CHAPTER 10 |
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Application of Carbon Nanotubes as a Solid-Phase Extraction Material for Environmental Samples
Krystyna Pyrzynska
Department of Chemistry, Warsaw University, Warsaw, Poland
Contents
1. Solid-Phase Extraction of Organic Compounds
2. Enrichment of Metallic Species
In spite of improvements in sensitivity due to modern analytical detection systems, conventional separation techniques are frequently used to overcome interferences from matrix elements and to improve detection limits through concentration of the analyte [1, 2]. Among different separation and enrichment techniques, batch and column techniques in which analytes are sorbed on different water-insoluble materials and eluted with acids or other reagents have been widely used [3]. For the materials to be beneficial in solid-phase extraction (SPE), given analytes should be easily, quantitatively, and reproducibly collected and eluted with minimum efforts exerted in experimental procedures. The SPE provides a number of important benefits in comparison with laborious classical liquid–liquid extraction, such as reduced solvent usage and exposure, low disposal costs, and short extraction times for sample preparation. Moreover, sorbent extraction can be used for selective retention of some particular chemical forms of a metal, thereby enabling speciation [4, 5].
Several materials have been proposed for SPE, such as C18-bonded silica [6, 7], polymeric sorbents [8], carbon-based materials [9], zeolites [10], or polyurethane foam [11]. These have been successfully applied for various organic compounds and metal ions in different kinds of samples. The mechanism of sorption depends on the nature of the sorbent and may include simple adsorption, ion exchange, chelation, or ion-pair formation. Adsorption occurs through van der Waals forces or hydrophobic interactions, which occur when the solid sorbent is highly nonpolar. The most common sorbents of this type include octadecyl-bonded silica and styrene-divinylbenzene copolymers that provide additional π–π interactions, when π electrons are present in the analytes. The presence of appropriate functional groups in solid matrices is needed for chelating trace elements. Binding of metal ions to these groups is dependent on nature, charge, and size of the given metal ion, the nature of the donor atoms present in the group, the nature of the solid support, and the buffering conditions. The sorbent may be packaged into different formats: filled microcolumns, cartridges, syringe barrels, or discs. The choice of specific sorbent should be based on analyte, sample matrix, and technique for final detection, whereas higher enrichment factors can be obtained using adequate experimental conditions (time of loading sample, sorbent mass, etc.).
Among carbon-based sorbents, activated carbon was certainly one of the first materials applied in SPE. It has been widely used in water and wastewater treatment, primarily as an adsorbent for the removal of organic and inorganic contaminants [12]. The acceptance of separation or concentration media based on the new generations of carbon sorbents, such as graphitized carbon black (GCB) and porous graphitized carbon, has been growing during the last decade as they were shown to be appropriate for trapping polar analytes [13]. More recently, fullerenes and carbon nanotubes (CNTs) were discovered, and this opened a new perspective beyond that of carbon materials based on flat graphite-like hexagonal layers [14, 15]. CNTs possess many unique electronic, mechanical and chemical properties, and larger surface area and excellent strength. Along with the improvement of production of CNTs and characterization, progress is being made in their application. Over the past 20 years, more and more potential applications such as catalyst supports, electronic and optical devices, reinforced materials, hydrogen storage, field-emission materials, or biomedical use have been found. In many applications, including the analytical ones (such as gas sensors, electrochemical detectors, and biosensors with immobilized biomolecules), modification of their surface and interfacial engineering are essential [16, 17].
The purpose of this chapter is to summarize the analytical applications of CNTs for the enrichment and separation of organic compounds, as well as metallic species using SPE technique. The study of adsorption properties of CNTs is important in both fundamental and practical point of view.
