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Competitive adsorption of Pb2+, Cu2+ and Cd2+ ions from aqueous solutions by multiwalled carbon nanotubes
Carbon 41 (2003) 2787–2792
Competitive adsorption of Pb 21 , Cu 21 and Cd 21 ions from aqueous solutions by multiwalled carbon nanotubes Yan-Hui Li a , *, Jun Ding a , Zhaokun Luan b , Zechao Di a , Yuefeng Zhu a , Cailu Xu a , Dehai Wu a , Bingqing Wei c a
Department of Mechanical Engineering, State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China b State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China c Department of Materials Science & Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Received 15 February 2003; accepted 12 August 2003
Abstract The individual and competitive adsorption capacities of Pb 21 , Cu 21 and Cd 21 by nitric acid treated multiwalled carbon nanotubes (CNTs) were studied. The maximum sorption capacities calculated by applying the Langmuir equation to single ion adsorption isotherms were 97.08 mg / g for Pb 21 , 24.49 mg / g for Cu 21 and 10.86 mg / g for Cd 21 at an equilibrium concentration of 10 mg / l. The competitive adsorption studies showed that the affinity order of three metal ions adsorbed by CNTs is Pb 21 .Cu 21 .Cd 21 . The Langmuir adsorption model can represent experimental data of Pb 21 and Cu 21 well, but does not provide a good fit for Cd 21 adsorption data. The effects of solution pH, ionic strength and CNT dosage on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 ions were investigated. The comparison of CNTs with other adsorbents suggests that CNTs have great potential applications in environmental protection regardless of their higher cost at present. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Oxidation; C. Adsorption; D. Functional groups
1. Introduction There has been increasing concern and more stringent regulation standards pertaining to the discharge of heavy metals to the aquatic environment, due to their toxicity and detriment to living species including humans. Metals are non-degradable and can accumulate in living tissues, so they must be removed from polluted water. Many treatment processes, such as chemical precipitation, evaporation, ion-exchange, adsorption, electrodialysis and reverse osmosis, are currently used [1]. Among these methods, adsorption is a promising and widely applied method due to its cost-effectiveness. A number of materials, including activated carbon (AC) [2,3], activated carbon cloths [4], fly ash [5], chitin [6], prawn shell [7], peanut hull pellets [8] and resins [9], have *Corresponding author. Tel.: 186-106-277-3641; fax: 186106-278-2413. E-mail address: [email protected] (Y.-H. Li).
been reported to be capable of adsorbing heavy metals from aqueous solutions. But efforts dedicated to exploring new effective adsorbents continue to grow. Carbon nanotubes, a new member in carbon family, were first reported following arc-discharge synthesis of C 60 by Iijima in 1991 [10]. These tubes consist of rolled up graphene sheets, which can be present as single-walled (SWCNT) or multiwalled (MWCNT) depending on their preparation conditions. They have been predicted and experimentally proven to possess exceptional mechanical properties, unique electrical properties, high chemical and thermal stability and a large specific surface area, and thus have attracted great attention in latent applications such as composite reinforcement [11], field emission [12], nanodevices [13], gas adsorption [14] and as catalyst supports [15]. One of these applications, hydrogen adsorption, has aroused considerable interest due to its potential to reduce problems related to the energy crisis. Dillon et al. reported that a gravimetric hydrogen storage density of 5–10 wt% in SWCNT was reached under H 2
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00392-0
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pressure at 300 Torr for 10 min at 277 K followed by 3 min at 133 K [14]. Li et al. showed that CNT supported alumina was a good anion adsorbent whose fluoride adsorption capacity was 15–25 times higher than that of an activated carbon [16]. CNTs oxidized with nitric acid also had a higher cation adsorption capacity; for example, their lead (II) and cadmium (II) adsorption capacities were 15.6 and 3.6 mg / g at metal ion equilibrium concentrations of 2.7 mg / l [17,18]. In this work, the adsorption of Pb 21 , Cu 21 and Cd 21 from single and ternary solutions by CNTs was studied. The effects of pH, ionic strength and CNT dosage on adsorption capacities, and the cost compared with other adsorbents, are discussed.
2. Experimental Multiwalled CNTs were prepared by dissociating methane in a hydrogen flow at 900 K using Ni nanoparticles supported on diatomites as catalysts. The as-prepared CNTs were immersed in hydrofluoric acid and concentrated nitric acid for 24 h, respectively, to dissolve the catalyst support and Ni particles [19] and then were dispersed in a mixture of concentrated nitric acid and refluxed for 1 h at 413 K. The treated CNTs were washed repeatedly using hot distilled water until the solution reached a pH value of 7. The purity of the oxidized CNTs is about 95% by volume. Analytical grade lead nitrate, copper chloride and cadmium chloride were used to prepare stock solutions of 1000 mg / l of the three metal ions, which were further diluted to the required concentrations before use. All experiments were carried out at room temperature. In single metal ion adsorption experiments, 0.05 g of treated CNTs were placed into 100-ml solutions with initial concentrations of Pb 21 from 10 to 60 mg / l, Cu 21 from 5 to 30 mg / l and Cd 21 from 2 to 15 mg / l, respectively. The initial and final pH values of the solution were adjusted with HNO 3 and NaOH solution and kept at 5.0. After the suspensions were shaken for 4 h, they were filtered through 0.45-mm membrane filters. The filtrates were immediately analyzed on an atomic absorption spectrometer. The amounts of metal ions adsorbed on CNTs were calculated by subtracting the equilibrium ion contents from the initial ion contents. In order to investigate the competitive adsorption of Pb 21 , Cu 21 and Cd 21 by CNTs, 0.1 g of acid oxidized CNTs were added into 100-ml solutions with equal initial concentrations of the three heavy metal ions from 5 to 30 mg / l, with other experimental details being similar to the above description. The same experimental conditions were adopted to study the influence of ionic strength on heavy metal ion competitive adsorption. The ionic strengths of the solutions were regulated at 0.01, 0.05 and 0.1 M, respectively, using NaNO 3 solution.
