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Separation and preconcentration of trace amounts of lead on octadecyl silica membrane disks modified with a new S-containing Schiff's base and its determination by flame atomic absorption spectrometry
Microchemical Journal 69 Ž2001. 1᎐6
Separation and preconcentration of trace amounts of lead on octadecyl silica membrane disks modified with a new S-containing Schiff’s base and its determination by flame atomic absorption spectrometry Omid Reza Hashemi a , Maryam Razi Kargar b, Farhad Raoufia , Aboulghasem Moghimi c , Hossein Aghabozorg d, Mohammad Reza Ganjali a,U a Department of Chemistry, Tehran Uni¨ ersity, Tehran, Iran Department of Chemistry, Islamic Azad Uni¨ ersity, Tehran, Iran c Department of Chemistry, Imam Hossein Uni¨ ersity, Tehran, Iran d Department of Chemistry, Tarbiat Moallem Uni¨ ersity, Tehran, Iran b
Received 24 March 2000; received in revised form 5 June 2000; accepted 6 June 2000
Abstract A synthetic procedure was developed for the preparation of a new S-containing Schiff’s base wethane amine, N, N⬘-bisŽ2-thienyl methylene.x. The resulting compound ŽL. was used as a modifier in octadecyl silica membrane disks for solid phase extraction and flame absorption spectrometric determination of lead in water samples. Extraction efficiency and influence of flow rates, pH, type and minimum amount of stripping acid were investigated. The maximum capacity of the membrane disks modified by 5 mg of the ŽL. used was found to be 700 g Pb 2q. The limit of detection of the proposed method is 16.7 ng mly1. The method was applied to the recovery of Pb 2q ions from different synthetic samples and spring water samples. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Lead ŽII.; Solid phase extraction; Bis-Schiff base; Flame atomic absorption spectroscopy
1. Introduction Lead is a trace heavy metal of major interest in U
Corresponding author.
environmental protection owing to its cumulative toxicity. Lead is still emitted into the biosphere in considerable amounts owing to its application as a fuel additive. Environmental lead results in a serious and well known health risk to animals and humans w1,2x. The presence of trace amounts of
0026-265Xr01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 0 . 0 0 0 5 4 - 0
2
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6
lead in many industrial streams is also undesirable, mainly because it may eventually be passed on to a food or other product used or consumed by people w3x. Hence, the development of new methods for selective separation, concentration and determination of lead in sub-micron levels is still a challenging task. The usual methods for determination of lead in solution involve spectrophotometric methods w4x, atomic absorption spectrometry w5x, inductively couple plasma-mass spectrometry w6,7x, electrothermal atomic absorption spectrometry w8x. However, due to the presence of lead in environmental samples at low levels, its separation from other elements present and also the use of a pre-concentration step prior to lead determination can be necessary. The use of ion exchange and chelating resin or the sorption of organic complexes on reversed phase silica or polystyrene has been applied for the pre-concentration of trace elements w9᎐14x. Solvent extraction of different metal ions has been widely employed in chemistry and industry for many years w15x. However, the use of classical extraction methods are usually time-consuming labor-intensive and require relatively large volumes of high purity solvents. Additional concern is the disposal of the solvent used, which can create a severe environmental problem. Thus, much interest has been recently focused in replacing conventional solvent extraction methods isolating environmental pollutants with solidphase extraction ŽSPE. techniques w16x. The SPE techniques are capable of highly selective removal of trace amounts of metal ions from solution containing matrices, which normally make separation difficult, with minimal usage of organic solvents. Schiff base as a classical ligand can be used as modifier in octadecyl silica membrane disks. Two groups of multi-dentate Schiff bases containing thiophen, furane, pyrrol, pyridine are known w17᎐22x. The reaction of 2-carbaldehyde and 2,6-dicarbaldehyde derivation of their five and six membrane ring heterocyclics with diamine results in open chain and macrocyclic Schiff bases, respectively. The number of studies performed on the synthesis and complexation studies of macrocyclic Schiff bases derived from 2,6-thiophene di-
carbaldehyde is considerably more than open chain bis-Schiff bases derived from 2-thiophene carbaldehyde. Among research performed on the synthesis and metal chelating properties of the open chain bis-Schiff bases, those concerned with 2-thiophenecarbaldehyde derivatives are very limited w18,20,22x. We have recently modified the octadecyl silica membrane disks with some classical ligands for selective extraction of ultra trace amounts of cerium w23x, lead w24x, copper w25x and silver w26x. In this paper we report the synthesis of a tetra dentate bis-Schiff base and its use as modifier for solid phase extraction of lead from water samples. To the best of our knowledge, octadecyl silica membrane disks modified by bis-Schiff base have not been employed previously for selective separation and concentration of lead from various samples.
