Photoproduction of Hydrated Electron from Constituents of Natural Waters

The wavelength dependence for the photoproduction of the hydrated electron (e-(aq)) from various humic and fulvic acids and from natural waters was determined, employing a method that converts e-(aq) to a methyl radical that is detected fluorimetrically as the O-methylhydroxylamine of a stable nitroxide. Quantum yields for e-(aq) production from potassium ferrocyanide and N,N-dimethylaniline are in agreement with previously reported values. The quantum yields for production of e-(aq) from colored dissolved organic matter (CDOM) decrease precipitously with increasing wavelength with the rate of decline increasing in the order: humic acid < fulvic acid < natural water in the UV-B region. For Suwannee River fulvic acid, quantum yields ranged from 7.9 × 10-6 at 366 nm to 1.9 × 10-4 at 296 nm indicating that previously reported values for e-(aq) production from CDOM involving laser sources of irradiation are high due to experimental artifact. Apparent natural water quantum yields at 296 nm are higher than those for humic substances, ranging from 9.4 × 10-5 to 3.7 × 10-3 depending on location. The highly absorbing waters of the Delaware and Chesapeake Bays show insignificant production of e-(aq). These results indicate that the hydrated electron, through its reaction with dioxygen, is not a significant source of hydrogen peroxide in many natural waters and that humic substances may not be the principal source of e-(aq) production.