Efficiency of Advanced Oxidation Processes for Treating Wastewater from Lithium-Ion Battery Recycling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The Organic materials in water by decomposing LIBs were well treated and studied by comparing with UV/H2O2, Fenton process and electrochemical oxidation.

Please check the following points.

 

Please write OH^o is hydroxyl radical in the sentence.

 

In line 51, please write the reference of AOP.

 

In Figure .2, please show the scale of equipment.

Also, the right letters of Figure b is missing from the manuscript.

 

In Figure 3,4 and 5, please show the error bars for measured plots.

 

In Figure 4. Please write the name of X axis.

 

In all sentences, please write the subscript or superscript. For example, FeSO4 ∙ 7 H2O

 

Please write the schematic graph of electrochemical oxidation equipment.

 

Please write the numbers in Table 3 to two decimal places.

 

In Figure 6, please write the target material and voltage of X ray.

 

Please use the capital letters for the first letter of Figure’s explanation.

Please check the English sentences by the native speaker.

 

In the economical consumption uses €/m3.　Please　write　the used year　for　the evaluation.

 

Please write the non-abbreviation word for the first appearance word. For example, HPLC/DAD , TOC, FSI,

 

Please move the “4.1. Electrochemical Oxidation” section in front of Conclusion. And please modify the conclusion. It is more understandable.

Comments on the Quality of English Language

Please correct some.

Author Response

Reviewer 1:

1. Please write OH• as hydroxyl radical in the sentence. (Line 59)

The term hydroxyl radical (OH•) has been written out at its first occurrence and clarified accordingly in line 59.

2. In line 51, please write the reference of AOP. (Line 49)

A suitable literature reference for advanced oxidation processes (AOPs) has been added at the first mention in line 49.

3. In Figure 2, please show the scale of equipment.

The dimensions of the electrohydraulic fragmentation (EHF) equipment have been added to the material and methods section (line 145).

4. Also, the right letter “b” of Figure 2 is missing from the manuscript.

The labeling of Figure 2 has been corrected, and the subfigure designation “b” has been added.

5. In Figure 4, please write the name of the X axis.

The X-axis label has been added to Figure 4 to ensure clarity and readability.

6. In all sentences, please write the subscript or superscript (e.g., FeSO₄·7H₂O).

All chemical formulas have been checked and corrected to ensure consistent and proper use of subscripts and superscripts throughout the manuscript.

7. Please write the schematic graph of electrochemical oxidation equipment.

A schematic illustration of the electrochemical oxidation setup has been added to the Appendix as Figure A1 and is referenced in the manuscript.

8. Please write the numbers in Table 3 to two decimal places.

All numerical values in Table 3 have been revised and formatted to two decimal places.

9. In Figure 6, please write the target material and voltage of X-ray.

Information on the target material (cobalt) and operating voltage has been added Figure description. 

10. Please use capital letters for the first letter of Figure explanations.

All figure captions have been revised to ensure capitalization of the first letter in accordance with journal style.

11. Please check the English sentences by a native speaker.

The manuscript has been thoroughly revised for grammar, style, and clarity, with particular attention to sentence structure, punctuation, and consistency.

12. In the economic consumption, €/m³ is used. Please write the year used for the evaluation.

The reference year for the economic evaluation has been added to the materials section (line 175).

13. Please write the non-abbreviated term at first appearance (e.g., HPLC/DAD, TOC, FSI).

All abbreviations are now written out in full at their first occurrence. TOC and HPLC are defined at first mention (lines 112 and 117, respectively). FSI has been removed from the manuscript.

14. Please move the “4.1. Electrochemical Oxidation” section in front of the Conclusion and modify the Conclusion.

The Conclusion has been rewritten for improved clarity and readability, and the Outlook section has been renamed and shortened.

