Trace Metal Enrichment in the Colloidal Fraction in Soils Developing on Abandoned Mine Spoils

Round 1

Reviewer 1 Report

The authors present a study on the metal distribution between colloid-size and bulk fractions in soils and mine residues. Although the topic of the paper is not new, the paper is interesting and offers detailed information on metal-mineral associations to potential readers. However, the paper should be shortened and more concise, especially the discussion and conclusion sections. The discussion should only mention those results more salient and restrict the section to the scope of their experimental work. In the conclusions, the authors could add a brief paragraph on the environmental implications of their study. The text should be corrected to improve the grammar and/or spelling mistakes. Additionally, I do have the following issues:

1. Why it is not included As in their study? it is a common element associated with pollution from mine sites.

2. A number of chemical extraction methods are well known to further study metal mineral association. For instance, noncrystalline and poorly crystalline Fe and Al in soils are typically extracted with ammonium oxalate solution. Why did not the authors use this or other methods?

3. what fraction size is considered the bulk soil? what is the depth of the O-horizon and at what depth was taken the soil samples? How many replicates subsamples were analyzed? 

4. There is a large list of articles dealing with colloid size soil particles. Your extraction method does not ensure the extraction of the <1micro size particles which could be the most extended size limit to nominate the colloid soil fraction (definitely, not <0.2mm as the authors claim in L392). How do you know the actual size of your extracted fraction. I suggest the reading of Gomez-Gonzalez et al (2016) 10.1016/j.chemosphere.2015.09.090; (2016b) 10.1007/s00216-016-9331-4; (2018) 10.1016/j.chemosphere.2018.01.100; and Serrano et al (2014) 10.1016/j.jhazmat.2014.12.025.

5. Within the Discussion section, the authors should not discuss their data in terms of colloid mobilization as their study does not assess this phenomenon. 

Author Response

1. Why it is not included As in their study? it is a common element associated with pollution from mine sites.

Other metal(loid)s of interest, including As, were not detected by SEM-EDS or in the acid extractions in the soil colloids, which we now note at the end of section 3.2. We know that As is present in both primary sulfides (Singer et al., 2020, GCA) and secondary Fe-oxides (Singer et al., 2020, Appl.Geochem.), but was not detected in colloids being transported in soil pore water (Chowdhury et al., 2021, Appl.Geochem.).

 

2. A number of chemical extraction methods are well known to further study metal mineral association. For instance, noncrystalline and poorly crystalline Fe and Al in soils are typically extracted with ammonium oxalate solution. Why did not the authors use this or other methods?

Our results (i.e. the XRD results in Figure 2) as well as previous work in our group (see previous note) have indicated that metals of interest are associated with multiple colloid and bulk phases including secondary oxides as well as primary sulfides and phyllosilicates. The extraction technique was intended to analyze metal content for all relevant phases.

 

3. what fraction size is considered the bulk soil? what is the depth of the O-horizon and at what depth was taken the soil samples? How many replicates subsamples were analyzed?

Section 2.1 has been revised to include the following information: (1) the O-horizon was approximately 2 cm thick; (2) samples were collected in triplicate at each location; and (3) bulk samples were sieved to < 1 mm after trying.  As noted in that section, the sample depth was 8.25 cm.

 

4. There is a large list of articles dealing with colloid size soil particles. Your extraction method does not ensure the extraction of the <1micro size particles which could be the most extended size limit to nominate the colloid soil fraction (definitely, not <0.2mm as the authors claim in L392). How do you know the actual size of your extracted fraction. I suggest the reading of Gomez-Gonzalez et al (2016) 10.1016/j.chemosphere.2015.09.090; (2016b) 10.1007/s00216-016-9331-4; (2018) 10.1016/j.chemosphere.2018.01.100; and Serrano et al (2014) 10.1016/j.jhazmat.2014.12.025.

