Review Reports - Electric Double Layers in Soils as Basis for Micro-Aggregation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper adopts classical moleculardynamics (MD) simulations. Taking kaolinite clay mineral together with four standard organicmatter samples from the International Humic Substances Society (IHSS), it investigates the regulatory mechanisms of water content and calciumion strength on electric doublelayer behaviors at the interfaces of soil organicmineral microaggregates, with a reasonable research framework. The study observes distinct “electrostatic fingerprint” phenomena for different organicmatter substances and can qualitatively correlate the chemical composition of organic matter with adsorptionaggregation behaviors. Nevertheless, several critical limitations exist:

① All simulations are performed under fixed neutralpH conditions, without considering widelydistributed acidicsoil environments. The substantial influences of pH on functionalgroup dissociation, calcium complexation and electric doublelayer properties are not included as variables, which restricts the extrapolation of conclusions to realworld soil scenarios.

② Although behavioral differences among the four humic substances are identified, the analysis remains merely qualitative. Quantitative interpretation linking compositional properties to microscale responses is absent. The contributions of different functional groups to innersphere complexes (ISC), outersphere complexes (OSC), hydrogen bonds and hydrophobic effects are not further dissected, leading to insufficient depth in exploring componentrelated mechanisms.

③ This is a purely numericalsimulationbased study lacking validation from experimental measured data. No corresponding laboratory soilaggregation experiments have been conducted to verify whether the “electrostatic fingerprint” can be observed in macroscale soil systems; therefore, the reliability of simulation results is difficult to evaluate. Constrained by MD timescale limits and idealized poremineral models, only qualitative mechanistic explanations can be achieved.

Author Response

Comment 1: This paper adopts classical molecular dynamics (MD) simulations. Taking kaolinite clay mineral together with four standard organic matter samples from the International Humic Substances Society (IHSS), it investigates the regulatory mechanisms of water content and calcium ion strength on electric double layer behaviors at the interfaces of soil organic mineral microaggregates, with a reasonable research framework. The study observes distinct “electrostatic fingerprint” phenomena for different organic matter substances and can qualitatively correlate the chemical composition of organic matter with adsorption aggregation behaviors.
Response 1: We thank the reviewer for the generally positive evaluation of our work.

Comment 2: Nevertheless, several critical limitations exist: All simulations are performed under fixed neutral pH conditions, without considering widely distributed acidic soil environments. The substantial influences of pH on functional group dissociation, calcium complexation and electric double layer properties are not included as variables, which restricts the extrapolation of conclusions to real world soil scenarios.
Response 2: The reviewer is correct that we have performed the simulations at neutral pH. We have studied the pH-dependent aggregation of similar systems in our previous work (Borisover et al 2023; doi:10.1016/j.colsurfa.2024.135507) and indeed found that the aggregation propensity may depend on pH. We have added a discussion on this behaviour and what it may mean to the current simulations on page 17, lines 429-437 of the revised manuscript. Repeating all simulations at a different pH value goes beyond the scope of the current work, which studies the differences between the four different organic materials under different electrostatic conditions.

Comment 3: Although behavioral differences among the four humic substances are identified, the analysis remains merely qualitative. Quantitative interpretation linking compositional properties to microscale responses is absent. The contributions of different functional groups to innersphere complexes (ISC), outersphere complexes (OSC), hydrogen bonds and hydrophobic effects are not further dissected, leading to insufficient depth in exploring component related mechanisms.
Response 3: We thank the reviewer for addressing a key missing point in our study. By using the principal component analysis data of the four different organic materials with their respective replicates, we generated a quantitative correlation between the organic composition and ISC+OSC, non-bonded interactions, hydrogen bonds, and occupancy of the surface in the Results subsections 3.4, 3.5, 3.6 and 3.7. We also elaborated on these findings in the Discussion section and included Pony Lake soil organic matter, which introduces another aspect where the balance between aromaticity and aliphaticity seem to play a relevant role.

Comment 4: This is a purely numerical simulation based study lacking validation from experimental measured data. No corresponding laboratory soil aggregation experiments have been conducted to verify whether the “electrostatic fingerprint” can be observed in macroscale soil systems; therefore, the reliability of simulation results is difficult to evaluate. Constrained by MD timescale limits and idealized pore mineral models, only qualitative mechanistic explanations can be achieved.
Response 4: To the best of our knowledge, it is not possible to access such fine scales at these complex interfaces from the experimental point of view nor to control all environmental variables to have similar conditions as in the simulation of such complex systems. Similarly, the idealized situations of a molecular simulation will not necessary be complete enough to correlate with experimental findings, but the semi-quantitative insights still help to interpret the effects of different soils and the driving mechanisms.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a well-designed molecular dynamics study on the interactions between kaolinite and four standardized soil organic matter (OM) samples, with particular attention to the role of the electric double layer (EDL) in micro-aggregation. The topic is timely and relevant, and the use of atomistic simulations to disentangle the complexity of organo-mineral interactions is commendable. However, after careful reading, I have several major concerns that, in my view, limit the current impact and clarity of the work.