1. SOLID-PHASE EXTRACTION OF ORGANIC COMPOUNDS
The characteristic structures and electronic properties of CNTs allow them to interact strongly with organic molecules via noncovalent forces, such as hydrogen bonding, π–π stacking, electrostatic forces, van der Waals forces, and hydrophobic interactions. These interactions, as well as hollow and layered nanosized structures, make them a good candidate for use as adsorbents. The surface, made up of hexagonal arrays of carbon atoms in graphene sheets, interacts strongly with the benzene rings of aromatic compounds. In 2001, Long and Yang [18] observed that dioxins, which have two benzene rings, were strongly adsorbed on CNTs. The amounts of dioxin adsorbed were 104 and 1017 times greater than that on activated carbon and γ-Al2O3, respectively. Dioxins after enrichment step could be removed from the adsorbent by temperature-programmed desorption. In several other works, sorption on CNTs has been examined for different aromatic compounds, such as chlorobenzenes [19–21], three endocrine disruptors (bisphenol A, 4-n-nonylphenol, and 4-tert-octylphenol) [22], several phthalate esters [23], triazine compounds [24], sulfonylurea herbicides [25, 26], phenoxyalkanoic acids [27, 28], and organochlorine pesticides [29], as well as polychlorinated dibenzo-p-dioxins and related compounds [30]. These works demonstrate the application of CNTs used as the packing of SPE cartridge in trace analysis of environmental pollutants in natural water samples in combination with high-performance liquid chromatography. Fang et al. [31] applied CNTs as a sorbent for online SPE of 10 different kinds of sulfonamides before their simultaneous determination in eggs and pork.
The CNTs, C18-bonded silica, and activated carbon were compared for their application for the enrichment of four chlorobenzenes under the same experimental conditions (columns packed with 15 mg of each sorbent materials, sample volume 100 mL, analytes at 5 ng/mL concentration level) [19]. The highest recoveries were found for CNTs (Fig. 10.1). Moreover, CNTs exhibit the highest adsorption ability toward these analytes, which was concluded from the obtained values of the apparent constant K in Freundlich model.
SPE of dicamba (3,6-dichloro-2-methoxybenzoic acid) using different polymeric sorbents (Oasis, Strata-X, Lichrolut SAX) and C18 Bond Elut or phenyl-silica gave unsatisfactory results [32]. This herbicide due to large scale of application, persistence, polar nature, and water solubility is present in several environmental compartments. CNTs have much greater herbicide adsorption capability than that of GCB and C18 adsorbents as it was shown in Fig. 10.2A [28]. The absorption capacity increases remarkably at lower pH of sample solution as dicamba is highly an acidic compound (pKa = 1.9). The decrease in pH leads to decrease in the ionization of the analyte and simultaneously to neutralization of the carbon surface charge. The HPLC chromatograms of river water samples spiked with dicamba and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) after preconcentration on C18 and CNT microcolumns are presented in Fig. 10.2B. The obtained results showed that CNTs were more effective than C18 (dicamba recovery was only 13%) for SPE of tested herbicides.
Figure 10.1 Recoveries of chlorobenzenes using CNTs, activated carbon, and C18 sorbents. Sample volume, 100 mL; sample concentration 5 ng/mL; eluent, 2 mL of methanol. DCB, dichlorobenzene; TCB, 1,2,4-trichlorobenzene; TeCB, 1,2,4,5-tetrachlorobenzene; HCB, hexachlorobenzene [19].
The thermodynamic characteristics of adsorption of several organic compounds on CNTs were determined by gas chromatography and compared with those for GBC and molecular crystals of fullerene C60 [33]. The comparison of adsorption data for compounds with different functional groups showed similarity of adsorption properties of CNTs and GCB, but there were significant differences in molecular interaction of adsorbed molecules with crystal C60.
Adsorption of naphthalene, phenanthrene, and pyrene from the class of polycyclic aromatic hydrocarbons (PAHs) on fullerenes, single-walled and multiwalled CNTs was investigated to study the relationship between sorbate and sorbent properties [34]. For different PAHs, adsorption was in correlation with their molecular size, while for different carbon nanomaterials, adsorption was in correlation with their surface area, micropore volume, and the volume ratios of mesopore to micropore.
Figure 10.2 Adsorption isotherms of dicamba and 2,4,5-T herbicides onto GCB, CNTs, and C18 adsorbents at pH 3, and SPE/HPLC chromatograms of river water samples (250 mL) spiked with 0.04 mg/mL of dicamba and 2,4,5-T and enriched on C18 and CNT microcolumns [28].
A novel, CNT-supported micro-SPE (µ-SPE) procedure has been developed [35]. A 6 mg sample of CNTs was packed inside a sheet (2 cm × 1.5 cm) of porous polypropylene membrane whose edges were heat-sealed to secure the contents. The µ-SPE, which was wetted with dichloromethane, was then placed in a stirred sewage sludge sample solution to extract organophosphorus pesticides, used as model compounds. After extraction, analytes were desorbed in hexane and analyzed using gas chromatography with mass spectrometry detection. Each µ-SPE device could be used for up to 30 extractions. The comparison of CNT-supported µ-SPE) procedure with hollow fiber membrane solid-phase microextraction (HFM-SPME) and headspace solid-phase microextraction (HS-SPME) showed that this procedure is accurate, fast, and could be used as cost-effective pretreatment method for sewage sludge samples.
