Harnessing Activated Hydrochars: A Novel Approach for Pharmaceutical Contaminant Removal

Round 1

Reviewer 1 Report

In the review article “Harnessing Activated Hydrochars: A Novel Approach for Pharmaceutical Contaminant Removal”, the authors presented a review regarding hydrochar utilization for pharmaceutical contaminants removal. It was organized into 10 subcategories. All the categories were well discussed. Not only, the hydrochars formation, and characterization, but also its adsorption mechanism and the interaction were described. Inclusively, the regulations and guidelines from international organizations were included.

Overall, it is a comprehensive, in-depth, and conclusive review paper which would greatly interest the readers. Moreover, it is a well-organized, well-written article. Therefore, this reviewer recommends its publication on Journal of Carbon Research after minor revision.

There are some minor issues that need to be considered.

Page 12, figure 3: As the author mentioned in the context the hydrochars are enriched with OH, CO, COOH… functional groups. In the graphics, only aromatic benzene rings were shown. It’s kind of misleading. The hydrogen bond interaction between hydrochars and heavy metals doesn’t make sense. Please reconsider it.

Page 20, line 730: The usage of “be harnessed to be recovered phosphorus” is inappropriate. It would be incredible to get the element phosphorus out of the sewage sludge. “The phosphorus-containing chemicals” would be good.

Author Response

Response to reviewers’ comments:

Title: Harnessing Activated Hydrochars: A Novel Approach for Pharmaceutical Contaminant Removal

Manuscript Id:  2732639

Dear Reviewers,

Thank you for your valuable suggestions. The manuscript has been amended according to your suggestions. The changes incorporated into the manuscript have been marked with a yellow-coloured font. I hope this revised manuscript will meet the standards of the “C Journal of Carbon Research”.

Reviewer 1: 

Comment 1: In the review article “Harnessing Activated Hydrochars: A Novel Approach for Pharmaceutical Contaminant Removal”, the authors presented a review regarding hydrochar utilization for pharmaceutical contaminants removal. It was organized into 10 subcategories. All the categories were well discussed. Not only, the hydrochars formation, and characterization, but also its adsorption mechanism and the interaction were described. Inclusively, the regulations and guidelines from international organizations were included.

Overall, it is a comprehensive, in-depth, and conclusive review paper which would greatly interest the readers. Moreover, it is a well-organized, well-written article. Therefore, this reviewer recommends its publication on Journal of Carbon Research after minor revision.

There are some minor issues that need to be considered.

Response: Thank you for your kind words, and I appreciate your recognition of the significance of the topic in our research.

Comment 2: Page 12, figure 3: As the author mentioned in the context the hydrochars are enriched with OH, CO, COOH… functional groups. In the graphics, only aromatic benzene rings were shown. It’s kind of misleading. The hydrogen bond interaction between hydrochars and heavy metals doesn’t make sense. Please reconsider it.

Response: Requisite revisions have been implemented in accordance with the comments provided by the reviewer.

Comment 3: Page 20, line 730: The usage of “be harnessed to be recovered phosphorus” is inappropriate. It would be incredible to get the element phosphorus out of the sewage sludge. “The phosphorus-containing chemicals” would be good.

Response: Changes have been made according to the suggestion of the reviewer.

Modified Sentence:

Within the context of wastewater treatment, hydrochar can be harnessed to recover the phosphorus-containing chemicals in sewage sludge.

