Adsorption of Patent Blue V from Textile Industry Wastewater Using Sterculia alata Fruit Shell Biochar: Evaluation of Efficiency and Mechanisms

Round 1

Reviewer 1 Report

The work does not show innovative research results. Standard description of the results has been used. The obtained sorption capacity of the tested sorbent is not particularly high. It seems, that it won't be possible to put the presented and described sorbent in practical use.

Author Response

1) The work does not show innovative research results. A standard description of the results has been used. The obtained sorption capacity of the tested sorbent is not particularly high. It seems, that it won't be possible to put the presented and described sorbent in practical use.

Author’s response: The authors thank the reviewer for the good suggestion concerning the requirement to show innovative results. Although the authors have done carefully planned and statistically significant experiments that used standard protocols and apparatuses, the results obtained from Sterculia alata fruit shell biochar is rather new and it has never been reported in the literature for Patent Blue dye. As advised, the authors have highlighted the novel/innovative aspect in the introduction and also facilitated in-depth mechanism-based discussion in the revised manuscript.  The authors have gone through the whole manuscript and improved the manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

1. "The biochar obtained after pyrolysis was washed with distilled water then dried at 110oC and finally impregnated with NaOH to improve its adsorption capacity (Park et al. 2013). " The authors did not specify how much NaOH has been dosed to the biochar and how does it affect the adsorption efficiency of biochar
2. "The effect on Patent Blue removal was investigated from pH 2 to 12 as presented in Figure 2b. Removal of dye decreased with the increase in pH and highest removal was observed at pH 2." Figure 2b does not reflect any information regarding pH changes
3. All figures quality need to be improved (for example, Figure 3 a,b, c, d, each subfigure either has a boarderline, or not have, not consistent at all)
4. "3.2.6. Development of adsorption isotherm" The authors have not included a discussion of the potential indication of a better fit of the data with Freundlich adsorption isotherm means.
5. The better adsorption capacity of 500 ppm of the dyes, does not mean it will be the optimum concentration, the authors also should try higher concentration of dyes to see what happens.

Author Response

2) "The biochar obtained after pyrolysis was washed with distilled water then dried at 110^oC and finally impregnated with NaOH to improve its adsorption capacity (Park et al. 2013)". The authors did not specify how much NaOH has been dosed to the biochar and how does it affect the adsorption efficiency of biochar

Author’s response: The authors thank the reviewer for this very good comment about the amount of NaOH dosing. The manuscript has been modified as follows: The NaOH activation method was used to improve the sorption properties of the tested biochar. Thermally treated biochar (3 g) was mixed with 40 mL of 4 M NaOH and incubated at room temperature for 2 h, under intermittent shaking (15 min interval) condition. After NaOH impregnation, the excess solution was discarded with vacuum filtering and the chemically treated solid was dried overnight in an oven at 105 ^oC. The dried sample was heated in a quartz-tube furnace to 800 ^oC, at a heating rate of 3 ^oC min^-1 under inert atmospheric conditions (N₂ flow = 2 L min^-1) for 2 h. After activation, the samples were taken out from the heating element and cooled down to ambient temperature under nitrogen flow. The activated samples were washed using 2 L of deionized (DI) water followed by 0.1 M HCl solution (200 mL) and washed again with DI water until the pH of filtrates was ~ 7.0. The washed activated carbon samples were dried in an oven at 105 ^oC and stored in a desiccator for further analysis. Each activated sample was denoted as ‘‘N-treatment temperature AC’’ (e.g. N-300AC for activated carbon from N-300).

3) "The effect on Patent Blue removal was investigated from pH 2 to 12 as presented in Figure 2b. Removal of dye decreased with the increase in pH and highest removal was observed at pH 2." Figure 2b does not reflect any information regarding pH changes.

Author’s response: The authors thank the reviewer for a very good and insightful scientific comment about the changes in pH. In the revised manuscript, Figure 2b does not show the effect of pH changes. Instead, it shows the variation of PB dye concentration at a fixed adsorbent dose of 4 g. Therefore, the authors have removed this figure citation from this line and it has been cited under the next sub-heading. As advised, the authors have provided some more data on the effect due to changes in pH and its influence on the removal of Patent blue dye. The manuscript has been revised as follows: When the adsorption of PB by the modified bael shell biochar (BSB) was studied in the pH range of 2.7 to 10.4, maximum sorption was perceived at pH 2.7. The percentage adsorption values of PB at seven of the tested initial pH values (i.e. 2.7, 4, 6, 7, 8, 9.2 and 10.4) were 74%, 62%, 58%, 55%, 47%, 40% and 39%, respectively. At lower pH values (i.e. 2.7), the surface charge density of BSB was found to be predominantly positive; thus, adsorbing high quantities of anionic PB dye molecules. The maximum adsorption capacity of BSB towards PB (V) was found to be 3.7 mg/g at a pH value of 2.7.

