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Ind. Eng. Chem. Res. 2006, 45, 1446-1453
Removal and Recovery of the Hazardous Azo Dye Acid Orange 7 through Adsorption over Waste Materials: Bottom Ash and De-Oiled Soya V. K. Gupta* Department of Chemistry, Indian Institute of Technology, Roorkee, Roorkee 247 667, India
Alok Mittal, Vibha Gajbe, and Jyoti Mittal Department of Applied Chemistry, Maulana Azad National Institute of Technology, Bhopal 462 007, India
An azo dye, Acid Orange 7 [p-(2-hydroxy-1 naphthylazo)benzene sulfonic acid] was removed by adsorption over two waste materials, namely, bottom ash, a power plant waste, and de-oiled soya, byproduct obtained during the processing of soybean in soya oil extraction mills. Both waste materials showed excellent adsorption abilities and can be treated as low-cost adsorbents. The adsorbents were characterized through IR spectroscopy and differential thermal analysis (DTA), and preliminary investigations were carried out by batch adsorption to examine the effects of pH, adsorbate concentration, sieve size, adsorbent dosage, contact time, and temperature. A plausible mechanism for the ongoing adsorption process and thermodynamic parameters were also obtained from Langmuir and Freundlich adsorption isotherm models. The kinetic measurements helped in determining the specific rate constant, confirming the applicability of the first-order rate expression. To identify whether the ongoing process is particle diffusion or film diffusion, the treatments given by Boyd et al. (Boyd, G. E.; Adamson, A. W.; Meyers, L. S. J. Am. Chem. Soc. 1947, 69, 2836) and Reichenberg (Reichenberg, D. J. Am. Chem. Soc. 1953, 75, 589) were employed. To assess the practical utility of the adsorbents, a fixed-bed column was designed, and necessary parameters were calculated by applying a masstransfer kinetic approach. Experiments were also performed for the recovery of loaded dye through chemical regeneration of spent columns, and an estimate of the operating costs was also calculated. Introduction Acid Orange 7 [p-(2-hydroxy-1-naphthylazo)benzene sulfonic acid] is a popular water-soluble dye that is used for dyeing a variety of materials such as nylon, aluminum, detergents, cosmetics, wool, and silk. Like most other azo dyes, it tends to be disposed in industrial wastewater and poses a severe health threat to humans.1,2 It is highly toxic, and its ingestion can cause eye, skin, mucous membrane, and upper respiratory tract irritation; severe headaches; nausea; water-borne diseases such as dermatitis; and loss of bone marrow leading to anemia.3 Its consumption can also prove fatal, as it is carcinogenic in nature and can lead to tumors.4 It has now been well established that the main cause of its chronic toxicity is the electron-withdrawing character of the azo group, which develops an electron deficiency and becomes reduced to carcinogenic amino compounds.5 The reduction of Acid Orange 7 produces 1-amino-2naphthol, which has been reported to induce bladder tumors.6 Acid Orange 7 can also easily undergo enzymatic breakdown along with reduction and cleavage to give aromatic amines, which, upon exposure, can cause methemoglobinemia. The intermediate amines thus formed also tend to oxidize the heme iron of hemoglobin from Fe(II) to Fe(III) and block oxygen binding, resulting in some characteristic symptoms such as cyanosis of lip and nose, weakness, and dizziness.7,8 When Acid Orange 7 enters the human body through ingestion, it is considered genotoxic; however, if some impurities, such as aromatic amines, are present, it shows mutagenic activity.9 Considering the toxicity and carcinogenic nature of the Acid Orange 7, removal of Acid Orange 7 has been attempted by * To whom correspondence should be addressed. E-mail: vinodfcy@ iitr.ernet.in. Fax: 0091-1332-285043.
