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Solid-phase extraction of trace Cu(II) Fe(III) and Zn(II) with silica gel modified with curcumin from biological and natural water samples by ICP-OES
Microchemical Journal 86 (2007) 189 – 194 www.elsevier.com/locate/microc
Solid-phase extraction of trace Cu(II) Fe(III) and Zn(II) with silica gel modified with curcumin from biological and natural water samples by ICP-OES Xiangbing Zhu a , Xijun Chang a,⁎, Yuemei Cui a , Xiaojun Zou a,b , Dong Yang a , Zheng Hu a a
Department of Chemistry, Lanzhou University, Lanzhou 730000, PR China b Shandong Lukang Pharmaceutical Company Limited, PR China
Received 20 January 2007; received in revised form 15 March 2007; accepted 15 March 2007 Available online 24 March 2007
Abstract Silica gel was firstly functionalized with aminopropyltrimethoxysilane obtaining the aminopropylsilica gel (APSG). The APSG was reacted subsequently with curcumin yielding curcumin-bonded silica gel (curcumin-APSG). This new bonded silica gel was used for separation, preconcentration and determination of Cu(II), Fe(III), Zn(II) in biological and natural water samples by inductively coupled plasma optical emission spectrometry (ICP-OES). Experimental conditions for effective adsorption of trace levels of metal ions were optimized with respect to different experimental parameters using batch and column procedures in detail. The optimum pH value for the separation of metal ions simultaneously on the newly sorbent was 4.0. Complete elution of the adsorbed metal ions from the sorbent surface was carried out using 2.0 mL of 0.1 mol L− 1 of HCl. Common coexisting ions did not interfere with the separation and determination at pH 4.0. The maximum static adsorption capacity of the sorbent at optimum conditions was found to be 0.63, 0.46 and 0.37 mmol g− 1 for Cu(II), Fe(III) and Zn(II) respectively. The time for 95% sorption for Cu(II) Fe(III) and Zn(II) was less than 2 min. The detection limits of the method defined by IUPAC was found to be 0.12, 0.15 and 0.40 ng mL− 1 for Cu(II), Fe(III) and Zn(II), respectively. The relative standard deviation (RSD) of the method under optimum conditions was lower 3.0% (n = 5). The procedure was validated by analyzing the certified reference river sediment material (GBW 08301, China), the results obtained were in good agreement with standard values. This sorbent was successfully employed in the separation and pre-concentration of trace Cu(II), Fe(III) and Zn(II) from the biological and natural water samples yielding 75-fold concentration factor. © 2007 Elsevier B.V. All rights reserved. Keywords: Silica gel; Curcumin; Heavy metal ions; Solid-phase extraction (SPE); ICP-OES
1. Introduction As the number of ecological and health problems associated with environmental contamination continues to rise, the determination of trace heavy metals in environmental samples is becoming more and more important [1]. So extraction and removal of toxic heavy metal ions from various matrices at trace level are of paramount importance [2,3]. Due to the very low concentration of most elements, including Cu(II), Fe(III) and Zn (II), in environmental samples, their separation and sensitive determination necessitate the use of a pre-concentration or trace ⁎ Corresponding author. E-mail address: [email protected] (X. Chang). 0026-265X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2007.03.002
enrichment method [4–6]. The classical liquid–liquid extraction and separation methods are usually time consuming and labor extensive and require relatively large volumes of high purity solvents. Of additional concern is disposal of the organic solvent used, which creates a severe environmental problem. Of all the pre-concentration methods, SPE method is currently being used as a pre-concentration or separation technique whenever there are complex matrices or low concentration because it can provide more flexible working conditions and simple operation [7]. SPE is mainly based on the utilization of inorganic and organic solid sorbents such as XAD resins [8–10], ion exchange resins [11], silica gel [12,13], cellulosic derivatives [14], polyurethane foam [15], active carbon [16], nanometer SiO2 [17] and rice husks [18]. Extraction and removal of metal ions by
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Table 1 Instrumental and operating conditions for ICP-AES measurements Parameter
Type or amount
R.F. power (kW) Carrier gas (Ar) flow rate (L min− 1) Auxiliary gas (Ar) flow rate (L min− 1) Coolant gas (Ar) flow rate (L min− 1) Nebulizer flow (psi) Pump rate (r min− 1) Observation height (mm) Integration time (s) On-axis Off-axis Wavelength (nm)
1.15 0.6 1.0 14 30 100 15 20 5 Cu 324.754, Fe 259.940, Zn 213.856
these sorbents is well known and mainly based on the possible surface reactivity and adsorptive characters incorporated into these solid phases [19]. However, the basic disadvantage of these solid sorbents is the lack of metal selectivity, which leads to high interference of other existing species with the target metal ion(s) [20]. To overcome this problem, a chemical or physical modification of the sorbent surface with some organic compounds, is usually used to load the surface with some donor atoms such as oxygen, nitrogen, sulfur and phosphorus [21]. Nowadays, silica has been adopted as a good alternating support for chelating substrate to be used as sorbent [22,23]. This is due to the availability of many silylating agents and silica phases of large specific surface area, such as silica gel (SG). SG is inert towards acids and organic solvents and has a high surface activity, good thermal and mechanical stability and resistance against swelling [24–26]. Polypherals have attracted attention as effective multinuclei chelating reagents for extraction and spectrophotometric determination of many metal ions. In this study, a method is described for the physical and chemical modification of silica gel with curcumin in an attempt to explore and study the selectivity characters incorporated into the modified silica gel for selective binding, extraction and pre-concentration of heavy metal ions. The procedure was validated by analysis of the standard reference materials (GBW 08301) and applied to the analysis of biological and natural water samples with satisfactory precision and accuracy. The results revealed that the proposed method is reliable. 2. Experimental 2.1. Apparatus An IRIS Advantage ER/S inductively coupled plasma spectrometer (TJA, USA) was used for all metal-determinations. The operation conditions and the wavelengths were summarized in Table 1. An electrothermal atomic absorption spectrometry (ETAAS) (WFX-1D, China) was used. The pH value was controlled with a pHs-10C digital pH meter (Xiaoshan Instrument Factory, China). Infrared spectra were recorded on a Nicolet NEXUS 670 FT-IR apparatus (U.S.A).
