A Comparison of New Gemini Surfactant Modified Clay with its

Article pubs.acs.org/jced

A Comparison of New Gemini Surfactant Modified Clay with its Monomer Modified One: Characterization and Application in Methyl Orange Removal Yujun Wang,† Xiaohui Jiang,*,† Limei Zhou,† Chunjie Wang,† Yunwen Liao,† Ming Duan,‡ and Xiaomin Jiang§ †

Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong, Sichuan, PR China 637009 ‡ State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan, PR China 610500 § Southwest Electric Power Design Institute, Chengdu, Sichuan, PR China, 610021 S Supporting Information *

ABSTRACT: New gemini surfactant, glycol bis-N-tetradecyl nicotinate dibromide (designated EG), and the corresponding monomer, methyl Ntetradecyl nicotinate bromide (ES), were synthesized and utilized to modify sodium bentonite (Na-Bt). EG-Bt and ES-Bt, the surfactant modified bentonites, were then used for methyl orange (MO) removal from the dye solution. EG was more effective than ES at expanding the interlayer space of Na-Bt. The adsorption of EG, ES and MO obeyed well the pseudo-secondorder kinetic model and Langmuir isotherms on Na-Bt or on the modified bentonite. However, the adsorption of EG was more spontaneous than that of ES, and EG replaced more small particles, such as Na+ and water, than ES did during the adsorption on Na-Bt. The elevated temperature impairs the adsorption of the surfactants, but enhances that of MO. MO absorbed more easily on EG-Bt than on ES-Bt. When the dosage of the surfactants used goes beyond a certain amount, the uptake of MO by EG-Bt/ES-Bt decreases slowly owing to desorption of the surfactants. EG and ES formed a complex with MO on the modified bentonite as evidenced by UV−vis spectra, and EG exhibited the stronger interaction with MO.

1. INTRODUCTION The majority of synthetic dyes are composed of aromatic rings, which endow the dye with tinctorial strength but make their metabolic products carcinogenic and nonbiodegradable when discharged into waste streams.1−3 For this reason, dye wastewater treatment is challenging for both the environmental scientists and chemists. There are several conventional methods available to deal with dye wastewater, of which adsorption is the most effective and convenient one.4,5 Because of its high adsorption capacity, clay is regarded as a potential candidate. However, the strong hydrophilicity of clay impairs the uptake capacity for organic contaminants. To address this weakness, surfactants have been employed to modify and convert the hydrophilic mineral surface to a hydrophobic one.6−8 In this practice selecting modifiers is crucial to obtain high effective organo-clays. Gemini-modified clay exhibits good efficacy in getting rid of organic contaminants from wastewater.9−12 Li and Rosen have reported that gemini surfactants, [C n H 2n+1 N + C(CH 3 ) 2 CH2CH2]22Br−, n = 10, 12, 14, and 16], are adsorbed onto sodium montmorillonite with one of the hydrophilic groups orienting on the clay surface and the other toward water,13 and © 2013 American Chemical Society

the gemini intercalated clay are effective and efficient for 2naphthol and 4-chlorophenol removal. Two gemini surfactants (1,3-bis (dodecyldimethylammonio)-propane dibromide and 1,3-bis (dodecyldimethylammonio)-2-hydroxypropane dichloride) modified montmorillonites were utilized to treat pnitrophenol solution.14 As a result, the organo-clay containing the gemini of a hydroxyl spacer was more effective than that without a hydroxyl spacer for removing p-nitrophenol from aqueous solution. Comparatively speaking, gemini-modified clay is less studied because commercially supplied gemini surfactants are devoid of category because preparation and purification of gemini surfactants are much more difficult than the traditional ones. However, different structures show different properties. Therefore we intend to compare the adsorption behavior on bentonite of a novel gemini and corresponding single chain surfactant, and methyl orange (MO) removal efficiency of the modified Received: February 19, 2013 Accepted: April 23, 2013 Published: May 3, 2013 1760

dx.doi.org/10.1021/je400164e | J. Chem. Eng. Data 2013, 58, 1760−1771

Journal of Chemical & Engineering Data

Article

bentonite. The present work may shed new light on developing new modified clay by gemini surfactants.

2. MATERIALS AND METHODS 2.1. Materials. The main reagents, such as nicotinic acid, thionyl chloride, glycol, bromotetradecane and MO were purchased from Chengdu Kelong Chemical Reagent Co., the reagents were all analytical grade and used without further purification. The composition of Na-bentonite (cation exchange capacity (CEC) of 66.00 mequiv/100 g) is listed in Table S1 in the Supporting Information. EG (C42H70Br2N2O4, Mw = 824.37, 97.058 % by gas chromatography in Figures S1 and S2) and ES (C21H36BrNO2, Mw = 413.19, 97.915 %) were prepared in our laboratory (Supporting Information, Figure S3 to Figure S10 and Table S2). Distilled water was used throughout the experiments. The simulated dye wastewater was prepared by dissolving MO into tridistilled water. 2.2. Preparation of Organobentonite and Adsorption Experiments. The preparation of organobentonite and sorption of EG/ES on Na-Bt and MO on ES-Bt and EG-Bt were done as described by Kan.15 The modified bentonites were denoted as xEG-Bt or xES-Bt, where x is the initial concentration of EG/ES used in the modification (10−3 mol·kg−1). The concentration of the adsorbates (Ct) after the adsorption was calculated from the standard equation (Supporting Information, Figure S11 and Table S3.). The amount of adsorbates (qt) adsorbed per gram of adsorbent was obtained according to eq 1.16,17 qt =

(C0 − Ct )V m

Figure 1. The relationship between the adsorption capacity and EG/ES concentration (square for ES and circle for EG).

