Ind. Eng. Chem. Res. 2006, 45, 1113-1122
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Removal of 2-Aminophenol Using Novel Adsorbents Vinod K. Gupta,*,† Dinesh Mohan,‡ Suhas,† and Kunwar P. Singh‡ Department of Chemistry, Indian Institute of Technology, Roorkee, Roorkee-247 667, Uttaranchal, India, and EnVironmental Chemistry DiVision, Industrial Toxicology Research Centre, Post Box No. 80, Mahatma Gandhi Marg, Lucknow-226001, India
Removal of toxic substances from wastewaters using low-cost alternatives to activated carbon is an important area in environmental sciences. Efforts have been made to convert a fertilizer waste and a steel industry waste into low-cost potential adsorbents. The developed products have been used for the removal of 2-aminophenol from aqueous solutions and wastewaters. Studies were conducted to delineate the effects of temperature, initial absorbate concentration, particle size of the adsorbent, and solid-to-liquid ratio. Equilibrium isotherms were determined at selected pH’s to assess the maximum adsorption capacity of the adsorbents. Both Freundlich and Langmuir models were used to interpret the adsorption data. The adsorption of 2-aminophenol is an endothermic process. Kinetic studies were performed, and various parameters such as mass transfer coefficient, effective diffusion coefficient, activation energy, and entropy of activation were evaluated to establish the mechanism of removal. Column studies were performed, and the breakthrough curves were used to optimize the contactors and identify a design correlation. Some feasibility experiments were also carried out with an aim to recover 2-aminophenol and demonstrate chemical regeneration of the spent columns. The column capacities of 312 and 30.00 mg/g were more than the batch capacities, which were 80.75 and 28.37 mg/g for activated carbon and activated slag, respectively. Overall, activated carbon developed from fertilizer waste exhibits better performance than activated slag developed from blast furnace slag. Introduction The presence of phenol and phenolic compounds in water and wastewater has been of great public concern. Phenol is one of the most frequent contaminants at hazardous waste sites and originates from various sources such as paper and pulp, pesticides, dyes, and chemical manufacturing industries. Besides these, wastewater originating from other industries such as resin manufacturing, gas and coke manufacturing, tanning, textile, plastic, rubber, pharmaceutical, and petroleum also contain different types of phenols. As per the World Health Organization regulation, 0.002 mg/L is the permissible limit for phenol concentration in potable water.1 Among the several currently known physical, chemical, and biological methods used for wastewater reuse, adsorption is one of the key processes used for the decontamination of organic pollutants. The adsorption of phenol and substituted phenols from aqueous solution on activated carbons has been intensively investigated.2-4 Although activated carbon is currently the most widely used adsorbent for wastewater treatment, because of its high cost there is a definite need for possible alternatives. The substitute for activated carbons should be easily available, economically feasible, and, above all, able to be regenerated physically/ chemically with simultaneous quantitative recovery of the adsorbate material. Recognizing the economic drawback of activated carbon, many investigators have studied the feasibility of less expensive materials such as clays,5 activated sludge,6 human hairs,7 rice husk,8 carbon cloth,9 rubber seed coat,10 coals,11 bagasse carbon,12 and various other adsorbents.13 For quite some time, we have been involved in developing some low-cost adsorbents for the removal and recovery of organics/metal ions from wastewater.14-16 Fertilizer plants * To whom correspondence should be addressed. Tel.: 0091-1332274458. Fax: 0091-1332-285801. E-mail:
[email protected];
[email protected]. † Indian Institute of Technology. ‡ Industrial Toxicology Research Centre.
