Separation/preconcentration of trace Pb(II) and Cd(II) with 2-mercaptobenzothiazole impregnated Amberlite XAD-1180 resin and their determination by flame atomic absorption spectrometry

2.1 Apparatus

A PerkinElmer model AAnalyst 800 flame atomic absorption spectrometer equipped with an air-acetylene burner was used for the analysis under the conditions suggested by the manufacturer. The operating conditions for Pb(II) and Cd(II) were as follows: wavelength, 283.3 and 228.8 nm; bandwidth, 0.7 and 0.7 nm, lamp current, 12 and 12 mA; acetylene and air flow rates, 2.0 and 17 L min⁻¹, respectively. All pH measurements were made with a Consort C533 model digital pH meter equipped with a combined pH electrode. A Fisons SGP-202–010J model mixer was used to prepare the impregnated resin and a Clifton model shaker was used for batch experiments.

2.2 Reagents

All chemicals were of analytical reagent grade. Distilled–deionized water was used in all experiments. Pb(II) and Cd(II) stock solutions (1000 mg L⁻¹) were prepared from the nitrate salts of metals in acidic medium. The working solutions of metals were obtained by diluting these stock solutions prior to use. The calibration curve was prepared using the standard solutions in 1 mol L⁻¹ HNO₃ by dilution from the stock solutions. 2-Mercaptobenzothiazole (MBT) solution of 0.5% (w/v) was prepared by dissolving 0.50 g of MBT (Rohm & Hass, Aldrich) in 100 mL of ethyl alcohol. The acetic acid/acetate buffer of pH 4–6 and ammonia/ammonium chloride buffer of pH 8–10 were used. Non-ionic Amberlite XAD-1180 resin (Acros Organics, NJ, USA) is a polystyrene divinylbenzene copolymer (surface area 500 m² g⁻¹, average porosity 400 Å and average diameter 530 μm). It was washed with 1 mol L⁻¹ hydrochloric acid and ethanol, respectively, and then rinsed with water until obtaining a neutral solution prior to use. It was dried at 105 °C in an oven before use. A glass column (10 cm long, 1 cm i.d.), with a reservoir of 200 mL on the top and a porous disk made from sintered glass just placed above the stopcock at the bottom, was used.

2.3 Preparation of MBT impregnated Amberlite XAD-1180 resin

15 mL of 0.5% (w/v) MBT solution and 0.5 g of AXAD-1180 resin were mixed in a mixer for 25 min. Then the prepared mixture was placed into the glass column (10 cm long, 1 cm i.d.) and the fluidic portion of the mixture was allowed to pass through the column at a flow rate of 1 mL min⁻¹. The flow rate was controlled by adjusting the stopcock, because this could be easily obtained gravitationally. The impregnated resin was rinsed with distilled water and conditioned with 10–15 mL of buffer solution (pH 9.5) prior to passage of the sample solution.

2.4 General preconcentration procedure

The column method was tested with model solutions prior to its application to the natural samples. The pH of model solutions (50 mL) including 20 μg of Pb(II) and 2.5 μg of Cd(II) was adjusted to 9.5 using ammonia/ammonium chloride buffer solution. The column was preconditioned by passing the buffer solution with pH 9.5 through the column and then the model solutions were passed through the column at a flow rate of 1 mL min⁻¹. The metal complexes retained on the resin were eluted with 20 mL of 2 mol L⁻¹ HNO₃ solution. The eluate was evaporated near to dryness on a hot plate. The residue was diluted to 5 mL with 1 mol L⁻¹ HNO₃ solution. The concentration of the metal ions in the final solution was determined by FAAS.

2.5 Analysis and preparation of samples

The sea water sample was taken from Aegean Sea, İzmir, Turkey and immediately filtered through a Millipore cellulose membrane filter (0.45 μm pore size), acidified to pH 2 with HNO₃, and stored in precleaned polyethylene bottles. Tap water samples taken from our research laboratory were used without pretreatment. The untreated waste water samples were taken from the Organized Industry District of Kayseri. They were acidified to pH 2 with HNO₃ and filtered through a blue ribbon filter paper. To oxidize organic matter, 50 mL of concentrated HNO₃ (65%, w/w) was added to 500 mL of water sample and heated for 2 h. After cooling, 50 mL of concentrated H₂O₂ (35%, w/w) was added to this sample solution and evaporated to about 10–15 mL of volume. The solution was filtered through a Millipore cellulose membrane filter (0.45 μm pore size) with 5–6 mL of hot 1 mol L⁻¹ HNO₃ and then diluted to 500 mL with distilled–deionized water. Aliquots of 25 mL of the sample solutions were analyzed by using the recommended procedure.

A 100-mg of the reference material (RM 8704 Buffalo river sediment, dried at 110 °C for 2 h) and sediment samples taken from the three main streets of Kayseri (Turkey) were dissolved in a beaker by adding 10 mL of aqua regia. The mixture was heated on a hot plate until to almost dryness. Then, 10 mL of aqua regia was added again to the residue and the mixture was evaporated to dryness. After dissolving the residue with about 10 mL of 1 mol L⁻¹ HNO₃, the solution was filtered through a filter paper with blue ribbon by using some hot 1 mol L⁻¹ HNO₃ for the separation of insoluble materials. 9 mL of 1 mol L⁻¹ NaF was added to the filtrate in order to prevent the precipitation of Fe(III) and Al(III), in hydroxide forms, found in the sample in excess amounts. So, Fe(III) and Al(III) ions have been masked by F⁻ in the solution as ${\text{FeF}}_6^{3-}$ and ${\text{AlF}}_6^{3-}$ anionic complexes. The pH of this solution was adjusted to pH 9.5 by adding NH₃/NH₄Cl buffer. The total volume was made up to 50 mL with distilled–deionized water. The column procedure described above was applied to separate and preconcentrate the metal ions from the sample solution.

