Photocatalytic Reduction of Cr(VI) and Pb(II) with Biogenically Synthesized Copper Oxide Nanoparticles Using an Extract of the Myriophyllum spicatum Plant

Round 1

Reviewer 1 Report

Authors need to provide the Tauch plot for fig.3 

Please provide the absorbance spectra of Fig.7 for  better understanding of the readers. 

Cycling Stability of the photocatalyst is important, provide the cycling stability of the photocatalysts . 

Please specify concentration of the cr and Pb in aqueous solution. 

specify the intensity of light. 

if it possible, provide the XPS analysis for photocatalysts. 

Authors need to write and discuss photocatalytic mechanism for both  pollutants individually. 

for sake of competence and readership of this manuscript, authors need to refer and cite these articles in revised manuscript: Ceramics International 44 (10) 11724-11729; Journal of Solid State Chemistry 262, 106-111; ACS Omega 2020, 5, 2, 1040–1051;  Materials Research Express 6 (11), 115044; Arabian Journal of ChemistryVolume 15, Issue 5, May 2022, 103739.

There are several typo and format errors please revise

In XRD, why the intensity peak at 30 degree appears. please explain and index carefully 

Need to improve and polish.

Author Response

-Authors need to provide the Tauch plot for fig.3 

Authors response: Tauc plot has been used to calculate the band gap energy of CuO and the plot has been added as Figure 3b.

-Please provide the absorbance spectra of Fig.7 for better understanding of the readers. 

Authors response: The absorption spectra of figure 7 has been added

-Cycling Stability of the photocatalyst is important, provide the cycling stability of the photocatalysts. 

Authors response: The reusability of biogenic synthesized copper oxide nanoparticles in Cr(VI) reduction was investigated and presented in Fig. 8

-Please specify concentration of the cr and Pb in aqueous solution.

 Authors response: The concentration of Pb and Cr(IV) in aqeous solution at every stage was specified in each Figure caption. The highest concentration investigated for both Pb and Cr(IV) was 100 mg/L and the lowest was 25 mg/L.

-specify the intensity of light. 

Authors response: In photocatalysis experiment, a xenon arc lamp is often used because of its ability to generate light of stable intensity throughout the wavelength range from 200 to 800 nm. Authors have used the same xenon arc lamp and have no means of determining its intensity.

-if it possible, provide the XPS analysis for photocatalysts. 

Authors response: The photocatalyst is a simple CuO nanoparticles, whose crystalline structure, morphology, and optical properties have been determined and discussed. Authors do not have the means of carrying out the XPS analysis of the metal oxide as further characterization. Furthermore, the compositional studies of the metal oxide, determined using EDX analysis in Figure 4g, has provided a confirmation of the atomic composition of the photocatalyst.

-Authors need to write and discuss photocatalytic mechanism for both pollutants individually. 

Authors response: The photocatalytic mechanism for both pollutants follow the same process. Hence, it would be superfluous and monotonous to repeat the same discussion twice. Therefore, the mechanism of both pollutants is included in the revised version.

-For sake of competence and readership of this manuscript, authors need to refer and cite these articles in revised manuscript: Ceramics International 44 (10) 11724-11729; Journal of Solid State Chemistry 262, 106-111; ACS Omega 2020, 5, 2, 1040–1051; Materials Research Express 6 (11), 115044; Arabian Journal of Chemistry Volume 15, Issue 5, May 2022, 103739.

Authors response: Some of the recommended papers have been cited in the revised version of the manuscript based on their relevance to the current study.

-There are several typo and format errors please revise

Authors response: The manuscript has been reviewed for possible typographical, and grammar errors.

-In XRD, why the intensity peak at 30 degree appears. please explain and index carefully 

Authors response: According to the JCPDS standard reference for CuO, the (110) peak is the first peak in the diffraction pattern. The peak around 2 θ= 30 could be attributed to the machine effect because a peat of the measurement has given a new diffraction pattern with no peak around 30.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors of this paper comprehensively present the synthesis of copper oxide nanoparticles through non-traditional synthesis. They then investigate the applicability of such nanostructures as catalysts for the reduction of heavy metals in water. The argumentation of the work is well developed and the introduction quite extensive. The study of the experimental conditions and characterisation of the system is comprehensive. 

