Modification of old corrugated container pulp with laccase and laccase–mediator system
Graphical abstract
Highlights
► Laccase treatment of OCC moderately improves fiber bonding properties. ► Laccase-mediator systems yield greater strength properties for OCC. ► Laccase and 1-hydroxybenzotriazole provides direct means of oxidizing lignocellulosic yielding improvements in strength.
Introduction
The utilization of recycled fibers has attained substantial interest globally due to increasing demand for plant resources and the great attention towards environmental sustainability. Old corrugated container (OCC) is one of main recycled fiber resources used in papermaking and accounts for about 40% of the total recycled paper (Guo et al., 2011). However, it is now well known that papermaking properties decay when recycled fibers go through the pulping and papermaking processes as exhibited by a decrease in fiber swelling ability and loss in strength properties (Chen et al., 2009, Chen et al., 2010b). In addition, these detrimental losses in physical properties, increase with each subsequent recycling sequence, which eventually prevents their utilization in papermaking.
In order to improve the papermaking properties of recycled fiber, researchers have begun to evaluate several technologies to enhance the fiber–fiber bonding properties of recycled pulp. Such as, mechanical refining (Jiao et al., 1998), enzyme treatments (Choi and Jong, 2001, Pala et al., 2001, Ruel et al., 2003) and chemical pretreatments (Waterhouse and Liang, 1995) such as a carboxymethyl treatment using chloroacetic acid (Rácz and Borsa, 1997) and the use of cellulose derivatives (i.e., carboxymethyl cellulose) (Blomstedt et al., 2007).
It is now well established that carboxylic acids groups are beneficial in the bonding of pulp fibers in paper and can increase the strength of paper (Zhang et al., 2007). These benefits have been attributed to the improved swelling and/or bonding ability of cellulosic fibers as the acid group content of cellulosic fibers is increased (Barzyk et al., 1997, Chen et al., 2010a). It has been reported that the fiber acid group content decreases when the fibers are once dried (Rácz and Borsa, 1997) which could lead to the decrease of the fiber swelling ability resulting in the decrease of fiber bonding ability and fiber strength (Chen et al., 2010a). Hence, one approach to optimize fiber strength retention during recycling is to enhance fiber charge as much as possible. Recently, the two chemical methods have been proposed to introduce pulp acid groups using either an alkaline chloroacetic acid treatment or by the addition of carboxymethyl cellulose (Blomstedt et al., 2007, Rácz and Borsa, 1997). However, chemical treatments involve harsh reaction conditions, loss of desirable components, and generation of new waste streams. An alternative approach to increase the fiber charge of recycled fiber, especially OCC is to use an oxidative enzymatic treatment. Laccase (p-diphenol:dioxygen oxidoreductase) is well known for its ability to oxidize various substrates including phenols, diphenols, aminophenols, polyphenols, polyamines, and lignin-related molecules (Burton, 2003, d’Acunzo et al., 2006, Galli and Gentili, 2004, Witayakran and Ragauskas, 2009, Zhang et al., 2002).
Indeed, Viikari et al. (1999) has reported that laccase oxidation of cellulosic fibers increased the acid group content of virgin fiber and this was shown to improve fiber swelling and flexibility. Chandra and Ragauskas (2002) have reported that the acid group content of high-yield kraft pulps increased onefold when treated with laccase and 4-hydroxybenzoic acid. Liu et al. (2009) showed that the wet tensile index of unbleached kraft pulp when treated with a combination laccase with methyl syringate was twice that of being treated solely laccase. In both these latter studies, laccase was employed to chemically graft the fiber with benzoic derivative onto the fiber.
Laccase–mediator treatment of kraft pulps with 2,2′-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid (ABTS), 1-hydroxybenzotriazole (HBT) or phenonthiazine-10-propionic acid (PPT) has been reported to beneficially improve wet tensile strength values of medium density fiberboards (Lund and Felby, 2001). Likewise, Zhang et al. (2005) found the carboxyl group content of masson pine stone groundwood pulp increased after treatment with laccase and violuric acid (VA). Witayakran and Ragauskas (2009) demonstrated the potential of laccase-facilitated grafting of amino acids to linerboard pulp increasing fiber charge and sheet strength properties. Chen et al. (2010a) also found the carboxyl group content, the tensile strength, and water retention value of old newspaper (ONP) increased when histidine was grafted onto pulp with laccase.
Researchers have also begun to examine the effect of laccase treatment on physical properties and surface property of OCC pulp. For example, Wang et al., 2005, Pei et al., 2006 reported the improvement of OCC strength properties when treated with laccase at laccase dose 16 U/g, pulp consistency 5%, pH 7, RT, reaction time 1 h and O2 atmosphere conditions. These result showed that wet ring crush compression strength (RCT) and wet tensile strength of increased by 34% and 35%, compared to control handsheet. Shan et al. (2009) studied the synergistic effect of laccase and hydrolase treatment on OCC pulp’s strength and fiber’s surface properties. Their results showed that the ring stiffness and tensile strength were increased by 27% and 18% after this enzymatic treatment. In contrast, the effects of laccase–mediator (HBT, VA, ABTS) bleaching on the properties of OCC pulp have not been examined.
This study examines several laccase and LMS treatments with OCC to enhance physical strength properties including STFI, tensile, ring crush, tear, and burst index. In addition, the beatability of laccase-treated OCC pulp was studied. The effects of these treatments were further quantified in terms of fiber charge, lignin content, and fiber surface morphology to yield a detailed understanding of the chemistry and physical properties of laccase/LMS treated OCC.
Section snippets
Materials
The pulp used was a commercial sample of old corrugated containers (OCC) pulp secured from a southeastern US mill. Laccase was provided by Novo Nordisk Biochem, NC and frozen until use. ABTS and HBT were purchased from Sigma–Aldrich, and VA was purchased from Tokyo into industries Co., Ltd. Graff “C” stain was purchased from IPS Testing Experts.
Enzyme activity assay
Laccase activity was determined by the oxidation of 2,2′-azinobis-(3-ethylbenzyl thiozoline-6-sulfonate) (ABTS) (Sealey and Ragauskas, 1998). The assay
Effect of laccase dose on the physical properties of OCC pulp
OCC pulp was preliminarily treated with different amounts of laccase using 40, 160, and 240 U/ o.d.g. pulp in a pH 7 sodium bicarbonate buffer solution at room temperature for 12 h and at 5% pulp consistency. The effect of the laccase dose on physical properties of OCC pulp is shown in Table 1. From these results it can be seen that apparent density, burst index, and ring crush of laccase-treated samples had little change with the increase in the laccase dose, compared with the control sample.
Conclusions
Comparing the control OCC pulp with the laccase and Lac-HBT treated pulps, the latter treatment provided the highest strength properties of the paper. The Lac-HBT treated OCC pulp gave a 21% increase of carboxyl group content compared with the control pulp, which led to the increase of fiber swelling resulting in the increase of the strength properties of paper. The fiber surface of OCC pulp after Lac-HBT treatment became rougher and more collapsed. The results from FT-IR analysis suggested
Acknowledgements
Y.M. Chen is grateful to China Scholarship Council for awarding a scholarship under the State Scholarship Fund to pursue her study. This work will be used by Y.M. Chen for partial fulfillment of the degree requirement for her doctoral research in the College of Environmental Science and Engineering at South China University of University, China. This work was partially supported by The National Natural Science Foundation of China (General Program) (No. 31170551).
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