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Enhanced removal of Cd(II) and Pb(II) by composites of mesoporous carbon stabilized alumina
Accepted Manuscript Title: Enhanced removal of Cd(II) and Pb(II) by composites of mesoporous carbon stabilized alumina Author: Weichun Yang Qiongzhi Tang Jingmiao Wei Yajun Ran Liyuan Chai Haiying Wang PII: DOI: Reference:
S0169-4332(16)00188-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.151 APSUSC 32383
To appear in:
APSUSC
Received date: Revised date: Accepted date:
23-9-2015 15-1-2016 18-1-2016
Please cite this article as: W. Yang, Q. Tang, J. Wei, Y. Ran, L. Chai, H. Wang, Enhanced removal of Cd(II) and Pb(II) by composites of mesoporous carbon stabilized alumina, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.151 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Enhanced removal of Cd(Ⅱ) and Pb(Ⅱ) by composites of mesoporous carbon stabilized alumina WeichunYang a,b, Qiongzhi Tang a, Jingmiao Wei a, Yajun Ran a, Liyuan Chai a,b, Haiying Wanga,b* a Department of Environmental Engineering, School of Metallurgy and Environment, Central South University, Lushan South Road 932, Changsha, 410017, P. R. China; b Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Lushan South Road 932,Changsha, 410017, P. R. China;
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To whom correspondence may be addressed: (Phone): 86-731-88830875; (fax):86-731-88710171; (e-mail): [email protected](H.Wang)
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Abstract: A novel adsorbent of mesoporous carbon stabilized alumina (MC/Al2O3) was synthesized through one-pot hard-templating method. The adsorption potential of MC/Al2O3 for Cd(Ⅱ) and Pb(Ⅱ) from aqueous solution was investigated compared with the mesoporous carbon. The results indicated the MC/Al2O3 showed excellent performance for Cd(Ⅱ) and Pb(Ⅱ) removal, the adsorption capacity reached 49.98 mg g-1 for Cd(Ⅱ) with initial concentration of 50 mg L-1 and reached 235.57 mg g-1 for Pb(Ⅱ) with initial concentration of 250 mg L-1, respectively. The kinetics data of Cd(Ⅱ) adsorption demonstrated that the Cd(Ⅱ) adsorption rate was fast, and the removal efficiencies with initial concentration of 10 and 50 mg L-1 can reach up 99% within 5 and 20 min, respectively. The pseudo-second-order kinetic model could describe the kinetics of Cd(Ⅱ) adsorption well, indicating the chemical reaction was the rate-controlling step. The mechanism for Cd(Ⅱ) and Pb(Ⅱ) adsorption by MC/Al2O3 was investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR), and the results indicated that the excellent performance for Cd(Ⅱ) and Pb(Ⅱ) adsorption of MC/Al2O3 was mainly attributed to its high surface area and the special functional groups of hydroxy-aluminum, hydroxyl, carboxylic through the formation of strong surface complexation or ion-exchange. It was concluded that MC/Al2O3 can be recognized as an effective adsorbent for removal of Cd(Ⅱ) and Pb(Ⅱ) in aqueous solution. Pb(Ⅱ);
Mesoporous carbon stabilized
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Keywords: Enhanced removal; Cd(Ⅱ); alumina; Hydroxy-aluminum (Al-OH)
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1. Introduction
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Contamination by heavy metals in the water is still a worldwide threat to public health due to their high toxicity and non-biodegradability[1-2]. Among the heavy metals, Cd(Ⅱ) and Pb(Ⅱ) are considered as most toxic metals, and could be released by various industries such as metal plating, metal finishing, battery manufacturing, and electronic industry[3]. Exposure to elevated level of Cd(Ⅱ) and Pb(Ⅱ) could cause severe damages to human health. The maximum allowable limits of cadmium and lead in drinking water set by the World Health Organization (WHO) are 3μg L-1 and 10 μg L-1, respectively [4]. Hence, the effective removal of these heavy metals is of great importance. Adsorption is one of the promising techniques for heavy metal ions removal due to its high efficiency, low-cost and simple operation[5-10]. Many adsorbents such as iron oxides[11-12], activated carbons[13-14], kaolinnite[15-16] and zeolite[17] have been reported for heavy metals removal. However, there were some problems, such as low adsorption capacity, aggregation and poor mechanical strength, limiting the application of these materials. Recently, mesoporous carbon is considered quite attractive in the field of adsorption because of its fascinating properties such as ordered pore structures, large pore size, high surface areas, abundant framework compositions and thermal and mechanical stability[18-22]. However, the hydrophobic surface of pristine mesoporous carbon was unsuitable for absorbing heavy metals from the aqueous solution. Thus, this material should be chemically modified or functionalized to reduce the hydrophobic nature for its application of heavy metals adsorption [23-26]. An effective method to enhance the adsorption ability is the formation of composites designed based on cooperative effects of the carbon mesostructures and the dispersed active component for adsorption [27-29]. For example, Wu et al.,[ 30] developed a novel composite using magnetic iron oxide deposited into the mesoporous carbon with very high arsenic adsorption capacities (up to 29.4 mg g−1) and fast adsorption rate. Liu et al. [31] have synthesized mesoporous carbon stabilized MgO nanoparticles exhibiting excellent performance in the CO2 capture process with the maximum capacity of 5.45 mol kg−1, much higher than many other MgO based CO2 trappers. However, far few relevant studies have been performed to simultaneously exploring and utilizing the high mesoporosities of carbon matrices and the dispersed active component for removal of heavy metals such as Cd(Ⅱ) and Pb(Ⅱ). Active alumina is one of the widely used adsorbents for metal removal because of its strong selectivity to metal ions[32-35] . Thus, it seems reasonable to hypothesize that ordered mesoporous carbon stabilized alumina is an effective adsorbent for Cd(Ⅱ) and Pb(Ⅱ) removal. In the present work, we have synthesized a novel adsorbent of mesoporous carbon stabilized alumina composites (MC/Al2O3) with high surface area by one-pot hard-templating method. The adsorption potential of the MC/Al2O3 for the removal of Cd( Ⅱ ) and Pb( Ⅱ ) from aqueous solution was investigated compared with
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mesoporous carbon (MC). The effects of initial concentration, pH and contact time on the adsorption of Cd(Ⅱ) and Pb(Ⅱ) were examined, and the mechanism of Cd(Ⅱ) and Pb(Ⅱ) adsorption on MC/Al2O3 was also discussed.
2. Material and methods
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2.1. Materials LUDOX® AS-40 colloidal silica (40 wt%, ~20nm) was purchased from Sigma-Aldrich Co., Ltd. All other chemicals were of analytical grade and were used as received without further purification. The Cd(Ⅱ) and Pb(Ⅱ) stock solution (1000 mg L-1) were prepared with deionized water using cadmium sulfate and lead nitrate, respectively. Experimental solutions for adsorption and analyses were prepared by diluting Cd(Ⅱ) and Pb(Ⅱ) stock solution with distilled water.
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2.2. Synthesis of adsorbents The mesoporous carbon stabilized active alumina composite (MC/Al2O3) was synthesized by one-pot hard-templating method according to a modified recipe of Li et al.[ 36]. Briefly, 15 g of soluble starch and 2.5g of aluminum sulfate were dissolved in 50 mL deionized water and mixed in a beaker and heated to form a homogeneous solution. Then, 20 g of colloidal silica was added dropwise to the solution with a rate of ~1 drop per second under vigorous stirring until the starch completely dissolved. After 20 min stirred, spread out the solution in a culture dish and naturally cooled down to form monolithic jelly-like gel. In the subsequent processes, the gel was dried in a forced air drying oven at 60℃ and carbonized in an nitrogen flow at 500℃ for 3 h. To obtain the final product, NaOH solution (30 wt%) was used to remove SiO2 in pyrolyticmaterial under 70℃water bath with strong stirring for 24 h. Afterwards, the resulting material was washed with ethanol and deionized water for several times, and dried at 60 ℃ for 24 h to obtain the MC/Al2O3. Moreover, the mesoporous carbon (MC) was synthesized by the method described above but without the addition of aluminum sulfate. 2.3. Characterization of adsorbents N2 adsorption–desorption measurements were employed to investigate the textural properties of the as-prepared adsorbents, which were performed on a Micromeritics ASAP 2050 instrument (Micromeritics, Norcross, GA, USA). The surface morphology was determined by scanning electron microscope (SEM, JSM-6360LV), and the microstructure was observed by transmission electron microscope (TEM, JEM-2100F, JEOL, Japan). The surface of adsorbent before and after Cd(Ⅱ) and Pb(Ⅱ) adsorption was analyzed using X-ray photoelectron spectroscopy (XPS, K-Alpha 1063 Ultra spectrometer, Thermo Fisher Scientific, UK) with amonochromatic Al KαX-ray source (1486.71 eV). The energy scale of the XPS spectra was calibrated with the binding energy of the C 1s peak due to the surface contamination. Fourier transformed infrared spectroscopy (FT-IR) spectra of the as-prepared adsorbents was obtained using a Nicolet iS10 spectrometer (Thermo
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Fisher Scientific Instruments, PA, USA) at 4 cm-1 resolution.
