The effects of sunlight on aqueous redox reactions between manganese oxides (MnO(x)) and selected organic substances are reported. No sunlight-induced rate enhancement was observed for the MnO(x) oxidation of substituted phenols, anisole, o-dichlorobenzene, or p-chloroaniline. On the other hand, solar radiation did accelerate the reduction of manganese oxides by dissolved organic matter (DOM) from aquatic environments. The photoreduction of MnO(x) by DOM was little affected by molecular oxygen in air-saturated water (250-mu-M), but was inhibited by 2,6-dichloroindophenol (0.5-6-mu-M), an excellent electron acceptor. MnO(x) reduction also was photosensitized by anthraquinone-2-sulfonate. These results indicate that the photoreduction probably involves electron transfer from excited states of sorbed DOM to the oxide surface. Wavelength studies indicated that ultraviolet-B radiation (280-320 nm) plays an important role in this photoreduction.