2. Experimental 2.1. Reagents Thiophen-2-carbaldehyde and 1,2-diaminoethane were purchased from Merck and used without further purification. All organic solvents were HPLC grade and from Merck Chemical company. All acids used were of the highest purity available from Merck. Analytical grade nitrate salts of lead, potassium, magnesium, calcium, strontium, cobalt, nickel, cadmium, copper, zinc and mercury Žall from Merck. were of the highest purity available and used without any further purification. Doubly distilled deionized water was used throughout. 2.2. Apparatus The NMR spectra were recorded on a Joel 90 MHz spectrometer, and TMS was used as internal reference. The FT IR spectra were recorded on a Shimatzo 4300 instrument using KBr pellets. The lead determination was carried out on a Perkin-Elmer 603 atomic absorption spectrometer with a hollow cathode lamp and a deuterium background corrector, at a wavelength of 217 nm
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6
Žresonance line. using an adjusted air᎐acetylene flame. The AAS determination of all other cations was performed under the recommended conditions for each metal. 2.3. Synthesis of S-containing Schiff base The S-containing Schiff’s base was synthesized in the usual manner by reduction of the 2thiophene carbaldehyde with the proper diamine in a 2:1 molar ratio as follows: a 50-ml round-balloon flask equipped with a condenser and magnetic bar was charged with 30 ml methanol, 0.007 M diamine and 0.014 M thiophene-2-carbaldehyde, the mixture was then refluxed for 1 h while stirring strongly. The solvent was then evaporated and a pale yellow solid was obtained. The crude solid product was purified by recrystallization from diethyl ether or a mixture of ethanolrdichloromethane. The pale yellow needle crystalline products were obtained in both cases. Ethane amine, N, N⬘-bis Ž2-thienyl methylene., mp: 92᎐94⬚C, 1 H-NMRŽCDCl 3 ., ␦ H : 8.3Ž2H, NsCH., 6.9᎐7.4Ž6H, thiophene CH., 3.8Ž4H, N᎐CH 2 . ppm. 13 C-NMRŽCDCl 3 ., ␦ C : 155.7, 142.2, 132.2, 130.3, 128.6, 78.4, 77.0, 75.6, 73.9, 70.3, 60.4 ppm. IR ŽKBr.: 2900, 2870, 2820, 1640 ŽCsN., 1490, 1455, 1430, 1320, 1220, 1120, 1040, 955, 905, 855, 845, 825, 755, 705 cmy1 . 2.4. Sample extraction Extractions were performed with 47 = 0.5 mm Ždiameter = thickness . Empore membrane disks containing octadecyl-bonded silica Ž8-m parti˚ pore size, 3M Co., Paul, MN.. The disk cles 60-A
3
was used in conjunction with a standard Millipore 47-mm filtration apparatus. After placing the membrane in the filtration apparatus, it was washed with 10 ml methanol and 10 ml acetonitrile to remove all contaminants arising from the manufacturing process and environment. After drying the disk by passing air through it for several minutes, a solution of 5 mg ŽL. dissolved in 2 ml CHCl 3 was introduced to the reservoir of the apparatus and was drawn slowly through the disk by applying a slight vacuum until the ligand penetrated the membrane completely. The solvent was evaporated at 50⬚C. Finally, the disk was washed with 25 ml water and dried by passing the air through it. The membrane disk modified by ŽL. was now ready for sample extraction. The general procedure for extraction of Pb 2q ions on the modified membrane disk was as follows: the modified disk was first washed with 25 ml water. This step pre-wets the surface of the disk prior to the extraction of Pb 2q ions from water, then 500 ml of the sample solution containing 20 g Pb 2q was passed through the membrane Žflow rate s 20 ml miny1 .. After the extraction, the disk was dried completely by passing air through it for a few minutes. The extracted lead was then stripped from the membrane disk using 10 ml of a 0.5 M solution of nitric acid into 10.0-ml volumetric flasks and the lead concentration was determined by FAAS.
