Abstract

Due to the high Brunauer–Emmett–Teller (BET) surface area of zeolitic imidazolate framework (ZIF)-8, a secondary crystallization method was used to prepare a particle electrode of γ-Al2O3@ZIF-8. According to the results from a field emission scanning electron microscope (SEM) and X-ray diffractometer (XRD), the particle electrode of γ-Al2O3 was successfully loaded with ZIF-8, and the BET surface area (1,433 m2/g) of ZIF-8 was over ten times that of γ-Al2O3. The key operation parameters of cell voltage, pH, initial RhB concentration and electrolyte concentration were all optimized. The observed rate constant (kobs) of the pseudo-first-order kinetic model for the electrocatalytic oxidation (ECO) system with the particle electrode of γ-Al2O3@ZIF-8 (15.2 × 10−2 min−1) was over five times higher than that of the system with the traditional particle electrode of γ-Al2O3 (2.6 × 10−2 min−1). The loading of ZIF-8 on the surface of γ-Al2O3 played an important role in improving electrocatalytic activity for the degradation of Rhodamine B (RhB), and the RhB removal efficiency of the three-dimensional (3D) electrocatalytic system with the particle electrode of γ-Al2O3@ZIF-8 was 93.5% in 15 min, compared with 27.5% in 15 min for the particle electrode of γ-Al2O3. The RhB removal efficiency was kept over 85% after five cycles of reuse for the 3D electrocatalytic system with the particle electrode of γ-Al2O3@ZIF-8.

INTRODUCTION

A printing and dyeing mill generates a large amount of dye-rich effluents which contain some complicated bio-refractory organic compounds. Even after biodegradation or chemical flocculation treatment (Holkar et al. 2016), there still exist some of these substances, such as Rhodamine B (RhB), methyl orange and malachite green (Yu et al. 2018). Considering the aesthetic and toxicity problem, it is indispensable to degrade these pigments completely. So far, attempts have been made to degrade and destroy those pigments using diverse advanced oxidation processes (AOPs), including photochemical or electrochemical reactions, ozonation, Fenton oxidation, and catalytic wet oxidation (He et al. 2019). In comparison with other AOPs, electrocatalytic oxidation (ECO) technologies have made great progress in wastewater treatment due to their high efficiency, environmental friendliness and versatility (Rana et al. 2019).

The present ECO technologies have developed from the two-dimensional (2D) system to the three-dimensional (3D) electrocatalysis oxidation system, which has become a promising technology and is widely employed in wastewater treatment. In 3D electrocatalysis oxidation systems, particle electrodes can be prepared using mesoporous material, such as granular active carbon (GAC), carbon aerogel (CA), metal particles, modified kaolin, and metal oxides (Zhang et al. 2010, 2013; Hardjono et al. 2011; Chu et al. 2016; Moreira et al. 2017; Long et al. 2019).

Considering the large Brunauer–Emmett–Teller (BET) surface area and electro-adsorption capacity of metal organic frameworks (MOFs), the preparation of a particle electrode using MOFs may be an attractive option (Duan et al. 2016; Ren et al. 2017). However, on account of the nanostructure of most MOF materials, it is difficult to prepare dimensionally stable particle electrodes using MOFs. Thus, as one of the mesoporous materials, dimensionally stable γ-alumina (γ-Al2O3) can be potentially chosen as the carrier of MOFs. Zeolitic imidazolate frameworks (ZIFs) belong to a special subclass of MOFs. In comparison with other MOFs, ZIF morphology is usually stable in aqueous solution and organic solvents, especially ZIF-8 (Wang et al. 2008). To the best of our knowledge, the application of MOFs in wastewater treatment has been reported mainly on adsorptive removal of methyl orange and methylene blue from aqueous solution (Haque et al. 2010). The active roles of ZIF-8 on the enhanced visible photocatalytic activity of Ag/AgCl have also been examined for RhB removal, which can be attributed to both adsorption and generation of superoxide radicals (Liu et al. 2017). An iron terephthalate metal–organic framework MIL-53(Fe) synthesized by a facile solvothermal reaction was capable of activating hydrogen peroxide (H2O2) to achieve high efficiency in a photocatalytic process (Ai et al. 2014). However, there are still some shortcomings limiting the industrial application of ECO technologies, such as the short lifetime of the electrode materials and low current efficiency due to some intrinsic drawbacks such as mass transfer limitation, small space–time yield, and low area–volume ratio. Therefore, it is important to develop new particle electrodes for improving the area–volume ratio and current efficiency.

