Copper at trace concentrations acts as both a micronutrient and a toxicant in marine and fresh water systems.1-8 This element is needed by plants at only very low levels and is toxic at higher levels. At these levels, copper can bind to the cell membrane and hinder the transport process through the cell wall. Copper at nearly 40 ng mL-1 is required for normal metabolism of many living organisms.9,10 On the other hand, copper is an important element in many industries. Thus, the development of new methods for selective separation, concentration and determination of it in sub-micro levels in different industrial, medicinal and environmental samples is of continuing interest. The determination of copper is usually carried out by flame and graphite furnace atomic absorption spectrometry (AAS)11,12 as well as spectrometric methods.13,14 However, due to the presence of copper in medicinal and environmental samples at low levels, its separation from other elements presents and also the use of a preconcentration step prior to its determination is usually necessary.
Different methods, especially Liquid- Liquid extraction of copper in the presence of various classical15-19 and macrocylic20,21 co-extractant ligands has attracted considerable attention. However, the use of classical extraction methods for this purpose is usually time-consuming, labor-intensive and requires large amounts of high purity solvents for extraction. Nevertheless, several other techniques for the preconcentration and separation of copper have been proposed including liquid chromatography22 supercritical fluid extraction,23 flotation,24 aggregate film formation,25 liquid membrane,26 column adsorption of pyrocatechol violet-copper complexes on activated carbon,27 ion pairing,28 ion pairing,29 preconcentration with yeast,30 and solid phase extraction using C18 cartridges and disks.31-33,56
Solid phase extraction (SPE) or liquid-solid extraction is poplar and growing techniques that are used to sample preparation for analysis. It is an attractive alternative for classical liquid-liquid extraction methods that reduce solvent usage and exposure, disposal costs and extraction time for sample separation and concentration purposed.34-36 In recent years, the octadecyl-bonded silica SPE disks have been utilized for the extraction and separation of different organic compounds from environmental matrices.37-40,46-47 Moreover, the SPE disks modified by suitable ligands are successfully used for selective extraction and concentration of metal ions.41-42
In a recent series of papers,43-45 we have described the application of metal-DNA conjugates to nucleic acid sequence determination with catalytic signal amplification; the assay relies on the esterase activity of a DNA-linked Cu complex. For optimization of the system and exploration of structure- activity relationships, a sensitive probe would be useful, which allows straightforward detection of esterase activity of ligated Cu2+ in low concentration. In this paper, we report a simple and novel approach forthe preconcentration of copper based on the adsorption of its 2-propylpiperidine-1-carbodithioate complex onto adsorbent alumina column. 2-propylpiperidine-1-carbodithioate is a ligand that gives a very sensitive color reaction with copper(Ⅱ). The adsorbed complex could be easily eluted using polyethylene glycol–sulfuric acid mixture and the recovery of copper was established using visible spectrophotometry. The effects of various experimental parameters such as sample volume, flow rate, effect of diverse ions, etc., were studied in detail. The validity of the proposed method was checked by applying it to the determination of copper in electroplating wastewater and spiked water samples.
Synthesis of sodium 2-propylpiperidine-1-carbodithioate reagent
Carbondisulphide (1.05 mol) was slowly added to a solution of 2-propyl-piperidine (1.43 mol) in 25 ml of water at 5℃ with constant stirring, followed by 1.0 mol of sodium hydroxide dissolved in 20 ml of water to form sodium 2-propylpiperidine-1- carbodithioate as shown in Fig. 1(a). The product was warmed to room temperature and washed 2~3 times with purified acetone. The reaction product was recrystallized from warm acetone. The purified compound has a melting point of 303~308℃ at 740mm pressure. Crystallization of water is less in 2-propylpiperidine-1- carbodithioate when compared with other carbodithioate, therefore, the extractability of the complex become easier. The metal salt of this reagent is a resonance hybrid of the structures, which contributed to the stability of metal complexes as shown in Fig. 1(b).
Analytical grade reagents were used in the preparation of all solutions. Triple distilled water was used for the preparation of solutions. The 1000 µgmL-1 Cu(Ⅱ) were prepared.
Fig. 1.(a) Synthesis of sodium 2-propylpiperidine-1-carbodithiodate reagent. (b) Resonance hybrid of the 2-propyl piperidine-1-carbodithioate with metal.