3. Results and discussion The ligand ŽL. is a 2N᎐2S donating Schiff’s base, which is insoluble in water at neutral pH. Our recent conductometric studies in acetonitrile revealed that it can form a fairly stable and selective complex with Pb 2q ion w27x. Thus, we decided to examine its capability as a suitable reagent for pre-concentration and separation of Pb 2q ions via solid extraction, by using octadecyl bonded silica membrane disks. Some preliminary experiments were carried out in order to choose a proper eluent for the retained Pb 2q ions after the extraction of 20 g
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6
4
Table 1 Percent recovery of lead from the modified membrane disks using different stripping acid solutions a Stripping acid solution
HNO3 Ž3 M. HNO3 Ž0.5 M. CH3 COOH Ž0.5 M. HCl Ž1 M. HBr Ž1 M. a
%Recovery ŽVolume Žml.. 5
10
15
25
81.2 74 79 45 68
100 100 100 77.3 80.3
100 100 100 79 82
100 100 100 81 88
Initial samples contained 20 g Pb 2q ion in 500 ml water.
lead in 500 ml of solution by the modified disks, the lead ions were stripped with varying volumes of varying concentrations of different acids ŽTable 1.. From the data given in Table 1, it is immediately obvious that among four different acid solutions used, 10 ml of 0.5 M nitric acid can accomplish the quantitative elution of lead from the membrane disk, while other acids used are ineffective for the complete elution of lead. The influence of flow rates of the sample and stripping solution from the modified membrane disk on the retention and recovery of lead ŽII. ion was investigated. It was found that, in the range of 1᎐35 ml miny1 , the retention of lead by the membrane disk was not affected by the sample solution flow rate considerably. Similar results for the extraction of organic w28,29x and inorganic materials w30,31x by octadecyl silica disks have already been reported in the literature. On the other hand, quantitative analysis of Pb 2q ions from the modified membrane disks was achieved in a flow rate range of 0.5᎐10 ml, using 10 ml of 0.5 M nitric acid as a stripping solution. At higher flow rates, larger volumes of 0.5 M nitric acid were necessary for quantitative stripping of Pb 2q ions. In order to investigate the optimum amount of ŽL. on the quantitative extraction of lead by the membrane disk, lead ion extraction was conducted by varying the amount of ligand from 0 to 20 mg ŽTable 2.. As can be seen, the extraction of lead is quantitative above 5 mg of ŽL.. Hence, subsequent extraction experiments were carried out with 5 mg of ŽL.. The influence of pH of aqueous samples on the recovery of 20 g Pb 2q from 500-ml solutions
was studied in the pH range 2.0᎐8.0. The pH was adjusted by using 0.1 M of either nitric acid or sodium hydroxide solutions. The results obtained indicated that the Pb 2q ion could be retained quantitatively by the modified membrane disk through the entire pH range studied. Higher pH values Ž) 8.0. were not tested because of the possibility of the hydrolysis of octadecyl silica in the disks. The break-through volume of sample solution was tested by dissolving 20 g of lead in 25, 50, 100, 250, 500, 1000 and 1200 ml water and the recommended procedure was followed. In all cases, the extraction by modified membrane disk was found to be quantitative. Thus, the breakthrough volume for the method should be greater than 1200 ml. The limit of detection ŽLOD. of the proposed method for the determination of lead was studied under the optimal experimental conditions. The LOD obtained from C LO D s K b S b my1 w32,33x for a numerical factor K b s 3, is 16.7 ng mly1 . Table 2 Effect of amount of ligand on Pb 2q ion extraction a Amount of L Žmg.
% Recovery
0 5 10 15 20
0 99.63 Ž1.9.b 99.33 Ž0.45.b 98.41 Ž1.8.b 20 99.67 Ž1.35.b
a Initial samples contained 20 g Pb 2q ion in 500 ml water. b Values in parentheses are R.S.D.s based on three replicate analyses.
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6 Table 3 Separation of lead from binary mixture a Diverse ion q
Na Kq Ca2q Sr2q Co2q Cd2q Ni2q Cu2q Zn2q Hg2q
Amount taken Žmg. 10 10 10 10 4 5 2 5 2 2
%Found
Table 5 Recovery of 20 g lead added to 500 ml solution of the synthetic and water samples %Recovery Sample
1.73 Ž0.06. 1.91 Ž0.1. 1.11 Ž0.088. 2.31 Ž0.41. 4.35 Ž0.49. 2.65 Ž0.06. 2.01 Ž0.2. 3.4 Ž0.28. 5.6 Ž0.31. 3.65 Ž0.53.
5
b
100.3 Ž1.3. 98.3 Ž0.97. 97.1 Ž1.07. 99.1 Ž1.08. 96.75 Ž2.7. 96.2 Ž1.5. 95.17 Ž1.6. 62.8 Ž1.19. 51.2 Ž2.05. 59.7 Ž1.2.
a
Initial samples contained 20 g Pb 2q ion in 500 ml water. b Values in parentheses are R.S.D.s based on three replicate analyses.
