Fixed Bed Adsorption of Benzene, Toluene, and Xylene (BTX

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Fixed Bed Adsorption of BTX Contaminants from Monocomponent and Multicomponent Solutions using a Commercial Organoclay Letícia Franzo Lima, Júlia Resende de Andrade, Meuris Gurgel Carlos da Silva, and Melissa Gurgel Adeodato Vieira Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Industrial & Engineering Chemistry Research

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Fixed Bed Adsorption of BTX Contaminants from Monocomponent and Multicomponent Solutions using a Commercial Organoclay Letícia F. Lima, Júlia R. de Andrade, Meuris G. C. da Silva, Melissa G. A. Vieira*

Department of Processes and Products Design, School of Chemical Engineering, University of Campinas - Albert Einstein Avenue, 500, 13083-852, Campinas, São Paulo – Brazil *Corresponding author. Telephone/Fax: +55.19.3521.0358 / +55.19.3521.3965. E-mail address: [email protected] Benzene, toluene and xylene (BTX) are potential contaminants of groundwater and there is a need to improve current remediation techniques, such as adsorption. Different materials can be applied in this process, like organoclays that have affinity for organic compounds. The aim of this work was to study BTX removal from mono-, bi- and tricomponent solutions in a dynamic fixed bed system filled with organoclay. For monocomponent system, using a 1.6 mmol/L adsorbate concentration, the useful removal quantities were 0.012 mmol/g, 0.030 mmol/g, 0.140 mmol/g for benzene, toluene and p-xylene respectively. Based on the results, the affinity order was pxylene>toluene>benzene. The multicomponent tests presented similar affinity tendency towards the organoclay. The mathematical model of Yan described the majority of the experimental breakthrough curves better than the model of Thomas. Partition was identified as the prevailing mechanism in BTX uptake and the main adsorption sites were associated with the nano-sized organic phases. Keywords: Adsorption; Organoclay; BTX compounds; Fixed bed.

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1.

INTRODUCTION Benzene, toluene and xylene (BTX) are typical volatile organic compounds

(VOCs) that are constituents of petroleum and industrial solvents widely employed in producing rubbers, lubricants, dyes, detergents, and drugs.1 BTX are some of the most common groundwater petroleum contaminants, especially due to leakages from underground oil storage tanks and discharge from refineries.2 BTX compounds are well known for their chronic toxicity and mutagenic potential.3 Benzene is among the most toxic compounds and causes in low concentrations haematological abnormalities, including leukemia. Acute exposure to high concentrations of benzene mainly affects the central nervous system.4 In general, two kinds of technologies are applicable for the control of VOCs: destruction (e.g. catalytic oxidation,5 thermal oxidation,6 and biofiltration7) and recovery (e.g. condensation,8 adsorption,9 membrane separation10). Adsorption stands out among these technologies, because it offers flexibility, low energy requirements and low operating costs.11 Activated carbon has been extensively used as adsorbent for BTX removal from water and wastewater; however, high price and regeneration cost of this material limit its further application.12-14 Clay minerals are considered effective alternative adsorbents for the removal of organic contaminants, due to high surface area and reactivity.15 Promising results have been obtained for BTX removal using different types of clay minerals.9, 16 Natural clay minerals typically possess layered structure with negative charge that is compensated by inorganic cations. The strong hydration of these cations creates a hydrophilic environment on raw clays, making them ineffective adsorbents for the removal of organic compounds.9,

17

The substitution of the exchangeable inorganic

cations by hydrophobic quaternary ammonium cations creates organoclay derivatives with organophilic properties.18 There are several studies on the removal of hydrocarbons from water and wastewater using smectite organoclays19 as well as bentonite organoclays.14, 17, 20-22 Process parameters for the design of adsorption systems are used for modelling the system and predicting results under a wide range of operating conditions. Key parameters, such as isotherm constants and mass transfer coefficients, are established by conducting experimental batch studies and subsequent application of mathematical models. The correlation of isotherm models, such as Langmuir and Freundlich, to the


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experimental adsorption data is used to assess suitability of an adsorbent for the adsorption system.23 In the case of multicomponent adsorption, the design of the process has to consider the interactions between the compounds of the mixture and several factors related to physical and chemical nature of both adsorbate and adsorbent. It is essential to evaluate these factors for the assessment of appropriate conditions in fixed bed for multicomponent adsorption in continuous process, indicated as secondary treatment of large volumes of low level polluting liquids.19 Considering that industrial effluents characteristically contain a combination of toxic compounds, it is of great importance the measurement of multicomponent adsorption equilibrium. However, due to its rather complex nature, only quite a few studies report multicomponent adsorption, especially using alternative adsorbents, such as organoclays. Therefore, the present work aims the investigation of mono- and multicomponent BTX adsorption onto organoclay. In this study, we examined the removal of BTX compounds by adsorption process through dynamic fixed bed system filled with the commercial organoclay Spectrogel (Type C). The adsorption tests were performed using mono-, bi- and tricomponent aqueous solutions in order to verify the competition of the BTX contaminants for the active sites of the adsorbent. In addition, the mathematical models of Thomas24 and Yan et al.25 were applied to describe the mono-, bi- and tricomponent adsorption behavior in the fixed bed. 2.

