ARTICLE pubs.acs.org/Langmuir
Synthesis of an Attapulgite Clay@Carbon Nanocomposite Adsorbent by a Hydrothermal Carbonization Process and Their Application in the Removal of Toxic Metal Ions from Water Li-Feng Chen, Hai-Wei Liang, Yang Lu, Chun-Hua Cui, and Shu-Hong Yu* Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, the School of Chemistry & Materials, University of Science and Technology of China, Hefei 230026, PR China
bS Supporting Information ABSTRACT: A new kind of attapulgite clay@carbon (ATP@C) nanocomposite adsorbent has been synthesized by a one-pot hydrothermal carbonization process under mild conditions using two cheap, ecofriendly materials (i.e., attapulgite clay (ATP), which is a magnesium aluminum silicate that is abundant in nature, and glucose, which is a green chemical obtained from biomass). Compared to carbon-based materials, this new ATP@C nanocomposite exhibits a high adsorption ability for Cr(VI) and Pb(II) ions with maximum adsorption capacities of 177.74 and 263.83 mg 3 g1, respectively. The results demonstrate that this nanocomposite is an exceptionally promising candidate as a low-cost, sustainable, and effective adsorbent for the removal of toxic ions from water.
’ INTRODUCTION Nowadays, the environment is globally polluted by toxic metals through industrial and agricultural processes and waste disposal.1 For example, a huge number of Cr(VI) species in drinking water increases the risk of severe diarrhea and bladder, liver, kidney, and skin cancers.2 Pb(II) ions can affect mental growth, red blood cells, the nervous system, and the kidneys in humans.3 Therefore, these have sparked immense interest in developing advanced technologies to remediate past contamination and prevent further discharge of these species. Various techniques have been employed for the treatment of Cr(VI)- and Pb(II)-contaminated water, including precipitation filtration, ion exchange, reverse osmosis, membrane separation, and electrochemical reduction and adsorption.3d,4 However, most of those methods were not widely used in many situations because of certain drawbacks such as high operational costs and the low feasibility for scale industries. In contrast, the adsorption technique, owing to its low initial cost, simplicity of design, ease of operation, and insensitivity to toxic substances, is considered to be by far the most versatile and promising method and has been studied for many years.5 Nevertheless, there are certain disadvantages in the present reported adsorbents, such as the use of environmentally unfriendly reagents (i.e., poisonous solvents, surfactants, or catalysts) in synthetic process, the complicated preparation methods, and the lower adsorption capacity.6 Therefore, it is urgent to develop a green synthesis strategy for the preparation of cheaper and more environmentally acceptable adsorbents for water decontamination. r 2011 American Chemical Society
Scheme 1. Schematic Illustration of the Synthesis Procedure of the ATP@C Nanocomposite
Recently, a series of functional carbon-based nanomaterials has been synthesized by the so-called hydrothermal carbonization (HTC) process from biomass.7 In contrast to the carbon nanotubes produced by high-temperature CVD techniques, the carbonaceous materials obtained from the HTC process at lower temperature possessed numerous oxygen-containing groups on their surfaces and hence could be used as adsorbents for water purification.6e,8 More recently, we have synthesized free-standing carbonaceous nanofiber membranes for the filtration and separation of nanoparticles from solution.9 Herein, we report a convenient, green synthesis route for preparing a cheap attapulgite clay@carbon (ATP@C) nanocomposite by the HTC process using two low-cost, environmentally friendly materials (i.e., ATP, a magnesium aluminum silicate rich in nature, and glucose, a cheap, green chemical). The obtained Received: May 9, 2011 Revised: June 13, 2011 Published: June 14, 2011 8998
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Figure 1. (a) SEM and (b) TEM images of natural ATP clay. (c) SEM and (d) TEM images of the ATP clay@C nanocomposite. Insets in (a) and (c) are photographs of the corresponding samples.
ATP@C nanocomposite exhibited a high adsorption capability for Cr(VI) and Pb(II) ions.