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2.4. Adsorption experiments All the adsorption experiments were performed in 100 mL polyethylene vials filled with 50 mL aqueous solution and 0.05 g adsorbent, and the vials were placed in thermostaticwater bath and shaken for 24 h at 25℃. Afterwards, the samples were filtered using a 0.45-μm membrane filter (Millipore, Billerica, MA, USA), the cadmium and lead concentration in solutions were determined using an atomic adsorption spectrophotometer (WFX-200, Beifen-Ruili Analytical Instrument Co., Ltd. Beijing, China). A quality control sample was analyzed every ten samples to insure that the calibration was valid for the analysis. The amount of heavy metals adsorbed was calculated from the measured aqueous concentration based on mass balance. Meanwhile blanks were analyzed to determine losses to reactor material and showed minimal effect. The effects of initial concentration on adsorption of Cd(Ⅱ) and Pb(Ⅱ) were investigated by varying initial concentration from 2 to 50 mg L-1 for Cd(Ⅱ) and from 10 to 250 mg L-1 for Pb(Ⅱ) at pH 5.0±0.1. The effects of pH on adsorption of Cd(Ⅱ) and Pb(Ⅱ) were investigated by adjusting initial solution pH from 2 to 6 for Cd(Ⅱ) and from1.8 to 5.5 for Pb(Ⅱ) using 0.1 M HCl and 0.1M NaOH with initial concentration of 20 mg L-1. The solution pH and concentrations of Cd(Ⅱ) and Pb(Ⅱ) were determined after adsorption. Kinetic experiment of Cd(Ⅱ) was conducted by batch experiment at pH 5.0 with several initial concentrations (10, 50 and 100 mg L-1). Each kinetics experiment involves a series of vials, and one of the vials was sacrificed to measure the residual aqueous Cd( Ⅱ ) concentration at selected time. Each adsorption experiment was carried out in duplicate to obtain reproducible results with an error of less than 5%.
3. Results and discussion
3.1. Characterization of adsorbents The morphology and structure of the MC and MC/Al2O3 were examined by SEM and TEM. Fig.1a and b display the SEM and TEM images of MC, from which numerous uniform mesoporous embedded in the carbon matrix, and the pores ranging from 10 to 20 nm in diameter with very thin carbon wall, which can provide sufficient space to stabilize alumna. As demonstrated in Fig.1c and d, MC/Al2O3 materials possessed a fluffy porous texture, and alumina was distributed on the carbon matrix. The information on surface elemental composition, surface area, and pore volume parameters of MC and MC/Al2O3 is summarized in Table 1. The results of surface elemental composition determined by XPS show that with the stabilization of alumina the surface carbon content decreased from 88.59% to 62.95% , while the surface oxygen content increased from 10.31% to 24.79%, and also aluminum atom was present at the surface, confirming that the coexistence of carbon and alumina for the MC/Al2O3. Based on pore volume information (Table 1), it is found that both MC and MC/Al2O3 were predominated by mesoporous, however with the stabilization of
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alumina, the pore volume of mesoporous for MC/Al2O3 decreased obviously. The BET surface area and total pore volume were 604.98 m2 g-1 and 1.80 cm3 g-1 for MC, and 415.24 m2 g-1 and 0.53 cm3 g-1 for MC/Al2O3, respectively. The distinct decrease in BET surface area and total pore volume for MC/Al2O3 might be due to the stabilization of alumina, partially occupying or even blocking mesoporous. The N2 adsorption–desorption isotherm plot for MC and MC/Al2O3 is shown in Fig. 2a. According to the IUPAC classification, the MC exhibited typical type IV isotherm plot with H1-type hysteresis loop, indicating MC was a typical mesoporous carbon material, while MC/Al2O3 still exhibited type IV isotherm plot, interestingly, a change in the type of hysteresis loop from H1 to H2 was perceived for MC/Al2O3, it seems to be related to the presence of alumina both inside and outside the pores [37]. According to Fig. 2b, the centered pore sizes decreased for MC/Al2O3, also confirming that alumina was successfully stabilized inside the pores of MC.
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The FT-IR spectra of MC and MC/Al2O3 are shown in Fig.3a and b. For MC, the band at 1600 cm-1 corresponded to -C=O of carboxylic groups. The characteristic peak at 3420 cm-1 was attributed to the water stuck in the samples, indicating that some hydroxyl groups were formed on the surface. The peak at 1383 cm-1 could be assigned to the stretching vibration of C-H. With the stabilization of alumina onto the surface of MC, the band at 3420 cm-1 was shifted to 3442 cm-1 and the peak broadened, indicating the interaction of metal ion with O–H groups of MC thereby increasing the proximity of Al–OH groups[38] . The sharp bands obtained at 996 and 472 cm−1 were stretching and bending modes of Al–O. From the above discussions, it was evident that MC/Al2O3 consisted both alumina and mesoporous carbon. 3.2. Comparison of adsorption performance between MC/Al2O3 and MC Fig.4 shows the adsorption performance of MC and MC/Al2O3 with initial concentration from 2 to 50 mg L-1 for Cd(Ⅱ) and from 10 to 250 mg L-1 for Pb(Ⅱ) at pH 5.0. As depicted in Fig.4a, the removal efficiency of Cd( Ⅱ ) decreased significantly with the increasing initial concentration for MC as the adsorbent. When the initial concentration was 36.6 mg L-1, the removal efficiency decreased to 32.4%. Interestingly, no obvious changes could be found in Cd(Ⅱ) adsorption efficiency for MC/Al2O3 with the increasing initial concentration, and the Cd(Ⅱ) adsorption efficiency reached up to 99% throughout the initial concentrations investigated in the present study, and the adsorption capacity reached 49.98 mg g-1 for Cd(Ⅱ) with initial concentration of 50 mg L-1. As shown in Fig.4b, both MC and MC/Al2O3 exhibited high removal efficiency towards Pb(Ⅱ) at the lower initial concentrations. However, with the initial concentration up to 50 mg L-1, these two materials differed greatly in the removal efficiency, and the removal efficiency for MC/Al2O3 was much higher than that for MC. Moreover it is shown only a very slight decrease in the Pb(Ⅱ) removal efficiency for MC/Al2O3 from 99.99% to 94.23% with the increasing initial concentration from 10 to 250 mg L-1, and adsorption capacity reached 235.57 mg g-1 for Pb(Ⅱ) with initial concentration of 250 mg L-1. Obviously, the adsorption performance of the MC/Al2O3 was significant prior to that of MC, indicating that the
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MC/Al2O3 materials with stabilization of alumna are beneficial to improve the adsorption performance. In comparison with Cd(Ⅱ) and Pb(Ⅱ) adsorption capacities of various adsorbents reported in the literature (Table 2), the MC/Al2O3 developed in the present study is highly competitive for removal of Cd(Ⅱ) and Pb(Ⅱ).
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3.3. pH effect As an important water chemistry parameter, pH can significantly affect the adsorption of Cd(Ⅱ) and Pb(Ⅱ). Fig. 5 shows the Cd(Ⅱ) and Pb(Ⅱ) removal efficiency on the MC/Al2O3 at different pH with initial concentration of 20 mg L-1. The pH was set from 2 to 6 for Cd(Ⅱ) and from1.8 to 5.5 for Pb(Ⅱ) to prevent precipitation of the metals in the form of hydroxides under alkaline pH. As shown in Fig.5a, the Cd(Ⅱ) removal efficiency increased greatly from pH 2 to 3, and considerably high removal efficiency up to 97% achieved over the pH range of 3-6. The similar trend of removal efficiency is also presented for Pb(Ⅱ) (Fig.5b) that the uptake of Pb(Ⅱ) increased sharply with increasing pH from 2.0 to 3.0 and reached up to 97% throughout the pH range of 3-5.5. At lower pH, the solution contains high concentration of H+ ions, which would strongly compete with Cd(Ⅱ) (or Pb(Ⅱ) ) for the active sites on the adsorbent, resulting in the weaker adsorption of Cd(Ⅱ) (or Pb(Ⅱ) ). As the pH of solutions increased, the competition of H+ ions for active sites decreased, meanwhile the degree of protonation of the surface function reduced, and hence adsorption capacity was high. The results indicate that the MC/Al2O3 can be applied within a wide pH range for Cd(Ⅱ) and Pb(Ⅱ) removal.
