On-line version ISSN 0104-6632
Braz. J. Chem. Eng. vol.18 no.3 São Paulo Sept. 2001
R.F.P.M.Moreira1*, J.L.Soares1, H.J.José1, and A.E.Rodrigues2
1Departamento de Engenharia Química e Engenharia de Alimentos,
Universidade Federal de Santa Catarina, Campus Universitário , Trindade,
88040-670, Fax: 55 48 331-9687, Florianópolis - Santa Catarina, Brazil
E-mail : email@example.com
2Laboratory of Separation and Reaction Engineering, Faculdade de
Engenharia da Universidade do Porto, Porto - Portugal
(Received: December 6, 2000 ; Accepted: August 9, 2001 )
The thermodynamics and kinetics of adsorption of reactive dyes on high-ash char was studied. Equilibrium data were obtained using the static method with controlled agitation at temperatures in the range of 30 to 60ºC. The Langmuir isotherm model was used to describe the equilibrium of adsorption, and the equilibrium parameters, RL, in the range of 0 to 1 indicate favorable adsorption. The amount of dye adsorbed increased as temperature increased from 30 to 40ºC, but above 40ºC the increase in temperature resulted in a decrease in the amount of dye adsorbed.
The kinetic data presented are for controlled agitation at 50 rpm and constant temperature with dye concentrations in the range of 10 ppm to50 ppm. The film mass transfer coefficient, Kf, and the effective diffusivity inside the particle, De, were fitted to the experimental data. The results indicate that internal diffusion governs the adsorption rate.
Keywords: color removal; dyestuffs; high ash, char, activated carbon.
There is no general method for the removal of color from dye wastewater. Methods of primary clarification, including sedimentation and flotation, are not effective for the removal of color without simultaneous chemical treatment. Processes such as membrane separation, coagulation and ion exchange are also used for the removal of color from dye wastewaters, but the cost of these processes is the main drawback of these techniques (Mishra & Tripathy, 1993).
Combined methods, such as activated sludge+coagulation, activated slugde+adsorption and coagulation+chemical oxidation, are being used by most industries (Lin & Chen, 1997). The experimental study of Singer and Little (1975), using textile wastewaters in a 21-day BOD test indicated that the color removal is less than 50%. Textile dyes are slowly degraded and some authors have reported that color removal is due to adsorption on micro-organisms (Pagga & Brown, 1986; Grau, 1991).
The economical decolourization of effluents by removal of dyes remains an important problem, although recently a number of successful systems using adsorption techniques have been developed. Activated carbon has been successfully used as an adsorbent for removal of dyes from wastewater. The performance of an activated carbon treatment process depends on the type of carbon and the characteristics of the wastewater in addition to the operating conditions.
Granular activated carbon (Mishra & Tripathy, 1993) is suitable for the removal of color and other organic compounds in effluents, and it is efficient when used in adsorption columns. However, granular char is an expensive material and regeneration results in a 10 to 15% loss of adsorbent (McKay, Otterburn & Sweeney, 1980).
Mixtures of fly ash and coal in different proportions have shown a high adsorption capacity for cationic dyes (DeJohn & Hutchins, 1976). By increasing the amount of coal in the mixture, the adsorption capacity is increased because of the high surface area available for adsorption. Comparative studies of adsorption capacity and costs of adsorbent, have shown that a fly ash/coal mixture in the proportion 1:1 could be used to replace the activated carbon .
One of the characteristics of Brazilian coal is its high ash content, which can reach 60%, but although this fact prevents its use for common adsorption processes, this characteristic could be exploited for the removal of dyes from wastewater (Mishra & Tripathy, 1993; Singh & Rawat, 1994; Kamel, Magda & Youssef, 1991). Previous studies have shown that high-ash char can be produced by pyrolysis at temperatures in the range of 600 to 800oC, and the proportion of transitional pores increases for the solid prepared at 600oC (Moreira et al., 1998c).
The objective of this study was to prepare a suitable and economical adsorbent to remove reactive dyes from aqueous solutions. A bituminous high-ash coal was used as the raw material, and efficiency of removal of reactive dyes used in textile industries (yellow and red monochlorotriazine) was studied.
In this work, two reactive dyes (yellow and red monochlorotriazine dyes MCT) were used to evaluate the efficiency of high-ash char as an adsorbent. These dyes are extensively utilized by textile industries in Brazil and were supplied by Quimisa S/A (Blumenau, Brazil). The molecular structure is shown in Table 1.
The adsorbent was prepared using mineral coal from Santa Catarina State, Brazil. After drying at 100ºC for 2 hours, the coal was placed in an oven and heated in a nitrogen atmosphere up to 600oC, using a heating rate of 120oC/min. The coal was kept in the oven for 5 minutes, and then the temperature was lowered to room temperature. The char was ground in a ball mill and classified by screening. The particles selected for this work correspond to the fraction 65/80 Mesh Tyler (0.18 mm). The characteristics of the materials used and the total pore surface area determined by mercury porosimetry (Poresizer 9320, Micromeritics) are shown in Table 2.
