A Critical Review of Nanobubble Flotation for Seawater Treatment Process

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review Article: “A Critical Review of Nanobubbles Flotation for Seawater Desalination”

1. Abstract (Lines 16-17):
• Introduce the nanobubble technique/process as a potential desalination method to provide clarity.
2. Language and Sentence Structure:
• Several sections of the manuscript require revision to improve sentence structure and language clarity. Some statements convey incorrect or misleading information. Examples include:
• Line 17: The term "salt water" should be replaced with "salt" in the sentence:
  “Commonly, salt water and heavy metal ions are dissolved in the water, and suspended solids exist to form as sediment.”
• Line 43: The sentence “Population growth, economic development, and increased freshwater consumption have exacerbated this problem.” is almost a repetition of Line 32 and should be revised to avoid redundancy.
• Line 52: When categorizing desalination technologies, Reverse Osmosis (RO) and Electrodialysis (ED) fall under electrically driven processes, while Mechanical Vapor Compression (MVC) should be mentioned under mechanically driven processes.
• Line 123: The phrase should read as “used in seawater desalination for drinking water production.”
• Line 178: The phrase should be revised to “the conversion of seawater to drinking water.”
• Line 184: The sentence “In addition, every kind of gas nanobubble has its application not only for seawater desalination but also for other applications.” lacks clarity and proper sentence structure. A revision is needed.
• Line 397: The statement “This review article requires advantages, especially for ion flotation.” is unclear and must be rewritten for coherence.
• Line 779: The statement “Confronting these intricate challenges is pivotal to establishing a solid theoretical foundation for the large-scale implementation of bulk nanobubbles in industrial settings. In industrial issues, it might be separated the heavy metal ions in the contaminant water.” is unclear and should be restructured for readability.
• Line 792: The phrase “Ion flotation has been used for the recovery of precious metals because it plays in the ionic ode and has physical and chemical interactions to separate the salt and metals ionic which are used the surfactants, as well as its characteristics and factors, are mentioned in the section the effect of surfactants on the ion flotation.” is not comprehensible and requires substantial revision.
• Line 800: The term “seawater conversion” is ambiguous—does the author mean “seawater desalination”? Please clarify.
3. Research Gap:
• The introduction does not explicitly highlight the existing research gap. While the review aims to provide an updated analysis, the specific gap in current knowledge should be clearly outlined.
4. Literature Search Criteria (Line 146):
• Clarify whether the search domains were applied consistently. Currently, the manuscript states:
  “The year of publication was limited to 2013 to January 2024 in ScienceDirect, whereas the year was limited to 2013 to 2024 in Google Scholar.”
• Were the same time constraints applied across both databases?
5. Terminology and Coherence (Line 309):
• The statement “Due to the lack of various methods in water treatment, ion flotation has been evaluated as an excellent alternative treatment process” lacks clarity and coherence. The term “lack of various methods” is vague and needs to be explicitly connected to prior discussions.
6. Figures and Citations:
• Ensure that the full terminology of the introduced techniques is included in the figure captions, including in Fig. 5.
• Verify that all figures are appropriately cited in the text and accompanied by relevant explanations, including in Fig. 5.
7. Table 4 – Comprehensive List of Membrane Technologies:
• The table titled “List of established or emerging industrial processes that use polymeric membranes” is not comprehensive. It excludes several emerging membrane technologies, such as:
• Membrane Distillation (MD)
• Forward Osmosis (FO)
• Membrane Capacitive Deionization (MCDI)
• Electrodialysis Reversal (EDR)
• Consider revising the table to include these emerging membrane systems.
8. Figure Citation Issue (Section 5, Line 773):
• Figure 12 is missing in the manuscript but is referenced in the text. Ensure that all figures exist and are correctly labeled.
9. Membrane Techniques for Salt Removal (Line 796):
• The statement “The ion separation with membranes integrated into nanobubbles needs to be investigated to produce pure and fresh water from seawater.” is vague and lacks elaboration.
• More details should be provided on potential membrane techniques for removing salt, including any recent advancements in membrane-nanobubble integration.
10. Conclusion Section:

• The conclusion is unclear and lacks structure. It does not adequately summarize:
• The key findings of the review
• The main contributions of the study
• Potential recommendations for future research
• The conclusion should be rewritten in a comprehensive and clear manner to effectively communicate the outcomes of the study.

Comments on the Quality of English Language

• • Line 17: The term "salt water" should be replaced with "salt" in the sentence:
    “Commonly, salt water and heavy metal ions are dissolved in the water, and suspended solids exist to form as sediment.”
  • Line 43: The sentence “Population growth, economic development, and increased freshwater consumption have exacerbated this problem.” is almost a repetition of Line 32 and should be revised to avoid redundancy.
  • Line 52: When categorizing desalination technologies, Reverse Osmosis (RO) and Electrodialysis (ED) fall under electrically driven processes, while Mechanical Vapor Compression (MVC) should be mentioned under mechanically driven processes.
  • Line 123: The phrase should read as “used in seawater desalination for drinking water production.”
  • Line 178: The phrase should be revised to “the conversion of seawater to drinking water.”
  • Line 184: The sentence “In addition, every kind of gas nanobubble has its application not only for seawater desalination but also for other applications.” lacks clarity and proper sentence structure. A revision is needed.
  • Line 397: The statement “This review article requires advantages, especially for ion flotation.” is unclear and must be rewritten for coherence.
  • Line 779: The statement “Confronting these intricate challenges is pivotal to establishing a solid theoretical foundation for the large-scale implementation of bulk nanobubbles in industrial settings. In industrial issues, it might be separated the heavy metal ions in the contaminant water.” is unclear and should be restructured for readability.
  • Line 792: The phrase “Ion flotation has been used for the recovery of precious metals because it plays in the ionic ode and has physical and chemical interactions to separate the salt and metals ionic which are used the surfactants, as well as its characteristics and factors, are mentioned in the section the effect of surfactants on the ion flotation.” is not comprehensible and requires substantial revision.
  • Line 800: The term “seawater conversion” is ambiguous—does the author mean “seawater desalination”? Please clarify.
  • Several sections of the manuscript require revision to improve sentence structure and language clarity. Some statements convey incorrect or misleading information. Examples include:

