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Separation and preconcentration system for lead and cadmium determination in natural samples using 2-aminoacetylthiophenol modified polyurethane foam
Desalination 249 (2009) 1199–1205
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Separation and preconcentration system for lead and cadmium determination in natural samples using 2-aminoacetylthiophenol modified polyurethane foam N. Burham ⁎ Chemistry Department, Faculty of Science, Fayoum University, Fayoum City, Egypt
a r t i c l e
i n f o
Article history: Accepted 22 April 2009 Available online 7 October 2009 Keywords: 2-Aminoacetylthiophenol Modified polyurethane foam Solid-phase extraction Cadmium Lead
a b s t r a c t A new modified polyurethane foam using 2-aminoacetylthiophenol (AATP) was employed for selective separation, preconcentration and determination of Cd and Pb ions in different samples by flame atomic absorption spectrometry (FAAS). This new modified foam was used as an effective sorbent for the solidphase extraction (SPE) of Cd and Pb ions from aqueous solutions from natural samples. Experimental conditions for effective separation of trace levels of the analyte ions were optimized with respect to different experimental parameters in batch process. The preconcentration factors are 167 and 250 for Pb and Cd respectively. The relative standard deviation (R.S.D.) under optimum conditions is < 10% (n = 5). The accuracy of the method was estimated by using a reference material BDH Cat no: 456422W real samples that were spiked with the analyte ions. The extraction isotherm of both Cd and Pb ions was obtained. This study indicated that the novel foam presents a high affinity for Cd and Pb due to the presence of good extractive sites (S, N and O) which are introduced to the foam material. The bonding of the studied metal ions by the foam is useful for the removal of metal contamination from real samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The determination of heavy metals, especially some toxic metals that play important roles in biological mechanisms, has been receiving much attention. Lead enters the organism primarily via the alimentary and/or the respiratory tract. The main sources of this metal intake are food, air and drinking water [1]. The major effects of cadmium poisoning are experienced in the lungs, kidneys and bones [2]. Briefly, it is seen that these two metals can dangerously affect human health even at ultra trace concentrations. Considering the low lead and cadmium concentration levels in natural samples, sensitive analytical techniques are required to obtain adequate detection limits. The contents of these ions at low concentrations have been determined by several techniques, including flame atomic absorption spectrometry (FAAS) [3–5], electrothermal atomic absorption spectrometry (ETAAS) [6], inductively coupled plasma mass spectrometry (ICP-MS) [7], inductively coupled plasma optical emission spectrometry (ICP-OES) [8], and X-ray fluorescence (XRF) [9], among others. In order to achieve accurate, reliable and sensitive results, preconcentrations and separations are needed when the concentrations of the analyte elements in the original material or the prepared solution are too low to be determined directly. Off-line solid-phase extraction (SPE) may be used as an alternative to flow injection liquid–liquid extraction, in order to improve the sensitivity and selectivity of Flam AAS. In addition, it has some extra advantages: ⁎ Tel.: +20183877751. E-mail address: [email protected]. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.04.009
large availability, easy recovery of the solid phase, attainability of large preconcentration factors and facility for separation using various systems. The most convenient sorbent materials for column packing are, C18 [10], amberlite or Chelex [11], activated carbon [12] polyurethane foam (PUF) [13,14], polytetrafluoroethylene (PTFE) [15] or polychlorotrifluoroethylene (PCTFE) [16]. Considering the high costs of other methods in comparison with flame AAS it is recognized that FAAS should be preferred to flameless AAS due to its advantages such as fast, cheap and no need of expert operators. Flame AAS has been successfully used in connecting with the enrichment procedures for determination of Cd, Pb and other ultra trace metals in various food and water samples [17–20]. For this purpose, solid-phase extraction (SPE) [17–19,21] is importantly interested in recent years. So, chelating sorbents obtained by immobilization of organic reagents on solid supports (adsorbed or chemically bonded) have found widespread application in preconcentration and separation of trace metals from a variety of matrices [17,20]. The PUF is an excellent sorbent material due to its high available surface area and extremely low cost. In addition, it is stable in acids and bases (except concentrated nitric), it will not change its structure when heated up to 180 °C [22]. Thus, it is a very suitable material for on-line and off-line preconcentration column packing. The aim of this study is first to highlight the method of modification of polyurethane foam with 2-aminoacetylthiophenol reagent, because the sulfur-containing chemical groups act as selective ligands with high bond stability for transition metal ions [23–25]. Particularly, high improving in the sensitivity of FAAS can be obtained in connecting with the enrichment method. The second aim is to develop a new, simple, cheap and rapid separation and determination procedure for Pb and Cd
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in trace level from natural samples using the modified PUF. The great advantage of this modified foam that it can extract metal ions without prior complexation. 2. Experimental 2.1. Instrumentation Flame atomic absorption spectrometric (FAAS) measurements of Pb and Cd were recorded on AAS5 FL instrument (Carl Zeiss Technology, Germany), equipped with standard burner for air–acetylene flame. The operational conditions for the measurements are depicted in Table 1. The FAAS method was used for the determination of the studied metal ions. The pH measurements were carried out using the microprocessor pH meter BT 500 BOECO, Germany, which was calibrated against two standard buffer solutions at pH 4 and 9. A mechanical shaker with up to 200 rpm (SL 350 Nüve San. Malz. Imal. Ve Tic. A.S., Akyurt, AnkaraTurkey) was used. Doubly distilled water (DDW) was obtained from two successive distillations using Hamilton laboratory glass instrument (Europe House, Sandwich Industrial Estate, Sandwich Kent, England). 2.2. Chemicals and solutions Unless otherwise stated, all reagents used were of analytical grade, all solutions were prepared with DDW. Laboratory glassware was kept overnight in chromic acid solution. Stock solutions of the studied metal ions were prepared by dissolving appropriate amounts of analytical reagent grade Lead (II) nitrate Pb(NO3)2 obtained from Aldrich (Milwaukee, USA), Cadmium nitrate Cd(NO3)2.4H2O obtained from Panreac (Barcelona, Spain). Sodium nitrite, sodium hydroxide were received from Winlab Company, UK for reagents fine chemicals. Hydrochloric acid used was delivered from Merck (Darmstadt, Germany), commercial, open-cell, polyether-type PUF (31.6 kg m− 3, supplied by the Egyptian company for foam production, Cairo, Egypt). All liquids were used without any further purification. 2.3. Synthesis of sulfur-containing ligands modified polyurethane foams 2.3.1. Synthesis of 2-aminoacetylthiophenol (AATP) This was prepared by modifying the method reported [26].2Aminothio-phenol 1.5 g and 10 mL of absolute ethanol were added to a three-necked flask, adjusted to pH 9 with Na2CO3. Fifty milliliters of acetic anhydride was added with a dropping funnel while stirring, maintaining the reaction temperature at 50–60 °C, then the precipitate was filtered, washed with DDW and dried. The yellow solid product was obtained have mp 145–147 °C. 2.3.2. Synthesis of AATP modified polyurethane foam (AATP-PUF) Polyurethane foam, commercial open-cell polyether type was cut into similar cubes (~0.125 cm), washed by a 0.1 mol L− 1 solution of HCl, followed by doubly distilled water and acetone then left to dry at room temperature. Finally, the foam was dried in a stove at 70 °C for 1 h and stored in a dark bottle. One-gram foam cubes were soaked in a HCl (1:1) for 1 h to liberate the maximum number of free amino
