A sensitive and simple cloud point extraction method was developed for simultaneous separation and preconcentration of cadmium and copper prior to their determination by flame atomic absorption spectrometry. 4-phenyl-3-thiosemicarbazide was used as complexing agent and the cadmium and copper complexes were extracted from the aqueous phase by Triton X-114 surfactant. The effects of parameters such as sample pH, ligand amount, concentration of surfactant, incubation temperature and time were optimized. For 20 mL of preconcentrated solution, the detection limits (3σ) were 0.20 and 0.49 μg L−1, and the enrichment factors were 20.7 and 19.9 for Cd(II) and Cu(II), respectively. In order to verify the accuracy of the developed method, certified reference materials (SLRS-5 river water and SPS-SW2 Batch 127 surface water) were analysed. Results obtained were in good agreement with the certified values. The proposed method was applied to tap water, river water and seawater samples with satisfactory results.

INTRODUCTION

Heavy metals are considered to be one of the main sources of pollution in the environment since they have a significant effect on its ecological quality (Tavallali et al. 2013). Cadmium is one of the most toxic elements among the heavy metals (Ning et al. 2014). It causes different damage and defects in lungs, kidneys and bones. Cadmium, with its high half-life time from 10 to 33 years, can accumulate in liver and kidneys (Ensafi et al. 2015). Its wide technological use as well as its production from burning oil and coal and incineration of waste causes an extensive anthropogenic contamination of soil, air and water (Tavallali et al. 2013). Although copper is one of the most essential elements in the body and plays an important role in many body functions, accumulation and an excess amount of it can directly affect the liver and nervous system which can lead to death (Baroumand et al. 2015). Excessive intake of copper would lead to accumulation of the metal in liver cells and haemolytic crisis, jaundice and neurological disturbances (Wen et al. 2011). These heavy metals may enter the food chain, accumulate in plants and animals, and may cause damage to human health. For these reasons cadmium and copper determination in water and biological matrices is a good tool for environmental and toxicological monitoring.

Several techniques, including flame atomic absorption spectrometry (FAAS) (Chen & Teo 2001; Tavallali et al. 2013; Ning et al. 2014; Baroumand et al. 2015; Naeemullah et al. 2016), electrothermal atomic absorption spectrometry (ETAAS) (Lopez-Garcia et al. 2014; Falahnejad et al. 2016), inductively coupled plasma optical emission spectrometry (ICP-OES) (Silva et al. 2009; Zhao et al. 2012), spectrophotometry (Wen et al. 2011; Yang et al. 2015), inductively coupled plasma mass spectrometry (ICP-MS) (Wang et al. 2015) and voltammetry (Abbasi et al. 2011; Ensafi et al. 2015) have been used for the determination of Cu(II) and Cd(II) in various types of sample. Among these techniques, FAAS has been widely used for the determination of heavy metals because of its low cost, ease of operation, high sample throughput and good selectivity (Wu et al. 2011). However, direct determination of heavy metal ions is generally difficult due to various factors, in particular their low concentrations and matrix effects. In order to solve these problems, separation, preconcentration and matrix elimination is usually required. Various preconcentration methods such as liquid–liquid extraction (LLE) (Karadaş & Kara 2014), solid phase extraction (SPE) (Moghadam Zadeh et al. 2015; Sheikhshoaie et al. 2015), co-precipitation (Prasad et al. 2006), cloud point extraction (CPE) (Chen & Teo 2001; Tavallali et al. 2013; Ning et al. 2014; Naeemullah et al. 2016), dispersive liquid–liquid microextraction (DLLME) (Wen et al. 2011; Lopez-Garcia et al. 2014), dispersive liquid–liquid microextraction based on solidification of floating organic drop (DLLME-SFO) (Wu et al. 2011) and ionic liquid-based single step microextraction (Khan et al. 2015) have been used for the separation and preconcentration of Cu(II) and Cd(II) from environmental matrices.

