A review on biocatalytic decomposition of azo dyes and electrons recovery

Enhanced photocatalytic degradation of norfloxacin and amido black 10B using bael gum-infused ZnO/Ag₂CO₃ nanocomposites for sustainable approach to environmental remediation and antimicrobial activity

The presence of antibiotics and dyes as organic contaminants has caused severe wastewater pollution, posing a significant environmental concern globally. To address this issue, this study highlights the green synthesis of ZnO/Ag₂CO₃ nanocomposites using bael gum (BG) for the photocatalytic degradation of norfloxacin (NOF) and Amido Black 10B (AB 10B), along with their antimicrobial activity. The synthesized BG-ZnO/Ag₂CO₃ nanocomposites were extensively characterized for their optical, structural, morphological, chemical purity, crystal structure, and surface area properties using XPS, XRD, FT-IR, FESEM, EDS, photoluminescence (PL) spectra, Raman analysis, HR-TEM, BET, and LC-MS. The nanocomposites materials exhibited excellent photocatalytic performance under visible light irradiation, achieving the degradation efficiencies of 91.03 % for AB 10B and 81.5 % for NOF,with corresponding rate constants of 0.017 min-1 and 0.0028 min-1. The study further evaluated the influence of catalyst dosage, effect of pH, radical scavengers, reusability, and the stability. Kinetic analysis revealed that the bio-nanocomposite follows first-order kinetics for both pollutants. Scavenger studies identified hydroxyl radicals (•OH) as the primary active species in the degradation process. LC-MS analysis confirmed the formation of degradation intermediates for NOF and AB 10B. Additionally, the bio-nanocomposite demonstrated significant antibacterial activity against various bacterial strains using the agar well diffusion method, exhibiting strong bactericidal effects against pathogenic bacteria. This study emphasizes the potential of bael gum-based bio-nanocomposites for improved photocatalytic degradation and antibacterial activity in wastewater treatment across various industries.

Microfibre pollution: An emerging contaminant, alarming threat to the global environment

Microfibres, mostly obtained from home laundry, textiles, industrial materials, sewage effluents, and sludge, are considered the main source of environmental pollution, which has become a prevalent threat to terrestrial and aquatic creatures. Global population growth and industrialization have led to a rise in fibre consumption and production, which spread its network in drinking water, beer, and seafood. Focusing on the alarming threat of microfibre towards the natural environment, we have penned an extensive review article about microfibre pollution. The manuscript is divided into various subparts, such as the introductory portion, which briefly summarizes the sources and presence of various hazardous pollutants in the environment, followed by a detailed discussion about microfibre. The second part elaborates on the sources and distribution of microfibreous pollutants and the third portion discloses the toxic chemicals utilized or produced from functionalization, as well as the negative implications of microfibre on the environment. The fourth part discloses the leading application strategies to diminish microfibre pollution by controlling the sources and the development of various remediations. The last portion deals with the future and critical aspects of microfibre contamination. The authors hope this review article will boost its domain in environmental chemistry, sustainable development and environmental engineering from academic to industrial level as it helps researchers in particular and academics in general.

Insights into the azo dye decolourisation and denitrogenation in micro-electrolysis enhanced counter-diffusion biofilm system

In this study, electron transport pathways were activated and diversified by coupling counter-diffusion biofilms with micro-electrolysis for Alizarin yellow R (AYR) denitrogenation. Due to the binding of AYR to two residues of EC 4.1.3.36 with higher binding energy, the expression of EC 4.1.3.36 was down-regulated, causing the EC 3.1.2.28 and EC 2.5.1.74 for menaquinone synthesis (redox mediator) undetectable in Membrane aerated biofilm reactors (MABR). Spontaneous electron generation in the micro electrolysis-coupled MABR (ME-MABR) significantly activated two enzymes. Activated menaquinone up-regulated decolourisation related genes expression in ME-MABR, including azoR (2.12 log2), NQO1 (2.97 log2), wrbA (0.45 log2), and ndh (0.47 log2). The diversified electron flow pathways also promoted the nitrogen metabolism coding genes up-regulation, accelerating further inorganic nitrogen denitrogenation after AYR mineralisation. Compared to MABR, the decolourisation, mineralisation, and denitrogenation in ME-MABR increased by 25.80 %, 16.53 %, and 13.32 %, respectively. This study provides new insights into micro-electrolysis enhanced removal of AYR.

Strong, tough, conductive and transparent nanocellulose hydrogel based on Ca2+-induced cross-linked double-networks and its adsorption of methylene blue dye

A novel multi-performance SHNC/SA/CaCl2 hydrogel with multi-performance was prepared via ultra-low-temperature freeze-thaw cycling and Ca2+ cross-linking for the removal of methylene blue (MB) from industrial wastewater. Various methods were used to characterize the structure and properties of hydrogel, and the internal structure of hydrogel showed a three-dimensional network with hydrogen and ester bonds. The SHNC/SA/CaCl2-15 hydrogel exhibited the highest tensile properties (elongation = 800 %), viscoelasticity (90 kPa), compressive strength (0.45 MPa), tensile strength (0.47 MPa) and ionic conductivity (4.34 S/cm). The maximum adsorption capacity of 2 g SHNC/SA/CaCl2-15 hydrogel was 608.49 mg/g at 40 °C, pH = 8 and adsorption 24 h. The adsorption process of hydrogel toward MB was more consistent with the second-order kinetic model and Langmuir isothermal adsorption model. According to the Langmuir isotherm model, the maximum monolayer adsorption capacity of SHNC/SA/CaCl2-15 hydrogel toward MB can reach 613.88 mg/g. Finally, it was found that the removal rate of SHNC/SA/CaCl2-15 hydrogel for MB was still as high as 90 % after five cycles of the adsorption-desorption test, and it could be reused. The hydrogel can be used as cheap and reusable adsorption material for cationic dyes. Our study provides a new perspective for the development of multifunctional cellulose hydrogel adsorbent materials.

