Unraveling the Impact of Copper Ions on Mineral Surfaces During Chalcopyrite–Molybdenite Flotation Separation Using Sodium Thioglycolate

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this manuscript, the authors studied the effect of copper ions on the selective flotation of molybdenite from chalcopyrite. In this study, sodium thioglycolate (STG) was used as the chalcopyrite depressant in selective moly flotation, and the authors demonstrated that moly flotation was significantly affected by the co-present of sodium thioglycolate and a sound amount of copper ion concentration. Then, the authors employed a series of characterisation methods, such as contact angle measurement, zeta potential measurement, UV-Vis analysis and XPS analysis to identify the chemisorption of STG and copper onto moly surface in the form of a stable molybdenite–Cu(I)–STG complex. The authors also ran a DFT simulation to demonstrate the enhanced reactivity between STG and moly surface by copper ions.

The copper adsorption on the moly surface in fact was studied previously on where sodium sulphide is used as copper depressant rather than STG. In moly circuit, sodium sulphide and sodium hydrosulphide are more commonly used compared to STG. Nevertheless, this is still an informative study for moly-copper circuit operator overall. The testworks were carried out systematically. The ‘promoting’ role of copper for the adsorption of STG to moly surface is quite an interesting finding.  

There are some parts to be clarified, which I have included in my comments below:

1. In the second last paragraph in the introduction, the research gaps on the effect of copper ions on selective flotation of moly in the presentation of chalcopyrite should be emphasised to the use of STC to distinguish the innovation of this work as compared to others.
2. Since in this moly system, STC was the key focus, while STC is not a very common reagent for chalcopyrite depressant in moly flotation. I would suggest clarifying/emphasising this in the title of the manuscript, so that the reader would know the authors are investigating the effect of copper ions on moly flotation when STC is in-use.

 

Author Response

Dear editor and reviewers,

Thank you very much for your attention and the referee’s evaluation and comments on our paper entitled “Unraveling the impact of copper ions on mineral surfaces during chalcopyrite-molybdenite flotation separation” (ID: applsci-3672766). Those comments are all valuable and very helpful for revising and improving our paper, as well as of important guiding significance to our researches. We have studied the comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as following:

Responds to the reviewer’s comments:

 

Reviewer #1: Reviewer's Comments and Suggestions

Comments 1: In the second last paragraph in the introduction, the research gaps on the effect of copper ions on selective flotation of moly in the presentation of chalcopyrite should be emphasised to the use of STC to distinguish the innovation of this work as compared to others.

Response 1: Dear reviewer, thank you for your valuable comments. We have revised the second last paragraph of the introduction to more clearly highlight the research gap regarding the effect of copper ions on the selective flotation of molybdenite in the presence of chalcopyrite, under conditions where sodium thioglycolate is used as a depressant.

 

Comments 2: Since in this moly system, STC was the key focus, while STC is not a very common reagent for chalcopyrite depressant in moly flotation. I would suggest clarifying/emphasising this in the title of the manuscript, so that the reader would know the authors are investigating the effect of copper ions on moly flotation when STC is in-use.

Response 2: Dear reviewer, thank you for your valuable comments. We agree that highlighting the role of sodium thioglycolate in the title would better reflect the novelty and focus of this study. Accordingly, we have revised the manuscript title to explicitly indicate the use of sodium thioglycolate in the flotation system. The revised title is: “Unraveling the impact of copper ions on mineral surfaces during chalcopyrite–molybdenite flotation separation using sodium thioglycolate.”

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for your submission. I would like to highlight a few methodological concerns and areas for improvement:

1. The use of DPPC and DOPC as stratum corneum (SC) surrogates is overly simplistic. The SC lipid matrix is primarily composed of ceramides, cholesterol, and free fatty acids, whereas phosphatidylcholines are more representative of cellular membranes than of the SC.
2. The monolayer studies were conducted at 20°C, which is significantly below the physiological skin temperature (32–37°C). This discrepancy can substantially influence lipid packing and phase behavior.
3. The attribution of specific interactions (e.g., hydrogen bonding) to the presence of PG/PGME appears speculative without proper reference controls, such as spectra of PG/PGME alone or lipid monolayers without enhancers.
4. The use of static, gas-phase DFT cluster models is a considerable oversimplification of membrane dynamics. These models are not suitable for inferring macroscale behavior such as permeability or membrane disruption.
5. Additionally, there is no mention of a solvent model, BSSE correction, or validation against experimental data, which undermines the reliability of the quantum chemical results.
6. The proposed mechanism of interaction remains partially addressed and insufficiently supported, particularly due to the lack of exploration of key processes such as:
   1. Lipid extraction
   2. Bilayer thinning
   3. Solvent-induced phase separation
   4. Water content modulation

Addressing these aspects would significantly strengthen the physiological relevance and scientific rigor of the study.

