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Modified ionic liquid-coated nanometer TiO2 as a new solid phase extraction sorbent for preconcentration of trace nickel
Accepted Manuscript Title: Modified ionic liquid-coated nanometer TiO2 as a new solid phase extraction sorbent for preconcentration of trace nickel Author: Mohammad Amjadi Azam Samadi PII: DOI: Reference:
S0927-7757(13)00371-3 http://dx.doi.org/doi:10.1016/j.colsurfa.2013.04.059 COLSUA 18390
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
19-2-2013 18-4-2013 23-4-2013
Please cite this article as: M. Amjadi, A. Samadi, Modified ionic liquid-coated nanometer TiO2 as a new solid phase extraction sorbent for preconcentration of trace nickel, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2013.04.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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Highlights
1-hexadecyl-3-methylimidazolium bromide was coated on TiO2 nanoparticles at pH 10.
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TAN was immobilized on IL-coated nanometer TiO2 and used as an efficient sorbent.
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TAN-IL-nanometer TiO2 has an extremely high adsorption capacity for Ni(II) ions.
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The sorbent was applied to the preconcentration of Ni from food and water samples.
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Modified ionic liquid-coated nanometer TiO2 as a new solid phase extraction sorbent for preconcentration of trace nickel
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Mohammad Amjadi*, Azam Samadi Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz
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5166616471, Iran
* Corresponding author
Email: [email protected]
Tel: +984113393109; Fax: +984113340191
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Abstract
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In this work, a high-capacity solid-phase extraction sorbent was developed for preconcentration
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of trace amounts of nickel ions prior to their determination by flame atomic absorption
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spectrometry (FAAS). The sorbent was prepared by immobilization of 1-(2-thiazolylazo)-2-
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naphthol (TAN) on nanometer-sized TiO2 coated with the ionic liquid, 1-hexadecyl-3-
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methylimidazolium bromide (C16mimBr). The chemical conditions for preparation of sorbent
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including pH and amounts of TAN and C16mimBr were optimized. Experimental conditions for
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preconcentration of Ni, elution conditions and the effect of interfering ions on the recovery of the
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analyte were also investigated. Under the optimum conditions, the calibration graph was linear in
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the range of 2.0 - 400 µg L-1 with a detection limit of 0.8 µg L-1. The adsorption capacity of the
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sorbent for Ni(II) was found to be 630 mg g-1. The preconcentration method coupled with FAAS
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was successfully applied to the determination of Ni(II) in various water and food samples.
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Keywords: Ionic liquid; TiO2 nanoparticles; Solid phase extraction; 1-(2-thiazolylazo)-2-
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naphthol; Nickel.
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1. Introduction
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Nickel is an abundant natural element with relatively high toxicity. It is known that exposure to
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nickel and its compounds can lead to serious health problems, including contact dermatitis and
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respiratory system diseases. Both noncancerous and cancerous respiratory effects have been
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observed in humans and animals exposed to airborne nickel compounds. Chronic bronchitis,
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emphysema, pulmonary fibrosis, and impaired lung function have been observed in nickel
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welders and foundry workers. The most commonly reported adverse health effect associated with
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nickel exposure is contact dermatitis. Nickel compounds are also known to be human
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carcinogens [1]. Therefore, monitoring of this element in environmental and food samples is an
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important analytical task. Flame atomic absorption spectrometry (FAAS) is a powerful and well-
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established technique for this purpose. But direct determination of Ni at low concentrations is
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difficult because of insufficient sensitivity of this technique as well as the matrix interferences
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occurring in real samples. Therefore‚ a preliminary separation and preconcentration step is often
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required.
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Solid phase extraction (SPE) is a cost-effective and important sample preparation technique that
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offers important advantages including low solvent usage, disposal costs and extraction time [2].
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Nanometer-sized metal oxides such as Al2O3, SiO2 and TiO2 have attracted increasing interest as
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sorbents in SPE due to their high surface area [3–10]. The basic disadvantage of these solid
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sorbents is the lack of metal selectivity, which leads to interfering of other species with the target
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metal ion(s). To overcome this problem, chemical or physical modification of the sorbent surface
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with some organic compounds, especially chelating agents, is often used to load the surface with
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some donor atoms such as oxygen, nitrogen, sulfur and phosphorus [11]. Metal oxides under
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normal pH conditions in water are hydrophilic and are not favorable for the adsorption of
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hydrophobic organic compounds. For this reason, a surface modification method based on
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hemimicelles or admicelles has been proposed [12,13]. Hemimicelles were generated by the
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adsorption of an ionic surfactant such as sodium dodecyl sulfate (SDS) on a metal oxide (such as
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alumina or silica) surface. Hence, hydrophobic chelating agents can be incorporated in the
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hemimicelles on which the desired trace metals were retained by complexation. Several
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nanometer-sized metal oxides coated by surfactants have been used as adsorbents for
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preconcentration of metal ions [5,14,15].
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Ionic liquids (ILs) are salts with melting points below ca. 100◦C [16]. They belong to a relatively
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new class of solvents which are ion pairs consisting an inorganic ion and an amphiphilic part
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with a hydrophilic polar head group (ionic in nature) and a hydrophobic hydrocarbon chain (the
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tail). In recent years, ILs have attracted great attentions due to their unique chemical and physical
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properties such as nonvolatility, excellent solvation qualities, non-flammability and high thermal
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stability [17,18]. ILs have been widely utilized in sample preparation processes, including liquid-
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liquid extraction [19–21], liquid-phase microextraction [22,23], solid-phase extraction [24,25]
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and solid-phase microextraction [26,27]. A new kind of ILs that is able to form micelles in
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aqueous solution has been recently reported [28–30] and used in analytical applications [31–35].
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Such ILs, which can be classified as cationic surfactants, constitute a new area of surfactant
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development; especially considering the limited number of traditional cationic surfactants. It is
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important to highlight the ease of tuning the properties of IL-based surfactants by simple
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chemical modifications of the cation/anion pair. Thus, up to 48 IL-based surfactants have been
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reported and their colloidal and interfacial behavior have been studied [31].
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In this work, a new sorbent with high adsorption capacity was prepared by immobilization of 1-
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(2-thiazolylazo)-2-naphthol (TAN) on nanometer TiO2 coated with surfactant-based ionic liquid,
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1-hexadecyl-3-methylimidazolium bromide (C16mimBr). The potential of the adsorbent for the
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preconcentration of trace Ni was assessed using column method.
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2. Experimental
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2.1. Instruments
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An Analytik jena flame atomic absorption spectrometer model Nova 400 (Jena, Germany)
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furnished with an air–acetylene flame and a nickel hollow cathode lamp, operated at 5 mA, was
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used. The instrument was set at a wavelength of 232.0 nm and slit width of 0.2 nm. A Metrohm
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model 654 was used for pH measurements. A peristaltic pump was used in the separation and
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preconcentration process and a laboratory-made microcolumn packed with 0.03 g of the sorbent
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was used for preconcentration.