3.1.2. The Elements and FTIR Analysis. The data from elemental analysis (Supporting Information, Table S4) unveil clearly that the amount of carbon and nitrogen elements is higher in the presence of EG/ES than in Na-Bt, and it is augmented with raising the dosage of EG/ES, which manifests that either EG or ES exists in the modified bentonite. The IR bands (Supporting Information, Figure S12 and Table S5) exhibit characteristic absorption of alkyl and ester groups in the samples of EG-Bt and ES-Bt besides characteristic Na-Bt bands,21 suggesting that EG/ES is in the interlayer of Na-Bt or adsorbed on the surface. 3.1.3. The Basal Spacing of EG-Bt and ES-Bt. The basal spacing of Na-Bt is 1.39 nm identical to the previously reported value.15 Figure 2 describes a stepwise expansion of the interlayer

(1)

Where C0 is the initial concentration of the absorbate, V is the initial volume of solution, and m is the mass of adsorbent. The relationship of decoloration efficiency (η) with the absorbance of MO solution untreated (A0) and treated by EG-Bt/ ES-Bt (A) is expressed as follow.15

η /% =

A0 − A 100 A0

(2)

2.3. Characterization. Nicolet-6700 FTIR spectrometer (Nicolet, USA) and elemental analyzer (EURO-EA3000, Italy) were employed to characterize the samples. Powder X-ray diffraction (XRD) patterns of different samples were recorded at different diffraction angles (2θ) using Rigaku D/max-rA powder diffractometer (Ultima, IV, Japan) with Cu Kα radiation (λ= 1.5418 Å). Thermogravimetric analyze (TGA) of the samples was performed on STA 449 F3 Jupiter thermal analyzer (Netzsch, Germany) in a platinum crucible at a ramp of 10 °C/min from room temperature to 800 °C and 20 mL/min of high-purity flowing argon atmosphere.18,19

Figure 2. The dependence of the basal spacing and the adsorption capacity of ES-Bt and EG-Bt on concentrations of ES/EG (square for ES and circle for EG).

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of EG-Bt and ESBt. 3.1.1. Intercalation of EG/ES on Na-Bt. Figure 1 depicts the varying adsorption capacity (qe) of EG/ES at equilibrium on NaBt with the initial concentration (C0). It is obviously that the qe raises with the increased EG/ES concentration in a linear fashion, and then plateaus, suggesting that EG/ES interacts with Na-Bt by strong electrostatic forces.20 The qe for EG is 1.63 mol·kg−1, that is twice as much as that of ES (0.82 mol·kg−1), indicating more EG molecules adsorbing on Na-Bt.

space with the intercalation of EG/ES into Na-Bt. The basal spacing of Na-Bt enlarges rapidly by E G /E S at lower concentration, and then up to 25.5 Å for EG-Bt and 19.6 Å for ES-Bt at 4.0·10−3 mol·kg−1 of ES/EG. When the concentration of ES is about 8.0·10−3 mol·kg−1, the maximum basal space (24.3 Å) is reached and afterward it is almost unchanged with further increasing the concentrations. The basal space of EG-Bt is 29.4 Å at 8.0·10−3 mol·kg−1 of EG, and then it rises to the maximum 31.1 Å. Compared with ES, we might conclude that gemini surfactant 1761



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°C for EG. The differential thermogravimetry (DTG) curve for ES is quite complicated: the mass loss peaks appear at (186, 253, and 438) °C, demonstrating that ES decomposes in a stepwise mode (see the thermogravimetry (TG) curves in Supporting Information, Figure S13). Na-Bt shows a peak of the volatilization of the free water at 84 °C (Figure 3b). However, the presence of EG/ES could turn the hydrophilic silicate surface into a hydrophobic one,24 so EG-Bt and ES-Bt have less free water to lose in the heating circumstance and present lower mass loss at this temperature. The thermogravimetric test indicates a mass loss of 15.55 % for 4.0ES-Bt, 26.97 % for 8.0ES-Bt, 30.12 % for 4.0EG-Bt, and 35.54 % for 8.0EG-Bt during the detachment of EG or ES, respectively. It seems that the higher is the dosage of EG/ES in Na-Bt, the larger is the mass loss of the modified bentonite, which agrees with that reported by Marras.25 EG shows less mass loss (in mole) during thermogravimetric measure considering that EG weighs about twice as much as ES. The peak at 186 °C (the initial mass loss temperature of free ES) disappears in the DTG curve for 4.0ES-Bt. The first peak at 331 °C indicates an improved thermal stability of ES by intercalation in bentonite. The mass loss peak at 229 °C seems more obvious for 8.0EG-Bt than for 4.0EG-Bt. On the basis of what Xi has reported,26 we may attribute the peak at 229 °C to the desorption of EG molecules on the clay surface, and deduce that the bigger head of EG hinders the intercalation and keeps the molecule on the surface of Na-Bt.27 3.1.5. Adsorption Isotherm of EG/ES on Na-Bt. Because the adsorption isotherm, for instance Langmuir, Freundlich, and Dubinin−Redushkevich (D-R) isotherms, provides a relationship of adsorbates in solution and on the solid phase when the adsorption is at equilibrium,28 we have fitted the obtained experiment data into following formulas.29,30

