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Speciation of Cr(III) and Cr(VI) by nanometer titanium dioxide micro-column and inductively coupled plasma atomic emission spectrometry
Spectrochimica Acta Part B 58 (2003) 1709–1714
Analytical note
Speciation of Cr(III) and Cr(VI) by nanometer titanium dioxide micro-column and inductively coupled plasma atomic emission spectrometry Pei Lianga,*, Taqing Shia, Hanbing Lua, Zucheng Jiangb, Bin Hub a
College of Chemistry, Central China Normal University, Wuhan 430079, PR China b Department of Chemistry, Wuhan University, Wuhan 430072, PR China Received 27 January 2003; accepted 16 June 2003
Abstract A sensitive and selective method has been developed for the speciation of Cr(III) and Cr(VI) in natural water using a nanometer titanium dioxide micro-column (20 mm=3.0 mm i.d.) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Under the optimized conditions (pH 6.0, flow rate 1.0 ml miny1 ), Cr(III) was retained on the column, then eluted with 2.0 mol ly1 HCl and determined by ICP-AES. Total chromium was determined after the reduction of Cr(VI) to Cr(III) by ascorbic acid as reducing reagent. The adsorption capacity of nanometer TiO2 for Cr(III) was found to be 7.6 mg gy1. The detection limit for Cr(III) was 0.32 mg ly1 and the relative standard deviation (R.S.D.) was 2.4% (ns11, Cs100 mg ly1 ) with an enrichment factor of 50. The proposed method has been applied to the determination and speciation of chromium in natural water samples with satisfactory results. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanometer titanium dioxide; Chromium; Speciation; ICP-AES
1. Introduction In recent years there is an increasing demand for the information about speciation because the toxicity of some elements depends on their chemical form w1x. Dissolved chromium is usually found in natural water in two different oxidation states, Cr(III) and Cr(VI), which have contrasting physiological effects. Cr(III) is considered an essential trace element for the maintenance of an effective *Corresponding author. Fax: q86-27-6786-7955. E-mail address: [email protected] (P. Liang).
glucose, lipid and protein metabolism in mammals w2x. On the other hand, Cr(VI) can be toxic for biological systems w3,4x, and water soluble Cr(VI) is extremely irritating and toxic to human body tissue owing to its oxidizing potential and easy permeating of biological membranes w5,6x. Therefore, it is of increasing importance to accurately define the individual quantity of both valence forms in environmental samples. A great number of speciation studies on chromium have been carried out in solid and liquid samples w7,8x. As the Cr content in natural waters is normally
0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547(03)00136-8
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at the low mg ly1 level, only a few analytical techniques are capable of direct differentiation of the chemical forms of Cr in water. As a result, preliminary species separation and preconcentration are required before detection w9x. Among the separation and preconcentration methods proposed for speciation of chromium, hyphenated technique of flow injection sorbent extraction preconcentration and separation coupled with atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometryymass spectrometry (ICP-AESyMS) is very attractive w10–17x. The main purpose of the use of flow analysis in speciation studies of trace metals is the possibility of improving the selective preconcentration of a given species on flow-through microcolumn with a solid sorbent prior to elution to the detector. Different solid sorbents have been used for flow injection separation and preconcentration, such as ion exchangers, chelate resin, immobilized chelating reagents on solid supports, silica and different oxides (Al2O3, MgO, ZnO, etc). Titanium dioxide, especially its low-temperature modification, anatase, is a promising material for preconcentration and separation of metal elements. Vassileva et al. systemically investigated the adsorption characteristics of transition metal ions on high area TiO2 and applied it for solid-phase extraction of heavy metal ions in water and AR grade alkali salts w18,19x. They also used it for chromium speciation analysis w20x and compared it with different oxides (MgO, Al2O3, ZrO2 and CeO2) w21x. Yu et al. w22x used TiO2 granules (0.5 mm diameter) for the speciation of chromium, at pH 2, Cr(VI) was retained in the TiO2 column, while Cr(III) was eluted and detected by ICP-MS. Nanometer material is a new solid material that gained importance in recent years due to its special properties w23,24x as ultrafine-grained particle. Nanoparticles are clusters of atoms or molecules of metal and oxide, ranging in size from 1 nm to almost 100 nm. One of its properties is that most of the atoms are on the surface of the nanoparticle. The surface atoms are unsaturated, and can therefore bind with other atoms, possessing highly chemical activity. Consequently, nanometer material can adsorb metal ions with high adsorption capacity. In a previous paper w25x, we studied the
Table 1 ICP-AES operating conditions and the analytical wavelength Parameters Incident poweryW Plasma gas (Ar) flow rateyl miny1 Auxiliary gas (Ar) flow rateyl miny1 Nebulizer gas (Ar) flow rateyl miny1 Observation heightymm Integration timeys Solution pump rateyml miny1 Wavelengthynm
1300 15 0.5 0.8 15 10 1.0 Cr 283.5
adsorption behavior of heavy metal ions on nanometer TiO2, and applied it to the determination of these elements in real samples with satisfactory results. In this work, the adsorption characteristics of nanometer TiO2 for Cr(III) and Cr(VI) were studied, and a new method was developed for the speciation of chromium by using a micro-column packed with nanometer TiO2 coupled with ICPAES. The proposed method has been applied to the determination and speciation of chromium in tap and lake water samples with satisfactory results. 2. Experimental 2.1. Apparatus A Perkin–Elmer Optima 2000DV ICP spectrometer equipped with the standard torch assembly and conventional cross flow nebulizer was used for the determination of chromium. The operation conditions and the analytical wavelengths are summarized in Table 1. The pH values were measured with a Mettler Toledo 320-S pH meter wMettler Toledo Instruments (Shanghai) Co. Ltdx supplied with a combined electrode. A HL-2 peristaltic pump (Shanghai Qingpu Instrument Factory, China) was used in separationypreconcentration process. A minimum length of PTFE tubing (i.d. 0.5 mm) was used for flow injection (FI) connections. A self-made PTFE micro-column (20 mm=3.0 mm i.d.) was used.
