Rapid industrialization leads to serious environmental hazards due to the increase in the release of pollutants into the environment. Industries that use synthetic dyes for different applications are a predominant source for dye contaminants by releasing the dye in wastewater with pretreatment or without treatment directly into the water bodies, making serious water pollution in the environment. Therefore, it is imperative to safeguard the environment from such contaminants and their associated negative impacts. The conventional treatment method that is used to treat dye-contaminated wastewater is generally costly and has a possibility to produce secondary metabolites. Due to the above problems, the biological method is preferable to treat effluent or dye-contaminated wastewater. Phycoremediation is an algae-based eco-friendly dye abatement technique from contaminated environments. This review highlights the phycoremediation of dyes and its underlying mechanisms along with the information on synthetic dyes, classification, hazardous effects, and other major techniques of dye abatement. This review provides a comprehensive insight into several influencing factors such as pH, temperature, contact time, the dose of algae biomass, and agitation speed, as well as functional groups involved in the phycoremediation process.
Dyes are becoming more widely used as a result of their numerous applications in various industries. Dyes are difficult to remove from wastewater due to the aromatic molecular structure that makes them much more stable to light, heat, and oxidizing agents [
Synthetic dyes are becoming more common in the textile and dyeing industries due to their ease and cost effectiveness in synthesis, as well as other advantages such as high light stability, firmness against temperature, detergents, and microbes, and color variety, when compared to natural dyes.
During the dyeing process, the main environmental problem associated with the use of dyes is their discharge because the fixation efficiency varies from 60 to 90% [
A dye is a coloring material that is used to provide color or to modify existing color for various substances. Dyes have chromophores and auxochromes that are substantively responsible for their color [
Characteristics of synthetic dyes [
Synthetic dyes | General description | Uses | Ionic nature | Light fastness | Washing fastness | Hot pressing fastness | Dry cleaning fastness | Seawater fastness | Solubility | Application pH |
---|---|---|---|---|---|---|---|---|---|---|
Acid dye | Originated from basic dye acidification; complete color range | Primarily for wool and silk; also, acetate, acrylic, and nylon | Anionic | Very good | Poor | Not affected | Good | Fair | Soluble in water | 4–5 |
Basic dye | First synthetic dye (1856), first coal tar dye | Primarily for wool, silk, nylon, and cotton | Cationic | Poor | Poor | Not affected | Mostly poor | Very poor | Soluble | 5–6 |
Direct dye | Dyes cellulosic fabrics directly; some dye wool and silk | Primarily for cellulosic fabrics | Anionic | Good to excellent | Poor | Good | Good | Poor to good | Depends on the types of direct dyes | 7 |
Disperse dye | Developed for acetate | Primarily for acetate, also, polyester, nylon, and cellulose fibers | Nonionic | Fair to excellent | Fair to good | Some color change is possible | Good | Good | Slightly soluble in water | 4–5 |
Reactive dye | Forms a covalent bond with the fiber | Primarily for cotton apparel | Anionic | Very good | Good | Not affected | Good | Good | Depends on the types of reactive dyes | 11–13 |
Sulfur dye | Insoluble in water, complete shade range | Used for linen, cotton, and jute | Nonionic | Poor to fair | Poor to good | Good | Good | Good | Insoluble | 10–11 |
Vat dye | Synthetic indigo original | Used for cotton and wool | Nonionic | Excellent | Good | Good | Good | Good | Insoluble, soluble leuco salts | 12–13 |
Color of the dye is a combination effect of chromophores and auxochromes. Chromophores describe the delocalized electron system with double bonds conjugated alternatively; auxochromes refer to electron-withdrawing or electron-donating substituents that enhance color of the chromophores by changing the electron system’s overall energy. Some of the essential chromophores include classes of N, CO, NO2, and quinoids and NH3, OH, SO3H, and CO2H. The bathochromic effect is shifting adsorption bands to longer wavelengths on a conjugated system of the dye that is advanced by the combination of both chromophores and auxochromes. Also, instead of increasing the chromophores in color processing, auxochromes are responsible for dye solubility and increasing its reactivity to fibers [
Dyes are broadly categorized based on (i) chromophoric groups in their chemical structures as azo dyes, anthraquinone dyes, phthalocyanine dyes, etc., (ii) their method of use or application as polyester dyes and cotton reactive dyes, and (iii) their dissociation into an aqueous solution as acidic, basic (cationic), direct reactive (anionic), and disperse/nonionic [
