Degradation of Toxic Organic Contaminants by Graphene Cathode in an Electro‐Fenton System

A novel composite electrode was constructed by pressing graphene and CuO, using a cathode in an electro‐Fenton (EF) system. Cyclic voltammetry, charge/discharge curve and electrochemical impedance spectroscopy (EIS) were used to characterize the com‐ posite electrode. The degradation of a toxic organic contaminant, Terramycin, by EF sys‐ tem was studied in an undivided electrolysis cell. The possible degradation products of Terramycin were studied by a Fourier transform‐infrared spectrum, and the findings showed that the structure of Terramycin was damaged. The variations of hydrogen per‐ oxide and the relative content of hydroxyl radical (.OH) during the degradation pro ‐ cess were traced by enzyme catalysis method and fluorescence spectrometry. The results showed that the electro‐catalytic degradation of Terramycin occurred by an ·OH radical mechanism. More importantly, this as‐prepared cathode was very stable and could be reused without any catalytic activity decrease, suggesting its potential application in the wastewater treatment.


Introduction
Persistent organic pollutants (POPs) resist conventional chemical and biological treatments and accumulate in the aquatic environment after discharge. They tend to be liposoluble and undergo food chain amplification and, if toxic, threaten human health [1,2].
Electro-Fenton (EF) reaction is one of the most widely used advanced oxidation processes (AOPs) for the destruction of a wide range of recalcitrant organic contaminants such as antibiotics that cannot be eliminated biologically in wastewater [3][4][5]. In the EF systems, they have the ability to in situ generate highly potent chemical oxidants such as ·OH, which is strong enough to non-selectively oxidize most organic as well as some inorganic compounds [6]. H 2 O 2 is simultaneously produced in an aqueous medium at the cathode by the 2e − reduction of the dissolved molecular oxygen (Eq. (1)), and ·OH is formed from the Fenton's reagent according to the following reaction (Eqs. (2, 3)) [7].
Therefore, cathode materials determine the efficiency of the EF system. Usually, carbon materials with excellent characteristics of conductivity, gas diffusion, adsorption and catalysis are chosen as cathode materials. Graphene, an excellent functional material, has been widely used in super capacitors [8], transistors [9,10], fuel cells [11], etc., but as electrode materials in electro-catalytic degradation of organic matter, very few have been reported. In this research, a new CuO-graphene electrode was constructed by pressing nano-CuO and pure graphene together. The electro-catalytic characteristics of the CuO-graphene electrode were studied using cyclic voltammetry, time power method and electrochemical impedance spectroscopy, respectively. In the EF system, the CuO-graphene electrode acted as the cathode and the pt net as the anode to degrade Terramycin. The concentration of Terramycin in the oxidative degradation was traced by HPLC. The variations of hydrogen peroxide and the relative content of hydroxyl radical during the degradation process were traced by enzyme catalysis method and fluorescence spectrometry, and the oxidation mechanism of Terramycin by EF system was studied.

