Application of Mineral Iron-Based Natural Catalysts in Electro-Fenton Process: A Comparative Study
and Mehmet A. Oturan 1,*Round 1
Reviewer 1 Report
The Authors proposed a interesting topic for research. The paper is publishable, but it is need to consider a few of uncertainties.
- Has the analysis of the content of this substance been considered, e.g. by chromatography? The TOC measurement is not fully reliable. Processes of this kind often result in intermediates that are often more toxic than the parent substances.
- Has the possibility of absorbing pollutants from the air in an open system been taken into account?
- How many repetitions were made for each analyzed sample?
- Has the amount of generated hydroxyl radicals been measured?
- On what basis were the individual process parameters selected?
- The discussion of the results with other studies is insufficient.
- Is the use of this process more economically justified compared to other also effective advanced oxidation processes?
- On what basis was the catalyst value selected?
- Were temperature measurements taken during the process?
Author Response
- Has the analysis of the content of this substance been considered, e.g. by chromatography? The TOC measurement is not fully reliable. Processes of this kind often result in intermediates that are often more toxic than the parent substances.
Response: We thank the Reviewer. We agree with his/her consideration. According to the toxicity assessment reported by our group [1-4], it was found that the toxic intermediates formed in the earlier electrolysis time; however, they are mineralized on longer electrolysis times. The toxicity of the treated solution decrease with the disappearance of cyclic/aromatic organic compounds along with the TOC decay. As can be seen in Fig.5, The residual TOC at the end of experiments is negligible, and mainly formed of carboxylic acids, which are not toxic and can be removed by biologically in the environment.
[1]. Barhoumi, N., Oturan, N., Olvera-Vargas, H., Brillas, E., Gadri, A., Ammar, S., Oturan, M. A., Pyrite as a sustainable catalyst in electro-Fenton process for improving oxidation of sulfamethazine. Kinetics, mechanism and toxicity assessment. Water Res., 2016. 94 (1): pp. 52-61.
[2]. Olvera-Vargas, H., Oturan, N., Buisson, D., Hullebusch, E.D.v., Oturan, M.A., Electro-oxidation of the pharmaceutical furosemide: Kinetics, mechanism, and by-products. CLEAN, Soil Air Water, 2015. 43 (11): pp. 1445-1558.
[3]. Dirany, A., Sires, I., Oturan, N., Ozcan, A., Oturan, M.A. Electrochemical treatment of the antibiotic sulfachloropyridazine: kinetics, reaction pathways,and toxicity evolution. Environ. Sci. Technol., 2012. 46, pp. 4074-4082.
[4]. El-Ghenymy, A., Rodríguez, R.M., Brillas, E., Oturan, N., Oturan, M.A., Electro Fenton degradation of the antibiotic sulfanilamide with Pt/carbon-felt and BDD/carbon-felt cells. Kinetics, reaction intermediates, and toxicity assessment. Environ. Sci. Pollut. Res., 2014. 21, pp. 8368–8378
- Has the possibility of absorbing pollutants from the air in an open system been taken into account?
Response: The pollutant studied, in particular the cefazolin, studied in this work, are relatively polar and soluble in water. Their solubility in air is considered negligible. There is no problem, particularly, in the case of pharmaceutical products since they are designed for human needs.
- How many repetitions were made for each analyzed sample?
Response: The experiments were performed in triplicate 3 and the average values were used in the graphs.
- Has the amount of generated hydroxyl radicals been measured?
Response: Thanks for this comment and suggestion. The electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy can be used for the confirmation of the presence of free radicals (such as hydroxyl radicals) in the advanced oxidation processes. This test is done via 5, 5-dimethyl-l-pyrroline N-oxide (DMPO) technique and in aqueous dispersion for DMPO-•OH. Un fortunately, this kind of test are not available in our lab. On the other hand, this kind of test were made several times and it is now considered that •OH are formed unambiguously in electro-Fenton process.
- On what basis were the individual process parameters selected?
Response: The solution pH, applied current, catalyst nature and its concentration are the most influent parameters in electro-Fenton process. The value of optimal value of solution pH (about three) and the nature of catalyst (ferrous iron) are now well established [5]. Therefore, in this work, the optimal value of two parameters; current and catalyst loading, was assessed regarding their influence on the electro-Fenton process efficiency. The applied current is one of the main key factors governing the effectiveness of electrochemical advanced oxidation processes because it regulates the amount of generated hydroxyl radicals that oxidizes organic matter present in the solution [5, 6]. The applied current controls the production of hydroxyl radicals (•OH) in the bulk solution through Eqs. (1), (2) and at the anode surface (BDD(•OH)) according to Eq. (4) [7].
