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Peer-Review Record

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

Water 2021, 13(2), 128; https://doi.org/10.3390/w13020128
Reviewer 1: Anonymous
Reviewer 2: Gangadhar Andaluri
Reviewer 3: Anonymous
Water 2021, 13(2), 128; https://doi.org/10.3390/w13020128
Received: 26 November 2020 / Revised: 5 January 2021 / Accepted: 5 January 2021 / Published: 7 January 2021
(This article belongs to the Section Wastewater Treatment and Reuse)

Round 1

Reviewer 1 Report

See attached file

Comments for author File: Comments.pdf

Author Response

Please see the attachment

December 31, 2020

Dear Editor,

Thanks a lot for your message regarding our manuscript entitled “Impact of Active Chlorines and •OH radicals on Degradation of Quinoline using Bipolar Electro-Fenton Process” (Manuscript ID: water-1033898). We would like to thank the reviewers for their helpful remarks and suggestions. We have reorganized the manuscript and the Electronic Supporting Information based on reviewers’ comments. All the questions and suggestions proposed by reviewers are carefully considered in the revised manuscript and answered bellow. The changes in revised manuscript were also highlighted by dark red color.

Below please find our point-by-point responses to the reviewers’ comments.

 

Reviewer #1:

Recommendation: Publish after major revisions.

Comments:

  1. It was quinoline solution that was tested – not technological wastewater. The matrix of wastewater which contains quinoline may vary. Therefore, I suggest to make an appropriate correction throughout the text and use the term “quinoline solution”.

Our response: Thanks for the indication. We have checked the manuscript and the “quinoline solution” also has been instead of “quinoline wastewater”. The changes in revised manuscript and supplementary materials have been highlighted by dark red color.

  1. Instead of 'COD removal', it's better to use 'COD decrease' (chemical oxygen demand is not a compound but the amount of oxygen that can be consumed by reactions in a measured solution).

Our response: Thanks for the suggestion. The manuscript has been modified and the 'COD decrease' has been instead of 'COD removal'. The changed part in the revised manuscript has been highlighted by dark red color.

  1. Why was 3g/L solution of quinoline used for testing?

Our response: Thanks for this issue. We have consulted the relevant literatures and studies. Quinoline, as a typical nitrogenous heterocyclic compound, with a N-atom incorporated in the ring system, has higher solubility in water than its homocyclic analogs. It is widely found in coking wastewater, dye wastewater, rubber wastewater, pharmaceutical wastewater [1]. In particular, quinoline is an important pharmaceutical intermediate and used to prepare nicotinic acid and hydroxyquinoline drugs. Its concentration in pharmaceutical wastewater is relatively high and the range is generally 50 mg/L~1000 mg/L [2,3,4]. However, pharmaceutical organic wastewater has the characteristics of high concentration, difficult degradation and poor biodegradability, which needs to be treated by a variety of processes. Generally speaking, after pretreatment, such as membrane concentration and evaporation, the concentration of quinoline in pharmaceutical wastewater can reach 1 g/L~5 g/L. In our work, according to a actual case, we mainly focused on the treatment of pharmaceutical wastewater after membrane concentration. To simulate the concentration of quinoline in the actual case, 3g/L solution of quinoline was used for testing.

[1] Jing, J.; Li, W.; Boyd, A.; Zhang, Y.; Colvin, V.L.; Yu, W.W. Photocatalytic degradation of quinoline in aqueous TiO2 suspension. J. Hazard. Mater. 2012, 237-238, 247-255.

[2] Chang, L.; Zhang, Y.; Gan, L.; Xu, H.; Rittmann, B.E. Internal loop photo-biodegradation reactor used for accelerated quinoline degradation and mineralization. Biodegradation 2014, 25, 587.

[3] Luo, Y.; Yue, X.; Wei, P.; Zhou, A.; Alimzhanova, S. A state-of-the-art review of quinoline degradation and technical bottlenecks. Sci. Total. Environ. 2020, 747, 141136.

[4] Weng, M.L.; Pei, J.C. Electrochemical oxidation of reverse osmosis concentrate using a novel electrode: parameter optimization and kinetics study. Desalination 2016, 399 ,21-28.

