Steel Wool for Water Treatment: Intrinsic Reactivity and Defluoridation Efficiency
, Mesia Lufingo 3, Tobias Licha 4 and Chicgoua Noubactep 1,5,*Round 1
Reviewer 1 Report
The manuscript ”Steel wool for water treatment: Intrinsic reactivity and defluoridation efficiency” has a very interesting topic: characterization of the reactivity of Fe(0)-based materials, and specifically, of steel-wool (SW). For the research group of Dr. Noubactep this work is the continuity of numerous previous investigations on developing and revisiting tools for the characterization of Fe(0) reactivity. With this new study, the authors present a reliable tool (the modified EDTA test) to assess the intrinsic reactivity of SW and, in the same time, conclude that defluoridation-test also appears to be a reliable method to evaluate the suitability of Fe(0) materials. The manuscript is written in a well-organized and systematic way, the objectives are clearly presented in the ”Introduction” section, and the results are consistent. Therefore, I would recommend publishing this paper after considering the following suggestions:
1) Line 20-21: ”A total of 15 Fe0 SW and one granular iron (GI) were tested in batch experiments”;
Please add the word ”specimens”: ”A total of 15 Fe0 SW specimens…..”
2) Line 72-75: ”On the other hand, the MB test relies on the evidence that MB adsorption onto positively charged iron corrosion products (FeCPs) is not favorable [46-48]. Accordingly, the kinetics of MB discoloration by Fe0 materials is slow and can be better characterized”
Even though it is mentioned at section 2.1.3., I believe it would be useful to remind also here that MB is a cationic dye; this will help readers to understand why MB adsorption onto positively charged iron corrosion products is not favorable (repetition is the mother of learning!).
3) Lines 98-101: ”The ethylenediaminetetraacetic acid (EDTA) solution used for the experiments was prepared by dissolving an analytical grade disodium salt of EDTA (Na2-EDTA) in tap water and diluting to a concentration of 0.002 M (2m M). The used Na2-EDTA salt from Merck (Darmstadt, Germany) has a molar weight of 336.28 g mol-1”;
First: Please mention why EDTA solution was prepared with tap water instead of deionized water;
Second. I believe the sentence ”The used Na2-EDTA salt from Merck (Darmstadt, Germany) has a molar weight of 336.28 g mol-1” can be omitted, since the molar weight of EDTA is not relevant to this study.
4) Lines 106-107: ” The mixture was heated and stirred with a magnetic stir rod”;
Please give the temperature and duration of heating.
5) Lines 114-115: ”MB has a maximum light absorption wavelength of 664.5 nm and a molecular mass of 319.85 g mol-1.”;
I believe the molecular mass of MB can be also omitted, since is not relevant to this study.
6) Lines 167-168: ”The columns were then intermittently charged with a gravity driven 2 mM EDTA solution”;
Please give the volume of the EDTA solution added to column in one leaching event.
7) Lines 205-206: ”The columns were then charged with a gravity driven water solution containing 25 mg L-1 of fluoride”;
Please give the volume of the fluoride solution added to column in one leaching event.
8) Lines 210-211: ” The pH was the effluent……”
Please replace the word ”was” with ”of”.
9) Line 319: ” lowering the Fe0 SW to 0.04 and 0.01, ……”
Please replace this with: ” lowering the mass of Fe0 SW to 0.04 or 0.01 g”.
10) Line 322-323: ” After achieving optimal test conditions with the 8 Fe0 SW the remaining 7 specimens and GI were tested using 0.01 g SW and 50 mL of the EDTA solution;
Is it 50 mL or 100 mL of EDTA solution? I understand that optimal test conditions (i.e., the modified EDTA test) were mentioned at lines 318-320: ”two major modifications were made to achieve reasonable kEDTA”: decreasing the Fe0 mass to 0.01 g and increasing the EDTA volume to 100 mL. Therefore, I believe the volume of EDTA in tests conducted under ”optimal conditions” was 100 mL, not 50 mL. If this is true, please make this correction here and also in figure 3b and Table 3.
