Kinetics and equilibriums for adsorption of poly(vinyl alcohol) from aqueous solution onto natural bentonite

Chemical Engineering Journal 214 (2013) 343–354

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Kinetics and equilibriums for adsorption of poly(vinyl alcohol) from aqueous solution onto natural bentonite Weishan Wang a,⇑, Baicun Zheng a, Zuiliang Deng b, Zhongjun Feng b, Lefeng Fu b a b

Research & Development Center for Sports Materials, East China University of Science and Technology, Shanghai 200237, PR China Shanghai Sunrise Polymer Co., Ltd., Shanghai 200232, PR China

h i g h l i g h t s " Kinetics and isotherms for adsorption of PVA onto bentonite were investigated systematically. " The adsorption process obeyed pseudo-second-order kinetic model. " Langmuir isotherm model fitted well with the experimental equilibrium data. " PVA molecules were adsorbed on the surfaces of bentonite particles with a certain degree of intercalation. " Zeta potential of bentonite particles decreased with increasing PVA concentration.

a r t i c l e

i n f o

Article history: Received 3 August 2012 Received in revised form 29 September 2012 Accepted 1 October 2012 Available online 10 November 2012 Keywords: PVA Bentonite Adsorption Kinetics Isotherm Zeta potential

a b s t r a c t Kinetics and isotherms for the adsorption of poly(vinyl alcohol) (PVA) from aqueous solution onto natural bentonite were investigated systematically with respect to initial concentration, contact time and temperature. The kinetics of the adsorption process were discussed using three kinetic models, viz., pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic model. The experimental data fitted well with pseudo-second-order kinetic model. The adsorption isotherms were also described by Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm model, respectively. It was found that the isotherms obeyed Langmuir model. FTIR results indicated the presence of PVA on the surface of bentonite/PVA complexes. XRD measurements showed that PVA molecules intercalated into the interlayers of bentonite. Zeta potential of bentonite particles decreased with increasing PVA concentration. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Stabilizing and flocculating properties of water-soluble polymers find practical applications in industry, agriculture, environment protection, etc. For example, stabilization effect is used to obtain stable suspensions and emulsions in the production of pigments [1,2], cosmetics [3] and pharmaceuticals [4]. Destabilization effect is helpful in water purification [5,6], mineral flotation [7], etc. All of which are ascribed to the interaction between polymers and solid surfaces, viz., steric stabilization and bridging flocculation. The wide applications of polymers mentioned above make it possible for polymer adsorption on the dispersed solid surfaces to attract attention. For which, investigation on polymer ⇑ Corresponding author. Tel./fax: +86 21 64251146. E-mail address: [email protected] (W. Wang). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.070

adsorption on the solid/liquid interface is of vital theoretical and practical importance. In this work, poly(vinyl alcohol) (PVA) and bentonite were chosen for studies because both of them find a wide practical application. PVA has main usage in adhesives, and is also very popular in many industrial branches as stabilizers and flocculants (e.g., production of cosmetics, pharmaceutics, paints and papers, etc.) [8]. The enormous amount of PVA discharged from industrial effluents has posed a significant threat to both human health and natural environment [9]. However, the conventional biological systems are not efficient for the degradation of PVA. Adsorption is one of the effective methods to remove contaminants from wastewaters. Even though the most promising adsorbent for adsorption is activated carbon with high surface area and high adsorption capacity, it is very expensive and has high operation costs. Therefore, there is a growing need to find adsorbents with high cost performance.

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Nomenclature

qm k1 k2 kp kf KL C

adsorption amount at time t (mg g1) adsorption amount at equilibrium (mg g1) monolayer adsorption capacity for Langmuir isotherm model (mg g1) adsorption capacity for D–R isotherm model (mg g1) rate constant for pseudo-first-order kinetic model (min1) rate constant for pseudo-second-order kinetic model (g mg1 min1) rate constant for intraparticle diffusion model (mg g1 min1/2) constant for Freundlich isotherm model (L g1) Langmuir isotherm model constant (L mg1) constant for intraparticle diffusion model (mg g1)

Recently, clay has been accepted as excellent adsorbents, catalysts, fillers in industry [10]. Among the clays studied, bentonite has received considerable recognition as an adsorbent because of its high adsorption capacity and low cost (compared with activated carbon). It is hydrated alumina-silicate clay primarily composed of the smectite group mineral montmorillonite. It is well known that the negative charge of clays, which is caused by isomorphous substitution of the layers by cations of lower valence, is balanced by exchangeable cations, such as Na+ and Ca2+ [11]. The wide usefulness of bentonite is essentially as a result of its high specific surface area, high chemical and mechanical stabilities, and a variety of surface and structural properties [12]. The adsorption of PVA onto clays has been studied by several researchers. Greeland [13] compared adsorption capacities of PVA on montmorillonite with different kinds of cations, and pointed out the polymer formed a layer of approximately 1 nm on the clay surface. Adsorption has also been studied for three different polymers (PVOH, HEC, HPMC) and four different types of solid (clay, mica, talc, limestone) by Chang et al. [14] without giving detailed analysis of equilibrium and kinetics. Emo Chiellini et al. [15] reported the adsorption and desorption of PVA on different solid substrates comprising montmorillonite, quartz sand, and farm solids as a function of PVA hydrolysis (72–98%), molecular weight and molecular weight distribution, and higher adsorption was detected on montmorillonite. Intercalation of PVA between the clay laminae was reported by De Bussetti and his coworker [16]. However, even though the adsorption of PVA onto clays is documented, there is still lack of understanding of the kinetics and equilibriums for adsorption of PVA onto bentonite. To clarify this point, we performed a systematic investigation of the adsorption of PVA from aqueous solution onto bentonite. The results provided an insight into the kinetics and isotherms for adsorption of PVA onto bentonite and allowed an assessment of the influence of temperature on the adsorption. Moreover, characterization of bentonite/PVA complexes (organo-bentonite) by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) allowed us to elucidate the interaction between bentonite and PVA. Finally, zeta potential of bentonite particles in aqueous suspensions was measured with different concentrations of PVA.

