PEER-REVIEWED ARTICLE bioresources.com Liang et al. (2019). “Bagasse alkali-oxygen pulping,” BioResources 14(1), 638-649. 638 Regulation of Superoxide Anion Radicals in Bagasse Alkali-Oxygen Pulping to Enhance Delignification Selectively Xing Liang, a Bing Sun, a, * Yuxin Liu, a, * Bao Zhang, a Yingzhen Shan, a Xueping Song, b and Shuangxi Nie b Alkali-oxygen pulping has received increasing attention as an environmentally compatible pulping process; however, the resulting pulp has low yield and properties due to carbohydrate degradation. In this work, the effect of regulating superoxide anion radicals (O2 -• ) on delignification selectivity in alkali-oxygen pulping was investigated. The pulp was characterized by Brunauer-Emmett-Teller (BET) analysis, scanning electron microscope (SEM), and X-ray diffraction (XRD). Lignin removal was improved by the regulation of O2 -• in the 1,2,3-trihydroxybenzene auto- oxidation system with addition of sodium 2-anthraquinonesulfonate, especially for the pulp from cooking time 120 min. The degree and selectivity of delignification were improved with alkali-oxygen pulping when sodium 2-anthraquinonesulfonate was added to facilitate the generation of superoxide anion radicals. The results showed that the increased fiber liberation was beneficial for the accessibility of O2 -• to lignin, which would improve the removal of lignin with negligible damage on cellulose, especially with regards to crystal area. Keywords: Superoxide anion radicals; Sodium 2-anthraquinonesulfonate; Alkali-oxygen pulping; Selectivity; Bagasse Contact information: a: Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China; b: Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, China; *Corresponding authors: sunbing_6@163.com; yuxinliukm@163.com INTRODUCTION Alkali-oxygen pulping is widely used to pulp non-wood. This method has low chemical costs and can be environmentally friendly (Chen and Hayashi 1997, 1999). Its other advantages include low alkali charge and low cooking temperature compared with conventional alkali pulping (Chen and Hayashi 1997; Hata et al. 2008; Zhang et al. 2008; Chen 2009). Sugarcane bagasse is an important agro-based lignocellulose that is renewable; it has loose fiber structure (Zhao et al. 2010; Kapoor et al. 2017) and has a high cellulose content of 40% (Saha 2003; Dinesh et al. 2006). These characteristics make bagasse suitable for alkali-oxygen pulping. Li et al. (2012) and Hata et al. (2008) have reported the use of NaOH and oxygen pulping of non-wood plant materials. Bagasse alkali-oxygen pulping involves two distinct delignification phases: the bulk phase and the residual phase (Fernandez et al. 1985; Sabatier et al. 1993). The ratio of p-hydroxyphenyl-to-guaiacyl-to-syringyl (H:G:S) units of bagasse lignin is 2:38:60 (Río et al. 2015); the high content of syringyl units is beneficial for improving the delignification rate (Yang 2010). In the bulk phase of alkali-oxygen pulping, primarily -O-4, -5 and other condensed substructures (Río et al. 2015) could be removed as much as possible due to lower oxygen delignification resistance (Agarwal et al. 1999; Poukka et al. 1999). In addition, the delignification selectivity is remarkable because the reactive activity of cellulose mainly affected by the accessibility of fiber (Lin et al. 2018) and less fiber was exposed during the initial cooking. However, the delignification