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 specie s by the photocatalytic action of TiO2, and the second on the photoche mical decomposition of hydrogen peroxide. Both stages are carried out under alkaline pH and at 85 degrees C in aqueous suspension at a consi stency of 5%. The experiments were performed on kraft (Eucalyptus gran dis, Pinus pinaster and Picea mariana), acesolv (E. grandis) and perox yformic acid (E. grandis) pulps. The presence of TiO, as photocatalyst showed several advantages, such as reduction of reaction time, preser vation of the pulp viscosity, increase of the selectivity during the p hotobleaching and decrease of the consumption of the bleaching chemica ls. UV/Vis and FTIR spectroscopies indicate a decrease of the conifera ldehyde structures during the TiO2 photocatalyzed stage, whereas quino nes entities were found to remain in the residual lignin even after th e final hydrogen peroxide stage. TiO2-photocatalyzed sequence was foun d to degrade more efficiently the chromophores, especially carbonyl gr oups, than the sequence carried out in its absence. (C) 1998 Elsevier Science S.A. All rights reserved.