Author Response

Comments and Suggestions for Authors

Point 1: Abstract (Lines 16-17):

Introduce the nanobubble technique/process as a potential desalination method to provide clarity.

 

Response 1: We would like to thank reviewer #1 for providing valuable comments and suggestions to improve the quality of the manuscript.

We added the nanobubble technique/process as a potential desalination method, as well as our following response, “to produce freshwater using the nanobubble technique, because it is necessary to modify the nanobubbles' flotation to separate the salt ions and suspended solids from water to get the freshwater. Nanobubbles flotation, especially ion flotation, is the formation of positively buoyant bubble-particles that agglomerates mixed with a recycling stream to saturate with air or carbon dioxide at high pressure to generate nanobubbles.”  

 

Point 2: Language and Sentence Structure:

Several manuscript sections require revision to improve sentence structure and language clarity. Some statements convey incorrect or misleading information. Examples include:

• Line 17: The term "salt water" should be replaced with "salt" in the sentence:
  “Commonly, salt water and heavy metal ions are dissolved in the water, and suspended solids exist to form as sediment.”

Response: We have revised the term “salt water” to “salt”. Thank you for your comment.

 

• Line 43: The sentence “Population growth, economic development, and increased freshwater consumption have exacerbated this problem.” is almost a repetition of Line 32 and should be revised to avoid redundancy.

Response: We have revised to avoid redundancy. As you advised, we deleted it and made a good statement. Thank you for your comment.

 

• Line 52: When categorizing desalination technologies, Reverse Osmosis (RO) and Electrodialysis (ED) fall under electrically driven processes, while Mechanical Vapor Compression (MVC) should be mentioned under mechanically driven processes.

Response: We have revised and mentioned the Mechanical Vapor Compression under the mechanically driven processes.

 

• Line 123: The phrase should read as “used in seawater desalination for drinking water production.”

Response: We have revised and changed the expression as you advised.

 

• Line 178: The phrase should be revised to “the conversion of seawater to drinking water.”

Response: We have revised and changed the expression conversion of seawater to drinking water.

 

• Line 184: The sentence “In addition, every kind of gas nanobubble has its application not only for seawater desalination but also for other applications.” lacks clarity and proper sentence structure. A revision is needed.

Response: We have revised and changed the expression as you advised.

 

• Line 397: The statement “This review article requires advantages, especially for ion flotation.” is unclear and must be rewritten for coherence.

Response: We have revised and changed the following statement,” This review article highlights the advantages of seawater desalination, particularly for ion exchange flotation. Ion flotation is one potential technique to remove hazardous ions from drinking water at low-level concentrations. It is environmentally friendly and biodegradable.”

 

• Line 779: The statement “Confronting these intricate challenges is pivotal to establishing a solid theoretical foundation for the large-scale implementation of bulk nanobubbles in industrial settings. In industrial issues, it might be separated the heavy metal ions in the contaminant water.” is unclear and should be restructured for readability.

Response: We have revised and changed the following statement,” The ion flotation derives from the mineral separation industry. This technique can remove organic and inorganic contaminants from wastewater in anionic or cationic forms. Currently, ion flotation is in use for the recovery of precious metals, ion separation, and wastewater treatment because of its low-cost ancillary devices, flexibility, low energy consumption, and a negligible amount of sludge.

 

• Line 792: The phrase “Ion flotation has been used for the recovery of precious metals because it plays in the ionic ode and has physical and chemical interactions to separate the salt and metals ionic which are used the surfactants, as well as its characteristics and factors, are mentioned in the section the effect of surfactants on the ion flotation.” is not comprehensible and requires substantial revision.

Response: We have revised and changed the following statement,” Ion flotation has been utilized to recover metals including arsenic, mercury, and lead. It functions in ionic mode and involves physical and chemical interactions to separate ionic salts and metals through surfactants. The section regarding the impact of surfactants on ion flotation explores its characteristics and influencing factors.

• Line 800: The term “seawater conversion” is ambiguous—does the author mean “seawater desalination”? Please clarify.

Response: We have revised the term “seawater conversion " to “ seawater desalination. Thank you for your advice.

 

Point 3: Research Gap:

The introduction does not explicitly highlight the existing research gap. While the review aims to provide an updated analysis, the specific gap in current knowledge should be clearly outlined.

 

Response 3: We have explicitly highlighted the existing research gap, and it has the specific gap clearly in the introduction

 

Point 4: Literature Search Criteria (Line 146):

Clarify whether the search domains were applied consistently. Currently, the manuscript states:
“The year of publication was limited to 2013 to January 2024 in ScienceDirect, whereas the year was limited to 2013 to 2024 in Google Scholar.”

