Department of Chemistry of Faculty of Science and Art, Sakarya University, Sakarya, Turkey

Transcription

1 CEJC 3(2) Determination of gold, palladium and copper by flame atomic absorption spectrometry after preconcentration on silica gel modified with 3-(2-aminoethylamino)propyl group Mustafa Imamoglu, Ali Osman Aydin, Mustafa S. Dundar Department of Chemistry of Faculty of Science and Art, Sakarya University, Sakarya, Turkey Received 10 November 2004; accepted 11 January 2005 Abstract: A preconcentration method of gold, palladium and copper based on the sorption of Au (III), Pd (II) and Cu (II) ions on a column packed with 3-(2-aminoethylamino)propyl bonded silica gel is described. The modified silica gel was synthesized and characterized by FT- IR and C, H, N elemental analysis. At column preconcentration, the effects of parameters such as ph, volume, flow rate, matrix constituents of solutions and type of eluent on preconcentration of gold, palladium and copper were studied. The recoveries of Au (III), Pd (II) and Cu (II) were 98.93±0.51, 98.81±0.36 and 99.21±0.42 % at 95 % confidence level, respectively. The detection limits (δ) of the elements were 0.032, and µg ml 1, respectively. The preconcentration method was applied for determination of gold and palladium in certified reference material SARM 7B and copper in river and synthetic seawater by FAAS. Gold, palladium and copper were determined with relative error lower than 10 %. c Central European Science Journals. All rights reserved. Keywords: Preconcentration, modified silica gel, flame atomic absorption spectrometry 1 Introduction The determination of trace metals in samples often requires a preconcentration and separation of trace metals from matrix constituents. For this purpose, several techniques; i.e. solvent extraction, ion-exchange, co-precipitation and sorption are used [1,2]. The sorption method is most suitable for trace analysis. In the sorption method, activated imamoglu m@yahoo.com

2 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) carbon, porous polymers, cellulose, silica gel and chitin are used. Silica gel is an excellent sorbent. It can be purchased in a wide specific surface area and porosity range. It has good mechanical strength and resistance to heat and acids, and does not swell in various solvents [3]. Silica gel has been modified with several organic functional groups for improving its utility as adsorbent for metal ions. Since 1975, modified silica gels have been used for adsorption or preconcentration of trace elements. Leyden et.al. used ethylenediamine immobilized silica gel and its derivates for the preconcentration of metal ions and anions [4-6]. Silica gel modified with 3-amino or thio- propyltrimethoxysilane was used for adsorption or preconcentration of metals [7-13]. Amine derivative bonded silica gels were synthesized and their adsorption properties have been investigated [14-16]. Organic functional groups, such as thiourea [17], 2-mercaptobenzothiazole [18] amidinothioureido [19] aminopropylbenzoylazo-1-(2-pyridylazo)-2-naphthol [20] p-dimethylaminobenzylidene rhodanine [21] were immobilized on silica gel surfaces, and thus modified silica gels were used for preconcentration of noble metals. Sorbents modified with NH 2 groups were used for the preconcentration of gold and platinum group metals. For example, Di and Davey developed preconcentration system for gold using α-aminopyridine resin [22]. Tong et.al. synthesized 3-aminopropyl bonded silica gel and used this modified silica gel for selective preconcentration of Au(III), Pt(IV) and Pd(II) [23]. Koklu and Tascioglu synthesized 3-aminopropyl and 3-mercaptopropyl bonded silica gel and used it for adsorption of copper (II), silver (I), and gold (III) [24]. In this study, preconcentration conditions of gold, palladium and copper ions on columns packed with 3-(2-aminoethylamino)propyl bonded silica gel were investigated. Then, gold, palladium and copper were determined by FAAS in several samples after preconcentration by this method. 2 Experimental 2.1 Apparatus and chemicals A Shimadzu AA6701F atomic absorption spectrometer was used for determination of gold. A Deuterium lamp was used for background correction. The flame type was airacetylene. IR spectra of modified and unmodified silica gels were obtained from KBr pellets using a Mattson 9495 FT-IR spectrometer. The ph of solutions was adjusted by adding HCl or NaOH solution and controlled with a Hanna 213 ph meter. All chemicals used were of analytical reagent grade (Merck, Darmstadt, Germany) unless otherwise specified. Deionized water was used throughout the experiments. Silica gel 60 (Merck), mm, was used. Spectroscopic grade standard solutions of gold, palladium and copper were used as ready and diluted to desired concentration with deionized water.

