(234d) Understanding the Combination of Titania and Laccase for Lignin Degradation

In the pulp and paper industry, for the conventional bleaching process, chlorine or chlorine-based chemicals are widely used as bleaching agents for chemically produced wood pulps. This leads to a chorine residual compounds that are toxic to environment such as chlorophenols, chloroguaiacols (Baggi et al., 2004). Recently, the enzymatic bleaching method has received much attention in the pulp and paper industry because of its being environmentally friendly nature (Archibald et al., 1997).

Laccases (EC 1.10.3.1) are multi copper enzymes found in white rot fungi. This enzyme catalyses the oxidation of the substrates molecules by accepting electrons from the copper irons, and then reducing the molecular oxygen to water (Riva, 2006). Laccase has been widely investigated for the biobleaching process of kraft pulp due to its ability to delignify (Archibald et al., 1997 and Bajpai, 1999). However, Laccase alone has a limit effect on bioremediation due to its specificity for phenolic compound in lignin. It has been reported that the inclusion of a mediator such as ABTS(2,2'-azonobis(3-ethylbenzthiazoline-6-sulfonate) (Hernandez Fernaud et al., 2006), HBT(1-hydroxybenzotriazole), VLA(violuric acid), and TEMPO(2,2',6,6'-tetramethyl-piperidine-N-oxyl) (Fabbrini et al., 2002) can extend the substrate range of laccase to non-phenolic subunits of lignin.

Titania (TiO₂) is a promising material for use as a photocatalyst. There has been increasing interest in its environmental applications due to its being inexpensive, strong oxidising and highly stable chemical. It is also capable of photooxidative destruction of most organics pollutants (Bizani et al., 2006 and Fujishima et al., 2000).

Some academic research has suggested improving enzyme biodegradation efficiency by using an application of photocatalytic processes (Peralta-Zamora et al., 2003). The photocatalytic reaction involves illumination of a large band-gap semiconductor. For instance, a significant enhancement on biological degradation efficiency of trinitrotoluene by Laccase from Phanerochaete chrysosporium was observed by application of a TiO₂-assisted photocatalytic pre-treatment (Hess et al., 1998). Furthermore, Paralta Zarnora et al.(2003) determined how the decolourisation efficiency of immobilised laccase using TiO₂ powder as photocatalytic pretreatment is affected by (1) support materials, (2) mediators and (3) colours.

In this study, the combination of laccase and titania was used as a bio-and chemi-catalyse in delignification. The degradation of lignin by using laccase, titania powder, titania with laccase in 1 and 2 steps reactions were investigated. The aim of this research work is to (1) identify various compounds in lignin; (2) evaluate the efficiency of organic degradation and lignin decolourisation by using a lignin modifying enzyme and titania powder; and, (3) provide an alternative route by using titania to increase the potential use of laccase in delignification.

The investigation was performed at an initial lignin concentration of 1 g/l, pH 5, 50^OC and agitation speed 250 rpm for 24 hr. The identification and quantitation of samples were carried out by GC/MS analysis before and after the degradation process. It was found that laccase could not remove colour from the lignin solution, while the efficiency of various organics degradation was about 30-40%. The lignin decolourisation and organics degradation by titania under UV irradiation was obtained at 75% and approximately 50-70%, respectively. The combination of laccase and lignin in 1 and 2 steps could moderately increase the degradation of lignin with little difference in performance between a 1 and 2 steps processes.