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Ultrasound-assisted removal of tetracycline from aqueous solution by mesoporous alumina Huiyuan Li, Xin Zhong, Hui Zhang, Luojing Xiang, Sebastien Royer, Sabine Valange and Joel Barrault
ABSTRACT This work describes the removal of tetracycline (TC) from aqueous solution using a mesoporous alumina (meso-Al2O3) as adsorbent in the presence of ultrasonic irradiation. Adsorption of TC was investigated under various operating conditions, including pH, adsorbent dosage, ultrasound power, and initial TC concentration. The results showed that the rate of TC sorption was enhanced with the assistance of ultrasound. The TC removal increased with the increase in sorbent dosage, pH and ultrasound power, but decreased with the increase in initial TC concentration. The adsorption isotherm data were fitted properly with the Freundlich model under ultrasonic irradiation, and the adsorption process followed a pseudo-second-order kinetic model. Key words
| adsorption, kinetics, meso-Al2O3, tetracycline, ultrasound
Huiyuan Li Xin Zhong Hui Zhang (corresponding author) Luojing Xiang Department of Environmental Engineering, Hubei Biomass-Resource Chemistry and Environmental Biotechnology Key Laboratory, Wuhan University, Wuhan 430079, China E-mail: [email protected] Xin Zhong Luojing Xiang Sebastien Royer Sabine Valange Joel Barrault Universite de Poitiers, IC2MP UMR 7285 CNRS, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France
INTRODUCTION The discovery and application of antibiotics have played a great role in human medicine and these substances are widely used in animal and plant disease control. Thus, the widespread use of antibiotics has led to the release of residues into the environment, causing increasingly prominent environmental pollution (Wang et al. ). Tetracycline (TC) is a class of antibiotics used extensively for disease control and against infections and has been detected in surface water in the environment (Zhang et al. ; Chen & Huang ). Long time exposure to antibiotics can cause drug tolerance of bacteria, leading to more and more intractable infections and an enormous cost of treatment of infectious diseases. Therefore, the removal of TC from aqueous solutions becomes a major environmental problem. So far, many studies have been focused on the treatment of TC in order to eliminate its impact on the environment. The adsorption process is one of the most effective and clean methods for the removal of hazardous compounds from aqueous solution (Chen & Huang ). Aluminum oxide has been reported as the most popular material in adsorption treatment for its meso/microporous structure, providing an extensively high surface area, large pore size doi: 10.2166/wst.2013.784
and high pore volume, and high adsorption capacity (Chen & Huang ). The adsorption of contaminants from aqueous solutions is a solid–liquid process, and ultrasound could improve the solid– liquid mass transfer due to the collapse of cavitation bubbles (Milenkovic´ et al. ). The enhancement of the adsorptive removal of organic pollutants has been observed under ultrasound (Hamdaoui et al. , ; Entezari & Al-Hoseini ; Hamdaoui & Naffrechoux ; Milenkovic´ et al. ). However, to our knowledge, there is no application of the sono-sorption process for the removal of TC from aqueous solution. Therefore, the main goal of the present study is to remove TC by the combination of ultrasound and mesoporous alumina sorbent. The effects of various conditions, i.e. pH values, ultrasound power, initial TC concentration and adsorbent dosage, were investigated, and the adsorption isotherm and the adsorption kinetics were determined.
METHODS The commercial TC hydrochloride (analytical reagent grade, 99%) was used without further purification, and the main
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Ultrasound-assisted removal of TC from aqueous solution
Water Science & Technology
characteristics are provided elsewhere (Hou et al. ). Cetyl-trimethylammonium bromide (CTABr) and aluminum-tri-sec-butoxide (Al[OCH(CH3)C2H5]3) employed in the preparation of the meso-Al2O3 were obtained from Sigma-Aldrich, and anhydrous ethanol (C2H5OH) was obtained from Alfa-Aesar. Pure mesoporous alumina material was prepared by the surfactant-assisted sol–gel procedure. The process involved the controlled hydrolysis of aluminum alkoxide precursor in the presence of CTABr (Lee et al. ), which has been described specifically in our previous study (Zhong et al. ). The BET (Brunauer–Emmett– Teller) surface area, pore size and pore volume of the synthesized sorbent were 531 m2 g–1, 6.1 nm, and 1.20 cm3 g1, respectively. The presence of peaks at 2θ values of 35 , 45 , and 61 indicated a poorly defined γ-Al2O3 crystal structure (Figure 1). The stock solution of TC was freshly prepared by dissolving a selected amount of TC in deionized water before each run. Sonication was generated by an ultrasonic cleaning instrument (100 W, 20 kHz, Kunshan Shumei Instrument Co., China). In a typical experiment, a given amount of adsorbent was added into 150 mL of TC aqueous solution. A mechanical stirrer was used to ensure a homogeneous mixing of the solution. Experiments were carried out at constant temperature of 20 C. At regular irradiation time intervals, a 1 mL aliquot was sampled, filtered through a membrane of pore size 0.22 μm and then analyzed using a Shimadzu-20A high performance liquid chromatography (HPLC) system (Hou et al. ). The amount of TC adsorbed at time t was calculated by Equation (1): W
W
W
W
q¼
(C0 C) × V M
Figure 1
|
(1)
X-ray diffraction patterns of the reference γ-Al2O3 sample (bar plots) and the calcined meso-Al2O3 material.
