Removal, preconcentration and determination of Ni(II) from different environmental samples using modified magnetite nanoparticles prior to flame atomic absorption spectrometry

2.1 Apparatus

A flame atomic absorption spectrophotometer (PG Instruments, England) was used with a nickel hollow-cathode-lamp, an operating current of 8 mA and wavelength and spectral bandwidth of 232.0 and 0.2 nm, respectively. A Fourier transform infrared spectrometer (IR Perresttige-21, Shimadzu) was used to determine the identity of the as-prepared nanoparticles and to characterize the coated Fe₃O₄ nanoparticles. The surface morphology of the powders was observed by the scanning electron microscope (LEO 1455VP SEM). Magnetic properties of the particles were determined by vibrating sample magnetometer (VSM 7400 Model Lake-Shore). pH measurements were made with a Metrohm Model 780 pH meter with a combination glass electrode. Other instruments used were: ultrasonic bath (S60H Elmasonic, Germany), mechanical stirrer (Heidolph, RZR2020), orbital shaker (Ika, KS130 Basic), and an electronic analytical balance (Adam, AA220LA) used for weighting the solid materials. In addition, for magnetic separations a strong neodymium-iron-boron (Nd₂Fe₁₂B) magnet (1.2 T, 2.5 × 5 × 10 cm) was used.

2.2 Reagents and solutions

All reagents used were of analytical grade and all solutions were prepared by using triple distilled and deionized water. A stock solution of 1000 μg mL⁻¹ of nickel(II) was prepared by dissolving 0.1238 g of NiNO₃ in 2 mL of concentrated nitric acid and was diluted to 100 mL. Working solutions were obtained by further diluting the stock solution to the required concentrations before use. Dimethylglyoxim (DMG), sodium dodecylsulfate (SDS), ferrous chloride (FeCl₂·4H₂O), ferric chloride (FeCl₃·6H₂O), aluminum isopropoxide, ethanol, nitric acid, hydrochloric acid and sodium hydroxide were used without further purification processes. The pHs of the solutions were adjusted with phosphate buffer. All of the chemicals were obtained from Merck.

2.3 Preparation of dimethylglyoxim/alumina-coated magnetite nanoparticles (DMG/SDS-ACMNPs)

The alumina-coated magnetite nanoparticles (ACMNPs) were prepared according to Karimi et al., (2011, 2012). A DMG/SDS solution was prepared by dissolving 150.0 mg DMG and 200.0 mg SDS in 100 mL deionized water. 10 mL of DMG/SDS solution was added to 10 mL water containing 0.2 g of ACMNPs. The pH of this suspension was adjusted to 2 by drop-wise addition of HNO₃ (0.1 mol L⁻¹) solution. The mixed solution was shaken for 15 min and then separated from the reaction medium under the magnetic field, and rinsed with 10 mL pure water. This product was used as sorbent for nickel ions.

2.4 General procedure

The procedure for the magnetic solid-phase extraction of nickel ion is as follows: 10 mL of nickel ion solution (5 μg mL⁻¹) was added to DMG/SDS-ACMNPs from section 2.3, subsequently the pH value was adjusted to 8.0–9.0 with phosphate buffer and the solution was shaken for 10 min to facilitate adsorption of the metal ions onto the nanoparticles. Then the magnetite adsorbents were separated easily and quickly using a magnet and decanted directly. Subsequently, 2 mL of 0.1 mol L⁻¹ HNO₃ solution was added as eluent. Finally, the magnet was used again to settle the magnetic nanoparticles and the eluate was separated for FAAS analysis.

2.5 Sample preparation procedure for soil, spinach, tomato, black tea, tobacco and different water samples

4–5 g of soil samples was collected from four different zones in Sirjan, Iran. The samples were digested with a mixture of 20 mL concentrated HCl and 5 mL of concentrated HNO₃ at room temperature, then they were heated to 100 °C for 5 h. The mixtures were evaporated to dryness and then 10 mL of the above acid mixture was added to the residue and evaporated to dryness again. After this, 20 mL of deionized water was added to the beaker and the insoluble parts were filtered. The pH was adjusted to 8.0 with phosphate buffer and the total volumes were made up to 20 mL with deionized water in four beakers and the solutions were taken for separation/preconcentration procedure and the solution was treated according to the general procedure prior to FAAS analysis.

2.0 g of dried powdered sample of spinach was digested in 20 mL of concentrated nitric acid for 30 min and diluted to 50 mL. After a day, the solution was filtered through a filter paper (Whatman, No. 2) and washed with 20 mL of 3.0 mol L⁻¹ HNO₃. The pH of the resultant solution was adjusted to 8.0 with phosphate buffer and the solution was treated according to the general procedure prior to FAAS analysis.