In the original manuscript, L392 (or the text in that section) does not refer to particle size analyses so we are not clear what the reviewer is specifically referring to.  However, the electron microscopy results of this work show evidence for < 0.2 mm particles (i.e. Figure 5-7), and the combination of SEM and TEM provided a confirmation of the particle size and morphology in the extracted fraction (see, for example, Figure S13). We appreciate being made aware of these relevant studies and now cite them in the introduction.

 

5. Within the Discussion section, the authors should not discuss their data in terms of colloid mobilization as their study does not assess this phenomenon.

We agree and have removed the first paragraph of section 4.1 (as well as changed the subsection title). In response to the Reviewer’s summary comments, we have also revised the conclusion section to be more concise.

Reviewer 2 Report

This article examines the distribution of trace elements in the colloidal fraction of soils compared to bulk soils from abandoned mine dumps. The combination of different methods of mineralogical and geochemical analysis is successfully presented. Overall, the article is written concisely and contextually. However, the Conclusion needs to be refined. The description of the scientific problem with the references at the beginning of the Conclusion is more appropriate for the Discussion (sentence 2-5 of the Conclusion; lines 509-518).  Focus on conclusions based on your own results. Also, the text is littered with minor misprints, some of which are noted below:

Line 64. It might be worth replacing the repeated word "colloids" to the word "grains" in the phrase "humic-coated kaolinite colloids".

Lines 145-147. Highlight the number of atoms in the molecules (HNO₃ and H₂O) with a subscript.

Lines 196-204. Dashes between intervals of minerals content are missing. Perhaps there is confusion over the values. 

Lines. 210-228. Check the particle size units throughout the text. In section 3.2, the diameter of colloids is given in metres (the letter Mu is missing).  Also, in the last sentence of this section, it is more likely that micrometres should be specified rather than millimetres (line 226).

Line 211. "A relatively"  is mentioned twice.

Line 278. Check the figures numbering. In section 3.4, figure 8 should be given.

Line 371. Correct 58 to 0.58.

Line 382. There  is an excessive space and dot in the section title.

Lines 341-357. Dashes between intervals of element concentrations are missing.

Line 391. "Of size"  is mentioned twice. Also, the size of the colloidal particles seems to be incorrect (micrometres instead of millimetres).

Author Response

This article examines the distribution of trace elements in the colloidal fraction of soils compared to bulk soils from abandoned mine dumps. The combination of different methods of mineralogical and geochemical analysis is successfully presented. Overall, the article is written concisely and contextually. However, the Conclusion needs to be refined. The description of the scientific problem with the references at the beginning of the Conclusion is more appropriate for the Discussion (sentence 2-5 of the Conclusion; lines 509-518).  Focus on conclusions based on your own results

The beginning of the conclusion section has been moved to the beginning of the discussion section and we have revised the conclusion section to be more concise. Grammatical and formatting errors have been fixed.

 

Also, the text is littered with minor misprints, some of which are noted below:

Line 64. It might be worth replacing the repeated word "colloids" to the word "grains" in the phrase "humic-coated kaolinite colloids".

We have replaced the second use of “colloids” with “grains”

 

Lines 145-147. Highlight the number of atoms in the molecules (HNO3 and H2O) with a subscript.

fixed

 

Lines 196-204. Dashes between intervals of minerals content are missing. Perhaps there is confusion over the values.

Average values were reported (with standard deviation values), not intervals. It seems that this was a formatting issue with the document with plus/minus signs which were not properly inserted giving the impression of ranges; these have been fixed.  (note to editor: this appears to have happened after submitting the document for submission)

 

Lines. 210-228. Check the particle size units throughout the text. In section 3.2, the diameter of colloids is given in metres (the letter Mu is missing).  Also, in the last sentence of this section, it is more likely that micrometres should be specified rather than millimetres (line 226).

these have been fixed (the Mu was included in the original manuscript but seems to have been lost during the uploading process).

