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2012 © The Japan Society for Analytical Chemistry
Study on Multi-walled Carbon Nanotubes On-line Separation/ Preconcentration of Chromium(III) and Chromium Speciation Hongmei YU,*† Wei SUN,** Xiuhui ZHU,* Xiaoming ZHU,* and Jianjun WEI*** *School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, P. R. China **Benxi Chemical Industry School, Benxi 117019, P. R. China ***107# (Department of Environmental Engineering), Beijing University of Chemical Technology, Beijing 100029, P. R. China
Using multi-walled carbon nanotubes (MWCNTs) as an adsorbent has been established for the on-line separation and preconcentration Cr(III) and chromium speciation. The surface functional groups and negative charges of MWCNTs are beneficial to the adsorption of Cr(III). At pH 3.0 – 6.0, a discrimination of Cr(III) and Cr(VI) is achieved on the MWCNTs surface. Cr(III) ions are adsorbed onto the oxidized MWCNTs surface, while Cr(VI) has no affinity for the MWCNTs. The adsorbed Cr(III) is quantitatively eluted by 10% (v/v) nitric acid with detection by flame atomic absorption spectrometry. By loading a 6.0-ml sample solution, an enrichment factor of 22, a detection limit (3σ) of 1.15 μg l–1 and a precision of 1.7% RSD at the 30 μg l–1 level (n = 7) are achieved for Cr(III) within a linear range of 5 – 200 μg l–1 (r = 0.9994). After Cr(VI) has been reduced to Cr(III) with hydroxylamine hydrochloride, the total amount of chromium is obtained, and the content of Cr(VI) is given by subtraction. The procedure is validated by analyzing chromium in a certified reference material, and is further applied for the speciation of chromium in electroplating wastewater samples with satisfactory results. (Received August 2, 2012; Accepted October 28, 2012; Published December 10, 2012)
Introduction As is well known, elements with different speciation have different toxicity. Chromium is a typical element characterized by its existence as various oxidation species. Two oxidation states of chromium, Cr(III) and Cr(VI) show different levels of toxicity to the environment, humans, animals and plants.1 Generally speaking, the toxicity of Cr(VI) is more than that of Cr(III). Cr(III) is considered to be an essential element in mammals for the maintenance of glucose, lipid and protein metabolism, whereas Cr(VI) is considered to be a toxin because of its ability to oxidize other species and its adverse impact on the lungs, liver and kidneys.2 Owing to these distinctly different physiological effects, it is particularly important to detect the species and to determine its concentration; and a great number of speciation studies have been performed.1–3 Due to the low level of chromium in water and/or matrix interferences, it is very difficult to quantify the chromium concentration with sufficient sensitivity. Therefore, it is essential to conduct sample enrichment and separation before testing. For this purpose, various separation and preconcentration methods, such as coprecipitation,7 liquid–liquid solid-phase extraction,4–6 8 and cloud-point exchange,9 have been developed. extraction Among these methods, the mini-column preconcentration technology is most widely employed because a variety of adsorption materials are used for adsorbing Cr(III) and/or To whom correspondence should be addressed. E-mail:
[email protected]
†
Cr(VI) selectively. Various new solid-phase extraction materials have been used for chromium speciation, such as modified silica gel,5 resin,6 ion exchanger,10 diatomite,11 polytetrafluoroethylene (PTFE),12 zeolite,13 oxides14 and biosorbent.15–17 Out of all the options, carbon nanotubes (CNTs) as an employed novel adsorbent has recently been widely investigated due to its Tuzen et al.24 have established a unique properties.18–23 solid-phase extraction procedure for chromium speciation based on the Cr(VI)-ammonium pyrrolidine dithiocarbamate (APDC) chelate on multi-walled carbon nanotubes (MWCNTs). However, an organic solvent was used to elute the adsorbed chelate on the nanotubes in the system. In the present work, we report on a novel method for the separation/preconcentration of Cr(III) ions and chromium speciation with oxidized MWCNTs as an adsorbent without any chelating reagent. The oxidized MWCNTs are packed into a mini-column in a flow injection (FI) system for on-line Cr(III) sorption, preconcentration and elution with flame atomic absorption spectrometry (FAAS) for monitoring. No organic solvent and/or organic reagent is consumed in the process. The adsorption mechanisms of Cr(III) on the oxidized MWCNTs have been investigated. We also validated the system by the preconcentration and determination of Cr(III) in a certified reference water sample and the spiking recovery in various electroplating wastewater samples. After the reduction of Cr(VI) to Cr(III), the total amount of chromium is obtained, and the content of Cr(VI) is given by subtraction.