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2.2. Reagents and solutions
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Doubly distilled de-ionized water (obtained from Ghazi Serum Co., Tabriz, Iran) was used for
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the preparation of all the solutions. The standard solution of Ni(II) (1000 mg L−1) was prepared
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by dissolving proper amount of Ni(NO3)2·6H2O from Merck (Darmstadt, Germany) in water and
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diluted to 100 mL in a volumetric flask. The required concentration of nickel solution was
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prepared by appropriate dilution of the stock solution. A 0.5 M nitric acid solution was prepared
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by appropriate dilution of concentrated HNO3 (Merck). A 2.0 g L-1 solution of 1-hexadecyl-3-
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methylimidazolium bromide (C16mimBr) was prepared by dissolving 0.2 g of IL (obtained from
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KimiaExir, Tehran, Iran) in water and diluted to 100 mL in a volumetric flask. A 0.01% (w/v) 1-
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(2-thiazolylazo)-2-naphthol (TAN) solution was prepared by dissolving 0.01 g of TAN (Fluka,
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Tokyo, Japan) in 50 mL of 0.1 M NaOH solution and making up to 100 mL with distilled
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deionized water. Nanometer-sized TiO2 with an average diameter of 21 nm was obtained from
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Evonik Industries AG Silica (Essen, Germany). Phosphate buffer solution (0.1 M, pH=7.0) was
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prepared by dissolving appropriate amount of sodium phosphate (Merck) in 100 mL of deionized
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water and adjusting the pH with hydrochloric acid.
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2.3. Preparation of sorbent
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A 0.5 g portion of TiO2 nanoparticles was placed in a 50 mL beaker. After adding 25 mL of 2.0 g
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L-1 of C16mimBr IL into the beaker, the pH was adjusted to 10.0 with 1 M NaOH while stirring
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the suspension with a stirrer. The mixture was sonicated for 5 min to suspend the NPs. Then 5
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mL of 0.01% TAN solution was added to the mixture. After mixing for 15 min, the modified
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nanometer-sized TiO2 was filtered off and washed with water then dried at room temperature for
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24 h.
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A similar procedure by using 25 mL of 2.0 g L-1 of cetyltrimethylammonium bromide (CTAB,
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Merck) was applied to prepare TAN-modified CTAB-coated TiO2 nanoparticles for comparison.
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2.4. Preparation of column
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A homogenous mixture of 0.03 g of modified nanometer-sized TiO2 and 0.03 g of glass beads
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(212-300 µm, Sigma, St. Louis, USA) was introduced into a polyethylene microcolumn (4 mm
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i.d., 3–4 cm length) plugged with a small portion of glass wool at both ends. Before use, 10 mL
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doubly distilled water was passed through the column in order to clean and condition it.
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2.5. General procedure
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Fifty mL aqueous sample solution containing 5–400 μg L−1Ni(II) was prepared and its pH value
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was adjusted to 7.0 with 1 mL of 0.1 M phosphate buffer. The solution was passed through the
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microcolumn by using a peristaltic pump adjusted to the desired flow rate. Afterwards, the
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retained nickel was eluted with 1.0 mL of 0.5 M HNO3 solution and nickel was determined by
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FAAS in the effluent.
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2.6. Real sample preparation
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Food samples including black tea, wheat flour, corn flour, maize starch and starch were
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purchased from local supermarkets. Dry ashing method was used for preparation of these
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samples [36]. A portion of 1.0 g of samples was accurately weighed into a crucible and heated
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gently on a hot plate at low temperature until a charred solid formed. It was then ashed in a
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muffle furnace at 500ºC for 3 h. After cooling, the resulted white ash was carefully moistened
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with 1.0 mL water, and then 2.0 mL of concentrated HNO3 (Suprapure, Merck) and 1.0 mL of
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concentrated HClO4 (Merck) were added in sequence. The solution was then heated to dryness
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on a hot plate at 200ºC. The resulting residue was treated with 1.0 mL concentrated HNO3 and
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5.0 mL water and then heated gently for about 5 min until the solution turned clear. After cooling
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at room temperature, the pH of obtained solution was adjusted at 7.0 with ammonia (5 M). By
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adding ammonia some interfering ions such as Zn(II) and Fe(III) were precipitated. After
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centrifugation and separation of precipitates, the supernatant was transferred into a 50 mL
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volumetric flask and 1.0 mL phosphate buffer (0.1 M) was added and diluted to the mark with
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water. Spiked samples were prepared by adding small volumes of Ni(II) standard solution (10
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mg L-1) to 1.0 g of powdered samples. After drying for 2 days at ambient temperature, samples
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were subjected to the same procedure described above.
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Two water samples including tap water and well water were selected and the proposed method
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was applied to determine their nickel contents. Tap water was collected from our laboratory
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(University of Tabriz, Tabriz, Iran) and well water was collected from Tabriz, Iran. The water
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samples were filtered through a Millipore 0.45 µm pore-size membrane into polyethylene
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bottles. Their pH values were adjusted to 7.0 by addition of phosphate buffer and analyzed
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according to the general procedure.
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3. Results and discussion
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3.1. Adsorption of TAN on IL-Coated Nanometer TiO2
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The studies conducted on TiO2 surface modification using organic agents were aimed at altering
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its surface character in the hydrophilic-hydrophobic system. The pH of the solution influences
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the surface charge of oxides, i.e. below the point of zero charge (pHpzc) metal oxides possess a
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positive surface charge, whereas above that pH they have a negative charge. The pHpzc is 6.2 for
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nanometer TiO2 [37]. 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) is considered as a
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surfactant with hydrophobic alkyl chain and hydrophilic group of imidazolium ring. The critical
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micelle concentration (CMC) value for this IL has been reported to be 0.76 mM. Above its CMC
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value, C16mimBr could assemble into micelles with spherical simple structures [28]. Based on
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these facts, it is expected that [C16mim]+ cations at concentrations lower than its CMC and at pH
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higher than 6.2 are adsorbed on the negatively charged nanometer TiO2 surface through
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electrostatic attraction. As a result of this interaction, which leads to the formation of
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hemimicelle on the nanometer TiO2 surface, the nanoparticles become hydrophobic and when
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chelating agent is added to them, retention of chelating molecules on the nanometer TiO2 surface
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occur via interactions between the hydrophobic groups of the chelating agent and the exposed
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hydrocarbon tails in the hemimicelles. 1-(2-Thiazolylazo)-2-naphthol (TAN) is a well-
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characterized organic chelating ligand which forms complexes with some transition metals [38].
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Since the hydrophilic groups of TAN are oriented towards the aqueous phase, metal ions were
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easily adsorbed on the nanometer TiO2-TAN surface.
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3.2. Optimization of conditions for preparation of the sorbent
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Optimized amounts of IL and TAN in sorbent preparation process were investigated with batch
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experiments. For this purpose, several sorbents with variable amounts of C16mimBr (10–70 mg)
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and TAN (0.1–2 mg) were prepared. Then 10 mg of each sorbent was added to 50 mL of 20 mg
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L-1 Ni(II) solution and stirred vigorously for 60 min so that equilibrium was attained. After
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centrifugation, the concentration of the Ni(II) ions in the solution was determined by FAAS and
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the adsorbed amount of Ni(II) was obtained.
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Fig. 1.a depicts the percentage of adsorbed Ni(II) as a function of the added IL amount. When
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the IL amount changed from 10 to 50 mg, the adsorption of Ni(II) on the nanometer TiO2
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increased because of the increase of adsorbed TAN molecules as a result of increasing the
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hydrophobicity of sorbent. With 50 mg of C16mimBr, the highest amount of adsorbed Ni(II) was
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achieved. When the amount of C16mimBr continued to be increased, there was no more increase
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in the adsorption amount, indicating that the surface of nanometer TiO2 was saturated by
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C16mimBr.
