Application of modified nano-alumina as a solid phase extraction sorbent for the preconcentration of Cd and Pb in water and herbal samples prior to flame atomic absorption spectrometry determination

Journal of Hazardous Materials 178 (2010) 900–905

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Application of modified nano-alumina as a solid phase extraction sorbent for the preconcentration of Cd and Pb in water and herbal samples prior to flame atomic absorption spectrometry determination M. Ezoddin a , F. Shemirani a,∗ , Kh. Abdi b , M. Khosravi Saghezchi a , M.R. Jamali c a

School of Chemistry, University College of Science, University of Tehran, Tehran, Iran Department of Medicinal Chemistry and Pharmaceutical, Faculty of Pharmacy, Tehran University of Medical Science, Tehran, Iran c Department of Chemistry, Payame Noor University, Behshahr, Iran b

a r t i c l e

i n f o

Article history: Received 29 June 2009 Received in revised form 6 February 2010 Accepted 9 February 2010 Available online 17 February 2010 Keywords: Solid phase extraction Nanometer-sized ␥-alumina Preconcentration Flame atomic absorption spectrometry

a b s t r a c t The first study on the high efficiency of nanometer-sized ␥-alumina coated with sodium dodecyl sulfate1-(2-pyridylazo)-2-naphthol (SDS-PAN) as a new sorbent solid phase extraction (SPE) has been reported. A microcolumn packed with modified nanometer-sized alumina was used to preconcentrate and separate Cd and Pb in water and herbal samples. The metals were eluted with 2 mL HNO3 directly and detected with the detection system flame atomic absorption spectrometry (FAAS). Various influencing parameters on the separation and preconcentration of trace metals, such as pH, flow rate, sample volume, amount of sorbent, and concentration of eluent, were studied. Under the optimized operating conditions, the sorption capacities of the modified nano-␥-alumina for Cd and Pb were 11.1 and 16.4 mg g−1 respectively. With 500.0 mL sample an enrichment factor of 250 was obtained. The detection limits of this method for Cd and Pb were 0.15 and 0.17 ␮g L−1 and the R.S.D.s were 2.8 and 3.2% (n = 10), respectively. The proposed method has been applied to the determination of these metal ions at trace levels in certified reference materials and real samples with satisfactory results. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In recent years, pollution of the environment by heavy metals has received considerable attention. Among heavy metals, Cd and Pb not only cause environmental pollution, but also bring harm to people’s health. Direct determination of these two heavy metals appears to be a difficult task as the concentration of them is close to or below the detection limits of most of the analytical instruments besides the real sample matrix may cause serious interference for their determination during the process. Separation and preconcentration methods can solve these problems. For this purpose, several procedures have been developed for the separation and preconcentration of contaminants from environmental matrices, such as: liquid–liquid extraction (LLE) [1–3] coprecipitation [4–6] and solid phase extraction (SPE) [7–15]. Among these procedures, the most commonly used method is solid phase extraction, which provides advantages such as preconcentration of trace metals from larger volumes of sample, reduction or elimination of matrix interfer-

∗ Corresponding author at: School of Chemistry, College of Science, University of Tehran, Enghelab Ave. P.O. Box 14155-64555, Tehran, Iran. Tel.: +98 21 61112481; fax: +98 21 66405141. E-mail address: [email protected] (F. Shemirani). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.02.023

ence, prevention of contamination and in situ or online coupling with various detectors. Many methods based on different SPE sorbents have been used for the preconcentration of metals. The main requirements for a substance to work effectively as a SPE sorption material are as follows: it should consist of a stable and insoluble porous matrix having suitable active groups, typically organic groups, which can interact with analytes, it should achieve fast and quantitative sorption, and it should have high sorption capacity, good regenerability, and surface area accessibility. To date, many sorbents, such as active carbon [16], modified resin [17] and nanometer-sized material [18], have been employed. Among these sorbents, Nanometer-sized materials have gained more attention due to their special properties [19]. One of the most interesting properties is that most atoms are on the surface of the nanoparticle. The unsaturated surface atoms can bind other atoms possessing strong chemical activities which produce a high sorption capacity [20]. Because nanometer-sized alumina has high surface area, high sorption capacity and high chemical activity, it could be successfully applied for the separation and preconcentration of trace metal ions in environmental samples. Chemical activation of the nanometer-sized alumina with functional groups containing N, S, O, and P atoms is highly efficient for the sorption of several metal ions. Metal chelates could provide higher selectivity and high