To study the effect of pH on competitive adsorption, 0.1 g of treated CNTs were dispersed into 100-ml solutions containing 30 mg / l of each of the heavy metal ions. The initial pH values of the solutions were adjusted from 2.3 to 11.0 using various concentrations of nitric acid and sodium hydroxide. The CNT dosage effects on heavy metal ion adsorption were carried out by adding 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3 g of treated CNTs to 100 ml solution with individual heavy metal ion concentrations of 30 mg / l at a pH value of 5.0.
3. Results and discussion Transmission electron microscopy images show that as-grown CNTs usually curve and entangle around each other with inner diameters of 6–12 nm, outer diameters of 20–30 nm, and lengths ranging from hundreds of nanometers to micrometers. After oxidation with nitric acid, the tips of CNTs were opened and fracture took place at positions where defects such as pentagons and heptagons existed. Hence oxidation improved the dispersivity and specific surface area of the CNTs. The most important effect of oxidation on CNTs was the large increase in the amount of functional groups on the surfaces of CNTs, which can cause an increase in their cation exchange capacity [17]. Single metal ion equilibrium sorption studies were conducted to investigate the maximum metal adsorption capacities of CNTs. It was observed from Fig. 1 that the amount of adsorbed Pb 21 and Cu 21 increased significantly in the range of low concentrations, then it increased gradually and reached 82 mg / g for Pb 21 and 29 mg / g for Cu 21 at an equilibrium concentration of 10 mg / l. The amount of adsorbed Cd 21 is the lowest in the whole range
Fig. 1. Adsorption isotherms for single ions of Pb 21 , Cu 21 and Cd 21 onto CNTs at room temperature and pH 5.0.
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of experimental equilibrium concentrations and attains 9.2 mg / g at an equilibrium concentration of 10 mg / l. At these conditions (pH 5), the adsorption capabilities of CNTs for the three heavy metal ions are in the order of Pb 21 . Cu 21 .Cd 21 . The experimental data for Pb 21 , Cu 21 and Cd 21 adsorption by CNTs were analyzed using the linear Langmuir sorption isotherm model as 1 1 1 ] 5 ] 1 ]] q qm bqm Ce where Ce is the equilibrium metal ion concentration (mg / l), q is the amount adsorbed (mg / g), qm is the maximum sorption capacity corresponding to complete monolayer coverage (mg / g) and b the Langmuir constant indirectly related to the energy of adsorption (l / mg). The Langmuir parameters were calculated and are listed in Table 1. The correlation coefficients, r 2 , are high and close to 1, suggesting that the experimental data can be represented by the Langmuir sorption model and the three adsorbed metal ions form a monolayer coverage on the surfaces of CNTs. The qm values calculated by the Langmuir equation are 97.08, 28.49 and 10.86 mg / g for Pb 21 , Cu 21 and Cd 21 , which follow the same sequence of Pb 21 .Cu 21 .Cd 21 . The competitive adsorption experiments of Pb 21 , Cu 21 and Cd 21 by CNTs were carried out by adding 0.1 g of CNTs into solutions with initial concentrations of the three heavy metal ions ranging from 5 to 30 mg / l. CNTs adsorbed Pb 21 , Cu 21 and Cd 21 ions simultaneously and showed the affinity in the order of Pb 21 .Cu 21 .Cd 21 (see Fig. 2), so the adsorption capacity of Pb 21 increased sharply and attained 27.6 mg / g at a Pb 21 equilibrium concentration of 2.4 mg / l. In comparison, it was only 17.6 mg / g for Cu 21 at a Cu 21 equilibrium concentration of 12.4 mg / l. The affinity of Cd 21 was the least among three heavy metal ions and the adsorption capacity attained the maximum of 7.1 mg / g at a Cd 21 equilibrium concentration of 2.9 mg / l, and then it decreased with increasing Cd 21 equilibrium concentration. The phenomenon of the decreased Cd 21 adsorption capacity may be attributed to the active adsorption sites on CNTs occupied mostly by Pb 21 and Cu 21 ions due to their affinity being higher than that of the Cd 21 ion. The linear Langmuir sorption isotherm model was used to fit the competitive adsorption
Fig. 2. Competitive adsorption isotherms for three ions of Pb 21 , Cu 21 and Cd 21 onto CNTs at room temperature and pH 5.0.
