Refractory organic pollutants originate from different industries [1] and can remain in the environment for a long time after their discharge. Because of the toxicity, carcinogenicity, mutagenicity, and teratogenicity of refractory organic pollutants, their presence in effluents may pose a great threat to human health and aquatic life, even at low levels [2]. Advanced oxidation processes (AOPs) have emerged as efficient processes for the complete degradation of organic compounds in aqueous media. These processes are based on the in situ generation of highly reactive radicals, such as the hydroxyl radical (OH•), O2-•, OOH•, and the sulfate radical (SO4-•), for mineralization of refractory organic pollutants [3-7]. Although the OH•-involved AOPs (e.g., Fenton reaction) are well established and widely used, SO4-•-involved AOPs have received increasing attention because of their advantages over the former [8-12]. For the generation of SO4-•, peroxymonosulfate (PMS, a component of Oxone) is recognized as the most common commercially available oxidant and is also an environmentally friendly reagent [13]. The generation of SO4-• from PMS can be achieved via several routes, such as thermal treatment [14], ultraviolet (UV) irradiation [15], sonication [16], and utilization of catalysts [17-19]; however, the use of catalysts appears to be the most feasible route [20]. Various catalysts, such as bases [21], transition metals [22, 23], zero-valent metals [24, 25], and metal-free materials [26], have been developed to activate PMS. Transition metals have been shown to be the most efficient catalysts, with Co ions specifically serving as the best catalysts [27]. However, neither homogeneous Co2+/PMS nor heterogeneous Co-supported catalysts can completely avoid the loss or leaching of Co ions, which thus leads to secondary heavy-metal pollution [26, 28]. Therefore, for environmental remediation, there is a strong need to develop novel efficient catalysts to activate PMS without the associated leaching of toxic metals.
Thus far, Mn oxides have attracted considerable attention as heterogeneous catalysts [29-33], adsorbents [34, 35], and battery materials [36-38] because these oxides, including MnO, MnO2, Mn2O3, and Mn3O4, are abundant on earth, almost non-toxic to the ecosystem, and possess a redox cycle between +2, +3, and +4 valences. Wang's group [39] fabricated Mn oxides in different oxidation states and used them for activating PMS for phenol degradation; they observed the following catalytic activity sequence: Mn2O3 > MnO > Mn3O4 > MnO2. Despite the high catalytic activity of single Mn oxide particles, especially nanoparticles, the extreme fineness of the oxides tends to cause particle aggregation into the bulk, thus depressing the catalytic activity and limiting their practical application. To overcome this difficulty, Mn-oxide-supported materials have been investigated. Han et al. [40] reported that Mn3O4-supported SBA-15 exhibited high activity for ethanol degradation in the presence of H2O2. In addition, Mn3O4-reduced graphene oxide hybrids have been fabricated and applied in the PMS-activated decomposition of Orange Ⅱ, displaying higher activity than single Mn3O4[41]. Similar composites involved in AOPs include graphene oxide/Mn3O4[42] and graphene/MnO2[43], where the aggregation of Mn oxide nanoparticles was efficiently reduced. In addition, Al2O3[44], SiO2[45], ZrO2[46], Fe3O4[47, 48], and TiO2[49] have also been used as supports for hybridization with Mn oxides, displaying the specific properties of catalysts, absorbents, or electrode materials.
