Review Reports - Removal of Thallium from Aqueous Solutions by Adsorption onto Alumina Nanoparticles

Round 1

Reviewer 1 Report

Title: " Removal of Thallium from Aqueous Solutions by Adsorption onto Alumina Nanoparticles".
After reviewing the present manuscript, reviewer found that the authors made interesting work and all required analysis for the removal of thallium from aqueous solutions. The reviewer found that this manuscript is fit with Processes MDPI and has following general comments and specific comments with regards to the improvement of the manuscript prior to the publication.

The manuscript is written in good English despite some minor errors. The content of the manuscript is well organized, and length and depth of the work is fair and sufficient for an article. The materials and methods section can be extended by including the supporting information to the manuscript. All the performed characterizations need to be discussed with figures in Results and Discussion section. This manuscript can be recommended for a publication after attending to major comments and suggestions below.

General observations - Manuscript may not have been proof-read sufficiently before the submission, and author comments were found in several places. Please remove them before submitting the revised version. Some formatting issues were observed.

Be consistent with the space between units and numbers. Use of stops in appropriate places and correct way of referencing must be carefully looked.

The comments below are organized in order as they appear in the manuscript.

1. Adding a graphical abstract will be a plus point for the manuscript.

2. It is suggested for the authors to include the highlights, with the most significant findings of the research project.

3. Line 45- Add more references to the sentence “In the United States, the maximum Tl level in drinking water and wastewater discharged has been set at 2 and 140 μg/L, respectively, by the USEPA”

4. Line 51- Add reference.

5. Line 91- How can you justify this sentence? Can you add a comparison table with surface area of different adsorbents? “In general, most of these adsorbents have insufficient porosity and surface area which limit their effectiveness in the adsorption process”.

6. Line 103- Rephrase the sentence “(ii) there are characterized by a good conductivity, a good selectivity and interesting catalytic, magnetic and optical properties.”

7. Line 168- “Chemicals utilized, and characterization methods need to be discussed in each manuscript and can’t just be cited from previous published work”.

8. Line 170- Provide complete details of experimental procedure.

9. Line 173- Provide complete details of pH_ZPC.

10. Line 210- Elaborate the discussion “Batch Adsorption Investigations of Thallium (I)”.

11. Line 226- Add figure for zero-charge point (pHzpc).

12. Line 234- Use numeric for pH four.

13. Line 312- Correct the subheading number to “3.1.3.”

14. Line 370- Add not less than ten more adsorbents to the comparison table.

15. All the performed characterizations need to be discussed with figures in Results and Discussion section.

16. References need to be adjusted according to alphabetical order.

Author Response

Removal of Thallium from Aqueous Solutions by Adsorption onto Alumina Nanoparticles

Ollé Rodrigue KAM¹, Corneille BAKOUAN^1,2, Inoussa ZONGO³ and Boubié GUEL^{1, *}

Response to Reviewer 1 Comments

General observations - Manuscript may not have been proof-read sufficiently before the submission, and author comments were found in several places. Please remove them before submitting the revised version. Some formatting issues were observed.

Response to General observations: We greatly appreciate the reviewer’s general observations. We have taken into account all these observations in the revised version.

Be consistent with the space between units and numbers. Use of stops in appropriate places and correct way of referencing must be carefully looked.

Response: Space between units and numbers has been taken into account. All references in the manuscript were numbered according to the authors ‘guidelines which are given by the Journal “Processes” as follows:

“References: References must be numbered in order of appearance in the text (including table captions and figure legends) and listed individually at the end of the manuscript.”

The comments below are organized in order as they appear in the manuscript.

Adding a graphical abstract will be a plus point for the manuscript.

Response to comment 1: A graphical abstract has been added.

It is suggested for the authors to include the highlights, with the most significant findings of the research project.

Response to comment 2: Here are the main highlights:

Low-cost synthesis of alumina nanoparticles from local bauxites.
Removal of thallium (I) at low concentration.
Coexistence of physisorption and chemisorption.
Adsorption behavior of alumina nanoparticles.

Most significant findings of the paper:

Previous investigations in the literature have used gamma-alumina nanoparticles, which were obtained from commercial routes, to investigate the removal of several metal ions, such as Ni(II), Zn(II), Cu(II) and Cd(II), and Tl(III). However, Tl(I) has not yet been the subject of any interactions with gamma-alumina nanoparticles which are synthesized from natural local bauxites. In this paper, the removal of Tl(I) ions, through adsorption onto synthesized gamma-alumina nanoparticles, was investigated for the first time.
We highlight the exceptional performance of the gamma-alumina nanoparticles, synthesized from local bauxite, which led to a deep purification of synthetic water contaminated with Tl(I). We were able to achieve a residual concentration of less than 2 µg/L at a pH of 8.5.

Line 45- Add more references to the sentence “In the United States, the maximum Tl level in drinking water and wastewater discharged has been set at 2 and 140 μg/L, respectively, by the USEPA”

Response to comment 3: More references have been added as follows:

“In the United States, the maximum Tl level in drinking water and wastewater discharged has been set at 2 and 140 μg/L, respectively, by the USEPA [6,16-19].”

Line 51- Add reference.

Response to comment 4: References have been added.

“However, to the best of our knowledge, gamma-alumina nanoparticles, synthesized from natural local bauxites, have not yet been used for the interaction with thallium (I) ions [62-66].”

Line 91- How can you justify this sentence? Can you add a comparison table with surface area of different adsorbents? “In general, most of these adsorbents have insufficient porosity and surface area which limit their effectiveness in the adsorption process”.

Response to comment 5: A comparison table (Table 1 in SI) has been added in Supplementary Information.

