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Water Research 39 (2005) 605–609 www.elsevier.com/locate/watres
Adsorption thermodynamic, kinetic and desorption studies of Pb2+ on carbon nanotubes Yan-Hui Lia,b,, Zechao Dib, Jun Dingb, Dehai Wub, Zhaokun Luanc, Yanqiu Zhua a
Advanced Materials, School of Mechanical, Materials and Manufacturing Engineering, the University of Nottingham, University Park, Nottingham NG7 2RD, UK b Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China c State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China Received 19 December 2003; received in revised form 2 November 2004; accepted 6 November 2004
Abstract Adsorption thermodynamics of Pb2+ on carbon nanotubes has been studied at various temperatures of 280, 298 and 321 K and the thermodynamic parameters, such as equilibrium constant (K0), standard free energy changes (DG0), standard enthalpy change (DH0) and standard entropy change (DS0), have been obtained. A pseudo-second-order rate model has been employed to describe the kinetic adsorption processes. Desorption studies reveal that Pb2+ can be easily removed from carbon nanotubes by altering the pH values of the solution using both HCl and HNO3, indicating that carbon nanotubes are a promising absorbent for wastewater treatment. r 2004 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Carbon nanotubes; Desorption; Kinetics; Lead; Thermodynamics
1. Introduction Recently, an increasing interest has been focused on removing Pb2+ ions from drinking water due to its supreme toxicity to our health. Drinking those that contain Pb2+ ions for a long term, even if in a very low concentration, could lead to a wide range of spectrum health problems, such as nausea, convulsions, coma, renal failure, cancer and subtle effects on metabolism and intelligence (Friberg et al., 1979; Rashed, 2001). Corresponding author. Advanced Materials, School of Mechanical, Materials and Manufacturing Engineering, the University of Nottingham, University Park, Nottingham NG7 2RD, UK. Tel.: +44 115 846 7257; fax: +44 115 951 3800. E-mail address: [email protected]
Different approaches to remove Pb2+ ions from wastewater, including chemical precipitation, ion exchange, reverse osmosis and adsorption, have been reported. One of which, adsorption method, is simple and cost-effective, thus has been widely used. Various adsorbents such as activated carbon (Lee et al., 1998; Reed et al., 1996), iron oxides (Liu and Huang, 2003), ﬁlamentous fungal biomass (Lo et al., 1999) and natural condensed tannin (Zhan and Zhao, 2003) have been explored and the results are promising. Carbon nanotubes (CNTs), a new member of the carbon family, have attracted special attentions to many researchers after their discovery in 1991 (Iijima, 1991) because they possess unique morphologies and have showed excellent properties and great potential for engineering applications such as composite reinforce-
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.11.004
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Y.-H. Li et al. / Water Research 39 (2005) 605–609
ment (Wagner et al., 1998), ﬁeld emission element (Wang et al., 1998), nanodevice component (Collins et al., 1997), gas adsorption materials (Dillon et al., 1997) and catalyst support phases (Planeix et al., 1994). CNTs are also good anion and cation adsorption materials for wastewater treatment, as they exhibit exceptionally large speciﬁc surface areas which are easy to decorate (Li et al., 2001, 2002, 2003). We have previously demonstrated that CNT supported alumina is a good ﬂuoride adsorbent whose adsorption capacity is 15–25 times higher than that of the activated carbon (Li et al., 2001). CNTs oxidized with nitric acid showed very good cation adsorption capability, e.g., their lead and cadmium (II) adsorption capacities being 15.6 and 3.6 mg/g, separately, at metal ion equilibrium concentration of 2.7 mg/ L (Li et al., 2002, 2003). In this paper, we will investigate the thermodynamic and kinetic of the Pb2+ absorption onto CNTs, in order to obtain the thermodynamic parameters, to establish the adsorption rate equation and to assess the reuse capacity of CNTs in wastewater treatment.
