Adsorption of Lead(II) Ions from Aqueous Solution on Low

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Adsorption of Lead(II) Ions from Aqueous Solution on Low-Temperature Exfoliated Graphene Nanosheets Zheng-Hong Huang,*,† Xiaoyu Zheng,‡ Wei Lv,‡ Ming Wang,† Quan-Hong Yang,*,‡ and Feiyu Kang† † ‡

Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ABSTRACT: Graphene nanosheets (GNSs) that were obtained by vacuum-promoted low-temperature exfoliation were used to adsorb lead ions from an aqueous system. The pristine and thermally modified GNSs were characterized with scanning electron microscopy observation and X-ray photoelectron spectroscopy analysis. It was interestingly found that the adsorption against lead ions was enhanced by heat treatment, although the oxygen complexes of GNSs showed a significant decrease. In addition, lead ion uptake resulted in an increase in the pH value of the solution. It is supposed that the Lewis basicity of GNSs is improved by heat treatment under a high vacuum, in favor of simultaneous adsorption of lead ions and protons onto GNSs.

’ INTRODUCTION Lead ions commonly exist in industrial and agricultural wastewater1 and acidic leachate from landfill sites in relatively high concentration. They are quite harmful to human and living things, leading to a wide range of spectrum health problems, such as nausea, convulsions, coma, renal failure, cancer, and subtle effects on metabolism and intelligence.2 Due to the importance of lead as a heavy metal ion contaminant in geochemical systems and its high toxicity, many techniques have been applied in the removal of Pb(II), such as flocculation,3 membrane filtration,4 solvent extraction,5 biosorption,6 chemical precipitation,7 reverse osmosis,8 adsorption,9 etc. Among these technologies, the adsorption method is considered to be highly effective and economical at present. In addition, it is an important method to understand the accumulation of metal ions at solid
liquid interfaces.10 Carbon nanotubes,2 activated carbon,9 clay minerals,10,11 microorganisms,12 plant wastes,6,13 and industrial and agricultural byproducts14 have been widely investigated as such adsorbents. Normally, adsorption is strongly dependent on the pore structure and surface area of the adsorbents,15 whereas metal ion uptake is largely ascribed to ion exchange or chemical adsorption on specific adsorption sites. At low concentrations, metal ion adsorption on solids is mainly a surface coordination process, which can be modeled thermodynamically as a complexation reaction between surface sites and metal ions. Thus, modification of the surface chemistry strongly influences the metal ion adsorption process.16 The graphene nanosheet (GNS), as an ideal two-dimensional material, is characterized by a large specific surface area (SSA), which is, theoretically, 2630 m2/g, calculated from the monolayer carbon structure. From this point, graphene is an ideal nonporous adsorbent, and adsorption mainly occurs on its planar surface r 2011 American Chemical Society

(external surface), escaping from intraporous diffusion like that in porous carbons but only controlled by external diffusion since powdered GNSs with randomly aggregated structures have open ion channels, which results in a short period to reach an adsorption equilibrium. Experimental investigations and theoretical calculations thus far have shown that GNS-based materials possess a high adsorption capacity.17
24 Yang et al. demonstrated that graphene oxide has a huge absorption capacity for Cu2þ, which is 10 times higher than that of active carbon.24 In our previous study, we found that graphene powder can adsorb a large amount of methylene blue and shows an enhanced adsorption capacity in solution.25 In the present work, the GNS was prepared by a lowtemperature exfoliation approach25 and further modified by heat treatment under a high vacuum. The adsorption characteristics of a pristine and thermally modified GNS toward lead ions in aqueous solution were investigated. The influence of heat treatment on the surface chemistry of the GNS and adsorption behavior for ions was studied.