Line 103- Rephrase the sentence “(ii) there are characterized by a good conductivity, a good selectivity and interesting catalytic, magnetic and optical properties.”

Response to comment 6: The sentence in line 103 has been rephrased as follows in the new version:

“As opposed to traditional adsorbents, Nano adsorbents show exceptional characteristics such as a large specific surface area, good conductivity and selectivity. They are chemical reactive and optically active. Moreover, they show interesting catalytic and magnetic properties.”

Line 168- “Chemicals utilized, and characterization methods need to be discussed in each manuscript and can’t just be cited from previous published work”.

Response to comment 7: We appreciate the reviewer comment. All the chemicals utilized and the characterization methods have been added in the revised version.

Line 170- Provide complete details of experimental procedure.

Response to comment 8: We appreciate the reviewer comment. Details of experimental procedure have been provided in the revised version.

Line 173- Provide complete details of pH_ZPC.

Response to comment 9: We appreciate the reviewer comment. Details of pH_ZPC.have been provided in the revised version.

Line 210- Elaborate the discussion “Batch Adsorption Investigations of Thallium (I)”.

Response to comment 10: We appreciate the reviewer comment. This section has been more elaborated in the revised version.

Line 226- Add figure for zero-charge point (pHzpc).

Response to comment 11: We appreciate the reviewer comment. Experimental figure for the zero point charge (pHzpc) has been added in the revised manuscript.

Line 234- Use numeric for pH four.

Response to comment 12: Numeric number “4” has been used for four.

Line 312- Correct the subheading number to “3.1.3.”

Response to comment 13: Subheading has been corrected to:

3.1.3. Effect of Adsorbent Dose

Line 370- Add not less than ten more adsorbents to the comparison table.

Response to comment 14: The comparison table has been completed with more than ten adsorbents as follows:

Table 3: Comparison of the present results with those reported in literature

Adsorbents	Adsorbate	Concentrations range (µg.L^-1)	Dosages (g.L^-1)	Residual concentrations of Tl (µg.L^-1)	References
pyrolysis residue	Tl(I)	1600	3	104.96	[18]
low-grade pyrolusite	Tl(I)	1600	3	17.5	[18]
Amorphous hydrous manganese dioxide (HMO)	Tl(I)	50,000	0.5	˂0.1	[40]
montmorillonite biochar composite	Tl(I)	800	-	33.2	[109]
montmorillonite biochar composite	Tl(I)	800	-	33.2	[109]
Magnetite (Fe₃O₄)	Tl(I)	450-10,000	2.5	29	[110]
MnO2@pyrite	Tl(I)	10,000-171,000	1	3.8	[111]
Fe-Mn binary oxides	Tl(I)	10,000	2	2	[112]
Nano-alumina	Tl(III)	10,000	3	4.4	[6, 113]
Modified ZnO Nanopowder	Tl(I)	5,000-50,000	2	3,600	[114]
Titanium peroxide	Tl(I)	46-20,000	0.2	˂2	[115]
modified activated carbon with rhodamine B	Tl(I)	30-50,000	40	3.23	[116]
biochar derived from watermelon rinds	Tl(I)	20,000-100,000	6	˂5	[117]
Synthesized γANPs	Tl(I)	20	1	0.97±0.23	Present study

All the performed characterizations need to be discussed with figures in Results and Discussion section.

Response to comment 15: We appreciate the reviewer comment. All the performed characterizations have been discussed with their corresponding figures in the “Results and Discussion section”.

References need to be adjusted according to alphabetical order.

Response to comment 16: According to the authors’ instructions given by the Journal “Processes”, References must be numbered in order of appearance in the text (including table captions and figure legends) and listed individually at the end of the manuscript. We have followed these instructions. All references in the manuscript were numbered according to the Journal’s instructions.

Author Response File: Author Response.docx

Reviewer 2 Report

The introduction presents important information about the manuscript. However, it is very extensive, making it exhausting for readers. Please summarize the introduction section and highlight the information that justify the innovation of the work.

The adsorption kinetics adjustments must be performed in the quadratic model. They can be inserted in Figure 2, discussing the contact time together with the kinetic models.

The adsorption isotherm adjustments must be performed in the quadratic model. They must be entered in figure 5.

The adsorption mechanisms must be discussed at the end of the analyses, compiling the information obtained from the pH variation, kinetic, isothermal and thermodynamic analysis. Characterization techniques, such as FTIR, SEM, TGA (before and after adsorption) can help in this discussion, increasing the value of the present work.

Author Response

Removal of Thallium from Aqueous Solutions by Adsorption onto Alumina Nanoparticles

Ollé Rodrigue KAM¹, Corneille BAKOUAN^1,2, Inoussa ZONGO³ and Boubié GUEL^{1, *}

Response to Reviewer 2 Comments

Comments and Suggestions for Authors

Suggestion 1: The introduction presents important information about the manuscript. However, it is very extensive, making it exhausting for readers. Please summarize the introduction section and highlight the information that justify the innovation of the work.

Response to suggestion 1: The introduction section has been summarized and we have highlighted the information that justifies the innovation of the work.

Suggestion 2: The adsorption kinetics adjustments must be performed in the quadratic model. They can be inserted in Figure 2, discussing the contact time together with the kinetic models.

Response to suggestion 2: The main objective of the study of the adsorption kinetics was to determine the minimum time necessary for a maximum adsorption of Tl(I) in solution (Figure 7). The analysis of Figure 7 shows that after 30 min, we have the maximum amount of Tl(I) in solution that has been adsorbed. In our case, the Tl(I) removal by gamma alumina nanoparticles constitutes a rapid process along with an adsorption equilibrium which is reached during the early 30 minutes of the process (Figure 7). Knowing this time, it is then used as the necessary time for carrying out the pseudo-first model and pseudo-second model of Tl(I) adsorption on γANPs (Figure 8).