Table 1 Conditions for flame atomic absorption spectrometer. Parameters
HC lamp current (mA) Slit width (nm) Wavelength (nm) Fuel flow (mL/h) Burner height (mm)
groups. Therewith, the foam was washed with water, placed into a 0.1 mol L− 1 HCl solution and then cooled in an ice bath. The foam was diazotized by the drop wise addition of 10 mL of 7 mol L− 1 of sodium nitrite to the cold solution containing the foam, and stirred vigorously until the yellow color appeared due to the formation of diazonium chloride. Then, it was left for 1 h at a temperature below 3 °C, as reported previously [23]. Next, the diazotized foam was filtered, washed with icecold water and coupled with 2-aminoacetylthiophenol 4.2 g in 100 mL ethanol and 1.0 mol L− 1 sodium acetate below 5 °C for 24 h following the method reported in [27]. The resulting brownish-orange foam was washed with DDW and air-dried. 2.4. Batch experiment Separation of Pb(II) and Cd(II) was carried out by a batch technique at 25 °C. 100 mg portion of the new PUF was mixed with 20 mL aliquot of the tested metal ion solution (1 µg mL− 1) in a shaker adjusted to the desired shaking speed. After a certain time, the solution was separated and the concentration of metal ion was determined. 2.4.1. Optimum pH of metal ion uptake Twenty milliliters of metal ion solutions containing 20 μg of Pb(II) and Cd(II) from single element aqueous solutions, were shaken with 0.1 g of foam for 1 h. The pH of the metal ion solution was adjusted before equilibration with HCl or NaOH over a range of 1–9. After the equilibration, the remained metal ions were determined by the recommended method. 2.4.2. Shaking time The effect of shaking time on the extraction efficiency of Pb(II) and Cd(II) was studied. For that purpose, 0.1 g of foam was added to 20 mL of sample containing 20 μg metal ions at the optimum pH and automatic shaking for different time intervals. The percentage of sorption (E%) was determined by using the following formula: E% = [(C0 − C)/C0] × 100. C0 and C are the initial and remaining concentrations (μg L− 1) of the metal ion, respectively. 2.4.3. Sorption capacity Determination of the sorbent capacity was carried out by the sorption of the selected metal ions from separate element aqueous solutions. Typically, 0.1 g of modified foam was equilibrated with 20–100 μg metal ion solution adjusted at the optimum pH, which was automatically shaken for 1 h at room temperature. After equilibration, the mixture was filtered and the remaining metal ion in the filtrate was determined. The sorption capacity per gram foam (Q, μg g− 1) was calculated from the equation: Q=[(C0 −C)×V]/m, where V is the sample volume in liter and m is the weight of the foam in gram. 2.5. Dynamic experiments In the dynamic experiments 1 g of the novel PUF was packed into the column (10 cm× 1 cm), with gentile pressure by a glass rod on the foam plugs onto the column filled with water to avoid air bubbles then adding glass beads on the top of foam bed to prevent foam floating upwards. The bed height of the foam column was about 60 mm. It was washed successively with DDW then stored in DDW for the next experiment. Test solutions were passed through the foam column at a flow rate of 3 mL min− 1. The stripping of the metal ions from the foam column was carried out with the suitable eluting agent and the amount of the metal ion was determined.
Metal ion Lead
Cadmium
3.0 1.2 217.0 65 5–10
2.0 1.2 228.8 50 4–12
2.5.1. Effect of flow rate on the retention and recovery The dependence of the uptake of the metal on the flow rate was studied for Pb (II) and Cd (II) at the pH chosen for maximum complexation, the sample (50 mL, 1µg mL− 1) flow rate being varied from 1 to 15 mL min− 1 and the retention percentage of the metal ion into the column was determined. The flow rates of the solutions were controlled
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by using the stopcock of the column. The flow rates of the eluent solutions were also investigated in the range 1–15 mL min− 1.
method. The experiment was repeated three times and the recovery percentage and RSD were calculated.
2.5.2. Breakthrough capacity 1000 mL samples containing Pb(II) or Cd(II) ions separately at concentrations (10 µg mL− 1), were passed through foam columns, at flow rates 3.0 mL min− 1. Each solution was adjusted to the optimum pH and the effluent was collected in 5 mL fractions where the amount of metal ions in each fraction was determined by the recommended method. The percentage of the metal in each effluent aliquot can be calculated and plotted against the volume of the effluent (Bed volume). The dynamic capacities with each metal ions could be calculated.
2.6.2. Analysis of spinach leaves The procedure was evaluated by the analysis of another real sample of spinach leaves. This sample was digested using the following procedure: a portion of 0.2 g of dry sample was precisely weighted; 4.0 mL of 1:1 (v/v) nitric acid solution was added and heated at 150 °C for 5 h [28]. After cooling at room temperature, the pH of the final digest was adjusted by suitable addition of a 10% (w/v) sodium hydroxide solution, and the mixture was finally diluted to 25 mL by doubly deionized water. The experiment was repeated three times and the recovery percentage and RSD were calculated.
2.5.3. Preconcentration Preconcentration of the studied metal ions was investigated from model solutions prior to the determination in the real samples. 25– 1000 mL Solutions containing Pb(II) and Cd(II) were passed through the column after adjusting to the optimum pH value and the flow rate. The stripping of the metal ions from the foam column was carried out by 0.1 mol L− 1 HNO3 solution and the amount of Pb(II) or Cd(II) in the eluent was determined by the recommended method. The concentration factor (CF) could be calculated from the ratio of initial volume of sample to the final volume after concentration. 2.6. Analysis of real samples 2.6.1. Real water analysis One-liter volume of tap water sample was collected from our research laboratory in Faculty of Science at Fayoum City or Qaroun lake water, Fayoum City spiked with 20 µg of standard solution of the metal ions, adjusted to pH 5 and passed through the modified foam columns at flow rate 3 mL min− 1. The columns were rinsed with 20 mL DDW and the metals were eluted by 0.1 mol L− 1 HNO3. The concentration of each metal ion was determined by the recommended
3. Results and discussion 3.1. IR analysis The proposed structure for AATP-PUF was confirmed by IR analysis. IR spectrum was obtained by grinding the dried foam with potassium bromide and then made into a pellet. The spectrum revealed that there are some changes were happened to the ligand functional groups. The most important change is the disappearance of the band at 3111– 3299 cm− 1 that is due to amino group of AATP. This indicates that the coupling occurred through the attack of the diazonium ion in the foam to the amino group of the AATP Scheme 1. The IR spectrum of the modified foam AATP-PUF was compared with that of the untreated foam. There are additional bands at 1660 (N=N) and 1711 cm− 1 (C=O), respectively. 3.2. Chemical stability of AATP-PUF The chemical stability of the modified foam was examined in acidic, basic and organic solutions. The new foam was mixed with 1–
Scheme 1. The proposed reactions involved in the preparation of the AATP-PUF.
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6 mol L− 1 HCl, 1 mol L− 1 H2SO4, 1–2 mol L− 1 NaOH, ethanol, methanol, isopropyl alcohol, n-butanol, chloroform, carbon tetrachloride and benzene, then stirred at room temperature during 24 h. The change in the degree of functionalization was calculated by the change in the metal uptake before and after the chemical treatment. It was found that there is no change in the metal uptake after stirring with the different solvents. Also, there is no leaching of the color after stirring for 24 h. This chemical stability enables the possibility of the application of AATP-PUF to different media.
exchange mechanism. With an increase in the pH, the metal ions species, mainly neutral, may be sorbed by hydrogen bonding mechanism along with ion exchange. Uptake of these metal ions from the solutions by the new solid-phase extractor can occur with the formation of surface complex with the built ligand (AATP) and the metal ions. However, the sites on the modified AATP-PUF can also contribute to the sorption process.