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adsorption by MC/Al2O3, X-ray photoelectron spectroscopy (XPS) was applied to study the surface chemical compositions of MC, MC /Al2O3 before and after adsorption, and the results are presented in Table 4 and Fig.7. Composition content relative to the total content of corresponding element was calculated from the peak area. The XPS spectrum of the C1s (Fig.7a) comprised four peaks with differentiated binding energy values, which can be assigned to the C-C(284.60 eV), C-OH(285.6 eV), C=O (287.5 eV) and O=C-O (288.3 eV), respectively. With the stabilization of alumina, the molar ratio of C-OH increased from 17.7% to 35.3%, while the molar ration of C-C decreased from 76.9% to 59.9%, implying the interaction between the mesoporous carbon and alumina. After adsorption, the molar ration of C-OH decreased from 35.3% to 20.6% for Pb(Ⅱ) adsorption, and decreased from 35.3% to 15.9% for Cd(Ⅱ) adsorption, implying that the C-OH function group is the active site for Cd(Ⅱ) and Pb(Ⅱ) adsorption. In Fig.7b, the O 1s spectrum can be well-resolved into three components, including C=O (531.1 eV), O-C-O or Al-OH (532.2 eV), O=C-O (533.5 eV). The content of O-C-O or Al-OH increased from 22.8% to 56.8%, suggesting that the Al-OH might be anchored to mesoporous carbon. In addition, after Cd(Ⅱ) and Pb(Ⅱ) adsorption the molar ration of O-C-O or Al-OH (532.2 eV) decreased from 56.8% to 31.5%, and also the binding energy value of Al 2p was shifted from 75.2 to 74.5 eV after Cd(Ⅱ) adsorption and from 75.2 to 75.0 eV after Pb(Ⅱ) adsorption(Fig.7c), confirming that the Al-OH groups might be involved in the adsorption process. This mechanism can also explain why the mesoporous carbon stabilized alumina shows higher adsorption capacity than MC. In addition, the binding energy of Cd 3d5/2 in the MC/ Al2O3 was 405.79eV(Fig.7d ), indicating the Cd(Ⅱ) adsorbed t could be assigned to –O-Cd. The peak of Pb 4f7/2 in the MC/Al2O3 was found in binding energy of 138.99eV (Fig.7e), which could be assigned to -O-Pb. To further confirm the adsorption mechanism of Cd(Ⅱ) and Pb(Ⅱ) onto MC/Al2O3, the changes of characteristic adsorption peaks before and after adsorption were investigated by FT-IR (Fig.3b, c and d). The bands at 3442(-OH), 1601(C=O), 996 and 472cm-1(Al–OH) were bathochromically shifted or disappear after the adsorption of Cd(Ⅱ) and Pb(Ⅱ), proving that hydroxyl, carboxylic and hydroxy-aluminum function groups were the sites for ion-exchange or complexation for the adsorption of Cd(Ⅱ) and Pb(Ⅱ) on MC/Al2O3[25, 47]. The presence of a narrow peak at 882cm-1 after Cd(Ⅱ) adsorption can be ascribed to the Cd-O bond vibration, meanwhile the bands between 685 to 472 cm-1 after Pb(Ⅱ) adsorption can be ascribed to the Pb-O bond vibration. It is suggested that the adsorbed Cd(Ⅱ) and Pb(Ⅱ) was bonded with the oxygen-containing functionalities of the MC/Al2O3 surface. Based on above discussion, three possible mechanism of Cd( Ⅱ ) and Pb( Ⅱ ) adsorption onto the MC/Al2O3 are proposed are proposed in Fig.8. And described as follow: (1) Cd(Ⅱ) and Pb(Ⅱ) ions form surface complexes with ≡AlOH by ; (2) the H+ from –COOH and C-OH on the MC/Al2O3 surface exchanges with Cd(Ⅱ) and Pb(Ⅱ) ions; (3) the high surface area and fluffy structure aid in accelerating the diffusion of the Cd(Ⅱ) and Pb(Ⅱ) ions into the MC/Al2O3.
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4. Conclusions
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We have developed a novel adsorbent of mesoporous carbon stabilized alumina (MC/Al2O3) by one-pot hard-templating method. The as-prepared MC/Al2O3 showed excellent performance for Cd(Ⅱ) and Pb(Ⅱ) adsorption, the adsorption efficiency reached up to 99% for Cd(Ⅱ) with initial concentration from 2 to 50 mg L-1 and 94.23% for Pb(Ⅱ) with initial concentration from 10 to 250 mg L-1, respectively. Kinetic study showed that the Cd(Ⅱ) adsorption rate is fast, and the removal efficiencies with initial concentration of 10 and 50 mg L-1 can reach up 99% within 5 and 20 min, respectively. The excellent performance for Cd(Ⅱ) and Pb(Ⅱ) adsorption of MC/Al2O3 were mainly attributed to its high surface area and the special functional groups of hydroxy-aluminum, hydroxyl, carboxylic through the formation of strong surface complexation or ion-exchange. Therefore, MC/Al2O3 can be recognized to be good adsorbent candidates when removing Cd(Ⅱ) and Pb(Ⅱ) in aqueous solution.
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This work was financial support by the National Natural Science Foundation of China (Grant No. 51304252) and Key Projects of Science and Technology of Hunan province (Grant No.2012FJ1010).
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[24] R. Moreno-Tovar, E. Terrés, J. Rene Rangel-Mendez, Oxidation and EDX elemental mapping characterization of an ordered mesoporous carbon: Pb(II) and Cd(II) removal, Appl. Surf. Sci. 303(2014) 373–380. [25] H. Yang, S. Li, H. Lin, J. Chen, Fabrication and characterization of mesoporous activated carbonfrom Lemna minor using one-step H3PO4 activation for Pb(II) removal, Appl. Surf. Sci. 317(2014) 422–431. [26] Y.Q. Wang, Z.B. Zhang, Y.H. Liu, X.H. Cao, Y.T. Liu, Q. Li, Adsorption of U(VI) from aqueous solution by the carboxyl-mesoporous carbon, Chem. Eng. J. 198–199(2012) 246–253. [27] D. Saha, S. Deng, Hydrogen adsorption on ordered mesoporous carbons doped with Pd, Pt, Ni, and Ru, Langmuir 25(2009) 12550–12560. [28] X. Ling, J. Li, W. Zhu, Y. Zhu, X. Sun, J. Shen, W. Han, L. Wang, Synthesis of nanoscale zero-valent iron/ordered mesoporous carbon for adsorption and synergistic reduction of nitrobenzene, Chemosphere 87(2012) 655–660.
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[29] L. Tang, G. Yang, G. Zeng, Y. Cai, S. Li, Y. Zhou, Y. Pang, Y. Liu, Y. Zhang, B. Luna, Synergistic effect of iron doped ordered mesoporous carbon on adsorption-coupled reduction of hexavalent chromium and the relative mechanism study, Chem. Eng. J. 239(2014) 114-122.
ip t
[30] Z. Wu, W. Li, P.A. Webley, D. Zhao, General and Controllable Synthesis of Novel Mesoporous Magnetic Iron Oxide@Carbon Encapsulates for Efficient Arsenic Removal, Adv. Mater. 24(2012) 485–491.
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[31] W. Liu, H. Jiang, K. Tian, Y. Ding, H. Yu, Mesoporous carbon stabilized MgO nanoparticles synthesized by pyrolysis of MgCl2 preloaded waste biomass for highly efficient CO2 capture, Environ. Sci. Technol. 47(2013): 9397−9403.
an
us
[32] A. Shokati, A. Nilchi, A.H. Hassani, M. Shariat, J. Nouri, A novel method for synthesis of nano-γ-Al2O3: study of adsorption behavior of chromium, nickel, cadmium and lead ions, Int. J. Environ. Sci. Technol. 12 (2015) 2003-2014.
