COMBINATION OF SOLID-PHASE EXTRACTION BASED ON NANO ALUMINA AND LIQUID-LIQUID EXTRACTION FOR SELECTIVE DETERMINATION OF PALLADIUM IN FOOD SAMPLES © 2015 R. Golshaei, F. Shemirani1, M. Davudabadi Farahani
Department of Analytical Chemistry, University College of Science, University of Tehran P.O. Box 14155-6455, Tehran, Iran 1E-mail: email@example.com Received 03.07.2013; in final form 30.01.2014
A new simple and reliable method for rapid and selective extraction and determination of the trace Pd(II) was developed. Combination of solid phase extraction (SPE) and liquid—liquid extraction (LLE) of trace amounts of palladium in food and water samples has been done successfully. By coupling of SPE with LLE, advantages of both can be combined including high extraction recovery and preconcentration factor (PF), environmental friendliness, low cost and appropriate time of analysis. All conditions for SPE and LLE were optimized in order to reach high PF and extraction recovery. The metal ion was eluted completely with 1.5 mL 2 M HClO4 in acetylacetone and measured by ame atomic absorption spectrometry (FAAS). The optimum experimental conditions such as pH, sample volume, amount of sorbent and concentration of eluent on the sorption of Pd(II) were evaluated. Under the optimized conditions, the sorption capacity of the modied nano-y-alumina for Pd(II) was 13.5 mg/g. Preconcentration from 200 mL sample solution permitted a preconcentration factor of 266. The limit of detection of this method for Pd(II) was 0.2 ^g/L and the relative standard deviation (RSD) of 100 ng/mL of Pd(II) was 2.3% (n = 10).
Keywords: solid phase extraction, liquid—liquid extraction, nano alumina, palladium, flame atomic absorption, food samples.
Pd(II) is an economically important metal due to it's extensive use in metallurgy, various chemical syntheses, production of medicinal devices and jewelry manufacturing . Since the introduction of catalytic converters into modern automobiles, the determination ofpalladi-um has received increasing attention due to it's release into the environment . Moreover, metallic Pd(II) has an allergenic potential on human, rhino conjunctivitis and other serious health problems. The monitoring of Pd(II) in environmental samples has a great importance with respect to estimation of the future risk of the human health and the ecosystem .
Different kinds of conventional analytical techniques, such as graphite furnace atomic absorption spectrometry (GFAAS) , inductively coupled plasma—atomic emission spectrometry (ICP—AES) , inductively coupled plasma—mass spectrometry , neutron activation analysis , ultraviolet-visible spectrometry  and FAAS has been used for determination of Pd(II). FAAS has been widely used for the determination of trace amounts of Pd(II) because of the low cost, operational facility and high sample throughput.
Due to low concentrations of metal ions in environmental samples and matrix interferences, the direct determination of Pd(II) in complex matrices is limited and requires preconcentration and separation procedure to improve sensitivity and selectivity of analyses .
Various separation-preconcentration techniques like solvent extraction, electro-deposition , co-precipitation, cloud point extraction, membrane filtration and solid phase extraction were used for that purpose . Among these techniques, solid phase extraction is the most common technique for preconcentration of analytes and has the advantages of high recovery, short extraction time, low cost, simplicity and low consumption of organic solvents over liquid—liquid extraction [10, 11].
Several solid sorbents have been used for preconcentration and determination of palladium such as activated carbon , dithiocarbamate-coated fullerene C60 , modified silica gel  and modified multi-walled carbon nanotubes . Among these adsorbents, alumina is an important sorbent in the solid phase ex-
traction studies of heavy-metal ions . Al2O3 nano-particles exhibit good adsorption efficiency especially due to high capacity, mechanical strength, low temperature modication and great active sites for interaction with metallic species. y-Alumina anticipated to be more adsorptive active sites than a-alumina . Generally, the alumina surface is hydrophilic and has low adsorption affinity for organic compounds; however, when it is treated with sodium dodecyl sulfate (SDS), alumina will acquire high adsorption capability.
In this research, SDS-modied y-alumina nanoparti-cles were employed for preconcentration and determination of Pd(II) in water and food samples after elution with HClO4 in acetylacetone. This eluent forms two immiscible phases and the Pd(II) ions are extracted to the upper phase, so we can obtain a preconcentration factor two times higher than with other extraction methods.