constant for D–R isotherm model (mg2 kJ2) time (min) initial concentration (g L1) concentration at time t (g L1) volume (mL) weight (g) wavelength of X-ray (nm) angle (°) temperature (K) Polanyi potential for D–R isotherm model correlation coefficient initial adsorption rate dimensionless constant for Langmuir isotherm

b t C0 Ct V m k h T

e r h RL

use. Chemical analysis of bentonite was performed by sequential X-ray fluorescence spectrometer (XRF-1800, Shimadzu Corporation) and the results are shown in Table 1. The mineralogical composition of the natural bentonite sample was determined by X-ray diffractograms (shown in Fig. 1). X-ray analysis of the sample was made using the three principal lines [17]. The following mineral phases were identified: montmorillonite, quartz, mica, magnesite, feldspar and calcite. The surface and pore size were summarized in Table 2 and the PSD of bentonite was shown in Fig. 2 (ASAP 2010N, Macromeritics). PVA with a degree of polymerization of 300 and degree of hydrolysis value of 87–89% was obtained from Kuraray Co., Ltd., Japan. Boric acid (99.5% pure) and potassium iodide (99.8% pure) were supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd., China and iodide (98.5% pure) was obtained from Shanghai Tianlian Fine Chemical Co., Ltd., China. The chemicals used for

Table 1 Main chemical composition of bentonite. Chemical composition (wt.%)

LOI (wt.%)

SiO2

Al2O3

MgO

CaO

Fe2O3

TiO2

K2O

Na2O

P2O5

72.99

16.33

4.67

3.18

1.78

0.35

0.17

0.04

0.05

11.70

LOI: Loss of ignition at 1273 K.

Q

Intensity (a.u.)

qt qe qmax

M-montmorillonite Q-quartz Mi-mica Mg-magnesite F-feldspar C-calcite

M Q

M

C F Mi

2. Materials and methods

Mg Q

Q

Mi

Q

Q

Mg

Q

2.1. Materials 10

Natural bentonite was kindly supplied by Huangshan Baiyue Activated Clay Co., Ltd. It was crushed, ground, sieved through a 200-mesh sieve and dried at 105 °C in an oven for 2 h prior to

20

30

40

50

o

2-Theta ( ) Fig. 1. XRD pattern of natural bentonite.

60

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In adsorption kinetics the adsorption amount of PVA at time t (min) was calculated by the following formula:

Table 2 Physical properties of bentonite. Sample

Bentonite 2

a

Microscope area (m /g) External surface area (m2/g)b BET surface area (m2/g)c Pore diameter (nm) a b c

qt ¼

11.8 26.8 38.6 7.6

ðC 0  C t ÞV m

ð1Þ

where qt is the adsorption amount of PVA at time t (mg g1), C0 is the initial concentration of PVA solution (mg L1), Ct is the PVA concentration at time t (mg L1), V is the suspension volume (mL), and m is the bentonite weight (g). According to the kinetic experiments, 600 min was enough for all the adsorption systems with different PVA concentrations. Consequently, the following equilibrium experiments were all conducted under the optimal conditions, in which the equilibrium time for the adsorption of PVA onto bentonite was 600 min, if not otherwise stated. Adsorption equilibrium study was carried out in order to confirm the relationship between adsorption amount and equilibrium concentration of PVA. The reproducibility during concentration measurements was ensured by repeating the experiments at least two times under identical conditions and the average values were reported.

Applying BJH model. External surface area = BET surface area  Microscope area. P/P0 ranges from 0.011 to 0.141.

2.0

1.5

2

3

Pore volume (×10 cm /g)

345

1.0

2.3. Characterization methods 0.5

0.0 100

1000

Pore diameter (A) Fig. 2. Pore size distribution of bentonite.

experiments were of analytical grade and were used without further purification. All the samples were prepared in deionized water (0.30 ls cm1). 2.2. Adsorption studies The adsorption experiments of PVA onto bentonite were carried out by using a horizontal thermostated shaker (SPH-103B, Shanghai Shiping Laboratory Equipment Co., Ltd.) operated at 300 rpm. A stock solution of 1 g L1 was prepared by dissolving a weighed amount of PVA in deionized water. The experimental solution was prepared by diluting the stock solution with deionized water if necessary. In each kinetic experiment, bentonite suspension, 0.1 wt.%, was prepared before 1 h. A series of PVA solutions with initial concentration range of 0.025–1.400 g L1 were obtained by diluting the stock solution above. Known volumes of PVA solutions and bentonite suspensions (100 mL, respectively) were transferred into 200 mL wide mouth bottle with glass stoppers to avoid evaporation. Then the dispersions were shaken in the shaker at the desired temperature. Kinetic studies were performed at 303–333 K. At various time intervals, the bottle was taken away from the shaker and aliquots of the suspension was collected in test tubes and was rapidly centrifuged using a laboratory centrifuge (TG16-WS, Shanghai Lu Xiangyi Centrifuge Instrument Co., Ltd.) at 12000 rpm (14,800g) for 10 min. The supernatant solutions were collected for further analysis. The PVA concentration before and after its adsorption was determined based on the reaction of PVA with H3BO3 and I2 solutions [18] according to the standard calibration curve. The complex PVA–H3BO3–I2 obtained colored the solution green. Its absorbance was colorimetrically measured at a kmax of 690 nm after 12 h from the start of the reaction with a UV–Vis spectrophotometer (UV-1800, Shanghai Mapada Instruments Co., Ltd.).

