Initial Study on Phytoextraction for Recovery of Metals from Sorted and Aged Waste-to-Energy Bottom Ash

Round 1

Reviewer 1 Report

There are some language and spelling mistakes that need to be checked

Reviewer 2 Report

This is a very interesting study and will be a valuable contribution.

Reviewer 3 Report

All of the concerns raised were addressed adequately in the revised version of the manuscript.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This paper describes several novel aspects pertaining to phytoextraction, recovery of metals and nutrient recycling. However, the manuscript in its present form is very weak and it requires a large reworking in order to be able to reach publishable standards of Soil Systems. Please consider the following points. 

• Certain parts of the abstract needs to be re-written. For example the first sentence (lines 10-11) does not make much sense. Please re-write.
• It raises a the valid question in the readers mind as to why the authors attempted to phytoextract metals from MIBA? Isn't direct extraction of these metals possible from MIBA? Is the extra phytoextraction step really necessary? These questions needs to be adequately justified in the manuscript. Otherwise, the work presented here makes little or no sense.
• Lines 90-94 very vaguely mentions the aim of this investigation. It has to be more robustly and explicitly stated along with any objectives of this work.
• This work lacks appropriate control experiments to compare the observed outcomes against.
• This work also lacks proper statistical scrutiny of the obtained results. There are no controls to compare the data against. Therefore, the conclusions drawn cannot be as robust and unambiguous compared to conclusions drawn when there are control experiments and proper statistical analysis are present. 
• Error bars for graphs presented in the Figure-2 should be included. Should the reader assume that SRef and Rref datasets are representing control experiments? Its is not clearly mentioned anywhere in the manuscript. Please clarify this.
• No statistical comparisons between datasets of figure-2 for example, has been made. Please use statistical significance values when making comparisons between datasets and drawing your conclusions.
• Graphs in figures 3 and 4 should be represented with error bars too.
• The conclusions section is weak and it needs to be further strengthened. Especially, the first paragraph (lines 372-374) are irrelevant and it can be removed entirely. Please remove the vague remarks in the conclusion, shorten it with only the salient findings of your work and finish it off with the potential implications.

Author Response

Reviewer 1

This paper describes several novel aspects pertaining to phytoextraction, recovery of metals and nutrient recycling. However, the manuscript in its present form is very weak and it requires a large reworking in order to be able to reach publishable standards of Soil Systems. Please consider the following points. 

1. Certain parts of the abstract needs to be re-written. For example the first sentence (lines 10-11) does not make much sense. Please re-write.

Response: Thank you for this comment! We have rewritten this sentence and revised the other parts of the abstract, please see lines 10-27.

 

2. It raises a the valid question in the readers mind as to why the authors attempted to phytoextract metals from MIBA? Isn't direct extraction of these metals possible from MIBA? Is the extra phytoextraction step really necessary? These questions needs to be adequately justified in the manuscript. Otherwise, the work presented here makes little or no sense.

Response: We have discussed and explained why direct metal extraction from MIBA is not suitable and why phytoextraction could be an alternative to recover the metals, please see the Introduction part, please see lines 53-67. We have now revised the text to further clarify this.

 

3. Lines 90-94 very vaguely mentions the aim of this investigation. It has to be more robustly and explicitly stated along with any objectives of this work.

Response: Thank you for your comment! We have revised the text to make the aims clearer, please see lines 98-104.

 

4. This work lacks appropriate control experiments to compare the observed outcomes against.

5. This work also lacks proper statistical scrutiny of the obtained results. There are no controls to compare the data against. Therefore, the conclusions drawn cannot be as robust and unambiguous compared to conclusions drawn when there are control experiments and proper statistical analysis are present. 

6. Error bars for graphs presented in the Figure-2 should be included. Should the reader assume that SRef and Rref datasets are representing control experiments? Its is not clearly mentioned anywhere in the manuscript. Please clarify this.

7. No statistical comparisons between datasets of figure-2 for example, has been made. Please use statistical significance values when making comparisons between datasets and drawing your conclusions.

 

Response: Thank you for these comments! As they are all within the same area i.e. how the data could be evaluated when there are no control experiments and that there is a lack of statistics for the results, we answer them together.

First, there are control experiments (SRef and RRef) that are treated exactly in the same way as the MIBA cultivation experiments but cultivated in soil. However, this was not clearly stated in the manuscript and we have now revised the text in several places to make this understandable, please see e.g. lines 116-118 and 137-140. We have also added statistics in Figure 2 and Table 2. Concerning Figures 3 and 4, please see the point below. We have revised and if needed adjusted the text to assure that all comparisons and conclusions are valid.

 

8. Graphs in figures 3 and 4 should be represented with error bars too.

Response: Thank you for the comment! The particle size distribution for the soil was analyzed in triplicates. However, PSD in MIBA was analyzed as singlicate, why no error bars could be applied. However, the method applied here (EN 933-1) uses 30 grams of the material, why also single samples could give important information. As the budget is limited and the purpose is not to evaluate the exact particle size distribution but to show the difference between MIBA and ordinary soil we decided to do this even though we agree that more tests give more exact results. We have clarified this in the text, lines… 177-178.

