Oleic Acid-Tailored Geopolymer Microspheres with Tunable Porous Structure for Enhanced Removal from Tetracycline in Saline Water

Round 1

Reviewer 1 Report

The manuscript highlights the potential benefit of using metakaolin-based geopolymer microspheres for tetracycline removal. Oleic acid was used as a surfactant to improve the adsorption process. In addition, the optimal dosage of such a surfactant was also found out. However, many detailed information are missing and they need to clarify these issues.

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1. A detailed information is missing. What is the minimum concentration of TC causing super-resistant bacteria? What kind of bacteria is produced during the accumulation of TC?
2. It is very confusing that on the one hand, the authors said that the adsorption method provides several advantages, such as convenient operation, low cost, low energy consumptions, etc., while on the other hand, most of the adsorbents they stated highly affect the production cost.
3. There are several techniques to produce filter-based membranes, for instance electrospinning. The technique also offers several advantages, including low-cost operation. However, the authors claim that the adsorption method is more advantageous than any other methods they stated. The authors must carefully compare various methods, including their advantages.
4. The authors said that clay becomes one of the limited materials to be applicable with the adsorption method. They also said that the same material is categorized as a costly natural material. As described elsewhere, clay is commonly used for low-cost buildings.
5. What was the weight ratio of industrial sodium silicate? (
6. What was the purity of metakaolin, tetracycline, and oleic acid?
7. The same point as no. 6, what was the purity of NaOH? And also HCl?
8. What was the weight or volume of PEG 2000 when the slurry being injected into it?
9. How many repetitions the authors performed sample characterizations? This must be provided.
10. How many were various doses of MM prepared?
11. In Fig. 2a, how many measurements were performed to obtain the given SD on the graphic bars?
12. The authors need also to specify different pH and different contact times they used.
13. What was the concentration of NaOH and HCl, respectively?
14. Excessive oleic acid results in a low pH. What concentration of oleic acid was? How low the pH was?
15. The MM3 was observed to have porous and loose structures, what about MM4? MM3 and MM4 were close to each other in SD? Did MM2 and MM5 have similar surface morphology? If not, why did the observed phenomena occur?
16. Following no.15, what was the percentage of porosity of MM3 in comparison to MM0? Did MM3 have the same percentage of porosity with MM4 material?
17. The authors stated that the oleic acid converted into surfactant in the encapsulation in situ. Did the CMC of oleic acid change during such a process?
18. In the FTIR characterization, was the strong adsorption observed for MM3 at the wavenumber of 3452 cm^-1 ? Were the FTIR curves normalized? What was the peak area of MM0 and MM3, respectively? How much in percentage they differed?
19. How reliable the zeta potential was? How many measurements were performed? The authors said that MM3 was more conducive to adsorption, due to electrostatic attraction. However, zeta potential of MM3 and TC was located at the same range. Indeed, the zeta potential of MM3 was much stronger, i.e. more negative than TC at the same measuring pH. The authors need to clarify more details on the electrostatic attraction, how the electrical double layers were formed between these two MM-based materials.
20. In addition to no.19, the authors briefly demonstrated the zeta potential measurements of their metakaolin-based microspheres. However, they did not exhibit that their microspheres were stable enough in natural rivers, lakes, wastewater, or simply in aquatic ecosystems.
21. The authors said that MM3 and TC were positively and negatively charged material, respectively, at pH 4. However, in Fig. 4d, the phenomena were not clearly shown since TC had a zeta potential of 0, while MM3 demonstrated a weak positive zeta potential. As a rule of thumb, the more positive zeta potential, the stronger the attraction or repulsion is.

 

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Author Response

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Reviewer 2 Report

The authors of the manuscript Oleic acid tailored geopolymer microspheres with tunable porous structure for enhanced removal of tetracycline in saline water carried out a large volume of experimental and theoretical work in order to establish the adsorption capacity of geopolymer microspheres.

However, there are some shortcomings that I noticed:

1. It would be good to indicate the pH of the solution before and after adsorption, because for such adsorbents the pH adjustment with HCl / NaOH is not correct, given that the acid / base will be neutralized by adsorbents that have amphoteric properties. It is best to work with buffer solutions.
2. Tetracycline can be considered an emerging pollutant that may be present in water in micro-quantities, so the use of such concentrated solutions seems unjustified.
3. Carefully check the units of measurement: page 6 - Qf is the maximum adsorption capacity, mg · L-1; fig. 4C- dV / dlogD the unit of measurement is not correct; fig. 8B the same;
4. Indicate the model you used to determine the PSD.
5. In tab. 5 you give reference to biochar, but in the text mention the activated carbon, see where the mistake is, because for activated carbon the adsorption value of TC is much higher.
6. page 18- the explanation regarding the role of salt is not convincing, and in addition it is not clear what you mean that in the presence of NaCl the solubility of TC decreases.
7. page 21 - correct Var der Waals

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Reviewer 3 Report

The authors studied the adsorption of the antibiotic drug tetracycline (TC) on oleic acid modified metakaolin-based microspheres, comparing the adsorption properties of the unmodified and modified microparticles. The study involves many aspects of the adsorption behavior: the optimization of the adsorption capacity by varying the dosage of the modifier, electron microscopic images, powder XRD, IR spectroscopy, the effect of pH on adsorption, determination of the thermodynamic and kinetic parameters, characterization of regeneration, etc. These results are interesting, worth for a publication in Sustainability, however, a thorough revision of the manuscript seems necessary.

- It is unusual that most of the equations used for the evaluation of the experimental data are presented only in the SI, whereas the parameters obtained by fitting to these equations are presented in the main article. I note here that eq. (4) is probably wrong: kyn and tau*kyn must have different dimensions

- (p. 7) the ‘adsorption equilibrium time’ is not defined

- (p. 9) an OH stretching band at 1636 cm-1 is improbable

- (p. 12) ‘R reached 100% when C0 was 50 mgL-1’ Looking at Fig. 6b, 500 mgL-1 seems to be the correct value

- (p. 19) the experimental conditions in the regeneration section are not clear (mass of MM3@TC, volume of NaOH solution)

- (p. 20) it is not clear what Y denotes

- (p. 21) …conducted to analyzed the…  unfinished sentence

I note that the English of the manuscript requires a substantial revision.

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Round 2

Reviewer 1 Report

The authors did not put what the reviewer asked, for instance the purity of chemicals. They said that they have revised the manuscript. However, the purity of the used chemicals were not included in it. Finally, they must check all the contents are available in the revised manuscript.

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Reviewer 3 Report

Eq (4) differs from the eq presented in the Author's response, In addition, the 1636 cm-1 peak in the IR spectrum belongs to an OH bending mode, not a stretching mode.

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Round 3

Reviewer 1 Report

Accepted