3.3. Effect of pH on the metal ions removal
The kinetics of sorption is important from the point of view that it controls the process efficiency. The curves are continuous and smooth suggesting a stable and uniform sorption process. The loading half time t½ defined as the time needed to reach 50% of the foam total loading capacity could be estimated from Fig. 2. From the kinetics of the studied analyte ions uptake, it is observed that an equilibrium time of about ≥ 15 min. is required for ≥ 90% uptake. However, the time of 50% sorption is ≤ 5 min for Pb and Cd. The faster uptake of these metal ions on the new modified PUF probably reflects a better accessibility of the studied analyst ions to the new chelating sites in the AATP-PUF. Also, the fact that this process is a surface phenomenon and that the surfaces are readily accessible to the ions in solution. The kinetics of the studied metal ions was subjected to Lagergren equation. The order of Cd and Pb uptake onto the PUF is evaluated by subjecting the data to the linear form of Lagergren equation [30] ln (qe − qt) = ln qe − kt where k is the first order rate constant of sorption and qt is the sorbed concentration of Cd and Pb ions at the time t, and a linear fit of ln (qe − qt) versus t is observed. This indicates that the kinetic of sorption is of the first order.
The effect of pH on the recovery of Cd and Pb by the new modified PUF is presented in Fig. 1. The pH of the aqueous solution was clearly an important parameter that controlled the uptake process. Maximum removal efficiency reached 96% for Cd and 100% for Pb at the pH ≥ 5. At pH, less than 2.0 the uptake of the studied metal ions is too small and then increases rapidly with increasing the pH value. The optimum pH value to remove the studied ions from aqueous solution is 5 Fig. 1. Based on the behavior of Cd and Pb extraction on the new PUF, we have speculated that ion exchange and hydrogen bounding may be the principal mechanism for the removal of these heavy metal ions [29]. There are a great deal of facts to support this speculation, including the components and the complexing properties of the new AATP-PUF, the properties of Cd and Pb ions and the uptake condition such as the effect of pH of the aqueous media. It has been recognized that heavy metal cations readily form complexes with oxygen, nitrogen or sulfurcontaining complexing agent built on the polymer. When these extractive sites are introduced to the PUF material, the binding ability of the modified PUF will be enhanced. Based on the electron donating nature of the oxygen, nitrogen and sulfur-containing groups in the PUF materials and the electron accepting nature of Cd and Pb ions, the ion exchange mechanism could be considered. For instance, Cd and Pb (divalent heavy metal) may attach itself two adjacent –NH group that can donate two piers of electrons to the metal ion, forming coordination compounds and releasing H+ into the solution. It is then readily understood that the equilibrium is quite dependent on the pH of the solution. At lower pH, the hydrogen ions compete with Cd and Pb ions for the exchange sites on the modified PUF, thereby partially releasing the latter. The studied metal cations are completely released under extreme acidic condition. In most cases, the percentage of metal uptake increases with increasing the pH up to certain value and then decreased with further increase of pH. Due to different properties of various heavy metal ions, the maximum uptake takes place in a slightly different pH range for different metals. In a certain pH range, for one specific heavy metal there may be a number of species present in the solution, such as M, MOH+, M(OH)2, etc. At lower pH values, the positive charged metal species may compete with the hydrogen ions and may be sorbed at the surface of AATP-PUF by ion
Fig. 1. Effect of pH on the recovery of Pb (II) and Cd (II) with AATP-PUF. The experiment was triplicated.
3.4. Effect of shaking time
3.5. Adsorption isotherm The relationship between the amount of a substance extracted at a constant temperature and its concentration in the equilibrium solution is called the adsorption isotherm Fig 3. The capacity of AATP-PUF was found to be 10,750 and 10,000 µg g− 1 for Pb and Cd respectively. This confirms the new modified foam has greater affinity against these metal ions. 3.6. Effect of the sample volume In order to explore the possibility of enriching low concentrations of analyte from large volume by the batch procedure, the effect of sample volume on the recovery of Pb and Cd ions was also investigated. For this purpose, 25–200 mL of sample solutions containing 20 µg of Pb or Cd was used at optimum conditions Fig 4. It was found that quantitative recovery (>95%) was obtained for Pb and Cd up to a sample volume of
Fig. 2. Loading rate of Pb (II) and Cd (II) with AATP-PUF. The experiment was triplicated.
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Fig. 3. Extraction isotherms of Pb (II) and Cd (II) with AATP-PUF. The experiment was triplicated.
Fig. 5. Breakthrough capacity of Pb (II) and Cd (II) with AATP-PUF. The experiment was triplicated.
100 mL. In this experiment, 100 mL of sample solution was adopted for the preconcentration of Pb or Cd ions from water samples, the extracted Pb or Cd ions can be eluted with 3 mL 0.1 mol L− 1 HNO3, so an enrichment factor of 33.3 is achieved by this method.
metal ions combine strongly with the new foam also Cd and Pb bond to the AATP-PUF are quite stable. Therefore, the sample and eluent flow rates are important parameters to obtain quantitative retention and elution of analyte ion. In consequence, a sample flow rate of 3 mL min− 1 was adopted as optimum for retention and elution.
4. Column procedure 4.1. Breakthrough curve The capacity of AATP-PUF column containing one gram of the foam could be determined from the curves obtained by plotting the amount of metal ion emerged from the column versus the volume of effluent collected Fig 5. AATP-PUF column is saturated with Pb or Cd under the optimum conditions. The capacity the AATP-PUF column for Pb and Cd are estimated to be 7000 and 6200 mg g− 1 respectively. The obtained dynamic capacity is smaller than the batch capacity this is because of the breakthrough capacity is not the total capacity; it is the working capacity of the column under the given conditions. 4.2. Effect of sample loading and elution The sample flow rate through the mini column is a very important parameter since it controls the time of analysis. The elution or loading of the tested analytes from AATP-PUF was examined at different flow rates between 1.0 and 15 mL min− 1. The studies show that the flow rate has more influence on the recovery of the metal ions. The foam at a flow rate of 3 mL min− 1 can extract the studded metal ions quantitatively. According to experimental results it was found that the recovery of Cd and Pd is 92%, at flow rate 7 mL min− 1 as these
4.3. Elution The effect of different solutions on the elution of the studied analyte ions on AATP-PUF was examined. The analyte ions sorbed quantitatively on the new modified PUF could be eluted with 0.1 mol L− 1 HNO3. On the contrary, to many other papers, it was not necessary to extract the analyte ions with HNO3 in acetone, where the eluent was evaporated to near dryness at 35 °C and diluted with HNO3 before the determination by AAS [17]. 4.4. Limit of detection The limit of detection (LOD) is defined as blank concentration + 3σ where σ is the standard deviation of blank determination. The proposed method for the determination of the investigated ions was studied under the optimal experimental conditions by applying the procedure for blank solutions. The detection limits were established by analyzing 17 blank solutions. The LOD of Pb and Cd were found to be 0.066 and 0.048µgL− 1 respectively. The LOD values for the studied ions with the new extractor enable the use of this material in collection of Pb and Cd ions at a trace concentration prior to their determination with high accuracy. 5. Analytical application of AATP-PUF 5.1. Preconcentration The preconcentration factor was calculated from the ratio of the initial volume to the final volume. The results are recorded in Table 2. Of all the preconcentration methods, chelating extractor recovery method is one of the effective multi element preconcentration methods, because it can provide more flexible working conditions together with good stability, selectivity, high concentration ability, high capacity of metal
Table 2 Preconcentration factor on AATP-PUF.
