4.2 Biomass Pretreatment
55
(Yuan et al. 2013). In addition, the cellulose hydrolysis depends greatly upon enzyme
adsorption onto the substrate surface and lignin may irreversibly adsorb enzymes
causing their inactivation. Based on these considerations, a pretreatment step is essential to achieve the effective enzymatic digestion of lignocellulosic biomass for the
production of biofuels. In recent years, innumerable pretreatment processes have
been developed to improve the lignocellulosic digestibility:
• Physical,
• Chemical [organosolv process; oxidative delignification; alkali pretreatment; acid
pretreatment; ionic liquids (ILs) pretreatment],
• Chemical–physical [steam explosion; hydrothermal pretreatment; supercritical
CO 2 (SC-CO 2 ) pretreatment; ammonia fibre explosion (AFEX)], and
• Biological pretreatment.
While each pretreatment process has its own specificity (Sun et al. 2016), chemical
and physicochemical pretreatment types are the most promising ones for industrial
applications.
However, a critical analysis of the literature highlights that pretreatment methods
can be ‘tailor-made’ for each individual biomass. Therefore, they should be meticulously selected and designed according to the specific properties of the biomass to
be achieved. In fact, a suitable pretreatment should not only significantly improve
the digestibility of biomass but also address the economical features of biofuel production (Taherzadeh and Karimi 2008). Furthermore, it can be stated that a single
pretreatment method authorizing a complete biomass delignification in an economic
and environmentally friendly manner has not yet been achieved (Iskalieva et al.
2012).
A number of researchers have recently exploited US for the intensification of
biomass delignification under alkaline conditions. In fact, hydroxyl ions are responsible for delignification during alkali pretreatment, based on the fragmentation, the
degradation and the dissolution of biomass lignin. Although a preliminary hypothesis being carried out, the exact physical mechanism of US-assisted delignification
under alkali condition has not yet been established.
In this context, Singh et al. (2014) have attempted to give mechanistic insight
into the US-assisted delignification of carrot grass (Parthenium hysterophorus) in
the presence of NaOH. They observed that both the physical and chemical effects of
transient cavitation contributed to the final delignification (90%), when the substrate
was pretreated with US (20 kHz, 500 W) under alkali conditions with NaOH (1.5%
w/v). This yield was double than that of the conventional pretreatment (mechanical
stirring). Furthermore, the physical effect of shock waves leads to the lignin matrix
depolymerization via the homolytic cleavage of phenyl ether α–O–4 and β–O–4
bonds. On the other hand, the chemical effect of radical generation promotes the
hydroxylation/oxidation of the aromatic moieties and the side chain elimination.
These peculiar mechanisms mean that US treatment provides effective delignification at ambient temperature, with low amounts of delignifying agents together with
decristallisation of cellulose through partial depolymerization.
55
(Yuan et al. 2013). In addition, the cellulose hydrolysis depends greatly upon enzyme
adsorption onto the substrate surface and lignin may irreversibly adsorb enzymes
causing their inactivation. Based on these considerations, a pretreatment step is essential to achieve the effective enzymatic digestion of lignocellulosic biomass for the
production of biofuels. In recent years, innumerable pretreatment processes have
been developed to improve the lignocellulosic digestibility:
• Physical,
• Chemical [organosolv process; oxidative delignification; alkali pretreatment; acid
pretreatment; ionic liquids (ILs) pretreatment],
• Chemical–physical [steam explosion; hydrothermal pretreatment; supercritical
CO 2 (SC-CO 2 ) pretreatment; ammonia fibre explosion (AFEX)], and
• Biological pretreatment.
While each pretreatment process has its own specificity (Sun et al. 2016), chemical
and physicochemical pretreatment types are the most promising ones for industrial
applications.
However, a critical analysis of the literature highlights that pretreatment methods
can be ‘tailor-made’ for each individual biomass. Therefore, they should be meticulously selected and designed according to the specific properties of the biomass to
be achieved. In fact, a suitable pretreatment should not only significantly improve
the digestibility of biomass but also address the economical features of biofuel production (Taherzadeh and Karimi 2008). Furthermore, it can be stated that a single
pretreatment method authorizing a complete biomass delignification in an economic
and environmentally friendly manner has not yet been achieved (Iskalieva et al.
2012).
A number of researchers have recently exploited US for the intensification of
biomass delignification under alkaline conditions. In fact, hydroxyl ions are responsible for delignification during alkali pretreatment, based on the fragmentation, the
degradation and the dissolution of biomass lignin. Although a preliminary hypothesis being carried out, the exact physical mechanism of US-assisted delignification
under alkali condition has not yet been established.
In this context, Singh et al. (2014) have attempted to give mechanistic insight
into the US-assisted delignification of carrot grass (Parthenium hysterophorus) in
the presence of NaOH. They observed that both the physical and chemical effects of
transient cavitation contributed to the final delignification (90%), when the substrate
was pretreated with US (20 kHz, 500 W) under alkali conditions with NaOH (1.5%
w/v). This yield was double than that of the conventional pretreatment (mechanical
stirring). Furthermore, the physical effect of shock waves leads to the lignin matrix
depolymerization via the homolytic cleavage of phenyl ether α–O–4 and β–O–4
bonds. On the other hand, the chemical effect of radical generation promotes the
hydroxylation/oxidation of the aromatic moieties and the side chain elimination.
These peculiar mechanisms mean that US treatment provides effective delignification at ambient temperature, with low amounts of delignifying agents together with
decristallisation of cellulose through partial depolymerization.