Preparation of neutral red functionalized Fe₃O₄@SiO₂ and its application to the magnetic solid phase extraction of trace Hg(II) from environmental water samples

Abstract

In this work, for the first time, neutral red functionalized Fe₃O₄@SiO₂ was prepared, characterized and subsequently used for magnetic solid phase extraction of Hg(II) from environmental water samples followed by inductively coupled plasma-mass spectrometry determination. TEM images shows uniform decoration of SiO₂ on the Fe₃O₄ surface (with a typical particle size of 220 nm). The FT-IR analysis indicates that neutral red was combined with the SiO₂ surface by chemical bonds. Trace levels of Hg(II) can be extracted efficiently and selectively due to the electrostatic and coordinate interactions of Hg(II) and neutral red. Parameters affecting the quantitative recovery of Hg(II) were investigated, and the optimized values were 6.0, 25 mg, 60 min, and 5 mL of 1.0 mol L⁻¹ HNO₃ for pH value of the solution, amount of the extractant, equilibration time and volume, concentration of eluent, respectively. Under the optimized conditions, at Hg(II) concentrations as low as 2 ng mL⁻¹, the recovery is >85%, and the enrichment factor is as high as 100. The commonly encountered ions show no interference with the extraction process. The material can be regenerated and reused at least ten times without loss of adsorption capacity (84 mg g⁻¹). In addition, magnetic separation greatly simplifies the extraction procedure and filtration and/or centrifugation processes were avoided. The material was successfully applied for the separation and pre-concentration of low levels of Hg(II) in environmental water samples with satisfactory results.

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Introduction

With the rapid development of industries, heavy metal pollution is becoming a more and more serious problem.^1,2 Mercury is one of the most dangerous heavy metals because of its high toxicity and accumulative properties within biological systems. Hg(II) is the main species of mercury in water, it may cause a variety of diseases, including renal dysfunction, serious cognitive and motion disorders, hepatic injury and minamata disease³ even at low concentration. Therefore, the monitoring of Hg(II) in environmental water samples is very important to ensure the quality of human health.⁴

Numerous instrumental technologies have been developed for the detection of Hg(II), such as atomic absorption spectrometry (AAS), inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS).^5–7 However, the concentration of Hg(II) is fairly low and matrices such as salt are very complex in environmental water samples. Therefore, sample preparation procedures are often required to improve detection sensitivity and selectivity.

Recently, a new solid phase extraction technique, called magnetic solid phase extraction (MSPE), has been developed for environmental sample preparation.^8–11 This technology is based on the use of a magnetic adsorbent for separation and pre-concentration of analytes from large volume of samples. When the magnetic adsorbent is added to the sample solution, the target analyte is adsorbed onto the surface of the magnetic adsorbent. Then, the material is separated and collected from the sample solution by applying an external magnet. After eluted with appropriate solvent, the analyte can be recovered and then determined.¹² Obviously, this method maintains the advantages of solid phase extraction, such as high recovery, high enrichment factor and low consumption of organic solvent.¹³ Meanwhile, the time consuming column filtration step encountered in traditional solid phase extraction can be avoided, especially for the extraction process using nano extractant.

Due to the key role of the adsorbent materials in MSPE, the exploration of new types of extractants becomes an active research field in analytical chemistry. So far, many magnetic particles, such as Fe₃O₄,¹⁴ Fe₃O₄/GO,¹⁵ Fe₃O₄@SiO₂ (ref. 16) and Fe₃O₄/CNT,¹⁷ have been used as supporter to prepare extractant materials for metal ions. Since Fe₃O₄@SiO₂ often exhibits advantages such as unique magnetic responsively, stable in solution and easily modifiable surface,¹⁸ the surface of this magnetic nanoparticle has been modified with specific ligand to make them as selective and efficient adsorbent. For example, Abolhasani et al.¹⁹ polymerized pyrrole on the surface of Fe₃O₄@SiO₂ and the material was used as a MSPE adsorbent for the efficient extraction of Cd(II) and Ni(II) from sea food samples. Tadjarodi et al.²⁰ functionalized Fe₃O₄@SiO₂ nanoparticles with phenyl isothiocyanate and then used for pre-concentration of Cd(II) and Pb(II) from fish, sediment, soil, and water samples. Amidoxime functionalized Fe₃O₄@SiO₂ was also prepared and used for highly efficient sorption of U(VI) from seawater and contaminated wastewater.²¹

