Electrokinetic Potential of Basic Zinc Sulfate and of Products of Its Ion Exchange

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study systematically investigates the synthesis and characterization of basic zinc sulfate (Zn₄(OH)₆SO₄·3.5H₂O) and its ion-exchange behavior with transition metal cations including Cu²⁺, Co²⁺, and Ni²⁺. The results demonstrate that basic zinc sulfate undergoes complete or partial conversion to brochantite when exposed to excess CuSO₄ solution, with the extent of conversion dependent on CuSO₄ concentration, reaction time, and temperature. This conversion reaction is exothermic in nature. In contrast, Co²⁺ and Ni²⁺ only alter the hydration level of basic zinc sulfate without forming crystalline basic sulfates of these metals under identical conditions. The fully converted brochantite exhibits higher specific surface area and an isoelectric point at pH 9.5-10, with distinctly different electrokinetic behavior compared to the pristine basic zinc sulfate. This work provides important experimental insights into the formation and transformation mechanisms of basic zinc sulfate precipitates in zinc-ion batteries. I would recommend it to Molecules after minor revisions. The comments and suggestions about this work are described as follows:

 

• The research motivation is insufficiently articulated in the introduction section. It is recommended that the authors elaborate on why Cu²⁺, Co²⁺, and Ni²⁺ were selected for comparative study and how this research relates to practical application scenarios of zinc-ion batteries.
• The TGA curve in Figure 2 lacks clear annotations. The authors mention that "horizontal lines correspond to a complete loss of water of hydration and of constitution," but these horizontal lines are not explicitly marked with mass loss stages. It is recommended to add clear annotations in the figure to indicate different dehydration stages, with detailed explanations of the corresponding chemical transformations in the figure caption, to enhance data readability and interpretability.
• In the XRD data analysis, the authors attribute the diffraction peaks of Z000 to a mixture of trihydrate and tetrahydrate of basic zinc sulfate, while simultaneously acknowledging that "the absence of peaks at 19.126°and 27.017°in Z000 is a weak point of this hypothesis." This contradiction undermines the reliability of the conclusion. It is recommended that the authors conduct more thorough phase analysis, such as Rietveld refinement, to confirm the actual phase composition of Z000, or at least discuss other possible phases that may be present.
• The authors do not provide a reasonable explanation for why Co²⁺ and Ni²⁺ fail to form corresponding basic salts. This phenomenon may involve multiple factors including ionic radius, coordination preference, and hydration energy. It is recommended that the authors explore the origin of differences in ion-exchange behavior between Cu²⁺ and Co²⁺/Ni²⁺, which would significantly enhance the theoretical depth of this study.
• Some recent literatures is well advised to be learned, i.e., 10.1039/D3EE03729K;1002/aenm.202404032. It is recommended that the authors appropriately cite these references in the introduction to broaden the theoretical background of this study.

Author Response

This study systematically investigates the synthesis and characterization of basic zinc sulfate (Zn₄(OH)₆SO₄·3.5H₂O) and its ion-exchange behavior with transition metal cations including Cu²⁺, Co²⁺, and Ni²⁺. The results demonstrate that basic zinc sulfate undergoes complete or partial conversion to brochantite when exposed to excess CuSO₄ solution, with the extent of conversion dependent on CuSO₄ concentration, reaction time, and temperature. This conversion reaction is exothermic in nature. In contrast, Co²⁺ and Ni²⁺ only alter the hydration level of basic zinc sulfate without forming crystalline basic sulfates of these metals under identical conditions. The fully converted brochantite exhibits higher specific surface area and an isoelectric point at pH 9.5-10, with distinctly different electrokinetic behavior compared to the pristine basic zinc sulfate. This work provides important experimental insights into the formation and transformation mechanisms of basic zinc sulfate precipitates in zinc-ion batteries. I would recommend it to Molecules after minor revisions. The comments and suggestions about this work are described as follows:

 

The research motivation is insufficiently articulated in the introduction section. It is recommended that the authors elaborate on why Cu²⁺, Co²⁺, and Ni²⁺ were selected for comparative study and how this research relates to practical application scenarios of zinc-ion batteries.

 

The following was added.