11) Line 445: ”and Fe0 nicht”
Please replace ”nicht” with ”not”
Author Response
Reviewer 1
The manuscript ”Steel wool for water treatment: Intrinsic reactivity and defluoridation efficiency” has a very interesting topic: characterization of the reactivity of Fe(0)-based materials, and specifically, of steel-wool (SW). For the research group of Dr. Noubactep this work is the continuity of numerous previous investigations on developing and revisiting tools for the characterization of Fe(0) reactivity. With this new study, the authors present a reliable tool (the modified EDTA test) to assess the intrinsic reactivity of SW and, in the same time, conclude that defluoridation-test also appears to be a reliable method to evaluate the suitability of Fe(0) materials. The manuscript is written in a well-organized and systematic way, the objectives are clearly presented in the ”Introduction” section, and the results are consistent. Therefore, I would recommend publishing this paper after considering the following suggestions:
Many thanks for this evaluation.
1) Line 20-21: ”A total of 15 Fe0 SW and one granular iron (GI) were tested in batch experiments”;
Please add the word ”specimens”: ”A total of 15 Fe0 SW specimens…..”
Done, thanks!
2) Line 72-75: ”On the other hand, the MB test relies on the evidence that MB adsorption onto positively charged iron corrosion products (FeCPs) is not favorable [46-48]. Accordingly, the kinetics of MB discoloration by Fe0 materials is slow and can be better characterized”
Even though it is mentioned at section 2.1.3., I believe it would be useful to remind also here that MB is a cationic dye; this will help readers to understand why MB adsorption onto positively charged iron corrosion products is not favorable (repetition is the mother of learning!).
Done, thanks! It now reads: “This observation is justified by the fact that sand, which is an excellent adsorbent for cationic MB is progressively coated by positively charged FeCPs.”
3) Lines 98-101: ”The ethylenediaminetetraacetic acid (EDTA) solution used for the experiments was prepared by dissolving an analytical grade disodium salt of EDTA (Na2-EDTA) in tap water and diluting to a concentration of 0.002 M (2m M). The used Na2-EDTA salt from Merck (Darmstadt, Germany) has a molar weight of 336.28 g mol-1”;
First: Please mention why EDTA solution was prepared with tap water instead of deionized water;
Second. I believe the sentence ”The used Na2-EDTA salt from Merck (Darmstadt, Germany) has a molar weight of 336.28 g mol-1” can be omitted, since the molar weight of EDTA is not relevant to this study.
Done thanks! It reads: “The ethylenediaminetetraacetic acid (EDTA) solution used for the experiments was prepared by dissolving an analytical grade disodium salt of EDTA (Na2-EDTA from Merck – Darmstadt/ Germany) in tap water and diluting to a concentration of 0.002 M (2m M). The tap water of the city of Göttingen has a very constant composition with low level of cations. For similar experiments in Cameroon or Tanzania our research group used deionized water.”
4) Lines 106-107: ” The mixture was heated and stirred with a magnetic stir rod”;
Please give the temperature and duration of heating.
This is really not important. Heating is just to accelerate dissolution of salts and the pH is monitored until the final pH value is obtained. As a rule this is done one day before fluoride analysis.
5) Lines 114-115: ”MB has a maximum light absorption wavelength of 664.5 nm and a molecular mass of 319.85 g mol-1.”;
I believe the molecular mass of MB can be also omitted, since is not relevant to this study.
We use to specify the MW of dyes because of the purity, for example reactive red 120 used in our previous works has a purity of just about 60 %. If someone want to strictly consider dye discoloration, he should consider this factor. In comparing independent data, it is also important to mention because of steric effects. No action!
6) Lines 167-168: ”The columns were then intermittently charged with a gravity driven 2 mM EDTA solution”;
Please give the volume of the EDTA solution added to column in one leaching event.
“About 300 mL of EDTA was added for each leaching event.” is added, Thanks!
7) Lines 205-206: ”The columns were then charged with a gravity driven water solution containing 25 mg L-1 of fluoride”;
Please give the volume of the fluoride solution added to column in one leaching event.
The columns were then charged with a gravity driven water solution containing 25 mg L-1 of fluoride (about 300 mL).
8) Lines 210-211: ” The pH was the effluent……”
Please replace the word ”was” with ”of”.
Done, thanks!
9) Line 319: ” lowering the Fe0 SW to 0.04 and 0.01, ……”
Please replace this with: ” lowering the mass of Fe0 SW to 0.04 or 0.01 g”.
Done, thanks!
10) Line 322-323: ” After achieving optimal test conditions with the 8 Fe0 SW the remaining 7 specimens and GI were tested using 0.01 g SW and 50 mL of the EDTA solution;
Is it 50 mL or 100 mL of EDTA solution? It is 50 mL, we maintain 50 mL to have the same volume than in previous works and the Phen test. Thanks!