A YL-110 peristaltic pump (The General Research Academe of Colored Metal, Beijing, China) was used in the separation/ pre-concentration process. A self-made glass microcolumn (45 mm × 2.5 mm i.d.) was used. 2.2. Reagents and solutions All reagents were of analytical grade and all solutions were prepared with double distilled water. Standard labware and glassware used were repeatedly cleaned with HNO3 and rinsed with double distilled water, according to a published procedure [27]. Metal ions (1 mg mL − 1) solutions were prepared by dissolving analytical grade salts in double distilled water with addition of hydrochloric acid and further diluted daily prior to use. Curcumin was used in this work (The chemical company, Beijing, China). Silica gel (80–120 mesh, Qingdao Ocean Chemical Company, Qingdao, China) and 3-aminopropyltrimethoxysilane (APS, Qingdao Ocean University Chemical Company, Qingdao, China) were used to prepare the ionimprinted functionalized sorbent. The reference materials (GBW 08301, river sediment) were obtained from the National Research Center for Certified Reference Materials (Beijing, China). 2.3. Synthesis of the sorbent Silica gel was first activated by refluxing with concentrated hydrochloric acid for 4 h, then it was filtered off and washed repeatedly with double distilled water several times until acidfree and dried in an oven at 160 °C for 8 h. The active silica gel was further homogenized by milling for 2 h. Activated silica gel was functionalized with 3-aminopropyltrimethoxysilane [28] to produce APSG. The sorbent was prepared by adding 4 g of APSG to 1 g curcumin, dissolved in 5 mL dimethylsulfoxide (DMSO) and 70 mL of methanol, and stirring the mixture under reflux for 8 h. The product of sorbent thus formed was washed thoroughly with DMSO, methanol and dried at 70 °C for 7 h. 2.4. Procedure 2.4.1. Batch method A series of standards solutions containing 1.0 μg mL− 1 Cu (II), Fe(III) and Zn(II) or sample solutions were transferred into a 10 mL beaker, and the pH value was adjusted to the desired value with 0.1 mol L− 1 HNO3 and 0.1 mol L− 1 NH3 · H2O, then the volume was adjusted to 10 mL with double distilled water. 25 mg of sorbent was added, and the mixture was shaken vigorously for 25 min to facilitate adsorption of the metal ions onto the sorbent. After the mixture was centrifuged and eluted with HCl solution, the concentrations of the metal ions in the elution were directly determined by ICP-OES. 2.4.2. Column method First, a homogenous mixture of 25 mg of sorbent and 50 mg of glass beads (40–60 mesh, The First Regent Factory, Shanghai,
X. Zhu et al. / Microchemical Journal 86 (2007) 189–194
China) were filled into a glass microcolumn (45 mm × 2.5 mm i.d.) plugged with a small portion of glass wool at both ends. Before use, HNO3 solution (pH 4) and double distilled water were sequentially passed through the column to equilibrate, clean and neutralize it. Portions 100 mL of aqueous standard or sample solutions containing 1.0 μg mL− 1 Cu(II), Fe(III) and Zn(II) were prepared, and the pH value was adjusted to the desired pH value with 0.10 mol L− 1 of HNO3 and 0.10 mol L− 1 of NH3 · H2O. Each solution was passed through the column at a flow rate of 1.0 mL min− 1 by a peristaltic pump. Afterwards, the metal ions retained on microcolumn were eluted with 2.0 mL of 0.5 mol L− 1 HCl solution and the analytes in the elution were determined by ICP-OES. 2.5. Sample preparation Yellow River water was collected from Yellow River, Lanzhou, China. The water sample was filtered through a 0.45 μm membrane filter (Tianjin, Jinteng Instrument Factory, Tianjin, China), and acidified to a pH of about 1 with concentrated HCl prior to storage for use. Tap water taken from our research laboratory (Lanzhou University, Lanzhou, China) was filtered through a Millipore filter and acidified to pH 2.0 with 2.0 mol L− 1 HNO3. The analysis was performed immediately. The usual general precautions were taken to avoid contamination. Pig liver was purchased from Binhe market, Lanzhou, China. Balsam pear leaves were obtained from Anning village, Lanzhou, China. Pig liver and balsam pear leaves were dried in an oven at 80 °C to constant weight. A 1000 g pig liver or balsam pear leaves sample was weighted and transferred into a digestion tube, and then 5 mL of concentrated HNO3 was added into it. The tube was left at room temperature for one night. Then it was placed in a digester block and heated slowly until the temperature was up to 165 °C. This temperature was maintained until ceasing the evolution of brown fumes. After the tube was cooled down, 1.3 mL perchloric acid was added into it. Then the temperature was raised to 210 °C until evolution of white fumes began. The volume was adjusted to 100 mL with double distilled water when the tube was cooled down [17].
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Fig. 1. Effect of pH on analytes recovery of 10 mL of 1.0 μg mL− 1 Cu(II), Fe (III) and Zn(II) after separation on curcumin-APSG at pH 4.0. Other conditions: 25 mg cucurmin-APSG, shaking time 25 min, temperature 25 °C.
in the curcumin. The peak at 1634.18 cm− 1 due to ν(C_O) and the peak at 1423.56 cm− 1 due to δ(OCH3) in the curcumin [30]. Consequently, the above analysis of IR spectrum suggests that strong interaction exists at the interface of APSG and curcumin, and APSG is successfully modified by curcumin. 3.2. Effect of pH The acidity of a solution has two effects on metal adsorption. Firstly, proton in acid solution can protonate binding sites of the chelating molecules. Secondly, hydroxide in basic solution may complex and precipitate many metals. Therefore, pH of a solution is the first parameter to be optimized. The adsorption capacity of each metal ion, determined by Eq. (1), as a function of pH is shown in Fig. 1. Nf ¼ ðni nf Þ=m
ð1Þ
where Nf is the adsorption capacity (mg metal/g phase Q); ni is the initial amount of metal ion (mg); nf is the amount of metal ion in supernatant (mg) and m is the amount of phase Q (g). It was found from Fig. 1 that quantitative extraction (N95%) could be obtained from pH 4–7. To avoid hydrolyzing at higher pH and determine these elements simultaneously, pH 4 was chosen for this experiment.
