Electrochemical Degradation of Nitrobenzene Wastewater: From Laboratory Experiments to Pilot-Scale Industrial Application

: In this study, the electrochemical degradation of nitrobenzene (NB) was conducted on the Ti/SnO 2 -Sb/Ce-PbO 2 anode with excellent functional performance. The effect of applied current density, electrode distance, pH value and initial concentration on the reaction kinetics of NB was sys-tematically studied. The total organic carbon (TOC) removal rate reached 91.5% after 60 min of electrolysis under optimal conditions. Eight aromatic intermediate products of NB were identified by using a gas chromatography coupled with a mass spectrometer, and two aliphatic carboxylic acids were qualitatively analyzed using a high-performance liquid chromatograph. The electrochemical mineralization mechanism of NB was proposed based on the detected intermediates and the identified key active oxygen specie. It was supposed that the hydroxyl radical produced on an anode attacked NB to form hydroxylated NB derivatives, followed by the benzene ring opening reactions with the formation of aliphatic carboxylic acids, which mineralized to CO 2 and H 2 O. In addition, NB was reduced to less stable aniline on the cathode surface, which resulted in actualized mineralization. The successful pilot-scale industrial application in combination with wastewater containing NB with the influent concentration of 80–120 mg L −1 indicated that electrochemical oxidation has great potential to abate NB in practical wastewater treatment.