The article can be published after minor revisions:

-When the authors show the photodegradation results figures 5-6-7, it would also be interesting if they showed the associated UV-VIS spectra and also Atomic Absorption Spectrophotometer results and not only the calculated percentage of degradation.

Author Response

Reviewer 2

The authors of this paper comprehensively present the synthesis of copper oxide nanoparticles through non-traditional synthesis. They then investigate the applicability of such nanostructures as catalysts for the reduction of heavy metals in water. The argumentation of the work is well developed and the introduction quite extensive. The study of the experimental conditions and characterization of the system is comprehensive. The article can be published after minor revisions:

-When the authors show the photodegradation results figures 5-6-7, it would also be interesting if they showed the associated UV-VIS spectra and also Atomic Absorption Spectrophotometer results and not only the calculated percentage of degradation.

Authors response: Authors have added the photodegradation graph from the absorption spectra to the figures.

-Comments on the Quality of English Language. Need to improve and polish.

Authors response: The manuscript has been reviewed for possible typographical, and grammar errors. Therefore, the English language of this manuscript has been polished and improved.

Author Response File: Author Response.pdf

Reviewer 3 Report

By utilizing the extract of the Myriophyllum spicatum plant, the authors have been able to produce CuO nanoparticles via a green synthesis route. An investigation of the photocatalytic properties of the nanoparticles was conducted on Cr(VI) and Pb(II). Although the study is interesting, it needs to be substantially revised before it can be accepted. Please take the time to consider the following comments.

1) L 64: This thus,....

2) L 66: Change TiO to TiO₂

3) L 89, 92, 93: Change Cr(IV) to Cr(VI)

4) L 93 closed bracket missing

5) L 101: check the salt of chromium. It is miswritten

6) L 115: degree sign missing

7) Fig 1: Check the y axis label

8) Why is there a peak around 32 degrees and it is not indexed by the authors?

9) Fig 2: "Transimance (%)" must be Transmittance. X-label "superscript cm-2" 

10) The sentence " The 183 spectra represent the characteristic absorbance peak at 384 nm indicating that copper oxide" need rewriting

11) L 190: How can SEM provide information on nanoparticle surface functionalization?

12) " The SEM micrographs at both low and high of the copper oxide nanoparticles are evenly distributed and homogeneous in nature" needs rewriting

13) L 198: "mela oxide"?

14) L 225: TiO₂

15) L 232: "C(VI)"?

16)"For example, when the pH of the solution was low, reaction (1) dominants and reduction of Pb(II) and Cr(VI) takes place. As the pH was increased, reaction (2) starts dominating the reaction (1), so that there will be an additional decrease in the amount of H ions."  The discussion is confusing and needs serious attention. The reaction 1 and 2 mentioned in the discussion does not relate to any reaction but the calculation of degradation. The authors should check this in the revised version.

17) Fig 6: remove from the caption "(conc. of photocatalyst = 3g/L"

18) Fig 7: Change the plotting method of Fig.7 and follow plots similar to Fig 6 and Fig.5.

19) Include a table that compares the photocatalytic performance with that reported in literature.

Misc: Check formatting errors in the reference section

"Myriophyllum spicatum" must be written as "Myriophyllum spicatum" in the manuscript.

Author Response

Reviewer 3

By utilizing the extract of the Myriophyllum spicatum plant, the authors have been able to produce CuO nanoparticles via a green synthesis route. An investigation of the photocatalytic properties of the nanoparticles was conducted on Cr(VI) and Pb(II). Although the study is interesting, it needs to be substantially revised before it can be accepted. Please take the time to consider the following comments.

• L 64: This thus,....