3. Results and discussion 3.1. FT-IR spectra
3.3. Effect of shaking time
The modified silica gel was confirmed by IR analysis. IR absorption spectrum of APSG shows the appearance of bands at 467.53, 808.27, 1102.98, 1630.13, 2925.80 and 3431.53 cm− 1, due to δ(Si–O–Si), ν(Si–O–Si), longitudinal SiO2 lattice vibration, δ(H2O), ν(CH3) and ν(Si–OH), respectively. The bands due to aminopropyl group appear at 1559.78 and 3436.12 cm− 1 assigned to δ(NH2) [29]. Comparing the IR spectrum of the curcumin-APSG with that of APSG, many new bands appeared at 1423.56, 1464.20, 1518.71, 1591.77 and 1634.18 cm− 1. The bands around 1464.20, 1518.71, 1591.77 cm− 1 were assigned to ν(C_C)
Modification of silica gel surface with organic complexing agents results in producing metal ion extractors that need only a few minutes to complete the metal ion extraction processes in comparison with other organic adsorbents [31] and this represents one of the advantages of using silica gel as inorganic solid support for immobilization of chelating compounds. The effect of shaking time on the recovery of the investigated metal ions at pH 4.0 was studied. As shown in Fig. 2, the adsorption of Cu(II), Fe(III) and Zn(II) was over 95% sorption during the first 2 min. It indicated that kinetics of equilibrium is very fast.
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Fig. 2. Effect of shaking time on analytes recovery of 10 mL of 1.0 μg mL− 1 Cu (II), Fe(III) and Zn(II) after separation on curcumin-APSG at pH 4.0. Other conditions: 25 mg curcumin-APSG, temperature 25 °C.
Fig. 4. Effect of flow rate on analytes recovery of Cu(II), Fe(III) and Zn(II). Other conditions: sample volume 50 mL, temperature 25 °C.
3.4. Effect of the mass of sorbent
adsorption, probably the metal ions could not sufficiently equilibrate with the sorbent. Thus, a flow rate of 1.0 mL min− 1 was selected as an optimum flow rate in this work.
To test the effect of extractant mass on quantitative retention of analytes, different amounts of sorbent (range from 5 to 40 mg) were added into solutions following the batch procedure. The results were shown in Fig. 3. Quantitative recoveries for the examined analytes were obtained in a range of 20–40 mg of sorbent. Quantitative retention was not obtained when mass of extractant was smaller than 20 mg. So 25 mg of sorbent was selected for further studies.
3.6. Maximum sample volume, elution condition and enrichment factor
The flow rate of sample solutions containing the Cu(II), Fe (III) and Zn(II) through the packed volume is a very important parameter because the retention of elements on adsorbent depends upon the flow rate of the sample solutions. Its effect was investigated under the optimum conditions (pH, eluent, etc.) by the column method. The flow rates were adjusted in a range of 0.5 to 2.5 mL min− 1. As shown in Fig. 4, the flow rate had a strong influence on the sorption of Cu(II), Fe(III) and Zn (II). The too small flow rates were not employed to avoid the longer extraction time. However, at flow rates greater than 1.5 mL min− 1, there was a decrease in the percentage of
When the total amount of investigated metal ions loaded kept constant to 1.0 μg, the maximum sample volume on the adsorption of Cu(II), Fe(III) and Zn(II) on 25 mg of sorbent was studied by passing sample volumes of 20–300 mL through the column by the recommended column procedure. As shown in Fig. 5, quantitative recovery (N 95%) was obtained for the sample volume of 150 mL for the Cu(II), Fe(III) and Zn(II) and at higher volume percent of recovery decreased. Therefore, 150 mL of sample solution was adopted for the pre-concentration of analytes from sample solutions. The elution condition was also studied by the batch and column procedure where various concentrations and volumes of HCl were used for the desorption of retained Cu(II), Fe(III) and Zn(II). The results showed that 2.0 mL of 0.1 mol L− 1 HCl was sufficient for 95% recovery for the investigated metal ions with a shaking time of 25 min. So, 2.0 mL of 0.1 mol L− 1 HCl was used as eluent in further experiments. Because the maximum
Fig. 3. Effect of sorbent mass on analytes recovery of 10 mL of 1.0 μg mL− 1 Cu (II), Fe(III) and Zn(II) after separation on curcumin-APSG at pH 4.0. Other conditions: shaking time 25 min, temperature 25 °C.
Fig. 5. Effect of the sample volume on analytes recovery of 1.0 μg mL− 1 Cu(II), Fe(III) and Zn(II) after separation on curcumin-APSG at pH 4.0. Other conditions: 25 mg curcumin-APSG, shaking time 25 min, temperature 25 °C.
3.5. Effect of flow rate
X. Zhu et al. / Microchemical Journal 86 (2007) 189–194 Table 2 Effect of foreign ions on percent recovery of 1.0 μg mL− 1 Cu(II), Fe(III) and Zn (II) on the sorbent followed by elution with 2.0 mL 0.1 mol L− 1 HCl Coexisting ions concentration (μg mL− 1) MgSO4 Ni2+ Cd2+ Na3PO4 Ca(NO3)2 Pb2+ Cr3+ Mn2+ Co2+ NaCl
500 1000 1000 500 2000 50 50 1000 200 2000
Recovery of analytes (%) Cu(II)
Fe(III)
Zn(II)
98.69 100 99.17 100 100 95.16 97.01 100 96.82 100
99.55 100 100 95.66 98.93 100 98.12 100 99.57 100
99.48 96.31 96.29 98.54 99.01 96.34 95.67 99.90 97.03 98.85
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Table 4 Analytical results of natural water samples (n = 5) Sample (ion)
(μg mL− 1) Found
Yellow River water Cu(II) 64.93 ± 1.08 Fe(III)
70.43 ± 1.18
Zn(II)
7.07 ± 1.02
Tap water Cu(II)
6.52 ± 0.60
Fe(III)
10.01 ± 0.50
Zn(II)
11.94 ± 1.02
sample volume is 150 mL, the high enrichment factor was calculated as 75.
