Removal of Mono- and Multicomponent BTX Compounds from

Article pubs.acs.org/IECR

Removal of Mono- and Multicomponent BTX Compounds from Effluents Using Activated Carbon from Coconut Shell as the Adsorbent Selene Maria de Arruda Guelli Ulson de Souza,* Adriana Dervanoski da Luz, Adriano da Silva, and Antônio Augusto Ulson de Souza Chemical Engineering Department, Laboratory of Numerical Simulation of Chemical Systems, Federal University of Santa Catarina, Campus Universitário, 88040-900 Florianópolis, Santa Catarina, Brazil ABSTRACT: The adsorption of benzene, toluene, and o-xylene (BTX) in monocomponent and multicomponent aqueous solutions onto activated carbon was studied in a batch reactor at 23 °C for wastewater reuse. The kinetics and thermodynamic equilibrium parameters were obtained for all compounds, where the competitiveness for the active site of adsorption was investigated. The results showed that the order of monocomponent adsorption of these three contaminants is o-xylene > toluene > benzene, and in the multicomponent mixture o-xylene is the most competitive contaminant for the active site of adsorption. The adsorption kinetics were fitted to the homogeneous diffusion model, giving linear correlation coefficients higher than 0.96. The Langmuir isotherm provided the best fit with the monocomponent experimental data. In this study, the multicomponent mixture isotherms were obtained and compared to the models found in the literature.

1. INTRODUCTION The petroleum and petrochemical industries are high water consumers generating high quantities of liquid effluent. Many toxic organic compounds are present in these effluents, notably hydrocarbons of high molecular mass which are difficult to remove. In particular, the compounds benzene, toluene, and xylene, known as BTX compounds, have a higher contamination potential due to the fact that they are the most hydrosoluble petroleum compounds.1 The BTX compounds are contaminants which significantly threaten human health and the environment due to their toxic and carcinogenic properties. Even in low concentrations BTX compounds may damage the liver, kidneys, and central nervous system.1 The high degree of toxicity of these compounds has led to increasing research on the processes aimed at their removal from effluents. The environmental laws and the costs associated with the use of water resources are becoming ever more present among the competitive factors of the industrial sector. Several studies have been directed toward the treatment of polluted streams close to the source point (integrated approach) and toward the treatment of the final effluents (end-of-pipe approach).2−7 Adsorption is a method which has been shown to be very efficient in removing organic compounds, since it reaches the limits established by legislation for the discharge of these effluents to water bodies.8 Adsorption onto activated carbon is used in the final stage of water and effluent treatments, in order to remove traces of hazardous contaminants not removed in the primary treatment.9,10 In general, studies on the removal of these compounds by adsorption have been carried out on pure components. However, in industrial effluents there is a mixture of toxic compounds to be removed. Experimental measurements of multicomponent adsorption equilibriums are complex and tedious to analyze, especially when there are more than two © 2012 American Chemical Society

components and when there is the influence of dissociation, ionic strength, and temperature.1 Shahalam et al.11 studied the competitiveness for the active site of adsorption of mixtures of petrochemicals (benzene, toluene, and xylene) dissolved in hexane in a fixed bed, using three types of sandy soils. The results showed that the greater the concentration of contaminants, the greater the adsorption. Toluene and xylene were adsorbed in greater quantity than benzene, indicating their greater adsorption potential. The authors affirm that the order of adsorption is influenced by the molar mass of each contaminant in the mixture, and the higher the molar mass the greater the tendency toward adsorption. According to Shahalam et al.,11 adsorption of benzene, toluene, and xylene onto sandy soils is approximately 30−200 times lower than the expected adsorption onto activated carbon. Yun et al.12 reported the experimental and theoretical study of benzene, toluene, and p-xylene and their binary and ternary mixtures using activated carbon at 28 °C. The data showed that both the extended Langmuir equation and the “ideal adsorbed solution theory” could predict the adsorption isotherm of the mixture with good precision. This study investigates the kinetics and thermodynamic equilibrium of the adsorption of BTX compounds in a batch reactor in monocomponent and multicomponent aqueous solutions using activated carbon as the adsorbent. The competitiveness for the active site of adsorption was also investigated. The kinetics data were fitted using the homogeneous diffusion model, and thermodynamic equilibrium Received: Revised: Accepted: Published: 6461

April 3, 2011 March 4, 2012 April 5, 2012 April 5, 2012 dx.doi.org/10.1021/ie2026772 | Ind. Eng. Chem. Res. 2012, 51, 6461−6469

Industrial & Engineering Chemistry Research

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data were fitted using four different adsorption isotherms given in the literature.

