E. Yilmaza,
R. M. Alosmanovb and
M. Soylak*a
aErciyes University, Fen Faculty, Department of Chemistry, 38039 Kayseri, Turkey. E-mail: soylak@erciyes.edu.tr; Fax: +90 352 4374933; Tel: +90 352 4374933
bBaku State University, Chemistry Department, Z. Khalilov str., 23, Baku, AZ1148, Azerbaijan
First published on 9th March 2015
A magnetic solid-phase extraction (M-SPE) procedure on a magnetic phosphorus-containing polymer (M-PhCP) was established for the separation/preconcentration of trace amounts of lead and cadmium in water samples prior to their microsampling flame atomic absorption spectrometric determination. The separation of lead(II) and cadmium(II) adsorbed as 2-(5-bromo-2-pyridylazo)-5-diethylamino-phenol (5-Br-PADAP) chelates on M-PhCP from aqueous solution was simply achieved by applying an external magnetic field via a permanent magnet. The important analytical factors governing the extraction efficiency such as pH, amount of adsorbent, eluent concentration and volume, vortex time, ligand volume, and sample volume were investigated and optimized. The influences of matrix components were also investigated. The limit of detection (LOD) of Pb and Cd was 2.7 and 1.1 μg L−1, respectively. The accuracy of the proposed method was proved by analysis of the TMDA-64.2 Lake Ontario water and SPS-WW2 waste water certified reference materials and addition–recovery tests. The method was successfully applied for the determination of lead and cadmium in water samples.
Due to the very low concentrations of heavy metals in real samples, their determinations demand very sensitive analytical techniques including inductively coupled plasma mass spectrometry (ICP-MS), electrothermal atomic absorption spectrometry (ET-AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and X-ray fluorescence (XRF).14–16 However, when compared to flame atomic absorption spectrometry (FAAS), the available analytical techniques have some disadvantages such as high cost, slowness and greater proneness to matrix interferences, as well as their high sensitivity advantages.16,17 FAAS is, in principle, a suitable technique due to the low cost and easy operation. However, FAAS does not have sufficient sensitivity and selectivity for the determination of heavy metals in real samples because of their low concentration, which may be lower than the detection limit of FAAS, and matrix effects.18,19
A separation/preconcentration procedure is generally required to solve sensitivity and selectivity problems. Several techniques have been developed for the separation and preconcentration of trace metals including: cloud point extraction,20 liquid–liquid extraction,21 precipitation/co-precipitation,13 and solid-phase extraction (SPE).17 SPE is considered superior to other techniques due to its simplicity, the consumption of small volumes of organic solvent, and the ability to obtain a higher preconcentration factor and greater speed.17,22,23
Magnetic adsorbents have attracted much attention because of their unique magnetic properties for SPE procedures.24–27 In a magnetic SPE procedure, a magnetic sorbent is added to the sample solution containing the target analyte. The analytes are adsorbed onto the magnetic adsorbent whether under mixing or standing conditions. The adsorbent with the captured analytes is then isolated from the sample solution by using an appropriate magnet. Finally, the target analytes adsorbed on the adsorbents are desorbed with suitable elution and analyzed using an appropriate method.28,29 The application of magnetic SPE simplifies the preconcentration/separation steps for analytes.28,29 The magnetic adsorbents do not need to be packed into a cartridge or column, as in traditional SPE procedures, and the phase separation can be realized easily by applying an external magnetic field.
In the present paper, a magnetic phosphorus-containing polymeric sorbent (M-PhCP) was used for the magnetic solid phase extraction (M-SPE) of lead(II) and cadmium(II) as 2-(5-bromo-2-pyridylazo)-5-diethylamino-phenol (5-Br-PADAP) chelates from natural water samples prior to their microsampling flame atomic absorption spectrometric determination.
Buffer solutions were prepared by using a combination of salts and solutions as follows: phosphate buffer solution (pH 2.0–4.0, sodium dihydrogen phosphate/phosphoric acid) and phosphate buffer solution (pH 5.0–7.0 sodium dihydrogen phosphate/disodium hydrogen phosphate). TMDA-64.2 Lake Ontario water (National Water Research Institute, Ontario, Canada) and SPS-WW2 waste water (Spectrapure Standards AS, Oslo, Norway) certified reference materials were used.
