Preparation and characterization of anion–cation organopalygorskite for 2-naphthol removal from aqueous solution

Journal of Molecular Liquids 195 (2014) 116–124

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Preparation and characterization of anion–cation organopalygorskite for 2-naphthol removal from aqueous solution Yanfang Tai a,b, Chunjie Shi a, Chuanhu Wang a,⁎ a b

School of Applied Chemistry and Environmental Engineering, Bengbu College, Bengbu 233030, Anhui, People's Republic of China School of Chemical Engineering, Hefei University of Technology, Hefei 230009, Anhui, People's Republic of China

a r t i c l e

i n f o

Article history: Received 10 August 2013 Received in revised form 17 January 2014 Accepted 5 February 2014 Available online 26 February 2014 Keywords: Palygorskite Organic modification 2-Naphthol Sorption Preconcentration mechanism

a b s t r a c t In this study, anion–cation organopalygorskite (ACOP) was prepared using microwave-assisted technique and characterized in detail. The elemental analysis and Fourier transform infrared (FTIR) spectroscopy analysis indicate that sodium dodecyl sulfate (SDS) and dioctadecyl dimethyl ammonium chloride (DDAC) have been successfully coated on the surface of palygorskite. The prepared ACOP was used for the preconcentration of 2naphthol from aqueous media. The effects of pH, ionic strength, agitation time and temperature were investigated. The sorption kinetic data can be well fitted by the pseudo-second-order model. The Langmuir, Freundlich and Linear models were applied to simulate the sorption isotherms and the results showed that the Freundlich model fitted the experimental data best. The thermodynamic parameters calculated from the temperature-dependent sorption isotherms indicated that the sorption of 2-naphthol on ACOP was an exothermic and spontaneous process. By integrating the above-mentioned laboratorial results together, the preconcentration of 2-naphthol on ACOP is considered to be a combination of surface sorption and partitioning mechanism. The findings herein suggested that ACOP can be potentially used as a cost-effective material for the preconcentration of actual organic contaminant-bearing effluents. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Clay minerals are abundant in nature. They are used as catalysts, photochemical reaction reagents, nanocomposites and sorbents in a variety of industries [1]. In addition, clays have been widely used as sorbents in environmental pollution management, and one of the most commonly used clays is palygorskite due to its high surface area and moderate cation exchange capacity (CEC) [2,3]. Palygorskite is a hydrated magnesium silicate, which exists in nature as a fibrous clay mineral with a structure consisting of parallel ribbons of 2:1 layers. The negatively charged layer surface of palygorskite is attributed to the isomorphous substitution within the layers (e.g. the replacement of Al3+ for Si4+ in the tetrahedral sheets) [4]. The above characteristics of palygorskite make it an excellent sorbent towards heavy metals [5], radionuclides [6] and cationic dyestuffs [7,8]. However, palygorskite only weakly sorbs hydrophobic organic compounds from aqueous solutions in its natural form because of the strong hydration of its inorganic exchangeable ions [9]. This deficiency can be remedied through organic modification of palygorskite with surfactants as the surfactants have long hydrophobic tails and can be adsorbed on the surface of palygorskite. ⁎ Corresponding author. Tel.: +86 552 3171286. E-mail address: [email protected] (C. Wang).

http://dx.doi.org/10.1016/j.molliq.2014.02.011 0167-7322/© 2014 Elsevier B.V. All rights reserved.

Researches on the organic modification of palygorskite mainly focus on palygorskite modified by the alkyl quaternary ammonium cationic surfactants. Many mechanisms have been postulated for hydrophobic organic compound removal on organic modified palygorskite, including partitioning, electrostatic attraction, ion exchange and van der Waals forces [10]. Sarkar et al. [11] investigated the synthesis and characterization of organopalygorskites by using dimethyldioctadecylammonium bromide (DMDAB) and cetylpyridinium chloride (CPC) with surfactant loadings equivalent to 100% and 200% CEC of the palygorskite, and then they used the organopalygorskites for adsorptive removal of p-nitrophenol (PNP). Huang et al. [12] used octodecyl trimethyl ammonium chloride (OTMAC) to modify the palygorskite for phenol removal. The results showed that phenol could be effectively removed from aqueous solutions by OTMAC modified palygorskite. Zhu and Chen have studied the organic modification of bentonite with the mixture of anionic and cationic surfactants and found that the mixture of anionic and cationic surfactants could form mixed micelles, which could produce synergy solubilization to organic compounds [13]. According to our literature survey, few researches focus on the modification of palygorskite by using anionic and cationic surfactants simultaneously, especially the study on the sorption behavior of hydrophobic organic compounds on palygorskite modified with the mixture of anionic and cationic surfactants are still scarce [14].

