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Adsorption of dyes using shale oil ash
PII: S0043-1354(00)00196-2
Wat. Res. Vol. 34, No. 17, pp. 4295±4303, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
www.elsevier.com/locate/watres
ADSORPTION OF DYES USING SHALE OIL ASH Z. AL-QODAH* Department of Chemical Engineering, Amman College for Engineering Technology, Al-Balqa Applied University, PO Box 340558, Amman, Marka 11134, Jordan (First received 1 March 1999; accepted in revised form 29 February 2000) AbstractÐThe adsorption of reactive dyes on shale oil ash has been investigated during a series of batch adsorption experiments.The adsorption isotherm data were ®tted to Langmuir isotherm. The two-resistance mass transfer model has been developed based on the ®lm resistance and homogeneous solid phase diusion. A computer program has been developed to estimate the theoretical concentration-time dependent curves and to compare them with the experimental curves by means of the best-®t approach. The model predicts that the external mass transfer coecient K was not aected by varying the initial dye concentration, but it increases when the agitation speed and temperature was raised. The diusion coecient D was found to increase when the initial dye concentration, and temperature was raised. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐadsorption kinetics, adsorption modeling, industrial wastewater treatment, reactive dyes, shale oil ash, adsorbents
NOMENCLATURE
isotherm constant for Langmuir equation, m3/kg solid aR isotherm constant for Redlich±Paterson equation a.s. agitation speed, rpm b isotherm constant for Langmuir equation, m3/kg dye Cb dye concentration in the bulk of the liquid phase, kg/ m3 Ce equilibrium dye concentration, kg/m3 D diusion coecient, m2/s K mass transfer coecient, m/s q average dye concentration in the particles, kg dye/m3 adsorbent Qe equilibrium dye concentration in the particle, kg dye/ kg adsorbent qi local dye concentration in the particle, kg dye/kg adsorbent r radial coordinate, m R radius of the particle, m t time, s a
Greek letters b exponent of Redlich±Paterson isotherm
INTRODUCTION
It is well known that textile and pulp mills, discharge highly colored industrial wastewater which contains appreciable concentrations of materials *Fax: +962-64-694294; e-mail: [email protected]
with high oxygen demand (BOD) and suspended solids (SS) (Mckay, 1984). These colored compounds are not only aesthetically displeasing, but also impede light penetration in the treatment pans, thus upsetting the biological treatment processes within the treatment plant. They also increase the BOD, and cause lack of dissolved oxygen to sustain aquatic life. In addition, many dyes are toxic to some microorganisms, and may cause direct destruction or inhibition of their catalytic capabilities (Asfour et al., 1985). Therefore, it is necessary to reduce dye concentration in the wastewater before their biological treatment process. Adsorption has been used extensively in industrial processes for many purposes of separation and puri®cation. The removal of colored and colorless organic pollutants from industrial wastewater is considered as an important application of adsorption processes using suitable adsorbent. At present, there is growing interest in using low cost, commercially available materials for the adsorption of dyes. A wide variety of materials such as ¯y ash (Mckay and McConvey, 1981; Banergee et al., 1997), peat (Mckay and Allen, 1983), phenolic resin (Kasaoka et al., 1984), wood (Gupta and Bhattacharya, 1985), maise cob (El-Guendi, 1991a), natural clays (Dweib, 1993), activated sludge (Pagga and Taeger, 1994), wood chips (Wang et al., 1995), jift (Haimour and Sayed, 1997), palm-fruit bunch particles (Nassar and Majdy, 1997), nanosize modi®ed silica (Wu et al.,
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1997), sugar beet pulp (Bousher et al., 1997), activated carbon from fertilizer waste (Gupta et al., 1997a), olive mill products (Gharaibeh et al., 1998), activated slag (Gupta et al., 1997a; Gupta, 1998), bagasse ¯y ash (Gupta et al., 1998) and diatomaceous silica (Al-Qodah, 1998), are being used as low cost alternatives to activated carbon. Shale oil ash (SOA) is an inorganic residue obtained after the direct combustion of the shale oil used as a source of energy. Shale oil reserves are abundant in 31 countries including Jordan. The utilization of shale oil has started in many places such as China, Estonia and Canada (Holopainen, 1991). The total residual ash material that might be generated from shale oil reserves has been estimated, to about 3:2 1011 tons. Therefore, an inexpensive residual management technology is needed for the disposal or bene®cial uses of this material. The amounts of chemical constituents in SOA is as follows: 32% CaMg (CO3), 16% CaCO3, 15% SiO2, 19% clay, 10% NaAlSi3O8, 6% KAlSi3O8 and 1% FeS2. It is evident that SOA contains a wide variety of acidic, basic and amphoteric oxides. In addition, this material which is produced from a high temperature process, is characterized by a high porosity. These chemical and physical properties suggest that SOA will have good adsorption behavior for both organic and inorganic pollutants found in wastewater. However, a review of the literature shows that SOA has not been used before as an adsorbent. For this reason, the intention of this paper is to study the adsorptive capabilities and kinetics of SOA as a possible cheap adsorption media for reactive dyes in order to reduce their concentrations in the euent produced by the Company of Textile, Zarka-Jordan or any other textile dying company. These dyes, which are used in coloring cotton and viscose ®bers usually, reduce the eciency of the biological treatment step in this factory owing to their reactivity. MATHEMATICAL MODELING
In most adsorption systems the mass transfer of the solute onto and within the adsorbent particle aects the adsorption rate. Models for predicting the performance of batch adsorbers are usually based on either a liquid ®lm resistance or pore diffusion resistance. The ®rst resistance describes the solute transfer rate from the bulk of the liquid phase, and the second one describes the solute transfer rate in the internal pores. It is clear that these models are limited to the range of the hydrodynamic conditions i.e. either liquid ®lm diusion control or solid side pore diusion control. In addition, these models are limited as they can only be applied over a short time period during the adsorption process. For this reason Mathews and Weber (1976) used a model based on a combination of the two resistances. After that, many researchers have
used the two resistance's model (Mckay, 1984, 1985; El-Guendi, 1991a; Dweib, 1993; Haimour & Sayed, 1997). In the two-resistance model, the single component adsorption rate includes the following mechanistic processes: . Solute diusion inside the liquid boundary layer from the liquid phase to the surface of the particle. . Solute adsorption on the surface of the particle. . Solute diusion in the pores of the particle. For spherical particles, the variation of the solute concentration q with distance r and time t is given by the following diusion equation: @ qi 1 @ @ qi Dr 2 2 1 r @r @t @r For concentration independent diusion coecient, equation (1) becomes: @ qi D @ @ qi r2 2 2 r @r @t @r The corresponding initial conditions are described by the following two equations: qi r, o o
3
cb 0 cbo
4
The boundary conditions are described by the following equations: qi R, t qs R, t qs t
5
@ qi 0, t 0 @r
6
The boundary condition described by equation (6) expresses the fact that the dye concentration at the surface of the particle is time dependent. Two of the most common isotherms are used to ®nd the relationship between the surface concentration qe and the equilibrium concentration Ce. These are the Langmuir isotherms, which assumes that equilibrium is attained when a monolayer of the adsorbate molecules saturates the adsorbent, and the three-parameter Redlich±Paterson isotherm. These two models can be represented by equations (7) and (8), respectively: qe
qe
aCe 1 bCe aCe
1 aR C be
7
8
where a, b, aR and b are constants. Since b approaches unity, as will be shown later,
Adsorption of dyes using shale oil ash
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Table 1. Some properties of the reactive dye used Dye Type Wave length pH
Drim yellow-K4G Azo dye 420 Acidic
only the Langmuir isotherm described by equation (7) was used in the model. The rate of change of the average concentrations of the solid phase at steady state conditions and the material balance equation that describes the dye concentration in the system are found elsewhere (Mckay, 1984, 1985; El-Guendi, 1991a; Dweib, 1993). After introducing the suitable dimensionless variables the resulting system of the dimensionless equations was solved numerically as they cannot be solved analytically. In the present investigation the implicit ®nite dierence scheme of Crank and Nicholsen used by El-Guendi (1991a) was employed. The general form of the solution is available elsewhere (Mckay et al., 1987; El-Guendi, 1991a).
