Indian Journal of Chemistry Vol. 51A, November 2012, pp. 1567-1573
Flow injection preconcentration system using a new functionalized resin for determination of cadmium(II) by flame atomic absorption spectroscopy Reena Saxena* & Suneeti Singh Department of Chemistry, Kirorimal College, University of Delhi, Delhi 110 007, India Email:
[email protected] Received 12 April 2012; revised and accepted 3 October 2012 A rapid and sensitive method for the determination of trace level of Cd(II) in industrial effluents by flame atomic absorption spectroscopy has been developed. The technique is based on the online flow injection preconcentration of Cd(II) ions on a mini column, packed with Amberlite XAD-2 functionalized with xylenol orange. Flow injection variables have been optimized for the determination and quantitative preconcentration of Cd(II). Cd(II) ions are effectively retained on the mini column at pH 5.0. A high sorption capacity of 348 µmol/g of dry resin is obtained and chelating resin can be reused for 70-80 cycles of sorption desorption without any significant change in the sorption capacity. With preconcentration times of 60 s and 120 s at a sample loading flow rate of 5 mL min-1, preconcentration factors of 17 and 32 are obtained. The calibration graph using the preconcentration system for Cd(II) is linear over the concentration range of 0-120 µg L-1. The precision for 5 replicate measurements are 1.68 and 1.12 for the determination of 20 and 50 µg L-1 Cd(II) respectively. The limits of detection obtained are 1.038 and 0.57 µg L-1 for 60 s and 120 s sorption time respectively. A sample throughput of 32 h-1 is obtained with 5 mL of sample solution. Spiked recovery studies in water samples with certified Cd(II) nitrate solution traceable to NIST shows recovery in the range of 98-99 %. The accuracy of the developed online FI-FAAS procedure has been validated by analyzing cadmium(II) in standard reference material (SRM-1643e). Keywords: Flame atomic absorption spectrometry, Online preconcentration, Amberlite XAD-2, Xylenol orange, Cadmium
Monitoring the presence of toxic trace elements in diverse matrices is an extremely important task to evaluate occupational and environmental exposure. In this context, cadmium is one of the most toxic elements and accumulates in humans mainly in the kidneys and liver and is classified as a prevalent toxic element1. It damages filtering mechanisms in kidney, which causes the excretion of essential proteins and sugars from the body and further kidney damage. According to the Central Pollution Control Board (CPCB) standard (EPA and IS:10500, 1992), the
permissible limit for Cd(II) in wastewater is 2 mg L-1 and the permissible level for Cd(II) in drinking water is 0.001 mg L-1 (ref. 2). Thus, cadmium(II) can be considered as a very serious water toxin and therefore, there is a critical need for determination of cadmium(II) at micro trace level in complex matrices which is a challenging problem. Often determination of trace elements in industrial effluents is associated with a preconcentration step since extremely low concentrations of metals are present in the samples. Some enrichment procedures have been developed for metal determination involving different analytical techniques such as co-precipitation3, liquid-liquid4, cloud-point5 and solid-phase extractions (SPE)6,7, etc. Solid phase extraction (SPE) has been extensively used for the separation and preconcentration of trace elements because this approach offers a several important benefits such as selectivity, eco-friendliness, reusability of resin, high preconcentration factors, achievement of high recoveries and easy recovery of the solid phase. Various solid phases including carbon nanotubes8, silica gel9, Amberlite XAD resins10,11, polyurethane foam12, and activated carbon13 have been used for preconcentrating traces of metals in environmental samples prior to their determination. Commercially available Amberlite XAD-2 and XAD-4 resins are polymeric sorbents based on polystyrenedivinylbenzene polymers with higher surface areas as compared with other chelating resins. The surface of this material is hydrophobic due to hydrocarbon chains. Thus, the retention of trace elements on such materials requires the addition of a ligand such as Quinoline-8-thiol14 to the sorbent. Among various analytical techniques used for metal determination, flame atomic absorption spectroscopy (FAAS) is widely used. However, this technique has low sensitivity for trace metals. The combination of flow injection online separation and preconcentration techniques which use solid-phase extraction has high potential for enhancing the relative sensitivity and selectivity of FAAS. The main benefits of this system are possibility of online sample treatment, high reproducibility, efficiency and sample throughput, low consumption of sample and reagent, cost effectiveness and minimum risk of contamination15.