CPE is an attractive method that reduces the consumption of and exposure to solvent, disposal costs and extraction time (Golbedaghi et al. 2012). This extraction method is based on the fact that most non-ionic surfactants in aqueous solutions form micelles and become turbid when heated to the cloud point temperature or in the presence of an electrolyte. Above the cloud point, the micellar solution separates into a surfactant-rich phase with a small volume and a diluted aqueous phase (Rezende et al. 2011; Zhao et al. 2012).

In this work, a new CPE method was developed for the preconcentration of Cu(II) and Cd(II) prior to FAAS determination. The reagent 4-phenyl-3-thiosemicarbazide was used as a chelating ligand. Triton X-114 was chosen as the non-ionic surfactant for the work because of its low cloud point temperature and the high density of the surfactant-rich phase as well as its low cost, commercial availability and lower toxicity. Some parameters that influence the extraction efficiency such as sample pH, ligand amount, concentration of surfactant, effect of salt addition and incubation temperature and time were investigated and optimized. The proposed method was then applied to the determination of Cu(II) and Cd(II) in tap water, river water and seawater samples. The accuracy of the developed method was verified by analysing certified reference materials (SLRS-5 river water and SPS-SW2 Batch 127 surface water).

EXPERIMENTAL

Instruments

A Perkin Elmer model AAnalyst 200 (Shelton, CT, USA) flame atomic absorption spectrometer equipped with deuterium background correction and appropriate hollow cathode lamps and an air–acetylene flame (air and acetylene flow rate 10 L min−1 and 2.3 L min−1, respectively) was used for determination of Cu(II) and Cd(II). The most sensitive wavelengths (nm) and lamp currents (mA) used for the determination of the analytes were as follows: Cu 324.75 and 30, and Cd 228.80 and 3, respectively. A Hanna Instruments model 221 (Cluj-Napoca, Romania) digital pH-meter with a combined glass electrode was used for all pH measurements. A Nuve ST 402 model thermostatic water bath (Ankara, Turkey) was used for controlling the temperature of the CPE experiments. A Hettich centrifuge model Rotafix 32 A (Germany) was used to accelerate the phase separation.

Reagents and solutions

All the reagents used were of analytical grade, and water purified by a reverse osmosis system (AquaTurk Reverse Osmosis System, HSC ARITIM, Istanbul, Turkey) was used to prepare all the solutions. Nitric acid, hydrochloric acid, sodium dihydrogen phosphate, sodium acetate, acetic acid, ammonium acetate, boric acid and ethanol from Sigma-Aldrich (St. Louis, MO, USA), Triton X-114 and sodium hydroxide from Fluka (Gillingham, Dorset, UK), and 4-phenyl-3-thiosemicarbazide and sodium tetraborate from Aldrich (St. Louis, MO, USA) were used in the experiments. The laboratory glassware used was kept in 10% (v/v) nitric acid overnight and rinsed with deionized water before use. Stock standard solutions (1,000 mg L−1) of the analytes were prepared by dissolving appropriate amounts of Cu(NO3)2.3H2O (Riedel-de haen from Sigma-Aldrich, St. Louis, MO, USA) and Cd(NO3)2.4H2O (Merck, Darmstadt, Germany) in 1% HNO3. Working standard solutions were prepared daily from the stock standard solutions by appropriate dilution with deionized water. Sodium dihydrogen phosphate/phosphoric acid buffers for pH 2–3, sodium acetate/acetic acid buffers for pH 4–5, ammonium acetate/acetic acid buffers for pH 6–7 and sodium tetraborate/boric acid buffers for pH 8–10 were used to adjust the pH of the solutions. The solution of the chelating ligand (2 × 10−2 M) was prepared by dissolving appropriate amounts of the reagent in ethanol. The non-ionic surfactant, 1.0% (w/v) Triton X-114 was prepared by dissolving 1.0 g of Triton X-114 in 100 mL of deionized water. The certified reference materials SPS-SW2 level 2 Batch 127 surface water (Spectrapure Standards AS, Oslo, Norway) and SRLS-5 river water (National Research Council, Ottawa, Canada) were used for verifying the accuracy of the proposed method.