A working solution of 10 µgmL−1 Cu(Ⅱ) was prepared by appropriate dilution. Polyethylene glycol and neutral alumina were from Merck. A 0.25 g of sodium 2-propylpiperidine-1-carbodithioate was taken in a volumetric flask and diluted to 100 mL using acetone. Sulfuric acid of the required concentrations was prepared from concentrated sulfuric acid by appropriate dilution. The alumina was prepared by treatment 2.5 mol L−1 sulfuric acid for 12 h at 50℃. After the completion of the acid activation process, the reaction mixture was transferred to 1000 mL of ice-cold water to quench the activation process.
A Jasco V-576 (Japan) model UV–visible spectrophotometer was used for absorbance measurements. One centimeter matched quartz cells were used to record the absorbances. The pH measurements were carried out by an ATC pH meter (EDT instruments, GP 353). Determination of Cu2+ contents in working samples were carried out by a Varian spectra.
A glass column 9.5 mm in diameter and 15 cm in length was used for preconcentration. A 3 g of alumina to form slurry with 25 mL water. The alumina adsorbent was poured into the column and packed to a height of 5 cm. Glass wool was placed at the bottom and at the top of the column for allowing the adsorbent to settle properly. The column was washed using triple distilled water followed by conditioning it using 0.1 mol L−1 sulfuric acid.
General procedure for adsorption
A 1 mL volume of 10 µgmL−1 solution of Cu(Ⅱ) was alumina with 2 mL of 0.5 mol L−1 sulfuric acid followed by the addition of 2 mL of 2-propylpiperidine-1-carbodithioate and the resulting volume was maintained at 100 mL. The sample solution was loaded on to the column containing alumina adsorbent maintaining a flow rate of 2 mL min−1. The reddish violet complex was adsorbed on to the column,which was evident from the absorbance of the resulting solution that emerged out of the column. The adsorbed complex was then eluted using 10 mL of polyethylene glycol-4.5 mol L−1 sulfuric acid mixture and the concentration of copper was determined by visible spectrophotometry at 598 nm.
RESULTS AND DISCUSSION
Extraction of copper(Ⅱ) from aqueous solutions Effect of acidity
The effect of acidity plays a significant role in the preconcentration studies. The complex once prepared is stable even at moderately low pH. The volume of 0.5 mol L−1 sulfuric acid was varied from 1to 3 mL for preparing different concentrations of the complex in 100 mL sample volume. The adsorbed complex was eluted using 10 mL of polyethylene glycol-sulfuric acid mixture. The results are presented in Fig. 2. As can be seen from the figure, it is evident that 2.5 mL of 0.5 mol L−1 sulfuric acid is required to achieve quantitative recovery of the copper-2-propylpiperidine-1-carbodithioate complex. Beyond 2 mL, there was a decrease in the recovery of copper.
Fig. 2.Effect of variation of volume of 0.5 mol L−1 sulfuric acid on the recovery of copper(Ⅱ).
Effect of volume of the eluent
The volume of polyethylene glycol-sulfuric acid mixture as the eluting agent was varied using glass wool as support for the adsorbent. Maximum quantitative recovery of copper(Ⅱ) was observed with 4:6 ratio of polyethylene glycol-4.5 mol L−1 sulfuric acid mixture. The results are presented in Table 1. The use of polyethylene glycol alone or sulfuric acid of lower concentrations was not effective in the elution of the complex.
Table 1.Variation of volume of PEG–sulfuric acid mixture
Effect of sample volume
The sample volume was varied from 50 to 500mL maintaining an overall acidity of 0.01 mol L−1 sulfuric acid. The resulting complex was eluted using 10 mLof polyethylene glycol-sulfuric acid mixture. The effect of the sample volume and the corresponding preconcentration factors are given in Figs. 3 and 4. As can be seen from the figures, it is evident that the recovery of copper is quantitative (>95%) up to 250 mL sample volume. A preconcentration factor of 25 could be attained for quantitative recovery (>95%) of Cu(Ⅱ) when the sample volume was 250 mL.
Fig. 3.Effect of variation of sample volume on the recover of copper(Ⅱ).
Fig. 4.Effect of sample volume on the preconcentration factor
Effect of the ratio of alumina loaded on the column
The amount of alumina was varied from 1.0 to 4.0 g. The concentration of copper(Ⅱ) was maintained at 10 µg in a 100 mL sample volume. The results are shown in Fig. 5. Quantitative recovery of copper(Ⅱ) could be attained in the range 2.5-4.0 g of alumina. For amounts less than 2.5 g there was a significant reduction in the recovery of copper.
