Sources and Magnitude of Heavy Metals in Sugarcane Plantation Soils with Different Agricultural Practices and Their Implications on Sustainable Waste-to-Foods Strategy in the Sugar–Ethanol Industry

Round 1

Reviewer 1 Report

The article is very interesting and was very well developed. The overall writing is good. The findings are very important and I think deserve publication in this journal.

Author Response

Thank you the reveiwer for your support.

Reviewer 2 Report

§  Line 55: “Since cane yield affects farmers’ incomes, the farmers typically apply mineral fertilizers and pesticides” sentence is vague give the reason why farmers apply mineral fertilizers and pesticides to make the statement complete and more scientific.

§  Line 56-58: is the importation of chemicals a problem needed to be addressed or the effect of the application of the imported chemicals on sugarcane production is the issue. Authors are recommended to revise the statement to connect with the problem statement of the study for coherent and easily understanding.

§  Section: 2.4. Sample preparation and analysis fix space at “HNO3 and 2 ml H2O2 at 200 °C for 15 min”.

§  Section: 3.2. Accumulations of heavy metals in the sugarcane plantation soils Check punctuations at “(Table 1).,”

§  Statement is unclear “Su (2014) found that the soil Hg was lower than the typical levels in general agricultural soils.” Authors are required to infer other studies appropriately.

§  “All the heavy metals were within the satisfactory limits set by the National Soil Standard for ….” And “The key concern was that the levels of soil as often exceeded the National Soil Standard for Agriculture, limited to 30 mg kg−1.” Section: 3.2. results discussion seems contradictory. Kindly revise for clarity.

§  Table 1: Are these references (Su, 2014) and (Zarcinas et al., 2004) used as a secondary or primary data standard for the study results interpretation or what? If the presence of these references seems to serve as a data standard for the interpretation of results, then it must be clearly stated prior to their use.

§  Hg accumulation in sugarcane as revealed by BCF in the study. To a great extent health issues related to the Hg residual effect are a potential challenge to sugarcane consumers. Therefore, the conclusion section must have some agricultural practices to mitigate Hg accumulation in sugarcane.

Author Response

Reviewer 2

1. Line 55: “Since cane yield affects farmers’ incomes, the farmers typically apply mineral fertilizers and pesticides

Response        Thank you for your suggestion. We have rewritten this sentence to make it more academic sound. The sentence (Line 52-53) is now modified to “Given the consequential impact of cane yield on farmers’ economic returns, it is customary for farmers to employ mineral fertilizers and pesticides.”

2. Line 56-58: Is the importation of chemicals a problem needed to be addressed or the effect of the application of the imported chemicals on sugarcane production is the issue. Authors are recommended to revise the statement to connect with the problem statement of the study for coherent and easily understanding.

Response        Thank you for your suggestions. In Line 56-58 in the introduction, we would like to elucidate the impact of chemical fertilizers on farmers' livelihoods. This paragraph might not directly relate to the objective of this work but it is leading to the next paragraph. The next paragraph, we introduce the utilization of industrial waste as a potential solution to enhance the adaptability to the volatile supply of chemical fertilizers. Finally, we outline our objectives, which include quantifying the levels of heavy metals contributed by industrial by-products to soils and sugarcane plants, and engage in a discussion on sustainability.

To be more convincing, rewrite the paragraph as followed:

Line 52-53:

 “Given the consequential impact of cane yield on farmers’ economic returns, it is customary for farmers to employ mineral fertilizers and pesticides. The chemicals are predominantly sourced through imports, with costs subject to fluctuations tiled to petroleum prices and global supply and demand.”

3. Section 2.4 Sample preparation and analysis fix space at “HNO3 and 2ml H2O2 at 200^oC for 15 min.”

Response        The space is now removed. Thank you for the correction.

4. Section 3.2 Accumulations of heavy metals in the sugarcane plantation soils. Check punctual at “Table 1.,“

Response        The comma sign is now removed. Thank once again for the correction.

5. Statement is unclear “Su (2014) found that the soil Hg was lower than the typical levels in general agricultural soils.” Authors are required to infer other studies appropriately.

Response        We are sorry for this misleading. We have modified this sentence to “The soil Hg was relatively lower than the typical agricultural soils worldwide, exhibiting the average content of 0.28 mg kg^-1 (Su, 2014).”

6. “All the heavy metals were within the satisfactory limits set by the National Soil Standard for ..” And “The key concern was that the levels of soil As often exceeded the National Soil Standard for Agriculture, limited to 25mg kg^-1” Section 3.2 Results Discussion seems contradictory. Kindly revise for clarity.

