Enzymatic Bioremediation of Dyes from Textile Industry Effluents

Abstract

The use of synthetic dyes began in 1865 with the discoveries of researcher William Henry Perkin. Its production and use only grew due to the high demand of several industrial sectors, mainly textiles. At the same time, concerns about environmental problems arose due to the disposal of wastewater with dyes, being the textile industry’s effluents the most polluting in the world. According to their structure, dyes can be more or less harmful, whereby azo dyes are the most worrisome from an environmental point of view. Problems, such as carcinogenicity, mutagenicity, and genotoxicity, are related to dyes, as well as contamination of water, and soil, and damages to agricultural plantations. Some of the methods used in the treatment of textile industrial effluents are membrane filtration, coagulation, chemical oxidation, biodegradation, photocatalytic degradation, phytoremediation, and enzymatic remediation. Enzyme remediation is considered an efficient, ecological, and innovative technique, through which enzymes can be used in free or immobilized form. The main enzymes involved in the degradation of azo dyes are azoreductases, laccases, and peroxidases. In some cases, harmful by-products are formed during the reactions and require proper management. Thus, this chapter addresses the main aspects of enzymatic bioremediation of dyes present in effluents from the textile industry.

1. Introduction

The history of dyes began over 4000 years ago, and for many years, dyes were extracted from natural sources, such as flowers, vegetables, wood, insects, and roots, among others [1]. The synthetic dye industry began with the synthesis of mauveine, by researcher William Henry Perkin, in 1865. This dye, which until then was extracted from coal tar, was synthesized by Perkin while the researcher was looking for a new synthetic route for quinine, a drug used to treat malaria [2]. Perkin’s discovery marked the creation of a new generation of dyes [3].

Synthetic dyes are organic compounds that are produced from raw materials of petrochemical origin. Such compounds may or may not be soluble in water, are generally easily absorbed, and quickly impart color to substrates [1]. Structurally, dyes contain three essential groups: the chromophore, which is the active site of dyes where atoms interacting with visible electromagnetic radiation are located [2]; auxochrome, which has functional groups that introduce the chromophore, increase the fiber’s affinity to color, and decrease its solubility in water [4] and conjugated aromatic structures, such as benzene, anthracene and perylene rings [2]. Dyes are classified according to their chemical structure and application mode. Thus, according to the chemical structure of the dye, this is classified into azo, anthraquinone, sulfur, phthalocyanine, and triarylmethane [2]. Depending on its method of application, the dye is classified as reactive, direct, dispersed, basic, and by vat dyeing [5].

The chemical composition of the dye reflects in its pigmentation (formation of its color), being also responsible for the lighter or darker tone of each dye. The coloring is due to the absorption of light of a certain wavelength in the visible range of the electromagnetic spectrum, that is, the dye is a molecule capable of absorbing certain light radiations and then reflecting the complementary colors [6]. Table 1 brings together the main classes of dyes used in the textile industry, the types of fiber or substrates to which the dyes of each class are applied, the types of interaction between dye and fiber or substrate, and the methods of application or dyeing.

Classes	Fiber type	Interaction between dye and fiber	Method of application
Acid dye	nylon, wool, silk	Electrostatics; hydrogen bond	Neutral to acid dye baths.
Basic dye	modified nylon, polyester	Electrostatics	Acid baths.
Direct dye	cotton, rayon, leather, nylon	Intermolecular forces.	Neutral or slightly alkaline baths containing additional electrolytes.
Dispersed dye	polyester, polyamide, acetate, plastic, acrylic	Hydrophobic - solid-state mechanism	High or low-temperature pressure transport methods.
Reactive dye	cotton, nylon, silk, wool	Covalent bond	Under the influence of heat and pH of the medium, which must be alkaline, the dye reacts with the fiber functional group, with which it covalently bonds.
Sulfur dye	cotton, rayon	Covalent bond	Aromatic substrate covered with sodium sulfide and reoxidized to sulfur-containing products, insoluble in fiber.
Vat dye	cotton, rayon	Impregnation and oxidation	Water-insoluble dyes are solubilized by reduction with sodium hydrosulfite and then exhausted into the fiber and reoxidized.

