Reduction of azo dyes by anaerobic bacteria: microbiological and biochemical aspects

Abstract

Azo dyes are recalcitrant pollutants commonly found in several industrial wastewaters, such as those originated from textile factories, which generally persist to biological transformation. Discharge of these effluents in open water bodies not only represents an aesthetic problem, but also may limit photosynthesis in aquatic plants. Furthermore, many azo dyes and products derived from their partial transformation in the environment (e.g. aromatic amines) may be toxic or carcinogenic. Biological wastewater treatment processes have emerged as promising technologies to remove azo dyes from industrial effluents and intensive research has been conducted during the last two decades in order to elucidate the mechanisms involved in the reductive decolourisation of azo dyes. The present work describes the main biochemical and microbiological aspects involved in the reductive decolourisation of azo dyes by anaerobic bacteria.

Effect of Anaerobic Azo Dye Reduction on Continuous Sludge Digestion

“…On the other hand, biogenic sulfide produced by SRB, had the capacity to reduce dye chemically at higher rates than biochemical reaction [27][28][29]. This phenomenon may in turn lead to decrease in azo dye concentrations that might be inhibitory to mixed anaerobic culture that produces the reducing equivalents necessary for biological dye reduction.…”

Efficient anaerobic treatment of synthetic textile wastewater in a UASB reactor with granular sludge enriched with humic acids supported on alumina nanoparticles

“…The HS-enriched consortium obtained by the granulation process was then evaluated for its capacity to achieve the reductive decolorization of the recalcitrant azo dye, reactive red 2 (RR2, Fig. 1), a model azo compound representative of textile wastewaters for its recalcitrance and toxicity (Beydilli et al 2000;Cervantes and Dos Santos 2011;Martínez et al 2013a). …”

Use of Multivariate Experimental Designs for Optimizing the Reductive Degradation of an Azo Dye in the Presence of Redox Mediators

“…Azo dyes (which contain at least one -N=N-bond) represent more than 70% (w/w) of the dyes produced worldwide that are used in the textile industry 1 and are regarded as highly stable and recalcitrant towards aerobic biodegradation. 2 The primary effects of the discharge of azo dyes into the environment are related to its direct and indirect (through the formation of aromatic amines) toxicity to aquatic organisms and to the coloring of water, which affects photosynthetic organisms.…”

“…In view of these considerations, different research groups have studied the anaerobic degradation of azo dyes. 1,6 The anaerobic degradation of azo dye implies the breakage of azo bonds by a co-metabolic reactor, where two reducing equivalents (e.g., NAD(P)H and FADH 2 ) donate electrons to the dye, thereby reducing the chromophore system. This process occurs in two steps (equations 1 and 2), and the electron transfer from the donor (the biologically degraded substrate) to the acceptor (the azo dye) is the rate-limiting step in the decolorization kinetics: 7,8 R 1 -N=N-R 2 + 2e -+ 2H + → R 1 -NH-NH-R 2 (hydrazo intermediate) (1) R 1 -NH-NH-R 2 + 2e -+ 2H + → R 1 -NH 2 + NH 2 -R 2 (aromatic amines) (2) One strategy employed by some authors 9,10 to accelerate the electron transfer and increase the removal efficiency of azo dye degradation in anaerobic environments involves the use of redox mediators (RMs) such as riboflavin and quinones.…”