Concerning Figure 4, standard deviations for the total amounts in original MIBA is given in Table 1, column 5, but due to limited sample amounts only single analyses were done for the residues. This is written in lines 172-174 and is now further clarified.

 

9. The conclusions section is weak and it needs to be further strengthened. Especially, the first paragraph (lines 372-374) are irrelevant and it can be removed entirely. Please remove the vague remarks in the conclusion, shorten it with only the salient findings of your work and finish it off with the potential implications.

Response: Thank you for your comment! We have now rewritten the conclusion section to make it more concise, please see lines 403-431.

Reviewer 2 Report

This study describes a phytoextraction approach to recovering metals from the mineral fraction of incinerator bottom ash that is generated as a waste product from Waste-to-Energy plants. The introduction to the paper provides a good justification for developing the phytoextraction technique which is the focus of this paper. The experimental design is sound and clearly demonstrates the ability of two plants, sunflower, and rapeseed, to extract metals from the mineral fraction of incinerator bottom ash.

Determining the metal content of bottom ash is a challenging analytical problem, and as the authors correctly describe, will require multiple approaches to get all metals into solution. Rather than simply provide a menu of analytical techniques that were used to dissolve the matrix I would prefer the authors to provide a table that lists which elements were analyzed by the various dissolution techniques described. ICP- MS is susceptible to a number of Interferences that must be corrected during the analysis to obtain quantitative data. Corrective methods for isobaric and matrix effects are not described in the Methods section and must be included before the manuscript should be published. Analytical methods are typically validated using certified reference materials and the authors do not describe the QA/QC procedures that were followed and do not list percent recoveries for elements using their analytical procedures. The extensive analytical data provided in this manuscript would be much more useful to the scientific community if this information was provided.

Author Response

Reviewer 2

This study describes a phytoextraction approach to recovering metals from the mineral fraction of incinerator bottom ash that is generated as a waste product from Waste-to-Energy plants. The introduction to the paper provides a good justification for developing the phytoextraction technique which is the focus of this paper. The experimental design is sound and clearly demonstrates the ability of two plants, sunflower, and rapeseed, to extract metals from the mineral fraction of incinerator bottom ash.

Determining the metal content of bottom ash is a challenging analytical problem, and as the authors correctly describe, will require multiple approaches to get all metals into solution. Rather than simply provide a menu of analytical techniques that were used to dissolve the matrix I would prefer the authors to provide a table that lists which elements were analyzed by the various dissolution techniques described.

ICP- MS is susceptible to a number of Interferences that must be corrected during the analysis to obtain quantitative data. Corrective methods for isobaric and matrix effects are not described in the Methods section and must be included before the manuscript should be published.

Analytical methods are typically validated using certified reference materials and the authors do not describe the QA/QC procedures that were followed and do not list percent recoveries for elements using their analytical procedures. The extensive analytical data provided in this manuscript would be much more useful to the scientific community if this information was provided.

Response: Thank you for your comments! We answer all comments together as they focus on the same theme i.e. validation of the analytical methods.

In this study, almost all analyses are done in duplicate or triplicate at accredited laboratories (Eurofins and Synlab) specialized in chemical analyses. The accreditation is a quality remark that the analyses are done in a correct way including the use of reference material/sample and interference corrections when needed. This was not clearly explained in the method part and we have now updated the text, please see lines 174-215. However, we have not included specific information about how each method is performed according to validation as this would prolong the manuscript and the information could be received from each laboratory.

Reviewer 3 Report

The paper titled “ Initial study on phytoextraction for recovery of metals from 2 sorted and aged waste-to-energy bottom ash” submitted by Karin Karlfeldt Fedje et al. is interesting, which compare the effects of two plants (rapeseed, and sunflower) on phytoextraction of metals in sorted and aged waste-to-energy bottom ash under the fertilization and without fertilization . However, Necessary revisions are required before the article can be accepted for publication.

1. Line 10-20: The test method is not clear and needs to be supplemented, including treatments etc.
2. Line 10-20:The test results are incomplete and need to be supplemented, such as fertilization effects,
3. Line 113-134. Was the experiment not repeated? If yes, how about number? And the standard deviation (SD) should be added in some Tables (2,3,4), and Figures (2,4).
4. Line 143: Did the box leak during watering and if so, was the solution put back into the box?
5. The processing color disposition in Figure 4 is too low and needs to be modified
6. The present conclusions is long-winded and needs refining, fertilization effects could be added in conclusion.

