Heavy Metal Sorption by Sludge-Derived Biochar with Focus on Pb2+ Sorption Capacity at μg/L Concentrations
Round 1
Reviewer 1 Report
Dear authors: the manuscript ID: processes-989402, presents Sorption of heavy metals in µg/L concentrations by sludge-derived biochar. After careful reading, I have several comments, questions and suggestions:
Comments, questions and suggestions:
- In the title you mention sludge-derived biochar, but you actually used also “Activated Carbon” I strongly suggest you change the title
- More results should be presented in abstract
- I suggest to not use the abbreviations in the abstract (SDBC etc.)
- The water “type 1” used in experiments should be describe more deeply
- Do you have information, if the samples were washed after the pyrolysis or not?
- How was provide the atmosphere in furnace? How was the product yield of prepared materials in compare to the initial dry precursory weight?
- Have you tried this sorption experiment just with the modelled solution or even onto real wastewater samples?
- How was the chemistry form of sorbed metals? If possible, I suggest you add pH/Eh diagram of these elements.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The authors (Sylwan et al) present an interesting work on the possible use of sludge-biochar for the sorption of heavy metals
General comment:
The topic is of clear interest and sufficient novelty. However, the manuscript presents numerous aspects that must be improved (or clarified) before it can be accepted for publication.
Specific comments:
Abstract:
I miss in the abstract one or two lines about the specific results obtained, which biochar has been most appropriate?, with which heavy metal has worked better/worse?
Introduction:
In relation to the introduction. There are numerous recent studies on trace element adsorption in water comparing the efficacy of different types of biochar. Please update the references, and incorporate the information provided information into the discussion of your results.
Table 1 is not complete. The EU has already published the EQS for As or Pb
For instance, revise the manuscripts:
Zhang et al., 2020. Adsorption of potentially toxic elements in water by modified biochar: A review. Journal of Environmental Chemical Engineering, 8, Issue 4, 104196. https://doi.org/10.1016/j.jece.2020.104196
Campos & de la Rosa. 2020. Assessing the Effects of Biochar on the Immobilization of Trace Elements and Plant Development in a Naturally Contaminated Soil. Sustainability 12(15), 6025; https://doi.org/10.3390/su12156025
Material and methods:
Please provide a detailed elemental composition of the 3 biochars including the total content of C, H P, Ash, As, Cd, Cu, Pb, Hg, Ni and Zn (provide the average and standard deviation-and number of replicates)
Please estate the reason of selecting pH=2.
For all the sections provide the number of replicates.
Results and discussion
The Surface area values measured for the SDBC and WDBC samples are much lower than those typically reported for biochars produced at 500-600 ºC. Normally, biochar has an SSA greater than 100-200 m2gr-1. This is a result of an inadequate process used to produce it, especially in the case of the Wood biochar, and this must be stated in the manuscript. In line with this, why did not you try to activate SDBC and WDBC?.
Please state the posible reasons and implications of the low Surface area for their sorption capacity.
Authors should warn that the concentrations of some metals in the sorbents are considerably high. Furthermore, Table 5 shows that SDBC released unacceptable amounts of Hg (3-7 mg/L Vs. EQS is 1.2-1-3). Thus the use of this biochar for the sorption of heavy metals should be discarded.
Concerning the sorption experiments. Please use different colors in all the figures for each treatment, it is really difficult to follow the results with the labels used. This will facilitate the reader's understanding of the figures.
Figure 1a shows that the removal of capacity of Pb+2 WAS NEGATIVE for SDBC and WDBC at 0.005, this results is maintained fultil 100 ppm of Pb for WDBC. This result is very negative and should be mentioned clearly. Please consider critically whether or not you believe with this result that this material should be used for the purpose studied.
Figure 1b. Why all the isotherm measurements were not performed at the same concentrations?.
Lines 337-342. Compare with more detail whether or not the results obtained are in line with those previously published, and in the case that SSA were not related with the heavy-metals sorption capacity of the chars, which stabilization mechanisms you suggest are avoiding the release of the heavy metals? In order to achieve this information, authors should determine the available contents of heavy metals at the biochars.
The removal results measured for the AC sample, a biochar of high quality, are susprisingly low. Could you provide posible explanations for that?.
Figures 2-to-6. Why SDBC and AC samples were not tested?
Section 3.4 and conclusions should be completely re-written.
The conclusions and some of the arguments provided in section 3.4 do not correspond to the results obtained. Authors should be critical. The conclusions are not a summary of introductory ideas and generalities. The results obtained indicate that SDBC released unacceptable amounts of Hg and the other two biochars did not show satisfactory results either.
The title does not correspond either to the aspects studied. Only the Pb analysis is completely executed and solely for 1 biochar sample.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
The manuscript has been considerably improved and all the questions proposed by this reviewer have been answered by the authors.
Just suggest a review of possible editing errors, capital letters, spaces, table formatting.
Revise possible improvements in the use of English languaje, for example in the abstract replace "had" by showed in line 23.
I only miss one sentence in the conclusions warning that not all biochars were so effective in the elimination of Pb+2 in wastewater. It is needed to warn as it has been seen in this study that some biochars do not work for this purpose.
Author Response
Thank you for your valuable input.
We have revised the manuscript and found some editing errors, English language errors and similar which have been corrected.
The conclusions have been amended according to the request, please see the updated manuscript.
