The textile industry is a diverse sector in terms of production of raw materials, operating processes, product development, and equipment (1). The industry is recognize for consuming large amounts of water, energy, and discharging high volumes of waste in to public sewage treatment plants (1-2). Wastewater generated from textile operating processes varies in composition due to the different chemical and physical treatments applied to different types of fibers (3). Textile pollutants of environmental concern include toxic organic compounds, color, suspended solids, and biochemical/chemical oxygen demand (BOD5/COD) (1-2). The removal of COD and BOD are important from an environmental point view since high levels can deplete the level of dissolved oxygen in the receiving rivers causing an increased amount of non-biodegradable organic matter (4).
Textile wet processing stages such as dyeing and finishing mills contribute to the major pollution loads in the industry sector since these processes use a wide range of chemicals to achieve the desired properties (luster, softness, etc.) on the textile product (1,3). The development of environmentally safe practices for dyeing textiles can impose technical and economical limitations given that both the wastewater quality and quantity depend to a considerable degree on the application used for a certain substrate (fiber or fabric) such as the dye type (e.g. reactive dyes) and auxiliary chemicals (e.g. wetting agents) (3,5). Dyes that are use in textile industry pose an environmental concern because they are design to produce colors that are resistant to oxidizing and reducing agents, washing, and light exposure (4-5). These properties make color removal from textile wastewater using biological treatments ineffective and most treated effluents are color upon leaving the plant (4).
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Different treatments and combinations of textile wastewater treatment technologies proposed in literature include biological treatments, activated carbon adsorption, electrocoagulation, electrochemical oxidation, ozone, and membrane filtration. Biological treatments are effective in treating effluent to governmental standards but ineffective in removing dyes (1, 4). Activated carbon adsorption is an alternative method to combine with biological treatments for effective removal of COD, heavy metals, and color however operating costs are high (2,4). Electrocoagulation is a low cost electrochemical wastewater treatment technology that can be use to remove color however an area of concern is the regular replacement of sacrificial electrodes due to oxidation in effluent stream (6). Electrochemical oxidation and ozone technologies are effective in removing color and organic pollutants in wastewater however capital and running costs are areas of concern (2). Membrane filtration processes are an advance treatment technology for treating water to be reuse and recycle in the industry sector (1,4).
The main challenges in the textile wet processing sector are modifying production methods to control pollution and disposal, reduce the cost of effluent treatment, and decontaminate effluent for reuse in textile operations (1,3). The primary aims of this paper are to describe two novel methods for reducing water consumption and pollution in textile dyeing process and review applications for textile wastewater treatment using electrocoagulation, electrochemical oxidation, and ozone. In addition, the treatment of textile wastewater for reuse using membrane filtration technologies such as nanofiltration and reverse osmosis will be discuss as a secondary treatment method.
Cotton, which is the worlds most widely used fiber, is a substrate that requires a large amount of water (70-150L for 1kg of cotton) for processing (1-2). The treatment of fiber or fabric with chemical pigments to impart color is call dyeing (1). Water used in the form of steam to transfer dyes on to substrate (1-2). Water is a "poor" medium for transferring dyes on to the fabric from an environmental viewpoint. The dyes used in coloring cotton are anionic (negatively charged) (5). Cotton develops a slightly negative charge in water because of the ionization of hydroxyl groups in cellulose (7) and therefore a large amount of salt (0.5-0.6kg NaCl) and alkali are added to the dye-bath to reduce the charge repulsion effects between the hydroxyl anions in cellulose and anionic dyes (e.g. reactive dyes) (1). At the end of dyeing process the dye-bath is heavily polluted with toxic organic compounds, electrolytes, and residual of dyes of which can be expensive to purify and recover (1,8). Effluent disposal is the primary option (1), since recycled water to be reuse in the wet processing stages needs to meet certain requirements such as no color, no suspended solids, low chemical oxygen demand (COD), and low conductivity (9).
