Removal of Arsenic(III) Ion from Aqueous Media Using Complex Nickel-Aluminum and Nickel-Aluminum-Zirconium Hydroxides

Round 1

Reviewer 1 Report

Manuscript Number: water-816395

Title: Removal of Arsenic(III) Ion from Aqueous Media Using Complex Nickel-Aluminum and Nickel-Aluminum-Zirconium Hydroxides

Article Type: article

 

In the manuscript the experimental research concerning removal of As(III) ions using nickel-aluminum and nickel-aluminum-zirconium complex hydroxides is presented. Authors investigated adsorbents physicochemical properties, adsorption kinetics, impact of pH on the phenomenon and adsorption isotherms. They proved that incorporation of zirconium into complex hydroxide resulted in higher specific surface area and in result larger amounts of As(III) could be adsorbed. According to WebOfKnowledge database the Authors have vast experience in this field of science. Their previous works were focused on the phosphate and borate ions removal. The manuscript is well written. Everything is clearly presented and discussed. Below I am presenting my remarks:

 

1. In the abstract Authors stated that they prepared hydroxides for As(III) removal yet in the “Materials and methods” section it is written that these substances were bought from Kansai Catalyst Co., Ltd. Therefore Authors have to explain whether the hydroxides were synthesized by them or they are a commercial product.
2. The purity grade of all reagents as well as the quality of water (conductivity) should be given in the text.
3. Equations 1 and 2 and their description have to be corrected. qe represents the amount of adsorbed solute at equilibrium and the qt is the amount of solute adsorbed at the given time t. I do not understand, why these equations use “experimental” and “calculated” amounts of As(III).
4. In the text Authors refer to the correlation coefficient. On the other hand in tables coefficient of determination is presented. Authors have to correct this issue.
5. Figure 5 and table 2: How was the equilibrium amount of As(III) determined? It seems that it was the value measured after 24 hours of experiment. But Figure 5 suggest that the equilibrium value for the NAZ1 sample will be higher than 15.2 mg/g. Could authors comment that issue?
6. Could authors comment such significant deviation of results in the case of NAZ1 sample in 25 C? What is the reason for such big deviation bars present in Fig. 7 for this sample?
7. This publication would be more interesting if the Authors would compare the obtained results with other publications dedicated to As(III) removal.

 

I think that the manuscript is very interesting and it can be published after minor revision.

Author Response

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Author Response File: Author Response.doc

Reviewer 2 Report

The manuscript explains the usage of complex metal hydroxides such as nickel-aluminum and nickel-aluminum-zirconium hydroxides for wastewater treatment namely arsenic ion removal from aqueous solution. The manuscript is well written and brings new insight into the field of metal hydroxides application for wastewater treatment. I recommend the manuscript for publications with minor revision if the author’s answers and comments with the points raised here.

• After “ The predominant forms of inorganic arsenic in aquatic systems are As(V) (arsenate) and As(III) (arsenite). “  Please mention “from where the arsenic gets to waters.
• Multi-metal hydroxides exhibit substantially different physicochemical characteristics than their single-metal counter parts, It is good to mention modified metal hydroxide with graphene oxide for removal of ions too as graphene oxide showed many application of removal of ions and its strong interaction with multiply charged ions ( Das. et al. Phys. Chem. C 2016, 120, 26, 14088-14100)
• Authors mentioned “It is clear that adsorption of As(III) onto NA11 and NAZ1 is complicated, because the time to equilibrium would be short if the process were controlled by a single factor such as ion exchange [16]. Is there any evidence that chemisorption also happened in this process?
• It is expected for adsorption process the amount of “point of zero charge” for different used compounds of the experiment to be reported, please provide such information or discuss it.
• “The pKa,1 of H3AsO3 is 9.2, which indicates that As(III) is a neutral species at pH 9.2. At pH >9.2, anionic forms of As(III) are produced that undergo ion exchange with sulfate ion from NA11 and NAZ11 under the experimental conditions”. Please correlate this information on the “point of zero charge” of surfaces and the functional groups such as –OH groups at the surface and pKa values.
• The quantities of As(III) adsorbed increase with increasing temperatures (10 < 25 < 45 °C), which indicates that chemisorption is involved in the adsorption of As(III) on NA11 and NAZ1.In which pH values? Please provide information for all pH values. Do the higher temperatures change the type of spices in the solutions such as different oxides in different pH values? Please discuss it.
• The results indicate that the release of sulfate from NA11 and NAZ1 by ion exchange has a role in the adsorption mechanism of As(III). Please mention the equation.
• Complex metal hydroxides have the ability to undergo ion exchange with charged species in the aqueous phase [19]. Isn’t it disagree with your chemisorption conclusion.

 

 

Author Response

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Author Response File: Author Response.doc

Reviewer 3 Report

This is an interesting article reporting the use of complex Ni-Al and Ni-Al-Zr hydroxides for the adsorption of As(III) from water. From my knowledge and research, there have been no studies yet on this topic, and although a lot of literature is available on different adsorbents for arsenic adsorption, it can be considered that this work is novel.

