Application of Waste Lemon Extract to Toxic Metal Removal through Gravitational Soil Flushing and Composting Stabilization

Round 1

Reviewer 1 Report

The authors are advised to take into consideration the following suggestions:

Line 15. “…was subjected…” rather than “…was subject…”

Line 140. Write H3PO4 using subscript for 3 and 4

Line 159. 60 cm high Χ 5 cm width Χ 5 cm length. What length stands for? Please add a Fifure of the experimental set-up?

Line 162-163. “The total volume of 8 L was used in the soil flushing treatment”. However, in Fig. 2 Effluent volume reaches 16 L.  please explain why.

Please add the soil column pore volume in paragraph 2.6. Soil column flushing experiment

Line 180. Separate 3.1 Paragraph to 3.1. Soil Properties and add a subsequent paragraph 3.2 Waste lemon extract and commercial citrus properties.

Line 214. Add the supplementary material here.

Author Response

Please see the attached file.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper reports the results of a study concerning the reclamation of soil samples contaminated by heavy metals by using solutions of commercial citric acid and lemon extract.

The clarity and the readability of the paper has improved over the first version but at least one main drawback remains. At line 219 the authors state that:

“Since the commercial citric solution contained little impurities (i.e., other co-existing trace chemicals), the flushing speed is about 8 L effluent per 90-120 minutes. On the contrary, the waste lemon extract has a rather complex composition, the flushing speed is about 8 L effluent per 300-360 minutes”.

I can conclude that the tests were carried out under different conditions of flow velocity into the soil column and the results cannot be compared.

Furthermore, there is an incongruence between what reported in the methods (it seemed that three systems were tested: with water, with the solution of citric acid and with the lemon extract) but in the results I do not find traces of the results obtained with the extraction with water. Conversely, it seems that 8 L of water were flushed after the citric acid or the lemon solution.

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

Sustainability

Manuscript number: sustainability-826635

Title: Application of waste lemon extract to toxic metal removal through gravitational soil flushing and composting stabilization.

In this work, the authors investigated the removal of toxic metals (Cu, Zn, Ni, Pb, Cd and Cr) from a polluted soil using soil flushing with waste lemon extract, and the results obtained were compared with those achieved using commercial citric acid.  The topic is appropriate, especially considering the high copper concentration of the polluted farmland treated, the experimental work is quite complete and the obtained results (removal of toxic metals, final pH of the soil, etc.) are interesting. However, although the authors have made an effort and have significantly improved the quality of the article compared to the initial version (incorporating valuable changes), several points should be properly addressed before being considered for publication in Sustainability.

General:

• Tables and figures, in general, are not well introduced and some results are not discussed. Please, improve this part.
• English should be revised (some typos and grammatical mistakes have been found).
• Unify the units: mg/kg or mg kg-1, mol/L or M, ml or L…
• For the sake of clarity, the abstract could be summarized (for example, it is not necessary to give two-time copper concentration values) and reorganized.
• What about the recovery of the soil flushing solution? Does any of the compounds present in the WLE get adsorbed in the soil? Does the effluent maintain its extraction capacity? Could it be reused in a new soil flushing experiment?
• Acronyms should be defined the first time they appear in the text. For example germination index (GI).

 

Specific comments:

• The values of reduction in soil copper concentration appearing in the abstract section are not included as such in the results section. These concentration values (1504 mg/kg and 1256 mg/kg for WLE and CCS experiments) refer to the final concentration of the soil (after the flushing treatments) or refer to the eliminated concentration ?. It is not clear.
• The objective and the principles of the stabilization step should be explained in depth.
• In the introduction section, the authors compare different works found in the literature attending to the “removal efficiency” of the soil washing experiments. However, although it could be calculated with the supplied data, the authors do not include the removal efficiency values obtained with the soil flushing solutions tested in the results section.
• “[…] a few studies also pointed out that citric acid may cause the soil acidification problem”, “suitable for soil application since it was considered mature from its appearance, odor and C/N ratio”. References supporting these statements should be included.
• In the experimental section, the authors said: “The compost was inspected by naked eyes and found to have the color of dark black or dark brown” and in the results section they stated that the color of the compost was dark gray.
• It would be interesting to discuss in depth how the soil flushing with WLE increases the pH of the soil (in contrast to CCS treatment).
• What is the concentration of TFC in the citric acid commercial solution?
• Figure 1 a and b could be grouped into a single figure to see more clearly the different signal intensity before and after the treatment.
• The procedure used for soil flushing experiments (8 L of extracting solution + 8 L of distilled water) is not clear in the experimental section.
• The flushing speed ratio obtained with WLE and CCS should be included in the figure caption of Figure 2.
• To be clearer, metals in Table 3 should appear in the same order than in Figure 2. Moreover, in Figure 3, the data corresponding to Cd is missing.
• I am sorry but I cannot see the difference between the data included in Figure 2 (concentration of metals in soil flushing solutions vs effluent volume) and the data included in Table 3 (concentration of metals in soil flushing solutions vs residence time). Values appearing in Table 3 are quite lower than those of Figure 2. I think it's a misunderstanding on my part. Please, explain better this point.

