Chin. J. Chem. Eng., 15(1) 132-137
Regulation of Superoxide Anion Radical During the Oxygen Delignification Process* CAO Shilin(@6$k)'b, ZHAN Huaiyu(&F
r)", FU Shiyu(f$Bfm)"**and CHEN Lih~i(P$&&)~
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510641, Fhina Institute of Material Engineering, Fujian Agriculture and Forestry University, Fuzhou 350002, China a
Abstract In this study, the superoxide anion radicals were generated by the auto-oxidation of 1,2,3-trihydroxybenzene and determined by UV spectrophotometry, and the reaction was found to be facilitated by anthraquinone-2-sulfonic acid sodium salt. The bamboo haft pulps were treated by the 1,2,3-trihydroxybenzene auto-oxidation method or the 1,2,3-trihydroxybenzene auto-oxidation combined with anthraquinone-2-sulfonic acid sodium salt to show the effect of the superoxide anion radicals during the oxygen delignification of bamboo haft pulp and the enhancing affect of anthraquinone compounds as an additive on delignification. The results indicated that the superoxide anion radicals could react with lignin and remove it from pulp with negligible damage on cellulose, and the anthraquinone-2-sulfonic acid sodium salt could facilitate the generation of superoxide anion radical to enhance delignification of pulps. The oxygen delignification selectivity could be improved using the 1,2,3-trihydroxybenzene auto-oxidation system combined with anthraquinone-2-sulfonicacid sodium salt. Keywords superoxide anion radical, anthraquinone-2-sulfonicacid sodium salt, oxygen delignification, selectivity
1 INTRODUCTION During the past two decades, environmental concerns regarding the impact of bleaching plant effluents have led to major modification of the bleaching processes for kraft pulps. Conventional bleaching sequences using chlorine are replaced by elemental chlorine-free (ECF) and total chlorine-free (TCF) sequences. This trend is expected to go on along with the emergence of new technologies. Oxygen delignification was introduced into pulp bleaching as a pre-bleaching stage all over the world due to its environmental benignness. A lower kappa number of pulps after the oxygen delignigication stage results in a lower active chemical charge required to bleach the pulp such as reducing chlorine dioxide charge to achieve the target brightness. Furthermore, oxygen delignification contributes to reduction of AOX, BOD, COD and color in the effluent across the bleaching plants[l], and the effluents from an oxygen delignification stage can be recycled to the chemical recovery system. Indeed, it has been reported that application of oxygen delignification is able to offset total bleaching costs by 20% on average[2,3]. However, there are some drawbacks of oxygen delignification, the major one of which is the low selectivity of the process as compared to chlorine dioxide bleaching. Carbohydrates in pulp can be degraded when lignin is removed during oxygen delignification, so that the commercial delignification process has to be terminated before the kappa number drops approximately to 50% of that for the original unbleached pulp[3,4]. Although extensive research has been carried out with the aim of suppressing the degradation of carbohydrates and understanding the chemistry in oxygen bleaching, this drawback has not been well
resolved. The degradation of carbohydrates during oxygen delignification is not caused by the direct attack of molecular oxygen, but by active oxygen spereactions initiated by species, such cies and chain-type as HOO., 0 2 . , HO. and so on, which are generated through the lignin reactions and the reduction of oxygen species during the process. Studies showed that these active oxygen species attack not only lignin, but also polysaccharides to result in random chain cleavage[5-71. Gierer et aZ. considered it was significant for improving the selectivity of oxygen delignification to regulate the reaction of 0 2 . and HO- with lignin. Reitberger et al.  believed that oxygen delignification was a free radical process governed by the interplay between superoxide and hydroxyl radical. Oxygen delignification should be carried out under milder conditions and this would require in turn development of specific oxidation catalysts. Fu et aZ. suggested that the increase of 0 2 . - can facilitate oxygen delignification of pulp with little effect on the degradation of carbohydrates so that the oxygen delignification selectivity improvement could be improved. In this study, the superoxide anion radical was generated by the 1,2,3-trihydroxybenzene auto-oxidation system and the relative concentration of radicals can be determined by UV spectrophotometry. The 1,2,3-trihydroxybenzene auto-oxidation system was applied to the treatment of bamboo kraft pulp to imitate the action of the superoxide anion radical during oxygen delignification and to find a way to improve the selectivity of oxygen delignification process so that oxygen delignification technology can be further improved. The effect of anthraquinone-2-sulfonic acid sodium salt as additive on the facilitation of superoxide
Received 2005-1 1-07, accepted 2006-04-24.
