Optimization and characterization of cellulose extraction from cotton straw using alkali pretreatment

Abstract

Cotton straw is an agricultural residue; its substantial cellulose content allows for the extraction of cellulose, which can subsequently be valorized to generate biofuels, biochemicals, and diverse high-value materials. Isolation of cellulose from cotton straw (CS) with acetic acid washing, ultrasonic-assisted alkali pretreatment, and bleaching treatment was optimized at ambient pressure. Sodium hydroxide/ethanol with ultrasound and hydrogen peroxide was used for pretreatment and bleaching, respectively. Experimental parameters for alkali pretreatment were optimized by the Plackett–Burman design (PBD) and subsequent Box-Behnken design (BBD) and obtained data was analyzed by the response surface modeling (RSM) method. The optimum conditions for alkali pretreatment were as follows: NaOH concentration, 10.5 wt%; reaction temperature, 90 ℃; reaction time, 7.2 h; C₂H₅OH concentration, 12 wt%; liquor ratio, 15:1; and ultrasonication time 2 min. Among them, the reaction temperature, NaOH concentration, and reaction time significantly influence the weight loss rate (WLR) of CS in alkali pretreatment. The optimum WLR of CS was 43.87%, and very close to predicted values of 43.92%. CS samples (untreated and treated) obtained from each stage were characterized by Fourier infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric (TG) analysis. Chemical composition analyses and various physicochemical characterization results show that the hemicellulose and lignin of CS were removed efficiently as well as cellulose content was increased simultaneously after alkali and bleaching treatments. This study would provide a theoretical basis for the potential utilization of cellulose-based materials in future applications. 

Graphical abstract

Multi-step pretreatment methods integrated acetic acid, NaOH-C₂H₅OH assisted with ultrasound, and H₂O₂ bleaching were carried out for the isolation of cellulose from CS in atmospheric pressure. The alkali pretreatment process was optimized using the RSM/BBD statistical model. The optimum WLR of CS is 43.87% very close to predicted values of 43.92%. Chemical composition analyses and various physicochemical characterization results show that the hemicellulose and lignin of CS were removed efficiently as well as cellulose content was increased simultaneously after multi-step pretreatments.