Added
Sum
Recovery (%)
0 10.0 0 10.0 0 10.0
64.93 ± 1.08 73.46 ± 1.43 70.43 ± 1.18 79.57 ± 1.01 7.07 ± 1.02 16.20 ± 0.20
98
0 10.0 0 10.0 0 10.0
6.52 ± 0.60 15.27 ± 1.05 10.01 ± 0.50 19.38 ± 1.09 11.94 ± 1.02 21.01 ± 0.95
92
99 95
97 96
3.7. Adsorption capacity 3.9. Analytical accuracy, precision and detection limit The capacity of the adsorbent is an important factor because it determines how much adsorbent is required to quantitatively remove a specific amount of metal ions from the solutions [32]. The adsorption capacities of various metal ions probably differ due to their size, degree of hydration and the value of their binding constant with the ligand immobilized onto matrix. The capacity study was adopted from the paper recommended by Maquieira et al. [33]. 20 mg of sorbent was equilibrated with a series of various concentrations of Cu(II), Fe(III) and Zn(II) ion solutions and the recommended procedure (batch method) described above was applied. In order to reach the “saturation”, the initial metal ion concentrations were increased till the plateau values (adsorption capacity values) were obtained. The maximum of adsorption capacities of the sorbent determined from the saturation condition of the isotherm were 0.63, 0.46 and 0.37 mmol g− 1 for Cu(II), Fe(III) and Zn(II), respectively. 3.8. Effect of coexisting ions The effect of different cations and anions on the adsorption of Cu(II), Fe(III) and Zn(II) on sorbent was studied using the batch procedure. As shown in Table 2, that in excess of 2000fold Ca(NO3)2, NaCl; 1000-fold Ni2+, Cd2+, Mn2+, 500-fold MgSO4, Na3PO4, 200-fold Co2+, and 50-fold Pb2+, Cr3+ ions do not affect the separation process.
The proposed procedure was applied to the analysis of Cu (II), Fe(III) and Zn(II) in the certified reference material that was river sediment (GBW 08301, China). The results shown in Table 3 were compared with the certified values using a t-test at 95% confidence limits [34]. Good agreement was obtained between the estimated content by the proposed method and the certified values. The results also indicated that the developed pre-concentration method for Cu(II), Fe(III) and Zn(II) were not affected by potential interferences from the major matrix elements of the analyzed standard materials. Under the selected conditions, eleven portions of standard solutions were enriched and analyzed simultaneously following the general procedure. The detection limits (3σ) of the method defined by IUPAC were found to be 0.12, 0.15 and 0.40 ng mL− 1 for Cu(II), Fe(III) and Zn(II), respectively. The relative standard deviation (RSD) of the eleven replicate determinations was lower than 3.0% for 1.0 μg mL− 1 Cu(II), Fe(III) and Zn(II), which indicated that the method had good precision for the analysis of trace Cu(II), Fe(III) and Zn(II) in solution samples. The minimum enrichment concentration of Cu(II), Fe(III) and Zn(II) is 6.67 ng mL− 1.
Table 5 Analytical results of biological sample (n = 5) Table 3 Analysis of standard reference material (GBW 08301) Analyte
Cu(II) Fe(III) Zn(II) a b
Found by present method
Certified value
(μg g− 1) a
(μg g−1)
50 ± 4.8 39,352 ± 126 254 ± 2.1
53 ± 6.0 39,400 ± 1300 251 b
x¯ ± s (n = 5). x¯ average value for five determinations, s standard deviation. Reference value.
Ion
Found by the proposed method
Found by ET-AAS method
Pig liver Cu Fe Zn Balsam pear leave Cu Fe Zn
(μg g− 1) 22.13 ± 0.16 4.37 ± 0.12 14.73 ± 0.09
21.54 ± 0.35 4.69 ± 0.10 15.02 ± 0.04
21.48 ± 0.16 3.01 ± 0.13 3.73 ± 0.05
23.06 ± 0.20 2.79 ± 0.07 4.14 ± 0.11
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3.10. Application of the method The proposed method was then applied for the determination of Cu(II), Fe(III) and Zn(II) in biological and natural water samples by ICP-OES. Meanwhile, for the analysis of natural water samples (Table 4), the standard addition method was used. The analytical results for biological samples (Table 5) were in agreement with the ET-AAS method. The results showed that the proposed method is suitable for the preconcentration of Cu(II), Fe(III) and Zn(II) in real samples of biological and natural water. 4. Conclusions Thus, a selective and sensitive method was established for determination of trace levels of Cu(II), Fe(III) and Zn(II) in biological samples as well as natural water samples based on silica gel modified with curcumin as a solid phase extractant. Quantitative enrichment of the analytes from very dilute aqueous was achieved with this method. The most important characteristic of the silica gel–curcumin is its excellent selectivity towards Cu(II), Fe(III) and Zn(II) over other ions. In addition, the preparation of silica gel–curcumin is relatively simple and rapid. The data in this paper revealed that the proposed method is simple, sensitive and reliable. References [1] Yongwen Liu, Ong Guo, Xijun Chang, Shuangming Meng, Dong Yang, Bingjun Din, Microchim. Acta 149 (2005) 95–101. [2] C. Kantipuly, S. Katragadda, A. Chow, H.D. Gesser, Talanta 37 (1990) 491. [3] P.E. Warwick, I.W. Croudace, A.G. Howard, Anal. Chem. 72 (2000) 3960. [4] A. Mitsuike, Methods for Preconcentration of Trace Elements, Khimiya, Moscow, 1986. [5] C.C. Huang, M.H. Yang, Automated on-line sample pretreatment system for the determination of trace metals in biological samples by inductively coupled plasma mass spectrometry, Anal. Chem. 69 (1997) 3930–3939. [6] C.H. Lee, J.S. Kin, M.Y. Suh, W. Lee, A chelating resin containing 4(2-thiazolylazo)resorcinol as the functional group synthesis and sorption behaviour for trace metal ions, Anal. Chim. Acta 339 (1997) 303–312.