M
[33] N. Sankararamakrishnan, M. Jaiswa , N. Verma, Composite nanofloral clusters of carbon nanotubes and activated alumina: An efficient sorbent for heavy metal removal, Chem. Eng. J. 235(2014) 1–9.
d
[34] M.Deravanesiyan, M. Beheshti, A. Malekpour, Alumina nanoparticles immobilization onto the NaX zeolite and the removal of Cr (III) and Co (II) ions from aqueous solutions, J. Ind. Eng. Chem. 21 (2015) 580–586.
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[35] T. S. Singh, K. K. Pant. Equilibrium, kinetics and thermodynamic studies for adsorption of As(III) on activated alumina, Sep. Sci. Technol. 36 (2004) 139-147. [36] J. Li, F. Qin, L. Zhang, K. Zhang, Q. Li, Y. Lai, Z. Zhang, J. Fang, Mesoporous carbon from biomass: one-pot synthesis and application for Li–S batteries, J. Phys. Chem. A 2(2014) 13916–13922. [37] A.García-Trenco, A. Martínez, A simple and efficient approach to confine Cu/ZnO methanol synthesis catalysts in the ordered mesoporous SBA-15 silica, Catal. Today 215 (2013) 152-161. [38] M. Barathi, A. Santhana Krishna Kumar, C.U. Kumar, N. Rajesh, Graphene oxide–aluminium oxyhydroxide interaction and its application for the effective adsorption of fluoride, RSC Adv. 4 (2014) 53711-53721. [39] Y. Li, J. De Wang, X.J. Wang, J.F. Wang, Adsorption-desorption of Cd(II) and Pb(II) on Ca-montmorillonite, Ind. Eng. Chem. Res. 51 (2012) 6520–6528. [40] C. Wang, J. Liu, Z. Zhang, B. Wang, H. Sun, Adsorption of Cd(II), Ni(II), and Zn(II) by Tourmaline at Acidic Conditions: Kinetics, Thermodynamics, and Mechanisms, Ind. Eng. Chem. Res. 51 (2012) 4397–4406.
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[41] B.C. Júnior, J.H.O. Nascimento, M.J. Quina, L.G. Ferreira, Determination of the Biosorption of Cd( ) by Coconut Fiber, J. Agric. Sci. Technol. 4 (2014) 291–298. [42] S.Z. Mohammadi, M.A. Karimi, D. Afzali, F. Mansouri, Removal of Pb(II) from aqueous solutions using activated carbon from Sea-buckthorn stones by chemical activation, Desalination. 262 (2010) 86–93. [43] H.T. Fan, J.B. Wu, X.L. Fan, D.S. Zhang, Z.J. Su, F. Yan, et al., Removal of cadmium(II) and lead(II) from aqueous solution using sulfur-functionalized silica prepared by hydrothermal-assisted grafting method, Chem. Eng. J. 198-199 (2012) 355–363.. [44]L. Chu, C. Liu, G. Zhou, R. Xu, Y. Tang, Z. Zeng, et al., A double network gel as low cost and easy recycle adsorbent: Highly efficient removal of Cd(II) and Pb(II) pollutants from wastewater, J. Hazard. Mater. 300 (2015) 153–160. [45]T.K. Naiya, A.K. Bhattacharya, S.K. Das, Adsorption of Cd(II) and Pb(II) from aqueous solutions on activated alumina., J. Colloid Interface Sci. 333 (2009) 14–26. [46] X. Deng, L. Lü, H. Li, F. Luo, The adsorption properties of Pb(II) and Cd(II) on functionalized graphene prepared by electrolysis method, J. Hazard. Mater. 1 (2010) 923–930.
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(a)
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[47] F. Zhao, E. Repo, D. Yin, M.E. Sillanpaa, Adsorption of Cd(II) and Pb(II) by a novel EGTA-modified chitosan material: kinetics and isotherms, J. Colloid Interface Sci. 409(2013)174–182.
(c)
(b)
(d)
Fig.1 SEM and TEM images of (a–b) MC, (c–d) MC/Al2O3
Page 13 of 25
Page 14 of 25
d
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an
M
cr
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1400
1000 MC MC/Al2O3
3
-1
Quanity Adsorbed (cm g STP)
(a) 1200
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800 600
cr
400
0.2
0.4
0 .8
0.6
Relative Pressure(P/P0)
MC MC/Al2O3
1.0
an
2.5
us
3.5 0 (b) 0.0 3.0
3
-1
dV/dlog(D) Pore Volume (cm g )
200
2.0
M
1.5 1.0
0.0 20
40
60
80
100
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0
d
Fig.2 (a) N2 adsorption–desorption isotherm plot and (b) BJH pore size distribution of the MC.5and MC/Al2O3
Pore diameter (nm)
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Transmittance (a.u.)
(a)
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(b)
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(c)
4000
3000
2000
1000
M
Wavenumbers(cm-1)
an
(d)
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Fig.3 FT-IR spectra of (a)MC, (b)MC/Al2O3,(c) MC/Al2O3 after Cd(Ⅱ) adsorption and (d) MC/Al2O3 after Pb(Ⅱ) adsorption
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Removal efficiency (%)
60
80
60
40
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Fig.4 Effect of initial concentration on (a) Cd(Ⅱ) and (b) Pb(Ⅱ) adsorption by the MC MC MC and MC/Al MC/Al2O3 MC/Al 2O3 2O3 0
0 10
20
30
40
50
0
-1
100
150
200
250
-1
Initial concentration (mg L )
M
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Initial concentration (mg L )
50
d
0
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20
20
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Removal efficiency (%)
80
40
(b)
100
(a)
100
Page 17 of 25
100
100
(a)
80
60
40
60
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Removal efficiency (%)
80
40
0
0 2
3
4
5
6
7
1
3
4
5
6
pH
M
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pH
2
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1
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20
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Fig.5 Effect of pH on the adsorption of (a) Cd(Ⅱ) and (b) Pb(Ⅱ) by MC/Al2O3 20
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Removal efficiency (%)
(b)
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100
2
(b)
(a) 1
10 mg/L 50 mg/L 100 mg/L
20 160
-1
-2 100
(c) 0
(d)
200
400
600
800
1000
1200
1400
-3 80 0
1600
qt (mg g-1)
10 mg L-1 50 mg L-1
80 60
0
800
1000
1200
1400
1600
t (min)
Fig.6 (a) Effect of contact time on Cd(Ⅱ) adsorption by MC/Al2O3; kinetics modeling of Cd( Ⅱ ) adsorption (b) pseudo-first-order kinetic plots; (c) 0 200pseudo-second-order 400 600 800 1000 1200plots; 1400 (d) 1600Elovich model plots. t is the reaction time kinetic 2 4 6 t (min) (min), qe (mg/g) and qt (mg/g) are the amount of adsorbed Cd at equilibrium and at In t any reaction time t.
d
0
600
40
20
20
400
60
M
100 mg L-1
40
200
an
100
100 mg L-1
10 mg L-1 50 mg L-1 -1 100 mg L
8
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t/qt (min g mg-1)
t (min)
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140 0 120
0
ip t
40
10 mg L-1 50 mg L-1
cr
60
log(qe-qt)
qt (mg/g)
80
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d
Fig.7 (a) XPS C 1s spectra of the MC, MC/Al2O3 before and after Cd(Ⅱ) or Pb(Ⅱ) adsorption; (b) XPS O 1s spectra of the MC, MC/Al2O3 before and after Cd(Ⅱ) or Pb(Ⅱ) adsorption; (c) XPS Al 2p spectra of the MC/Al2O3 after Cd(Ⅱ) or Pb(Ⅱ) adsorption; (d)XPS Cd 3d spectra of the MC/Al2O3 after Cd(Ⅱ) adsorption; (e) XPS Pb 4f spectra of the MC/Al2O3 after Pb(Ⅱ) adsorption
OH
OH COOH
OH
O
COO
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Cd(Ⅱ) (or Pb(Ⅱ))
Al O
surface
ion ex
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Fig.8 Mechanism illustration of Cd(Ⅱ) (or Pb(Ⅱ)) adsorption by the MC/Al2O3
11.8 5.06
Vtot
Vmic
Vmes
1.796 0.525
0.136 0.106
1.660 0.419
Ac ce pt e
MC/Al2O3
604.98 415.24
d
MC
C% O% Al% 88.59 10.31 62.95 24.79 3.28
Diameter (nm)
M
g-1)
an
Table 1 Surface Elemental Composition, Surface Area, and Pore Volume Parameters for MC and MC/Al2O3 Adsorbents surface elemental SBETb Average Pore volume (cm3g-1) (m2 pore compositiona
a Determined by X-ray photoelectron spectroscopy (XPS). b Determined by N2 adsorption using the Brunauer-Emmett-Teller (BET) method. c Total pore volume, determined at P/P0= 0.985. d Micropore volume, calculated using the t-plot method. e Mesoporous volume, calculated by Vt - Vmic.
