Quantify Mercury Sulfide in Sediments for Bioavailability Assessment
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn a previous study Hsieh (Water 2024, 16, 70) investigated the chemical nature of Hg, Cu and Zn sulphides precipitated in solutions and the acid-extractable and non-acid-extractable fraction of these metals in surface sediments from the Apalachicola Bay. He found that Hg and Cu sulphides were bi-sulphides and the nonreactive fractions of Hg and Cu in sediments resulted much higher than that of the mono-sulphide Zn. Based on these preliminary results, in this work the content of total Hg, non-sulphide Hg, sulphide Hg, total sulphides, and organic matter were determined in samples of surface and deeper sediments from 25 stations in Apalachicola Bay. Essentially, the results showed that in this aquatic environment, with low Hg concentrations and high organic matter content, most of the metal is sequestered into non-bioavaialble sulphide form. In general, adopted methodologies are innovative and seems suitable to answer questions posed by the research; the results are clearly reported and interpreted. However, in the Discussion section it would seem appropriate to explain that they refer to a particular environment with low levels of total Hg and high organic matter content. In fact, sulphides are one the main factors affecting the bioavailability of Hg, but other parameters such as pH, redox potential, the occurrence of Fe and Mn oxides, clay, and microorganisms can also play an important role. For instance, in Hg contaminated sediments Oliveri et al (Mar. Chem. 2016, 186, 1-10) found very low concentrations of the bioavailable fraction (< 2%), but in deeper sediments the Hg mobilization increased, probably due to the effects of Fe and Mn redox dynamic and the sulphide oxidation. It is known that even the dissolved organic matter of the aromatic group can promote the release of Hg from insoluble Hg sulphides (e.g. Zhao et al., Toxic 2024, 12,715). Therefore, further information on the study area in section Materials and methods would be useful. For instance: were your sediment samples in most of the study area anoxic and dark grey in colour?
Specific comments: L. 22: “sequestrate most of it as sulphide”. In the environmental conditions of your study area?
L. 53: by determining?
L. 198: probably, other factors could contribute to the low Hg concentrations in sediments, for example, the uptake of soluble forms of the metal by organisms.
L. 211: Does most of the organic matter in the bay come from the river inlets? There are no autochthonous sources? L. 221: Perhaps, possible variations in the Hg bioavailability could be more accurately assessed by analysing its bioaccumulation in suitable species of organisms.
Comments for author File: 
Comments.pdf
Author Response
However, in the Discussion section it would seem appropriate to explain that they refer to a particular environment with low levels of total Hg and high organic matter content. In fact, sulphides are one the main factors affecting the bioavailability of Hg, but other parameters such as pH, redox potential, the occurrence of Fe and Mn oxides, clay, and microorganisms can also play an important role. For instance, in Hg contaminated sediments Oliveri et al (Mar. Chem. 2016, 186, 1-10) found very low concentrations of the bioavailable fraction (< 2%), but in deeper sediments the Hg mobilization increased, probably due to the effects of Fe and Mn redox dynamic and the sulphide oxidation. It is known that even the dissolved organic matter of the aromatic group can promote the release of Hg from insoluble Hg sulphides (e.g. Zhao et al., Toxic 2024, 12,715). Therefore, further information on the study area in section Materials and methods would be useful. For instance: were your sediment samples in most of the study area anoxic and dark grey in colour?
Response: Thank you for your comments. The sediments in our study sites are all grey, anoxic and highly sulfidic. The high level of sulfides we reported in the article confirms that. Indeed, the suggestions you made are our future direction of study.
Specific comments: L. 22: “sequestrate most of it as sulphide”. In the environmental conditions of your study area?
Response: Yes. The sulfide S/Hg molar ratio is about one million and mercury sulfide has the priority to be sequestered among all metals according to the solubility products.
- 53: by determining? Yes. Thank you for your correction.
- 198: probably, other factors could contribute to the low Hg concentrations in sediments, for example, the uptake of soluble forms of the metal by organisms.
Response: Agree. Uptake by organisms is possible. Just don’t have data to show that.
- 211: Does most of the organic matter in the bay come from the river inlets? There are no autochthonous sources? L. 221: Perhaps, possible variations in the Hg bioavailability could be more accurately assessed by analysing its bioaccumulation in suitable species of organisms.