M. Ezoddin et al. / Journal of Hazardous Materials 178 (2010) 900–905

enrichment factors for such a preconcentration and separation. PAN is known as a famous compound that has a strong ability to form chelates with many metal ions. It is one of the most extensively used complexing agents for trace element analysis. Recent studies on the use of surfactant-coated mineral oxide columns for SPE have demonstrated these new sorbent materials to be a promising tool for the extraction/preconcentration of organic compounds in a wide polarity range [31]. For these purposes, we have developed nano-sized alumina coated with SDS-PAN, as a new and effective sorbent for the preconcentration and determination of Cd and Pb by FAAS in real samples with satisfactory results. 2. Experimental 2.1. Apparatus The determination of Pb and Cd was carried out using the Spectra AA from Varian 220, equipped with an air-acetylene burner. Measurements were carried out in the peak area mode at 228.8 and 217.0 nm, using a spectral bandwidth of 1.0 and 0.5 for Cd and Pb, respectively. Background correction has been used with deuterium-lamp. The lamp currents were set at 4 and 5 mA for Cd and Pb, respectively. The pH values were measured with a pHmeter Model 692 from Metrohm supplied with a glass-combined electrode. Separation was assisted using a refrigerated centrifuge (Hettich, Universal 320 R) equipped with an angle rotor (6-place, 9000 rpm, Cat. No. 1620A).

The nano-alumina could be modified repeatedly by SDS and PAN after each desorption step by acid. 2.5. Preparation of Valeriana officinalie The plant sample was prepared according to AOAC 975.03 and 985.01 methods [21]. An accurately weighed sample (∼5 g) was slightly heated on a hot plate at 100–120 ◦ C for 15 min, then it was placed into a furnace and was further heated at 500 ◦ C for 2 h (temperature was hermostatically controlled). After cooling, 10 drops of deionized water and then 4.0 mL of 8 mol L−1 HNO3 were added to the sample, slightly heated on the hot plate to dryness and then was placed in the furnace at 500 ◦ C for 1 h. After cooling, 10 mL of 1 mol L−1 HNO3 was added to the sample and the content was transferred into 50 mL normal flasks. The sample was then treated according to the given procedure. 2.6. Lettuce sample preparation 5 g of herbal sample was digested with 5 mL of 16 mol L−1 HNO3 in a covered beaker to near dryness. If necessary, another 5 mL portion of 16 mol L−1 HNO3 was added as often until the sample solutions became clear. 1 mL of 0.12 mol L−1 HCl was then added to ensure complete digestion. After cooling the sample to room temperature, the digested solution was diluted to 100 mL with deionized water [22]. Aliquots of the obtained clear solution were analyzed according to the prescribed.

2.2. Reagents

3. Results and discussion

All chemicals used were of analytical-reagent grade. The stock standard solutions containing 1000 mg L−1 of Cd(II) and Pb(II) were prepared by dissolving appropriate amounts of their nitrate salts in distilled water and working standard solutions were obtained by appropriate stepwise dilution of the stock standard solutions. Nano-␥-alumina (Al2 O3 -gamma powder, 40–80 nm, purity: >99% NanoAmor, Los Alamos, NM, USA) was activated by shaking with 5 mol L−1 nitric acid and washed three times with distilled water. SDS (Schuchardt, Germany) was used without further purification. The chelating solution was prepared by dissolving 0.25 g of PAN in 100 mL of 95% ethanol.

