Various Approaches for the Detoxification of Toxic Dyes in Wastewater

Abstract

Use of dyes as well as colorants in industrial processes has extensively increased. Effluents from various industries such as textile, paint, food, etc. are reported to have a diverse range of colorants. The effluents from these industries are often released into natural water bodies, causing serious water and environmental pollution, to which humans and other species are constantly exposed. Continued changes in climate have also affected water availability for people around the world. Thus, advanced treatments and removal of harmful contaminants from municipal and industrial wastewater are becoming increasingly important. Removal of dyes and colorants from wastewater can be done in a variety of ways, including physical, chemical, and biological treatments. These technologies, however, differ in terms of efficiency, cost, and environmental effect. There are many technological and economic challenges for the wastewater treatment methods currently available. The search for the most suitable strategy for successful degradation or removal of dyes from effluents is an urgent requirement. Previously published research suggests that the use of enzymes for dye removal is a more economic and effective strategy as compared to traditional techniques. Nanoparticles, with their exceptional physicochemical features, have the potential to tackle the problem of wastewater purification in a less energy-intensive way. However, extensive standardization would be a necessity for the use of different nanoparticles. Therefore, intense research in the use of enzymes and nanoparticle-based technologies may provide much needed technological solution for the remediation of a diverse range of dyes from wastewater.

1. Background

Rapid urbanization, shifting consumption patterns, population expansion, and accelerated rates of socioeconomic development have all contributed to increased water pollution in the biosphere, [1]. Dyes, heavy metals, cyanide, toxic organics, nitrogen, phosphorus, phenols, and suspended particles are all prominent causes of pollution in natural water from residential and industrial operations, which are capable of changing the color of water [2]. This pollution load is a significant burden in terms of wastewater management since it not only raises treatment costs but also introduces a variety of chemical and microbiological contaminants into water sources [2].

The colorants are chemical compounds that are capable of imparting color to the materials to which they are applied. These can be used together with various components to either change or contribute color to a product. Colorants are applied in various industries including textiles, paints, plastics, prints, etc. As a result, a massive flow of sewage contaminated with these dyes have been discovered in many sectors [3]. According to several studies, colors at concentrations higher than 1 mg/L have been reported [1,4,5,6]. Different colorants used in the textile industry have an important role in value addition, appearance, and consumer satisfaction. Colorants can be classified into two groups, namely pigments and dyes. Based on the source of origin, dyes can be classified as natural or synthetic. Natural dyes are obtained from plants and other natural sources. However, synthetic dyes are man-made (Figure 1) [7]. Due to their cost-effectiveness in synthesis and great stability, synthetic dyes have seen a significant surge in demand and application in the textile industry [1,2,7].

Dyes polluting water prevent penetration of sunlight and a slower rate of photosynthesis. Furthermore, colorants interfere with gas solubility, and hence, they affect the growth of aquatic vegetation and animal life [8]. There are many reports regarding the carcinogenicity and mutagenicity of several colorants [9,10,11]. The dye Red 3 can cause cancer in animals. Another carcinogen, benzidine, has also been detected in Red 40 and Yellow 5 and 6 [10]. Food dyes Tartazine, Erythrosine, and Carmoisine can cause liver cancer [11]. Water pollution caused by both natural and synthetic dyes has become a crucial issue for human health and poses serious ecological hazards. The unchecked usage of dyes because of excessive industrialization has resulted in subsequent exposure of humans to these toxicants.

Pigments and dyes are different based on their solubility and particle size. Pigments are bigger in size as compared to dyes and are able to maintain their crystalline structures during application [12]. Combination with another medium is required before application to a surface. Dyes are soluble colorants and have the ability to diffuse into the material and to become an integral part [12]. Dyes are combustible, but pigments are not. Pigments are usually applied in paints, inks, and polymers. The application of dyes is not only restricted to textiles. They also used in the coloration of paper, leather, food, and various other products [13]. The color of dyes is due to their ability to absorb light in the range of 400–700 nm, i.e., in the visible spectrum. It has been assumed that the color of dyes is correlated with the structure of the dye. There are certain molecular structures or groups with multiple bonds, which when present in dyes impart color [14]. These molecular structures are called chromophores. A chromophore is a conjugated structure containing double and single bonds that alternate [12]. The presence of chromophores is of utmost importance for a dye. A dye must have at least one chromophore. The color of a dye increases with an increase in the number of chromophores.

Table 1. The structure of chromophores.

S. N.	Chromophore	Structure	Wavelength (nm)
1	Carbonyl	>C=O	280
2	Azo	-N=N-	262
3	Nitro	-O-N=O	270
4	Nitroso	-N=O	330
5	Triphenylmethane	-NO2	230
6	Conjugated diene	-C=C-C=C-	233
7	Conjugated triene	-C=C-C=C-C=C-	268
8	Conjugated tetraene	-C=C-C=C-C=C-C=C-	315
9	Benzene		261

shows various chromophores with their structure and the wavelength associated with them.

2. Production and Discharge of Dyes

Currently, a little over 10,000 dyes are commercially available including acidic, basic, reactive, azo, and diazo dyes [15]. These are used in the coloration of various materials such as paper, plastic, textiles, leather, food, drugs, and cosmetics [16]. Worldwide, approximately 7 × 10⁷ tons of dye are produced annually [17]. Textile industries have historically been the world’s major consumers of dyes. It is expected that the demand for dyes would continue to rise in the coming years, as fiber consumption has expanded at a higher pace with population growth [17]. Reactive dyes have witnessed an increase in usage due to factors such as an increase in use of cellulose-containing fibers as well as technical limitations posed by the use of other dyes [18]. Almost 2% of dyes produced annually are discharged in processing waters from manufacturing operations, with textile and related sectors accounting for 10–25% of dyes spilled [19]. Therefore, a large amount of dye enters into the wastewaters. This wastewater is usually treated with physical and chemical methods. However, high cost and disposal problems often result in industries discharging this effluent-laden wastewater, untreated. The discharge of highly colored effluent has become one of the leading causes of environmental problems.

It has been reported that reactive dyes have rather low rates of fixation as compared with the higher fixation rates of basic dyes. Basically, up to 800 mg/L of hydrolyzed dyes may remain in the bath after the completion of the reactive dyeing process [20]. Therefore, a large volume of reactive dyes is discharged into the effluent and poses a serious issue to human health and quality of water. Furthermore, because reactive dyes are not easily biodegradable, they may persist in the effluent despite intensive treatment [21]. It has been noted that textile industries use large volumes of potable water and chemicals. In consequence, wastewater dumped from textile dyeing operations brings all of the unused dye(s) and auxiliary chemicals to the wastewater streams. According to recent reports, partial dye exhaustion accounts for around 8–20% percent of the overall pollutant burden. The presence of even a small fraction of dye in water at concentrations of less than 1 ppm is normally considered unacceptable due to the color, which affects transparency and purity of streams and other water resources [5,6,22].

3. Toxicity of Dyes

Dyes may affect living organisms as well as the environment adversely, due to their toxic effects. U.S. regulatory organizations (US Environmental Protection Agency, the Consumer Protection and Safety Commission, and the Occupational Safety and Health Agency) consider several of the dyes and pigments to be carcinogenic. Benzidine and benzidine congener dyes such as CI Direct Black 38, CI Acid Red 114, CI Direct Blue 15, and CI Direct Blue 218 are examples [23]. Azo dyes have one or more nitrogen–nitrogen double bonds (–N=N–) in their chemical structure [24]. These are converted to aromatic amines by intestinal microflora or liver azoreductases after oral administration. Mammalian microsomal enzymes further convert aromatic amines to genotoxic chemicals [25].