Introduction
Nitrobenzene (C6H5NO2, NB) is widely used as a raw material and an intermediate product for the manufacture of anilines, pesticides, pharmaceuticals, explosives and dyes [1,2]. NB cannot be efficiently removed by traditional activated sludge technology due to its strong electronegativity of the nitro-group which decreases the electron density of the benzene ring [3,4]. In addition, NB and its transformation metabolites have an inhibition effect on microbial activity in the biological treatment process due to toxicity and mutagenicity [5][6][7], which leading to the accumulation of NB in the environment. A previous study reported that it accounted for 80% of surface water contaminated by NB in China, with the residual concentration ranging from <0.3 ng L −1 to 8450.0 ng L −1 [8].
The ubiquity of NB has drawn increasing attention due to its potential hazard to aquatic systems and to human beings [9][10][11]. The 96 h sublethal concentration (LC50) values of NB on Daphnia pulex and Ciprinus carpio were determined to be 0.760 mg L −1 and 1.907 mg L −1 , respectively [12]. The genotoxicity of NB, which could induce micronuclei in mammalian cells, has also been reported [13]. NB is listed as a priority pollutant [14] and is classified as a Group B2 carcinogen [15]. The permission concentration for NB in the surface water is restricted to 17 μg L −1 in China for environmental security [16]. Therefore, it is crucial to seek a dependable way to eliminate NB from wastewater.
Advanced oxidation processes (AOPs) have recently been utilized for treatment of refractory organic pollutants, mainly due to their ability to produce highly oxidizing radicals, e.g., hydroxyl radical (•OH) [17,18]. It has been demonstrated that NB could be efficiently removed by ozonation [19][20][21], photocatalysis [22,23] and the Fenton reaction [18,24]. Nevertheless, strict operational conditions constrained the application of these AOPs, or the subsequent sludge disposal increased the invested cost of the wastewater treatment plants. Thus, it is still urgent to develop an efficient, feasible alternative approach to decompose NB in wastewater.
Electrochemical oxidation is considered an environmental-friendly technology for the purification of chemical industry wastewater [17]. The degradation efficiency of electrochemical oxidation primarily depends on the choice of anode materials on which the pollutants can be degraded by direct and indirect oxidation reactions [25,26]. Ce-doped PbO2 electrodes with SnO2-Sb as the middle layer (Ti/SnO2-Sb/Ce-PbO2) have been proven to possess high chemical stability and good performance on the removal of a variety of recalcitrant pollutants, e.g., perfluorocarboxylic acids [27,28], sulfamethoxazole [29], ibuprofen [26], and phenol [30] from aqueous solutions. However, it is yet to be determined whether NB can be degraded by the Ti/SnO2-Sb/Ce-PbO2 anode. Moreover, laboratory experiments applying the electrochemical oxidation process to decontaminate various wastewaters, including dye wastewater [31,32], pharmaceuticals and personal care products [33,34] and coking wastewater [35], have been extensively conducted and have effectively clarified its advantages, but very little information is available on the industrial application.
This paper presents the feasibility of the electrochemical oxidation process using Ti/SnO2-Sb/Ce-PbO2 anode for the treatment of NB wastewater on an industrial scale. To optimize the process parameters, electrolysis of NB was carried out under different applied current densities, electrode distances, solution pHs, and initial NB concentrations in the lab-scale experiment. The mineralization mechanism of NB was revealed based on identified intermediate organic byproducts, the trace of nitrogen atom, and the confirmed key active oxygen specie. Furthermore, a pilot-scale industrial electrochemical reaction system was developed and applied to clarify the potential practical application value for NB contaminated water. Figure 1a shows that the surface layer of the Ti/SnO2-Sb/Ce-PbO2 electrode is covered with dense polyhedron grains and that most of the grain sizes of particles are less than 1 μm. The nanostructure of the electrode can provide a striking number of reaction sites for the degradation of target pollutants. The lateral view SEM image of the prepared Ti/SnO2-Sb/Ce-PbO2 anodes is shown in Figure 1b. It was estimated that the average coating thickness of the anode was about 175 μm. Further energy dispersive spectrometer (EDS) element mapping (Figure 1c) revealed the uniform distribution of Pb, O and Ce, with the Ce/Pb stoichiometric ratios of about 0.1/23.3. Figure 1d shows that the characteristic peaks located at 2θ = 25.5, 32.1, 36.3, 49.2, 52.3, 59.0 and 62.6, correspond to the lattice planes (110), (101), (200), (211), (221), (310) and (301), respectively. The peak positions of the electrode corresponding to card number 73-0851 in the database are consistent with the tetragonal crystal structure of β-PbO2. There are no obvious CeO2 diffraction peaks in the XRD pattern, indicating a low doping level or that the rare earth element Ce is doped into the PbO2 lattice. Figure 1c demonstrates that the oxygen evolution potential (OEP) of the prepared electrode is up to 1.92 V vs. SCE. Generally, the high OEP could effectively inhibit the oxygen evolution side-reaction which can enhance the degradation performance of the anode during an electrocatalytic degradation process. The mechanical and chemical stability is important in applications. The long-term electrochemical performance of Ti/SnO2-Sb/Ce-PbO2 anode was scrutinized by an accelerated life test conducted in extreme conditions (0.5 mol L −1 H2SO4 solution and high current density of 1 A cm −2 ). As shown in Figure 1f, the accelerated service lifetime of the anode was 340 h. Obviously, the anode would have a much longer lifetime under mild conditions for wastewater treatment.

Electrochemical Degradation Kinetics of NB
In all laboratory experiments, the degradation of NB followed pseudo-first-order kinetics which can be described as follows: where kNB and C are the pseudo-first-order rate constant (min −1 ) and NB concentration (mg L −1 ), respectively and t is the electrolysis time (min). Parameters including kNB, half-lives (t1/2) and average voltage and correlation coefficient (R 2 ) are summarized in Table 1. Figure  2 presents the removal efficiency of NB as a function of current density, electrode distance, solution pH and initial NB concentration during the electrochemical oxidation process.

Effect of Applied Current Density
As shown in Figure 2a, increasing the applied current density can significantly enhance the NB removal rates. The kNB values were measured to be 2.7 × 10 −2 , 3.3 × 10 −2 , 4.5 × 10 −2 , 5.9 × 10 −2 , 7.8 × 10 −2 and 8.8 × 10 −2 min −1 under the current density of 1, 2, 5,10, 20 and 30 mA cm −2 , and the corresponding t1/2 values were 25.7, 21.0, 15.4, 11.7, 8.9 and 8.0 min, respectively (see Table 1). It was apparent that the degradation efficiency of NB improves with increasing the current density. It was conjectured that a tremendous amount of •OH production in the reaction system under a high current density resulted in NB molecules being instantaneously degraded [36,37]. It was noted that the removal rate of NB was slightly increased 0.3% (from 99.4% to 99.7%) when the current density was increased from 20 mA cm −2 to 30 mA cm −2 , whereas the energy cost (calculated by Equation(S1) in Text S1 in Supplementary Materials) was greatly increased by 20% (from 86.3 ± 3 Wh L −1 to 103.5 ± 2 Wh L −1 ). It could be conjectured that undesirable side reactions, such as water splitting reaction and oxygen evolution, were particularly pronounced as the current density increased; the observation of intensive bubbles on electrode surface verified this conjecture [38].

