PEER-REVIEWED ARTICLE bioresources.com Dai et al. (2016). “Xylanase for reduction of AOX,” BioResources 11(1), 3204-3214. 3204 Xylanase-Aided Chlorine Dioxide Bleaching of Bagasse Pulp to Reduce AOX Formation Yi Dai, a,c,1 Xueping Song, a,1 Cong Gao, a Sha He, a Shuangxi Nie, a,c, * and Chengrong Qin a,b,c, * Xylanase pretreatment was used to improve the chlorine dioxide bleaching of bagasse pulp. The pulp was pretreated with xylanase, which was followed by a chlorine dioxide bleaching stage. The HexA content of the pulp and the AOX content of the bleaching effluent were measured using UV-Vis and GC-MS methods, respectively. The results showed that a good correlation occurred between HexA and kappa number. HexA content of the pulp decreased significantly after the xylanase pretreatment. The AOX content of the bleaching effluent decreased as HexA was removed from the pulp. It was found that AOX could be reduced by up to 29.8%, comparing XD0 with a D0 stage. Fourier transform infrared spectroscopy (FTIR) was employed to determine the breakage of chemical bonds in the pulp. It revealed that some lignin and hemicellulose were removed after xylanase treatment. The GC-MS results showed that some toxic chloride such as 2,4,6-trichlorophenol could be completely removed after xylanase pretreatment. Keywords: Xylanase; Hexenuronic acid; AOX; FTIR; GC-MS Contact information: a: College of Light Industry and Food Engineering, Guangxi University, Nanning, 530004, PR China; b: The Guangxi Key Laboratory of Environmental Engineering, Protection and Assessment. Guilin 541004, PR China; c: Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, PR China; *Corresponding authors: nieshuangxi061@163.com; qin_chengrong@163.com. 1 These authors contributed equally to this work INTRODUCTION In the past two decades, the global pulp and paper industry has undergone great changes in response to environmental pressure. Chlorinated bleaching agents reacting with kraft pulp generate a complex mixture of degradation products, such as adsorbable organic halogens (AOX), which adversely impact the environment (Sharma et al. 2015). Meanwhile, AOX is regarded as an extremely important parameter to test and as an indicator of environmental influence (Lehtimaa et al. 2010). Rigorous environmental laws promote technological developments that would reduce chlorine from emissions of AOX in bleaching effluents. With this background, biotechnology surrounding the pulping process has been developed rapidly. Lots of studies have been conducted dealing with enzyme bleaching, including xylanase (Shatalov and Pereira 2007; Fillat and Roncero 2009), laccases, and two enzymes treated jointly (Aracri and Vidal 2011; Fonseca- Maldonado et al. 2014), which provided the possibility for industrial application in the pulp bleaching process. Many studies have focused on the effect of xylanase pretreatment in the context of bio-bleaching. Some advantages of xylanase application are low capital investment, higher pulp brightness, reduction of bleaching chemicals [15 to 20%], and low AOX concentration in the final effluent. Xylanase use has saved on chemical costs for mills without changing the existing process. Sharma et al. (2015) observed that bio-bleaching, using xylanase- producing Bacillus halodurans FNP 135 through submerged (SmF) and solid state