1.The Abstract and Conclusions are overly general and lack quantitative support.

Both the Abstract and the Conclusions section present broad claims about "unique fingerprints" and "reproducible behavior" of each OM sample, but they fail to provide any specific numerical or statistical evidence to substantiate these statements.

2 The role of Ca2+ ions between the interactions between kaolinite and soil organic matter (OM). The role of Ca2+ ions—which is one of the central themes in the Results and Discussion—is almost entirely absent from both the Abstract and Conclusions. Given that Ca2+ bridging is repeatedly highlighted as a key mechanism for OM adsorption and aggregation (e.g., Figures 5, 8, and 9), the failure to mention this in the concluding remarks leaves a significant gap.

3.The Discussion section feels underdeveloped and somewhat descriptive.

While the Results section is rich in data and figures, the Discussion largely reiterates the observations without deeply interpreting the underlying physical or chemical principles, nor does it adequately connect the findings to broader soil science contexts.

4.The role of Cl- ions is introduced but not fully explored

Chloride ions are mentioned in the context of screening and substitution of OM on the aluminol surface (Figures 4 and 10, and Discussion). However, their distribution, coordination environment, and dynamics are not systematically analyzed. Since Cl- is a weakly coordinating anion, its competitive adsorption with anionic OM is an important aspect of ionic strength effects. The current treatment is rather cursory.

Does Cl- preferentially occupy specific surface sites?

Does Cl- affect the hydration layer or the mobility of Ca2+?

Is the substitution of OM by Cl- reversible or concentration-dependent?

5.The Gini coefficient analysis is mentioned but not explained

In the Discussion, the Gini coefficient is introduced as a metric to quantify surface coverage equality. However, no definition, formula, or interpretation of the Gini values is provided in the main text.

6.Others

Change “3.5. Surface occupancy” to “3.7. Surface occupancy”.

Vertical ordinate‌ of Figure 10: Change “h-bonds” to “”H-bonds.

Line 196: change “_Partially filled” to “Partially filled”

Author Response

Comment 1: The manuscript presents a well-designed molecular dynamics study on the interactions between kaolinite and four standardized soil organic matter (OM) samples, with particular attention to the role of the electric double layer (EDL) in micro-aggregation. The topic is timely and relevant, and the use of atomistic simulations to disentangle the complexity of organo-mineral interactions is commendable. However, after careful reading, I have several major concerns that, in my view, limit the current impact and clarity of the work.
Response 1: We thank the reviewer for the positive evaluation of our work.

Comment 2: The Abstract and Conclusions are overly general and lack quantitative support.
Both the Abstract and the Conclusions section present broad claims about "unique fingerprints" and "reproducible behavior" of each OM sample, but they fail to provide any specific numerical or statistical evidence to substantiate these statements.
Response 2: We improved the Abstract and Conclusions sections by using the principal component analysis data of the different organic material samples to correlate organic composition with the variables, allowing us to numerically quantify the effect of the organic composition and elaborate further the analysis in the Discussion section (pages 17 and 18) in terms of aromaticity and acidity of soil samples.

Comment 3: The role of Ca2+ ions between the interactions between kaolinite and soil organic matter (OM). The role of Ca2+ ions—which is one of the central themes in the Results and Discussion—is almost entirely absent from both the Abstract and Conclusions. Given that Ca2+ bridging is repeatedly highlighted as a key mechanism for OM adsorption and aggregation (e.g., Figures 5, 8, and 9), the failure to mention this in the concluding remarks leaves a significant gap.
Response 3: We thank the reviewer pointing us to this missing element. We now included the role of Ca ions in terms of cation bridges, clustering and adsorption to mineral surfaces in the Abstract and the Conclusions section and reflect on the formation of aggregates.

Comment 4: The Discussion section feels underdeveloped and somewhat descriptive.
While the Results section is rich in data and figures, the Discussion largely reiterates the observations without deeply interpreting the underlying physical or chemical principles, nor does it adequately connect the findings to broader soil science contexts.
Response 4: We have significantly rewritten the Discussion section. We now more deeply elaborate our findings and by using numerical correlations, we changed from a qualitative to a quantitative assessment that links the organic composition to ISC+OSC, non-bonded interactions, hydrogen bonds, and occupancy of the surface. Moreover, our findings and conclusions have high value in broader scales in soil science.

Comment 5: The role of Cl- ions is introduced but not fully explored
Chloride ions are mentioned in the context of screening and substitution of OM on the aluminol surface (Figures 4 and 10, and Discussion). However, their distribution, coordination environment, and dynamics are not systematically analyzed. Since Cl- is a weakly coordinating anion, its competitive adsorption with anionic OM is an important aspect of ionic strength effects. The current treatment is rather cursory.
Does Cl- preferentially occupy specific surface sites?
Response 5: We thank the reviewer for these questions which allow us to have a bigger picture of the competing species. We added the influence of chloride ions as relevant part of our study in subsections 3.4, 3.5, 3.6 and 3.7 and explored in more detail the impact of the ions on the already mentioned properties and the environment in the Discussion section (page 18 subsection 4.3).