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Table 2 Comparison of Cd(Ⅱ) and Pb(Ⅱ) adsorption capacity of various adsorbents
Cd(II)
Pb(II)
Ca-Montmorillonite
5.0
13.0
tourmaline coconut Fiber sea-buckstone activated carbon sulfur-functionalized silica
9.56 31.12 -
-
30.7
62.2
polyving alcohol/polyacrylic acid activated alumina
115.88
194.99
6.164
9.146
mesoporous carbon stabilized alumina (MC/Al2O3)
49.98
235.57
Initial concentration
ref.
Cd(II): 50 mg L−1 Pb(II): 808.6 mg L−1 Cd(II): 50 mg L−1 Cd(II):10-140 mg L-1 Pb(II):50-300 mg L-1
[39] [40] [41] [42]
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adsorption capacity (mg g-1)
cr
51.81
us
Cd(II): 50–1000 mg L-1 Pb(II):50–1000 mg L-1 Cd(II): 30–160 mg L-1 Pb(II):100–350 mg L-1 Cd(II): 50 mg L−1 Pb(II): 50 mg L−1 Cd(II): 50 mg L−1 Pb(II):250 mg L−1
[43] [44] [45] This study
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Table 3 Kinetic and statistical parameters of the three kinetic models Data set Pseudo-first-order model Pseudo-second-order model
Elovich model +
R2
R2
qe
k2
-1
0.0457
0.00069
0.8122
9.980
0.6877
50 mg L-1
1.0035
0.0035
0.4544
50.000
0.0171
100mg L-1
17.006
0.0037
0.8574
90.909
0.0014
10 mg L
α
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k1
β
1.0000
9.935
0.0061
1.0000
36.898
2.2283
0.9999
51.263
6.0088
cr
qe,
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d
M
an
us
Note: t is the reaction time (min), qe (mg/g) and qt (mg/g) are the amount of adsorbed fluoride at equilibrium and at any reaction time t, k1(/min) and k2 (g·/mg/min) are the equilibrium rate constants for pseudo-first-order and pseudo-second-order models respectively, and the Elovich constant α is related to the sorption rate while β is related to the surface coverage, R2 is correlation coefficient.
Table 4 Elemental composition of oxygen and carbon atoms present on the materials surface C 1s O 1s
Materials
C-C (284.6 eV)
C-OH (285.6 eV)
C=O (287.5 eV)
O=C-O (288.3 eV)
C=O (531.1 eV)
O=C-O (533.5 eV)
28.6%
O-C-O or Al-O(H) ( 532.2 eV) 22.8%
MC
76.9%
17.7%
1.5%
3.8%
MC /Al2O3
59.9%
35.3%
-
4.8%
15.9%
56.8%
27.3%
MC/Al2O3 79.4% after Cd(Ⅱ) adsorption
15.9%
2.4%
2.4%
21.6%
31.5%
46.9%
48.5%
24
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20.6%
4.4%
1.5%
13.0%
Highlight 0 0
Pb
M
60
40
20
d
20
80
an
Removal efficiency (%)
60
MC MC/Al2O3
0
10 20carbon 30 40 alumina 50 was prepared 0 50 one-pot 100 hard-templating 150 200 Mesoporous stabilized by -1 -1 Initial concentration (mg L ) method. Initial concentration (mg L ) MC/Al2O3 showed excellent performance for Cd(Ⅱ) and Pb(Ⅱ) adsorption. Enhanced adsorption was due to the high surface area and special functional groups.
Ac ce pt e
Removal efficiency (%)
Cd 80
us
100
100
MC MC/Al2O3
50.0%
cr
Graphical abstract
40
37.0%
ip t
MC/Al2O3 73.5% after Pb(Ⅱ) adsorption
250
25
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S0169-4332(16)00188-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.151 APSUSC 32383
To appear in:
APSUSC
Received date: Revised date: Accepted date:
23-9-2015 15-1-2016 18-1-2016
Please cite this article as: W. Yang, Q. Tang, J. Wei, Y. Ran, L. Chai, H. Wang, Enhanced removal of Cd(II) and Pb(II) by composites of mesoporous carbon stabilized alumina, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.151 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Enhanced removal of Cd(Ⅱ) and Pb(Ⅱ) by composites of mesoporous carbon stabilized alumina WeichunYang a,b, Qiongzhi Tang a, Jingmiao Wei a, Yajun Ran a, Liyuan Chai a,b, Haiying Wanga,b* a Department of Environmental Engineering, School of Metallurgy and Environment, Central South University, Lushan South Road 932, Changsha, 410017, P. R. China; b Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Lushan South Road 932,Changsha, 410017, P. R. China;
*
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To whom correspondence may be addressed: (Phone): 86-731-88830875; (fax):86-731-88710171; (e-mail): [email protected](H.Wang)
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Abstract: A novel adsorbent of mesoporous carbon stabilized alumina (MC/Al2O3) was synthesized through one-pot hard-templating method. The adsorption potential of MC/Al2O3 for Cd(Ⅱ) and Pb(Ⅱ) from aqueous solution was investigated compared with the mesoporous carbon. The results indicated the MC/Al2O3 showed excellent performance for Cd(Ⅱ) and Pb(Ⅱ) removal, the adsorption capacity reached 49.98 mg g-1 for Cd(Ⅱ) with initial concentration of 50 mg L-1 and reached 235.57 mg g-1 for Pb(Ⅱ) with initial concentration of 250 mg L-1, respectively. The kinetics data of Cd(Ⅱ) adsorption demonstrated that the Cd(Ⅱ) adsorption rate was fast, and the removal efficiencies with initial concentration of 10 and 50 mg L-1 can reach up 99% within 5 and 20 min, respectively. The pseudo-second-order kinetic model could describe the kinetics of Cd(Ⅱ) adsorption well, indicating the chemical reaction was the rate-controlling step. The mechanism for Cd(Ⅱ) and Pb(Ⅱ) adsorption by MC/Al2O3 was investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR), and the results indicated that the excellent performance for Cd(Ⅱ) and Pb(Ⅱ) adsorption of MC/Al2O3 was mainly attributed to its high surface area and the special functional groups of hydroxy-aluminum, hydroxyl, carboxylic through the formation of strong surface complexation or ion-exchange. It was concluded that MC/Al2O3 can be recognized as an effective adsorbent for removal of Cd(Ⅱ) and Pb(Ⅱ) in aqueous solution. Pb(Ⅱ);
Mesoporous carbon stabilized
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Keywords: Enhanced removal; Cd(Ⅱ); alumina; Hydroxy-aluminum (Al-OH)
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1. Introduction
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Contamination by heavy metals in the water is still a worldwide threat to public health due to their high toxicity and non-biodegradability[1-2]. Among the heavy metals, Cd(Ⅱ) and Pb(Ⅱ) are considered as most toxic metals, and could be released by various industries such as metal plating, metal finishing, battery manufacturing, and electronic industry[3]. Exposure to elevated level of Cd(Ⅱ) and Pb(Ⅱ) could cause severe damages to human health. The maximum allowable limits of cadmium and lead in drinking water set by the World Health Organization (WHO) are 3μg L-1 and 10 μg L-1, respectively [4]. Hence, the effective removal of these heavy metals is of great importance. Adsorption is one of the promising techniques for heavy metal ions removal due to its high efficiency, low-cost and simple operation[5-10]. Many adsorbents such as iron oxides[11-12], activated carbons[13-14], kaolinnite[15-16] and zeolite[17] have been reported for heavy metals removal. However, there were some problems, such as low adsorption capacity, aggregation and poor mechanical strength, limiting the application of these materials. Recently, mesoporous carbon is considered quite attractive in the field of adsorption because of its fascinating properties such as ordered pore structures, large pore size, high surface areas, abundant framework compositions and thermal and mechanical stability[18-22]. However, the hydrophobic surface of pristine mesoporous carbon was unsuitable for absorbing heavy metals from the aqueous solution. Thus, this material should be chemically modified or functionalized to reduce the hydrophobic nature for its application of heavy metals adsorption [23-26]. An effective method to enhance the adsorption ability is the formation of composites designed based on cooperative effects of the carbon mesostructures and the dispersed active component for adsorption [27-29]. For example, Wu et al.,[ 30] developed a novel composite using magnetic iron oxide deposited into the mesoporous carbon with very high arsenic adsorption capacities (up to 29.4 mg g−1) and fast adsorption rate. Liu et al. [31] have synthesized mesoporous carbon stabilized MgO nanoparticles exhibiting excellent performance in the CO2 capture process with the maximum capacity of 5.45 mol kg−1, much higher than many other MgO based CO2 trappers. However, far few relevant studies have been performed to simultaneously exploring and utilizing the high mesoporosities of carbon matrices and the dispersed active component for removal of heavy metals such as Cd(Ⅱ) and Pb(Ⅱ). Active alumina is one of the widely used adsorbents for metal removal because of its strong selectivity to metal ions[32-35] . Thus, it seems reasonable to hypothesize that ordered mesoporous carbon stabilized alumina is an effective adsorbent for Cd(Ⅱ) and Pb(Ⅱ) removal. In the present work, we have synthesized a novel adsorbent of mesoporous carbon stabilized alumina composites (MC/Al2O3) with high surface area by one-pot hard-templating method. The adsorption potential of the MC/Al2O3 for the removal of Cd( Ⅱ ) and Pb( Ⅱ ) from aqueous solution was investigated compared with
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mesoporous carbon (MC). The effects of initial concentration, pH and contact time on the adsorption of Cd(Ⅱ) and Pb(Ⅱ) were examined, and the mechanism of Cd(Ⅱ) and Pb(Ⅱ) adsorption on MC/Al2O3 was also discussed.