EXPERIMENTAL
Materials The SOA used in this study was obtained from the Authority of Natural Resources-Jordan. Samples of this material were burned at 8008C for 1 h. The residual ash was cooled, then milled and sieved to several fractions. Three fractions of 53-15, 150-250 and 250-355 mm diameter were used. The bulk density of the ash particles is 1450 kg/m3 and the porosity is about 0.42. The SOA was ®rst mixed with warm water and stirred for 15 min in order to dissolve the soluble portion of the ash. The ash was then ®ltered and dried in order to calculate its density and voidage. The reactive dyes used in this study are listed in Table 1. These dyes were used as received from the Company of Textiles, Zarka-Jordan.
Drim blue-KBL Anthraquanine dye 620 Acidic
Drim red K4BL Azo dye 530 Basic
until no signi®cant change in the dye concentration was measured. Spectroscopic analysis was carried out using PV 8700 visible spectrophotometer (Philips Scienti®c). Samples of the ®ltrate dye solution of about 1 ml were diluted with distilled water if necessary, then the concentration was measured with 1.0 cm light path glass cells. RESULTS AND DISCUSSION
Adsorption isotherms The adsorption isotherm indicates how the adsorbate molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. The analysis of the isotherm data by ®tting them to dierent isotherm models is an important step to ®nd the suitable model that can be used for design purposes (ElGuendi, 1991a). Figure 2 shows a plot of the dye loading on the adsorbent against the dye equilibrium concentration in the liquid phase for three dierent dyes at 258C. It is evident that drim yellow-K4G and drim blueKBL can easily be removed from the liquid phase by the ash particles. For example, a solution containing 30 mg/l of drim yellow-K4G or drim blueKBL produced an equilibrium loading of 120 mg of drim yellow-K4G and 88 mg of drim blue-KBL per 1 g of the adsorbent. The ash maximum adsorption capacity from drim yellow-K4G, Drim blue-KBL
Methods Contacting single component dye solutions of dierent concentrations such as 20, 40, 60, 80, 100, 150, 200, 250 300 and 340 ppm, with ®xed amounts of the ash examined the adsorption capacity of SOA. These experiments were conducted in a shaker path at constant temperature. In each experiment, 100 ml from the dye solution was placed in a 500-ml conical ¯ask containing 0.17 g of the ash. After closing the ¯asks, they were mounted in the orbital shaker for 24 h. The solutions were then ®ltered and analyzed. This procedure was repeated at four dierent temperatures of 20, 25, 35 and 458C. The dimension of the isothermal stirred batch adsorber used to carry out the kinetic study is shown in Fig. 1. In each experiment, 1.7 l of the dye solution was continuously stirred with a certain amount of the ash. Samples of the solution were continuously withdrawn from the adsorber by a suitable syringe. Each experimental run continued
Fig. 1. Schematic diagram of the batch adsorber: (1) stainless steel variable speed agitator; (2) sampling hole; (3) cover; (4) baes.
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Fig. 4. Redlich±Paterson plot of the adsorption of drim yellow-K4G with the ash at 258C. Fig. 2. Adsorption isotherm for drim dyes with SOA of 53±150 mm diameter.
and Drim red K4BL were 160, 140 and 100 mg dye/g adsorbent. The adsorption capacities of commercial activated carbon for the three dyes at the same conditions were 180, 150 and 144 mg dye/g adsorbent. On the other hand, the adsorption capacity of SOA is about 25% higher than that of Pentonite for the same dyes at the same conditions. These results indicate that SOA has good capabilities as an adsorbent for dyes. One more advantage of this adsorbent is the ease of its regeneration. It was indicated by some regeneration tests that if hot gases of 300±4008C were eluted through a dry bed of the exhausted adsorbent for a period of 20 min the adsorbed dye would be oxidized and the adsorbent becomes fresh again (Fig. 2). Figures 3 and 4 show a plot of Langmuir isotherm and a plot of Redlich±Paterson isotherm, respectively. The linear form of Langmuir isotherm and that of Redlich±Paterson can be represented by equations (9) and (10), respectively,
Ce 1 b Ce a a qe ln
aCe ÿ1 qe
9
b ln Ce ln aR
10
While Langmuir isotherm parameters a and b were obtained by plotting Ce/qe vs Ce, Redlich±Paterson isotherm parameters were obtained by plotting ln(aCe/qeÿ1) vs lnCe. After ®tting the data using the computer program ``Origin'', equations (21) and (22) become as equations (11) and (12), respectively, Ce 0:1442 4:395Ce qe ln
6:935Ce ÿ1 qe
11
0:9889 ln Ce ÿ 29
12
The values of R 2 for equations (11) and (12) are 0.9957 and 0.9834, respectively. Table 2 shows the values of the parameters of the two models at 258C. It is evident from these values that b approaches 1. Consequently, the Langmuir equation can accurately describe the equilibrium data over the concentration range used in this investigation. For this reason, the Langmuir model was used in the developed kinetic model. Kinetic studies The proposed model has been tested using a number of experiments. The model system of equations was solved numerically using the implicit ®nite dierence scheme. A computer program was Table 2. Values of the two isotherm parameters of SOA (dp=150± 250 mm) with drim-yellow at 258C Model Langmuir
Fig. 3. Langmuir isotherm of the drim dyes with the ash.
