On the Role of the Cathode for the Electro-Oxidation of Perfluorooctanoic Acid

Round 1

Reviewer 1 Report

The work relates to the development of highly efficient and cost-effective processes for the elimination of perfluorooctanoic acid using a BDD anode coupled with cathodes made of BDD, Pt, Zr and stainless steel.

The authors consider a pure mass transport controlled reaction for the first exchange of charge between a molecule of PFOA and the anode surface; in this case, the limiting current density is calculated using the equation established from the Nernst diffusion model. How the authors calculate the mass transfer coefficient? Please, specify the procedure and refer to the work Photoelectrochemical oxidation of phenol with nanostructured TiO2-PANIelectrodes under solar light irradiation, Separation and Purification Technology 208 (2019) 153–159154

In fig 1 is reported the trend of lnC vs t; please, report lnC/C₀ vs t to obtain a k_deg (min^-1), as in table 2.

The authors asses that the cathode plays an important role, but they don’t give enough emphasis in the discussion to the behaviour of different cathodes (specify if they are commercial ones or synthesised) . How the authors selected the cathodes and planned the experimental campaign to perform the electrochemical oxidation of PFOA? Please refer to Experimental study on the optimisation of azo-dyes removal by photo-electrochemical oxidation with TiO2 nanotubes, Chemosphere, Volume 248, June 2020, Article number 125938 and Design of experiment for the optimization of pesticide removal from wastewater by photo-electrochemical oxidation with tio2 nanotubes, Catalysts, Open AccessVolume 10, Issue 5, May 2020, Article number 512.

Please, report a sketch of the apparatus used for the electrochemical experiments.

Author Response

The work relates to the development of highly efficient and cost-effective processes for the elimination of perfluorooctanoic acid using a BDD anode coupled with cathodes made of BDD, Pt, Zr and stainless steel.

First of all, we would like to thank the reviewer for the time devoted to revise and correct our manuscript. We have taken all his/her comments into consideration, introducing the pertinent modifications on the manuscript and here on the response to reviewers. For the sake of clarity, changes on the manuscript have been introduced using blue font and highlighting in yellow. We have also indicated these modifications in this answer to reviewers document.

 

• The authors consider a pure mass transport controlled reaction for the first exchange of charge between a molecule of PFOA and the anode surface; in this case, the limiting current density is calculated using the equation established from the Nernst diffusion model. How the authors calculate the mass transfer coefficient? Please, specify the procedure and refer to the work Photoelectrochemical oxidation of phenol with nanostructured TiO2-PANIelectrodes under solar light irradiation, Separation and Purification Technology 208 (2019) 153–159154

Thanks for remarking this issue. We had not included the calculation of the mass transfer coefficient. Based on the literature proposed by the reviewer we have extended the information as follows:

k_m=2.7·10^-5 m/s (determined experimentally using the ferri/ferro system, as described elsewhere²⁶) for a flow rate of 0.360 m³/h ²⁷.

26. Mais, L.; Mascia, M.; Palmas, S.; Vacca, A., Photoelectrochemical oxidation of phenol with nanostructured TiO2-PANI electrodes under solar light irradiation. Sep. Purif. Technol. 2019, 208, 153-159.

 

• In fig 1 is reported the trend of lnC vs t; please, report lnC/C0 vs t to obtain a kdeg (min-1), as in table 2.

Following the reviewer’s suggestion, we have changed Fig 1a. which now shows ln C_PFOA/C_PFOA,0, as shown below.

Figure 1. Influence of the cathode material in a) PFOA removal (symbols: experimental data, lines: kinetic fitting), b) TOC depletion, c) released fluorine (symbols: experimental data, lines: kinetic fitting). Operating conditions: [PFOA]₀: 100 mg/L, j: 7.9 mA/cm², electrolyte: 3.5 mM Na₂SO₄, T: 25ºC, pH₀: 4.

• The authors asses that the cathode plays an important role, but they don’t give enough emphasis in the discussion to the behaviour of different cathodes (specify if they are commercial ones or synthesised) . How the authors selected the cathodes and planned the experimental campaign to perform the electrochemical oxidation of PFOA? Please refer to Experimental study on the optimisation of azo-dyes removal by photo-electrochemical oxidation with TiO2 nanotubes, Chemosphere, Volume 248, June 2020, Article number 125938 and Design of experiment for the optimization of pesticide removal from wastewater by photo-electrochemical oxidation with tio2 nanotubes, Catalysts, Open AccessVolume 10, Issue 5, May 2020, Article number 512.

Thanks for remarking this subject. We forgot to include where the cathodes were purchased. We have included the information on the manuscript as follows:

BDD (Adamant Technologies, Switzerland),  Zirconium, Stainless steel, and Pt (5µm) on Titanium substrate (provided by MAGNETO special anodes B.V, Netherlands) were employed as cathodes.

Regarding the selection of the cathodes and experimental conditions, we did not follow the DOE model as in the papers kindly proposed for citation by the reviewer. Therefore,  we have not included those references.

In brief, we did a screening using the different available materials that we had in the lab. With previous knowledge on the Spanish group on the hydrodechlorination of organic molecules using Pt and Pd catalysts, we thought that the change of cathode could have a significant repercussion on the molecule defluorination. Hence, the selection of the Pt cathode. Previous works from the French group had obtained excellent results using both Zr and steel cathodes. Therefore, for the sake of comparison, we used those materials as well as BDD.

The selection of the experimental conditions (type of electrolyte, working pH, temperature, etc.) was also based on previous experience of both the research group and also on the existing literature on the degradation of organic pollutants in aqueous phase through electrooxidation processes, as cited throughout the text.

• Please, report a sketch of the apparatus used for the electrochemical experiments.

