Impact of Active Chlorines and OH Radicals on Degradation Of

water

Article Impact of Active Chlorines and •OH Radicals on Degradation of Quinoline Using the Bipolar Electro-Fenton Process

Wenlong Zhang 1,2, Jun Chen 2,*, Jichao Wang 2 , Cheng-Xing Cui 2, Bingxing Wang 2 and Yuping Zhang 2

1 College of Chemistry, Zhengzhou University, Zhengzhou 450001, China; [email protected] 2 College of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China; [email protected] (J.W.); [email protected] (C.-X.C.); [email protected] (B.W.); [email protected] (Y.Z.) * Correspondence: [email protected]; Tel.: +86-13-598-692-578

Abstract: Quinoline is a typical nitrogenous heterocyclic compound, which is carcinogenic, teratogenic, and mutagenic to organisms, and its wastewater is difficult to biodegrade directly. The bipolar electro-Fenton process was employed to treat quinoline solution. The process/reaction conditions were optimized through the single factor experiment. The degradation kinetics of chemical oxygen demand (COD) was analyzed. To get the degradation mechanism and pathways of quinoline, the intermediate products were identified by gas chromatograph–mass spectrometer (GC–MS). By using sodium chloride as supporting electrolyte in the electro-Fenton reaction system with initial 2 pH 3.0, conductivity 15,800 µs/cm, H2O2 concentration 71 mmol/L, current density 30.5 mA/cm , and applied voltage 26.5 V, 75.56% of COD was decreased by indirect oxidation with electrogen- eration of hydroxyl radicals (•OH) and active chloric species in 20 min. The COD decrease of quinoline solution followed the first order reaction kinetic model. The main products of quinoline degradation were 2(1H)-quinolinone, 4-chloro-2(1H)-quinolinone, 5-chloro-8-hydroxyquinoline, and 5,7-dichloro-8-hydroxyquinoline. Furthermore, two possible degradation pathways of quinoline were proposed, supported with Natural charge distribution on quinoline and intermediates calculated at the theoretical level of MN15L/6-311G(d). Citation: Zhang, W.; Chen, J.; Wang, J.; Cui, C.-X.; Wang, B.; Zhang, Y. Keywords: quinoline; bipolar eletro-Fenton; active chlorines; hydroxyl radicals; kinetic model; Impact of Active Chlorines and •OH degradation pathways; mechanism Radicals on Degradation of Quinoline Using the Bipolar Electro-Fenton Process. Water 2021, 13, 128. https:// doi.org/10.3390/w13020128 1. Introduction

Received: 26 November 2020 With rapid development of economy and industry, water environment pollution has Accepted: 5 January 2021 become increasingly serious. Treatment of hardly-degradable organic wastewater has Published: 7 January 2021 attracted a variety of research focuses. Quinoline, as an important chemical raw material, is often used in the manufacture of drugs, dyes, herbicides, and pesticides, etc. [1]. The Publisher’s Note: MDPI stays neu- resulting wastewater containing quinoline has become a common organic pollutant in water tral with regard to jurisdictional clai- and soil environments. Quinoline, a nitrogen-containing heterocyclic aromatic compound, ms in published maps and institutio- is carcinogenic, teratogenic, and mutagenic to organisms, and can accumulate in advanced nal affiliations. animals along the food chain, seriously threatening human health [2]. However, due to its stable structural properties and toxicity, it is difficult to directly and effectively degrade with conventional physicochemical and biological methods. Therefore, efficient treatment techniques to remove quinoline in wastewater are definitely needed. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. Recently, advanced oxidation processes (AOPs), including wet air oxidation [3], su- This article is an open access article percritical water oxidation [4], Fenton reagent oxidation [5], photocatalytic oxidation [6], distributed under the terms and con- and electrochemical oxidation [7] have been widely utilized to treat industrial organic ditions of the Creative Commons At- wastewater. Among them, electro-Fenton (E-Fenton) oxidation technology can effectively tribution (CC BY) license (https:// remove recalcitrant organic pollutants in wastewater. The E-Fenton process is a new ad- creativecommons.org/licenses/by/ vanced oxidation water treatment technology developed based on the traditional Fenton 4.0/). method. It generally has four different categories: (1) inert electrodes with high catalytic

Water 2021, 13, 128. https://doi.org/10.3390/w13020128 https://www.mdpi.com/journal/water Water 2021, 13, x FOR PEER REVIEW 2 of 15

Water 2021, 13, 128 2 of 14 method. It generally has four different categories: (1) inert electrodes with high catalytic activity are used as anodes and Fenton reagents are added to electrolyzer from outside, (2)activity ferrous are ions used (Fe as2+) anodesare externally and Fenton added, reagents and hydrogen are added peroxide to electrolyzer (H2O2) is fromproduced outside, on 2+ the(2) cathodes ferrous ions, (3) (Fesacrificial) are externallyiron anodes added, are taken and hydrogenas the Fe2+ peroxideion source (H while2O2) isH produced2O2 is ex- 2+ ternallyon the cathodes, injected, (3)and sacrificial (4) Fe2+ and iron H anodes2O2 are aregenerated taken as through the Fe sacrificialion source anodes while Hand2O 2airis 2+ spargingexternally cathodes, injected, respectively and (4) Fe [8].and The H2O E2-areFenton generated process through is favored sacrificial due to anodes a wide andrange air ofsparging applicable cathodes, wastewater, respectively short treatment [8]. The E-Fenton time, no processsecondary is favoredpollution, due small to a equipment wide range footprint,of applicable and wastewater,low operating short cost. treatment This process time, is no usually secondary employed pollution, in combination small equipment with biochemicalfootprint, and methods low operating for wastewater cost. This treatment. process is usually employed in combination with biochemicalTo date, methods the E-Fenton for wastewater process treatment. has been extensively applied in treatment of wastewaterTo date, containing the E-Fenton phenol, process nitrobenzene, has been extensively anilinen, and applied azo dyes, in treatment etc. [9–12 of]. wastewater However, researchcontaining on degradation phenol, nitrobenzene, of nitrogenous anilinen, heterocyclic and azo compounds dyes, etc. [9 –by12 the]. However, E-Fenton researchprocess wereon degradation scarce, and ofthe nitrogenous mechanism heterocyclicof its application compounds to quinoline by the degradation E-Fenton processwas still were not clearscarce,. In and this the paper, mechanism we utilized of its a application bipolar E-Fenton to quinoline process degradation to treat quinoline was still solution not clear., whereIn this constant paper, we current utilized remained a bipolar in E-Fentonthe electrolytic process bath to treatwith quinolineiron plates solution, as anode where and cathode,constant respectively. current remained Its unique in the structure electrolytic could bath reduce with iron the platescell voltage as anode and andinhibit cathode, side reactions.respectively. Sodium Its unique chloride structure (NaCl) couldsolution reduce was theused cell as voltagethe supporting and inhibit electrolyte side reactions. to im- proveSodium the chloride treatment (NaCl) efficiency. solution We wasinvestigated used as thethe supportingeffects of initial electrolyte pH value, to improve conductiv- the treatment efficiency. We investigated the effects of initial pH value, conductivity, H2O2 ity, H2O2 concentration, applied voltage, current density, and reaction time on the chemi- concentration, applied voltage, current density, and reaction time on the chemical oxygen cal oxygen demand (COD) decrease rate of quinoline solution. The kinetic model of COD demand (COD) decrease rate of quinoline solution. The kinetic model of COD degradation degradation was established to make it predictable in practical applications. Moreover, to was established to make it predictable in practical applications. Moreover, to elucidate elucidate the degradation mechanism and pathways of quinoline by the E-Fenton process, the degradation mechanism and pathways of quinoline by the E-Fenton process, the the presence of hydroxyl radicals (•OH) was verified with fluorescence spectroscopy and presence of hydroxyl radicals (•OH) was verified with fluorescence spectroscopy and the intermediate products were identified with GC–MS. Finally, the possible degradation the intermediate products were identified with GC–MS. Finally, the possible degradation pathways of quinoline with the action of •OH radicals and active chlorines, were sug- pathways of quinoline with the action of •OH radicals and active chlorines, were suggested gested with theoretical calculation. with theoretical calculation.

