Removal of phenol from aqueous solution by adsorption onto OTMAC-modified attapulgite

Removal of phenol from aqueous solution by adsorption onto OTMAC-modified attapulgite

ARTICLE IN PRESS Journal of Environmental Management 84 (2007) 229–236 www.elsevier.com/locate/jenvman Removal of phenol from aqueous solution by ad...

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ARTICLE IN PRESS

Journal of Environmental Management 84 (2007) 229–236 www.elsevier.com/locate/jenvman

Removal of phenol from aqueous solution by adsorption onto OTMAC-modified attapulgite Jianhua Huang, Xingguo Wanga,b,, Qingzhe Jina,b, Yuanfa Liua,b, Ying Wangb a

The Key Laboratory of Food Science and Safety, Ministry of Education, Southern Yangtze University, 170 Huihe Road, Wuxi 214036, Jiangsu Province, China b School of Food Science and Technology, Southern Yangtze University, 170 Huihe Road, Wuxi 214036, Jiangsu Province, China Received 26 May 2005; received in revised form 19 May 2006; accepted 25 May 2006 Available online 21 July 2006

Abstract The potential of octodecyl trimethyl ammonium chloride (OTMAC)-modified attapulgite (AT) for phenol adsorption from aqueous solutions was studied. The comparison of natural AT and modified AT showed that it is possible to utilize the sonication-modified OTMAC-AT in the treatment of phenol-contaminated wastewaters. Batch sorption studies were carried out to evaluate the effect of contact time, shaking frequency, temperature and the amount of AT. The results showed that in a lab-scale reactor, at room temperture, with an amount of the modified AT added (2.5 g), and a shaking frequency of 140 rev/min, the adsorption rate of phenol could be 60.4% for a duration of 60 min. The sorption kinetics were described by a pseudo-second-order model, and the values of k and qe were 1.367 mg/ ig min and 0.7901 ig/mg, respectively. The analysis of equilibrium data showed that the Freundlich isotherms were found to be applicable for the adsorption equilibrium data. K and 1/n were estimated to be 14.53 and 0.8438, respectively. r 2006 Elsevier Ltd. All rights reserved. Keywords: Sonicate; Attapulgite; Octodecyl trimethyl ammonium chloride; Adsorption

1. Introduction Phenol and its derivatives are aromatic molecules containing hydroxyl, methyl, amide or sulfonic groups attached to the benzenoid ring structure. Phenols are widely distributed as environmental pollutants due to their common presence in the effluents of many industrial processes, including oil refineries, petrochemical plants, ceramic plants, steel plants, coal conversion processes and phenolic resin industries. Phenols are considered as priority pollutants since they are harmful to organisms at low concentrations and many of them have been classified as hazardous pollutants because of their potential harm to human health. Stringent US Environmental Protection Agency (EPA) regulations call for lowering phenol content in the wastewater to less than 1 mg/l (Dutta et al., 1992). Phenol is a widespread and highly toxic component of Corresponding author. Tel.: +86 510 587 67 99; fax: +86 510 587 67 99. E-mail address: [email protected] (X. Wang).

0301-4797/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2006.05.007

water and soil pollutants. Phenol concentrations of over 2 mg/l are toxic to fish and concentrations between 10 and 100 mg/l result in death of aquatic life within 96 h (Lanouette, 1977). Wastewater containing phenols and other toxic compounds need careful treatment before discharge into the receiving bodies of water. Adsorption onto activated carbons in the form of grains or powder is a well-known process for organic contaminant removal (Kyuya et al., 2004). The porous nature of this adsorbent material and its high internal surface area are favorable properties for adsorption. However, the cost is high and recovering activated carbon particles from treated water may be difficult. Removal of phenols from wastewater using combined microfiltration and biological processes has been reported (Campos et al., 2002). The use of membrane technology for biocatalysis has a number of advantages including high volume capacities, the possibility of performing a reaction simultaneously with a separation function, giving rise to multiple biochemical chain reactions, lower susceptibility to process disturbances and depending on the application, reduced sterilization

