Aox Formation Mechanisms During The Chlorine Dioxide Bleaching Of Bagasse Pulp

As the main reagent of ECF bleaching technology, chlorine dioxide has a good delignification selectivity. But the absorbable organic halogens (AOX) will still be produced during the bleaching. AOX contains more than 300 different organochlorines and some compounds of the AOX are toxic and show the tendency to biological accumulation because of their lipophilic nature. These parts of AOX are the uppermost Persistent Organic Pollutants (POPs) during the ECF bleaching. The pulp bleaching effluent pollution problems have become increasingly prominent these years. The emission standard of AOX has been adjusted as a mandatory assessment indicator in the papermaking industrial pollutants emission standards of China.With regard to the reasons of AOX formation during the bleaching of pulp, many scholars believe that residual lignin of the unbleached pulp is the main source of AOX. In fact, formation of AOX can also be effected by hemicellulose of the pulp. During the alkali and high temperature cooking, 4-O-methylglucuronic acid of the hemicellulose will be turned to unsaturated hexenuronic acid (HexA). In addition to the contribution of lignin to AOX formation, research on influence of on AOX formation will provide theoretical basis for the controlling of the AOX formation during the ECF bleaching of bagasse pulp.This study employed the response surface method optimizing for the AOX formation during the ECF bleaching of bagasse pulp. The macrokinetics model was established and the Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and X-ray Photoelectron Spectroscopy (XPS) surface analysis methods were used to study the kinetics and mechanisms of AOX formation. The lignin model compound was employed to investgate the kinetic reaction of lignin and chlorine dioxide, and mechanisms of AOX formation was then expounded. The xylanase was employed to pretreat for the bagasse pulp, and influence of HexA on AOX formation were investgated. The major studies and results are as follows:The first chlorine dioxide stage (DO) in the ECF bleaching of a soda AQ bagasse pulp has been investigated using response surface methodology (RSM) for experimental design. The effects and interactions of the available chlorine dosage, the reaction temperature and the pH value, on the AOX content of wastewater as well as the brightness and viscosity of the pulp were examined. The optimal reaction conditions to achieve a balance of reduced AOX content, higher brightness and higher viscosity were:available chlorine dosage of 3.7%, reaction temperature of 76℃ and pH 8.8. Under these conditions, AOX content of 14.7 mg/L, brightness of 52.4% ISO and viscosity of 929 mL/g were observed. To the authors’knowledge these results provide the only detailed study of AOX formation in ECF bleaching of Soda AQ pulp from bagasse. By and large the effects of process variables were similar to those seen in ECF bleaching of kraft pulps.A kinetic model of the first chlorine dioxide bleaching stage (DO) in an ECF bleaching sequence is presented for bagasse pulps. The model is based on the rate of AOX formation. The effects of the chlorine dioxide dosage, the sulfiiric acid dosage, and the reaction temperature on the AOX content of wastewater are examined. The reaction of AOX formation could be divided into two periods. A large amount of AOX was formed rapidly within the first 10 min. Ten minutes later, the AOX formation rate significantly decreased. The kinetics could be expressed as dW/dt=660.8·eT/997.98·[ClO2]0.877·[H2SO4]0.355 ·W-1.065, where W is the AOX content, t is the bleaching time (min), T is the temperature (K), [ClO2] is the dosage of chlorine dioxide (kg/odt), and [H2SO4]is the dosage of sulfuric acid (kg/odt). The fit of the experiment results obtained for different temperatures, initial chlorine dioxide dosages, initial sulfuric acid dosages, and AOX content were very good, revealing the ability of the model to predict typical mill operating conditions.The aim of this work was to obtain knowledge on how chlorine dioxide bleaching change the surface chemistry of bagasse pulp, as well as of how the surface chemistry affect the AOX formation, by using chemical analysis in combination with Attenuated Total Reflectance-Fourier Transform Infrared Spectrum (ATR-FTIR) and X-ray Photoelectron Spectroscopy (XPS). Both ATR-FTIR and XPS analysis indicated that surface oxidation took place immediately during the chlorine dioxide bleaching. Lignin of the fiber surface obviously decreased during the bleaching, especially for the first 10 minutes. Ratio of aromatic carbons to aliphatic carbons decreased during the bleaching also indicated that the lignin was degraded and some new aliphatic carbons formed, which might noticeably deposit on the surface of the fiber. As the bleaching time went on, more and more acidic function groups turned up on the fiber surface, (A/B) ratios constantly increased. The acidic function groups can also react with the electron donor, which thus affect the whole bleaching efficiency.Kinetics study of oxidation of 1-(3,4-Dimethoxyphenyl)ethanol, a lignin model compounds, with chlorine dioxide under simulated bleaching conditions were investigated. The UV-vis spectrophotometer was employed to characterize the reaction products of chlorine dioxide and 1-(3,4-Dimethoxyphenyl)ethanol. A possible reaction mechanism was proposed. The results showed the chlorination and oxidizing reaction of 1-(3,4-Dimethoxyphenyl)ethanol coexisted. The hypochlorous acid was the main reason to form AOX. An overall second-order rate equation was also established from UV-vis spectra results. It was shown as:-d[ClO2]/dt=k’[MVA][ClO2], where k’ refers to observed second-order rate constants. The results showed the kinetics model correlated well with the experimental data, so the degree of 1-(3,4-Dimethoxyphenyl)ethanol oxidized by chlorine dioxide can be quantitative evaluated based on this model. AOX formation characteristic and practical implications of this work were investigated based on the kinetics and mechanism, which might bring a new door open for resolving the major problems of environmental pollution in ECF bleaching of pulp.Xylanase was evaluated in chlorine dioxide bleaching of bagasse pulp. Pretreated with xylanase could obviously reduce AOX and HexA in the same dosage of chlorine dioxide. Compared to the control pulp, AOX could be reduced about 21.4% to 26.6% after xylanase treatment. Chlorine dioxide demand could be reduced about 12.5% to 22% to achieve the same brightness. The ATR-FTIR and XPS were employed to determinate the surface chemistry of the control pulp, xylanase treated and chlorine dioxide treated pulps. It showed that lignin and hemicellulose (mainly HexA) were the chief culprits resulting in AOX formation. Xylanase pretreatment could remove HexA and expose more lignin, which thus reduced the chlorine dioxide demand.