The experimental setup employed in this work has been thoroughly described on the materials and methods section. Nonetheless, to comply the reviewer’s requirements, we have included a reference to a previous work from the research group in which the sketch of the apparatus is given.

A detailed scheme of the experimental set-up can be found elsewhere ⁴².

42. Lan, Y.; Coetsier, C.; Causserand, C.; Serrano, K. G., An experimental and modelling study of the electrochemical oxidation of pharmaceuticals using a boron-doped diamond anode. Chem. Eng. J. 2018, 333, 486-494.

Author Response File: Author Response.pdf

Reviewer 2 Report

The Authors proposed a interesting topic for research. The paper is publishable, but it is need to consider a few of uncertainties.

• The 'results' section cannot be presented in this form - only tables and figures. It requires a description of the processes, presenting the results also in a descriptive form.
• Chapter 4.1 requires the location of the manufacturers from which the materials and reagents were purchased.
• Please enter the origin of all devices.
• How many repetitions were made?
• On what basis were the individual process parameters selected?
• Please present the methodology of sample preparation for analysis.
• Editing the text requires attention in the chapter "Discussion".
• Is it profitable to run the processes at such a time?

Author Response

The Authors proposed a interesting topic for research. The paper is publishable, but it is need to consider a few of uncertainties.

We would like to thank the reviewer for his/her comments and the time devoted to the revision of our manuscript. We have thoroughly revised the document following his/her suggestions and have introduced all the proposed changes, which are signaled in the revised manuscript (as well as in this response) in blue font highlighted in yellow.

• The 'results' section cannot be presented in this form - only tables and figures. It requires a description of the processes, presenting the results also in a descriptive form.

Following the reviewer’s suggestion and revisiting the instructions for authors of the journal, we have combined the results and discussion sections.

• Chapter 4.1 requires the location of the manufacturers from which the materials and reagents were purchased.

As requested by the reviewer, we have specified the location of the manufacturer of the reagents used in section 4.1 (Germany)

• Please enter the origin of all devices.

Following the reviewer’s petition, we have included the following information in the revised manuscript in the materials and methods section:

The electrochemical cell is a one-compartment flow filter-press reactor which was operated under galvanostatic conditions using an ELCAL 924 power supply (Italy).

BDD (Adamant Technologies, Switzerland),  Zirconium, Stainless steel, and Pt (5µm) on Titanium substrate (provided by MAGNETO special anodes B.V., Netherlands) were employed as cathodes.

PFOA concentration was measured by high performance liquid chromatography connected with an ultraviolet–visible spectrometry detector (HPLC-UV Agilent 1200 Series HPLC, USA). An ion-exclusion column (ZORBAX Eclipse Plus C18, 100 mm, 1.8µm, Agilent, USA) was used as stationary phase. As mobile phase mixture of ACN/4mM H₂SO₄ aqueous solution with a ratio: 3/2 was employed and the column temperature was set to 50 °C.  a 60% ACN – 40% mixture was employed at 0.5 mL/min. The detection UV wavelength was set to 206 nm. Total Organic Carbon was quantified using a TOC analyzer (Shimadzu TOC-VSCH, Japan). Fluoride was analyzed in an ion chromatograph with chemical suppression (Metrohm 790 IC, Switzerland) using a conductivity detector. A Metrosep A supp 5-250 column (25 cm long, 4 mm diameter, Switzerland) was used as stationary phase and 0.7 mL/min of a 3.2 mM/1 mM aqueous solution of Na₂CO₃ and NaHCO₃, respectively, as mobile phase.

• How many repetitions were made?

Thanks for pointing out this issue, which we forgot to indicate in the original manuscript. We have included the following information on the materials and methods section:

All runs were performed by triplicate with a deviation lower than 5% in all cases.

 

• On what basis were the individual process parameters selected?

For the cathode selection, we performed a screening using the different available materials that we had available. With previous knowledge on the Spanish group on the hydrodechlorination of organic molecules using Pt and Pd catalysts, we started with the hypothesis that the change of cathode could have a significant repercussion on the molecule defluorination. Hence, the selection of the Pt cathode. Previous works from the French group had obtained excellent results using both Zr and steel cathodes. Therefore, for the sake of comparison, we used those materials as well as BDD.

The selection of the experimental conditions (type of electrolyte, working pH, temperature, etc.) was also based on previous experience of both the research group and also on the existing literature on the degradation of organic pollutants in aqueous phase through electrooxidation processes, as cited throughout the text.

• Please present the methodology of sample preparation for analysis.

Following the reviewer’s advice, we have included the following information on the manuscript:

Samples were periodically withdrawn from the reactors, filtered through 0.2 µm nylon syringe plug-in filters and immediately analyzed, without any further manipulation.

• Editing the text requires attention in the chapter "Discussion".

As indicated by the reviewer, after merging the results and discussion sections in one, we have revised the text, correcting minor grammar mistakes and simplifying certain sentences throughout the document.

• Is it profitable to run the processes at such a time?

This is indeed a very interesting issue, as pointed out by the reviewer. It is true that 6h electrooxidation may seem a long reaction time for pollutant degradation. The high stability of the PFOA molecule, as well as the high concentration tested (PFOA₀: 100 mg/L) require 360 min to be completely eliminated.

Nonetheless we remarked in the manuscript that our BDD-Pt configuration is, for the moment, the most cost-efficient in terms of defluorination against applied electric charge (as shown in Fig 7).

We should also point out that a high PFOA concentration was used in this work, which aims to obtain a better understanding on the defluorination mechanism and the role of the cathode in PFOA electrooxidation. The selected concentration is way above the one present in real water streams. Therefore, the application of this technology in real wastewater would be cheaper in terms of electricity consumption with shorter reaction times. Future works will apply this technology in more diluted real streams, as indicated in the conclusions of the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I recommend for publication.