| With the growth of constraints in resources, energy, environment, the most important step to achieve sustainable development of the pulp and paper-making industry is to make full use of the plant resources. In recent years, high-yield pulp has been rapidly developed. However, compared with chemical pulp, high-yield pulp has the disadvantages of low fiber strength, low stiffness and susceptibility to photo-yellowing and so on. Therefore, in order to make high-yield pulp more widely used, modification is imperative. In this study, APMP pulp was used as raw material. Laccase and laccase mediator system (LMS) are used for modifying APMP, respectively. And the influences of laccase and LMS on the postcolour number of APMP pulp were studied. Enzyme-mild acidic hydrolysis was adoped to isolate and purify the residual lignin in APMP pulp, then laccase and LMS were applied to modified lignin to analyze their impacts on the properties of lignin. It is important to reveal mechanism of laccase and LMS for APMP photo-yellowing.In this paper, laccase and LMS were used to modify the APMP before the treatment of UV light irradiation and an aging treatment at 105℃.The main purpose of the modification was to analyse the changes of brightness. After that, the effects of laccase and LMS on the photo-yellowing were studied. The optimal modified condition:dosage of laccase was 4 IU/g, dosage of HBT was 0.5%, pulp consistence was 4%, time was 60 min, temperature was 50℃, and the pH was 4.5. The APMP treated by laccase and LMS showed an effective resistant of photo-yellowing. And the brightness was slightly lower.In the experiment, thermal gravimetric analysis method was used to test laccase and LMS’s impacts on thermal stablility of lignin. Experiments showed that the modification can increase the rate of thermal degradation of lignin and reduce lignin’s thermal stability. After treatment of LMS, the initial temperature of substantial weight loss is lower than that of the laccase treatment, and the degradation rate is also higher, indicating that the addition of mediator could reduce the thermal stability of the ligninThis paper selected 13C-NMR and 31P-NMR to analyze the content of functional groups and connecting structures in lignin structure.13C-NMR test results found that laccase and LMS had a significant impact on the lignin’s structure. After laccase treated, the contents of COOH, C3-OH, C1-C and Ca in β-O-4 had increased, while the contents of C6-H, C2-H, Cβinβ-O-4 and OCH3 were slightly lower. After LMS treatment, the contents of COOH, C3-OH increased slightly, and the contents of C1-C, Ca and Cp in β-O-4, C6-H, C2-H and OCH3 reduced significantly.31P-NMR analysis showed that, after the modification of laccase and LMS, the aliphatic hydroxyl group and the total phenolic hydroxyl contents were reduced, and contents of condensation type phenolic hydroxyl, S-type phenolic hydroxyl group, G-type phenolic hydroxyl group and a phenolic hydroxyl group were also significantly decreased. Among them, in addition to S-type phenolic hydroxyl, the contents had larger declination after LMS post-processingIn addition, Gel Permeation Chromatography (GPC) is one of the important means for detecting the lignin performance. Studies had shown that the molecular weight of lignin after laccase treatment have a more significant increase due to the possible polymerization reaction among lignin. The molecular weight of the lignin after treatment of LMS is significantly lower. The process may result in a degradation of the lignin macromolecules... |
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