Author Response

Please see the attached file.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I appreciated the efforts of the authors in replying to my comments. You can proceed with the publication of the paper. 

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

The authors are advised to take into consideration the following comments while revising their manuscript.

The manuscript title refers to the rehabilitation of copper contaminated soil, however five other metals have been investigated in the study.  Thus, the title is suggested to be amended to “…rehabilitate heavy metal contaminated soil…”.

Please provide info in Materials and Methods on the preparation of the heavy metal flushing solution, heavy metals concentration, etc.

Line 106. Please provide information on how the waste lemon was crushed, e.g. using a blender, something else?

Line 111. Please remove the word “basic” from the subtitle and throughout the rest of the manuscript.

Line 139-144. Please provide more detailed information of columns operation, e.g. were they operated in a batch mode, fed by gravity, manually or using a pump? How was the effluent collected? 

 

Author Response

Reviewer 1

The authors are advised to take into consideration the following comments while revising their manuscript.

Response:

The authors appreciated the efforts of the reviewers and editor for improving the submitted manuscript. All the comments were considered and the modifications were made accordingly.

 

Comment 1:

The manuscript title refers to the rehabilitation of copper contaminated soil, however five other metals have been investigated in the study. Thus, the title is suggested to be amended to “…rehabilitate heavy metal contaminated soil…”.

Response:

Waste lemon extract application to rehabilitate heavy metal contaminated soil through gravitational soil flushing and composting stabilization.

 

Please refer to the title of the revised manuscript.

 

Comment 2:

Please provide info in Materials and Methods on the preparation of the heavy metal flushing solution, heavy metals concentration, etc.

Response:

Three 800 g soil samples (containing heavy metal as the following, Cu: 2487±139 mg/kg; Zn: 403.7±3.1 mg/kg; Ni: 140.9±4.6 mg/kg; Pb: 108.0±0.4 mg/kg; Cd: 1.94±0.04 mg/kg; Cr: 204.0±11 mg/kg) which were air-dried with a 10-mesh screen (pore size 2 mm) were placed into soil columns (60 cm high × 5 cm width × 5 cm length, with 30 cm in height of tested soil sample), saturated the soil with 400 ml DI water, and flushed with 8000 ml (1) DI water (control group), (2) commercial citric solution (containing 10 g/L of citric acid) and (3) waste lemon extract solution (containing 9.9±0.1 g/L of citric acid and 46 mg/L of rutin, a citrus flavonoid glycoside). During the flushing process, the pH was measured and heavy metal concentrations of each liter in the flushed liquid were analyzed using ICP-OES.

 

Please refer to lines 151-158 for this amendment.

 

Comment 3:

Line 106. Please provide information on how the waste lemon was crushed, e.g. using a blender, something else?

Response:

The Electrolux EBR-3416 blender was used to crush the waste lemon.

 

Please refer to line 110 for this amendment.

 

Comment 4:

Line 111. Please remove the word “basic” from the subtitle and throughout the rest of the manuscript.

Response:

The authors appreciated the reviewer’s suggestion, and the modification was made by removing the “basic”.

 

Please refer to lines 116 and 175.

 

Comment 5:

Line 139-144. Please provide more detailed information of columns operation, e.g. were they operated in a batch mode, fed by gravity, manually or using a pump? How was the effluent collected? 

Response:

The tested soil column was mounted on a wooden stand, with a beaker at the bottom outlet for filtrate collection after gravitational flushing.

 

Please refer to lines 157-158 for this amendment.

 

 

Reviewer 2 Report

Summary Comments

The paper describes a lab study of the potential for leaching heavy metals from contaminated soils by a waste lemon extract with and without added waste lemon compost.

At first, the paper seems to be very well written. However, after some scrutiny, the paper raises at least five major issues.

1. Variations in soil texture?

Flushing the soil with commercial citric solution and waste lemon extract with or without added compost will not change soil texture, i.e., the proportions of sand, silt and clay. Nevertheless, the soil samples have significantly different texture after the flushing treatments, see Table 6. These differences must be discussed and explained by the authors. To this reviewer, the most plausible explanation seems to be that the soil was not homogenized prior to subdivision for the various treatments. Interestingly, the data for plant available Cu, Zn and Cr after flushing correlate inversely with the clay content. See attached graphs.