When the adsorption process was over, the dispersion was filtered using sintered disk filter funnel. The bentonite/PVA complexes were washed several times with deionized water until free of PVA in the filtrate detected by UV–vis spectrophotometer. The complex was resuspended in deionized water and centrifuged. The obtained complex was dried at 353 K in a hot air oven, and then gently crushed, ground in an agate mortar to pass through a 200 mesh sieve, and kept in a sealed bottle for further characterization. FTIR spectra of samples were acquired by a 6700 Fourier transform infrared spectrometer (Nicolet) using KBr pressed disk technique. The prepared sample and KBr were weighted and then were ground in an agate mortar prior to pellet making. The spectra were obtained by accumulating 32 scans at a resolution of 2 cm1 in the range of 4000–400 cm1. The X-ray studies were performed using the powder diffraction technique. The analysis was conducted using a D/MAX 2550 VB/PC diffractometer (Rigaku). The source of X-ray radiation was a sealed tube with a copper anode and nickel filter supplied by the generator (40 kV, 100 mA). Diffraction measurements were conducted with the 2h angle of 2–80° at the scanning rate of 0.02°/min. The basal spacing was calculated by using Bragg’s equation (Eq. (2)).

nk ¼ 2d sin h

ð2Þ

where n is an integer (n = 1), k is the wavelength of incident wave (k = 0.15418 nm), d is the basal spacing between the layers in the bentonite lattice and h is the angle between the incident ray and the scattering planes. Zeta potential of the suspensions was measured with a ZEN3600 Zeta Potential Analyzer (Malvern, UK). The zeta potential value for each sample was taken as an average of 24 measurements. 3. Results and discussion 3.1. Effect of initial PVA concentration, contact time and temperature The influence of initial PVA concentrations in the suspensions on the rate of adsorption onto bentonite was investigated between 0.025 and 1.40 g L1. As shown in Fig. 3, when the initial PVA concentration was increased from 0.025 to 1.40 g L1, the adsorption amount of PVA increased from 16.4 to 301.8 mg g1 at 303 K, from 20.6 to 618.7 mg g1 at 313 K, from 20.8 to 780.6 mg g1 at 323 K, and from 31.8 to 1031.8 mg g1 at 333 K, respectively. These

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0.025g/L 0.25g/L 1.00g/L

1200

1000

0.05g/L 0.40g/L 1.20g/L

0.10g/L 0.60g/L 1.40g/L

0.15g/L 0.80g/L

333 K

800

600

400

200

0 800

323 K

700 600 500 400 300 200

-1

Adsorption amount (mg g )

100 0 600

313 K

500

400

300

200

100

indicated the initial PVA concentration played an important role in the adsorption capacities of PVA onto bentonite. Meanwhile, in the first 60 min, the initial adsorption rate was greater for higher initial PVA concentration. Because the diffusion of PVA molecules through solution to the surface of bentonite was affected by the PVA concentration, since the shaking speed was constant. The increment in PVA concentration accelerated the diffusion of PVA molecules from solution onto bentonite surfaces due to the increase in driving force of the concentration gradients [19,20]. The effect of contact time on the amount of PVA adsorbed onto bentonite at various temperatures (shown in Fig. 3) was also investigated at the range of initial PVA concentration from 0.025 to 1.40 g L1. The rate of PVA adsorbed onto bentonite was rapid initially and then slowed down gradually until it attained an equilibrium beyond which there was no significant increase in the adsorption rate. When the equilibrium time was reached, the adsorption amount of PVA was not increased. The equilibrium time for the adsorption of PVA onto bentonite was 300–420 min. At the beginning, the PVA molecules were adsorbed on the exterior surface of bentonite, the adsorption rate of which was fast. When this adsorption process reached saturation, the PVA molecules entered into the pores of bentonite and were adsorbed by the interior surface of bentonite particles [21] or intercalated into the interlayers of bentonite. In this case a long time was needed. The adsorption curves were single, smooth and continuous leading to saturation. Based on this result, the contact time was fixed at 600 min for the rest of the batch experiments to make sure that equilibrium was reached in all cases. The adsorption amount of PVA onto bentonite at equilibrium was also affected by temperature from 303 to 333 K (shown in Fig. 4). As depicted in Fig. 4, the adsorption amount increased as the temperature increased, indicating favorable adsorption occurs at higher temperatures. The phenomenon may be due to the acceleration of originally slow adsorption or the creation of some new active sites on the adsorbent surface [22]. Similar results have been reported for the adsorption of Congo red onto Ca–bentonite [23] and for the adsorption of reactive red onto sonication-surfactantmodified attapulgite clay [24]. In general, before the equilibrium time, the increment in PVA adsorption owing to the increase of temperature showed a kinetically controlling process. After the equilibrium achieved, the uptake increased with increasing temperature indicated the adsorption of PVA onto bentonite was controlled by an endothermic process.

0 300

303 K 303 K 313 K 323 K 333 K

-1

Adsorption amount (mg g )

1000

200

100

800

600

400

200

0 0

0

100

200

300

400

500

600

Time (min) Fig. 3. Effect of contact time on the adsorption of PVA onto natural bentonite at 303–333 K.

0

200

400

600

800

1000

1200

-1

Ce (mg g ) Fig. 4. Adsorption isotherms of PVA onto bentonite at 303–333 K.

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3.2. Kinetic modeling

After integration and applying boundary conditions t = 0 to t = t and qt = 0 to qt = t, the integration form of Eq. (3) becomes

Generally, three steps are involved during the process of adsorption by porous adsorbent particles: external mass transfer, interparticle transport and chemisorption [25]. Several kinetic models have been proposed to clarify the mechanism of a solute sorption from aqueous solution onto an adsorbent: pseudo-firstorder kinetic model of Lagergren [26,27], pseudo-second-order kinetic model of Ho [26,28] and intraparticle diffusion model of Weber and Morris [29]. There is no study about kinetic modeling of adsorption of PVA from aqueous solution onto bentonite. In this work, adsorption kinetics of PVA onto bentonite was studied in terms of pseudofirst-order kinetic model, pseudo-second-order kinetic model and intraparticle diffusion model. 3.2.1. Pseudo-first-order kinetic model The pseudo-first-order equation of Lagergren [26,27] is expressed as follows:

dqt ¼ k1 ðqe  qt Þ dt

ð3Þ

ln

qe ¼ k1 t qe  qt

ð4Þ

Eq. (4) can be arranged to obtain a linear form:

lnðqe  qt Þ ¼ ln qe  k1 t

ð5Þ

where qt (mg g1) and qe (mg g1) are the adsorption amounts of PVA at time t and equilibrium, and k1 (min1) is the pseudo-first-order rate constant for the adsorption process. The values of qe, k1 and the correlation coefficients were determined from the linear plots of ln(qe  qt) versus t for various initial PVA concentrations (plots not shown) and presented in Table 3. The correlation coefficients for the pseudo-first-order kinetic model obtained at all the studied PVA concentrations were relatively high (see Table 3). However, although r2 values were reasonably high in some cases, the calculated qe values obtained from this equation did not give reasonable values, which were too low compared with the experimental qe values. This finding indicated that the adsorption process did not follow the pseudo-first-order kinetic model, even though these plots

