Journal of the Chilean Chemical Society
versión ISSN 0717-9707
J. Chil. Chem. Soc. vol.56 no.2 Concepción 2011
J. Chil. Chem. Soc., 56, N° 2 (2011), págs.: 649-652
PRECONCENTRATION OF Cd2+ IN DIFFERENT SAMPLES BY CHEMICALLY MODIFIED SIO2-DHAQ NANOPARTICLES
ANUPREET KAURa, USHA GUPTAa
aDepartment of Chemistry, Punjabi University, Patiala, Punjab-INDIA. e-mail: firstname.lastname@example.org
A new analytical method using 1,8-dihydroxyanthraquinone modified SiO2 nanoparticles as solid-phase extractant has been developed for the preconcentration of trace amounts of Cd2+ in different samples. Conditions of the analysis such as preconcentration factor, effect of pH, sample volume, shaking time, elution conditions and effects of interfering ions for the recovery of analyte were investigated. The adsorption capacity of nanometer SiO2-DHAQ was found to be 85.83 μmol/g at optimum pH and the detection limit (3σ) was 0.60ng/mL. The extractant showed rapid kinetic sorption. The adsorption equilibrium of Cd2+ on nanometer SiO2-DHAQ was achieved within 20 min. Adsorbed Cd2+ was easily eluted with 6mL of 1M hydrochloric acid. The maximum preconcentration factor has been found to be 66.70. The method was applied for the determination of trace amounts of Cd2+ in different samples.
Keywords: Chemically modified SiO-nanoparticles, 1,8-dihydroxyanthraquinone, Preconcentration, Separation.
In recent years, the toxicity and effects of trace elements to the danger of public health and environment have attracted increasing attention in the fields of pollution and nutrition. Metals are prevalent in the environment. There are hundred of natural and anthropogenic sources of heavy metal pollution including the mining coal, natural gas, paper and cholro-alkali industries. They are derived from natural and anthropogenic sources. Cadmium is considered a non- essential and highly toxic element with a series of cumulative effect. Its toxicity is comparable to that of arsenic and mercury but its lethal potential is higher than that of any other metallic element. In the recent years there has been an increased concern over the concentration of cadmium in drinking and natural water due to its high toxicity and pollution to the environment especially the aquatic system. Nowadays there are many known sources of contamination of cadmium owing to the large number of its inorganic salts, which are used in catalytic and synthetic reactions, in Ni-Cd battery manufacturing and as stabilizers for plastics and additives. The FAO-WHO joint expert committee on food additives recommended a provisional maximum tolerance daily in take for cadmium of 1-1.2 μg/Kg body mass from all sources Therefore, accurate and reliable methods sometimes must be developed for cadmium determination. Because of their extremely low concentration, a preliminary concentration step i.e. preconcentration is usually necessary prior to the determination of the metals.[2-3] Nowadays, liquid- liquid extraction [4-6], ion-exchange resins7 and solid- phase extraction [8-9] are used for the preconcentration of different metal ions. Of all these methods, solid phase extraction has been widely used since it is simple, rapid and inexpensive, less polluting to the environment and can be easily automated. Many materials such as organic chelate resin, silica gel, activated carbon, activated alumina, zeolites and microcrystalline materials are commonly used as adsorbents.
Nowadays, nanometer materials have become more important owing to its special physical and chemical properties. The field of nanocomposite materials has received the attention, imagination and close scrutiny of scientists and engineer in recent years. These particles fall within the colloidal range, exhibiting typical colloidal properties. The surface atoms are unsaturated and can therefore bind with other atoms, possess high chemical activity. Nanoparticles exhibit intrinsic surface reactivity and high surface areas and can strongly chemisorb many substances. The size, surface structure and interparticle interaction of nanomaterials determine their unique properties and the improved performances and make their potential application in many areas. Nanoparticles such as TiO2  Al2O3 ZrO2, CeO2, and modified silica nanoparticles have been used for the preconcentration of many metal ions and give promising results when used for trace element analysis of different samples. In present work, chemically grafted SiO2-DHAQ nanoparticles were prepared by sol-gel  method and characterized by Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR). These nanoparticles have been used for the preconcentration and separation of cadmium prior to their determination by spectrophotometric method. 
