Formation and involvement of superoxide (O-2 center dot(-)/HO2 center dot) and hydroxyl (OH center dot) radicals in TCF bleaching processes: A review

The chemical reactions taking place during TCF bleaching processes are reviewed with special reference to the integral role of the radical species (OH . and O2 .(-)/HO2 .) involved. It is shown that these radical species are responsible for different reaction types: Hydroxyl radicals introduce radical sites into the substrate. These substrate radicals may be oxidized, may disproportionate or may combine with superoxide radicals. Superoxide radicals, after coupling with substrate radicals, bring about fission of C-C bonds and thereby open aromatic rings and cleave ring-conjugated double bonds. In addition, they are intermediates in the formation of hydrogen peroxide and hydroxyl radicals. The generation of these two radical species in the course of the four one-electron reduction steps of oxygen to water and their involvement in oxygen and hydrogen peroxide bleaching is tentatively summarized in Scheme 1. Experimental support for the validity of this scheme is provided. Scheme 2 describes the course of events during ozone bleaching. This scheme includes, in addition to the main oxygen bleaching reactions outlined in Scheme 1, also the reactions specific for ozone. Based on the reaction modes of superoxide and hydroxyl radicals, the common features of TCF bleaching processes are elaborated.