Biomedical Engineering Reference
In-Depth Information
case of high brightness, the brightness value added was about 2 % ISO (Interna-
tional Organization for Standardization)). When achieving the same brightness, the
amount of Cl
2
was decreased by 14.2 %, and Cl
2
O was decreased by 16.1-18.7 %,
decreasing the pollution load.
(2) LCC is decomposed by hemicellulase
LCC is a complex of lignins and carbohydrates. Part of the LCC naturally exists
in pulp materials, and the other part is formed in the pulping process. The other
role of hemicellulase in the biobleaching process is to decompose the hemicellulose
in LCC, reducing the degree of polymerization and volume and contributing to the
dissolution of the residual lignin. Some scholars treated spruce KP with xylanase
or acid and extracted the KP by alkali or DMSO, then studied the molecular
weight distribution of carbohydrates and lignin. They found that the molecular
weight of the treated samples was smaller than the untreated samples, indicating
that hemicellulose in the LCC can be degraded by the xylanase and was easier to
dissolve.
(3) Synergism of xylanase and other enzymes
①
Synergism of xylanase and galactoglucomannan enzyme
The previous discussion indicated that the recovery and deposition of galac-
toglucomannan are less on the surface of KP fiber. The dissolution rate of
galactoglucomannan pretreated by galactoglucomannan enzyme was low (about
5 %). The dissolution rate of galactoglucomannan pretreated by xylanase and
then galactoglucomannan enzyme was six times higher than the dissolution rate
without xylanase pretreatment. The reason is that hemicellulose is absorbed and
deposited in KP fiber surface mainly contains xylan. The xylan is dissolved by
xylanase, increasing the permeability of the fiber surface and then is treated by
galactoglucomannan enzyme, which is conducive to dissolving galactoglucoman-
nan in the LCC and KP fibers.
②
Synergism effect of xylanase and ligninolytic enzymes
The xylan of pulp fibers is pretreated by xylanase to increase the permeability of
the fiber surface and reduce the limitation and shielding effects of the diffusion
of enzymes. Then, the ligninolytic enzyme promotes the decomposition of lignin
and LCC, which is conducive to the dissolution of the residual lignin.
As can be seen from the results in Table
8.6
, xylanase treatment may improve
the brightness by 2.6 %, and then ligninolytic enzyme treatment may improve
the brightness by 3.6 %. Ligninolytic enzyme treatment of lignin alone may
increase the brightness only by 2.3 %. This shows that xylanase is helpful for
delignification [
34
].
8.3.2.2
Mechanism Model of Xylanase-Assisted Delignification
From the previous analysis, in the pulping process, when the solution concentration
is high, the xylan first would be dissolved in the cooking black liquor. In later
