the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 May 2025
The Effect of Amino Acids on the Fenton and photo-Fenton Reactions in Cloud Water: Unraveling the Dual Role of Glutamic Acid
Abstract. In this work, Glutamic acid (Glu) was selected as a model amino acid (AAs) to investigate its complexation with Fe(III) and Fe(II), focusing on its impact on the Fenton reaction and the photolysis of Fe(III) in cloud aqueous phase. Glu was found to enhance the rate constant for the reaction of Fe(II)-Glu with H2O2 to 1.54±0.13×104 M-1 s-1, which is significantly higher than that of classic Fenton reactions (~50–70 M-1 s-1). In contrast, the photolysis quantum yield of Fe(III)-Glu complex was determined to be 0.037 under solar simulated irradiation, largely lower than Fe(III)-hydroxy complexes (0.216). In the overall process (Fenton or Fe(III) photolysis), it was found that •OH formation decreased in the presence of Glu. Additionally, the fate of Glu in the presence of Fe(III) was investigated as well as the oxidation process (driven by •OH and ligand-to-metal charge transfer (LMCT) reaction) led to the formation of short-chain carboxylic acids and ammonium under simulated solar light. Interestingly, these two processes generated different primary short-chain carboxylic acids, indicating distinct mechanisms. This study provides valuable insights into the role and fate of amino acids in atmospheric chemistry, helping to further understand their impact on atmospheric processes.
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RC1: 'Comment on egusphere-2025-1744', Anonymous Referee #1, 27 May 2025
This is a well-written and comprehensive study investigating the complexation of glutamic acid (Glu) with Fe(II)/Fe(III) and its implications for atmospheric chemistry, particularly in cloud water. The experiments are well-designed, and the results provide valuable insights into the role of amino acids in modifying Fenton and photo-Fenton reactions. The findings are novel and contribute significantly to our understanding of atmospheric oxidative processes.
The atmospheric relevance could be further emphasized by discussing how variations in cloud pH, light intensity, or iron/ligand ratios might influence the observed processes.
It would be better to explicitly state in the first paragraph of “Introduction’ that the role of amino acids in modifying iron redox chemistry and OH production remains poorly understood.
Were the pH conditions (3.8–5.6) chosen to represent specific atmospheric scenarios (e.g., polluted vs. remote clouds)? A brief justification would be useful.
For the photolysis experiments, was the light spectrum adjusted to match real solar conditions?
The reported rate constant (1.54 × 10⁴ M-1 s-1) is a key finding. However, how does this compare with other Fe(II)-organic complexes (e.g., oxalate, citrate)? A brief discussion would be useful.
The detection of formate/acetate as primary LMCT products is interesting. Could these compounds further complex Fe(III) and influence subsequent reactions?
The discussion of Fe-Glu fractions in cloud water is insightful, but how might these vary in highly polluted vs. marine environments?
How might these findings affect our understanding of SOA formation?
Citation: https://doi.org/10.5194/egusphere-2025-1744-RC1 -
AC1: 'Reply on RC1', Marcello Brigante, 23 Jun 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1744/egusphere-2025-1744-AC1-supplement.pdf
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AC1: 'Reply on RC1', Marcello Brigante, 23 Jun 2025
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RC2: 'Comment on egusphere-2025-1744', Anonymous Referee #2, 10 Jun 2025
The article presents a detailed investigation into the potential effect of glutamic acid (Glu)-iron complexes on Fenton and photo-Fenton reactions in cloud water and on the hydroxyl radical yield. Using Glu as a model amino acid, the study offers compelling insights into how these organic compounds may interfere with iron (photo)chemistry leading to an impact on the oxidative potential of cloud water.
Overall, this study makes a valuable contribution to our understanding of aqueous-phase atmospheric chemistry. I have just few comments.
- It would be interesting to compare the results with other well-known iron complexes with organic ligands that might be relevant for cloud water (for example Fe-oxalate)
- It would be good to further discuss the potential of forming the Glu-iron complexes in real cloud water. Are there other compounds that may complex strongly the iron? Since the Glu is only a model for AAs, I think the work is relevant because other AA-iron complexes may behaviour in the same way.
- In section 2.2.1 please add the concentration of H2O2 used in the experiments (line 131).
- In Figure 1 the letters (a, b, c and d) referred to the different plots are not very visible, probably it would be good to put them outside the plots.
- From Figure 3 and Figure MS6 the degradation of Glu with and without H2O2 seems very similar, please report also the data about the irradiation of Glu alone.
- In the figure caption for Figure 3 is described only the left panel and not the right panel, please add it.
- Page 19, lines 389-391, please add the reactivity constant between formic acid and hydroxyl radical.
