Ionic Polyacrylamides as Stability-Modifying Substances of Soil Mineral Suspensions Containing Heavy Metal Impurities

Round 1

Reviewer 1 Report

The paper examines ionic polyacrylamides (PAMs) adsorption on clay minerals (montmorillonite, type 2:1) and kaolinite, type 1:1, without or with heavy metal ions (Cr(VI), Pb(II)). To demonstrate how this paper will bridge the gap, a novelty and relevance statement must be presented. A poor materials and methods section was written without a single subsection. There needs to be more effort put into making the authors' method more understandable and reasonable through the use of concentrations, especially the ratio of pollutant to adsorbate. Moreover, the paper presents only simple results without critically analyzing the literature or discussing it deeply. I recommend the manuscript to be published by Processes only after a major revisions.

Comments:

Title: Why did the authors use heavy metal impurities?

Line 10: mechanism, where is this mentioned in the R&D section?

The results of the electrokinetic and stability tests should be added to lines 17-20.

Replace "hazardous impurities separation" with more relevant keywords.

In what way did the authors categorize their work? Is it a soil washing method? What are the possibilities for implementing the work in the real world? At the end of the R&D section, this aspect of the work must be discussed in depth.

Literature reviews are not included in the introduction. It would be helpful to add specific studies that have used PAM + heavy metal ions on surfaces of montmorillonite and kaolinite, such as j.Chemosphere.2021.130088 and 10.1007/s11368-020-02677-1.

The authors should explain how this paper differs from their previous publications in this field to justify its novelty.

Throughout the introduction, the citing style was applied only at the end of each paragraph in the group format. It is better to avoid this style and list each reference properly rather than group them all together. Avoid citing if it is not necessary.

A subsection must be added to section 2, such as 2.1, 2.2, etc.

Please write the manufacture name, the purity, and the particle sizes of the aluminosilicates on lines 79-87 instead of presenting general information.  

Table 1: What is the difference in measurement methods for pHzpc and pHiep?  

Table 2: Weigh average molecular weight [Da]?

Table 2: please remove the Molecular structure from the Table and present it as separate graphs.

Line 126: “Pb2+ cations and chromium(VI)” Please either give the full name or the abbreviate form through the manuscript.

Table 3 and related information in the text can be given in Appendix.

Lines 181-189: Please transfer this paragraph into section 2.

Line 193: [33, 34] Why does this sentence require citations at the end?

Please change PPM (non-SI unit) to mg/L (SI unit).

There was no discussion of the results obtained in Section 3.1. There should have been citations to similar studies to support the results in lines 194-197. This section along with the other subsections in R&D need to be thoroughly improved.

Line 205: The sentence looked unfished. Please complete.

Please add a and b to figures 6 and 7.

Figure 7: Did the authors use the same ratio of adsorbates to pollutants (100 ppm PAM: 100 ppm Cr(IV))? If this ratio is used, how can such a low heavy metal capacity be justified? It is important to discuss this ratio and the capacity obtained.

The conclusion should be rewritten in a quantitative manner to include all the significant findings.

Author Response

Reviewer 4

1. Title: Why did the authors use heavy metal impurities?

The heavy metal impurities were used, because both chromium(VI) and led(II) are elements which in a minimum amount has definitely adverse effect on animals and plants. Cr(VI) occurring as a chromate or dichromate anions in aqueous solutions, is very strong oxidant with high bioavailability and solubility in a wide pH range. Thus, it penetrates biological membranes readily and oxidizes unsaturated bonds in fatty acids, nucleic acids and proteins (it damages the gastrointestinal tract and skin). Both examined metals shows mutagenic, teratogenic and carcinogenic effects relative to humans and animals. In plant organisms they may damage to the photosynthetic apparatus.

Such explanation was added in the Introduction section.

 

2. Line 10: mechanism, where is this mentioned in the R&D section?

The various mechanism responsible for adsorbates binding with the solid surface were mentioned in the Results and discussion section, e.g. electrostatic interactions, formation of hydrogen and chemical bonds between the polymer chains and the solid surface, polymer-metal complexes formation, competition of solid active sites and intercalation in inter-package spaces of montmorillonite.