CNTs can also remove and preconcentrate volatile organic compounds. Saridara et al. [36] described a microtrap operating as a nanoconcentrator and injector for gas chromatography. A thin layer of CNTs was deposited by catalytic chemical vapor deposition on the inside wall of a steel capillary to fabricate the microtrap. The obtained film provides an active surface for fast adsorption and desorption of small organic molecules such as hexane and toluene. Stronger sorption of toluene in comparison with hexane could be attributed to the π–π interaction between the CNTs’ side-wall and the aromatic ring.
The purge-and-trap system was used for the evaluation of CNTs as an adsorbent for trapping 16 volatile organic compounds from gaseous mixtures and indirectly from water samples [37]. Due to their porous structure, CNTs were found to have much higher breakthrough volumes than that of GCB (Carbopack B) with the same surface area.
2. ENRICHMENT OF METALLIC SPECIES
CNTs have also great analytical potential as an SPE adsorbent for enrichment and recovery of metal ions [38–50]. The chemical and thermal treatment processing could have great impact on the adsorption capability of CNTs for the removal of metal ions, because the performance of carbon materials is mainly determined by the nature and concentration of the surface functional groups. Gas-phase oxidation of activated carbon increases mainly the concentration of hydroxyl and carbonyl surface groups, while the liquid-phase oxidation increases particularly the content of carboxylic acids [51]. Oxidation of CNTs with nitric acid is an effective method to remove the amorphous carbon, carbon black, and carbon particles introduced during their preparation process [38]. It is known that the oxidation of carbon surface can offer not only a more hydrophilic surface structure but also a larger number of oxygen-containing functional groups, which increase the ion-exchange capability of carbon materials. The amount of carboxyl and lactone groups on the CNTs treated with nitric acid was higher than the amount on CNTs oxidized with H2O2 and KMnO4 [38]. At the same pH value, the zeta potential for these oxidized CNTs increased in the following order: H2O2 < KMnO4 < HNO3. This order indirectly suggests that the amount of acid functional groups increases in the same way. However, Cd(II) adsorption capacity for CNTs treated with KMnO4 was greater (11.0 mg/g) than that for treated with H2O2 (2.6 mg/g) and HNO3 (5.1 mg/g). This may be due to the adsorption role of the residual MnO2 particles. However, the process is still unclear and needs more explanations.
Figure 10.3 shows the effect of pH on the adsorption of Co(II) on untreated CNTs and CNTs treated with HNO3 [44]. The functional groups introduced by oxidation improve the ion-exchange capabilities of CNTs and make cobalt(II) adsorption capacity increase correspondingly. The adsorption of cobalt species increased with the increase of pH from 3 to 9, but more sharp increase was observed for oxidized CNTs. The low adsorption that took place in acidic region can be attributed, in part, to competition between H+ and Co2+ ions on the same sites. Furthermore, the charge of CNT surface becomes more negative with the increase of pH, which causes electrostatic interactions and, thus, results in higher adsorption of metal species.
Figure 10.3 Effect of pH on the adsorption of Co(II) with untreated CNTs and CNTs oxidized with HNO3. Initial metal concentration is 10 mg/L Adapted from Li YH [43].
The pH of the solution influences the surface charge of CNTs. When the pH is higher than the isoelectric point of the oxidized CNTs, the negative charge on the surface provides electrostatic attractions that are favorable for adsorbing cations. The affinity order of the metal ions toward CNTs at pH ≥ 7 is Cu(II) > Pb(II) > Zn(II) > Co(II) > Ni(II) > Cd(II) > Mn(II) (Fig. 10.4) [44]. This order agrees very well with the electronegativity of metal ion and the first stability constant of the associated metal hydroxide [51]. The general trend is that higher electronegativity and higher first constant correspond to a higher sorption level of a given metal ion.
Quantitative sorption of metal ions was usually obtained at pH ≥ 6 [41–45] although quantitative adsorption (>95%) of rare earth elements was obtained when the pH exceeds 3.0 [39]. As metal ions are poorly adsorbed at low pH, acid solutions such as HNO3 or HCl have been used for their desorption and recovery.