Unlike zinc and copper basic sulfates, which have been extensively studied, the basic sulfates of other transition metals are less well-documented. It can be speculated that reactions analogous to (1) with ZnSO₄ replaced by another transition metal sulfate may produce basic sulfates of transition metals. Saarinen et al. [16] mixed a NiSO₄ solution with understoichiometric amounts of NaOH at different conditions. Their precipitates contained only 1.8 and 1.2% of sulfur, respectively, and their XRD patterns matched that of Ni(OH)₂, except that in one precipitate the reflection corresponding to a d-spacing of 0.4605 nm was split into two reflections corresponding to a d-spacing of about 0.42 and 0.52 nm, respectively. The present authors are not aware of analogous experiments with Co(II). Basic sulfates of cobalt were mentioned in several studies reporting on MOFs [17], and references therein, but investigation of basic sulfates of cobalt was not the main goal of these studies. Moreover Co(II) was partially oxidized to Co(III). A mixed Zn-Co basic sulfate has been studied [18]. The unsuccessful attempts of having obtained basic sulfates of transition metals other than Zn or Cu does not imply that such compounds cannot be obtained. Cation exchange is among the potential methods to synthesize such compounds. Basic sulfates of Co and Ni are not directly related to the aforementioned rechargeable, mildly acidic zinc ion batteries. On the other hand mixed electrolytes like ZnSO₄ + MnSO₄ have been proposed for those batteries. The Mn cations (and other cations) may partially replace Zn and thus affect the crystallization of Zn basic sulfates, their degree of hydration etc. Therefore fundamental research of cation-exchange, also in the systems, which are not directly related to the batteries may lead to a better understanding of zinc basic sulfates and thus, indirectly to a better understanding of rechargeable, mildly acidic zinc ion batteries.

and

The electrokinetic properties of basic zinc sulfate at low pH may be relevant to mildly acidic zinc ion batteries, namely the stability of colloidal dispersions against coagulation is governed by the z potential. This makes a difference if the basic zinc sulfate in the batteries is present as a stable colloidal dispersion or it rapidly coagulates and settles down.

 

The TGA curve in Figure 2 lacks clear annotations. The authors mention that "horizontal lines correspond to a complete loss of water of hydration and of constitution," but these horizontal lines are not explicitly marked with mass loss stages. It is recommended to add clear annotations in the figure to indicate different dehydration stages, with detailed explanations of the corresponding chemical transformations in the figure caption, to enhance data readability and interpretability.

 

The Figure is replaced with a new version and the caption was amended.

 

In the XRD data analysis, the authors attribute the diffraction peaks of Z000 to a mixture of trihydrate and tetrahydrate of basic zinc sulfate, while simultaneously acknowledging that "the absence of peaks at 19.126° and 27.017° in Z000 is a weak point of this hypothesis." This contradiction undermines the reliability of the conclusion. It is recommended that the authors conduct more thorough phase analysis, such as Rietveld refinement, to confirm the actual phase composition of Z000, or at least discuss other possible phases that may be present.

 

Rietveld refinement is added in the corrected Ms. as the Appendix.

This is a big piece of text with 4 figures so we do not copy it into our replies.

On top of this the following was added.

The XRD pattern presented in Fig. 1 is complicated, and it is due to the presence of multiple phases. Moreover the proportions between particular phases are affected by the humidity level and temperature, and they may even change in course of collecting the XRD pattern. Interestingly enough .an alternative explanation of the XRD pattern of Z001 can be offered when the “new phase” (2-2.25 hydrate) recently discovered by Stanimirova et al. [12] is taken into account on top of the other hydrates, which are well-established in the literature. An extended analysis of Fig. 1 is presented in the Appendix.

 

The authors do not provide a reasonable explanation for why Co²⁺ and Ni²⁺ fail to form corresponding basic salts. This phenomenon may involve multiple factors including ionic radius, coordination preference, and hydration energy. It is recommended that the authors explore the origin of differences in ion-exchange behavior between Cu²⁺ and Co²⁺/Ni²⁺, which would significantly enhance the theoretical depth of this study.

 

The following was added.