Metal-organic frameworks (MOFs) have received considerable attention for catalytic application because of their large surface area, uniform but tunable cavity, and tailorable chemistry [50-52]. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs with zeolite-type topology and outstanding thermal and chemical stabilities [53]. To date, a number of ZIFs with rich structural and topological diversity have been prepared by virtue of the flexibility of the metal and link variation [54]. Compared with SBA-15 [55], graphene oxide [42], Fe3O4[47], and ZrO2[46], which require high energy consumption during preparation or complicated synthesis procedures, ZIFs are much easier to prepare with mild reaction conditions. Some studies have demonstrated the use of ZIFs and ZIF-derived materials as heterogeneous catalysts for activating PMS. For instance, Lin's group [56] reported for the first time that ZIF-67 (a Co-based MOF) could efficiently activate PMS to degrade rhodamine B (RhB) in water. They also reported several MOF-derived materials for activating PMS to degrade RhB, acid yellow, and caffeine in water [57-59]. One of the ZIFs with sodalite topology, ZIF-8 (Zn(C4H5N2)2) with a molecular weight of 227 g/mol, cavity of 11.6 Å, and pore aperture of 3.4 Å [60], has attracted considerable attention because its hydrothermal stability is better than those of other MOFs. Au, Ag, Pd, and Ru nanoparticles have been supported on ZIF-8 using different processes, and the resulting structures have been applied in catalytic reactions other than PMS activation [61-63]. Additionally, some metal oxide (ZnO, TiO2, etc.)-supported ZIF-8 structures acting as photocatalysts to degrade methylene blue or RhB have also been investigated [64, 65]. Nevertheless, to the best of our knowledge, there have been no investigations on the preparation of Mn3O4/MOFs or their applications.
RhB is a carcinogenic, toxic, and highly water-soluble xanthene organic compound with a complex structure and high stability that is widely used in various industries, resulting in a recognized refractory-organic-pollutant-containing effluent [66, 67]. The major techniques employed to remove RhB from water include adsorption [2], photocatalytic oxidation [68], the Fenton process [69], ultrasonic degradation [70], ozonation [71], and electrochemical oxidation [72]. Adsorption is easy to achieve with a high removal efficiency; however, this process only transfers the pollutants from the liquid to solid phase, resulting in secondary pollution. Other techniques require special equipment or devices or high energy consumption, resulting in high capital and operating costs. The Fenton process is a rapid and inexpensive process; however, it has several significant disadvantages such as the acidic pH (pH = 2-4), production of large amounts of iron sludge, and low total organic carbon removal efficiency.
In this study, an environmentally friendly composite, Mn3O4/ZIF-8, was first prepared by a solvothermal method and then utilized for the PMS-activated degradation of RhB in aqueous solutions. The effects of the preparation conditions on the catalyst performance and of several reaction parameters on the catalytic degradation of RhB were examined. Furthermore, a mechanism for the catalyst preparation and RhB degradation was proposed, and the regeneration and reusability of the composite were investigated.
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 2-methylimidazole (C4H6N2, 2-HMIM), sodium formate (NaCOOH·2H2O), N, N-dimethylformamide (DMF), potassium permanganate (KMnO4), sodium nitrite (NaNO2), methanol (MeOH), ethanol (EtOH), and tert-butyl alcohol (TBA) were purchased from Sinopharm Chemical Reagent Co., Ltd. Oxone (2KHSO5·KHSO4·K2SO4, KHSO5, with PMS as an active component) was purchased from Shanghai Future Chemical Technology Co., Ltd. All the reagents were used as-received without further purification. Deionized water was used throughout the investigation.
ZIF-8 was prepared using a previously reported facile room-temperature colloidal chemistry route with modifications [73-75]; sodium formate was used to enhance the heterogeneous nucleation and intergrowth of ZIF-8 crystals [76]. A typical synthesis is described as follows. Zn(NO3)2·6H2O (8.77 g) was dissolved in 50 mL of DMF, and 9.20 g NaCOOH·2H2O and 9.68 g 2-HMIM were mixed and dissolved in 25 mL of DMF. Then, the former solution was added to the latter. The molar ratio of the final growth solution was 1:4:3:33 (Zn(NO3)2·6H2O: 2-HMIM:NaCOOH·2H2O:DMF). The solution turned milky after several minutes and was stirred at room temperature for 4 h for ZIF-8 growth. The resulting solid products were recovered by centrifugation at 3000 r/min for 5 min. Once separated, they were alternatively resuspended in methanol and deionized water for rinsing and centrifugation. This operation was repeated 5-6 times; finally, the solid materials were dried in air at 70 ℃ overnight.
