Aerobic decolorization, degradation and detoxification of azo dyes by a newly isolated salt-tolerant yeast Scheffersomyces spartinae TLHS-SF1

Removal of food dyes using biological materials via adsorption: A review

It is alarming that synthetic food dyes (FD) are widely used in various industries and that these facilities discharge their wastewater into the environment without treating it. FDs mixed into industrial wastewater pose a threat to the environment and human health. Therefore, removing FDs from wastewater is very important. This review explores the burgeoning field of FD removal from wastewater through adsorption using biological materials (BMs). By synthesizing a wealth of research findings, this comprehensive review elucidates the diverse array of BMs employed, ranging from algae and fungi to agricultural residues and microbial biomass. Furthermore, this review investigates challenges in practical applications, such as process optimization and scalability, offering insights into bridging the gap between laboratory successes and real-world implementations. Harnessing the remarkable adsorptive potential of BMs, this review presents a roadmap toward transformative solutions for FD removal, promising cleaner and safer production practices in the food and beverage industry.

Exploring the decolorization efficiency and biodegradation mechanisms of different functional textile azo dyes by Streptomyces albidoflavus 3MGH

Efficiently mitigating and managing environmental pollution caused by the improper disposal of dyes and effluents from the textile industry is of great importance. This study evaluated the effectiveness of Streptomyces albidoflavus 3MGH in decolorizing and degrading three different azo dyes, namely Reactive Orange 122 (RO 122), Direct Blue 15 (DB 15), and Direct Black 38 (DB 38). Various analytical techniques, such as Fourier Transform Infrared (FTIR) spectroscopy, High-Performance Liquid Chromatography (HPLC), and Gas Chromatography-Mass Spectrometry (GC-MS) were used to analyze the degraded byproducts of the dyes. S. albidoflavus 3MGH demonstrated a strong capability to decolorize RO 122, DB 15, and DB 38, achieving up to 60.74%, 61.38%, and 53.43% decolorization within 5 days at a concentration of 0.3 g/L, respectively. The optimal conditions for the maximum decolorization of these azo dyes were found to be a temperature of 35 °C, a pH of 6, sucrose as a carbon source, and beef extract as a nitrogen source. Additionally, after optimization of the decolorization process, treatment with S. albidoflavus 3MGH resulted in significant reductions of 94.4%, 86.3%, and 68.2% in the total organic carbon of RO 122, DB 15, and DB 38, respectively. After the treatment process, we found the specific activity of the laccase enzyme, one of the mediating enzymes of the degradation mechanism, to be 5.96 U/mg. FT-IR spectroscopy analysis of the degraded metabolites showed specific changes and shifts in peaks compared to the control samples. GC-MS analysis revealed the presence of metabolites such as benzene, biphenyl, and naphthalene derivatives. Overall, this study demonstrated the potential of S. albidoflavus 3MGH for the effective decolorization and degradation of different azo dyes. The findings were validated through various analytical techniques, shedding light on the biodegradation mechanism employed by this strain.

Biotransformation of the azo dye reactive orange 16 by Aspergillus flavus A5P1: Performance, genetic background, pathway, and mechanism

This study reports the strain Aspergillus flavus A5P1 (A5P1), which is with the capable of degrading the azo dye reactive orange 16 (RO16). The mechanism of RO16 degradation by A5P1 was elucidated through genomic analysis, enzymatic analysis, degradation pathway analysis and oxidative stress analysis. Strain A5P1 exhibited aerobic degradation of RO16, with optimal degradation at an initial pH of 3.0. Genomic analysis indicates that strain A5P1 possesses the potential for acid tolerance and degradation of azo dye. Enzymatic analysis, combined with degradation product analysis, demonstrated that extracellular laccase, intracellular lignin peroxidase, and intracellular quinone reductase were likely key enzymes in the RO16 degradation process. Oxidative stress analysis revealed that cell stress responses may participate in the RO16 biotransformation process. The results indicated that the biotransformation of RO16 may involves biological processes such as transmembrane transport of RO16, cometabolism of the strain with RO16, and cell stress responses. These findings shed light on the biodegradation of RO16 by A5P1, indicating A5P1's potential for environmental remediation.

Toxic effects and mechanisms of cationic blue SD-GSL on Chlorella vulgaris before and after the biological decolorization process

Biodegradation is regarded as an efficient way to decolorize azo dyes. However, the changes in the algal toxicity of azo dyes during biodecolorization are still unclear. In this study, the physiological responses of Chlorella vulgaris to the hydrophobic and hydrophilic components of cationic blue SD-GSL (a typical monoazo dye) and its biodecolorization products were investigated. The toxicity of each component to Chlorella vulgaris and the sources of the toxicity were analyzed. The cationic blue SD-GSL components inhibited the algal cell division and superoxide dismutase (SOD) activity at all concentrations, and inhibited the synthesis of chlorophyll-a (Chl-a) at concentrations >100 mg/L, whereas increased the malondialdehyde (MDA) content. The hydrophobic and hydrophilic components of its biodecolorization products had enhanced inhibition rates on cell density (10.7% and 15.6%, respectively), Chl-a content (31.2% and 8.4%, respectively), and SOD activity (13.5% and 1.9%, respectively) of Chlorella vulgaris, and further stimulated an increase in MDA content (4.4% and 7.0%, respectively), indicating that the biodecolorization products were more toxic than the pristine dye. Moreover, the toxic effect of hydrophobic components on Chlorella vulgaris was stronger than that of hydrophilic components. The sensitivity sequence of Chlorella vulgaris to the toxicity of cationic blue SD-GSL and its biodecolorization product components was: Chl-a synthesis > SOD activity > cell division. SUVA analysis and 3D-EEM analysis revealed that the enhanced algal toxicity of the biodecolorization products of cationic blue SD-GSL was attributed to the aromatic compounds, which were mainly concentrated in the hydrophobic components. UPLC-Q-TOF-MS was used to verify dye biodecolorization byproducts. The information obtained from this study helps to understand the decolorization products toxicities of the biologically treated azo dyes, thereby providing new insights into the environmental safety of textile wastewater after traditional biological treatment.

Comparing biofilm reactors inoculated with Shewanella for decolorization of Reactive Black 5 using different carrier materials

This study assessed the performance of biofilm reactors inoculated with azo dye degrading Shewanella for the decolorization of Reactive Black 5 (RB5), using three different carrier materials, namely almond shell biochar, moving bed biofilm reactor (MBBR), and polypropylene carrier (PPC). The reactors were fed with low-nutrient artificial wastewater containing RB5, and all three carriers showed good RB5 decolorization performance, with varying efficiencies. Liquid Chromatography-Mass Spectrometry analysis revealed distinct RB5 degradation pathways associated with each carrier, influenced by carrier materials and microbial communities. The MBBR carrier exhibited good stability due to its rough surface and microbial aggregates. Sequencing results highlighted differences in the microbial community structures among the carriers. Shewanella predominated the functional bacteria in the MBBR and PPC carriers, while the biochar carrier fostered highly efficient degrading microbial communities. The physicochemical properties of carrier materials significantly influenced the microbial community and RB5 degradation efficiency. These findings provide valuable insights for optimizing biofilm reactors for dye-containing wastewater treatment.

Anaerobic biodegradation of azo dye reactive black 5 by a novel strain Shewanella sp. SR1: Pathway and mechanisms

The efficiency of microbial populations in degrading refractory pollutants and the impact of adverse environmental factors often presents challenges for the biological treatment of azo dyes. In this study, the genome analysis and azo dye Reactive Black 5 (RB5) degrading capability of a newly isolated strain, Shewanella sp. SR1, were investigated. By analyzing the genome, functional genes involved in dye degradation and mechanisms for adaptation to low-temperature and high-salinity conditions were identified in SR1. The addition of co-substrates, such as glucose and yeast extract, significantly enhanced RB5 decolorization efficiency, reaching up to 87.6%. Notably, SR1 demonstrated remarkable robustness towards a wide range of NaCl concentrations (1-30 g/L) and temperatures (10-30 °C), maintaining efficient decolorization and high biomass concentration. The metabolic pathways of RB5 degradation were deduced based on the metabolites and genes detected in the genome, in which the azo bond was first cleaved by FMN-dependent NADH-azoreductase and NAD(P)H-flavin reductase, followed by deamination, desulfonation, and hydroxylation mediated by various oxidoreductases. Importantly, the degradation metabolites exhibited reduced toxicity, as revealed by toxicity analysis. These findings highlighted the great potential of Shewanella sp. SR1 for bioremediation of wastewaters contaminated with azo dyes.