MATERIALS AND METHODS 2.1.

Adsorbent

The adsorbent used in this study was the commercial organoclay Spectrogel (Type C), generously provided by SpectroChem Company from Brazil. As informed by the manufacturers, this organoclay is of bentonite type and functionalized by the surfactant dialkyl dimethyl ammonium. The material was grinded and sieved to obtain 0.655 mm average particle size (fraction between 24 and 28 Tyler mesh sieves). Previously, Stofela et al.26 carried out the characterization of the commercial organoclay and reported that the surface area is about 2.7 m²/g and that the solid may be classified as non-porous or macroporous. The bulk density and true density were found to be 1.0474 and 1.6516 g/cm3, respectively, while the organoclay’s porosity was estimated as 36.58%. The authors verified the presence of micro- and mesopores, which volumes were estimated as 0.284 and 2.298 cm3/g, and the majority of pores has the



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diameter of 0.2 mm. In addition, Stofela and Vieira27 found out the d001 basal spacing to be 21.74 Å by X-ray diffraction (XRD) and the analysis of energy-dispersive X-ray spectroscopy (EDS) showed that the approximate chemical composition of this commercial organoclay is: O (43.08%); C (31.44%); Si (13.80%); Al (5.06%); Na (2.02%); Fe (1.71 %); Cl (1.44 %); Mg (0.90%); S (0.21%); Ca (0.16%) and Ti (0.15%). The following vibrational stretching of were provided by infrared spectroscopy with Fourier transform (FT-IR): O-H; C-H; H-O-H; CH2; Si-O; Si-O-Si and Si-O-Al.

According to Zhu et al.28,

29

, clays can be classified in type I or type II,

depending on their synthesizing method. While type I are synthetized by the intercalation of small organic cations, type II are obtained by the modification with cationic surfactants, such as dialkyl dimethyl ammonium used to obtain the organoclay Spectrogel of this study. Considering that type II organoclays can assume different interlayer arrangements, the d basal spacing of Spectrogel reaching over 20 Å implies in the pseudotrimolecular layer arrangement.30 2.2.

Adsorbate solutions

Benzene, toluene and p-xylene were purchased from Sigma-Aldrich, Brazil (purity of 99.9%) and the solutions were prepared using Milli-Q water (Millipore, Brazil). Given that p-xylene is more commercially sought than m- or o-xylenes,31 this isomer was employed in the present work, as well as in previous studies regarding kinetic and equilibrium of BTX adsorption onto the organoclay.32, 33 As presented in Table 1, the initial concentrations evaluated in monocomponent tests were 0.6, 1.2 and 1.6 mmol/L, similarly to Stofela et al.26 and Stofela et al.32 The maximum concentration investigated was chosen based on the water solubility of pxylene (175 mg/L ~ 1.6 mmol/L), which is the least soluble BTX compound.34 For multicomponent experiments, equimolar solutions were prepared with fixed initial concentration of 0.9 mmol/L. This value was selected because it is in the range of the calibration curve for all BTX compounds in High Performance Liquid Chromatography (HPLC), used for chemical quantification.


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Table 1. Initial concentrations and BTX molar fractions used in mono-, bi- and tricomponent continuous adsorption tests. Initial concentration Molar Fraction Composition (mmol/L) Benzene Toluene p-Xylene Benzene (B) 0.6; 1.2; 1.6 1 Monocomponent Toluene (T) 0.6; 1.2; 1.6 1 p-Xylene (X) 0.6; 1.2; 1.6 1 B+T 0.9 ½ ½ Bicomponent B+X 0.9 ½ ½ T+X 0.9 ½ ½ Tricomponent B+T+X 0.9 ⅓ ⅓ ⅓ 2.3.

Column system assembly

Continuous adsorption studies were performed in a column which properties are listed in Table 2.