generate waste slurry due to liquid fuel combustion, and this causes a disposal problem. The waste slurry is converted into a cheap carbonaceous adsorbent and has previously been used for the removal of metal ions, phenols, and dyes, .1,17,18 Steel plants produce granular blast furnace slag as a byproduct, and this material also causes a disposal problem. Presently, this is being used as filler. Efforts have also been made to covert this waste into a low-cost adsorbent. This has also been used previously for the removal of metal ions and dyes.19-21 Continuing our activities in this direction, we have used the activated carbon derived from fertilizer waste material and activated slag from steel industry waste material for the removal and recovery of a substituted phenol from water and wastewater. Among the various substituted phenols, the removal of 2-aminophenol has not yet been fully explored in all aspects. Attempts are made to assess the ability of activated slag and carbonaceous material developed, respectively, from waste generated by blast furnaces and fertilizer plants for the removal of 2-aminophenol. Experimental Section All reagents were of analytical reagent (AR) grade. Stock solutions were made by dissolving 2-aminophenol in doubly distilled water. Instrumentation. The pH measurements were made using a pH meter (model CT No. CL46, Toshniwal, India). The concentration of 2-aminophenol (2AP) was measured with a Shimadzu UV-visible spectrophotometer, model 2100 (double beam), and a 1 cm light-path cell, with an absorbance accuracy of 0.004 at λmax of the adsorbate. The wavelength, corresponding to a maximum absorbance of 2-aminophenol, was found to be 430 nm with an accuracy of (0.3 nm. Absorbance was found to vary linearly with concentration, and dilutions were undertaken when it exceeded 0.8. Biological degradation of 2AP was also taken into account by running blank determinations. X-ray measurements were done using a Phillips X-ray diffractometer employing nickel-filtered Cu KR radiation. A carbon-hydrogen-
10.1021/ie051075k CCC: $33.50 © 2006 American Chemical Society Published on Web 01/04/2006
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nitrogen-sulfur (CHNS) analyzer (Elementar Vario EL III) was used for determining carbon and sulfur content. A model QS-7 Quantasorb surface area analyzer measured the surface area. The IR spectra of the samples were recorded on a Perkin-Elmer FTIR spectrophotometer model 1600 using pellet (pressed-disk) technique. The porosity and density of the adsorbents were determined by mercury porosimetery and by specific gravity bottles, respectively. The chemical constituents of carbon and slag were analyzed following the standard methods of chemical analysis.22 Material Development. (i) Preparation of Activated Carbon. The raw material, a waste product from National Fertilizer Limited, Bhatinda, India, was in the form of small, spherical, black, greasy granules. This plant has a production capacity of 900 MTPD of liquid ammonia. At 100% load, the plant produces waste slurry at the rate of 60 ton/h of carbonaceous material that must be disposed of. A sample of the waste slurry was converted into an adsorbent by treating it with hydrogen peroxide1,18 to oxidize the adhering organic material and then heated to 200 °C in air until the emission of black soot stopped. The activation was performed in a muffle furnace by heating the sample in the presence of air at 450 °C for 1 h. The material was then treated with 1.0 M HCl to remove the ash content and then washed with distilled water. It was dried at 100 °C for 24 h and sieved before use. Adsorption tests were performed on 150-200 mesh particles (unless stated otherwise). Finally, the product was stored in a vacuum desiccator until required. (ii) Preparation of Activated Slag. The waste obtained from the Tata Iron and Steel Company, Limited, Jamshedpur (India), was in the form of small, spherical, soft granules. The waste was treated as described earlier20,21 to impart adsorption characteristics. Sorption Procedure. Batch studies were performed to obtain the data on the rate and extent of sorption. For isotherm studies, a series of 50 mL test tubes were employed. Each test tube was filled with 10 mL of phenolic solution of varying concentrations (10-4-10-3 M in the case of activated carbon and 10-5-10-4 M in the case of activated slag) and adjusted to the desired pH and temperature. A predecided amount of adsorbent was added into each test tube and agitated intermittently for the desired time periods, up to a maximum of ∼24 h. The contact time and other conditions were selected on the basis of preliminary experiments that demonstrated that the equilibrium was established in 6-8 h. Equilibration for longer times that is between 10 and 24 h gave practically the same uptake. Therefore, the contact period was 24 h in all equilibrium tests. The sorption studies were also carried out at different temperatures i.e., 30, 40, and 50 °C, to determine the effect of temperature. The effect of an anionic surfactant (Manoxol-1B) on the adsorption of 2AP was also studied. Stoppered glass tubes containing adsorbate solution along with the detergent and fixed amount of adsorbent were equilibrated for 24 h. The supernatant liquid was centrifuged and analyzed for the phenol uptake. The phenol concentration retained in the adsorbent phase was calculated by using eq 1,
qe )
(Co - Ce)V W
(1)
where Co and Ce are the initial and equilibrium concentrations, respectively, of phenol in solution; V is the volume; and W is the weight of the adsorbent. The experiments were repeated a number of times, and average values are reported. Standard deviations were found to be within (2.0%. Further, the error bars for the figures were smaller than the symbols used to plot the graphs and, hence, are not shown.
