The Combination of Nitrogen (N2) Pyrolysis and Carbon Dioxide (CO2) Activation for Regenerating Spent Activated Carbon
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this article, the authors have systematically studied the regeneration of spent GAC using a combination of pyrolysis and CO2 activation. However, the article needs to state the novelty of the work explicitly and clearly explain how the study differs from previous works on the topic. Other comments are given below.
Abstract - "Pore properties were...significantly higher" - please edit.
Introduction - Language could be improved. Some examples are given below.
L41 - When PAC was... Please rewrite.
L53 - "several researches..." Please edit.
L172 - ... it was obviously...
Materials and methods—A TGA-GC MS could provide clarity. I recommend using it to show which gas is being produced upon pyrolysis.
Results - Figure 4 - The image is overflowing into the margin.
The EDS data for the KNC sample is missing and should be included.
Author Response
Q1. In this article, the authors have systematically studied the regeneration of spent GAC using a combination of pyrolysis and CO2 activation. However, the article needs to state the novelty of the work explicitly and clearly explain how the study differs from previous works on the topic.
Reply: Although the topic is not of academic value, the regeneration of spent GAC by using a combination of pyrolysis and CO2 activation was adopted in this work that was very different from previous works on the topic. Therefore, this novelty statement has been added to the third paragraph of the Introduction.
Q2. Abstract - "Pore properties were...significantly higher" - please edit.
Reply: As suggested by the reviewer, the English language and spell errors in the Abstract have been checked and revised by a native English speaker.
Q3. Introduction - Language could be improved. Some examples are given below.
L41 - When PAC was... Please rewrite.
L53 - "several researches..." Please edit.
L172 - ... it was obviously...
Reply: As suggested by the reviewer, the English language and writing in the Introduction and other sections have been checked and revised by a native English speaker.
Q4. Materials and methods—A TGA-GC MS could provide clarity. I recommend using it to show which gas is being produced upon pyrolysis.
Reply: Indeed, the TGA-GC/MS will be a useful technique for analyzing the gas compositions of vent stream that are being produced upon thermal regeneration. However, the TGA used in this work was not coupled to GC/MS (one of expensive and sophisticated instruments). Moreover, the present study focused on the pore properties, textural and chemical characteristics of resulting GAC products by the combined pyrolysis-activation method.
Q5. Results - Figure 4 - The image is overflowing into the margin.
Reply: The images in Figure 4 have been adjusted to make them consistent.
Q6. The EDS data for the KNC sample is missing and should be included.
Reply: As suggested by the reviewer, we re-measured the EDS data of three samples (i.e., KN, KN-900-00 and KNC-900-00) by using another EDS instrument (Model: X-stream-2; Oxford Instruments, Abingdon, UK). The EDS data for the KNC sample (KNC-900-00) has been added to make them consistent in Figure 4 and Figure 5.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article is written clearly and contains all the necessary information. However, it lacks a direct indication of the scientific aspect of the study. There is no information regarding the purpose for which the activated carbon was used prior to regeneration, nor about the substances it may have contained. The impact of the regeneration method on the adsorption properties should be described in more detail. A significant advantage of the study would be a repeated analysis of the activated carbon's performance after regeneration.
Iine 100: What is exactly Spent GAC ? Please provide more information about it.
line 181-184: Please provide references for this explanation
line 263: Please add some information in the SEM pictures.
line 314: Please add some references from 2023-2024.
Author Response
Q1. The article is written clearly and contains all the necessary information. However, it lacks a direct indication of the scientific aspect of the study.
Reply: Although the topic is not of academic value, the regeneration of spent GAC by using a combination of pyrolysis and CO2 activation was adopted in this work that was very different from previous works on the topic. Therefore, this novelty statement has been added to the third paragraph of the Introduction.
Q2. There is no information regarding the purpose for which the activated carbon was used prior to regeneration, nor about the substances it may have contained.