’ EXPERIMENTAL SECTION Materials. GNSs were prepared by a vacuum-promoted lowtemperature exfoliation (for the details see ref 25). Further heat treatment at 500 and 700 °C for GNSs was performed to obtain modified GNSs, denoted as GNS-500 and GNS-700, respectively. Adsorption Test. Aqueous solutions of lead nitrate were prepared (AR), with initial concentrations ranging between 5 and 80 mg/L. Batch adsorption tests for lead ions were performed as follows. Using one set of Received: February 17, 2011 Revised: April 18, 2011 Published: May 19, 2011 7558

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sealed flasks, equal masses (0.0125 g) of GNS samples were added to 25 mL of lead ion solution with different initial concentrations. Subsequently, the mixtures in the flasks were ultrasonicated for a while to guarantee a good dispersion of graphene and then placed in a water bath with stirring at a constant temperature (30 ( 0.5 °C) for 15 h to allow complete equilibration. The GNS/solution mixtures were filtered with a microsyringe filter, and the concentrations of the residual lead ions were analyzed by an Optima 3000 inductively coupled plasma (ICP) spectrometer (IRIS Advantage, Thermo Elemental). In addition, the impact of the pH value on adsorption was also investigated. The adsorption test was performed by adding 0.0125 g of graphene to a 40 mg/L lead ion solution for 15 h at 30 °C. The initial pH values of the solution were adjusted from 2 to 8 with 0.1 M HCl and 0.1 M NaOH. The adsorption capacity (qe, mg/g) and the removal percentage (A, %) were calculated from the following equations: C0
Ce qe ¼ V ms A¼

C0
Ce  100 C0

where C0 and Ce (mg/L) are the initial and final concentrations of adsorbate in the flasks, respectively, V is the volume of the solution (L), and ms is the mass of dry adsorbent used (g).

Figure 1. SEM image of a low-temperature exfoliated GNS.

Nitrogen adsorption was measured at 77 K by using a BEL miniinstrument, and the SSA of graphene was obtained by Brunauer
Emmett
Teller (BET) analysis of the adsorption isotherm. The SSA was also measured by a methylene blue (MB) adsorption method, where MB dye was employed as a molecular probe for the measurement of the adsorption amount and UV
vis spectroscopy was used to measure the concentration change of MB before and after the adsorption by graphene.26 Since graphene tends to aggregate together, the adsorption-based SSA measurement values are normally lower than the theoretical value and gas adsorption (dense aggregation) gives a much lower SSA value than liquid adsorption (partly dispersed in solvent).

’ RESULTS AND DISCUSSION Surface Chemistry of Low-Temperature Exfoliated Graphenes. Our previous study reveals that a high vacuum and heat

treatment at a low temperature of 200 °C can exert an outward drawing force on the expanding graphene and remove oxygen, resulting in an effective exfoliation and formation of few-layered graphenes. Previous data have shown that over 60% of these graphene sheets are single-layered, and for the sample used in this study, the SSAs are ∼400 m2/g based on N2 adsorption and ∼1000 m2/g based on liquid adsorption where MB is employed as the probe adsorbate. The random overlayering of graphene sheets does not lay great hurdles for a liquid adsorbate such as MB in the liquid phase (dispersed in the liquid phase subject to ultrasonication) at room temperature but hinders nitrogen diffusion onto part of the graphene surface (in powdered aggregation) at cryotemperature (77 K), which is a possible reason for the difference in SSA values obtained by gas and liquid adsorption. From this point, the SSA value based on liquid adsorption is more useful for understanding Pb2þ adsorption in the liquid phase.25 Figure 1 shows a representative scanning electron microscopy (SEM) image of a low-temperature exfoliated sample. After heat treatment at different temperatures, the morphologies and SSAs of graphene do not show apparent differences. However, the chemical compositions present a difference which was characterized by the X-ray photoelectron spectroscopy (XPS) technique. Heat treatment at higher temperature resulted in a significant decrease in the oxygen content, as seen in the survey XPS spectra (Figure 2a). The C 1s XPS spectra, as shown in Figure 2b, can be deconvoluted into four components corresponding to carbon atoms in different oxygen-containing

Figure 2. XPS survey spectra of pristine and heat-treated GNSs (left) and example of the C 1s spectrum of a GNS sample with the results of curve fitting (right). 7559

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Table 1. Curve Fitting Results of XPS Analysis functional group

GNS

GNS500

GNS700

graphitic

64.21

71.32

70.67

alcohol, ether group

19.91

18.16

18.29

carbonyl groups

6.01

3.85

2.71

carboxyl groups

4.17

5.03

5.25

π
π* shake-up

5.7

1.64

3.08

Figure 4. Effect of the initial concentration on the removal percentage.