  1. Line 133: please provide information on to what level of pH the solution was acidified at the beginning of the experiment (up to pH=3 or lower?). How much H2SO4 was added?

Our response: Thanks for this question. The amount of H2SO4 added was provide at the beginning of the experiment and the value of pH could reach about 3 in catalytic system.

The manuscript has been modified and the changes in revised manuscript have been highlighted by dark red color.

As follows:

Line 132: Then H2O2 was added and agitation was done with the magnetic stirrer. Under the 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 resulting solution).

  1. What was the concentration of NaCl?

Our response: Thanks for the question. The manuscript has been modified and the changed part in revised manuscript has been highlighted by dark red color.

As follows:

Line 133: Under the optimum conditions, the amount of NaCl added was 3 g/L,

In our work, the NaCl was used as supporting electrolyte to improve conductivity of solution and treatment efficiency of E-Fenton process. Under the optimum conditions, total conductivity of quinoline solution was adjusted to 15800 µs/cm, while the corresponding amount of NaCl added was 3 g/L. If the concentration of NaCl is too high, it may affect subsequent biological treatment. One of the water quality characteristics for most industrial wastewater is that wastewater contains dissolved salts with high concentration, which is the major factor causing the malfunctions in many biological treatment systems. Chen G H found that the salinity of 4.12 g/L NaCl was favorable for nitrification, but it would decrease beyond this concentration [1]. Lei Z F reported that the concentration of NaCl should be less than 4 g/L, which ensured the operation of subsequent biological treatment system [2]. In the E-Fenton process, treatment efficiency can be improved by properly increasing the concentration of NaCl, but the concentration should not be too high.

[1] Chen, G.H.; Wong, M.T.; Okabe, S.; Watanabe, Y. Dynamic response of nitrifying activated sludge batch culture to increased chloride concentration. Water. Res. 2003, 37, 3125-3135.

[2] Lei, Z.F. Malfunction effects of sodium salts with high concentration on existing wastewater biological treatment systems. Industrial. Water. Treatment. 2000, 20, 06-10.

  1. Are the derivatives of quinoline, the products of its degradation, less harmful than quinoline itself?

Our response: Thanks for the opinion. We have consulted the relevant literatures and studies. In our study, four main intermediates, including 2(1H)-quinolinone, 4-chloro-2(1H)-quinolinone, 5-chloro-8-hydroxyquinoline, 5,7-dichloro-8-hydroxyquinoline, were indentified in the process of quinolone degradation. They are hydroxylated or chlorinated derivatives of quinoline and most of them are less harmful than quinoline itself, toxicological data of quinoline and its degradation products are as follows. Upon the attack of •OH radicals and active chloric species, the derivatives further fragmented into small molecules such as 2-picolinic acid and benzene. These compounds contain only one ring, which are less harmful and easier to degrade than quinoline. With the reaction going on, benzene ring or pyridine ring would be cleaved and further oxidized to carboxylic acids small molecule substance such as formic acid, acetic acid and oxalic acid [1]. It is well known that these small molecules substance can provide carbon sources for subsequent biological treatment.

Toxicological data of quinoline and its degradation products.

Compounds

Toxicity

quinoline

LD50: 460 mg/kg (rat oral)

8-hydroxyquinoline

LD50: 1200 mg/kg (rat oral)

5-chloro-8-hydroxyquinoline

LD50: 1200 mg/kg (rat oral)

5,7-dichloro-8-hydroxyquinoline

LDLo: 2000 mg/kg (cat oral)

2-picolinic acid

LD50: 562 mg/kg (qal oral)

benzene

LD50: 3306 mg/kg (rat oral)

All data in the table are from MSDS database

[1] Wang, C.; Ma, K.; Wu, T.; Ye, M.; Tan, P.; Yan, K. Electrochemical mineralization pathway of quinoline by boron-doped diamond anodes. Chemosphere 2016, 149, 219-223.

 

 

Thank you very much for your time and consideration. We hope the revised manuscript will be more appropriate for publication in Water.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper talks about e-fenton process for quinoline. Here are my few minor suggestions:

  1. Sec 2.3: What was the basis for 3g/L concentration. Please include some literature on concentrations found in the wastewater
  2. Was there any production of sludge from the Fenton process? If yes, how do we handle sludge at large volumes (in a wastewater treatment plant for example?)
  3. The addition of H2SO4 leads to the formation of sulfates in treated wastewater and there are generally limitations on how much sulfates etc are present in the effluent. Was the sulfate concentration measure? How does this number translate to a full-scale operation?
  4. COD was monitored instead of concentration of quinoline, was there any reason for that?
  5. How will this process work in actual wastewater where there are multiple components? 
  6. Graphs should have error bars, all experiments should be performed in triplicates
  7. pH reduction to 3 at a full-scale operation is expensive. How would you justify the treatment process?
  8. Water start splitting at higher voltages, generally, lower voltages are fine. did you see any water splitting, since there is NaCl, there could be chlorine gas release
  9. Mass balance and Cost calculations would be important.

Author Response

Please see the attachment

December 31, 2020

Dear Editor,

Thanks a lot for your message regarding our manuscript entitled “Impact of Active Chlorines and •OH radicals on Degradation of Quinoline using Bipolar Electro-Fenton Process” (Manuscript ID: water-1033898). We would like to thank the reviewers for their helpful remarks and suggestions. We have reorganized the manuscript and the Electronic Supporting Information based on reviewers’ comments. All the questions and suggestions proposed by reviewers are carefully considered in the revised manuscript and answered bellow. The changes in revised manuscript were also highlighted by dark red color.

Below please find our point-by-point responses to the reviewers’ comments.

 

Reviewer #2:

Recommendation: Publish after major revisions.

Comments:

1 Sec 2.3: What was the basis for 3g/L concentration. Please include some literature on concentrations found in the wastewater.

Our response: Thanks for this issue. We have consulted the relevant literatures and studies. Quinoline, as a typical nitrogenous heterocyclic compound, with a N-atom incorporated in the ring system, has higher solubility in water than its homocyclic analogs. It is widely found in coking wastewater, dye wastewater, rubber wastewater, pharmaceutical wastewater [1]. In particular, quinoline is an important pharmaceutical intermediate and used to prepare nicotinic acid and hydroxyquinoline drugs. Its concentration in pharmaceutical wastewater is relatively high and the range is generally 50 mg/L~1000 mg/L [2,3,4]. However, pharmaceutical organic wastewater has the characteristics of high concentration, difficult degradation and poor biodegradability, which needs to be treated by a variety of processes. Generally speaking, after pretreatment, such as membrane concentration and evaporation, the concentration of quinoline in pharmaceutical wastewater can reach 1 g/L~5 g/L. In our work, according to a actual case, we mainly focused on the treatment of pharmaceutical wastewater after membrane concentration. To simulate the concentration of quinoline in the actual case, 3g/L solution of quinoline was used for testing.

[1] Jing, J.; Li, W.; Boyd, A.; Zhang, Y.; Colvin, V.L.; Yu, W.W. Photocatalytic degradation of quinoline in aqueous TiO2 suspension. J. Hazard. Mater. 2012, 237-238, 247-255.

[2] Chang, L.; Zhang, Y.; Gan, L.; Xu, H.; Rittmann, B.E. Internal loop photo-biodegradation reactor used for accelerated quinoline degradation and mineralization. Biodegradation 2014, 25, 587.

[3] Luo, Y.; Yue, X.; Wei, P.; Zhou, A.; Alimzhanova, S. A state-of-the-art review of quinoline degradation and technical bottlenecks. Sci. Total. Environ. 2020, 747, 141136.

[4] Weng, M.L.; Pei, J.C. Electrochemical oxidation of reverse osmosis concentrate using a novel electrode: parameter optimization and kinetics study. Desalination 2016, 399 ,21-28.

2 Was there any production of sludge from the Fenton process? If yes, how do we handle sludge at large volumes (in a wastewater treatment plant for example?)

Our response: Thanks for the question. In the process of Fenton oxidation, sludge would be produced, which mainly came from the sedimentation of iron ions. However, compared with the traditional Fenton reagent method, the amount of sludge produced by E-Fenton process was much less [1]. In a wastewater treatment plant, for the E-Fenton process, a small amount of sludge would be deposited at the bottom of the electrolyzer. The sludge was pumped out by the sludge pump, combined with a large amount of sludge in the flocculation tank, and filtered by plate and frame filter press, concentrated and dehydrated, then classified and recycled after drying.