Preparation and characterization of CuO-graphene cathode
In this section, we will describe the preparation and the properties of CuO-graphene cathode and discuss its electro-catalytic characterization. Graphene (layer of 8-10, diameter of 20-40 μm, thickness of 6-15 nm, purity 99.5%) was purchased from Ningbo Morsh Technology Company in China. Nano-CuO was prepared by the hydrothermal method. The CuO-graphene composite cathode was prepared by loading CuO onto graphene powder. CuO and graphene were mixed in an ethanol/PTFE emulsion (60% by volume), sonicated for 5 min to disperse the mixture completely, and then dried at 50°C to form a dough-like paste. Two pieces of the paste were fixed on a stainless steel mesh current collector and pressed at 30 MPa for 2 min. The electrode was refluxed in acetone for 24 h to remove ethanol and surface PTFE. The dimensions of the composite cathode were 1.0 cm 2 × 5.0 mm (see Figure 1; the composite cathode was firmly compressed with the edge sealed with epoxy coating to yield an exposed area of 1 cm 2 ).
CHI-660E electrochemical station was employed to study the electro-catalytic characteristics of the CuO-graphene cathode at room temperature. A Pt net (99.99% purity, Tianjin Aida Technology Co., Ltd.) of 2.0 cm 2 in area was used as the counter electrode and a saturated calomel electrode (SCE) equipped with a Luggin capillary as the reference electrode. Figure 2 displayed the cyclic voltammetry curves of the CuO-graphene electrode under neutral pH conditions after being scanned eight times. The electrode showed good electro-catalytic properties under the working voltage. There were two pairs of redox peaks in the system medium.
The current peak at 0.246 V vs. SCE is produced by the cathodic reduction of O 2 to H 2 O 2 and that at −0.231 V vs. SCE is produced by the cathodic reduction of H + to H 2 [12]. Thus, there are two degradation pathways that may occur in the EF system. One is the anodic electrochemical oxidation process on the Pt anode [13] and the other is the cathodic ·OH oxidation process in which the CuO-graphene cathode catalyzes the two-electron reduction of O 2 to H 2 O 2 under certain potential, and the H 2 O 2 may be converted to ·OH [14].  The specific capacitance is the ratio of capacity and mass of the electrode, which could represent the reversibility and stability of the as-prepared electrode while charging/discharging. Figure 3 showed the charge/discharge curve of CuO-graphene cathode in a 10 g/L Na 2 SO 4 solution with a scan rate of 1 mV/s, a current of 10 mA and a potential range of −0.8 to +0.8 V (vs. SCE). After nine repeated cycles, the charging and discharging curve still showed good symmetry, the potential was essentially linear with time and the charge-discharge curve slope dV/dt was still constant, suggesting that the CuO-graphene cathode material had a good reversibility [15].
EIS is a rapid, in situ and non-destructive technique for investigating the electrochemical properties of the electrode surface and affording information on the impedance changes of the electrode surface during the modification process [16,17]. Figure 4 illustrated the Nyquist diagrams and Bode plots of the CuO-graphene cathode under different reduction potentials in 10 g/L Na 2 SO 4 solution. It can be seen that all the impedance spectra exhibited a capacitive semicircle at the medium-high frequency and a bigger capacitive semicircle at the medium-low frequency. Two-time constants were observed in the Bode plots under different reduction potential. R s is the solution resistance and the medium-high frequency circuit Q m -R m corresponds to the capacitance and the ohmic process within the porous structure; the medium-low frequency circuit Q dl -R ct is the double-layer capacitance and the kinetic process of electrochemical reaction (the O 2 reduction into H 2 O 2 ) [18,19]. For the capacitive loops, the coefficients n 1 and n 2 represent a depressed feature in the Nyquist diagram. The EIS data was simulated by ZSimpWin software through Figure 5 [20,21]. It was seen that the equivalent circuit fitted the experimental data well in most of the frequency ranges (Figure 4), suggesting that the equivalent circuit ( Figure 5) was suitable.
According to the fitted results in Table 1, the values of R s remained constant as the reduction potential increased. When the reduction potential changed from 0 to −250 mV, the R ct values decreased from 420.5 to 30.12 Ω cm 2 , and R m values decreased from 9.51 to 3.45 Ω cm 2 , indicating that the reduction rate of O 2 into H 2 O 2 on CuO-graphene cathode was very rapid under higher reduction potential, that is to say, the degradation rate based on Cuo-graphene cathode of EF system increased with reduction potential. It should be noted that higher reduction potential might cause the electrolysis of water [22].