In this study, a particle electrode developed from ZIF-8 material was first proposed and the preparation process was optimized. Characterization and performance research of the particle electrode were also carried out. Moreover, this study was committed to supplying a useful base and reference for practical application of 3D electrochemical degradation on dyeing wastewater treatment. RhB was used as the representative target substance to prepare the simulated wastewater to evaluate the electrochemical performance of the prepared particle electrodes. The effects of operating parameters, such as cell voltage, pH, initial RhB concentration and electrolyte concentration, on RhB degradation were all examined. The mechanisms for RhB removal and the reusability of the particle electrodes were both investigated.

EXPERIMENTAL SECTION

Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), methanol, zinc chloride, sodium formate and sodium sulphate anhydrous were purchased from Guangzhou chemical reagent factory, and 2-methylimidazole was purchased from Macklin. All of these reagents were of analytical purity grade. The γ-Al2O3 (BET surface area: 140 m2/g, pore volume: 0.386 cm3/g) with diameters of 3–5 mm was purchased from Dongzi Science and Technology Co., Ltd, China. A titanium electrode (5 cm × 5 cm × 1 mm) and Ti/RuO2 (5 cm × 5 cm × 1 mm) electrode were purchased from Zhao Bond Metal Composite Material Co., Ltd, China.

Preparation of particle electrode γ-Al2O3@ZIF-8

In this study, γ-Al2O3 was used as the support for the synthesis of the particle electrode. The γ-Al2O3 particles were washed with tap-water and deionized water three times under ultrasonication, and were then baked in a muffle furnace at 823 K for 6 h at 1 K/min heating rate.

In a standard synthesis of ZIF-8 crystals (Venna et al. 2010), 70 mL methanol containing 3.3 g 2-methylimidazole was poured into an equal volume solution containing 1.5 g (Zn(NO3)2·6H2O). The mixture was stirred for 24 h at room temperature (298 ± 1 K). The product was collected by centrifuging, washed with methanol three times, and then dried at 353 K overnight in a drying oven for further research.

The powdered sample of ZIF-8 was dissolved in methanol in a certain proportion and the well-distributed suspended solution was obtained under ultrasound in 10 minutes. The dry γ-Al2O3 was dipped into the above-mentioned suspended solution for 5 min. After that period, the particles were taken out from the solution and dried at room temperature for 3 hours, then dried at 393 K for 24 h. Thus, the particles loading the ZIF-8 seed layer were obtained, to be used as the support for the secondary growth of ZIF-8 membranes (Xu et al. 2011).

The particles loading the ZIF-8 seed layer were tiled at the bottom in a Teflon-lined autoclave, followed by slowly pouring ZIF-8 synthesis solution containing ZnCl2, 2-methylimidazole, sodium formate and methanol into the autoclave, and then sealed and heated in a stainless steel autoclave at 393 K for 4 h, then cooled down to room temperature. The resulting particles were washed with methanol three times and dried at room temperature for 24 h, and then the γ-Al2O3@ZIF-8 was obtained.