Kinetic Studies. Evaluating the performance of unit processes utilizing adsorption requires an understanding of the kinetics of uptake or the time dependence of the concentration distribution of the organic solute in both bulk solution and solid adsorbent and identification of the rate-determining step. For kinetic studies, the batch technique was selected because of its simplicity. A number of stoppered Pyrex glass tubes (50 mL capacity), containing a 10 mL solution of 2-aminophenol of known concentration, were placed in a thermostat shaking assembly. When the desired temperature was reached, 0.01 g of carbon and 0.1 g of slag was added into the required tube by mechanical shaking of the solutions. At predecided intervals of time, the solutions of the specified tubes were separated from the sorbent material and analyzed for the uptake of 2AP. Column Studies. Adsorption isotherms have traditionally been used for the preliminary investigations and the fixing of the operational parameters, but in actual practice, continuous operations normally use column-type operations. Moreover, the isotherms cannot give accurate scale-up data in fixed-bed systems, and so the practical applicability of the product in column operations has also been ascertained. A glass column (40 × 0.5 cm) was filled with activated carbon and activated slag (mesh size 200-250) on a glass wool support. A weighed quantity of adsorbent was made into slurry with hot water and fed slowly into column, displacing a heel of water as outlined by Fornwalt and Hutchins.23 The technique avoids air entrapment. The column was loaded with the appropriate 2AP solution at a flow rate of 0.4 mL/min. The operation on the column was stopped when 90% of the capacity was used. Column Regeneration. Recovery of the adsorbate material as well as the regeneration of adsorbent is an important process in wastewater treatment. Consequently, experiments have been carried out in which adsorbents were loaded with 2-aminophenol and subjected to elution of phenol with acetone. Results and Discussion Characterization. The samples of activated carbon (fertilizer plant waste) and activated slag (blast furnace waste) were chemically analyzed, and the results indicate that carbon (9092%) is the main constituent of activated carbon, which also includes Al (0.4-0.6%) and Fe (0.6-0.8%). Thus, activated carbon from fertilizer waste consisting mainly of carbon may be treated as organic in nature. As compared to activated carbon, the analytical data of activated slag indicates silica and calcium oxides (30.8 and 30.5%, respectively) are the main constituents, with other constituents including Al2O3 (23.3%), MgO (9.9%), S (0.9%), MnO (0.6%), and FeO (0.5%). The activated slag, having mainly inorganic constituents, may be said to be inorganic in nature. The loss on ignition was found to be 12.1 and 6.2% and the density was found to be 1.30 and 2.36 g/cm3 for activated carbon and activated slag, respectively. The samples of activated carbon and activated slag (1.0 g each) were stirred with deionized water (100 mL, pH 6.8) for 2 h and left for 24 h in an airtight, stoppered conical flask. A lowering of pH was observed in the case of activated carbon, but in the case of activated slag, some enhancement takes place. Therefore, according to the Steenberg classification,24 the activated carbon in our case can be termed as “L” carbon. Both activated carbon and activated slag are quite stable in water, salt solutions, acids, bases, and organic solvents. The surface areas of the samples activated in air and determined by nitrogen gas adsorption were found to be 629 and 107 m2/g for activated carbon and activated slag, respec-
Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006 1115
Figure 1. Effect of pH on the adsorption of 2-aminophenol on activated carbon and activated slag.