The maximum capacity of the membrane disk modified by 5 mg of ŽL. was determined by passing 500-ml portions of an aqueous solution containing 3000 g lead through the disk, followed by determination of the retained metal ions using FAAS. The maximum capacity was found to be 700 g of Pb 2q ions on the disk. In order to investigate the selective separation and determination of Pb 2q ion from its binary mixtures with diverse metal ions, an aliquot of aqueous solution Ž500 ml. containing 20 g Pb 2q and milligram amounts of other cations was taken and the recommended procedure was followed, the results are summarized in Table 3. The results show that the lead ions in the binary mixTable 4 Separation of lead from binary mixtures a in presence of ammonia Diverse ion
Amount taken Žmg.
%Found
%Recovery
Cu2q Zn2q Hg2q
5 2 2
NADPc NADPc NADPc
99.23 Ž3.26.b 99 Ž5.2.b 99.4 Ž3.5.b
a Initial samples contained 20 g Pb 2q ion in 500 ml water. b Values in parentheses are R.S.D.s based on three replicate analyses. c No adsorption, passes through disk in the presence of 2.5= 10y4 M ammonia as masking agent.
Synthetic sample 1 ŽCd2q, Co2q and Ni2q, 2 mg of each cation. Synthetic sample 2 ŽNaq, Kq and Ca2q, 5 mg of each cation. Synthetic sample 3 ŽCu2q, Zn2q and Hg2q, 2 mg of each cation. Spring water Ž1. Spring water Ž2.
%Recovery of Pb2q ions 97.97 Ž1.02.a 99.2 Ž0.5.a 97.1 Ž0.3.a 100.23 Ž0.91.a 95.6 Ž5.2.a
a
Values in parentheses are R.S.D.s based on three replicate analyses.
tures are retained almost completely by the modified membrane disk. Meanwhile, retention of other cations by the disk is very low and they can be separated from Pb 2q ion ŽTable 4.. It should be noted that, in the case of copper, zinc and mercury ions, some interfering effect of the cations on the separation of Pb 2q ions was completely removed by addition of 2.5= 10y4 M ammonia ŽpH 8.0᎐8.5. as a suitable masking agent. In order to assess the applicability of the method to real samples, with different matrices containing varying amounts of a variety of diverse ions, it was applied to the separation and recovery of lead ions from three synthetic samples as well as two different water samples ŽDamavand spring, Iran. ŽTable 5.. As seen, the results of three analyses of each sample show that, in all cases, the lead recovery is almost quantitative.
4. Conclusions The proposed method has the following advantages: the method is rapid, the time taken for the separation and analysis of lead in a 500-ml water sample is at the most 20 min; it can selectively separate Pb 2q ions from other metal ions associated, even in much higher concentrations; the method can be successfully applied to the separation and determination of lead in real samples; and it is a simple method for the separation of lead.
6
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6
References w1x T.E. Berma, Toxic Metals and their Analysis, Heyden & Sons, London, 1980. w2x C.D. Klassen, M.D. Andur, J. Dull, Casarett and Dulls Toxicology, 3rd ed, MacMillan, New York, 1986. w3x R.M. Izatt, J.S. Bradshaw, R.L Bruening, Pure Appl. Chem. 68 Ž1996. 1237. w4x Z. Marczenko, Separation and Spectrophotometric Determination of Elements, Ellis Horwood, London, 1986. w5x B. Welz, Atomic Absorption Spectrometry, VCH, Germany, 1985. w6x K.E. Murphy, P.J. Paulsen, J. Fresenius Anal. Chem. 352 Ž1995. 203. w7x S. Savin, T. Petrova, T. Dzherayan, M. Relkhstat, J. Fresenius Anal. Chem. 340 Ž1997. 217. w8x I. Sekerka, I. Lechner, Anal. Chim. Acta 254 Ž1991. 99. w9x J.F. Tyson, Analyst 110 Ž1985. 419. w10x S. Olsen, L.C.R. Pessenda, J. Ruzicka, E.H. Hansen, Analyst 108 Ž1983. 905. w11x S. Caroli, A. Alimanti, F. Petrucci, Anal. Chim. Acta 254 Ž1991. 99. w12x A.M. Naghmush, K. Pyrznska, M. Trojanowicz, Talanta 42 Ž1995. 85. w13x K. Grudpan, S. Lawraugrath, P. Sookamiti, Analyst 120 Ž1995. 2107. w14x N.L. Lancaster, G.D. Marshall, E.R. Gonzalo, J. Ruzicka, G.D. Christian, Analyst 119 Ž1994. 1459. w15x Method for chemical analysis of water and waste, EPA60014-79-02 Environmental Protection Agency, Research Triangle Park, NC,1983. w16x Y. Yamini, N. Alizadeh, M. Shamsipur, Anal. Chim. Acta 355 Ž1997. 69.