Table 2. Properties of fixed bed packed with organoclay. Property Value Bed height (cm) 6.5 Bed intern diameter (cm) 0.65 Bed volume (cm³) 2.16 Packed density (g/cm³) 0.93 Mass of adsorbent (g) 2.0 Bed porosity (-) 0.44 Six liters of stock BTX solution was continuously agitated, to assure homogeneity, prior to be fed to the bottom of the bed by a peristaltic pump at constant up flow rate of 5 mL/min (approximately 15.07 mL/min.cm²). Previously, Lima et al.35 and Lima et al.36 studied the adsorption fluid dynamics of toluene and p-xylene in fixed bed packed with the same commercial organoclay. The flow rates of 5; 10 and 20 mL/min were tested and lower values of mass transfer zone heights were verified at 5 mL/min, which was then selected as the operating flow rate of the present work. During the tests, aliquots of 2 mL were periodically collected at the entrance and exit of the fixed bed, being the outlet solutions filtrated using 0.22 µm Millex hydrophilic membrane filters (Millipore, Brazil). The inlet and outlet concentrations of the BTX compounds were determined by HPLC connected to a UV/vis detector (model SPD-10AV) and a Phenomenex C18 column of 150 mm, with an internal diameter of 4.6 mm and particle size of 5 µm. The BTX compounds were identified at a wavelength of 206 nm. The mobile phase was composed of 28% acetonitrile (J. T. Baker, Brazil), 35% methanol (J. T. Baker, Brazil) and 37% Milli-Q water at 1 mL/min flow rate. The



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samples were immediately analyzed without storage and the injected volume of each filtrated sample in HPLC was 30 µL. 2.4.

Column system analysis

Efficiency and mass transfer parameters were calculated to evaluate the adsorption of the BTX compounds in the fixed bed column. The capacity of BTX

adsorption was determined by the total amount removed, (mmol/g), and by the useful amount removed until the breakthrough time, (mmol/g), calculated by Equations 1

and 2, respectively. The breakthrough time, (min), is the time in which the outlet concentration reaches 5% of the feed solution concentration.37 = =

. 1 −

. 1 −

(1)

(2)

Where is the adsorbate initial concentration (mmol/L); is the adsorbate

concentration at time (mmol/L); is the flow rate (L/min); is the adsorbent

mass (g).

Another important parameter is the mass transfer zone height, ℎ (cm), which

is related to the effects of mass transfer. The lower the ℎ value, the closer the system

is to ideality, indicating greater removal process efficiency. The ℎ parameter can be calculated by Equation 3, where ℎ (cm) is the bed height.38 ℎ = 1 −

.ℎ

(3)

The total removal percentage of the fixed bed, % (%), is calculated as the

ratio between the total amount removed ( ) and the total amount of adsorbate fed to the

bed up to its saturation point, (mmol). On the other hand, the useful removal percentage, % (%), is related to the amount removed up to the bed breakthrough

point. These parameters are presented in Equations 4 and 5, where (g) is the adsorbent mass.

% =

. . 100


(4)

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% =

. . 100

(5)

An economic parameter of great importance to separation processes is selectivity, α (-). In multicomponent adsorption, it indicates how the adsorption of one component is favored in relation to another. The selectivity can be defined by Equation 6, in which X and Y are, respectively, the component molar fraction in solid and fluid phases at equilibrium. The subscriptions 1 and 2 identify the mixture compounds being compared.39

!"

2.5.

=

#! $# " %! $% "

(6)

Mathematical modelling

The mathematical models of Thomas24 and Yan et al.25 were employed to describe the breakthrough curves of the fixed bed BTX adsorption process. Equation 7 describes Thomas model: 1 = 1 + exp *+ . . − + . . ,

(7)

Where and are, respectively, the outlet and inlet adsorbate concentration in the

fixed bed (mg/mL); is the operating time (min); + is the rate constant of adsorption

(mL/mg/min); is the maximum solid-phase solute concentration (mg/g); is the amount of adsorbent in column (g) and is the flow rate (mL/min).

The error that arises from Thomas model utilization, particularly at very low and

high operation times, can be reduced by the modified dose-response model, proposed by Yan et al.25 This model is formulated by Equation 8: =1−

1

./ .

1 + . . -

(8)

Where 01 is the Yan model constant; 1 is the maximum adsorption capacity estimated

(mg/g), whereas the other parameters are as defined above.

The models of Thomas and Yan were adjusted to the experimental results of BTX continuous adsorption using the software Maple 17. The best fitting models were



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identified based on the values of correlation coefficient (R²), adjusted correlation coefficient (R²adj), and Akaike Information Criterion (AIC) presented in Equations 9, 10 and 11, respectively. The best correlations are those with the highest values of R² and R²adj and the lowest values for AIC. " = 1 −

∑9 :! * $

345

∑9 :! * $

²7=> = 1 −

9?!

9?5

345

6786 "

,

;;;;;; " − $ ,

(9)

. (1 − " )

9

0B = C. ln FG *$ :!

− $

345

(10) − $

6786 "

, H + 2. J

(11)

345 6786 Where C is the number of data points, $ is the experimental value, $ is

;;;;;; the value predicted by the model, $ is the average of the experimental values and J

is the number of adjustable parameters. 3.

