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 (Phi(MHP)) vary from (1.09 +/- 0.09) x 10(-6) to (4.95 +/- 0.11) x 10(-6) in the tested CDOM solutions, including four reference natural organic matters and two effluent organic matters. The quantum yields of H2O2 (Phi H2O2) are simultaneously measured, and the ratios of Phi H2O2 to Phi(MHP), range from 147 to 676. The formation of MHP is highly related to the presence of superoxide radical ions (O-2(center dot-)) and methyl radicals (CH3 center dot); therefore, a photoformation mechanism of MHP has been proposed. The photochemically generated CH3 center dot reacts with O-2 to yield CH3OO center dot. Subsequently, CH3OO center dot is reduced to MHP by O-2(center dot-). Our results also suggest that the yield of CH3 center dot 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 photoFenton reactions. This study can be useful to understand the photochemical formation of organic peroxides in surface waters.

Automated summary

This study found that under simulated solar irradiation, methylhydroperoxide (MHP) is the only identified organic hydroperoxide (ROOH) formed in chromophoric dissolved organic matter (CDOM) solutions, with varying quantum yields. The formation mechanism of MHP involves the presence of superoxide radical ions and methyl radicals. Additionally, the degradation of MHP and H2O2 is similar in terms of photodegradation, hydrolysis, Fenton, and photoFenton reactions, providing insights into the photochemical formation of organic peroxides in surface waters.