Evaluation of BTEX and phenol removal from aqueous solution by multi-solute adsorption onto smectite organoclay

Evaluation of BTEX and phenol removal from aqueous solution by multi-solute adsorption onto smectite organoclay

Journal of Hazardous Materials 239–240 (2012) 95–101 Contents lists available at SciVerse ScienceDirect Journal of Hazardous Materials journal homep...

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Journal of Hazardous Materials 239–240 (2012) 95–101

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Evaluation of BTEX and phenol removal from aqueous solution by multi-solute adsorption onto smectite organoclay M.N. Carvalho, M. da Motta ∗ , M. Benachour, D.C.S. Sales, C.A.M. Abreu Departamento de Engenharia Química, Universidade Federal de Pernambuco, 50.740-521 Recife, PE, Brazil

h i g h l i g h t s     

The removal process of BTEX and phenol was evaluated. Smectite clay was converted to organophilic adsorbent by Na2 CO3 and HDTMA treatment. BTEX were removed from the liquid phase with approximately the same specific rates. Phenol was an exception, adsorbing slowly. The removal efficiencies in the multicomponent system was: E > X > T > P ≈ B.

a r t i c l e

i n f o

Article history: Received 11 April 2012 Received in revised form 28 July 2012 Accepted 31 July 2012 Available online 29 August 2012 Keywords: Adsorption Multi-solute Organoclays Wastewater

a b s t r a c t The removal process of BTEX and phenol was evaluated. The smectite organoclay for single-solute system reached removal was evaluated by adsorption on smectite organoclay adsorbent by kinetic and equilibrium efficiencies between 55 and 90% while was reached between 30 and 90% for multi-solute system at 297 K and pH 9. The Langmuir–Freundlich model was used to fit the experimental data with correlation coefficient between 0.98 and 0.99 providing kinetic and equilibrium parameter values. Phenol and ethylbenzene presented high maximum adsorbed amount, 8.28 and 6.67 mg/g, respectively, compared to the other compounds for single-solute. Toluene and p-xylene presented high values of adsorption constant which indicates a high adsorption affinity of compounds to organoclay surface and high binding energy of adsorption. Phenol presented low kinetic adsorption constant value indicating slow rate of adsorption. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Phenol and BTEX (benzene, toluene, ethylbenzene and xylenes) are often present in wastewater from chemical and petrochemical industries. Because of their high toxicity to human health and for the environment, stringent regulations have been imposed on concentration of these compounds inside wastewaters for the safe discharge [1]. Phenol and BTEX can be included in the list of organic compounds identified as hazardous chemicals due to adverse effects on human health at very low concentrations [2]. Different methods such as thermal oxidation, catalytic oxidation, absorption, condensation, membrane separation and adsorption have been used for removal of organic compounds from wastewaters. The combined processes also are widely used for the treatment of organic contaminants in wastewater. The combined carbon-activated sludge process has been used in order to improve

∗ Corresponding author. Tel.: +55 81 2126 7268; fax: +55 81 2126 7278. E-mail address: [email protected] (M. da Motta). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.07.057

the efficiency of this process [3] considering that the process has great potential to control effluent toxicity from oil refineries. The adsorption process is one of the most efficient methods to remove pollutants in wastewater. This process is indicated as a secondary treatment for low level polluting liquids and through continuous process (fixed bed of adsorbent or moving bed of adsorbent) can be used to treat large volumes of liquid effluents. Removal of phenol and BTEX from chemical or petrochemical liquid effluents by adsorption can be evaluated as a multicomponent adsorption problem. Thus the solute–adsorbent interactions must be considered taking into account the different adsorbent sites and the adsorption selectivity of the solutes. Attentions have been focused on adsorbent performances that depend on their natures and are related to adsorption capacity and regeneration. Some materials such as resin [1,3], Moringa oleifera pods [4], rice bran [5], and bagasse fly ash [6] have been used as adsorbent for removal of organic compound by adsorption. Recently, there has been an increasing interest in the study of adsorption process for removing aqueous organics species using organoclays [7–22]. These adsorbent materials are obtained from