2.2. Materials Materials and and Methods Methods 2.1.2.1. Target Target Pollutant Pollutant Quinoline,Quinoline, a a heterocyclic heterocyclic aromatic aromatic organic organic compound, compound, is is a a weak weak base, base, colorless colorless liquid liquid atat room room temperature. temperature. Because Because a a nitrogen nitrogen atom atom is is incorporated incorporated in in the the ring, ring, its its solubility solubility in in waterwater is is higher than itsits homocyclichomocyclic analogous, analogous, and and it it is is prone prone to to accumulate. accumulate. The The molecular molec- ularformula formula of quinoline of quinoline is C is9H C79NH7 withN with molecular molecular weight weight of of 129.16. 129.16. Its Its chemical chemical structure structure is isshown shown in in Figure Figure1. 1.

FigureFigure 1. 1. TheThe chemical chemical structure structure of of quinoline. quinoline.

2.2.2.2. Reagents Reagents and and Instruments Instruments Quinoline (C9H7N, analytical pure grade) was purchased from Shanghai Macklin Bio- Quinoline (C9H7N, analytical pure grade) was purchased from Shanghai Macklin Bi- chemical Co., Ltd. (Shanghai, China). Sulfuric acid (H2SO4, analytical pure grade) and hydro- ochemical Co., Ltd. (Shanghai, China). Sulfuric acid (H2SO4, analytical pure grade) and gen peroxide (H2O2, 30 wt%) were obtained from Yantai Shuangshuang Chemical Co., Ltd. hydrogen peroxide (H2O2, 30 wt%) were obtained from Yantai Shuangshuang Chemical (Yantai, China). Sodium hydroxide (NaOH, analytical pure grade) and sodium chloride Co., Ltd. (Yantai, China). Sodium hydroxide (NaOH, analytical pure grade) and sodium (NaCl, guaranteed pure grade) were purchased from Tianjin Damao Chemical Reagent chloride (NaCl, guaranteed pure grade) were purchased from Tianjin Damao Chemical Factory (Tianjin, China). Polyacrylamide (PAM, anionic) and terephthalic acid (TA, analyt- Reagent Factory (Tianjin, China). Polyacrylamide (PAM, anionic) and terephthalic acid ical pure grade) were purchased from Shanghai Aiaddin Biochemical Technology Co., Ltd. (TA, analytical pure grade) were purchased from Shanghai Aiaddin Biochemical Technol- (Shanghai, China). Dichloromethane (CH2Cl2, analytical pure grade) was obtained from Tian- ogy Co., Ltd. (Shanghai, China). Dichloromethane (CH2Cl2, analytical pure grade) was jin Deen Chemical Reagent Co., Ltd. (Tianjin, China). All the chemicals were used without further purification and prepared with deionized water. A high-frequency pulse switching power supply (NHWYM500-50, Jinan Nenghua Electromechanical Equipment Co., Ltd., Jinan, China), ultra-quiet small air pump (MA-1000,