Many thanks,

Author Response

Dear editor and reviewers,

Thank you very much for your attention and the referee’s evaluation and comments on our paper entitled “Unraveling the impact of copper ions on mineral surfaces during chalcopyrite-molybdenite flotation separation” (ID: applsci-3672766). Those comments are all valuable and very helpful for revising and improving our paper, as well as of important guiding significance to our researches. We have studied the comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as following:

Responds to the reviewer’s comments:

 

Reviewer #2: Reviewer's Comments and Suggestions

Comments 1: The use of DPPC and DOPC as stratum corneum (SC) surrogates is overly simplistic. The SC lipid matrix is primarily composed of ceramides, cholesterol, and free fatty acids, whereas phosphatidylcholines are more representative of cellular membranes than of the SC.

Comments 2: The monolayer studies were conducted at 20°C, which is significantly below the physiological skin temperature (32–37°C). This discrepancy can substantially influence lipid packing and phase behavior.

Comments 3: The attribution of specific interactions (e.g., hydrogen bonding) to the presence of PG/PGME appears speculative without proper reference controls, such as spectra of PG/PGME alone or lipid monolayers without enhancers.

Comments 4: The use of static, gas-phase DFT cluster models is a considerable oversimplification of membrane dynamics. These models are not suitable for inferring macroscale behavior such as permeability or membrane disruption.

Comments 5: Additionally, there is no mention of a solvent model, BSSE correction, or validation against experimental data, which undermines the reliability of the quantum chemical results.

Comments 6: The proposed mechanism of interaction remains partially addressed and insufficiently supported, particularly due to the lack of exploration of key processes such as:

1. Lipid extraction
2. Bilayer thinning
3. Solvent-induced phase separation
4. Water content modulation

Response: Dear reviewer, thanks for your valuable comments. We would like to respectfully clarify that the issues raised in your comment do not appear to be relevant to the focus or content of our manuscript. It is possible that the comment was inadvertently associated with our submission. Nonetheless, we sincerely appreciate your careful review and the time you dedicated to evaluating our work.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for your submission. The topic is indeed of scientific interest. However, I would like to highlight several methodological concerns and areas that would benefit from clarification and further validation.
Please accept my apologies for the previous review, which did not accurately reflect a proper assessment of this manuscript.

1. Assigning Cu(I) at 933.03 eV and Cu(II) at 934.37 eV without accompanying Cu LMM Auger analysis remains inconclusive. Although Cu²⁺ reduction to Cu⁺ on sulfide surfaces is plausible, the absence of characteristic shake-up satellites for Cu(II) undermines the reliability of the oxidation state assignment.
2. The use of CuOH⁺ as the aqueous copper species is not chemically justified. At pH 8, the dominant species are likely to be Cu(OH)₂(aq), Cu₂(OH)₂²⁺, or similar hydrolyzed forms. A clear rationale or speciation diagram should be provided to support this choice.
3. The reported positive adsorption energy for STG (+264.94 kJ/mol) is not only unusually high for surface adsorption (physisorption typically < 50 kJ/mol), but also contradicts the manuscript’s conclusion that "STG cannot adsorb without Cu." This discrepancy suggests potential issues in model stability, energy minimization, or basis set convergence.
4. While the XPS results indicate Cu adsorption and some S–C bond interactions, they do not definitively support the formation of a stable ternary molybdenite–Cu(I)–STG complex. Alternative pathways—such as Cu–STG complexation followed by physical adsorption onto molybdenite—should be explored or ruled out.
5. The interaction model focuses on Cu(I) coordination with thiol (–SH) groups (Figs. 10–11), which is chemically reasonable. However, the role of carboxylate (–COO⁻) moieties, known for metal coordination and surface affinity, is overlooked in the discussion of hydrophilicity changes and adsorption behavior.
6. Although Fig. 5 shows a reduction in surface hydrophobicity, contact angle measurements alone do not demonstrate displacement of kerosene by STG. Adsorption isotherms, surface coverage quantification, or interfacial tension data would help substantiate this claim.
7. There is no discussion of Cu²⁺ hydrolysis (e.g., formation of Cu(OH)⁺ or Cu(OH)₂) or potential STG–Cu complexation in solution, both of which could significantly influence adsorption behavior. The use of UV-Vis spectroscopy provides data on residual STG concentration but not on chemical speciation.
8. The study focuses exclusively on molybdenite, yet Fig. 4b briefly mentions the impact of Cu²⁺ on chalcopyrite depression. This point is dismissed without sufficient experimental support or surface characterization, which limits the broader applicability of the findings.
9. The –SH group in STG is prone to oxidation, particularly in the presence of Cu²⁺. This factor could critically affect the stability and performance of the depressant, but is not discussed.

Addressing these issues would significantly enhance the scientific rigor and practical relevance of the study. I encourage the authors to consider these aspects carefully in a revised version.