Table 3 Kinetic parameters for the adsorption of PVA onto bentonite at 303–313 K. T (K) C0 (g/L)

qexp Pseudo-first-order model (mg/g) ksa1 qe r2 k1 (103 1/min) (104 L/min m2) (mg/g)

Pseudo-second-order model

Intraparticle diffusion model

k2 ksa2 qe r2 (104 g/mg min) (L/min m2) (mg/g)

kp ksap C r2 (mg/g min1/2) (L/min m2) (mg/g)

303

0.025 0.05 0.10 0.15 0.25 0.40 0.60 0.80 1.00 1.20 1.40

16.4 30.3 57.2 79.4 117.8 143.6 191.2 248.6 290.6 299.6 301.8

9.06 7.48 9.27 9.60 5.91 9.01 6.60 8.82 7.23 8.06 7.20

4.72 3.89 4.82 4.97 3.06 4.66 3.42 4.56 3.73 4.20 3.73

4.6 6.3 12.5 15.8 26.4 35.6 45.2 40.6 55.8 62.1 43.6

0.9568 0.8373 0.8147 0.7350 0.6823 0.8399 0.9442 0.8028 0.9357 0.9873 0.9536

61.98 39.17 23.44 18.20 7.84 7.83 4.72 7.52 4.40 4.19 5.45

32.11 20.30 12.15 9.43 4.06 4.06 2.45 3.90 2.28 2.17 2.82

16.6 30.6 57.8 80.3 113.6 145.1 193.1 250.6 293.3 302.1 312.5

0.9996 0.9995 0.9999 0.9999 0.9985 0.9990 0.9984 0.9999 0.9996 0.9995 0.9998

0.23 0.38 0.84 1.25 1.68 1.57 2.14 2.63 2.99 3.00 2.27

0.12 0.20 0.44 0.65 0.87 0.82 1.11 1.36 1.55 1.56 1.18

11.7 22.7 41.1 55.9 78.1 111.1 145.0 197.7 229.0 237.6 254.7

0.5368 0.3460 0.3027 0.2392 0.3830 0.4568 0.7725 0.3568 0.6555 0.7420 0.7390

313

0.025 0.05 0.10 0.15 0.25 0.40 0.60 0.80 1.00 1.20 1.40

19.4 8.24 40.8 8.08 72.0 9.20 98.4 9.28 153.7 8.12 242.3 8.69 364.6 8.31 451.0 9.61 576.2 11.44 609.1 7.10 618.7 9.62

4.25 4.20 4.77 4.82 4.20 4.51 4.30 4.97 5.91 3.68 4.97

5.3 7.0 15.1 14.2 39.1 28.0 40.0 24.3 35.6 28.6 43.7

0.9632 0.8753 0.8269 0.7494 0.8899 0.8432 0.7833 0.6204 0.6454 0.4834 0.7434

48.08 40.18 19.55 21.86 6.57 10.99 7.61 11.68 10.25 9.12 7.51

24.91 20.82 10.13 11.33 3.40 5.69 3.94 6.05 5.31 4.73 3.89

19.7 41.1 72.7 99.1 156.3 243.9 366.3 452.5 578.0 609.8 621.1

0.9995 0.9997 0.9998 0.9999 0.9996 0.9999 1.0000 1.0000 0.9998 1.0000 1.0000

0.28 0.39 0.94 1.12 2.28 1.85 2.74 2.97 3.72 2.72 2.94

0.14 0.20 0.48 0.58 1.18 0.96 1.42 1.54 1.93 1.41 1.52

13.8 33.1 53.8 77.3 108.3 206.5 311.5 396.6 508.2 558.0 562.2

0.6663 0.4335 0.3802 0.2160 0.3870 0.2601 0.3000 0.1803 0.1442 0.3901 0.4836

323

0.025 0.05 0.10 0.15 0.25 0.40 0.60 0.80 1.00 1.20 1.40

26.8 8.08 53.6 8.67 96.2 11.35 134.3 9.63 192.8 7.10 334.4 7.28 470.8 8.30 657.4 7.31 742.8 9.48 768.2 6.70 780.6 12.40

4.20 4.51 5.91 4.97 3.68 3.78 4.30 3.78 4.92 3.47 6.42

6.9 11.8 16.5 22.2 56.4 62.3 100.9 87.8 114.8 95.2 95.0

0.9449 0.8956 0.7872 0.7456 0.8001 0.7126 0.9176 0.7486 0.9512 0.7626 0.9225

37.85 23.58 20.83 14.01 3.91 3.89 2.74 3.07 2.81 2.71 4.55

19.61 12.22 10.79 7.26 2.03 2.02 1.42 1.59 1.46 1.40 2.36

27.1 54.2 97.1 135.5 196.1 344.8 476.2 662.3 746.3 769.2 787.4

0.9994 0.9998 1.0000 1.0000 0.9984 0.9997 0.9998 0.9999 0.9999 0.9999 1.0000

0.34 0.68 1.28 1.73 3.02 4.32 5.75 5.96 5.84 6.11 4.92

0.18 0.35 0.66 0.90 1.56 2.24 2.98 3.09 3.02 3.16 2.55

19.8 40.2 72.4 101.7 130.9 249.3 356.5 539.7 626.9 645.0 687.0

0.6332 0.5685 0.2426 0.2673 0.6549 0.2950 0.5443 0.4065 0.4758 0.4926 0.3712

333

0.025 31.8 7.93 0.05 67.0 10.02 0.10 113.6 8.35 0.15 168.4 9.84 0.25 255.4 7.10 0.40 415.2 10.89 0.60 597.6 7.12 0.80 869.2 9.43 1.00 986.8 7.60 1.20 1014.4 5.70 1.40 1031.2 8.30

