Reactive Red 2 Decolorization Research Articles

Immobilized redox mediators on modified biochar and their role on azo dye biotransformation in anaerobic biological systems: Mechanisms, biodegradation pathway and theoretical calculation

In this study, a novel solid-phase redox mediator (RM) was manufactured via immobilizing anthraquinone-2-sulfonate (AQS) onto modified biochar (BC). The RM was used to enhance the anaerobic granular sludge (AGS) driven anaerobic biodegradation of azo dyes. Prepared at the optimal pyrolysis temperature of 550 ℃, the AQS modified BC550 exhibits a higher adsorption capacity of about 12.81% toward AQS compared to that of commercial granular activated carbon (GAC). X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses showed that BC550-AQS contained more redox moieties (quinone and C = O) than GAC-AQS, indicating that the BC550 is suitable as a carrier of immobilized AQS. During the 36-h reaction, incubation of BC550-AQS as the solid-phase RM obviously enhanced the capacity of AGS (BC550-AQS + AGS), evidenced by the maximum percentage of Reactive Red 2 (RR2) decolorization (92.27%), which increased up by 80.55% and 14.48% compared to the controls (BC550-AQS, 17.95%) and (GAC-AQS + AGS, 78.61%). Besides, BC550-AQS exhibits good reusability and a higher RR2 decolorization rate (per AQS dosage) over 15 consecutive cycles. The mechanism of biodegradation pathway of RR2 initiated by BC550-AQS was studied using liquid chromatography-mass spectrometry and chemical computation. Density-functional theory calculations successfully predict that the RR2 biodegradation pathway mainly includes azo bond cleavage and electron attraction with a stronger electron affinity. This work not only proposes a deep insight into the biodegradation pathway of RR2 by employing BC550-AQS in an anaerobic biological system but also develops effective solid-phase RMs for efficient removal of contaminants from wastewaters by biological treatment.

Biological detoxification and decolorization enhancement of azo dye by introducing natural electron mediators in MFCs

Reactive red 2 (RR2) is a highly recalcitrant and toxic azo dye that can cause the collapse of biological treatment system. Although MFC can decolorize RR2 effectively, its performance is still inevitably affected by toxicity. Anthraquinone can enhance MFCs’ performance through mediating electron transfer. In this study, an anthraquinone-rich natural plants (B.rheum (Rheum offcinale Baill)) was extracted and then added to MFCs. The optimal dosage was selected and the enhanced effects were investigated. The results showed that adding 5%(V/V) extract resulted in the optimal performance elevation of MFC. When 5% extract was added together with RR2, 15.63% and 1.33-fold improvement in RR2 decolorization efficiency and rate were achieved compared with the control group. Meanwhile, higher power density (2.75 W/m3), coulombic efficiency (6.45%), and lower internal resistance (233.69 Ω) were also observed when 5% B.rheum extract and RR2 were added. B.rheum extract in MFCs enhanced microbial activity and enriched the dye-degrading microorganisms, such as Enterobacter, Raoultella, Comamonas and Shinella. B.rheum extract acts as “antidote” in alleviating the biotoxicity of RR2 was firstly illustrated in this study. The results provided a new strategy for using plant-source electron mediators to simultaneously improve biological detoxification, bioelectricity generation and dye decolorization in bioelectrochemical system.

Decolorization of high-concentration Reactive Red 2 in water using UV and persulfate in a 3-liter photoreactor