Water 2021, 13, x FOR PEER REVIEW 3 of 15

obtained from Tianjin Deen Chemical Reagent Co., Ltd. (Tianjin, China). All the chemicals were used without further purification and prepared with deionized water. Water 2021, 13, 128 3 of 14 A high-frequency pulse switching power supply (NHWYM500-50, Jinan Nenghua Electromechanical Equipment Co., Ltd., Jinan, China), ultra-quiet small air pump (MA- 1000, Zhongshan Chuangmei Electric Appliance Co., Ltd., Zhongshan, China), and elec- tronicZhongshan analytical Chuangmei balance Electric (JJ224BC, Appliance Changshu Co., Ltd., Shuangjie Zhongshan, Testing China), Instrument and electronic Factory, ana- Changshu,lytical balance China) (JJ224BC, were Changshu used. A DDS Shuangjie-307A Testingconductivity Instrument meter Factory, and PHSJChangshu,-4F pH Chinameter) were purchased used. A DDS-307A from Sha conductivitynghai Yidian meter Scientific and PHSJ-4FInstrument pH Co., meter Ltd., were (Shanghai, purchased China) from. AnShanghai LH-25A Yidian intelligent Scientiﬁc multi Instrument-parameter Co., digestion Ltd., (Shanghai, instrument China). and 5B An-3B(V8) LH-25A multi intelligent-param- etermulti-parameter water quality digestion analyzer instrumentwere obtained and 5B-3B(V8)from Beijing multi-parameter Lianhua Yongxing water Technology quality an- Developmentalyzer were obtained Co., Ltd., from (Beijing,Beijing China). Lianhua Agilent Yongxing Technologies Technology’ Cary Development Eclipse Fluorescence Co., Ltd., spectrophotometer(Beijing, China). Agilent (No. G9800A, Technologies’ Agilent Cary Technologies Eclipse Fluorescence Inc, Santa Clara spectrophotometer, CA, USA), rotary (No. evaporatorG9800A, Agilent (RE2000A, Technologies Shanghai Inc, Yarong Santa Clara, Biochemical CA, USA), Instrument rotary evaporator Factory, (RE2000A, Shanghai, China),Shanghai nitrogen Yarong Biochemical blower (MTN Instrument-2800W, ShandongFactory, Shanghai, Aobi Technology China), nitrogen Co., Ltd., blower Dezhou (MTN-, China),2800W, Shandongand gas chromatograph Aobi Technology-mass Co., spectrometer Ltd., Dezhou, (Agilent China), 7890B/5977A and gas chromatograph- GC–MS, Ag- ilentmass Technologies spectrometer Inc, (Agilent Santa 7890B/5977A Clara, CA, USA) GC–MS, were Agilentalso used Technologies in the experiments. Inc, Santa Clara, CA, USA) were also used in the experiments. 2.3. Experimental Methods 2.3. Experimental Methods 2.3.1. Preparation of Quinoline Solution 2.3.1. Preparation of Quinoline Solution The quinoline solution (about 3 g/L) was obtained by adding quinoline into deion- The quinoline solution (about 3 g/L) was obtained by adding quinoline into deionized ized water with stirring for 30 min. water with stirring for 30 min.

2.3.2. E E-Fenton-Fenton Experimental Experimental Apparatus A s schematicchematic of the E E-Fenton-Fenton experimental apparatus isis shown in Figure2 2.. ItIt consistedconsisted of an undividedundivided electrolyticelectrolytic bath, bath, a a high-frequency high-frequency pulse pulse stabilized stabilized DC DC power power supply, supply and, anda small a small air pump. air pump. The The electrolyzer electrolyzer was was made made of polypropylene of polypropylene with with the the effective effective volume vol- umeof 3 L.of The3 L. Q345E The Q345E iron plates iron plates (15.5 cm(15.5× 10.5cm × cm) 10.5 were cm) usedwere asused anode as anode and cathode. and cat Threehode. Threebipolar bipolar iron plates iron plates were sandwichedwere sandwiched between between the two the electrodestwo electrodes with with a 3-cm a 3 distance-cm dis- tancebetween between adjacent adjacent plates. plates. All parameters All parameters of the of high-frequency the high-frequency pulse stabilizedpulse stabilized DC power DC powersupply supply were tunable were tunable (voltage, (voltage 0–500 V;, 0 current,–500 V; current 0–50 A;, frequency,0–50 A; frequency 0–20,000, Hz;0–20 and,000 duty Hz; andcycle, duty 10–100%). cycle, 10 The–100%). output The waveform output waveform was a pulse was square a pulse wave. square A wave. small pumpA small with pump air withsparger air wassparger used was for used continuous for continuous supply of supply air in theof air electrolyzer, in the electrolyzer, with discharge with discharge pressure pressureof 0.025 MPaof 0.025 and MPa discharge and discharge capacity ofcapacity 0.010 m of3/min. 0.010 m3/min.

Figure 2. SchematicSchematic of of the the E E-Fenton-Fenton process process set set up. up. (1 (1) )High High-frequency-frequency pulse pulse stabilized stabilized DC DC power power supplysupply,, (2) thethe E-FentonE-Fenton cell,cell, ((3)) anode,anode, ((44)) cathode,cathode, ((55)) ironiron plates,plates, andand ((66)) airair pump.pump.

InIn the E E-Fenton-Fenton process, with external direct current, Fe2+ ionsions are are produced produced by by dis- dis- solution of anode plates, and H2O22 isis yielded yielded through through the the oxygen reduction reaction on thethe cathodes as as shown in Equations (1) and (2) [[13].13]. Highly Highly reactive reactive •OH radicals are 2+ generated through the interactioninteraction betweenbetween HH22OO22 and Fe 2+ ioionsns ( (EquationsEquations (3) (3) and and (4)) (4)) [14][14].. The h hydroxylydroxyl radical, known as thethe strongeststrongest oxidizingoxidizing speciesspecies after/exceptafter/except fluorine ﬂuorine has high-standard redox potential (E0 = 2.80 V/SHE) [15], which reduces the selectivity to degrade organic pollutants. •OH can react with the macromolecular refractory organics in wastewater and turn them into readily biodegradable intermediates as shown in Equation (5), which can even directly oxidize them into carbon dioxide (CO2), water, and inorganic ions [16]. Moreover, Fe2+ ions can be regenerated through the reduction of Fe3+ ions at the cathode as shown in Equation (6). When NaCl is used as the supporting Water 2021, 13, 128 4 of 14

electrolyte, the conductivity of wastewater is improved. Active chloric species can be generated by reactions (7) and (8) [17], which potentially contribute to the removal of organic pollutants. Fe − 2e− → Fe2+ (1) + − O2 + 2H + 2e → H2O2 (2) 2+ 3+ − Fe + H2O2 → Fe + OH + •OH (3) 3+ 2+ + Fe + H2O2 → Fe + HO2 • + H (4)

RH+•OH → •R + H2O (5) Fe3+ + e− → Fe2+ (6) + − Cl2 + H2O → H + Cl + HClO (7) HClO ↔ H+ + ClO− (8) At room temperature, 3 L quinoline solution was transferred to a container. NaCl was added into the quinoline solution as the supporting electrolyte to improve the conductivity. Then H2O2 was added and agitation was done with the magnetic stirrer. Under optimum conditions, the amount of NaCl added was 3 g/L, and initial pH value was adjusted to 3.0 with 9 mL 5 M H2SO4 (the concentration of H2SO4 was 15 mmol/L in the resulting solution). The resulting solution was poured into the E-Fenton reactor. Air was bubbled from the bottom of the reactor to provide oxygen and generate stirring. Subsequently the reaction was triggered by switching on the DC current. After a specific reaction time, the solution was taken out and the pH was adjusted to 8–9 using the 5 M NaOH. Next, 2 wt% polyacrylamide was added as the flocculant, the solution was stirred for 10 min and left standing for 30 min. The supernatant was withdrawn followed by filtered with a 0.45 µm filter paper to analyze the water quality. The E-Fenton process was carried out to investigate the effect of operating parameters on COD decrease. The specific experimental conditions were as follows: reaction time, 5–30 min; initial pH value, 2.5–5.1; concentration of H2O2, 17.5–106.5 mmol/L; conductivity, 5570–29,300 µs/cm; applied voltage, 12.9–6.2 V, current density, and 12.2–42.7 mA/cm2. Under the previous optimized process parameters, we sequentially changed each of the conditions in turn, and kept the other optimal conditions the same.