If the variations in plant available Cu, Zn and Cr after flushing are influenced – or determined – by preexisting differences in clay content, then there is not a scientific basis for making the following statements in the Conclusions chapter (and similar statement in the Abstract):

“The results showed that soil flushing using waste lemon extract for copper removal is more effective than using commercial citric solution ...”

and

“Additionally, the waste lemon extract can not only flush copper but also cadmium, zinc, nickel, lead, and chromium”.

 

2. Statistically significant differences?

According to Table 7, the plant available Cu after flushing with citric solution ranged from 217.3 to 249.1 mg/kg (233.2±15.9). Flushing with waste lemon extract resulted in plant available Cu ranging from 205.4 to 237.2 mg/kg (223.1±15.9). In other words, the standard deviation for the means for the two flushing solutions overlapped by 19.9 mg/kg. Based on these data, how can the authors make the conclusion that “... waste lemon extract for copper removal is more effective than using commercial citric solution”?

 

3. Data for root length?

Data for root length are missing. Table 8, which is referred to, shows above-ground biomass – not root length.

 

4. Abstract

The Abstract should contain the data for the most important findings that answer the research objectives. The pH values are not among them. Reduced heavy metal availability appears to be at the core of the study. The Abstract should give concrete answers to the research questions stated in the research objective.

 

5. Research objective

Reduction of heavy metal availability is not mentioned in the research objective (Lines 93–95) despite the fact it seems to be the main topic of the paper.

The research objective includes soil structure damage, fertility decline and cost reduction. None of these topics are covered in the paper.

 

Detailed Comments

Line 91–92: The sentence starting with “Even though ...” is incomplete. It might be better to merge the clause with the preceding sentence and use ‘although’ instead of ‘even though’. To avoid too long sentence, maybe write: “Therefore, the citric acid has been proposed to replace hydrochloric acid. It has been demonstrated that citric acid can effectively remove the heavy metal from the treated soil [24], although a few studies also pointed out that citric acid may cause the soil acidification problem”.

Line 94: The term “improve soil acidification” implies making the soil more acid. The authors probably mean “alleviate soil acidity”, which is the opposite.

Lines 95–96: Move the last sentence to the Subchapter 2.3 Analysis of basic soil properties.

Line 100: Specify the meaning of the term “DI water”. Deionized?

Line 112: Insert the last sentence in the Introduction. Explain how the soil was sampled, how it was homogenized and how it was divided into subsamples for the different treatments. In other words, what steps were taken to assure that the soil samples exposed to the varioius treatments were identical, particularly with respect to organic matter content and texture?

Line 115: Write “particle-size distribution” with hyphen.

Line 131: Write “FTIR spectroscopy”.

Line 137 and elsewhere: Space between number and unit: “25 °C” (as done above).

Line 146 and elsewhere: Space between number and unit: “10 %”.

Table 1. Remove paragraph spacing within rows (rows 3, 4, 6).

Line 169: Lowercase ‘c’ in commercial.

Line 213: Write number in letters: “four”.

Line 237–238: How do the authors explain the fact that the soil pH increased from 4.56 to 5.70 after flushing with an acid lemon extract having a pH of 3.4?

Table 6: The proportions of sand, silt and clay should be the same for the different flushing and stabilization treatments. Was the soil homogenized properly before samples were divided for the different treatments? If not, parts of the measured treatment effects might actually be due to preexisting differences in soil properties.

Line 254: The term “soil structure” should probably be “soil texture”?

Line 266 and 269–270: “Brassica chinensis” in italic.

Lines 266–267: The statement “The root length flushed by waste lemon extract was longer than flushed by citric acid after three days of cultivation” must be documented by a table and statistical analysis. (Note also that the lengths were not longer – the roots were longer).

Lines 268–269: Write “nutrient absorption” instead of “nutrients absorption”. Grammar rule: Nouns used as modifiers (adjectives) shall normally be written in singular form.

Table 8: This table is a figure, isn’t it?

Table 8: Data for root length should be provided and correlated with treatments instead of showing photos of above-ground biomass. Obviously, the plants grow better with compost added. What was the effect of flushing?

Line 284: Write “nutrient elements” instead of “nutritious elements”.

Comments for author File: Comments.docx

Author Response

Reviewer 2

General comment:

The paper describes a lab study of the potential for leaching heavy metals from contaminated soils by a waste lemon extract with and without added waste lemon compost.