The tomato samples once in the laboratory were washed with deionized water and 40 g of sample was heated in silica crucible for 3 h on a hot plate and the charred material was transferred to furnace for overnight heating at 650 °C. The residue was cooled, treated with 10 mL concentrated nitric acid and 3 mL 30% (w/v) H₂O₂ again kept in furnace for 2 h at the same temperature so that no organic compound traces are left. The final residue was treated with 3 mL concentrated hydrochloric acid and 3 mL 70% (w/v) perchloric acid and evaporated to fumes, so that all the metals change to their respective ions. The dissolved solid residue was filtered and its pH was adjusted to 8.0 and made up to 100 mL. Then SPE procedure and FAAS analysis given above was applied to the solution.

4.0 g of dried powdered sample of black tea was digested in 30 mL of concentrated nitric acid and diluted to 50 mL. After a day, the solution was filtered through a filter paper (Whatman, No. 2) and washed with 20 mL of 3.0 mol L⁻¹ HNO₃. The pH of the resultant solution was adjusted to 8.0 with phosphate buffer and the solution was treated according to the general procedure prior to FAAS analysis.

1.0 g of tobacco was digested with 6 mL of concentrated HNO3 and 2 mL of 30% (w/v) H₂O₂ in microwave system. After digestion of the samples, the pH of the digested samples was adjusted to 8.0 with phosphate buffer. Then the volume of the digested sample was made up to 30.0 mL with distilled water. Blanks were prepared in the same way as the sample, but omitting the sample. The separation/preconcentration procedure given above was applied to the solution prior to FAAS analysis.

Water samples (i.e., tap water, spring water, river water and mineral water), were filtered after collection through filter paper (Whatman, No. 4) to remove suspended particulate matter and acidified 5.0 mL of concentrated HNO₃ prior to storage in polyethylene containers for use. The solutions were neutralized with concentrated NH₃ and then pH of solutions was adjusted to 8.0 with phosphate buffer. The SPE procedure was carried out as described in the general procedure.

3.1 Characterization of MNPs, ACMNPs and DMG/SDS-ACMNPs

To enable practical application of ACMNPs, it is most important that the sorbents should possess superparamagnetic properties. Magnetic properties were characterized by measuring the hysteresis and remanence curves by means of a vibrating sample magnetometer (VSM). The magnetization curves show that both MNPs and ACMNPs exhibit typical superparamagnetic behavior due to no hysteresis (Karimi et al., 2012). However, these ACMNPs are sufficient for magnetic separation with a conventional magnet. SEM images of MNPs and ACMNPs also showed the uniform size distribution of the nanoparticles (Karimi et al., 2012).

The XRD pattern for the Fe₃O₄ NPs in Fig. 1a shows six characteristic peaks for Fe₃O₄ (2θ = 30.08, 35.42, 43.08, 53.56, 56.98 and 62.62) marked by their indices (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0). These peaks are consistent with the database in D8ADVANCE file (PDF No. 03-0862) and revealed that the resultant nanoparticles were pure Fe₃O₄ with a spinal structure and the immobilized process did not change its crystal phase. Fig. 1b shows the ACMNP XRD pattern, as compared to naked MNPs a series of peaks were added that attributed to alumina according to the software database file. The average crystallite size (D) is calculated to be 5.7 ± 3 nm for MNPs and 18.9 ± 2 nm for ACMNPs (after coating the MNPs with alumina), using the Debye–Sherrer formula D = Kλ/(β cosθ), where K is the Sherrer constant (K = 0.89), λ is the X-ray wavelength (λ = 1.5406Å), β is the full peak width at half maximum (FWHM), and the θ, Bragg diffraction angle (Hong et al., 2007). The result is well consistent with that of SEM analysis (Karimi et al., 2012).

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Figure 1

XRD patterns for the naked Fe₃O₄ (a) and alumina coated Fe₃O₄ nanoparticles (b).