Table 1. Adsorption Kinetics for Adsorption of Binary Combinations of BTX Compounds onto Activated Carbon

2. MATERIAL AND METHODS 2.1. Materials. The adsorbent used was activated carbon “Carbon 119” of plant origin produced from coconut shell, obtained from the physical activation process. The solvents used were the following: distilled water, to prepare solutions of the BTX compounds; Milli-Q water and HPLC-grade methanol of the Carlo Elba Gold series for HPLC (high-performance liquid chromatography). HPLC-grade benzene (Fluka), HPLC/UV spectroscopy grade toluene (VETEC), and 98% HPLC-grade o-xylene (Aldrich) were used. 2.2. Methods. 2.2.1. Experimental Procedure. Prior to the laboratory tests the samples passed through a treatment which consisted of the granular adjustment of the activated carbon (18/20 MESH) to approximately 0.85 mm, washing for a period of 10 days, and drying at 110 °C for 3 h, in order to carry out the kinetics and thermoequilibrium experiments. To determine the concentration of BTX compounds, highperformance liquid phase chromatography on an HPLC (CG) connected to a UV/visible detector (Model CG 437-B and 250 mm Nucleosil C18 reverse phase column, internal diameter of 4.6 mm) was used. The mobile phase was HPLC-grade methanol (Carlo Erba) and Milli-Q water (80:20), at a flow rate of 0.8 mL/min. The BTX compounds were identified at a wavelength of 254 nm. All experiments were performed in triplicate. 2.2.2. Characterization of the Adsorbent. The characterization of the adsorbent was carried out from the following tests: particle size, hardness, humidity, volatile material, ash content, and fixed carbon.13 The BET (Brunauer, Emmett, and Teller) and BJH (Barrett, Joyner, and Halenda)14 tests were carried out to determine the surface area, pore volume, pore size distribution, and particle irregularity of the material. Scanning electron microscopy (SEM) was carried out to obtain micrographs of the physical structure of the activated carbon. 2.2.3. Adsorption Kinetics. Each isolated compound solution was prepared with a different initial concentration of the BTX compounds, containing the adsorbate of interest, placed in 250 mL Erlenmeyer flasks, closed with a Teflon stopper to avoid volatilization of the adsorbate, and then placed on a shaker tray inside a thermostatic bath, at a temperature of 23 °C and shaken at 120 rpm, both controlled. The adsorbent mass used in the experiment was 1 g. The initial pH of the adsorption remained at around 6.4. The curves for the adsorption kinetics were obtained by removing at regular time intervals 0.5 mL aliquots of the solutions in the Erlenmeyer flasks. For the monocomponent solutions the adsorption kinetics was determined for initial concentrations ranging from 15 to 150 mg/L. Table 1 shows the adsorption kinetics for the adsorption of binary combinations of the BTX compounds on activated carbon. Table 2 shows the combinations used for the tricomponent study. 2.2.4. Adsorption Isotherms. The studies on the thermodynamic equilibrium between the adsorbent and the adsorbate were carried out for each compound in isolation or in mixtures. The tests were carried out at 23 °C and 120 rpm, adding 0.5 g of adsorbent to seven 250 mL Erlenmeyer flasks, with Teflon stoppers. To ensure that the equilibrium of the solution was reached, a thermostatted shaker tray was used for 24 h.

benzene (mg/L)

toluene (mg/L)

toluene (mg/L)

o-xylene (mg/L)

benzene (mg/L)

o-xylene (mg/L)