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where q is the amount of TC adsorbed at time t (mg g1), C0 and C are the concentration of TC at time t0 and t (mg L1), V is the volume of TC solution, and M is the weight of the adsorbent (g).
RESULTS AND DISCUSSION TC removal in different systems The removal of TC from aqueous solution by the mesoporous alumina adsorbent was conducted in the presence and the absence of ultrasonic irradiation. Compared with silent condition (conventional method), TC removal was increased by 37.2% with the assistance of ultrasound after 60 min adsorption, as shown in Figure 2. A longer time (210 min) was required for the conventional method to achieve similar removal efficiency with respect to the sono-sorption process. Hamdaoui & Naffrechoux () thought the organic removal by sorbent in the ultrasoundassisted technique is due to both adsorption and ultrasonic degradation. To investigate the removal mechanism of TC, tert-butyl alcohol (TBA), a typical hydroxyl radical scavenger, was added to the ultrasonic-assisted system. It can be seen from Figure 2 that the presence of TBA had very little effect on TC removal. Moreover, no degradation products in the solutions were detected from the HPLC chromatograms (data not shown). Therefore, TC removal by the ultrasound-assisted technique was solely attributed to the adsorption of TC within the pore volume of the meso-Al2O3 sorbent rather than oxidation. Ultrasound-enhanced adsorptive removal of TC from aqueous solution could be related to the cavitation process. It is accepted that the liquid–solid sorption process involves
Figure 2
|
TC removal by meso-Al2O3 in the absence and presence of ultrasound (C0 ¼ 50 mg L1, pH0 4, [meso-Al2O3] ¼ 0.3 g L1, ultrasound power ¼ 80 W).
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several consecutive steps such as transport of the pollutant from the bulk solution to the external surface of the sorbent, diffusion of the pollutant from the external surface and into the pores of the sorbent, and adsorption of the pollutant on the adsorption sites on the internal surface of the pores (Hamdaoui & Naffrechoux ). Microjets and shockwaves generated by the cavitation can affect the particle size of the sorbent (Zhong et al. ), leading to an increased surface area that could promote a larger amount of available sites for sorption, thereby increasing the removal of pollutant from aqueous solution (Hamdaoui et al. ). In addition, the strong turbulence induced by ultrasound irradiation reduces the thickness of liquid films surrounding the sorbent and increases the rate of external diffusion (Hamdaoui & Naffrechoux ). In the meantime, microjets may induce turbulence and additional convective mass transport inside the pores (Hamdaoui et al. ). To verify this assumption, the film diffusion coefficient (Hamdaoui & Naffrechoux ) and intra-particle diffusion coefficient (Weber & Morris ) were determined according to Equations (2) and (3), respectively: ln
C ka ¼ f t C0 V
(2)
pffiffi q ¼ kp t þ Cq
(3)
where kfa and kp are film diffusion coefficient and intra-particle diffusion coefficient, respectively, and Cq is the intercept. The values of film diffusion coefficient and intra-particle diffusion coefficient obtained under silent and sonicated conditions are summarized in Tables 1 and 2. As can be seen from Table 2, the intercept Cq under silent condition was negative (2.59 mg g1). It indicated that the film thickness retarded intra-particular diffusion (McKay et al. ). The film thickness would be reduced under ultrasonic irradiation and the film diffusion coefficient was 2.42 times higher than the silent system. In the meantime, the induced turbulence and additional convective mass transfer inside the pores caused by microjets led to the increase of
Table 1
|
Film diffusion coefficient in the silent and sonicated systems
Silent system
Sonicated system
kfa × 104 (L min1)
R2
kfa × 104 (L min1)
R2
9.386
0.9930
22.68
0.9848
Table 2
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Intra-particle diffusion coefficient in the silent and sonicated systems
Silent system
Sonicated system Cq
kp
Cq
(mg g1 min1/2)
(mg g1)
R2
12.87
0.9624
kp (mg g1 min1/2)
(mg g1)
R2
7.91
2.59
0.9879 14.52
