Mercury Fraction and Transformation in Sediment Cores of the Eutrophic Estuary in Northern Taiwan

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Overall Evaluation:

This study conducts a fractionation analysis of different mercury species in the sediments of a eutrophic estuary in northern Taiwan using a sequential extraction method, exploring the transformation of mercury during sediment burial. The article is well-structured, the research design is reasonable, and the experimental data are thorough. The study provides in-depth discussions on the distribution and geochemical cycling of mercury in estuarine sediments. However, there are still areas that could be improved to enhance the persuasiveness and scientific rigor of the article.

Suggestions for Improvement:

1. Optimization of Figure Layout and Color Usage:

   In Figure 6, the distribution percentages of mercury fractions lack clear visual differentiation. It is recommended to improve the color scheme to make the fractions  more distinguishable, thereby enhancing the readability of the figure. Consider using different saturation levels or fill patterns for differentiation. Additionally, all figures should include a legend or brief explanations to help readers quickly understand the figure content.

2. Depth of Discussion:

   - Further Explanation of the Role of Organic Matter and Microorganisms in Mercury Fractionation: In Section 4.2, the authors mention that organic chelated mercury (F3) may be transformed into methylmercury under microbial influence, but the discussion is brief. It is recommended to elaborate on the role of organic matter in the methylation process of mercury, especially in organic-rich regions of the sediments where methylation may be more likely to occur, affecting the bioavailability of active mercury. Citing relevant studies on how organic content and microbial activity influence mercury methylation would deepen the discussion.

3. Refinement of Ecological Risk Assessment:

   - Quantification of Ecological Risk: The article mentions that active mercury may pose risks to the ecosystem, but the risk is not quantified. It is suggested to incorporate ecological risk evaluation standards (such as ERL and ERM values) and compare the study data with these standards in tables or figures. For example, the proportions of active mercury fractions (F1 and F2) at each sampling point could be calculated and compared with EPA sediment quality guidelines (such as ERL/ERM) to quantify the potential ecological risks of active mercury. Additionally, the potential risks of methylmercury, considering its possible biomagnification and bioaccumulation in the food chain, should also be discussed.

   - Exploration of Risk Management Suggestions: When discussing ecological risks, it would be beneficial to suggest risk management strategies based on the actual situation in the DRE. For example, for areas with higher proportions of active mercury in upstream regions, measures to reduce anthropogenic mercury inputs, such as limiting industrial emissions and improving wastewater treatment standards, could be recommended to reduce ecological risk. This would not only enhance the practical applicability of the article but also align with the journal's focus on environmental management and policy.

4. Supplementing References:

   - Citing Recent Relevant Literature: Although several important references are cited in the literature review, there is a lack of sufficient literature support when discussing the effects of salinity, pollution sources, and microbial influences on mercury fractionation. It is suggested to include recent studies on the effects of salinity, mercury fractionation behaviors, and ecological risk assessments, especially those related to mercury geochemical behavior in estuarine or marine environments. This would enrich the scientific basis for the discussion and demonstrate the article's relevance in the current research context.

Overall Recommendation:

The research is novel, and the experimental design is well thought out, with high academic value. However, there is room for improvement in data interpretation, background introduction, and the expansion of conclusions. I recommend that the article be revised and resubmitted for further review.

Author Response

Thank you for your valuable comment, please check the author reply by attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this study, the authors investigated the Hg fraction and transformation in three sediment cores of the eutrophic estuary (Danshuei River Estuary, DRE) in Northern Taiwan by the sequential extraction methods. Although there are some ambiguities in this paper, the reviewer thinks that the results presented here may be valuable in understanding the geochemical behavior of Hg during the sediment burial processes. My comments are given below.

 1.      The authors collected the sediment cores with a length of 50 cm at three stations in the DRE. The reviewer thinks that the vertical profiles of Hg fraction and transformation at the three stations depend on the sedimentation rates. The authors should give information on the sedimentation rates at the three stations.

2.      What are the water depths of the sampling stations of sediment cores in the DRE? Especially when the water is shallow, the sedimentary strata in upper layer may be significantly disturbed by physical and/or biological mixing. This probably affects the vertical profiles of Hg fraction and transformation in the sediment cores. This should be discussed.

