Photochemical bleaching of chemical pulps catalyzed by titanium dioxide

A two-stage process for photochemical bleaching of cellulosic pulps is presented. The first, based on the generation of oxygen active species by the photocatalytic action of TiO2, and the second on the photochemical decomposition of hydrogen peroxide. Both stages are carried out under alkaline pH and at 85 degrees C in aqueous suspension at a consistency of 5%. The experiments were performed on kraft (Eucalyptus grandis, Pinus pinaster and Picea mariana), acesolv (E. grandis) and peroxyformic acid (E. grandis) pulps. The presence of TiO, as photocatalyst showed several advantages, such as reduction of reaction time, preservation of the pulp viscosity, increase of the selectivity during the photobleaching and decrease of the consumption of the bleaching chemicals. UV/Vis and FTIR spectroscopies indicate a decrease of the coniferaldehyde structures during the TiO2 photocatalyzed stage, whereas quinones entities were found to remain in the residual lignin even after the final hydrogen peroxide stage. TiO2-photocatalyzed sequence was found to degrade more efficiently the chromophores, especially carbonyl groups, than the sequence carried out in its absence. (C) 1998 Elsevier Science S.A. All rights reserved.