Azo dyes are hardly ever directly mutagenic or carcinogenic. However, there are some dyes having free amino groups with reported carcinogenicity [26]. The azo groups can be split to create two aromatic amines under reductive conditions. Toxic amines are released into the water once azo links are cleaved [25]. These intermediate products have serious negative consequences for humans and aquatic life. These intermediates cause harm to important organs in humans, including the brain, liver, kidneys, central nervous system, and reproductive system [27]. They also reduce light penetration and so limit photosynthetic activity. As a result, toxicity of a dye percolates from their discharge point to the receiving water. As a result, researchers are advised to discover a method for removing such harmful components in industrialized nations across the world [28]. Due to the dyes’ complicated molecular structure, wastewater treatment using traditional methods such as physical, chemical, and biological is challenging.

The colors used in textiles have been shown to be extremely carcinogenic and highly toxic. Furthermore, because the color can be seen by human eyes even when the concentration is less than 1 ppm, the colorful effluent has a negative aesthetic effect on the wastewater [5,6,22]. Additionally, the absorption and reflection of sunlight by colored effluents has an impact on water clarity and gas solubility [8]. This pollution load is a significant burden in terms of wastewater management, since it not only raises treatment costs but also exposes water to a wide range of chemical and microbiological contaminants.

Heavy metal ions from textile effluents have been found in significant amounts in algae and aquatic plants [29]. Worse, certain colors, as well as their biodegraded derivatives, are poisonous, carcinogenic, and mutagenic [30]. Decentralized treatment approaches for wastewater sectors have been introduced to improve the accessibility of small-scale enterprises [31]. Since the correct treatment of such harmful pollutants is costly, they are finally disposed of untreated into the environment, accounting for around 40% of total industrial wastewater [32]. The research and development in the field of environmental protection is a necessary and an urgent issue pace with industrial development. This aids the development of environmentally friendly technologies that minimize freshwater use and wastewater production. The emission of large volumes of synthetic dyes into the environment is a source of public concern, and it exacerbates legislative issues [33,34].

4. Environmental Impacts of Dyes

Despite the fact that color is the most appealing aspect of any textile material, it may pose a threat to the environment and living things. Pollution from dyeing, printing, and finishing industries has become a serious problem.

4.1. Air Pollution

The majority of procedures in the wet processing industry emit gaseous emissions [35]. For the dyeing and printing sectors, gaseous air has been identified as the second most significant pollution source [36]. Different types of gases, such as CO₂, NO₂, SO₂, and others, pollute the air. Fabric preparation, dyeing, resin finishing, and printing wastewater treatment plants are all important sources of air emissions in textile operations [37].

4.2. Soil Pollution

Textile dyeing and printing yield/plant massive amounts of effluents at various stages of the textile processing process due to the usage of excessive amounts of dyes. Underground leakage and accidental spillage of toxic compounds from textile industries may cause soil pollution. Furthermore, long-term irrigation of agricultural fields, as well as use of sewage sludge from water sources polluted with textile effluents, are the main reasons for the transfer of dye components to the soil [15]. Inorganic and organic pollutants in dye waste pose a threat to the soil as it contains allergy, mutagenic and cancer-causing properties [38]. Dyes pose an ecological danger as soils lose their physicochemical features, making them vulnerable to erosion, productivity loss, reduced sustainability, and plant growth rate reduction. Water and soil are contaminated by the textile dye waste created during manufacturing. Soil dye contamination has a negative impact on agricultural production [39].

4.3. Water Pollution

Textile effluents are significant pollutants. Textile production, which also includes cotton farming, utilizes about 93 billion cubic meters of water every year, which represents around 4% of fresh water used globally [40]. Moreover, wet processing of textiles consumes a lot of water and chemicals in the textile industry. Following processing, these industries release raw effluents into local waterways. There is a significant amount of infiltration and percolation of harmful substances into the soils during the flow of effluents, damaging soil, subterranean water, pools, and plants [37]. Various industries release toxic effluents including azo dyes, which have a negative impact on water resources, soil fertility, aquatic species, and ecosystem stability [41]. From washing raw wool or manufactured fiber creation through garment manufacturing, the textile sector uses a lot of water. Textile dyes hinder photosynthesis, restrict plant development, penetrate the food chain, lead to bioaccumulation, and may enhance toxicity, mutagenicity, carcinogenicity, and increased oxygen demand ((biochemical oxygen demand (BOD) and chemical oxygen demand (COD)) [39].

5. Negative Effects of Dyes on Various Populations

Dyes are hazardous to aquatic species (fish, algae, bacteria, etc.) as well as mammals (lethal impact, genotoxicity, mutagenicity, and carcinogenicity) [42]. Dyes are not easily degradable in natural environments and cause severe environmental pollution. The toxins included in textile industry effluents have an impact on the habitat and survival of animal species. Toxicant bioaccumulation is influenced by the pollutants’ availability and persistence in water, food, and physiochemical characteristics.

5.1. Effect on Microbial Population

Dyes bioaccumulate in sediments and soil and are transported to public water, leading to environmental deterioration and destruction and manipulation of microbial populations [43]. Microbes have evolved an enormous variety of novel metabolic pathways or a library of catabolic enzymes throughout millennia, allowing them to adapt to a wide range of circumstances [39]. Drainage of wastewater into a water body enhances its food stock, promoting microbial development with or without the ability to breakdown or consume xenobiotic and recalcitrant chemicals for their energy needs, resulting in a complicated shift in microbial diversity. Dyes endure as environmental contaminants and span whole food chains, leading to biomagnification and simultaneously resulting in larger levels of contamination in higher trophic levels relative to their prey [39,44].

5.2. Impact of Dyes on Fish

Fish suffer from a variety of physiological and biochemical abnormalities as a result of changes in water quality [45]. Fish are vulnerable to a wide variety of toxins including dyes from industrial effluents [46]. Dyes regularly come into close contact with fish during respiration through gills. Therefore, fish are considered to be useful models for assessing the impact of various dyes in their surroundings. Contaminants infiltrate the organs of fish, causing biochemical and hematological harm. In the fish, these organs such as the kidneys, liver, and gills perform critical tasks such as xenobiotic excretion, respiration, distribution, and biotransformation [47]. These toxins and dyes can harm nephrons in the kidneys, resulting in renal failure. In a recent in vitro study, hematoxylin dye showed several changes in the kidney (glomerulonephritis, tubular degeneration, etc.) of a fish Cirrhinus mrigala [46]. Previous studies on Gambusia affinis have demonstrated significant cytotoxic effects on RBCs. Another study on Mastacembelus armatus, found that exposure to textile effluents including dyes had the potential to alter the ionic regulation of organs such as the liver, kidney, and muscle by lowering salt and chloride ion concentrations [44].

5.3. Impact on Algae

Increasing levels of dyes present in water bodies affect many aspects of algae, such as growth protein content, pigment content, and other nutrient content [48]. Different colors can have a variety of effects on algae. Algae are 50 percent more susceptible to pollutants than organisms normally utilized in toxicological experiments for measuring contamination in aquatic environments. Increasing the concentration of dyes in water inhibits the development of Spirulina platensis and lowers its nutritional content [44]. Dyestuff pollution leads to inhibition of microalgae growth and an imbalance in the tropic transfer nutrients and energy in aquatic ecosystem [49].

5.4. Effect on Agriculture

Diverse industrial effluents can contaminate agricultural fields with a significant impact on soil quality and plant growth [50]. The chemical and biological condition of soil and water can be altered by dissolved compounds in industrial effluents, which may affect plant growth and productivity [50]. Dyes are discharged into wastewater, which is frequently utilized in agriculture for irrigation [39]. The germination rate of okra crop can be significantly inhibited by the use of textile effluent-containing water [51]. Some dyes present in effluents have been shown to inhibit seed germination and agricultural plant growth. The elongation of seedling shoots and roots is inhibited by a greater concentration of untreated effluents. Further, the high percentage of particles in effluent limit the amount of dissolved oxygen in the water, limiting seedling growth and development. It is possible that dissolved solids in the wastewater prevent seed germination [38].