Reviewer 2 Report

In this review paper, the authors introduced the utilization of hydrochar in removing the pharmaceutical contaminants from water. The review comes from the following points of what is hydrochar, how produce hydrochar, how to evaluate hydrochar, how hydrochar remove contaminants from water. And then followed by how the pharmaceutical continents could be related to these. The review is logical and presented systematically. And the final discussion and suggestions are constructive. This manuscript is recommended to be accepted after minor revisions. The detailed suggestions are below:

1.       Please provide more chemical characteristics and process parameters in the introduction of hydrochar. This information will help readers understand the chemical mechanism of absorbent. (section 2, 1^st paragraph)

2.       What is the overall yield of biochar from biomass?

3.       In table 1, please cite the references in the table inside by the specific information.

4.       Please revise the writing of these sentences. “Brunauer-Emmett-Teller (BET) Analysis, which gauges the specific sur-310 face area of activated hydrochars, thus imparting insights into their adsorption capacity 311 [45]. Pore Size Distribution Analysis, employing techniques such as Barrett-Joyner-312 Halenda (BJH) or Horvath-Kawazoe (HK) methods, which determine the pore size distri-313 bution, including micropores, mesopores, and macropores [46]. Scanning Electron Mi-314 croscopy (SEM), which furnishes visual data pertaining to surface morphology and pore 315 structure [47]….” These sentences do not contain proper verbs.

5.       The content of regulation and guidance could be non-related to the scientific review of hydrochar utilization. This part could be condensed in other section.

6.       The picture quality in Figure 2 could be increased.

7.       In Figure 3, please increase the text content to make it more noticeable.

The only part that has an English writing issue is the characterization of hydrochar introduction.

Author Response

Response to reviewers’ comments:

Title: Harnessing Activated Hydrochars: A Novel Approach for Pharmaceutical Contaminant Removal

Manuscript Id:  2732639

Dear Reviewers,

Thank you for your valuable suggestions. The manuscript has been amended according to your suggestions. The changes incorporated into the manuscript have been marked with a yellow-coloured font. I hope this revised manuscript will meet the standards of the “C Journal of Carbon Research”.

Reviewer 2: 

Overall Comments: In this review paper, the authors introduced the utilization of hydrochar in removing the pharmaceutical contaminants from water. The review comes from the following points of what is hydrochar, how produce hydrochar, how to evaluate hydrochar, how hydrochar remove contaminants from water. And then followed by how the pharmaceutical continents could be related to these. The review is logical and presented systematically. And the final discussion and suggestions are constructive. This manuscript is recommended to be accepted after minor revisions. The detailed suggestions are below:

Comment 1:       Please provide more chemical characteristics and process parameters in the introduction of hydrochar. This information will help readers understand the chemical mechanism of absorbent. (section 2, 1^st paragraph)

Response: In accordance with the valuable suggestions provided by the esteemed reviewer, relevant content has been incorporated into the first paragraph of Section 2.

Added Text:

Hydrochar generally exhibits non-toxicity and hydrophobicity. The characteristics of hydrochar, including surface chemistry, porosity, particle size, and specific surface area, are contingent upon the temperature and reaction duration applied during hydrothermal carbonization (HTC). The hydrochar surface is typically endowed with numerous oxygen-containing functional groups that manifest favorable adsorption affinities towards both polar and non-polar functional groups, thereby resulting in elevated adsorption capacity. It is noteworthy, however, that such advantageous properties may be compromised during gas-phase activation aimed at augmenting specific surface area. Consequently, judicious selection of processing conditions is imperative to preserve the desired attributes of hydrochar.

The hydrochar synthesis process occurs in an aqueous environment, typically employing a stainless steel autoclave loaded with biomass and a specified quantity of water (typically within the range of 1:3 to 1:10 ratios of biomass to water). In comparison to biochar, hydrochar exhibits a slightly acidic nature attributed to a higher presence of oxygenated functional groups. Pyrolysis-induced loss of carboxyl and hydroxyl groups’ renders biochar alkaline, with alkalinity influenced by inorganic and metal compounds like Ca and Mg. Hydrothermal carbonization (HTC) results in the removal of some inorganic components in the aqueous medium, contributing to the acidic pH of hydrochar .

