Adsorption Kinetics and Isotherm Study of Basic Red 5 on Synthesized Silica Monolith Particles

Round 1

Reviewer 1 Report

The authors in their manuscript report the synthesis and the adsorptive performance of Silica monolithic particles (SMPs) for the removal of textile (Congo Red) dye using batch experiments. The material is characterized by analytical methods that provide the basic data to understand the adsorption process. They concluded that silica-based adsorbent owns reasonably high adsorption capacities for the applied. Their findings are interesting and original, overall, the manuscript is relatively well-written, brings valuable information and it is suitable for publication in Water. However, Language must be tighten up and there are spelling mistakes. Most importantly, some revisions are highly necessary before accepting this, otherwise, interesting study.

1. Lines 74-87 do not contribute in the scope of the work. The basic information on dyes can be written in 3-4 sentences.
2. Line 102-111. It is unclear why the adsorption process is preferred over other physicochemical processes (e.g., membrane separation, oxidation, Biodegradation, photocatalytic reactions).
3. Knowledge gap of the study is unclear? The author shall discuss it more critically. Huge variety of adsorbents, e.g., activated carbon derived from low-cost biomaterials and waste materials are available with superior adsorption capacity and rapid adsorption kinetics.
4. Line 112-119: The objective of the study is unclear and there are spelling mistakes and sentence structure is incorrect.
5. Line 170: “V constitutes to the volume of the adsorbate” is wrong.
6. Line 258-259: what is the physical meaning of 2nd order kinetic model (and comparably of 1st order) and how this can be interpreting to the dye adsorption. Likewise, among other kinetics models, what are boundary conditions to use such kinetic models? How about the assumptions? how they are helpful to describe adsorption process.
7. Table 2: The units of k1 and k2 are lacking.
8. Tables 3 and 4 can be combined. The units of quantities are missing.
9. In abstract, the dye adsorption capacities are stated. How these adsorption capacities of the adsorbent are estimated? How the adsorption sites on the adsorbent can be measured with SEM technique? What is meant by “BJH pore size distribution was at 87.15? What kind of adsorption parameters are estimated by adsorption kinetics models?
10. How the SMPs be applied in wastewater treatment. The commonly applied adsorption filters cannot be adopted for nanoparticles. Can these nanoparticles be applied in adsorption-assisted membrane systems (e.g., org/10.1002/jctb.6728)?
11. The authors may explain the economics of the process if applied for the removal of arsenic from drinking water. What are the key arguments to apply this adsorbent over other superior dye adsorbing materials?

Author Response

Reviewer = 01 Comments

The authors in their manuscript report the synthesis and the adsorptive performance of Silica monolithic particles (SMPs) for the removal of textile (Congo Red) dye using batch experiments. The material is characterized by analytical methods that provide the basic data to understand the adsorption process. They concluded that silica-based adsorbent owns reasonably high adsorption capacities for the applied. Their findings are interesting and original, overall, the manuscript is relatively well-written, brings valuable information and it is suitable for publication in Water. However, Language must be tighten up and there are spelling mistakes. Most importantly, some revisions are highly necessary before accepting this, otherwise, interesting study.

1. Lines 74-87 do not contribute in the scope of the work. The basic information on dyes can be written in 3-4 sentences.

Ans: The unnecessary information were accordingly removed.

1. Line 102-111. It is unclear why the adsorption process is preferred over other physicochemical processes (e.g., membrane separation, oxidation, Biodegradation, photocatalytic reactions).

Ans: Worthy reviewer, the mentioned processes are not cost effective and you know membrane process needs continuous supply of electricity and frequent cleaning. Similarly, all of the other mentioned processes are costly and cannot be applied everywhere especially in third world countries. Therefore, we have used the cheapest source, silica. The required detail has been incorporated in the revised paper. 

1. Knowledge gap of the study is unclear? The author shall discuss it more critically. Huge variety of adsorbents, e.g., activated carbon derived from low-cost biomaterials and waste materials are available with superior adsorption capacity and rapid adsorption kinetics.

Ans: Worthy reviewer, it has already been explained from line 97 to 105 and the main reason of using silica is positive interactions between adsorbent and adsorbate as our dye is cationic while silica has negatively charged surface. If we want to use activated carbon we are not sure whether the surface charges will be positive or negative.

1. Line 112-119: The objective of the study is unclear and there are spelling mistakes and sentence structure is incorrect.

Ans: The whole section was rephrased in a meaningful way. Hopefully it will be ok now.

1. Line 170: “V constitutes to the volume of the adsorbate” is wrong.

Ans: Worthy reviewer, the section was rephrased accordingly.

1. Line 258-259: what is the physical meaning of 2nd order kinetic model (and comparably of 1st order) and how this can be interpreting to the dye adsorption. Likewise, among other kinetics models, what are boundary conditions to use such kinetic models? How about the assumptions? how they are helpful to describe adsorption process.

Ans: Worthy reviewer, the information needs to be incorporated as pointed out by you, are repeatedly reproduced in many research papers. If we incorporate such information it will raise the similarity index of the paper and you know it better that journals publishes those papers having low similarity index. We have tried to incorporate some of the details. Hopefully it will now be ok.

1. Table 2: The units of k1 and k2 are lacking.

Ans: The units of k1 and k2 are inserted in table 2 accordingly.

1. Tables 3 and 4 can be combined. The units of quantities are missing.

Ans: Tables 3 and 4 were combined.

1. In abstract, the dye adsorption capacities are stated. How these adsorption capacities of the adsorbent are estimated? How the adsorption sites on the adsorbent can be measured with SEM technique? What is meant by “BJH pore size distribution was at 87.15? What kind of adsorption parameters are estimated by adsorption kinetics models?

