Using Steel Slag for Dissolved Phosphorus Removal: Insights from a Designed Flow-Through Laboratory Experimental Structure

Round 1

Reviewer 1 Report

Article Title: Using steel slag for dissolved phosphorus removal: Insights from a designed flow-through laboratory experimental structure

Abstract: there are a few English grammatical errors, but reads well. Mention is made of a maximum removal of 61 mg/kg. It would be helpful to also have data on average P removal.

Tables and Graphics:
*Note: The tables and graphs were reviwed before reading the article text to determine how easy it is to follow them as stand alone parts.

• Figure 1 - in the illustration parts 1 and 2 seem reversed if the flow is from left to right, i.e., 1 would be the top and 2 would be the bottom.
• Table 1 - variables k and b seem to be based on a best fit line approach to the data; however it is unclear what b means. Maybe the definition of each of these should be included in the Table title. Also it is unclear why there is a descrepancy between the single single filter segment 1 and segment 4 unless this is a calculation exercise based on the results of the 4 single segments placed in series.
• Figure 2 - based on these results it remains unclear how this system was operated.These numbers appear to be post-treatment numbers and only make sense if the system is run in series. After reading through the manuscript, it seems that this graph would benefit from some indication of where the retention times are (33, 66, 99 min) otherwise it is not apparent why the pH jumps around.
• Table 2 - based on these values, it does appear that the segments were operated in series, which less calcium used and an increase in pH each stage
• Figure 3 - difficult to compare P removed because the y axis limits are not the same. At first glance the higher concentration appears to yield better removal. However this may not be the case.
• Figure 4 - since these images are setup for comparisons they should have the same x-axis and y-axis scales.Otherwise it is difficult to understand the comparisons.
• Figure 5 - unclear how this figure is useful unless the line at the bottom is a best fit line. Otherwise not sure how to adjust the removal data for a different mg/kg added value.
• Table 3 - It would be nice if this information is ordered according to one of the columns, i.e., smallest to largest P conc or smallest to largest retention time. In current form that data seems too random.
• Figure 6 - regression for % removal seems to fit better than the regression for mg/kg removed.

Introduction:
- line 54 technically the acronym RT has not yet been introduced, so this would be a good place to first spell it out.
- good explanation of the problem of P transport, primarily through tile drainage.
- focus presented as P removal as a function of slag mass and retention time

Materials and Methods:

• Experimental equipment description - experimental setup is well thought and appropriate for these experiments
• Flow-through experiments- it does appear that the segments are set up in series;
• Data analysis - proposed calculations seem reasonable; this helps to better explain some of the numbers found in table 1 for the multiple segment calculations

Results:
Dynamic changes of P removal
- it appears that within the 2-hr run time steady-state conditions were reached
- however, neither the introduction nor the materials and methods explained the reasoning for measuring calcium concentrations nor pH (written before reading table 2)
- to address this above concern reaction 1 should appear much earlier in the manuscript to explain both of these measurements (Ca2+ and pH)
- not sure that Figure 4 provides any additional information to the discussion (particularly since the scales on the axes are inconsistent); at this point in the discussion this reader did not feel the need to see these graphs to be convinced that the first segment's removal efficiency was lower.

Total P removal under varied mass & P input
- a good discussion that does not seem to require figure 5 in order to be understood and/or accepted by readers.
- based on the discussion table 3 would be better presented if ordered (from least to greatest) by the study retention times. This version of the presentation would more closely align with the focus of the discussion about table 3.
- the calculation provided as an example may need a little further explanation, i.e., 50% removal in figure 6 corresponds to a 85 mg/kg added. 160g*4.86ha*5yr = 3888g /(85g/Mg) = 46 Mg

Conclusions:
- comment about downstream segments being more efficient than upstream segments still seems a bit inaccurate since the initial P concentration in each of the downstream segments is lower than the previous segment. This statement may need to be worded differently. Comparing the "efficiency of removal" does not seem like the correct approach to describe what is taking place here.

References:
good references. no further comments on this section

Author Response

Dear Reviewer:

Thank you for your comments concerning our manuscript entitled " Using steel slag for dissolved phosphorus removal: Insights from a designed flow-through laboratory experimental structure". We have studied comments carefully and have made corrections. Revised portion are marked and highlighted in response letter. The point-by-point corrections in the manuscript according to the reviewers’ comments. The language thoroughly improved by the native speaker, Chad Penn, one of the co-authors. The attachment includes the response letter and the new version manuscript. Thank you!

Best wishes

Linhua Wang

Author Response File: Author Response.pdf

Reviewer 2 Report

The present manuscript evaluates the performance of phosphorus removal by steel slag using an innovative designed with multi-segments flow-through structure under laboratory simulation conditions. The introduction clearly states the background of the study with well-described aims, while the materials and methods section needs some improvement for a better understanding of the process. Although the results and their discussion are consistent and follow a linear structure, some discussion on the fate of “calcium phosphate precipitates” and the effects of steel slug on other elements will highlight the significance of the study.

Specific Comments:

1. L54: Please explain “RT” at the time of first use.
2. L84: “….. flow-through ….”, better to rewrite as “….. multi-segment flow-through ….”
3. In the materials and methods section, the authors can consider adding a schematic diagram of the experimental equipment for a better understanding of the design.
4. Please specify the composition of steel slag used in this experiment.
5. L126: The experiment was conducted for 2 hours. How was the duration of the experiment selected?
6. Please explain the formula used in Table 1.
7. Please replace the word “Four” with “Quadruple” in Table 1 and throughout the manuscript.
8. L175: The section heading should be “Results and Discussion”
9. Figure 2(a) shows some negative values. Please explain the reason.
10. A brief discussion on the fate of “calcium phosphate precipitates” would be better.
11. How about the effects of steel slug on other elements when used in field-scale?

