Adsorption of Aromatic Compounds on Activated Carbons from Lignin

Ind. Eng. Chem. Res. 2007, 46, 2853-2860

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Adsorption of Aromatic Compounds on Activated Carbons from Lignin: Kinetic Study Luis M. Cotoruelo,*,† Marıá D. Marque´ s,† Jose´ Rodrı´guez-Mirasol,† Toma´ s Cordero,† and Juan J. Rodrı´guez‡ Departamento de Ingenierıá Quı´mica, UniVersidad de Ma´ laga, 29071 Ma´ laga, Espan˜ a, and Ingenierıá Quı´mica, UniVersidad Auto´ noma de Madrid, 28049 Madrid, Espan˜ a

The kinetics of adsorption of five aromatic compounds in aqueous solution was studied in a batch system using activated carbons from eucalyptus kraft lignin as adsorbents. The batch adsorption processes were carried out in a range of temperatures; the values for the experimental variables were selected from previous tests. The rate and the adsorption yield were the main output information. The extent of adsorption is reported as plots of the adsorbate concentration on the solid phase versus time; an empirical equation has been applied for the experimental kinetic data fittings. The apparent mass-transfer order has been determined by applying a pseudo-nth-order rate equation. In addition, effective intraparticle diffusion coefficients have been estimated for all the studied cases. Introduction The adsorption of aromatic compounds in aqueous phase has been studied using a great variety of adsorbate-adsorbent systems. They include silica gel to adsorb basic dyes,1-2 m-phenylendiamine on heat-treated sepiolite,3 phenols on fly ash,4 dyes onto perlite.5 and methylene blue on sewage sludges.6 In addition, polymeric adsorbents can be made from monomers such as styrene, divinylbenzene, and acrylic esters. The former two monomers result in adsorbents that have an affinity for nonpolar organics in aqueous solution, whereas the acrylic esters result in adsorbents with an affinity for polar solutes.7-12 Nevertheless, activated carbon (AC) is perhaps one of the most widely used adsorbents in the industry. The peat, lignite, and lignite-derived activated carbon also exhibit adsorption of dyes and organics such as benzoic acid and chlorobenzoic acids and reduce the COD of an effluent.13 Although the adsorption with AC cannot remove all organics, is an efficient process for removing undesirable constituents. Some of the most widespread activated carbons uses for liquid-phase adsorption are those in water treatment for the removal of color, odors, metals, pesticides, and organic pollutants from aqueous solutions.14-20 In addition, ACs are widely used for separations in the chemical, petroleum, and pharmaceutical industries, as well as in the removal of pollutants from air and industrial gaseous streams. Wastewaters from industries include a broad range of toxic organic compounds that are persistent in the environment due to their low biodegradability. The treatment of wastewaters with activated carbon avoids pollution and in some cases provides valuable products to be recycled into the manufacturing process.21-22 All of the fundamental and practical industrial applications of adsorption techniques are mainly due to the well-developed internal porous structure of activated carbons and their various surface chemical groups. The surface functional groups on the activated carbon can strongly influence the chemical affinity for sorption of any types of adsorbates; those are carboxylic, phenolic, hydroxyl, carbonyl, and peroxide groups.23-25 * To whom correspondence should be addressed. E-mail: lcot@ uma.es. Phone: +34 952 13 20 37. Fax: +34 952 13 20 38. † Universidad de Ma ´ laga. ‡ Universidad Auto ´ noma de Madrid.