Electrocatalytic setup and operation

The 3D-electrochemical degradation of RhB was carried out in a rectangular undivided organic glass cell of 585 mL (13 cm × 9 cm × 5 cm) capacity. In the system, a Ti/RuO2 electrode and a Ti electrode were used as the anode and the cathode, respectively. The available working surface of the two electrodes was 15 cm2 (5 cm × 3 cm). The anode and cathode were situated vertically and parallel to each other with an inner gap of 4 cm to position a given volume of the particle electrodes, which were immersed in the simulated wastewater for 24 h prior to its use. Every particle with high independence would serve as an electrolysis cell in the system, and thus the electrode areas were increased greatly. The particle electrodes were placed in a support netted container in the 3D electrocatalytic system, 450 mL of RhB solution was transferred into the reactor, and Na2SO4 was added into the RhB solution to enhance the conductivity and adjust the initial solution pH before electro-degradation. A DC power supply (RXN 305D, China) was employed to connect with anode and cathode. Compressed air was sparged into the 3D electrode reactor from the bottom at 0.04 m3/h; the compressed air was used to provide oxygen and accelerate the mass transfer of RhB.

Optimization of influence factors for RhB degradation

The effects of cell voltage, pH, initial RhB concentration and electrolyte concentration on the electrocatalytic degradation of RhB were examined. The design of laboratory batch tests is shown in Table 1. Typically, 450 mL of RhB solution was used for tests and all samples were measured in triplicate. To exclude the effect of adsorptive removal on RhB degradation, the 3D electrocatalytic system was immersed in the simulated wastewater for 24 h each test prior to its use. And the RhB samples were collected every 5 min after the start of the treatment until complete removal of RhB (the results are shown in the Supplementary Material, available with the online version of this paper).

Table 1

Design of the batch tests for study on influence factors of RhB degradation

GroupInfluence factorsOther operation parameters
1. Effect of cell voltage (V) Initial pH of 2.0, 20 mg/L RhB, 8 g/L of Na2SO4 
10 
15 
20 
25 
2. Effect of pH 2.0 Cell voltage of 20 V, 20 mg/L RhB, 8 g/L of Na2SO4 
4.0 
6.0 
9.0 
11.0 
3. Effect of initial RhB concentration (mg/L) 10 Cell voltage of 20 V, initial pH of 2.0, 8 g/L of Na2SO4 
20 
40 
60 
4. Effect of electrolyte concentration (g/L) Cell voltage of 20 V, initial pH of 2.0, 20 mg/L RhB 
16 
GroupInfluence factorsOther operation parameters
1. Effect of cell voltage (V) Initial pH of 2.0, 20 mg/L RhB, 8 g/L of Na2SO4 
10 
15 
20 
25 
2. Effect of pH 2.0 Cell voltage of 20 V, 20 mg/L RhB, 8 g/L of Na2SO4 
4.0 
6.0 
9.0 
11.0 
3. Effect of initial RhB concentration (mg/L) 10 Cell voltage of 20 V, initial pH of 2.0, 8 g/L of Na2SO4 
20 
40 
60 
4. Effect of electrolyte concentration (g/L) Cell voltage of 20 V, initial pH of 2.0, 20 mg/L RhB 
16 

Comparison of electrocatalytic activity for RhB degradation and kinetics analysis

Under the obtained optimum operation parameters, the degradation of RhB was comparatively investigated with particle electrodes of γ-Al2O3 and γ-Al2O3@ZIF-8. In order to compare the electrocatalytic activity, the degradation of RhB was fitted using a pseudo-first-order kinetic model, as described by the following equation (Diao et al. 2017): 
formula
(1)
where Ct is the concentration of RhB at a selected time (mg/L), C0 is the initial RhB concentration (mg/L), kobs is the observed rate constant (min−1), and t is time (min).

Experiments for mechanisms of RhB degradation

In order to demonstrate the mechanisms of RhB degradation with the particle electrode of γ-Al2O3@ZIF-8, two identical electrocatalytic systems were comparatively examined. One was used as control without tertiary butyl alcohol (TBA) addition, and to the other was added 1% (v/v) of TBA as a hydroxyl radical (·OH) quencher. The experiment was carried out at the optimum conditions that were determined based on the above experiments: cell voltage of 20 V, pH of 2.0, initial RhB concentration of 20 mg/L and electrolyte concentration of 8 g/L. The collection and measurement of samples were the same as in the above-mentioned steps.

