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

Abstract

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.

Introduction

Dyes are colored organic compounds which impart color to the product upon application and are mostly used substances in textile industries. Approximately 100,000 dyes are available commercially with annual production of more than 7 × 10⁵ tones (Sudha et al. 2014). Dyes are variously classified into categories depending on their structure, viz; acidic, basic, disperse, azo, diazo, anthraquinone etc. (Campos et al. 2001). Among various available dyes, azo dyes are one of the most widely used textile dyes consisting of various organic aromatic compounds with one or more azo bond (–N=N–) and their chemical structure allows them to absorb visible light spectrum (Yamjala et al. 2016). The azo group is either substituted by phenyl or naphthyl group which in turn may be substituted by one or more moieties like chloride (–Cl), amino (–NH₂), hydroxyl (–OH), carboxylic (–COOH), nitro (–NO₂) and methyl (–CH₃) groups creating diversity of different azo dyes (Fatima et al. 2017). The structure of some of common dyes has been previously reported (Rauf et al. 2010; Ali 2010; Ishchi and Sibi 2020) and are presented in Fig. 1. More than 3000 different azo dyes are produced because of their ease of preparation and cost effectiveness in comparison to natural dyes. Azo dyes are used in industries including paper, clothing, food, cosmetic and pharmaceutical industries, rendering them responsible for the generation of large quantities of dye-rich wastewaters (Rani et al. 2014).

Fig. 1.

Structure of some of common dyes

Approximately 10–15% of dyes are discharged into effluents from dyeing process which are resistant to be degraded under the influence of factors namely water, sweat, exposure to light and many oxidizing agents as well as by microbial attack (Lellis et al. 2019). This colored wastewater from the textile industry is a complex mixture of many pollutants like degradable organics, pH altering agents, nutrients, salts, sulfur, toxicants refractory organics, organochlorine-based pesticides, high chemical oxygen demand (COD), total organic carbon (TOC), biological oxygen demand (BOD), total suspended solids (TSS), total dissolved solids (TDS), and heavy metals associated with dyes and the dyeing process (Al-Amrani et al. 2014). Discharge of effluent containing azo dye and their metabolites cause the contamination of the natural groundwater reservoirs and impart adverse effects on aquatic ecosystem, eutrophication and perturbations. Furthermore azo dyes containing textile effluents retard the growth and germination of various ecologically important biomasses which in turn trigger soil erosion, deteriorates the soil fertility and even destruction of the wildlife (Varjani et al. 2020). Moreover, most of the azo dyes are toxic, carcinogenic and mutagenic (Chung 2016).

There are many physical, chemical, and photochemical methods namely coagulation, flocculation, filtration, photodegradation, adsorption, chemioxidation membrane processes and reverse osmosis to treat toxic and azo dyes from textile effluents (Du et al. 2017; Sehar et al. 2019; Chen et al. 2019; Sehar et al. 2016, 2021a, b; Al-Najar et al. 2022; Younis and Loucif 2021; Younis et al. 2018). However, these technologies have certain limitations of being expensive, less proficient, generation of huge quantity of toxic sludge, aromatic amines and intermediate waste products (Kurade et al. 2011). The new closed-loop technologies such as the reuse of microbially or enzymatically treated dyeing effluents could help to delimit enormous water consumption. Owing to increasing stringent environmental legislation across the world, it is imperative to devise effective wastewater remediation techniques, especially those that allow color removal. The discharge of dye-contaminated wastewater into natural waterways presents a substantial risk to human and environmental health. Therefore the removal of toxic dyes from colored wastewaters is very important to protect ecosystem from hazardous effects of colored wastewater. With this background, the current review presents a deep insight into various biological methods such as through the use of plant phytoremediation), animal wastes, fungi, yeast, algae and bacteria for the discoloration and degradation of textile dyes.

Biological methods for decolorization and degradation of textile effluents

Different bioremediation strategies have been proposed and employed for the efficient and ecofriendly degradation of azo dyes because huge numbers of plants, animal wastes and microorganisms such as bacteria, fungi, yeasts, actinomycetes and algae can degrade these azo dyes in different ways. These organisms have the ability to mineralize azo dyes into CO₂ and H₂O under certain environmental conditions. The use of different biological methods for degradation of textile effluents has been schematically presented in Fig. 2.