Parameter Value
a (l/g) 6.935
b( l/g) 30.478
Redlich±Paterson a (l/g) 6.935
aR (l/g) 29
b 0.9889
Adsorption of dyes using shale oil ash
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developed to ®t the experimental curve to the theoretical one. By altering the values of K and D it was possible to obtain the best ®t between the experimental and theoretical curves of the batch adsorption. This approach for obtaining the values of K and D at dierent experimental conditions has been employed by many researchers (Mathews and Weber, 1976; Mckay et al., 1984; Mckay, 1985; ElGuendi, 1991a). The eect of the initial dye concentration, adsorbent mass, particles size, mixing speed, and temperature on the rate and extent of adsorption by SOA were studied.
was equal to 2.5 10ÿ6 m/s and remained constant for all values of initial concentration. The diusion coecient inside the particle pores, D increased from 7 10ÿ11 to 12.7 10ÿ11 m2/s as the initial dye concentration changed from 60 to 380 mg/l. This behavior of concentration dependent diusivity agrees with investigations obtained by Hu et al. (1994), for the adsorption of hydrocarbons by activated carbon and those obtained by Haimour and Sayed (1997), for the adsorption of methylene blue by jift.
Eect of initial concentration
Eect of adsorbent mass
The eect of initial dye concentration on the rate of adsorption by SOA is shown in Fig. 5 where the experimental results are shown as discrete points and those obtained from the model by solid lines. It is evident from Fig. 6 that, in a series of experiments in which the initial concentration of drim yellow-K4G ranged from 50 to 380 mg/l, the correlation between the experimental and the theoretical data is excellent. For a particular experiment, the rate of adsorption decreased with time until it gradually approached a plateau, owing to the continuous decrease in the concentration driving force. In addition, the initial rate of adsorption was greater for higher initial dye concentration, because the resistance to the dye uptake decreased as the mass transfer driving force increased. The kinetic parameters, K and D, predicted by the model are listed in Table 3. It was found that the mass transfer coecient K was not aected by changing dyes initial concentration, and its value
The eect of varying the ash mass on dye adsorption is shown in Fig. 6. It is evident from this ®gure that the correlation between the experimental and theoretical results is good. It can be seen and, as expected, that the dye concentration in the solution decreased at a faster rate as the adsorbent mass increased. For example, the relative dye concentration after 40 min from start up decreased from 0.93 to 0.4 as the adsorbent concentration increased from 0.5 to 2 g/l. The mathematical model predicts that the mass transfer coecient, K, and the diusivity coecient, D do not change as the adsorbent mass changes. The values of K and D were 2.5 10ÿ6 m/s, and 1 10ÿ10 m2/s. These results are in agreement with those of Mckay (1985) for the adsorption of acid dye 25, and El-Guendi (1991a). The results obtained from this section of experiments indicate that the SOA has a large potential as an adsorbent for dyes. It was found that, if 2 g of the adsorbent were mixed with 1 l of the dye sol-
Fig. 5. Eect of initial concentration on the adsorption rate of drim yellow-K4G with the ash (dp=200 mm, T = 258C, M/V = 1 kg/m3, a.s.=400 rpm).
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Fig. 6. Eect of adsorbent mass on the adsorption rate of drim yellow with the ash (dp=200 mm, Co=200 ppm, V = 1.7 l, T = 258C, a.s.=400 rpm).
ution, more than 90% of the dye molecules would be adsorbed. Eect of agitation speed The eect of contact time on adsorption of drim yellow-K4G with SOA at dierent agitator speeds is shown in Fig. 7. The correlation between the experimental with the theoretical results is again excellent. The data shown in Fig. 8 indicates that the rate of adsorption increases as the agitator speed increases. This eect can be attributed to the increased turbulence and as a consequence, the decrease boundary layer thickness around the
adsorbent particles is a result of increasing the degree of mixing. The mass transfer coecient K increased from 2 10ÿ6 to 3.3 10ÿ6 m/s as the agitator speed was increased from 250 to 600 rpm. These results are at agreement with those of Asfour et al., 1985 for the removal of dyes with sawdust and those of Haimour and Sayed, 1997 for the adsorption of methylene blue with jift. On the other hand, the diusion coecient inside the particle pores remained constant for all values of agitator speed and equal to 1 10ÿ10 m2/s. In large-scale continuous adsorption processes there are two possible modes of operation. These
Table 3. Various experimental conditions for the adsorption of drim yellow-K4G with SOA Set of experiments 1
2
3 4 5
M/V kg/m3
Temperature (8C)
a.s. (rpm)
CO (ppm)
Dp (mm)
K 105 (m/s)
D 1010 (m2/s)
1 1 1 1 1 0.5 1 1.5 2 1 1 1 1 1 1 1 1 1 1
25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 20 25 35 45
400 400 400 400 400 400 400 400 400 250 400 600 400 400 400 400 400 400 400
50 100 200 300 380 200 200 200 200 200 200 200 200 200 200 200 200 200 200
200 200 200 200 200 200 200 200 200 200 200 200 100 200 300 200 200 200 200
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.2 0.25 0.33 0.25 0.25 0.25 0.2 0.25 0.37 0.42
0.7 0.8 1.10 1.1 1.27 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.9 1.0 1.14 1.2
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Fig. 7. Eect of agitation speed on the adsorption rate of drim yellow with the ash (dp=200 mm, T = 258C, M/V = 1 kg/m3, CO=200 ppm).
are the use of a batch stirred adsorber or a continuous ®xed bed adsorber. The above results indicate that the rate of adsorption in a stirred adsorber was greater with an increase in the degree of mixing. In continuous ®xed bed operations, the boundary layer thickness can be reduced by increasing the dye solution ¯ow rate by circulating part of the euents
or by using ¯uidized bed adsorbers, as these multiphase contactors are characterized by intensive mixing and high mass transfer rates. These two alternatives will be examined in our future studies. Eect of particle size Figure 9 depicts the eect of contact time on the
Fig. 8. Eect of particle size (dp) on the adsorption rate of the dye with the ash (CO=200 ppm, T = 258C, M/V = 1 kg/m3, a.s.=400 rpm).