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The effect of amount of TAN in sorbent preparation process in the range of 0.1 to 2 mg was
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investigated and the results are shown in Fig. 1.b. Adsorption percentage of Ni increased with
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increasing TAN amount up to 0.5 mg and with further increase in the amount of TAN, the
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adsorption amount did not increase significantly. It could be considered that the surface of
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nanometer TiO2-C16mimBr was saturated by TAN.
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pH is one of the important influencing factors on the adsorption behavior of the hemimicelles
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system because the surface charge of nanometer TiO2 is pH dependent. To investigate the effect
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of pH on sorbent preparation process, several sorbents with fixed amount of TiO2 were prepared
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in IL-TAN solutions with different pH values in the range of 7-11. After drying the sorbents,
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they were used for preconcentration of Ni. As shown in Fig. 1.c, nanometer TiO2–IL-TAN
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exhibited low adsorption for Ni when the pH was between 7.0 and 8.0. With the increase of pH,
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the adsorption amount obviously increased and reached the maximum value at pH 10.0. This can
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be attributed to the fact that the surface of nanometer TiO2 is negatively charged when the pH
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value is quite higher than the point of zero charge (6.2), so the [C16mim]+ ions was easily
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adsorbed onto the surface of TiO2. Based on the results, pH 10.0 was chosen as the optimum
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value for sorbent preparation.
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3.3. Optimization of extraction conditions
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Extraction experiments were performed in a continuous flow mode by passing sample solutions
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through a microcolumn packed with the sorbent at flow rate of 1.0 mL min-1.
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The effect of pH on the adsorption of Ni ions onto the sorbent was investigated in the range of
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3.0–9.0. The results, shown in Fig. 2.a., revealed that a quantitative recovery (≥98%) occurs at
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the pH range of 6.0–9.0. Hence, pH 7.0 was selected for further studies. It was found that
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phosphate buffer with final concentration of 0.02 M was adequate for adjusting pH at this value.
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Elution of Ni from the nanometer TiO2-TAN adsorbent was investigated by using HCl, HBr and
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HNO3 as eluent following the general procedure. The obtained results showed that HNO3 is the
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better eluent because of better reproducibility and highest recovery of determinations with this
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eluent. Hence, the effect of HNO3 volume on the recovery of analyte was studied. It was found
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that with 1.0 mL of 0.5 M HNO3 quantitative recoveries (ca. 100%) could be obtained. The
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obtained results are given in Fig. 2.b.
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Since the retention of elements on the sorbent depends on the flow rate of the sample solution, its
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effect was examined by passing 50 mL of sample solution through microcolumn with a
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peristaltic pump. The flow rates were adjusted in the range of 0.5–2.5 mL min-1. As shown in
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Fig. 2.c., recovery of Ni ions decreased by increasing the flow rate beyond 1.5 mLmin-1. Thus, a
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flow rate of 1.0 mL min-1 was selected for subsequent experiments.
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In order to explore the possibility of enriching low concentrations of analyte from large volumes
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of solution, the effect of sample volume on the retention of Ni(II) ions was also investigated. For
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this purpose, volumes of 25, 50, 100, 150 and 200 mL of sample solution containing 2.0 µg of Ni
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were passed through the column at the optimum flow rate. As shown in Fig. 2.d., quantitative
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recovery (>98%) was obtained for sample volume of 50 mL for Ni ions. Therefore, this volume
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was adopted for the preconcentration of analyte from sample solutions.
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3.4. Adsorption capacity
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The adsorption capacity is an important factor, because it determines how much sorbent is
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required to quantitatively concentrate the analyte from a given solution. The adsorption capacity
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of the sorbent was determined using the batch technique [14]. A portion of 10 mg of sorbent was
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placed into a 50 mL beaker, 25 mL of nickel solution (with concentrations in the range of 1.0–
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800 mg L-1) was added and the pH was adjusted at 7.0 with phosphate buffer. The resulting
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mixture was stirred for 60 min. The solid phase was separated by centrifugation and the metal
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ion was determined in the supernatant solution by FAAS. The results are shown in Fig. 3a. The
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analysis of the isotherm data is important in order to develop an equation that accurately
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represents the results. The Langmuir isotherm is valid for monolayer sorption onto a surface
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containing a finite number of identical sites: q=(qmbCeq)/(1+bCeq), where Ceq is the concentration
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of the analyte in solution (mg L−1), q is adsorbed metal ion by the sorbent in mg g−1, qm is the
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maximum adsorption capacity and b is a constant. Langmuir linear regression can be written as:
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(Ceq/q) = (Ceq/qm)+(1/bqm). A plot of (Ceq/q) versus Ceq yields a slope=1/qm and an intercept=
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1/(bqm). So, the maximum adsorption capacity (qm) can be obtained by slope of Langmuir linear
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regression. According to Fig.3.b, the adsorption capacity of ionic liquid-coated nanometer TiO2
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equals 630 mg g-1. To clarify the advantages of IL-based surfactant, the adsorption capacity of
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IL-modified nanometer TiO2 was compared with that of a nanometer TiO2 modified with a
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conventional cationic surfactant, cetyltrimethylammonium bromide (CTAB). Fig.3.a shows that
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the maximum adsorption capacity in this case is 384 mg g-1, which is much lower than that of
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C16mimBr-coated TiO2. The different adsorbed amounts are probably related to the structure of
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CTAB and C16mimBr. The positively-charged head group of C16mimBr is an imidazole ring,
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while that of CTAB is a quaternary ammonium cation. Because of the larger charge density of
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the imidazole ring, C16mimBr is able to interact with the negatively-charged nanometer TiO2
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surface more strongly [33]. Also, as shown in Table 1, compared to some other sorbents, IL-
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modified nanometer-sized TiO2 has very high capacity for nickel.
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3.5. Effect of potentially interfering ions
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The effects of potentially interfering ions on the determination of Ni were examined using 40 μg
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L−1 of Ni containing the added interfering ions. The tolerance limit was set as the nickel
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equivalent concentration of the diverse ions required to cause more than ±5% errors in the
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determination of nickel. According to the results (Table 2) Fe(III) and Zn(II) ions have the
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highest interferences. For determination of Ni(II) in food samples, these interfering ions were
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removed via their precipitation by ammonia.
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3.6. Analytical figures of merit
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Calibration graphs were obtained both with and without preconcentration. While the linear range
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without preconcentration was 0.2 - 15 mg L−1, the calibration graph after preconcentration by
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using the proposed method was linear in the range of 2.0 - 400 µg L−1 with a correlation
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coefficient of 0.9977. The amount of Ni(II) ions in 50 mL was measured after elution of
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adsorbed ions by 1.0 mL of eluent, therefore the maximum preconcentration factor for this
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method is 50. The detection limit according to the definition of IUPAC (3Sb/b, where Sb is the
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standard deviation of blank and b is the slope of calibration graph) is 0.8 µg L−1. A study of
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precision was performed by carrying out five independent measurements of solutions of Ni(II) at
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40 μg L−1 and gave a relative standard deviation of 2.0%.
