Assessing enzymatic deinking for secondary fibers paper recycling in the presence of flexographic inks
Graphical abstract
Introduction
Recycled paper has become an important source of raw material for the pulp and paper industry. Generally, the industrial process for removing contaminants involves pulping, screening, cleaning and flotation. Although current flotation technologies are efficient for offset printed paper deinking, the use of environmentally friendly flexographic inks presents some recycling difficulties, due to the small size of the pigments and its hydrophilic character [1], [2]. This fact poses a problem for the paper recycling process, since ink particles resettle in fibers causing waste waters issues.
Laccases are multicopper oxidases that catalyze the oxidation of phenolic compounds and aromatic amines using oxygen as its electron acceptor. The driving force of the enzymatic mechanism lays on a difference between the redox potential of the reducing substrate and that of the oxidation site in the enzyme’s active center, the copper T1. Most laccases only allow direct oxidation of low-redox-potential compounds, but high-redox potential laccases produced by basidiomycetes (E° ⩾ 0.70 V) are of great biotechnological interest due to their higher oxidative capabilities, including the oxidation of mediator compounds with high-redox potentials [3]. Both high- and low-redox fungal laccases catalyze the oxidation of a wide range of phenolic compounds, using molecular oxygen as the electron acceptor, but in the presence of low molecular mass compounds (acting as mediators) their substrate range is extended to non-phenolic substrates [3], [4]. These compounds are oxidized by laccases to stable radicals, which act on compounds that are not directly oxidizable by these enzymes. It has been reported that the laccase-mediator system degrades not only lignin but also a great variety of recalcitrant and environmentally harmful compounds [5], [6], [7]. HBT (1-hydroxybenzotriazole) has been described as an efficient synthetic mediator for laccases in pulp delignification [8], pitch removal [9] and oxidation of polycyclic aromatic compounds [10]. Nevertheless, natural substances, as methyl syringate, are being tested as mediators since they are environmentally-friendly and could be obtained as by-products of the pulp industry or from industrial effluents [4], [11], [12], [13]. In the last years, many studies have been conducted on laccase-producing strains and on the application of purified laccases, in both free and immobilized forms, for the removal of synthetic dyes [12], [14], [15], [16], [17], inkjet [18] and flexographic inks [4].
The industrial use of laccases from white rot fungi is currently restricted due to its limited heterologous expression [19], [20], [21]. However, a laccase from the ascomycete Myceliophthora thermophila has been successfully expressed in industrial heterologous hosts with high yields [22].
In a previous work we demonstrated the feasibility of decolorizing flexographic inks using laccases from three white rot fungi Trametes villosa, Coriolopsis rigida, and Pycnoporus coccineus and from one ascomycete, M. thermophila, in the presence of synthetic and natural mediators [4]. The highest decolorization values were obtained in reactions with the three basidiomycete laccases and HBT, but good results were also obtained using the M. thermophila laccase in the presence of syringyl-type natural mediators.
In this paper a new method for biodeinking commercial pulp pre-dyed with flexographic inks at lab scale is proposed. Decolorization by C. rigida and M. thermophila laccases has been assayed in the presence of HBT or methyl syringate as synthetic and natural mediators. Moreover, influence of different paper additives commonly present in secondary fibers has been evaluated.
Section snippets
Flexographic inks, laccases and mediators
The flexographic inks, Blue 1 (B1), Red 48:4 (R48) and Flexiprint Magenta HX-E (MG), were supplied by Quimovil SA and Flint Group Iberia SA (Spain). Laccase from M. thermophila (Novozym 51003) was supplied by Novozymes® (Denmark) and laccase from C. rigida was produced in a basal medium with glucose supplemented with copper plus ethanol [23]. The mediators used were HBT (Fluka) and methyl syringate (Novozymes®).
Laccase activity was determined at 25 °C by measuring the oxidation of 5 mM
Biodeinking with laccase-mediator system
Before ink decolorization, the pH of the reaction media were adjusted to values previously determined [4] as a good compromise between enzyme’s stability and activity. C. rigida laccase assays were carried out at pH 4 since this enzyme had maximum activity levels against DMP in a pH range of 3.5–4.5 and was stable for 24 h at room temperature at pH 4. When using M. thermophila laccase the treatments were carried out at pH 6, at which the enzyme is more stable and works efficiently.
Biodeinking of
Conclusion
Flexographic paper cannot be deinked by the flotation methods currently applied in recycled paper production. In this study, we demonstrate the feasible decolorization of flexographic inks in the presence of pulps by using a high-redox fungal laccase through the laccase-mediator system. Calcium carbonate interferes with laccase biodeinking, which explains the differences between the pulps studied and indicates that the presence of mineral charge in old paper may difficult biodeinking and slow
Acknowledgements
This work has been funded by PROLIPAPEL II S-2009AMB-1480 and PRI-PIBAR-2011-1402 projects. The authors thank ENCE for providing pulp, and TORRASPAPEL S.A. for pulps and additives, Novozymes® for supplying enzymes and to Quimovil S.A. and Flint Group Iberia S.A. for providing flexographic inks.
References (29)
- et al.
Oxidation of non-phenolic substrates: an expanded role for laccase in lignin biodegradation
FEBS Lett.
(1990) - et al.
Biodeinking of flexographic inks by fungal laccases using synthetic and natural mediators
Biochem. Eng. J.
(2012) - et al.
Environmental biocatalysis: from remediation with enzymes to novel green processes
Trends Biotechnol.
(2006) - et al.
White-rot fungi and their enzymes for the treatment of industrial dye effluents
Biotechnol. Adv.
(2003) - et al.
Optimization of laccase-mediator system in producing biobleached flax pulp
Bioresour. Technol.
(2010) - et al.
Towards industrially-feasible delignification and pitch removal by treating paper pulp with Myceliophthora thermophila laccase and a phenolic mediator
Bioresour. Technol.
(2011) - et al.
A new approach to the biobleaching of flax pulp with laccase using natural mediators
Bioresour. Technol.
(2010) - et al.
Paper pulp delignification using laccase and natural mediators
Enzyme Microb. Technol.
(2007) - et al.
The enzymatic decolorization and detoxification of synthetic dyes by the laccase from a soil-isolated ascomycete, Paraconiothyrium variabile
Int. Biodeterior. Biodegrad.
(2013) - et al.
Biodegradation of textile dyes by immobilized laccase from Coriolopsis gallica into Ca-alginate beads
Int. Biodeterior. Biodegrad.
(2014)
Survey of recent trends in biochemically assisted degradation of dyes
Chem. Eng. J.
Isolation of two laccase genes from the white-rot fungus Pleurotus eryngii and heterologous expression of the pel3 encoded protein
J. Biotechnol.
Role of Pycnoporus coccineus laccase in the degradation of aromatic compounds in olive oil mill wastewater
Enzyme Microb. Technol.
Involvement of ligninolytic enzymes of Myceliophthora vellerea HQ871747 in decolorization and complete mineralization of Reactive Blue 220
Chem. Eng. J.
Cited by (0)
- 1
Current address: Centro de Investigación forestal, INIA, Carretera de La Coruña km 7,5, E-28020 Madrid, Spain.