Adsolubilization of 2,4,6-trichlorophenol from aqueous solution by surfactant intercalated ZnAl layered double hydroxides

Chemical Engineering Journal 279 (2015) 597–604

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Adsolubilization of 2,4,6-trichlorophenol from aqueous solution by surfactant intercalated ZnAl layered double hydroxides Peiwen Zhao, Xiaohua Liu, Weiliang Tian, Dongpeng Yan, Xiaoming Sun, Xiaodong Lei ⇑ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, PO Box 98, Beijing 100029, China

h i g h l i g h t s  Two surfactant intercalated LDHs were used for removal of 2,4,6-trichlorophenol.  The adsorbents have large adsolubilization capacities for 2,4,6-trichlorophenol.  The organo-LDH shows good adsolubilization ability because of

p–p interaction.

 Both of the adsorbents can be regenerated and reused at least 10 times.

a r t i c l e

i n f o

Article history: Received 22 March 2015 Received in revised form 10 May 2015 Accepted 11 May 2015 Available online 27 May 2015 Keywords: Adsolubilization Layered double hydroxide Adsorption 2,4,6-Trichlorophenol Intercalation

a b s t r a c t Adsolubilization is an efficient adsorptive way to remove organic pollutants from natural environment. By intercalation of anionic organics, a three-dimensional hydrophobic interlayer region can be obtained in the gallery of layered double hydroxides (LDHs), which can be further used as outstanding adsolubilization materials. In this work, dodecylbenzenesulfonate (DBS) and dodecylsulfate (DS) intercalated ZnAl layered double hydroxides (DBS-LDH and DS-LDH) were prepared as adsorbents toward removing 2,4,6-trichlorophenol (2,4,6-TCP) from aqueous solutions. Batch adsolubilization experiments were conducted to study several influence factors on 2,4,6-TCP removal. DBS-LDH and DS-LDH have the adsorption capacities of 166.8 mg/g and 96.7 mg/g at 298 K under pH = 3, respectively. DBS-LDH presented better adsolubilization property than that of DS-LDH. The adsorption kinetics was found to follow the pseudo-second order model and the spontaneous adsorption process was verified by the negative value of DG . The equilibrium isotherms for organic contaminants uptake were fitted well with the linear model. The LDH-based adsorbents still have good adsorption performance after 10 adsorption cycles. Therefore, the two surfactant intercalated LDHs can be considered as potential adsorbents in environmental applications for the removal of 2,4,6-TCP pollutant from aqueous solutions. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction 2,4,6-trichlorophenol (2,4,6-TCP) is known as a representative toxic chlorophenol compound, which has been widely used in many domains such as paper manufacturing, petrochemical industry and pesticide industry [1–6]. As one of the main toxic, carcinogenic and mutagenic chlorophenol organic pollutants [7,8], 2,4,6-TCP is existed in industrial wastewater and could lead to great harm to both the ecosystem and human beings. It has been recorded as a priority pollutant by the US Environmental Protection Agency [9] and European Regulatory Authorities [10]. Therefore, it is highly important to remove 2,4,6-TCP compound from aqueous solution. Currently, a number of methods have been ⇑ Corresponding author. Tel.: +86 10 64455357; fax: +86 10 64425385. E-mail address: [email protected] (X. Lei). http://dx.doi.org/10.1016/j.cej.2015.05.037 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

developed to eliminate 2,4,6-TCP from wastewater [11–15]. However, these methods and materials usually involve high cost, complex process, and even may lead to secondary pollution during degradation process. Therefore, some researchers have paid their attention to adsorption methods (especially adsolubilization) to solve the issue of water contamination by phenol and its derivatives including 2,4,6-TCP [16–20]. During past few years, layered double hydroxide (LDH) materials have attracted more attention because of their potential applications, such as catalysis [21], photochemistry [22], electroche mistry [23], polymerization [24], environmental applications [25], and so on. The structure of LDHs consists of positively charged mixed-metal hydroxide layers separated by charge-balancing anions and water molecules. They are expressed by the formula 3+ x+ n [M2+ )x/nmH2O [26], where the cationic M2+ and 1xM x(OH)2] (A 3+ M occupy the octahedral holes in a brucite-like layer and the An