According to the hard–soft acid–base theory,²² soft acid like Hg metal would react favorably with N-containing ligands. In other words, organic ligands containing N atoms may have strong affinity for Hg(II). Considering the fact that neutral red is a cheap and easily available N-containing organic compound, it may serve as an efficient ligand for Hg(II). Herein, neutral red functionalized Fe₃O₄@SiO₂ (Fe₃O₄@SiO₂-NR) was prepared and applied for the separation and pre-concentration of Hg(II) from water. The objectives of this work are (i) to characterize the morphology, nanostructures and magnetic properties of Fe₃O₄@SiO₂-NR using XRD, FT-IR, TEM and vibration sample magnetometer (VSM) techniques; (ii) to investigate the effect of pH value of the solution, amount of the extractant, equilibration time and volume, concentration of eluent on the quantitatively recovery of Hg(II); (iii) to elucidate the extraction mechanism of Hg(II) on Fe₃O₄@SiO₂-NR; and (iv) to apply Fe₃O₄@SiO₂-NR for separation and pre-concentration of Hg(II) from environmental water samples followed by ICP-MS determination.

Experimental section

Chemicals

FeCl₃·6H₂O, FeSO₄·7H₂O, NH₃·H₂O, tetraethoxysilane (TEOS), 3-chloropropyl triethoxysilane (CPTMS) and HgCl₂ were purchased from Aladdin Co., Ltd. Neutral red was acquired from Tianjin Kemiou Chemical Reagent Co., Ltd. These chemicals were used as received without further purification except for drying of TEOS, CPTMS and HgCl₂ before use. The stock standard solution of Hg(II) (1.0 mg mL⁻¹) was prepared by dissolving HgCl₂ in aqueous solution. Dilute solutions were prepared by appropriate dilution of the stock solution. Analytical grade hydrochloric acid and sodium hydroxide were used to adjust pH of the aqueous phases. Water used in the experiments was prepared by redistillation of deionized water. All the other chemicals were commercially available analytical reagents.

Preparation of neutral red functionalized Fe₃O₄@SiO₂

The synthetic routes for neutral red functionalized Fe₃O₄@SiO₂ were illustrated in Scheme 1. The Fe₃O₄ nanoparticles were prepared via chemical co-precipitation of FeCl₂ and FeCl₃ according to the procedures described in literature.²³ Silica coated magnetic nanoparticles with a dense silica shell on the surface of Fe₃O₄ nanoparticles were prepared by the Stöber method through sol–gel reaction of tetraethoxysilane (TEOS).²⁴ Subsequently, the chlorine silicate shell was formed on the dense silica shell using 3-chloropropyl triethoxysilane (CPTMS) in the dry toluene. In doing so, Fe₃O₄@SiO₂ nanoparticles (1 g) were dispersed into dry toluene (15 mL) and CPTMS (1 mL) in toluene (5 mL) was added. Then the mixture was stirred for 12 h at 80 °C. The resulting microspheres were washed four times with dry toluene and ethanol, respectively, and then dried under vacuum for 12 h at 60 °C. Thus prepared magnetic nanoparticles were designated as Fe₃O₄@SiO₂-Cl.

Scheme 1 Preparation of neutral red functionalized Fe₃O₄@SiO₂.

Next, Fe₃O₄@SiO₂-Cl was dispersed in N,N-dimethylformamide (100 mL) containing neutral red (0.1 g), and anhydrous potassium carbonate (0.4 g) was added to promote the reaction. The mixture was agitated and stirred for 12 h at 60 °C. Then, it was cooled to room temperature, and the product was separated using a magnet. The resulting magnetic microspheres were washed with ethanol and deionized water for several times, and dried under vacuum for 12 h at 60 °C. The obtained nanoparticles were denoted as Fe₃O₄@SiO₂-NR.

Characterization of the material

The crystal structure of Fe₃O₄ and neutral red functionalized Fe₃O₄@SiO₂ were analyzed by X-ray diffraction (XRD). The patterns were recorded in the 2θ range of 20–80° with a scan rate of 0.02°/0.4 s by using a Bruker D8-AXS diffraction system equipped with a Cu K_α radiation (λ = 0.15406 Å). The organic groups on the surface of Fe₃O₄, Fe₃O₄@SiO₂-Cl and Fe₃O₄@SiO₂-NR were examined by using a Bio-Rad FT-IR spectrometer (America). The size and morphology of Fe₃O₄ and Fe₃O₄@SiO₂-NR samples were characterized with transmission electron microscopy (TEM) by using a JEM 2100 microscope (JEOL, Japan). Magnetic properties were measured at room temperature in a vibration sample magnetometer (VSM7407, America).