Reuse tests

In order to examine the reusability of the novel particle electrode of γ-Al2O3@ZIF-8 in a 3D electrocatalytic system for RhB degradation, as the typical steps for preparation of 450 ml of RhB solution and the 3D electrocatalytic system, the optimum operation parameters of cell voltage of 20 V, pH of 2.0, initial RhB concentration of 20 mg/L, and 8 g/L of Na2SO4 were used and the treatment time was 60 min. The RhB degradation tests were carried out repeatedly five times, and the collection and measurement of samples were the same as in the above-mentioned steps (the results are shown in the online Supplementary Material).

Analytical methods

The morphology and energy dispersive spectrometry (EDS) of the particle electrode were characterized by a field emission scanning electron microscope (SEM, ZEISS Ultra 55, Germany, Carl Zeiss). BET surface area and micropore distribution of ZIF-8 were measured by N2 adsorption at 77 K on an ASAP2020 instrument. An X-ray diffractometer (XRD, Rigaku MiniFlex 600) was employed to analyze the crystal structures of the particle electrodes. Fourier transform infrared (FTIR) spectra of the particle electrodes were investigated with a Fourier transform infrared spectrometer (FTIR Spectrometer, Nicolet 6700, Thermo Nicolet). Before measurement, the particle electrodes were dehydrated under vacuum at 393 K for 12 h. The RhB concentration was measured with a UV-vis spectrometer (UV6000PC, China) at its maximum absorption wavelength of 554 nm. The initial pH of the simulated wastewater was measured with a DZS-706 multi-parameter water quality meter (Rex Electric Chemical, Shanghai, China).

RESULTS AND DISCUSSION

Characterization

The SEM images of the particle electrode are shown in Figure 1. Apparently, no matter from the top view or the cross-section view, compact hexahedral crystals could be observed on the outer surfaces of the γ-Al2O3@ZIF-8. The images also show that the average crystal sizes and the film thickness are about 15 μm and 30 μm, respectively. That indicated a large quantity of ZIF-8 crystals had been growing on the surface of the γ-Al2O3 successfully. Figure 2 shows (a) the N2 adsorption–desorption isotherms and (b) micropore distribution of ZIF-8. On account of the nano-structure and high BET surface area of ZIF-8 (1,433 m2/g) compared with that of γ-Al2O3 (140 m2/g), the adsorption capacity of the γ-Al2O3@ZIF-8 was increased, and correspondingly, the performance of the particle electrode might be improved.

Figure 1

SEM images of the particles: (a) top view of the γ-Al2O3, (b) top view of the γ-Al2O3@ZIF-8, (c) cross-section of the γ-Al2O3@ZIF-8.

Figure 1

SEM images of the particles: (a) top view of the γ-Al2O3, (b) top view of the γ-Al2O3@ZIF-8, (c) cross-section of the γ-Al2O3@ZIF-8.

Figure 2

(a) N2 adsorption–desorption isotherms and (b) micropore distribution of ZIF-8.

Figure 2

(a) N2 adsorption–desorption isotherms and (b) micropore distribution of ZIF-8.

It is well known that as one of the MOFs, ZIF-8 has the property of permanent microporosity and tremendous BET specific surface areas. Considering the theory of carbon fiber felt (ACF) used as electrodes in electro-catalysis (Huang & Su 2010), large BET specific surface areas and electrical conductivity could improve efficiency and reproducibility.

The impregnation process is the key to obtaining γ-Al2O3@ZIF-8 in our synthesis method. To ensure that the ZIF-8 crystals have been successfully loaded on the γ-Al2O3 particles, XRD and IR were employed to analyze the structures and functional groups of the γ-Al2O3 and γ-Al2O3@ZIF-8. Figures 3 and 4 show XRD patterns and IR spectrograms of the particles before and after loading, respectively. Both of the spectrograms of γ-Al2O3@ZIF-8 seem to be homologous with γ-Al2O3 substantially and yet some main characteristic peaks appear of ZIF-8, which are pointed out in the figures. The diffraction peaks of γ-Al2O3@ZIF-8 are relatively weak, which results from the relatively lower content of ZIF-8. Another possible reason is being weakly crystalline during secondary growth.

