Yellowing Inhibition Of Poplar Mechanical Pulp And Wheat Straw Chemical Pulp By Biological Pretreatment And Its Mechanism

China is a major country of production,trade and consumption of paper products,and the traditional pulp was often produced from wood as raw material,which brings serious pressure to the limited forest resources.At the same time,the extensive use of chemical reagents in pulping process also brings serious environmental problems.High yield pulp and the non-wood pulp,partly eased the contradiction between the supply and demand of wood resources.However,the characteristics of straw pulp and high yield pulp,which are easy to yellow under the light or heat induction,limit their application.Up to now,the some researches about the pulp yellowing inhibition have been published,and the reported methods in the literatures were focused on lignin modification,addition of inhibitor and whitening agent,etc.Due to the disadvantages of high cost and high toxicity,however,these methods have not been widely used.The application of biotechnology in paper industry is of great significance to suppress the yellowing of pulp,improve the quality of pulp,and reduce production costs and pollutant emissions.In our previous work,a strain,Fusarium concolor X4,was screened and found that it had obvious inhibition effect on the heat induced yellowing of pulps.The strain has relative low cellulase activity,which was little effected on pulp yield after biological treatment.On the basis of previous studies,this paper studied the yellowing inhibition effect of Fusarium concolor X4 treatment on high yield pulps,and discussed its inhibition yellowing mechanism.Based on rules of enzymes production during the Fusarium concolor X4 treatment on pulp,some main enzymes such as xylanase,laccase,etc.were screened out and used to treat wheat straw chemical pulp or high yeild pulp for studying their inhibition effect on heat-and light-induced yellowing of the pulps.Based on the above research,it is expected to provide references for the modification of microbial enzymes and the production of compound enzyme preparations to inhibit the yellowing of pulp.The main research contents and results are as follows:1.Mechanism of inhibition of yellowing of poplar CTMP by the treatment with Fusarium concolor X4The brightness decrease of the pulps was related to the light source and the type of pulps.Compared with the chemical pulp,light induced yellowing of CTMP was more likely to occur.In the 24 hours of light aging,the brightness of pulp decreased rapidly,then,the yellowing trend slowed down.The brightness of pulps decreased with the decrease of light wavelength when light intensity was same.Based on yellowing effect and treatment time,the 340 nm light source and 24 h of aging time were chosen as conditions of light aging for light induced yellowing of pulp.Dry-heat aging method has unique advantages in the condition control and reproducibility.In the study,the dry-heat at 105? was chosen for studying on heat induced yellowing of pulp.By studying the changes on pulp brightness and PCN of poplar CTMP and wheat straw CP during heat aging in different time,we determined that dry-heat aging time was 24 h.Fusarium concolor X4 treatment can improve the brightness of poplar CTMP and has good inhibition effect on yellowing.On the 2nd day and the 6th day of the treatment,the brightness of pulp increased by 5%and 12.2%,and PCNs decreased by 14.5%and 16.8%,respectively.Lignin peroxidase,manganese peroxidase,xylanase,and a very small amount of FPA,endo-1,4-?-glucanase(CMCase),and ?-xylosidase had been secreted during the bio-treatment.Lignin peroxidase and xylanase were detected at the 2nd day of treatment,but manganese peroxidase and laccase appeared relatively late,and reached to the peak at 10th day and 8th day of biological treatment respectively.Although lignin peroxidase was secreted earlier,the maximum activity of the enzyme was reached at 14th day.The analysis of diffuse reflectance spectrum,chemical compositions and infrared spectrum showed that Fusarium concolor X4 treatment decreased the content of lignin,hemicellulose and HexA(hexenuronic acid)in poplar CTMP.In which,the hemicellulose content greatly decreased after biological treatment,and lignin content,especially for S lignin,was reduced to some extent.The decrease of these substances reduced the sensitivity of pulp to light,thereby reduced the formation of chromophores in pulps after light aging.2.Mechanism and inhibition of yellowing of pulps by the treatment of hemicellulaseThe heterologous expressed xylanase was purified and used to treat poplar CTMP and wheat straw CP,showed that xylanase treatment had effect on brightness improvement and yellowing inhibition.And the effect on brightness and yellowing inhibition of wheat straw CP was better than that of poplar CTMP.After enzyme treatment at xylanase dosage of 40 IU/g,brightness of wheat straw CP increased by 3%compared with