Figure 10.4 Sorption of metal ions on CNTs in the function of pH [44].
Single-metal-ion equilibrium studies usually have been conducted to investigate the maximum metal adsorption capacities of CNTs. This factor determines how much sorbent is required to quantitatively concentrate the analytes from a given solution. CNTs exhibit high affinity toward Zn(II) and Pb(II) as the adsorption capacities for these metal ions are equal to 34.4 mg/g [41] and 31.8 mg/g [49], respectively. For other divalent metals, much lower values have been reported: Cu(II), 7.5 mg/g [45]; Ni(II), 6.9 mg/g [38]; Cd(II), 5.1 mg/g [38]; Mn(II), 4.9 mg/g [42]. The adsorption capacity for each rare earth element was found to be in the range from 7.2 (for gadolinium) to 9.9 mg/g (for europium) [39]. However, the reported values depend also on the surface area of the used CNTs, as well as on experimental conditions. By applying the Langmuir equation to single-ion adsorption isotherms, Li et al. [43] calculated the maximum sorption capacities for lead, copper, and cadmium ions as 97.1, 24.5, and 10.9 mg/g, respectively. Similar experiments conducted for competitive adsorption (from the metal mixtures) showed the same affinity order of tested metal ions, e.g., Pb(II) > Cu(II) > Cd(II). However, the calculated maximum sorption capacity, corresponding to complete monolayer coverage, for Pb(II) decreased from 97.1 to 34.0 mg/g. The adsorption isotherms for single cases and for the mixture of these three ions onto CNTs are shown in Fig. 10.5. On the contrary, adsorption of Cu(II) in the presence of other divalent metal ions decreased only slightly, being the most pronounced by Pb(II) (Fig. 10.6). The decrease in absorption capacity for Cu(II) in the presence of Mn(II) was only approximately 12 and 2% at their initial concentration of 2 and 10 mg/L, respectively [44]. Thus, CNTs have good adsorption properties and high capacity for Cu(II) and can be applied for the preconcentration and purification of this element in aqueous solutions containing other metals.
Figure 10.5 Adsorption isotherms of Pb(II), Cu(II), and Cd(II) on CNTs for single ions (A) and competitive adsorption (B); 100 mL of sample at pH 5.0 was shaken with 50 mg of CNTs for 4 h [42].
Figure 10.6 Adsorption capacity of CNTs for Cu(II) in the presence of other divalent metal ions [43].
For pure analytical application, CNTs packed into microcolumns have been applied for preconcentration of rare earth elements [39], Pb(II) [47], Cd(II) [48, 50], Cu(II) [50], as well as Mn(II) and Ni(II) [52], prior to their determination by flame atomic absorption spectrometry or inductively coupled plasma atomic emission spectrometry in several environmental samples. The proposed methods were significant with respect to the good performance and preconcentration system including satisfactory preconcentration factor and enrichment efficiency, thus resulting in a low limit of detection. Moreover, addition of chelating agents and organic solvents for elution, which are most commonly used in SPE preconcentration systems with modified octadecyl silica (C18) or polymeric sorbents, is not necessary.
The potential uses of fullerenes and CNTs as sorbents for preconcentration of lead, mercury, and tin organometallic compounds have been investigated [53]. These metals were complexed with diethyldithiocarbamate in a flow system and, after simultaneous enrichment on a microcolumn, were determined by gas chromatography. Comparative studies showed that CNTs, as well as C60 and C70 fullerenes, were superior to GCB and C18-silica for the extraction of the 11 compounds that are studied. The sensitivity for the determination of these compounds (slope of calibration graph) was higher for carbon sorbents because they allow higher sample volume (25 mL) to be preconcentrated. The sensitivity achieved with C18-silica was the lowest due to the smaller sample volume (15 mL) and lower efficiency of sorption (lower column capacity). The molecular surface area and volume for CNTs are larger than those of fullerenes, and thus, the highest sensitivities for the determination of organometallic compounds were observed for CNTs. The selectivity was lowest for GCB and C18-silica; the metal ions posing the most severe interference were Cu(II) and Co(II) for all organotin species, as well as Fe(III) and Zn(II) for trimethyl lead. The proposed method was successfully applied for the determination of lead, mercury, and tin compounds in water and coastal sediment samples.
