Characterization of the Physical, Chemical, and Adsorption Properties of Coal-Fly-Ash–Hydroxyapatite Composites

Round 1

Reviewer 1 Report

Comments on the paper by Soco et al on Coal Fly-Ash-Hydroxyapatite Composites

The paper concerns the metal or dye removal abilities of some supposedly novel coal flyash / hydroxyapatite composites. It represents a study in the repurposing of waste, this waste primarily being the flyash byproduct from various industries in Poland.

In general, although much data has been obtained, I do not find the paper publishable in its present form as I have concerns about 1) some of its scientific design especially its rationale in some procedures and 2) concerns about interpretations in the FTIR analysis which are plainly wrong in one section of the paper towards the end.

I feel some corners have been cut in doing the work and that more awareness or attention should have been paid to some aspects. It wasn’t clear why some procedures were followed or at least the reasons were somewhat superficial and did not fully take into account consequences.

Finally, English is problematic in some parts. It needs the attention of a proficient or native English speaker to go over the paper and check the grammar and idiomatic use of some of the English expressions used. Finally it would appear that residual Polish language appeared in some tables that had not been translated into English. Some instrumental technique names were also not described correctly. The paper does need substantial revision.

I make the following suggestions for improving the paper.  In doing so my preliminary decision is to reject the paper but to invite resubmission once they have addressed these points.

Introduction: Overall good but please check the English. The biggest issue is the identification of research gaps in the study which were not clear hence the aims though stated were not indicative of where this current work stands in relation to other work in the scientific literature. Surely others have considered making these fly-ash/HAp composites? Where are the previous studies and what were their results? A more concerted effort should have been made to search for these studies. Then the placement of the current study in relation to existing studies could then be made clear.

Materials and Methods:

Some of my most fundamental questions arose in this section.

The description of the coal fly ash was brief. We only got a classification but this said nothing about its physical state or appearance. We were told about a characterization even before we were informed about how it was analyzed. This seemed too brief.

Synthesis: These composites were made by chemical precipitation. It was not clear how the fly-ash once received from the supplier had been processed. Did it need washing or was it used as received? Details are needed. “White and grey” CFA-HAP ppts are referred to. Presumably, this was how the product appeared though its description was somewhat ambiguous. The white is presumably HAp and the grey is flyash. This wasn’t helped by the fact that flyash was not described formerly.

“The reagents were characterized by a theoretical Ca/P ratio of 1.67” …this suggests that the Ca and P amounts were in this mole ratio. The question begs …were these Ca:P mole ratios checked in the final product or were they just taken for granted? I am guessing some analysis took place because we get proportions of oxides present but it was not clear if Ca:P mol ratio was checked in the HAp/CFA precipitates. What it seemed was that these materials were just made and the assumption made that they were Ca:P 1.67: 1 …there was one vague sentence stated that this was mostly true but largely we are left wondering what the Ca:P mole is… what we would have expected was that (at least) the CaP part of the composite was dissolved any chemically analyzed to give the Ca:P mole ratio by some technique like  ICP-MS. FTIR analysis on some precipitates made without the flyash would have been instructive because one would assume the flyash does not have too much of an effect on the formation of these. At least then we would have some idea of the true Ca:P mole ratio in these ppts.  I put some emphasis on this because the FTIR indicates there are carbonates present in the ppts which can affect Ca:P mole ratio so making an assumption scientifically that it is “All Ca:P mole ratio 1.67” is perhaps misleading and scientifically naïve.

The next part of the experimental studies that I found unclear as to purpose and rationale was the soaking experiments. The composite was added to the solutions of Cu(II), pH was adjusted up to 11 and then agitated. I was somewhat surprised because I thought surely that the Cu(II) would precipitate out and simply stick to the composites without the Cu(II) actually adsorbing. Now there does appear to be some awareness of this in discussion though it is not well discussed. Their rationale for adjusting the pH to 11 is not clear but the reason of the HAP not dissolving was given. However is this a good rationale in relation to how it might be used in real life? Also, how can you divorce the effect of the simple precipitation process and sticking to the composite or remaining suspended in water from actual adsorption? I did see a discussion on this when comparing adsorption and ion exchange and there was even a table illustrating it but the explanation of how those results were arrived at was completely lacking ….Why not do studies at different pH where Cu(II) is not precipitating? Is there a practical use at pH 11? Actually there already exist methods for precipitating out metal ions using alums. It would be more useful to consider these composites at neutral pH at least for the metal ion removals.