Effect of Electrode Distance and pH
The electrode distance affects the diffusion rate of the target pollutant in the electrochemical oxidation system, which has an impact on its degradation efficiency [38]. Figure  2b suggests that complete removal of NB can be achieved with an electrode distance of 0.5 cm and 1.0 cm with 50 min electrolysis. When the electrode distance is 2.0 cm or 3.0 cm, NB cannot be entirely removed after 60 min. Table 1 shows that the kNB value decreased significantly with increased electrode distance, which can be explained by the fact that NB molecules can rapidly diffuse from bulk solution to anode surface at shorter electrode distance, and thus improve the electrochemical selectivity of •OH for NB [29]. Nevertheless, taking various factors, e.g., treatment efficiency, space efficiency, the short-circuit problem of the system and the investment costs, into consideration for the practical application, a relative larger electrode distance (i.e., 1.5 cm) is required.
The solution pH typically influences the degradation efficiency, and the pH fluctuation is commonly found for practical industrial wastewater [9]. Hence, the influence of the solution pH was investigated at a wide range (2.0-12.0) ( Figure 2c). As shown in Table S1, there was little difference in the degradation kinetics of NB at the investigated initial pH values. A slight decrease was observed at the initial pH of 2.0 (kNB = 6.4 × 10 −2 min −1 ) and 12.0 (kNB = 5.6 × 10 −2 min −1 ). It was because the generation of •OH was inhibited both in strong acidic and alkaline environments, resulting in the decrease of NB removal efficiency [9]. This indicated that the degradation of NB on the Ti/SnO2-Sb/Ce-PbO2 anode can be performed at wide pH ranges, which will undoubtedly apply to NB wastewater.

Effect of Initial NB Concentration
The effect of the initial NB concentration, ranging from 5.0 mg L −1 to 200.0 mg L −1, was investigated, and the result is shown in Figure 2d. It is clearly that entire elimination of NB was accomplished within 60 min electrolysis for all test levels. Table 1 highlights that the degrade rate constant of NB was slightly decreased from 9.2 × 10 −2 to 7.7 × 10 −2 min −1 with the initial concentration increasing from 5 to 50 mg L −1 . This phenomenon was because the amount of •OH radicals produced in this reaction system were constant under certain current density [4], resulting in a higher probability that NB molecules would be oxidized by •OH at relative lower initial NB concentrations. At the same time, the kNB values were similar for higher NB concentrations ranging from 50 to 200 mg L −1 . Despite a lower ratio of •OH to NB at higher initial concentrations, more NB molecules can react with the •OH radicals on the anode surface, leading to a fast degradation rate.
Considering the requirement of high removal efficiency of NB and low energy cost, the current density of 20 mA cm −2 was recommended in the pilot-scale industrial application. The electrode distance of 1.5 cm was adopted based on the treatment efficiency and practical problems which have been stated in Section 2.2.2. The solution pH has little effect on the degradation efficiency of NB, however, overly acidic and alkaline conditions cause corrosion of electrodes and other devices, resulting in increased operating costs. Therefore, the aqueous solution needs to be properly adjusted in practical applications if the pH of the wastewater is overly acidic or alkaline. The NB concentration in the influent of the designed electrochemical reaction unit was about 80~120 mg L −1 within the investigated range, which made it possible for high performance of the electrochemical system using the Ti/SnO2-Sb/Ce-PbO2 anode.

