Photochemical Formation of Methylhydroperoxide in Dissolved Organic Matter Solutions

Although it is known that the solar irradiation of chromophoric dissolved organic matter (CDOM) solutions generates H₂O₂, whether or not organic hydroperoxides (ROOHs) are photochemically formed remains unclear. This study employs high-performance liquid chromatography with the postcolumn enzymatic derivatization method to examine whether ROOHs can be formed in CDOM solutions under simulated solar irradiation. Methylhydroperoxide (MHP) is the only identified ROOH under our experimental conditions, and the quantum yields of MHP (Φ_MHP) vary from (1.09 ± 0.09) × 10^-6 to (4.95 ± 0.11) × 10^-6 in the tested CDOM solutions, including four reference natural organic matters and two effluent organic matters. The quantum yields of H₂O₂ (Φ_H) are simultaneously measured, and the ratios of Φ_H to Φ_MHP range from 147 to 676. The formation of MHP is highly related to the presence of superoxide radical ions (O₂^•-) and methyl radicals (CH₃^•); therefore, a photoformation mechanism of MHP has been proposed. The photochemically generated CH₃^• reacts with O₂ to yield CH₃OO^•. Subsequently, CH₃OO^• is reduced to MHP by O₂^•-. Our results also suggest that the yield of CH₃^• to MHP under air-saturated conditions is 52% and increases to 98% under oxygen-saturated conditions. The decays of MHP and H₂O₂ are very similar in terms of photodegradation, hydrolysis, Fenton, and photo-Fenton reactions. This study can be useful to understand the photochemical formation of organic peroxides in surface waters.