1. Introduction

Presently,determination of trace heavy and precious metals in environmental samples is essential,because they have negative effects on human health ^{[1, 2]}.

Copper is one of the most widely distributed elements in the environments of industrialized countries. It is present in all organisms,land and marine. It has been shown that copper is an essential element in many biological processes,such as blood formation and the function of many important enzymes. Copper is classified as a biogenic element,playing a significant role in photosynthesis,metabolism of nitrogen compounds,or regulation of RNA and DNA transcription process ^[3].

Silver is an industrially important element. The widespread use of silver compounds and silver-containing procedures in industry, medicine,jewelry,cloud seeding,and in the disinfection of drinking water has resulted in increasing silver content of environmental samples. It is used for the preparation of corrosion- resistant alloys,and its compounds are extensively used in the processing of foods,drugs,beverages,and in filters and other equipment to purify water ^[4].

The importance of palladium has grown in recent years due to its increasing use in the production of dental and medical devices, jewelry,and catalytic converters. Although the benefits of car catalysts are indisputable,the emission of Pd into the environment is largely associated with the production and recycling of catalytic converters in the metal finishing industry as well as the operation of vehicle catalysts ^[5].

All of these issues emphasize the importance of identifying and quantifying Cu,Ag,and Pd to provide comprehensive information about their properties and human healthrelevance. For thispurpose, several analytical methods have been developed to measure these ions in clinical,environmental,industrial,and pharmaceutical samples including: spectrophotometry,flame atomic absorption spectrometry (FAAS),inductively coupled plasma-mass spectrometry (ICP-MS),inductively coupled plasma-atomic emission spectrometry (ICP-AES),and electrochemical methods.

Electrothermal atomic absorption spectrometry (ETAAS) is a good technique for the determination of ultra-trace amounts of heavy metals in several types of samples due to its sensitivity ^[6]. But by reason of ultra-low concentration of metals and high concentration of interfering matrix components in most real samples,ETAAS often requires a suitable pretreatment step. Various techniques have been used for the separation and preconcentration of Cu,Ag,and Pd,such as dispersive liquid- liquid microextraction (DLLME),solidified floating organic drop microextraction (SFODME),and cloud point extraction (CPE).

To the best of our knowledge,there was no previous literature on the application of modified Al₂O₃ and ETAAS for simultaneous column preconcentration and determination of ultra-trace amounts of copper,silver,and palladium. In this work,the adsorption behavior of Cu,Ag,and Pd on modified Al₂O₃ with polyethylenimine (PEI) as a novel solid phase extractor was studied. Experimental parameters affecting the column preconcentration and determination of metals,such as ETAAS temperature program,pH,eluent type,sample and eluent flow rates, sample volume,and interfering ions were studied and optimized. The proposed method has been applied for the determination of trace amounts of Cu,Ag,and Pd in sea water,synthetic samples, and standard reference materials with satisfactory results. 2. Experimental 2.1. Apparatus

Copper,silver,and palladium measurements were performed with a Varian Spectra AA 220 atomic absorption spectrometer (Varian,Australia) with a deuterium lamp background correction and equipped with a graphite furnace (GTA-110 series). Optimum operating parameters for ETAAS are given in Table 1. A Metrohm 713 pH meter (Metrohm,Switzerland) was used for pH measurements with a combined glass calomel electrode. Thermogravimetric analysis (TGA) was accomplished with a Perkin-Elmer TGAUSA (pyris) (Perkin-Elmer,USA). Also a mechanical shaker with speed control was used for preparation of the adsorbent. A Thermo Finnigan Flash EA1112 microanalyzer (Finnigan,Germany) was used for determination of C,H,N percentage.

Table 1

Table 1 Instrumental parameters and thermal programs of ETAAS for determination of Cu, Ag and Pd. a Cu. b Ag. c Pd.

Table 1 Instrumental parameters and thermal programs of ETAAS for determination of Cu, Ag and Pd.

