Effects of pretreatment on morphology, chemical composition and enzymatic digestibility of eucalyptus bark: a potentially valuable source of fermentable sugars for biofuel production - part 1

Abstract

Background: In recent years, the growing demand for biofuels has encouraged the search for different sources of underutilized lignocellulosic feedstocks that are available in sufficient abundance to be used for sustainable biofuel production. Much attention has been focused on biomass from grass. However, large amounts of timber residues such as eucalyptus bark are available and represent a potential source for conversion to bioethanol. In the present paper, we investigate the effects of a delignification process with increasing sodium hydroxide concentrations, preceded or not by diluted acid, on the bark of two eucalyptus clones: Eucalyptus grandis (EG) and the hybrid, E. grandis x urophylla (HGU). The enzymatic digestibility and total cellulose conversion were measured, along with the effect on the composition of the solid and the liquor fractions. Barks were also assessed using Fourier-transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (NMR), X-Ray diffraction, and scanning electron microscopy (SEM).

Results: Compositional analysis revealed an increase in the cellulose content, reaching around 81% and 76% of glucose for HGU and EG, respectively, using a two-step treatment with HCl 1%, followed by 4% NaOH. Lignin removal was 84% (HGU) and 79% (EG), while the hemicellulose removal was 95% and 97% for HGU and EG, respectively. However, when we applied a one-step treatment, with 4% NaOH, higher hydrolysis efficiencies were found after 48 h for both clones, reaching almost 100% for HGU and 80% for EG, in spite of the lower lignin and hemicellulose removal. Total cellulose conversion increased from 5% and 7% to around 65% for HGU and 59% for EG. NMR and FTIR provided important insight into the lignin and hemicellulose removal and SEM studies shed light on the cell-wall unstructuring after pretreatment and lignin migration and precipitation on the fibers surface, which explain the different hydrolysis rates found for the clones.

Conclusion: Our results show that the single step alkaline pretreatment improves the enzymatic digestibility of Eucalyptus bark. Furthermore, the chemical and physical methods combined in this study provide a better comprehension of the pretreatment effects on cell-wall and the factors that influence enzymatic digestibility of this forest residue.

Figure 1

Remaining glucose, xylose and total lignin content in the solid fraction from HGU and EG barks after each pretreatment step.

Figure 2

FTIR-PCA scores plot of PC-1 and PC-2 obtained for HGU and EG eucalyptus clones bark undergoing different pretreatment conditions.

Figure 3

Loadings of PC-1 of FTIR of HGU and EG eucalyptus clones bark.

Figure 4

CPMASTOSS NMR spectra of EG and HGU bark samples without pretreatment (a), after acid (b) and alkaline (NaOH 4%) (c) pretreatments.

Figure 5

CPMASTOSS NMR spectra of eucalyptus bark samples after and before pretreatment.

Figure 6

CPMASTOSS NMR spectra of lyophilized hydrolysates (liquor fraction) from the EG and HGU treated with NaOH concentrations of 0.25% (a) and 4.0% (b) after acid treatment, and only 4% NaOH (c).

Figure 7

Determination of crystallinity index of HGU and EG eucalyptus clones bark, before and after different pretreatment condition.

Figure 8

Surface images of eucalyptus bark obtained by SEM: (a) and (b) samples extracted with hot water only; (c) and (d) samples which underwent the acid treatment.

Figure 9

Surface images obtained by SEM on eucalyptus bark samples treated different NaOH concentrations: (a) and (b) NaOH 0.5%; (c) and (d) NaOH 1.0%. The amount of surface residues decrease with the alkaline treatment and neighboring cell bundles start to separate in the longitudinal direction.

Figure 10

SEM images of eucalyptus bark samples treated with different NaOH concentrations: (a) NaOH 2.0%; (b)NaOH 4.0%.

Figure 11

SEM images of eucalyptus bark samples treated with 2.0% of NaOH. (a) & (c): EG clone; (b) & (d): HGU clone. Globular structures assigned to lignin agglomerates are more frequently observed on EG than on HGU samples.

Figure 12

Enzymatic hydrolysis yield obtained for eucalyptus barks after acid and/or alkali treatments along 48 h.

Figure 13

Total glucose released from both eucalyptus clones barks after 48 h hydolysis taking into account the losses during each pretreatment step.