Presence of Humic Acids in Landfill Leachate and Treatment by Flocculation at Low pH to Reduce High Pollution of This Liquid

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the properties of humic acid in garbage filtrate and the method of changing pH were used to treat high-pollution garbage leachate by flocculation. The method and treatment technology have certain novelty and practicability. Various characterization techniques were used to verify the humic acid recovered from flocculation. A major repair is recommended, and the following issues are supplemented.

1.     The color of the filtrate varies with the different organic matter (inorganic matter) in the filtrate. How to ensure the consistency of the treatment process by spectrophotometry?

2.     The content of Cl ^- in the filtrate is very high, how to avoid the interference of Cl ^- in the determination of COD?

3.     What are the differences in preparation between HA1 and HA2 samples? Why two samples? Please add clarification in the text.

4.     Please add the conditions and effects of different batches of waste filtrate treatment to explain the effectiveness of the technology.

Author Response

Please, find the responses in the attach document (Word)

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

In the paper was presented a new method for the treatment of high polluted landfill leachate consisting in the flocculation at low pH. High polluted landfill leachate has a high proportion of composting leachate (composting of solid wastes), which means the abundance of humic acids in this liquid. In view of the chemical properties in aqueous solutions of these HAs, which are protonated at low pH and are ionized at medium-high pH, landfill leachate is effectively flocculated at pH 2.0. Under acidic conditions, phenolic alcohols present in HAs are protonated at pH be low 5.5 and carboxylic acids at pH below 3.5. The protonation of the functional groups of the HA molecules reduces highly Zeta potential absolute value to zero and conducts to the formation of intermolecular H-bonds, resulting in big aggregates which are unstable in solution and precipitate. During the precipitation process of HAs, other organic molecules are dragged, and COD and color are reduced in high extent (84-86 %). Mean diameter of the molecules has been measured during flocculation at low pH and the formation of big aggregates has been detected at pH 3.0 and 2.0 and also in the precipitate. This precipitate was analyzed by CHNS chemical analysis and infrared spectrometry (FT-IR), resulting very closed in chemical composition to commercial and natural HAs and showing in the FT-IR spectrum significative signals coincident with commercial HA, especially with carboxylic acids and phenolic alcohols which are characteristic of the chemical structure of a humic acid. The formation of intermolecular H-bonds between HAs also has been detected by FT-IR spectrometry.

This is very valuable work. It should be published after minor revisions as below.

 

General Remarks

 

Abstract

Please do not use abbreviation in abstract or provide the full name.

 

Detailed remarks

 

1.      Point 6. …”Patents’…??? Explain please

2.        Figure 2,4,7 should be self-explaining. Please add proper information.

3.        Table 1 should be self-explaining. Please add proper information.

Author Response

Please, read the attach document (Word)

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

After analyzing the manuscript, I believe that the research has the potential to be published in Sustainability, provided that the suggestions and corrections indicated are effectively applied throughout the manuscript, since it is necessary to: a) expand the discussion on scalability, costs and sustainability; b) explore applications in leachate from different sources; 3) address the destination or potential reuse of the waste generated; 4) consolidate redundant points and include explicit limitations in the study and, finally; 5) Update and expand the literature review.

Based on the analysis of the manuscript, I highlight below the main points to be improved that can be adjusted to qualify for publication in Sustainability. To assist the authors, I have separated the adjustments to be made according to the topic addressed in the manuscript, such as: 1) Insufficient discussion on Scalability; 2) Applicability to different types of leachate; 3) insufficient details on costs and sustainability; 4) Limited discussion on precipitate management; 5) Redundancy in the discussion section; 6) Improve discussion of graphs and tables; 7) Inclusion of study limitations; 8) Update of references.

 

1. Insufficient Discussion on Scalability

• Problem: Although the proposed method is promising, there is no detailed discussion on the practical feasibility and scalability of the low pH flocculation process on an industrial scale. The absence of such analysis may limit the impact and practical applicability of the work.

• Suggestion: Include a section or paragraph in the discussion addressing the challenges and requirements for large-scale implementation, such as:

1.       Costs associated with the use of H₂SO₄ in large volumes;

2.       Management of waste generated during flocculation, including precipitates;

3.       Possible impacts on secondary effluent treatment (if any).

 

2. Applicability to Different Types of Leachate

• Problem: The study focuses on compound leachate from the Salamanca Waste Treatment Center, but there is no mention of how the methodology would behave in leachate from different sources (varying chemical compositions). The lack of generalizability can be seen as a limitation.

• Suggestion: Explore, even theoretically, how the method would be adapted to leachates with different levels of organic load, pH or metal composition. Provide more comparative references with other leachates from the literature.

 

3. Insufficient Detailing of Costs and Sustainability

• Problem: The paper claims that flocculation is a cost-effective approach compared to other methods (Fenton oxidation, ozonation, etc.), but does not present quantitative data to justify this claim.

• Suggestion: Add a preliminary analysis of the operational costs of the proposed approach, including:

1. Consumption of reagents (H₂SO₄ and others).

2. Equipment needed for pH and precipitation control.

3. Comparison with costs of conventional treatments.

4. Sustainability assessment considering the waste generated.

 

4. Limited Discussion on Precipitate Management

• Problem: The precipitates formed during flocculation (rich in humic acids) are chemically characterized, but the manuscript does not address the fate or potential reuse of these solids.

• Suggestion: Discuss the possibilities for reusing the precipitates, such as:

1. Use in agriculture (organic amendments).

2. Combustion or energy recovery.

3. Additional treatment for safe disposal.

 

 

 

           

5. Redundancy in Some Parts of the Discussion

• Problem: Certain aspects of flocculation, such as the explanation of protonation of functional groups at low pH and its relationship with zeta potential, are repeated throughout the text. This may hinder the fluidity of the reading.

• Suggestion: Consolidate the chemical explanations in a more succinct manner, maintaining clarity but reducing redundancy.

 

6. Need for Better Exploration of Graphs and Tables

• Problem: Although the data are well presented, the interpretation of some graphs and tables could be further explored. For example, the results on zeta potential and aggregate formation could include more detailed inferences about their practical relevance.

• Suggestion: Add interpretations that link the graphical data to the concrete advantages of the proposed method, reinforcing the impact of the results.

 

7. Inclusion of Limitations in the Study

• Problem: The article does not explicitly address the limitations of the method or potential problems in practical application. This may give the impression that the method is completely free of challenges.

• Suggestion: Dedicate a paragraph in the discussion section to limitations, such as:

1. The need for strict pH control.

2. Generation of potentially corrosive secondary effluents due to the use of H₂SO₄.

3. Limitations in efficiency for leachate with different chemical compositions.

 

8. Outdated or Insufficient References

• Problem: Some references are relatively old (e.g., Christensen et al., 1998), which may weaken the justification for the study. In addition, the literature review on emerging leachate treatment methods could be more comprehensive.

• Suggestion: Incorporate more recent references (last 5 years) that address advances in sustainable leachate treatment methods, such as:

1. Use of natural flocculants.

2. Comparisons with hybrid systems (e.g., biofiltration combined with physicochemical processes).

Author Response

Please, read the attach document for responses and corrections (Word)

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I examined the revised manuscript and felt that the author could solve the reviewer's problem. Therefore, I think this manuscript is now ready for publication.

Author Response

Thank you for reading and improving this manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

After evaluating the adjusted version of the manuscript, I consider it suitable for publication in Sustainability.

Best Regards,

Reviewer

Author Response

Thank you for participating in make this article better. Best regards.