Table 1.

Main classes of dyes used in the textile industry, types of fiber to which the dyes of each class are applied, types of interaction between dye and fiber, and methods of application or dyeing [7].

Dyes are materials of great importance in different industrial sectors, such as fabric production, papermaking, plastics, cosmetics, as well as in medicine and biology [8]. Currently, the world production of dyes is about 800 tons a year and most of the dyes produced, about 70 million tons a year, are used in the textile industry [1].

With high world production, the textile industry occupies the second place among the industrial sectors that most pollute since during the dyeing stage a large amount of dyes is released into the environment due to the nonadhesion of the dye to the substrate to be dyed [2]. Therefore, the search for economically viable and ecologically sustainable alternatives for the treatment of effluents containing textile dyes is of extreme importance and interest, whereupon bioremediation is a process that can help to solve this industrial problem.

This chapter brings together the main and most recent information reported in the scientific literature on the enzymatic bioremediation of dyes from textile industry effluents. In this context, the negative impacts of dyes used in this industrial segment on human and animal health are discussed, as well as methods conventionally used for the treatment of industrial effluents containing dyes, the principles of enzymatic bioremediation, the enzymes used in this process, and their by-products.

2. Negative impacts of textile dyes on human and animal health

Textile industry effluents are considered the most polluting compounds both by the volume generated and discarded and by their toxicity [9]. Wastewater from the textile industry is estimated to contain between 10 and 200 mg L⁻¹ of dyes, as well as other organic chemicals, inorganic compounds, and additives. Even after the treatment of such effluents, about 90% of the dyes are still dumped in water bodies without undergoing chemical changes [1]. The biodegradation of such dyes is hampered by their xenobiotic nature, aromatic structure, high thermal resistance, and photostability [4].

In recent studies, Gita et al. [9] have observed that the toxicity of dyes is generally low for mammals and aquatic organisms, however, secondary products formed by biodegradation, especially aromatic amines from anaerobic dye reduction, can be harmful. In addition, these authors found that the concomitant presence of dyes and other pollutants in textile wastewater, such as heavy metals, can have a synergistic effect, causing considerable damage to the aquatic environment.

The main concern about the discharge of dyes is the presence of genotoxic, mutagenic, teratogenic, and carcinogenic effects, observed in animal studies [9]. Carcinogenicity is related to the formation of ions that bind to DNA and RNA, causing mutations and leading to the formation of tumors. In this sense, benzidine and 2-naphthylamine dyes are associated with a high incidence of bladder cancer [10]. Azure-B dye is capable of interspersing in the helical structure of the DNA and may have cytotoxic effects since it is an inhibitor of monoamine oxidase A (MAO-A), an enzyme that acts on the central nervous system and is important to human behavior [10]. Sudan 1 dye, widely used in the textile industry, although illegal in many European countries and the US, is also used in foods, such as paprika. Such dye, when present in the body of humans and animals, is transformed by the action of enzymes in carcinogenic aromatic amines [10]. Furthermore, human exposure to dyes can still generate skin and lung irritations, headaches, congenital malformation, and nausea [11].

Triphenylmethane dyes are phytotoxic to agricultural plantations, cytotoxic to mammals, and generate tumors in several fish species [10]. The violet crystal dye is also a powerful carcinogen, capable of inducing tumors in fish, such as hepatocellular carcinoma and reticular cell sarcoma in several organs [10].

Some of the main environmental problems related to the disposal of synthetic dyes are—i. contamination of surface water, which leads to decreased penetration of light, with damage to photosynthesis and consequent oxygen deficiency; ii. accumulation of nonbiodegradable organic dyes along with the food chain; iii. Soil and water contamination, and iv. inhibition of growth and development of various crops of agricultural interest [4].

In the literature, a correlation is described between the increase in the concentration of dyes and the decrease in the growth of microalgae, reaching the total suppression of their growth [9]. In that study, different concentrations of three dyes were used to evaluate the specific growth rate of green algae Chlorella vulgaris exposed to dyes. Such findings are important because the inhibition of microalgae growth causes disturbances in the trophic transfer of energy and nutrients in aquatic environments [4].