is more efficient at enlarging the basal spacing of Na-Bt than the corresponding monomer. Xi et al. have suggested the conformations of cationic surfactants ODTMA (octadecyltrimethylammonium bromide), DODMA (dimethyldioctadecylammonium bromide), and TOMA (methyltrioctadecylammonium bromide) in montmorillonite.22 They have proposed that a paraffin-type bilayer arrangement of the surfactant in DM2.0CEC is responsible for 39.83 Å of basal spacing, a paraffin-type monomolecular arrangement for (22 and 32.9) Å, but the surfactant is much tilted in the interlayer of the organo-clay for 22 Å. Ignoring the different head groups or spacer of the surfactants, ES is comparable to ODTMA because they belong to mono-alkyl chain surfactants while EG is comparable to DODMA because double alkyl chains exist in their molecule. Hence, we speculate, EG and ES may appear in an arrangement of a paraffin-type monomolecular in EG-Bt and ES-Bt. A comparison of the curves in Figure 1 and Figure 2 shows that the basal spacing enlarges ∼2 Å when the amount of EG is from (6.0·10−3 to 1.6·10−2) mol·kg−1, while the qe continuously augments with raising EG concentration up to 1.4·10−2 mol·kg−1. It seems that the increased EG adsorbs on the surface of Na-Bt to form hemimicelle or micelle instead of inserting into the interlayer of Na-Bt.23 3.1.4. Thermogravimetric Analysis of Na-Bt, EG-Bt, and ESBt. Figure 3a shows a small peak at 69 °C, which is due to the water loss in the sample, and much bigger mass loss peaks at 257

Langmuir isotherm: Ce C 1 = + e qe bqm qm

(3)

Freundlich isotherm: log qe = log k f +

1 log Ce n

(4)

D-R isotherm: ln qe = ln qm − K ′ε 2

(5)

Where qm and b represent the monolayer adsorption capacity and Langmuir isotherm coefficient, respectively. ε is Polanyi potential and equal to RT ln(1 + 1/Ce). K′ is related to mean adsorption energy (E) by the following expression: E=

1 2K ′

(6)

RL, known as the separation factor, is defined by formula 7: RL =

1 1 + bC0

31

(7)

Figure 4 graphs the plots of data fitting by Langmuir isotherm (those by Freundlich and D-R isotherms available in Supporting Information, Figure S14). Table 2 summarizes the corresponding parameters. R2 of the plots demonstrate that the Langmuir isotherm can better describe the adsorption of EG/ES on Na-Bt. The calculated

Figure 3. DTG curves of EG/ES and EG-Bt/ES-Bt/Na-Bt: (a) solid line for EG, dash line for ES; (b) 8.0EG-Bt, 4.0EG-Bt, 8.0ES-Bt, 4.0ES-Bt, and Na-Bt are arranged from the top to the bottom in turn. 1762



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where k1 and k2 represent the pseudo-first-order and pseudosecond-order rate constants, respectively. The Elovich equation is often used to describe activated chemisorption,38 and the linearized Elovich equation can be expressed as follows after being rearranged and simplified: 1 1 qt = ln(αβ) + ln t β β (10) where α is the initial sorption rate and β related to the surface coverage and activation energy for chemisorption. Figure 5 depicts the pseudo-second-order plots of EG/ES on the bentonite (the plots by pseudo-first-order kinetics and

Figure 4. Langmuir isotherm for the adsorption of EG/ES on Na-Bt: the horizontal coordinates on the top and the vertical coordinates on the right are for EG (●), and the others are for ES(■).

qm is 0.82 mol·kg−1 for ES and 1.72 mol·kg−1 for EG, respectively, and they agree well with the experimental values (0.82 mol·kg−1 and 1.63 mol·kg−1, respectively). RL values in Table 1 for EG-Bt/ES-Bt range from 0 to 1, indicating the favorable adsorption characterized by a relatively high surfactant uptake at low concentration in the solution.32 In addition, a diminishing RL value with the surfactant concentration means that the higher the concentration is, the more favorable is the adsorption.33 In Freundich isotherm, kf, is essentially related to the capacity of the adsorbent while n is a function of the adsorption strength. kf is 0.82 for ES and 1.46 for EG, respectively, it means that Na-Bt absorbs more EG than ES. The n value being greater than 1 indicates again a favorable process happening. Furthermore, the higher the n value is, the better is the adsorption, the stronger is the bond that forms between the adsorbate and adsorbent.34 So ES adsorbs stronger than EG on Na-Bt, which is consistent with the thermogravimetric result. The mean adsorption energy (E) in the D-R isotherm can act as a ruler to distinguish chemical and physical adsorption.35 The E value is 2.50 kJ·mol−1 for ES-Bt and 2.13 kJ·mol−1 for EG-Bt, respectively, which manifests that ES/EG absorbing on Na-Bt can be characterized as physical adsorption. 3.1.6. Adsorption kinetics of ES/EG on Na-Bt. A pseudo-firstorder equation and pseudo-second-order reaction kinetics are widely used for probing the adsorption of an adsorbate from an aqueous solution.36,37 log(qe − qt ) = log qe −

k1t 2.303

Figure 5. The pseudo-second-order kinetic for the adsorption of EG/ES onto the Na-Bt: square for 4.0ES, circle for 8.0ES, upward triangle for 4.0EG, downward triangle for 8.0EG, and the vertical coordinates on the right for 4.0ES.