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Nomenclature benzene, toluene, ethylbenzene and xylenes component: b (benzene); t (toluene); e (ethylbenzene); x (xylenes) and p (phenol) CEC cation exchange capacity HDTMA hexadecyl trimethyl ammonium chloride XRD X-ray diffraction HPLC high-performance liquid chromatography component initial concentration in the liquid phase C0i (mg/L) Ci component concentration in the liquid phase (mg/L) ˚ d0 0 1 basal spacing (A) ka kinetic adsorption constant (Ln /(g min mgn−1 )) kinetic desorption constant (mg/(g min)) kd K adsorption constant (L/mg)n Kd multi-solute partition coefficient octanol–water partition coefficient Kow k1 constant of pseudo-first-order adsorption rate (min−1 ) k2 constant of pseudo-second-order adsorption rate (g/mg)/min ms mass of adsorbent (g) heterogeneity factor ni q adsorbed amount (mg/g) qe adsorbed amount at equilibrium (mg/g) qm,i maximum BTEX and phenol amount adsorbed at saturation conditions (mg/g) specific surface area (m2 /g) Sp T temperature (K) Vp total pore volume (cm3 /g) BTEX i

modifications to allow the development of organophilic properties of clay minerals. The organoclays are usually prepared using a quaternary ammonium cation [(CH3 )3NR]+ or [(CH3 )2NR2 ]+ , where R is an organic radical. The physical properties and adsorptive capacity of organoclays depend on the molecular size of the R group [23]. Some researchers [2,8,12,18–21] have published studies about phenol and BTEX adsorption by organoclays in which the affinity of these components is discussed. The adsorption affinities of BTEX compounds for organoclays were studied and the preference order of adsorption was related to physicochemical properties of compounds: partition coefficient, hydrophobicity, polarity, structure and molecular size [18–21]. Smectite clays and their modified forms can present superficial characteristics that indicate a variety of different adsorption sites. Silanol groups, aluminum, silicon and organic radicals in organoclays can promote adsorption allowing interactions with nonionic polar organic compounds. Affinity interactions between the clay adsorbent and a different nonionic aromatic organic compound can be related to their polarities. The BTEX compounds can present polarity due to ␲ electrons in an aromatic ring by adding inductive effects promoted by methyl and ethyl groups. Adsorption isotherms of decolorization of maize oil follow the Freundlich equation, indicating the existence of heterogeneous adsorption sites on the solid’s surface. Heterogeneity was attributed both to different active centers on the smectite surface (Brönsted and Lewis centers) and to the different phases present in the adsorbent bentonite, such as illitic layers and clinoptilolite, which also have active centers on their surfaces [24]. The present work investigated the removal of aromatic compounds, phenol and BTEX, by adsorption process through the contact between single-solute, multi-solute system and a smectite organoclay. Evaluations were performed considering