Magnetic solid phase extraction procedures

All the experiments for the MSPE of Hg(II) were conducted in a glass vessel. A certain amount of Fe₃O₄@SiO₂-NR and aqueous Hg(II) were added into the vessel, and the initial pH value of the aqueous solutions was adjusted to 1.9–11.0 with negligible volume of aqueous HCl (0.1 mol L⁻¹) or aqueous NaOH (0.01 mol L⁻¹). Then the mixture was shaken by a water thermostatic shaker. After extraction equilibrium, the adsorbent was collected by a hand held magnet, and the concentration of Hg(II) in the supernatants was determined by ELAN DRC-e inductively coupled plasma mass spectrometer (PerkinElmer, USA) or TU-1810 UV-Vis spectrophotometer (Persee General Instrument Co., Ltd., China). The adsorption capacity and extraction percentage of Hg(II) was calculated by eqn (1) and (2):where q_e represents the equilibrium adsorption capacity (mg g⁻¹), C₀ and C_e are the concentrations of Hg(II) in the aqueous phase before and after extraction (mg L⁻¹), m is the mass of the adsorbent (mg), V is the volume of Hg(II) solution (mL), and E (%) stands for the extraction percentage.

Sample preparation

Several natural water samples were prepared for Hg(II) determination, namely, rain water, snow water, river water and sea water. Rain water and snow water were collected in Xinxiang city. River water was collected from Yihe River in the front of Luoyang Longmen Grottoes, and sea water was collected in Huanghai Sea at Qingdao area. For natural water sampling, 1 L of polyethylene bottles was used. The bottles were rinsed three times with the water sample before use. Then, the polyethylene bottles were filled with water samples and aqueous HNO₃ was added to adjust pH < 2. Then, the water samples were kept at 4 °C. Just before use, all samples were filtered through 0.45 μm membrane filters and the pH was adjusted to 6.0.

Results and discussion

The characteristics of the Fe₃O₄@SiO₂-NR

The structures of Fe₃O₄ and Fe₃O₄@SiO₂-NR nanoparticles were analyzed by XRD and the results were shown in Fig. 1A. The relatively strong diffraction peaks at 2θ = 30.3°, 35.5°, 43.2°, 53.7°, 57.3°, 62.7° and 74.5° correspond to the characteristics of Fe₃O₄ nanoparticles (JCPDS no. 65-3107) reported previously.²⁵ In Fe₃O₄@SiO₂-NR, the presence of peaks at 2θ = 30.3°, 35.5°, 43.2°, 57.3° and 62.7° suggests that the Fe₃O₄ nanoparticles were well retained after the functionalization of neutral red. Therefore, the as-prepared material maintains the magnetic property of Fe₃O₄ and can be separated easily from solution by an external magnet.

Fig. 1 (A) X-ray powder diffraction patterns of Fe₃O₄ and Fe₃O₄@SiO₂-NR; (B) FT-IR spectra of Fe₃O₄, Fe₃O₄@SiO₂-Cl and Fe₃O₄@SiO₂-NR; TEM images of (C) Fe₃O₄ and (D) Fe₃O₄@SiO₂-NR; (E) magnetic curves of Fe₃O₄ and Fe₃O₄@SiO₂-NR.

The FT-IR spectrum of Fe₃O₄, Fe₃O₄@SiO₂-Cl and Fe₃O₄@SiO₂-NR were displayed in Fig. 1B. It is shown that for Fe₃O₄, the peak observed at 575 cm⁻¹ is the characteristic of Fe–O vibration. Compared with Fe₃O₄, 2955 cm⁻¹ and 1096 cm⁻¹ in the spectrum of Fe₃O₄@SiO₂-Cl can be ascribed to the stretching vibration of –CH₂– and Si–O–Si, respectively. After the functionalization of neutral red (Fe₃O₄@SiO₂-NR), the bands at 3227 cm⁻¹ and 1605 cm⁻¹ have been assigned to the bending vibration and the stretching vibration of –NH of neutral red molecule, while the peaks at 1500 cm⁻¹ and 1437 cm⁻¹ are the adsorption of –C=NH and –CH₃ of this molecule, respectively. The FT-IR results confirm that the uniform silica shell was coated on Fe₃O₄ surface, and neutral red was functionalized on the surface of the material.