’ EXPERIMENTAL SECTION The ATP clay was kindly supplied by Guoxing Colloidal Co. Ltd. (Anhui Province, China) and was milled into about 320 mesh. All other chemicals were analytical grade and commercially available from Shanghai Chemical Reagent Co. Ltd. and used as received without further purification. Synthesis of the ATP@C Nanocomposite. In a typical procedure, 0.5 g of ATP clay was dispersed in 38 mL of glucose solution (2.5 g glucose) under vigorous magnetic stirring to form a homogeneous dispersion at room temperature. Then, the mixed solution was put into a Teflon-lined stainless steel autoclave with a volume capacity of 50 mL. Finally, the container was closed and maintained at 160 °C for 48 h. After that, the autoclave was cooled to room temperature naturally. Then, the as-prepared nanocomposite were obtained by filtering, rinsing, and drying. Batch Adsorption Experiments. The Cr(VI) and Pb(II) solutions were prepared from known quantities of analytical-grade potassium dichromate (K2Cr2O7) and Pb(NO3)2 dissolved in deionized water, respectively. The pH values were adjusted with HCl or NaOH solution, and all of the pH measurements were carried out using a pH meter (Mettler Toledo Delta 320). The quantity of metal ions adsorbed by per unit mass of the sample, Qe (mg/g), is calculated using the following equation Qe ¼
ðCi Ce ÞV M
ð1Þ
in which Ci is the initial concentration of metal ions (mg/L), Ce is the equilibrium concentration of the metal ions (mg/L), M is the mass of the sample (g), and V is the volume of the metal ion solution (L). To study the adsorption kinetic of Cr(VI) and Pb(II) ions, 50.0 mg (or 30.0 mg for Pb2+) of ATP@C nanocomposites was added to 75.0 mL
Figure 2. XRD patterns of (a) the ATP@C nanocomposite and (b) natural ATP clay. of an 81.93 mg 3 L1 Cr(VI) (or 150.0 mL of a 114.9 mg 3 L1 Pb2+) aqueous solution at pH 2.0 (or pH 6.0 for Pb2+). The adsorption experiments were carried out in conical flasks mounted on a shaker at 260 rpm under ambient conditions for a specified time. Then, the suspension was filtered with a 220 nm membrane filter. All of the adsorption tests were performed in duplicate. To examine the effect of different pH values on adsorption, 500.0 mg of ATP and 10.0 mg of ATP@C were added to 15.0 mL of Cr2O72 aqueous solutions with initial concentrations of 5.52 and 100.35 mg 3 L1 at different pH values for 30 h. For Pb(II), 10.0 mg of ATP and ATP@C was added to 50.0 mL of Pb(II) aqueous solutions with an initial concentration of 108.8 and 164.6 mg 3 L1, respectively. The experiments were carried out in a manner similar to that described above, except that the reaction went on at different pH values for 30 h. To study the equilibrium adsorption isotherm tests, 500.0 mg of ATP or 10.0 mg of ATP@C was added to 8999
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Figure 3. (a) FT-IR spectra and (b) TGA curves obtained in air for ATP and the ATP@C nanocomposite. carried out using an Atomscan Advantage (Thermo Ash Jarrell Corporation, USA) spectrometer.
