ARTICLE pubs.acs.org/IECR
Fabrication of Hierarchical Layered Double Hydroxide Framework on Aluminum Foam as a Structured Adsorbent for Water Treatment Shan He, Yufei Zhao, Min Wei,* David G. Evans, and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
bS Supporting Information ABSTRACT: A hierarchical MgAl layered double hydroxide (LDH) framework has been fabricated via the in situ crystallization technique on aluminum foam, which can be used to remove heavy metal ion Cr(VI) and anionic dye (Remazol Brilliant Blue R, denoted as RBBR) from aqueous solutions. The as-prepared LDH/Al-foam displays flowerlike LDH microspheres composed of numerous LDH nanoplatelets, which was confirmed by XRD and SEM. The sorption kinetics of hierarchical LDH framework for Cr(VI) was appropriately described by a pseudo-second-order model. Sorption isotherms of both Cr(VI) and RBBR were also studied, which can be fitted by the Langmuir model more satisfactorily than the Freundlich one. It was found that the sorption capacities (qMAX) of the LDH framework reached 27.8 mg g1 for Cr(VI) and 212.8 mg g1 for RBBR, respectively, much larger than those of the corresponding LDH powder sample (21 mg g1 for Cr(VI) and 166.7 mg g1 for RBBR). Furthermore, the hierarchical LDH framework exhibits excellent sorptionregeneration performances compared with the powder sample, which facilitates its repeatable and cyclic usage over a long period. Owing to the high sorption capacity, low-cost preparation, convenient manipulation, and easy regeneration of the hierarchical LDH framework, it can be potentially used as a structured adsorbent in the field of water treatment.
1. INTRODUCTION Heavy metal ions and synthetic dyes entering the environment as toxic wastes give rise to potentially serious environmental problems throughout the world.1,2 Heavy metal ions, widely existing in the effluents of the electroplating, tanning, mining, and fertilizer industries, are carcinogenic contaminants to humans.3 Synthetic dyes applied in the textile industry are carcinogenic and mutagenic pollutants for humans and aquatic organisms.4 It is therefore urgent to develop new materials and techniques to settle these issues. Conventional water treatment methods for the removal of heavy metal ions and synthetic dyes involve oxidation, reduction, precipitation, membrane filtration, biological process, ion exchange, and sorption.5,6 Sorption is believed to be a facile and convenient approach among these techniques.6 Therefore, the development of new sorption materials that show a high sorption capacity at low cost is a promising route to selectively and efficiently remove heavy metal ions and synthetic dyes from aqueous solutions. In recent years, layered double hydroxides (LDHs) as a class of anionic clays have attracted considerable attention from both industry and academia because of their potential applications in areas including catalysis and sorption.7,8 LDHs can be generally expressed by the formula [M2+1‑xM3+x(OH)2](An)x/n 3 mH2O, where M2+ and M3+ are di- and trivalent metal cations and An is the anion compensating for the positive charge of the hydroxide layers.9 LDH materials have been widely used as adsorbents to remove heavy metal ions10,11 and other contaminants12 from wastewater owing to their large specific surface areas and high anion exchange capacities. However, the application of LDH powder sorbents shows the following drawbacks: the formation of aggregates; difficulties in subsequent separation, regeneration, r 2011 American Chemical Society
and recycling processes. From this viewpoint, hierarchical adsorbents with controllable morphology, orientation, and dimensionality have evoked considerable interest owing to their unique properties superior to traditional ones.13 Various chemical and physicochemical methods (Ostwald ripening, self-assembly method, template-sacrificial method, and chemically induced self-transformation) have been developed to fabricate adsorbents with hierarchical nanostructures (e.g., Fe2O3, γ-Al2O3, and CeO2) which show excellent sorption behaviors.1315 In our previous work, LDH films with hierarchical morphologies have been fabricated on different templates via the solgel nanocopying and in situ growth process, exhibiting high adaptabilities in water treatment.16 Open-celled aluminum foams as supports of structured catalysts and sorbents have been