Activated carbon frequently exhibits high removal efficiency for most dissolved organic compounds; the removal efficiency is influenced by the characteristics of the activated carbon nature. In general, manufacturing processes and precursor determine the characteristics of the produced activated carbon. Thus, the surface properties of the activated carbon can be varied depending on the raw materials, the nature of the activation agent, and the activation process conditions. Activated carbons with a high surface area usually show a high adsorption capacity. Three main zones in the heterogeneous internal surface of the activated carbons can be distinguished: the carbon basal planes, the chemical groups, and the inorganic ash content. The majority of adsorption sites for organic liquids are on the basal planes, which form more than 90% of the activated carbon surface. However, the high activity of the surface chemical groups can lead to a significant effect on the overall adsorption capacity.26 Study of the adsorption equilibrium and kinetics is essential to supply the basic information required for the design and operation of adsorption equipment. Adsorption techniques have gained favor in recent years because they are considered efficient for the removal of trace organic pollutants from water that cannot be removed using other treatment processes. In addition, adsorption and desorption kinetics are technologically important, because the diffusion within solid particles is a phenomenon of great importance in catalysis, metallurgy, microelectronics, materials science, and other numerous scientific and technological applications. A mass transfer occurs during the adsorption process; the first step is the solute transfer through the adsorbent external surface film, and the others are the solute fluid diffusion into the pore holes and the adsorbed molecules’ migration along the pore surfaces, if it takes place. The former is characterized by the external mass-transfer coefficient and the last ones by the internal pore and surface diffusivities. Available bulk adsorbate concentration in the liquid phase and adsorbed solute concentration on the solid phase are considered time-dependent. So, it is possible to see how the rate of adsorption changes with time by plotting the concentration decay curve in the liquid phase or the adsorbate concentration growth curve on the solid phase. Mathematical models for diffusion and adsorption into activated carbon have achieved good success in simulating kinetic data.27-32 There are many different factors influencing

10.1021/ie061445k CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007

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Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007

Table 1. Physical Properties and Elementary Analysis of the Activated Carbons ABET (m2/g) As (m2 /g) burnoff (%) Vmicrop N2 (cm3/g) Vmicrop CO2 (cm3/g) Vmesop (cm3/g) Vmacrop (cm3/g) Vtotal (cm3/g) average particle radius (µm) particle density (g/cm3) ultimate analysis %C %H %N %O

AC23

AC41

AC60

753 46 23.1 0.315 0.187 0.043 0.046 0.404 3.22 0.74

847 162 41.1 0.333 0.245 0.301 0.096 0.730 3.35 0.46

1200 246 60.3 0.379 0.298 0.491 0.144 1.014 2.49 0.40

96.2 0.77 0.00 3.03

95.4 0.70 0.00 3.90

94.7 0.68 0.00 4.62

the adsorption rate for a dissolved solute on a porous media. Adsorbent parameters such as particle size, particle density, and porosity are included in mathematical models for kinetics. On the other hand, in a good contact system, operating conditions as pH, pressure, and temperature determine the equilibrium state. For the present work, activated carbons from eucalyptus kraft lignin have been put in contact in a discontinuous batch system with several aromatics in aqueous solutions; the results are reported in kinetic terms. In a previous work,33 equilibrium and thermodynamics related to these same systems were studied. Experimental Section The activated carbons have been prepared in our laboratory according to the procedure described in a previous work.34 Eucalyptus kraft lignin was supplied by the Empresa Nacional de Celulosas as obtained by acid precipitation of kraft black liquors. The lignin sample was carbonized under N2 atmosphere in a laboratory horizontal tubular furnace. The heating rate was 10 K/min until reaching 623 K, a temperature that was held for 2 h. The carbonized product obtained was washed with 1% H2SO4 aqueous solution. The activation was carried out by CO2 partial gasification at 1073 K in the same furnace during different times to obtain activated carbons with different burnoff (AC23, AC41, and AC60). A high porosity and internal surface were developed during the activation process. Ultimate analyses of activated carbons were carried out in a Perkin-Elmer (model 2400 CHN) analyzer (Table 1). The porous structure of the activated carbons was characterized by means of adsorption-desorption of N2 at 77 K and CO2 at 273 K using a Quantachrome apparatus (Autosorb-1 model) and by mercury porosimetry using a Carlo Erba mercury Porosimeter 4000. The N2 adsorption at 77 K was used to calculate the apparent surface area ABET, external area AS, the micropore, and the narrow mesopore (2 nm < d < 8 nm) volumes. The CO2 adsorption was used to calculate the narrow micropore by applying the Dubinin-Radushkevich equation. Mercury porosimetry was used to calculate the wide mesopore volume (8 nm < d < 50 nm) and the macropore volume. Particle densities were determined from the mercury displacement method. Physical properties of activated carbons are given in Table 1. According to the IUPAC classification, the pores may be subdivided in broad terms according to their width (d) in macropores (d > 50 nm), mesopores (2 < d < 50 nm) and micropores (d < 2 nm). As shown in Table 1, AC41 and AC60 correspond to a mesoporous (and microporous) carbon, but AC23 may be classified mainly as microporous carbon. The morphology of the external surface of the activated carbons was studied by scanning electron microscopy (SEM).