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Experimental Apparatus and measurements An AAnalyst-200 atomic absorption spectrometer (AAS) (PerkinElmer, Waltham, USA) with a chromium hollow cathode lamp (Vigous Instrument Co., Ltd., Beijing, China) is used for absorbance measurements of chromium. The lamp is used as the light source at a wavelength of 357.9 nm, and is operated at 5 mA with a 0.8-mm spectral bandpass. A burner height of 10 mm, an air flow rate of 4.4 l min–1 and an acetylene flow rate of 3.46 l min–1 are employed. An LZ-2000 flow injection (FI) system (Zhaofa Institute for Laboratory Automation, Shenyang, China) is employed for the on-line separation and preconcentration of Cr(III). The FI system consists of two peristaltic pumps, an injection valves and a laboratory-made PTFE mini-column packed with the oxidized MWCNTs, which is located in the loop of the injection valves. A silicone pump tube (4.0 mm o.d. × 2.0 mm i.d.) is used to deliver the sample solution. Other external channels are connected with a 0.5-mm i.d. PTFE tube. Characterization of the oxidized MWCNTs is performed by recording the FT-IR spectra with a WQF-200 spectrometer (The Second Optical Instrument Factory, Beijing, China). Zeta potentials of the oxidized MWCNTs are measured by a Malvern zetameter (Zetasizer Nano ZS). pH measurements are performed using a pHS-3C digital pH meter (Hangzhou East Star Equipment Factory, Zhejiang, China). Reagents and chemicals All chemicals used in this work were of at least of analytical reagent grade, and doubly deionized water of 18 MΩ cm was used throughout. A 1.000 g l–1 Cr(III) stock solution was prepared by dissolving 0.5124 g of CrCl3·6H2O (AR, Shenyang Chemicals Reagents Co., Shenyang, China) in a 100-ml solution of 0.1 mol l–1 hydrochloric acid (GR, Shenyang Chemicals Reagents Co., China), and was stored in the dark after preparation. A 10.0 μg ml–1 Cr(III) solution was obtained by dilution of this stock solution with the same concentration as the HCl solution. Standard solutions of various concentrations were obtained daily by a step-wise dilution of the 10.0 μg ml–1 Cr(III) solution with doubly deionized water. A 1.000 g l–1 Cr(VI) stock solution was prepared by dissolving 0.2829 g of K2Cr2O7 dried at 120° C for 1 h (AR, Beijing Chemicals Reagents Co., Beijing, China) in 100 ml doubly deionized water, and was stored in the dark after preparation. Standard solutions of various concentrations were obtained daily by a step-wise dilution of this solution with doubly deionized water. A 10% (v/v) HNO3 solution (GR, Shenyang Chemicals Reagents Co., China) was used as the eluent, and a 0.1 g ml–1 hydroxylamine hydrochloride solution (AR, Beijing Chemical Reagent Co., China) was used as a reducing agent. The pH value of the sample solution was adjusted with either HNO3 or NaOH (GR, Shenyang Pharmaceutical Co., Ltd., Shenyang, China) of 0.1 mol l–1. A chromium component analysis of an analog natural water reference material (GBW (E) 080403) was from the National Research Center for Certified Reference Materials (Beijing, China). Mini-column packing The pretreatment of MWCNTs was as follows: 0.2 g of MWCNTs (95% purity, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China) was soaked in 10 ml of concentrated HNO3 for 24 h to remove any residual metal catalyst. The MWCNTs was then treated by refluxing in C for 2 h. Finally, concentrated HNO3 as an oxidant at 120°