Response: I think there are autochthonous sources but most of the organic matter was from the river input according to the literature cited and we observed very little submerged vegetation in the bay. You are right. Analyzing Hg level of local organisms would be another way to assess the Hg bioavailability. The problem is that not all the organisms have the same level of Hg in a location. Their level of Hg depends many factors such as their species, diet, ages, metabolic pathways, and mobility around the location.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper is of great interest. A new method for determining bioavailable mercury is proposed. The method has been applied in practice to sediments collected at 25 stations in the Apalachicola bay.
Unfortunately, the authors do not provide any information about what these sediments are. What is their grain size distribution, what are their ORP values? How does the content of organic matter and bioavailable mercury change in the vertical profile of the sediment cores? What is the salt content (TDS values) in the water and sediments? It would be useful to include photos of the sediment cores. This data could make the article even better.
Author Response
The paper is of great interest. A new method for determining bioavailable mercury is proposed. The method has been applied in practice to sediments collected at 25 stations in the Apalachicola bay.
Unfortunately, the authors do not provide any information about what these sediments are. What is their grain size distribution, what are their ORP values? How does the content of organic matter and bioavailable mercury change in the vertical profile of the sediment cores? What is the salt content (TDS values) in the water and sediments? It would be useful to include photos of the sediment cores. This data could make the article even better.
Response: Thank you very much for your comments. We didn’t do the particles size or ORP analyses but noticed that almost all the sediment samples were grey and clayey and highly sulfidic indicating anoxic conditions. Our analysis showed that the sulfide-S/Hg molar ratio was about one million (7x105). The salinity range was 0-34 ppt (we mentioned this later in the manuscript). I include two pictures of our core sample and the sections after being sealed into the cut-off syringes for your reference. The pictures are not very clear and probably not appropriate to be published with the article.
Reviewer 3 Report
Comments and Suggestions for AuthorsSimplify the background to make it more accessible, particularly for readers who may not be familiar with mercury chemistry.
Provide a clearer research gap statement, focusing on why current methods are insufficient and how this new approach improves bioavailability assessment.
Expand on the public health and ecological relevance of the study, emphasizing why assessing bioavailable mercury is critical for aquatic systems.
In the study site description, connect ecological features (e.g., river discharge, water mixing) more directly to mercury cycling in the sediments.
Include details on replication, quality control, and detection limits for analytical methods to provide a stronger validation of the results.
Clarify the difference method (HgT – AE-Hg = non-AE-Hg) and whether any error propagation is considered when calculating the non-sulfide mercury.
Mention if redox potential of the sediments was measured in situ, as it significantly influences mercury sulfide stability.
Provide statistical analysis beyond correlation, such as regression models or principal component analysis (PCA), to better explain the variance in mercury distribution across the sediment samples.
Include error bars in figures where appropriate to show variability in measurements.
Compare the findings with other estuaries or sediment studies, both nationally and internationally, to place the results in a global context.
Emphasize the implications for mercury methylation since methylmercury is the most toxic and bioavailable form in the food web.
Discuss how climate change, storms, or dredging may affect sediment redox conditions and consequently alter mercury bioavailability.
Expand the comparison of the proposed method with other bioavailability assessments for mercury, particularly focusing on its advantages over conventional methods like sequential extractions.
Include policy and practical recommendations. How could this new method inform regulatory guidelines or assist in monitoring strategies for mercury contamination in aquatic systems?
Correct the typo: “exclution” → “exclusion”.
Ensure uniformity in the use of abbreviations (e.g., AE-Hg vs. non-AE-Hg).
Improve the clarity of figure captions by making them self-explanatory, without referencing the text.
Add a graphical abstract summarizing the method and key findings to attract a broader audience.
Author Response
Simplify the background to make it more accessible, particularly for readers who may not be familiar with mercury chemistry.
Response: Do you mean explain the mercury sulfide chemistry? It has been explained in detail in the paper of Hsieh, 2024 (cited in Ref #16). It is too lengthy to be repeated here. However, we did describe it briefly in the manuscript (Line 45-48) that mercury sulfide precipitated in sediments is exclusively mercury bi-sulfide (Hg2+S2=) rather than mercury mono-sulfide (Hg++S=) as traditionally thought. Mercury bi-sulfide is acid non-extractable under nitrogen while mercury mono-sulfide is acid extractable under nitrogen.
