Strengthen Air Oxidation of Refractory Humic Acid Using Reductively Etched Nickel-Cobalt Spinel Catalyst

Round 1

Reviewer 1 Report

1. There are some minor typographic mistakes which should be corrected before final acceptance.
2. Why only one concentration of Na₂SO₃ was examined in the experiments?
3. The highest TOC removal percentage obtained in the current work is 77% which is lower than the authors’ previous work, namely 93.4%. Although that value was obtained at higher temperature, how can the authors ensure that the modified catalysts can perform better even in high temperatures?

Author Response

My co-authors and I sincerely thank the reviewers for the time spent evaluating our manuscript and for their helpful comments. We addressed all comments when revising the manuscript, as described below. We also highlighted the changes in yellow in the revised version.

 

Point 1: There are some minor typographic mistakes which should be corrected before final acceptance.

 

Response 1: The minor typographic mistakes have been corrected.

 

Point 2: Why only one concentration of Na2SO3 was examined in the experiments?

 

Response 2: We noticed that the improvement of NCO-RS₂₀ on the basic characteristics (such as specific surface aera, XPS) and catalytic performance were limitted compared with NCO. The modification effect of Na₂SO₃ on NCO was relatively weaker than that of NaBH₄. Thus, we studied more on the modification of NCO using NaBH₄.

 

Point 3: The highest TOC removal percentage obtained in the current work is 77% which is lower than the authors’ previous work, namely 93.4%. Although that value was obtained at higher temperature, how can the authors ensure that the modified catalysts can perform better even in high temperatures?

Response 3: Thank you for your constructive comments. We supplemented two sets of catalytic air oxidation (CAO) experiments conducted in 60°C or 90°C using NCO-RB₂₀ as the catalyst. The results were added in our manuscript (line 257−267) and the support information (Figure S1 and S2). The TOC removal reached 90.0% at the 720th min in 90°C CAO using NCO-RB₂₀ at an initial TOC of 123.8 mg/L. In our previous study, after 24-hour CAO in 90°C, the TOC removal reached 93.4% at an initial TOC of 28.8 mg/L, and 48.0% at an initial TOC of 107.8 mg/L. The weight hourly space velocity (WHSV, which refers to the mass of pollutants treated by per unit mass of catalyst in per unit time) of NCO-R achieved 0.00186 h^-1, 3.3 times higher than the WHSV (0.00043 h^-1) of the NCO applied in our previous work. Thus, we have conformed that the modified catalysts can perform better even in high temperatures.

 

Author Response File: Author Response.docx

Reviewer 2 Report

In my opinion, information on the following issues should be added to the article.

What caused the increase in the background on the XRD graphs? Could this be caused, for example, by the appearance of an amorphous phase after processing, and if so, has this been taken into account?

Was the pH measured during the acid degradation experiments and did the reductive treatment affect the pH of the suspensions used for testing?

Author Response

My co-authors and I sincerely thank the reviewers for the time spent evaluating our manuscript and for their helpful comments. We addressed all comments when revising the manuscript, as described below. We also highlighted the changes in yellow in the revised version.

 

Point 1: What caused the increase in the background on the XRD graphs? Could this be caused, for example, by the appearance of an amorphous phase after processing, and if so, has this been taken into account?

 

Response 1: Thank you so much for your constructive comments. We conducted the same XRD test method for the catalysts studied in this work. Thus, we belived that the appearance of an amorphous phase in modified catalysts caused the increase in the background on the XRD graphs. The amorphous structures in NCO-R might increase the defect sites at its surface and change the surface properties, such as surface acidity. The surface properties and other chemical properties were more related to the catalytic performance of catalysts, which were studied in the following parts of our manuscript.

 

Point 2: Was the pH measured during the acid degradation experiments and did the reductive treatment affect the pH of the suspensions used for testing?

 

Response 2: This comment is helpful for us to improve results. We supplemented pH results and explanation in our manuscript (line 278−289, line 304−305) and the support information (Table S1 and Figure S3).

The pH of the suspensions containing NCO-RB was around 8.8−9.1, higher than the initial pH 8.6 of the HA solution, while the pH of the suspensions containing NCO or NCO-RS₂₀ remained at 8.6 (Table S1). Due to the enhancement in the surface acidity and ASOs , NCO-RB had different surface hydroxylation with NCO. Therefore, the groups of HA combining with catalysts changed, promoting the hydrolysis of HA anions and increasing the pH of suspensions. Along with the removal of HA in CAO at 25℃, the pH of solution decreased from 8.6 to around 8.0 for the first 10 min and then changed slightly (Figure S3). The solution pH in CAO using NCO-R was higher than that in CAO using NCO at the 420th min. The pH of solution basically unchanged at 60℃ using NCO-RB₂₀. However, at 90℃ using NCO-RB₂₀, the pH of solution increased to 9.3 in the first 120 min, and then decreased to 8.4 at the 420th min (Figure S3). The change of pH was limited and had little effect on the overall CAO reactions. Heating would probably accelerate the oxidation of nitrogenous organic groups of HA at the surface of NCO-R, producing more NH4+ and increasing the solution pH.

 

 

Author Response File: Author Response.docx