Elovich equation are shown in Supporting Information, Figure S15), and the corresponding kinetic parameters are tabulated in Table 2. The R2 is about 1 for the pseudo-second-order kinetic model fitting, and the qe,cal values agree very well with the qe,exp, validating that this model describes quite well the adsorption of EG /ES onto the bentonite. Although raising the initial concentration of EG/ES results in qe,cal augmenting, it leads to the rate constants (k2) values gradually declining because of the steric hindrance at higher concentration of the surfactant.39 The k2 for ES adsorption is five times more than that for EG in 4.0EGBt/ES-Bt; however, it is almost the same for 8.0EG-Bt/ES-Bt. The cmc value of EG (1.2·10−4 mol·kg−1) is almost seven times lower than that of ES (8·10−4 mol·kg−1), which means that EG is more ready for micellization than ES. So we could deduce the reason for the smaller k2 value of EG; that is, micelles of EG on the surface of Na-Bt interfere with the intercalation, so the k2 for EG-Bt is much smaller than that for ES-Bt. 3.1.7. Adsorption Thermodynamics of ES/EG on Na-Bt. Thermodynamic parameters can be employed to probe the

(8)

t 1 t = + 2 qt q k 2qe e

(9)

Table 1. The Parameters by Langmuir, Freundlich, and D-R Isotherms for EG/ES Adsorption on Na-Bt Langmuir isotherm b surfactant ES EG

Freundlich isotherm

D-R isotherm

qm

kg· mol−1 4

5.84·10 6.24·103

E

K′

qm

RL

mol·kg−1

R2

χ2

kf

n

R2

χ2

KJ·mol−1

mol2·KJ−2

mol·kg−1

R2

χ2

0.0085 to 0.0011 0.0742 to 0.0099

0.82 1.72

0.999 0.986

0.27 1.02

0.82 1.46

14.91 2.77

0.903 0.963

0.05 0.12

2.50 2.13

0.08 0.11

1.18 1.58

0.979 0.946

0.02 0.24

1763


0.899 0.950 0.957 0.894

nature of a process. The following relationships are used to evaluate the enthalpy ΔH0ad, Gibbs free energy ΔG0ad, and entropy ΔS0ad.40

3.19 0.12 0.65 1.25

ln Kd =

0 0 ΔSad ΔHad − R RT

(11)

(12)

The expanded uncertainty was calculated using a coverage factor of 2, which gives a level of confidence of approximately 95 %.

Figure 6. Plot of Van’t Hoff equation for adsorption of EG/ES onto NaBt (the same symbols as those in Figure 5).

The data in Table 3 manifest that theΔG0ad for EG is more negative than for ES, suggesting that the intercalation of the gemini is more spontaneous than that of the monomer. It can be seen clearly that ΔH0ad is converted from negative to positive with a rise in temperature for 8.0EG/8.0ES, which means a less spontaneous intercalation. We know that the configuration of the surfactants is more disordered at higher temperature than at lower temperature, which may retard the intercalation and result in declining ΔG0ad. The positive ΔS0ad hints at a favorable process for EG/ES adsorption on Na-Bt. As we know, the clay surface is covered with water before the adsorption of EG/ES molecules. Upon adding the surfactants, EG/ES will substitute the adsorbed water molecules because EG/ES is repelled by water molecules in bulk solution and would rather stay on the Na-Bt surface than in aqueous phase due to the amphoteric characteristics of surfactants. Although EG/ES molecules undergo the entropy decline during the adsorption, water and Na+ restore their freedom due to the replacement by EG/ES, increasing the entropy. On the whole, ΔS0ad is positive in the course of EG/ES adsorption and water and Na+ desorption. The larger ΔS0ad value for EG indicates that EG may substitute more water and Na+ than ES does during the intercalation. The value of ΔH0ad is found to be negative in (293 to 342) K, which means that the stability of EG/ES may increase due to the intercalation, the adsorption is physical in nature involving weak forces of attraction and the adsorption process is also exothermic,41 and the elevating temperature will impair the intercalation.

a

0.499 0.908 0.496 0.965 1.252 0.060 0.252 0.058 0.047 0.093 0.026 0.049 4.0 8.0 4.0 8.0 4.0ES 8.0ES 4.0EG 8.0EG

0.89 30.96 5.76 3.74

0.879 0.891 0.965 0.949

0.498 ± 0.003 0.907 ± 0.002 0.495 ± 0.003 0.964 ± 0.004

1 0.9999 1 0.9999

0.20 0.11 0.20 0.10

5.7·10 5.0·1041 1.3·10133 1.1·10520

Kd, the thermodynamic equilibrium constant, is calculated from the relation of ln qe/Ce vs qe at different temperature and extrapolating to zero.38 A plot of ln Kd versus 1/T (Figure 6) is linear with a slope of ΔH0ad/R and with intercept of ΔS0ad/R.