Aquatic macrophytes are used as natural ecological markers to quantify the phytotoxicity of textile dyes when exposed to effluents that contain those since there is a change in all their parameters [4]. In the presence of two textile dyes, Lemna giba, an aquatic macrophyte, had its growth rate and photosynthetic pigment content decreased. The authors of the study concluded that this species can be used as a bioindicator of polluting dyes [12]. High concentrations of dyes are reported to decrease vital elements, such as P, Mg, Ca, S, and Ca in plants of Eichhornia crassipes and Salvinia natans, which also presented damaged roots, chlorosis, and necrosis in leaves [13].

Among thousands of dyes studied, found in effluents, more than 100 have the potential to form carcinogenic amines. However, these potentially toxic dyes are still marketed and used, especially in small textile factories. In several places around the world, the demands of export and cheap labor sustain the existence of factories with a small-scale activity that clandestinely releases toxic dyes into water bodies [10].

2.1 Textile industry effluents: Composition and conventional methods of treatment

Textile industry effluents contain large quantities of biodegradable organic compounds and nonbiodegradable compounds [14]. According to the literature, there are more than 8000 substances, such as acids, surfactants, salts, metals, oxidizing agents, reducing agents, as well as dyes and their auxiliaries [15]. Wastewater from the textile industry contains characteristic color, resulting from the mixture of dyes, in addition to the presence of metals, organic carbon, ammonium salts, nitrate, and orthophosphate [5].

Due to the environmental impact of this type of effluent, pretreatment is necessary before such compounds are released into natural water bodies, and the textile industry shows interest in controlling this problem [14]. However, even after treatment, effluents are still discarded in rivers with up to 90% of dyes that have not undergone chemical changes [1]. Table 2 shows information related to the studied treatment processes for the removal of textile dyes from industrial effluents and the main results obtained, as reported in the literature.

Name of dyes	Treatment Method	Main Results	Reference
Reactive Yellow 138, Reactive Red 231, and Navy HEXL® Procion	Electrolysis, carried out in a filter-press cell, under galvanostatic conditions.	Complete discoloration (99%) was observed in all cases.	[14]
Reactive Red 120	Biodegradation and dye biosorption by Pseudomonas guariconensis.	The immobilized VITSAJ5 bacterium exhibited maximum adsorption of 87%.; There was only 37% of removal without immobilization of the microorganism.	[15]
Malachite Green, Reactive Red 198, and Direct Yellow 31	Chitosan adsorption.	The amount of dye adsorbed depends on the mass of the adsorbent and decreased with its increase.	[16]
Basic Blue 9 (MB), Basic Green 4 (MG), and Acid Orange 52 (MO)	Adsorption using synthesized materials	Fast adsorption of MB, MG, and MO in the initial 60 min.; After 240 min, adsorption equilibrium is reached.	[17]
Basic Blue 26 (BB26), Basic Green 1 (BG1), Basic Yellow 2 (BY2), and Basic Red 1 (BR1)	Adsorption on carbonaceous materials (acai seeds and Brazil nut shells), activated in the following ways: chemical activation with H3PO4, heat treatment, and oxidation with HNO3.	The adsorbents activated by heat treatment showed good performance for the removal of BB26 (87 and 85%) and BG1 (100 and 99%) but were not efficient for the removal of BY2 and BR1.; Chemical activation was the most efficient for all dyes tested.; Oxidation with HNO3 showed the worst results.	[18]
Diamine Green B (DG-B), Acid Black 24 (AB-24), and Congo Red (CR)	Cellulose adsorption on cationized rice husk (CRHC).	Maximum adsorption capacities of DG-B, AB-24, and CR: 207.15, 268.88, and 580.09 mg g^−1 at pH = 8, respectively, following the order CR > DG-B > AB-24.	[19]
Methylene Blue (MB)	Photocatalytic degradation of organic dyes with nanocomposites	Synthesized nanocompounds showed high catalytic activity for the reduction of methylene blue under solar irradiation, efficiency of up to 90.1%, simple and low-cost method.	[20]
Basic Yellow 28 (BY28), Acid Brown 75 (AB75)	Adsorption of cationic and anionic dyes by natural clays rich in smectite.	BY28: removal efficiency increased (97%) with increasing pH.; AB75 anionic dye: adsorption was high in acidic medium (86%).	[21]
Reactive Violet 5 (RV5)	Decolorization of azo-reactive dyes using sequential chemical treatment and activated sludge.	Almost complete decolorization was obtained for dye concentrations up to 300 mg L^−1.; Fenton’s reagent was unable to decolorize at concentration ≥ 500 mg L^−1 (87.4% dechlorination).	[22]
Procion Red HE-3B (RR120)	Photoelectrocatalysis	Treatment proved to be efficient, with up to 100% of decolorization in 30 min, concentration 10 mg L^−1 of the dye RR120.; The efficiency is only effective at low concentrations, with increasing concentration the decolorization occurs to a certain extent, then stabilizes.	[23]
Reactive Red 120	Simultaneous adsorption, filtration, and photoelectrocatalytic oxidation processes	The simultaneous performance of the treatments demonstrated that the dye was completely removed in solution.; No pretreatment of intermediate by-products was necessary.	[24]
Acid Blue 25	Adsorption	The absorbent material was shown to reach an equilibrium constant in 270 min, as was observed to reduce absorption with alkaline solutions.; The mortality rate of Daphnia similis reduced from 50–10% using 30 mg of quaternary chitosan granules when compared to the control.	[25]
Acid Blue 25	Adsorption Chitosan beads (CB) and chitosan beads with immobilized Saccharomyces cerevisiae by zeta potential (CBY)	The adsorbent with immobilized S. cerevisiae reached equilibrium faster than the chitosan polymer alone.; The adsorption capacity increased in both treatments with acidification, and also varied with temperature.; There was a significant decrease in toxicity with the CBY treatment.	[26]