Authors response: The grammatical error has been corrected to “This is an indication that”

• L 66: Change TiO to TiO₂

Authors response: Changed in the revised version

• L 89, 92, 93: Change Cr(IV) to Cr(VI)

Authors response: We thank the reviewer for this observation, Cr(IV)  has been changed to Cr(VI) in the revised manuscript

• L 93 closed bracket missing

Authors response: The bracket is now closed in L93

• L 101: check the salt of chromium. It is miswritten

Authors response: The salt used for the preparation of Cr(VI) has been corrected in revised manuscript.

• L 115: degree sign missing

Authors response:  Degree sign is now included as “90 °C”

• Fig 1: Check the y axis label

Authors response: The y axis label has been checked and corrected as “(intensity (a.u))

• Why is there a peak around 32 degrees and it is not indexed by the authors?

Authors response: The peak around 2 θ= 32 could be attributed to the machine effect because a repeat of the measurement has given a new diffraction pattern with no peak around 30.

• Fig 2: "Transimance (%)" must be Transmittance. X-label "superscript cm-2" 

Authors response: In Figure 2, Transimance (%)" has been corrected as Transmittance and the X-label corrected to "superscript cm-1" 

• The sentence " The 183 spectra represent the characteristic absorbance peak at 384 nm indicating that copper oxide" need rewriting

Authors response: The sentence in L183, has been corrected as “Therefore, the absorbance peak at 384 nm attributed to copper oxide, which is in agreement with some related studies on plant mediated biosynthesis of CuO NPs”

• L 190: How can SEM provide information on nanoparticle surface functionalization?

Authors response: L190 has been corrected as “SEM analysis also displays the structure, and degree of agglomeration of nanoparticles”

• " The SEM micrographs at both low and high of the copper oxide nanoparticles are evenly distributed and homogeneous in nature" needs rewriting

Authors response: The sentence has been rewritten as” The SEM micrographs of copper oxide nanoparticles at different magnifications are evenly distributed and homogeneous in nature”

• L 198: "mela oxide"?

Authors response: mela oxide has been corrected to metal oxide

• L 225: TiO₂
• Authors response: TiO² is now corrected as TiO₂
• L 232: "C(VI)"?

Authors response: C(VI) is corrected now as Cr(VI)

16)" For example, when the pH of the solution was low, reaction (1) dominants and reduction of Pb(II) and Cr(VI) takes place. As the pH was increased, reaction (2) starts dominating the reaction (1), so that there will be an additional decrease in the amount of H ions."  The discussion is confusing and needs serious attention. The reaction 1 and 2 mentioned in the discussion does not relate to any reaction but the calculation of degradation. The authors should check this in the revised version.

Authors response: The discussion has been revised as “when the pH of the solution was low, the higher reduction of both Pb(II) and Cr(VI) took place. As the pH increases, the reduction of both metal ions decreases and this might be due to the decrease in the amount of H⁺ ions in the solution.”

• Fig 6: remove from the caption "(conc. of photocatalyst = 3g/L"

Authors response: This is an oversight, "(conc. of photocatalyst = 3g/L") is now removed from Fig. 6 caption

• Fig 7: Change the plotting method of Fig.7 and follow plots similar to Fig 6 and Fig.5.

Authors response: Figure 7 plot has been changed to plotting method used in Fig. 5 and 6

19) Include a table that compares the photocatalytic performance with that reported in literature.

Misc: Check formatting errors in the reference section

Authors response:  The comparison table of photocatalytic performance of similar catalyst has been included in revised manuscript (Table 1). The formatting errors has been checked and corrected. Formatting errors has been checked and corrected accordingly.

20) "Myriophyllum spicatum" must be written as "Myriophyllum spicatum" in the manuscript.

Authors response: Myriophyllum spicatum has been rewritten in italic in the revised version of the manuscript

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Authors improved the quality of the manuscript

Reviewer 2 Report

After the correction of the original paper, this new version is suitable for publication

Reviewer 3 Report

The manuscript can be accepted for publication.

Check Fig 2: (cm^-1)