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The aim of the present work is to develop an online flow injection based preconcentration system for the determination of trace levels of Cd(II) in industrial effluents by employing a mini column packed with xylenol orange modified Amberlite XAD-2 and by using flame atomic absorption spectroscopy (FAAS) for detection. The same resin has already been used for preconcentration of Pb(II) and shows good results16. Experimental A Perkin Elmer (Shelton, CT, USA) FIAS-400, flow injection analysis system was coupled with AAnalystTM 400 flame atomic absorption spectrometer for automatic processing of the method and operated in pre-concentration mode. The entire system was controlled by a personal computer and winlab32 (ver. 6.5.0.0266) application software. The FIAS-400 system17 consisted of two peristaltic pumps P1, P2, a 5-port 2-position injection valve and a pre-concentration column. It was connected to the spectrometer’s nebuliser, using a short PTFE capillary (20.0 cm length and 0.35 mm i.d.) in order to minimise the dead volume and eluent dispersion. A pH-meter (Elico Ltd, India) was used for pH measurements. CHNS elemental analysis was carried out on an Elementor (model Vario EL-III) analyser. Thermal analysis was carried out using a Perkin Elmer Diamond DSC. IR spectra (4000-400 cm-1) were recorded on a Perkin Elmer (model Spectrum RX-1) spectrophotometer. A stock solution of 1000 mg L-1 Cd(II) was prepared by dissolving appropriate amount of analytical reagent grade cadmium(II) nitrate in doubly distilled water and the metal ion solution was standardized by complexometric titration with EDTA before use. Standard solutions were prepared by dilution of the stock solutions. The pH adjustments were made with acetate and phosphate buffers (1–5 mL). The laboratory glassware was kept overnight in 10 % nitric acid solution and washed before use with deionised water. Doubly distilled water was used to wash the glassware and to prepare all solutions. A mixture of KCl (0.2 mol L-1), HCl (0.2 mol L-1), sodium acetate (0.1 mol L-1) and acetic acid (0.1 mol L-1) in appropriate ratio was used for making acidic buffer solutions. Buffer solution of pH 7 was prepared by mixing sodium hydroxide (0.1 mol L-1) and KH2PO4. Basic buffer solutions were prepared by mixing Na2HPO4 (0.1 mol L-1), HCl (0.1 mol L-1) and NaOH (0.1 mol L-1) in appropriate ratio. Amberlite XAD-2 resin (Sigma-Aldrich) used has surface area
approximately 300 m2/g, bead size 20–60 mesh, pore diameter 90 Å (mean pore size) and a pore volume of ~0.65 mL/g. Xylenol orange used was of analytical reagent grade. All analytical reagents were obtained either from E. Merck or Thomas Baker. Certified Cd(II) nitrate solution traceable to NIST was procured from E. Merck. Industrial effluents used in this study were collected from Rishikesh, UP, Mayapuri Industrial Area, Delhi and Noida Industrial Area, UP, India. The amino polymer was prepared by nitrating Amberlite XAD-2 with HNO3 and H2SO4 mixture followed by reducing the nitro derivative with SnCl2 and conc. HCl. The amino polymer was filtered off and washed with water and 2 mol L-1 NaOH, which released the free amino polymer according to the following equation: (RNH3 )2 SnCl6 + 8NaOH 2RNH2 + Na2SnO3 + 6NaCl + 5H2O The amino polymer was treated with 2 mol L-1HCl, washed with water to remove the excess HCl, suspended in 300 cm3 of ice-water mixture and mixed with small amounts (1 mL ) of 1 mol L-1 HCl and 1 mol L-1 NaNO2 until the reaction mixture began to give a permanent blue colour with starch-iodide paper18-20. Thereafter, the diazotized polymer was filtered, washed with cold water and reacted at 0–3 °C for 24 h with 2 g of xylenol orange, dissolved in 400 mL of water and 200 mL of glacial acetic acid. The dark brown coloured beads were filtered washed with 4 mol L-1 HCl and water successively and dried in air. The proposed structure of Amberlite XAD-2 functionalized with xylenol orange (AXAD-2-XO) is shown in structure (I). Anal. (%): Found: C, 58.99; H, 5.37; N, 6.88; calc. for C31H28N2O13SNa4N2C8H7.1H2O: C, 57.28; H, 4.53; N, 6.85. The elemental data confirm one water molecules per repeat unit of chelating resin.