Evolution of TOC and Nitrogen-Containing Final Products
Though some techniques could also largely remove NB from water, they may yield toxic byproducts, such as nitrosobenzene and aniline [5][6][7]. To examine the mineralization degree of NB in the optimized electrochemical oxidation system (i.e., current density of 20 mA cm −2 , the electrode distance of 1.0 cm, pH without adjustment, and the initial concentration of 100 mg L −1 ), the TOC removal rate and the evolution of nitrogen-containing inorganic products were fully examined at optimized conditions. As is evident from Figure 3, the TOC removal rate achieved 91.5% after 60 min of electrolysis, which implies that most NB molecules and their intermediate byproducts were simultaneously oxidized by the electrochemical oxidation process. The mineralization process of heterocyclic compounds is typically accompanied by the formation of inorganic ions. Ammonium (NH4 + ), nitrate (NO3 − ), and nitrite (NO2 − ) ions were observed during the electrochemical oxidation of NB, and the concentration trends of NO3 − -N, NO2 − -N, NH4 + -N and total nitrogen (TN) are shown in Figure 3. The denitration reaction of nitro groups resulted in the occurrence of NO2 − , which was further converted to NO3 − on the anode surface. The generation of NH4 + can be explained by consecutive reductive reactions of nitro group or the direct reduction process of the produced NO2 − on the cathode surface. The concentration of NO3 − -N was accumulated to 6.2 mg L −1 ; nevertheless, the concentrations of NO2 − -N and NH4 + -N were much lower compared with those of NO3 − -N, which can be partially due to the instability of the NO2 − and NH4 + ions. Notably, the mass balance of TN gradually decreased with electrolysis time, and the TN concentration was progressively reduced from 11.3 to 7.6 mg L −1 . We hypothesize that the fraction of the nitrogen atom left the solution in the form of N2 and/or was adsorbed on electrode surface might be responsible for this phenomenon.

Qualitative and Quantitative Determination of •OH
Generally, •OH is considered as the key active oxygen specie in the electrochemical oxidation system. It was noted that Na2SO4 is routinely added to the solution as a supporting electrolyte to increase ionic conductivity. It is worth highlighting that when sulfates are used, the persulfates may be generated at non-active anodes [39]. In order to discern the key active oxygen free radical that causes the mineralization of NB, tertiary butyl alcohol (TBA) and methanol (MeOH) were added into the system as scavengers, and the result is shown in Figure 4a. Apparently, the removal rate of NB was significantly decreased when adding 50 mM TBA/MeOH to the reaction electrolyte, with the kNB decreased from 7.8 × 10 −2 min −1 to 2.4 × 10 −2 min −1 and 2.1 × 10 −2 min −1 , respectively. Further increasing the scavenger concentration to 500 mM caused a notable decrease of reduction in NB oxidation kinetics (3.3 × 10 −3 min −1 with 500 mM TBA, 2.8 × 10 −3 min −1 with 500 mM MeOH). By comparing the reaction rate constant of scavengers and active oxygen species, TBA was generally observed as •OH scavenger and MeOH was observed as •OH and SO4 •− scavenger [40][41][42]. The above analyses indicate that •OH was the main active oxygen specie during the electrochemical oxidation reaction. Of particular note, the removal rate of NB was still able to reach 15.4% despite the existence of 500 mM MeOH, which might be due to the effect of other active oxygen species, e.g., O2 •− , or the direct oxidation of NB on the anode. On the other hand, the active oxygen species would not be completely quenched by the scavenger, leading to small degradation of NB.
Salicylic acid was utilized as a probe to quantify the concentration of •OH generated in this electrochemical oxidation system with an applied current density of 20 mA cm −2 (detailed see Text S2). As shown in Figure 4b, the generation of •OH in this process followed typical pseudo-first-order kinetics, and the theoretical production rate constant of •OH was calculated to be 47.1 mM min −1 m −2 . •OH was a non-specific oxidant, which could react with most organic pollutants [43]. The aforementioned results suggest that •OH was the main active specie, and the high production could ensure the high mineralization rate of NB.