successfully used in environmental separation and industrial catalysis processes. These materials display low pressure drops, high geometric surface areas, and good heat and mass transfer characteristics due to their threedimensional cellular structures composed of interconnected channels or macropores.17,18 This gives us impetus to take the challenge of fabricating a hierarchical LDH framework adsorbent on the support of open-celled aluminum foam. The resulting adsorbent would show the following advantages: the threedimensional cellular structure of Al foam provides support for the immobilization of LDH building blocks so as to largely prevent aggregation and to deliver necessary mechanical robustness Received: July 22, 2011 Accepted: November 23, 2011 Revised: November 21, 2011 Published: November 23, 2011 285
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against wear and tear; the nanostructure of LDH supplies high surface area and abundant sorption sites. In the present work, we report the preparation of an Mg/ Al-LDH hierarchical framework as a structured adsorbent via the in situ growth process of LDH film on the open-celled aluminum foam (denoted as LDH/Al-foam), and demonstrate its sorption performance for Cr(VI) and an anionic dye in water treatment. The as-prepared LDH/Al-foam shows high-dispersive flowerlike LDH microspheres which are composed of numerous LDH nanoplatelets. Cr(VI) and Remazol Brilliant Blue R dye (RBBR) were chosen as models of highly toxic pollutants in water, and their sorption behaviors by the LDH/Al-foam were studied thoroughly. The pseudo-second-order model can be used to describe the sorption kinetics of Cr(VI); the Langmuir model yields a much better fit than the Freundlich one to reveal the sorption equilibrium for Cr(VI). It was found that the LDH/Alfoam exhibits much a higher sorption capacity, more stable sorptionregeneration behavior, and convenient manipulation compared with the corresponding LDH powder sample. In addition, the LDH/Al-foam shows excellent sorption performance for the anionic dye RBBR. Therefore, this work provides a facile and effective method for the fabrication of a hierarchical LDH framework which can be used as a structured adsorbent in water treatment with low cost, high capability, and recycling usage.
The sorption capacity (qe) of Cr(VI) can be calculated by the following equation: qe ¼
ðc0 ce ÞV m
ð1Þ
where c0 is the initial concentration of Cr(VI) in solution (mg L1), ce is the equilibrium concentration (mg L1), qe is the equilibrium sorption capacity (mg g1), m is the mass of adsorbent (g), and V is the volume of solution (L). Sorption Isotherms for Cr(VI). The sorption isotherms for Cr(VI) were established by batch type sorption experiments. The LDH/Al-foam (with 45 mg of LDH sorbent approximately accounting for 4 wt % of the LDH/Al-foam) was immersed into 90 mL of Cr(VI) solution (from 5 to 80 mg L1) at ambient temperature with constant shaking until the sorption equilibrium was obtained, and a 1 mL aliquot was filtered from the solutions using a 0.45 μm membrane filter. The concentration of Cr(VI) in the filtrate was analyzed by using ICP-AES. The pH of the Cr(VI) solution was controlled at 6.0 during the process of sorption. For comparison, the LDH powder sorbent (45 mg) and pristine Al foam (the same weight as in the LDH/Al-foam sample) were also evaluated in the same way, respectively. Test for the SorptionRegeneration Recycles. Desorption of Cr(VI) from the LDH/Al-foam (desorption/regeneration measurements) was performed by immersing the Cr(VI)-loaded LDH/Al-foam into 200 mL of Na2CO3 solution (0.01 mol L1) with constant shaking at 30 °C for 3 h.11 The loaded Cr(VI) was released into the Na2CO3 solution via the anion exchange between Cr2O72 and CO32 owing to the particularly high affinity of carbonate to the brucite-like sheets of LDH material. Subsequently, the LDH/Al-foam was separated from the Na2CO3 solution, washed thoroughly, and reused for the following sorptionregeneration cycles for 10 times. Sorption of RBBR. Sorption experiments of RBBR were carried out in a 500 mL stoppered conical flask at room temperature. The LDH/Al-foam with 150 mg of LDH sorbent was dipped into 300 mL of RBBR solution (50 mg L1, a typical dye wastewater concentration) with constant shaking. The pH of RBBR solution was ∼7.5 without control during the process of sorption. At specified time intervals, 2 mL of solution was removed and filtered through a 0.45 μm membrane filter. The concentration of RBBR in the filtrate was determined by UVvis spectroscopy at 593 nm. The removal percentage (R%) of RBBR can be calculated by the following equation: c0 ce ð2Þ R% ¼ 100 c0