Amberlite XAD-2 functionalized with xylenol orange resin
(I)
NOTES
The AXAD-2-XO resin (160 mg) was packed in a mini glass column (3.0 cm length and 3.0 mm internal diameter). The two ends of the column were sealed with cotton. The prepared column was treated with 2.0 mol L-1 HNO3 and washed with doubly distilled water until the resin was free from acid. A suitable aliquot of the solution containing Cd(II) in the concentration range 5–200 µg L-1 was passed through this column after adjusting its pH to an optimum value. The column had a constant performance during all experiments, and there was no need for any regeneration or repacking. Acid resistant tubes (Tygon) were used to pump the sample through the minicolumn with a peristaltic pump. Another peristaltic pump was used for elution of the enriched Cd(II) on the AXAD-2-XO resin complex with HNO3 at an optimum flow rate of 5.0 mL min-1 and then subjected to flame atomic absorption spectrometric determination. The FI-FAAS process was controlled by a computer program21 which includes pre-filling, filling , loading and elution steps . Each preconcentration cycle starts with loading time of 60 s and elution includes 20 s as reading time . The flow rate in the elution step was set at 5 mL min-1 to ensure recovery of maximum metal. Absorbance peak was recorded with its height taken as the analytical signal. The calibrations were linear in the range 0–120 µg L-1 of Cd(II). Three replicates of measurements were carried out for every standard and sample solutions. Each measurement was followed by a blank check.
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Results and discussion FT-IR spectrum of AXAD-2-XO resin (Fig. 1(a)) has additional bands at 3382 cm-1 (O-H stretching), 2925 cm-1 (C-H), 1651 cm-1 (COO-), 1519 cm-1 (N=N), 1341 cm-1 (C-N) and 1269 cm-1 (C-OH aromatic). This supports the loading of xylenol orange onto Amberlite XAD-2 with diazotized (N=N) coupling. Comparative analysis of the FT-IR spectra of AXAD-2-XO resin before and after Cd(II) sorption showed that C-OH and C-N of AXAD-2-XO resin at 1269 and 1341 cm-1 vibrations were shifted by 5–10 cm-1, suggest that chelation involving O-H group and N of iminodiacetate group in xylenol orange is at least partly responsible for the sorption of Cd(II). Conclusive evidence of the bonding is also shown by the observation that new bands in the FT-IR spectra of the Cd(II) metal complexes at 520 and 425 cm-1 assigned to ʋ(Cd-O) and ʋ(Cd-N) stretching vibrations respectively (Fig. 1(b)). Thermogravimetric analysis (TGA) curve of freshly prepared AXAD-2-XO resin shows a very slow but steady weight loss up to 500 °C . The observed weight loss is 2.3 %, up to 120 °C (Fig. 2). This is due to the physisorbed water on the resin, which supports the presence of one water molecule per repeat unit of the chelating resin. It is reasonable to assume that with each ring one dye molecule is covalently bonded through an azo group. It probably occupies a position para to the –CH-CH2– group, owing to electronic effect and steric reasons.
Fig. 1—FT-IR spectra of Amberlite XAD-2 functionalized with xylenol orange (a) before, and, (b) after sorption of cadmium.
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Fig. 2—TGA of xylenol orange.
