Improved electrochemical oxidative degradation of Reactive Red 24 dye by BDD anodes coupled with nitrate: operating parameters, kinetics, and degradation pathways

: The electrochemical oxidation (EO) process coupled with BDD anode and nitrate was used to improve 5 Reactive Red 24 (RR24) removal efficiency in wastewater treatment. The effects of operating parameters on the 6 decolorization efficiency of RR24 were discussed, and the optimal operating parameters were obtained as 7 follows: 45 mA cm -2 , 100 mM SO 42- , 7 mM NO 3- , 60°C, pH 5.88, and 100 mg L -1 RR24 initial concentration. 8 The energy consumption for 100% decolorization within 15 min is 0.92 kWh m -3 , and the total organic carbon 9 (TOC) reaches 51.35% within 90 min. Through the effect of quenchers on RR24 decolorization efficiency, 10 various active species in the EO process were studied. It was found that • OH was closely related to the 11 decolorization degradation of RR24, reaching a contribution rate of 99.47%. Finally, we propose the degradation 12 pathways of RR24 by UV-Vis spectrum and LC-MS test. In summary, the proposed treatment process could be 13 applied to treat RR24 dyes as an efficient method. 14


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There are many kinds of dyes, allowing for a colorful world in people's lives and producing huge economic 17 benefits. At the same time, a large amount of dye wastewater is discharged into the environmental water, 18 reducing the transparency and oxygen content of the water body and affecting the growth of aquatic organisms 19 and microorganisms. Azo dyes are primary used in the printing and dyeing industries, accounting for more than 20 70% of the dyes. Azo dyes are aromatic compounds containing azo groups (-N = N -). Some dyes and their 2 degradation products are toxic and threaten human and animal health (Nidheesh et al., 2018). As a typical azo 22 dye, Reactive Red 24 (RR24) is widely used in the printing and dyeing industries because of its strong 23 chromaticity stability and easy production. Unfortunately, RR24 is difficult to degrade, remains in the water for 24 a long time, and has a greater hidden danger of environmental pollution (Brillas and Martinez-Huitle, 2015). 25 Therefore, an environmentally friendly way for RR24 treatment is necessary. 26 In recent years, traditional physicochemical and biological treatment methods have been adopted in dye 27 wastewater treatment. However, these technologies have some unavoidable disadvantages (Brillas and 28 Martinez-Huitle, 2015). For example, physical adsorption produces a lot of sludge, which brings secondary 29 pollution; although biodegradation has good prospects, the degradation of macromolecules is more complex, 30 the retention time is extended, and the decolorization efficiency is low (Moussa et al., 2017). Thus, the 31 electrochemical advanced oxidation process (EAOP) can be used as an auxiliary or alternative to traditional 32 methods due to its many advantages (Wu et al., 2012). EAOP can oxidize organic pollutants directly by electron 33 transfer and indirectly oxidize organic pollutants on the anode surface by electrochemical active substances 34 (such as •OH) and improves oxidation efficiency (Florenza et al., 2014;He et al., 2015). Therefore, a suitable 35 anode material in the EAOP is critical. 36

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The electrochemical degradation RR24 simulated wastewater experimental device uses a 150 mL beaker 80 as an electrolytic cell, with Pt as the cathode and Ti/BDD as the anode. DC stabilized power supply (WLS-05, 81 Sangli, China) controlled the current density. A magnetic stirrer controlled the stirring speed. The electrode area 82 is 2 cm 2 , and the distance between the electrodes is 2 cm. It adopts intermittent measurement and uses NaOH 83 and H2SO4 to adjust the pH (pH-3C, Shanghai, China). 84 Cyclic voltammetry (CV) test used an electrochemical workstation (CHI660E, Shanghai, China) with a 85 three-electrode system (Ti/BDD as a working electrode, saturated calomel electrode (SCE) as the reference 86 electrode, and Pt served as a counter electrode). 87

