Carbon nanostructures for separation, preconcentration and speciation of metal ions

Carbon nanostructures for separation, preconcentration and speciation of metal ions

Trends Trends in Analytical Chemistry, Vol. 29, No. 7, 2010 Carbon nanostructures for separation, preconcentration and speciation of metal ions Krys...

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Trends in Analytical Chemistry, Vol. 29, No. 7, 2010

Carbon nanostructures for separation, preconcentration and speciation of metal ions Krystyna Pyrzynska Novel carbon-based nanomaterials with unique properties find increasing use in analytical science. This article presents an up-to-date overview of recent applications of carbon nanotubes (CNTs), metal oxide-CNT nanocomposites and carbon-encapsulated magnetic nanoparticles for enrichment and separation of metal ions, and speciation. The sorption mechanism appears to be mainly attributable to chemical interactions between metal ions and the functional groups on the surface. I address the effects of surface oxidation and chemical functionalization, sorption capacities and process parameters, and discuss the application of these new nanomaterials to metal speciation. ª 2010 Elsevier Ltd. All rights reserved. Keywords: Carbon nanostructure; Carbon nanotube; Carbon encapsulation; Magnetic nanoparticle; Metal ion; Nanocomposite; Preconcentration; Solid-phase extraction; Separation; Speciation

1. Introduction Krystyna Pyrzynska* University of Warsaw, Department of Chemistry, Pasteura 1, 02-093 Warsaw, Poland


Fax: +4822 8223532; E-mail: [email protected]


Determination of metal ions at trace levels is very important in the context of environmental protection, food and agricultural chemistry, and high-purity materials. Metals are introduced into the environment by natural and anthropogenic sources. The major man-made sources of are soils and dusts from traffic, industry and weathered materials. Due to low concentrations of metal ions in environmental samples and matrix interferences, the determination of metal ions in complex matrices is limited and requires preconcentration and/or separation of analytes to improve sensitivity and selectivity of analyses [1,2]. Solid-phase extraction (SPE), a low-cost extraction technique, has been developed and widely used for this purpose due to its selectivity and high preconcentration efficiency [3– 5]. The choice of selective sorbent should be based on analyte, sample matrix and technique for final detection, whereas higher enrichment factors can be obtained

using adequate experimental conditions (e.g., time of loading sample, sorbent mass, and volume of eluent). The sorbent may be packaged in different formats: filled microcolumns; cartridges; syringe barrels; or, discs. Several materials (e.g., C18 bonded silica [6,7], polymeric sorbents [8], carbon-based materials [9], zeolites [10] or polyurethane foam [11]) have been proposed. 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 removal of organic and inorganic contaminants [12]. The use of a newer generation of carbon sorbents (e.g., graphitized carbon black and porous graphitized carbon) has been growing during the past decade, since they were shown to be appropriate for trapping polar analytes [13]. More recently, much attention has been paid to carbon nanostructure materials of different chemical composition, produced as nanoparticles, nanowires or nanotubes. The uniqueness of these materials is due to their mechanical, electrical, optical, catalytic, magnetic and photonic properties, and extremely large surface area [14]. With improvement in the production of carbon nanostructures, progress is being made in their characterization and application [15–17]. With regard to the numerous possibilities of technical applications, it is not surprising that their use for environmental purposes has also been considered [18]. The large specific surface area, the outstanding thermal and chemical stabilities,

0165-9936/$ - see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2010.03.013

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and the recent development in large-scale synthesis make carbon nanostructures attractive as interesting possible SPE materials for detection and remediation of various organic compounds, metal ions and their complexes [19–22]. This article presents an up-to-date overview, focused mainly on recent analytical applications of carbon nanotubes (CNTs), metal oxide-CNT nanocomposites and carbon-encapsulated magnetic nanoparticles (CEMNPs) for the enrichment and separation of metal ions using SPE. These carbon sorbents could also be used for speciation. I discuss papers published over the past five years in more detail. I emphasize the application of carbon nanostructures for on-line automated preparation of environmental, food and biological samples.