All chemicals were of analytical reagent grade,and deionized water was used in all experiments. Stock solutions of copper (1000 mg/L) and silver (1000 mg/L) were prepared by dissolving the appropriate amount of copper nitrate (Cu(NO₃)₂,Merck, Germany) and silver nitrate (AgNO₃,Merck,Germany) in deionized water,respectively. A stock solution of Pd²⁺ (1000 mg/L) was prepared by dissolving a proper amount of palladium chloride (PdCl₂,Merck,Germany) in 10.0 mL of HCl (1.0 mol/L) and diluted to 250.0 mL in a standard flask. Solutions of lower concentrations were prepared daily by a suitable dilution of the stock solutions with deionized water. Also,a solution of polyethylenimine (5%) (Molecular weight: 600,000-1,000,000) (Fluka,Switzerland) was prepared in deionized water. Al₂O₃ (Fluka,Switzerland) was used as an adsorbent. A 1.0 mol/L solution of thiourea (Merck,Germany) was prepared by dissolving a proper amount of thiourea in deionized water. Buffer solution was prepared from 0.1 mol/L of KH₂PO₄/K₂HPO₄ (Merck,Germany) for pH7. HCl,HNO₃,and NaNO₃ were purchased from Merck (Germany). 2.3. Preparation of modified adsorbent

The modified alumina with PEI (PEI/Al₂O₃) was prepared as following: 1.0 g of alumina,0.5 g of NaNO₃,and 5.0 mL of PEI (5%) was mixed,and the pH was adjusted to about 7.0. The obtained suspension was shaken for 24 h. Then,the adsorbent was filtered, washed several times with deionized water and finally dried in a desiccator. It was characterized by elemental analysis (C,H,N) (Table 2) and TGA. The thermogravimetric analysis curve of the PEI/Al₂O₃ adsorbent shows four mass loss steps. The 1.7% mass loss up to 88.6 ℃ in the first step is due to adsorbed water. In the second step,the mass loss is 3.9% up to 296.4 ℃. In the third step,mass loss is 3.9% up to 506.5 ℃,and in the fourth step,mass loss is 1.53% up to 723.1 ℃. The mass losses in the second,third and fourth steps correspond with PEI.

Table 2

Table 2 Elemental analysis (C, H, N) for two kinds of adsorbent. aAverage of two measurements.

Table 2 Elemental analysis (C, H, N) for two kinds of adsorbent.

A total of 50.0 mg of modified adsorbent (PEI/Al₂O₃) was slurred in water and then poured into a funnel-tipped glass tube (length: 80 mm,diameter: 5 mm). Before use,the column was conditioned with the buffer solution at pH 7.0. Cotton was placed at the bottom and the top of the column to allow the adsorbent to settle properly. 2.5. Recommended procedure

100.0 mL of the standard multi-element solution containing 100.0 ng of Cu,Ag,and Pd and 10.0 mL of buffer solution (pH = 7) was passed through the column with the desired flow rate (2.0 mL/ min). Then,the formed metal chelates that settled on the column were desorbed with two 5.0 mL of eluent solution at the flow rate of 4.0 mL/min. Each 5.0 mL of eluent contains 3.0 mL of thiourea (1.0 mol/L) and 2.0 mL of HCl (1.0 mol/L). Finally,20.0 μL of each 5.0 mL of eluate solution was automatically injected by the autosampler into the graphite tube and then the absorbance of Cu, Ag,and Pd were measured under the operating conditions summarized in Table 1. 2.6. Samples preparation 2.6.1. Sea water

The selected water sample (Caspian Sea water) was filtered through a Millipore filter to remove suspended particulate matter. Then,2 mL of nitric acid was added to prevent adsorption of the metallic ions onto the flask walls,and the solution was stored at 4 ℃ in a refrigerator,and the suggested method was applied to the determination of copper,silver,and palladium ^[7]. For preconcentration, the pH of the samples was adjusted to 7.0 before analyzing by the described procedure. 2.6.2. Standard reference material

In order to confirm the validity of the developed procedure,this method has also been applied for the determination of the content of Cu,Ag,and Pd in a Standard Reference Material; SRM 1643d.

