From Methyl Thioethers to Methanesulfonic Acid: Direct and Indirect Photodegradation Pathways in Aquatic Environments

Abstract

The purpose of this research is to investigate the photochemical degradation of methyl thioethers in aquatic systems, with a specific focus on understanding the mechanisms leading to methanesulfonic acid (MSA) production. The model methyl thioethers selected for this purpose were, methionine (Met) and 3-(methylthio)benzoic acid (3-BA-SMe), the first is natural and alkyl while the second is synthetic and aromatic. The motivation beyond this research comes from the observation that sulfur-containing compounds are rapidly photochemically degraded in natural waters. In most cases, sulfate is the dominant sulfur-containing product, but methyl thioethers have been shown to generate MSA as the main photostable sulfur product. This represents a previously unrecognized formation route for MSA in the environment, which had been thought to primarily be derived from gas-phase oxidation of dimethylsulfide. Given the novelty and potential importance of this aqueous-phase route to MSA, the goal of this thesis is to better understand the underlying chemical mechanisms involved in this transformation. This thesis explores the aquatic formation pathways of MSA through the photosensitized degradation of Met, detailed in Chapter 2. An investigation in the involvement of the triplet excited states and singlet oxygen pathways in Met photodegradation using two model aromatic ketone photosensitizers was elaborated in Chapter 3. Additionally, Chapter 4 focuses on the MSA formation through direct photolysis of 3-BA-SMe and its sulfoxide derivative. This research uncovers that both direct and indirect photolysis are possible pathways for MSA formation. Chapter 2 examines Met's photodegradation in diverse dissolved organic matter (DOM) systems. The consistent formation of MSA and sulfate as photoproducts underscores the generality of the sensitizing capacities of DOM across different environmental contexts. It was found that different DOM samples gave similar product patterns with differences in the relative yields of MSA and other products. Further experiments with aromatic ketone sensitizers that model DOM’s photoreactivity validated the role of the S-methyl group of Met in MSA production. In addition, considerable effort was put into identifying as many products from Met photodegradation as possible using both HPLC- and NMR-based methods. Kinetic modeling of the product development was used to derive a detailed mechanism for Met's degradation, emphasizing the involvement of triplet excited states and singlet oxygen pathways. These findings contribute to a deeper understanding of the complex interplay between organic matter and photochemical processes controlling the fate of Met in natural waters. Chapter 3 delves into the impact of aromatic ketone sensitizers, specifically 3-methoxyacetophenone (3MAP) and 4-carboxybenzophenone (4-CBBP), on Met's degradation kinetics. The relative role of triplet states of the sensitizers was tested by changing the concentration of the main triplet state quencher, dissolved oxygen. Varying the oxygen levels revealed that Met reacts directly with the excited triplet state of 4-CBBP. The role of singlet oxygen was similarly examined by changing the solvent to a H2O/D2O mixture, which increases the singlet oxygen concentration in solution. These studies showed that 3MAP promotes Met’s degradation by acting as a singlet oxygen sensitizer. Transient absorption measurements gave additional insights into the reactivity of 3MAP with Met. The findings in this chapter underscore the importance of considering different sensitizing mechanisms when studying photochemical processes in aquatic environments. Chapter 4 showed that direct photolysis is the principal photoprocess for 3-BA-SMe, resulting in the formation of its sulfoxide counterpart, 3-(methylsulfinyl)benzoic acid (3-BA-SOMe), and subsequent MSA production. Product identification and kinetic modeling proved valuable in elucidating key aspects of the chemical mechanism, including determining the contributions from both singlet and triplet excited states. These findings deepen our understanding of the photochemical fate of methyl thioethers and highlight the complex interplay between environmental factors and photodegradation processes. In conclusion, this thesis provides a comprehensive examination of the photodegradation of methyl thioethers and the subsequent formation pathways of methanesulfonic acid in aquatic environments. By elucidating the mechanisms involved and highlighting the influence of environmental factors, this research contributes to our understanding of photochemical processes and their implications for environmental chemistry of sulfur-containing compounds. Furthermore, these findings underscore the need for continued research in this area to address remaining questions regarding the role of photochemistry in the aquatic sulfur cycle.