Accessing the Efficacy of Sargassum-Based Aqueous Phase Products Derived from Hydrothermal Carbonisation and Hydrothermal Liquefaction on Plant Growth

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript of Smith et al., investigated the application of aqueous extracts obtained through hydrothermal carbonization and liquefaction for the use as fertilizer and plant growth product. The use of seaweed is a widely studied topic that is highly relevant, especially in today’s global situation (potential climate mitigating feedstock). Therefore this paper deserves recognition in this field of research. The approach of the authors is well performed, the article is well written and the applied methodology merits publication in Phycology. However, after reading the manuscript I have some minor and major comments which ought to be addressed before publication:

-      -   Line 19: “-“ missing after macro.

-      -   Please be consistent in terminology throughout the article (seaweeds vs macroalgae, space between number and unit, …).

-    -     Abbreviation and full name are mentioned several times in the article (abstract section, abbreviation list, introduction section, materials and methods section, Table1, Table2, …). The reviewer suggests only mentioning the full name once and then use the abbreviation.

-    -     Regarding the hydrothermal reactor; what was the stirring rate of the reactor? Did the authors investigate potential mass transfer limitation effects? The treatment time was between 15min - 1h, was heating and cooling time significant (compared to the treatment time)? What were the post-processing steps? Was the reactor content cooled (If yes, how?)? How were the different phases separated (centrifugation, filtration, …). Please elaborate and add details.

-     -    What was the solid/liquid ratio that was applied in the reactor? Usually wet biomass is used for hydrothermal carbonization. However, the authors freeze-dried the samples. So is extra water added to the reactor? Please elaborate.

-     -    The authors analyzed the extracts and seaweed for fertilizer relevant compounds (PO₄³-, NH₄⁺, TN and TOC) and heavy metals. However, did the authors consider measuring other relevant compounds too? For example, all brown seaweed polysaccharides have proven properties such as antimicrobial, antibacterial,… that could also potentially influence plant growth one way or the other. Did the authors consider these tests? Based only on the measurements in the manuscript it might be difficult to reproduce the findings as the seaweed composition is highly variably based on several factors (time of harvest, geographic location, pre-processing, …). For this reason it is highly advisable to present as much unknowns as possible (such as the complete analysis of the extracts and/or seaweed composition).

-    -     Did the authors also consider aluminum (as a heavy metal) release? Aluminum is also an interesting metal, especially in the case of brown seaweeds. This is presented in the table (table 1) but not really discussed in the manuscript.

-    -     What kind of ICP analysis was used? OES or MS? What was the range of the detection limit for these metals?

-    -     Please mind significant figures in Table 1 and Table 2.

-   -      Please increase font size in Figure 2, 3, 4 and 5 to facilitate reading.

-   -      Line 298, “3+” in superscript.

-    -     Is it possible to make a mass balance to assess in which fraction the heavy metals end up? As they are not present in the aqueous extracts (or not in that high concentrations), they are probably present in the solid char? Is this confirmed via (ICP) analysis? In general it would be interesting to know what the yields of the different fractions are based on the process that is applied. Please elaborate.

Author Response

Thank you very much for taking the time to read through our manuscript and providing us with useful feedback. Please the responses addressed below:

 -   Line 19: “-“ missing after macro.
-Changes made.

  -   Please be consistent in terminology throughout the article (seaweeds vs macroalgae, space between number and unit, …).
-Changes made.

  -     Abbreviation and full name are mentioned several times in the article (abstract section, abbreviation list, introduction section, materials and methods section, Table1, Table2, …). The reviewer suggests only mentioning the full name once and then use the abbreviation.
-Changes made.

 -     Regarding the hydrothermal reactor; what was the stirring rate of the reactor? Did the authors investigate potential mass transfer limitation effects? The treatment time was between 15min - 1h, was heating and cooling time significant (compared to the treatment time)? What were the post-processing steps? Was the reactor content cooled (If yes, how?)? How were the different phases separated (centrifugation, filtration, …). Please elaborate and add details.

The seaweed was milled to below 10 um particle size following recommendations from other works in the research group. On average, heating time was 45min for the 250C reactions. Cooling time took around 20-25 mins for both temperatures. After cooling, the products were separated by filtration using gravity. The solid phase was collected on pre-dried and weighed filter paper (Fisherbrand® QL100 papers) while the liquid phase was collected on a shake flask placed below the funnel. The solid phase was dried at 60C for 48h, weighed and stored at room temperature and the liquid phase was weighed and stored at -20C.  The reactor was cooled via an in-built cooling coil connected to a water chiller. The water temperature was  −4 °C. Have updated this to manuscript

  -    What was the solid/liquid ratio that was applied in the reactor? Usually wet biomass is used for hydrothermal carbonization. However, the authors freeze-dried the samples. So is extra water added to the reactor? Please elaborate.

