Mineralization of High-Concentration Aqueous Aniline by Hybrid Process

The efficient mineralization of high-concentration aqueous aniline (HCAA) is an issue needing to be resolved. In this study, a hybrid process of ozonation and electrochemical oxidation (ECO) was proposed for improving the mineralization of HCAA (1000 mg·L−1). The results indicated that chemical oxygen demand (COD) removal by the hybrid process was far greater than that of a single ozonation or ECO process, revealing that the hybrid process might avoid low efficiency in late ozonation and initial ECO. Thus, a subsequent combination effect clearly existed. In this hybrid process, ozonation stage time was selected as 60 min for optimal COD removal. The main products of the ozonation stage were maleic and succinic acids, with declining pH which was beneficial to the following ECO stage. Nitrite and nitrate formed during ozonation, which acted as electrolytes for the ECO stage, in which maleic and succinic acids were fully degraded and pH thus increased. Moreover, the aniline degradation mechanism of the hybrid process was deduced, demonstrating the superiority of this hybrid process. Finally, more than 95% COD removal was achieved, which met the COD limit requirement and achieved pH control simultaneously, according to the discharge standards of water pollutants for dyeing and finishing of the textile industry in China (GB 4287–2012).


Introduction
Aniline is commonly used by the chemical industry in several processes, such as for producing synthetic paints, antioxidants, herbicides, and varnishes [1][2][3]. Table 1 exhibits the main sources of aniline pollution in water. This chemical exhibits significant accumulation in nature due to its low degradability [4][5][6]. The carcinogenicity, teratogenicity, and mutagenicity of aniline have been widely reported due to its long-term persistence in the environment and potential for accumulating in organisms [7][8][9]. Strict standards have been set for the discharge of aniline wastewater (AW) in many countries and it is necessary for AW to be thoroughly treated before release to nature.
The treatment methods for AW mainly include adsorption, biodegradation, membrane separation, and photocatalytic oxidation [10][11][12][13][14]. Adsorption technology is suitable for low-concentration AW due to its limited adsorbing capacity [15], such that its adsorbing material requires frequent regeneration for treating high-concentration AW. Biodegradation needs a long culture cycle to achieve suitable results with existing biological strains and high-concentration AW and, thus, the degradation rate is slow [16]. Membrane separation can enrich low-concentration AW and act as a pretreatment process [17], but it rarely used for high-concentration AW. Photocatalytic oxidation requires additional artificial UV light to increase aniline mineralization, leading to high treatment costs [18]. Advanced oxidation processes (AOPs) have attracted increasing interest in recent years, owing to their high-degradation efficiency and short treatment period for organic wastewater treatment [19][20][21]. Among AOPs, ozonation and electrochemical oxidation (ECO) processes are widely used in wastewater treatment. In general, ozonation efficiency is high in its initial Based on the characteristics of ozonation and ECO for treating HCAA, a hybrid process of ozonation and ECO was examined here for possible improvement of the oxidation efficiency for HCAA. To the best of our knowledge, a hybrid process of ozonation and ECO has been little reported regarding the oxidation of such solutions and thus first proposed here. Additionally, the intermediate products and basic mechanism of this hybrid process needed to be revealed.
In this study, HCAA was degraded by a hybrid process, with the purposes of: (1) comparing the effects of the single processes and hybrid process; (2) investigate and optimize the key factors of the hybrid process; and (3) explore the decomposition mechanism of HCAA by the hybrid process.

Materials
Aniline (C 6 H 7 N, M av 93.13 g/mol) sodium hydroxide (NaOH, M av 40.01 g/mol) and activated carbon (AC; particle size 200 ± 50 µm) were obtained from Aladdin Industrial Corp., City Industry, CA, USA. All other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All chemicals were analytical grade and used as received without further purification. All solutions were prepared using ultrapure water.

