The manufacture and use of dyes and pigments is a multibillion-dollar industry. The use of these substances is an integral part of almost all manufacturing processes. Interestingly, the widespread use of synthetic colorants and the modern dye industry dates only to 1856 with the synthesis of mauveine by W.H. Perkin (Kirk-Othmer Encyclopedia of Chemical Technology 1992). Of all the different types of dyes, azo dyes are the most useful and widely used colorants. In 1858, J. P. Griess synthesized a yellow azo dye that was commercialized briefly (Kirk-Othmer Encyclopedia of Chemical Technology 1992). Other azo dyes that saw early commercialization included Chrysodine (in 1875), Congo Red (in 1884), and Bismark Brown. Following these early syntheses, numerous azo dyes have been made and it is estimated that over 2,000 are in use (Colour Index, 3rd Ed.). Wastewaters are produced during the synthesis and use of dyes. Such wastewaters must be treated. The most important criterion for treatment of wastewater is that associated toxicity is reduced to acceptable levels. Similarly, odors must be reduced and the amount of dissolved material must be reduced. Decolorization of water is also a primary goal of water purification processes and one that is critical to dye-containing wastewaters.
There are several reasons for the popularity and use of azo dyes. Azo dyes are available in almost every color across the spectrum, as a group they are colorfast and many can be structurally modified to bind to a variety of natural and synthetic fabrics (Kirk-Othmer Encyclopedia of Chemical Technology 1992). Unfortunately, azo dyes are resistant to biological degradation. This is likely due to the fact that azo linkages are rarely found in nature. Thus micro-organisms have not often been exposed to compounds containing this functional group and there have been few instances in which selective pressures on micro-organisms would have favored survival of those species that evolved enzymes capable of specifically destroying azo dyes. The toxicity of azo dyes makes it important that ways be found to treat wastewaters that contain these substances.
This review focuses on the ability of fungi to degrade azo dyes. Before, 1990, there were few reports of the biodegradation of azo dyes by aerobic micro-organisms. Cripps et al. (1990) showed that three azo dyes were extensively, degraded by the white rot fungus Phanerochaete chrysosporium under aerobic conditions. Since that time, many manuscripts have been published documenting degradation of azo dyes by P. chrysosporium and other fungi. Like P. chrysosporium, most of the other fungi studied and shown to degrade azo dyes are white rot fungi. Interestingly, however, a few fungi that are not white rot fungi have been reported to have some ability to degrade this important class of compounds. The ability of white rot fungi and other micro-organisms to degrade azo dyes was reviewed by this author several years ago (Bumpus 1995). A number of reviews addressing, at least in part, the biodegradation and/or biodecolorization of azo dyes in water by white rot fungi have appeared recently (Stolz 2001; Fu and Viraraghavan 2001; Knapp et al. 2001; Robinson et al. 2001; McMullan et al. 2001). This review summarizes this area of research and important recent observations while attempting to keep overlap with other reviews at a minimum.
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