Synthetic dyes are extensively used in textile dyeing, paper printing, color photography, pharmaceutical, food, cosmetics and other industries. During textile dyeing, the amount of dye lost in the effluents is dependent upon the class of dye used, varying from only 2% loss when using basic dyes to a 50% loss when reactive dyes are used . Approximately 20% of the losses enter the environment through effluents from wastewater treatment plants .
Azo compounds are a broad class of organic compounds with formula R-N=N-R′, in which R and R′ can be either aryl or alkyl groups. Azo compounds are solids of varying color from yellow to red and violet to blue. The N=N group is called an azo group and parent compound, HNNH, is called diimide. Synozol Red HF-6BN, an azo reactive dye, is a representative of a dye class to be recalcitrant with a conventional wastewater treatment system. Reactive dyes are easily soluble in water, therefore they have little affinity to be adsorbed on biomass and generally pass through activated sludge systems .
It is quite undesirable to discharge azo dyes with different colors into the environment due to their color pollution, biorecalcitrance and toxic intermediates, since the cleavage of azo bonds produces aromatic amines which are considered mutagenic and carcinogenic [4, 5]. In addition, some azo dyes or their metabolites because of low biodegradability may be mutagens or carcinogens to humans as well as to other animals [5, 6]. Therefore, considerable attention has been given to evaluating the fate of azo dyes during wastewater treatment and in the natural environment. The effluents of these industries are highly colored and disposal of these wastes into natural waters causes damage to the environment [7, 8].
Decolorization of these dyes by physical or chemical methods including flotation, hyper filtration, , adsorption, coagulation-flocculation, ion-exchange, oxidation, electrochemical methods [10, 11] and precipitation methods, chemical degradation or photodegradation is financially and often also methodologically demanding, time-consuming and mostly not very effective . The above mentioned ways for clean-up are expensive, coupled with the formation of large amount of sludge and the emission of toxic substances , which limit their application .
Compared with chemical and physical methods, a number of studies have focused on biological treatment, in which microorganisms which are able to decolorize and biodegrade these azo dyes are used, thus producing lower costs and fewer toxic resultants . Several combined anaerobic and aerobic microbial treatments have been suggested to enhance the degradation of azo dyes . Alternatively, dye decolorization using microbial enzymes has received great attention in recent years due to its efficient application [16–18].
Color removal processes with active microorganisms have two different simultaneous steps: adsorption of dyes on the surface of the organisms and degradation of dyes by the enzymes produced by these organisms [19–21]. Decolorization of textile dye effluent does not occur when treated aerobically by municipal sewage systems . Brightly colored, water-soluble reactive and acid dyes are the most problematic, as they tend to pass through conventional treatment systems unaffected . The possibility of using fungi to decolorize wastewater containing dyes has received much attention because their ligninolytic enzymes have the ability to degrade many recalcitrant pollutants, including synthetic dyes. Biocatalytic processes based on fungi provide alternative methods to decolorize textile effluents [23–25].
The aim of this work was to exploit the biodecolorization of Synozol red HF-6BN by fungi with the following objectives: (1) to assess the ability of the fungal cultures to decolorize the actual dye industry waste, (2) to confirm the degradation of the dye, and (3) to assess the toxicity of the degraded products.