50 + 50 + 50 + 50 + toluene (mg/L)

0 30 50 100 benzene (mg/L)

50 + 50 + 50 + 50 + o-xylene (mg/L)

0 30 50 100 toluene (mg/L)

50 + 50 + 50 + 50 + o-xylene (mg/L)

0 30 50 100 benzene (mg/L)

50 50 50 50

+ + + +

0 30 50 100

50 50 50 50

+ + + +

0 30 50 100

50 50 50 50

+ + + +

0 30 50 100

Table 2. Ternary Combinations Used To Determine the Adsorption Kinetics of BTX Compounds on Activated Carbon benzene (mg/L)

toluene (mg/L)

o-xylene (mg/L)

50 30 30 50

30 50 30 50

30 30 50 50

For the tests on the monocomponent equilibrium, the solutions were prepared in the following concentrations: 150, 130, 110, 90, 70, 50, and 30 mg of BTX/L and distilled water. For the tricomponent mixtures, the ratios used were the following: 150/150/150, 130/130/130, 110/110/110, 90/90/90, 70/70/70, 50/50/50, and 30/30/30 mg/L. The initial adsorption pH was 6.4.

3. RESULTS AND DISCUSSION 3.1. Characterization of Activated Carbon. From the analysis of the physical and chemical characteristics, it was verified that the activated carbon used for the adsorption of BTX compounds had a low humidity content (0.03% dry basis), low ash content (1.4% dry basis), and high fixed carbon content (94.99% dry basis). The results for the textural characterization of the adsorbent which included the determination of the surface area, extent of microporosity, and pore size distribution are presented in Table 3. Table 3. Textural Characterization of Adsorbent under Study textural characteristic

value

surface area pore volume average pore diameter micropore volume micropore area pore distribution of particles

724 m2/g 0.39 cm3/g 21.35 Å 0.31 cm3/g 614 m2/g min 18 Å; max 400 Å

Figure 1 shows the results for the scanning electron microscopic (SEM) analysis of the activated carbon used. The magnifications were 30 and 125 times. It is clear in Figure 1 that there are a large number of pores, consistent with the data in Table 3. The pore size distribution varies between 18 and 400 Å, with micropores and mesopores (average diameter of around 21 Å) being predominant. 6462

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McKay16 related the internal diffusion to the ratio between the molecular diameter of the adsorbate and the adsorbent pore diameter and proposed that the intraparticle diffusivity and adsorbent porosity are interdependent. On observing the adsorption kinetics curves (Figure 2), it can be verified that the diffusion in the micropore controls the mass transfer process. This can be confirmed by the large quantity of micropores present in the adsorbent, which can also be seen in Table 3. They also observed a drop in the exponential effective diffusivity as the initial concentration of the contaminant increased. The equilibrium times for the kinetics tests for the 150 mg/L concentration of the compounds benzene, toluene, and o-xylene were approximately 700 min (≅12 h), 500 min (≅8.5 h), and 750 min (≅13 h), respectively. The experimental data for the adsorption kinetics of the monocomponent BTX compounds (Figure 2) were fitted to the model for homogeneous diffusion within a particle, according to Ruthven.14

3.2. Adsorption Kinetics. 3.2.1. Adsorption Kinetics for the Monocomponent System. Figure 2 shows the monocomponent adsorption kinetics for the BTX compounds. For each compound the concentrations studied were 15, 50 100, and 150 mg/L, with a constant temperature maintained at 23 °C and initial adsorption pH 6.4. On comparing the BTX compounds for a concentration of 15 mg/L, it can be noted that the efficiency of the adsorbent was greater for the removal of o-xylene. At 15 mg/L, the concentration of contaminant remaining at equilibrium was 4.03 mg/L for benzene, 2.76 mg/L for toluene, and 0.7 mg/L for o-xylene. According to Shahalam et al.11 and Yun et al.,12 the adsorption of monocomponent BTX compounds occurs in the following order: xylene > toluene > benzene. The adsorption favoring this order of compounds can be explained by the reduction in solubility and increase in molecular mass, since o-xylene is the compound which has the lowest solubility in water and the greatest molecular mass, while benzene is the most soluble and has the lowest molecular mass. On evaluating Figure 2, it can be observed that there is a stage where there is a rapid drop in concentration, where the kinetics is governed by the diffusion at the boundary layer, and, for the rest of the curve, where the velocity is lower, there is a strong influence from the internal diffusion.15 Al-Duri and