CPE procedure

An aliquot of the sample solution containing Cu(II) and Cd(II) ions was transferred to a 50 mL polyethylene centrifuge tube. Borate buffer (1.0 mL, pH 8.0), 0.5 mL of 2 × 10−2mol L−1 ligand and 1.0 mL of 1.0% (w/v) Triton X-114 solution were added. The mixture was diluted to 20 mL with deionized water. The mixture was manually shaken for 5–6 sec and left to stand for 10 min in a thermostated water bath set at 50 °C. The resulting solution was centrifuged at 4,000 rpm for 10 min to obtain phase separation. It was then cooled at +4 °C in a refrigerator for 10 min to increase the viscosity of the surfactant-rich phase. The aqueous phase was carefully removed with a Pasteur pipette and, to decrease its viscosity, the surfactant-rich phase was diluted to 1.0 mL with 1.0 mol L−1 HNO3. The final solution was aspirated directly into the FAAS instrument. The CPE procedure described above was also applied to the blank and calibration standards.

Sample preparation

The proposed method was applied to natural water samples. Tap water sample was collected directly at the laboratory (Balıkesir University), river water was collected from Küçük Bostancı River and seawater was collected from the Aegean Sea close to the Edremit coast. The river water and seawater samples were filtered through a cellulose membrane of 0.45 μm pore size, acidified to pH 2 using nitric acid and stored in pre-cleaned polyethylene bottles. The pH of the water samples (20 mL) was adjusted to pH 8.0 using a few drops of 10% (w/v) sodium hydroxide solution and then maintained using borate buffer solution. The proposed method was applied to the samples.

RESULTS AND DISCUSSION

Optimization of the experimental variables

The analytical parameters that affect the performance of CPE, such as sample pH, ligand amount, concentration of surfactant, incubation temperature and time and effect of salt addition were investigated and optimized. Standard solution (10 mL) containing 100 μg L−1 of Cu(II) and 50 μg L−1 Cd(II) was used in these experiments. A univariate optimization procedure was undertaken, i.e., varying one parameter at a time, keeping the others constant. All the experiments were carried out in triplicate.

The extraction of metal ions by surfactant micelles generally occurs after the formation of a complex with sufficient hydrophobicity (Silva et al. 2009). Since the pH is one of the main parameters for chelation reactions, the effect of pH on CPE procedure was investigated over the pH range of 2.0–10.0. The effect of the sample solution pH on the recovery of Cu(II) and Cd(II) is shown in Figure 1. Quantitative recoveries were obtained at pH ranges 6.0–10.0 for Cu(II) and 8.0–10.0 for Cd(II). Therefore, all further experiments were performed at pH 8.0 for the simultaneous extraction of the Cu(II) and Cd(II).
Figure 1

Effect of sample pH on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, 0.5 mL of 2 × 10−2 mol L−1 ligand solution, 0.5 mL of 1.0% (w/v) Triton X-114, incubation temperature 70 °C, incubation time 30 min.

Figure 1

Effect of sample pH on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, 0.5 mL of 2 × 10−2 mol L−1 ligand solution, 0.5 mL of 1.0% (w/v) Triton X-114, incubation temperature 70 °C, incubation time 30 min.

The concentration of the ligand has a direct effect on the formation of metal–ligand complex as well as its extraction. The effect of ligand amount on the recovery of Cu(II) and Cd(II) ions was examined over the ligand amount range 0.0–4.2 mg. For this purpose, 0.5 mL of different concentrations of ligand between 0.0 and 5 × 10−2 mol L−1 was added to 10 mL of standard solution containing 100 μg L−1 of Cu(II) and 50 μg L−1 Cd(II) at pH 8.0. The results are given in Figure 2. The recovery of Cu(II) and Cd(II) was quantitative in the ligand amount ranges 0.04–4.2 mg and 1.7–4.2 mg, respectively. Therefore, 1.7 mg of ligand (0.5 mL of 2 × 10−2 mol L−1) was used for further experiments.
Figure 2

Effect of ligand amount on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 1.0% (w/v) Triton X-114, incubation temperature 70 °C, incubation time 30 min.

Figure 2

Effect of ligand amount on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 1.0% (w/v) Triton X-114, incubation temperature 70 °C, incubation time 30 min.