4.09 5.18 4.35 5.08 3.68 5.65 3.68 4.87 3.94 2.95 4.30

11.3 12.6 20.9 36.0 63.0 118.8 112.3 163.8 111.4 109.5 137.8

0.9043 20.25 0.8082 25.46 0.7219 12.92 0.7882 8.39 0.9419 3.69 0.8145 2.95 0.8879 2.22 0.8935 1.92 0.7993 2.55 0.6501 2.17 0.9438 2.11

10.05 13.19 6.69 4.35 1.91 1.53 1.15 0.99 1.32 1.12 1.09

32.4 67.6 114.7 170.4 257.7 420.2 602.4 877.2 990.1 1016.5 1036.9

0.9980 0.9999 0.9998 0.9999 0.9989 0.9996 0.9997 0.9999 0.9999 0.9998 0.9999

0.56 0.69 1.58 2.56 0.14 5.06 6.49 9.80 7.37 7.45 7.19

0.29 0.36 0.82 1.33 0.07 2.62 3.66 5.08 3.82 3.86 3.72

20.3 25.2 83.3 119.9 189.5 314.3 465.6 678.9 841.6 862.3 886.2

0.5944 0.2837 0.2191 0.2345 0.6772 0.5269 0.5734 0.4382 0.4041 0.4072 0.5685

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had high correlation coefficients with the experimental data [30]. In addition, it was observed that, at all the PVA concentrations, the adsorption data were well represented by the pseudo-first-order kinetic model only in the first 60 min and thereafter it deviated from theory. The adsorption data were well represented only in the region where rapid adsorption took place. This confirmed that it was not appropriate to use the pseudo-first-order kinetic model to predict the adsorption kinetics of PVA onto bentonite for the entire adsorption process. Consequently, this kinetic model cannot describe the adsorption of PVA onto bentonite adequately during the whole adsorption period.

25

0.025g/L 0.25 g/L 1.00 g/L

20

0.05g/L 0.40g/L 1.20g/L

0.10g/L 0.60g/L 1.40g/L

0.15g/L 0.80g/L

333 K 15

10

5

3.2.2. Pseudo-second-order kinetic model The pseudo-second-order equation [26,28] is given by 0

dqt ¼ k2 ðqe  qt Þ2 dt

ð6Þ 323 K

Integration Eq. (6) for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt gives

20

1 1 ¼ þ k2 t qe  qt qe

15

ð7Þ

Eq. (7) can be rearranged to obtain the following linear from:

t 1 1 ¼ þ t qt k2 q2e qe

10

ð8Þ

-1

t/qt (min g mg )

5

where k2 (g mg1 min1) is the equilibrium rate constant of pseudosecond-order kinetic equation. The curve plots of t/qt versus t for the adsorption of PVA onto bentonite were drawn at various initial PVA concentrations (shown in Fig. 5), from which the kinetic data were calculated and given in Table 3. The values for r2 and qe also indicated that this equation produced better results (see Table 3): at all PVA concentrations, r2 for this kinetic model were found to be extremely high (>0.998), and the calculated qe values were also close to experimental data generally. These results implied that the adsorption system studied obeyed the pseudo-second-order kinetic model. The pseudo-second-order kinetic rate constants were used to calculate the initial adsorption rate given by h ¼ k2 q2e . The calculated h values were plotted against temperature in Fig. 6. The initial adsorption rate, h, was found to increase initially and then decrease generally. Increase in the initial adsorption rate at the beginning was ascribed to the endothermic nature of the adsorption system. Decrease at higher temperature meant too high temperature was not suitable for enhancing the initial adsorption rate.

0

30

313 K

25

20

15

10

5

0

35

3.2.3. Intraparticle diffusion model The adsorption of organic molecules onto solid surface is usually governed by either liquid phase mass transport or intraparticle mass transport rate. When the diffusion (internal surface and pore diffusion) of organic molecules inside the adsorbent is the ratelimiting step, then adsorption date can be presented by the following intraparticle diffusion model equation [28,29]: 1 2

qt ¼ kp t þ C

ð9Þ

303 K

30 25 20 15 10

1

where C is the intercept (mg g ) and kp is the rate constant of intraparticle diffusion (mg g1 min1/2). The pseudo-first-order and pseudo-second-order kinetic model cannot identify the diffusion mechanism, and then the kinetic data were analyzed by using the intraparticle diffusion model. According to this model, the plot of uptake, qt, versus the square root of time, t1/2, should be linear if the intraparticle diffusion is involved in the adsorption, and if these lines pass through the origin, then intraparticle diffusion is the rate

5 0 0

100

200

300

400

500

600

Time (min) Fig. 5. Pseudo-second-order kinetic model for the adsorption of PVA onto bentonite at 303–333 K.

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W. Wang et al. / Chemical Engineering Journal 214 (2013) 343–354

0.025g/L 0.25g/L 1.00g/L

0.8

0.6

1200

0.80g/L 1.00g/L 1.20g/L 1.40g/L

1000

0.05g/L 0.40g/L 1.20g/L

0.10g/L 0.60g/L 1.40g/L

0.15g/L 0.80g/L

333 K

800

0.4

600

400

0.2 200

0

0.0 8

323 K

600

450

-1

h (mg g min )

6

750

0.15g/L 0.25g/L 0.40g/L 0.60g/L

-1

4 300

2 -1

Adsorption amount (mg g )

150

0 60

45

0.025g/L 0.05 g/L 0.10 g/L

0

600

313 K

500

400

300

30

200

15

100

0

0 300

300

310

320

303 K

330

Temperature (K) Fig. 6. Initial adsorption rate versus temperature for adsorption of PVA onto bentonite at 303–333 K.

controlling step, otherwise this indicates that two or more steps occurred in the adsorption process. The plots (shown in Fig. 7) presented multilinearity, indicating that three steps took place. As can be seen in Fig. 7, the first sharper step was not observed and completed before 10 min, which may be considered as the external surface adsorption. The second step described the gradual adsorption from 10 to 60 min, where intraparticle diffusion was rate-controlling. The third step was attributed to the final equilibrium stage, in which intraparticle diffusion started to slow down due to the decrease of PVA concentration in solution. The slope of the second step characterized the rate parameter corresponding to the intraparticle diffusion, and the intercept was proportional to the boundary layer thickness. The r2 values of the intraparticle diffusion model were lower than that of the pseudo-second-order kinetic model, but this model indicated that the adsorption of PVA onto

200

100

0 5

10

15 1/2

t

20

25

1/2

(min )

Fig. 7. Intraparticle diffusion model for the adsorption of PVA onto bentonite at 303–333 K.

bentonite may be followed by an intraparticle diffusion model up to 60 min. This implied that although intraparticle diffusion was

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involved in the adsorption process, it was not the only rate-controlling step. Additionally, the specific surface area normalized rate constants (ksa) were calculated by the following equation:

k

6.0

ð10Þ

qa

303 K 313 K 323 K 333 K

6.5

5.5

where qa is the surface area concentration of bentonite in dispension (m2 L1), and ksa is the rate constant normalized to the specific surface area concentration (L min1 m2). The results were listed in Table 3.

