Organo-Modified Vermiculite: Preparation, Characterization, and Sorption of Arsenic Compounds

Round 1

Reviewer 1 Report

This article is mostly well prepared. Defects/errors and suggestions are listed below for the authors to improve.

Line 66,67 - HDBAC-Cl, the “C” is supposed to represent Cl, hence the abbreviation should be HDBA-Cl (in comparison with HDTMA-Br) or HDBAC (and through all this article).

Line 83 - add the source or manufacturer of the vermiculite used in this

                 study.  

Line 93 - CHNS Elementar Vario EL III’ s manufacturer and country.

Line 115 - “2.2 Sorption experiments” A flow-chart may help the reader to catch the complicated procedures more clearly. (This is up to the choice of the authors).

Line 148 – Eqs. (2) I suggest that replacing C_s by n(number of moles/kg). The symbol C_s is confusing, because it has different unit (mmole/kg) with C_eq(mmol/L).

Line 149 – Eqs. (3) If the change is taken, then “1/n” can be replaced to “1/ν”

Line Line 182 – “for the peak at 14Å, the intensity of reflection decreased (Fig. 1c, 1d).” Fig. 1c does not show DECREASE of the peak at 14Å. This sentence should be restated.

Line 186 – “… higher sharpness …” Comparing 1d, “sharpness” should be replaced by a more descriptive term.

Line 203 – “1.0 HDBAC” should be “2.0 …”

Line 278 – “(>10 nm)”, < 10 nm? See Fig. 5.

Line 283 – Fig. 4, will an ordinary plot better than this 3-D drawing?

Line 303 – “at 990°C”, Fig. 6(a) shows 950°C.

Line 305 – “quadrupole mass spectra (OMS)”. Instrument? Manufacturer?  

Line 335 – in [49,56], cannot find referred HDBAC data.

Line 346 - 1. Add DTA/TG analyses of HDTMA-Br and HDBA-Cl to show the 

                      effects of adsorption on vermiculite (delaying of disintegration).

                 2. Fig. 6(b) 27% is not correct (it's 30%).

Line 376 – “the adsorption of As(V) on the 2.0 HDTMA – the removal was significantly less if the pH was <6, …” However, I see Fig.8(a), the adsorption of As(V) on 2.0 HDTMA(pH<6) are 40-50 mmol/kg, which are the highest of all adsorbents… Thus I have the suggestion below(Line 376-387). 

Line 376-387 This paragraph should be rewritten.

Line 461 Fig.11 should be plotted in the linear form of Langmuir and Freundlich isotherms (Eqs. 4 and 5). Fig. 11(a),(b),(c) and (d) should be enlarged to show the details of the plot (linearity).

Line 588 – Conclusions are required.

Author Response

Point 1: Line 66,67 - HDBAC-Cl, the “C” is supposed to represent Cl, hence the abbreviation should be HDBA-Cl (in comparison with HDTMA-Br) or HDBAC (and through all this article).

Response 1: The abbreviation has been corrected throughout the article.

Point 2: Line 83 - add the source or manufacturer of the vermiculite used in this study.

Response 2: The manufacturer has been added (Lines 83-84).

Point 3: Line 93 - CHNS Elementar Vario EL III’ s manufacturer and country.

Response 3: The manufacturer and country have been added (Lines 94-95).

Point 4: Line 115 - “2.2 Sorption experiments” A flow-chart may help the reader to catch the complicated procedures more clearly. (This is up to the choice of the authors).

Response 4: We are not sure that adding the flow-chart would be necessary. We clearly emphasized that experiments were carried out in the two sections and described each of them.

Point 5: Line 148 – Eqs. (2) I suggest that replacing C_s by n(number of moles/kg). The symbol C_s is confusing because it has a different unit (mmole/kg) with C_eq(mmol/L).

Response 5: The change of Cs to n has been done. It has been emphasized that n means an adsorbed amount of As(III) or As(V) onto unmodified and modified vermiculite (Line 149).

