fermentation process (Seok et al. 2015). However, the high
operational cost and requirement of an excess amount of
energy (varies with reagent) make the process less desirable
(Badiei et al. 2014; Brodeur et al. 2011). Cui et al. pretreated
1 g of dry grass with 20 mL of dilute NaOH (0.5, 1, 2, 4,
and 8% (w/v)) at 35 °C for 30 min resulting in a maximum
total reducing sugars (TRS) of 330.8 mg/g of sugar and
maximum hydrogen formation of 19.25 mL at 0.5% NaOH
(Cui and Shen 2012). Wang et al. have reported 85% lignin
removal from coastal bermudagrass samples with an optimal
TRS production of 71% of the maximum theoretical value
after sodium pretreatment at 0.75% NaOH for 15 min at
121 °C (Wang et al. 2010). The concentration of sodium
hydroxide plays an important role in removing lignin and
forming waste after pretreatment. Thus, based on the type of
substrate, the concentration of NaOH should be chosen for
effective alkaline pretreatment (Ren et al. 2016). In comparison to NaOH, lime (calcium hydroxide) can also be used
to treat some of the lignocellulose materials (Ren et al. 2016;
Yadav et al. 2019). Cao et al. have reported the efficient
utilization of lime pretreatment for the corn stalk, which
helped to expose more cellulose to surface by disrupting
rigid structures and increasing the biodegradability of the
substrate that ultimately enhanced H 2 production (2012).
Apart from this, lime kiln technology can also be employed
for the recovery of lime, making the use of lime as a cheap
and effective method (Yadav et al.2019).
Ionic Liquid Treatment
Ionic liquids (ILs) have ions (cation and anion) as their main
composition. The properties of ionic liquids include high
polarity, thermally stability, low melting point (less than
100 °C), and negligible vapor pressure (Kumar and Sharma
2017; Literature review of physical and chemical pretreatment processes for lignocellulosic biomass n.d.). Moreover,
ILs are inflammable and odorless while having high solubility and less toxicity. They have the capacity to dissolve
lignin or cellulose (Salakkam et al. 2019). Fu et al. reported
the use of 1-ethyl-3-methylimidazolium acetate and water
for pretreatment of triticale straw, obtaining a yield of 81%
fermentable sugar (Fu and Mazza 2011). Talbot et al.
reported the triethylammonium hydrogen sulfate as an ionic
liquid for pretreatment of miscanthus giganteus grass. The
results unveiled that around 85% lignin and up to 100%
hemicellulose was solubilized into the ionic liquid while
99% ionic liquid was recovered each time (Brandt-Talbot
et al. 2017). However, the high cost of ILs is a major factor
that restricts its use for wider adoption in chemical pretreatment methods.
Organosolv Treatment
In the organosolv treatment, organic solvents, such as acetone, ethanol, organic acid, methanol, and ethylene glycol
have been used to selectively extract lignin from the lignocellulosic biomass in the presence of a catalyst (acid, base, or
salt) at the appropriate temperature and pressure (Kumar and
Sharma 2017). The method can obtain high-quality lignin
while making the cellulosic fibers easily accessible for
chemical interactions (Literature review of physical and
chemical pretreatment processes for lignocellulosic biomass
n.d.). An increase in the accessible surface area and pore
volume of cellulose is mainly due to the solvent assisted
breaking of hemicellulose and lignin linkages. However, the
process demands expensive solvents, which are flammable.
Also, solvent presence adversely affects the microorganism
growth, enzymatic hydrolysis, and fermentation process
(Kumar and Sharma 2017). The various factors, such as
catalyst type, reaction time, temperature, and concentration
of the solvent, define the physical characteristics of pretreated biomass that ultimately determines the extent of
hydrogen production (Kumar and Sharma 2017). The process requires the additional solvent recovery steps while
handling a large volume of organic solvents, which ultimately limits organosolv process utilization on a large scale.
2.3 Physiochemical Pretreatments
The combination of physical and chemical pretreatment is
referred to as physiochemical pretreatment, and it aims to
increase the efficacy of hydrolysis by enhancing the lignin
removal. Various physicochemical pretreatments, such as
ammonia fiber explosion, steam explosion, sulfite pretreatment, and liquid hot water, have been used for pretreatment
of lignocellulose.