The morphology and size of Fe₃O₄ and Fe₃O₄@SiO₂-NR were observed by TEM. It is shown that the pristine Fe₃O₄ have uniform size with an average diameter of about 200 nm (Fig. 1C), and the core–shell structure of the particles prepared in this work is monodisperse, homogeneous and spherical. The average size of Fe₃O₄@SiO₂-NR magnetic microspheres is about 220 nm (Fig. 1D). Therefore, the thickness of SiO₂ shell is about 10 nm. These results indicate that the coating of magnetic nanoparticles with SiO₂ was done successfully.

As shown in Fig. 1E, the magnetic saturation of Fe₃O₄@SiO₂-NR was about 45 emu g⁻¹, which was lower than that of Fe₃O₄ microspheres (75 emu g⁻¹). This might be related to the magnetic inactive layer containing SiO₂. However, the decrease in magnetic saturation did not seriously affect the magnetic separation of Fe₃O₄@SiO₂-NR, and this magnetic material still exhibited strong magnetism and could be collected within 10 s in solution under a conventional magnet and dispersed quickly with a slight shake once the magnetic field was removed. This indicates that Fe₃O₄@SiO₂-NR was successfully synthesized with high magnetic responsivity. In addition, no remanence was detected after removal of the applied magnetic field, suggesting that Fe₃O₄@SiO₂-NR was super paramagnetic.

Comparison of extraction efficiency for different sorbents

Here, the extraction efficiencies of Hg(II) by Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-NR were evaluated under the same experimental conditions (Fig. 2). It was shown that the extraction percentage of Hg(II) by the three kinds of nanoparticles follows the order Fe₃O₄ < Fe₃O₄@SiO₂ < Fe₃O₄@SiO₂-NR, and the extraction efficiency with Fe₃O₄@SiO₂-NR was remarkably higher than that with Fe₃O₄ and Fe₃O₄@SiO₂. This is certainly due to the functionalization of neutral red on the surface of Fe₃O₄@SiO₂, which has more active adsorption sites than the other two materials. Thus it can be concluded that Fe₃O₄@SiO₂-NR maintains the property of neutral red and has strong affinity towards Hg(II). Therefore, Fe₃O₄@SiO₂-NR was employed for the extraction of Hg(II) in the next studies.

Fig. 2 Extraction efficiency of Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-NR for Hg(II).

Optimization of magnetic solid phase extraction parameters

The pH of sample solution is an important factor that affects the extraction of metal ions at trace levels. To evaluate the effect of pH on the extraction of Hg(II), 25 mL of solutions containing 50 μg of Hg(II) was studied within the pH value of 1.9–11.0 and then the extraction procedure described above was applied. As can be seen from Fig. 3A, quantitative recoveries for Hg(II) were obtained at pH 6.0. Hence, pH 6.0 is considered as the optimum pH for further experiments.

Fig. 3 (A) The influence of pH value on the extraction of Hg(II) by Fe₃O₄@SiO₂-NR; (B) effect of adsorbent dosage on Hg(II) extraction by Fe₃O₄@SiO₂-NR.

To get a higher extraction percentage, the effect of adsorbent dosage was investigated by adding 5–50 mg of magnetic adsorbent into 25 mL of Hg(II) solution (Fig. 3B). The extraction percentage for Hg(II) was increased and reached quantitative values at 25 mg. Therefore, 25 mg of adsorbent was selected for further experiments.

From Fig. 3A, it can be seen that the extraction efficiency of Hg(II) by Fe₃O₄@SiO₂-NR decreases with the decrease of the pH value when the pH is lower than 6.0. Therefore, acid was chosen to elute Hg(II) retained on Fe₃O₄@SiO₂-NR. Aqueous HNO₃ is a recommended acid for ICP-MS determination.²⁶ For this reason, various concentrations and volumes of HNO₃ were studied for the desorption of Hg(II). The results indicate that 5 mL of HNO₃ (1.0 mol L⁻¹) is sufficient to recover Hg(II) from Fe₃O₄@SiO₂-NR. Thus, this concentration of HNO₃ was employed as the eluent for the desorption of Hg(II).

