Efficient Adsorption Performance of Lithium Ion onto Cellulose Microspheres with Sulfonic Acid Groups
Round 1
Reviewer 1 Report
In this work a cellulose microsphere adsorbent with sulfuric acid was used to adsorb Li+ ions from aqueous solutions. As discussed in the introduction, the separation of Li+ in an environmentally friendly manner has many practical applications. The work is scientifically sound, and very straightforward. The authors performed a straightforward study to understand the adsorption of Li+ and the effect of various environmental factors. As such there is no real criticism to offer.
My only comment has to do with the reproducibility of the study. From ready, I am given the impression that each experiment was only performed once. Were multiple trials made? If so, could you please mention so. If not, then why not? Multiple trials should be performed if possible to ensure the reproducibility of the results. Error bars should be added to all figures, and uncertainties added to the model parameters based on your multiple trials.
Author Response
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Author Response.pdf
Reviewer 2 Report
This is an interesting work, potentially publishable in QBS. While the level of scientific novelty is medium, the results are of considerable practical importance and can be indeed useful for future technologies of lithium removal. The main ideas, experimental plan, results and discussion are, in most part, fine. However, there are some issues that, in my opinion, should be dealt with before publication.
While the text is in general relatively well written, it contains numerous language errors (mostly in grammar). I suggest having the text checked and corrected by someone fully proficient in English or by professional editing service.
Also some polymer names are misspelled (e.g. there should be “poly(vinyl chloride)” and “poly(vinylidene fluoride)” in the Introduction).
The Authors use many abbreviations, which is fine, but care should be taken to explain every abbreviation where it appears for the first time in the text (e.g. CMS, ICP-AES).
It is not obvious if the obtained products can be named “bio-absorbents”. Actually they seem to be hybrid structures combining the core of natural origin with synthetic substituents. Also the synthetic method is not fully “green”, involving a synthetic monomer, synthetic surfactant, organic solvents (acetone, 2-propanol), etc.
There is no universal agreement on how the synthesis method employed in this work should be called. The name “pre-irradiation grafting”, albeit often found in literature, may suggest that the grafting takes place before (“pre”) irradiation, which is not true. Actually “post-irradiation grafting” seems to be more logical, since grafting takes place after (“post”) irradiation. But I understand if the Authors prefer to follow the traditional name (“pre-irradiation”). Nevertheless, even if we adopt this name for the whole process, the first step shown in Scheme 1 is simply “irradiation”.
In the Introduction the Authors should mention that radiation grafting of glycidyl methacrylate on cellulose has already some history (at least from 2005, see the paper of Alberti et al. in Eur. Polym. J., till more recent works, for instance the 2012 paper of Takacs et al. in Radiat. Phys. Chem., 2015 paper of Madrid and Abad and 2016 paper of Barsbay et al. in the same journal).
I understand that the procedures described in chapters 2.3.1 and 2.3.2 are not invented in this work, but rather adapted from ref. [27]. However, if the Authors have decided to describe them here again (which is convenient for the readers), they should give all details. Were the cellulose microparticles dried before irradiation (it is well known that moisture modifies the process) ? It should be stated in 2.3.1 that samples were irradiated by gamma rays (irradiator type ?). Was the 5 % GMA emulsion an aqueous emulsion ? How was the efficiency of the washing process checked ? Why no homopolymerization inhibitor was used in 2.3.1 ? What was the role of the 2-propanol used in 2.3.2 ?
In 2.4 it should be stated why were (almost) all absorption tests done at pH 10.
There is some apparent discrepancy as to the regeneration procedure mentioned in chapters 2.5 and 3.3.4. In the former, the regenerating liquid is stated as 1 mol/L HCl, while in the latter the acid concentration is said to be 2 mol/L. Which is correct ?
What was the rationale behind running the column experiment using a different lithium salt than for the batch experiments ?
In the Experimental is should be started how many runs of each experiment were performed. Error bars should be provided in the graphs.