A Potentially Practicable Halotolerant Yeast Meyerozyma guilliermondii A4 for Decolorizing and Detoxifying Azo Dyes and Its Possible Halotolerance Mechanisms

In this study, a halotolerant yeast that is capable of efficiently decolorizing and detoxifying azo dyes was isolated, identified and characterized for coping with the treatment of azo-dye-containing wastewaters. A characterization of the yeast, including the optimization of its metabolism and growth conditions, its detoxification effectiveness and the degradation pathway of the target azo dye, as well as a determination of the key activities of the enzyme, was performed. Finally, the possible halotolerance mechanisms of the yeast were proposed through a comparative transcriptome analysis. The results show that a halotolerant yeast, A4, which could decolorize various azo dyes, was isolated from a marine environment and was identified as Meyerozyma guilliermondii. Its optimal conditions for dye decolorization were ≥1.0 g/L of sucrose, ≥0.2 g/L of (NH4)2SO4, 0.06 g/L of yeast extract, pH 6.0, a temperature of 35 °C and a rotation speed of ≥160 rpm. The yeast, A4, degraded and detoxified ARB through a series of steps, relying on the key enzymes that might be involved in the degradation of azo dye and aromatic compounds. The halotolerance of the yeast, A4, was mainly related to the regulation of the cell wall components and the excessive uptake of Na+/K+ and/or compatible organic solutes into the cells under different salinity conditions. The up-regulation of genes encoding Ca2+-ATPase and casein kinase II as well as the enrichment of KEGG pathways associated with proteasome and ribosome might also be responsible for its halotolerance.

The synergistic activation of peroxymonosulfate for the degradation of Acid Scarlet GR by palygorskite/MnO2/Fe3O4 nanocomposites

A heterogeneous Fenton-like system comprising palygorskite/MnO₂/Fe₃O₄ (PMM) as a superior, low-cost, and eco-friendly ternary catalyst for the activation of peroxymonosulfate (PMS) was investigated with regard to its ability to degrade Acid Scarlet GR in an aqueous solution. Under the optimum catalytic oxidation conditions of 1 g L^-1 PMM, 0.7 g L^-1 PMS, and an initial pH value of 5, 200 mg L^-1 Acid Scarlet GR was completely degraded within 300 min. PMM exhibited outstanding magnetic recovery ability and reusability after nine cycles with a degradation efficiency of up to 95.4%. The PMM catalyst had a broad working pH range. Singlet oxygen ¹O₂ was considered to play the principal role in the degradation of Acid Scarlet GR in the PMM/PMS system. The synergistic effect between MnO₂, Fe₃O₄, and PG substantially accelerated the catalytic activity of PMM, and MnO₂ was identified to be the primary active site. These findings indicate that PMM is a nanocatalyst that can efficiently activate PMS for the treatment of dye-containing wastewater.

Efficient Degradation of Printing and Dyeing Wastewater by Lotus Leaf-Based Nitrogen Self-Doped Mesoporous Biochar Activated Persulfate: Synergistic Mechanism of Adsorption and Catalysis

The discharge of printing and dyeing wastewater has been increasing, causing serious environmental pollution with the rapid development of the industry. Based on this, an N self-doped mesoporous lotus leaf biochar (LLC800) was prepared from lotus leaves as raw material for the activation of Persulfate (PS) to degrade wastewater from printing and dyeing. The removal rate of AO7 by PS, LLC800 and LLC800/PS systems were 0.84%, 31.11% and 99.46%, respectively. Electron paramagnetic resonance spectroscopy (EPR) and quench tests showed the presence of free radicals (•OH, SO4●− and O2●−) and nonradical (1O2) in the LLC800/PS system, where nonradicals (1O2) play an important role in the degradation of AO7. The “N self-doped” effect formed by the high N content of lotus leaves is the main factor leading to the high adsorption and catalytic performance of lotus leaf biochar. The effect of pyrolysis temperature on the performance of biochar can be attributed to the change of N content and conformation and specific surface area in biochar. Moreover, the LLC800/PS system has a strong resistance to interference. This work can provide technical support for the preparation of high-performance adsorption-catalytic biochar and the development of high-performance activation materials for persulfate.

Composition and Ecological Functionality of Fungal Communities Associated with Smokeless Tobacco Products Mainly Consumed in India

The microbial communities present in smokeless tobacco products (STPs) perform critical steps in the synthesis of carcinogens, mainly tobacco-specific nitrosamines (TSNAs). Most studies emphasize the bacterial component, and the mycobiome of STPs has not been well characterized. In this study, we investigated the fungal communities in the different categories of STPs by sequencing the internal transcribed spacer (ITS) rRNA region of the fungal genome. The ecological character of the fungal community associated with STPs was determined by using FUNGuild. Our results indicated that Ascomycota and Basidiomycota were the most abundant fungal phyla across all STPs. The predominant fungal genera in STPs were Pichia, Sterigmatomyces, and Mortierella. The α-diversity varied significantly across the STPs based on observed, Fisher, and Shannon indices. Using SparCC cooccurrence network analysis, significant positive correlations of 58.5% and negative connections of 41.5% were obtained among fungal genera identified in STPs. Furthermore, the functional predictions by FUNGuild determined that STPs possessed high abundances of saprotroph and pathotroph-saprotroph-symbiotroph fungal trophic groups. At the functional guild level, the qiwam samples contained high abundances of soil saprotrophs, while plant pathogens were prevalent in pan-masala samples. These results suggest that various fungal populations reside in STPs and interrelate with each other and can contribute to the synthesis of TSNAs. This study has established the basis for future large-scale investigations of STP-associated mycobiota and the impact of such mycobiota in oral carcinogenesis in STP users via inflammation and carcinogens (TSNAs and mycotoxins). IMPORTANCE Smokeless tobacco products (STPs) contain complex microbial communities that influence the synthesis of carcinogens, such as tobacco-specific nitrosamines (TSNAs). Research on STP-associated bacterial populations revealed connections between bacterial metabolism and TSNA synthesis. The abundance of the fungal population may also have an impact on the production of TSNAs. This study examined STPs popularly used in India, and diverse fungal communities were identified in these STPs. Pichia, Sterigmatomyces, and Mortierella were the predominant fungal genera in the STPs. High abundances of saprotroph and pathotroph-saprotroph-symbiotroph trophic groups in STPs could affect the degradation of tobacco products and the synthesis of TSNAs.

Biodecolorization and Ecotoxicity Abatement of Disperse Dye-Production Wastewater Treatment with Pycnoporus Laccase

The biological treatment efficiency of dye wastewater using activated sludge (AS) is largely limited to the chromaticity and ecotoxicity of dyestuff. To alleviate this limitation, eleven industrial-grade disperse dyes were obtained from a fiber-dyeing factory, and for the first time, we studied the decolorization and detoxification effects of using the Pycnoporus laccase enzyme. Efficient decolorization was achieved with the following conditions: dye concentration 50 mg/L, 1-hydroxybenzotriazole (HBT) 0.15 mM, temperature 65 °C, pH 4, and laccase 0.33 U/mL. The decolorization rate of disperse dyes, ranging from 51 to 96% in this investigation, was highly dependent on the dye type, concentration, laccase loading, and HBT. The ecotoxicity of dyes was evaluated by studying the germination/growth of wheat seed as well as the respiratory rate of aerobic AS. Laccase treatment mitigated the phytotoxicity of dyes because of the higher wheat germination (e.g., increase of 38% for Black ECT 200%) and growth rate (e.g., increase of 91% for Blue 2BLN 200%). The reduced ecotoxicity of decolorized dye solution towards microorganisms was also confirmed by the finding that the oxygen uptake by aerobic AS was increased relative to that of the untreated samples (e.g., increase of 14 folds for Blue HGL 200%). In addition, the chemical oxygen demand (COD) of decolorized dye solution was slightly lower than that without decolorization during the respiratory test. The experimental results suggest that enzymatic decolorization and detoxification can be potentially used as a pretreatment method for disperse dye wastewater followed by AS treatment.

