Aox Reduction Study During The Hing Temperature Chlorine Dioxide Bleaching Of Bagasse Pulpr

Hexenuronic acid(HexA)not only influences the determination of kappa number,but it is also one of the major factors producing absorbable organic halogens(AOX)during the chlorine dioxide bleaching of pulps.The efficient removal of the HexA from cooked pulp is important to minimize the need for bleaching and the pollution load from the mill effluent.In this paper,using bagasse pulp as raw material,the main factors influencing HexA degradation in the high-temperature chlorine dioxide bleaching(D-hot)process were analyzed by single factor experiment.The degradation conditions were optimized using response surface methodology(RSM).Then the impact of residual HexA in the pulp on the D-hot was analyzed.The degradation kinetics of HexA in D-hot process was studied by constructing a mathematical model.By comparing conventional chlorine dioxide bleaching(D0),the effects of D-hot on bagasse fiber and the micro-distribution of lignin in the process of delignification was investigated with FT-IR,XPS,and SEM-EDS,and the AOX components in bleaching wastewater were also investigated.The mechanism of AOX reduction and chemical changes of bagasse fibers during the D-hot were revealed.The main research contents and results are as follows:The RSM was applied to determine the optimal oxidative degradation conditions for HexA in bagasse pulp.Based on the previous single-variable test results,reaction temperature,pH,and reaction time were chosen as the independent variables,with the amount of degradation of HexA being the response value in the RSM.According to the central composite design(CCD)principles,the type of RSM with three factors and three levels was adopted and analyzed to determine the significant factors and the strength of interactions between factors.The optimal conditions determined were as follows:a reaction temperature of 94.7 ?,a pH of 3.7,and a reaction time of 124 min,which resulted in the amount of degradation of HexA at 10.5 ?mol g-1.The effect of the hemicellulose-linked HexA to chlorine dioxide content,kappa number,brightness reversion and effluent properties under the D-hot stage compared to a typical D0 was investigated.After bleaching,the amount of HexA in the bleached pulp was determined by UV-Vis.The AOX,COD and TOC in the bleached effluent were analyzed using the AOX analyzer,COD analyzer and TOC analyzer,respectively.The results showed that the amount of chlorine dioxide content could previously be reduced after D-hot.Kappa number and brightness reversion of bleached pulp obviously improved.Compared to control,the AOX content decreased dramatically with an increase in temperature.However,the COD and TOC increase were significant.A kinetic model considering the removal of hexenuronic acid and their degradation is proposed.Degradation reactions of hexenuronic acid are first-order with respect to their content.The pH influences the degradation of hexenuronic acid.The activation energy is 2.23 kJ·mol-1 when pH is 3.2 indicating that the degradation of hexenuronic acid is very rapid in the condition.The effect of the D-hot on the properties of bagasse fiber and the formation of AOX was studied.The bagasse pulp was subjected to both D-hot and D0,and the AOX contents in the effluent were analyzed respectively.The gas chromatography mass spectrometry(GC-MS)results suggested that the main components of the D0 stage wastewater were chlorinated hydrocarbons and chlorinated diphenyls.In contrast,those AOXs in the D-hot stage wastewater were very few.The GC-MS,ATR-FTIR,and XPS results showed the D-hot process is more effective in the removal of the residual phenolic lignin and the hemicellulose-linked HexA compared with D0.Furthermore,in comparison,the AOX content could be reduced by 50%with D-hot?The fully bleached pulp obtained via D-hotEpD process has a higher brightness than that obtained by D0EpD,which provides a reliable theoretical basis for industrial application.Pretreatment of bagasse pulp by D-hot has been performed to study delignification and structural changes in lignocelluloses.The compositions of the products were measured by GC-MS.The structure and morphology of the D-hot pretreated bagasse pulps were characterized by FT-IR,SEM,X-ray diffraction(XRD),Metso Kajaani FS300 fiber analyzer and optical analyzer.Temperature played a crucial role in the lignin depolymerization and hemicellulose hydrolysis.The results show that the lignin content was reduced by 28.39%in the P layer and 20.93%in the S layer with increasing temperature from 60 ? to 95 ?.The resulting compositions of D-hot stage effluent was mainly included oxidized lignin and derivatives of furfural.Compared with the control,the category of oxidized lignin can be increased with D-hot pretreatment.Also,pretreated bagasse pulp has high brightness of 59.71%ISO,crystallinity index of 70.84%,L/W of 50.84 and outstanding optical properties.Therefore,bleached bagasse pulps might have the opportunities for further application in paper and paper-based materials.