Fixed-bed adsorption of reactive azo dye onto granular activated carbon prepared from waste
Introduction
Azo dyes are synthetic organic compounds widely used in textile dyeing, paper printing and other industrial processes such as the manufacture of pharmaceutical drugs, toys and foods [1]. This chemical class of dyes, which is characterized by the presence of at least one azo bond (–NN–) bearing aromatic rings, dominates the worldwide market of dyestuffs with a share of about 70% [2]. Reactive dyes are the most common dyes used due to their advantages, such as bright colors, excellent color fastness and ease of application [3], [4]. They exhibit a wide range of different chemical structures, primarily based on substituted aromatic and heterocyclic groups. A large number of reactive dyes are azo compounds that are linked by an azo bridge [5]. Many reactive dyes are toxic to some organisms and may cause direct destruction of creatures in water [6]. In addition, since reactive dyes are highly soluble in water, their removal from effluent is difficult by conventional physicochemical and biological treatment methods [7], [8].
Batch experiments are usually done to measure the effectiveness of adsorption for removing specific adsorbates as well as to determine the maximum adsorption capacity. The continuous adsorption in fixed-bed column is often desired from industrial point of view. It is simple to operate and can be scaled-up from a laboratory process [9]. A continuous packed bed adsorber does not run under equilibrium conditions and the effect of flow condition (hydrodynamics) at any cross-section in the column affects the flow behaviour downstream. The flow behaviour and mass transfer aspects become peculiar beyond a particular length to diameter ratio of the column [10]. In order to design and operate fixed-bed adsorption process successfully, the breakthrough curves under specified operating conditions must be predictable. The shape of this curve is influenced by the individual transport process in the column and in the adsorbent [11]. Breakthrough determines bed height and the operating life span of the bed and regeneration times [12]. Adsorption in fixed-bed columns using activated carbon has been widely used in industrial processes for the removal of contaminants from aqueous textile industry effluents, since it does not require the addition of chemical compounds in the separation process [13].
Activated carbon adsorption has been found to be superior for wastewater treatment compared to other physical and chemical techniques, such as flocculation, coagulation, precipitation and ozonation as they possess inherent limitations such as high cost, formation of hazardous by-products and intensive energy requirements [14]. However, commercially available activated carbons are still considered expensive [15]. This is due to the use of non-renewable and relatively expensive starting material such as coal, which is unjustified in pollution control applications [16]. Therefore, in recent years, this has prompted a growing research interest in the production of activated carbons from renewable and cheaper precursors which are mainly industrial and agricultural by-products. The methods of activation commonly employed can broadly be divided into two main types: thermal (or physical) activation and chemical activation. Thermal activation involves primary carbonization (below 700 °C) followed by controlled gasification under the action of oxidizing gases at high temperature (up to 1100 °C). In chemical activation the precursor is mixed with a chemical restricting the formation of tars (e.g. ZnCl2, H3PO4, etc.), after kneading carbonized and washed to produce the final AC. The chemical incorporated into the interior of the precursor particles reacts with the thermal decomposition products reducing the evolution of volatiles and inhibits the shrinkage of the particles. In this way, the conversion of the precursor to carbon is high, and once the chemical is eliminated after the heat treatment, a large internal porosity is formed [17]. Phosphoric acid activation only involves a single heat treatment step and is achieved at lower temperatures (400–600 °C), higher yields are obtained and most of the phosphoric acid can be recovered after the process is completed. The chemical activation of lignocellulosic materials with phosphoric acid has been extensively investigated from the development of porosity [18], [19], [20], [21] and mechanism of degradation from the precursor point of view [22], [23]. Phosphoric acid incorporated into the interior of the precursor particle restricted the formation of tar as well as other liquids such as acetic acid and methanol and inhibited the particle shrinkage or volume contraction during heat treatment. Phosphoric acid changed or modified the surface chemistry of adsorbent due to the formation of acidic oxygen-contained complexes by strong oxidization [22].
In this work, the removal efficiency of bamboo waste based granular activated carbon (BGAC) used RB5 in the textile industry by fixed-bed column was investigated. The important design parameters such as inlet concentration of dye solution, flow rate of fluid and column bed height [8], [24], [25], [26] were investigated using a laboratory scale fixed-bed column. The breakthrough curves for the adsorption of RB5 were analyzed using Adam's–Bohart, Thomas and Yoon–Nelson models. Further, modeling on the adsorption dynamics of the fixed bed was presented and finally the correlation between the model and the experimental data was compared.
Section snippets
Adsorbate
Chemazol Black B (C.I. Reactive Black 5) (RB5) used in this study was purchased from Sigma–Aldrich (M) Sdn Bhd, Malaysia. RB5 has molecular formula C26H21N5Na4O19S6 (Mol. wt. 991.82 g/mol). The maximum wavelength of this dye is 579 nm. The dye stock solution was prepared by dissolving accurately weight dye in distilled water to the concentration of 1 g/L. The experimental solutions were obtained by diluting the dye stock solution in accurate proportions to needed inlet concentrations. The chemical
Effect of initial dye concentration
The effect of a variation of the inlet RB5 concentration from 50 to 200 mg/L used with the same adsorbent bed height of 60 mm and solution flow rate of 10 mL/min is shown by the breakthrough curve in Fig. 2. As shown in Fig. 2, in the interval of 50 min, the value of Ct/Co reached 0.87, 0.96 and 0.98 when inlet concentration was 50, 100 and 200 mg/L, respectively. It is illustrated that the breakthrough time slightly decreased with increasing inlet RB5 concentration. At lower inlet RB5
Conclusion
This investigation showed that the granular activated carbon prepared from bamboo waste by chemical activation using phosphoric acid was a promising for removing RB5 from aqueous solutions using fixed-bed adsorption column. The fixed-bed adsorption system was found to perform better with lower RB5 inlet concentration, lower feed flow rate and higher activated carbon bed height. The column experimental data were analyzed by the Adam's–Bohart, Thomas and Yoon–Nelson models. For RB5 adsorption,
Acknowledgement
The authors acknowledge the research grant provided by the Universiti Sains Malaysia under the Research University (RU) Scheme (Project No. 1001/PJKIMIA/814005).
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