The investigation of removal of dye by laccase immobilized on modified chitosan derivatives

Synthetic dyes are widespreadly used as coloring agents in the textile, paper, cosmetics, leather dyeing, color photography, pharmaceutical and food industries. Synthetic dyes are also increasingly used in the textile and dyeing industries because of their ease and cost-effectiveness in synthesis, high stability to light, temperature, variety in color compared with natural dyes. These dyes are toxic, mutagenic and carcinogenic. Furthermore, synthetic dyes are caused serious environmental pollution. There are several physical and/or chemical processes for dye removal from wastewater. However, several commercially available methods are expensive, which limit their application. A secondary pollution problem in these processes will arise because of excessive chemical use. Therefore researchers are investigating an economic and effective ways. Alternatively, enzymatic dye decolorization applications have received great attention in recent years. To improve enzyme activity, stability and selectivity, enzymes have generally been studied with the enzymes immobilized on a solid support. Laccase is highly specific and have a wide range of substances are suitable for different application processes. The ability of laccase enzymes to highly useful biocatalyst for various biotechnological applications such as dye decolorization, removal of phenol, bleaching. In this study, three synthetic textile dyes, Methyl Red (MR) Methylene Blue (MB) and Crystal Violet (CV) were removed by immobilized laccase from aqueous solutions in a batch system. MWCNT-chitosan composite beads were used for immobilization of laccase (benzenediol:oxygen oxidoreductase, EC 1.10.3.2) as a carrier. Optimum pH was found as pH 4.0 for free laccase and found as pH 5.0 for immobilized laccase. Optimum temperature was found as 40°C for free and immobilized laccase. It was observed that resistance of immobilized laccase to changes in temperature is more than free laccase. In the storage stability studies, free and immobilized laccase enzyme was stored at 4 °C and 25°C for enzyme activities were measured at regular times. As a result of storage studies, immobilize laccase was found to be more stabile than free laccase. Effect of pH, temperature, incubation time and dye concentration was investigated on enzymatic dye decolorization. The optimum pH for maximum decolorization of methyl red, methylene blue and crystal violet was found as pH 6.0, pH 5.0 and pH 4.0 respectively. Dye decolorization percentage was found to be as 64.2% for methyl red, %72.3 for methylene blue and %40.2 for crystal violet after 120 min. incubation. Under the experimental condition showed 96,9% decolorization of a toxic azo dye MR (pH 6.0), 93,1% decolorization of a basic dye MB (pH 5.0) and 79.5% decolorization of a basic dye CV (pH 4.0) were achieved by using immobilized laccase within 24 h at 35 °C (130 rpm).