In order to obtain higher enrichment factor, the magnetic solid phase extraction method was applied to 20–500 mL of the sample solution with 25 mg of Fe₃O₄@SiO₂-NR. 85% recovery of Hg(II) could be obtained even when the sample volumes was 500 mL. As a result, the enrichment factor is found to be 100. Therefore, the developed method is very suitable for the pre-concentration of trace Hg(II) from large volumes of sample solution.

The effects of common interfering ions on the extraction efficiency of Hg(II) were also investigated. The results are given in Table 1. The recoveries of Hg(II) were quantitative for the given concentration of interfering ions. It can be concluded that large amount of interfering ions have no considerable effect on the magnetic solid phase extraction of Hg(II), and the developed method has a good tolerance to the interferences.

Table 1 Influences of interfering species on the recovery of Hg(II)

Ions	Concentration mg L^−1	Added as	Recovery, %
K^+	2000	KNO3	99 ± 2
Na^+	2000	NaNO3	97 ± 2
Ca^2+	1000	CaCl2	97 ± 1
Mg^2+	1000	MgCl2	98 ± 2
Cu^2+	25	Cu(NO3)2·3H2O	95 ± 2
Pb^2+	25	Pb(NO3)2·6H2O	93 ± 1
Cd^2+	25	Cd(NO3)2·4H2O	94 ± 2
Zn^2+	25	Zn(NO3)2·6H2O	92 ± 2
NO3^−	2000	NaNO3	101 ± 2
PO4^3−	2000	Na3PO4	96 ± 1
Cl^−	1750	NaCl	97 ± 2
SO4^2−	250	Na2SO4	96 ± 2

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Adsorption kinetics and adsorption isotherms

A series of experiments have been performed to investigate the kinetics for the adsorption of Hg(II) by Fe₃O₄@SiO₂-NR (Fig. 4A). It can be seen that quantitative recovery of Hg(II) can be achieved in 60 min. The pseudo first order and pseudo second order models (see ESI†) are used to elucidate the adsorption processes of Hg(II) on Fe₃O₄@SiO₂-NR. The kinetic parameters obtained from fitting results are summarized in Table S1.† It can be seen that the pseudo second order model provides better correlation coefficients than the pseudo first order model, and the calculated equilibrium adsorption capacities (q_e,cal) from the pseudo second order model agreed better with the experimental values. This suggests that the pseudo second order model is more suitable for describing the adsorption kinetics of Hg(II) by Fe₃O₄@SiO₂-NR. It is reported that adsorption behavior of the pseudo second order model suggests chemisorptions' process.²⁷ Therefore, the adsorption rate of Hg(II) on Fe₃O₄@SiO₂-NR was controlled by chemisorption mechanism.

Fig. 4 (A) Effect of shaking time on the extraction of Hg(II) by Fe₃O₄@SiO₂-NR; (B) adsorption isotherm of Hg(II) by Fe₃O₄@SiO₂-NR.

The equilibrium experiments were performed at 25 °C with different Hg(II) solutions. The Langmuir and Freundlich models (see ESI†) are employed to fit the data of adsorption isotherms. The more description and the corresponding parameters of Langmuir and Freundlich models are presented in Table S2.† It can be seen from Fig. 4B and Table S2† that the Langmuir model gives a better fit than the Freundlich model, suggesting that the adsorption of Hg(II) on Fe₃O₄@SiO₂-NR is monolayer coverage. It is also shown that the maximum adsorption capacities of Fe₃O₄@SiO₂-NR for Hg(II) calculated from Langmuir is 83.7 mg g⁻¹. The high adsorption capacity of Fe₃O₄@SiO₂-NR may be attributed to the abundant N-containing functional groups on the surface of Fe₃O₄@SiO₂-NR. The –NH₂ groups on the surface of Fe₃O₄@SiO₂-NR can form strong surface complexes with Hg(II) ions.