5.5. Impact on Human Health

The use of contaminated ground water causes debilitating health issues in humans, and in some cases, death [52]. Reactive dyes are one of the major areas of concern of textile effluents, as they are non-biodegradable, mutagenic and carcinogenic, and untreatable through conventional treatment systems [53]. Workers in the textile business are exposed to a variety of chemicals, including dyes, solvents, optical brighteners, finishing agents, and a variety of natural and synthetic fiber dusts, all of which can be harmful to their health [54]. Moreover, dyes containing effluent enter aquatic species, transit through the food chain, and eventually reach humans, causing hypertension, occasional fever, kidney damage, cramping, and other physiological problems [44].

Those who work in the finishing industry are routinely exposed to crease-resistant chemicals. These agents have the potential to release formaldehyde, which is known to be hazardous [54]. Flame retardants, such as organophosphorus and organobromine chemicals, are also present in the workplace [55]. Textile manufacturers utilize a variety of dyes, including azo dyes, which are aromatic hydrocarbon derivatives of benzene, toluene, naphthalene, phenol, and aniline [54].

5.5.1. Cancer

Cancer is among the leading causes of death across the world. There are several carcinogenic substances in dyestuff that cause genetic changes and disrupt cell signaling pathways [56]. Several dyes used in the textile industry, such as azo dyes, cause malignancies [25,26].

Azo dyes are used in coloring garments and directly come in contact with skin [57]. There is a high chance of absorption by the skin, depending on the conditions. Under certain conditions, azo dyes undergo breakdown, forming aromatic amines [58,59]. Some of these amines are proven to be potential carcinogenic agents [60,61] and cause cancer in humans [62]. The EU has prohibited the use of azo dyes in all textiles since they emit cancer-causing amines. Workers in the textile sector are exposed to a variety of chemicals that are known to cause cancer. The results of a review of 54 research studies on textile industry workers found that they are susceptible to a variety of malignancies [54]. Long-term exposure to various chemicals and physical elements in the textile sector may result in occupational cancer among textile workers [54]. Alternative non-toxic textile chemicals should be developed and used.

The usage of food colors in human diets has sparked growing concern about their health effects. Azo dyes are widely utilized in the cosmetics, pharmaceutical, textile, printing, and food sectors. They are used as biological stains or pH indicators in laboratories. However, there might be a link between the usage of azo dyes and the rise in cancer incidence [10,11]. Intestinal cancer is more frequent in highly industrialized nations. Aromatic amines, frequently reported in hair dyes, are carcinogenic in experimental animals and found to be mutagenic by in vitro studies [63]. During routine usage, small quantities of these possibly carcinogenic chemicals are absorbed via the skin. Manuela Gago-Dominguez [63] found two cohort studies and five case-control studies that offered some evidence on personal hair coloring usage and bladder cancer risk. Ever-users had a slightly increased risk of basal cell carcinoma (the most prevalent kind of skin cancer) than non-users when it comes to particular malignancies. The use of permanent dye for a longer period of time appears to increase the risk of several breast and ovarian cancers. Women with naturally dark hair are more likely to develop Hodgkin lymphoma, whereas women with naturally light hair are more likely to develop basal cell carcinoma [64].

5.5.2. Effect on Liver

Various food colorants are reported to induce hepatic damage due to their ability to decrease the activities of various antioxidant enzymes and increase oxidative stress [65]. Various diseases have been related to damage to the antioxidative defense system and, as a result, increased oxidative stress [66].

Humans are regularly exposed to various food colorants as well as organic solvents used in different industrial processes associated with hepatotoxicity [65]. It has already been reported that employees in the textile processing and dyeing sector have encountered harmful effects on the liver and kidneys [67]. Acrylic fabrics contain dimethylformamide, which can cause liver damage and other health problems when it comes into contact with the skin [68,69]. As a result, those who work with acrylics wear clothing to protect themselves.

5.5.3. Allergies

Textile dye allergy is more common than previously reported. It can cause marked dermatitis and widespread autoeczematization reactions [70]. The most frequent allergens are Disperse Blue 106 and 124, which are frequently found in 100% acetate and 100% polyester liners used in women’s clothing. Disperse Blue 106 or 124 can serve as a screening allergen for textile dye dermatitis. The American Contact Dermatitis Society designated disperse blue dyes the Contact Allergen of the Year in 2000 [70].

When unbound colors from colored clothing flow into skin, allergies to textile dyes can cause skin symptoms such as severe eczema/dermatitis [71]. Many colors are water soluble, and perspiration from the body can have the same effect, resulting in dye leaching and an increased risk of allergy development. The most allergenic dyes are azo dyes [72]. Due to the rising frequency of allergies, these dyes are no longer commonly used in fabric dyeing. Dispersion dyes have also been linked to allergic contact dermatitis.

5.5.4. Effect on Hormones and Central Nervous System

In textile industries, chlorinated solutions are used to dissolve many compounds during the production and cleaning of garments [73,74]. In addition, chlorobenzene is used as a solvent for dyes, pesticides, biocides, and chemical mediators in the production of textile colors [73,74]. The consequences of exposure vary depending on the kind of chlorobenzene, but the toxic effect on liver, thyroid, and central nervous system are the most prevalent [75]. Hexachlorobenzene has been reported to disrupt human hormones [76]. Inhalation of chlorobenzene is known to cause narcosis, restlessness, tremors, and muscular spasms in animals. Numbness, cyanosis, hyperesthesia (enhanced sensitivity), and muscular spasms are all symptoms of neurotoxicity in humans. Trichloroethylene, a chemical that depletes the ozone layer, has been shown to have negative effects on various organ systems if present in the environment [77]. Heavy metals such as mercury and lead build up in the body over time, causing lasting health problems [78].

6. Dye removal Techniques

Clean water is one of the most essential natural resources on the earth since it is necessary for life. Wastewater is also an important resource, especially in locations that suffer from frequent droughts with water shortages. The wastewater includes numerous dangerous and toxic substances. Therefore, it is very necessary to treat wastewater before it is discharged back into the environment. As a result, the treatment of wastewater is very significant due to two reasons: to reestablish the supply of water, and to protect the environment from pollutants.

The dye-containing wastewater should be treated in an efficient way in order to protect the environment and water resources from adverse effects [79]. From a purely engineering standpoint, the technique for removing dyes and pigments from textile effluents is basically a simple separation process. Technically, there are various strategies or separation techniques that may be used to remove a certain component from dye containing wastewater (Figure 2). The treatment strategies are grouped into physical methods, chemical methods, nanoparticles-based method, and biological methods. The physical methods are further grouped into adsorption, coagulation–flocculation, and filtration methods [80,81,82]. The chemical methods are sub-grouped into oxidation, coagulation, electrochemical oxidation, and photochemical degradation [83]. However, biological methods are two types, including plant-based and microbe-based. Microbe-based are further divided into aerobic and anaerobic.

Physical treatment methods use physical forces to separate colors from wastewater. Physical processes include coagulation, flocculation, and sedimentation (as well as other precipitation methods), adsorption (on a range of inorganic and organic substrates), filtration, reverse osmosis, ultra-filtration, and nano-filtration [84].

Various chemical reactions such as reduction, oxidation, ion exchange, and neutralization reactions or the introduction of chemicals are used to create the chemical technique [85]. Biological therapies, on the other hand, are often carried out aerobically or anaerobically, depending on the presence or lack of oxygen in the system. Biodegradation refers to biological treatments that occur in the presence of a biological catalyst that promotes deterioration. Chemical treatment procedures such as reduction, oxidation, complicated metric methods, ion exchange, and neutralization are often carried out by adding chemicals through chemical reactions [85].

