Abstract
The use of nonconventional adsorbents, particularly those that can be easily regenerated, to replace activated carbon in the removal of color from dye wastewaters has been recently proposed. This work shows a thermodynamic and kinetic study of the adsorption of reactive dyes (yellow monochlorotriazine and yellow dichlorotriazine), in liquid phase, on commercial alumina. The basic thermodynamic data were obtained using the static method, with a thermostatic bath at four different temperatures (30, 40, 50 and 60oC) and different pH values. The kinetic data were obtained by adding a known quantity of adsorbent to a dye solution at a constant temperature and under controlled stirring conditions. It was possible to draw the uptake curves, using the effects of the stirring on the adsorption rate. The intraparticle effective diffusivity was estimated using the film and pore diffusion model. The results were compared with the data obtained using a commercial activated carbon.
Dye wastewater; activated alumina
Adsorption of textile dyes on alumina. Equilibrium studies and contact time effects
R.F.P.M. MOREIRA** To whom correspondence should be addressed. To whom correspondence should be addressed., M.G. PERUCHand N. C. KUHNEN
Chemical Engineering Department, Federal University of Santa Catarina(UFSC), Campus Universitário, Trindade, Mail Box 476, CEP 88040-900 Florianópolis - SC, Brazil
(Received: May 7, 1997; Accepted: September 22, 1997)
Abstract - The use of nonconventional adsorbents, particularly those that can be easily regenerated, to replace activated carbon in the removal of color from dye wastewaters has been recently proposed. This work shows a thermodynamic and kinetic study of the adsorption of reactive dyes (yellow monochlorotriazine and yellow dichlorotriazine), in liquid phase, on commercial alumina. The basic thermodynamic data were obtained using the static method, with a thermostatic bath at four different temperatures (30, 40, 50 and 60oC) and different pH values. The kinetic data were obtained by adding a known quantity of adsorbent to a dye solution at a constant temperature and under controlled stirring conditions. It was possible to draw the uptake curves, using the effects of the stirring on the adsorption rate. The intraparticle effective diffusivity was estimated using the film and pore diffusion model. The results were compared with the data obtained using a commercial activated carbon.
Keywords: Dye wastewater, activated alumina.
INTRODUCTION
Color is a characteristic of wastewater which is easily detected. Some dyes are stable and do not suffer biodegradation and those containing heavy metals are toxic. Consequently, it is important to remove these pollutants from the wastewaters before their final disposal. In most situations, the use of a combination of different methods of treatment is necessary in order to remove all the contaminants present in the wastewater (Hamza and Hamoda, 1980; Shaul et al.; 1982; Shelley et al., 1976). Therefore, adsorption became one of the most effective methods of decolorization of textile wastewater (McKay, 1980; Yeh et al.; 1993; McKay and Al-Duri, 1990). Activated carbon is, by and large, the most commonly used adsorbent (McKay and Al-Duri, 1990), although other materials such as activated clay, wood and different types of cellulose-based materials have also been recently investigated (Yeh et al.; 1993; Nassar and Geundi, 1990; Yeh and Thomas, 1995a). One important point to be considered when choosing an adsorbent is the possibility of easy regeneration.
The aim of this work is to determine the efficiency of the removal of reactive dyes, namely yellow mono and dichlorotriazine from water solutions, using commercial activated alumina as an adsorbent.
METHOD
The dyes used in this work represent two of the most commonly used dyes in the textile industry and belong to the reactive class. The yellow dyes from the group of mono and dichlorotriazine were used.
The adsorbent was a commercial activated alumina with a particle size of 0.123mm (70-230 mesh Tyler), and was characterized by mercury porosimetry and BET surface area. The average pore size is 560 A and the superficial area is 182.4 m2/g. Activated alumina used as adsorbent has typical surface area range from 200 to 500m2/g and their surface area is generated by the removal of water of constitution from hydrated aluminas.
The dye concentrations in solution were measured by UV-visible spectrophotometry using a CELM-E Model 225D spectrometer. The wavelength was selected so as to obtain the maximum absorbance, l max, for each dyestuff used.
The equilibrium data were obtained using a static method. A known amount of adsorbent, dried at 100oC for 24 hours, was added to an aqueous solution of the dyestuff whose initial concentration had been previously determined. The adsorption isotherms were taken in the temperature range of 30 to 60oC. The solution pHs varied between 4.4 to 4.7. The 250 ml flasks were kept in a thermostatic bath and stirred at a controlled speed.
The effects of contact time on the removal of color were studied by adding 5.00 g of adsorbent to 100 ml of distilled water containing 50 ppm of dyestuff at a constant temperature and a controlled stirring speed. After regular intervals of time, the slurry solution was filtrated and the concentration of the dye was determined.
The solid adsorbent was analyzed by Fourier Transform infrared spectroscopy with (FTIR) in a BOMEM equipment, using a KBr disk in air as a reference.