Were the same time constraints applied across both databases?

 

 

Response 4: We have updated the literature review by including the manuscript states and highlighted in blue: The year of publication was limited to 2013 to January 2024 in ScienceDirect, whereas the year was also limited to 2013 to January 2024 in Google Scholar.

Yes, the same constraints applied across both databases.

 

Point 5:             Terminology and Coherence (Line 309):

The statement “Due to the lack of various methods in water treatment, ion flotation has been evaluated as an excellent alternative treatment process” lacks clarity and coherence. The term “lack of various methods” is vague and needs to be explicitly connected to prior discussions.

 

 

Response 5: We have updated the terminology and coherence and highlighted in blue as follows:

 

Given the numerous drawbacks of various water treatment methods, ion flotation has been considered an excellent alternative for wastewater treatment due to its low energy consumption, quick operation, minimal space requirements, and simplicity.

 

Point 6: Figures and Citations:

• Ensure that the introduced techniques' terminology is included in the figure captions, including in Fig. 5.
• Verify that all figures are appropriately cited in the text and accompanied by relevant explanations, including in Fig. 5.

 

Response 6: We have updated the figure 5 and its citations, then highlighted in blue as follows:

According to Figure 5, Electrochemical technologies have become essential for removing organic substances from water and wastewater. Below are some key methods:

• Electrochemical Oxidation: This process involves the direct transfer of electrons from pollutants to the anode, leading to their degradation. Innovations in electrocatalytic anode materials have significantly improved the efficiency and selectivity of this process [119].
• Electrogenerated Active Chlorine: This method electrochemically produces active chlorine species in water by utilizing chloride ions, which then oxidize organic pollutants. It is effective against a wide range of contaminants [121].
• Electrocatalytic Reduction: This technique reduces pollutants at the cathode, making it particularly effective for treating compounds that are challenging to oxidize. Recent advancements in cathode materials have enhanced their efficiency [122].
• Electrocoagulation: This process uses electric current to remove suspended solids, heavy metals, and various contaminants from water. It is highly effective for treating wastewater with elevated levels of organic pollutants [122], [123].
• Electrodialysis: This method employs an electric field to drive ions through selective membranes, effectively separating them from water. It is advantageous for desalination and removing ionic contaminants [124].
• Electrochemical Advanced Oxidation Processes (EAOPs): These processes combine electrochemical oxidation with various advanced oxidation techniques, such as Fenton's reaction, to enhance the breakdown of complex organic compounds [52].

These technologies continually evolve, and research is ongoing to improve their efficiency, scalability, and sustainability. Thus, they promise a bright future for water treatment.

 

Point 7: Table 4 – Comprehensive List of Membrane Technologies:

• The table titled “List of established or emerging industrial processes that use polymeric membranes” is not comprehensive. It excludes several emerging membrane technologies, such as:
• Membrane Distillation (MD)
• Forward Osmosis (FO)
• Membrane Capacitive Deionization (MCDI)
• Electrodialysis Reversal (EDR)
• Consider revising the table to include these emerging membrane systems.

 

 

Response 7: We have updated several emerging membrane systems technologies, which are also the type of process desalination technologies that have the membrane systems. We added one row for the pictures of the membrane systems we put on that table row.

 

 

Point 8: Figure Citation Issue (Section 5, Line 773):

• Figure 12 is missing in the manuscript but is referenced in the text. Ensure that all figures exist and are correctly labeled.

 

Response 8: We have updated pictures and their references in the text.

Point 9: Membrane Techniques for Salt Removal (Line 796):

• The statement “The ion separation with membranes integrated into nanobubbles needs to be investigated to produce pure and fresh water from seawater.” is vague and lacks elaboration.
• More details should be provided on potential membrane techniques for removing salt, including any recent advancements in membrane-nanobubble integration.

 

Response 9: We have updated the statement and highlight in blue as follows:

 

Salt is an ion dissolved in water, and in certain types of seawater, it is bonded to heavy metals. The separation of ions using membranes integrated with nanobubbles must be explored to produce pure, fresh water from seawater. For further investigation, according to Figure 4, especially on the membrane systems, it can be done in a multi-membrane system to produce fresh and pure water from seawater [177].

 

Point 10: Conclusion Section:

• The conclusion is unclear and lacks structure. It does not adequately summarize:
• The key findings of the review
• The main contributions of the study
• Potential recommendations for future research
• The conclusion should be rewritten comprehensively and clearly to communicate the study's outcomes effectively.

 

Response 10: We have updated the conclusions section in blue as follows:

Seawater desalination offers a continuous and reliable source of high-quality water, functioning within the natural freshwater ecosystems' ongoing water cycle. Nevertheless, the challenge is vast. Only 2.7% of the planet's water is freshwater, and a mere 0.3% is readily available for human use, making freshwater scarcity a critical global issue. This predicament has intensified due to rising freshwater usage. By 2050, three-quarters of the worldwide population is expected to experience freshwater shortages. Extracting freshwater from seawater through desalination has emerged as an essential solution. Desalination facilities that eliminate salt and minerals from seawater are vital in addressing water scarcity. Over 19,000 plants worldwide produce more than 100 million cubic meters of fresh water daily, highlighting the situation's urgency. A recent study on seawater desalination utilizing nanobubble flotation has been evaluated.

