Reduction Of Azo Dyes Research Articles

Kinetic investigations on the reduction of azo dyes in absence and presence of micelle forming surfactants

Kinetic investigations on the reduction of azo dyes in absence and presence of micelle forming surfactants

Electrode as sole electrons donor for enhancing decolorization of azo dye by an isolated Pseudomonas sp. WYZ-2

Pseudomonas sp. WYZ-2 was isolated from a biocathode which accelerating azo dye decolorization. When the electrode was polarized at -0.8 V (vs. SCE), WYZ-2 could exist on electrode, because the current of working electrode stabilized at -0.35 mA from -0.13 mA after inoculation. Moreover, cyclic voltammetry scanned an unidentified redox-active molecule which involved in the electron charge transfer potentially. On azo dye decolorization experiments by WYZ-2 modified electrode, electrochemical tests also indicated that the catalytic ability of WYZ-2 modified electrode was improved because charge transfer resistance decreased to 255 Ω from 720 Ω, azo dye reduction potential was shifted to -0.78 V from -0.89 V, and the maximum decolorization efficiency of azo dye was increased to 93.4% from 53.2%, comparing with unmodified electrode. Although numerous studies on azo dye decolorization employed biological agents, electrochemical activity bacteria accelerate the decolorization process using electrode as sole electron source has seldom been reported.

Characterization of a new oxygen-insensitive azoreductase from Brevibacillus laterosporus TISTR1911: Toward dye decolorization using a packed-bed metal affinity reactor

This study reports the identification of a new bacterial azoreductase from Brevibacillus laterosporus TISTR1911, its heterologous production in Escherichia coli, the biochemical characterization and immobilization for use in dye biodegradation processes. The recombinant azoreductase (BrAzo) is a monomeric FMN oxygen-insensitive enzyme with a molecular mass of 23 kDa showing a broad specificity for the reduction of synthetic azo dyes. Double hexahistidine-tagged BrAzo was immobilized onto a nickel chelating column and methyl orange was used to assess its degradation potential using a packed-bed reactor. The dye degradation is described by an exponential model in a downstream batchwise continuous flow mode operated with recycling. The complete degradation of methyl orange (170 μM at 600 mL/h) was achieved in 3 h and continued over 9 cycles. Coupling the immobilized BrAzo with glucose dehydrogenase for NADH regeneration yielded a shorter 1.5 h-degradation period that was maintained throughout 16 cycles.

Enzymatic transformation of nitro-aromatic compounds by a flavin-free NADH azoreductase from Lysinibacillus sphaericus

Azo dyes and nitro-aromatic compounds are the largest group of pollutants released in the environment as industrial wastes. They create serious health and environmental problems. Azoreductases catalyze the reduction of azo dyes and nitro compounds to their respective amines. AN azoreductase was purified up to 12-fold from Lysinibacillus sphaericus using ion-exchange and size exclusion chromatography. It was optimally active at pH 7.4 and 75°C. It was stable at 70°C for 30min. The purified enzyme utilized NADH rather than NADPH as an electron donor to reduce substrates. The molecular weight of the purified enzyme was ~29kDa. The enzyme also acted as nitroreductase and could selectively reduce the nitro group of 2-nitrophenol, 4-nitrobenzoic acid, 2-nitro-benzaldehyde and 3-nitrophenol. Reduction products of these compounds were identified by IR and NMR.

The non-enzymatic reduction of azo dyes by flavin and nicotinamide cofactors under varying conditions