2. Oxidation and functionalization of CNTs CNTs can be described as a graphite sheet rolled up into a nanoscale tube; they are typically several nanometers (nm) in diameter and both ends are normally capped by fullerene-like structures. There are two main types of CNT: single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs), named according to the number of carbon-atom layers in the wall of the nanotubes. At present, the three main methods for CNT synthesis are arc discharge, laser ablation and chemical-vapor deposition [15,23]. The last method seems to be the most promising for possible scale up due to the relatively low growth temperature, high yields and high purities that can be achieved. However, low synthesis temperature often results in high defect density in the CNTs obtained. Because such CNTs usually contain carbonaceous or metallic impurities, purification is an essential issue to be addressed. Considerable progress in the purification of CNTs has been made and a number of purification methods, including chemical oxidation, physical sepa-


ration, and combinations of chemical and physical techniques, have been developed to obtain CNTs with the desired purity [24]. A problem of considerable interest is that of determining the specific surface area and accessible internal pore volume of real CNT samples, since many applications of CNTs require more detailed information on the structural properties of these materials. The method most widely used for determining surface area and poresize distribution of CNTs is based on the adsorption of nitrogen at 77 K using the Brunnauer–Emmett–Teller (BET) model and Barrett-Joyner-Halenda (BJH) method. BET measurements showed that the surface area of SWCNTs available for liquid-phase mass transfer is higher than that of MWCNTs, so the metal-sorption capacity of SWCNTs is greater [23]. The characterization of CNTs is usually by thermogravimetric analysis, X-ray diffraction, scanning/ transmission electron microscopy and/or Raman and infrared spectroscopy [24–26]. Strong adsorptive interaction between CNTs and organic aromatic pollutants is attributed to p-p electrondonor-acceptor interaction between aromatic molecules (electron acceptors) and the highly-polarizable graphene sheets (electron donors). The main adsorption mechanism of metal ions to CNTs is considered to be surface complexation with functional groups (Fig. 1), so the performance is mainly determined by the nature and the concentration of these groups. Acid treatment is one of the methods most commonly employed to remove the amorphous carbon and metaloxide impurities introduced by the preparation process [26,27]. It is known that oxidation of carbon surfaces can offer not only a more hydrophilic surface structure, but also a larger number of oxygen-containing functional groups (–OH, -C=O and –COOH), which increase the ion-exchange capability of carbon material. This process also increases the possibility of further

Figure 1. Major mechanism for sorption of divalent metal ions onto the surface of a carbon nanotube (CNT).



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modification and functionalization of the graphitic surface. Gas-phase oxidation of activated carbon mainly increases the concentration of hydroxyl and carbonyl surface groups, while oxidation in the liquid phase particularly increases the content of carboxylic acids [28]. The amount of carboxyl and lactone groups on the CNTs treated with nitric acid was higher than when this process was conducted using H2O2 and KMnO4 [29]. However, one of the main drawbacks of acid-oxidation methods is CNT fragmentation (shortening) and defect generation in the graphitic network [24]. Besides acid concentration and exposition time, the high ultrasonic power typically employed to disperse CNT agglomerates during oxidation has been identified as an additional source of fragmentation [26,27]. There therefore needs to be a compromise between functionalization parameters (acid concentration, treatment time and sonication power) and CNT damage. Aviles et al. [26] found that low-power sonochemical treatment employing HNO3 (3 mol/L) for 2 h, followed by identical treatment with H2O2 (30% v/v), proved to be most effective for oxidation, minimizing damage to CNTs. Oxidized CNTs can be further functionalized. When a cationic surfactant (e.g., cetyltrimethylammonium chloride) is added to oxidized CNTs, hydrophobic and ionic interactions may lead to formation of hemimicelle/ admicelle aggregates on the CNT surface; in this way, a new kind of adsorbent, namely hemimicelle-capped carboxy-modified CNTs, was obtained [30]. This new SPE material exhibits good retention ability for metal oxyanions (e.g., H2AsO4). NaF was used as masking reagent for elimination of lead, iron and other coexisting transition-metal interferences. Tan et al. [31] proposed enrichment of nickel ions onto CNTs in the presence of an anionic surfactant, sodium dodecylbenzene sulfonate. The sulfonate molecules, adsorbed on the carbon-nanoparticle surfaces, cause an attraction between the anionic head groups and the positively-charged Ni(II) ions. Sorption of nickel was also enhanced in the presence of poly(acrylic acid), used as a model for natural organic materials, which can be present in wastewater [32]. For simultaneous separation and preconcentration of trace amounts of Au(III) and Mn(II), CNTs were modified with N,NÔ-bis(2-hydroxybenzylidene)-2,2 0 (aminophenylthio)ethane [33]. Doyle and Tour [34] proposed environmentallyfriendly functionalization of CNTs in molten urea by diazonium salts formed in situ. Biomolecule-functionalized CNTs are expected to be more selective than untreated and oxidized CNTs for the SPE of metal ions, as they possess different functional groups with various binding capacities [35,36]. Liu et al. [35] applied L-cysteine-functionalized CNTs as a selective sorbent for Cd(II) enrichment. The mercapto content in this sorbent was 3.0 mmol/g, which is much greater 720

