On the Role of Poly-Glutamic Acid in the Early Stages of Iron(III) (Oxy)(hydr)oxide Formation

Round 1

Reviewer 1 Report

This is interesting and well-done work, and represent a valuable incremental contribution to the literature and a step toward further studies.  My comments are relatively minor.

I suspect that most readers of Minerals will need to be reminded of the structure of (poly) glutamic acid, so it might be worth including a structure diagram.  Which form was used?  It wasn’t gamma, but one of the others?

Sodium salts of poly(glutamic acid) were used; does the Na play any role, or is that the subject of a future study?  It is gratifying that akaganeite is the primary product, as Cl is present in the system.  I was at first surprised at the relatively high (10 degee) lower limit for the powder diffraction measurements, but there seem to be no peaks from metal complexes, so the range seems fine.

In line 246, I don’t think we mean Figure 4(a).  It is awkward that the scales of Figures 4 differ; that makes comparison more difficult.

Do the authors plan any XAFS and/or Pair Distribution Function studies to gain insight into the Fe coordination?  The 4:1 carboxyl:Fe ratio is interesting; can the authors provide a picture of their best guess at the Fe environment?

Author Response

This is interesting and well-done work, and represent a valuable incremental contribution to the literature and a step toward further studies.  My comments are relatively minor.

We thank the Reviewer very much for their appreciation of our work and their valuable comments. We address the comments point-by-point in the following. Corresponding changes are highlighted in yellow in the revised manuscript.

I suspect that most readers of Minerals will need to be reminded of the structure of (poly) glutamic acid, so it might be worth including a structure diagram.  Which form was used?  It wasn’t gamma, but one of the others?

In the study, we have used poly-L-glutamic acid sodium salt with two different molecular weights. The structure of the used polyamino acid is shown in the revised paper, and it is the alpha form. We added the chemical structure and the notation to the revised manuscript.

Sodium salts of poly(glutamic acid) were used; does the Na play any role, or is that the subject of a future study?  It is gratifying that akaganeite is the primary product, as Cl is present in the system.  I was at first surprised at the relatively high (10 degee) lower limit for the powder diffraction measurements, but there seem to be no peaks from metal complexes, so the range seems fine.

In regards to the use of sodium salt poly(glutamic acid), our study does not give any direct insight into any possible effects of the Na⁺ ions. Also, we used sodium hydroxide for the counter-titration and keeping the pH value constant, so there is an additional source for sodium ions. We do not expect any decisive influence of Na⁺ ions on the nucleation behavior of Fe(III) (oxy)(hydr)oxide in this experimental approach, but the role of different ions in general should indeed not be neglected. This will be subject to future studies.

Chloride ions are indeed present in the system. We have acquired the XRD data from a lower limit of 5 degree 2θ, but there were no additional reflections below 10 degrees 2θ. In the revised manuscript, we have included the broadened 2θ range.

In line 246, I don’t think we mean Figure 4(a).  It is awkward that the scales of Figures 4 differ; that makes comparison more difficult.

Yes, we agree, it should be Fig. 3(a). This has been corrected. The axis of TG measurements in the revised manuscript are now plotted on the same scale, facilitating comparison.

Do the authors plan any XAFS and/or Pair Distribution Function studies to gain insight into the Fe coordination?  The 4:1 carboxyl:Fe ratio is interesting; can the authors provide a picture of their best guess at the Fe environment?

XAFS and/or Pair Distribution Function studies would certainly provide new insight into the Fe(III) coordination in the presence of polyglutamic acid and we would be happy to perform such measurements in the future. The present study does not provide data on the Fe(III) coordination, and we prefer to not add speculation about this at this point.

Reviewer 2 Report

This work deals with the mechanism of iron oxide formation in presence of additives, polyglutamic acid additive is considered in here. Early stages of the nucleation of Fe(III) oxide are examined. Authors provide sound and consistent set of experiments that merit to be published.

However, before publication, the presentation of the results could be improved according to remarks hereafter.

*The Materials and Methods section is not perfectely clear. In particular, the description of the titration experiment is somewhat confusing. Iron solution is added to solution with or without polymer while only polymer solutions are indicated (line 95). Figure 1 shows Cex(OH-) vs Cadded(Fe3+) plots: both Cex et Cadded have to be defined (the latter concentration is also designated by “[Fe3+]added”. Please choose only one notation). In the same way the quantity “[H+]released” have to be defined.

*Results section

-The physical meaning of the quantity “[H+]released/[Fe3+]added” should be given.

-The XRD pattern of iron oxide after treatment at 1000°C are shown in figure 3. The interest of these diffractograms is not obvious.

*Discussion

-It is necessary to further explain the following conclusion:

Line 274: This ratio does not correspond to the simple binding of monomeric Fe(III) species to carboxyl groups but rather implies that Fe(III) species are present as highly dynamic olation PNC clusters, which have been detected in earlier work [20], that effectively interact with the additive as a whole.

Why cluster have to be considered?

-I recommend to add schemes to illustrate the proposed mechanism.

Author Response

This work deals with the mechanism of iron oxide formation in presence of additives, polyglutamic acid additive is considered in here. Early stages of the nucleation of Fe(III) oxide are examined. Authors provide sound and consistent set of experiments that merit to be published.