Fig. 4. Effect of sample volume on the recovery of Pb and Cd with AATP-PUF. The experiment was triplicated.
Foam type
Metal ion
Initial volume mL
Final volume mL
Recovery %
CF
ATP-PUF
Pb (II) Cd (II)
1000 1000
6 4
96 96.5
167 2.4 250 3.2
CF: concentration factor; RSD: relative standard deviation.
RSD %
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ions and simple operation [3,4]. However, at the same time, its demands for trace element determination and concentration using the new modified extractors are ever increasing. This is because such functional polymers can be employed for the preconcentration in water systems [5,6]. If the analyte concentration in the sample is too low to be detected by flame AAS, a preconcentration step is necessary. This was achieved by increasing the ratio of the sample to eluent volumes as needed. The recoveries and the preconcentration factors achieved at the lowest concentration limit below which the recoveries become non-quantitative are listed in Table 2. Twenty micrograms of the analyte ions in different sample volumes 25–1000 mL were allowed to pass through the columns at flow rate of 3 mL min− 1. The elution of the studied ions from AATP-PUF column occurred with recovery percentages of 96– 96.5% in final solutions between 4 and 6 mL with preconcentration factors 167–250 and RSD less than 10% which indicates adequate accuracy and good precision of the developed method. These results show that Pb and Cd can be concentrated effectively from the dilute aqueous solution using AATP-PUF columns.
5.2. Recycling of the sorbent To test the recyclability of the new modified foam one gram of the foam was used to extract the analyte ions through extraction striping cycle's column operation. The stripping agent used in this experiment was 0.1 mol L− 1 HNO3. The results of the recyclability studies indicated that the modified foam was found to be able to remove more than 96% ± 1% of the analyte ions from solution through more than 17 extraction cycles. The repeated use of the sorbent is likely to be a key factor in improving wastewater process economics. Various factors are probably involved in determining rates of Pb and Cd desorption, such as the extent of hydration of the heavy metal ions and sorbent structure. However, an important factor appears to be the binding strength. In this work, the extraction time was found to be <4 min with 4–6 mL 0.1 mol L− 1 HNO3. Therefore, the modified foam showed excellent reusability and stability towards the studied analyte ions. These results show that the AATP-PUF has enough stability to be reused at least 17 times, and it is a good chelating extractor for preconcentration of the studied analyte ions. Based on it, a new system for the determination of these metal ions by flam AAS after their preconcentration on a short column packed with AATP-PUF is established.
5.3. Validation of the method A reference material (Quality control standard #1, BDH Cat no: 456422 W) was used for the method validation. As seen in Table 3, the results were compared with the certified values. Good agreement was obtained between the estimated content by the proposed method and the certified values for Pb and Cd.
5.4. Determination of Pb and Cd in natural samples 5.4.1. In natural water The proposed column method was applied for the separation and determination of analyte ions in tap water and lake water samples. Lake or tap water sample were adjusted to the optimum conditions. The standard addition method was applied to check the selectivity of AATP-PUF for the analyte ions against matrix elements and to determine the accuracy and the precision of the new extraction method. The results are listed in Table 3, the water samples were spiked with 20 µg L− 1 for each of the studied metal ions then treated by AATP-PUF and then desorbed by 0.1 mol L− 1 HNO3. The recoveries are all in the range of 96–104% with R.S.D less than 10%, demonstrating that the interference species in the matrix were eliminated satisfactory after the extraction and preconcentration procedure by the column method. The results also indicate the suitability of the AATP-PUF for the preconcentration and quantitative recovery of the studied ions from natural water samples. 5.4.2. Determination of cadmium and lead in spinach leaves The proposed analytical procedure was applied to the determination of Cd and Pb in spinach leaves. The results are described in Table 3. Recoveries of spike were quantitative. The results indicate that the proposed method can be reliably used for the determination of these analyte ions in natural sample. 6. Comparison with other preconcentrating extractors A comparison of the proposed system with other preconcentration procedures is given in Table 4. Some of the parameters obtained were comparable to those presented by other methods described in the literature. As seen from the data in Table 4, the proposed method developed by using AATP-PUF system has high preconcentration factor when compared to other methods reported in Table 4. 7. Conclusion 2-Aminoacetylthiophenol modified polyurethane foam was successfully synthesized by coupling the foam through 2-aminoacetylthiophenol. The prepared sorbent showed several good characteristics, such as fast extraction kinetics, low price, low LOD and proper selectivity for Cd and Pb. This modified foam can be potentially used as a solid-phase extraction material for the selective preconcentration, separation and determination, when coupled with flame AAS, of trace Cd and Pb in environmental samples. The new extractor has good chemical and physical stability feature, rapid equilibration in the recovery of the studied analyte ions and can be used many times without losing its uptake capacity. This extractor expected to be useful material for the extraction and preconcentration of Pb and Cd ions.
Table 3 Addition–recovery tests in the experiments for Pb (II) and Cd (II) in different real samples. Sorbent
Sample
Metal ion
Added µg L− 1 or µg g− 1
Found µg L− 1 or µg g− 1
Recovery %
RSDa %
Materials sources
AAT-PUF
Quality Control Standard #1, No: 456422W Tap water
Pb (II) Cd (II) Pb (II) Cd (II) Pb (II) Cd (II) Pb (II) Cd (II)
20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0
19.89 20.1 20.1 19.2 20.8 19.4 19.1 19.2
99.45 100.5 100.5 96.0 104.0 97.2 95.5 96.0
2.4 4.8 2.1 5.1 4.5 3.9 6.2 5.3
No:456422W
Lake water Spinach leaves a
Based on three replicate measurements in the same loaded column.
