Hierarchical Superwetting ZOMO-PAA@CuC₂O₄ Nanorod-Coated Copper Mesh for Robust and Efficient Oily Wastewater Treatment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. In the Results and Discussion section, the morphology of Na-ZnM ZOMO nanoparticles and the copper mesh should be discussed together, rather than separately introducing the nanoparticles first. The main figure should appear earlier than the supporting figures.

2. When describing the SEM images, quantitative data such as pore size, fiber diameter, etc., of the original copper mesh should be provided, along with specific data such as the length of nanowires on the modified mesh.

3. Figures should be placed after they are mentioned in the text; the current placement of figures is inappropriate. Additionally, the clarity of the figures is low.

4. It is recommended to include a video of the emulsion separation process as supporting material.

Author Response

Reviewer:

1. In the Results and Discussion section, the morphology of Na-ZnM ZOMO nanoparticles and the copper mesh should be discussed together, rather than separately introducing the nanoparticles first. The main figure should appear earlier than the supporting figures.

Response:

We sincerely thank the reviewer for the constructive and valuable comments, which have significantly improved the quality of the manuscript. Accordingly, a combined discussion of the morphology of the Na–ZnM ZOMO nanoparticles and the coated copper mesh has been included. In addition, following the reviewer’s suggestion, the main Figure 1 has been repositioned earlier in the manuscript, ahead of the supporting figures. The corresponding descriptions have updated in the revised manuscript as follows: The morphology of the synthesized ε-Keggin-type Na-ZnM ZOMO nanoparticles were examined by SEM (Fig. S1). SEM imaging (Fig. S1a) shows densely packed, uniformly dispersed sub-micron particles (50–80 nm) without large voids or agglomerates, indicating structural homogeneity (see page 4, lines 154-157).

Reviewer:

2) When describing the SEM images, quantitative data such as pore size, fiber diameter, etc., of the original copper mesh should be provided, along with specific data such as the length of nanowires on the modified mesh.

Response:

We sincerely appreciate the reviewer’s insightful and constructive comments. In response, we have quantitatively analyzed the lengths and diameters of the Cu(OH)₂ nanowires and the diameters of the CuC₂O₄ nanorods using SEM image analysis software. The corresponding data have been provided in the revised manuscript. The corresponding descriptions have been updated in the revised manuscript as follows: The Cu(OH)₂ nanowires have lengths of approximately 5–9 μm and average diameters of 120 ± 30 nm. Subsequent immersion in oxalic acid converts the nanowires into thicker CuC₂O₄ nanorods with an average diameter of 240 ± 45 nm (Fig. 1d₂) (see page 4, lines 151-153).

Reviewer:

3) Figures should be placed after they are mentioned in the text; the current placement of figures is inappropriate. Additionally, the clarity of the figures is low.

Response:

We sincerely appreciate the reviewer’s useful comments. In response, all figures have been relocated to appear after the corresponding text. Moreover, all low-resolution figures have been replaced with high-resolution versions to improve clarity and readability.

Reviewer:

4) It is recommended to include a video of the emulsion separation process as supporting material.

Response:

We are grateful for the reviewer’s insightful comments. In response, a video demonstrating the separation of a surfactant-stabilized oil-in-water emulsion has been included in the revised manuscript. The corresponding descriptions have been updated in the revised manuscript as follows: Moreover, a video demonstrating the separation of a SSE emulsion is provided as ESI Video S1 (see page 9, lines 303-304).

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript reports the fabrication of a hierarchically structured ZOMO–PAA@CuC₂O₄ nanorod-coated copper mesh for oil-in-water emulsion separation. While the topic is relevant and the experimental work is extensive, the manuscript in its current form raises significant concerns regarding novelty, conceptual clarity, data interpretation, and overstatement of performance.