w17x R.H. Holm, Prog. Inorg. Chem. 7 Ž1996. 83. w18x M.P. Cookley, L.H. Young, R.A. Gallagher, J. Inorg. Nucl. Chem. 31 Ž1969. 1449. w19x J.H. Weber, Inorg. Chem. 6 Ž1967. 258. w20x G.L. Eichhorn, J.C. Bailar, J. Am. Chem. Soc. 75 Ž1953. 2905. w21x A.E. Frost, H.H. Freedwan, J. Am. Chem. Soc. 24 Ž1959. 1905. w22x R.R. Shukla, N. Ahmad, S. Chandra, S. Misra, R. Rastogi, G. Narian, J Indian Chem. Soc. L = V Ž1988. 214. w23x Z. Ghasemi, L. Hajy-Agha Babaei, M. Yousefi, M. Shamsipur, M.R. Ganjali, Anal. Chem., submitted. w24x O.R. Hashemi, F. Raoufi, M. R. Ganjali, A. Moghimi, M. Kargar-Razi, M. Shamsipur, J. Radio Anal. Nucl. Chem., submitted. w25x L. Haj-Agha Babaei, Z. Ghasemi, H. Sharghi, M.R. Ganjali, Pol. J. Chem., submitted. w26x M.R. Ganjali, Z. Ghasemi, L. Haj-Agha Babaei, F. Darviche, O.R. Hashemi, F. Raoufi, Anal. Chim. Acta, submitted. w27x Z. Porghobadi, M.S. Thesis, Tehran University, Tehran, Iran, 1999. w28x Y. Yamini, M.J. Ashraf-Khorassani, J. High Resol. Chromatogr. 17 Ž1994. 634. w29x Y. Yamini, M. Shamsipur, Talanta 43 Ž1996. 2117. w30x Y. Yamini, N. Alizadeh, M. Shamsipur, Sep. Sci. Technol. 32 Ž1997. 2078. w31x Y. Yamini, N. Alizadeh, M. Shamsipur, Anal. Chim. Acta 355 Ž1997. 69. w32x ACS Committee on Environmental Improvement, Anal. Chem. 52 Ž1980. 2242. w33x J.D. Ingle, S.R. Crouch, Spectrochemical Analysis, Prentice Hall, Englewood Cliffs NJ, 1988.
Separation and preconcentration of trace amounts of lead on octadecyl silica membrane disks modified with a new S-containing Schiff’s base and its determination by flame atomic absorption spectrometry Omid Reza Hashemi a , Maryam Razi Kargar b, Farhad Raoufia , Aboulghasem Moghimi c , Hossein Aghabozorg d, Mohammad Reza Ganjali a,U a Department of Chemistry, Tehran Uni¨ ersity, Tehran, Iran Department of Chemistry, Islamic Azad Uni¨ ersity, Tehran, Iran c Department of Chemistry, Imam Hossein Uni¨ ersity, Tehran, Iran d Department of Chemistry, Tarbiat Moallem Uni¨ ersity, Tehran, Iran b
Received 24 March 2000; received in revised form 5 June 2000; accepted 6 June 2000
Abstract A synthetic procedure was developed for the preparation of a new S-containing Schiff’s base wethane amine, N, N⬘-bisŽ2-thienyl methylene.x. The resulting compound ŽL. was used as a modifier in octadecyl silica membrane disks for solid phase extraction and flame absorption spectrometric determination of lead in water samples. Extraction efficiency and influence of flow rates, pH, type and minimum amount of stripping acid were investigated. The maximum capacity of the membrane disks modified by 5 mg of the ŽL. used was found to be 700 g Pb 2q. The limit of detection of the proposed method is 16.7 ng mly1. The method was applied to the recovery of Pb 2q ions from different synthetic samples and spring water samples. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Lead ŽII.; Solid phase extraction; Bis-Schiff base; Flame atomic absorption spectroscopy
1. Introduction Lead is a trace heavy metal of major interest in U
Corresponding author.