688

(·103) kg·mol−1

β α

mol·kg−1·s−1

qe,cal

mol·kg−1

k2

kg·mol−1·s−1

qe,exp

R2 qe,cal k1

min−1

C0

(·10−3) mol·kg−1 code

(·10−3) mol·kg−1

Article

0 ΔGad = −RT ln Kd

mol·kg−1

R2

χ

2

pseudo-second-order kinetic

a

pseudo-first-order kinetic

Table 2. The Kinetic Parameters by the Three Models for EG/ES Adsorption on Na-Bt

(·10−5)

Elovich equation

R2


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Table 3. Thermodynamic Parameters for the Adsorption of EG and ES on Na-Bt 4.0ES

8.0ES

4.0EG

8.0EG

T

ΔG0ad

ΔS0ad

ΔH0ad

ΔG0ad

ΔS0ad

ΔH0ad

ΔG0ad

ΔS0ad

ΔH0ad

ΔG0ad

ΔS0ad

ΔH0ad

K

kJ mol−1

J mol−1

J mol−1 K−1

kJ mol−1

J mol−1

J mol−1 K−1

kJ mol−1

J mol−1

J mol−1 K−1

kJ mol−1

J mol−1

J mol−1 K−1

293 303 313 323 333 343

−5.951 −6.038 −4.923 −4.042 −3.796 −2.636

20.219 19.840 15.646 12.433 11.322 7.610

−26.310

−0.457 −0.353 −0.314 −0.135 0.025 0.051

1.549 1.154 0.991 0.406 −0.086 −0.158

−3.689

−9.759 −8.630 −7.728 −6.541 −5.700 −4.434

33.171 28.348 24.561 20.126 16.997 12.810

−40.359

−9.405 −6.986 −5.806 −2.566 0.155 1.055

32.244 31.847 22.811 18.313 −0.687 −3.293

−73.928

3.2. MO Removal by EG-Bt/ES-Bt. 3.2.1. Optimization of MO Adsorption. Figure 7 displays that the incremental

Supporting Information, Figure S16) specify that the amount of EG/ES desorbed from the modified clay increases with prolonging the immersion time and so does the MO amount in the supernates. It is clear that the decreasing amount of MO is due to the desorption of EG/ES, which increases the solubility of MO in water due to the solubilization of surfactants for dyes, and so decreases the amount of MO on EG-Bt/ES-Bt. So the amount of the modifier should be taken into account during the preparation of organo-clays. 3.2.1.1. Effect of Weight Ratio. As shown in Figure 8, the decolorization of MO is varied with the weight ratio of EG-Bt/ES-

Figure 7. Variation of MO amount at equilibrium with the concentration of EG/Es on Na-Bt: square for ES-Bt, circle for EG-Bt. Conditions: 3.05·10−4 mol·kg−1 of MO, 0.06 g of EG-Bt/Es-Bt, 30 °C, pH 6.3, and 1.8·103 s.

concentration of EG/ES on Na-Bt raises the amount of MO adsorbed by EG-Bt/ES-Bt, MO removal reaches the highest (1.02·10−1 mol·kg−1 for EG-Bt and 7.71·10−2 mol·kg−1 for ES-Bt) at the concentration of 4.0·10−3 mol·kg−1 of EG and 8.0·10−3 mol·kg−1 of ES, then it declines a little with further elevated concentration of EG/ES on Na-Bt. It is also noticeable that at the maximum uptake of MO, the amount needed for EG is about half of that for ES. Therefore, the gemini surfactant modified clay is more efficient in getting rid of the colorants from the aqueous solution of dye contaminant. As shown in Figure 7, beyond the optimum amount of EG/ES in the modified bentonite, the uptake of MO by EG-Bt/ES-Bt decreases slowly with the further addition of the surfactants, which, we speculate, might be due to the desorption of the surfactants. So we conducted the measurement of UV−vis spectra for the supernates of 10.0EG-Bt and 10.0ES-Bt in water and in MO solution, respectively. The results (Table 4 and

Figure 8. The relationship between decoloration rate of MO and weight ratio of EG-Bt and Es-Bt: square for 4.0 ES-Bt, circle for 8.0ES-Bt, upward triangle for 4.0EG-Bt, downward triangle for 8.0EG-Bt. Condition: 3.05·10−4 mol·kg−1 of MO, 30 °C, pH 6.3, and 1.8·103 s.

Bt. The MO removal was enhanced with raising the weight ratio of 4.0EG-Bt/8.0ES-Bt; it was 98.68 % for 8.0ES-Bt at a weight ratio of 0.35 % and 99.36 % for 4.0EG-Bt at a weight ratio of 0.2 %, respectively, and was almost unchanged when the concentration further increased. This phenomenon indicates that the geminimodified bentonite possesses higher uptake capacity for dye than the monomer-modified one. It should be noticed that MO removal is 98.68 % for 8.0ES-Bt and 8.81 % for 4.0ES-Bt in the weight ratio range employed, suggesting that a little more surfactant loading in the bentonite will improve dye removal efficiency greatly. 3.2.1.2. Effect of Reaction Time. The time profiles in Figure 9 expose an obviously enhanced uptake of MO with the elongated reaction time for both 4.0EG-Bt and 8.0Es-Bt. The decolorization rate is augmented to 99.68 % and 81.34 % in the first 1.8·103 s for 4.0EG-Bt/8.0ES-Bt. With further elongated time, the uptake of