2. Material and methods
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2.1. Materials LUDOX® AS-40 colloidal silica (40 wt%, ~20nm) was purchased from Sigma-Aldrich Co., Ltd. All other chemicals were of analytical grade and were used as received without further purification. The Cd(Ⅱ) and Pb(Ⅱ) stock solution (1000 mg L-1) were prepared with deionized water using cadmium sulfate and lead nitrate, respectively. Experimental solutions for adsorption and analyses were prepared by diluting Cd(Ⅱ) and Pb(Ⅱ) stock solution with distilled water.
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2.2. Synthesis of adsorbents The mesoporous carbon stabilized active alumina composite (MC/Al2O3) was synthesized by one-pot hard-templating method according to a modified recipe of Li et al.[ 36]. Briefly, 15 g of soluble starch and 2.5g of aluminum sulfate were dissolved in 50 mL deionized water and mixed in a beaker and heated to form a homogeneous solution. Then, 20 g of colloidal silica was added dropwise to the solution with a rate of ~1 drop per second under vigorous stirring until the starch completely dissolved. After 20 min stirred, spread out the solution in a culture dish and naturally cooled down to form monolithic jelly-like gel. In the subsequent processes, the gel was dried in a forced air drying oven at 60℃ and carbonized in an nitrogen flow at 500℃ for 3 h. To obtain the final product, NaOH solution (30 wt%) was used to remove SiO2 in pyrolyticmaterial under 70℃water bath with strong stirring for 24 h. Afterwards, the resulting material was washed with ethanol and deionized water for several times, and dried at 60 ℃ for 24 h to obtain the MC/Al2O3. Moreover, the mesoporous carbon (MC) was synthesized by the method described above but without the addition of aluminum sulfate. 2.3. Characterization of adsorbents N2 adsorption–desorption measurements were employed to investigate the textural properties of the as-prepared adsorbents, which were performed on a Micromeritics ASAP 2050 instrument (Micromeritics, Norcross, GA, USA). The surface morphology was determined by scanning electron microscope (SEM, JSM-6360LV), and the microstructure was observed by transmission electron microscope (TEM, JEM-2100F, JEOL, Japan). The surface of adsorbent before and after Cd(Ⅱ) and Pb(Ⅱ) adsorption was analyzed using X-ray photoelectron spectroscopy (XPS, K-Alpha 1063 Ultra spectrometer, Thermo Fisher Scientific, UK) with amonochromatic Al KαX-ray source (1486.71 eV). The energy scale of the XPS spectra was calibrated with the binding energy of the C 1s peak due to the surface contamination. Fourier transformed infrared spectroscopy (FT-IR) spectra of the as-prepared adsorbents was obtained using a Nicolet iS10 spectrometer (Thermo
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Fisher Scientific Instruments, PA, USA) at 4 cm-1 resolution.
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2.4. Adsorption experiments All the adsorption experiments were performed in 100 mL polyethylene vials filled with 50 mL aqueous solution and 0.05 g adsorbent, and the vials were placed in thermostaticwater bath and shaken for 24 h at 25℃. Afterwards, the samples were filtered using a 0.45-μm membrane filter (Millipore, Billerica, MA, USA), the cadmium and lead concentration in solutions were determined using an atomic adsorption spectrophotometer (WFX-200, Beifen-Ruili Analytical Instrument Co., Ltd. Beijing, China). A quality control sample was analyzed every ten samples to insure that the calibration was valid for the analysis. The amount of heavy metals adsorbed was calculated from the measured aqueous concentration based on mass balance. Meanwhile blanks were analyzed to determine losses to reactor material and showed minimal effect. The effects of initial concentration on adsorption of Cd(Ⅱ) and Pb(Ⅱ) were investigated by varying initial concentration from 2 to 50 mg L-1 for Cd(Ⅱ) and from 10 to 250 mg L-1 for Pb(Ⅱ) at pH 5.0±0.1. The effects of pH on adsorption of Cd(Ⅱ) and Pb(Ⅱ) were investigated by adjusting initial solution pH from 2 to 6 for Cd(Ⅱ) and from1.8 to 5.5 for Pb(Ⅱ) using 0.1 M HCl and 0.1M NaOH with initial concentration of 20 mg L-1. The solution pH and concentrations of Cd(Ⅱ) and Pb(Ⅱ) were determined after adsorption. Kinetic experiment of Cd(Ⅱ) was conducted by batch experiment at pH 5.0 with several initial concentrations (10, 50 and 100 mg L-1). Each kinetics experiment involves a series of vials, and one of the vials was sacrificed to measure the residual aqueous Cd( Ⅱ ) concentration at selected time. Each adsorption experiment was carried out in duplicate to obtain reproducible results with an error of less than 5%.
3. Results and discussion
3.1. Characterization of adsorbents The morphology and structure of the MC and MC/Al2O3 were examined by SEM and TEM. Fig.1a and b display the SEM and TEM images of MC, from which numerous uniform mesoporous embedded in the carbon matrix, and the pores ranging from 10 to 20 nm in diameter with very thin carbon wall, which can provide sufficient space to stabilize alumna. As demonstrated in Fig.1c and d, MC/Al2O3 materials possessed a fluffy porous texture, and alumina was distributed on the carbon matrix. The information on surface elemental composition, surface area, and pore volume parameters of MC and MC/Al2O3 is summarized in Table 1. The results of surface elemental composition determined by XPS show that with the stabilization of alumina the surface carbon content decreased from 88.59% to 62.95% , while the surface oxygen content increased from 10.31% to 24.79%, and also aluminum atom was present at the surface, confirming that the coexistence of carbon and alumina for the MC/Al2O3. Based on pore volume information (Table 1), it is found that both MC and MC/Al2O3 were predominated by mesoporous, however with the stabilization of
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alumina, the pore volume of mesoporous for MC/Al2O3 decreased obviously. The BET surface area and total pore volume were 604.98 m2 g-1 and 1.80 cm3 g-1 for MC, and 415.24 m2 g-1 and 0.53 cm3 g-1 for MC/Al2O3, respectively. The distinct decrease in BET surface area and total pore volume for MC/Al2O3 might be due to the stabilization of alumina, partially occupying or even blocking mesoporous. The N2 adsorption–desorption isotherm plot for MC and MC/Al2O3 is shown in Fig. 2a. According to the IUPAC classification, the MC exhibited typical type IV isotherm plot with H1-type hysteresis loop, indicating MC was a typical mesoporous carbon material, while MC/Al2O3 still exhibited type IV isotherm plot, interestingly, a change in the type of hysteresis loop from H1 to H2 was perceived for MC/Al2O3, it seems to be related to the presence of alumina both inside and outside the pores [37]. According to Fig. 2b, the centered pore sizes decreased for MC/Al2O3, also confirming that alumina was successfully stabilized inside the pores of MC.
