Mechanism of Removal of Hexavalent Chromium from Aqueous Solution by Fe-Modified Biochar and Its Application

Round 1

Reviewer 1 Report

Dear Authors.

• Regarding to the adsorbent dose, and considering the used Cr(VI) initial concentration, I think that an adsorbent dose of 8-10 g/L is quite high.
• The dry conditions of Fe-modified biochar should be indicated in the manuscript.
• The authors should assure that the equilibrium time has been achieved within the kinetic adsorption experiments. In this sense, experimental kinetic and adsorption isotherms must be included in the manuscript.
• Finally, the specific surface area values of the adsorbents (Table 1) must not be provided with 4 decimals, since usually the error of this value (reported by the equipment) is about 5-10 m2/g.

 

Author Response

Dear Reviewer,

Thanks very much for taking your time to review this manuscript. I really appreciate your comments and suggestions. Please find my itemized  responses below and my revisions in the revised manuscript.

Point 1: Regarding to the adsorbent dose, and considering the used Cr(VI) initial concentration, I think that an adsorbent dose of 8-10 g/L is quite high.


Response 1:  I agree with this comment that the adsorbent dose is quite high. The biochar used in our study was purchased from a company. Under its manufacturing  conditions,  the specific surface area and pore volume of the biochar are not very large (Please kindly refer to Table 1 in the revised manuscript). Thus, this may cause a higher adsorbent dose in the study. In terms of this aspect, we plan to select commercial biochar with large surface area and porosity for further research, so as to reduce the adsorbent dose.

Point 2: The dry conditions of Fe-modified biochar should be indicated in the manuscript.

 Response 2: The drying conditions of Fe-modified biochar is at 105℃,which has been added in the revised manuscript. Please kindly refer to Page 2, line 84.

Point 3：The authors should assure that the equilibrium time has been achieved within the kinetic adsorption experiments. In this sense, experimental kinetic and adsorption isotherms must be included in the manuscript.

Response 3:  The adsorption was continued for 720 minutes, in which it has already achieved equilibrium in the kinetic adsorption experiments. Experimental kinetic and adsorption isotherms have been added in the revised manuscript. Please kindly refer to Figure 8 on Page 9 and Figure 9 on Page 10.

Point 4: The specific surface area values of the adsorbents (Table 1) must not be provided with 4 decimals, since usually the error of this value (reported by the equipment) is about 5-10 m²/g.

Response 4:  The specific surface area values of the adsorbents (Table 1) with 4 decimals has been replaced with 2 decimals. Please kindly refer to Table 1 on Page 4.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Authors in paper presented method obtained of FeBC by the impregnation method using rice straw biochar. Material was characterization shows that iron oxides were loaded on the surface of FeBC, which had more active sites than that of BC. The surface functional groups of FeBC also changed, facilitating the removal of Cr(VI). Adsorption experiments show that the removal efficiency of Cr(VI) by FeBC was significantly improved from 4.57% to 99.5%.  Mechanism of process was described. This study shown that the FeBC could be used as an efficient adsorbent for the Cr(VI) removal from aqueous solutions. The paper is in the mainstream of green chemistry research in which absorbents are made of biomaterials and are easily biodegradable. In order for the presented publication to present a very good scientific level, it is necessary to add to it data and a discussion on the possibility of multiple applications of the absorption material for the adsorption of Cr (VI) from aqueous solutions. How to remove absorbed ions from the absorbent? How do the adsorption properties of the material change in the subsequent adsorption cycles? When this process is feasible, how manage the toxic FeBC-Cr material with adsorbed of Cr (VI) ions?

Author Response

Dear Reviewer,

Thanks very much for taking your time to review this manuscript. I really appreciate your comments and suggestions. Please find my itemized  responses below and my revisions in the revised manuscript.

Point 1: It is necessary to add to it data and a discussion on the possibility of multiple applications of the absorption material for the adsorption of Cr (VI) from aqueous solutions.

Response 1: In China, there are many electroplating enterprises, and thus the volume of wastewater containing Cr (VI) is huge. Our study on the appilication of the modified biochar to electroplating wastewater is representative among different types of industrial wastewater.   In addition, modified biochar have multiple applications for chromium containing wastewater. In the revised manuscript, some researches about the appilication of the modified biochar on other enterprise wastewater with Cr (VI) are added. Yan Yang et al. investigate the feasibility of FSBC to adsorb Cr in actual tanning wastewater. After adsorbing, the total Cr and Cr(VI) removal efficiency reach 98.50% and 99.60%, respectively, and the concentration of Total Cr and Cr(VI) all were in standard. Please kindly refer to 3.8 on page 12.

To extend and verify the application of the modified biochar, further study on different industrial wastewater will be conducted.

 Point 2: How to remove absorbed ions from the absorbent? How do the adsorption properties of the material change in the subsequent adsorption cycles? When this process is feasible, how manage the toxic FeBC-Cr material with adsorbed of Cr (VI) ions?

 Response 2: Some studies indicate that alkali solution or Na₂EDTA solution can desorb Cr from FeBC, which may help to realize the recycling and reuse of the adsorbent. As the adsorption cycles increase, the Cr (VI) removal rate decreases. Finally, the toxic FeBC-Cr material is included in hazardous waste management and collected for incineration treatment. After incineration, ash containing chrome is collected for recycling. In the revised manuscript, management about the toxic FeBC-Cr material with Cr (VI) has been added, Please kindly refer to 3.8 on page 12.

Author Response File: Author Response.pdf

Reviewer 3 Report

Manuscript titled “Mechanism of Removal of Hexavalent Chromium from Aqueous Solution by Fe-modified Biochar and Application” presents synthesis and characterization of Fe-modified biochar derived from rice straw biochar and its application as an adsorbent for removing Cr(VI) from aqueous solutions. So, I think that the subject is relevant to Applied Sciences. I will recommend publication in this Journal, after the correction of some minor issues which are listed below.

p.1 line 8

“This study discussed the mechanism of Fe-modified biochar (FeBC) derived from rice straw biochar (BC) as an adsorbent for removing Cr(VI) from aqueous solution and the applicabilityin actual industrial wastewater was assessed.” - This sentence is illogical.

The manuscript title should also be revised!

 

p. 3 line 104

“(…) and an adsorbent concentration of 8 g·L^-1.”  We cannot determine the concentration of the adsorbent because it is insoluble in the tested solution. Please correct throughout the document.

 

p. 3 line 119

Correct the index C0.

 

p. 4 line 135

Correct the indexes K1 and K2.

K2 should have the following unit: (g mg⁻¹ min⁻¹)

 

Figure 2 is illegible. Please improve the quality.

There is an error in the axis caption of Figure 5.

There is an incorrect caption for Figure 8.

What is further fate of the used sorbents? How can we recover Cr?

Author Response

Dear Reviewer,

Thanks very much for taking your time to review this manuscript. I really appreciate your comments and suggestions. Please find my itemized  responses  in the attachment and my revisions in the revised manuscript.

Author Response File: Author Response.pdf