P. Liang et al. / Spectrochimica Acta Part B 58 (2003) 1709–1714
2.2. Standard solution and reagents Stock solution (1.000 g ly1) of Cr(III) was prepared by dissolving CrCl3Ø6H2O (The First Reagent Factory, Shanghai, China) in 0.1 mol ly1 hydrochloric acid. Stock solution (1.000 g ly1) of Cr(VI) was prepared by dissolving K2Cr2O7 (The First Reagent Factory, Shanghai, China) in 0.1 mol ly1 nitric acid. 10% aqueous ascorbic acid solution was prepared fresh daily. HCl, NH3ØH2O and NH4Cl were of analytical reagent grade. Doubly distilled deionized water was used throughout. Nanometer TiO2 (the diameter is from 10 to 50 nm, and the specific surface areas determined by the BET method was 208 m2 gy1) was provided by the Laboratory of Inorganic Chemistry, Department of Chemistry, Wuhan University, and the synthesis method and characters of nanometer TiO2 was reported previously w26x. 2.3. Sample preparation Tap water was collected in our laboratory. Lake water was collected from East Lake, Wuhan, China. All water samples were filtered through a 0.45 mm membrane filter (Tianjin Jinteng Instrument Factory, Tianjin, China) and storage at 4 8C. The water sample must not be acidified before storage, because this would change the chemical species w27x. Water sample was divided into two parts: ● Cr(III) determination: water sample (49 ml) was adjusted to pH 6.0 with dilute ammonia, and then diluted to 50 ml with deionized water in a calibrated flask. ● Total chromium determination: To 49 ml of water sample 0.5 ml aqueous ascorbic acid was added. The pH was adjusted to pH 6.0 with dilute ammonia, and then diluted to 50 ml with deionized water in a calibrated flask. 2.4. Column preparation One hundred milligrams of nanometer TiO2 was filled into a PTFE micro-column (20 mm=3.0 mm i.d) plugged with a small portion of glasswool at both end. Before use, 1.0 mol ly1 HCl solution
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and doubly distilled deionized water were passed through the column in order to clean and condition it. Then, the column was conditioned to the desired pH with 0.1 mol ly1 NH4Cl buffer solution. 2.5. General procedure A portion of aqueous sample solution containing Cr(III) was prepared, and the pH value was adjusted to the desired pH with 0.1 mol ly1 HCl and 0.1 mol ly1 NH3ØH2O. The solution was passed through the column at a flow rate of 1.0 ml miny1 by using a peristaltic pump. Afterwards, the Cr(III) retained on micro-column was eluted with 2.0 mol ly1 HCl solution. The chromium in the elution was determined by ICP-AES. 3. Results and discussion 3.1. Effect of pH on adsorption of Cr(III) and Cr(VI) The effect of pH on the retention of Cr(III) and Cr(VI) on to the column of nanometer TiO2 has been investigated separately. pH values of sample solutions were adjusted to a range of 1;8 with 0.1 mol ly1 HCl or NH3ØH2O and passed through the column. The retained ions were stripped off from the column and determination by ICP-AES as described in the recommended procedure. The results of the effect of pH on the recoveries of Cr(III) and Cr(VI) were shown in Fig. 1. As can be seen, quantitative recovery ()90%) was found for Cr(III) when the pH exceed 5.5 while the recovery of Cr(VI) was rather low (-5%). This could make it possible to separate Cr(III) and Cr(VI). A pH of 6.0 was selected for subsequent work. 3.2. Elution of Cr(III) Elution of Cr(III) from the nanometer TiO2 microcolumn was investigated by using hydrochloric acid solution at different concentrations as stripping agent. Elution of Cr(III) was quantitative with 1.0 ml of G2.0 mol ly1 HCl, lower concentrations gave only 60–80% recovery. To ensure
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In this work, 50 ml of sample solution was adopted for the preconcentration of Cr(III) from water sample, the adsorbed Cr(III) can be eluted with 1.0 ml 2.0 mol ly1 HCl, so an enrichment factor of 50 was achieved by this method. 3.5. Adsorption capacity
Fig. 1. Effect of pH on the adsorption (R%) of Cr(III) and Cr(VI) on nanometer TiO2. Cr(III) and Cr(VI): 1.0 mg mly1.