data. It can be seen in Table 1 that the experimental data for Pb 21 and Cu 21 ion adsorption can be well represented by the Langmuir equation with r 2 of 0.9672 and 0.9667, respectively. In contrast, for Cd 21 ion adsorption r 2 is only 0.2053, so the Langmuir monolayer adsorption model cannot be used to interpret Cd 21 ion adsorption under competitive adsorption conditions. The ionic strengths of 0.01, 0.05 and 0.1 M were chosen to investigate their effect on the Pb 21 , Cu 21 and Cd 21 ion adsorption by CNTs. Fig. 3 shows that the adsorption capacities of Pb 21 , Cu 21 and Cd 21 ions by CNTs decreased with increasing ionic strength. This phenomenon can be attributed to two factors. First, the Pb 21 , Cu 21 and Cd 21 ions form electrical double layer complexes with the CNTs, which favor the adsorption when the concentration of the competing salt is decreased. This might indicate that the adsorption interaction between the functional groups of the adsorbent and the metal cations was mainly ionic in nature, which is consistent with an ion-exchange mechanism. The second factor is the influence of the ionic strength on the activity coefficients of Pb 21 , Cu 21 and Cd 21 ions, which limit their transfer to the CNT surfaces [20]. The effect of pH on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 by CNTs is shown in Fig. 4. It can been
Table 1 Langmuir parameters of single and competitive adsorption isotherm models for Pb 21 , Cu 21 and Cd 21 ions onto CNTs Type of ions
Pb 21 Cu 21 Cd 21
Single ion adsorption
Competitive ion adsorption 2
qm (mg / g)
B (l / mg)
r
97.08 28.49 10.86
1.51 3.82 0.29
0.9873 0.9821 0.993
qm (mg / g)
b (l / mg)
r2
34.01 17.04 3.3
1.84 4.28 26.26
0.9672 0.9667 0.2053
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seen that at low pH the adsorption percentages are negligible. In between pH 1.8 and 6 the percentages of Pb 21 and Cu 21 increase sharply and almost attain values of 100%, while only a small increase is noted for Cd 21 . The higher adsorption percentages of Pb 21 and Cu 21 at lower pH are mainly due to the ionizable surface charge of oxidized CNTs. The zeta potential, which is usually used to quantify the adsorbent surface charge, was measured
and showed that the isoelectric point of the oxidized CNTs shifts to a lower pH value compared with that of as-grown CNTs [18] due to the higher surface functionalization by acidic groups introduced by nitric acid oxidation. The negatively charged surfaces of oxidized CNT offer electrostatic attractions which are favorable for adsorbing Pb 21 , Cu 21 and Cd 21 ions. As the affinities of Pb 21 and Cu 21 by CNTs are stronger than that of Cd 21 , the Pb 21 and Cu 21 ions are adsorbed preferentially, which lead to the lower adsorption percentage of Cd 21 at lower pH values. In the pH range from 6 to 11 the adsorption percentage of Cd 21 increases rapidly due to the combined role of adsorption and precipitation. Fig. 5 shows the effect of the CNT dosage on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 by CNTs. The adsorption capacities of all three heavy metal ions increase with increasing CNT dosages. The adsorption percentages of Pb 21 , Cu 21 and Cd 21 are 56.1, 23.7 and 1.3%, respectively, at a CNT dosage of 0.05 g. The former two reached almost 100% adsorption at a dosage of CNTs of 0.3 g. The increase of Cd 21 adsorption percentage was slow at first and attained 75.4% at a CNT dosage of 0.3 g. At that point, Pb 21 and Cu 21 ions are almost adsorbed completely and there are more vacant active adsorption 21 sites used for Cd sorption. In other words, the affinities 21 21 21 of CNTs for Pb , Cu and Cd follow the order of 21 21 21 Pb .Cu .Cd . 21 21 21 The adsorption capacities of Pb , Cu and Cd by CNTs were compared with those of other adsorbents (see Table 2, with the data having been collected at different conditions). The three metal adsorption capacities of algae 21 were the highest among all adsorbents, and the Cd
Fig. 4. Effect of pH on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 ions onto CNTs at room temperature and initial ion concentrations of 30 mg / l.
Fig. 5. Effect of CNT dosage on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 ions onto CNTs at room temperature and initial ion concentrations of 30 mg / l.
Fig. 3. Effect of ionic strength on the competitive adsorption isotherms of Pb 21 , Cu 21 and Cd 21 ions onto CNTs at room temperature and pH 5.0.
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Table 2 Comparison of individual Pb 21 , Cu 21 and Cd 21 ion adsorption capacities of different adsorbents Pb 21 (mg / g)
Adsorbent
Cu 21 (mg / g)
Cd 21 (mg / g)
CNTs
97.08
28.49
10.86
Biosorbent Granular AC Powdered AC Fly ash
73.69 15.58 26.9
21.14 5.08 4.45 8.1
24.3 3.37 3.37 8.0
5.8
4.2
0.18
16.4 331.52 1.21
– 82.55 0.259
–
Scolecite Microbead Algae Iron-coated sand
4.3 134.88 –
adsorption capacity of the biosorbent was also higher than that of our HNO 3 oxidized CNTs. The metal removal capacities of the other adsorbents such as GAC, PAC, fly ash, scolecite, microbead and iron coated sand were lower than those of our CNTs. The comparison suggests that CNTs have great potential for use as heavy metal ion adsorbents in wastewater treatment. The most important factor currently limiting the use of CNTs in practical environmental protection applications is their high cost. Like most new materials, carbon nanotubes are expensive, with costs usually being higher than $1 / g (see Table 3), which are 1000 times greater than those of typical ACs. Many methods of preparing CNTs have been used, including arc discharge, laser ablation and chemical vapor deposition (CVD), and each method has its advantages and defects. The two former methods can prepare CNTs of high quality, but their production quantities are relatively low. The third method (CVD) can produce CNTs in larger batches (despite containing a large number
Conditions
Refs.