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requirements. However, the cost is high. Horseradish peroxidase (HPR) catalyses the oxidation of phenols and can be used to remove phenols from wastewater (Cooper and Nicell, 1996). This enzymatic method has many advantages, including broad substrate specificity, effectiveness over a wide range of pH, temperatures and phenol concentrations. The main drawback of this method is that it requires a large amount of enzyme to achieve a high removal efficiency due to enzyme deactivation. Other methods of phenol removal are: the degradation of phenol through the low-temperature oxidation of phenol in an aqueous phase in the presence of Ni-oxide (Christoskova and Stoyanova, 2001), the electrochemical treatment of phenolic wastewater (Bahadir and Abdurrahman 2003), and treatment of aqueous phenol solutions by ultraviolet (UV), g-ray and electron beams in the presence of the TiO2 nanoparticles (Chitose et al., 2003). Although these methods are effective, they suffer from the shortcoming of high cost. Attapulgite (AT) is a hydrated magnesium aluminum silicate present in nature as a fibrillar silicate clay mineral. AT particles, seen under an electronic microscope, are associated in fibrous bundles similar to those of hay (Cao et al., 1996; Haden and Schwint, 1967). AT has permanent negative charges on its surface, which enable it to be modified by cationic surfactants, to enhance contaminant retention and retard contaminant migration. There are large reserves of AT in South China (Jiang Su, Zhe Jiang and An Hui province) and in the USA (Florida). AT was first utilized in the 1940s, and has been mainly used as an absorbent, adsorbent, catalyst carrier, densifying agent, adhesive and food additive (Yan, 1981). It is also used as a filler to reduce the cost of polymer materials (Peng, 1998; Sheng et al., 1996). Compared to other adsorbents, the adsorption onto AT is more attractive because the AT is much cheaper than the others. Adsorption of phenanthrene on different organoclay complexes from distilled and saline water was reported. The adsorbed amounts of phenanthrene on montmorillonite exchanged by organic cations were several times higher than those obtained using montmorillonite clay without surface modification (El-Nahhal and Safi, 2004). Sorption of phenol and 2-, 3- and 4-chlorophenol from water by tetramethylammonium-smectite and tetramethylphosphonium-smectite was also reported (Lawrence et al., 1998). The adsorption–desorption of the herbicide fluridone on Na-montmorillonite and several organo-montmorillonite complexes was studied at a variety of loadings of the organic cation and pH levels (Yaron-Marcovich et al., 2004). Removal of phenol from water by adsorption– flocculation using bentonite modified with short chain cationic surfactant was also studied (Shen, 2002). The sorptive capability of clay minerals for anionic radionuclides was improved substantially by exchanging the natural inorganic interlayer cations with certain organic cations such as hexadecylpyridinium, hexadecyltrimethylammonium, benzethonium and dipyridinododecane ca-

tions (Riebe et al., 2005; Dultz et al., 2005). In this paper, AT modified with octodecyl trimethyl ammonium chloride (OTMAC) is studied for the adsorption of phenol.

2. Materials and methods 2.1. Chemicals A quaternary ammonium compound, OTMAC was obtained from the Feixiang Chemical Co. and used as received. Phenol, HCl and NaCl were obtained from Chinese Chemical Company and were of analytical grade.

2.2. Sorbent (Zhang et al., 1993; Alvatez et al., 1987) The AT used in this study was purchased from Oilbetter Co. in China, with a particle size p0.075 mm (200-mesh) as reported by the supplier. Its composition is given in Table 1. The AT was stirred in dilute HCl to exchange the interlayer cation with H+, separated by centrifugation, and mixed with NaCl solution to exchange the interlayer cation with Na+. Then, the mixture was centrifuged to separate the AT, which was washed, dried and crushed. The AT was added to OTMAC solution before the mixture was sonicated. Next, the AT was separated, washed, dried and crushed. To compare the adsorption efficiency, the mixture of AT and OTMAC without being sonicated was stirred at 60 1C for 60 min, then separated, washed, dried and crushed. The OTMAC-modified AT was used in the batch experiments.