5.0

lnqe

ksa ¼

7.0

4.5 4.0

3.3. Adsorption isotherms

3.5

Equilibrium adsorption isotherm is one of the most important data to elucidate the adsorption mechanism. Several isotherm equations were selected to describe the adsorption process, namely Langmuir [31], Freundlich [32] and Dubinin–Radushkevich [33] isotherms. The Langmuir equation is probably the best known and most widely applied adsorption isotherm. Langmuir isotherm models the single coating layer on the adsorbent surface, supposing that there exists specific adsorptive sites on the adsorbent surface. The linear form of Langmuir isotherm equation is represented as follows:

1 1 1 ¼ þ qe qmax qmax K L C e

ð11Þ

where qe (mg g1) is the adsorption amount of PVA at equilibrium, Ce (mg L1) is the PVA concentration in solution at equilibrium, qmax (mg g1) is the monolayer adsorption capacity of the adsorbent and the KL (L mg1) is the Langmuir constant related to the free energy of the adsorption. Plots of 1/qe versus 1/Ce for the adsorption of PVA onto bentonite (Fig. 8) gave a straight line of slope 1/(qmaxKL) and intercept 1/qmax. Freundlich’s empirical equation for the description of the adsorption process is based on the assumption that the adsorbent has a heterogeneous surface composed of different adsorptive sites. Adsorption on homogeneous surface follows the Langmuir isotherm. The Freundlich equation can be represented in the linear form as:

ln qe ¼ ln kf þ

1 ln C e n

ð12Þ

where kf (L g1) and n are Freundlich isotherm constants, being indicative of the extent of the adsorption and the degree of

0.06

303 K 313 K 323 K 333 K

0.05

-1

1/qe (g mg )

0.03

0.02

0.01

0.00 0.02

2.5 2

0.04

0.06

0.08

0.10

-1

1/Ce (L mg ) Fig. 8. Langmuir plots for the adsorption of PVA onto bentonite.

3

4

5

6

7

lnCe Fig. 9. Freundlich plots for the adsorption of PVA onto bentonite.

nonlinearity. qe and Ce reflect the same parameters described in the Langmuir equation. The plots of ln qe versus ln Ce for the adsorption of PVA onto bentonite (Fig. 9) were employed to generate the intercept value of kf and the slope of 1/n, and the constants were given in Table 4. The D–R isotherm is more general than the Langmuir isotherm, because it dose not assume a homogeneous surface or constant sorption potential [34,35]. The linear form of D–R equation is:

ln qe ¼ ln qm þ be2

ð13Þ

where b is a constant related to the adsorption energy (mg2 kJ2) and e is the Polanyi potential equal to RT ln(1 + 1/Ce), where R (J/ K mol) is the gas constant and T (K) is the absolute temperature. The slope of the plots of ln qe versus e2 for the adsorption of PVA onto bentonite (not shown) gives b and the intercept yields the adsorption capacity qm (mg g1). The D–R parameters were listed in Table 4. As can be seen from the data listed in Table 4 that the adsorption of PVA onto bentonite was better fitted to the Langmuir isotherm model than the Freundlich and D–R isotherm model as indicated by the r2 values, which was indicative of the homogeneity of the adsorptive sites on the bentonite particles. It was also noted that the values of n were bigger than 1, reflecting the beneficial adsorption. The essential features of the Langmuir isotherm can be expressed in terms of a dimensionless constant called equilibrium parameter (RL), which is defined by the following equation [29]:

RL ¼

0.04

0.00

3.0

1 1 þ K LC0

ð14Þ

where C0 (mg g1) is the highest initial PVA concentration and KL reflects the same parameter in the Langmuir equation. The values of RL indicates the shape of the isotherms to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0). The effect of isotherm shape on whether adsorption was favorable or unfavorable was evaluated. For a Langmuir-shaped adsorption process, the isotherm shape can be classified by Eq. (14). It was observed that the RL values in the range of 0–1 confirmed that the adsorption of PVA onto bentonite was favorable. Plots of the calculated RL values versus initial PVA concentrations were shown in Fig. 10. Lower RL values at higher initial PVA concentrations indicated that the adsorption process was more favorable at higher concentrations.

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W. Wang et al. / Chemical Engineering Journal 214 (2013) 343–354 Table 4 Adsorption isotherm constants for the adsorption of PVA onto bentonite. T (K)

Langmuir

Freundlich

qmax (mg g 303 313 323 333

1

)

319.5 632.9 806.5 1063.0

3

KL (10 L mg

1

)

r

3.09 1.10 0.52 0.25

2

0.9983 0.9911 0.9914 0.9718

RL

kf (L g

0.188–0.928 0.394–0.973 0.579–0.987 0.741–0.994

2.880 2.237 4.267 6.273

Dubinin–Radushkevich (D–R)

1

)