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a thorough rinse with 500 ml of doubly deionized water was performed to remove any excessive acid, followed by drying at 100° C for 2 h. The oxidized MWCNTs was stored for further studies. Further, 2.0 mg of the oxidized MWCNTs was packed into a piece of PTFE tube (30 mm o.d. × 2.0 mm i.d.) to form a mini-column with an effective length of approximately 3.0 mm. The mini-column was blocked at both ends with a small amount of glass wool to keep the MWCNTs in place. Before use, the mini-column was activated by passing through 10% (v/v) HNO3 and doubly deionized water at a flow rate of 2.0 ml min–1 for 1 and 2 min, respectively, and finally the mini-column was evacuated by air. General procedure During the sample loading process, a two-position valve was set at the loading position. A sample or a standard solution at pH 4.0 was driven by a peristaltic pump at a flow rate of 2.0 ml min–1 to flow through the mini-column for 3 min; during this process Cr(III) ions were retained onto the mini-column. In the meantime, a doubly deionized water flow was used to clean the nebulizer of the atomic-absorption spectrometer. After evacuating the sample loading tube and the mini-column for 10 s, the two-position valve was switched to the injection position. The dissolution of the retained analyte was facilitated by pumping a HNO3 solution (10%, v/v) to flow through the mini-column in the opposite direction at a flow rate of 5.2 ml min–1 for 4 s, and the eluate was directly transferred into the flame atomic-absorption spectrometer for quantification. The mini-column was flushed with doubly deionized water for a period of 8 s at a flow rate of 5.2 ml min–1 after each cycle of the preconcentration-elution operation. It was evacuated with air before the next operation started. Determination of Cr(VI): After taking another water sample, adding 1 ml of 0.1 g ml–1 hydroxylamine hydrochloride into the sample solution to convert Cr(VI) to Cr(III), the total amount of chromium was obtained according to the above-mentioned experimental method, and the concentration of Cr(VI) was given by subtraction.
Results and Discussion Oxidation of MWCNTs and the adsorption mechanisms of Cr(III) by MWCNTs The pretreatment of CNTs significantly influences its adsorption capability.18 Literature25 has reported that the increases of surface area and the pore volume of CNTs processed by the concentrated HNO3 are superior to that of other oxidants, such as H2O2 and KMnO4. Thus, concentrated HNO3 as an oxidant is chosen to process the MWCNTs. After the oxidation treatment, the MWCNTs surface can be introduced into a large number of oxygen-containing functional groups26 and negative charges,25 which are conducive to improve the adsorption capacity of MWCNTs. In our study, MWCNTs were treated by C for a period of refluxing in the concentrated HNO3 at 120° time. Moreover, the effects of the reflux time within a range of 1 – 3 h on the adsorption efficiencies of Cr(III) adsorbed on the oxidized MWCNTs surface were investigated. The results show that the adsorption efficiencies of Cr(III) increase with the extension of the reflux time up to 2 h, and thereafter decrease with the further prolongation of the reflux time for up to 3 h. Thus, a reflux time of 2 h was chosen to process the MWCNTs. The adsorption of Cr(III) onto the oxidized MWCNTs surface is attributed to the functional groups of the MWCNTs surface
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Fig. 2 Zeta potential curve vs. pH of the oxidized MWCNTs.
Fig. 1 FT-IR spectra of the oxidized MWCNTs. a, Before adsorption of Cr(III); b, after adsorption of Cr(III); c, after elution of Cr(III).