where C0 and Ct are the Pb2+concentrations contained in the original solution and after time t, respectively; V is the volume of the solution and W represents the weight of the CNT used. In order to investigate the desorption capacity of Pb2+ from CNTs, 0.05 g CNTs were introduced to 100 ml solution whose initial Pb2+ concentration is 55 mg/L and pH is 5.0. As the adsorption reaches equilibrium, the Pb2+ concentration of the solution was measured, and then the solution was ﬁltrated using a membrane to recover the CNT sample. These CNTs were dried at 60 1C and dispersed into 100 ml deionized water. The pH values of the solution were adjusted, from 1.6 to 5.4, using HCl and HNO3 solutions, respectively. After the solutions reach equilibrium, the Pb2+ concentrations were remeasured and, the desorption results were then obtained. These adsorption/ desorption processes were repeated for three times, to further ascertain the desorption capability of CNTs.
3. Results and discussion 2. Experimental CNTs were prepared by Ni nanoparticle catalysed pyrolysis of propylene in a hydrogen ﬂow at 750 1C. The as-grown CNTs were dispersed in concentrated HNO3 and were reﬂuxed for 30 min at 140 1C, to remove the catalyst. By dissolving lead nitrate in deionized water, we obtained the lead containing stock solution (1000 mg/L), which was further diluted to the required concentrations before being used. To evaluate the thermodynamic properties, we ﬁrst prepared various solutions with initial Pb2+ concentration ranging from 10 to 80 mg/L (10 mg/L interval, 100 ml, pH ¼ 5), and then added 0.05 g CNTs to each solution. These samples were then mounted on a shaker (HZQ-C) and shaken continuously for 6 h at 280, 298 and 321 K, respectively. The suspensions were ﬁltered using a 0.45 mm membrane, and the ﬁltrates were immediately measured using an atomic adsorption spectroscopy. The differences between the initial and the equilibrium Pb2+ concentrations determine the amount that Pb2+ adsorbed by CNTs. Adsorption kinetics samples were prepared by adding 1 g of CNTs into 2000 ml solution (pH ¼ 5.0), and the Pb2+ concentration was 10, 20 and 30 mg/L, separately, at 298 K. At predetermined time intervals, samples were collected utilizing a 0.45 mm membrane ﬁlter and then analyzed by an atomic adsorption spectroscopy. The Pb2+ adsorption amount at time t, qt (mg/g), was calculated by qt ¼ ðC 0 C t ÞV =W ;
The CNT starting materials usually exhibit curved features, with ca. 30 nm o.d. and 10 nm i.d., and hundreds of nm to mm long. Their speciﬁc surface areas are 122 and 153 m3/g, respectively, before and after oxidation treatment by nitric acid. Fig. 1 shows the adsorption isotherms of Pb2+ on CNTs at 280, 298 and 321 K. These data could be approximated by the Freundlich isotherm model, which depicts the relationship between the amount of Pb2+ adsorbed by per unit mass of CNTs (qe, mg/g) and the equilibrium concentration of Pb2+ (Ce, mg/L) in solution: qe ¼ K F C l=n e ;
where KF and n are Freundlich constants related to the adsorption capacity and adsorption intensity, respec-
Fig. 1. Adsorption isotherms of Pb2+ onto CNTs at different temperatures.
