A Critical Review on PFAS Removal from Water: Removal Mechanism and Future Challenges

Round 1

Reviewer 1 Report

The authors present an important review of current literature on PFAS, with a focus on removal and remediation challenges. The review topic is important, however, the writing is poor in places, there are unnecessary details, inconsistencies (units used include °C, C, and K; some headings have all uppercase, some are lower case, some headings have erroneous periods; acronym issues; chemical formula do not always use subscripts consistently, for example TiO2; and more). There are some important considerations which should be addressed prior to publication. Of importance is a highlight on the short-chain PFAS mentioned in the abstract. As the authors state, the short-chain PFAS were originally developed as environmentally safer and healthier alternatives to long chain based on lower bioaccumulation potential; however, the authors state that they are more dangerous and more bioaccumulative than PFOA, which is not supported by references and citations and is not expanded on more in the review.

Additionally, some specific comments are included below. Note, that in lieu of line numbers, which were not provided in the review version, section titles and sentences will be pasted.

Some examples of problematic areas include:

In Section 1. Introduction:

1.      The line “Based on their end functional groups, PFAS can be distinguished into two compounds: perfluoro carboxylic acids (PFCAs) and perfluoroalkane sulfonic acids (PFSAs) [5].” can be misleading. While carboxylic acids and sulfonic acids are the more well studied, there are more than just two functional groups, including alcohols, sulfonamides, ether acids, phosphates, etc.

2. There is a typo at line “All the sorbents decreased the PFAS leaking by 99%.”, leaking should be replaced with leaching.

3. There are two issues in the sentence “Short-chain PFAS (C2–C7) have been used as an alternative to PFOS and PFOA (C8) due to their decreased demand and production (Gewurtz et al., 2019).” The first issue is formatting in regards to the citation (author, date is used here instead of citation/reference number). The second issue is with reference to “decreased demand”. Commercial demand for PFAS has not decreased, rather, alternative PFAS have been developed and alternative/replacement PFAS uses have been expanded due to continued demand for the products.

4. There is a typo in the sentence “Both PFAS and GenX are very difficult to remove from the environment due to their persistent physio-chemical and thermostable nature, however, owing to their increased solubility and mobility, the shorter ones are more challenging to eliminate [2, 8]. Numerous techniques have been used to remove the PFAS and GenX from the water and soil, including electrooxidation [24], activated carbon [27], membrane technologies [11, 28], electrochemical degradation, clay minerals [8], ion-exchange technologies [7], nanoparticles [29, 30], reverse osmosis [31], biochar [32, 33], etc” It is not correct to state both PFAS and GenX or to state remove the PFAS and GenX, because GenX is PFAS.

5.      There are grammatical errors in the sentences “In 1947, another company named as 3M invented its trademark PFOA with the invention of numerous chemical compounds having 8 carbons tied to fluorine [35]. In 1963, the scientists in the US Navy worked with the 3M to manufacture aqueous film-forming foams and patented it in 1967 [36].” Remove “as” and “the” in the phrases “another company named as 3M” and “US Navy worked with the 3M to manufacture”. Also, PFOA is a single compound containing 8 carbons, 7 of which are fluorinated. The sentence is poorly written and the references are poor. Also, reference 35 is an online article which does not appear to be peer-reviewed; the authors should consider finding better references.

6.      Why did the authors change from using PFAS as singular and plural to PFASs for plural in the sentence “The two most produced and utilized PFASs are PFOA and PFOS.”?

7.      The section on direct and indirect emission sources (“There are two types of PFAS emission sources: direct emission sources, which include emissions made at any point in the PFAS product cycle, and indirect emission sources, which include emissions made during the transition of the precursor. Primary PFAS pro-ducers, AFFF users, and downstream sectors (such as leather, metal plating, semiconduc-tor, and textile) are all direct contributors to PFAS contamination [43]. The two main indi-rect sources of PFAS contamination are water treatment plants and municipal landfills [43]. Some common sources of PFAS are exhibited in (Figure 2). “ is written in a confusing manner. To clarify, direct emission sources include emission at anytime in the lifecycle when products contain the substance as a primary or residual/impurity, whereas indicted sources refer to the formation of the product from transformation of the products precursors in the environment or biota.

8.      The following section seems out of place “For Class B hazardous fuel fires, AFFF formulations often include PFAS because of how well they perform as surfactants [44]. The AFFF that 3M produced using the electrochemical fluorination (ECF) technology is known as legacy PFOS AFFF [45]. The military, the aviation industry, the oil and gas industry, and the firefighting sector were among the first to use AFFF due to its superior efficacy and efficiency in combating fires [45, 46].”

