Determination of copper, nickel, manganese and cadmium ions in aqueous samples by flame atomic absorption spectrometry after simultaneous coprecipitation with Co(OH)₂

2.1 Instrumentation

A SensAA GBC atomic absorption spectrometer (Dandenong, Australia) equipped with deuterium background correction and air–acetylene burner was used for absorbance measurements. Copper, nickel, manganese and cadmium hollow cathode lamps were used as light source at wavelengths of 324.8, 232.0, 279.5 and 228.8 nm, respectively. The instrumental parameters are listed in Table 1. The acetylene flow rate and burner height were adjusted in order to obtain the maximum absorbance signal, while aspirating the analyte solution. A Metrohm 692 pH meter (Herisau, Switzerland) was used for pH measurements. A Centurion scientific centrifuge model 1020 D.E. (West Sussex, United Kingdom) was used to accelerate the phase separation.

2.2 Reagents and solutions

Deionized water was used throughout the experimental work. Standard solutions of copper (0.5–200.0 ng mL⁻¹), nickel (0.5–200.0 ng mL⁻¹), manganese (0.5–250.0 ng mL⁻¹) and cadmium (0.3–80.0 ng mL⁻¹) were prepared by diluting stock solutions of 1000.0 mg L⁻¹ of these ions supplied by Merck (Darmstadt, Germany). The working reference solutions were prepared daily by stepwise dilution from stock solutions with deionized water. For preparation of blank solution 50 mL distilled water was given and the pH was adjusted to 11 by NaOH solution. The laboratory glassware was soaked overnight in a 1.4 mol L⁻¹ HNO₃ solution. Before using, all of the glasswares were washed with deionized water and dried. Solutions of alkali metal salts (1% w/v) and various metal nitrate and acetate salts (0.1% w/v) were used to study the interference of anions and cations, respectively. HNO₃ and NaOH solutions were used for pH adjustments.

2.3 Coprecipitation procedure

The coprecipitation procedure was optimized with 50 mL of an aqueous solution containing 5.0 ng mL⁻¹ of Cu(II), Ni(II), Mn(II) and 1.0 ng mL⁻¹ of Cd(II) prior to its application to the real samples. First, the pH of solution was adjusted to 11.0 by NaOH solution. Then 1.25 mL of 1000 mg L⁻¹ of Co(II) was poured into the centrifuge tube. After 10 min, the mixture was centrifuged at 4000 rpm for 5 min. The supernatant was removed and the precipitate was dissolved with 2.0 mL of 4 mol L⁻¹ HNO₃. The final solution was aspirated directly into the FAAS for determination of the analyte ions.

2.4 Preparation of real samples

Two certified reference materials (CRMs) supplied by the National Institute for Environment Studies (NIES) No. 1 Pepperbush and NIES No. 7 Tea Leaves have been analyzed. Approximately 0.5 g of NIES No. 1 and 1.0 g of NIES No. 7 were weighted accurately into two Teflon cup (individually) and dissolved in concentrated nitric acid (∼10 mL) with heating on a water bath at 80 °C for 120 min. The solution was cooled and 50 mL deionized water was added to it and filtered by using cellulose membrane of 0.45 μm pore size (Millipore). The filtrate (approximately 60 mL) was made to 100.0 mL with deionized water in a calibrated flask. An aliquot (50 mL) of the sample solution was taken individually, and the analyte ions were determined by the coprecipitation procedure.

Water samples were collected in acid leached polyethylene bottles. The only pretreatment was acidification to pH 2 with nitric acid, which was performed immediately after collection, in order to prevent adsorption of the metal ions on the flask walls. The samples were filtered before analyses through a cellulose membrane of 0.45 μm pore size (Millipore).

3.1 Effects of pH

The pH of the sample solutions was adjusted in a range of 7–12 by using diluted NaOH and the coprecipitation procedure was applied to this solutions. As can be seen in Fig. 1, the absorbance of the analyte ions increased with increase in pH and the maximum absorbance for copper, nickel and manganese obtained at the pH range 10.5–12, whereas the maximum absorbance for cadmium was obtained at the pH range 10.5–11.5. After pH 11.5, the absorbance of Cd(II) decreased with increasing of pH. Therefore, pH 11 was selected as optimum pH in all subsequent works for separation and preconcentration of the analyte ions.

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Figure 1 Effect of pH on the coprecipitation procedure, Conditions: sample volume, 50.0 mL, 5.0 ng mL⁻¹ of Cu(II), Ni(II), Mn(II) and 1.0 ng mL⁻¹ of Cd(II). Carrier element (Co(II)), 1.25 ml of 1000 mg L⁻¹; standing time, 10 min; centrifugation time, 5 min at 4000 rpm; final solution, 2.0 mL of 4 mol L⁻¹ HNO₃.

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3.2 Effect of amount of carrier element

The influence of amount of carrier element is an essential consideration for efficient extraction in coprecipitation procedure. For investigating the influence of amount of Co²⁺ as a carrier element on the coprecipitation procedure performance, several experiments were performed by adding varying amounts of Co²⁺ from 0.0 to 3.0 mg. The rest of the experimental conditions were kept constant. The results showed that, the absorbance of the analyte ions increased and reach to quantitative value for the analyte ions at the range of 1.0–3.0 mg. Therefore, 1.25 mg Co²⁺ was selected in all subsequent works.

