Environmental Engineering Reference
In-Depth Information
A result of this unique interaction is that, at 220°C under microwave treatment,
it was found that glucose yield was nearly 50 times higher than that under
similar conventional hydrolysis conditions. According to the proposed model,
the yield of glucose obtained below 220°C is only related to the depolymeriza-
tion of amorphous cellulose and not the crystalline content that becomes active
above 220°C.
It has also been found that glucose yield under microwave conditions is power
density dependent, suggesting that the microwave activation/decomposition of
cellulose has a strong kinetic dimension. These are thought to result from two
competitive processes determined by the speed of the CH
2
OH group rotation: (1)
acceleration by interaction with microwave photons; and (2) deceleration through
interaction (e.g., collision, electromagnetic) with neighboring groups. The
dominance of either process depends on the degree of freedom of the CH
2
OH
groups. For depolymerization of cellulose to occur, the CH
2
OH groups need to
acquire the activation energy necessary to provoke the proposed SN
2
reaction
described above. This can already be achieved at high microwave power densities,
while more elevated temperatures are required to liberate the CH
2
OH groups
when using lower microwave densities.
3.2.5
Biological Pretreatment
3.2.5.1
Fungal Digestion
Although thermal chemical methods provide efficient means for biomass pretreat-
ment to reduce the recalcitrance of the lignocellulosic biomass, these methods
require expensive equipment, the use of corrosive chemicals, and intensive energy
consumption, which limits their industrial applications and may contribute to
environmental pollution. In contrast, biological pretreatments that utilize wood-
degrading microorganisms for lignin removal can offer a safer, cheaper, and more
environmentally friendly biomass pretreatment process. White-rot fungi are
regarded as the most effective microorganisms for delignification of many differ-
ent lignocullulosic biomass including wood chips, wheat straw, grass and soft-
wood [42]. This is due to its remarkable ability to secrete a series of extracellular
lignin-degrading enzymes such as lignin peroxidases, manganese peroxidases,
and laccases to degrade lignin and hemicellulose [43, 44]. These can degrade
lignin completely into carbon dioxide and water.
Phanerochaetechrysosporium
,
Ceriporiosissubvermispora
,
Cyathusstercoreus
,
Dichomitussqualens
,
Phlebia
radiate
,
Pleurotllsostreatus
, and
Trametesversicolor
are some widely studied
white-rot fungi that show high delignification efficiency [45, 46].
Solid-state fermentation is a preferred process to perform biological delignifi-
cation over submerged fermentation because it leads to lower costs for product
recovery, drying and lower risk of contamination [47]. Delignification of lignocel-
lulosic biomass with white-rot fungi can be conducted at solid state in a bioreactor
