The effect of amino acids on the Fenton and photo-Fenton reactions in cloud water: unraveling the dual role of glutamic acid

In this work, glutamic acid (Glu) was selected as a model amino acid (AA) 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 the 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 the 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), which 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.