Faculty of Science, Fayoum City Qaroun Lake water Fayoum City
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Table 4 Comparative data from some recent studies for preconcentration of trace metals. System
Studied metals
pH
Eluent
Flow rate mL min− 1
CF
LOD µg L− 1
Reference
Diaion SP-850/Alpha-benzoin oxime Amberlite XAD-2000/8-hydroxyquinoline Dowex Optipore V-493/dibenzyldithiocarbamate Amberlite XAD-2010/DDTC Polyurethane foam/acetylacetone Polyurethane foam/2-aminoacetylthiophenol (AATP-PUF)
Cd, Pb Cd, Pb Cd, Pb Cd, Pb Cd, Pb Cd, Pb
8.0 6.0 2.0 6.0 6.0 5.0
1 mol L− 1 HNO3 1 mol L− 1 HNO3 in acetone 1 mol L− 1 HNO3 in acetone 1 mol L− 1 HNO3 in acetone 0.1 mol L− 1 HNO3 0.1 mol L− 1 HNO3
5.0 10.0 4.0 10.0 3.0 3.0
50 100 50 100 288,224 250,167
0.28–0.73 0.3–2.2 0.43–0.65 0.08–0.26 0.07–0.09 0.048–0.066
[31] [32] [33] [34] [35] This Work
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Separation and preconcentration system for lead and cadmium determination in natural samples using 2-aminoacetylthiophenol modified polyurethane foam N. Burham ⁎ Chemistry Department, Faculty of Science, Fayoum University, Fayoum City, Egypt
a r t i c l e
i n f o
Article history: Accepted 22 April 2009 Available online 7 October 2009 Keywords: 2-Aminoacetylthiophenol Modified polyurethane foam Solid-phase extraction Cadmium Lead
a b s t r a c t A new modified polyurethane foam using 2-aminoacetylthiophenol (AATP) was employed for selective separation, preconcentration and determination of Cd and Pb ions in different samples by flame atomic absorption spectrometry (FAAS). This new modified foam was used as an effective sorbent for the solidphase extraction (SPE) of Cd and Pb ions from aqueous solutions from natural samples. Experimental conditions for effective separation of trace levels of the analyte ions were optimized with respect to different experimental parameters in batch process. The preconcentration factors are 167 and 250 for Pb and Cd respectively. The relative standard deviation (R.S.D.) under optimum conditions is < 10% (n = 5). The accuracy of the method was estimated by using a reference material BDH Cat no: 456422W real samples that were spiked with the analyte ions. The extraction isotherm of both Cd and Pb ions was obtained. This study indicated that the novel foam presents a high affinity for Cd and Pb due to the presence of good extractive sites (S, N and O) which are introduced to the foam material. The bonding of the studied metal ions by the foam is useful for the removal of metal contamination from real samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The determination of heavy metals, especially some toxic metals that play important roles in biological mechanisms, has been receiving much attention. Lead enters the organism primarily via the alimentary and/or the respiratory tract. The main sources of this metal intake are food, air and drinking water [1]. The major effects of cadmium poisoning are experienced in the lungs, kidneys and bones [2]. Briefly, it is seen that these two metals can dangerously affect human health even at ultra trace concentrations. Considering the low lead and cadmium concentration levels in natural samples, sensitive analytical techniques are required to obtain adequate detection limits. The contents of these ions at low concentrations have been determined by several techniques, including flame atomic absorption spectrometry (FAAS) [3–5], electrothermal atomic absorption spectrometry (ETAAS) [6], inductively coupled plasma mass spectrometry (ICP-MS) [7], inductively coupled plasma optical emission spectrometry (ICP-OES) [8], and X-ray fluorescence (XRF) [9], among others. In order to achieve accurate, reliable and sensitive results, preconcentrations and separations are needed when the concentrations of the analyte elements in the original material or the prepared solution are too low to be determined directly. Off-line solid-phase extraction (SPE) may be used as an alternative to flow injection liquid–liquid extraction, in order to improve the sensitivity and selectivity of Flam AAS. In addition, it has some extra advantages: ⁎ Tel.: +20183877751. E-mail address: [email protected]. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.04.009
large availability, easy recovery of the solid phase, attainability of large preconcentration factors and facility for separation using various systems. The most convenient sorbent materials for column packing are, C18 [10], amberlite or Chelex [11], activated carbon [12] polyurethane foam (PUF) [13,14], polytetrafluoroethylene (PTFE) [15] or polychlorotrifluoroethylene (PCTFE) [16]. Considering the high costs of other methods in comparison with flame AAS it is recognized that FAAS should be preferred to flameless AAS due to its advantages such as fast, cheap and no need of expert operators. Flame AAS has been successfully used in connecting with the enrichment procedures for determination of Cd, Pb and other ultra trace metals in various food and water samples [17–20]. For this purpose, solid-phase extraction (SPE) [17–19,21] is importantly interested in recent years. So, chelating sorbents obtained by immobilization of organic reagents on solid supports (adsorbed or chemically bonded) have found widespread application in preconcentration and separation of trace metals from a variety of matrices [17,20]. The PUF is an excellent sorbent material due to its high available surface area and extremely low cost. In addition, it is stable in acids and bases (except concentrated nitric), it will not change its structure when heated up to 180 °C [22]. Thus, it is a very suitable material for on-line and off-line preconcentration column packing. The aim of this study is first to highlight the method of modification of polyurethane foam with 2-aminoacetylthiophenol reagent, because the sulfur-containing chemical groups act as selective ligands with high bond stability for transition metal ions [23–25]. Particularly, high improving in the sensitivity of FAAS can be obtained in connecting with the enrichment method. The second aim is to develop a new, simple, cheap and rapid separation and determination procedure for Pb and Cd
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in trace level from natural samples using the modified PUF. The great advantage of this modified foam that it can extract metal ions without prior complexation. 2. Experimental 2.1. Instrumentation Flame atomic absorption spectrometric (FAAS) measurements of Pb and Cd were recorded on AAS5 FL instrument (Carl Zeiss Technology, Germany), equipped with standard burner for air–acetylene flame. The operational conditions for the measurements are depicted in Table 1. The FAAS method was used for the determination of the studied metal ions. The pH measurements were carried out using the microprocessor pH meter BT 500 BOECO, Germany, which was calibrated against two standard buffer solutions at pH 4 and 9. A mechanical shaker with up to 200 rpm (SL 350 Nüve San. Malz. Imal. Ve Tic. A.S., Akyurt, AnkaraTurkey) was used. Doubly distilled water (DDW) was obtained from two successive distillations using Hamilton laboratory glass instrument (Europe House, Sandwich Industrial Estate, Sandwich Kent, England). 2.2. Chemicals and solutions Unless otherwise stated, all reagents used were of analytical grade, all solutions were prepared with DDW. Laboratory glassware was kept overnight in chromic acid solution. Stock solutions of the studied metal ions were prepared by dissolving appropriate amounts of analytical reagent grade Lead (II) nitrate Pb(NO3)2 obtained from Aldrich (Milwaukee, USA), Cadmium nitrate Cd(NO3)2.4H2O obtained from Panreac (Barcelona, Spain). Sodium nitrite, sodium hydroxide were received from Winlab Company, UK for reagents fine chemicals. Hydrochloric acid used was delivered from Merck (Darmstadt, Germany), commercial, open-cell, polyether-type PUF (31.6 kg m− 3, supplied by the Egyptian company for foam production, Cairo, Egypt). All liquids were used without any further purification. 2.3. Synthesis of sulfur-containing ligands modified polyurethane foams 2.3.1. Synthesis of 2-aminoacetylthiophenol (AATP) This was prepared by modifying the method reported [26].2Aminothio-phenol 1.5 g and 10 mL of absolute ethanol were added to a three-necked flask, adjusted to pH 9 with Na2CO3. Fifty milliliters of acetic anhydride was added with a dropping funnel while stirring, maintaining the reaction temperature at 50–60 °C, then the precipitate was filtered, washed with DDW and dried. The yellow solid product was obtained have mp 145–147 °C. 2.3.2. Synthesis of AATP modified polyurethane foam (AATP-PUF) Polyurethane foam, commercial open-cell polyether type was cut into similar cubes (~0.125 cm), washed by a 0.1 mol L− 1 solution of HCl, followed by doubly distilled water and acetone then left to dry at room temperature. Finally, the foam was dried in a stove at 70 °C for 1 h and stored in a dark bottle. One-gram foam cubes were soaked in a HCl (1:1) for 1 h to liberate the maximum number of free amino