Steam Explosion Treatment
The steam explosion pretreatment method is a commonly
used process, which is a combination of chemical, thermal,
and mechanical treatments. In this method, the residues are
exposed to high-pressure steam for a few seconds, followed
by sudden depressurization (explosion). The parameters that
affect the method are moisture content, residence time,
particle size, and temperature (Salakkam et al. 2019; Literature review of physical and chemical pretreatment processes for lignocellulosic biomass n.d.). The method is an
energy-efficient and environment-friendly method that
requires fewer chemicals. However, the formation of
Bioconversion of Lignocellulosic Residues into Hydrogen
63
operational cost and requirement of an excess amount of
energy (varies with reagent) make the process less desirable
(Badiei et al. 2014; Brodeur et al. 2011). Cui et al. pretreated
1 g of dry grass with 20 mL of dilute NaOH (0.5, 1, 2, 4,
and 8% (w/v)) at 35 °C for 30 min resulting in a maximum
total reducing sugars (TRS) of 330.8 mg/g of sugar and
maximum hydrogen formation of 19.25 mL at 0.5% NaOH
(Cui and Shen 2012). Wang et al. have reported 85% lignin
removal from coastal bermudagrass samples with an optimal
TRS production of 71% of the maximum theoretical value
after sodium pretreatment at 0.75% NaOH for 15 min at
121 °C (Wang et al. 2010). The concentration of sodium
hydroxide plays an important role in removing lignin and
forming waste after pretreatment. Thus, based on the type of
substrate, the concentration of NaOH should be chosen for
effective alkaline pretreatment (Ren et al. 2016). In comparison to NaOH, lime (calcium hydroxide) can also be used
to treat some of the lignocellulose materials (Ren et al. 2016;
Yadav et al. 2019). Cao et al. have reported the efficient
utilization of lime pretreatment for the corn stalk, which
helped to expose more cellulose to surface by disrupting
rigid structures and increasing the biodegradability of the
substrate that ultimately enhanced H 2 production (2012).
Apart from this, lime kiln technology can also be employed
for the recovery of lime, making the use of lime as a cheap
and effective method (Yadav et al.2019).
Ionic Liquid Treatment
Ionic liquids (ILs) have ions (cation and anion) as their main
composition. The properties of ionic liquids include high
polarity, thermally stability, low melting point (less than
100 °C), and negligible vapor pressure (Kumar and Sharma
2017; Literature review of physical and chemical pretreatment processes for lignocellulosic biomass n.d.). Moreover,
ILs are inflammable and odorless while having high solubility and less toxicity. They have the capacity to dissolve
lignin or cellulose (Salakkam et al. 2019). Fu et al. reported
the use of 1-ethyl-3-methylimidazolium acetate and water
for pretreatment of triticale straw, obtaining a yield of 81%
fermentable sugar (Fu and Mazza 2011). Talbot et al.
reported the triethylammonium hydrogen sulfate as an ionic
liquid for pretreatment of miscanthus giganteus grass. The
results unveiled that around 85% lignin and up to 100%
hemicellulose was solubilized into the ionic liquid while
99% ionic liquid was recovered each time (Brandt-Talbot
et al. 2017). However, the high cost of ILs is a major factor
that restricts its use for wider adoption in chemical pretreatment methods.
Organosolv Treatment
In the organosolv treatment, organic solvents, such as acetone, ethanol, organic acid, methanol, and ethylene glycol
have been used to selectively extract lignin from the lignocellulosic biomass in the presence of a catalyst (acid, base, or
salt) at the appropriate temperature and pressure (Kumar and
Sharma 2017). The method can obtain high-quality lignin
while making the cellulosic fibers easily accessible for
chemical interactions (Literature review of physical and
chemical pretreatment processes for lignocellulosic biomass
n.d.). An increase in the accessible surface area and pore
volume of cellulose is mainly due to the solvent assisted
breaking of hemicellulose and lignin linkages. However, the
process demands expensive solvents, which are flammable.
Also, solvent presence adversely affects the microorganism
growth, enzymatic hydrolysis, and fermentation process
(Kumar and Sharma 2017). The various factors, such as
catalyst type, reaction time, temperature, and concentration
of the solvent, define the physical characteristics of pretreated biomass that ultimately determines the extent of
hydrogen production (Kumar and Sharma 2017). The process requires the additional solvent recovery steps while
handling a large volume of organic solvents, which ultimately limits organosolv process utilization on a large scale.
2.3 Physiochemical Pretreatments
The combination of physical and chemical pretreatment is
referred to as physiochemical pretreatment, and it aims to
increase the efficacy of hydrolysis by enhancing the lignin
removal. Various physicochemical pretreatments, such as
ammonia fiber explosion, steam explosion, sulfite pretreatment, and liquid hot water, have been used for pretreatment
of lignocellulose.
Steam Explosion Treatment
The steam explosion pretreatment method is a commonly
used process, which is a combination of chemical, thermal,
and mechanical treatments. In this method, the residues are
exposed to high-pressure steam for a few seconds, followed
by sudden depressurization (explosion). The parameters that
affect the method are moisture content, residence time,
particle size, and temperature (Salakkam et al. 2019; Literature review of physical and chemical pretreatment processes for lignocellulosic biomass n.d.). The method is an
energy-efficient and environment-friendly method that
requires fewer chemicals. However, the formation of
Bioconversion of Lignocellulosic Residues into Hydrogen
63