- I suggest to change the colours of the lines in the figure 4a, to avoid confusion with the other part of the figures (Figure4b,c,d) where for each colour indicates a specific carboxylic acid.
- In page 22 you discuss the TOC results, have you also performed IC (inorganic carbon) analysis? From that you could also directly measure the mineralization of the Glu. If you have these data, please add them in the manuscript (or in the SI).
- Page 23, lines 456-457, please add a reference for the deamination process as you did for the other reaction involved in the overall mechanism.
Citation: https://doi.org/10.5194/egusphere-2025-1744-RC2 -
AC2: 'Reply on RC2', Marcello Brigante, 23 Jun 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1744/egusphere-2025-1744-AC2-supplement.pdf
Status: closed
-
RC1: 'Comment on egusphere-2025-1744', Anonymous Referee #1, 27 May 2025
This is a well-written and comprehensive study investigating the complexation of glutamic acid (Glu) with Fe(II)/Fe(III) and its implications for atmospheric chemistry, particularly in cloud water. The experiments are well-designed, and the results provide valuable insights into the role of amino acids in modifying Fenton and photo-Fenton reactions. The findings are novel and contribute significantly to our understanding of atmospheric oxidative processes.
The atmospheric relevance could be further emphasized by discussing how variations in cloud pH, light intensity, or iron/ligand ratios might influence the observed processes.
It would be better to explicitly state in the first paragraph of “Introduction’ that the role of amino acids in modifying iron redox chemistry and OH production remains poorly understood.
Were the pH conditions (3.8–5.6) chosen to represent specific atmospheric scenarios (e.g., polluted vs. remote clouds)? A brief justification would be useful.
For the photolysis experiments, was the light spectrum adjusted to match real solar conditions?
The reported rate constant (1.54 × 10⁴ M-1 s-1) is a key finding. However, how does this compare with other Fe(II)-organic complexes (e.g., oxalate, citrate)? A brief discussion would be useful.
The detection of formate/acetate as primary LMCT products is interesting. Could these compounds further complex Fe(III) and influence subsequent reactions?
The discussion of Fe-Glu fractions in cloud water is insightful, but how might these vary in highly polluted vs. marine environments?
How might these findings affect our understanding of SOA formation?
Citation: https://doi.org/10.5194/egusphere-2025-1744-RC1 -
AC1: 'Reply on RC1', Marcello Brigante, 23 Jun 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1744/egusphere-2025-1744-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Marcello Brigante, 23 Jun 2025
-
RC2: 'Comment on egusphere-2025-1744', Anonymous Referee #2, 10 Jun 2025
The article presents a detailed investigation into the potential effect of glutamic acid (Glu)-iron complexes on Fenton and photo-Fenton reactions in cloud water and on the hydroxyl radical yield. Using Glu as a model amino acid, the study offers compelling insights into how these organic compounds may interfere with iron (photo)chemistry leading to an impact on the oxidative potential of cloud water.
Overall, this study makes a valuable contribution to our understanding of aqueous-phase atmospheric chemistry. I have just few comments.
- It would be interesting to compare the results with other well-known iron complexes with organic ligands that might be relevant for cloud water (for example Fe-oxalate)
- It would be good to further discuss the potential of forming the Glu-iron complexes in real cloud water. Are there other compounds that may complex strongly the iron? Since the Glu is only a model for AAs, I think the work is relevant because other AA-iron complexes may behaviour in the same way.
- In section 2.2.1 please add the concentration of H2O2 used in the experiments (line 131).
- In Figure 1 the letters (a, b, c and d) referred to the different plots are not very visible, probably it would be good to put them outside the plots.
- From Figure 3 and Figure MS6 the degradation of Glu with and without H2O2 seems very similar, please report also the data about the irradiation of Glu alone.
- In the figure caption for Figure 3 is described only the left panel and not the right panel, please add it.
- Page 19, lines 389-391, please add the reactivity constant between formic acid and hydroxyl radical.
- I suggest to change the colours of the lines in the figure 4a, to avoid confusion with the other part of the figures (Figure4b,c,d) where for each colour indicates a specific carboxylic acid.
- In page 22 you discuss the TOC results, have you also performed IC (inorganic carbon) analysis? From that you could also directly measure the mineralization of the Glu. If you have these data, please add them in the manuscript (or in the SI).
- Page 23, lines 456-457, please add a reference for the deamination process as you did for the other reaction involved in the overall mechanism.
Citation: https://doi.org/10.5194/egusphere-2025-1744-RC2 -
AC2: 'Reply on RC2', Marcello Brigante, 23 Jun 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1744/egusphere-2025-1744-AC2-supplement.pdf
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