complete elution, 1.0 ml of 2.0 mol ly1 HCl was used as eluent in the following experiments. 3.3. Effect of flow rate of sample solution The effect of flow rate of sample solution on the retention of Cr(III) on nanometer TiO2 was examined under the optimum conditions (pH, eluent, etc.). The flow rate was adjusted in a range of 0.5–2.0 ml miny1. It was found that the retention of Cr(III) was practically not changed up to 1.0 ml miny1 flow rate. However, the retention of Cr(III) will decrease with further increasing of the flow rate after it is over 1.0 ml miny1, due to a decrease in the adsorption kinetics at a high flow rate. Thus, a flow rate of 1.0 ml miny1 is employed in this work.
The capacity study used was adapted from that recommended by Maquieira et al. w28x, 25 ml aliquots of a series of concentrations (5;50 mg mly1) was adjusted to the appropriate pH, the proposed separation and preconcentration procedure described above has been applied. A breakthrough curve was gained by plotting the concentration (mg mly1) vs. the micrograms of Cr(III) adsorbed per gram. From the breakthrough curve, the adsorption capacity of nanometer TiO2 for Cr(III) was found to be 7.6 mg gy1. 3.6. Effects of coexisting ions The effects of common coexisting ions on the determination of Cr(III) were investigated. In these experiments, solutions of 1.0 mg mly1 of Cr(III) containing the added interfering ions were treated according to the recommended procedure. The tolerance of the coexisting ions, defined as the largest amount making the recovery of Cr(III) less than 90%, were Naq, Kq, 5.0 g ly1; Ca2q, Mg2q, 2.0 g ly1; Cu2q, Zn2q, Mn2q, Ni2q, Co2q, 0.5 g ly1; Al3q, Pb2q, 0.25 g ly1; Fe3q,
3.4. Effect of the sample volume In order to explore the possibility of enriching low concentrations of analytes from large volumes, the maximum applicable sample volume must be determined. For this purpose, 20, 50, 100, 150 and 200 ml of sample solutions containing 10 mg of Cr(III) were passed through the microcolumn under the optimum conditions. As shown in Fig. 2, the recovery of Cr(III) was quantitative () 90%) up to 50 ml of sample volume.
Fig. 2. Effect of the sample volume on the recovery of Cr(III). pH: 6.0; Cr(III): 10 mg.
P. Liang et al. / Spectrochimica Acta Part B 58 (2003) 1709–1714
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Table 2 Determination of Cr(III) and Cr(VI) in natural water samplesa Samples
GSBZ 5009-88 Tap water
Lake water
b
Added (mg ly1)
Foundc (mg ly1)
Cr(III)
Cr(VI)
Cr(III)
0 500e 0 0.50 1.00 0 0.50 1.00
0 0 0.50 1.00 0 0.50 1.00
0.78"0.04 1.26"0.11 1.80"0.12 0.57"0.05 1.06"0.09 1.61"0.14
Recovery (%) Cr(VI)d
Total
Cr(III)
Cr(VI)
0.32"0.03 0.82"0.05 1.29"0.10 0.99"0.07 1.47"0.12 1.97"0.12
645"40 1124"54 1.10"0.06 2.08"0.17 3.09"0.18 1.56"0.12 2.53"0.19 3.58"0.20
96e – 96 102 – 98 104
– 100 97 – 96 98
a
Sample volume: 50 ml. National reference material (China): the certified value of Cr concentration is 640"36 mg ly1. c Mean of five determinations. d Calculated value. e the result for total Cr. b
2y y1 50 mg ly1; SiO2y ; PO43y, 0.5 g 4 , SO4 , 1.0 g l y1 l . It can be seen that the presence of major cations and anions has no obvious influence on the determination under the selected conditions.