Table 2. Adsorption Constants for the Langmuir and Freundlich Isotherm Models Langmuir sample

Figure 3. Adsorption isotherms of GNSs toward Pb2þ at 30 °C and pH 4.

functional groups:27 (a) nonoxygenated C at 284.8 eV, (b) carbon in C
O at 286.2 eV, (c) carbonyl carbon (CdO, 287.9 eV), and (d) carboxylate carbon (O—CdO, 289.0 eV). In addition, there is an additional component at 291 eV corresponding to the π
π* transition peak. Curve fitting results of C 1s XPS spectra are summarized in Table 1. It is interestingly found that the amount of alcohol, ether, and carbonyl groups decreased but the amount of carboxyl groups increased with increasing heat treatment temperature, although the amount of total oxygen-containing functional groups decreased. Adsorption of Lead(II) Ions on GNSs. Figure 3 shows the adsorption isotherms of pristine and thermally modified GNSs toward Pb2þ at 30 °C and pH 4. For GNSs, the adsorption capacity increased with increasing equilibrium concentration, which belongs to an L-type isotherm. It is interestingly found that the adsorption against lead ions within the experimental concentration ranges is enhanced after heat treatment at higher temperature. Figure 4 shows the effect of the initial concentration on the removal percentage of Pb(II) ions by GNSs using a 0.0125 g adsorbent dose. The removal percentage of Pb(II) decreased with increasing initial concentration. At low initial solution concentration, the surface area and the availability of adsorption sites were relatively high, and the adsorbate was easily adsorbed and removed.28 At higher initial solution concentration, the total available adsorption sites are limited, thus resulting in a decrease in the percentage removal of adsorbate. On the other hand, similar to the change of adsorption capacity, the removal percentage increased with increasing treatment temperature. The XPS analysis in Table 1 reveals that the fraction of the functional group
COOH increased. This is likely to refer to the importance of the carboxylic group over other surface oxygen containing functional groups in the enhancement of lead uptake. Langmuir and Freundlich isotherms which are often used to describe and understand the mechanism of the adsorption were

qm

KL

Freundlich 2

r

n

KF

r2

GNS

22.42

65.01

0.9934

2.81

4.89

0.9455

GNS500 GNS700

35.21 35.46

414.74 5719.5

0.9946 0.9994

3.65 5.18

12.08 12.59

0.8949 0.9123

used in the present study. The experimental data for Pb2þ adsorption on different absorbents were analyzed by using the isotherm models of both Langmuir and Freundlich:29 qe ¼

q m K L Cc 1 þ K L Ce

qe ¼ KF Ce 1=n where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium (mg/g), qm and KL are Langmuir constants related to the adsorption capacity and energy of adsorption, respectively, and KF and n are Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. As presented in Table 2, a high value of the coefficient of correlation indicates better agreement between experimental and predicted data using the Langmuir equation than the Freundlich equation. As discussed above, in our samples, single-layered GNSs are dominant and Pb2þ adsorption mostly occurs onto the external surface of graphene planes (subject to ultrasonication) dispersed in the liquid phase. Since the fraction of multilayered graphenes is relatively low, the contribution of the ion intercalation into the interlayer spacing of tightly stacked graphenes to the total adsorption amount is limited and not considered here when Pb2þ adsorption is discussed. Figure 5 shows that the adsorption of lead ions occurred very fast initially, with about 85% of total lead ions being removed within 5 min. Thereafter, the adsorption proceeded slowly with contact time before reaching a plateau value after 6 h. The high adsorption rate within the initial 5 min was attributed to the nonporous laminated structure and large external surface of GNSs. Lead ions do not have to traverse farther and deeper into the pores, which does not need a strong driving force. In sharp contrast, equilibrium was attained after 50
60 min for Pb(II) adsorbed by activated carbon from hazelnut husks.9 7560

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Table 3. Change of the pH Value after Adsorption of Lead Ions

Figure 5. Effect of the contact time on adsorption.