3.1. Effect of pH

2.3. Preparation of modified nano--alumina 1.5 g of activated nano-␥-alumina was added to 50 mL solution containing 100 mg SDS and 1 mL PAN solution in a 100 mL flask. The pH of solution was adjusted to 2–2.5 with 3 mol L−1 HCl solutions, and then the flask was shaked mechanically for 15 min [30]. After this step, modified nano-␥-alumina powder was packed into a microcolumn (20 mm × 2.0 mm i.d.) plugged with small portion of glass–wool at both ends. Before using column, it was washed with methanol and high purity deionized water. The peristaltic pump was connected to the sample up take capillary. 2.4. Producer A known volume of sample solution containing Pb(II) and Cd(II) in the range of 0.5–200 ␮g L−1 were prepared and the pH value was adjusted to 7–8 with 0.1 mol L−1 HCl and NaOH. The solution was passed through the microcolumn packed with modified nano-␥-alumina sorbent at a flow rate of 3 mL min−1 controlled by a peristaltic pump. Afterwards, the metal ions retained on the sorbent, were eluted using 2 mL of 2 mol L−1 HNO3 at a flow rate of 4 mL min−1 . The analytes in the effluent were determined by FAAS.

901

The sorption of SDS on nano-sized alumina is highly dependent on the pH of solution. Positively charged nano-␥-alumina surfaces effectively sorbed negatively charged SDS at lower pH values. Therefore, retention of organic compounds on SDS-coated nano-␥-alumina occurs. The anionic surfactant SDS is effectively retained on positively charged nano-␥-alumina surfaces via formation of self-aggregates [15,23] over a wide pH range (1–6), whereas very little amount of SDS could be retained on inert surface of nano-␣-alumina. Maximum sorption of SDS on nano-␥-alumina was achieved at pH 1–2.9 by shaking the solution containing SDS and nano-␥-alumina for 15 min. When solution was acidified, SDS would form hemi-micelles on nano-␥-alumina by strong sorption. The micelles could trap PAN molecules homogeneously, which cause nano-␥-alumina to change in color from white to orange (Fig. 1) [30]. The pH value plays an important role in the sorption of different ions on the sorbents. The sorption behavior of Cd and Pb on the modified nano-␥-alumina in water samples was studied in the pH range of 3.0–10.0. The pH of the solution was adjusted at the required value by the addition of 1.0 mol L−1 NaOH or 1.0 mol L−1 HCl. As can be seen in Fig. 2, a quantitative recovery (>95%) was found for Cd and Pb at the pH range of 7–8. In order to preconcentrate ions simultaneously, a pH of 7–8 was selected as the compromise condition. 3.2. Effect of the amount of modified nano--alumina In order to investigate the effect of the amount of modified nano-␥-alumina on the quantitative extraction of Cd and Pb, the extraction was conducted by varying the amounts of the modified nano-␥-alumina from 10 to 300 mg. The results indicated that the quantitative recovery (>95%) of Cd and Pb was obtained with increasing of modified nano-␥-alumina amount up to 30 mg. Hence,

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M. Ezoddin et al. / Journal of Hazardous Materials 178 (2010) 900–905

Fig. 3. Effect of flow rate on analyte recovery; pH (7–8); concentrations: 60 ␮g L−1 Cd (II) and Pb (II).

Fig. 1. A suggested binding mode of SDS-PAN to nano-alumina surface, M: metal ion.

50 mg of the modified nanometer-sized alumina was used for further experiments due to the greater availability of the surface area at high amounts of the sorbent and to account for other extractable species. 3.3. Sample flow rate and sample volume The retention of metal ions on the sorbent depends on the flow rate of the metal ions solution. The influence of metal ions sorption on modified nanometer-sized alumina was investigated by varying the flow rate of the sample solution in the range of 1–6 mL min−1 , and passing the solution through the microcolumn using a peristaltic pump. The effect of flow rate on the sorption of metal ions was shown in Fig. 3. It was found that the retention of the studied ions was practically not changed up to 4.0 mL min−1 of flow rate. However, the recoveries of the analytes decreased when the flow rate was increased over than 4.0 mL min−1 . Hence, a flow rate of 3.0 mL min−1 was employed in this work. Sample volume is one of the important parameters for obtaining high preconcentration factor. For that reason, the volumes of sample solution containing 3 ␮g each of Pb(II), and Cd(II) were varied from 10 to 2000 mL. The metals were preconcentrated on the modified nano-␥-alumina by applying the proposed procedure. As shown in Fig. 4, quantitative recovery (>95%) of Cd and Pb was obtained up to 500 mL of sample solution. The preconcentration factor was 250 for sample solutions since the final elution volume was 2 mL. The decreased recovery with sample volume may be due