Regeneration of Fe₃O₄@SiO₂-NR

The regeneration ability of the adsorbent is an important factor to evaluate the cost effectiveness of the material. To examine the reusability of the adsorbent developed in this work, Fe₃O₄@SiO₂-NR was used in at least ten cycles at pH 6.0 for the extraction of Hg(II). Fig. 5 shows the extraction percentage for each adsorption/desorption cycle. Obviously, the Hg(II) adsorption efficiency is still higher than 95.1% after ten cycles, which proves the good reusability of Fe₃O₄@SiO₂-NR. In addition, three batches of Fe₃O₄@SiO₂-NR prepared at different times were used to test the reproducibility of the adsorbent. It is demonstrate that the results are good enough to be reproduced by these particles.

Fig. 5 Adsorption–desorption cycle of Fe₃O₄@SiO₂-NR for Hg(II). Temperature, 25 °C; shaking time, 60 min; desorption reagent, 1.0 mol L⁻¹ HNO₃.

Extraction mechanism

In this work, Hg(II) can be extracted efficiently and selectively due to the electrostatic and coordinate interaction. It is observed that at pH = 1.9, the extraction percentage of Hg(II) is about 60%. Clearly, Fe₃O₄@SiO₂-NR exhibits better extraction performance than some other adsorbents based on coordination adsorption mechanism.^28–30 This indicates that other affinity may also exist between the material and Hg(II) except for coordination, such as the electrostatic attraction between positively charged Fe₃O₄@SiO₂-NR and negatively charged HgCl₃⁻ or HgCl₄²⁻. In order to confirm this speculation, Hg(NO₃)₂ was used instead of HgCl₂, and the pH value of solution was adjusted to 1.9 by aqueous HNO₃. Thus, Hg(II) exists in cationic form (Hg²⁺) but not anionic form (HgCl₃⁻ or HgCl₄²⁻). It was observed that only 3% of Hg(II) could be extracted by Fe₃O₄@SiO₂-NR. Therefore, it seems that the electrostatic interaction between positively charged Fe₃O₄@SiO₂-NR and negatively charged mercury (HgCl₃⁻ and HgCl₄²⁻) played a dominant role in strong acidic conditions. Since in the MSPE procedure, aqueous HCl was used for adjusting solution pH value, the concentration of Cl⁻ decreases with decreasing H⁺ concentration. Therefore, with the increase of pH value, less and less Hg(II) exists in anionic form, and the electrostatic interaction can be neglected in strong basic systems. On the contrary, the coordinate interaction is enhanced due to the decrease of H⁺ concentration and becomes a dominant factor for the extraction of Hg(II). Thus, the extraction percentage increases as the pH value is increased and then remains almost constant at pH > 6.0.

Analytical applications

To examine and evaluate the applicability of Fe₃O₄@SiO₂-NR, several environmental water samples including rain water, snow water, river water and sea water were chosen for MSPE-ICP-MS determination of Hg(II). After collection, the samples were filtered through a 0.45 μm of membrane filter and the pH values of the samples were adjusted to 6.0 before magnetic solid phase extraction. The relative recovery (RR) was calculated based on the following equation:³¹where C_found is the concentration of Hg(II) in the final solution of spiked sample which is calculated from the calibration curve, C_real is the concentration of Hg(II) in the final solution of non-spiked sample, and C_added is the concentration of standard Hg(II) solution that was spiked into the real sample. It was shown in Table 2 that the recoveries of all spiked samples were in the range from 94% to 110%. This indicates that the proposed method is suitable for determination of Hg(II) in environmental water samples.

Table 2 Determination of Hg(II) in environmental water samples (n = 3)

Sample	Added (μg L^−1)	Found (μg L^−1)	Recovery (%)	RSD (%)
Rain water	0	—	—	—
	31.3	30.8	98.4	2.13
Snow water	0	—	—	—
	30.5	28.9	94.7	2.38
River water	0	—	—	—
	32.6	35.2	107.9	2.16
Sea water	0	0.05	—	—
	30.1	33.4	110.9	1.64

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Comparison with other extractants for solid phase extraction of Hg(II)

Comparative information from some studies on solid phase extraction of Hg(II) by various methods for the figure of the merits was given in Table 3. It was found that Fe₃O₄@SiO₂-NR exhibited higher solid phase extraction capacity and better adsorption selectivity for Hg(II) than most of the other adsorbents. Furthermore, the preparation of Fe₃O₄@SiO₂-NR was relatively simple, convenient and low cost. Thus, Fe₃O₄@SiO₂-NR should be a good choice for magnetic solid phase extraction of Hg(II) from environmental water samples.