Intermediate Analysis and Degradation Pathway
Some intermediate byproducts, e.g., nitrophenols, aniline and organic acids, which have adverse effects on environmental health and human beings, are normally detected during the degradation of NB by AOPs [44][45][46]. To better elucidate the mineralization mechanism of NB by electrochemical oxidation on the fabricated anode, the intermediates were recognized with the assistance of a gas chromatograph coupled with a mass spectrometer (GC-MS) and a high-performance liquid chromatograph (HPLC). Eight aromatic intermediates of NB were detected based upon the MS fragmentation pattern, "nitrogen rule" and relevant literatures. Detailed information of the detected intermediates is shown in Table S1. The transformations of the intermediate structures included the denitration and substitution of nitro group by hydroxyl group (phenol), the formation of a ketone group on the benzene ring (benzoquinone) and the hydroxylation of the benzene ring (o-Nitrophenol, m-Nitrophenol and p-Nitrophenol). Meanwhile, two isomers, resorcinol and hydroquinone were also detected. The entities of the above intermediates are commonly reported byproducts by other oxidation processes, e.g., anodic oxidation [17], Fenton-like catalytic degradation with Fe-Cu bimetallic catalysts [3], and photodegradation with ZnO2 nanoparticles [47]. It was noted that aniline was quantified by GC-MS, which implies that the reduction reaction of NB also exists in this degradation system. Moreover, two aliphatic carboxylic acid intermediates, including maleic acid and oxalic acid, were identified by HPLC, indicating that aromatic intermediates were further decomposed into low molecular-weight carboxylic acids.
Based on the identified intermediates and the qualitative key reactive specie (•OH), the possible degradation pathway of NB was proposed (see Figure 5). The α-, β-and γcarbons of the aromatic ring of NB were attacked by •OH, accompanied by the formation of three nitrophenol isomers (o-Nitrophenol, m-Nitrophenol and p-Nitrophenol). Subsequently, the nitro group of nitrophenol was further attacked by •OH, resulting in denitration and the formation of dinitrophenols. The released NO2 − ion might have simultaneously oxidized or reduced on the anode or cathode. The presence of NO3 − and NH4 + could indirectly verify this hypothesis. On the other hand, the nitro group of NB with strong electron-withdrawing characteristics could be electrophilic and attacked by •OH to form phenol. It is noteworthy that the reduction of NB also occurred on the cathode leading to the formation of aniline, which was also identified in other synchronistic oxidation and reduction systems [9,48]. Then, the substitution of -NH2 by the -OH group could generate the product phenol with the release of NH4 + . The further oxidation of phenol by •OH led to the formation of dinitrophenols. Then, hydroquinone was dehydrogenated to benzoquinone. In the subsequent stage, the aromatic intermediates underwent a series of oxidation reactions by •OH attack, resulting in the ring cleavage to form aliphatic carboxylic acids, including maleic and oxalic acids. These small molecule acids were finally oxidized into CO2 and H2O, which was verified by the high mineralization rate of NB.

Pilot-Scale Industrial Application
To explore the feasibility of electrochemical oxidation using Ti/SnO2-Sb/Ce-PbO2 anode in industrial applications, a pilot-scale application was investigated in one chemical plant which is described in greater detail in Section 3.2. The static degradation experiment was conducted under optimized laboratory conditions, i.e., applied current density of 20 mA cm −2 and electrode distance of 1.5 cm. Figure 6 shows that the degradation rate of NB and COD in the wastewater were 96.2% and 68.8%, respectively. Obviously, compared to a laboratory experiment, a real NB wastewater experiment involves difficult challenges. However, it could improve the biodegradability subsequent biological treatment. Significantly, there was no significant change for the COD removal after 90 min of electrolysis, so the hydraulic retention time was kept at 90 min in the sequential treatment. Although existing processes of Fenton oxidation and flocculant precipitant can remove most organic pollutants and meet the quality requirement of the influent into a biological unit, their disadvantages, such as unstable water quality, extra additives of Fenton's reagent and flocculant, and large amounts of chemical sludge, are many. Hoping to resolve the above-mentioned problems, the electrochemical oxidation equipment was installed to replace original Fenton and flocculant units. As shown in Figure 7, the concentration of NB in the effluent was 2.14-4.73 mg L −1 and the COD was 1106-1520 mg L −1 at the average inlet flow of 0.55 m 3 h −1 within a 30 d pilot run time. Obviously, the effectiveness of electrochemical oxidation was superior to the replaced Fenton-flocculation process. In addition, Table S2 gives a rough estimate of the economic costs of the electrochemical oxidation unit and the replaced Fenton-flocculant precipitant process. Results show that the operating cost for the electrochemical oxidation unit accounted for only 18.8% of the cost of the replaced Fenton-flocculant precipitant process, showing the economic feasibility of the application of electrochemical oxidation in this chemical plant.

