Review Reports - Efficient Removal of Hexavalent Chromium from Wastewater with Electro-Reduction

Round 1

Reviewer 1 Report

This article presents an electro-reduction method to reduce hexavalent chromium (Cr (VI)) to trivalent chromium (Cr (III)). The author’s present different parameters affect the reaction as well as reaction time, current intensity, reaction temperature and dosage of H₂SO₄. The reduction efficiency was evaluated. The electro-coagulation methods are always of great interest to area of water treatment (indifferent industry like: tannery industry and mechanical industry). In this respect, the topic of this study is interesting and the approach is valuable in the aspect of practical application. It is well organized. However, it still requires revision before being accepted; the details are listed below.

Comment:

1. Introduction

It is short! Bring up gap of the knowledge and problem statement. Highlight the novelty of current study.

2. Materials and methods

- Please add some information concerning your experimental steps. . This will greatly help readers understand the process along with the results in this article.

- In page 2, line 53 and 54: The anode and cathode used in the experiments was a steel slice (2 × 2 cm). This choice is arbitrary or based on the corrosion reduction phenomenon.

3. Results and discussion

Please, add error bars for Figures 2, 3 and 4.

Author Response

Reviewer: 1

This article presents an electro-reduction method to reduce hexavalent chromium (Cr (VI)) to trivalent chromium (Cr (III)). The author’s present different parameters affect the reaction as well as reaction time, current intensity, reaction temperature and dosage of H2SO4. The reduction efficiency was evaluated. The electro-coagulation methods are always of great interest to area of water treatment (indifferent industry like: tannery industry and mechanical industry). In this respect, the topic of this study is interesting and the approach is valuable in the aspect of practical application. It is well organized. However, it still requires revision before being accepted; the details are listed below.

Comment:

1. Introduction

It is short! Bring up gap of the knowledge and problem statement. Highlight the novelty of current study.

Response：We had rewritten the Introduction.

Chromium is an important strategic metal widely used in many fields due to its excellent properties [1-5]. In the earth, chromium is mainly existed in the oxidative states of hexavalent chromium and trivalent chromium. The chromium (III) compounds are relatively stable and have low solubility and mobility. Other way, the chromium (VI) mainly existed as chromate (CrO₄^2-, HCrO₄^-) and dichromate (Cr₂O₇^2-), which has high solubility. The removal of chromium (VI) has attracted much more attention as it is a hazardous contaminant [6, 7].

Currently, many remediation techniques have been developed to remove heavy metal ions from waste water. It can be divided into three species. The traditional one is physical methods like adsorption [8-11], ion exchange [12], membrane filtration [7], which are mainly conducted by their physical properties. Due to the different chemical properties of chromium (VI) and chromium (III), chemical precipitation [13], coagulation [14], and chemical reduction [15, 16] are often used to remove chromium (VI). Also some biological treatments [17, 18] (phytoremediation [19]) are also applied to remove chromium (VI) in waste water and groundwater. Lead sulfate as a precipitation is used to precipitate chromium (VI) and it can remove chromium (VI) from 0.2mol/L to 0.15 mmol/L in recent study [13], but lead sulfate is a second pollutant which is harmful for environment. In the same time, difficulties and challenges like high costs, large scale application and causing secondary pollution are still remained in current technologies. It is need to overcome these challenges and develop new effective techniques for removal of chromium (VI).

Electricity acted as a clean source is widely used in hydrometallurgy like electro-oxidation technology [20-22] and treatment of different wastewater like electrocoagulation [23-25]. The electrocoagulation involves the generation of coagulants by dissolving metal electrode or free electron. The metal ions are formed their hydroxide, like Al(OH)²⁺, Al(OH)₂⁺, Al(OH)₄^- for Al electrode, and Fe(OH)²⁺, Fe(OH)₃ for Fe electrode. These metal hydroxides have large surface areas and can adsorb soluble organic compounds. In this paper, an electrocoagulation-like technology electro-reduction is applied to reduce Cr (VI) to Cr (III). The mechanism and parameters including dosage of H₂SO₄, reaction time, reaction temperature and current intensity affect the reaction are investigated. Response surface methodology [26-28] is also used to predict the optimal reaction conditions and observe the interactions between parameters.

2. Materials and methods

-Please add some information concerning your experimental steps. This will greatly help readers understand the process along with the results in this article.

Response：We had added the detailed experimental steps in the revision.

“All chemicals were of analytical grade, including potassium dichromate (K₂Cr₂O₇) and sulfuric acid (H₂SO₄), which were collected from Kelong Co., Ltd, Chengdu, China, and used as received without purification.