Table 4. The Concentration of ES/EG in Water and MO in the Modified Bentonite Solution ES (mol·kg−1)

EG (mol·kg−1)

MO (mol·kg−1)

immersion time (s)

in H2O

in H2O

in ES-Bt

in EG-Bt

1.8·103 2.4·103 3.0·103

0.152 0.174 0.191

0.193 0.221 0.238

0.084 0.170 0.176

0.011 0.013 0.016 1765



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Figure 9. The dependence of reaction time on decoloration rate of MO by EG-Bt and Es-Bt (the same symbols as those in Figure 8). Conditions: 3.05·10−4 mol·kg−1 of MO, 0.04 g of modified bentonite, pH 6.3, and at 30 °C.

MO is leveling out. In addition, MO removal by 8.0EG-Bt and 4.0ES-Bt seems independent of time. 3.2.1.3. Effect of Temperature. The relationship between MO removal and the temperature is drawn in Figure 10. The elevated

Figure 11. Variation of decoloration rate of MO upon the initial pH value of the solution (a) and the hydrolysis of NG and NS (b) (the same symbols as those in Figure 8). Conditions: 3.05·10−4 mol·kg−1 of MO, 0.04 g of modified bentonite, 25 °C, and 1.8·103 s).

and the practical conditions must be taken into consideration when modifying and applying bentonite. The uptake of MO by 4.0ES-Bt is little affected by pH value of the solution. We deduce the reason being as most molecules of the ES added stay in the interlayer of 4.0ES-Bt, and they are indifferent of acid or alkaline circumstances. 3.2.2. Adsorption Isotherm of MO on EG-Bt/ES-Bt. Langmuir fitting of MO adsorption on EG-Bt/ES-Bt is graphed in Figure 12 (Freundlich and D-R fittings in Supporting Information, Figure S17), and the high regression coefficient suggests that this model describes the adsorption very well. Analyzing the parameters in Table 5 manifests that EG-Bt is more effective for MO removal. The maximum adsorption capacity (qm) of MO by 4.0EG-Bt (1.81·10−1 mol·kg−1 and 3.25·10−1 mol·kg−1) is a magnitude larger than that by 4.0ES-Bt

Figure 10. The dependence of temperature on decoloration rate of MO by EG-Bt and Es-Bt (the same symbols as those in Figure 8). Conditions: 3.05·10−4 mol·kg−1 of MO, 0.04 g of modified bentonite, pH 6.3, and 1.8·103 s.

temperature slowly rises up the decolorization rate of MO. It is 99.22 % and 88.81 % for 4.0EG-Bt and 8.0ES-Bt at 25 °C, respectively, and then almost unchanged with temperatures, specifying that the uptake capacity of MO by EG-Bt and ES-Bt is almost temperature-independent and that EG-Bt can be used all year around. 3.2.1.4. Effect of pH. Figure 11a shows that EG-Bt and ES-Bt function effectively for MO removal in pH 2 to pH 7, but MO removal drops drastically in alkaline condition except for 4.0ESBt. In a previous study,15 we have proved that the homologue of EG can be hydrolyzed in alkaline solution, the resulted carboxyl group is negatively charged and will hinder the adsorption of MO onto the surfactant modified bentonite. So, we may attribute the reduced removal efficiency of EG-Bt/ES-Bt to the alkaline hydrolysis of EG/ES (Figure 11b). Thereby it should be emphasized that the structural characteristics of the surfactants

Figure 12. Langmuir adsorption isotherm for MO adsorption on modified bentonite (the same symbols as those in Figure 8, the vertical coordinates on the right for 4.0ES-Bt (■), and the one on the left for the others). 1766



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Table 5. The Parameters by Langmuir, Freundlich, and D-R Isotherms for MO Adsorption on ES-Bt/EG-Bt Langmuir isotherm b surfactant 4.0ES-Bt 8.0ES-Bt 4.0EG-Bt 8.0EG-Bt

Freundlich isotherm

D-R isotherm

qm

kg· mol−1 5

2.09·10 5.249·104 1.07·106 2.01·106

E

K′

qm

RL

mol·kg−1

R2

χ2

kf

n

R2

χ2

KJ·mol−1

mol2·KJ−2

mol·kg−1

R2

χ2

0.0617 to 0.0231 0.0080 to 0.0035 0.0028 to 0.0021 0.0020 to 0.0014

0.02 0.19 0.18 0.33

0.989 0.978 0.999 0.999

0.16 0.43 0.11 0.35

2.68 11.66 50.25 93.32

2.79 1.88 18.84 16.52

0.894 0.997 0.942 0.711

0.19 0.06 0.11 0.88

0.99 0.72 1.83 3.16

0.51 0.96 0.15 0.05

0.02 0.11 0.18 0.33

0.964 0.857 0.887 0.822

0.22 1.67 2.71 0.36

(1.96·10−2 mol·kg−1 and 1.90·10−1 mol·kg−1), which agrees with that by the adsorption experiment. 3.2.3. Adsorption Kinetics of MO on EG-Bt/ES-Bt. Figure 13a displays the time profile for MO adsorbing on ES-Bt and EG-Bt in

