Combining subcritical water extraction and bleaching with hydrogen peroxide to obtain cellulose fibres from rice straw

Cellulose fibres from rice straw (RS) were obtained by applying a combined and green process consisting of subcritical water extraction (SWE) (at 160 and 180 °C), followed by bleaching with hydrogen peroxide (H₂O₂). The bleaching variables were optimised for both extraction residues (R160 and R180), obtaining optimum conditions for 4 % H₂O₂ at pH 12 for 1 h of reaction. Under these conditions, four successive bleaching cycles were applied to promote cellulose purification in both R160 and R180 lignocellulosic samples, which were characterised as to their yield, whiteness index (WI), chemical composition, microstructural characteristics, degree of crystallinity, and thermal stability. The samples pre-extracted with SWE at 180 °C were more susceptible to bleaching than those pre-treated at 160 °C, being richer in cellulose (86.2 vs. 72.4 %), with higher WI (80.1 vs. 78) and yield (46.2 vs. 36.7 %) after the four bleaching cycles. Likewise, the R180 bleached fractions had higher crystallinity (67 vs. 63%) and were more thermostable than the R160 bleached fibres. Therefore, the SWE at 180 °C, followed by four bleaching cycles with H₂O₂ (4 %, at pH 12, for 1 h), can be considered a green and more sustainable alternative for obtaining cellulose fibres from RS with high cellulose purity.