From a conceptual standpoint, the claimed novelty is not convincingly established. The authors state that systematic investigations of ZOMO–PAA@CuC₂O₄ nanorod-based membranes are scarce; however, the manuscript does not critically distinguish this system from a large body of closely related literature, including several prior studies from the same research group that already employ CuC₂O₄ nanostructures on copper meshes combined with highly hydrophilic inorganic coatings for underwater superoleophobic separation. The practical and mechanistic advantage of introducing ZOMO, as opposed to previously reported oxides, MOFs, or tungstate-based systems, is not clearly articulated. What specific property of ZOMO is essential here, beyond general hydrophilicity, and why could similar performance not be achieved with simpler or already established materials?

Throughout the manuscript, there is a recurring lack of terminological consistency that undermines the scientific rigor. Most notably, oil-in-water (O/W) and water-in-oil (W/O) emulsions are used interchangeably in several sections. For instance, the Results section refers to “W/O emulsion separation” while all experimental descriptions, figures, and performance metrics clearly correspond to O/W systems. The same inconsistency appears in the experimental section describing the separation setup, where 100 mL of “W/O emulsion” is reportedly used. This is not a trivial typographical issue, as the separation mechanism, wetting model, and performance criteria differ fundamentally between O/W and W/O emulsions. The authors should carefully revise the manuscript to ensure complete consistency and clarify whether any W/O systems were actually tested.

The interpretation of droplet size distribution data obtained by DLS is another major concern. The manuscript repeatedly claims “near-complete demulsification” after filtration, yet DLS measurements show that oil droplets in the filtrate remain in the range of approximately 45–300 nm, particularly for surfactant-stabilized emulsions. The presence of stable submicron droplets in the permeate suggests partial passage of emulsified oil rather than complete separation. On what basis do the authors equate a reduction in droplet size with demulsification rather than membrane leakage or surfactant-stabilized droplet penetration? Additionally, no discussion is provided regarding the detection limits of DLS, potential interference from surfactants, or whether blank measurements were conducted.

The reported separation efficiencies above 99% rely solely on COD measurements, yet no complementary oil concentration analysis (e.g., TOC, infrared oil analysis, or extraction-based quantification) is provided. COD can be strongly influenced by residual surfactants and dissolved organics. How do the authors ensure that the COD reduction reflects oil rejection rather than surfactant retention or dilution effects? This issue is particularly critical for the surfactant-stabilized emulsions, where COD values remain relatively high even after separation.

Although flux values as high as 2600 L·m⁻²·h⁻¹ are reported under gravity-driven conditions, the experimental configuration uses an extremely small effective membrane area (3.14 cm²). The manuscript does not address how edge effects, short flow paths, or scale-dependent hydraulic resistance may artificially inflate flux values at such small scales. How do the authors expect the flux to change when the membrane area is increased by one or two orders of magnitude, and is there any preliminary evidence supporting scalability beyond laboratory-scale demonstrations?

Claims regarding durability, robustness, and environmental stability are also insufficiently supported. Stability tests are limited to 24-hour immersion experiments, and evaluation is restricted to underwater oil contact angle measurements. No post-treatment structural or chemical characterization (such as SEM, XRD, or FTIR after pH or salinity exposure) is provided to demonstrate that the ZOMO–PAA layer remains intact. Given the known water solubility and swelling behavior of PAA, how do the authors exclude partial dissolution or gradual loss of the coating during prolonged operation?

The recyclability tests show a clear decline in flux over repeated cycles, especially for surfactant-stabilized emulsions, yet the manuscript describes the performance as “excellent recyclability.” No quantitative flux recovery ratio is calculated, and no cleaning protocol efficiency is assessed. At what point does flux stabilization occur, and how many cycles can the membrane realistically sustain before performance drops below an acceptable threshold?

In addition to these major issues, several concrete errors and weaknesses should be corrected. There is repeated text in the description of the anti-oil-adhesion test in Figure 5. Symbol definitions in the Young–Laplace equation are inconsistent, with θₐ and θw used ambiguously. Abbreviations such as UWOCA are not always used consistently. Furthermore, the reference list is heavily dominated by the authors’ own publications, while several relevant recent studies from other groups on polymer-bound inorganic superhydrophilic membranes are not discussed.

