Dyes that are used in textile industry pose an environmental concern because of they are design to produce colors that are resistant to oxidizing and reducing agents, washing and light exposure. Natural fibers such as cotton are primarily color with azo dyes of which are environmental concerns due to their chemical and photolytic stability to degradation in water, which are ideal properties for clothes maker [Gharbani, 2008]. These properties make color removal from textile wastewater using biological treatments ineffective and most treated effluents are color upon leaving the plant. Often determining the type of chemicals and dyes used for textile dyeing can impose technical and economical limitations [Neill et al., 1999] when considering, changing conventional production methods to environmental friendly practices for reducing pollution [Ren, 2000; Babu et al., 2007].
Textile wet processing stages such as dyeing and finishing stages contribute to the major pollution loads in the industry sector since these processes use a wide range of chemicals to achieve the desired properties (e.g. luster) of the textile product [3-5]. Major pollutants of environmental concern in textile wastewater include toxic organic compounds, color, suspended solids, and biochemical/chemical oxygen demand (BOD5/COD) [1-2]. The disposal of textile effluent in the municipal STP is an environmental concern because these industrial pollutants may pass through unchanged and enter the receiving rivers or streams potentially harming the welfare of aquatic life . The adverse effect of these pollutants on the aquatic environment include depletion levels in dissolved oxygen, reduction in photosynthetic activity, and increase susceptibility for organisms to acids and bases [2,6].
Textile wastewater treatment technologies proposed in literature include biological treatments, electrocoagulation, electrochemical oxidation, ozone, and membrane filtration. Biological treatments are effective in treating effluent to governmental standards but ineffective in removing dyes which are complex structures with high molecular weights and not easily biodegradable [9, 7]. A low cost electrochemical wastewater treatment technology that can be use to remove color is electrocoagulation. Electrochemical oxidation and ozone technologies are effective in removing color and organic pollutants in wastewater . Membrane filtration processes are an advance treatment technology for the purification of water to be reuse in the industry sector [3,9]. The review paper is center on textile dyeing and effluent treatment and hence organized in two sections. The first section describes two novel methods for dyeing cotton (1) pre-treating cotton with cationic reagents to enhance dye fixation, and (2) replacing water with supercritical carbon dioxide (CO2) as a dye transfer medium. The second section involves a detail description on the treatment of textile wastewater with an emphasis on dye removal using electro-coagulation, electrochemical oxidation, ozone, and membrane filtration technologies. [Revise thesis statement]
Dyes, Dye Fixation and Wash Fastness
Two types of dyes mainly used to color cotton and other fabrics are direct dyes and fiber reactive dyes. Both types are anionic. Direct dyes create a relatively weak hydrogen bond with fabric cellulose polymer forming a semi durable attachment. Direct dyes are easier to use and less expensive however they are not as wash fast as reactive dyes. Direct dyes exhibit good light fastness and are used on textiles such as curtains and upholstery that are seldom or never launder. Cotton and other cellulose fabrics is colored with reactive dyes because these dyes have good light stability and wash fastness characteristics but poor dye-fixation yields (60-70%) . Dye fixation describes the rate of dye adsorption on the substrate after rinsing off with water and consequently gets discharge in dye bath . 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 [8-9]. The dye-fiber reaction is facilitated by large amount of salt and electrolytes that reduce the charge repulsion forces between the negatively charge dye molecules and the negatively charge hydroxyl groups in cellulose [9-10,12].
Wash fastness is an important factor to weigh into consideration when determining the durability of the product . 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 . Approximately 40% of hydrolyzed (un-fixed) dye remains in the treatment bath at the end of dyeing process as a result 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 [1,11,14]. Therefore, an extensive demand for wash-off is required to achieve the desired wash fastness characteristics on the product . 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) . Dyes go through a water-soluble phase then individual dye molecules penetrate into the cellulose fibers and finally the dye attaches to the polymer chain that makes up the cotton fiber. Dye houses confront energy use issues. In order to dye fabric evenly the fabric and the dye solution is in constant motion. This requires pumps to keep the solution circulating and some form of mechanical process to pull the fabric through in a continuous motion.
The development of environmentally-friendly practices for reducing water consumption and industrial pollutants creates a challenge since different textile processes differ in their composition due to the different chemical or physical methods used on fabrics and machinery . 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. The treatment of fiber or fabric with chemical pigments to impart color is call dyeing. Water used in the form of steam to transfer dyes on to substrate (fiber) . Cotton develops a slightly negative charge in water because of the ionization of hydroxyl groups in cellulose. The dyes used in coloring cotton are anionic (negatively charged) . Therefore, special treatments are required to exhaust (fix) anionic dyes on to the substrate  These special treatments include a large amount of salt (0.5-0.6kg NaCl) and alkali to reduce the charge repulsion effects between the hydroxyl anions in cellulose and anionic dyes (e.g. reactive dyes) . The treatment bath at the end of dyeing process is heavily polluted with toxic organic compounds, electrolytes, and residual of dyes of which can be expensive to purify and recover [3,5]. Effluent disposal is the primary option , 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 .
