Abstract

At different calcination conditions, titanium-based manganese oxides (MnOx) electrodes were fabricated by spraying method without adhesive. The MnOx/Ti electrodes were applied in electrochemical oxidation of wastewater treatment for the first time. The surface morphologies of electrodes were tested by scanning electron microscopy. The formation of different manganese oxidation states on electrodes was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The electrochemical properties of the electrodes have been performed by means of cyclic voltammetry and electrochemical impedance spectroscopy. The characterizations revealed that the MnOx/Ti-350(20) electrode, prepared at calcination temperature of 350 °C for 20 min, exhibited fewer cracks on the electrode surface, larger electrochemically effective surface area and lower charge transfer resistance than electrodes prepared at other calcination conditions. Moreover, Acid Red B was used as target pollutant to test the electrode activity via monitoring the concentration changes by UV spectrophotometer. The results showed that the MnOx/Ti-350(20) electrode presented the best performance on decolorization of Acid Red B with the lowest cell potential during the process of electrochemical oxidation, and the chemical oxygen demand (COD) conversion was 50.7%. Furthermore, the changes of Acid Red B during the electrochemical oxidation process were proposed by the UV–vis spectra.

INTRODUCTION

The typical treatment of dye wastewater includes absorption (Zhang et al. 2014a, 2014b), membrane separation (Mo et al. 2008), aerobic or anaerobic treatment (Hosseini Koupaie et al. 2011), electrochemical treatment (Körbahti et al. 2011) and so on. Adsorption has been studied popularly due to the simplicity of operation. Activated carbon (Namasivayam & Kavitha 2002), minerals (Ozdemir et al. 2004), solid and agricultural wastes (Namasivayam et al. 2001) and bionanomaterials (Kumar et al. 2016) have been used as adsorbents. However, the regeneration of adsorbents limited for its application because of the high cost for dye removal.

Electrochemical oxidation, as one of the advanced oxidation processes (AOPs), has gained considerable attention for organic wastewater treatment (Chen 2004; Hou et al. 2017; Singh & Mishra 2018). However, the main defect is its relatively high costs, especially for the energy consumption until the complete mineralization (Oller et al. 2011). As a core component of electrochemical oxidation reaction,the electrodes have significant effect both on the degree of mineralization and energy consumption. Different types of electrodes have been developed for electrochemical oxidation of pollutant throughout the years, including graphite (Zhang et al. 2008), glassy-carbon (Ureta-Zanartu et al. 2002), Pt (Alaoui et al. 2015), metal oxides electrodes and boron-doped diamond (BDD) (Alfaro et al. 2006; Cui et al. 2009). The metal oxides electrodes are mainly composed of PbO2 (Tong et al. 2008) and dimensionally stable anode (DSA) electrode. The DSA electrodes, such as titanium coated with PbO2 (Polcaro et al. 1999), RuO2 (Kaur et al. 2019), IrO2 (Chatzisymeon et al. 2010), SnO2 (Yang et al. 2015) and two or more of them in complexes (Song et al. 2010; Gao et al. 2017) etc., have been extensively studied for wastewater treatment. However, the industrial application of these electrodes is limited due to leaching of metal cations during preparation and electrochemical corrosion, which will cause secondary pollution in the water system (Kong et al. 2007; Xiang et al. 2015). Therefore, it is necessary to develop environmentally friendly electrodes. Due to the low cost and toxicity, manganese oxides have been applied extensively in electrochemical fields, such as batteries (Salgado et al. 2003; Xu & Song 2015), supercapacitors (Jiang & Kucernak 2002; Wei et al. 2011), oxygen reduction reaction (Lee et al. 2011) and water splitting reaction (Wiechen et al. 2012; Nam et al. 2015). The manganese oxides have been investigated as interlayers or dopant for other metal oxides coatings to improve the performance in most of studies, while electrode with MnOx coated on Ti directly was rarely studied.