3.      In lines 533–535, the authors mentioned that the labile Hg (F1+F2) and moderately labile Hg (F3) fractions may be transformed into the non-labile Hg (F4+F5). Does this suggest that elemental Hg (Hg(0)) is newly formed during the diagenetic processes within the sediment? This mechanism should be explained. The reviewer thinks that HgS but not elemental Hg is rather easily formed and fixated under anoxic conditions because of its extremely low solubility in water, similar to CuS and ZnS (e.g., Kitano et al., Geochim. Cosmochim. Acta, 44, 1279–1285, 1980). In addition, the authors are encouraged to measure sulfide-S in the sediment core samples.

Other comments

Lines 14–16: Explain briefly the sequential extraction method (namely Bloom method) used, as stated in lines 312–314.

Lines 28–29: The reviewer does not understand the meaning of this sentence. Further explanation is needed.

Line 57: “Hg2+” is correct.

Lines 97–98: Nowadays, mercury has been very strictly regulated. Nevertheless, what are industries that discharge wastewater containing Hg?

Line 132: Eliminate “the” before “examine”.

Line 215: Specify whether wet or dry sample was used.

Lines 236–239: The significant figures of these values are too many. They are at most three based on analytical error.

Line 328: Figure 3 shows much higher TOC concentrations than 0.04–0.39%.

Lines 413–414: The reviewer does not understand why the partition dynamics of Hg between water and solid phase in the water column are   more complicated than those in the sediment core. Further explanation is needed.

Line 535: Eliminate “that” before “the transformation”.

Author Response

Thank you for your valuable comment, please check the author reply by attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

To facilitate review, revisions in the original manuscript should have been highlighted in red. My comments on the revised manuscript are given below.

 Major comment

In lines 533–535 (line number of the original manuscript), the authors mentioned that the labile Hg (F1+F2) and moderately labile Hg (F3) fractions may be transformed into the non-labile Hg (F4+F5). Does this suggest that elemental Hg (Hg(0)) is newly formed during the diagenetic processes within the sediment? This mechanism should be explained. The reviewer thinks that HgS but not elemental Hg is rather easily formed and fixated under anoxic conditions because of its extremely low solubility in water, similar to CuS and ZnS (e.g., Kitano et al., Geochim. Cosmochim. Acta, 44, 1279–1285, 1980). In addition, the authors are encouraged to measure sulfide-S in the sediment core samples.

→ The authors replied that the F4 and F5 fractions of sedimentary Hg analyzed by the Bloom SEM are the Hg compounds associated with the crystalline in Fe-Mn oxides and organosulfur, and bond to HgS as well as crystal lattice, respectively. If so, why is “F4 Elemental Hg” indicated in Table 1? Moreover, the authors replied that in contrast to FeS and FeS2, HgS was very refractory, and this result can partly explain why the contribution of F5 to the total Hg pool was much lower than those of F3 and F4 fractions in the sediment cores of the DRE. Does this mean that HgS is not completely dissolved in aqua regia? Is this true? In the authors’ reply, they mentioned that the present study was unable to examine the role of sulfite in controlling the sedimentary Hg fraction in the sediment core samples due to without analyzing the sulfite contents in the samples. What role does the authors think sulfite plays? If this is not clear, the above sentence should be eliminated.    

 Other comments (Line number indicates the number of the original manuscript.)

Lines 14–16: Explain briefly the sequential extraction method (namely Bloom method) used, as stated in lines 312–314.

→ No reply or revision

Lines 28–29: The reviewer does not understand the meaning of this sentence. Further explanation is needed.

→ No reply or revision

Line 57: “Hg2+” is correct.

→ No reply or correction

Line 132: Eliminate “the” before “examine”.

→ No reply or correction

Line 215: Specify whether wet or dry sample was used.

→ The authors replied “dry sample”. They should specify this in the paper. How did the authors dry the sediment? If the sediment was dried by heating, doesn’t it physically or chemically change? This may strongly affect the Hg fractions in the sediment.  

Lines 236–239: The significant figures of these values are too many. They are at most three based on analytical error.

→ No reply or correction

Line 328: Figure 3 shows much higher TOC concentrations than 0.04–0.39%.

→ No reply or correction

Line 535: Eliminate “that” before “the transformation”.

→ No reply or correction

Author Response

Thank you for your suggestions, we attached the author reply.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

I think that the revised manuscript has been satisfactorily improved. Hence, this is acceptable for publication.