Fenton sludge contains a large amount of iron hydroxide precipitation and organic substances. Iron hydroxide can be converted into iron oxide or ferric oxide by incineration, while organic pollutants can be converted into inorganic salt, water and gas after high temperature incineration, which greatly reduces the volume of Fenton sludge and is an efficient treatment method. The global consensus on sludge disposal is to recycle sludge. Fenton sludge is rich in iron, which can be enriched and recovered by acid leaching and lime calcination to realize resource reuse. Using Fenton sludge as raw material, PFS and MPFs flocculants can be prepared, which is used for comprehensive utilization of iron sludge resources.

[1] Kim, H.G.; Ko, Y.J.; Lee, S.; Hong, S.W.; Lee, W.S.; Choi, J.W. Degradation of organic compounds in actual wastewater by electro-Fenton process and evaluation of energy consumption. Water. Air. Soil. Pollut. 2018, 229, 335.

3 The addition of H2SO4 leads to the formation of sulfates in treated wastewater and there are generally limitations on how much sulfates etc are present in the effluent. Was the sulfate concentration measure? How does this number translate to a full-scale operation?

Our response: Thanks for the reminding. According to the suggestion, relevant informations on the concentration of H2SO4 were provide.

The content of the manuscript has been modified, and the changed part in revised manuscript has been highlighted by dark red color.

As follows:

Line 134: and initial pH value was adjusted to 3.0 with 9 mL 5 M H2SO4 (the concentration of H2SO4 was 15 mmol/L in resulting solution).

The concentration of sulfates in wastewater should not be too high, because sulfate radical in wastewater is easily reduced to hydrogen sulfide by sulfate reducing bacteria (SRB) during transportation, which seriously corrodes treatment facilities and drainage pipes. High concentration of sulfates in wastewater is also harmful to the normal metabolism of microorganisms, which is not conducive to the subsequent biological treatment.

In the wastewater discharge standard, the sulfate discharge concentration is required to be less than 1500 mg/L. In this work, Considering high volatility of hydrochloric acid, sulfuric acid was used to adjust the pH of quinoline solution. Under the optimum process conditions, initial pH value was adjusted to 3.0 with 9 mL 5 M H2SO4 in 3 L solution. The concentration of sulfate was calculated as 15 mmol/L in resulting solution, which is within the wastewater discharge standard.

According to this number, the amount of sulfuric acid added in per ton of wastewater can be calculated. At a full-scale operation, total sulfuric acid added can be estimated by enlarging a certain proportion. Sulfuric acid will be mixed with wastewater through pipeline mixer, and the pH value of wastewater will be monitored by pH sensor.

4 COD was monitored instead of concentration of quinoline, was there any reason for that?

Our response: Thanks for the issue. There are many kinds of organic pollutants in actual wastewater, COD value is one of the important detection indexes, which can comprehensively reflect the amount of organic pollutants in wastewater. In this work, quinoline was degraded by E-Fenton process, and multiple organic intermediates were produced. To comprehensively analyze the removal of organic pollutants in the process of quinoline degraded by E-Fenton, COD was selected as the detection index. In addition, there are many instruments for COD monitored, method of COD monitored is simple, and the results can be obtained in a short time.

5 How will this process work in actual wastewater where there are multiple components?

Our response: Thanks for the question. The actual wastewater contains multiple components. Degradation mechanism of organic pollutants by E-Fenton process is complex, besides the oxidation of •OH radicals, there are also anodic oxidation and electrocoagulation. •OH radicals produced by Fenton process can attack the organic pollutants without selectivity in the wastewater and degrade them into small molecular substances, which can be removed by flocculation. In the process of actual wastewater treatment. According to initial pH of the wastewater, NaOH or H2SO4 is used to adjust wastewater to the appropriate acidic conditions. Through small-scale test, various parameters of E-Fenton process will be optimized to achieve high removal efficiency. In addition, the cost of wastewater treatment should be considered, appropriate process parameters are selected.

6 Graphs should have error bars, all experiments should be performed in triplicates.

Our response: Thanks for the suggestion. We have consulted the relevant literatures and the results in the manuscript and supplementary materials have been modified. The changed parts in revised manuscript have been highlighted by dark red color. All catalytic performances were performed in triplicates of parallel experiment and the error bars in the graph have been added.

As follows:

Figure 3. Effect of various process conditions on the COD decrease efficiency of quinoline solution in electro-Fenton reaction. (a) Reaction time; (b) pH value; (c) Electrical conductivity; (d) H2O2 concentration; (e) Current density; (f) Voltage.

Figure 4. (a) The COD for different reaction time; (b) Linear dependence of first-order reaction based on COD; (c) Linear dependence of second-order reaction based on COD; (d) Fluorescence test for hydroxyl radicals measurement.

Supplementary Materials:

Table S1. Relationship between the COD of quinoline solution and reaction time.

Reaction time

Ct (mg/L)

ln(C0/Ct)

1/Ct-1/C0

0

6976

0.0000

0.0000000

2

5821

0.1810

0.0000284

5

4578

0.4212

0.0000751

10

3217

0.7740

0.0001675

15

2238

1.1369

0.0003035

20

1707

1.4077

0.0004425

Table S2. Kinetic parameters of COD degradation by E-Fenton process.

Reaction orders

Linearized form of kinetic model

Kinetic equations

k

R2

First-order kinetics

ln(C0/Ct)=k1t

lnCt=8.8092-0.0707t

0.0707

0.9946

Second-order kinetics

1/Ct-1/C0=k2t

1/Ct=(2.2074×10-5)t+(1.2154×10-4)

2.2074×10-5

0.9813

 

7 pH reduction to 3 at a full-scale operation is expensive. How would you justify the treatment process?

Our response: Thanks for the question. The initial pH value of wastewater is one of the factors affecting the removal efficiency of organic pollutants by E-Fenton process. Acidic condition is suitable for Fenton oxidation reaction, and the most suitable pH value is usually 2~4. The initial pH value 3 has been widely used as the optimum condition for wastewater treatment by E-Fenton process [1]. At a full-scale operation, pH values of actual wastewater are different. According to initial pH of actual wastewater, pH reduction to 3, the amount of H2SO4 or NaOH added is different, so operating costs are also different. Considering the cost and the removal efficiency of organic pollutants comprehensively, process parameters can be adjusted appropriately to reduce operating costs.

In this work, the results of cost calculations in E-Fenton process under the optimal process conditions were shown in Table S13 of supplementay materials. At a full-scale operation, pH reduction to 3, the cost of H2SO4 added in per ton wastewater is about 0.6 RMB, which is within the acceptable range.

In fact, the optimal pH indicates a disadvantage of electro-Fenton process, because the pH of most waters is not within the optimal range. There are two ways to decrease the pH of wastewater. One is to add acid, and the other is to mix the target wastewater with some acidic wastewater. Some researchers investigated the wastewater treatment in neutral pH and the organics could also be removed successfully. But, in that case, the wastewater is depolluted mainly by coagulation rather than by degradation of •OH [2,3].

[1] Zhou, M.H.; Tan, Q.Q.; Wang, Q.; Jiao, Y.L.; Oturan, N.; Oturan, M.A. Degradation of organics in reverse osmosis concentrate by electro-Fenton process. J. Hazard. Mater. 2012, 215-216, 287-293.

[2] Chen, Z.; Chen, X.; Zheng, X.; Chen, R.Y.; Lin, Z.H.; Chen, Y.F.; Zhang, Y.K. Influence of pH and current concentration on electrochemical-generated hydroxyl radical for degradation and decolorization of dye waste-water. Res. Environ. Sci. 2002, 15, 42-52.

[3] Jiang, C.C.; Zhang, J.F. Progress and prospect in electro-Fenton process for wastewater treatment. J. Zhejiang. Univ. Sci. A. 2007, 8, 1118-1125.

8 Water start splitting at higher voltages, generally, lower voltages are fine, did you see any water splitting, since there is NaCl, there could be chlorine gas release.

Our response: Thanks for the comments. The applied voltage directly determines the electrode potential of anode and cathode, and affects the E-Fenton reaction efficiency. When applied voltage is too low, current density is low, the release of Fe2+ from electrode plates becomes slow, and Fenton reaction efficiency is low. When the applied voltage is greater than a certain value, water start splitting at higher voltages, the side reactions such as hydrogen evolution and oxygen evolution will be aggravated, which is not conducive to the removal of organic pollutants.