This investigation examines the effectiveness of using UV and persulfate (PS) in a 3-liter photoreactor in the decolorization of high-concentration Reactive Red 2 (RR2) in water. At 25 °C, the effects of UV wavelength, UV power, and Na2S2O8 dosage on the decolorization of high-concentration RR2 (100 mg/L) by the UV/PS process were studied. With a UV power of 16 W, the efficiency and rate of decolorization of RR2 in the UV-254 nm/PS process were much larger than those in the UV-365 nm/PS process. The UV-254 nm/PS process with a UV power of 16 W provided a higher efficiency and faster rate of decolorization of RR2 than did the UV-254 nm/PS process with a UV power of 8 W. In the UV-254 nm/PS process (16 W), the efficiency and rate of decolorization of RR2 were enhanced by increasing the Na2S2O8 dosage, suggesting that the decolorization of RR2 was not limited by excess Na2S2O8 dosage. This investigation concerns the sustainable decolorization of high-concentration RR2 (100 mg/L) by the UV-254 nm/PS process in a 3-liter photoreactor. After 60 min, the RR2 (100 mg/L) was completely decolorized and 88% mineralized at 70 °C and an Na2S2O8 dosage of 1.92 g/L in the UV-254 nm/PS process (16 W). Accordingly, the UV-254 nm/PS process in a 3-liter photoreactor at high temperature can be used to decolorize strongly high-concentration RR2 in water.

Azo dye biotransformation mediated by AQS immobilized on activated carbon cloth in the presence of microbial inhibitors

In this work, anthraquinone-2-sulfonate (AQS) was covalently immobilized onto activated carbon cloth (ACC), to be used as redox mediator for the reductive decolorization of reactive red 2 (RR2) by an anaerobic consortium. The immobilization of AQS improved the capacity of ACC to transfer electrons, evidenced by an increment of 3.29-fold in the extent of RR2 decolorization in absence of inhibitors, compared to incubations lacking AQS. Experiments conducted in the presence of vancomycin, an inhibitor of acidogenic bacteria, and with 2-bromoethane sulfonic acid (BES), an inhibitor of methanogenic archaea, revealed that acidogenic bacteria are the main responsible for RR2 biotransformation mediated by immobilized AQS. Nonetheless, the results also suggest that some methanogens are able to maintain their capacity to use immobilized AQS as an electron acceptor to sustain the decolorization process, even in the presence of BES.

AQDS immobilized solid-phase redox mediators and their role during bioelectricity generation and RR2 decolorization in air-cathode single-chamber microbial fuel cells

The application of immobilized redox mediators (RMs) in microbial fuel cells (MFCs) is an emerging technology for electricity generation with simultaneous azo dye decolorization due to facilitated electrons transfer from bacteria to anodes and azo dyes. The use of immobilized RMs avoids the requirement of their continuous dosing in MFCs, which has been the main limitation for practical applications. Two strategies of anthraquinones-2,6-disulphonic salt (AQDS) immobilization, AQDS immobilized with polyvinyl alcohol particles and AQDS immobilized on anodes by electropolymerization, were evaluated and compared to achieve simultaneous reactive red 2 (RR2) dye reduction and bioelectricity generation. The AQDS immobilized by electropolymerization showed the highest power density (816±2mW/m2) and extent of RR2 decolorization (89±0.6%). This power density is one of the highest values yet achieved in the presence of a recalcitrant pollutant, suggesting that immobilization was important for enabling current generation in the presence of RR2.

Decolorization of Reactive Red 2 in aqueous solutions using RPB-prepared nanoscale zero-valent iron

This investigation assesses the feasibility of decolorizing Reactive Red 2 (RR2) in aqueous solutions using nanoscale zero-valent iron (nZVI). nZVI was prepared in a rotating packed bed (RPB) at a rotational speed of 1400rpm and liquid flow rates of 0.6L/min. The effects of pH, nZVI dosage, and temperature on the decolorization of RR2 were experimentally investigated. The results thus obtained importantly reveal that a lower pH, higher nZVI dosage, and higher temperature yielded more efficient decolorization of RR2. Furthermore, the decolorization of RR2 using the prepared nZVI during the initial process was consistent with the pseudo-first-order kinetic model. Additionally, the prepared nZVI was more reactive in decolorizing RR2 than ZVI. With an nZVI dosage of 0.1g/L, the prepared nZVI effectively decolorized RR2, yielding a decolorization efficiency of around 88% in 45min at pH 4 and 50°C. These promising results obviously demonstrate the potential of the prepared nZVI for effectively decolorizing dyes in aqueous solutions.