biodegradation10 and photosensitization on TiO2 particles.11 Unfortunately, metabolic intermediates after biodegradation of Acid Orange 7 have been found to be carcinogenic,12 and photosensitization produces 1,2-naphthoquinone and phthalic acid during the course of degradation, which are also toxic to the humans.11 Biofilm systems have also been used for aerobic nitrification, anoxic denitrification, and anaerobic digestion to decolorize Acid Orange 7.12 The only work reported so far on the removal of Acid Orange 7 from aqueous solutions was carried out through adsorption over spent brewery grains.13 Additional reports are available in which sorption of the dye was achieved with wool fabric at low temperature14 and chitin/ cellulose composite fiber.15 However, both of these reports mainly attribute the sorption to the dyeing property of Acid Orange 7. Thus, it can be judged that, despite its high toxicity, very limited attempts have been made so far to remove Acid Orange 7 from aqueous solutions. Even common chemical/physical methods such as coagulation, flocculation, ozonation, reverse osmosis, electrolysis, ultrachemical filtration, and chemical treatments have not been tried.16-21 This might be due to either the high solubility of the dye in water or the possibility of generating toxic intermediates/products during the course of the process. On the other hand, adsorption, which is influenced by the chemical properties and structure of the dye and also does not produce any toxic products, is deliberately used for the eradication of Acid Orange 7 from wastewater. For the past few years, our laboratory has exploited inexpensive adsorbents such as bottom ash and de-oiled soya as efficient and suitable adsorbents for the removal and recovery of hazardous dyes. The developed methods have been found to be easy, versatile, and economical because of their easy operation, simple design, and low investment costs. As a result,
10.1021/ie051111f CCC: $33.50 © 2006 American Chemical Society Published on Web 01/12/2006
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these potential adsorbents were utilized here to assess their proficiency for the removal of the toxic dye Acid Orange 7. The adsorbents under consideration are bottom ash, a waste product of power-generation plants, and de-oiled soya, a waste product produced at soybean oil extraction mills. The bottom ash is a coarse, granular, incombustible byproduct of coal-fired power plants, obtained after the combustion of coke. It is an undesired collected material, whose disposal has always been a matter of concern to the station authorities, as the dumped ash is considered highly undesirable for agricultural lands. After numerous tests carried out under stringent Environmental Protection Agency guidelines, it has now been well confirmed that bottom ash is completely nontoxic and thus can be used as an adsorbent in aqueous media.22,23 The other valuable adsorbent, de-oiled soya, is the processed waste material from the soya oil industries, which is obtained after extracting all possible nutrients of soybeans. India is one of the leading producers of the soybean crop, and our city contains many soya oil extraction plants, which drew our attention to use their waste as a potential adsorbent for the removal of toxic dyes.23,24 Materials and Methods Acid Orange 7 [p-(2-hydroxy-1-naphthylazo)benzene sulfonic acid, I], molecular formula C16H11N2O4SNa, was obtained from M/S Merck, and a 0.01 M stock solution was prepared in doubly distilled water. All other reagents used were of AR grade.
The adsorbent bottom ash was procured from the thermal power station (TPS) of M/s Bharat Heavy Electrical Limited (BHEL) in Bhopal, India. The type of coal used by the TPS is subbituminous and was obtained from M/s South Eastern Coal Field Limited, Chirmiri, Sarguja, India. The other adsorbent, de-oiled soya, was a kind gift from M/s Surya Agro Oils, Bhopal, India. A microprocessor-based pH meter (model HI 8424, M/s Henna Instruments, Italy) was used for pH measurements. IR spectral absorption studies were carried out on an HP FT-IR spectrometer, and Quantasorb model QS-7 surface area analyzer was used to measure the surface area of the adsorbent. X-ray measurements were carried out on a Philips X-ray diffractometer, and a Philips SEM 501 electron microscope was used for scanning electron microscopy. All absorbance measurements were recorded on UV/vis spectrophotometer (model 117, M/s Systronics, Ahmedabad, India) over the wavelength range 200-500 nm. Material Development. For the present investigation, the adsorbents bottom ash and de-oiled soya were first washed with doubly distilled water and dried. This dried material was then treated with hydrogen peroxide solution for 24 h to oxidize the adhering organic material, after which both adsorbents were kept in an oven at 100 °C to remove the moisture. The de-oiled soya was sieved to the desired particle sizes of 0.425-0.15 mm (36 mesh), 0.15-0.08 mm (100 mesh), and e0.08 mm (170 mesh). The bottom ash was further activated in a muffle furnace at 500 °C for 15 min in the presence of air and then sieved to different sizes. The final products of both adsorbents were stored in separate vacuum desiccators until required.