 

Various nanobubbles display traits like the free movement of water molecules, mass transfer, the electrical charge at the surface (zeta potential), and the chemical breakdown of wastewater. The key aspect of the nanobubble flotation process is the interaction between bubbles and pollutant particles. These interactions involve ion flotation in the seawater process with the exchange of ions and the principles of ion flotation. Notably, nanobubbles exhibit distinctive characteristics when compared to micro-nano bubbles (MNBs), which include enhanced mass transfer, prolonged stability, elevated zeta potential, a high surface-to-volume ratio, and the production of free radicals upon their collapse. Thanks to their capability to generate highly reactive free radicals, nanobubbles are particularly effective in treating wastewater and drinking water. Various types of nanobubbles have different uses. Hydrogen nanobubbles in fuel mixtures can improve combustion efficiency compared to conventional gas. Nitrogen nanobubbles can enhance the hydrolysis of waste-activated sludge and boost methane production during anaerobic digestion. Oxygen nanobubbles facilitate methane generation during the anaerobic digestion of cellulose, while bulk carbon dioxide nanobubbles find applications in the food processing industry. Ozone micro-nanobubbles can enhance ozone mass transfer, achieving high levels of dissolved ozone concentration in the water phase and extending the reactivity of ozone in that phase. They are also extensively utilized for breaking down organic contaminants.

The influence of salt on the characteristics of nanobubbles over time was examined and analyzed. The extended DLVO theory was utilized to interpret and elaborate on our experimental results concerning the stability of nanobubbles. Nanobubbles improve the membrane's capability to filter out salt and contaminants. The fourth point focuses on lowering chemical reliance; the effectiveness of nanobubbles in purification reduces the need for chemical additives typically utilized in conventional desalination. This decrease in chemical reliance leads to cost reductions and diminishes the environmental impact of the water treatment procedure. In conclusion, these tiny bubbles play a crucial role in the progress of desalination technology.

 

 

 

Comments on the Quality of English Language

 

• Line 17: The term "salt water" should be replaced with "salt" in the sentence:
  “Commonly, salt water and heavy metal ions are dissolved in the water, and suspended solids exist to form as sediment.”

Response: We have revised the term “salt water” to “salt”. Thank you for your comment.

 

• Line 43: The sentence “Population growth, economic development, and increased freshwater consumption have exacerbated this problem.” is almost a repetition of Line 32 and should be revised to avoid redundancy.

Response: We have revised to avoid redundancy. As you advised, we deleted it and made a good statement. Thank you for your comment.

 

• Line 52: When categorizing desalination technologies, Reverse Osmosis (RO) and Electrodialysis (ED) fall under electrically driven processes, while Mechanical Vapor Compression (MVC) should be mentioned under mechanically driven processes.

Response: We have revised and mentioned the Mechanical Vapor Compression under the mechanically driven processes.

 

• Line 123: The phrase should read as “used in seawater desalination for drinking water production.”

Response: We have revised and changed the expression as you advised.

 

• Line 178: The phrase should be revised to “the conversion of seawater to drinking water.”

Response: We have revised and changed the expression conversion of seawater to drinking water.

 

• Line 184: The sentence “In addition, every kind of gas nanobubble has its application not only for seawater desalination but also for other applications.” lacks clarity and proper sentence structure. A revision is needed.

Response: We have revised and changed the expression as you advised.

 

• Line 397: The statement “This review article requires advantages, especially for ion flotation.” is unclear and must be rewritten for coherence.

Response: We have revised and changed the following statement,” This review article highlights the advantages of seawater desalination, particularly for ion exchange flotation. Ion flotation is one potential technique to remove hazardous ions from drinking water at low-level concentrations. It is environmentally friendly and biodegradable.”

 

• Line 779: The statement “Confronting these intricate challenges is pivotal to establishing a solid theoretical foundation for the large-scale implementation of bulk nanobubbles in industrial settings. In industrial issues, it might be separated the heavy metal ions in the contaminant water.” is unclear and should be restructured for readability.

Response: We have revised and changed the following statement,” The ion flotation derives from the mineral separation industry. This technique can remove organic and inorganic contaminants from wastewater in anionic or cationic forms. Currently, ion flotation is in use for the recovery of precious metals, ion separation, and wastewater treatment because of its low-cost ancillary devices, flexibility, low energy consumption, and a negligible amount of sludge.

 

• Line 792: The phrase “Ion flotation has been used for the recovery of precious metals because it plays in the ionic ode and has physical and chemical interactions to separate the salt and metals ionic which are used the surfactants, as well as its characteristics and factors, are mentioned in the section the effect of surfactants on the ion flotation.” is not comprehensible and requires substantial revision.

Response: We have revised and changed the following statement,” Ion flotation has been utilized to recover metals including arsenic, mercury, and lead. It functions in ionic mode and involves physical and chemical interactions to separate ionic salts and metals through surfactants. The section regarding the impact of surfactants on ion flotation explores its characteristics and influencing factors.

• Line 800: The term “seawater conversion” is ambiguous—does the author mean “seawater desalination”? Please clarify.

Response: We have revised the term “seawater conversion " to “ seawater desalination. Thank you for your advice.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

When I accepted this paper for review based on the title and the abstract I wondered, being a desalination person, how was it possible that I had no idea about this new desalination technique of desalination via nano-bubbles. Well, after reading the text I am less perplex, because, as opposed to the title, this piece does not ready talk about desalination at all and definitely not about seawater desalination, as implied by its caption. This is some kind of review on nanobubbles, like which there are a few already. It discusses all the regular angles, "desalination" not being one of them. As a regular review it is not bad although it tends to be too general and somewhat unfocused. In many places the English usage is not polished enough and in some places it is even worse than that (see Section 4.2, for example). Section 4.4 and 4.5, that supposedly address the issue of nanobubbles and desalination, are almost entirely void. What information do Fig 4, Fig 5 and Fig. 6 add to the readers?