Optimization of reactive black 5 decolorization by the newly isolated Saccharomyces cerevisiae X19G2 using response-surface methodology

In the current investigation, the capacity of different yeast strains to decolorize reactive black 5 (RB-5) was assessed. A comparative study between the different strains demonstrated that Saccharomyces cerevisiae X19G2 exhibited the highest decolorization rate (69.20 ± 1.16%) after 48 h of incubation. This strain was selected to optimize the medium components' concentrations for maximum RB-5 decolorization. Response-surface methodology (RSM) was tested for the most significant parameters (glucose, yeast extract and RB-5 dye concentrations) that were previously determined by Plackett-Burman design. A dye decolorization rate of 99.59 ± 0.24% was achieved within 48 h using a maximum RB-5 concentration (0.15 g/L) with glucose and yeast extract concentrations equalling to 10.5 g/L and 1 g/L, respectively. Experimental data results proved to fit well with the pseudo-second order kinetics model. The phytotoxicity assessment was carried out using Raphanus sativus seeds to determine the toxicity of RB-5 before and after treatment by S. cerevisiae. Results suggested that germination rate and the length of seeds radical irrigated with 0.15 g/L of RB-5 decreased by 30 and 53%, compared to those irrigated with treated solution. Therefore, metabolites derived from decolorization of RB-5 by S. cerevisiae X19G2 were significantly less toxic than the original dye.

Biodegradation and Detoxification of Azo Dyes by Halophilic/Halotolerant Microflora Isolated From the Salt Fields of Tibet Autonomous Region China

This study aimed to decolorize azo dyes in high-salt industrial wastewater under high-salt and low oxygen conditions using extreme halophilic/halotolerant bacteria screened from the salt fields of Tibet, which consisted of Enterococcus, unclassified Enterobacteriaceae, Staphylococcus, Bacillus, and Kosakonia. Under the optimal conditions, 600 mg/l Congo red, Direct Black G (DBG), Amaranth, methyl red, and methyl orange could be completely decolorized in 24, 8, 8, 12, and 12 h, respectively. When the DBG concentration was 600 mg/l, NADH–DCIP, laccase, and azo reductase were confirmed to be the primary reductase and oxidase during the degradation process, and the degradation pathways were verified. The microflora could not only tolerate changes in salt concentrations of 0–80 g/l, but also displayed strong degradative ability. Under high-salt concentrations (≥ 60 g/l NaCl), NADH–DCIP reductase was primarily used to decolorize the azo dye. However, under low salt concentrations (≤ 40 g/l NaCl), azo reductase began to function, and manganese peroxidase and lignin peroxidase could cooperate to participate in DBG degradation. Additionally, the halophilic/halophilic microflora was shown to convert the toxic DBG dye to metabolites of low toxicity based on phytotoxicity analysis, and a new mechanism for the microflora to degrade DBG was proposed based on intermediates identified by liquid chromatography-mass spectrometry (LC–MS). This study revealed that the halophilic/halophilic microflora has effective ecological and industrial value for treating wastewater from the textile industry.

Yeasts Inhabiting Extreme Environments and Their Biotechnological Applications

Yeasts are microscopic fungi inhabiting all Earth environments, including those inhospitable for most life forms, considered extreme environments. According to their habitats, yeasts could be extremotolerant or extremophiles. Some are polyextremophiles, depending on their growth capacity, tolerance, and survival in the face of their habitat’s physical and chemical constitution. The extreme yeasts are relevant for the industrial production of value-added compounds, such as biofuels, lipids, carotenoids, recombinant proteins, enzymes, among others. This review calls attention to the importance of yeasts inhabiting extreme environments, including metabolic and adaptive aspects to tolerate conditions of cold, heat, water availability, pH, salinity, osmolarity, UV radiation, and metal toxicity, which are relevant for biotechnological applications. We explore the habitats of extreme yeasts, highlighting key species, physiology, adaptations, and molecular identification. Finally, we summarize several findings related to the industrially-important extremophilic yeasts and describe current trends in biotechnological applications that will impact the bioeconomy.

A critical review on the treatment of dye-containing wastewater: Ecotoxicological and health concerns of textile dyes and possible remediation approaches for environmental safety

The synthetic dyes used in the textile industry pollute a large amount of water. Textile dyes do not bind tightly to the fabric and are discharged as effluent into the aquatic environment. As a result, the continuous discharge of wastewater from a large number of textile industries without prior treatment has significant negative consequences on the environment and human health. Textile dyes contaminate aquatic habitats and have the potential to be toxic to aquatic organisms, which may enter the food chain. This review will discuss the effects of textile dyes on water bodies, aquatic flora, and human health. Textile dyes degrade the esthetic quality of bodies of water by increasing biochemical and chemical oxygen demand, impairing photosynthesis, inhibiting plant growth, entering the food chain, providing recalcitrance and bioaccumulation, and potentially promoting toxicity, mutagenicity, and carcinogenicity. Therefore, dye-containing wastewater should be effectively treated using eco-friendly technologies to avoid negative effects on the environment, human health, and natural water resources. This review compares the most recent technologies which are commonly used to remove dye from textile wastewater, with a focus on the advantages and drawbacks of these various approaches. This review is expected to spark great interest among the research community who wish to combat the widespread risk of toxic organic pollutants generated by the textile industries.

Microbe mediated remediation of dyes, explosive waste and polyaromatic hydrocarbons, pesticides and pharmaceuticals

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Environmental pollutants dyes, pesticides, pharmaceuticals, explosive waste and polyaromatic hydrocarbons.

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Environmental pollutants toxicity.

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Possible microbial biodegradation pathways of environmental pollutants.

Industrialization and human activities have led to serious effects on environment. With the progress taking place in the biodegradation field, it is important to summarize the latest advancement. In this review, we intend to provide insights on the recent progress on the biodegradation of environmental contaminants such as dyes, pesticides, pharmaceuticals, explosive waste and polyaromatic hydrocarbons by microorganisms. Along with the biodegradation of environmental contaminants, toxicity effects have also been discussed.

Biological remediation technologies for dyes and heavy metals in wastewater treatment: New insight

The pollution of the environment caused by dyes and heavy metals emitted by industries has become a worldwide problem. The development of efficient, environmentally acceptable, and cost-effective methods of wastewater treatment containing dyes and heavy metals is critical. Biologically based techniques for treating effluents are fascinating since they provide several benefits over standard treatment methods. This review assesses the most recent developments in the use of biological based techniques to remove dyes and heavy metals from wastewater. The remediation of dyes and heavy metals by diverse microorganisms such as algae, bacteria, fungi and enzymes are depicted in detail. Ongoing biological method's advances, scientific prospects, problems, and the future prognosis are all highlighted. This review is useful for gaining a better integrated view of biological based wastewater treatment and for speeding future research on the function of biological methods in water purification applications.