Linear or quadratic adjustments are used to verify the best fitting model in terms of coefficient of determination. In our study, the linear adjustment fits very well with the kinetic data. In contrast, the quadratic model is generally used in the case of a modeling. Since we are not dealing with data modeling in our study, the choice was to only use the linear adjustment.

Suggestion 3: The adsorption isotherm adjustments must be performed in the quadratic model. They must be entered in figure 5.

Response to suggestion 3: As we have already stated above, in this case the best fit of the isotherm date was obtained with the linear adjustment. Since we are not dealing with data modeling in our study, the choice was to only use the linear adjustment.

Suggestion 4: The adsorption mechanisms must be discussed at the end of the analyses, compiling the information obtained from the pH variation, kinetic, isothermal and thermodynamic analysis. Characterization techniques, such as FTIR, SEM, TGA (before and after adsorption) can help in this discussion, increasing the value of the present work.

Responses to suggestion 4: The adsorption mechanisms have been discussed at the end of the analyses, combining the information obtained from the pH variation, kinetic, isothermal, and thermodynamic analysis. As a matter of fact, we would like to note that this work is part of a global research investigation on thallium removal by using alumina nanoparticles, synthesized from local bauxites. The present paper constitutes the first set of investigation to state the removal efficiency of these alumina nanoparticles as for thallium elimination. The second set of investigations will deal with surface characterizations on spent alumina nanoparticles and thallium (I) removal in real contaminated waters in order to fully assess the type of binding at the Nano alumina surface and the fate of the pollutant in the environment, respectively.

Author Response File: Author Response.docx

Reviewer 3 Report

Journal: Processes

Title: Removal of Thallium from Aqueous Solutions by Adsorption onto Alumina Nanoparticles

The authors investigate the removal of Tl on ANP sorbent by examining the influence of pH, mass, contact time, concentration and by analyzing two kinetic reaction models and two isothermal models.

The benefit of the paper is that the removal of Tl below the maximum permissible concentrations according to WHO was achieved. The results and discussion as well as conclusions should be strengthened (details in the review).

It should also clearly define the optimal conditions, especially pH and mass based on the criteria of removal efficiency or adsorbent capacity. Therefore, it should be discussed in Figures 1, 2, 4 and 5 through the removal efficiency.

COMMENTS FOLLOW:

2. Materials and Methods

Chapter 2.2.2. in the manuscript, i.e. chapter 2.2. in SI it is exactly the same, do not duplicate.

Explain in detail the experimental conditions for each experiment performed (co, pH, mass, time, rpm...) in SI

3.1.1. Effect of initial pH

There are contradictory thoughts in this section.

The authors found that the pHpzc of ANP is 8, that is, the surface of ANP becomes more positive with a decrease in pH, or more negative with an increase in pH compared to pHpzc=8. So the surface is more negative at pH=12 than at pH=10.

The authors say: "Line 221-223: Figure 1 shows that the quantity of Tl(I) adsorbed by the alumina nanoparticles increases with the pH increase up to 8 and stays at a constant level when the pH value varies between 8 and 10." also explain that beyond the pH pzc there is an influence of electrostatic attraction „line 230-231: … favor the sorption of Tl + , as a result of the electrostatic attraction which involves Tl+ ions and the negative surface of alumina…“.

According to this statement, it seems that at pH>pHpzc thallium exists in cationic form as Tl+. Therefore as stated in lines 223-224 " When the pH value is greater than 10, a decrease in the quantity of Tl(I) adsorbed is observed. " it is not clear why the capacity is decreasing because according to the above logic as explained, the capacity should be increasing.

On the other hand, the sentence in lines 241-242 "When the pH values are above 10, the adsorption capacity decreases." Indeed, at pH values higher than 10 (pH > 10) is not adequately explained. Why?? Furthermore it seems that there are also negative species of Tl like Tl(OH) - or maybe Tl(OH)4- (check well).

Lines 243-244: „As a result, the adsorbent surface becomes negatively charged, which induces a competition between the latter and the Tl( OH)- ions.“ - Competition of Tl(OH)- and OH- for the negative surface???

At what pH does the precipitation of Tl occur? Maybe 10-11?

In line 218 you state pH in the range 2-10, and in Figure 1 pH=2-12.

So, systematize this chapter and explain well. Clearly state the optimum pH. Also in Figure 1, add the removal efficiency vs pHo curve.

3.1.2. Effect of the Agitation Time on Tl(I) Adsorption and Adsorption Kinetics

Lines 251-252 “The effect of the agitation time on the adsorption process was studied to evaluate the quantity of Tl(I) which is adsorbed at different times (Figure 2).” vs. lines 256-258: “The results obtained, plotted as the evolution of the amount of thallium (I) Q t (µg/g) adsorbed by the alumina nanoparticles as a function of the contact time t (min), Q t = f(t), are illustrated in Figure 2.” - There is no need to write the same thing in different ways.

Lines 254-256: “It should be noted that the adsorption is carried out at pH 8.5 and at room temperature. The samples are taken at predetermined intervals time and are separated by centrifugation and filtration.” - This part belongs to section 2. Materials and methods, i.e. SI section.

Line 261: “…a decrease is observed” - I don't see this from Figure 2, I see stagnation, i.e. an imperceptible increase after 30 minutes.

I think that explaining the adsorption mechanism only by applying kinetic and isothermal models is not enough, it can be helpful but not enough, I repeat.