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2. Experimental section 2.1. Materials Zn(NO3)26H2O, Al(NO3)39H2O and NaOH were purchased from Beijing Chemical Co., Ltd. Sodium dodecylbenzenesulfonate (SDBS) and sodium dodecylsulfate (SDS) were received from Sinopharm Chemical Reagent Co., Ltd. 2,4,6-TCP was obtained from TCI Chemicals (Shang hai) Pvt. Ltd. All the reagents were analytical grade chemicals and used without any purification. 2.2. Preparation of adsorbents

Fig. 1. XRD patterns of DS-LDH (a) and DBS-LDH (b).

is the interlayer exchangeable anions, which is located in the hydrate layered galleries, x is the layer charge density (x = [M2+]/ ([M2+] + [M3+])) [27,28]. LDHs possess many excellent properties like interlayer anion-exchange capacity, larger surface area and surface positive charge. These characteristics make LDHs and their calcined products become effective adsorbents to concentrate some anionic inorganic and organic contaminants from wastewater [29–34]. With further study of LDHs, anionic surfactants with long alkyl chain can be intercalated into the interlayer. The intercalation of anionic surfactants between layers of LDHs can change the original hydrophilic surface to hydrophobic accompanied by the increased interlayer spacing [35–43]. These significant variations endow modified LDHs new adsorptive properties for some organic molecules that general LDH materials do not. The adsorption process is called ‘‘adso lubilization’’, i.e. the dissolution of hydrophobic organics in the three-dimensional (3D) hydrophobic organic phase, rather than the uptake on the LDH external surface [44,45]. The concept of adsolubilization has been extensively used in a range of organic-anion intercalated LDHs adsorption investigations [46–50]. In this work, modified ZnAl-LDHs intercalated with DBS and DS were prepared and used as adsorbents for adsorbing 2,4,6-TCP from aqueous solution, respectively. The adsorption capacities including the effect of different factors of the adsorbents for 2,4,6-TCP were systematically investigated. The results show that the DBS-LDH and DS-LDH can serve as efficient and low-cost adsorbents for the removal of 2,4,6-TCP from wastewater.

Fig. 2. FT-IR spectra of DS-LDH (a) and DBS-LDH (b).

Surfactant modified ZnAl-LDHs were prepared by a co-precipitation method [47,50]. Solution A: 0.02 mol of Zn(NO3)26H2O and 0.01 mol of Al(NO3)39H2O was dissolved in 100 mL of deionized water (Zn2+:Al3+ molar ratio = 2:1). Solution B: 2 M of NaOH solution was obtained by dissolving NaOH in deionized water. 30 mL of solution B was added dropwise to 100 mL of solution A in a flask. After stirring for 2 min, 100 mL of SDBS (0.15 M) solution was added into the flask. During the synthesis process, temperature and pH of the mixture were maintained at about 298 K and 10, respectively. The resulting suspension was stirred at 353 K for 8 h. At the end, the resulting slurry was separated by centrifugation, washed with deionized water for several times and dried at 338 K for 24 h, denoted as DBS-LDH. It may be noted that deionized water was decarbonated and nitrogen was introduced throughout the whole process to reduce interference of CO2 from atmosphere. In addition, DS intercalated ZnAl-LDH was also obtained by the similar method. The product was denoted as DS-LDH. 2.3. Batch adsolubilization tests Batch adsolubilization tests were carried out in conical flasks. A certain amount of DS-LDH and DBS-LDH (30–700 mg) was added into 50 mL of 2,4,6-TCP solution. The initial concentration of 2,4,6-TCP ranged from 30 to 300 mg/L (1.5 to 15 mg per 50 mL solution). The initial pH of solution (3, 4, 5, 7, 9, 11) was adjusted by dropwise adding HCl or NaOH solution. The mixture was stirred with an agitation speed of 100 rpm during adsolubilization process. Temperature was varied to study its influence on the adsorption capacity. Isotherm data were obtained at 298, 308, 318 and 328 K in a constant temperature bath which controlled the

Fig. 3. Effect of contact time on adsolubilization ability of (a) DBS-LDH and (b) DSLDH. (Vsol = 50 mL, adsorbent mass = 30 mg, initial 2,4,6-TCP concentration Co = 300 mg/L and pH = 7).