Point 6: Line 149 – Eqs. (3) If the change is taken, then “1/n” can be replaced to “1/ν”

Response 6: The change of 1/n to 1/v has been done (Line 150). 

Point 7:  Line 182 – “for the peak at 14Å, the intensity of reflection decreased (Fig. 1c, 1d).” Fig. 1c does not show a DECREASE of the peak at 14Å. This sentence should be restated.

Response 7: You are right. It was a mistake. For the peak at 14Å, the intensity of reflection decreased in case of 2.0 HDTMA. It has been corrected (Line 186-187). 

Point 8: Line 186 – “… higher sharpness …” Comparing 1d, “sharpness” should be replaced by a more descriptive term.

Response 8: The sentence has been changed to be more descriptive (Line 190). 

Point 9: Line 203 – “1.0 HDBAC” should be “2.0 …”

Response 9: It has been corrected (Line 207). 

Point 10: Line 278 – “(>10 nm)”, < 10 nm? See Fig. 5.

Response 10: It has been corrected (Line 282).

Point 11: Line 283 – Fig. 4, will an ordinary plot better than this 3-D drawing?

Response 11: Considering the number of curves, the 2D chart would be illegible. While separating them into more charts would generate too many figures. 

Point 12: Line 303 – “at 990°C”, Fig. 6(a) shows 950°C.

Response 12: It has been corrected (Line 307).

Point 13: Line 305 – “quadrupole mass spectra (OMS)”. Instrument? Manufacturer?

Response 13: The missing part has been supplemented in Analytical Methods (Line 175-176).

Point 14: Line 335 – in [49,56], cannot find referred HDBAC data.

Response 14: This sentence was incomplete and references were related to other surfactants. Authors corrected it (Line 349-350).

Point 15: Line 346 - 1. Add DTA/TG analyses of HDTMA-Br and HDBA-Cl to show the effects of adsorption on vermiculite (delaying of disintegration).

Response 15: DTA/TG results for HDTMA-Br and HDBA-Cl have been added (Line 313-322).

Point 16: Fig. 6(b) 27% is not correct (it's 30%).

Response 16: It has been corrected (Line 362).

Point 17: Line 376 – “the adsorption of As(V) on the 2.0 HDTMA – the removal was significantly less if the pH was <6, …” However, I see Fig.8(a), the adsorption of As(V) on 2.0 HDTMA(pH<6) are 40-50 mmol/kg, which are the highest of all adsorbents… Thus I have the suggestion below(Line 376-387).

Response 17: You are right. It was a mistake, that none of the authors noticed. It should be: “the removal was significantly less if the pH was > 6”. It has been corrected. Thank you for pointing it out (Line 393-394).

Point 18: Line 376-387 This paragraph should be rewritten.

Response 18: The paragraph has been corrected (Line 393-402).

Point 19: Line 461 Fig.11 should be plotted in the linear form of Langmuir and Freundlich isotherms (Eqs. 4 and 5). Fig. 11(a),(b),(c) and (d) should be enlarged to show the details of the plot (linearity).

Response 19: The linearization forms of Freundlich and Langmuir isotherms have been added in Supplementary.

Point 20: Line 588 – Conclusions are required.

Response 20: Conclusion section has been added (Line 613-637)

Reviewer 2 Report

The article is quite interesting, especially considering the using of organo-vermiculites modified with benzyldimethylhexadecylammonium chloride (HDBAC). Therefore, it is very relevant and I would recommend it for publication after the elimination of the following comments: 

280 "aand" 

355 It is written here that heat-treated vermiculite is used, although earlier in the article, especially in the section with TG/DTG I did not see a mention of this. 

377 Maybe at pH>6? 

378 Again, maybe pH >2? 

385 The effect of the initial pH on the equilibrium is described here, but where is it shown? 

389 There's a lot of points that overlap. Maybe you should separate them, for example, As(III) and As(V) build on different charts? 

397 "This effect is due to an increase in the number of active centers that are located on the surface of the vermiculite." How it is possible if you do not increase the dosage of sorbent? 