Fig. 4. Effect of sample volume on recovery Cd and Pb, pH (7–8); concentrations: 60 ␮g L−1 Cd (II), Pb (II), and flow rate: 3 mL min−1 .

to the low concentration of metal ions in the solution when sample volume was increased. 3.4. Choice of eluent and flow rate Some experiments were carried out in order to choose a proper eluent for the adsorbed Cd and Pb ions by modified nano␥-alumina. The Cd and Pb ions were stripped with different concentrations (0.1–4 mol L−1 ) of various acids. From the data given in Table 1, it is obvious that 2.0 and 5.0 mL of 2.0 mol L−1 nitric acid could accomplish the quantitative elution of Cd and Pb from the modified nano-␥-alumina. Thus, we selected 2.0 mL of 2.0 mol L−1 nitric acid as eluent for stripping of studied metal ions. The effect of elution flow rate on the recovery of analytes was investigated. Quantitative recovery was obtained at flow range of 1–6 mL min−1 . The best flow rate of 4 mL min−1 was selected. Table 1 Effect of type, concentration and volume of eluent on the adsorption of metal ions. Eluent

Volume (mL)

Recovery (%) Cd

Fig. 2. Effect of pH on sorption Cd and Pb on modified nano-␥-alumina, concentrations: 60 ␮g L−1 Cd (II), Pb (II), and flow rate: 3 mL min−1 .

1 mol L−1 HNO3

2

2 mol L−1 HNO3

0.5 1 2 5 10

4 mol L−1 HNO3 1 mol L−1 HCl 2 mol L−1 HCl

2 2 2

Values in the parentheses are R.S.D. (N = 3).

89.2 (2.8) 48.9 (3.1) 68.2 (2.8) 99.4 (2.6) 98.2 (2.6) 93.92 (2.5) 90.0 (2.8) 65.2 (2.7) 84.7 (2.8)

Pb 81.9 (2.7) 76.6 (2.9) 72.6 (2.9) 99.5 (2.7) 109.1 (2.7) 97.65 (2.4) 87.3 (3.1) 68.4 (3.5) 81.9 (2.7)

M. Ezoddin et al. / Journal of Hazardous Materials 178 (2010) 900–905 Table 6 Physical properties of nano/microsized [23].

Table 2 Tolerance limits for coexisting ions in adsorption of 60 ␮g L−1 of Cd and Pb. Foreign ions

Li+ Po4 −3 Ni2+ Cr6+ Fe3+ Ag+ Mg2+ Cu2+ Sn2+ Zn2+ Hg2+

Interferent/ion ratio

Microsized alumina

Recovery (%) 2

Pb

Cd

Cd

Pb

1000 500 100 100 100 100 100 50 50 50 10

1000 500 100 100 100 100 500 100 50 10 10

96.3 (2.8) 95.9 (2.8) 97.2 (2.4) 100.1 (1.8) 95.2 (2.7) 96.2 (2.3) 95.1 (2.7) 99.3 (2.8) 98.4 (2.7) 99.3 (1.9) 99.3 (1.9)

98.4 (3.4) 105.4 (2.8) 105.1 (2.8) 100.2 (3) 103.2 (3.1) 99.2 (3.3) 95.1 (3.2) 95.4 (3.4) 102.2 (2.8) 104.0 (2.7) 95.6 (3.1)

Surface area (m /g) Mean particle diameter (␮m) PZC Density (g/cm3 )

Pb

A = 0.001C + 0.007 0.9992 0.15 2.8 250

A = 0.0036C + 0.0073 0.9998 0.17 3.2 250

155 100 8.5 3.97

235 30–40 8.2 3.65

Metal cations and anions were added individually to sample solutions containing 60 ␮g L−1 of Cd and Pb in order to examine the effect of common coexisting ions on the sorption of studied metal ions. The tolerance limit was considered if it resulted in a ±5% variation in sorption efficiency of Cd and Pb. As can be seen in Table 2, most of examined cations and anions did not interfere with the extraction and determination. It has been found that recovery of Cd(II) and Pb(II) was almost quantitative in the presence of foreign ions and therefore useful for the analysis of Cd and Pb in real samples.