Due to the lower temperature of the HTC process, carbon conversion is reduced compared to pyrolysis, yielding higher atomic ratios of H/C and O/C in hydrochar. Consequently, hydrochar demonstrates elevated atomic ratios of hydrogen to carbon and oxygen to carbon in contrast to biochar . The increased hydrogen content in hydrochar, known for its involvement in polar interactions, may enhance its adsorption capacity for pharmaceutical compounds exhibiting polar or hydrogen-bonding functionalities. Additionally, the oxygen-containing functional groups in hydrochar can engage in various chemical interactions, including hydrogen bonding and Lewis acid-base interactions, potentially influencing the adsorption of pharmaceutical compounds with oxygen-binding sites or those susceptible to such interactions.

Comment 2: What is the overall yield of biochar from biomass?

Response:  The biochar yield during the pyrolysis process depends on the type and nature of biomass used. In general the overall yield of biochar from biomass is 10-80%.

Reference: Yaashikaa, P. R., Kumar, P. S., Varjani, S., & Saravanan, A. (2020). A critical review on the biochar production techniques, characterization, stability and applications for circular bioeconomy. Biotechnology Reports, 28, e00570.

Comment 3:       In table 1, please cite the references in the table inside by the specific information.

Response: Requisite revisions have been implemented in accordance with the comments provided by the reviewers.

Comment 4: Please revise the writing of these sentences. “Brunauer-Emmett-Teller (BET) Analysis, which gauges the specific sur-310 face area of activated hydrochars, thus imparting insights into their adsorption capacity 311 [45]. Pore Size Distribution Analysis, employing techniques such as Barrett-Joyner-312 Halenda (BJH) or Horvath-Kawazoe (HK) methods, which determine the pore size distri-313 bution, including micropores, mesopores, and macropores [46]. Scanning Electron Mi-314 croscopy (SEM), which furnishes visual data pertaining to surface morphology and pore 315 structure [47]….” These sentences do not contain proper verbs.

Response: Adjustments have been made based on the reviewer's insightful suggestion, involving the incorporation of proper verbs into the sentences.

Revised Text:

The Brunauer-Emmett-Teller (BET) analysis is employed to quantify the specific surface area of activated hydrochars, thereby yielding valuable insights into their adsorption capacity [46]. Pore Size Distribution Analysis utilizes techniques such as Barrett-Joyner-Halenda (BJH) or Horvath-Kawazoe (HK) methods to ascertain the pore size distribution, encompassing micropores, mesopores, and macropores [47]. Scanning electron microscopy (SEM) provides visual data concerning surface morphology and pore structure [48].  Elemental analysis (CHN) determines the carbon, hydrogen, and nitrogen content, revealing alterations in composition resulting from activation [49]. Fourier transform infrared spectroscopy (FTIR) is utilized to identify functional groups on the surface, offering insights into the chemical composition [50]. X-ray photoelectron spectroscopy (XPS) plays a crucial role in analysing the elemental composition and chemical states of elements on the surface of activated hydrochars [51]. Thermogravimetric analysis (TGA) quantifies weight changes as the activated hydrochar is heated, thereby furnishing information about its thermal stability and organic content [52]. Batch Adsorption Studies evaluate the adsorption capacity of activated hydrochars for specific contaminants by analyzing equilibrium and kinetic adsorption data [53]. Isotherm modelling involves fitting adsorption data to isotherm models, such as Langmuir and Freundlich, to elucidate adsorption behaviour [54]. Kinetic studies are designed to assess the rate at which adsorption occurs, facilitating an understanding of the adsorption mechanism [55]. X-ray diffraction (XRD) is instrumental in determining the crystalline structure of activated hydrochars [56]. N₂ Adsorption-Desorption Isotherms serve a dual purpose by aiding BET analysis and providing insights into surface area and pore size [57]. Solid-state nuclear magnetic resonance (NMR) offers insights into the distribution of carbon species and functional groups on the surface [58]. Transmission electron microscopy (TEM) provides detailed imagery of the nanoscale structure and morphology of activated hydrochars [59]. Atomic Force Microscopy (AFM) quantifies surface roughness and topography [60].