In Section 2. Treatment techniques for the removal of PFAS:

1.      The heading for section 2.1 has an error with an extra period. “2.1. . Sorption”

2.      The sentence “hydrophobic and electrostatic interactions are the main mechanisms of PFAS removal using biochar [59].” Needs to be capitalized.

3.      Why is the acronym GAC finally defined in section 2.1.2? GAC was referenced multiple times throughout the review and should be defined the first time it is presented. Also, the heading “2.1.2.. Activated carbon” has an extra period.

In Figures and Tables:

1.      There is a grammatical error/typo in the Figure 1 legend. The sentence “History of the PFAS with discovery in different form and transformation with passage of time from 1938 to 2024” is poorly written and grammatically incorrect.

2.      In Table 1, the incorrect acronym is given for PFOS, perfluorooctanesulfonic acid. Instead of PFOS, the authors listed “PFOSA”, which is used for a different PFAS known as Perfluorooctanesulfonamide containing the formula C8H2F17NO2S. Also, the preferred IPAC name is 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-Heptadecafluorooctane-1-sulfonic acid.

3.      In Table 1, PFOS is incorrectly listed under the “PFCAs”. In addition to being a PFSA, this is the second time this compound is listed in the table.

4.      In Table 1, the last compound listed (PFO2HxA) is an ether, and should be listed under the PFECAs category, not under the PFCAs category.

There are minor grammatical errors in the manuscript.

Author Response

The authors present an important review of current literature on PFAS, with a focus on removal and remediation challenges. The review topic is important, however, the writing is poor in places, there are unnecessary details, inconsistencies (units used include °C, C, and K; some headings have all uppercase, some are lower case, some headings have erroneous periods; acronym issues; chemical formula do not always use subscripts consistently, for example TiO₂; and more). There are some important considerations which should be addressed prior to publication. Of importance is a highlight on the short-chain PFAS mentioned in the abstract. As the authors state, the short-chain PFAS were originally developed as environmentally safer and healthier alternatives to long chain based on lower bioaccumulation potential; however, the authors state that they are more dangerous and more bioaccumulative than PFOA, which is not supported by references and citations and is not expanded on more in the review.

The statement is supported using reference in the introduction and removed from the abstract. All inconsistencies have been addressed.

Additionally, some specific comments are included below. Note, that in lieu of line numbers, which were not provided in the review version, section titles and sentences will be pasted.

Some examples of problematic areas include:

In Section 1. Introduction:

1. The line “Based on their end functional groups, PFAS can be distinguished into two compounds: perfluoro carboxylic acids (PFCAs) and perfluoroalkane sulfonic acids (PFSAs) [5].” can be misleading. While carboxylic acids and sulfonic acids are the more well studied, there are more than just two functional groups, including alcohols, sulfonamides, ether acids, phosphates, etc.

The line is corrected to “Perfluoro carboxylic acids (PFCAs) and perfluoroalkane sulfonic acids (PFSAs) are the two well studied major subclasses of PFAS”.

2. There is a typo at line “All the sorbents decreased the PFAS leaking by 99%.”, leaking should be replaced with leaching.

The sentence is deleted, and the paragraph is modified accordingly to point out the soil as a route of PFAS exposure. The references are rearranged accordingly.

3. There are two issues in the sentence “Short-chain PFAS (C2–C7) have been used as an alternative to PFOS and PFOA (C8) due to their decreased demand and production (Gewurtz et al., 2019).” The first issue is formatting in regard to the citation (author, date is used here instead of citation/reference number). The second issue is with reference to “decreased demand”. Commercial demand for PFAS has not decreased, rather, alternative PFAS have been developed and alternative/replacement PFAS uses have been expanded due to continued demand for the products.

Thanks for highlighting this important point. The word “decreased demand” is deleted, and the reference is modified accordingly.

4. There is a typo in the sentence “Both PFAS and GenX are very difficult to remove from the environment due to their persistent physio-chemical and thermostable nature, however, owing to their increased solubility and mobility, the shorter ones are more challenging to eliminate [2, 8]. Numerous techniques have been used to remove the PFAS and GenX from the water and soil, including electrooxidation [24], activated carbon [27], membrane technologies [11, 28], electrochemical degradation, clay minerals [8], ion-exchange technologies [7], nanoparticles [29, 30], reverse osmosis [31], biochar [32, 33], etc” It is not correct to state both PFAS and GenX or to state remove the PFAS and GenX, because GenX is PFAS.