Provide a clearer research gap statement, focusing on why current methods are insufficient and how this new approach improves bioavailability assessment.
Response: We did that. Please see Line 31-41 of the manuscript.
Expand on the public health and ecological relevance of the study, emphasizing why assessing bioavailable mercury is critical for aquatic systems.
Response: In response to your comment, we added a reference to the first paragraph of the introduction.
In the study site description, connect ecological features (e.g., river discharge, water mixing) more directly to mercury cycling in the sediments.
Response: We have the river discharge in the study site description (Line 66-67) and the organic matter distribution pattern in Fig. 3. How ecological features affecting mercury cycling is not clear at present. Those need to be studied in the future.
Include details on replication, quality control, and detection limits for analytical methods to provide a stronger validation of the results.
Response: Duplication of core and grab samples were described in sample collection section. Detection limit and quality control of the Hg analyses were described in the analytical sections (Line 117-127).
Clarify the difference method (HgT – AE-Hg = non-AE-Hg) and whether any error propagation is considered when calculating the non-sulfide mercury.
Response: The results were reported with the range and the standard error (Table 1). The detection limit and quality control of the analyses were described in the manuscript.
Mention if redox potential of the sediments was measured in situ, as it significantly influences mercury sulfide stability.
Response: We didn’t determine redox potential of the sediments. The main thrust of this paper is quantitation of mercury sulfide in sediments, which had never been achieved in the past. How redox potential affect mercury sulfide formation in sediments is not clear at the present stage because in the past we didn’t even have a method for the mercury sulfide analysis in sediments.
Provide statistical analysis beyond correlation, such as regression models or principal component analysis (PCA), to better explain the variance in mercury distribution across the sediment samples.
Response: The statistical procedures we used for data analysis should be sufficient for this report. Please suggest how PCA can better improve the finding of this study.
Include error bars in figures where appropriate to show variability in measurements.
Response: We show the 95 % confidence intervals in Fig. 2. The contour plot does not allow to show error bars. All the standard errors were listed in Table 1.
Compare the findings with other estuaries or sediment studies, both nationally and internationally, to place the results in a global context.
Response: The bioavailable mercury assessed here is the “potentially bioavailable mercury”. For example, methyl mercury is only a fraction of the potentially bioavailable mercury. We could not find any literature which assess “potentially bioavailable” mercury in sediments. There are articles tried to assess mercury bioavailability in sediments but they are all based on non-rigorous methods with non-conclusive and ambiguous results. It does not make too much sense to compare with non-conclusive and ambiguous results. (Please see the review by Wallschläger et al. cited in #7 reference for detail.)
Emphasize the implications for mercury methylation since methylmercury is the most toxic and bioavailable form in the food web.
Response: Yes. Methyl mercury is bioavailable but it is only a fraction of the potentially bioavailable mercury in sediments. We mention that in the introduction.
Discuss how climate change, storms, or dredging may affect sediment redox conditions and consequently alter mercury bioavailability.
Response: This is a good topic for discussion. Unfortunately, we know almost nothing about it because we didn’t even have a method to study it until now.
Expand the comparison of the proposed method with other bioavailability assessments for mercury, particularly focusing on its advantages over conventional methods like sequential extractions.
Response: They are a few studies that tried to assess Hg bioavailability in sediments but none of them are conclusive. (Please see the review of Reference #7)
Include policy and practical recommendations. How could this new method inform regulatory guidelines or assist in monitoring strategies for mercury contamination in aquatic systems?
Response: This is a bit out of the scope of this article, although we hint this a little bit in the conclusion section.
Correct the typo: “exclution” → “exclusion”.
Response: OK. Thank you.
Ensure uniformity in the use of abbreviations (e.g., AE-Hg vs. non-AE-Hg).
Response: OK. Thank you.
Improve the clarity of figure captions by making them self-explanatory, without referencing the text.
Response: Ok. We will do just that.
Add a graphical abstract summarizing the method and key findings to attract a broader audience.
Response: Thank you for your suggestion. We submitted a graphic abstract later to the editor but apparently it did not show in the reviewer version.
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsAccept
