Preconcentration of Heavy Metals Using Chemically Modified Submicron Nanoparticles
by Anupreet Kaur and Usha Gupta,
Department of Chemistry, Patiala, Punjab, India
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 Ni(II), Co(II) and Cu(II). 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. Under optimum pH conditions adsorption capacity and limit of detection were found to be 64.66, 44.84 and 37.66 µmol/g and 0.48, 0.54 and 0.71 µg/L for Ni(II), Co(II) and Cu(II), respectively. The maximum preconcentration factor was 60, 62 and 50 for Ni(II), Co(II) and Cu(II). The method has been applied for the determination of trace amounts of Ni(II), Co(II) and Cu(II) in various food and pharmaceutical samples.
Heavy metal ion contamination represents a significant threat to the ecosystem because of the severe toxicological effects on living organisms, even at very low concentrations. The determination of metal ions at micro and trace level is very important in the context of environmental pollution protection, food and agriculture chemistry, and high-purity materials development. The main goal today is to adopt appropriate methods and to develop suitable sensitive analytical techniques, and there is a crucial need for preconcentration of trace elements prior to analysis because of their frequent low concentrations in various matrices [1-6].
Table1: Effect of concentration of hydrochloric acid solution on elution of Ni(II), Co(II) and Cu(II) (n=3).
Solvent extraction and solid phase extraction are arguably the most commonly executed forms of preconcentration and for many years they have dominated approaches to the enrichment of pesticides, drugs and trace compounds. Nowadays, separation/preconcentration techniques for metal ion determination include liquid-liquid extraction, ion-exchange resins and solid-phase extraction. Solid phase extraction has been widely used in comparison with traditional extraction techniques, because it is simple, rapid and inexpensive, less polluting to the environment and can be easily automated. Nowadays, nanometer materials have become more important owing to their special physical and chemical propertities. 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 improved performances and make them suitable for application in many areas [7-8]. Nanoparticles such as TiO2, Al2O3, ZrO2, and CeO2 have been used for the preconcentration of many metal ions and give promising results when used for trace element analysis of different samples. In this present work, chemically grafted SiO2-DHAQ nanoparticles have been used for the preconcentration and separation of nickel, cobalt and copper prior to their determination by spectrophotometry.
Apparatus : Absorbance of Ni(II), Co(II) and Cu(II) was measured with a UV-Vis Shimadzu-1700 spectrophotometer. The pH values were controlled by a Century Cp-901 digital pH meter. An infrared spectrum was recorded on a PerkinElmer FT-IR system.
Reagents and standard solutions : Unless otherwise stated, all reagents used were of analytical reagent grade and all solutions were prepared with double distilled deionized water. The 3-aminopropyltriethoxysilane of GR grade was supplied by Acros Organics (USA).1,8-dihydroxyanthraquinone (DHAQ) was obtained from Merck (Germany). Nanometer SiO2 was synthesized according to the method reported in . The glassware was washed with chromic acid and soaked in 5% nitric acid overnight and then cleaned with double distilled water before use.
Modification process : Surface modification of SiO2 nanoparticles were performed in a 250 mL flask. Nanometer SiO2 (1 g) was dispersed into dry toluene (30 mL), and then 3-aminopropyltriethoxysilane (4 mL) was gradually added, with continuous stirring. The mixture was refluxed for 6 h. The silylated nanometer SiO2 was filtered off, washed with toluene and ethanol and dried at 60 °C for 3 h. The product was transferred into the flask, and then 30 mL diethyl ether was added followed by 2 g of DHAQ and refluxed at 72 °C for 4 h. Reaction mixture was filtered under vaccum.
General procedure : Aliquots of sample solutions containing Ni(II), Co(II) and Cu(II) were prepared and pH was adjusted to the desired value with ammonia/ammonium chloride, boric acid/borax buffer. Then, 20 mg, 40 mg and 30 mg at pH 6.5, 7.0 and 7.62 were adjusted for the analysis of Ni(II), Co(II) and Cu(II), respectively. These metal ions were eluted with hydrochloric acid and then filtered from grafted nanoparticles and determined by standard spectrophotometry .
Results and Discussion
Scanning electron eicroscopy (SEM) : The average diameter of the nanoparticles SiO2 and SiO2-DHAQ were 100 nm, and 2 μm confirmed by scanning electron microscopy. Figures 1 and 2 reveal the average size of SiO2 nanoparticle and SiO2-DHAQ nanoparticles, respectively.
FT-IR spectrum analysis : The chemical grafting of 1,8-dihydroxyanthraquinone on to the surface of nanometer SiO2 was confirmed by FT-IR. It revealed that 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 . Many new peaks appeared in FT-IR 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.
Effect of pH on enrichment recovery : The adsorption of Ni(II), Co(II) and Cu(II) on nanometer SiO2-DHAQ was studied at different pH values (3.4-10.0) by general procedure. The results of the recoveries of the metal ions are shown in Figure 3. Quantitative recovery (>95%) for Ni(II) in pH range of 6.5-7.0, for Co(II) and Cu(II) in the pH range of 7.0-8.0 was obtained.
Effect of eluent concentration and volume : Because the adsorption of Ni(II), Co(II) and Cu(II) on nanometer SiO2-DHAQ at pH Effect of nanometer SiO2-DHAQ amount : To test the effect of amount of extractant on quantitative retention of analyte, different amounts (5-45mg) of nanometer SiO2-DHAQ were added into the solution following the experimental procedure. Quantitative extraction of the Ni(II), Co(II) and Cu(II) was obtained in the range of 5-45 mg of nanometer SiO2-DHAQ. 20 mg, 40 mg and 30 mg amounts of nanometer SiO2-DHAQ as extractant were found to be sufficient for further studies of Ni(II), Co(II) and Cu(II) metal ions . The results are shown in Figure 6.