Comment 5: The content of regulation and guidance could be non-related to the scientific review of hydrochar utilization. This part could be condensed in other section.

Response: The content of “Regulations and guidelines for pharmaceutical contaminants” has been condensed according to the suggestion of the reviewer and added in Section 6.

Condensed part:

6.6. Regulations and guidelines for pharmaceutical contaminants

Regulations and guidelines for pharmaceutical contaminants in water vary significantly across countries and regions. Noteworthy regulatory bodies include the United States Environmental Protection Agency (EPA), which, under the Safe Drinking Water Act, establishes maximum contaminant levels (MCLs) for substances, including pharmaceuticals [119]. The World Health Organization (WHO) provides international guidance through its "Guidelines for Drinking-water Quality," addressing the evaluation and control of pharmaceutical contaminants [120]. The European Union (EU) outlines parameters in the Drinking Water Directive, encompassing criteria for water quality, including pharmaceuticals [121]. Various countries, through their environmental agencies, set standards and regulations, discernible in thresholds for pharmaceutical contaminants in surface waters [122]. Additionally, pharmaceutical industry regulations, such as those from the U.S. Food and Drug Administration (FDA), indirectly impact contaminant prevalence by mitigating environmental impact [123]. Several international agreements and organizations are dedicated to addressing concerns associated with pharmaceutical contaminants in water. Among them, the United Nations Environment Programme (UNEP) and the Organization for Economic Co-operation and Development (OECD) are notable participants [124].

Alongside regulations and guidelines, research initiatives and monitoring programs aim to identify and understand pharmaceutical contaminants in water, forming the foundation for regulatory decisions. It is crucial to recognize substantial disparities in regulations contingent on specific pharmaceutical contaminants, concentrations, contamination sources, and intended water use. The field of pharmaceutical contaminant regulation is dynamic, evolving with advancements in research and an enhanced understanding of associated risks, leading to the emergence of new guidelines and standards. Local and regional authorities play a vital role in enforcing these regulations, emphasizing the importance of consulting relevant bodies for the latest information on pharmaceutical contaminant regulations and guidelines.

Comment 6: The picture quality in Figure 2 could be increased.

 Response: The picture quality of Figure 2 has been increased.

Comment 7: In Figure 3, please increase the text content to make it more noticeable.

Response: In response to the reviewer's guidance, enhancements have been applied to the textual content of Figure 3 to enhance its visibility.

Comments on the Quality of English Language: The only part that has an English writing issue is the characterization of hydrochar introduction.

Response: According to the suggestion of the reviewer, the English writing on the characterization of hydrochar introduction has been improved.

Reviewer 3 Report

As a review paper, this manuscript reasonably covers the topic of using hydrochars/biochars in water treatment applications for removal of pharmaceutical compounds. There are some points of weakness that require significant improvement to add more depth and value, as follows:

1) The manuscript requires a methodology section that clearly explains how articles reviewed were found, filtered, screened and selected for inclusion and discussion. For example, if a database like Scopus or Web of Science was used, what procedure for searching and retrieving articles was used. If a database was not use, then how was the review able to comprehensively cover the topic? It is not enough to simply "Google" for papers, and even the use of Google Scholar has limited ability to be used for accurate searching and filtering of results. Also, if in now preparing a methodology section additional papers are found that should have been included, they should now be included.

2) As it is too easy to write portions of a review using artificial intelligence nowadays, the authors should include statements about its use or lack thereof.

3) Considering that this is a manuscript about pharmaceutical compounds, it is light on the chemistry. There is no mention about the chemical structure of such pharmaceutical compounds, and how the sorbents interact with chemicals of various classes. Likewise, what makes these sorbents any more selective for pharmaceutical compounds versus any other organic in waters and wastewaters? There should be deeper discussion of these chemistry and surface chemistry topics, including chemical formulas and structures.