Table 2.

Examples of treatment processes used to remove textile dyes.

The composition, as well as the standards allowed for each substance present in the composition of effluents from textile factories, aiming at its release in surface water bodies, vary according to the standards of each country. In China, the chemical oxygen demand (COD) and chrominance of wastewater from dyeing and finishing processes cannot exceed 80 mg L⁻¹ and 60, respectively, so that such effluents can be released into the environment. In the United States, according to the Environmental Protection Agency (EPA), the limit value for COD is 163 kg per ton of fabric, however, in practice, cod effluents are up to 15 times higher than the legal standard [27]. Therefore, it is essential to apply efficient treatment strategies that ensure the complete removal of pollutants or that ensure the sustainability of the environment for future generations through physical, chemical, and biological technologies or a combination of them [10].

Physical methods, such as membrane filtration (nanofiltration, reverse osmosis, electrodialysis), sorption techniques, or chemical methods, such as coagulation or flocculation combined with flotation and filtration, flocculation by precipitation, electroflotation, and electrokinetic coagulation, considered for the removal of various dyes, do not degrade them. Such methods simply promote the reduction of the concentration of dyes, converting them from one chemical way to another, thus creating secondary pollution [6]. Among the several processes used for the removal of wastewater dyes, such as chemical oxidation, biodegradation, electrochemical treatment, adsorption, and photocatalytic degradation, the use of photocatalyst provides good results with high efficiency, low cost, speed, and better performance in environmental conditions when sunlight is used in the process [28].

Several natural materials, such as chitosan, are used in physical dye adsorption processes. Chitosan is a modified natural biopolymer, derived from the deacetylation of chitin, which is the most abundant polymer on the planet, derived from important biomass produced by inferior plants and animals, such as arthropods, shells of crustaceans, lobsters, shrimps, crabs, and squid [16]. Adsorption is one of the most efficient methods for removing dyes, however, there is a need for further treatment of the residue resulting from the process.

In addition to the physical and chemical processes aimed at the removal of dyes from wastewater, biological processes also play an important role. Among the biological methods that can be used to remove dyes from industrial wastewater, phytoremediation is a process that has advantages compared to chemical and physical methods of removal. The removal of textile dyes by plants occurs by adsorption, accumulation, and subsequent degradation, mediated by enzymes [29].