2.4. Analytical Test Methods The pH value and conductivity of solution were measured by PHSJ-4F pH meter and DDS-307A conductivity meter, respectively. The COD was measured with the digestion instrument and the multi-parameter water quality analyzer. The •OH was monitored by means of terephthalic acid fluorescent probe method on Agilent Technolgies Cary Eclipse fluorescence spectrometer (excitation wavelength 315 nm, and emission wavelength 425 nm) [18–20]. For intermediate identification, samples were extracted with CH2Cl2 and concentrated using the rotary evaporator, then determined with an Agilent 7890B gas chromatograph (GC) interfaced with a 5977A mass selective detector (MS) equipped with an Agilent 7683B auto sampler and HP-5MS capillary column. Helium was used as the carrier gas with a flow rate of 1 mL min−1. The COD decrease rate of solution could be calculated according to Equation (9):

C − C η = i t × 100% (9) Ci

where η is the COD decrease rate (%), and Ci and Ct denote the concentration of COD in the feed solution and in the E-Fenton treated solution, respectively (mg/L). Water 2021, 13, 128 5 of 14

The linear forms of the ﬁrst-order and second-order kinetic models are shown in Equations (10) and (11): C0 ln = k1t (10) Ct 1 1 − = k2t (11) Ct C0

where C0 and Ct represent the concentration of COD at 0 min and time t, respectively −1 (mg/L), k1 is the ﬁrst-order rate constant (min ), and k2 is the second-order rate constant (mg−1·L−2·min−1).

3. Results and Discussion 3.1. E-Fenton Single-Factor Experimental Results 3.1.1. The Effect of Reaction Time on COD Decrease Figure3a shows that the COD decrease efficiency was improved with the reaction time. The highest decline of COD occurred at 20 min with a COD decrease rate of 75.56%. After that, COD decrease efficiency remained stable over time. The rapid COD reduction during the first 20 min was attributed to oxidation of quinoline. The degradation rate increased slowly after 20 min, likely due to the formation of hard-to-degrade by-products. When hydrogen peroxide was consumed completely, organic compounds could not be decomposed even with increasing time. Therefore, the optimal reaction time of the E-Fenton process in this study was 20 min.

3.1.2. The Effect of Initial pH on COD Decrease The pH value is well known to play an important role in the E-Fenton process since it can affect iron solubility, complexation, and redox cycling between Fe2+ and Fe3+ [21]. Initial pH strongly affects the degradation performance of the E-Fenton process. The pH value 3 has been widely used as the optimum condition for wastewater treatment [22]. The effect of initial pH on COD decrease efficiency is shown in Figure3b. In agreement with the previous report, the COD decrease efficiency was highest at initial pH 3.0, then declined from 72.94% to 33.19% with pH values from 3.0 to 5.1. The lower COD decrease efficiency at higher pH could be attributed to the instability of the ferrous ions and the formation of hydroxides. Iron ions would form precipitates with the increasing pH value, resulting in 2+ fewer free Fe ions to react with H2O2, and consequently causing a reduction in the •OH generation rate [23]. However, when the pH decreased from 3.0 to 2.5, the COD decrease rate decreased to 57.73%. At low pH, the excess H+ could react with •OH and terminate the reaction as shown in Equation (12) [24], diminishing the COD decrease rate.

+ − H + •OH + e → H2O (12)

3.1.3. The Effect of Conductivity on COD Decrease In this study, NaCl was used as the supporting electrolyte to improve COD decrease efficiency of wastewater (Figure S1). As shown in Figure3c, the COD decrease rose from 30.65% to 73.03% when the conductivity increased from 5570 µs/cm to 15,800 µs/cm. How- ever, as the conductivity increased further to 29,300 µs/cm, the COD decrease was slightly decreased to 65.25%. When the NaCl was added into the electrolyzer, active chloric species like hypochlorite, chlorine dioxide, and chlorine could be generated electrochemically, leading to indirect oxidation in favor of the COD decrease [25]. Moreover, the conductivity of the solution was increased to facilitate the electron transfer in the E-Fenton reaction [26]. However, the current was shared by the reaction intermediates and supporting electrolytes. When the concentration of electrolytes exceeded a certain amount, the proportion of the reaction to be carried out by the degradation products was reduced, thus the COD decrease rate decreased. In wastewater treatment, excessive supporting electrolyte does not efficiently increase the COD decrease efficiency. Here, the conductivity was optimized to be 15,800 µs/cm for the COD decrease. Water 2021, 13, 128 6 of 14 Water 2021, 13, x FOR PEER REVIEW 6 of 15

Figure 3. Effect of various process conditions on the chemical oxygen demand (COD) decrease efficiency of quinoline Figure 3. Effect of various process conditions on the chemical oxygen demand (COD) decrease efﬁciency of quinoline solution in electro-Fenton reaction. (a) Reaction time, (b) pH value, (c) electrical conductivity, (d) H2O2 concentration, (e) solution in electro-Fenton reaction. (a) Reaction time, (b) pH value, (c) electrical conductivity, (d)H O concentration, current density, and (f) voltage. 2 2 (e) current density, and (f) voltage. 3.1.2. The Effect of Initial pH on COD Decrease 3.1.4. The Effect of H2O2 Concentration on COD Decrease The performancepH value is well of wastewater known to play treated an important by the E-Fenton role in processthe E-Fenton is deemed process to besince re- 2+ 3+ latedit can to affect hydrogen iron solubility, peroxide, whichcomplexation, determines and the redox amount cycling of hydroxyl between radicalsFe and generated. Fe [21]. Initial pH strongly affects the degradation performance of the E-Fenton process. The pH Figure3d displays the effect of H 2O2 concentration on COD decrease efﬁciency. With the increasingvalue 3 has concentration been widely ofused hydrogen as the peroxide,optimum thecondition COD decrease for wastewater rate gradually treatment rose [22]. and The effect of initial pH on COD decrease efficiency is shown in Figure 3b. In agreement then fell. The COD decrease rate reached the highest value with H2O2 concentration of 71wit mmol/L.h the previous The more report, hydrogen the COD peroxide decrease resulted efficiency in the was more highest hydroxyl at initial radicals pH with 3.0, then Fe2+ asdeclined catalyst, from the 72.94% more mineralization to 33.19% with occurred, pH value ands from the COD3.0 todecrease 5.1. The lower rate increased. COD decrease How- efficiency at higher pH could be attributed to the instability of the ferrous ions and the ever, excessive H2O2 would generate less reactive hydroperoxyl radical (HO2•) instead of formation of hydroxides. Iron ions would form precipitates with the increasing pH value, resulting in fewer free Fe2+ ions to react with H2O2, and consequently causing a reduction in the •OH generation rate [23]. However, when the pH decreased from 3.0 to 2.5, the