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adsorption of drim yellow-K4G with the ash of dierent particle sizes. Three dierent fractions of average sizes ranging between 100 and 300 mm were used in this phase of experiments. The results shown in Fig. 8 indicate that the rate and extent of dye adsorption by SOA was decreased by increasing the particle size. This behavior can be attributed to the relationship between the eective speci®c surface area of the adsorbent particles and their sizes. The eective surface area decreased as the particles size increased and as a consequence, the saturation adsorption per unit mass of the adsorbent decreased (Haimour & Sayed, 1997; Banergee et al., 1997; Wu et al., 1997). The particle size was found to have no eect on the mass transfer and the diusion coecients. Table 3 shows that the mass transfer coecient K and the diusion coecient D remain constants and equal to 2.5 10ÿ6 m/s and 1 10ÿ10 m2/s when the particle size increases from 100 to 300 mm. This behavior is in consistence with the theory, because the variations in the particle size in this study did not aect the hydrodynamics as the total adsorbent concentration is very low. Cooney and Adesanya (1995) have shown by modeling some experimental data that, particles size variations in a batch adsorption process will have a signi®cant eect on K and D if the fractional mass uptake values are greater than 0.7. This value may occur either at higher solid concentrations than those used in the present investigation or at very low adsorbate initial concentration. These results are not in agreement with some of
those found in the literature. Mckay and McConvey (1981) reported that the range of particle sizes had little in¯uence on the mass transfer coecient in the adsorption of acidic dyes with wood. On the other hand, Haimour and Sayed (1997), who used relatively low adsorbate concentrations, reported that, the mass transfer coecient decreased and the diusion coecient increased as the particle size increased in the adsorption of methylene blue with jift. Eect of temperature The temperature has two major eects on the adsorption process. Increasing the temperature is known to increase the rate of diusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particle, owing to the decrease in the viscosity of the solution. In addition, changing the temperature will change the equilibrium capacity of the adsorbent for a particular adsorbate. In this phase of study, a series of experiments were conducted at 20, 25, 35, and 458C to study the eect of temperature on the adsorption rate and the kinetic coecients K and D. Figure 9 depicts the eect of contact time on the rate of adsorption of drim yellow-K4G with the ash at four dierent temperatures. In addition to the excellent correlation between the theoretical and the experimental data, Fig. 9 shows that the rate of adsorption increases as the temperature increased. Furthermore, the mass transfer coecient increased from 2 10ÿ6 to 4.2 10ÿ6 and the diusion coecient D increased from
Fig. 9. Eect of temperature on the adsorption capacity and rate of the dye with the ash (dp=200 mm, M/V = 1 kg/m3, CO=200 ppm, a.s.=400 rpm).
Adsorption of dyes using shale oil ash
0.9 10ÿ10 to 1.2 10ÿ10 as the temperature increases from 20 to 458C. These results agree with those of Haimour and Sayed (1997) and Mckay and Allen (1980). CONCLUSIONS
Based upon the experimental and the theoretical results in this investigation, the following conclusions can be drawn: . SOA has high potential to adsorb reactive dyes from aqueous solutions; . the adsorption isotherms of the reactive dyes with the ash were ®tted to Langmuir isotherm; . the initial rate of adsorption of reactive dyes with the ash was high, and then it declines with time until it reaches a plateau; . the adsorption of reactive dyes with the ash can easily be described by the two-resistance's model; . the rate of adsorption was found to increase by increasing the initial concentration, adsorbent mass, agitation speed and temperature; . in addition, it decreases by increasing by the particle size; . a feasible process option would comprise the adsorption of the dye from the warm solution euents from the mills by the ®nally divided ash, and then desorbing it at relatively low temperatures; . the results of this research study were compared to the published data in the same ®eld, and found to be in agreement with most of them.
AcknowledgementsÐThe Author is grateful to the administration of the Higher Council of Science and Technology, Amman-Jordan, for funding this research. The Author also wishes to thank the Authority of Natural Resources, Jordan, for providing the shale oil samples, and the Company of Textile, Zarka-Jordan, for providing the reactive dye samples. REFERENCES
Al-Qodah Z. (1998) Adsorption of methylene blue with diatomite. J. Eng. Technology 17(9), 128±137. Asfour H. M., Nassar M. M., Fadali O. A. and El-Guendi M. S. (1985) Color removal from textile euents using hardwood saw dust as an adsorbent. J. Chem. Technol. Biotechnol. 35A, 28±35. Banergee K., Cheremisino P. N. and Cheng L. S. (1997) Adsorption kinetics of O-Xylene by ¯yash. Wat. Res. 31, 249±261. Bousher A., Shen X. and Edyvean R. G. J. (1997) Removal of colored organic matter by adsorption onto low-cost materials. Wat. Res. 31(8), 2084±2092. Cooney D. O. and Adesanya (1995) Eect of particle size distribution on adsorption kinetics in stirred batch systems. Chem. Eng. Sci. 38(9), 1535±1541. Dweib M. A. (1993) Adsorption of dyes from their solutions using natural clays. M.Sc. thesis, University of Jordan.