Response:

The author would like to appreciate the review’s effort for the manuscript improvement. All the comments were responded and modifications were made. We believe that the revised manuscript shall meet the standard of Sustainability for publication.

 

Comment 1:

At first, the paper seems to be very well written. However, after some scrutiny, the paper raises at least five major issues.

Variations in soil texture?

Flushing the soil with commercial citric solution and waste lemon extract with or without added compost will not change soil texture, i.e., the proportions of sand, silt and clay. Nevertheless, the soil samples have significantly different texture after the flushing treatments, see Table 6. These differences must be discussed and explained by the authors. To this reviewer, the most plausible explanation seems to be that the soil was not homogenized prior to subdivision for the various treatments. Interestingly, the data for plant available Cu, Zn and Cr after flushing correlate inversely with the clay content. See attached graphs.

If the variations in plant available Cu, Zn and Cr after flushing are influenced – or determined – by preexisting differences in clay content, then there is not a scientific basis for making the following statements in the Conclusions chapter (and similar statement in the Abstract):

 

“The results showed that soil flushing using waste lemon extract for copper removal is more effective than using commercial citric solution ...”

and

“Additionally, the waste lemon extract can not only flush copper but also cadmium, zinc, nickel, lead, and chromium”.

Response:

The reviewer’s comment is appreciated. The soil samples which were air-dried with a 10-mesh screen (pore size 2 mm), and were homogenized properly before experimentation. As shown in Table 6, the soil texture changed after the flushing. As compared to the original sample, the SOM, EC, sand%, and silt% decreased at various degrees after flushing treatment while clay% increased. Such observation may due to the fact that the silt and clay might move downwards in the CCS and WLE flushing process, causing the redistribution of soil constituents. After the composting treatment, nutrients were introduced to enhance the microbial activity, causing further variation in soil structure. Although the soil texture variation occurred, the soil samples before and after experiments were classified as “loamy sand” or “sand”, due to high and low content of sand and clay according to the soil classification ternary diagram, respectively, despite the fact that the differentiating criterion between is not so clear.

 

Similar studies regarding the soil texture variation before and after flushing treatment have been reported. In the studies by Wang et al. (2017), Guo et al. (2018) and Zhai et al. (2018), soil structures varied after soil flushing, and the extent of variation depended on the characteristics of flushing solutions. A more detail discussion was included in the revised manuscript.

 

Please refer to lines 261-269 for the amendment.

 

Table 2 in the study by Wang et al. (2017)

Table 1 in the study by Guo et al. (2018)

Table 1 in the study by Zhai et al. (2018),

References:

Wang, Y., Ma, F., Zhang, Q., Peng, C., Wu, B., Li, F., Gu Q., 2017. An evaluation of different soil washing solutions for remediating arsenic-contaminated soils. Chemosphere 173, 368-372.

 

Guo, X., Zhao, G., Zhang, G., He, Q., Wei, Z., Zheng, W., Qian, T., Wu, Q., 2018. Effect of mixed chelators of EDTA, GLDA, and citric acid on bioavailability of residual heavy metals in soils and soil properties. Chemosphere 209, 776-782.

 

Zhai, X., Li, X., Huang, B., Luo, N., Huang, M., Zhang, Q., Zeng, G., 2018. Remediation of multiple heavy metal-contaminated soil through the combination of soil washing and in situ immobilization. Science of the Total Environment 635, 92-99.

 

Comment 2:

Statistically significant differences?

According to Table 7, the plant available Cu after flushing with citric solution ranged from 217.3 to 249.1 mg/kg (233.2±15.9). Flushing with waste lemon extract resulted in plant available Cu ranging from 205.4 to 237.2 mg/kg (223.1±15.9). In other words, the standard deviation for the means for the two flushing solutions overlapped by 19.9 mg/kg. Based on these data, how can the authors make the conclusion that “... waste lemon extract for copper removal is more effective than using commercial citric solution”?

Response:

The author appreciated the reviewer’s comment. In the first submission, we mistakenly input the standard deviation of CCS for Cu, and it should be 5.75, instead of 15.9. This correction has been made in the revised submission.