control,and poplar CTMP was increased by 0.6%.The PCN of wheat straw CP decreased by 14.3%and 34.1%compared with control after light and heat induced yellowing,while poplar CTMP had no significant change after enzyme treatment.The analysis of the supernatant from treatment liquor and chemical composition of pulps after xylanase treatment indicated that xylanase had different effect on hemicellulose,lignin and HexA content of different pulp.Compared with CTMP,the related components in wheat straw CP were more likely to be degraded by xylanase treatment.Diffuse reflectance spectra and infrared spectra showed that xylanase treatment decreased the content of hemicellulose and lignin in the pulp,and effectively reduced the formation of chromophores during light-and heat-induced yellowing.The mixed treatment with xylosidase(Abf and Xyl)and xylanase(Xyn)was helpful to improve the brightness and the brightness stability of wheat straw CP,but the synergistic effect on poplar CTMP was weak.Based on the infrared spectra of pulps and treatment liquor components analysis before and after enzyme treatment,it was found that mixed enzyme treatment promoted the degradation of hemicellulose and lignin in pulp,and the effect on wheat straw CP was more obvious.3.Mechanism and inhibition of yellowing of poplar CTMP and wheat straw CP by laccase and laccase/mediator treatmentIt was found that laccase treatment affected brightness and brightness stability of poplar CTMP,and the brightness stability could be greatly improved by the addition of medium,but it had an adverse effect on the brightness of bleached poplar CTMP.Compared with mediator ABTS,acetosyringone and lilac aldehyde,mediator HBT addition resulted in the greatest reduction in the PCN of poplar CTMP,but not significantly affected the brightness of bleached pulp.When 2%HBT(w/w,oven dry weight of pulp)was added to the laccase system(20IU/g),PCN of poplar CTMP decreased by 17.8%and 21.2%after light and heat induced yellowing,respectively.The treatment of laccase and laccase/mediator was beneficial to the improvement of brightness and brightness stability of bleached wheat straw CP,and the increase of brightness after LMS treatment was higher than that of brightness after L treatment.For light-induced yellowing,PCN of wheat straw CP was reduced by about 15%after L and LMS treatment compared with the respective control,and for the heat induced yellowing,PCN of LMS treated pulp was reduced by 14.3%compared with the control,while the L treatment did not improve the heat stability of wheat straw CP.Analysis of the hydrolysate fractions,diffuse reflectance spectra and infrared spectra of poplar CTMP and wheat straw CP showed that the dissolution of hemicellulose and lignin by laccase mediator(LMS)treatment were higher than that by laccase(L)treatment,and the effect of the treatment with L and LMS on wheat straw CP was greater than that on CTMP.XPS analysis of wheat straw CP before and after enzyme treatment showed that treatment with L and LMS could reduce the lignin content on pulp surface,and LMS treatment was better than L treatment.The analysis about infrared spectrum of milled wood lignin(WML)from poplar CTMP and wheat straw showed that L and LMS treatment could reduce the aromatic carbonyl and ether structure in WML,and inhibit the formation of aromatic carbonyl and ether in WML of poplar CTMP during light and heat aging.These structures in lignin from wheat straw were also weakened during light aging.The UV-vis spectra of WML from poplar CTMP and wheat straw before and after treatment with laccase and LMS showed that the both L and LMS treatment resulted in MWLs degradation,and LMS led to more degradation.Under the same treatment conditions,the effect of laccase and LMS on the lignin of wheat straw was greater than that of poplar CTMP,and LMS effect on lignin was greater than laccase.4.Mechanism and inhibition of yellowing of poplar CTMP and wheat straw CP by mixed enzyme of laccase and hemicellulaseThe mixed treatment of laccase,laccase/mediator and hemicellulase could significantly improve the brightness and brightness stability of poplar CTMP and wheat straw CP.The treatment with Xyn/Abf-LMS system had the greatest influence on pulp brightness and brightness stability,and the effect on wheat straw CP was better than that of poplar CTMP.Based on the analysis of the hydrolysate components of poplar CTMP and wheat straw CP,diffuse reflectance spectra and infrared spectra,it was found that the treatment of hemicellulase and mixed enzyme of xylanase and laccase can lead to hemicellulose and lignin dissolution.The effect of Xyn/Abf-LMS and Xyn/Xyl-LMS system was the most obvious,and the changes of components of wheat straw CP was greater than that of poplar CTMP.XPS analysis of wheat straw CP before and after enzyme treatment showed that Xyn/Abf-LMS treatment could reduce the content of lignin and increase the content of aldehyde and ketone groups on the pulp surface.