The FT-IR spectra of the phosphorus-containing polymeric sorbent (PhCPS) and magnetic phosphorus-containing polymeric sorbent (M-PhCP) were recorded on a Perkin-Elmer Spectrum 400 ATR-FT-IR spectrometer (Waltham, MA, USA). Scanning electron microscopy (SEM) images were obtained on a LEO 440 scanning electron microscope with an accelerating voltage of 20 kV. For the SEM measurements, the samples were covered with Au/Pd. X-ray diffractions (XRD) pattern were taken with Bruker AXS D8 advanced diffractometer. The surface area, pore volume and pore width of the magnetic phosphorus-containing polymeric sorbent (M-PhCP) were determined by the BET-N2 method using a Micromeritics Gemini VII analyzer. A pH meter, Sartorious PT-10 (Germany) was used for measuring the pH values. A VWR international model (Germany) vortex mixer was used during the experimental period.
The developed magnetic SPE method was also applied to TMDA 64.2 Lake Ontario water and SPS-WW2 waste water certified reference materials. The analytes in the eluate were determined using flame AAS and a microsampling system.
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Fig. 1 The FT-IR spectra of the phosphorus-containing polymeric sorbent (PhCPS) and magnetic phosphorus-containing polymeric sorbent (M-PhCP). |
Scanning electron microscopy (SEM) micrographs presented in Fig. 2 show the porous surface structures of the PhCPS (A) and M-PhCP (B). There are some differences between the morphology of the PhCPS and the M-PhCP. The PhCPS surface is quite rough, providing a large exposed surface area for the adsorption of metal–ligand complexes. To investigate, the presence and distribution of magnetic particles in the magnetic phosphorus-containing polymer, the elemental mapping image (EMI) was carried out using SEM-EDX analysis. For this purpose, the elemental mapping image of the iron element was performed (Fig. 3). The obtained elemental mapping image indicates that the magnetic iron particles are homogeneously dispersed on the phosphorus-containing polymer. The elemental composition of the magnetic phosphorus-containing polymer was also analyzed using SEM-EDX. The sample was covered with Au. The obtained spectrum is given in Fig. 4. The amount of iron found was 6.8 wt%.
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Fig. 2 SEM images of the phosphorus-containing polymeric sorbent (PhCPS) (A) and magnetic phosphorus-containing polymeric sorbent (M-PhCP) (B). |
The structures of the phosphorus-containing polymer (A) and the magnetic phosphorus-containing polymer (B) were characterized by XRD and the diffractograms are given in Fig. 5. There are four types of iron oxides commonly formed including magnetite (Fe3O4), maghemite (γ-Fe2O3), hematite (α-Fe2O3) and goethite (FeO(OH)). Among them, only magnetite and maghemite are magnetic. The XRD pattern of the magnetic phosphorus-containing polymer displays five main diffraction peaks at 2θ = 31.8°, 34.3°, 45.5°, 55.3° and 62.4° that can be assigned to maghemite or magnetite. Other peaks are also observed at 2θ = 19.1°, 33.3°, 42.2° and 59.5° for the magnetic phosphorus-containing polymer, which may be related to the presence of goethite. The diffraction pattern of the magnetic phosphorus-containing polymer is close to the standard pattern for JCPD. It is clearly seen that the particles do not show sharp diffraction peaks. Instead, a broad band appears. This is typical for amorphous materials and also for ultrafine crystalline materials where diffraction peaks cannot be well resolved.
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Fig. 5 XRD patterns of the phosphorus-containing polymer (A) and the magnetic phosphorus-containing polymer (B). |
From the IR spectra it is clear that the typical bands for the phosphorus-containing polymer (O–H, C–H, O–H (P–O–H): PO (resonance state) P
O: 1183 cm−1, C–O (P–C–O) and C–Cl: 495 cm−1) also appeared for the magnetic phosphorus-containing polymer. The IR spectrum for the magnetic phosphorus-containing polymer shows that the polymeric parts of the magnetic phosphorus-containing polymer are effective for the adsorption of analytes as their 5-Br-PADAP chelates. Van der Waals interactions are important for adsorption studies. The functional groups of the phosphorus-containing polymer provide effective van der Waals interactions between analyte–5-Br-PADAP chelates and the adsorbent.