4) The introduction section, towards its end, should include a paragraph that explains what other similar review papers existing related to the topic of this review, and how this review differs from those existing reviews. This not only helps to explain the value of this review for readers, but also helps to connect this review to other reviews through citations networks.

Language is ok, but it is necessary to indicate if any part of the manuscript was written by AI tools.

Author Response

Response to reviewers’ comments:

Title: Harnessing Activated Hydrochars: A Novel Approach for Pharmaceutical Contaminant Removal

Manuscript Id:  2732639

Dear Reviewers,

Thank you for your valuable suggestions. The manuscript has been amended according to your suggestions. The changes incorporated into the manuscript have been marked with a yellow-coloured font. I hope this revised manuscript will meet the standards of the “C Journal of Carbon Research”.

Reviewer 3: 

As a review paper, this manuscript reasonably covers the topic of using hydrochars/biochars in water treatment applications for removal of pharmaceutical compounds. There are some points of weakness that require significant improvement to add more depth and value, as follows:

Comment 1: The manuscript requires a methodology section that clearly explains how articles reviewed were found, filtered, screened and selected for inclusion and discussion. For example, if a database like Scopus or Web of Science was used, what procedure for searching and retrieving articles was used. If a database was not use, then how was the review able to comprehensively cover the topic? It is not enough to simply "Google" for papers, and even the use of Google Scholar has limited ability to be used for accurate searching and filtering of results. Also, if in now preparing a methodology section additional papers are found that should have been included, they should now be included.

Response: Requisite methodology sections have been implemented in accordance with the comments provided by the reviewers.

Added Methodology:

The investigation was done throughout the years by focusing on hydrochar and pharmaceutical contaminants and involved a systematic search on Scopus databases utilizing the combined keywords "hydrochar" and "pharmaceutical contaminants" on a single day, specifically December 2, 2023, to minimize potential fluctuations. The initial search yielded a solitary paper associated with the specified keywords. Subsequent modifications to the search, incorporating the keywords "biochar" and "pharmaceutical contaminants," revealed 12 relevant papers. Further exploration was conducted on the ScienceDirect database using the combined keywords "hydrochar" and "pharmaceutical contaminants," yielding 571 results. Subsequently, a meticulous filtering process was implemented, resulting in the exclusion of 123 articles deemed irrelevant or unrelated. This left a total of 136 articles. Among these, 39 were initially categorized as review papers, and their inclusion was subtracted, culminating in the identification of 84 research papers. Throughout the manuscript preparation, the synthesis of information drew not only from these 84 selected research papers but also from additional pertinent content obtained from Google Scholar.

Comment 2:  As it is too easy to write portions of a review using artificial intelligence nowadays, the authors should include statements about its use or lack thereof.

Response: There is no use of artificial intelligence to write the portions of the review paper.

Comment 3:  Considering that this is a manuscript about pharmaceutical compounds, it is light on the chemistry. There is no mention about the chemical structure of such pharmaceutical compounds, and how the sorbents interact with chemicals of various classes. Likewise, what makes these sorbents any more selective for pharmaceutical compounds versus any other organic in waters and wastewaters? There should be deeper discussion of these chemistry and surface chemistry topics, including chemical formulas and structures.

Response: In accordance with the guidance provided by the reviewer, pertinent content has been integrated into the manuscript. The chemical architecture of pharmaceutical contaminants and their potential interaction mechanisms with hydrochar are delineated in tabular format (Table 2). Sorbents selectivity for pharmaceutical compounds over other organics in waters and wastewater has been written in section 3.