Chemical methods may produce harmful intermediate compounds. Colored wastewater treatment is influenced not only by ecological factors such as COD, BOD, total organic carbon, absorbable organic halide, temperature, and pH, but also by the original dye concentrations in the wastewaters [85]. When the by-product stream has minor environmental consequences, the overall process is called ecologically acceptable. If not acceptable, other added treatments are required, and the problem of removing hazardous compounds from raw influent is resolved.

6.1. Physical Treatment

6.1.1. Filtration Technology

Filtration technology is one of the most important physical treatment methods for the purification of wastewater [86]. Some aspects of this technology include techniques such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis [71,87]. This method exhibited some promising results in the elimination of color. The importance of individual membranes in the water treatment process cannot be overstated. Due to its huge pore size, microfiltration is not suitable for wastewater treatment; however, nanofiltration and ultrafiltration may effectively remove many colors. The dye molecule clogs the membrane in most cases, limiting the separation process for dyeing wastewater treatment [88]. The high pressure, significant energy consumption, high cost of the membrane, and limited life duration of this process are all limitations, and these qualities make it difficult to treat dyestuff or remove organic pollutants [89].

In terms of eliminating salts, reverse osmosis is more successful. The high pressure, significant energy consumption, high cost of the membrane, and limited life duration of this process are all limitations, and these properties make it difficult to treat dyestuff or remove organic pollutants using filtration method. It has better decolorizing and desalting properties against different dye effluents and may be recycled. The treated wastewater is as close to clean water as possible [85].

6.1.2. Adsorption

Adsorption is the process through which chemical and physical forces are used to attach dissolved molecules onto the surface of an adsorbent material [90]. Wastewater treatment is possible through adsorption by activated carbon. Such materials remove dyes present in wastewater through the process of adsorption alone or through the combination of ion exchange and adsorption [90,91]. Due to favorable results, high adsorption ability, and ease of operation, this technique is preferred to remove different types of dyes [92]. However, high costs have led to limited use [93]. Hence, scientists are constantly on the lookout for low-cost alternatives to activated carbon.

6.2. Chemical Treatment

Several chemical processes are extensively used for wastewater treatment to enhance the quality and purity of water. The most used chemical processes in wastewater treatment are oxidation, coagulation–flocculation, electrochemical oxidation, and photocatalytic degradation.

6.2.1. Oxidation

Due to its ease of use, oxidation is one of the most regularly utilized chemical decolorization processes. Biological organisms, ozone, sodium hypochlorite, hydrogen peroxide, and even acids are all used in the oxidation processes. Since the dyes are broken down, the oxidative process produces smaller molecules [18]. Traditional oxidation procedures are unable to thoroughly oxidize dyes (primarily for color removal) and hazardous organic compounds from textile effluents. The aforesaid constraint can be solved by developing advanced oxidation processes (AOPs) that generate free hydroxyl radicals (OH). When compared to typical oxidants, these free radicals obviously boost the pace of reaction by many orders of magnitude. OHs have the ability to oxidize colors as well as hazardous chemical molecules that are generally resistant to oxidation by conventional oxidants [18]. Oxidizing agents like ozone and hydrogen peroxide are employed with catalysts (such Fe, Mn, and TiO₂) in AOPs, either with or without an irradiation source [94]. Chemical oxidation is used to remove colors from dye-containing effluents, resulting in dye molecule aromatic ring cleavage [95].

6.2.2. Coagulation–Flocculation Method

Disperse and reactive dyes are often used in the textile industry and have become an important environmental concern due to their widespread use in dyeing polyester and cotton [96]. Nonionic disperse dyes exhibited very low water solubility, and hence the coagulation–flocculation method can effectively remove these dyes [97]. High concentration of impurities and toxins from wastewater can be treated before its disposal with coagulants. Metal salts and polymers can be used as coagulants whereas, flocculants serve as polymers which can increase folcs aggregation, leading to easy separation [98]. In the first phase, coagulants are added during vigorous shaking to allow neutralization or reduction of charge on the finely dispersed particles. In final phase, flocculants are mixed with the fine dispersed particles, and the resultant large particles can easily be separated by the process of sedimentation [99]. Natural coagulants such as Azadirachta indica and Tamarindus indica can efficiently remove dye from the textile wastewater effluents [100]. There are several known chemical coagulants used in the coagulation–flocculation process such as ferric sulfate, ferric chloride and etc. These can be used to maintain the purity of the removal system. However, other chemical coagulants such as hydrated lime and magnesium carbonate help in absorbing dyes and dye products [101,102,103]. In this procedure, a coagulant is added to a solution and mixed vigorously, which leads to the formation of precipitates, leading to trapping of organic contaminants and impurities, providing treated clear water.

6.2.3. Electrochemical Oxidation

This procedure is used to remove color from dye solutions as an alternate treatment [87]. This is carried out in a Batch type split electrolytic cell, using a platinum (Pt) cage as an anode and a Pt foil as a cathode under constant voltage. By using a continuous electrolysis voltage, complete color removal was achieved in a very short period (16 min) at pH 2. Extending the treatment time to 40 min resulted in a significant proportion of COD removal (46%) [104].

6.2.4. Photocatalytic Degradation

Photocatalytic degradation is a novel advanced oxidation process used for mineralizing dye compounds. One of the key advantages of photo catalytic degradation over previous technologies is that no secondary disposal processes are required. For the breakdown of Methylene Blue dye, photo catalysis is a very successful method. In the presence of UV/H₂O₂ and UV/TiO₂, photo catalytic degradation can occur in both homogeneous and heterogeneous ways. Photo catalytic activity was improved by using an electron scavenger. Variable parameters such as starting dye concentration, pH, and H₂O₂ concentration influence degradation [104].

6.3. Biological Techniques

Recent application of several physicochemical methods is expensive and produces large amounts of sludge after treatment. When compared to other physical and chemical procedures, biological treatment is frequently the most cost-effective. Numerous microorganisms such as bacteria, yeast, algae, and fungus are capable of collecting and breaking down different dyes [105]. Therefore, methods including fungal decolorization, microbial decolorization, adsorption by (living or dead) microbial biomass, and bioremediation systems are routinely used to remediate industrial emissions. Biological therapy methods are divided into aerobic and anaerobic treatment categories based on the oxygen requirement. Due to its great effectiveness and extensive use, aerobic biological treatment is the most common biological treatment [85].

The biological process is less expensive than other approaches. Biological processes have investment costs that are 5–20 times lower and operational costs that are 3–10 times lower than chemical techniques [87]. Coagulation or flocculation procedures generate significant volumes of sludge that require safe disposal. Secondary waste streams are created by adsorption and, to a lesser degree, membrane filtering processes, which require additional treatment. There are numerous reports on the use of physicochemical methods for color removal from dye-containing effluents [106,107,108]. Due to their ecologically friendly character, economic efficiency, and capacity to create less sludge, the biological treatment strategies are the most appropriate and commonly employed in the current context [109,110].

Bioremediation is often carried out by using microorganisms to remove contaminants from the environment. In such methods, microorganisms adapt to hazardous wastes and spontaneously generate new resistant strains, which subsequently convert various dangerous compounds into less damaging forms. The activity of biotransformation enzymes is responsible for the biodegradation of refractory chemicals in the microbial system [111].

Several studies have shown that enzymatic pathways, such as those associated with laccase, lignin peroxidase, NADH-preferring 2,6-dichloroindophenol (NADH–DCIP) reductase, tyrosinase, hexane oxidase, and aminopyrine N-demethylase [112,113,114,115,116,117] can cause the decomposition of complex organic compounds. In order to combat azo dye pollution in an environmentally friendly manner, biotechnologists have successfully utilized different treatment technologies to remove colors from effluent streams.

The adaptability and activity of the chosen microorganisms determine the efficiency of microbial decolorization. As a result, in recent years, a significant number of species have been examined for decolorization and mineralization of different dyes [118]. One of the fascinating biological features of wastewater treatment is the separation of powerful species and their destruction [119]. Bacteria [112], fungus [120], yeasts [121], actinomycetes [121,122], and algae [92,98,123] are among the microorganisms capable of decolorizing a wide spectrum of colors.