Y. Tai et al. / Journal of Molecular Liquids 195 (2014) 116–124

In this paper, anion–cation organopalygorskite (ACOP) was modified with sodium dodecyl sulfate (SDS) and dioctadecyl dimethyl ammonium chloride (DDAC). 2-Naphthol has been selected as a model of hydrophobic organic compound to investigate its sorption behavior on ACOP. 2-Naphthol, mainly discharged from industries related with medicine, dyestuff, photograph, and agrochemical industries, is of particular concern due to its acute toxicity, low biodegradation and negative environmental impacts [15]. For the purpose of understanding the mechanisms of 2-naphthol preconcentration from aqueous media, the equilibrium data was fitted to evaluate isotherms (Langmuir, Freundlich, and Linear), kinetic and thermodynamic parameters, since the modeling of sorption isotherms, kinetics and thermodynamics is important for predicting the preconcentration mechanisms of 2-naphthol on ACOP. 2. Materials and methods 2.1. Materials and reagents The starting palygorskite sample was obtained from Xuyi county (Jiangsu, China) and milled through a 200-mesh screen prior to its use in the experiments. 2-Naphthol (molecular formula: C10H7OH, molecular weight: 144.2), SDS, DDAC and other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China) in analytical purity and used directly without any further purification. Deionized water was used in all experiments. 2.2. Preparation of ACOP The natural palygorskite was treated with 1.0 mol·L−1 HCl for 12 h at room temperature. The obtained solid was separated from water by vacuum filtration and washed with deionized water for several times until Cl− was not tested with 0.1 mol·L−1 AgNO3. After drying treatment, the solid was milled through a 200-mesh screen and modified with a 1.0 mol·L−1 NaCl solution to prepare Na-palygorskite. Excess NaCl and other exchangeable cations were removed from Napalygorskite by filtering. Then it was washed with deionized water until no Cl− was detected with 0.1 mol·L−1 AgNO3. The obtained Napalygorskite was dried at 105 °C for 2 h to eliminate the free water. The cation exchange capacity (CEC) of Na-palygorskite was measured to be 0.385 mmol·g−1 by using an ethylenediamine complex of Cu [16]. SDS and DDAC, as the anionic surfactant and cationic surfactant, respectively, were used for the organic modification of Na-palygorskite. In order to prepare the ACOP, SDS and DDAC were added into 2% (weight ratio) Na-palygorskite suspension simultaneously. The ratios of DDAC/ Na-palygorskite and SDS/Na-palygorskite were 0.77 mmol·g− 1 and 0.19 mmol·g−1, respectively. The mixture was put into a microwave reactor and irradiated at 800 W for 5 min. Then the mixture was separated from water by vacuum filtration and washed with deionized water for several times. The obtained ACOP sample was dried at 80 °C for 4 h and milled through a 200-mesh screen. 2.3. Characterization ACOP was characterized by XRD, FTIR, SEM, elemental analysis and zeta potential measurement. The carbon, nitrogen and sulfur contents were determined quantitatively via elemental analyses in a CNHS analyzer (Vario EL IIIEl emental Analyzer, Germany). X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max diffractometer using Cu Kα radiation over 2θ from 3° to 70° and the XRD device was operated at 40 kV and 100 mA. The Fourier transform infrared (FTIR) spectroscopy of Na-palygorskite and ACOP were recorded from 400 cm−1 to 4000 cm−1 on a Fourier FTIR spectrometer (Nicolet 67) by KBr pellet method. N2 adsorption–desorption isotherms (at 77.3 K) were obtained using a Tristar II 3020 automatic surface area and pore analyzer. The adsorption data were then employed to determine surface area using the

117

Brunauer, Emmett, and Teller (BET). Zeta potentials of ACOP in solid/ water suspensions at variety of pH were measured by a Zetasizer Nano ZS90 to analyze the charging property. The scanning electron microscopy (SEM) images of Na-palygorskite and ACOP were recorded by a field emission scanner (FE-SEM, JEOL JSM-6700 F). 2.4. Batch sorption of 2-naphthol onto ACOP Batch sorption experiments were carried out in 10 mL glass centrifuge tubes equipped with Teflon-lined screw caps under ambient conditions. ACOP, NaCl and 2-naphthol stock solutions were added to achieve the desired concentrations of individual components. The effect of pH on the sorption of 2-naphthol was determined at initial concentrations of 30, 50 and 70 mg·L−1, respectively, and the equilibrium pH was set over a range of 2–12. The suspension pH was adjusted to desired values by adding negligible volumes of 0.01 or 0.1 mol·L−1 HCl or NaOH solution. The errors for adjusting various pH values were ± 0.1. Then the centrifuge tubes containing suspensions with various reaction conditions were oscillated on a rotary shaker at 150 rpm for 2 h. For sorption kinetic study experiments, the contact time was ranging from 5 min to 120 min and the pH was close to 6.5 ± 0.1. The reason for carrying out the experiments at pH = 6.5 ± 0.1 is that the natural pH of the suspensions is close to 6.5 ± 0.1. For thermodynamic study, the suspensions were shaken continuously on a thermostatic rotary shaker at three different temperatures. After reaching equilibrium, the glass centrifuge tubes were centrifuged at 4000 rpm for 30 min to separate the solid from liquid phases. The supernatant was filtered through 0.45 μm cellulose acetate membrane filter. The concentration of 2-naphthol in the supernatant was determined by UV–vis spectrophotometer operated at the wavelength of 290 nm. In order to further enhance the determination sensitivity, the analytical solution was basified to pH ~ 11 with 0.1 mol·L−1 NaOH to ensure that 2-naphthol is present in the dissociation state. The detection limit using this protocol was approximately 0.05 mg·L−1. The percentage (sorption% = (C0 − Ce) / C0 × 100%), amounts of 2naphthol adsorbed by ACOP (qe = V(C0 − Ce) / m) and distribution coefficient (Kd = (C0 − Ce) / Ce × V/m) were derived from the difference of initial concentration (C0), the final concentration in supernatant (Ce), the mass of ACOP (m) and the volume of suspension (V). 2.5. Regeneration experiments The repeated availability of ACOP through many cycles of sorption/ desorption was investigated to evaluate the application potential of this material in the removal of 2-naphthol from wastewater in real work. Desorption was measured immediately after sorption with the initial 2-naphthol concentration of 150 mg·L−1 at 298 K. After centrifugation treatment, all the supernatants were pipetted out, then the 2naphthol-loaded ACOP was left in contact with deionized water. Desorption was continued for 4 h on the same rotary shaker at a rotating speed of 150 rpm at 298 K. The supernatants were separated by centrifugation at 4000 rpm for 30 min. The equilibrium concentration of 2naphthol in the supernatant was determined spectrophotometrically. The sorption/desorption test was conducted consecutively for 5 times. 3. Results and discussion 3.1. Characterization of ACOP The SEM micrographs of Na-palygorskite and ACOP are shown in Fig. 1. One can see that the morphological differences between Napalygorskite and ACOP are distinct. The Na-palygorskite is present in the form of dispersive fibers with a high length-diameter ratio, while ACOP is present in the form of fiber aggregate with a rough surface. This is because the fibers are bound together by surfactant molecules. The results indicate that Na-palygorskite has been modified by