Water 2021, 13, 128 7 of 14

the highly reactive •OH species (Equation (13)) [27]. Therefore, high H2O2 concentration conversely lessened the COD decrease efﬁciency.

•OH + H2O2 → HO2• + H2O (13)

3.1.5. The Effect of Current Density on COD Decrease Current density is an important parameter and strongly affects the degradation performance of the E-Fenton process. As shown in Figure3e, the COD decrease efficiency obviously increased from 22.97% to 68.93% with current density from 12.2 mA/cm2 to 30.5 mA/cm2. Subsequently the COD decrease efficiency slightly decreased as the current density increased further. In the E-Fenton process, a high enough concentration of Fe2+ ions, produced from the sacrificed anode, was crucial. Increasing the current density generated more ferrous ions, and more hydroxyl radicals were produced by the Fenton reaction, leading to the high COD decrease efficiency. However, with further increase of current density, side reactions occurred as shown in Equations (14) and (15), reducing H2O2 amount/concentration, which was unfavorable to COD decrease [28,29].

+ − H2O2 → HO2• + H + e (14)

− − 2H2O + 2e → H2 + 2OH (15) The optimal current density was 30.5 mA/cm2 for 3 g/L quinoline, where the E-Fenton process provided the highest COD decrease efﬁciency.

3.1.6. The Effect of Voltage on COD Decrease Figure3f reveals the effect of applied voltage on COD decrease. The COD decrease gradually rose from 22.97% to 66.64% as the applied voltage increased from 12.9 V to 26.5 V. The COD decrease was greatest with the voltage 26.5 V. Further growth of the applied voltage did not amplify COD decrease efﬁciency along with high power consumption. In this study, the optimal voltage was 26.5 V.

3.2. Kinetics Analysis of COD Degradation The establishment of a kinetic model can provide support for the scale-up and industry. We investigated the COD degradation kinetics with quinoline in the E-Fenton system. Based on the above optimum parameters, we investigated the kinetics of COD decrease in the following experimental conditions: initial pH 3.0, conductivity 15,800 µs/cm, H2O2 concentration 71 mmol/L, current density 30.5 mA/cm2, voltage 26.5 V, and reaction time 20 min. The relationship between the COD of quinoline solution and reaction time is shown in Table S1. The kinetic rate constants were obtained by adopting the linearized forms of first-order and second-order kinetic models (Equations (10) and (11)). The linear plots are depicted in the Figure4 and the results are shown in Table S2. COD degradation of quinoline solution in E-Fenton system was checked for linear fitting based on two different reaction orders (first and second orders). Figure4a is the COD degradation curve of the batch that had the highest efficiency among the considered runs of quinoline degradation in the study. The linear regression testing for first-order reaction and second-order reaction are shown in Figure4b,c, respectively. The results showed that the R-squares were 0.9946 and 0.9813 in the two reaction orders and the first-order reaction had the higher R-square value. Thus, the COD degradation of quinoline solution by the E-Fenton process matched the first-order kinetics. The apparent rate constant was determined to be 0.0707 min−1. Water 2021, 13, x FOR PEER REVIEW 8 of 15

3.1.6. The Effect of Voltage on COD Decrease Figure 3f reveals the effect of applied voltage on COD decrease. The COD decrease gradually rose from 22.97% to 66.64% as the applied voltage increased from 12.9 V to 26.5 V. The COD decrease was greatest with the voltage 26.5 V. Further growth of the applied voltage did not amplify COD decrease efficiency along with high power consumption. In this study, the optimal voltage was 26.5 V.

3.2. Kinetics Analysis of COD Degradation The establishment of a kinetic model can provide support for the scale-up and industry. We investigated the COD degradation kinetics with quinoline in the E-Fenton system. Based on the above optimum parameters, we investigated the kinetics of COD decrease in the following experimental conditions: initial pH 3.0, conductivity 15,800 µs/cm, H2O2 concentration 71 mmol/L, current density 30.5 mA/cm2, voltage 26.5 V, and reaction time 20 min. The relationship between the COD of quinoline solution and reaction time is Water 2021, 13, 128 shown in Table S1. The kinetic rate constants were obtained by adopting the linearized8 of 14 forms of first-order and second-order kinetic models (Equations (10) and (11)). The linear plots are depicted in the Figure 4 and the results are shown in Table S2.

FigureFigure 4.4. ((aa)) TheThe CODCOD forfor differentdifferent reactionreaction times,times, ((bb)) linearlinear dependencedependence ofof ﬁrst-orderfirst-order reactionreaction basedbased onon COD,COD, ((cc)) linearlinear dependence of second-order reaction based on COD, and (d) fluorescence test for hydroxyl radicals measurement. dependence of second-order reaction based on COD, and (d) ﬂuorescence test for hydroxyl radicals measurement.

3.3. MeasurementCOD degradation of Hydroxyl of quinoline Radicals solution in E-Fenton system was checked for linear fitting based on two different reaction orders (first and second orders). Figure 4a is the The •OH radicals are well known as the dominative active species responsible for the COD degradation curve of the batch that had the highest efficiency among the considered E-Fenton oxidative reactions [30,31]. To explore the reaction mechanism of the E-Fenton process,runs of quinoline the amount degradation of •OH in in this the E-Fenton study. The system linear was regression determined testing by measuringfor first-order the fluorescentreaction and intensity second- oforder 2-hydroxyterephthalic reaction are shown acid in (TAOH) Figure 4b deriving,c, respectively. from the reactionThe results be- tweenshowed TA that and the•OH R-squares radicals were at different 0.9946 and electrolysis 0.9813 in time. the two The reaction fluorescence orders peaks and of the TAOH first- appearedorder reaction at 425 had nm, the and higher the fluorescence R-square value. intensity Thus, gradually the COD increased degradation with of electrolysis quinoline time, as shown in Figure4d. This indicated that •OH radicals progressively increased with electrolysis time in 20 min.