Laboratory Experiments
The preparation process of the Ti/SnO2-Sb/Ce-PbO2 electrode was identical to the previous study [26]. Briefly, titanium sheets were sanded with 200, 600 and 800 grit sandpaper, and then immersed in 10% NaOH solution and etched with a 5% boiling oxalic acid solution. Subsequently, the middle layer (SnO2-Sb) was prepared by the sol-gel technique with the coating solution (the molar ratio of ethylene glycol, citric acid, SnCl4·4H2O and SbCl3 was 140:30:9:1). The Ce-PbO2 layer was acquired by electrodeposition with Ti/SnO2-Sb as the anode. The electrodeposition was conducted under 20 mA cm −2 for 60 min, and the electrolyte consisted of 0.1 M HNO3, 200 g L −1 Pb(NO3)2, 0.4 g L −1 Ce(NO3)3 and 0.5 g L −1 NaF. The relevant preparation details are provided in Text S3. In the lab-scale electrochemical experiments, the prepared electrode (50 mm × 50 mm × 1 mm) served as the anode, and a titanium sheet (purity 99.9%) with the same dimensions was used as the cathode. The surface morphology of the anode was investigated using scanning electron microscopy (SEM/EDS, S4800, Hitachi, JPN). The crystal phase composition and crystallinity of the anode were examined with X-ray diffraction (XRD) (PANalytical, Almelo, NL). The electrochemical measure was conducted in a conventional three-electrode cell system on a CHI660E electrochemical workstation (CH Instruments, Shanghai, CHN). The prepared anode, a platinum plate, and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. The linear sweep voltammetry (LSV) test was conducted in 0.5 mol L −1 Na2SO4 solution with a scan rate of 50 mV s −1 . The accelerated service lifetime tests were performed at the current density of 1 A cm −2 in the 0.5 mol L −1 H2SO4 solution. Inactivation of electrode occurred when the recorded cell voltage was 5 V higher than the initial value.
The effect of applied current density (1-30 mA cm −2 ) and initial pH values (2.0-12.0) were investigated in a 30 mL reaction solution with initial NB concentration of 100 mg L −1 .
To examine the effect of electrode distances (0.5-3.0 cm), 30-100 mL of 100 mg L −1 NB solution was used to conduct the experiment. Furthermore, the effect of initial NB concentrations (5.0-200.0 mg L −1 ) was investigated to effectively evaluate the application scope of the electrochemical oxidation. To increase the conductivity, 35 mM Na2SO4 was added in all solutions as a supporting electrolyte.

Pilot-Scale Industrial Application
The chemical plant is located in Huaian, Jiangsu province in China, and its main commercial products are dinitrotoluene and toluidine. The load of industrial wastewater is about 30 m 3 h −1 , and the dominant organic pollutant is NB. Original wastewater treatment processes mainly included Fenton oxidation, flocculant precipitant, bio-contact oxidation and ozone discoloring. The influent concentrations of NB and COD are 80-120 mg L −1 and 4000~6000 mg L −1 , respectively, for the Fenton oxidation unit. The scheme of the pilot-scale electrochemical reaction system is shown in Figure 8. The electrolytic cells were made of polymethyl methacrylate undivided with the dimensions of 1.5 m × 0.8 m × 0.8 m. The electrodes were connected in the series mode. The Ti/SnO2-Sb/Ce-PbO2 anode (50 cm × 50 cm × 0.1 cm) and titanium sheet with the same size were fixed on the reactor. The top of the reactor set up A skimming dish was set up on the top of the reactor; and a dustpan shaped sludge collecting hopper was set up on the bottom. The wastewater was pumped from the bottom, with an outlet on the top of the reactor. Other operation parameters were designed on the basis of the results of laboratory experiments. The flowcharts of the wastewater treatment units before and after the application are represented in Figure S1. The requirement of the chemical plant was that the concentrations of NB and COD be lower than 5 mg L −1 and 1500 mg L −1 in the effluent of the electrochemical oxidation unit. Figure 8. The scheme of the pilot-scale industrial electrochemical reaction system: 1-the shell of the reaction; 2-anodes; 3-cathodes; 4-grille; 5-inlet pipe; 6-dustpan shaped collecting hopper; 7-wash water pumps; 8-deslagging pipe; 9-DC power supply; 10-skimming dish; 11-fixed link; 12-outlet pipe.