Table 1 Conditions for flame atomic absorption spectrometer. Parameters
HC lamp current (mA) Slit width (nm) Wavelength (nm) Fuel flow (mL/h) Burner height (mm)
groups. Therewith, the foam was washed with water, placed into a 0.1 mol L− 1 HCl solution and then cooled in an ice bath. The foam was diazotized by the drop wise addition of 10 mL of 7 mol L− 1 of sodium nitrite to the cold solution containing the foam, and stirred vigorously until the yellow color appeared due to the formation of diazonium chloride. Then, it was left for 1 h at a temperature below 3 °C, as reported previously [23]. Next, the diazotized foam was filtered, washed with icecold water and coupled with 2-aminoacetylthiophenol 4.2 g in 100 mL ethanol and 1.0 mol L− 1 sodium acetate below 5 °C for 24 h following the method reported in [27]. The resulting brownish-orange foam was washed with DDW and air-dried. 2.4. Batch experiment Separation of Pb(II) and Cd(II) was carried out by a batch technique at 25 °C. 100 mg portion of the new PUF was mixed with 20 mL aliquot of the tested metal ion solution (1 µg mL− 1) in a shaker adjusted to the desired shaking speed. After a certain time, the solution was separated and the concentration of metal ion was determined. 2.4.1. Optimum pH of metal ion uptake Twenty milliliters of metal ion solutions containing 20 μg of Pb(II) and Cd(II) from single element aqueous solutions, were shaken with 0.1 g of foam for 1 h. The pH of the metal ion solution was adjusted before equilibration with HCl or NaOH over a range of 1–9. After the equilibration, the remained metal ions were determined by the recommended method. 2.4.2. Shaking time The effect of shaking time on the extraction efficiency of Pb(II) and Cd(II) was studied. For that purpose, 0.1 g of foam was added to 20 mL of sample containing 20 μg metal ions at the optimum pH and automatic shaking for different time intervals. The percentage of sorption (E%) was determined by using the following formula: E% = [(C0 − C)/C0] × 100. C0 and C are the initial and remaining concentrations (μg L− 1) of the metal ion, respectively. 2.4.3. Sorption capacity Determination of the sorbent capacity was carried out by the sorption of the selected metal ions from separate element aqueous solutions. Typically, 0.1 g of modified foam was equilibrated with 20–100 μg metal ion solution adjusted at the optimum pH, which was automatically shaken for 1 h at room temperature. After equilibration, the mixture was filtered and the remaining metal ion in the filtrate was determined. The sorption capacity per gram foam (Q, μg g− 1) was calculated from the equation: Q=[(C0 −C)×V]/m, where V is the sample volume in liter and m is the weight of the foam in gram. 2.5. Dynamic experiments In the dynamic experiments 1 g of the novel PUF was packed into the column (10 cm× 1 cm), with gentile pressure by a glass rod on the foam plugs onto the column filled with water to avoid air bubbles then adding glass beads on the top of foam bed to prevent foam floating upwards. The bed height of the foam column was about 60 mm. It was washed successively with DDW then stored in DDW for the next experiment. Test solutions were passed through the foam column at a flow rate of 3 mL min− 1. The stripping of the metal ions from the foam column was carried out with the suitable eluting agent and the amount of the metal ion was determined.
Metal ion Lead
Cadmium
3.0 1.2 217.0 65 5–10
2.0 1.2 228.8 50 4–12
2.5.1. Effect of flow rate on the retention and recovery The dependence of the uptake of the metal on the flow rate was studied for Pb (II) and Cd (II) at the pH chosen for maximum complexation, the sample (50 mL, 1µg mL− 1) flow rate being varied from 1 to 15 mL min− 1 and the retention percentage of the metal ion into the column was determined. The flow rates of the solutions were controlled
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by using the stopcock of the column. The flow rates of the eluent solutions were also investigated in the range 1–15 mL min− 1.
method. The experiment was repeated three times and the recovery percentage and RSD were calculated.
2.5.2. Breakthrough capacity 1000 mL samples containing Pb(II) or Cd(II) ions separately at concentrations (10 µg mL− 1), were passed through foam columns, at flow rates 3.0 mL min− 1. Each solution was adjusted to the optimum pH and the effluent was collected in 5 mL fractions where the amount of metal ions in each fraction was determined by the recommended method. The percentage of the metal in each effluent aliquot can be calculated and plotted against the volume of the effluent (Bed volume). The dynamic capacities with each metal ions could be calculated.
2.6.2. Analysis of spinach leaves The procedure was evaluated by the analysis of another real sample of spinach leaves. This sample was digested using the following procedure: a portion of 0.2 g of dry sample was precisely weighted; 4.0 mL of 1:1 (v/v) nitric acid solution was added and heated at 150 °C for 5 h [28]. After cooling at room temperature, the pH of the final digest was adjusted by suitable addition of a 10% (w/v) sodium hydroxide solution, and the mixture was finally diluted to 25 mL by doubly deionized water. The experiment was repeated three times and the recovery percentage and RSD were calculated.
2.5.3. Preconcentration Preconcentration of the studied metal ions was investigated from model solutions prior to the determination in the real samples. 25– 1000 mL Solutions containing Pb(II) and Cd(II) were passed through the column after adjusting to the optimum pH value and the flow rate. The stripping of the metal ions from the foam column was carried out by 0.1 mol L− 1 HNO3 solution and the amount of Pb(II) or Cd(II) in the eluent was determined by the recommended method. The concentration factor (CF) could be calculated from the ratio of initial volume of sample to the final volume after concentration. 2.6. Analysis of real samples 2.6.1. Real water analysis One-liter volume of tap water sample was collected from our research laboratory in Faculty of Science at Fayoum City or Qaroun lake water, Fayoum City spiked with 20 µg of standard solution of the metal ions, adjusted to pH 5 and passed through the modified foam columns at flow rate 3 mL min− 1. The columns were rinsed with 20 mL DDW and the metals were eluted by 0.1 mol L− 1 HNO3. The concentration of each metal ion was determined by the recommended
3. Results and discussion 3.1. IR analysis The proposed structure for AATP-PUF was confirmed by IR analysis. IR spectrum was obtained by grinding the dried foam with potassium bromide and then made into a pellet. The spectrum revealed that there are some changes were happened to the ligand functional groups. The most important change is the disappearance of the band at 3111– 3299 cm− 1 that is due to amino group of AATP. This indicates that the coupling occurred through the attack of the diazonium ion in the foam to the amino group of the AATP Scheme 1. The IR spectrum of the modified foam AATP-PUF was compared with that of the untreated foam. There are additional bands at 1660 (N=N) and 1711 cm− 1 (C=O), respectively. 3.2. Chemical stability of AATP-PUF The chemical stability of the modified foam was examined in acidic, basic and organic solutions. The new foam was mixed with 1–
Scheme 1. The proposed reactions involved in the preparation of the AATP-PUF.
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6 mol L− 1 HCl, 1 mol L− 1 H2SO4, 1–2 mol L− 1 NaOH, ethanol, methanol, isopropyl alcohol, n-butanol, chloroform, carbon tetrachloride and benzene, then stirred at room temperature during 24 h. The change in the degree of functionalization was calculated by the change in the metal uptake before and after the chemical treatment. It was found that there is no change in the metal uptake after stirring with the different solvents. Also, there is no leaching of the color after stirring for 24 h. This chemical stability enables the possibility of the application of AATP-PUF to different media.
exchange mechanism. With an increase in the pH, the metal ions species, mainly neutral, may be sorbed by hydrogen bonding mechanism along with ion exchange. Uptake of these metal ions from the solutions by the new solid-phase extractor can occur with the formation of surface complex with the built ligand (AATP) and the metal ions. However, the sites on the modified AATP-PUF can also contribute to the sorption process.