Reviewer 2 Report

Comments and Suggestions for Authors

In the manuscript Efficiency of Advanced Oxidation Processes for Treating Wastewater from Lithium-Ion Battery Recycling, the authors compared three advanced oxidation processes: oxidation with ultraviolet light and hydrogen peroxide (UV/H₂O₂), the Fenton process, and electrochemical oxidation to treat the LIB recycling process water. After the comparison, the electrochemical oxidation process superior than the rest two in terms of efficiency, chemical cost, and performance. While authors clearly illustrate their design of experiments, problems raised while I reviewing the manuscript.

(1), There is no SI affiliated with the manuscript at all.

(2), There are too many typos regarding the units and punctuation marks, together with mistakes of wrong section numbers, incomplete figures, and leftover nonsense template contents.

(3), Why the main element of the precipitate is calcium and there is no lithium?

(4), “The experiment with 3g/L was aborted prematurely” Why?

(5), What is the pH during the UV/H2O2 reaction? Is this pH the best for UV/H2O2 reaction?

(6), How fair that you compare the UV/H2O2 process at different wastewater treatments to this research?

(7), You claimed that part of the UV energy is wasted on heating the water during the process. How does the temperature of reaction system affect the removal efficiency?

(8), You claimed that the expensive glass coating is a must so the process is unfavored. Why not use cheap polymer composite materials?

(9), The conclusion part is unorganized and wordy. It must be rewritten.

Author Response

(1), There is no SI affiliated with the manuscript at all.

We apologize for the missing Supplementary Information in the initial submission. As this was our first submission to an MDPI journal, we did not fully realize that the Supplementary Information needed to be explicitly referenced and integrated into the manuscript, even though the files were uploaded separately during the submission process.

We sincerely apologize for this oversight and would like to thank Reviewer II for pointing this out. The Supplementary Information has now been properly included (with the exception of Excel file) and clearly referenced in the revised manuscript.

(2), There are too many typos regarding the units and punctuation marks, together with mistakes of wrong section numbers, incomplete figures, and leftover nonsense template contents.

Thank you for pointing this out. We sincerely apologize for the number of typographical and formatting issues in the original submission. All units, punctuation marks, and section numbering have been carefully checked and corrected throughout the manuscript. In addition, incomplete figures, incorrect cross-references, and leftover template or placeholder content have been removed or replaced.

The manuscript has been thoroughly revised to ensure consistency with journal formatting requirements and to improve overall clarity and readability. We appreciate the reviewer’s careful reading and helpful feedback, which significantly improved the quality of the revised version.

(3), Why the main element of the precipitate is calcium and there is no lithium?

Thank you for this interesting question. The predominance of calcium in the precipitate and the absence of lithium are mainly related to the very different solubilities of their respective carbonate compounds. Calcium carbonate (CaCO₃) has a very low solubility in water (approximately 14 mg/L at 20 °C), which favors its precipitation under the applied conditions. In contrast, lithium carbonate (Li₂CO₃) is highly soluble (approximately 13.3 g/L at 20 °C) and therefore remains in the aqueous phase rather than forming a solid precipitate.

As a result, calcium preferentially precipitates as CaCO₃, while lithium stays dissolved in the treated wastewater.

(4), “The experiment with 3g/L was aborted prematurely” Why?

Thank you for this question. The experiment at an H₂O₂ concentration of 3 g/L was aborted prematurely because the sample was accidentally destroyed during handling. A repetition of the experiment was not possible, as a limited amount of real process wastewater was available and had been fully consumed by the end of the experimental campaign.

Based on the observed trend prior to termination, no significant additional insights were expected from continuing this experiment, as the degradation rate was clearly lower than for higher H₂O₂ concentrations. We have clarified this point in the revised manuscript.

(5), What is the pH during the UV/H2O2 reaction? Is this pH the best for UV/H2O2 reaction?

The UV/H₂O₂ experiments were conducted at pH 4, which represents a compromise between favorable conditions for hydroxyl radical formation and operational constraints related to material compatibility and fluoride hydrolysis. Although UV/H₂O₂ oxidation can be effective over a wider pH range, a systematic pH optimization was beyond the scope of this study.

(6), How fair that you compare the UV/H2O2 process at different wastewater treatments to this research?