However, before publication, the presentation of the results could be improved according to remarks hereafter.

We are grateful to the Reviewer for recognizing our research achievements and suggesting publication after addressing their questions, which is done point-by-point in the following. Corresponding changes are highlighted in yellow in the revised manuscript.

*The Materials and Methods section is not perfectely clear. In particular, the description of the titration experiment is somewhat confusing. Iron solution is added to solution with or without polymer while only polymer solutions are indicated (line 95). Figure 1 shows Cex(OH-) vs Cadded(Fe3+) plots: both Cex et Cadded have to be defined (the latter concentration is also designated by “[Fe3+]added”. Please choose only one notation). In the same way the quantity “[H+]released” have to be defined.

We have corrected the noted issues in the revised manuscript (please see line 95 in the revised text.) Also, in the experimental part of the revised manuscript, we have explained the quantities we used to present the obtained titration curves (please see lines 102-107 in the revised manuscript). Also, all quantities are now consistently given.

*The physical meaning of the quantity “[H+]released/[Fe3+]added” should be given.

The quantity [H⁺]_released/[Fe³⁺]_added in the original manuscript, or c_released(H⁺)/c_added(Fe³⁺) in the revised manuscript, is a unitless quantity giving the ratio of produced protons per hexa-aqua iron(III) added, at any time during the titrations. Thereby, this quantity reflects the changes in chemical equilibrium upon Fe(III) dosing. c(H⁺)_released is equal to c_ex(OH^-), the quantity that we measure. In the Fe(III) system without additive, the released H⁺ ions originate only from Fe(III) hydrolysis, while in the system with additive, they originate from both Fe(III) hydrolysis and binding to the additive (i.e., due to releasing protons from the carboxylic acid functions of the polymer). We have added this clarification to the revised manuscript (please see lines 138 – 142 in the revised manuscript).

The XRD pattern of iron oxide after treatment at 1000°C are shown in figure 3. The interest of these diffractograms is not obvious.

The XRD patterns of the samples obtained in titration experiments with and without the polymer after the TG treatment are provided to show that the phase transformation and formation of the hematite phase occurred during the heat treatment, which is also indicated in the TG/DTA measurements. Although in this case the final product has the same crystal phase, another additive may yield different phases after a respective thermal treatment. We thus believe that the data is useful, and we prefer to keep it in the revised manuscript.

-It is necessary to further explain the following conclusion:

Line 274: This ratio does not correspond to the simple binding of monomeric Fe(III) species to carboxyl groups but rather implies that Fe(III) species are present as highly dynamic olation PNC clusters, which have been detected in earlier work [20], that effectively interact with the additive as a whole.

Why cluster have to be considered?

We appreciate this comment. The Reviewer is correct that our observation does not strictly prove the interaction of the additive with clusters, so we have re-phrased this section accordingly. Rather, the data indicates that the interaction with clusters is likely and this also provides a potential rationalization of the inhibiting effect that is discussed in the final paragraphs of the manuscript.

We now write, “In the case of electrostatic binding of monomeric Fe(III) species, a maximum n(COOH):n(Fe³⁺)_added mole ratio of 3:1 would be expected, that is, one iron(III) ion per three carboxylic acid functions for a very strong interaction. We observed that the actual number of iron(III) ions per carboxylic acid function of the polymer is 0.25 (i.e., the above-mentioned ratio is 4:1). Although it could be ascribed to a weaker interaction in terms of monomeric binding, where the average number of carboxylic acids functions interacting with one iron(III) would be higher than for the above-mentioned scenario, our titration results in the system without additive are in line with the previous study by Scheck et al. [20], where the existence of olation PNCs and the different regimes in the titration curves were established. Further characterization data, i.e., XRD, FTIR, TG-DTA, indicates the formation of Fe(III) (oxy)(hydr)oxide-pGlu composites. Also, the titration data show that no interaction with the additive occurs after the nucleation point, implying that the additive is actively involved in the nucleation event. This suggests an interaction of the additive with molecular FeOx precursors toward the formation of the solid. Having in mind the hydrolyzing nature of the system, binding of single Fe(III) ions to carboxyl groups up to the saturation level may induce polyelectrolyte iron salt precipitation rather than the observed formation of a composite with 85 wt.% of inorganic phase. The observed n(COOH):n(Fe³⁺)_added binding ratio may also be related to the structure and the Fe(III) coordination in the highly-dynamic olation Fe(III) PNCs, namely, some of the carboxyl groups may be sterically inaccessible to Fe(III).”

Indeed, referring back to the question about XAFS measurements and the Fe(III) coordination environment of Reviewer #1, this study provides basic insights into the early stages of additive-controlled nucleation in the iron(III) system, and it requires further more detailed characterization to assess the exact nucleation mechanism on the molecular level, which is beyond scope for the present work.

I recommend to add schemes to illustrate the proposed mechanism.

The recommendation is absolutely reasonable and we do support it in principle. However, we have thought about a possible scheme and in this particular case, we would have to add too much speculation about the molecular details of the interaction mechanism. Thus, since we prefer to not include speculation, we decided to not add a schematic illustration.