Reagents. All chemicals used were of analytical reagent grade. A stock solution ofPd(II) (1000 mg/L) was prepared by dissolving of the proper amount of PdCl2 (Merck, Darmstadt( in double distilled water. The working standard solutions were prepared by appropriate step by step dilution. y-Alumina nanoparticles (NP) (Al2O3-gamma powder, 40—80 nm, nano Amor, Los Alamos, NM, USA) was activated by shaking with 5 M nitric acid and washed three times with distilled water. SDS (Schuchardt, Germany) was used without further purification. The chelating solution was prepared by dissolving 0.25 g of1-(2-pyridylazo)-2-naphthol (PAN) in 100 mL of ethanol. A buffer solution (pH 4; 4 M) was prepared by mixing of 82 mL of 4 M acetic acid and 18 mL of4 M sodium acetate. High purity HNO3 (65%, Suprapur, Merck, Darmstadt, Germany) and hydrogen peroxide were used for digesting of lettuce sample.
Instruments. The determination of Pd(II) was carried out using the Spectra AA from Varian 400, equipped with an air-acetylene burner. The lamp current was set at 5 mA. Measurement was carried out in the peak height mode at 244.8 nm, using a spectral band width of 0.2 nm and background correction with deuterium Lamp. The pH values were measured with a pH meter Model 692 from Metrohm (Herisau, Switzerland) supplied with a glass-combined electrode. Separation was assisted using a refrigerated centrifuge (Hettich, Universal 320R) equipped with an angle rotor (6-place, 9000 rpm, cat.no.1620A).
Preparation of modified nano alumina. Generally, the alumina surface is hydrophilic and has low adsorption affinity for organic compounds, however, when it is treated with SDS, alumina acquires high adsorption capability. Negatively charged SDS at lower pH is
sorbed on positively charged nano alumina, which helps the retention of hydrophobic compounds . SDS was sorbed on Al2O3 by shaking the solution containing SDS and nano-y-alumina for 15 min. However, by adding nitric acid, SDS forms hemimicelles on the surface of nano alumina, and this is the best way to homogeneously trap PAN. Because the solution pH is below the point of zero charge of alumina (pH 8.5), the alumina surface is positively charged, and anionic surfactants such as SDS will adsorb onto the surface.
Hydrophilic Hydrophobic ©yW4/\
Positivery charged surface Alumina particle
1.5 g of activated nano-y- alumina was added to 50 mL of solution containing 100 mg SDS and 1 mL of PAN solution in a 50 mL flask. The pH of the solution was adjusted to 2—2.5 with 3 M HCl and then the flask was shaken mechanically for 15 min . After this step, it was centrifuged, washed with distilled water
and then dried, yielding a fine powder.
Determination procedures. A known volume of sample solution containing Pd(II) in the range of 0.56— 45 ^g/L was prepared and the pH value was adjusted to
4 with acetate buffer. Solution was mixed with modified nano-y-alumina sorbent and the suspension was shaken for 5 min, then mixture centrifuged at 5000 rpm for
5 min. Afterwards, the metal ions retained on the sorbent, were eluted by using 1.5 mL of 2 M HClO^acety-lacetone. The extractant formed a binary immiscible solution, and finally the analyte in the upper phase of eluent was determined by FAAS.
Sample preparation. 0.75 g of lettuce sample was oxidized in the presence of 2.0 mL of H2O2 (30%) and 5.0 mL of HNO3 (65%). Then 20 mL of deionized water was added until the sample solution became limpid. Then the residues were cooled to room temperature and dissolved in 10 mL of triply distilled water, filtered through a filter paper and transferred into a flask.
RESULTS AND DISCUSSION
Extraction and preconcentration of precious metals, such as Pd(II), from nature is important. In order to obtain highly sensitive, accurate and reproducible results, the analytical parameters, including pH, extraction time, amount and capacity of the sorbent, desorption conditions, type and volume of eluent, were optimized for the preconcentration of Pd(II) ions. In all optimization steps, the concentration ofPd(II) was 100 ^g/L.
Effect of pH. Due to the sorption is dependant on pH, we have evaluated the influence of this parameter on the adsorption of Pd(II) ions. The effect of pH on the recovery of Pd(II) ions was studied with pH varying from 3.0 to 10.0. As shown in Fig. 1, the maximum
recovery of sorption is at pH 4. This is due to the combined effects of the formation of metal complexes and the electrostatic attraction between the positively charged surface of Al2O3 NPs and the anionic surfactants. In highly acidic conditions, the electrostatic attraction between negative charges of SDS and positive charges of the surface of Al2O3 NPs was not strong because most of the functional groups of the components were protonated leaving few available ionized groups, and the adsorption of SDS ion reduced. Therefore, pH 4 was selected for the next studies.
Extraction time and amount of modified nano-y-alumina. The total extraction time rel
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