ARTICLE IN PRESS Y.-H. Li et al. / Water Research 39 (2005) 605–609
tively. The Freundlich equation is applicable to highly heterogeneous surfaces, and an adsorption isotherm lacking a plateau indicates a multi-layer adsorption (Daifullah et al., 2004). The parameter KF refers to the relative adsorption capacity, i.e., CNTs exhibit higher Pb2+ adsorption capacity at 321 K than that at 280 K, according to the KF data shown in Table 1. Based on Eq. (2), we obtained a straight line when plotting ln qe vs. ln Ce (Fig. 2). The constants, KF and n, are then deﬁned by the intercepts and slopes of the line, respectively. It appears that the Freundlich model agrees well with our experimental data (Table 1), with the correlation coefﬁcient values being close to 1 at different temperatures. The Pb2+ adsorption capacity increases with the rise of temperatures, indicating an endothermic reaction. Thermodynamic parameters can be calculated from the variation of the thermodynamic equilibrium constant K0 with the change in temperature (Niwas et al., 2000). For adsorption reactions, K0 is deﬁned as follows: K0 ¼
as vs C s ¼ ae ve C e
where as is the activity of adsorbed Pb2+, ae is the activity of the Pb2+ in solution at equilibrium, Cs is the amount of Pb2+ adsorbed by per mass of CNTs (mmol/ g) and Ce is the Pb2+ concentration in solution at equilibrium (mmol/ml), vs is the activity coefﬁcient of the adsorbed Pb2+ and ve is the activity coefﬁcient of the Pb2+ in solution. As the Pb2+concentration in the
solution decreases and approaches to zero, K0 can be obtained by plotting ln(Cs/Ce) vs. Cs (Fig. 3) and extrapolating Cs to zero (Niwas et al., 2000). The straight line obtained is ﬁtted to the points based on a least-squares analysis. Its intercept with the vertical axis gives the values of K0. The adsorption standard free energy changes (DG0) can be calculated according to DG0 ¼ RT ln K 0 ;
where R is the universal gas constant (1.987 cal/deg/mol) and T is the temperature in kelvin. The average standard enthalpy change (DH0) is obtained from Van’t Hoof equation DH 0 1 1 ; (5) ln K 0 ðT 3 Þ ln K 0 ðT 1 Þ ¼ R T3 T1 where T3 and T1 are two different temperatures. The standard entropy change (DS0) can be obtained by DS0 ¼
DG 0 DH 0 : T
The thermodynamic parameters are listed in Table 2. A positive standard enthalpy change suggests that the interaction of Pb2+ adsorbed by CNTs is endothermic, which supported by the increasing adsorption of Pb2+ with the increase in temperature; a negative adsorption
Table 1 Parameters of Freundlich adsorption isotherm models for Pb2+ on CNTs Temperature (K)
280 298 321
12.4100 15.5646 16.1448
4.5269 4.4802 4.3440
0.9869 0.9659 0.9755
Fig. 3. Plots of ln Cs/Ce vs. Cs at various temperatures.
Table 2 Values of various thermodynamic parameters for adsorption of Pb2+ on CNTs Thermodynamic constant
Fig. 2. Linearized Freundlich isotherms for Pb2+ adsorption by the CNTs at different temperatures.
K0 DG0 ( 1000 cal mol1) DH0 ( 1000 cal mol1) DS0 (cal mol1 K1)
Temperature (K) 280
10.891 1.329 0.441 6.321
11.628 1.453 0.441 6.356
12.052 1.588 0.441 6.321
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standard free energy change and a positive standard entropy change indicate that the adsorption reaction is a spontaneous process (Niwas et al., 2000). The positive standard entropy change may be due to the release of water molecule produced by ion exchange reaction between the adsorbate and the functional groups on the surfaces of CNTs (Liang and Chen, 2004; Li et al., 2002). Adsorption kinetics, demonstrating the solute uptake rate, is one of the most important characters which represent the adsorption efﬁciency of the CNTs and therefore, determines their potential applications. According to Fig. 4, the Pb2+ adsorption rates increase dramatically in the ﬁrst 10 min for various initial concentrations, and they reach equilibrium gradually at 20, 50 and 60 min, corresponding to Pb2+ initial concentrations of 10, 20 and 30 mg/L, respectively. To analyze the adsorption rate of Pb2+ onto the CNTs, the pseudo-second-order rate equation was evaluated based on the experimental data (Benguella and Benaissa, 2002) t=qt ¼
1=ð2K 0 q2e Þ
þ t=qe ;
where K0 stands for the pseudo-second-order rate constant of adsorption (g/(mg min)). Linear plot feature of t/qt vs. t (Fig. 5) was achieved and the K0 values calculated from the slopes and intercepts were summarized in Table 3. The correlation coefﬁcients of the pseudo-second-order rate model for the linear plots are very close to 1, thus suggesting that kinetic adsorption can be described by the pseudosecond-order rate equation. The repeated availability is an important factor for an advanced adsorbent. Such adsorbent not only possess higher adsorption capability, but also shows better desorption property, which will signiﬁcantly reduce the overall cost for adsorbent. Fig. 6 shows the Pb2+ desorption curves with regard to various solutions of pH values. It is apparent that the Pb2+ desorption increases
Fig. 5. Test of pseudo second-order rate equation for adsorption of different concentrations of Pb2+ by CNTs (pH ¼ 5.0, at 298 K).