Response        We appreciate your comments. Both sentence is not actually contradicting. In line 261-263, We have said that “All the heavy metals were within the satisfactory limits set by the National Soil Standard for Agriculture (Notification of the National Environment Board on Soil Quality Standard, issued on March 11, 2021), except for As.” In line 266-267, we have emphasized the concern level of Arsenic (As) that “The key concern was that the levels of soil As often exceeded the National Soil Standard for Agriculture, limited to 25 mg kg^-1.”

7. Table 1: Are these references (Su, 2014) and (Zarcinas et al., 2004) used as a secondary or primary data standard for the study results interpretation or what? If the presence of these references seems to serve as a data standard for the interpretation of results, then it must be clearly stated prior to their use.

Response        We are sorry to make you misunderstanding. Both studies, (Su, 2004) and (Zarcinas et al., 2004) were not involved with this study and they use different datasets to interpretation. For the sentence referred to Su (2004), we have modified it for better clarification (see comment 5).

For the Zarcinas et al. (2004), it is the study in Thailand using soil samples from various part of the country, covering diverse land covers. We therefore use their levels as the background soil levels for Thailand. We have mentioned details of Zarcinas et al. (2004) dataset in Line 218-220.

Line 218-220:

“In this study, the background levels of the soils in Thailand were obtained from Zarcinas et al. (2004). The soils were sampled from forested, uncultivated, and remote from industry areas across the country (Zarcinas et al., 2004).”

8. Hu accumulation in sugarcane as revealed by BCF in the study. To a great extent health issues related to the Hg residual effect are a potential challenge to sugarcane consumers. Therefore, the conclusion section must have some agricultural practices to mitigate Hg accumulation in sugarcane.

Response        Thank you so much for the recommendation. We add that the solution proposed in this study could more or less mitigate the health risk associated with Hg.

Line 550-554:

“The combined application of vinasse and filter cake, as a source of organic matter, showed dual benefits by increasing soil metal content and reducing Zn bioconcentration, thereby enhancing cane yield. Additionally, reduced reliance on chemical fertilizers contributes to a diminished Hg bioconcentration, consequently mitigating health risks.”

Reviewer 3 Report

The manuscript requires a correction. Detailed comments:

1.      Abstract needs to modify and to be revised to be quantitative. You can absorb readers' consideration by having some numerical results in this section.

2.      The introduction was prepared in a too superficial manner, the introduction should be expanded and completed.

3.      Introduction- The authors could better emphasize the novelty of the research carried out.

4.      Introduction - briefly explain the motivation for undertaking this research, its relevance and originality, where it fits into the development of the field, and why it should be of interest to readers.

5.      Methodology - 2.1. Sampling sites - geographic coordinates of the sampling locations for testing should be added.

6.      Methodology - Write the LOD and LOQ of the devices used.

7.      The weaknesses and limitations of this study compared to other studies should be considered.

8.      It is important to check that the writing text clearly expresses and explains each idea and result obtained.

9.      A better discussion would be necessary in order to emphasize the main findings. The discussion section of the article is poorly written. Must be upgraded with new articles.

10.   References to literature - should be corrected according to the journal requirements (see Instructions for Authors) - references must be numbered in order of appearance in the text.

11.   Conclusions should state the main findings. The conclusions needs improvement - highlight the most important findings and identify the added value of the main finding.

12.   Conclusions - consider adding numerical results to this section.

Author Response

Reviewer 3

1. Abstract needs to modify and to be revised to be quantitative. You can absorb readers’ consideration by having some numerical results in this section.

Response Thank you for the suggestions. We have modified the abstract to be more concise and adding some quantitative information on BCF to make it more interesting. However, we have done multiple analyses and all quantitative results could not be shown here, but interpretation of the results, due to limited word count Therefore, we also provide more of these quantitative findings in the conclusion.