10 mL of the solution (SRM 1643d) was poured into a 100.0 mL measuring flask,diluted to the mark with deionized water,and the analyte was determined by the proposed method.

Palladium (from its standard stock solution) was added to the SRM 1643d before its preparation. 2.6.3. Synthetic sample

Since no standard samples were accessible for the simultaneous determination of copper,silver,and palladium with the developed method (according to Cu,Ag and Pd linear ranges),the method was applied to a synthetic mixture. Hence,a synthetic mixture containing different cations was prepared. Aliquots of the synthetic sample were taken and the general procedure was applied. 3. Results and discussion

Chelating agents with nitrogen atoms have excellent adsorption properties for heavy metal ions and are applied in the separation,enrichment,and removal of heavy metal ions ^[8].

Polyethylenimine (PEI) is a polyamine and has a large quantity of nitrogen atoms from amino groups on the line-type and branched structure of PEI,which can produce very strong chelating action. Silver and palladium ions are soft cations; for them,the following order of donor atom affinity is observed: O < N < S. Copper ions are in the borderline cation group that possesses strong affinity for intermediate (N) and soft (S) ligands. Coating alumina with PEI is a cheap and simple method (one step). The results of elemental analysis of the dried adsorbent showed that N and C sites exist,thus alumina was coated with PEI. The experiments showed that the PEI/Al₂O₃ in neutral media has more adsorption capacity than in acidic or alkaline media. Increasing the ionic strength caused an increase in the amount of PEI at alumina surface at all measured pH values. Therefore, NaNO₃ was used to increase the ionic strength. The amount of NaNO₃ was optimized,and it was observed that 0.5 g of NaNO₃ was suitable for 1.0 g of alumina (Table 2).

3.1. Effect of pH

The reaction between metal ions (Cu,Ag,and Pd) and the chelating agent can be influenced by changes in pH. In order to optimize this parameter,the pH of the sample solution was studied in the range of 2.0-11.0,using of HNO₃ and KOH for pH adjustment. The results showed that the copper,silver and palladium ions were completely adsorbed on the adsorbent over the pH range of 5.5- 8.0,4.5-8.0 and 6-8.0,respectively (Fig. 1). At very acidic pHs,a majority of the amine sites are protonated and cannot take part in the chelation process,but above pH 8.0,the recoveries are not quantitative because of hydroxide formation of metal ions. So,for simultaneous preconcentration of these three ions,pH 7.0 was chosen as the optimum pH for further studies. Also,for pH adjusting,KH₂PO₄/K₂HPO₄ buffer with pH 7.0 was used.

Fig. 1

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Fig. 1.Effect of pH on the extraction efficiency of Cu, Ag and Pd. Experimental conditions were the same as in Table 3 except the pH.

A series of different amounts of the adsorbent (PEI/Al₂O₃) were used for simultaneous adsorption of 100.0 ng of Cu,Ag,and Pd from 100.0 mL of sample solution. The results showed that for the quantitative extraction of three ions,at least 50.0 mg of the adsorbent is needed. 3.3. Adsorption capacity

The capacity of the adsorbent is an important factor because it determines how much adsorbent is required to quantitatively extract a specific amount of metal ions from the sample solutions. The adsorption capacity of PEI/Al₂O₃ was determined by passing 100.0 mL of solution containing 1.0 mg of copper,silver,and palladium from 50.0 mg of adsorbent. The amount of Cu,Ag,and Pd ions adsorbed on to the PEI/Al₂O₃ were calculated by the difference between the initial and the final concentrations of analyte ions in the solution. The maximum adsorption capacity has been found to be 17.0,13.5 and 13.0 mg/g for copper,silver,and palladium, respectively. The difference between the adsorption capacities of the three metal ions can be due to their sizes,degree of hydration, and their charge/radius ratio.