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3.7. Analytical applications
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The method was applied to the determination of nickel in various water and food samples. Table
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3 shows the obtained results. The recovery tests were performed by spiking the samples with a
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known amount of nickel before any pretreatment. As can be seen, recoveries between 97 and
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106% were obtained, which confirm the accuracy of the method.
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Conclusions
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Immobilized TAN on IL-coated nanometer TiO2 was prepared as a novel sorbent, for
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preconcentration of Ni(II) ions from water and food samples. C16mimBr as an IL-based
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surfactant is a cationic surfactant but their behavior and properties differ from conventional
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cationic surfactants such as CTAB as a result of difference in their structures. The IL-coated
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nanometer TiO2 exhibited much higher extraction capacity (630 mg g-1) than CTAB-coated
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nanometer TiO2 (384 mg g-1) and other reported sorbents. This work introduces a simple method
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for sorbent preparation, and reveals the tremendous application potentials of IL-coated
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nanometer-sized TiO2 in environmental sample preparation.
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[30] R. Vanyúr, L. Biczók, Z. Miskolczy, Micelle formation of 1-alkyl-3-methylimidazolium bromide ionic liquids in aqueous solution, Colloids Surf. A 299 (2007) 256–261. [31] V. Pino, M. Germán-Hernández, A. Martín-Pérez, J.L. Anderson, Ionic liquid-based surfactants in separation science, Sep. Sci. Technol. 47 (2012) 264–276.
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magnetic
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chlorophenols in environmental water samples with 1-hexadecyl-3-methylimidazolium nanoparticles
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chromatographic analysis, Anal. Chim. Acta 715 (2012) 113–119. [35] M. Germán-Hernández, V. Pino, J.L. Anderson, A.M. Afonso, A novel in situ
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preconcentration method with ionic liquid-based surfactants resulting in enhanced sensitivity for the extraction of polycyclic aromatic hydrocarbons from toasted cereals, J.
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by FAAS, Food Anal. Methods 5 (2011) 1070–1078. [40] M. Tuzen, M. Soylak, L. Elci, Multi-Element preconcentration of heavy metal ions by solid
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phase extraction on chromosorb 108, Anal. Chim. Acta, 548 (2005)101.
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determination by FAAS in environmental samples, Turk. J. Chem. 31 (2007) 631. [42] V.A. Lemos, A.S. dos Passos, G. dos Santos Novaes, D. de Andrade Santana, A.L. de
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Carvalho, D.G. da Silva, Determination of cobalt, copper and nickel in food samples after pre-concentration on a new pyrocatechol-functionalized polyurethane foam sorbent, React.
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Funct. Polym. 67 (2007) 573–581.
[43] O. Sadeghi, N. Tavassoli, M.M. Amini, H. Ebrahimzadeh, N. Daei, Pyridine-functionalized
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vegetables grown in close proximity by electrothermal atomic adsorption spectroscopy, Food Chem. 127 (2011) 364–368. [44] G. Ozcelik, M. Imamoglu, S.Z. Yildiz, D. Kara, Chemically modified silica gel with N-(2aminoethyl)-salicylaldimine for simultaneous solid phase extraction and preconcentration of Cu(II), Ni(II), Cd(II) and Zn(II) in waters, Water, Air, Soil Pollut. 223 (2012) 5391– 5399.
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[44] Z.A. Alothman, M. Habila, E. Yilmaz, M. Soylak, Solid phase extraction of Cd(II), Pb(II), Zn(II) and Ni(II) from food samples using multiwalled carbon nanotubes impregnated with 4-(2-thiazolylazo)resorcinol, Microchim. Acta 177 (2012) 397–403.
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[46] Ö. Yalçınkaya, H. Erdoğan, Preconcentration and determination of manganese and nickel from various water samples by nano zirconium oxide/boron oxide, Spectrosc. Lett. 45
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M
an
us
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(2012) 602–608.
23
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Table 1
Adsorption capacity (mg g−1) 75.1
Reference
85.1
[39]
4.9
[40]
cr
Comparison of adsorption capacities and detection limits of various sorbents.
48.5
[41]
0.6
1.4
[42]
0.14
-
[43]
0.3
15.31
[44]
4.3
4.6
[45]
4.9
168.4
[46]
0.8
630
This work
Detection limit (µg L-1)
MCM-41
N-[3-(triethoxysilyl) propyl]isonicotinamide
3.50
MCM-48
N-[3-(triethoxysilyl) propyl]isonicotinamide
3.25
Chromosorb 108
Bathocuproinedisulfonic acid
0.44
Amberlite XAD-4
1,2-bis(o-aminophenylthio) Ethane
3.0
Polyurethane foam
Pyrocatechol
Silica
Pyridine
Silica Gel
N-(2-aminoethyl)salicylaldimine
Carbon nanotubes
4-(2-thiazolylazo)resorcinol
Nano(ZrO2/B2O3)
-
Nanometer-sized TiO2
1-(2-thiazolylazo)-2-naphthol
ip t
Ligand
[39]
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Solid support
24
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Table 2 Effect of foreign ions on the recovery of nickel (40 µg L-1). Tolerance limit
Na+, Ca2+ ,Cd2+,Ba2+,As5+,Mo6+, NO32+
-
-
K ,Mg ,Cl ,F ,SO4 2+
Cu ,Co ,Cr
1000 800
3+
300
2+
Pb
200
Mn
100
Zn2+, V5+
50
Fe3+
20
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M
an
2+
cr
2+
2-
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+
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Interfering ion
25
Page 25 of 30
Table 3 Results obtained for nickel determination in various water and food samples (n = 3). Added a
Found b
Recovery (%)
t-Statistic. c
Black tea
0
0.46±0.05
-
-
0.5
0.97±0.03
102
2
2.42±0.03
98
0
NDd
-
0.5
0.50±0.01
100
2
2.12±0.10
0
0.22±0.04
0.5
0.73±0.03
2
2.28±0. 10
0
ND
0.5 2 Maize starch
cr
us
0
2.08
-
-
103
1.04
-
-
0.53±0.03
106
1.73
2.05±0.05
102
1.73
ND
-
-
0.49±0.05
98
0.35
2.10±0.09
105
1.92
ND
-
-
10
10.0±0.5
100
0
40
39.6±2.0
99
0.35
0
ND
-
-
10
9.7±2.0
97
0.26
40
41.3±3.0
103
0.75
0
Ac ce
0
pt
2
Well water
-
0.58
0.5
Tap water
2.31
106
an
Starch
0.58
102
M
Wheat flour
ed
Corn flour
ip t
Sample
a
The added and found values have μg L-1 unit in the case of water samples and μg g-1 unit in the case of food samples. b Averages of three determinations ± standard deviation. c Critical t-value at %95 confidence level is 3.18. d Not detected.
26
Page 26 of 30
Figures captions: Fig. 1. Effect of (a) amount of C16mimBr, (b) amount of TAN and (c) pH of IL solution on the adsorption of the Ni(II) ions (20 mg L-1). Conditions: for (a) TAN: 0.5 mg, pH: 10.0; for (b) IL:
ip t
50 mg, pH: 10.0 and for (c) TAN: 0.5 mg, IL: 50 mg.
cr
Fig. 2. (a) Effect of pH of sample solution on the analyte recovery; Ni: 40 μg L−1; sample flow
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rate: 1.0 mL min-1 (b) Effect of volume of eluent on the analyte recovery; Ni: 40 μg L−1, pH: 7; sample volume: 50 mL; sample flow rate: 1.0 mL min-1 (c) Effect of sample flow rate on the
an
analyte recovery; Ni: 40 μg L−1 ; pH: 7; sample volume: 50 mL (d) Effect of volume of sample on
M
analyte recovery; Ni: 2.0 µg; pH: 7.