The work is well structured and well supported and integrated in the existing literature. The English expression throughout the paper is good. In global terms, the methodology is well thought out, but there are issues with the presentation and interpretation of results that, in my opinion, need to be addressed before the manuscript is suitable for publication:

• In section 3.1 and Table 1, it is shown clearly that NAZ1 is the adsorbent with the higher specific surface area, and in section 3.2 this is correlated with the As(III) adsorption capacity, making the results coherent. However, in lines 136-138 you state that the incorporation of Zr⁴⁺ was what caused the increase of the specific surface area. However, NAZ2 adsorbent has an even higher ratio of Zr to the other metals (namely Al), and we do not observe the same effect. How do you explain this?
• In lines 151-152, and reflecting to the rest of the text, you select NA11 and NAZ1 to evaluate As(III) in the following experiments. I can understand why you chose NAZ1 as it is clearly the best performing material, but why NA11 since it was the worst performing adsorbent?
• The results of XPS need to be better explained and pu tinto contexto. Arsenic is usually identified in XPS by the As3d peak, which is at around 45 eV, not 262 eV. What does 262 eV apply to? You should also identify the peaks close to it in Fig. 4, which also suffer changes before/after adsorption.
• In section 3.3, you state that equilibrium was reached within 24 h, but was it really reached? It doesn’t seem so by observing Fig. 5, since a plateau does not seem to have been reached, at least for NAZ1. The curve for NAZ1 also does not seem to correspond exactly to that which can be derived from the parameters presented in Table 2. For NA11, qe,exp is identified as being 9.1 mg/g but by the graph it seems more like 8 mg/g. Please review these results.
• Both in kinetic and Langmuir fitting, it would be preferable to perform non-linear regressions rather than linear simplifications, which introduce a lot of error in the fits. Please check Simonin (2016), “On the comparison of pseudo-first order and pseudo-second order rate laws in the modeling of adsorption kinetics”, Chemical Engineering Journal 300, 254-263.
• Regarding the adsorption mechanism: In lines 161-163, when interpreting adsorption kinetics, you say that ion exchange cannot be the main adsorption mechanism otherwise the equilibrium time would be shorter; then by analysis of the kinetic models, you conclude that adsorption occurs by chemisorption (line 170); however, in section 3.6 you state that by the correlation with the release of sulfate, the main mechanism of As(III) uptake is ion Exchange. How do you reconcile all these different views?
• Conclusions need to be thoroughly reviewed. Line 234 is not a conclusion, since the incorporation of Zr⁴⁺ into the adsorbent is not shown in this work. Lines 238-239 is a vague and speculative statement. Kinetic data do not show anything regarding the thermodynamic nature of the process.

Some other minor corrections I suggest:

• Lines 22-23: instead of “…were prepared for removal of arsenic ion, As(III)…” write “… were prepared for the removal of arsenite ions, As(III)…”
• Line 27: instead of “XRS” the acronym should be “XPS”
• Insert references for the statement on the IARC (line 44) and of WHO limits (line 45).
• Lines 91-92: please explain in more detail in what consists the fluoride-ion adsorption method. The reference was not very clear.
• Line 162: instead of “the time to equilibrium” write “the time to reach equilibrium”
• Line 194: write “Isotherms” instead of “Istherms”
• Figure 7: what pH were these isotherms carried out?
• Line 236: instead of “also was greater” write “was also greater"

I hope you find my comments of interest for the improvement of your manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.doc

Round 2

Reviewer 3 Report

I appreciate the response the authors have given and the effort they have put in improving the manuscript. However, there are a few points that although a reply was given, in my opinion, the modifications were insufficient:

• The response given for the As peak was a weak justification. As LMM peaks between 150 and 350 eV overlap with many other elements (check https://xpssimplified.com/elements/arsenic.php ). If you cannot detect As 3d peaks, I think it is speculative to identify As in the surface just by this 262 eV peak. I suggest you remove XPS results entirely.
• The response given for the choice of NA11 along with NAZ1 is valid, but was not included in the text, so it is not available to the reader. I suggest some explanation is included after line 174.
• The maximum adsorbed capacities of As(III) reported in the Langmuir isotherm and in Table 4 are not consistent with those cited on the Abstract (line 25) and the conclusions (lines 273-274). In fact, the adsorbed capacities you cite there, which for NAZ1 are larger, correspond to the adsorbed capacity at equilibrium from the kinetic model. There is a contradiction here between the isotherm and the kinetic models, since the kinetic model report a q higher than the maximum allowed by the isotherm. There are two explanations for these results not to be consistent:
  • In fact, despite your explanations, 24 h is not time enough to reach equilibrium, leading the results of the isotherm to be lower;
  • There are too few experimental points collected for the isotherm studies (Fig. 7 is clear that none of them reaches the plateau for maximum adsorption), and more points need to be acquired, for higher initial concentrations.

Other than that, I have another few minor suggestions of correction due to the alterations done in the manuscript:

• Line 47: instead of “occurred natural sources” write “occurring from natural sources”
• Lines 62-64: I do not know why this reference was added but it seems out of context here, since it refers to graphene oxide (not a metal hydroxide) and not to the removal of arsenic. I suggest its removal.
• Line 86-87: instead of “were synthesized the following method” write “were synthesized by the following method”
• Line 88: “at heating” – please be more clear, provide temperature. Instead of “the reaction mixture added” write “the reaction mixture was added”
• Line 89: instead of “The solution pH was adjusted by 25%” write “The solution pH was adjusted using a 25%”
• Line 197: instead of “previous study pointed out” write “a previous study pointed out”
• Lines 199-200: instead of “indicates that the experimental data is fitted to the kinetic model” write “indicates that the kinetic model fits to the experimental data”.
• Lines 201-202: instead of “indicates that the obtained data is considered to be better much for the pseudo-second-order model” write “indicates that the pseudo-second-order model is considered to be better for the obtained data”
• Line 266: define WTR

I hope you find my comments useful for the improvement of the paper.

Author Response

Please see the attachment. 

Author Response File: Author Response.doc