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P. Zhao et al. / Chemical Engineering Journal 279 (2015) 597–604 Table 1 Kinetic model parameters obtained in adsolubilization of 2,4,6-TCP by DBS-LDH and DS-LDH. Adsorbents

Qe,exp (mg/g)

Pseudo-first-order 1

k1 (min DBS-LDH DS-LDH

126.5 83.5

)

2

6.13  10 7.20  102

Pseudo-second-order Qe,cal (mg/g) 145.7 118.9

R

2

0.992 0.960

k2 (g mg1 min1) 4

8.81  10 7.40  104

Qe,cal (mg/g)

R2

139.1 94.1

0.999 0.998

conditions, contact time, solution pH, initial 2,4,6-TCP concentration, and dosage of adsorbents on adsolubilization were investigated. 2.4. Regeneration tests Regeneration tests were carried out in order to investigate the reutilization performance of the adsorbents. After accomplishing the first equilibrium adsorption process at room temperature, the powders of DBS-LDH and DS-LDH after adsorbing 2,4,6-TCP were immersed in 30 mL acetone by ultrasonic treatment for 30 min. Then recovered materials were redispersed in 2,4,6-TCP aqueous solution of identical concentrations. This procedure on adsorption-regeneration cycle was repeated 10 times. The concentration of residual 2,4,6-TCP after each adsorption cycle was detected by the same method. Fig. 4. Effect of pH on adsolubilization ability of (a) DBS-LDH and (b) DS-LDH (Vsol = 50 mL, adsorbent mass = 30 mg and initial 2,4,6-TCP concentration Co = 300 mg/L).

temperature to within ±1 K for various initial 2,4,6-TCP concentrations. Two-hour was found to be enough to reach adsolubilization equilibrium. After adsorbing, the suspension was filtered with 0.22 lm organic membrane filter. The residual 2,4,6-TCP concentration was detected by UV–vis absorption spectra at 290 nm for acidic condition and at 314 nm for alkaline condition, respectively [20,39]. The amount of 2,4,6-TCP adsolubilised per unit mass of adsorbents (Qt, mg/g) at a certain time was calculated by the following equation:

Qt ¼

ðC o  C t Þ V m

2.5. Characterization Powder X-ray diffraction (XRD) data were collected on a Shimadzu Model XRD-6000 powder diffractometer, using Cu Ka radiation handling at 30 mA and 40 kV in the 2h range of 2–70°

ð1Þ

where V (L) is the volume of solution; Co and Ct (mg/L) are the 2,4,6-TCP concentration at initial and time t respectively; m (g) is the mass of adsorbents; when adsorption equilibrium was reached, Qt become Qe and Ct turned to Ce. For exploring the optimized

Fig. 5. Effect of temperature on adsolubilization ability of (a) DBS-LDH and (b) DSLDH (Vsol = 50 mL, adsorbent mass = 30 mg, initial 2,4,6-TCP concentration Co = 300 mg/L and pH = 3).

Fig. 6. Equilibrium isotherms of DBS-LDH (A) and DS-LDH (B) for 2,4,6-TCP at varied temperatures.