Added Text in section 3:

Hydrochar's selectivity for pharmaceutical compounds over other organics in waters and wastewaters can be attributed to its porous structure, chemical functional groups, surface chemistry, electrostatic interactions, aromaticity, and the potential for specific affinity. The porous nature of hydrochar provides an effective medium for adsorption, while its surface features and chemical composition may favor interactions with pharmaceutical molecules. The presence of aromatic structures in hydrochar aligns with the aromatic rings often found in pharmaceutical compounds. Additionally, tailored modifications to the hydrochar surface can enhance its selectivity for pharmaceuticals. These combined factors contribute to the effectiveness of hydrochar as a selective adsorbent for pharmaceutical compounds in water treatment applications.

Comment 4: The introduction section, towards its end, should include a paragraph that explains what other similar review papers existing related to the topic of this review, and how this review differs from those existing reviews. This not only helps to explain the value of this review for readers, but also helps to connect this review to other reviews through citations networks.

Response: Modifications have been implemented in accordance with the valuable suggestions provided by the reviewer.

Added text in the Introduction Part

As we conclude the introduction, it is crucial to contextualize this review within the broader landscape of existing literature on pharmaceutical contaminants in water. While numerous review papers have addressed the removal of emerging contaminants, including pharmaceuticals, through adsorption methods, they often provide a generalized overview without a specific focus on hydrochar [18-21]. It is noteworthy that existing works typically discuss the broader category of adsorbents rather than exclusively delving into the unique attributes of hydrochar for pharmaceutical contaminant removal. In this vein, our review distinguishes itself by offering an exclusive exploration of hydrochar as a specialized adsorbent for the removal of pharmaceutical contaminants. By narrowing the focus to hydrochar, we aim to provide a comprehensive and in-depth analysis of its distinct properties—such as high surface area and exceptional adsorption capabilities—that render it a promising and environmentally friendly solution for addressing the complex challenges posed by pharmaceutical contaminants in water sources. This deliberate focus serves to contribute novel insights to the existing body of literature and positions our review as a specialized and valuable resource for those interested in the application of hydrochar in pharmaceutical contaminant remediation.

Comments on the Quality of English Language

Language is ok, but it is necessary to indicate if any part of the manuscript was written by AI tools.

Response: There is no use of artificial intelligence to write portions of the review paper.

Reviewer 4 Report

The manuscript by Gautam et al. is focused on the approach to mitigate the challenges posed by pharmaceutical contaminants in water sources. This review paper has cast light upon the substantial and promising role of activated hydrochars. The manuscript is well written. It is possible to find all the necessary details: starting from the emergence of hydrochar as a sustainable adsorbent and finishing with current challenges and future Directions. Even Regulations and guidelines for pharmaceutical contaminants in various countries are introduced by authors.

The manuscript is interesting for environmental experts, policymakers, and industry stakeholders and deserves to be published in MDPI C Journal of Carbon Research. 

Author Response

Response to reviewers’ comments:

Title: Harnessing Activated Hydrochars: A Novel Approach for Pharmaceutical Contaminant Removal

Manuscript Id:  2732639

Dear Reviewers,

Thank you for your valuable suggestions. The manuscript has been amended according to your suggestions. The changes incorporated into the manuscript have been marked with a yellow-coloured font. I hope this revised manuscript will meet the standards of the “C Journal of Carbon Research”.

Reviewer 4: 

The manuscript by Gautam et al. is focused on the approach to mitigate the challenges posed by pharmaceutical contaminants in water sources. This review paper has cast light upon the substantial and promising role of activated hydrochars. The manuscript is well written. It is possible to find all the necessary details: starting from the emergence of hydrochar as a sustainable adsorbent and finishing with current challenges and future Directions. Even Regulations and guidelines for pharmaceutical contaminants in various countries are introduced by authors.

The manuscript is interesting for environmental experts, policymakers, and industry stakeholders and deserves to be published in MDPI C Journal of Carbon Research. 

Response: We are expressing our gratitude for your kind and thoughtful comments.

Round 2

Reviewer 3 Report

The authors have addressed my comments and the revised manuscript has significantly improved.