1.0

2

n

r

1.478 1.207 1.272 1.267

0.9724 0.9881 0.9786 0.9668

qm (mg g1)

b (104) (mg2 kJ2)

r2

157.6 265.3 347.8 452.2

1.21 1.10 0.59 0.36

0.3193 0.2785 0.2641 0.2847

4.2

3.9

RL

lnK2

0.8

3.6

0.6 3.3

303 K 313 K 323 K 333 K

0.4

0.0

0.3

3.0 0.6

0.9

1.2

0.00300

1.5

0.00306

0.00312

3.4. Thermodynamic parameters

ð15Þ

where k2 (g mg1 min1) is the rate constant of pseudo-second-order kinetic model, Ea (kJ mol1) is the Arrhenius activation energy of adsorption and A is the Arrhenius factor, R is the gas constant which is equal to 8.314 J mol1 K1, and T (K) is the solution temperature. The linear plot of ln k2 versus 1/T (shown in Fig. 11) gives a slope of Ea/R. The magnitude of Ea gives an opinion about the mechanism of adsorption of 5–40 kJ mol1 for physical adsorption and 40– 800 kJ mol1 for chemical adsorption [36]. The activation energy obtained in this study was 30.36 kJ mol1 indicating that PVA adsorption onto natural bentonite corresponded to physisorption. Hence, the affinity of PVA molecules onto bentonite may be attributed to Van de Waals forces or hydrogen bonds between PVA molecules and the surface of bentonite particles. The thermodynamic parameters such as Gibbs free energy change DG0, standard enthalpy DH0, and standard entropy DS0 were also evaluated to understand the influence of temperature on the adsorption process better. Calculations were performed using 0.1 g L1 PC solution at various temperatures. The thermodynamic parameters were determined by the following equations and were presented in Table 5,

KC ¼

C ads C sol

Fig. 11. Plot of ln k2 versus 1/T for PVA adsorption onto bentonite.

Temperature (K)

Kc

DG 0 (kJ mol1)

D H0 (kJ mol1)

DS0 (J K1 mol1)

303 313 323 333

0.4481 0.6340 1.1552 1.7473

2.02 1.19 0.39 1.54

39.17

138.79

where KC is the equilibrium constant, Cads is the amount of PVA (mg) adsorbed on the bentonite per liter of the solution at equilibrium, and Csol is the equilibrium concentration (mg L1) of the PC in the solution. T is the absolute temperature of the solution (K) and R is the gas constant with the value of 8.314 J mol1 K1. DH0 and DS0 were calculated from the slope and intercept of van’s Hoff plots of ln KC versus 1/T (see Fig. 12). The results were listed in Table 5. The decrease of the change in Gibbs free energy change from 2.02 to 1.54 kJ mol1 with the increase of temperature from 303 K to 333 K indicated that the presence of an energy barrier at low temperature in the adsorption and the adsorption was favorable at high temperature. Generally, the absolute magnitude of the change in adsorption enthalpy for physisorption is in the range of 20 to 40 kJ mol1, and chemisorption is between 400 and 80 kJ mol1. The positive DH0 (39.17 kJ mol1) shown in Table 5 revealed the adsorption was endothermic and physical in nature. Furthermore, positive DS0 (138.79 J K1 mol1) of the adsorption process meant an irregular increase of the randomness at the PVA/bentonite interface during adsorption process.

ð16Þ

DG0 ¼ RT ln K C 0

ln K C ¼

0.00330

Table 5 Thermodynamic parameters for the adsorption of PVA onto bentonite.

The Arrhenius equation was applied to evaluate the activation energy of the adsorption process, the Arrhenius equation was applied by the following relationship:

Ea RT

0.00324

1/T(K )

Fig. 10. Plots of equilibrium parameter (RL) versus initial PVA concentration at 303– 333 K.

ln k2 ¼ ln A 

0.00318 -1

-1

Co (g L )

ð17Þ

3.5. FTIR spectra

ð18Þ

FTIR is a sensitive method to probe the molecular environment of organic groups with the organoclay [37]. FTIR spectra of PVA,

0

DS DH  R RT

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W. Wang et al. / Chemical Engineering Journal 214 (2013) 343–354

the Si–O stretching vibration, Si–O–Al (octahedral) bending vibration and Si–O–Si bending vibration, respectively. Compared to natural bentonite, the spectra of bentonite/PVA complexes showed two additional peaks at 2920 and 2880 cm1, which were attributed to the asymmetric and symmetric stretching vibrations of the methyl and methylene groups. This observation indicated the presence of the PVA molecules on the surface of bentonite/complexes.

0.6

0.3

lnKC

0.0

-0.3

3.6. X-ray diffraction -0.6

-0.9 0.00301

0.00308

0.00315

0.00322

0.00329

-1

1/T (K ) Fig. 12. Plot of ln Kc versus 1/T for estimation of thermodynamic parameters.

PVA

Transmittance (a.u.)

NBT OBT-0.025-PVA

3500

3000

The most widely used method for the study of intercalation surfactants in the galleries of phyllosilicates is XRD, which provides information on the interlayer structure of surfactant [39]. The XRD patterns of natural bentonite and bentonite adsorbed different initial concentrations of PVA were shown in Fig. 14. An intense reflection at 2h = 7.14° (corresponding interlayer distance was 1.24 nm (see Table 6)) was observed for natural bentonite, which was attributed to the d001 plane of montmorillonite. In comparison with natural bentonite, the d001 peak of organo-bentonite shifted towards lower, viz., decreased from 5.88° to 5.70° with the increment in the initial PVA concentration from 0.025 to 0.400 g/L, and the corresponding basal spacing increased from 1.50 to 1.55 nm (shown in Table 6). These results indicated the expansion of the interlayer space of bentonite due to the intercalation of PVA molecules.