3.4. Degradation Pathways of Quinoline To explore the oxidation degradation pathways, GC–MS was employed to identify intermediate compounds produced during the catalytic oxidation degradation process of quinoline. Figure5a showed the GC–MS chromatogram of quinoline solution after being treated with the E-Fenton process for 20 min. A series of peak spectra were obtained and the highest peak in the spectrum was assigned to quinoline, the amount of other intermediates was relatively low. The products of other intermediates have been mineralized by electrolysis, and therefore the lower peaks were presented. Mass spectra of the main intermediate products are illustrated in Figure5b–f. The main intermediates during quinoline degradation are listed in Table S3. Four main intermediates, including 2(1H)-quinolinone, 4-chloro-2(1H)-quinolinone, 5-chloro-8-hydroxyquinoline, and 5,7-dichloro-8-hydroxyquinoline, were identiﬁed, and the reliability was above 85%. WaterWater2021 2021,,13 13,, 128 x FOR PEER REVIEW 109 ofof 1415

FigureFigure 5.5.( a(a)) GC–MS GC–MS chromatogram chromatogram of o quinolinef quinoline solution solution treated treated by the by E-Fenton the E-Fenton process process for 20 for min, 20 and min (b,– andf) mass (b– spectraf) mass spectra of main intermediates during quinoline degradation. of main intermediates during quinoline degradation.

••OHOH radicalsradicals are are well well known known to to be be responsible responsible for for the the efﬁcient efficient degradation degradation of organic of or- pollutants.ganic pollutants. (It was (It well was accepted well accepted that the that E-Fenton the E- processFenton wasprocess founded was founded on •OH radicals,on •OH whichradicals, could which degrade couldorganic degrade pollutants organic pollutants efﬁciently.) efficiently.) When NaCl When was used NaCl as was electrolyte used as inelectrolyte the electro-Fenton in the electro reaction,-Fenton active reaction, chloric active species, chloric such species, as hypochlorous such as hypochlorous acid (HClO), acid hypochlorite(HClO), hypochlorite ions (ClO ions−), and(ClO chlorine−), and chlorine could be could electrochemically be electrochemically generated. generated Both .• BothOH radicals•OH radicals and active and active chloric chloric species species could reactcould with react organics with organics in wastewater. in wastewater. The removal The re- of organicmoval of pollutants organic pollutants was attributed was attributed to the electro-Fenton to the electro reaction-Fenton and reaction the indirect and the oxidation indirect ofoxidation active chloric of active species chloric [32 species]. Based [32]. on Based the intermediate on the intermediate products products detected bydetected GC–MS, by reactionGC–MS, mechanismreaction mechanism and degradation and degradation pathways pathways of quinoline of quinoline in the E-Fenton in the E system-Fenton were sys- deducedtem were and deduced supported and supported with theoretical with theoretical calculation, calculation, as shown inas Figureshown6 .in Figure 6.

Water 2021, 13, 128 10 of 14 Water 2021, 13, x FOR PEER REVIEW 11 of 15

FigureFigure 6. The6. The Natural Natura chargel charge on on each each atom atom calculated calculated atat thethe theoretical level of of MN15L/6 MN15L/6-311G(d)-311G(d) along along with with two two possible possible degradationdegradation pathways pathways of of quinoline quinoline in in E-Fenton E-Fenton system. system. TheThe electrostatic potential potential energy energy surface surface of ofquinoline quinoline is shown is shown at at the upper right corner/inset. the upper right corner/inset.

Water 2021, 13, 128 11 of 14

Quinoline is a type of nitrogen heterocyclic compound and it lacks a π electron. The lone-pair electrons at the nitrogen atom does not conjugate with the heterocyclic ring, Water 2021, 13, x FOR PEER REVIEW 12 of 15 Water 2021, 13, x FOR PEER REVIEW which reduces the interaction of the heterocyclic ring12with of 15 the electrophile reagent. Instead,