Decolorization of C.I. Reactive Red 2 by a large specific surface area In2TiO5: photodegradation and adsorption

The sol–gel method was used to synthesize nano-scale In2TiO5 with a large specific area. The X-ray diffraction patterns show that the crystalline phase of the prepared composite was In2TiO5 and rutile. According to TEM measurements, the In2TiO5 was spherical and the particle size was about 30–50 nm. The point of zero charge, specific surface area, pore volume, and pore diameter of In2TiO5 was pH 4.2, 588 m2/g, 0.62 cm3/g, and 5.2 nm. The titanium and indium cations in the prepared In2TiO5 were in oxidative state IV and III, respectively. The parent compound was C.I. Reactive Red 2 (RR2), which was used to evaluate the photodegradation and adsorption efficiency of In2TiO5. Under 0.2 g/L In2TiO5 addition, RR2 decolorization by photodegradation and adsorption at pH 3 was 79 and 70 %, and at pH 7 it was 50 and 27 %. The photodegradation rate of RR2 at pH 3 and 7 was 0.0076 and 0.0037 min−1. The maximum RR2 adsorption capacity of In2TiO5 at pH 3 and 7 was 70.5 and 27.3 mg/g, respectively. The RR2 removal rate by In2TiO5 under visible-light irradiation was higher than that of adsorption.

Decolorization of C.I. Reactive Red 2 by UV/TiO2/PAC and visible light/TiO2/PAC systems

AbstractIn this work, the sol-gel method is utilized to synthesize TiO2/powder-activated carbon (PAC) from TiCl4 and PAC. The TiO2/PAC system was used to decolorize C.I. Reactive Red 2 (RR2) under ultraviolet (UV) and visible light irradiation. The surface characteristics of TiO2 and TiO2/PAC were analyzed by X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, UV-vis spectroscopy, BET surface area analysis, and X-ray photoelectron spectroscopy (XPS). The effects of the C/Ti ratio, calcination temperature, photocatalyst dose, RR2 concentration, wavelength of light, and pH on the decolorization of RR2 by TiO2/PAC were evaluated. The optimal calcination temperature for forming TiO2 and TiO2/PAC was 400°C; moreover, the surface areas of TiO2 and TiO2/PAC were 42 and 108 m2/g, respectively. The sizes of the aggregates of TiO2 and TiO2/PAC particles were approximately 40 and 30 nm, respectively. Experimental results reveal that the percentage of anatase increased with the amoun...

Effects of oxidant concentration and temperature on decolorization of azo dye: comparisons of UV/Fenton and UV/Fenton-like systems

This study applies photo-Fenton and photo-Fenton-like systems to decolorize C.I. Reactive Red 2 (RR2). The oxidants were H(2)O(2) and Na(2)S(2)O(8); Fe(2+), Fe(3+), and Co(2+) were used to activate these two oxidants. The effects of oxidant concentration (0.3-2 mmol/L) and temperature (25-55 °C) on decolorization efficiency of the photo-Fenton and photo-Fenton-like systems were determined. The decolorization rate constants (k) of RR2 in the tested systems are consistent with pseudo-first-order kinetics. The rate constant increased as oxidant concentration and temperature increased. Activation energies of RR2 decolorization in the UV/H(2)O(2)/Fe(2+), UV/H(2)O(2)/Fe(3+), UV/Na(2)S(2)O(8)/Fe(2+) and UV/Na(2)S(2)O(8)/Fe(3+) systems were 32.20, 39.54, 35.54, and 51.75 kJ/mol, respectively.