The XRD pattern presented in Fig. 1 is complicated, and it is due to the presence of multiple phases. Moreover, the proportions between particular phases are affected by the humidity level and temperature, and they may even change in course of collecting the XRD pattern. Interestingly enough, an alternative explanation of the XRD pattern of Z001 can be offered when the “new phase” (2-2.25 hydrate) recently discovered by Stanimirova et al. [12] is taken into account on top of the other hydrates, which are well-established in the literature. An extended analysis of Fig. 1 is presented in the Appendix.

 

and

On the other hand the XRD patterns of basic zinc sulfates are strongly affected by a preferential orientation of platelets in specimens prepared for XRD measurements, and very weak reflexes of higher order are commonplace in these compounds.

and

This result is not much surprising for the following reasons:

Lack of evidence for basic sulfates of Co and Ni in the literature.

Jahn-Teller effect dependent on the number of d-electrons in divalent cations.

Difference in ionic radii between Zn and Cu on the one hand and Co and Ni on the other.

 

Some recent literatures is well advised to be learned, i.e., 10.1039/D3EE03729K;1002/aenm.202404032. It is recommended that the authors appropriately cite these references in the introduction to broaden the theoretical background of this study.

 

This is a very important review, but it is not directly related to our research.

Reviewer 2 Report

Comments and Suggestions for Authors

The study systematically investigates the ion-exchange behavior and electrokinetic properties of basic zinc sulfate, Zn₄(OH)₆SO₄·xH₂O, and reports that Cu²⁺ can effectively transform it into brochantite, Cu₄(OH)₆SO₄, accompanied by changes in surface properties and specific surface area. The experimental design is relatively systematic, involving various salts and reaction conditions, and the materials are characterized using XRD, BET, and ζ potential measurements. However, the study mainly remains at a phenomenological level, and the underlying mechanism of the ion-exchange reaction is not explored in sufficient depth. In particular, key compositional and structural evidence—such as elemental quantification and morphological characterization (e.g., ICP, SEM, or XPS)—is missing, leaving the Zn–Cu exchange process and the exact composition of the products insufficiently verified. Therefore, although the work presents a set of useful experimental observations, further improvements in characterization and mechanistic discussion are necessary to strengthen the scientific rigor of the manuscript. Overall, the manuscript shows potential for publication, but substantial revisions and additional analyses are recommended before it can be considered suitable for publication.

Q1: The manuscript claims that Zn in basic zinc sulfate is replaced by Cu through ion exchange to form brochantite. However, the evidence is mainly based on XRD patterns. Considering that hydration changes can also modify XRD reflections of Zn₄(OH)₆SO₄ hydrates, additional compositional analyses are required to confirm the cation exchange. Techniques such as ICP-OES, EDS mapping, or XPS would provide quantitative information on Zn/Cu ratios and help verify the completeness of the exchange reaction.

Q2:The structural evolution during ion exchange is discussed only based on XRD and BET measurements. Morphological characterization (e.g., SEM or TEM) is missing. Such data would help clarify whether the transformation from Zn₄(OH)₆SO₄ to brochantite occurs through dissolution–reprecipitation or through a topotactic ion-exchange mechanism. Providing microstructural images would significantly strengthen the structural interpretation.

Q3:The authors suggest that the Zn–Cu exchange reaction is exothermic because higher temperatures lead to incomplete conversion. However, this conclusion is only inferred qualitatively. A more rigorous discussion of the thermodynamic driving force and possible reaction pathways would improve the scientific depth of the manuscript. For example, the authors may discuss the relative stability of Zn and Cu hydroxy sulfates, or provide equilibrium considerations that explain why Cu²⁺ uniquely enables complete transformation.

Author Response

The study systematically investigates the ion-exchange behavior and electrokinetic properties of basic zinc sulfate, Zn₄(OH)₆SO₄·xH₂O, and reports that Cu²⁺ can effectively transform it into brochantite, Cu₄(OH)₆SO₄, accompanied by changes in surface properties and specific surface area. The experimental design is relatively systematic, involving various salts and reaction conditions, and the materials are characterized using XRD, BET, and ζ potential measurements. However, the study mainly remains at a phenomenological level, and the underlying mechanism of the ion-exchange reaction is not explored in sufficient depth. In particular, key compositional and structural evidence—such as elemental quantification and morphological characterization (e.g., ICP, SEM, or XPS)—is missing,

 

We added SEM.