RESULT AND DISCUSSION 3.1.

Effect of inlet BTX concentration on monocomponent adsorption

At constant flow rate of 5 mL/min, the performance of the fixed bed was evaluated for three different BTX inlet concentrations (0.6; 1.2 and 1.6 mmol/L) in monocomponent tests. Figure 1 shows the breakthrough curves obtained for benzene, toluene and pxylene. The curves are expressed in terms of normalized concentration, that is the ratio between outlet and inlet concentrations of the BTX compound (/ ), as a function of time. Table 3 presents the efficiency and mass transfer parameters for each investigated condition.


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9 Figure 1. Effect of inlet concentration on column breakthrough curves for monocomponent adsorption of: (a) benzene, (b) toluene and (c) p-xylene (L = 5 mL/min). 1.0

1.0

0.6 mmol/L 1.2 mmol/L 1.6 mmol/L

0.9 0.8


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Time (min)

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0.5 0.4 0.3 0.2 0.1 0.0 0

200

400

600

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1200

1400

Time (min)

(c)

Table 3. Effect of inlet BTX concentration on the efficiency and mass transfer parameters for monocomponent adsorption onto commercial organoclay. ℎ % % (mmol/L) (min) (mmol/g) (mmol/g) (cm) (%) (%) 0.6 73.419 0.160 0.027 0.500 8.720 22.550 Benzene 1.2 14.446 0.760 0.018 2.740 25.610 43.010 1.6 11.309 0.430 0.012 4.490 31.250 70.390 0.6 11.580 0.330 0.009 2.905 15.270 45.900 Toluene 1.2 4.806 0.490 0.008 2.901 11.990 42.630 1.6 16.480 0.365 0.030 2.970 11.350 50.660 0.6 14.182 0.390 0.012 4.200 27.740 58.040 p-Xylene 1.2 248.508 0.840 0.250 0.330 31.920 33.060 1.6 56.678 1.450 0.140 3.800 38.300 63.600 From Figure 1, it can be noticed that the breakthrough curves for any of the BTX compounds did not reach adsorption saturation, which would occur only if the level / = 1 was reached. In the same way, the saturation of the fixed bed was not verified

by da Luz et al.3 for BTX monocomponent adsorption onto activated carbon. In the



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present work, extremely high periods would be required for saturation attainment, indicating that adsorption occurs very slowly in certain regions of the fixed bed. Tests longer than 1,132 min were prohibitive due to the elevated volatility of the BTX compounds, which could interfere in the final adsorption results. According to Table 3, highest total amounts adsorbed ( ) were obtained at the

intermediate inlet concentration of 1.2 mmol/L for benzene and toluene; in the case of

p-xylene, it happened at the highest inlet concentration. The fact that higher inlet concentrations did not imply in the highest amounts of BTX adsorbed can be associated to the non-saturation of the fixed bed column. Consequently, the number of adsorption sites occupied in each assay was not the same for all the compounds, leading to distinct adsorption behavior. It is important to say that the same experimental procedure to avoid losses due volatilization was adopted in all assays (sealing the stock solution flask). Therefore, the obtained values may not be related to volatilization fenomena.

No regular trend was verified for the breakthrough time ( ) with varying inlet

concentrations. Nevertheless, higher percentages of useful BTX removal were obtained for the higher 1.6 mmol/L inlet concentration. In this condition, the useful amounts removed from monocomponent solutions were of 0.012, 0.030 and 0.140 mmol/g of benzene, toluene and p-xylene, respectively. It means that the removal capacity of the organoclay follows the order: p-xylene > toluene > benzene. Similarly, Carvalho et al.19 investigated the adsorption of BTX compounds, as well as ethylbenzene and phenol, onto smectite organoclay and the following removal efficiency order was obtained for single-solute batch system: ethylbenzene > p-xylene > toluene > benzene > phenol. The differences in the affinities between the BTX contaminants and the organoclay material can be related to the physicochemical properties of the compounds (Table 4), such as structure, molecular weight, solubility, and hydrophobicity.

Table 4. Physicochemical properties of benzene, toluene and xylene. Compound Benzene Toluene Xylene

Molecular weight34 (g/mol) 78.11 92.14 106.17

Solubility in water at 25 °C34 (mg/L) 1780 515 175

Boiling point34 (°C) 80.1 110.8 144.4


log +OP (-) 2.13 2.73 3.15

9, 40

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11 From Table 4, the greater molecular weight of p-xylene, in conjunction with its

lower solubility and hydrophilicity (based on log +OP ), contribute for this BTX contaminant to be the one that interacts most with the organoclay.41 Besides that, p-

xylene presents two methyl groups on the aromatic ring that promote stronger dispersion interaction and increase the affinity for the solid phase.42 The models of Thomas and Yan were applied to the experimental breakthrough curves of Figure 1. Figure 2 shows the curves of these models for the condition of 1.2 mmol/L inlet concentration of benzene, toluene and p-xylene and it can be noted that only p-xylene satisfactorily fit the models for the entire evaluated region. In the case of benzene and toluene, this happens solely for Yan model until the times of 200 and 600 min, respectively. Table 5 lists the values of the fitted parameters, and of R², R²adj and AIC.