Table 3 The comparison for the adsorption of Hg(II) using different adsorbents

Adsorbent	Adsorbed ions	pH	Adsorption capacity of Hg(II) mg g^−1	Ref.
Oxidised carbon nanotubes	Hg(II)	5.0	3.2	32
2-(2-oxoethyl)hydrazine carbothioamide modified silica gel	Hg(II)	3.0	37.5	33
1-Acylthiosemicar bazide modified activated carbon	Cu(II), Hg(II), Pb(II)	3.0	67.8	34
Hg-PAN ion-imprinted polymethacrylic microbeads	Hg(II)	7.0	64.2	35
Ethylene glycol bis-mercaptoacetate modified 3-(trimethoxysilyl)-1-propanethiol coated Fe3O4	Ag(I), Cd(II), Cu(II), Hg(II), Pb(II)	6.0	41.6	36
Fe3O4@SiO2	Hg(II), Pb(II)	6.0	—	37
Ethylacrylate modified wool fibers	Cu(II), Hg(II), Ni(II)	6.0	66.7	38
Fe3O4@SiO2-NR	Hg(II)	6.0	83.7	Present work

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Conclusion

In this work, for the first time, the core–shell Fe₃O₄@SiO₂-NR magnetic microspheres were prepared, characterized and used as an adsorbent for the magnetic solid phase extraction of Hg(II) prior to its ICP-MS determination. Compared with Fe₃O₄ and Fe₃O₄@SiO₂, the Fe₃O₄@SiO₂-NR composite exhibited remarkably higher extraction efficiency towards Hg(II) due to the functionalization of neutral red. In addition, the Fe₃O₄@SiO₂-NR composite maintained the magnetic property of Fe₃O₄, and could be separated easily from aqueous solution in only 10 seconds. Adsorption isotherms were well fitted by Langmuir model and the kinetics of adsorption followed the pseudo second order model. This suggests that monolayer coverage of Hg(II) was formed on the surface of Fe₃O₄@SiO₂-NR, and chemical adsorption might be the rate controlling step. Mechanism analysis indicates that Hg(II) was extracted through the electrostatic and coordinate interactions. Moreover, an excellent selectivity, sensitivity, good reusability and high enrichment factor was also found for Fe₃O₄@SiO₂-NR. Thus Fe₃O₄@SiO₂-NR was successfully applied as MSPE adsorbents for the pre-concentration of trace Hg(II) in environmental water samples, which paved a way to highly effective cleanup and enrichment of trace Hg(II) in complicated matrices.

Acknowledgements

This work was supported financially from the National Natural Science Foundation of China (No. 21377036), and the Innovation Scientists Projects of Henan Province (No. 144200510004).