In order to confirm the activity of PEI in the extraction and confirm its deposition onto the surface of Al₂O₃,the outlined method was tested using unmodified Al₂O₃. The maximum adsorption capacity of unmodified Al₂O₃ was found to be 6.6 mg/g for Cu,4.8 mg/g for Ag,and 5.1 mg/g for Pd,which confirms that the PEI was adsorbed on the surface of alumina and the adsorption capacity of PEI-Al₂O₃ is much higher than unmodified Al₂O₃. 3.4. Effect of eluent type

In order to select the most effective eluent for desorption of the retained metal ions,a series of various eluent solutions were studied under the optimum conditions (Table 3). The results showed that quantitative recoveries were obtained using two cycles with 5.0 mL eluent including 3.0 mL of thiourea (1.0 mol/L) followed by one cycle of 2.0 mL of HCl (1.0 mol/L).

Table 3

Table 3 The eluent solution type and recovery of copper, silver and palladium ions. Conditions: Original samples containing 100.0 ng of copper, silver and palladium, pH 7.0; buffer volume, 10.0 mL; sample flow rate, 2.0 mL/min; eluent flow rate, 4.0 mL/min; amount of adsorbent, 50 mg.

Table 3 The eluent solution type and recovery of copper, silver and palladium ions.

The sample and eluent flow rates are important parameters to obtain quantitative retention and elution,respectively. The influence of flow rates was investigated in the range of 0.2- 10 mL/min for both the sample and eluent. The retentions for analytes (including 100.0 ng of Cu,Ag,and Pd at pH 7.0) were quantitative up to a sample flow rate of 2.0 mL/min. Also,in the range of 0.2-4.0 mL/min for eluent flow rate,there was no significant difference in the recovery of all three metal ions. Therefore,all further studies were performed at 2.0 mL/min sample flow rate and 4.0 mL/min eluent flow rate. 3.6. Breakthrough volume

When dealing with real samples containing very low concentrations of element ions,in order to obtain high preconcentration factors,the maximum applicable sample volume must be determined. Thus the effect of sample volume on the extraction of 100.0 ng of Cu,Ag,and Pd was studied by taking different samples volumes (500-2500 mL). The extraction was carried out as described earlier. The results show that the recovery of the analytes did not decrease significantly up to 2000,1500,and 2000 mL for Cu,Ag,and Pd,respectively. By using 10.0 mL of eluent,the preconcentration factors of 200,150,and 200 were obtained for Cu,Ag,and Pd,respectively. 3.7. Reusability of modified adsorbent

In order to investigate the effectiveness of the modified adsorbent,PEI/Al₂O₃ was reused. The experimental results indicate that the recovery of analytes decreases slightly when the adsorbent is reused 8,6,and 9 times for Cu,Ag,and Pd, respectively. 3.8. Effect of interfering ions on recovery

The efficiency of the suggested method in the extraction and preconcentration of the three ions in the presence of various cations and anions was examined by using a solution containing 100.0 ng of Cu,Ag and Pd with addition of various concentrations of potential interferences. The tolerance level was defined as the maximum amount of foreign species producing an error of ±5% in Cu,Ag,and Pd determination. The tolerance level of each potentially interfering ion species was tested,and if interference occurred,the ratio was reduced until it ceased. As can be seen from Table 4,several species did not interfere even at high concentrations,showing that the method described is applicable to the analysis of Cu,Ag,and Pd ions in different samples.

Table 4

Table 4 Effect of potentially interfering ions on the extraction efficiency of Cu, Ag and Pd. Experimental conditions were the same as in Table 3.

Table 4 Effect of potentially interfering ions on the extraction efficiency of Cu, Ag and Pd.