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Table 2 Isotherm parameters for 2,4,6-TCP adsolubilization on DBS-LDH and DS-LDH. Adsorbents

T (K)

Langmuir model Qm

DBS-LDH

DS-LDH

298 308 318 328 298 308 318 328

Freundlich model KL

R 3

3.0  10 1.3  103 2.2  103 2.0  103 1.1  105 0.6  103 2.4  103 3.5  103

93.28 387.6 121.9 87.87 4.1  104 490.2 69.25 22.46

2

0.593 0.792 0.631 0.811 0.550 0.810 0.721 0.723

(k = 1.542 Å) , with a scanning rate of 10° min1. FT-IR spectra were obtained on a Bruker Vector 22 spectrometer in the range between 400 and 4000 cm1 by using a standard KBr disk method (sample/KBr = 1/100). Zn and Al elemental analysis was performed with inductively coupled plasma emission spectroscopy (ICP) (Shimadzu ICPS-7500). C, H and N elemental analysis was carried out using an elemental analyzer (Carlo Erba 1106). UV–vis absorption spectra were recorded on a Shimadzu UV-2501PC spectrophotometer. A calibration curve of 2,4,6-TCP solution was performed before each analytical test, which ensured the linear correlation of absorbance and concentration with the correlation coefficient of R2 > 0.99.

3. Results and discussion 3.1. Structure of adsorbents The powder X-ray diffraction patterns of DBS-LDH and DS-LDH are given in Fig. 1, which shows typical, comparatively ordered structures with basal spacing d003 = 2.65 nm of DS-LDH (Fig. 1a) and d003 = 2.92 nm of DBS-LDH (Fig. 1b), respectively. Similar results had been reported by several literatures [51,52]. It can be observed that basal (0 0 3) reflections of the two samples move to a lower 2h degree compared with the basal (0 0 3) reflection of CO3-LDH, indicating the successful intercalation of the surfactant anions in the interlayer. This is consistent with that DBS and DS anions could be intercalated into the LDH with the form of antiparallel perpendicular monolayer arrangement [47,52]. The feature structure of DBS-LDH and DS-LDH were also confirmed by FT-IR spectra (Fig. 2) that could supply more information on interlayer characters. DS-LDH and DBS-LDH samples showed the common features. For example, broad bands of O–H stretching vibration occurred at 3518 cm1 due to water molecules; stretching vibration bands of C–H appeared at 2957, 2921, 2852 cm1, and C–H bending vibration bands appeared at 1469 cm1. Both of samples had no strong absorption peak at 1356 cm1, accounting for the absence of CO2 anions into the interlayer. Besides, for 3 DS-LDH (Fig. 2a), the sulfate S = O stretching vibration bands at 1219 and 1081 cm1 were observed. Stretching vibration bands of C–O and S–O emerged at 992 cm1 and 826 cm1 respectively. These characteristic peaks were proved that the DS was intercalated into LDH interlayer. For DBS-LDH (Fig. 2b), the characteristic

