Magnetic dispersive micro solid-phase extraction for trace mercury pre-concentration and determination in water, hemodialysis solution and fish samples

    Magnetic dispersive micro solid-phase extraction for trace mercury preconcentration and determination in water, hemodialysis solution and fish samples Zarrin Es’haghi, Ghasem Rezanejade Bardajee, Salameh Azimi PII: DOI: Reference:

S0026-265X(16)30002-9 doi: 10.1016/j.microc.2016.03.005 MICROC 2431

To appear in:

Microchemical Journal

Received date: Revised date: Accepted date:

15 November 2015 15 March 2016 15 March 2016

Please cite this article as: Zarrin Es’haghi, Ghasem Rezanejade Bardajee, Salameh Azimi, Magnetic dispersive micro solid-phase extraction for trace mercury pre-concentration and determination in water, hemodialysis solution and fish samples, Microchemical Journal (2016), doi: 10.1016/j.microc.2016.03.005

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ACCEPTED MANUSCRIPT Magnetic dispersive micro solid-phase extraction for trace mercury pre-concentration and determination in water, hemodialysis solution and fish samples

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Zarrin Es’haghi*, Ghasem Rezanejade Bardajee, and Salameh Azimi

Department of Chemistry, Payame Noor University, Tehran 19395-4697, Iran,

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Corresponding author: Zarrin Es'haghi

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Tel:+985118691088, Fax: +985118683001, E-mail: [email protected]

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Abstract

A new preconcentration method, magnetic dispersive micro solid phase extraction (MDMSPE)

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developed for separation and determination of mercury. In this technique, an appropriate mixture of extraction solvent, disperser solvent and nanomagnetite (Fe3O4)/chelating agent 1-(2-

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ethoxyphenyl) -3-(4-ethoxyphenyl) triazene functionalized multi-walled carbon nanotubes with silica shell, as an adsorbent injected rapidly into an aqueous solution containing mercury. After the proper contact time, the nano-adsorbent separated from the aqueous phase by applying magnetic field outside of the vial and transferred to another vial with the elution solvent. The residual solution determined by cold vapour atomic absorption spectroscopy. The main factors affected the preconcentration of mercury investigated and optimized, such as extraction and disperser solvent type, adsorbed amount, sample pH value, effluent concentration, extraction time, the volume of chelating agents and temperature. The adsorption equilibrium data obeyed the Langmuir isotherm models and the kinetic data were well suited to the pseudo-second-order model. Thermodynamic studies revealed the endothermic nature of the procedure. Under the optimum experimental conditions, the detection limit for Hg (II) found to be 1.5 ±0.27 ng mL-1 1

ACCEPTED MANUSCRIPT and its limit of quantification (LOQ) was 5.0±0.32 ng mL-1 (n=5). The linear range of the calibration curve was 9±0.51-1000±0.03 ng mL-1with a correlation coefficient of 0.9994.

absorption

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atomic

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vapour

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Keywords: Magnetic dispersive micro solid phase extraction, Mercury, Chelating agent, Cold

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spectroscopy.

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1. Introduction

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Industrial growth and urbanization cause major environmental pollution by various agents such

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as heavy metals [1]. Mercury is one of the most pollutant heavy metals, which is imputable to its toxicity, persistent character in the environment and biology as well as bioaccumulation along

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the food chain [2]. Mercury has been widely used in several different kinds of products such as,

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plastic, paper, paint and pharmaceutical products [3-5]. It has the negative effects on human health such as kidney and liver toxicity [6, 7], neurological damage [8], chromosome breakage

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and birth defects [9]. Some other important contributors of mercury to the environment are anthropogenic activities like increased mining and high rate of fossil-fuel burning [10].