Azo dyes are ubiquitous in products and often become environmental pollutants due to their anthropogenic nature. Azoreductases are enzymes which are present within many bacteria and are capable of breaking down the azo dyes via reduction of the azo bond. Often, though, carcinogenic aromatic amines are formed as metabolites and are of concern to humans. Azoreductases function via an oxidation–reduction reaction and require cofactors (a nicotinamide cofactor and sometimes a flavin cofactor) to perform their function. Non-enzymatic reduction of azo dyes in the absence of an azoreductase enzyme has been suggested in previous studies, but has never been studied in detail in terms of varying cofactor combinations, different oxygen states or pHs, nor has the enzymatic reduction been compared to azoreduction in terms of dye reduction or metabolites produced, which was the aim of this study. Reduction of azo dyes by different cofactor combinations was found to occur under both aerobic and anaerobic conditions and under physiologically-relevant pHs to produce the same metabolites as an azoreductase. Our results show that, in some cases, the non-enzymatic reduction by the cofactors was found to be equal to that seen with the azoreductase, suggesting that all dye reduction in these cases is due to the cofactors themselves. This study details the importance of the use of a cofactor-only control when studying azoreductase enzymes.

Treatment of azo dye-containing synthetic textile dye effluent using sulfidogenic anaerobic baffled reactor

This study aims at investigating azo dye reduction performance of a sulfidogenic anaerobic baffled reactor (ABR) for around 400 days. ABR was operated at 30 °C in a temperature-controlled room and hydraulic retention time (HRT) was kept constant at 2 days. The robustness of ABR was assessed under varying azo dye loadings and COD/sulfate ratios. Additionally, oxygen was supplied (1-2 L air/m(3)reactor min) to the last compartment to investigate the removal of azo dye breakdown products. ABR performed well in terms of COD, sulfate and azo dye removals throughout the reactor operation. Maximum azo dye, COD and sulfate removals were 98%, 98% and 93%, respectively, at COD/sulfate ratio of 0.8. Aeration created different redox conditions in last compartment, which enhanced the removal of COD and breakdown products. The adverse effects of aeration on azo dye reduction were eliminated thanks to the compartmentalized structure of the ABR.

Reductase-like Activity of Silicon Nanowire Arrays

The MTT (3-(4,5-dimethyl-2-thiazol)-2,5-diphenyl-2H-tetrazolium bromide) reduction method is widely used for measuring cell viability and proliferation. However, when MTT was used to study cells on silicon nanowire arrays (SiNWAs), the measured viability was much higher than normal values, resulting in a misleading estimate of cell viability. Our results demonstrated that the apparent high viability of cells is due to the fact that the SiNWAs itself was capable of reducing MTT in the absence of cells. In the presence of coenzyme, its reducing capacity was enhanced, thus showing the reductase-like function of SiNWAs. Furthermore, the chemical composition and nanostructure of Si surface had a strong influence on MTT reduction with the HF-treated SiNWAs (H-SiNWAs) showing significant reducing capacity. For example, the reduction capacity of H-SiNWAs samples was significantly higher than that of HF-treated planar silicon, whereas Piranha-treated SiNWAs and planar silicon did not reduce MTT. H-SiNWAs were also used for the reduction of azo dyes and showed a decolorization rate of more than 65% and as high as 90%. These findings suggest the potential use of SiNWAs as enzyme-mimics in biotechnology and environmental chemistry.

The effect of salt on the performance and characteristics of a combined anaerobic–aerobic biological process for the treatment of synthetic wastewaters containing Reactive Black 5

The findings of the present work showed that variation in NaCl concentration, in the range 0–100g/l, has a significant effect on the performance and characteristics of a combined anaerobic–aerobic process used for treatment of a synthetic RB5 containing wastewater. The effect of increasing NaCl concentration on anaerobic decolourisation efficiency was time dependent and two different types of relationships were observed for the initial and the later part of the process. Anaerobic decolourisation data from runs carried out under low salinity (NaCl concentration⩽20g/l) was successfully modelled using the second order kinetic rate equation, whereas at high salinity (NaCl concentration=100g/l) the first order kinetic model gave a better fit of the experimental data. Increase in salinity resulted in increase in Extracellular Polymeric Substances concentration throughout the anaerobic process and Soluble Microbial Products during the early stages of the process. UV–Vis spectral and FTIR analysis showed no effect of salt on the mechanism of anaerobic azo dye reduction whereas it lead to retardation or inhibition of the autooxidation of one of anaerobic RB5 reduction metabolites during the aerobic stage. The latter has implications regarding the use of the combined anaerobic–aerobic process for treatment of azo dye containing wastewaters.