pH 5.0; room temperature pH 4.7; 293 K pH 5.0; 303 K pH 5.0; 303 K pH 5.0; room temperature pH 6.0; room temperature pH 6.8; 293 K pH 5.0; 294 K pH 6.0; 298 K
Our data in the paper [20] [20] [20] [21] [22] [23] [24] [25]
of defects), and is deemed to be a promising route to reduce the CNT cost in the future, which would increase the use of CNTs in environmental protection applications.
4. Conclusion The single and competitive adsorption of Pb 21 , Cu 21 and Cd 21 by CNTs was studied, and two kinds of experimental data sets showed that the adsorption affinity of Pb 21 , Cu 21 and Cd 21 by CNTs followed the order Pb 21 .Cu 21 .Cd 21 . The competitive adsorption capacities of the three metal ions increased with increasing pH and CNT dosage and decreased with increasing ionic strength. Although HNO 3 oxidized CNTs show higher adsorption capacities for heavy metal ions compared with other adsorbents and would find many uses in environmental protection applications, their high cost currently limits their practical use.
Table 3 Comparison of price and other data of MWCNT with AC offered by some corporations Adsorbent
Method
Diameter or size (nm)
Output
Purity (%)
Price
Source
MWCNT
CVD
10–40
10 kg / day
95
$20 / g
MWCNT
CVD
20–30
4 kg / day
.80
$2 / g
MWCNT
2–15
–
10–40
$14 / g
Bamboo AC
Arc process –
2–5 mm
1000 kg / year
87 wt%
$1 / kg
Granular AC
–
4–12 mm
2000 kg / year
–
$0.67 / kg
http: / / www. gzenergy.com http: / / www. sunnano.com http: / / www. mercorp.com http: / / www. tuttglobal.com http: / / www. acticarbon.com
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Acknowledgements This work was financially supported under the State Key Program for Fundament Research of MOST, China, Grant no. G20000264-04.
References [1] An HK, Park BY, Kim DS. Water Res 2001;35:3551–6. [2] Gabaldon C, Marzal P, Ferrer J, Seco A. Water Res 1996;30:3050–60. [3] Faur-Brasquet C, Kadirvelu K, Le Cloirec P. Carbon 2002;40:2387–92. [4] Bayat B. J Hazard Mater 2002;95:251–73. [5] Kadirvelu K, Faur-Brasquet C, Le Cloirec P. Langmuir 2000;16:8404–9. [6] Benguella B, Benaissa H. Water Res 2002;36:2463–74. [7] Chu KH. J Hazard Mater 2002;90:77–95. [8] Brown P, Jefcoat IA, Parrish D, Gill S, Graham E. Adv Environ Res 2000;4:19–29. [9] Diniz CV, Doyle FM, Ciminelli VST. Sep Sci Technol 2002;37:3169–85. [10] Iijima S. Nature 1991;354:56–8. [11] Wagner HD, Lourie O, Feldman Y, Tenne R. Appl Phys Lett 1998;72:188–90. [12] Wang QH, Setlur AA, Lauerhaas JM, Dai JY, Seelig EW, Chang RPH. Appl Phys Lett 1998;72:2912–3.
[13] Collins PG, Zettl A, Bando H, Thess A, Smalley RE. Science 1997;278:100–3. [14] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Nature 1997;386:377–9. [15] Planeix JM, Coustel N, Coq B, Brotons V, Kumbhar PS, Futartre R, Geneste P, Bernier P, Ajayan PM. J Am Chem Soc 1994;116:7935–6. [16] Li Y, Wang S, Cao A, Zhao D, Zhang X, Xu C, Luan Z, Ruan D, Liang J, Wu D, Wei B. Chem Phys Lett 2001;350:412–6. [17] Li Y, Wang S, Wei J, Zhang X, Xu C, Luan Z, Wu D, Wei B. Chem Phys Lett 2002;357:263–6. [18] Li Y, Wang S, Luan Z, Ding J, Xu C, Wu D. Carbon 2003;41(5):1057–62. [19] Li Y, Xu C, Wei B, Zhang X, Zheng M, Wu D, Ajayan P. Chem Mater 2002;14:483–5. [20] Reddad Z, Gerente C, Andres Y, Le Cloirec P. Environ Sci Technol 2002;36:2067–73. [21] Ayala J, Blanco F, Garcia P, Rodriguez P, Sancho J. Fuel 1998;77:1147–54. [22] Bosso ST, Enzweiler J. Water Res 2002;36:4795–800. [23] Salih B, Denizli A, Kavakli C, Say R, Piskin E. Talanta 1998;46:1205–13. [24] Yu QM, Matheickal JT, Yin PH, Kaewsarn P. Water Res 1999;33:1534–7. [25] Lai CH, Chen CY. Chemosphere 2001;44:1177–84.
