Arsenic Fixation in Polluted Soils by Peat Applications

Round 1

Reviewer 1 Report

Arsenic Fixation in Polluted Soils by Peat

By Antonio Aguilar-Garrido and colleagues

 

This manuscript examines 3 soils (AZN: mine tailing pond burst impacted, SC: a calcaric soil, SNC: regosol from quartzites) amended with or without peat (0, 2, 5%) examining changes in soil water chemistry and Arsenic mobility. The two non-mine waste impacted soils were previously treated with 0, 300 or 600 mg / kg As. Peat additions reduced by half water extractable As in unpolluted soils, while at higher As burdens (300 and 600 mg/kg treatments) water extractable As reductions were proportionally far greater (~98%). Peat additions were shown to increase soil respiration amounts by 2-3 fold over non peat amended soils. Lactuca sativa germination and root elongation in the SC and SNC soils was negatively affected in the As treated soils. Peat amendment was more beneficial in the calcareous soil (SC) than SNC for restoring L. sativa germination and root elongation though both improved. The AZN soil showed poor germination and growth of L. sativa and peat amendments did not improve the situation even though As _w was greatly decreased. Toxic effects of phenolic compounds from the peat are hypothesized to account for this, with those compounds being effectively neutralized in calcareous soils but not in non-calcareous soils.  Authors concluded that for calcareous soils 5% peat amendments were recommended to lower As mobility and toxicity while for non calcareous soils, lower peat amendments should be used and that a combination of peat and carbonate additions should be studied as that may ameliorate some of the negative actions of the peat.

 

 

Overall, this is a well formulated study designed to give targeted information for soil remediation projects which are increasingly important globally and is generally very well written paper.  I recommend acceptance with minor revisions. There are areas where the language could be improved and tightened-up in places, some of which but not all are mentioned in the specific edit suggestions below.

 

Edit suggestions:

Recommend using “calcareous” as opposed to “carbonated” soil as the former is a more widely used and understood term.

While I appreciate the authors use of Acronyms such as LS for root elongation make more sense in Spanish, in an English paper “RE” or even “RL” would be easier for the reader to follow.

The writing in the results and discussion section could use some further polishing in places as some sentences.

 

Specific edit suggestions below:

Line 32

While the statement is fine it should be supported by a reference or two.

Line 37 “a labor-“ is unnecessary, delete.

 

Line 80

“This high potential may be due to the fact that peat has a large active surface area, which is a consequence of the high organic matter content, and thus a large number of adsorption sites. In addition, adsorption is not the only feasible immobilization mechanism of PHEs, retention processes such as precipitation, coprecipitation and complex formation are also [37,39].”

Suggest instead:

“This high potential is likely due to peat having large active surface area, a consequence of the high organic matter content, and thus a large number of adsorption sites. Additionally, adsorption is not the only feasible immobilization mechanism of PHEs as processes such as precipitation, coprecipitation and complex formation are also expected and would aid retention [37,39].”

 

 Line 257

Generally it is not needed to state “ Figure X shows…” as the results can be describe and the Figure referenced within the sentence describing the results. Generally, it saves a bit of space and comes across as more professional to avoid the former approach.

“Figure 2 shows the seed germination percentage of Lactuca sativa L. (SG%) for the different studied soils and treatments. AZN soil showed a low germination rate for AZN-0 and AZN-2, respectively, and even lower for the higher peat treatment (AZN-5), but without significant differences between peat treatments. In SC soils, without As addition, reported germination of 100%, whereas with As addition completely inhibits the Lactuca sativa L. seed germination.”

Suggest the alternate:

“AZN soil showed a low germination rate of Lactuca sativa L. (SG%) for AZN-0 and AZN-2, respectively, and even lower for the higher peat treatment (AZN-5), but without significant differences between peat treatments (Figure 2). In SC soils, without As addition, reported germination of 100%, whereas with As addition completely inhibits the Lactuca sativa L. seed germination (Figure 2).”