Figure 1. SEM observation of a CO2 activated carbon.

SEM micrographs were obtained from selected activated carbons by means of the JSM840 Jeol apparatus. A typical micrograph is shown in Figure 1, which shows that the activated carbons have an irregular and porous external surface with lots of cavities, cracks, and irregular protrusions. The organic compounds used in the present study as adsorbate were as follows: nitrobenzene (NB), p-nitroaniline (p-NA), and p-nitrotoluene (p-NT) supplied by Aldrich; aniline (A) and toluene (T) supplied by Panreac, in the highest purity available. The batch technique was selected in these studies because of its simplicity. A number of stoppered Pyrex glass bottles containing 100 mL of NB, A, p-NA, T, and p-NT aqueous solutions with known concentrations were placed in an orbital incubator (Gallenkamp, model INR-250). The solution pH was, in all the cases, that of the distilled water used (6.8-7.2). Temperatures for the different experiments varied in the range of 278-308 K. Typically, 10 mg of activated carbon was poured into each bottle and the solutions were agitated by mechanical shaking; agitation velocity in the kinetic experiments was 150 rpm equivalent. The maximum contact time was 240 min. The solutions were separated from the adsorbent and analyzed to determine the adsorbate uptake at predicted intervals of time. Adsorbates concentrations were determined by UV spectroscopy at the following wavelengths: NB, 268 nm; A, 280 nm; p-NA, 379 nm; T, 206 nm; and p-NT, 284 nm, corresponding to the maximum absorption values. A UV-visible (Varian, model Cary 1E) spectrophotometer was used for these analysis. The amount of adsorbed compound on activated carbon was calculated as the change in the aqueous-phase concentration from the initial value according to the following equation:

qt ) (Co - Ct)/w

(1)

where Co is the adsorbate initial concentration (mmol/L), Ct is the adsorbate concentration (mmol/L) at a certain contact time t (s), and w is the activated carbon dose (g/L). Results and Discussion Both, equilibrium and kinetic data help to measure the yield and the rate of the adsorption process. In the present work, in order to arrange the data, we have considered the kinetic studies in terms of three aims: (a) to achieve an empirical equation to reproduce the concentration decay curve for each experiment; (b) to determine, by using the fitted data, the apparent kinetic constant and order in relation to the available active sites on the adsorbent surface; (c) to estimate the effective diffusion

Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007 2855

Figure 2. Kinetics of toluene adsorption on AC23. Effect of temperature.

Figure 3. Kinetics of nitrobenzene adsorption on AC23, AC41, and AC60 at 278 K.

coefficients assuming the intraparticle mass transfer as the ratecontrolling step. Empirical Model for the Concentration in the Liquid Phase. The removal of NB, A, p-NA, T, and p-NT from their aqueous solutions by adsorption on AC23, AC41, and AC60 has been studied at 278, 293, and 308 K. Selected results of the kinetics at the three different temperatures with the three activated carbons and the five compounds are shown in Figures 2 -4. The experimental results were fitted to a doublehyperbolic (D-H) model:

qt )

L3t L1t + L2 + t L4 + t

L3L4 L1L2 dqt + ) dt (L + t)2 (L + t)2 2

(2)

(3)

4

where qt (mmol/g) is the amount of adsorbate on the adsorbent at time t (min); L1, L2, L3, and L4 are empirical coefficients. Calculated coefficients for the empirical equations are shown in Table 2. It includes the χ2 (average quadratic deviation) values, which indicated a good fits. The model fittings (lines) are shown in Figures 2-4. It is evident that the model was able to reproduce the adsorption rate under a range of contact time and it is clear that the adsorption rate and yield increase with increasing temperature. This is due, as can be seen later, to the increase of the diffusion coefficients with the temperature.