Figure 3

X-ray diffraction patterns of the particles.

Figure 3

X-ray diffraction patterns of the particles.

Figure 4

IR spectra of the particles.

Figure 4

IR spectra of the particles.

Comparison of RhB degradation with γ-Al2O3, γ-Al2O3@ZIF-8 and kinetics analysis

The degradation efficiency of RhB with the prepared particle electrode of γ-Al2O3@ZIF-8 was investigated, compared with the traditional particle electrode of γ-Al2O3. As shown in Figure 5(a), the particle electrode of γ-Al2O3@ZIF-8 decolorized RhB more effectively than the electrode of γ-Al2O3. The RhB removal rate for the system with particle electrode of γ-Al2O3@ZIF-8 reached over 93.5% in 15 min. In contrast, the removal rate of RhB for the electrocatalytic system with particle electrode of γ-Al2O3 only reached 27.5% in 15 min and reached 90% after treatment for 90 min. Ai et al. (2014) employed an iron terephthalate metal–organic framework MIL-53(Fe) to achieve high efficiency in photocatalytic degradation of RhB, with the presence of hydrogen peroxide (H2O2), under visible light irradiation within 50 min, and found that the removal efficiency of RhB was less than 85% in 20 min. Liu et al. (2017) prepared a novel Ag/AgCl/ZIF-8(50%) composite photocatalyst for the removal of RhB from aqueous solution, and found that the removal efficiency of RhB was less than 80% in 30 min with the synthesized Ag/AgCl/ZIF-8(50%). It was also found that both the adsorption ability of ZIF-8 and the ability to form O2.− contributed to the degradation of RhB. Compared with the previous studies, the higher RhB removal efficiency was obtained in a shorter reaction time for the 3D electrocatalytic system with the particle electrode of γ-Al2O3@ZIF-8 addition.

Figure 5

Time-course variation with different particle electrodes: (a) C/C0 of RhB solution and (b) pseudo-first-order kinetics for the degradation of RhB.

Figure 5

Time-course variation with different particle electrodes: (a) C/C0 of RhB solution and (b) pseudo-first-order kinetics for the degradation of RhB.

As shown in Figure 5(b), the kinetics of electrocatalytic degradation of RhB were pseudo-first-order, and the observed rate constant (kobs) of the ECO system with particle electrode of γ-Al2O3@ZIF-8 (15.2 × 10−2 min−1) was over five times higher than that of the system with traditional particle electrode of γ-Al2O3 (2.6 × 10−2 min−1), which suggested that the loading of ZIF-8 on the surface of a particle electrode of γ-Al2O3 enhanced greatly the electrocatalytic activity of the 3D ECO system.

Mechanisms for RhB degradation with particle electrode γ-Al2O3@ZIF-8

As shown in Figure 6, the absorption spectrum of RhB was characterized by its maximum absorbance at 554 nm and the absorbance peaks declined obviously with prolonged reaction time due to electrochemical degradation. Natarajan et al. (2011) also reported a study on a UV-LED/TiO2 process for degradation of RhB, and found that the formed intermediate compounds were similar to previously reported literature on the degradation of RhB, and the formed oxidized products were mineralized into CO2, H2O, NO3 and NH4+. It was inferred that the degradation of RhB by the photogenerated active species such as •OH and holes could attack the central carbon of RhB to decolorize the dye and it was further degraded via the N-de-ethylation process. The adsorptive removal of RhB was excluded through immersing the particle electrode for 24 h prior to its use. However, the direct quantitative comparison of RhB removal between ECO and the available technologies is difficult since lots of factors (e.g. initial RhB concentration, operating conditions, among others) could affect the results. The higher RhB removal efficiency of the 3D electrocatalytic system with particle electrode of γ-Al2O3@ZIF-8 is favorable for increasing the treatment capacity for dyeing wastewater.