2. EXPERIMENTAL METHOD 2.1. Materials. RBBR (CI 61200, Reactive Blue 19) was purchased from Sigma-Aldrich Co. Analytical grade chemicals including Mg(NO3)2 3 6H2O, urea, NaOH, K2Cr2O7, and anhydrous ethanol were purchased from the Beijing Chemical Co. Ltd. and used without further purification. Aluminum foams were purchased from the Institute of Solid State Physics, Chinese Academy of Sciences. Deionized and decarbonated water was used in all the experimental processes. 2.2. Fabrication of the LDH/Al-Foam by an in Situ Growth Technique. Aluminum foam (100 mm 50 mm 50 mm) was ultrasonically cleaned in ethanol and deionized water, respectively. The LDH/Al-foam was fabricated by the in situ growth technique on the aluminum foam by means of urea hydrolysis similar to the previous report by our group.19 The in situ crystallization technique involves direct growth of LDH crystallites on an alumina substrate with their ab-facet perpendicular to the substrate surface, which can be explained by a homogeneous nucleation mechanism. In a typical procedure, a piece of aluminum foam was immersed into a solution of Mg(NO3)2 3 6H2O (0.027 mol) and urea (0.162 mol) dissolved in 500 mL of deionized water in a glass vessel. The glass vessel was sealed and maintained at 80 °C for 48 h. After cooling, the substrate was washed with ethanol and dried at 70 °C for 12 h. 2.3. Evaluation of the LDH/Al-Foam as Adsorbent. Sorption of Cr(VI). The LDH/Al-foam with 150 mg of LDH sorbent was immersed into 300 mL of Cr(VI) solution (from 5 to 80 mg L1) at ambient temperature with constant shaking. The pH of the Cr(VI) solution was adjusted to 6.0 and maintained constant during the process of sorption. A 1 mL aliquot was filtered from the solution at specific time intervals using a 0.45 μm membrane filter. The concentration of Cr(VI) in the filtrate was analyzed by using inductively coupled plasma atomic emission spectroscopy (ICP-AES).
where c0 is the initial concentration of RBBR in solution (mg L1) and ce is the equilibrium concentration (mg L1). Sorption Isotherms for RBBR. The sorption isotherms for RBBR were established by batch type sorption experiments. The LDH/Al-foam with 50 mg of LDH sorbent was immersed into 100 mL of RBBR solution (from 25 to 600 mg L1) at ambient temperature with constant shaking, until sorption equilibrium was obtained. For comparison, the LDH powder sorbent (50 mg) and pristine Al foam (the same weight as in the LDH/ Al-foam sample) were evaluated in the same way, respectively. 2.4. Characterization Technique. The powder X-ray diffraction (XRD) measurements were performed on a Rigaku XRD6000 diffractometer, using Cu Kα radiation (λ = 0.154 18 nm) at 40 kV, 30 mA, with a scanning rate of 10 deg min1, and a 286
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Moreover, the appearance of (015), (110), and (113) reflections in Figure 1, curves b and c, indicates the formation of a wellcrystallized hydrotalcite-like LDH phase. Photographs and a low-magnification SEM image of pristine Al foam are shown in Figure 2a and Figure 2b, respectively, from which a framework consisting numerous open cells and pores was observed (average cell size 10 ppi). A high-magnification SEM image (Figure 2c) shows that casting defects (such as cracks, cavities, and heaves) were dispersed on the surface of Al foam. Figure 2d exhibits the surface view of the resulting LDH/ Al-foam, and abundant particles can be observed. A highmagnification SEM (Figure 2e) shows that the LDH microspheres with a diameter of 5 μm are highly dispersive, and are composed of numerous nanoplatelets (∼2.5 μm in width and 100 nm in thickness) intercrossed with each other. The energy dispersive X-ray spectrometry (EDX) analysis of the Mg/AlLDH framework on Al foam (Figure 2f) reveals the presence of Mg, Al, and O, with an Mg/Al molar ratio of ∼2.0. Based on the results above, it can be concluded that an LDH framework immobilized on the surface architecture of Al foam can be fabricated via the in situ growth procedure. The mass percentage of the LDH sorbent in the LDH/Al-foam was found to be approximately 4 wt % by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES). Moreover, it was found that vigorous ultrasonic treatment could not lead to the peeling or delamination of LDH film from the Al foam, indicating strong adhesion of LDH material to the substrate (Figure S1 in the Supporting Information). 