In situations where the application of chemical products must be continuous, the use of microorganisms may be considered a simpler and low-cost process, since microorganisms can be added only once in the effluent to be treated, as they have the potential to multiply [30]. Within this context, the activated sludge is commonly used in bioreactors for effluent treatment, which is one of the most used processes by the textile industry [10]. Another possible biological method for the treatment of effluents is the use of bacterial cultures. The isolation of pure cultures from textile wastewater is usually not performed, as it can be a slow and laborious process. Thus, mixed bacterial cultures are commonly used, which, due to cooperation to achieve a potentiated effect, provide better results in discoloration and mineralization of toxic aromatic amines [1].

3. Principles of enzymatic bioremediation

Bioremediation techniques have been gaining increasing prominence worldwide due to high public acceptance, low cost compared to conventional remediation methods, high availability of enzymes, and minimal impact on the environment [31]. The exploration of enzymes for bioremediation has been of great interest due to their ability to function in wider ranges of pH and temperature, in the presence of contaminants and saline concentrations [32]. Enzymatic bioremediation is an ecological, economical, promising, and innovative technique. The process consists of exploring the typical characteristics of microorganisms or genetically modified organisms capable of producing specific enzymes to catalyze or metabolize the pollutant, transforming the toxic form into a nontoxic form and sometimes into new products [33].

Among the enzymes involved in bioremediation processes are laccases, dehalogenases, and hydrolases. Laccases are enzymes capable of catalyzing the oxidation of phenolic compounds, aromatic amines, and their compounds. Dehalogenases degrade a wide range of halogenated compounds by cleaving C – X bonds (X = halogen atom, such as Cl). Hydrolases break chemical bonds using water and convert larger molecules into smaller molecules, decreasing their toxicity. These enzymes facilitate the cleavage of C – C, C – O, C – N, S – S, S – N, S – P, C – P bonds [33].

Enzymes can be used in free or immobilized form, the latter having the following advantages—long-term operational stability, easy recovery, and reuse in industrial applications, which improve process performance and lower overall cost [34]. Immobilization consists of coupling the enzyme with an insoluble support matrix to maintain an adequate geometry, which guarantees greater stability to the enzyme [32]. The bioremediation process using microbial enzymes can be slow and so far, only a few bacterial species have been able to produce enzymes with potent biodegradation capacity. Thus, the use of genetically modified organisms is more common due to their ability to produce large amounts of enzymes under optimized conditions [33].

Enzymes from aerobic bacteria, such as Pseudomonas, Alcaligenes, Sphingomonas, Rhodococcus, and Mycobacterium, are often used in the bioremediation of pesticides and hydrocarbons, while those produced by anaerobic bacteria are more used in bioremediation of polychlorinated biphenyls (PCBs), trichloroethylene (TCE) decolorization, and chloroform. The main enzymes used in bioremediation processes include those of the cytochrome P450 family, laccases, hydrolases, dehalogenases, dehydrogenases, proteases, and lipases [33]. Fungi can also biodegrade, generally mediated by enzymes, such as azoreductases, lignin peroxidases, manganese peroxidases, and laccases. White rot fungi, for example, are capable of degrading textile dyes through peroxidases and laccases [10].

In the treatment of effluents from the textile industry, enzymes act on the dyes, generating precipitates that can be easily removed or chemically transformed into easy-to-treat compounds [35]. The rate of dye degradation by enzymes will depend on the chemical structure of the dye, salt content, the concentration of metal ions, pH, and temperature of the wastewater [36]. The enzymatic degradation of pollutants in textile effluents has several advantages, such as specificity and selectivity to the substrate, in addition to being an accessible, efficient method that meets the principles of green chemistry [37]. The requirement of large amounts of enzyme, high cost, thermal instability, inhibition of enzymatic activity, attack of certain enzymes by proteases, and the formation of undesirable by-products are the main difficulties or challenges related to the use of enzymatic degradation for wastewater treatment [30].

Some of the problems listed can be solved, at least partially, by immobilizing effective enzymes in low-cost matrices, leading to their separation and reuse, in addition to application in continuous bioreactors [30]. To control the reactions in the biodegradation process, the use of enzymes is often more advantageous than the use of cells [37]. As for the high cost of the enzymes themselves due to the fact of trying to obtain an enzymatic solution as pure as possible, the tendency is that it will decrease as technologies and techniques advance and the exploration of cheaper growth substrates for the reproduction of microorganisms increases.