Ozonation Device
The removal of aniline from aqueous solution by ozonation was carried out in a cylindrical laboratory-scale reactor (800 mL). In each experiment, the reactor was filled with 500 mL of 1000 mg·L −1 aqueous aniline (AA), with ozone produced from pure oxygen via an ozone generator (BMT 802X, BMT Messtechnik GmbH, Stahnsdorf, Germany). Experiments were performed at constant gas flow (measured at 25 • C and 1 atm) and constant inlet ozone concentration (50 g·m −3 ). Ozone entered the reactor through a gas distributor in the reactor bottom and was agitated in the solution. AA was prepared in purified water (Milli-Q Advantage A10, Millipore Corp., Billerica, MA, USA) and input to the reactor by a peristaltic pump. The ozone concentration was measured by an ozone analyzer (BMT 964, BMT Messtechnik GmbH, Stahnsdorf, Germany). In an experiment, 350 mg of modified AC (described below) were introduced in the reactor for promotion of ozonation reactions [32]. Ozone gas leaving the reactor was removed before venting in gas-washing bottles filled with 10% potassium iodide solution.

Electrochemical Oxidation Device
A 500 mL electrochemical reactor was assembled with a plate-shaped anode (Ti/SnO 2 -Sb 2 O 3 , 10 × 10 cm, Xinhengtai New Material Technology Co., Ltd., Xi'an, China) and cathode (SS316 plate, 10 × 10 cm, Xinhengtai New Material Technology Co., Ltd., Xi'an, China), which was vertically positioned in the electrochemical reactor, spaced 3 cm apart. One end of each electrode was connected to a direct current (DC) power supply, which supplied voltage and current density at 0-30 V and 25 mA·cm −2 . Compressed air was introduced at the reactor bottom as the oxygen supply through a gas distributor and solution stirred at room temperature (25 • C). For the hybrid process, the entering solution was from the ozonation device. For the single ECO process, the aniline concentration was the same as that in the ozonation experiment.

The Hybrid Process
The hybrid process was assembled by connecting the ozonation and ECO processes ( Figure 1, dotted line). Initially, AA is treated by ozonation and then the resulting solution mineralized by ECO.

Electrochemical Oxidation Device
A 500 mL electrochemical reactor was assembled with a plate-shaped anode (Ti/SnO2-Sb2O3, 10 × 10 cm, Xinhengtai New Material Technology Co., Ltd., Xi'an, China) and cathode (SS316 plate, 10 × 10 cm, Xinhengtai New Material Technology Co., Ltd., Xi'an, China), which was vertically positioned in the electrochemical reactor, spaced 3 cm apart. One end of each electrode was connected to a direct current (DC) power supply, which supplied voltage and current density at 0-30 V and 25 mA•cm −2 . Compressed air was introduced at the reactor bottom as the oxygen supply through a gas distributor and solution stirred at room temperature (25 °C ). For the hybrid process, the entering solution was from the ozonation device. For the single ECO process, the aniline concentration was the same as that in the ozonation experiment.

The Hybrid Process
The hybrid process was assembled by connecting the ozonation and ECO processes ( Figure 1, dotted line). Initially, AA is treated by ozonation and then the resulting solution mineralized by ECO.  Figure 1. Schematic view of the ozonation process, ECO process, and hybrid process.

Modification of Activated Carbon (AC)
AC was modified using the following steps: 1. The material was washed with deionized water until the washing effluent was nearly colorless and then dried in a hot air oven at 105 °C for 2 h; 2. A 40 mL volume of 30% (wt%) NaOH aqueous was added to 20 g of dried AC, oscillated for 2 h at 30 °C , allowed to sediment for 24 h, and then filtered to remove retrieve the solid;

Modification of Activated Carbon (AC)
AC was modified using the following steps: 1.
The material was washed with deionized water until the washing effluent was nearly colorless and then dried in a hot air oven at 105 • C for 2 h; 2.
A 40 mL volume of 30% (wt%) NaOH aqueous was added to 20 g of dried AC, oscillated for 2 h at 30 • C, allowed to sediment for 24 h, and then filtered to remove retrieve the solid; 3.
The modified AC was dried in an oven at 100 • C for 2 h, rinsed with ionized water to neutrality, dried for 24 h at 105 • C, and then dried.
The modified AC was labeled ACm. Before an ozonation experiment, ACm was soaked in the aqueous aniline (1000 mg·L −1 ) for 24 h to eliminate adsorption effects.