3.2. Sorption Behavior of Cr(VI) on the LDH/Al-Foam. Sorption Kinetics. The sorption performance of LDH/Al-foam was investigated for Cr(VI) in this work, and Figure 3A shows the effect of initial concentration on the sorption capacity for Cr(VI). The sorption capacity increased along with time and reached a constant value after 480 min, indicating sorption equilibration for Cr(VI) was achieved. Upon increasing the initial concentration of Cr(VI) from 5 to 80 mg L1, the sorption capacity increased from 5.5 to 23.4 mg g1 (Figure 3A). The maximum Cr(VI) sorption capacity of the LDH/Al-foam (23.4 mg g1) is much larger than that of the powder sample (17.8 mg g1) (as shown in Figure S2 in the Supporting Information). Sorption kinetics was studied to explain the sorption mechanism and characteristics of Cr(VI) by the LDH/Al-foam. Ho’s pseudo-second-order model21,22 was chosen for the description of the sorption process for Cr(VI):
Figure 1. XRD patterns of (a) Al foam, (b) LDH/Al-foam, and (c) LDH powder sample. The rhombic symbol indicates the reflections from the Al foam.
Figure 2. (a) Optical photograph of the Al foam sample (inset: corresponding cross-section picture). SEM images of Al foam at (b) low magnification and (c) high magnification (inset: micrograph of the surface of Al foam); SEM images of resulting LDH/Al-foam at (d) low magnification and (e) high magnification; (f) EDX of the LDH material on the Al foam.
2θ angle ranging from 3 to 70°. The morphology of the LDH film was investigated by using a Hitachi S-4700 scanning electron microscope (SEM) fitted with an Oxford Instruments Isis-300 energy dispersive X-ray (EDX) analyzer operating at 20 kV. A UVvis spectrophotometer (Beijing PGENERAL TU-1901) was employed to measure the adsorption spectra of RBBR in the wavelength range 200700 nm. Inductively coupled plasma atomic emission spectroscopy (ICP-AES, Shimadzu ICPS-7500) was used to measure the concentration of Cr(VI) in the solution.
t 1 t ¼ þ qt k2 qe 2 qe
ð3Þ
where qe and qt (mg g1) are the amounts of Cr(VI) adsorbed on the LDH/Al-foam at equilibrium and at time t (min), respectively; k2 (g mg1 min1) is the rate constant of the pseudosecond-order kinetic model. The linear relationship of t/qt vs t is presented in Figure 3B. The values of qe and k2 can be calculated from the slope and intercept of the plot, respectively, which are listed in Table 1. It can be seen that the pseudo-second-order kinetic model can be satisfactorily used to describe the sorption behavior of Cr(VI) on the LDH/Al-foam in terms of high correlation coefficients (R2 > 0.99). In addition, the calculated sorption capacity (qe,cal) obtained from the model is quite close to the experimental one (qe,exp). As a result, the sorption kinetics fitted well with the pseudo-second-order model, suggesting that the sorption process of Cr(VI) on the LDH/Al-foam involves a chemisorption process that is the rate-limiting step.21,22
3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of the LDH/Al-Foam by an in Situ Growth Technique. Figure 1 illustrates the XRD patterns
of the pristine Al foam and the resulting Mg/Al-LDH on the Al foam by the in situ growth method. The characteristic reflections of Al were clearly observed from curve a, while the XRD pattern of LDH/Al-foam (curve b) exhibits a superimposition of the Al substrate (denoted by the rhombic symbol) and the LDH structure with a series of (00l) reflections appearing as narrow, symmetric, strong lines at low angles. The basal spacing for the LDH material immobilized on Al foam is 0.767 nm, close to the value reported for CO3-containing LDHs.20 The XRD pattern is also in accordance with the corresponding Mg/Al-LDH powder sample prepared by the coprecipitation method (curve c). 287
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Figure 3. (A) Effect of initial concentration on the sorption capacity for Cr(VI). (B) Pseudo-second-order kinetic plots for sorption of Cr(VI) by LDH/ Al-foam. Initial Cr(VI) concentration: (a) 5, (b) 10, (c) 20, (d) 40, and (e) 80 mg L1; the adsorbent amount was 0.57 g L1.