References

1. N. Abdullah, R. J. Gohari, N. Yusof, A. F. Ismail, J. Juhana, W. J. Lau and T. Matsuura, Chem. Eng. J., 2016, 289, 28–37.
2. Y. Xie, J. Wang, M. Wang and X. Ge, J. Hazard. Mater., 2015, 297, 66–73.
3. S. Guha, S. Lohar, I. Hauli, S. K. Mukhopadhyay and D. Das, Talanta, 2011, 85, 1658–1664.
4. Y. Date, A. Aota, S. Terakado, K. Sasaki, N. Matsumoto, Y. Watanabe, T. Matsue and N. Ohmura, Anal. Chem., 2013, 85, 434–440.
5. Z. A. Chandio, F. N. Talpur, H. Khan, H. I. Afridi, G. Q. Khaskheli and M. A. Mughal, RSC Adv., 2014, 4, 3326–3331.
6. V. C. G. dos Santos, M. T. Grassi, M. S. de Campos, P. G. Peralta-Zamora and G. Abate, Analyst, 2012, 137, 4458–4463.
7. H. Wang, Z. Wu, B. Chen, M. He and B. Hu, Analyst, 2015, 140, 5619–5626.
8. L. Hao, C. Wang, Q. Wu, Z. Li, X. Zang and Z. Wang, Anal. Chem., 2014, 86, 12199–12205.
9. L. S. Qing, Y. Xue, Y. M. Liu, J. Liang, J. Xie and X. Liao, J. Agric. Food Chem., 2013, 61, 8072–8078.
10. P. Yan, M. He, B. Chen and B. Hu, Analyst, 2015, 140, 4298–4306.
11. Y. Cai, Z. Yan, M. NguyenVan, L. Wang and Q. Cai, J. Chromatogr. A, 2015, 1406, 40–47.
12. C. Herrero-Latorre, J. Barciela-García, S. García-Martín, R. M. Peña-Crecente and J. Otárola-Jiménez, Anal. Chim. Acta, 2015, 892, 10–26.
13. Q. Zhou and Z. Fang, Talanta, 2015, 141, 170–174.
14. M. Safdarian, P. Hashemi and M. Adeli, Anal. Chim. Acta, 2013, 774, 44–50.
15. E. Ziaei, A. Mehdinia and A. Jabbari, Anal. Chim. Acta, 2014, 850, 49–56.
16. S. Su, B. Chen, M. He, B. Hu and Z. Xiao, Talanta, 2014, 119, 458–466.
17. C. Wu, J. Fan, J. Jiang and J. Wang, RSC Adv., 2015, 5, 47165–47173.
18. Y. Deng, D. Qi, C. Deng, X. Zhang and D. Zhao, J. Am. Chem. Soc., 2008, 130, 28–29.
19. J. Abolhasani, R. H. Khanmiri, E. Ghorbani-Kalhor, A. Hassanpour, A. A. Asgharinezhad, N. Shekari and A. Fathi, Anal. Methods, 2015, 7, 313–320.
20. A. Tadjarodi, A. Abbaszadeh, M. Taghizadeh, N. Shekari and A. A. Asgharinezhad, Mater. Sci. Eng., C, 2015, 49, 416–421.
21. Y. Zhao, J. Li, L. Zhao, S. Zhang, Y. Huang, X. Wu and X. Wang, Chem. Eng. J., 2014, 235, 275–283.
22. Y. Chen, H. Wang, Y. Pei, J. Ren and J. Wang, ACS Sustainable Chem. Eng., 2015, 3, 3167–3174.
23. N. Ma, L. Zhang, R. Li, Y. Zhou, Z. Cai, C. Dong and S. Shuang, Anal. Methods, 2014, 6, 6736–6744.
24. S. Sadeghi, H. Azhdari, H. Arabi and A. Z. Moghaddam, J. Hazard. Mater., 2012, 215–216, 208–216.
25. C. Yang, W. Guo, L. Cui, N. An, T. Zhang, H. Lin and F. Qu, Langmuir, 2014, 30, 9819–9827.
26. H. Peng, N. Zhang, M. He, B. Chen and B. Hu, Talanta, 2015, 131, 266–272.
27. L. H. Jiang, Y. G. Liu, G. M. Zeng, F. Y. Xiao, X. J. Hu, X. Hu, H. Wang, T. T. Li, L. Zhou and X. F. Tan, Chem. Eng. J., 2016, 284, 93–102.
28. M. Hadavifar, N. Bahramifar, H. Younesi and Q. Li, Chem. Eng. J., 2014, 237, 217–228.
29. N. Caner, A. Sarı and M. Tüzen, Ind. Eng. Chem. Res., 2015, 54, 7524–7533.
30. J. Fan, Y. Qin, C. Ye, P. Peng and C. Wu, J. Hazard. Mater., 2008, 150, 343–350.
31. H. Sereshti, M. Vasheghani Farahani and M. Baghdadi, Talanta, 2016, 146, 662–669.
32. B. Parodi, A. Londonio, G. Polla, M. Savio and P. Smichowski, J. Anal. At. Spectrom., 2014, 29, 880–885.
33. X. Chai, X. Chang, Z. Hu, Q. He, Z. Tu and Z. Li, Talanta, 2010, 82, 1791–1796.
34. R. Gao, Z. Hu, X. Chang, Q. He, L. Zhang, Z. Tu and J. Shi, J. Hazard. Mater., 2009, 172, 324–329.
35. I. Dakova, I. Karadjova, V. Georgieva and G. Georgiev, Talanta, 2009, 78, 523–529.
36. M. H. Mashhadizadeh, M. Amoli-Diva, M. R. Shapouri and H. Afruzi, Food Chem., 2014, 151, 300–305.
37. H. Hu, Z. Wang and L. Pan, J. Alloys Compd., 2010, 492, 656–661.
38. M. Monier, D. M. Ayad and A. A. Sarhan, J. Hazard. Mater., 2010, 176, 348–355.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15999k

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