Mn3O4 samples were prepared using the facile solvothermal method reported by Zhang et al. [77] with some modifications. Briefly, 1.04 g KMnO4 was dissolved in 120 mL of aqueous ethanol (60%) at room temperature to form a homogeneous solution. The solution was then transferred into a 200-mL Teflon-lined stainless steel autoclave and heated at 120 ℃ for 8 h. After naturally cooling to room temperature, the resulting precipitates were recovered by centrifugation, washed with deionized water, and finally dried in air at 70 ℃ overnight.
We successfully prepared Mn3O4/ZIF-8 as a novel composite. In a typical synthesis, 0.50 g of the prepared ZIF-8 was dispersed into 120 mL of aqueous ethanol (60%) with stirring for 1 h. Then, 1.04 g KMnO4 was added to the suspension, which was magnetically stirred for 1 h to ensure that the KMnO4 was completely dissolved. Next, the mixture was transferred into a 200-mL Teflon-lined stainless steel autoclave, which was sealed and heated to 120 ℃. After incubation for 8 h, the autoclave was naturally cooled to room temperature. The resulting precipitates were separated by centrifugation, washed with deionized water, and finally dried in air at 70 ℃ overnight. The prepared sample was labeled 0.5-Mn/ZIF-120.
By modifying the ZIF-8 and KMnO4 dosages and the solvothermal temperature, we obtained different Mn3O4/ZIF-8 composites, which were labeled n-Mn/ZIF-120, where n represents the Mn3O4 loading on ZIF-8 (mass ratio of Mn3O4 to ZIF-8), with values of 0.25, 0.5, 1.0, and 1.5.
For comparison, a mechanical mixture of ZIF-8 and Mn3O4 was also prepared and was labeled Mn3O4+ZIF.
X-ray diffraction (XRD) patterns of the samples were obtained using a Rigaku D/Max-2200X powder X-ray diffractometer with a Cu Kα radiation source of wavelength 1.54056 Å at 40 kV and 40 mA. The wide-angle data were collected from 5° to 80° (2θ) with a scan speed of 8°/min. The external morphologies of the samples were examined using scanning electron microscopy (SEM; JEOL JSM-6700F). The internal structures of the samples were characterized using high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2010F) with a field-emission gun at 200 kV. The surface element compositions and chemical states of the samples were analyzed using X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi) with 300-W Al Kα radiation. Fourier-transform infrared (FT-IR) spectra were recorded from KBr pellets in the range of 4000-400 cm-1 on a Nicolet 6700 spectrometer.
The catalytic activities of the prepared materials were evaluated based on the degradation of RhB in aqueous solutions at room temperature. The RhB degradation tests were performed in an 800-mL glass reactor containing 500 mL of the RhB solutions. The reactants were stirred at 4000 r/min. Unless specifically stated, the reaction temperature was maintained at 23 ℃, with an initial solution pH of 5.18 and RhB concentration of 10 mg/L. In a typical test, a fixed amount of catalyst was added to the RhB solution and stirred for 30 min to achieve adsorption-desorption equilibrium; then, a known amount of PMS was added to the reactor to initiate the degradation. At fixed intervals, 6-mL samples of the suspension were removed with a syringe and quenched with excess sodium nitrite to prevent further reaction. The suspensions were separated by centrifugation at 3000 r/min for 5 min to obtain the supernatant for subsequent analysis.
For the catalyst recyclability tests, the used solid material was recovered by filtration, washed with deionized water, and then dried in an oven at 70 ℃ for 12 h before the next use. Several parallel tests were conducted for every run before the last run to ensure that there was sufficient catalyst for the next run.
The RhB concentrations were quantified using a ultraviolet-visible (UV-vis) spectrophotometer (UV-5300PC, Shanghai) at 552 nm. The Mn concentration in the aqueous solution was measured using inductively coupled plasma (ICP) emission spectrometry (Prodigy). UV-vis spectra were obtained using a UV-vis spectrophotometer (UV-5300PC, Shanghai) at different reaction times, with the spectra scanned from 300 to 650 nm. The principal degradation products of RhB were detected by gas chromatography-mass spectrometry (GC-MS) using an Agilent 7890A gas chromatograph equipped with an HP-5 MS capillary column (30 m × 0.25 mm × 0.25 μm) combined with an Agilent 5975C mass spectrometer. For the gas chromatography, the injection temperature was 250 ℃; the initial temperature of 70 ℃ was maintained for 2 min and increased to 300 ℃ at a heating rate of 20 ℃/min, which was maintained for another 6 min; the flow rate of He was 1 mL/min; and the sample injected was 1 μL. For the mass spectrometry, an HP-5 MS capillary column was used, along with electrospray ionization and an ionizing energy of 70 eV, scanning range of 41-450 amu, and ion source and transmission line temperatures of 250 ℃.