Competitive adsorption of Pb 21 , Cu 21 and Cd 21 ions from aqueous solutions by multiwalled carbon nanotubes Yan-Hui Li a , *, Jun Ding a , Zhaokun Luan b , Zechao Di a , Yuefeng Zhu a , Cailu Xu a , Dehai Wu a , Bingqing Wei c a
Department of Mechanical Engineering, State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China b State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China c Department of Materials Science & Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Received 15 February 2003; accepted 12 August 2003
Abstract The individual and competitive adsorption capacities of Pb 21 , Cu 21 and Cd 21 by nitric acid treated multiwalled carbon nanotubes (CNTs) were studied. The maximum sorption capacities calculated by applying the Langmuir equation to single ion adsorption isotherms were 97.08 mg / g for Pb 21 , 24.49 mg / g for Cu 21 and 10.86 mg / g for Cd 21 at an equilibrium concentration of 10 mg / l. The competitive adsorption studies showed that the affinity order of three metal ions adsorbed by CNTs is Pb 21 .Cu 21 .Cd 21 . The Langmuir adsorption model can represent experimental data of Pb 21 and Cu 21 well, but does not provide a good fit for Cd 21 adsorption data. The effects of solution pH, ionic strength and CNT dosage on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 ions were investigated. The comparison of CNTs with other adsorbents suggests that CNTs have great potential applications in environmental protection regardless of their higher cost at present. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Oxidation; C. Adsorption; D. Functional groups
1. Introduction There has been increasing concern and more stringent regulation standards pertaining to the discharge of heavy metals to the aquatic environment, due to their toxicity and detriment to living species including humans. Metals are non-degradable and can accumulate in living tissues, so they must be removed from polluted water. Many treatment processes, such as chemical precipitation, evaporation, ion-exchange, adsorption, electrodialysis and reverse osmosis, are currently used [1]. Among these methods, adsorption is a promising and widely applied method due to its cost-effectiveness. A number of materials, including activated carbon (AC) [2,3], activated carbon cloths [4], fly ash [5], chitin [6], prawn shell [7], peanut hull pellets [8] and resins [9], have *Corresponding author. Tel.: 186-106-277-3641; fax: 186106-278-2413. E-mail address: [email protected] (Y.-H. Li).
been reported to be capable of adsorbing heavy metals from aqueous solutions. But efforts dedicated to exploring new effective adsorbents continue to grow. Carbon nanotubes, a new member in carbon family, were first reported following arc-discharge synthesis of C 60 by Iijima in 1991 [10]. These tubes consist of rolled up graphene sheets, which can be present as single-walled (SWCNT) or multiwalled (MWCNT) depending on their preparation conditions. They have been predicted and experimentally proven to possess exceptional mechanical properties, unique electrical properties, high chemical and thermal stability and a large specific surface area, and thus have attracted great attention in latent applications such as composite reinforcement [11], field emission [12], nanodevices [13], gas adsorption [14] and as catalyst supports [15]. One of these applications, hydrogen adsorption, has aroused considerable interest due to its potential to reduce problems related to the energy crisis. Dillon et al. reported that a gravimetric hydrogen storage density of 5–10 wt% in SWCNT was reached under H 2
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00392-0
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pressure at 300 Torr for 10 min at 277 K followed by 3 min at 133 K [14]. Li et al. showed that CNT supported alumina was a good anion adsorbent whose fluoride adsorption capacity was 15–25 times higher than that of an activated carbon [16]. CNTs oxidized with nitric acid also had a higher cation adsorption capacity; for example, their lead (II) and cadmium (II) adsorption capacities were 15.6 and 3.6 mg / g at metal ion equilibrium concentrations of 2.7 mg / l [17,18]. In this work, the adsorption of Pb 21 , Cu 21 and Cd 21 from single and ternary solutions by CNTs was studied. The effects of pH, ionic strength and CNT dosage on adsorption capacities, and the cost compared with other adsorbents, are discussed.
2. Experimental Multiwalled CNTs were prepared by dissociating methane in a hydrogen flow at 900 K using Ni nanoparticles supported on diatomites as catalysts. The as-prepared CNTs were immersed in hydrofluoric acid and concentrated nitric acid for 24 h, respectively, to dissolve the catalyst support and Ni particles [19] and then were dispersed in a mixture of concentrated nitric acid and refluxed for 1 h at 413 K. The treated CNTs were washed repeatedly using hot distilled water until the solution reached a pH value of 7. The purity of the oxidized CNTs is about 95% by volume. Analytical grade lead nitrate, copper chloride and cadmium chloride were used to prepare stock solutions of 1000 mg / l of the three metal ions, which were further diluted to the required concentrations before use. All experiments were carried out at room temperature. In single metal ion adsorption experiments, 0.05 g of treated CNTs were placed into 100-ml solutions with initial concentrations of Pb 21 from 10 to 60 mg / l, Cu 21 from 5 to 30 mg / l and Cd 21 from 2 to 15 mg / l, respectively. The initial and final pH values of the solution were adjusted with HNO 3 and NaOH solution and kept at 5.0. After the suspensions were shaken for 4 h, they were filtered through 0.45-mm membrane filters. The filtrates were immediately analyzed on an atomic absorption spectrometer. The amounts of metal ions adsorbed on CNTs were calculated by subtracting the equilibrium ion contents from the initial ion contents. In order to investigate the competitive adsorption of Pb 21 , Cu 21 and Cd 21 by CNTs, 0.1 g of acid oxidized CNTs were added into 100-ml solutions with equal initial concentrations of the three heavy metal ions from 5 to 30 mg / l, with other experimental details being similar to the above description. The same experimental conditions were adopted to study the influence of ionic strength on heavy metal ion competitive adsorption. The ionic strengths of the solutions were regulated at 0.01, 0.05 and 0.1 M, respectively, using NaNO 3 solution.