Advancements in application of modified biochar as a green and low-cost adsorbent for wastewater remediation from organic dyes

Synthetic organic dyes, which are resistant to biodegradation, pose a notable health risk, potentially leading to cancer and respiratory infections. Researchers have addressed this concern by exploring physicochemical methods to remove organic dyes from wastewater. A particularly promising solution involves modified biochar adsorbents, which demonstrate high efficiency in organic dye removal. Biochar, a charcoal-like material derived from biomass pyrolysis, offers advantages such as low cost, eco-friendliness, high efficiency and reusability. Beyond its role in sustainable soil remediation, biochar proves effective in removing organic dyes from wastewater after undergoing physical or chemical modification. Acid-base activation or metal-heteroatom impregnation enhances biochar's adsorption capacity. This comprehensive review examines the attributes of biochar, common methods for production and modification, and the impacts of raw materials, pyrolysis temperature, heating rate and residence time. It further elucidates the biochar adsorption mechanism in the removal of organic dyes, assessing factors influencing efficiency, including biochar feedstock, solution pH, adsorption temperature, particle size, initial dye concentration, biochar dosage and reaction time. It explores challenges, opportunities, reusability and regeneration methods of biochar in treating organic dye wastewater. It also discusses recent advances in organic dye removal using adsorption-based biochar. The review ultimately advocates for enhancing biochar's adsorption performance through post-modification.

Electrogenic engineered flow through tri-phasic wetland system for azo dye treatment: Microbial dynamics and functional metagenomics

Electrogenic engineered flow through tri-phasic wetland (EEFW) system based on nature-based ecological principles was studied by integrating successive biological microenvironments. The potential mechanism of the plant root-based microbial community and its functional diversity with the influence of plant-microbe-electrode synergism towards dye degradation was evaluated. The EEFW system was operated at three varied dye loads of 10, 25 and 50 mg L-1, where the results from the cumulative outlets revealed a maximum dye removal efficiency of 96%, 96.5% and 93%, respectively. Microbial community analysis depicted synergistic dependence on the plant-microbe-electrode interactions, influencing their functional diversity and metabolism towards detoxification of pollutants. The core microbial taxa enriched against the microenvironment variation were mostly associated with carbon and dye removal viz., Desulfomonile tiedjei and Rhodopseudomonas palustris in Tank 1 and Chloroflexi bacterium and Steroidobacter denitrificans in Tank 2. The degradation of polycyclic aromatic hydrocarbons, chloroalkane/chloroalkene, nitrotoluene, bisphenol, caprolactam and 1,1,1-trichloro-2,2-bis(4-chlorophenyl) ethane (DDT) were observed to be predominant in Tank 1. EEFW system could be one of the option for utilizing nature-based processes for the treatment of wastewater by self-induced bioelectrogenesis to augment process efficiency.

Development of carboxymethyl cellulose-graphene oxide biobased composite for the removal of methylene blue cationic dye model contaminate from wastewater

Utilizing Glutaraldehyde crosslinked sodium carboxymethyl cellulose (CMC-GA) hydrogel and its nanographene oxide composite (CMC-GA-GOx), an effective carboxymethyl cellulose-graphene oxide biobased composites adsorbent was developed for the adsorption removal of methylene blue (MB) cationic dye contaminate from industrial wastewater. The CMC-GA-GOx composites developed were characterized using FTIR, RAMAN, TGA, SEM, and EDX analysis instruments. Through batch experiments, several variables affecting the removal of MB dye, including the biocomposites GO:CMC composition, adsorption time, pH and temperature, initial MB concentration, adsorbent dosage, and NaCl concentration, were investigated under different conditions. The maximum dye removal percentages ranged between 93 and 98%. They were obtained using biocomposites CMC-GA-GO102 with 20% GO weight percent, adsorption time 25 min, adsorption temperature 25 °C, MB concentrations 10-30 ppm, adsorption pH 7.0, and 0.2 g adsorbent dose. The experimental data of the adsorption process suit the Langmuir isotherm more closely with a maximal monolayer adsorption capacity of 76.92 mg/g. The adsorption process followed the kinetic model of pseudo-second order. The removal of MB was exothermic and spontaneous from a thermodynamic standpoint. In addition, thermodynamic results demonstrated that adsorption operates most effectively at low temperatures. Finally, the reusability of the developed CMC-GA-GO102 has been proved through 10 successive cycles where only 14% of the MB dye removal percentage was lost. These results suggest that the developed CMC-GA-GO102 composite may be an inexpensive and reusable adsorbent for removing organic cationic dyes from industrial wastewater.

In situ treatment of real textile effluent in constructed furrows using consortium of Canna indica and Saccharomyces cerevisiae and subsequent biochemical and toxicity evaluation

Emerging contaminants removals like dyes and heavy metals from the textile effluent have an immense challenge. The present study focuses on the biotransformation and detoxification of dyes and in situ textile effluent treatment by plants and microbes efficiently. A mixed consortium of perennial herbaceous plant Canna indica and fungi Saccharomyces cerevisiae showed decolorization of di-azo dye Congo red (CR, 100 mg/L) up to 97% within 72 h. Root tissues and Saccharomyces cerevisiae cells revealed induction of various dye-degrading oxidoreductase enzymes such as lignin peroxidase, laccase, veratryl alcohol oxidase and azo reductase during CR decolorization. Chlorophyll a, Chlorophyll b and carotenoid pigments were notably elevated in the leaves of a plant during the treatment. Phytotransformation of CR into its metabolic constituents was detected by using several analytical techniques, including FTIR, HPLC, and GC-MS and its non-toxic nature was confirmed by cyto-toxicological evaluation on Allium cepa and on freshwater bivalves. Mix consortium of plant Canna indica and fungi Saccharomyces cerevisiae efficiently treated textile wastewater (500 L) and reduced ADMI, COD, BOD, TSS and TDS (74, 68, 68, 78, and 66%) within 96 h. In situ textile wastewater treatment for in furrows constructed and planted with Canna indica, Saccharomyces cerevisiae and consortium-CS within 4 days reveals reduced ADMI, COD, BOD, TDS and TSS (74, 73, 75, 78, and 77%). Comprehensive observations recommend this is an intelligent tactic to exploit this consortium in the furrows for textile wastewater treatment.