Absorbance of Cd2+ was measured with UV-Vis Shimadzu-1700 spectrophotometer. The pH values were controlled by century Cp-901 digital pH meter. Infrared spectra was recorded on a Perkin Elmer FT-IR analysis.
Unless otherwise stated, all reagents used were of analytical reagent grade and all solutions were prepared with double distilled water. 3-aminopropyltriethoxysilane of GR grade was supplied by Acros Organics (USA). 1,8-dihydroxyanthraquinone(DHAQ) was obtained from Merck (Mumbai). Nanometer SiO2 was synthesized according to the method reported.14 The average diameter of the silica nanoparticles is 100nm as confirmed by Scanning Electron Microscopy. Stock solution of Cd2+ was prepared by dissolving spectral pure-grade CdSO4.8H2O and diluted as and when required. The glassware was washed with chromic acid and soaked in 5% nitric acid overnight and then cleaned with double distilled water before use.
Surface modification of SiO2 nanoparticles were performed in a 250mL flask. Nanometer SiO2 (1g) was dispersed into dry toluene (30mL), and then 3-aminopropyltriethoxysilane (4mL) was gradually added, with continuous stirring. The mixture was refluxed for 6h. The silylated nanometer SiO2 was filtered off, washed with toluene and ethanol and dried at 60°C for 3h. The product was transferred into the flask, and then 30mL diethyl ether was added followed by 2g of DHAQ and refluxed at 72°C for 4h. Reaction mixture was filtered under vaccum and the chemically modified nanoparticles were stored in dessicator.
Aliquots containing 0.5 μg of Cd2+ were prepared and pH was adjusted to 7.0 with 0.2 M Sodium tetraborate/boric acid buffer, 25mg of SiO2-DHAQ particles were added, and the mixture was shaken vigorously for 20mins to facilitate adsorption of metal ion onto the adsorbent. 6mL of 1M hydrochloric acid was used for elution of Cd2+ from the adsorbent. Eluent was filtered with cellulose nitrate membrane (47mm and 0.1um) and determined by standard spectrophotometric method.15 The results of analysis are given in Table 1.
RESULTS AND DISCUSSION
The modification of nanometer-sized material is usually required in order to prevent a conglomeration of particles and to improve its consistency in relation to other materials, such as organic polymers. In addition, the modification of nanometer-sized materials can improve the selectivity of nanometer-sized materials towards metal ions. Organosiloxane is the most often used modifiers, in which one side is linked with the inorganic nanometer-sized materials, and the exposed end is the analytical functional group. The modified nanoparticles SiO2-DHAQ were characterized by FT-IR and SEM.
The average diameter of the nanoparticles SiO2, SiO2-APTES and SiO2 -DHAQ was 100nm, 1um and 2um confirmed by Scanning Electron Microscopy. Figure 1, 2 and 3 show the scanning electron micrographs of these nanoparticles respectively.
The chemical grafting of 1,8-dihydroxyanthraquinone on the surface of nanometer SiO2 was confirmed by FT-IR spectrum. The main absorption peaks of nanometer SiO2 (3448.0, 1642.5, 1404, 1070.2, 964.2, 798.8 cm-1) are in agreement with standard spectrum of SiO2. 16 Many new peaks appeared in FT-IR spectrum by grafting with DHAQ as peak at 1652 cm-1 arises both from C=O and C=N stretching. The phenyl ring vibrations appear at 1500, 1467.5, 1377.5 and 1349.7 cm-1.
The adsorption of Cd2+ on SiO2-DHAQ nanoparticles was studied in pH range of 3.4 to 10.0 following the recommended procedure. The results of effect of pH on the recoveries of the metal ions are shown in Figure 4. It can be seen that a quantitative recovery (≥95%) was found for Cd2+ in the pH range of 7.0-10.0. Cd(II) is a soft acid and has good affinity towards hard base ligands containing the oxygen, sulphur and nitrogen groups.
The buffer solution containing increases the selectivity and does not affect the sensitivity. Therefore, the progressive decrease in the progressive decrease in the retention of Cd(II) at low pH is due to competition between the hydrogen ion and Cd(II) for adsorption on SiO2-DHAQ Nanoparticles.