2.3. Batch adsorption test (Wu, 2003; Li et al., 2001) Sorption experiments were carried out by allowing different amounts of modified AT to reach equilibrium with phenol solutions of known concentration. All the phenol solutions were prepared to give initial phenol concentrations of 50 ig/ml. Each batch test sample in the sorption experiments was prepared in a 250 ml iodine flask, with 50 ml of 50 ig/ml phenol solution together with modified AT. The amount of modified AT was varied in different experiments. Next, the iodine flasks were shaken at different temperatures for different times in the shakingthermostatic bath. The aqueous phase was separated by centrifugation at 2000 rev/min for 10 min. Aliquots of supernatant were withdrawn from each sample for phenol analysis. Table 1 Chemical composition of attapulgite Composition MgO Al2O3 SiO2 SO2 K2O CaO TiO2 MnO Fe2O3 Content (%) 8.2

11.9

63.9 1.0

1.9

1.6

1.4

0.5

9.7

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2.4. Analysis of phenol concentration When water was the solvent, the concentration of phenol was determined following the method of Gales, Jr. and Booth (1976), which is based on spectrophotometric analysis of the developed color resulting from the reaction of phenol with 4-aminoantipyrine. The final equilibrium concentrations were determined spectrophotometrically using a Spectronic UNICO UV-2100 spectrophotometer (China). The amount of phenol adsorbed was determined from the initial and final concentrations of phenol in the liquid phases. All experiments were run in triplicate to ensure reproducibility. A calibration curve of the absorbance versus phenol concentration was obtained. The percentage of phenol removal was calculated as follows: X% ¼

C0  C1  100%. C0

(1)

231

100

(a)

95

(b)

90 (c)

85 80 75 70 65 60 55 3500

3000

2500 2000 1500 Wavenumber (cm-1)

1000

500

Fig. 1. FIIR spectra of: (a) unmodified attapulgite, (b) sonicating modified OTMAC-attapulgite and (c) stirring modified OTMAC-attapulgite.

70

2.5. Isothermal adsorption experiments

60

a high temperature activated attapulgite b stirring modified attapulgite

ðC 0  CÞV Q¼  103 . W

(2)

Phenol removal, %

c Sonicating modified attapulgite

Batch shaking sorption experiments were conducted under the optimal conditions of the above mentioned phenol batch sorption experiments, with the initial phenol concentration varying as 25, 50, 100, 200, 300, 400, 450, 500 ig/ml. The adsorption capacity in the studies was calculated as follows:

50 40 30 20 10 0 a

2.6. Fourier-transform infrared (FTIR) spectroscopy The FTIR spectra of AT were obtained in a 470-IR spectrometer (Japan). Bulk materials were dried and finely ground. The samples were prepared for analysis by mixing 100 mg of KBr with about 2 mg of the material and then compressing the mixture to pellets. 2.7. Surface area measurements Surface areas were investigated using BET analysis. In the BET method, surface areas were calculated from the adsorbed nitrogen volume by an automatic volumetric apparatus. 3. Results and discussion 3.1. Comparison of natural attapulgite and modified attapulgite To compare the differences among the unmodified AT, stirring modified OTMAC-AT and sonication-modified OTMAC-AT, the FTIR spectra of the three kinds of AT are shown in Fig. 1. Two bands at 2700–3000 cm1 of

b

c

Fig. 2. Percentage of phenol removal by high-temperature-activated attapulgite, stirring modified attapulgite and sonicating modified attapulgite.