Reactive dyes are the only textile dyes designed during the application process to form a covalent bond with the substrate. The reactive dyes offer a wide range of shades of good light fastness and excellent cotton wash fastness. These properties situate this class of dyes at the end of the market quality [
Acid dyes are used to color nylon, wool, silk, leather, paper, food, and cosmetics. Acid dyes include chemical groups of azo, anthraquinone, triphenylmethane, azine, xanthene, nitro, and nitroso [
Azo dyes are the largest group of dyes with a functional group of N=N as a chromophore in an aromatic system. Chromophores contain functional groups such as –N═N–, –C═O, –NO2, and O = (C6H4) = O (quinoid assemblies), and auxochromes contain functional groups such as –NH3, –COOH, –OH, and –SO3H [
Triarylmethane dyes are hydrocarbon derivatives that contain two types such as acidic and basic. Triarylmethane acid dyes contain at least two groups of SO3H which are used to indicate wool and silk dye fibers. Besides, dyes that comprise only one group of SO3H are used as indicators, such as phenolphthalein. On the contrary, basic triarylmethane dyes are massively used in the production of stamping inks, writing, and printing [
The sulfonic acid group in anthraquinone dyes is responsible for their water solubility. Mainly, these groups of dyes are used to dye in wool and silk industries due to their ability towards auxiliary binding agents. Food coloring dyes are a subclass of acid dyes that are used to dye protein fibers and certain nylon fibers at high temperatures [
Direct dyes are massively used to dye protein fibers and dye synthetic fibers such as nylon and rayon. These dyes lack fast-drying features after applied to fabrics. Basic dyes formed a colored cationic salt when dissolved in water. These cationic salts bind with an anionic substrate. Basic dyes are also known as cationic dyes and are found to be powerful coloring agents for acrylic fibers [
Disperse dyes are water-insoluble nonionic dyes that are commonly used on polyester, nylon, cellulose, and acrylic fibers. They encompass several groups such as azo, anthraquinone, styryl, nitro, and benzodifuranone. The dyes used for plastics, gasoline, lubricants, oils, and waxes are solvent soluble. These dyes are principally azo and anthraquinone chemical classes. The dyes used for coloring cotton, rayon, silk, leather, paper, and wood are called sulfur dyes. Vat dyes are insoluble in water and used to dye cellulosic fibers. The main chemical groups of these dyes are anthraquinone and indigoids [
The effluent discharged from the dyeing industries is the major cause of water pollution because it contains a huge volume of dyes. The high concentration of dyes in water bodies blocks sunlight and reduces the oxygenation potential of receiving water, affecting aquatic biodiversity and photosynthesis. Blue, green, or brown color of watercourses is accepted somehow by the public, but red and purple colors in water bodies receive people’s concern [
The industrial effluents containing dyes affect the growth of microorganisms and photosynthetic activities of aquatic flora by reducing light penetration in receiving water bodies. Moreover, discharged dyes reduce the amount of dissolved oxygen in water by forming a thin layer on the surface of the receiving water body. Furthermore, this kind of effluent increases the chemical oxygen demand (COD) which is an indication of high-level pollution [
The hazardous effects of synthetic dyes on the environment from different industries [
Azo dyes can cause multiple health issues to humans such as skin irritation, chemosis, contact dermatitis, exophthalmos, lacrimation, rhabdomyolysis, permanent blindness, vomiting gastritis, acute tubular necrosis supervene, hypertension, vertigo, and, upon ingestion, edema of the neck, face, tongue, pharynx, and larynx along with respiratory distress [
Different conventional technologies such as chemical, physicochemical, and biological methods which are used to treat wastewater contaminated with dyes are summarized in Table
Methods of synthetic dye biodegradation and decolorization [
Adsorption | Refers to a process where a substance or material is concentrated at a solid surface from its liquid of the gaseous surrounding. |
Irradiation | Involves the use of radiations usually obtained from monochromatic UV lamps working under 253.7 nm. It is a simple and effective technique for removing a wide variety of organic contaminants and disinfecting harmful microorganisms. |
Filtration processes | Microfiltration: it is mostly employed for the treatment of dye baths containing pigment dyes. |
Ultrafiltration: this technique can remove polluting substances such as dyes only 31–76% but can be used to eliminate macromolecules and particles. | |