kinetic and equilibrium approaches. The removal efficiencies were quantified and related to the pH of aqueous solution. The Langmuir–Freundlich model was used to fit the experimental data providing the values for kinetics and equilibrium parameters. Based on the order of magnitude of the parameters from single-solute system, predictions were made to justify the behavior of the multicomponent system. 2. Materials and methods 2.1. Preparation of adsorbent The organoclay was synthesized by an ion exchange between smectite clay and quaternary ammonium salt. The smectite clay presenting cation exchange capacity (CEC) value of 78.3 mequiv./100 g was obtained from Paraiba, Brazil and ground to 200 mesh size for further use. The clay preliminary was converted into polycationic clay by adding sodium salt and then converted into organophilic by the treatment with the cationic surfactant hexadecyltrimethylammonium chloride (HDTMA). Initially, 10 mL of an aqueous solution of sodium carbonate (5.00 mequiv. of sodium) was added to a dispersion formed by 50.0 g of clay and water. The mixture was placed in a glass batch reactor at 800 rpm stirring speed for 3 h at 318 K. After reaction the Na-saturated clay was vacuum filtered, washed with deionized water for removal of excess sodium and dried at 373 K for 12 h. The organoclay was prepared by ion exchange between Na+ and HDTMA+ in relative concentration 150% CEC. Then, the material was dried at 333 K for 48 h, ground to 200 mesh and ready to be used as adsorbent. 2.2. Adsorbate solutions Phenol and the BTEX chemicals (purity: 99.95–99.99 wt.%; Ultra Scientific, USA) were used to prepare the adsorbate stock solutions in 0.20 wt.% methanol. There is no meaningful interference from methanol on the adsorption measurements of the compounds studied over the concentration range [25]. The solutions were used for both kinetic and equilibrium isotherm experiments, in order to quantify the adsorbent capacity and process efficiency. 2.3. Analysis Standard X-ray diffraction analysis (XRD) was performed for the samples of modified and ordinary smectite clay. The basal spacing (d0 0 1 ) of surfactant-modified smectite was determined by a Rigaku diffractometer with Cu K␣ radiation. The quantitative analysis of BTEX and phenol aqueous solution was performed by high performance liquid chromatography (HPLC, Schimadzu, LC Solution) analysis using the following operational characteristics: 210 nm wavelength UV detector, 20 ␮L injection volume; C18 chromatographic column; 313 K column temperature, acetonitrile/methanol/water (28:35:37, v/v) mobile phase according to the method used by Breitkreitz et al. [26]. 2.4. Effect of pH The influence of pH was investigated on the removal of BTEX and phenol by the smectite organoclay. The system adsorbent/adsorbate was evaluated at pH 4, 7 and 9 where the concentration of each compound in single-solute and in the mixture was equivalent to 10.0 mg/L. BTEX and phenol solutions were prepared at controlled temperature (296 ± 1 K) to avoid evaporation losses. The solutions were kept under constant stirring for 15 min to complete homogenization of the mixture. A volume of 50.0 mL of each solution was placed in contact with 1.0 g of organoclay. The pH

M.N. Carvalho et al. / Journal of Hazardous Materials 239–240 (2012) 95–101 Table 1 Textural and XRD characteristics of the smectite clays.

97

an organoclay was higher at pH between 7 and 9, after evaluations in the pH range of 3–11.

Smectite clay

Sp (m2 /g)

Vp × 10 (cm3 /g)

˚ d0 0 1 (A)

Untreated smectite clay HDTMA smectite clay

32.98 61.66

1.25 2.13

14.73 22.08

of the suspension was adjusted by addition of 0.1 M HCl or 0.1 M NaOH. 2.5. Kinetic and equilibrium experiments The kinetic experiments were carried in 125 mL Erlenmeyer flasks sealed with parafilm in order to avoid the volatilization of contaminants. A constant volume of 50.00 mL of BTEX and phenol with the following initial concentrations: 10.82 mg/L benzene, 29.06 mg/L toluene, 8.58 mg/L ethylbenzene, 8.55 mg/L m-xylene, 8.52 mg/L p-xylene and 10 mg/L phenol was added to Erlenmeyer flask containing 1.0 g of smectite organoclay and the suspension was kept under constant stirring (300 rpm) at 296 ± 1 K. Samples of the liquid phase were withdrawn at time intervals ranging from 0.5 min to 240 min and analyzed by HPLC immediately. The equilibrium experiments were also carried out in 125.00 mL Erlenmeyer flasks with parafilm. 50.00 mL of BTEX and phenol solutions with initial concentration varying from 2.00 to 29.00 mg/L was mixed with a constant mass of samples (1.0 g). The dispersions (constant pH 9) were shaken (300 rpm) at constant temperature of 296 ± 1 K for 240 min and the final BTEX and phenol concentrations were determined by HPLC. 3. Results and discussion 3.1. Characterization of the adsorbents The textural and XRD characteristics of the smectite clays such as specific surface area (Sp ), total pore volume (Vp ) and basal spacing (d0 0 1 ) related to the adsorption capacities are presented in Table 1. The treatment of the smectite clay with quaternary ammonium salt (HDTMA) to produce the smectite organoclay (2 = 5.96) increased the distance of the basal spacing (d-spacing, d0 0 1 ) from ˚ as well as promoted the increase of the specific 14.73 A˚ to 22.08 A, surface area. Although the literature has reported more high values for specific surface area of smectite clay [27–29] some authors [30–32] have found values between 4.57 m2 /g and 60.7 m2 /g. The values of total pore volume and basal spacing obtained are in agreement with that reported in the literature [31,32]. The cationexchange reactions have been traditionally exploited as an effective method to replace inorganic ions with organic cationic surfactant molecules which intercalate into the clay gallery, resulting in expansion of the interlayer spacing thereby leading to an increase in the basal spacing [33]. 3.2. Effect of pH on BTEX and phenol adsorption The effect of pH on the adsorption and removal efficiencies of BTEX and phenol of the smectite organoclay is shown in Fig. 1. The adsorbed amounts obtained under equilibrium conditions were higher at pH 9 for all aromatics studied in this work and the removal efficiencies were in the range of 70–90%. On the other hand, the lower adsorbed amounts observed in acid medium may be associated with the presence of H+ ions that bind to the surface silanol groups of the organoclay by hydrogen bonding making some active sites inaccessible to reactant molecules. Similar results were found by Mangrulkar et al. [21], where the removal of o-chlorophenol by