In summary, while the manuscript presents a technically competent fabrication process and extensive characterization, the scientific contribution is weakened by unclear novelty, inconsistent terminology, questionable interpretation of separation data, and overstatement of durability and performance. Substantial revision is required to clarify the conceptual advance, correct fundamental inconsistencies, and provide more rigorous validation of the claimed separation mechanism and long-term applicability before the work can be considered for publication.

Author Response

Reviewer:

1. The manuscript reports the fabrication of a hierarchically structured ZOMO–PAA@CuC₂O₄ nanorod-coated copper mesh for oil-in-water emulsion separation. While the topic is relevant and the experimental work is extensive, the manuscript in its current form raises significant concerns regarding novelty, conceptual clarity, data interpretation, and overstatement of performance.

Response:

We sincerely appreciate the reviewer’s constructive and valuable comments, which have greatly contributed to improving the quality of our manuscript. In response, we have carefully revised the manuscript in accordance with the reviewer’s suggestions.

Reviewer:

2. From a conceptual standpoint, the claimed novelty is not convincingly established. The authors state that systematic investigations of ZOMO–PAA@CuC₂O₄ nanorod-based membranes are scarce; however, the manuscript does not critically distinguish this system from a large body of closely related literature, including several prior studies from the same research group that already employ CuC₂O₄ nanostructures on copper meshes combined with highly hydrophilic inorganic coatings for underwater superoleophobic separation. The practical and mechanistic advantage of introducing ZOMO, as opposed to previously reported oxides, MOFs, or tungstate-based systems, is not clearly articulated. What specific property of ZOMO is essential here, beyond general hydrophilicity, and why could similar performance not be achieved with simpler or already established materials?

Response:

We sincerely appreciate the reviewer’s insightful and constructive comments. We agree that the novelty of the present work required clearer articulation and more explicit differentiation from prior CuC₂O₄-based membranes, including our own previous studies. Accordingly, we have substantially strengthened the discussion of novelty in the revised manuscript by explicitly clarifying the functional role of ZOMO within the composite architecture, beyond its contribution to general hydrophilicity. The corresponding descriptions have been updated in the revised manuscript as follows: Unlike previously reported oxide-, tungstate-, or MOF-modified CuC₂O₄ systems that mainly rely on surface roughness effects, ZOMO acts as a multifunctional interfacial modifier by enhancing surface hydration, increasing water affinity through polar metal-oxygen sites, and improving coating uniformity when integrated with a binding layer [41, 42]. These combined effects facilitate the formation of a stable hydration layer under water, which effectively suppresses oil droplet penetration, particularly in surfactant-stabilized O/W emulsions. With a well-defined geometric structure [42,43], ZOMOs provide robust frameworks featuring multiple active sites, which have been successfully exploited in catalysis, environmental remediation, and oil–water separation [44-46] Hierarchical micro-/nanoscale surfaces, critical for achieving special wettability, are often constructed by assembling particles of varying sizes [47]. When integrated with CuC₂O₄ nanorods, ZOMOs not only reinforce hierarchical surface roughness but also promote the formation of a dense hydration layer, thereby stabilizing underwater superoleophobicity against diverse oil types, including surfactant-stabilized emulsions. Furthermore, poly(acrylic acid) (PAA), known for its strong hydrophilicity, excellent blending ability, and superior water solubility [48,49], serves as an effective binder that enhances interfacial adhesion between ZOMOs and CuC₂O₄ nanorods, yielding a robust and durable hybrid coating. Based on these considerations, we hypothesize that this synergistic architecture, combining CuC₂O₄-induced roughness with ZOMO–PAA-derived hydrophilicity, offers an effective strategy for fabricating superhydrophilic and underwater superoleophobic copper mesh membranes with high separation efficiency, antifouling stability, and recyclability for oil-in-water emulsion separation. To the best of our knowledge, systematic investigations of such integrated ZOMO–PAA@CuC₂O₄ nanorod-based membranes remain limited, underscoring the novelty of this approach (see pages 2-3, lines 90-114).