-Hydrothermal conversion of wet biomass was not considered at this point. It would be beneficial from a scale up point of view to remove the need for drying large amounts of biomass at scale. For the reactions mentioned in the manuscript the biomass was dried and additional water in excess was added to the reactor for the initiation of hydrothermal carbonisation. The liquid to solid ratio was kept constant  at 4.7 (g/g) across all HTC reactions while temperature was varied (180C, 250C) for the Mexican and British Sargassum. - have added to manuscript

 

  -    The authors analyzed the extracts and seaweed for fertilizer relevant compounds (PO₄³-, NH₄⁺, TN and TOC) and heavy metals. However, did the authors consider measuring other relevant compounds too? For example, all brown seaweed polysaccharides have proven properties such as antimicrobial, antibacterial,… that could also potentially influence plant growth one way or the other. Did the authors consider these tests? Based only on the measurements in the manuscript it might be difficult to reproduce the findings as the seaweed composition is highly variably based on several factors (time of harvest, geographic location, pre-processing, …). For this reason it is highly advisable to present as much unknowns as possible (such as the complete analysis of the extracts and/or seaweed composition).
-Our collaborative paper; Tonon 2022, addresses compounds found in Sargassum in the Caribbean Sea, including the Mexican sargassum used in our manuscript. Such analysis of compounds includes vitamin contents, fatty acid and amino acid compositions. This has been referenced in line 327. https://www.mdpi.com/2673-9410/2/1/11

 -     Did the authors also consider aluminum (as a heavy metal) release? Aluminum is also an interesting metal, especially in the case of brown seaweeds. This is presented in the table (table 1) but not really discussed in the manuscript.
-Agree, have added into manuscript

  -     What kind of ICP analysis was used? OES or MS? What was the range of the detection limit for these metals?
ICP-OES was used to measure elements and heavy metals, have added this to the manuscript. Levels of elements were detected as low as 0.01ppm

  -     Please mind significant figures in Table 1 and Table 2.
-Have changed to superscript ^a notes instead.

 -      Please increase font size in Figure 2, 3, 4 and 5 to facilitate reading.
-Have increased font size in these figures.

  -      Line 298, “3+” in superscript.
-Changes made.

 -     Is it possible to make a mass balance to assess in which fraction the heavy metals end up? As they are not present in the aqueous extracts (or not in that high concentrations), they are probably present in the solid char? Is this confirmed via (ICP) analysis? In general it would be interesting to know what the yields of the different fractions are based on the process that is applied. Please elaborate.
-Heavy metals should be fractioned off near completely in solid char, as found in another paper published by co-authors of this manuscript. Hydrothermal liquefaction was used and found 99% of heavy metals were partitioned off into the solid phase, using Sargassum as the feedstock. Heavy metals were measured using ICP-MS analysis, and for all samples out of calibration range of ICP-MS, ICP-OES was used. Paper can be found here-https://pubs.rsc.org/en/content/articlehtml/2019/se/c8se00408k  

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript developed a strategy to utilize hydrothermal aqueous phase as a fertilizer for crop growth. The subject is meaningful. However, before possible publication, the authors need to address the following issues:

1.     The authors are suggested to point out clearly which fraction of the hydrothermal products could be used as fertilizer, in both title and abstract.

2.     In the introduction part, the authors only listed heavy metals that are not suitable for crop growth. It is also suggested to provide nutrient elements/metals required for crop growth.

3.     Literature survey for utilizing hydrothermal aqueous phase should be expanded, examples are:

https://www.sciencedirect.com/science/article/pii/S004896972302541X

https://www.sciencedirect.com/science/article/pii/S0165237022002480

https://www.sciencedirect.com/science/article/pii/S1385894723035337

https://www.sciencedirect.com/science/article/pii/S0016236122033129

4.     The authors stated that hydrothermal processing showed the ability to separate heavy metals to the solid phase. For comparison, the quantitative analysis of heavy metal elements in the seaweed feedstock and resulting hydrochar is required.

5.     Fig. 2-5, the characters are too small. It is recommended to increase the font size.

Comments on the Quality of English Language

Minor English editing is required

Author Response

Thank you very much for taking the time to read through our manuscript. Please find my responses to  your feedback below:

-The authors are suggested to point out clearly which fraction of the hydrothermal products could be used as fertilizer, in both title and abstract.
-Have added into abstract

-In the introduction part, the authors only listed heavy metals that are not suitable for crop growth. It is also suggested to provide nutrient elements/metals required for crop growth.
-Have updated and expanded on this point a bit now in introduction. This is already discussed in more detail in the results.

-Literature survey for utilizing hydrothermal aqueous phase should be expanded, examples are:

https://www.sciencedirect.com/science/article/pii/S004896972302541X

https://www.sciencedirect.com/science/article/pii/S0165237022002480

https://www.sciencedirect.com/science/article/pii/S1385894723035337

https://www.sciencedirect.com/science/article/pii/S0016236122033129

-Have expanded this in intro

 

 

 

-The authors stated that hydrothermal processing showed the ability to separate heavy metals to the solid phase. For comparison, the quantitative analysis of heavy metal elements in the seaweed feedstock and resulting hydrochar is required.

Heavy metals should be fractioned off near completely in solid char, as found in another paper published by co-authors of this manuscript. Hydrothermal liquefaction was used and found 99% of heavy metals were partitioned off into the solid phase, using Sargassum as the feedstock. Paper can be found here-https://pubs.rsc.org/en/content/articlehtml/2019/se/c8se00408k 

 

-Fig. 2-5, the characters are too small. It is recommended to increase the font size.
-Have increased font size in these figures.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors addressed most of the comments raised by the reviewer and therefore, the reviewer accepts this manuscript for publication in Phycology.

Reviewer 2 Report

Comments and Suggestions for Authors

The Manuscript is publishable