3.1 Effect of pH

One of the most significant parameters, actually the first, is to investigate the effect of pH of sample solution during the optimization studies of a method. The effect of pH on the preconcentration of metal ions onto MBT coated Amberlite XAD-1180 was investigated by passing the model solution (50 mL) containing 20 μg of Pb(II) and 2.5 μg of Cd(II) adjusted at pH 9.5 through the column. The retained metal ions were eluted with 20 mL of 2 mol L⁻¹ HNO₃. The eluate was evaporated near to dryness on a hot plate and it was diluted to 5 mL with 1 mol L⁻¹ HNO₃ solution. The concentration of the metal ions in the final solution was determined by FAAS. The effect of pH on the preconcentration of metal ions is shown in Fig. 2. Quantitative recovery values (⩾95%) were found in the pH ranges 9–10 for Pb(II) and 8–10 for Cd(II). Hence, the pH 9.5 was selected for further studies.

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Figure 2

Effect of pH on the recovery of Pb(II) and Cd(II) on a column filled with the impregnated resin (sample volume: 50 mL, eluent: 20 mL of 2 mol L⁻¹ HNO₃).

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3.2 Elution

In order to obtain the quantitative recovery of retained metal ions on MBT coated Amberlite XAD-1180, the different type (HCl and HNO₃), concentrations (1–3 mol L⁻¹) and volumes (5, 10 and 20 mL) of eluents were examined. During these studies, 50 mL aliquots of the sample solutions containing 20 μg of Pb(II) and 2.5 μg of Cd(II) were used at pH 9.5. Quantitative recovery values were obtained by using 20 mL of 2 mol L⁻¹ nitric acid solution. The recoveries for the other eluting solutions were in the range of 52–83%. The effect of volume of 2 mol L⁻¹ nitric acid solution was also studied. In the present investigation, 20 mL of 2 mol L⁻¹ nitric acid solution was the best and thus selected as eluent for the elution of Pb(II) and Cd(II).

3.3 Effect of sample volume

The effect of sample volume on the retention of metal ions was studied. For this purpose, 50–150 mL of the model solutions adjusted to pH 9.5 and containing 20 μg of Pb(II) and 2.5 μg of Cd(II) was preconcentrated by passing through the column. To elute the metal ions retained on the column, 20 mL of 2 mol L⁻¹ HNO₃ was used. As shown in Fig. 3, the recoveries of Pb(II) and Cd(II) were quantitative for the sample volumes up to 100 mL. When the final measurement volume is 5 mL, the preconcentration factor is 20. The decreased recovery with increasing sample volumes may be due to the low concentration of metal ions in the solution and/or the elution of the analytes by the sample solution itself.

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Figure 3

Effect of sample volume on the recovery of Pb(II) and Cd(II).

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3.4 Effect of coexisting ions

The effects of coexisting ions such as sodium, potassium, calcium, magnesium, chloride and sulfate usually found in natural samples were examined using model solutions containing 20 μg of Pb(II) and 2.5 μg of Cd(II) and excess matrix ions in 50 mL of model solutions. The experimental results are given in Table 1. The results show that the presence of high concentrations of major cations and anions has no obvious effect on the studied metal ion recoveries.

3.5 Precision and detection limit of the method

The precision of the method was determined by performing successive ten retention and elution cycles followed by FAAS. It was found that the recoveries (R% ± s) of Pb(II) and Cd(II) were 97.8 ± 2.4, and 99.0 ± 2.6, respectively. In order to determine the detection limit (DL, n = 20) of the proposed method, a 50-mL of blank solution was passed through the column under the optimal experimental conditions and the retained metal ions were eluted. The DLs obtained as concentration corresponding to three times the standard deviation of the blank solution were 5.0 μg L⁻¹ for Pb(II) and 0.35 μg L⁻¹ for Cd(II). In the calculation of the DLs of the method, the preconcentration factor (PF) of 20 was taken into consideration. The limit of quantification (LOQ, 10 s) values were found to be 16.5 μg L⁻¹ for Pb(II) and 1.2 μg L⁻¹ for Cd(II). The calibration graphs for Pb(II) and Cd(II) were found to be A = 0.0017 + 0.008 C in the interval of 1–12 μg mL⁻¹ (r = 0.9992) and A = 0.0032 + 0.152 C in the interval of 0.1–2.0 μg mL⁻¹ (r = 0.9995), respectively. The calibration data were obtained without application of the preconcentration procedure for FAAS.

3.6 Accuracy of the proposed method and application to real samples

The accuracy of the proposed method was evaluated by determination of the metal ions in the standard reference material, RM 8704-Buffalo river sediment (Table 2) and recovery studies in the water samples (Table 3). The results in Table 2 show that there is no significant difference between the results obtained by the proposed method and certified values. The accuracy of the method was also checked by measuring the recoveries for Pb(II) and Cd(II) in the spiked water samples. As can be seen from Table 3, the recovery values changing from 90% to 105% demonstrate that the described method can be applied to the analysis of the water samples without interference. The Pb(II) and Cd(II) results ( $\overline{x}\pm s$ ) obtained from the analysis of the three main street sediment samples (n = 3) were found to be 274 ± 4 and 5.55 ± 0.54, 258 ± 5 and 4.30 ± 0.93, 241 ± 6 and 5.34 ± 0.95 μg g⁻¹, respectively.