Fig. 5.Effect of amount of alumina on the recovery of copper (alumina, 10 µg copper(Ⅱ) in a sample volume of100 mL).
Effect of flow rate
The flow rate of 1-4 mLmin−1 was found to be suitable for optimum loading of the Cu(Ⅱ2-propylpiperidine-1-carbodithioate complex on the alumina adsorbent column. At higher flow rates, there was a reduction in the percentage adsorption of copper. This could be probably due to the insufficient contact time between the sample solution and the adsorbent. A flow rate of 2 mL min−1 was maintained for the elution of the complex.
Stability of the column
The stability of the column was tested using 10 µg Cu(Ⅱ) maintaining a sample volume of 100 mL. The adsorbed Cu(Ⅱ)-2-propylpiperidine-1-carbodithioate complex was eluted using 10 mL of polyethylene glycol-4.5 mol L−1 sulfuric acid mixture. The column could be used with good precision and quantitative recovery (>95%) for 10 cycles. Beyond 10 cycles, there was a considerable reduction in the recovery of copper.
Precision and detection limit
The precision studies were carried out at 0.2 µgmL−1 level of copper by carrying out 10 separate determinations using the above-mentioned procedure. The sample volume was maintained at 100 mL. The relative standard deviation of the method was found to be 4.0%. The sensitivity of the developed method is reflected by the limit of detection (LOD) studies, defined as the lowest concentration of copper(Ⅱ) below which quantitative sorption of the metal ion by the adsorbent is not perceptibly seen. The LOD for Cu(Ⅱ) was found to be 5 µgL−1.
Effect of diverse ions
The effect of diverse ions was studied at varying concentrations. The solid phase extraction studies were carried out as mentioned above using 10 µg Cu(Ⅱ) maintaining a sample volume of 100 mL. The studies indicated that Mg2+, Ca2+, SO42 , Cl, NO3, Zn2+, Co2+ and Ni2+ did not cause any significant reduction in the recovery of copper. The results are presented in Table 2 showing the recovery of copper with varying concentrations of metal ions. Except Fe3+, the recovery was found to be quantitative in the concentration range of the metal ions that was investigated. Since, the ions that are commonly present in water samples did not affect significantly the recovery of copper the method was applied to study the recovery of copper in electroplating wastewater and spiked tap water and well water samples.
Table 2.Effect of diverse ions on the recovery of 10 μg copper(Ⅱ) in a volume of 100 mL
Analysis of copper(Ⅱ) in electroplating wastewater and spiked water samples
The proposed method was applied to study the recovery of copper(Ⅱ) in electroplating wastewater sample. The electroplating wastewater sample had the following characteristics-pH 2.8, calcium: 65 mg L−1, magnesium: 40.5 mg L−1, chloride: 970 mg L−1, sulfate: 780 mg L−1. The wastewater sample was diluted to the required concentration and the preconcentration procedure was applied as mentioned above. The recovery of copper was found to be quantitative and the results are presented in Table 3. The proposed method was also applied to tap water (Tehran, 20 January, 2007) and well water (Saveh, 8 February, 2007) samples by spiking known concentrations of copper in varying sample volumes. The recovery of copper was found to be quantitative with an average relative standard deviation of 3.5% on triplicate measurements and the results are shown in Table 4.
Table 3.Analysis of copper(Ⅱ) in electroplating wastewater
Table 4.Recovery study in water samples
The proposed method for copper is simple, novel and could be effectively used for the solid phase extraction of copper. The preconcentration factor was 25 for a 250 mL sample volume. The method showed minimum interferences with commonly found ions in real samples and the recovery of copper was quantitative. The quantitative recovery of copper(Ⅱ) in electroplating wastewater and spiked water samples with a relative standard deviation of 3.5% reflects the validity and accuracy of themethod when applied to real samples. The adsorbent alumina exhibits better or comparable capacity values in comparison to most of the chelating matrices reported in the literature.48-53 The adsorbent exhibited good stability under the experimental conditions. The important features of the proposed method are its high sorption capacity with good enrichment factor. The developed method is sensitive in detecting copper(Ⅱ) at ppb levels. The alumina column was stable with the data reproducibility up to 10 cycles of continuous usage. Hence, it can be concluded that the proposed method can be used for the effective solid phase extraction of copper(Ⅱ).