the intra-particle diffusion coefficient from 7.91 to 14.52 mg g1 min1/2. This resulted in a more rapid adsorption rate and the intercept Cq became positive accordingly (Wu et al. ). Effect of pH on TC removal The effect of pH was examined in the range of 2–7 when the initial TC concentration was 50 mg L1, the sorbent dosage was 0.3 g L1, and the ultrasound power was 80 W. As can be observed in Figure 3(a), the removal of TC increased with increasing initial pH. The change of pH affects not only the molecular speciation of TC, but also the surface charge state of alumina. The zero-point-of-charge (pHzpc) of Al2O3 was reported to be in the range of 7.5–8.7 (Chen & Huang ), whereas the Al2O3 surface was positively charged when pH was below its pHzpc in this study. TC is predominantly in its protonated form (TCHþ 3 ) at pH 2, as indicated in Figure 3(b). Electrostatic repulsion between positive charges of TC and Al2O3 surfaces would hinder the adsorption of TC on alumina. As pH rose to 3, the fraction of TC in positive form (TCHþ 3 ) decreased, while that in neutral form (TCH2) increased. This resulted in the increase of TC removal, but the increase was limited since the major form of TC was still TCHþ 3 . TC removal increased significantly at pH 4, as the main species of TC changed from being positive to neutral. Furthermore, at pH 7, about 20% of TC is in a negative form and electrostatic attraction led to a higher TC removal efficiency than at pH 4 and 5. Effect of sorbent dosage on TC removal Figure 4 illustrates that the removal of TC increased with the increase in sorbent content. This was attributed to an increase of the sorbent surface area and to the availability of more sorption sites on account of the increase of the sorbent dosage (Entezari & Al-Hoseini ; Hamdaoui et al. ). However, the amount of TC sorbed per mass unit of sorbent decreased with the increase in sorbent content. The sorption was decreased from 262.56 to 78.03 mg g1 when sorbent dosage was increased from 0.1 to 0.5 g L1.
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Figure 3
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Figure 4
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Ultrasound-assisted removal of TC from aqueous solution
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The removal (a) and speciation (b) of TC under different initial pH values (C0 ¼ 50 mg L1, [meso-Al2O3] ¼ 0.3 g L1, ultrasound power ¼ 80 W).
Effect of sorbent amount on TC removal (C0 ¼ 50 mg L1, pH0 4, ultrasound power ¼ 80 W).
Hamdaoui et al. () attributed it to the split in the flux or the concentration gradient between adsorbate concentrations in the solution and on the sorbent surface. Effect of ultrasound power on TC removal The removal of TC by mesoporous alumina was investigated in the presence of ultrasonic irradiation under five different powers. Although the presence of ultrasound could effectively enhance adsorption of TC (Figure 1), TC removal increased insignificantly when ultrasound power rose from 50 to 99 W (Figure 5). The number of cavitation events, the intensity of the high-speed microjets, and the intensity of the high-pressure shock waves induced by acoustic cavitation mostly depend on the power delivered to the system (Hamdaoui et al. ). A strong ultrasound power would lead to greater effects on the sorption process than would a weaker ultrasound power. But, meanwhile, more cavitation events would give rise to more molecules to be
Figure 5
|
Effect of ultrasound power on TC removal (C0 ¼ 50 mg L1, pH0 4, [mesoAl2O3] ¼ 0.3 g L1).
desorbed (Hamdaoui et al. ). In addition, when ultrasound power exceeded a certain value, an insufficient collapse of the cavitation bubbles and an acoustic screen could be generated, and the scattering effect would occur, which consumed a part of ultrasonic power and translated it into heat (Zhong et al. ). Therefore, ultrasound power in the range of 50–99 W had a limited effect on TC removal. Effect of initial TC concentration on TC removal Different initial concentrations, ranging from 50 to 150 mg L1, were investigated (Figure 6). When the initial TC concentration increased from 25 to 150 mg L1, the removal efficiency decreased from 82.7 to 42.8%. While the amount of sorbent remains constant and the TC concentration increases, the ratio of sorbate to sorbent increases. By increasing the amount of TC, the TC removal fraction should decrease due to the saturation of the sorption sites (Entezari & Al-Hoseini ). It should be
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Table 3 C0 (mg L1)
Figure 6
|
Effect of initial TC concentration on TC removal (pH0 4, [meso-Al2O3] ¼ 0.3 g L1, ultrasound power ¼ 80 W).