3.1 Main enzymes used in the bioremediation of textile dyes and toxicity of degradation by-products

Enzyme-mediated bioremediation has gained notoriety due to its versatility and efficiency in the degradation of persistent organic pollutants, thus being applied in industrial, biotechnological, and environmental processes [38]. These enzymes can be obtained from the extraction of intracellular and extracellular metabolites from cultures of certain species of bacteria, fungi, algae, and plants [39].

Study objective(s)	Results and by-products of degradation	Reference
Use of ionic liquids (ILs) with surfactant characteristics in the degradation of Indigo Carmine (IC) dye by laccase.	Rapid and significantly higher discoloration of the IC dye in 0.5 h.; Color removal percentage: 82% (against 6% obtained without ionic liquids).; By-products from IC oxidation induced by laccase: indole-2,3-dione, which is decomposed into aminobenzoic acid. Both are less toxic than the IC.	[38]
Use of the isolate of Oudemansiella canarii to produce laccase and evaluation of its potential in the degradation of Congo Red (CR).	The O. canarii laccase was efficient in decolorizing the red of the dye.; Accumulation of various intermediates during degradation as naphthalene derivatives, for example. These products are less toxic than CR.	[40]
Validation of a novel bioinformatics amalgamation and bacterial remediation approach using non-native strains for decolorization and degradation of azo dyes: Drimaren Red CL-5B (Reactive Red 195).	The gas chromatography–mass spectrometry (GC–MS) analysis of the degradation products indicated the formation of low molecular weight metabolites, confirming the dye degradation.; Need to carry out microbial toxicity, cytotoxicity, and phytotoxicity tests before large-scale bioremediation.	[41]
Development of an airlift bioreactor for the use of copper alginate laccase in the degradation of dyes: Indigo Carmine (IC), Remazol Brilliant Blue R (RBBR), Bromophenol Blue (BB), Crystal Violet (CV), Malachite Green (MG), Congo Red (CR), Direct Blue 15 (DB) and Direct Red 23 (DR).	100% decolorization of IC and RBBR, quickly.; Discoloration percentages of MG, BB, and CV: 82; 64.4, and 48.5%; respectively.; Percentages of discoloration of azo dyes CR, BD, and DR: 64, 54, and 22%, respectively.; Isatin sulfonic acid was confirmed as the main degradation product.	[36]
Development of a hydrogel blended with an agarose-chitosan polymer for plant-based horseradish peroxidase (HRP) immobilization and its use in the degradation of synthetic textile dye RB-19.	During the degradation process, the chromophore was fragmented into respective smaller fractions, leading to discoloration.; The RB-19 has degraded into its possible daughter compounds.; There is no result of toxicity studies of these compounds.	[42]
Use of a packed bed reactor equipped with polyacrylamide gel-immobilized horseradish peroxidase (PAG-HRP) for the purpose of sequentially degrading the Methyl Orange (MO) dye.	PAG-HRP biocatalytic system: efficient in biologically based degradation.; The MO degradation efficiency was 93.5% at pH 6.; Significant reduction in the toxicity of azo textile dyes according to the results of acute toxicity bioassays together with phytotoxicity.	[43]
Study the potential of Aspergillus niger for detoxification and discoloration of Congo Red (CR) dye.	High CR removal (85%).; 97% of discoloration results from the combination of two processes: adsorption and enzymatic biodegradation.; Detoxification by A. niger indicates degradation of amines in solution.; According to phytotoxicity and microtoxicity analysis results, the metabolites generated after the CR biodegradation are less toxic than the crude dye.	[44]
