Aquatic Humic Substances as Sources and Sinks of Photochemically Produced Transient Reactants

In sunlit surface waters aquatic humic substances and nitrate act as sensitizers or precursors for the production of photoreactants such as singlet oxygen, humic-derived peroxy radicals, hydrogen peroxide, solvated electrons, and ·OH radicals. Lifetimes of the various reactants are controlled by their reactions with aquatic humic substances (·OH radicals), by solvent quenching (singlet oxygen), by reactions with molecular oxygen (solvated electron), or by other processes (peroxy radicals). The steady-state concentration of each transient formed during solar irradiation was determined from the apparent first-order disappearance rate of added organic probe compounds. The probe compounds used had selective reactivities with the individual transient species of interest. Effects of the photoreactants on the elimination of micropollutants and on chemical transformations of DOM are discussed.