Figure 6. Effect of the pH on the adsorption capacity.

The pH value of the solution has been identified as the most important variable governing metal adsorption on the adsorbent. The effect of the pH value on the adsorption capacity for lead ions from aqueous solutions is illustrated in Figure 6. When the pH value was lower than 3, the adsorption of lead ions was almost not observed. However, the adsorption capacity for lead sharply increased when pH increased from 3 to 5. At a low pH value, the low adsorption capacity was due to the competition of lead ions with hydrogen ions. On the other hand, the high positive charge density on the surface of GNSs at low pH was another cause. With an increase of the pH value, the number of hydrogen ions and the potential decreased, resulting in an increase of the adsorption capacity for lead ions. When the pH was higher than 7.6, the removal percentage was approximately 100%, which is due to the cooperating role of adsorption and precipitation. The adsorption of lead ions in acidic solution with carbon adsorbent is commonly regarded as ion exchange and/or complex formation. It is surprisingly found that the pH value of the solution increased after adsorption of lead ions (Table 3). Moreover, the increase in the pH value increased with increasing lead uptake. Thus, ion exchange is not the main cause. However, the delocalized π electron systems of graphene layers act as Lewis bases in aqueous solution and form electron donor
acceptor complexes with H2O molecules and lead ions.30 In the Lewis definition, acids are electron pair acceptors and bases are electron pair donors. The fundamental reaction between Lewis acid A and Lewis base B is the formation of a complex (or an adduct, a coordination compound, or an addition compound) A
B. In this reaction, the unshared electron pair of the base forms a coordination bond (or dative bond or dipolar bond) with an

pH 2

pH 3

pH 4

pH 4.8

pH 5.6

pH 7.6

GNS

1.96

3.10

4.87

5.24

5.41

5.72

GNS500 GNS700

2.00 1.98

3.35 3.39

5.70 6.11

4.93 6.27

6.13 6.25

6.57 6.77

electron-deficient atom of the acid.31 In the case of graphene and a heavy metal ion, graphene is the Lewis base and the heavy metal ion is the Lewis acid. A complex between graphene and the heavy metal ion can occur through Lewis acid
base interaction. Thus, it is regarded that lead ions and protons simultaneously adsorbed onto graphene, resulting in an increase in the pH value. On the other hand, it is known that Pb(II) adsorption onto the outgassed carbons proceeds via electrostatic interaction between Pb(II) cations and C π electrons other than the ionic exchange mechanism.1 For our study, electrostatic interaction could be another cause. The heat treatment at high temperature in a vacuum can eliminate the oxygen-containing groups, leading to increases in both Lewis basicity and electrostatic attraction. Thus, heat treatment for graphene favors adsorption toward lead and hydrogen ions. High lead uptake is accompanied by a high increase in the pH value.

’ CONCLUSIONS Low-temperature exfoliated GNSs were heat-treated under a high vacuum, leading to a significant decrease in the oxygen complexes. The pristine and thermally treated GNSs exhibit similar adsorption characteristics toward lead ions in aqueous solution, which is strongly dependent on the pH value. However, it is interestingly found that the adsorption against lead ions is enhanced by heat treatment, which is ascribed to the increase in the Lewis basicity and electrostatic attraction of graphene. Due to its large external surface, the GNS exhibits a high initial adsorption rate. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.-H.H.); qhyangcn@tju. edu.cn (Q.-H.Y.). Phone: þ86-10-62773752 (Q.-H.Y.). Fax: þ86-10-62771160 (Q.-H.Y.).