Namely, the authors in line 265 "diffuse into pores of the adsorbent" clearly indicate that there is probably also a mechanism of diffusion into the adsorbent. Why did the authors use only kinetic reaction models and not diffusion models? For example, in lines 268-269, they also mention active sites "adsorbent active sites".

Therefore, I ask the authors to clearly explain the mechanism of adsorption and show it with a chemical reaction. It would also be useful to show the SEM-EDS analysis of the adsorbent saturated with Tl ions. What active sites does the adsorbent have? Briefly describe the characterization of the sample published in the previous paper.

4. Conclusion

Lines 451-452: “We found that electrostatic interactions, at pH values ranging from 8 to 10, governed the adsorption process.” - Is this correct, i.e. contrary to the explanations from section 3.1.1.?

Lines 452-455: “The adsorbent surface dissolved for pH values greater than 10. As a result, we observed a weakening of the interaction between the adsorbent surface and the thallium ions (Tl + ) ions. A decrease in the adsorption rate was then observed.” - The style of expression is not the best.

SPECIFIC COMMENTS:

Line 395-396: “The values of the different parameters indicate an efficiency of thallium (I) removal, suggesting that the synthesized alumina nanoparticles have a better adsorption capacity.” - in relation to what?

Carefully read the instructions on citing references. For example reference No. 1 is not cited correctly, Bozena is a first name, and K i.e. Karbowska is a surname. Review and correct all references in detail.

The writing of measuring units should be uniform throughout the entire manuscript.

Author Response

Removal of Thallium from Aqueous Solutions by Adsorption onto Alumina Nanoparticles

Ollé Rodrigue KAM¹, Corneille BAKOUAN^1,2, Inoussa ZONGO³ and Boubié GUEL^{1, *}

Response to Reviewer 3 Comments

The authors investigate the removal of Tl on ANP sorbent by examining the influence of pH, mass, contact time, concentration and by analyzing two kinetic reaction models and two isothermal models.

COMMENTS FOLLOW:

Materials and Methods

Question 1: Chapter 2.2.2. in the manuscript, i.e. chapter 2.2. in SI it is exactly the same, do not duplicate.

Question 2: Explain in detail the experimental conditions for each experiment performed (co, pH, mass, time, rpm...) in SI

Responses to 1 and 2: We greatly appreciate the reviewer comments. The “Materials and Methods” section has now been elaborated in the revised version.

3.1.1. Effect of initial pH

There are contradictory thoughts in this section.

Question 3: The authors found that the pHpzc of ANP is 8, that is, the surface of ANP becomes more positive with a decrease in pH, or more negative with an increase in pH compared to pHpzc=8. So the surface is more negative at pH=12 than at pH=10.

Question 4: The authors say: "Line 221-223: Figure 1 shows that the quantity of Tl(I) adsorbed by the alumina nanoparticles increases with the pH increase up to 8 and stays at a constant level when the pH value varies between 8 and 10." also explain that beyond the pH pzc there is an influence of electrostatic attraction „line 230-231: … favor the sorption of Tl + , as a result of the electrostatic attraction which involves Tl+ ions and the negative surface of alumina…“.

Question 5: According to this statement, it seems that at pH>pHpzc thallium exists in cationic form as Tl+. Therefore as stated in lines 223-224 " When the pH value is greater than 10, a decrease in the quantity of Tl(I) adsorbed is observed. " it is not clear why the capacity is decreasing because according to the above logic as explained, the capacity should be increasing.

Question 6: On the other hand, the sentence in lines 241-242 "When the pH values are above 10, the adsorption capacity decreases." Indeed, at pH values higher than 10 (pH > 10) is not adequately explained. Why?? Furthermore it seems that there are also negative species of Tl like Tl(OH) - or maybe Tl(OH)4- (check well).

Question 7: Lines 243-244: „As a result, the adsorbent surface becomes negatively charged, which induces a competition between the latter and the Tl( OH)- ions.“ - Competition of Tl(OH)- and OH- for the negative surface???

Question 8: At what pH does the precipitation of Tl occur? Maybe 10-11?

Responses to 3, 4, 5, 6, 7 and 8: We greatly appreciate the reviewer comments. This section has been completely revised to explain why Tl(I) is adsorbed at pH values from 4 to 10, and why there is a decrease after pH 10. The revised version is given in red in the manuscript as follows:

“pH values ranging from 2 to 12 were selected to investigate the influence of the pH suspension on the optimum condition for the adsorption process. Figure 6 shows that the quantity of Tl(I) adsorbed by the alumina nanoparticles increases with the pH increase up to 8 and stays at a constant level when the pH value varies between 8 and 10. When the pH value is greater than 10, a decrease in the quantity of Tl(I) adsorbed is observed. It is worth noting that Tl(I) is soluble as the species of Tl⁺ or Tl(OH)_aq at a wide pH range (0-14) [34,42,92,93,]. However, Tl⁺ is the species which is likely to be more easily adsorbed, through electrostatic attraction on a negatively charged surface.

Figure 6: Effect of initial pH on the adsorption of Tl(I) by γANPs.

Physical and chemical properties of alumina play an important role in understanding the adsorption behavior in terms of active sites involved in adsorption process. It is known that the active sites on alumina surface are reactive hydroxyls groups. Moreover, the properties of the surface of alumina bearing reactive hydroxyl groups strongly depends on pH. Indeed, in acidic medium when the pH values are smaller than the pHzpc value of the investigated alumina nanoparticles, which is 8, the surface is positively charged; at a basic medium (pH > pHzpc), the surface is charged negatively (Figure 7).

Figure 7: Properties of γANPs.surface: (a) acidic pH, positive charge; (b) point of zero charge; (c) basic pH, negative charge.

