Biodegradability of kraft mill TCF biobleaching effluents: application of enzymatic laccase-mediator system

Biobleaching: An eco-friendly approach to reduce chemical consumption and pollutants generation

The pulp and paper industry is known to be a large contributor to environmental pollution due to the huge consumption of chemicals and energy. Several chemicals including H2SO4, Cl2, ClO2, NaOH, and H2O2 are used during the bleaching process. These chemicals react with lignin and carbohydrates to generate a substantial amount of pollutants in bleach effluents. Environmental pressure has compelled the pulp and paper industry to reduce pollutant generation from the bleaching section. Enzymes have emerged as simple, economical, and eco-friendly alternatives for bleaching of pulp. The pretreatment of pulp with enzymes is termed as biobleaching or pre-bleaching. Different microbial enzymes such as xylanases, pectinases, laccases, manganese peroxidases (MnP), and lignin peroxidases are used for biobleaching. Xylanases depolymerize the hemicelluloses precipitated on pulp fiber surfaces and improves the efficiency of bleaching chemicals. Xylanase treatment also increases the pulp fibrillation and reduces the beating time of the pulp. Pectinases hydrolyze pectin available in the pulp fibers and improve the papermaking process. Laccase treatment is found more effective along with mediator molecules (as a laccase-mediator system). Biobleaching of pulp results in the superior quality of pulp along with lower consumption of chlorine-based chemicals and lower generation of adsorbable organic halidesadsorbable organic halides (AOX. An enzyme pretreatment reduces the kappa number of pulp and improves ISO brightness significantly. Better physical strength properties and pulp viscosity have also been observed during biobleaching of pulp.

Immobilized laccase on bentonite-derived mesoporous materials for removal of tetracycline

Bentonite is a natural and environmentally clay mineral, and bentonite-derived mesoporous materials (BDMMs) were obtained conveniently from the alkali and acid treatment of bentonite. In the present study, BDMMs were explored for immobilization of laccase obtained from Trametes versicolor. As a result, bentonite-derived mesoporous materials-Laccase (BDMMs-Lac) was developed for the removal of tetracycline (TC). The enzyme immobilization process was carried out through physical adsorption contact (ion exchange adsorption, hydrogen bond adsorption, and Van der waals adsorption) between the BDMMs and laccase. The process of immobilization remarkably increased its operating temperature. The BDMMs-Lac exhibited over 60% removal efficiency for TC within 3 h in the presence of 1-hydroxybenzotriazole (HBT). In conclusion, BDMMs-Lac showed more promising potential than free laccase for practical continuous applications.

New alternative energy pathway for chemical pulp mills: From traditional fibers to methane production

Chemical pulp mills have a need to diversify their end-product portfolio due to the current changing bio-economy. In this study, the methane potential of brown, oxygen delignified and bleached pulp were evaluated in order to assess the potential of converting traditional fibers; as well as microcrystalline cellulose and filtrates; to energy. Results showed that high yields (380mL CH₄/gVS) were achieved with bleached fibers which correlates with the lower presence of lignin. Filtrates from the hydrolysis process on the other hand, had the lowest yields (253mL CH₄/gVS) due to the high amount of acid and lignin compounds that cause inhibition. Overall, substrates had a biodegradability above 50% which demonstrates that they can be subjected to efficient anaerobic digestion. An energy and cost estimation showed that the energy produced can be translated into a significant profit and that methane production can be a promising new alternative option for chemical pulp mills.

Xylanase and laccase based enzymatic kraft pulp bleaching reduces adsorbable organic halogen (AOX) in bleach effluents: a pilot scale study

In present study, xylanase and laccase were produced in a cost-effective manner up to 10 kg substrate level and evaluated in elemental chlorine free bleaching of Eucalyptus kraft pulp. Compared to the pulp pre-bleached with xylanase (15%) or laccase (25%) individually, the ClO2 savings were higher with sequential treatment of xylanase followed by laccase (35%) at laboratory scale. The sequential enzyme treatment when applied at pilot scale (50 kg pulp), resulted in improved pulp properties (50% reduced post color number, 15.71% increased tear index) and reduced AOX levels (34%) in bleach effluents. The decreased AOX level in effluents will help to meet AOX discharge limits, while improved pulp properties will be value addition to the paper.