3.05·10−4 mol·kg−1 of MO. It is obvious that MO uptake by EGBt/ES-Bt rises up rapidly in the first 5.0·102 s, then slowly, and then plateaus. However, the time needed to reach the maximum uptake of MO is decreased with the incremental concentration of the surfactants in the modified clay. Figure 13b depicts the data fitting by the pseudo-second-order kinetics for MO adsorption on ES-Bt/EG-Bt (the data fitting by other kinetics models in Supporting Information, Figure S18) and Table 6 summarizes the corresponding kinetics parameters. The high R2 value for the pseudo-second-order kinetics (close to 1), as well as the concordance of qe,cal and qe,exp, indicates that pseudo-second-order kinetics describes rather well the adsorption of MO on ES-Bt/EG-Bt. 3.2.4. Adsorption Thermodynamics of MO on EG-Bt/ES-Bt. The plots by Van’t Hoff equation are depicted in Figure 14 for MO adsorption on EG-Bt/ES-Bt, and the corresponding thermodynamic parameters are summarized in Table 7.

Figure 13. (a) Time profile of MO adsorption on modified bentonite and (b) the pseudo-second order-kinetic plots (the same symbols as those in Figure 8, the vertical coordinates on the right for 4.0ES-Bt(■), and the one on the left for the others).

Figure 14. Plots of Van’t Hoff equation for adsorption of MO on modified bentonite (the symbols are the same as those in Figure 8).

The ΔG0ad value is negative and becomes more and more negative with elevated temperatures for the modified bentonites except 4.0ES-Bt, suggesting that MO adheres voluntarily onto the

Table 6. The Kinetic Parameters for MO Adsorption on ES-Bt/EG-Bt pseudo-first-order kinetic

pseudo-second-order kinetic a

Elovich equation α

β

k1

qe,cal

k2

qe,cal

surfactant−bentonite

min−1

mol·kg−1

R2

mol·kg−1

kg·mol−1·s−1

mol·kg−1

R2

χ2

mol·kg−1·s−1

kg·mol−1

R2

4.0ES-Bt 8.0ES-Bt 4.0EG-Bt 8.0EG-Bt

0.1477 0.1409 0.1659 0.3361

0.011 0.103 0.174 0.136

0.972 0.965 0.973 0.968

0.016 ± 0.505 0.116 ± 0.205 0.143 ± 0.007 0.143 ± 0.005

0.0626 0.0056 0.0032 0.0152

0.017 0.131 0.152 0.150

0.999 0.999 0.996 0.999

0.032 0.497 0.205 0.094

0.10 0.39 0.30 3.02

0.95 0.11 0.08 0.13

0.973 0.996 0.988 0.876

a

qe,exp

The expanded uncertainty was calculated using a coverage factor of 2, which gives a level of confidence of approximately 95 %. 1767



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Table 7. Thermodynamic Parameters for the Adsorption of MO on EG-Bt/ES-Bt 4.0ES -Bt

8.0ES-Bt

4.0EG-Bt

8.0EG-Bt

T

ΔG0ad

ΔS0ad

ΔH0ad

ΔG0ad

ΔS0ad

ΔH0ad

ΔG0ad

ΔS0ad

ΔH0ad

ΔG0ad

ΔS0ad

ΔH0ad

K

kJ mol−1

J mol−1

Jmol−1K−1

kJ mol−1

J mol−1

J mol−1K−1

kJ mol−1

J mol−1

Jmol−1K−1

kJ mol−1

J mol−1

Jmol−1K−1

283 288 293 298

8.086 7.142 6.984 7.021

−28.430 −24.660 −23.699 −23.426

39.927

−2.237 −3.624 −6.376 −8.073

8.302 12.976 22.144 27.469

112.503

−4.665 −7.152 −8.281 −10.105

16.818 25.161 28.586 34.224

94.089

−11.500 −11.778 −12.062 −12.351

40.651 40.913 41.183 41.462

4.559

modified clays, much preferably onto EG-Bt. The positive ΔS0ad for MO adsorption on the modified clays except on 4.0ES-Bt gets more positive with the raising temperature, suggesting an enhanced affinity of the modified clays toward MO.42 Contrary to the adsorption of EG/ES on the bentonite, the ΔH0ad value for MO sorption by EG-Bt/ES-Bt is positive in the temperature range of 283 to 298 K, indicating an endothermic and entropy driving process happening during the adsorption. However, positive ΔG0ad and negative ΔS0ad are observed for MO in 4.0NS-Bt, which is responsible for the poor uptake capacity of the clay toward MO. Linearizing the isotherms and kinetics equations will result in the inherent bias of the methods. We use the nonlinear regression Chi-square (χ2) eq 13 to value the fitting quality.43 ⎡ (q − qe,cal)2 ⎤ e,exp ⎥ ⎢ χ =∑ ⎥ ⎢ qe,cal ⎦ ⎣ 2

(13)

Figure 15. Thermogravimetric curves of EG-Bt-MO and ES-Bt-MO: 8.0EG-Bt-MO, 4.0EG-Bt-MO, 8.0ES-Bt-MO, and 4.0ES-Bt-MO is arranged from the top to the bottom, respectively.