Nanofiltration: it is employed for the treatment of colored effluents from the textile industry mostly in a combination of adsorption and nanofiltration as NF modules are very sensitive to fouling by colloidal materials and macromolecules. | |
Reverse osmosis | This technique is used to eliminate hydrolyzed reactive dyes, most types of ionic compounds, and chemical auxiliaries in a single step. |
Electrochemical | This process is very simple and is based on applying an electric current to wastewater by using sacrificial iron electrodes to produce ferrous hydroxide. |
Oxidative processes | Oxidation by ozonation is a technology initially used in the 1970s, and it is carried out by ozone generated from oxygen. |
Chemical oxidation is the conversion of pollutants by chemical oxidizing agents (such as chlorine, ozone, Fenton reagents, UV/peroxide, and UV/ozone). | |
Oxidation with sodium hypochlorite: in this treatment, azo bond cleavage is initiated and accelerated by the attack of the dye molecule by Cl+ at the amino group. | |
The oxidation processes with hydrogen peroxide (H2O2) can be used as wastewater treatment in two systems: (1) homogenous systems based on using visible or ultraviolet light, soluble catalysts such as Fenton reagents which are strong oxidants compared to H2O2 and other chemical activators such as ozone and peroxidase. (2) Heterogenous systems based on using semiconductors, zeolites, and clays with or without ultraviolet light. | |
Photochemical oxidation: the UV treatment of dye-containing wastewater in the presence of H2O2 can break down the dye molecules into smaller organic molecules or even ultimate products such as CO2 and H2O and other inorganic oxides. | |
Coagulation | Coagulation of dyes and other auxiliaries in textile effluents has been successfully done by aluminum, iron slats, organic polymer, flocculants, etc. |
Electrocoagulation | Electrocoagulation is an advanced electrochemical treatment for dye and color removal. It involves processes such as electrolytic reactions at electrodes, coagulation in the aqueous effluent and adsorption of soluble pollutants on coagulants, and, finally, their removal by sedimentation |
Bacterial | Aerobic biological treatment: use bacteria and oxygen (from injected air) to remove dissolved organic load (COD/BOD) from dye-containing wastewater. The process is controlled by oxygen sensors in the activated sludge (aeration tank), and residual bacteria (waste activated sludge) can be separated in various ways. |
Anaerobic biological treatment: anaerobic biodegradation of water-soluble dyes including azo dyes is mainly reported to take place by a redox reaction with hydrogen leading to the formation of methane, carbon dioxide, hydrogen sulfide, and other gaseous compounds and releasing electrons. | |
Fungal | Fungal organisms can decolorize a wide range of dyes. |
Microalgal | Algae are capable of decolorizing colored wastewater through mechanisms of enzymatic pathways as well as adsorption on algal biomass. |
Enzymatic | The enzyme that is produced from microbes and plants is used to degrade or decolorize dyes. |
Conventionally, several physical and chemical methods including ultrafiltration, adsorption, flocculation, coagulation, ozonation, advanced oxidation processes, photocatalytic oxidation, Fenton process, and chemical and electrochemical coagulation were applied for the removal of color [
The biological method (bioremediation) is considered the best alternative over conventional physicochemical treatment due to its potential advantages of being inexpensive and nonhazardous. Bioremediation (using microbes) is a pollution control technology where the biological systems are used to drive the degradation or transformation of various toxic chemicals into less harmful forms. Biological treatment methods are eco-friendly methods that are gaining importance in today’s scenario. Microorganisms such as bacteria, fungi, algae, yeast, and their enzymes can be successfully utilized to remove color of a wide range of dyes through anaerobic, aerobic, sequential anaerobic-aerobic treatment, and biosorption processes [
Phycoremediation means the use of macroalgae, microalgae, and cyanobacteria for the removal or biotransformation of contaminants, containing nutrients, synthetic dyes, heavy metals, and xenobiotics from dye-contaminated wastewater and CO2 from waste air (for environmental cleanup) [
Algae are derived from the Greek word “alga” which means “phyco.” These organisms are heterogeneous, predominantly eukaryotic, and aquatic organisms that differ from microscopic cells to highly differentiated plants. Algae are categorized as either aquatic or freshwater plants which have very high carbon trapping and photosynthetic efficiencies when compared to terrestrial plants [
Abatement of various synthetic dyes by algae.