1/2 q C −C 6 ⎛ Dst ⎞ ⎜ ⎟ = = o π ⎝ r2 ⎠ qe C − Ce

(1)

where q/qe is the fraction adsorbed in the solid phase, (Co − C)/ (C − Ce) is the fraction removed in the fluid phase, r is the position on the radius in relation to the center of the particle considered spherical (cm), t is the time (s), and Ds is the homogeneous diffusion coefficient (cm2/s). The homogeneous diffusion coefficients are calculated using eq 1, and they are given in Table 4. It can be observed that the Ds values obtained for the monocomponent systems show good linear correlation coefficients. These values are in agreement with the values found in the literature for other solute/adsorbent systems.17 In Table 4 it can be verified that the concentration of oxylene is independent of the initial concentration, compared with benzene and toluene which have a greater resistance as a

Figure 1. SEM micrographs of the activated carbon obtained from coconut shell.

Figure 2. Effect of different concentrations on the kinetics of (a) benzene, (b) toluene, and (c) o-xylene adsorption onto activated carbon. 6463



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a lower resistance to diffusion than benzene and toluene, independently of the concentration. 3.2.2. Adsorption Kinetics for the Bicomponent System. A series of experiments, with the binary combinations shown in Table 1, was investigated for the analysis of competiveness for the active site of adsorption. The kinetics experiments were conducted using Erlenmeyer flasks of 250 mL, initial pH 6.4 at 23 °C, with a mass of 1 g of activated carbon. 3.2.2.1. Effect of Toluene and o-Xylene on the Adsorption of Benzene. The results for the adsorption of benzene with the simultaneous presence of toluene and o-xylene are shown in parts a and b, respectively, of Figure 3. From Figure 3a, it can be noted that the adsorption of benzene was inhibited by the presence of high concentrations of toluene, at approximately 100 mg/L. Similarly, it can be seen in Figure 3b that the adsorption of benzene is again inhibited, this time by the presence of o-xylene, with this effect being greater as the o-xylene concentration increases. At all of the initial benzene concentrations evaluated, its concentration

Table 4. Values Obtained for the Homogenous Diffusion Coefficients for Monocomponent Compounds benzene Co (mg/L)

Ds (×107) (cm2/s)

15 50 100 150

10.42 9.17 5.50 4.83

o-xylene

toluene

R2

Ds (×107) (cm2/s)

0.9654 0.9679 0.9701 0.9790

10.28 7.17 6.83 3.83

R2

Ds (×107) (cm2/s)

R2

0.9773 0.9864 0.9854 0.9870

10.92 10.8 10.75 10.65

0.9891 0.9942 0.9956 0.9923

function of the increase in the concentration used. A possible explanation for this is that benzene and toluene have similar molecular configurations and they can be adsorbed in the micropores of the carbonaceous material, presenting a greater resistance to mass transfer as the concentration is increased. Since o-xylene has a larger configuration, it can be adsorbed in a greater quantity in the mesopores of the adsorbent, presenting

Figure 3. Adsorption kinetics of benzene in bicomponent mixture with toluene and o-xylene on activated carbon: (a) 50 mg/L benzene and different concentrations of toluene; (b) 50 mg/L benzene and different concentrations of o-xylene.

Figure 4. Adsorption kinetics of toluene in bicomponent mixtures with benzene or o-xylene on activated carbon: (a) 50 mg/L toluene at different concentrations of benzene; (b) 50 mg/L toluene and different concentrations of o-xylene.