Spraying method is an important method for industrial coating preparation. Pawar et al. (2009) prepared boron-doped ZnO electrodes with high growth rate and uniform coating by spray pyrolysis for dye-sensitized solar cells. Kim et al. (2006) prepared a carbon nano-film electrode by electrostatic spray deposition technique which showed well interconnected porous structures and adhered onto the substrate without adhesive. The reported preparation process was with low temperature and easy to control. Martos et al. (2001) obtained lead oxide thin film electrode by spray pyrolysis method as the negative electrode for secondary lithium batteries. They proposed that spray pyrolysis method was not only easy to operate, but also had low cost, which can be used to deposit large-area membrane electrodes for online production applications. Yao (2011) prepared Ti/SnO2-Sb electrode by ultrasonic spray pyrolysis which exhibited the uniformity of the microstructure on the electrode film. In summary, the spraying method have been widely used due to its advantages, such as simple and controllable operation, uniform coating layer, the ability to prepare large-area electrodes, etc. Although the spraying method is commonly used in the fields of capacitors, batteries, membrane electrodes, etc., the application of Ti-based MnOx electrodes for dye wastewater treatment has not been reported yet.

This work describes the fabrication and characterization of manganese oxides on titanium-based substrate without adhesive for the electrochemical oxidation of Acid Red B (AR B) in wastewater. The influence of different calcination temperature and calcination time on electrodes during the prepared process were investigated by various physic-chemical and electrochemical characterizations to optimize the preparation conditions. Furthermore, the degradation kinetics and degradation pathway of AR B on the optimized electrode were also analyzed.

EXPERIMENTAL

Electrode preparation

Pre-treatment of Ti substrate

The titanium plates (TA1, 2 cm × 3 cm × 0.1 cm, Suzhou Shuertai Industrial Technology Co., Ltd, China) were polished with 400-grit and 800-grit sandpaper and washed by ultrasonic. Then the washed titanium plates were degreased in 10% NaOH solution at 90 °C for 1 h. After rinsing with deionized water, the Ti plates were etched in a boiled 10% oxalic acid solution for 2 h. All the treated Ti plates were stored in 1% oxalic acid solution.

Preparation of MnOx coating

0.25 mol/L Mn(NO3)2 solution was prepared with isopropanol as solvent and ultrasonicated for 30 min. 4 mL of Mn(NO3)2 solution was sprayed onto the pretreated Ti substrate by the spray gun (caliber: 0.2 mm, pressure: 4 kPa), which was connected with oil-free air compressor (600 W-8 L, Aotusi Industry Co., Ltd, China). The sprayed Ti plate was dried at 100 °C for 10 min and then thermally treated at 200 °C for 5 min. The above processes were repeated four times. Finally, the electrode was obtained after sintering at a (a = 250, 350, 450, 550, 650) °C for b (b = 20, 60) min, named MnOx/Ti-a(b). The active area of prepared electrodes was 4 cm2.

Electrode characterization

Physic-chemical characterization

Scanning electron microscopy (SEM) was performed on a field-emission JSM-7500F microscope (Japan Electronics Co., Ltd) operating at accelerating voltage of 20 kV. X-ray diffraction (XRD) was carried out on X'PERT PRO diffractometer (Netherlands Spectris) using CuKα radiation (λ = 1.5418 Å) over the range of 5–90° with a scan speed of 5°/min at room temperature. X-ray photoelectron spectroscopy (XPS) was measured on ESCALAB250 spectrometer (American Thermo VG Co., Ltd) Infrared spectrometer (IR) was conducted on a Nicolet iS50 FT-IR (Thermo Fisher Scientific) in a KBr matrix in the range 400–4,000 cm−1.

Electrochemical characterization

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out in a conventional three-electrode cell system, containing the prepared electrode as working electrode, platinum electrode (20 mm × 30 mm × 0.1 mm) as the counter electrode, and Ag/AgCl/0.1M KCl (Shanghai Ciyue Electronic Technology Co., Ltd) as the reference electrode at CHI660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd, China). The CV was conducted in 0.2 M sodium sulfate solution and the scan rate was 50 mV/s. The EIS was tested at open circuit potential with the frequency range from 1 × 105 to 1 × 10−2 Hz and the amplitude of 10 mV.

The lifetime of the electrode was performed by anodic polarization at 40 mA/cm2 in 0.5 M H2SO4 solution. The accelerated service lifetime of the electrode was defined as the duration from the initial value to the cell potential increased to 5 V (Correa-Lozano et al. 1997; Zhao et al. 2009). The cell potentials were recorded by data recorder (34970A, Keithley Instruments, USA).