Algae | Synthetic dyes | Experimental conditions: initial dye conc. (mg/L), pH, time (hr), temp (oC), biomass (g/L) | % removal | References |
---|---|---|---|---|
Aniline blue | 25, —, —, 264, —, — | 58 | [ | |
Reactive Blue 220 | —, 8, 336, —, — | 84.2 | [ | |
Phenoxyalkanoic acid herbicide 2,4-D | —, —, —, —, — | 47 | [ | |
Malachite green | 100, —, —, —, 1 | 67 | [ | |
Blue dye | —, 10, 336, —, 3 | 76.48 | [ | |
Red dye | —, 10, 336, —, 3 | 62.63 | ||
Blue dye | —, 10, 336, —, 3 | 78.29 | ||
Red dye | —, 10, 336, —, 3 | 64.21 | ||
Monoazo and diazo | —, —, 48, 25, — | 68 | [ | |
Malachite green | —, 8.5, 17.5, —, — | 85.9 | [ | |
Rhodamine B | 100, 8, 480, 30, — | 80 | [ | |
Methylene blue | 100, —, —, —, — | 83.04 | [ | |
Methylene blue | 25, 10, —, —, 2.5 | 4.012 | [ | |
Acid Black 1 | 40, 4.2, 0.5, —, — | 96.8 | [ | |
Methyl orange | 500, 6.5, 132, —, 0.4 | 97 | [ | |
Malachite green | 5, 8, 2.5, 25, 2 | 98.3 | [ | |
Yellow dye | 10, —, 336, —, — | 3.12 | [ | |
Yellow dye | 10, —, 336, —, — | 45.03 | ||
Malachite green | 10, 9, —, —, — | 87.1 | [ | |
Methylene blue | 20, —, 144, —, — | 98.6 | [ | |
Malachite green | ||||
Reactive blue azo dyes | 100, 7, 168, 30, — | [ | ||
Acid Black 210 | 125, 2, 1, 60, 0.5 | 98.55 | [ | |
Acid Blue 7 | 125, 2, 1.25, 60, 0.5 | 97.05 | ||
Reactive Black 5 | 200, 5, 240, 40, — | 80 | [ | |
Direct Blue 71 | 200, 8, 240, 40, — | 78 | ||
Disperse Red 1 | 300, 8, 240, 40, — | 84 | ||
Malachite green | 5, —, 120, —, 0.2 | 93 | [ | |
Methylene blue | 5, —, 120, —, 0.2 | 66 | ||
Safranin | 5, —, 120, —, 0.2 | 52 | ||
Malachite green | 100, 5, —, 50, — | 97.13 | [ | |
Methylene blue | 100, 6, 1, —, 0.1 | 67 | [ | |
Malachite green | 78 | |||
Methylene blue | 25, 8, 2.83, —, 1.25 | 91.92 | [ | |
Synazol | —, 3, 18, 30, 8 | 85 | [ | |
Malachite green | —, 10, 2.5, 25, - | 80.7 | [ | |
Methylene blue | 90, 7.9, 0.5. 37, 1 | 86.1 | [ | |
Acid Blue 9 | 100, 1, 3.75, 33, 3 | 87.64 | [ | |
Methyl violet | 10, 8, 1.33, 25, 2 | 98.85 | [ | |
Malachite green | 9.7, 6.8, 1.25, —, 3.9 | 57.81 | [ | |
Malachite green | 6, 6, 1.15, —, 0.004 | 73.49 | [ | |
Malachite green | 6, 6, 1.15, —, 0.004 | 91.61 | ||
Congo red | 50, —, 216, 35, — | 100 | [ |
As the name specifies, these are microscopic algae and can be motile or nonmotile depending on the presence of the flagella. Microalgae are unicellular and photosynthetic microorganisms, usually in the size range of 1–400
Macroalgae are multicellular organisms and are generally seen without the aid of a microscope. Seaweeds, also known as macroalgae, are macroscopic multicellular algae with defined tissues and specialized cells. Macroalgae have cell types that are similar to those of terrestrial vascular plants. Furthermore, the macroalgae may be either marine or freshwater [
The molecular structure and microbial activity are influenced by the culture condition that is necessary for effective abatement of dyes. The algae efficiency of biosorption can be affected by optimizing the operating conditions such as pH, temperature, biosorbent dosage, and agitation [