Microfiber Emissions from Functionalized Textiles: Potential Threat for Human Health and Environmental Risks

The growing worldwide population is directly responsible for the increased production and consumption of textile products. One of the key reasons for the generation of microfibers is the use of textiles and garment materials, which is expected to increase. The textile industry is responsible for the invisible pollution that is created by textile microfibers, which have been detected in marine sediments and organisms. The present review paper demonstrates that the microfibers discharged from functionalized textiles exhibit non-biodegradable characteristics and that a considerable proportion of them possess toxic properties. This is primarily attributed to the impact of textiles' material functionalization on their biodegradability. The potential for these microfibers, which are released from textiles that contain a variety of dyes, toxic chemicals, and nanomaterials, to pose a variety of health risks to both humans and other living organisms is discussed in this paper. In addition, this paper covers a wide variety of preventative and minimizing measures for reduction, which are discussed in terms of several phases ranging from sustainable production through the consumer, end of life, domestic washing, and wastewater treatment phases.

Performance and mechanism of azo dyes degradation and greenhouse gases reduction in single-chamber electroactive constructed wetland system

A single-chamber microbial fuel cell-microbial electrolytic cell with a novel constructed wetland system was proposed for synergistic degradation of congo red and reduction in emissions of greenhouse gases. The closed-circuit system showed higher chemical oxygen demand and congo red removal efficiencies by 98 % and 96 % on average, respectively, than traditional constructed wetland. It could also significantly reduce the emissions of CH₄ and N₂O (about 52 % CO₂-equivalents) by increasing the electron transfer. Microbial community analysis demonstrated that the progressive enrichment of dye-degrading microorganisms (Comamonas), electroactive bacteria (Tolumonas, Trichococcus) and denitrifying microorganisms (Dechloromonas) promoted pollutant removal and electron transfer. Based on gene abundance of xenobiotics biodegradation, the congo red biodegradation pathway was described as congo red → naphthalene and alcohols → CO₂ and H₂O. In summary, the single-chamber closed-circuit system could significantly improve the degradation of congo red and reduce the emissions of greenhouse gases by influencing electron transfer and microbial activity.

Recent advances in biodecolorization and biodegradation of environmental threatening textile finishing dyes

Organic nature of dyes and their commercially made products are widely utilized in many industries including paper, cosmetics, pharmaceuticals, photography, petroleum as well as in textile manufacturing. The textile industry being the top most consumer of a large variety of dyes during various unit processes operation generates substantial amount of wastewater; hence, nominated as "Major Polluter of Potable Water". The direct discharge of such effluents into environment poses serious threats to the functioning of biotic communities of natural ecosystems. The detection of these synthetic dyes is considered as relatively easy, however, it is extremely difficult to completely eliminate them from wastewater and freshwater ecosystems. Aromatic chemical structure seems to be the main reason behind low biodegradability of these dyes. Currently, various physiochemical and biological methods are employed for their remediation. Among them, microbial degradation has attracted greater attention due to its sustainability, high efficiency, cost effectiveness, and eco-friendly nature. The current review presents recent advances in biodegradation of industrial dyes towards a sustainable and tangible technological innovative solutions as an alternative to existing conventional physicochemical treatment processes.

Construction of double tube granular sludge microbial fuel cell and its characteristics and mechanism of azo dye degradation

Microbial fuel cells (MFCs) can obtain electrical energy from extensive organic matter and complete wastewater treatment at the same time. The principal purpose of the research is to find a solution to the biodegradation of X-3B in a double tube MFC with graphite fiber brush as the anode and carbon cloth as the cathode. The anaerobic, aerobic, and electrochemical processes in the MFC were investigated. The effects of dye concentration and circuit connectivity on the performances of MFCs were explored. The degradation efficiency of X-3B in the anode region (85.56%) was higher than that in the cathode region (14.16%) within 24 h under the optimal voltage of 0.43 V, indicating a synergistic effect between electrode reaction and biodegradation. The power density increased from 12.12 mW/m³ to 60.45 mW/m³ with the addition of X-3B from 50 to 200 mg/L, because of the reduced ohmic and polarization resistance. Intermediate productions such as aniline were manufactured with the conjugated double bond of X-3B broken, and the intermediates were degraded into small molecular products like phenol during further degradation processes. Moreover, dye concentration and circuit connection had significant effects on the relative abundance of the microbial community at phylum and genus levels. In general, MFC is a good approach to energy generation and azo dye treatment at the same time.

Degradation of acid red 73 wastewater by hydrodynamic cavitation combined with ozone and its mechanism

Many azo dyes are consumed in the textile and dyeing industry, which makes the wastewater recalcitrant and toxic to the aquatic environment. Dye degradation by the combination of hydrodynamic cavitation and ozone (HC + O₃) has caused extensive interest. The degradation mechanism of the hybrid system needs further investigation. This study investigated the degradation of acid red 73 (AR73) by HC + O₃. Meanwhile, the degradation pathways and mechanisms were present. The optimal operation parameters were: inlet pressure of 0.15 MPa, O₃ dosage of 45 mg/min, initial dye concentration of 10 mg/L, and initial pH at 7.5. As a result, the decolorization rate, removal of UV₂₅₄ and NH₃-N were 100%, 71.28%, and 87.36% in 30 min, respectively. Humic acid and most of the co-existing anions (HCO₃^-, SO₄^2-, Cl^-, PO₄^3-, NO₃^-) played a positive role in the degradation of AR73, while NO₂^- restrained. The reactive species of singlet oxygen (¹O₂), hydroxyl radicals (·OH) and super oxygen radicals (·O₂^-) showed synergism in the hybrid system, and the decolorization was attributed to the fracture of azo bonds by ¹O₂. Meanwhile, aromatic amines were generated and further degraded into small molecule compounds. The research certificated that the HC + O₃ can be an effective technology for azo dye degradation.