Analytical Methods and Procedures
Conductivity was measured using a conductivity meter (Multi 3630, WTW, Xylem, Inc., Rye Brook, NY, USA). Aniline, COD, ammonia nitrogen (NH 4 + -N), nitrate-nitrogen (NO 3 − -N), and nitrous nitrogen (NO 2 − -N) were measured according to the standard protocol of the State Environmental Protection Administration of China [33].
Polymeric products formed in the initial ECO process were detected by gas chromatography with MS (GC-MS; Agilent 7890a, Agilent Technologies, Inc., Santa Clara, CA, USA and Waters GCT Premier, Waters Corp., Milford, MA, USA, respectively). Carrier gas was 99.999% nitrogen at 1 mL/min. Sample quantity was 0.2 µL and inlet temperature 280 • C. The chromatographic column was a fused quartz capillary column (HP-5MS, 30 m × 0.25 mm × 0.25 µm, Agilent Technologies, Inc., Santa Clara, CA, USA). The column's initial temperature was 40 • C for 5 min, programmed heating at 8 • C/min to 240 • C, and then retained for 20 min.
The treatment effect of AA was assessed using the discharge standards of water pollutants for dyeing and finishing of textile industry in China (GB 4287-2012). The main indices are listed in Table 2. Table 2. Main index limited in discharge standard (GB 4287-2012).

Index
Indirect Discharge Limited Value It should be noted that indirect discharge means the act of enterprises discharging pollutants into a public sewage treatment system. In most cases, enterprises are inclined to indirect discharge and, therefore, meeting the indirect discharge standard is the treatment goal for AA in this study. In the discharge standard, COD is the important index and COD removal thus used here to evaluate oxidation effects.