n 0.90 0.87 0.76 0.77 1.10 1.11 0.73 0.58

Kf

Linear model R

0.42 0.31 0.10 0.06 0.45 0.24 0.05 0.06

2

Kd 3

0.997 0.998 0.986 0.995 0.998 0.999 0.983 0.990

0.78  10 0.69  103 0.48  103 0.35  103 0.41  103 0.36  103 0.35  103 0.35  103

b

R2

4.96 5.03 4.61 7.27 0.87 1.79 5.89 10.32

0.995 0.999 0.998 0.999 0.998 0.999 0.998 0.999

vibration bands of the sulfonate groups were shown at 1177, 1039, 1011 and 831 cm1; framework vibration bands in the range 1600–1400 cm1 come from benzene ring. These characteristic peaks proved the intercalation of DBS into LDH interlayer [49,53]. Based on elemental analyses, the Zn:Al:DS molar ratio was about 2.00:1.13:0.98 for DS-LDH and Zn:Al:DBS molar ratio was about 2.00:0.97:0.91 for DBS-LDH, respectively. Both of them were close to 2:1:1. It is indicated that the experimental results are consistent with calculated compositions according to LDH structural data [28]. 3.2. Effect of contact time on 2,4,6-TCP adsolubilization The effect of contact time on 2,4,6-TCP adsolubilization was performed at room temperature and pH 7. Fig. 3 shows that 2,4,6-TCP adsorption quantities increased considerable in the first 40 min, and reached adsorption equilibrium at about 60 min. The adsolubilization capacity of DBS-LDH was higher than that of DS-LDH, mainly due to the hydrophobic interlayer spacing of DBS-LDH was larger and the strong p–p interaction between the phenyl rings of both 2,4,6-TCP and DBS-LDH. After reaching equilibrium, the adsorption capacity of DBS-LDH and DS-LDH toward 2,4,6-TCP can be evaluated as 126.5 mg/g and 83.5 mg/g, respectively. Kinetics models were also studied. As is indicated in Table 1, the adsolubilization processes of the adsorbents can be fitted well by the pseudo-second order model, based on the correlation coefficients R2. In addition, the calculated Qe,cal values of samples by using this model are also closer to the experimental Qe values. The model is based on the adsolubilization capacity of the hydrophobic solid phase and infers the behavior over the adsorption process, gives priority to diffusion process [54,55]. 3.3. Effect of initial solution pH on 2,4,6-TCP adsolubilization The pH value was one of the critical parameters to adsorption process and could influence the hydrophobicity of adsorbents and the solubility of contaminants. It is well known that 2,4,6-TCP is a kind of ionizable hydrophobic organic compound. The dissociation constant (pKa) of 2,4,6-TCP is 6.23 [20]. When the pH was lower than 6.23, 2,4,6-TCP is ionized difficultly and could be presented as the molecular form containing –OH group.

Table 3 Thermodynamic parameters for 2,4,6-TCP adsolubilization on DBS-LDH and DS-LDH. Adsorbents

DBS-LDH

T (K)

298

308

318

328

298

DS-LDH 308

318

328

DGo (kJ mol–1) DSo (J mol–1 k–1) DHo (kJ mol k1)

16.49 18.8 22.27

16.74

16.34

15.97

14.93 4.7 34.08

15.11

15.47

15.96

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of 2,4,6-TCP over DBS-LDH and DS-LDH at 298, 308, 318 and 328 K were also investigated (Fig. 6). With the temperature increasing, the movement speed of organic molecules got faster, which is a disadvantage for 2,4,6-TCP molecules adsorbed into the hydrophobic interlayer [55,56]. Both of the adsolubilization processes mainly present a linear adsorption character within the concentration range. The linear model (Fig. 6) gives the best fitting results with Langmuir and Freundlich models according to the correlation coefficient R2, as is shown in Table 2. The linear adsorption behavior can be described as follows [48]:

Q e ¼ K d Ce þ b

Fig. 7. Effect of initial 2,4,6-TCP concentration on the adsorption ability of (a) DBSLDH and (b) DS-LDH (Vsol = 50 mL, adsorbent mass = 30 mg and T = 298 K).

On the contrary, it is ionized easily and could stay at the ionic form with –O group when the pH was higher than 6.23. The effect of pH on the adsolubilization process was studied at 298 K and the adsorption time was 120 min, as is shown in Fig. 4. For DBS-LDH and DS-LDH, the adsolubilization capacities of 2,4,6-TCP decreased with the initial pH values from 3 to 11. Adsorbents were not obviously dissolved under this initial pH range. This phenomenon can be interpreted by the structure and adsolubilization mechanism. When the solution was acid, hydrophobic 2,4,6-TCP was molecular form and could be adsorbed easily into the hydrophobic 3D interlamination formed by surfactant anions in the LDH interlayer. On the contrary, 2,4,6-TCP was mainly presented as ionic form when the solution was alkaline, so the hydrophilic deprotonated part is relative difficult to be adsorbed into the hydrophobic interlayer of adsorbents. Consequently, DBS-LDH and DS-LDH display the best adsolubilization capacities for molecular type of 2,4,6-TCP at pH = 3, and the adsolubilization capacities were 166.8 mg/g and 96.7 mg/g, respectively. 3.4. Effect of temperature on 2,4,6-TCP adsolubilization The effect of temperature on 2,4,6-TCP adsolubilization was investigated at initial pH = 3 and adsorption for 120 min. As is shown in Fig. 5, for both DBS-LDH and DS-LDH, the adsolubilization capacities of 2,4,6-TCP decreased with the temperature. It is associated with the 2,4,6-TCP molecular movement rate during the adsolubilization process. The equilibrium adsorption isotherms