3 254 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) Preparation of 3-(2-aminoethylamino)propyl bonded silica gel Silica gel (25 g) was purified with concentrated hydrochloric acid for 4 hours under reflux condition, then filtered off and washed several times with deionized water until washing contains no chloride. The silica gel was dried at 150 C for 12 hours. For synthesis of 3-chloropropyl bonded silica gel (CPSG), 20 grams dry silica gel were mixed with 150 ml dry toluene and 20 ml 3-chloropropyltrimethoxysilane in a roundbottomed flask and refluxed for 24 hours under nitrogen atmosphere. Then the reaction mixture was filtered off and washed with toluene, ethanol and diethyl ether and dried at 60 C for 6 hours. The reaction is shown in Scheme 1. Scheme 1 Synthesis of 3-chloropropyl bonded silica gel. For synthesis of 3-(2-aminoethylamino)propyl bonded silica gel (AEAP-SG), 20 grams CPSG were mixed with 8 ml ethylenediamine and 10 ml triethylamine in dry toluene and refluxed for 18 hours under nitrogen atmosphere. At the end, modified silica gel was filtered off, washed with toluene, ethanol and diethyl ether and dried at 60 C for 6 hours. The reaction is presented in Scheme 2. Scheme 2 Synthesis of 3-(2-aminoethylamino)propyl bonded silica gel. 2.3 Procedure of column preconcentration A column (15 cm length and 0.8 mm i.d.) filled with 0.2 g of 3-(2-aminoethylamino)propyl bonded silica gel was used for preconcentration. A sample solution (100 ml) including 0.05 mg metal ions was passed through the column with 7 ml min 1 flow rate. The ph of the sample solution was adjusted to desired value with hydrochloric acid and sodium hydroxide. Then, metal ions sorbed on the modified silica gel were eluted by 0.1 mol l 1 potassium cyanide solution. Eluate was collected in a 10 ml calibrated flask and metal ions were determined by flame atomic absorption spectrometry. The recovery of metals was calculated from the ratio of the metal amount in the eluate to the metal amount in sample solution. Blanks and samples were treated in the same manner.

4 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) Sample preparation The certified platinum ore SARM 7B from The South African Bureau of Standards was used to test the accuracy of the preconcentration method described. 2 g of platinum ore was weighed accurately into a beaker. In order to decompose the sample 40 ml of reverse aqua regia was added to the beaker. This mixture stirred for 6 hours without heating and then evaporated to dryness. This process was repeated once more. Then, 20 ml of 1/1 HCl-HNO 3 solution was added to the beaker and stirred for 2 hours at C, and then evaporated to dryness. At the end, 30 ml of 10 % HCl solution was added, heated and hot filtered. The filtrate was collected in a 100 ml calibrated flask and NaOH solution was used to adjust the ph of solution to 1 or 2.5. Gold (III) and palladium (II) ions in the solutions were preconcentrated on a AEAP-SG loaded column at the optimum conditions. 3 Results and discussion 3.1 Characterization of modified silica gel In order to confirm the modification of silica gel with ethylenediamine, infrared spectrometric analysis was carried out. The band of silanol groups at 3466 cm 1 in unmodified silica gel was decreased compared to AEAP-SG. The bands observed in the modified silica gel are C-H stretching at 2957 and 2851 cm 1, and N-H bendings at 1490 cm 1 and 1567 cm 1. Moreover, the results of elemental analysis are 5.90 % C, 2.59 % N, 1.66 % H. Quantity of 3-(2-aminoethylamino)propyl group attached to the silica gel surface was calculated from the elemental analysis and found as 0.98 mmol g Effect of ph The solutions of metal ions were adjusted to various ph and passed through a column filled with modified silica gel at flow rate of 7 ml min 1. Then, the adsorbed metal ions were eluted from the column and determined by FAAS. The recoveries of Au (III), Pd (II) and Cu (II) were calculated from the results. The effect of solution ph on the recoveries of metal ions (Recovery, %) is shown in Fig. 1. Gold (III), palladium (II) and copper (II) ions have been recovered quantitatively at sample ph between 2 and 3, 1 and 2, and 5 and 6, respectively. ph 2.5 for gold (III), ph 1.0 for palladium (II) and ph 5.5 for copper (II) ions were selected as the optimum ph for subsequent experiments. In addition, the modified silica gel can be used for separation of gold (III) and palladium (II) from copper (II) ions by controlling solution ph value.