Aniline Concentration and COD Removal
In previous studies, phosphate buffer has been frequently added to AA to control ozonation pH [32,34,35]. However, added phosphate buffer increases the treatment cost and leads to the subsequent difficulties of biological dephosphorization. Therefore, in this study, all experiments were conducted without an aqueous buffer. Sodium chloride (NaCl, 0.5 g/L) was added as the electrolyte for ECO. In the hybrid process, the initial ozonation stage lasted 40 min and the following 120 min was the ECO stage. The evolution of the dimensionless aniline concentration and COD removal with time are shown in Figure 2.
(NaCl, 0.5 g/L) was added as the electrolyte for ECO. In the hybrid process, the initial ozonation stage lasted 40 min and the following 120 min was the ECO stage. The evolution of the dimensionless aniline concentration and COD removal with time are shown in Fig  In all cases, a fast decay of aniline concentration was observed and complete removal achieved after ~60 min ( Figure 2a). Considering only aniline removal, there was no great advantage in the hybrid process. However, when observing COD removal, the advantage of the hybrid process became clear ( Figure 2b). When the effect of COD removal by the single ozonation process was compared with the single ECO process, it was observed that COD removal by the single ozonation process was higher than that of the single ECO in the first 75 min, while the opposite trend occurred after this period. According to previous reports, aniline is catalytically decomposed by ·OH radicals in the initial single ozonation period. The oxidant species is highly reactive and degrades aniline in a short time, which shows that the indirect ozonation reaction has a dominant role. However, here, the direct ozonation reaction gradually took the leading role as intermediates formed which lowered the pH over time. As is well known, the reaction rate of direct ozonation is much less than that of indirect ozonation. Thus, in the single ozonation process of HCAA, COD removal was higher in the initial period than in later stages. In the single ECO process, COD removal was very low (<15%) in 40 min. It was found that polymeric products formed in the initial ECO stage might result in electrode fouling as a consequence of the adsorption of polymeric oxidation products on their surfaces, which can lead to low COD removal in initial times of single ECO ( Figure 3) [36]. COD removal increased significantly once polymeric products were oxidized on the electrode surface from a constant electrode current supply over time.  In all cases, a fast decay of aniline concentration was observed and complete removal achieved after~60 min (Figure 2a). Considering only aniline removal, there was no great advantage in the hybrid process. However, when observing COD removal, the advantage of the hybrid process became clear (Figure 2b). When the effect of COD removal by the single ozonation process was compared with the single ECO process, it was observed that COD removal by the single ozonation process was higher than that of the single ECO in the first 75 min, while the opposite trend occurred after this period. According to previous reports, aniline is catalytically decomposed by ·OH radicals in the initial single ozonation period. The oxidant species is highly reactive and degrades aniline in a short time, which shows that the indirect ozonation reaction has a dominant role. However, here, the direct ozonation reaction gradually took the leading role as intermediates formed which lowered the pH over time. As is well known, the reaction rate of direct ozonation is much less than that of indirect ozonation. Thus, in the single ozonation process of HCAA, COD removal was higher in the initial period than in later stages. In the single ECO process, COD removal was very low (<15%) in 40 min. It was found that polymeric products formed in the initial ECO stage might result in electrode fouling as a consequence of the adsorption of polymeric oxidation products on their surfaces, which can lead to low COD removal in initial times of single ECO (Figure 3) [36]. COD removal increased significantly once polymeric products were oxidized on the electrode surface from a constant electrode current supply over time. In all cases, a fast decay of aniline concentration was observed and complete removal achieved after ~60 min (Figure 2a). Considering only aniline removal, there was no great advantage in the hybrid process. However, when observing COD removal, the advantage of the hybrid process became clear (Figure 2b). When the effect of COD removal by the single ozonation process was compared with the single ECO process, it was observed that COD removal by the single ozonation process was higher than that of the single ECO in the first 75 min, while the opposite trend occurred after this period. According to previous reports, aniline is catalytically decomposed by ·OH radicals in the initial single ozonation period. The oxidant species is highly reactive and degrades aniline in a short time, which shows that the indirect ozonation reaction has a dominant role. However, here, the direct ozonation reaction gradually took the leading role as intermediates formed which lowered the pH over time. As is well known, the reaction rate of direct ozonation is much less than that of indirect ozonation. Thus, in the single ozonation process of HCAA, COD removal was higher in the initial period than in later stages. In the single ECO process, COD removal was very low (<15%) in 40 min. It was found that polymeric products formed in the initial ECO stage might result in electrode fouling as a consequence of the adsorption of polymeric oxidation products on their surfaces, which can lead to low COD removal in initial times of single ECO (Figure 3) [36]. COD removal increased significantly once polymeric products were oxidized on the electrode surface from a constant electrode current supply over time.  COD removal of 1000 mg·L −1 AA by the single ozonation, single ECO, and hybrid processes in 160 min was 62, 73, and 81%, respectively. This indicated that a subsequent combined process effect clearly existed. In the hybrid process, COD removal was higher than that of single ozonation and ECO processes after 40 min. The hybrid process was observed to combine the merits of the two processes and promote COD removal.

Variation of pH
Examination of pH evolution over time during ozonation, ECO, and hybrid processes showed that, due to aniline oxidation products, including carboxylic acid, organic aldehyde, or alcohol, pH significantly decreased with ozonation reactions carried out without buffer conditions, which coincided with preliminary results (Figure 4) [37]. In the ECO reaction, H + was reduced to hydrogen gas on the cathode over time and OH − accumulation raised alkalinity. In the hybrid process, pH was lowered in the ozonation stage and then gradually increased in the ECO stage, as H + was reduced to a gas. In ECO under low pH, the oxygen evolution reaction might be inhibited, as the oxidation potential of ·OH was higher under acidic conditions (+2.85 V) than under alkaline conditions (+2.02 V). Under acid conditions, ·OH has a stronger oxidation ability, which was beneficial for the following ECO stage and thus improved degradation efficiency [38].

Variation of pH
Examination of pH evolution over time during ozonation, ECO, and hybrid processes showed that, due to aniline oxidation products, including carboxylic acid, organic aldehyde, or alcohol, pH significantly decreased with ozonation reactions carried out without buffer conditions, which coincided with preliminary results (Figure 4) [37]. In the ECO reaction, H + was reduced to hydrogen gas on the cathode over time and OH − accumulation raised alkalinity. In the hybrid process, pH was lowered in the ozonation stage and then gradually increased in the ECO stage, as H + was reduced to a gas. In ECO under low pH, the oxygen evolution reaction might be inhibited, as the oxidation potential of ·OH was higher under acidic conditions (+2.85 V) than under alkaline conditions (+2.02 V). Under acid conditions, ·OH has a stronger oxidation ability, which was beneficial for the following ECO stage and thus improved degradation efficiency [38].