Fig. 2.

Biological methods for decolorization and degradation of textile effluents

Decolorization and degradation by plants (phytoremediation)

The use of plants for the treatment of the toxic pollutants is termed as phytoremediation. It is plausible and emerging techniques in the new era of environmental sciences for the remediation of soil and ground water contaminated with heavy metals and organic pollutants. Besides, plants are autotrophic in nature, so they require a very limited amount of nutrients and maintenance cost. Being eukaryotic, they are more efficient and their efficiency increase with the passage of time due to increase in the biomass. In addition to these advantages, phytoremediation has benefits of aesthetic appeal and sustainability. Plant based biosorbent/adsorbent for the removal of dyes from aquatic effluents can be a better alternative to reduce the large quantity of waste by waste, thus helps in maintaining circular economy. Besides, plant based adsorbents can be effectively utilized as an active site for biosorption as they are not only composed of lignin, cellulose and hemicelluloses containing various important functional groups such as hydroxyl, carboxyl, and others. Therefore, it is mandatory to select a suitable biosorbent after assessing the surface characteristics for effective sorption process. Different plants parts such as leaves, stem bark, seeds, fruit peel, flower etc., can be used as an efficient biosorbent for the degradation of wide variety of dyes (Table 1). Nouren and Bhatti (2015) reported the potential of partially purified Citrus limon (lemon) peroxidase for decolorizing Basic violet 3 dye which is a common synthetic dye. Besides, they also monitored the effect of metal ions and mediators, pH, concentration of dye and enzyme dose during decolorization. According to their results, 96.34% of decolorization was attained at 42 U/mL of peroxidase enzyme dose 0.5 mM of p-coumaric acid and 0.25 mM of H₂O₂. Various physical parameters such as temperature, pH and time period were optimized to be 45 °C, 4.5 and 5 min respectively for maximum decolorization. They found that the presence of metal ions had no significant effect on decolorization process, thus it can be suitable for industrial purposes. Moreover, the degradation products were also identified through advanced techniques like UPLC/MS and proposed mechanistic pathway for efficient degradation of Basic violet 3 dye also. It was also reported that degradation products of Basic violet 3 had lower toxic effects on Zee mays (maize) showed in comparison to dye itself. The study concluded that phytoremediation could be an effective tool for degradation of toxic dyes.

Table 1.

Biodegradation of dyes by plant and animal waste

Plant waste
Name of dye	Parts of plants	Decolorization (%)	Optimal experimental conditions	References
Methylene blue	Banana pseudo-stem	98.2	100 mg/L, pH 7, 25 °C, 120 rpm, 450 min	Bello et al. (2018)
Reactive red- 2	Stem of Hymenaea courbaril L	91.7	60 mg/L, 0.5 g/L, pH 4.47, 25 °C, 400 rpm, 24 h	Mendonça et al. (2019)
Methylene Blue	Leaf powder of Butea monosperma	98.7	dye concentration of 100 mg/L, pH 8, 120 min	Das et al. (2020)
Crystal violet dye	Stem-barks of Rhizophora mucronata	99.8	Initial dye concentration 1.0 × 10−5 M, 25 °C, pH 7, 60 min	Oloo et al. (2020)
Reactive yellow-160 dye	Citrullus colocynthis stem	91.4	0.02% dye concentration, pH 7, temperature 40 °C, and 9 mg/L concentration of adsorbent	Kiran et al. (2020)
Animal waste
Acid Blue -80	Chicken feathers	80	pH 6, 50 °C, 3000 min	García-Sabido et al. (2016)
Methylene Blue	Snail shell (Helix aspersa)	82	pH 7, 15 °C, 250 rpm, 20 min	Erradi et al. (2019)
Acid Yellow- 127	Pogonias cromis	93	pH 2.5, 60 °C, 1000 rpm, 60 min	Kabir et al. (2019)
Methylene Blue	Chicken feathers	100	pH 4, 281 K, 24 h	Pradhan and Bajpai (2020)