3. CONCLUSIONS
The need for efficient methods for sample concentration and cleanup in environmental application is constantly growing. In recent years, the SPE method has become the most often used technique for trace analysis of metals and organic environmental pollutants. SPE technique is fast, reproducible, cleaner extracts are obtained, and solvent consumption is reduced. Moreover, SPE can be easily incorporated into automated analytical procedures. Thus, efforts dedicated to exploring new effective adsorbents continue to grow.
CNTs have been proved to be effective adsorbents for the removal of organic compounds, particularly with those containing benzene rings. Their large sorption capacity is linked to well-developed internal pore structure and surface area. In recent years, the analytical potential of CNTs as the solid-phase adsorbents for metal ions and their chelates has been explored. The presence of the active sites on the surface, inner cavities, and internanotube space contribute to the high metal removal capability of CNTs. Moreover, their strong sorption properties could be utilized in voltammetric stripping determination of transition metal cations with electrodes modified with CNTs [54]. CNTs express also high fluoride removal efficiency in a broad pH value range [55]. The comparison with other commercially available adsorbents suggests that CNTs are promising sorbents for environmental protection applications. The sorption properties of CNTs, including the possibility of their chemical functionalization [56], can be utilized in developing new microseparation methods and techniques.
References
[1] Brown RJC, Milton MJT. Analytical techniques for trace element analysis; an overview. Trends Analyt Chem 2005;24:266–74.
[2] Becker JS. Trace and ultratrace analysis in liquids by atomic spectrometry. Trends Analyt Chem 2005;24:243–54.
[3] Camel V. Solid phase extraction of elements. Spectrochim Acta Part B 2003;58:1177–233.
[4] Hirose K. Chemical speciation of trace metals in seawater: a review. Anal Sci 2006; 22:1055–63.
[5] Pyrzynska K, Trojanowicz M. Functionalized cellulose sorbents for preconcentration of trace metals in environmental analysis. Crit Rev Anal Chem 1999;29:313–21.
[6] Jak PK, Patel S, Mishra BK. Chemical modification of silica surface by immobilization of functional groups for extractive concentration of metal ions. Talanta 2004; 62:1005–28.
[7] Spivakov BY, Malofeeva GL, Petruskhin OM. Solid-phase extraction on alkyl-bonded silica gels in inorganic analysis. Anal Sci 2006;22:503–19.
[8] Rao TP, Praven RS, Daniel S. Sorption of divalent metal ions from aqueous solution by carbon nanotubes: A review. Crit Rev Anal Chem 2004;34:177–93.
[9] Pyrzynska K. Application of carbon sorbents for the concentration and separation of metal ions. Anal Sci 2007;23:631–7.
[10] Valdes MG, Perez-Cordoves AI, Diaz-Garcia ME. Zeolites and zeolite-based materials in analytical chemistry. Trends Analyt Chem 2006;25:24–30.
[11] Lemos VA, Santos MS, Santos ES, Santos MJS, dos Santos WNL, Souza AS, et al. Application of polyurethane foam as a sorbent for trace metal preconcentration — a review. Spectrochim Acta part B 2007;62:4–12.
[12] Strelko Jr V, Malik DJ, Streat M. Interpretation of transition metal sorption behaviour by oxidized active carbons and other adsorbents. Sep Sci Technol 2004;39(9):1885–905.
[13] Forgács E. Retention characteristics and practical applications of carbon sorbents. J Chromatogr A 2002;975:229–43.
[14] Trojanowicz M. Analytical applications of carbon nanotubes: a review. Trends Analyt Chem 2006;25:480–9.
[15] Valcárcel M, Simonet BM, Cárdenas S, Suárez B. Present and future applications of carbon nanotubes to analytical science. Anal Bioanal Chem 2005;382:1783–90.
[16] Lin T, Bajpai V, Ji T, Dai L. Chemistry of carbon nanotubes. Aust J Chem 2003;56: 635–51.
[17] Sun YP, Fu K, Huang W. Functionalized carbon nanotubes: properties and applications. Acc Chem Res 2002;35:1096–104.
[18] Long RQ, Yang RT. Carbon nanotubes a superior sorbent for dioxin removal. J Am Chem Soc 2001;123:2058–9.
[19] Liu G, Wang J, Zhu Y, Zhang X. Application of multiwalled carbon nanotubes as a solid-phase extraction sorbent for chlorobenzenes. Anal Lett 2004;37:3085–104.