Comments on the Quality of English Language

See comments above

Author Response

Comments and Suggestions for Authors

Point 1: When I accepted this paper for review based on the title and the abstract, I wondered, being a desalination person, how was it possible that I had no idea about this new desalination technique of desalination via nano-bubbles. Well, after reading the text I am less perplexed, because, as opposed to the title, this piece does not really talk about desalination at all and not about seawater desalination, as implied by its caption. This is some kind of review on nanobubbles, like there are a few already. It discusses all the regular angles, "desalination" not being one of them. As a regular review, it is not bad, although it tends to be too general and somewhat unfocused. In many places, the English usage is not polished enough; in some areas, it is even worse than that (see Section 4.2, for example). Sections 4.4 and 4.5, which supposedly address the issue of nanobubbles and desalination, are almost entirely void. What information do Figures 4, 5, and 6 add to the readers?

 

Response 1: We would like to thank reviewer #2 for providing valuable comments and suggestions to improve the quality of the manuscript.

We added statements on section 4.2 about the effect of the Nanobubbles to enhance the desalination process in blue as follows:

Impact of the NBs to Enhance the Desalination Process

In extracting pollutants from water, ion flotation does not offer a notable method for separating heavy metals [23,68]. This method can eliminate organic and inorganic contaminants, whether anionic or cationic, from wastewater and ion flotation. Although its explanation is distinct, it is similar to froth flotation. [91]. The essential process involves the adsorption of surfactants onto the nanobubbles at the base of the column flotation, which will engage with the heavy metal ions for adequate collection. As the bubbles ascend, surfactant ions are captured in the froth. One of the most commonly used synthetic surfactants in various industries is sodium dodecyl sulfate (SDS) [93], [94], [95]. This surfactant is also known as sodium lauryl sulfate.

Figure 1. The molecular structure of Sodium Dodecyl Sulfate (SDS)

Figure 4 illustrates the molecular structure of this anionic surfactant for the desalination process with heavy metal.  The SDS is a surfactant that is effective in removing heavy metals. Kukizaki M. et al. reported that applying the SDS as the collector in an ion flotation process removes the Mn²⁺, Cu^2+, and Zn²⁺ from water. [96]. Table 3 shows the results obtained from various studies of the various pollutants and removal using the SDS to remove different kinds of heavy metal ions through ion flotation.

 

 

We also have updated the explanation on section 4.4, including figure 5, as follows:

In section 4.4, we have updated the technology and contents of this section.

 

Nanobubbles will be employed more frequently as preparation technology and research advances. Developing and optimizing processes for the stable and effective production of nanobubbles will be crucial.

Figure 2. The most critical electrochemical technologies used to remove organic substances from water and wastewater.

According to Figure 5, Electrochemical technologies have become essential for removing organic substances from water and wastewater. Below are some key methods to describe Figure 5.

• Electrochemical Oxidation: This process involves the direct transfer of electrons from pollutants to the anode, leading to their degradation. Innovations in electrocatalytic anode materials have significantly improved the efficiency and selectivity of this process [119].
• Electrogenerated Active Chlorine: This method electrochemically produces active chlorine species in water by utilizing chloride ions, which then oxidize organic pollutants. It is effective against a wide range of contaminants [121].
• Electrocatalytic Reduction: This technique reduces pollutants at the cathode, making it particularly effective for treating compounds that are challenging to oxidize. Recent advancements in cathode materials have enhanced their efficiency [122].
• Electrocoagulation: This process uses electric current to remove suspended solids, heavy metals, and various contaminants from water. It is highly effective for treating wastewater with elevated levels of organic pollutants [122], [123].
• Electrodialysis: This method employs an electric field to drive ions through selective membranes, effectively separating them from water. It is advantageous for desalination and removing ionic contaminants [124].
• Electrochemical Advanced Oxidation Processes (EAOPs): These processes combine electrochemical oxidation with various advanced oxidation techniques, such as Fenton's reaction, to enhance the breakdown of complex organic compounds [52].

These technologies continually evolve, and research is ongoing to improve their efficiency, scalability, and sustainability. Thus, they promise a bright future for water treatment.

 

Response for Figure 6: We have updated the explanation in addition to the physical and chemical parameters as follows:

Based on Figure 6, separations using electrical potentials and differences in ion electrophoretic mobilities are promising but relatively unexplored. [144], [147] While carrier-mediated transport offers high selectivity in liquid membranes, these systems are unstable, and selective transport via hopping between anchored carriers remains elusive. According to the ion separation in Figure 6, the physical and chemical parameters involved in the desalination process are size, charge, hydration energy, electrophoretic mobility (porosity), and molecular recognition.

Figure 3. Illustration of ion separation

Desalination of brackish water and seawater has become an increasingly popular solution to global water scarcity. [148] As involved as in the ion separation. According to Table 4, reverse osmosis (RO) is the most widely utilized desalination technology due to its energy efficiency and space-saving design. Also, it shows us that in Table 4, the systems membranes have been established for industrial processes. In recent years, numerous efforts have been made to enhance membrane performance, specifically higher permeability, to improve RO's energy efficiency and the performance of the ion separation.