pH is one of the most important parameters that affects the biosorption efficiency of algae [
Temperature plays a great role in the biodegradation of dyes. The viscosity of the solution containing the dyes decreased with an increase in temperature. The amount of adsorbed dye decreased with increasing temperature, indicating the exothermic nature of the biosorption process [
The dye uptake rate of sorbent species is rapid at the beginning of the contact time; however, as the contact time reaches the equilibrium, the uptake rate is reduced or halted. For instance, in the study of Pratiwi et al. [
Dye concentration has a major influence on the dye abatement process. The adsorption efficiency is directly proportional to the pollutant concentration. Hence, the efficiency is generally found to be higher in lower dye concentration and subsequently reduced with an increment in dye concentration [
Agitation is crucial for equal mixing of the medium components, dispersion of cells, and nutrients, as well as mass transfer phenomena in the dye abatement process. In addition to this, agitation also leads to shear force, which affects microbes in various dimensions that include morphological changes, rupture of the cell wall, variation in their growth, and product formation [
The dosage of the biosorbent (biomass concentration) is also another factor that influences the dye abatement capacity of biosorbents. There are direct relationships between biomass concentration and biosorption, which mean the number of biosorption sites increases with an increased dosage of biomass concentration that leads to efficient biosorption of dyes [
Different microorganisms may have different pathways for degrading different dyes depending upon the dye structure, strategy of the microbial system for dye degradation, and many other factors. Even small structural differences can affect the decolorization process [
Mechanisms of algal decolorization can involve enzymatic degradation, adsorption, or both. Color removal by algae is mainly due to three intrinsically different mechanisms such as assimilative utilization of chromophores for the production of algal biomass, CO2 and H2O transformation of colored molecules to noncolored ones, and adsorption of chromophores on algal biomass (Figure
Mechanisms of dye adsorption by algae [
Algae have a high surface area and high binding affinity during biosorption that facilitate the biosorption capacity. The electrostatic attraction and complexation are known to take place during algal biosorption [
Fourier-transform infrared (FTIR) spectroscopy will be used to determine the infrared spectrum of the major functional groups of the microbes. FTIR spectroscopy has the advantages of its simple, inexpensive, fast, noninvasive, and multiplex measurements. FTIR spectroscopy has already been used to study the biosorption of dyes from wastewaters using algae. The selected algal functional groups involved in the decolorization of different synthetic dyes are summarized in Table
Algal functional group involved in decolorization of different synthetic dyes.
