Transformation of Pb, Cd, and Zn Minerals Using Phosphates

Round 1

Reviewer 1 Report

Dear Authors

After corrections, I'm pleased to infor that your manuscript is suitable for publication in Minerals

 

Author Response

Thank you for the information

Reviewer 2 Report

The authors have extensively edited the manuscript and addressed most of the suggestions brought up in my previous review.  I still think it is important to acknowledge in the text the potential drawbacks of using phosphate in situ and indicate that more studies are needed to assess the mobility and leachability of phosphate from treated samples.

 

There were a few grammatical edits necessary.  

Line 68 - Add "of" after immobilization.

Line 74 - Change "researches" to "research" and add "to" after "pointed."

Line 76 - "have been published" instead of "has been published"

Line 117 - "kept in the dark" instead of "kept at dark"

Line 125 - "digesting" instead of "digests"

Line 150 and 162 - HCl = hydrochloric acid 

Line 156 - NaHCO3 = sodium bicarbonate

Line 210 - "enabled decrease in content" instead of "enabled to reduce the content"

Line 226 - Remove the word "tested"

Line 233 - Use "adsorb" instead of "absorb"

Line 306 - Remove the comma and "to" after the word besides.

Author Response

The authors have extensively edited the manuscript and addressed most of the suggestions brought up in my previous review. I still think it is important to acknowledge in the text the potential drawbacks of using phosphate in situ and indicate that more studies are needed to assess the mobility and leachability of phosphate from treated samples.

An additional paragraph dealing with mentioned issues has been added to the manuscript. Page 11, lines 325-337.

 

There were a few grammatical edits necessary. 

Line 68 - Add "of" after immobilization.

Corrected. Page 2, line 68.

 

Line 74 - Change "researches" to "research" and add "to" after "pointed."

Corrected. Page 2, line 74.

 

Line 76 - "have been published" instead of "has been published"

Corrected. Page 2, line 76.

 

Line 117 - "kept in the dark" instead of "kept at dark"

Corrected. Page 4, line 117.

 

Line 125 - "digesting" instead of "digests"

Corrected. Page 4, line 125.

 

Line 150 and 162 - HCl = hydrochloric acid

Corrected. Page 4, lines 150 and 163.

 

Line 156 - NaHCO3 = sodium bicarbonate

Corrected. Page 4, line 157.

 

Line 210 - "enabled decrease in content" instead of "enabled to reduce the content"

Corrected. Page 7, line 211.

 

Line 226 - Remove the word "tested"

Corrected. Page 9, line 227.

 

Line 233 - Use "adsorb" instead of "absorb"

Corrected. Page 9, line 234.

 

Line 306 - Remove the comma and "to" after the word besides.

Corrected. Page 11, line 308.

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

Dear authors

Please, find in attached file my remarks and suggestions.

 

Comments for author File: Comments.pdf

Reviewer 2 Report

The authors present their work investigating the use of phosphate solutions to decrease accessibility of Pb, Cd, and Zn in soils from a mining and smelting site.  The authors assess the immobilization of Pb, Cd, and Zn through TCLP (leaching method), sequential extraction, XRD, and SEM. Through this work, they have shown that phosphate solutions can be effective at decreasing the leachable Pb, Cd, and Zn in soils through formation of insoluble mineral phases.  While this paper presents some interesting results, there are several places where the text should be clarified. Additional information or considerations should also be included. If the authors are proposing that this method of adding phosphate to soil for metal immobilization should be considered at the field scale, the lability or leachability of phosphate into the environment should be considered and discussed.  Specific comments are included below. If the clarity of the text can be improved and additional considerations addressed, I would support the publication of this paper. 

 

Abstract

Line 13 - Replace “reduce” with “decrease.”  This comes up at several points in the paper but I won’t list them all in my comments.  Since you are dealing with metals that all have multiple oxidation states, using “decrease” will clarify your meaning as you are not referring to redox processes here.  

 

Line 17 - Please clarify.  You mention decrease in total metal concentration - is it total leachachable metal as assessed by TCLP?  

 

Line 19 - Again please clarify.  “Most effective immobilization of stable compounds was noted for lead” but how was this assessed?  Sequential extraction?

 

Introduction

Line 34 - Are these effects for chronic or acute exposure to Cd?

 

Line 65 - Suggested edit for clarity. Remove “readily available” and replace with “as.” 

 

Line 74-75 - You indicate that there isn’t much information about conditions/time under which the newly formed phases remain insoluble.  Did you test the stability over time with additional leaching tests or sequential extractions? If not, that would be a benefit to this paper.

 

Lines 76 - 80 - These two sentences should  be clarified. Perhaps - "There were two primary objectives of these studies: (1) to examine the effectiveness of immobilization of Zn, Cd and Pb by potassium phosphate additions to natural soil and (2) to identify and characterize the newly formed metal phosphate precipitates."

 

Materials and Methods

Materials

Line 85-88 - Awkward phrasing.  Please clarify.

 

Line 96 - Why did you choose a borderline between the layers of 16 cm?

 

Line 101 - Consider also adding a text statement (in addition to the table) that the samples had been previously characterized (and list the citation).

 

Immobilization of heavy metals with phosphorus compounds

Line 112 - How many replicates?  

 

Line 113 - Edit to say “kept in the dark at room temperature.”  

 

Line 114 - Why did you choose potassium phosphate?  Were samples agitated during phosphate exposure?  

 

Line 121-122 - Fix the degree symbols.

 

Line 131 - What happened to the samples after they were centrifuged?  ...and the supernatant was filtered and analyzed by atomic absorption spectroscopy?