Table 1. Kinetic Parameters for the Sorption of Cr(VI) by the LDH/Al-Foam Based on the Pseudo-Second-Order Model C0
qe,exp
k2
qe,cal
(mg g1)
(mg g1)
(g mg1 min1)
(mg g1)
R2
5
5.5
2.00 103
6.4
0.9913
10
8.6
2.03 103
9.1
0.9933
20 40
13.2 20.3
1.04 103 0.89 103
14.0 21.2
0.9901 0.9911
80
23.4
1.13 103
24.6
0.9961
Table 2. Langmuir and Freundlich Isotherm Parameters for the Sorption of Cr(VI) Langmuir qm
Sorption Isotherms. Sorption isotherms were investigated to show the sorption capacity of the LDH/Al-foam for Cr(VI) (Figure 4). Two commonly used sorption isotherm models were used to fit the equilibrium data: the Langmuir (eq 4) and Freundlich models (eq 5):23
ln qe ¼ ln KF þ
1 ln ce n
KF R2
n
(L g1)
R2
LDH/Al-foam
27.8
0.071
0.9877 2.244 3.731 0.9794
LDH powder sample
21.0
0.079
0.9993 2.132 2.746 0.9701
monolayer sorption capacity; KL (L mg1) is the Langmuir sorption constant; KF (L g1) is the Freundlich constant; 1/n is the heterogeneity factor. It was found that the self-sorption of Cr(VI) on the pristine Al foam was hardly observed. The Langmuir model yields a much better fit than the Freundlich model based on the values of R2 listed in Table 2. In addition, the sorption capacity (qMAX) reaches ∼27.8 mg g1 for Cr(VI), which is much larger than that of the corresponding LDH powder sample (∼21.0 mg g1). The lower sorption capability of the LDH powder sample is probably due to the formation of aggregates, which is generally inevitable for nanoscale materials. In contrast, well-dispersed microcrystals of LDH on Al foam without aggregation as well as the existence of LDH nanoplatelets (Figure 2e) provide a mechanical stability and large surface area, resulting in the superior sorption performance of LDH/Al-foam. SorptionRegeneration of the LDH/Al-Foam for Recycle Application. Since the sorptionregeneration of adsorbents is the most difficult part in the wastewater treatment systems, it is very important to gain an efficient and low-cost sorption regeneration process.24 The regeneration and recycle application of the LDH/Al-foam were demonstrated through the Cr(VI)containing solution. Desorption of Cr(VI) from the LDH/Alfoam was carried out in a Na2CO3 solution in order to evaluate the regeneration behavior of the adsorbent, and Figure 5 shows the desorption percentage of Cr(VI) as a function of time from the LDH/Al-foam and the powder adsorbent, respectively. It was found that the LDH powder sample shows a desorption percentage of 87% after 120 min. In contrast, the LDH/Al-foam exhibits a faster desorption rate and a higher desorption capacity compared with the powder sample. A desorption percentage of 98% was achieved after 90 min. The consecutive sorptionregeneration cycles for Cr(VI) by the LDH/Al-foam and powder sample, respectively, were
Figure 4. Sorption isotherms for Cr(VI): (a) LDH/Al-foam; (b) LDH powder sample; (c) pristine Al foam.
ce 1 ce ¼ þ KL qMAX qe qMAX
KL
(mg g1) (L mg1)
adsorbent
Freundlich
ð4Þ ð5Þ
where qe (mg g1) is the amount of Cr(VI) adsorbed on the LDH/Al-foam at equilibrium; Ce (mg L1) is the concentration at equilibrium; qMAX (mg g1) is the theoretical maximum 288
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Figure 5. Desorption percentage of Cr(VI) from (a) LDH/Al-foam and (b) LDH powder sample as a function of time (200 mL of 0.01 M Na2CO3 solution was used at 30 °C).