Current perspective of innovative strategies for bioremediation of organic pollutants from wastewater

Organic contaminants in water are a growing environmental threat to sustainable development, with detrimental effects on the biosphere. In recent years, researchers have increasingly focused their attention on the area of bioremediation as an important tool to eliminate harmful pollutants from the environment. This review examines the application of bioremediation technologies to the removal of organic pollutants, with an emphasis on hydrocarbons and textile dyes. It applies a descriptive bibliometric analysis to study statistical practicality-vs-applicability of bioremediation of emerging organic pollutants. The paper identifies efficient pathways for bioremediation of different types of organic pollutants and outlines the potential for an eco-friendly and economical approach for the biological remediation of micropollutants by microalgae. Facts and figures on various hazardous pollutants, constraints in their current removal from water at an industrial level, and promising future solutions are carefully presented here.

Wood-feeding termite gut symbionts as an obscure yet promising source of novel manganese peroxidase-producing oleaginous yeasts intended for azo dye decolorization and biodiesel production

BACKGROUND

The ability of oxidative enzyme-producing micro-organisms to efficiently valorize organic pollutants is critical in this context. Yeasts are promising enzyme producers with potential applications in waste management, while lipid accumulation offers significant bioenergy production opportunities. The aim of this study was to explore manganese peroxidase-producing oleaginous yeasts inhabiting the guts of wood-feeding termites for azo dye decolorization, tolerating lignocellulose degradation inhibitors, and biodiesel production.

RESULTS

Out of 38 yeast isolates screened from wood-feeding termite gut symbionts, nine isolates exhibited high levels of extracellular manganese peroxidase (MnP) activity ranged between 23 and 27 U/mL after 5 days of incubation in an optimal substrate. Of these MnP-producing yeasts, four strains had lipid accumulation greater than 20% (oleaginous nature), with Meyerozyma caribbica SSA1654 having the highest lipid content (47.25%, w/w). In terms of tolerance to lignocellulose degradation inhibitors, the four MnP-producing oleaginous yeast strains could grow in the presence of furfural, 5-hydroxymethyl furfural, acetic acid, vanillin, and formic acid in the tested range. M. caribbica SSA1654 showed the highest tolerance to furfural (1.0 g/L), 5-hydroxymethyl furfural (2.5 g/L) and vanillin (2.0 g/L). Furthermore, M. caribbica SSA1654 could grow in the presence of 2.5 g/L acetic acid but grew moderately. Furfural and formic acid had a significant inhibitory effect on lipid accumulation by M. caribbica SSA1654, compared to the other lignocellulose degradation inhibitors tested. On the other hand, a new MnP-producing oleaginous yeast consortium designated as NYC-1 was constructed. This consortium demonstrated effective decolorization of all individual azo dyes tested within 24 h, up to a dye concentration of 250 mg/L. The NYC-1 consortium's decolorization performance against Acid Orange 7 (AO7) was investigated under the influence of several parameters, such as temperature, pH, salt concentration, and co-substrates (e.g., carbon, nitrogen, or agricultural wastes). The main physicochemical properties of biodiesel produced by AO7-degraded NYC-1 consortium were estimated and the results were compared to those obtained from international standards.

CONCLUSION

The findings of this study open up a new avenue for using peroxidase-producing oleaginous yeasts inhabiting wood-feeding termite gut symbionts, which hold great promise for the remediation of recalcitrant azo dye wastewater and lignocellulosic biomass for biofuel production.

Decolorization mechanism, identification of an FMN-dependent NADH-azoreductase from a moderately halotolerant Staphylococcus sp. MEH038S, and toxicity assessment of biotransformed metabolites

The application of halotolerant microorganisms capable of decolorizing is attractive. Decolorization mechanism, the effect of different parameters on the decolorization percentage, and toxicity analysis of Reactive Black 5 before and after decolorization were investigated in the present study. The decolorization percentage for live cells of Staphylococcus sp. strain MEH038S was more than dead cells, which demonstrated that Reactive Black 5 was decolorized through the degradation process. The results confirmed that an FMN-dependent NADH-azoreductase gene was responsible for the decolorization and then was identified as Staphylococcus sp. EFS01 azoreductase from a moderately halotolerant Staphylococcus strain for the first time. The maximal decolorization of 98.15% was observed at pH 6.5 and 35 ^° C for 50 mg/L of Reactive Black 5. In addition, more than 90% decolorization was achieved with 5-40 g/L of NaCl. The results of Gas chromatography-mass spectrometry and Fourier transform infrared spectroscopy showed that Reactive Black 5 was broken to the lower molecular weight compounds without any chromophoric azo groups. Phytotoxicity and fish toxicity proved that the biotransformed metabolites of Reactive Black 5 degradation were more toxic than the original dye. The moderate halotolerant strain exhibited a remarkable decolorization capability and can be applied for textile wastewater treatment. PRACTITIONER POINTS: An azoreductase gene from a moderately halotolerant Staphylococcus was identified. More than 90% decolorization efficiency was observed under high-salt conditions. Biotransformed metabolites of RB5 degradation were identified. Toxicity analysis of biotransformed metabolites was investigated.

Biosorption mechanisms of cationic and anionic dyes in a low-cost residue from brewer's spent grain

The brewer's spent grain (BSG) is a byproduct of the brewing industry produced in large quantities and with few ecological disposal options. The use of this low-cost residue was investigated for the removal of methylene blue (MB) and tartrazine yellow (TY) dyes. The BSG has been extensively characterized to obtain its physicochemical characteristics. Batch experiments were conducted to investigate the effects of biosorption parameters: initial pH, kinetics, equilibrium isotherms and adsorption thermodynamics. The characterization showed high carbon content and heterogeneous morphology with the presence of meso and macropores. The best experimental conditions were obtained as pH 11 for MB and pH 2 for TY. Kinetics resulted in an equilibrium time of 240 min for MB and 300 min for TY and was best represented by the pseudo-second order model. Different interaction mechanisms were suggested, such as electrostatic interactions, electron donors and electron acceptors, hydrogen bonds, π-π dispersion interactions and the dye molecules aggregation. Equilibrium data were better represented by Langmuir isotherm. The maximum adsorbed amount of MB and TY was 284.75 and 26.18 mg/g, respectively, in each better experimental condition. Through the thermodynamic analysis, it was observed that the adsorption of the dyes was spontaneous and favourable. MB is preferentially retained through chemisorption, whereas TY followed a physical process. Considering the characteristics and results found compared to the recent literature, it was verified that BSG can be used as an effective and innovative biosorbent for removal purposes of dyeing effluent.

Recent advances in the biodegradation of azo dyes

As dye demand continues to rapidly increase in the food, pharmaceutical, cosmetic, paper, textile, and leather industries, an industrialization increase is occurring. Meanwhile, the degradation and removal of azo dyes have raised broad concern regarding the hazards posed by these dyes to the ecological environment and human health. Physicochemical treatments have been applied but are hindered by high energy and economic costs, high sludge production, and chemicals handling. Comparatively, the bioremediation technique is an eco-friendly, removal-efficient, and cost-competitive method to resolve the problem. This paper provides scientific and technical information about recent advances in the biodegradation of azo dyes. It expands the biodegradation efficiency, characteristics, and mechanisms of various microorganisms containing bacteria, fungi, microalgae, and microbial consortia, which have been reported to biodegrade azo dyes. In addition, information about physicochemical factors affecting dye biodegradation has been compiled. Furthermore, this paper also sketches the recent development and characteristics of advanced bioreactors.