Figure 5. Adsorption kinetics of o-xylene in bicomponent mixture with benzene and toluene on activated carbon: (a) 50 mg/L o-xylene at different concentrations of benzene; (b) 50 mg/L o-xylene at different concentrations of toluene. 6464



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decreases rapidly in the first 400 min (≅6.5 h) and then slowly reaches equilibrium around 700 min (≅12 h). 3.2.2.2. Effect of Benzene and o-Xylene on the Adsorption of Toluene. The adsorption of toluene with different concentrations of benzene and o-xylene is shown in parts a and b, respectively, of Figure 4. In parts a and b of Figure 4, it can be observed that the presence of benzene and the presence of o-xylene, respectively, have an inhibitory effect on the adsorption of toluene. This effect becomes greater with an increase in o-xylene concentration. Also, for toluene the presence of another contaminant in the same solution resulted in a lower adsorption rate, due also to the occupation of the adsorption sites by the other contaminant. 3.2.2.3. Effect of Benzene and Toluene on the Adsorption of o-Xylene. Figure 5 shows the o-xylene adsorption kinetics in the presence of the contaminants benzene (Figure 5a) and toluene (Figure 5b). From the profiles in Figure 5, it can be observed that the influence of benzene and toluene had an effect which favored the o-xylene adsorption. This behavior can be explained by the occurrence of a reaction between the contaminants and the surface of the adsorbent.

On comparing the results of Figures 3, 4, and 5, in which the bicomponent kinetics results for the BTX compounds are given, it can be noted that o-xylene is the most competitive for the active site of adsorption. This finding was also reported by Shahalam et al.11 and Yun et al.12 One of the reasons for this high affinity for the adsorbent is the polarity, since activated carbon is a material with an apolar surface, and of the BTX compounds o-xylene has the lowest polarity, followed by toluene and benzene. The experimental data for the bicomponent systems were fitted to the homogeneous diffusion model, eq 1, in order to estimate the surface diffusion coefficients, according to Table 5. It was observed that the surface diffusion coefficients for the bicomponent systems had good linear correlation coefficients, as shown in Table 4. 3.2.3. Tricomponent Adsorption Kinetics. Figure 6 shows the adsorption kinetics for the BTX compounds, for the tricomponent mixture. Through analysis of Figure 6a, it can be verified that with an increase in the concentration of the BTX compounds in the mixture there is a greater inhibition effect of the contaminant benzene, which is found in greater quantity in the solution. This occurs because at the beginning of the adsorption there is a large quantity of available active sites, when all of the contaminants are adsorbed. Over time the quantity of active sites decreases and competition between the contaminants with

Table 5. Comparison of Experimental Values Obtained for Homogeneous Diffusion Coefficients for Bicomponent Systems mixture

Co (mg/L)

Ds (×107) (cm2/s)

R2

B (B/T) B (B/X) T (T/B) T (T/X) X (X/B) X (X/T)

50/50 50/50 50/50 50/50 50/50 50/50

10.55 10.05 10.4 10.33 20.43 20.57

0.9875 0.9605 0.9812 0.9871 0.9901 0.9904

Table 6. Comparison of Experimental Values for Surface Diffusion Coefficients for the Tricomponent System mixture

Co (mg/L)

Ds (×107) (cm2/s)

R2

B (B/T/X) T (B/T/X) X (B/T/X)

50/50/50 50/50/50 50/50/50

9.12 10.53 10.68

0.9658 0.9836 0.9511

Figure 6. Adsorption kinetics of tricomponent mixture of BTX compounds, on activated carbon, evaluating the different initial concentrations of carbon: (a) benzene, (b) toluene, and (c) o-xylene. 6465



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Table 7. Adsorption Isotherm Models for Experiments with Monocomponent and Multicomponent Systems isotherm model Langmuir (Sulaymon and Ahmed18) Freundlich (Ruthven14)

Langmuir−Freundlich (Hernández et al.1)

Radke−Prausnitz (Ruthven14)

monocomponent

qe =

qmax bLCe 1 + bLCe

qe = kFCe1/ nF

qe = qe =

multicomponent

qei =

(3a)

qe1 =

(4a)

qmLFbLFCe m 1 + bLFCe m

qei =

(5a)