Figure 4. Nitrogen adsorptiondesorption isotherms of (a) ATP and (b) the ATP@C nanocomposite. 15 mL of aqueous Cr(VI) solutions with different concentrations at pH 2.0. For Pb(II), 10.0 mg of ATP or ATP@C was added to 50.0 mL of Pb(II) solutions at pH 6.0. The experiments were carried out in a manner similar to that described above, except that all samples were agitated for 30 h at room temperature. Characterization. SEM was performed on a JEOL JSM-6700F scanning electron microscope operating at 10 kV and a Zeiss Supra 40 high-resolution field-emission scanning electron microscope operating at 5 kV. Transmission electron microscope (TEM) images were obtained with a Hitachi H7650 transmission electron microscope operated at an acceleration voltage of 120 kV and equipped with CCD imaging system. High-resolution transmission electron microscope (HRTEM) images were performed on a JEOL-2010 transmission electron microscope operated at an acceleration voltage of 200 kV. The FTIR spectra were characterized on a Bruker Vector-22 FT-IR spectrometer from 4000 to 500 cm1 at room temperature. Thermogravimetric analysis (TGA) was conducted on a DTG-60H thermal analyzer (Shimadzu) at a heating rate of 10.00 K 3 min1 from room temperature to 800 °C in a flow of air. N2 adsorption measurements were determined by an ASAP 2020 accelerated surface area and porosimetry instrument (Micromeritics), equipped with automated surface area, at 77 K using BarrettEmmett Teller (BET) calculations for the surface area. X-ray photoelectron spectra (XPS) were obtained on an X-ray photoelectron spectrometer (ESCALab MKII) using Mg KR radiation (1253.6 eV) as the excitation source. The zeta potential values were determined on a Delsa Nano C zeta potential instrument (Beckman Coulter). The inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements were
’ RESULTS AND DISCUSSION ATP with an ideal molecular formula of [Mg5Si8O20(OH)2(OH2)4 3 4H2O] is present in a type of hydrous, porous magnesium aluminum silicate fibrillar mineral containing ribbons.10 ATP mines are abundant in nature, and their price is quite low.10c In terms of its cheap and nanostructured features, ATP is an ideal template for preparing rodlike nanocomposites. In the present study, we used ATP as a substrate to prepare the ATP@C nanocomposite by a simple and absolutely green synthesis strategy, as shown in Scheme 1. Typical field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of the representative product, ATP@C, clearly revealed the rodlike nanocomposite (Supporting Information, Figure S1a, d and Figure 1c,d), which has a diameter of 4080 nm and a length of 2001000 nm, similar to the original ATP template (Figure 1a,b). It is obvious that there are a large number of granular nanospheres with a size of several tens of nanometers dispersed on the surface of the ATP (Figure 1c,d). According to the previous study,6e the presence of templates in the HTC process can effectively prevent the usual homogeneous nucleation of carbon species from the bulk solution and promote the heterogeneous deposition of carbonaceous matter on the external templates for the formation of well-defined nanostructured composites. On the contrary, without the external template, only microsized carbon spheres were obtained,11 which exhibited lower adsorption capacities.6e By comparing the XRD patterns (Figure 2a,b), there is no obvious difference among the scattering peaks from the two samples,12 which demonstrates that the natural ATP still exists in ATP@C and plays the role as a template. Moreover, the color change of the sample after the HTC process further confirms the successful synthesis of ATP@C nanocomposite (insets in Figure 1a,c). The control experiment revealed that most of the carbon nanospheres remain on the ATP host after ultrasonication treatment of ATP@C, indicating that the carbonaceous compound adhere to the ATP rods firmly and the as-synthesized ATP@C nanocomposite is an excellent integrated nanocomposite. In addition, another two control samples, namely, ATP@C-1 and ATP@C-2, were also synthesized by the hydrothermal carbonization of 1.5 and 4.0 g of glucose in the presence of 9000
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0.5 g of ATP, respectively. The FE-SEM image of ATP@C-1 shows that the morphology is similar to that of ATP@C with the exception of the smaller diameter owing to containing fewer carbonaceous compounds (Supporting Information, Figure S1b,e). Once the amount of glucose is increased to 4.0 g (ATP@C-2), not only rodlike nanocomposites but plenty of micrometer-sized spheres were yielded (Supporting Information, Figure S1c,f) as a result of the higher concentration of glucose in the reaction mixture. Figure 3a shows the FT-IR spectra of the original ATP templates13 and the ATP@C nanocomposite. Our previous studies indicated that a carbohydrate such as glucose could not be carbonized completely at so low a temperature as 160 °C, thus resulting in the formation of a large number of oxygen-containing groups on the surfaces of the carbonization products.14 After ATP was coated with carbonaceous components, several new Table 1. Physicochemical Characterization of ATP Clay and the ATP@C Nanocomposite item 2
1
BET surface area (m 3 g ) t-method external surface area (m2 3 g1) t-method pore area (m2 3 g1)
ATP
ATP@C
169.426
61.648
156.247
61.158
13.179
0.490
total pore volume (cm3 3 g1)
0.577
0.208
average pore diameter (nm)
13.629
13.465
Figure 5. Zeta potential vs pH of (a) ATP clay and (b) the ATP@C nanocomposite.