Georgin et al. 2019 used Cedar bark (Cedrela fissilis) as an adsorbent for the removal of red 97 dye from effluents. Cedar bark being considered as waste product of wood processing industry possess an irregular surface, amorphous structure, and is mainly composed of holocellulose and lignin. Results from fixed bed adsorption experiments were achieved at pH 2.0 with maximum adsorption capacity of 422.87 mg g⁻¹ at 328 K. Experimental kinetic data explains the general order model and the equilibrium was attained within 30 min. Overall the adsorption process was spontaneous and endothermic as suggested by standard Gibbs free energy change (ΔG°) values ranging from − 21 to − 26 kJ mol⁻¹ and the enthalpy change (ΔH°) of 18.98 kJ mol⁻¹ respectively. Thus, Cedar bark served as an efficient adsorbent (color removal efficiency of 86.6%) to treat a simulated effluent and can be safely applied as a low-cost adsorbent for the treatment of colored effluents in batch and continuous adsorption systems.

Decolorization and degradation by animal waste

Numerous studies traced the biosorption/adsorption potential of dyes by using animal waste biomass including shells, feathers, fish scales etc., and surprisingly promising results have been reported as shown in Table 1. Marrakchi et al. 2017 used carbonized fish scales (CFS) of Labeo rohita to investigate the adsorption potential of reactive orange-16 dye through batch processing. Their findings revealed that maximum dye sorption capacity of 114.2 mg/L was attained 50 °C and in alkaline medium suggesting that the nature of the adsorption was endothermic. Besides, their data best fitted with the Freundlich isotherm and pseudo-second-order models. Niero et al. (2019), have investigated the removal efficiency of reactive Turquoise Blue 15 (RTB15) and Reactive Red 120 (RR120) dyes from textile wastewater by using powdered fish scales of Sardinella brasiliensis as low-cost absorbents. Their results revealed that approximately 90% removal efficiency of both dyes was achieved by using fish scales biosorbent having surface area of 69.0 m²/g at 25 °C, slightly acidic medium, 15 min contact time in the presence of hydroxyl and carboxylic functional groups.

Yang et al. (2019) successfully utilized frass of yellow mealworms (larvae of Tenebrio molitor L.) to produce dye-removing biochar. Afterwards, this biochar was used as an adsorbent to investigate the sorption of cationic malachite green dye from aqueous medium. Initially, the larvae were fed on wheat straw for 32 days for biomass production, which were then utilized for the production of biochars through pyrolysis. Their findings revealed that biochar generated from wheat straw exhibited maximum sorption capacity of 1738.6 mg/g for malachite green dye as compared to that produced from frass fed with bran. The main mechanism behind this adsorption was found to be chemisorption and electrostatic interactions. According to their results, the adsorption isotherm and kinetics data were best fitted to Langmuir sorption model and pseudo-second-order model respectively. Thus, they concluded that frass of yellow mealworms larvae can be served to generated high quality bioadsorbents.

Pradhan and Bajpai (2020) utilized chicken feathers for preparation of adsorbent films and investigated their adsorption potential for removal of dyes (methylene blue and crystal violet) from aqueous medium. The batch adsorption studies suggested that maximum adsorption capacity for methylene blue and crystal violet was 714.3 mg/g at 281 K and 555.6 mg/g at 301 K respectively after 24 h. They also found that chemical structure of dye influences the adsorption process.

Decolorization and degradation by algae

Algae are photosynthetic autotrophic microorganisms that are widely distributed on the planet earth. Algae are widely used in various biotechnological and industrial sectors including cosmetics, pharmaceuticals, food industry, production of bioactive substances and pigments, single cell proteins, biofuels etc. due to their rapid growth, increased photosynthetic efficiencies, higher biomass output (Mishra et al. 2021). Besides, algae are also widely used for the degradation of complex azo dyes due to the production of azo reductase enzyme that helps in reduction of azo bonds. In this regard, blue-green algae (cyanobacteria), green algae and diatoms are most widely utilized for decolorization and degradation of textile dyes (Singh and Singh 2017). This algal degradation may be achieved by enzymatic degradation by azoreductases, adsorption or combination of both (Tang et al. 2019). The presence of extensive surface area and elevated binding abilities enables algal biomass to achieve maximum biosorption efficiencies against various contaminants present in wastewater from textile and other industrial origin. The color removal by algae involves intrinsically three different mechanisms, namely, (1) Assimilative utilization of chromophores to produce algal biomass. (2) CO₂ and H₂O conversion of colored molecules to non-colored molecules and (3) chromophores adsorption on algal biomass (Sharif et al. 2020).

