Characterisation of selective biodelignification of trembling aspen refiner mechanical pulp by Rigidoporus ulmarius
Creators: name: Dawson-Andoh, Benjamin Ebo
Publisher: name: University of British Columbia
Year: 2010

description: Rigidoporus ulmarius (FT.) Imaz. has been reported briefly in the literature to selectively biodelignify wood both naturally in the field, and also under laboratory conditions. Selective biodelignification of lignocellulosic materials by fungi is influenced to a great extent by the environmental or cultural factors employed. The effect ofthe following cultural factors: aeration; exogenous addition of hydrogen peroxide; concentration of mineral solution; addition of a surfactant (tween 80); and exogenous addition of veratryl alcohol on selective biodelignification of aspen (Populus tremuloides Michx.) refiner mechanical pulp (RMP) by R. ulmarius was studied. The objective was to optimise the depletion of lignin by R. ulmarius while maintaining the carbohydrate level, especially cellulose, at its original level. The quantitative success of this objective was defined as the selective biodelignification index (SBI), defined as the ratio of total carbohydrates to the total residual lignin. Fungal-treated pulps with the highest SBI and the greatest lignin depletion were compared with controls by physical means on handsheets. This study represents the first systematic attempt to determine the effect of combinations of several cultural factors on selective biodelignification of a lignocellulosic substrate by R. ulmarius. Also, a novel aspect was the exogenous addition of veratryl alcohol and hydrogen peroxide to the culture at the estimated beginning of the secondary metabolic phase of the fungus, because lignin biodegradation is hypothesized to commence during this phase, which is represented by the cessation of fungus growth. Optimal conditions for selective biodelignification of aspen RMP included addition of mineral solution (Kirk-Schultz mineral solution, eight-fold concentration) and oxygen flushing in the absence of other chemical additives or nitrogen supplements. Pulps given this treatment were characterised by residual contents of 14.8% lignin, 51.5% glucose and 15.1% xylose. This was equivalent to a lignin reduction of 30.8%, and a xylan depletion of 23.7%, without proportional reduction in cellulose content. Consequently, the SBI was 4.5, versus 3.2 for the untreated controls. Other cultural conditions caused even greater lignin loss(up to 50.4 %), but these treatments were accompanied by severe cellulose and hemicellulose reduction, leading to pulps with impaired strength properties. In general, mineral solution (eight-fold concentration) enhanced selective biodelignification and lignin biodegradation under oxygen flushing. Similarly, exogenous addition of veratryl alcohol to the culture also enhanced selective biodelignification. However, exogenous addition of hydrogen peroxide to treatments containing other factors had a common effect of limiting selective biodelignification. Under high oxygen flushing, both lignin and carbohydrate biodegradation was enhanced to the same extent by exogenous addition of tween 80 (0.05%), with the result that SBI was not much affected. The optimum pH and incubating temperature for selective biodelignification were 4.5 and 28 °C, respectively. Biodegradation of pulp was also attended by acidification of medium. Under optimal selective biodelignification conditions, significant changes in some optical properties of pulps were noted. Brightness increased by 7.4%,while opacity decreased by 0.3%. Modest but significant improvement in some strength properties of handsheets made from this pulp was observed. Dry zero-span breaking length increased by 4.4%, while burst and tensile indices also increased by 16.8% and 28.9% respectively. The weighted average fiber length of pulps was reduced from 1.17 mm for the controls to 1.12 mm for pulps given the optimal treatment. This represented 4.2% reduction in weighted average fiber length. Fines content of such pulps were reduced by a negligible 3.2%.