than the sulfhydryl analogues grafted on silica substrate (0.3 mmol/g) [37]. CNTs possess larger specific surface area and offer greater potential to host cysteine. L-tyrosine immobilized on CNTs was studied for preconcentration of some divalent metal ions [36]. The sorption decreased in the order: Cu(II) > Ni(II) > Zn(II) >> Co(II), and this tendency shows that bigger cations are more sorbed on L-tyr-CNTs. However, even for cobalt ions, which exhibit the lowest affinity to that new sorbent, the retention capacity was about twice that of un-functionalized CNTs. Biosorbent obtained by immobilizing microorganisms of Pseudomonas aeruginosa onto CNTs was studied for recovery of some metal ions (Co, Cd, Pb, Mn, Cr and Ni) from large sample volumes [38]. The adsorption capacities of this biosorbent for cobalt, cadmium, lead and nickel ions were lower than the values obtained for sorption of metal chelates with ammonium pyrrolidine dithiocarbamate (APDC) onto CNTs [39].

3. Effect of pH and sorption efficiency of metal ions Solution pH influences the surface charge of CNTs. Fig. 2 shows the distribution of the main functional groups (e.g., carboxyl and hydroxyl) as a function of pH calculated from the acid-base titration [40]. At pH > pHPZC (point of zero charge), the positively-charged metal ions can be adsorbed on the negatively-charged oxidized CNTs, so sorption of metal ions generally increases with increasing pH. The low adsorption that takes place in acidic solutions can be attributed in part to the competition between hydrogen and metal ions on the same CNT sites. Usually, pH > 6 is selected for sorption, although quantitative recovery of rare-earth elements was obtained at pH 3 [41]. The affinity order of divalent metal ions towards CNTs at pH > 7 is: Cu(II) > Pb(II) > Zn(II) > Co(II) > Ni(II) > Cd(II) > Mn(II) [42]. This order agrees very well with metal-ion electronegativity and the first stability constant of the associated metal hydroxide [40]. In some cases, when a higher pH is employed, both sorption and precipitation are involved in the removal of metal ions from aqueous solution [31,36,43,44]. It was suggested that the use of appropriate dimensions of CNTs may support the trapping process of the precipitated metal hydroxides [45]. CNTs of 5–15 lm length and with external diameters of 10–30 nm gave the highest enrichment efficiency towards Cu(II), Zn(II) and Pb(II) at pH 9, but, for Cd(II), the highest recovery was achieved with external diameters of 20–40 nm [46]. The use of carbon materials is often problematic due to the difficulties of desorbing some polar organic analytes [47,48]. Common solvents (e.g., methanol, acetonitrile and dimethylformamide) were shown to be inadequate for quantitative recovery of organic pollutants, so a

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Figure 2. The distribution of oxidized surface sites of carbon nanotubes (CNTs) as a function of pH. Conditions: m/V = 5 g/L; I = 0.01 mol/L NaClO4; T = 20C [40].