Elution of Cd2+ from SiO2-DHAQ nanoparticles extractant was investigated by using various concentrations of hydrochloric acid. Quantitative recoveries (≥95%) of Cd2+ were obtained using 6mL of 1M hydrochloric acid as eluent. Therefore, 6mL of 1M of hydrochloric acid was used as eluent in subsequent experiments. The results of effect of eluent concentration and volume are given in Table 2 and Table 3.
To test the effect of amount of extractant on quantitative retention of analyte, different amount of nanometer SiO2-DHAQ were taken by the general procedure. Quantitative adsorption of the Cd2+ was obtained in the range of 5-40mg of SiO2-DHAQ nanoparticles.25mg of adsorbent was found to be sufficient for further studies. The results are shown in Figure 5.
The adsorption of Cd2+ on 25mg of nanometer SiO2-DHAQ was studied for different shaking time (10-45min). The results indicated that within 20mins the extraction percentage of Cd2+ ≥95% was achieved. The results are shown in Figure 6.
The adsorption capacity is an important factor as it determines how much adsorbent is quantitatively required to concentrate the analytes from a given solution. A breakthrough curve was obtained by plotting the concentration (mg/L) vs. the mmol of Cd2+ adsorbed per gram. From the breakthrough curve the amount of modified nanometer SiO2-DHAQ for Cd2+ was found to be 85.83μmol/g at pH 7.0. The results are shown in Figure 7.
In order to explore the possibility of concentrating low concentration of analytes from large volumes, the effect of sample volume on the retention of metal ions was also investigated. For this purpose 50, 100, 150, 200, 250, 300 and 400mL of the sample solutions containing 1.0 μg Cd2+ was shaken, quantitative recoveries (≥95%) were obtained for sample volume of ≤300mL for Cd2+. Therefore, 50mL of sample volume solution was adopted for the preconcentration of analytes from sample solutions. The results are shown in Figure 8.
The effect of common coexisting ions on the sorption of Cd2+ was investigated. In these experiments, a solution of 5.0 mgmL-1 of each analyte that contains the added interfering ion was analyzed according to the recommended procedure. The tolerance limit (mgL-1) for anions such Cl-1, Br- 1, NO3-1 , SO42-, PO43- and EDTA were 0.12, 0.008, 0.06, 0.07, 0.04 and 1.1 respectively. The tolerance limits in mgmL-1 for Ca2+, Mg2+, Co2+, Ni2+, Fe3+, Al3+, Cr3+, Pb2+, Mn2+ and Zn2+ were 0.41, 0.40, 0.67, 0.02, 0.03, 0.24, 0.11, 0.07, 0.06 and 0.07 respectively. These results demonstrate that SiO2-DHAQ nanoparticles can be used for the preconcentration Cd2+ in different samples, because common cations and anions at their normal levels do not affect the sorption efficiency of nanoparticles for the cadmium ions.
Under the optimized conditions, five portions of Cd2+ standard solutions were enriched and analyzed simultaneously following the experimental method. The relative standard deviation (RSD) of the method was 2.2 % for the determination of 5.0 μg Cd2+ in 100mL water samples respectively. The detection limit of this method for Cd2+ was 0.60 ng/mL.
The developed method has been applied for the determination of trace amounts of Cd2+ in food samples (potato chips, biscuits) and spiked tap water, mineral water, waste water, synthetic (nail paints). For analysis, 200mL of sample solution was taken and studied by general procedure.
In order to establish the validity of the proposed procedure, the method was applied to the determination of the content of the studied elements in the standard reference material - Oriental Tobacco Leaves. The results are given in Table 4, 5 and Table 6.
The preconcentration method described by using 1,8-dihydroxyanthra-quinone anchored silica nanoparticles for the determination of Cd2+ in water samples has a good accuracy, repeatability and sensitivity. The preparation of sorbent is easy and the preconcentration factors obtained are sufficiently large. The results of comparison of preconcentration factor are given in Table 6.
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(Received: March 18, 2010 - Accepted: March 18, 2010).