OBT-0.05-PVA

3.7. Zeta potential measurement

OBT-0.25-PVA

The zeta potential is an electrical potential in the double layer at the interface between a particle, which moves in an electrical field and the surrounding liquid. The estimation of the zeta potential in bentonite dispersion is complicated because bentonite has two different charges-negative charges in large surfaces and positive

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 13. FTIR spectra of PVA, natural bentonite and bentonite adsorbed different initial concentrations of PVA.

natural bentonite and bentonite adsorbed several initial concentrations of PVA (NBT represents natural bentonite, and OBT-x-PVA represents bentonite adsorbed PVA with the concentration of x) were shown in Fig. 13, depicting the major changes of bentonite before and after the adsorption of PVA. After the adsorption of PVA, the spectra of the OBT not only had characteristic bentonite bands, but also exhibited some new characteristic bands. As can be seen from Fig. 9, the adsorption peak at 3620 cm1 was assigned to the –OH stretching vibration of water molecules within the bentonite interlayer and weakly bonded to the Si–O surface, and the broad peak centered at 3470 cm1 was due to the – OH stretching vibration of adsorbed water. The peak at around 1640 cm1 corresponded to the –OH deformation of water in both natural bentonite and organo-bentonite. The bands observed at around 1110 and 1040 cm1 represented the Si–O coordination vibrations and the stretching vibrations of Si–O in the Si–O–Si groups of the tetrahedral sheet respectively, which corresponded to the characteristic band of montmorillonite [38]. The –OH bending vibration peaks of octahedral layer was observed at 914 cm1 (AlAlOH) and 847 cm1 (AlMgOH), which pointed out the octahedral substitution process. The peaks originating from the external bentonite components (for example, quartz) were located at 796, 779 and 694 cm1. Also, 625, 521 and 467 cm1 were assigned to

OBT-6 OBT-5 OBT-4 OBT-3 NBT-2 OBT-1 NBT

120

100

80

60 o

40

20

2-Theta ( C) Fig. 14. XRD patterns of natural bentonite and bentonite–PVA complexes.

Table 6 Basal spacings of NBT and OBT. Sample

Initial PVA concentration (g L1)

d001 (nm)

NBT OBT-1 OBT-2 OBT-3 OBT-4 OBT-5 OBT-6

0.000 0.025 0.050 0.100 0.150 0.250 0.400

1.24 1.50 1.50 1.52 1.53 1.54 1.55

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W. Wang et al. / Chemical Engineering Journal 214 (2013) 343–354 0

-26

-32 -34

-6

-36 -38

-8 -40

zeta potential (mV)

-30 -4

-10

-28

Zeta potential (mV)

without PVA 1.0 g/L PVA

-2

Zeta potential (mV)

-5

-24

-15

-20

-25

-42 -10 -44 2

4

6

8

-30

10

pH

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

-1

Fig. 15. Zeta potentials of bentonite dispersions as a function of pH value in the absence and presence of PVA.

charges in narrow edges respectively. Fig. 15 showed the zeta potentials of bentonite dispersions as a function of pH value in the absence and presence of PVA. A cation-exchanging reaction depends on the negative charge of the clay minerals, which can be divided into two types: permanent charge independent of pH value and variable charge, which depends strongly on pH value of the solution [40]. When acid was added into the dispersion, the proton of Al–OH group at the edge surface of bentonite crystal was not easily dissociated and even excess proton was adsorbed as follows:

Al—OH þ Hþ ! Al—OH2 Hence, the negative charge on the edge surface was reduced. Correspondingly, zeta potential of bentonite particles decreased in Fig. 15. Meanwhile, net charge of bentonite particles was negative. Hence, no isoelectric point was found over the pH range examined. When the dispersion was made basic by adding NaOH solution, the proton of Al–OH group dissociates as follows: 

Al—OH þ OH ! AlO þ H2 O Thus, the negative charge increased. In other words, the polarity of the electrical double layer on the edge surface of bentonite crystal changes with pH value in the dispersion. PVA is a nonionic structural polymer that is dissolvable in water, which does not interact electrostatically with charged surface of bentonite particles. PVA molecules can attach or anchor on bentonite particles surfaces by Van de Waals forces or hydrogen bonds and intercalate into the interlayers. Adsorption of PVA onto the charged surfaces of bentonite particles leads to a significant change in the charge distribution of the electrical double layers. As can be seen from Fig. 15, the zeta potential for bentonite dispersions in the presence of PVA was higher than that without PVA over the pH range studied. This indicated that bentonite is negatively charged to a much smaller extent after PVA adsorption. The variation of zeta potential of bentonite with different PVA concentrations (0.025–1.40 g L1) was presented in Fig. 16. The magnitude of the zeta potential decreased dramatically with increasing PVA concentration at the beginning and then slowed down, the trend of which was consistent with the adsorption data above (Fig. 4). This could be explained by a screening effect on the electrical charges of edges of bentonite particles, which cause a decrease in electrostatic interactions between particles. As a result of the screening of the surface charge by the adsorption of PVA, electrostatic interactions would become less, and there would also be a

PVA concentration (g L ) Fig. 16. Zeta potential of bentonite suspension with different concentrations of PVA.

decrement in zeta potential [41]. The other reason may be increasing viscosity of the dispersion (caused by high concentration of PVA) lowered the mobility of the bentonite particles and made the zeta potential decrease. 4. Conclusions (1) The initial PVA concentration played an important role in the adsorption capacities of PVA onto bentonite. The adsorption rate was rapid initially and then slowed down gradually until it attained an equilibrium. The equilibrium time was fixed at 600 min. The adsorption amount of PVA onto bentonite increased as the temperature increased, indicating favorable adsorption occurs at higher temperature. (2) The results of kinetic modeling for the data of adsorption of PVA onto bentonite indicated the adsorption system studied obeyed the pseudo-second-order kinetic model more than the pseudo-first-order kinetic model. The plots of intraparticle diffusion model implied the adsorption process occurs by means of three steps and it was not the only rate-controlling. (3) The isotherms study showed that Langmuir isotherm model fitted well with the experimental equilibrium data, indicating the homogeneity of the adsorptive sites on the bentonite particle surface. The thermodynamic parameters indicated adsorption of PVA onto bentonite corresponded to physisorption. (4) The two additional peaks centered at 2920 and 2880 cm1 of bentonite/PVA complexes compared with natural bentonite indicated the presence of PVA molecules on the surface of bentonite/PVA complex. The increment in the basal spacings of bentonite/PVA complexes with several PVA concentrations implied the PVA molecules intercalated into the interlayers of bentonite. The zeta potential of bentonite particles decreased with increasing PVA concentration because of a layer of PVA chains on the surface of bentonite particle.

Acknowledgements The financial support (11nm0501900, 10DZ2290600) from Science and Technology Commission of Shanghai Municipality was gratefully acknowledged. The research was also partially supported by the science and technology cooperation project between Yunnan Province and University/China Academy (2009AD008).

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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2012.10.070.

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