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Y. Tai et al. / Journal of Molecular Liquids 195 (2014) 116–124

Fig. 1. Scanning electron microscopy (SEM) images of Na-palygorskite (A) and ACOP (B).

surfactants and the surface properties of Na-palygorskite have been changed obviously. Similar observations were found in the SEM images of organoclays investigated by Bhatt et al. [17]. The XRD patterns of Na-palygorskite and ACOP are shown in Fig. 2A. It is observed that attapulgite is the main component phase in the clay used in this study, with appreciable amounts of montmorillonite and quartz. The characteristic diffraction peak is found in the patterns of Napalygorskite and ACOP at about 2θ = 8.3°, corresponding to (1 1 0) reflections. The characteristic peak positions and d-spacing of samples do not change obviously after surface modification, indicating that the structure and crystallinity of palygorskite are maintained in the ACOP [18]. Fig. 2B shows the FTIR spectra of Na-palygorskite and ACOP. As shown in Fig. 2B, compared with the absorption band Na-palygorskite, the absorption bands at 3428 and 1650 cm−1 of the modified

(A)

palygorskite sample, corresponding to the \OH stretching vibration and bending vibration of H2O of palygorskite, weaken and shift to the lower wavenumber. This result indicates that the H2O content is reduced with the replacement of the surfactant ions, resulting in the surface properties of Na-palygorskite changing from hydrophilic to hydrophobic. The peaks at 1018 cm−1 and 475 cm−1 are attributed to Si\O\Si bonds. Moreover, the surfactant modified palygorskite shows the characteristic absorption bands of symmetric and asymmetric stretching vibration of the CH2 and CH3 at around 2924 cm−1 and 2855 cm−1. The results indicate that DDAC and SDS have been successfully coated on the surface of palygorskite. The N2 adsorption–desorption isotherms of Na-palygorskite and ACOP in Fig. 2C represent type II isotherm with a narrow hysteresis loop according to the IUPAC classification [19]. A summary of the surface

(B) Q

Relative Intensity

P P P

Q

M

Q

2855 2924

Transmittance (%)

P: Palygorskite Q: Quartz M: Montmorillonite

Na-palygorskite

ACOP

1650 474

3428

1019

Na-palygorskite

1655

35603425

475

ACOP

0

10

20

30

40

50

60

1018

70

4000 3500 3000 2500 2000 1500 1000

500

Wavenumber (cm-1)

2 Theta (degree)

(C)

(D)

300 Na-palygorskite ACOP

30

Zeta potential (mV)

Quantity adsorbed (cm3 g-1)

40 ACOP Na-palygorskite

250 200 150 100 50

20 10 0 pHiep

pHiep

-10 -20 -30

0 0.0

0.2

0.4

0.6

0.8

Relative pressure (P/P0)

1.0

-40 0

2

4

6

8

10

12

pH

Fig. 2. Characterization of Na-palygorskite and ACOP: XRD patterns of Na-palygorskite and ACOP (A); FTIR spectra of Na-palygorskite and ACOP (B); N2 adsorption–desorption isothermal curves of Na-palygorskite and ACOP (C); and Zeta potentials of Na-palygorskite and ACOP as a function of pH (D).

