Biomedical Engineering Reference
In-Depth Information
new reaction channels and the synthesis of new cellulose functional materials.
To improve the enzymatic hydrolysis rate of straw and extracted xylose from
the hydrolysates, Zhu et al. [
22
,
23
] adopted three ways to deal with straw:
microwave/alkali, microwave/acid/alkali, and microwave/acid/alkali/H
2
O
2
.Straw
pretreated by microwave/acid/alkali/H
2
O
2
had the highest weight loss, cellulose
content, and enzymatic hydrolysis rate. In microwave/alkali pretreatment, xylose
cannot be recycled, and in microwave/acid/alkali and microwave/acid/alkali/H
2
O
2
pretreatment, xylose crystals can be obtained from hydrolysates. Tang et al. [
24
]
pointed out that ultrasound pretreatment changes the morphological structure,
accessibility, and oxidative activity of the lignocellulose, and the accessibility
of cellulose increased with microwave processing time.
For now, this method is still in the laboratory stage. Because of higher processing
costs, microwave pretreatment is difficult to realize for industrial applications.
4.3.1.4
Radiation Treatment
In the production process of viscose fiber and acetate fiber, high-energy rays such as
electron beam and
-rays are widely used to pretreat the lignocellulose raw materials
to obtain the desired polymerization degree and increase the activity of cellulose,
which would reduce wastewater and environmental pollution caused by the addition
of chemicals. The role of ionizing radiation, on the one hand, is to depolymerize the
cellulose, decrease the degree of polymerization (DP), change the molecular weight
distribution, and make the molecular weight distribution more concentrated than for
ordinary cellulose. On the other hand, ionizing radiation increases the activity and
accessibility of cellulose by loosening the cellulose structure and affecting its crystal
structure [
25
]. Therefore, in the production of viscose fibers, radiation treatment
with the dissolution pulp can improve the reaction activity of the cellulose-generated
viscose. The content of NDF cellulose, acid detergent fiber (ADF), acid-insoluble
lignin (ADL), and reducing sugar in the cell wall would be reduced under high-
degree radiation, thereby enhancing the digestibility of straw [
26
]. Stavtsov [
27
]
studied the molecular weight distribution of irradiated cellulose and found it is
more homogeneous than that without irradiation. Owing to the irregular cleavage
of radiation, the electron beam can penetrate into not only the noncrystalline
regions of cellulose but also the crystalline regions. Radiation (or electron beam)
combined with chemical methods to pretreat straw and other agricultural waste can
increase the degradation of cellulose, hemicellulose, and lignin better than one of the
methods adopted separately. The same conclusion was obtained no matter whether
the agricultural waste was pretreated with high radiation doses (up to 500 kG) and
low concentrations of chemical reagents (up to 5 %) or a low radiation dose and high
concentrations of chemical reagents. Siriwattana [
28
]used10kGy
”
-ray to pretreat
agricultural waste, which was dealt with by 20 % urea in advance; the degradation
of NDF, ADF, ADL, cellulose, hemicellulose, lignin, and cutin were significantly
higher than by urea pretreatment.
”