3.3. Kinetic and equilibrium evaluations The adsorbed amounts and removal efficiencies of single-solute BTEX and phenol as a function of time (contact time) between solid and liquid phase are shown in Fig. 2. The adsorption process of single-solute and multi-solute solutions was quite fast for all compounds due to high adsorption rate within the first 10 min of contact reaching the equilibrium about 60 min after the beginning of the process. Based on experimental data, the removal efficiencies for single-solute system follow the order: ethylbenzene > xylenes > toluene > benzene > phenol. The experimental evaluations for multi-solute system are shown in Fig. 3, providing the following order of removal efficiencies: ethylbenzene > xylenes > toluene > phenol > benzene. The ethylbenzene and xylenes were the most adsorbed compounds in both single-solute and multi-solute systems. A similar study of BTEX adsorption by Lake and Rowe [11] also presented higher adsorption capacities for ethylbenzene and xylenes. The low adsorption of phenol was suggested to be due to the repulsion between the negative surface charges of adsorbent and the polarity of hydroxyl group present in the phenol. In the case of multi-solute system the competitive adsorption and molecular sizes of BTEX compounds play an important role. Large molecules can occupy a high number of available active sites while small molecules occupy low number of active sites including that allocated in the interlamellar region of the clay. According to McBride [29] the interactions between organic molecules and water disrupt the normal hydrogen-bonded arrangement of water molecules. The increase of this effect is higher for large molecules of high molecular weights. Because of this, a large molecule of high molecular weight destabilizes the aqueous solution to a greater extent than a small molecule and larger nonpolar molecules are “pushed” out of solution onto surfaces more completely than small ones. Experimental evaluations for the multi-solute system indicate lower removal efficiencies than those obtained by single-solute system. This result was observed for all compounds evaluated. A similar work developed by Gitipour et al. [19] evaluated the adsorption of BTEX compounds present in aqueous solution organoclays, indicating that the adsorption affinity of BTEX occurs in the following order: ethylbenzene > oxylene > toluene > benzene. This fact was related to molecular size of the compounds in which ethylbenzene with a large molec˚ was the most adsorbed. Benzene with a molecular ular size (9.5 A) size of 6.6 A˚ was less adsorbed by the organoclay. In Tables 2 and 3 the values of the adsorbed amount of BTEX and phenol from this work and by two other studies [34,35], obtained in the same adsorption equilibrium conditions, and expressed in terms of the mass of adsorbent and by specific surface area are shown. The smectite organoclay presented similar values of adsorbed amount per mass of adsorbent by comparing to those obtained with nanozeolites and modified clay (diatomite). Considering the amount adsorbed related to the specific area of the adsorbents the nanozeolites presented the higher values. However, the organoclay meets the growing need for materials with low production costs (activation at 318 K) and specific organophilic properties, while the nanozeolites and diatomites require high activation temperatures, 773 K and 823 K, respectively. The costs of some adsorbents indicated by Gupta et al. [37] were taken as reference to the cost of the smectite organoclay employed in this work. The indication of an intermediate level of price (0.98 US $/kg) for the smectite, placed between the adsorbents bagasse fly ash (0.01 US $/kg) and activated carbon of almond shell (1.54–2.03 US $/kg) was obtained.