3.7. Detection limits and precision According to the definition of IUPAC, the detection limit (3s) of this method for Cr(III) with an enrichment factor of 50 is 0.32 mg ly1; and relative standard deviation (R.S.D.) is 2.4% (ns11, Cs 100 mg ly1). 3.8. Analysis application The accuracy of the proposed method was examined by analyzing total chromium in environmental water reference materials (ERMs, GSBZ 500988). In Table 2, the analytical value was in agreement with the certified value of chromium in the standard reference material. The proposed method has been applied to the speciation of Cr(III) and Cr(VI) in tap and lake water samples collected in Wuhan. The standard addition method was used, and the analytical results and the recovery were given in Table 2. The results indicated that the recoveries were reasonable for trace analysis, in a range of 96;104%.
4. Conclusion In this work, the adsorption behaviors of Cr(III) and Cr(VI) on nanometer TiO2 were systematically investigated. Based on its high adsorption selectivity for Cr(III) and Cr(VI), a sensitive and selective method for the separation and determination of chromium species in natural water using nanometer TiO2 packed micro-column coupled with ICPAES was developed. Compared with the mm sized TiO2 used for chromium speciation w20,22x, the adsorption capacity of nanometer TiO2 for Cr is higher (7.6 mg gy1 vs. 5.0 mg gy1 w20x and 4.0 mg gy1 w22x), mainly due to its high surface areas (208 m2 gy1). The developed method has been successfully applied to the speciation of chromium in tap and lake water samples, and the precision and accuracy of the method are satisfactory. The method may be used for the speciation of chromium in various matrices other than water. References w1x A. Kot, J. Namiesnik, The role of speciation in analytical chemistry, Trends Anal. Chem. 19 (2000) 69–79. w2x C. Barnowski, N. Jakubowski, D. Stuewer, J.A.C. Broekaert, Speciation of chromium by direct coupling of ion exchange chromatography with inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 12 (1997) 1155–1161. w3x A.S. Prasad, D. Oberleas, Trace Elements in Human Health and Disease, Academic Press, 1976, p. 79.
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w4x A.B. Gwizala III, S.K. Johnson, S. Mollah, R.S. Houk, Speciation of chromium(VI) and chromium(III) using pneumatically assisted electrospray mass spectrometry, J. Anal. At. Spectrom. 12 (1997) 503–506. w5x C.M. Andrles, N. Jakubowski, J.A.C. Broekaert, Speciation of chromium using reversed phase-high performance liquid chromatography coupled to different spectrometric detection methods, Spectrochim. Acta Part B 52 (1997) 189–200. w6x R.K. Mugo, K.J. Orians, Seegoing method for the determination of chromium(VI) and total chromium in sea water by electron-capture detection gas chromatography, Anal. Chim. Acta 271 (1993) 1–9. w7x M.J. Marques, A. Salvador, A.E. Morales-Rubio, M. de la Guardia, Analytical methodologies for chromium speciation in solid matrices: a survey of literature, Fresenius J. Anal. Chem. 362 (1998) 239–248. w8x M.J. Marques, A. Salvador, A.E. Morales-Rubio, M. de la Guardia, Chromium speciation in liquid matrices: a survey of literature, Fresenius J. Anal. Chem. 367 (2000) 601–613. w9x M. Sperling, X.F. Yin, B. Welz, Differential determination of chromium(VI) and total chromium in natural water using flow injection on-line separation and preconcentration electrothermal atomic absorption spectrometry, Analyst 117 (1992) 629–635. w10x A.G. Cox, L.G. Cook, C.W. McLeod, Rapid sequential determination of chromium(III)- chromium(VI) by flow injection analysis-inductively coupled plasma atomic emission spectrometry, Analyst 110 (1985) 331–333. w11x K. Isshiki, Y. Sohrin, H. Karatani, E. Nakayama, Preconcentration of chromium(III) and chromium(VI) in sea water by complexation with quinolin-8-ol and adsorption on macroporous resin, Anal. Chim. Acta 224 (1989) 55–64. w12x M. Sperling, S.K. Xu, B. Welz, Determination of chromium(III) and chromium(VI) in water using flow injection on-line preconcentration with selective adsorption on activated alumina and flame atomic absorption spectrometric detection, Anal. Chem. 64 (1992) 3101–3108. w13x J.L. Manzoori, M.H. Sorouraddin, F. Shemirani, Chromium speciation by a surfactant-coated alumina microcolumn using electrothermal atomic absorption spectrometry, Talanta 42 (1995) 1151–1155. w14x R.M. Cespon-Romero, M.C. Yebra-Biurrun, M.P. Bermejo-Barrera, Preconcentration and speciation of chromium by the determination of total chromium and chromium(III) in natural waters by flame atomic absorption spectrometry with a chelating ion-exchange flow injection system, Anal. Chim. Acta 327 (1996) 37–45.