Adsorption Studies. Adsorption of Acid Orange 7 was carried out by the batch technique in aqueous suspensions of bottom ash and de-oiled soya, and experiments were conducted to observe the effects of pH, temperature, particle size, amount of adsorbent, concentration, and contact time. Aqueous solutions of the dye exhibit a red-orange color, which gradually disappears after adsorption. Adsorption isotherms were recorded at equilibrium conditions for concentration of dyes over the range from 3 × 10-5 to 1 × 10-4 M at a fixed pH in the range of 2-10. The selected concentration range was ascertained after a good deal of examination. For the adsorption studies, 25 mL of the dye solution of desired concentration was added to a series of 100-mL volumetric flasks, and a known amount of adsorbent of particle size 100 BSS mesh was added to each flask. The flasks were then mechanically agitated intermittently to achieve equilibrium. When equilibrium was thought to have been established, the supernatant was carefully filtered through Whattmann filter paper (no. 41) and analyzed spectrophotometrically by measuring the absorbance at λmax ) 490 nm. Kinetic Studies. Kinetic measurements were also carried out through the batch technique in airtight 100-mL conical flasks after a known amount of bottom ash and de-oiled soya and 25 mL of dye solution had been added. The flasks were then kept in a water bath maintained at the desired temperature and agitated mechanically. After a fixed time interval, the adsorbent was separated by filtration, and the filtrate thus obtained was analyzed spectrophotometrically to determine the equilibrium concentration of the dye. The kinetic studies were also carried out at different adsorbate concentrations. Column Studies. For column operations, two glass columns, each of total length 30 and 1-cm internal diameter, were packed separately over a glass wool support with a known amount of bottom ash and de-oiled soya of specific mesh size (both 100 mesh); the length of the column bed was kept at 1.5 cm. To minimize air entrapment in the column, the adsorbents were filled in the form of a slurry over water. Each column was then loaded with dye solution of appropriate concentration and percolated at a flow rate of 0.5 mL min-1 under the influence of gravity. In each case, several 10-mL aliquots of effluent were collected and analyzed spectrophotometrically at the dye absorption maximum. Once the effluent concentration matched the concentration of loaded dye, column operations were shut down. Column Regeneration. As an acidic azo dye, Acid Orange 7 exhibits much better affinity toward NaOH solution than water. Hence, to achieve faster recovery of the dye, each exhausted column was regenerated by eluting dilute NaOH (pH 12). Total recovery of the dye was achieved by percolating a few aliquots of NaOH (10 mL each) through the column at a flow rate of 0.5 mL/min. The selection of NaOH as the eluent was made after the complete recovery of the dye; the column was finally washed with hot water. Results and Discussion Characterization of Adsorbent Material. Both activated adsorbents, i.e., bottom ash and de-oiled soya, were characterized by standard chemical analysis methods. The chemical composition and physicochemical properties obtained are presented in Table 1. The scanning electron microscopic images reveal that the particles of bottom ash and de-oiled soya are porous and can be approximated as spheres. The differential thermal analysis (DTA) curves plotted for activated bottom ash exhibited the thermal stability of the material, and even at high temperature, negligible weight loss was observed. The d
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Table 1. Chemical Constituents of Adsorbents bottom ash
Table 2. Effect of Amount of Adsorbent on the Rate of Uptake of Acid Orange 7
de-oiled soya
constituent
wt %
constituent
wt %
moisture SiO2 Al2O3 FesO3 CaO MgO Na2O
15 45.4 19.3 9.7 15.3 3.1 1.0
moisture fiber SiO2 Ca P
11 6 2 0.2 0.7
adsorbent bottom ash de-oiled soya
adsorbent amount (g)
amount adsorbed ×10-4 (g)
t1/2 (s)
0.1 0.2 0.01 0.05 0.1
5.60 5.93 6.65 7.39 7.61
56.23 52.65 45.88 40.02 38.88
