Azo Dyes

Azo dyes account for >70% of the global industrial demand (∼9 million tons). Owing to their genotoxic/carcinogenic potential, the annual disposal of ∼4,500,000 tons of dyes and/or degraded products is an environmental and socio-economic concern. In comparison to physico-chemical methods, microbe-mediated dye degradation is considered to be low-input, cost-effective and environmentally-safe. However, under different environmental conditions, interactions of chemically diverse dyes with metabolically diverse microbes produce metabolites of varying toxicity. In addition, majority of studies on microbial dye-degradation focus on decolorization with least attention towards detoxification. Therefore, the environmental significance of microbial dye detoxification research of past >3 decades is critically evaluated with reference to dye structure and the possible influence of microbial interactions in different environments. In the absence of ecosystem-based studies, the results of laboratory-based studies on dye degradation, metabolite production and their genotoxic impact on model organisms are used to predict the possible fate and consequences of azo dyes/metabolites in the environment. In such studies, the predominance of fewer numbers of toxicological assays that too at lower levels of biological organization (molecular/cellular/organismic) suggests its limited ecological significance. Based on critical evaluation of these studies the recommendations on inclusion of multilevel approach (assessment at multiple levels of biological organization), multispecies microcosm approach and native species approach in conjunction with identification of dye metabolites have been made for future studies. Such studies will bridge the gap between the fundamental knowledge on dye-microbe-environment interactions and its application to combat dye-induced environmental toxicity. Thus an environmental perspective on dye toxicity in the background of dye structure and effects of environmental processes has been developed. Based on past 3 decades of research on microbial dye detoxification, the current state of knowledge has been analyzed, environmental relevance of these studies was ascertained, research gaps in microbe-mediated azo dye detoxification have been identified and a research framework emphasizing a better understanding of complex interactions between dye-microbe and environmental processes has been proposed. It provides directions for undertaking environmentally sound microbial dye detoxification research.

The objective of this work was to model and optimize the degradation of black azo dye Naphthol Blue Black (NBB) by the Fenton process (advanced oxidation) using a minimum of experiments. A Plackett-Burman screening design was first applied to determine the main factors influencing the process. The dye discoloration efficiency gave a yield approximately equal to 97 % in the best conditions of several operating parameters used. The variance analysis (ANOVA) showed the effects of all different factors and deduced the most important ones. Subsequently, a second quadratic design of experiments central composite type (CCD) was applied using the response surface methodology (RSM) to optimize
the most important parameters deduced by the first design cited above, in order to obtain the best performance of the discoloration of NBB with this process. So from the contour plots and the response surfaces, the discoloration yield enhanced to the maximum and the optimization plot given by the Mnitab software, showed the following optimized parameters [NBB]=37.5 mg/L, [H2O2]=66.5 mg/L, [Iron]=3.5 mg/L and pH=3.4 for a yield of 100 % with a desirability of 1.0000. At last, to confirm that the discoloration was due to the degradation of the dye, the chemical
oxygen demand (COD) was studied and in the optimized conditions, the
degradation reached 94.78 % after 120 min of treatment. The kinetics of the dye degradation showed by the COD abatement was relatively slow compared to the kinetics of the dye discoloration.

Water pollution is a major global problem. Organic pollutants are one of the major groups of toxic and carcinogenic contaminants that, due to their complex structures, show resistance to biodegrada-tion processes. The pulsed corona discharge (PCD) advanced oxidation technology, with a point-to-point configuration of the electrodes immersed in the aqueous solution, was used for decomposing Acid Black 1 (AB1) as a representative of synthetic organic azo dyes. The effects of electrical field frequencies (60 and 120 Hz) and electrode gap spaces (2, 4 and 6 mm) on decomposition of AB1 were investigated. The largest decomposition achieved was 99.93% with optimal conditions of electrical field frequencies of 120 Hz and an 8 mm electrode gap space. Also, the effect of the catalytic properties of titanium dioxide nanoparticles on the treatment process of the PCD reactor was investigated. Different concentrations of TiO 2 NPs (0.075, 0.3, 0.08 and 3 gL −1) were tested. A concentration of 0.08 gL −1 TiO 2 was found to be the optimal concentration that increased the dye degradation from 52.62% (with no titanium dioxide nano particles) to 94.14% in 15 min. However, a higher concentration of TiO 2 showed the adverse effect and decreased the degradation level of AB1. Finally, the pulsed corona discharge as an oxidation process was compared with the photocatalytic processes of UV, UV/H 2 O 2 and UV/TiO 2 in terms of removing AB1 from an aqueous solution, and promising results were reported. Published by Elsevier Ltd.

Bisazo Tröger base analogs are claw-looking molecules that carry two identical but independent photoswitching groups. These bisazo compounds, like dual switches, can be at EE, EZ, and ZZ photoisomerization states while the ∧-shape void of Tröger base scaffold may also introduce an additional possibility of stereoisomerism by holding the photoisomerizable groups in endo or exo positions. To date, the exact molecular geometries of the isomerizable analogs of Tröger's base have not been elaborated. Owing to the special features and potential applications of these molecules, herein their photoisomerization is probed. The gleaned data from 15N isotope labeling and a variety of spectroscopic, spectrophotometric, crystallographic, and computational techniques have revealed the structure of the studied photoisomers and suggested their use in the design of molecular pull and push mechanisms, light driven actuators, and artificial muscles.

This paper describes research on natural azo compounds isolated from fungi, plant, bacteria, and invertebrates. More than 120 biologically active diazene containing alkaloids demonstrate confirmed pharmacological activity, including antitumor, antimicrobial, and antibacterial effects. The structures, origin, and biological activities of azo compounds are reviewed. Utilizing the computer program PASS, some structure–activity relationship new activities are also predicted, pointing toward possible new applications of these compounds. This article emphasizes the role of natural azo compounds as an important source of drug prototypes and leads for drug discovery.

A biosurfactant producing Gram positive bacterium isolated from anodic biofilm of textile wastewater fed MFC was identified as Bacillus sp. MFC (Accession number: MT322244). Scanning Electron Microscopy of the bacterium showed appendages, the bacterium forms biofilm on Congo red agar medium. The obtained results showed that the addition of 5 mg/l endogenous biosurfactant to the bacterial cells resulted in 19-fold increase in bacterial surface-bound exopolysaccharides (EPS) and 1.94-fold increase in biofilm. However, when the bio-surfactant concentration increased to 20 and 40 mg/l, EPS and biofilm decreased and the cells lost their colony forming ability. The dielectric properties of the bacterial cells showed increase in conductivity and relative permittivity with increasing biosurfactant concentrations. The shape of the voltammogram currents peak, their location and Electrochemical impedance spectroscopy (EIS) suggest the involvement of biofilm as direct electron transfer pathway. The average voltage obtained was 0.65 V as compared to 0.45 V for the control MFC. Decolourization was tested for Congo red in a double chamber Microbial Fuel Cell (MFC), the results showed 2-fold increase in decolourization when biosurfactant is added post biofilm formation. The results confirm that Bacillus sp. MFC possess electrogenic properties and that adding low concentrations of endogenous biosurfactant to 24 h biofilm accelerates electron transfer by inducing perforations in the cell wall and increasing EPS as an electron transfer transient medium. Therefore, MFC performance can be enhanced.