The concentration of surfactant used in CPE is a critical factor. In order to raise the efficiency of the extraction procedure, the concentration of Triton X-114 in the sample solution was optimized evaluating concentrations between 0.005% and 0.375% (w/v). As shown in Figure 3, quantitative recoveries of the analytes were obtained at concentrations between 0.025% and 0.1%. Above 0.1% (w/v), the recovery of the analytes slowly decreased. Therefore, a concentration of 0.05% (w/v) Triton X-114 was selected for subsequent experiments.
Figure 3

Effect of Triton X-114 concentration on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 2 × 10−2 mol L−1 ligand, incubation temperature 70 °C, incubation time 30 min.

Figure 3

Effect of Triton X-114 concentration on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 2 × 10−2 mol L−1 ligand, incubation temperature 70 °C, incubation time 30 min.

The effect of ionic strength on the extraction efficiency of analytes was examined using NaCl at concentrations from 0.0 to 0.5 mol L−1. The results are given in Figure 4. According to the results obtained, salt addition has no significant effect on the extraction efficiency of Cu(II) and Cd(II). Therefore, all the extraction experiments were carried out without the addition of salt.
Figure 4

Effect of NaCl concentration on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 2 × 10−2 mol L−1 ligand, 0.5 mL of 1.0% (w/v) Triton X-114, incubation temperature 70 °C, incubation time 30 min.

Figure 4

Effect of NaCl concentration on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 2 × 10−2 mol L−1 ligand, 0.5 mL of 1.0% (w/v) Triton X-114, incubation temperature 70 °C, incubation time 30 min.

The shortest incubation time and the lowest possible equilibration temperature are very important for completion of the reaction and efficient separation of the phases (Naeemullah et al. 2016). The dependence of extraction efficiency upon incubation temperature and time was studied over the range of 30–70 °C and 5–30 min, respectively. The results are given in Figures 5 and 6. Quantitative recoveries were obtained between 40 and 70 °C. It was observed that an incubation time of 10 min is enough for quantitative extraction of the analytes. Therefore, an incubation temperature of 50 °C and an incubation time of 10 min were selected as optimum.
Figure 5

Effect of incubation temperature on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 2 × 10−2 mol L−1 ligand, 0.5 mL of 1.0% (w/v) Triton X-114, incubation time 30 min.

Figure 5

Effect of incubation temperature on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 2 × 10−2 mol L−1 ligand, 0.5 mL of 1.0% (w/v) Triton X-114, incubation time 30 min.

Figure 6

Effect of incubation time on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 2 × 10−2 mol L−1 ligand, 0.5 mL of 1.0% (w/v) Triton X-114, incubation temperature 50 °C.

Figure 6

Effect of incubation time on the recovery of Cu(II) and Cd(II). Conditions: sample volume 10 mL, sample pH 8.0, 0.5 mL of 2 × 10−2 mol L−1 ligand, 0.5 mL of 1.0% (w/v) Triton X-114, incubation temperature 50 °C.

Effect of the volume of sample solutions

In order to obtain a high preconcentration factor, the sample volume is a key consideration. Under optimized conditions, the effect of sample volume on the extraction of Cu(II) and Cd(II) was examined. To evaluate the effect of sample volume on the recovery of Cu(II) and Cd(II), 10, 20 and 30 mL of sample solutions containing 200 ng Cu(II) and 100 ng Cd(II) were used. The results obtained are given in Table 1. Quantitative recoveries were obtained for the sample volumes studied. The preconcentration factors obtained were 10, 20 and 30 for a 10, 20 and 30 mL sample solution, respectively.