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The most usual methods for determination of mercury are X-ray fluorescence (XRF) [11], atomic absorption spectrometry [12], voltammetry [13] and inductively coupled plasma (ICP)

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[14]. Among these techniques, in the present study, cold vapour atomic absorption spectroscopy (CVAAS) selected for magnetic dispersive micro solid phase extraction of mercury. Flame atomic absorption spectroscopy is not sufficiently sensitive for the determination of mercury. Thus, atomic absorption employing the cold trap generation technique has been developed to improve the sensitivity for mercury. In the CVAAS, an acidified solution containing mercury reacted with Tin(II) chloride in a vessel external to the atomic absorption instrument. The produced mercury atoms transported to an absorption cell installed in the instrument by the air flow. So, traditional flame atomic absorption spectrometer sensitivity increased about up to five times. Dispersive solid phase extraction (dSPE) [15] is a promising sample pre-handling technique. In dSPE, an SPE adsorbent dispersed in a sample solution containing the target analytes. After 3

ACCEPTED MANUSCRIPT extraction, the adsorbent settled by centrifugation. This approach enables the analytes to interact equally with all the adsorbent particles, achieving greater capacity per amount of adsorbent and

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avoiding common problems of conventional SPE method such as channelling or blocking of

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cartridges or disks.

Multi-walled carbon nanotubes (MWCNTs) have drawn much interest due to their strong

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adsorption power, exceptional mechanical properties, high chemical stability and a large specific

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surface area [16], besides the advantages, they suffer some disadvantages which limit the application of MWCNTs such as poor dispersion and tedious isolation in aquatic phase. At

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present, Fe3O4 magnetic nanoparticles have become more and more important as adsorbents for solid phase micro extraction [17], because they easily isolated using a magnetic field placed

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outside of the extraction container. Therefore, combine magnetic properties into MWCNTs system has the advantages of high adsorption capacity of MWCNTs and the separation

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convenience of magnetic materials.

In this work, the magnetic functionalized MWCNTs modified with silica compound and a new triazen ligand (1-(2-ethoxyphenyl)-3-(4-ethoxyphenyl) triazene (EET), (MWCNTs-Fe3O4 MNPs- silica-EET) to the selective Hg(II) adsorption from the solution. Triazen compounds have recently drawn attention due to a miscellany of interesting properties [18, 19]. Triazene's characterized by having a diazoamino group (–N=N–N–) studied for over one hundred years on their interesting structural and anticancer properties [20]. In our experiments, during the sol-gel reaction of 3-(trimethoxy silyl) 1-propane thiol (TMOSPT) with triazen solutions and magnetic multi-walled carbon nanotubes (MWCNTsFe3O4 MNPs), the silica particles formed interconnecting rigid networks and immobilized on the surface of the MWCNTs-Fe3O4 MNPs nanoparticles, while the triazen (EET) and MWCNTs4

ACCEPTED MANUSCRIPT Fe3O4 MNPs were simultaneously fixed on the silica particle surface. MWCNTs-Fe3O4 MNPssilica-EET applied for separation and enrichment of trace amounts of Hg (II) by MDMSPE and

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then desorbed Hg (II) with HNO3, determined by using CVAAS.

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The developed method used for mercury determination in fresh and spring water, Caspian Sea, hemodialysis solution and canned tuna fish samples.

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2. Experimental

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2.1 Material and solutions

Ferric chloride hexa-hydrate (FeCl3·6H2O), ferrous chloride tetra-hydrate (FeCl2·4H2O),

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ammonium hydroxide (NH4OH), sodium hydroxide (NaOH), hydrochloric acid (HCl), mercury nitrate di-hydrate (Hg (NO3)2. 2H2O, stannous chloride, tetramethoxyorthosilicate (TMOS),

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tetraethylorthosilicate (TEOS) ,and 3-(trimethoxy silyl) 1-propane thiol (TMOSPT) were of analytical grade and all obtained from Merck (Darmstadt, Germany). All aqueous solutions

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prepared with de-ionized water. Tin(II) chloride solution (25 % w/v) as the reducing agent prepared by adding 25.00 g of stannous chloride to a 20 mL of conc. hydrochloric acid. The mix heated to dissolve the stannous chloride then allowed to cool before diluting the solution to 100 ml with water and mixing.