w15x S. Hirata, K. Honda, O. Shikino, N. Maekawa, M. Aihara, Determination of chromium(III) and total chromium in seawater by on-line column preconcentration inductively coupled plasma mass spectrometry, Spectrochim. Acta Part B 55 (2000) 1089–1099. w16x B. Demirata, Speciation of Cr(III) and Cr(VI) by means of melamine-urea-formaldehyde resin and FAAS, Mikrochim. Acta 136 (2001) 143–146. w17x A. Tunceli, A.R. Turker, Speciation of Cr(III) and Cr(VI) in water after preconcentration of its 1, 5diphenylcarbazone complex on Amberlite XAD-16 resin and determination by FAAS, Talanta 57 (2002) 1199–1204. w18x E. Vassileva, I. Proinova, K. Hadjiivanov, Solid-phase extraction of heavy metal ions on a high surface area titanium dioxide (anatase), Analyst 121 (1996) 607–612. w19x E. Vassileva, K. Hadjiivanov, Determination of trace elements in AR grade alkali salts after preconcentration by column solid-phase extraction on TiO2 (anatase), Fresenius J. Anal. Chem. 357 (1997) 881–885. w20x E. Vassileva, Use of high surface area TiO2 for preconcentration and following determination of Cr species by on-line flow injection inductively coupled plasma atomic emission spectrometry, Analusis 28 (2000) 878–884. w21x E. Vassileva, K. Hadjiivanov, T. Stoychev, C. Daiev, Chromium speciation analysis by solid-phase extraction on a high surface area TiO2, Analyst 125 (2000) 693–698. w22x J.C. Yu, X.J. Wu, Z.L. Chen, Separation and determination of Cr(III) by titanium dioxide-filled column and inductively coupled plasma mass spectrometry, Anal. Chim. Acta 436 (2001) 59–67. w23x A. Henglein, Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles, Chem. Rev. 89 (1989) 1861–1873. w24x Q.J. Xue, K. Xu, Nanochemistry, Progress in Chem. (Chinese) 12 (2000) 431–444. w25x P. Liang, Y.C. Qin, B. Hu, T.Y. Peng, Z.C. Jiang, Nanometer-size titanium dioxide microcolumn on-line preconcentration of trace metals and their determination by inductively coupled plasma atomic emission spectrometry in water, Anal. Chim. Acta 440 (2001) 207–213. w26x L. Zan, J.C. Zhong, Q.R. Luo, Influence factors on preparing process of TiO2 nanoscale particles, J. Inorg. Mater. (Chinese) 14 (1999) 264–270. w27x T. M. Florence, The speciation of trace elements in waters, Talanta 29 (1982) 345–364. w28x A. Maquieira, H.A.M. Elmahadi, R. Puchades, Immobilized cyanobacteria for on-line trace metal enrichment by flow injection atomic absorption spectrometry, Anal. Chem. 66 (1994) 3632–3638.
Analytical note
Speciation of Cr(III) and Cr(VI) by nanometer titanium dioxide micro-column and inductively coupled plasma atomic emission spectrometry Pei Lianga,*, Taqing Shia, Hanbing Lua, Zucheng Jiangb, Bin Hub a
College of Chemistry, Central China Normal University, Wuhan 430079, PR China b Department of Chemistry, Wuhan University, Wuhan 430072, PR China Received 27 January 2003; accepted 16 June 2003
Abstract A sensitive and selective method has been developed for the speciation of Cr(III) and Cr(VI) in natural water using a nanometer titanium dioxide micro-column (20 mm=3.0 mm i.d.) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Under the optimized conditions (pH 6.0, flow rate 1.0 ml miny1 ), Cr(III) was retained on the column, then eluted with 2.0 mol ly1 HCl and determined by ICP-AES. Total chromium was determined after the reduction of Cr(VI) to Cr(III) by ascorbic acid as reducing reagent. The adsorption capacity of nanometer TiO2 for Cr(III) was found to be 7.6 mg gy1. The detection limit for Cr(III) was 0.32 mg ly1 and the relative standard deviation (R.S.D.) was 2.4% (ns11, Cs100 mg ly1 ) with an enrichment factor of 50. The proposed method has been applied to the determination and speciation of chromium in natural water samples with satisfactory results. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanometer titanium dioxide; Chromium; Speciation; ICP-AES
1. Introduction In recent years there is an increasing demand for the information about speciation because the toxicity of some elements depends on their chemical form w1x. Dissolved chromium is usually found in natural water in two different oxidation states, Cr(III) and Cr(VI), which have contrasting physiological effects. Cr(III) is considered an essential trace element for the maintenance of an effective *Corresponding author. Fax: q86-27-6786-7955. E-mail address: [email protected] (P. Liang).