Freshwater and marine micro/macro algae has been extensively studied by various researchers as efficient biosorbents to decolorize the synthetic dyes from the waste effluents. Besides living and dead algal biomass (free/immobilized) can be used for dye decolorization (Table 2). Daneshvar et al. (2017) studied desorption capacity of Methylene blue (MB) dye by using Nizamuddinia zanardinii (brown macroalga). Their results suggested that dye desorption efficiency is directly related to particle size of algal biomass and maximum desorption efficiency of 68.70 ± 2.03% was attained with particle size > 250 µm. increased by increasing the alga particle sizes from 250 mm. Besides, the reusability of algal biomass was successfully attained during five consecutive sorption and desorption cycles. Therefore, they concluded that brown macroalga Nizamuddinia zanardinii can be efficiently used as environmentally safe sorbent for the removal of basic dyes from aqueous solution.

Table 2.

Biodegradation of dyes by algal, fungal and yeast biomass

Algal biomass
Name of dye	Algae	Decolorization (%)	Experimental conditions	References
Textile dyes (Indigo)	Phormidium autumnale UTEX1580	98	14 days incubation time	Dellamatrice et al. (2017)
Methylene Blue	C. pyrenoidosa	98.20	1 day incubation time	Lebron et al. (2018)
Methyl orange, Methyl red, Pb2 + ions	Sargassum fluitans	25 68 95	30 min incubation time 30 min incubation time 80 min incubation time	López-Miranda et al. (2020)
Fungal biomass
Acid red 88	Achaetomium strumarium	99	10 mg/L of adye, pH 4, temperature 40 °C, 96 h	Bankole et al. (2018)
Congo Red	Aspergillus niger	97	pH 5, 200 mg/L of dye, 6 days incubation, temperature 28 °C, 120 to 150 rpm	Asses et al. (2018)
Acid Blue 161 Procion Red MX-5B	Aspergillus terreus	100	336 h of treatment, temperature 30 °C	Almeida and Corso (2019)
Direct Blue-1	Aspergillus terreus GS28	98.4	168 h 100 mg/l L, temperature 30 °C	Singh and Dwivedi (2020)
Yeast biomass
Red B (ARB) A detoxification	Pichia occidentalis	98	2.0 g/L glucose, 0.6 g/L ammonium sulfate, 0.08 g/L yeast extract, 30 g/L NaCl, 30 °C, pH 5.0, 16 h	Song et al. (2017)
Congo red dye	Saccharomyces cerevisiae	90.7	40 ppm (0.004 g/100 mL), pH 4, 18 h	Mathivanan et al. (2018)
Acid Scarlet GR	Galactomyces geotrichum GG	92	0.1 g/L of yeast extract, 2.0 g/L of glucose, 0.4 g/L of ammonium sulfate, ≤ 40 g/L of NaCl, pH 7.0–8.0. 100 mg/L of dye, 10 h	Guo et al. (2019)

Chen et al. (2018) studied the removal of malachite green, dye from aqueous solution through pyrolysis using biochar derived from algal biomass, Ulothrix zonata. They found that maximum sorption capacity of 5306.2 mg/g was achieved for malachite green at high temperature, i.e., 308 K. Besides, the study also indicated that adsorption ability of algal derived biochar was several times higher than that of other adsorbents from previous studies. Also, relatively less processing time is required for algal derived biochar as compared to nonbiochar adsorbents. Thus, algal biomass can be a better choice for biochar production as they are not only easily available in diverse habitats, less time consuming, and environmentally safe in comparison to other adsorbents.