From the literature, it is also known that the spatial distribution of the reactive hydroxyl groups is rather complex [94]. Characterizing alumina by fluorescence spectroscopy, Rémy et al. demonstrated that the reactive hydroxyl groups are not homogenously distributed on alumina surface, but they are rather clustered into regions of high density [94]. In addition, in our context ICP results show that although the predominant oxides of the γANP samples include aluminum oxide, they contain other alkali oxides present in trace amounts. As a result, the surface properties of the synthesized Nano alumina will depend on reactive hydroxyl groups not homogeneously distributed as well as the impurities which are also present at the surface. The surface charge formation and the strong dependence of the properties on the pH are considered in the following discussion.

For pH values below the pHzpc value, the surface of the Nano alumina is supposed to carry a positive charge due to the surface protonation (Figure 7). In this context, attraction of Tl⁺ ions is not favored on the positive reactive sites. However, due the complexity of the surface charge formation, Tl⁺ ions are likely to be adsorbed at a small extent on other negative sites which originate from impurities present at the surface. Hence, by increasing the pH from 4 up to 8, the adsorption rate gradually increases with increasing pH up to 8. When the pH values are greater than the pHzpc, the negative charge of alumina nanoparticles favor the sorption of Tl⁺, as a result of the electrostatic attraction which involves Tl⁺ ions and the negative surface sites of alumina nanoparticles. Thus, in the pH range from 8 to 10, a constant and significant adsorption of Tl⁺ on the surface of γ-alumina was noted, suggesting the predominance of Tl⁺ in this range of pH. The optimum pH for the removal of Tl(I) on γ-alumina is therefore in the pH range from 8 to 10. Similar results were reported by Yubing Pu et al. who investigated the removal of thallium (I) by multiwall carbon nanotubes [34]. In addition, for pH values smaller than 4, it is noted that the elimination of Tl⁺ ions from the aqueous medium gradually decreases, indicating that the removal of Tl⁺ by is highly influenced in an acidic medium. As a matter of fact, for pH values smaller than 4 (pH < 4), the gamma-alumina nanoparticles dissolve and the consequence is a modification of the surface properties of the materials [95], which thus brings about a drastic reduction of the adsorption of . Xiaoliu Huangfu et al. reported similar results on the removal of thallium (I) by manganese dioxide nanoparticles, with aggregation of MnO₂ and leaching of Mn under acidic conditions at pH 4.0 [42]. When the pH values are above 10, the adsorption capacity decreases. Indeed, at pH values higher than 10 (pH > 10), the aqueous solution is enriched with hydroxyl ions (). This enrichment of the solution in hydroxyl leads to the conversion of part of the Tl⁺ ions in solution into TlOH(aq) [96]. According to Lin and Nriagu, Tl⁺ is the dominant species of Tl(I) for pH < 11.7, and at pH > 11.7, Tl⁺ is converted into TlOH(aq) [96]. In our context, we found an adsorption decrease from the pH value of 10, with a Tl(I) concentration taken as 20 mg.L^-¹ (or 9.78.10^-⁸ mol L^-¹). In principle, the same observation could have been made here if the pH step variation was set smaller. However, the pH step variation was set at a value of 2 in the present experiment, which causes a lower pH value than 11.7. Further experiments are needed in the pH range from 10 to 12 to assess the process. Indeed, when describing Tl species, the real difficulty is the diverse values which are used as for the hydrolysis constants of Tl(I) [6]. An accurate calculation which takes into account the concentration of Tl⁺ ions and the real part of Tl⁺ which is converted into TlOH(aq) will certainly lead to an exact pH value of conversion depending on the ratio log [Tl(OH)aq]/[Tl⁺][SI]. The decrease in the adsorption of Tl⁺ on the gamma alumina is therefore justified by the coexistence of the Tl⁺ and Tl(OH)(aq) species in the solution where only the Tl⁺ species are likely to interact with the negatively charged gamma alumina surface.”

Question 9: In line 218 you state pH in the range 2-10, and in Figure 1 pH=2-12.

Response to 9: The correct pH range is 2 to 12.

Question 10: So, systematize this chapter and explain well. Clearly state the optimum pH. Also in Figure 1, add the removal efficiency vs pHo curve.

Response to 10: The optimum pH is located between 8 and 10 where a maximum of adsorption. Then for the experiments, the value of 8.5 has been chosen.

3.1.2. Effect of the Agitation Time on Tl(I) Adsorption and Adsorption Kinetics

Question 11: Lines 251-252 “The effect of the agitation time on the adsorption process was studied to evaluate the quantity of Tl(I) which is adsorbed at different times (Figure 2).” vs. lines 256-258: “The results obtained, plotted as the evolution of the amount of thallium (I) Q t (µg/g) adsorbed by the alumina nanoparticles as a function of the contact time t (min), Q t = f(t), are illustrated in Figure 2.” - There is no need to write the same thing in different ways.

Response to 11: Correction has been done in the text.

Question 12: Lines 254-256: “It should be noted that the adsorption is carried out at pH 8.5 and at room temperature. The samples are taken at predetermined intervals time and are separated by centrifugation and filtration.” - This part belongs to section 2. Materials and methods, i.e. SI section.

Response to 12: Correction has been done in the text.

Question 13: Line 261: “…a decrease is observed” - I don't see this from Figure 2, I see stagnation, i.e. an imperceptible increase after 30 minutes.

Response to 13: Correction has been done in the text.

Question 3: I think that explaining the adsorption mechanism only by applying kinetic and isothermal models is not enough, it can be helpful but not enough, I repeat.