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The FT-IR spectra of MC and MC/Al2O3 are shown in Fig.3a and b. For MC, the band at 1600 cm-1 corresponded to -C=O of carboxylic groups. The characteristic peak at 3420 cm-1 was attributed to the water stuck in the samples, indicating that some hydroxyl groups were formed on the surface. The peak at 1383 cm-1 could be assigned to the stretching vibration of C-H. With the stabilization of alumina onto the surface of MC, the band at 3420 cm-1 was shifted to 3442 cm-1 and the peak broadened, indicating the interaction of metal ion with O–H groups of MC thereby increasing the proximity of Al–OH groups[38] . The sharp bands obtained at 996 and 472 cm−1 were stretching and bending modes of Al–O. From the above discussions, it was evident that MC/Al2O3 consisted both alumina and mesoporous carbon. 3.2. Comparison of adsorption performance between MC/Al2O3 and MC Fig.4 shows the adsorption performance of MC and MC/Al2O3 with initial concentration from 2 to 50 mg L-1 for Cd(Ⅱ) and from 10 to 250 mg L-1 for Pb(Ⅱ) at pH 5.0. As depicted in Fig.4a, the removal efficiency of Cd( Ⅱ ) decreased significantly with the increasing initial concentration for MC as the adsorbent. When the initial concentration was 36.6 mg L-1, the removal efficiency decreased to 32.4%. Interestingly, no obvious changes could be found in Cd(Ⅱ) adsorption efficiency for MC/Al2O3 with the increasing initial concentration, and the Cd(Ⅱ) adsorption efficiency reached up to 99% throughout the initial concentrations investigated in the present study, and the adsorption capacity reached 49.98 mg g-1 for Cd(Ⅱ) with initial concentration of 50 mg L-1. As shown in Fig.4b, both MC and MC/Al2O3 exhibited high removal efficiency towards Pb(Ⅱ) at the lower initial concentrations. However, with the initial concentration up to 50 mg L-1, these two materials differed greatly in the removal efficiency, and the removal efficiency for MC/Al2O3 was much higher than that for MC. Moreover it is shown only a very slight decrease in the Pb(Ⅱ) removal efficiency for MC/Al2O3 from 99.99% to 94.23% with the increasing initial concentration from 10 to 250 mg L-1, and adsorption capacity reached 235.57 mg g-1 for Pb(Ⅱ) with initial concentration of 250 mg L-1. Obviously, the adsorption performance of the MC/Al2O3 was significant prior to that of MC, indicating that the
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MC/Al2O3 materials with stabilization of alumna are beneficial to improve the adsorption performance. In comparison with Cd(Ⅱ) and Pb(Ⅱ) adsorption capacities of various adsorbents reported in the literature (Table 2), the MC/Al2O3 developed in the present study is highly competitive for removal of Cd(Ⅱ) and Pb(Ⅱ).
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3.3. pH effect As an important water chemistry parameter, pH can significantly affect the adsorption of Cd(Ⅱ) and Pb(Ⅱ). Fig. 5 shows the Cd(Ⅱ) and Pb(Ⅱ) removal efficiency on the MC/Al2O3 at different pH with initial concentration of 20 mg L-1. The pH was set from 2 to 6 for Cd(Ⅱ) and from1.8 to 5.5 for Pb(Ⅱ) to prevent precipitation of the metals in the form of hydroxides under alkaline pH. As shown in Fig.5a, the Cd(Ⅱ) removal efficiency increased greatly from pH 2 to 3, and considerably high removal efficiency up to 97% achieved over the pH range of 3-6. The similar trend of removal efficiency is also presented for Pb(Ⅱ) (Fig.5b) that the uptake of Pb(Ⅱ) increased sharply with increasing pH from 2.0 to 3.0 and reached up to 97% throughout the pH range of 3-5.5. At lower pH, the solution contains high concentration of H+ ions, which would strongly compete with Cd(Ⅱ) (or Pb(Ⅱ) ) for the active sites on the adsorbent, resulting in the weaker adsorption of Cd(Ⅱ) (or Pb(Ⅱ) ). As the pH of solutions increased, the competition of H+ ions for active sites decreased, meanwhile the degree of protonation of the surface function reduced, and hence adsorption capacity was high. The results indicate that the MC/Al2O3 can be applied within a wide pH range for Cd(Ⅱ) and Pb(Ⅱ) removal.
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3.4. Adsorption kinetics Fig.6 shows the time dependence of Cd(Ⅱ) adsorption on the MC/Al2O3 at various initial concentrations. It can be seen the Cd(Ⅱ) adsorption on the adsorbent exhibited an initial rapid uptake followed by a slower removal rate that gradually removed completely. The removal efficiencies with initial concentration of 10, 50 and 100 mg L-1can reach 99% within 5, 20 and 300 min, respectively. The fast adsorption rate might be caused by the small diffusion path as MC/Al2O3 might block the pores and the diffusion of metal ions into inner surface and pores would be shortened[46]. Three kinetic models namely pseudo-first-order, pseudo-second-order and Elovich models have been used to describe the kinetics of Cd(Ⅱ) adsorption on the MC/Al2O3. The fit of the three models to the Cd(Ⅱ) adsorption kinetic data are shown in Fig. 6b-d, and the model parameters obtained by curve-fitting kinetic data are listed in Table 3. Pseudo-second-order model was considered more suitable in describing the adsorption kinetics of Cd(Ⅱ) on the adsorbent according to the related coefficient (R2) (R2> 0.9999) (Table 3), indicating the chemical reaction was the rate-controlling step for Cd(Ⅱ) adsorption on MC/Al2O3. 3.5. Mechanism for Cd(Ⅱ) and Pb(Ⅱ) adsorption by MC/Al2O3 We have demonstrated the high Cd(Ⅱ) and Pb(Ⅱ) adsorption performance of the as-prepared MC/Al2O3. To find out the main mechanism of Cd(Ⅱ) and Pb(Ⅱ)
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adsorption by MC/Al2O3, X-ray photoelectron spectroscopy (XPS) was applied to study the surface chemical compositions of MC, MC /Al2O3 before and after adsorption, and the results are presented in Table 4 and Fig.7. Composition content relative to the total content of corresponding element was calculated from the peak area. The XPS spectrum of the C1s (Fig.7a) comprised four peaks with differentiated binding energy values, which can be assigned to the C-C(284.60 eV), C-OH(285.6 eV), C=O (287.5 eV) and O=C-O (288.3 eV), respectively. With the stabilization of alumina, the molar ratio of C-OH increased from 17.7% to 35.3%, while the molar ration of C-C decreased from 76.9% to 59.9%, implying the interaction between the mesoporous carbon and alumina. After adsorption, the molar ration of C-OH decreased from 35.3% to 20.6% for Pb(Ⅱ) adsorption, and decreased from 35.3% to 15.9% for Cd(Ⅱ) adsorption, implying that the C-OH function group is the active site for Cd(Ⅱ) and Pb(Ⅱ) adsorption. In Fig.7b, the O 1s spectrum can be well-resolved into three components, including C=O (531.1 eV), O-C-O or Al-OH (532.2 eV), O=C-O (533.5 eV). The content of O-C-O or Al-OH increased from 22.8% to 56.8%, suggesting that the Al-OH might be anchored to mesoporous carbon. In addition, after Cd(Ⅱ) and Pb(Ⅱ) adsorption the molar ration of O-C-O or Al-OH (532.2 eV) decreased from 56.8% to 31.5%, and also the binding energy value of Al 2p was shifted from 75.2 to 74.5 eV after Cd(Ⅱ) adsorption and from 75.2 to 75.0 eV after Pb(Ⅱ) adsorption(Fig.7c), confirming that the Al-OH groups might be involved in the adsorption process. This mechanism can also explain why the mesoporous carbon stabilized alumina shows higher adsorption capacity than MC. In addition, the binding energy of Cd 3d5/2 in the MC/ Al2O3 was 405.79eV(Fig.7d ), indicating the Cd(Ⅱ) adsorbed t could be assigned to –O-Cd. The peak of Pb 4f7/2 in the MC/Al2O3 was found in binding energy of 138.99eV (Fig.7e), which could be assigned to -O-Pb. To further confirm the adsorption mechanism of Cd(Ⅱ) and Pb(Ⅱ) onto MC/Al2O3, the changes of characteristic adsorption peaks before and after adsorption were investigated by FT-IR (Fig.3b, c and d). The bands at 3442(-OH), 1601(C=O), 996 and 472cm-1(Al–OH) were bathochromically shifted or disappear after the adsorption of Cd(Ⅱ) and Pb(Ⅱ), proving that hydroxyl, carboxylic and hydroxy-aluminum function groups were the sites for ion-exchange or complexation for the adsorption of Cd(Ⅱ) and Pb(Ⅱ) on MC/Al2O3[25, 47]. The presence of a narrow peak at 882cm-1 after Cd(Ⅱ) adsorption can be ascribed to the Cd-O bond vibration, meanwhile the bands between 685 to 472 cm-1 after Pb(Ⅱ) adsorption can be ascribed to the Pb-O bond vibration. It is suggested that the adsorbed Cd(Ⅱ) and Pb(Ⅱ) was bonded with the oxygen-containing functionalities of the MC/Al2O3 surface. Based on above discussion, three possible mechanism of Cd( Ⅱ ) and Pb( Ⅱ ) adsorption onto the MC/Al2O3 are proposed are proposed in Fig.8. And described as follow: (1) Cd(Ⅱ) and Pb(Ⅱ) ions form surface complexes with ≡AlOH by ; (2) the H+ from –COOH and C-OH on the MC/Al2O3 surface exchanges with Cd(Ⅱ) and Pb(Ⅱ) ions; (3) the high surface area and fluffy structure aid in accelerating the diffusion of the Cd(Ⅱ) and Pb(Ⅱ) ions into the MC/Al2O3.