Table 1

The effect of sample volume on the recovery of Cu(II) and Cd(II)

Sample volume (mL) Analyte concentration (μg L−1)
 
Recovery (%)
 
Cu(II) Cd(II) Cu(II) Cd(II) 
10 20 10 97.2 ± 3.5 102.6 ± 3.3 
20 10 5.0 96.7 ± 1.7 97.1 ± 5.4 
30 6.7 3.3 98.3 ± 5.4 100.5 ± 4.5 
Sample volume (mL) Analyte concentration (μg L−1)
 
Recovery (%)
 
Cu(II) Cd(II) Cu(II) Cd(II) 
10 20 10 97.2 ± 3.5 102.6 ± 3.3 
20 10 5.0 96.7 ± 1.7 97.1 ± 5.4 
30 6.7 3.3 98.3 ± 5.4 100.5 ± 4.5 

Interference studies

In order to demonstrate the selectivity of the developed method for determination of the analytes at trace levels, the effects of common coexisting ions on the extraction of Cd(II) and Cu(II) were studied. In these experiments, 10 mL of solution containing 100 μg L−1 Cu(II) and 50 μg L−1 Cd(II) ions were added to interfering ions at a concentration of 10 mg L−1 and treated according to the recommended extraction procedure. The results are given in Table 2. The presence of the tested ions does not affect the recovery of Cu(II) and Cd(II) ions. In addition, the effect of the major matrix ions present in waters (Mg2+, Ca2+, Na+, K+, SO42−, Cl, CO32− and NO3) on the recoveries of the analytes was investigated using a synthetic seawater sample containing 1,270 mg L−1 Mg2+, 400 mg L−1 Ca2+, 10,800 mg L−1 Na+, 390 mg L−1 K+, 5,100 mg L−1 SO42−, 600 mg L−1 CO32−, 16,600 mg L−1 Cl, and 620 mg L−1 NO3. A 10 mL aliquot of the synthetic seawater solution spiked with 100 μg L−1 of Cu(II) and 50 μg L−1 Cd(II) was analysed according to the recommended procedure. The results are given in Table 2. It can be seen that the seawater matrix ions have no significant effect on the recovery of Cu(II) and Cd(II) ions. Therefore, the proposed method can be applied to samples containing high amounts of salt.

Table 2

Effect of interfering ions on the recovery of Cu(II) and Cd(II)

Interfering iona Added as Recovery (%)
 
Cu(II) Cd(II) 
Pb2+ Pb(NO3)2 99.7 ± 5.3 105.8 ± 2.6 
Cr3+ Cr(NO3)3. 9H297.7 ± 0.8 96.8 ± 3.6 
Mn2+ Mn(NO3)2. 4H2100.0 ± 3.3 98.0 ± 0.7 
Zn2+ Zn(NO3)2. 6H297.9 ± 2.5 98.1 ± 4.7 
Fe3+ Fe(NO3)3. 9H2100.0 ± 3.6 104.6 ± 6.2 
Ba2+ Ba(NO3)2 96.9 ± 3.1 102.4 ±2.5 
Al3+ Al(NO3)3. 9H297.2 ± 2.0 101.0 ± 1.3 
Sr2+ Sr(NO3)2 100.3 ± 0.4 98.5 ± 0.5 
Ni2+ Ni(NO3)2. 6H298.3 ± 5.5 96.6 ± 1.5 
Synthetic seawater KNO3, NaCl, MgSO4. 7H2O, CaCO3 101.8 ± 0.9 101.7 ± 2.8 
Interfering iona Added as Recovery (%)
 
Cu(II) Cd(II) 
Pb2+ Pb(NO3)2 99.7 ± 5.3 105.8 ± 2.6 
Cr3+ Cr(NO3)3. 9H297.7 ± 0.8 96.8 ± 3.6 
Mn2+ Mn(NO3)2. 4H2100.0 ± 3.3 98.0 ± 0.7 
Zn2+ Zn(NO3)2. 6H297.9 ± 2.5 98.1 ± 4.7 
Fe3+ Fe(NO3)3. 9H2100.0 ± 3.6 104.6 ± 6.2 
Ba2+ Ba(NO3)2 96.9 ± 3.1 102.4 ±2.5 
Al3+ Al(NO3)3. 9H297.2 ± 2.0 101.0 ± 1.3 
Sr2+ Sr(NO3)2 100.3 ± 0.4 98.5 ± 0.5 
Ni2+ Ni(NO3)2. 6H298.3 ± 5.5 96.6 ± 1.5 
Synthetic seawater KNO3, NaCl, MgSO4. 7H2O, CaCO3 101.8 ± 0.9 101.7 ± 2.8 

aConcentration of the interfering ions = 10 mg L−1.