Parametric Study of Methyl Orange Removal Using Metal–Organic Frameworks Based on Factorial Experimental Design Analysis

Wastewater treatment plants (WWTPs) are one of the most energy-intensive industries. Every stage of wastewater treatment consumes energy, which is the primary contributor to WWTP costs. Adsorbents and process optimization are critical for energy savings. The removal of dyes from industrial wastewater by adsorption using commercially available adsorbents is inefficient. Metal–organic frameworks (MOFs) have outstanding properties that can improve separation performance over current commercial adsorbents, and thus, these materials represent a milestone in improving dye removal in water treatment methods. In this work, three types of metal–organic frameworks (Fe-BTC, Cu-BTC, and ZIF-8) have been investigated as prospective adsorbents for methyl orange removal from water in batch setups. The results showed that at 15 mg/L MO initial concentration and 100 mg dosage, Fe-BTC had the highest removal efficiency of 91%, followed by ZIF-8 (63%), and finally Cu-BTC (35%), which exhibited structural damage due to its instability in water. Fe-BTC maintained consistent adsorption capacity over a wide range of pH values. Furthermore, a 23 full factorial design analysis was implemented to evaluate the conditions for maximum MO-removal efficiency. The main effects, interaction effects, analysis of variance (ANOVA), and the Pareto chart were reported. The statistical analysis demonstrated that the MOF type was the most significant factor, followed by dosage and initial concentration. The analysis indicated that the type of MOF and dosage had a positive effect on the removal efficiency, while the initial concentration had a negative effect. The two-way and three-way interactions were also found to be significant.

Microbial fuel cells for mineralization and decolorization of azo dyes: Recent advances in design and materials

Microbial fuel cells (MFCs) exhibit tremendous potential in the sustainable management of dye wastewater via degrading azo dyes while generating electricity. The past decade has witnessed advances in MFC configurations and materials; however, comprehensive analyses of design and material and its association with dye degradation and electricity generation are required for their industrial application. MFC models with high efficiency of dye decolorization (96-100%) and a wide variation in power generation (29.4-940 mW/m²) have been reported. However, only 28 out of 104 studies analyzed dye mineralization - a prerequisite to obviate dye toxicity. Consequently, the current review aims to provide an in-depth analysis of MFCs potential in dye degradation and mineralization and evaluates materials and designs as crucial factors. Also, structural and operation parameters critical to large-scale applicability and complete mineralization of azo dye were evaluated. Choice of materials, i.e., bacteria, anode, cathode, cathode catalyst, membrane, and substrate and their effects on power density and dye decolorization efficiency presented in review will help in economic feasibility and MFCs scalability to develop a self-sustainable solution for treating azo dye wastewater.

Adsorptive Behavior of Tartaric Acid Treated Holarrhena antidysenterica and Citrullus colocynthis Biowastes for Decolourization of Congo Red Dye from Aqueous Solutions

The aim of the present work is to eradicate Congo red (CR) dye from aqueous solutions since the dye compounds are harmful to human life and the environment leading to detrimental results. For this purpose, Holarrhena antidysenterica (HA) and Citrullus colocynthis (CC) adsorbents were used for the adsorptive removal of Congo red dye from wastewaters. The unmodified adsorbents (U-HA and U-CC) were chemically modified using tartaric acid (TA). Morphological structures were examined by FTIR and SEM. Batch adsorption studies were tested at a variety of pH, time exposure, temperatures, and adsorbent dosages. Thermodynamic parameters such as Gibbs free energy ( $∆G^{°}$ ), enthalpy ( $∆H^{°}$ ), entropy changes ( $∆S^{°}$ ), and energy of activation ( $E_{\text{a}}$ ) were also calculated. The results revealed that tartaric acid-Citrullus colocynthis (TA-CC) gave optimum conditions of time of contact (35 min), temperature conditions (40°C), pH (3), and dosage of adsorbent (1.6 g) for maximum dye removal. Tartaric acid-Holarrhena antidysenterica (TA-HA) gave equilibrium time of contact (30 min), temperature (40°C), and pH optimum (2) along with a 1.6 g dosage of adsorbent. Mechanistic understanding of adsorption isotherm provided that the Langmuir model was followed by raw and modified adsorbents. Maximum adsorption capacities $Q_{\max }$ attained were 60.61 (mg g-1), 128.21 (mg g-1), 87.71 (mg g-1), and 131.57 (mg g-1), respectively, for U-HA, TA-HA, U-CC, and TA-CC. The results of kinetic modeling displayed a high value of $R^2$ (0.99) along with minimal error (RMSE) for dye removal showing that the pseudo-second-order kinetic model has acceptable accuracy. Fourier transform infrared proposed the electrostatic, pi-pi interactions, and hydrogen bonding as dominant adsorption mechanisms at acidic pH, respectively. Rate-determining steps comprise both surface and intraparticle diffusions. Thermodynamics indicated that the dye adsorption of CR is spontaneous, exothermic, and favorable in nature. These agricultural wastes due to specific points such as low cost, availability, and high removal rates of adsorption are highly competent for the expulsion of anionic dye like CR from wastewaters.

Efficient decolorization of azo dye wastewater with polyaniline/graphene modified anode in microbial electrochemical systems

Azo dye pollution has become a worldwide issue, and the current treatment methods can hardly meet the expected emission standards. Microbial electrochemical systems (MESs) show promising applications for decolorization, but their performance critically depends on the microorganisms. Electrode modification is an interesting method of improving decolorization performance. However, the mechanisms of how the modification can affect microbial communities and the decolorization process remain unclear. Here, a modified anode with polyaniline (PANI) and graphene was fabricated via electro-deposition. Consequently, the highest decolorization efficiency was obtained. The Congo red (CR) decolorization rate of the MESs with the PANI/graphene-modified electrode (PG) reached 90% at 54 h. By contrast, the CR decolorization rates of the MESs with the PANI-modified electrode (P) and those of the MESs with the unmodified electrode (C) only reached 68% and 79%, respectively. Results of the microbial community analysis showed abundant Methanobrevibacter arboriphilus in PG (11%), which was 5.5 times that in C (2%) at 18 h. This phenomenon may be related to the rapid decolorization. The upregulated metabolism pathways, including arginine and proline metabolism, purine metabolism, arginine biosynthesis, and riboflavin metabolism, provided more electron shuttles and redox mediators that facilitated the extracellular electron transfer. Therefore, the PG-modified electrode facilitated the decolorization by altering certain metabolic pathways. This study can help to improve the guideline on the potential application of MESs for wastewater treatment.