Surface area measurements: It was good that these were obtained but are there issues with doing these measurements on composite materials? There was no detail given on how samples were prepared for this or what possible complications could be. (line 184)

I did not understand their pzc measurement method insofar as the determination from the equation. That needs more explanation. (line 195)

It is not clear how they calculated Ca:P ratios from their data for their products. They give a table of oxides but no discussion on how they did the calculation. It was completely unclear and vague.

FTIR analysis…these were only conducted on the composites …and they could not see the CFA in the spectra. This can be understood from their explanations but then why did they not do FTIR spectra of the flyash itself? It seems strange not to do this. In addition, they could have easily generated HAp without the flyash present and done the FTIR of that. It is not difficult. Then there could have been better clarity of the individual components. In some FTIR spectra, the detected carbonate component was quite variable in relative intensity to the phosphate peaks. They distinguished between the carbonate types but did not explain so much why this had occurred. Could this have been due to variable pH values in the suspension mixtures when they made their composites? We are left with little details. It is important to make some level of observation to try and explain these differences otherwise that information is lost and people coming along later to make similar materials cannot reproduce the work... The caption in Figure 1 is plainly wrong …it is an FTIR spectrum not what the authors have written.

Their pzc data and interpretations seem OK. It is not always calculated or discussed so their inclusion of this was good.

I also had no issue with the adsorption modelling except that the data acquisition for constructing the adsorption isotherm should have been better explained. I also did not pick up the significance of the Redlich Peterson isotherm. How this is seen from a physical point of view was not explained clearly in the manuscript.

Comparisons of adsorption data with HAP alone, CFA alone …from the paper and analyses it seemed that the composites were “mostly” HAp …there was some attempt to make comparisons with HAp alone studies and some CFA alone studies but it was not very clear. This was not helped by the unclear English expression used in this section (line 283-290) They should tabulate the comparisons with their work …and perhaps put it in Table 5.

Desorption (line 300): This was studied which was also interesting. However, their discussions in relation to Cu(II) removal need clarifying as I indicated earlier …especially in relation to how they came to construct Figure 4. It is not at all clear how they got the data for this from their work.

FTIR after adsorption and desorption. … Given this is mostly a bulk analysis technique the fact they saw little change in their spectra is hardly surprising. However, in this section, they make a major mistake in interpretation and state that a band at 2924 cm-1 is due to “C-N”  binding….this is plainly wrong and raises concerns about their ability to interpret FTIR spectra. 2924 cm^-1 is a C-H stretching vibration. It is probably the C-H band from the dye adsorption but nothing to do with C-N. The discussion referring to this needs to be completely changed (basically removed and revised). What seemed interesting was that the carbonate content in their composites increased and that was probably due to the (in my opinion) unjustified experiment occurring at pH 11. It would seem that doing the adsorption at that pH would have simply led to the increased propensity of the suspension to produce calcium carbonate merely by carbon dioxide/alkali interactions. They did not seem to show awareness of this in their discussion

Also, statements are made about adsorption in relation to the “undissociated lactone” which is really very vague. I see other statements invoking “lactone” and there is no reference made to any molecular structure of the dye ( a picture of which would help) so that statements like these are unhelpful to the reader.

In conclusion, the work should have been described with more chemical interpretation and better explanation. There seemed to be a lack of awareness of this in some aspects of the study.  It is important also to place their work in the context of the present state of the art. There must be numerous studies on these composites so an awareness of the previous literature should have been demonstrated.

Author Response

Dear Reviewer,

we are grateful for your discerning comments and positive critique leading to improving the manuscript. We appreciate the time you spend on our manuscript. We are aware of the shortcomings in our English, but the text of the manuscript was translated by a native speaker (commissioned work). Of course, we are willing to outsource linguistic proofreading at the final stage. We tried to clear up the doubts and to fill the gaps. Please see the revised manuscript with corrections marked red. The answers are in the attachment.