Y. Tai et al. / Journal of Molecular Liquids 195 (2014) 116–124

properties of the two samples is listed in Table 1. Compared with Napalygorskite, the BET surface area of ACOP decreases obviously. Such a reduction can be mainly attributed to the coating of Na-palygorskite surface pores by surfactant molecules, which prevents the access of nitrogen molecules into some of the pores [20]. It is interesting to note that the total pore volume of ACOP is higher than that of Napalygorskite, which can be interpreted by the formation of a new void in the aggregates [17]. These observations clearly demonstrate that the surfactant molecules are grafted on the outer surface of palygorskite fibers. The zeta potentials of Na-palygorskite and ACOP were observed over pH 1.0 to 11.0. The Na-palygorskite shows an isoelectric point (pHiep) at pH 4.5 in Fig. 2D. The zeta potential of ACOP becomes less negative as the pH values of the suspension increase and the pHiep of ACOP is about 8.5. The differences of zeta potentials between Na-palygorskite and ACOP indicate that there are amounts of positively charged cations and part of negatively charged anions coating on the surface of Napalygorskite after modification with DDAC and SDS. The carbon, nitrogen and sulfur contents of Na-palygorskite and ACOP are listed in Table 2. The carbon content of ACOP is measured to be 18.46%; in contrast, the Na-palygorskite possesses much less carbon content (0.25%). In addition, the nitrogen and sulfur contents of ACOP are 0.52% and 0.33%, respectively. The results suggest that some DDAC and SDS molecules are present on the surface of palygorskite, which is consistent with the results of FTIR analysis. 3.2. Effect of pH The removal of a pollutant from an aqueous medium by sorption is highly dependent on the solution pH, which affects the surface charge of the adsorbent and the ionization degree of the adsorbate. Fig. 3 shows the pH-dependent sorption of 2-naphthol on ACOP at three different initial concentrations. It is found that the sorption of 2-naphthol increases a little with increasing pH at pH b 8.5, while the sorption of 2-naphthol decreases with increasing pH at pH N 8.5. Generally, solution pH can significantly affect the protonation–deprotonation transition of surface groups on ACOP such as \OH. At pH b 8.5, the surface of ACOP is served as Lewis acid and 2-naphthol is considered to be Lewis bases [21]. Thus, at pH b 8.5, many electron-depleted surface sites that served as Lewis acid become available on ACOP and then the sorption of 2-naphthol becomes strong. However, at pH N 8.5, 2naphthol can be dissociated to anions and the surface of ACOP can be negatively charged [22]. Herein, the decrease of 2-naphthol sorption at pH N 8.5 can be attributed to the electronic repulsion force between the dissociated species of adsorbates and negatively charged ACOP. From the discussion above, one can infer that the influence of electrostatic interaction is one of the main mechanisms in the sorption process. 3.3. Effect of ionic strength 2-Naphthol or any other contaminant is always present in contaminated effluents or soils together with large amounts of soluble salts. The sorption of 2-naphthol on ACOP as a function of NaCl concentration at pH 5.0 and 9.0 are depicted in Fig. 4. One can see that the sorption percentage of 2-naphthol decreases as the background NaCl concentration increases from 0 to 0.1 mol·L−1 at pH 5.0 and 9.0. A reduction in the amount of 2-naphthol sorption on ACOP at the two pH values can be attributed to changes in the surface potential of the adsorbent, due to an

119

Table 2 Carbon, nitrogen and sulfur contents of Na-palygorskite and ACOP. Sample

C (%)

N (%)

S (%)

C/N(S)

Na-palygorskite ACOP

0.25 18.46

– 0.52

– 0.33

– 35.50(55.94)

increase in ionic strengths in the medium [23]. In addition, the degree of 2-naphthol sorption decrease is lower than that at pH 9.0. It is observed that about 10% and 25% reduction in sorption occurs at pH 5.0 and 9.0, respectively, as the ionic strength increases from 0 to 0.1 mol·L−1. The results of this work are similar with the results reported by Sarkar et al. [11]. As explained earlier, at pH 9.0, 2-naphthol can be dissociated to anions, and the repulsive interactions between the anions and the negatively charged surface of ACOP can be influenced obviously by increasing ionic strength of the solution due to a screening effect of the surface charge produced by the salt added [24]. Consequently, the ionic strength-dependent sorption of 2-naphthol on ACOP suggests that the sorption process may occur on the surface of ACOP. 3.4. Effect of agitation time Fig. 5A shows the sorption of 2-naphthol on ACOP at three different initial concentrations of 2-naphthol (C0 = 30, 50 and 70 mg·L−1) as a function of agitation time. One can see that the sorption of 2-naphthol increases fast in the first 20 min, and then slows down until the sorption process reaches equilibrium after 60 min. The whole sorption dynamics process can be divided into two distinct steps: an initial fast sorption, followed by a much slower sorption. The fast 2-naphthol removal rate in the beginning is attributed to the rapid diffusion of 2-naphthol from the solution to the external surfaces of ACOP. Moreover, an increase in the initial 2-naphthol concentration leads to an increase in the amount of 2-naphthol adsorbed on ACOP, indicating that the initial concentration of 2-naphthol is the important driving force to overcome all mass transfer resistance between solid and liquid phases [25]. In order to investigate the mechanism of sorption and the potential rate-controlling steps, the experimental data were simulated by three kinetic models, i.e., the pseudo-first-order model proposed by Lagergren [26], the pseudo-second-order kinetic model proposed by Ho and McKay [27] and the intraparticle diffusion model proposed by Weber and Morris [28]. The respective linear forms of the equations are given as follows:Pseudo-first-order model: lnðqe –qt Þ ¼ lnqe –kt

ð1Þ

Pseudo-second-order model: t 1 1 ¼ þ t qt Kqe 2 qe

ð2Þ

Intraparticle diffusion model: qt ¼ ki t

1=2

þC

ð3Þ

where t (min) is the agitation time, qt (mg·g− 1) is the amount of 2naphthol adsorbed on ACOP at time t, qe (mg·g−1) is the equilibrium sorption capacity, k is the rate constant of the pseudo-first-order sorption (min−1), K (g·mg− 1·min− 1) is the pseudo-second-order rate

Table 1 Surface properties of Na-palygorskite and ACOP. Sample

BET surface area (m2·g−1)

Pore volume (cm3·g−1)

Average pore diameter (nm)a

Na-palygorskite ACOP

185.86 88.55

0.32 0.38

6.89 17.16

a

Average pore diameter calculated using the relationship 4 × Pore volume / BET surface area.