Reviewer:

3. Throughout the manuscript, there is a recurring lack of terminological consistency that undermines the scientific rigor. Most notably, oil-in-water (O/W) and water-in-oil (W/O) emulsions are used interchangeably in several sections. For instance, the Results section refers to “W/O emulsion separation” while all experimental descriptions, figures, and performance metrics clearly correspond to O/W systems. The same inconsistency appears in the experimental section describing the separation setup, where 100 mL of “W/O emulsion” is reportedly used. This is not a trivial typographical issue, as the separation mechanism, wetting model, and performance criteria differ fundamentally between O/W and W/O emulsions. The authors should carefully revise the manuscript to ensure complete consistency and clarify whether any W/O systems were actually tested.

Response:

We sincerely appreciate the reviewer for pointing out the inconsistency between O/W and W/O terminology. In response, we have carefully corrected all instances of “water-in-oil (W/O)” to “oil-in-water (O/W)” throughout the manuscript. We confirm that all separation experiments, figures, and performance metrics exclusively correspond to O/W emulsions, and not W/O systems. The manuscript has been comprehensively revised to ensure full terminological consistency, and the separation mechanism is now discussed strictly within the O/W framework.

Reviewer:

4. The interpretation of droplet size distribution data obtained by DLS is another major concern. The manuscript repeatedly claims “near-complete demulsification” after filtration, yet DLS measurements show that oil droplets in the filtrate remain in the range of approximately 45–300 nm, particularly for surfactant-stabilized emulsions. The presence of stable submicron droplets in the permeate suggests partial passage of emulsified oil rather than complete separation. On what basis do the authors equate a reduction in droplet size with demulsification rather than membrane leakage or surfactant-stabilized droplet penetration? Additionally, no discussion is provided regarding the detection limits of DLS, potential interference from surfactants, or whether blank measurements were conducted.

Response:

We thank the reviewer for the insightful and critical observation. We concur that the term “near-complete demulsification” lacks sufficient precision and may be misleading, especially in the context of surfactant-stabilized emulsions. In response, the manuscript has been revised to clarify that the decrease in droplet size following filtration arises from partial droplet rejection and structural rearrangement of the emulsified oil, rather than complete demulsification. The corresponding descriptions have been updated in the revised manuscript as follows: The disappearance of micron-scale peaks in the droplet size distributions indicates significant reduction in droplet size and partial rejection of emulsified oil droplets (see pages 8-9, lines 295-297).

DLS provides qualitative insight into droplet size evolution but cannot be regarded as direct evidence of complete oil removal, particularly in the presence of surfactants (see page 9, lines 305-306).

Reviewer:

5. The reported separation efficiencies above 99% rely solely on COD measurements, yet no complementary oil concentration analysis (e.g., TOC, infrared oil analysis, or extraction-based quantification) is provided. COD can be strongly influenced by residual surfactants and dissolved organics. How do the authors ensure that the COD reduction reflects oil rejection rather than surfactant retention or dilution effects? This issue is particularly critical for the surfactant-stabilized emulsions, where COD values remain relatively high even after separation.

Response:

We thank the reviewer for raising this important concern regarding the use of COD measurements. While COD was employed as a commonly accepted bulk parameter to assess separation performance at the laboratory scale, it does not allow for the unambiguous differentiation of oil droplets, surfactants, and dissolved organic compounds. Accordingly, the revised manuscript now explicitly discusses the limitations of COD analysis and highlights the necessity of complementary oil quantification approaches, such as TOC or extraction-based methods, as avenues for future investigation. The corresponding descriptions have been updated in the revised manuscript as follows: It is worth noting that although COD was employed as a commonly accepted bulk parameter to evaluate separation performance at the laboratory scale, it does not allow for the unambiguous differentiation among oil droplets, surfactants, and dissolved organic compounds. Complementary oil quantification approaches, such as total organic carbon (TOC) analysis or extraction-based methods, represent important directions for future work (see pages 10-11, lines 341-346).