Reply: As suggested by the reviewer, the information about the purpose for which the activated carbon was used prior to regeneration has been added to the third paragraph of the Introduction. Moreover, the present study focused on the pore properties, textural and chemical characteristics of regenerated GAC products. On the other hand, the substances in the Spent GAC which may have contained or adsorbed were not determined in this work due to the trade secret.
Q3. The impact of the regeneration method on the adsorption properties should be described in more detail. A significant advantage of the study would be a repeated analysis of the activated carbon's performance after regeneration.
Reply: The present study focused on the pore properties, textural and chemical characteristics of regenerated GAC products because they are closely related their adsorption properties. As suggested by the reviewer, the impact of the regeneration method on the adsorption properties has been described in more detail (seen in Sec. 3.2).
“In the previous studies [26, 27], a spent bleaching earth (SBE)was thermally regenerated at 500-800°C under pyrolysis gas nitrogen (N2) and 800-900°C under CO2 activation. Compared to untreated SBE, the regenerated material exhibited enhanced pore properties due to new pore formation from the carbonization and activation of non-desorbed organics [28]. Although regeneration of spent GAC is a well-explored topic, this work uniquely focuses on using a combination of Nâ‚‚ pyrolysis and COâ‚‚ activation, making it different from most prior studies. …..”
Q4. line 100: What is exactly Spent GAC ? Please provide more information about it.
Reply: The term “Spent GAC” has be revised to make it clear.
Q5. line 181-184: Please provide references for this explanation.
Reply: As suggested by the reviewer, the relevant references have been cited to make it consistent.
“….. Three distinct regions were observed in the derivative thermogravimetry (DTG) curve. In the temperature range of 100-300°C, the observed weight loss is likely attributed to the thermal desorption of moisture, light gases (e.g., CO2) and low- molecular-weight organics [31]. At the higher temperature range, 300-500°C, it is like due to the thermal desorption or decomposition of heavier organics adsorbed onto the sample. The thermal decomposition that occurred above 700°C is a significant observation which may be due to the breakdown of the limited inorganics with moderate melting points and adsorbed organics with high decomposition features [32], or the char forming of residual/trapped organics [33].”
Q6. line 263: Please add some information in the SEM pictures.
Reply: Regarding the information about the SEM pictures (Figure 4), it has been added to the first paragraph of the Sec. 3.4.
“SEM and EDS analyses were employed to examine the surface morphology and elemental composition of the spent and regenerated GACs. Figure 4, shows the SEM images of the spent GAC and two of the regenerated samples (KN-900-00 and KNC-900-00) at 1000× and 3000× magnifications. It can be seen that fine particles were loosely deposited or packed on the GAC surface. Although the samples exhibited similar porous textures, the COâ‚‚-activated product (KNC-900-00) appeared more porous as seen in Figure 4(c), which corresponds with its higher measured pore properties compared to those of spent GAC and pyrolysis-regenerated GAC (i.e., KN-900-00).”
Q7. line 314: Please add some references from 2023-2024.
Reply: As suggested by the reviewers, three references (Refs. 31-33) have been added to the References, making it valuable.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe manuscript deals with the recycling and improvement of old adsorbent, which is why I believe it a topic that is very useful and important. However, I have a few suggestions and comments.
- Although it is not the topic of the manuscript, what was GAC used for, or more precisely, I am interested in whether additional pollution occurs during regeneration? Have you taken into account that nothing is being released in the form of pollutants?
- What did it look like in practice to get the KN-temperature-time obtained from spent GAC pyrolysis under N2 flow of 500 mL/min at different temperatures for holding times 0 minutes? More precisely, how can it be obtained at zero minutes?
- In the paragraph Lines 172-189, please support your claims and discussion with literature. Example `` In the temperature range of 100-300°C, it may refer to the thermal desorption of moisture and light organics adsorbed[?].``
- Has the TGA/DTG been done for the samples after regeneration/activation? Can we see any difference in composition that could help in improvement of the new material?