Table 3 Kinetic parameters of Pb2+ adsorbed on CNTs at different initial Pb2+ concentrations Initial Pb2+ concentration (mg/L)
10 20 30
17.09 23.41 30.32
0.0092 0.0116 0.0053
0.9989 0.9999 0.9987
Fig. 6. Desorption of Pb2+ from CNTs by adjusting the pH values of the solution using HCl and HNO3 solutions.
Fig. 4. Effect of contact time on Pb2+ absorption rate for different concentrations (pH ¼ 5.0, at 298 K).
when the pH value of the solution is reduced using HCl and HNO3. The desorption percentages for the two acids are ca. zero at pH ¼ 5.4, increases sharply at pH ¼ 4.0, and eventually reaches 100% at pH ¼ 2.0. These results show that the Pb2+ adsorbed by CNTs can easily be desorbed, thus the CNTs can be employed repeatedly in heavy metal adsorption.
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4. Conclusion Adsorption thermodynamics and kinetics of Pb2+ on CNTS have been studied. A positive value of the standard enthalpy change suggests that the interaction of Pb2+ adsorbed by CNTs is endothermic. The negative adsorption standard free energy changes and positive standard entropy changes indicate that the adsorption reaction is spontaneous process. The kinetic adsorption process can be well described by the pseudosecond-order rate model. Pb2+ can be easily desorbed from CNTs by adjusting the solution pH values, thus CNTs exhibit promising application potentials as an adsorbent in wastewater treatment. Acknowledgements This work was funded by the State Key Project for the Fundamental Research of MOST of China (Grant No. 20000264-04) and by the open funds of State Key Laboratory of Environmental Aquatic Chemistry. References Benguella, B., Benaissa, H., 2002. Cadmium removal from aqueous solutions by chitin: kinetic and equilibrium studies. Water Res. 36, 2463–2474. Collins, P.G., Zettl, A., Bando, H., Thess, A., Smalley, R.E., 1997. Nanotube nanodevice. Science 278, 100–103. Daifullah, A.A.M., Girgis, B.S., Gad, H.M.H., 2004. A study of the factors affecting the removal of humic acid by activated carbon prepared from biomass material. Colloid Surf. A: Physicochem. Eng. Aspects 235, 1–10. Dillon, A.C., Jones, K.M., Bekkedahl, T.A., Kiang, C.H., Bethune, D.S., Heben, M.J., 1997. Storage of hydrogen in single-walled carbon nanotubes. Nature 386, 377–379. Friberg, L., Nordberg, G.F., Vouk, B., 1979. Handbook on the Toxicology of Metals. Elsevier, North-Holland, Biomedical Press, Amsterdam. Iijima, S., 1991. Helical microtubules of graphitic carbon. Nature 354, 56–58. Lee, M.Y., Shin, H.J., Lee, S.H., Park, J.M., Yang, J.W., 1998. Removal of lead in a ﬁxed-bed column packed with activated carbon and crab shell. Sep. Sci. Technol. 33, 1043–1056.
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