Sustainable remediation of hazardous environmental pollutants using biochar-based nanohybrid materials

Biochar is a well-known carbon material with diversified functionalities and excellent physicochemical characteristics with high wastewater treatment potential. This review aims to summarize recent advancements in the development of biochar and biochar-based nanohybrid materials as a potential tool for the removal of harmful organic compounds such as synthetic dyes/effluents. The formation of biochar using pyrolysis of renewable feedstocks and their applications in various industries are explained hereafter. The characteristics and construction of biochar-based hybrid materials are explained in detail. Diversity of feedstocks, including municipal wastes, industrial byproducts, agricultural, and forestry residues, endows different biochar types with a wide structural variety. The production of cost-effective biochar drives the interest in manipulating biochars and induces desire functionality using nanoscale reinforcements. Various types of biochars, such as magnetic biochar, layered nanomaterial coated biochar, nanometallic oxide composites, chemically and physically functionalized biochar, have been produced. With the aid of nanomaterial, hybrid biochar exhibits a high potential to remove toxic contaminants. Depending upon biochar type, dyes/effluents can be removed via different mechanisms, including the Fenton process, photocatalytic degradation, π-π interaction, electrostatic interaction, and physical adsorption. In conclusion, desired physicochemical features, and tunable surface properties of biochar present high potential material in removing organic dyes and other effluents. The blended biochar with different materials/nanomaterials endows broader development and multi-functional opportunities for treating dyes/effluents.

A detailed insight on fabricated porous chitosan in eliminating synthetic anionic dyes from single and multi-adsorptive systems with related studies

Chitosan was fabricated via gelation method using CaBr₂.xH₂O/methanol solution and was studied as a potential adsorbent (MCh) in adsorbing anionic synthetic dyes like Bromophenol blue (BB), Direct blue 6 (DB) and Congo red (CR) from single (one dye species at a time) and multi (having two dyes; binary and all three dyes; tertiary) adsorptive systems. Physico-chemical modifications of MCh surface prior and post modification and dye adsorption were evaluated using scanning electron microscopy, Energy-dispersive X-ray spectroscopy, powder X-ray diffraction analysis, surface area analysis and Fourier-transformed infrared spectroscopy. Influential parameters influencing the adsorption process viz. initial pH of dye solution, MCh dosage, adsorption temperature and initial concentration of dye species were optimised. Adsorptive studies involving single adsorptive setups verified formation of sorbate's (dye species) monolayer over the sorbent's (MCh) surface via chemisorption; as established by Langmuir isotherm and pseudo-second order kinetics model analysis. Theoretical maximum adsorption capacities of MCh for BB, DB and CR was found to be 81.301 mg/g, 163.934 mg/g and 75.758 mg/g, respectively. Meanwhile, for all multi-adsorptive systems, competitive Langmuir isotherm model verified antagonistic behaviour of an individual dye over other dye adsorption over MCh surface in their respective adsorptive systems. Thermodynamics of the sorbate-sorbent interaction was exothermic, spontaneous, with elevated degree of disorderedness; concluding the interaction as thermodynamically favourable. Co-existing metal cations and anionic salts had minimal effect on MCh's adsorption efficiency. Phytotoxicity assay via germination of Vigna mungo seeds verified the efficacy of the adsorbent in eliminating the dye species from single and multi-adsorptive systems.

Recent advancements in azo dye decolorization in bio-electrochemical systems (BESs): Insights into decolorization mechanism and practical application

Recent advances in bio-electrochemical systems (BESs) for azo dye removal are gaining momentum due to having electrode biocarrier and electro-active bacteria that could stimulate decolorization via extracellular electron transfer. Enhanced decolorization performance is observed in most laboratory studies, indicating the great potential of BESs as an alternative to the traditional biological processes or serving as a pre-/post-processing unit to improve the performance of biological processes. It is proven more competitive in environmental friendly than physicochemical methods. While, the successful application of BESs to azo dye-containing wastewater remediation requires a deeper evaluation of its performance, mechanism and typical attributes, and a comprehensive potential evaluation of BESs practical application in terms of economic analysis and technical optimizations. This review is organized to address BESs as a practical option for azo dye removal by analyzing the decolorization mechanisms and involved functional microorganisms, followed by the comparisons of device configurations, operational conditions, and economic evaluation. It further highlights the current hurdles and prospects for the abatement of azo dyes via BES related techniques.

Porous g-C3N4 with defects for the efficient dye photodegradation under visible light

Porous graphitic carbon nitride (p-C₃N₄) was fabricated via simple pyrolyzing treatment of graphitic carbon nitride (g-C₃N₄). The defects could be introduced into the structure of g-C₃N₄ by breakage of some bonds, which was beneficial for the generation of electron-hole pairs and inhibiting their recombination. Compared with g-C₃N₄, p-C₃N₄ showed a narrow band gap to promote the utilization of visible light. Furthermore, the porous structure also increased the specific surface area to maximize the exposure of active sites and promote mass transfer during photodegradation. As a result, the as-reported p-C₃N₄ exhibited considerably higher degradation efficiency for Rhodamine B (RhB) and Methyl Orange (MO) than that of the original g-C₃N₄. Moreover, the photocatalyst showed high durability and stability in recycling experiments.

