Investigating the structure–effect relationships of various natural phenols used as laccase mediators in the biobleaching of kenaf and sisal pulps

Nine phenol derivatives, p-coumaric acid (PC), vanillin (V), acetovanillone (AV), acetosyringone (AS), syringaldehyde (SA), coniferaldehyde (CLD), ferulic acid (FRC), sinapic acid (SNC), and sinapyl aldehyde (SLD) were assayed as laccase redox mediators in the biobleaching of kenaf and sisal pulps. As a general behaviour, the phenolic mediators increased the kappa number (KN) and reduced the brightness of pulps. In particular, these changes were found to depend in a linear manner on the energy of the highest occupied molecular orbital (EHOMO) of the mediators. The phenolic mediator with the lowest EHOMO (PC) led to the highest increase of KN and the lowest reduction of brightness. On the contrary, syringyl derivatives (i.e. SA) with high EHOMO values caused small KN increases and significant losses of brightness. This behaviour was explained on the basis of a competition between grafting and polymerisation processes. The former basically affects KN, whereas the latter affects pulp brightness.