 

Line 211. "A relatively"  is mentioned twice.

fixed

 

Line 278. Check the figures numbering. In section 3.4, figure 8 should be given.

fixed

 

Line 371. Correct 58 to 0.58.

fixed

 

Line 382. There  is an excessive space and dot in the section title.

fixed

 

Lines 341-357. Dashes between intervals of element concentrations are missing.

See previous note about plus/minus signs being lost during file transfer; these have been added back in.

 

Line 391. "Of size"  is mentioned twice. Also, the size of the colloidal particles seems to be incorrect (micrometres instead of millimetres).

This text has been removed

Reviewer 3 Report

The main objectives of this study was to determine the mineralogy and composition of phases in the colloidal fraction of soils developed in abandoned coal mine spoil, and determine the size-dependent enrichment of trace metals. Overall, this study is a good attempt to elucidate the morphology, and elemental and mineral composition of the colloids at the micro- and nanoscales, using a combination of X-ray diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), and Focused Ion Beam (FIB) sectioning of selected colloids for Transmission Electron Microscopy (TEM). The article could be of interest to researchers in mineralogy and environmental pollution, particularly metal contamination.

However, the manuscript need polishing before the acceptance. Fortunately, it can be addressed in subsequent revisions.

Specific comments :

-The abstract is well written and organized. However quantitative results must be added in abstracts for the benefit of readers.

-Line 28-30: Rewrite following sentence, meaning is not clear. "Mine spoils often contain pyrite (FeS2) and other metal sulfides, which formed under reducing conditions during."

- The information about the number of experimental replicates is missing in the materials and method section.

-Paragraph 3.2 micro (μ) is missing 

-Figure 8: Error bar is missing is some of the charts.

-The study suggest enrichment of the trace metals in the colloidal portion. Most of the metals except Al and Mn were higher in colloidal fraction then the bulk soil. Any possible reason for this?

-The authors mentioned that there was a age difference of  50-75 years between 11 sites/piles. Also some of the piles were at the bottom while others towards the top of hill. Is there any statistical difference or any relation observed between the trace metal concentration and topology/age of the sampling sites.

-The manuscript needs to be improved in terms of English language. Spelling mistakes and grammatical errors can be noted. Help of an English editor or native speaker may be taken. 

Author Response

-The abstract is well written and organized. However quantitative results must be added in abstracts for the benefit of readers.

Quantitative data has been added to the abstract regarding: (1) the mineralogical composition of Fe-bearing colloids; and (2) the correlation between trace metals and Fe via the acid extractions.

 

-Line 28-30: Rewrite following sentence, meaning is not clear. "Mine spoils often contain pyrite (FeS2) and other metal sulfides, which formed under reducing conditions during."

“during” has been removed from the end of the sentence

 

- The information about the number of experimental replicates is missing in the materials and method section.

fixed

 

-Paragraph 3.2 micro (μ) is missing

fixed

 

-Figure 8: Error bar is missing is some of the charts.

The figure caption has been updated to note that some error values are small enough in some cases to not be visualized.

 

-The study suggest enrichment of the trace metals in the colloidal portion. Most of the metals except Al and Mn were higher in colloidal fraction then the bulk soil. Any possible reason for this?

Some of the metals (i.e. Na, Ca, and Si) are likely primarily associated with phyllosilicates, whereas Mn and other metals are likely associated with secondary oxides; we have revised the text (L491) to make it clear that metals could be associated with both types of colloids. However, further work would be needed to clarify the speciation of metals like Na and Ca.

 

-The authors mentioned that there was a age difference of  50-75 years between 11 sites/piles. Also some of the piles were at the bottom while others towards the top of hill. Is there any statistical difference or any relation observed between the trace metal concentration and topology/age of the sampling sites.

This question is the focus of on-going work and investigated as a part of a MS thesis (Smart, 2021) and is currently being developing into a manuscript for submission; we believe it is beyond the scope of the current work.

 

-The manuscript needs to be improved in terms of English language. Spelling mistakes and grammatical errors can be noted. Help of an English editor or native speaker may be taken.

Grammatical and formatting errors have been fixed throughout the manuscript.