modified OTMAC-ATs are related to the stretching vibrations of C–H groups. And the transmittance spectra at 2700–3000 cm1 of the sonication-modified OTMACAT is stronger than that of the stirring modified OTMACAT while the other transmittance spectra of stirring modified OTMAC-AT is stronger than that of the sonication-modified OTMAC-AT. The free C–H stretching band occurs at approximately 3000 cm1. However the FTIR spectra of unmodified AT did not peak. Fig. 2 shows the percentage of phenol removal by hightemperature-activated attaplgite, stirring modified AT and sonication-modified AT. AT used as an adsorbent is usually high temperature activated to increase the adsorption. The sonication-modified OTMAC-AT was the best of the three used in the absorption experiments. Surface areas for the samples were investigated using BET analysis. Specific surface areas for the natural AT and sonication-modified OTMAC-AT are listed in Table 2.

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Table 2 Specific surface areas for the quartz sand and various iron-coated sands

Surface areas (m2/g)

Natural attapulgite

Stirring modified OTMAC-attapulgite

Sonicating modified OTMAC-attapulgite

167.24

83.67

63.96

46

180 y = 1.2657x + 1.1719

140

44

2 R = 0.9995

120 t /qt

Phenol removel, %

160

42

100 80 60

40

40 0

50

100

150

20

Contact time, min

0

Fig. 3. Percentage of phenol removal by modified attapulgite as a function of contact time (50 1C, 60 rev/min, 1.5 g).

0

50

100

150

t, mins Fig. 4. Pseudo-second-order sorption kinetics of phenol (50 1C, 60 rev/ min, 1.5 g).

3.2. Effect of contact time Fig. 3 shows the effect of contact time on phenol removal. The rate of adsorption of phenol was rapid initially and then slowed down gradually until it attained an equilibrium beyond which the rate of adsorption decreased. The initial rapid sorption was perhaps due to participation of special functional groups of OTMAC. The results also indicate that the equilibrium time required for the adsorption of phenol on modified AT was almost 60 min and much longer contact time was noticed for the desorption of phenol adsorbed by AT. Therefore, considering economic and practical aspects, the contact time of 60 min was employed in all subsequent experiments. To determine the order of adsorption kinetics of phenol, the first-order kinetics equation was tested but straight lines were not obtained. Then, t/qt versus t graphs were plotted (Fig. 3). The sorption kinetics may be described by a pseudo-second-order model. The differential equation is as follows: dq ¼ kðqe  qt Þ2 , dt

(3)

where qe is the amount of phenol adsorbed at equilibrium (ig/mg); qt is the amount of phenol adsorbed at time t (ig/ mg); and k is the equilibrium rate constant of pseudosecond-order sorption (mg/ig min). Integrating Eq. (3) for the boundary conditions t ¼ 0–t and qt ¼ 0–qt gives 1 1 ¼ þ kt, ðqe  qt Þ qe

(4)

which is the integrated rate law for a pseudo-second-order reaction. Eq. (4) can be rearranged to obtain a linear form t 1 1 ¼ þ t qt kq2e qe

(5)

and h ¼ kq2e , where h is the initial sorption rate (ig/mg min). The rate parameters k and qe can be directly obtained from the intercept and slope of the plot of t/qt against t. Fig. 4 shows the pseudo-second-order kinetics of phenol adsorption onto modified AT. Values of k and qe computed from Fig. 4 are qe ¼ 0.7901 ig/mg and k ¼ 1.367 mg/ig min. It is clear that the kinetics of phenol adsorption onto modified AT follows this model, with regression coefficients higher than 0.999. The calculated values agree very well with the experimental data, indicating that the sorption system studied belongs to the second-order kinetic model. 3.3. Effect of attapulgite amount In order to investigate the effect of adsorbent amount on the removal of phenol, a series of experiments was performed for a wide range of AT mass. The relationship between adsorption rate and AT amount used is given in Fig. 5. The adsorption rate increases with increasing amount of AT added. The rate of sorption was very rapid when the amount of AT was less than 2.5 g, and slowed down with more AT. The relationship between the phenol adsorbed per

