Ti/RuO2-IrO2-SnO2 Anode for Electrochemical Degradation of Pollutants in Pharmaceutical Wastewater: Optimization and Degradation Performances

Electrochemical oxidation technology is an effective technique to treat high-concentration wastewater, which can directly oxidize refractory pollutants into simple inorganic compounds such as H2O and CO2. In this work, two-dimensionally stable anodes, Ti/RuO2-IrO2-SnO2, have been developed in order to degrade organic pollutants from pharmaceutical wastewater. Characterization by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) showed that the oxide coating was successfully fabricated on the Ti plate surface. Electrocatalytic oxidation conditions of high concentration pharmaceutical wastewater was discussed and optimized, and the best results showed that the COD removal rate was 95.92% with the energy consumption was 58.09 kW·h/kgCOD under the electrode distance of 3 cm, current density of 8 mA/cm2, initial pH of 2, and air flow of 18 L/min.


Introduction
In the past several decades, pharmaceutical industry has played an important role in social development, but it also produces a lot of environmental pollution. The pharmaceutical industry wastewater is highly complex and contains a large number of toxic and poorly biodegradable compounds. Due to the unique properties of the pharmaceutical industry, it is recognized as one of the difficult high-concentration organic wastewater [1,2]. Traditional treatment methods (such as adsorption, cohesion, filtration, ion exchange, etc.) are not enough to completely purify the mineralized pharmaceutical wastewater, and the post-treatment procedures are too complicated, which limits the application in actual engineering [3,4]. In recent years, many researchers have devoted themselves to the development of advanced oxidation methods to treat wastewater, mainly including chemical oxidation, Fenton oxidation, wet air oxidation, supercritical water oxidation, photochemical oxidation and electrochemical oxidation [5][6][7][8][9]. Among many advanced oxidation methods, electrocatalytic oxidation technology, as the green chemical technology, can degrade complex organic pollutants in wastewater into non-toxic small molecular organic compounds or carbon dioxide and water through mineralization of various organic pollutants [10][11][12][13][14][15]. A large number of studies have shown that electrocatalytic oxidation technology is one of the effective methods for the treatment of high concentration pharmaceutical wastewater, but the technology is still in the initial stage [16].
In the electrocatalytic oxidation technology, the electrode is the key factor to complete the electrochemical reaction and improve the current efficiency. Different electrode materials can change the electrochemical reaction rate by orders of magnitude. Therefore, choosing the appropriate electrode material is an effective way to improve the efficiency of the electrochemical reaction. Since the development of the dimensionally stable anodes (DSA) electrode in the 1960s, it has good stability and catalytic activity and quickly gained popularity and has been widely used. The DSA electrode is a kind of metal oxide (such as Ti/SnO 2 ) coating electrode with excellent electrocatalytic activity. This electrode has the advantages of high oxygen evolution potential, low cost, and easy operation, but also has low stability and short life.
Nowadays, to prolong the electrode life, researchers usually doped noble metal oxides (such as RuO 2 and IrO 2 ) with strong corrosion resistance and high stability into the titanium substrate anode coating [17,18] [21]. Obviously, the doping of noble metal oxides is beneficial to improve the electrochemical performance and the stability and the electrode life.
Hence, in this work, we used the ternary oxide composite Ti/RuO 2 -IrO 2 -SnO 2 as the anode for electrocatalytic oxidation of pharmaceutical wastewater for the first time. The effects of electrode distance, current density, initial pH, and air-flow on the electrocatalytic degradation performance of organic from pharmaceutical wastewater were systematically investigated. The optimal reaction conditions were determined and the energy consumption of the reaction was investigated. In addition, the SEM, XRD, and EDS techniques are employed to analyze the surface morphology, structure, and composition of Ti/RuO 2 -IrO 2 -SnO 2 anode before and after the electrocatalytic reaction.

Reagent and Materials
All chemicals were purchased commercially and can be used without further purification.

Characterization
The surface morphology of the electrodes was observed by SEM (ULTRA plus, Carl Zeiss AG, Jena, German) and quantitative analyze trace element on the electrode surface by EDS (ULTRA plus, Carl Zeiss AG, Jena, German). Under 30 kw, 60 kV and 60 mA Cu Kα (λ = 1.5406 Å), the phase formation on the electrode was investigated via powder X-ray diffraction (XRD) technology on X'pert pro X-Ray Diffractometer. Diffraction patterns in the 20 to 80 • regions were recorded at a rate of 5 • /min. Track box furnace (KSL1400X, Kejing, Hefei, China) was used for the sintering of anodic oxide coating. Reactor power supply was provided by DC stabilized power supply (APS3005S-3D, Gratten, Nanjing, China). The pH of raw water was measured by PHS-3C pH meter (Yoke Instrument, Shanghai, China). The content of COD in water samples before and after reaction was measured by UV-vis spectrophotometer (UV-2800, UNICO, Shanghai, China).