- Please the author to place Table 2 before the discussion regarding Figure 2, for easier following of the text and connection with the Table.
- Section 3.3. why didn't you take it for 30 min for this comparison same as for N2 adsorption-desorption and pore size distribution? To praise, the SEM figures are clear.
- At Figure 5, why didn't you show the Elemental composition after CO2 activation?
Author Response
Q1. Although it is not the topic of the manuscript, what was GAC used for, or more precisely, I am interested in whether additional pollution occurs during regeneration? Have you taken into account that nothing is being released in the form of pollutants?
Reply: The present study focused on the pore properties, textural and chemical characteristics of regenerated GAC products which were produced by a combination of pyrolysis and CO2 activation. Indeed, the pollutants could be released from the vent gas upon performing the thermal regeneration. These thermally desorbed and/or decomposed substances may pose the gaseous pollutants, but they were not determined in this work.
Q2. What did it look like in practice to get the KN-temperature-time obtained from spent GAC pyrolysis under N2 flow of 500 mL/min at different temperatures for holding times 0 minutes? More precisely, how can it be obtained at zero minutes?
Reply: In the thermal regeneration experiments at the holding time of zero minute, the profile was heated to reach the specified temperature under the ramp rate of 10°C/min. Afterward, the power was automatically turned off, thus causing the reduction in the temperature of process system due to no heat input and heat output by passing gas (N2) still.
Q3. In the paragraph Lines 172-189, please support your claims and discussion with literature. Example `` In the temperature range of 100-300°C, it may refer to the thermal desorption of moisture and light organics adsorbed[?].
Reply: As suggested by the reviewer, the relevant reference have been cited to support the discussion on the TGA/DTG results.
“….. Three distinct regions were observed in the derivative thermogravimetry (DTG) curve. In the temperature range of 100-300°C, the observed weight loss is likely attributed to the thermal desorption of moisture, light gases (e.g., CO2) and low- molecular-weight organics [31]. At the higher temperature range, 300-500°C, it is like due to the thermal desorption or decomposition of heavier organics adsorbed onto the sample. The thermal decomposition that occurred above 700°C is a significant observation which may be due to the breakdown of the limited inorganics with moderate melting points and adsorbed organics with high decomposition features [32], or the char forming of residual/trapped organics [33].”
Q4. Has the TGA/DTG been done for the samples after regeneration/activation? Can we see any difference in composition that could help in improvement of the new material?
Reply: As suggested by the reviewer, the TGA/DTG of a regenerated GAC (i.e., KNC-900-30) was done and depicted below (not seen in the revised manuscript, but shown here). It indicated that the sample after regeneration/activation was very thermally stable, implying an obvious difference from those of the spent GAC in Figure 1.
Q5. Please the author to place Table 2 before the discussion regarding Figure 2, for easier following of the text and connection with the Table.
Reply: As suggested by the reviewer, the sequence between Table 2 and Figure 2 has been revised to make it logical.
Q6. Section 3.3. why didn't you take it for 30 min for this comparison same as for N2 adsorption-desorption and pore size distribution? To praise, the SEM figures are clear.
Reply: Figure 2 and Figure 3 showed the N2 adsorption-desorption and pore size distribution curves of the spent GAC (i.e., KN) and optimal products (i.e., KN-900-30 and KNC-900-30), respectively. However, the SEM images of the spent GAC (i.e., KN) and regenerated GAC products almost indicated similar pore textures. The clear SEM images of the regenerated GAC products (i.e., KN-900-00 and KNC-900-00) can be seen in Figure 4.
Q7. At Figure 5, why didn't you show the Elemental composition after CO2 activation
Reply: As suggested by the reviewer, the EDS data (elemental compositions) for the KNC sample (KN-900-00) has been added to make them consistent in Figure 4 and Figure 5.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsNo comments. Good job!
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript has been revised in accordance with the comments sent. I recommend it for publication.
Reviewer 3 Report
Comments and Suggestions for AuthorsThank the authors for appreciating my comments