Figure 4. Kinetics of NB, A, p-NA, T, and p-NT adsorption on AC41 at 278 K.

Preliminary investigations on the rate of adsorption on AC23, AC41, and AC60 indicated that the processes are quite rapid, and typically 65-75% (depending on adsorbate, adsorbent, and temperature) of the ultimate adsorption occurs during the first hour of contact. Figure 5 shows the values of dqt/dt versus t for NB on AC23 adsorption at different temperatures. For this plot, data were provided using eq 3 from the D-H model. A sharp decrease in the adsorption rate is observed for the first minutes of contact time; this step corresponds to the surface coverage in the widest pores and the more active sites. A second step develops for 1 h approximately with an intermediate decreasing rate; this step is the most interesting to kinetic control for its relation to the narrow mesopore and micropore coverage. The last part ends at the equilibrium conditions, after several days of contact time. The structurally different activated carbons also showed varied adsorption rates for the same adsorbate molecules (Figure 3). The uptake rate of NB is much higher on AC60 than on AC23 and AC41. It can be explained because of the higher surface area and pore volume of AC60 (Table 1); for all the studied cases, the highest adsorption rates were obtained at the highest temperatures and active carbon burnoff grades. The chemical characteristics of the activated carbon surface have been described in a previous work.33 It is very important that the surface of the activated carbons contains different types of surface groups such as carboxylic, lactones, phenolic, and carbonyl. This diversity of oxygen surface groups on the activated carbon makes the surface chemistry much more versatile than that of other adsorbents.35 The physisorption of aromatics on activated carbon takes place mainly through van der Waals dispersive interactions between the molecules and the carbon basal planes. Heterogeneous oxygen surface groups attract and locate the electrons of the basal planes, hence forming partially positive islands in the basal planes. So, the surface chemistry of activated carbons has an important role in adsorption equilibrium and kinetics. On the other hand, the functional group of the aromatic adsorbate can activate or deactivate the benzene ring to which it is attached. NB, A, p-NA, T, and p-NT have different functional chemical groups, molecular sizes, and water solubilities. They determine the nature of mechanisms as well as the extent and the strength of adsorption.25,36 Activating groups (OH, NH2, CH3) act as electron donors, which create a partially negative benzene ring by pushing the electrons toward the ring. Deactivating groups (Cl, NO2) attract the electrons and produce a partially positive ring. For this reason, the electronic interactions

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Table 2. Empirical Coefficients for D-H Fitting Equation AC23 T (K)

AC41

278

293

308

L1 L2 L3 L4 χ2

1.300 122.0 0.188 2.350 2 × 10 -6

1.655 49.00 0.570 0.370 2 × 10 -6

1.800 36.00 0.560 0.110 6 × 10 -6

L1 L2 L3 L4 χ2

1.100 32.00 0.196 0.120 3 × 10 -6

1.000 30.20 0.300 0.600 3 × 10 -6

L1 L2 L3 L4 χ2

0.530 64.00 0.203 0.300 3 × 10 -6

L1 L2 L3 L4 χ2 L1 L2 L3 L4 χ2

278

293

AC60 308

278

293

308

Nitrobenzene Co: 0.407 1.215 1.240 94.00 63.00 0.530 1.118 0.840 0.270 4 × 10 -6 3 × 10 -6