 

	Original	CCS	WLE	Original+ C	CCS + C	WLE + C
Cd	0.08±0.01	0.04±0.01	0.04±0.01	0.04±0.01	0.03±0.01	0.07±0.01
Cu	677.7±36.9	233.2±5.75	221.3±15.9	612.5±11.3	218.9±1.8	156.6±9.3
Zn	42.6±2.8	33.7±0.7	33.0±1.8	41.7±0.7	30.6±1.5	30.9±3.9
Ni	29.2±1.9	29.0±0.3	21.4±1.2	24.2±0.2	23.7±0.1	13.9±1.3
Pb	18.6±0.5	15.8±0.4	15.1±2.2	11.8±0.1	11.8±0.1	11.3±0.5
Cr	11.80±0.07	9.39±0.89	9.31±0.64	20.23±1.14	14.28±2.36	12.07±1.1

 

Through calculating in a similar way after CCS and WLE flushing, the Cu plant availability for CCS flushing ranged from 227.45-238.95 and those for WLE flushing was between 205.4 and 237.2, which have the occurring probability of 68.2% (i.e., ± one standard deviation) according to the standard normal distribution. By recalculating the upper and lower limits considering half of one standard deviation (i.e., ± half of one standard deviation), the Cu plant availability for CCS flushing ranged from 230.33-236.08 and those for WLE flushing was between 213.35 and 229.25, which have the occurring probability of 38.2%. For CCS flushing, the probability for Cu plant availability higher than 230.33 is 69.1% (i.e., only considering one tail end probability). Similarly, the probability for Cu plant availability lower than 229.25 is 69.1% for WLE flushing. In short, comparing the observed plant observed results after CCS and WLE flushing, soil flushing using waste lemon extract for copper removal has the possibility of 69.1% to be more effective than using commercial citric solution because waste lemon extract contained other substances that might also chelate heavy metals. A detail discussion was added and the statement in the conclusion is modified.

 

Please refer to lines 284-290 and 314 for the amendments.

 

 

Comment 3:

Data for root length?

Data for root length are missing. Table 8, which is referred to, shows above-ground biomass – not root length.

Response:

We appreciated the suggestion of the reviewers. Table 8 has been revised to show the root lengths of tested species subject different flushing treatments and composting after one-, two-, and three-day germination. The manuscript has been revised accordingly.

 

Please refer to line 300 and Table 8 for the amendments.

 

Data of root length (Table 8)

	After one day	After two days	After three days
Original	0.20 - 0.25 cm	0.25 - 0.30cm	0.45 - 0.50cm
CCS	0.20 - 0.25cm	0.25 - 0.30cm	0.55 - 0.65cm
WLE	0.25 - 0.30cm	0.30 - 0.40cm	0.70 - 0.80cm
Original + C	0.15 - 0.20cm	0.15 - 0.70cm	0.50 - 2.00cm
CCS + C	0.15 - 0.20cm	0.20 - 0.75cm	1.60 - 2.45cm
WLE + C	0.25 - 0.30cm	0.45 - 0.60cm	2.40 - 2.70cm

 

 

Comment 4:

Abstract

The Abstract should contain the data for the most important findings that answer the research objectives. The pH values are not among them. Reduced heavy metal availability appears to be at the core of the study. The Abstract should give concrete answers to the research questions stated in the research objective.

Response:

The reviewer’s comment is appreciated. In the present study, the soil flushing efficiency using waste lemon extract for heavy metal contaminated soil was evaluated, and the alleviation of acidification and plant availability improvement after soil flushing and composting treatments were investigated. The research objectives were rephrased.

 

In addition to the heavy metal removal and plant availability improvement, another important finding is the pH increase for using waste lemon extract to flush heavy metal contaminated soil while using commercial citric solution may decrease the pH in the tested soil environment. Therefore, the abstract was revised accordingly.

 

Please refer to lines 98-100 for the amendment.

 

Comment 5:

Research objective

Reduction of heavy metal availability is not mentioned in the research objective (Lines 93–95) despite the fact it seems to be the main topic of the paper.

 

The research objective includes soil structure damage, fertility decline and cost reduction. None of these topics are covered in the paper.

Response:

The reviewer’s comment is appreciated. The purpose of this study is to investigate the heavy metal removal efficiency, extent of acidification and the resulting plant availability using waste lemon extract to flush the contaminated soil. In line with the above objectives, the research objective has been revised.

 

Please refer to lines 98-100 for the amendment.

 

Comment 6:

Detailed Comments

 

Line 91–92: The sentence starting with “Even though ...” is incomplete. It might be better to merge the clause with the preceding sentence and use ‘although’ instead of ‘even though’. To avoid too long sentence, maybe write: “Therefore, the citric acid has been proposed to replace hydrochloric acid. It has been demonstrated that citric acid can effectively remove the heavy metal from the treated soil [24], although a few studies also pointed out that citric acid may cause the soil acidification problem”.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to lines 95-97 for the amendment.