Figure 2. Experimental and predicted monocomponent breakthrough curves of: (a) benzene, (b) toluene and (c) p-xylene (QR =1.2 mmol/L; L=5 mL/min). 0.8

0.5

Experimental Yan Thomas

0.4


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(a)

(b) 0.5


0.4

0.3

C/Co

C/Co

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.2

0.1

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(c)


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Table 5. Effect of inlet concentration in Yan and Thomas model parameters for BTX monocomponent breakthrough curves. Yan Model Inlet BTX " 7=> concentration 1 (mg/g) 01 " 0B Compound (mmol/L) 0.6 0.6902 1.9248 0.9806 0.9589 -101.5540 Benzene 1.2 0.6629 1.9875 0.8750 0.7526 -59.8331 1.6 0.6933 0.4889 0.9382 0.8744 -58.1337 0.6 0.8897 0.5282 0.9539 0.9025 -54.9323 Toluene 1.2 0.3863 1.9382 0.9773 0.9525 -98.2529 1.6 0.5374 0.6261 0.9503 0.8944 -47.6324 0.6 1.4588 0.5099 0.9662 0.9287 -58.4261 p-Xylene 1.2 1.8141 1.7404 0.9382 0.8735 -78.5804 1.6 1.0532 1.0155 0.9630 0.9238 -68.4052 Thomas Model Inlet BTX + " 7=> concentration 0B " Compound (mL/mg/min) (mg/g) (mmol/L) 0.6 11.5839 0.5929 0.7712 0.5677 -60.9560 Benzene 1.2 6.7319 0.9174 0.9174 0.8240 -69.2174 1.6 6.5769 0.6443 0.8621 0.7309 -39.5185 0.6 13.8841 0.4464 0.8057 0.6198 -36.3323 Toluene 1.2 4.5697 0.9193 0.7034 0.4666 -47.7824 1.6 3.9539 0.8022 0.5772 0.2726 -19.5087 0.6 13.6197 0.4941 0.9394 0.8742 -51.6236 p-Xylene 1.2 7.1533 1.3658 0.9765 0.9509 -101.5070 1.6 5.2127 1.0852 0.9533 0.9043 -64.5611 The rate constant of Thomas model (+ ) characterizes the rate of solute transfer

from the liquid to the solid phase. From Table 5, + values diminished with increase in the inlet concentration, indicating a greater mass transfer resistance. As can be verified in Equation 7 of Thomas model, it is especially deficient when experimental time is zero since it has a fixed value, contrary to real conditions.43 The error from Thomas

model is minimized in Yan model, which was able to describe the entire experimental breakthrough curve of p-xylene, of benzene until 200 min and of toluene until 600 min for 1.2 mmol/L inlet concentration, as presented in Figure 2. Regarding Thomas model, although being one of the most general and widely used for column performance prediction, it did not well represent the experimental data of the present work, with the exception of p-xylene assay for =1.2 mmol/L. In this

case, the high R²adj value of 0.9509 indicated the satisfactory fit of Thomas model. In


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13 the assay of benzene for =1.2 mmol/L, even though Thomas model represented the

breakthrough curve better than Yan model, it may not be considered satisfactory, since R²adj value was rather low (0.8240). For 1.6 mmol/L inlet concentration (maximum evaluated concentration), the values of predicted BTX uptake of Yan model (1 ) followed the order p-xylene > toluene > benzene, similarly to the experimental observations. 3.2.

Competition analysis on multicomponent BTX adsorption

Multicomponent adsorption of BTX compounds in fixed bed was studied to verify the effects of competition for the active sites of the organoclay. Figure 3 presents the breakthrough curves for bicomponent and tricomponent systems and Table 6 describes the parameters of efficiency and mass transfer obtained.