1.045 26.00 1.300 0.024 3 × 10 -6

1.600 30.00 1.126 0.270 6 × 10 -6

1.540 19.8 1.282 0.008 6 × 10 -6

1.200 26.80 1.715 0.043 4 × 10 -6

1.080 16.00 0.230 0.120 1 × 10 -5

Aniline Co: 1.000 1.585 1.500 14.70 15.20 0.110 0.255 0.001 0.070 8 × 10 -6 5 × 10 -6

1.800 17.30 0.290 0.020 3 × 10 -6

1.750 13.30 0.236 0.003 3 × 10 -5

1.691 11.40 0.370 0.002 1 × 10 -5

1.460 16.90 0.789 0.120 2 × 10 -6

0.510 53.00 0.243 0.050 2 × 10 -7

0.590 40.00 0.250 0.030 9 × 10 -7

p-Nitroaniline Co: 0.217 0.690 0.713 31.40 28.50 0.388 0.450 0.047 0.008 -6 1 × 10 7 × 10 -7

0.600 27.70 0.610 0.009 1 × 10 -6

0.870 46.70 0.832 0.450 3 × 10 -6

0.780 73.20 1.000 0.312 9 × 10 -7

0.830 52.40 0.980 0.100 5 × 10 -6

1.106 26.50 1.106 26.50 3 × 10 -5

2.200 27.90 0.599 0.016 1 × 10 -5

2.212 37.80 0.960 0.002 2 × 10 -6

Toluene Co: 0.435 3.043 3.231 25.30 21.90 0.600 0.500 3.360 0.140 1 × 10 -5 1E-04

2.550 27.40 1.304 0.040 4 × 10 -5

3.000 24.60 1.160 0.280 2 × 10 -5

3.054 17.80 1.130 0.008 2 × 10 -5

2.570 17.00 1.664 0.020 3 × 10 -5

0.733 32.71 0.281 0.305 2 × 10 -5

0.785 25.93 0.421 0.251 1 × 10 -5

0.892 18.80 0.484 0.205 4 × 10 -5

p-Nitrotoluene Co: 0.196 0.756 0.930 41.58 24.60 0.514 0.581 0.263 0.050 3 × 10 -6 7 × 10 -7

1.000 10.74 0.630 0.031 9 × 10 -5

0.850 22.06 0.600 0.170 1 × 10 -6

0.955 19.50 0.730 0.100 8 × 10 -6

1.000 17.00 0.850 0.060 2 × 10 -5

of the benzene ring with the surface basal planes influence the adsorption mechanism.37 Therefore, it is expected that the adsorption behavior of the adsorbates can differ from each other. Figure 6 shows the equilibrium approach percentage versus time; equilibrium data (qe) are included as well. It is observed that the removal of NB, T, and p-NT were far larger than that of the other adsorbates (p-NA and A). The adsorption capacity of NB (a compound with a deactivating group) was higher than the other adsorbates, as expected. Being a deactivating group, the nitro (NO2) group withdraws electrons from the aromatic ring, making it partially positive, which favors the formation of donor-acceptor interactions between the oxygenated groups of the activated carbon surface (donor) and the aromatic ring (acceptor).

The (CH3) group is an activating group. This means that the aromatic ring of the toluene molecule has a partial negative charge. Toluene adsorption suggests that it occurs due to the attractive interactions between the benzene ring and the positive islands in the basal planes of the activated carbons. Furthermore, when we compare the adsorption rate among adsorbates, we must consider the strength and the nature of the adsorbent-adsorbate interactions altogether. Nevertheless, physical concerns such as molecular size and solubility of the adsorbates are very important as well. Kinetic Model for the Concentration on the Solid Phase. The adsorption kinetic data have been analyzed by the pseudofirst-order Lagergren equation,38 by numerous authors.39-41 This differential equation generalized for order n is

Figure 5. Nitrobenzene adsorption rate on AC23 at different temperatures. Data from D-H model.