mentioned that the actual amount of TC sorbed per mass unit of sorbent increased with the increase of TC concentration in the solution. As the concentration of TC in the solution was increased from 25 to 150 mg L1, the sorption amount was increased from 68.90 to 213.84 mg g1. This shows that by increasing TC concentration sixfold, the amount of sorption was also increased 3.1-fold. Adsorption isotherm and kinetics The Freundlich isotherm is an empirical equation describing heterogeneous surface adsorption. The linear form of the equation is expressed as: 1 ln qe ¼ ln KF þ ln ce n
(4)
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Parameters of pseudo-second-order kinetic models qe, exp (mg g1)
k2 × 104 (mg1 g1 min1)
qe,cal (mg g1)
R2
25
68.90
38.00
73.24
0.9999
50
117.98
7.23
134.41
0.9655
100
183.92
2.19
233.80
0.9701
150
213.84
2.08
270.40
0.9697
where k2 (g mg1 min1) is the pseudo-second-order rate constant, which can be determined from the intercept of the plot of t/q versus t. The fitted values for k2 and qe are summarized in Table 3. In all cases, the correlation coefficient values (R 2) were more than 0.9, indicating that the data exhibit a good compliance with the pseudo-second-order kinetic model.
CONCLUSION The mesoporous alumina (meso-Al2O3) has high surface area, large pore size and high pore volume, making this material suitable for achieving a high TC removal in the ultrasound-assisted adsorption process. The adsorption efficiency of TC was significantly enhanced in the presence of ultrasonic irradiation. Equilibrium and kinetic data fit with the Freundlich and the pseudo-second-order expression, respectively. Consequently, the ultrasound-assisted adsorption appears as a promising process for the treatment of wastewaters contaminated with TC.
where ce is TC equilibrium concentration (mg L1), qe is the amount of TC at equilibrium (mg g1), KF is the Freundlich constant related to the adsorption capacity of the adsorbent (mg11/n L1/n g1), and n is the heterogeneity factor indicating the adsorption intensity of the adsorbent. Based on the slope and intercept of the linear plot of lnqe versus lnce, the values of KF and n were obtained to be 40.42 mg11/n L1/n g1 and 2.59, respectively. The high R 2 value obtained (0.9905) indicated the Freundlich model fitted well with the adsorption isotherm data. In an attempt to examine the adsorption kinetics of TC onto the meso-Al2O3 adsorbent, the pseudo-second-order model, which is given by Equation (5), is used to fit the experimental data:
X. Zhong greatly acknowledges the academic award for excellent PhD candidates funded by the Ministry of Education of China (Grant No. 5052011205013). We appreciate the financial support by the Fundamental Research Funds for the Central Universities, China, (Grant No. 201120502020004) and Natural Science Foundation of China (Grant Nos. 20977069, 21211130108). We appreciate the valuable comments and suggestion of the anonymous reviewers.
t 1 t ¼ þ q k2 q2e qe
Chen, W. R. & Huang, C. H. Adsorption and transformation of tetracycline antibiotics with aluminum oxide. Chemosphere 79 (8), 779–785.
(5)
ACKNOWLEDGEMENTS
REFERENCES
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Ultrasound-assisted removal of TC from aqueous solution
Entezari, M. H. & Al-Hoseini, Z. S. Sono-sorption as a new method for the removal of methylene blue from aqueous solution. Ultrasonics Sonochemistry 14, 599–604. Hamdaoui, O. & Naffrechoux, E. Adsorption kinetics of 4chlorophenol onto granular activated carbon in the presence of high frequency ultrasound. Ultrasonics Sonochemistry 16 (1), 15–22. Hamdaoui, O., Naffrechoux, E., Tifouti, L. & Pétrier, C. Effects of ultrasound on adsorption–desorption of pchlorophenol on granular activated carbon. Ultrasonics Sonochemistry 10 (2), 109–114. Hamdaoui, O., Chiha, M. & Naffrechoux, E. Ultrasoundassisted removal of malachite green from aqueous solution by dead pine needles. Ultrasonics Sonochemistry 15 (5), 799–807. Hou, L. W., Zhang, H., Wang, L. G. & Chen, L. Ultrasoundenhanced magnetite catalytic ozonation of tetracycline in water. Chemical Engineering Journal 229, 577–584. Lee, H. C., Kim, H. J., Rhee, C. H., Lee, K. H., Lee, J. S. & Chung, S. H. Synthesis of nanostructured γ-alumina with a cationic surfactant and controlled amounts of water. Microporous and Mesoporous Materials 79 (1–3), 61–68. McKay, G., Otterburn, M. S. & Sweeney, A. G. The removal of colour from effluent using various adsorbents – III. Silica: Rate processes. Water Research 14 (1), 15–20.