tively. The experimental value of methylene blue number was also worked out and found out to be 163 and 18, for activated carbon and activated slag, respectively. However, the theoritical value of methylene blue number for activated carbon and activated slag (surface area ) 629 and 107 m2/g, respectively) by taking the dye molecule area as 197 Å2 was also calculated and found to be 189 and 32 for activated carbon and activated slag, respectively. This difference in theoretical and experimental values indicates that some pores of activated carbon and activated slag are not accessible to methylene blue, which indicates the presence of micropores. The X-ray diffraction patterns of both adsorbents do not show any peaks, indicating the amorphous nature of activated carbon and activated slag. The IR spectra of activated carbon, not shown, indicated weak and broad peaks in the region of 1800-1600 cm-1. The band at 1700 cm-1 corresponded to the normal carbonyl group, while the one at 1605 cm-1 may be due to conjugated hydrocarbonbonded carbonyl groups. Although some inference can be made about the surface functional groups from the IR spectra, the weak and broad band do not provide any authentic information about the nature of surface oxides. The data, however, indicate the presence of some surface groups on the adsorbent material activated in air. Infrared spectra of activated slag indicated broad and weak peaks in the region 4000-500 cm-1. The adsorption bands in the region 3700-3500 cm-1 were assigned to a free hydroxyl group, while the band at 3622 cm-1 indicated the presence of interlayer hydrogen bonding. The peaks at 3427, 3290, 30503095, and 2985-2883 cm-1 indicated the presence of nordstrandite, brucite, bochmite, and diaspore. The band at 2857 cm-1 confirms the presence of γ-FeOOH, while those at 937 and 733 cm-1 suggested the presence of braunite and goethite in the sample.19 Sorption Studies. The change in adsorption of 2AP on activated carbon and activated slag over a pH range of 2-10 is depicted in Figure 1. It is found that the uptake of 2AP stays constant up to pH 9.0, and beyond that, a decline in adsorption is observed. Thus, the separation process of 2-aminophenol, with the help of these adsorbents, can be best achieved over a pH range of 2-9. The surface of activated carbon is negatively charged due to the existence and dissociation of a variety of organic functional groups such as carboxyl, phenolic, alcholic, and quinone, while that of activated slag would very much depend on the pHzpc of SiO2 (∼2.3) and Al2O3 (∼8.3). It can be inferred that the surface of activated slag is positive in the low pH region (pH < 2.3) and negative in the higher pH region (pH > 8.3). The phenol
Figure 2. Adsorption isotherms of 2-aminophenol at different temperatures (a) on activated carbon and (b) on activated slag.
molecule, which gets protonated in the low pH range, would be taken up more on the negatively charged carbon surface. Slag, being positively charged in an acidic medium, would attract a lesser amount of adsorbate. The pKa value of the adsorbate is 9.72. Naturally, at high pH, 2-aminophenol gets deprotonated, and its uptake on both the adsorbents goes down due to the formation of negatively charged phenoxide ion. The isotherms for adsorption of the 2AP at their optimum pH (4.0 for both the adsorbents) of adsorption on activated carbon and activated slag are shown in parts a and b, respectively, of Figure 2 at three different temperatures. The isotherms are positive, regular, and concave to the concentration axis. The uptake of the 2AP increased with an increase in temperature, thereby indicating the process is endothermic. The uptake of 2AP on activated slag as well as activated carbon is 70-90% at low adsorbate concentrations. The uptake at higher 2AP concentrations is 75% on activated carbon and 54% on activated slag. The data for the adsorption of 2AP on activated carbon and activated slag were fitted to the Freundlich and Langmuir isotherms. The linear forms of the Freundlich and
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Table 1. Freundlich and Langmuir Parameters for the Adsorption of 2-Aminophenol on Activated Carbon and Activated Slag Freundlich Parameters temp (°C)
activated carbon KF (mol/g) 3.98 ×
30
10-5
activated slag R2
n 1.75
0.9836
n
R2
1.11
0.9262
KF (mol/g) 1.77 ×
10-8
Langmuir Parameters activated carbon
activated slag
temp (°C)
Q0 × 104 (mol/g)
b × 10-4 (L/mol)
R2
30
8.33 7.40a 9.09 8.04a
1.54
0.9240
2.20
0.9276
40 a
Q0 × 106 (mol/g)
b × 10-4 (L/mol)
R2
6.66 5.30a 10.0 5.90a
1.80
0.9452
1.90
0.9431
Experimentally observed values.