and the role of electrostatic attraction. When Cr(III) in the solution flows through the MWCNTs mini-column, the Cr(III) ions are retained on the MWCNTs surface via interaction with the oxygen-containing functional groups on its surfaces. This is illustrated by the change in the FT-IR spectra of the oxidized MWCNTs before and after the adsorption of Cr(III) (Figs. 1a and 1b). These stretching vibration bands of –OH at 3461 cm–1, >C=O at 1744 cm–1, –CO at 1097 cm–1 and the bending vibration band of –OH at 1639 cm–1 are observed for the oxidized MWCNTs. After the adsorption of Cr(III), the red shifts from 3461 to 3440 cm–1 and from 1639 to 1602 cm–1 of the absorption band for –OH is recorded. At the same time, the stretching vibration bands of >C=O and –CO are also shifted from 1744 to 1669 cm–1 and 1097 to 1084 cm–1, respectively, which is attributed to the coordination interaction. The FT-IR spectrum of the oxidized MWCNTs after elution of the Cr(III) is shown in Fig. 1c. The FT-IR spectrum is similar to that of the oxidized MWCNTs before the adsorption of Cr(III). This indicates that the adsorbed Cr(III) on the oxidized MWCNTs is completely eluted. The negative charges of the oxidized MWCNTs surface are also favorable to the adsorption of Cr(III), due to the role of electrostatic interactions. Figure 2 shows the variation in the zeta potential of the oxidized MWCNTs dependence of the pH. The results show that the zeta potentials of the oxidized MWCNTs are negative values in the whole pH range of 1 – 12. As observed, a significant fall in the zeta potential of the oxidized MWCNTs is recorded as the pH values of the sample solution increase from 1.0 to 3.0. The zeta potential of the most negative interval is observed within a range of pH from 3.0 to 6.0. After that, the zeta potential values slightly add in the range of pH 7.0 – 12.0. The role of electrostatic attraction is favorable to the Cr(III) adsorbed onto the MWCNTs surface. Especially in the range of pH 3.0 – 6.0, a potential of approximately –34.4 mV is more beneficial to the adsorption of Cr(III) on the oxidized MWCNTs. pH dependence of chromium adsorption on MWCNTs The pH value of a sample solution has a significant influence on the adsorption of Cr(III) and Cr(VI) ions, because it affects not only the sorbent surface properties, but also the distribution of metal species.16 The adsorption efficiencies of Cr(III) and Cr(VI) on the oxidized MWCNTs surface were investigated by varying the pH values of the sample solutions in the range of
Fig. 3 Effects of the pH on the sorption efficiencies of Cr(III) and Cr(VI) on MWCNTs. Mass of MWCNTs, 2.0 mg; sample volume, 6 ml; Cr(III) concentration, 100 μg l–1; eluent, 10% (v/v) HNO3; sampling flow rate, 2.0 ml min–1; elution flow rate, 5.2 ml min–1; elution time, 4 s.
1.0 – 11.0 according to the recommended procedure. The results are shown in Fig. 3. As observed, a significant increment in the sorption efficiencies of Cr(III) is recorded as pH values of the sample solution increase from 1.0 to 3.0. However, the adsorption efficiencies of Cr(VI) decrease. The reason is that the electronegativity of the oxidized MWCNTs enhances with the increase of the pH, and the role of electrostatic attraction is favorable to the adsorption of Cr(III); however, the role of the electrostatic repulsion is not beneficial to the adsorption of Cr(VI). The maximum retention of Cr(III) is found at pH 3.0 – 6.0 with adsorption efficiencies of 98% and higher; however, the adsorption efficiencies of Cr(VI) are lower than 7%. This indicates that at pH 3.0 – 6.0, discriminations of Cr(III) and Cr(VI) are achieved on the oxidized MWCNTs surface. With a further increase of the pH up to 11.0, the retention of Cr(III) decreases because of its hydrolysis, but its adsorption efficiencies are still much higher than that of Cr(VI), like 75% and above. Thus, pH 4.0 was selected as the enrichment acidity for further study. A novel procedure for the on-line separation/preconcentration of Cr(III) and chromium speciation was developed by employing a flow-injection mini-column system. The method includes a separation and preconcentration of Cr(III) on the oxidized MWCNTs at pH 4.0, and its subsequent detection with FAAS after elution, followed by converting Cr(VI) to Cr(III) and total chromium analysis, and Cr(VI) is obtained by subtraction.
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Table 1 Characteristic performance data of the procedure for Cr(III) analysis
Fig. 4 Effects of the enrichment time. Mass of MWCNTs, 2.0 mg; Cr(III) concentration, 100 μg l–1; eluent, 10% (v/v) HNO3; sampling flow rate, 2.0 ml min–1; elution flow rate, 5.2 ml min–1; elution time, 4 s.