To study the effect of pH on competitive adsorption, 0.1 g of treated CNTs were dispersed into 100-ml solutions containing 30 mg / l of each of the heavy metal ions. The initial pH values of the solutions were adjusted from 2.3 to 11.0 using various concentrations of nitric acid and sodium hydroxide. The CNT dosage effects on heavy metal ion adsorption were carried out by adding 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3 g of treated CNTs to 100 ml solution with individual heavy metal ion concentrations of 30 mg / l at a pH value of 5.0.
3. Results and discussion Transmission electron microscopy images show that as-grown CNTs usually curve and entangle around each other with inner diameters of 6–12 nm, outer diameters of 20–30 nm, and lengths ranging from hundreds of nanometers to micrometers. After oxidation with nitric acid, the tips of CNTs were opened and fracture took place at positions where defects such as pentagons and heptagons existed. Hence oxidation improved the dispersivity and specific surface area of the CNTs. The most important effect of oxidation on CNTs was the large increase in the amount of functional groups on the surfaces of CNTs, which can cause an increase in their cation exchange capacity [17]. Single metal ion equilibrium sorption studies were conducted to investigate the maximum metal adsorption capacities of CNTs. It was observed from Fig. 1 that the amount of adsorbed Pb 21 and Cu 21 increased significantly in the range of low concentrations, then it increased gradually and reached 82 mg / g for Pb 21 and 29 mg / g for Cu 21 at an equilibrium concentration of 10 mg / l. The amount of adsorbed Cd 21 is the lowest in the whole range
Fig. 1. Adsorption isotherms for single ions of Pb 21 , Cu 21 and Cd 21 onto CNTs at room temperature and pH 5.0.
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of experimental equilibrium concentrations and attains 9.2 mg / g at an equilibrium concentration of 10 mg / l. At these conditions (pH 5), the adsorption capabilities of CNTs for the three heavy metal ions are in the order of Pb 21 . Cu 21 .Cd 21 . The experimental data for Pb 21 , Cu 21 and Cd 21 adsorption by CNTs were analyzed using the linear Langmuir sorption isotherm model as 1 1 1 ] 5 ] 1 ]] q qm bqm Ce where Ce is the equilibrium metal ion concentration (mg / l), q is the amount adsorbed (mg / g), qm is the maximum sorption capacity corresponding to complete monolayer coverage (mg / g) and b the Langmuir constant indirectly related to the energy of adsorption (l / mg). The Langmuir parameters were calculated and are listed in Table 1. The correlation coefficients, r 2 , are high and close to 1, suggesting that the experimental data can be represented by the Langmuir sorption model and the three adsorbed metal ions form a monolayer coverage on the surfaces of CNTs. The qm values calculated by the Langmuir equation are 97.08, 28.49 and 10.86 mg / g for Pb 21 , Cu 21 and Cd 21 , which follow the same sequence of Pb 21 .Cu 21 .Cd 21 . The competitive adsorption experiments of Pb 21 , Cu 21 and Cd 21 by CNTs were carried out by adding 0.1 g of CNTs into solutions with initial concentrations of the three heavy metal ions ranging from 5 to 30 mg / l. CNTs adsorbed Pb 21 , Cu 21 and Cd 21 ions simultaneously and showed the affinity in the order of Pb 21 .Cu 21 .Cd 21 (see Fig. 2), so the adsorption capacity of Pb 21 increased sharply and attained 27.6 mg / g at a Pb 21 equilibrium concentration of 2.4 mg / l. In comparison, it was only 17.6 mg / g for Cu 21 at a Cu 21 equilibrium concentration of 12.4 mg / l. The affinity of Cd 21 was the least among three heavy metal ions and the adsorption capacity attained the maximum of 7.1 mg / g at a Cd 21 equilibrium concentration of 2.9 mg / l, and then it decreased with increasing Cd 21 equilibrium concentration. The phenomenon of the decreased Cd 21 adsorption capacity may be attributed to the active adsorption sites on CNTs occupied mostly by Pb 21 and Cu 21 ions due to their affinity being higher than that of the Cd 21 ion. The linear Langmuir sorption isotherm model was used to fit the competitive adsorption
Fig. 2. Competitive adsorption isotherms for three ions of Pb 21 , Cu 21 and Cd 21 onto CNTs at room temperature and pH 5.0.