Reduction experiments were carried out in a batch mode to evaluate the reduction efficiency of electricity for chromium (VI) ions from aqueous solutions. In the batch experiments, a stock solution of 1.000 g/L chromium (VI) was prepared by dissolving a certain K₂Cr₂O₇ in distilled water, the acidic medium was prepared by adding different volumes of H₂SO₄, the current was from a set of electric supply, the anode and cathode used in the experiments was steel slice (2×2 cm) and the distance of two electrodes were 2 cm. Further, in each set of experiments, 100 mL of chromium (VI) was added into a beaker and agitated in a thermostat water-bath shaker at 500 rpm. During the experiments, the samples were collected at different intervals (5 min), and analyzed for residual concentration of chromium (VI) in the solution [13]. ”

-In page 2, line 53 and 54: The anode and cathode used in the experiments was a steel slice (2 × 2 cm). This choice is arbitrary or based on the corrosion reduction phenomenon.

Response：The area of the steel slice chose 2 × 2 cm was based on the volume of the reaction medium. The slice was inserted into the reaction medium; the area of the steel slice submerged in the reaction medium was 2 × 2 cm.

3. Results and discussion

Please, add error bars for Figures 2, 3 and 4.

Response：We had added the error bars for Figures 2, 3, 4.

Figure 2 Effect of volume of H₂SO₄ on reduction efficiency of chromium

(Initial concentration of Cr (VI) = 1.000 g/L, Current intensity = 0.05 A)

Figure 3 Effect of reaction temperature on reduction efficiency of chromium

Figure 4 Effect of current intensity on reduction efficiency of chromium

(Volume of H₂SO₄ = 0.50 mL, initial concentration of Cr (VI) = 1.000 g/L)

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript presents an experimental study to remove hexavalent Chromium to trivalent Chromium using an electro-reduction method. The effect of four operating parameters including concentration of H₂SO₄, reaction time, reaction temperature, and current intensity was investigated using central composite design (CCD) approach.

The manuscript requires significant improvement and does not meet the expected quality to be published in the journal of Processes at this version.

Here is a list of major comments:

General comments:

- All the grammatical errors should be corrected. Examples: Page 2: Line 53 and Line 64, Page 8: Line 191

- Verb tense: All the verbs in the Introduction section should be in “presence” instead of “past”. Example: Page 1, Line 26

In the earth, chromium “is” mainly existed, instead of “was” mainly existed

Specific Comments:

Title: The title should be revised to present information about the technique which has been used for the removal of the Cr elements.

Abstract: Experimental methodology, specially the design of experiment approach, needs to be presented in the Abstract briefly.

Introduction:

Page 1, line 40: explain briefly some of the “difficulties and challenges” of the current technologies.

Page 2, line 43-46: Review and present briefly the recent studies reported about electrocoagulation and electro-reduction which are done using design of experiment approach.

Materials and Methods:

- Present the complete details of the chemicals (e.g. grade, supplier), experimental measurement like collecting intervals, and experimental setup such as electrode distance.

- How was the current applied to the reaction cell? Was it through the constant current or constant voltage device?

- Present the experimental table, listing all the parameters and their variation range.

- Explain the Design of Experiment methodology used in this study.

- In Equation (1), how was the volume before and after each experiment measured? How much was the measurement accuracy in order to provide a precise value for reduction efficiency?

Results and Discussion

- Figure 2: Explanation is needed to justify why the slope of the lines for 0.75 mL and 1.0 mL is similar.

- The pH of the solution should be reported.

- What would be the possible effect of solution conductivity as it increases with the concentration of H₂SO₄? Did the authors measure it?

- Figure 3: How did the authors maintain the temperature of the solution at desired temperatures? Comments are needed about heat loss from the reaction cell and Joule Heating.

- Figure 4: Steel electrode is commonly used in electrocoagulation cell as an inert cathode. The authors need to explain why they considered steel instead of Iron electrode if the target is to produce Fe²⁺ ions. Can the similar trend in Figure 4 be related to choice of material instead of intensity of current?

- Table 2: All the statistical parameters need to be introduced and explained briefly in the Materials and Methods section.

- Figure 7: The explanation presented in Page 6, Lines 163-165 is not clear. What is the measure of “randomness” for the data in this figure? A more convincing justification is needed for this Figure.

- Legend should be added to Figure 5 to 7.

- Figure 8: How the authors develop the Figure 8? Why did the plot of current intensity have curvature? Replace the “leaching efficiency” with the “reduction efficiency” to be consistent with the terminologies.

Author Response

Reviewer: 1

The manuscript presents an experimental study to remove hexavalent Chromium to trivalent Chromium using an electro-reduction method. The effect of four operating parameters including concentration of H2SO4, reaction time, reaction temperature, and current intensity was investigated using central composite design (CCD) approach.