Fig. 3. (a) Sorption isotherms of Ni (II) on nanometer TiO2 modified by C16mimBr and CTAB
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pt
ed
and (b) corresponding Langmuir linear plots.
27
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Fig 1.
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an
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cr
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a
28
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Fig.2
d
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M
an
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cr
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a
29
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Fig. 3
M
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a
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b
30
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S0927-7757(13)00371-3 http://dx.doi.org/doi:10.1016/j.colsurfa.2013.04.059 COLSUA 18390
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
19-2-2013 18-4-2013 23-4-2013
Please cite this article as: M. Amjadi, A. Samadi, Modified ionic liquid-coated nanometer TiO2 as a new solid phase extraction sorbent for preconcentration of trace nickel, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2013.04.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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Highlights
1-hexadecyl-3-methylimidazolium bromide was coated on TiO2 nanoparticles at pH 10.
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TAN was immobilized on IL-coated nanometer TiO2 and used as an efficient sorbent.
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TAN-IL-nanometer TiO2 has an extremely high adsorption capacity for Ni(II) ions.
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The sorbent was applied to the preconcentration of Ni from food and water samples.
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Modified ionic liquid-coated nanometer TiO2 as a new solid phase extraction sorbent for preconcentration of trace nickel
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Mohammad Amjadi*, Azam Samadi Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz
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5166616471, Iran
* Corresponding author
Email: [email protected]
Tel: +984113393109; Fax: +984113340191
3
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Abstract
2
In this work, a high-capacity solid-phase extraction sorbent was developed for preconcentration
3
of trace amounts of nickel ions prior to their determination by flame atomic absorption
4
spectrometry (FAAS). The sorbent was prepared by immobilization of 1-(2-thiazolylazo)-2-
5
naphthol (TAN) on nanometer-sized TiO2 coated with the ionic liquid, 1-hexadecyl-3-
6
methylimidazolium bromide (C16mimBr). The chemical conditions for preparation of sorbent
7
including pH and amounts of TAN and C16mimBr were optimized. Experimental conditions for
8
preconcentration of Ni, elution conditions and the effect of interfering ions on the recovery of the
9
analyte were also investigated. Under the optimum conditions, the calibration graph was linear in
10
the range of 2.0 - 400 µg L-1 with a detection limit of 0.8 µg L-1. The adsorption capacity of the
11
sorbent for Ni(II) was found to be 630 mg g-1. The preconcentration method coupled with FAAS
12
was successfully applied to the determination of Ni(II) in various water and food samples.
13
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Keywords: Ionic liquid; TiO2 nanoparticles; Solid phase extraction; 1-(2-thiazolylazo)-2-
15
naphthol; Nickel.
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1. Introduction
17
Nickel is an abundant natural element with relatively high toxicity. It is known that exposure to
18
nickel and its compounds can lead to serious health problems, including contact dermatitis and
19
respiratory system diseases. Both noncancerous and cancerous respiratory effects have been
20
observed in humans and animals exposed to airborne nickel compounds. Chronic bronchitis,
21
emphysema, pulmonary fibrosis, and impaired lung function have been observed in nickel
22
welders and foundry workers. The most commonly reported adverse health effect associated with
23
nickel exposure is contact dermatitis. Nickel compounds are also known to be human
24
carcinogens [1]. Therefore, monitoring of this element in environmental and food samples is an
25
important analytical task. Flame atomic absorption spectrometry (FAAS) is a powerful and well-
26
established technique for this purpose. But direct determination of Ni at low concentrations is
27
difficult because of insufficient sensitivity of this technique as well as the matrix interferences
28
occurring in real samples. Therefore‚ a preliminary separation and preconcentration step is often
29
required.
30
Solid phase extraction (SPE) is a cost-effective and important sample preparation technique that
31
offers important advantages including low solvent usage, disposal costs and extraction time [2].
32
Nanometer-sized metal oxides such as Al2O3, SiO2 and TiO2 have attracted increasing interest as
33
sorbents in SPE due to their high surface area [3–10]. The basic disadvantage of these solid
34
sorbents is the lack of metal selectivity, which leads to interfering of other species with the target
35
metal ion(s). To overcome this problem, chemical or physical modification of the sorbent surface
36
with some organic compounds, especially chelating agents, is often used to load the surface with
37
some donor atoms such as oxygen, nitrogen, sulfur and phosphorus [11]. Metal oxides under
38
normal pH conditions in water are hydrophilic and are not favorable for the adsorption of
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Page 5 of 30
hydrophobic organic compounds. For this reason, a surface modification method based on
40
hemimicelles or admicelles has been proposed [12,13]. Hemimicelles were generated by the
41
adsorption of an ionic surfactant such as sodium dodecyl sulfate (SDS) on a metal oxide (such as
42
alumina or silica) surface. Hence, hydrophobic chelating agents can be incorporated in the
43
hemimicelles on which the desired trace metals were retained by complexation. Several
44
nanometer-sized metal oxides coated by surfactants have been used as adsorbents for
45
preconcentration of metal ions [5,14,15].
46
Ionic liquids (ILs) are salts with melting points below ca. 100◦C [16]. They belong to a relatively
47
new class of solvents which are ion pairs consisting an inorganic ion and an amphiphilic part
48
with a hydrophilic polar head group (ionic in nature) and a hydrophobic hydrocarbon chain (the
49
tail). In recent years, ILs have attracted great attentions due to their unique chemical and physical
50
properties such as nonvolatility, excellent solvation qualities, non-flammability and high thermal
51
stability [17,18]. ILs have been widely utilized in sample preparation processes, including liquid-
52
liquid extraction [19–21], liquid-phase microextraction [22,23], solid-phase extraction [24,25]
53
and solid-phase microextraction [26,27]. A new kind of ILs that is able to form micelles in
54
aqueous solution has been recently reported [28–30] and used in analytical applications [31–35].
55
Such ILs, which can be classified as cationic surfactants, constitute a new area of surfactant
56
development; especially considering the limited number of traditional cationic surfactants. It is
57
important to highlight the ease of tuning the properties of IL-based surfactants by simple
58
chemical modifications of the cation/anion pair. Thus, up to 48 IL-based surfactants have been
59
reported and their colloidal and interfacial behavior have been studied [31].
60
In this work, a new sorbent with high adsorption capacity was prepared by immobilization of 1-
61
(2-thiazolylazo)-2-naphthol (TAN) on nanometer TiO2 coated with surfactant-based ionic liquid,
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Page 6 of 30
62
1-hexadecyl-3-methylimidazolium bromide (C16mimBr). The potential of the adsorbent for the
63
preconcentration of trace Ni was assessed using column method.