Facile synthesis of recyclable laccase-mineral hybrid complexes with enhanced activity and stability for biodegradation of Evans Blue dye

Two morphologies of laccase-mineral hybrid complexes, i.e., laccase-mineral hybrid nanoflowers (La-HNF) and nanopetals (La-HNP), were synthesized via biomineralization using Cu₃ (PO₄)₂·3H₂O as the mineral for Evans Blue (EB) dye biodegradation. XRD patterns and FT-IR spectra results revealed the successful immobilization of laccase via in-situ formed Cu₃(PO₄)₂·3H₂O crystals. Compared with free laccase, laccase-mineral hybrid complexes showed higher enzymatic activity due to the activation effect induced by copper ions of Cu₃(PO₄)₂·3H₂O, further, the improved kinetic parameters of laccase-mineral hybrid complexes could be ascribed to nanoscale-dispersed laccase molecules within hybrid complexes. For EB dye biodegradation, the reason why the biodegradation efficiency (94.9%) of La-HNF was higher than that (86.8%) of La-HNP could be synergistic effect of immobilized laccase within 3D hierarchical structure of La-HNF. In addition, the optimized biodegradation conditions (pH 4.6 and 40 °C) of La-HNF were obtained, moreover, 93.2% and 48.1% of EB dye were biodegraded by La-HNF after stored for 30 days and reused for 10 cycles, respectively, demonstrating La-HNF have good practicability.

Recent Advances in Biopolymer-Based Dye Removal Technologies

Synthetic dyes have become an integral part of many industries such as textiles, tannin and even food and pharmaceuticals. Industrial dye effluents from various dye utilizing industries are considered harmful to the environment and human health due to their intense color, toxicity and carcinogenic nature. To mitigate environmental and public health related issues, different techniques of dye remediation have been widely investigated. However, efficient and cost-effective methods of dye removal have not been fully established yet. This paper highlights and presents a review of recent literature on the utilization of the most widely available biopolymers, specifically, cellulose, chitin and chitosan-based products for dye removal. The focus has been limited to the three most widely explored technologies: adsorption, advanced oxidation processes and membrane filtration. Due to their high efficiency in dye removal coupled with environmental benignity, scalability, low cost and non-toxicity, biopolymer-based dye removal technologies have the potential to become sustainable alternatives for the remediation of industrial dye effluents as well as contaminated water bodies.

Polymer Nanocomposites for Photocatalytic Degradation and Photoinduced Utilizations of Azo-Dyes

Specially designed polymer nanocomposites can photo-catalytically degrade azo dyes in wastewater and textile effluents, among which TiO2-based nanocomposites are outstanding and extensively explored. Other nanocomposites based on natural polymers (i.e., chitosan and kaolin) and the oxides of Al, Au, B, Bi, Fe, Li, and Zr are commonly used. These nanocomposites have better photocatalytic efficiency than pure TiO2 through two considerations: (i) reducing the hole/electron recombination rate by stabilizing the excited electron in the conducting band, which can be achieved in TiO2-nanocomposites with graphene, graphene oxide, hexagonal boron nitride (h-BN), metal nanoparticles, or doping; (ii) decreasing the band energy of semiconductors by forming nanocomposites between TiO2 and other oxides or conducting polymers. Increasing the absorbance efficiency by forming special nanocomposites also increases photocatalytic performance. The photo-induced isomerization is exploited in biological systems, such as artificial muscles, and in technical fields such as memory storage and liquid crystal display. Heteroaryl azo dyes show remarkable shifts in photo-induced isomerization, which can be applied in biological and technical fields in place of azo dyes. The self-assembly methods can be employed to synthesize azo-dye polymer nanocomposites via three types of interactions: electrostatic interactions, London forces or dipole/dipole interactions between azo dyes, and photo alignments.

Couple of graphene oxide and functionalized carbon nanotubes for dye degradation enhancement of anatase under visible light and solar irradiation

Graphene oxide sheets (GO) were coupled with carbon nanotubes (CNTs) to enhance the photoactivity of anatase under visible and solar irradiation. The carbon nanotube surface was functionalized in the acidic reflux condition before coupling with GO and decoration of anatase by the sol-gel method. A modified kinetic model was appropriately applied to predict the breakthrough in the methylene blue degradation yield and determine the constant rate which was clearly affected by coupling architecture. The nanocomposite fabricated by the same proportions of GO and CNTs, 3.33%, exhibited the maximal degradation yield, 96.5%, in the dye solution with the initial concentration of 3.0 mg l-1. The characterizations based on X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and field emission scanning electron microscopy (FESEM) revealed that the functionalized CNTs could create the appropriate space between the graphene sheets for uniformly interconnection of anatase via oxygen-containing groups onto the material surfaces. This enhancement in the degradation efficiency could be ascribed to the unique architecture, leading to a decrease in bandgap energy, 2.2 eV, which facilitated the electron-hole separation. Besides of breakthrough in the photoreaction rate, the adequate architecture led to an efficient reduction in the content of carbon-based materials. Also, the performance of mentioned nanocomposite under sunlight photons was effectively higher than that under UV irradiation. The hybrid nanocomposite provided a large number of active sites for photoreactions to facilitate the treatment of wastewater under solar irradiation.

Ethanol as an efficient cosubstrate for the biodegradation of azo dyes by Providencia rettgeri: Mechanistic analysis based on kinetics, pathways and genomics

Azo dyes pose hazards to ecosystems and human health and the cosubstrate strategy has become the focus for the bioremediation of azo dyes. Herein, Brilliant Crocein (BC), a model pollutant, was biodegraded by Providencia rettgeri domesticated from activated sludge. Additional ethanol, as a cosubstrate, could accelerate P. rettgeri growth and BC biodegradation, as reflected by the Gompertz models. This phenomenon was attributed to the smaller metabolites and greater number of potential pathways observed under the synergistic effect of ethanol. Genomic analysis of P. rettgeri showed that functional genes related to azo bond cleavage, redox reactions, ring opening and hydrolysis played crucial roles in azo dye biodegradation. Furthermore, the mechanism proposed was that ethanol might stimulate the production of additional reducing power via the expression of related genes, leading to the cleavage of azo bonds and aromatic rings. However, biodegradation without ethanol could only partly cleave the azo bonds.