ð2Þ

where Qe (mg/g) is the amount of 2,4,6-TCP adsolubilised per unit mass of adsorbents at equilibrium, Ce (mg/L) is the equilibrium concentration in aqueous solution, Kd (dm3/Kg) is the distribution coefficient and b is the linear regression constant. The values of Kd, b and R2 of the linear model of adsorption were given in Table 2. Thermodynamic parameters such as Gibbs free energy (DG ), enthalpy change (DH ), and entropy change (DS ) were determined using the following equations [55]:

DG ¼ RT ln K ¼ DH  T DS

ð3Þ

DH DS þ RT R

ð4Þ

ln K ¼ 

where R is the universal gas constant (8.314 J/mol K) and T (K) is the temperature. The values of DH and DS were derived from the slope and intercept of the van’t Hoff plot according to the Eq. (4), respectively. As is given in Table 3, the negative values of DG at different temperatures reflected that the adsolubilization processes were spontaneous. The change of free energy for physisorption is usually between –20 and 0 kJ mol1, whereas chemisorption is in the range from –80 to 400 kJ mol1 [55–57], implying that the adsolubilization processes were physical adsorption and hydrophobic force was strong to 2,4,6-TCP and drive the 2,4,6-TCP adsorbed into the hydrophobic interlayer region of the adsorbents. The heat changes are different when the interlayer anion is varied suggesting different kind of mechanism over these two samples. The p–p interaction, feasible with DBS-LDH, favors adsolubilization better compared to DS-LDH and thus exothermic adsorption was seen. Owing to this interaction, the degree of freedom is restricted for DBS-LDH and thus lesser entropy for this sample, as observed, compared to DS-LDH. 3.5. Effect of initial concentration and dosage on 2,4,6-TCP adsolubilization Fig. 7 exhibits the effect of initial 2,4,6-TCP concentration on the adsolubilization abilities of DBS-LDH and DS-LDH. The adsolubilization capacities of samples showed the rising trend and representative linear curves with the initial 2,4,6-TCP concentration [49], indicating the adsolubilization processes were partitioning. The organic compound 2,4,6-TCP was dissolved into the 3D hydrophobic organic interlayers rather than adsorbed on the external surface of DBS-LDH and DS-LDH. In our laboratory, we had also observed the similar adsolubilization process previously [49,58]. Fig. 8 shows the removal percentages of 2,4,6-TCP by different dosages of DBS-LDH and DS-LDH from aqueous solutions. Within a certain range, the maximum removal percentages of 2,4,6-TCP by both of the modified-LDHs can reach above 80%. 3.6. Adsorption mechanism

Fig. 8. Effect of adsorbent dosage on 2,4,6-TCP adsolubilization ability of (a) DBSLDH and (b) DS-LDH (Vsol = 50 mL, initial 2,4,6-TCP concentration Co = 300 mg/L).

The surfactant anions with the hydrophobic alkyl-chains are intercalated into the LDH interlayer region, due to hydrogen

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Fig. 9. The proposed mechanism of 2,4,6-TCP adsolubilization onto a DBS-LDH. The four interactions are thought to be (A) p–p interaction, (B) non-polar interactions, (C) hydrogen bonds and (D) electrostatic interactions.

Fig. 10. XRD spectra of DBS-LDH (a), DS-LDH (b) after adsorption.

