Fenton-like Reactions in Acidic Environments: New Mechanistic Insights and Implications to Atmospheric Particle-Phase Chemistry

Fenton and Fenton-like processes are important oxidation cycles in the atmospheric aqueous particle phase, yet their mechanisms in the presence of iron complexes remain incompletely understood. This study investigated Fenton-like reactions in the presence of oxalate from pH 2 to 5. The rate constant for the reaction of FeC2O4 with H2O2 (Fenton-like) was derived as (3.2 ± 0.3) × 103 M–1 s–1. Thermochemical analysis indicated a significant increase in activation entropy (ΔS‡ = −6.0 ± 0.8 for Fenton-like versus −87 ± 9 J K–1 mol–1 for the Fenton in the absence of oxalate). We propose the formation of a monodentate [Fe(HC2O4)]+ complex with estimated log K = 1.3 ± 0.2 to account for the behavior of measured second-order rate constants at pH ≤ 3. The rate constant for the reaction between [Fe(HC2O4)]+ and H2O2 was estimated at (2.7 ± 0.8) × 103 M–1 s–1. The Fe(II) regeneration observed in the presence of phenolic compounds was found to be relevant only at lower initial oxalate concentrations, when oxalate complexes represented less than 1% of the total Fe(II). These findings demonstrate how oxalate modifies the Fenton-like mechanism, amplifying its role in the aqueous particle-phase chemistry.