Variation of Nitrogen and Conductivity
The concentration variations of NH4 + -N, NO2 − -N, and NO3 − -N were detected in the oxidation process ( Figure 5). In all cases, NH4 + -N concentrations were low, which indicated that only a little ammonia was converted to NH4 + and that most oxidized to NO2 − or NO3 − . The NO2 − -N concentration first increased and then declined over time, as in the initial stage, the formation amount of NO2 − -N was higher than the conversion to NO3 − -N, which led to NO2 − -N accumulation. After 80 min, most NO2 − -N was converted to NO3 − -N, with the concentration of NO3 − -N finally rose to ~140 mg L −1 (Figure 6). At the same time, most OH − formed in ECO stage was neutralized by H + dissociated from carboxylic acid, which has little help to conductivity improvement.

Variation of Nitrogen and Conductivity
The concentration variations of NH 4 + -N, NO 2 − -N, and NO 3 − -N were detected in the oxidation process ( Figure 5). In all cases, NH 4 + -N concentrations were low, which indicated that only a little ammonia was converted to NH 4 + and that most oxidized to NO 2 − or NO 3 − . The NO 2 − -N concentration first increased and then declined over time, as in the initial stage, the formation amount of NO 2 − -N was higher than the conversion to NO 3 − -N, which led to NO 2 − -N accumulation. After 80 min, most NO 2 − -N was converted to NO 3 − -N, with the concentration of NO 3 − -N finally rose to~140 mg·L −1 (Figure 6). At the same time, most OH − formed in ECO stage was neutralized by H + dissociated from carboxylic acid, which has little help to conductivity improvement.    Simultaneously, the conductivity of the solution was measured (Figure 7). In gle ozonation process, conductivity increased with increased NO2 − -N and NO3 − -N trations. In the single ECO process, the initial conductivity was high, which decre the first 40 min, and then increased over time. NaCl addition provided the hig conductivity and the formation of chlorine or hypochlorous acid (Cl2 or HClO, tively; Equations (1)-(3) caused decreased conductivity in the first 40 min, but th quent generation of NO2 − and NO3 − finally improved conductivity. These results that the conductivity difference between single ozonation and ECO was 200-300 after 40 min. Thus, the formation of NO2 − -N and NO3 − -N played an important ro late period of the single ozonation process and was beneficial for enhancing cond for ECO. Simultaneously, the conductivity of the solution was measured (Figure 7). In the single ozonation process, conductivity increased with increased NO 2 − -N and NO 3 − -N concentrations. In the single ECO process, the initial conductivity was high, which decreased in the first 40 min, and then increased over time. NaCl addition provided the high initial conductivity and the formation of chlorine or hypochlorous acid (Cl 2 or HClO, respectively; Equations (1)-(3) caused decreased conductivity in the first 40 min, but the subsequent generation of NO 2 − and NO 3 − finally improved conductivity. These results showed that the conductivity difference between single ozonation and ECO was 200-300 µs·cm −1 after 40 min. Thus, the formation of NO 2 − -N and NO 3 − -N played an important role in the late period of the single ozonation process and was beneficial for enhancing conductivity for ECO. In the hybrid process, during the ozonation stage, conductivity g for the formation of NO2 − and NO3 − . During the hybrid ECO stage, con greatly from added NaCl and the following variation trend the same a In the hybrid process, during the ozonation stage, conductivity gradually increased for the formation of NO 2 − and NO 3 − . During the hybrid ECO stage, conductivity increased greatly from added NaCl and the following variation trend the same as for the single ECO process: 2Cl − → Cl 2 + 2e − (1)