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(a) 1.4

(b) 100

1.3

pH 4 pH 6 pH 9

1.2

pH 4 pH 6 pH 9

90

1.1

80

1.0

70

0.9

60

X (%)

q (mg/g)

0.8 0.7 0.6

50 40

0.5 0.4

30

0.3

20

0.2

10

0.1 0.0

0 benzene

toluene

ethylbenzene

xylenes

phenol

benzene

toluene

ethylbenzene

xylenes

phenol

Fig. 1. Effect of pH on the adsorption and removal efficiencies of BTEX and phenol of the smectite organoclay: (a) equilibrium adsorption capacity, (b) equilibrium removal efficiency. Conditions: 1.0 g, 50 mL, 296 K and 120 min.

(a) 1.0

(b) 100

0.9

90

0.8

80 benzene toluene ethylbenzene m-xylene p-xylene phenol

q (mg/g)

0.6 0.5

70 60 X (%)

0.7

0.4

50 40

0.3

30

0.2

20

0.1

10

0.0

benzene toluene ethylbenzene m-xylene p-xylene phenol

0 0

50

100

150

200

250

50

0

100

t (min)

150

200

250

t (min)

Fig. 2. Kinetic and equilibrium of adsorption of single-solute BTEX and phenol on smectite organoclay: (a) adsorbed amounts, (b) removal efficiencies. Conditions: 1.0 g, 50 mL, 296 K and pH 9.

(a) 1.0

(b) 100

0.9

90

0.8

80

0.7

0.5

60 X(%)

0.6 q (mg/g)

70

benzene toluene ethylbenzene xylenes phenol

0.4

50 40

0.3

30

0.2

20

0.1

10

0.0

benzene toluene ethylbenzene xylenes phenol

0 0

20

40

60

80 t (min)

100

120

140

0

20

40

60

80

100

120

140

t (min)

Fig. 3. Kinetic and equilibrium of adsorption of multi-solute BTEX and phenol on smectite organoclay: (a) adsorbed amounts, (b) removal efficiencies. Conditions: 1.0 g, 50 mL, 296 K and pH 9.

M.N. Carvalho et al. / Journal of Hazardous Materials 239–240 (2012) 95–101

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Table 2 Amount of adsorbed BTEX and phenol at equilibrium in terms of the mass of adsorbent. Conditions: 296 K and pH 9. qe (mg/g) B

T

E

o-X

m-X

p-X

P

0.52 0.30 1.10

0.69 0.30 0.83

0.72 0.60 0.67

– 0.70 –

0.76 – –

0.75 0.90 –

0.51 – –

Material

Author

Smectite organoclay Thermally modified diatomite Treated nanozeolites

This work [34] [35]

Table 3 Amount of adsorbed BTEX and phenol at equilibrium in terms of the specific area of adsorbent. Conditions: 296 K and pH 9. qe (mg/m2 ) B

T

E

o-X

m-X

p-X

P

0.84 0.94 2.26 3.50

1.11 0.94 1.71 3.23

1.16 2.26 1.37

– 2.20 – –

1.22 – – –

1.21 2.83 – –

0.82 – – 1.12

Table 4 Costs of several adsorbents and smectite organoclay. Adsorbent

Price (US $/kg)

Bentonitea Bagasse fly asha Coconut shell charcoal (as received)a Activated carbon (almond shell)a Smectite organoclay

∼0.05 0.009 0.34 1.54–2.93 0.98

a

Gupta et al. [37].