3.3. Effect of pH on the metal ions removal
The kinetics of sorption is important from the point of view that it controls the process efficiency. The curves are continuous and smooth suggesting a stable and uniform sorption process. The loading half time t½ defined as the time needed to reach 50% of the foam total loading capacity could be estimated from Fig. 2. From the kinetics of the studied analyte ions uptake, it is observed that an equilibrium time of about ≥ 15 min. is required for ≥ 90% uptake. However, the time of 50% sorption is ≤ 5 min for Pb and Cd. The faster uptake of these metal ions on the new modified PUF probably reflects a better accessibility of the studied analyst ions to the new chelating sites in the AATP-PUF. Also, the fact that this process is a surface phenomenon and that the surfaces are readily accessible to the ions in solution. The kinetics of the studied metal ions was subjected to Lagergren equation. The order of Cd and Pb uptake onto the PUF is evaluated by subjecting the data to the linear form of Lagergren equation [30] ln (qe − qt) = ln qe − kt where k is the first order rate constant of sorption and qt is the sorbed concentration of Cd and Pb ions at the time t, and a linear fit of ln (qe − qt) versus t is observed. This indicates that the kinetic of sorption is of the first order.
The effect of pH on the recovery of Cd and Pb by the new modified PUF is presented in Fig. 1. The pH of the aqueous solution was clearly an important parameter that controlled the uptake process. Maximum removal efficiency reached 96% for Cd and 100% for Pb at the pH ≥ 5. At pH, less than 2.0 the uptake of the studied metal ions is too small and then increases rapidly with increasing the pH value. The optimum pH value to remove the studied ions from aqueous solution is 5 Fig. 1. Based on the behavior of Cd and Pb extraction on the new PUF, we have speculated that ion exchange and hydrogen bounding may be the principal mechanism for the removal of these heavy metal ions [29]. There are a great deal of facts to support this speculation, including the components and the complexing properties of the new AATP-PUF, the properties of Cd and Pb ions and the uptake condition such as the effect of pH of the aqueous media. It has been recognized that heavy metal cations readily form complexes with oxygen, nitrogen or sulfurcontaining complexing agent built on the polymer. When these extractive sites are introduced to the PUF material, the binding ability of the modified PUF will be enhanced. Based on the electron donating nature of the oxygen, nitrogen and sulfur-containing groups in the PUF materials and the electron accepting nature of Cd and Pb ions, the ion exchange mechanism could be considered. For instance, Cd and Pb (divalent heavy metal) may attach itself two adjacent –NH group that can donate two piers of electrons to the metal ion, forming coordination compounds and releasing H+ into the solution. It is then readily understood that the equilibrium is quite dependent on the pH of the solution. At lower pH, the hydrogen ions compete with Cd and Pb ions for the exchange sites on the modified PUF, thereby partially releasing the latter. The studied metal cations are completely released under extreme acidic condition. In most cases, the percentage of metal uptake increases with increasing the pH up to certain value and then decreased with further increase of pH. Due to different properties of various heavy metal ions, the maximum uptake takes place in a slightly different pH range for different metals. In a certain pH range, for one specific heavy metal there may be a number of species present in the solution, such as M, MOH+, M(OH)2, etc. At lower pH values, the positive charged metal species may compete with the hydrogen ions and may be sorbed at the surface of AATP-PUF by ion
Fig. 1. Effect of pH on the recovery of Pb (II) and Cd (II) with AATP-PUF. The experiment was triplicated.
3.4. Effect of shaking time
3.5. Adsorption isotherm The relationship between the amount of a substance extracted at a constant temperature and its concentration in the equilibrium solution is called the adsorption isotherm Fig 3. The capacity of AATP-PUF was found to be 10,750 and 10,000 µg g− 1 for Pb and Cd respectively. This confirms the new modified foam has greater affinity against these metal ions. 3.6. Effect of the sample volume In order to explore the possibility of enriching low concentrations of analyte from large volume by the batch procedure, the effect of sample volume on the recovery of Pb and Cd ions was also investigated. For this purpose, 25–200 mL of sample solutions containing 20 µg of Pb or Cd was used at optimum conditions Fig 4. It was found that quantitative recovery (>95%) was obtained for Pb and Cd up to a sample volume of
Fig. 2. Loading rate of Pb (II) and Cd (II) with AATP-PUF. The experiment was triplicated.
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Fig. 3. Extraction isotherms of Pb (II) and Cd (II) with AATP-PUF. The experiment was triplicated.
Fig. 5. Breakthrough capacity of Pb (II) and Cd (II) with AATP-PUF. The experiment was triplicated.
100 mL. In this experiment, 100 mL of sample solution was adopted for the preconcentration of Pb or Cd ions from water samples, the extracted Pb or Cd ions can be eluted with 3 mL 0.1 mol L− 1 HNO3, so an enrichment factor of 33.3 is achieved by this method.
metal ions combine strongly with the new foam also Cd and Pb bond to the AATP-PUF are quite stable. Therefore, the sample and eluent flow rates are important parameters to obtain quantitative retention and elution of analyte ion. In consequence, a sample flow rate of 3 mL min− 1 was adopted as optimum for retention and elution.
4. Column procedure 4.1. Breakthrough curve The capacity of AATP-PUF column containing one gram of the foam could be determined from the curves obtained by plotting the amount of metal ion emerged from the column versus the volume of effluent collected Fig 5. AATP-PUF column is saturated with Pb or Cd under the optimum conditions. The capacity the AATP-PUF column for Pb and Cd are estimated to be 7000 and 6200 mg g− 1 respectively. The obtained dynamic capacity is smaller than the batch capacity this is because of the breakthrough capacity is not the total capacity; it is the working capacity of the column under the given conditions. 4.2. Effect of sample loading and elution The sample flow rate through the mini column is a very important parameter since it controls the time of analysis. The elution or loading of the tested analytes from AATP-PUF was examined at different flow rates between 1.0 and 15 mL min− 1. The studies show that the flow rate has more influence on the recovery of the metal ions. The foam at a flow rate of 3 mL min− 1 can extract the studded metal ions quantitatively. According to experimental results it was found that the recovery of Cd and Pd is 92%, at flow rate 7 mL min− 1 as these
4.3. Elution The effect of different solutions on the elution of the studied analyte ions on AATP-PUF was examined. The analyte ions sorbed quantitatively on the new modified PUF could be eluted with 0.1 mol L− 1 HNO3. On the contrary, to many other papers, it was not necessary to extract the analyte ions with HNO3 in acetone, where the eluent was evaporated to near dryness at 35 °C and diluted with HNO3 before the determination by AAS [17]. 4.4. Limit of detection The limit of detection (LOD) is defined as blank concentration + 3σ where σ is the standard deviation of blank determination. The proposed method for the determination of the investigated ions was studied under the optimal experimental conditions by applying the procedure for blank solutions. The detection limits were established by analyzing 17 blank solutions. The LOD of Pb and Cd were found to be 0.066 and 0.048µgL− 1 respectively. The LOD values for the studied ions with the new extractor enable the use of this material in collection of Pb and Cd ions at a trace concentration prior to their determination with high accuracy. 5. Analytical application of AATP-PUF 5.1. Preconcentration The preconcentration factor was calculated from the ratio of the initial volume to the final volume. The results are recorded in Table 2. Of all the preconcentration methods, chelating extractor recovery method is one of the effective multi element preconcentration methods, because it can provide more flexible working conditions together with good stability, selectivity, high concentration ability, high capacity of metal
Table 2 Preconcentration factor on AATP-PUF.