A 1000 mg.L-1 Hg (II) stock solution prepared by dissolving accurate amounts of their salts in water. The daily working solutions were ready by appropriate dilutions of the stock solution immediately prior to use in the experiments. 2.2 Canned tuna fish sample preparation and digestion Persian Gulf canned tuna fish samples purchased from popular supermarkets in Ghazvin, Iran. The contents of cans of tuna homogenized thoroughly in a food blender. The 2 ± 0.001 g of homogenized mixture then digested according to the following procedure: the test sample 5

ACCEPTED MANUSCRIPT weighed into a 500 mL glass digestion tube, and 10 ml of conc. HNO3 and 5.0 mL of H2SO4 added drop wise. The tube was then placed on a steam bath to complete dissolution. It was then

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removed from the steam bath, and after cooling the solution transferred into a 50 ml volumetric

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flask. This solution is placed under the extraction process, according to the procedure described in section 2.8. Then, mercury determined in the digested test sample using cold vapour atomic

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absorption spectrophotometry. The other real samples analysed after the extraction, without any

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treatment. 2.3 Instruments

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An atomic absorption spectrometer (Spectra AA 220 FS, Varian) equipped with a vapour generation accessory (VGA-77, Varian) and a T-shaped quartz absorption cell used for mercury

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determination.

A mercury hollow cathode lamp applied as light source. The operating condition of the

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instrument adjusted according to the manufacturer’s instruction as follow; lamp current 4.0 mA, absorbance wavelength at 253.7 nm, spectral bandwidth at 0.5 nm, vapour Type cold vapour, flow rate of Tin (II) chloride solution as the reducing agent 1.0 mL/min, flow rate of samples 7.0 mL/min.

2.4 (2-ethoxyphenyl)-3-(4-ethoxyphenyl) triazene (EET) Synthesis A 1000 ml flask charged with 100 g of ice and 100 ml of water and then cooled to 0 ºC in an icebath. After that, a solution containing 6.9 g (0.05 mol) of 4-ethoxyaniline in 25 ml of methanol and 10 ml (0.12 mol) of hydrochloric acid (d = 1.18 g.ml−1) added to the reaction flask. Then, a solution containing 3.45 g (0.05 mol) of NaNO2 in 25 ml of water added during 15 min under stirring. Afterwards, 6.38 g (0.05 mol) of 2-ethoxyaniline added to the mixture for a period of 30 min under vigorous stirring. Finally, the sodium acetate solution (18% of concentration) added to 6

ACCEPTED MANUSCRIPT a pH between 7 and 8 was obtained, and then stirred for 2 h. The yellow colour residue filtered and dissolved in diethyl ether. After evaporation of diethyl ether, the purified orange crystals

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obtained (yield, 97%) [21].

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2.5 Fabrication of MWCNTs-Fe3O4 MNPs-silica-EET

MWCNTs oxidized as previously reported, according to the following procedure [22]. Untreated

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MWCNTs heated in an oven at 550 ºC for 45 minutes to remove amorphous carbon. After

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thermal treatment, a 500 mg of MWCNTs was dispersed into a flask containing 20 ml of a 70% sodium hypochlorite solution (6mL of H2O + 14mL of NaClO). The result was then rocked in an

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ultrasonic cleaning bath for 120 minutes. The solution filtered through a 0.45 µm nylon fiber filter, moving over the activated MWCNTs. Finally, the MWCNTs washed thoroughly with

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double distilled water several times until the pH of the filtrate was neutral. The filtered solid dried in the oven at 70 ºC, obtaining carboxylic acid-functionalized MWCNTs (MWCNT–

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COOH).