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4. Conclusions
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We have developed a novel adsorbent of mesoporous carbon stabilized alumina (MC/Al2O3) by one-pot hard-templating method. The as-prepared MC/Al2O3 showed excellent performance for Cd(Ⅱ) and Pb(Ⅱ) adsorption, the adsorption efficiency reached up to 99% for Cd(Ⅱ) with initial concentration from 2 to 50 mg L-1 and 94.23% for Pb(Ⅱ) with initial concentration from 10 to 250 mg L-1, respectively. Kinetic study showed that the Cd(Ⅱ) adsorption rate is fast, and the removal efficiencies with initial concentration of 10 and 50 mg L-1 can reach up 99% within 5 and 20 min, respectively. The excellent performance for Cd(Ⅱ) and Pb(Ⅱ) adsorption of MC/Al2O3 were mainly attributed to its high surface area and the special functional groups of hydroxy-aluminum, hydroxyl, carboxylic through the formation of strong surface complexation or ion-exchange. Therefore, MC/Al2O3 can be recognized to be good adsorbent candidates when removing Cd(Ⅱ) and Pb(Ⅱ) in aqueous solution.
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Acknowledgments
References
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This work was financial support by the National Natural Science Foundation of China (Grant No. 51304252) and Key Projects of Science and Technology of Hunan province (Grant No.2012FJ1010).
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[47] F. Zhao, E. Repo, D. Yin, M.E. Sillanpaa, Adsorption of Cd(II) and Pb(II) by a novel EGTA-modified chitosan material: kinetics and isotherms, J. Colloid Interface Sci. 409(2013)174–182.
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Fig.1 SEM and TEM images of (a–b) MC, (c–d) MC/Al2O3
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Fig.3 FT-IR spectra of (a)MC, (b)MC/Al2O3,(c) MC/Al2O3 after Cd(Ⅱ) adsorption and (d) MC/Al2O3 after Pb(Ⅱ) adsorption
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Fig.7 (a) XPS C 1s spectra of the MC, MC/Al2O3 before and after Cd(Ⅱ) or Pb(Ⅱ) adsorption; (b) XPS O 1s spectra of the MC, MC/Al2O3 before and after Cd(Ⅱ) or Pb(Ⅱ) adsorption; (c) XPS Al 2p spectra of the MC/Al2O3 after Cd(Ⅱ) or Pb(Ⅱ) adsorption; (d)XPS Cd 3d spectra of the MC/Al2O3 after Cd(Ⅱ) adsorption; (e) XPS Pb 4f spectra of the MC/Al2O3 after Pb(Ⅱ) adsorption
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Fig.8 Mechanism illustration of Cd(Ⅱ) (or Pb(Ⅱ)) adsorption by the MC/Al2O3
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g-1)
an
Table 1 Surface Elemental Composition, Surface Area, and Pore Volume Parameters for MC and MC/Al2O3 Adsorbents surface elemental SBETb Average Pore volume (cm3g-1) (m2 pore compositiona
a Determined by X-ray photoelectron spectroscopy (XPS). b Determined by N2 adsorption using the Brunauer-Emmett-Teller (BET) method. c Total pore volume, determined at P/P0= 0.985. d Micropore volume, calculated using the t-plot method. e Mesoporous volume, calculated by Vt - Vmic.
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Table 2 Comparison of Cd(Ⅱ) and Pb(Ⅱ) adsorption capacity of various adsorbents
Cd(II)
Pb(II)
Ca-Montmorillonite
5.0
13.0
tourmaline coconut Fiber sea-buckstone activated carbon sulfur-functionalized silica
9.56 31.12 -
-
30.7
62.2
polyving alcohol/polyacrylic acid activated alumina
115.88
194.99
6.164
9.146
mesoporous carbon stabilized alumina (MC/Al2O3)
49.98
235.57
Initial concentration
ref.
Cd(II): 50 mg L−1 Pb(II): 808.6 mg L−1 Cd(II): 50 mg L−1 Cd(II):10-140 mg L-1 Pb(II):50-300 mg L-1
[39] [40] [41] [42]
ip t
adsorption capacity (mg g-1)
cr
51.81
us
Cd(II): 50–1000 mg L-1 Pb(II):50–1000 mg L-1 Cd(II): 30–160 mg L-1 Pb(II):100–350 mg L-1 Cd(II): 50 mg L−1 Pb(II): 50 mg L−1 Cd(II): 50 mg L−1 Pb(II):250 mg L−1
[43] [44] [45] This study
Ac ce pt e
d
M
an
Adsorbent
Page 22 of 25
Page 23 of 25
d
Ac ce pt e us
an
M
cr
ip t
Table 3 Kinetic and statistical parameters of the three kinetic models Data set Pseudo-first-order model Pseudo-second-order model
Elovich model +
R2
R2
qe
k2
-1
0.0457
0.00069
0.8122
9.980
0.6877
50 mg L-1
1.0035
0.0035
0.4544
50.000
0.0171
100mg L-1
17.006
0.0037
0.8574
90.909
0.0014
10 mg L
α
ip t
k1
β
1.0000
9.935
0.0061
1.0000
36.898
2.2283
0.9999
51.263
6.0088
cr
qe,
Ac ce pt e
d
M
an
us
Note: t is the reaction time (min), qe (mg/g) and qt (mg/g) are the amount of adsorbed fluoride at equilibrium and at any reaction time t, k1(/min) and k2 (g·/mg/min) are the equilibrium rate constants for pseudo-first-order and pseudo-second-order models respectively, and the Elovich constant α is related to the sorption rate while β is related to the surface coverage, R2 is correlation coefficient.
Table 4 Elemental composition of oxygen and carbon atoms present on the materials surface C 1s O 1s
Materials
C-C (284.6 eV)
C-OH (285.6 eV)
C=O (287.5 eV)
O=C-O (288.3 eV)
C=O (531.1 eV)
O=C-O (533.5 eV)
28.6%
O-C-O or Al-O(H) ( 532.2 eV) 22.8%
MC
76.9%
17.7%
1.5%
3.8%
MC /Al2O3
59.9%
35.3%
-
4.8%
15.9%
56.8%
27.3%
MC/Al2O3 79.4% after Cd(Ⅱ) adsorption
15.9%
2.4%
2.4%
21.6%
31.5%
46.9%
48.5%
24
Page 24 of 25
20.6%
4.4%
1.5%
13.0%
Highlight 0 0
Pb
M
60
40
20
d
20
80
an
Removal efficiency (%)
60
MC MC/Al2O3
0
10 20carbon 30 40 alumina 50 was prepared 0 50 one-pot 100 hard-templating 150 200 Mesoporous stabilized by -1 -1 Initial concentration (mg L ) method. Initial concentration (mg L ) MC/Al2O3 showed excellent performance for Cd(Ⅱ) and Pb(Ⅱ) adsorption. Enhanced adsorption was due to the high surface area and special functional groups.
Ac ce pt e
Removal efficiency (%)
Cd 80
us
100
100
MC MC/Al2O3
50.0%
cr
Graphical abstract
40
37.0%
ip t
MC/Al2O3 73.5% after Pb(Ⅱ) adsorption
250
25
Page 25 of 25