Preparation of the Ti/RuO 2 -IrO 2 -SnO 2 Electrode
The Ti/RuO 2 -IrO 2 -SnO 2 electrode was prepared by a simple thermal decomposition method according to the previous literatures [22,23]. Typically, Ti plates with dimensions of 250 × 250 × 2 mm were used as the substrates. The Ti plates was etched in 20% boiling oxalic acid for 2 h, washed with deionized water and stored in ethanol solution. This pre-treatment served to remove the surface oxide layer and improve the coating adhesion of Ti plates. Ruthenium chloride, chloroiridic acid, and tin tetrachloride were used as the precursor materials. Using the Pechini method, the polymeric precursor solution was prepared by mixing citric acid (CA) and ethylene glycol (EG) at 60-70 • C. A certain molar ratio of Ru-Ir-Sn precursor was added to the polymer solution, and it was kept at 60-70 • C for 30 min to dissolve. The obtained precursor solution was evenly coated on both sides of the Ti substrate with a brush, dried at 80 • C for 8 min and then sintered in track box furnace at 300 • C for 10 min. The above steps were repeated until the precursor solution was used up. Finally, the sample was put into track box furnace and annealed at 550 • C for 1 h [24].

Experimental Device
The experimental reactor consisted of a power supply, Ti/RuO 2 -IrO 2 -SnO 2 anode, Ti cathode, aerator, and an electrocatalytic cell as shown in Figure 1. The electrocatalytic oxidation cell is made of quartz glass with a size of 250 × 160 × 280 mm. The built-in anode is a self-made Ti/RuO 2 -IrO 2 -SnO 2 electrode plate. The cathode is a Ti plate purchased from Baoji Changli Special Metal Co., Ltd. The electrodes were connected to power supply to install the reactor. The reactor is an improved parallel structure reactor with a shared-anode and double electrolytic cells, that is, it is composed of one anode plate and two cathode plates. The effective area of the anode plate immersed in solution is 250 × 250 × 2 mm. This device can double the treatment capacity of organic pollutants without increasing the area of the electrolytic cell, and effectively improve the utilization efficiency of the anode plate.

Experimental Procedure and Analytical Methods
The COD value in the system was determined by the fast confined catalytic digestionspectrophotomete. The detection limit of this method for COD is 50~1000 mg/L [25]. The water samples in this experiment were measured after dilution. Generally, experiments were proceeding three times in parallel for each process, and the final data was taking the average values.
The electrocatalytic performance of the reactor was evaluated according to the COD removal rate and energy consumption. Determination of COD using fast closed digestion method. The COD removal rate and energy consumption were calculated using the following formula [26]: where COD 0 is the COD value of raw pharmaceutical wastewater, mg/L; COD 1 is the COD value of the treated pharmaceutical wastewater, mg/L; U is the reaction voltage, V; I is the reaction current, A; T is the reaction time, h; V is the volume of pharmaceutical wastewater, L.

Characterization of the Electrodes
The stability and lifetime of the electrode is a significant factor affecting the capability of the electrocatalytic oxidation reactor. The configuration of surface and constituent elements of electrodes are often served as precise and straightforward indicators for evaluating electrode stability [27]. As shown in Figure 2, the surface morphology of electrode was studied by SEM and EDS. In Figure 2a, the surface of the titanium electrode is generally flat with a few small particles. In Figure 2b, the anode (Ti/RuO 2 -IrO 2 -SnO 2 ) was inerratic with many cracks that is representative "cracked clay" characteristic of electrodes produced by thermal decomposition, which may be caused by the high-temperature calcination of the precursor coating of the metal oxide formed on the surface of the Ti plates. The calcination of the electrode surface not only facilitates the formation of oxide coatings, but also increases the electrochemically active surface area [28]. The EDS spectrum of the coating on the RuO 2 -IrO 2 -SnO 2 anode showed that Sn, Ru, Ir, Ti, C and O were on the electrode surface (as shown in Figure 3). This indicated that RuO 2 , IrO 2 , and SnO 2 were successfully coated on the Ti plates. The C signal comes from the presence of adventitious carbon on the electrode surface. The observed titanium signal may be due to the inability of the oxide coating to completely cover the surface of the titanium substrate. In addition, we obtain the percentage of element content on the electrode surface through EDS analysis (as shown in Table 2). The weight percentages of Ti, Ru, Ir and Sn are 27.37, 8.71, 4.56 and 7.47%, respectively. After calculation, it is found that there are 17.16% of RuO 2 , 0.33% of IrO 2 , and 14.19% SnO 2 on the surface of the Ti plate, respectively. The high concentration of ruthenium oxide is located in the electrode, which not only provides more active sites for electrocatalytic oxidation, but also enhances the stability and life of the electrode.  Furthermore, the crystal structure and chemical composition of the electrode was also evaluated by the XRD technique, and given in Figure 4