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Milenkovic´, D. D., Bojic´, A. L. & Veljkovic´, V. B. Ultrasoundassisted adsorption of 4-dodecylbenzene sulfonate from aqueous solutions by corn cob activated carbon. Ultrasonics Sonochemistry 20 (3), 955–962. Wang, X., Wang, Y. & Li, D. Degradation of tetracycline in water by ultrasonic irradiation. Water Science and Technology 67 (4), 715–721. Weber, W. J. & Morris, J. C. Kinetics of adsorption on carbon from solutions. Journal of the Sanitary Engineering Division, American Society of Civil Engineers 89 (17), 31–60. Wu, F. C., Tseng, R. L. & Juang, R. S. Initial behavior of intraparticle diffusion model used in the description of adsorption kinetics. Chemical Engineering Journal 153 (1–3), 1–8. Zhang, H., Liu, F., Wu, X. G., Zhang, J. H. & Zhang, D. B. Degradation of tetracycline in aqueous medium by electrochemical method. Asia-Pacific Journal of Chemical Engineering 4 (5), 568–573. Zhong, X., Xiang, L., Royer, S., Valange, S., Barrault, J. & Zhang, H. Degradation of C.I. Acid Orange 7 by heterogeneous Fenton oxidation in combination with ultrasonic irradiation. Journal of Chemical Technology and Biotechnology 86 (7), 970–978.
First received 8 July 2013; accepted in revised form 29 November 2013. Available online 13 December 2013
© IWA Publishing 2014 Water Science & Technology
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69.4
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2014
Ultrasound-assisted removal of tetracycline from aqueous solution by mesoporous alumina Huiyuan Li, Xin Zhong, Hui Zhang, Luojing Xiang, Sebastien Royer, Sabine Valange and Joel Barrault
ABSTRACT This work describes the removal of tetracycline (TC) from aqueous solution using a mesoporous alumina (meso-Al2O3) as adsorbent in the presence of ultrasonic irradiation. Adsorption of TC was investigated under various operating conditions, including pH, adsorbent dosage, ultrasound power, and initial TC concentration. The results showed that the rate of TC sorption was enhanced with the assistance of ultrasound. The TC removal increased with the increase in sorbent dosage, pH and ultrasound power, but decreased with the increase in initial TC concentration. The adsorption isotherm data were fitted properly with the Freundlich model under ultrasonic irradiation, and the adsorption process followed a pseudo-second-order kinetic model. Key words
| adsorption, kinetics, meso-Al2O3, tetracycline, ultrasound
Huiyuan Li Xin Zhong Hui Zhang (corresponding author) Luojing Xiang Department of Environmental Engineering, Hubei Biomass-Resource Chemistry and Environmental Biotechnology Key Laboratory, Wuhan University, Wuhan 430079, China E-mail: [email protected] Xin Zhong Luojing Xiang Sebastien Royer Sabine Valange Joel Barrault Universite de Poitiers, IC2MP UMR 7285 CNRS, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France
INTRODUCTION The discovery and application of antibiotics have played a great role in human medicine and these substances are widely used in animal and plant disease control. Thus, the widespread use of antibiotics has led to the release of residues into the environment, causing increasingly prominent environmental pollution (Wang et al. ). Tetracycline (TC) is a class of antibiotics used extensively for disease control and against infections and has been detected in surface water in the environment (Zhang et al. ; Chen & Huang ). Long time exposure to antibiotics can cause drug tolerance of bacteria, leading to more and more intractable infections and an enormous cost of treatment of infectious diseases. Therefore, the removal of TC from aqueous solutions becomes a major environmental problem. So far, many studies have been focused on the treatment of TC in order to eliminate its impact on the environment. The adsorption process is one of the most effective and clean methods for the removal of hazardous compounds from aqueous solution (Chen & Huang ). Aluminum oxide has been reported as the most popular material in adsorption treatment for its meso/microporous structure, providing an extensively high surface area, large pore size doi: 10.2166/wst.2013.784
and high pore volume, and high adsorption capacity (Chen & Huang ). The adsorption of contaminants from aqueous solutions is a solid–liquid process, and ultrasound could improve the solid– liquid mass transfer due to the collapse of cavitation bubbles (Milenkovic´ et al. ). The enhancement of the adsorptive removal of organic pollutants has been observed under ultrasound (Hamdaoui et al. , ; Entezari & Al-Hoseini ; Hamdaoui & Naffrechoux ; Milenkovic´ et al. ). However, to our knowledge, there is no application of the sono-sorption process for the removal of TC from aqueous solution. Therefore, the main goal of the present study is to remove TC by the combination of ultrasound and mesoporous alumina sorbent. The effects of various conditions, i.e. pH values, ultrasound power, initial TC concentration and adsorbent dosage, were investigated, and the adsorption isotherm and the adsorption kinetics were determined.