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Y. Tai et al. / Journal of Molecular Liquids 195 (2014) 116–124

20

the origin, intraparticle diffusion was not the only rate-limiting step. The results confirmed that the sorption of 2-naphthol onto ACOP was a multi-step process, involving sorption onto the surface and diffusion into the interior [30].

qe (mg g-1)

18 16

3.5. Sorption isotherms

14

-1

C0 = 30 mg L

-1

C0 = 50 mg L

12

-1

C0 = 70 mg L

10 8 2

4

6

8

10

12

pH Fig. 3. Sorption of 2-naphthol on ACOP as a function of pH. m/V = 2.0 g·L−1, T = 298 K, I = 0.01 mol·L−1 NaCl.

To evaluate the sorption capacity of ACOP, the sorption of 2naphthol on ACOP at three temperatures (298, 318 and 338 K) was carried out. Fig. 6A presents the sorption isotherms of 2-naphthol on ACOP at pH 6.5. It is clear that quantity of 2-naphthol adsorbed on ACOP increases with increasing its concentration in the equilibrium solution. The sorption of 2-naphthol decreases with increasing temperature, indicating that the sorption process is promoted at lower temperature. For isotherm fitting, herein, Langmuir, Freundlich and Linear models are conducted to fit the experimental data and to establish the relationship between the amount of 2-naphthol adsorbed on ACOP and its concentration remained in solution [31–33]. Langmuir model: qe ¼

−1

−1

−1/2

constant, C (mg·g ) is a constant, and ki (mg·g ·min ) is the diffusion rate constant. The curve fitting plots of the pseudo-first-order model, pseudosecond-order model and intraparticle diffusion model for 2-naphthol sorption onto ACOP are shown in Fig. 5B–D, respectively. Based on the above kinetic models, the corresponding kinetic parameters are listed in Table 3. The sorption data of 2-naphthol can be well described by the pseudo-second-order model since the correlation coefficients are close to 1. In contrast, the values of the correlation coefficient R2 for the pseudo-first-order model changed in the range 0.852–0.895 (Table 3). Furthermore, the experimental values of qe,exp (mg·g−1) are far from the calculated qe,cal (mg·g−1). This suggests that the sorption of 2-naphthol onto ACOP does not follow the pseudo-first-order kinetic model, and it is not a diffusion-controlled phenomena [29]. The intraparticle diffusion model was used to determine if particle diffusion is the rate-limiting step. A plot of qt versus t1/2 should be linear if intraparticle diffusion is involved in the sorption process and if the plot passes through the origin then intraparticle diffusion is the sole rate-limiting step. The plot for the sorption of 2-naphthol onto ACOP (Fig. 5D) suggests that the sorption occurred in a multi-step process. The values of ki and C, reported in Table 3, were calculated according to the first linear part of the curves. As the plot did not pass through

pH = 5.0 pH = 9.0

16

qe (mg g-1)

15 14 13

K L qm C e 1 þ K LCe

ð4Þ

Freundlich model: qe ¼ k F C e

n

ð5Þ

Linear model: qe ¼ K P C e þ b

ð6Þ

where qm (mg·g−1) is the maximum sorption capacity, qe (mg·g−1) is the equilibrium sorption capacity, KL (L·mg−1) is the Langmuir constant that relates to the heat of sorption process, kF (mg1 − ng−1·Ln) represents the sorption capacity when the equilibrium concentration of adsorbates equals to 1, n represents the degree of sorption dependence at equilibrium concentration, and KP (L·g− 1) is partition coefficient and b is a constant. The experimental data of 2-naphthol sorption are regressively simulated with Langmuir, Freundlich and Linear models and the results are given in Fig. 6B–D. The relative values calculated from the models are listed in Table 4. One can see from the correlation coefficient (R2) values (Table 4) that Freundlich model yields a better fit than Langmuir and Linear models, which indicates that heterogeneous sorption sites exist on ACOP. It is worth noting that the correlation coefficients for Langmuir and Freundlich models are very close to 1, revealing that the preconcentration of 2-naphthol by ACOP from aqueous solutions is controlled by multiple mechanisms such as hydrophobic partitioning and electrostatic attraction [11]. In addition, Fig. 7 presents the sorption isotherms of Na-palygorskite and ACOP toward 2-naphthol at 298 K. The results show that the sorption capacity of 2-naphthol on ACOP is much higher than that on Napalygorskite, which can be also illustrated from the Langmuir sorption capacities of ACOP (33.65 mg·g−1) and Na-palygorskite (6.40 mg·g−1). From the results, one can see that the organic modification of palygorskite is of significant influence for the enhancement of 2naphthol removal.

