Adsorption/Desorption Capability of Potassium-Type Zeolite Prepared from Coal Fly Ash for Removing of Hg²⁺

Round 1

Reviewer 1 Report

The authors report their findings on using fly-ash zeolites for removing Hg2+ ions from water. It is claimed that prepared zeolites are effective sorbents. Their applicability for regeneration is also marked with desorption capacity of the most studied sample here (FA48). The article is well written, it concerns 6 figures, references 46 articles.

The biggest flaw in this article is in my opinion not satisfactory characterization of the samples under study. I understand that authors already published the results from other methods (ref. 27), but for me there is no a direct indication why the sample FA48 has the highest CEC. Do you know how much Al atoms are in the structure? It directly impacts the adsorption capacity.

What is iCAP 7600 Duo – is it ICP?

There should be error bars in Figure 1.

Why initial concentration is so broad in results in Figure 2?

You write that the adsorption capacity in binary solutions is influenced by the radius of hydrated ion and electronegativity – what are those for Hg2+?

Author Response

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Author Response File: Author Response.doc

Reviewer 2 Report

The following manuscript "Adsorption/Desorption Capability of Potassium-type Zeolite 2 prepared from Coal Fly Ash for Removing of Hg²⁺" is an interesting manuscript about investigating the possibility of Hg²⁺ removal from aqueous media by using K-type zeolite prepared from coal fly ash, especially since the extraction of mercury from aqueous media is necessary for human health and conservation of water environment.

On one hand, The effects of contact time, pH, temperature, and selectivity on the adsorption of Hg²⁺ were appraised. The authors found that the value of FA48 (potassium-type zeolite) was greater than that of sodium-type zeolite, as a result the adsorbed Hg²⁺ onto FA48 was easily desorbed using a sodium hydroxide solution. Also, the adsorption temperature did not strongly affect the adsorption capability of FA48. These techniques may aid in mitigating heavy metal pollution and thus contribute to the establishment of a sustainable society. On the other hand, I noticed that authors have published reference 27 in a sister journal, viz., WATER, which is entitled as "Removal of Pb2+ from Aqueous Solutions Using K-Type Zeolite Synthesized from Coal Fly Ash", according to that, slight changes and very similar behavior is found in these two manuscripts. Accordingly, I do not think this is a very important contribution to be considered in your respected Journal.

Overall, this study is well designed. However, kindly take the following comments into consideration:

1. Please note that the journal requires selective keywords; words that are precise and do not already feature in the title are ideal.

2. Over use of the conjunction word (and).

3. Please note that the journal requires the results to be at a separate point from the discussion.

4. Which program is used for the used figures? Is it excel, SPSS, etc...

5. You mentioned the (big three) heavy metals mercury, lead and cadmium. But you only referenced the lead and cadmium.

6. Kindly edit the following references [8, 13]:

7. Although the references are up to date, these two references are not [29, 36].

Author Response

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Author Response File: Author Response.doc

Reviewer 3 Report

Manuscript ID: sustainability-1158245

Title: Adsorption/Desorption Capability of Potassium-type Zeolite Prepared from Coal Fly Ash for Removing of Hg²⁺

Authors: Yuhei Kobayashi et al.

Line 51. Add information from 2019-2020. 2011 is so old information.

Line 53-55. Reference [13] from 2005. Today only in China generated more than 550 Mt of CFA per year. I think in all world this amount Is about 800-900 Mt per year, not 120 Mt.

Line 55, 58. Use CFA for all text – coal fly ash, not fly ash.

Line 67-70. Add references for each method, because it is hard to understand what references are refer to which methods.

Line 98. Why even 93 ºC used in zeolite preparation experiments?

Line 118-120. Add formula for this calculation.

Line 157. Write full name of “CEC” abbreviation.

Line 174. Is it possible to write “hydrothermal synthesis” at 93 ºC? If authors use atmospheric pressure it is not hydrothermal process.

Figure 1. Why FA samples better that FA1? Add data for FA1 to the figure 1.

Figure 2. How was 7 ºC of solution was obtained? Add this information to Methods section.

Line 207-208. Is it possible to use Sips equation to represent adsorption properties?

Figure 3. Zoom in on the figure. Take the range on the Y-axis 0.03-0.055, and on the X-axis 0.1-0.13.

Figure 5. X-axis title do not write “Elapsed time”. Write “adsorption duration” or “adsorption time”.

Table 3. Is it possible to use Elovich model? Q_t =   ln (αβ) +  lnt.

1. Is it possible to use a high-pressure reactor to intensification reaction time of the zeolite preparation? 48 hours is a very long process.
2. Authors can add a discussion about the application of the results for the purification of solutions from mercury. Where can these zeolites be used in Japan? For what type of water purification?

Author Response

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Author Response File: Author Response.doc

Reviewer 4 Report

In this work K-type zeolite samples prepared from coal fly ash were used as adsorbents to remove Hg²⁺ from the aqueous media. This is a very interesting work that aims the valorization of an industrial waste - coal fly ash – to remove an important water contaminant – Hg²⁺.

The experiments were well conducted, and the paper is written in a comprehensive way. In my opinion the authors only need to perform minor modification upon acceptance:

• Results and discussion: Although the authors mention that the characterization of the zeolite samples is published previously and a summary of the main properties is described in the text, I consider that an easier reading could be attained if the information is preset on a Table.
• Figure 1 - Why the adsorption experiments were performed after 24 h contact if the kinetic results presented ahead show that after approximately 3 h for FA48. Was it expected that the kinetic curves of the other samples present a different behaviors?
• Figure 5 – Caption: contact time: 24 h ? I believe is a typo.

Author Response

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Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

In my opinion, the article could be now considered for publication in this Journal.

Reviewer 3 Report

The authors responded in detail to all comments. The quality of the figures has been significantly improved. The authors added a more detailed discussion of the results using additional equations.

The article in this form can be accepted to Sustainability.