bonding interaction and electrostatic attraction, which further leads to the formation of a tight and well-aligned structure [36,51]. Consequently, 2,4,6-TCP molecules could be captured and solubilized in the hydrophobic interlayer region due to adsolubilization. A schematic representation of the suggested interactions between 2,4,6-TCP molecules and a DBS-LDH particle is shown in Fig. 9. We believed there are some types of interactions,

including p–p interaction (region A in Fig. 9), non-polar attractions (region B), hydrogen bonds (region C) and electrostatic interactions (region D). According to the discussion above, the adsolubilization process was influenced by the pH, indicated that electrostatic interaction is not a key factor in these interactions. However, there are no p–p interaction between 2,4,6-TCP and DS-LDH because of no aromatic ring in DS-LDH. DBS-LDH shows the advantage of the adsolubilization ability because of its larger interlayer spacing and stronger binding force with 2,4,6-TCP due to p–p interaction between benzene rings. The XRD patterns of the samples after 2,4,6-TCP adsolubilization are shown in Fig. 10. The (0 0 3), (0 0 6) and (0 0 9) peaks of the 2,4,6-TCP adsorbed samples appeared at the same degrees of the original adsorbents (Fig. 1). This phenomenon indicated that 2,4,6-TCP adsolubilization over DBS-LDH and DS-LDH is a physisorption process and the adsorbed 2,4,6-TCP molecules have not changed the chemical structure of adsorbents. In addition, the physisorption is beneficial for facile adsorbent regeneration and recycling [59]. However, the presence of (1 1 0) and (0 1 2) reflections in DS-LDH sample (Fig. 10b) after adsolubilization indicated the structure undergoes some modifications that has influenced the capacity of the adsorbent during regeneration and recycling (Fig. 11). 3.7. Recycling of adsorbents The development of reusable adsorbent is economically important. By extraction with acetone, adsorbed 2,4,6-TCP can be almost eliminated and removed, and thus the DBS and DS intercalated LDH can be reconverted as adsolubilization materials for reuse. Consecutive adsolubilization–regeneration cycles for 2,4,6-TCP with the LDH adsorbents were repeated 10 times under the same experimental conditions. Fig. 11 shows that at the first regeneration cycle of adsorbents, 2,4,6-TCP removal percentages still have more than 82% and 73% correspond to DBS-LDH and DS-LDH. In addition, the adsolubilization capacities of the two adsorbents diminish progressively with each cycle of regeneration. This may be attributed to slight loss and structure modifications of DBS-LDH and DS-LDH when the extraction of 2,4,6-TCP from acetone treatment. However, at the last recycle, they still have more than 57% of the 2,4,6-TCP removal percentage. 4. Conclusions

Fig. 11. Recycling experiment with 10 cycles of removing 2,4,6-TCP by (A) DBSLDH, (B) DS-LDH (Vsol = 50 mL, adsorbent mass = 700 mg, initial 2,4,6-TCP concentration Co = 300 mg/L, pH = 7 and T = 298 K).

Two types of ZnAl-LDHs intercalated with surfactant anions (DBS and DS) were synthesized by the co-precipitation method. The composites were good adsorbents for the adsolubilization of