Reviewer:

6. Although flux values as high as 2600 L·m⁻²·h⁻¹ are reported under gravity-driven conditions, the experimental configuration uses an extremely small effective membrane area (3.14 cm²). The manuscript does not address how edge effects, short flow paths, or scale-dependent hydraulic resistance may artificially inflate flux values at such small scales. How do the authors expect the flux to change when the membrane area is increased by one or two orders of magnitude, and is there any preliminary evidence supporting scalability beyond laboratory-scale demonstrations?

Response:

We sincerely appreciate the reviewer’s comment regarding the scale dependence of flux measurements. In response, we recognize that the relatively small effective membrane area (3.14 cm²) employed in this work may give rise to edge effects and reduced hydraulic resistance, thereby overestimating apparent flux values. The reported fluxes therefore represent intrinsic permeability under laboratory-scale, gravity-driven conditions rather than directly translatable industrial performance.

Reviewer:

7. Claims regarding durability, robustness, and environmental stability are also insufficiently supported. Stability tests are limited to 24-hour immersion experiments, and evaluation is restricted to underwater oil contact angle measurements. No post-treatment structural or chemical characterization (such as SEM, XRD, or FTIR after pH or salinity exposure) is provided to demonstrate that the ZOMO–PAA layer remains intact. Given the known water solubility and swelling behavior of PAA, how do the authors exclude partial dissolution or gradual loss of the coating during prolonged operation?

Response:

We sincerely appreciate the reviewer for raising concerns about durability. While post-treatment structural characterization was not conducted in this study, coating stability is inferred from the strong coordination interactions between ZOMO, PAA, and the Cu-based substrate, supported by previous reports on the aqueous stability of PAA-bound inorganic coatings. Nonetheless, the revised manuscript now explicitly recognizes the potential for PAA swelling and gradual dissolution as a limitation and highlights long-term structural characterization as a critical direction for future investigation. The corresponding descriptions have been updated in the revised manuscript as follows: It is worth noting that, given the known water solubility and swelling behavior of PAA, the potential for PAA swelling and gradual dissolution is recognized as a limitation of the ZOMO–PAA@CuC₂O₄ NR coatings during prolonged operation (see page 13, lines 402-404).

Reviewer:

8. The recyclability tests show a clear decline in flux over repeated cycles, especially for surfactant-stabilized emulsions, yet the manuscript describes the performance as “excellent recyclability.” No quantitative flux recovery ratio is calculated, and no cleaning protocol efficiency is assessed. At what point does flux stabilization occur, and how many cycles can the membrane realistically sustain before performance drops below an acceptable threshold?

Response:

We are grateful for the reviewer’s careful observation and insightful comments. In response, we acknowledge that describing the recyclability as “excellent” was overly optimistic in light of the observed flux decline, particularly for surfactant-stabilized emulsions. Accordingly, the manuscript has been revised to remove qualitative descriptors and instead focus on the observed flux evolution trends.

Reviewer:

9. In addition to these major issues, several concrete errors and weaknesses should be corrected. There is repeated text in the description of the anti-oil-adhesion test in Figure 5. Symbol definitions in the Young–Laplace equation are inconsistent, with θₐ and θw used ambiguously. Abbreviations such as UWOCA are not always used consistently. Furthermore, the reference list is heavily dominated by the authors’ own publications, while several relevant recent studies from other groups on polymer-bound inorganic superhydrophilic membranes are not discussed.

Response:

We sincerely thank the reviewer for the careful and detailed evaluation. All issues identified, including repeated text in Figure 5, inconsistencies in symbol definitions within the Young–Laplace equation, abbreviation usage, and reference balance, have been addressed. Furthermore, recent publications from other research groups on polymer-bound inorganic superhydrophilic membranes have been added to enhance the literature coverage and ensure a balanced discussion (see reference 6, 12, 24, 36, 41, 42, 57).