64
2. Experimental
66
2.1. Instruments
67
An Analytik jena flame atomic absorption spectrometer model Nova 400 (Jena, Germany)
68
furnished with an air–acetylene flame and a nickel hollow cathode lamp, operated at 5 mA, was
69
used. The instrument was set at a wavelength of 232.0 nm and slit width of 0.2 nm. A Metrohm
70
model 654 was used for pH measurements. A peristaltic pump was used in the separation and
71
preconcentration process and a laboratory-made microcolumn packed with 0.03 g of the sorbent
72
was used for preconcentration.
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73
2.2. Reagents and solutions
75
Doubly distilled de-ionized water (obtained from Ghazi Serum Co., Tabriz, Iran) was used for
76
the preparation of all the solutions. The standard solution of Ni(II) (1000 mg L−1) was prepared
77
by dissolving proper amount of Ni(NO3)2·6H2O from Merck (Darmstadt, Germany) in water and
78
diluted to 100 mL in a volumetric flask. The required concentration of nickel solution was
79
prepared by appropriate dilution of the stock solution. A 0.5 M nitric acid solution was prepared
80
by appropriate dilution of concentrated HNO3 (Merck). A 2.0 g L-1 solution of 1-hexadecyl-3-
81
methylimidazolium bromide (C16mimBr) was prepared by dissolving 0.2 g of IL (obtained from
82
KimiaExir, Tehran, Iran) in water and diluted to 100 mL in a volumetric flask. A 0.01% (w/v) 1-
83
(2-thiazolylazo)-2-naphthol (TAN) solution was prepared by dissolving 0.01 g of TAN (Fluka,
84
Tokyo, Japan) in 50 mL of 0.1 M NaOH solution and making up to 100 mL with distilled
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Page 7 of 30
deionized water. Nanometer-sized TiO2 with an average diameter of 21 nm was obtained from
86
Evonik Industries AG Silica (Essen, Germany). Phosphate buffer solution (0.1 M, pH=7.0) was
87
prepared by dissolving appropriate amount of sodium phosphate (Merck) in 100 mL of deionized
88
water and adjusting the pH with hydrochloric acid.
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85
89
2.3. Preparation of sorbent
91
A 0.5 g portion of TiO2 nanoparticles was placed in a 50 mL beaker. After adding 25 mL of 2.0 g
92
L-1 of C16mimBr IL into the beaker, the pH was adjusted to 10.0 with 1 M NaOH while stirring
93
the suspension with a stirrer. The mixture was sonicated for 5 min to suspend the NPs. Then 5
94
mL of 0.01% TAN solution was added to the mixture. After mixing for 15 min, the modified
95
nanometer-sized TiO2 was filtered off and washed with water then dried at room temperature for
96
24 h.
97
A similar procedure by using 25 mL of 2.0 g L-1 of cetyltrimethylammonium bromide (CTAB,
98
Merck) was applied to prepare TAN-modified CTAB-coated TiO2 nanoparticles for comparison.
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cr
90
2.4. Preparation of column
101
A homogenous mixture of 0.03 g of modified nanometer-sized TiO2 and 0.03 g of glass beads
102
(212-300 µm, Sigma, St. Louis, USA) was introduced into a polyethylene microcolumn (4 mm
103
i.d., 3–4 cm length) plugged with a small portion of glass wool at both ends. Before use, 10 mL
104
doubly distilled water was passed through the column in order to clean and condition it.
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105 106 107
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Page 8 of 30
2.5. General procedure
109
Fifty mL aqueous sample solution containing 5–400 μg L−1Ni(II) was prepared and its pH value
110
was adjusted to 7.0 with 1 mL of 0.1 M phosphate buffer. The solution was passed through the
111
microcolumn by using a peristaltic pump adjusted to the desired flow rate. Afterwards, the
112
retained nickel was eluted with 1.0 mL of 0.5 M HNO3 solution and nickel was determined by
113
FAAS in the effluent.
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114
2.6. Real sample preparation
116
Food samples including black tea, wheat flour, corn flour, maize starch and starch were
117
purchased from local supermarkets. Dry ashing method was used for preparation of these
118
samples [36]. A portion of 1.0 g of samples was accurately weighed into a crucible and heated
119
gently on a hot plate at low temperature until a charred solid formed. It was then ashed in a
120
muffle furnace at 500ºC for 3 h. After cooling, the resulted white ash was carefully moistened
121
with 1.0 mL water, and then 2.0 mL of concentrated HNO3 (Suprapure, Merck) and 1.0 mL of
122
concentrated HClO4 (Merck) were added in sequence. The solution was then heated to dryness
123
on a hot plate at 200ºC. The resulting residue was treated with 1.0 mL concentrated HNO3 and
124
5.0 mL water and then heated gently for about 5 min until the solution turned clear. After cooling
125
at room temperature, the pH of obtained solution was adjusted at 7.0 with ammonia (5 M). By
126
adding ammonia some interfering ions such as Zn(II) and Fe(III) were precipitated. After
127
centrifugation and separation of precipitates, the supernatant was transferred into a 50 mL
128
volumetric flask and 1.0 mL phosphate buffer (0.1 M) was added and diluted to the mark with
129
water. Spiked samples were prepared by adding small volumes of Ni(II) standard solution (10
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Page 9 of 30
mg L-1) to 1.0 g of powdered samples. After drying for 2 days at ambient temperature, samples
131
were subjected to the same procedure described above.
132
Two water samples including tap water and well water were selected and the proposed method
133
was applied to determine their nickel contents. Tap water was collected from our laboratory
134
(University of Tabriz, Tabriz, Iran) and well water was collected from Tabriz, Iran. The water
135
samples were filtered through a Millipore 0.45 µm pore-size membrane into polyethylene
136
bottles. Their pH values were adjusted to 7.0 by addition of phosphate buffer and analyzed
137
according to the general procedure.
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3. Results and discussion
140
3.1. Adsorption of TAN on IL-Coated Nanometer TiO2
141
The studies conducted on TiO2 surface modification using organic agents were aimed at altering
142
its surface character in the hydrophilic-hydrophobic system. The pH of the solution influences
143
the surface charge of oxides, i.e. below the point of zero charge (pHpzc) metal oxides possess a
144
positive surface charge, whereas above that pH they have a negative charge. The pHpzc is 6.2 for
145
nanometer TiO2 [37]. 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) is considered as a
146
surfactant with hydrophobic alkyl chain and hydrophilic group of imidazolium ring. The critical
147
micelle concentration (CMC) value for this IL has been reported to be 0.76 mM. Above its CMC
148
value, C16mimBr could assemble into micelles with spherical simple structures [28]. Based on
149
these facts, it is expected that [C16mim]+ cations at concentrations lower than its CMC and at pH
150
higher than 6.2 are adsorbed on the negatively charged nanometer TiO2 surface through
151
electrostatic attraction. As a result of this interaction, which leads to the formation of
152
hemimicelle on the nanometer TiO2 surface, the nanoparticles become hydrophobic and when
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Page 10 of 30
chelating agent is added to them, retention of chelating molecules on the nanometer TiO2 surface
154
occur via interactions between the hydrophobic groups of the chelating agent and the exposed
155
hydrocarbon tails in the hemimicelles. 1-(2-Thiazolylazo)-2-naphthol (TAN) is a well-
156
characterized organic chelating ligand which forms complexes with some transition metals [38].