[20] Cai Y, Cai Y, Mou S, Lu Y. Multi-walled carbon nanotubes as a solid-phase extraction adsorbent for the determination of chlorophenols in environmental water samples. J Chromatogr A 2005;1081:245–7.
[21] Peng X, Li Y, Luan Z, Di Z, Wang H, Tian B, et al. Adsorption of 1,2-dichlorobenzene from water to carbon nanotubes. Chem Phys Lett 2003;376:154–8.
[22] Cai Y, Jiang G, Zhou Q. Multiwalled carbon nanotubes as a solid-phase extraction adsorbent for the determination of bisphenol A, 4-n-nonylphenol and 4-tert-octylphenol. Anal Chem 2003;75:2517–21.
[23] Cai YQ, Jiang GB, Liu JF, Zhou QX. Multi-walled carbon nanotubes packed cartridge for the solid-phase extraction of several phthalate esters from water samples and their determination by high performance liquid chromatography. Anal Chim Acta 2003;494:149–56.
[24] Zhou Q, Xiao J, Wang W, Liu G, Shi Q, Wang J. Determination of atrazine and simazine in environmental water samples using multiwalled carbon nanotubes as the adsorbents for preconcentration prior to high performance liquid chromatography with diode array detector. Talanta 2006;68:1309–15.
[25] Zhou Q, Wang W, Xiao J. Preconcentration and determination of nicosulfuron, thifen-sulfuron-methyl and metsulfuron-methyl in water samples using carbon nanotubes packed cartridge in combination with high performance liquid chromatography. Anal Chim Acta 2006;559:200–6.
[26] Zhou Q, Xiao J, Wang W. Trace nalysis of triasulfuuron and bensulfuron-methyl in water samples using carbon nanotubes packed cartridge in combination with high-performance liquid chromatography. Microchim Acta 2007;157:93–8.
[27] Biesaga M, Pyrzynska K. The evaluation of carbon nanotubes as a sorbent for dicamba herbicide. J Sep Sci 2006;29:2241–4.
[28] Pyrzynska K, Stafiej A, Biesaga M. Sorption behaviour of acidic herbicides on carbon nanotubes. Microchim Acta 2007;159:293–8.
[29] Zhou Q, Xiao J, Wang W. Using multi-walled carbon nanotubes as solid phase extraction adsorbents to determine dichlorodiphenyltrichloroethane and its metabolites at trace level in water samples by high performance liquid chromatography with UV detection. J Chromatogr A 2006;1125:152–8.
[30] Fugetsu B, Satoh S, Iles A, Tanaka K, Nishi N, Watari F. Encapsulation of multi-walled carbon nanotubes (MWCNTs) in Ba2+-alginate to form coated micro-beads and their application to the pre-concentration/elimination of dibenzo-p-dioxin, dibenzofuran, and biphenyl from contaminated water. Analyst 2004;129:565–6.
[31] Fang GZ, He JX, Wang S. Multiwalled carbon nanotubes as sorbent for on-line coupling of solid-phase extraction to high-performance liquid chromatography for simultaneous determination of 10 sulfonamides in eggs and pork. J Chromatogr A 2006;1127:12–7.
[32] Biesaga M, Jankowska A, Pyrzynska K. Comparison of different sorbents for solid-phase extraction of phenoxyalkanoic acid herbicides. Microchim Acta 2005;150:317–22.
[33] Ya Davydov V, Kalashnikova EV, Karnatsevich VI, Kirillov I. Adsorption properties of multi-wall carbon nanotubes. Full Nano Carb Nanostruct 2004;12:513–8.
[34] Yang K, Zhu L, Xing B. Adsorption of polycyclic aromatic hydrocarbons by carbon nanomaterials. Environ Sci Technol 2006;40:1855–61.
[35] Basheer C, Alnedhary AA, Rao BSM, Valliyaveettli S, Lee HK. Development and application of porous membrane-protected carbon nanotube micro-solid-phase extraction combined with gas chromatography/mass spectrometry. Anal Chem 2006;78: 2853–6.
[36] Saridara C, Brukh R, Iqbal Z, Mitra S. Preconcentration of volatile organics on self-assembled carbon nanotubes in a microtrap. Anal Chem 2005;77:1183–7.