data. It can be seen in Table 1 that the experimental data for Pb 21 and Cu 21 ion adsorption can be well represented by the Langmuir equation with r 2 of 0.9672 and 0.9667, respectively. In contrast, for Cd 21 ion adsorption r 2 is only 0.2053, so the Langmuir monolayer adsorption model cannot be used to interpret Cd 21 ion adsorption under competitive adsorption conditions. The ionic strengths of 0.01, 0.05 and 0.1 M were chosen to investigate their effect on the Pb 21 , Cu 21 and Cd 21 ion adsorption by CNTs. Fig. 3 shows that the adsorption capacities of Pb 21 , Cu 21 and Cd 21 ions by CNTs decreased with increasing ionic strength. This phenomenon can be attributed to two factors. First, the Pb 21 , Cu 21 and Cd 21 ions form electrical double layer complexes with the CNTs, which favor the adsorption when the concentration of the competing salt is decreased. This might indicate that the adsorption interaction between the functional groups of the adsorbent and the metal cations was mainly ionic in nature, which is consistent with an ion-exchange mechanism. The second factor is the influence of the ionic strength on the activity coefficients of Pb 21 , Cu 21 and Cd 21 ions, which limit their transfer to the CNT surfaces [20]. The effect of pH on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 by CNTs is shown in Fig. 4. It can been
Table 1 Langmuir parameters of single and competitive adsorption isotherm models for Pb 21 , Cu 21 and Cd 21 ions onto CNTs Type of ions
Pb 21 Cu 21 Cd 21
Single ion adsorption
Competitive ion adsorption 2
qm (mg / g)
B (l / mg)
r
97.08 28.49 10.86
1.51 3.82 0.29
0.9873 0.9821 0.993
qm (mg / g)
b (l / mg)
r2
34.01 17.04 3.3
1.84 4.28 26.26
0.9672 0.9667 0.2053
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seen that at low pH the adsorption percentages are negligible. In between pH 1.8 and 6 the percentages of Pb 21 and Cu 21 increase sharply and almost attain values of 100%, while only a small increase is noted for Cd 21 . The higher adsorption percentages of Pb 21 and Cu 21 at lower pH are mainly due to the ionizable surface charge of oxidized CNTs. The zeta potential, which is usually used to quantify the adsorbent surface charge, was measured
and showed that the isoelectric point of the oxidized CNTs shifts to a lower pH value compared with that of as-grown CNTs [18] due to the higher surface functionalization by acidic groups introduced by nitric acid oxidation. The negatively charged surfaces of oxidized CNT offer electrostatic attractions which are favorable for adsorbing Pb 21 , Cu 21 and Cd 21 ions. As the affinities of Pb 21 and Cu 21 by CNTs are stronger than that of Cd 21 , the Pb 21 and Cu 21 ions are adsorbed preferentially, which lead to the lower adsorption percentage of Cd 21 at lower pH values. In the pH range from 6 to 11 the adsorption percentage of Cd 21 increases rapidly due to the combined role of adsorption and precipitation. Fig. 5 shows the effect of the CNT dosage on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 by CNTs. The adsorption capacities of all three heavy metal ions increase with increasing CNT dosages. The adsorption percentages of Pb 21 , Cu 21 and Cd 21 are 56.1, 23.7 and 1.3%, respectively, at a CNT dosage of 0.05 g. The former two reached almost 100% adsorption at a dosage of CNTs of 0.3 g. The increase of Cd 21 adsorption percentage was slow at first and attained 75.4% at a CNT dosage of 0.3 g. At that point, Pb 21 and Cu 21 ions are almost adsorbed completely and there are more vacant active adsorption 21 sites used for Cd sorption. In other words, the affinities 21 21 21 of CNTs for Pb , Cu and Cd follow the order of 21 21 21 Pb .Cu .Cd . 21 21 21 The adsorption capacities of Pb , Cu and Cd by CNTs were compared with those of other adsorbents (see Table 2, with the data having been collected at different conditions). The three metal adsorption capacities of algae 21 were the highest among all adsorbents, and the Cd
Fig. 4. Effect of pH on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 ions onto CNTs at room temperature and initial ion concentrations of 30 mg / l.
Fig. 5. Effect of CNT dosage on the competitive adsorption of Pb 21 , Cu 21 and Cd 21 ions onto CNTs at room temperature and initial ion concentrations of 30 mg / l.
Fig. 3. Effect of ionic strength on the competitive adsorption isotherms of Pb 21 , Cu 21 and Cd 21 ions onto CNTs at room temperature and pH 5.0.
Y.-H. Li et al. / Carbon 41 (2003) 2787–2792
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Table 2 Comparison of individual Pb 21 , Cu 21 and Cd 21 ion adsorption capacities of different adsorbents Pb 21 (mg / g)
Adsorbent
Cu 21 (mg / g)
Cd 21 (mg / g)
CNTs
97.08
28.49
10.86
Biosorbent Granular AC Powdered AC Fly ash
73.69 15.58 26.9
21.14 5.08 4.45 8.1
24.3 3.37 3.37 8.0
5.8
4.2
0.18
16.4 331.52 1.21
– 82.55 0.259
–
Scolecite Microbead Algae Iron-coated sand
4.3 134.88 –
adsorption capacity of the biosorbent was also higher than that of our HNO 3 oxidized CNTs. The metal removal capacities of the other adsorbents such as GAC, PAC, fly ash, scolecite, microbead and iron coated sand were lower than those of our CNTs. The comparison suggests that CNTs have great potential for use as heavy metal ion adsorbents in wastewater treatment. The most important factor currently limiting the use of CNTs in practical environmental protection applications is their high cost. Like most new materials, carbon nanotubes are expensive, with costs usually being higher than $1 / g (see Table 3), which are 1000 times greater than those of typical ACs. Many methods of preparing CNTs have been used, including arc discharge, laser ablation and chemical vapor deposition (CVD), and each method has its advantages and defects. The two former methods can prepare CNTs of high quality, but their production quantities are relatively low. The third method (CVD) can produce CNTs in larger batches (despite containing a large number
Conditions
Refs.