spacings obtained from the X-ray spectrum of the bottom ash indicate the presence of mainly alumina (Al2O3), gypsum (CaSO4‚2H2O), beaverite [Pb(Cu,Fe,Al)3(SO4)2(OH)6], borax (Na2B4O7‚10H2O), and kaolinite {2[Al2Si2O5(OH)4]}. From the IR spectrum, bands at 3467, 2930, 2676, 1502, 1097, and 790 cm-1 were obtained, which indicate the presence of laumonite, amber, mulite, azurite, bavenite, and kaolinite in the bottom ash. Sharp adsorption bands in the region of 3700-3500 cm-1 apparently reveal the presence of free hydroxyl groups. In the case of de-oiled soya, the presence of gorthite [4(FeO‚OH)], corundum [2(R-Al2O3)], coesite (SiO2), and laumonite [4(CaAl2Si4O12‚4H2O)] was supported by the bands obtained at 479.6, 779.1, 1113.5, and 3459 cm-1. Adsorption Studies. To determine the optimum pH range for the removal of the dye, the adsorption of both adsorbents was studied at varying pH in the range of 2.0-10.0 (Figure 1). The nature of the graph clearly indicates that, in each system, the maximum uptake of the dye takes place at around pH 2.0. It was observed that, in both cases, the uptake of the dye decreases with incrwasing pH to 4 and thereafter remains constant. Hence, all subsequent investigations were performed at pH 2.0 for both adsorbents. To optimize the adsorbent dose for the removal of Acid Orange 7 from the solutions, adsorption investigations were carried out with different adsorbent doses. The amounts of dye removed by adsorption on bottom ash and de-oiled soya are summarized in Table 2. It is observed that, for both adsorbents, the uptake of the dye increases with increasing amount. A significant increase in the adsorption process was observed, where the adsorption amount increased from 0.025 to 0.300 g in the case of bottom ash and from 0.01 to 0.10 g in the case of deoiled soya, and any further addition of adsorbent did not cause any significant change in the rate of adsorption. It was observed
that, when the amount of adsorbent was increased by a factor of 2, the corresponding rate of adsorption doubles. Almost optimum adsorption was observed in the case of 0.1 g of bottom ash and 0.05 g of de-oiled soya. Thus, in all subsequent kinetic studies, the amounts of bottom ash and de-oiled soya were chosen as 0.10 and 0.05 g, respectively. The half-life of the process was also determined at varying doses for each adsorbent and was found to increase with increasing amount. The adsorption experiments were then carried out using different concentrations of dye ranging from 3 × 10-5 to 10 × 10-5 M at a fixed pH of 2.0 and different temperatures (30, 40, and 50 °C). An increase in the efficiency of the adsorbents with increasing concentration of the dye in solution is evident in Figure 2. It is clear from this figure that, for the bottom ash, the uptake is almost 78% at low concentration and about 68% at higher concentration of the dye at all temperatures. However, in the case of de-oiled soya at 30, 40, and 50 °C, the uptakes of the dye are about 70%, 75%, and 80% at low concentration and 58%, 60%, and 67% at high concentration, respectively. These results also signify good efficacy of both adsorbents toward Acid Orange 7. The rate of removal of the dye with both adsorbents at optimum pH and adsorbate concentration was found to be quite high, and kinetic experiments showed that adsorption equilibrium was attained within about 2 h for the bottom ash and in about 1 h for the de-oiled soya (Figure 3a and b, respectively). Thus, in the entire temperature range, almost 82-86% and 8892% adsorptions of the dye were accomplished over bottom ash and de-oiled soya, respectively. On the basis of the above results, the half-lives of both adsorptions were also calculated and found to increase with increasing time. Furthermore, in the batch adsorption experiments, three different particle sizes, 36, 100, and 170 BSS mesh, were tested for both adsorbents, and it was observed that, in each case, the uptake of the dye capacity increases with increasing mesh size (Table 3). Thus, in the present studies, adsorbents (bottom ash
Figure 1. Effect of pH on percentage removal of Acid Orange 7 by bottom ash and de-oiled soya [adsorbent dose ) 0.1 g (BA) or 0.05 g (DOS), sieve size ) 0.08-0.15 mm, concentration of AO7 ) 9 × 10-5 M, equilibration time ) 24 h].