I understand that optimal test conditions (i.e., the modified EDTA test) were mentioned at lines 318-320: ”two major modifications were made to achieve reasonable kEDTA”: decreasing the Fe0 mass to 0.01 g and increasing the EDTA volume to 100 mL. Therefore, I believe the volume of EDTA in tests conducted under ”optimal conditions” was 100 mL, not 50 mL. If this is true, please make this correction here and also in figure 3b and Table 3.
11) Line 445: ”and Fe0 nicht”
Please replace ”nicht” with ”not”
Done, thanks!
Reviewer 2 Report
The authors of the manuscript provided experimental results of the different Fe0 containing steel wool samples corrosion in aqueous media in presence of sodium-EDTA. Additionally they performed decolorization of methylene blue and defluoridation experiments. The main aim of the work was to show that the kinetics of iron corrosion in aqueous media in presence of Na-EDTA ("intrinsic reactivity" according to authors' terminology) can be used as the universal method of Fe0 efficiency characterization (independent of contaminant the Fe0 is used for).
According to the presented results there is no direct correlation between the iron corrosion rates and methylene blue decolorization as well as defluoridation. Thus, the main aim was not achieved. Although negative result is also worth of being published, authors should explain why they got such unexpected result. Also, as authors mentioned in the manuscript, the iron corrosion rate depends on oxygen dissolved in water. However, this parameter was not controlled during the Na-EDTA tests.
Different results presented by the authors were not explained from the point of view of the materials they used. Why one SW material was more efficient than the other?
In my opinion, the idea of developing some standard method of evaluating intrinsic activity is good, however, authors should pay careful attention to the experimental realization as well as to explanation of the observed relationships.
Author Response
The authors of the manuscript provided experimental results of the different Fe0 containing steel wool samples corrosion in aqueous media in presence of sodium-EDTA. Additionally they performed decolorization of methylene blue and defluoridation experiments. The main aim of the work was to show that the kinetics of iron corrosion in aqueous media in presence of Na-EDTA ("intrinsic reactivity" according to authors' terminology) can be used as the universal method of Fe0 efficiency characterization (independent of contaminant the Fe0 is used for).
Not really! The starting point is that current approaches are species-specific! Our goal is to improve that and EDTA and Phen tests are not specific. While MB discoloration and water defluoridation are somewhat specific but in combination with the first test on the path to universality. The precision is essential!
According to the presented results there is no direct correlation between the iron corrosion rates and methylene blue decolorization as well as defluoridation (was not sought, altogether they demonstrated that more attention should be paid to the long-term corrosion rate). Thus, the main aim was not achieved. (we strongly disagree, please read our conclusions one more time!) Although negative result is also worth of being published, authors should explain why they got such unexpected result. Also, as authors mentioned in the manuscript, the iron corrosion rate depends on oxygen dissolved in water. However, this parameter was not controlled during the Na-EDTA tests. (Sorry this is the chemistry of the system as demonstrated by other researchers and repeated in our manuscript with Eq. 2 to 4!)
Different results presented by the authors were not explained from the point of view of the materials they used. Why one SW material was more efficient than the other? This conclusion derives from the misunderstanding of the objectives as already explained.
In my opinion, the idea of developing some standard method of evaluating intrinsic activity is good, however, authors should pay careful attention to the experimental realization as well as to explanation of the observed relationships. (We fully agree with this. Our data just demonstrated the necessity of such procedure as called by Kim et al. (2014) or Li et al. (2016) but base d on more reliable data and longer-lasting experiments)
Many thanks for this evaluation.
Reviewer 3 Report
The report by Benjamin Hildebrant and co-authors, “Steel wool for water treatment: Intrinsic reactivity and defluoridation efficiency”, is a study of the intrinsic reactivity of Fe0-bearing steel wool (Fe0 SW) materials using the ethylenediaminetetraacetate method to use these materials in sewage filters.
In terms of content, the use of SW as filters has been extensively studied in the prior literature. Nevertheless, these authors point to the role of the intrinsic reactivity of the proposed materials of different brands in the efficiency of water purification from methylene blue and fluorine, and this is an intriguing aspect. This article contains a sufficient amount of experimental work; the stages of sample preparation and analysis conditions are well described.
This study may be beneficial in creating long-term filters for the purification of aqueous solutions and gases. I recommend the article for publication. My comments are advisory in nature:
In lines 23 and 165: mass values are indicated with an accuracy of three digits (0.100 g and 0.500 g). Perhaps it is better to write 0.1 g and 0.5 g, respectively.