Effect of enrichment variables With a sample volume of 6.0 ml, the adsorption efficiencies of Cr(III) ions were investigated by varying the sample loading flow rate within 20 – 60 rpm. The results show that the adsorption efficiency is the best at a loading flow rate of 20 rpm; however, the adsorption efficiencies gradually become lower as the sampling flow rate increases from 20 up to 60 rpm, indicating that an excessive flow rate leads to a lack of adsorption time and a decrease in the adsorption capacity of the mini-column. Therefore, a sampling flow rate of 2.0 rpm, i.e., 2.0 ml min–1, has been selected for further investigations. The effects of the sampling time from 1 to 6 min were also investigated by fixing the other experimental parameters, as illustrated in Fig. 4. It shows that the absorbance significantly increases at a greater slope of the linearity with an increasing enrichment time of up to 3 min. However, the absorbance afterwards increases at a lower slope of the linearity, i.e., the rate of increase per unit time decreases. Meanwhile, the sampling frequency becomes lower with an increase of enrichment time. The results show that a sampling time of 3 min is appropriate when assessing the entire procedure in terms of the sensitivity, enrichment factor and analytical frequency. Thus, a sampling time of 3 min, i.e., a sampling volume of 6.0 ml, is used during testing. Effects of elution variables The effects of the same concentrations of nitric acid, hydrochloric acid, phosphoric acid, oxalic acid, and acetic acid as eluents on the elution efficiencies of Cr(III) retained on the oxidized MWCNTs were examined under the same elution volume condition. The results show that an organic weak acid is not capable of completely eluting the retained Cr(III) from the surface of the oxidized MWCNTs, as compared with inorganic acids. Among the three inorganic acids, the elution efficiency of HNO3 is the best. The effects of the HNO3 concentration were also investigated within the range of 2 – 12% (v/v). The results indicate that the elution efficiencies are gradually improved with an increase of the HNO3 concentration up to 10% (v/v), and level off afterwards. Therefore, the retained Cr(III) is considered to have been completely eluted by HNO3. A 10% (v/v) HNO3 solution is thus employed. With a fixed Cr(III) concentration of 100 μg l–1 and an elution time of 4 s, the effects of the elution flow rate in the range of 2.2 – 6.2 ml min–1 on the elution efficiency of the retained Cr(III) were investigated. The results show that the elution
Parameter
Value
Sample volume Linear range Regression equation Correlation coefficient Enrichment factor Detection limit (3σ, n = 11) RSD (30 μg l–1, n = 7) Dynamic sorption capacity Sampling frequency
6.0 ml 5 – 200 μg l–1 A = 0.9745 × 10–3C (μg l–1)+ 0.0238 0.999 22 1.15 μg l–1 1.7% 7.1 mg g–1 16 h–1
Table 2 Comparisons of the analytical performance of the different sorbents preconcentration of chromium with detection by FAAS
Adsorbent
S. carlsbergensis SG-p-DMABD Fiber Nb2O5/SiO2 Chelating resin XAD-7 PS-NAPdien Oxidized MWCNTs
Detection Sample RSD, % limit/μg l–1 volume/ ml Cr(III) Cr(VI) Cr(III) Cr(VI) 100 50 25 13.2 100 30 10 6.0
7.4 1.1 50 0.34 1.58 3.27 0.6 1.2
— — — — — — 2.5 —
5.0 5.0 4.3 4.6 — 5.0 2.55 1.7
— — — — — — 0.8 —
Ref.
3 5 27 28 29 30 31 This method
SG, Silica gel; p-DMABD, p-dimethylaminobenzaldehyde; PS-NAPdien, polystyrene functionalized with N,N-bis(naphthylideneimino)diethylenetriamine.
efficiencies gradually improve with the increase of the elution flow rate, however, they significantly decrease when the elution flow rate is more than 5.2 ml min–1. The reason is that the elution flow rate is too fast to insufficiently elute Cr(III) retained on the oxidized MWCNTs. Therefore, an elution flow rate of 5.2 ml min–1 is adopted for further investigations. Under the above optimum elution flow rate condition, the effects of the elution time in the range of 2 – 10 s on the elution efficiencies of the retained Cr(III) were also investigated. The results show that elution efficiencies of 95% and higher can be achieved during the whole range of elution times, indicating that the elution process is fairly fast. In practice, a fixed elution time of 4 s is used. Interference effects of foreign species The potential interference effects of some foreign species, which are frequently encountered in environmental water samples, were tested using the presented procedure by gradually increasing the amount of foreign ions. At a Cr(III) concentration level of 30 μg l–1 and within a ±5% error range, 10000-fold of Na+, K+, 5000-fold of Cr2O72–, 500-fold of Ca2+, Mg2+, 300-fold of Ag+, 200-fold of Ni2+, Zn2+, Co2+, 100-fold of Pb2+, Cu2+, 10-fold of Al3+ pose no interferences on the determination of Cr(III). Large anions, Cl–, PO43–, CO32–, SO42–, I–, Ac–, etc., also pose no interferences on the Cr(III) concentration determination. Therefore, this procedure can be directly employed without any further treatment or use of masking reagents.