6.3.1. Decolorization and Degradation of Dyes by Plants (Phytoremediation)

Phytoremediation is used to treat dye waste contaminants produced from a variety of sources. Phytoremediation is a new method that claims to be a cost-effective and efficient way to clean up heavy metals and organic contaminants from soils and groundwater [124]. Plants and bacteria linked with plant root systems are used in phytoremediation to safeguard the environment by removing contaminants in the form of inorganic and organic wastes. The key benefits of phytoremediation are that it is an autotrophic process that uses minimal nutrient supply and that it is easy to maintain and accept by the public owing to its aesthetic appeal and environmental sustainability [111,125]. Plants are able to handle toxins from contaminated sites because of adaptation at the genetic level. Narrow-leaved cattails have been tested in synthetic reactive dye wastewater treatment under caustic circumstances [126,127], where Coco yam plants may reduce the amount of synthetic reactive dye wastewater by 72–77 percent.

The decolorization effectiveness of azo dyes in textile effluent has been evaluated using three plant species including Brassica juncea, Sorghum vulgare, and Phaseolus mungo, all of which have different agronomic repercussions. B. juncea, S. vulgare, and P. mungo were found to be 79%, 57%, and 53% efficient, respectively [111,128]. Similarly, a plant called Blumea malcommi has been discovered to breakdown textile colors (Reactive Red 5B).

Hairy root cultures of Tagetes patula L. have been found to be efficient in decolorizing Reactive Red 198, and the enzyme system involved has been identified [125]. Large-scale phytoremediation depends on the plant’s resistance to pollutants, the bioavailable percentage of contaminants, the transpiration of volatile organic pollutants, and vast treatment areas [112,129,130].

6.3.2. Treatment by Fungus

Due to their rapid adaptability and metabolism of diverse nitrogen and carbon sources, filamentous fungi can flourish in a range of habitats, including live plants, soil, and organic waste. Fungi generate a large number of extracellular and intracellular enzymes capable of digesting a wide range of organic pollutants, including dye effluents, organic waste, steroid compounds, and polyaromatic hydrocarbons. Using white-rot fungus to biodegrade azo dyes has been described in several publications [131,132]. Dye removal by mycoremediation has been found to be a safe, low-cost, and natural process [133]. Under static conditions of 25 °C and 6.5 pH, Penicillium ochrochloron decolorizes cotton blue dye in 2 h. It has been reported that after six days of incubation under ideal experimental circumstances, A. flavus strain F10 was able to decrease malachite green by up to 98–99 percent (150 mg/L MG). Enzymatic events including hydroxylation, demethylation, and ring breakage are among the malachite green decolorization methods authorized by A. flavus. For decolorization and dye degradation, immobilized growing cells are more stable and reusable than free cells [134].

6.3.3. Treatment by Algae

Algae-mediated biodegradation of dye-containing water is thought to be a preferable option since it produces no secondary waste products and may remove a variety of nutrients from the water. Furthermore, unlike traditional treatment techniques, algae-based bioprocesses may work under typical air conditions, are inexpensive, and environmentally beneficial. Many pollutants have been observed to be digested and absorbed by algal biomass [80]. Enteromorpha flexuosa has been used to produce monolithic algal green powder (MAGP). Fabricated MAGP has been reported for the removal of crystal violet, methylene blue, and reactive black 5 by 90.3%, 93.4%, and >90%, respectively [135].

6.3.4. Treatment by Lichen

Lichens are one of nature’s most important examples of symbiotic living. The combination of a mycobiont (fungus) and a photobiont (green alga or cyanobacterium) is known as lichen. Many lichen species can adapt to diverse environmental circumstances because the photobiont and mycobiont play complementary functions. Lichens are suggested as heavy metal contamination indicators [136]. Only one investigation on the dye-removal capabilities of the lichen Parmelia perlata has been published [137]. C. convoluta was shown to be a suitable biosorbent for removing textile colors from industrial effluent at a reasonable cost [136].

6.3.5. Microbial Treatment

Biological wastewater treatment is a common technique employed in dye treatment [17]. Alternative dye removal methods, such as microbial biocatalysts to decrease the dyes in effluent, have the potential to outperform physio-chemical techniques. Several papers have described the utilization of a large number of species for the removal and full mineralization of various colors. Microorganisms already present in wastewater treatment feed on complex chemicals in wastewater, reducing them to simpler molecules and therefore improving treatment. The fundamental benefit of this method is that it is economical, has minimal operating expenses, and produces benign end products. These processes, on the other hand, can be aerobic (in the presence of oxygen), anaerobic (in the absence of oxygen), or mixed aerobic–anaerobic. Due to their capacity to treat colored wastewaters, bacteria and fungus are commonly utilized in aerobic treatment [138].

Aerobic Treatment

Under aerobic circumstances, bacteria and fungus are the most widely utilized microorganisms for dye decolorization. For more than three decades, aerobic microorganisms capable of degrading different colors have been utilized. Numerous microorganisms that can degrade colors have been discovered. In this kind of treatment, the enzymes released by microorganisms present in the wastewater break down the organic molecules [139,140]. Researchers have been attempting to discover and isolate aerobic bacteria capable of breaking down different dye. The strain Kurthia sp. has been reported to efficiently decolorize a range of triphenylmethane dyes (crystal violet, ethyl violet, magenta, brilliant green, and malachite green). The level of COD reduction of cell free extracts of triphenylmethane colors following biotransformation was greater than 88 percent in all dyes except ethyl violet (70%), according to the researchers [141]. Chemical compounds present in spent coffee ground (SCG) affects respiratory activity of sewage sludge. SCG can enhance the microbial respiration in activated sludge, leading to aerobic degradation or utilization by a sewage sludge bacterial consortium [142]

Various researchers have studied many fungi extensively for their capacity to decolorize a broad spectrum of colors [143]. Effective dye decolorization has also explored by using Cyathus bulleri, Rhyzopus oryzae, Coriolus versicolor, Streptomyces sp., Laetiporous sulphureus, Funalia trogii, Trametes versicolor, and other microorganisms [141]. However, the dye removal process has been observed to be affected by many operational parameters such as the initial pH, the beginning concentration of pollutants, and the temperature of the effluent. It has been found that the effluent’s treatability by other microbes can be improved for effective dye removal. Although these procedures are plainly appropriate for some colors, the majority of dyes are resistant to biological degradation or are not transformable under aerobic conditions [138].

Anaerobic Treatment

Anaerobic decolorization utilizes a hydrogen-based oxidation-reduction process instead of free molecular oxygen in an aerobic environment, enabling azo and other water-soluble dyes to be decolorized. Anaerobic reduction of azo dyes could be a cost-effective and efficient way to remove color from textile effluent. The effectiveness of different anaerobic treatment methods for the breakdown of a wide range of dyes has been frequently shown [141]. Furthermore, several anaerobic bacteria may decolorize azo dyes by reducing the azo bond in anaerobic conditions [144]. For the breakdown of a wide spectrum of synthetic colors, the anaerobic treatment appears to be rather promising. Some dyes have been reported to be destroyed or mineralized in anaerobic circumstances in the literature. Researchers have observed that under anaerobic circumstances employing methanogenic granular sludge, decolorization of azo dyes showed some promising results in the case of mordant orange-1 and azo-disalicylate, which could be decreased and decolorized. Another study demonstrated the viability of using anaerobic granular sludge for complete removal of 20 azo dyes [141].

When compared to aerobic systems, an anaerobic pre-treatment step might be a less expensive option because aeration is not required and difficulties with sludge bulk are avoided. It has been observed that anaerobic effluent treatment for dye removal is effective; however, heavy metals can be retained by sulphate reduction [138]. Furthermore, the key constraint for dye degradation is foaming difficulties, associated for surfactants, and high effluent temperatures along with high pH. However, BOD removal may be inadequate. Additionally, the dyes and other refractory organics are not mineralized. It is also reported that nutrients (N and P) are not removed, and sulphates may lead to formation of sulfides [138].