“Driven by Thailand's Bio-Circular Green Economy strategy, this study explores industrial waste utilization as a solution to mitigate impacts of climate change and economic insecurity. This study involved interviews with farmers and field sampling across 131 sugarcane plantations, primarily in four districts in Kanchanaburi, western Thailand. The investigation aimed to assess heavy metal levels (As, Fe, Mn, Zn, Cu, Cr, Pb, Cd, Hg) in the plantation soils and their accumulations in soils and biomass under various agricultural practices, including the uses of filter cake and vinasse (industrial wastes from sugar-ethanol industries). Spatial analysis showed that As often exceeded national soil-quality standards for agriculture. The Cd, As, and Zn tended to accumulate at a high level in most soils, whereas Pb accumulated locally. Factors like clay content, soil alkalinity (for As and Mn), soil organic matters (As, Mn, and Hg), and no/minimum tillage (Zn and Cd) promoted the metal accumulation. Factor analysis showed that natural geochemical processes govern the spatial variations of the metals. The application of filter cake led to soils with elevated Cr, Fe, As, Cd, and Mn content and a clayey organic-rich composition, while the vinasse resulted in soils with higher levels of Zn, Cu, Fe, and clayey saline. The bioconcentration factors (BCF) revealed that sugarcane generally accumulated Hg (BCF ~2.32-35.72), whereas Cu (2.67) and As (1.04) tended to accumulate in sandy-soil farmlands. The waste utilization, however, neither contributed significantly to the concern levels of soil metals nor enhanced the bioconcentration of the soil metals into the sugarcane biomass. Utilizing the wastes as fertilizers could benefit cane yield, but further studies should focus on proper fertilization rates and food safety.”

2. The introduction was prepared in a too superficial manner. The introduction should expanded and completed.
3. The Introduction- The authors could better emphasize the novelty of the research carried out.
4. Introduction- Briefly explain the motivation for undertaking this research, its relevance and originality, where it fits into the development of the field, and why it should be of interest to readers.

Response for 2-4        Thank you for your series of suggestions on the introduction. Yes, our introduction is not completed during lack of novelty, motivation, and outcomes responding to the readers. We therefore add sentences in the introduction section to fill all the gaps

Line 116-121: Explaining novelty and motivation

“Previous studies often overlooked variabilities in soil physicochemical properties, fertilization practices, and soil conservation efforts within sugarcane plantation areas. These factors significantly influence heavy metal dynamics in soils and biomass. Moreover, there are gaps in the literature regarding investigation of waste utilization sustainability and a lack of comprehensive integration of these factors affecting cane yield, a pivotal aspect for economic sustainability.”

Line 126-131: Explaining the outputs and outcomes

“The findings suggest the sustainability of utilizing wastes from the sugar-ethanol industry in sugarcane cultivation. These outcomes have the potential to drive positive changes in agricultural practices, environmental stewardship, food safety assurance, policy formation, and academic knowledge advancement based in agriculture and environmental science. They collectively contribute to the overall sustainability and resilience of sugarcane farming systems.”

5. Methodology-2.1. Sampling sites-Geographical coordinates of the sampling locations for testing should be added.

Response        We acknowledge your comment. The details on geographical cooridation of the sampling area is given in Line 136-137

“They are situated between latitudes from 13.7 to 14.0°N and longitude from 99.0 to 99.9°E.”

6. Methodology-Write the LOD and LOQ of the devices used.

Response        Thank you so much for your comments. We have added the detection limit of the ICP used for the analysis. Furthermore, we provide calibration details to showR2 of the calibration curve.

Line 206-208

“In this study, there are nine metals included in the discussion: Arsenic (As), Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Chromium (Cr), Lead (Pb), Cadmium (Cd), and Mercury (Hg). The ICP-OES detection limits are 1.0, 0.08, 0.026, 0.07, 0.26, 0.17, 1.3, 0.1, and 1.0 mg L^-1, respectively. Calibration concentrations ranged from 0.5 mg L^-1 to 10 mg L^-1, resulting in a coefficient of determination, R², of 0.999669 or higher.”

7. The weaknesses and limitations of this study compared to other studies should be considered.

Response        We acknowledge your comments and the research limitations are given as the last section of Result and Discussion (3.8 Limitations) in Line 509-516:

3.8. Limitations

        “It is noted that this study carries certain weakness and limitations. The study was focus on the sugarcane fields in western Thailand. Though it addresses diverse soil properties but may not capture the full spectrum of soil conditions found in various regions. The study also provides limited exploration of socioeconomic factors and may not fully account for seasonal variations. The emphasis on short-term outcomes might overlook longer-term trends. These limitations present opportunities for future research to enhance understanding of heavy metal dynamics in sugarcane plantation systems.”

8. It is important to check that the writing text clearly expresses and explains each idea and result obtained.

Response        We have checked the entire manuscript and make some text modifications for better clarification and concise.

9. A better discussion would be necessary in order to emphasize the main finding. The discussion section of the article is poorly written. Must be upgraded with new articles.

Response        We appreciate your valuable suggestions. In response, we have integrated the results and discussion sections. Within each sub-section, we typically start with the presentation of the study's findings, followed by their interpretation, and subsequently engage in a comprehensive discussion. This discussion incorporates insights gleaned from prior research to elucidate points of concurrence or disparity. We adhere to this writing pattern to facilitate readers' enhanced comprehension of the implications derived from the results.