The degradation process of Terramycin in EF system
The EF system degradation of Terramycin was performed in an undivided cell (50 mL) with a two-electrode system at room temperature. A Pt net (99.99% purity, Tianjin Aida Technology Co., Ltd) of 1.0 cm 2 in area was used as the anode. The as-prepared CuO-Graphene electrode was employed as the cathode. The initial concentration of Terramycin was 20 ug/L. Na 2 SO 4 aqueous solution of 10 g/L was used as the electrolyte to increase the conductivity. The initial pH of the Terramycin solution was neutral at 7.0. The solution was magnetically stirred at room temperature during the whole reaction period. Before degradation experiments, the system was stirred for 40 min to establish adsorption/desorption equilibrium between the solution and electrodes, and then different voltages were applied and 1 mL samples were collected and analyzed with HPLC spectra to examine the decomposition of Terramycin.
Analysis of HPLC: The mobile phases were pump-mixed dynamically from oxalic buffers (including 1 mmol/L of EDTA solution) and acetonitrile at specified compositions. The flow rate was 1.0 mL/min and the injection volume was 20 mL. The UV detection was at 254 nm, and the column temperature was maintained at 35°C [23]. Figure 6 showed the changes of HPLC chromatograms of Terramycin in the EF system under 5 V in 10 g/L of Na 2 SO 4 aqueous solution. The mobile phase was V(acetonitrile): V(oxalic buffers) = 30: 70 of mixture, adding a little EDTA in the mobile phase in order to chelate with Cu 2+ /Cu + which were corroded from the CuO-Graphene electrode. It was observed that the peak of Terramycin at the retention time of 3.06 min gradually decreased with time, and disappeared thoroughly after 180 min, which means that the tetracycline had degraded completely. Figure 7 depicted the degradation rate of Terramycin in the EF system under different applied potentials. As we know, the generation rate of H 2 O 2 on the cathode depended on the applied potential, and the degradation efficiency of the EF system is proportional to the quantity of ·OH produced by Fenton's reaction [24,25]. When no power was applied, no H 2 O 2 was Figure 6. HPLC spectra of Terramycin during degradation in the EF system with the CuO-Graphene cathode: initial concentration of Terramycin, 20 ug/L; electrolyte, 10 g/L of Na 2 SO 4 ; applied potential, 5 V. generated in EF system and there was no Terramycin degradation in the blank solution [26,27].

Effect of applied potential on degradation of Terramycin in EF system
The degradation efficiency of Terramycin increased with the addition of applied potentials; only 60.6% of Terramycin decomposed in 180 min at 4 V, and when the potential reached 5 V, Terramycin decomposed 100% in 180 min. This phenomenon can be ascribed to the fact that a higher potential leads to higher H 2 O 2 formation rate, resulting in an increased rate of ·OH formation from Fenton's reaction. When the applied potential exceeded 5 V, the electrolysis of water increased.

Concentration of oxidizing species during degradation of Terramycin
The effectiveness of the Fenton reaction depends on the catalytic generation of H 2 O 2 , which yields the highly reactive species, ·OH radical, that oxidizes organic material non-selectively. In the EF system, the electrogenerated levels of H 2 O 2 and ·OH determine the rate and efficiency of Terramycin degradation. Figure 9 displayed the concentration changes of H 2 O 2 and relative content of ·OH as the degradation reaction proceeded.
Analysis of H 2 O 2 : The concentration of H 2 O 2 was determined by the oxidation of peroxidase catalytic method. The reaction was carried out in a colorimetric tube (10 mL). 30 μL of 1.0 mg/ mL horseradish peroxidase (POD), 1 mL of NaH 2 PO 4 -NaOH buffer solution (pH = 6.80), 150 μL of 10.0 mg/mL N,N-diethyl-p-phenylenediamine (DPD) and 1 mL sample were added into the colorimetric tube in turns. After oxidation for 15 min, the concentration of H 2 O 2 was analyzed spectroscopically at the wavelength of 510 nm [28]. Analysis of ·OH: The relative content of ·OH was determined by fluorescence spectrometry. 1 mL of 0.01 mol/L benzoic acid and 1 mL sample from Fenton's reaction system was added into a 10 mL colorimetric tube and the volume was kept constant at 5mL; after a reaction time of 10 min, the content of ·OH was analyzed with an excitation wavelength of 309 nm and an emission wavelength of 410 nm, and the slit width of excitation and emission was 5 nm [29]. Little amounts of H 2 O 2 and ·OH were detected in the blank solution (the applied potential was 0 V). With the electrolysis performed, the amount of H 2 O 2 concentration accumulated in the EF system was calculated. As 5 V was applied, the concentration of H 2 O 2 , generated by cathodic reduction, increased with time. After 90 min, the H 2 O 2 concentration reached 2.3 × 10 −5 mol/L and fluctuated over time. This indicates that the H 2 O 2 production increased in the beginning and then was consumed in the degradation of Terramycin. These results were in good agreement with the previous experimental studies by Wang [14], who conducted diethyl phthalate degradation using a self-made Pd/C gas-diffusion cathode and a Ti/IrO 2 / RuO 2 anode. By comparing Figure 8a and b, it can be seen that the electrogenerated H 2 O 2 in the EF system was quickly converted to ·OH. The content of ·OH increased by prolonging the electrolysis time and then reached a steady state after about 120 min [30,31]. The degradation of Terramycin in the EF system was in accordance with the characteristic of the Fenton reaction, indicating that the degradation process involves ·OH [32,33].