Amberlite
XAD-2
functionalized
with
The pH plays a major role in the preconcentration mechanism for metal ions. The influence of pH on the preconcentration of Cd(II) on to the resin was investigated in the pH range 2–11 with 0.02 mg L-1 Cd(II) solution and by keeping all other parameters constant. The influence of the solution pH on analytical signal indicates that sorption is effective within pH range 4.5–6.0 and is maximum at pH 5.0. Therefore, pH 5 was selected as the optimum pH for further studies. The adsorption process involves the release of H+ ions to allow the firm complexation of metal ions to AXAD-2-XO resin as follows: M2+ + 2xylenol orange → M(xylenol orange)2 + 2H+. Some earlier reports on separation of Cd(II) with xylenol orange have reported the optimum pH as 6 (ref. 22), 7-8 (ref. 23) and 8-9 (ref. 24). The difference in optimum pH may be because of the different polymeric supports used in these studies. It may also be noted that Tiwari & Singh25 and Ion & Barbu26 have reported the optimum pH to be 4.5–5.0 and 5 respectively. The effect of nitric acid concentration was studied in the range of 0.01–1.5 mol L-1. The results show that with increase in the nitric acid concentration up to 0.5 mol L-1, absorbance increases and then decreases at higher concentrations. Therefore, 0.5 mol L-1 nitric acid was chosen as the optimum concentration of the eluent, while the elution time was fixed at 20 s in order to ensure complete elution of adsorbed Cd(II) ions. Online time-based preconcentration systems are influenced by the loading flow rate because it regulates maximum mass transfer from liquid to solid phase. The amount of sample varies with flow rate if preconcentration time is constant, for this experiment. Therefore, at fixed eluent flow rate of 5 mL min−1, the
effect of the sample flow rate on the preconcentration of 4.0 mL of 0.2 mg L-1 Cd(II) solution was investigated in the range of 2–7 mL min−1. Analytical signals decreased at flow rates higher than 5 mL min−1. Moreover, high flow rates increase back pressure and which could cause leakage. Thus, a flow rate of 5 mL min−1 was chosen for subsequent experiments as a compromise between efficiency and stability of system. A study on the influence of preconcentration time for Cd(II) shows that analytical signal increased almost linearly up to 180 s. This indicates that the transfer phase factor is constant within this time interval. Higher preconcentration times could be beneficial to the determination of the samples with lower Cd(II) concentrations. However, long periods are not suitable for flow injection preconcentration systems as sample throughput is low. Hence, a preconcentration time of 60 s was used for the determination of Cd(II) in water samples. The influence of eluent flow rate in the elution step was also studied within the range 2.0-7.0 mL min−1, as it establishes the velocity of acid solution through the minicolumn. The eluent was pumped into the column through the peristaltic pump of the FIAS-400 system. Results obtained shows that the analytical signal is maximum at eluent flow rate of 5.0 mL min-1. The rate of uptake of metal ions on AXAD-2-XO resin was studied by batch method. The resin beads (0.5 g) were stirred with 50 mL of 50 mg L-1 solution of Cd(II) at room temperature for 2-120 min. The adsorbed metal ion on the resin was eluted with 30 mL of 2 mol L-1 HNO3 and after appropriate dilution determined by FAAS. The amount of metal ion sorbed on the resin phase was calculated in as percentage of saturation of resin with cadmium. The loading half time (t1/2) required to reach 50 % sorption of the total loading capacity of resin was found to be < 7 min. This shows resin-metal interaction is sufficiently rapid for Cd(II) ions at optimum pH. The adsorptive capacity of the AXAD-2-XO sorbent for the retention of Cd(II) was also determined. About 100 mg of the resin was equilibrated with 100 mL of 100 mg L-1 Cd(II) at pH 5.0. After shaking for 6 h, the resin was filtered and the metal ions from the resin were desorbed by shaking the resin beads with 30 mL of 2 mol L-1 HNO3. The filtrate was aspirated into the flame of pre-standardized FAAS, after suitable dilution to calculate the sorption capacity. The results demonstrate that the sorbent has a sorption capacity of 348 µmol/g of dry resin, which is much higher as compared to other resins27. The reason for this high
NOTES
