Plasma-Assisted Abatement of Per- and Polyfluoroalkyl Substances (PFAS): Thermodynamic Analysis and Validation in Gliding Arc Discharge
, Jimmy Murillo-Gelvez 2, Mobish A. Shaji 3, Alexander A. Fridman 3, Alexander Rabinovich 3, Erica R. McKenzie 2, Gregory Fridman 4 and Christopher M. Sales 1Round 1
Reviewer 1 Report
In ight of my own research expertise, I an discussing only the thermochemical aspects of the current study. To begin with, i do not understand the variations given for the various bond strengths as well as their presumed precision in their various tables and associated discussions. Certainly, the recommended values are much more (and I would say necessarily) sloppy. For example, we find the following bond energy quantities: C–F (3) 446.77 [46] C–F (4) 447.73 [46]? Why are they different. Why are they given with two decimal places when the input study (ref. 6 itself. Giving the values as 447 and 448 (why different) should be adequate, or even say, the values are ca. 450 kJ mol-1. The authors equate the S0 and Cp of octanoic acid and perfluoroctanoic acid. What is found for perfluorinated species and hydrocarbons for which these data are known. Indeed, perhaps the entire thermochemical analysis should be accompanied by a corresponding analysis of C7H16 and C7F16, or even C6H6 and C6F6..
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Reviewer 2 Report
The authors presented a very interesting study on the treatment of PFAS-contaminated water by non-equilibrium plasmas. By thermodynamic calculations, they estimated the enthalpies of the reaction and formation of perfluorooctanoic acid (PFOA, a long-chain legacy PFAS) in water. Then, they fed these enthalpies to HSC Chemistry software to produce a model of PFOA defluorination in an aqueous solution. The model results were compared to experimental results from gliding arc treatment of PFOA or PFOS-contaminated water using argon and air. The comparison showed that the “experimental” PFOA and PFOS mineralization required more energy than predicted by thermodynamics, which was anticipated as the model did not take kinetics into account. Finally, the activation energies for hydroxyl radicals and hydrated electrons to react with the alpha C–F bond in PFOA were roughly estimated.
The study is well-written and systematic. Minor changes are proposed before publication according to the following comments:
1) It is not clear how the HSC Chemistry software works, or better: What type of calculations are performed to come up with the moles of each compound in the reaction and the enthalpy of the reaction system? The authors should add a short description for these calculations.
2) Table 3: Are the “plasma power” values refer to the power absorbed by the plasma or to the total consumed power? The authors should clarify, and if these values correspond to absorbed power, how did the authors measure the absorbed power?
3) Did the authors measure the real temperature in the water in contact with the gliding arc? Is this compatible with the temperature of 195 oC (relevant temperature requirement for the plasma-water surface area, where most of the mineralization process is hypothesized to occur)?
4) Line 106: it -> its
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