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The concentration of RR24 was obtained by UV-Vis spectrophotometer (Shimadzu UV-2600, Japan) (Txt 89 SM-1, Fig. SM-1). 90 The color removal is calculated via the following Eq.  The free radical quencher MeOH, TBA, and phenol were added to RR24 simulated wastewater to study 94 free radicals' influence and contribution rate. 95 The energy consumption in the degradation process of RR24 is estimated by following Eq. (5) (Uranga-96 where U is mean voltage (V) during the electrolysis; I is current (A); t represents the electrolysis time (h); V 99 represents the volume of wastewater being treated (m 3 ). 100 Whether the electrochemical degradation process of RR24 satisfies the pseudo-first-order kinetics can be 101 relatively complete. In other words, BDD film is of good quality. We knew that the BDD electrode has excellent 127 physical properties and excellent electrochemical properties. As the scan rate (ν) increases, the oxidation peak 128 current (Ip) increases, and there is a good linear relationship between v 1/2 and Ip (Fig. 1c, and 1d), which means 129 the diffusion process is the control step of the EO reaction. Moreover, the electrochemical window is an 130 important indicator to measure the electrocatalytic performance of electrode material. As Fig. 1e shown that the 131 electrochemical window of the Ti/BDD electrode is 3.50 V, which is much larger than that of Pt or DSA electrode 132 cm -2 , the RR24 decolorization rate improves significantly, reaching almost 100% at 25 min. In the electrolysis 12 process, ln(A0/At)-t accords with the pseudo-first-order kinetic relationship, and the apparent kinetic constants 175 k are 0.064 min -1 , 0.12 min -1 , 0.22 min -1 , 0.36 min -1 (R 2 , 0.96), respectively, which means the decolorization rate 176 of RR24 is mainly controlled by current density. A high current density accelerates the electron transport rate; 177 therefore, the direct oxidation rate will be accelerated. (He et al., 2015). At the same time, the • OH 178 concentration will continue to increase, speeding up the oxidation of organic matter, which accelerates the 179 breaking of -N=N-in RR24 molecular, leading to decolorization. The current efficiency decreases as the current 180 increases. A lower current density is more conducive to reducing energy consumption and increasing current 181 efficiency and the degradation time. In terms of energy consumption, 45 mA cm -2 is appropriate. 182

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The addition of supporting electrolytes can significantly change the electrochemical reaction process and 184 increase the degradation efficiency. As Fig. 3b shown that the decolorization rate of RR 24 in the electrochemical 185 system when the SO4 2-concentration is 25, 50, 75, 100 mM at 15 min is 83.15%, 94.77%, 97.61%, 98.76%, 186 respectively. As the SO4 2-concentration increases, the decolorization rate of RR 24 has been greatly improved. 187 This is because the increase in SO4 2-concentration increases the conductivity of the solution; that is to say, it 188 promotes the transfer rate of electrons in the electrolyte, which is beneficial to greatly increase the generation 189

The effect of NO3concentration 194
The addition of NO3 -has a significant effect on the decolorization rate of RR24 (Fig. 3c). The  conditions of 20, 30, 40, and 60°C at 2.5 min. In other words, the temperature has a large influence on the 216 decolorization rate of RR24, which increases with the increase of temperature. At 40°C and 60°C, due to the 217 higher temperature, the dye molecules are more reactive and easier to react with •OH, resulting in higher 218 degradation efficiency. Moreover, RR24 dyeing generally adopts a high temperature (80-90°C) dyeing process, 219 and the discharge temperature of wastewater is close to 60°C. Therefore, 60℃ is selected in the degradation 220 process, which is convenient and energy-saving. 221 222

The effects of different initial pH 223
The effect of pH on the decolorization of RR24 was studied. As shown in Fig.4b, at 20  At 15 min, the decolorization efficiency of RR24 was 99.91%, 98.94%, and 97.98%, respectively, which 237 decreased with the increase of its initial concentration, but the overall decolorization efficiency was excellent. 238 In the electrolysis process, ln(A0/At)-t satisfies the pseudo-first-order kinetic relationship; At 50 mg L -1 and 239 100 mg L -1 , the apparent kinetic constants are close, 0.52 min -1 and 0.50 min -1 , respectively. Under the same 240 other conditions, the amount of •OH produced on the electrode surface is certain; As the initial concentration 241 increases, the concentration of radicals is diluted, and the probability of collision with dye molecules decreases, 242 and the entire electrochemical oxidation process turns into diffusion control, which reduces the decolorization rate. Within a certain range, the amount of •OH on the surface of BDD is constant. The higher the initial 244 concentration of RR24, the more the number of -N = N-fractured simultaneously, the subsequent accumulation 245 of intermediate products will inevitably require more free radical which prolonged degradation time increased 246 energy consumption. Therefore, the initial concentration is 100 mg L -1 . 247 248