Electrode activity test

The electrode activity tests were conducted in a stirred cell containing 10 mL of 300 mg/L Acid Red B (AR B) and 0.2 mol/L sodium sulfate as supporting electrolyte at room temperature. All the tests were carried out with the current density of 3 mA/cm2 in constant current system over a reaction time of 150 min. The prepared electrode was used as anode and pure titanium plate was used as cathode with the distance between them of 1 cm. The concentration of AR B was analyzed by UV spectrophotometer (UV-2600, Shimadzu Corporation, Japan) at the absorbance wavelength of 515 nm. Chemical oxygen demand (COD) analyses were carried out by benchtop VIS spectrophotometer (DR3900, Hach Company, USA) and digital reactor block (DRB200, Hach Company, USA). The intermediates during electrochemical oxidation process were detected by LCMS (Q Exactive UHMR Hybrid Quadrupole-Orbitrap mass spectrometer, Thermo Fisher Scientific, USA).

RESULTS AND DISCUSSION

XRD analysis

Figure 1 presents the XRD patterns of MnOx/Ti-a(b) electrodes prepared at different calcination conditions. As shown in Figure 1, a group of peaks appeared at of 38.4°, 40.2°, 70.7°, 76.2°, 77.4° and 82.3° in all the patterns were corresponding to the typical reflections of Ti substrate (JCPDS No.44-1294). The peaks at of 53.2° and 62.3° in all the patterns were indexed to (116) and (214) reflections of Mn2O3 (JCPDS No.33-0900). For the MnOx/Ti-250(20) electrode (Figure 1(a)), additional diffraction peaks at of 36.1° and 59.8° were attributed to (211) and (224) reflections of Mn3O4 (JCPDS No.24-0734), and the peak at of 40.6° was assigned to (113) reflection of Mn2O3 (JCPDS No.33-0900). The peak at of 35.6° for MnOx/Ti-350(20) (Figure 1(b)) and other electrodes except for MnOx/Ti-250(20) was assigned to (110) reflection of Mn2O3 (JCPDS No.33-0900). The diffraction peaks were identical for MnOx/Ti-350(20) and MnOx/Ti-350(60) (Figure 1(f)), indicating that the coating phases of the MnOx/Ti-350(20) and MnOx/Ti-350(60) were the same. The diffraction peaks at of 33.6° and 55.2° for the MnOx/Ti-450(20) electrode (Figure 1(c)) were corresponding to (211) and (411) reflections of TiO2 (JCPDS No.33-1381), respectively, which could be observed in the pattern of MnOx/Ti-550(20) (Figure 1(d)) and MnOx/Ti-650(20) (Figure 1(e)) as well. For MnOx/Ti-550(20) (Figure 1(d)) and MnOx/Ti-650(20) (Figure 1(e)), the diffraction peak at of 23.7° was corresponding to (012) reflection of Mn2O3 (JCPDS No.33-0900). Two additional peaks at of 28.8° and 66.8° appeared in the pattern of MnOx/Ti-650(20) (Figure 1(e)) were assigned to the (110) and (310) reflections of MnO2 (JCPDS No.50-0866). XRD results suggested that the MnOx coating layer of MnOx/Ti-250(20) electrode was made of Mn3O4 and Mn2O3 phase. When calcined at 350 °C for 20 min and 60 min, the MnOx coating layer of electrodes was mainly made of Mn2O3 phase. TiO2 was formed on the surface coating of electrodes prepared at the calcination temperature raised to above 450 °C. The formation of TiO2 indicated that the Ti substrates were oxidized into TiO2, which would have a negative effect on catalytic oxidization of organic pollutants due to the poor conductive ability of TiO2 (Zhang et al. 2014a, 2014b). The MnOx coating layer of MnOx/Ti-650(20) electrode was made of Mn2O3 and MnO2 phase. The XRD results indicated that the calcination temperature during the preparation process had significant effect on the composite of MnOx coating layer in this study.

Figure 1

XRD patterns of the Ti based MnOx electrodes prepared at different calcination temperatures for 20 min (a) MnOx/Ti-250(20), (b) MnOx/Ti-350(20), (c) MnOx/Ti-450(20), (d) MnOx/Ti-550(20), (e) MnOx/Ti-650(20) and at 350 °C for 60 min (f) MnOx/Ti-350(60).