mixture of dichloromethane and methanol [48] or acetonitrile and NH3 [49] was recommended. Also, the backflush desorption (in the opposite direction to loading) has to be used when a small volume of eluent (5–10 mL) is applied. Metal ions are poorly adsorbed onto CNTs at low pH, so acid solutions (e.g., HNO3 or HCl) in the concentration range 0.5–2 mol/L have been used for their desorption and recovery. Metal ions can be preconcentrated on CNTs as their chelates. In this case, the sample pH is adjusted to its optimal value to assure the complexation reaction. Cu(II), Cd(II), Pb(II), Zn(II), Ni(II) and Co(II) were quantitatively retained as neutral APDC chelates in the pH range 2–6 [39], while Cr(VI) was recovered at pH 2–4 as an APDC complex [46]. Nitric acid with acetone was found to be the most satisfactory eluent in the experiments using APDC [39,46]. Rhodium ions were complexed with 1-(2-pyridylazo)-2-naphthol (PAN) using an acetate buffer of pH 3.7 [50]. The sorbed Rh-PAN complex was eluted using N,N 0 -dimethylformamide. To investigate the maximum metal-adsorption capacities of CNTs, single metal-ion-equilibrium studies are usually conducted. The equilibrium factor determines how much sorbent is required to concentrate the analytes from a given solution. CNTs exhibit high affinity toward Cu(II), Zn(II) and Pb(II); the adsorption capacities for these metal ions are 47 mg/g [51], 41–44 mg/g [52,53] and 83 mg/g [54], respectively. The adsorption capacity for each rare-earth element was found to be in the range 7.2 (gadolinium)–9.9 (europium) mg/g [41].

Wang et al. [55] found that 1 g of CNTs adsorbs at least 40 mg of radionuclide 243Am(III). However, the reported values depend on the surface area of the CNTs used, their pretreatment step and the experimental conditions. By applying the Langmuir Equation to single ionadsorption isotherms, Li et al. [56] calculated the maximum sorption capacities for lead, copper and cadmium ions as 97 mg/g, 25 mg/g and 11 mg/g, respectively. Similar experiments conducted for competitive adsorption for these three ions (from the metal mixtures) showed the same affinity order of tested metal ions [i.e. Pb(II) > Cu(II) > Cd(II)]. However, the calculated maximum sorption capacity, corresponding to complete monolayer coverage, decreased about three times. By contrast, adsorption of Cu(II) in the presence of other divalent metal ions decreased only slightly, and was most pronounced in the case of Pb(II) (Fig. 3) [57]. The reported sorption capacities for Cu(II) [39,42,51] are higher than those of various chelate-functionalized SPE materials {e.g., calix[4]arene-o-vanillin thiosemicarbazone Merrifield resin [58], Amberlite XAD-2 loaded with calmagite [59] or o-aminophenol [60]} and the recently reported polymethacrylic microbeads imprinted with 4-(2-pyridylazo)resorcinol as a specific Cu(II) ligand [61]. The presence of the active sites on the surface, inner cavities and inter-nanotube space contributes to the high metal-sorption capability of CNTs. Table 1 shows recent applications of CNTs for removal and enrichment of metal ions. It is also worth noting that carbon nanofibers oxidized with nitric acid have been successfully applied for the preconcentration of



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Adsorption capacity. mg/g

1.8 Cu - Co Cu - Mn Cu - Pb Cu - Zn Cu

1.5 1.2 0.9 0.6 0.3 0.0

2 mg/L

10 mg/L

Initial Cu concentration Figure 3. Adsorption capacity of carbon nanotubes (CNTs) for Cu(II) in the presence of other divalent metal ions at the same concentration [57].

Table 1. Recent applications of carbon nanotubes (CNTs) for preconcentration of metal ions Metal Mn(II) Au(III) Cd Co Cu, Cd, Pb, Zn, Ni, Co Eu, Gd, Ho, La, Sm, Tb, Yb Cr(VI) Rh(III) Zn Co Cu Ni Pb Cu, Cd Cd Cu, Zn, Mn, Pb Cu Co Cu, Co, Pb Cu, Co, Ni, Pb







L-cysteine L-tyrosine APDC APDC PAN PAN o-cresolphthalein

5.5–8 9 2 3 2 3.7 8 8.8 7


4.7 6 4.9 6 5-8 5-8.5 7-9 7



LODb (lgnL)


250 250 33 180 80 50 100 120 19 60 44.2 24 51 14-20 150 300 50 40


0.01 0.03 0.28 0.05 0.3–0.6 0.003–0.06 0.90 0.01 0.01 1.46 2.6 0.11–0.3 0.01 0.28–1.0 0.42 0.55 0.001–0.04 1.6–5.7