The manuscript requires significant improvement and does not meet the expected quality to be published in the journal of Processes at this version.

Here is a list of major comments:

General comments:

- All the grammatical errors should be corrected. Examples: Page 2: Line 53 and Line 64, Page 8: Line 191

- Verb tense: All the verbs in the Introduction section should be in “presence” instead of “past”. Example: Page 1, Line 26

In the earth, chromium “is” mainly existed, instead of “was” mainly existed

Response：We had revised the whole manuscript and corrected some grammatical errors and Verb tense.

Specific Comments:

Title: The title should be revised to present information about the technique which has been used for the removal of the Cr elements.

Response：We had changed the Title of the manuscript

“Efficient Removal of Hexavalent chromium from Wastewater with Electro-reduction”

Abstract: Experimental methodology, specially the design of experiment approach, needs to be presented in the Abstract briefly.

Response：We had revised the Abstract and added some information indeed.

“Removal of hexavalent chromium had attracted much more attention as it was a hazardous contaminant. An electrocoagulation-like technology electro-reduction was applied. The chromium (VI) in the wastewater was reduced to chromium (III) by the electron supplied by electricity power and Fe²⁺ formed of corrosion of steel electrode in acid conditions. The mechanism and parameters affect the reaction were investigated. The results optimized by response surface methodology indicated that the influence of single factor on the reduction efficiency followed the order: A: dosage of H₂SO₄ > C: reaction time > D: reaction temperature > B: current intensity. The reduction efficiency was hardly affected by current intensity, while it was increased with the increasing of reaction time and acid concentration. The reducing agent, Fe²⁺ and extra free electron, were acted as reducing agent and could easily reduce hexavalent chromium to trivalent chromium at high temperature in acidic medium.”

Introduction:

Page 1, line 40: explain briefly some of the “difficulties and challenges” of the current technologies.

Response：Difficulties and challenges like high costs, large scale application and causing secondary pollution are still remained in current technologies.

Page 2, line 43-46: Review and present briefly the recent studies reported about electrocoagulation and electro-reduction which are done using design of experiment approach.

Response：We had rewritten the Introduction.

Materials and Methods:

- Present the complete details of the chemicals (e.g. grade, supplier), experimental measurement like collecting intervals, and experimental setup such as electrode distance.

Response：We had added the detailed of the chemicals and the detailed experimental measurement in the revision.

- How was the current applied to the reaction cell? Was it through the constant current or constant voltage device?

Response：The current was provided by an electric supply. During the experiment process, the current kept as constant.

- Present the experimental table, listing all the parameters and their variation range.

Response：The variation ranges of the parameters were all detailed in the Figures. It could be seen the variation ranges clearly in Figure 2 to 4.

Figure 2 Effect of volume of H₂SO₄ on reduction efficiency of chromium

(Initial concentration of Cr (VI) = 1.000 g/L, Current intensity = 0.05 A)

Figure 3 Effect of reaction temperature on reduction efficiency of chromium

(Volume of H₂SO₄ = 0.50 mL, initial concentration of Cr (VI) = 1.000 g/L, Current intensity = 0.05 A)

Figure 4 Effect of current intensity on reduction efficiency of chromium

(Volume of H₂SO₄ = 0.50 mL, initial concentration of Cr (VI) = 1.000 g/L)

- Explain the Design of Experiment methodology used in this study.

Response：The detailed experiment methodology was as followed:

- In Equation (1), how was the volume before and after each experiment measured? How much was the measurement accuracy in order to provide a precise value for reduction efficiency?

Response：The volume of the reaction medium was measure by a volumetric, and the concentration of residual chromium (VI) in the reaction medium was determined by titration with ammonium ferrous sulfate according to the reference (H. Peng, J. Guo, B. Li, Z. Liu, C. Tao, High-efficient recovery of chromium (VI) with lead sulfate, Journal of the Taiwan Institute of Chemical Engineers,2018,85149-154, 10.1016/j.jtice.2018.01.028)

Results and Discussion

- Figure 2: Explanation is needed to justify why the slope of the lines for 0.75 mL and 1.0 mL is similar.

Response：Figure 2 showed the Effect of volume of H₂SO₄ on reduction efficiency of chromium. The reduction efficiency of chromium (VI) was increased with the increase of dosage of H₂SO₄. The effect of volume of H₂SO₄ was significant at a certain range. Continue increasing volume of H₂SO₄ out of range had no obvious increment. So the slope of the lines for 0.75 mL and 1.0 mL was similar.

- The pH of the solution should be reported.