 

Solid-Association of Zn, Cd, and Pb

General - Give chemical names instead of formulas (or list both).  Use mL instead of cm3.  Give citations for the specific steps of the extraction.

 

Line 144 - DCB method?  Include the full name, not just the acronym.

 

Methods of Analysis

Line 153 - Can you comment on the data quality?  Validation samples were within a certain spec? Or calibration curves had an R2 > 0.99?

 

Line 159 - You list XRF as a method of analysis.  My understanding was that the chemical composition of the samples was performed previously.  Perhaps this information would be best in the Materials section.

 

Results and Discussion

Line 170 - Figure 1 - Consider moving this figure to supplemental information.  

 

Line 187-190 - Remove sentence.  It is awkward and seems redundant.  

 

Line 195 - Clarify when you are talking about before phosphate treatment and after in this paragraph.  For BII, you list 9.42 mg/dm3 leachable lead in line 195 but 9.43 in line 197.  Keep values consistent.

 

Line 197 - Consider editing phrase to be…”phosphate enabled decrease of leachable lead in sample BII…”

 

Line 198-199 - Be clear that you are talking about leachable Cd.

 

Line 201 - Consider editing phrase to be …”The content of leachable Zn in the control soil samples varied from…”

 

Line 203 - Consider editing sentence to be…”Exposure of Zn to K2HPO4 influenced its mobility; however, the limit value was still slightly exceeded for experimental sample BIII (250.8 mg/dm3).”

 

Figure 3 - Mark the EPA limit on the graph.  In the figure, the concentrations are listed as mg/L.  In the text, they are listed as mg/dm3.  Use consistent units throughout to enhance clarity.  Make it clear in the figure caption that this data is based on TCLP.

 

Line 212 - Consider editing to the following… “A higher percentage decrease in leachable concentration was achieved for…”

 

Line 215 - You mention there are competitive reactions of Pb, Zn, and Cd with phosphates.  Are there other metals that might be competing? Can you calculate whether all of the phosphate would have been used if you assume that the decrease in leachable concentration is all due to metal phosphate formation?

 

Line 218 - entrapping?

 

Line 220 - “on the basis of”

 

Line 221 - The modeling exercises start with a lot of lead in the system - orders of magnitude higher than what is in your samples.  Do you get similar results if you start with a more realistic amount of lead? It would be helpful to discuss the modeling results in the context of the samples from your study.  

 

Line 222 - Pb concentration was decreased to ~0.1 µg/L - is this leachable Pb? Pb in the water column?  What is the relevance of this number?

 

Line 235 - Indicate that SEM results will be presented later.

 

Line 236 - “the complete formation may take…”  (omit “of”)

 

Lines 239-240 - Omit “At this stage of the experiments”

 

Lines 241-243 - Consider editing to…”More detailed studies are required to unambiguously confirm the occurrence of these mechanisms and phrases.”

 

Line 244 - Omit “The results showed that”

 

Line 245 - Reference Fig 4.

 

Figure 4 - In grayscale Zn and Pb look the same.  Consider making one much darker or much lighter.  

 

Line 253 - Consider “allows estimation of…” instead of “allows estimating the number…”

 

Line 255 - Consider “accepted” instead of “agreed”

 

Line 280-281 - Consider the following edit.  “Increase in carbonate-bound Cd is probably caused by the dissolution of the carbonate matrix under more acidic conditions in the presence of phosphate.”

 

Line 282 - Use “for” instead of “about”

 

Line 286 - Consider “in addition to” instead of “except for”

 

Line 293 - “Similar” instead of “Accordingly”

 

Line 299 - “conversion of” instead of “converting”

 

Fig 5 - Consider making colors easier to distinguish in grayscale.

 

General - Can you comment on how this work extends to other sites or types of samples?

 

Line 311 - Clarify whether you are referring to before or after phosphate treatment in this part of the discussion.  Did you compare samples before and after?

 

Line 327 - Consider the following edit “Chemically unstable limestones released zinc into the solution during the grain digestion process.”  

 

Line 329 - “Newly formed mineral substances…” Clarify this sentence.

 

Line 349-351 - Consider the following edits “...was detected (Figure 6b).  The percent of lead in the residuum ranged from 25% to 50%. The most common, stable minerals of lead are lead silicates found in the form of small, “jagged” plaques as well as spherical grains.”

 

Line 351-352 - “On the other hand…” Please clarify this sentence.  

 

Line 353 - Consider the following edit.  “The concentration of lead and zinc potentially available…”

 

Line 354-357 - Consider the following edit “With such low lead concentration, detecting the products of Pb reaction with phosphate ions by XRD and SEM was impossible.  The effectiveness of lead immobilization in the soil can be evaluated using the results from the TCLP-based procedure, as well as selective extraction methods.”

 

Line 358 - Consider “stable” instead of “permanent forms”

 

Line 360-362 - “Mobilization of metals…”  Awkward sentence, please clarify.

 

Conclusions

Line 374 - Consider “decrease in leachable metal concentration” instead of “reduction of concentration.”  Saying reduction of concentration makes it sounds like you have removed metal from the system entirely.   

 

Line 378 - Consider the following edit…”occur as infiltrates and needles on soluble carbonate rock fragments.”

 

Line 384 - Consider the following edit…”However, new, more detailed studies are required to determine concentrations of phosphates needed for the most effective immobilization of heavy metals.”

 

General - It is important to discuss the mobility of phosphate in the environment if you are proposing to use this in situ.  It would be good to have data showing leachability of phosphate from these samples. I think people would be concerned about phosphate pollution when using a technique like this in situ.

 

References

Check formatting - Ref 1 is indented differently than the rest.

Check ref 53 - It seems like you are missing information.