Isolation of cellulose fibers from wetland reed grass through an integrated subcritical water hydrolysis-pulping-bleaching process

Cellulose fibers were isolated from wetland reed grass (Phragmites karka) by consolidated subcritical water hydrolysis, pulping and bleaching process. Phragmites biomass was treated with subcritical water at variable temperatures of 130–210 °C at constant pressure, reaction time and the biomass-to-water ratio of 3.5 MPa, 30 min and 1:50, respectively. The optimized temperature was found to be 170 °C with a total reducing sugars yield of 15.5 wt% and minimal charring of biomass. The treated biomass was carried out through pulping and bleaching process to isolate cellulose fibers. The highest cellulose yield was found to be 35.1 wt% with a residual lignin content of 0.4 wt% at 0.5 M NaOH. In a comparative study, the untreated biomass, subcritical water hydrolyzed biomass and cellulose were characterized by several physicochemical characterization tools such as proximate, ultimate and compositional analysis, thermogravimetric analysis, Fourier-transmission infrared, Fourier-transmission near-infrared and Raman spectroscopy, X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, scanning electron microscopy, transmission electron microscopy and atomic force microscopy to estimate the crystallinity index, carbon content, delignification, cellulose recovery as well as thermal stability and morphology of cellulose fibers. With the inclusion of subcritical water hydrolysis, a higher yield of cellulose was obtained from Phragmites with a crystallinity index of 65.3%. Due to the effective hydrolysis process, subsequent processes such as pulping and bleaching required less severe conditions.