12 3.6. Sorption thermodynamics

11 0.00

0.02

0.04

0.06

0.08

0.10

C[NaCl] (mol L-1) Fig. 4. Effect of ionic strength on the sorption of 2-naphthol on ACOP. C0 = 50 mg·L−1, m/V = 2.0 g·L−1, T = 298 K.

In order to gain insight into the possible mechanisms involved in the removal process, the thermodynamic parameters were calculated from the sorption isotherms of 2-naphthol on ACOP at three different temperatures. As shown in Fig. 6, the 2-naphthol sorption decreases continuously as the temperature increases from 298 K to 338 K, indicating that the sorption reaction is an exothermic process. The Gibbs free energy

Y. Tai et al. / Journal of Molecular Liquids 195 (2014) 116–124

(A)

121

(B) 20

ln (qe - qt)(mg g-1)

qt (mg g-1)

16 14 12 10

-1

C0 = 70 mg L

8

C0 = 30 mg L

-1

C0 = 50 mg L

0

-1

C0 = 70 mg L

-1 -2

-1

C0 = 50 mg L

6 4

-1

1

18

-3

-1

C0 = 30 mg L

0

20

40

60

80

100

120

0

20

40

t (min)

(C)

80

100

120

(D) 20

12

-1

C0 = 70 mg L

18

-1

10

C0 = 50 mg L

8

C0 = 30 mg L

-1

qt (mg g-1)

t/qt (min g mg-1)

60

t (min)

6 4

16 -1

C0 = 30 mg L

14

-1

C0 = 50 mg L

12

-1

C0 = 70 mg L

10

2

8

0 0

20

40

60

80

100

120

2

4

t (min)

6

8

10

12

t0.5 (min0.5)

Fig. 5. Sorption of 2-naphthol on ACOP as a function of agitation time (A); Linear plots of pseudo-first-order (B), pseudo-second-order (C) and intraparticle diffusion (D) for the sorption of 2-naphthol onto ACOP. pH = 6.5 ± 0.1, C0 = 50 mg·L−1, m/V = 2.0 g·L−1, T = 298 K, I = 0.01 mol·L−1 NaCl.

change (ΔG°), standard entropy change (ΔS°) and standard enthalpy change (ΔH°) are derived from the following relationship [34]: 

ΔG ¼ −RT lnK o



lnK o ¼

ð7Þ



ΔS ΔH − R RT

ð8Þ

where Ko is the sorption equilibrium constant, R (8.314 J·mol−1·K−1) is the ideal gas constant, and T (K) is the temperature in Kelvin. Values of lnKo are obtained by plotting lnKd versus Ce and extrapolating Ce to zero (Kd is the distribution coefficient) [35]. Its intercept with vertical axis gives the value of lnKo. ΔS° and ΔH° can be calculated from the slope and intercept of the plot of lnKo versus 1/T by using Eq. (8) (see Fig. 8B). The distribution coefficients as a function of solute final concentration at 298, 318, and 338 K are shown in Fig. 8A. Sorption thermodynamic parameters are listed in Table 5. The values of ΔG° and ΔH° of the process at three temperatures are negative, thus the sorption reaction is a spontaneous and exothermic process. The positive ΔS° indicates that the reorientation or restructuring of water around the solute

or surface is unfavorable, since it disturbs the existing water structure and imposes a new and more ordered structure on the surrounding water molecules. As a result of 2-naphthol sorption onto ACOP surface, the liberation of the solvent molecules from the solvated shells is more predominate in the studied system, thus, the freedom degree of the water molecule increases [12]. It is necessary to note that the value of ΔH° is −13.72 kJ·mol−1, indicating the physical nature of the sorption process, involving hydrophobic interaction forces, van der Waal's forces, weak forces of electrostatic attraction, dipole bond forces and hydrogen bond forces [36]. 3.7. Regeneration studies To make the sorption process more economical, it is necessary to study regeneration of the sorbent. A sorbent material is acceptable for application in removing contaminants from waste water only when it does not pose potential risk of immediate release of the contaminants back into the environment. Regeneration experiment is conducted for the initial 2-naphthol concentration of 150 mg∙L−1. The sorption capacities for ACOP through 5 recycling times are 27.02, 25.88, 25.45, 24.34 and 23.75 mg∙g−1 respectively. With increasing the recycling times,

Table 3 Kinetic parameters for the sorption of 2-naphthol on ACOP. Adsorbate concentration

C0 = 30 mg/L C0 = 50 mg/L C0 = 70 mg/L

qe,exp

10.35 15.61 18.86

Pseudo-first-order model

Pseudo-second-order model 2

Intra-particle diffusion model 2

k

qe,cal

R

K

qe,cal

R

ki

C

R2

0.030 0.038 0.038

6.79 8.82 10.65

0.864 0.895 0.852

0.061 0.045 0.042

10.48 15.82 19.08

0.999 0.999 0.999

0.274 0.640 0.692

8.27 11.61 14.42

0.925 0.987 0.946

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Y. Tai et al. / Journal of Molecular Liquids 195 (2014) 116–124

(A)

(B) 30 298 K 318 K 338 K

5 25

Ce/qe(g L-1)

qe (mg g-1)

20 15 298 K 318 K 338 K

10

4 3 2

5 1 0 0

20

40

60

80

100

120

0

20

40

80

100

120

80

100

120

(D)