Figure 3. Breakthrough curves for equimolar BTX solutions: (a) benzene and toluene, (b) toluene and p-xylene, (c) benzene and p-xylene, and (d) benzene, toluene and p-xylene (L = 5 mL/min; QR = 0.9 mmol/L). 1.0

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Toluene Xylene

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(d)


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Table 6. Efficiency and mass transfer parameters for multicomponent adsorption onto commercial organoclay. ℎ % % System Component (min) (mmol/g) (mmol/g) (cm) (%) (%) Benzene 2.127 0.158 0.002 4.097 13.521 62.854 B+T Toluene 4.309 0.149 0.001 2.788 14.338 20.931 Toluene 10.789 0.247 0.008 4.262 22.002 67.830 T+X p-Xylene 119.73 0.496 0.131 1.552 25.351 47.792 Benzene 17.211 0.264 0.023 4.241 10.211 43.691 B+X p-Xylene 12.999 0.694 0.015 6.276 36.657 48.282 Benzene 4.355 0.140 0.002 3.423 5.461 14.310 B+T+X Toluene 61.294 0.155 0.025 1.610 18.109 40.289 p-Xylene 65.162 0.163 0.023 1.106 14.325 33.089 Figure 3 reveals the competition of BTX components for the available sites of the organoclay. It happens that, at the begging of multicomponent adsorption, the active sites of the organoclay are taken by both strongly and weakly adsorbed components without restrictions. In the course of time, the bulk fluid carries the weakly adsorbed components, which firstly take the active sites of the final part of the fixed bed. Though, these sites are displaced by the strongly compounds that have adsorption favored instead of the weakly ones. Therefore, the measured concentration of weakly adsorbed components is higher within the fixed bed.23 On the basis of Figure 3a, the concentration of benzene leaving the column is higher than of toluene, due to the higher affinity of toluene for the adsorbent. Therefore, the adsorption of toluene is favored in the presence of benzene. On the other hand, as shown in Figure 3b, the adsorption of toluene is hindered by the competition in the presence of p-xylene, which has greater affinity for the stationary phase than toluene. In Figure 3c, the adsorption of benzene and p-xylene is compared and it can be noted that the competition favored p-xylene removal. In the case of the ternary system, the competition in the presence of benzene and

toluene favors p-xylene adsorption, as presented in Figure 3d. Furthermore, the

values of Table 6 demonstrate that the BTX adsorption capacity onto the commercial organoclay follows the order: p-xylene > toluene > benzene, what is in agreement with the results from the monocomponent study (Section 3.1). Luz et al.44 employed activated carbon for BTX multicomponent adsorption in fixed bed and also reported xylene as being the most competitive compound for the available active sites of the adsorbent and that it displaces the other BTX contaminants. Stofela et al.33 performed batch studies on binary BTX adsorption onto the same


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organoclay of the present work and verified that toluene was preferentially adsorbed in B+T system and p-xylene in B+X and T+X systems. Selectivity coefficients were estimated for the binary adsorption tests and are presented in Table 7. It is important to mention that the molar fraction of each

component in the solid phase was calculated from values; while the molar fraction in

the fluid phase at equilibrium was determined from the concentration of the solution at the outlet of the fixed bed.

Table 7. Selectivity of the adsorbent for BTX compounds in binary tests. System B+T T+X B+X Benzene Toluene Toluene p-Xylene Benzene p-Xylene Compound (1) (2) (1) (2) (1) (2) 0.6933 1.4422 0.2488 4.0179 0.7714 1.2962 (-) !" The higher the selectivity coefficient, the higher the preference of the organoclay for a particular adsorbate. Therefore, the obtained selectivity values corroborate the affinity order verified for binary experiments, as shown in Figures 3a, 3b and 3c. In B+T system, toluene is preferentially adsorbed in comparison with benzene, and pxylene is more competitive for the active sites of the organoclay than toluene (T+X system) and benzene (B+X). The models of Yan and Thomas were applied to the experimental data of multicomponent BTX adsorption and the predicted curves are shown in Figure 4 for the binary tests and in Figure 5 for the ternary test. Table 8 displays the parameters of the adjusted models, along with R², R²adj and AIC values.



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16 Figure 4. Experimental and predicted bicomponent breakthrough curves of (a) benzene and toluene, (b) toluene and p-xylene, and (c) benzene and p-xylene adsorption onto organoclay. 1.0

Benzene

0.8 0.6 0.4

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0.0 1.0

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Thomas

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17 Figure 5. Experimental and predicted tricomponent breakthrough curves of benzene, toluene and p-xylene adsorption onto organoclay. 0.6

