Removal of Dye by Adsorption: A Review

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 thepHfinal values againtpHinitialwhere pHpzcis the point when pHinitial = pHfinal [27]. The value of pH is used to describe pHpzconly for the systems in which H/OH are the potential determining ions. Due to the presence of functional groups such as OH, COO groups, cationic dye adsorption is favoured at pH ˃ pHpzc, whereas, anionic dye adsorption is favoured at pH ˂ pHpzc where the surface becomes positively charged [26]. Acevedo et al. [28] studied the effect of solution pH on the adsorption of two commercial dyes, Basic Astrazon Yellow 7GLL and Reactive Rifafix Red 3BN on activated carbons made up of reinforcing fibres from tyre waste and low-rank bituminous coal. The results obtained shown that the adsorption of reactive dye was more favoured in solution of pH 2, whereas the basic dye was adsorbed more easily in a solution of pH 12. Dawood and Sen [29] studied the effect of solution pH on the adsorption of Congo red by pine cone and they noticed that the adsorption was maximum at pH 3.5. Another study conducted by Aksu and Isoglu [30] reported the effect of solution pH on the adsorption of Gemazol turquoise blue-G as a reactive dye using sugar beet pulp and they International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 4 (2016) pp 2675-2679 © Research India Publications. http://www.ripublication.com 2677 noticed that the adsorption was at maximum at pH 2 where the adsorption capacity was 83.7 mg/g and then decreased with a further increase in pH and reached zero at pH 6. Effect of initial dye concentration The dye removal efficiency is highly dependent on the initial dye concentration. The effect of initial dye concentration relies on the immediate relation between the dye concentration and the available binding sites on the adsorbent surface. The removal efficiency will decrease with an increase in the initial dye concentration due to the saturation of adsorption sites on the adsorbent surface [31]. There will be unoccupied binding sites on the adsorbent surface at a low dye concentration, and when the initial dye concentration increases, there will be insufficient sites for the adsorption of dye molecules, thus decreasing the dye removal efficiency [24]. On the other hand, the increase in initial dye concentration will cause an increase in the loading capacity of the adsorbent and this may be due to the high driving force for mass transfer at a high initial dye concentration [32]. Cheng et al. [33] investigated the effect of initial dye concentration on the adsorption capacity of the Cu(II)-AMTD xerogelunder equilibrium conditions with a concentration of 0.5 g/L of adsorbent. The results shown that the adsorption capacity increased with an increase in the dye concentration. The maximum adsorption capabilities of the three dyeson the Cu(II)-AMTD xerogel reached 80, 96 and 105 mg/g forMethyl orange, Thymol blue and Acid fuchsin, respectively. They propose that an increase in the initial dye concentration leads to anincrease in the mass gradient between the solution and the adsorbent and thus acts as a driving force for the transfer of dye moleculesfrom solution to the surface of Cu(II)-AMTD xerogel. Another study conducted by Garg et al. [34] reported the adsorption of Methylene blue by sulphuric acid treated sawdust (SDC) at an adsorbent dose of (0.4 g/100 mL), at a temperature of (26 ± 1 °C) and at pH (7.0) and they found that the unit adsorption for SDC increased from 12.49 mg/g to 51.4 mg/g as the Methylene blue concentration was increased from 50 mg/L to 250 mg/L, while the percentage of dye removal decreased from 99.9 % to 82.2 % as the Methylene blue concentration was increased from 50 mg/L to 250 mg/L.Yagub et al. [35] studied the effect of initial dye concentration on the adsorption of methylene blue (MB) by pine leaves and they found that as the initial dye concentration increased from 10 to 90 mg/L, the percentage of dye removal decreased from 96.5% to 40.9% after 240 min of adsorption process. Effect of adsorbent dosage Adsorbent dosage is an important parameter in order to determine the adsorbent’s capacity for a given amount of the adsorbate at the operating conditions. In order to study the effect of adsorbent dosage on the adsorption process, it can be carried out by prepare adsorbent-adsorbate solution with different amount of adsorbents added to fixed initial dye concentration then shaken together until equilibrium time [24]. Generally the dye removal increases with increasing adsorbent dosage, where the amount of sorption sites at the surface of adsorbent will increase by increasing the dose of adsorbent, and as a result increase the percentage of dye removal from the solution [36]. By analysing the effect of adsorbent dosage, it gives an idea for the ability of a dye adsorption to be adsorbed with the minimum amount of adsorbent, so as to identify the ability of a dye from an economic point of view [24]. Hassani et al. [37] studied the adsorption of two cationic dyes, Basic Green 4 (BG4) and Basic Yellow 28 (BY28) by chemically modified nanoclay. They reported that a relatively strong increase in adsorbent dosage resulted in the increase of the removal efficiencies of both dyes at