The textile industry uses the electrochemical techniques both in textile processes (such as decolorizing fabrics and dyeing processes) and also in wastewaters treatments (color removal from waste water).The industry is in the need of New Green Technologies for different Textile processes and waste water treatment. There is growing awareness and readiness to adapt new green technologies for Cleaner Production methods. Such new green technologies help industries to achieve green production and cost reduction at the same time. Therefore there is an urgent need to promote new green technologies in textile processes.Electrochemical reduction reactions are mostly used in sulfur and vat dyeing, but in some cases, they are applied to effluents discoloration. These electrogenerated species are able to bleach indigo-dyed denim fabrics and to degrade dyes in wastewater in order to achieve the effluent color removal. The main objective of this paper is to review the electrochemical techniques applied to textile industry.
KEY WORDS: Electrochemical, Textile, dyeing, sulfur dye, vat dye.
Electrochemistry refers to the use of electrical energy in initiating chemical reactions, replacing traditional aid agents in direct chemical reactions. Traditionally, the electrochemical techniques have been used for the synthesis of compounds or for metal recovery treatments. But now a days electrochemical techniques are used in the bleaching of textile materials. Their application in sulfur- and vat-dyeing processes is also interesting. In this case, dyes are reduced by means of an electrochemical reaction (instead of sodium dithionite). In this way, sulfur and vat dyeing become cleaner processes as the addition of chemical reagents is not required.
Although the electrochemical methods play an important role in the different textile processes listed above, their wider range of applications are related to color removal in wastewater treatments in particular, in the degradation of nonbiodegradable dyes (such as reactive dyes). This kind of dyes requires additional treatments to obtain uncolored effluents. In general, the electrochemical methods are cleaner than physicochemical and membrane technologies because they use the electron as unique reagent and they do not produce solid residues.
THE ELECTROCHEMISTRY IN THE TEXTILE PRODUCTION PROCESSES
BLEACHING OF TEXILE MATERIALS
Cotton bleaching takes place after the scouring process with the aim of destroying the natural raw color of this fiber. The most common reactive to provide whiteness to cotton is hydrogen peroxide. Chong and chu reported the use of electrochemical techniques to generate in situ this oxidant required for cotton bleaching by the electrolysis of oxygen in the presence of an alkaline electrolyte. This electrolyte proceeds from the scouring process. They propose the use of the electrolysis process in a combined scouring and bleaching process, and they concluded that the whiteness obtained in the combined method is comparable to that obtained with conventional methods. Although the electrochemical techniques have been applied to bleach raw fibers, their main application in bleaching field is the discoloration of indigo–denim-dyed fabrics. An important step in the processing of indigo-dyed textiles is the finishing of the garment to obtain the required visual effect. The removal or destruction of part of indigo requires a combination of mechanical agitation and chemical attack, mainly with oxidizing agents.
The most useful oxidant for bleaching indigo denims is hypochlorite. The conventional method to obtain the decolorized effect of these denims is based on the addition of this chemical reagent to the dye bath, but recently the generation in situ of the hypochlorite by an electrochemical oxidation is becoming a more attractive method, because it offers several advantages with respect to the conventional method:
- Improvement in the process control and consistency,
- Lower-process costs due to the production of more regular shades, the possibility of bleaching bath regeneration and the lower amount of effluent generated.
DYEING PROCESS: DYES REDUCTION
Vat dyes, especially indigo, play an important role in textile industry. They are insoluble in water and cannot dye fibers directly. They must be reduced in alkali medium to become soluble in water. When the dyes are absorbed onto the fiber, they return to their original form by a subsequent reoxidation. Sulfur dyes also are water-insoluble dyes, containing sulfur as an integral part of the chromophore group. The alkaline-reduced form is required for the dyeing process and subsequently, when they are added to the fiber, they are oxidized to the insoluble form. In attempt to increase the ecoefficiency of these dyeing processes, electrochemical techniques have been investigated in the reduction of such dyes, which avoids the addition of reducing agents as sodium dithionite. Sodium dithionite (Na2S2O4) is the most used reducing agent in the industrial dyeing process with this kind of dyes, but after its reaction, it cannot be recycled. It also produces large amounts of sodium sulfate and toxic sulfite products. For this reason, the treatment of dyeing effluents requires the addition of hydrogen peroxide, which also causes high costs and other additional problems.
The most attractive new procedures to reduce vat and sulfur dyes are electrochemical reduction methods, because the addition of reducing agents is not required. This method also avoids the generation of toxic products due to the reaction between the added reagents and the dye molecules. For all these reasons, electrochemical reduction processes are considered more suitable: no reagents addition is required, no byproducts are formed and no tertiary treatments are necessary to treat the final effluents. The energy is the only requirement of electrochemical methods. Electrochemical techniques constitute a promising field for the different steps of textile process, but their application to the dyeing of vat and sulfur dyes is specially interesting to avoid the use of reducing reagents.
WASTEWATER COLOR REMOVAL
The textile industry produces large volumes of wastewater in its dyeing and finishing processes. These effluents have as common characteristic their high coloration. Colorants, the additive substances that cause a variation in color, can be divided in dyes or pigments. Pigments in general are insoluble substances which have not the chemical affinity to the substrate to be colored; otherwise, dyes are generally soluble (or partially soluble) in organic compounds
Several methods are used for the removal of organic dyes from wastewaters. Most of dyes are only partially removed under aerobic conditions in conventional biological treatments. As biological treatment is insufficient to remove color and to accomplish with current regulations, the application of Some electrochemical color removal methods have been applied to industrial effluents. The current physico-chemical methods, based on the separation of dyes from the effluents, produce a residue which requires an additional treatment to be destroyed. Also, the absorbent materials require their regeneration after several treatments, and the filtration and membranes methods need cleaning treatments. Chemical oxidation methods are rather expensive and involve some operational difficulties. Biological treatments are a simple method but supply inefficient results in discoloration because dyes have aromatic rings in their large molecules that provide them chemical stability and resistance to the microbiological attack. Enzymatic decomposition requires further investigation in order to know which enzymatic process takes place; moreover, temperature and pressure have to be controlled to avoid enzymes denaturalization.