’ ACKNOWLEDGMENT We are grateful for financial support from the Program for New Century Excellent Talents in University (Grant NCET10-0496) and the National Natural Science Foundation of China (Grant 51072091). ’ REFERENCES (1) Machida, M.; Mochimaru, T.; Tatsumoto, H. Lead(II) adsorption onto the graphene layer of carbonaceous materials in aqueous solution. Carbon 2006, 44 (13), 2681–2688. (2) Li, Y. H.; Di, Z. C.; Ding, J.; Wu, D. H.; Luan, Z. K.; Zhu, Y. Q. Adsorption thermodynamic, kinetic and desorption studies of Pb2þ on carbon nanotubes. Water Res. 2005, 39 (4), 605–609. (3) Karbassi, A. R.; Nadjafpour, S. Flocculation of dissolved Pb, Cu, Zn and Mn during estuarine mixing of river water with the Caspian Sea. Environ. Pollut. 1996, 93 (3), 257–260. 7561

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Langmuir (4) Soylak, M.; Unsal, Y. E.; Kizil, N.; Aydin, A. Utilization of membrane filtration for preconcentration and determination of Cu(II) and Pb(II) in food, water and geological samples by atomic absorption spectrometry. Food Chem. Toxicol. 2010, 48 (2), 517–521. (5) Makrlik, E.; Vanura, P. Solvent extraction of lead using a nitrobenzene solution of strontium dicarbollylcobaltate in the presence of polyethylene glycol PEG 400. J. Radioanal. Nucl. Chem. 2005, 267 (1), 233–235. (6) Ofomaja, A. E.; Naidoo, E. B.; Modise, S. J. Biosorption of copper(II) and lead(II) onto potassium hydroxide treated pine cone powder. J. Environ. Manage. 2010, 91 (8), 1674–1685. (7) Matlock, M. M.; Howerton, B. S.; Atwood, D. A. Chemical precipitation of lead from lead battery recycling plant wastewater. Ind. Eng. Chem. Res. 2002, 41 (6), 1579–1582. (8) Rao, M. M.; Ramana, D. K.; Seshaiah, K.; Wang, M. C.; Chien, S. W. C. Removal of some metal ions by activated carbon prepared from Phaseolus aureus hulls. J. Hazard. Mater. 2009, 166 (2
3), 1006–1013. (9) Imamoglu, M.; Tekir, O. Removal of copper (II) and lead (II) ions from aqueous solutions by adsorption on activated carbon from a new precursor hazelnut husks. Desalination 2008, 228 (1
3), 108–113. (10) Fan, Q. H.; Li, Z.; Zhao, H. G.; Jia, Z. H.; Xu, J. Z.; Wu, W. S. Adsorption of Pb(II) on palygorskite from aqueous solution: Effects of pH, ionic strength and temperature. Appl. Clay Sci. 2009, 45 (3), 111–116. (11) Xu, D.; Tan, X. L.; Chen, C. L.; Wang, X. K. Adsorption of Pb(II) from aqueous solution to MX-80 bentonite: Effect of pH, ionic strength, foreign ions and temperature. Appl. Clay Sci. 2008, 41 (1
2), 37–46. (12) Majumdar, S. S.; Das, S. K.; Chakravarty, R.; Saha, T.; Bandyopadhyay, T. S.; Guha, A. K. A study on lead adsorption by Mucor rouxii biomass. Desalination 2010, 251 (1
3), 96–102. (13) Zhao, J. O.; Liu, L.; Guo, Q. G.; Shi, J. L.; Zhai, G. T.; Song, J. R.; Liu, Z. J. Growth of carbon nanotubes on the surface of carbon fibers. Carbon 2008, 46 (2), 380–383. (14) Saeed, A.; Iqbal, M.; Holl, W. H. Kinetics, equilibrium and mechanism of Cd2þ removal from aqueous solution by mungbean husk. J. Hazard. Mater. 2009, 168 (2
3), 1467–1475. (15) Kuchta, B.; Firlej, L.; Maurin, G. Modeling of adsorption in nanopores. J. Mol. Model. 2005, 11 (4
5), 293–300. (16) Zhang, J. N.; Huang, Z. H.; Lv, R.; Yang, Q. H.; Kang, F. Y. Effect of growing CNTs onto bamboo charcoals on adsorption of copper ions in aqueous solution. Langmuir 2009, 25 (1), 269–274. (17) Leenaerts, O.; Partoens, B.; Peeters, F. M. Adsorption of H2O, NH3, CO, NO2, and NO on graphene: A first-principles study. Phys. Rev. B 2008, 77 (12), 6. (18) Balog, R.; Jorgensen, B.; Wells, J.; Laegsgaard, E.; Hofmann, P.; Besenbacher, F.; Hornekaer, L. Atomic hydrogen adsorbate structures on graphene. J. Am. Chem. Soc. 2009, 131 (25), 8744–8745. (19) Kim, G.; Jhi, S. H. Ca-decorated graphene-based threedimensional structures for high-capacity hydrogen storage. J. Phys. Chem. C 2009, 113 (47), 20499–20503. (20) Petit, C.; Bandosz, T. J. Graphite oxide/polyoxometalate nanocomposites as adsorbents of ammonia. J. Phys. Chem. C 2009, 113 (9), 3800–3809. (21) Romero, H. E.; Joshi, P.; Gupta, A. K.; Gutierrez, H. R.; Cole, M. W.; Tadigadapa, S. A.; Eklund, P. C. Adsorption of ammonia on graphene. Nanotechnology 2009, 20 (24), 8. (22) Lee, H.; Ihm, J.; Cohen, M. L.; Louie, S. G. Calcium-decorated graphene-based nanostructures for hydrogen storage. Nano Lett. 2010, 10 (3), 793–798. (23) Wuest, J. D.; Rochefort, A. Strong adsorption of aminotriazines on graphene. Chem. Commun. 2010, 46 (17), 2923–2925. (24) Yang, S. T.; Chang, Y. L.; Wang, H. F.; Liu, G. B.; Chen, S.; Wang, Y. W.; Liu, Y. F.; Cao, A. N. Folding/aggregation of graphene oxide and its application in Cu2þ removal. J. Colloid Interface Sci. 2010, 351 (1), 122–127. (25) Lv, W.; Tang, D. M.; He, Y. B.; You, C. H.; Shi, Z. Q.; Chen, X. C.; Chen, C. M.; Hou, P. X.; Liu, C.; Yang, Q. H. Low-temperature