Kinetics during the redox biotransformation of pollutants mediated by immobilized and soluble humic acids

The aim of this study was to elucidate the kinetic constraints during the redox biotransformation of the azo dye, Reactive Red 2 (RR2), and carbon tetrachloride (CT) mediated by soluble humic acids (HAs) and immobilized humic acids (HAi), as well as by the quinoid model compounds, anthraquinone-2,6-disulfonate (AQDS) and 1,2-naphthoquinone-4-sulfonate (NQS). The microbial reduction of both HAs and HAi by anaerobic granular sludge (AGS) was the rate-limiting step during decolorization of RR2 since the reduction of RR2 by reduced HAi proceeded at more than three orders of magnitute faster than the electron-transferring rate observed during the microbial reduction of HAi by AGS. Similarly, the reduction of RR2 by reduced AQDS proceeded 1.6- and 1.9-fold faster than the microbial reduction of AQDS by AGS when this redox mediator (RM) was supplied in soluble and immobilized form, respectively. In contrast, the reduction of NQS by AGS occurred 1.6- and 19.2-fold faster than the chemical reduction of RR2 by reduced NQS when this RM was supplied in soluble and immobilized form, respectively. The microbial reduction of HAs and HAi by a humus-reducing consortium proceeded 1,400- and 790-fold faster than the transfer of electrons from reduced HAs and HAi, respectively, to achieve the reductive dechlorination of CT to chloroform. Overall, the present study provides elucidation on the rate-limiting steps involved in the redox biotransformation of priority pollutants mediated by both HAs and HAi and offers technical suggestions to overcome the kinetic restrictions identified in the redox reactions evaluated.

Immobilized redox mediator on metal-oxides nanoparticles and its catalytic effect in a reductive decolorization process

Different metal-oxides nanoparticles (MONP) including α-Al2O3, ZnO and Al(OH)3, were utilized as adsorbents to immobilize anthraquinone-2,6-disulfonate (AQDS). Immobilized AQDS was subsequently tested as a solid-phase redox mediator (RMs) for the reductive decolorization of the azo dye, reactive red 2 (RR2), by anaerobic sludge. The highest adsorption capacity of AQDS was achieved on Al(OH)3 nanoparticles, which was ∼0.16mmolg−1 at pH 4. Immobilized AQDS increased up to 7.5-fold the rate of decolorization of RR2 by anaerobic sludge as compared with sludge incubations lacking AQDS. Sterile controls including immobilized AQDS did not show significant (<3.5%) RR2 decolorization, suggesting that physical–chemical processes (e.g. adsorption or chemical reduction) were not responsible for the enhanced decolorization achieved. Immobilization of AQDS on MONP was very stable under the applied experimental conditions and spectrophotometric screening did not detect any detachment of AQDS during the reductive decolorization of RR2, confirming that immobilized AQDS served as an effective RMs. The present study constitutes the first demonstration that immobilized quinones on MONP can serve as effective RMs in the reductive decolorization of an azo dye. The immobilizing technique developed could be applied in anaerobic wastewater treatment systems to accelerate the redox biotransformation of recalcitrant pollutants.

Immobilized humic substances on an anion exchange resin and their role on the redox biotransformation of contaminants

A novel technique to immobilize humic substances (HS) on an anion exchange resin is presented. Immobilized HS were demonstrated as an effective solid-phase redox mediator (RM) during the reductive biotransformation of carbon tetrachloride (CT) and the azo model compound, Reactive Red 2 (RR2). Immobilized HS increased ∼4-fold the extent of CT reduction to chloroform by a humus-reducing consortium in comparison to incubations lacking HS. Immobilized HS also increased 2-fold the second-order rate constant of decolorization of RR2 as compared with sludge incubations lacking HS. To our knowledge, the present study constitutes the first demonstration of immobilized HS serving as an effective solid-phase RM during the reductive biotransformation of priority contaminants. The immobilizing technique developed could be appropriate for enhancing the redox biotransformation of recalcitrant pollutants in anaerobic wastewater treatment systems.