 

3. The results of the electrokinetic and stability tests should be added to lines 17-20.

The Abstract was completely re-written and this fragment was changed.

 

4. Replace "hazardous impurities separation" with more relevant keywords.

This keyword was changed to “lead(II) and chromium(VI) separation”.

 

5. In what way did the authors categorize their work? Is it a soil washing method? What are the possibilities for implementing the work in the real world? At the end of the R&D section, this aspect of the work must be discussed in depth.

The definition of conformation of the adsorbed PAM macromolecules as well as the structure of the polymer-metal complexes in the surface layer allowed for a detailed characterization of the processes taking place at the clay mineral-solution interface. Soil flocculants selected for research are widely used in agriculture. The addition of such macromolecular compounds affects the stability of colloidal mineral particle suspensions and their rheological properties. As a result of the flocculation process, the soil cohesion, its structure and the infiltration of rainwater are improved. The use of polymer-containing structure-binding agents reduces the transport of sediments, and thus slows down the soil erosion process. On the other hand, the use of polyacrylamide flocculants changes the bioavailability of heavy metal ions present in soil to living organisms. The conducted research may contribute to a more conscious use of polyacrylamide flocculants in agriculture and will allow to predict the mechanisms of the immobilization process and bioavailability of heavy metal ions for plants.

The above explanation was presented at the end of the R&D section.

 

6. Literature reviews are not included in the introduction. It would be helpful to add specific studies that have used PAM + heavy metal ions on surfaces of montmorillonite and kaolinite, such as j.Chemosphere.2021.130088 and 10.1007/s11368-020-02677-1.

The detailed literature review was presented in our previous paper: Fijałkowska, G., Szewczuk-Karpisz, K., Wiśniewska, M., Polyacrylamide Soil Conditioners: The Impact of Nanostructured Clay Minerals’ Aggregation and heavy Metals’ Circulation in the Soil Environment. In Nanomaterials and Nanocomposites, Nanostructure Surfaces, and Their Applications: Selected Proceedings of the 7th International Conference Nanotechnology and Nanomaterials (NANO2019), Fesenko, O., Yatsenko, L. Eds.; Springer Proceedings in Physics, 2020, Volume 246, pp. 111-127, which was cited as reference [4]. To avoid repetition we limited to demonstrating gaps in knowledge and research related to the discussed issue (paragraph 2 in the Introduction section).

 

7. The authors should explain how this paper differs from their previous publications in this field to justify its novelty.

The aim of the present study was to compare the adsorption mechanism of polyacrylamide (PAM) flocculants of various ionic nature in the presence of heavy metal ions (i.e. Cr(VI) and Pb(II)) on the surface of selected soil minerals - montmorillonite and kaolinite. The present publication is a collection, comparison and summary of the detailed research presented in our previous papers. The collective analysis of the obtained adsorption, electrokinetic and stability results allowed for a comprehensive and systematic characterization of the mechanism of adsorption of both heavy metal ions and polyacrylamide structure-forming compounds on the surface of soil minerals, and the determination of the influence of these substances on the accumulation and immobilization of ionic forms of heavy metals in the soil environment.

 

8. Throughout the introduction, the citing style was applied only at the end of each paragraph in the group format. It is better to avoid this style and list each reference properly rather than group them all together. Avoid citing if it is not necessary.

The citation style was corrected, as it was possible.

 

9. A subsection must be added to section 2, such as 2.1, 2.2, etc.

The subsections 2.1-2.3 were added.

 

10. Please write the manufacture name, the purity, and the particle sizes of the aluminosilicates on lines 79-87 instead of presenting general information.

These general information were removed and the appropriate details were added.

 

11. Table 1: What is the difference in measurement methods for pH_zpc and pH_iep?

The values of the point of zero charge (pzc) - pH_pzc and the isoelectric point (iep) - pH_iep were determined using potentiometric titration and electrophoretic mobility measurements, respectively. The pH_pzc point is the pH value at which the concentrations of the positively and negatively charged surface groups are equal, so that the total surface charge of the solid is 0. The pH_iep point is the pH value at which the electrokinetic (zeta) potential of a solid particle is zero, so that due to the same number of positive and negative groups located in the slipping plane area of electrical double layer.

