Green biocatalytic approach for textile wastewater decolorization using versatile peroxidase

Colored wastewater generated by textile industries is enormous due to dyeing, washing, printing and finishing processes. The waste-water may contain reactive dyes, azo dyes, salts, surfactants and other processing chemicals. The conventional Effluent Treatment Plant (ETP) systems are capable of reducing suspended solids and part of the organic load, however they are not always able to completely remove the residual color. This means that treated water can remain colored and could exert environmental stress on rivers, canals and other surface water bodies. Synthetic dyes are hard to break down as they are meant not to wash out, withstand heat, light, or microbial attack. One particularly difficult dye to remove is the azo dye, which features a strong azo bond (–N=N–) that gives the dye strong color intensity and chemical stability. These dyes, when left in water, have a negative impact on light penetration in water, affect aquatic life and increase the pollution load. Water discharges from textile industries can have severe ecological effects on receiving water bodies if there is residual color discharged. Dye molecules decrease the penetration of light reaching aquatic plants and algae, thereby hindering growth of aquatic plants and algae and causing changes in the dissolved oxygen content of water that can impact aquatic microorganisms, plankton and fish. Azo dyes can be reductively split under environmental conditions releasing aromatic amine intermediate products, some of which have toxic and carcinogenic effects. Azo dyes may break down to release toxic aromatic amine intermediates; other dyes do not produce any known toxic byproducts in their manufacture. So, minimizing the colour residues as a prior step to discharge is significant to preserve water quality and protect aquatic ecosystems. Thus, more efficient and clean technologies are required for the final removal of color in textile industries. Enzymatic treatment is now emerging as a green technology for treating wastewater for polishing. Versatile Peroxidase (VP) was regarded as promising among the various enzymes due to its wide range of oxidation activity. VP is primarily produced by white-rot fungi, and has catalytic activity characteristic of both manganese peroxidase and lignin peroxidase. This can be used to oxidize a broad spectrum of dye molecules and other complex organic compounds. Laboratory experiments revealed that ligninolytic enzymes like Versatile Peroxidase can be used for decoloration of synthetic dyes. Approximately 70–95% of the different synthetic dyes have been reported to be decolorized by VP based systems depending on dye structure and operating conditions. The primary mechanism in azo dyes is the cleavage of the azo bond (–N=N–) in the dye molecule, leading to the removal of the visible coloring azo chromophore. Nevertheless, the removal of colour does not necessarily mean complete mineralization and additional studies on COD removal and toxicity removal is required for real wastewater applications. Hydrogen peroxide (H₂O₂) is essential to the working of VP. VP generates ROS intermediates in the presence of H₂O₂. These intermediates react with visible color chromophores such as azo bonds and aromatic structures. These structures are broken or altered and resulting in decolorization and transformation of dye molecules into smaller and potentially less persistent compounds. Thus VP can serve as a complementary material for decolorization and minimize the recalcitrant organic matter of textile effluent.

VP possesses certain distinct merits over other enzymes. The use of laccase for the removal of dyes has been extensively investigated, though in many cases mediators are needed for the degradation of complex non-phenolic dyes. Several dye structures can be broken down by manganese peroxidase; however, the effectiveness of this depends on the availability of manganese and wastewater properties. Lignin peroxidase is pH and temperature sensitive with high oxidation ability. VP is attractive because it has both wider oxidative properties in the same enzyme system, and can be used for more complex textile effluents. Industrially, VP can be applied as a polishing process following the conventional biological treatment. Further, VP technology is more feasible as an advanced polishing stage after the existing ETP technology for textile dyeing industries in Bangladesh which are export oriented. The salt content of textile effluents is high and the presence of mixed reactive dyes, detergents, and other chemicals used in the textile processes can affect the enzyme activity in industrial conditions. Thus, the crude enzyme extract obtained from the fermentation with white-rot fungi, which has a high VP content, could be used in a more economical manner than the commercially available purified enzymes. It can enhance the quality of the final effluent, decrease chemical reliance and promote sustainable wastewater management practices. It can also be used to help reduce the water reuse targets by color removal prior to discharge and/or recycling. But some troubles need to be addressed before the large-scale application. The dosing of hydrogen peroxide needs to be controlled as too much H₂O₂ can decrease the activity of enzymes. The stability of enzyme in high pH, salt, temperature variation and mixed chemical conditions is also important. Another factor is the cost of producing it on a large scale. VP application is from an economic point of view, an extra investment in enzyme production, enzyme stabilization, and hydrogen peroxide (H₂O₂) supply. Using purified commercial VP enzymes can add significantly to the cost of treatment; however, production of crude VP rich enzyme via fungal fermentation can decrease costs of production. VP polishing may be a little more expensive than conventional ETP for a large export oriented dyeing factory, but the extra cost can be offset by higher quality effluent, meeting international environmental standards and greater possibility for water reuse. Enhancement in enzyme immobilization and recovery techniques, and development of low-cost enzyme production methods are anticipated to improve the economic viability of this technology. Further studies are required for immobilization of the enzymes, low-cost production of the enzymes, genetic improvement and pilot-scale studies with real textile wastewater. By designing the process carefully, Versatile Peroxidase can be utilized as a valuable green biocatalyst for upgrading textile ETP systems and for cleaner production in the textile industry.

References

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