Figure 6. Equilibrium approach for AC41 adsorption on the five aromatics at 293 K. Data from D-H fittings; qe in mmol/g.


dqt ) k(qm - qt)n dt

(4)

where k is the adsorption rate constant and n is the apparent order of the process related to the available adsorbent concentration (qm - qt), which is the driving force of the process, that is, the vacant and accessible active sites in the surface of the adsorbent, for any time of contact. In this work, qm (achievable coverage at the maximum contact time) was calculated from the fitting eq 2 as qm ) L1 + L3. At the equilibrium time, qm becomes qe. The adsorption rate denoted by (dqt/dt) shows how much adsorbate can be adsorbed on the adsorbent from the liquid phase per unit time. Figure 5 reveals that the adsorption rate (dqt/dt) decreases with time as it gradually approaches the equilibrium condition due to the continuous fall of the driving force (qm - qt). By combining eq 3 and eq 4, it becomes in logarithmic terms

log

(

L1L2

(L2 + t)

2

+

)

L3 L4 (L4 + t)2

Figure 7. Adsorption rate of p-NT on AC41 related to the AC available active sites. Ce (mmol/L): 0.009, 0.020, and 0.023 at 278, 293, and 308 K.

) log k + nlog (qm - qt) (5)

As two examples, Figures 7and 8 show log (dqt/dt) versus log (qm - qt) for p-NT on AC41 at different temperatures and for toluene at 308 K on different activated carbons, respectively. It is observed that the slope (n value) in the first minutes is very high (high (qm - qt) values), which means this corresponds to a very high initial adsorption rate that is strongly dependent on the high initial concentration of the available active sites. This behavior leads to a high apparent kinetic order. At the following (intermediate) minutes, the apparent kinetic order becomes close to second order in relation to the empty active centers on the activated carbon (n ≈ 2). Good linear adjustments were observed for all temperatures, adsorbates, and adsorbents. The adsorption rate constants k were calculated from the ordinate of the plots, and their values were between 0.02 and 0.08 g/mmol‚min for all the adsorbates, temperatures, and activated carbons. Those are the valid average values for the experimental contact time range (0-240 min). The adsorption rate increased with the temperature, which was due to the increase of the diffusion coefficient with temperature. Values of apparent activation energy can be obtained by applying the Arrhenius equation; values of 5-15 kJ/mol have been obtained, the lower ones for the most favored adsorption process (T) and the higher for the less favored ones (p-NA). This kinetic model proposed for a pseudo second order has been used successfully by other authors.3,42-45 Estimation of the Effective Diffusion Coefficients. Masstransfer processes in multiporous solids are always complicated because of molecular sieve and diffusional effects. The adsorption into the smallest pores can be excluded, because adsorbate molecules can be too large to enter into the volume of these elements. Diffusion takes place when the dimensions of the adsorbate molecules are only slightly smaller than the pore diameter. For nonporous and macroporous solids, the internal diffusion may be neglected and the control is determined by external diffusion processes. On the other hand, adsorbent particles become heterogeneous systems formed by a porous solid phase and a fluid phase filling the void fraction of the solid. A number of models have been postulated, which differ in their description of the diffusion process occurring within the particle.46 Intraparticle diffusion depends on many factors such as the structure of the sorbent, physical properties of the sorbent and adsorbate, chemical properties of the sorbate, equilibrium behavior, and the system

Figure 8. Adsorption rate of toluene on AC23, AC41, and AC60 at 308 K related to the ACs’ available active sites. Ce (mmol/L): 0.101, 0.064, and 0.027 for AC23, AC41, and AC60.

conditions. The internal diffusion can be expressed by the two possible simultaneous mechanisms of diffusion inside the particles that can be distinguished as pore diffusion in the fluid phase filling the pores and surface diffusion on the solid phase.46 The combined model can be described by the general equation29

P

[(

)]

