Carbon Material-Based Flow-Electrode Capacitive Deionization for Continuous Water Desalination
, Dezhen Wu 1, Jialu Li 1, Rongxuan Xie 1 and Zhenmeng Peng 1,*
Round 1
Reviewer 1 Report
processes-2098490
Carbon material-based Flow-Electrode Capacitive Deionization for Continuous Water Desalination
Authors have systematically investigated the effects of carbon loading in the slurry electrode, slurry electrode flow rate, and cell voltage on the rate, energy consumption, charge efficiency, and water recovery percentage of the FCDI system. The reviewer would like to see the issues listed below be addressed first, before reaching a final decision.
1- [Page 2, Line 88] "The ink was further stirred overnight at room temperature to improve the 88 slurry stability for longer than the duration of experimental test." Please clarify the stirring method used overnight. It is a well-known fact that the applied mixing method dramatically affects the homogeneity and stability of the mixed slurry.
2- [Page 6, Lines 164 and 170] Authors state they compared six samples, yet they have only five shown on the graphs.Please make the necessary corrections.
3- [Page 6, Lines 169-170] Authors only give the carbon loading in the slurry electrode and the applied cell potential. However, they do not mention the flow rate of the water passing through the system, and it will have a tremendous impact on the rate performance of the system. Furthermore, for section 3.2, the flow rate of the slurry electrode and operation time were not listed. Please provide the listed and any other missing operating parameters in the body of the manuscript.
4- [Page 6, Lines 181-183] "This can ascribe to that it has a poor ion transport through mesopores within individual MC particles that slows the charging/discharging process." This statement needs a reference. Please cite an appropriate reference.
5- [Page 8, Lines 223-225] “This result is ascribed to that as a higher carbon particles concentration in the flow electrode promotes the charge transfer rate, which lowers down the required cell overvoltage and thus improves the energy utilization efficiency.” This statement needs a reference. Please cite an appropriate reference.
6- [Pages 7 and 8] The flow rate of water during operation was not given for sections 3.3, 3.4, 3.5, and 3.6. Please add it.
7- [Page 8, Line 234] “The obtained I-t data are plotted Fig. 5a, in which the 1.5 V run showed the…” After 1.23 V water starts to dissociate, authors should experimentally show if any parasitic reactions occur during this experiment. The given data is not enough to reach a conclusion since the CV test’s voltage window is -1, 1 V. Please run the necessary tests and back the necessity of this 1.5 V experiment.
8- [Page 9, Lines 247-250] “However, it decreased slightly to 84.9% when the applied voltage increased to 1.5 V. It was reported in literature studies that exceeding 1.2 V cell voltage would lead to unwanted faradaic reactions, like water electrolysis and carbon electrode oxidation, which cause a decline in the charge efficiency.” If this is the case, then why did authors run a test at 1.5 V instead of 1.2 V. Also, authors must run a test with 1.2 V operating voltage for section 3.4.
9- [Page 10, Lines 282 and 283] “The test was run with a flow electrode containing 1.5 M CB BP, a cell voltage of 1.5 V, and 20 mL min-1 flow rate of the flow electrode.” After stating that 1.5 V will lead to unwanted faradaic reaction, why did authors choose to run the long test at 1.5 V. Reviewer suggests authors to reconsider another voltage value for the long test and rerun the test with that voltage value.
10- [Page 11, Lines 312-314] “Effects of carbon electrode concentration, cell voltage, and flow rate of the flow electrode on the FCDI performance were investigated, which showed different influences on the desalination performances.” Authors must clearly state the conclusion of the effects of each investigated operating parameter.
11- General comment #1: Conventionally, the concentration of slurry electrodes is reported in wt% or vol%. Authors reported their slurry electrode concentrations in M. Please convert them to wt% or vol% for easier comparison with the literature.
12- General comment #2: For 1.5 M CB BP loading, 30 minutes operation, 20 mL min-1 flow rate, and 1 V cell voltage operating conditions the following ASSR and SRE 48.3 μg cm-2 min-1 and 24.8%, (in Sec 3.3), 13.5 μg cm-2 min-1 and 6.9% (in Sec 3.4), and 22.1 μg cm-2 min-1 and 11% (in Sec 3.5). Please clarify why is there such a big performance fluctuation for the same operating condition?
13- General comment #3: FCDI system runs for an hour in Sec 3.2, 30 minutes for Secs 3.3, 3.4, and 3.5, and for 1.4 hours for Section 3.6. One of the four metrics that was employed by authors is salt removal efficiency (SRE), which is simply the ratio of the change in the salt concentration of the feed water to the initial concentration. Yet, when the run times of the tests are not the same, it can be misleading, especially for the FCDI system since fresh slurry is continuously supplied. Reviewer suggests Authors to report SRE values either per unit time, or to report SRE values for a given time (e.g., 10th minute SRE values).
14- General comment #4: Authors have investigated 5 different carbon materials, yet, they have not put some of the commonly used carbon materials in their tests (i.e., Graphene, Activated Carbon, Carbide Derived Carbon, etc.). Reviewer suggests authors to consider these carbon materials for a future study, since they are already proven to perform better than the ones they have investigated when in film electrode form.