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Solid-phase extraction of trace Cu(II) Fe(III) and Zn(II) with silica gel modified with curcumin from biological and natural water samples by ICP-OES Xiangbing Zhu a , Xijun Chang a,⁎, Yuemei Cui a , Xiaojun Zou a,b , Dong Yang a , Zheng Hu a a
Department of Chemistry, Lanzhou University, Lanzhou 730000, PR China b Shandong Lukang Pharmaceutical Company Limited, PR China
Received 20 January 2007; received in revised form 15 March 2007; accepted 15 March 2007 Available online 24 March 2007
Abstract Silica gel was firstly functionalized with aminopropyltrimethoxysilane obtaining the aminopropylsilica gel (APSG). The APSG was reacted subsequently with curcumin yielding curcumin-bonded silica gel (curcumin-APSG). This new bonded silica gel was used for separation, preconcentration and determination of Cu(II), Fe(III), Zn(II) in biological and natural water samples by inductively coupled plasma optical emission spectrometry (ICP-OES). Experimental conditions for effective adsorption of trace levels of metal ions were optimized with respect to different experimental parameters using batch and column procedures in detail. The optimum pH value for the separation of metal ions simultaneously on the newly sorbent was 4.0. Complete elution of the adsorbed metal ions from the sorbent surface was carried out using 2.0 mL of 0.1 mol L− 1 of HCl. Common coexisting ions did not interfere with the separation and determination at pH 4.0. The maximum static adsorption capacity of the sorbent at optimum conditions was found to be 0.63, 0.46 and 0.37 mmol g− 1 for Cu(II), Fe(III) and Zn(II) respectively. The time for 95% sorption for Cu(II) Fe(III) and Zn(II) was less than 2 min. The detection limits of the method defined by IUPAC was found to be 0.12, 0.15 and 0.40 ng mL− 1 for Cu(II), Fe(III) and Zn(II), respectively. The relative standard deviation (RSD) of the method under optimum conditions was lower 3.0% (n = 5). The procedure was validated by analyzing the certified reference river sediment material (GBW 08301, China), the results obtained were in good agreement with standard values. This sorbent was successfully employed in the separation and pre-concentration of trace Cu(II), Fe(III) and Zn(II) from the biological and natural water samples yielding 75-fold concentration factor. © 2007 Elsevier B.V. All rights reserved. Keywords: Silica gel; Curcumin; Heavy metal ions; Solid-phase extraction (SPE); ICP-OES
1. Introduction As the number of ecological and health problems associated with environmental contamination continues to rise, the determination of trace heavy metals in environmental samples is becoming more and more important [1]. So extraction and removal of toxic heavy metal ions from various matrices at trace level are of paramount importance [2,3]. Due to the very low concentration of most elements, including Cu(II), Fe(III) and Zn (II), in environmental samples, their separation and sensitive determination necessitate the use of a pre-concentration or trace ⁎ Corresponding author. E-mail address: [email protected] (X. Chang). 0026-265X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2007.03.002
enrichment method [4–6]. The classical liquid–liquid extraction and separation methods are usually time consuming and labor extensive and require relatively large volumes of high purity solvents. Of additional concern is disposal of the organic solvent used, which creates a severe environmental problem. Of all the pre-concentration methods, SPE method is currently being used as a pre-concentration or separation technique whenever there are complex matrices or low concentration because it can provide more flexible working conditions and simple operation [7]. SPE is mainly based on the utilization of inorganic and organic solid sorbents such as XAD resins [8–10], ion exchange resins [11], silica gel [12,13], cellulosic derivatives [14], polyurethane foam [15], active carbon [16], nanometer SiO2 [17] and rice husks [18]. Extraction and removal of metal ions by
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Table 1 Instrumental and operating conditions for ICP-AES measurements Parameter
Type or amount
R.F. power (kW) Carrier gas (Ar) flow rate (L min− 1) Auxiliary gas (Ar) flow rate (L min− 1) Coolant gas (Ar) flow rate (L min− 1) Nebulizer flow (psi) Pump rate (r min− 1) Observation height (mm) Integration time (s) On-axis Off-axis Wavelength (nm)
1.15 0.6 1.0 14 30 100 15 20 5 Cu 324.754, Fe 259.940, Zn 213.856
these sorbents is well known and mainly based on the possible surface reactivity and adsorptive characters incorporated into these solid phases [19]. However, the basic disadvantage of these solid sorbents is the lack of metal selectivity, which leads to high interference of other existing species with the target metal ion(s) [20]. To overcome this problem, a chemical or physical modification of the sorbent surface with some organic compounds, is usually used to load the surface with some donor atoms such as oxygen, nitrogen, sulfur and phosphorus [21]. Nowadays, silica has been adopted as a good alternating support for chelating substrate to be used as sorbent [22,23]. This is due to the availability of many silylating agents and silica phases of large specific surface area, such as silica gel (SG). SG is inert towards acids and organic solvents and has a high surface activity, good thermal and mechanical stability and resistance against swelling [24–26]. Polypherals have attracted attention as effective multinuclei chelating reagents for extraction and spectrophotometric determination of many metal ions. In this study, a method is described for the physical and chemical modification of silica gel with curcumin in an attempt to explore and study the selectivity characters incorporated into the modified silica gel for selective binding, extraction and pre-concentration of heavy metal ions. The procedure was validated by analysis of the standard reference materials (GBW 08301) and applied to the analysis of biological and natural water samples with satisfactory precision and accuracy. The results revealed that the proposed method is reliable. 2. Experimental 2.1. Apparatus An IRIS Advantage ER/S inductively coupled plasma spectrometer (TJA, USA) was used for all metal-determinations. The operation conditions and the wavelengths were summarized in Table 1. An electrothermal atomic absorption spectrometry (ETAAS) (WFX-1D, China) was used. The pH value was controlled with a pHs-10C digital pH meter (Xiaoshan Instrument Factory, China). Infrared spectra were recorded on a Nicolet NEXUS 670 FT-IR apparatus (U.S.A).