Figure 1

XRD patterns of the Ti based MnOx electrodes prepared at different calcination temperatures for 20 min (a) MnOx/Ti-250(20), (b) MnOx/Ti-350(20), (c) MnOx/Ti-450(20), (d) MnOx/Ti-550(20), (e) MnOx/Ti-650(20) and at 350 °C for 60 min (f) MnOx/Ti-350(60).

SEM analysis

The surface morphologies of MnOx/Ti-a(b) electrodes prepared at different calcination conditions were observed by SEM. As displayed in Figure 2(a) and 2(b), the surface of MnOx/Ti-250(20) and MnOx/Ti-350(20) was relatively complete. In comparison, the surface of MnOx/Ti-450(20) (Figure 2(c)) and MnOx/Ti-550(20) (Figure 2(d)) exposed some cracks, and the surface of MnOx/Ti-650(20) (Figure 2(e)) was completely cracked. More cracks appeared on the surface of electrodes as the calcination temperature raised during the preparation of electrodes. The incomplete surface was easy to be damaged during electrochemical test, which led to poor electrochemical activity (Santos et al. 2014; Shao et al. 2016). Compared with MnOx/Ti-350(20), the surface of MnOx/Ti-350(60) was also complete. However, a lot of agglomeration revealed on the surface as the calcination time prolonged, which might reduce the active sites for electrochemical oxidation (Xu & Song 2015).

Figure 2

SEM images of the Ti based MnOx electrodes prepared at different calcination temperatures (a) MnOx/Ti-250(20), (b) MnOx/Ti-350(20), (c) MnOx/Ti-450(20), (d) MnOx/Ti-550(20), (e) MnOx/Ti-650(20) and at 350 °C for 60 min (f) MnOx/Ti-350(60).

Figure 2

SEM images of the Ti based MnOx electrodes prepared at different calcination temperatures (a) MnOx/Ti-250(20), (b) MnOx/Ti-350(20), (c) MnOx/Ti-450(20), (d) MnOx/Ti-550(20), (e) MnOx/Ti-650(20) and at 350 °C for 60 min (f) MnOx/Ti-350(60).

XPS analysis

The surface compositions of electrodes were carried out by XPS, and Mn 2p spectra of the prepared MnOx/Ti-a(b) electrodes were presented in Figure 3. All the measurements were carried out with reference to C 1s binding energy (BE) (284.5 eV) as internal standard. The binding energy of Mn 2p1/2 and Mn 2p3/2 for the prepared electrodes were located at about 653.5 eV and 641.5 eV, respectively. The coexistence of Mn(II), Mn(III) and Mn(IV) in all the prepared electrodes was evidenced by fitting the peaks, which could be assigned to MnO (Lee et al. 2011), Mn2O3 (Wang et al. 2017) and MnO2 (Li et al. 2011), respectively, as reported in the literatures. The relative mass ratio of MnOx phase calculated by Mn 2p3/2 is displayed in Table 1. From Table 1, the relative mass of MnO was 27.9%, 26.4%, 30.8%, 34.8%, 24.3% and 26.8% for MnOx/Ti-250(20), MnOx/Ti-350(20), MnOx/Ti-450(20), MnOx/Ti-550(20), MnOx/Ti-650(20) and MnOx/Ti-350(60), respectively. The relative mass of Mn2O3 was 28.8%, 28.0%, 28.3%, 23.8%, 28.2% and 28.7% for MnOx/Ti-250(20), MnOx/Ti-350(20), MnOx/Ti-450(20), MnOx/Ti-550(20), MnOx/Ti-650(20) and MnOx/Ti-350(60), respectively. The relative mass of MnO2 was 43.4%, 45.6%, 40.9%, 41.4%, 44.5% and 44.5% for MnOx/Ti-250(20), MnOx/Ti-350(20), MnOx/Ti-450(20), MnOx/Ti-550(20), MnOx/Ti-650(20) and MnOx/Ti-350(60), respectively. The content of Mn2O3 was almost the same for different electrodes. Compared with other electrodes, MnOx/Ti-350(20) was with the lowest amount of MnO and highest amount of MnO2.

