langmuir | Freundlich |
| Qm (mg/g) | KL (L/mg) | R2 | KF (L/g) | | R2 |
PC | 1050.15 | 0.53 | 0.99 | 431.27 | | 0.97 |
ZIF-67/PC | 1639.42 | 0.34 | 0.99 | 812.73 | | 0.97 |
ZIF-67/PC@C | 1644.52 | 0.33 | 0.99 | 814.21 | | 0.97 |
3.5 Recyclability
Reusability is an important factor for practical use of adsorbents. After each adsorption, ZIF-67/PC and ZIF-67/PC@C were washed with ethanol and dried for next use. Figure 7 shows the adsorption efficiency of ZIF-67/PC and ZIF-67/PC@C after five adsorption and desorption cycles. ZIF-67 /PC has poor cycling performance, with the adsorption efficiency remaining only 42% after one cycle and almost disappearing after four cycles. the adsorption efficiency of ZIF-67/PC@C is 82% of the initial adsorption efficiency after five cycles. The results shows that the cyclic properties of ZIF-67/PC could be significantly improved after high temperature pyrolysis and can be reused for the adsorptive removal of MG. As shown in Fig. 7c, the magnetic characterization of ZIF-67/PC@C composite was determined by using VSM at room temperature. The saturation magnetization (Ms) of ZIF-67/PC@C is 5.419 emu/g, which allows ZIF-67/PC@C to be quickly and easily separated from aqueous solution with an external magnetic field.
3.6 Adsorption mechanism
The adsorption of MG by PC not only depends on the pore filling caused by high porosity, but also has many adsorption mechanisms. MG with positive charge is easily attracted by electrostatic adsorption on the surface of PC, resulting in electrostatic adsorption. There are a large number of carboxyl and hydroxyl groups on the surface of PC, which can provide a large number of hydrogen donors and combine with the hydrogen bond acceptor of MG to produce hydrogen bond interaction, resulting in the adsorption of MG to PC. The benzene ring on PC can produce π-π interaction with MG, which is helpful for the adsorption of MG on PC. In addition, MG molecule has the flat ring structure energy and the van der Waals force exists on PC surface to produce the adsorption effect. Based on the analyses of the kinetics and adsorption isotherm, the adsorption capacity of PC to MG was significantly improved after ZIF-67 is successfully loaded onto PC. ZIF-67 was sheet structure and these nanosheets consist of 2-MIM, which are connected to cobalt ions as illustrated in Fig. 8. The structure of imidazole rings is similar to that of aromatic hydrocarbons, which can be tightly bound by π-π stacking interactions. This special interaction may enable ZIF-67 to greatly improve the adsorption performance of MG on the surface of PC.
3.7. Catalytic degradation of ZIF-67/PC@C
The use of ZIF-67/PC@C as heterogeneous catalyst to activate PMS for the degradation of MG was investigated. The degradation kinetics is shown in Fig. 9a. It can be seen that PC shows weak catalytic activity for the degradation of MG, with a low removal rate of 26.3%. The degradation performance of ZIF-67 modified PC on MG is significantly improved and the degradation rate gradually improves for the prolonged contact time. The MG removal rate reaches 100% after adsorption for 8 min, indicating that ZIF-67/PC@C could activate PMS efficiently for the MG decolonization in aqueous solution. This is resulted from the degradation of MG by SO4−· radical dot and generated from the reaction between HSO5−and Co2+ incorporated in ZIF-67/PC@C[34]. Besides, SO4−· radical dot could also react with water to generate ·OH radical dot, which is also able to degrade MG. Additionally, the coexistence of Co and N on the surface of ZIF-67/PC may have synergistic effect for PMS activation, which further enhances its degradation performance. The time-dependent MG concentration profile was measured with different initial concentrations of PMS as shown in Fig. 9b. The removal efficiency of MG increases from 41–100% when the PMS concentration increases from 200 mg/L to 600 mg/L, indicating that increase of initial PMS concentration can promote the removal rate of MG, because catalyst could have more sufficient contact with PMS and generate more SO4−· radical dot in the solution.
