Biobleaching chemistry of laccase-mediator systems on high-lignin-content kraft pulps 1 Fadi S. Chakar and Arthur J. Ragauskas Abstract: A high-lignin-content softwood kraft pulp was reacted with laccase in the presence of 1-hydroxybenzotriazole (HBT), N-acetyl-N-phenylhydroxylamine (NHA), and violuric acid (VA). The biodelignification response with violuric acid was superior to both 1-hydroxybenzotriazole and N-acetyl-N-phenylhydroxylamine. NMR analysis of residual lignins isolated before and after the biobleaching treatments revealed that the latter material was highly oxidized and that the magnitude of structural changes was most pronounced with the laccase – violuric acid biobleaching system. An increase in the content of carboxylic acid groups and a decrease in methoxyl groups were noted with all three laccase- mediator systems. The oxidation biobleaching pathway is directed primarily towards noncondensed C5 phenolic lignin functional structures for all three laccase-mediated systems. The laccase – violuric acid system was also reactive to- wards C5-condensed phenolic lignin structures. Key words: laccase, mediator, lignin, pulp, biobleaching, phenolics. Résumé : On a soumis de la pulpe de papier kraft ayant un degré élevé de lignine à une réaction avec de la laccase en présence de 1-hydroxybenzotriazole (HBT), de N-acétyl-N-phénylhydroxylamine (NHA) et d’acide violurique (AV). La réponse de biodélignification avec de l’acide violurique est supérieure celle observée avec le 1-hydroxybenzotriazole ou la N-acétyl-N-phénylhydroxylamine. L’analyse RMN des lignines résiduelles isolées avant et après les traitements de bioblanchiment révèle que ces derniers matériaux sont fortement oxydés et que l’amplitude des changements structu- raux est maximale avec le système de bioblanchiment avec de la laccase et de l’acide violurique. Avec chacun des sys- tèmes utilisés, on a noté une augmentation de la teneur en groupes acides carboxyliques et une diminution en groupes méthoxyles. Pour les trois systèmes catalysés par la laccase, la voie d’oxydation de bioblanchiment est orientée princi- palement vers les structures fonctionnelles de la lignine phénolique en C5 non condensée. Le système de laccase – acide violurique est aussi réactif vis-à-vis des structures fonctionnelles de la lignine phénolique en C5 condensée. Mots clés : laccase, médiateur, lignine, pulpe, bioblanchiment, phénoliques. [Traduit par la Rédaction] Chakar and Ragauskas 352 Introduction The bleaching of kraft pulps is an integral component of manufacturing high-value products in the pulp and paper in- dustry. Historically, removal of lignin from kraft pulps was accomplished primarily using chlorine or hypochlorous acid. However, because of environmental concerns, these chemi- cal agents have been replaced with the use of chlorine diox- ide, hydrogen peroxide, and oxygen (1). Nonetheless, each of these chemical agents has certain limitations, and research is ongoing for improved lignin removal treatments. One promising future strategy for bleaching kraft pulps involves the use of biological systems (2). Currently, several enzy- matic technologies are being developed for pulp-bleaching operations, including manganese peroxidase, lignin peroxi- dase, and laccase (3, 4). Although all of these biobleaching technologies have advanced over the past decade, laccase biobleaching treatments have achieved some of the most no- table results, yielding +50% delignification of low-lignin content kraft pulps with little oxidative degradation towards pulp carbohydrates (5–7). The high oxidative selectivity of a laccase biobleaching system towards lignin has been shown to provide distinct pulp-yield benefits when high-lignin- content kraft pulps are biodelignified (8–10). It is now well established that laccase delignification of kraft pulps can only occur in the presence of a low- molecular-weight chemical mediator. This is commonly attributed to pulp fiber dimensions, which prevent the diffu- sion of laccase into the fiber to allow for lignin oxidation (11). Hence, laccase biobleaching of pulp fibers involves the oxidation of laccase by O 2 , laccase oxidation of a mediator, and the diffusion of oxidized mediator into the fiber, which then oxidizes the lignin. The majority of the oxidized lignin is removed in a subsequent alkaline extraction stage. The efficacy of a laccase-mediator system (LMS) for delignification is critically dependent on the type of media- tor employed. The first reported mediator for LMS delignification was 2,2′-azinobis(3-ethylbenzothiazoline-6- sulfonate); its bleaching chemistry has been extensively Hexplored (12–14). Additional mediators have since been discovered that significantly improve the overall deligni- fication effect of an LMS-stage (15). Many of these media- Can. J. Chem. 82: 344–352 (2004) doi: 10.1139/V03-195 © 2004 NRC Canada 344 Received 6 June 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 23 February 2004. F.S. Chakar and A.J. Ragauskas. 2 Institute of Paper Science and Technology, Atlanta, GA 30318, U.S.A. 1 This article is part of a Special Issue dedicated to Professor Ed Piers. 2 Corresponding author (e-mail: arthur.ragauskas@ipst.edu).