Evidence for photochemical generation of superoxide ion in humic waters

Abstract

On irradiation with visible or near UV light the coloured substances in natural humic waters can undergo several reactions. They may be bleached and partially degraded1–3, perhaps by a cyclic mechanism involving oxidation and reduction of iron4, or they may generate a variety of free radicals5 or serve as sensitizers for the generation of singlet oxygen6–8. They may also bring about the generation of hydrogen peroxide9–11. It has been suggested10,11 that the actual photochemical process may be the generation of superoxide ion, and that hydrogen peroxide is produced by the dismutation of this. We present here more direct evidence for the photochemical generation of superoxide ion, O2·−, in natural waters.