Figure 2. Effect of concentration on the adsorption of Acid Orange 7 over bottom ash and de-oiled soya at different temperatures [pH ) 2, adsorbent dose ) 0.1 g (BA) or 0.05 g (DOS), sieve size ) 100 mesh, equilibration time ) 24 h].
Ind. Eng. Chem. Res., Vol. 45, No. 4, 2006 1449
Figure 3. Effect of time on uptake of Acid Orange 7 by (a) bottom ash and (b) de-oiled soya at different temperatures [pH ) 2, adsorbent dose ) (a) 0.1 or (b) 0.05 g, sieve size ) 100 mesh, concentration of AO7 ) 9 × 10-5 M]. Table 3. Effect of Adsorbent Sieve Size on the Rate of Adsorption of Acid Orange 7 adsorbent bottom ash de-oiled soya
particle size (mm)
amount adsorbed ×10-4 (g)
t1/2 (s)
0.15-0.425 0.08-0.15 0.08 0.15-0.425 0.08-0.15 0.08
0.99 1.16 1.22 5.61 5.70 5.75
338.06 350.06 332.38 56.03 55.13 54.44
and de-oiled soya) each of 100-mesh (0.15-mm) particle size were chosen. Adsorption Isotherms. The Freundlich and Langmuir adsorption isotherm models were successfully applied to the adsorption data obtained at 30, 40, and 50 °C for Acid Orange 7 adsorption over bottom ash and de-oiled soya, and accordingly, thermodynamic parameters were calculated. For the equilibrium concentration of adsorbate (Ce) and amount adsorbed at the equilibrium (qe), the following linear forms of the Langmuir and Freundlich adsorption isotherm equations were used
1 1 1 ) + qe Q0 bQ0Ce
(1)
log qe ) log KF + (1/n) log Ce
(2)
where Q0 and b are Langmuir constants, whereas KF and n are Freundlich constants.
Figure 4. Langmuir adsorption isotherms for the (a) Acid Orange 7-bottom ash and (b) Acid Orange 7-de-oiled soya systems at different temperatures.
For both adsorbents, at all temperatures, plots of 1/Ce versus 1/qe (Figure 4a and b) and log Ce versus log qe (Figure 5a and b) gave straight lines and corroborate the applicability of the Langmuir and Freundlich adsorption models in each case. The intercept and slope of each straight line were used to obtain the Langmuir and Freundlich constants (Table 4). Table 4 indicates that, for both adsorbents, the value of Q0 increases with increasing temperature, which confirms an endothermic process in both cases. The thermodynamic parameters, i.e., the changes in the Gibb’s free energy (∆G°), enthalpy (∆H°), and entropy (∆S°), for each system were then evaluated from the Langmuir constants using the equations
∆G°) -RT ln b ∆H° ) -R ∆S° )
(
)()
T2T1 b2 ln T2 - T1 b1
∆H° - ∆G° T
(3) (4)
(5)
where b, b1, and b2 are the equilibrium constants at 30, 40, and 50 °C, respectively, and R is the universal gas constant. For each system, the feasibility of the adsorption process is corroborated from the negative values of ∆G°. The positive values of enthalpy change (∆H°) for the processes signify the endothermic nature, and the positive values of entropy change
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Figure 6. Lagergren plot for the adsorption of Acid Orange 7 on bottom ash and de-oiled soya at different temperatures.
