Investigating the Effects of Gliding Arc Plasma Discharge’s Thermal Characteristic and Reactive Chemistry on Aqueous PFOS Mineralization

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors report on the mineralization of the PFOS in water by means of GAP reactor. 200 mL of aqueous solution with 100 mg/L PFOS is treated in the way that solution droplets are injected in the region containing gliding arc. The falling droplets coming out from the source are then collected in glass beaker. The effect of discharge gas (air, nitrogen, argon), applied power, and also the treatment time are studied. The solutions were analyzed for HF or F^- with fluoride selective electrode to estimate the defluorination rate and LC-QTOF-MS was used to analyze PFOS and short chain PFAS to get information about the PFOS removal and presence of the short chain products. Additionally, the treatment performed with hot air was performed to estimate the pure gas temperature effect in the process. The selected topic is highly timely due to problems with highly persistent fluorinated molecules and the authors demonstrate that the GAP can be an effective tool for its demineralization. However, there are several issues, which should be addressed before publication.

1) The definition of the defluorination is not clear. I understand it the way that there is no free F^- in the solution before the treatment, because all F atoms are bonded to PFOS. The initial F^- mass (which is used in the equation (1)) should be, therefore, zero. I would expect here the mass of all F atoms in the solution before the treatment, considering that there are 17 of them per one PFOS.

2) The figure 5 should use the same percentage scale for both plotted trends. It will much better demonstrate that there is an issue with missing F in the F mass balance.

3) The experiments with the hot gas have demonstrated that the defluorination of PFOS is not a temperature effect. On the other hand, the similar PFOS removal is observed for the both methods and since no short chain PFAS are detected in any of the treatments, evaporation seems to be a serious issue. PFOS is a surfactant being located mainly at the droplet surface and it is probably preferentially evaporated in any of the treatments – see the uncomplete F-mass balance especially for the hot air treatment – where I expect higher temperature compared to the GAP treatment since the used temperature has been calculated as “worst case scenario” temperature from the power balance. It is very important to knwo what is happening with the “evaporated” PFOS. It is probably also modified by the GAP, but if not, or if it is only cut into shorter chains, the effectivity of the GAP treatment will be significantly reduced. Is the process open to the ambient atmosphere and are the potential gaseous products such as evaporated PFOS, water vapor, F₂ and/or CO/CO₂ just leaving the system? Can these volatile products be collected for example in a cold trap? Analysis of this collected products would resolve these questions.

4) The authors state that they have used LC-QTOF-MS to detect many possible short chain species, but only the PFOS was detected. Is there any indication what is happening with the carbon and the sulfonyl hydroxide group from PFOS? What is the expectation of the authors?

5) it would be also very useful to know the site of the droplets entering the GAP region. The amount of the PFOS at the surface of the droplet compared to droplet volume could be tuned by changing this size, enhancing potentially the treatment efficiency. Was the droplet size optimized?

6) There are few typos in the text:
- page 6, line 225-226”: … as shown equation (2) below:” – “in” missing

- page 13, line 480: “…such as as DC…” –  “as” two times here…

Author Response

Dear Reviewer, 

Thank you for your valuable comments, please see the attached word document (Response to reviewer 1.docx) for our responses and summary of changes we made in the manuscript to address your comments. 

Best, 

The authorship team

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The paper of Shaji et al. entitled "Investigating the effects of gliding arc plasma discharge’s thermal characteristic and reactive chemistry on aqueous PFOS mineralization" explores the efficacy of gliding arc plasma (GAP) discharges in mineralizing perfluorooctanesulfonic acid (PFOS), a persistent environmental contaminant belonging to the class of per-and polyfluoroalkyl substances (PFAS). The study investigates the impact of plasma gas temperatures and reactive species on PFOS degradation using different gas environments (air, nitrogen, and argon) in GAP discharges. In general the paper is well-written and the subject is of interest of Plasma MDPI journal. Some improvements are suggested in order to improve paper quality:

(a) Please, remove the dot at the end of the title. Also remove the dot of subtitles along the text;

(b) In topic 2.2., please inform the manufacturer and modelo of the power supply and liquid pump;

(c) Figure 4: Please show the photography of the system, together with themal image;

(d) While the study suggests the involvement of reactive oxygen and nitrogen species, further investigations (such as UV-Vis spectrofotometry) can be interesting to identify specific reactive species and their contributions to PFOS degradation. 

Author Response

Dear Reviewer, 

Thank you for your valuable comments, please see the attached word document (Responses to reviewer 2. docx) for our response and summary of changes we made in the manuscript to address your comments. 

Best, 

The authorship team

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have responded correspondingly to all my issues and the manuscriot can be published in the current form.