Application of laccase-based systems for biobleaching and functionalization of sisal fibres

Abstract

This research project originated from interest in assessing the potential of enzyme technology (particularly laccase-based systems) for the biomodification of sisal specialty fibres by using environmentally friendly processes. This doctoral work focused on two different research lines, namely: biobleaching and enzymatic functionalization of sisal pulp fibres. The study was started by assessing the use of natural, potentially cost-effective phenolic compounds as substitutes for expensive, potentially toxic laccase mediators. The tendency of natural phenols to either promote delignification or couple onto pulp was examined with a view to assessing their potential for either bleaching or functionalizing sisal fibres. In the biobleaching study, totally chlorine free (TCF) sequences were implemented in order to compare the efficiency of a selected natural mediator and a well-known synthetic mediator, both in the presence and absence of a xylanase pre-treatment. The effluents resulting from each stage in the sequence were analysed with a view to assessing the environmental impact of the laccase treatments ¿a scarcely explored aspect of biobleaching sequences. The xylanase stage proved highly efficient in reducing the HexA content of sisal fibres and in boosting the bleaching effect of the laccase treatments. The proposed TCF sequences provided high-cellulose sisal pulp with brightness above 80% ISO and a reduced HexA content; also, they exhibited improved performance and a reduced impact on effluent properties relative to the use of the synthetic mediator. Two different approaches to fibre functionalization were explored, namely: lignin modification (biografting) and cellulose modification (laccase¿TEMPO oxidation). Biografting of phenolic compounds was for the first time studied in sisal pulp. Covalent binding of the originally assayed phenolic compounds to sisal fibres during the laccase treatment was exposed by a novel analytical approach based on pyrolysis-GC/MS. The phenolic compound showing the highest tendency to couple to fibres was selected to investigate biografting under different reaction conditions and to evaluate the extent of phenol coupling via various pulp properties. Biografting efficiency was enhanced by refining the fibres prior to the enzyme treatment, which provided improved strength-related properties in the resulting paper. The use of the laccase-TEMPO system to oxidatively modify cellulose and improve strength-related properties in sisal pulp was for the first time evaluated as an environmentally friendly alternative to existing halide-based systems. The first part of this study revealed that the laccase¿TEMPO system considerably improved wet strength in sisal pulp by effect of the formation of a substantial amount of aldehyde groups in cellulose chains that facilitated inter-fibre bonding through hemiacetal linkages. The influence of process variables on various properties of the oxidized fibres and resulting paper was assessed by using a three-variable statistical plan. The conditions maximizing functionalization and the improvement in paper strength properties were used to design treatments of increased efficiency that exposed the potential of laccase¿TEMPO oxidation for biorefining pulp fibres. Analytical methods including pyrolysis-GC/MS, polyelectrolyte titration, conductimetric titration, carbohydrate determination by HPLC, fibre morphology analysis by SEM and thermogravimetry were used to both characterize the raw material and gain a better understanding of the reaction mechanisms behind the different laccase-based treatments. Some of the analyses were performed by collaborating research groups at IRNAS (Seville, Spain) and the Department of Chemical Engineering of the University of Huelva (Spain). Also, part of this doctoral work was conducted at the Institute of Paper Science and Technology of the Georgia Institute of Technology (Atlanta, USA).