Figure 5. Freundlich Adsorption Isotherms for the (a) Acid Orange 7-bottom ash and (b) Acid Orange 7-de-oiled soya systems at different temperatures. Table 4. Freundlich and Langmuir Constants of Acid Orange 7 Adsorption over Different Adsorbents at Different Temperatures Langmuir Constants Q0 × 10-5 (mol/g)
b × 105 (L/mol)
adsorbent
30 °C
40 °C
50 °C
30 °C
40 °C
50 °C
bottom ash de-oiled soya
3.572 2.535
3.624 2.622
3.782 2.737
3.001 3.081
3.019 3.630
3.056 4.719
Freundlich Constants n
Kf
adsorbent
30 °C
40 °C
50 °C
30 °C
40 °C
50 °C
bottom ash de-oiled soya
0.1423 0.1672
0.1424 0.1704
0.1502 0.1750
1.000 1.000
1.000 1.000
1.000 1.000
Table 5. Values of Thermodynamics Parameters for the Adsorption of Acid Orange 7 on Different Adsorbents -∆G° (kJ mol-1) adsorbent
30 °C
40 °C
50 °C
∆H° (kJ‚mol-1)
∆S° (J K-1 mol-1)
bottom ash de-oiled soya
31.77 31.83
32.84 33.32
33.92 35.08
0.744 17.470
107.31 162.56
(∆S°) indicate the excellent affinity of the dye toward the adsorbents. The results obtained are represented in Table 5. To recognize the favorability of the adsorption process, the dimensionless separation factor (r) was calculated by the wellknown Langmuir isotherm using equation. The separation factor
(r) was found to be less than unity for both bottom ash and de-oiled soya and indicates highly favorable sorption for both adsorbents. Adsorption Rate Constant Study. To determine the rate constant for each adsorption process, the well-known Lagergren rate equation25 was applied. The time versus log(qe - qt) plots (Lagergren plots) were found to be linear for both systems (Figure 6) and clearly demonstrate the first-order nature of the adsorption process in each case. The kad values evaluated from these straight line plots at 30, 40, and 50 °C were found to be -2.556 × 10-2, -2.671 × 10-2, and -2.856 × 10-2, respectively, for bottom ash and -5.527 × 10-2, -5.600 × 10-2, and -6.356 × 10-2, respectively, for de-oiled soya. For wastewater treatment, the rate at which dissolved organic substances are removed from dilute aqueous solutions by solid adsorbents is an extremely significant aspect, and the mechanism of the involved process also plays an equally important role. Thus, to identify whether the rate-determining step of the underlying process is particle diffusion or film diffusion, the mathematical treatments prescribed by Boyd et al.26 and Richenberg27 were applied. During the adsorption of organic substances from aqueous solution through porous adsorbents, the following three consecutive steps might occur: (i) film diffusion, where transport of the adsorbate through a surface film to the exterior of the adsorbent takes place; (ii) particle diffusion, where transport of the adsorbate within the pores of the adsorbent with the exception of a small amount of adsorption that occurs on the exterior surface of the adsorbent takes place; and (iii) adsorption of the adsorbate on the interior surface of the adsorbent. Of these, the third step is very fast and cannot control the overall rate of uptake by porous adsorbents, whereas, of the remaining two steps, external transport of the dye might be greater than, smaller than, or almost the same as the internal transport. When external transport is greater than internal transport, the rate of the adsorption process is controlled by particle diffusion; when external