The costs of several adsorbents were compared to the smectite organoclay as shown in Table 4. The use of low-cost adsorbents such as clay, coal, peat, zeolites, biomass, fly ashes and for water treatment depends strongly on their origin. The adsorption process will provide an attractive technology if the low-cost adsorbent is ready for use. However, physical and chemical treatments are proposed for improving their adsorption capacity. Often these treatment methods are not cost effective at large scale [38]. Based on the experimental data of BTEX and phenol adsorption for multi-solute system the partition coefficients (Kd ) were quantified under equilibrium conditions (Table 5). Value of Kd ∼ =1, Kd < 1 or Kd > 1 corresponds to linear, unfavorable and favorable isotherms, respectively. Ethylbenzene and xylenes for multi-solute system had more high values of Kd indicating high removal efficiencies. The results indicate low water solubility values for the compounds with high partition coefficient, except for benzene and phenol. This fact is related to the sorption process that is driven more by the low solubility of nonpolar molecule in solution than by a significant force of attraction to the organic matter. Chemically similar nonpolar molecules such as benzene, naphthalene, and pyrene have Table 5 Partition coefficients (Kd ) for BTEX and phenol adsorption on smectite organoclay. Conditions: 1.0 g, 50 mL, 296 K and pH 9. Compound

Water solubilityb 25◦ (g/L)

Octanol–water partition coefficientb (Kow )

Multi-solute partition coefficientc (Kd )

Benzene Toluene Ethylbenzene Xylenesa Phenol

0.180 0.627 0.208 0.193 8.200

2.13 2.73 3.15 3.15 1.48

0.91 0.86 2.73 2.50 1.22

a b c

Average values. Lide [36]. This work.

Material

Author

Smectite organoclay Thermally modified diatomite Treated nanozeolites Montmorillonite

This work [34] [35] [9]

affinities for soil organic matter that increase with molecular weight because of their different hydrophobic surface areas [29]. Phenolic compounds removal from aqueous solutions is more difficult due to their high solubility. 3.4. Adsorption rate of BTEX and phenol on organoclay The phenomenological behavior of fluid–solid adsorption process was focused on the Langmuir–Freundlich model. The expression for the net adsorption rate (ri ) is given by: ri =



ka,i q Cini (qm,i − qi ) − i qm,i Ki



(1)

where ka,i is the kinetic adsorption constant, Ki is the adsorption constant, ni is the heterogeneity factor, Ci is the BTEX and phenol concentration in the liquid phase, qi is the BTEX and phenol concentration on the organoclay, qm,i is the maximum BTEX and phenol amounts adsorbed at saturation conditions on the organoclay surface and i = benzene, toluene, ethylbenzene, xylenes or phenol. 3.4.1. Modeling of BTEX and phenol adsorption in a batch adsorber The batch adsorption operation for solute–organoclay system is represented by following mass balance equation: −V

dCi = ms ri dt

(2)

where V is the volume of the liquid phase and ms is the mass of the solid. The relationship between Ci and qi is given by Eq. (3). qi =

C0,i − Ci V ms

(3)

The solution of mass balance equations was obtained by fourthorder Runge–Kutta method and the hybrid fractional error function (HYBRID) was used to obtain the best-fitting data. The curve fitting for equilibrium and kinetic data of single-solute adsorption is shown in Figs. 4 and 5, respectively. The Langmuir–Freundlich model used in this work was compared to the pseudo-first-order and pseudo-second-order kinetic models given by Eqs. (4) and (5), respectively. ri = k1,i (qe,i − qi ) ri = k2,i (qe,i − qi )

(4) 2

(5)

where k1,i and k2,i are the constants of first and second-order adsorption rate. The comparative results are shown in Table 6.