 

12. Table 2: Weigh average molecular weight [Da]?

In the case of macromolecular compounds four types of average molecular weights are used, dependently on the method which was applied for their determination. There are: number average molecular weight, weight average molecular weight, z- average molecular weight and viscosity average molecular weight.

13. Table 2: please remove the Molecular structure from the Table and present it as separate graphs.

If it is not a problem, I I would like to leave these molecular structures in Table 2. It will not disturb the order of data presentation in the next three tables.

 

14. Line 126: “Pb²⁺ cations and chromium(VI)” Please either give the full name or the abbreviate form through the manuscript.

This sentence was: “lead(II) forming Pb²⁺ cations and chromium(VI) occurring mainly in the form of CrO₄^2- and HCrO₄^- anions”. I think that it is correct.

I changed these abbreviations in other fragments of the manuscript.

 

15. Table 3 and related information in the text can be given in Appendix.

I think there is no point in creating Appendix section only for Table 3. If is it possible, I would leave Table 3 as it is.

 

16. Lines 181-189: Please transfer this paragraph into section 2.

This paragraph was transferred.

 

17. Line 193: [33, 34] Why does this sentence require citations at the end?

This sentence require citations at the end, because these papers contain some data concerning the solution pH influence on the adsorbed amount of PAM and heavy metals on the montmorillonite surface.

 

18. Please change PPM (non-SI unit) to mg/L (SI unit).

The ppm units were changed in whole manuscript to mg/L units.

 

19. There was no discussion of the results obtained in Section 3.1. There should have been citations to similar studies to support the results in lines 194-197. This section along with the other subsections in R&D need to be thoroughly improved.

The discussion of obtained adsorbed amounts of AnPAM and CTPAM, as well as heavy metal ions as a function of solution pH was presented. The conformational changes of ionic polymer chains were described and connected with the structure of adsorption layers.

 

20. Line 205: The sentence looked unfished. Please complete.

This sentence was re-written.

 

21. Please add a and b to figures 6 and 7.

The a and b were added to Figures 6 and 7.

 

22. Figure 7: Did the authors use the same ratio of adsorbates to pollutants (100 ppm PAM: 100 ppm Cr(IV))? If this ratio is used, how can such a low heavy metal capacity be justified? It is important to discuss this ratio and the capacity obtained.

Figure 6 presents the adsorption results obtained for polymer samples with an initial concentration of 100 mg/L and chromate anions with concentrations of 1, 10 and 100 mg/L. Both adsorbates were added to the systems simultaneously or 1 hour apart. Such low adsorption of metal ions is result of their low initial concentration. In the cases of samples containing Cr(VI) with initial concentrations of 1 and 10 ppm, their almost 100% adsorption is observed.

 

23. The conclusion should be rewritten in a quantitative manner to include all the significant findings.

The Conclusions section was re-written.

Reviewer 2 Report

In my opinion, the conclusion is extensive. My recommendation is to redraft more concise and precise.

 

The topic of the paper is interesting and is well-written. The results and the experimental planning are appropriate.

Author Response

In my opinion, the conclusion is extensive. My recommendation is to redraft more concise and precise.

The Conclusions section was re-written to be more concise and precise.

Reply : Thank you for your review. We have revised the manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

Dear editor,

The manuscript (processes-1834229) aims to uncover the mechanism of adsorption of ionic PAMs on clay minerals w and w/o heavy metal ions. The study presents a new perspective towards PAM induced stabilization of clay colloid when considering the involvement of heavy metal ions. Moreover, the manuscript reads well. However, there are several major comments that should be properly addressed to make the work re-considerable for publication:

1- Title; instead of "stability modifying agent" stabilizer could be a better term.

2- Page 2, line 46; this statement needs references.

3- Materials and methods; this part should solely describe the experiments. Information about kaolinite and montmorillonite are unnecessary.