value may be due to high surface area of Amberlite XAD-2 and more chelating sites of xylenol orange. The sorption capacity of AXAD-2-XO is ~80 times higher than that of AC-XO. This may be due to chemical immobilization of XO on Amberlite XAD-2, due to which binding with Cd(II) is stronger as compared to binding with physisorbed XO on activated carbon22. Under the optimised conditions the detection limit was calculated by the 3σ criterion, defined as concentration that gives a response equivalent to three times the standard deviation of the blank and found to be 1.038 and 0.57 µg L-1 at 60 s and 120 s preconcentration times respectively. The calculation of the enhancement factor (EF) was based on the ratio of the slopes of the calibration curves, obtained with and without pre-concentration, using FAAS28. It is proposed that the detection limits and enrichment factors will increase with a longer preconcentration time. For standards buffered to pH 5 and 60 s loading time, EF was 17. Although the enrichment factor was higher when 120 s loading time was applied (EF 32), for satisfactory sample throughput (32 h-1), in further studies 60 s loading time was accepted. The linear equations with regression were as follows for calibration curves, obtained without and with preconcentration at 60 s and 120 s preconcentration time respectively: A = –0.00011 + 0.000392C, A60 = –0.03148 + 0.006994C and A120 = −0.01110 + 0.01261C. The correlation coefficients are 0.99381, 0.99306 and 0.99838, where A, A60 and A120 are the absorbances at 0, 60 and 120 preconcentration times respectively, and C is the concentration of cadmium(II) in µg L-1.The precision (RSD) was found to be 1.68 % and 1.12 % for 20 and 50 µg L-1 of Cd(II) respectively. Preconcentration procedures for trace elements in high salt content samples can be strongly affected by matrix constituents of the sample. Prior to application Table 1—Effects of matrix ions on the recoveries of Cd(II) ions Electrolytes NaCl Na2SO4 NaNO3 Na3PO4 NaBr NaI MgCl2 Citric acid Tartaric acid Ascorbic acid EDTAa a
In mmol L
-1
Tolerance limit Electrolytes Tolerance limit (mg L-1) (mol L-1) 15 0.2 K(I) 0.03 Pb(II) 20 0.015 Fe(III) 5 10 0.08 Cu(II), Ca(II) 0.03 Al(III) 8 0.1 Zn(II) 15 0.02 0.2 0.2 0.4 0.8
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of the preconcentration method for the determination of Cd(II) ions in samples with complicated matrix, the influence of some alkali, alkaline earth and transition metal ions on the recoveries of the analyte ions were also investigated. The results demonstrate that the presence of large amounts of these metal ions in the sample have no significant effect on the analytical signal of Cd(II) (Table 1). The chelating resin was studied in 1.0–5.0 mol L-1 HNO3 acid. Resin was kept in acid for 24 h and then filtered. The solid was washed with doubly distilled water until free from acid and then air dried. Its total sorption capacity determined as described above. The sorption capacity was found to vary < 2.0 % than that of the untreated one. Thus, the present resin can withstand acid concentration up to 5.0 mol L-1. The chelating resin can be reused in ~70-80 cycles of sorption-desorption without any significant change in sorption capacity. To check the applicability of the proposed method for the separation and pre-concentration of Cd(II) ions, the modified resin AXAD-2-XO was tested for analysis of different water samples. Industrial effluents were collected from three different places, Rishikesh (Ganges water), Noida and Mayapuri industrial areas. The samples were filtered through a 0.45 µm pore size filter to remove any suspended particulate matter prior to its preconcentration. Then 5 mL (sample flow rate 5 mL min-1 and sample loading time 60 s) of each sample was preconcentrated and analysed by the proposed method. The levels of Cd(II) concentration increased slightly in industrial waste water samples as this waste water was collected from near Noida and Mayapuri industrial area. The accuracy of the preconcentration procedure was checked by the recovery experiments for spiked water sample using certified Cd(II) nitrate solution traceable to NIST. As shown in Table 2, the recoveries of 10 µg L-1 Cd(II) spiked in the samples ranged from 98-99.5 %. Table 2—Analytical results for the determination of Cd(II) in water samples Sample
Cd(II) (µg L-1)
After addition of 10 µg L-1 Cd(II)a Mayapuri industrial 2.92 ± 0.02 12.74 ± 0.32 area, Delhi, India Noida industrial 1.74 ± 0.01 11.69 ± 0.19 area, UP, India Rishikesh industrial 1.35 ± 0.01 11.18 ± 0.32 area, UP, India
Recovery (%)
Initial
a
99.0 99.5 98.5
Spiked with certified Cd(II) nitrate solution (E. Merck) traceable to NIST Gaithersburg, USA.