(C) 1.6

30 298 K 318 K 338 K

1.4

298 K 318 K 338 K

25

1.2

qe (mg g-1)

logqe (mg g-1)

60

Ce(mg L-1)

Ce(mg L-1)

1.0 0.8 0.6

20 15 10 5

0.4 0.4

0.8

1.2

1.6

2.0

0 0

20

40

60

Ce(mg L-1)

logCe(mg L-1)

Fig. 6. Sorption isotherms of 2-naphthol (A), fitting lines of the Langmuir sorption isotherms of 2-naphthol (B), fitting lines of the Freundlich sorption isotherms of 2-naphthol (C), and fitting lines of the Linear sorption isotherms of 2-naphthol (D) at three different temperatures. m/V = 2.0 g·L−1, pH = 6.5 ± 0.1, I = 0.01 mol·L−1 NaCl.

3.8. Simulated 2-naphthol-bearing wastewater treatment In view of the complicacy and heterogeneity of aquatic environment, we tested the sorption performance of ACOP towards a simulated wastewater containing various environmental contaminants such as phosphate (5 mg∙L−1), Pb(II) (10 mg∙L−1), naphthalene (25 mg∙L−1), methylene blue (MB) (40 mg∙L−1) and 2-naphthol (50 mg∙L−1). The above components are commonly present in the effluent discharged from electroplating, textile printing, fertilizer processing, oil refining, etc. The simulated wastewater had an original pH value of about 5.5. The experimental treatment process was conducted by adding 100 mL simulated wastewater into a 200 mL beaker containing 2.0 g∙L−1 ACOP at pH 5.5. The mixed suspensions were continuously stirred for 4 h by using a rotary shaker and filtered through a 0.45 μm membrane

filter. The obtained supernatants were used for concentration measurement. The removal efficiency of ACOP towards 2-naphthol-bearing simulated wastewater is shown in Fig. 9. As can be seen from Fig. 9, the removal percentages of ACOP towards phosphate, Pb(II), naphthalene, MB and 2-naphthol at pH 6.0 were 70%, 81%, 47%, 65% and 52%, respectively. The obtained results indicate that ACOP has satisfying removal

30 25 20

qe (mg g-1)

the sorption capacity for ACOP decreased in a limited range. The excellent regeneration property of ACOP suggests that it can be used as a cost-effective material for the removal of 2-naphthol from aqueous solution.

ACOP Na-palygorskite

15 10 5

Table 4 The parameters for Langmuir, Freundlich and Linear isotherms at 298, 318 and 338 K.

0 Langmuir model

298 K 318 K 338 K

Freundlich model

Linear model

0

qm

KL

R2

KF

n

R2

KP

C

R2

33.65 32.73 28.37

0.043 0.029 0.025

0.992 0.986 0.979

2.63 1.87 1.54

0.552 0.585 0.577

0.995 0.996 0.996

0.239 0.211 0.169

8.21 6.36 5.21

0.887 0.875 0.908

20

40

60

Ce (mg

80

100

120

L-1)

Fig. 7. Sorption isotherms of 2-naphthol on Na-palygorskite and ACOP at 298 K. m/V = 2 g·L−1, pH = 6.5 ± 0.1, I = 0.01 mol·L−1 NaCl.

Y. Tai et al. / Journal of Molecular Liquids 195 (2014) 116–124

(A)

80

7.2 298 K 318 K 338 K

70

Removal Efficiency (%)

lnKd (mL g-1)

6.8 6.4 6.0 5.6

60 50 40 30 20 10

5.2 0

20

40

60

80

100

120

0 phosphate

Ce(mg L-1)

Pb(II)

naphthalene

MB

2-naphthol

Fig. 9. Removal efficiency of ACOP towards simulated 2-naphthol-bearing wastewater.

(B) 7.0

lnKo (mL g-1)

123

6.8

6.6

6.4 0.0029

0.0030

0.0031

1/T

0.0032

0.0033

0.0034

(K-1)

Fig. 8. Linear plots of lnKd vs. Ce (A) and lnKo vs. 1/T (B) for the sorption of 2-naphthol on ACOP at 298, 318 and 338 K. m/V = 2 g·L−1, pH = 6.5 ± 0.1, I = 0.01 mol·L−1 NaCl.

efficiency towards both inorganic and organic contaminants. The removal efficiency of ACOP towards three organic contaminants, i.e., naphthalene, MB and 2-naphthol, are lower than that of two inorganic contaminants (phosphate and Pb(II)). The results can be explained from the difference of initial concentrations of inorganic and organic contaminants, which presents that the initial concentrations of phosphate and Pb(II) are obviously lower than that of naphthalene, MB and 2-naphthol. Furthermore, It is found that the removal efficiency of 2-naphthol from the simulated wastewater is slightly lower than that from single-component system from the experimental data as mentioned earlier. This can be attributed to the combined influence of coexistent electrolyte ions and organic matters from simulated wastewater [37]. The findings herein suggest that ACOP could be potentially used as an effective material for actual effluent disposal.