The samples of high-ash char were characterized by FTIR spectroscopy in a BOMEM spectrometer. The KBr disk method was used, and the concentration of the sample in the disk was 0.2%. The spectra were taken using air as a reference (Chang, 1981). FTIR spectra were taken in the range of 4000 to 400 cm-1 in order to observe changes in the solid structure before and after the adsorption of dyes on char (Figure 1).
The concentration of dyes was measured by UV/Vis spectrophotometry at the wavelength selected for maximum absorbance of the dyes, 420 nm and 506 nm for the yellow and red MCT, respectively.
Equilibrium adsorption isotherms were determined at temperatures in the range of 30 to 60ºC in an orbital shaker. A set of 400 mL Erlenmeyer flasks with 100 mL of dyestuff solution and different amounts of adsorbent were shaken for 24 hours at a given temperature. For each dyestuff solution, a blank test was run under the same conditions. At the end, color and pH were measured after centrifugation for 30 minutes. The pH value remained constant at 6.5. Previous tests had shown that the adsorption of these dyes on char is fast and thermodynamic equilibrium is reached in 10 hours (Peruch, 1997).
The adsorption kinetics was measured by contacting a 400 mL solution of dyestuff with a known mass of solids at constant temperature and agitation. After regular intervals of time, a 1 mL aliquot was taken and its concentration was measured. The amount of dye retained in equilibrium per mass of adsorbent, qe, in each flask was calculated with Equation 1.
where V is the volume of the solution, Co is the initial concentration of the solution, Ce (g/L) is the liquid concentration at equilibrium, and W is the mass of the solids.
RESULTS AND DISCUSSION
Thermodynamics of Adsorption
The experiments were performed to measure the isotherms of adsorption of yellow and red MCT on char in a temperature range of 30 to 60oC. According to the shape of the curves, the Langmuir (Eq.2) model was fitted (Figure 2).
Figure 2 shows that when temperature increases from 30 to 40ºC, the amount of adsorbed dye increases, as reported by Moreira et al. (1998a; 1998b) for adsorption of the same dyes on activated alumina and commercial activated carbon. At temperatures higher than 40ºC, however, it was observed that adsorption capacity decreases as temperature increases in a behavior typical of exothermic adsorption.
McKay et al. (1982) reported on the adsorption of dyes on chitin and observed that an increase in temperature leads to an increase in adsorption capacity. This phenomenon, which was unexpected, has still not sufficiently studied.
It has been suggested that organic molecules are adsorbed on carbons due to the van der Waals forces between benzenic arrays of carbons and organic molecules (Peruch, 1997). Considering the molecular structures of yellow and red MCT, it is possible that the orientation of these molecules on the surfaces of solids could be dependent on temperature, and different orientations might explain the results found in this work. At 30oC, the orientation of dye molecules on the surface could lead to a lower adsorption capacity when compared with the test at 40oC. In this case, a different orientation of the dye molecules would then lead to a decrease in the entropy of adsorption and consequently an increase in the adsorption capacity. At temperatures higher than 40ºC, after all adsorbed molecules have achieved the same orientation, the adsorption capacity decreases, as described in the literature on exothermic processes. This behavior can be confirmed by the results found in the FTIR spectra, showing that the adsorption form of dye molecules at 30ºC is only a little different from the adsorption form at 40ºC (Figure 1). The spectra of samples of saturated yellow and red MCT/char are shown in contrast to the spectrum of the original char.
The primary amines show two characteristic peaks: one near 3500 cm-1 and the other near 3400 cm-1, as can be observed in Figure 1, for the saturated yellow and red MCT/char obtained at 40ºC. In the spectrum of saturated yellow MCT/char obtained at 30ºC, the peak ascribed to primary amine is not observed. Secondary amines show a peak in the range of 3350 to 3310 cm-1, although this is not abundantly clear in Figure 1. This result indicates that, at 30ºC, the primary amine group is involved in the adsorption, while in the experiment at 40ºC, this group is not involved. Although this behavior is not clear for red MCT/char, it can be observed that the peak intensity ascribed to primary amines for red MCT/char obtained at 30ºC is lower than that obtained at 40ºC. The other peaks appearing in the spectra are the same at both temperatures, indicating that the other bonds involved in adsorption at these two temperatures are similar.
The Henry constant (KL) and the apparent enthalpy of adsorption were calculated with Equations 3 and 4, respectively.
The dimensionless separation factor, RL, was defined by Equation 5 and was used to evaluate whether the isotherms were favorable (Hall et al., 1966).
The Langmuir parameters fitted to experimental data at each temperature are shown in Table 3. The RL values (Langmuir equilibrium parameter) indicate that the behavior of the adsorption is favorable.
The apparent enthalpy of adsorption was calculated from the Henry constant, KL, in the temperature range of 40 to 60oC; the values obtained were 36.9 KJ/mole for yellow MCT and 38.6 KJ/mole for red MCT, low values which are typical of physical and exothermic adsorption.