Ans: The adsorption capacity is determined from Langmuir constant which is estimated from intercept of the graph provided in the paper. Through adsorption sites estimation is not possible it is determined through BJH and surface area analysis. SEM only provides information about surface morphology. The BJH pore distribution information are already there in the manuscript. Rate constants of the kinetics equation have been calculated from kinetics models whose values are there table.

 

1. How the SMPs be applied in wastewater treatment. The commonly applied adsorption filters cannot be adopted for nanoparticles. Can these nanoparticles be applied in adsorption-assisted membrane systems (e.g., org/10.1002/jctb.6728)?

Ans: Worthy reviewer, the study is about adsorption not membrane. Presently it will apply as adsorbent in industry effluent tank. Your suggestion is valuable it can be applied in a hybrid manner with membrane process, especially ultrafiltration membrane process. We will apply it in our future study.

1. The authors may explain the economics of the process if applied for the removal of arsenic from drinking water. What are the key arguments to apply this adsorbent over other superior dye adsorbing materials?

Ans: Being a negatively charged adsorbent, it can be used effectively for arsenic (being positively charged) adsorption. Being low-cost material, it will be economical. The key argument why it has been used has already been explained in introduction section. The dye is cationic while the adsorbent negatively charged. The interactions were considered to to be favorable that’s why used rather than super adsorbents. 

 

Reviewer 2 Report

This manuscript demonstrates the adsorption of basic red 5 by synthesized silica monolith particles. The adsorption kinetics and isotherm were particularly studied. It can be published after a major revision based on following comments.

1. Line 130, Figure 1a and Figure 1b are separated and should be rearranged.?
2. Figure 1b did not show the adsorption capacity versus time at different temperatures. So, the discussion at Line 179-182 is confusing.
3. XRD should be carried out to confirm the crystalline of the prepared adsorbent.
4. Kinetics and isotherm study needs critical supports from previous reports, for example, Environmental Science & Technology 2021 55 (8), 4287-4304; ACS ES&T Engineering 2021 1 (4), 623-661; Journal of Cleaner Production 2019, 232, 774-783.
5. Comparison with other adsorbents should be added and discussed.
6. The regeneration ability should be studied.

Author Response

Reviewer 02 Comments

This manuscript demonstrates the adsorption of basic red 5 by synthesized silica monolith particles. The adsorption kinetics and isotherm were particularly studied. It can be published after a major revision based on following comments.

1. Line 130, Figure 1a and Figure 1b are separated and should be rearranged?

Ans: the figures were separated accordingly with required rearrangements.

1. Figure 1b did not show the adsorption capacity versus time at different temperatures. So, the discussion at Line 179-182 is confusing.

Ans: Now it is shown in figure 3

1. XRD should be carried out to confirm the crystalline of the prepared adsorbent.

Ans: Worthy reviewer, at this stage we do not have the sample to carry out XRD analysis. Also we do not have XRD available in our lab.

1. Kinetics and isotherm study needs critical supports from previous reports, for example, Environmental Science & Technology 2021 55 (8), 4287-4304; ACS ES&T Engineering 2021 1 (4), 623-661; Journal of Cleaner Production 2019, 232, 774-783.

Ans: worthy reviewer, the suggested citations were informative and were cited accordingly in the respective sections.

1. Comparison with other adsorbents should be added and discussed.

Ans: worthy reviewer, for comparison table .. have been accordingly inserted. 

1. The regeneration ability should be studied.

Ans: required details were inserted accordingly in the revised paper.

Round 2

Reviewer 1 Report

The authors have provided the revision but there are still some mistakes.

Table 4: Units of K_L and K_f are lacking.

Table 6: How the adsorption capacity for the applied adsorbent is estimated as 576. 7 mg/g. If I see from Table 4 that the applied adsorbent has a maximum Langmuir adsorption capacity of 0.0425 mg/g. This is a quite big discrepancy.

Line 243-244: The adsorption equilibrium is achieved in 45 min but if I look at 4 the true equilibrium is achieved after 300 min. The authors shall critically discuss the results concerning adsorption equilibrium.

Section 3.2.3: How the results of the Intraparticle diffusion model describe the fast adsorption kinetic behavior of the synthesized adsorbent.

The grain size of the synthesized adsorbent is missing.

 

 

Author Response

able 4: Units of K_L and K_f are lacking.

The unit was incorporated accordingly

Table 6: How the adsorption capacity for the applied adsorbent is estimated as 576. 7 mg/g. If I see from Table 4 that the applied adsorbent has a maximum Langmuir adsorption capacity of 0.0425 mg/g. This is a quite big discrepancy.

The mistake was corrected accordingly

Line 243-244: The adsorption equilibrium is achieved in 45 min but if I look at 4 the true equilibrium is achieved after 300 min. The authors shall critically discuss the results concerning adsorption equilibrium.

worthy reviewer, the y axis of the graph do not starts from zero thats why it look like that otherwise it is 45 min. In case we put graph with 0 y axis the graph seem some awkward 

Section 3.2.3: How the results of the Intraparticle diffusion model describe the fast adsorption kinetic behavior of the synthesized adsorbent.

The description in the respective section was revised according to the graph obtained

The grain size of the synthesized adsorbent is missing.

it was accordingly mentioned in the SEM section

Reviewer 2 Report

I am satisfied with the revisions that the authors have made. The manuscript can be accepted for publication as it is.

Author Response

Thanks worthy reviewer