A comprehensive review on magnetic carbon nanotubes and carbon nanotube-based buckypaper for removal of heavy metals and dyes

Industrial effluents contain several organic and inorganic contaminants. Among others, dyes and heavy metals introduce a serious threat to drinking waterbodies. These pollutants can be noxious or carcinogenic in nature, and harmful to humans and different aquatic species. Therefore, it is of high importance to remove heavy metals and dyes to reduce their environmental toxicity. This has led to an extensive research for the development of novel materials and techniques for the removal of heavy metals and dyes. One route to the removal of these pollutants is the utilization of magnetic carbon nanotubes (CNT) as adsorbents. Magnetic carbon nanotubes hold remarkable properties such as surface-volume ratio, higher surface area, convenient separation methods, etc. The suitable characteristics of magnetic carbon nanotubes have led them to an extensive search for their utilization in water purification. Along with magnetic carbon nanotubes, the buckypaper (BP) membranes are also favorable due to their unique strength, high porosity, and adsorption capability. However, BP membranes are mostly used for salt removal from the aqueous phase and limited literature shows their applications for removal of heavy metals and dyes. This study focuses on the existence of heavy metal ions and dyes in the aquatic environment, and methods for their removal. Various fabrication approaches for the development of magnetic-CNTs and CNT-based BP membranes are also discussed. With the remarkable separation performance and ultra-high-water flux, magnetic-CNTs, and CNT-based BP membranes have a great potential to be the leading technologies for water treatment in future.

Application of magnetic fields to wastewater treatment and its mechanisms: A review

Magnetic field (MF) has been applied widely and successfully as an efficient, low-cost and easy-to-use technique to enhance wastewater treatment (WWT) performance. Although the effects of MF on WWT were revealed and summarized by some works, they are still mysterious and complex. This review summarizes the application of MF in magnetic adsorption-separation of heavy metals and dyes, treatment of domestic wastewater and photo-magnetic coupling technology. Furthermore, the mechanisms of MF-enhanced WWT are critically elaborated from the perspective of magnetic physicochemical and biological effects, such as magnetoresistance, Lorentz force, and intracellular radical pair mechanism. At last, the challenges and opportunities for MF application in WWT are discussed. For overcoming the limitations and taking advantages of MFs in WWT, fundamental research of the mechanisms of the application of MFs should be carried out in the future.

Sustainable strategy for the enhancement of hazardous aromatic amine degradation using lipopeptide biosurfactant isolated from Brevibacterium casei

The application of biosurfactants for the degradation of various toxic compounds has received much attention among researchers worldwide. A stimulated degrading method was carried out in this research to determine the efficiency of surfactant on the biodegradation of aromatic amine 4-Aminobiphenyl (4-ABP). The biosurfactant mediated process is an alternative strategy for chemical surfactants because chemical surfactants are toxic and nonbiodegradable. The bacterium was isolated through the enrichment process and identified using 16S rRNA sequencing method. The molecular characterization showed that the isolate belongs to Brevibacterium casei-4AB. Biosurfactant produced in this study was examined through screening activities like oil spreading, emulsification activity and surface tension measurement. Instrumental characterization like Fourier Transform Infrared Spectrophotometer (FT-IR) results suggested that there is a presence of NH group, aliphatic hydrocarbons, ester groups, amide and alkenes and further Gas chromatography- Mass Spectrometry (GC-MS) results confirmed the presence of fatty acids such as Hexadecanoic and Octadecadienoic acid which showed that the produced surfactant is lipopeptide. Protein content and lipid content in the biosurfactant was found to be 18 ± 0.8% and 30 ± 0.1%. The degraded metabolites of 4-ABP were analyzed through the GC-MS process which revealed the presence of metabolites such as 5-Amino-2-methoxy phenol.

Magnetically stimulated azo dye biodegradation by a newly isolated osmo-tolerant Candida tropicalis A1 and transcriptomic responses

A recently isolated osmo-tolerant yeast Candida tropicalis A1, which could decolorize various azo dyes under high-salinity conditions, was systematically characterized in the present study. Stimulating dye-decolorization effectiveness and osmo-tolerance of the yeast by static magnetic field (SMF) was investigated and transcriptomic responses of the yeast to SMF was analyzed to propose possible mechanisms. The results demonstrated that the yeast A1 effectively decolorized (≥ 97.50% within 12 h) and detoxified (from high toxicity to low toxicity within 24 h) 70 mg/L Acid Red B (ARB) under the optimized conditions through a series of steps including naphthalene-amidine bond cleavage, reductive or oxidative deamination/desulfurization, open-loop of hydroxy-substituted naphthalene or benzene and TCA cycle. Moreover, dye decolorization performance and osmo-tolerance of the yeast A1 were further improved by 24.6 mT SMF. Genes encoding high-affinity hexose/glucose transporter proteins and NADH-ubiquinone oxidoreductase were up-regulated by 24.6 mT SMF, which might be responsible for the increase of dye decolorization. Significant up-regulation of glycerol-3-phosphate dehydrogenase and cell wall protein RHD3 suggested that osmo-tolerance was enhanced by 24.6 mT SMF through promoting production and intracellular accumulation of glycerol as compatible solute, as well as regulation of cell wall component. In conclusion, 24.6 mT SMF led to the up-regulation of related genes resulting in enhanced dye biodegradation efficiency and osmo-tolerance of the yeast A1.

Blue dye degradation in an aqueous medium by a combined photocatalytic and bacterial biodegradation process

This paper aimed at implementing a treatment system for polluted water with textile dyes, starting with a photocatalytic decomposition process using sunlight as a source of energy and continuing with a bacterial biodegradation process, in order to reach degradation percentages higher than those obtained using only one of the processes mentioned above. When water treatment with the dye in the combined system was over, an acute ecotoxicity test was performed to make sure that toxic metabolites were not produced due to biodegradation. Solophenyl Blue azoic dye, and Erionyl Blue and Terasil Blue anthraquinone dye-colored solutions were treated with the Pd/Al ₈₀ Ce ₁₀ Zr ₁₀ catalyst in a solar collector for the photocatalytic process. On the other hand, the waste dye, which was obtained from photocatalysis with a bacterial consortium from polluted areas by metals and hydrocarbons in aerobic conditions, was inoculated for biodegradation. Biodegradation was obtained for the dyes after both processes as 90.91% for the Solophenyl Blue azoic dye, and 87.80% and 87.94%, respectively, for the Erionyl Blue and Terasil Blue anthraquinone dyes. After the degradation processes, it was proven, via an ecotoxicity test with Daphnia magna , that toxic metabolites had not been produced.

Dye-decolorization of a newly isolated strain Bacillus amyloliquefaciens W36

Dye-decolorization is one of the most important steps in dye-polluted wastewater treatment. The dye-decolorization bacteria were isolated from active sludge collected from wastewater treating pond of a dyeing and printing plant using serial dilution method. Among the 44 bacteria isolates from the active sludge, the strain Bacillus amyloliquefaciens W36 was found to have strong ability in dye-decolorization. The effects of carbon source, nitrogen sources, C/N, metal ions, temperature, pH, and rotation speed for dye-decolorization were investigated. The optimum decolorization conditions were that the strain was grown in enriched mineral salt medium (EMSM) using maltose 1 g/L, (NH₄)₂SO₄ 1 g/L as carbon and nitrogen source respectively, supplemented with 100 mg/L different dyes (pH 6.0), at 30 °C, 200 rpm from 48 to 96 h. The bacteria could aerobically decolorize dyes, such as Coomassie brilliant blue (95.42%), Bromcresol purple (93.34%), Congo red (72.37%) and Sarranine (61.7%), within 96 h. The dyes decolorization products were analyzed by ultra-violet and visible (UV-vis) spectroscopy before and after decolorization, which indicated that the four dyes were significantly degraded by the strain. The results indicated that the bacteria Bacillus amyloliquefaciens W36 could be used in dye-polluted wastewater treatment.

A comprehensive review on the integration of advanced oxidation processes with biodegradation for the treatment of textile wastewater containing azo dyes

The threat of dye contamination has achieved an unsurpassed abnormal state lately due to their massive consumption in several enterprises including textile, leather, cosmetic, plastic, and paper industries. This review focuses on the integrations of various advanced oxidation processes (AOPs), such as Fenton, photocatalysis, and ozonation, with biodegradation for the treatment of textile azo dyes. Such integrations have been explored lately by researchers to bring down the processing cost and improve the degree of mineralization of the treated dyeing wastewater. The review refers to the basic mechanisms, the influence of various process parameters, outcomes of recent works, and future research directions. All the three AOPs, independently, demonstrated substantial color reduction of 54–100%. The ozonation process, stand-alone, showed the most efficient decolorization (of 88–100%) consistently in all reviewed research works. In contrast, all three AOPs independently offered varied and inadequate COD reduction in the range of 16–80%. The AOPs, after getting integrated with biodegradation, yielded an additional reduction (of 11–70%) in the COD-levels and (of 16–80%) in the TOC-levels. Further, the integration of AOPs with biodegradation has potential to significantly reduce the treatment costs. The review suggests further research efforts in the direction of sequencing chemical and biological routes such that their synergistic utilization yield complete detoxification of the textile azo dyes economically at large-scale.