* Supported by the National Natural Science Foundation of China (No.20477046), the Natural Science Foundation of Fujian Province of China (No.2004HZ03-5) and the Young Scientist Innovation Foundation of Fujian Province of China (No.2006F3009).
** To whom correspondence should be addressed. E-mail: [email protected]
Regulation of SuperoxideAnion Radical During the Oxygen Delignification Process
anion radicals was also investigated.
2 MATERIALS AND METHODS 2.1 Materials 2.1.1 Preparation of bamboo pulp Mixed bamboos, provided by Shaowu Zhongzhu Pulp&Paper Co., Fujian, were cultivated in the northern Fujian of China, including mainly Bambus Schrebel; Dendrocalamopsis (Chia et Fung) Keng f., Sinobambusa Makino, Phyllostachys Sieb. et Zucc. and so on. Bamboo pulp for oxygen delignification was cooked by the extended modified continuous cooking method with the cooking conditions: 14% (expressed as Na20) of active alkali charge at the impregnating zone and the current zone, 3Og-L-' of warm white liquor concentration at the countercurrent zone, 25% of sulfidity, 160°C of maximum cooking temperature[111. 2.1.2 Reagent 1,2,3-ttihydroxybenzeneand anthraq~one-2-sulfonic acid sodium salt were purchased from China Medicine Shanghai Chemical Reagent Corporation. Hydroxyme was purchased from Sigma. Other chemicals used were of analytical reagent grade. lOOmmol.L-' Tris-HC1 buffer solution at pH=8.2 was prepared by mixing 250ml of 0.4mol.L-' Tris and 200ml of 0.2mol.L-' HC1, and then diluted to lOOOml in a volumetric flask with double distilled water. The pH value of Tris-HC1 buffer solution was adjusted to 8.20 k0.02 with a pHS-3C digital pH meter. ~ m m o l-- ~ 1,2,3-trihydroxybenzene solution was prepared by dissolvFg 0.1261g of 1,2,3-ttihydroxybenzene with 10 mm01.L- HC1 and diluted to lOOOml in a volumetric flask. 1mmol.L - anthraquinone-2-sulfonic acid sodium solution was prepared by dissolving 0.3283g of anthraquinone-2-sulfonicacid sodium salt with double distilled water and diluted to lOOOml in a volumetric flask.
2.2 Methods 2.2.1 Determination of superoxide anion radical 0 2 . 5.0ml of Tris-HC1 buffer solution (lOOmmol.L-', pH=8.2) and an appropriate volume of 1,2,3-trihydroxybenzene solution (lmmol.L-') were added into a lOml colorimetric tube and diluted to the mark with double distilled water. After shaking, the solution was poured into the cuvette and the absorbance at 325nm was measured with a UV Spectrophotometer at an interval of 30s against a blank of deionized water. The rate of auto-oxidation of 1,2,3-trihydroxybenzene to produce superoxide anion radical was regulated to approximately 0.070minp1 by changing the concentration of 1,2,3-trihydroxybenzene. The temperature during measurement was kept at 25°C by a constant temperature water bath[ 121. 2.2.2 Facilitation of superoxide anion radical 0 2 . by anthraquinone-2-sulfonicacid sodium 5.0ml of Tris-HC1 buffer solution (lOOmmol.L-', pH = 8.2), 1.Oml of anthraqyinone-2-sulfonic acid sodium solution (lmmol-L- ) and 0 . 8 p of 1,2,3-trihydroxybenzene solution (1mmol.L- ) were added sequentially into a lOml colorimetric tube, and
then diluted to the mark with double distilled water. The absorbance at 325nm was measured as above. The solution of anthraquinone-2-sulfonic acid sodium mixed with double distilled water instead of 1,2,3-trihydroxybenzene solution was used as a reference since anthraquinone-2-sulfonicacid sodium had an absorbance at 325nm (see Fig.1). 0.60 I
0.12 0 300
340 360 380 wavelength, nm
Figure 1 The spectrum of the althraquinone-2-sulfonic acid sodium salt (20mmol.L- , aqueous solution)