∂CP ∂CP ∂q(r) 1 ∂ 2 ∂q(r) + FP ) 2 + D S FP r PDP ∂t ∂t ∂r ∂r r ∂r

(6)

where CP and q(r) are the solute concentrations in the fluid inside pore and in the solid as adsorbed phase, respectively; r is the radial spatial coordinate; t is the time; P is the particle porosity; FP is the particle density; and DP and DS are the pore diffusion coefficient and the surface diffusion coefficient, respectively. General assumptions for this combined model were the following: (1) uniform concentration of adsorbate in the external solution; (2) adsorbent particles of spherical shape, diffusion taking place in the radial direction; (3) negligible mass flux compared to transport by diffusion; (4) concentrations on the surface remain at equilibrium during the entire period of adsorption (instantaneous adsorption); and (5) constant temperature.47 If we consider the solid homogeneous diffusion in a sphere with a constant diffusivity across the particle, then the equation is

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[

]

∂q(r) DS ∂ 2 ∂q(r) r ) 2 ∂t ∂r r ∂r

(7)

This approach side steps trying to differentiate among diffusion into the pore spaces, diffusion along the surfaces, which can occur subsequently to adsorption (surface diffusion), and diffusion in the solid material itself. Crank32 gave an exact solution to this equation when the following occurred: (a) external film resistance was negligible, (b) the sphere was initially free of solute, and (c) the concentration of the solute on the surface remained constant. This situation is called the “infinite bath” case, since an infinite surrounding medium would result in a constant concentration of solute on the surface of the solid (the amount of solute in the surrounding fluid is very large compared with the amount of solute that the solid can adsorb). However, we are really not only interested in how q varies with radial position r, but also in the average amount of solute in the solid at any particular time (qt). Crank developed the following equation:

qt qe

)1-

6

∞

∑

1

π2 n)1 n2

(

)

exp -

DSn2π2t R2

(8)

where R is the particle radius and qe is the average equilibrium concentration of the adsorbate in the solid phase. DS becomes an apparent or effective diffusivity (Di) if we consider that the surface diffusion dominates the transport. In this case, adsorbate molecules in the bulk phase surrounding the particle will locally equilibrate with adsorbed molecules at the mouth of the pore, and adsorbed molecules diffuse into the particle under its own gradient. We can very often find in the literature that shorttime solutions are needed to investigate the behavior of adsorption during the initial stage of adsorption. For linear isotherms, this can be analytically achieved by taking the Laplace transform of a model equation.28 So, for small times, or, more precisely, for qt/qe < 0.3, eq 8 may be written as

Di )

()

qt 2 πR2 qe 36t

Figure 9. Effective diffusion coefficients for aniline adsorption (Co, 1.000 mmol/L) on AC41 (w, 0.100 g/L) at different temperatures.

Figure 10. Effective diffusion coefficients for p-NT adsorption (Co, 0.217 mmol/L) on AC41 (w, 0.100 g/L) at different temperatures.

(9)

When the adsorption isotherm is not linear, the linear model is also satisfactory for adsorbates that are not very strongly adsorbed and that show adsorption isotherms with smooth and moderate change of slope.48 Nevertheless, apparent diffusion coefficients generally vary with the surface concentration. In this work, we have calculated the effective diffusion coefficients from the kinetic results by using eq 9. The equilibrium data qe were determined in a previous work.33 The average sizes (radius) of the activated carbons particles were estimated from SEM micrographs by an image analysis procedure (Table 1). The qt values corresponding to different times were supplied by the D-H model. To illustrate the results, several cases of varied activated carbons, temperatures, and adsorbates were selected, as examples. We can plot the diffusion coefficients in terms of time or adsorbed amount. The effect of the temperature on the diffusion coefficients is shown in Figures 9 -12. In all the studied cases, an increase of temperature induces an increase of the mass-transfer parameters, as a consequence of a higher thermal agitation; consequently, an increase in the adsorption rate is observed. The continuous decrease of Di values as contact time grows is due to the consumption of the high-energy adsorption sites and the spatial difficulties enhancement. Those lead to a drop

Figure 11. Effective diffusion coefficients for aniline adsorption (Co, 1.000 mmol/L) on AC60 (w, 0.100 g/L) at different temperatures.