The tests were generally conducted in duplicate, and the experimental errors were all below 3%.
The crystal structures of the synthesized ZIF-8 and Mn3O4/ZIF-8 composites were determined using powder XRD analysis. As observed in Fig. 1(a), the characteristic peaks of the as-synthesized ZIF-8 matched very well with the simulated ZIF-8 pattern, indicating that the sample consisted of a single ZIF-8 phase [54]. Fig. 1(b) displays the XRD patterns of the n-Mn/ZIF-120 composites with different Mn3O4 loadings. The characteristic peaks of both Mn3O4 and ZIF-8 were detected in the composite samples, implying that Mn3O4 was successfully dispersed onto the surface of ZIF-8 without destruction of the ZIF-8 structure, verifying the high hydrothermal stability of ZIF-8 during the synthesis process. With increasing Mn3O4 loading, the intensities of the Mn3O4 peaks increased, demonstrating the improved crystallinity with higher Mn3O4 loading. Moreover, no additional peaks were detected, suggesting the high purity of the Mn3O4/ZIF-8 composites as end products.
SEM images of the prepared samples are presented in Fig. 2(a) and (b). The 0.5-Mn/ZIF-120 sample exhibited a cauliflower-like surface morphology, and Mn3O4 sheets with sizes ranging from a few tens to hundreds of nanometers were observed on the surface of ZIF-8. The average particle size of ZIF-8 was approximately 250 nm. To obtain insight into the structure of the 0.5-Mn/ZIF-120 composite, TEM and HRTEM images were obtained, as shown in Fig. 2(c) and (d). Fig. 2(c) clearly shows that the Mn3O4 nanoparticles with sizes of 50-150 nm were closely deposited on the surface of ZIF-8. Fig. 2(d) presents an HRTEM image of the 0.5-Mn/ZIF-120 composite with well-defined lattice fringes, which indicates that the Mn3O4 nanoparticles were highly crystallized. The lattice spacings were approximately 0.31, 0.25, 0.24, 0.22 and 0.15 nm, corresponding to the (112), (211), (202), (220) and (321) planes of Mn3O4 (JCPDS No. 24-0734), respectively, which is in accordance with the XRD results. No evidence of the presence of Mn3O4 inside the ZIF-8 was observed in any of the TEM images. Fig. 2(e)-(g) presents energy-dispersive X-ray spectrometry (EDX) elemental mappings of Fig. 2(c). Most of the Mn was located in the outer layer of the sample, whereas Zn was mainly concentrated in the core region, indicating the dispersion of Mn3O4 nanoparticles on the external surface of ZIF-8 instead of in its internal structure.