Langmuir models are represented by eqs 2 and 3,
1 log qe ) log KF + log Ce n
( ) ( )
Ce 1 1 ) 0 + 0 Ce qe Qb Q
(2) (3)
where qe is the amount of solute adsorbed per unit weight of adsorbate (mol/g); KF, (1/n), and b are characteristic constants; Ce is the equilibrium concentration or the concentration in the bulk fluid phase (mol/L); and Q0 is the solid-phase concentration corresponding to complete coverage of available adsorption sites. The value of KF has been used as a relative measure of adsorption capacity; (1/n) and b are related to enthalpy and intensity of adsorption. Results obtained show the applicability of both models over a wide range of concentrations and are given in Table 1. The validity of the Langmuir model suggests the adsorption process to be monolayer (homogeneous sites), which is equally valid to the Freundlich model in the case of activated slag. However, the Freundlich model fits slightly better (as evident from correlation coefficients) in the case of activated carbon, suggesting some heterogeneity as compared to the case of activated slag. Further, it can be seen from the table that the values of Q0 are found to be more in the case of the carbon2AP system. The same trend was also observed for the Freundlich plots. Also, the value of the adsorption capacity increases with an increase in temperature, and this is higher in the case of carbon system. The dimensionless constant separation factor, RL,25 was determined at 30 and 40 °C in the broad concentration range, and all the values of RL are found to be 0, indicating the favorable adsorption of 2-aminophenol on activated carbon as well as on activated slag. Natural water bodies can be contaminated by surfactants and other substances due to the discharge of wastewater from commercial and domestic sectors. Most commonly used detergents are anionic types. Therefore, the adsorption of 2-aminophenol has also been observed in the presence of an anionic surfactant Manoxol-1B. It is found that the surfactant does not affect the uptake of the 2AP significantly, and on both
Figure 3. Effect of amount (a) of activated carbon and (b) of activated slag on the rate of uptake of 2-aminophenol.
adsorbents, the uptake is reduced by 1-2% in the presence of Manoxol-1B. Thermodynamic parameters, i.e., free energy (∆G°), enthalphy (∆H°), and entropy (∆S°) changes, were also calculated using equations as described earlier19 and are given in Table 2. The negative values of ∆G° indicate the spontaneous nature of 2AP adsorption on both the adsorbents. The change in enthalpy ∆H° for 2AP adsorption on activated carbon and activated slag was found to be positive. The positive values confirm the endothermic nature of adsorption. These values are consistent with the results presented in Table 1. The positive values of entropy show the increased randomness at solid/ solution interface with some structural changes in the adsorbate and adsorbent and the affinity of adsorbents toward 2AP. Kinetic Studies. The effect of contact time, mass of adsorbent, particle size of adsorbent, and concentration of the adsorbate solution on the uptake of 2AP on carbonaceous material and blast furnace slag were investigated from the kinetic point of view. Preliminary investigations on the rate of uptake of 2-aminophenol on activated carbon and activated slag indicated that the processes are quite rapid and typically 40-50% of the ultimate adsorption occurs within the first hour of contact. This
Table 2. Thermodynamic Parameters, Rate Constants (Kad), and Mass Transfer Coefficients (βL) of the Adsorption of 2-Aminophenol on Activated Carbon and Activated Slag -∆G (kJ/mol) adsorbents
30 °C
40 °C
∆H (kJ/mol)
∆S (J/(mol‚K))
βL (cm/s)
Kad (/min)
activated carbon activated slag
24.04 24.68
26.01 25.63
28.12 4.28
175 95
2.61 × 10-6 2.71 × 10-7
0.39 × 10-2 0.82 × 10-2
Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006 1117
Figure 5. Effect of adsorbate concentrations on the rate of uptake of 2-aminophenol (a) on activated carbon and (b) on activated slag. Figure 4. Effect of temperature on the rate of uptake of 2-aminophenol (a) on activated carbon and (b) on activated slag.