Many thanks and best regards,

Author Response

Dear editor and reviewers,

Thank you very much for your attention and the referee’s evaluation and comments on our paper entitled “Unraveling the impact of copper ions on mineral surfaces during chalcopyrite-molybdenite flotation separation” (ID: applsci-3672766). Those comments are all valuable and very helpful for revising and improving our paper, as well as of important guiding significance to our researches. We have studied the comments carefully and have made correction which we hope meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer’s comments are as following:

Responds to the reviewer’s comments:

 

Reviewer #2: Reviewer's Comments and Suggestions

Comments 1: Assigning Cu(I) at 933.03 eV and Cu(II) at 934.37 eV without accompanying Cu LMM Auger analysis remains inconclusive. Although Cu²⁺ reduction to Cu⁺ on sulfide surfaces is plausible, the absence of characteristic shake-up satellites for Cu(II) undermines the reliability of the oxidation state assignment.

Response 1: Dear reviewer, thank you for your valuable comments. We agree that it is indeed difficult to accurately distinguish between Cu(I) and Cu(II) based solely on the Cu 2p3/2 binding energies at 933.03 eV and 934.37 eV, particularly in the absence of Cu LMM Auger analysis. However, the assignment of these peaks in our study was made with reference to previously published literature (Minerals Engineering 2018, 115, 44-52; Minerals Engineering 2019, 130, 101-109). In addition, we re-fitted the Cu 2p spectra and clearly observed representative Cu(II) satellite features in the 940–945 eV region, providing strong evidence for the presence of Cu(II).

Figure 1 Cu 2p XPS spectra of molybdenite: (a) untreated, (b) exposed to Cu²⁺, (c) exposed to STG, and (d) exposed to both Cu²⁺ and STG.

 

Comments 2: The use of CuOH+ as the aqueous copper species is not chemically justified. At pH 8, the dominant species are likely to be Cu(OH)₂(aq), Cu₂(OH)₂²⁺, or similar hydrolyzed forms. A clear rationale or speciation diagram should be provided to support this choice.

Response 2: Dear reviewer, thank you for your attention to the hydrolyzed species of copper ions. For the 8 mg/L Cu²⁺ solution system, we conducted a speciation analysis and have included the corresponding aqueous chemistry diagram in the revised manuscript (Figure 2). The calculation results indicate that CuOH⁺ is the predominant species at around pH 8, where its relative abundance reaches a maximum. In contrast, Cu(OH)₂(aq) becomes the dominant species only at a higher pH range (approximately 9.2). Therefore, considering CuOH⁺ as the primary hydrolyzed species under pH 8 conditions is both reasonable and theoretically justified. The relevant figure has been incorporated into the main text to further support and strengthen the discussion.

Figure 2 Distribution diagrams of 8mg/L Cu²⁺ solution as a function of pH.

 

Comments 3: The reported positive adsorption energy for STG (+264.94 kJ/mol) is not only unusually high for surface adsorption (physisorption typically < 50 kJ/mol), but also contradicts the manuscript’s conclusion that "STG cannot adsorb without Cu." This discrepancy suggests potential issues in model stability, energy minimization, or basis set convergence.

Response 3: Dear reviewer, thank you for your valuable comments. According to the DFT calculation results, the adsorption energy of STG on the untreated molybdenite surface is highly positive (+264.94 kJ/mol), indicating a significant repulsive interaction between STG and molybdenite. However, after the mineral surface is modified by copper ions, the adsorption energy of STG becomes negative, suggesting that effective interaction between STG and molybdenite only occurs in the presence of copper ions. This result is consistent with the conclusion presented in our manuscript that “STG cannot be adsorbed on the hydrated molybdenite (001) surface when no Cu ions are present.”

 

Comments 4: While the XPS results indicate Cu adsorption and some S–C bond interactions, they do not definitively support the formation of a stable ternary molybdenite–Cu(I)–STG complex. Alternative pathways—such as Cu–STG complexation followed by physical adsorption onto molybdenite—should be explored or ruled out.

Response 4: Dear reviewer, thank you for your valuable comments. We conducted additional single-mineral flotation experiments, in which Cu²⁺ was pre-mixed with STG before being introduced into the flotation system, to investigate the effect of Cu–STG complexation on the flotation behavior of molybdenite. The recovery of molybdenite remained at 91.79%, indicating that the pre-formed Cu–STG complex did not exhibit any inhibitory effect on molybdenite, and suggesting that this complex does not adsorb onto the mineral surface. Therefore, the mechanism by which copper ions and STG influence molybdenite flotation is likely as follows: copper ions first adsorb onto the molybdenite surface, providing active sites that facilitate subsequent interaction with STG, ultimately leading to the formation of a molybdenite–Cu(I)–STG complex.