K rCe 1+

K r 1 − Nr C Fr e

qei =

(6a)

qmax iCe1 n

1 + ∑i = 1 biCei

(3b)

a1Ce1b1+ b11 Ce1b11 + a12Ce2b12

qe2 =

a 2Ce2b2 + b22 Ce2b22 + a 21Ce1b21

(4b)

qoibiCei mi n

1 + ∑i = 1 biCei mi

(5b)

K iCei n

1 + ∑i = 1

Ki C 1 − Ni Fi ei

(6b)

Figure 7. Monocomponent adsorption isotherms for benzene, (a), toluene (b), and o-xylene (c) on activated carbon.

greater affinity for the active sites of the adsorbent occurs.18 This effect is greater after approximately 300 min (≅5 h). In Figure 6b the adsorption of toluene is evaluated and it can be verified that this competition for the active sites occurs earlier, at approximately 125 min (≅2 h). In Figure 6c, the adsorption of o-xylene is analyzed in the presence of benzene and toluene, where it can be observed that there is a small interference in the o-xylene curve. In the case of o-xylene the presence of benzene and toluene had little effect on the adsorption since it is possible that the o-xylene was adsorbed in the mesopores and macropores of the adsorbent while the benzene and toluene were adsorbed in the micropores. Thus, the o-xylene will be adsorbed simultaneously as if it were alone in the solution and the benzene and toluene compete for the active sites available in the adsorbent. For all of the mixtures there was a change in the adsorption rate and quantity adsorbed compared with the data for the monocomponent compounds, as can be seen in Table 6. The results for the fitting of the kinetics data show good linear correlation coefficients. 3.3. Adsorption Equilibrium Study. The quantity of BTX compounds adsorbed, qe (mg/g), for each test, is calculated through the following mass balance, considering that the

contaminant not found in solution is adsorbed on the solid phase.

qe =

V (Co − Ce) M

(2)

where V (L) is the initial solution volume, Co (g/L) is the initial solution concentration, Ce (g/L) is the solution concentration obtained at equilibrium, and M (g) is the adsorbent mass present in each experiment. All of the results obtained experimentally for the adsorption equilibrium of the BTX compounds were fitted by the leastsquares method using the software program STATISTICA 7.0, according to the Langmuir (eqs 3a and 3b), Freundlich (eqs 4a and 4b), Langmuir−Freundlich (eqs 5a and 5b), and Radke− Prausnitz (eqs 6a and 6b) models for the monocomponent and multicomponent systems, as shown in Table 7. All of the experimental results for the concentration curves versus time and the thermodynamic equilibrium curves were obtained in triplicate, plotting the average value for the concentrations. The confidence level adopted for the fitting of the models to the experimental data was 95%, and the software STATISTICA 7.0 was used. 6466



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separation factor, and nF, Freundlich model parameter14 were calculated. The adimensional separation factor, commonly called the Langmuir equilibrium parameter, RL, varied from 0.1198 to 0.1413. The parameter nF (Freundlich) revealed values of between 1 and 10. These results are given in Table 8, and they indicate favorable adsorption for all the adsorption tests. This favorable behavior indicated by the isotherms can also be observed in Figures 5−7. On evaluating the parameters shown in Table 8, for the adsorption of the BTX compounds, it can be verified that the maximum adsorption capacity is obtained for o-xylene, which has a larger structure, greater molar mass, and lower solubility in water. 3.3.2. Multicomponent Adsorption Isotherms. The adsorption isotherms of the experiments carried out with the tricomponent mixture of the BTX compounds on activated carbon are shown in Figure 8. In Figure 8 it can be verified that the models shown for the adsorption of the three components did not fit well the experimental data, especially when the concentration was high. This may have occurred due to the use of monocomponent data for the fitting of the tricomponent model. In Figure 9 a comparison between the isotherms for the monocomponent and tricomponent adsorption of the BTX compounds on activated carbon is shown. It can be verified that the presence of more than one contaminant negatively influences the adsorption process, since there is greater competition for the active site of the activated carbon. In the tricomponent mixture, benzene is the compound with the lowest adsorption onto the solid phase, followed by toluene and then o-xylene. Due to a higher affinity of o-xylene for the activated carbon, as shown in the monocomponent results, with o-xylene being a