Surface treated acid-activated carbon for adsorption of anionic azo dyes from single and binary adsorptive systems: A detail insight

Current study deals with the surface modification of acid activated carbon (prepared from Pongamia pinnata shells) with Cetyltrimethylammonium bromide (CTAB) and its role as an adsorbent in eliminating anionic azo dyes viz. Congo red (CR) and Direct blue 6 (DB) from single and binary adsorptive systems. Binary adsorptive system involved the synergistic and antagonistic influence of one dye over the adsorption of other dye. Physico-chemical alterations due to surfactant modification and post adsorption were studied using atomic force microscopy (AFM), Zeta Potential, scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), surface area analysis and Fourier-transformed infrared spectroscopy (FTIR). Process parameters influencing efficient adsorption of CR and DB species viz. initial pH of dye solution, adsorbent dosage, incubation temperature and initial concentration of dye species were optimised. Sorbate-sorbent interaction studies for single adsorptive system revealed sorbate's monolayer formation over adsorbent's surface and the involvement of chemisorption, as verified by Langmuir isotherm model and pseudo-second order model, respectively. Langmuir maximum adsorption capacity of the adsorbent was 555.56 mg/g for CR and 625.00 mg/g for DB. Meanwhile, for binary adsorptive system, competitive Langmuir model verified both CR and DB had antagonistic/competitive effect over each other's adsorption. Thermodynamic analysis revealed the adsorptive process as exothermic, spontaneous and thermodynamically favourable with an elevated degree of dis-orderedness. Co-existing cations and anions has nominal effect on the adsorption capacity of dyes. Recyclability studies verified a modest efficiency of 62.52% for CR and 50.47% for DB species after the end of 4th adsorption-desorption cycle; thus affirming its recyclability potential. Phytotoxic assay affirmed the effectivity of the adsorbent in adsorbing dye species from aqueous solutions using Vigna mungo seeds as the model.

Constructed wetland-microbial fuel cell for azo dyes degradation and energy recovery: Influence of molecular structure, kinetics, mechanisms and degradation pathways

Complete degradation of azo dye has always been a challenge due to the refractory nature of azo dye. An innovative hybrid system, constructed wetland-microbial fuel cell (CW-MFC) was developed for simultaneous azo dye remediation and energy recovery. This study investigated the effect of circuit connection and the influence of azo dye molecular structures on the degradation rate of azo dye and bioelectricity generation. The closed circuit system exhibited higher chemical oxygen demand (COD) removal and decolourisation efficiencies compared to the open circuit system. The wastewater treatment performances of different operating systems were ranked in the decreasing order of CW-MFC (R1 planted-closed circuit) > MFC (R2 plant-free-closed circuit) > CW (R1 planted-open circuit) > bioreactor (R2 plant-free-open circuit). The highest decolourisation rate was achieved by Acid Red 18 (AR18), 96%, followed by Acid Orange 7 (AO7), 67% and Congo Red (CR), 60%. The voltage outputs of the three azo dyes were ranked in the decreasing order of AR18 > AO7 > CR. The results disclosed that the decolourisation performance was significantly influenced by the azo dye structure and the moieties at the proximity of azo bond; the naphthol type azo dye with a lower number of azo bond and more electron-withdrawing groups could cause azo bond to be more electrophilic and more reductive for decolourisation. Moreover, the degradation pathway of AR18, AO7 and CR were elucidated based on the respective dye intermediate products identified through UV-Vis spectrophotometry, high-performance liquid chromatography (HPLC), and gas chromatograph-mass spectrometer (GC-MS) analyses. The CW-MFC system demonstrated high capability of decolouring azo dyes at the anaerobic anodic region and further mineralising dye intermediates at the aerobic cathodic region to less harmful or non-toxic products.

Microwave assist sorption of crystal violet and Congo red dyes onto amphoteric sorbent based on upcycled Sepia shells

A new sorbent based on Sepia shells (cuttlefish bones) has been synthesized (SSBC) and tested for the sorption of cationic dye (crystal violet, CV) and an anionic dye (congo red, CR). SSBC was produced by reaction of sepia shells powder with urea in the presence of formaldehyde. In the first part of the work, the sorbent was characterized using scanning electron microscopy, energy dispersive X-ray analysis, Fourier-transform infra-red spectrometry and titration (for determining pH_PZC). In a second step, sorption properties were tested on the two dyes through the study of pH effect, sorbent dosage, temperature and ionic strength; the sorption isotherms and uptake kinetics were analyzed at the optimum pH: Langmuir equation fits isotherm profiles while the kinetic profile can be described by the pseudo-second order rate equation. Maximum sorption capacities reach up to 0.536 mmol g^-1 for CV and 0.359 mmol g^-1 for CR, at pH 10.6 and 2.4, respectively. The comparison of sorption properties at different temperatures shows that the sorption is endothermic. Processing to the sorption under microwave irradiation (microwaved enforced sorption, MES) increases mass transfer and a contact time as low as 1 min is sufficient under optimized conditions (exposure time and power) reaching the equilibrium, while 2-3 h were necessary for "simple" sorption. Dye desorption was successfully tested using 0.5 M solutions of NaOH and HCl for the removal of CR and CV, respectively. The sorbent can be re-used for a minimum of four cycles of sorption/desorption. Finally, the sorbent was successfully tested on spiked tap water and real industrial wastewater.

Spectral deconvolution associated to the Gaussian fit as a tool for the optimization of photovoltaic electrocoagulation applied in the treatment of textile dyes

The objective of this study was to investigate the color removal in a binary mixture of azo dyes from the photovoltaic electrocoagulation (EC) technique, using spectral deconvolution and the Gaussian fit for qualitative and quantitative determination of the physical color parameter. Initially, a conventional energy source was used to feed the EC reactor and the experimental design was conducted according to the Rotational Central Compound Design (RCCD). The spectral deconvolution method associated to the Gaussian fit aided in the description of the composition of the sample matrix, In the first step, through the Analysis of Variance, the RCCD and the three-dimensional surface response graphs, the optimized operating conditions were identified, which corresponded to 1320 A m^-2 with an reaction time of 16.6 min, and an expected removal of 98.40% for Scarlet Red (SR) dye and 1160 A m^-2 with a run time of 15.7 min and 97.9% removal for Turquoise Blue (TB) dye. Using the photovoltaic module as the power source of the EC reactor, a maximum removal of 97 ± 0.43% for TB dye and 98% ± 0.81 for SR was obtained. The results encourage the applicability of photovoltaic module-fed EC technology as a promising alternative for the treatment of effluents containing textile dyes, as well as the use of the spectral deconvolution method associated with the Gaussian fit, for the reliability and precision of the results.