Author Response

Dear Reviewer:

Thank you for your comments concerning our manuscript entitled " Using steel slag for dissolved phosphorus removal: Insights from a designed flow-through laboratory experimental structure". We have studied comments carefully and have made corrections. Revised portion are marked and highlighted in response letter. The point-by-point corrections in the manuscript according to the reviewers’ comments. The language thoroughly improved by the native speaker, Chad Penn, one of the co-authors. The attachment includes the response letter and the new version manuscript. Thank you!

Best wishes

Linhua Wang

Author Response File: Author Response.pdf

Reviewer 3 Report

 

• Page 1, Line 13: Please, introduce the steel slag as a by-product of steel making process.
• Page 1, Line 14-16: Please, re-write the sentence without using “will” in describe the experimental variables.
• Page 1, Line 16-18: is the objective of this study to investigate the performance of P removal by steel slag? Or to examine the design of multi-segments flow-through structure under laboratory simulation conditions? Please, clarify. Since, the second sentence should be in methodology.
• Page 1, Line 21: Please, re-write the sentence avoiding to use first person (plural) possessive adjective in “Our findings”.
• Page 1, Line 27: please, avoid to use abbreviations in keywords.
• Page 2, Line 64-65: Please, provide a reference for the sentence “It has been estimated that 42-86% of agricultural stream water comes from tile drainage.”
• Page 2, Line 71: Please, correct the citation style for “(Sharpley, 1994)” as a number.
• Page 2, Line 76-79: The author must not show the findings of current study in (Introduction section), which is for pervious works. Please, remove the paragraph “This study revealed that the contribution by tile drainage was 13-45% and increased with discharge. These results demonstrated that tile drainage may transport P with a greater rate and become an important source of P to streams in agricultural watersheds. Thus, tile drains represent a potential interception point with regards to reducing dissolved P loads.”
• Page 2, Line 83: Please, re-write the sentence avoiding to use first person (plural) subject pronoun “we”.
• Page 3, Line 95: mention to figure “Figure 1a” has no meaning.
• Page 3, Line 95-97: please, consider the following questions to answer in the revised manuscript:
• Has the water analysed in ICP machine or any technique for phosphorus contents (the dissolved phosphorus)?
• Has the steel slag been washed before using in the experiment? or,

• Has the steel slag been socked in de-ionised water over night and dried? If yes,
• Has the de-ionised water been analysed to investigate the possibility of release any chemical elements from steel slag?
• Has the steel slag been analysed for chemical composition before and after the experiment?
• Has the steel slag released any metal into the water in the experiment?

• What is the chemical source of calcium in steel slag?
• Is the steel slag an adsorbent or an absorbent material?
• Has the experiment investigated the accumulated phosphorus on the surface of a slag material?
• Is there any sedimentation (from the precipitation process)? If yes, has the sedimentation been analysed chemically?

• Page 3, Line 98: Please, re-write the sentence avoiding to use “we”.
• Page 3, Line 103: what is the meaning of “Between”? Is it (Sampling chamber was designed to be a 30 cm-long segment at the middle of the 1 m segment)?
• Page 4, Line 134-136: please, provide the wavelength for phosphorus and calcium ion measured by the ICP-OES machine.
• Page 4, Table 1: please, define all the abbreviations in the Table1.
• Page 5, Line-175: Section “3. Results” should be as (Results and Discussion).
• Page 5, Line 176-193: The results showing the figure 2 have not been explained and discussed well.
• Discussion and the interpretation of the results are not satisfied, for example:
• Page 5, Line 179: What are the possible reasons of the low phosphorus concentrations before 15 mins of the experiment?
• In parallel with the above, why there was a decreeing in concentration of calcium ion before 15 mins of the starting time of the test?

• Page 5, Line 182-184: Dose the steel slag work as filter? It could be the high removal of phosphors occurs at the beginning 92% - 85%, and then the removal decreased with time 15% - 9%.
• Page 5, Line187-188: what is the meaning of “normalized” in the “when normalized for slag mass, the P removal efficiency was similar for the two different inflow P concentrations.”
• Page 6, Lines 196 - 203: the explanation is not clear for “a lesser P concentration has less chemical potential for precipitating calcium phosphate.” It should be because depletion of soluble Ca ion.
• Page 7, Line 220-223: This statement contrasts with the above explanation in (page 6 Line 196-210).

• Page 7, Line 223-224: please, provide the calcium content per kg of steel slag.
• Page 7, Line 223-224: FYI, buffer means keeping pH at a nearly constant value or reducing the high value of pH, since current study considering water.
• Page 8, Line 243: Please, re-write the sentence avoiding to use first person (plural) possessive adjective in “our study”.

 

Author Response

Dear Reviewer:

Thank you for your comments concerning our manuscript entitled " Using steel slag for dissolved phosphorus removal: Insights from a designed flow-through laboratory experimental structure". We have studied comments carefully and have made corrections. Revised portion are marked and highlighted in response letter. The point-by-point corrections in the manuscript according to the reviewers’ comments. The language thoroughly improved by the native speaker, Chad Penn, one of the co-authors. The attachment includes the response letter and the new version manuscript. Thank you!

Best wishes

Linhua Wang

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have addressed the queries appropriately. The work as a whole is now more logically described with a consistent purpose and easy to understand.

Author Response

Dear Reviewer:

Thanks for your affirmation.

Reviewer 3 Report

No comments,

Revised Manuscript is much better in the current form.

Author Response

Dear Reviewer:

Thanks for your affirmation.