Arsenate Removal from the Groundwater Employing Maghemite Nanoparticles

Round 1

Reviewer 1 Report

This manuscript demonstrates arsenate removal from the groundwater with maghemite nanoparticles, focusing on the optimization of the removal process parameters using orthogonal array method. In addition, the maghemite nanoparticles were well characterized and investigated for the arsenate removals under various conditions, like pH ranging from 7.0 to 9.0. The effect of matrice in the simulation and collected groundwater on the arsenate removal is also evaluated. This manuscript can be published after a major revision with following comments.  

1. The quantitative analysis should be added to the Abstract to highlight the effectiveness of maghemite nanoparticles for arsenate removal.

2. Please revise the wrong serial number in the subtitle experimental section.

3. Taguchi's design of experimental methodology should be more concise.

4. It is better to investigate the adsorption kinetics of arsenate by maghemite nanoparticles, go to Environmental Science & Technology 2021 55 (8); 4287-4304, Nano Res. 15, 2961–2970 (2022).

5. how about the surface area of maghemite nanoparticles before and after adsorption.

Author Response

The quantitative analysis should be added to the Abstract to highlight the effectiveness of maghemite nanoparticles for arsenate removal.

Response: The comment is rectified by adding quantitative data in the abstract.

Please revise the wrong serial number in the subtitle experimental section.

Response: The comment is rectified.

Taguchi's design of experimental methodology should be more concise.

Response: Following changes were made to make it more concise to a possible extent:

• Removed lines from section 2.6 (discussed in the introduction)

It is a useful statistical tool in developing a suitable experimental approach for laboratory investigations by selecting a minimum number of experiments. It overcomes the drawbacks related to conventional experimental designs.

• Removed lines from section 2.6.2

Taguchi proposed several orthogonal arrays in selecting the experimental design based on the number of parameters and the interaction.

• Removed lines in section 2.6.5

Among three types of S: N ratio responses (smaller-is-better, nominal-the-best, larger-is-better), the larger-is-better characteristic was used in this study.

• Removed lines in section 2.6.5 (discussed in section 3.5 as well)

For example, the parameters total dissolved solids (B) and shaking speed (C) were found to be significant at the levels of B3 and C2, respectively; after that,

It is better to investigate the adsorption kinetics of arsenate by maghemite nanoparticles, go to Environmental Science & Technology 2021 55 (8); 4287-4304, Nano Res.15, 2961–2970 (2022).

Response: I thank the reviewer for the suggestion. We also did experiments exploring the equilibrium isotherm and kinetic models. Considering the manuscript length, we have not included these studies in the present manuscript. This is a part of our another manuscript. We are highlighting a few points highlights related to our studies in response to the above-mentioned references:

1. Maghemite NPs and their functionalized nanostructures.
2. Incorporation of industry waste (organics rich) for preparing modified MNPs for enhanced efficiency.
3. Exploring their efficiency for groundwater representing real-world conditions.
4. Isotherm and kinetic studies.
5. 1D experiments by supporting these NPs on sand and pumice (under conditions of constant porosity).
6. 3D experiments, representing in-situ application through injections mode.

5. How about the surface area of maghemite nanoparticles before and after adsorption.

Response: We have not calculated the surface area after adsorption. Because of the large experimental size, measuring the surface area for both treated and untreated MNPs for all the investigated ranges is not economical. However, we have measured the surface area of untreated MNPs, which comes to around 59 m² g^-1. This is comparatively higher among those nanoadsorbents utilized for arsenic removal. 

Author Response File: Author Response.docx

Reviewer 2 Report

Title: Arsenate removal from groundwater using maghemite nanoparticles
After reviewing the present manuscript, I found that the authors made very interesting work and all required analysis for the prepared maghemite nanoparticles adsorbents were determined.
I found that this manuscript is very fit with the Water MDPI and need minor revision for publication

1)      Graphical abstract not included- Adding a graphical abstract would help add weightage to the manuscript.

2)      Include highlights to the manuscript, to present the most significant findings of the research.

3)      Throughout the manuscript the numbering of heading and subheading is incorrect, rearrange them in an order.

4)      There are many published papers on Arsenate removal from groundwater. The mechanisms have also been well documented. The authors need to summarize those studies in the introduction section and thus point out the knowledge gaps and why this work is necessary.

5)      Table numbers need to be rearranged in an ordered: Table 7 is missing

6)      Page2-Para2- Try to mention all the different arsenic removal techniques rather than just citing the references. “Among arsenic removal technologies”

7)      Rearrange heading number "Experimental"

8)      Rearrange subheading number "Materials and reagents"

9)      Page4-Para1-Use a comma after “solution” from the sentence- "The Na2HAsO4.7H2O (M.W. = 312.01 g mol-1) salt was utilized to prepare the arsenic working solution was purchased from"

10)   Rearrange subheading number "Instrumentations and Equipment"

11)   Rearrange subheading number "Synthesis of nanostructured maghemite"

12)   Rearrange subheading number "Sample collection and analysis"

13)   Rearrange subheading number "Formulation of synthetic water"