A comparison of the adsorption capacities of the char and a commercial activated carbon is shown in Figure 3. The char used in this study shows an adsorption capacity about 10 times higher than that of the commercial activated carbon, and it could be used as an adsorbent for color removal from textile effluents.
A comparison of the adsorption dyes on char and ash only is shown in Figure 4. The ash was obtained by complete combustion of char at 600ºC for 12h in air. The adsorption isotherms obtained at 30oC show that the amount adsorbed on ash is smaller than that adsorbed on char, as also reported by DeJohn and Hutchins (1976) for the adsorption of cationic dyes on fly ash. This fact shows that the presence of mineral matter is not the only factor responsible for the high adsorption capacity observed for char, but rather it is the combined effect of ash composition (Julien et al. (1998)), surface chemistry and surface area available for adsorption.
Kinetics of Adsorption
The kinetics of adsorption on char for yellow and red MCT at different initial dye concentrations is shown in Figure 5.
The initial decrease in the concentration of dye with time is governed by diffusion in the boundary layer, and the remainder of the curve, where the rate of adsorption is diminished, has an influence on internal diffusion. The first mechanism is fast, followed by diffusion of dyes in the pores and capillaries of the structure of the char, as also reported by Banerjee et al. (1997) and McKay et al. (1980).
In Figure 5, it can be observed that when the initial concentration of dye is higher, the curve decreases more slowly and the fractional adsorption is small. However, at lower concentrations, the initial rate of adsorption is faster, indicating fast interaction between the dye and the solid.
The experimental kinetic data were fitted to the film and pore diffusion model in order to evaluate the mass transfer coefficients, Kf, and De, and the mass transfer coefficients in the film and in the pores, respectively.
The mass balance in bulk liquid-phase concentration is given by Equation 6.
with the initial condition t=0; Cb=Co
The mass balance inside the particle is described by Equation 7.
with the following boundary conditions:
where qi is the amount of adsorbed dye in the particle relative to the mass of adsorbent, Ci is the concentration of dye in the liquid phase inside the particle, Cb is the bulk concentration of the dye, r is the radial position, t is time, De is the effective diffusivitiy , ep is the particle porosity, rapp is the apparent density, R is the radius of the particle, and Kf is the coefficient of mass transfer in the film. We should note that
rappqi = (rappq epCi)
The values for Kf and De were calculated by the numerical method of finite differences fitting adjusted to the experimental data, using the film and pore diffusion model to obtain values for the yellow MCT and red MCT adsorption established in each experiment (Table 4 and Figure 5). The high Biot numbers (Biot = Kf.dp/De), in the range of 36 to 415, indicate that resistance to diffusion in the boundary layer is low and resistance to diffusion in the pores of the particles governs the rate of adsorption.
The experimental results showed that the high-ash char used is able to remove reactive dyestuff solutions. The adsorption isotherms were determined and data was analyzed according to the Langmuir model. Equilibrium data showed that the amount of dye adsorbed increased as the temperature rose from 30 to 40ºC, and above 40ºC the increase in temperature resulted in a decrease in the amount of dye adsorbed. The apparent enthalpy of adsorption, calculated from the Langmuir equilibrium constant at temperatures in the range 40 to 60ºC were 36.9 KJ/mol and 38.6 KJ/mol for the yellow monochlorotriazine-char and red monochlorotriazine-char systems respectively, indicating physical adsorption. The maximum amount of dye adsorbed on the char used in this work at equilibrium was higher than that adsorbed on commercial activated carbon. The contribution of the amount of dye adsorbed on ash was evaluated and found to be much smaller than that adsorbed on char. The adsorption capacity of high-ash char observed is due to the combined effect of ash content and composition, besides physical structure and surface chemistry.
The rate at which reactive dye adsorption occurs can be described using the film and pore diffusion model. The film mass transfer coefficients, Kf, and the effective diffusivity inside the particle, De, were fitted to the experimental data, and the results indicated that internal diffusion governed the adsorption rate.
|b||Langmuir equilibrium constant, L/g|
|Bi||Biot number, dimensionless|
|Co||Initial concentration in liquid phase, g/L|
|Cb||Concentration of dye in liquid phase, g/L|
|Ce||Concentration of dye in liquid phase in equilibrium, g/L|
|Ci||Concentration inside the particle, g/L|
|De||Effective diffusivity, cm2/min|
|Kf||Mass transfer coefficient in the film, cm/s|
|KL||Henry constant, L/g|
|q||Concentration of dye retained in the particle, mg/g|
|qi||Concentration of dye adsorbed, mg/g|
|qe||Amount of dye adsorbed in equilibrium, mg/g|
|qo||Maximum amount of dye adsorbed in equilibrium, mg/g|
|r||Radial position of the particle, cm|
|R||Radius of the particle|
|RL||Dimensionless separation factor|
|V||Volume of the solution, L|
|Va||Volume of the adsorbent, L|
|W||Total mass of the adsorbent, g|
|rapp||Apparent density, g/cm3|
|ep||Porosity of the particle|
|DH||Enthalpy of adsorption, KJ/mol|
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