The biosorptive capacity of Sargassum muticum algae against methylene blue and lead (II) ions removal from aqueous solution was recently investigated by Hannachi and Hafidh (2020). Their results suggested that very little amount of algal biomass (0.3 g) as an adsorbent gave maximum removal of methylene blue (93.633%) and Pb²⁺ ions (96.129%) against an initial concentration of 35 mg/L and 25 mg/L respectively. Their investigations revealed that biosorption of methylene blue may be due to electrostatic attractions, n-π interactions and hydrogen bonds while chelation reaction may be responsible for lead uptake. Thus algal biomass Sargassum muticum proved to be an ecofriendly and budget friendly biosorbent for treatment of textile wastewater.

El-Mekkawi et al. (2021) investigated biosorbent potential of microalgae, Chlorella vulgaris for the removal of Acid Red 1 dye from aqueous solution simulated to textile dye industry wastewater. Their results revealed that maximum 96% removal efficiency of Acid Red dye was achieved after 1 h at pH 3 by utilizing 15 mg/L initial dye concentration. Further, SEM analysis revealed that the consistent and small homogeneous algal biomass helps to increase the surface area that favors adsorption capacity. Similarly, the long regular fiber bundles having smooth surface of dye-loaded biomass helps in the essential absorption inside the biosorbent particle. Thus, the reusability of dye-loaded algal biomass is an important economical step towards an integrated sustainable solution applicable to treat industrial wastes through a zero-waste process.

Decolorization and degradation by fungi

Filamentous fungi can grow on range of ecological niches like living plants, soil and organic waste because of their speedy adaptation and metabolism on varying nitrogen and carbon sources. Fungi produce a huge quantity of extracellular and intracellular enzymes with degrading capability of many types of organic contaminants, like dye effluents, organic waste, steroid compounds and polyaromatic hydrocarbons and lignocelluloses (Shah and Banerjee 2020). Moreover, fungal biomasses are also composed of different components such as glucans, chitin and proteins etc., that contains various functional groups (hydroxyl, amines and carboxyl). These functional groups provides active sites for uptake of different molecules, thus fungal biomasses have the potential to act as efficient biosorbent (Escudero et al. 2017).

Various reports have been published that showed fungal biomass possess potential to degrade and decolorize wide variety of dyes from textile industry as shown in Table 2. Naskar and Majumder (2017) utilized Aspergillus niger for removal of textile dye acid yellow-99 dye from aqueous medium. Their results revealed that Aspergillus niger is capable of exhibiting highest dye adsorption capacity of 544.30 mg/g with biomass concentration 2 g/L at 30 °C and pH 3. Moreover, 98% removal efficiency of dye was obtained by using 0.1 M NaOH medium. Their findings suggested that rapid and efficient dye adsorption process was attained by the complex electrostatic and intermolecular interactions due to the presence of carboxyl and amine groups onto the surface of biomass.

Drumm et al. (2019) investigated the potentiality of phytopathogenic fungal biomass Phoma sp. for the treatment of colored wastewater containing Acid Red 18 dye. Results obtained from batch experiments suggested that Phoma sp. efficiently decolorize 90% of dye under optimum concentration of 1.25 g/L, pH of 2.0 and 298 K temperature. Moreover, adsorption capacity of 56.6 mg/g was attained in around 180 min. Thus, they concluded that Phoma sp. inactive biomass can be a cheap alternative treatment of colored effluents in both continuous as well as discontinuous biosorption modes.

Riegas-Villalobos et al. 2020 investigated the role of T. versicolor fungal biomass as well as laccase in decolorization of Orange II dye from liquid culture. Biomass and laccase were produced with three different carbon sources viz., Their results suggested that in vivo treatment of fungal biomass and lacasses exhibited more than 84% dye removal against various carbon sources (bran flakes, wheat bran and wheat flour) used in the study. While, on the other hand, in vivo treatment with fungal biomass without laccase enzyme showed 30 to 72% dye removal. Thus, their study concluded that both fungal biomass and laccase enzyme played an influential role in decolorization of Orange II dye.