Figure 6

UV–vis spectra of RhB with γ-Al2O3@ZIF-8 electrodes at various treating times: (a) 0 min; (b) 5 min; (c) 10 min; (d) 15 min; (e) 20 min; (f) 25 min; (g) 30 min. Cell voltage of 20 V, pH of 2.0, initial RhB concentration of 20 mg/L, Na2SO4 of 8 g/L as supporting electrolyte.

Figure 6

UV–vis spectra of RhB with γ-Al2O3@ZIF-8 electrodes at various treating times: (a) 0 min; (b) 5 min; (c) 10 min; (d) 15 min; (e) 20 min; (f) 25 min; (g) 30 min. Cell voltage of 20 V, pH of 2.0, initial RhB concentration of 20 mg/L, Na2SO4 of 8 g/L as supporting electrolyte.

TBA is a commonly used ·OH radical scavenger and reacts with hydroxyl radicals with a rate constant of 6 × 108 M−1·s−1 (Ikhlaq et al. 2012). As shown in Figure 7, the experimental results showed that the RhB degradation efficiency for the system with the addition of TBA was obviously lower than that with no TBA addition, which also suggested that hydroxyl radicals played an important role in the electrocatalytic degradation of RhB. It could be attributed to the disintegration of the azo linkage (conjugated xanthene ring), which acts as the chromophore of RhB (Yan et al. 2011). This is probably because ·OH radicals attacked the N = N bond of the azo dye, which may be the most active site. According to the SEM (Figure 1) and XRD (Figure 3) results, ZIF-8 was successfully supported on the outer surface of the γ-Al2O3.

Figure 7

Effect of 1% (v/v) TBA addition on RhB removal.

Figure 7

Effect of 1% (v/v) TBA addition on RhB removal.

Figure 8 shows the mechanisms of hydroxyl radical generation in the 3D electrocatalytic system with particle electrode of γ-Al2O3@ZIF-8. There were two pathways to generate hydroxyl radicals: on the anode and particle electrode. At an appropriate voltage, the particles were polarized to form lots of charged microelectrodes with one surface as anode while the other was charged the opposite. The generation of hydroxyl radicals on the surface of the particle electrode can be expressed as Equation (2) and Equation (3) (Zhang et al. 2013). On the other hand, the BET surface area of the ZIF-8 was 1,433 m2/g and was much higher than that of the carrier γ-Al2O3, which could greatly enhance the adsorption capacity for RhB and accelerate the mass transfer rate of RhB at the liquid–solid interface, resulting in a high efficiency of RhB degradation. 
formula
(2)
 
formula
(3)
Figure 8

Mechanisms of hydroxyl radical generation in the 3D electrocatalytic system with particle electrode of γ-Al2O3@ZIF-8.

Figure 8

Mechanisms of hydroxyl radical generation in the 3D electrocatalytic system with particle electrode of γ-Al2O3@ZIF-8.

CONCLUSIONS

In summary, we demonstrated that the novel particle electrode of γ-Al2O3@ZIF-8 showed good electrocatalytic activity and reusability for RhB degradation in a 3D electrochemical system. The electrocatalytic activities depended greatly on the various operating parameters. The results on the hydroxyl radical quencher by TBA showed that the loading of ZIF-8 on the surface of γ-Al2O3 played an important role in improving electrocatalytic activity for RhB degradation, and the RhB removal efficiency of the electrocatalytic system with the particle electrode of γ-Al2O3@ZIF-8 was 93.5% in 15 min. The high RhB removal efficiency and reusability of the particle electrode of γ-Al2O3@ZIF-8 in the ECO system make it a promising treatment technology for dye wastewater.

ACKNOWLEDGEMENTS

This research was financially supported by the Nature Science Foundation of Guangdong Province (No. 2016A030313432) and the Outstanding Young Innovative Talent Training Plan of Guangdong Universities (No. 2012LYM_0050). Dr Qilin Wang acknowledges the support of Australian Research Council Discovery Early Career Researcher Award (DE160100667).

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Supplementary data