 

Regarding the other analyses. We agree that extra information would be interesting, but we have a limited budget (we have to pay for these analyses) and time (we only have 10 days for corrections). Our system is heterogeneous as indicated by XRD, so the problem of taking a representative sample for certain analysis is not trivial, and the received information is not necessarily representative for the entire material. For example most surface analyses are related to a thin layer, dependent on the depth of penetration of certain type of radiation. Finally our system is sensitive to the level of humidity and vacuum (as in XPS) destroys our systems.

In other words we focused on more detailed analysis of our data (which we already have) in our correction rather than on collecting more (probably useless) data.

leaving the Zn–Cu exchange process and the exact composition of the products insufficiently verified. Therefore, although the work presents a set of useful experimental observations, further improvements in characterization and mechanistic discussion are necessary to strengthen the scientific rigor of the manuscript. Overall, the manuscript shows potential for publication, but substantial revisions and additional analyses are recommended before it can be considered suitable for publication.

 

Q1: The manuscript claims that Zn in basic zinc sulfate is replaced by Cu through ion exchange to form brochantite. However, the evidence is mainly based on XRD patterns. Considering that hydration changes can also modify XRD reflections of Zn₄(OH)₆SO₄ hydrates, additional compositional analyses are required to confirm the cation exchange. Techniques such as ICP-OES, EDS mapping, or XPS

 

We do not think that detailed analyses requested by the Referee are useful or realistic in our systems. We can always present more results but we ask first: what for?

ICP-OES: we work with analytical grade reagents so the level of trace elements is low. We do not need detailed information on trace elements: their concentration is low, and the likelihood that they substantially affect the studied phenomena is low. 

XPS, EDS: the vacuum will destroy our samples.

would provide quantitative information on Zn/Cu ratios and help verify the completeness of the exchange reaction.

We already know a lot from XRD (Appendix in the revised version). In case of incomplete exchange, the Zn/Cu ratios different from one grain to another and even within one grain.

 

Q2:The structural evolution during ion exchange is discussed only based on XRD and BET measurements. Morphological characterization (e.g., SEM or TEM) is missing.

 

SEM is included.

 

Such data would help clarify whether the transformation from Zn₄(OH)₆SO₄ to brochantite occurs through dissolution–reprecipitation or through a topotactic ion-exchange mechanism. Providing microstructural images would significantly strengthen the structural interpretation.

The following was added.

As discussed in the Introduction, isomorphic substitution of Zn by Cu in zinc basic sulfates is only possible up to certain Cu level, and further cation exchange results in a crystallization of a new phase. Brochantite (monoclinic) differs from zinc basic sulfates in its structure (namuwite is hexagonal and osakaite and lahnsteinite are triclinic) and in the hydration level (brochantite is anhydrous and zinc basic sulfates are hydrated), so the ion-exchange is due to dissolution-precipitation. It cannot be fully excluded that the initial stage of the ion exchange may go via a topotactic substitution.

 

 

Q3:The authors suggest that the Zn–Cu exchange reaction is exothermic because higher temperatures lead to incomplete conversion. However, this conclusion is only inferred qualitatively. A more rigorous discussion of the thermodynamic driving force and possible reaction pathways would improve the scientific depth of the manuscript. For example, the authors may discuss the relative stability of Zn and Cu hydroxy sulfates, or provide equilibrium considerations that explain why Cu²⁺ uniquely enables complete transformation.

 

The following was added.

There are no reliable data on the solubility product or on other standard thermodynamic functions of Zn₄(OH)₆SO₄∙x H₂O in the literature. The level of hydration of Zn₄(OH)₆SO₄ is very difficult to control, and these thermodynamic functions certainly depend on the level of hydration. Therefore the direction of Zn-Cu exchange cannot be predicted / verified using the chemical thermodynamics. We also emphasize that only replacement of Zn by Cu was studied, and replacement of Cu by Zn was not. In other words the present results do not imply that brochantite cannot be converted into a corresponding Zn-salt with a sufficient excess of Zn ions in solution.

 

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors almost anwsered my questions, I agree to accept it.