Competing Sorption of Se(IV) and Se(VI) on Schwertmannite

Round 1

Reviewer 1 Report

Comprehensive research was carried out for elucidating the mechanism of Se(IV) and Se(VI) sorption on schwertmannite specimens. While sorption of single oxyanion species on the iron mineral has been reported repeatedly, the results include new outcomes in respect with the competitive sorption kinetics between the two species. These could be invaluable for considering application of the mineral to wastewaters as well as the geochemical implications.

Specific comments:

1. L147-149. SorbP and others are stated to contain small amount of goethite. Can authors rule out the possibility that the trace goethite affect the Se sorption kinetics? This may be stated in Discussion.

2. The sorption experiment was conducted over 56 days. Given that schwertmannite may transform into goethite within several weeks to months, I suggest additional XRD data showing whether the initial mineral phase has maintained during the sorption experiment.

Author Response

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Reviewer 2 Report

I very much pleased this work, whose objective was to study the competitive sorption of Se(IV) and Se(VI) on schwertmannite under highly acidic conditions. This experimental study highlights the effect of the presence of single and mixed species of Se(IV) and Se(VI) on partitioning of such species into schwertmannite by using spectroscopic techniques. The manuscript is well organised and I consider this to be an excellent paper. The experimental work is precise and the applied methodology is adequate to obtain interesting results. All the figures appropriately demonstrate results described in the text.

In my opinion, this is a useful research to understand the competitive sorption of Se(IV) and Se(VI) onto schwermannite. From a fundamental research point of view, I miss the XRD characterization of the solid phases obtained after adsorption, that is at the end of the reaction time to check the possibility that mineral transformation occurs after aging. It would be interesting planning future research to address the study of the competition among S(VI), Se(VI) and Se(IV) to evaluate the efficiency of schwertmannite as sequester of selenium species.

 

I would recommend this study for publication without changes.

 

 

 

 

 

Author Response

Thank you for your comments. We have added additional XRD data for samples after 48h and 56d as Figure S2 in the Supplementary Materials. The results showed that the initial mineral phase was maintained during the sorption experiment

Reviewer 3 Report

The article presents the results on Se sorption on three different schwertmannite samples, which possess certain scientific novelty and may be potentially used for development of water treatment systems. The study presented is well-structured and the results support the conclusions. 

Thus, the article may be accepted after minor revisions.

1. Check the reference 23 and other:

Page 2: Three different SHM specimen were used in the current study: SorbB was generate by microbial oxidation of Fe(II) in a mine water Fe removal plant operated at pH 2.9–3.2 74 [23]……

Page 4: Sulfate concentrations were measured spectrophotometrically by the BaCl2-Gel-129 atin turbidimetry method [23] at 420 nm.

References: 23. Tabatabai, M.A. A Rapid Method for Determination of Sulfate in Water Samples. Environ. Lett. 1974, 7, 237–243.

The reference 23 is cited twice (pp. 2 and 4). The reference regards to the description of the sulfate determination method. Probably, SorbB description should be in other work.

2. Some differences between SHM samples in Se adsorption were observed (Fig. 2). Could you add some assumptions regarding the reasons of these differences based on the results obtained on the structure and composition of the studied SHM samples?

Author Response

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Reviewer 4 Report

General comments

 

The manuscript investigates the sorption of selenium when Se ⁴⁺ and Se⁶⁺ coexist in schwertmannite. The authors used three samples of schwertmannite obtained in differents ways and compare the adsorption of Se⁴⁺ and S⁶⁺  on them. The manuscript is of interest and complkements thoose on the adsorption of selenium in oxides and schwertmannite.

The information provided is detailed and authors made a deep study presented in a well-structured manuscript. I made sufgggestions for minor  changes and additiuons.  

Specific comments

It is important to note that this also is an essential nutrient.

It should be important for the readers to know why you choose schwertmannite?

Figure 1. please, indicate which is the position of the main peaks of schwertmannite and goethite.

In lines 152-154  you comment the morphology and size of the schwertmannite materials used in the experiments. How did you observe this?. You could indicate this aspect in the materials and methods section.  I also consider that the Figure S2 should be interesting to be in the main body of the manuscript.

I know that in Minerals a conclusions section is not mandatory but in this case this should be useful for the readers.

Author Response

Thank you for your comments. Please see the attachment

Author Response File: Author Response.docx