Bouras et al. 2021 investigated the biosorption characteristics of methylene blue dye from aqueous solutions by two fungal species, viz., Aspergillus carbonarius and Penicillium glabrum. The maximum biosorption capacities of A. carbonarius (21.88 mg/g) and P. glabrum (16.67 mg/g) were attained within 120 min at 30 °C and pH 8.2. Their findings revealed that interactions between dye molecules and fungal biomasses is responsible for adsorption as depicted by adsorption kinetics that fit the pseudo-second-order equation. Moreover, equilibrium is consistent with Langmuir's model. Thus, both fungal biomasses can serve as budget and environmental friendly biosorbents for the removal of methylene blue from aqueous solutions.

Decolorization and degradation by yeasts

Yeasts are eukaryotic, unicellular microorganisms classified as members of the fungus kingdom. Yeast cells are easily cultivated in laboratory by using various growth media and are easily available for the bioaccumulation of pollutants from industrial waste water even at lower pH values. Yeast cultures generally produces higher biomass production per unit time in comparison to fungi and bacteria. Besides, yeast cultures play a very important role in dye degradation process by providing carbon substrate for growth, vitamins, and additional compounds that may serve as electron shuttles (Imran et al. 2016). Table 2 shows degradation of different dyes using yeast.

Bankole et al. (2017) characterized newly isolated yeast, Diutina rugosa for decolorization of indigo dye. Their results revealed that almost complete decolorization of indigo dye with 99.97% dye removal was attained after 5 days with 10 mg/L dye concentration, pH 2 and temperature 30 °C. Their findings also suggested that yeast exhibited enzyme-mediated system that resulted in degradation of indigo dye with the production of intermediate metabolites, viz., 2-dihydro-3H-indol-3-one and cyclopentanone. In addition, Langmuir’s and Temkin’s isotherm studies confirm monolayer and heterogeneous biosorption of Diutina rugosa that is dependent on dye concentrations and temperature. Thus, they concluded that Diutina rugosa can efficiently be utilized as an environment and budget-friendly alternative to remove indigoid dyes from contaminated environment.

Contreras et al. (2019) isolated yeast strain Galactomyces geotrichum KL20A from Colombian natural fermented milk known as Kumis and investigated its potential for decolorization of Methylene Blue. Their findings suggested that Galactomyces geotrichum KL20A was able to decolorize 76.6% within 48 h, at 35 °C and with a dye concentration of 50 ppm. Besides, pseudo-first-order model revealed a rate constant of 2.2 × 10 − 2/h and a half time for biotransformation of 31.2 h. In addition, cytotoxicity assay based on the hemolytic reaction was also performed. Their results showed that by-products generated as a result of bioremoval process showed 22% hemolysis in comparison to hemolytic activity (100%) of the negative control. Thus, it was inferred that Galactomyces geotrichum KL20A and G. geotrichum KL20 is efficient enough to remove significant amounts of methylene blue from wastewater effluents as well as substantial reduction of the cytotoxicity to human erythrocytes.

Al-Tohamy et al. (2020) isolated novel yeast strain Sterigmatomyces halophilus SSA-1575 from the gut of a wood-feeding termite Reticulitermes chinensis. This novel strain was subjected to trace decolorizing and detoxifying potential of Reactive Black 5 dye (RB5). According to their results, Sterigmatomyces halophilus SSA-1575 showed optimal decolorizing performance of more than 95% within 24 h at 30 °C, pH and optimum dye concentration of 50 mg/L. Two important enzymes, viz., NADH-dichlorophenol indophenol reductase (NADH-DCIP) and lignin peroxidase (LiP) produced by S. halophilus were found to be responsible for the decolorization process. Moreover, it was observed that decolorization process was enhanced with the addition of various carbon and energy sources such as glucose, yeast extract, sulfate and ammonium. Finally, toxicity assay was performed to evaluate the safety of its metabolic intermediate products after decolorization of RB5. Their finding showed that after decolorization, inhibition ratios of 25 and 50 mg/L of dye were reduced from 69 to 7% and 85% to 9% 50 mg/L dye respectively against V. fischeri after 30 min. Thus they concluded that the yeast strain, S. halophilus SSA-1575 is ecofriendly alternate in bioremediation of toxic pollutants and textile dye wastewater.