[35] [36] [39] [41] [46] [50] [52,53] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72]


PF, Preconcentration factor (ratio of highest sample volume to volume of eluent). LOD, Limit of detection. Compounds: PAA, Polyacrylic acid; APDC, Ammonium pyrrolidine dithiocarbamate; PAN, 1-(2-pyridylazo)-2-naphthol; DMF, N,N 0 -dimethylformamide. b

metal ions in environmental and biological samples [44,73,74]. The systems based on on-line preconcentration and determination of metallic species increase 722

sample throughput and decrease sample and reagent consumption [65–68,75]. The efficiency of metal preconcentration essentially depends on the time taken for

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the sample solution to pass through the microcolumn; the use of a longer loading time (greater sample volume) or higher flow rate improves enrichment but simultaneously leads to a decrease in the sampling rate. Very large preconcentration factors (PFs) were reported – 300 for cobalt [70] and 250 for gold and manganese [33] – when 1 L water samples were preconcentrated. However, such large PF values do not always properly illustrate the effectiveness of enrichment in a given setup as a sampling rate of only 0.02/h was obtained [33]. The sensitivity of metal determination depends mainly on the kind of detection technique used (Table 1). The examples presented show that preconcentration utilizing CNTs is an efficient approach to improving the sensitivity of FAAS determination of metal ions. El-Sheikh [76] compared the efficiency of activated carbon (AC) and CNTs (prepared by oxidation with nitric acid) for the enrichment of Mn(II), Cr(III), Pb(II), Cd(II), Cu(II) and Zn(II) from water. For CNTs, lower limits of detection (LODs) were obtained with electrothermal atomic absorption spectrometry (ETAAS), particularly for zinc, while the linear ranges of detection were generally greater for oxidized AC.

4. Application of CNTs to speciation analysis of metals The environmental and biological activity of metal ions depends on their chemical properties, including oxidation state, solubility and degree of formation of complexes with different inorganic and organic ligands existing in the water environment. Thus, speciation analysis that relates to these properties is essential to solve the problems with metal-ion detection in many fields (e.g., environment, biology, biochemistry, clinical chemistry, nutrition and toxicology). As the direct determination of metallic species is impossible by the analytical methods most commonly used, separation procedures are usually required prior to final measurements. The speciation procedures for chromium are generally based on separation and preconcentration of one of its species – Cr(III) or Cr(VI). Total chromium content is then determined by reduction of Cr(VI) or oxidation of Cr(III) [77]. Hu et al. [40] found that the maximum sorption of Cr(VI) on oxidized CNTs occurred at a pH below 2 and was independent of ionic strength. Cr(VI) anions are adsorbed by the protonated groups and the electrophilic surface groups of CNTs, then part of Cr(VI) is reduced to Cr(III) cations, which are released back into solution. Cr(III) ions are captured by sorption and ion exchange on the weak acid surface groups [40]. Tuzen and Soylak [46] introduced another approach to chromium speciation using CNTs. Cr(VI) was recovered quantitatively at pH 2–4 as an APDC chelate, while


the sorption of Cr(III) was below 10% under these conditions. After oxidation of Cr(III) to Cr(VI) with hydrogen peroxide in a basic medium, the method was applied to the determination of total chromium. The two inorganic vanadium species, V(IV) and V(V), exhibited strong affinity to the oxidized CNTs in the pH range 3.5–5.0; these can therefore be used for total vanadium preconcentration [75]. For speciation purposes, V(IV) ions were masked with 1,2-cyclohehanediaminetetraacetic acid (CDTA) in order to determine the V(V) concentration selectively after elution with HCl. For a sample volume of 1 mL, a PF of 20 was obtained and the LOD was 19 ng/L. The potential uses of fullerenes and CNTs as sorbents for preconcentration of lead, mercury and tin organometallic compounds have been studied [78]. These metals were complexed with sodium diethyldithiocarbamate in a flow system, and, after simultaneous enrichment on a microcolumn, were determined by gas chromatography with mass spectrometric detection (GCMS). Comparative studies showed that CNTs and C60 fullerenes were superior to graphitized carbon black (GCB) and C18-silica for the extraction of the 11 compounds studied. As can be seen from the results presented in Table 2, 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 the highest sensitivities for the determination of organometallic compounds were observed for