Algae | Synthetic dyes | Functional groups involved | References |
---|---|---|---|
Methylene blue | C=C, C=O, C-H, C-O, Fe-O, and C-C | [ | |
Methyl violet | |||
Blue dye | OH, ≡ C-H, N-H, and C-X(X = Cl, Br) | [ | |
Red dye | OH, ≡ C-H, N-H, C=O, C-H, and C-X (X = Cl, Br) | ||
Methylene blue | O-H —NH2, C=O, and C–O | [ | |
Remazol Black B | C-H and CH3 | [ | |
Methyl orange | O-H, N-H, and ―N=N― | [ | |
Reactive Red 120 | O-H, NH2, CH2, C-N, P-O, S-O, and C-H | [ | |
Tartrazine and allura red | O-H, NH2, CH2, C=C, –S–O, –P–O, and –CH | [ | |
Methylene blue | O-H, C–H, C=O, –C=C-, and –CH3 | [ | |
Methylene blue | N-H, C-H, R-N=C=S, C=C, and S-S | [ | |
Methylene blue | –OH, –NH, C–H, HC=O, and R2C = O | [ | |
Malachite green | -OH, -NH, CH, -COO–,-SO3, and -C-O | [ | |
Malachite green | -OH, -NH, CH, -COO–,-SO3, -C-O, and -C = O | ||
Malachite green | OH, -NH, CH, -SO3, -C-O, and -C = O | ||
Methylene blue | N–H, O–H, –CH, C–CH3, C=O, C–OH, P–O–C, S O, and C–S–O | [ | |
Sandocryl Golden Yellow C-2G | –OH, –COOH, C=O, and C–O | [ | |
Rhodamine B | N-H, H-O/N, COO−, C=N, C-O-C, N-O, -OH, C-H, C=O, C=C, C-N, C-O, and C-Cl | [ | |
Rhodamine B | N-H, H-O/N, COO−, C=N, C-O-C, N-O, -OH, C-H, C=O, C=C, C-N, C-O, and C-Cl | ||
Rhodamine B | N-H, H-O/N, COO−, C=N, C-O-C, N-O, -OH, C-H, C=O, C=C, C-N, C-O, and C-Cl | ||
Direct Fast Scarlet 4BS | O-H, N-H, -SO3, C=O, C-N, and PO4 | [ | |
Brown algae | Crystal violet | O–H, N–H, C–H, C=O, C–OH, O–C–O, and –C–O | [ |
Methylene blue | O–H, C–H, C=O, N–H, and C–O | [ | |
Methylene blue | O–H, C–H, C=O, N–H, and C–O | ||
Methylene blue | O–H, C–H, C=O, N–H, and C–O | ||
Methylene blue | –OH, –NH, –CH2, –CH3, C=O, C-O, –COOH, and C-O-C | [ | |
Direct Yellow 12 | OH, C=C, C-H, and C=C | [ | |
Methyl violet | –OH, –NH2, –C–H, –C=O, C–O, –C–N–C, and S=O | [ | |
Methylene blue | –OH, –NH2, –CH, COOH, >C=O, C–O, >S=O, and C–N | [ | |
Malachite green | C=C, C-N, C-H, -CH3, and –C–N– | [ | |
Malachite green | –OH, –NH, and COOH | [ | |
Malachite green | –OH, –NH, and COOH | ||
Malachite green | NH2, C=O, C–O, C–O, and C–H | [ | |
Malachite green | –OH, –NH, C–H, C=O, and C–O | [ | |
Remazol Black 5 and Remazol Brilliant Blue | –OH, –NH, –C=O, N-H, –C-O, and –C-N | [ | |
Methyl orange | C-H, =C-H, C=O, C=C, NO2, P-OH, S=O, O–H, N–H, P-O, and C-S | [ | |
Malachite green | C=C, C–N, C–H, and –CH3 | [ |
The use of dyes is increasing day by day due to their multiple applications in different industries. The discharge of dyes into the environment not only produces an aesthetically unpleasing effect but also creates serious environmental concerns. Remediation of dye-contaminated wastewater by phycoremediation technology has gained much emphasis in recent years. Accordingly, phycoremediation technology plays a vital role principally in developing and underdeveloped countries due to the advantages such as ease of availability, high efficiency, cost effectiveness, large specific surface area, environmentally friendly, and chemical and physical stability. Algae have different functional groups on their cell wall such as amino, carboxyl, hydroxyl, and phosphate groups, which are responsible for the dye removal process. Various operational conditions such as solution pH, contact time, initial dye concentration, adsorbent dosage, and temperature are the crucial elements for the removal of dyes. Hence, these factors are taken under consideration during evaluating the performance of algal capacity regarding that of dye abatement. Despite the progressive developments in alga-based removal technologies, considerable limitations still exist which demand future research for the utilization of holistic phycoremediation removal techniques in an effective manner on a large scale.
The authors declare that they have no conflicts of interest.
The authors wish to acknowledge all who had been instrumental in the creation of this review article.