transport is smaller than internal transport, the rate is governed by film diffusion; and when external transport is almost the same as internal transport, the transport of the adsorbate ions to the boundary at a significant rate is not be possible because a liquid film develops around the solid adsorbent. Rate Expression and Treatment of Data. Thus, to precisely interpret the experimental findings, a quantitative treatment of the sorption data based on the model proposed by Richenberg27 was used, and various parameters were calculated. For each
Ind. Eng. Chem. Res., Vol. 45, No. 4, 2006 1451 Table 6. Values of the Effective Diffusion Coefficient (Di), Preexponential Constant (D0), Activation Energy (Ea), and Entropy of Activation (∆S#) for the Diffusion of Acid Orange 7 in Bottom Ash and De-Oiled Soya Di (m2/s) adsorbent
30 °C
40 °C
50 °C
D0 (m2/s)
Ea × 104 (J mol-1)
∆S# (J K-1 mol-1)
bottom ash de-oiled soya
1.48 × 10-11 2.38 × 10-11
1.56 × 10-11 3.14 × 10-11
1.69 × 10-11 4.28 × 10-11
2.9 × 10-9 3.2 × 10-7
1.0760 2.1037
-60.82 -21.91
Table 7. Fixed-Bed Adsorber Calculations for Bottom Ash and De-Oiled Soya Columns adsorbent
C0 (mg/mL)
Cx (mg/mL)
Cb (mg/mL)
Vx (mL)
Vb (mL)
V x - Vb (mL)
Fm (mg/cm2)
D (cm)
bottom ash de-oiled soya
9.0 × 10-5 9.0 × 10-5
8.0 × 10-5 8.7 × 10-5
6.7 × 10-7 1.2 × 10-5
170 560
40 380
130 180
0.02 0.02
1.5 2.0
calculated value of F, the corresponding value of Bt was derived from Reichenberg’s table.27 The plots of Bt versus time clearly make a distinction between film- and particle-diffusioncontrolled mechanisms. For both adsorbents, at lower concentrations, the Bt versus time plots were linear and did not pass through the origin, but at higher concentrations, curves were obtained that also did not pass through the origin. The natures of the graphs indicate that the ongoing adsorption processes are governed by a film-diffusion mechanism in the entire concentration range and at each temperature. The values of the effective diffusion coefficient, Di, for each adsorption process at the three different temperatures 30, 40, and 50 °C are presented in Table 6. It is clear from the presented data that the value of Di increases with increasing temperature. This might be due to the increase in mobility of ions entering pores of different widths and different electronic fields along the diffusion path. Moreover, as the temperature increases, a gradual decline in the retarding forces acting on the diffusing ions takes place, which, in turn, increases the value of Di. The plot of log Di versus 1/T shown in the Figure 7 was found to be linear for each system. To the determine energy of activation (Ea) and change in entropy of activation (∆S#) of the ongoing adsorption processes, the well-known Arrhenius equation was employed. The values of ∆S#, Ea, and the preexponential constant (D0) are listed in Table 6. The negative ∆S# values indicate that no significant change occurs in the internal structure of either adsorbent throughout the adsorption process. Column Studies. In column-type continuous-flow operations, because the rate of adsorption depends on the concentration of solute in the solution being treated, such operations have been found to be advantageous over batch operations. The method
Figure 7. Plot of log Di vs 1/T for the Acid Orange 7-bottom ash and Acid Orange 7-de-oiled soya systems.