 

We have changed the subtitle of section 4.5 to “Effect of gas nanobubbles on the bulk nanobubbles” because this section describes the impact of the various contaminants applied to some gas nanobubbles and their effect on the physical and chemical processes and phenomena, including absorption, precipitation, and so on, to extract the heavy metal.

 

Point 2: Language and Sentence Structure:

Several manuscript sections require revision to improve sentence structure and language clarity. Some statements convey incorrect or misleading information. Examples include:

• Line 17: The term "salt water" should be replaced with "salt" in the sentence:
  “Commonly, salt water and heavy metal ions are dissolved in the water, and suspended solids exist to form as sediment.”

Response: We have revised the term “salt water” to “salt”. Thank you for your comment.

 

• Line 43: The sentence “Population growth, economic development, and increased freshwater consumption have exacerbated this problem.” is almost a repetition of Line 32 and should be revised to avoid redundancy.

Response: We have revised to avoid redundancy. As you advised, we deleted it and made a good statement. Thank you for your comment.

 

• Line 52: When categorizing desalination technologies, Reverse Osmosis (RO) and Electrodialysis (ED) fall under electrically driven processes, while Mechanical Vapor Compression (MVC) should be mentioned under mechanically driven processes.

Response: We have revised and mentioned the Mechanical Vapor Compression under the mechanically driven processes.

 

• Line 123: The phrase should read as “used in seawater desalination for drinking water production.”

Response: We have revised and changed the expression as you advised.

 

• Line 178: The phrase should be revised to “the conversion of seawater to drinking water.”

Response: We have revised and changed the expression conversion of seawater to drinking water.

 

• Line 184: The sentence “In addition, every kind of gas nanobubble has its application not only for seawater desalination but also for other applications.” lacks clarity and proper sentence structure. A revision is needed.

Response: We have revised and changed the expression as you advised.

 

• Line 397: The statement “This review article requires advantages, especially for ion flotation.” is unclear and must be rewritten for coherence.

Response: We have revised and changed the following statement,” This review article highlights the advantages of seawater desalination, particularly for ion exchange flotation. Ion flotation is one potential technique to remove hazardous ions from drinking water at low-level concentrations. It is environmentally friendly and biodegradable.”

 

• Line 779: “Confronting these intricate challenges is pivotal to establishing a solid theoretical foundation for the large-scale implementation of bulk nanobubbles in industrial settings. In industrial issues, it might be separated the heavy metal ions in the contaminant water.” is unclear and should be restructured for readability.

Response: We have revised and changed the following statement,” The ion flotation derives from the mineral separation industry. This technique can remove organic and inorganic contaminants from wastewater in anionic or cationic forms. Currently, ion flotation is in use for the recovery of precious metals, ion separation, and wastewater treatment because of its low-cost ancillary devices, flexibility, low energy consumption, and a negligible amount of sludge.

 

• Line 792: The phrase “Ion flotation has been used for the recovery of precious metals because it plays in the ionic ode and has physical and chemical interactions to separate the salt and metals ionic which are used the surfactants, as well as its characteristics and factors, are mentioned in the section the effect of surfactants on the ion flotation.” is not comprehensible and requires substantial revision.

Response: We have revised and changed the following statement,” Ion flotation has been utilized to recover metals including arsenic, mercury, and lead. It functions in ionic mode and involves physical and chemical interactions to separate ionic salts and metals through surfactants. The section regarding the impact of surfactants on ion flotation explores its characteristics and influencing factors.

• Line 800: The term “seawater conversion” is ambiguous—does the author mean “seawater desalination”? Please clarify.

Response: We have revised the term “seawater conversion " to “ seawater desalination. Thank you for your advice.

 

Point 3: Research Gap:

The introduction does not explicitly highlight the existing research gap. While the review aims to provide an updated analysis, the specific gap in current knowledge should be clearly outlined.

 

Response 3: We have explicitly highlighted the existing research gap, and it has the specific gap clearly in the introduction

 

Point 4: Literature Search Criteria (Line 146):

Clarify whether the search domains were applied consistently. Currently, the manuscript states:
“The year of publication was limited to 2013 to January 2024 in ScienceDirect, whereas the year was limited to 2013 to 2024 in Google Scholar.”

Were the same time constraints applied across both databases?

 

 

Response 4: We have updated the literature review by including the manuscript states and highlighted in blue: The year of publication was limited to 2013 to January 2024 in ScienceDirect, whereas the year was also limited to 2013 to January 2024 in Google Scholar.

Yes, the same constraints applied across both databases.

 

Point 5:             Terminology and Coherence (Line 309):

The statement “Due to the lack of various methods in water treatment, ion flotation has been evaluated as an excellent alternative treatment process” lacks clarity and coherence. The term “lack of various methods” is vague and needs to be explicitly connected to prior discussions.

 

 

Response 5: We have updated the terminology and coherence and highlighted in blue as follows:

 

Given the numerous drawbacks of various water treatment methods, ion flotation has been considered an excellent alternative for wastewater treatment due to its low energy consumption, quick operation, minimal space requirements, and simplicity.

 

Point 6: Figures and Citations:

• Ensure that the introduced techniques' terminology is included in the figure captions, including in Fig. 5.
• Verify that all figures are appropriately cited in the text and accompanied by relevant explanations, including in Fig. 5.