To understand the composition and chemical bonding of the 0.5-Mn/ZIF-120 composite, XPS measurements were performed (Fig. 3). Strong peaks of Mn 2p, Zn 2p, C 1s, N 1s, and O 1s are observed in Fig. 3(a). In the Mn 2p region (Fig. 3(b)), two peaks are observed at binding energies of 641.5 and 652.9 eV, which correspond to the Mn 2p1/2 and Mn 2p3/2levels, respectively, and the splitting width (11.4 eV) is consistent with a previous report on Mn3O4 [78]. The Mn 2p3/2 peak could be further divided into two peaks. The peak at 640.6 eV was assigned to the Mn 2p3/2 peak of MnO and the other peak at 641.8 eV coincided with the Mn 2p3/2peak of Mn2O3. Based on the peak areas, Mn(Ⅱ) and Mn(Ⅲ) accounted for 37.82 and 62.18 of the total Mn3O4 supported on ZIF-8, respectively. In the Zn 2p profile (Fig. 3(c)), two peaks are observed at 1020.8 (Zn 2p3/2) and 1044.0 eV (Zn 2p1/2), which are compatible with Zn2+ of the ZIF-8 structure [79]. In the C 1s region, three peaks are observed: the carbon peak at lower binding energy, 284.4 eV, is consistent with adventitious carbon, which accumulates on nearly all samples exposed to air [80], and the carbon peaks at 284.9 and 286.0 eV are attributed to the presence of methyl imidazole groups and carbonates, respectively [81, 82]. Moreover, by fitting the N 1s region (Fig. 3(e)), two peaks were obtained: the main peak at 398.7 eV and a smaller peak at 399.9 eV, which were assigned to the imidazole groups and uncoordinated methyl imidazole linkers [81-83], respectively. For oxygen (Fig. 3(f)), three different peaks located at 529.3, 530.7, and 531.9 eV, respectively, were observed, which correspond to Mn-O-Mn, Zn-O, and carbonates in the Mn3O4/ZIF-8 composite, respectively [81]. The XPS analyses are in good agreement with the XRD results.
To further identify the molecular structure and functional groups ofthe as-synthesized ZIF-8 and n-Mn/ZIF-120, FT-IR spectra were recorded, and the results are presented in Fig. 4. For ZIF-8, the peak at 420 cm-1 is attributed to the Zn-N stretching mode. In addition, the peaks in the 500-1350 and 1350-1500 cm-1 spectral regions can be assigned as plane bending and stretching of the imidazole ring, respectively. Peaks for the C=N stretching mode at 1580 cm-1 and for the aromatic and aliphatic C-H stretching at 2930 and 3140 cm-1 are also observed [84]. Two new peaks appeared for the n-Mn/ZIF-120 composites at 494 and 484 cm-1, representing the vibration of the Mn-O stretching modes of tetrahedral and octahedral sites, respectively. In addition, in the FT-IR spectra of the n-Mn/ZIF-120 composites with different Mn3O4 loadings, the diffractions of both ZIF-8 and Mn3O4 can be detected. These results support the formation of a Mn3O4/ZIF-8 structure.
Based on the preparation procedures and characterization of the Mn3O4/ZIF-8 composites, the Mn3O4/ZIF-8 preparation route is presented in Scheme 1. First, ZIF-8 was synthesized at room temperature by dispersing Zn(NO3)2·6H2O in DMF and 2-HMIM and NaCOOH·2H2O in DMF and then mixing the two solutions together, resulting in the formation of ZnO, which acted as a heterogeneous nucleation site as well as a secondary metal source. Once the ZIF crystals were nucleated, NaCOOH served as a deprotonator, promoting the intergrowth of ZIF crystals [85]. Second, n-Mn/ZIF-120 composites with different Mn3O4 loadings were fabricated using a solvothermal process, where the prepared ZIF-8 was first dispersed in 60% EtOH solution to create a suspension. Then, a certain amount of KMnO4 was dissolved in the suspension to release MnO4-, which was able to coordinate the Zn2+ ions in ZIF-8 through electrostatic interaction. During the solvothermal reaction, MnO4- was reduced by EtOH as described in Eq. (1) [86], which led to the final inorganic product of crystalline Mn3O4.
The catalytic activity and stability of the prepared n-Mn/ZIF-120 composites were evaluated based on the RhB degradation and Mn leaching in solutions in the presence of PMS, respectively. Fig. 5 shows the effect of the Mn3O4 loading in n-Mn/ZIF-120 on the RhB degradation and Mn leaching. As observed in Fig. 5(a), the effect of Mn3O4 loading on the RhB degradation was significant, with the RhB degradation efficiency increasing with increasing Mn3O4 loading from 0.25 to 0.5 and then slightly decreasing with further increase of the Mn3O4 loading to 1.0. The optimal RhB degradation of 99.4% was achieved in 60 min at 0.5 Mn3O4 loading. However, further increase of the Mn3O4 loading led to a decreased RhB degradation efficiency because of the "covering effect"; that is, loading of a large amount of Mn3O4 onto ZIF-8 can result in heavy coverage of the active sites on the support surface. The Mn leaching results are presented in Fig. 5(b); upon increasing the Mn3O4 loading from 0.25 to 1.0, the Mn leaching slightly increased and then significantly increased at higher loadings. Thus, 0.5-Mn/ZIF-120 is regarded as the best catalyst to achieve the highest RhB degradation with the least Mn leaching and, thus, the highest catalytic activity and stability.