Effect of shaking time : The adsorption of Ni(II), Co(II) and Cu(II) on nanometer SiO2-DHAQ were studied for different shaking time (10-45 mins). The results are shown in Figure 7.
Adsorption capacity (QS) : 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 µmol of Ni(II), Co(II) and Cu(II) adsorbed per gram. From Figure 8 the breakthrough curve the adsorption capacities for Ni(II), Co(II) and Cu(II) were found to be 64.66, 44.84 and 37.66 µmol/g, respectively.
Effect of sample volume : In order to explore the possibility of concentrating trace analytes from large volumes, the effect of sample volume on the retention of metal ions was also investigated. For this purpose 50, 100, 125, 150, 200, 250, 300, 350 and 400 mL of the sample solutions containing 1.0 µg Ni(II), Co(II) and 5.0 µg Cu(II) were shaken, and quantitative recoveries (>95%) were obtained for sample volume of ≤300 mL for Ni(II), Co(II) and Cu(II) (Figure 9). Therefore, the concentration factors were 60, 62 and 50 for Ni(II), Co(II) and Cu(II), respectively.
Table 2: Effect of volume of hydrochloric acid solution on elution of Ni(II), Co(II) and Cu(II) (n=3).
Effect of coexisting ions : The effect of common coexisting ions on the sorption of Ni(II), Co(II) and Cu(II) was investigated. In these experiments, a solution of 5.0 µg/mL of each analyte that contains the added interfering ion was analysed according to the recommended procedure. The tolerance limit (mg/L) for anions such Cl-1, Br-1, NO3-1, SO42-, PO43- and EDTA were 0.20, 0.26, 0.24, 0.15, 0.05 and 0.01, respectively. The tolerance limits in mgmL-1 for Ca(II), Mg(II), Cr(III), Pb(II), Mn(II), Zn(II), Cd(II), Fe (II) and Hg (II), were 0.25, 0.01, 0.010, 0.08, 0.075, 0.011, 0.07, 0.08 and 0.011, respectively. These results demonstrate that SiO2-DHAQ nanoparticles can be used to preconcentrate Ni(II), Co(II) and Cu(II) in food samples, because common cations and anions at their normal levels do not adversely affect the sorption efficiency of nanoparticles for these three metal ions.
Analytical Precision and Detection Limit
Under the optimum conditions, three portions of Ni(II), Co(II) and Cu(II) standard solutions were enriched and analysed simultaneously following the experimental method. The relative standard deviation (RSD) of the method was 3.1%, 3.7% and 4.3% for the determination of 0.60 µg, 0.45 µg and 0.54 µg for Ni(II), Co(II) and Cu(II) in 100 mL food samples respectively. The detection limits were 0.48, 0.54 and 0.71µg/L for Ni(II), Co(II) and Cu(II), respectively.
The developed method has been applied to the determination of trace amounts of Ni(II), Co(II) and Cu(II) in food samples (biscuits and potato chips).For food samples, a mixture of nitric acid and hydrochloric acid (8:4) was applied to all the samples of potato chips and biscuits, which were then digested.  The results of analysis are given in Table 1.
Determination of nickel in edible oil : A 0.2-15 g sample of oil (vegetable oil) was placed in a beaker and decomposed with concentrated nitric acid. The dried sample was heated in a muffle furnace at 600 ˚C for 1 hour and then allowed to cool. After addition of a few drops of conc. nitric acid, it was dried and again heated for 1 hour at 700 ˚C in a muffle furnace. The resultant ash was dissolved in concentrated hydrochloric acid, diluted with double distilled water, and the final volume was made up to 100 mL in a standard flask. An aliquot of the resulting solution was taken and determined by the general procedure. The results of the determination are given in Table 2.
Table 3: Determination of heavy metal ions concentration (μg /g) in different brands of potato chips (PC-1 to PC-2) and biscuits (BC-1 to BC-2) available in Patiala City, India.
Determination of copper in milk : 500 mL of milk was placed in a beaker and dissolved in concentrated nitric acid and evaporated to dryness. The residue was dissolved in hydrochloric acid, filtered, neutralized with ammonia solution and then diluted with double distilled water in a standard flask. The results of the determination are given in the Table 2.
The preconcentration method described using 1,8-dihydroxyanthraquinone anchored silica nanoparticles for the determination of Ni(II), Co(II) and Cu(II) 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 3.
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Anupreet Kaur is a Research Scholar in the Department of Chemistry in Punjabi University, Patiala. She joined her Ph. D under the supervision of Dr. Usha Gupta. Her work in Nanotechnology and Material Science includes Synthesis of Nanoparticles and analytical applications of these in preconcentration and separation of environmental pollutants. She has ten publications her credit in the field of nanotechnology.
Usha Gupta done her M.Sc, and M.Phil from Punjabi University, Patiala (India) and received her Ph.D degree in 1986 from the same University. From 1986 to 1988 she worked as a lecturer in M.M Degree College, Patiala. From 1988 to 1993 she worked as a post doctoral research associate at Punjabi University, Patiala and currently working as a Sr. lecturer of Inorganic Chemistry at the same University. Her fields of research work in Nanotechnology and Analytical Chemistry are synthesis of chemically modified nanoparticles for the removal of toxic pollutants in natural water and industrial effluent samples and simultaneous determination of toxic metal ions and pesticides using H-Point Standard Addition Method.