In E-Fenton process, according to the power supply mode of electrolyzer, it can be divided into monopole and bipolar. In monopole E-Fenton process, the circuit is in parallel, so sum of current through each cell is total current and voltage of each cell is equal, monopole cell has the characteristics of low current and high voltage. Bipolar cell is just the opposite, the circuit is in series, so current of each cell is equal and total voltage is the sum of each cell voltage, bipolar cell has the characteristics of low current and high voltage operation. In our work, the circuit of bipolar E-Fenton process was in series, three iron plates were sandwiched between two electrodes with distance between adjacent plates 3 cm. The optimal total voltage was 26.5 V and the voltage of adjacent cell was controlled at about 6 V~7 V, which was in a reasonable range. The cell voltage of conventional E-Fenton process is generally 3 V~10 V.

NaCl was used as the supporting electrolyte to improve the treatment efficiency. Active chloric species could be generated electrochemically, such as hypochlorous acid (HClO), hypochlorite ions (ClO-) and chlorine. Mogyoródy investigated the chlorine–water equilibrium on the electrochemical destruction, and reported that the role of HClO as the predominant reagent was more distinct in a less acidic medium (pH 3.5~6.5) [1]. When the pH value was above 3, the principal anodic oxidation products should be HClO. The concentration of NaCl was 3 g/L under the optimum conditions, active chloric species produced by electrochemistry was microscale. In addition, the solubility of chlorine in water was very high, which was not enough to produce a large amount of chlorine gas release.

[1] Mogyoródy, F. Influence of chlorine–water equilibria on the electrochemical destruction of thiocarbamate herbicides in NaCl solutions. J. Appl. Electrochem. 2006,.36, 765–771.

9 Mass balance and Cost calculations would be important.

Our response: Thanks for the indication. We have finished the Mass balance and Cost calculations and the results were performed.

As follows:

Table S13. The results of Mass balance and Cost calculation in E-Fenton process under the optimal process conditions.

Experimental reagent

Reagent dosage per liter solution

Types of industrial reagents

Industrial reagents consumption

Market price of industrial reagents

Wastewater treatment cost per ton

5 M H2SO4

3.0 ml/L

98 wt% H2SO4

1.501 kg/t

400 RMB/t

0.60 RMB/t

30 wt% H2O2

7.2 ml/L

30 wt% H2O2

7.992 kg/t

800 RMB/t

6.39 RMB/t

NaCl (AR)

3.0 g/L

NaCl (TP)

3.000 kg/t

500 RMB/t

1.50 RMB/t

5 M NaOH

5.0 mL/L

NaOH (TP)

2.415 kg/t

2500 RMB/t

2.50 RMB/t

2 wt PAM

10.0 mL/L

PAM (TP)

20 g/t

9000 RMB/t

0.18 RMB/t

Electricity consumption

2.2 kW•h

Electricity cost

2.20 RMB/t

Estimated cost per ton of wastewater treatment

13.37 RMB/t=2.05 dollars/t

The content of the manuscript has been modified, and the changes in revised manuscript and supplementary materials have been highlighted by dark red color. Table S13 was added to supplementary materials.

Line 344:3.5. Mass balance and Cost calculations

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

Line 367: Table S13: The results of Mass balance and Cost calculation in E-Fenton process under the optimal process conditions.

 

Thank you very much for your time and consideration. We hope the revised manuscript will be more appropriate for publication in Water.

Author Response File: Author Response.pdf

Reviewer 3 Report

See attachment.

Comments for author File: Comments.pdf

Author Response

Please see the attachment

December 31, 2020

Dear Editor,

Thanks a lot for your message regarding our manuscript entitled “Impact of Active Chlorines and •OH radicals on Degradation of Quinoline using Bipolar Electro-Fenton Process” (Manuscript ID: water-1033898). We would like to thank the reviewers for their helpful remarks and suggestions. We have reorganized the manuscript and the Electronic Supporting Information based on reviewers’ comments. All the questions and suggestions proposed by reviewers are carefully considered in the revised manuscript and answered bellow. The changes in revised manuscript were also highlighted by dark red color.