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Table 6 Langmuir–Freundlich, pseudo-first-order and pseudo-second-order kinetic parameters for adsorption rate of BTEX onto smectite organoclay. Component

Langmuir–Freundlich

Benzene Toluene Ethylbenzene m-Xylene p-Xylene Phenol

Pseudo-first-order

Pseudo-second-order

ka × 102 [(Ln /g min mgn−1 )]

qm (mg/g)

n

K (L/mg)n

R2

k1 × 102 (min−1 )

R2

k2 × 102 [(g/mg)/min]

R2

5.19 9.30 5.04 5.97 6.05 0.34

2.04 1.27 6.67 3.83 2.09 8.28

1.16 1.23 0.70 1.46 1.12 1.05

3.58 94.92 5.43 9.76 19.74 0.51

0.9976 0.9976 0.9778 0.9997 0.9992 0.9191

0.07 −28.97 0.08 −13.17 −17.43 −8.43

0.7228 0.9107 0.9957 0.9685 0.8384 0.8910

52.59 6.73 6.05 3.40 7.03 38.72

0.9998 0.8693 0.7940 0.5960 0.9963 0.7651

(b) 35

(a) 20 18

30 16 25

14

1/q (g/mg)

1/q (g/mg)

12 10 8 6

20

15

10

4

benzene ethylbenzene p-xylene model

2

toluene m-xylene phenol model

5

0

0 0

1

2

3 n

4

5

0

n

5

10

15 n

(1/C) (L/mg)

20

25

30

n

(1/C) (L/mg)

Fig. 4. Single-solute adsorption equilibrium of BTEX and phenol on smectite organoclay: (a) benzene, ethylbenzene and p-xylene; (b) m-xylene, phenol and toluene. Conditions: 1.0 g, 50 mL, 296 K and pH 9.

Besides the aspects of affinity and selectivity, other approach considering the adsorption rates is a useful indicator in compoundorganoclay interactions. The values for ka,i parameter indicate that the phenol have a slow rate of adsorption, however, with a high coverage on organoclay surface indicated by the qm,i parameter. On the other hand, toluene presented low qm,i indicating a selective adsorption on active sites, but high values of Ki and kai which suggest meaningful binding energy and fast adsorption on organoclay surface, respectively.

30 benzene toluene ethylbenzene m-xylene p-xylene phenol model

25

C (mg/L)

20

15

10

4. Conclusions

5

0 0

5

10

15

20

25

30

35

40

45

50

55

60

65

t (min) Fig. 5. Single-solute adsorption kinetics of BTEX and phenol on smectite organoclay. Conditions: 1.0 g, 50 mL, 296 K and pH 9.

The experimental data for each of the adsorbates examined best fit the Langmuir–Freundlich kinetic model, providing correlation coefficient (R2 ) values ranging from 0.9191 (for phenol) up to 0.9997 (for m-xylene). Phenol and ethylbenzene presented high maximum amounts adsorbed (qm,i ) indicating that they cover many active sites of organoclay and that they are not selective for one particular active site. Toluene and p-xylene present high values of Ki which indicates a high adsorption affinity of compounds to organoclay surface and high binding energy of adsorption.

The process of removal of BTEX and phenol in aqueous solutions was developed by adsorption on smectite organoclay. The adsorbent material, based on smectite clay was obtained by the treatment with the cationic surfactant hexadecyltrimethylammonium chloride (HDTMA) to improve the adsorption properties. Textural modifications were observed and related to increasing the specific surface area from 32.68 to ˚ Removal 61.66 m2 /g and basal spacing from 14.73 to 22.08 A. efficiencies between 55 and 90% were obtained by single-solute system, whereas removal efficiencies from 30 to 90% were reached by multi-solute system. Based on experimental data, the removal efficiencies for single-solute system follows the order: ethylbenzene > xylenes > toluene > benzene > phenol. The removal efficiencies for multi-solute system follows the order: ethylbenzene > xylenes > toluene > phenol > benzene.Evaluation of the adsorption kinetics and equilibrium for the BTEX and phenol removal in single-solute and multi-solute systems were performed at which adsorption equilibrium was established after 60 min. The Langmuir–Freundlich model fitted to the experimental data provided kinetic and equilibrium parameters for single-solute

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