[37] Li QL, Yuan DX, Lin QM. Evaluation of multi-walled carbon nanotubes as an adsorbent for trapping volatile organic compounds from environmental samples. J Chromatogr A 2004;1026:283–8.
[38] Li YH, Wang S, Luan Z, Ding J, Xu C, Wu D. Adsorption of cadmium(II) from aqueous solution by surface oxidized carbon nanotubes. Carbon 2003;41:1057–62.
[39] Liang P, Liu Y, Guo L. Determination of trace rare earth elements by inductively coupled plasma atomic emission spectrometry after preconcentration with multiwalled carbon nanotubes. Spectrochim Acta Part B 2005;60:125–9.
[40] Lu C, Chu H, Liu C. Removal of zinc(II) from aqueous solution by purified carbon nanotubes: kinetics and equilibrium studies. Ind Eng Chem Res 2006;45: 2850–5.
[41] Lu C, Chiu H. Adsorption of zinc(II) from water with purified carbon nanotubes. Chem Eng Sci 2006;61:1138–45.
[42] Liang P, Liu Y, Guo L, Zeng J, Lu H. Multiwalled carbon nanotubes as solid-phase extraction adsorbent fort the preconcentration of trace metal ions and their determination by inductively coupled plasma atomic emisssion spectrometry. J Anal At Spectrom 2004;19:1489–92.
[43] Li YH, Ding J, Luan Z, Di Z, Zhu Y, Xu C, et al. Competitive adsorption of Pb2+, Cu2+ and Cd2+ ions from aqueous solutions by multiwalled carbon nanotubes. Carbon 2003;41:2787–92.
[44] Stafiej A, Pyrzynska K. Adsorption of heavy metal ions with carbon nanotubes. Sep Sci Technol 2007;58:49–52.
[45] Liang P, Ding Q, Song F. Application of multiwalled carbon nanotubes as solid phase extraction sorbent for preconcentration of trace copper in water samples. J Sep Sci 2005;28:2339–43.
[46] Li YH, Wang S, Wei J, Zhang X, Xu C, Luan Z, et al. Lead adsorption on carbon nanotubes. Chem Phys Lett 2002;357:263–6.
[47] Barbosa AF, Segatelli MG, Pereira AC, de Santana Santos A, Kubota LT, Luccas PO, et al. Solid-phase extraction system for Pb(II) ions enrichment based on multiwall carbon nanotubes coupled on-line to flame atomic absorption spectrometry. Talanta 2007;71:1512–9.
[48] Tarley CRT, Barbosa AF, Segatelli MG, Figueiro EC, Luccas PO. Highly improved sensitivity of TS-FF-AAS for Cd(II) determination at ng L–1 levels using a simple flow injection microcolumn preconcentration system with multiwall carbon nanotubes. J Anal At Spectrom 2006;21:1305–13.
[49] Li YH, Di Z, Ding J, Wu D, Luan Z, Zhu Y. Adsorption thermodynamic, kinetic and desorption studies of Pb2+ on carbon nanotubes. Water Res 2005;39:605–9.
[50] Liang HD, Han DM. Multi-walled carbon nanotubes as sorbent for flow injection on-line microcolumn preconcentration coupled with flame atomic absorption spectrometry for determination of cadmium and copper. Anal Lett 2006;39:2285–95.
[51] Dastgheib SA, Rockstraw DA. A model for the adsorption of single metal ion solutes in aqueous solution onto activated carbon produced from pecan shells. Carbon 2002;40:1843.
[52] Figueiredo JL, Pereira MFR, Freitas MMA, Órfão JJM. Modification of the surface chemistry of activated carbons. Carbon 1999;37:1379–89.
[53] Muñoz J, Gallego M. Valcárcel M. Speciation of organometallic compounds in environmental samples by gas chromatography after flow preconcentration on fullerenes and nanotubes. Anal Chem 2006;77:5389–95.
[54] Wu K, Hu S, Fei J, Bai W. Mercury-free simultaneous determination of cadmium and lead at a glassy carbon electrode modified with multi-wall carbon nanotubes. Anal Chim Acta 2003;489:215–21.
[55] Li YH, Wang S, Zhang X, Wie J, Xu C, Luan Z, et al. Adsorption of fluoride from water by aligned carbon nanotubes. Mater Res Bull 2003;38:469–76.
[56] An X, Zeng H. Functionalization of carbon nanobeads and their use as metal ion adsorbents. Carbon 2003;41:2889–96.