The sentence is changed from PFAS and GenX to “PFAS are very difficult to remove from the environment due to their persistent physio-chemical and thermostable nature”

5. There are grammatical errors in the sentences “In 1947, another company named as 3M invented its trademark PFOA with the invention of numerous chemical compounds having 8 carbons tied to fluorine [35]. In 1963, the scientists in the US Navy worked with the 3M to manufacture aqueous film-forming foams and patented it in 1967 [36].” Remove “as” and “the” in the phrases “another company named as 3M” and “US Navy worked with the 3M to manufacture”. Also, PFOA is a single compound containing 8 carbons, 7 of which are fluorinated. The sentence is poorly written and the references are poor. Also, reference 35 is an online article which does not appear to be peer-reviewed; the authors should consider finding better references.

“as” and “the” is removed in the phrases “another company named as 3M” and “US Navy worked with the 3M to manufacture”. The reference of a peer reviewed article is added.

6. Why did the authors change from using PFAS as singular and plural to PFASs for plural in the sentence “The two most produced and utilized PFASs are PFOA and PFOS.”?

All plural PFASs are changed to PFAS

7. The section on direct and indirect emission sources (“There are two types of PFAS emission sources: direct emission sources, which include emissions made at any point in the PFAS product cycle, and indirect emission sources, which include emissions made during the transition of the precursor. Primary PFAS producers, AFFF users, and downstream sectors (such as leather, metal plating, semiconductor, and textile) are all direct contributors to PFAS contamination [43]. The two main indirect sources of PFAS contamination are water treatment plants and municipal landfills [43]. Some common sources of PFAS are exhibited in (Figure 2). “is written in a confusing manner. To clarify, direct emission sources include emission at any time in the lifecycle when products contain the substance as a primary or residual/impurity, whereas indicted sources refer to the formation of the product from transformation of the products precursors in the environment or biota.

Thanks for pointing out this important point. The sentence is rewritten accordingly to make the meaning clear.

8. The following section seems out of place “For Class B hazardous fuel fires, AFFF formulations often include PFAS because of how well they perform as surfactants [44]. The AFFF that 3M produced using the electrochemical fluorination (ECF) technology is known as legacy PFOS AFFF [45]. The military, the aviation industry, the oil and gas industry, and the firefighting sector were among the first to use AFFF due to its superior efficacy and efficiency in combating fires [45, 46].”

This section describes a source of PFAS. It is rewritten to make it clear “AFFF formulations often include PFAS due to their performance as surfactants for Class B hazardous fuel fires”

In Section 2. Treatment techniques for the removal of PFAS:

1. The heading for section 2.1 has an error with an extra period. “2.1. . Sorption”

>Corrected

2. The sentence “hydrophobic and electrostatic interactions are the main mechanisms of PFAS removal using biochar [59].” Needs to be capitalized.

The Hydrophobic is capitalized.

3. Why is the acronym GAC finally defined in section 2.1.2? GAC was referenced multiple times throughout the review and should be defined the first time it is presented. Also, the heading “2.1.2.. Activated carbon” has an extra period.

>Extra period has removed.

In Figures and Tables:

1. There is a grammatical error/typo in the Figure 1 legend. The sentence “History of the PFAS with discovery in different form and transformation with passage of time from 1938 to 2024” is poorly written and grammatically incorrect.

The title is rewritten from “History of the PFAS with discovery in different form and transformation with passage of time from 1938 to 2024” to “Timeline of the PFAS transformation with passage of time from 1938 to 2024”

2. In Table 1, the incorrect acronym is given for PFOS, perfluorooctanesulfonic acid. Instead of PFOS, the authors listed “PFOSA”, which is used for a different PFAS known as Perfluorooctanesulfonamide containing the formula C8H2F17NO2S. Also, the preferred IPAC name is 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-Heptadecafluorooctane-1-sulfonic acid.

The PFOSA is changed to PFOS and IUPAC name is added in the table

3. In Table 1, PFOS is incorrectly listed under the “PFCAs”. In addition to being a PFSA, this is the second time this compound is listed in the table.

It is deleted from PFCAs list

4. In Table 1, the last compound listed (PFO2HxA) is an ether, and should be listed under the PFECAs category, not under the PFCAs category.

PFO2HxA is placed under PFCAs category

Author Response File: Author Response.docx

Reviewer 2 Report

Author Response

In their work, the authors presented an overview of knowledge about the risks of polyfluoroacrylic substances. The work analyzed various issues related to PFAS - such as removal mechanism, treatment techniques and cost analysis. Interesting diagrams and charts deserve attention. The tables summarize and compare information presented by other authors. The work is interesting and may be published in an approved form.

Thanks, for valuable comments and recommendation about our manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

"Comments for review regarding PFAS removal": The following issues must be cleared:

I am confused whether this review has only focused on the removal of PFAS from water or all the compartments of the environment. The title is not in line with the content and conclusion. 

