Study On Laccase/Mediator System Deinking And Mechanism Of Old Newsprint

The Recycling of wastepaper is an effective way to resolve those problems that pulp and paper industry now is facing, such as shortage of raw materials, lacking of energy, pollution, et al. Deinking is the key of recycling. Contrary to conventional chemical deinking, enzymatic deinking has many priorities. Now deinking with cellulase and hemicellulase has been industrialized. It is significant to search other enzymes that could be used in deinking.Ligninases that catalyze the removal of surface lignin are promising for deinking of old newsprint that contains lignin-rich mechanical pulp. Ligninases include three enzymes-LiP, MnP and laccase. Laccase(EC1.10.3.2)is a polyphenol oxidase, and it can degrade the lignin when used in the presence of a chemical mediator and oxygen.It is important to evaluate the quality of deinking pulp quickly and correctly to control and optimize deinking technology, and to ensure paper made from the secondary fiber could meet the needs. So it is significant to study methods of qulity evaluation of deinked pulps.Different specimen preparation methods were used to evaluate deinked pulp quality and deinking efficiency. To ONP chemically and enzyme deinked pulp, wire-blotter difference of specimen prepared using standard methods, tap water, no adjustment of pH, adding PAM was the least and retention of ink particles was the highest. To MOW chemical and enzyme deinked pulp, wire-blotter difference of specimen prepared using buchner funnel procedure, distilled water, adding PAM and Al₂(SO₄)₃ was the least and retention of ink particles was the highest. Quality of deinked pulps could be evaluated quickly and correctly to combining brightness with image analysis and ERIC. Calculating practical and theoretical deinking efficiency during deinking process, results demonstrated that practical deinking efficiency was always smaller than theoretical deinking efficiency which clarified that deinking technology could be further optimized.Deinking technology of ONP with Laccase mediator system(LMS) was studied. Results showed that it was practicable to deink ONP with LMS. The optimum pulping conditions were: laccase charge 10 LAMU/g, mediator (violuric acid) charge 0.5%, time of pulping 30 min, and keeping the pulp at the optimum temperature for 15 min. yield of LMS deinked pulp was higher than the control pulp. Its flotation selectivity was also higher.Deinking technology of ONP by cellulase/hemicellulase combined with LMSwas studied. Results showed that cellulase/ hemicellulase combined with LMS could deink ONP synergistically. The optimum conditions of LMS /cellulase deinking were: first deinking with 0.2 ECU/g cellulase for 25 min, and then keeping temperature at 55 "C with 3 LAMU/g laccase and 0.5% mediator for 30 min. The optimum conditions of LMS /hemicellulase deinking were: first deinking with 0.5 AXU/g cellulase for 25 min, and then keeping temperature at 55 °C with 3 LAMU/g laccase and 0.5% mediator for 30 min. Yield and flotation selectivity of LMS deinked pulp were higher than those of the control.Fiber morphology of deinked pulps were studied by ESCA, IR, SEM. Fiber average length and coarseness, specific surface area and specific volume were determined. ESCA analysis showed that O/C of LMS deinked pulp was higher than that of the control pulp;surface lignin content decreased after LMS treatment. IR analysis showed that some lignin was dissolved during LMS treatment. Fiber average length and coarseness changed little even though a large dosage of laccase was used, which declared that laccase action mainly occurred at the surface of fiber, and could not touch inner of the fiber. Specific surface area and specific volume analysis of LMS deinked pulp were higher than those of control pulp. SEM Results showed that defiberaion took place at the fiber surface.Dioxane lignin of deinked pulp was extracted and its structure was analyzed by elemental analysis, functional groups determination, GPC, IR, 'H-NMR and 2D-NMR. Comparing with lignin from control pulp, phenolic hydroxy and methoxyl content of lignin from LMS deinked pulp decreased, while content of carboxyl and carbonyl groups increased. IR results showed that lignin was degraded during LMS treatment. Content of methoxyl, phenolic hydroxy, hydroxy of lignin were calculated through 'H-NMR. They all decreased after laccase treatment. 2D-NMR results showed that guaiacyl unit was oxidized by LMS. Stilbene, dibenzene methane and 5-5'type sub-structure in lignin were stable to LMS treatment, while P -O-4 and P - P type sub-structure were degraded.Combining fiber morphology with lignin structure analysis, probable mechanism of LMS deinking could be inferred. First, mediator was oxidized to mediator complex by laccase with the presence of oxygen. This mediator complex was very small, so it could osmose into the fiber and oxidativly degraded lignin. The bond between ink and fiber became loose. Ink could be peeled off the fiber at the aid of shear force.