Table 1

The characterization and electrochemical results for MnOx/Ti-a(b)

SampleRelative mass ratio of MnOx phase (%)
AR B removal efficiency at 30 minCell potentialak constantEnergy consumptionbRct
Mn2+Mn3+Mn4+(%)(V)(min−1)(W·h/L)(Ω)
MnOx/Ti-250(20) 27.9 28.8 43.4 46.2 6.90 0.01693 8.93 3332.00 
MnOx/Ti-350(20) 26.4 28.0 45.6 66.8 4.96 0.02766 4.31 81.61 
MnOx/Ti-450(20) 30.8 28.3 40.9 54.3 6.34 0.02208 5.79 653.76 
MnOx/Ti-550(20) 34.8 23.8 41.4 50.4 7.20 0.01933 8.33 5666.00 
MnOx/Ti-650(20) 27.3 28.2 44.5 15.1 >25 0.00293 >100 – 
MnOx/Ti-350(60) 26.8 28.7 44.5 57.4 5.57 0.02364 5.20 82.66 
SampleRelative mass ratio of MnOx phase (%)
AR B removal efficiency at 30 minCell potentialak constantEnergy consumptionbRct
Mn2+Mn3+Mn4+(%)(V)(min−1)(W·h/L)(Ω)
MnOx/Ti-250(20) 27.9 28.8 43.4 46.2 6.90 0.01693 8.93 3332.00 
MnOx/Ti-350(20) 26.4 28.0 45.6 66.8 4.96 0.02766 4.31 81.61 
MnOx/Ti-450(20) 30.8 28.3 40.9 54.3 6.34 0.02208 5.79 653.76 
MnOx/Ti-550(20) 34.8 23.8 41.4 50.4 7.20 0.01933 8.33 5666.00 
MnOx/Ti-650(20) 27.3 28.2 44.5 15.1 >25 0.00293 >100 – 
MnOx/Ti-350(60) 26.8 28.7 44.5 57.4 5.57 0.02364 5.20 82.66 

aThe cell potential was determined at 150 min of AR B degradation.

bThe energy consumption was calculated at AR B removal efficiency of 80%.

Figure 3

Mn 2p spectra of electrodes (a) MnOx/Ti-250(20), (b) MnOx/Ti-350(20), (c) MnOx/Ti-450(20), (d) MnOx/Ti-550(20), (e) MnOx/Ti-650(20) and (f) MnOx/Ti-350(60).

Figure 3

Mn 2p spectra of electrodes (a) MnOx/Ti-250(20), (b) MnOx/Ti-350(20), (c) MnOx/Ti-450(20), (d) MnOx/Ti-550(20), (e) MnOx/Ti-650(20) and (f) MnOx/Ti-350(60).

Electrochemical activity of electrodes

AR B degradation

The electrochemical oxidation test of electrodes were performed with 3 mA/cm2 for AR B concentration of 300 mg/L. The removal efficiency of AR B and cell potential during the degradation process are displayed in Figure 4(a) and 4(b). The removal efficiency of AR B at 30 min and the cell potential at 150 min for AR B degradation are summarized in Table 1. MnOx/Ti-350(20) displayed the highest AR B degradation efficiency and lowest cell potential. When calcined over 350 °C for 20 min and 60 min, the removal efficiency of AR B decreased and the cell potential increased distinctly. Combined with XRD results, the removal efficiency of AR B for MnOx/Ti-250(20) was lower than that for MnOx/Ti-350(20), indicating that Mn3O4 phase was less efficient than Mn2O3 phase for AR B degradation. As the calcination temperature raised over 350 °C, the content of MnO2 was increased based on the XPS results. It has been reported that MnO2 was more beneficial for the degradation of pollutants (Peng et al. 2016; Massa et al. 2017), however, the reduction of removal efficiency and increase of cell potential might attribute to the formation of TiO2 on the basis of XRD results, which had a negative effect on electrochemical oxidization of AR B. Moreover, the difference of AR B removal efficiency between MnOx/Ti-350(20) and MnOx/Ti-350(60) was slight, but the cell potential at the final reaction for MnOx/Ti-350(20) (4.96 V) was lower and more stable than that for MnOx/Ti-350(60) (5.57 V). MnOx/Ti-350(20) was with the most stable cell potential during the degradation process of AR B, which attributed to the relatively complete surface in the SEM results. It was confirmed that the complete surface of electrode could keep the performance of electrode stable (Wang et al. 2009; Yao 2011). MnOx/Ti-350(60) revealed some agglomeration in SEM image, so that the active sites on the surface of MnOx/Ti-350(60) were less than that of MnOx/Ti-350(20), which was not beneficial for the AR B degradation. The lowest cell potential of MnOx/Ti-350(20) also could be beneficial to decrease the required energy consumption. According to the XPS, XRD and SEM characterizations, the superior performance of MnOx/Ti-350(20) was mainly attributed to high relative mass of MnO2, high conductivity without TiO2 formation, smooth and complete surface.