Analytical performance of the method and comparison with other methods

The analytical performance of the proposed method was evaluated under the optimized conditions. Calibration graphs were constructed using 20 mL of the standard solutions buffered at pH 8.0 and containing the concentration ranges of 2.5–100 μg L−1 Cu(II) and 1.25–50 μg L−1 Cd(II). The standard solutions were processed by the optimized CPE method. Table 3 summarizes the analytical characteristics of the optimized method, including linear working range, correlation coefficient (R), limit of detection (LOD), relative standard deviation (RSD), equation of calibration curves, preconcentration factor (PF) and enrichment factor (EF). The limits of detection, defined as LOD = 3 Sb/m and where Sb is standard deviation of ten replicate blank signals and m is slope of the calibration curve obtained with 20-fold preconcentration, were found to be 0.49 μg L−1 for Cu(II) and 0.20 μg L−1 for Cd(II). The precision of the method was evaluated for a solution containing 5.0 μg L−1 of Cu(II) and 2.5 μg L−1 of Cd(II), and their RSD values were found to be 3.1% for Cu(II) and 2.4% for Cd(II) (n = 10). The preconcentration factor for the proposed method is calculated by the ratio of the sample volume (20 mL) to the final volume (1 mL). Enrichment factors were calculated as the ratio of the slopes of calibration graphs obtained using the preconcentration method and direct aspiration.

Table 3

Analytical characteristics of the proposed CPE method

Parameters Cu(II) Cd(II) 
Sample volume (mL) 20 20 
Calibration equation (with preconcentration) A = 4.05 × 10−3C + 3.32 × 10−3 A = 1.19 × 10−2C + 5.17 × 10−3 
Working range (μg L−12.5–100 1.25–50 
Correlation coefficient (R) 0.9999 0.9998 
Detection limit (LOD) (μg L−10.49 0.20 
RSD (%) (5.0 μg L−1 Cu(II) and 2.5 μg L−1 Cd(II)) 3.1 2.4 
Calibration equation (direct aspiration) A = 2.03 × 10−4C + 2.92 × 10−3 A = 5.76 × 10−4C + 6.89 × 10−3 
Working range (μg L−150–2,000 25–1,000 
Preconcentration factor 20 20 
Enrichment factor 19.9 20.7 
Parameters Cu(II) Cd(II) 
Sample volume (mL) 20 20 
Calibration equation (with preconcentration) A = 4.05 × 10−3C + 3.32 × 10−3 A = 1.19 × 10−2C + 5.17 × 10−3 
Working range (μg L−12.5–100 1.25–50 
Correlation coefficient (R) 0.9999 0.9998 
Detection limit (LOD) (μg L−10.49 0.20 
RSD (%) (5.0 μg L−1 Cu(II) and 2.5 μg L−1 Cd(II)) 3.1 2.4 
Calibration equation (direct aspiration) A = 2.03 × 10−4C + 2.92 × 10−3 A = 5.76 × 10−4C + 6.89 × 10−3 
Working range (μg L−150–2,000 25–1,000 
Preconcentration factor 20 20 
Enrichment factor 19.9 20.7 

A comparison of the characteristic data obtained using the method developed with other reported preconcentration methods for Cu(II) and Cd(II) determination is summarized in Table 4. The limits of detection of the analytes are lower than or comparable to those obtained with other separation/preconcentration methods.

Table 4

Comparative data from recent studies on preconcentration–separation of copper and cadmium

