Carbamazepine Removal by Clay-Based Materials Using Adsorption and Photodegradation
Round 1
Reviewer 1 Report
The manuscript reports application of natural and modified clays for adsorption and photo-oxidation of carbamazepine, an important pollutant of emerging concern. The study is rather broad and performed with care, but there are several points that can be addressed before acceptance for publication;
1) The proposed amounts of clays to be used per volume of water - about 1% (10 g/l) is tremendously high. Was it really necessary? How can this be realized practically in water purification?
2) The adsorption isotherms are fitted quite well to Langmuir model, however, this model is describing monolayer adsorption with uniform distribution of active centers, while the authors describe a complex process with pre-adsorption of an active layer.
3) The description of photocatalytic degradation leaves several questions. It seems that the best effect is coming just from reactive oxygen species originating from photo decomposition of hydrogen peroxide itself. Does the clay has any own photocatalytic activity? Is there any specific bandgap effect or is it coming just from surface reactivity?
4) In the introduction, it is reasonable to mention even the enzymatic degradation of carbamazepine, for example from Nanomaterials 2020, 10(2). E282.
5) It is important in photocatalytic degradation to identify the products of oxidation and check or at least comment on their possible toxicity.
Author Response
Attached a file with detailed answers
Author Response File: 
Author Response.docx
Reviewer 2 Report
Levakov et al. studied the relative effectiveness of bentonite and montmorillonite clays and their modifications with thiaminea and benzalkonium for adsorption of carbamazepine (CBZ). Moreover, the degradation of CBZ by photooxidation/photocatalysis with the addition of H2O2 as oxidant and synthetic mica-montmorillonite and laponite as (photo)catalysts was carried out under UVC irradiation. Overall, the present work seems like a patchwork of two individual parts, which on their own are relatively lackluster for standalone papers. This is evident from the discontinuation of the use of clays for the adsorption and those for the photodegradation experiments. I have some pressing concerns about the quality of the results presented and/or the way the data are presented, which has a strong impact on the discussion, more on that can be found in my comments. Due to the extent of the required corrections and further experiments, I recommend that the submitted manuscript be rejected.
1. The claim that TiO2 or ZnO are very expensive (page 2 line 81) is not entirely true. When strictly comparing unit price per weigh, this is true, since photocatalysts cost ~100-250 $/kg, whereas clays cost ~0,5-25$/kg, but the practical aspect is disregarded entirely. First, photocatalysts are used in much smaller mass loadings than adsorbents. Typical photocatalyst loadings are up to ~1 g/L, whereas for clays 10 g/L or more is not uncommon. In addition, the adsorbent needs to be regenerated somehow or disposed of, imparting additional costs, whereas photocatalysis does not produce secondary waste. The difficulties of separating the photocatalyst and clay from suspension are more-the-less the same.
2. What is DDW? Please define the abbreviation (page 3, line 123).
3. “Optical density” (page 4 line 151) is an unusual construct. Did the authors mean “absorption”?
4. Please amend the description of the non-linear curve fitting procedure (page 5 line 216). scipy.optimize.curve_fit is a function within SciPy, a library for Python. scipy.optimize.curve_fit is not a part of Python itself. Please mention the version of SciPy utilized and properly cite SciPy: https://scipy.org/citing-scipy/
5. I would strongly refrain from using "P25 analog" in the manuscript. Hombikat is not a P25 analog. First, Hombikat is only anatase and has some amorphous titanium dioxide, while P25 is ~70% anatase and 30% rutile and is crystalline. In addition, there is a wealth of literature showing that the activity is not the same, to name a few:
Mehrvar et al., Int. J. Photoenergy 04 (2002) 141-146
Alhakimi et al., J. Photochem. Photobiol. A 157 (2003) 103-109
Alonso-Tellez et al., J. Photochem. Photobiol. A 250 (2012) 58-65.
I understand the desire to compare to the P25, but that is misleading if P25 itself was not used. Tell like it is and amend the manuscript accordingly.
6. A schematic representation of the experimental photoreactor setup would be very useful.
7. What do the authors mean by “covered at the edge (4.6 mm*25 cm)” (page 6 lines 254-255). I have never come across this.
8. The volume ratio of the mobile phase A and B in the total flow of the mobile phase is not specified. That is an important parameter.
9. In relation to the conclusion of the adequacy of the HPLC method for the determination of CBZ (page 6 lines 258-259), please mention the number and concentrations of calibration points used and the value of the coefficient of determination. Was the calibration curve forced through zero?