existence of DDAC and SDS on the surface of ACOP after modification, there are four potential mechanisms controlling 2-naphthol uptake on ACOP: (i) partitioning of 2-naphthol molecules into DDAC and SDS layers; (ii) hydrophobic interactions between the hydrophobic tails of DDAC and SDS and the hydrophobic benzene ring of 2-naphthol molecules; (iii) electrostatic interactions between the positively charged outward-pointing head groups of the DDAC and the negatively charged C10H7O−; and (iv) the hydrogen bonding of 2-naphthol with Si(or Al)\ O on ACOP surface. Taking all the results that have been discussed above into account, it is proposed that enhanced hydrophobic interaction is the predominant mechanism contributing to the increased 2-naphthol sorption on ACOP, compared with Na-palygorskite. This mechanism can also explain the following observations from sorption tests: the weak pH dependency of 2-naphthol sorption on ACOP can be explained by the strengthened hydrophobic interactions [38]. It is known that clay minerals modified with the mixture of anionic and cationic surfactants can substantially enhance the sorption of ionizable organic solutes from aqueous solutions, as the mixture of anionic and cationic surfactants can form mixed micelles, which can produce synergies solubilization to organic compounds [14,39]. Herein, Na-palygorskite was modified with the mixture of DDAC and SDS, which have long hydrophobic tails, thus the surface of ACOP became hydrophobic and 2-naphthol molecules were adsorbed into the micro-organic phase of micelles by partitioning/ extraction. Furthermore, electrostatic interactions and hydrogen bond forces are other possible interaction mechanisms that are responsible for the preconcentration of 2-naphthol on ACOP. In the pH range of our experiment, part of 2-naphthol exists as anionic form C10H7O−, thus the electrostatic interaction between C10H7O− and positively nitrogen atoms of head groups of the cationic surfactant (DDAC) exists. It is noticeable that the hydrogen bond forces cannot be ignored, as Na-palygorskite has certain sorption capacity for 2-naphthol from the previous experimental data.

3.9. Potential preconcentration mechanisms Several possible interactions between organoclays and hydrophobic aromatic compounds are responsible for the preconcentration of aromatic compounds, including hydrogen bonds, electrostatic interactions and hydrophobic partitioning interactions [10]. Depending on the Table 5 Thermodynamic parameters of 2-naphthol sorption on ACOP at 298, 318 and 338 K.

ΔG° (kJ·mol−1) ΔH° (kJ·mol−1) ΔS° (J·mol−1·K−1)

298 K

318 K

338 K

−17.47 −13.72 12.52

−17.67

−17.98

4. Conclusions In the present study, ACOP was found to be an effective sorbent obtained from organic modification of Na-palygorskite by microwaveassisted technique for the preconcentration of 2-naphthol from aqueous solutions. The prepared ACOP was characterized by elemental analysis, XRD, FTIR, N2 adsorption–desorption, zeta potential measurement and SEM. The effects of pH, ionic strength, agitation time and temperature were investigated by batch method to reveal the potential mechanisms of 2-naphthol preconcentration onto ACOP. The results showed that the sorption data was satisfactorily explained by Freundlich isotherm and maximum sorption capacity of 2-naphthol onto ACOP was found to be

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Y. Tai et al. / Journal of Molecular Liquids 195 (2014) 116–124

33.65 mg·g−1. The rate of sorption was found to obey pseudo-secondorder model with a good correlation coefficient. Taking all the results that have been discussed above into account, it is proposed that the preconcentration of 2-naphthol by ACOP from aqueous solutions is controlled by multiple mechanisms such as hydrophobic partitioning, electrostatic attraction and hydrogen bond forces. On the basis of the results obtained, it can be concluded that ACOP can act as a potential low-cost and efficient sorbent for the preconcentration of hazardous 2-naphthol from aqueous environment. Acknowledgment The Natural Science Foundation of Anhui Higher Education Institution of China (Grant No. KJ2012B093) is acknowledged. References [1] L.B. Paiva, A.R. Morales, F.R. Valenzuela Díaz, Appl. Clay Sci. 42 (2008) 8–24. [2] S.D. Miao, Z.M. Liu, Z.F. Zhang, B.X. Han, Z.J. Miao, K.L. Ding, G.M. An, J. Phys. Chem. C 111 (2007) 2185–2190. [3] P. Liu, T.M. Wang, Ind. Eng. Chem. Res. 46 (2007) 97–102. [4] Q.H. Fan, D.D. Shao, J. Hu, W.S. Wu, X.K. Wang, Surf. Sci. 602 (2008) 778–785. [5] H. Chen, Y.G. Zhao, A.Q. Wang, J. Hazard. Mater. 149 (2007) 346–354. [6] Q.H. Fan, X.L. Tan, J.X. Li, X.K. Wang, W.S. Wu, G. Montavon, Environ. Sci. Technol. 43 (2009) 5776–5782. [7] Y. Liu, W.B. Wang, A.Q. Wang, Powder Technol. 225 (2012) 124–129. [8] H. Chen, J. Zhao, A.G. Zhong, Y.X. Jin, Chem. Eng. J. 174 (2011) 143–150. [9] Y. Park, G.A. Ayoko, R.L. Frost, J. Colloid Interface Sci. 360 (2011) 440–456. [10] J. Xie, W.N. Meng, D.Y. Wu, Z.J. Zhang, H.N. Kong, J. Hazard. Mater. 231–232 (2012) 57–63. [11] B. Sarkar, Y.F. Xi, M. Megharaj, G.S.R. Krishnamurti, R. Naidu, J. Colloid Interface Sci. 350 (2010) 295–304.

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