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El-Guendi M. (1991a) Homogeneous surface diusion model of basic dyestus onto natural clay in batch adsorbers. Adsorption Sci. Technol. 8(2), 217±225. Gharaibeh S. H., Moore S. V. and Buck A. (1998) Eect of industrial wastewater using processed solid residue of olive mill products and commercial activated carbon. J. Chem. Technol. Biotechnol. 71(1), 180±189. Gupta M. P. and Bhattacharya P. K. (1985) Studies on color removal from bleach plant euent of a Kraft pulp mill. J. Chem. Tech. Biotechnol. 35B, 23±32. Gupta V. K., Srivastava S. K. and Mohan D. (1997a) Equilibrium uptake, sorption dynamics, optimization and column operation for the removal and recovery of malachite green from wastewater using activated carbon activated slag. Ind. Eng. Chem. Res. 36(6), 2207±2218. Gupta V. K., Sharma S., Yadav I. S. and Mohan D. (1998) Utilization of bagasse ¯y ash generated in the sugar industry for the removal and recovery of phenol and p-nitrophenol from wastewater. J. Chem. Tech. Biotechnol. 71, 180±186. Gupta V. K. (1998) Equilibrium uptake, sorption dynamics, process development and column operation for the removal of copper and nickel from aqueous solution and wastewater using activated slag, a low-cost adsorbent. Ind. Eng. Chem. Res. 37(1), 192±202. Haimour N. and Sayed S. (1997) The adsorption kinetics of methylene blue dye on jift. Dirsat, Natural and Eng. Sci. 24(2), 215±224. Holopainen H. (1991) Experience of oil shale combustion in ahlistorm pyro¯ow CFB-boiler. Oil Shale 8(3), 194± 205. Hu X., Okamota A. and Kataoka T. (1994) Experimental concentration dependence of surface diusivity of hydrocarbons in activated carbon. Chem. Eng. Sci. 49(13), 2145±2152. Kasaoka S., Sakata K., Tanaka E. and Naitoh R. (1984) Design of molecular-sieve carbons, studies on the adsorption of various dyes in the liquid phase. Inter. Chem. Eng. 24, 734±742. Mathews A. P. and Weber W. J. (1976) Eects of external mass transfer and inter-particle diusion on adsorption. AIChE Symp. Ser. 73, 91±98. Mckay G. (1984) Two-resistance mass transfer models for the adsorption of dyestus from solutions using activated carbon. J. Chem. Tech. Biotechnol. 34A, 294±310. Mckay G. (1985) The adsorption of dyestus from aqueous solutions using activated carbon: an external mass transfer and homogeneous surface diusion model. AIChE J. 31(2), 335±338. Mckay G. and Allen S. J. (1980) Single resistance mass transfer models for adsorption of dyes on peat. J. Separ. Process. Technology 4(3), 1±7. Mckay G., Allen S. J., McConvey I. F. and Walters J. H. R. (1984) External mass transfer and homogeneous solid-phase diusion during the adsorption of dyestus. Ind. Eng. Chem. Process Des. Dev. 23, 221±226. Mckay G. and McConvey I. F. (1981) The external mass transfer of basic and acidic dyes on wood. J. Chem Tech. Biotechnol. 31, 401±408. Nassar M. M. and Majdy Y. H. (1997) Removal of dierent basic dyes from aqueous solutions by adsorption on palm-fruit bunch particles. Chem. Eng. J. 66, 223±339. Pagga U. and Taeger T. (1994) Development of a method for adsorption of dyestus on activated sludge. Wat. Res. 28(5), 1051±1057. Wang K., Furney T. D. and Halway M. C. (1995) Modeling the HF adsorption process on wood chips in a packed-bed reactor. Chem. Eng. Sci. 50, 2883±2897. Wu G., Kaliadima A., Her Y. and Matijevic E. (1997) Adsorption of dyes on nanosize modi®ed silic particles. J. Coll. Interface Sci. 195, 222±228.
Wat. Res. Vol. 34, No. 17, pp. 4295±4303, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
www.elsevier.com/locate/watres
ADSORPTION OF DYES USING SHALE OIL ASH Z. AL-QODAH* Department of Chemical Engineering, Amman College for Engineering Technology, Al-Balqa Applied University, PO Box 340558, Amman, Marka 11134, Jordan (First received 1 March 1999; accepted in revised form 29 February 2000) AbstractÐThe adsorption of reactive dyes on shale oil ash has been investigated during a series of batch adsorption experiments.The adsorption isotherm data were ®tted to Langmuir isotherm. The two-resistance mass transfer model has been developed based on the ®lm resistance and homogeneous solid phase diusion. A computer program has been developed to estimate the theoretical concentration-time dependent curves and to compare them with the experimental curves by means of the best-®t approach. The model predicts that the external mass transfer coecient K was not aected by varying the initial dye concentration, but it increases when the agitation speed and temperature was raised. The diusion coecient D was found to increase when the initial dye concentration, and temperature was raised. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐadsorption kinetics, adsorption modeling, industrial wastewater treatment, reactive dyes, shale oil ash, adsorbents
NOMENCLATURE
isotherm constant for Langmuir equation, m3/kg solid aR isotherm constant for Redlich±Paterson equation a.s. agitation speed, rpm b isotherm constant for Langmuir equation, m3/kg dye Cb dye concentration in the bulk of the liquid phase, kg/ m3 Ce equilibrium dye concentration, kg/m3 D diusion coecient, m2/s K mass transfer coecient, m/s q average dye concentration in the particles, kg dye/m3 adsorbent Qe equilibrium dye concentration in the particle, kg dye/ kg adsorbent qi local dye concentration in the particle, kg dye/kg adsorbent r radial coordinate, m R radius of the particle, m t time, s a
Greek letters b exponent of Redlich±Paterson isotherm
INTRODUCTION
It is well known that textile and pulp mills, discharge highly colored industrial wastewater which contains appreciable concentrations of materials *Fax: +962-64-694294; e-mail: [email protected]
with high oxygen demand (BOD) and suspended solids (SS) (Mckay, 1984). These colored compounds are not only aesthetically displeasing, but also impede light penetration in the treatment pans, thus upsetting the biological treatment processes within the treatment plant. They also increase the BOD, and cause lack of dissolved oxygen to sustain aquatic life. In addition, many dyes are toxic to some microorganisms, and may cause direct destruction or inhibition of their catalytic capabilities (Asfour et al., 1985). Therefore, it is necessary to reduce dye concentration in the wastewater before their biological treatment process. Adsorption has been used extensively in industrial processes for many purposes of separation and puri®cation. The removal of colored and colorless organic pollutants from industrial wastewater is considered as an important application of adsorption processes using suitable adsorbent. At present, there is growing interest in using low cost, commercially available materials for the adsorption of dyes. A wide variety of materials such as ¯y ash (Mckay and McConvey, 1981; Banergee et al., 1997), peat (Mckay and Allen, 1983), phenolic resin (Kasaoka et al., 1984), wood (Gupta and Bhattacharya, 1985), maise cob (El-Guendi, 1991a), natural clays (Dweib, 1993), activated sludge (Pagga and Taeger, 1994), wood chips (Wang et al., 1995), jift (Haimour and Sayed, 1997), palm-fruit bunch particles (Nassar and Majdy, 1997), nanosize modi®ed silica (Wu et al.,
