A Review of Recent Advances in Detection and Treatment Technology for Perfluorinated Compounds

Round 1

Reviewer 1 Report

   The manuscript is written in good, understandable English with only few grammatical or improper wording errors, which are acceptable in the case of non-native English speakers.  However, there are also some meritorious errors, often copied from the cited original articles.  In a review like this, errors committed by the original authors should be either omitted or corrected by the authors of the review, not repeated.  In my opinion, the following is wrong and needs to be corrected to make the review acceptable for publication:

1. The abstract of a review should summaries briefly the paper contents, and possibly include some final conclusions.  There is no place in the abstract for general information nor discussion.  Hence, the fragment of the abstract starting from "The techniques..." in line 19 to "...persistence of PFCs." in line 24 does not fit to the abstract and should be deleted.

2. lines 205, 252, 271, 311, 329, 332, 333, 345 and Table 1:  By definition, maximum recovery value of an analyte can be 100%.  The recoveries higher than 100% make no sense, because one cannot recover more analyte than it was present in the system.  Experimental values of the recovery exceeding 100% are the results of experimental errors.  Such values should be rounded to 100% or, when reported from some literature source, standard deviation of the result should be reported too to point out that the experimental value is within accuracy limits of the analytical method used.  In a review, when no experimental errors are available, the recoveries exceeding 100% should be rounded to 100% or omitted (as non-reliable).

3. line 249:  Reporting ultrasonic radiation power as 65%, without reporting what was the maximum power of the ultrasound transducer makes no sense, because different transducers emit different ultrasound powers.  This value should be converted to power units (i.e., Watts).

4. lines 315/316:  The statement starting as: "Since PFCs are not volatile..." is not true, because in general PFCs (i.e., perfluorinated compounds) are  much more volatile than the corresponding hydrogen-substituted compounds (e.g., octanoic acid boils at 237 C , while perfluorooctanoic acid (PFOA) boils at 189-192 C).  Only some PFCAs (i.e., perfluorocarboxylic acids) and  PFSA (i.e., perfluoroalkylsulphonic acids) need derivatization to make them volatile enough for analysis by GC-MS.

5. line 330: 89.3-116.3% accuracy would not be satisfactory.  These numbers probable were not "accuracy" values.

6. line 360: The acronym MOF should be defined at its first use, because it may not be clear to all readers.  Same applies to other acronyms that are not defined in the text (e.g., some of those reported in Tables 1-3,  in line 518 and ROS in line 686)

7. Chemical reactions explaining a reaction mechanism has to be well balanced to make sense (i.e., the number of electrons and total charge on both sides of the reaction equation have to be equal), while in the equation (9) there is an unpaired electron on the perfluoroheptyl radical on the left side and no unpaired electron on the right side of the equation.  Instead, there is H⁺ cation, while there is no positive charge on the left.  That equation would be balanced if there was hydrogen radical on the right side instead of H⁺.  However, conversion of a free radical into an alcohol would require participation of an oxidant, while water is a very poor oxidant. So, the reaction (9) is not possible (i.e., even when balanced correctly). Nonetheless, formation of the alcohol is possible by direct combination of the perfluoroalkyl radical with hydroxyl radical, as shown in the following equation:

^●C₇F₁₅ + ^●OH → C₇F₁₅OH

8. TiO₂ is missing in equation (3).  The correct equation would be:

TiO₂(e^-cb) + O₂ → TiO₂ + ^●O₂^-

9. Equation (4) does not make sense, because at close to neutral pH, concentration of H⁺ in water is very low, too low to react fast enough with very low concentration of oxygen radical anions (^●O₂^-), for kinetic reasons. Hence, direct protonation of the radical anions with water (which is present at high concentration) is more likely.  Consequently, the correct elementary reaction would be:

^●O₂^- + H₂O → HOO^● + OH^-

10. It is not clear where the electron in equation (6) comes from.  The correct form of that mechanistic step would be:

H₂O₂ + TiO₂(e^-cb) → ^●OH + OH^- +TiO₂

11. Equation (7) does not make sense.  Why should uncharged species (i.e. the perfluorooctanoic acid and the hydroxyl radical) generate charged species (i.e., the radical cation of perfluorooctanoic acid and the hydroxyl anion)?  This would be a redox reaction, where the hydroxyl radical would have to act as an oxidant and the perfluorooctanoic acid would have to act as a reducing agent, while the hydroxyl radical is a poor oxidant and saturated carboxylic acids are not reducing agents.  Such reaction would have very high activation energy and would not occur at ambient temperatures.   Direct hydrogen exchange between the perfluorooctanoic acid and the hydroxyl radical is more likely, as shown in the following equation:

C₇F₁₅COOH + ^●OH → C₇F₁₅COO^● + H₂O

12. As the formation of perfluorooctanoic acid radical cations is not possible, the reaction (8) does not make sense.  If the reaction shown in the previous comment occurs, then the carboxyl radicals may decarboxylate quickly as follows:

C₇F₁₅COO^● → C₇F₁₅^● + CO₂

Erroneous mechanisms should not be repeated in a review. Consequently, the mechanism depicted by the reactions (1)-(11) should be either corrected (with appropriate comments) or neglected (in order not to proliferate information noise).

