A comparative assessment of biomass pretreatment methods for the sustainable industrial upscaling of rice straw into cellulose

Prior to the bioconversion of low cost, renewable lignocellulosic residues such as rice straw into value added bio-products, their recalcitrant structure needs to be fractionated by using various pretreatment methods. This study is aimed at the comparison of pretreatment protocols that will enable more efficient degradation and conversion of lignocellulosic biomass into cellulose-rich fractions. In this study, the cellulose-rich fractions of rice straw were obtained using the following pretreatment methods: alkali pretreatment using 5% (w/w) NaOH at 121 °C for 1 h, alkali treatment followed by bleaching pretreatment using acidified NaClO₂ at 75 °C for 30 min, organosolvent pretreatment using formic acid: ethanol (3:1) at 160 °C for 1 h and steam explosion pretreatment at 180 °C for 5 min. The compositional analysis showed that alkali pretreatment followed by bleaching had remarkable fractionation efficiency with an increase in the relative cellulose concentration from 37.2 to 64.3%. The organosolvent and steam explosion pretreatment methods are green alternatives for the fractionation of lignocellulosic components. However, in comparison to the alkali bleaching pretreatment, these methods are less efficient with a relative increase in cellulose concentration from 37.2% in untreated rice straw to 46.5 and 43.7% in organosolvent and steam explosion pretreatment, respectively. The pretreated rice straw fractions were then investigated on the basis of various physicochemical characterization techniques such as Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric Analysis (TGA). Further the economic feasibility of these pretreatment methods needs to be considered to estimate the related cost for technology transfer. This paper gives a comprehensive characteristic comparison of cellulose-rich variants obtained from the rice straw using different pretreatment methods along with the preliminary cost analyses indicating their potential economic feasibility.