14)   Rearrange subheading number "Taguchi's design of experimental methodology"

15)   Rearrange subheading number "Design of experiment (Phase 1)"

16)   Rearrange subheading number "Orthogonal array (OA) and assignment of parameters"

17)   Rearrange subheading number "Batch removal experiments (Phase 2)"

18)   Rearrange subheading number "Evaluation of outputs and performance assessment (Phase 3)"

19)   Rearrange subheading number "Prediction of average adsorption capacity"

20)   Rearrange subheading number "Determination of confidential interval"

21)   Rearrange subheading number "Confirmation experiments (Phase 4)"

22)   Rearrange subheading number "Modelling of adsorption processes"

23)   Rearrange subheading number "Artificial neural network (ANN) for predictive modeling"

24)   Rearrange heading number "Results and discussion"

25)   Rearrange subheading number "Characterization of maghemite nano-particles"

26)   Page17-Para1- Elaborate discussion about FESEM

27)   Page17-Fig4- FTIR image quality can be improved

28)   Page 17-Para1- Is it possible for the authors to compare all the Characterization of maghemite nano-particles with untreated nano-particles with figures to show the difference after treatment?

29)   Rearrange subheading number "Elemental characterization of groundwater and its formulation"

30)   Rearrange subheading number "Characteristics of arsenic removal in the multi-ionic system"

31)   Rearrange subheading number "Effects of process parameters: Shaking speed, temperature, and contact time"

32)   Rearrange subheading number "Effects of process parameters: As^V concentration, TDS, and pH"

33)   Rearrange subheading number "Analysis of inter-parametric interactions"

34)   Page20-Para2-Point of zero charge figure can be included if figure limit is not exceeding

35)   Rearrange subheading number "Selection of optimal levels and estimation of response characteristics"

36)   Rearrange subheading number "Confirmation experiments"

37)   Rearrange subheading number "Analysis using Visual MINTEQ"

38)   Rearrange subheading number "Surface complexation models (SCMs) for adsorption behavior"

39)   Rearrange subheading number "ANN Predictions"

40)   Rearrange heading number “Conclusions"

41)   The maximum ANN model adsorption capacities of the MNPs to As^V only reached 4.2 mg/g. These values are low for nano-particle adsorbents. The authors need to modify the discussion to address this.

42)   Authors should add table to compare the adsorption efficiency of the γ-Fe2O3 (maghemite) nano-particles(MNPs) with other adsorbents?

43)   References need to be arranged in alphabetical order

Author Response

Thanks for reviewing the manuscript. The response sheet is attached.

Author Response File: Author Response.docx

Reviewer 3 Report

This is a well written paper with good content and flow. Some minor edits are suggested in the annotated PDF version. Please check and follow the suggestions.

Some concerns I had-

The effect of pH on the removal of arsenic seems increasing with increase of pH from 7 to 9 in this study but this seems decreasing in another similar study.

Please provide some detailed info on effects of competing ions on the removal of arsenite.

Comments for author File: Comments.pdf

Author Response

This is a well written paper with good content and flow. Some minor edits are suggested in the annotated PDF version. Please check and follow the suggestions.

Some concerns I had-

Comment: The effect of pH on the removal of arsenic seems increasing with increase of pH from 7 to 9 in this study but this seems decreasing in another similar study.

Response: This study investigates the arsenic removal from the aqueous solution containing elemental ions representing the real-world composition. We could not locate studies in literature exploring removal behavior in arsenic-contaminated groundwater while simulating its actual composition employing maghemite NPs. Apparently, the presence of various ions in the solution in the real world scenario may cause a change/shift in adsorption behavior compared to those demonstrated in the literature which may be due to the formation of the secondary site and complex species formed onto the NP's surface contributing toward arsenic adsorption.  This study also demonstrates that similar NPs can behave differently in an aqueous solution containing single arsenic ions or arsenic along with other ions.

Comment: Please provide some detailed info on effects of competing ions on the removal of arsenite.

Response: The details explaining the impact of various ions on arsenic adsoprtion are as follows:

1. The phosphate ions affect the As^V adsorption (negative) in pH range 7-9 and concentrations under investigation, i.e., 1.0-1.75 ppm. Its complex species also affect the As^V
2. Ca²⁺ ions above the concentration of 28 ppm show an insignificant impact on AsV adsorption, whereas <28 ppm, these ions partially compete with As^V during adsorption. This competing effect increases as pH increases from 7 to 9.
3. Moreover, the complex species of Ca2+, such as calcium hydrogen phosphate and calcium, compete with the AsV during adsorption.
4. Mg²⁺ ions have a negligible impact on As^V adsorption for the investigated concentration range. However, its complexes, such as magnesium phosphate and magnesium hydrogen phosphate, compete with AsV during adsorption.
5. The nitrate ion's interactions with NPs surface at high concentrations (>10 ppm) increase as pH varies from 7 to 9. In literature, these ions are also reported to provide secondary sites for arsenic adsorption. This interpretation also justify an increase in As^V adsorption as pH increases from 7 to 9.

Author Response File: Author Response.docx