adsorption subsequently gives way to a very slow approach to equilibrium, and the equilibrium is achieved in 6-8 h. The effect of the amount of adsorbents on the rate of uptake of 2-aminophenol is shown in Figure 3. The total uptake increases with an increase in adsorbent dose. There is a substantial increase when carbon dosage increases from 0.5 to 1.0 g/L while the removal efficiency is increased; upon introducing additional an 0.5 g/L of carbon, the increase is not so significant. Keeping this in view, the amount of carbon, taken in all subsequent kinetic studies, is 1.0 g/L; see Figure 3. In the case of slag, the adsorbed amount in the first hour of contact increases proportionally with an increase in the slag dosage. Upon doubling the adsorbent amount from 10 to 20 g/L, the amount of phenol adsorbed also increases by almost two-fold. When the amount of adsorbent is increased 3× (from 10 to 30 g/L), the removal of adsorbate also increases in the same proportion (5.0 × 10-6 to 15 × 10-6 moles). It is quite evident that, after 6 h of equilibrium, 27% of the total 2-aminophenol is removed by 10 g/L of the adsorbent slag, while 20 g/L of slag removed 37% of 2-aminophenol and 30 g/L of adsorbent adsorbs 42% under identical experimental conditions. Thus, there is an increase in phenol adsorption only by 10% upon enhancing the amount of slag from 10 to 20 g/L, and a further increase in the slag amount increases the percent removal of 2-aminophenol only by 5%. Keeping this in view, the amount of slag taken in all subsequent studies is 20 g/L.
The removal of 2-aminophenol has also been studied at 30, 40, and 50 °C and is presented in Figure 4. The extent of adsorption of 2-AP and its rate of removal are found to increase with temperature, indicating the process is endothermic in nature. Phenol removal in the first hour of the process goes up from 1.2 × 10-4 to 1.7 × 10-4 and 2.5 × 10-4 mol/g in the case of carbon and from 5.0 × 10-7 to 6.0 × 10-7 and 10 × 10-7 mol/g in the case of slag as the temperature rises from 30 to 40 and 50 °C, respectively. The time required for 50% of the total adsorption decreases with an increase in temperature, i.e., t50 is found to be 2.8, 2.6, and 2.3 h in the case of carbon and 2.5, 2.4, and 1.8 h in the case of slag at 30, 40, and 50 °C, respectively. There is a direct relationship between the initial adsorbate concentration and the removal rate. The amount of solute removed in the first hour of contact increases as the concentration of adsorbate increases. Increasing concentration of adsorbate from 10-4 to 10-3 M, for carbonaceous adsorbent, enhances the removal rate from 0.35 × 10-4 to 1.2 × 10-4 mol/g (Figure 5). Similarly, in the case of slag, the adsorption rate increases from 0.7 × 10-7 to 5.0 × 10-7 mol/g as the concentration is increased from 10-5 to 10-4 M (Figure 5). However, the time required for 50% of the ultimate adsorption is more or less the same at different adsorbate concentrations. The independence of the time required to achieve a definite fraction of equilibrium adsorption on initial concentration indicates that the adsorption process is first order, which is confirmed by Lagergren’s plots, discussed later.