Reduction of azo dyes by flavin reductase from Citrobacter freundii A1

<em>Citrobacter freundii </em>A1 isolated from a sewage treatment facility was demonstrated to be able to effectively decolorize azo dyes as pure and mixed culture. This study reports on the investigation on the enzymatic systems involved. An assay performed suggested the possible involvement of flavin reductase (Fre) as an azo reductase. A heterologouslyexpressed recombinant Fre from <em>C. freundii </em>A1 was used to investigate its involvement in the azo reduction process. Three model dyes were used, namely Acid Red 27 (AR27), Direct Blue 15 (DB15) and Reactive Black 5 (RB5). AR27 was found to be reduced the fastest by Fre, followed by RB5, and lastly DB15. Redox mediators nicotinamide adenine dinucleotide (NADH) and riboflavin enhance the reduction, suggesting the redox activity of the enzyme. The rate and extent of reduction of the model dyes correlate well with the reduction potentials (<em>E</em><sub>p</sub>). The data presented here strongly suggest that Fre is one of the enzymes responsible for azo reduction in <em>C. freundii </em>A1, acting via an oxidation-reduction reaction.

Reductive Degradation of Organic Compounds Using Microbial Nanotechnology

Members of the genus Clostridia can reduce Pd(II) ions to form metallic Pd nanoparticles (bio-Pd). Cultures of C. pasteurianum BC1 were used to generate bio-Pd, which is primarily formed on the microbial cell wall. Batch experiments using C. pasteurianum BC1 cells loaded with bio-Pd showed efficient reduction of the organic azo dyes, methyl orange and Evans blue, while little reduction of dyes was observed in control experiments using Pd-containing heat-killed microbial cultures or Pd-free viable bacterial cultures. Degradation of azo dyes was found to occur via reductive hydrogenation of the azo-linkage. Molecular hydrogen, which is concomitantly generated by C. pasteurianum, is used in the reduction reaction. The process described in this study is a potentially viable alternative to current groundwater and wastewater treatment technologies that fail to adequately degrade the large quantities of hazardous spent textile dyes that are discharged into the environment each year.

Effect of nitrate on anaerobic azo dye reduction

The aim of the study was to investigate the effect of nitrate on anaerobic color removal efficiencies. For this aim, anaerobic-aerobic sequencing batch reactor (SBR) fed with a simulated textile effluent including Remazol Brilliant Violet 5R azo dye was operated with a total cycle time of 12 h, including anaerobic (6 h) and aerobic cycles (6 h). Microorganism grown under anaerobic phase of the reactor was exposed to different amounts of competitive electron acceptor (nitrate) and performance of the system was determined by monitoring color removal efficiency, nitrate removal, nitrite formation and removal, oxidation reduction potential, color removal rate, chemical oxygen demand (COD), specific anaerobic enzyme (azo reductase) and aerobic enzyme (catechol 1,2 dioxygenase), and formation and removal of aromatic amines. Variations of population dynamics of microorganisms exposed to various amount of nitrate were identified by denaturing gradient gel electrophoresis (DGGE). It was found that nitrate has adverse effect on anaerobic color removal efficiency and color removal was achieved after denitrification process was completed. It was found that nitrate stimulates the COD removal efficiency and accelerates the COD removal in the first hour of anaerobic phase. About 90 % total COD removal efficiencies were achieved in which microorganism exposed to increasing amount of nitrate. Population dynamics of microorganisms exposed to various amount of nitrate were changed and diversity was increased.

Comparison of Different Types of Redox Mediators on the Chemical Reduction of Azo Dye by Sulfide

Azo dyes are extensively used in the industry, resulting in the large amount of wastewater. Redox mediators (R.M.) can accelerate the chemical reduction of azo dye by shuttling electrons from primary electron donors such as sulfide to azo dyes. Three different types of R.M. were studied on the enhancement of chemical reduction of Acid Orange 7. The electron shuttle ability of these types of R.M. with 2.5 mM of initial sulfide concentration at pH 7.5, either dissolved or undissolved type, followed in order by riboflavin, humic acid and activated carbon. The potential engineering application of these R.M. lie on the superior catalytic ability for riboflavin, wide existence in the environment for humic acid, and retaining in the bioreactors all the times for activated carbon.