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Table 3 Determination of the speciation analysis of chromium in electroplating wastewater samples (sample volume, 6 ml; n = 3; 95% confidence level) Determined Spiked Cr(III) Cr(VI) Sample No. –1 –1 –1 –1 –1 –1 Cr(III)/μg l Cr(VI)/μg l Found/μg l RSD, % Recovery, % Found/μg l RSD, % Recovery, % Cr(III)/μg l Cr(VI)/μg l 1 2 3
38.69 ± 2.9 29.60 ± 0.6 38.17 ± 0.6
35.41 ± 3.0 89.10 ± 0.5 75.10 ± 0.5
40.0 40.0 40.0
40.0 80.0 40.0
Analytical performances The analytical performance data obtained under the aforementioned experimental conditions are summarized in Table 1 concerning the preconcentration of trace Cr(III) by using the oxidized MWCNTs as an adsorbent with detection by FAAS. Obviously, the presented procedure provides a low detection limit of 1.15 μg l–1, along with a precision of 1.7% RSD at 30 μg l–1 (n = 7). When a 5.0 mg l–1 Cr(III) solution at a flow rate of 2.0 ml min–1 flows through the mini-column packed with 2.0 mg of the oxidized MWCNTs for 5 min, a maximum dynamic sorption capacity of 7.1 mg g–1 for Cr(III) is achieved. A comparison of the characteristic performance data of the system with some of the reported procedures based on solid-phase extraction with detection of FAAS is summarized in Table 2. The detection limit and the RSD obtained by this approach are at the same level as those of the reported methodologies in the case of low sample consumption. Speciation analysis of chromium in water samples The method was validated by analyzing chromium in a chromium component analysis of an analog natural water reference material, i.e., GBW (E) 080403 (0.50 ± 0.04 mg l–1). The obtained value for the chromium concentration 0.506 ± 0.021 mg l–1 agrees well with the certified value. It was further applied for the determination of Cr(III) and Cr(VI) by spiking recoveries in three electroplating wastewater samples. Before analysis, these water samples were filtered through a cellulose membrane of 0.45-μm pore size, and adjusted to pH 4.0 with HNO3. The results are summarized in Table 3. It can be seen that satisfactory spiking recoveries within the range of 100.0 – 102.2% for Cr(III), and 95.7 – 108.0% for Cr(VI) were achieved, which demonstrates the practical usefulness of this approach.
Conclusions We established an on-line separation/preconcentration procedure for the selective sorption of Cr(III) and chromium speciation based on FAAS detection with oxidized MWCNTs used as an absorbent. The MWCNTs oxidized with concentrated HNO3 can not only provide a large number of oxygen-containing functional groups, but can also provide a large number of negative charges. Both help in the retention of Cr(III) onto the oxidized MWCNTs surface, while Cr(VI) remains in the solution. The detection limit and the RSD obtained by this approach are at the same level as those reported for the chromium separation/preconcentration methods in the case of low sample consumption, and does not need to consume any organic solvent and/or organic reagent. This system can be used for chromium speciation analysis in various environmental water samples.
40.91 ± 3.0 40.00 ± 0.4 40.43 ± 0.4
1.2 1.5 1.5
102.2 100.0 101.1
38.30 ± 3.0 6.40 ± 0.8 38.60 ± 0.8
1.7 1.9 1.9
95.7 108.0 96.5
Acknowledgements This work is financially supported by the Natural Science Foundation of China (No. 20807044), the Science and Technology Foundation of Education Department of Liaoning Province (No. L2011043) and the Science and Technology Foundation of Anshan City, China (No. 08SF05).
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