Magnetic carboxylic acid-functionalized multi walled carbon nanotubes (MWCNTs-Fe3O4 MNP) prepared as reported by Gong et al. [23]. For this purpose 500 mg of MWCNT–COOH suspended in 250 ml water containing 700 mg (2.5 mM) of iron (II) chloride tetrahydrate and 1350 mg of iron (III) chloride hexahydrate (5 mM). The temperature of the suspension raised to 50 ºC under argon atmosphere followed by the slow addition of 5 ml of 8 mol.L-1 ammonium hydroxide solution with stirring. The pH of the suspension controlled in the range of 10–11. After complete addition of ammonium hydroxide solution, the temperature was brought up to 80 ºC and reaction allowed to be continued for 30 min. The suspension cooled to room temperature and MWCNTs-Fe3O4 MNP isolated from the mixture with the help of a permanent magnet.

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ACCEPTED MANUSCRIPT Separated MWCNTs-Fe3O4 MNP washed thrice with deionized water, followed by ethanol. Finally, MWCNTs-Fe3O4 MNP dried under vacuum.

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The MWCNTs-Fe3O4 MNP modified sequentially with TEOS, TMOS, TMOSPT and EET to

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introduce amine and thiol groups as previously mentioned by Zhang et al [24]. Typically, 20 mL magneto- suspension (10 mg of MWCNTs-Fe3O4 MNP, which dispersed in 10 ml water) diluted

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with 150 mL ethanol, and the mixture was homogenized by ultrasonication for 15 min prior to

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the addition of 1 mL NH3-H2O. After being stirred vigorously for 30 min at 30 ºC, 1.0 ml TMOSPT dropped into the solution. The reaction performed for 45 min and then 0.2-2.0 ml

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concentrated EET solution introduced and lasted reaction for another 4 h; the suspension cooled to room temperature and MWCNTs-Fe3O4 MNPs-silica-EET was isolated from the mixture with

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the aid of a permanent magnet. Separated MWCNTs-Fe3O4 MNPs-silica-EET washed thrice with deionised water, followed by ethyl alcohol. Finally, MWCNTs-Fe3O4 MNPs-silica-EET dried

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under vacuum. During the mentioned stages, the amount of EET and the effect of silica compounds (TMOS, TEOS, and TMOSPT) were systematically investigated to correlate the dependence of physicochemical properties of MWCNTs-Fe3O4 MNPs-silica-EET on the key preparation parameters.

The structure and morphology of MWCNTs-Fe3O4 MNPs-silica-EET characterized by FT-IR (Jasco 4200), X-ray diffraction (XRD-D8, BRUKER), transmission electron microscopy (TEMCM10, Philips). Magnetic properties of the nano adsorbent were investigated using a vibrating sample magnetometer (VSM) with an applied field between−1000 and 10000 Oe at room temperature (MDKF, Iran). The thermal stability of MWCNTs-Fe3O4 MNPs-silica-EET was investigated by thermogravimetric analysis (STA 503, Bahr, Germany) at a heating rate of 10 °C/min under N2 flow (10 ml.min-1). 8

ACCEPTED MANUSCRIPT 2.6 EET Characterization Infrared, HNMR,

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CNMR spectra and CHN analysis confirmed the EET structure. 1H NMR

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(300 MHz, DMSO-d6, 298 K, relative to Me4Si): 1.37 (CH3, 6), 4.13 (CH2, 4), 6.94-8.06 (phenyl

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protons, 8), 12.89 (NH, 1), 13CNMR (300 MHz, DMSO-d6, 298 K, relative to Me4Si): 14.3 (CH3 aliphatic), 65.1 (CH2 aliphatic), 115, 125.8, 120.2, 120.9 (C-H benzene ring), 145.8 (CCO-),

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134.4 and 155.4 (C-N=N-N). FT-IR (KBr, cm−1): 3304.43 (N-H), 3073.15 (C-H-, sp2), 1591.67

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(C= C), 1397.96, 1164.27 (N= N), 844.78. Elemental analysis, Found: (CHN: (C16H19N3O2) C: 67.32%, H: 6.68%, N: 14.70%. which is calculated for (C16H19N3O2): C, 67.35; H, 6.71; N,