2.2.3 Reaction of bamboo krafi pulp with 0 2 . - generated by 1,2,3-trihydroxybenzene auto-oxidation system The reaction of bamboo kraft pulp with the 1,2,3-trihydroxybenzene auto-oxidation system was performed in a plastic bag placed in a water bath held at 60°C. 20g of bamboo kraft pulp (dry base) mixed with variable amount of 1,2,3-trihydroxybenzene solution (1Og.L-') (ranging from 0 to 2.0% based on oven dry weight of bamboo kraft pulq) and lOOml of Tris-HC1 buffer solution (1OOmmol.L- , pH=8.2) were adjusted to a 10% consistency and loaded into a plastic bag. The plastic bag was sealed and immersed under the water for 90min. After reaction, the pulp was completely washed with double distilled water, dewatered to a 20%-30% consistency, dispersed and equilibrated in a sealed plastic bag (at least for 12h) for measurement. 2.2.4 Reaction of bamboo kraft pulp with 1,2,3trihydroxybenzene auto-oxidation system in the presence of the anthraquinone-2-sulfonicacid sodium salt The reaction 1,2,3-trihydroxybenzeneauto-oxidation system with bamboo h a f t pulp was carried out in a plastic bag placed in a water bath at 60°C. 20g of bamboo kraft pulp (dry base) mixed with variable amount of anthraquinone-2-sulfonic acid sodium salt (ranging from 0.05% to 0.2% based on dry weight of bamboo kraft puip), 2.0% of 1,2,3-trihydroxybenzene solution (1Og.k- ) and lOOml Tris-HC1 buffer solution (1OOmmol.L- , pH=8.2) was adjusted to a 10% consistency with double distilled water and reacted for 90min. After reaction, the pulp was washed well with double distilled water, dewatered to a 20%-30% consistency, dispersed and equilibrated in a sealed plastic bag (at least for 12h) for measurement. 2.2.5 Oxygen delignification of bamboo krafi pulp with the anthraquinone-2-sulfonicacid sodium salt Oxygen delignification was carried out in a one-liter stainless steel reactor with Teflon lining equipped with a back-to-back anchor stirrer with a Chin. J. Ch. E. 15(1) 132 (2007)
Chin. J. Ch. E. (Vol. 15, No.1)
diameter of lOOmm and an external oil mantle heater. 50g of bamboo haft pulp mixed with appropriate chemicals, additives and double distilled water was adjusted to 10% consistency and put into the reactor under the temperature of 90°C and oxygen pressure of 0.5MPa. The NaOH charge on oven dry pulp was 3%, the reaction time was 80min. The MgS04 charge (on oven dry pulp) was 0.5%. The stirring speed of the reactor was 400r/min. The amounts of anthraquinone-Zsulfonic acid sodium salt were varied from 0 to 0.2% based on oven dry weight of bamboo haft pulp. After oxygen delignification, the pulp was washed thoroughly with double distilled water, dewatered to a 20%-30% consistency, dispersed and equilibrated in a sealed plastic bag (minimum 12h) for measurement. 2.2.6 Measurement and analysis The Kappa number and viscosity were determined according to TAPPI T238 and SCAN-CM15:88, respectively. The whiteness was measured with a Technibrite Micro TB-1C. The calculation of the ratio of delignification, the ratio of viscosity reduction and the selectivity of delignification were done according to Ref.[l3].
ates, such as purpurogallin, semiquinonate, quinone and other oxides[l4,15]. The amount of superoxide anion radical is positively related to the amount of absorbent compounds, which can be determined by UV spectrophotometry. There is a linear relationship between the absorbance of the intermediates and reaction time from OSmin to 5min. However, the wavelength for measurement varied in different reports: 325nm[12,17], 337nm[l8] and 420nm[19,20]. In order to define a wavelength for the determination, the absorbance of 1,2,3-trihydroxybenzene solution and the absorbance of the 1,2,3-trihydroxybenzene auto-oxidation system were measured on the UV Spectrophotometer in the range from 200nm to 500nm. The results were showed in Figs.2, 3 and 4. The experimental results showed that the pure 1,2,3-trihydroxybenzenehad a maximum absorbance at about 220nm (Fig.2) and the intermediates generated in the 1,2,3-trihydroxybenzene auto-oxidation system had a maximum absorbance round 325nm (Fig.3) without any peak at 337nm and 420nm. The absorbance of the intermediates at 325nm increases with the processing of 1,2,3-trihydroxybenzene auto-oxidation, and reached to a maximum absorbance in about 25 min. Thereafter, the absorbance of the intermediates at 325nm decreased and the absorbance of the intermediates at 420nm increased gradually due to the further oxidation of intermediates (Fig.4), at which stage no more superoxide anion radical was formed. Therefore, the wavelength of 325nm was selected for measurement of superoxide anion radical instead of 337nm or 420nm.