Response to 14: We greatly appreciate the reviewer comment. It can be helpful to assess the adsorption mechanism by including deep surface characterizations. As a matter of fact, we would like to note that this work is part of a global research on thallium removal by using alumina nanoparticles, synthesized from local bauxites. The present paper constitutes the first set of investigations to state the removal efficiency of these alumina nanoparticles as for thallium elimination by using synthesized alumina nanoparticles. The second set of investigations will deal with surface characterizations on spent alumina nanoparticles (Tl(I)-loaded alumina nanoparticles) and thallium (I) removal in real contaminated waters in order to fully assess the type of binding at the Nano alumina surface and the fate of the pollutant in the environment, respectively.

Question 15: Namely, the authors in line 265 "diffuse into pores of the adsorbent" clearly indicate that there is probably also a mechanism of diffusion into the adsorbent. Why did the authors use only kinetic reaction models and not diffusion models? For example, in lines 268-269, they also mention active sites "adsorbent active sites".

Response to 15: The intraparticle diffusion model has been used to extend the mechanism as follows:

“Intraparticle diffusion of thallium adsorption by nano-alumina

Generally, sorption processes have a complex nature, in which both surface sorption and intraparticle diffusion can occur. In order to determine the limiting step of the Tl(I) sorption process on nanoaluminas, the experimental data were analyzed by applying the Weber-Morris method. This model can be used to determine whether external transport or intraparticle transport governs the rate of sorption processes.

Figure 10 represents the intraparticle diffusion modeling curve for gamma alumina nanoparticles for 100 mg as adsorbent dose. Analysis of the curve (Figure 10) shows that two linear parts are involved. The first part ranges from 10 to 30 min and the second part from 30 min to 120 min. While the steeply rising first part defines an initial rapid absorption by boundary layer effects, the plateau-like second part, which is associated with intraparticle diffusion, takes place after completion of outer surface coverage by the first process. This multilinearity of the curve suggests that external mass transfer and intraparticle diffusion are involved in different phases of the Tl sorption process on gamma nanoparticles [42,93,95,96]. Senol, Z.M et al. (2010) [100] obtained similar results on the elimination of Tl(I) onto polyacryamide-aluminosilicate composites. The fact that the straight line is not passing through the origin indicates that the intraparticle diffusion could not be considered as the limiting step, and that other processes are involved in the sorption of Tl by gamma alumina nanoparticles [xx]. As a matter of fact, the correlation coefficient (R²) of the intraparticle diffusion model is equal to 0.5662. This value suggests that the adsorption process of Tl(I) is more complex and that other mechanisms than intraparticle diffusion are involved in the sorption of Tl(I) on the studied gamma alumina nanoparticles [115, 117].”

Figure 9: Intra-particle diffusion modeling for the adsorption of Tl(I) on the gamma alumina nanoparticles

Question 16: Therefore, I ask the authors to clearly explain the mechanism of adsorption and show it with a chemical reaction. It would also be useful to show the SEM-EDS analysis of the adsorbent saturated with Tl ions. What active sites does the adsorbent have? Briefly describe the characterization of the sample published in the previous paper.

Response to 16: The characterization of the sample published in our previous paper has been added. As already stated, the active sites are the reactive hydroxyl groups which can interact with Tl(I) ions depending on the pH. The revised manuscript takes all this into account. Moreover, a section entitled “Mechanism of Thallium Adsorption onto γANPs” has been proposed.

In adsorption studies, the most informative techniques used to characterize adsorbents before and after pollutant removal include XRD, FT-IR, and SEM-EDS. These techniques were used in the present work to characterize the alumina nanoparticles before thallium adsorption. As for the characterization after thallium adsorption, they are currently being carried out on exhausted alumina nanoparticles surface. Indeed, we did not obtain satisfactory results during the first set of analyzes. One of the reasons may be the low concentration of Tl used in this work (20 µg/L or 9.78.10^-8 mol/L). The choice of this concentration is related to the actual value found in one of our study where the thallium concentration in groundwater was found to be around 10^-8 mol/L. As a matter of fact, the characterization of spent alumina nanoparticles surface, i.e. exhausted alumina nanoparticles which are obtained after thallium adsorption, is part of our research program. We are currently carrying out XPS measurement to assess the oxidation state of the adsorbed thallium.

Conclusion

Question 17: Lines 451-452: “We found that electrostatic interactions, at pH values ranging from 8 to 10, governed the adsorption process.” - Is this correct, i.e. contrary to the explanations from section 3.1.1.?

Response to 17: Correction has been done in the revised version.

Question 18: Lines 452-455: “The adsorbent surface dissolved for pH values greater than 10. As a result, we observed a weakening of the interaction between the adsorbent surface and the thallium ions (Tl + ) ions. A decrease in the adsorption rate was then observed.” - The style of expression is not the best.

Response to 18: Correction has been done in the revised version as follows:

“At pH greater than 10, Tl(OH) influences the adsorption process, and as a result, we observed a decrease of the interaction between the adsorbent surface and the thallium ions (Tl⁺) ions.”

SPECIFIC COMMENTS:

Quesstion 19: Line 395-396: “The values of the different parameters indicate an efficiency of thallium (I) removal, suggesting that the synthesized alumina nanoparticles have a better adsorption capacity.” - in relation to what?

Response to 19: In this section, the parameters we are talking about are the following:

The determination coefficient R² which is 0.9831 for the Freundlich model.
The n value is such that 1/n is smaller than 1.
It should also be noted that the value of the n parameter of the Freundlich equation is greater than 1, reflecting favorable adsorption

We obtained an approximated value of Qm in the range of 338 mg/g for the Freundlich model. Since we are working with trace concentration in the present investigation, this value of Qm cannot be compared with other Qm values which were obtained in investigations dealing with high concentration of thallium.