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The best operating parameters are current density 45 mA cm -2 , supporting electrolyte concentration 100 250 mM, NO3 -concentration 7 mM, temperature 60°C, initial pH is not adjusted, initial dye concentration 100 mg 251 L -1 . The energy consumption for 100% decolorization within 15 min is 0.92 kWh m -3 , and the TOC reaches 252 51.35% within 90 min (Fig. 4d). The energy consumption is low, and the mineralization rate is good. The 253 selected treatment method has greater advantages compared with other methods, as shown in Table 2. 254    [NaNO3] = 7 mM; original pH; 60 °C) 283 The UV-Vis spectra at different electrolysis times (Fig. 6a) shows that after 2.5 min of reaction, the 284 absorption peaks at 510 nm and 534 nm are significantly reduced, the naphthalene ring absorption peaks at 370 285 nm are reduced, and the benzene ring absorption peaks at 238 nm and 287 nm are dropping slightly. After 10 286 min, the characteristic absorption peaks at 510 nm and 534 nm disappeared completely, and the conjugated 287 chromonic system containing azo bonds in the RR24 molecule was destroyed and decolorized. In the ultraviolet 288 region, the benzene ring absorption peak at 287 nm and the naphthalene ring absorption peak at 370 nm 289 disappeared, leaving only the benzene ring absorption peak at 238 nm. The results show that in a short reaction 290 time, EAOP can rapidly degrade the azo group and naphthalene ring in the dye wastewater into benzene ring 291 products, and finally be oxidized to CO2 and H2O. 292 aniline, benzoquinone, phenol, 6-chloro-1,3,5-triazine-2,4-diol, 4-amino-6-chloro-1,3,5-triazin-2-ol, nitrate, 299 and sulfate ions. Table SM-2 and Figure SM-2 have the mass spectrum data of the above products. Figure 7  300 shows the proposed degradation pathways of RR24. The azo bond break first, then the cleavage of the C-N 301 bond between the benzene or naphthalene rings and the triazine ring, followed by the C-N bond between the 302 amide and the amide group and the naphthalene or triazine rings (Xikui Wang et al., 2011). The triazine 303 compound is oxidized further to 6-chloro-1,3,5-triazine-2,4-diol, 4-amino-6-chloro-1,3,5-triazin-2-ol and the 304 amino converted to a hydroxyl group. Aniline compounds are turned into phenolic compounds, oxidized to 305 benzoquinone, and finally oxidized to CO2 and H2O (Ruiz et al., 2011;Asghar et al., 2015). Naphthalene 306 compounds are converted into 2, 4-(benzodiazepine) phthalic acid, and 1-isocyanate naphthalene; forming 307 sulfate ions through the sulfonic acid group cut from the naphthalene ring, and finally the nitrogen in the RR24 308 molecule is oxidized to nitrate ion (Thiam et al., 2016). 309

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First, for high-efficiency decolorization and degradation, the NO3 -+ SO4 2-system is determined to be a 311 suitable electrochemical system for RR24 degradation by comparing the oxidative degradation systems of 312 different electrolytes. Secondly, the effect of several operating parameters on RR24 decolorization is discussed. 313 Among them, the current density and NO3 -concentration have the greatest impact. The best operating process 314 parameters for RR24 electrochemical degradation are obtained: current density 45 mA cm -2 , 100 mM support 315 electrolyte , 7 mM NO3 -, at 60°C, initial pH is not adjusted, and RR24 initial concentration 100 mg L -1 . Under 316 this condition, the energy consumption for 100% decolorization within 15 min was 0.92 kWh m -3 , and the TOC 317 reached 51.35% within 90 min. The quencher on the decolorization of RR24 electrochemical decolorization was 318 also studied. It was found that •OH was closely related to the decolorization degradation of RR24, reaching a 319 contribution rate of 99.47%. Finally, the samples were scanned by UV-Vis at different electrolysis times, and 320 LC-MS tested the samples after 40 min of electrolysis to deduce the possible degradation route of RR4 by 321 analyzing the intermediate products in the degradation process. To achieve higher dye wastewater treatment 322 efficiency, the EO/ NO3 -+SO4 2-system can perform advanced treatment on the effluent after biological treatment and return to the biological treatment system through the system or secondary biological treatment with carbon 324