Biobleaching of kraft pulp with recombinant manganese peroxidase

Recombinant manganese peroxidase (rMnP) produced from the yeast Pichia pastoris in high cell density, fed-batch fermentations was tested for its ability to degrade lignin in kraft pulps. Softwood kraft pulp, hardwood kraft pulp and oxygen delignified hardwood kraft pulp were used to investigate the enzyme effectiveness in treating pulps with different amount of initial lignin or lignin structure. In all cases, a significant reduction in kappa number was observed by including rMnP at a dosage of 30U/g pulp. No difference was observed between softwood kraft pulp and hardwood kraft pulp in the delignification rates. High lignin content (lightly cooked) kraft pulps showed similar deligninfication rates as the normal kraft pulps. However, the highly delignified pulps, whether it is oxygen delignified or overcooked to kappa numbers below 10, are less susceptible to delignification by rMnP compared to the normal pulps. Repeated rMnp treatments combined with alkaline extraction gave more extensive delignification. It reduced kappa number by 61%, which is about three times of a single rMnP treatment. A sequential enzymatic beaching with xylanase and rMnp provided higher kappa number reduction compared to the individual enzymatic treatment and the simultaneous treatment with both enzymes.; The effects of Manganese ion, hydrogen peroxide on the oxidation of 2,6-dimethoxyphenol (DMP) were investigated to establish the optimal application of rMnP oxidative system. Manganese ion showed a positive effect on the reaction rate, which was dependent on the concentration of hydrogen peroxide. High concentration of hydrogen peroxide caused partial inactivation of enzyme and as a result reducing the positive effect of manganese concentration on the oxidation rate. Concentrations of 0.8 mM manganese ion and 0.05 mM hydrogen peroxide were found to maximize the catalytic properties of rMnP for the oxidation of DMP.; The effects of hydrogen peroxide, Mn2+ concentration, organic acid chelating agent, surfactant, pH, enzyme dosage and pulp concentration on the lignin degradation during biobleaching of hardwood kraft pulp with rMnP were evaluated. 10mM H2O2 supplied semi-continuously (pulse concentration 0.1mM) provided the best bleaching results in terms of kappa number reduction and pulp brightness improvement. Higher concentrations of H2O2 caused a significant loss of rMnP activity during bleaching. The optimal Mn2+ concentration for lignin oxidation was in the range of 0.1∼0.2 mM. Organic acids such as malonate and oxalate stimulated lignin degradation, whereas succinate and acetate showed no effect on delignification. The optimal pulp concentration for rMnP biobleaching was below 2%. Higher pulp concentration resulted in less bleaching efficiency by rMnP.; rMnP also were tested for its ability to bleach kraft pulp in total chlorine free (TCF) and elemental chlorine free (ECF) bleaching sequences. For TCF bleaching, the sequence used was O(rMnP)QP where O stands for oxygen delignification, rMnP for the enzymatic treatment, Q for the chelating agent stage and P for the alkaline peroxide stage. The inclusion of the enzymatic treatment significantly improved the final brightness of the hardwood kraft pulp compared to the control pulp. Furthermore, the treatment with rMnP resulted in a significant energy saving during pulp refining with PFI mill with a slight improvement in pulp strength properties such as tensile index and tear index. For ECF bleaching, the sequence used was DED for hardwood kraft pulp and DEDED for softwood kraft pulp, where D stands for chlorine dioxide stage and E for alkaline extraction stage. Three-time repeated rMnP treatments combined with alkaline extraction were applied as pretreatment prior to DED or DEDED bleaching sequences, and the total effective chlorine required to obtain a pulp of 88% ISO brightness was reduced by 41% and 30% for hardwood kraft pulp and softwood kraft pulp respectively, compared to the conventional...