RESULTS AND DISCUSSION
FTIR Characterization
An infrared analysis were done in order to observe spectroscopic changes in the adsorbent material before and after adsorption (Figure 1).
The FTIR spectrum of the fresh alumina sample showed absorbance at 3700-3400 cm-1 due to the vibration of the OH group and a 1020-975 cm-1 due to the angular deformation of the OH bond and, although absorbance in the 770-720 cm-1 region was expected due to the Al-OH groups, such absorbance was not registered in the fresh sample but it appeared in the saturated sample.
The FTIR spectrum of the sample saturated with a 0.1% (wt) solution of yellow monochlorotriazine did not present any significant spectroscopic change, showing that adsorption is probably a physical process or, if it is chemical, the adsorbent-adsorbate bond energy is very low.
Equilibrium Studies - Temperature Effects
Adsorption isotherms were taken for the yellow mono and dichlorotriazine dyes in the temperature range of 30 to 60oC. The results are shown in figures 2 and 3. The experimental data were fitted to the Langmuir equation (eq. 1) and the Langmuir model constant values are presented in Table 1.
The Langmuir equilibrium constant, KL (Table 1), measured at different temperatures was used to determine the adsorption enthalpy using the Clausius -Clapeyron equation (eq.2)
An increase in the Qo value was observed as the temperature increased, for both the yellow mono and the dichlorotriazine dyes. This in an unexpected behavior, but it has been reported by some authors when studying the adsorption of different type of dyes and other organic compounds over several adsorbents (McKay et al. (1980): McKay et al. (1982); Nakhla et al. (1994); Asfour et al. (1994); Gayle (1994). McKay et al. (1980, 1982) suggested that this behavior is due to the possibility of an increase in the porosity and in the total pore volume of the adsorbent with the increase of the temperature. Achife and Ibemesi (1989) suggested the possibility of the increase of the number of active sites for the adsorption with the increase of the temperature. The increase of the Qo values with the increase of the temperature is still not get well understood and it is a phenomenon worthy of further investigation.
Many variables such as the molecular volume of the dye, its planarity and its ability to bind to the adsorbent, among others, can affect the degree of adsorption. More dichlorotriazine is adsorbed than monochlorotriazine. This difference must be related to the higher affinity of the yellow dichlorotriazine dye for the alumina surface, as the molecules of the two dyes have similar molecular volumes.
FTIR spectra of fresh (a) and saturated alumina with a 0.1% solution of yellow monochlorotriazine (b).
Adsorption isotherms of yellow monochlorotriazine on alumina at pH 4.5. The solid lines denote the theoretical isotherms calculated according Langmuir equation.
Adsorption isotherms of yellow dichlorotriazine on alumina at a pH of 4.5. The solid lines denote the theoretical isotherms calculated according to Langmuir equation.
Table1: Langmuir isotherms at different temperatures for the adsorption of yellow mono and dichlorotriazine on alumina
The essential characteristics of a Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, RL, (Weber and Chakarvorti, 1974) which is defined by equation (3).
RL=1/(1+bCo) (3)
The parameter indicates the isotherm shape as follows:
RL value Type of isotherm
RL > 1 Unfavorable
RL=1 Linear
0< RL< 1 Favorable
RL=0 Irreversible
The RL values are listed in Table 2 and they
are near zero for both dyes used, and it is a typical behavior of an irreversible isotherm.
The apparent adsorption enthalpy is -1.52Kcal/mol for the yellow monochlorotriazine dye, and +5.00 Kcal/mol for the dichlorotriazine dye. The apparent adsorption enthalpy of these dyes on activated carbon (Moreira and Peruch, 1996) showed a similar negative value for the monochlorotriazine dye and a similar positive value for the dichlorotriazine dye. The low value of adsorption enthalpy for the yellow monochlorotriazine indicates a physical adsorption, which agrees with the results obtained from the characterization by FTIR. Although the enthalpy of adsorption is almost always a negative value showing that adsorption is an exotermic process, the literature has shown apparent enthalpy with positive values (McKay et al., 1982).
Equilibrium parameter, RL, for the adsorption of the yellow mono and dichlorotriazine dyes.
McKay et al.(1982) reported an apparent heat of adsorption of +13.3 Kcal/mol for the adsorption of Direct Red 84 on chitin and this author attributed this positive apparent adsorption enthalpy value to geometric factors that allow chitin to adsorb more dye. In this present study, for the yellow dichlorotriazine dye, the positive value for the apparent adsorption enthalpy could also be attributed to geometric factors that would allowed more adsorption at high temperature.
Yeh and Thomas(1995a) recently published results on Disperse Red - 60 dye on alumina, showing that less than 1g of alumina per 100 ml of a solution with 150 ppm of dye is necessary to obtain maximum removal of the color and thereafter the amount of dye removed is independent of the amount of adsorbent. The results of our work show typical irreversible adsorption isotherms, by contrast Yeh and Thomas (1995a)results suggest that color removal is dependent on the amount of alumina.