4) All figures quality need to be improved (for example, Figure 3 a, b, c, d, each subfigure either has a border line, or not have, not consistent at all)

Author’s response: The authors thank the reviewer for the good and most important comment about the quality of the figures. As advised, all the figures have been modified and numbered accordingly.

5) "3.2.6. Development of adsorption isotherm" The authors have not included a discussion of the potential indication of a better fit of the data with Freundlich adsorption isotherm means.

Author’s response: The authors appreciate the reviewer’s good comments on the adsorption isotherm model fitting. As advised by the reviewer, the results and discussion section has been modified as follows: Ahmad et al. (2020) has reported the adsorption isotherm for Methylene Blue (MB) dye using cow dung biochar, rice husk biochar and sludge biochar. After model fitting, the authors reported that the adsorption behaviour of MB was better explained by the Langmuir model (R² = 0.996, 0.998 and 0.998) when compared to the Freundlich isotherm model (R²=0.972, 0.952 and 0.715). In another study, Khan et al. (2020) reported the MB removal using cow dung biochar, rice husk biochar and sludge biochar and ascertained the log q_e values for the two models as 17.50, 17.97, 19.21 mg/g and 1.243, 1.254, and 1.283 mg/g, respectively. Goswami et al. (2020) studied the adsorption of Congo red dye using Arjuna seed biochar and reported > 90.0 % removal in a hybrid treatment system containing biochar, microorganisms and ozone treatment. In another study, Gong et al. (2013) reported maximum equilibrium absorption capacities of 934.58 and 581.40 mg/g for Methyl Orange and MB dye, respectively. Mittal and Mishra (2014) reported nearly similar values of MB in Gumghatti and Fe₃O₄ nanocomposite as ~ 654 mg/g.

6) The better adsorption capacity of 500 ppm of the dyes, does not mean it will be the optimum concentration, the authors also should try higher concentration of dyes to see what happens.

Author’s response: The authors thank the reviewer for the good suggestion to increase the concentration of the dye in solution. Honestly, as we have finished these experiments in lab with our research students, it is difficult to continue additional experiments. We will certainly consider this good recommendation when we start continuous adsorption experiments using flow through columns.

Author Response File: Author Response.docx

Reviewer 3 Report

Page 2

Line 62 Chemical modificationof biochar with the use of acid, bases or polymer may result in

Line 64 but cost of operation may also be more

Result and Discussion

Page 5

Line 235 in order to confirm the nature of adsorption process

Conclusion

Page 7

Line 303 biochar is able to remove the dye more than  80% at 500 mg/L

Author Response

7) Page 2; Line 62 Chemical modification of biochar with the use of acid, bases or polymer may result in

Line 64 but cost of operation may also be more.

Author’s response: The reviewer’s good comment about the cost is really well-appreciated. Indeed, from a practical perspective, pre-treatment of the biomass (i.e. the Sterculia alata fruit shell biochar) will result in additional operating costs that includes the costs to operate physical and chemical unit processes. Besides, operational costs in terms of energy consumption should also be considered when the adsorption process is scaled up from the lab to pilot and industrial scales. In the revised manuscript, the authors have added this important information.

8) Result and Discussion; Page 5; Line 235 in order to confirm the nature of adsorption process.

Author’s response: The authors thank the reviewer for the good sentence structure suggestion. The manuscript has been modified accordingly.  

9) Conclusion; Page 7; Line 303 biochar is able to remove the dye more than 80% at 500 mg/L.

Author’s response: As advised by the reviewer, this sentence has been modified as follows in the revised manuscript: “The results obtained from this study are quite promising because the modified biochar was able to remove > 80%, at an initial PB dye concentration of 500 mg/L.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

1. Please double check grammar again.
2. The figure of pH variation effects on the absorption capacities should be included.

Author Response

Dear Sir, 

Thanks for your kind comments and authors have modified the manuscript as per your comments. The responses of the comments are given below:

1. Please double-check grammar again.

Reply: The whole manuscript was checked by an English language expert and modified the grammar of the manuscript.

2. The figure of pH variation effects on the absorption capacities should be included.

Reply: Author has incorporated the figure of pH variation effects on the absorption capacities 

Reviewer 3 Report

Corrections have been taking care of.

Author Response

Dear Sir, 

Thanks for your appreciation and we enjoyed a lot during the preparation of the revision of this manuscript.