Fig. 4. Effect of sample volume on the recovery of Pb and Cd with AATP-PUF. The experiment was triplicated.
Foam type
Metal ion
Initial volume mL
Final volume mL
Recovery %
CF
ATP-PUF
Pb (II) Cd (II)
1000 1000
6 4
96 96.5
167 2.4 250 3.2
CF: concentration factor; RSD: relative standard deviation.
RSD %
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ions and simple operation [3,4]. However, at the same time, its demands for trace element determination and concentration using the new modified extractors are ever increasing. This is because such functional polymers can be employed for the preconcentration in water systems [5,6]. If the analyte concentration in the sample is too low to be detected by flame AAS, a preconcentration step is necessary. This was achieved by increasing the ratio of the sample to eluent volumes as needed. The recoveries and the preconcentration factors achieved at the lowest concentration limit below which the recoveries become non-quantitative are listed in Table 2. Twenty micrograms of the analyte ions in different sample volumes 25–1000 mL were allowed to pass through the columns at flow rate of 3 mL min− 1. The elution of the studied ions from AATP-PUF column occurred with recovery percentages of 96– 96.5% in final solutions between 4 and 6 mL with preconcentration factors 167–250 and RSD less than 10% which indicates adequate accuracy and good precision of the developed method. These results show that Pb and Cd can be concentrated effectively from the dilute aqueous solution using AATP-PUF columns.
5.2. Recycling of the sorbent To test the recyclability of the new modified foam one gram of the foam was used to extract the analyte ions through extraction striping cycle's column operation. The stripping agent used in this experiment was 0.1 mol L− 1 HNO3. The results of the recyclability studies indicated that the modified foam was found to be able to remove more than 96% ± 1% of the analyte ions from solution through more than 17 extraction cycles. The repeated use of the sorbent is likely to be a key factor in improving wastewater process economics. Various factors are probably involved in determining rates of Pb and Cd desorption, such as the extent of hydration of the heavy metal ions and sorbent structure. However, an important factor appears to be the binding strength. In this work, the extraction time was found to be <4 min with 4–6 mL 0.1 mol L− 1 HNO3. Therefore, the modified foam showed excellent reusability and stability towards the studied analyte ions. These results show that the AATP-PUF has enough stability to be reused at least 17 times, and it is a good chelating extractor for preconcentration of the studied analyte ions. Based on it, a new system for the determination of these metal ions by flam AAS after their preconcentration on a short column packed with AATP-PUF is established.
5.3. Validation of the method A reference material (Quality control standard #1, BDH Cat no: 456422 W) was used for the method validation. As seen in Table 3, the results were compared with the certified values. Good agreement was obtained between the estimated content by the proposed method and the certified values for Pb and Cd.
5.4. Determination of Pb and Cd in natural samples 5.4.1. In natural water The proposed column method was applied for the separation and determination of analyte ions in tap water and lake water samples. Lake or tap water sample were adjusted to the optimum conditions. The standard addition method was applied to check the selectivity of AATP-PUF for the analyte ions against matrix elements and to determine the accuracy and the precision of the new extraction method. The results are listed in Table 3, the water samples were spiked with 20 µg L− 1 for each of the studied metal ions then treated by AATP-PUF and then desorbed by 0.1 mol L− 1 HNO3. The recoveries are all in the range of 96–104% with R.S.D less than 10%, demonstrating that the interference species in the matrix were eliminated satisfactory after the extraction and preconcentration procedure by the column method. The results also indicate the suitability of the AATP-PUF for the preconcentration and quantitative recovery of the studied ions from natural water samples. 5.4.2. Determination of cadmium and lead in spinach leaves The proposed analytical procedure was applied to the determination of Cd and Pb in spinach leaves. The results are described in Table 3. Recoveries of spike were quantitative. The results indicate that the proposed method can be reliably used for the determination of these analyte ions in natural sample. 6. Comparison with other preconcentrating extractors A comparison of the proposed system with other preconcentration procedures is given in Table 4. Some of the parameters obtained were comparable to those presented by other methods described in the literature. As seen from the data in Table 4, the proposed method developed by using AATP-PUF system has high preconcentration factor when compared to other methods reported in Table 4. 7. Conclusion 2-Aminoacetylthiophenol modified polyurethane foam was successfully synthesized by coupling the foam through 2-aminoacetylthiophenol. The prepared sorbent showed several good characteristics, such as fast extraction kinetics, low price, low LOD and proper selectivity for Cd and Pb. This modified foam can be potentially used as a solid-phase extraction material for the selective preconcentration, separation and determination, when coupled with flame AAS, of trace Cd and Pb in environmental samples. The new extractor has good chemical and physical stability feature, rapid equilibration in the recovery of the studied analyte ions and can be used many times without losing its uptake capacity. This extractor expected to be useful material for the extraction and preconcentration of Pb and Cd ions.
Table 3 Addition–recovery tests in the experiments for Pb (II) and Cd (II) in different real samples. Sorbent
Sample
Metal ion
Added µg L− 1 or µg g− 1
Found µg L− 1 or µg g− 1
Recovery %
RSDa %
Materials sources
AAT-PUF
Quality Control Standard #1, No: 456422W Tap water
Pb (II) Cd (II) Pb (II) Cd (II) Pb (II) Cd (II) Pb (II) Cd (II)
20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0
19.89 20.1 20.1 19.2 20.8 19.4 19.1 19.2
99.45 100.5 100.5 96.0 104.0 97.2 95.5 96.0
2.4 4.8 2.1 5.1 4.5 3.9 6.2 5.3
No:456422W
Lake water Spinach leaves a
Based on three replicate measurements in the same loaded column.
Faculty of Science, Fayoum City Qaroun Lake water Fayoum City
N. Burham / Desalination 249 (2009) 1199–1205
1205
Table 4 Comparative data from some recent studies for preconcentration of trace metals. System
Studied metals
pH
Eluent
Flow rate mL min− 1
CF
LOD µg L− 1
Reference
Diaion SP-850/Alpha-benzoin oxime Amberlite XAD-2000/8-hydroxyquinoline Dowex Optipore V-493/dibenzyldithiocarbamate Amberlite XAD-2010/DDTC Polyurethane foam/acetylacetone Polyurethane foam/2-aminoacetylthiophenol (AATP-PUF)
Cd, Pb Cd, Pb Cd, Pb Cd, Pb Cd, Pb Cd, Pb
8.0 6.0 2.0 6.0 6.0 5.0
1 mol L− 1 HNO3 1 mol L− 1 HNO3 in acetone 1 mol L− 1 HNO3 in acetone 1 mol L− 1 HNO3 in acetone 0.1 mol L− 1 HNO3 0.1 mol L− 1 HNO3
5.0 10.0 4.0 10.0 3.0 3.0
50 100 50 100 288,224 250,167
0.28–0.73 0.3–2.2 0.43–0.65 0.08–0.26 0.07–0.09 0.048–0.066
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