environmental protection owing to its cumulative toxicity. Lead is still emitted into the biosphere in considerable amounts owing to its application as a fuel additive. Environmental lead results in a serious and well known health risk to animals and humans w1,2x. The presence of trace amounts of
0026-265Xr01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 0 . 0 0 0 5 4 - 0
2
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6
lead in many industrial streams is also undesirable, mainly because it may eventually be passed on to a food or other product used or consumed by people w3x. Hence, the development of new methods for selective separation, concentration and determination of lead in sub-micron levels is still a challenging task. The usual methods for determination of lead in solution involve spectrophotometric methods w4x, atomic absorption spectrometry w5x, inductively couple plasma-mass spectrometry w6,7x, electrothermal atomic absorption spectrometry w8x. However, due to the presence of lead in environmental samples at low levels, its separation from other elements present and also the use of a pre-concentration step prior to lead determination can be necessary. The use of ion exchange and chelating resin or the sorption of organic complexes on reversed phase silica or polystyrene has been applied for the pre-concentration of trace elements w9᎐14x. Solvent extraction of different metal ions has been widely employed in chemistry and industry for many years w15x. However, the use of classical extraction methods are usually time-consuming labor-intensive and require relatively large volumes of high purity solvents. Additional concern is the disposal of the solvent used, which can create a severe environmental problem. Thus, much interest has been recently focused in replacing conventional solvent extraction methods isolating environmental pollutants with solidphase extraction ŽSPE. techniques w16x. The SPE techniques are capable of highly selective removal of trace amounts of metal ions from solution containing matrices, which normally make separation difficult, with minimal usage of organic solvents. Schiff base as a classical ligand can be used as modifier in octadecyl silica membrane disks. Two groups of multi-dentate Schiff bases containing thiophen, furane, pyrrol, pyridine are known w17᎐22x. The reaction of 2-carbaldehyde and 2,6-dicarbaldehyde derivation of their five and six membrane ring heterocyclics with diamine results in open chain and macrocyclic Schiff bases, respectively. The number of studies performed on the synthesis and complexation studies of macrocyclic Schiff bases derived from 2,6-thiophene di-
carbaldehyde is considerably more than open chain bis-Schiff bases derived from 2-thiophene carbaldehyde. Among research performed on the synthesis and metal chelating properties of the open chain bis-Schiff bases, those concerned with 2-thiophenecarbaldehyde derivatives are very limited w18,20,22x. We have recently modified the octadecyl silica membrane disks with some classical ligands for selective extraction of ultra trace amounts of cerium w23x, lead w24x, copper w25x and silver w26x. In this paper we report the synthesis of a tetra dentate bis-Schiff base and its use as modifier for solid phase extraction of lead from water samples. To the best of our knowledge, octadecyl silica membrane disks modified by bis-Schiff base have not been employed previously for selective separation and concentration of lead from various samples.
2. Experimental 2.1. Reagents Thiophen-2-carbaldehyde and 1,2-diaminoethane were purchased from Merck and used without further purification. All organic solvents were HPLC grade and from Merck Chemical company. All acids used were of the highest purity available from Merck. Analytical grade nitrate salts of lead, potassium, magnesium, calcium, strontium, cobalt, nickel, cadmium, copper, zinc and mercury Žall from Merck. were of the highest purity available and used without any further purification. Doubly distilled deionized water was used throughout. 2.2. Apparatus The NMR spectra were recorded on a Joel 90 MHz spectrometer, and TMS was used as internal reference. The FT IR spectra were recorded on a Shimatzo 4300 instrument using KBr pellets. The lead determination was carried out on a Perkin-Elmer 603 atomic absorption spectrometer with a hollow cathode lamp and a deuterium background corrector, at a wavelength of 217 nm
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6
Žresonance line. using an adjusted air᎐acetylene flame. The AAS determination of all other cations was performed under the recommended conditions for each metal. 2.3. Synthesis of S-containing Schiff base The S-containing Schiff’s base was synthesized in the usual manner by reduction of the 2thiophene carbaldehyde with the proper diamine in a 2:1 molar ratio as follows: a 50-ml round-balloon flask equipped with a condenser and magnetic bar was charged with 30 ml methanol, 0.007 M diamine and 0.014 M thiophene-2-carbaldehyde, the mixture was then refluxed for 1 h while stirring strongly. The solvent was then evaporated and a pale yellow solid was obtained. The crude solid product was purified by recrystallization from diethyl ether or a mixture of ethanolrdichloromethane. The pale yellow needle crystalline products were obtained in both cases. Ethane amine, N, N⬘-bis Ž2-thienyl methylene., mp: 92᎐94⬚C, 1 H-NMRŽCDCl 3 ., ␦ H : 8.3Ž2H, NsCH., 6.9᎐7.4Ž6H, thiophene CH., 3.8Ž4H, N᎐CH 2 . ppm. 13 C-NMRŽCDCl 3 ., ␦ C : 155.7, 142.2, 132.2, 130.3, 128.6, 78.4, 77.0, 75.6, 73.9, 70.3, 60.4 ppm. IR ŽKBr.: 2900, 2870, 2820, 1640 ŽCsN., 1490, 1455, 1430, 1320, 1220, 1120, 1040, 955, 905, 855, 845, 825, 755, 705 cmy1 . 2.4. Sample extraction Extractions were performed with 47 = 0.5 mm Ždiameter = thickness . Empore membrane disks containing octadecyl-bonded silica Ž8-m parti˚ pore size, 3M Co., Paul, MN.. The disk cles 60-A
3
was used in conjunction with a standard Millipore 47-mm filtration apparatus. After placing the membrane in the filtration apparatus, it was washed with 10 ml methanol and 10 ml acetonitrile to remove all contaminants arising from the manufacturing process and environment. After drying the disk by passing air through it for several minutes, a solution of 5 mg ŽL. dissolved in 2 ml CHCl 3 was introduced to the reservoir of the apparatus and was drawn slowly through the disk by applying a slight vacuum until the ligand penetrated the membrane completely. The solvent was evaporated at 50⬚C. Finally, the disk was washed with 25 ml water and dried by passing the air through it. The membrane disk modified by ŽL. was now ready for sample extraction. The general procedure for extraction of Pb 2q ions on the modified membrane disk was as follows: the modified disk was first washed with 25 ml water. This step pre-wets the surface of the disk prior to the extraction of Pb 2q ions from water, then 500 ml of the sample solution containing 20 g Pb 2q was passed through the membrane Žflow rate s 20 ml miny1 .. After the extraction, the disk was dried completely by passing air through it for a few minutes. The extracted lead was then stripped from the membrane disk using 10 ml of a 0.5 M solution of nitric acid into 10.0-ml volumetric flasks and the lead concentration was determined by FAAS.