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60 62

1.4 1.2

40

1 0.8

30

0.6

20

0.4

Phenol absorbed / attapulgite

1

2 3 Amount of attapulgite, g

56

53

50 290

0

0 0

59

10

Phenol removal

0.2

Phenol removal, %

50 Phenol removal, %

Phenol absorbed / attapul gite ig/mg

1.6

233

4

300

5

Fig. 5. Percentage of phenol removal by modified attapulgite as a function of amount of attapulgite (501C, 60 rev/min, 60 min).

310 Temperature, K

320

330

Fig. 6. Percentage of phenol removal by modified attapulgite as a function of temperature (2.5 g, 60 rev/min, 60 min).

0.6

3.4. Effect of temperature The relationship between the absorption rate and temperature is given in Fig. 6. The results indicated a low temperature of the shaking bath favors the absorption of phenol by AT and the sorption rate reached the maximum at 25 1C. This suggests the optimum temperature was 25 1C To determine the thermodynamic parameters of adsorption the thermodynamic aspects of adsorption are also displayed in Fig. 7. The equilibrium constant for the adsorption reaction of phenol on modified AT, K, was defined as follows: K¼

M ad C 0  C e C 0 ¼ ¼  1, Ce Ce Ce

(6)

where Mad is the adsorbed amount of adsorbate at equilibrium, C0 the initial concentration of adsorbate

y = 0.8836x - 2.5225 R2 = 0.9126

0.5 0.4 InK

mg of AT and the amount is also given in Fig. 5. Obviously, the phenol adsorbed per mg of AT decreased rapidly with an increase in the amount of AT. The results indicate that the phenol adsorption by the adsorbent would be limited as phenol adsorption attained equilibrium. A higher adsorbent concentration resulted in a higher phenol removal. The adsorption rate was affected by the concentration of the phenol solution. Even the additional AT did not increase the absorption rate rapidly when the phenol was too diluted. This phenomenon can be explained in this way: since the volume of solution used for each adsorption experiment was the same, the average distance between adsorbate and adsorbent would be increased as the phenol concentration was decreased. Consequently, the dispersive force between the phenol and the surface of the AT was less strong so that less adsorption would take place. At the same time, the decreased diffusivity of the adsorbate at its lower concentration was also thought to partly contribute to the decreased adsorption. So the optimum AT amount added was 2.5 g.

0.3 0.2 0.1 0 3

3.1

3.2

3.3

3.4

3.5

1/T (x10-3) Fig. 7. Linear plots of ln K versus 1/T (2.5 g, 60 rev/min, 60 min).

(mg/l), Ce the equilibrium concentration of adsorbate (ig/mg). The change in the Gibbs free energy for a reaction is expressed as DG ¼ DG  þ RT ln Q.

(7)

Here, DG1 and Q are the standard Gibbs free energy change and reaction quotient, respectively. When the reaction reaches the equilibrium state, DG becomes zero so that DG1 is equal to RT ln K, where K denotes the equilibrium constant. Therefore, DG1 for an adsorption reaction will be estimated if K for adsorption is known, which can be calculated from the experimental results using Eq. (6). Moreover, using the relationship, DG1 ¼ DH1TDS1, ln K can be expressed in Eq. (8). Thus, if the equilibrium constants for an adsorption reaction at different temperatures are known, the standard enthalpic and entropic changes for adsorption can be also estimated from the slope and intercept of a linear plot of ln K versus 1/T   1 1 ln K ¼  (8) DH  þ DS . RT R

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234

Fig. 7 shows the plots of ln K versus 1/T and it can be seen that the linearity of the plots is good. Table 3 represents the estimated thermodynamic parameters for phenol adsorption based on the thermodynamic