initial dye concentration of 30 mg/L, pH 6 and contact time of 35 min. They suggested that increasing adsorbent dosage will provides more surface area, thereby leading to more binding sites for the adsorption of target pollutants onto the modified nanoclay. Another research carried out by Sonawane and Shrivastava [38] analysed the effect of adsorbent dose on the removal of Malachite green by maize cob and they concluded that at 20 mg/L of dye, pH of 8 and a contact time of 25 min, the increase of percentage of dye removal from 90.0% to 98.5% when the adsorbent dose increased from 0.5 to 12 g/L. Effect of temperature Temperature is an important factor that serves as an indicator as to whether the adsorption is an exothermic or endothermic process. If the adsorption is an endothermic process, the adsorption capacity will increases with increasing temperature. This may possibly due to the increase in the number of active sites and the mobility of the dye molecules at higher temperature [39]. In contrast, if the adsorption is an exothermic process, the adsorption capacity will decrease with increasing temperature. In this case, higher temperature may decrease the adsorptive forces between the dye molecules and the active sites on the adsorbent surface [40]. In a study carried out by Fu et al. [41], polydopamine (PDA) microspheres, synthesised by a facile oxidation polymerisation route, were evaluated as a potential adsorbent for selective adsorption and separation of organic dyes. The adsorption processes towards nine water-soluble dyes (anionic dyes: methyl orange (MO), eosin-Y (EY), eosin-B (EB), acid chrome blue K (ACBK), neutral dye: neutral red (NR), and cationic dyes: rhodamine B (RhB), malachite green (MG), methylene blue (MB), safranine T (ST)) were thoroughly investigated. The results obtained showed that the temperature plays an important role in adsorption process. The adsorption capacities of 3 representative dyes (MG, MB and NR) on PDA microspheres increase as the experimental temperature ascends from 15 °C to 45 °C, which indicates that the adsorption is more favorable on comparatively high temperature. It is because that dyes molecular diffuse more quickly as the temperature rises, resulting in more opportunity to connect with adsorbents through specific sites. Hameed and Ahmad [42] investigated the adsorption of Methylene blue (MB) by garlic peel and they found that the adsorption capacity increased from 82.64 to 142.86 mg/g when the temperature increased from 30 °C to 50 °C indicating that the adsorption is endothermic in nature. Another research conducted by Senthilkumaar et al. [39] discussed the adsorption of Crystal violet (CV) on phosphoric and sulphuric acid activated carbons (PAAC and SAAC), prepared from International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 4 (2016) pp 2675-2679 © Research India Publications. http://www.ripublication.com 2678 male flowers coconut tree. The effect of temperature on the adsorption rate was studied by carrying out a series of experiments at 28, 33, 38, 43 and 48 °C for both the carbons. The adsorption of Crystal violet on PAAC and SAAC increased from 19.8% to 96.80% and 7.02% to 48.83%, respectively. This suggest that the adsorption process is endothermic in nature when temperature was increased from 28 to 48 °C at pH 6 and initial dye concentration of 40 mg/L. They concluded that increasing temperature may produce a swelling effect within the internal structure of the carbons enabling more dye molecules diffusion into carbon. Further, they also believed that the possibility of increase of the number of active sites for the adsorption with the increase of temperature. This may also be a result of an increase in the mobility of the dye molecule with the rise of temperature. Conclusion During the last few years many articles discussed about the adsorption of dyes have been published. This review is an attempt to highlight the potential of adsorption process for the elimination of dye molecules from industrial wastewater. This paper presented the effect of various physico-chemical experimental conditions that will affect the dye adsorption such as solution pH, initial dye concentration, adsorbent dosage, and temperature. The most important parameter is the solution pH, where a high pH value is preferred for cationic dye adsorption while a low pH value is more suitable for anionic dye adsorption. For the effect of initial dye concentration, the removal efficiency will decrease with an increase in the initial dye concentration due to the saturation of adsorption sites on the adsorbent surface. It was also noticed that the dye removal efficiency increases with increasing adsorbent dosage where the amount of sorption sites available for the adsorption of dye molecules will increase by increasing the dose of adsorbent. As for the effect of temperature, if the adsorption is an endothermic process, the adsorption capacity will increase with increasing temperature. In contrast, if the adsorption is an exothermic process, the adsorption capacity will decrease with increasing temperature. Therefore, as a conclusion all these factors should be taken into account while evaluating the adsorption capacity of different adsorbents from the economic point of