Ozonation Time
In the hybrid process, ozonation time was crucial for aniline conversion and also impacted the ECO stage efficiency. The influence of different ozonation times on COD removal was examined and maximal COD removal in the hybrid process found to be with 60 min of ozonation (Figure 8), with COD removal at 95% of 1000 mg·L −1 AA and at 81 and 91% with 40 and 80 min, respectively. It may be seen from Figure 2b, when ozonation time was 40 min, 60 min, and 80 min in the single ozonation process, the COD removal was 37%, 47%, and 50% respectively. The COD removal improvement was 10% when ozonation time from 40 min prolonged to 60 min, but it was only 3% when ozonation time prolonged from 60 min to 80 min. It revealed that these ozonation products formed at 40 min ozonation may be degraded further by ozonation, while those products formed at 60 min ozonation were difficult to be degraded further by ozonation. On the other hand, Figure 8 indicated that these products formed in 40 min ozonation were more difficult to be degraded further in the next ECO stage than those products formed in 60 min or 80 min ozonation, which led to a stronger decrease of COD for 60 min and 80 min than that of 40 min. Therefore, the products formed with 60 min ozonation were ideal intermediates for the hybrid process. It was more efficient to select 60 min as the ozonation time for the whole COD removal in the same reaction time. At the same time, Figure 5 showed most of amino were converted to NH 4 + , NO 2 − , or NO 3 − in 60 min ozonation stage which was close to these formed in 80 min ozonation, while these ions formed in 40 min ozonation were only half of those formed in 60 min ozonation, which would decrease the conductivity obviously. Thus, the 60 min was selected as the best ozonation time in the hybrid process.

Nitrite and Nitrate as Electrolyte
In the ozonation stage, most of the amino groups were oxidized to nitrite and nitrate, with total concentrations of nitrite and nitrate increased to~600 mg·L −1 in the ECO stage, which also evidently enhanced conductivity. Thus, it was feasible that nitrite and nitrate replaced NaCl as the electrolyte at the ECO stage of the hybrid process. Comparison experiments with no electrolyte addition and different NaCL additions indicated that COD removal with 1 g·L −1 NaCl was superior to that added of 0.5 g·L −1 or no addition, particularly at 80 to 100 min, from promotion effects by HOCl (Equations (1)-(3); Figure 9). However, this superiority gradually shrank over time and, finally, COD removal differences between 1 g·L −1 and no addition were~2% by 160 min. In consideration of corrosion resulting from chloride and desalination cost in the next treatment, the hybrid process with no electrolyte was concluded to be the best.
In the ozonation stage, most of the amino groups were oxidized to with total concentrations of nitrite and nitrate increased to ~600 mg•L − which also evidently enhanced conductivity. Thus, it was feasible tha replaced NaCl as the electrolyte at the ECO stage of the hybrid proce periments with no electrolyte addition and different NaCL additions i removal with 1 g•L −1 NaCl was superior to that added of 0.5 g•L −1 or n larly at 80 to 100 min, from promotion effects by HOCl (Equations (1)-( ever, this superiority gradually shrank over time and, finally, COD r between 1 g•L −1 and no addition were ~2% by 160 min. In considerati sulting from chloride and desalination cost in the next treatment, the h no electrolyte was concluded to be the best.

Assessment on the Hybrid Process
The performance of the hybrid process on 1000 mg•L −1 AA was ass the hybrid process without electrolyte. The ozonation stage was 60 m min. Except for total nitrogen, the main indices of AA treated by the h in the indirect discharge-limited range (Table 3). Notably, for COD r was difficult to achieve high COD removal and maintain pH at 6-9 sim

Assessment on the Hybrid Process
The performance of the hybrid process on 1000 mg·L −1 AA was assessed by operating the hybrid process without electrolyte. The ozonation stage was 60 min and ECO for 100 min. Except for total nitrogen, the main indices of AA treated by the hybrid process were in the indirect discharge-limited range (Table 3). Notably, for COD removal and pH, it was difficult to achieve high COD removal and maintain pH at 6-9 simultaneously for the single processes (Figure 3). In the single ozonation process, pH decreased over time as COD removal increased. In contrast, in the single ECO process, pH increased to >9 when COD removal reached the discharge standard. However, the hybrid process appeared to improve COD removal and held the pH in the desired range simultaneously. This fortuitously settled the conundrum regarding high COD removal and pH controlled in a limited range.