Method Technique Analyte Detection limit (LOD) (μg L−1) Preconcentration factor Sample volume (mL) Time of analysis (min) Reference 
CPE FAAS Cu, Cd 0.49 (Cu), 0.20 (Cd) 20 20 30 This work 
CPE FAAS Cu, Cd, Pb, Zn 0.27 (Cu), 0.099 (Cd) 64.3 (Cu), 57.7 (Cd) 50 At least 30 Chen & Teo (2001)  
CPE ICP-OES Cu, Zn, Cd, Ni 1.2 (Cu), 1.0 (Cd) 10 15 65 Silva et al. (2009)  
IL-SSME FAAS (microinjection system) Cd 0.35 50 10 Data not available Khan et al. (2015)  
Micelle-mediated extraction FI-FAAS Cd, Co, Cu, Mn, Ni, Pb, Zn 3.2 (Cu), 0.39 (Cd) 8.4 4.2 Kara (2009)  
MDSPE FAAS Cd, Pb 0.16 (Cd) 25 50 Ezoddin et al. (2015)  
SPE FAAS Cd 0.5 50 50 20 Mehdinia et al. (2015)  
SPE FAAS Cd 3.71 100 100 Mirabi et al. (2015)  
USAE-SFODME FAAS Fe, Cu 4.1 (Cu) 13.4 6.7 28 Khayatian & Hassanpoor (2013)  
DLLME FAAS Cu 3.0 10 Farajzadeh et al. (2008)  
SDSPE FAAS Co, Ni, Cu 1.5 (Cu) 20 40 8.5 Meng et al. (2015)  
Method Technique Analyte Detection limit (LOD) (μg L−1) Preconcentration factor Sample volume (mL) Time of analysis (min) Reference 
CPE FAAS Cu, Cd 0.49 (Cu), 0.20 (Cd) 20 20 30 This work 
CPE FAAS Cu, Cd, Pb, Zn 0.27 (Cu), 0.099 (Cd) 64.3 (Cu), 57.7 (Cd) 50 At least 30 Chen & Teo (2001)  
CPE ICP-OES Cu, Zn, Cd, Ni 1.2 (Cu), 1.0 (Cd) 10 15 65 Silva et al. (2009)  
IL-SSME FAAS (microinjection system) Cd 0.35 50 10 Data not available Khan et al. (2015)  
Micelle-mediated extraction FI-FAAS Cd, Co, Cu, Mn, Ni, Pb, Zn 3.2 (Cu), 0.39 (Cd) 8.4 4.2 Kara (2009)  
MDSPE FAAS Cd, Pb 0.16 (Cd) 25 50 Ezoddin et al. (2015)  
SPE FAAS Cd 0.5 50 50 20 Mehdinia et al. (2015)  
SPE FAAS Cd 3.71 100 100 Mirabi et al. (2015)  
USAE-SFODME FAAS Fe, Cu 4.1 (Cu) 13.4 6.7 28 Khayatian & Hassanpoor (2013)  
DLLME FAAS Cu 3.0 10 Farajzadeh et al. (2008)  
SDSPE FAAS Co, Ni, Cu 1.5 (Cu) 20 40 8.5 Meng et al. (2015)  

CPE: cloud point extraction; IL-SSME: ionic liquid-based single step microextraction procedure; SPE: solid phase extraction; MDSPE: magnetic-dispersive solid phase extraction; DLLME: dispersive liquid–liquid microextraction; USAE-SFODME: ultrasound-assisted emulsification solidified floating organic drop microextraction; SDSPE: suspension dispersive solid phase extraction.

Accuracy of the method

In order to evaluate the accuracy of the proposed method, two certified reference materials, SLRS-5 river water and SPS-SW2 Batch 127 surface water were analysed using the developed method. The analytical results are given in Table 5. The results obtained by the proposed method were in good agreement with the certified values and the precision was between 2.2 and 3.7% RSD. As seen in Table 5, the values obtained for the certified reference samples were between 95.0 and 108.0% of certified values. These results show the accuracy and repeatability of the developed method for the determination of Cu(II) and Cd(II) in water samples. In order to test the significance of differences between certified and obtained values, the Student's t-test was applied to results of the proposed method and certified values. For 2 degrees of freedom at the 95% confidence level, critical t value is 4.30. As can be seen, t values are smaller than the critical value of t at 95% confidence level for all certified samples, indicating that there is no evidence of systematic error in the proposed method.

