• Corpus ID: 37775854

Removal of Dye by Adsorption : A Review

@inproceedings{Seow2016RemovalOD,
  title={Removal of Dye by Adsorption : A Review},
  author={Ta Wee Seow and Chi Kim Lim},
  year={2016}
}

• T. W. Seow, C. K. Lim
• Published 2016
• Chemistry, Engineering

Dye removal from industrial wastewater is an important environmental concern. So far, various physical and chemical treatment methods have been reviewed for the removal of dyes such as biological degradation, ion exchange, chemical precipitation, reverse osmosis, coagulation, flocculation, etc. However, these treatment processes have their limitations such as high cost, generation of toxic sludge, etc. Adsorption which is the most economical and effective has become the most preferred method for the dyes removal. This review will presented the application of adsorption in the removal of dyes from aqueous solution. The paper provides the literature information about the dyes as well as its toxicity and classification. Further, the adsorption factors that will affect the process such as solution pH, initial dye concentration, adsorbent dosage and temperature have also been reported. Conclusions have been drawn from the literature review regarding the effects of those adsorption factors toward the dyes adsorption. Introduction Dyes are basically natural or synthetic, organic compounds that can connect themselves to surfaces or fabrics to provide bright and lasting colour. They are applied in various industries such as leather, textile, paper, rubber, cosmetics, plastic, pharmaceuticals and food industries. Most of them are complex organic molecules and are resistant to many things such as the action of detergents [1-2]. Coloured dye wastewater is regarded as a direct result of the production of the dye and also as a consequence of its use in the textile and related industries. There are more than 100,000 commercial dyes are known with an annual production of over 7 X 10tonnes per year [3]. It is estimated that 2% of the dyes are discharged in effluent from manufacturing operation, while 10% was discharged from textile and associated industries [4]. Discharge of these dyes into the water streams will affects the people who may use these effluents for living purposes such as washing, bathing and drinking [5]. Some dyes can cause allergy, dermatitis, skin irritation, cancer and mutations in humans [6]. Apart from that, dyes also associated with environmental concern with their absorption and reflection of sunlight entering the water, which will inhibit the growth of bacteria, limiting it to levels insufficient to biologically degrade impurities in the water [7]. Furthermore, dyes can also affect the aquatic plants as they reduce sunlight transmission through water. Therefore, the removal of such coloured compounds from waste effluents becomes environmentally important because even a small amount of dye in water can be toxic and highly visible [8]. Different physic-chemical processes have been applied for the treatment of coloured wastewater. These include precipitation, flocculation, electro-kinetic coagulation, electro-flotation, ion exchange, membrane filtration, electro-chemical destruction, irradiation and ozonation. However, all these treatment methods are costly, suffering from many restrictions and cannot be utilised by small industries to treat the wide range of wastewater [9]. Hence, adsorption process has been preferred for the treatment of the wastewater due to its cheapness, simple design and easy operation, less energy intensiveness, no effect by toxic substances and high quality of the treated effluents particularly for well-designed sorption processes [10-11]. Therefore, this review article was undertaken in order to provide comprehensive and critical review information on the application of the adsorption process for the treatment of dye wastewater and to study the effects of adsorption factors (solution pH, initial dye concentration, adsorbent dosage and temperature) on the dye uptake. Adsorption Over the last few years, a sizeable research investigations have been undertaken by various researchers for wastewater treatment using adsorption process. The adsorption process is utilised as a stage of integrated chemical-physical-biological process for the treatment of wastewater [12-13], or simultaneously with a biological process [14]. Adsorption is a surface phenomenon in which a multi-components fluid (gas or liquid) mixture is attached to the surface of a solid adsorbent to form attachments via physical or chemical bond [15]. The substance that providing the solid surface is termed as adsorbent while material removed from the liquid phase is known as adsorbate. Tiny chemical particles suspended in another phase of matter, meaning in the air as a gas or in water as a liquid, are considered as contaminants. These contaminants can be separated from those phases through adsorption process that bonded onto solid phase [16]. Thus, decontaminating the air and liquid phases. The adsorption process of dye molecules usually consists of four consecutive steps [17]. The first step involves the diffusion or convection of dye molecules through the bulk of International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 4 (2016) pp 2675-2679 © Research India Publications. http://www.ripublication.com 2676 solution. In the next stage, the dye molecules will diffuse through a diffusional boundary layer (film diffusion). This is followed by the diffusion of dye molecules from the surface into the interior of the adsorbent materials. Lastly, the dye molecules will attach to the surface of the materials through molecular interactions. The dye concentration and agitation force may affect step 2. Step 3 is the rate determining stage which will affect the adsorption of dye molecules on the substrate. While step 4 is dependent on the nature of the dye molecules, such as anionic and cationic structures. It is important to highlight that step 3 could involve two different phenomena. The first is porous diffusion (the adsorbate first diffuse in the liquid filling the pores and then is adsorbed), another is surface diffusion (the adsorbate is first adsorbed then diffuses from one site to another) [18]. Adsorption can be divided into two types i.e. chemical sorption and physical sorption. For chemical sorption or chemisorption, it is defined by the formation of strong chemical associations between molecules or ions of adsorbate, which is generally due to the exchange of electrons where the process generally is irreversible. Whereas physical sorption or physisorption involves the weak van der Waals intraparticle bonds between adsorbate and adsorbent and thus physisorption generally is reversible in most cases [19]. Classification of dyes There are different ways for classification of dye molecules. It can be classified in terms of colour, structure or application methods [20]. Due to the complexities of the colour nomenclature from the chemical structure system, the classification in terms of application is often favourable. The common dyes applied in the textile industry are acid dyes, basic dyes, direct dyes, azo dyes, reactive dyes, mordant dyes, vat dyes, disperse dyes and sulfur dyes [21], where azo derivatives are the major class of dyes that are used in the industry today. Table 1 represents the dyes that commonly used in textile dyeing operations. Table 1: Typical dyes used in textile dyeing industries [21] Dye class Description Acid Water-soluble anionic compounds Basic Water-soluble, applied in weakly acidic dyebaths; very bright dyes Direct Water-soluble, anionic compounds; can be applied directly to cellulosics without mordants (or metals like chromium and copper) Disperse Water-insoluble Reactive Water-soluble, anionic compounds; largest dye class Sulfur Organic compounds containing sulfur or sodium sulfide Vat Water-insoluble; oldest dyes; more chemically complex Other than that, dyes are also usually classified based on their particle charge upon dissolution in aqueous application medium such as cationic dyes which are basic dyes while the anionic dyes include direct, acid and reactive dyes, and nonionic dyes (dispersed dyes) [22]. Factors affecting adsorption of dye There are many factors will affect the adsorption of dye molecules such as solution pH, initial dye concentration, adsorbent dosage and temperature. In-depth study and optimisation of these parameters will greatly help in the development of industrial-scale treatment process for the dye removal. Thus, these factors will be discussed in the next section. Effect of solution pH The pH of the solution is a very important parameter in the adsorption process, particularly for dye adsorption. The magnitude of electrostatic charges which are imparted by the ionised dye molecules is controlled by the solution pH. As a result the rate of adsorption will vary with the pH of the medium used [23]. In general, at low solution pH, the percentage of dye removal will decrease for cationic dye adsorption, while for ionic dyes the percentage of removal will increase. In contrast, high solution pH is preferable for cationic dye adsorption but shows a lower efficiency for anionic dye adsorption [24]. At high solution pH, the positive charge at the solution interface will decrease while the adsorbent surface appears negatively charged [25]. As a result, the cationic dye adsorption will show an increase and the anionic dye adsorption will decrease. At low pH solution, the positive charge on the solution interface will increase and the adsorbent surface will appear positively charged, which results in decrease in cationic dye adsorption and an increase in anionic dye adsorption [24]. The adsorption ability of the surface and the type of surface active centres are indicated by the significant factor which is known as point of zero charge (pHpzc) [26]. In order to determine the pHpzc, dye solution with different pH should prepared and considered as pHinitial and then the fix amount of adsorbent will be added to the dye solution. The dye solution will be shaken until the equilibrium is achieved where the pH at equilibrium is regarded as pHfinal, then plot