glucose, lipid and protein metabolism in mammals w2x. On the other hand, Cr(VI) can be toxic for biological systems w3,4x, and water soluble Cr(VI) is extremely irritating and toxic to human body tissue owing to its oxidizing potential and easy permeating of biological membranes w5,6x. Therefore, it is of increasing importance to accurately define the individual quantity of both valence forms in environmental samples. A great number of speciation studies on chromium have been carried out in solid and liquid samples w7,8x. As the Cr content in natural waters is normally
0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547(03)00136-8
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at the low mg ly1 level, only a few analytical techniques are capable of direct differentiation of the chemical forms of Cr in water. As a result, preliminary species separation and preconcentration are required before detection w9x. Among the separation and preconcentration methods proposed for speciation of chromium, hyphenated technique of flow injection sorbent extraction preconcentration and separation coupled with atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometryymass spectrometry (ICP-AESyMS) is very attractive w10–17x. The main purpose of the use of flow analysis in speciation studies of trace metals is the possibility of improving the selective preconcentration of a given species on flow-through microcolumn with a solid sorbent prior to elution to the detector. Different solid sorbents have been used for flow injection separation and preconcentration, such as ion exchangers, chelate resin, immobilized chelating reagents on solid supports, silica and different oxides (Al2O3, MgO, ZnO, etc). Titanium dioxide, especially its low-temperature modification, anatase, is a promising material for preconcentration and separation of metal elements. Vassileva et al. systemically investigated the adsorption characteristics of transition metal ions on high area TiO2 and applied it for solid-phase extraction of heavy metal ions in water and AR grade alkali salts w18,19x. They also used it for chromium speciation analysis w20x and compared it with different oxides (MgO, Al2O3, ZrO2 and CeO2) w21x. Yu et al. w22x used TiO2 granules (0.5 mm diameter) for the speciation of chromium, at pH 2, Cr(VI) was retained in the TiO2 column, while Cr(III) was eluted and detected by ICP-MS. Nanometer material is a new solid material that gained importance in recent years due to its special properties w23,24x as ultrafine-grained particle. Nanoparticles are clusters of atoms or molecules of metal and oxide, ranging in size from 1 nm to almost 100 nm. One of its properties is that most of the atoms are on the surface of the nanoparticle. The surface atoms are unsaturated, and can therefore bind with other atoms, possessing highly chemical activity. Consequently, nanometer material can adsorb metal ions with high adsorption capacity. In a previous paper w25x, we studied the
Table 1 ICP-AES operating conditions and the analytical wavelength Parameters Incident poweryW Plasma gas (Ar) flow rateyl miny1 Auxiliary gas (Ar) flow rateyl miny1 Nebulizer gas (Ar) flow rateyl miny1 Observation heightymm Integration timeys Solution pump rateyml miny1 Wavelengthynm
1300 15 0.5 0.8 15 10 1.0 Cr 283.5
adsorption behavior of heavy metal ions on nanometer TiO2, and applied it to the determination of these elements in real samples with satisfactory results. In this work, the adsorption characteristics of nanometer TiO2 for Cr(III) and Cr(VI) were studied, and a new method was developed for the speciation of chromium by using a micro-column packed with nanometer TiO2 coupled with ICPAES. The proposed method has been applied to the determination and speciation of chromium in tap and lake water samples with satisfactory results. 2. Experimental 2.1. Apparatus A Perkin–Elmer Optima 2000DV ICP spectrometer equipped with the standard torch assembly and conventional cross flow nebulizer was used for the determination of chromium. The operation conditions and the analytical wavelengths are summarized in Table 1. The pH values were measured with a Mettler Toledo 320-S pH meter wMettler Toledo Instruments (Shanghai) Co. Ltdx supplied with a combined electrode. A HL-2 peristaltic pump (Shanghai Qingpu Instrument Factory, China) was used in separationypreconcentration process. A minimum length of PTFE tubing (i.d. 0.5 mm) was used for flow injection (FI) connections. A self-made PTFE micro-column (20 mm=3.0 mm i.d.) was used.