Mutual effect between electrochemically active bacteria (EAB) and azo dye in bio-electrochemical system (BES)

Herein, the mutual effect between azo dye and the performance of electrochemically active bacteria (EAB) is investigated in detail, which is crucial to understand and control the bio-electrochemical systems (BESs) operation for azo dye containing wastewater treatment. EAB is enriched at controlled potential of -0.2 V vs Ag/AgCl in single-chamber BESs. Over 95% azo dye (alizarin yellow R (AYR)) was decolorized regardless of the initial AYR concentration ranging from 30 to 120 mg/L within 24 h. The fastest decolorization rate was obtained at AYR initial concentration of 70 mg/L, which was 4.25 times greater in the closed circuit BESs than that in the open circuit one. 16S rRNA gene based microbial community analysis showed that Geobacter was dominant in EAB with relative abundance increased from 77.98% (0 mg/L AYR) to 92.22% (70 mg/L AYR), indicating that azo dye selectively boosts the growth of exoelectrogens in electrode biofilm communities. Under electricity stimulation, extracellular process can be mutually conducted by azo dye compounds, which is favorable for accelerating reaction rate and avoiding of significant toxic effect on EAB.

Microbial degradation of azo dye carmoisine in aqueous medium using Saccharomyces cerevisiae ATCC 9763

Carmoisine is an azo dye widely used in many industries, and therefore frequently occurs in the effluent of many factories. To our knowledge, biological degradation of carmoisine has received little attention. The present study investigates the capability of Saccharomyces cerevisiae ATCC 9763 for degradation of carmoisine. Spectrophotometry data indicates that carmoisine (50 mg/l) was eliminated from the aqueous medium after approximately 7 h of incubation with Saccharomyces under anaerobic shaking conditions. Thin layer chromatography (TLC) revealed the removal of carmoisine as well as the appearance of aromatic amines in samples collected from the decolourized medium by S. cerevisiae and this was subsequently confirmed by Fourier transform infrared (FTIR) spectroscopy. Liquid chromatography mass spectrometry (LC/MS) was carried out on fractions from consecutive column chromatography and two-dimensional (2D) chromatography. LC/MS indicated degradation of carmoisine into its constituent aromatic amines. In addition, investigating the effect of environmental conditions on the decolourization process indicated that yeast extract could positively affect decolourization rates; shaking significantly accelerated decolourization and shortened the time required for complete biodecolourization from ≃ 8 days to ≃ 7 h; and Saccharomyces was able to consume sucrose as a carbon source and remove the carmoisine despite the presence of sunset yellow, which remained unaffected.

Dye Adsorbent Materials Based on Porous Ceramics from Glass Fiber-Reinforced Plastic and Clay

We investigated the use of waste glass fiber-reinforced plastic (GFRP) to remove dye from industrial wastewater. The dye adsorbent material, based on GFRP/clay ceramics, was produced by mixing crushed GFRP with clay and firing the resulting mixture. Several types of ceramics were produced by adjusting the mixing ratio of clay, crushed 40% GF/GFRP, and firing atmosphere. Adsorption tests with methylene blue (MB) dye were performed by mixing the ceramics into an MB solution while controlling the stirring speed and measuring the decrease in MB dye concentration over time. These results showed that GFRP/clay ceramics reductively fired at 1073 K had a higher MB dye adsorption ability than that of the clay ceramic. The MB dye absorptivity of the reductively fired ceramics increased as we increased the mixing ratio of GFRP. We attribute this result to the high plastic carbide content in the ceramic, which has excellent dye absorbability. Furthermore, these particles had a comparatively high specific surface area and porosity.

From biotechnology principles to functional and low-cost metallic bionanocatalysts

Chemical, physical and mechanical methods of nanomaterial preparation are still regarded as mainstream methods, and the scientific community continues to search for new ways of nanomaterial preparation. The major objective of this review is to highlight the advantages of using green chemistry and bionanotechnology in the preparation of functional low-cost catalysts. Bionanotechnology employs biological principles and processes connected with bio-phase participation in both design and development of nano-structures and nano-materials, and the biosynthesis of metallic nanoparticles is becoming even more popular due to; (i) economic and ecologic effectiveness, (ii) simple one-step nanoparticle formation, stabilisation and biomass support and (iii) the possibility of bio-waste valorisation. Although it is quite difficult to determine the precise mechanisms in particular biosynthesis and research is performed with some risk in all trial and error experiments, there is also the incentive of understanding the exact mechanisms involved. This enables further optimisation of bionanoparticle preparation and increases their application potential. Moreover, it is very important in bionanotechnological procedures to ensure repeatability of the methods related to the recognised reaction mechanisms. This review, therefore, summarises the current state of nanoparticle biosynthesis. It then demonstrates the application of biosynthesised metallic nanoparticles in heterogeneous catalysis by identifying the many examples where bionanocatalysts have been successfully applied in model reactions. These describe the degradation of organic dyes, the reduction of aromatic nitro compounds, dehalogenation of chlorinated aromatic compounds, reduction of Cr(VI) and the synthesis of important commercial chemicals. To ensure sustainability, it is important to focus on nanomaterials that are capable of maintaining the important green chemistry principles directly from design inception to ultimate application.

Recent advancements in bioremediation of dye: Current status and challenges

The rampant industrialization and unchecked growth of modern textile production facilities coupled with the lack of proper treatment facilities have proliferated the discharge of effluents enriched with toxic, baleful, and carcinogenic pollutants including dyes, heavy metals, volatile organic compounds, odorants, and other hazardous materials. Therefore, the development of cost-effective and efficient control measures against such pollution is imperative to safeguard ecosystems and natural resources. In this regard, recent advances in biotechnology and microbiology have propelled bioremediation as a prospective alternative to traditional treatment methods. This review was organized to address bioremediation as a practical option for the treatment of dyes by evaluating its performance and typical attributes. It further highlights the current hurdles and future prospects for the abatement of dyes via biotechnology-based remediation techniques.