In table 1: the “size” column contains descriptive characteristics. Is it possible to replace this information with more accurate quantitative data on samples?
On line 148: "... chopping the SW samples into pieces of 1-2 mm in length ...", and on line 195: "The SW was cut into small pieces (1 - 2 cm) ...". Is it about the same operation? If so, why different sizes?
On lines 280 - 281: “Dannenberg and Potter [15] reported on specific surface areas (SSA) of 120, 100, and 50 cm2 for grades 0 (d = 50 μm), 1 (d = 60 μm), and 2 ( d = 75μm) Fe0 SW. " Was the real specific surface areas of your samples measured? This source is more than 50 years old.
Figure 3a: The legend to the graph needs to be modified. This placement seems unsuccessful.
Lines 344 - 348: Statements about the level of reactivity ("low", "middle", "very reactive") should preferably be supported by references.
Lines 429 - 430: Was the pH of the solution measured during the experiments? A change in the pH of the solution may also indirectly indicate the contribution of the co-precipitation mechanism to the overall cleaning process.
Author Response
Reviewer 3
The report by Benjamin Hildebrant and co-authors, “Steel wool for water treatment: Intrinsic reactivity and defluoridation efficiency”, is a study of the intrinsic reactivity of Fe0-bearing steel wool (Fe0 SW) materials using the ethylenediaminetetraacetate method to use these materials in sewage filters.
In terms of content, the use of SW as filters has been extensively studied in the prior literature. Nevertheless, these authors point to the role of the intrinsic reactivity of the proposed materials of different brands in the efficiency of water purification from methylene blue and fluorine, and this is an intriguing aspect. This article contains a sufficient amount of experimental work; the stages of sample preparation and analysis conditions are well described.
This study may be beneficial in creating long-term filters for the purification of aqueous solutions and gases. I recommend the article for publication. My comments are advisory in nature:
Many thanks for this evaluation.
In lines 23 and 165: mass values are indicated with an accuracy of three digits (0.100 g and 0.500 g). Perhaps it is better to write 0.1 g and 0.5 g, respectively.
The meaning is that an analytical balance should be used.
In table 1: the “size” column contains descriptive characteristics. Is it possible to replace this information with more accurate quantitative data on samples?
Actually no, because even this semi-quantitative data are rarely specified (as we have specified).
On line 148: "... chopping the SW samples into pieces of 1-2 mm in length ...", and on line 195: "The SW was cut into small pieces (1 - 2 cm) ...". Is it about the same operation? If so, why different sizes?
Many thanks cm is correct, we have fixed!
On lines 280 - 281: “Dannenberg and Potter [15] reported on specific surface areas (SSA) of 120, 100, and 50 cm2 for grades 0 (d = 50 μm), 1 (d = 60 μm), and 2 ( d = 75μm) Fe0 SW. " Was the real specific surface areas of your samples measured? This source is more than 50 years old.
No, but it is the only source comparing the efficiency of SW while taking into account the difference in grade.
Figure 3a: The legend to the graph needs to be modified. This placement seems unsuccessful.
It reads now:
Figure 3. Comparison of the dissolution rate of eight steel wool specimens (SW1 to SW8) in 50 mL of a 2 mM EDTA solution under quiescent conditions for up to 70 h. Experimental conditions: (a) mSW = 0.10 g, and (b) mSW = 0.01 g. The represented lines are not fitting functions; they just connect the points to ease visualization.
Lines 344 - 348: Statements about the level of reactivity ("low", "middle", "very reactive") should preferably be supported by references.
No, this is a relative classification “created” in this work and not comparable to any other data. As comparison efforts are rare, the best one been from Dannenberg and Potter (1968) – 52 years old!.
Lines 429 - 430: Was the pH of the solution measured during the experiments? A change in the pH of the solution may also indirectly indicate the contribution of the co-precipitation mechanism to the overall cleaning process.
Yes but not considered in the discussion as it was already done in previous works. In particular, Heimann (2018) demonstrated that fluoride removal was “quantitative” at pH0 = 4.0 and “insignificant” for higher values. Considering these aspects herein would just lengthen the text. Hildebrant (2018) also gives a more detailed discussion.
Reviewer 4 Report
The study is of high quality and provide useful knowladges about both iron oxide as sorbent and methodology for its reactivity testing.
Author Response
Many thanks for this evaluation.