Influence of Cationization process for Dyeing Cotton
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 [9-13]. 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 [11-12]. 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 . 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 .
The dyeing process can occur under neutral or mild acidic conditions without the use of electrolytes and salts [14-15]. 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 [12-14]. Severe wash-off procedures are eliminating since hydrolysis of dyes normally occurs under alkaline conditions (pH 11) [12-14]. 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). 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 . Subramanian et al (2006) demonstrated that the optimal temperature for cationization process was at 70ºC; at higher temperatures the rate of penetration of the cationic reagent for the substrate will be absorbed less and influence total dye utilization absorbing on to the substrate.
“The ratio of K/S is proportional to the concentration of dye molecules in textile; it is a measure for the coloration of the textile .” “The ratio of light absorption (K) and scatter (S) can be calculated using kubelka-munk function .” Montazer et al. (2007) reported that the color strength (K/S) values for dyeing cationized cotton fiber were 2-4 times higher than untreated fiber where K/S values range from 1-4. “The ratio of K/S is proportional to the concentration of dye molecules in textile; it is a measure for the coloration of the textile .” The higher K/S values suggest a higher level of dye fixation was obtain with the treated fabric than without pretreatment . “The most beneficial part of the cationization technique is the reduction of dissolved solids in the effluent as this cannot be removed from the effluent easily, which need capital intensive and cost consuming treatments like Reverse osmosis, Nano Filtration, ion-exchange, etc.” [Kannan et al., 2006].
Textile Dyeing in Supercritical Carbon Dioxide
Supercritical fluid technology is a promising application in textile processing industry particularly dyeing stage for the reason that it can be environmentally friendly, energy saving, increase productivity, eliminate effluent disposal and treatment [14-15]. The development of a water-free dyeing procedure can be accomplished using supercritical carbon dioxide (SC-CO2) [14-18]. 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 . In addition, no additional energy is required to dry the fabric after the dyeing process . Supercritical fluids are described as “the temperatures and pressures of a substance above its critical point and have densities and viscosities between those of the gas and liquid states in the substance .” 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 [15,17]. A general illustration of the equipment used for dyeing textiles in SC-CO2 is shown in Fig.3 .
SC-CO2 has been successfully employed as a solvent system in the dyeing and finishing processes for synthetic fibers [14-15]. In dyeing polyester textiles, SC-CO2 penetrates inside the fibers causing them to swell for increasing the accessibility of dye molecules to the substrate . As the pressure is lowered, the dye molecules are trapped inside the shrinking polyester fibers and no waste is generated since the dye molecules cannot be hydrolyze . 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 used in dyeing natural textiles (e.g. cotton) and fixed on to substrate by chemical (e.g. covalent bonds) or physical (e.g. van der Waal forces) bonds . 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 [Beltrame]; (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) ; (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 . 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 .
Fernandez et al. (2005) investigated use of non-polar reactive dyes with monofluorotriazine reactive group for dyeing cotton in supercritical carbon dioxide. Prior to dyeing cotton a solution of methanol was applied to swell the fibers. Cosolvents applied during dyeing process which are isopropanol and methanol “The hydrophobic part of methanol will make diffusion of the hydrophobic non-polar reactive dyes into the cotton possible. This pretreatment in combination with a cosolvent, applied during dyeing, is essential to facilitate the transport and diffusion of the dye across the fiber to reach the reactive sites of the cotton” . “The effect of adding acids to the reaction or dyeing medium on the kinetics and dyeing performance on cotton were investigated and discussed”. “The pretreatment of cotton prior to dyeing and the use of cosolvent during the dyeing process cannot be eliminated. Pretreatment of the cotton and cosolvent are required for making cotton more accessible to the dye and favoring dye transport and diffusion across the fiber…the pretreatment does not involve any chemical reactions with the cotton, methanol interacts physically with the cotton, creating new H-bonds within adjacent cellulose chains.” “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.” Acids= H3PO4 and acetic acid. The color strength measured values with monofluorotriazines in the range of 9.8 and 17.3. “The results were reproduced at larger scale with a significant reduction of the amount of dye and acid used. A fixation of 100% ”.
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