Analytical Methods
The concentrations of NB were measured using HPLC Dionex U3000 (Thermo Fisher, Waltham, MA, USA) with a dual absorbance detector (DAD). The separation was performed on an Athena C18-WP (4.6 mm × 250 mm, 5 μm particle size) column. The mobile phase was ultrapure water and methanol (30/70, v/v). The detected ultraviolet absorption wavelength was 265 nm. and the flow velocity was set at 1 mL min −1 with an injection volume of 20 μL.
Intermediate products of NB formed in the reaction system were characterized by GC-MS (7890A-5975C, Agilent, Santa Clara, CA, USA) equipped with a DB-5MS column (30 m × 0.25 mm, 0.25 μm particle size). The initial GC column was kept at 50 °C for 2 min, then increased to 300 °C with a rate of 20 °C min −1 . Notably, each individual sample was extracted dichloromethane three times before the GC-MS analysis. In addition, qualitative determination of the generated carboxylic acids was performed by HPLC-UV, using a HSS XSelect Τ3-C18 column (2.1 mm × 100 mm, 2.5 μm particle size). The ultraviolet absorption wavelength was 220 nm, and the mobile phase was 4 mmol L −1 H2SO4 with a flow velocity of 0.4 mL min −1 .
The TOC and TN of the electrolyte were measured on a TOC-L-CPH/CPN analyzer (Shimadzu, Kyoto, Japan). Concentrations of ammonia nitrogen (NH4 + -N) were measured using HJ 535-2009 Water Quality Determination of Ammonia Nitrogen-Nessler's Reagent Spectrophotometry. The concentrations of nitrate ion (NO3 − ) and nitrite ion (NO2 − ) in the electrolyte were detected using an ion chromatography system Dionex ICS-1000 ( Thermo Fisher, Waltham, MA, USA).

Conclusions
NB can be effectively degraded and mineralized by electrochemical oxidation using a Ti/SnO2-Sb/Ce-PbO2 anode. The electrochemical degradation of NB followed pseudofirst-order kinetics and the kNB increase with increasing current density and decreasing electrode distance. The solution pH and initial concentration of NB exhibited less impact on the removal of NB. Eight aromatic intermediates and two aliphatic carboxylic acid byproducts of NB were quantified using GC-MS and HPLC, respectively. A plausible electrochemical mineralization pathway of NB was proposed based on the identified intermediates, the qualitative and quantitative determination of the key active oxygen specie. The pilot-scale industrial experiment indicated that the concentration of NB and COD decrease to 2.14-4.73 mg L −1 and 1106-1520 mg L −1 , respectively, which effectively improves the biodegradability of wastewater and the stability of the effluent. Our findings highlight the potential of a Ti/SnO2-Sb/Ce-PbO2 electrode for remediation of NB wastewater and provide support information for further industrial application of electrochemical oxidation technology.
Supplementary Materials: The following are available online at www.mdpi.com/article/10.3390/catal12020190/s1, Text: S1, Text: S2, Text: S3, Figure S1: The flowcharts of the wastewater treatment processes before (a) and after (b) reform in the chemical plant; Table S1: Intermediates of NB identified by GC-MS/MS in electrospray ionization positive mode, Table S2: The operating costs of electrochemical oxidation unit and the replaced Fenton-flocculant precipitant process.