3. Results and discussion The ligand ŽL. is a 2N᎐2S donating Schiff’s base, which is insoluble in water at neutral pH. Our recent conductometric studies in acetonitrile revealed that it can form a fairly stable and selective complex with Pb 2q ion w27x. Thus, we decided to examine its capability as a suitable reagent for pre-concentration and separation of Pb 2q ions via solid extraction, by using octadecyl bonded silica membrane disks. Some preliminary experiments were carried out in order to choose a proper eluent for the retained Pb 2q ions after the extraction of 20 g
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6
4
Table 1 Percent recovery of lead from the modified membrane disks using different stripping acid solutions a Stripping acid solution
HNO3 Ž3 M. HNO3 Ž0.5 M. CH3 COOH Ž0.5 M. HCl Ž1 M. HBr Ž1 M. a
%Recovery ŽVolume Žml.. 5
10
15
25
81.2 74 79 45 68
100 100 100 77.3 80.3
100 100 100 79 82
100 100 100 81 88
Initial samples contained 20 g Pb 2q ion in 500 ml water.
lead in 500 ml of solution by the modified disks, the lead ions were stripped with varying volumes of varying concentrations of different acids ŽTable 1.. From the data given in Table 1, it is immediately obvious that among four different acid solutions used, 10 ml of 0.5 M nitric acid can accomplish the quantitative elution of lead from the membrane disk, while other acids used are ineffective for the complete elution of lead. The influence of flow rates of the sample and stripping solution from the modified membrane disk on the retention and recovery of lead ŽII. ion was investigated. It was found that, in the range of 1᎐35 ml miny1 , the retention of lead by the membrane disk was not affected by the sample solution flow rate considerably. Similar results for the extraction of organic w28,29x and inorganic materials w30,31x by octadecyl silica disks have already been reported in the literature. On the other hand, quantitative analysis of Pb 2q ions from the modified membrane disks was achieved in a flow rate range of 0.5᎐10 ml, using 10 ml of 0.5 M nitric acid as a stripping solution. At higher flow rates, larger volumes of 0.5 M nitric acid were necessary for quantitative stripping of Pb 2q ions. In order to investigate the optimum amount of ŽL. on the quantitative extraction of lead by the membrane disk, lead ion extraction was conducted by varying the amount of ligand from 0 to 20 mg ŽTable 2.. As can be seen, the extraction of lead is quantitative above 5 mg of ŽL.. Hence, subsequent extraction experiments were carried out with 5 mg of ŽL.. The influence of pH of aqueous samples on the recovery of 20 g Pb 2q from 500-ml solutions
was studied in the pH range 2.0᎐8.0. The pH was adjusted by using 0.1 M of either nitric acid or sodium hydroxide solutions. The results obtained indicated that the Pb 2q ion could be retained quantitatively by the modified membrane disk through the entire pH range studied. Higher pH values Ž) 8.0. were not tested because of the possibility of the hydrolysis of octadecyl silica in the disks. The break-through volume of sample solution was tested by dissolving 20 g of lead in 25, 50, 100, 250, 500, 1000 and 1200 ml water and the recommended procedure was followed. In all cases, the extraction by modified membrane disk was found to be quantitative. Thus, the breakthrough volume for the method should be greater than 1200 ml. The limit of detection ŽLOD. of the proposed method for the determination of lead was studied under the optimal experimental conditions. The LOD obtained from C LO D s K b S b my1 w32,33x for a numerical factor K b s 3, is 16.7 ng mly1 . Table 2 Effect of amount of ligand on Pb 2q ion extraction a Amount of L Žmg.