Under the optimum conditions,performance characteristics were obtained by processing standard solutions of copper,silver, and palladium,and the data is illustrated in Table 5.

Table 5

Table 5 Analytical characteristics of suggested method.

Table 5 Analytical characteristics of suggested method.

To test the reproducibility of the proposed column solid phase extraction method,the suggested procedure was repeated seven times under optimum conditions. The relative standard deviation (R.S.D.) was measured to be ±5.4%,±4.7%,and ±5.2% for Cu,Ag,and Pd,respectively. The calibration curves for the determination of Cu, Ag,and Pd under the optimized conditions show linearity over the range of 15.0-140 ng/L (R² = 0.9988) for Cu,4.0-93 ng/L (R² = 0.9991) for Ag,and 7.5-125 ng/L (R² = 0.9983) for Pd in the original solutions. Also,limit of detections based on 3S_b/m (where S_b is the standard deviation of the blank signals and m is the slope of the calibration curve after extraction) were calculated to be 3.9,1.1,and 2.0 ng/L for Cu,Ag,and Pd,respectively. The enrichment factor,which was calculated based on the slopes of the calibration curves with or without the extraction,was 195 for copper,147 for silver,and 197 for palladium. Also,the preconcentration factors are 200,150,and 200 for Cu,Ag,and Pd,respectively. 3.10. Analytical applications

In order to establish the validity of the procedure,the proposed method was applied to the simultaneous extraction of copper, silver,and palladium in Caspian Sea water. The reliability of the method was checked by analysis of the samples spiked with known amounts of Cu,Ag,and Pd in the sea water sample. The results presented in Table 6 reveal that recovery at 95% confidence level is satisfactory.

Table 6

Table 6 Analysis of copper, silver and palladium in sea water.

Table 6 Analysis of copper, silver and palladium in sea water.

Also,to verify the accuracy of the method,this procedure was also applied to the determination of Cu,Ag,and Pd in a standard reference material (SRM 1643d) and a synthetic sample. The analytical results are given in Table 7. As can be seen,the obtained results are in good agreement with the reference values,and there is no significant difference between the results and the accepted values. Thus,the procedure is reliable for analysis of a wide range of samples.

Table 7

Table 7 Analysis of copper, silver and palladium in synthetic sample and certified reference material.

Table 7 Analysis of copper, silver and palladium in synthetic sample and certified reference material.

Table 8 compares the characteristic data of the suggested method with other methods for determination of Cu,Ag,and Pd which have been reported in the literature ^{[5, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34]}. As can be seen from Table 8,for the three elements,the recommended method possesses a wide linear dynamic range,high sampling volume, high preconcentration factor,and high sensitivity. The proposed method has also the lowest detection limit except the results reported in the following literature ^{[20, 27, 29]} and best enrichment factor except the following reported ^{[12, 15, 16, 33]},for simultaneous extraction of copper,silver,and palladium ions. Moreover,in all of these reported articles,only one or two elements were determined,whereas the recommended procedure can extract and determine of three metal ions,simultaneously. Finally because of the low toxicity of this method,it is proper to mention this method as a green and environmentally friendly method.

Table 8

Table 8 Comparison of the proposed method with other reported methods for preconcentration and determination of copper, silver and palladium. a Preconcentration factor. b Enrichment factor. c Adsorptive stripping voltammetry. d Inductively coupled plasma-optical emission spectrometry.

Table 8 Comparison of the proposed method with other reported methods for preconcentration and determination of copper, silver and palladium.

In this study,a new adsorbent (PEI/Al₂O₃) in combination with ETAAS was applied for simultaneous preconcentration and determination of copper,silver,and palladium. It has been shown that these three metal ions can form the complexes with PEI and then,adsorbed on the surface of modified alumina. Besides considerably high preconcentration ability,some other benefits of the system were the simultaneous determination of three elements,enhancement of ETAAS sensitivity,its simplicity and speed of analysis.