56
4 Effective Biomass Valorization Procedures Using Ultrasound …
He et al. (2017) have also investigated the effects of US pretreatment on structural changes in biomass. In particular, eucalyptus wood (particle size 0.5 mm) was
pretreated (0.5–1.5 h) under alkali condition (NaOH) using an US bath working at
28 kHz and 300 W. US pretreatment had a strong influence on the tissue structure of
biomass, as it reduced its lignin content and increased its crystallinity (up to 35.5%),
making it prone to subsequent enzymatic hydrolysis.
Since it is well known that biomass delignification under alkaline conditions
can be improved by increasing pretreatment temperature, the heat produced under
US irradiation may be a profitable way to save energy and time. Wu et al. (2017)
have demonstrated that the heat dissipated from a horn-type sonicator, operating at
22 kHz and 300 W for 70 min, led to a reliable and effective methodology for rice
straw alkali pretreatment. This protocol provided 3.5-fold higher concentrations of
reducing sugars after 48 h of enzymatic saccharification (2.91 vs. 0.85 g L
−1 ) than in
the absence of US. These results can be attributed to a higher cellulose content present
in US pretreated samples and lower hemicellulose and lignin contents, together with
higher surface area and porosity as compared to untreated samples. However, it
should be noted that the porosity, surface area and cellulose content of pretreated
straw samples increased with increasing alkali concentration (NaOH w/v: 0.4, 0.6,
0.8 and 1.0%,) with or without US.
Another effective US-assisted alkali pretreatment of chilli post-harvest residue has
been reported by Sindhu et al. (2017). Biomass US pretreatment (80 kHz, 0–60 min)
was performed in the presence of either KOH, NaOH, or Ca(OH) 2 and was followed by hydrothermal treatment in an autoclave (121 °C, 60 min) before enzymatic
hydrolysis. The authors studied the interaction between sonication time and biomass
loading on enzymatic hydrolysis. Maximum reducing sugar yield (0.44 g/g) was
reported at high levels of sonication time (up to 60 min) and middle levels of lignocellulosic loading (11.0–15.0% w/w). This US pretreatment was able to effectively
remove lignin and hemicelluloses from chilli post-harvest residue and improve the
final sugar yield. Furthermore, an inhibitor analysis of the hydrolyzate revealed that
furfural, 5-hydroxymethylfurfural and organic acids, such as citric acid, propionic
acid and succinic acid, were not present. This means that fermentation could be carried out without the prior detoxification of the hydrolysate, yielding 1.94% ethanol
with a fermentation efficiency of 70.08%.
Sugarcane bagasse was also investigated for the production of ethanol after pretreatment (Kandasamy et al. 2017). The raw biomass was first pretreated under acidic
(H 2 SO 4 2.5–3.5%), alkali (NaOH 22–25%) and US conditions (20 kHz, 100 W) used
either alone or combined. Using SEM analyses, it was established that the complex
bagasse structure was destroyed after combined acid/US pretreatment (Fig. 4.1). The
pretreated bagasse was then used as substrate for fermentation to biofuels without
prior enzymatic hydrolysis, yielding higher ethanol production as compared to the
previous results reported in the literature. An ethanol concentration of 911 mg/L was
achieved combining all the pretreatment types; the bagasse was treated with acid
(3%) and US (50 or 109 W), and then with alkaline (23%) and US (50 or 109 W).
Another interesting application of US concerns the delignification of waste newspaper (Subhedar and Gogate 2014a). A ca. twofold increase in delignification (around
4 Effective Biomass Valorization Procedures Using Ultrasound …
He et al. (2017) have also investigated the effects of US pretreatment on structural changes in biomass. In particular, eucalyptus wood (particle size 0.5 mm) was
pretreated (0.5–1.5 h) under alkali condition (NaOH) using an US bath working at
28 kHz and 300 W. US pretreatment had a strong influence on the tissue structure of
biomass, as it reduced its lignin content and increased its crystallinity (up to 35.5%),
making it prone to subsequent enzymatic hydrolysis.
Since it is well known that biomass delignification under alkaline conditions
can be improved by increasing pretreatment temperature, the heat produced under
US irradiation may be a profitable way to save energy and time. Wu et al. (2017)
have demonstrated that the heat dissipated from a horn-type sonicator, operating at
22 kHz and 300 W for 70 min, led to a reliable and effective methodology for rice
straw alkali pretreatment. This protocol provided 3.5-fold higher concentrations of
reducing sugars after 48 h of enzymatic saccharification (2.91 vs. 0.85 g L
−1 ) than in
the absence of US. These results can be attributed to a higher cellulose content present
in US pretreated samples and lower hemicellulose and lignin contents, together with
higher surface area and porosity as compared to untreated samples. However, it
should be noted that the porosity, surface area and cellulose content of pretreated
straw samples increased with increasing alkali concentration (NaOH w/v: 0.4, 0.6,
0.8 and 1.0%,) with or without US.
Another effective US-assisted alkali pretreatment of chilli post-harvest residue has
been reported by Sindhu et al. (2017). Biomass US pretreatment (80 kHz, 0–60 min)
was performed in the presence of either KOH, NaOH, or Ca(OH) 2 and was followed by hydrothermal treatment in an autoclave (121 °C, 60 min) before enzymatic
hydrolysis. The authors studied the interaction between sonication time and biomass
loading on enzymatic hydrolysis. Maximum reducing sugar yield (0.44 g/g) was
reported at high levels of sonication time (up to 60 min) and middle levels of lignocellulosic loading (11.0–15.0% w/w). This US pretreatment was able to effectively
remove lignin and hemicelluloses from chilli post-harvest residue and improve the
final sugar yield. Furthermore, an inhibitor analysis of the hydrolyzate revealed that
furfural, 5-hydroxymethylfurfural and organic acids, such as citric acid, propionic
acid and succinic acid, were not present. This means that fermentation could be carried out without the prior detoxification of the hydrolysate, yielding 1.94% ethanol
with a fermentation efficiency of 70.08%.
Sugarcane bagasse was also investigated for the production of ethanol after pretreatment (Kandasamy et al. 2017). The raw biomass was first pretreated under acidic
(H 2 SO 4 2.5–3.5%), alkali (NaOH 22–25%) and US conditions (20 kHz, 100 W) used
either alone or combined. Using SEM analyses, it was established that the complex
bagasse structure was destroyed after combined acid/US pretreatment (Fig. 4.1). The
pretreated bagasse was then used as substrate for fermentation to biofuels without
prior enzymatic hydrolysis, yielding higher ethanol production as compared to the
previous results reported in the literature. An ethanol concentration of 911 mg/L was
achieved combining all the pretreatment types; the bagasse was treated with acid
(3%) and US (50 or 109 W), and then with alkaline (23%) and US (50 or 109 W).
Another interesting application of US concerns the delignification of waste newspaper (Subhedar and Gogate 2014a). A ca. twofold increase in delignification (around