The adsorption efficiency is related to the internal surface area and pore volume. The BET surface area, pore volume and pore width of the M-PhCP were found to be 7.53 m2 g−1, 0.00535 cm3 g−1 and 2.84 nm, respectively. The BET isotherm of the M-PhCP sample in Fig. 6 shows that the contribution of mesopores to the total surface area and pore volume is significantly higher than that of macropores.34
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Fig. 6 Nitrogen adsorption/desorption isotherm of the magnetic phosphorus-containing polymeric sorbent (M-PhCP). |
Compared to conventional adsorbents, magnetic adsorbents have some advantages including high extraction capacity, rapid extraction dynamics and high extraction efficiency.29,40–44 The amount of the magnetic adsorbent was studied in the range of 25–100 mg of magnetic adsorbent. The recoveries for Pb(II) and Cd(II) were increased and reached quantitative values at 100 mg of magnetic adsorbent. Therefore, 100 mg of adsorbent was selected for further experiments.
The recoveries of the analytes by the iron species only were 75% and the recoveries of the analytes on the phosphorus-containing polymer with a ligand without magnetic properties was lower than 50%. The recoveries of the analytes on the magnetic phosphorus-containing polymer with ligand (presented system) were quantitative (>95%).
The influences of various eluents on the recoveries of Pb(II) and Cd(II) ions from the magnetic phosphorus-containing polymeric sorbent were also examined. The results are summarized in Table 1. It should be pointed out that the regeneration of the magnetic property of the adsorbent is possible due to the dissolution of Fe3O4 in a high acid concentration. Hence, low concentrations of HNO3 were examined in this study. The recoveries of the analyte ions were quantitative with 0.5 M HNO3 in acetone. The recoveries were not quantitative for the other eluents. 0.5 M HNO3 in acetone was used as the eluent in all further works for the quantitative recovery of metal ions from M-PhCP. At the same time, the effect of the eluent volume on the recoveries of Cd(II) and Pb(II) was studied. It was found that quantitative recoveries could be obtained with 5.0 mL of 0.5 M HNO3 in acetone solution and more than 5.0 mL. Therefore, 5.0 mL of eluent was used in the following experiments.
Eluent type | Recovery, % | |
---|---|---|
Pb(II) | Cd(II) | |
a N: number of samples analyzed. | ||
7 mL 0.05 mol L−1 HNO3 in acetone | 77 ± 1 | 69 ± 4 |
7 mL 0.1 mol L−1 HNO3 in acetone | 73 ± 6 | 69 ± 3 |
7 mL 0.25 mol L−1 HNO3 in acetone | 70 ± 10 | 46 ± 2 |
7 mL 0.5 mol L−1 HNO3 in acetone | 95 ± 2 | 99 ± 2 |
1 mL 0.5 mol L−1 HNO3 in acetone | 44 ± 0 | 50 ± 0 |
2 mL 0.5 mol L−1 HNO3 in acetone | 67 ± 0 | 45 ± 1 |
3 mL 0.5 mol L−1 HNO3 in acetone | 104 ± 6 | 45 ± 0 |
4 mL 0.5 mol L−1 HNO3 in acetone | 89 ± 2 | 45 ± 0 |
5 mL 0.5 mol L−1 HNO3 in acetone | 95 ± 2 | 101 ± 3 |
In order to obtain a hydrophobic metal complex, 5-Br-PADAP was used as the complexing agent. The effects of the amounts of 5-Br-PADAP on the recoveries of the analytes were also examined with model solutions containing 20 μg lead(II) and 10 μg cadmium(II) in the range of 0.0–0.5 mg of 5-Br-PADAP. The results are shown in Fig. 8. The results are not quantitative without the ligand. Quantitative recoveries were obtained using 0.1 mg of 5-Br-PADAP. Hence, 0.1 mg of 5-Br-PADAP was used for further study.
In order to obtain a high preconcentration factor,45–52 the magnetic SPE method was applied to 10, 15, 20, 30 and 40 mL of sample solution. Quantitative recoveries of the analytes could be obtained using 100 mg of the M-PhCP when the sample volume was 40 mL. The preconcentration factor was 80. Therefore, the developed method is very suitable for the preconcentration of trace lead and cadmium from large volumes of sample solution.
The effects of common interfering ions on the recoveries of the Pb(II) and Cd(II) ions were also examined. The obtained results are given in Table 2. The recoveries of all analyte ions were quantitative for the given concentrations of interfering ions. The results demonstrate that thousand fold excesses of the analyte ions have no effect on the preconcentration and determination of the Pb(II) and Cd(II).