4- Table 1 and 2 are results of characterizations? If so, they should be moved to results and discussion part. Otherwise, there is a need to reference for reporting the values.

5- Table 1; why there are two values for mean pore diameter of montmorillonite?

6- Page 4, line 138; how realistic is the selected concentration range for the heavy metal ions?

7- Page 4, line 139; how concentration of adsorbates was correlated to absorbance of the color solution? Any pre-constructed calibration curve?

8- Figure 1; the columns need error bars. Why AnPAM5 shows a better adsorption performance than AnPAM30?

9- Page 6, discussions are reference-less. The claims and discussions should be supported by referral to the literature.

10- I was wondering why pH 5 and 7 have been selected for this study? The pH difference is not significant to clearly indicate the effect of surface charge.

11- Page 9, line 322; formation of hydrogen bonding between polymer and clay needs to be validated through FTIR. As esterification could be another likely bonding mode.

12- Page 10, line 367; what about chelation of heavy metal ions by PAM?

13- Conclusions; what above-mentioned factors? Conclusion can be more concise and not review every findings of the research. 

Author Response

1. Title; instead of "stability modifying agent" stabilizer could be a better term.

I believe the current title is appropriate. As research has shown, ionic PAM can act as stabilizer or destabilizer, depending on the system conditions, influencing the stability of soil mineral suspensions.

I changed also one word in the title: “Ionic polyacrylamides as stability-modifying substances of soil mineral suspensions containing heavy metal impurities”.

 

2. Page 2, line 46; this statement needs references.

The references were added to this statement.

 

3. Materials and methods; this part should solely describe the experiments. Information about kaolinite and montmorillonite are unnecessary.

Information about kaolinite and montmorillonite was removed from Materials and methods section.

 

4. Table 1 and 2 are results of characterizations? If so, they should be moved to results and discussion part. Otherwise, there is a need to reference for reporting the values.

Tables 1-3 were moved to Results and discussion section.

 

5. Table 1; why there are two values for mean pore diameter of montmorillonite?

The montmorillonite is characterized by high specific surface area equal to 259 m²/g, which is associated with its specific layered structure (2:1 clay). The observed isotherms of nitrogen adsorption of IV type with H3 hysteresis loop indicate a multilayer adsorption on the mesopores. Moreover, the obtained hysteresis is generated by slit-shaped pores and/or panel-shaped particles. The mesoporous character of the montmorillonite surface is also visible in the pore size distribution (PSD) diagram. The PSD parameter is bimodal, i.e. there are two groups of mesopores: the first of the average diameter equal to 3.8 nm and the second one – 5.6 nm.

 

6. Page 4, line 138; how realistic is the selected concentration range for the heavy metal ions?

The initial concentration range of heavy metals was 1 - 100 ppm. A contamination level of 100ppm can be found locally in areas where these metals are mined and used in industrial applications. Lower concentrations may be recorded in nearby groundwater.

 

7. Page 4, line 139; how concentration of adsorbates was correlated to absorbance of the colour solution? Any pre-constructed calibration curve?

The concentration of individual adsorbates was determined spectrophotometrically by measuring the absorbance of the color solution formed after adding the appropriate indicator. Of course, the appropriate calibration curves (shoving the dependencies of colored solution absorbancies as a function of their concentration) were prepared at the same beginning of each measurement series. This information was added in the text.

 

8. Figure 1; the columns need error bars. Why AnPAM5 shows a better adsorption performance than AnPAM30?

Instead of error bars (data labels are presented) in the figures showing the adsorbed amounts, I added information about the measurement error Materials and methods section. The adsorption tests were repeated 3 times and the final result was the mean value of these 3 measurements. The measuring error did not exceed 5%.

AnPAM-5 shows a better adsorption performance than AnPAM-30 due to the lower content of carboxyl groups which are the source of negative charge of macromolecules. The electrostatic repulsion between these groups causes polymer chain development. In the case of AnPAM-5 this development is limited and the adsorbed macromolecules assumes more coiled conformation, which enable denser packing of adsorption layer and G increase.