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2,4,6-TCP in aqueous solution. It was found that the optimum adsorption equilibrium can be reached at a short time period under acidic condition and at room temperature. DBS-LDH has high adsolubilization capacity due to p–p interaction between benzene rings and large interlayer spacing. The adsolubilization kinetics was found to follow the pseudo-second-order model with correlation coefficient R2 of 0.998. The experimental equilibrium presented a linear adsorption character. The negative values of DG suggested that the processes of adsolubilization were spontaneous physisorption. Furthermore, regeneration study revealed that DS-LDH and DBS-LDH can be partially regenerated and reused at least 10 times. It can be concluded that these anionic surfactants modified LDHs may be considered as potential adsorbents in water treatment for the removal of organic pollutants. Acknowledgments This work was supported by the 973 Program (No. 2014CB932104), National Natural Science Foundation, Program for New Century Excellent Talents in Universities, Fundamental Research Funds for the Central Universities (Nos. ZZ1501 and YS1406) and Program for Changjiang Scholars, Innovative Research Team in University (No. IRT1205) and Beijing Engineering Center for Hierarchical Catalysts of P.R. China. References [1] S. Pan, L. Zhou, Y. Zhao, X. Chen, H. Shen, M. Cai, M. Jin, Amine-functional magnetic polymer modified graphene oxide as magnetic solid-phase extraction materials combined with liquid chromatography–tandem mass spectrometry for chlorophenols analysis in environmental water, J. Chromatogr. A 1362 (2014) 34–42. [2] B.H. Hameed, I.A.W. Tan, A.L. Ahmad, Adsorption isotherm, kinetic modeling and mechanism of 2,4,6-trichlorophenol on coconut husk-based activated carbon, Chem. Eng. J. 144 (2008) 235–244. [3] G. Chen, X. Shan, Y. Wang, B. Wen, Z. Pei, Y. Xie, T. Liu, J.J. Pignatello, Adsorption of 2,4,6-trichlorophenol by multi-walled carbon nanotubes as affected by Cu[II], Water Res. 43 (2009) 2409–2418. [4] G. Lente, J.H. Espenson, Oxidation of 2,4,6-trichlorophenol by hydrogen peroxide. Comparison of different iron-based catalysts, Green Chem. 7 (2005) 28–34. [5] S.G. Chung, Y.S. Chang, J.W. Choi, K. Youl, Baek, S.W. Hong, S.T. Yun, S.H. Lee, Photocatalytic degradation of chlorophenols using star block copolymers: removal efficiency, by-products and toxicity of catalyst, Chem. Eng. J. 215 (2013) 921–928. [6] S. Zheng, Z. Yang, D. Jo, Y. Park, Removal of chlorophenols from groundwater by chitosan sorption, Water Res. 38 (2004) 2315–2322. [7] F.W. Wiese, H.C. Chang, R.V. Lloyd, J.P. Freeman, V.M. Samokyszyn, Peroxidasecatalyzed oxidation of 2,4,6-trichlorophenol, Arch. Environ. Contam. Toxicol. 34 (1998) 217–222. [8] M.A. Sánchez, M. Vásquez, B. González, A previously unexposed forest soil microbial community degrades high levels of the pollutant 2,4,6trichlorophenol, Appl. Environ. Microbiol. 70 (2004) 7567–7570. [9] K.C. Christoforidis, M. Louloudi, Y. Deligiannakis, Complete dechlorination of pentachlorophenol by a heterogenous SiO2–Fe–porphyrin catalyst, Appl. Catal. B 95 (2010) 297–302. [10] P.T. Marc, G.M. Verónica, A.B. Miguel, G. Jaime, E. Santiago, Degradation of chlorophenols by means of advanced oxidation processes: a general review, Appl. Catal. B 47 (2004) 219–256. [11] N.N.M. Zain, B.N.K. Abu, S. Mohamad, N.M. Saleh, Optimization of a Greener method for removal phenol species by cloud point extraction and spectrophotometry, Spectrochim. Acta A 118 (2014) 1121–1128. [12] S.L. Liu, S. Li, H.Y. Niu, T. Zeng, Y.Q. Cai, C.H. Shi, B.H. Zhou, F.C. Wu, X.L. Zhao, Facile synthesis of novel flowerlike magnetic mesoporous carbon for efficient chlorophenols removal, Micropor. Mesopor. Mater. 200 (2014) 151–158. [13] R. Xu, C.L. Chi, F.T. Li, B.R. Zhang, Laccasepolyacrylonitrile nanofibrous membrane: highly immobilized, stable, reusable, and efficacious for 2,4,6trichlorophenol removal, ACS Appl. Mater. Interfaces 5 (2013) 12554–12560. [14] M.S. Miao, Y.J. Zhang, L. Shu, J. Zhang, Q. Kong, N. Li, Development and characterization of the 2,4,6-trichlorophenol (2,4,6-TCP) aerobic degrading granules in sequencing batch airlift reactor, Int. Biodeterior. Biodegrad. 95 (2014) 61–66. [15] L. Tian, Y. Zhao, S. He, M. Wei, X. Duan, Immobilized Cu–Cr layered double hydroxide films with visible-light responsive photocatalysis for organic pollutants, Chem. Eng. J. 184 (2012) 261–267. [16] S. Pura, G. Atun, Enhancement of nitrophenol adsorption in the presence of anionic surfactant and the effect of the substituent position, Colloids Surf. A 253 (2005) 137–144.

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