Below please find our point-by-point responses to the reviewers’ comments.

 

Reviewer #3:

Recommendation: Publish after major revisions.

Comments:

(1) First and foremost, the conclusions drawn are based on the Mulliken charge analysis. It is well known, that such analysis is highly erroneous, since its results are extremely dependent on the functional basis used in the calculations. Thus, may the authors repeat their computations to obtain more reliable values? I may suggest NBO analysis, available in the commonly used packages, like Gaussian. If molecular geometries are already in place, such a computations should not be problematic.

Our Response: Thanks for this suggestion. We replaced the “Mulliken Charges” on each structure in Figure 6 by “Natural Charges” that are obtained from Natural Population Analysis in Gaussian 16. The revised Figure 6 is as follows:

The manuscript has been modified and the changes in revised manuscript have been highlighted by dark red color.

Line 292: We performed theoretical investigations for the properties of quinoline and the intermediates at the theoretical level of MN15L/6-311G(d) with Gaussian 16 c, which could provide state-of-the-art results for electronic structure modeling [34].

Figure 6. The Natural charge on each atom calculated at the theoretical level of MN15L/6-311G(d) along with two possible degradation pathways of quinoline in E-Fenton system. The electrostatic potential energy surface of quinoline was shown at the upper right corner/inset.

 

 

(2) In the same vein, the conclusions based solely on the net atomic charges may not be reliable. It would be good idea to leverage them with more advanced examination, based on reactivity indices, like Fukui function indices (see for example https://doi.org/10.3390/w12123398).

Our Response: Thanks for this suggestion. Based on understanding the paper (Water 2020, 12, 3398), we calculated Fukui function indices and depicted double descriptor isosurface with the grid data of Fukui function. The results were collected in Table 1 as follows. The Fukui function indices for all intermediates except for N-phenylformamide (free radical) were also calculated for the intermediates in Table S4-S12. Fukui function only supports the calculation of closed shell system, N-phenylformamide is a free radical gene and belongs to open shell system, so its Fukui function indices are not calculated. Moreover, we revised the discussion of reactivity based on the new calculated results, which have been highlighted with red color in the revised manuscript.

Table 1. Results of computational analysis for quinoline.

Structure and Isosurface

Atoms

Fukui Function Indices

f +

f -

f ave

(a)

(b)

C1

0.08230

0.08188

0.08209

C2

0.08467

0.08017

0.08242

C3

0.08359

0.09483

0.08921

C4

0.07142

0.08377

0.07759

C5

0.07145

0.07644

0.07394

C6

0.08744

0.08955

0.08849

N7

0.09924

0.19795

0.14859

C8

0.10085

0.07889

0.08987

C9

0.09294

0.08442

0.08868

C10

0.11795

0.06248

0.09021

H11

0.01491

0.00823

0.01157

H12

0.01469

0.00854

0.01161

H13

0.01184

0.01110

0.01147

H14

0.01550

0.01045

0.01297

H15

0.01495

0.01341

0.01418

H16

0.01615

0.00989

0.01302

H17

0.02015

0.00804

0.01409

(a) The optimized geometry of quinoline and the numeration of atoms in quinoline; (b) The calculated double descriptor isosurface with the grid data of Fukui function. The blue color indicates positive region and the red color indicates negative region.

Line 294: The optimized geometry of quinoline and the double descriptor isosurface depicted with the grid data of Fukui function, along with the calculated Fukui function indices were shown in Table 1. Fukui function is dependable for prediction of reacitivity [35].

Line 301: The f + of carbon atoms 8 and 10 is the largest and regions on the two carbon atoms are positive (seen isosurface in Table 1), which indicates that the two atoms are prone to electrophile substitution reaction. Comparatively, f - of N7 is 0.19795, 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 substitution reaction. Figure 6 showed Natural charge distribution of each atom in quinoline calculated at the theoretical level of MN15L/6-311G(d). The possible degradation pathways of quinoline in E-Fenton system were presented in Figure 6, where the Natural charge distribution on each atom of all intermediates was also presented.