Figure 3 Different Interaction Modes with Molybdenite.

 

Comments 5: The interaction model focuses on Cu(I) coordination with thiol (–SH) groups (Figs. 10–11), which is chemically reasonable. However, the role of carboxylate (–COO⁻) moieties, known for metal coordination and surface affinity, is overlooked in the discussion of hydrophilicity changes and adsorption behavior.

Response 5: Dear reviewer, thank you for your insightful suggestion. Due to the relatively stronger interaction capacity of –SH group with metal sites, the current research primarily focused on the coordination mechanism between –SH groups and Cu(I). Additionally, we also fully agree that the –COO^– group possesses certain metal-coordinating capabilities and exhibits affinity toward mineral surfaces, potentially playing a role in regulating adsorption behavior and surface hydrophobicity. To improve the accuracy of our experimental results and the comprehensiveness of our conclusions, we have revised the manuscript to refine the proposed mechanism involving –SH groups and have also included the potential contribution of –COO^– groups.

 

Comments 6: Although Fig. 5 shows a reduction in surface hydrophobicity, contact angle measurements alone do not demonstrate displacement of kerosene by STG. Adsorption isotherms, surface coverage quantification, or interfacial tension data would help substantiate this claim.

Response 6: Dear reviewer, thank you for your valuable comments. We have revised the relevant statements in the manuscript to clarify that the reduction in the contact angle of the molybdenite surface only reflects changes in surface hydrophobicity and should not be interpreted as direct evidence of reagent adsorption or displacement of kerosene action. We apologize for any ambiguity caused by the original wording and sincerely appreciate your constructive suggestion.

 

Comments 7: There is no discussion of Cu²⁺ hydrolysis (e.g., formation of Cu(OH)⁺ or Cu(OH)₂) or potential STG–Cu complexation in solution, both of which could significantly influence adsorption behavior. The use of UV-Vis spectroscopy provides data on residual STG concentration but not on chemical speciation.

Response 7: Dear reviewer, thank you for your valuable comments. We acknowledge that Cu²⁺ may undergo hydrolysis or form complexes with STG in solution, and these processes could influence its adsorption behavior. To explore this, we measured the UV-Vis spectra of a solution containing only Cu²⁺ and STG. The results showed that the residual concentration of STG remained at 61.59% in the presence of Cu²⁺, indicating that copper species in solution can interact with a portion of the STG. However, when Cu²⁺ and STG were sequentially added to a molybdenite suspension, the residual STG concentration dropped to 20.81%, suggesting that Cu²⁺ significantly promotes the adsorption of STG onto the molybdenite surface, which supports our proposed conclusion. Furthermore, as mentioned in our response to Comment 4, we also investigated the effect of Cu–STG complexation on molybdenite flotation. The results demonstrated that the Cu–STG complex had no significant impact on molybdenite flotation performance, further validating the interaction mechanism we proposed.

 

Comments 8: The study focuses exclusively on molybdenite, yet Fig. 4b briefly mentions the impact of Cu²⁺ on chalcopyrite depression. This point is dismissed without sufficient experimental support or surface characterization, which limits the broader applicability of the findings.

Response 8: Dear reviewer, thank you for your valuable comments. In the original description of the results in Fig. 4b, our intention was to emphasize that chalcopyrite remained strongly depressed under the combined action of Cu²⁺ and STG, suggesting that Cu²⁺ had a negligible effect on the depressive behavior of STG toward chalcopyrite. To eliminate ambiguity, we have revised two relevant sentences in the manuscript as follows:

1. “The sequential addition of Cu²⁺ and STG still remained the chalcopyrite recovery below 10%, indicating Cu²⁺ negligibly influenced the depression behavior of STG for chalcopyrite.”
2. “These findings revealed that the presence of Cu²⁺ in the STG system significantly suppressed the flotation performance of molybdenite, affecting the separation efficiency between molybdenite and chalcopyrite.”

 

Comments 9: The –SH group in STG is prone to oxidation, particularly in the presence of Cu²⁺. This factor could critically affect the stability and performance of the depressant, but is not discussed.

Response 9: Dear reviewer, thank you for your professional suggestion. We agree that, in theory, the –SH group has the potential to undergo oxidation in the presence of Cu²⁺. However, under the experimental conditions of this study, the flotation reaction time was relatively short, and most of the Cu²⁺ in solution was reduced to Cu⁺ on the molybdenite surface, which has a certain reducing capability that may help stabilize the –SH group. Moreover, the experimental results showed that the depressive effect of STG was not significantly weakened during flotation process. Its adsorption behavior, contact angle variation, and flotation selectivity all exhibited good stability, indicating that the –SH group maintained high activity and functionality in the current system.

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Author,

You have successfully addressed all the requested improvements, and I sincerely appreciate your effort.

Many thanks again for your hard work.

Cheers