Structure Change And Removal Mechanism Of Residual Lignin In Bagasse Pulp During Chlorine Dioxide Bleaching

As the main producer of non wood pulp in the world,China has rich experience in non wood fiber papermaking.Sugarcane is widely planted in South and southwest China due to its short growth cycle and high economic value.Bagasse pulp plays an indispensable role in non wood pulp.The residual lignin?UBPRL?in unbleached bagasse pulp not only affects the brightness of pulp,but also is one of the main source substances that can adsorb organic chloride?AOX?during chlorine dioxide bleaching.In this paper,the chlorine dioxide clean bleaching technology?ECF?and its mechanism of bagasse pulp were studied systematically,in order to provide some theoretical basis and technical support for mature and perfect ECF Bleaching of bagasse pulp.In this paper,the unbleached bagasse pulp was used as the starting material.Firstly,the main factors affecting the purity and yield of lignin in the extraction process of lignin were analyzed by single factor experiment,and the reaction conditions were optimized.Secondly,the structural changes of UBPRL in ECF bleaching process were discussed by FT-IR and NMR,and the AOX components and lignin degradation products in bleaching wastewater were analyzed by GC-MS.Thirdly,we synthesized 5,5'-diallyl-2,2'-dihydroxy-3,3'-dimethoxybiphenyls?DDD?through experiments,and built a mathematical model to study the degradation kinetics of DDD and Cl O₂.Finally,the reaction conditions of Fenton degradation of AOX were optimized by RSM.The research contents and results are as follows:Two-stage Enzymatic-Weak acidolysis method was used to separate lignin from bagasse,unbleached bagasse pulp and ECF bleached pulp.The effect of the amount of complex cellulase on the extraction of residual lignin from bagasse pulp by enzymatic weak acidolysis was studied.The results showed that when the enzyme dosage was 3 m L/g,the residual lignin had high yield and purity,which could well reflect the original structure of lignin.In the process of conventional soda AQ cooking and bleaching,the average molecular weight of lignin first increases and then decreases,in which the cooking process makes the molecular weight of lignin increase,and the bleaching process makes the molecular weight of lignin decrease.The polydispersity?PDI?of lignin decreased with the order of bagasse,unbleached pulp and bleached pulp.Residual lignin was extracted from unbleached sugarcane bagasse pulp via the sequential Enzymatic-Weak acid hydrolysis method.Subsequently,chlorine dioxide delignification was applied to the extracted lignin.The structural changes of lignin during chlorine dioxide delignification were characterized by Fourier transform infrared spectroscopy?FT-IR?,¹³C-nuclear magnetic resonance(¹³C NMR),¹H-¹³C heteronuclear single quantum coherence?2D HSQC?,and quantitative ³¹P-nuclear magnetic resonance(³¹P-NMR)spectroscopy.The degradation products of lignin dissolved in the simulated bleaching liquor,extracted,and quantified via gas chromatography-mass spectrometry?GC-MS?.Lignin was significantly degraded during the delignification process,showing cleavage of the aromatic ring,removal of the methoxyl group,and cleavage of major inter-unit linkages.However,the carboxylic acid content increased and GC-MS analysis indicated that lignin was degraded into several organic substances dissolved in the liquor.However,only a few absorbable organic halides?AOX?were generated,confirming that chlorine dioxide bleaching is an ideal bleaching method.Using 5,5'-diallyl-2,2'-dihydroxy-3,3'-dimethoxybiphenyl?DDD,a laboratory-synthesized biphenyl lignin model compound?,the activity,kinetics and mechanisms in aqueous solution were investigated for the delignification of Cl O₂.The influence of the process operating conditions?initial p H and temperature?on the reaction pathway and product distribution were studied as well.The DDD instant concentration was calculated by a special differential ultraviolet-visible?UV-vis?method to exclude the effect caused by newly generated products.Kinetic parameters for the reaction in aqueous solution were determined for DDD and Cl O₂ with a kinetic model by regression analysis.The reaction with excess Cl O₂ at a p H of 3.32 was monitored to elucidate the reaction kinetics.The reaction was proven to be first-order with respect to DDD.The overall kinetic order of the reaction obeys the characteristics of a second-order kinetic reaction at lower p H values,but the fit to a second-order rate law is gradually lost with an increase in the p H.Under the experimental conditions first-order rate constants?k?,second-order rate constants?K?and reaction stoichiometries were affected by the initial p H and temperature.Gas chromatography-mass spectrometry?GCMS?,UV-vis spectroscopy and an AOX analyzer were employed to characterize the newly generated products.The activation energy was calculated to be 141.58 k J mol^-1 by the Arrhenius equation.The potential reaction pathway of one of the newly generated products??1Z,3Z,5Z,7Z?-2,7-diallylocta-1,3,5,7-tetraene-1,4,5,8-tetracarboxylate detected by GCMS?is proposed.The effects of four main factors on the removal of absorbable organic halogens were evaluated by responsesurfacemethodology,and the changes of absorbable organic halogens components were characterized by gas chromatography-mass spectrometer during the Fenton reaction.The high regression coefficient?R²=0.9028?and the low coefficient of variation?7.45%?indicated that the model was accurate in predicting the experimental results.Thewere respectively 3.42,16.3 m M,22.5 and 1.2 h.Consequently,93.8%of absorbable organic halogens were removed under the optimized condition.The initial p H is the most important factor impacting the absorbable organic halogens removal.Furthermore,gas chromatography-mass spectrometer revealed that 25 out of 28 organic compounds including 7 absorbable organic halogens were thoroughly removed.Conclusively,Fenton reaction can effectively remove absorbable organic halogens from the simulated bleaching effluent.In this paper,the structure of the lignin in bagasse pulp and its changes in the process of pulp bleaching were systematically discussed,and the response surface system was introduced into the Fenton reaction to remove AOX to optimize the reaction conditions,which laid a theoretical foundation for the systematic ECF bleaching process.