A YL-110 peristaltic pump (The General Research Academe of Colored Metal, Beijing, China) was used in the separation/ pre-concentration process. A self-made glass microcolumn (45 mm × 2.5 mm i.d.) was used. 2.2. Reagents and solutions All reagents were of analytical grade and all solutions were prepared with double distilled water. Standard labware and glassware used were repeatedly cleaned with HNO3 and rinsed with double distilled water, according to a published procedure [27]. Metal ions (1 mg mL − 1) solutions were prepared by dissolving analytical grade salts in double distilled water with addition of hydrochloric acid and further diluted daily prior to use. Curcumin was used in this work (The chemical company, Beijing, China). Silica gel (80–120 mesh, Qingdao Ocean Chemical Company, Qingdao, China) and 3-aminopropyltrimethoxysilane (APS, Qingdao Ocean University Chemical Company, Qingdao, China) were used to prepare the ionimprinted functionalized sorbent. The reference materials (GBW 08301, river sediment) were obtained from the National Research Center for Certified Reference Materials (Beijing, China). 2.3. Synthesis of the sorbent Silica gel was first activated by refluxing with concentrated hydrochloric acid for 4 h, then it was filtered off and washed repeatedly with double distilled water several times until acidfree and dried in an oven at 160 °C for 8 h. The active silica gel was further homogenized by milling for 2 h. Activated silica gel was functionalized with 3-aminopropyltrimethoxysilane [28] to produce APSG. The sorbent was prepared by adding 4 g of APSG to 1 g curcumin, dissolved in 5 mL dimethylsulfoxide (DMSO) and 70 mL of methanol, and stirring the mixture under reflux for 8 h. The product of sorbent thus formed was washed thoroughly with DMSO, methanol and dried at 70 °C for 7 h. 2.4. Procedure 2.4.1. Batch method A series of standards solutions containing 1.0 μg mL− 1 Cu (II), Fe(III) and Zn(II) or sample solutions were transferred into a 10 mL beaker, and the pH value was adjusted to the desired value with 0.1 mol L− 1 HNO3 and 0.1 mol L− 1 NH3 · H2O, then the volume was adjusted to 10 mL with double distilled water. 25 mg of sorbent was added, and the mixture was shaken vigorously for 25 min to facilitate adsorption of the metal ions onto the sorbent. After the mixture was centrifuged and eluted with HCl solution, the concentrations of the metal ions in the elution were directly determined by ICP-OES. 2.4.2. Column method First, a homogenous mixture of 25 mg of sorbent and 50 mg of glass beads (40–60 mesh, The First Regent Factory, Shanghai,
X. Zhu et al. / Microchemical Journal 86 (2007) 189–194
China) were filled into a glass microcolumn (45 mm × 2.5 mm i.d.) plugged with a small portion of glass wool at both ends. Before use, HNO3 solution (pH 4) and double distilled water were sequentially passed through the column to equilibrate, clean and neutralize it. Portions 100 mL of aqueous standard or sample solutions containing 1.0 μg mL− 1 Cu(II), Fe(III) and Zn(II) were prepared, and the pH value was adjusted to the desired pH value with 0.10 mol L− 1 of HNO3 and 0.10 mol L− 1 of NH3 · H2O. Each solution was passed through the column at a flow rate of 1.0 mL min− 1 by a peristaltic pump. Afterwards, the metal ions retained on microcolumn were eluted with 2.0 mL of 0.5 mol L− 1 HCl solution and the analytes in the elution were determined by ICP-OES. 2.5. Sample preparation Yellow River water was collected from Yellow River, Lanzhou, China. The water sample was filtered through a 0.45 μm membrane filter (Tianjin, Jinteng Instrument Factory, Tianjin, China), and acidified to a pH of about 1 with concentrated HCl prior to storage for use. Tap water taken from our research laboratory (Lanzhou University, Lanzhou, China) was filtered through a Millipore filter and acidified to pH 2.0 with 2.0 mol L− 1 HNO3. The analysis was performed immediately. The usual general precautions were taken to avoid contamination. Pig liver was purchased from Binhe market, Lanzhou, China. Balsam pear leaves were obtained from Anning village, Lanzhou, China. Pig liver and balsam pear leaves were dried in an oven at 80 °C to constant weight. A 1000 g pig liver or balsam pear leaves sample was weighted and transferred into a digestion tube, and then 5 mL of concentrated HNO3 was added into it. The tube was left at room temperature for one night. Then it was placed in a digester block and heated slowly until the temperature was up to 165 °C. This temperature was maintained until ceasing the evolution of brown fumes. After the tube was cooled down, 1.3 mL perchloric acid was added into it. Then the temperature was raised to 210 °C until evolution of white fumes began. The volume was adjusted to 100 mL with double distilled water when the tube was cooled down [17].
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Fig. 1. Effect of pH on analytes recovery of 10 mL of 1.0 μg mL− 1 Cu(II), Fe (III) and Zn(II) after separation on curcumin-APSG at pH 4.0. Other conditions: 25 mg cucurmin-APSG, shaking time 25 min, temperature 25 °C.
in the curcumin. The peak at 1634.18 cm− 1 due to ν(C_O) and the peak at 1423.56 cm− 1 due to δ(OCH3) in the curcumin [30]. Consequently, the above analysis of IR spectrum suggests that strong interaction exists at the interface of APSG and curcumin, and APSG is successfully modified by curcumin. 3.2. Effect of pH The acidity of a solution has two effects on metal adsorption. Firstly, proton in acid solution can protonate binding sites of the chelating molecules. Secondly, hydroxide in basic solution may complex and precipitate many metals. Therefore, pH of a solution is the first parameter to be optimized. The adsorption capacity of each metal ion, determined by Eq. (1), as a function of pH is shown in Fig. 1. Nf ¼ ðni nf Þ=m
ð1Þ
where Nf is the adsorption capacity (mg metal/g phase Q); ni is the initial amount of metal ion (mg); nf is the amount of metal ion in supernatant (mg) and m is the amount of phase Q (g). It was found from Fig. 1 that quantitative extraction (N95%) could be obtained from pH 4–7. To avoid hydrolyzing at higher pH and determine these elements simultaneously, pH 4 was chosen for this experiment.