Where qe,exp and qe,cal are the experimental data and the calculated equilibrium capacity by isotherms and kinetics equations, respectively. It is recognized that lower value of χ2 indicates better fitting. So the lower values of χ2 in Tables 1, 2, 5, and 6 validate our results. 3.3. The Mechanism of MO on Modified Bentonite. To understand the mechanism, XRD, TGA, and UV−vis were employed to probe ES-Bt-MO/EG-Bt-MO (MO adsorbing on ES-Bt/EG-Bt). A slightly enlarged basal spacing of ES-Bt/EG-Bt is observed upon the adsorption of MO (Supporting Information, Figure S19a,b), indicating that a small portion of MO goes into the interlayer of ES-Bt/EG-Bt while the majority of MO molecules stay on the surface of ES-Bt/EG-Bt. The basal spacing of ES-Bt is enlarged by 1.2 Å for 4.0ES-Bt and by 1.6 Å for 8.0ES-Bt with the adsorption of MO. However, it is almost unchanged for 4.0EG-Bt and diminished by 1.5 Å for 8.0EG-Bt with the adsorption of MO. Frost has reported the similar phenomenon.44−46 On the basis of their view about this issue, we could speculate that the inserted MO molecules may substitute some of EG molecules in the layers of 8.0ES-Bt, causing the rearrangement of EG, and diminishing the basal spacing. The thermogravimetric analysis on EG-Bt-MO/ES-Bt-MO is graphed in Figure 15 and Supporting Information, Figure S20. A comparison of the TG curves in Figure 15 with those in Figure 3b show that the change in these plots is pronounced, which reminds us that on the surface or in the interlayer of the bentonite, MO might interact with the adsorbed EG/ES. It is widely accepted that introducing organic cations can convert the hydrophilic clay into a hydrophobic clay and the formed organic phase acts as a partition medium of organic contaminants on the clay.47,48 According to this view, surfactantmodified bentonite ridding dye wastes relies on the fact that surfactants can solubilize organic compounds. However, it has

been widely accepted that surfactants can enhance sensitivity of trace pollutant detection, and that the strengthened sensitivity is rooted in the formation of a surfactant−contaminant complex.49−52 Bahrami investigated the combination of anionic azo dyes and gemini surfactants. He suggested that a dye−surfactant complex was assembled by the long-range electrostatic and shortrange hydrophobic forces between the dyes and the cationic gemini surfactants.53 So surfactants may bind strongly with dyes in solution. Could dyes interact with surfactants on the surface of Na-Bt? The interaction of molecules can be evaluated by many technologies including UV−vis spectroscopy. The changed structure or the formation of complexes characterizes the shifting position and varying absorbance in the UV−vis spectrum. Therefore, a solid-state UV−vis spectrum test was conducted to probe the interaction. Na-Bt shows a humpy peak at 258 nm, ES and EG have a unsymmetrical peak around 300 nm. 8.0Es-Bt, 8.0EG-Bt, and MO show three peaks, (221, 263, 486; 225, 264, 414; 256, 345 with 315 as shoulder, and 417) nm, with intensities decreasing in turn (Figure 16). However the second peak of MO diminishes as a shoulder in EG-Bt-MO/ES-Bt-MO, and the third peak in MO becomes strong and red-shifted. It is interesting that the intensity of the peaks in EG-Bt-MO is stronger than that in ES-Bt-MO, hinting that EG-Bt combines with MO more strongly than ES-Bt does. Therefore we could conclude that surfactants on organoclays may interact with dyes to form complexes besides solubilizing dyes in their micelle.

4. CONCLUSIONS From above discussion following conclusions can be drawn. 1768



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Article

ASSOCIATED CONTENT

* Supporting Information S

Additional tables and figures as described in the text. This material is available free of charge via the Internet at http://pubs. acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 817 2314508. Fax: +86 817 2568625. E-mail: [email protected]. Funding

We appreciate the Educational Department of Sichuan Province (No. 10ZA022) and the National Natural Science Foundation of China (21176201) for financial supports. Notes

The authors declare no competing financial interest.

■

REFERENCES

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Figure 16. Solid-state UV−vis spectra of the samples employed in this study (1) for MO, (2) for 8.0EG-Bt-MO (a) and 8.0ES-Bt-MO (b), (3) for EG (a) and ES (b), (4) for 8.0EG-Bt (a) and 8.0ES-Bt (b), (5) for NaBt.

(1) EG and ES behave differently in modifying Na-Bt. ES intercalates into the interlayer of the bentonite while EG molecules mainly stay on the surface due to the big head groups of EG. Although both surfactants are in an arrangement from bilayer to paraffin-type monolayer in Na-Bt, the gemini surfactant EG is more efficient at expanding the basal spacing of bentonite. (2) The intercalation of EG is more spontaneous than that of ES, and EG substitutes more water and Na+ than ES does during the intercalation. So the intercalation of EG on NaBt exhibits larger negative ΔG0ad and larger positive ΔS0ad. MO adheres voluntarily onto the modified clays, however, much preferably onto EG-Bt. The temperature affects oppositely the adsorption of the surfactants on Na-Bt and MO on EG-Bt/ES-Bt: the elevated temperature impairs the adsorption of the surfactants, but enhances that of MO. (3) Both EG and ES may interact with MO to form a complex on Na-Bt, but the gemini reacts more strongly with dyes. Generally speaking, gemini surfactants are more competent than the corresponding monomer in dye wastewater disposal. 1769



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A Comparison of New Gemini Surfactant Modified Clay with its

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