bands, including the 1400 and 1700 cm1 bands of carbonyl groups, the 1512 cm1 band of a ketone, and the 1620 cm1 band of CdC,6e emerge in the FT-IR spectrum of the ATP@C nanocomposite. These results indicate that the ATP clay has been successfully modified by the functional carbonaceous species. The successful coating of carbon onto ATP clay was further confirmed by thermal gravimetric analysis (TGA) performed under air. The mass loss curves for ATP and ATP@C are shown in Figure 3b. For pure ATP, the weight losses below 120 °C and at about 200 °C are assigned to the release of free water and the structural water of ATP,10b,15 respectively. For the ATP@C composites, the main weight loss below 200 °C is similar to that of ATP, but that between 200 and 550 °C can be assigned to the oxidative degradation of the carbonaceous nanocomposite under air. The weight of ATP@C remains almost constant above 550 °C owing to the disappearance of carbon species in the composites, which approximately hold a proportion of 20 wt % as calculated from TGA analysis. The above results further reveal that the glucose-derived carbon has been successfully grafted onto the ATP clay. Figure 4 shows the N2 adsorptiondesorption isotherms of ATP and ATP@C. The ATP clay exhibits a N2 isotherm close to type IV (in the IUPAC classification) with one clear H3-type hysteresis loop from P/P0 = 0.30 to 0.90 owing to an interconnected mesoporous system (250 nm).16 After the HTC reaction, the area of the hysteresis loops became smaller. The isotherm of ATP@C shows an intermediate shape between types II and IV,17 and the N2 adsorbed amount of ATP@C near P/P0 = 1.0 decreases compared with that of ATP. The porous properties of the ATP@C nanocomposite and ATP clay are summarized in Table 1, including the specific surface area, t-method pore volume, total pore volume, and pore diameter. It can be seen that the specific surface area decreased from 169.43 m2 3 g1 for the ATP clay to 61.65 m2 3 g1 for ATP@C, and the pore volume of ATP@C become obviously smaller than that of ATP (Table 1), probably because the pores of ATP were covered, filled, or partially blocked by the carbonaceous nanoparticles during the HTC reaction process.18 The above results revealed that the carbonaceous nanomaterials, which resulted in a significant change in the porosity, really are integral parts of the ATP@C nanocomposite. To obtain information about the surface charge of this material, zeta potential measurements were carried out at various pH values as shown in Figure 5 using variable amounts of 0.1 M NaOH and 0.1 M HCl to adjust the pH of the samples. These
Figure 6. Adsorption kinetics curve of (a) Cr(VI) onto the ATP@C nanocomposite and (b) Pb(II) onto the ATP@C nanocomposite. 9001
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Figure 7. Equilibrium adsorption isotherms of Cr(VI) on (a) ATP and (b) ATP@C. The dashed lines represent Langmuir modeling of the data.
Figure 8. Equilibrium adsorption isotherms of (a) Pb(II) onto ATP and (b) Pb(II) onto ATP@C. Dashed lines represent Langmuir modeling of the data.