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pores of the particles, especially when their size is large. However, the access to all pores is facilitated as particle size becomes smaller. The adsorption of 2-aminophenol from liquid to solid phase can be considered as a reversible reaction with equilibrium established between the two phases. The following Lagergren’s first-order rate equation26,27 is applied for the determination of the rate constant.
log (qe - qt) ) log qe -
Kad t 2.303
(4)
where qe and qt are the amounts of phenol adsorbed (mg/g) at equilibrium and at time t, respectively, and Kad is the first-order rate constant. A plot of log(qe - qt) vs t gives a straight line, as can be seen in Figure 7, confirming the applicability of the firstorder rate expression of Lagergren. The rate constant, Kad, was calculated from the slope of the plot, and the values are presented in Table 2. The mass transfer analysis of 2AP during the adsorption process at 30 °C was studied by using the diffusion equation20,28
(
ln
)
Ct 1 + mk 1 mk βLSst ) ln Co 1 + mk 1 + mk mk
(5)
Here, Ct ) concentration of adsorbate after time t (mol/L); Co ) initial concentration of adsorbate, (mol/L); k ) constant obtained by multiplying Langmuir constants, Q0 and b; βL ) mass transfer coefficient (cm/s); and m ) mass of adsorbent per unit volume of particle-free adsorbate solution (g/L).
m)
Figure 6. Effect of particle size (a) of activated carbon and (b) of activated slag on the rate of uptake of 2-aminophenol.
W V
(6)
Ss ) outer surface of the adsorbent per unit volume of particle)free slurry (per cm) and is calculated as
Ss )
Figure 7. Lagergren’s plots for the adsorption of 2-aminophenol at 30 °C on activated carbon and activated slag.
The adsorption of 2-aminophenol has been studied at different particle sizes of the adsorbent, viz., 100-150, 150-200, and 200-250 mesh. The behavior of 2-aminophenol on both the adsorbents is almost the same, and its uptake on particles of different sizes is depicted in Figure 6. The rate of removal increases with decreasing size of the adsorbent particles, and the time required for 50% of the total adsorption is also less with particles of smaller size. This may be due to the fact that phenol molecules are not able to penetrate to some of the interior
6m (1 - P)dpFp
(7)
where dP is the particle diameter (cm), Fp is the density of adsorbent (g/cm), and p is the porosity of adsorbent particles. The values of mass transfer coefficient (βL) were determined from the slopes and the intercepts of the straight-line plots in Figure 8. The linear nature of the plots confirms the validity of the diffusional model for the 2-aminophenol-carbon and 2-aminophenol-slag systems. The results in Table 2 indicate that the velocity of the phenol transport from the bulk to the solid phase is quite rapid. Thus, the two adsorbents can be useful for the removal of 2-aminophenol from wastewater, which is in conformity with the results obtained by other workers.28 Kinetic data obtained in this work were analyzed by applying the Boyd et al.,29 Reichenberg,30 and Helfferich31 mathematical models using eqs 8-11
F)1-
[ ]
(8)
exp(-n2Bt)
(9)
2 2 1 -Ditπ n
∞
6
∑
π2 n)1 n2
r02
or
F)1-
6 π
∞
∑ 2 n)1
1
n
2
where F is the fractional attainment of equilibrium at time t
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Figure 8. Plots of ln[(Ct/C0) - {1/(1 + mk)}] versus time for the mass transfer of 2-aminophenol at 30 °C for (a) activated carbon and (b) activated slag.
and is obtained by the expression
F)
Qt Q0
(10)
where Qt is the amount of adsorbate taken up at time t, Q0 is the maximum equilibrium uptake, and
B)
π2Di ro2
) time constant
(11)
where Di is the effective diffusion coefficient of ion in the adsorbent phase and ro is the radius of the adsorbent particle, assumed to be spherical. The term n is an integer that defines the infinite series solution. Bt values were obtained for each observed value of F, from Reichenberg’s table,30 and the results are plotted in Figure 9. The linearity test of Bt versus time plots was employed to distinguish between the film-diffusion-controlled and particlediffusion-controlled adsorption. If the plot of Bt vs time (having slope B) is a straight line passing through the origin, then the adsorption rate is governed by the particle-diffusion mechanism; otherwise, it is governed by film diffusion. The adsorption kinetics of 2-aminophenol on carbonaceous material and slag presents some interesting results. The Bt versus time plots (Figure 9) at low concentrations, viz., e5 × 10-4 M in the case of carbon and