the electrophile attack occurs readily on the benzene ring [33]. We performed theoretical Quinoline investigationsis a type of nitrogen forheterocyclic the properties compound and of it quinoline lacks a π electron. and theThe intermediates at the theoretical Quinoline is a type of nitrogen heterocyclic compound and it lacks a π electron. The lone-pair electronslevel at ofthe MN15L/6-311G(d) nitrogen atom does not conjugatewith Gaussian with the heterocyclic16, which ring, could provide state-of-the-art results lone-pair electrons at the nitrogen atom does not conjugate with the heterocyclic ring, which reduces the interaction of the heterocyclic ring with the electrophile reagent. In- which reduces forthe interaction electronic of the structure heterocyclic modeling ring with the [34 electrophile]. The optimized reagent. In- geometry of quinoline and the stead, the electrophile attack occurs readily on the benzene ring [33]. We performed theo- stead, the electrophile attack occurs readily on the benzene ring [33]. We performed theoretical investigationsdouble for the descriptor properties of isosurface quinoline and depicted the intermediates with the at the grid theoreti- data of Fukui function, along with the retical investigations for the properties of quinoline and the intermediates at the theoretical level of MN15L/6calculated-311G(d) Fukui with Gau functionssian 16, which indices could are provide shown state in-of Table-the-art1 re-. The Fukui function is dependable cal level of MN15L/6-311G(d) with Gaussian 16, which could provide state-of-the-art results for electronic structure modeling [34]. The optimized geometry of quinoline and the sults for electronicfor structure prediction modeling of reactivity [34]. The optimized [35]. geometry of quinoline and the double descriptor isosurface depicted with the grid data of Fukui function, along with the double descriptor isosurface depicted with the grid data of Fukui function, along with the calculated Fukui function indices are shown in Table 1. The Fukui function is dependable calculated Fukui function indices are shown in Table 1. The Fukui function is dependable for prediction of reactivityTable [35]. 1. Results of computational analysis for quinoline. for prediction of reactivity [35]. Table 1. Results of computational analysis for quinoline. Fukui Function Indices Structure and IsosurfaceTable 1. Results of computationalAtoms analysis for quinoline. Fukui Functionf + Indices f − f ave Structure and Isosurface Atoms Fukui Function Indices Structure and Isosurface Atoms f + f − f ave C1 f + 0.08230f − f ave 0.08188 0.08209 C1 0.08230 0.08188 0.08209 CC1 0.08230 0.084670.08188 0.08209 0.08017 0.08242 C2 2 0.08467 0.08017 0.08242 C2 0.08467 0.08017 0.08242 CC3 3 0.08359 0.083590.09483 0.08921 0.09483 0.08921 C3 0.08359 0.09483 0.08921 CC4 4 0.07142 0.071420.08377 0.07759 0.08377 0.07759 C4 0.07142 0.08377 0.07759 CC5 5 0.07145 0.071450.07644 0.07394 0.07644 0.07394 C5 0.07145 0.07644 0.07394 CC6 6 0.08744 0.087440.08955 0.08849 0.08955 0.08849 C6 0.08744 0.08955 0.08849 NN7 0.09924 0.099240.19795 0.14859 0.19795 0.14859 N7 7 0.09924 0.19795 0.14859 8 CC 8 0.10085 0.100850.07889 0.08987 0.07889 0.08987 (a) C8 0.10085 0.07889 0.08987 (a)(a) C9 0.09294 0.08442 0.08868 CC9 9 0.09294 0.092940.08442 0.08868 0.08442 0.08868 C10 0.11795 0.06248 0.09021 CC10 10 0.11795 0.117950.06248 0.09021 0.06248 0.09021 H11 0.01491 0.00823 0.01157 HH11 11 0.01491 0.014910.00823 0.01157 0.00823 0.01157 H12 0.01469 0.00854 0.01161 HH12 12 0.01469 0.014690.00854 0.01161 0.00854 0.01161 H13 0.01184 0.01110 0.01147 HH13 13 0.01184 0.011840.01110 0.01147 0.01110 0.01147 H14 0.01550 0.01045 0.01297 HH14 0.01550 0.015500.01045 0.01297 0.01045 0.01297 H15 14 0.01495 0.01341 0.01418 H15 0.01495 0.01341 0.01418 HH16 15 0.01615 0.014950.00989 0.01302 0.01341 0.01418 (b) H16 0.01615 0.00989 0.01302 (b) HH17 16 0.02015 0.016150.00804 0.01409 0.00989 0.01302 (b) H17 0.02015 0.00804 0.01409 (a) Optimized geometry of quinoline and the numeration of atoms inH quinoline17 , (b) calculated double0.02015 descriptor isosurface 0.00804 0.01409 (a) Optimized geometry of quinoline and the numeration of atoms in quinoline, (b) calculated double descriptor isosurface with the grid data of Fukui function. The blue color indicates positive regions and the red color indicates negative regions. with the(a grid) Optimized data of Fukui geometry function. ofThe quinoline blue color indicates and the positive numeration region ofs and atoms the red in color quinoline, indicates (b negative) calculated region doubles. descriptor isosurface with the grid data of Fukui function.The The negative blue color electrostatic indicates potential positive energy regions mainly and the locates red coloron the indicates heterocyclic negative ring, regions. The negative electrostatic potential energy mainly locates on the heterocyclic ring, which induces the low electron cloud density on the heterocyclic ring and higher cloud which induces the low electron cloud density on the heterocyclic ring and higher cloud density on the benzene ring. The f + of carbon atoms 8 and 10 is the largest and regions on density on the benzeneThe ring. negative The f + of carbon electrostatic atoms 8 and potential 10 is the largest energy and reg mainlyions on locates on the heterocyclic ring, the two carbon atoms are positive (see isosurface in Table 1), which indicates that the two the two carbon atoms are positive (see isosurface in Table 1), which indicates that the two atoms are pronewhich to electrophile induces substitution the low reaction. electron Comparatively, cloud density f − of N7 on is 0.19795, the heterocyclic ring and higher cloud atoms are prone to electrophile substitution reaction. Comparatively, f − of N7 is 0.19795, which is the largestdensity one among on the all atoms. benzene Moreover, ring. the The regionsf + inof the carbon vicinity of atoms N7 (seen 8 and 10 is the largest and regions which is the largest one among all atoms. Moreover, the regions in the vicinity of N7 (seen isosurface in Table 1) is negative, which suggests that the N atom prefers to nucleophilic isosurface in Tableon the1) is negative, two carbon which atomssuggests arethat positivethe N atom(see prefers isosurface to nucleophilic in Table1), which indicates that the substitution reaction. Figure 6 shows Natural charge distribution of each atom in quino- substitution reaction. Figure 6 shows Natural charge distribution of each atom in quinoline calculated two at the atoms theoretical are level prone of MN15L/ to electrophile6-311G(d). The substitution possible degradation reaction. Comparatively, f − of N7 is line calculated at the theoretical level of MN15L/6-311G(d). The possible degradation pathways of quinoline0.19795, in the which E-Fenton is system the largest are presented one among in Figure all6, where atoms. the Natu- Moreover, the regions in the vicinity pathways of quinoline in the E-Fenton system are presented in Figure 6, where the Natu- ral charge distribution on each atom of all intermediates is also presented. As known, the ral charge distributionof N7 on(seen each isosurfaceatom of all intermediates in Table 1is) also is negative, presented. As which known, suggests the that the N atom prefers to electron cloud density on the benzene ring is higher than that on the pyridine ring, leading electron cloud density on the benzene ring is higher than that on the pyridine ring, leading to electrophile attacknucleophilic on the former. substitution The hydroxyl radical reaction. has strong Figure electronegativity6 shows Natural and charge distribution of each to electrophile attack on the former. The hydroxyl radical has strong electronegativity and electron affinityatom [36]. The in calculated quinoline gapcalculated between the highest at the occupied theoretical molecular level orbital of MN15L/6-311G(d). The possible electron affinity [36]. The calculated gap between the highest occupied molecular orbital (HOMO) of quinoline and the lowest unoccupied molecular orbital (LUMO) of •OH is (HOMO) of quinolinedegradation and the lowest pathways unoccupied of quinolinemolecular orbital in the(LUMO) E-Fenton of •OH is system are presented in Figure6, 10.7 eV, while the gap between the LUMO of quinoline and HOMO of •OH is −11.3 eV. 10.7 eV, while thewhere gap between the Natural the LUMO charge of quinoline distribution and HOMO on of each •OH atomis −11.3 of eV. all intermediates is also presented. Consequently, the electron could transfer from the HOMO of •OH to the LUMO of quin- Consequently, the electron could transfer from the HOMO of •OH to the LUMO of quinoline. The NaturalAs charges known, were the−0.18 electron on carbon 3 cloud and −0.17 density on carbon on 6, respectively. the benzene The ring is higher than that on the oline. The Natural charges were −0.18 on carbon 3 and −0.17 on carbon 6, respectively. The electrophilic addition of •OH was excited to attack the benzene ring of quinoline, leading electrophilic additionpyridine of •OH ring, was excited leading to attack to electrophilethe benzene ringattack of quinoline, on theleading former. The hydroxyl radical has strong electronegativity and electron afﬁnity [36]. The calculated gap between the highest