The removal of RhB based on adsorption with ZIF-8 and 0.5-Mn/ZIF-120, single chemical oxidation with PMS, and catalytic degradation with different catalysts in the presence of PMS were evaluated, and the results are presented in Fig. 6. In the single chemical oxidation, the degradation of RhB in 60 min was less than 20%, suggesting that PMS alone cannot efficiently oxidize RhB. Moreover, 0.5-Mn/ZIF-120 and ZIF-8 showed very limited adsorption for RhB. For the RhB catalytic degradation in the presence of PMS with 0.5-Mn/ZIF-120, Mn3O4, ZIF-8, and the mixture of Mn3O4 and ZIF-8 (Mn3O4+ZIF), 0.5-Mn/ZIF-120 exhibited the best catalytic activity with almost complete RhB degradation in 60 min. Meanwhile, the RhB degradation in the presence of ZIF-8 and PMS was greater than that in the presence of PMS alone, possibly because of the extra contribution of adsorption by ZIF-8. Notably, the Mn3O4+ZIF mixture contributed approximately 65% of the RhB degradation in the presence of PMS in 60 min, less than that contributed by Mn3O4, which may be due to PMS consumption by porous ZIF-8 adsorption.
Fig. 7 shows the effect of various 0.5-Mn/ZIF-120 dosages on RhB degradation in the presence of PMS. As observed in Fig. 7(a), the RhB degradation efficiency increased with increasing 0.5-Mn/ZIF-120 dosage. Almost complete degradation of RhB was achieved in 60 min for the 0.3 g/L of 0.5-Mn/ZIF-120 dosage. The inset in Fig. 7(b) presents the kinetic curves of RhB degradation, illustrating that the RhB catalytic degradation follows a first-order kinetics model ln C/C0 = -kt, where C and C0 represent the RhB concentration at time (t) and t = 0, respectively, and k (min-1) is the apparent reaction rate constant. As observed in Fig. 7(b), when the 0.5-Mn/ZIF-120 dosage increased from 0.1 to 0.4 g/L, k increased almost linearly from 0.0367 to 0.1520 min-1, which can be attributed to the increased numbers of adsorption sites for RhB and active sites for activation of PMS and, hence, more SO4-•.
The catalytic degradation of RhB was also affected by the PMS dosage, as shown in Fig. 8. The RhB degradation efficiency significantly increased with increasing PMS dosage from 0.1 to 0.3 g/L and only slightly changed at higher PMS dosages. Fig. 8(b) shows the corresponding relationship between the initial rate constant k and PMS dosage, with the inset presenting the kinetic fitting curves. The RhB degradation clearly fits the first-order kinetics model well. As observed in Fig. 8, the RhB degradation efficiency and degradation rate constant both increased first and then decreased with further increased PMS dosage. With increasing PMS dosage from 0.1 to 0.5 g/L, the RhB degradation efficiency in 40 min increased from 25% to 92%. Because PMS is the source of the reactive sulfate radicals, the increase of the PMS dosage promotes the generation of sulfate radicals and, hence, faster RhB degradation. However, because of the self-quenching of sulfate radicals by PMS, as indicated in Eq. (2), a further increase of the PMS dosage leads to lower RhB degradation rates [87].