3. Results and discussion 3.1. FT-IR spectra
3.3. Effect of shaking time
The modified silica gel was confirmed by IR analysis. IR absorption spectrum of APSG shows the appearance of bands at 467.53, 808.27, 1102.98, 1630.13, 2925.80 and 3431.53 cm− 1, due to δ(Si–O–Si), ν(Si–O–Si), longitudinal SiO2 lattice vibration, δ(H2O), ν(CH3) and ν(Si–OH), respectively. The bands due to aminopropyl group appear at 1559.78 and 3436.12 cm− 1 assigned to δ(NH2) [29]. Comparing the IR spectrum of the curcumin-APSG with that of APSG, many new bands appeared at 1423.56, 1464.20, 1518.71, 1591.77 and 1634.18 cm− 1. The bands around 1464.20, 1518.71, 1591.77 cm− 1 were assigned to ν(C_C)
Modification of silica gel surface with organic complexing agents results in producing metal ion extractors that need only a few minutes to complete the metal ion extraction processes in comparison with other organic adsorbents [31] and this represents one of the advantages of using silica gel as inorganic solid support for immobilization of chelating compounds. The effect of shaking time on the recovery of the investigated metal ions at pH 4.0 was studied. As shown in Fig. 2, the adsorption of Cu(II), Fe(III) and Zn(II) was over 95% sorption during the first 2 min. It indicated that kinetics of equilibrium is very fast.
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Fig. 2. Effect of shaking time on analytes recovery of 10 mL of 1.0 μg mL− 1 Cu (II), Fe(III) and Zn(II) after separation on curcumin-APSG at pH 4.0. Other conditions: 25 mg curcumin-APSG, temperature 25 °C.
Fig. 4. Effect of flow rate on analytes recovery of Cu(II), Fe(III) and Zn(II). Other conditions: sample volume 50 mL, temperature 25 °C.
3.4. Effect of the mass of sorbent
adsorption, probably the metal ions could not sufficiently equilibrate with the sorbent. Thus, a flow rate of 1.0 mL min− 1 was selected as an optimum flow rate in this work.
To test the effect of extractant mass on quantitative retention of analytes, different amounts of sorbent (range from 5 to 40 mg) were added into solutions following the batch procedure. The results were shown in Fig. 3. Quantitative recoveries for the examined analytes were obtained in a range of 20–40 mg of sorbent. Quantitative retention was not obtained when mass of extractant was smaller than 20 mg. So 25 mg of sorbent was selected for further studies.
3.6. Maximum sample volume, elution condition and enrichment factor
The flow rate of sample solutions containing the Cu(II), Fe (III) and Zn(II) through the packed volume is a very important parameter because the retention of elements on adsorbent depends upon the flow rate of the sample solutions. Its effect was investigated under the optimum conditions (pH, eluent, etc.) by the column method. The flow rates were adjusted in a range of 0.5 to 2.5 mL min− 1. As shown in Fig. 4, the flow rate had a strong influence on the sorption of Cu(II), Fe(III) and Zn (II). The too small flow rates were not employed to avoid the longer extraction time. However, at flow rates greater than 1.5 mL min− 1, there was a decrease in the percentage of
When the total amount of investigated metal ions loaded kept constant to 1.0 μg, the maximum sample volume on the adsorption of Cu(II), Fe(III) and Zn(II) on 25 mg of sorbent was studied by passing sample volumes of 20–300 mL through the column by the recommended column procedure. As shown in Fig. 5, quantitative recovery (N 95%) was obtained for the sample volume of 150 mL for the Cu(II), Fe(III) and Zn(II) and at higher volume percent of recovery decreased. Therefore, 150 mL of sample solution was adopted for the pre-concentration of analytes from sample solutions. The elution condition was also studied by the batch and column procedure where various concentrations and volumes of HCl were used for the desorption of retained Cu(II), Fe(III) and Zn(II). The results showed that 2.0 mL of 0.1 mol L− 1 HCl was sufficient for 95% recovery for the investigated metal ions with a shaking time of 25 min. So, 2.0 mL of 0.1 mol L− 1 HCl was used as eluent in further experiments. Because the maximum
Fig. 3. Effect of sorbent mass on analytes recovery of 10 mL of 1.0 μg mL− 1 Cu (II), Fe(III) and Zn(II) after separation on curcumin-APSG at pH 4.0. Other conditions: shaking time 25 min, temperature 25 °C.
Fig. 5. Effect of the sample volume on analytes recovery of 1.0 μg mL− 1 Cu(II), Fe(III) and Zn(II) after separation on curcumin-APSG at pH 4.0. Other conditions: 25 mg curcumin-APSG, shaking time 25 min, temperature 25 °C.
3.5. Effect of flow rate
X. Zhu et al. / Microchemical Journal 86 (2007) 189–194 Table 2 Effect of foreign ions on percent recovery of 1.0 μg mL− 1 Cu(II), Fe(III) and Zn (II) on the sorbent followed by elution with 2.0 mL 0.1 mol L− 1 HCl Coexisting ions concentration (μg mL− 1) MgSO4 Ni2+ Cd2+ Na3PO4 Ca(NO3)2 Pb2+ Cr3+ Mn2+ Co2+ NaCl
500 1000 1000 500 2000 50 50 1000 200 2000
Recovery of analytes (%) Cu(II)
Fe(III)
Zn(II)
98.69 100 99.17 100 100 95.16 97.01 100 96.82 100
99.55 100 100 95.66 98.93 100 98.12 100 99.57 100
99.48 96.31 96.29 98.54 99.01 96.34 95.67 99.90 97.03 98.85
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Table 4 Analytical results of natural water samples (n = 5) Sample (ion)
(μg mL− 1) Found
Yellow River water Cu(II) 64.93 ± 1.08 Fe(III)
70.43 ± 1.18
Zn(II)
7.07 ± 1.02
Tap water Cu(II)
6.52 ± 0.60
Fe(III)
10.01 ± 0.50
Zn(II)
11.94 ± 1.02
sample volume is 150 mL, the high enrichment factor was calculated as 75.