The above just allows for more compact writing style. I suggest you try to do similar with your other results lead in sections.

 

 

From Line 299:

“In SC samples the increase in As solubility implied a decrease in all studied endpoints of toxicity bioassays (BR, SG and LS) (p < 0.01).”

This sounds like more soluble As decreases toxicity.

Suggest the alternate:

“In SC samples, higher As solubility (Table 2) is implicated in greater soil toxicity seen from all studied endpoints of toxicity bioassays (BR, SG and LS) (p < 0.01).”

 

Line 386 is a bit hard to follow

“Although soils fix some of the As added, being higher the amount of As fixed when it is higher the content of iron oxides in soil and provided that pH is between 3.8 and 6.3, additions of 2 and 5% of peat enhance arsenic retention above 98% in most cases.”

Suggest the alternate:

While soils fix some of the added As, increasingly so with higher soil iron oxide content, additions of 2 and 5% of peat enhance arsenic retention greatly, above 98% in most cases.

 

As pH gradients effects weren’t really investigated here and it simplifies the sentence, I omitted that point.

 

Line 388:

“This significant reduction in As solubility is motivated by the consequent increase of OC, total humic extract and humic and fulvic acids from the peat addition, which provides an important content of colloidal reactive fractions that allow the complexation of the As compounds.”

Suggest the alternate:

Peat addition increase of OC, total humic extract and humic and fulvic acids providing important colloidal reactive fractions that allow the complexation of the As compounds, reducing As solubility.

Line 391

“The peat addition also has an effect on the biological response of soils to As toxicity, which differs according to soil properties. In carbonated soils, which have been most affected by As treatments, the biological response has been worse (lower soil respiration and inhibition of Lactuca sativa L. seed germination) than in low or non carbonated soils. However, the addition of peat improves the biological response to As toxicity.”

Suggest the alternate:

Soil As toxicity differs according to soil properties and peat addition. Biological response to Arsenic additions was worse (lower soil respiration and inhibition of Lactuca sativa L. seed germination) in calcareous soils than non-calcareous soils and the former soil showed the greatest improvement in biological response / lowered As toxicity with peat amendments.

 

Good work, 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The present work focuses on the application of peat as remediation technique for arsenic contaminated soil in order to decrease the arsenic mobility. The overall work is well written and organized. Results are properly presented. However, some main concern about the work can be pointed out.  

• Introduction: it is not clear the novelty of the work compared to previous studies on this topic. Authors should better state this perspective highlighting the main points which differ from previous works.
• Figures 2 and 3: some data display very high error bars making the data itself not precisely reliable. 
• Conclusions: this section should be more concise. 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

1. The soil samples indicated as AZN, SC, SNC were taken from the different horizons. Why is that? Were they taken from the same spot but at different depths? What were the depths that the soil samples were taken?
2. The pH values of the soluble extract shown in Table 2 seemed to be very different from the soil pH values shown in Table 1. Why did the authors observe such differences? Also, the As contamination seemed to have some effects on the extract pH values as the pH of the control soils were different from the pH of the As-contaminated soils. This might need more explanation, since pH is an important factor for As mobility. What were the soil pH of each treated soil samples?
3. As treated soils are different from the control soil (i.e., 0 mg/kg of As). Then the results of the As contaminated soil and the control soil cannot be compared, since pH can be an important factor that influence the As mobility as well as the toxicity. Therefore, comparing the results of the different soil samples and carrying out the correlation analysis might not be appropriate. Please revise the manuscript.
4. Also, the AZN As concentration is about 120 mg/kg, so in order to compare the results with different soil types, it will be more appropriate to artificially contaminate the SC and SNC soil samples to have 120 mg/kg of As. Please revise the manuscript so that the comparisons and correlation analysis become appropriate.
5. The seed germination/root elongation test was carried out using the soluble extract, not the soil. The results could be different if the test was done in the soil environment as there are other factors that affect the seed germination/root elongation. Why did the authors only use the extracts?

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors revised the manuscript reflecting some of the comments given. No further comments.