Evaluation of Adsorption Mechanism of Chromium(VI) Ion Using Ni-Al Type and Ni-Al-Zr Type Hydroxides

Round 1

Reviewer 1 Report

I carefully read the manuscript “Removal of Chromium(VI) Ion from Aqueous Media Using Ni-Al Type and Ni-Al-Zr Type Hydroxides” and I found it well-structured and interesting. For this reason, I recommend its publication, but some aspects should be revised.

1. The title refers to Ni-Al type hydroxides, beyond the Ni-Al-Zr ones, but that kind of materials are described as the subject of a previous paper and with the same application. Thus, I would suggest to focalize the title on the novelties and to specify which are the new/previous results along the text.
2. In the abstract, the materials are labelled as NA and NAZ, but the presence of numerical codes (11, 12, 1, 2) should be justified, i.e. indicating that the differences are in terms of component ratios.
3. The choice of the compositions is not introduced by any premise.
4. No mention about the synthesis method is reported and the presence of sulphates is quite unclear if not supported by synthesis details.
5. The release of metals and its detection is not mentioned in the “Materials and Methods” section.
6. Why are only NA11 and NAZ1 deeply investigated?
7. In the summary of characterization results from previous studies, authors should describe the physical chemical features, distinguishing the type of properties (i.e. morphological, crystallographic, etc.). This could make the description more ordered.
8. Line 168: there is a mistake in the formula MgAk (Al?). Moreover, regarding the comparison between different works about Cr(VI) adsorption, the value of 30.28 mg/g (ref 30) can be considered of the same order of magnitude of the present paper results, although, in the comment, the authors underline a significant difference.
9. The discussion about pH influence is the weakest point of this manuscript. The overall comment is not very clear; the results obtained at pH 7 and 9 are not so different. Lastly, when the materials acquire a positive charge, shouldn’t Cr-anions be more attracted by the surface? This part should be strongly revised, since it is the core of adsorption mechanism explanation.
10. How can the authors know that the adsorption is a chemisorption?
11. In the pictures of elemental distribution, NA11 shows a remarkable difference with respect to the other sample and this aspect is not evidenced and commented.
12. The results should be indicated with the standard errors, as reported in the method description, but these values are not present. In particular, in the case of temperature effect, the differences between the results do not seem significant, especially without the error bars.
13. Line 276-277: In the sentence “These phenomena agree with the adsorption behaviors  of  Cr(VI)  using  NAZ1”, which is the term of comparison?
14. The conclusions may be more impactful.

Author Response

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Author Response File: Author Response.doc

Reviewer 2 Report

This manuscript synthesized two double hydroxides and two triple hydroxides, and studied Cr(VI) removal as a function of pH, Cr(VI) concentration, temperature and so on. This study matches the scope of Water. However, the manuscript is not well-written and some complementary work is required. I would recommend the publication of manuscript after a major revision.

 

1 Although the English-writing of this manuscript is generally standard, the improvement of English is still required

 

2 Authors indicated that aluminum released from nickel-aluminum hydroxides is higher that form nickel-aluminum-zirconium hydroxides, which means that the latter is more stable. This should be the highlight of this paper, however, authors did not explain the reason.

 

3 Authors used mg/g to describe the Cr(VI) adsorption and evaluated the performance of adsorbents. However, this is unreasonable, because the mass of nickel-aluminum-zirconium hydroxide is higher than nickel-aluminum hydroxides. Maybe authors should try with mg/m².

 

4 Line 152-153, “The correlation coefficient between the quantity of Cr(VI) adsorbed, and the surface hydroxyl group was 0.890”. This is very confused, because quantify surface hydroxyl group is difficult and I did not find how these were quantified in this study.

 

5 Line 155-156 “other factors, such as specific surface area and pHpzc, did not affect the adsorption capability of Cr(VI) from aqueous media”. This is very confused too. Specific surface area is defined to be unimportant unless authors can demonstrate Cr(VI) adsorption on these sorbents is controlled only by anion exchange

 

6 Fig.5 is important because it elucidated that adsorption is almost controlled by anion exchange. I did not agree that XPS analysis can be called as “mechanism study”, because authors only find Cr spectral existence after adsorption experiments. Authors should try more discussion on adsorption on interlayer sites and/or edge sites of sorbents.

Author Response

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Author Response File: Author Response.doc

Reviewer 3 Report

The Manuscript “Removal of Chromium (VI) Ion from Aqueous Media Using Ni-Al Type and Ni-Al-Zr Type Hydroxides” requires revision before accepted for publication. The specific comments are given below.

1. Clearly define the research hypothesis.
2. What methods were used to determine the statistical significance of differences between the analyzed variables? What level of significance was adopted?
3. Statistical research is very important in experiments. For example, the hypothesis on distribution of each analyzed variable can be verified with a Shapiro-Wilk test. One-way analysis of variance (ANOVA) can be used to determine the significance of the difference between variables. Variance homogeneity in groups can be checked with Levene's test, whereas the significance of differences between the analyzed variables can be determined with a Tukey’s HSD test. Complete the methodology.
4. Ln 44, 166, 197 Avoid group citations.
5. Add the standard deviations in the results listed in the text.
6. The manuscript discussion part needs more discussion and compared with other research results.
7. Add „Abbreviations” before „References”.

Author Response

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Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

The manuscript was revised in accordance to my comments, but the added parts need to be checked in terms of English form.

Reviewer 2 Report

Revised manuscript is ready for pubilication.

Reviewer 3 Report

Thank you for considering my comments.