P. Liang et al. / Spectrochimica Acta Part B 58 (2003) 1709–1714
2.2. Standard solution and reagents Stock solution (1.000 g ly1) of Cr(III) was prepared by dissolving CrCl3Ø6H2O (The First Reagent Factory, Shanghai, China) in 0.1 mol ly1 hydrochloric acid. Stock solution (1.000 g ly1) of Cr(VI) was prepared by dissolving K2Cr2O7 (The First Reagent Factory, Shanghai, China) in 0.1 mol ly1 nitric acid. 10% aqueous ascorbic acid solution was prepared fresh daily. HCl, NH3ØH2O and NH4Cl were of analytical reagent grade. Doubly distilled deionized water was used throughout. Nanometer TiO2 (the diameter is from 10 to 50 nm, and the specific surface areas determined by the BET method was 208 m2 gy1) was provided by the Laboratory of Inorganic Chemistry, Department of Chemistry, Wuhan University, and the synthesis method and characters of nanometer TiO2 was reported previously w26x. 2.3. Sample preparation Tap water was collected in our laboratory. Lake water was collected from East Lake, Wuhan, China. All water samples were filtered through a 0.45 mm membrane filter (Tianjin Jinteng Instrument Factory, Tianjin, China) and storage at 4 8C. The water sample must not be acidified before storage, because this would change the chemical species w27x. Water sample was divided into two parts: ● Cr(III) determination: water sample (49 ml) was adjusted to pH 6.0 with dilute ammonia, and then diluted to 50 ml with deionized water in a calibrated flask. ● Total chromium determination: To 49 ml of water sample 0.5 ml aqueous ascorbic acid was added. The pH was adjusted to pH 6.0 with dilute ammonia, and then diluted to 50 ml with deionized water in a calibrated flask. 2.4. Column preparation One hundred milligrams of nanometer TiO2 was filled into a PTFE micro-column (20 mm=3.0 mm i.d) plugged with a small portion of glasswool at both end. Before use, 1.0 mol ly1 HCl solution
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and doubly distilled deionized water were passed through the column in order to clean and condition it. Then, the column was conditioned to the desired pH with 0.1 mol ly1 NH4Cl buffer solution. 2.5. General procedure A portion of aqueous sample solution containing Cr(III) was prepared, and the pH value was adjusted to the desired pH with 0.1 mol ly1 HCl and 0.1 mol ly1 NH3ØH2O. The solution was passed through the column at a flow rate of 1.0 ml miny1 by using a peristaltic pump. Afterwards, the Cr(III) retained on micro-column was eluted with 2.0 mol ly1 HCl solution. The chromium in the elution was determined by ICP-AES. 3. Results and discussion 3.1. Effect of pH on adsorption of Cr(III) and Cr(VI) The effect of pH on the retention of Cr(III) and Cr(VI) on to the column of nanometer TiO2 has been investigated separately. pH values of sample solutions were adjusted to a range of 1;8 with 0.1 mol ly1 HCl or NH3ØH2O and passed through the column. The retained ions were stripped off from the column and determination by ICP-AES as described in the recommended procedure. The results of the effect of pH on the recoveries of Cr(III) and Cr(VI) were shown in Fig. 1. As can be seen, quantitative recovery ()90%) was found for Cr(III) when the pH exceed 5.5 while the recovery of Cr(VI) was rather low (-5%). This could make it possible to separate Cr(III) and Cr(VI). A pH of 6.0 was selected for subsequent work. 3.2. Elution of Cr(III) Elution of Cr(III) from the nanometer TiO2 microcolumn was investigated by using hydrochloric acid solution at different concentrations as stripping agent. Elution of Cr(III) was quantitative with 1.0 ml of G2.0 mol ly1 HCl, lower concentrations gave only 60–80% recovery. To ensure
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In this work, 50 ml of sample solution was adopted for the preconcentration of Cr(III) from water sample, the adsorbed Cr(III) can be eluted with 1.0 ml 2.0 mol ly1 HCl, so an enrichment factor of 50 was achieved by this method. 3.5. Adsorption capacity
Fig. 1. Effect of pH on the adsorption (R%) of Cr(III) and Cr(VI) on nanometer TiO2. Cr(III) and Cr(VI): 1.0 mg mly1.
complete elution, 1.0 ml of 2.0 mol ly1 HCl was used as eluent in the following experiments. 3.3. Effect of flow rate of sample solution The effect of flow rate of sample solution on the retention of Cr(III) on nanometer TiO2 was examined under the optimum conditions (pH, eluent, etc.). The flow rate was adjusted in a range of 0.5–2.0 ml miny1. It was found that the retention of Cr(III) was practically not changed up to 1.0 ml miny1 flow rate. However, the retention of Cr(III) will decrease with further increasing of the flow rate after it is over 1.0 ml miny1, due to a decrease in the adsorption kinetics at a high flow rate. Thus, a flow rate of 1.0 ml miny1 is employed in this work.