Decoloration of Orange G (OG) using electrochemical reduction

The electrocatalytic hydrogenation of Orange G is investigated using spectrophotometric experiments in laboratory cells. The working electrode consists of a thin grid coated with a layer of nickel in which fine particles of Raney nickel are dispersed. The optimal conditions of decoloration are as follows: basic pH, 0.05 g/L of dye concentration and 0.05 A of current density. Under these conditions, the OG decoloration efficiency reached 100% after only 1800 s of reaction. The observed values of the maximum absorbance in the spectra of the reaction mixture fitted well the polynomials of the fifth degree with respect to reaction time. The initial degradation rate of the dye is obtained easily as the differential coefficient of the functions at initial time. The degradation rate of the dye in the initial stage of the reaction is given by the first-order rate equation. The instantaneous current efficiency was calculated and the results indicated that cathodic reduction was the main contributor to the decoloration of OG. Direct cathodic reduction of azo dyes allows decolorization of intensively coloured textile wastewater without addition of chemicals or formation of sludge. The technique is of particular interest for the treatment of concentrated dye baths. The effect of current density, dye concentration, and concentration and nature of the supporting electrolyte on the reduction of the Orange G are reported.

Towards advanced aqueous dye removal processes: A short review on the versatile role of activated carbon

During the last decade, several physico-chemical and biological techniques have been developed to remove colour from textile wastewaters. Some of these techniques rely on and many will profit from activated carbon (AC). The role of AC is versatile: (1) it acts as a dye adsorbent, not only in straightforward adsorption processes but also in AC-enhanced coagulation and membrane filtration processes; (2) it generates strong oxidising agents (mostly, hydroxyl (OH) radicals) in electrochemical dye oxidation; (3) it catalyses OH production in advanced oxidation processes; (4) it catalyses anaerobic (azo) dye reduction and supports biofilm growth in microbial dye removal. This paper reviews the role of AC in dye decolourisation, evaluates the feasibility of each AC-amended decolourisation technique and discusses perspectives on future research.

Effect of dye structure and redox mediators on anaerobic azo and anthraquinone dye reduction

We investigated the biological decolourisation of dyes with different molecular structures. The kinetic constant values (k1) achieved with azo dye Reactive Red 120 were 7.6 and 10.1 times higher in the presence of RM (redox mediators) AQDS and riboflavin, respectively, than the assays lacking RM. The kinetic constant achieved with the azo dye Congo Red was 42 times higher than that obtained with the anthraquinone dye Reactive Blue 4. The effect of RM on dye reduction was more evident for azo dyes resistant to reductive processes, and ineffective for anthraquinone dyes because of the structural stability of the latter.

The effect of perspiration on photo-induced chemical reaction of azo dyes and the determination of aromatic amine products

In order to investigate the impact of perspiration on photo-induced chemical reaction of azo dyes and the carcinogenic aromatic amine products produced from the reaction, we have carried out experimental studies on the photochemical reaction of C.I. Reactive Red 2 mixed with American Association of Textile Chemists and Colorists（AATCC）standard artificial perspiration. UV–vis spectroscopic technique was employed to monitor the reaction processes, and the reaction products were analyzed by hollow fiber protected liquid-liquid-liquid phase micro-extraction with capillary electrophoresis（HF-LLLME-CE). The results showed that perspiration had remarkable influence on the photochemical reaction of azo dyes. Aromtic amines formed during the photochemical process as a result of reduction of azo dyes by organic components in perspiration. The HF-LLLME-CE methodology was validated in analyzing aromatic amines produced from the photochemical degradation of azo dye C.I. Reactive Red 2 and C.I. Acid Red 35 mixed with artificial perspiration.