We also included new references to show variations of the results among different studies. There are:

Demir, A. (2021). Speciation, risk assessment and bioavailability of metals in the agricultural soils of the Göksu Delta, Turkey. Soil and Sediment Contamination: An International Journal, 30(3), 292-313. https://doi.org/10.1080/15320383.2020.1839740

Tian, K., Li, M., Hu, W., Huang, B., & Zhao, Y. (2022). Environmental capacity of heavy metals in intensive agricultural soils: Insights from geochemical baselines and source apportionment. Science of The Total Environment, 819, 153078. https://doi.org/10.1016/j.scitotenv.2022.153078

Zwolak, A., Sarzyńska, M., Szpyrka, E., & Stawarczyk, K. (2019). Sources of soil pollution by heavy metals and their accumulation in vegetables: A review. Water, air, & soil pollution, 230, 164. https://doi.org/10.1007/s11270-019-4221-y

10. References to literature should be corrected according to the journal requirements (see Instruction for Authors) - references must be numbered in order of appearance in the text.

Response        We are thankful for the comments. The instruction recommended ACS ref style and we have to modified accordingly.

11. Conclusions should state the main finding. The conclusion needs improvement - highlight the most important findings and identify the added value of the main finding.
12. Conclusions - consider adding numerical results to this section.

Response for 11-12:   We appreciate your suggestions. We modified the conclusion by providing the quantitative values and limiting word count to 500 or less.

“Conclusion

Thailand, a major sugar exporter, heavily relies on cane sugar for its economy. However, most sugarcane farmers use costly agrochemicals, like mineral fertilizers and pesticides, to boost yield. To align with the national strategy for sustainable growth, there is a growing emphasis on using industrial waste, driving the Bio-Circular Green Economy. This study examines heavy metals levels in the sugarcane plantation soils, considering various fertilization methods, including industrial wastes from sugar-ethanol industries (filter cake and vinasse). We also assess the impact of soil properties and conservation practices on metal accumulations, bioaccumulation in sugarcane leaves, and the promotion of sustainability through waste-based fertilizers.

The study covers 131 sampling sites in four districts in Kanchanaburi, western Thailand, dominated by sugarcane plantations. Spatial analysis revealed that Arsenic (As) often exceeded the national soil quality standard for agriculture of 25 mg kg^-1. According to the Geoaccumulation index (I_geo), soil accumulations of Cd, As, and Zn (median I_geo values of 1.35, 0.04, -0.69, respectively) scattered across the studied area, whereas Pb was concentrated in Bo Phloi sub-district. Factor analysis highlighted natural geochemical processes’ substantial influence on spatial variations in soil metal distribution. Applications of mineral fertilizers and agrochemicals elevated levels of Cd, Hg, As, and Zn in soil. The use of filter cake increased soil content of Cr, Fe, As, Cd, and Mn and altered the soil physicochemical properties towards an organic-rich clayey composition. The molasses-based vinasse enriched soils with more Zn, Cu, and Fe, along with alterations in soil properties towards saline clayey soil.

Spearman correlation analysis indicated that inorganic ions and complexes of clay fraction significantly accounted for the high metal accumulations in soils, primarily through adsorption or precipitation processes. Alkaline and organic-rich soils were strongly associated with As and Mn accumulations, whereas organic matter was solely correlated with soil Hg accumulation. No-tillage or minimum tillage practices strongly associated with high soil Zn and Cd levels, whereas other conservation measures showed less pronounced effects.

Bioconcentration factor (BCF) revealed that the sugarcane generally acted as a Hg accumulator (BCF ~2.32 to 35.72), while the biomass accumulation for Cu (~2.67) and As (~1.64) tended to be prominent in sandy soil. Mineral fertilizers and agrochemicals were the main contributors to these metal accumulations. The BCFs for the other metals in sugarcane leaves were generally < 1, indicating limited phytoextraction potential.

Based on the findings of this investigation, the waste utilization did not significantly impact soil metal levels or enhance the bioconcentration in sugarcane biomass. The combined application of vinasse and filter cake, as a source of organic matter, showed dual benefits by increasing soil metal content and reducing Zn bioconcentration, thereby enhancing cane yield. Additionally, reduced reliance on chemical fertilizers contributes to a diminished Hg bioconcentration, consequently mitigating health risks. However, further research is needed to determine optimal waste application quantities for the optimizing cane yield, ensuring food safety, safeguarding environmental health.”

Round 2

Reviewer 3 Report

All the queries have been adressed by the authors and all the comments have been satisfactorily adressed  with supporting literature.