It has long been known that azo dyes can be reduced by anaerobic bacteria in the human intestine to aromatic amines (Dieckhues 1960). The reduction equivalents gained by the oxidation of an auxiliary substrate through NADH2 reduce the azo bond to form aromatic amines and thus decolorize the solution (Fig. 9.7b). We use the simplified notation of the energy carrier here as NADH2, as opposed to the proper notation as NAD(P)H + H+ (see Chapter 3).
Putting it into the perspective of the classic three-step anaerobic degradation of complex carbon substrates, the anaerobic reduction of the dye is thought to take place in the first of the three steps by acidogenic bacteria.
The hypothesis for the mechanism of the biological reduction of azo dyes through the redox equivalents varies, depending on the enzyme (azo reductase) which participates in the decolorization. The enzymatic reduction theory was proposed by Zimmermann et al. (1982), Rafii et al. (1990), Haug et al. (1991), and Chung and Stevens (1993). Although most of the microorganisms reported to produce azo reductase are facultative anaerobic bacteria (Chung and Stevens 1993), some obligate anaerobic bacteria have been isolated from human intestinal microflora which produce azo reductase (Rafii et al. 1990). They found that the enzyme was extracellular and did not require induction by an azo dye for production.
Yoo et al. (2000) presented evidence that sulfur-reducing bacteria (SRB) can play an important role in the reduction of azo dyes. The azo bond is most likely chemically reduced through the sulfide produced by Desulphovibri from the sulfate often found in dye wastewaters. The biomediated chemical reduction rate was much faster than the rate found for fermenting bacteria.
The inhibition of Vibrio fischeri by the products of anaerobically treated RB 5 was studied by Libra et al. (2004). The EC50 value was decreased from 29.6 mg L-1 (completely hydrolyzed RB 5) to 1.5 mg L-1 (partly hydrolyzed RB 5; Libra et al. 2004).
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