15- General comment#5: Authors should check the way units are represented at some parts they used “mL/min” formatting and at some other places “mL min-1” please check the author’s manual and make the necessary corrections.
16- General comment #6: The language of the manuscript is good and easy to read as it is now. Also, the formatting of the equations and figures are clear and consistent.
The reviewer would like to see the revised version of the manuscript after the above-mentioned issues are addressed before making the final decision.
Author Response
We thank the reviewer for valuable comments, which are very helpful to improve the quality of this work. We have revised our manuscript carefully and correspondingly. Attached please find our point-by-point response to the reviewer's comments.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments for author File: 
Comments.pdf
Author Response
We thank the reviewer for valuable comments, which are very helpful to improve the quality of this work. We have revised our manuscript carefully and correspondingly. Submitted please find our point-by-point response to the reviewer's comments.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Authors did a great job addressing reviewer's previous comments.
Reviewer has another minor suggestion and believes that its is a necessity to have in flowable systems. That is the energy cost of pumping the slurry and the water to the system. Since, the author's evaluated the energy consumption of these systems, to provide an improved evaluation the pumping energy cost should be added to the electrical energy requirements for operation. Authors may check any of the following two papers for the calculation of energy cost of pumping. All that is experimentally needed is the measure of the pressure drop occurring during operation.
The reviewer believes after this minor modification paper will be much more comprehensive and will be of interest to a wider researcher community and will be ready for acceptance.
https://iwaponline.com/aqua/article/69/2/134/72295/Impact-of-flow-configuration-on-electrosorption
https://www.sciencedirect.com/science/article/abs/pii/S0378775317303816?via%3Dihub
Author Response
Reviewer 1:
Authors did a great job addressing reviewer's previous comments.
Response: We thank the reviewer for the valuable comments, which were very helpful to improve the quality of this work.
Reviewer has another minor suggestion and believes that its is a necessity to have in flowable systems. That is the energy cost of pumping the slurry and the water to the system. Since, the author's evaluated the energy consumption of these systems, to provide an improved evaluation the pumping energy cost should be added to the electrical energy requirements for operation. Authors may check any of the following two papers for the calculation of energy cost of pumping. All that is experimentally needed is the measure of the pressure drop occurring during operation.
The reviewer believes after this minor modification paper will be much more comprehensive and will be of interest to a wider researcher community and will be ready for acceptance.
https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fiwaponline.com%2Faqua%2Farticle%2F69%2F2%2F134%2F72295%2FImpact-of-flow-configuration-on-electrosorption&data=05%7C01%7Cksa27%40uakron.edu%7C1524a819d6a84903276808dae50a5b23%7Ce8575dedd7f94ecea4aa0b32991aeedd%7C0%7C0%7C638074130476823427%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=6c2t3xQzv%2Fjda%2BoTj1Qz%2Behs%2B9fTKKbncCE5ujpTwHo%3D&reserved=0
https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fabs%2Fpii%2FS0378775317303816%3Fvia%253Dihub&data=05%7C01%7Cksa27%40uakron.edu%7C1524a819d6a84903276808dae50a5b23%7Ce8575dedd7f94ecea4aa0b32991aeedd%7C0%7C0%7C638074130476823427%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=Tlxd5WvAmKXn9%2FMjAPt1T3UmwwUb%2FI2ZIfZ9MG3oXa0%3D&reserved=0
Response: We thank the reviewer for this suggestion. We agree with the reviewer that the energy cost of pumping the slurry and the water to the system would ideally be included to calculate the total energy consumption of the whole desalination system. We have careful read the two papers the reviewer mentioned and conducted a literature search on proper evaluation of the pumping energy component. In order to accurately calculate the pumping energy and pumping cost, both the pressure drop and pump efficiency data are required. Unfortunately, we are unable to determine the pressure drop because this requires measuring the viscosity and the density of the slurry. However, our lab does not have the needed equipment (i.e. viscometer) to conduct the measurements. It was also difficult for us to find a group that can help measuring the viscosity because of the holiday season. On the other hand, we went through the pump manual and did not find the pump efficiency value. We managed to find the pump power consumption which as stated is < 30W. Based on this information, we tried to estimate the pumping energy by multiplying this value by the duration of the experiments. The energy consumption would be <30 W-hr for section 3.2 (1 hour duration), <15 W-hr for sections 3.3 to 3.5 (0.5 hour duration), and <41.7 W-hr for section 3.6 (1.38 hours duration). However, we do not think these rough estimations are reliable and can be used as reference by other researchers, because (1) the pump power is dramatically higher than the needed power to pump the slurry and the water to the small-scale desalination device, and (2) the pump power energy is not equivalent to, but should be significantly higher than the pumping energy. The use of pump power for evaluating the pumping energy consumption would cause a large error. Thus we did not add these estimated values to the revised manuscript. We revised the manuscript to clarify that the energy consumption calculation was for NaCl removal, without inclusion of the pumping energy. We revised the manuscript in line 138: “energy consumption of NaCl removal (EC, kJ molNaCl-1),“ and in lines 144-146: “The EC is the energy consumption for the process of removing NaCl from the desalinating stream. The pumping energy is not included in this calculation.” on page 5.