3 RESULTS AND DISCUSSION It is widely believed that active oxygen species are formed during the oxygen delignification, which correspond to the lignin removal in pulp. The more the active oxygen species are in the reaction system, the more the lignin release from pulp. However, not all those active oxygen species are good for pulp because some of them, such as HO-, are high reactive chemically so as to destroy the cellulose in pulp. It was reported that the superoxide anion radical is possible to react with lignin by ring opening, but not break cellulose chains in pulp[ 101. Therefore, the selectivity of oxygen delignification can be improved by regulation of the active oxygen species in the oxygen delignification system. The aim - of present article is to study the generation of 0 2 . and the reaction of 0 2 . - with bamboo haft pulp, as well as the effect of anthraquinone-Zsulfonic acid sodium to facilitate 02.generation so as to increase the selectivity of delignification during the oxygen delignification process.
3.1 Generation and determination of 0 2 3.1.1 The wavelength for measurement of 1,2,3-trihydroxybenzene auto-oxidation system The auto-oxidation of 1,2,3-trihydroxybenzene can produce superoxide anion radical, accompanying the formation of absorbent compounds of intermediFebruary, 2007
230 250 270 wavelength, nm
Figure 2 The UV-visible spectrogram of 1,2,3-trihydroxybenzene(0.25mol.L-', pH=8.2 Tris-HC1 buffer solution) -18.1 1.2 $ 0.9
,Smin f i j 5 m i ,25min n
n 200 250 300 350 400 450 500 wavelength, nm
Figure 3 The UV-visible spectrogram of 1,2,34rihydroxybenzene auto-oxidationsystem from Smin to 25min (O.lrnolK', pH=8.2 Tris-HC1buffer solution)
Regulation of SuperoxideAnion Radical During the Oxygen DelignificationProcess
11.8 1.5 -
8 1.2 s -E 0.9 -
3 % 0.6 0.3 -
200 250 300 350 400 450 500 wavelength, nm
Figure 4 The UV-visiblespectrogram of 1,2,34rihydroxybenzeneauto-oxidationsystem from 25min to 125min (O.lmol.L-', pH=8.2 Tris-HC1 buffer solution)
3.1.2 Effect of the concentration of 1,2,3-trihydroxybenzene In order to examine the auto-oxidation rate of 1,2,3-trihydroxybenzene by UV spectrophotometry, the concentration of 1,2,3-trihydroxybenzene should be controlled in a suitable range because the absorbance of 1,2,3-tnhydroxybenzene varied widely with the increase of its concentration, which would cause large measuring error and a narrower linear range between absorbance and reaction time. On the other hand, measurement would be difficult for the low sensitivity if the concentration of 1,2,3-trihydroxybenzene was too low. The literatures showed that the concentration of 1,2,3-trihydroxybenzene should be controlled so that auto-oxidation rate of 1,2,3-trihydroxybenzene varied around 0.070min-' during its auto-oxidation to form superoxide anion radical[ 121. The present experimental results showed that the change of absorbance of 1,2,3-trihydroxybenzene auto-oxidation system with time at various concentration in Fig.5.
3.2 The facilitation of anthraquinone-2-sulfonic acid sodium salt to the generation of 0 2 It would be desired if the superoxide anion radical can be facilitated during oxygen delignification. In this article, the effect of anthraquinone-2-sulfonicsodium on the generation of superoxide anion radical was probed by adding anthraquinone-2-sulfonic sodium in the l ,2,3-trihydroxybenzene auto-oxidation solution. Anthraquinone, is an interesting compound, which can improve lignin dissolving during pulp cooking. In the oxygen delignification experiment, it was tested that whether anthraquinone was an efficient agent for advancing delignification. Because it was poor dissolved in water, anthraquinone-2-sulfonicacid sodium salt was used instead of anthraquinone. The experimental results in Fig.6 showed that the absorbance of the 1,2,3-trihydroxybenzene auto-oxidation system increased with the addition of anthraquinone-2-sulfonic acid sodium, which indicated the concentration of 02.- increased in the system. Therefore, the anthraquinone-2-sulfonic acid sodium can be used to facilitate the generation of 0 2 . h oxygen delignification system. The reason for facilitation of anthraquinone-2-sulfonicacid sodium to the formation of 02.-is not clear now, and the research is continuing.