Construction of a new lipase- and xylanase-producing oleaginous yeast consortium capable of reactive azo dye degradation and detoxification

A new oleaginous yeast consortium Y-BC-SH which stands for molecularly identified species Yarrowia sp., Barnettozyma californica and Sterigmatomyces halophilus was successfully constructed in this study. This multipurpose oleaginous yeast consortium was developed based on its higher ability to accumulate large amounts of lipids in the form of triacylglycerol, grow on xylose, produce lipase and xylanase and it could rapidly decolorize and degrade commonly-used textile reactive azo dyes. The specific enzyme activities of lipase, xylanase, xylan esterase, β-xylosidase, CMCase, β-glucosidase and cellobiohydrolase produced by Y-BC-SH were significantly higher than that of individual strains. As chemical oxygen demand reduction had occurred in the dye mixture solutions, it was evidence of their color removal and mineralization by Y-BC-SH. The significant induction of oxidoreductive enzymes by Y-BC-SH was probably due to the coordinated metabolic interactions of the individual strains. Phytotoxicity assay confirmed that metabolites generated after dye degradation by Y-BC-SH are non-toxic.

Immobilization interaction between xenobiotic and Bjerkandera adusta for the biodegradation of atrazine

The aim of the present work is to evaluate the ability of 'fungi' for the biodegradation of recalcitrant xenobiotic compound, 'Atrazine' in batch liquid cultures. Different parameters like pH (2.0-8.0) temperature (16-32 °C), biomass (1-5 g), and concentration (25-100 ppm) were optimized for the efficient degradation of atrazine. The decomposition behavior of atrazine is analyzed with the help of Fourier Transform Infrared (FTIR) spectroscopy. Herein, we have reported that the Bjerkandera adusta possess high removal efficiency of the xenobiotic compound (atrazine) up to 92%. The fungal strain investigated could prove to be a valuable active pesticide degrading micro-organism, with high detoxification values. These results are useful for improved understanding and prediction of the behavior and fate of B. adusta in the bio-purification of wastewater contaminated with xenobiotics. Thus providing a new and green approach for the remediation of toxicants without altering the environmental components.

Degradation and Toxicity Analysis of a Reactive Textile Diazo Dye-Direct Red 81 by Newly Isolated Bacillus sp. DMS2

An efficient diazo dye degrading bacterial strain, Bacillus sp. DMS2 was isolated from a long-term textile dye polluted environment. The strain was assessed for its innate ability to completely degrade and detoxify Direct Red 81 (DR81) textile dye under microaerophilic conditions. The degradation ability of strain showed significant results on optimizing the nutritional and environmental parameters. Based on statistical models, maximum efficiency of decolorization achieved within 24 h for 100 mg/l of dye supplemented with glucose (0.02%), MgSO₄ (0.002%) and urea (0.5%) at 30°C and pH (7.0). Moreover, a significant catabolic induction of a laccase and azoreductase suggested its vital role in degrading DR81 into three distinct metabolites (intermediates) as by-products. Further, toxicity analysis of intermediates were performed using seeds of common edible plants, aquatic plant (phytotoxicity) and the nematode model (animal toxicity), which confirmed the non-toxic nature of intermediates. Thus, the inclusive study of DMS2 showed promising efficiency in bioremediation approach for treating industrial effluents.

Ecofriendly biodegradation of Reactive Black 5 by newly isolated Sterigmatomyces halophilus SSA1575, valued for textile azo dye wastewater processing and detoxification

A total of seven yeast strains from 18 xylanolytic and/or xylose-fermenting yeast species isolated from the wood-feeding termite Reticulitermes chinenesis could efficiently decolorize various azo dyes under high-salt conditions. Of these strains, a novel and unique azo-degrading and halotolerant yeast, Sterigmatomyces halophilus SSA1575, has been investigated in this study. This strain could significantly decolorize four combinations of a mixture of dyes. It showed a high capability for decolorizing Reactive Black 5 (RB5) even at 1,500 mg L⁻¹. The strain SSA1575 still showed a high capability for decolorizing a 50 mg L⁻¹ RB5 with a salt mixing at a NaCl concentration of up to 80 g L⁻¹. It also exhibited significant ability to decolorize repeated additions of dye aliquots, with a reduction in time of up to 18 h. Most of the tested carbon and nitrogen sources could significantly enhance a RB5 decolorization. However, this process was inhibited by the addition of sucrose and sodium nitrate. NADH-dichlorophenol indophenol (NADH-DCIP) reductase and lignin peroxidase were determined as the key reductase and oxidase of S. halophilus SSA1575. Finally, strain SSA1575, can effectively detoxify RB5 into non-toxic products. Overall, S. halophilus SSA1575, might be a promising halotolerant yeast valued for the treatment of various textile effluents with high salinity.

Integrated metagenomic and metaproteomic analyses reveal potential degradation mechanism of azo dye-Direct Black G by thermophilic microflora

Direct Black G (DBG) is a typical toxic azo dye with extensive applications but it poses a serious threat to the aquatic ecosystem and humans. It is necessary to efficiently and safely remove DBG from environments by the application of various treatment technologies. A thermophilic microflora previously isolated from the soil can effectively metabolize DBG. However, the molecular basis of DBG degradation by this thermophilic microflora remains unknown. In this study, metagenomic sequencing technology and qRT-PCR have been used to elucidate the functional potential of genes and their modes of action on DBG. A quantitative metaproteomic method was further utilized to identify the relative functional proteins involved. Subsequently, the possible co-metabolic molecular mechanisms of DBG degradation by candidate genes and functional proteins of the thermophilic microflora were illustrated. The combination of metagenomics and metaproteomics to investigate the degradation of DBG by a microflora was reported for the first time in recent literature; this can further provide a deep insight into the molecular degradation mechanism of dye pollutants by natural microflora.

Performance of a Newly Isolated Salt-Tolerant Yeast Strain Sterigmatomyces halophilus SSA-1575 for Azo Dye Decolorization and Detoxification

The effective degradation of hazardous contaminants remains an intractable challenge in wastewater processing, especially for the high concentration of salty azo dye wastewater. However, some unique yeast symbionts identified from the termite gut system present an impressive function to deconstruct some aromatic compounds, which imply that they may be valued to work on the dye degradation for various textile effluents. In this investigation, a newly isolated and unique yeast strain, Sterigmatomyces halophilus SSA-1575, was identified from the gut system of a wood-feeding termite (WFT), Reticulitermes chinensis. Under the optimized ambient conditions, the yeast strain SSA-1575 showed a complete decolorization efficiency on Reactive Black 5 (RB5) within 24 h, where this azo dye solution had a concentration of a 50 mg/L RB5. NADH-dichlorophenol indophenol (NADH-DCIP) reductase and lignin peroxidase (LiP) were determined as the key reductase and oxidase of S. halophilus SSA-1575. Enhanced decolorization was recorded when the medium was supplemented with carbon and energy sources, including glucose, ammonium sulfate, and yeast extract. To understand a possible degradation pathway well, UV-Vis spectroscopy, FTIR and Mass Spectrometry analyses were employed to analyze the possible decolorization pathway by SSA-1575. Determination of relatively high NADH-DCIP reductase suggested that the asymmetric cleavage of RB5 azo bond was mainly catalyzed by NADH-DCIP reductase, and finally resulting in the formation of colorless aromatic amines devoid of any chromophores. The ecotoxicology assessment of RB5 after a decolorization processing by SSA-1575, was finally conducted to evaluate the safety of its metabolic intermediates from RB5. The results of Microtox assay indicate a capability of S. halophilus SSA-1575, in the detoxification of the toxic RB5 pollutant. This study revealed the effectiveness of halotolerant yeasts in the eco-friendly remediation of hazardous pollutants and dye wastewater processing for the textile industry.