Evaluate the performance of a new Meyerozyma guilliermondii, Yarrowia sp. and Sterigmatomyces halophilus (MG-Y-SH) oleaginous yeast consortium in the decolorization and detoxification of textile dyes Reactive Black 5 (RB5), Reactive Red 120 (RR120), Reactive Blue 19 (RB19), Reactive Green 19 (RG19), Blue Remazol R (RBBR), Bromophenol Blue (BPB), Azure B (AB), Methylene Blue (MB), Methyl Red (MR), Malachite Green (MG), Congo Red (CR) and Scarlet GR (SGR).	Maximum decolorization efficiency: ranged between 55.81 (blend III) and 80.56% (blend VI) in 24 h of treatment with MG-Y-SH at 18°C and static conditions.; Maximum decolorization efficiency by MG-Y-SH reached 100% for 100 mg L^−1 of RR120 in 3 h.; Phytotoxicity results indicate the ability of MG-Y-SH to convert the toxic azo dye RR120 into non-toxic metabolites.	[45]
Test a new consortium of oleaginous yeasts that produce lipase and xylanase in the removal of Sigma-Aldrich, Reactive Black 5 (RBB), Reactive Green 19 (G19R), Reactive Red 120 (HE3B), Reactive Blue 19 (B19R), Reactive Violet 5 (V5R) and Reactive Orange 16 (O3R) textile dyes.	Discoloration rate obtained by the Yarrowia sp., Barnettozyma californica, and S. halophilus (Y-BC-SH) consortium: higher than that of pure yeast cultures.; Phytotoxicity assay results: metabolites generated after biodegradation of RBB are less toxic when compared to the original dye.	[46]
Examine Methylene Blue (MB) dye removal performance by an immobilized enzyme.	The immobilized enzyme showed the highest removal efficiency (99%) compared to the pure nanocarrier and the free enzyme (81 and 36% removal, respectively).; No result of toxicity analysis of by-products was presented.	[47]
Evaluation of a new strain of white-rot fungus, Ceriporia lacerata, of its ability to discolor Congo Red (CR) in a statically open system and the effect of toxicity of degradation products.	The discoloration occurred by the absorption of mycelia and by degradation by manganese peroxidase (MnP) and laccase enzymes.; By-products or intermediates identified: naphthylamine and benzidine (very toxic to an organism).; At 48 h the by-products were more toxic than the original dye, demonstrating the dye can take a long time to become harmless.	[48]
Immobilization of lignin peroxidase (LiP) on Ca-alginate granules, its application in the degradation of dyes, and its potential for reducing the cytotoxicity of Reactive Red 195a (VR), Reactive Blue 21 (AR21), Reactive Blue 19 (AR19); Reactive Yellow 154a (AR154); Sandal-Fix Black CKF.	Discoloration efficiencies: 66, 59, 52, 40, and 48% were observed for VR, AR21, AR19, sandal-fix black CKF, and AR154, respectively with free LiP, which increased to 93, 83, 89, 70, and 80% with immobilized LiP. It was an efficient catalyst for the decolorization and detoxification of synthetic dye solutions.; Results of the hemolytic and brine shrimp lethality tests—they showed that Ca-alginate beads entrapped LiP may be an effective biocatalyst for bioremediation of dye-based textile industry effluents.	[49]
Biochemical characterization of stable azoreductase enzyme from Chromobacterium violaceum and its application in the degradation of Methyl Red, Methyl Orange, and Amaranth dyes present in industrial effluent.	The lower value of the Michaelis–Menten constant (KM) indicates a very high affinity of the three dyes with the azoreductase enzyme.; Azo dye metabolites resulted from the action of enzyme: they had reduced toxicity on fibroblast cell lines (L929) as compared to raw and intact dye.	[50]