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1997), sugar beet pulp (Bousher et al., 1997), activated carbon from fertilizer waste (Gupta et al., 1997a), olive mill products (Gharaibeh et al., 1998), activated slag (Gupta et al., 1997a; Gupta, 1998), bagasse ¯y ash (Gupta et al., 1998) and diatomaceous silica (Al-Qodah, 1998), are being used as low cost alternatives to activated carbon. Shale oil ash (SOA) is an inorganic residue obtained after the direct combustion of the shale oil used as a source of energy. Shale oil reserves are abundant in 31 countries including Jordan. The utilization of shale oil has started in many places such as China, Estonia and Canada (Holopainen, 1991). The total residual ash material that might be generated from shale oil reserves has been estimated, to about 3:2 1011 tons. Therefore, an inexpensive residual management technology is needed for the disposal or bene®cial uses of this material. The amounts of chemical constituents in SOA is as follows: 32% CaMg (CO3), 16% CaCO3, 15% SiO2, 19% clay, 10% NaAlSi3O8, 6% KAlSi3O8 and 1% FeS2. It is evident that SOA contains a wide variety of acidic, basic and amphoteric oxides. In addition, this material which is produced from a high temperature process, is characterized by a high porosity. These chemical and physical properties suggest that SOA will have good adsorption behavior for both organic and inorganic pollutants found in wastewater. However, a review of the literature shows that SOA has not been used before as an adsorbent. For this reason, the intention of this paper is to study the adsorptive capabilities and kinetics of SOA as a possible cheap adsorption media for reactive dyes in order to reduce their concentrations in the euent produced by the Company of Textile, Zarka-Jordan or any other textile dying company. These dyes, which are used in coloring cotton and viscose ®bers usually, reduce the eciency of the biological treatment step in this factory owing to their reactivity. MATHEMATICAL MODELING
In most adsorption systems the mass transfer of the solute onto and within the adsorbent particle aects the adsorption rate. Models for predicting the performance of batch adsorbers are usually based on either a liquid ®lm resistance or pore diffusion resistance. The ®rst resistance describes the solute transfer rate from the bulk of the liquid phase, and the second one describes the solute transfer rate in the internal pores. It is clear that these models are limited to the range of the hydrodynamic conditions i.e. either liquid ®lm diusion control or solid side pore diusion control. In addition, these models are limited as they can only be applied over a short time period during the adsorption process. For this reason Mathews and Weber (1976) used a model based on a combination of the two resistances. After that, many researchers have
used the two resistance's model (Mckay, 1984, 1985; El-Guendi, 1991a; Dweib, 1993; Haimour & Sayed, 1997). In the two-resistance model, the single component adsorption rate includes the following mechanistic processes: . Solute diusion inside the liquid boundary layer from the liquid phase to the surface of the particle. . Solute adsorption on the surface of the particle. . Solute diusion in the pores of the particle. For spherical particles, the variation of the solute concentration q with distance r and time t is given by the following diusion equation: @ qi 1 @ @ qi Dr 2 2 1 r @r @t @r For concentration independent diusion coecient, equation (1) becomes: @ qi D @ @ qi r2 2 2 r @r @t @r The corresponding initial conditions are described by the following two equations: qi r, o o
3
cb 0 cbo
4
The boundary conditions are described by the following equations: qi R, t qs R, t qs t
5
@ qi 0, t 0 @r
6
The boundary condition described by equation (6) expresses the fact that the dye concentration at the surface of the particle is time dependent. Two of the most common isotherms are used to ®nd the relationship between the surface concentration qe and the equilibrium concentration Ce. These are the Langmuir isotherms, which assumes that equilibrium is attained when a monolayer of the adsorbate molecules saturates the adsorbent, and the three-parameter Redlich±Paterson isotherm. These two models can be represented by equations (7) and (8), respectively: qe
qe
aCe 1 bCe aCe
1 aR C be
7
8
where a, b, aR and b are constants. Since b approaches unity, as will be shown later,
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Table 1. Some properties of the reactive dye used Dye Type Wave length pH
Drim yellow-K4G Azo dye 420 Acidic
only the Langmuir isotherm described by equation (7) was used in the model. The rate of change of the average concentrations of the solid phase at steady state conditions and the material balance equation that describes the dye concentration in the system are found elsewhere (Mckay, 1984, 1985; El-Guendi, 1991a; Dweib, 1993). After introducing the suitable dimensionless variables the resulting system of the dimensionless equations was solved numerically as they cannot be solved analytically. In the present investigation the implicit ®nite dierence scheme of Crank and Nicholsen used by El-Guendi (1991a) was employed. The general form of the solution is available elsewhere (Mckay et al., 1987; El-Guendi, 1991a).
EXPERIMENTAL
Materials The SOA used in this study was obtained from the Authority of Natural Resources-Jordan. Samples of this material were burned at 8008C for 1 h. The residual ash was cooled, then milled and sieved to several fractions. Three fractions of 53-15, 150-250 and 250-355 mm diameter were used. The bulk density of the ash particles is 1450 kg/m3 and the porosity is about 0.42. The SOA was ®rst mixed with warm water and stirred for 15 min in order to dissolve the soluble portion of the ash. The ash was then ®ltered and dried in order to calculate its density and voidage. The reactive dyes used in this study are listed in Table 1. These dyes were used as received from the Company of Textiles, Zarka-Jordan.
Drim blue-KBL Anthraquanine dye 620 Acidic
Drim red K4BL Azo dye 530 Basic
until no signi®cant change in the dye concentration was measured. Spectroscopic analysis was carried out using PV 8700 visible spectrophotometer (Philips Scienti®c). Samples of the ®ltrate dye solution of about 1 ml were diluted with distilled water if necessary, then the concentration was measured with 1.0 cm light path glass cells. RESULTS AND DISCUSSION
Adsorption isotherms The adsorption isotherm indicates how the adsorbate molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. The analysis of the isotherm data by ®tting them to dierent isotherm models is an important step to ®nd the suitable model that can be used for design purposes (ElGuendi, 1991a). Figure 2 shows a plot of the dye loading on the adsorbent against the dye equilibrium concentration in the liquid phase for three dierent dyes at 258C. It is evident that drim yellow-K4G and drim blueKBL can easily be removed from the liquid phase by the ash particles. For example, a solution containing 30 mg/l of drim yellow-K4G or drim blueKBL produced an equilibrium loading of 120 mg of drim yellow-K4G and 88 mg of drim blue-KBL per 1 g of the adsorbent. The ash maximum adsorption capacity from drim yellow-K4G, Drim blue-KBL
Methods Contacting single component dye solutions of dierent concentrations such as 20, 40, 60, 80, 100, 150, 200, 250 300 and 340 ppm, with ®xed amounts of the ash examined the adsorption capacity of SOA. These experiments were conducted in a shaker path at constant temperature. In each experiment, 100 ml from the dye solution was placed in a 500-ml conical ¯ask containing 0.17 g of the ash. After closing the ¯asks, they were mounted in the orbital shaker for 24 h. The solutions were then ®ltered and analyzed. This procedure was repeated at four dierent temperatures of 20, 25, 35 and 458C. The dimension of the isothermal stirred batch adsorber used to carry out the kinetic study is shown in Fig. 1. In each experiment, 1.7 l of the dye solution was continuously stirred with a certain amount of the ash. Samples of the solution were continuously withdrawn from the adsorber by a suitable syringe. Each experimental run continued
Fig. 1. Schematic diagram of the batch adsorber: (1) stainless steel variable speed agitator; (2) sampling hole; (3) cover; (4) baes.