13. As most of the reactions (1)-(11) were incorrect, the reactions shown in Figure 1 also require appropriate correction. Moreover, the reaction in the upper left corner of the picture suggests that hydroxyl radicals (^●OH) are formed from oxygen on the surface of Pb-TiO₂ particles, while hydroperoxyl radicals  (^●OOH) are generated instead, as shown by the equations (3)-(4).

14. line 557: When reporting an electrode potential, it is always necessary to add in parenthesis relative to what reference electrode it was measured, because the electrode potentials can be referred to Saturated Calomel Electrode (SCE), Normal Hydrogen Electrode (NHE), Ag/AgCl electrode, etc.  Consequently, reporting the electrode potential value alone, without specifying the type of reference is useless.

15. Figure 3: It is not clear what the numbers in parenthesis are referred to.

16. Figure 4 is also not free of errors, because it implies that the perfluorinated heptyl radicals are converted into perfluoroheptanoic acid just by reaction with water, which does not make sense, for the reasons explained in my Comment 7.

17. Reactions (13) and (14):  It is well known that three-molecular reactions are very rare and inefficient, because collision of three molecules (including particles such as an electron or H⁺) at the same time and at an appropriate arrangement in space to form products has very low probability.  So, the reactions (13) and (14) cannot be elementary reactions involved in the mechanism of PFOA degradation.  The correct elementary reactions would be:

For the reaction (13):   C₇F₁₅COOH + e^- → C₇F₁₅^● + HCOO^-

For the reaction (14):   C₇F₁₅COOH + CO₂^●- → C₇F₁₅^● +HCOO^- + CO₂

18.  The reaction (15) is not possible for the reasons explained in my Comment 7.

19.  Reaction (16) is messed up (i.e., the reaction arrow is in wrong place).

20. Reaction (18): Chemical reactions should contain integer coefficients at the reagent formulas to make sense.  While the formula  written as Fe₂O₃∙0.5H₂O is acceptable (because it shows the contents of water in the ferrihydrite crystalline lattice), specifying 8.5H₂O in a chemical reaction makes no sense and is not acceptable in chemistry.  The correctly written Feammox reaction would be:

6(Fe₂O₃∙0.5H₂O) + 20H⁺ + 2NH₄⁺ → 12Fe²⁺ 17H₂O + 2NO₂^-

 

Moreover, the following replacements or deletions will correct improper wording:

line 103: there is: perfluoronanoic acid -> there should be: perfluorononanoic acid

line 108: rate -> recovery

line 118: aggregate -> accumulate

lines 223/224: because it is highly soluble in PFC salts and has -> because PFCs are highly soluble in it and it has

lines 241, 246, Table 1: recovery rate -> recovery (because "recovery rate" would be in % per unit time)

line 285: boiling reflux -> boiling, reflux (or just: reflux)

line 292: more -> most

line 308: derived -> derivatized

line 311: quantitative limit -> quantification limit

lines 393, 406: of -> on

lines 410, 411: delete:  that of

line 419: a more -> an

line 514: adding TiO₂ particles to Pb -> you probably meant: adding Pb to TiO₂ particles

line 532: precipitate -> be released (because oxygen is a gas and does not "precipitate"; solids precipitate)

line 639: C₇F₁₅OH₈ under the further catalysis of ^●OH -> C₇F₁₅OH by combination with ^●OH

line 670: A6, is -> A6, which is

line 680: can be to be -> can be

Table 1: Recovery rate -> Recovery

Table 3: Removal rate -> Removal degree; Defluorination rate – Defluorination degree; Mineralization rate -> Mineralization degree (because those data are not "rates")

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The review summarizes the data on the important environmental problem. In general, the work is suitable for the publication, but some improvements are required.

1. Graphical abstract – flip signatures “Pretreatment and detection” and “Treatment technologies”

2. Page 2, Line 34. “As we all know, organic compounds are mainly composed of carbon and hydrogen, which may include oxygen, nitrogen, sulfur, and other elements.” Rewrite the sentence. Organic compounds must always (not mainly) contain carbon-hydrogen or carbon-carbon bonds.

3. Page 2, Line 72, Subsection 2.1. Pollution status of PFCs. Include information on the main sources of the pollutants in air/water, etc.

4. Page 3, Line 112, Subsection 2.2. Toxicity analysis of PFCs. The data shown in the subsection are mainly based on the effect of PFCs on living organisms under laboratory conditions. Could you characterize real harmful environmental effect of PFCs based on the data on PFC pollution in different sites (water bodies, atmosphere, etc.)? Did you find the information on threshold limit values for PFCs?

5. Page 8, Line 383. “granular activated carbon” – add the abbreviature (GAC), as it is used in the manuscript.

6. Page 9, Line 422. “PH can affect” à “The pH can affect”

7. Page 16, Line 670. Eq.(18). Move the equation closer to the sentence. It is not obvious the relationship of the Feammox pathway and PFC biodegradation.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The article may be accepted as the authors improved the manuscript.