 

9. Page 6, discussions are reference-less. The claims and discussions should be supported by referral to the literature.

The discussion was enriched with additional literature reports.

 

10. I was wondering why pH 5 and 7 have been selected for this study? The pH difference is not significant to clearly indicate the effect of surface charge.

Adsorption measurements were performed at the two solution pH values - neutral and slightly acidic conditions typical of clay soils, i.e. 5 and 7 ± 0.1. Pb(II) ions adsorption measurements were not performed at pH 7 due to the precipitation of lead hydroxide in solutions under such conditions and initial concentration of lead equal to 100 ppm. What is more at pH values lower than 5 the reduction of Cr(VI) to Cr(III) can occur in adsorption layers of polyacrylamide.

 

11. Page 9, line 322; formation of hydrogen bonding between polymer and clay needs to be validated through FTIR. As esterification could be another likely bonding mode.

The surface functional groups of adsorbents were identified using the Fourier-transform infrared spectroscopy (FTIR). For this purpose, the sample was mixed with KBr and the tablet was formed. They were analysed using the FTIR spectrometer (Nicolet 8700A, Thermo Scientific). The spectra were recorded in the range of 400 – 4000 cm^-1. These results were described in the cited paper: Wiśniewska, M., Fijałkowska, G., Szewczuk-Karpisz, K., The mechanism of anionic polyacrylamide adsorption on the montmorillonite surface in the presence of Cr(VI) ions. Chemosphere 2018, 211, 524-534.We attempted to perform FTIR tests of polyacrylamide modified minerals, but probably due to the insufficient amount of adsorbed polymer for this method, we did not obtain changes in spectra for unmodified and polymer-modified samples.

 

12. Page 10, line 367; what about chelation of heavy metal ions by PAM?

The chelation possibilities of heavy metal ions were described in the in the sentences that precede this fragment: “Neutral amide groups present in CtPAM macromolecules, namely free electron pairs on nitrogen atoms of these groups, are responsible for the formation of these chemical bonds. In turn, the formation of the Cr(VI)‑CtPAM complex is associated with the electrostatic interaction between negative chromate ions and positive polymer segments containing quaternary amine groups.”

 

13. Conclusions; what above-mentioned factors? Conclusion can be more concise and not review every findings of the research.

The Conclusions section was re-written.

Reviewer 4 Report

Dear Authors,

Please find my comments as below.

Best regards

 

1-      The Abstract does not show the aim and result of the research. I highly recommend to re-write the Abstract.

2-      The manuscript has many grammatic errors. The file should be checked carefully.

3-      I suggest to add negative effect of heavy metals into the introduction.

4-      Adding references to the section of “effect of the solution pH” is necessary.

5-      I suggest to add a schematic presentation of the method under the optimum condition.

6-      The references are so old, 1999, 1985 !!! change the references with new ones.

Author Response

1. The Abstract does not show the aim and result of the research. I highly recommend to re-write the Abstract.

The Abstract was completely re-written.

 

2. The manuscript has many grammatic errors. The file should be checked carefully.

The manuscript was checked with the respect of correctness of the English language. The grammatic errors was corrected.

 

3. I suggest to add negative effect of heavy metals into the introduction.

The negative effect of examined heavy metals in living organisms were presented in the Introduction section.

 

4. Adding references to the section of “effect of the solution pH” is necessary.

The appropriate references were added in this section.

 

5. I suggest to add a schematic presentation of the method under the optimum condition.

The schematic representation of the effect of mineral type on the binding mechanisms of both adsorbates (in example of AnPAM-30 and Pb(II)) on the both aluminosilicates surface was added as Fig. 4.

 

6. The references are so old, 1999, 1985 !!! change the references with new ones.

Unfortunately, I cannot change these references as they refer to the basics of research methods or original computer programs that were used in the experiments. In other cases, the most recent literature is cited.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

After corrections of the above-mentioned points the paper can be accepted for publication in the journal of Processes.

Reviewer 3 Report

Dear editor,

Considering the applied corrections, the manuscript is publishable now.