Table 5

Results for the certified reference materials

Certified reference material Analyte Certified value (μg L−1Found valuea (μg L−1Recovery (%) RSDb (%) tc 
SPS-SW2 Batch 127 surface water Cu(II) 100 ± 1 95.0 ± 2.6 95 2.2 3.3 
Cd(II) 2.50 ± 0.02 2.60 ± 0.13 104 5.0 1.3 
SLRS-5 River water Cu(II) 17.4 ± 1.3 18.8 ± 0.7 108 3.7 3.5 
Cd(II) 0.0060 ± 0.0014 <LOD – – – 
Certified reference material Analyte Certified value (μg L−1Found valuea (μg L−1Recovery (%) RSDb (%) tc 
SPS-SW2 Batch 127 surface water Cu(II) 100 ± 1 95.0 ± 2.6 95 2.2 3.3 
Cd(II) 2.50 ± 0.02 2.60 ± 0.13 104 5.0 1.3 
SLRS-5 River water Cu(II) 17.4 ± 1.3 18.8 ± 0.7 108 3.7 3.5 
Cd(II) 0.0060 ± 0.0014 <LOD – – – 

aMean value ± standard deviation based on three replicate determinations.

bRSD: relative standard deviation.

ct = , where t is statistical value (for 2 degrees of freedom, the critical value of t at the 95% confidence level is 4.30), s is the standard deviation, N is number of independent determinations, x is the experimental mean value, and μ is the certified value.

Application of the method to real samples

The proposed method was applied to tap water, river water and seawater samples. The applicability of the method was evaluated by spiking of these water samples with 10 μg L−1 of Cu(II) and 5 μg L−1 of Cd(II). The results are given in Table 6. The average percentage recovery values of Cu(II) and Cd(II) were 96 and 108 for tap water samples, 98 and 104 for river water samples, and 101 and 96 for seawater samples, respectively. These results demonstrate the reliability and accuracy of the method for the determination of Cu(II) and Cd(II) in natural water samples.

Table 6

Analytical results of water samples and the recovery of spiked analytes

Sample Analyte Added (μg L−1Founda (μg L−1Recovery (%) RSD (%) 
Tap water Cu(II) – 7.2 ± 0.2 – 2.8 
10 16.8 ± 0.7 96 4.2 
Cd(II) – <LOD – – 
5.4 ± 0.1 108 1.9 
River water Cu(II) – 2.1 ± 0.2 – 9.5 
10 11.9 ± 0.2 98 1.7 
Cd(II) – <LOD – – 
5.2 ± 0.2 104 3.8 
Seawater Cu(II) – 5.2 ± 0.2 – 3.8 
10 15.3 ± 0.6 101 3.9 
Cd(II) – <LOD – – 
4.8 ± 0.3 96 6.3 
Sample Analyte Added (μg L−1Founda (μg L−1Recovery (%) RSD (%) 
Tap water Cu(II) – 7.2 ± 0.2 – 2.8 
10 16.8 ± 0.7 96 4.2 
Cd(II) – <LOD – – 
5.4 ± 0.1 108 1.9 
River water Cu(II) – 2.1 ± 0.2 – 9.5 
10 11.9 ± 0.2 98 1.7 
Cd(II) – <LOD – – 
5.2 ± 0.2 104 3.8 
Seawater Cu(II) – 5.2 ± 0.2 – 3.8 
10 15.3 ± 0.6 101 3.9 
Cd(II) – <LOD – – 
4.8 ± 0.3 96 6.3 

aMean value ± standard deviation based on three replicate determinations.

CONCLUSIONS

The proposed method for simultaneous separation and preconcentration of Cu(II) and Cd(II) by CPE, using Triton X-114 as surfactant and 4-phenyl-3-thiosemicarbazide as complexing agent, has shown to be an efficient, simple, accurate, precise, inexpensive, green and safe method. Triton X-114 is of relatively low cost and low toxicity. The surfactant-rich phase can easily be introduced into the nebulizer of the spectrometer after dilution with 1.0 mol L−1 HNO3 and determined directly by FAAS. The method requires nearly 30 min of sample preparation time per sample. However eight samples can be prepared for analysis simultaneously. The developed method can be considered to be an alternative to other more sensitive analytical techniques, such as GFAAS, ICP-OES and ICP-MS of which the latter two instruments, especially ICP-MS, are not found in many laboratories due to their price.

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