Table 2. Sensitivitya for the determination of lead, mercury and tin compounds after preconcentration on various sorbents [76] Compoundb +

MeHg TML+ EtHg+ Hg2+ DML2+ Sn2+ TEL+ DEL2+ MBT3+ DBT2+ TBT+


C60 fullerene



445 800 420 455 770 820 999 605 640 620 690

400 700 370 420 690 740 900 540 570 560 630

350 670 320 375 630 600 820 510 535 515 590

185 390 190 220 350 375 450 280 300 270 340


Slope of the calibration graphs: analyte area-to-internal standard area · 103, concentration in ng/mL, sample volume 25 mL (excepting 15 mL for C18-silica). GCB – graphitized carbon black. b Compounds: MeHg+, Methylmercury; TML+, Trimethyllead; EtHg+, Ethylmercury; DML2+, Dimethyllead; TEL+, Triethyllead; DEL2+, Diethyllead; MBT3+, n-Butyltin; DET2+, Di-n-butyltin; TBT+, Tri-n-butyltin.



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CNTs. Moreover, the method proved quite selective with CNTs and fullerenes, since most of the foreign metals exhibited no interference at concentrations 1000 times greater than those of the analytes. The selectivity was lowest for GCB and C18-silica; the metal ions posing the most severe interference were Cu(II) and Co(II) for organotin species and Fe(III) and Zn(II) for trimethyllead. The proposed method was successfully applied for the determination of lead, mercury and tin compounds in water and coastal sediment samples. The enrichment ability of CNTs was also applied for the preconcentration of thallium(I) at pH 5 and its separation from the matrix [79]. The total thallium content was then determined by reducing Tl(III) with hydroxylamine. The PF, calculated as the ratio between the slopes of the calibration curves, obtained applying the enrichment procedure and directly by ETAAS was 20. The proposed on-line system was able to determine thallium at ng/L levels in drinking water.

5. CNT-based nanocomposites CNT-based composites can combine the properties of CNTs with those of their guest components and have been studied extensively in recent years [80,81]. Encapsulating polymers and metal oxides in hollow

cavities of CNT is an interesting route for preparing new functional materials. A high CNT volume fraction allows the properties of the CNTs to dominate the composite properties, while the matrix provides support (e.g., resisting buckling under shear or compression), protection and a way of sharing load between the CNTs [81]. The encapsulation approach primarily protects the nanoparticles against the external environment, hampers aggregation and may improve the dispersion stability of core-shell nanomaterials in a wide range of suspending solvents. It is very interesting that the cavities of CNTs can be filled with ferromagnetic materials (e.g., Fe, Co or Ni) [82,83]. CEMNPs are core-shell materials with surface characteristics similar to CNTs; this similarity enables their use as solid sorbents. A unique, attractive property of CEMNPs is that magnetic nanoparticles can readily be isolated from a sample solution by applying an external magnetic field. Their surface can be functionalized with appropriate functional groups (Fig. 4) [84]. Briefly, CNTs were dispersed in a Fe(NO3)3 solution with the help of an ultrasonic bath. After draining excess water on a rotary evaporator with a vacuum pump, the resulting materials were reduced using hydrogen at, successively, 560C and 900C. In this way, Fe nanoparticles were deposited inside the inner cavities of CNTs. They were then attacked by carbon radicals generated by the thermal

Figure 4. Scheme for the preparation of carbon-encapsulated magnetic Fe nanoparticles [73].


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decomposition of azodiisobutyronitrile. The Fe-CNTcyano material produced was refluxed in an aqueous sodium hydroxide-methanol mixture. CNT-iron oxide magnetic composites showed good sorption properties for the removal of transition-metal ions (Cu, Co, Pb) from aqueous solutions [85,86]. The forces of attraction between the anionic surface sites and the cationic metal ions easily result in the formation of metal-ligand magnetic composite complexes. The sorbents used, due to their excellent magnetic performance, are easily mobile and separable from the solvent in a weak magnetic field. It was found that the adsorption capacity of the CNTiron oxide magnetic composites for Sr(II) is higher than that of CNTs and iron oxides at pH >7, even though the preparation process of the iron oxides was the same as that of the magnetic composite [87]. Probably, the synergistic effect between the oxidized CNTs and the iron oxides improves the adsorption capacity of the magnetic composites. Chen et al. [88] examined the interaction between Eu(III) and a CNT-iron oxide magnetic composite in the absence and the presence of poly(acrylic acid) (PAA) as a surrogate for natural organic matter. They found that the presence of PAA resulted in strong enhancement of Eu(III) adsorption and adsorbed PAA anions can be considered as a ‘‘bridge’’ between the magnetic com-