METHODS The commercial TC hydrochloride (analytical reagent grade, 99%) was used without further purification, and the main
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H. Li et al.
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Ultrasound-assisted removal of TC from aqueous solution
Water Science & Technology
characteristics are provided elsewhere (Hou et al. ). Cetyl-trimethylammonium bromide (CTABr) and aluminum-tri-sec-butoxide (Al[OCH(CH3)C2H5]3) employed in the preparation of the meso-Al2O3 were obtained from Sigma-Aldrich, and anhydrous ethanol (C2H5OH) was obtained from Alfa-Aesar. Pure mesoporous alumina material was prepared by the surfactant-assisted sol–gel procedure. The process involved the controlled hydrolysis of aluminum alkoxide precursor in the presence of CTABr (Lee et al. ), which has been described specifically in our previous study (Zhong et al. ). The BET (Brunauer–Emmett– Teller) surface area, pore size and pore volume of the synthesized sorbent were 531 m2 g–1, 6.1 nm, and 1.20 cm3 g1, respectively. The presence of peaks at 2θ values of 35 , 45 , and 61 indicated a poorly defined γ-Al2O3 crystal structure (Figure 1). The stock solution of TC was freshly prepared by dissolving a selected amount of TC in deionized water before each run. Sonication was generated by an ultrasonic cleaning instrument (100 W, 20 kHz, Kunshan Shumei Instrument Co., China). In a typical experiment, a given amount of adsorbent was added into 150 mL of TC aqueous solution. A mechanical stirrer was used to ensure a homogeneous mixing of the solution. Experiments were carried out at constant temperature of 20 C. At regular irradiation time intervals, a 1 mL aliquot was sampled, filtered through a membrane of pore size 0.22 μm and then analyzed using a Shimadzu-20A high performance liquid chromatography (HPLC) system (Hou et al. ). The amount of TC adsorbed at time t was calculated by Equation (1): W
W
W
W
q¼
(C0 C) × V M
Figure 1
|
(1)
X-ray diffraction patterns of the reference γ-Al2O3 sample (bar plots) and the calcined meso-Al2O3 material.
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where q is the amount of TC adsorbed at time t (mg g1), C0 and C are the concentration of TC at time t0 and t (mg L1), V is the volume of TC solution, and M is the weight of the adsorbent (g).
RESULTS AND DISCUSSION TC removal in different systems The removal of TC from aqueous solution by the mesoporous alumina adsorbent was conducted in the presence and the absence of ultrasonic irradiation. Compared with silent condition (conventional method), TC removal was increased by 37.2% with the assistance of ultrasound after 60 min adsorption, as shown in Figure 2. A longer time (210 min) was required for the conventional method to achieve similar removal efficiency with respect to the sono-sorption process. Hamdaoui & Naffrechoux () thought the organic removal by sorbent in the ultrasoundassisted technique is due to both adsorption and ultrasonic degradation. To investigate the removal mechanism of TC, tert-butyl alcohol (TBA), a typical hydroxyl radical scavenger, was added to the ultrasonic-assisted system. It can be seen from Figure 2 that the presence of TBA had very little effect on TC removal. Moreover, no degradation products in the solutions were detected from the HPLC chromatograms (data not shown). Therefore, TC removal by the ultrasound-assisted technique was solely attributed to the adsorption of TC within the pore volume of the meso-Al2O3 sorbent rather than oxidation. Ultrasound-enhanced adsorptive removal of TC from aqueous solution could be related to the cavitation process. It is accepted that the liquid–solid sorption process involves
Figure 2
|
TC removal by meso-Al2O3 in the absence and presence of ultrasound (C0 ¼ 50 mg L1, pH0 4, [meso-Al2O3] ¼ 0.3 g L1, ultrasound power ¼ 80 W).