3.8. Reaction mechanism and degradation pathways
It has been reported that ·OH and SO4−· are important reactive radicals in ZIF-67/PC@C and PMS system for dyes degradation[35]. Therefore, in order to verify radicals generated during the MG degradation process, tert-butanol (TBA), Ethanol (EtOH), and methanol (MeOH) were employed as probes to quench radicals involved in the ZIF-67/PC@C and PMS system. MeOH can be used as a radical scavenger for SO4−·. TBA without α-H can be used as a radical scavenger for radical ·OH Ethanol (EtOH) is used as a radical scavenger to quench ·OH and SO4−· in the reaction solution[36]. 100% MG could be removed without any scavenger for 10 min as shown in Fig. 10. However, the degradation rate of MB decreased from 100–91% and 56% when 1 M TBA and MeOH are added to the aequous solution of MB, respectively. Obviously, the free radical quenching effect of TBA is less than that of MeOH, indicating that ·OH had little effect on MG degradation, and SO4−· was the main active free radical in MB degradation. In addition, the addition of 1 M EtOH can still achieve the degradation efficiency up to 38% because there are also non-free radical reactions in the degradation process of MG. The self-decomposition of PMS will produce certain singlet oxygen (1O2), and 1O2 can also promote the decomposition of MG.
Combined with the experiment results, the possible mechanism of PMS activation by ZIF-67/PC@C for MG degradation is shown in Fig. 11. MG could be oxidized through radical and non-radical oxidation pathways in the ZIF-67/PC@C and PMS system. For the radical oxidation pathway, the Co nanoparticles contribute to the generation of SO4−· and ·OH, while Co is oxidized to Co2+ (Eqs. 7). After that, PMS quickly oxidizes the released Co2+ to Co3+ and generated SO4−· (Eqs. 8). Co3+ could also react slowly with HSO5− and be reduced to Co2+ (Eqs. 9). Some of the produced SO4−· radicals could react with H2O to generate ·OH (Eqs. 10).
\(\text{C}\text{o}+{2\text{H}\text{S}\text{O}}_{5}^{-}\) →\({Co}^{2+}+{2OH}^{-}\)+\({2SO}_{4}^{-·}\) (7)
\({Co}^{2+}+{\text{H}\text{S}\text{O}}_{5}^{-}\) →\({Co}^{3+}+{OH}^{-}\)+\({SO}_{4}^{-·}\) (8)
\({Co}^{3+}+{\text{H}\text{S}\text{O}}_{5}^{-}\) →\({Co}^{2+}\)+\({SO}_{5}^{-·}\)+\({H}^{+}\) (9)
\({SO}_{4}^{·-}+{H}_{2}O\) →\({SO}_{4}^{2-}\)+\(\bullet OH\)+\({H}^{+}\) (10)
For the non-radical oxidation pathway, the generation of 1O2, as the dominant radicals, originates from the self-decomposition of PMS (Eqs. (11), (12) and (13)). Additionally, the graphitic N had a strong adsorption ability to catch PMS molecules[37] and the generation of 1O2 would be accelerated by the PMS accumulation (Eqs. 11). Besides, the C = O bond at the defective boundary of graphene could activate PMS through nucleophilic addition to generate 1O2 (Eqs. 14)[38].
\({SO}_{5}^{2-}+{\text{H}\text{S}\text{O}}_{5}^{-}\) →\({SO}_{4}^{2-}+{\text{H}\text{S}\text{O}}_{4}^{-}\)+1O2 (11)
\({SO}_{4}^{·-} +{\text{H}\text{S}\text{O}}_{5}^{-}\) →\({SO}_{5}^{-·}+{\text{H}\text{S}\text{O}}_{4}^{-}\) (12)
\({2SO}_{5}^{·-}+{H}_{2}O\) →\({SO}_{4}^{2-}+{2\text{H}\text{S}\text{O}}_{5}^{-}\)+1O2 (13)
\(\text{C}=\text{O}+{\text{H}\text{S}\text{O}}_{5}^{-}\) +\({OH}^{-}\)→\(\text{C}=\text{O}+2{SO}_{4}^{-·}+{H}_{2}O\)+1O2 (14)