Figure 4

(a) The removal efficiency, (b) cell potential, (c) logarithmic relationship between concentration of AR B and electrolysis time and (d) evolution of energy consumption by electrodes prepared at different calcination conditions during the degradation of AR B.

Figure 4

(a) The removal efficiency, (b) cell potential, (c) logarithmic relationship between concentration of AR B and electrolysis time and (d) evolution of energy consumption by electrodes prepared at different calcination conditions during the degradation of AR B.

The electrochemical oxidation process of AR B by all the prepared MnOx/Ti electrodes was considered to follow the pseudo-first-order reaction kinetics model, calculated by the Equation (1), where C0 was the initial concentration of AR B, Ct was the concentration of AR B at time t, and k was the first-order kinetic rate constant. The fitting results are shown in Figure 4(c). The value of kinetic rate constant (k) is displayed in Table 1, and the correlation coefficient (R2) for all the electrodes was greater than 0.990. Obviously, the kinetic rate by electrodes prepared at different calcination conditions was also confirmed that MnOx/Ti-350(20) performed the best activity for electrochemical oxidation of AR B. 
formula
(1)

Energy consumption

The economic feasibility of AR B degradation by all the prepared electrodes was evaluated by determining the energy cost. The time needed during the degradation of AR B was calculated by the k values in Table 1, since this could simulate the treatment of the wastewater in real life. As reported by Lin et al. (2013), the following equation was used to determine the time: 
formula
(2)
where t is the time (min) needed to degrade AR B, k is the first order rate constant (min−1), C0 is the influent concentration and Ct is the effluent concentration (mg/L). The energy consumption (W·h/L) can be determined by the time: 
formula
(3)
where S is the electrode surface area (cm2), I is the applied current density (mA/cm2), U is the applied voltage (V), and V is the volume of the solution in the reactor (mL). The evolution of energy consumption for AR B degradation by different electrodes was presented in Figure 4(d). As shown in Table 1, the energy consumptions for different electrodes were calculated when the removal efficiency of AR B reached 80%. The energy consumption was increased when the calcination temperature raised to above 350 °C and the calcination time was prolonged to 60 min. The energy consumption for MnOx/Ti-350(20) was remarkably lower than that of other electrodes during all the reaction time.

Electrochemical characterization of electrodes

Cyclic voltammetry

The cyclic voltammetry of electrodes prepared at different calcination conditions was tested in 0.2 M sodium sulfate solution at scan rate of 50 mV/s and is shown in Figure 5. No additional peaks appeared in the CV curves except the one formed by the oxygen evolution reaction, which indicated that all the prepared electrodes are non-active electrodes in this test situation (Duan et al. 2014). The electrochemical activity of electrodes, which was related to the geometric area of cyclic voltammetry, shown in the order of MnOx/Ti-350(20) > MnOx/Ti-350(60) > MnOx/Ti-450(20) > MnOx/Ti-550(20) > MnOx/Ti-250(20) > MnOx/Ti-650(20). The larger current density at the same potential meant the higher electrochemical activity (Duan et al. 2012; Xu et al. 2015). The current density of MnOx/Ti-350(20) was much higher than that of other electrodes, indicating that MnOx/Ti-350(20) was with the most active for water splitting. Water splitting was beneficial for the organics degradation on account of the generation of hydroxyl radicals (Hernández et al. 2016). Therefore, MnOx/Ti-350(20) presented the best electrochemical performance for AR B degradation in all of the electrodes. Combined with the physico-chemical characterization results, it was inferred that the MnOx phases had significant effect on activity of electrodes and MnOx coating with the predominant presence of higher Mn valence state was more favorable for the degradation of AR B.

Figure 5

Cyclic voltammetry of electrodes prepared at different calcination conditions at a scan rate of 50 mV/s in a 0.2 M Na2SO4 solution.

Figure 5

Cyclic voltammetry of electrodes prepared at different calcination conditions at a scan rate of 50 mV/s in a 0.2 M Na2SO4 solution.