 

Response 6: We have updated the figure 5 and its citations, then highlighted in blue as follows:

According to Figure 5, Electrochemical technologies have become essential for removing organic substances from water and wastewater. Below are some key methods:

• Electrochemical Oxidation: This process involves the direct transfer of electrons from pollutants to the anode, leading to their degradation. Innovations in electrocatalytic anode materials have significantly improved the efficiency and selectivity of this process [119].
• Electrogenerated Active Chlorine: This method electrochemically produces active chlorine species in water by utilizing chloride ions, which then oxidize organic pollutants. It is effective against a wide range of contaminants [121].
• Electrocatalytic Reduction: This technique reduces pollutants at the cathode, making it particularly effective for treating compounds that are challenging to oxidize. Recent advancements in cathode materials have enhanced their efficiency [122].
• Electrocoagulation: This process uses electric current to remove suspended solids, heavy metals, and various contaminants from water. It is highly effective for treating wastewater with elevated levels of organic pollutants [122], [123].
• Electrodialysis: This method employs an electric field to drive ions through selective membranes, effectively separating them from water. It is advantageous for desalination and removing ionic contaminants [124].
• Electrochemical Advanced Oxidation Processes (EAOPs): These processes combine electrochemical oxidation with various advanced oxidation techniques, such as Fenton's reaction, to enhance the breakdown of complex organic compounds [52].

These technologies continually evolve, and research is ongoing to improve their efficiency, scalability, and sustainability. Thus, they promise a bright future for water treatment.

 

Point 7: Table 4 – Comprehensive List of Membrane Technologies:

• The table “List of established or emerging industrial processes that use polymeric membranes” is not comprehensive. It excludes several emerging membrane technologies, such as:
• Membrane Distillation (MD)
• Forward Osmosis (FO)
• Membrane Capacitive Deionization (MCDI)
• Electrodialysis Reversal (EDR)
• Consider revising the table to include these emerging membrane systems.

 

 

Response 7: We have updated several emerging membrane systems technologies, which are also the type of process desalination technologies that have the membrane systems. We added one row for the pictures of the membrane systems we put on that table row.

 

 

Point 8: Figure Citation Issue (Section 5, Line 773):

• Figure 12 is missing in the manuscript but is referenced in the text. Ensure that all figures exist and are correctly labeled.

 

Response 8: We have updated pictures and their references in the text.

Point 9: Membrane Techniques for Salt Removal (Line 796):

• The statement “The ion separation with membranes integrated into nanobubbles needs to be investigated to produce pure and fresh water from seawater.” is vague and lacks elaboration.
• More details should be provided on potential membrane techniques for removing salt, including recent advancements in membrane-nanobubble integration.

 

Response 9: We have updated the statement and highlight in blue as follows:

 

Salt is an ion dissolved in water, and in certain types of seawater, it is bonded to heavy metals. The separation of ions using membranes integrated with nanobubbles must be explored to produce pure, fresh water from seawater. For further investigation, according to Figure 4, especially on the membrane systems, a multi-membrane system can produce fresh and pure water from seawater. [177].

 

Point 10: Conclusion Section:

• The conclusion is unclear and lacks structure. It does not adequately summarize:
• The key findings of the review
• The main contributions of the study
• Potential recommendations for future research
• The conclusion should be rewritten comprehensively and clearly to communicate the study's outcomes effectively.

 

Response 10: We have updated the conclusions section in blue as follows:

Seawater desalination offers a continuous and reliable source of high-quality water, functioning within the natural freshwater ecosystems' ongoing water cycle. Nevertheless, the challenge is vast. Only 2.7% of the planet's water is freshwater, and a mere 0.3% is readily available for human use, making freshwater scarcity a critical global issue. This predicament has intensified due to rising freshwater usage. By 2050, three-quarters of the worldwide population is expected to experience freshwater shortages. Extracting freshwater from seawater through desalination has emerged as an essential solution. Desalination facilities that eliminate salt and minerals from seawater are vital in addressing water scarcity. Over 19,000 plants worldwide produce more than 100 million cubic meters of fresh water daily, highlighting the situation's urgency. A recent study on seawater desalination utilizing nanobubble flotation has been evaluated.

 

Various nanobubbles display traits like the free movement of water molecules, mass transfer, the electrical charge at the surface (zeta potential), and the chemical breakdown of wastewater. The key aspect of the nanobubble flotation process is the interaction between bubbles and pollutant particles. These interactions involve ion flotation in the seawater process with the exchange of ions and the principles of ion flotation. Notably, nanobubbles exhibit distinctive characteristics when compared to micro-nano bubbles (MNBs), which include enhanced mass transfer, prolonged stability, elevated zeta potential, a high surface-to-volume ratio, and the production of free radicals upon their collapse. Thanks to their capability to generate highly reactive free radicals, nanobubbles are particularly effective in treating wastewater and drinking water. Various types of nanobubbles have different uses. Hydrogen nanobubbles in fuel mixtures can improve combustion efficiency compared to conventional gas. Nitrogen nanobubbles can enhance the hydrolysis of waste-activated sludge and boost methane production during anaerobic digestion. Oxygen nanobubbles facilitate methane generation during the anaerobic digestion of cellulose, while bulk carbon dioxide nanobubbles find applications in the food processing industry. Ozone micro-nanobubbles can enhance ozone mass transfer, achieving high levels of dissolved ozone concentration in the water phase and extending the reactivity of ozone in that phase. They are also extensively utilized for breaking down organic contaminants.