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Table 3—Comparison of the performance characteristics of selected online SPE methods and the present method for Cd(II) determination with FAAS Support
Chelating agent
AXAD-2 Pyrocatechol AXAD-2 Pyrocatechol Human hair Ammonium pyrrolidine dithiocarbamate AXAD-2 2-Aminothiophenol AXAD-4 Diethyldithiophosphate Polyurethane 2-(6′-Methyl-2′foam benzothiazolylazo) chromotropic acid Silica gel Niobium(V) oxide AXAD-2 Xylenol orange
Eluent EF
LOD (µgL−1)
Ref.
HCl 22 HNO3 16 HNO3 19
0.27 0.31 0.77
29 30 31
HNO3 28 HNO3 20
0.14 1.0
32 33
2.04
34
0.02 0.57
35 This work
-
22
loading capacity, the adsorption time of resin is also very fast and good reusability. The resin can be recycled 70-80 times without affecting its sorption capacity. The sorbed ions can readily be desorbed with common mineral acids with 98 % recovery. The proposed method has been successfully applied for the determination of trace Cd(II) in industrial waste water samples. Acknowledgement The authors are thankful to Department of Science and Technology, New Delhi, India, for financial assistance. References
HNO3 28.9 HNO3 32
In order to evaluate the accuracy of developed procedure, standard reference material NIST SRM 1643e (supplied by National Institute of Standard and Technology (NIST), Gaithersburg, MD, USA) was analyzed for cadmium(II). The certified concentration was 6.568 ± 0.073 µg L-1 and the recovery was 98.9 % (6.50 ± 0.38 µg L-1, n = 3). It was found that there is no significant difference between results obtained by the proposed method and the certified values. The performance characteristics of the proposed method has been compared with other online SPE preconcentration FAAS methods reported in the literature (Table 3). The proposed method shows good limit of detection (LOD) with reasonable enhancement factor (EF) over other online preconcentration methods. The proposed method shows the applicability of resin AXAD-2-XO packed column in flow injection online sorbent extraction preconcentration system coupled with FAAS for Cd(II) determination in industrial effluents. The parameters including pH of the sample, sample volume, type and volume of eluents and effect of foreign ions were studied. The principle interest in using this chelating resin in trace analysis is based on the simplicity of the method for separation and preconcentration of the Cd(II) ions. The main advantages of the proposed method are the low detection limit, high preconcentration factor, short analysis time and high selectivity. This proposed online system has high tolerance to interferences from the matrix ions and thus allows the application of the proposed procedure for Cd(II) determination in a large range of samples. AXAD-2-XO resin has good
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18 19 20 21 22 23 24 25
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Ion A C & Barbu L, Separation and Preconcentration of Cu2+, Cd2+, Zn2+ on the Chelating Resin Amberlite XAD-4 Loaded with Xylenol Orange, Proc of 13th Romanian Int Conf on Chemistry and Chem Eng, Vol. I, (Editura Printech, Bucharest) Sept 2003. 27 Venkatesh G & Singh A K, Sep Sci Technol, 42 (2007) 3429. 28 Zhao S, Liang H, Yan H, Yan Z, Che S, Xiand Z & Miaoxian C, Clean, 38 (2010) 146. 29 Lemos V A, Gama M E & Lima da Silva A, Microchim Acta, 153 (2006) 179.
30 31 32 33 34 35
1573 Lemos V A, DaSilva G D, Carvalho L A, Santanade A D, Novaes D G & Passos D A, Microchem J, 84 (2006) 14. An-Na, Tang Yun-Fei & Hu , Instrument Sci Tech, 39 (2011) 110. Lemos V A & Baliza Xavier P, Talanta, 67 (2005) 564. Santos E J, Herrmann A B, Ribeiro A S & Curtius A J, Talanta, 65 (2005) 593. Gama M E, Lima da Silva A & Lemos V A, J Hazard Mater, B136 (2006) 757. Maltez F H, Vieira A M, Ribeiro S A, Curtius J A & Carasek E, Talanta, 74 (2008) 586.
