Redox Mediated Decolorization and Detoxification of Direct Blue 80 by Partially Purified Ginger (Zingiber officinale) Peroxidase
Abstract
Textile industries are releasing a large number of toxic synthetic dyes into waste waters. Hence, the removal of such compounds from environment prior to t heir final disposal is necessary. In the present study, potential use of ginger (Zingiber officinale ) peroxidase in decolorization and detoxification of direct blue 80 has been investigate d. It was found that only 0.166 U/ ml of ginger peroxidase was sufficient for maximum decolorization of dye (25 mg/L). H 2O2 was requi red in low concentration (0.3 mM) in the presence of 0.6 mM 1-hydroxybenzotriazole . Direct blue 80 was also successfully removed in stirred batch process. It was observed that ginger peroxidase was highly stable over a wide range of pH and temperatures. Km and V max of the enzyme for direct blue 80 was found to be 27.8 mg/L and 2.09 mg/L/min, respectively. In UV-visible spectra l analysis a sharp decline in peak was observed for the treated direct blue 80 which substantiates the breakdown of chromophore group of dye. Genotoxicity assessment by comet assay and chromosomal aberration test confirmed that the direct blue 80 was succe ssfully detoxified by ginger peroxidase. Other direct and acid dyes were also treated either as a single or a mixture of different dyes and it was observed that these dyes were also decolorized significantly under similar experimental conditions. Our study suggests that this enzyme -redox mediator system constitutes a cost effective model which can decolorize the industrial textile effluents and also can reduce the toxic load of environment.
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Introduction
Textile industries are most concerned problem of the environment, because they release a wide range of colored pollutants into wastewater. Around 5-40% of the total dyes are released in the environment. 1 Colored water obstructs the absorption of sunlight and thus it can hamper the photosynthetic activity of the aquatic plants.2 Some of the dyes are the derivatives of the benzidine, which is a potential human carcinogen.3 Therefore, the removal of such harmful aromatic compounds is an urgent need of time. Many conventional methods have been employed for the treatment of such pollutants which involves physicochemical and chemical processes like adsorption, chemical reduction, coagulation, electrolysis and precipitation.4 But these methods have several inherent problems such as applicability to limited concentration ranges, high cost, incomplete removal and introduction of hazardous materials to the environment.5 In conventional biological method, microbes have been utilized for the treatment of such compounds in wastewater.6 However, microbial method have also many demerits like acclimatization of microbes in the dye effluent, need of substrate to support the microbial culture and production of large amount of solid wastes.7 Recently, enzymatic method is gaining much interest for the removal of aromatic compounds from the contaminated effluents due to their merits over the other known classical and biological methods.8
Enzymatic methods have a number of advantages over the known conventional treatments like operation at high and low contaminant concentration; applicable over the wide range of pH, temperature and salinity; short time of treatment; reduction in sludge volume and minimum compounds are required.9 Enzymes involved in the removal of aromatic compounds from various contaminated sites are mainly oxidoreductive enzymes such as peroxidases and polyphenol oxidases.10 Several dyes are recalcitrant to the action of peroxidase, therefore redox mediators are required along with enzyme for the removal of colored pollutants.11 Peroxidases used for the removal of dyes have been isolated from many plant sources like horseradish roots, whiteradish, turnip roots, tomato, soybean, bitter gourd and Saccharum uvarum.7 However, due to high cost of purification and low enzyme activity, these enzymes have not been utilized at a large scale.12 To ensure the removal of toxic pollutants from wastewater, ecotoxicological tests via biological assays are required.13 Most of the animal assays are costly, therefore they are not used in routine monitoring.14 Plant bioassays are less expensive and have been used to monitor the environmental pollution.15
In the present study an attempt has been made to utilise the partially purified ginger peroxidase (GP) for the removal of DB 80 which is a benzidine based dye and expected to be metabolized into benzidine which is a known human carcinogen.3 The decoloriztion of DB 80 has been done under different optimized parameters like pH; temperature; time; concentration of enzyme, H O and HOBT. In order to evaluate the affinity of enzyme for DB 80, kinetic parameters of the GP were 2 2 determined. UV-visible spectral analysis was also done to confirm the breakdown of chromophoric group. Genotoxic assessment test was performed for DB 80 and its decolorized product by comet assay and Allium cepa chromosomal aberration assay. Some other dyes and their mixtures were also treated under identical treatment conditions.
Conclusion
This study demonstrated that synthetic dyes released by textile industries were effectively removed by GP in conjunction with redox mediator HOBT. The enzyme was quite active over a wide range of pH and temperatures which makes the GP applicable at industrial level for the decolorization of dyes. H 2O2, which is an expensive co -substrate, was required in very low concentration for optimum decolorization of dyes. DB 80 was successfully decolorized in a stirred batch process in a very short time period which can make the process feasible for treatment of textile ef fluents. The enzyme was partially purified from easily available and inexpensive source that can overcome the high cost of purification of enzyme which acts as a barrier for its application in wastewater treatment. Also the DB 80 was significantly detoxifi ed which implies that environment can be free of hazardous dyes by this redox mediated enzyme system.