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14.73%. 2.7 Adsorbent characterization

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FT-IR and TGA spectra used for comparing the amount of the triazen loaded ligand on different silica surface. By comparing different spectra, TMOSPT was selected as the best silica

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compound for more chelating of triazen compound, therefore more and easily adsorption of Hg (II) would occur. TMOSPT was selected for further experiments. Fig. 1a shows the FT-IR spectra of the synthesized MWCNTs-Fe3O4 MNPs-TMOSPT-EET and MWCNTs-Fe3O4 MNP-TMOSPT. The peak at 588 cm-1 is assigned to the stretching of Fe–O bond. The bands at 972 and 1065 cm-1 were assigned to the vibration of the Si-O bonds, which indicate, the formation of silica shells on the surface of Fe3O4. The broadband at 3400 cm-1, can be assigned to the hydrogen-bonding silanol groups and adsorbed water and the band at 2950 cm1

is associated with the CH2 vibrations corresponding to the C-H stretching. The observed

absorption peak at 1602 cm-1 is connected to the N–H bending vibration and those appeared at 1410 and 1644 cm-1 are assigned to the =C-H bending and C=C stretching, respectively. These

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ACCEPTED MANUSCRIPT outcomes suggest that the amine and vinyl functional groups are present on the airfoil of the modified MNPs.

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Fig.1a Fig.1b, displays the XRD pattern of the MWCNTs-Fe3O4 MNPs -TMOSPT-EET, which presents

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almost the same feature of unmodified Fe3O4 MNPs. Six characteristic peaks (2θ = 30.1◦, 35.8◦,

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43.3◦, 53.7◦, 57.7◦, and 63.1◦), related to their corresponding indices ((2 2 0), (3 1 1), (4 0 0), (4 2

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2), (5 1 1) and (4 4 0)) were observed in the case of Fe3O4 nanoparticles respectively. Nevertheless, the diffraction peak at 2θ = 26.87° is assigned to (002) plane of MWCNT–COOH

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and the broad peak at 2θ= 18–24◦ corresponded to the amorphous structure of silica shell [25]. Fig.1b

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The particle size of nanoparticles (NPs) determined through Debye-Scherrer's equation. The Fe3O4 nanoparticles were spherical and the particle sizes were approximately 10 nm (9 ± 2 nm).

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After MWCNTs functionalization, Fe3O4-MWCNTs appear quasi-spherical in shape with an average diameter of about 20 nm. The MWCNTs-Fe3O4MNPs-TMOSPT-EET were approximately 36.92 nm.

The magnetic properties of MWCNTs-Fe3O4 MNPs-TMOSPT-EET and MWCNTs-Fe3O4 MNPs were characterized using a vibrating sample magnetometer (Fig.1c). They exhibited super paramagnetic behavior and had little hysteresis, remanence and coercivity due to the fact that the particles are composed of ultrafine magnetite nanocrystals. The magnetic saturation (Ms) values are 24.03 and 19.04 emu g-1, for MWCNTs-Fe3O4 MNPs and MWCNTs-Fe3O4 MNPsTMOSPT-EET respectively. As can be understood, in the presence of silica shell, the magnetic potency of the nanocomposite reduced [25].

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ACCEPTED MANUSCRIPT The coating on the iron oxide core, which includes the silica polymeric layer, is typically a few nm thick. Since this thick coating increases the distance separating the magnetic core from the

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solution, and the saturation magnetization of the magnetic core are significantly reduced. This

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reduces the potency of the nanocomposites as a magnetic sorbent. The mechanism by which the coating decreases the saturation magnetisation of the magnetite core is not well understood. For

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such nanocomposites, the decrease in magnetism may be attributed the ensuing distortion of the

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structure. Despite this, it should be noted that the modified Fe3O4 MNPs, still demonstrate the strong magnetization, which indicates their suitability for magnetic separation and targeting.