in driving force. This is especially notorious when, at advanced contact times, the adsorption on macro- and mesopores is very slow and the adsorbate net flow within micropores becomes difficult because of the tortuosity as well as the pore mouth obstruction. Figure 13 compares the active carbons in terms of their diffusion coefficients falling as the adsorbate (NB) concentration on adsorbent progresses. High activation grade leads to a higher available surface area and pore volume, which improve the mass-transfer capabilities.


adsorbates’ affinity toward the activated carbon surface. A decrease in the adsorbent-adsorbate affinity leads to a decrease in the adsorption rate and capacity. Conclusions

Figure 12. Effective diffusion coefficients for p-NT adsorption (Co, 0.196 mmol/L) on AC60 (w, 0.100 g/L) at different temperatures.

The empirical model for the concentration in the liquid phase (D-H) provides a good description of the kinetics of aromatic compound adsorption onto activated carbons. This model has been used to determine qm and qt (achievable coverage at the maximum contact time and coverage at t time, respectively), which have been used in the driving force determination for the kinetic model for the concentration on the solid phase. From this model, the adsorption rate constants, the apparent order, and the activation energies have been estimated. The temperature effect on the adsorption of aromatics from water solutions is observed with an increase of the rate of adsorption, as a result of the increase of the diffusion coefficients when temperature increases. Acknowledgment The authors acknowledge the Spanish DGICYT-MEC (Project PPQ2003-07160) and the Junta Andalucıá (TEP-184) for the financial support. Nomenclature

Figure 13. Effective diffusion coefficients for NB adsorption (Co, 0.407 mmol/L) on AC23, AC41, and AC60 (w, 0.100 g/L) at 293 K.

Figure 14. Effective diffusion coefficients for NB, A, p-NA, T, and p-NT adsorption on AC41 (w, 0.100 g/L) at 293 K; Co in mmol/L.

Finally, in relation to the adsorbate comparison, we observed (in Figure 14) how diffusion coefficients decrease in the course of the first 5 min of contact time. Adsorbates with an NO2 group in its molecule (NB, p-NA, p-NT) exhibit the highest values. As has been said, the NO2 group deactivates the benzene ring. This provides a high uptake capacity for the adsorbate on the negative charged groups of the carbon surface. For this reason, we can expect both high adsorption rate and capacity. On the other hand, the NH2 group (A, p-NA) activates the electronic cloud of the aromatic ring, which reduces the

ABET ) apparent surface area (m2/g) AC ) activated carbon As ) external area (m2 /g) Co ) initial concentration of adsorbate (mmol/L) CP ) concentration of solute in the fluid inside pore (mmol/L) Ct ) concentration of adsorbate in the fluid phase at a certain contact time (mmol/L) Di ) apparent or effective diffusivity (cm2/s) DP ) pore diffusion coefficient (cm2/s) DS ) surface diffusion coefficient (cm2/s) d ) pore width (nm) k ) adsorption rate constant (g/mmol min) L1, L2, L3, L4 ) empirical coefficients in D-H model n ) order in the pseudo-nth-order kinetic equation qe ) equilibrium concentration of the adsorbate in the solid phase (mmol/g) qm ) concentration of the adsorbate in the solid phase at the maximum contact time (mmol/g) q(r) ) concentration of the adsorbate in the solid phase at the radial coordinate (mmol/g) qt ) concentration of the adsorbate on the adsorbent at time t (mmol/g) R ) radius of the adsorbent particle (cm) r ) radial spatial coordinate T ) temperature (K) t ) time (s, min) w ) adsorbent dose (g/L) χ2 ) average quadratic deviation P ) particle porosity FP ) particle density (g/cm3) Adsorbates A ) aniline p-NA ) p-nitroaniline NB ) nitrobenzene

2860


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ReceiVed for reView November 12, 2006 ReVised manuscript receiVed February 27, 2007 Accepted March 5, 2007 IE061445K

Adsorption of Aromatic Compounds on Activated Carbons from Lignin

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