Table 3 Analytical characteristics of proposed method at the optimum conditions. Cd

Nano-sized alumina

3.6. Interference effects

Values in the parentheses are R.S.D. (N = 3).

Regression equation Correlation coefficient (r) LOD (␮g L−1 ) (n = 10) R.S.D.a (%) (n = 10)b Enrichment factorc

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3.7. Analytical performance

Cd and Pb (II) concentration was 60 ␮g L−1 for which R.S.D. was obtained. b Values in parenthesis are the Cd and Pb concentrations (␮g L−1 ) for which the R.S.D. was obtained. c Ratio of the initial volume to the final volume. a

3.5. Effect of ionic strength The influence of ionic strength on the extraction of Cd and Pb was studied in the sodium nitrate solution with various concentrations from 0.01 to 1.0 mol L−1 . The results (data not shown) indicated that ionic strength has no significant effect on extraction efficiency up to 1.0 mol L−1 of NaNO3 . These observations showed the specific tendency of modified nano-␥-alumina for Cd and Pb ions and the possibility of using this method for the separation of Cd and Pb from the solution with high ionic strength.

Table 3 shows the analytical characteristics of the method. Under the optimum experimental conditions, limit of detection (S/N = 3) was calculated to be 0.15 ␮g L−1 for Cd and 0.17 ␮g L−1 for Pb. A good correlation coefficient was obtained and relative standard deviation (R.S.D.) for 10 replicate measurements of 60 ␮g L−1 Cd(II) and Pb(II) was 2.8 and 3.2% respectively. The calibration curves were investigated up to 200 ␮g L−1 which were linear. As the amount of Cd and Pb in the sample solution was measured after a final volume of nearly 2 mL, the solution was concentrated by a factor of 250. 3.8. Sorption capacity The capacity of the sorbent is an important factor that determines how much sorbent is required to remove a specific amount of metal ions from the solution quantitatively. In order to study the sorption capacity of modified nano-sized alumina, 50 mg sorbent

Table 4 Determination of Cd and Pb in a standard reference material in this work. Concentration (␮g kg−1 )

Reference material

Founda

Certified

SRM1640 a

Pb

Cd

Pb

Cd

27.89 ± 0.14

22.79 ± 0.96

26.21 ± 0.91

21.23 ± 0.80

Mean ± S.D. (n = 5).

Table 5 Determination of Cd and Pb in real and spiked samples. Sample

b

Tap water Mineral waterd Valeriana officinalie Lettuce Lettucee Valeriana officinaliee a b c d e

Concentrationa (␮g L−1 )

Added

Cd

Pb

Cd

Pb

N.D. N.D. 24.7 ± 1.3 25.4 ± 1.2 23.1 ± 1.1 22.9 ± 1.9

60 60 60 60 – –

60 60 60 60 – –

c

N.D. N.D. N.D. 2.5 ± 1.9 2.0 ± 0.4 N.D.

Mean ± S.D. (n = 5). From drinking water system of Tehran, Iran. Not detected. Damavand mineral water. The data obtained by electrothermal atomic absorption spectrometry (ET AAS).

Founda (␮g L−1 ) Cd 59.1 ± 61.5 ± 60.0 ± 61.3 ± – –

Recovery (%) Pb

1.5 1.7 1.4 1.5

61.0 ± 59.8 ± 83.4 ± 85.9 ± – –

1.2 1.8 1.2 1.5

Cd

Pb

105.4 100.1 100.6 98.0 – –

101.6 99.6 97.3 100.9 – –

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Table 7 Comparison of the proposed method with other methods reported. Sorbents

Technique

Silica gel SP70-benzoine oxime resin MWCNTs MWNTs SP-850 resin Modified silica gel Alumina Surfactant-coated alumina (PAN) Surfactant-coated alumina Nanometer-sized alumina Modified nanometer alumina

FAAS FAAS FAAS FAAS FAAS FAAS FAAS FAAS FAAS ICP FAAS

a b c

Sorption capacity (mg g−1 )

LOD (␮g L−1 )