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Ultrasound-assisted removal of TC from aqueous solution
Water Science & Technology
several consecutive steps such as transport of the pollutant from the bulk solution to the external surface of the sorbent, diffusion of the pollutant from the external surface and into the pores of the sorbent, and adsorption of the pollutant on the adsorption sites on the internal surface of the pores (Hamdaoui & Naffrechoux ). Microjets and shockwaves generated by the cavitation can affect the particle size of the sorbent (Zhong et al. ), leading to an increased surface area that could promote a larger amount of available sites for sorption, thereby increasing the removal of pollutant from aqueous solution (Hamdaoui et al. ). In addition, the strong turbulence induced by ultrasound irradiation reduces the thickness of liquid films surrounding the sorbent and increases the rate of external diffusion (Hamdaoui & Naffrechoux ). In the meantime, microjets may induce turbulence and additional convective mass transport inside the pores (Hamdaoui et al. ). To verify this assumption, the film diffusion coefficient (Hamdaoui & Naffrechoux ) and intra-particle diffusion coefficient (Weber & Morris ) were determined according to Equations (2) and (3), respectively: ln
C ka ¼ f t C0 V
(2)
pffiffi q ¼ kp t þ Cq
(3)
where kfa and kp are film diffusion coefficient and intra-particle diffusion coefficient, respectively, and Cq is the intercept. The values of film diffusion coefficient and intra-particle diffusion coefficient obtained under silent and sonicated conditions are summarized in Tables 1 and 2. As can be seen from Table 2, the intercept Cq under silent condition was negative (2.59 mg g1). It indicated that the film thickness retarded intra-particular diffusion (McKay et al. ). The film thickness would be reduced under ultrasonic irradiation and the film diffusion coefficient was 2.42 times higher than the silent system. In the meantime, the induced turbulence and additional convective mass transfer inside the pores caused by microjets led to the increase of
Table 1
|
Film diffusion coefficient in the silent and sonicated systems
Silent system
Sonicated system
kfa × 104 (L min1)
R2
kfa × 104 (L min1)
R2
9.386
0.9930
22.68
0.9848
Table 2
|
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Intra-particle diffusion coefficient in the silent and sonicated systems
Silent system
Sonicated system Cq
kp
Cq
(mg g1 min1/2)
(mg g1)
R2
12.87
0.9624
kp (mg g1 min1/2)
(mg g1)
R2
7.91
2.59
0.9879 14.52
the intra-particle diffusion coefficient from 7.91 to 14.52 mg g1 min1/2. This resulted in a more rapid adsorption rate and the intercept Cq became positive accordingly (Wu et al. ). Effect of pH on TC removal The effect of pH was examined in the range of 2–7 when the initial TC concentration was 50 mg L1, the sorbent dosage was 0.3 g L1, and the ultrasound power was 80 W. As can be observed in Figure 3(a), the removal of TC increased with increasing initial pH. The change of pH affects not only the molecular speciation of TC, but also the surface charge state of alumina. The zero-point-of-charge (pHzpc) of Al2O3 was reported to be in the range of 7.5–8.7 (Chen & Huang ), whereas the Al2O3 surface was positively charged when pH was below its pHzpc in this study. TC is predominantly in its protonated form (TCHþ 3 ) at pH 2, as indicated in Figure 3(b). Electrostatic repulsion between positive charges of TC and Al2O3 surfaces would hinder the adsorption of TC on alumina. As pH rose to 3, the fraction of TC in positive form (TCHþ 3 ) decreased, while that in neutral form (TCH2) increased. This resulted in the increase of TC removal, but the increase was limited since the major form of TC was still TCHþ 3 . TC removal increased significantly at pH 4, as the main species of TC changed from being positive to neutral. Furthermore, at pH 7, about 20% of TC is in a negative form and electrostatic attraction led to a higher TC removal efficiency than at pH 4 and 5. Effect of sorbent dosage on TC removal Figure 4 illustrates that the removal of TC increased with the increase in sorbent content. This was attributed to an increase of the sorbent surface area and to the availability of more sorption sites on account of the increase of the sorbent dosage (Entezari & Al-Hoseini ; Hamdaoui et al. ). However, the amount of TC sorbed per mass unit of sorbent decreased with the increase in sorbent content. The sorption was decreased from 262.56 to 78.03 mg g1 when sorbent dosage was increased from 0.1 to 0.5 g L1.
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Figure 3
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Figure 4
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Ultrasound-assisted removal of TC from aqueous solution
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The removal (a) and speciation (b) of TC under different initial pH values (C0 ¼ 50 mg L1, [meso-Al2O3] ¼ 0.3 g L1, ultrasound power ¼ 80 W).
Effect of sorbent amount on TC removal (C0 ¼ 50 mg L1, pH0 4, ultrasound power ¼ 80 W).