Author Response

Reviewer 3

The paper titled “ Initial study on phytoextraction for recovery of metals from 2 sorted and aged waste-to-energy bottom ash” submitted by Karin Karlfeldt Fedje et al. is interesting, which compare the effects of two plants (rapeseed, and sunflower) on phytoextraction of metals in sorted and aged waste-to-energy bottom ash under the fertilization and without fertilization . However, Necessary revisions are required before the article can be accepted for publication.

1. Line 10-20: The test method is not clear and needs to be supplemented, including treatments etc.

Response: Thank you for this comment! We have added text about the method and revised the results as well as the other parts of the abstract within the total allowed maximum of 200 words, please see lines 10-27.

 

 

2. Line 10-20:The test results are incomplete and need to be supplemented, such as fertilization effects,

Response: Please, see point 1 above!

 

3. Line 113-134. Was the experiment not repeated? If yes, how about number? And the standard deviation (SD) should be added in some Tables (2,3,4), and Figures (2,4).

Response: Thank you for your comment! The experiment was done once during one season, but control plants were used to compare cultivation during ordinary circumstances in soil with cultivation in MIBA. However, this was not clearly stated in the manuscript and we have now revised the text in several places to make this obvious, please see e.g. lines 116-118 and 137-140.

Standard deviations are already present in Table 3 and have been added to Table 2. Concerning Table 4, this is only calculated indicative amounts based on average contents in the rapeseed roots, why we think that the addition of SDs is not needed. Error bars are added to Figure 2. Concerning Figure 4, SDs for the total amounts in original MIBA is given in Table 1, column 5, but due to limited sample amounts only single analyses were done for the residues. This is written in lines 184-187 and is now further clarified.

 

4. Line 143: Did the box leak during watering and if so, was the solution put back into the box?

Response: Some water was collected in the bottom of the boxes, but no leachate was observed. The collected water was not reachable to the roots. Due to practical limitations, the water at the bottom of the boxes was not returned to the plants. We have added information about this, please see lines 159-160.

 

5. The processing color disposition in Figure 4 is too low and needs to be modified

Response: We have now modified the colouring.

 

6. The present conclusions is long-winded and needs refining, fertilization effects could be added in conclusion.

Response: That you for this comment! We have revised and rewritten the conclusions and added text about the fertilization effect, please see lines 404-432.

Reviewer 4 Report

The present paper describes a study on phytoextraction realized on bottom ashes in order to recover metals.

The main objective is to evaluate plants (sunflower and rapeseed) capable of both growing on bottom ashes in Nordic climate and accumulating Zn.

The manuscript can be accepted with major revision due to lack of information and of statistical data.

Material and method

Cultivation experiment

Lines 120 – 122: the supplier of Sunflowers and Rapeseeds should be indicated.

Line 127 – 128: composition of reference soil should be indicated.

What are the water holding capacities of the different soils?

Chemical analyses

Lines 185-186: unsuitable fonts.

Results and discussion

Plant growth and biomass

Figure 2 and corresponding text: without statistical confirmation, results only show trends and corresponding conclusion should be much more cautious.

Lines 268-269: no information is given on the water holding capacity of the “soils”.

Lines 314-315: bioaccumulation factors should be calculated.

Author Response

Reviewer 4

The present paper describes a study on phytoextraction realized on bottom ashes in order to recover metals.

The main objective is to evaluate plants (sunflower and rapeseed) capable of both growing on bottom ashes in Nordic climate and accumulating Zn.

The manuscript can be accepted with major revision due to lack of information and of statistical data.

Material and method

Cultivation experiment

1. Lines 120 – 122:the supplier of Sunflowers and Rapeseeds should be indicated.

Response: Thank you for noticing this. We have added this information, please see lines 147-148.

 

2. Line 127 – 128:composition of reference soil should be indicated.

Response: This is given in Table 1, 2^nd and 3^rd columns.

 

3. What are the water holding capacities of the different soils?

Response: Unfortunately, due to limited budget we were not able to do these analyses but based on evaluation of the particle size distribution curve the water holding capacity of MIBA is not optimal for cultivation. However, in further research analysis of water holding capacity will be included.

 

Chemical analyses

4. Lines 185-186: unsuitable fonts.

Response: Thank you for noticing this. We have now corrected the font.

 

Results and discussion

Plant growth and biomass

5. Figure 2 and corresponding text: without statistical confirmation, results only show trends and corresponding conclusion should be much more cautious.

Response: Thank you for this comment! We have added error bars in Figure 2 and revised and if needed adjusted the text to assure that all comparisons and conclusions are valid.

 

6. Lines 268-269: no information is given on the water holding capacity of the “soils”.

Response: Please, see point 3 above.  

 

7. Lines 314-315:bioaccumulation factors should be calculated.

Response: Thank you for this comment! The bioaccumulation factors could be calculated by the reader if using the data in Tables 1 and 3. Initially, we planned to include this in Table 3 but due to space limitation and the fact that BAC<1 for most elements we decided not to. Instead, we have added some text about this, please see lines 340-345 and 365-366.