7. Enzyme-Mediated Dye Removal

The use of enzymatic proteins may be a viable option to overcome the majority of the problems associated with the employment of microbes. Enzymes have a number of helpful properties. They can selectively decompose a pollutant without impacting the effluent’s other components. As a result, enzymatic treatment is appropriate for effluents with a high concentration of the refractory target pollutants in contrast to other contaminants [145]. They are catalysts with either limited (chemo-, region-, and stereo-selectivity) or broad specificity, and may thus be used with a wide variety of chemicals in mixtures. For the decolorization of synthetic azo dyes, enzymatic methods hold a lot of promise.

White-rot fungi generate nonspecific extracellular ligninolytic enzymes that effectively remove synthetic dyes [146]. Lignin peroxidase, manganese peroxidase, and copper-containing laccase are some examples of such enzymes [139]. Laccases have the most bioremediation potential due to their capacity to oxidize a wide range of substrates. Laccases belong to the oxidase family, which have a catalytic site with four copper atoms. The capability of laccases to digest phenolic compounds enables them to breakdown xenobiotic chemicals in wastewater treatment [139]. Due to their potential industrial uses, a lot of research is currently being conducted on laccase-mediated dye removal utilizing wild strains.

The microorganism employed to make laccase should be able to provide acceptable yields, while also avoiding the creation of toxins or other undesirable products. The availability of powerful microbial strains and the use of this biocatalyst for industrial-scale dye removal are the key obstacles with wild strain [147]. It is also worth noting that wild strains should hold up well in industrial settings. The lack of batch-to-batch fluctuation in laccase synthesis reported in wild strains is favorable. The biggest disadvantage of laccase synthesis utilizing wild strain is the poor yield [139]. However, isolating a powerful strain for dye removal is time-consuming and difficult. The microbial diversity during dye degradation under natural settings must be assessed, and new well-adapted microbial strains must be isolated, screened, and characterized in order to possibly boost enzyme production.

Recombinant DNA technology-assisted laccase creation with high yield and reliability under industrial conditions are some of the key solutions. The enzyme that is most suited for industrial applications has been chosen in this strategy [141]. The major difficulty for recombinant DNA technology is to introduce genes into genetically altered organisms to improve their fermentation characteristics. Engineered laccase enzymes’ robustness has been discovered to be important for industrial applications. Cloning of the laccase gene, random mutagenesis, site-specific mutagenesis, or a combination of the two methods have all been utilized to produce robust engineered laccase enzymes for industrial purposes [148].

Iterative saturation mutagenesis (ISM) is a method of directed evolution used to enhance enzyme properties. In ISM, repetitive cycles of saturation mutagenesis are used at two or three amino acid positions in the protein and its structure. Three to four rounds of ISM were used to find advantageous mutations, which were then systematically included into the libraries [149]. The development of bioprocesses in the twenty-first century has centered on enzyme-mediated bioprocesses, which are an appealing tool for achieving economic and ecological goals. In addition, it reduces the cost of the enzymes that are used which is the major objective to reduce the cost of total process of bioprocess.

Several publications have been reported on cellular recognition of dyes via genetic regulation and laccase enzyme expression in the presence of complex organic compounds. Laccases were discovered to be formed when microorganisms were cultivated on dyes because the colors increased enzyme complex activity in the microorganisms [150]. It’s indeed worthwhile to note that dyes did not directly penetrate the cell to affect gene regulation and laccase enzyme production. Because enzyme secretion is an induction process, a physical interaction between a component of the cell’s regulatory apparatus and the inducer is essential. On the cell surface, the inducers have a recognition site that controls the process. The expressed enzyme that are secreted extracellularly, have the capability to hydrolyze the complex structure, making it easier to move inside [151].

Microbial fermentation requires raw materials, biomass, treatment, and other ingredients for cells to grow well. In most cases, a pure strain of a microbe is needed in the vessel. The bioreactor aids the natural process by providing ideal circumstances for cell growth and the formation of metabolites and enzymes, such as optimal temperature, pH, and nutritional ingredients. After a given amount of lag time, the cells will begin to proliferate exponentially and achieve a maximum cell concentration once the medium is exhausted. Furthermore, the fermentation process can account for anywhere from 5% to 50% of the overall fixed and operational expenses of the process, which vary greatly based on the kind of product, concentration level produced, and purity needed [152]. As a result, a bioreactor’s ideal design and operation affects the process’ total technological and economic performance, making it more effective as compared to the conventional activated sludge process. It is important to explore and develop three main areas in order to carry out a bioprocess on a wide scale. Optimization is a major requirement for a fermentation process in order to obtain a powerful biocatalyst (such as microorganisms, animal cells, plant cells, or enzyme) and medium [153]. Furthermore, it is very necessary to provide the optimum possible environment for the catalyst to perform through bioreactor design. Unfortunately, the application of free enzyme for dye removal is very limited because of poor stability and high production cost. The immobilized laccase-based approach can overcome drawbacks such as limited stability and high manufacturing costs. The fundamental advantage of an immobilized system is the stability of the enzyme in harsh environmental conditions or in the presence of chemicals. It is also obvious that immobilized laccase may be easily detached from the process, allowing the enzymes to be utilized indefinitely [153,154].

Kaushik et al. conducted solid state fermentation experiments with Aspergillus lentulus to produce xylanase [155]. Various low-cost agricultural leftovers were employed as substrate in this experiment. On the fourth day of incubation, when wheat bran was utilized as the substrate, the highest xylanase output (158.4 U/g) was recorded. However, in the presence of maize cob, sugarcane bagasse, and wheat straw as substrates, xylanase production was 153.0 U/g, 129.9 U/g, and 49.4 U/g, respectively. It was tested to alleviate the problem of pollution caused by pulp bleaching and the emission of colored effluent in the pulp and paper industries [155]. At high pH and temperature, the enzyme demonstrated good stability (>75% activity at pH 9 and 70 °C) [156]. It was discovered that dye removal may be used to suit the needs of the pulp and bleaching sectors in a cost-effective and long-term manner [141].

7.1. Types of Enzymes Participating in the Decolorization and Degradation of Dyes

Several enzymes have been reported to be involved in the removal of dyes, which proved to be an effective molecular weapon for dye decolorization and degradation. Enzymes (laccases, peroxidases, peroxidases, manganese, lignin peroxidases, microperoxidase-11, polyphenol oxidases, and azoreductases) have been used to decolorize and degrade dyes [157].

7.1.1. Oxidative Enzymes

Several oxidases have been identified in bacteria, filamentous fungi, yeast, and plants. These enzymes catalyze the conversion of a wide range of substrates into less toxic insoluble compounds. Toxic compounds are removed from waste through a mechanism that involves the formation of free radicals followed by insoluble product. Oxidative enzymes for the breakdown of azo dyes include polyphenol oxidases (PPO), manganese peroxidase (MnP), lignin peroxidase (LiP), laccase (Lac), tyrosinase (Tyr), and N-demethylase [158].

7.1.2. Reductive Enzymes

Catalytic proteins called azoreductases are produced by microorganisms such as bacteria, algae, and yeast [159]. Numerous bacterial species are capable of breaking down azo dyes in anaerobic (reduced oxygen) environments. Under reduced conditions, the azo bond is first broken during the dye decolorization process by the bacterial enzyme azoreductase, which results in the formation of toxic colorless aromatic amines. To completely decolorize azo dyes, azoreductases rely on reducing equivalents (such as NADPH, NADH, and FADH) [158].