417 In my opinion, the concentration range is poorly chosen here, as for example can be read in the work of "Hristovski, K.D.; Markovski, J. Engineering metal (hydr)oxide sorbents for removal of arsenate and similar weak-acid oxyanion contaminants: A critical review with emphasis on factors governing sorption processes. Sci.Total Environ. 2017, 598, 258–271."

And "Figure 9. Sorption of As(III) or As(V) ions as a function of initial concentration for..."? or equilibrium? 

439 It is surprising that such a significant increase in the dose of sorbent, especially 2.0 HDTMA, does not lead to a significant change in the degree of arsenic extraction from the solution. Even if you look at the sorption isotherm in figure 9 for 2.0 HDTMA, you can see that the degree of arsenic extraction from the solution at an equilibrium concentration of 4 mmol/L is about 33%, and at 8 mmol/L - 24%. Changing the dosage should lead to the same results? 

466 The Discussion section contains many repetitions from the Results section and is very extensive, in fact, it contains a conclusion as well. All this leads to the fact that the article is hard to read. I would recommend to combine the discussion and results and make a separate Conclusion section.

Author Response

Point 1: 280 "aand"

Response 1: It has been corrected (Line 284).

Point 2: 355 It is written here that heat-treated vermiculite is used, although earlier in the article, especially in the section with TG/DTG I did not see a mention of this.

Response 2: In Abstract, and other sections, particularly section 2.1 Materials, has been emphasized that expanded vermiculite has been investigated.

Point 3: 377 Maybe at pH>6?

Response 3: It has been corrected (Line 394). 

Point 4: 378 Again, maybe pH >2?

Response 4: It has been corrected (Line 395).

Point 5: 385 The effect of the initial pH on the equilibrium is described here, but where is it shown?

Response 5: Figure reflecting the effect of the initial pH on the equilibrium pH has been added to the supplementary (Fig. S1).

Point 6: 389 There's a lot of points that overlap. Maybe you should separate them, for example, As(III) and As(V) build on different charts?

Response 6: We wanted to condense results and avoid a large number of figures. The figure is quite complex, however, we believe that the general trend is clearly visible. Moreover combining the results of As(III) and As(V) sorption allow to easily compare and point the differences between As(III) and As(V) sorption.

Point 7:  397 "This effect is due to an increase in the number of active centers that are located on the surface of the vermiculite." How it is possible if you do not increase the dosage of sorbent?

Response 7: You are right. This sentence was incorrect and has been removed (Line 413-414).

Point 8: 417 In my opinion, the concentration range is poorly chosen here, as for example can be read in the work of "Hristovski, K.D.; Markovski, J. Engineering metal (hydr)oxide sorbents for removal of arsenate and similar weak-acid oxyanion contaminants: A critical review with emphasis on factors governing sorption processes. Sci.Total Environ. 2017, 598, 258–271."

Response 8: The article describes fundamental research, whose purpose was, among others, to check what will be the size of the sorption for several selected concentrations. Thank you for the suggestion, your comment certainly will be considered in the next stages of the research.

Point 9: And "Figure 9. Sorption of As(III) or As(V) ions as a function of initial concentration for..."? or equilibrium?

Response 9: Yes, the correct description is “Sorption of As(III) or As(V) ions as a function of equilibrium concentration for (a) VER, 0.5 HDTMA, 1.0 HDTMA, and 2.0 HDTMA, and (b) VER, 0.5 HDBA, 1.0 HDBA, and 2.0 HDBA” (Line 434-436).

Point 10: 439 It is surprising that such a significant increase in the dose of sorbent, especially 2.0 HDTMA, does not lead to a significant change in the degree of arsenic extraction from the solution. Even if you look at the sorption isotherm in figure 9 for 2.0 HDTMA, you can see that the degree of arsenic extraction from the solution at an equilibrium concentration of 4 mmol/L is about 33%, and at 8 mmol/L - 24%. Changing the dosage should lead to the same results?

