Barley Straw Cellulose Fiber Isolation Using Pressurized Aqueous Ethanol, Ultrasound, and Bleaching Processes

The main objectives of this study were to determine the optimum conditions of ultrasound (US) treatment to remove non-cellulosic biomass from barley straw residues, evaluate the effect of US treatment on the characteristics of acidified sodium chlorite (ASC)- or alkaline hydrogen peroxide (AHP)-bleached cellulose fibers, and compare the characteristics of cellulose fibers obtained after AHP and ASC bleaching. First, barley straw was treated by pressurized aqueous ethanol (PAE), then US treatment (400–1200 W/20 kHz/10–40 min) and ASC (1.7%/75 °C/2–6 h) or AHP bleaching (20%/75 °C/2–6 h) were performed to obtain cellulose fibers. Chemical composition, color, crystallinity, changes of functional groups, and morphology of cellulose fibers obtained were determined. The PAE treatment at 200 °C and 50 bar using 20% ethanol followed by US treatment at 1200 W/40 min removed most of the hemicellulose (92.9 \(\pm\) $\pm$ 0.1%) and half of the initial lignin content, resulting in a cellulose-rich residue. Then, 99.1 \(\pm\) $\pm$ 1.1 of hemicellulose and 86.5 \(\pm\) $\pm$ 0.6% of lignin were removed after bleaching for 6 h. The highest purity of the barley straw cellulose fiber was 91.1 \(\pm\) $\pm$ 2.3%, obtained after PAE + US + AHP bleaching for 6 h. At this optimum condition, the resultant cellulose fibers had crystallinity index of 38.9% without changing cellulose I structure. Morphology images revealed that the US treatment reduced the diameters of cellulose fibers to 3.54 and 4.42  \(\upmu\) $\mathrm{\mu }$ m after PAE + US + ASC and PAE + US + AHP bleaching for 6 h, respectively.