On the other hand, the catalyst loading (to provide appropriate ferrous iron concentration) is another important parameter affecting the performance of the electro-Fenton process, since the generation rate of •OH from Fenton's reaction (4) is dependent on the availability of free Fe2+ ions as catalyst [5].
[5]. Brillas, E., Sires, I., Oturan, M.A., Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry. Chem. Rev. 2009. 109 (12), pp. 6570-6631.
[6]. Sires, I., Brillas, E., Oturan, M.A., Rodrigo, M.A., Panizza, M., Electrochemical advanced oxidation processes: today and tomorrow. A review. Environ. Sci. Pollut. Res., 2014. 21 (14),p p. 8336-8367
[7]. N. Barhoumi, H. Olvera-Vargas, N. Oturan, D. Huguenot, A. Gadri, S. Ammar, E. Brillas, M. A. Oturan, Appl. Catal. B, 2017. 209, pp. 637-647.
- The discussion of the results with other studies is insufficient.
Response: This comment is greatly appreciated. There is a lack of comprehensive study on degradation of CFZ from wastewater by electro-Fenton processes. Therefore, we added a new sub-section (sub-section 8.2) and a new Table (Table 2) to the manuscript to compare the results of this study with some other degradation strategies (Lines 278-291).
- Is the use of this process more economically justified compared to other also effective advanced oxidation processes?
Response: The electrochemical advanced oxidation processes (in particular anodic oxidation and electro-Fenton) have the advantage to generate in situ the reagents to generate active radical species avoiding thus the cost of chemicals and their transport and/stockage problems. On the other hand, the implementation of catalysts for high-catalytic efficiency in advanced oxidation processes is important but remain a big challenge. One of the challenges ahead is the economic aspect. In this study, we used available, cheap and mineral catalysts instead of synthetic catalysts which are more expensive. The use of natural Fe-based catalysts allows recovering and reusing it several times inversely to the classical electro-Fenton, which can be considered as beneficial in economical point of view.
- On what basis was the catalyst value selected?
Response: In order to find the best levels for each factor, some preliminary experiments were done prior to the main electrolysis tests and the levels were selected based on these preliminary experiments. It was found that very high amount of catalysts loading reduced the process efficiency through parasitic reactions which waste the generated •OH by extra Fe2+ (Eq. (10)).
Therefore, a certain range of catalysts dosage was defined thanks to preliminary experiments.
- Were temperature measurements taken during the process?
Response: According to the mentioned condition in the manuscript (Lines: 298 and 329) the EF tests were performed at room temperature which means no heating or cooling were needed for the experiments for two reasons: (i) The Fenton’s reaction needs low energy for activation, and (ii) high temperatures hinder the efficiency of the process since the lead to the decrease of O2 solubility in water which control the rate of •OH formation through the Fenton reaction.
Reviewer 2 Report
- Caption to the table 1 contains "Sulfasalazine" instead of "Cefazolin". Also, chemical structure for cefazolin in the table is obviously wrong and correspond to sulfasalazine.
- Minor style editing is required. Example: "Raman spectoscopy armed with diode light".
- It was stated in paragraph 3.2.1. that the current of 200 mA is optimum for cefazoline degradation for all catalysts. Why current of 100 mA was used for all of the other series of experiments?
Author Response
- Caption to the table 1 contains "Sulfasalazine" instead of "Cefazolin". Also, chemical structure for cefazolin in the table is obviously wrong and correspond to sulfasalazine.
Response: Thanks for this kind consideration. Sulfasalazine was corrected as Cefazolin in the Table 3.
- Minor style editing is required. Example: "Raman spectroscopy armed with diode light".
Response: The manuscript was revised and some style and typos have been corrected.
- It was stated in paragraph 3.2.1. that the current of 200 mA is optimum for cefazoline degradation for all catalysts. Why current of 100 mA was used for all of the other series of experiments?
Response: According to the pre-experiments, the recognition of optimum amount of catalysts loading in 200 mA was complicated. Therefore, to evaluate the effect of catalysts loading on the process efficiency, the lower current (100 mA) was selected to provide better understanding about optimum catalysts loading.
Reviewer 3 Report
attached
Comments for author File: 
Comments.pdf
Author Response
We appreciate the reviewer’s comments and points that helped us to improve the quality of our paper. We believe that the revised manuscript has benefitted from an improvement in the overall presentation and clarity.
Round 2
Reviewer 1 Report
I recommend it for publication