10. UVC/H2O2 degradation experiments are not photocatalytic, page 6 lines 261-264, that is photooxidation. There is no photocatalyst to do photocatalysis in this case! In addition, the sentence is written in such a way as if TiO2 and the clays were used in concentrations of 0.2 to 1 mg/L, which I am quite certain is not the case. Or is it? Did the authors mean to say mg/mL?
11. According to page 7, lines 329-331: “B1 release interfered with UV Visible measurements, and in order to overcome this problem a simple mathematical spectra separation was performed, based on two well-known spectrums of the components (B1 and carbamazepine) at known concentrations, and calculating the mix spectrum by superposition”, it would seem that thiamine and CBZ co-eluted during HPLC analysis? Why didn’t the authors try to alter the mobile phase composition or utilize gradient elution to separate them? Possible errors due to peak deconvolution could have been avoided then.
12. Please compare the adsorbents in terms of relative cost-effectiveness for the same removal of CBZ. Thiamine and benzalkonium chloride are not cheap in relation to clays themselves.
13. Please specify the range from-to of CBZ addition in Fig 2.
14. What to the authors mean by a-priori saturation of CEC with Ca2+ for Ca-SWy1? If calcium salts were added to the CBZ solution, salting out of CBZ could have occurred! That could contribute to the high adsorption in the case of Ca-SWy1. Desorption tests of CBZ need to be done.
15. I find the notion of the presence of CaCO3 in CBZ purchased from Sigma very strange. Are the authors sure that this is not a consequence of contaminated labware, or some cross-contamination? The purity of CBZ according to Sigma’s specification sheet should be greater than 98% and the content of inorganics negligible. Furthermore, FTIR alone is not enough to confirm the presence of CaCO3 in CBZ. Even less so, because the spectra shown don’t extend to 500 cm-1, where inorganic peaks should be present. Thermogravimetric analysis of the CBZ powder should be done at least on in conjunction with ICP-OES analyses of the stock solution in well cleaned labware. In addition, if CaCO3 were present, would be hardly soluble anyway. Since even no pH measurements were reported, all of this is too speculative.
16. Please check the values reported for the photocatalyst/adsorbent loadings in Table 3. They do not match with values shown in Table 1.
17. Why different results were obtained for the degradation of equimolar CBZ amounts with the addition of 0.5 mg/L H2O2 under UVC irradiation according to Figs 5. and 6?! In Fig. 5. there is no degradation at 20 minutes, whereas in Fig. 6 nearly 15% degradation of CBZ was achieved.
18. Why were different clays used for the adsorption and photodegradation experiments? Bentonite and Ca-montmorillonite and their thiamine (B1) and benzalkonium (bzk) modified variants were used for the adsorption experiments, while synthetic mica-montmorillonite and laponite (RD ) were used for photocatalysis. As a result, the discussion is not coherent.
19. I would strongly suggest to measure the concentration of transition metal ions in solution after the addition of clays by ICP-OES. Photo-Fenton process may play a role. Once again, pH values for the solutions should have been presented.
20. Please double check that the mass concentration of catalysts reported should have been in mg/ml. It is not plausible that such small amounts, as given in the discussion (0.2 mg/L photocatalyst?!), resulted in such large differences of observed CBZ degradation.
21. The removal of CBZ in the presence of clays in the photodegradation experiments is not only due to direct degradation of CBZ, but also due to adsorption. But again, if 0.5 mg/L was indeed used, the results presented herein would be truly ground-breaking.
22. The abstract needs to be amended according to my previous comments. In addition, desorption of CBZ by washing was not described in the experimental section and discussion.
Author Response
Attached a file with detailed answers
Author Response File: Author Response.docx
Reviewer 3 Report
Dear authors, The present study proved that, two treatments that each of them by itself are completely ineffective, due to a synergistic effect, it leads to an effective process.
Could you please explain in more details why did you consider that CBZ might have been behave like a cation?
In general, the manuscript is well organized and the study has been performed systematically. I would add that the testing on some real water samples, one from industrial area and another from a municipal wastewater, would have been very valuable.
In conclusions, although the authors are rather pessimistic on the large-scale applications of these methods, in general, I think that the results from this study worth of being presented. The adsorption followed by photodegradation came like a very efficicient solution.
Also, in the continuation of these findings, I may suggest that a feature study to include some low angle diffraction methods, in order to prove if or not the drug adsorption took place in the internal pores.
Author Response
Attached a file with detailed answers
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
I feel that after the profound revision performed the article is worth publishing in its present form.
Reviewer 2 Report
The authors have addressed my initial concerns and have resolved the pressing issues. I therefore propose to accept the submitted manuscript for publication in Water after minor revision (minor text editing improvements necessary).