The deep oxidation of Terramycin in EF system
In order to measure the extent of Terramycin mineralization during the degradation process, the TOC change of solution was tracked during degradation, and the results were displayed in Figure 9. The TOC did not change when no power was applied to the electrode, but as a voltage of 5 V was applied, the mineralization rate of Terramycin was 59.8% in 10 h. These results indicated that the EF system not only degraded the Terramycin but also oxidized over half of the Terramycin into CO 2 (mineralization) [34,35].

IR spectra of the Terramycin in EF system during degradation
IR spectra were used to identify non-mineralized products as the degradation proceeded and the results are displayed in Figure 10 [36][37][38]. Initially, the bands 1450 and 930 and 750-800 cm −1 corresponded to benzene ring vibration and C─H in the benzene ring plane bending vibration, respectively. The in-plane bending vibration absorption peak at 1620-1650 cm −1 indicated the existence of O═C─NH 2 , and the bands at 1310 and 1180-1240 cm −1 indicated the existence of ( ). With the photoelectron-catalytic reaction proceeding, bands of ( ) vibrations disappeared after 10 h, and at the same time, a new band at 1110 cm −1 emerged, which indicated that the tertiary amine was wiped off from the skeleton molecule of Terramycin and degraded into small amine molecules. Meanwhile, the 750-800 and 930 cm −1 bands of characteristic vibrations in the benzene ring bond disappeared. These results showed that the Terramycin aromatic structure was damaged.

The stability of CuO-graphene electrode in EF system
The stability of the electrode is very important for its application in environmental technology [39,40]. In order to study the stability of CuO-graphene electrode, the degradation of Terramycin with the same CuO-graphene electrode under the same conditions is also evaluated. Figure 11 showed the degradation stability of the CuO-graphene cathode over five cycles in the EF system. After being reused five times, the degradation rates of Terramycin for cycle one, two, three, four and five were 97.84, 95.67, 95.21, 96.54 and 93.88%, respectively, indicating that the CuO-graphene electrode was stable and reusable.

Conclusion
A new cathode-CuO-graphene electrode was prepared by pressing nano-CuO and graphene. The degradation of Terramycin was investigated in an undivided electrolysis cell with a CuOgraphene cathode and Pt net anode. The electro-catalytic performance of the CuO-graphene cathode was studied by cyclic voltammetry, charge/discharge curve and electrochemical impedance spectroscopy. The primary conclusions were listed as follows: (1) The CuO-graphene cathode was stable and exhibited high catalytic activity in neutral Na 2 SO 4 solution.
(2) Under the optimal conditions of using 10 g/L of supporting electrolyte Na 2 SO 4 at 5 V with a pH of 7.0 and reaction time of 180 minutes, respectively, up to 100% of 20 ug/L of Terramycin degraded.
(3) Terramycin can be mineralized to carbon dioxide and small molecules; this new EF system may soon be developed for other toxic organic contaminants degradation.