Ions | Concentration, μg mL−1 | Added as | Recovery, % | |
---|---|---|---|---|
Pb(II) | Cd(II) | |||
Na+ | 10![]() |
NaNO3 | 100 ± 0 | 99 ± 2 |
K+ | 10![]() |
KCl | 95 ± 4 | 97 ± 1 |
Co2+ | 25 | Co(NO3)2·6H2O | 98 ± 5 | 101 ± 1 |
Cu2+ | 25 | Cu(NO3)2·3H2O | 95 ± 2 | 99 ± 2 |
Zn2+ | 25 | Zn(NO3)2·6H2O | 97 ± 2 | 101 ± 3 |
Fe3+ | 25 | Fe(NO3)3·9H2O | 96 ± 0 | 102 ± 2 |
Mn2+ | 25 | Mn(NO3)2·4H2O | 95 ± 2 | 100 ± 1 |
Cd2+ | 25 | Cd(NO3)2·4H2O | 99 ± 5 | — |
Ni2+ | 25 | Ni(NO3)2·6H2O | 100 ± 4 | 103 ± 1 |
Pb2+ | 25 | Pb(NO3)2·6H2O | — | 99 ± 2 |
Cr3+ | 25 | Cr(NO3)3·9H2O | 100 ± 4 | 97 ± 2 |
Cl− | 10![]() |
KCl | 95 ± 4 | 97 ± 1 |
SO42− | 5000 | Na2SO4 | 98 ± 3 | 96 ± 6 |
The calibration equations were [A = 0.0023 + 0.0053C] for lead and [A = 0.0002 + 0.2216C] for cadmium where A is the absorbance and C is the analyte concentration in mg L−1 linear with correlation coefficients of 0.9980 and 0.9978, respectively. The detection limits of the analytes, defined as 3 times the signal/slope (slope of the calibration curve), were 2.7 and 1.1 μg L−1 for Pb(II) and Cd(II). The quantification limits, which were defined as 10 times the signal/slope (slope of the calibration curve), were 9.1 and 3.6 μg L−1 for Pb(II) and Cd(II). The relative standard deviations (RSD, %), evaluated using the results of the analysis of seven replicates containing 30 μg L−1 of Pb(II), and Cd(II), were 3.7 and 4.6, respectively. The preconcentration factor (PF = the ratio of the highest sample volume to the eluent volume) was 80. Consumptive index (CI) is another effective way to evaluate the analytical performance of a preconcentration system. CI was also determined by using the slope of the sample volume (SV) to the experimental preconcentration factor (CI = SV/PF). CI was found as 0.5 mL.
Certified reference material | Analyte | Found, μg L−1 | Certified value, μg L−1 | Recovery, % |
---|---|---|---|---|
TMDA-64.2 fortified water | Pb | 280 ± 27 | 288 | 97 |
Cd | 266 ± 6 | 266 | 100 | |
SPS-WW2 waste water | Pb | 513 ± 1 | 500 | 103 |
Cd | 103 ± 5 | 100 | 103 |
To further ensure the accuracy of the magnetic SPE method, the developed method was also applied to the analysis of four water samples corresponding to the CRMs and the analytical results along with the recoveries for the spiked waters are given in Table 4. As can be seen in Table 4, a good correlation exists between the added and recovered amounts of lead and cadmium ions. The results for the analysis of the certified reference materials and the addition–recovery studies show that the developed magnetic SPE method is reliable and independent from effects of the matrix for the determination of lead and cadmium in water samples.
Sample | Added, μg L−1 | Pb(II) | Cd(II) | ||
---|---|---|---|---|---|
Found, μg L−1 | Recovery, % | Found, μg L−1 | Recovery, % | ||
a BDL: below the detection limit.b Mean ± standard deviation. | |||||
River water | 0 | 52 ± 6a | — | BDLb | |
40 | 93 ± 9 | 101 | 41 ± 0 | 103 | |
Dam water | 0 | BDL | — | BDL | — |
133 | 133 ± 0 | 100 | 141 ± 0 | 106 | |
Well water | 0 | 45 ± 0 | — | 45 ± 0 | — |
133 | 178 ± 0 | 100 | 178 ± 0 | 100 | |
Waste water | 0 | 158 ± 31 | — | 18 ± 0 | — |
200 | 347 ± 9 | 97 | 214 ± 5 | 98 |
The most important advantages of the developed method compared with other preconcentration methods in the literature are: rapid collection of the analyte from the adsorbent by use of magnet elution, which eliminates the time-consuming column passing or filtration operation when compared with column and filtration operations, few adsorbent requirements and a low limit of detection for analytes. The analyses of certified reference materials and recovery tests show that the developed method can be used for the rapid preconcentration/separation of Pb(II) and Cd(II) in various water samples.
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