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Cotton and other cellulose fabrics are mainly color with reactive dyes because these dyes have good light stability and wash fastness characteristics but poor dye-fixation yields (60-70%) (5). Dye fixation describes the rate of dye adsorption on the substrate after rinsing off with water and consequently gets discharge in dye bath (3). Reactive dyes attach on the fiber via a covalent bond formation between the reactive group of the dye and the nucleophilic group in the fiber (7,10). Wash fastness is an important factor to take into consideration when determining the durability of the product (3). Wash fastness is dependent on the covalent bond strength between the fiber and dye against alkaline and acid hydrolysis, and the efficient use of water to remove unreacted dye from the substrate (3). Approximately 40% of hydrolyzed (un-fixed) dye remains in the treatment bath at the end of dyeing process because of the competitive reaction between the hydroxyl anions (OH-) in the alkaline bath and the anionic dye molecules for the nucleophiles in the cellulose fibers (11). Therefore, an extensive demand for wash-off is required to achieve the desired wash fastness characteristics on the product (3,11). Several factors may influence the amount of dye loss such as dye application technique, dept of shade, material to liquor ratio (amount of liquid need to dye a given weight of goods) (3-4).
Influence of Cationization for Dyeing Cotton with Reactive Dyes
The application of pretreatment agents in a process known as cationization is an environmentally friendly approach proposed in literature to increase dye utilization, lower water consumption, and reduce waste in treatment baths (7-8,11-13). Cationization is a process that introduces amino groups in the cellulose fiber through a reaction between the reactive group of quaternary cationic agents (e.g. epoxy and 4-vinylpyridine) and the hydroxyl groups in the cellulose fiber (7,11). A combination of electrostatic interactions such as ion-dipole forces, hydrogen bonds, and van der Waal dispersion forces influence the adsorption of the reactive group from cationic agent on the negatively charged hydroxyl group in the cellulose fiber (11). The pretreatment agent poly(4-vinylpyridine) quaternary ammonium compound (shown in Figure 1) is an example of a cationic nucleophilic polymer applied to cotton prior to dyeing with reactive dyes. Blackburn and Burkinshaw (2003) proposed to possible reactions take place between the pretreatment agent and cellulose (1) an ion-dipole interaction between the lone pair electrons on the oxygen atom of the hydroxyl group in cellulose to the positively charged nitrogen atom in pyridine ring, or (2) a strong hydrogen bond interaction between the hydrogen atom of hydroxyl group in cellulose with the pi-electron system of the pyridine ring. The reaction between the reactive group of dye molecules and the amino-functional nucleophiles of the cationized fiber occurs via a nucleophilic substitution mechanism or a Michael addition to a double bond on the dye molecule (11).
The dyeing process can occur under neutral or mild acidic conditions without the use of electrolytes and salts (8,11-13) Dye adsorption will increase because the nucleophiles on the substrate will be highly reactive for the dye molecules because of the columbic attraction between the anionic dye molecules for the positively charge nucleophiles (7,11-12). Severe wash-off procedures are eliminating since hydrolysis of dyes normally occurs under alkaline conditions (pH 11) (12-13). Blackburn and Burkinshaw (2003) reported that using cationization process prior to dyeing with reactive dyes reduced the level of water consumption to nearly half of that applied to conventional dyeing process (<100L per 1kg of cotton). Subramanian et al. (2006) reported that the effluent of cationized cotton dyeing method substantially reduced the level of total dissolved solids (TDS) in the dye-bath (2265ppm) compared to conventional dyeing method (15200ppm).
The Kubelka-Munk equation  is use to measure color strength (K/S) values (13). The color strength (K/S ratio) is proportional to the concentration of dye molecules adsorbed in the substrate (7). The maximum wavelength of absorbency on the dye cloth can be use to calculate the reflectance value (R) (11,13).
Montazer et al. (2007) reported that the color strength (K/S) values for the treated cotton fiber were 2-4 times higher than untreated fiber where K/S values range from 1-4. Kanik and Hauser (2004) concluded that the amount of cationic reagent required for the cationization process depends on the dye concentration and dye substantivity (dye affinity to fiber). If the dye has low substantivity then additional cationic reagent and dye concentration is required for the dyeing process (12). Subramanian et al. (2006) investigated the effect of temperature during the cationization process. The authors determine that at higher temperatures the rate of penetration of the cationic reagent for the substrate will be adsorb less and hence influence the dye adsorption on the substrate. The optimal temperature for cationization process suggested at 70°C (13).