Equilibrium Studies - Effect of Ph
The hydrogen ion concentration (pH) primarily affects the degree of ionization of the yellow monochlorotriazine sorbate and the surface properties of the sorbent. These, in turn, lead to alterations in the amount of sorbate removed in such away that in pH 7.3 the adsorption is practically zero. From Figure 4 it is clear that the amount of yellow monochlorotriazine removed varies with pH, i.e., lower pH value produces a larger adsorbed quantity. This can be explained on the basis of formation of a positively charged surface on alumina. A low pH quite probably results in a lowering of the decrease of the negative charge on the adsorbent, thus enhancing the adsorption of the negatively charged adsorbate (Weber et al., 1963).
Contact Time Studies
The mechanism of color removal can be described in four steps:
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migration of the dye molecules from the solution to the film around the particle;
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diffusion through the liquid film to the surface;
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intraparticle diffusion;
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adsorption on an active site.
An analytical solution (equation 4) for the film and pore diffusion model (McKay, 1984; Yeh and Thomas, 1995b) was used. This model is based on the mass transfer model in which the process starts on the particle surface, forming a reacted zone that moves to the center of the particle at a given velocity. Thus, during the entire process, the nonconverted nucleus is decreasing in size as the reaction is occurring.
(4)
where
B = 1-1/Bi Bi = Kf.R/Deff
X = (1-h )1/3 h = q/q*
a = [(1-Ch)/Ch]1/3 t = (Co/r p.q*) (Deff.t/R2)
Ch = q* W/CoV
Equation 4 was linearizated to determinate the Deff e Kf values.
The kinetic results are presented as rate of uptake curves which plot the amount of dye adsorbed per unit mass of adsorbent against time.
In general, an increase in stirring causes a decrease in the width of the hydrodynamic limit layer, where the external resistance to the mass transfer is located. Figure 5 shows that the adsorption of yellow monochlorotriazine on alumina is very fast and that upon increasing the stirring from 70 to 150 rpm, the adsorption rate increases.
The adjusted values for the effective diffusivity and mass transfer coefficient in the film, according to the pore and film diffusion model, are presented in Table 3.
A comparison between the adsorption rate of yellow monochlorotriazine on alumina and on activated carbon (Figure 6) shows that the dye adsorption on alumina is much faster than on granular activated carbon, and the effective diffusivities calculated from film and pore model (Equation 4) are 2.37.10-6 cm2/s and 6.42.10-5 cm2/s for the adsorption on activated carbon and activated alumina respectively. Both adsorbents achieve approximately the same maximum dye removal.
Preliminary studies on alumina regeneration for further use show that the alumina could be reactivated at 400oC, and so it is a potential substitute for the activated carbon.
Figure 4: Sorption isotherms of yellow monochlorotriazine on alumina at 30oC and different pHs values. The solid lines denote the theoretical isotherms calculated according to Langmuir equation.
Adsorption kinetics of yellow monochlorotriazine on alumina at different stirring speeds. The solid lines denote the theoretical kinetics calculated according equation 4. (Mass of alumina, 5.0 g).
Comparison of adsorption kinetics of yellow monochlorotriazine on alumina and on activated carbon. The solid lines denote the theoretical isotherms calculated according equation 4. (150 rpm, 5.0g of adsorbent in 100 ml of 50 ppm solution).
CONCLUSION
The factors that affect the adsorption of dyes in aqueous solution were studied for the yellow mono and dichlorotriazine dyes, using activated alumina as an adsorbent. Activated alumina was shown to be an excellent adsorbent. The Langmuir isotherm adjusted well to the experimental data and indicated an irreversible adsorption isotherm of the dye on alumina. The enthalpy of adsorption on alumina was calculated and suggested a physical adsorption. The pore and film diffusion model described the mass transfer of the dyes on alumina and the internal diffusion coefficient was calculated.
NOMENCLATURE
Bi Biot number
C Liquid phase concentration, g/l
Ce Equilibrium concentration, g/l
Co Initial concentration, g/l
Deff Effective diffusivity, cm2/s
Dp Particle diameter, cm
Kf External mass transfer coefficient, cm/s
KL Langmuir equilibrium constant, l/g
q Amount of dye removed per unit of solid mass, mg/g
q* Amount of dye removed per unit of solid mass in equilibrium, mg/g
Qo Maximum amount of dye removed per unit of solid mass, mg/g
R Average particle radius, cm
V Volume of solution, l
W Mass of adsorbent, g
r p Particle density, g/cm3
t time, s
l max Walength of maximum absorbance, cm-1
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Publication Dates
-
Publication in this collection
09 Oct 1998 -
Date of issue
Mar 1998
History
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Accepted
22 Sept 1997 -
Received
07 May 1997