5 256 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) Fig. 1 The effect of ph on the recovery of gold (III), palladium (II) and copper (II) (sample volume, 100 ml; amount of each metal, 0.05 mg; eluent, 10 ml of 0.1 mol l 1 potassium cyanide; flow rate of the solution, 7 ml min 1 ). 3.3 Effect of eluent For elution of gold (III), palladium (II) and copper (II) ions adsorbed on column, hydrochloric acid, nitric acid, potassium iodide, potassium cyanide solutions at various concentrations were tested. The flow rates of eluents were 3 ml min 1. The recovery of metal ions with various eluents is shown in Table 1. Gold and palladium ions adsorbed on the column could be quantitatively eluted with 5 or 10 ml of 0.1M potassium cyanide solutions. The copper could be eluted from the column with 5 or 10 ml of 0.1M potassium cyanide or 10 ml of 1 M HNO 3 and 2 M HCl solutions. 3.4 Effect of sample flow rate The metal solutions were percolated through the column at flow rates varying from 1.45 to 40 ml min 1. As can be seen in Figure 2, quantitative recoveries were obtained at flow rates from 1.45 to 20 ml min 1. For subsequent experiments 7 ml min 1 flow rate was chosen as the optimum flow rate. 3.5 Effect of sample volume For obtaining maximum applicable sample volume, the effect of solution volume on recovery of gold, palladium and copper was studied. The metal solutions containing 0.05 mg metal at the optimum ph and various volumes from 100 ml to 1000 ml were passed through the column. The results are shown in Fig. 3. By using 10 ml of eluent, the enrichment factor of can be obtained. The recoveries were gradually decreased with increasing sample volume. Although, at high sample volume (i.e ml), quantitative recovery of the metal ions was achieved. It can be said that the modified silica gel can adsorb these metal ions at very low concentration from large volume of solution.

6 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) Eluent Recovery, % Au (III) Pd (II) Cu (II) 10 ml 0.1 M KCN ml 0.1 M KCN ml 0.1 M KI ml 0.1 M KI ml 1 M HNO ml 1 M HCI ml 2 M HCI Table 1 The effect of eluent on the recovery of gold, palladium and copper (sample volume, 100 ml; amount of each metal, 0.05 mg; flow rate of the solution, 7 ml min 1 ; ph of gold (III) solution 2.5; ph of palladium (II) solution 1.0; ph of copper (II) solution 5.5; flow rates of each eluent, 3 ml min 1 ). Fig. 2 The effect of flow rate on the recovery of gold (III), palladium (II) and copper (II) (sample volume, 100 ml; amount of each metal, 0.05 mg; eluent, 10 ml of 0.1 mol l 1 potassium cyanide; ph of gold (III) solution 2.5; ph of palladium (II) solution 1.0; ph of copper (II) solution 5.5). 3.6 Precision of method and limit of detection Precision of the preconcentration of gold, palladium and copper on the AEAP-SG column was investigated at the optimum conditions (sample volume, 100 ml; amount of metal, 0.05 mg; eluent, 10 ml 0.1 M potassium cyanide; flow rate, 7 ml min 1 ; ph of gold (III) solution 2.5; ph of palladium (II) solution 1.0; ph of copper (II) solution 5.5). For this purpose, eight solutions of each metal were percolated through the column and metals in the eluate were determined by FAAS. The recoveries of Au (III), Pd (II) and Cu (II) were 98.93±0.51, 98.81±0.36 and 99.21±0.42 % at 95 % confidence level, respectively. The recoveries are quantitative and the described method can be reliably used for the preconcentration of gold, palladium and copper. The stability of AEAP-SG is very good. Adsorption properties of the modified silica gel were not changed after 10 times using. The detection limits (δ) of gold, palladium and copper based on three times the standard deviation of the blanks (n=10) were 0.032, and µg ml 1, respectively.

