The Impact of Dissolved Organic Matter on Arsenic Mobilization from Goethite in the Presence of Silicic Acid and Phosphate under Reducing Conditions

Round 1

Reviewer 1 Report (Previous Reviewer 1)

This study shows interesting results in the field of the impact of Fe oxides reduction on As moility in presence of different compounds that might affect the As biogeochemical behavior. A few improvements are suggested below:

Lines 178, 189, 397: problem with the reference.

Lines 204-205: I cannot see the increase of As during the first 168h in figure 1.  The graphs start at 25 h, the value at T0 could be useful to see an increase.

Line 220: 168 "what?"

Line 354 : H4, put the "4" in indice

Line 361: remove "can be" to imporve the sentence

Lines 379-380: it depends on As speciation, true for AsV

Figure 4: only 2 values on a graph, it would be better shown in a table

 Lines 413-415 : SO4 reduction not possible? Didn't you observe a darkening of the flask that might be related to FeS formation? Possibility to precipitate As2S3?

Line 469 : space is lacking

Author Response

For further details and our response to your valuable comments, please see the attached file.

Reviewer 2 Report (Previous Reviewer 2)

The researchers have revised most of my comments, some of the minor comments remains to be addressed, which include:

The introduction section severely lacks in indicating the relevant literature studies. For instance, the researchers are claiming that DOM might interfere in redox reactions but didn’t provide any solid proof. For such purpose, research article “ https://doi.org/10.3390/molecules24081619” may be referred. Because strong justification is always the key requirement for any statement and researchers claim is not wrong but still they need to specifically provide the evidences from literature if redox transformation is occurring then provide the proof.

In addition, in results and discussion section, the researchers didn't quantitatively explored the presence of As(III) and As(V) species. They are just claiming that transformation of As species are taking place in controlled conditions. How much transformation? The researchers should include it as well. 

Conclusion section should be brief enough to give broad understanding of current research study.

Author Response

For further details and our response to your valuable comments, please see the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report (Previous Reviewer 3)

I have some suggestions to improve it before its publication:

Line 178, 189 and 397: the link does not work, the reference is missing, please correct the mistake

Line 314: Please, write the chemical formulas correctly. Special care should be taken to distinguish sub and superscripts.

Figure 3: The quality of the figure needs to be improved. Please, provide a figure with higher resolution (at least 300 dpi). Remove unnecessary boxes in the figure.

The bibliography must be properly reviewed and completed. The journals must be completed with doi. Please include the link.

Author Response

For further details and our response to your valuable comments, please see the attached file.

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The manuscript entitled "The redox impact of dissolved organic matter on arsenic mobilization from goethite in the presence of silicilic acid and phosphate" reports results of experiments designed to evaluate the influence of P and Si anions on As behavior in presence of increasing concentrations of goethite (Fe oxide) with natural organic matter and its endogenous microflora under anaerobic conditions. The experimental design is ingenious, and the style of paper is of good quality. However, Authors should answer to some questions and apply the corresponding corrections or clarifications before publication.

First, the title does not reflect exactly the content of the research. As a fact, redox potential values are not highlignted in the manuscript. We know that experiments were performed in anaerobic conditions and that anaerobic bio-processes (Fe bioreduction, methanogenesis) occured, however, evolution of redox potential is not presented. Thus I suggest to change the title, perhaps indicating the reducing or anaerobic conditions but not "redox impact". Secondly, whereas microbial activities (Fe reduction, AsV reduction, methanogenesis) play a major role in the processes, they are absent in the abstract, they should be mentioned. The material and methods section is very clear, no comment! 

Concerning the reading and interpretation of results, I don't agree with the authors, or misinterpreted something. From the first sentence of the results section, you mention "mobilization of As" and comment this mobilization. However, you added soluble As and after you observed (figure 1) that this As was more or less immobilized according o the conditions. When I see the figure 1 (a, b and c), I see that this adsorption is maximum with the "high" Fe addition, and is decreased by the presence of P and/or Si anions. I don't understand why you interpret this "obstacle to adsorption" as a "mobilization". Fe is mobilized by solubilization (fig 1 j, k, l), yes, but As is not mobilized. It is soluble from the beginning, and just more or less adsorbed.

The second main point to be clarified is linked to the effect of DOM (section 4.4, lines 403-404). You write "high DOM availability increased concentrations of As and Fe in the solution". In Figure 3 (s, t, u) we can see that the evolution of DOC is similar with the 3 Fe concentrations. However, As and Fe evolution in Figure 1 are very different according to Fe concentrations. For example, high DOC in Fig 3s, with Si + P anions, does not correspond to higher As concentrations in Fig 1d.  So this sentence could apply only for the high Fe concentration.

These two important points should drive the major corrections/clarification to be applied both in the results and discussion/conclusion sections. Other minor points have to be corrected:

• problems with "error! reference source" lines 172, 390
• line 249: WHO guideline value is 10 µg/L and not > 10 µg/L
• legend of Figure 2: "total dissolved arsenic"

Reviewer 2 Report

The manuscript “water-1667199” entitled “The redox impact of dissolved organic matter on arsenic mobilization from goethite in the presence of silicic acid and phosphate” deals with As mobilization process in simulated environmental conditions. Following are my comments:

• Line 25-26! We attribute this increase in As mobilization primarily to the high availability of DOM in the presence of H4SiO4. How comes author make such judgement at their own. We all know that DOM itself pose antagonistic effect on As remediation and is responsible for mobility of As species in environment. Apart, silicates are also competing species in As remediation. Then what’s new here? It becomes most obvious that in cooccurring environment, As mobilization must increase.
• The researchers are talking about redox processes in soils and sediments. On other hand they didn’t define whether its trivalent As species or pentavalent species in such soils. The fraction of such species in such environment will further strengthen the understanding on the redox transformation. I can’t believe such errors can be seen in a research manuscripts. The researchers must first develop their understanding on the speciation and redox species type first before proceeding to any such study.
• Line 30! Is it really arsenate species not arsenite. As far as research studies are concerned and even soil environment is concern, arsenite i.e. As(III) species must be dominant owing to anoxic condition of soil until or unless some surface soil is concerned or even if altered manually by aerators. Even surface soils can contain both As(III) and As(V). This needs to be clarified with strong justification.
• The introduction section severely lacks in indicating the relevant literature studies rather hardly any relevant literature has been presented in current manuscript. For instance, the researchers are claiming that DOM might interfere in redox reactions but didn’t provide any solid proof. For such purpose, research article “ https://doi.org/10.3390/molecules24081619” may be referred. Because strong justification is always the key requirement for any statement and researchers claim is not wrong but still they need to specifically provide the evidences from literature if redox transformation is occurring then attach the proof. Further there are quite other lacking points. Like they rarely referred to any study where any such evidence is mentioned like redox transformation, how occurring, why occurring, what force As species to transform, is it environmental conditions or specifically DOM itself. In addition, how with geological conditions, transformation of As species are related with. So, now researchers must have gain idea, what is exactly expected in introduction section and how can they improve it.
• Another strong issue in introduction section is gap analysis and objectives. At here, any reader can be lost and the purpose of whole manuscript becomes useless. These needs serious attention. Even the novelty of manuscript is question mark from current content of manuscript. Do mention the novelty of current manuscript. For instance, the researchers are on one side claiming it’s DOM which is responsible for redox reactions and transformation and on other side they are saying microbially mediated redox transformation. Appropriately address this comment.
• Section 2.1. The redox form of As in peat is As(III) or As(V) or combination of both?
• Line 147-148! If you are purging nitrogen gas in system, then it’s mean anaerobic condition prevails in bottle atmosphere and anoxic condition prevails in soil, then isn’t it clearly means that As(III) will be dominant there owing to adjustment of physical environment. In such condition, how DOM can oxidize As(III)? Need to provide strong evidences by spectroscopic techniques. Only experimental results in such scenario can’t be justified.
• As a minor comment, keep homogeneity whenever referring to any figure in the main text. At one point, it’s mentioned like Figure 1A and on other side it’s mentioned Figure 1a. Keep same formatting throughout the manuscript.
• As per Table 1, initial As concentration in soil is 0.53 mg/Kg, however while presenting results, researchers are using different units like ug/L. Can researchers provide details on conversion factor i.e. from ug/L to mg/Kg since main aim is to provide data on As mobilization or leaching. It’s always good to present it in the form of mg/Kg. In the revised version, I would be happy to see the calculations on conversion as a response from researchers. Similar scenario for presenting results of DOC, phosphate as well silicates will enhance content as well as significance of current manuscript.
• In the whole manuscript, researchers are continuously talking about arsenite and arsenate, it’s always good to present the exact fraction in terms of mg/Kg of arsenite as well as arsenate. This will serve the purpose of LC-ICP-MS technique.
• Line 172, 389! Issue exists in citation of references, make correction.
• Line 430-431! If silicates somehow increase dissolution of DOM from soil and somehow enhance microbial activity, it’s itself presence always hinders microbial activity> How researchers will justify such point.
• Line 437-441! The mechanism is unjustifiable and researchers can’t predict such mechanisms just based on series of experiments, Evidence using advance analytical techniques is a must requirement for such purpose. In addition, there is already anoxic environment, but researchers are claiming that presence of phosphate and silicates ultimately release DOM from soil and resultingly microbial reduction of As(V) to As(III) occurs. It is not possible at all, since as per evidences from literature DOM are somehow responsible for oxidation of pollutants and here the condition is already reducing environment means As(III) is already prevailing, from where does As(V) came in such environment. That’s a big question mark. Even if the pH is increasing, that itself is a indicator that some oxidative environment is approaching. Here, the concepts of researchers are totally not understandable and not justifiable as per scientific point of view.
• Based on all these comments, abstract as well as conclusion section needs significant improvement and should be brief enough to give broad understanding of current research study.

Reviewer 3 Report

No supplementary material can be accessed. Authors must solve this, is important in order to evaluate the rest of work. I advance some indications in order to improved the fisrt part

Line 24: since it is not specified that PO₄ is phosphate until line 60, please use the correct form of oxoanion in abstract (include charges)

Line 90: if peat is taken in 2014, it strikes me that they keep it at room temperature until the experiments are carried out

Table 1: Please keep in mind that all devices used in this study should include information such as model, manufacturer, and country. In addition, all methods should provide references.

Line 114: Please check NO^3- expression

Line 115: Please check conductivity units (could be mS cm^-1?)

Line 125: Does 4.4 125 mg g-1 goethite (medium goethite: mFe) mean 4,4 mg goethite per gram of fresh peat or dry weight peat? "? If yes, please add it; if not, please use an alternative expression

Line 135: no supplementary material can be accessed. Please, solve this, is important to evaluate the rest of work

Table 2: add 1 superscript to first column (Treatments¹)

2.4 sample collection: Indicate if the volume was replaced or final concentration was corrected based on the extracted volume when a sample volume was extracted at each time

Line 172: the reference is missing, please correct the mistake

Line 184: information should be included in the reference section

Line 231-32:  Please, include data or graphs to support this affirmation

To continue reviewing the work I need to access the supplementary material