3 4 5 time, min
Figure 6 The facilitation of anthraquinone-2-sulfonicacid sodium salt to the generation of superoxide anion radicals in the 1,2,3-trihydroxybenzeneauto-oxidationsystem NaAQ, ml: 0 0; A 1.0
3 0.3 -2 0.2 0.1
3 4 5 time, min
Figure 5 The auto-oxidation rate of 1,2,3-trihydroxybenzenewith different concentrationat the wavelength of 325nm concentration, mo1.L-l: + 0.04; rn 0.06; A 0.08; 0 0.10
It was demonstrated in Fig.5 that the rate of the auto-oxidation of 1,2,3-trihydroxybenzeneincreased with the increasing concentration of 1,2,3-trihydroxybenzene, and the concentration of 1,2,3-trihydroxybenzene between O.O8mol-L-' and O.lOmol.L-' was suitable for the measurement of the auto-oxidation system by W spectrophotometry. It was obvious that there existed good linear relationship between absorbance and time in 3min. Therefore, the concentration of 1,2,3-trihydroxyin the reaction system would be 0.08mol.L- in the subsequent experiment.
3.3 The reaction of 0 2 . generated in the 1,2,3trihydroxybenzene auto-oxidation system with bamboo kraft pulp The bamboo kraft pulp was treated with the 1,2,3-trihydroxybenzene auto-oxidation system to show the action of 0 2 . - in the delignification process, the results were shown in Table 1. It can be seen in Table 1 that the Kappa number of bamboo h a f t pulp decreased after reacting with the 1,2,3-trihydroxybenzene auto-oxidation system, and furthermore the degree of delignification increased with an increase in the concentration of 1,2,3-&ihydroxybenzene, which means that the part of lignin in pulp was removed by 02. . The viscosity of pulp reduced only 4.6% while the bamboo kraft pulp was delignified from Kappa number 17 to 15.2. This results also suggested that 02.- degraded the carbohydrates only slightly while it was reacting with lignin. Chin. J. Ch. E. 15(1) 132 (2007)
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The reason for 0 2 . - removing lignin was that it can react with the phenolic lignin unit to break its aromatic structure and form muconic acid derivatives. Therefore, the lignin fragments became water-soluble and could be removed from the pulp. Fu et aZ. proposed that the formation of methanol during the reaction of 0 2 . - with the phenolic lignin unit confirmed the demethylation of lignin, which led to an increase in phenolic content.
anthraquinone-2-sulfonic sodium to generation of 02.resulted in the increase of the amount of 0 2 . - , which is good oxygen active species to react with lignin in bamboo haft pulp, and accelerated removal of lignin with very little effect on the carbohydrates.
3.5 The effect of anthraquinone-2-sulfonic sodium to the oxygen delignification of tlie bamboo kraft pulp In order to study the effect of an3.4 Effect of anthraquinone-2-sulfonic acid sothraquinone-2-sulfonic sodium on the oxygen deligdium to the reaction of the superoxide anion radinification of the bamboo h a f t pulp, the oxygen deligcals with bamboo kraft pulp nification of the bamboo haft pulp was performed by From above results, anthraquinone-2-sulfonic soadding auxiliary agent anthraquinone-2-sulfonicsodium may improve formation of superoxide anion radicals. Furthermore, the effect of anthraquinone-2-s~dfonic dium. The results were shown in Table 3. It was displayed in Table 3 that the Kappa numsodium on the reaction system of the superoxide anion ber of the bamboo haft pulp decreased, the brightness aft pulp was studied by adding radicals with bamboo h of the pulps and the degree of delignification inanthraquinone-2-sulfonic sodium to the mixture of pulp creased, and the viscosity of the pulp also increased and 1,2,3-trihydroxybenzene. with an increase of the charge of anThe experiment results in Table 2 showed that the thraquinone-2-sulfonic acid sodium, so that the selecdegree of delignification and the selectivity of deligAs tivity of oxygen delignification was improved. nification for bamboo haft pulp increased gradually shown by the above experiments, 0 2 - generated with a little loss of viscosity as the charge of anduring the delignification process can react with lignin thraquinone-2-sulfonic sodium increased. The reason and remove it, which was facilitated by anfor such beneficial results is that the facilitation of thraquinone-2-sulfonic sodium. In addition, negligible Table 1 The reaction of the superoxide anion radicals with kraft pulp 1,2,3-Trihydroxybenzene charge, %
Bamboo haft pulp after reaction KaDDa number viscositv.
