Enzymes in bleaching of kraft pulp: Dissertation

Cellulose pulp produced by the kraft pulping process is the predominant raw material for production of paper. Although most of the wood lignin is removed during kraft pulping the residual brown coloured lignin must be removed and the brightness of the pulp increased in a multistage bleaching processes prior to paper production. Due to their environmental impacts the use of chlorine chemicals in bleaching is decreasing and new, alternative bleaching chemicals and processes are actively being studied. The most feasible biotechnical alternative in the bleaching of kraft pulp is treatment with enzymes. In this work the use of enzymes affecting either lignin or hemicellulose was studied as a pretreatment for bleaching of kraft pulp. Lignin modifying enzymes, lignin peroxidase and laccase, were produced by the white rot fungus Phlebia radiata in carrier-attached submerged bioreactor cultivation. For the production of lignin peroxidase P. radiata required a surfactant and the use of veratryl alcohol increased the production of both activities. P. radiata produced and degraded veratryl alcohol in a similar manner as the most widely studied white rot fungus Phanerochaete chrysosporium. The lignin peroxidases of P. radiata were shown to have similar immunological epitopes as the major peroxidase of P. chrysosporium. The main difference in the lignin degradation systems of the two white rot fungi is the laccase activity which is not produced by commonly used P. chrysosporium strains. The lignin modifying enzymes were shown to effect only slight modification of two macromolecular lignin preparations originating from wood and from the kraft process. Laccase was more effective than lignin peroxidase in degradation of lignin and modification of the water soluble fraction of lingin preparations. These modifications were, however, less pronounced compared with the growing fungus P. radiata. Pretreatment of pulp with lignin modifying enzymes did not increase lignin removal in a subsequent peroxide delignification. On the contrary laccase and lignin peroxidase actually reduced the brightness of kraft and peroxyformic acid pulps, respectively. In the sequential use of hemicellulase and lignin modifying enzymes, however, laccase slightly increased the delignification of kraft pulp in a subsequent chemical bleaching. The idea of using hemicellulases prior to bleaching of kraft pulp was introduced in 1986 as a result of joint research between VTT and KCL (Finnish Pulp and Paper Research Institute). In this work the use of hemicellulases was further studied using several hemicellulase preparations and individual enzymes. Endo-ß-xylanase was shown to be the major enzyme component increasing the bleachability of pulps. Other isolated hemicellulases studied in this work did not affect lignin removal to the same extent as xylanase. A proposal for the mechanism of xylanase aided bleaching was presented. It is known that during kraft pulping xylan is first solubilized in the cooling liquor. In the later stages of the cook xylan is reprecipitated on the pulp fibres. Partial hydrolysis of the reprecipitated xylan fraction was proposed to render the pulp surface more permeable for lignin removal. This hypothesis was verified by comparison of the enzymatic hydrolysis and peroxide delignification of conventional kraft pulps and those produced by the continuous liquor flow-through method (FTD pulps). Although higher amounts of xylo oligomers and xylose were liberated in the enzymatic hydrolysis of FTD pulps, hemicellulase treatment increased the bleachability of conventional kraft pulps more than that of FTD pulps. Further evidence for the validity of the hypothesis was obtained in alkali extraction of hemicellulase treated craft pulp. More and higher molecular mass lignin was extracted from the treated pulp than from the reference pulp. The hydrolysis of reprecipitated xylan was further studied with a model substrate, isolated from a flow through cook. The enzymatic solubilization of the reprecipitated xylan was somewhat lower than that of isolated xylans originating from wood and from kraft pulp. However, the solubilization of isolated xylans was always higher than that of fibre bound xylans. Although processing of wood results in removal of substituents and changes in the arrangement of xylan chains, it was obvious that the main obstacle to the efficient hydrolysis and solubilization of fibre bound xylans was poor penetration of enzymes into the fibre matrix. In xylanase aided bleaching, however, only a limited hydrolysis of xylan at the fibre surface is required for increased extractability of lignin.