Response：In order to quantify the effect of acid, we used the actual dosage of H₂SO₄to replace the pH.

- What would be the possible effect of solution conductivity as it increases with the concentration of H2SO4? Did the authors measure it?

Response：The Equation (12) and (13) showed that chromium (VI) was easily reduced in acidic conditions. Other way, the conductivity of the solution was also increased with the concentration of H₂SO₄.

“ II) The corrosion of steel slice was accelerated at high acid concentration, which made the Fe²⁺ more exposed and reacted with chromium (VI) like Equation (6); III) the acidic conditions also promoted the conductivity of the solution, which then increased the reduction of chromium because of the increasing amount of extra free-electron (Equation (11)). ”

- Figure 3: How did the authors maintain the temperature of the solution at desired temperatures? Comments are needed about heat loss from the reaction cell and Joule Heating.

Response：Figure 3 showed that the reduction efficiency was increased with ten increase of reaction temperature. It could be concluded that while the reaction temperature increased to 80 ℃ or 90 ℃, the reduction efficiency of chromium would also be increased and also the reaction time could be reduced. In the paper, we chose 70 ℃ as the highest reaction temperature was because the reaction time in 70 ℃ was enough to analyze experimental data. If the reaction temperature was higher than 70 ℃, the reduction efficiency of chromium (VI) was easy up to 100%, there was no much time to analyze results.

- Figure 4: Steel electrode is commonly used in electrocoagulation cell as an inert cathode. The authors need to explain why they considered steel instead of Iron electrode if the target is to produce Fe2+ ions. Can the similar trend in Figure 4 be related to choice of material instead of intensity of current?

Response：The original intention of this paper was to investigate the effect of current intensity on the reduction efficiency of chromium. While during the experimental process, the steel slice was corroded, some Fe²⁺ was formed and acted as reducing agent.

- Table 2: All the statistical parameters need to be introduced and explained briefly in the Materials and Methods section.

Response：In order to reduce the similarly of the paper to references, the statistical parameters were not detailed explained in the paper. If reader wanted to know the detailed statistical parameters could get some more information in the references [29-31].

[29] Y. Yuan, L. Tan, Y. Xu, J. Dong, Y. Zhao, Y. Yuan, Optimization of Processing Parameters for Lettuce Vacuum Osmotic Dehydration Using Response Surface Methodology, Polish Journal of Food and Nutrition Sciences,2018,68, 10.1515/pjfns-2017-0013

[30] H. Nawaz, M.A. Shad, A. Rauf, Optimization of extraction yield and antioxidant properties of Brassica oleracea Convar Capitata Var L. leaf extracts, Food Chem,2018,242182-187, 10.1016/j.foodchem.2017.09.041

[31] S.J. Mat Rosid, W.A. Wan Abu Bakar, R. Ali, Characterization and modelling optimization on methanation activity using Box-Behnken design through cerium doped catalysts, Journal of Cleaner Production,2018,170278-287, 10.1016/j.jclepro.2017.09.073

Response：In this study, the Response Surface Methodology simulated in Design-Expert software was used to simulate the experiments. In the software, the response value of the reduction efficiency of chromium could be predicted by the software and also be calculated by the experiments. Figure 6 showed the difference between the predicted values and experimental results.

- Legend should be added to Figure 5 to 7.

Response：The legends about Figure 5 to 7 are all included in the Figures

Figure 5 Normal probabilities of the residuals

Figure 6 Plot of the predicted responses vs. the actual values

Figure 7 Plot of the residual responses vs. the predicted responses

Response：(1) Figure 8 was obtained by the software of Design-expert. It showed a comparison of all the parameters at a selected point in a chosen design space. The reduction efficiency was modeled by changing only one parameter at its range which others held constant. The results showed in Figure 4 indicated that the current intensity in the experiments had no obvious effect on the reduction efficiency. While Figure 8 showed that the current intensity had curvature effects, it might be the variation range during the simulated process was bigger than the experimental process.

(2) We had changed the “leaching efficiency” to “reduction efficiency” in the figure title of Figure 8.

Figure 8 Perturbation plot for the reduction efficiency of vanadium in the design space; A: dosage of H₂SO₄; B: current intensity; C reaction time; D: reaction temperature

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The revised manuscript still needs significant improvement in Introduction and Materials and Methods section. I strongly encourage the authors to address the major concerns and revise the manuscript accordingly.

Following up the previous comments:

- In the introduction section, review and present briefly the recent studies reported on electrocoagulation and electro-reduction which are done using design of experiment approach.

- All the statistical parameters used in CCD model need to be introduced and explained briefly in the Materials and Methods section.

Author Response

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

The manuscript meets the requirements to be published in the journal of Processes.