Fig. 9 shows the effect of various initial RhB concentrations on the RhB degradation and degradation kinetics in the presence of 0.5-Mn/ZIF-120 and PMS. As observed in Fig. 9(a), the RhB degradation efficiency decreased with increasing initial RhB concentration. The inset in Fig. 9(b) presents the kinetic curves for RhB degradation, demonstrating that RhB degradation obeys a first-order kinetics model with respect to the substrate RhB. As observed in Fig. 9(b), the kinetic rate constant linearly decreased from 0.0745 to 0.0394 min-1 with increasing initial RhB concentration from 10 to 25 mg/L. The increase in the initial RhB concentration led to the adsorption of an increased number of RhB molecules on the surface of 0.5-Mn/ZIF-120 and the production of many intermediates and carbonaceous deposits on the catalyst surface during RhB degradation. The RhB degradation in the presence of 0.5-Mn/ZIF-120 and PMS was dependent on the sulfate radicals generated. Because the concentrations of the catalyst and oxidant were the same, the sulfate radical concentrations produced in solution were the same. Therefore, for a higher concentration of RhB in the solution, a longer degradation time was needed [88]. A few researchers have drawn a similar conclusion that a higher dye concentration results in greater suppression of dye removal [39, 89, 90].
The RhB degradation in the presence of 0.5-Mn/ZIF-120 and PMS under different reaction temperatures is shown in Fig. 10. At room temperature (23 ℃), the complete degradation of RhB was achieved in 40 min, whereas at 45 ℃, almost complete degradation of RhB was achieved in 20 min, indicating that the RhB degradation by 0.5-Mn/ZIF-120-activated PMS is an endothermic reaction and can be substantially accelerated at elevated temperature [57]. Because the kinetic rate constant k increased with increasing temperature, the relationship between k and temperature can be correlated using the Arrhenius equation presented in Eq. (3):
where A is the pre-exponential factor (min-1), R is the universal gas constant, Ea is the activation energy, and T is temperature in Kelvin (K). A plot of lnk versus 1/T is presented in the inset of Fig. 10, with the data points perfectly fit by linear regression with R2 of 0.983. This finding indicates that k can be closely associated with temperature by the Arrhenius equation and that k at other temperatures can be predicted.
Using the Arrhenius equation, the activation energy of the degradation reaction was calculated to be 39.17 kJ/mol. Various reaction activation energies have been reported for Mn-based catalysts used for activating PMS to degrade organics, and these findings are summarized in Table 1. The RhB degradation reaction by 0.5-Mn/ZIF-8-activated PMS has a lower activation energy (and hence a more rapid reaction rate) than those of the reactions initiated by most other Mn-based catalysts.
The activation of PMS by metal-containing catalysts generates two major reactive radicals, SO4-• and HO• [93]. To identify the principal radicals for the proposed degradation mechanism, the effect of radical scavengers on RhB catalytic degradation was examined, and the results are presented in Fig. 11. Various quenchers including EtOH and TBA were used. EtOH with alpha hydrogen can readily react with SO4-• and HO•, whereas TBA without alpha hydrogen is inert to SO4-• and mainly reacts with HO• [94]. As observed in Fig. 11, the RhB degradation profile without the addition of a quencher differed significantly from the profiles with the addition of EtOH or TBA at molar ratios of EtOH:PMS or TBA:PMS of 2000:1 or 4000:1. Moreover, little difference between the two degradation profiles was observed with the addition of EtOH or TBA. The RhB degradation in 20 min in the absence of a quencher reached approximately 86.4%, whereas those with the addition of EtOH or TBA at the above molar ratios decreased to approximately 74% and 56%, respectively, suggesting that EtOH and TBA indeed functioned as quenchers during the reaction. Accordingly, it can be concluded that both SO4-• and HO• are involved in the attack of RhB in aqueous solutions; however, HO• is the predominant active species. Nevertheless, HO• is mainly derived from SO4-• and hence PMS, as indicated by Eq. (4).
Fig. 12 shows the temporal absorption spectrum changes of RhB degradation by 0.5-Mn/ZIF-120-activated PMS. It is generally accepted that two competitive processes exist for RhB degradation: N-de-ethylation and the destruction of the conjugated structure [95, 96]. The intensity of the characteristic peak of RhB at 552 nm clearly decreased to half of its original value in 10 min, indicating the destruction of the conjugated xanthene structure. In addition, a blue shift of the maximum absorption band of the solution was observed because of the formation of a series of N-de-ethylated intermediates. Thus, it can be deduced that the ring opening simultaneously occurred with N-de-ethylation during the RhB degradation process.