ARTICLE

exfoliated graphenes: Vacuum-promoted exfoliation and electrochemical energy storage. ACS Nano 2009, 3 (11), 3730–3736. (26) McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem. Mater. 2007, 19 (18), 4396–4404. (27) Stankovich, S.; Piner, R. D.; Chen, X. Q.; Wu, N. Q.; Nguyen, S. T.; Ruoff, R. S. Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate). J. Mater. Chem. 2006, 16 (2), 155–158. (28) Wang, F. Y.; Wang, H.; Ma, J. W. Adsorption of cadmium(II) ions from aqueous solution by a new low-cost adsorbent-bamboo charcoal. J. Hazard. Mater. 2010, 177 (1
3), 300–306. (29) Li, Y. H.; Wang, S. G.; Wei, J. Q.; Zhang, X. F.; Xu, C. L.; Luan, Z. K.; Wu, D. H.; Wei, B. Q. Lead adsorption on carbon nanotubes. Chem. Phys. Lett. 2002, 357 (3
4), 263–266. (30) Radovic, L. R.; Moreno-Castilla, C.; Rivera-Utrilla, J. In Chemistry and Physics of Carbon; Radovic, L. R., Ed.; Marcel Dekker: New York, 2001; Vol. 27, p 227. (31) Laurence, C.; Gal, J. F. Lewis Basicity and Affinity Scales: Data and Measurement; Wiley: New York, 2009.

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