7.2. Immobilized Enzymes

Due to a number of limitations, some enzymes cannot be used in the dye oxidation and degradation due to their stability and lack of reusability [157]. With increasing effluent complexity, enzyme stability and catalytic efficiency decline. The use of enzymes has helped to overcome some of these restrictions. Enzymes can be used as catalysts with a longer lifetime in the immobilized form. They can be stabilized and have their lives extended by enzymatic immobilization. New methods are constantly being developed for the immobilization of enzymes with superior utilization and efficiency. Immobilization of oxidoreductases in industrial applications would permit the enzyme’s reuse and lower the price of these procedures. Recent reviews of oxidoreductases immobilized on various supports have covered a variety of applications [160].

8. Nanoparticle-Based Dye Degradation

In some studies, nanomaterials (NPs) were used to remove metals, microbes, and oil from contaminated water [136,161]. Contaminant removal has been replaced by the use of nanomaterials. Nanoparticles are particles with a diameter of fewer than 100 nanometers. Materials used in this method have at least one component with a dimension of less than 100 nm. Due to their nanoscale size, these materials exhibit mechanical, electrical, optical, and magnetic properties that are very distinct from those of traditional materials. Due to their small size and large surface area, nanomaterials are highly adsorbent and reactive [162]. Furthermore, nanomaterials are highly mobile in solution. Metallic nanoparticles have been shown to have physical and chemical characteristics that enable researchers to use them in a variety of disciplines, including biosensors, electronics, food, textiles, the environment, healthcare, and agriculture [163]. Research interest in metallic nanoparticles has garnered interest due to their catalytic activity in the breakdown of hazardous dyes. Nanoparticles for catalytic degradation of textile dyes can be made using a variety of methods, including physical, chemical, and biological methods [164,165,166]. The size and shape of the nanoparticles, dye concentration, alkali addition pH, nanoparticle concentration, salt addition, and duration affect the decolorization significantly.

8.1. Types of Nanomaterials Used in the Treatment of Water and Wastewater

Various types of nanomaterials have been observed to remove heavy metals, organic pollutants, inorganic anions, and bacteria. Nanomaterials, such as metal oxide nanoparticles, carbon nanotubes, and nanocomposites have all been thoroughly studied for their potential use in the treatment of water and wastewater.

8.1.1. Zero-Valent Meal Nanoparticles

Studies have been conducted on zero-valent nanoparticles (nZVI), which hold great promise for environmental cleanup [167].

Silver Nanoparticles

Silver is an essential noble metal with substantial disinfectant and antibacterial capabilities, as well as enormous therapeutic value. Large surface area, great dispersion, and small size of silver nanoparticles (AgNps) contribute to their powerful biological characteristics such as antibacterial, anticancer, antidiabetic, antioxidant, and anti-inflammatory actions. Biosynthesized silver nanoparticles have been reported to have a wide range of applications in biomedical sciences [168].

Iron Nanoparticles

Zero-valent metal nanoparticles, such as Fe, Zn, Al, and Ni, have recently attracted a lot of attention in the field of water treatment [169]. The most widely studied nanoparticles for environmental remediation are nZVI due to their purportedly low cost, highly reactive surface sites, and high in-situ reactivity. Despite the fact that the majority of the lab-scale, pilot-scale, and survey observation studies focused on soil and groundwater restoration, nZVI has also been efficaciously used for other applications, including the removal of phosphorus, the stabilization of biosolids, the decolorization of dyes, as a membrane anti-fouling agent, and the treatment of nuclear waste, explosives, and herbicides [170].

8.1.2. Metal Oxide Nanoparticles

A wide range of applications, such as catalysis, sensors, (opto)electronic materials, and environmental remediation, are formulated from metal oxide nanoparticles [171]. Successful application of metal oxide nanoparticles requires controlled synthesis, and solution-phase methods offer a significant level of control over the synthesis products.

Iron Oxides Nanoparticles

Due to their simplicity and availability, iron oxide nanoparticles are gaining popularity as adsorbents for heavy metal removal [172]. Magnetite (Fe₃O₄), hematite (Fe₂O₃), and maghemite (Fe₂O₃) are the most likely common and significant iron oxides in terms of technology. The magnetic properties of iron oxide nanoparticles have been found to be strongly influenced by surface effects. It is challenging to recover the adsorbents after treatment is complete because iron-oxide-based magnetic materials’ responses to an external magnetic field decreased as their surface area decreased. In recent years, nonmagnetic hematite (Fe₂O₃) has also been used for heavy metal removal. Generally speaking, because of their small size, nanosorbent materials can be challenging to separate and recover from contaminated water [173].

Zinc Oxide Nanoparticles

Due to their distinctive properties, such as high oxidation capacity and good photocatalytic property, zinc oxide ZnO nanoparticles (ZnO NPs) have become a valuable photocatalytic candidate in the water and wastewater treatment industry in addition to TiO₂ nanoparticles [174]. ZnO NPs are suitable for sewage treatment due to their compatibility with organisms and environmental friendliness. Since their band gap energies are similar, their photocatalytic capacity is comparable to that of TiO₂ NPs. In comparison to TiO₂ NPs, ZnO NPs are less expensive [175].

Titanium dioxide Nanoparticles

Due to its high photocatalytic activity, affordable price, photostability, and chemical and biological stability, titanium dioxide (TiO₂) has received the most attention in recent years. Furthermore, TiO₂ is one of the best photocatalysts on the planet due to its low cost, toxic-free property, chemical stability, and easy availability [176]. TiO₂ exists in three natural states: anatase, rutile, and brookite. Anatase is still thought to be a good material for nanophotocatalysis [176]. Large quantities of TiO₂ NPs are produced globally for use in a variety of applications. TiO₂ NPs differ from their fine particle analogues in terms of their physicochemical characteristics, which could affect how active they are biologically [177]. Other characteristics of TiO₂ catalysts, such as thermal stability and strong mechanical properties, have enormous potential for use in the photocatalytic treatment of contaminated water. TiO₂ is effectively used because it has a large catalyst surface area and is dispersed directly into wastewater. It raises the possibility of photocatalytic reactions [177].

8.1.3. Carbon Nanotubes

These materials have distinctive structural and electronic characteristics. As a result, they are a fascinating class of materials that have piqued the interest of scientists for fundamental research and a variety of applications, including sorption processes [178]. They are highly effective for the treatment of water and wastewater due to their high capacity for adsorbing a wide range of contaminants, fast kinetics, large specific surfaces, and selectivity towards aromatics. There are many different types of carbon nanomaterials (CNMs), including carbon beads, fibers, and nanoporous carbon, in addition to carbon nanotubes (CNTs). Among these, the most advancement and attention has been given to CNTs in recent years [179].

8.1.4. Nanocomposite

As compared to nanoparticles, nanocomposites are more stable, selective, and able to adsorb more material. A lot of research using nanocomposites has recently been performed on various heavy metal detection techniques in water [180]. The use of nanocomposites for the adsorption of different pollutants, including heavy metal ions and dyes from contaminated water, has garnered a lot of interest. The production of various nanocomposites has increased dramatically in recent years. Real composite materials should be bulky, immobile, smooth materials with nano reactivity that are anchored to or impregnated into a parent material structure. Furthermore, nontoxic, long-term stable, low-cost materials are required for water and wastewater treatment [181].

8.2. Type of Nanomaterials in Wastewater Treatment

Based on the materials used, nanotechnology can be divided into three categories. Nano-adsorbents, nano-catalysts, and nano-membranes are all examples of nanomaterials.

8.2.1. Nano-Adsorbents

Adsorbent nanoparticles are nano-sized particles made from organic or inorganic materials that have a strong affinity for adsorbing substances [182]. This means that they are able to remove a lot of pollutants. It is possible to use these nanoparticles in the removal of different kinds of pollutants due to their important characteristics, such as their catalytic potential, small size, high reactivity, and their higher surface energy [182].