posite and Eu(III). However, above pH 4, differences were observed in the decreasing trend of Eu adsorption when different addition sequences for reagent were applied (Fig. 5). The highest adsorption was obtained when the magnetic composite and PAA were preequilibrated before addition of Eu(III). These results are important for estimating and optimizing the removal of organic and inorganic pollutants by the magnetic composite. CNTs can also be used as support for other metal oxides. An Al2O3-CNT nanocomposite was studied as a sorbent in a flow preconcentration method for nickel determination by flame AAS (FAAS) [89], while MnO2coated CNTs were proposed for the enrichment of Pb(II) from aqueous solution [90]. These nanocomposites were shown to be very stable after several preconcentration and elution cycles, and did not need any ligand complexation of the analytes. Approximately 200 cycles without sorption capacity loss could be achieved even when working at a flow preconcentration rate of 10 mL/min [89]. Cerium nanoparticles supported on CNTs were developed for the removal of arsenate [91] and chromium [92]. Calcium and magnesium ions significantly enhance the adsorption capacity of CeO2-CNTs due to a ternary surface complex reaction between solid surface, cations and As(V) anions.

Figure 5. Percentage adsorption of Eu(III) on the magnetic composite in the presence of PAA as a function of pH with different sequences of reagent addition. (1) Eu(III) and PAA were pre-equilibrated before addition of the magnetic composite; (2) the magnetic composite and Eu(III) were pre-equilibrated before addition of PAA; and, (3) the magnetic composite and PAA were pre-equilibrated before addition of Eu(III). Conditions: Eu(III) = 4 · 105 mol/L; PAA = 60 mg/L; I = 0.1 mol/L NaClO4; m/V = 0.6 g/L [88].



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TiO2-CNT nanocomposites have been studied for the removal of Cr(VI) through adsorption and photocatalytic reduction to Cr(III) under ultraviolet irradiation [93]. 6. Conclusions

[4] [5] [6] [7] [8] [9] [10]

The need for efficient methods for sample concentration and clean-up in environmental analysis is constantly growing. The new carbon nanoparticles have been shown to be effective adsorbents for the removal of metal ions as well as their complexes. They are also very useful in speciation analysis. The presence of the active sites on the surface, inner cavities and inter-CNT space contributes to the high metal-removal capability. The possibility of chemical functionalization can be utilized in developing new microseparation methods and techniques. Whilst the above studies indicate that CNTs are potentially efficient adsorbents for metal ions in different kinds of environmental samples, their practical application may be hampered by their high cost and the difficulty in collecting them from their dispersing medium. Lu et al. [64] performed statistical analysis on the replacement cost of CNTs, which had been reported to be effective Ni(II) sorbents and could be re-used through several cycles of water treatment and regeneration. The results of their analysis revealed that such re-usable carbonaceous sorbents could be used as cost-effective sorbents regardless of their relatively high unit cost. CNTs are now commercially available with different outer diameters and lengths. Moreover, CNTs showed better reversibility of metal-ion sorption and less weight loss after repeated sorption/desorption processes than granulated activated carbon [56,64]. Tuzen et al. [39] reported that 300 mg of CNTs could be used for more than 250 experiments without any changes in their sorption behavior. Nanocomposites can combine the properties of CNTs with those of their guest components (e.g., metal oxides or chelating polymers). They exhibit intrinsic surface reactivity and large surface areas, and can strongly chemisorb several substances. These sorbents, due to their excellent magnetic performance, are easily mobile and separable from the solvent under low magnetic field. The preparation of these adsorbents is very simple and cost efficient, compared with other commercially-available SPE materials [94]. The modification with complexing reagents could increase the number of binding sites to interact with metal ions, thus increasing their selectivity.




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