Added
Sum
Recovery (%)
0 10.0 0 10.0 0 10.0
64.93 ± 1.08 73.46 ± 1.43 70.43 ± 1.18 79.57 ± 1.01 7.07 ± 1.02 16.20 ± 0.20
98
0 10.0 0 10.0 0 10.0
6.52 ± 0.60 15.27 ± 1.05 10.01 ± 0.50 19.38 ± 1.09 11.94 ± 1.02 21.01 ± 0.95
92
99 95
97 96
3.7. Adsorption capacity 3.9. Analytical accuracy, precision and detection limit The capacity of the adsorbent is an important factor because it determines how much adsorbent is required to quantitatively remove a specific amount of metal ions from the solutions [32]. The adsorption capacities of various metal ions probably differ due to their size, degree of hydration and the value of their binding constant with the ligand immobilized onto matrix. The capacity study was adopted from the paper recommended by Maquieira et al. [33]. 20 mg of sorbent was equilibrated with a series of various concentrations of Cu(II), Fe(III) and Zn(II) ion solutions and the recommended procedure (batch method) described above was applied. In order to reach the “saturation”, the initial metal ion concentrations were increased till the plateau values (adsorption capacity values) were obtained. The maximum of adsorption capacities of the sorbent determined from the saturation condition of the isotherm were 0.63, 0.46 and 0.37 mmol g− 1 for Cu(II), Fe(III) and Zn(II), respectively. 3.8. Effect of coexisting ions The effect of different cations and anions on the adsorption of Cu(II), Fe(III) and Zn(II) on sorbent was studied using the batch procedure. As shown in Table 2, that in excess of 2000fold Ca(NO3)2, NaCl; 1000-fold Ni2+, Cd2+, Mn2+, 500-fold MgSO4, Na3PO4, 200-fold Co2+, and 50-fold Pb2+, Cr3+ ions do not affect the separation process.
The proposed procedure was applied to the analysis of Cu (II), Fe(III) and Zn(II) in the certified reference material that was river sediment (GBW 08301, China). The results shown in Table 3 were compared with the certified values using a t-test at 95% confidence limits [34]. Good agreement was obtained between the estimated content by the proposed method and the certified values. The results also indicated that the developed pre-concentration method for Cu(II), Fe(III) and Zn(II) were not affected by potential interferences from the major matrix elements of the analyzed standard materials. Under the selected conditions, eleven portions of standard solutions were enriched and analyzed simultaneously following the general procedure. The detection limits (3σ) of the method defined by IUPAC were found to be 0.12, 0.15 and 0.40 ng mL− 1 for Cu(II), Fe(III) and Zn(II), respectively. The relative standard deviation (RSD) of the eleven replicate determinations was lower than 3.0% for 1.0 μg mL− 1 Cu(II), Fe(III) and Zn(II), which indicated that the method had good precision for the analysis of trace Cu(II), Fe(III) and Zn(II) in solution samples. The minimum enrichment concentration of Cu(II), Fe(III) and Zn(II) is 6.67 ng mL− 1.
Table 5 Analytical results of biological sample (n = 5) Table 3 Analysis of standard reference material (GBW 08301) Analyte
Cu(II) Fe(III) Zn(II) a b
Found by present method
Certified value
(μg g− 1) a
(μg g−1)
50 ± 4.8 39,352 ± 126 254 ± 2.1
53 ± 6.0 39,400 ± 1300 251 b
x¯ ± s (n = 5). x¯ average value for five determinations, s standard deviation. Reference value.
Ion
Found by the proposed method
Found by ET-AAS method
Pig liver Cu Fe Zn Balsam pear leave Cu Fe Zn
(μg g− 1) 22.13 ± 0.16 4.37 ± 0.12 14.73 ± 0.09
21.54 ± 0.35 4.69 ± 0.10 15.02 ± 0.04
21.48 ± 0.16 3.01 ± 0.13 3.73 ± 0.05
23.06 ± 0.20 2.79 ± 0.07 4.14 ± 0.11
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3.10. Application of the method The proposed method was then applied for the determination of Cu(II), Fe(III) and Zn(II) in biological and natural water samples by ICP-OES. Meanwhile, for the analysis of natural water samples (Table 4), the standard addition method was used. The analytical results for biological samples (Table 5) were in agreement with the ET-AAS method. The results showed that the proposed method is suitable for the preconcentration of Cu(II), Fe(III) and Zn(II) in real samples of biological and natural water. 4. Conclusions Thus, a selective and sensitive method was established for determination of trace levels of Cu(II), Fe(III) and Zn(II) in biological samples as well as natural water samples based on silica gel modified with curcumin as a solid phase extractant. Quantitative enrichment of the analytes from very dilute aqueous was achieved with this method. The most important characteristic of the silica gel–curcumin is its excellent selectivity towards Cu(II), Fe(III) and Zn(II) over other ions. In addition, the preparation of silica gel–curcumin is relatively simple and rapid. The data in this paper revealed that the proposed method is simple, sensitive and reliable. References [1] Yongwen Liu, Ong Guo, Xijun Chang, Shuangming Meng, Dong Yang, Bingjun Din, Microchim. Acta 149 (2005) 95–101. [2] C. Kantipuly, S. Katragadda, A. Chow, H.D. Gesser, Talanta 37 (1990) 491. [3] P.E. Warwick, I.W. Croudace, A.G. Howard, Anal. Chem. 72 (2000) 3960. [4] A. Mitsuike, Methods for Preconcentration of Trace Elements, Khimiya, Moscow, 1986. [5] C.C. Huang, M.H. Yang, Automated on-line sample pretreatment system for the determination of trace metals in biological samples by inductively coupled plasma mass spectrometry, Anal. Chem. 69 (1997) 3930–3939. [6] C.H. Lee, J.S. Kin, M.Y. Suh, W. Lee, A chelating resin containing 4(2-thiazolylazo)resorcinol as the functional group synthesis and sorption behaviour for trace metal ions, Anal. Chim. Acta 339 (1997) 303–312.
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