EFa

Pb

Cd

Pb

Cd

Pb

Cd

Pb

Cd

19.76 – – 10.3 0.02c 12.63 13.3 – 13.3 15.4 16.4

12.5 – – 9.5 0.06c 6.09 – 2.7 – 10.3 11.1

0.51 16 2.6 0.60 0.50 0.58 2.8 – 1.6 0.53 0.17

0.96 4.2 – 0.45 0.47 0.65 – 24.3 – 0.14 0.15

300 75 44.2b 80 50 200 63 – 300 50 250

200 100 – 80 50 100 – 100 – 50 250

3000 150 20 400 250 2000 500 – 1800 100 500

2000 200 – 400 250 1000 – 1000 – 100 500

Volume sample (mL)

Flow rate (mL min−1 )

2 7 5 5 5 2 – 5 4 1 3

References

[24] [25] [26] [27] [28] [29] [15] [30] [31] [32] This work

Enrichment factor. EF was calculated by ratio of slopes of analytical curve with and without preconcentration step. mmol g−1 .

lyzed. The certified values and the analytical results were presented in Table 4. The results found were in good agreement with the certified values of CRM. This method was applied to determine of Cd and Pb in water and herbal samples. In order to test the reliability of the proposed methodology, some of the samples were analyzed by electrothermal atomic absorption spectrometry (ET AAS). Applying the paired t-test no significant difference at 95% confidence level was observed. The data obtained with the proposed and ET AAS methods for spiked and real samples were presented in Table 5. The results of analysis of samples showed that the proposed method can be reliably used for the determination of Cd and Pb in different matrices. Fig. 5. Adsorption isotherm of Cd and Pb ions from aqueous solution on the surface of sorbent at pH 7–8. Table 8 Comparison of the modified nano-alumina with modified alumina sorbents. Sorbenta

Capacity factor (mg g−1 )

Flow rate (mL min−1 )

References

Alumina Alumina Alumina Alumina Nano-alumina

2.7 3.07 0.93 1.36 11.1–16.4

5 5 6 5 3

[30] [33] [34] [35] This work

a

Modified with SDS-PAN.

was added to 50 mL of solution with different concentrations of Cd and Pb at pH 7–8. After shaking for 1 h, the mixture was filtered and the supernatant aspirated to flam atomic absorption spectrometry. The sorption capacity (aE , mg g−1 ) was calculated as

5. A comparison of analytical performance data with literatures The physical properties of nano/microsized alumina are given in Table 6 [23]. A comparison of the represented method with other SPE methods with different adsorbents reported in the literature was given in Table 7. Higher sorption capacity, higher preconcentration factor, lower detection limits and good R.S.D. values are some of the advantages of the proposed method. Modified nanoalumina coated with SDS-PAN has higher sorption capacity than modified micro-particle alumina with SDS-PAN [30,33–35], however flow rate is a little lower than that of micro-particle alumina due to back pressure of smaller particle size of nano-alumina was given in Table 8. Modifying nano-sized alumina is very simple and takes a short time for preparation. Therefore, nanometer-sized alumina is a good choice for the separation and preconcentration of metal ions in environmental samples due to its low cost when compared to commercially available sorbents.

aE = (C0 − CE )Vm−1 where C0 and CE are the initial and equilibrium concentrations (mg L−1 ) of analyte ions in the solution, respectively. According to these results (Fig. 5), the maximum amounts of Cd and Pb ions that can be sorbed by modified nano-␥-alumina were found to be 11.1 and 16.4 mg g−1 respectively. Modified nano-alumina coated with SDS-PAN has higher sorption capacity than modified micro-particle alumina with SDS-PAN [30,33–35]. 4. Real sample analysis and analytical performance In order to validate the method for accuracy and precision, a certified reference material (SRM 1640, for Natural water) was ana-

6. Conclusion A new sorbent of modified nano-␥ alumina has been reported. It was found that the nanometer-sized alumina modified with SDSPAN in this study, is stable and has efficient analytical performance in preconcentration Cd and Pb samples. The simple, rapid and simultaneous determination of elements with high sensitivity and reproducibility are the advantages of modified nano-sized alumina. This methodology gives good accuracy, low limits of detection, excellent precision and relatively high kinetic sorption on the target analytes, which show its potentiality in trace analysis in various samples with complicated matrix.

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