3.3 Effect of standing time, centrifugation rate and time

The standing time, centrifugation rate and time were also examined. For investigating the influence of the standing time, 1.25 mL of Co²⁺ 1000.0 mg L⁻¹ was added into 50 mL of an aqueous solution containing 5.0 ng mL⁻¹ of Cu(II), Ni(II), Mn(II) and 1.0 ng mL⁻¹ of Cd(II). Then, the standing time was investigated in the range of 0–20 min. The results showed that after 8 min, quantitative recoveries were obtained. Therefore, 10 min was selected as optimum standing time for all subsequent experiments. The effect of the centrifugation speed on the extraction of the analyte ions was investigated in the range of 1000–5000 rpm for 5 min. The results showed that, the maximum extraction was obtained for the analyte ions at 4000 rpm. Therefore, all further studies were performed at 4000 rpm.

The effect of centrifugation time on the extraction of the analyte ions was tested in the range of 2–15 min at 4000 rpm. The results showed that maximum extraction was obtained for the analyte ions in the range of 4–15 min. Therefore, 5 min was selected as optimum centrifugation time for all subsequent experiments.

3.4 Effect of sample volume

The effect of sample volume is a very important parameter in coprecipitation procedure (Sun, 1993; Uluozlu et al., 2009). To this purpose, the influence of the sample volume on the coprecipitation efficiency of the analyte ions was examined. In order to explore the possibility of enriching analytes from natural water with a high preconcentration factor, a series of sample solutions, containing 250.0 ng of Cu(II), Ni(II), Mn(II) and Cd(IfI) were studied in the volume range 20–600 mL at the optimum conditions. The results showed that the quantitative extractions were obtained for the analyte ions in the range of 20–500 mL. Therefore, 25 mL was selected as sample volume for all subsequent experiments. With respect to the final elution volume of 2.0 mL of HNO₃ and a breakthrough volume of 500 mL, a preconcentration factor of 250 was achievable.

3.5 Effect of foreign ion

Matrix effect is an important problem in the determination of heavy metals in the real samples. The effect of some important diver’s ions on the absorbance of the analyte ions was also examined. For this study, 50 mL of an aqueous solution containing 5.0 ng mL⁻¹ of Cu(II), Ni(II), Mn(II) and 1.0 ng mL⁻¹ of Cd(II) was taken and different amounts of foreign ions were added to it and coprecipitation procedure was followed. Tolerable limit was defined as the highest amount of foreign ions that produced an error not exceeding ±5% in the determination of Cu(II), Ni(II), Mn(II) and Cd(II) ions. The results are given in Table 2. The results showed that large numbers of anions and cations used have no considerable effect on the determination of Cu(II), Ni(II), Mn(II) and Cd(II) ions.

3.6 Calibration, precision and detection limits

Calibration curves were obtained by coprecipitation 50.0 mL of a sample solution containing of known amounts of four analyte ions under the optimized conditions. Enrichment factor was calculated as the ratio of the slope of calibration curve after and before of microextraction. The enrichment factors for Cu(II), Ni(II), Mn(II) and Cd(II) ions were 24.7, 22.7, 23.9 and 24.4, respectively. The reproducibility and the repeatability were evaluated with 50 mL of an aqueous solution containing 5.0 ng mL⁻¹ of Cu(II), Ni(II), Mn(II) and 1.0 ng mL⁻¹ of Cd(II). The repeatability (intra-day) and reproducibility (inter-day) of the method were evaluated by carrying out seven replicate extraction and determination of the analyte ions during a day (intra-day) and seven replicates at seven subsequent days (inter-day). The results were reported in Table 3. The detection limits based on 3S_b/m for Cu(II), Ni(II), Mn(II) and Cd(II) in the original solution were 0.2, 0.2, 0.3 and 0.07 ng mL⁻¹, respectively. Analytical parameters of the method are given in Table 3.

3.7 Accuracy of the method

The accuracy and applicability of the proposed coprecipitation procedure has been applied to the determination of Cu(II), Ni(II), Mn(II) and Cd(II) ions in National Institute for Environment Studies (NIES) No. 1 Pepperbush and NIES No. 7 Tea Leaves. Results are given in Table 4. It was found that there is no significant difference between results obtained by the coprecipitation procedure and the certified results. These results indicate the applicability of the coprecipitation procedure for copper, nickel, manganese and cadmium determination is free of interference.

3.8 Applications

The proposed coprecipitation procedure has been also applied to different natural waters for the enrichment and separation of the analyte ions. The results are given in Table 5. The utility of the coprecipitation procedure was also evaluated by examining the recoveries of the analyte ions from water samples spiked with the standard analytes. According to this Table, the added Cu(II), Ni(II), Mn(II) and Cd(II) ions can be quantitatively recovered from the water samples by the coprecipitation procedure. These results demonstrate the applicability of the procedure for copper, nickel, manganese and cadmium ions determination in water samples. The recovery of copper, nickel, manganese and cadmium added to the samples demonstrates the efficiency of the proposed procedure.

3.9 Comparison of the proposed method with the other methods

A comparison of the proposed coprecipitation procedure with the other reported preconcentration methods (Saracoglu et al., 2003; Uluozlu et al., 2010; Doner and Ege, 2005; Soylak and Onal, 2006; Ghaedi et al., 2008) for the copper, nickel, manganese and cadmium extraction from water samples are given in Table 6. The obtained detection limits, RSD% and linear range by the coprecipitation procedure are comparable or better than most of those reported in the literature.