Ratio of delignification, %
Ratio of viscosity reduction, %
Selectivity of delignification
Note: Kappa number of original bamboo haft pulp, 17.2; viscosity, lllOml.g-'; reaction temperature, 60°C; time, 90min.
Table 2 The effect of anthraquinone-2-sulfonicacid sodium salt on the reaction of 02.-with bamboo kraft pulp Anthraquinone-2-sulfonic acid sodium charge, /%
Bamboo haft pulp after reaction K~~~~ number viscosity,
Ratio of delignification, %
Ratio of viscosity reduction, %
Selectivity of delignification
4.4 14.1 1040 18.0 6.3 0.2 Note: 1. Kappa number of original bamboo haft pulp, 17.2; viscosity, lllOml.g-'; reaction temperature, 60°C; time, 90min. 2. The 1,2,3-trihydroxybenzenecharge, 2.0% (based on 0.d. mass of bamboo pulp).
Table 3 The effect of anthraquinone-2-sulfofonicacid sodium on the oxygen delignification of the bamboo kraft pulp Anthraquinone-2-sulfonic acid sodium charge, %
Bamboo haft pulp after reaction K~~~~ number viscosity, d . g - l
Ratio of Ratio of viscosity Selectivity of Whiteness, % delignification, % reduction, % delignification (ISO)
0.2 7.5 954 56.4 14.1 6.2 33.7 Note: Kappa number of original bamboo haft pulp, 17.2; viscosity, 1llOmbg-'; reaction temperature, 90°C; reaction time, 80min; oxygen pressure, O.5MPa; pulp consistency, 10%; MgS04 charge, 0.5%; stirring speed, 400rmK'.
Regulation of SuperoxideAnion Radical During the Oxygen DelignificationProcess
reduction of viscosity of bamboo h a f t pulp was observed since the degradation of carbohydrates was mild due to the lower reactivity of 0 2 . - compared to HO.. HO- is a very strong oxygen active species, which can react with both lignin and cellulose. It would be much beneficial if 0 2 . - can be facilitated and HO. can be restrained during oxygen delignification. It was concluded that the selectivity of delignification during oxygen delignification was improved by adding anthraquinone-2-sulfonicsodium.
4 CONCLUSIONS The oxygen delignification is a chemically efficient and environmentally benign technology for offsetting effluent loading during the pulp bleaching. Regulation of active oxygen species including 0 2 . , HO., etc. becomes important in the oxygen delignification for reducing degradation of carbohydrates and improving the selectivity of delignification. The 1,2,3-tnhydroxybenzene auto-oxidation system can form 02+accompanying some absorbent compounds, so that the 0 2 . - can be indirectly surveyed using UV spectrophotometry. The 0 2 . - showed its possibility to delignification with negligible cellulose loss. Besides, a anthraquinone compound, the anthraquinone-2-sulfonic sodium can facilitate 02. generation in the 1,2,3-trihydroxybenzene auto-oxidation system and improve the selectivity of lignin degradation from bamboo h a f t pulp. The increase of both the delignification rate and selectivity of oxygen delignification can be obtained by adding anthraquinone-2-sulfonic sodium in the bamboo h a f t pulp oxygen delignification process. NOMENCLATURE KO the Kappa number before oxygen delignification K1 the Kappa number after oxygen delignification L the ratio of delignification S the selectivity of delignification V the ratio of viscosity reduction V, the viscosity before oxygen delignification, mly-’ V1 the viscosity after oxygen delignification, ml.g-
2 3 4
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