Table 2. Langmuir Equation Parameters of the Adsorption of Cr(VI) and Pb(II) on ATP and ATP@C Cr(VI) sample
Qm
b
Pb(II) R2
Qm
b
R2
ATP
0.036
0.108
0.9857
105.25
0.1270
0.9685
ATP@C
177.74
0.03286
0.9814
263.83
0.0280
0.9938
results showed that the zeta potential values for the surface of the ATP@C nanocomposite are negative over the entire pH range from 1.0 to 9.0 and even more negative than those of ATP at a higher pH value, which may be attributed to the fact that some of the carboxylic groups are located at the surface of the ATP@C nanocomposite. Furthermore, the zeta potential of ATP@C is negative above pH 2 and decreased toward acidic pH values because of the protonation of the hydroxide ions on the surface. This very low isoelectric point is indicative of existing abundant acid sites, which play an important role in retaining the ionic contaminants. The optimal material, the ATP@C nanocomposite, was applied as a new absorbent for the removal of Cr(VI) and Pb(II) from aqueous solution. It is known that both the pH value of the aqueous solution and the contact time are key factors in the removal of metal ions. Thus, the optimum experimental conditions need to be acquired. The effects of the initial pH value on
the adsorption capacities of Cr(VI) and Pb(II) on ATP and ATP@C were studied first. It is evident that the adsorption characteristics of the materials were highly pH-dependent and that the maximum adsorption capacity of Cr(VI) and Pb(II) on the ATP@C nanocomposite occurred at pH 2.0 and 6.0, respectively (Supporting Information, Figures S2 and S3). As we discussed above, the surface of ATP@C was negatively charged and thus could capture Pb2+ ions via an electrostatic interaction. Besides, the amount of Pb(II) adsorbed by per unit mass of ATP@C increases with the increase in pH in the range of 1.06.0 (Supporting Information, Figure S3b) because the surface charge on the ATP@C is larger at a higher pH value (Figure 5). Thus, the mechanism of Pb(II) adsorption onto the ATP@C nanocomposite is believed to be a ion-exchange process.6h However, the adsorption behavior of Cr(VI) on ATP@C is obviously different from that of Pb(II) because Cr(VI) mainly exists in the form of an anion at acidic pH, which cannot be captured by acidic groups on the ATP@C nanocomposite by electrostatic attraction. A probable mechanism for the removal of Cr(VI) was proposed to be the synergistic effect of surface complexation, redox reaction, and cation exchange. Similar behaviors of Cr(VI) removal were observed by other groups as well.6c,19 The surface complexation between oxygen-containing groups of ATP@C and Cr(VI) occurs primarily at low pH values and weakens gradually with increasing pH values because the more negatively charged surface of ATP@C inhibits the complexation with Cr(VI) anions at higher pH values. Meanwhile, the Cr(III) cation, which was 9002
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Table 3. Metal Adsorption Capacity of the Carbonaceous Materials material
Cr(VI) (mg/g)
Pb(II) (mg/g) 21.88
21 and 22
∼35.0
23 6e
activated carbon (AC)
57.769.3
oxidized AC carbon aerogel
10.815.5
CNTs
31.0
composite chitosan
153.85
44.5
reference
9 6f
biosorbent pure C ATP@C
177.74
60.8
6e
263.83
present work
formed through the reducing Cr(VI) anion by the reducing substrate (CxOH) on the ATP@C nanocomposite, can be captured by a cation-exchange mechanism. Therefore, the ATP@C nanocomposite exhibited a maximum adsorption capacity for Cr(VI) at pH 2.0 (Supporting Information, Figure S2b). In addition, X-ray photoelectron spectroscopy (XPS) analysis suggests that chromium was adsorbed on ATP@C as both Cr(III) and Cr(VI) (Supporting Information, Figure S4),5 which also indicated that part of the adsorbed Cr(VI) on ATP@C was indeed reduced to Cr(III). For the purpose of assessing the adsorption rates and obtaining the equilibrium time required for the adsorption isotherm, adsorption kinetics studies of Cr(VI) and Pb(II) on the ATP@C nanocomposite were performed. The adsorption rate curves of Cr(VI) and Pb(II) on the ATP@C composites are shown in Figure 6. The adsorption capacities of Cr(VI) and Pb(II) on the ATP@C nanocomposite rise steeply with the increase in contact time from the beginning to 5 h. Thereafter, the adsorption uptake continues to increase at a slower rate and then approaches the adsorption equilibrium point after 30 h, following a constant adsorption with further increasing time. Hence, 30 h was fixed as the equilibrium time throughout this study. The adsorption isotherms of Cr(VI) and Pb(II) on the sample are shown in Figures 7 and 8. The adsorption behavior was described by the Langmuir isotherm equation,20 which describes a homogeneous adsorption surface, meaning that all of the adsorption sites have equal adsorbate affinities. The isotherm equation gives Qe in the form of Qe ¼