Indeed, the discussion about the adsorption capacity of the alumina nanoparticles is only based on the Freundlich parameters (Table 4), namely n greater than 1 which indicates favorable adsorption. In addition, isotherms with n > 1 are classified as L-type isotherms reflecting a high affinity between adsorbate and adsorbent.

Question 20: Carefully read the instructions on citing references. For example reference No. 1 is not cited correctly, Bozena is a first name, and K i.e. Karbowska is a surname. Review and correct all references in detail.

Response to 20: Correction has been done.

Question 21: The writing of measuring units should be uniform throughout the entire manuscript.

Response to 21: Correction has been done.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

After reviewing the revised version of manuscript, reviewer found that the authors made all the required changes according to the suggested comments.

This manuscript can be recommended for a publication.

Author Response

According to the Reviewer N°1, all the suggestions and questions have been taken into account. Therefore, the Reviewer N°1 has recommended the present paper for a publication in the Journal Processes. There is no need to give an answer.

Author Response File: Author Response.docx

Reviewer 2 Report

In recent years it has been observed by several authors that using linearized models can "force" an adjustment with high R² in aqueous effluents. Therefore, I insist that the adjustments be made in the quadratic model.

Author Response

Removal of Thallium from Aqueous Solutions by Adsorption onto Alumina Nanoparticles

Ollé Rodrigue KAM¹, Corneille BAKOUAN^1,2, Inoussa ZONGO³ and Boubié GUEL^{1, *}

Response to Reviewer 2 Comments

Response to comment:

We greatly appreciate the reviewer’s comment. The isotherm and kinetic data have been fitted using both linear and quadratic models. As a matter of fact, both linear and quadratic models lead to high values of the coefficient of determination in the following cases: pseudo-second order kinetic and Freundlich isotherm models. Yet, only the linear fitting leads to a straight line as expected by the theoretical expressions. The following modifications were added in the manuscript.

Adsorption Kinetics using linear and quadratic models

Adsorption kinetics data were investigated through pseudo-first order and pseudo-second order models. The kinetic data of the adsorption of Tl⁺ by γAl₂O₃were fitted using linear and quadratic models (Figures 9-10).

(a)

(b)

Figure 9: Pseudo-first order model of Tl(I) adsorption on the γANPs: (a) linear model and (b) quadratic model.

(a)

(b)

Figure 10: Pseudo-second order model of Tl(I) adsorption on the γANPs: (a) linear model and (b) quadratic model

Figure 9 indicates that the coefficients of determination of both linear and quadratic models are very low, indicating a poor correlation between the ordinate (Ln (Qe - Qt) and the abscissa (time). As a result, the linear and quadratic models are not appropriate to describe the pseudo-first-order model. Furthermore, with an R² value equals to 0.5579 the linear pseudo-first order model cannot be considered for the fitting of the kinetic data in the present work.

Figure 10 shows that the coefficients of determination (R²) obtained with both linear and quadratic models are almost similar and very high, 0.992 and 0.9966, respectively, indicating a strong correlation between the ordinate (t/Qe) and the abscissa (time). Although both linear and quadratic models lead to high values of the coefficients of determination (R²), only the linear adjustment provides a straight line as expected by the theoretical expression (Equation 4). Since the accuracy of the fit of an adsorption model to experimental data is usually assessed as a function of the magnitude of the coefficient of determination, the linear pseudo-second order model with an R² value being close to unity is retained in the present work for the description of the kinetic data.

Table 2 summarizes the experimental kinetic constants and determination coefficients as calculated using the linear adjustment for both pseudo-first and pseudo-second order models, for the elimination of Tl⁺ onto 0.1 g of as adsorbent dose. As already stated, the experimental results are better fitted by the linear pseudo-second order kinetic model (Figure 10a) than the linear pseudo-first order model. This observation is assessed by the coefficient of determination of the pseudo-second order kinetic model which is greater than 0.99 [100, 101]. It is therefore suggested that chemisorption is involved in the adsorption process [100, 101]. This result is similar to those reported for the adsorption of thallium (I) on the following adsorbents: multiwall carbon nanotubes [34], polyacriamide-aluminosilicate composites [100] and FeOOH-MnO₂ nanocomposites [101]. Generally, the experimental data of the adsorption of Tl(I) on several adsorbents are better interpreted by the pseudo-second order kinetic model [34,100,101]. Taking into account the work by Vithanage, M. et al. (2016) describing the fact that kinetic order is dependent on initial concentrations, it is suggested that the pseudo-second-order kinetic model is more adaptable to experimental data when polluting species are less abundant (low initial concentration) than the available adsorption sites [102]. In other words, adsorption kinetics of Tl⁺ at /(Tl⁺ solution) interface could be expressed in terms of Tl(I) concentration.

Table 2: Kinetic constants of Tl(I) removal on γANPs.

Sorbent

Pseudo-First-Order

Pseudo-Second-Order

K₁(min⁻¹)

Qe₁(mg/g)

R²

K₂(g/mg.min)

Qe₂(mg/g)

R²

γANPs

0.0006

1666.666

0.5579

6.506

0.023

0.992

Adsorption Isotherms using linear and quadratic models

The relationship between Qe (amount of adsorbate adsorbed by the adsorbent) and Ce (concentration of adsorbate remaining in the solution after the system has reached equilibrium at a constant temperature) is described in terms of adsorption isotherms. The equilibrium data were adjusted to the Langmuir and Freundlich models using linear and quadratic models. The Langmuir and Freundlich models are shown in Figure 14 and 15, respectively.

(a)

(b)

Figure 14: Langmuir isotherm of Tl(I) adsorption on γANPs: (a)Linear model and (b) quadratic model.