Electrochemical impedance spectroscopy

The electrochemical impedance spectroscopy (EIS) is an electrochemical measurement to characterize electrochemical system by equivalent circuit method. The EIS test was conducted in 0.2 M Na2SO4 with AR B solution at open circuit potential. Both the measured and stimulated results, and the equivalent circuit model was shown in Figure 6. Rs and Rct represented the ohmic resistance and charge transfer resistance, respectively. According to the fitting results, the Rct value of all the prepared electrodes is illustrated in Table 1. The Rct value of MnOx/Ti-350(20) was much smaller than that of other electrodes prepared at different calcination temperatures, while the Rct value of MnOx/Ti-350(20) and MnOx/Ti-350(60) was similar. The reduction of the semicircle's diameter in the Nyquist plots indicated a lower resistance to the charge transfer, which could enhance the reaction kinetics. It has been reported that the charge transfer during the AR B degradation depended on the surface properties of the MnOx-based electrode materials (Hernández et al. 2016; Massa et al. 2017). So MnOx/Ti-350(20) with the smallest value of Rct showed the largest k and lowest cell potential, which was beneficial for the reduction of energy consumption during the electrochemical oxidation of AR B.

Figure 6

Electrochemical impedance spectroscopy of electrodes prepared at different calcination conditions.

Figure 6

Electrochemical impedance spectroscopy of electrodes prepared at different calcination conditions.

Electrode stability

The accelerated service life experiments were carried out by MnOx/Ti-350(20) in 0.5 M sulfuric acid solution at 40 mA/cm2. The experimental result was illustrated in Figure 7(a). According to the empirical formula proposed by B. Correa-Lozano et al. (1997), the actual electrode life (τ1) had a relationship between the life of the accelerated life test (τ2), actual current density (i1) and accelerated current density (i2). The actual lifetime of MnOx/Ti-350(20) calculated by the Equation (4) was 17 h. 
formula
(4)
Figure 7

(a) The accelerated lifetime tests of MnOx/Ti-350(20) electrode. (b) SEM images of deactivated MnOx/Ti-350(20) electrode after electrochemical test.

Figure 7

(a) The accelerated lifetime tests of MnOx/Ti-350(20) electrode. (b) SEM images of deactivated MnOx/Ti-350(20) electrode after electrochemical test.

The characterizations of the MnOx/Ti-350(20) electrode after accelerated service life experiments were tested by SEM, XRD and IR. As shown in Figure 7(b), the surface morphology of the MnOx/Ti-350(20) electrode after electrochemical tests was characterized by SEM. Some cracks appeared on the surface of the MnOx/Ti-350(20) electrode after electrochemical tests, which might cause the MnOx/Ti-350(20) electrode to be deactivated. The XRD patterns of MnOx/Ti-350(20) electrode before and after electrochemical test were shown in Figure S1 in the Supplementary Materials (available with the online version of this paper). The diffraction peaks corresponded to Ti substrate and Mn2O3 were observed in MnOx/Ti-350(20) electrodes before and after accelerated service life experiments. The positions of peaks and the relative intensities were identical in both patterns, indicating the MnOx phase of MnOx/Ti-350(20) electrode was not changed after accelerated service life experiments. As shown in Figure S2 in the Supplementary Materials (available online), the electrodes before and after the accelerated service life experiments were characterized by IR. The bands appeared at 3,200–3,500 cm−1 and 1,250–1,650 cm−1 were attributed to the O-H stretching vibration (Abou-El-Sherbini et al. 2002; Eren et al. 2009) and the O-H bending vibration (Pagnanelli et al. 2007), respectively. The Mn–O vibrations can be observed in the region of 500–700 cm−1. The bands were not changed in both spectra, which suggested that the functional groups of electrodes were maintained after accelerated service life experiments.

COD conversion and decolorization of AR B

COD conversion, Faradaic efficiency and specific energy consumption

COD conversion reflected the electrochemical capabilities of electrode, and the Faradaic efficiency (FE) was calculated from the COD conversion by Equation (5): 
formula
(5)
where CODt and COD0 are the COD of AR B before and after the test, I is the electrolysis current (A), F is Faraday's constant (96,487 C/mol), V is the reaction volume (L) and Δt is the reaction time (s).
The specific energy consumption (SEC) was the energy consumption for the removal of 1 kg of COD and calculated as Equation (6): 
formula
(6)

where I and U are the average cell potential (V) and current (A), t is the electrolysis time (h), CODt and COD0 are the COD of AR B at time t and initial time and V is the sample volume (L).