Honeycombed Au@C-TiO2-Xcatalysts for enhanced photocatalytic mineralization of Acid red 3R under visible light

The mineralization of organic pollutants under visible light is challenging, limiting the practical application of photocatalytic technology in wastewater treatment. To achieve the efficient mineralization of Acid red 3R (AR3R), a series of honeycombed catalysts (TiO₂, C-TiO_2-X, Au@TiO₂ and Au@C-TiO_2-X) were prepared via a facile in situ synthetic method and characterized by XRD, TEM, BET, XPS and DRS, respectively. The introduction of C and Au species promote the simultaneous generation of •O₂^- and •OH over Au@C-TiO_2-X under visible light radiation. The Au@C-TiO_2-X catalyst showed superior performance for the deep mineralization of AR3R, affording a TOC removal rate larger than 90 % within 240 min under visible light (> 420 nm). The photocatalytic degradation mechanism of AR3R is proposed according to UV-vis and in situ DRIFTS analysis. The superior photocatalytic activity of Au@C-TiO_2-X is attributed to the synergistic effect of •O₂^- and •OH owing to C doping and Au deposition.

Improving Azo Dye Decolorization Performance and Halotolerance of Pichia occidentalis A2 by Static Magnetic Field and Possible Mechanisms Through Comparative Transcriptome Analysis

A halotolerant yeast, Pichia occidentalis A2, was recently isolated that can decolorize various azo dyes. The azo dye decolorization performance of this strain was characterized, including the degradation pathway and detoxification effects of this yeast. Additionally, the effect of static magnetic field (SMF) on this decolorization process was investigated. Activities of key enzymes were analyzed to estimate the change of metabolic activity. Furthermore, possible mechanisms were analyzed through detecting differentially expressed genes between yeast A2 in the absence and presence of SMF. The results indicated that yeast A2 displayed the optimal decolorization performance when the concentrations (in g/L) of glucose, (NH4)2SO4, yeast extract, and NaCl were 4.0, 1.0, 0.1, and ≤30.0, respectively. Meanwhile, the optimal rotation speed, temperature, and pH were 160 rpm, 30°C, and 5.0, respectively. Acid Red B was decolorized and detoxified by yeast A2 through successive steps, including cleavage of the naphthalene-amidine bond, reductive deamination, oxidative deamination/desulfurization, open-loop of hydroxy-substituted naphthalene, and tricarboxylic acid cycle. The dye decolorization efficiency and halotolerance of yeast A2 were enhanced by 206.3 mT SMF. The activities of manganese peroxidase, and laccase were elevated 1.37- and 1.16-fold by 206.3 mT SMF, but lignin peroxidase activity showed little change. It was suggested from the transcriptome sequence that the enhanced halotolerance might be related to the upregulated genes encoding the enzymes or functional proteins related to intracellular synthesis and accumulation of glycerol.

Azo dye decolorization by a halotolerant consortium under microaerophilic conditions

As a result of the use of a large amount of salt in dye industries, azo dye decolorization is often needed under hypersaline environments and low dissolved oxygen. Consortium GG-1, which is able to decolorize azo dyes in high salt concentrations and microaerophilic conditions, can be enriched using Metanil Yellow. Consortium GG-1 is mainly composed of Zobellella (62.25%), Rheinheimera (12.4%) and Marinobacterium (9.44%) and is able to decolorize azo dyes under 1%-10% salinity. The activities of azoreductase, laccase and lignin peroxidase were also measured. Together with the detected intermediates and the results obtained from FTIR, the decolorization process of Metanil Yellow was proposed. The influences of pH, initial concentration of azo dyes and concentration of yeast extract on the decolorization rate were also detected. Meanwhile, consortium GG-1 was identified with wide substrate specificity to dyes such as Direct Blue B, Acid Black ATT, and Acid Violet 7. Therefore, consortium GG-1 was identified with potential use in azo dye elimination.

Intermittent aeration strategy for azo dye biodegradation: A suitable alternative to conventional biological treatments?

Most solutions for biological treatment of azo dyes are based on conventional anaerobic-aerobic processes, but transition to full scale demands technology simplification and cost reductions. We suggest a new approach, in which aeration is intermittently supplied for simultaneous removal of color and toxic metabolites in a single compartment. Effects of aeration strategy and glucose concentration on decolorization and organic matter removal were assessed using factorial design (32) and response surface analysis. Bioreactors were inoculated with microorganisms previously acclimated to Direct Black 22 (DB22), which was the azo compound used in this study. Assays performed with synthetic textile wastewater showed that long-term decolorization was not impaired at a moderate level of aeration (4 hourly-cycles per day). Aerated batches presented lower color removal velocities, but these negative impacts were offset by increasing initial glucose concentration. Higher degrees of mineralization of the azo compound and higher organic matter removals were achieved in intermittently aerated experiments, which led to lower toxicity to Daphnia magna. Biomolecular analysis revealed that the microbial community structure was strongly associated with operational efficiency parameters. These findings suggest intermittent aeration can be implemented to accomplish enhanced azo dye biodegradation.

Silver nanoparticles in dye effluent treatment: A review on synthesis, treatment methods, mechanisms, photocatalytic degradation, toxic effects and mitigation of toxicity

The current scenario of water resources shows the dominance of pollution caused by the draining of industrial effluents. The polluted waters have resulted in severe health and environmental hazards urging for a suitable alternative to resolve the implications. Various physical and chemical treatment steps currently in use for dye effluent treatment are more time consuming, cost-intensive, and less effective. Alternatively, nanoparticles due to their excellent surface properties and chemical reactivity have emerged as a better solution for dye removal and degradation. In this regard, the potential of silver nanoparticles in dye effluent treatment was greatly explored. Efforts were taken to unravel the kinetics and statistical optimization of the treatment conditions for the efficient removal of dyes. In addition, the role of silver nanocomposites has also experimented with colossal success. On the contrary, studies have also recognized the mechanisms of silver nanoparticle-mediated toxicity even at deficient concentrations and their deleterious biological effects when present in treated water. Hence, the fate of the silver nanoparticles released into the treated water and sludge, contaminating the soil, aquatic environment, and underground water is of significant concern. This review summarizes the current state of knowledge regarding the use of silver nanoparticles and silver-based nanocomposites in effluent treatment and comprehends the recent research on mitigation of silver nanoparticle-induced toxicity.

Electrochemical oxidation of Acid Orange 7 azo dye using a PbO2 electrode: Parameter optimization, reaction mechanism and toxicity evaluation

In this study, electrochemical oxidation of Acid Orange 7 (AO 7) azo dye has been investigated using a Fe-doped PbO₂ electrode. The degradation of AO 7 followed pseudo-first-order reaction kinetics. The removals of AO 7, chemical oxygen demand (COD) and total organic carbon (TOC) were 87.15%, 49.88% and 44.94% after 60 min of electrolysis at the optimal conditions (Na₂SO₄ concentration 0.1 M, initial pH 5, initial AO 7 concentration 100 mg L^-1 and applied current density 20 mA cm^-2), respectively. And the corresponding degradation rate constant was 0.035 min^-1. The intermediates formed during electrochemical process were identified, and a possible degradation pathway was proposed, which was initiated by the oxidation of azo bond (-NN-), hydroxylation and substitution reaction of -NH₂ and -SO₃H under the attack of OH, and ended with the formation of mineralization products such as NH₄⁺, NO₃^-, SO₄^2-, CO₂ and H₂O. The toxicity of treated AO 7 solution towards Vibrio fischeri increased slightly at first and then rapidly reduced to non-toxicity with prolonging time. The results indicate that electrochemical oxidation of AO 7 using Fe-doped PbO₂ electrode is a promising way.