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Fig. 4. Redlich±Paterson plot of the adsorption of drim yellow-K4G with the ash at 258C. Fig. 2. Adsorption isotherm for drim dyes with SOA of 53±150 mm diameter.
and Drim red K4BL were 160, 140 and 100 mg dye/g adsorbent. The adsorption capacities of commercial activated carbon for the three dyes at the same conditions were 180, 150 and 144 mg dye/g adsorbent. On the other hand, the adsorption capacity of SOA is about 25% higher than that of Pentonite for the same dyes at the same conditions. These results indicate that SOA has good capabilities as an adsorbent for dyes. One more advantage of this adsorbent is the ease of its regeneration. It was indicated by some regeneration tests that if hot gases of 300±4008C were eluted through a dry bed of the exhausted adsorbent for a period of 20 min the adsorbed dye would be oxidized and the adsorbent becomes fresh again (Fig. 2). Figures 3 and 4 show a plot of Langmuir isotherm and a plot of Redlich±Paterson isotherm, respectively. The linear form of Langmuir isotherm and that of Redlich±Paterson can be represented by equations (9) and (10), respectively,
Ce 1 b Ce a a qe ln
aCe ÿ1 qe
9
b ln Ce ln aR
10
While Langmuir isotherm parameters a and b were obtained by plotting Ce/qe vs Ce, Redlich±Paterson isotherm parameters were obtained by plotting ln(aCe/qeÿ1) vs lnCe. After ®tting the data using the computer program ``Origin'', equations (21) and (22) become as equations (11) and (12), respectively, Ce 0:1442 4:395Ce qe ln
6:935Ce ÿ1 qe
11
0:9889 ln Ce ÿ 29
12
The values of R 2 for equations (11) and (12) are 0.9957 and 0.9834, respectively. Table 2 shows the values of the parameters of the two models at 258C. It is evident from these values that b approaches 1. Consequently, the Langmuir equation can accurately describe the equilibrium data over the concentration range used in this investigation. For this reason, the Langmuir model was used in the developed kinetic model. Kinetic studies The proposed model has been tested using a number of experiments. The model system of equations was solved numerically using the implicit ®nite dierence scheme. A computer program was Table 2. Values of the two isotherm parameters of SOA (dp=150± 250 mm) with drim-yellow at 258C Model Langmuir
Fig. 3. Langmuir isotherm of the drim dyes with the ash.
Parameter Value
a (l/g) 6.935
b( l/g) 30.478
Redlich±Paterson a (l/g) 6.935
aR (l/g) 29
b 0.9889
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developed to ®t the experimental curve to the theoretical one. By altering the values of K and D it was possible to obtain the best ®t between the experimental and theoretical curves of the batch adsorption. This approach for obtaining the values of K and D at dierent experimental conditions has been employed by many researchers (Mathews and Weber, 1976; Mckay et al., 1984; Mckay, 1985; ElGuendi, 1991a). The eect of the initial dye concentration, adsorbent mass, particles size, mixing speed, and temperature on the rate and extent of adsorption by SOA were studied.
was equal to 2.5 10ÿ6 m/s and remained constant for all values of initial concentration. The diusion coecient inside the particle pores, D increased from 7 10ÿ11 to 12.7 10ÿ11 m2/s as the initial dye concentration changed from 60 to 380 mg/l. This behavior of concentration dependent diusivity agrees with investigations obtained by Hu et al. (1994), for the adsorption of hydrocarbons by activated carbon and those obtained by Haimour and Sayed (1997), for the adsorption of methylene blue by jift.
Eect of initial concentration
Eect of adsorbent mass
The eect of initial dye concentration on the rate of adsorption by SOA is shown in Fig. 5 where the experimental results are shown as discrete points and those obtained from the model by solid lines. It is evident from Fig. 6 that, in a series of experiments in which the initial concentration of drim yellow-K4G ranged from 50 to 380 mg/l, the correlation between the experimental and the theoretical data is excellent. For a particular experiment, the rate of adsorption decreased with time until it gradually approached a plateau, owing to the continuous decrease in the concentration driving force. In addition, the initial rate of adsorption was greater for higher initial dye concentration, because the resistance to the dye uptake decreased as the mass transfer driving force increased. The kinetic parameters, K and D, predicted by the model are listed in Table 3. It was found that the mass transfer coecient K was not aected by changing dyes initial concentration, and its value
The eect of varying the ash mass on dye adsorption is shown in Fig. 6. It is evident from this ®gure that the correlation between the experimental and theoretical results is good. It can be seen and, as expected, that the dye concentration in the solution decreased at a faster rate as the adsorbent mass increased. For example, the relative dye concentration after 40 min from start up decreased from 0.93 to 0.4 as the adsorbent concentration increased from 0.5 to 2 g/l. The mathematical model predicts that the mass transfer coecient, K, and the diusivity coecient, D do not change as the adsorbent mass changes. The values of K and D were 2.5 10ÿ6 m/s, and 1 10ÿ10 m2/s. These results are in agreement with those of Mckay (1985) for the adsorption of acid dye 25, and El-Guendi (1991a). The results obtained from this section of experiments indicate that the SOA has a large potential as an adsorbent for dyes. It was found that, if 2 g of the adsorbent were mixed with 1 l of the dye sol-
Fig. 5. Eect of initial concentration on the adsorption rate of drim yellow-K4G with the ash (dp=200 mm, T = 258C, M/V = 1 kg/m3, a.s.=400 rpm).