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The modified ACMNPs were also confirmed by FT-IR analysis, as shown in Fig. 2. As can be seen in Fig. 2a, a broad band exists around 588.182 cm⁻¹, assignable to the Fe–O–Fe of the MNPs. The peak at about 1632.45 cm⁻¹ can be assigned to the stretching vibration of N₂ adsorbed on the surfaces of the nanoparticles. The flexing vibration peak of hydroxyl, resulting from the adsorbed water, can be observed at 3433.64 cm⁻¹ (Hong et al., 2008). In the spectrum of ACMNPs (Fig. 2b), compared with the spectrum of MNPs, after binding alumina, and the broadening of the peak at 584.325 cm⁻¹ can be assigned to Al–O, that overlapped with Fe–O characteristic peak. Comparison of the FT-IR spectra of ACMNPs and DMG/SDS-ACMNPs (Fig. 2c) also shows a new sharp peak at 1067.41 cm⁻¹ , it was due to that the C–N stretching peak of DMG stabilized on ACMNPs. Consequently, the FT-IR data suggest that DMG is successfully immobilized on the ACMNPs’ surface.

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Figure 2

FTIR spectra of the Fe₃O₄ nanoparticles (a), ACMNPs (b), and DMG/SDS-ACMNPs (c).

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3.2 Amounts of DMG and SDS

The anionic surfactant of SDS is effectively sorbet on the positively charged alumina surface to form aggregates (Karimi et al., 2012). Therefore, negatively charged SDS surfactant ions are adsorbed at low pH values due to increased interaction between the alumina and SDS. We tried to coat alumina surfaces with SDS on which DMG could be subsequently trapped (Fig. 3). The SDS would form hemimicelles or admicelles on alumina by strong adsorption and the micelles could trap DMG molecules, homogeneously (Hong et al., 2007). The influence of various amounts of DMG and SDS on the adsorption of nickel ions on ACMNPs was investigated. The results showed that maximum adsorption obtained when 60 and 50 mg of SDS and DMG were used, respectively (Figs. 4 and 5). Thus, these concentrations were selected as the optimum concentrations of SDS and DMG for further studies.

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Figure 3

A suggested schematic for adsorption of Ni(II) to the DMG/SDS-ACMNPs.

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Figure 4

Effect of DMG concentration on adsorption of nickel(II). Conditions: ACMNPs (0.2 g), SDS (10 mL, 100 mg g⁻¹), Ni(II) solution (10 mL, 5 μg mL⁻¹, pH 8).

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Figure 5

Effect of SDS concentration on adsorption of nickel(II). Conditions as in Fig. 3 except 10 mL DMG (75 mg g⁻¹) was added.

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3.3 Effect of pH, standing and magnetic separation time

The effect of pH is the main investigated factor for the extraction studies. The effect of pH on the adsorption of nickel by DMG/SDS-ACMNPs at 25 °C showed that the adsorption of nickel is quantitative (100%) in the pH range of 7.0–10.0 (Fig. 6). At acidic media the nitrogen atom could be protonated and at basic media the hydroxyl proton dissociates. This reveals that the mechanism of sorption of nickel ion is pH dependent and pH could affect the stability of the complex. Thus, the pH 8.0 was used as optimum pH for further studies.

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Figure 6

Effect of pH on adsorption of nickel(II). Conditions: Conditions as in Fig. 3 except 10 mL DMG (75 mg g⁻¹) was added.

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In order investigation effect of time on adsorption nickel on the DMG/SDS-ACMNPs was studied. In the experiment, DMG/SDS-ACMNPs possessed large saturation magnetization and super-paramagnetism properties, which enabled them to be completely isolated at less than 1 min by a strong magnet. When the ACMNPs were isolated immediately without a standing process, the recovery of Ni(II) ions was only 58%. But, when the standing time was adjusted to 5, 10, 20, 30, 40 and 50 min, recoveries were improved to 78, 82, 95.5, 95 and 95%, respectively. Standing time of 20 min was sufficient to achieve satisfactory adsorption and better recovery of nickel ions.

3.4 Effect of sample volume

In order to obtain a preconcentration factor, a larger volume of sample solution is required. To study the effect of sample volume on the recovery of nickel ions, the sample solutions in the range of 50–1000 mL containing 50.0 ng of nickel ions were operated according to the general procedure and eluted using 2.5 mL of nitric acid (1.0 mol L⁻¹). At sample volumes higher than 800 mL, the percent of recovery decreased. Consequently, a preconcentration factor of 320 could be attained for quantitative recovery of 95 ± 2% of Ni(II) when the sample volume was 800 mL.

3.5 Effect of nanoparticles’ amount

Fig. 7 shows the effect of nanoparticles’ amount for quantitative extraction of Ni(II). Various amounts of ACMNPs (from 10 to 250 mg) were investigated. The extraction was found to be quantitative when the amount was 180 mg or more. Experiments were carried out with 200 mg modified nanoparticles. Thus, 200 mg of ACMNPs was selected as the optimum amount for further studies.