Table 8. Parameters for Fixed-Bed Adsorbers of Bottom Ash and De-Oiled Soya Columns adsorbent
tx (min)
td (min)
tf (min)
f
d (cm)
saturation (%)
bottom ash de-oiled soya
84661 28000
64741 9000
80 760
0.999 0.916
0.7653 0.3304
99.95 98.60
chosen for application in the present research was therefore a fixed-bed adsorber column. The removal of dyes by columntype continuous-flow operations reaches equilibrium conditions when the adsorbent mixture stops adsorbing the solute dye. The equilibrium point is indicated by the concentration of dye in the effluent, and at this point, the concentration of dye in the effluent becomes equal to the initial concentration. Different parameters were then calculated for each system as described earlier.22,23 Plots of C/C0 against the eluted volume (V), referred to as breakthrough curves, exhibit a characteristic S shape for most adsorption operations in wastewater treatments. The plot of concentration of effluent versus eluted volume is depicted in Figure 8, which represents a typical breakthrough curve for each system. The curves were used to determine the values of Vb, Vx, Cb, and Cx, which were then used to evaluate tx, tf, f, δ, and the percentage saturation at the breakpoint. The results obtained are reported in Tables 7 and 8. The breakthrough curves were then compared to obtain the column capacity at complete exhaustion. It was thus observed that the breakthrough capacity is higher than the batch capacity. The higher capacity of column operations might be due to the continuous increasing concentration gradient at the interface of the adsorption zone as it passes through the column, whereas in batch experiments, the concentration gradient decreases with time. Column Regeneration and Dye Recovery. Regeneration of the adsorbents and recovery of the dye are extremely important
Figure 8. Breakthrough curves for the removal of Acid Orange 7 by bottom ash and de-oiled soya columns.
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(4) The column capacity for each process was found to be higher than the batch capacity. The recovery of the dye was achieved by eluting dilute NaOH through the column, and both adsorbents can be regenerated. (5) From this study, it can be concluded that both materials are inexpensive and can be used as excellent adsorbers. Acknowledgment The authors are thankful to the Council of Scientific and Industrial Research, CSIR, New Delhi, India, for providing financial assistance to undertake the work. Literature Cited
Figure 9. Desorption of Acid Orange 7 by bottom ash and de-oiled soya columns.
considerations for wastewater treatment. The subsequent studies were carried out for the desorption operation on each system using NaOH, because it shows an extremely good solubility toward basic media. The desorption process for the two systems was examined under identical conditions of flow rate, column length, etc. It was observed that 250 mL of dilute NaOH (pH 12) was found to be sufficient for almost complete desorption of the dye. For the bottom ash system, five 10-mL aliquots elute almost 96% of the adsorbate, and the rest is desorbed in 12 10-mL increments , whereas for the de-oiled soya system, the first five 10-mL aliquots desorbed only 57% of the dye, and the rest was desorbed in 17 increments of 10 mL each (Figure 9). The percentage recovery of the dye was almost 90.43% for BA and 95.27% for DOS. The column was then washed with about 50 mL of hot water in 10-mL fractions at a flow rate of 0.5 mL/min. The adsorption efficiencies of both systems were then reviewed by refilling the column with dye solution of the appropriate concentration. The breakthrough capacities for the two systems in the first, second, third, fourth, and fifth cycles were found to be almost 85%, 78%, 72%, 64%, and 55% and 92%, 88%, 82%, 74%, and 65% for the bottom ash and deoiled soya columns, respectively. Conclusions The research work presented here shows that dissolved dyes can be successfully removed from aqueous solution by adsorption on bottom ash and de-oiled soya. These adsorbents were found to be useful and valuable means for controlling water pollution due to dyes. The chief characteristics and results of the study are as follows: (1) The batch adsorption experiments show that the adsorption of the azo dye Acid Orange 7 over bottom ash and de-oiled soya is dependent on pH, particle size, amount of adsorbent, concentration, contact time, and temperature, and almost 78% adsorption could be accomplished at low concentrations, whereas at higher concentrations, the adsorption was slightly decreased to 68% for bottom ash at all temperatures. In the case of de-oiled soya, almost 70%, 75%, and 80% adsorption could be achieved at low concentrations and 58%, 60%, and 67% at high concentrations at 30, 40, and 50 °C, respectively. (2) The thermodynamic parameters obtained in both cases confirm the feasibility of the process at each concentration. (3) The results of kinetic experiments show that, for both adsorbents, the adsorption proceeds via film diffusion at higher and lower concentrations.
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ReceiVed for reView October 5, 2005 ReVised manuscript receiVed December 8, 2005 Accepted December 9, 2005 IE051111F