Response 10: Changing the dosage not necessarily should lead to the same results. These results may be surprising at first, however, after a close look, there is a logical explanation, why increasing the dosage of sorbent does not lead to a significant change in the degree of arsenic extraction. Please see the Discussion section lines 581-592.

Point 11: 466 The Discussion section contains many repetitions from the Results section and is very extensive, in fact, it contains a conclusion as well. All this leads to the fact that the article is hard to read. I would recommend to combine the discussion and results and make a separate Conclusion section.

Response 11: Instructions for authors state that research manuscript should be divided into Introduction, Results, Discussion, Materials and Methods, Conclusions (optional). Thus we decided to keep that section division. However, we added the Conclusions section and partially rewritten the Discussion section (Line: 593-637).

Round 2

Reviewer 2 Report

Despite the significant changes, the work still has some shortcomings.

129 What pH=5 you are talking about here? The initial or equilibrium? Need to clarify. If the initial, then why on the Figure S2 (a) reference point 5.88, and S2 (b) - 4.78, and not 5 as indicated?

135 The same. It is indicated that the pH value is fixed at level 6, and on the Figure S3 the reference point is 6.38 and 5.78 for As(III) and As(V), respectively. In addition, why such a high pH value was chosen, if the Figure 8 shows that at this pH begins a sharp decrease in sorption capacity for 2.0 HDTMA? And just as I pointed out earlier, as a result of the choice of high initial arsenic concentration and pH, it is necessary to take large dosages, which, in my opinion, leads to contradictory data in figs 9 and 10 and low degree of arsenic removal. Which subsequently also reflected in the conclusions. I would recommend expanding experiments with the effects of dosage under different conditions (lower initial pH and lower concentration) to show that conclusions are made objectively, taking into account the possible effects of other factors.

Figures 8, 9, 11. It is better to name the y-axis "Equilibrium adsorption capacity..." as given in equation 5. 

479 Flattening or flattering?

Author Response

Point 1: 129 What pH=5 you are talking about here? The initial or equilibrium? Need to clarify. If the initial, then why on the Figure S2 (a) reference point 5.88, and S2 (b) - 4.78, and not 5 as indicated?

Response 1: The word „constant” suggests maintaining constant pH during experiments. But actually, pH has been set at value 5 at the start of the experiments and then not modified. We only measure the pH after experiments (Fig. S2). To clarify we changed „constant” to „initial”.  

Point 2: 135 The same. It is indicated that the pH value is fixed at level 6, and on the Figure S3 the reference point is 6.38 and 5.78 for As(III) and As(V), respectively. In addition, why such a high pH value was chosen, if the Figure 8 shows that at this pH begins a sharp decrease in sorption capacity for 2.0 HDTMA? And just as I pointed out earlier, as a result of the choice of high initial arsenic concentration and pH, it is necessary to take large dosages, which, in my opinion, leads to contradictory data in figs 9 and 10 and low degree of arsenic removal. Which subsequently also reflected in the conclusions. I would recommend expanding experiments with the effects of dosage under different conditions (lower initial pH and lower concentration) to show that conclusions are made objectively, taking into account the possible effects of other factors.

Response 2: As above. We changed „constant” to „initial”. Thank you very much for suggestions regarding the selection of other experimental conditions. Experiments in suggested conditions would mean creating another work because all experiments, analyses, and research would have to be done again. The choice of pH 6 is dictated by the potential use of organo-vermiculites to remove arsenic compounds in a specific industrial application for wastewater treatment from the paint industry. Wastewater containing arsenic that we want to purify has pH 5-7 and contains up to 1000 mg As/L. It would be the goal of our future studies.

Point 3: Figures 8, 9, 11. It is better to name the y-axis "Equilibrium adsorption capacity..." as given in equation 5. 

Response 3: According to the reviewer suggestion we changed the name of the y-axis in figures 8,9,11.

Point 4: 479 Flattening or flattering?

Response 4: Thank you. It should be „flattening”. We corrected the words in lines 473 and 476.