% Recovery
0 5 10 15 20
0 99.63 Ž1.9.b 99.33 Ž0.45.b 98.41 Ž1.8.b 20 99.67 Ž1.35.b
a Initial samples contained 20 g Pb 2q ion in 500 ml water. b Values in parentheses are R.S.D.s based on three replicate analyses.
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6 Table 3 Separation of lead from binary mixture a Diverse ion q
Na Kq Ca2q Sr2q Co2q Cd2q Ni2q Cu2q Zn2q Hg2q
Amount taken Žmg. 10 10 10 10 4 5 2 5 2 2
%Found
Table 5 Recovery of 20 g lead added to 500 ml solution of the synthetic and water samples %Recovery Sample
1.73 Ž0.06. 1.91 Ž0.1. 1.11 Ž0.088. 2.31 Ž0.41. 4.35 Ž0.49. 2.65 Ž0.06. 2.01 Ž0.2. 3.4 Ž0.28. 5.6 Ž0.31. 3.65 Ž0.53.
5
b
100.3 Ž1.3. 98.3 Ž0.97. 97.1 Ž1.07. 99.1 Ž1.08. 96.75 Ž2.7. 96.2 Ž1.5. 95.17 Ž1.6. 62.8 Ž1.19. 51.2 Ž2.05. 59.7 Ž1.2.
a
Initial samples contained 20 g Pb 2q ion in 500 ml water. b Values in parentheses are R.S.D.s based on three replicate analyses.
The maximum capacity of the membrane disk modified by 5 mg of ŽL. was determined by passing 500-ml portions of an aqueous solution containing 3000 g lead through the disk, followed by determination of the retained metal ions using FAAS. The maximum capacity was found to be 700 g of Pb 2q ions on the disk. In order to investigate the selective separation and determination of Pb 2q ion from its binary mixtures with diverse metal ions, an aliquot of aqueous solution Ž500 ml. containing 20 g Pb 2q and milligram amounts of other cations was taken and the recommended procedure was followed, the results are summarized in Table 3. The results show that the lead ions in the binary mixTable 4 Separation of lead from binary mixtures a in presence of ammonia Diverse ion
Amount taken Žmg.
%Found
%Recovery
Cu2q Zn2q Hg2q
5 2 2
NADPc NADPc NADPc
99.23 Ž3.26.b 99 Ž5.2.b 99.4 Ž3.5.b
a Initial samples contained 20 g Pb 2q ion in 500 ml water. b Values in parentheses are R.S.D.s based on three replicate analyses. c No adsorption, passes through disk in the presence of 2.5= 10y4 M ammonia as masking agent.
Synthetic sample 1 ŽCd2q, Co2q and Ni2q, 2 mg of each cation. Synthetic sample 2 ŽNaq, Kq and Ca2q, 5 mg of each cation. Synthetic sample 3 ŽCu2q, Zn2q and Hg2q, 2 mg of each cation. Spring water Ž1. Spring water Ž2.
%Recovery of Pb2q ions 97.97 Ž1.02.a 99.2 Ž0.5.a 97.1 Ž0.3.a 100.23 Ž0.91.a 95.6 Ž5.2.a
a
Values in parentheses are R.S.D.s based on three replicate analyses.
tures are retained almost completely by the modified membrane disk. Meanwhile, retention of other cations by the disk is very low and they can be separated from Pb 2q ion ŽTable 4.. It should be noted that, in the case of copper, zinc and mercury ions, some interfering effect of the cations on the separation of Pb 2q ions was completely removed by addition of 2.5= 10y4 M ammonia ŽpH 8.0᎐8.5. as a suitable masking agent. In order to assess the applicability of the method to real samples, with different matrices containing varying amounts of a variety of diverse ions, it was applied to the separation and recovery of lead ions from three synthetic samples as well as two different water samples ŽDamavand spring, Iran. ŽTable 5.. As seen, the results of three analyses of each sample show that, in all cases, the lead recovery is almost quantitative.
4. Conclusions The proposed method has the following advantages: the method is rapid, the time taken for the separation and analysis of lead in a 500-ml water sample is at the most 20 min; it can selectively separate Pb 2q ions from other metal ions associated, even in much higher concentrations; the method can be successfully applied to the separation and determination of lead in real samples; and it is a simple method for the separation of lead.
6
O.R. Hashemi et al. r Microchemical Journal 69 (2001) 1᎐6
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