Best regards,

Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

This study characterized the physical and chemical properties of four composites prepared by the hydroxyapatite wet precipitation method under different reaction conditions and their ability to sorb Cu(II) and rhodamine B dye. This manuscript was within the scope of our journal. Overall, the manuscript is interesting and well prepared. Therefore, I would suggest it for publication after moderate revision. There are suggested changes to clarify and some parts and help the reader, as follows:

1. Coal fly ashes can retain them as mentioned by authors. However, they generally contain potentially harmful elements such as Cr, As, etc. The environmental risks for usage of coal fly ashes should be touched somewhere in the main body.
2. L141–143: The desorption test was conducted under pH of 2. The pH value was maintained during the test or not. This should be clarified in the main body because the composite materials prepared in this study have an alkaline pH. If did not, it is better that the equilibrium pH value after the test is added.
3. L313–325 and Fig. 4: this study just conducted desorption test under pH of 2, and did not divide the sorption mechanisms into chemisorption and ion exchange. At least, there were not any chemical evidences. Authors should clearly separate the results from discussion. In Fig. 4, the legends should be changed to “desorbed under pH 2” and “not desorbed under pH 2” from “ion exchange” and “chemisorption”, respectively.

Author Response

Dear Reviewer,

thank you very much for your commitment to improve the manuscript. We appreciate your insights. We tried to clear up the doubts and to fill the gaps. Please see the revised manuscript with corrections marked red. The answers are in the attachment.

Best regards,

Authors

Author Response File: Author Response.pdf

Reviewer 3 Report

Coal fly ash-hydroxyapatite composites were synthesized for adsorbing heavy metals and dyes. The topic of this is study is meaningful. However, several problems exist in this manuscript:

1) How about the mineral composition of coal fly ash and the composites?

The structure of hydroxyapatite in the composites should be further characterized and verified. XRD analysis should be done.

2) I think that the physical-chemical characteristics and the adsorption performance of the raw coal fly ash should also be investigated. e.g. FTIR, pH_PZC, BET specific surface area and BJH total pore volume, Cu(II) and RB adsorption equilibrium for raw coal fly ash should be done.

Author Response

Dear Reviewer,

thank you very much for your commitment to improving the manuscript. We appreciate your insights. We tried to clear up the doubts and to fill the gaps. Please see the revised manuscript with corrections marked red. The answers are in the attachment.

Best regards,

Authors

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Regrettably I found the rebuttal to my points more difficult to follow than the original manuscript!  I think the revision attempt is poor and has not been seriously executed with a view to clarity

I am left none the wiser about the revised manuscript. 

Abstract has poor English still which confuses people .. "lactone" appears without support  as to what it means in the abstract ...despite my saying it needed explanation. It was done in the manuscript but in the abstract it remains

I still cannot understand why the experiment was done at pH 14 apart from a reason about the pH of dye solutions but what about Cu(II) solutions?  The comments on this were not formulated with any clarity in mind. The revisions don't help

What the authors need to understand is that the paper should be 100% clear as to what the study was about to a reader without any vagueness

It is really hard to follow.  The English still needs improving

The mode of sampling of the flyash is really vague ...it was done to a Polish standard ....so what does that actually mean? 

The 2924 cm-1 peak was stated as being due to "Aromatic" hydrocarbons ....how can that be?  Aromatic C-H stretches appear above 3000 cm-1

Reading through the revised paper .... does not aid the understanding. 

Author Response

Dear Reviewer,

thank you for your commitment to improving the manuscript. At the same time, we are worried that we did not meet your expectations. We tried to clear up the doubts and to fill the gaps. Please see the revised manuscript. The amendments are implemented by red.

Best regards, 

Authors

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors have made some revisions and improvements in the revised paper. However, I still feel that the manuscript need further modification.

1) The author did not investigate the adsorption property of original CFA on RB and Cu ions.

For Cu ions adsorption: Although some Refs. were employed for comparison, the adsorption conditions were different, e.g. the pH values in Ref. [82] and this study were 1 and 11, respectively.

Therefore, the advantage of this composite material is not so much clear.

Author Response

Dear Reviewer,

thank you for your comments. Please see the revised manuscript. The amendments are implemented by red. The proofreading has been made by the MDPI English Editing Service (specialist edit).

Best regards,

Authors

Author Response File: Author Response.pdf