 

Comment 7:

Line 94: The term “improve soil acidification” implies making the soil more acid. The authors probably mean “alleviate soil acidity”, which is the opposite.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 99 for the amendment.

 

Comment 8:

Lines 95–96: Move the last sentence to the Subchapter 2.3 Analysis of basic soil properties

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to lines 117-119 for the amendment.

 

Comment 9:

Line 100: Specify the meaning of the term “DI water”. Deionized?

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 104 for the amendment.

 

Comment 10:

Line 112: Insert the last sentence in the Introduction. Explain how the soil was sampled, how it was homogenized and how it was divided into subsamples for the different treatments. In other words, what steps were taken to assure that the soil samples exposed to the various treatments were identical, particularly with respect to organic matter content and texture?

 

Response:

The soil samples were collected from a contaminated farmland using random sampling strategy following the protocol (NIEA S102.63B) proposed by Taiwan Environmental Protection Administration (EPA), in which the copper concentration was measured as high as 2487 mg/kg. Soil from ten sampling locations was collected using a spade, sieved through 2-mm screen to remove large objects such as branches, twigs, rocks. The soil was mixed completely to assure the homogeneity and stored at room temperature for later use.

 

Please refer to lines 117-122 for the amendment.

 

Comment 11:

Line 115: Write “particle-size distribution” with hyphen.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 126 for the amendment.

 

Comment 12:

Line 131: Write “FTIR spectroscopy”.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 142 for the amendment.

 

Comment 13:

Line 137 and elsewhere: Space between number and unit: “25 °C” (as done above).

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 148 for the amendment.

 

Comment 14:

Line 146 and elsewhere: Space between number and unit: “10 %”.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 162 for the amendment.

 

Comment 15:

Table 1. Remove paragraph spacing within rows (rows 3, 4, 6).

Response:

The reviewer’s comment is appreciated. The modification was made as suggested if possible.

 

Please refer to Table 1 for the amendment.

 

Comment 16:

Line 169: Lowercase ‘c’ in commercial.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 183 for the amendment.

 

Comment 17:

Line 213: Write number in letters: “four”.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 229 for the amendment.

 

Comment 18:

Line 237–238: How do the authors explain the fact that the soil pH increased from 4.56 to 5.70 after flushing with an acid lemon extract having a pH of 3.4?

Response:

The reviewer’s comment is appreciated. During the soil flushing process, the aqueous citric acid might act as the chelating agent to capture the metal ions adsorbed on the surface of soil particles. The hydrogen ions might take up the soil active sites that used to occupied by heavy metals, resulting in a decrease in hydrogen ion concentration. More explanation was added in the revised manuscript.

 

Please refer to lines 254-257 for the amendment.

 

Comment 19:

Table 6: The proportions of sand, silt and clay should be the same for the different flushing and stabilization treatments. Was the soil homogenized properly before samples were divided for the different treatments? If not, parts of the measured treatment effects might actually be due to preexisting differences in soil properties.

Response:

The reviewer’s comment is appreciated. This comment is similar to Comment 1. Please refer to the response to Comment 1 for the detail.

 

Comment 20:

Line 254: The term “soil structure” should probably be “soil texture”?

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 282 for the amendment.

 

Comment 21:

Line 266 and 269–270: “Brassica chinensis” in italic.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to lines 300 and 305 for the amendments.

 

Comment 22:

Lines 266–267: The statement “The root length flushed by waste lemon extract was longer than flushed by citric acid after three days of cultivation” must be documented by a table and statistical analysis. (Note also that the lengths were not longer – the roots were longer).

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to lines 301, 305 and Table 8 for the amendments.

 

Comment 23:

Lines 268–269: Write “nutrient absorption” instead of “nutrients absorption”. Grammar rule: Nouns used as modifiers (adjectives) shall normally be written in singular form.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 304 for the amendment.

 

Comment 24:

Table 8: This table is a figure, isn’t it?

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to Table 8 for the amendment.

 

Comment 25:

Table 8: Data for root length should be provided and correlated with treatments instead of showing photos of above-ground biomass. Obviously, the plants grow better with compost added. What was the effect of flushing?

Response:

The reviewer’s comment is appreciated. The modification was made as suggested. In addition to the discussion in the original submission, a sentence was added.

 

Among the roots observed for different treatments in the present study, the root after WLE + C treatment has the longest length and the one without any treatment has the shortest.

 

Please refer to Lines 300-302 for the amendment.