Polymerisation of Kraft lignin from black liquors by laccase from Myceliophthora thermophila: effect of operational conditions and black liquor origin

The capacity of laccase from Myceliophthora thermophila to promote the oxidative polymerisation of Kraft lignin (KL) was evaluated in several conditions of pH, temperature, enzyme dosage and treatment time. Moreover, different black liquors from the Kraft cooking of Eucalyptus globulus and mixture of Pinus pinaster/E. globulus were evaluated in order to determine the effect of the KL source on the polymerisation reaction. Furthermore, one of these black liquors was fractionated by sequential organic solvent fractionation and the polymerisation of the corresponding fractions was tested. Polymerisation products were analysed by size exclusion chromatography and Fourier transform infrared spectroscopy. The results provide evidence of notable lignin modifications after incubation with laccase. Structural oxidation and a notably molecular weight increase were attained, reaching a polymer of 69-fold its initial molecular weight depending on the raw lignin. Moreover, optimum values of reaction conditions were obtained: pH 7.3, 70°C, 2UmL(-1) and 2h.

Assessing the environmental impact of biobleaching: effects of the operational conditions

The environmental impact of enzyme bleaching stages applied to oxygen-delignified eucalypt kraft pulp was assessed via the chemical oxygen demand (COD), color, absorbance spectrum, residual enzyme activity and Microtox toxicity of the effluents from a laccase-HBT (1-hydoxybenzotriazole) treatment. The influence of the laccase and HBT doses, and reaction time, on these effluent properties was also examined. The laccase dose was found to be the individual variable most strongly affecting COD, whereas the oxidized form of HBT was the main source of increased color and toxicity in the effluents. Moreover, it inactivated the enzyme. Oxidation of the mediator was very fast and essentially dependent on the laccase dose. Using the laccase-mediator treatment after a xylanase stage improved pulp properties without affecting effluent properties. This result holds great promise with a view to the industrial implementation of biobleaching sequences involving the two enzymes in the future.

Reutilization of effluents from laccase-mediator treatments of kraft pulp for biobleaching

Several effluents from laccase-mediator treatments of kraft pulp were recovered and subsequently reused with fresh pulp in order to simulate recirculation of effluents during biobleaching. The effluents were used as a new bleaching stage without any modification except enzyme addition. Pulp treated with effluents were afterwards chemically bleached by using the simple sequence LQPo, where L represents the treatment with effluent and laccase addition, Q is a chelating stage and Po is an alkaline peroxide stage. This system showed a promising potential on delignification, with kappa number ranging from 5.5 to 6.6 after LQPo sequence, depending on the type of effluent employed in L stage. Improvements on pulp brightness were also reported compared with control experiment.

In situ encapsulation of laccase in microfibers by emulsion electrospinning: preparation, characterization, and application

Laccase from Trametes versicolor was successfully in situ encapsulated into the poly(D,L-lactide) (PDLLA)/PEO-PPO-PEO (F108) electrospun microfibers by emulsion electrospinning. The porous morphology of electrospun microfibers was observed with scanning electron microscope, and the core-shell structure of microfibers and existence of laccase in microfibers were proved by laser confocal scanning microscopy micrograph. In this study, fibrous porosity and core-shell structure are advantageous to the activity and stability preservation of immobilized laccase. The activity of immobilized laccase could retain over 67% of that of the free enzyme. After 10 successive runs in the enzyme reactor, the immobilized laccase could also maintain 50% of its initial activity. Crystal violet dye was successfully degraded by the PDLLA/F108-laccase electrospun microfiber membranes. It was observed that the immobilized laccase possessed a broadening pH range of catalysis activity compared to free laccase.

Biobleaching of eucalypt kraft pulp with a two laccase-mediator stages sequence

A new biobleaching sequence, with two enzymatic stages based on the application of laccase-mediator systems, was tested (L(1)EL(2)QPo) in order to increase the effectiveness of enzyme delignification on eucalypt kraft pulp. Different synthetic -1-hydroxybenzotriazole (HBT) and violuric acid (VA) - and natural - syringaldehyde (SyAl) - mediators were used in the laccase stages and the biobleached pulp were compared in terms of chemical, optical and physico-mechanical properties. The pulp bleached with HBT or VA showed similar delignification (64.1% and 65.9% respectively) and optical properties (86.4% and 86.1% ISO brightness respectively) than an industrial TCF pulp (68.3% delignification and 84.8% ISO brightness). SyAl improved these properties in a lower extent (56.71% delignification and 80.52% ISO brightness). Regarding physico-mechanical properties of pulp, the biobleaching sequence had no a negative effect, even some slight improvements were observed in very specific cases.