Effects of TiO 2 dosage, pH and temperature on decolorization of C.I. Reactive Red 2 in a UV/US/TiO 2 system

This study elucidates the decolorization of C.I. Reactive Red 2 (RR2) in ultrasound (US), US/TiO 2, ultraviolet (UV)/TiO 2 and UV/US/TiO 2 systems. The effects of TiO 2 dosage, pH and temperature on RR2 decolorization were determined in a UV/US/TiO 2 system. The pseudo-first-order decolorization rate constants of 0.5, 1 and 2 g/l TiO 2 in the UV/US/TiO 2 system were 0.45, 0.57 and 0.94 h −1, respectively. In the UV/US/TiO 2 system, the decolorization rate of RR2 declines as pH increases, and increases as temperature increasing. At pH 7, the decolorization rates followed the order of UV/US/TiO 2 (0.94 h −1) > UV/TiO 2 (0.85 h −1) > US/TiO 2 (0.25 h −1) > US. The activation energy of the UV/US/TiO 2 system at pHs of 4, 7 and 10 was 6.58, 11.62 and 21.32 kJ/mol, respectively. Experimental results suggest that the RR2 decolorization in the UV/US/TiO 2 system was a diffusion-controlled reaction.

Photodegradation of C.I. Reactive Red 2 in UV/TiO2-based systems: Effects of ultrasound irradiation

This investigation elucidated the decolorization of C.I. Reactive Red 2 (RR2) in US/TiO(2), UV/TiO(2) and UV/US/TiO(2) systems and evaluated the effect of ultrasound (US) irradiation in photocatalysis. The effects of RR2 concentration, temperature and the addition of NaCl, Na(2)S(2)O(8) and radical scavenger were determined. The decolorization reactions obeyed the pseudo-first-order kinetics in all tested systems. In US-related systems, the decolorization rate of RR2 declines as RR2 concentration increases. At pH 7, the decolorization rates followed the order UV/US/TiO(2) (0.94 h(-1))>UV/TiO(2) (0.85 h(-1))>US/TiO(2) (0.25 h(-1)). The promotion efficiencies of adding NaCl in US/TiO(2), UV/TiO(2) and UV/US/TiO(2) systems were 16%, 18% and 29%, respectively. The decolorization rate increased with the temperature; additionally, the decolorization rate in UV/US/TiO(2)/Na(2)S(2)O(8) exceeded that in UV/US/TiO(2). The inhibition of RR2 decolorization by adding 1-butanol reveals that the primary decolorization pathway involves hydroxyl radicals, and that direct oxidation by photogenerated holes is probably important in the UV/TiO(2)-based system. After 120 min of the reaction, the TOC degradation efficiencies of UV/TiO(2) and UV/US/TiO(2) systems were 47% and 63%, respectively.

Prevenient dye-degradation mechanisms using UV/TiO 2/carbon nanotubes process

Photocatalysis research heavily emphasizes increasing photo-efficiency. This study presents the application of carbon nanotubes (CNTs) to increase the photocatalytic activity of TiO 2. It elucidates the effect of CNTs dose on the decolorization efficiency of aqueous azo dye, C.I. Reactive Red 2 (RR2), determines the effects of SO 4 2− formation and removal of total organic carbon (TOC), and measures the effects of various ultraviolet wavelengths. Scanning electron microscopy was used to elucidate the mixing phenomenon and the size of TiO 2 and CNTs; X-ray diffraction was used to determine crystallinity; a BET meter was used to measure surface area and a spectrophotometry was used to determine the decolorization of RR2. Experimental results indicated significant effects of photodegradation on the combination of TiO 2 with CNTs and electron transfer is higher for 410 nm irradiation than for 365 nm, revealing that solar light can be used. The electron transfer in the TiO 2/CNTs composites reduced the electron/hole recombination and increased the photon efficiency and the prevenient dye-degradation mechanisms using UV/TiO 2/CNTs were established.