Figure 7. SEM images of LDH/Al-foam after 10 consecutive cycles of sorptionregeneration: (a) low magnification; (b) high magnification. SEM images of powder sample after (c) 1 cycle and (d) 10 cycles, respectively (insets show high-magnification SEM images).
Figure 6. Regeneration ratio of LDH/Al-foam and powder adsorbent, respectively, for 10 consecutive cycles of sorptionregeneration.
repeated 10 times under uniform experimental conditions, and the results are shown in Figure 6. The regeneration ratio of the LDH powder sample is ∼65% for 10 cycles. Compared with the LDH powder sample, the LDH/Al-foam maintains a higher regeneration ratio (∼90%) in the consecutive cycles of sorptionregeneration. The superior sorptionregeneration properties of the LDH/Al-foam are attributed to the hierarchical structure which facilitates a high dispersion of LDH nanoplatelets; moreover, no obvious change in its morphology was observed after 10 cycles (Figure 7a,b), owing to the strong mechanical stability of the LDH/Al-foam. For the powder sample, however, serious aggregation of LDH nanoparticles occurs (Figure 7c,d), which imposes a great influence on the sorption desorption process and results in the low regeneration ratio. 3.3. Removal of RBBR by LDH/Al-Foam. The removal of dye pollutants is also of great importance in water treatment. In this study, the sorption behavior of the LDH/Al-foam for a dye, RBBR, was further studied. The characteristic absorption of RBBR at 593 nm was chosen to monitor the adsorption process, and the removal percentage (R%) of RBBR was used to characterize the sorption capacity (as shown in eq 2). For a typical dye solution (300 mL, 50 mg L1), the removal percentage for RBBR was 49% after 140 min (as shown in Figure S3 in the Supporting Information) for the LDH powder sample (150 mg). In the case
Figure 8. (A) UVvis absorption spectra of RBBR solution along with time. (B) Photographs of RBBR aqueous solution in the presence of LDH/Al-foam sorbent after different adsorption times.
of the LDH/Al-foam, a rapid decrease of absorption intensity at 593 nm along with time can be observed (Figure 8A), and the removal percentage of RBBR reaches as high as 94% after 140 min. This adsorption can be further visually observed by the decolorization process of the RBBR aqueous solution (Figure 8B). The results demonstrate that the LDH/Al-foam exhibits a better removal performance for RBBR compared with the powder sorbent. Figure 9 shows the sorption isotherms of RBBR. The self-sorption of RBBR on the pristine Al foam was hardly observed (Figure 9c); the Langmuir (eq 4) and Freundlich (eq 5) models were used to simulate the sorption performance of the LDH/Al-foam and the powder sample, respectively. It was 289
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the LDH/Al-foam adsorbent displays excellent sorption regeneration performances, which guarantees its repeatable and cyclic usage over a long period. Therefore, the LDH/Al-foam in this work exhibits the advantages of large sorption capability, strong mechanical robustness, convenient manipulation, and easy regeneration. It is expected that the LDH/Al-foam can be potentially used as an efficient and recycling adsorbent in water treatment.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures showing SEM images of the LDH/Al-foam tested for adhesion, equilibrium sorption capacity of the LDH/Al-foam and powder adsorbent for Cr(VI) with different initial Cr(VI) concentrations, and UVvis absorption spectra of RBBR aqueous solution in the presence of LDH powder sorbent; table listing Langmuir and Freundlich isotherm parameters for the sorption of RBBR. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 9. Sorption isotherms for RBBR: (a) LDH/Al-foam, (b) LDH powder sample, and (c) pristine Al foam.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86-10-64412131. Fax: +86-10-64425385. E-mail: weimin@ mail.buct.edu.cn.