Line 322: The Fukui function indices for all intermediates except for N-phenylformamide (free radical) were also calculated for the intermediates in Table S4-S12, which indicates similar results for the degradation process to the above discussion. (3) May the authors make (at least) a try to provide a more detailed routes for both considered channels? All the elementary steps proposed are going through transition states, reactive intermediates etc. For example, steps (1) and (6) ( OH additions) start from prereactive complexes (E<0) and, later, adducts are created through transition state. Similarly for other elementary processes. These important facts are not even mentioned in the manuscript!

Our Response: Many thanks for this suggestion. The location of transition states is especially important for understanding the reaction mechanisms, which could give the kinetic information of a reaction. Four main intermediates, including 2(1H)-quinolinone, 4-chloro-2(1H)-quinolinone, 5-chloro-8-hydroxyquinoline, 5,7-dichloro-8-hydroxyquinoline, were indentified by GC-MS during quinoline degradation. Based on the intermediate products, two possible degradation pathways of quinoline could be speculated. We performed theoretical investigations for the properties of quinoline and these intermediates at the theoretical level of MN15L/6-311G(d) with Gaussian 16. The Natural charge on each atom of these intermediates and Fukui function indices were obtained, which could provide state-of-the-art results for electronic structure modeling which could help us obtain a large part of the mechanisms. The theoretical investigation about the electronic structure of reactants and intermediates is sufficient for us to give a relatively clear degradation pathway herein. Consequently, we did not try to locate the transition states for each elementary step.

(4) The software used for the electronic structure calculations should be specified.

Our Response: We performed all the calculations in Gaussian 16 software program, which has been specified in the revised manuscript.

 

 

 

 

 

We have carefully examined the manuscript, and other modifications were also marked in dark red in the revised manuscript.

Thank you very much for your time and consideration. We hope the revised manuscript will be more appropriate for publication in Water.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have addressed all my comments. The manuscript can be accepted for publication.

Author Response

Thanks for the reviewr's kind comment.

Reviewer 3 Report

The changes made by the authors are satisfactory. The only small issue to be improved is the accuracy (5 digits after dot) of the partial atomic charges reported (Figure 6 and the text explaining). Such a precision has no physical justification, it should be cut to maximum 2 significant digits. Furthermore, charges listed on Figure 6 (light green)  are barely  readable. Can you make them more visible?

No further review is needed.

Author Response

Please see the attachment

January 5, 2021

Dear Editor,

Thanks a lot for your message regarding our manuscript entitled “Impact of Active Chlorines and •OH radicals on Degradation of Quinoline using Bipolar Electro-Fenton Process” (Manuscript ID: water-1033898). We would like to thank the reviewers for their helpful remarks and suggestions. We have reorganized the manuscript and the Electronic Supporting Information based on reviewers’ comments. All the questions and suggestions proposed by reviewers are carefully considered in the revised manuscript and answered bellow. The changes in revised manuscript were also highlighted by dark red color.

Below please find our point-by-point responses to the reviewers’ comments.

 

Reviewer #3:

Recommendation: Publish after major revisions.

Comments:

The changes made by the authors are satisfactory. The only small issue to be improved is the accuracy (5 digits after dot) of the partial atomic charges reported (Figure 6 and the text explaining). Such a precision has no physical justification, it should be cut to maximum 2 significant digits. Furthermore, charges listed on Figure 6 (light green) are barely readable. Can you make them more visible?

No further review is needed.

Our Response: Thanks for the reviewer’s kind comment. The accuracy of atomic charges reported (Figure 6 and the text explaining) has been cut to 2 significant digits, and charges listed in Figure 6 have been highlighted with red color. We think Figure 6 is clear and easy to read now.

The manuscript has been modified and the changes in revised manuscript have been highlighted by dark red color.

As follows:

Figure 6. The Natural charge on each atom calculated at the theoretical level of MN15L/6-311G(d) along with two possible degradation pathways of quinoline in E-Fenton system. The electrostatic potential energy surface of quinoline was shown at the upper right corner/inset.

Line314: The Natural charges were -0.18 on carbon 3 and -0.17 on carbon 6, respectively.

Line316: The Natural charge on position 6 in 8-hydroxyquinoline was -0.22, lower than other positions on benzene ring.

 

Thank you very much for your time and consideration. We hope the revised manuscript will be more appropriate for publication in Water.

Author Response File: Author Response.pdf

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