(a)

(b)

Figure 15: Freundlich isotherm of Tl(I) adsorption on γANPs: (a) Linear model and (b) quadratic model

Figure 14 indicates that the coefficients of determination of both linear and quadratic models are very low, indicating a poor correlation between the ordinate (Ce/Qe) and the abscissa (Ce). As a result, the linear and quadratic models are not appropriate to describe the Langmuir isotherm model. Furthermore, the linear adjustment of the Langmuir isotherm model (R² equals to 0.1762) cannot be considered for the fitting of the isotherm data in the present work.

Figure 15 shows that the coefficients of determination (R²) obtained with both linear and quadratic models are almost similar and very high, 0.9831 and 0.9873, respectively, indicating a strong correlation between the ordinate (LnQe) and the abscissa (LnCe). Although both linear and quadratic models lead to high values of the coefficients of determination (R²), only the linear adjustment of the Freundlich isotherm provides a straight line as expected by the theoretical expression (Equation 6). The linear fitting of the Freundlich isotherm model is therefore retained for the description of the isotherm data in the present work.

The parameters of thallium (I) adsorption isotherms are presented in Table 4. The values obtained for the coefficient of determination (R²) indicate that the thallium (I) adsorption data correspond better to the Freundlich isotherm (Figure 15a) than to the Langmuir isotherm (Figure 14a) at low initial concentrations. The Freundlich isotherm is suitable for data obtained at low Ce values (contaminant concentration in equilibrium solution). These results are in agreement with other similar investigations dealing with the removal of thallium (I) by zeolites and manganese dioxide nanoparticles [95,101]. The value of n determined in the Freundlich isotherm is greater than one, which implies 1/n < 1. This result supports the fact that the adsorption process is better described by the Freundlich isotherm and would indicate a chemisorption process [118,119]. In addition, isotherms with n > 1 are classified as L-type isotherms reflecting a high affinity between adsorbate and adsorbent and indicating a process of chemisorption in the Tl(I) removal [119-121].

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript has been significantly improved, but some small corrections are still needed.

2.2.2. Determination of pH ZPC of the adsorbent (γANPs)

The sentences in lines 198-200 and 205-208 are redundant in my opinion, since in this chapter it is important only to describe the experiment, and not to explain how pHpzc is determined graphically.

4. Conclusion

Previous comment:

New comment:

Lines 741-742: “We found that electrostatic interactions, at pH values ranging from 4 to 10, governed the adsorption process.” -This statement is not correct according to the explanations in the results and discussion chapter. Namely, my previous comment refers to the previous explanation when you stated in the discussion that from pH=4-10 there is an electrostatic attraction. That's why I said that the conclusion is contradictory. To summarize, above pH=8 the ANP surface becomes negative, and electrostatic attraction plays a role at pH=8-10. Therefore, the corrected sentence is not correct, but the sentence stated in the first version of the paper is correct.

Previous comment:

New comment: his reference is again not correctly cited, write the correct last name. Be sure to cite the other references correctly.

Author Response

Removal of Thallium from Aqueous Solutions by Adsorption onto Alumina Nanoparticles

Ollé Rodrigue KAM¹, Corneille BAKOUAN^1,2, Inoussa ZONGO³ and Boubié GUEL^{1, *}

Response to Reviewer 3 Comments

2.2.2. Determination of pH ZPC of the adsorbent (γANPs)

Question 1: The sentences in lines 198-200 and 205-208 are redundant in my opinion, since in this chapter it is important only to describe the experiment, and not to explain how pHpzc is determined graphically.

Response to question 1: The sentences referring to lines 198-200 and 205-208 have been suppressed to remove the redundancy.

Conclusion

Previous comment:

New comment (Question 2):

Question 2: Lines 741-742: “We found that electrostatic interactions, at pH values ranging from 4 to 10, governed the adsorption process.” -This statement is not correct according to the explanations in the results and discussion chapter. Namely, my previous comment refers to the previous explanation when you stated in the discussion that from pH=4-10 there is an electrostatic attraction. That's why I said that the conclusion is contradictory. To summarize, above pH=8 the ANP surface becomes negative, and electrostatic attraction plays a role at pH=8-10. Therefore, the corrected sentence is not correct, but the sentence stated in the first version of the paper is correct.

Response to question 2: We appreciate the reviewer’s comment. The electrostatic interactions occur between the negatively ANP surface and the positively Tl⁺ ions, in the pH range from 8 to 10. Under pH = 8, the surface phenomena are more complex and need further surface characterizations.

In consequence, the sentence in the conclusion has been revised as follows:

“We found that electrostatic interactions, at pH values ranging from 8 to 10, governed the adsorption process.”

Previous comment:

Carefully read the instructions on citing references. For example reference No. 1 is not cited correctly, Bozena is a first name (given name), and K i.e. Karbowska is a surname (last name). Review and correct all references in detail.

New comment (Question 3):

Question 3: his reference is again not correctly cited, write the correct last name (or surname). Be sure to cite the other references correctly.

Response to question 3: We greatly apologize for this mistake in writing the last name (or surname) Karbowska. Instead of writing “Karbowsk, B.”, the correct reference is “Karbowska, B.” Finally the correct reference, according to the guidelines of authors, is the following:

“1. Karbowska, B. Presence of thallium in the environment: sources of contaminations, distribution and monitoring methods. Environ. Monit. Assess. 2016, 188, 1–19”

Subsequently, taking into account this suggestion, we revised all the cited references according to the following guidelines:

Lastname, F.; Lastname, F.; Lastname, F. Title. Processes 2022, 10, pages range.

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

The authors have improved discussion and manuscript writing. Now, I recommend this work for publication.