3.3.1. Monocomponent Adsorption Isotherms. The adsorption isotherms for the monocomponent experiments were obtained for the BTX compounds individually. Parts a, b, and c of Figure 7 show the results for the adsorption isotherm fitted with the four models shown in Table 7 for benzene, toluene, and o-xylene, respectively. The model parameters for the BTX compounds were calculated and are given in Table 8. In order to evaluate the essential characteristics of the isotherm and to obtain its form, the parameter RL, adimensional Table 8. Values for Equilibrium Adsorption Isotherms for the BTX Compounds in Monocomponent Systems Langmuir qmax (mg/g) bL (L/g) RL R2 Freundlich nF kF R2 Langmuir−Freundlich qmLF (mg/g) bLF (L/g) m R2 Radke−Prausnitz Kr(m3/kg) Fr Nr R2

benzene

toluene

o-xylene

114.7721 0.049 0.1198 0.9921

125.0972 0.0497 0.1182 0.9896

141.3028 0.0405 0.1413 0.9962

1.4700 7.7100 0.9887

1.4900 9.6200 0.9932

1.4300 9.5800 0.9954

72.902 24 0.021 083 1.759 077 0.9986

75.211 67 0.019 801 1.863 521 0.9886

121.810 1 0.011 149 1.090 888 0.9988

4.368 445 2052.234 −0.810 452 0.9918

4.688 764 7068.454 −1.178 45 0.9901

5.240 231 314.7915 −0.225 624 0.9848

Figure 8. Adsorption isotherms obtained with tricomponent mixture of benzene, toluene, and o-xylene on activated carbon: (a) benzene, (b) toluene, and (c) o-xylene. 6467



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Figure 9. Comparison between monocomponent and tricomponent adsorption isotherms for BTX compounds on activated carbon: (a) benzene, (b) toluene, and (c) o-xylene.

less soluble and less polar compound with a greater molecular mass, it is the contaminant which interacts most with the solid phase, showing a higher adsorption capacity than the other compounds.

In the case of the ternary mixture, there was inhibition for each component in the mixture. Regarding the competition for the active site of adsorption, the most competitive of the BTX contaminants is o-xylene, due to its higher molecular mass and lower water solubility compared with the other compounds.

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4. CONCLUSIONS In this study the adsorption process of BTX compounds in aqueous solution using activated carbon as the adsorbent was investigated. Monocomponent and multicomponent kinetics and thermodynamic equilibrium studies were carried out in a batch system in order to study the competitiveness for the active site of adsorption. Few studies on the multicomponent adsorption of BTX compounds can be found in the literature. An understanding of the phenomena which occur during the tricomponent adsorption process is fundamental to the design of adsorption columns, particularly considering the competitiveness of the BTX compounds for the active sites. This study aims to fill this gap in the existing literature and will be useful to researchers studying the competition for active sites in binary and ternary mixtures. The adsorbent used in this study showed good adsorption capacity compared with others reported in the literature and could be employed in the removal of these toxic multicomponent compounds present in industrial effluents. The adsorption kinetics of individual BTX compounds, for a concentration of 50 mg/L, reaches equilibrium at around 7 h for benzene, 6 h for toluene, and 8 h for o-xylene. For the multicomponent system, the adsorption kinetics is rapid, with the homogeneous diffusion values found being very close to those obtained for the respective monocomponent systems. The adsorption isotherm models used for the monocomponent systems showed a good fit with the experimental results. The Langmuir isotherm provided the best fit with the monocomponent experimental data and was used to describe the equilibrium.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +55 (48) 3721 5231. Fax: +55 (48) 3721 9687. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was carried out with the financial support of ANP (Brazilian Petroleum Agency) through the Human Resources Program of ANP for the natural gas and petroleum sector: PRH09-ANP/MME/MCT.

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REFERENCES

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Removal of Mono- and Multicomponent BTX Compounds from

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