7 258 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) Fig. 3 The effect of the solution volume on the recovery of gold (III), palladium (II) and copper (II) (amount of each metal, 0.05 mg; eluent, 10 ml of 0.1 mol l 1 potassium cyanide; flow rate of the solution, 7 ml min 1 ; ph of gold (III) solution 2.5; ph of palladium (II) solution 1.0; ph of copper (II) solution 5.5). 3.7 Effect of matrix The effect of matrix ions on the recoveries of gold, palladium and copper were investigated because of real samples such as ore containing alkali, alkaline earth and some heavy metals. For this purpose, 100 ml of 0.5 mg l 1 gold, palladium and copper solutions at the optimum ph were prepared containing different metal ions at various concentrations. The prepared solutions were passed through the column at the optimum conditions. The results are shown in Table 2. As can be seen, the recovery of Au (III), Pd (II) and Cu (II) decrease by increasing matrix ions concentration. However, the recovery of the elements is at least 90.6 %. The recovery of gold (III) and palladium (II) ions from the solution including 500-fold excess of iron (III) or copper (II) ions is about 91 %. An AEAP-SG loaded column can be used for preconcentration of gold and palladium from ore or environmental samples. The recovery of copper (II) ions is not affected significantly by the alkali and alkaline earth metals. The modified silica gel can be used for the preconcentration of copper from natural water samples. 3.8 Analytical application For validation of the preconcentration method described in this study as an analytical technique, gold and palladium content of certified platinum ore SARM 7B from The South African Bureau of Standards was determined. The results are given in Table 3. As can be seen, the values found corresponded well with the certified values. Copper was determined in river and synthetic seawater. Synthetic seawater was prepared according to literature [25] and spiked with copper. The results obtained are shown in Table 4. As can be seen, AEAP-SG can successfully be used for preconcentration copper.

8 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) Matrix Ions Concentration Recoveries, % (mg l 1 ) Au (III) Pd (II) Cu (II) Na K Mg Ca Cu Fe Table 2 The effect of matrix ions on the recovery of Au (III), Pd (II) and Cu (II) (sample volume, 100 ml; amount of gold, palladium and copper 0.05 mg; flow rate of the solution, 7 ml min 1 ; ph of gold (III) solution 2.5; ph of palladium (II) solution 1.0; ph of copper (II) solution 5.5; eluent, 10 ml of 0.1 mol l 1 potassium cyanide). Metal Certified Value (ppm) Found Value (ppm) Relative Error, % Gold 0.27± ± Palladium 1.54± ± Average and standard deviation of three determinations. Table 3 Determination of gold and palladium in certified platinum ore SARM 7B. 4 Conclusions This study demonstrated that 3-(2-aminoethylamino)propyl bonded silica gel loaded columns offer a method for the preconcentration of Au (III), Pd (II) and Cu (II). Trace levels of gold (III) and palladium (II) in ore or environmental samples can be preconcentrated and separated by using an AEAP-SG column and determined with AAS. The modified silica gel can be used for separation of gold (III) and palladium (II) from copper (II) ions. Preparation of 3-(2-aminoethylamino)propyl bonded silica gel is simple and easy. The modified silica gel can adsorb gold (III), palladium (II) and copper (II) ions at high flow

9 260 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) Sample Added Copper Found Value Relative Error, Amount (µg) (µg) ± s % Sea water ± ± ± River water ± ± Average and standard deviation of three determinations. Table 4 Determination of copper in river and sea water. rates and from large volumes of solution. Also, the recovery is not affected by matrix ions. The proposed method provides simple, accurate and efficient preconcentration methods for gold, palladium and copper. Acknowledgment This work was supported partly by Sakarya University Scientific Research Foundation with Project Number We give thanks to Professor E.M. Soliman for advice on the modification of silica gel. References [1] Z.B. Alfassi and C.M. Wai: Preconcentration techniques for trace elements, CRC Press Inc., USA, [2] B.S. Garg, R.K. Sharma, N. Bhojak and S. Mittal: Chelating resins and their applications in the analysis of trace metal ions, Microchem J., Vol. 61(2), (1999), pp [3] K. Terada: Preconcentration of trace elements by sorption, Analytical Sciences, Vol. 7, (1991), pp [4] D.E. Leyden, G.H. Luttrell, W.K. Nonidez and D.B. Werho: Preconcentration of certain anions using reagents immobilized via silylation, Anal. Chem., Vol. 48(1), (1976), pp [5] D.E. Leyden and G.H. Luttrell: Characterization and application of silylated substrates for the preconcentration of cations, Anal. Chim. Acta, Vol. 84, (1976), pp [6] D.E. Leyden and G.H. Luttrell: Preconcentration of trace metals using chelating goups immobilized via silylation, Anal. Chem., Vol. 47(9), (1975), pp [7] C. Ekinci and U. Koklu: Determination of vanadium, manganese, silver and lead by graphite furnace atomic absorption spectrometry after preconcentration on silica-gel modified with 3-aminopropyltriethoxysilane, Spectrochim. Acta B, Vol. 55(9), (2000), pp

