Chemical Pretreatment of Sunflower Straw Biomass: The Effect on Chemical Composition and Structural Changes

In this study, various pretreatment methods, such as thermal (24 h at 80 °C; 1 h at 120 °C), alkaline (2–20 g ΝaΟΗ/100 g TS, 24 h at 80 °C) and acid (2–20 g Η2SO4/100 g TS, 1 h at 120 οC; 2–20 g Η3PO4/100 g TS, 1 h at 120 °C; 2–20 g HCl/100 g TS, 1 h at 120 °C), were applied on sunflower straw biomass and the effect of each pretreatment method on the fractionation of the lignocellulosic content was evaluated. Experiments showed that pretreatment with the acids led to higher hemicellulose solubilization, while lignin degradation was achieved when the sunflower was treated with NaOH. Higher acids concentration led to higher solubilization of hemicellulose and higher concentrations of released furaldehydes (5-hydroxymethylfurfural and furfural) and aliphatic acids (i.e. formic and acetic acid). On the contrary, the higher the NaOH concentration, the higher was the phenolics concentration observed. Scanning electron microscopy images revealed a change in morphology after different pretreatment methods, compared to the raw substrate. Thus, disruption of the outer layers and removal of mass from the initial connected structure were observed, for all pretreated feedstocks. Complete analysis of the liquid fractions obtained after pretreatment, led to the estimation of the ratio of sum of soluble sugars concentration to the sum of inhibitors concentration (Σ(soluble sugars)/Σ(inhibitors)), which characterizes the fermentability of the sunflower straw hydrolysates.