Benzene

0.4 0.2 0.0 0.6

Toluene

0.4

C/Co

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.2 0.0 0.6

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Thomas

Table 8. Yan and Thomas model parameters for BTX binary and ternary breakthrough curves. Yan Model " 7=> System Component 1 (mg/g) 01 " 0B Benzene 0.5221 0.0011 0.9828 0.9639 -90.3110 B+T Toluene 0.6947 0.0047 0.9709 0.9393 -89.6938 Toluene 1.0501 0.0015 0.9901 0.9792 -86.0850 T+X p-Xylene 1.2417 0.0033 0.9904 0.9798 -99.0670 Benzene 0.7084 0.0007 0.9370 0.8729 -63.5350 B+X p-Xylene 1.2746 0.0055 0.8760 0.7577 -77.2890 Benzene 0.6489 0.0022 0.9836 0.9657 -99.9300 B+T+X Toluene 0.8616 0.0033 0.9556 0.9086 -85.7070 p-Xylene 0.8506 0.0047 0.9586 0.9146 -95.5110 Thomas Model + " 7=> System Component (mg/g) 0B " (mL/mg/min) Benzene 20.1842 0.1244 0.6653 0.4097 -32.5560 B+T Toluene 17.3399 0.2971 0.8279 0.6669 -57.7810 Toluene 25.5679 0.1675 0.8644 0.7324 -36.0850 B+X p-Xylene 20.1110 0.3198 0.8945 0.7885 -54.3000 Benzene 26.4742 0.0906 0.6841 0.4459 -19.5670 T+X p-Xylene 19.6654 0.4079 0.9425 0.8837 -101.9300 Benzene 31.7073 0.1369 0.8270 0.6674 -51.2610 B+T+X Toluene 28.8458 0.2029 0.7996 0.6204 -54.5030 p-Xylene 23.9752 0.2724 0.8095 0.6371 -64.2070



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18 Based on the values of R², R²adj and AIC, it can be said that the behavior of multicomponent BTX adsorption processes was better represented by Yan model rather than Thomas model. Values of R²adj up to 0.9798 were found, even though Yan model has been developed for monocomponent systems. The previous observations on the BTX preferential removal by the organoclay were confirmed by the higher values of maximum adsorption capacity (1 ) obtained for toluene (B+T system) and for p-xylene

(T+X and B+X systems). In the case of ternary system, the highest 1 was verified for

p-xylene, followed by toluene and benzene. 3.3.

Comparison between monocomponent and multicomponent tests

Figure 6 shows a column graph with comparative values of total amounts removed ( ) of each BTX compound from monocomponent ( = 0.6 mmol/L) and multicomponent solutions.

Figure 6. Comparison of the total quantity removed of benzene, toluene and pxylene from monocomponent (QR = 0.6 mmol/L) and multicomponent solutions. 1.4

P-Xylene Toluene Benzene

1.3 1.2 1.1 1.0 0.9

qt (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

B

T

X

BT

TX

BX

BTX

It can be noticed that values obtained for toluene are greater in single-

component than in multicomponent assays, due to the competition for active sites in multicomponent systems that can cause a decrease in the total quantity removed of the compound. Conversely, the adsorption of p-xylene was greatly favored in the presence of benzene or toluene, being that the amount of p-xylene removed in bicomponent tests was superior to the monocomponent. In the case of ternary adsorption, all the BTX contaminants had lower values in comparison with those of monocomponent tests.

The different behavior of the three different compounds in mono- and

multicomponent can also be observed in Figure 7, from which it is possible to compare


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19 the breakthrough curves of the BTX contaminants adsorbed from single, binary and ternary solutions. Figure 7. Comparison of breakthrough curves for monocomponent (QR = 0.6 mmol/L) and multicomponent (equimolar, QR = 0.9 mmol/L) adsorption of: (a) benzene, (b) toluene and (c) p-xylene. 1.0

1.0

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C/Co

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5 0.4 0.3 0.2 0.1 0.0 0

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Time (min) Mono-component Binary X+T

Binary X+B Ternary X+B+T

(c)

The analysis of Figure 7 confirms the competition between compounds when present in solution as mixtures and evidences that different combinations of BTX compounds produce different effects on the adsorption of each contaminant. From Figure 7a, it is clear that benzene is the compound with adsorption most influenced by the presence of others, since its breakthrough curves for mono- and multicomponent systems are quite distinct. Regarding toluene adsorption, Figure 7b reveals that the majority of breakthrough curves are very close to each other, indicating that the adsorption of this



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compound is not strongly influenced in mixtures. Only for the binary adsorption of toluene and p-xylene, this latter caused significant modifications in the breakthrough behavior of toluene. Figure 7c shows that p-xylene’s breakthrough curves are quite similar and concentrated in the same graphical region, indicating that this compound is the BTX contaminant with the greatest affinity for the organoclay. As pointed out in previous section, the behavior of the breakthrough curves can be related to the physicochemical properties of the BTX molecules. Having the highest molecular weight (larger molecular size), p-xylene does not suffer negative influence from the presence of other compounds, since it occupies active sites of different sizes than benzene and toluene molecules do. Thus, these last two compounds compete for the same remaining active sites. As demonstrated in single adsorption studies, toluene has higher affinity for the solid phase compared to benzene, particularly because of the side chain (methyl group) on its aromatic ring.42 This explains why toluene adsorption remains almost the same for mono- and multicomponent assays, being only expressively influenced in the case of binary mixture with p-xylene. 3.4.