occupied molecular orbital (HOMO) of quinoline and the lowest unoccupied molecular orbital (LUMO) of •OH is 10.7 eV, while the gap between the LUMO of quinoline and HOMO of •OH is −11.3 eV. Consequently, the electron could transfer from the HOMO of •OH to the LUMO of quinoline. The Natural charges were −0.18 on carbon 3 and −0.17 on carbon 6, respectively. The electrophilic addition of •OH was excited to attack the benzene ring of quinoline, leading to formation of hydroxylated derivatives, such as 8-hydroxyquinoline. The Natural charge on position 6 in 8-hydroxyquinoline was Water 2021, 13, 128 12 of 14

−0.22, lower than other positions on benzene ring. The oxidation of active chloric species produced 5-chloro-8-hydroxyquinoline and further 5,7-dichloro-8-hydroxyquinoline. The derivatives were further attacked by •OH radicals and active chloric species, leading to cleavage of the benzene ring and yielding nitrogen-containing intermediate compounds, such as 2-picolinic acid. These nitrogen-containing intermediates then formed small molecules and mineralized to form CO2 and H2O. The Fukui function indices for all intermediates except for N-phenylformamide (free radical) were also calculated for the intermediates in Tables S4–S12, which indicated similar results to the above discussion for the degradation process. Based on the identiﬁcation of intermediates, another possible degradation pathway of quinoline by the E-Fenton process has been proposed. First, the pyridine ring was attacked by hydroxyl radicals to form 2-hydroxyquinoline, with is rapidly oxidized into 2(1H)-quinolinone. The Natural charges on atoms 3 and 8 turn positive because of the large electrophilicity of oxygen atom. The calculated energy of 8-hydroquionline is slightly higher than that of 2-hydroquionline by 5.4 kcal mol−1. The introduction of an oxygen group at position 8 in pyridine ring led to the increase of charge density on carbon atom 10, which was prone to electrophilic reaction. The 2(1H)-quinolinone was oxidized by active chloric species into 4-chloro-2(1H)-quinolinone. Upon the further attack of hydroxyl radicals and active chloric species, the pyridine ring fragmented into N-phenylformamide. The N-phenylformamide is in turn oxidized into aniline and benzene. Finally, the benzene ring fragmented into small molecules, which were mineralized to CO2 and H2O. When nitrogenous heterocyclic compounds were oxidized and degraded, oxidants with strong electronegativity would attack carbon atoms with high electron cloud density and broke aromatic rings upon effective collision. As •OH radicals and active chlorines attacked aromatic rings, hydroxylation and chlorination ﬁrst took place. Next, hydroxylated derivatives and chlorinated derivatives fragmented into intermediates containing aromatic rings, which were further oxidized to produce organic acids or other small molecule substances. In the end, the small molecule substances were mineralized into CO2 and H2O.

3.5. Mass Balance and Cost Calculations The results of mass balance and cost calculations in the E-Fenton process under the optimal process conditions are shown in Table S13. At full-scale operation, the treatment cost per ton of wastewater is about 2.05 dollars.

4. Conclusions We found that the bipolar E-Fenton process could effectively degrade quinoline in wastewater. The optimal conditions for quinoline degradation by the E-Fenton process were determined. With the optimal conditions, the COD decrease efficiency of quinoline solution could reach 75.56%. The current density had a great influence on the COD decrease efficiency of quinoline. Kinetics analysis showed that the COD degradation of quinoline solution by the E-Fenton process followed the first-order kinetics. The addition of NaCl was confirmed to have a synergistic effect in the E-Fenton system. By using NaCl as electrolyte, hydroxyl radicals and active chloric species were the dominant oxidants. Upon the attack of •OH radicals and active chloric species derived from the oxidation of chloride ions, two possible degradation pathways of quinoline were proposed. Quinoline molecules were firstly attacked to produce hydroxylated derivatives and chlorinated derivatives. The derivatives further fragmented into small molecules. In the end, the small molecule substances were mineralized into CO2 and H2O. The present work demonstrated the potential of the E-Fenton process using iron electrodes to mineralize quinoline and provided a foundation for the degradation of quinoline substances in wastewater by the E-Fenton process in practical applications. Water 2021, 13, 128 13 of 14

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-444 1/13/2/128/s1: Figure S1. The effect of different supporting electrolytes on COD decrease efficiency of quinoline solution by the E-Fenton process; Table S1. Relationship between the COD of quinoline solution and reaction time; Table S2. Kinetic parameters of COD degradation by the E-Fenton process; Table S3. The main intermediates during quinoline degradation identified by GC–MS; Tables S4–S12. Results of computational analysis for the intermediates; Table S13. The results of mass balance and cost calculation in the E-Fenton process under the optimal process conditions. Author Contributions: Conceptualization, W.Z., J.C., and C.-X.C.; methodology, W.Z. and J.C.; soft- ware, W.Z. and C.-X.C.; investigation, W.Z.; data curation, W.Z., J.W., and C.-X.C.; writing—original draft preparation, W.Z.; writing—review and editing, J.C., J.W., C.-X.C., and B.W.; supervision, J.C.; funding acquisition, J.C., J.W., and Y.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the National Natural Science Foundation of China (No.51802082), and by the Program for Science & Technology Innovation Talents in Universities of Henan Province (21HATIT016). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the Supplementary Material. The data presented in this study are available in Supplementary Material. Conflicts of Interest: The authors declare no conflict of interest.

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