’ ACKNOWLEDGMENT This work was supported by the 973 Program (Grant 2011CBA00504), the National Natural Science Foundation of China, the 111 Project (Grant B07004), and the Collaboration Project from the Beijing Education Committee.
Figure 10. Regeneration ratio of LDH/Al-foam adsorbent for RBBR during 10 consecutive cycles of sorptionregeneration.
’ REFERENCES (1) Zhong, L. S.; Hu, J. S.; Cao, A. M.; Liu, Q.; Song, W. G.; Wan, L. J. 3D Flowerlike Ceria Micro/Nanocomposite Structure and Its Application for Water Treatment and CO Removal. Chem. Mater. 2007, 19, 1648–1655. (2) Fei, J. B.; Cui, Y.; Yan, X. H.; Yang, Y.; Wang, K. W.; He, Q.; Li, J. B. Controlled Preparation of MnO2 Hierarchical Hollow Nanostructures and Their Application in Water Treatment. Adv. Mater. 2008, 20, 452–456. (3) Cieslak-Golonka, M. Toxic and Mutagenic Effects of Chromium (VI). Polyhedron 1995, 15, 3667–3689. (4) Correia, V. M.; Stephenson, T.; Judd, S. J. Characterization of Textile Wastewaters, a Review. Environ. Technol. 1994, 15, 917–919. (5) Zhu, M. X.; Li, Y. P.; Xie, M.; Xin, H. Z. Sorption of an Anionic Dye by Uncalcined and Calcined Layered Double Hydroxides: a Case Study. J. Hazard. Mater. B 2005, 120, 163–171. (6) Hu, J. S.; Zhong, L. S.; Song, W. G.; Wan, L. J. Synthesis of Hierarchically Structured Metal Oxides and their Application in Heavy Metal Ion Removal. Adv. Mater. 2008, 20, 2977–2982. (7) Liu, H. C.; Min, E. Z. Catalytic Oxidation of Mercaptans by Bifunctional Catalysts Composed of Cobalt Phthalocyanine Supported on MgAl Hydrotalcite-derived Solid Bases: Effects of Basicity. Green Chem. 2006, 8, 657–662. (8) Dadwhal, M.; Ostwal, M. M.; Liu, P. K. T.; Sahimi, M.; Tsotsis, T. T. Adsorption of Arsenic on Conditioned Layered Double Hydroxides: Column Experiments and Modeling. Ind. Eng. Chem. Res. 2009, 48, 2076–2084. (9) Evans, D. G.; Slade, R. C. T. Structural Aspects of Layered Double Hydroxides. Struct. Bonding (Berlin) 2005, 119, 1–87. (10) Yang, L.; Shahrivari, Z.; Liu, P. K. T.; Sahimi, M.; Tsotsis, T. T. Removal of Trace Levels of Arsenic and Selenium from Aqueous
found that the Langmuir model can be used to describe the sorption behavior more satisfactorily than the Freundlich model does, based on the values of R2 listed in Table S1 in the Supporting Information. The sorption capacity (qMAX) of the LDH/Al-foam reaches ∼212.8 mg g1 for RBBR, which is much larger than that of the corresponding LDH powder sample (∼166.7 mg g1). In addition, when the LDH/Al-foam was regenerated and reused (see detailed information in the Supporting Information), it maintained a large and stable sorption capacity for RBBR over 10 consecutive cycles (Figure 10), demonstrating that it can serve as a stable, effective, and recyclable sorbent.
4. CONCLUSIONS We have demonstrated a facile approach for the fabrication of a hierarchical LDH adsorbent on Al foam, which exhibits a high dispersion of LDH nanoplatelets and strong adhesion to the substrate. The LDH/Al-foam shows a superior sorption capability for Cr(VI) as well as RBBR compared with the corresponding LDH powder sample, owing to its hierarchical structure. The sorption kinetics of the hierarchical LDH/Alfoam for Cr(VI) was appropriately described by the pseudosecond-order model. The sorption isotherms of both Cr(VI) and RBBR were also studied, which can be fitted by the Langmuir model more satisfactorily than by the Freundlich one. Furthermore, 290
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