To provide more clarity and consequence, and to keep in line with notation used in Eqns. (3) and (4), I would suggest to modify the Y-label descriptions in Fig. 3 A and B. Either there should be “Li uptake” and “Equilibrium Li uptake”, or “qt” and “q”, respectively. Consequently, the same notation should be used in Figs. 4 and 5 A. Since most data are presented as regular plots, there is apparently no reason why data in Fig. 4 are presented as a histogram.
There is one more problem regarding Fig. 3 A and B. It is clear that the points represent the experimental results. But what are the solid lines ? Are they fittings to these data with functions described by Eqns. (3) and (4) (for Fig. 3 A) and by Eqns. (5) and (6) (for Fig. 3 B) ?
It is not clear why shall we expect a strong increase of the particle size (from 340 to 390 micrometer) upon substituting the methacrylate groups in the side chains with sulfonate functions.
The first sentence of chapter 3.3.1 is not clear. What is actually meant by “time of absorption equilibrium” ?
The description of processes taking place during and after irradiation (chapter 3.1) is superficial and actually not precise. Why should peroxides be generated only in the crystalline region (actually they cannot be formed inside the crystallites; they could be formed at the surface of crystallites and in the amorphous phase). Is it true that heating cellulose to a moderate temperature of 50 ˚C allows the radicals (not peroxides) trapped inside the crystalline phase to migrate to the surface and initiate polymerization ? I strongly suggest to re-write this paragraph taking care to make the statements precise (probably by describing the role of radicals and peroxides separately) and to cite relevant references.
What is obviously missing in the results of this work is the quantification of the sulphate groups. The Authors measure and calculate the DS, but there is no information regarding the number of sulfate groups (e.g. in mmol/g of the product). From the practical point of view the latter data (missing) are much more important than the former (provided). The paper would gain a lot if quantitative data on the sulfate group content are provided. As it stands, lack of this information actually leads to potential misunderstandings. For instance, in lines 205-206, we read that “absorption capacity increased with increasing the content of sulfonic acid groups”. This is not a justified conclusion when no quantitative data of the sulfonic acid content of CGS-I and CGS-II are shown.
Results described in chapter 3.3.3 indicate that the product suffers from the common drawback of simple (in the chemical sense) absorption systems, namely lack of selectivity. Are there any perspectives to improve the product with this respect ?
In my opinion, it is disputable whether the data presented in Fig. 6 B indicate that the product has truly a good stability. What is actually causing as much as 17 % percent of activity loss after 5 cycles ? What is the main problem here ? How this could be possibly prevented ?
While I understand why the Authors have measured the effluent concentration for a long time (ca. 8 hours), the current form of Fig. 7 B does not allow to see the short-time-scale details discussed in the text (lines 263-266). I would suggest to keep the main figure as it is, but add an inset showing what happens at short time (up to 15-20 min. ?).
In the first part of the Introduction the Authors suggest that they are seeking for a low-cost method of lithium removal. But in the further text there is no comment whether their method is expected to be much cheaper than the existing ones.
Reference section needs attention. As it stands, the citation format is not uniform, in particular with respect to the usage of capital letters in paper titles.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript entitled “Efficient adsorption performance of lithium-ion onto cellulose microsphere with sulfonic acid groups” investigated the adsorptive separation of Li+ from aqueous solution using a self-prepared cellulose microsphere adsorbent. The adsorption isotherm and kinetics, effect of pH and the presence of K+ and Na+, adsorption mechanism, and column adsorption were systematically studied. The results are interesting, and the manuscript is well-written. The manuscript can be published on Quantum Beam Science with minor revision.
1. Page 6, line 179 to 181. Although pseudo-first and -second-order rate equations have been widely used for adsorption kinetic data fittings, the misapplications of those equations have been found (AIChE Journal, 64(5) 2018, 1793, DOI 10.1002/aic.16051). I suggest the authors add a short discussion about this issue by citing the highlighted paper.
2. In Fig 3, the plots of the data fitting (both isotherm and kinetics) should be presented.
3. Tables 1 and 4 need to be merged into one table. In addition, when comparing the adsorption results with the data in the literature, it would be better to show the test conditions (pH, temperature, initial concentration, etc.) for adsorption data, because those conditions have significant effects on adsorption capacity and kinetics.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Thank you for your response. Nice work.