Comment 6: Does Cl- affect the hydration layer or the mobility of Ca2+?
Response 6: Water molecules are not affected by the presence of chloride ions due to the existing screening of charged species already present. Chloride ions only interact with other ions by replacing OM molecules at the surface and forming pairs far from the EDL region.

Comment 7: Is the substitution of OM by Cl- reversible or concentration-dependent?
Response 7: We thank the reviewer for this very deep question. We added the following paragraph to the Discussion in page 18, lines 476-483: “Additionally, it is worth noting that weakly coordinating Cl– ions, competing with OM molecules, exhibit high mobility over the aluminol surface. Thus, Cl– ions distribute across the mineral surface not occupied by OM molecules We performed additional simulations to verify if OM molecules replaced by Cl– ions could be re-adsorbed on the mineral surface reversibly by removing all Cl– ions with their respective counterions. As Cl– ions only contribute to the charged species of the EDL, without disturbing the water environment, removal of them allow adsorption reversibility, achieving OM coverage recovery of 98%.”

Comment 8: The Gini coefficient analysis is mentioned but not explaine
In the Discussion, the Gini coefficient is introduced as a metric to quantify surface coverage equality. However, no definition, formula, or interpretation of the Gini values is provided in the main text.
Response 8: The definition, interpretation, and references of the Gini coefficient can be found in section 2.4. We have made efforts to enhance the clarity of this text to prevent any misunderstandings.

Comment 9: Change “3.5. Surface occupancy” to “3.7. Surface occupancy”.
Response 9: We fixed the section numbering.

Comment 10: Vertical ordinate‌ of Figure 10: Change “h-bonds” to “”H-bonds.
Response 10: We changed Figure 10 accordingly.

Comment 11: Line 196: change “_Partially filled” to “Partially filled”
Response 11: We performed a proof reading and fixed all typos found.

Reviewer 3 Report

Comments and Suggestions for Authors

The “Results” and “Discussion” section should be put together in order to helps author to realy understand which results is discussed.
The conclusion section should be lengthy with essential results
In section 3, line 196, remove the symbol “_” at the beginning of the sentence.
Line 283, author should write “Figure 8” instead of “Figure 2”.
They should also improve the discussion section.
The authors should check in the overall manuscript some typos errors.

Author Response

Comment 1: The “Results” and “Discussion” section should be put together in order to helps author to realy understand which results is discussed.
Response 1: We thank the reviewer for the suggestion. We quantified the effect of organic composition and added it to our study after which we reformulated the Results and Discussion section. This reordering improved the readability and understanding of our work.

Comment 2: The conclusion section should be lengthy with essential results
Response 2: In sight of the added interpretation, we elaborated more deeply on the Abstract and Conclusions section making it not only more lengthy, but also more informative and with a wider perspective focused in how our results are reflected on soil science.

Comment 3: In section 3, line 196, remove the symbol “_” at the beginning of the sentence.
Response 3: We removed the extra symbol.

Comment 4: Line 283, author should write “Figure 8” instead of “Figure 2”.
Response 4: We corrected the Figure label accordingly.

Comment 5: They should also improve the discussion section.
Response 5: We improved the Discussion section by adding the organic composition as independent variable of formed complex via aggregates or sorbed species, stabilization of structures via non-bonded interactions, hydrogen bonds and the occupied mineral surface as dependent variables.

Comment 6: The authors should check in the overall manuscript some typos errors.
Response 6: We performed a proof reading and fixed all typos found.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Please check Figure 21 (Figure 9) in Results subsection 3.5. The data for the Partially, Fully and Ions conditions look very similar; please verify whether there is any error.

Numerous figure citations in the text refer to figure numbers that do not appear in the manuscript. The actual figures only go up to Figure 12, whereas in‑text references reach Figure 24.

Moreover, Figure 11 suffers from very poor image resolution.

Author Response

Comment 1: Please check Figure 21 (Figure 9) in Results subsection 3.5. The data for the Partially, Fully andIon conditions look very similar; please verify whether there is any error.

Response 1: We thank the reviewer for the careful review, we checked the data and the numbers are quite comparable, which is why the plots appear very similar.

Comment 2: Numerous figure citations in the text refer to figure numbers that do not appear in the manuscript. The actual figures only go up to Figure 12, whereas in‑text references reach Figure 24.

Response 2: We are grateful to the reviewer for identifying this issue. The error arose primarily from the conversion process from Word to PDF; we have endeavored to rectify the numbering accordingly.

Comment 3: Moreover, Figure 11 suffers from very poor image resolution.

Response 3: We changed the format file and increased the resolution of all Figures.