The capacity study used was adapted from that recommended by Maquieira et al. w28x, 25 ml aliquots of a series of concentrations (5;50 mg mly1) was adjusted to the appropriate pH, the proposed separation and preconcentration procedure described above has been applied. A breakthrough curve was gained by plotting the concentration (mg mly1) vs. the micrograms of Cr(III) adsorbed per gram. From the breakthrough curve, the adsorption capacity of nanometer TiO2 for Cr(III) was found to be 7.6 mg gy1. 3.6. Effects of coexisting ions The effects of common coexisting ions on the determination of Cr(III) were investigated. In these experiments, solutions of 1.0 mg mly1 of Cr(III) containing the added interfering ions were treated according to the recommended procedure. The tolerance of the coexisting ions, defined as the largest amount making the recovery of Cr(III) less than 90%, were Naq, Kq, 5.0 g ly1; Ca2q, Mg2q, 2.0 g ly1; Cu2q, Zn2q, Mn2q, Ni2q, Co2q, 0.5 g ly1; Al3q, Pb2q, 0.25 g ly1; Fe3q,
3.4. Effect of the sample volume In order to explore the possibility of enriching low concentrations of analytes from large volumes, the maximum applicable sample volume must be determined. For this purpose, 20, 50, 100, 150 and 200 ml of sample solutions containing 10 mg of Cr(III) were passed through the microcolumn under the optimum conditions. As shown in Fig. 2, the recovery of Cr(III) was quantitative () 90%) up to 50 ml of sample volume.
Fig. 2. Effect of the sample volume on the recovery of Cr(III). pH: 6.0; Cr(III): 10 mg.
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Table 2 Determination of Cr(III) and Cr(VI) in natural water samplesa Samples
GSBZ 5009-88 Tap water
Lake water
b
Added (mg ly1)
Foundc (mg ly1)
Cr(III)
Cr(VI)
Cr(III)
0 500e 0 0.50 1.00 0 0.50 1.00
0 0 0.50 1.00 0 0.50 1.00
0.78"0.04 1.26"0.11 1.80"0.12 0.57"0.05 1.06"0.09 1.61"0.14
Recovery (%) Cr(VI)d
Total
Cr(III)
Cr(VI)
0.32"0.03 0.82"0.05 1.29"0.10 0.99"0.07 1.47"0.12 1.97"0.12
645"40 1124"54 1.10"0.06 2.08"0.17 3.09"0.18 1.56"0.12 2.53"0.19 3.58"0.20
96e – 96 102 – 98 104
– 100 97 – 96 98
a
Sample volume: 50 ml. National reference material (China): the certified value of Cr concentration is 640"36 mg ly1. c Mean of five determinations. d Calculated value. e the result for total Cr. b
2y y1 50 mg ly1; SiO2y ; PO43y, 0.5 g 4 , SO4 , 1.0 g l y1 l . It can be seen that the presence of major cations and anions has no obvious influence on the determination under the selected conditions.
3.7. Detection limits and precision According to the definition of IUPAC, the detection limit (3s) of this method for Cr(III) with an enrichment factor of 50 is 0.32 mg ly1; and relative standard deviation (R.S.D.) is 2.4% (ns11, Cs 100 mg ly1). 3.8. Analysis application The accuracy of the proposed method was examined by analyzing total chromium in environmental water reference materials (ERMs, GSBZ 500988). In Table 2, the analytical value was in agreement with the certified value of chromium in the standard reference material. The proposed method has been applied to the speciation of Cr(III) and Cr(VI) in tap and lake water samples collected in Wuhan. The standard addition method was used, and the analytical results and the recovery were given in Table 2. The results indicated that the recoveries were reasonable for trace analysis, in a range of 96;104%.
4. Conclusion In this work, the adsorption behaviors of Cr(III) and Cr(VI) on nanometer TiO2 were systematically investigated. Based on its high adsorption selectivity for Cr(III) and Cr(VI), a sensitive and selective method for the separation and determination of chromium species in natural water using nanometer TiO2 packed micro-column coupled with ICPAES was developed. Compared with the mm sized TiO2 used for chromium speciation w20,22x, the adsorption capacity of nanometer TiO2 for Cr is higher (7.6 mg gy1 vs. 5.0 mg gy1 w20x and 4.0 mg gy1 w22x), mainly due to its high surface areas (208 m2 gy1). The developed method has been successfully applied to the speciation of chromium in tap and lake water samples, and the precision and accuracy of the method are satisfactory. The method may be used for the speciation of chromium in various matrices other than water. References w1x A. Kot, J. Namiesnik, The role of speciation in analytical chemistry, Trends Anal. Chem. 19 (2000) 69–79. w2x C. Barnowski, N. Jakubowski, D. Stuewer, J.A.C. Broekaert, Speciation of chromium by direct coupling of ion exchange chromatography with inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 12 (1997) 1155–1161. w3x A.S. Prasad, D. Oberleas, Trace Elements in Human Health and Disease, Academic Press, 1976, p. 79.
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