 

Data of root length (Table 8)

	After one day	After two days	After three days
Original	0.20 - 0.25 cm	0.25 - 0.30cm	0.45 - 0.50cm
CCS	0.20 - 0.25cm	0.25 - 0.30cm	0.55 - 0.65cm
WLE	0.25 - 0.30cm	0.30 - 0.40cm	0.70 - 0.80cm
Original + C	0.15 - 0.20cm	0.15 - 0.70cm	0.50 - 2.00cm
CCS + C	0.15 - 0.20cm	0.20 - 0.75cm	1.60 - 2.45cm
WLE + C	0.25 - 0.30cm	0.45 - 0.60cm	2.40 - 2.70cm

 

Comment 26:

Line 284: Write “nutrient elements” instead of “nutritious elements”.

Response:

The reviewer’s comment is appreciated. The modification was made as suggested.

 

Please refer to line 322 for the amendment.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper could be of interest for the journal’s readers but, in the present form, it cannot deserve publication. Here some suggestions to improve the quality (and readability) of the paper

Abstract

Line 12 – please add the information concerning the standard deviation. It is not clear to me if the target of the study was only the copper or also other heavy metals. In my opinion there is some contradiction between the title and the abstract.

Lines 13-14 – in my opinion the concentration of citric acid in the solutions from both the origins (lemon waste and commercial citric acid) is necessary

Line 16 – please quantify the result obtained after 15 minutes.

Lines 19-20 – please quantify.

Line 21 – not clear to me the effect of the “retention time” on the flushing process

Line 22 – not clear the mention to the pH: which is the sense of this information?

The last sentence of the abstract is very hard to be understood; please provide an introductive sentence concerning the use of the composted lemon waste (after citric acid extraction) as a soil stabilizer.

In my opinion the abstract must be rewritten in order to highlight the key results of the study.  

Line 32 - Bunches of citations should be avoided. Please say why each study deserves be cited and which is its relation with your study or with the subject you are treating in the paper

The novelty of this study over the previous state of the art must be underlined.

Line 104 – when you refer to the ratio between water and lemon waste, do you refer to a raw or dried lemon waste? This is because the amount of total solids (or water) in the lemon waste can change from one stock to another. In my opinion an information concerning the amount of TS in the lemon waste was necessary. Same line: was the water used deionized or distilled?

Section 2.2 – not clear how long was the composting process. In the authors’opinion, is this process reproducible at a full scale?

Line 117 – how many soil samples did you collect and analyze? Please anticipate the information reported in line 120.

Line 119 – not clear to me the precise mention to copper. Why?

Section 2.8 – not clear to me on which soil samples the germination tests were carried out. On the raw soil? On the soil after flushing treatment? On the soil after compost addition?

Table 4 – this legend is not clear. Furthermore, it is not clear the sense of the germination test carried out in section 3.2

Lines 222 – 223 – not clear to me how the flushing time or residence time was assessed.

Not clear how results of Figure 4 were obtained.

The quality of the language must be improved; a lot of errors are present. A careful revision of the language and sentence structure by an English native speaker is strongly advised to improve the readability of the manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments to second review

The paper has still significant weaknesses that must be addressed before potential publishing. Below are the most essential issues.

1. All data must undergo appropriate statistical analyses with predefined confidence level (P < 0.05) to guide the authors’ interpretation of the results. This applies to the data shown in Table 7 and Table 8.

The current reference to “half of one standard deviation” and “probability for Cu plant availability higher than 230.33 is 69.1%” is unconventional data analysis and data reporting.

2. The data (measurement ranges) in Table 8 are not informative. The table must show averages and statistically significant differences at P < 0.05.

In addition, the method for measuring and analyzing root lengths must be described in the M&M chapter.

3. The new explanation for the observed rise in soil pH from 4.56 to 5.70 after leaching the soil with the lemon extract holding a pH of 3.4, appears to be speculative. To be reliable, the explanation needs a reference to at least one published study showing that an acid chelating solution will increase the soil pH due to chelation of heavy metals. If such studies do not exist, it is more likely that the elevated pH values are due to measuring error.

4. The authors refer to three papers to justify the observed changes in particle-size distribution in the current study after leaching with lemon juice. It is important to keep in mind that the current study adds relatively weak acids (citric acid, lemon juice) to a very strongly acidic (USDA classification) soil (pH 4.56). Comparing this situation to the situation described by e.g., Wang et al. (2017) (as the authors do in their response) is irrelevant since Wang et al. leached a slightly alkaline soil with 2M phosphoric acid with pH 0.9 (!) and a dithionite/EDTA solution. In an alkaline soil, the pH-0.9 acid will obviously dissolve minerals, and dithionite/EDTA solution is, in fact, specifically used to dissolve cementing iron hydroxides to separate mineral particles. It is essential that the data for the current study are interpreted in the context of the current soil and solution characteristics. The fact that the citric acid and the lemon juice treatments increased the clay content from 4 % to 11 % raises another question of measurement error. Likewise, the reduction in silt content from 14 % to 3 % by simply adding compost (no acids), strengthens the question of measurement error even more. A reliable explanation is needed. Reference to the three papers by Wang et al., Guo et al. and Zhai et al. does not shed much light on the current issues since conditions are different.