Q m Ce Ce + ð1=bÞ
ð2Þ
where Qe is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g) when the equilibrium concentration is Ce (mg/L), Qm refers to the maximum adsorption capacity corresponding to the quantity of adsorbate required to form a monolayer adsorbed per unit mass of adsorbent (mg/g), and b is the equilibrium constant (L/mg). It is clear that the adsorption data fit Langmuir isotherm models well and the estimated maximum uptakes of Cr(VI) calculated from the Langmuir isotherm were 177.74 and 0.036 mg 3 g1 for ATP@C and ATP, respectively, and the maximum adsorption capacities of Pb(II) on ATP@C and ATP were 263.83 and 105.25 mg 3 g1, respectively (Table 2). The adsorption capacities of Cr(VI) and Pb(II) ions on the ATP@C nanocomposite are approximately 4930 times and 2.5 times as much as that of ATP. That is to say, a dramatic increase in the adsorption capacities of Cr(VI) and Pb(II) ions was observed upon decorating ATP with carbonaceous species. Moreover, the
adsorption capacity of ATP@C is larger than those of ATP@C-1 and ATP@C-2 (Figure S5, Table S1) for the less oxygen-containing carbonaceous compounds (Figure S6) and for a large number of carbonaceous spheres (Figure S1c,f), respectively. The values are also higher than those of other carbon-based adsorbents reported recently (Table 3), which underlines that the ATP@C nanocomposite prepared here is one potential adsorbent material for removing toxic metal ions.
’ CONCLUSIONS In this paper, one new attapulgite@C (ATP@C) nanocomposite can be synthesized by a one-pot hydrothermal process using two environmentally friendly materials, namely, ATP clay and glucose. Compared with other carbon-based adsorbents, the ATP@C nanocomposite shows obvious advantages and possesses a higher adsorption capacity for the removal of Cr(VI) and Pb(II) ions in water with maximum adsorption capacities of 177.74 and 263.83 mg 3 g1, respectively. This new nanocomposite is qualified for wastewater treatment as a low-cost, sustainable, effective adsorbent. Furthermore, the surface of a hydrothermal carbon nanolayer could be further engineered or modified to increase its adsorption capability and selectively remove other contaminants in water. ’ ASSOCIATED CONTENT
bS
Supporting Information. FE-SEM images of ATP@C, ATP@C-1, and ATP@C-2. Effect of pH on the adsorption of Cr(VI) and Pb(II) on ATP and ATP@C. XPS spectra. Equilibrium adsorption isotherms of Cr(VI) and Pb(II) on ATP@C, ATP@C-1, and ATP@C-2. Langmuir equation parameters of the adsorption of Cr(VI) and Pb(II) on ATP@C-1, ATP@C-2, and ATP@C. TGA curves obtained under air for ATP@C-1 and ATP@C. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: + 86 551 3603040. E-mail:
[email protected].
’ ACKNOWLEDGMENT S.-H.Y. acknowledges funding support from the National Basic Research Program of China (2010CB934700), the National Natural Science Foundation of China (nos. 91022032 and 50732006), and the International Science & Technology Cooperation Program of China (2010DFA41170). ’ REFERENCES (1) (a) Nriagu, J. O.; Pacyna, J. M. Nature 1988, 333, 134–9. (b) Barnhart, J. J. Soil. Contam. 1997, 6, 561–568. (c) Psenner, R. Water Air Soil Pollut. 1998, 104, 221–221. (2) (a) Jerome, O. Nriagu, E. N. Chromium in the Natural and Human Environments: Wiley: New York, 1988. (b) Costa, M. Crit. Rev. Toxicol. 1997, 27, 431–442. (c) Levy, L. S.; Venitt, S. Carcinogenesis 1986, 7, 831–835. (3) (a) Low, K. S.; Lee, C. K.; Liew, S. C. Process Biochem. 2000, 36, 59–64. (b) Dietrich, K. N.; Succop, P. A.; Bornschein, R. L.; Krafft, K. M.; Berger, O.; Hammond, P. B.; Buncher, C. R. Environ. Health Perspect. 1990, 89, 13–19. (c) Goldstein, G. W. Environ. Health Perspect. 9003
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