Table 3 Thermodynamic parameters of the adsorption reaction of phenol onto modified attapulgite Temperature (1C)

DG1 (kJ/mol)

DH1 (kJ/mol)

DS1 (J/mol K)

20 30 40 50

1.198 0.989 0.779 0.569

7.346 7.346 7.346 7.346

20.972 20.972 20.972 20.972

3.5. Effect of shaking frequency Fig. 8 illustrates the relationship between the adsorption rate and the shaking frequency. The adsorption rate increases as shaking frequency increases, at low values of frequency. The sorption rate was almost unchanged and even slightly decreased when the shaking frequency was more than 60 rev/min. The results indicated that increasing shaking leads to the desorption of phenol adsorbed in the AT. The optimum shaking frequency was 60 rev/min.

64

Phenol removal, %

calculations and results in Fig. 7. It was noted that for all of the considered experimental conditions DG1 became less negative as the temperature rose and DH1 had a negative value, which implied that the adsorption reaction of phenol was radiative. Therefore, the experimental results in Fig. 6 that indicate the increase in adsorbability of phenol with temperature were substantiated thermodynamically. The negative DS1’s for adsorption indicate the higher order of reaction during the adsorption of phenol onto the adsorbent and also reflect the affinity of the adsorbent materials for phenol. The phenol was limited on the surface of the adsorbent and moved less freely than in the water.

62

3.6. The orthogonal experiment evaluation

60

58 0

50 100 Shaking frequency, r/min

150

Fig. 8. Percentage of phenol removal by modified attapulgite as a function of shaking frequency (25 1C, 60 min, 2.5 g).

The evaluation and results of the orthogonal experiments are shown in Table 4. Also the changing trend of adsorption rate with the changing of shaking time, amount of AT added, temperature, and shaking frequency is shown in Fig. 9. The results indicate that the effects of the four factors above on the adsorption were different, and the amount of AT gives the strongest effect, then the shaking frequency, the temperature, and the shaking time had almost no effect. A shaking time of 60 min, an AT amount of 2.5 g, a temperature of 25 1C and a shaking frequency of 140 rev/min were suggested as the optimum conditions.

Table 4 Orthogonal experiment List

Shaking time (min)

Amount of attapulgite added (g)

Temperature (1C)

Shaking frequency (rev/min)

Adsorption rate (%)

1 2 3 4 5 6 7 8 9 K1 K2 K3 k1 k2 k3 R

1(45) 1 1 2(60) 2 2 3(75) 3 3 167.34 172.54 167.34 55.78 57.51 55.78 5.20

1(2) 2(2.5) 3(3) 1 2 3 1 2 3 151.07 173.20 182.95 50.36 57.73 60.98 31.88

1 (25) 2 (30) 3 (35) 2 3 1 3 1 2 171.24 168.64 167.34 57.08 56.21 55.78 3.90

1(60) 2 (100) 3(140) 3 1 2 2 3 1 165.71 167.66 173.85 55.24 55.89 57.95 8.14

49.38 56.54 61.42 52.96 57.19 62.39 48.73 59.47 59.14

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235

200

Phenol removal, %

175

150

125

100 45

60 75 Time, min

2

2.5 3 Amount of attapulite, g

25

30 35 Temperature, °C

50

100 150 frequency, r/min

Fig. 9. Effect of time, attapulgite amount, temperature, and shaking frequency on phenol removal.

the adsorption energy of phenol in the modified AT decreases on completion of the sorptional centers of the adsorbent. Banat et al. (2000) reported a similar observation for sorption of phenol onto bentonite. The Freundlich model can be expressed as the following equation:

4.5 y = 10.8438x + 1.6123 4

R2 = 0.9716

lg qe

3.5

qe ¼ KC 1=n e ,

3

2.5

0 1

1.5

2 lg Ce

2.5

3

Fig. 10. Freundlich isotherm for the adsorption of phenol (2.5 g, 60 rev/ min, 60 min).

