Oxidation of Trivalent Arsenic to Pentavalent Arsenic by Means of a BDD Electrode and Subsequent Precipitation as Scorodite
Round 1
Reviewer 1 Report
This work studies the removal of As (III) from wastewater by combining its electro-oxidation to As (IV) and the subsequent co-precipitation with Fe (II) into scorodite. A commercial BDD electrode was proven effective for the electro-oxidation of As (III). The effects of As (III) concentration, electrode area, and other ions (Fe and Cu) were investigated. An economic analysis was carried out, showing a potential advantage over the prevailing H2O2 oxidation process for As (III) removal. I recommend accepting the manuscript after revisions to address the following issues:
1. A summary of the economic analysis should be included in the Abstract, as that is a key result of this work.
2. The results here are based on the treatment of a concentrated As (III) solution (several grams per liter). What is the typical concentration of As (III) in real wastewater? Is a pre-concentrating process necessary in order to implement the electro-oxidation of As (III) efficiently? These issues should be discussed in the main manuscript.
3. What is the typical voltage for the electro-oxidation of As (III)? I see the Authors assumed it 5 V for electrolysis at 100 mA/cm2. Is it close to the true value as observed experimentally?
4. Line 291: The Authors cited the 10-year service time in acid from the manufacturer. But in reality, the BDD is subjected to a positive potential when being used as the electrode, which could largely reduce its service time (see the reference: 10.1021/acsami.5b11638). Long-term stability and/or reusability of the BDD electrode for As (III) oxidation should be studied.
5. Line 293: What is the meaning of “18.8 T USD”?
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript entitled “Oxidation of Trivalent Arsenic to Pentavalent Arsenic by Means of BDD Electrode and Subsequent Precipitation as Scorodite” contains interesting results regarding practical aspects of arsenic removal in the copper’s industry. However, major revisions are needed especially regarding the discussion of the results. Please find details here below:
1. (Abstract) The abstract is too short and simplified. It does not provide any information regarding the novelty of this work. References should be not present in the abstract.
2. (Introduction) The problem in copper industry is well described but, the state of the art concerning arsenic oxidation processes is limited. Please add information regarding advanced oxidation processes (AOP) used to oxidize As(III) to As(V) such as:
- Photocatalysis (DOI: 10.1021/es0489238)
- Photoelectrocatalysis (DOI: 10.1016/j.electacta.2018.10.003)
- Ozonation (DOI: 10.5004/dwt.2011.2372)
- Fenton process (DOI: 10.1021/es026208x)
- Sonochemical oxidation (DOI: 10.1016/j.watres.2010.04.003)
Please cite suggested references possibly with others.
3. (Line 48-51) What is the novelty compared to the previous literature? This information is not clear in the manuscript.
4. (Line 60-62) This part of the text should be moved in the discussion section.
5. (Line 63) The purity of all reagents should be provided
6. (Line 94) Please explicit the meaning of 1 A and 2 A.
7. (Line 97-102) & (Line 253) This equation cannot be theoretically used if the proposed mechanism is an indirect process via hydroxyl radicals formation. This can be used for the faradic process, that is the formation of hydroxyl radicals: the recorded current is not directly related to the As(III)-to-As(V) reaction. Based on which theoretical assumption this law was used to estimate the consumption of arsenic?
8. (Line 127) What is the cathode reaction?
9. (Figure 2) The oxidation state of Cu and Fe ions should be provided. Why 100% oxidation of As(III) is not reached when adding Cu and Fe?
10. (Line 144-149) Cu(II) is reduced to Cu(I) or Cu(0) at the cathode? Fe(II) is already in a reduced oxidation state, this probably explains why it is not depleted at the cathode. Why not using Fe(III) ions? Please also add data showing Cu(II) and Fe(II) temporal trends. How the concentration of Fe(II) and Cu(II) is estimated?
11. (Line 234-238) Please add references and more details (e.g., peak reflection positions) to state that the provided XRD corresponds to scorodite.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
All required revisions were successfully made. The revised version of the manuscript can be accepted for publication.