The authors need to convince the readers that the observed differences in heavy metal concentrations are not an effect of preexisting differences in particle-size distributions (ref. the graphs provided in my first review).

5. Throughout the paper, the authors seem to use the terms “soil structure” and “soil texture” interchangeably when, apparently, soil texture is intended.

Soil structure = aggregation and arrangement of primary soil particles into peds with characteristic size, shape, and degree of distinctness resulting from pedogenic processes.

Soil texture = relative proportions of sand, silt and clay.

The new statement in Line 268–269 regarding the ternary soil texture diagram “... despite the fact that the differentiating criterion between is not so clear” may no longer be relevant. The National Soil Survey Center, USDA-NRSC issued a clarification of the soil texture class boundaries in 2009 (see https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_031477.pdf).

Author Response

please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The quality of the paper has improved after revision but some issues remain to be addressed.

First of all, the quality of the language must be improved; a lot of errors are still present. A careful revision of the language and sentence structure by an English native speaker is strongly advised to improve the readability of the manuscript, with a special attention to the abstract.

Punctual remarks:

Lines 19-21: the sentence is not clear, especially the word “decreased” (line 20). Are the values into parenthesis the residual Cu concentration after treatment? If yes, the residual concentration after treatment with the lemon extract is higher than that obtained with the commercial solution, so I understood that the commercial solution performed better.

Lines 25-26: To which do you refer with “this finding”? The sentence is not clear.

Line 142: please give some details concerning the method used for total flavonoids determination.

Section 2.6: you must decide if put the characterization of the soil and the extracts into the Materials and Methods section or present them as a results and put them in the Results and Discussion section.

Line 205: not completely clear the mention to the “germination index” in this section. Please spell CCQC or say if it is an agency, an institution or so on.

Section 3.3: at the very beginning of the section please say what Figure 2 is. The start of the section is too rude.

Lines 225-227: I understand that the impurities (which kind of impurities: small particulates? Other chemical compounds?) affect the conditions of the test (see also lines 230-231). This is an important point, because if the operating conditions of the two tests are not the same, you cannot compare the results and draw conclusions.

Line 291: unit of measurements are missing.

Round 3

Reviewer 2 Report

I regret to report that the paper appears to lack the scientific reliabilty needed for international publishing. I would like to highlight three issues:

Issue 1: In my first review, I pointed out that data interpretation must be based on conventional statistical tests. The authors answered that "The statistical analysis was conducted based on the observed data, which showed an apparent overlap between plant availabilities of copper using waste lemon extract (WLE) and commercial citric solution (CCS) if P < 0.05 is considered. In order to explore the metal removal efficiencies using different citric solutions, “half of one standard deviation” was specifically considered in the present study." This means that since there was no statistically significant difference between the two treatments, the authors adjusted the statistical test until they could "prove" a difference. In the first version of the paper, this manipulation with statistics was, in fact, hidden. In my view, research results emanating from manipulated, unconventional statistical tests should not be published.

Issue 2: Data on changes in soil pH are unreasonable and hence, require a logical explanation (example: adding an acid with pH 3.4 to a soil with pH 4.56 increased the soil to pH 5.70). The attempted explanation by the authors is not scientifically sound. My request for supportive references for their explanation led the authors to refer to a few previous studies. However, the data given by these references are all reasonable and logical, and they do not at all justify the unreasonable data presented the current paper. The authors seem to be more more concerned with concealing the issue than clarifying it.

Issue 3: This issue is similar to Issue 2. Unreasonable data for particle-size distributions before and after flushing require scientifically sound explanation. The heavy metal concentrations are more strongly correlated with clay content than flushing agents. As under Issue 2, the provided references show reasonable changes in particle-size distributions, whereas the current paper shows changes that are not reasonable. Following my question, the authors admit that “For the tests involving in-situ soil samples, each sample has its own properties, and the non-homogeneity among the collected samples is unavoidable. The experiment results may exhibit apparent differences, even among the samples collected in the same areas.” Nevertheless, the authors stand by their initial interpretations.

In conclusion, I find the paper to contain a series of unscientific elements that combined should bar it from being published in a reputable scientific journal.