10 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) [8] H. Ince, S. Akman and U. Koklu: Sorption and preconcentration of copper and cadmium on silica-gel modified with 3-aminopropyltriethoxysilane, Fresenius Journal of Analytical Chemistry, Vol. 342(7), (1992), pp [9] S. Akman, H. Ince and U. Koklu: 3-(Trimethoxysilyl)-1-propanethiol supported on silica-gel for the separation and preconcentration of copper and cadmium in water, Analytical Sciences, Vol. 7(5), (1991), pp [10] A.G. Howard, M. Volkan and O.Y. Ataman: Selective preconcentration of arsenite on mercapto-modified silica gel, Analyst, Vol. 112, (1987), pp [11] M. Volkan, O.Y. Ataman and A.G. Howard: Preconcentration some metals from sea water on a mercapto-modified silica gel, Analyst, Vol. 112, (1987), pp [12] A. Korez, A.E. Eroglu, M. Volkan and O.Y. Ataman: Speciation and preconcentration of inorganic tellurium from waters using a mercaptosilica microcolumn and determination by hydride generation atomic absorption spectrometry, J. Anal. Atom. Spectrom., Vol. 15(12), (2000), pp [13] G. Gokturk, M. Delzendeh and M. Volkan: Preconcentration of germanium on mercapto-modified silica gel, Spectrochim. Acta B, Vol. 55(7), (2000), pp [14] E.M. Soliman: Synthesis and metal collecting properties of mono, di, tri and tetramine based on silica gel matrix, Anal. Lett., Vol. 30(9), (1997), pp [15] M.E. Mahmoud: Comparison of metal uptake properties of silica gel-bound ion exchangers and some amine derivatives, Anal. Lett., Vol. 29(10), (1996), pp [16] C. Airoldi and A.S. Gonçalves: Immobilized 3-aminopyridine on silica: adsorption of some metal (II) Chlorides in non-aqueous solutions, Colloids and Surface, Vol. 28, (1987), pp [17] P. Liu, Q.S. Pu and Z.X. Su: Synthesis of silica gel immobilized thiourea and its application to the on-line preconcentration and separation of silver, gold and palladium, Analyst, Vol. 125(1), (2000), pp [18] Q.S. Pu, Z.X. Su, Z.H. Hu, X.J. Chang and M. Yang: 2-Mercaptobenzothiazolebonded silica gel as selective adsorbent for preconcentration of gold, platinum and palladium prior to their simultaneous inductively coupled plasma optical emission spectrometric determination, J. Anal. Atom. Spectrom., Vol. 13(4), (1998), pp [19] S.M. Zhang, Q.H. Pu, P. Liu, Q.Y. Sun and Z.X. Su: Synthesis of amidinothioureidosilica gel and its application to flame atomic absorption spectrometric determination of silver, gold and palladium with on-line preconcentration and separation, Anal. Chim. Acta, Vol. 452(2), (2002), pp [20] F. Liu, K.A. Li, Y.S. Wu, X. Wang and S.Y. Tong: Study on preconcentration and separation of trace Pd (II) and Pt (IV) with silica gel bonded by aminopropylbenzoylazo-1-(2-pyridylazo)-2-naphthol, Microchem. J., Vol. 52(3), (1995), pp [21] K. Terada, T. Kiba and K. Morimoto: Preconcentration of silver (I), gold (III) and palladium (II) in sea-water with para-dimethylaminobenzylidenerhodanine supported on silica-gel, Anal. Chim. Acta, Vol. 116(1), (1980), pp [22] P. Di and D.E. Davey: Online preconcentration and separation of palladium, platinum and iridium using alpha-amino pyridine resin with flame atomic-absorption spectrometry, Talanta, Vol. 42(5), (1995), pp

11 262 M. Imamoglu et al. / Central European Journal of Chemistry 3(2) [23] A. Tong, Y. Akama and S. Tanaka: Selective preconcentration of Au (III), Pt (IV) and Pd (II) on silica-gel modified with -aminopropyltriethoxysilane, Anal. Chim. Acta, Vol. 230(1), (1990), pp [24] U. Koklu and S. Tascioglu: Copper (II), silver (I), and gold (III) adsorptions on the amino- and thiol-modified silicas, Chimica Acta Turcica, Vol. 16, (1988), pp [25] J.E. Cantle (Ed.): Atomic absorption spectrometry, 2nd Ed., Elsevier Scientific Publishing Company, The Netherlands, 1986, pp