Is this the main scope of this paper? Long- and short-chain PFAS, such as GenX are the main focus of this assessment,?

Why Table 1 is important for this review? Has this table been created by the authors or copied from other sources?

Do not summarize the studies in the body of the review. It makes it difficult to understand. 

The conclusion is poorly written. The authors must have a conclusion for each table separately in the conclusion section. You should conclude which removal method was more effective and why. 

Now, the future recommendation is not clear. Which method can be suggested by the authors?

Needs some modifications. 

Author Response

"Comments for review regarding PFAS removal":

>The following issues must be cleared: I am confused whether this review has only focused on the removal of PFAS from water or all the compartments of the environment.

This review focused on the removal of PFAS from water

>The title is not in line with the content and conclusion.

In the title, the word “environment” has been replaced with “water”.

>Is this the main scope of this paper? Long- and short-chain PFAS, such as GenX are the main focus of this assessment,

The main scope of paper is to compare the different treatment techniques for both Long- and short-chain PFAS. In short chain PFAS, this study specifically focused on GenX.

>Why Table 1 is important for this review? Has this table been created by the authors or copied from other sources?

This table has been created by authors but the images of the chemical structures have been copied from an open access “Pubchem” website PubChem (nih.gov).

>Do not summarize the studies in the body of the review. It makes it difficult to understand.

Thanks for your suggestion. In a review article, we read research article and write their finding in a review article and compare with other studied. It can only be done by summarizing their studies.

>The conclusion is poorly written. The authors must have a conclusion for each table separately in the conclusion section. You should conclude which removal method was more effective and why.

The conclusion has been improved by adding “The MOFs exhibited higher adsorption capacity compared to biochar and AC. The reverse osmosis and nanofiltration both found to be effective techniques for PFAS removal, however, the need to clean or change filter and degradation of membrane is costly. Destruction technique is the most effective one, but its use is limited due to formation of PFAS precursors during treatment techniques. Therefore, it is recommended to use combined treatment techniques for PFAS removal.” as per your valuable suggestion

>Now, the future recommendation is not clear. Which method can be suggested by the authors?

The destruction techniques with combined treatment technique targeting the elimination of long and short-chain PFAS is suggested after reviewing the relevant technologies.

Author Response File: Author Response.docx

Reviewer 4 Report

In this study, the effectiveness of various removal techniques for PFAS (Per- and polyfluoroalkyl substances) was evaluated. The most efficient methods were found to be the destruction techniques, but they also had some limitations, such as the formation of PFAS precursors and high operational costs. Based on these findings, it is expected that a combination of methods will be necessary to effectively remediate water contaminated with PFAS. This is a very good summary review that covers a lot of content and can be accepted with minor revision.

 It is recommended that abbreviations be listed separately.

 It is recommended that short or long chain PFAS be listed or labeled separately or removed from the headings in Tables 2, 3 and 4.

 Page 7, 2.1 2.1.1.. 2.1.2.. It is recommended to remove all periods after subheadings.

Page 13, Adsorption capacity/Kd (((mg g−1)?

Table 2. It is recommended that all units be placed on the first row.

Page 27 PFOS and PFOA in Figure 9 are hard to see.

Author Response

In this study, the effectiveness of various removal techniques for PFAS (Per- and polyfluoroalkyl substances) was evaluated. The most efficient methods were found to be the destruction techniques, but they also had some limitations, such as the formation of PFAS precursors and high operational costs. Based on these findings, it is expected that a combination of methods will be necessary to effectively remediate water contaminated with PFAS. This is a very good summary review that covers a lot of content and can be accepted with minor revision.

>It is recommended that abbreviations be listed separately.

Abbreviations has been added

>It is recommended that short or long chain PFAS be listed or labeled separately or removed from the headings in Tables 2, 3 and 4.

The word “long and short chain” has been removed from the recommended table headings

>Page 7, 2.1 2.1.1.. 2.1.2.. It is recommended to remove all periods after subheadings.

All periods after subheadings have been removed

>Page 13, Adsorption capacity/Kd (((mg g−1)?

Corrected with (mg g^-1)

>Table 2. It is recommended that all units be placed on the first row.

I appreciate your valuable suggestion. The authors have used different units for dosage, initial concentration, and adsorption capacity. Therefore, the units cannot be removed from the Table 2 body.

>Page 27 PFOS and PFOA in Figure 9 are hard to see.

The figure 9 has been taken from an article in Journal of Hazardous Materials with copyright permission, therefore, it cannot be edited

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The paper is acceptable in its current form. 

Author Response

Thanks for accepting our manuscript.