Thank you for raising this point. We acknowledge that direct quantitative comparisons between UV/H₂O₂ oxidation applied to different types of wastewater are inherently limited, as wastewater composition, initial COD levels, target removal efficiencies, and reactor configurations vary significantly across studies.

The literature comparison in this work is therefore intended to provide an order-of-magnitude context for energy and oxidant demand rather than a direct performance benchmark. To ensure transparency, we explicitly discuss the wide ranges reported in the literature and emphasize that meaningful conclusions can only be drawn from experiments conducted under comparable boundary conditions. This limitation has been clearly stated in the manuscript, and the comparison is used solely to contextualize the observed energy and chemical requirements of LIB recycling wastewater relative to other industrial effluents.

(7), You claimed that part of the UV energy is wasted on heating the water during the process. How does the temperature of reaction system affect the removal efficiency?

Thank you for this comment. In the present study, the reaction system was actively cooled, and the bulk temperature remained close to ambient conditions during UV/H₂O₂ oxidation. Therefore, temperature was not used as an independent process parameter, and its direct influence on removal efficiency was not systematically investigated.

The statement regarding energy losses refers to the strong absorption of UV radiation by the wastewater matrix at 254 nm, which leads to non-photochemical energy dissipation as heat rather than to hydroxyl radical generation. While elevated temperatures can, in principle, enhance reaction kinetics or H₂O₂ decomposition, this effect is typically secondary compared to photon availability in UV-based AOPs. Under the conditions applied here, increasing temperature would not compensate for the limited photon utilization caused by matrix absorption and would further increase overall energy demand.

We have clarified this aspect in the revised manuscript to emphasize that the observed efficiency limitations are primarily related to optical absorption rather than thermal effects.

 

(8), You claimed that the expensive glass coating is a must so the process is unfavored. Why not use cheap polymer composite materials?

Thank you for this valuable suggestion. Polymer-based or composite materials may indeed represent potential alternatives for UV reactor components and could reduce material costs in certain applications. However, under the operating conditions of UV/H₂O₂ oxidation, particularly at 254 nm and in the presence of oxidizing agents and fluoride-containing species, the long-term stability of many polymeric materials remains uncertain. UV-induced degradation and chemical aging may lead to material embrittlement or particle release, which could pose an additional risk of secondary contamination.

As a systematic assessment of material durability, potential microplastic formation, and long-term performance was beyond the scope of this study, and quantitative cost data were not available, we decided to remove the corresponding statement from the manuscript. We believe that a dedicated material compatibility study would be required to reliably evaluate polymer-based solutions for this application.

(9), The conclusion part is unorganized and wordy. It must be rewritten.

The conclusion was rewritten reorganized and shortend.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript is scientifically strong and addresses an important and timely topic related to wastewater treatment in LIB recycling. The comparative evaluation of three AOPs is well designed, and the results are supported by experimental data. However, several revisions are needed to improve clarity, readability, presentation quality, and depth of discussion. Please address the points below:

1. Avoid using bullet points in the abstract. The abstract should be written as a single, coherent paragraph that summarizes the study.
2. The manuscript contains several grammatical errors, overly long sentences, and instances of awkward phrasing (such as misuse of “were” vs. “where,” missing articles, and repeated words).
3. In Sections 3.2 and 3.3, TOC removal approaches 100%, while COD removal plateaus at about 70%. Please provide a clearer mechanistic explanation for this trend, particularly how inorganic species contribute to COD during dichromate digestion and why this results in lower COD removal despite nearly complete TOC removal.
4. Several figures, especially Figure 3, appear to have low resolution. Please improve the figure quality to ensure readability.
5. The manuscript identifies CaCO₃ precipitation, but the implications for long-term system operation are not sufficiently discussed. Please expand on potential scaling challenges with BDD electrodes, including impacts on mass transfer, efficiency, and maintenance requirements.
6. Please include a brief explanation of why higher current densities lead to salt precipitation, such as effects related to local pH changes, ion migration, or supersaturation near the electrode surface.

 

Author Response

1. Avoid using bullet points in the abstract. The abstract should be written as a single, coherent paragraph that summarizes the study.