Biodegradation of Reactive Orange 16 azo dye by simultaneous action of Pleurotus ostreatus and the yeast Candida zeylanoides

The purpose was to investigate a simultaneous biodegradation of the recalcitrant monoazo dye Reactive Orange 16 (RO16) in a mixed culture consisting of a biofilm of Pleurotus ostreatus-colonizing polyamide carrier and a suspension of the yeast Candida zeylanoides to see their biological interactions and possible synergistic action during degradation. Decolorization in the mixed culture was more effective than in the fungal monoculture, the respective decolorizations reaching 87.5% and 70% on day 11. The proliferation of yeast was reduced compared with the C. zeylanoides monoculture but enabled the yeast to participate in decolorization. The interaction of P. ostreatus with the yeast resulted in a gradual decrease of fungal manganese-dependent peroxidase (MnP) and laccase activities. Gas chromatography-mass spectrometry (GC-MS) analysis of the degradation products brought evidence that P. ostreatus split the dye molecule asymmetrically to provide 4-(ethenylsulfonyl) benzene whose concentration was much decreased in the mixed culture suggesting its increased metabolization in the presence of the yeast. In contrast, C. zeylanoides split the azo bond symmetrically producing the metabolites 4-(ethenylsulfonyl) aniline and α-hydroxybenzenepropanoic acid. Those metabolites were rapidly degraded in the mixed culture. A novel aspect is represented by the evidence of a mutual cooperative action of the fungal and yeast microorganisms in the mixed culture resulting in rapid decolorization and degradation of the dye.

Co-metabolic degradation of refractory dye: A metagenomic and metaproteomic study

Fructose was utilized as an additional co-substrate to systematically investigate the molecular mechanism of its boosting effect for the degradation of refractory dye reactive black 5 (RB5) by a natural bacterial flora DDMZ1. A decolorizing rate of 98% was measured for sample YE + FRU(200) (with 3 g/L fructose additionally to yeast extract medium, 10% (v/v) inoculation size of flora DDMZ1, 200 mg/L RB5) after 48 h. This result was 21% and 77%, respectively, higher than those of samples with only yeast extract or only fructose. Fructose was found to significantly stimulated both intracellular and extracellular azoreductase secretion causing enhanced activity. Metagenomic sequencing technology was used to analyze the functional potential of genes. A label-free quantitative proteomic approach further confirmed the encoding of functional proteins by the candidate genes. Subsequently, the molecular mechanism of RB5 degradation by candidate genes and functional proteins of the dominant species were proposed. This study provides important perspectives to the molecular mechanism of co-metabolic degradation of refractory pollutants by a natural bacterial flora.

Enhanced azo dye biodegradation performance and halotolerance of Candida tropicalis SYF-1 by static magnetic field (SMF)

Enhancing Acid Red B (ARB) decolorization by growing cells of a halotolerant yeast Candida tropicalis SYF-1 with static magnetic field (SMF) was investigated. Activity of key enzymes and membrane phospholipid fatty acids (PLFAs) were analyzed for estimating the change of metabolic activity and membrane salt-stress response, respectively. Possible enhancement mechanisms were revealed through comparative transcriptome analysis. The results showed that 95.0 mT SMF enhanced ARB decolorization by growing cells of a yeast SYF-1, as well as cell growth and halotolerance capability. Activity of intracellular lignin peroxidase (LiP) and laccase (Lac) was 1.51- and 1.47-fold higher with 95.0 mT SMF than that without SMF, respectively. Unsaturation degree and chain length of dominant PLFAs was increased by 95.0 mT SMF treatment. Several functional protein encoding unigenes related to organics biodegradation, cell growth and halotolerance were 1.17- to 4.19-fold up-regulated in response to 95.0 mT SMF.

Decolourization and biodegradation of azo dye methyl red by Rhodococcus strain UCC 0016

In the present study, locally isolated Rhodococcus strains were attempted as biological tools for methyl red removal, a mutagenic azo dye posing threat to the environment if left untreated. Rhodococcus strain UCC 0016 demonstrated superior methyl red-decolourizing activity of 100% after 24 h at static condition in comparison to Rhodococcus strain UCC 0008 which recorded 65% decolourization after 72 h. Optimization of physicochemical parameters at 30°C, pH 7 and supplementing glucose as the carbon source resulted in improved methyl red-decolourizing activity at static condition and reduced the time taken to achieve complete decolourization by 80%. Higher concentration of methyl red (5 g/L) was able to be decolourized completely within 10 h by adopting the technology of immobilization. The encapsulated cells of Rhodococcus strain UCC 0016 demonstrated higher substrate affinity (K_m = 0.6995 g/L) and an accelerated rate of disappearance of methyl red (V_max = 0.3203 g/L/h) compared to the free cells. Furthermore, the gellan gum beads could be reused up to nine batches without substantial loss in the catalytic activity indicating the economic importance of this protocol. Analysis of methyl red degradation products revealed no germination inhibition on Triticum aestivum and Vigna radiata demonstrating complete toxicity removal of the parent dye after biological treatment. The occurrence of new and altered peaks (UV-Vis and FTIR) further supported the notion that the removal of methyl red by Rhodococcus strain UCC 0016 was indeed through biodegradation. Therefore, this strain has a huge potential as a candidate for efficient bioremediation of wastewater containing methyl red.

Azo dyes decolorization under high alkalinity and salinity conditions by Halomonas sp. in batch and packed bed reactor

Biodecolorization and biodegradation of azo dyes are a challenge due to their recalcitrance and the characteristics of textile effluents. This study presents the use of Halomonas sp. in the decolorization of azo dyes Reactive Black 5 (RB5), Remazol Brilliant Violet 5R (RV5), and Reactive Orange 16 (RO16) under high alkalinity and salinity conditions. Firstly, the effect of air supply, pH, salinity and dye concentration was evaluated. Halomonas sp. was able to remove above 84% of all dyes in a wide range of pH (6-11) and salt concentrations (2-10%). The decolorization efficiency of RB5, RV5, and RO16 was found to be ≥ 90% after 24, 13 and 3 h, respectively, at 50 mg L^-1 of dyes. The process was monitored by HPLC-DAD, finding a reduction of dyes along the time. Further, Halomonas sp. was immobilized in volcanic rocks and used in a packed bed reactor for 72 days, achieving a removal rate of 3.48, 5.73, and 8.52 mg L^-1 h^-1, for RB5, RV5 and RO16, respectively, at 11.8 h. The study has confirmed the potential of Halomonas sp. to decolorize azo dyes under high salinity and alkalinity conditions and opened a scope for future research in the treatment of textile effluents.

Decolorization of mordant yellow 1 using Aspergillus sp. TS-A CGMCC 12964 by biosorption and biodegradation

In this report, the decolorization features of extracellular enzymes and mycelia separately prepared from Aspergillus sp. TS-A CGMCC 12,964 (120 h) were investigated. The fermentation broth of TS-A degraded 98.6% of Mordant Yellow 1 (50 mg/L) at an initial pH 6 within 1 h with over 70% of the dye (50 mg/L) degraded by extracellular enzymes and 18.8% removed by live mycelia. The degradation products of the dye were analyzed by UV-Vis and FTIR spectra. The decolorization rates of extracellular enzymes and mycelia were examined under different contact periods, dye concentrations and pH values. The extracellular enzymes exhibited excellent degradation activity under weak acidic conditions. In addition, biosorption models of mycelia fitted well the Langmuir isotherm model and the pseudo-second-order kinetic equation. Although the decolorization process was achieved through the synergistic effects of mycelia and extracellular enzymes, decolorization was dominated by the biodegradation activity of the extracellular enzymes from TS-A.