Effect of Electrode Distance
For the electrocatalytic oxidation, it was of interest to investigate the effect of electrode distance. At a current density of 1 mA/cm 2 , air flow of 12 L/min, pH of 7, as shown in Figure 5, When the electrode distance increased from 3 to 5 cm, the COD removal rate dropped by 10.2%. The experimental results indicated that the electrocatalytic performance with a plate spacing of 3 cm higher than a plate spacing of 5 cm. The reason was that when the distance between the plates is 5 cm, the distance between the two plates is too large, and the ion transmission distance between the two plates becomes longer, which increases the mass transfer resistance and weakens the mass transfer effect, thereby resulting in reduced electrochemical reaction efficiency [29,30]. Enhancing the electrode distance requires increasing the battery voltage output to maintain a constant current density. Under high voltage, not only energy consumption increase, but side reactions such as the release of hydrogen and oxygen will occur on the surface of the anode and cathode, which will reduce the efficiency of organic degradation [31,32]. In addition, Sun et al. found that the electrocatalytic oxidation performance is significantly reduced, the operating cost is increased with the distance between the anode and cathode decreases (especially less than 3 cm). It may be because the distance between the two electrodes is small, which makes it difficult for the solution between the plates to exchange with the solution outside the plates, which leads to polarization of wastewater concentration. This situation will affect the mass transfer efficiency, resulting in a decrease in the degradation rate [33]. At the same time, when the distance between the electrodes is very close, the surface of the electrode cannot be supplemented with oxygen, resulting in a reduction in the generation efficiency of OH and the degradation efficiency of pollutants. Therefore, the optimal electrode distance is 3 cm.

Effect of Current Density
Current density affects electrocatalytic performance by affecting electron transfer in oxidation reactions, and proper current density is critical for electrocatalytic removal of COD in wastewater. In Figure 6a, the degradation rate of COD increases obviously when the current density gradually increases in the range of 1-8 mA/cm 2 . When the current density was 8 mA/cm 2 , the COD removal rate was 90.52%, with the energy consumption of 81.71(kW·h)/kg COD. As the current density reached 10 mA/cm 2 , the COD removal rate was 90.48%. The current density has a significant impact on the COD removal effect of the electrocatalytic system, and directly affects the current transmission rate and treatment effect in the electrochemical reaction process. As shown in Figure 6b, in the process of enhancing the current density from 1 to 8 mA/cm 2 , the degradation efficiency showed the simultaneously increasing trend. With the increase in current density, the rate and intensity of charge transfer also increase, thereby promoting the formation of active intermediates and increasing the degradation efficiency of pollutant in wastewater [34,35]. It may be that the side reactions also occur on the electrode surface under too high current density, and the concentration of the active group decreases, thereby reducing the efficiency of reaction electrolysis and increasing energy consumption. At the same time, the tank voltage is too high, the bypass current and short-circuit current increase significantly, the temperature of the wastewater rises, and part of the electrical energy is converted into heat. Therefore, the best current density is 8 mA/cm 2 .

Effect of Air Flow
In general, the degradation efficiency of organic pollutants is affected by the reactive oxygen radicals in the catalytic oxidation reaction system, and the formation of reactive oxygen radicals (e.g., superoxide radical, hydroxyl radical) is related to the oxygen of reaction system, as shown in the Equations (3) and (4) [36,37].
Obviously, a systematic investigation of the oxygen (or air) concentration in the reaction system is of great significance for improving the performance of electrocatalytic oxidation. Figure 7 shows the law of COD removal efficiency and energy consumption in wastewater when the air flow rate is increased from 6 L/min to 30 L/min. With the increase in air flow rate, the removal rate of COD in wastewater is increasing because the concentration of H 2 O 2 produced by the system increases. The increase in air velocity increases the dissolved oxygen in the water, increases the cathode efficiency, and produces more H 2 O 2 , which promotes an increase in the removal rate of wastewater COD. When the rate reaches 18 L/min, the best COD removal rate is 94.06%, and the energy consumption is 74.63 (kW·h)/kgCOD. When the air velocity is higher than 18 L/min, the wastewater COD removal rate does not increase significantly with the increase in the air velocity, indicating that when the air velocity is higher than 18 L/min, the wastewater COD removal rate is mainly controlled by the electrochemical kinetics that produces H 2 O 2 . In addition, increasing the oxygen/air concentration in the catalytic reaction system promotes the formation of active oxygen species, thereby improving the removal efficiency of COD [38].
Excessively increasing the air flow makes it difficult for the organic pollutants in the wastewater to be effectively adsorbed on the electrode surface and react with active oxygen species, which causes the COD removal rate to gradually decrease [39,40]. The results further imply that the electrocatalytic degradation rate is the largest when the aeration amount is 18 L/min.