Biodecolorization of the Azo Dye Reactive Red 2 by a Halotolerant Enrichment Culture

The decolorization of the azo dye Reactive Red 2 (RR2) under anoxic conditions was investigated using a mesophilic (35 degrees C) halotolerant enrichment culture capable of growth at 100 g/L sodium chloride (NaCl). Batch decolorization assays were conducted with the unacclimated halotolerant culture, and dye decolorization kinetics were determined as a function of the initial dye, biomass, carbon source, and an externally added oxidation-reduction mediator (anthraquinone-2,6-disulphonic acid) concentrations. The maximum biomass-normalized RR2 decolorization rate by the halotolerant enrichment culture under batch, anoxic incubation conditions was 26.8 mg dye/mg VSSxd. Although RR2 decolorization was inhibited at RR2 concentrations equal to and higher than 300 mg/L, the halotolerant culture achieved a 156-fold higher RR2 decolorization rate compared with a previously reported, biomass-normalized RR2 decolorization rate by a mixed mesophilic (35 degrees C) methanogenic culture in the absence of NaCl. Decolorization kinetics at inhibitory RR2 levels were described based on the Haldane model (Haldane, 1965). Five repetitive dyeing/decolorization cycles performed using the halotolerant culture and the same RR2 dyebath solution demonstrated the feasibility of biological renovation and reuse of commercial-strength spent reactive azo dyebaths.

Homogeneous catalytic ozonation of C.I. Reactive Red 2 by metallic ions in a bubble column reactor

This study elucidates the decolorization of C.I. Reactive Red 2 (RR2) by homogeneous catalytic ozonation. The effects of pH and catalyst dosage were evaluated in O 3/Mn(II), O 3/Fe(II), O 3/Fe(III), O 3/Zn(II), O 3/Co(II) and O 3/Ni(II) systems. In O 3/Mn(II), O 3/Fe(II) and O 3/Fe(III) systems, increasing the catalyst concentration increased the rate of RR2 decolorization; however, further increasing the catalyst concentration caused no further significant increase. When 0.6 mM catalyst was added, the decolorization rates of O 3/Mn(II), O 3/Fe(II), O 3/Fe(III), O 3/Zn(II), O 3/Co(II) and O 3/Ni(II) systems at pH 2 were 3.295, 1.299, 1.278, 1.015, 0.843 and 0.822 min −1, respectively. Under all of the experimental conditions, the decolorization efficiency of catalytic ozonation exceeded that of ozonation alone. The decolorization rate markedly exceeds the TOC removal rate in all tested systems. The effect of the radical scavenger on the catalytic ozonation processes suggests that the decolorization reaction in catalytic ozonation systems proceeds by mainly radical-type mechanisms, except in the O 3/Mn(II) system.

Decolorization of C.I. Reactive Red 2 by catalytic ozonation processes

This study adopted O 3, UV/TiO 2/O 3, O 3/Mn(II) and O 3/MnO 2 systems to assess the decolorization efficiency of C.I. Reactive Red 2 (RR2). The decolorization rate increased with concentrations of Mn(II) and MnO 2 in the ranges 0.05–0.1 and 0.05–0.8 g/l, respectively. However, when 0.5–3 g/l TiO 2 was added, the effect of TiO 2 dosage for RR2 decolorization was insignificant in the UV/TiO 2/O 3 system. At pH 2, the decolorization rate constants of O 3, O 3/Mn(II) (0.05 g/l), O 3/Mn(II) (0.1 g/l), O 3/Mn(II) (0.15 g/l), O 3/MnO 2 (0.05 g/l) and O 3/MnO 2 (0.8 g/l) were 0.816, 2.001, 3.173, 3.087, 1.040 and 1.648 min −1, respectively. After 5 min of reaction, the decolorization rates followed the order O 3/Mn(II) > O 3/MnO 2 > O 3 > UV/TiO 2/O 3; however, the TOC removal did not vary among these systems. Adding ethanol reduced the decolorization rate of the UV/TiO 2/O 3 and O 3/MnO 2 systems and did not affect the decolorization rate of O 3/Mn(II). Decolorization in UV/TiO 2/O 3, O 3/Mn(II) and O 3/MnO 2 systems is suggested to proceed by mainly radical-, surface- and radical-type mechanisms, respectively. Additionally, direct ozonation cannot be ignored in O 3/Mn(II) and O 3/MnO 2 systems.

Application of redox mediators to accelerate the transformation of reactive azo dyes in anaerobic bioreactors.

