the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
pH regulates the formation of organosulfates and inorganic sulfate from organic peroxides reaction with dissolved SO2 in aquatic media
Xiaofan Lv
Makroni Lily
Narcisse Tsona Tchinda
Abstract. Organic peroxides (OPs) are an important component of dissolved organic matter (DOM), detected in various aquatic media. Despite their unique functions as redox agents in water ecosystems, the complete mechanisms and factors controlling their transformation are not explicitly established. Here, we evaluate the pH effect on the aqueous-phase reaction of three selected OPs (methyl hydroperoxide (MHP), peracetic acid (PAA) and benzoyl peroxide (BZP)) with dissolved SO2. Results show that due to the presence of hydroperoxyl group in their structures, MHP and PAA preferably form inorganic sulfate and organosulfate (methyl sulfate for MHP and acetyl sulfate for PAA) depending on the pH, while BZP exclusively forms organosulfate (benzoyl sulfate) in the pH range investigated. Moreover, it is seen that the ability for PAA to form inorganic sulfate relative to organosulfate is more pronounced, which is supported by a previous experimental observation. The effective rate constants of the transformation of these peroxides within pH 1 – 10 and 240 K – 340 K ranges exhibit positive pH and temperature dependencies, and BZP is seen to degrade more effectively than MHP and PAA. In addition to the pH impact, it is highlighted that the formation of organic and/or inorganic sulfate strongly depends on the nature of the substituents on the peroxy function. Namely, PAA and BZP are more reactive than MHP, which may be attributed to the electron-withdrawing effects of -C(O)R (R = -CH3 and -C6H5 for PAA and BZP, respectively) substituents that activate the peroxy function. The results further indicate that the aqueous-phase degradation of OPs can adequately drive the change in the chemical composition of DOM, both in terms of organic and inorganic sulfate mass fractions.
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Lin Du et al.
Status: open (until 24 Oct 2023)
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RC1: 'Comment on egusphere-2023-2044', Anonymous Referee #1, 03 Oct 2023
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In this work the authors carried out quantum calculations to explore the aqueous-phase reactions of organic peroxides (MHP, PAA and BZP) with dissolved SO2 or S(IV) species under various pH conditions. The simulated results clearly demonstrated the effects of pH on the major reaction pathways between various organic peroxides with S(IV) species leading to form inorganic sulfate and organosulfate. The potential reaction mechanisms were discussed with details and in general agreed with the existing experimental results. The paper is well written and provide greater mechanistic understanding of the organic peroxides chemistry in aspect of inorganic and organic sulfur formation. I only have a few minor comments
General comments:
For the quantum chemical calculations, what are the concentrations of the reactants used in the calculations? Would the reaction pathways and kinetics potentially affect by the reactant concentrations?
For the reaction pathways, would there be any other possible reaction pathways between the organic peroxides and S(IV) in addition to the ones discussed and considered in the calculations?
In the atmospheric implications, it is very nice the authors to show and discuss the effective rate constants for different reaction systems under different values of pH and temperature and their corresponding lifetimes. the authors have pointed out such reactions may be important for sulfate formation under oxidant-limiting conditions. Can the authors further elaborate this point? Also, could the authors comment the yield of inorganic sulfate and organosulfates in different reaction systems and environmental conditions.
Minor comments.
Line 100, “The diffusion coefficient for a reactant is related to its radius in any medium of viscosity 𝜂 by the Stokes-Einstein approach (Einstein, 1905).” It is not clear what is the viscosity of the aqueous solutions. How the diffusion would affect the reaction pathways if the solutions or aqueous aerosols were hlghly viscous?
Line 115, “The formation of SO2•H2O•MHP is relatively endergonic at 298.15 K and standard concentration of 1 M.” Can the authors elaborate why the concentration of 1M was chosen for the calculations? Would the concentration of the reactants affect the reaction pathways and kinetics?
Line 155, “The high proportion of sulfate relative to methyl sulfate observed by Lind et al. can further be explained by the demonstrated fast hydrolysis of methyl sulfate at acidic pH (Hu et al., 2011) and its effective oxidation by OH radicals (Kwong et al., 2018) to form inorganic sulfate.” Could the authors comment how significance of these two processes in the formation of inorganic sulfate and organosulfates relative to the reactions between organic peroxides and S(IV) species?
Line 183, “In general, the reaction of PAA is much more favorable to the formation of inorganic sulfate than the reaction of MHP at all pH ranges, while the formation of organosulfate is slightly prevented. This is in line with the experimental observation that the reaction of PAA with dissolved SO2 almost exclusively forms inorganic sulfate (Lind et al., 1987).” What would be the ratios of the inorganic sulfate to organosulfate formed upon the reaction of PAA and dissolved SO2 at pH 1.81 – 6.97 and pH > 6.97? Also, what would be ratios for other reaction systems?
Citation: https://doi.org/10.5194/egusphere-2023-2044-RC1
Lin Du et al.
Lin Du et al.
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