Thank you for this suggestion. The abstract has been revised and is now presented as a single, coherent paragraph without bullet points, in accordance with the journal’s guidelines.

2. The manuscript contains several grammatical errors, overly long sentences, and instances of awkward phrasing (such as misuse of “were” vs. “where,” missing articles, and repeated words).

Thank you for this comment. The manuscript has been thoroughly revised to correct grammatical errors, reduce overly long sentences, and improve clarity and readability. Specific issues such as incorrect word usage (e.g., “were” vs. “where”), missing articles, awkward phrasing, and repeated words have been carefully addressed throughout the text. We appreciate the reviewer’s careful reading, which helped improve the overall quality of the manuscript.

3. In Sections 3.2 and 3.3, TOC removal approaches 100%, while COD removal plateaus at about 70%. Please provide a clearer mechanistic explanation for this trend, particularly how inorganic species contribute to COD during dichromate digestion and why this results in lower COD removal despite nearly complete TOC removal.

Thank you for this important comment. The apparent discrepancy between nearly complete TOC removal and a COD removal plateau at approximately 78% can be explained by the contribution of inorganic species to the dichromate-based COD measurement. While TOC selectively quantifies organic carbon, the COD test also accounts for the oxidation of reduced inorganic compounds that act as electron donors under the strongly oxidative digestion conditions.

In the investigated LIB recycling wastewater, inorganic species such as reduced phosphorus and sulfur compounds, likely originating from the hydrolysis and degradation of electrolyte conducting salts (e.g., PF₆⁻ and related intermediates), can be oxidized during dichromate digestion and are therefore detected as oxygen demand, despite not containing organic carbon. In addition, carbonate and bicarbonate species may partially contribute to COD under acidic digestion conditions, particularly when inorganic carbon is mobilized or precipitated during treatment.

As a result, even when organic carbon is almost completely removed, a residual COD remains due to the oxidation of these inorganic constituents. A corresponding clarification has been added to the Discussion section of the revised manuscript to better explain this mechanistic difference between TOC and COD behavior (see line 246-250).

4. Several figures, especially Figure 3, appear to have low resolution. Please improve the figure quality to ensure readability.

Thank you for this comment. All figures, including Figure 3, have been revised and replaced with high-resolution versions to ensure improved readability. Axis labels, legends, and symbols have been checked and optimized in accordance with the journal’s requirements.

5. The manuscript identifies CaCO₃ precipitation, but the implications for long-term system operation are not sufficiently discussed. Please expand on potential scaling challenges with BDD electrodes, including impacts on mass transfer, efficiency, and maintenance requirements.

Thank you for this important comment. The implications of CaCO₃ precipitation for long-term system operation have been expanded in the Discussion section of the revised manuscript. We now explicitly address potential scaling on boron-doped diamond (BDD) electrodes and downstream components, including its effects on the electrochemically active surface area, mass transfer limitations, cell voltage, and overall process efficiency.

In addition, maintenance aspects are discussed, highlighting that electrode scaling can increase cleaning requirements but can be effectively mitigated in electrochemical systems by operational strategies such as periodic polarity reversal and regular rinsing, which enable in situ removal of mineral deposits. These additions clarify how CaCO₃ precipitation may influence long-term performance and how associated operational challenges can be managed in future pilot- and full-scale applications. This has been included in the manuscript in line 388-392.

6. Please include a brief explanation of why higher current densities lead to salt precipitation, such as effects related to local pH changes, ion migration, or supersaturation near the electrode surface.

Thank you for this suggestion. A brief mechanistic explanation has been added to the Discussion section. At higher current densities, intensified electrochemical reactions generate pronounced local pH gradients at the electrode surfaces, particularly due to cathodic hydroxide formation. Simultaneously, enhanced ion migration increases the local concentrations of calcium and carbonate species near the electrodes, which promotes supersaturation and favors CaCO₃ precipitation (line 385-388)

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript was well corrected.

This article should be published.

Comments on the Quality of English Language

Please correct some.

Reviewer 2 Report

Comments and Suggestions for Authors

Accept in present form