Optimisation of process parameters for bioreduction of azo dyes using Bacillus firmus under batch anaerobic condition

A new dye decolourising bacterial strain was isolated from textile wastewater and identified as Bacillus firmus. The study indicated that the bacterium could efficiently decolourise different azo dyes under static culture conditions. Characterisation of the efficiency of azo dye reduction by this isolate using both spectral and HPLC analysis was found to be a function of process parameters which include dye concentration, culture broth pH, incubation temperature, aeration as well as nitrogen source. For decolourisation, the optimal pH and temperature were 7–8 and 20–35°C respectively, while remarkable dye degradation was obtained within 18 h for dye concentrations below 100 mg L−1. With the addition of yeast extract and under optimal conditions, dye reduction was enhanced and complete colour removal was achieved within 12 h. Colour removal was shown to be due to biodegradation rather than adsorption of dyes on bacterial cells. This study confirms the ability of the new dye‐degrading strain, Bacillus firmus, to decolourise and degrade different azo dyes and highlights its high biotechnology potential for the eco‐friendly treatment of textile wastewater when optimal conditions are applied.

Molecular determinants of azo reduction activity in the strain Pseudomonas putida MET94

Azo dyes are the major group of synthetic colourants used in industry and are serious environmental pollutants. In this study, Pseudomonas putida MET94 was selected from 48 bacterial strains on the basis of its superior ability to degrade a wide range of structurally diverse azo dyes. P. putida is a versatile microorganism with a well-recognised potential for biodegradation or bioremediation applications. P. putida MET94 removes, in 24 h and under anaerobic growing conditions, more than 80% of the majority of the structurally diverse azo dyes tested. Whole cell assays performed under anaerobic conditions revealed up to 90% decolourisation in dye wastewater bath models. The involvement of a FMN dependent NADPH: dye oxidoreductase in the decolourisation process was suggested by enzymatic measurements in cell crude extracts. The gene encoding a putative azoreductase was cloned from P. putida MET94 and expressed in Escherichia coli. The purified P. putida azoreductase is a 40 kDa homodimer with broad substrate specificity for azo dye reduction. The presence of dioxygen leads to the inhibition of the decolourisation activity in agreement with the results of cell cultures. The kinetic mechanism follows a ping-pong bi-bi reaction scheme and aromatic amine products were detected in stoichiometric amounts by high-performance liquid chromatography. Overall, the results indicate that P. putida MET94 is a promising candidate for bioengineering studies aimed at generating more effective dye-reducing strains.

Azo and anthraquinone dye decolorization in relation to its molecular structure using soluble Trichosanthes dioica peroxidase supplemented with redox mediator

In this study we investigated the decolorizing ability of Trichosanthes dioica -derived soluble peroxidase on structurally complex azo and anthraquinone dyes in the presence of redox mediators. Our results show that riboflavin acted as a better redox mediator than antraquinone-2, 6-disulfonate (AQDS). Riboflavin served as an efficient electron transferor than AQDS in the reduction of azo dyes, by contrast with anthraquinone dyes. Although the extent of decolorization (expressed as percent dye decolorization) varied from one dye to the other, maximum decolorization was achieved for the case when suspensions containing 0.45 EU (Enzyme Units)/ml and 0.2 mM riboflavin at pH 5.0 were incubated for 2 h at 40 °C. Redox mediator supplementation remarkably improved the decolorizing potential of T. dioica peroxidase. Azo dyes exhibited higher decolorization as compared to anthraquinone dyes in the presence of riboflavin. On the contrary, percent decolorization in the absence of redox mediators was inadequate and slow. Redox mediators studied herein facilitated decolorization of stable anthraquinone dyes. Therefore, the effect of redox mediators on T. dioica peroxidase-dye decolorization is affected by stability, thus the molecular structure. ► Riboflavin enhances decolorization of azo and anthraquinone dyes more than AQDS. ► Anthraquinone dyes decolorized at higher concentrations of redox mediators. ► Cleavage of chromophore for azo dyes is favorable as compared to anthraquinone. ► Redox mediated dye decolorization is related to the dye molecular structure.

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

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.