Effect of Initial pH
The initial pH value in the electrocatalytic reaction determines the reaction process and the direction of the chemical reaction, and the rate of hydrogen peroxide or hydroxyl radical at the cathode is affected by the initial pH (as shown in Figure 8). At electrode distance of 3 cm, the current density of 8 mA/cm 2 , air flow of 18 L/min, the COD removal rate increased with the decreasing pH as shown in Figure 8. The removal rate of COD was 91.45% at pH 7, and the energy consumption was 53.93(kW·h)/kgCOD. In this process, hydroxyl radicals play an important role in the removal of hard-degradable pollutants [41]. Under the dual action of the electric field and the DSA electrode, a large amount of ·OH is generated in the reactor [42,43]. Compared with pH 7, the COD removal rate was 95.92% and the energy consumption was to 58.09(kW·h)/kgCOD at pH 2. It may be because H 2 O 2 is more easily produced in the reaction system at the lower pH (O 2 +2H + + 2e − → H 2 O 2 ), thereby further increasing the degradation efficiency [44].

Characterization of the Electrode after Reaction
To further demonstrate the Ti/RuO 2 -IrO 2 -SnO 2 electrode has excellent electrocatalytic ability and stability, we checked the surface morphology of the Ti/RuO 2 -IrO 2 -SnO 2 electrode after long-term operation by SEM and EDS technology, and the results are given in Figures 9 and 10. On the whole, the surface of the anode and cathode electrodes has undergone significant changes for a long time. For the cathode (Figure 9a), the smoothness of the surface of the Ti plate is reduced, and the surface has a coral-like structure. This may be due to the accumulation of organic molecules during the reaction. For the anode (Figure 9b), the surface smoothness of the Ti/RuO 2 -IrO 2 -SnO 2 electrode also decreases, and a large number of granular structures are accumulated on the surface and cracks, with a particle diameter of about 300 nm. This may be due to the adsorption of a large amount of particulate organic on the surface of the oxide coating of the anode during the long-term reaction.  To verify this, we conducted EDS analysis for the anode material Ti/RuO 2 -IrO 2 -SnO 2 , and the results are shown in Figure 10. Obviously, Ti, Sn, Ru, Ir and other metal elements still exist on the electrode surface. In addition, compared with fresh electrodes (see Figure 3), the content of C element on the electrode surface greatly increases after a long time of reaction, which is caused by the adsorption of a large number of organic pollutants in the wastewater on the electrode surface. In addition, according to the results of the weight percentage of each element (as shown in Table 3), the carbon content has increased from 9.86 to 54.85%, which further proves that our inference is reasonable. The above results further suggested that the Ti/RuO 2 -IrO 2 -SnO 2 electrode exhibited excellent activity and stability for the electrocatalytic oxidation of pharmaceutical wastewater during the long-term reaction.

Conclusions
In summary, we use a simple thermal decomposition method to prepare an oxide coating (RuO 2 -IrO 2 -SnO 2 ) on the Ti substrate as an anode material for the electrocatalytic oxidation of pollutants in pharmaceutical wastewater. Experimental results show that the ternary oxide composite coating electrode has an excellent performance in the electrocatalytic oxidation of pharmaceutical wastewater. COD removal efficiency and energy consumption were used as evaluation indexes, and various parameters (such as electrode distance, current density, air flow, and pH) were optimized. The optimal reaction conditions are that the electrode distance is 3 cm, the current density is 8 mA/cm 2 , the air flow rate is 18 L/min, and the pH is 2. Under these conditions for 24 h of operation, the COD removal rate was 95.92%, and the energy consumption was 58.09 kW·h/kgCOD. No other pollutants are generated during the reaction, the pollutant concentration of the effluent is greatly reduced, and there is no secondary pollution. It is an environment-friendly, efficient and stable method for the degradation of organic pollutants in various industrial wastewaters.
But compared with the traditional water treatment technology, the operating cost of the electrocatalytic oxidation method has a certain increase, but with the development of new electrocatalytic materials, the production cost will be greatly reduced. The TiO 2 used in this article also reduces the cost of the electrode plate to a certain extent. This work helps to understand the good oxidative degradation performance of the electrocatalytic oxidation system on high-concentration wastewater, the feasibility of the electrocatalytic oxidation method used in actual industrial wastewater treatment is explored, which provides certain technical support for the practical application of this technology.

DSA
Dimensionally Stable Anodes COD Chemical Oxygen Demand EC Energy Consumption