Saravanan et al. 2021 investigated a comparison on the accumulation of two dyes, viz., Reactive Red 11 and Acid Green 1 dyes using live yeast culture Pichia pastoris. According to their results, pH 2 was found to be optimum where maximum bioaccumulation concentrations, color removal efficiency and uptake of dye took place at 30 °C, the uptake efficiency for both the dyes was optimal. While, maximum dried biomass concentration took place at pH 3. However, dye concentration was found to be inversely related to color removal efficiency, growth rate and concentrations of dried biomass. On the other hand, the capacity of dye uptake was found to be directly related to the initial concentration of dye. In conclusion, P. pastoris was found to be more suitable for accumulation of Acid Green 1 dyes in comparison to Reactive Red 11.

Decolorization and degradation by bacteria

Bacteria possess a high degree of biodegradation and mineralization as well as they exhibit huge diversity towards a variety of azo dye and have extra advantage of being environmentally friendly and less sludge production (Saratale et al. 2011). In general, discoloration and degradation of azo dyes occur under anaerobic, anoxic and aerobic condition by a variety of bacterial species. The basic mechanism in the degradation of azo dyes is the reductive cleavage of azo bond (–N=N) with the help of azo reductases in the absence of oxygen (Solís et al. 2012). It results in the formation colorless solution containing aromatic amines, which are potentially hazardous and are carcinogenic. However, these intermediate metabolites can be degraded aerobically or anaerobically. Table 3 showed the use of variety of bacterial species for degradation of wide range of dyes.

Table 3.

Biodegradation of dyes by bacterial isolates

Name of dye	Dye degrading bacteria	Decolorization (%)	Optimal condition	References
Reactive Red 239	Bacillus sp. strain CH12	100	pH 10, 30 °C, 24 h	Guadie et al. (2017)
Sudan dye	Shewanella putrefaciens CN32	90.23	6 °C, initial pH 7.0–8.0 and NaCl concentrations 0–20 g/L, 108 h	Liu et al. (2018)
Congo red	Brevibacillus parabrevis	95.71	Temperature: 30 °C, Time 6 days	Talha et al. (2018)
Orange M2R	Bacillus farraginis	98	pH7.0, initial dye concentration 1%, temperature 37 °C	Siddique and Alif (2018)
Reactive Red 198	Enterococcus faecalis-Klebsiella variicola bacterial consortium	99.26	pH 8.0, 37 °C, 72 h of incubation	Eslami et al. (2019)
Methyl orange	M. yunnanensis	95	pH 7, temperature 30 °C, dye concentration 100 mg/L, 1% glucose and 1% peptone	Carolin et al. (2021)

Bacterial decolorization and degradation under aerobic and anaerobic conditions

Several bacterial species have been isolated and characterized that can degrade the azo dyes aerobically. Such strains normally require an organic carbon source because they cannot use dyes as the growth substrate. These bacteria are capable of cleaving –N=N– bonds and utilize amines for their growth, for instance, Pigmentiphaga kullae K24 and Xenophilus azovorans KF 46 Oxidoreductive enzymes are present in aerobic bacteria that helps in breakdown of dye molecules symmetrically or asymmetrically. They could also bring about deamination, desulfonation, hydroxylation, etc. (Jamee and Siddique 2019).

Under anaerobic conditions, methanogens carry out the degradation of aromatic amines. Methanogenesis is brought about by a coordinated participation of acidogenic, acetogenic and methanogenic bacteria (Khan et al. 2013). Reduction of azo dyes under anaerobic conditions seems to be non-specific and most of the azo groups are decolorized. The rate of the decolorization is independent of the molecular weight of the azo dye, indicating that decolorization is not a cell specific process and cell permeability has no effect (Khandare and Govindwar 2015). The enzyme azoreductase is responsible for the degradation of azo dyes under anaerobic conditions, whereas Nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH) serves as the reducing agents. During this process of degradation, intermediates that are formed can be degraded either aerobically or anaerobically.