Table 3.

Main results of studies on dye bioremediation by enzymes and degradation by-products.

Table 3 shows some studies related to the degradation of dyes by enzymes produced by microorganisms. As it is shown, many of the tested can decolorize the dyes, as well as provide a decrease in their toxicity, as in the case, for example, of horseradish peroxidase, which promotes the decrease in the toxicity of the methyl orange dye.

As reported in the literature, dye-decolorizing microorganisms produce a variety of enzymes, including azoreductase, riboflavin reductase, laccase, peroxidases, NADH-DCIP reductase, tyrosinase, reductase, and aminopyrine N-demethylase, lignin peroxidase, and veratryl alcohol oxidase [39]. Among those enzymes, the main ones responsible for the discoloration of azo dyes are azoreductases, laccases, and peroxidases [35].

Azoreductases are considered the main degradation enzymes produced by bacteria [30]. Such enzymes can be of two types—i. membrane-bound or ii. intracellular, which are thermostable, hydrophilic, and, in general, capable of degrading azo dyes more efficiently [51]. Laccases are copper-dependent and use oxygen to degrade lignin and other aromatic compounds, including textile dyes [32]. Because of their higher redox potential, fungal laccases, when compared to plant or bacterial laccases, are used to treat various xenobiotics, including textile dyes, present in water and soils [40].

Peroxidases also play a role in the degradation of the azo dye and are oxidoreductases, which contain heme. Peroxidases are present in plants, microorganisms, and animals. The mechanism of action of such enzymes is similar to that of laccases, providing the degradation of the dye without the production of toxic by-products [30]. Peroxidases act especially on synthetic dyes, degrading their respective constituents through the oxidative polymerization of phenolic compounds to form insoluble polymers [52]. An association between oxide-reducing enzymes can significantly reduce the toxicity of dyes [39].

Enzymes are proteins easily affected by changes in pH, and small variations in the medium’s pH can result in changes in the ionization phase of the active site and the distribution of charge in the protein structure, possibly affecting its affinity for the substrate [52]. Thus, one of the main challenges of enzymatic treatment is the deactivation of the biocatalyst caused, mainly, by the denaturation of the enzyme, due to the pH of the medium or extreme temperatures, which can alter the conformation of the enzyme’s active site [53]. Despite the many advances in enzymatic engineering, enzymes are still expensive and/or labile and, as a result, the industrial application of enzymes often requires their immobilization in a matrix (support) [54].

It is essential to evaluate the toxicity of effluents containing dyes after they have undergone enzymatic biodegradation, as some degradation products are mutagenic and carcinogenic, which represents a threat to human and animal health [30]. Thus, phytotoxicity tests are widely used and, according to the literature, among the bioindicators considered suitable for the detection of environmental toxicity, Artemia salina and Daphnia magna are cited [43].

Ali et al. [55] performed phytotoxicity studies, whose results indicate that MG-Y-SH can convert the toxic azo dye RR120 into nontoxic metabolites. However, many studies reported in the literature lack further tests to evaluate the by-products of enzymatic dye degradation, as well as the effects of these by-products on the environment.

4. Conclusions

Much of the textile dyes are still discharged into rivers without undergoing chemical changes, even with conventional effluent treatments. Pollution generated by dyes from textile industry effluents is harmful to human and animal health, presenting carcinogenic, genotoxic, mutagenic effects, in addition to having direct effects on the survival of aquatic species, as such dyes can accumulate in the food chain, conferring toxicity to water and soil and interfere with the development of crops of agricultural interest.

A more rigorous inspection of the release of dyes is important given its potential toxicity, as well as the factories that may be clandestinely dumping effluents containing toxic dyes in water bodies, without any treatment. Studies must be carried out to optimize effluent treatment methods, which must be ecological and efficient, making use of new technologies provided by modern science.

Among the methods currently used, photocatalytic degradation presents good results, is cheap, and uses sunlight, a clean source of energy. In addition to this method, there is phytoremediation, considered an ecologically correct process, and enzymatic remediation. The enzymes used in the enzymatic bioremediation of textile industry effluents are mainly azoreductases, laccases, and peroxidases.

Enzymatic bioremediation or even conventional treatment can generate by-products that are equally toxic to the starting compounds. But in some cases, less toxic intermediate compounds are generated, such as those presented in this chapter. Therefore, due importance must be given to these secondary products or by-products, identifying them, quantifying them, and subjecting them to proper handling and treatment.

The key point for the treatment of dyes is to have greater investment by companies to put the results of scientific research into practice. An alternative would be to carry out tests in simulation stations, as if on an industrial scale. In addition, genetic engineering has significantly revolutionized the field of bioremediation, with the possibility of modifying organisms or their metabolites so that they are more efficient in degrading pollutants.

Acknowledgments

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brazil (CAPES) – Finance Code 001. We are grateful to this research funding agency and the Agricultural and Livestock Graduation Program, São Paulo State University (UNESP), School of Agricultural and Veterinarian Sciences (FCAV).

Conflict of interest

The authors declare no conflict of interest.