The influence of salt on the characteristics of nanobubbles over time was examined and analyzed. The extended DLVO theory was utilized to interpret and elaborate on our experimental results concerning the stability of nanobubbles. Nanobubbles improve the membrane's capability to filter out salt and contaminants. The fourth point focuses on lowering chemical reliance; the effectiveness of nanobubbles in purification reduces the need for chemical additives typically utilized in conventional desalination. This decrease in chemical reliance leads to cost reductions and diminishes the environmental impact of the water treatment procedure. In conclusion, these tiny bubbles play a crucial role in the progress of desalination technology.

 

 

 

Comments on the Quality of English Language

 

• Line 17: The term "salt water" should be replaced with "salt" in the sentence:
  “Commonly, salt water and heavy metal ions are dissolved in the water, and suspended solids exist to form as sediment.”

Response: We have revised the term “salt water” to “salt”. Thank you for your comment.

 

• Line 43: The sentence “Population growth, economic development, and increased freshwater consumption have exacerbated this problem.” is almost a repetition of Line 32 and should be revised to avoid redundancy.

Response: We have revised to avoid redundancy. As you advised, we deleted it and made a good statement. Thank you for your comment.

 

• Line 52: When categorizing desalination technologies, Reverse Osmosis (RO) and Electrodialysis (ED) fall under electrically driven processes, while Mechanical Vapor Compression (MVC) should be mentioned under mechanically driven processes.

Response: We have revised and mentioned the Mechanical Vapor Compression under the mechanically driven processes.

 

• Line 123: The phrase should read as “used in seawater desalination for drinking water production.”

Response: We have revised and changed the expression as you advised.

 

• Line 178: The phrase should be revised to “the conversion of seawater to drinking water.”

Response: We have revised and changed the expression conversion of seawater to drinking water.

 

• Line 184: The sentence “In addition, every kind of gas nanobubble has its application not only for seawater desalination but also for other applications.” lacks clarity and proper sentence structure. A revision is needed.

Response: We have revised and changed the expression as you advised.

 

• Line 397: The statement “This review article requires advantages, especially for ion flotation.” is unclear and must be rewritten for coherence.

Response: We have revised and changed the following statement,” This review article highlights the advantages of seawater desalination, particularly for ion exchange flotation. Ion flotation is one potential technique to remove hazardous ions from drinking water at low-level concentrations. It is environmentally friendly and biodegradable.”

 

• Line 779: “Confronting these intricate challenges is pivotal to establishing a solid theoretical foundation for the large-scale implementation of bulk nanobubbles in industrial settings. In industrial issues, it might be separated the heavy metal ions in the contaminant water.” is unclear and should be restructured for readability.

Response: We have revised and changed the following statement,” The ion flotation derives from the mineral separation industry. This technique can remove organic and inorganic contaminants from wastewater in anionic or cationic forms. Currently, ion flotation is in use for the recovery of precious metals, ion separation, and wastewater treatment because of its low-cost ancillary devices, flexibility, low energy consumption, and a negligible amount of sludge.

 

• Line 792: The phrase “Ion flotation has been used for the recovery of precious metals because it plays in the ionic ode and has physical and chemical interactions to separate the salt and metals ionic which are used the surfactants, as well as its characteristics and factors, are mentioned in the section the effect of surfactants on the ion flotation.” is not comprehensible and requires substantial revision.

Response: We have revised and changed the following statement,” Ion flotation has been utilized to recover metals including arsenic, mercury, and lead. It functions in ionic mode and involves physical and chemical interactions to separate ionic salts and metals through surfactants. The section regarding the impact of surfactants on ion flotation explores its characteristics and influencing factors.

• Line 800: The term “seawater conversion” is ambiguous—does the author mean “seawater desalination”? Please clarify.

Response: We have revised the term “seawater conversion " to “ seawater desalination. Thank you for your a

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Perhaps I was not clear in my previous review: I do not think that nanobubbles can desalinate seawater (which means that they can reduce the TDS from 35-40 g/L to ~0.3 g/L). This review has not convinced me differently (in fact, to the contrary). I think that this is a decent review on water treatment mechanisms through the application of NB, but it has nothing to do with the word "desalination", and even less than that with the phrase "seawater desalination". I would thus ask to change the title and simply call it a NB/water treatment review. The way it is now, is is misleading and utterly incorrect. Removing heavy metals from water through coagulation and flotation is NOT desalination.

Author Response

Point 1: Perhaps I was not clear in my previous review: I do not think that nanobubbles can desalinate seawater (which means that they can reduce the TDS from 35-40 g/L to ~0.3 g/L). This review has not convinced me differently (in fact, to the contrary). I think this is a decent review on water treatment mechanisms through applying NB. Still, it has nothing to do with the word "desalination", and even less than that with the phrase "seawater desalination". I would thus ask to change the title and simply call it a NB/water treatment review. The way it is now, is misleading and utterly incorrect. Removing heavy metals from water through coagulation and flotation is NOT desalination.

 

Response 1: We would like to thank reviewer for providing valuable comments and suggestions to improve the quality of the manuscript.

I agree that this is something strange but it has been changed the terminology of the seawater treatment process. Again, thank you for critically supporting details on my manuscript.

We added/eliminated the words, statements, paraphases and conjungtions with some revisions about the entire of manuscripts. For the word “desalination”, we have changed including the tittle and it simply call as follows: A Critical Review of Nanobubbles flotation for seawater treatment process. Overall, we have done the revision as follow as in the separated documents, which is written in the manuscript.

Author Response File: Author Response.pdf