Textile Dyeing in Supercritical Carbon Dioxide
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The application of supercritical fluid technology in textile wet processing industry can be environmentally friendly, energy saving, increase productivity, eliminate effluent disposal and treatment (14-19). Supercritical fluids have temperatures and pressures above its critical point including densities and viscosities between those of the gas and liquid state of the substance (19). The development of a water-free dyeing procedure can be accomplished using supercritical carbon dioxide (SC-CO2) (14). Dyeing in SC-CO2 has several beneficial properties these include low in cost; CO2 can be recycled after use, non-toxic, not volatile, and non-flammable (15). In addition, no additional energy is required to dry the fabric after the dyeing process (14). SC-CO2 exhibits densities and solvating powers similar to liquid solvents adding to its advantage in textile processing since its low viscosity and rapid diffusion properties allow the dye to diffuse faster into the textile fibers (14,16). An illustration of the equipment used for dyeing textiles in SC-CO2 is shown in Figure 2 (16). It consists of a vessel
The application of SC-CO2 for dyeing synthetic fibers has been successful (16-17). In dyeing polyester textiles, SC-CO2 penetrates inside the fibers causing them to swell for increasing the accessibility of dye molecules to the substrate (14,16). As the pressure is lower, the dye molecules are trap inside the shrinking polyester fibers and no waste is generate since the dye molecules cannot be hydrolyze (16). On the other hand, dyeing cotton and other natural fibers in SC-CO2 medium has been unsuccessful primarily due to the fact that most dyes used in SC-CO2 medium are nonionic (e.g. disperse dyes) dyes thus limiting their application for dyeing polar substrates (fibers). Water-soluble dyes are mainly use in dyeing cotton and fix on to substrate by chemical (e.g. covalent bonds) or physical (e.g. van der Waal forces) bonds (15-16). Therefore, further research is required to enhance the affinity of nonpolar or polar dyes for polar substrates in SC-CO2 medium.
Van der Kraan et al. (2003) reported that four factors influence the role of supercritical CO2 dyeing on natural fibers (1) dye substantivity (affinity) for the substrate; (2) dye solubility of SC-CO2 at operating pressure and temperature; (3) fiber accessibility to allow diffusion of dye molecules on substrate pores; (4) the reactivity of dye with the textile. Based on the author's studies of synthetic and natural textiles with reactive dichlorotriazine dyes in SC-CO2 the limiting factors for supercritical dyeing are dye substantivity and accessibility of the dye molecules for the substrate pores; the influence of pressure and temperature on coloration was not observed. Ozcan et al (1997) investigated the solubility of eight disperse dyes (non-ionic) with different structures in SC-CO2 medium and reported that there was no correlation between the molecular properties (e.g. molecular weight, polarity) of the dyes and solubility.
Several methods have been investigated to improve dyeing of cotton and other natural fibers in SC-CO2 medium: (a) pretreatment of cotton with a swelling agent polyethylene glycol (PEG) in SC-CO2 medium and benzamide crystals to act as a "synergistic agent" for increasing dye fixation on to cotton fiber (19); (b) development of SC-CO2/pentaethylene glycol n-octyl ether (C8H5) system using 1-pentanol as a co-surfactant to increase solubility of ionic dyes (e.g. reactive dyes) (18); (c) dyeing cotton and other natural fibers (e.g. wool and silk) with dichlorotrazine dyes to facilitate the dye-fiber reaction and enhance fiber accessibility for dye molecules (17). However, these methods add additional chemicals and extra processing stages that may not be necessarily ecological and economical for development of a green process. In addition, satisfactory light fastness and wash fastness characteristics are obtained (14).
Fernandez et al. (2005) investigated the use of non-polar reactive dyes with a monofluorotriazine reactive group for dyeing cotton in supercritical carbon dioxide. The effects of adding acids such as acetic acid and phosphoric acid along with cosolvents in the SC-CO2 dyeing medium were analyzed. The authors determine that two critical steps for dyeing cotton in supercritical carbon dioxide is the pretreatment step prior to dyeing and cosolvent applied during the dyeing process. A solution of methanol was added to cotton to swell the fibers. This pretreatment step modifies the properties of cotton such that the hydrophobic part of methanol will allow the cotton fibers to be more accessible to the non-polar reactive dyes and diffuse across the fiber to contact sites of cellulose (14). Methanol was the primary cosolvent used to act as "dye carriers" and ensure swelling of the fibers (14). The authors concluded that "an outstanding dye fixation of 99% on cotton dyed in SC-CO2 was achieved using monofluorotriazine reactive dyes and by adding small quantities of acids" (14). High color strength values in the range of 9.8 and 17.3 were achieved using monoflurotriazine reactive dyes. In addition, the authors demonstrated that these results are reproducible at a larger scale with less dye and acid consumption.