Adsorption of basic dyes on granular acivated carbon and natural zeolite
Introduction
The removal of dyes and organics in an economic way remains an important problem although a number of systems have been developed with adsorption techniques. Adsorption has been found to be superior to other techniques for water re-use in terms of initial cost, simplicity of design, ease of operation and insensitivity to toxic substances.
Granular activated carbon is the most popular adsorbent and has been used with great success (McKay, 1982), but is expensive. Consequently, new materials as chitin (McKay, 1982), silica gel (McKay et al., 1980), natural clay (El-Geundi (1991), El-Geundi (1993a), El-Geundi (1993b)), bagasse pith (McKay, 1998) are being studied. A very limited amount of information is available on the use of natural zeolites as a method for dye removal (Meshko et al., 1999). A full description of low cost adsorbents for waste and wastewater treatment: a review has been presented by Pollard et al. (1992).
A number of studies have been reported with regard to the adsorption equilibrium of dye removal processes using various adsorbents. In most adsorption systems dyestuffs-adsorbents Langmuir (McKay et al (1982), McKay et al (1980); El-Geundi, 1993b; Meshko et al., 1999), Freundlich (McKay et al (1982), McKay et al (1980); El-Geundi, 1993a; Meshko et al., 1999), and Redlich–Peterson (McKay, 1998) isotherms have been applied to describe equilibrium between liquid and solid phases.
The prediction of adsorbate uptake rates by adsorbent pellets is important for adsorption calculations. For the transport of adsorbates from the bulk of the fluid phase to the interior of a pellet before adsorption takes place, the following mass-transfer processes may be present: film mass transfer and intraparticle mass transfer.
Two intraparticle diffusion mechanisms are involved in the adsorption rate (a) diffusion within the pore volume known as pore diffusion, and (b) diffusion along the surface of pores known as surface diffusion (Tien, 1994). Some of the investigators have applied the pore diffusion model with (Ruthven, 1984; McKay et al., 1987) and without film resistance (El-Geundi, 1993b; Ruthven, 1984). McKay has developed “homogeneous solid phase diffusion” model to describe the adsorption of dyes on bagasse pith (1998). The “homogeneous solid phase diffusion” model has been developed based on external mass transfer and surface diffusion by El-Geundi (1991). He has applied this model for adsorption of basic dyes onto natural clay in batch adsorber. The branched pore kinetic model was used to describe the adsorption of cobalt phthalocyanine dye onto activated carbon and basic dyes onto natural clay (El-Geundi, 1993a).
The aim of this work is to study the ability of granular activated carbon and natural zeolite to remove basic dyes from aqueous solutions. The equilibrium and kinetic studies have been performed. The importance of obtaining isotherms and kinetics curves lies in developing a model which accurately represents the results obtained and could be used for design purposes.
Section snippets
Mathematical model
The “homogeneous solid diffusion” model basically assumes that the adsorbent particles are homogeneous. The adsorbate molecule is transferred through the adsorbent particles by “creeping” from one adsorption site to another on the solid surface. This is mathematically described by Fick's second law of diffusion. Equilibrium is set between solid- and liquid-phase concentrations at the surface of the adsorbent particle. Although the particle is assumed to be homogeneous, this does not preclude
Materials
Adsorbates. Two basic dyes, supplied by Ciba–Geigy, were used as adsorbates. The dyestuffs were used as commercial salts without purification. The dye Maxilon Schwarz FBL-01 300% (MS-300) contains 30–40% component A and 20–30% component B. The dye Maxilon Goldgelb GL EC 400% (MG-400) contains more than 90% component A.
Component A is 2–[[(4–methoxyphenyl)methylhydrazono]methyl]- 1,3,3-trimethylindolium methosulfat and component B is 2–[[4–[ethyl(2–hydroxyethyl)
Adsorption isotherms
The distribution of dye between the adsorbent and dye solution, when the system is at equilibrium, is important to obtain the capacity of the granular activated carbon and the natural zeolite for the dyes.
A number of equations exist which enable the equilibrium data to be correlated and two most frequently used, for dilute solutions, are the Langmuir and Freundlich isotherms represented by , respectively. The parameters in these equations are very useful for predicting adsorption capacities
Conclusions
Certain general points may be deduced from the experimental and theoretical analysis on predicting kinetics for adsorption of basic dyes onto granular activated carbon and natural zeolite:
- 1.
The equilibrium studies have shown that both dyes have higher adsorption capacity for the granular activated carbon than for the natural zeolite;
- 2.
In the homogeneous solid diffusion model, the range of varying the external mass transfer coefficient kf has been estimated assuming a linear isotherm and negligible
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