157
Since the hydrophilic groups of TAN are oriented towards the aqueous phase, metal ions were
158
easily adsorbed on the nanometer TiO2-TAN surface.
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3.2. Optimization of conditions for preparation of the sorbent
161
Optimized amounts of IL and TAN in sorbent preparation process were investigated with batch
162
experiments. For this purpose, several sorbents with variable amounts of C16mimBr (10–70 mg)
163
and TAN (0.1–2 mg) were prepared. Then 10 mg of each sorbent was added to 50 mL of 20 mg
164
L-1 Ni(II) solution and stirred vigorously for 60 min so that equilibrium was attained. After
165
centrifugation, the concentration of the Ni(II) ions in the solution was determined by FAAS and
166
the adsorbed amount of Ni(II) was obtained.
167
Fig. 1.a depicts the percentage of adsorbed Ni(II) as a function of the added IL amount. When
168
the IL amount changed from 10 to 50 mg, the adsorption of Ni(II) on the nanometer TiO2
169
increased because of the increase of adsorbed TAN molecules as a result of increasing the
170
hydrophobicity of sorbent. With 50 mg of C16mimBr, the highest amount of adsorbed Ni(II) was
171
achieved. When the amount of C16mimBr continued to be increased, there was no more increase
172
in the adsorption amount, indicating that the surface of nanometer TiO2 was saturated by
173
C16mimBr.
174
The effect of amount of TAN in sorbent preparation process in the range of 0.1 to 2 mg was
175
investigated and the results are shown in Fig. 1.b. Adsorption percentage of Ni increased with
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increasing TAN amount up to 0.5 mg and with further increase in the amount of TAN, the
177
adsorption amount did not increase significantly. It could be considered that the surface of
178
nanometer TiO2-C16mimBr was saturated by TAN.
179
pH is one of the important influencing factors on the adsorption behavior of the hemimicelles
180
system because the surface charge of nanometer TiO2 is pH dependent. To investigate the effect
181
of pH on sorbent preparation process, several sorbents with fixed amount of TiO2 were prepared
182
in IL-TAN solutions with different pH values in the range of 7-11. After drying the sorbents,
183
they were used for preconcentration of Ni. As shown in Fig. 1.c, nanometer TiO2–IL-TAN
184
exhibited low adsorption for Ni when the pH was between 7.0 and 8.0. With the increase of pH,
185
the adsorption amount obviously increased and reached the maximum value at pH 10.0. This can
186
be attributed to the fact that the surface of nanometer TiO2 is negatively charged when the pH
187
value is quite higher than the point of zero charge (6.2), so the [C16mim]+ ions was easily
188
adsorbed onto the surface of TiO2. Based on the results, pH 10.0 was chosen as the optimum
189
value for sorbent preparation.
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176
3.3. Optimization of extraction conditions
192
Extraction experiments were performed in a continuous flow mode by passing sample solutions
193
through a microcolumn packed with the sorbent at flow rate of 1.0 mL min-1.
194
The effect of pH on the adsorption of Ni ions onto the sorbent was investigated in the range of
195
3.0–9.0. The results, shown in Fig. 2.a., revealed that a quantitative recovery (≥98%) occurs at
196
the pH range of 6.0–9.0. Hence, pH 7.0 was selected for further studies. It was found that
197
phosphate buffer with final concentration of 0.02 M was adequate for adjusting pH at this value.
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191
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Page 12 of 30
Elution of Ni from the nanometer TiO2-TAN adsorbent was investigated by using HCl, HBr and
199
HNO3 as eluent following the general procedure. The obtained results showed that HNO3 is the
200
better eluent because of better reproducibility and highest recovery of determinations with this
201
eluent. Hence, the effect of HNO3 volume on the recovery of analyte was studied. It was found
202
that with 1.0 mL of 0.5 M HNO3 quantitative recoveries (ca. 100%) could be obtained. The
203
obtained results are given in Fig. 2.b.
204
Since the retention of elements on the sorbent depends on the flow rate of the sample solution, its
205
effect was examined by passing 50 mL of sample solution through microcolumn with a
206
peristaltic pump. The flow rates were adjusted in the range of 0.5–2.5 mL min-1. As shown in
207
Fig. 2.c., recovery of Ni ions decreased by increasing the flow rate beyond 1.5 mLmin-1. Thus, a
208
flow rate of 1.0 mL min-1 was selected for subsequent experiments.
209
In order to explore the possibility of enriching low concentrations of analyte from large volumes
210
of solution, the effect of sample volume on the retention of Ni(II) ions was also investigated. For
211
this purpose, volumes of 25, 50, 100, 150 and 200 mL of sample solution containing 2.0 µg of Ni
212
were passed through the column at the optimum flow rate. As shown in Fig. 2.d., quantitative
213
recovery (>98%) was obtained for sample volume of 50 mL for Ni ions. Therefore, this volume
214
was adopted for the preconcentration of analyte from sample solutions.
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198
216
3.4. Adsorption capacity
217
The adsorption capacity is an important factor, because it determines how much sorbent is
218
required to quantitatively concentrate the analyte from a given solution. The adsorption capacity
219
of the sorbent was determined using the batch technique [14]. A portion of 10 mg of sorbent was
220
placed into a 50 mL beaker, 25 mL of nickel solution (with concentrations in the range of 1.0–
13
Page 13 of 30
800 mg L-1) was added and the pH was adjusted at 7.0 with phosphate buffer. The resulting
222
mixture was stirred for 60 min. The solid phase was separated by centrifugation and the metal
223
ion was determined in the supernatant solution by FAAS. The results are shown in Fig. 3a. The
224
analysis of the isotherm data is important in order to develop an equation that accurately
225
represents the results. The Langmuir isotherm is valid for monolayer sorption onto a surface
226
containing a finite number of identical sites: q=(qmbCeq)/(1+bCeq), where Ceq is the concentration
227
of the analyte in solution (mg L−1), q is adsorbed metal ion by the sorbent in mg g−1, qm is the
228
maximum adsorption capacity and b is a constant. Langmuir linear regression can be written as:
229
(Ceq/q) = (Ceq/qm)+(1/bqm). A plot of (Ceq/q) versus Ceq yields a slope=1/qm and an intercept=
230
1/(bqm). So, the maximum adsorption capacity (qm) can be obtained by slope of Langmuir linear
231
regression. According to Fig.3.b, the adsorption capacity of ionic liquid-coated nanometer TiO2
232
equals 630 mg g-1. To clarify the advantages of IL-based surfactant, the adsorption capacity of
233
IL-modified nanometer TiO2 was compared with that of a nanometer TiO2 modified with a
234
conventional cationic surfactant, cetyltrimethylammonium bromide (CTAB). Fig.3.a shows that
235
the maximum adsorption capacity in this case is 384 mg g-1, which is much lower than that of
236
C16mimBr-coated TiO2. The different adsorbed amounts are probably related to the structure of
237
CTAB and C16mimBr. The positively-charged head group of C16mimBr is an imidazole ring,
238
while that of CTAB is a quaternary ammonium cation. Because of the larger charge density of
239
the imidazole ring, C16mimBr is able to interact with the negatively-charged nanometer TiO2
240
surface more strongly [33]. Also, as shown in Table 1, compared to some other sorbents, IL-
241
modified nanometer-sized TiO2 has very high capacity for nickel.