Potential interaction mechanism between BTX and the organoclay

In the present work, the word adsorption was employed to describe the process of BTX removal onto the organoclay. Chiou45 affirms that surface adsorption, partition, surface precipitation, and structural incorporation are the main related mechanisms for contaminants uptake. The adsorption behavior onto type II organoclays, such as Spectrogel of this study, is quite complex, since they generally present small surface area and pore volume and interlayers that can expand in the presence of water. According to Zhu et al.28, the uptake of hydrophobic organic contaminants (HOCs) by type II organoclays is controlled by multiple mechanisms, but the typical dominant mechanism in is partition, in which the contaminants do not concentrate onto the surface of the adsorbent, but penetrate into the entire network of a bulk phase. The main sorption sites for HOCs uptake through partition mechanism are considered as the nano-sized organic phases that can be formed by the cationic surfactant aggregates on type II organoclays. Intrinsically, linear isotherms are generally obtained over relatively wide range of solute concentrations for the HOCs removal by type II organoclays. Indeed, linear isotherms have been reported in previous studies employing Spectrogel organoclay as BTX adsorbent.26, 32, 33 Therefore, in this work, partition can be regarded as the prevailing mechanism in BTX uptake and the main adsorption sites can be


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associated with the nano-sized organic phases formed by the surfactant dialkyl dimethyl ammonium used in the functionalization of the organoclay.

4.

CONCLUSION Column investigations on mono- and multicomponent BTX adsorption were carried

out using the commercial organoclay Spectrogel type C. In monocomponent studies, for 1.6 mmol/L inlet concentration, the useful amounts removed of benzene, toluene and xylene were of 0.012; 0.03 and 0.14 mmol/g, respectively. Therefore, removal capacity of the organoclay follows the order p-xylene > toluene > benzene, which could be associated to the physicochemical properties of the BTX contaminants. Greater molecular weight lower solubility and hydrophilicity makes p-xylene the compound that interacts most with the organoclay. Concerning percentage removal, the highest values obtained in monocomponent systems were of 31.25 % for benzene, 15.27% for toluene and 38.30% for p-xylene. The majority of the experimental breakthrough curves presented better modeling adequacy to Yan model, as was already expected since it was originally designed to address limitations of Thomas model. The obtained R² values of Yan model were up to 0.9806 in monocomponent assays. Multicomponent assays evidenced the competition of BTX compounds for the available sites of the organoclay and revealed that different combinations produce different effects on the adsorption of each contaminant. The highest percentage removal value was of 36.657% obtained for p-xylene removal in the presence of benzene. Toluene was better removed than benzene in B+T binary system; while in B+X and T+X systems, p-xylene was shown to have greater affinity for the organoclay. The estimated selectivity values corroborated these results. Ternary adsorption studies indicated the compounds are preferentially adsorbed following the order: p-xylene > toluene > benzene. The multicomponent experimental data was also best represented by Yan model, with R² values of up to 0.99. ACKNOWLEDGEMENTS The authors thank the SpectroChem Company for providing the organoclay, CNPq (Proc. 300986/2013-0) and CAPES for financial support.



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FIGURE CAPTIONS Figure 1. Effect of inlet concentration on column breakthrough curves for monocomponent adsorption of: (a) benzene, (b) toluene and (c) p-xylene (L = 5 mL/min). Figure 2. Experimental and predicted monocomponent breakthrough curves of: (a) benzene, (b) toluene and (c) p-xylene (QR =1.2 mmol/L; L = 5 mL/min). Figure 3. Breakthrough curves for equimolar BTX solutions: (a) benzene and toluene, (b) toluene and p-xylene, (c) benzene and p-xylene, and (d) benzene, toluene and pxylene (L = 5 mL/min; QR = 0.9 mmol/L). Figure 4. Experimental and predicted bicomponent breakthrough curves of (a) benzene and toluene, (b) toluene and p-xylene, and (c) benzene and p-xylene adsorption onto organoclay. Figure 5. Experimental and predicted tricomponent breakthrough curves of benzene, toluene and p-xylene adsorption onto organoclay. Figure 6. Comparison of the total quantity removed of benzene, toluene and p-xylene from monocomponent (QR = 0.6 mmol/L) and multicomponent solutions. Figure 7. Comparison of breakthrough curves for monocomponent (QR = 0.6 mmol/L) and multicomponent (equimolar, QR = 0.9 mmol/L) adsorption of: (a) benzene, (b) toluene and (c) p-xylene.



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GRAPHICAL ABSTRACT


Fixed Bed Adsorption of Benzene, Toluene, and Xylene (BTX

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