Hamdaoui et al. () attributed it to the split in the flux or the concentration gradient between adsorbate concentrations in the solution and on the sorbent surface. Effect of ultrasound power on TC removal The removal of TC by mesoporous alumina was investigated in the presence of ultrasonic irradiation under five different powers. Although the presence of ultrasound could effectively enhance adsorption of TC (Figure 1), TC removal increased insignificantly when ultrasound power rose from 50 to 99 W (Figure 5). The number of cavitation events, the intensity of the high-speed microjets, and the intensity of the high-pressure shock waves induced by acoustic cavitation mostly depend on the power delivered to the system (Hamdaoui et al. ). A strong ultrasound power would lead to greater effects on the sorption process than would a weaker ultrasound power. But, meanwhile, more cavitation events would give rise to more molecules to be
Figure 5
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Effect of ultrasound power on TC removal (C0 ¼ 50 mg L1, pH0 4, [mesoAl2O3] ¼ 0.3 g L1).
desorbed (Hamdaoui et al. ). In addition, when ultrasound power exceeded a certain value, an insufficient collapse of the cavitation bubbles and an acoustic screen could be generated, and the scattering effect would occur, which consumed a part of ultrasonic power and translated it into heat (Zhong et al. ). Therefore, ultrasound power in the range of 50–99 W had a limited effect on TC removal. Effect of initial TC concentration on TC removal Different initial concentrations, ranging from 50 to 150 mg L1, were investigated (Figure 6). When the initial TC concentration increased from 25 to 150 mg L1, the removal efficiency decreased from 82.7 to 42.8%. While the amount of sorbent remains constant and the TC concentration increases, the ratio of sorbate to sorbent increases. By increasing the amount of TC, the TC removal fraction should decrease due to the saturation of the sorption sites (Entezari & Al-Hoseini ). It should be
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Table 3 C0 (mg L1)
Figure 6
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Effect of initial TC concentration on TC removal (pH0 4, [meso-Al2O3] ¼ 0.3 g L1, ultrasound power ¼ 80 W).
mentioned that the actual amount of TC sorbed per mass unit of sorbent increased with the increase of TC concentration in the solution. As the concentration of TC in the solution was increased from 25 to 150 mg L1, the sorption amount was increased from 68.90 to 213.84 mg g1. This shows that by increasing TC concentration sixfold, the amount of sorption was also increased 3.1-fold. Adsorption isotherm and kinetics The Freundlich isotherm is an empirical equation describing heterogeneous surface adsorption. The linear form of the equation is expressed as: 1 ln qe ¼ ln KF þ ln ce n
(4)
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Parameters of pseudo-second-order kinetic models qe, exp (mg g1)
k2 × 104 (mg1 g1 min1)
qe,cal (mg g1)
R2
25
68.90
38.00
73.24
0.9999
50
117.98
7.23
134.41
0.9655
100
183.92
2.19
233.80
0.9701
150
213.84
2.08
270.40
0.9697
where k2 (g mg1 min1) is the pseudo-second-order rate constant, which can be determined from the intercept of the plot of t/q versus t. The fitted values for k2 and qe are summarized in Table 3. In all cases, the correlation coefficient values (R 2) were more than 0.9, indicating that the data exhibit a good compliance with the pseudo-second-order kinetic model.
CONCLUSION The mesoporous alumina (meso-Al2O3) has high surface area, large pore size and high pore volume, making this material suitable for achieving a high TC removal in the ultrasound-assisted adsorption process. The adsorption efficiency of TC was significantly enhanced in the presence of ultrasonic irradiation. Equilibrium and kinetic data fit with the Freundlich and the pseudo-second-order expression, respectively. Consequently, the ultrasound-assisted adsorption appears as a promising process for the treatment of wastewaters contaminated with TC.
where ce is TC equilibrium concentration (mg L1), qe is the amount of TC at equilibrium (mg g1), KF is the Freundlich constant related to the adsorption capacity of the adsorbent (mg11/n L1/n g1), and n is the heterogeneity factor indicating the adsorption intensity of the adsorbent. Based on the slope and intercept of the linear plot of lnqe versus lnce, the values of KF and n were obtained to be 40.42 mg11/n L1/n g1 and 2.59, respectively. The high R 2 value obtained (0.9905) indicated the Freundlich model fitted well with the adsorption isotherm data. In an attempt to examine the adsorption kinetics of TC onto the meso-Al2O3 adsorbent, the pseudo-second-order model, which is given by Equation (5), is used to fit the experimental data:
X. Zhong greatly acknowledges the academic award for excellent PhD candidates funded by the Ministry of Education of China (Grant No. 5052011205013). We appreciate the financial support by the Fundamental Research Funds for the Central Universities, China, (Grant No. 201120502020004) and Natural Science Foundation of China (Grant Nos. 20977069, 21211130108). We appreciate the valuable comments and suggestion of the anonymous reviewers.
t 1 t ¼ þ q k2 q2e qe
Chen, W. R. & Huang, C. H. Adsorption and transformation of tetracycline antibiotics with aluminum oxide. Chemosphere 79 (8), 779–785.
(5)
ACKNOWLEDGEMENTS
REFERENCES
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First received 8 July 2013; accepted in revised form 29 November 2013. Available online 13 December 2013