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Fig. 6. Eect of adsorbent mass on the adsorption rate of drim yellow with the ash (dp=200 mm, Co=200 ppm, V = 1.7 l, T = 258C, a.s.=400 rpm).
ution, more than 90% of the dye molecules would be adsorbed. Eect of agitation speed The eect of contact time on adsorption of drim yellow-K4G with SOA at dierent agitator speeds is shown in Fig. 7. The correlation between the experimental with the theoretical results is again excellent. The data shown in Fig. 8 indicates that the rate of adsorption increases as the agitator speed increases. This eect can be attributed to the increased turbulence and as a consequence, the decrease boundary layer thickness around the
adsorbent particles is a result of increasing the degree of mixing. The mass transfer coecient K increased from 2 10ÿ6 to 3.3 10ÿ6 m/s as the agitator speed was increased from 250 to 600 rpm. These results are at agreement with those of Asfour et al., 1985 for the removal of dyes with sawdust and those of Haimour and Sayed, 1997 for the adsorption of methylene blue with jift. On the other hand, the diusion coecient inside the particle pores remained constant for all values of agitator speed and equal to 1 10ÿ10 m2/s. In large-scale continuous adsorption processes there are two possible modes of operation. These
Table 3. Various experimental conditions for the adsorption of drim yellow-K4G with SOA Set of experiments 1
2
3 4 5
M/V kg/m3
Temperature (8C)
a.s. (rpm)
CO (ppm)
Dp (mm)
K 105 (m/s)
D 1010 (m2/s)
1 1 1 1 1 0.5 1 1.5 2 1 1 1 1 1 1 1 1 1 1
25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 20 25 35 45
400 400 400 400 400 400 400 400 400 250 400 600 400 400 400 400 400 400 400
50 100 200 300 380 200 200 200 200 200 200 200 200 200 200 200 200 200 200
200 200 200 200 200 200 200 200 200 200 200 200 100 200 300 200 200 200 200
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.2 0.25 0.33 0.25 0.25 0.25 0.2 0.25 0.37 0.42
0.7 0.8 1.10 1.1 1.27 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.9 1.0 1.14 1.2
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Fig. 7. Eect of agitation speed on the adsorption rate of drim yellow with the ash (dp=200 mm, T = 258C, M/V = 1 kg/m3, CO=200 ppm).
are the use of a batch stirred adsorber or a continuous ®xed bed adsorber. The above results indicate that the rate of adsorption in a stirred adsorber was greater with an increase in the degree of mixing. In continuous ®xed bed operations, the boundary layer thickness can be reduced by increasing the dye solution ¯ow rate by circulating part of the euents
or by using ¯uidized bed adsorbers, as these multiphase contactors are characterized by intensive mixing and high mass transfer rates. These two alternatives will be examined in our future studies. Eect of particle size Figure 9 depicts the eect of contact time on the
Fig. 8. Eect of particle size (dp) on the adsorption rate of the dye with the ash (CO=200 ppm, T = 258C, M/V = 1 kg/m3, a.s.=400 rpm).
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adsorption of drim yellow-K4G with the ash of dierent particle sizes. Three dierent fractions of average sizes ranging between 100 and 300 mm were used in this phase of experiments. The results shown in Fig. 8 indicate that the rate and extent of dye adsorption by SOA was decreased by increasing the particle size. This behavior can be attributed to the relationship between the eective speci®c surface area of the adsorbent particles and their sizes. The eective surface area decreased as the particles size increased and as a consequence, the saturation adsorption per unit mass of the adsorbent decreased (Haimour & Sayed, 1997; Banergee et al., 1997; Wu et al., 1997). The particle size was found to have no eect on the mass transfer and the diusion coecients. Table 3 shows that the mass transfer coecient K and the diusion coecient D remain constants and equal to 2.5 10ÿ6 m/s and 1 10ÿ10 m2/s when the particle size increases from 100 to 300 mm. This behavior is in consistence with the theory, because the variations in the particle size in this study did not aect the hydrodynamics as the total adsorbent concentration is very low. Cooney and Adesanya (1995) have shown by modeling some experimental data that, particles size variations in a batch adsorption process will have a signi®cant eect on K and D if the fractional mass uptake values are greater than 0.7. This value may occur either at higher solid concentrations than those used in the present investigation or at very low adsorbate initial concentration. These results are not in agreement with some of
those found in the literature. Mckay and McConvey (1981) reported that the range of particle sizes had little in¯uence on the mass transfer coecient in the adsorption of acidic dyes with wood. On the other hand, Haimour and Sayed (1997), who used relatively low adsorbate concentrations, reported that, the mass transfer coecient decreased and the diusion coecient increased as the particle size increased in the adsorption of methylene blue with jift. Eect of temperature The temperature has two major eects on the adsorption process. Increasing the temperature is known to increase the rate of diusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particle, owing to the decrease in the viscosity of the solution. In addition, changing the temperature will change the equilibrium capacity of the adsorbent for a particular adsorbate. In this phase of study, a series of experiments were conducted at 20, 25, 35, and 458C to study the eect of temperature on the adsorption rate and the kinetic coecients K and D. Figure 9 depicts the eect of contact time on the rate of adsorption of drim yellow-K4G with the ash at four dierent temperatures. In addition to the excellent correlation between the theoretical and the experimental data, Fig. 9 shows that the rate of adsorption increases as the temperature increased. Furthermore, the mass transfer coecient increased from 2 10ÿ6 to 4.2 10ÿ6 and the diusion coecient D increased from
Fig. 9. Eect of temperature on the adsorption capacity and rate of the dye with the ash (dp=200 mm, M/V = 1 kg/m3, CO=200 ppm, a.s.=400 rpm).
Adsorption of dyes using shale oil ash
0.9 10ÿ10 to 1.2 10ÿ10 as the temperature increases from 20 to 458C. These results agree with those of Haimour and Sayed (1997) and Mckay and Allen (1980). CONCLUSIONS
Based upon the experimental and the theoretical results in this investigation, the following conclusions can be drawn: . SOA has high potential to adsorb reactive dyes from aqueous solutions; . the adsorption isotherms of the reactive dyes with the ash were ®tted to Langmuir isotherm; . the initial rate of adsorption of reactive dyes with the ash was high, and then it declines with time until it reaches a plateau; . the adsorption of reactive dyes with the ash can easily be described by the two-resistance's model; . the rate of adsorption was found to increase by increasing the initial concentration, adsorbent mass, agitation speed and temperature; . in addition, it decreases by increasing by the particle size; . a feasible process option would comprise the adsorption of the dye from the warm solution euents from the mills by the ®nally divided ash, and then desorbing it at relatively low temperatures; . the results of this research study were compared to the published data in the same ®eld, and found to be in agreement with most of them.
AcknowledgementsÐThe Author is grateful to the administration of the Higher Council of Science and Technology, Amman-Jordan, for funding this research. The Author also wishes to thank the Authority of Natural Resources, Jordan, for providing the shale oil samples, and the Company of Textile, Zarka-Jordan, for providing the reactive dye samples. REFERENCES
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