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Figure 7

Effect of nanoparticles’ amount on adsorption of nickel(II). Conditions as in Fig. 3 except 10 mL DMG (75 mg g⁻¹) was added.

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3.6 Desorption conditions

A variety of eluents were tested in order to elute the adsorbed nickel from the sorbent. Different eluents of thiourea, thiosulphate, HNO₃, H₂SO₄ and NaOH at various concentrations were examined so that the most effective eluent for the quantitative recovery of nickel ions could be chosen. It was found that 2.5 mL of nitric acid with a concentration of 1.0 mol L⁻¹ was sufficient for quantitative recovery of adsorbed Ni(II).

3.7 Interference study

The study of interference ions was performed by a standard mixture solution containing 50.0 ng mL⁻¹ of nickel ion and a certain amount of foreign ions. The following excesses of ions do not interfere (i.e., caused a relative error of less than 5%): more than a 1000-fold (largest amount tested) amount of Na⁺, I⁻, F⁻, Cl⁻, Br⁻, ${\text{NO}}_3^{-}$ , ${\text{SO}}_4^{2-}$ , ${\text{ClO}}_4^{-}$ , CH₃COO⁻; a 100-fold amount of ${\text{NH}}_4^{+}$ ^, Mg²⁺, Fe²⁺, Zn²⁺, Co²⁺, Pb²⁺, Cr³⁺; a 50-fold amount of Co²⁺, Cu²⁺, Bi³⁺, Al³⁺, Fe³⁺ and a 15-fold amount of Pd²⁺.

3.8 Sorption capacity and sorbent regeneration

Sorption capacity study used here was adapted from the method recommended by Maquieira et al. (1994). The static adsorption capacity of DMG-SDS/ACMNPs was found to be 9.72 mg g⁻¹ for nickel ions. Regeneration is one of the key factors for evaluating the performance of the adsorption material. In this work, it was found that the sorbent can be re-used up to 3 times without loss of analytical performance. Considering that 4.0 g of ACMNPs could be prepared in one batch and only 200 mg of ACMNPs was used for one extraction operation, this reusability time is acceptable.

3.9 Adsorption isotherms

The equilibrium adsorption isotherm is principle in depicting the reciprocal behavior between adsorbates and adsorbent and is important for understanding the model of adsorption systems. The analysis of the isotherm data by fitting them to different isotherm models is an important step to find the suitable model that can be used for design purposes (El-Guendi, 1991). The adsorption studies were investigated at fixed adsorbent portion (0.5 g of damped nanoparticles), temperature (25 °C) and varying Ni(II) concentrations (2.0–20.0 μg mL⁻¹). The concentration of Ni(II) in the liquid phase (C_e) was determined by FAAS. Fig. 8a shows the adsorption isotherm, which was fitted to the Langmuir model using the nonlinear regression method. This isotherm relates metal per unit weight of adsorbent (Q_e, mg/g) to (C_e). The linear form of Langmuir isotherm equation is represented by the Eq. (1) (Langmuir, 1918):

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Figure 8

Equilibrium adsorption isotherm of Ni(II) on DMG/SDS-ACMNPs (a) and linearized Langmuir isotherm for Ni(II) by this sorbent (b). Conditions: ACMNPs (0.2 g), DMG (10 mL, 75 mg g⁻¹), SDS (10 mL, 100 mg g⁻¹), Ni(II) solution (10 mL, 2–20 μg mL⁻¹, pH 8), equilibrium time: 10 h, temperature: 25 °C.

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3.10 Analytical performance and method validation

In order to show the validation of the proposed method, under the optimal experimental conditions, the analytical features of the method such as limit of detection (LOD), linear range of the calibration curve and precision of Ni(II) were examined. The LOD of the proposed method based on 3 times the standard deviation of the blank (3S_b) was 4.6 ng mL⁻¹. The linear range of calibration curve for Ni(II) was 10.0–100.0 ng mL⁻¹ with a correlation coefficient of 0.9985. The regression equation for the line was A = 0.0894 C_Ni + 0.0184 (n = 7), where C_Ni is the concentration of Ni(II) in μg mL⁻¹ and A is the absorbance. The relative standard deviation (RSD) for 7 replicate measurements of 50.0 ng mL⁻¹ of nickel ion was 1.9%.

3.11 Analytical applications

In order to assess the applicability of the method to real samples, it was applied to the SPE and determination of nickel in different samples of tap water, spring water and plants of tea and spinach Table 1. The reliability was checked by spiking experiments. As the results show, the proposed method is suitable for the separation/preconcentration and determination of Ni(II).