A larger amount of AT was not suggested considering the saving of adsorbent. The adsorption rate was 60.4% under the optimum conditions.

(9)

where qe is the amount of equilibrium adsorption (ig/mg), Ce the equilibrium concentration of adsorbate (ig/ml), K and 1/n are the Freundlich constants related to adsorption capacity and adsorption intensity. By taking the logarithm for both sides of Eq. (9), the linear form of the Freundlich model can be obtained   1 lg qe ¼ lg K þ (10) lg C e . n The plot of lg qe versus lg Ce is given in Fig. 10. It showed good linearity with an R2 of 0.9716. The values for K and 1/n can be obtained from the intercept on the y-axis and slope of the linear line. They were estimated to be 14.53 and 0.8438, respectively. 4. Conclusions

3.7. Adsorption isotherms Adsorption isotherms are useful in understanding the sorption interaction. Several models have been published in the literature to describe experimental data of adsorption isotherms, which show the different adsorption mechanisms. The Langmuir and Freundlich models are the most frequently employed models. We attempted to fit the equilibrium data to an adsorption isotherm and found that the equilibrium adsorption of phenol on the modified AT followed the Freundlich isotherm well. Fitting of the data to the Freundlich adsorption isotherm model indicates that

The potential of OTMAC-modified attapulgite (AT) to remove phenol from aqueous solution was assessed. Comparison of the unmodified AT, stirring modified OTMAC-AT and sonication-modified OTMAC-AT indicates that the sonication-modified OTMAC-AT was the superior adsorbent among these three samples. It is, therefore, possible to use it in the treatment of phenolcontaminated wastewaters. The adsorption rate of phenol on the sonication-modified OTMAC-AT was 60.4% under the optimum conditions: shaking time of 60 min, AT amount of 2.5 g, temperature of 25 1C and shaking

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frequency of 140 rev/min. The sorption kinetics were described by a pseudo-second-order model, and the values of k and qe were 1.367 mg/ig min and 0.7901 ig/mg, respectively. The analysis of equilibrium data showed that the Freundlich isotherm was found to be applicable for the adsorption equilibrium data. K and 1/n were estimated to be 14.53 and 0.8438, respectively. Acknowledgments The authors express their gratitude to the NSFC (National Natural Science Foundation of China), for its financial support (Contract No: 20376028). We also thank the Testing & Analysis Center of South Yangtze University. References Alvatez, B.A., Perez, C.R., Ruiz, H.E., 1987. Process for manufacturing organophilic fibrous clays. EP0221225. Bahadir, K.K., Abdurrahman, T., 2003. Continuous electrochemical treatment of phenolic wastewater in a tubular reactor. Water Research 37, 1505–1514. Banat, F.A., Al-Bashir, B., Al-Asheh, S., Hayajneh, O., 2000. Adsorption of phenol by bentonite. Environmental Pollution 107, 391–398. Campos, J.C., Borges, R.M.H., Oliveira Filho, A.M., Nobrega, R., Sant’Anna Jr., G.L., 2002. Oilfield wastewater treatment by combined microfiltration and biological processes. Water Research 36, 95–104. Chitose, N., Ueta, S., Seino, S., 2003. Radiolysis of aqueous phenol solutions with nanoparticles, 1: phenol degradation and TOC removal in solutions containing TiO2 induced by UV, c-ray and electron beams. Chemosphere 50, 1007–1013. Christoskova, S.T., Stoyanova, M., 2001. Research note degradation of phenolic wasters over Ni-oxide. Water Research 35, 2073–2077. Cooper, V.A., Nicell, J.A., 1996. Removal of phenols from a foundry wastewater using horseradish peroxidase. Water Research 30, 954–964. Dultz, S., Riebe, B., Bunnenberg, C., 2005. Temperature effects on iodine adsorption on organo-clay minerals, II: structural effects. Applied Clay Science 28, 17–30.

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