For MnOx/Ti-350 (20), COD conversion was achieved 50.7% in 150 min, besides, the FE and SEC listed in Table 2 was 79.21% and 150.21 k·Wh/kg COD, respectively. The difference between the decolorization of AR B and COD conversion implied that the AR B could be readily converted into colourless molecules which were difficult for further degradation.

Table 2

The COD conversion, FE and SEC value of AR B degradation for MnOx/Ti-350 (20) at 150 min

ElectrodeCOD conversion (%)FE (%)SEC (k·Wh/ kg COD)
MnOx/Ti − 350 (20) 50.7 79.21 150.21 
ElectrodeCOD conversion (%)FE (%)SEC (k·Wh/ kg COD)
MnOx/Ti − 350 (20) 50.7 79.21 150.21 

UV–vis spectra of AR B

The changes in the UV–vis spectra of AR B during the electrochemical oxidation process from 200 to 800 nm were measured and shown in Figure 8(a). The UV–vis spectrum of AR B mainly consisted of three well-resolved bands, which were located at 515, 320 and 220 nm, respectively. The absorption peak at 515 nm, which was used to determine the concentration of AR B, was assigned to the n–π* transition in –N = N– group (Solozhenko et al. 1995). The peaks at 220 and 320 nm were originated from the absorption of the ππ* transition related to the naphthalene rings in the dye molecule (Wu et al. 2000; Xiong et al. 2001). The obvious decrease of absorption peak at 515 nm and 320 nm could be observed as the reaction proceeded. No other new absorbance peak appeared in the UV–vis region.

Figure 8

(a) UV–vis spectra of AR B solution and (b) LCMS analysis of AR B degradation at 90 min during the electrochemical oxidation process by MnOx/Ti-350(20).

Figure 8

(a) UV–vis spectra of AR B solution and (b) LCMS analysis of AR B degradation at 90 min during the electrochemical oxidation process by MnOx/Ti-350(20).

During the electrochemical oxidation process at 90 min, the sample of AR B degradation was conducted by LCMS analysis. As shown in Figure 8(b), the results of LCMS analysis inferred the possible pathway for AR B degradation. Combined with the UV–vis spectra, the break of –N = N– led to the decolorization of AR B first. The possible intermediates were some hydrazo derivatives (Hu et al. 2015), which could be oxidized to some naphthalene-type intermediates or compounds containing benzene ring, such as phthalic acid (Siddique et al. 2011). These compounds could be further degraded to form oxalic acid, and mineralized to form CO2 and H2O finally.

CONCLUSION

In conclusion, MnOx/Ti electrodes were synthesized at different calcination conditions by the spraying method. The surface morphology of electrodes and phases of manganese oxides were employed by SEM, XRD and XPS. Then all the prepared electrodes were used for electrochemical oxidation of AR B. The results showed that the electrochemical oxidation of AR B by MnOx/Ti-350(20) presented the highest removal efficiency with the lowest cell potential during the degradation process, which was related to the relative complete surface of MnOx/Ti-350(20) and predominant content of MnO2. The electrochemical characterization test confirmed that MnOx/Ti-350(20) was with the highest current response and lowest charge transfer resistance, which facilitated the degradation of AR B. The COD conversion reached 50.7% while the removal efficiency was 93.6% at 150 min during the electrochemical oxidation of AR B by MnOx/Ti-350(20). At the same time, Faradaic efficiency and specific energy consumption was 79.21% and 150.21 k·Wh/ kg COD, respectively. Moreover, the degradation process of AR B was detected by UV–vis spectra, which was confirmed that the –N = N– group and naphthalene ring were all electrochemical oxidized during 150 min. This research presents a facile spraying method for the preparation of Ti-based MnOx electrodes with low toxicity and high stability, which is a promising candidate applied in real wastewater treatment.

ACKNOWLEDGEMENTS

This research was financially supported by the National Natural Science Foundation of China (Grant No. 21503217) and Fundamental Research Funds for the Central Universities (Nos. 3132016059, 3132019334).

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Supplementary data