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242 243
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Page 14 of 30
3.5. Effect of potentially interfering ions
245
The effects of potentially interfering ions on the determination of Ni were examined using 40 μg
246
L−1 of Ni containing the added interfering ions. The tolerance limit was set as the nickel
247
equivalent concentration of the diverse ions required to cause more than ±5% errors in the
248
determination of nickel. According to the results (Table 2) Fe(III) and Zn(II) ions have the
249
highest interferences. For determination of Ni(II) in food samples, these interfering ions were
250
removed via their precipitation by ammonia.
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244
251
3.6. Analytical figures of merit
253
Calibration graphs were obtained both with and without preconcentration. While the linear range
254
without preconcentration was 0.2 - 15 mg L−1, the calibration graph after preconcentration by
255
using the proposed method was linear in the range of 2.0 - 400 µg L−1 with a correlation
256
coefficient of 0.9977. The amount of Ni(II) ions in 50 mL was measured after elution of
257
adsorbed ions by 1.0 mL of eluent, therefore the maximum preconcentration factor for this
258
method is 50. The detection limit according to the definition of IUPAC (3Sb/b, where Sb is the
259
standard deviation of blank and b is the slope of calibration graph) is 0.8 µg L−1. A study of
260
precision was performed by carrying out five independent measurements of solutions of Ni(II) at
261
40 μg L−1 and gave a relative standard deviation of 2.0%.
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262
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252
263
3.7. Analytical applications
264
The method was applied to the determination of nickel in various water and food samples. Table
265
3 shows the obtained results. The recovery tests were performed by spiking the samples with a
15
Page 15 of 30
266
known amount of nickel before any pretreatment. As can be seen, recoveries between 97 and
267
106% were obtained, which confirm the accuracy of the method.
268
Conclusions
270
Immobilized TAN on IL-coated nanometer TiO2 was prepared as a novel sorbent, for
271
preconcentration of Ni(II) ions from water and food samples. C16mimBr as an IL-based
272
surfactant is a cationic surfactant but their behavior and properties differ from conventional
273
cationic surfactants such as CTAB as a result of difference in their structures. The IL-coated
274
nanometer TiO2 exhibited much higher extraction capacity (630 mg g-1) than CTAB-coated
275
nanometer TiO2 (384 mg g-1) and other reported sorbents. This work introduces a simple method
276
for sorbent preparation, and reveals the tremendous application potentials of IL-coated
277
nanometer-sized TiO2 in environmental sample preparation.
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ip t
[46] Ö. Yalçınkaya, H. Erdoğan, Preconcentration and determination of manganese and nickel from various water samples by nano zirconium oxide/boron oxide, Spectrosc. Lett. 45
Ac ce
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M
an
us
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(2012) 602–608.
23
Page 23 of 30
Table 1
Adsorption capacity (mg g−1) 75.1
Reference
85.1
[39]
4.9
[40]
cr
Comparison of adsorption capacities and detection limits of various sorbents.
48.5
[41]
0.6
1.4
[42]
0.14
-
[43]
0.3
15.31
[44]
4.3
4.6
[45]
4.9
168.4
[46]
0.8
630
This work
Detection limit (µg L-1)
MCM-41
N-[3-(triethoxysilyl) propyl]isonicotinamide
3.50
MCM-48
N-[3-(triethoxysilyl) propyl]isonicotinamide
3.25
Chromosorb 108
Bathocuproinedisulfonic acid
0.44
Amberlite XAD-4
1,2-bis(o-aminophenylthio) Ethane
3.0
Polyurethane foam
Pyrocatechol
Silica
Pyridine
Silica Gel
N-(2-aminoethyl)salicylaldimine
Carbon nanotubes
4-(2-thiazolylazo)resorcinol
Nano(ZrO2/B2O3)
-
Nanometer-sized TiO2
1-(2-thiazolylazo)-2-naphthol
ip t
Ligand
[39]
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pt
ed
M
an
us
Solid support
24
Page 24 of 30
Table 2 Effect of foreign ions on the recovery of nickel (40 µg L-1). Tolerance limit
Na+, Ca2+ ,Cd2+,Ba2+,As5+,Mo6+, NO32+
-
-
K ,Mg ,Cl ,F ,SO4 2+
Cu ,Co ,Cr
1000 800
3+
300
2+
Pb
200
Mn
100
Zn2+, V5+
50
Fe3+
20
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pt
ed
M
an
2+
cr
2+
2-
us
+
ip t
Interfering ion
25
Page 25 of 30
Table 3 Results obtained for nickel determination in various water and food samples (n = 3). Added a
Found b
Recovery (%)
t-Statistic. c
Black tea
0
0.46±0.05
-
-
0.5
0.97±0.03
102
2
2.42±0.03
98
0
NDd
-
0.5
0.50±0.01
100
2
2.12±0.10
0
0.22±0.04
0.5
0.73±0.03
2
2.28±0. 10
0
ND
0.5 2 Maize starch
cr
us
0
2.08
-
-
103
1.04
-
-
0.53±0.03
106
1.73
2.05±0.05
102
1.73
ND
-
-
0.49±0.05
98
0.35
2.10±0.09
105
1.92
ND
-
-
10
10.0±0.5
100
0
40
39.6±2.0
99
0.35
0
ND
-
-
10
9.7±2.0
97
0.26
40
41.3±3.0
103
0.75
0
Ac ce
0
pt
2
Well water
-
0.58
0.5
Tap water
2.31
106
an
Starch
0.58
102
M
Wheat flour
ed
Corn flour
ip t
Sample
a
The added and found values have μg L-1 unit in the case of water samples and μg g-1 unit in the case of food samples. b Averages of three determinations ± standard deviation. c Critical t-value at %95 confidence level is 3.18. d Not detected.
26
Page 26 of 30
Figures captions: Fig. 1. Effect of (a) amount of C16mimBr, (b) amount of TAN and (c) pH of IL solution on the adsorption of the Ni(II) ions (20 mg L-1). Conditions: for (a) TAN: 0.5 mg, pH: 10.0; for (b) IL:
ip t
50 mg, pH: 10.0 and for (c) TAN: 0.5 mg, IL: 50 mg.
cr
Fig. 2. (a) Effect of pH of sample solution on the analyte recovery; Ni: 40 μg L−1; sample flow
us
rate: 1.0 mL min-1 (b) Effect of volume of eluent on the analyte recovery; Ni: 40 μg L−1, pH: 7; sample volume: 50 mL; sample flow rate: 1.0 mL min-1 (c) Effect of sample flow rate on the
an
analyte recovery; Ni: 40 μg L−1 ; pH: 7; sample volume: 50 mL (d) Effect of volume of sample on
M
analyte recovery; Ni: 2.0 µg; pH: 7.
Fig. 3. (a) Sorption isotherms of Ni (II) on nanometer TiO2 modified by C16mimBr and CTAB
Ac ce
pt
ed
and (b) corresponding Langmuir linear plots.
27
Page 27 of 30
Fig 1.
Ac ce
pt
ed
M
an
us
cr
ip t
a
28
Page 28 of 30
Fig.2
d
Ac ce
pt
ed
M
an
us
cr
ip t
a
29
Page 29 of 30
Fig. 3
M
an
us
cr
ip t
a
Ac ce
pt
ed
b
30
Page 30 of 30