GC-MS analysis of the supernatant phase is very useful for obtaining information about the degradation products. Before the GC-MS analysis, sample derivatization of the resultant solution coupled with extraction with organic solvent methylene chloride was performed. The total ion chromatogram (TIC) spectrum and GC-MS analysis results of the principal degradation by-products of the 0.5-Mn/ZIF-120-PMS-RhBaq system are presented in Fig. 13 and Table 2, respectively. The degradation products could be divided into two groups: aromatic compounds with different substituents and organics with lower molecular weights, most of which consisted of organic acid and alcohol. Among the degradation products, the glycerol content was the highest, implying that the conjugated structure of RhB was destroyed and decomposed into simple organics, which could be further converted into CO2 and H2O.
Based on previous investigations [97] and the quenching test results, the following mechanism of PMS activation by 0.5-Mn/ZIF-120 for RhB degradation was proposed:
First, the activation of PMS by 0.5-Mn/ZIF-120 occurs to generate SO4-• and •OH (Eqs. (5) and (6)), and most of the produced SO4-• radicals are rapidly depleted by reacting with RhB. During the reaction, the generated •OH radicals also attack RhB but at a moderate reaction rate (Eq. (7)). Simultaneously, some of the produced SO4-• radicals react with H2O via Eq. (4) to generate •OH. After the depletion of SO4-•, •OH becomes the single reactive species to attack RhB. Meanwhile, the Mn2O3 reacts with PMS, resulting in the regeneration of the original Mn3O4 and •OH (Eq. (6)). Based on these comprehensive considerations, it is suggested that the hydroxyl radicals generated in the presence of 0.5-Mn/ZIF-120 and PMS play a key role in the RhB degradation.
The reusability of a catalyst is an important factor reflecting its performance and potential for practical application. The as-synthesized catalyst Mn3O4/ZIF-8 was easily separated from aqueous solution using simple filtration with a 0.22-μm membrane and regenerated by washing with deionized water three times and then drying in an oven at 60 ℃ overnight. Fig. 14(a) and (b) show the catalytic activity and corresponding Mn leaching of the reused catalyst, respectively. The catalytic activity of 0.5-Mn/ZIF-120 remained high for five runs, with a RhB degradation efficiency of more than 96% and Mn leaching of less than 5%, indicating the good reusability of the catalyst. In addition, no significant changes were observed in the XRD patterns of the catalyst before and after use (Fig. 14(c)), indicating no significant crystal structure changes of the recycled catalyst.
A Mn-oxide-supported MOFs material, Mn3O4/ZIF-8, was successfully synthesized using a facile solvothermal method for the first time. This material was characterized using several techniques and tested for its ability to activate PMS for the degradation of RhB in aqueous solutions. The Mn3O4/ZIF-8 composite prepared with a Mn3O4:ZIF-8 mass ratio of 0.5 to 1 exhibited the best catalytic activity and negligible Mn leaching. In addition, the RhB degradation reached 98% in 40 min under the following conditions: 0.4 g/L catalyst dosage, 0.3 g/L PMS dosage, 10 mg/L initial RhB concentration, unadjusted initial solution pH (5.18), and reaction temperature of 23 ℃. The catalytic oxidative degradation of RhB in the 0.5-Mn/ZIF-120-PMS system obeyed first-order kinetics, and the degradation rate increased with increasing 0.5-Mn/ZIF-120 and PMS dosages and reaction temperature and decreasing initial RhB concentration. Quenching tests demonstrated that the •OH radicals primarily derived from SO4-• (and hence PMS) were the predominant active radicals attacking RhB. Finally, the catalyst 0.5-Mn/ZIF-120 was effectively recovered and reused for activating PMS to degrade RhB in water for at least five runs.
The characterization of prepared samples was supported by the Analytical and Testing Center of Shanghai University, Shanghai, China.
2017,
Vol. 38 Issue (8): 1360-1372 
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2017,
Vol. 38 Issue (8): 1360-1372 PDF