The Merits and Mechanism of Hybrid Process
During the single ozonation process, the intermediate product variation was observed to include maleic and succinic (butanedioic) acids as the main products after 60 or 120 min ozonation ( Figure 10, Table 4). At low pH, the formation of secondary oxidants, such as ·OH, were inhibited and direct oxidation acts as the main reaction for aniline degradation allowed [39]. However, the reaction rate of direct oxidation was much lower than that of ·OH in indirect oxidation. At the same time, aniline was present in its molecular form in neutral or basic conditions. When the solution was acidic, protonated aniline predominated in solution, as the cation anilinium, which was less adsorbed by AC m than the corresponding molecular form. Thus, aniline degradation efficiency in ozonation declined significantly when maleic and succinic acids accumulated in late ozonation.  In the hybrid process, the main products of the ozonation stage were maleic and succinic acids, which clearly decreased the pH and inhibited radical ·OH formation from O3. Simultaneously, low pH might inhibit the oxygen evolution reaction and improve the oxidation potential of ·OH. Thus, the ozonation stage laid a perfect foundation for the ECO stage. In the ECO, in which maleic and succinic acids were completely mineralized to CO2 and H2O, providing excellent degradation of carboxylic acids [40,41], and avoiding the formation of polymeric products. The decomposition mechanism of the hybrid process is  In the initial times of single ECO, AA (1000 mg·L −1 ) reactions were apt to form polymeric products ( Table 5). The polymerization reactions were hardly conducive to COD removal, which resulted in electrode fouling and led to increased electrical potential of ECO (from 12.2 to 15.6 V). These polymeric products might have been decomposed to CO 2 and H 2 O with a constant current supply. However, the potential of ECO was still high (from 14.7 to 15.5 V) due to fouled electrodes, which meant that increased energy consumption was needed. Thus, the formation of polymeric products in the initial period of the single ECO process was a critical issue for HCAA treatment. In the hybrid process, the main products of the ozonation stage were maleic and succinic acids, which clearly decreased the pH and inhibited radical ·OH formation from O 3 . Simultaneously, low pH might inhibit the oxygen evolution reaction and improve the oxidation potential of ·OH. Thus, the ozonation stage laid a perfect foundation for the ECO stage. In the ECO, in which maleic and succinic acids were completely mineralized to CO 2 and H 2 O, providing excellent degradation of carboxylic acids [40][41][42], and avoiding the formation of polymeric products. The decomposition mechanism of the hybrid process is shown in Figure 11. In the ozonation stage, aniline first reacted with ·OH to form p-aminophenol, which was then gradually oxidized to p-nitrophenol, hydroquinone, and benzoquinone, with maleic and succinic acids the main products by the end of the ozonation stage. In the ECO stage, maleic and succinic acids were decomposed thoroughly to CO 2 and H 2 O. The hybrid process combined the merits of the ozonation and ECO processes and effectively avoided the low COD removal efficiency in late ozonation as well as in initial ECO.  Figure 11. Aniline decomposition mechanism in the hybrid process.

Conclusions
A hybrid process, including ozonation and ECO stages, was successfully u eralize HCAA and the overall process investigated. The subsequent combina on COD removal of the hybrid process were clearly observed. The ozonation Figure 11. Aniline decomposition mechanism in the hybrid process.

Conclusions
A hybrid process, including ozonation and ECO stages, was successfully used to mineralize HCAA and the overall process investigated. The subsequent combination effects on COD removal of the hybrid process were clearly observed. The ozonation stage time was selected to be 60 min for optimal COD removal. Nitrite and nitrate were considered to be excellent electrolytes for the hybrid process because of self-formation and no corrosion. The results showed that the hybrid process achieved 95% COD removal and settled the issue between COD removal and pH range (6-9) limits in the discharge standards. The aniline decomposition mechanism of the hybrid process indicated that maleic and succinic acids were the main products by the end of the ozonation stage, which were subsequently thoroughly decomposed to CO 2 and H 2 O in the ECO stage. The hybrid process combined the merits of single ozonation and ECO processes and effectively avoided low efficiency in late ozonation and initial ECO.