Azo dyes are nonspecifically reduced under anaerobic conditions but the slow rates at which reactive azo dyes are converted presents a serious problem for the application of anaerobic technology as a first stage in the complete biodegradation of these compounds. As quinones have been found to catalyze reductive transfers by acting as redox mediators, the application of anthraquinone-2,6-disulfonic acid (AQDS) during continuous anaerobic treatment of the reactive azo dye, Reactive Red 2 (RR2), was evaluated. A mixture of volatile fatty acids was used as the electron-donating primary substrate. Batch experiments demonstrated that AQDS could increase the first-order rate constant of RR2 reductive cleavage by one order of magnitude. In the continuous experiment, treatment of RR2 containing synthetic wastewater in a lab-scale upflow anaerobic sludge blanket (UASB) reactor yielded low dye removal efficiencies (<30%). Consequently, severe toxicity problems occurred, eventually resulting in almost complete inhibition of the methanogenic activity. Addition of catalytic concentrations of AQDS (19 microM) to the reactor influent caused an immediate increase in the dye removal efficiency and recovery of biological activity. Ultimately, RR2 removal efficiency stabilized at 88%, and higher AQDS loads resulted in higher RR2 removal efficiencies (up to 98% at 155 microM AQDS). Examination of the RR2 decolorizing properties of dye-adapted reactor sludge and of nonadapted reactor seed sludge revealed that RR2 decolorization was principally a biologically driven transfer of reducing equivalents from endogenous and added substrates to the dye. Hydrogen, added in bulk, was clearly the preferred electron donor. Bacteria that couple dye decolorization to hydrogen oxidation were naturally present in seed sludge. However, enrichment was required for the utilization of electrons from volatile fatty acids for dye reduction. The stimulatory effect of AQDS on RR2 decolorization by AQDS-unadapted sludge was mainly due to assisting the electron transfer from endogenous substrates in the sludge to the dye. The stimulatory effect of AQDS on RR2 decolorization by sludge from the AQDS-exposed reactor was, in addition, strongly associated with the transfer of electrons from hydrogen and acetate to the dye, probably due to enrichment of specialized AQDS-reducing bacteria.

Biological Decolorization of the Azo Dye Reactive Red 2 Under Various Oxidation–Reduction Conditions

The decolorization of the azo dye reactive red 2 (RR2) was investigated under aerobic, anoxic, and methanogenic conditions. The dye was not decolorized by an aerobic culture kept under aerobic conditions for 7 days. However, incubation of the same culture under anoxic conditions resulted in RR2 decolorization with an initial rate of 40 mg/L·d and to an extent of 91.7% after the culture oxidation–reduction potential (ORP) dropped to less than ‐50 mV. A second addition of RR2 to the same culture kept under anoxic conditions resulted in a greater initial RR2 decolorization rate (86 mg/L·d) at an ORP value of –250 mV. Decolorization of RR2 at an initial dye concentration of 300 mg/L was achieved by an unacclimated methanogenic culture at an initial rate of 83 mg/L·d and without any inhibition. However, as a result of acclimation, initial RR2 decolorization rates of 523 and 1050 mg/L·d were achieved by the same methanogenic culture after 5 months and 2 years of weekly additions of 300 mg/L RR2, respectively. Dye decolorization followed Michaelis–Menten kinetics. Under methanogenic conditions and at an initial dye concentration of 300 mg/L, the maximum RR2 decolorization rate per unit biomass was 0.12 and 0.70 mg RR2/mg volatile suspended solids d without acclimation and after a 2‐year acclimation, respectively. Lack of significant RR2 decolorization was observed in autoclaved methanogenic culture controls. Therefore, the reductive cleavage of the dye azo bond resulting in the decolorization of RR2 and formation of aromatic amines was biologically mediated and is attributed to the reduced conditions created and maintained by the anoxic and anaerobic cultures. Thus, biological decolorization of reactive azo dyes is feasible under anoxic–anaerobic conditions and should be further explored for textile wastewater management.