8.2.2. Nano-Catalysts

In nano-catalysis, metallic nanoparticles and light energy interact, producing strong and widespread photocatalytic activities. Due to its high and broad photocatalytic activity, this treatment is gaining popularity. Inorganic components such as semiconductors and metal oxides are commonly found in nano-catalyst materials [174].

8.2.3. Nano-Membranes

The separation of particles from wastewater is the responsibility of a nano-membrane [169]. These filters are very effective at removing dyes, heavy metals, and other contaminants. Nanomaterials used as nano-membranes include nanotubes, nanoribbons, and nanofibers. Nanoparticles integrated into membranes are more practical and useful than nano-adsorbents, nano-catalysts, or nano-membranes because they not only involve an efficacious physical treatment but also nanoparticles to enhance the quality of the treatment [169].

8.3. Use of Nanoparticles in Waste Water Treatment

Several studies have undertaken use of nanoparticle catalytic activity against a variety of poisonous dyes, including Auramine O, Tymol Blue, Rhodamine B dye, Congo red, Phloxine B, Methyl orange, and many others [166]. Through the use of silver nanoparticles, up to 90–100% degradation of several colors was observed. Furthermore, the dye degradation process employing nanoparticles is rapid and does not include any toxic chemicals commonly seen in biological and chemical wastewater treatment. Researchers recently highlighted the efficient dye degradation potential of green synthesised AgNPs for methyl orange, methyl red, and Congo red (individually), and synthesised AgNPs from Sophora mollis leaf extract indicated 88 percent methylene blue degradation in 160 min in another study [163].

Electron transport may be used to explain the dye degradation reaction. The catalytic activity of chemically synthesized AgNPs (CH-AgNPs) is directly dependent on the capacity of NaBH₄ to donate electrons and the dye molecule to receive electrons [163]. To begin, the NaBH₄ and dye molecule is absorbed into the surface of CH-AgNPs. NaBH₄ functions as a strong nucleophilic agent after absorption, whereas the dye molecule works as an electrophilic agent. CH-AgNPs operate as a relay system in the solution, assisting in the transmission of the electron needed for dye degradation from NaBH₄ to the dye molecule [183,184,185]. The dye molecules decomposed into little colorless compounds such as SO4₂⁻, CO₂, H₂O, and others during the dye degradation process [186].

9. Reuse of Industrial Wastewater

Water costs have risen as a result of increased industrial water demand and shortage in some areas. Furthermore, the new environmental rules emphasize the recycling and reuse of water. Reusing wastewater has both environmental and economic benefits [187].

On the one hand, it reduces pollution discharge into the environment, while on the other, it allows for reduced water usage and depuration process costs. If all types of wastewater are decreased, firms can save up to 20–50% on water and effluent treatment costs, boosting profitability. The next stage is to implement suitable water conservation measures, such as water reuse, which may be performed in a succession of water tanks that are gradually utilized in product rinsing. Treating and reusing of the industrial wastewater have been suggested to be important strategies to conserve water, and the reduction of dye induced pollution and health hazards. If the cleaned water is to be reused in an industrial operation, it is usually pumped into a holding tank and utilized as needed [188].

Traditional wastewater treatment system consists of primary, secondary, and tertiary units to remove toxic dyes from effluents. However, some kind of inorganic as well as organic material remains in treated industrial water. Therefore, it is very necessary to use high end technologies for their effective removal and reuse of water. In broad terms, water reuse refers to the process of reusing treated water for beneficial purposes such as non-potable urban applications (such as toilet flushing, street washing, and fire protection), agricultural and landscape irrigation, industrial processes, groundwater recharge, recreation, and direct or undirected water supply. Water reuse can be seen as a long-term strategy that is both environmentally protective and cost-effective. It was reported that wastewater treated by combined chemical coagulation and electrocoagulation shows a decrease in pH values. However, an increasing trend was seen in cases of COD, TDS, and EC, which was observed after the treatment [189].

10. Wastewater Reuse: Advantages for Textile Industries

10.1. Lesser Environmental Issues

The most significant benefit that wastewater reuse will provide to the textile industry is a reduction in environmental impact. Textile sector wastewaters do really contain considerable amounts of phosphates, dyes, oils, metals, and other contaminants [190]. Obviously, all of these might pose a serious environmental threat, with negative consequences for the surrounding areas. Such dangers may always be prevented with suitable wastewater treatment procedures [191].

10.2. Improved Performance

Efficient wastewater treatment arrangements can help organizations improve their performance. Leading wastewater reuse technology manufacturers, such as ground water technologies (GWT), provide innovative and environmentally friendly solutions that have been proved to improve the performance of wastewater businesses. Furthermore, they are compatible with existing systems and do not need extensive modification [192].

10.3. Economically Beneficial

It really is critical to recognize that appropriate wastewater reuse may be transformed into a valuable resource with major economic benefits. Many industries are reliant on wastewater that has been treated. Textile businesses, which generate large amounts of wastewater, can certainly meet the needs of others. Normal wastewaters, not merely cleaned water, are also being sought for diverse applications to suit varied demands [193].

10.4. Dealing with the Issue of Rising Water Demand

Waters produced after wastewater treatment are critical for sectors such as textiles, which require large amounts of water for manufacture. It implies that these treated fluids may be utilized in regular manufacturing or production processes, which will be very cost-effective, especially with the support of renowned service providers such as GWT.

10.5. Remedy for a Drought-Stricken Area

For its wide applications, the textile industry is heavily reliant on freshwater. Wastewater reuse may considerably reduce the issues associated with freshwater, as well as the reliance on it. Proper wastewater treatment is especially important for textile businesses, which require a large quantity of fresh water, on hot days or in drought-affected areas, when the hazards to freshwater are greater.

10.6. Better Quality Product

The use of fully treated water improves the finished product’s quality since it is completely free of dangerous chemicals. In today’s world, wastewater reuse is purely a mechanical process. Its implementation necessitates the least amount of people. It indicates that the industry will not have to devote any additional resources or people to improve the quality of its output.

10.7. Zero Liquid Discharge (ZLD)

If an industrial site dumps its effluents into streams, deep wells, sewers, and/or other waterways, there are certain rules in place. Freshwater is becoming increasingly limited, which has negative consequences for the local economy, water security, and ecology. By investing in a ZLD wastewater treatment system, water can be conserved, which is critical to industrial operations, while also safeguarding vital community resources [194]. Many industrial facilities and effluent regulators are bolstering initiatives to decrease industrial process waste in response to an increased push toward environmental protection.

A ZLD treatment system makes use of innovative scientific water treatment procedures to keep liquid waste at the conclusion of your industrial process to a bare minimum, as the name implies. A properly designed and effective ZLD treatment system should be able to accommodate fluctuations in waste contamination and flow, as well as modify chemical amounts as needed.

11. Conclusions

Due to the discharge of harmful and dangerous compounds, environmental pollution is becoming a huge problem all over the world. Dye decolorization and degradation are the most pressing challenges in textile manufacturing today. The goal of this study was to evaluate published studies to gain insight of various procedures and strategies for removing pollutants or decolorization of textile industry effluent, particularly dyestuffs. However, these techniques are unable to totally remove the color from effluent. Therefore, there is an urgent need to establish an economical, environmentally friendly, and efficient procedure for the decolorization of wastewater. In this literature survey, industrial-scale enzyme-mediated dye removal techniques were found to be very promising. However, it is necessary to solve issues including the selection of strains for optimum enzyme production and their effective large-scale culture. Furthermore, silver nanoparticles-based dye removal techniques have proven to be effective for wide range of dyes. However, the nanoparticle-based approach also faces some key challenges in dye wastewater treatment which need to be addressed urgently. Considering these drawbacks, this review suggests that the future solution to the issue of toxic dye-based wastewater from textile industries could be the combination of novel nanotechnologies and enzyme degradation techniques. Environmental conservation will require the synchronic use of both conventional enzyme degradation techniques and nanotechnology.