Improvement of As(V) Adsorption by Reduction of Granular to Micro-Sized Ferric Hydroxide

Round 1

Reviewer 1 Report

It is always interesting to study new alternatives for arsenic removal from aqueous solutions. The authors introduced a new and interesting alternative that can certainly be of interest to the target audience. I recommend accepting the manuscript after major revision:

1. The introduction is not enough to show the importance of this study. I recommend including a short review of some of the most important arsenic removal processes, such as arsenic abatement,  to strengthen the importance of this work.
2. Before any kind of adsorption test, it is very important to study the chemistry of the working solution and the ionic exchange system. The chemical analysis must be present at the beginning of the tests, and not cited in another work. It is very important to include a speciation study of the arsenic in the aqueous medium, considering concentration of ions, such as sulfates, pH, potential and ionic strength. I assume the arsenic is present as AsO4-, but arsenic is an electro-active species, and its redox state can change to (+3).
3. For a research article, it is important to include comments over the chemical phenomenon taking place at the adsorbent-solution interphase.  I recommend at least proposing a reaction mechanism for the capture of arsenic by the adsorbent. Would it be possible, under these circumstances, for the formation of scorodite to occur?
4. Finally, I recommend focusing the analysis less on mathematical adjustments and more on interpreting the data. For example, table 1 provides a lot of information regarding the process itself. Parameter "b" not only provides quadrature data, but also accounts for the reversibility of the process, that is, the feasibility of removing arsenic from the adsorbent and reusing it continuously. If "b">1, the process tends to be irreversible, which is the case for this process. I recommend that the authors include what would be the next step in arsenic removal in this study. Would it be possible to extract the arsenic for reuse, or should the adsorbent be discarded once saturated?

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Economic and efficient arsenic removal is of practical interest, the presented paper is well written and should be of interest to the Readers. However a critical part is not presented (milling procedure), which is a vital part of the work and should be amended before publication.

 

• Introduction chapter, lines 46-57: description of other methods available for arsenic removal could be supplemented, as options are only partially mentioned in the text.
• line 60: q_max is not defined upon the first use in the text (only in the abstract).
• chapter 2.3, lines 106-112: applied instrumental parameters should be given in the text, not only as a reference
• line 135: why was the 800nm line selected for UV-Vis determination? Please amend the applied instrumental parameters.
• lines 149-150: what frequency was applied for the sonication?
• lines 158-159: since RC filters are not inert, was the retention of the filter investigated?
• lines 160-162: please describe the applied instrumental parameters for ICP and ICP/MS methods.
• A key information, namely the description of the applied milling procedure is missing from the manuscript. Please amend and present the milling procedure and applied conditions!
• lines 230-232: the OF-U sample morphology should be adequately described. It seems aggregated particles are visible, the “intermediate situation” is not informative and should be explained. The displayed image of Figure S2/c should also be considered as the magnification and lighting conditions differs from ‘a-b’.
• Figure 1: quality should be improved, lines are shaky and their visibility could be improved. How was the fitting done for OF-U sample, as there are no measured point above cc. 700 C_e ?
• Figure 2, Figure 4, Figure 5: quality should be improved, lines are sometimes shaky and their visibility could be improved.
• The results are promising, however, the stability and the sedimentability (separation) of the milled adsorbents should be determined for a proper assessment of their practical application. The zeta-potential values suggest well-separability, however, was the it investigated under real conditions (with different pH, T or in presence of other ions) and after adsorption?

 

 

Mino issues, typos:

• line 79-80: “industrial byproduct based in Fe(OH)3“ should be “based on” or “made of”. Please revise.
• line 139-140: “was the stirring system chosen” should be “the stirring system was chosen”
• line 158: decimal points should be used (0,2 μm should be 0.2 μm)
• line 226: the term “big boulders” might be unfortunate to describe microstructures observed in SEM.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The last two paragraphs of the introduction should be improved. The innovation of the work must be presented.

It is common knowledge that, generally, a smaller particle size of adsorbent presents a greater surface area of ​​contact with the adsorbate. Thus, the work should investigate the adsorbent-adsorbate relationships more deeply, since it is already expected in most adsorption tests and there is no innovation in this sense.

In lines 114-118, two types of solvents are mentioned for carrying out the adsorption tests, the reason for this solvent change must be described in the methodology and not in the results.

Section 2.4 (113-162) presents the information in a confusing way to the reader, please improve the description of the methods.

Please explain why, on line 243, R² > 0.8 is considered a good fit (change the comma to ".").

On line 272, insert the author of the reference.

Other kinetics models can help to improve the discussion of results, such as the general order model. In addition, thermodynamic tests and characterizations such as FT-IR before and after adsorption must be carried out. The adsorption mechanisms must be discussed in depth.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The Manuscript has improved considerably. Since almost all of my comments were answered, I recommend accepting it in its current condition.

Reviewer 3 Report

After reviewing the updated version of the manuscript, I recommend its publication in the journal Processes. The authors were dedicated to filling the main gaps in the present work.