For these reasons, the electrochemical methods are nowadays the subject of a wide range of investigations at laboratory and pilot-plant scale. The advantage of these electrochemical techniques is that electron is a clean reagent. They also have good versatility and high-energy efficiency. They are easy for automation and safety because it is possible to operate at smooth conditions. The main types of electrochemical methods applied to wastewater treatment, briefly described below.
Electrocoagulation systems provide electrochemical aggregation of heavy metals, organic and inorganic pollutants, to produce a coagulated residue to be separated or removed from water.
This technique is an indirect electrochemical method which produces coagulant agents (Fe3+ or Al3+) from the electrode material (Fe or Al) in hydroxide medium. These species, that is, Fe(OH)3, can remove dissolved dyes by precipitation or by flotation. These complexed compounds are attached to the bubbles of H2 (gas) evolved at the cathode and transported to the top of solution. The inconvenient of the Electrocoagulation in comparison to the other electrochemical methods is that it produces secondary residues (the complex formed with pollutant and hydroxide) which implies in the usage of tertiary treatments.
Indirect Oxidation Methods
The indirect electro-oxidation occurs when strong oxidants are generated in situ during the electrolysis and react with the organic pollutants such as dyestuffs, producing its total or partial degradation.
- Mainly two methods one used:
- The first one is the electro-oxidation with active chlorine, which is the major oxidizing agent. In this case, free-chlorine gaseous and/or the generated chlorine-oxygen species such as hypochlorous acid (HClO) or hypochlorite ions (ClO−) depending on the pH, oxidize the organic matter present in the effluents.
- The second one is the electro-Fenton process , where organics degradation occurs by hydroxyl radicals (OH•) formed from Fenton’s reaction between catalytic Fe2+ and H2O2, this hydrogen peroxide is also electrogenerated from O2 reduction.
This technique has an important inconvenience: a strong acidic medium is required. As the reactive dyeing process is carried out in basic medium (generally pH > 10), a high amount of acid has to be added before the treatment. Subsequently, the treated effluent must be neutralized to be discharged. Consequently, the whole process produces a high increase of the wastewater salinity.As some industrial wastewaters contain large amounts of chloride, the first approach is more suitable to treat this kind of effluents, because the addition of any chemical product is not required whereas in second case, Fenton reagent is needed. In contrast, the combination of electrochemistry and chloride can produce haloforms such as chloroform, although it is not an inconvenient if the treated water is degraded lately in a biological plant to accomplish its mineralization.
Color Removal from Textile and other Industrial Wastewater using Ozone
Ozone has been used for successfully for removal of color from textile wastewater streams in plants around the world as well as in other industrial wastewater processes. In wastewater treatment, ozone is often used in conjunction with biological treatment systems such as activated sludge. Organic dyes are mostly refractory due to their large molecular size and they can be poorly removed by adsorption on activated sludge. In some cases ozone has been used before the biological process, but mainly after biological treatment. If the wastewater is hardly biodegradable or toxic to activated sludge pretreatment is an option.
Ozone can be used prior to a biological process since it has a tendency to convert organic molecules into smaller more biodegradable species. This can enhance the efficiency of the biological process. In addition, ozone treatment of wastewater increases the oxygen content of the water (unconverted oxygen and ozone that decomposes back to oxygen that was mixed with the water) which results in improvement in aerobic processes. While this benefit is well known in the literature it is difficult to practically apply since the amount of improvement is difficult to predict and pilot studies involving ozone and biological processes are difficult to carry out. The effect of ozone on improving biodegradability and reducing toxicity is worth noting in terms of the effect of the treated water on the receiving stream. Where the treated water is tested for toxicity, the impact of the treatment process on this parameter must be considered. Destroying one organic molecule, but creating more toxic ones in a treatment process has been observed, for example the ozonation of MTBE without any additional agents or treatment processes can result in a more toxic wastewater. Another consideration is the presence of surfactants and the need to remove these compounds from the water. In some locales surfactant concentrations are tightly controlled and must be kept under 1 ppm. This creates an additional demand for oxidant. Some textile waste waters contain both color and surfactants.
Ozone is effective in removing the color from all dyes used in textile processing. The amount of ozone can vary depending on a number of factors: how much color was removed in the biological process, the type of dye used, where ozone is applied in the process, etc. Knowing the proper amount of ozone required to meet the color removal objective for the receiving water body is critical to the economics of the ozone system. In general it is not easy to predict the amount of ozone required, so in virtually all cases where specific previous experience is not available, pilot testing is employed.
The electrochemical techniques have been proved to be efficient in different oxidation or reduction steps of the textile processes such as: bleaching denim fabrics or reduction of sulfur and vat dyes, where their applications are available in both natural and synthetic fibers. They constitute a less harmful alternative than the traditional processes. In addition, the electrochemical treatments have been extensively applied to the decontamination of wastewaters from the textile processes. The possibility of reusing dyeing effluents treated by electrochemical methods is particularly interesting and it implies an important saving of water and salt. This kind of studies is especially important in Mediterranean countries, where the river flow rates are low and their salinity is nowadays an increasing environmental problem.
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