Decolorization using pure bacterial culture and consortium of bacteria

Decolorization of textile effluent containing azo dyes was studied previously by using pure bacterial cultures, the most prominent being Pseudomonas luteola, Bacillus cereus, Bacillus subtilis and Aeromonas hydrophila, Pseudomonas sp., SUK1, and Proteus mirabilis, Proteus mirabilis, Pseudomonas luteola and Rhizobium radiobacter (Madhushika et al. 2019; Joshi et al. 2020; Shantkriti and Senthil 2021). The capacity and efficiency of the selected pure strain depend on its adaptability and activity There are many advantages of using pure culture over mixed culture, like pure strains produce a reproducible data so it becomes easier to interpret and explain the observations of the experimental results. In pure strains, it is easy to find the mechanism of the degradation, easy to implement tools of biochemistry and molecular biology on pure cultures. In order to produce modified strains with better enzyme activity, the knowledge of pure strain is necessary.

Normally the rate of bacterial decolorization is higher than fungal decolorization. This is because the multiplication or growth rate of bacteria is higher than fungi. However the pure strains of bacteria alone cannot mineralize the azo compounds, rather they result in the formation of intermediates which are more toxic than the dyes itself. The metabolic intermediates are actually aromatic mines, which are the result of reductive cleavage of Azo bond (–N=N–). Thus, these aromatic amines need immediate further decomposition. So the treatment process containing a mixed population of different bacterial strains shows a higher and efficient rate of decolorization. In addition, these activities, of complex microbial communities cause the complete mineralization of the azo dye. This is due to the fact that in synergistic microbial systems the different microorganisms attack on the different bonds of the same dye molecule (Saratale et al. 2011). For example, in general the azo dyes are degraded into aromatic amines by one type bacterial strain. The same bacterial strains are mostly unable to further degrade. So it will retard the mineralization process. While in case of consortium the aromatic amines produced by one kind of bacteria are immediately taken up by another group of bacteria. Hence it results in a faster and efficient rate of decolorization. Furthermore, pure strains require a longer time to adapt in the hostile environment, hard to isolate, characterize and gives a slower rate of decolorization.

For a large number of pollutants, microbial consortia have been used for bioremediation for the lab as well as at industrial scale. In general it is accepted that for biodegradation, consortia has a better efficiency for biodegradation than pure strains. The reason is that in case of consortia broader enzymatic capacity is present and toxic metabolites are used by co metabolism and in consortia bacteria express their full capacity to degrade pollutant in optimal conditions. However, various environmental factors such as pH, temperature and presence or absence of oxygen affect the growth of the bacteria. Consortium of bacterial cultures immobilized on the solid surface was employed for the degradation of a mixture of sulphonated azo dyes such as Acid Orange 7 & Acid Red 88. For the efficient treatment of textile effluent, a combined system of plant and bacteria was developed. The consortia of Glandularia pulchella (sweet) Tronc., P. monteilli ANK revealed a 100% decolorizing efficiency for a mixture of dyes (Kabra et al. 2013).

Conclusions

There is no doubt in the toxic and recalcitrant nature of textile effluents that generate disastrous effects on natural aquatic ecosystems. To mitigate the harmful effects of these toxic textile effluents, biological approaches are critically useful and effective as they involve natural processes of plants, bacteria, extremophiles and fungi biomasses to decolor or even mineralize the textile dyes. Based on a comprehensive literature survey, it was concluded that there are some certainties achieved for the treatment of textile waste with biological approaches with some challenges needed to be overcome. It has been established that the microbial communities are effective for biodegradation of complex compounds. It is vital to determine the efficiency of the applied treatments with the use of combined toxicity assays. Both aerobic and anaerobic treatment systems are highly effective for industrial wastewater treatment except for those with bleaching kraft effluents. Such effluents are more toxic to anaerobic bacteria that is why their treatment by anaerobic means was less effective than aerobic biological techniques. Due to presence of high residual COD, a further treatment is prerequisite for anaerobic treatment of high strength wastewater. Fungal treatment is very effective in decolorization of dyes in conjunction with coagulation, chemical oxidation, and ozonation. However, the development of integrated treatments through combined physicochemical and biological treatment processes with process optimization allowed to not only to reduce dye concentration and COD, but also for reducing pH, BOD and toxicity. Therefore, to promote sustainable development, further research work should involve interdisciplinary approach, where scientific evidence, engineering dimensions are considered.

Funding

This research receives no external funding.

Declarations

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.