pH 5.0; room temperature pH 4.7; 293 K pH 5.0; 303 K pH 5.0; 303 K pH 5.0; room temperature pH 6.0; room temperature pH 6.8; 293 K pH 5.0; 294 K pH 6.0; 298 K
Our data in the paper [20] [20] [20] [21] [22] [23] [24] [25]
of defects), and is deemed to be a promising route to reduce the CNT cost in the future, which would increase the use of CNTs in environmental protection applications.
4. Conclusion The single and competitive adsorption of Pb 21 , Cu 21 and Cd 21 by CNTs was studied, and two kinds of experimental data sets showed that the adsorption affinity of Pb 21 , Cu 21 and Cd 21 by CNTs followed the order Pb 21 .Cu 21 .Cd 21 . The competitive adsorption capacities of the three metal ions increased with increasing pH and CNT dosage and decreased with increasing ionic strength. Although HNO 3 oxidized CNTs show higher adsorption capacities for heavy metal ions compared with other adsorbents and would find many uses in environmental protection applications, their high cost currently limits their practical use.
Table 3 Comparison of price and other data of MWCNT with AC offered by some corporations Adsorbent
Method
Diameter or size (nm)
Output
Purity (%)
Price
Source
MWCNT
CVD
10–40
10 kg / day
95
$20 / g
MWCNT
CVD
20–30
4 kg / day
.80
$2 / g
MWCNT
2–15
–
10–40
$14 / g
Bamboo AC
Arc process –
2–5 mm
1000 kg / year
87 wt%
$1 / kg
Granular AC
–
4–12 mm
2000 kg / year
–
$0.67 / kg
http: / / www. gzenergy.com http: / / www. sunnano.com http: / / www. mercorp.com http: / / www. tuttglobal.com http: / / www. acticarbon.com
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Acknowledgements This work was financially supported under the State Key Program for Fundament Research of MOST, China, Grant no. G20000264-04.
References [1] An HK, Park BY, Kim DS. Water Res 2001;35:3551–6. [2] Gabaldon C, Marzal P, Ferrer J, Seco A. Water Res 1996;30:3050–60. [3] Faur-Brasquet C, Kadirvelu K, Le Cloirec P. Carbon 2002;40:2387–92. [4] Bayat B. J Hazard Mater 2002;95:251–73. [5] Kadirvelu K, Faur-Brasquet C, Le Cloirec P. Langmuir 2000;16:8404–9. [6] Benguella B, Benaissa H. Water Res 2002;36:2463–74. [7] Chu KH. J Hazard Mater 2002;90:77–95. [8] Brown P, Jefcoat IA, Parrish D, Gill S, Graham E. Adv Environ Res 2000;4:19–29. [9] Diniz CV, Doyle FM, Ciminelli VST. Sep Sci Technol 2002;37:3169–85. [10] Iijima S. Nature 1991;354:56–8. [11] Wagner HD, Lourie O, Feldman Y, Tenne R. Appl Phys Lett 1998;72:188–90. [12] Wang QH, Setlur AA, Lauerhaas JM, Dai JY, Seelig EW, Chang RPH. Appl Phys Lett 1998;72:2912–3.
[13] Collins PG, Zettl A, Bando H, Thess A, Smalley RE. Science 1997;278:100–3. [14] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Nature 1997;386:377–9. [15] Planeix JM, Coustel N, Coq B, Brotons V, Kumbhar PS, Futartre R, Geneste P, Bernier P, Ajayan PM. J Am Chem Soc 1994;116:7935–6. [16] Li Y, Wang S, Cao A, Zhao D, Zhang X, Xu C, Luan Z, Ruan D, Liang J, Wu D, Wei B. Chem Phys Lett 2001;350:412–6. [17] Li Y, Wang S, Wei J, Zhang X, Xu C, Luan Z, Wu D, Wei B. Chem Phys Lett 2002;357:263–6. [18] Li Y, Wang S, Luan Z, Ding J, Xu C, Wu D. Carbon 2003;41(5):1057–62. [19] Li Y, Xu C, Wei B, Zhang X, Zheng M, Wu D, Ajayan P. Chem Mater 2002;14:483–5. [20] Reddad Z, Gerente C, Andres Y, Le Cloirec P. Environ Sci Technol 2002;36:2067–73. [21] Ayala J, Blanco F, Garcia P, Rodriguez P, Sancho J. Fuel 1998;77:1147–54. [22] Bosso ST, Enzweiler J. Water Res 2002;36:4795–800. [23] Salih B, Denizli A, Kavakli C, Say R, Piskin E. Talanta 1998;46:1205–13. [24] Yu QM, Matheickal JT, Yin PH, Kaewsarn P. Water Res 1999;33:1534–7. [25] Lai CH, Chen CY. Chemosphere 2001;44:1177–84.