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Fig. 1c

TGA pplied to measure the organic content over the surface of the synthesized magnetic

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nanocomposites. The TGA (Fig. 1d) curve for the MWCNTs-Fe3O4 MNPs-TMOSPT-EET

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presents the first weight loss below 120 °C and this can be explained by the evaporation of the

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adsorbed solvent (water and ethanol) attached to the particle surfaces. Later on this little weight loss, an unexpected weight gain is taking place within the temperature range from 200 to 300 °C. As reported by Caruntu et al. [26], this weight gain can be ascribed to the oxidation of Fe3O4 to Fe2O3. The approximately 12% weight loss from 460 °C to 700 °C is presumably due to the decay of organic groups, while relatively slow weight loss at elevated temperatures can be affiliated to the decomposition of silica scale. Fig. 1d Transmission electron microscopy image (TEM) (Fig. 1e) of MWCNTs-Fe3O4MNPs-TMOSPTEET indicates the formation of spherical particles and the silica coated magnetic nanoparticles which exhibit perfectly spherical with smooth surface and represent clear core–shell structure.

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ACCEPTED MANUSCRIPT The core–shell MWCNTs-Fe3O4 MNPs-TMOSPT-EET are uniform with a size of about 39 nm and the silica layer is about 9 nm in thickness.

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Fig.1e 2.8 Magnetic dispersive micro- solid phase extraction procedure

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The extraction method which used in this research is based on dispersive-nanoparticle assisted –

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liquid phase microextraction for the preliminary preconcentration and determination of trace

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amounts of mercury. The application of nanoparticles permits the easy separation and extraction of analyte from the real samples. The analyte was accumulated on a magnetic nanomaterial

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sorbent and identified by atomic cold trap-absorption spectroscopy technique. In this procedure, 10.0 mL of ultra-pure water placed in a 40 mL glass tube with conical bottom

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and spiked at the level of 1 mgL−1 of Hg (II). MWCNTs-Fe3O4 MNPs-TMOSPT-EET (3mg) as adsorbent, Methanol (1.8 mL) as disperser solvent and 60 µL chloroform as organic solvent,

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injected rapidly into the sample solution using a 5.0 mL gastight Hamilton syringe (Bonaduz, Switzerland). The produced cloudy solution sonicated for 2 minute, and then the adsorbents isolated by super magnet by applying it outer place of the tube. After isolating, the settled phase dried in water bath at 50 ºC in order to evaporate the extraction solvent. Then, 5 mL of 1molL−1 solutions of HNO3 (as elution solvent) added. The obtained solution introduced into CVAAS. All the experiments were done in triplicates and means of the results were calculated and described.

3. Results and discussion 3.1 Optimization of MDMSPE effective parameters Extraction solvent used in the MDMSPE should have higher density than water, excellent extraction ability for the target analytes and low solubility in water. In this work, chloroform, 12

ACCEPTED MANUSCRIPT carbon tetrachloride benzyl alcohol and chlorobenzene were used as extraction solvents. The experiments were carried out by using 1.8 mL of methanol and different volumes of the

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extraction solvent. The results revealed that chloroform has the highest extraction efficiency in

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comparison with the other three common extraction solvents. In this study chloroform applied as liquid extraction solvent which accommodated adsorbing more Hg (II) from the aqueous solution

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and transferred it to the adsorbent easily. Therefore it causes more extraction efficiency.

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Disperse solvent must be miscible in both aqueous and extraction phase and should accept the good dispersive ability. Methanol, ethanol, acetone, acetonitrile, methanol: acetone (1:1),

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methanol: acetonitrile (1:1) were therefore used as disperser solvents and the effect of these solvents on the performance of MDMSPE investigated. The obtained results show that methanol

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has the highest extraction efficiency of mercury solution among these disperser solvents. The amount of MWCNTs-Fe3O4 MNPs-TMOSPT-EET in the extraction phase was evaluated in

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the range 1–10 mg. The results obtained are shown in Fig.2a. Equally it can be ascertained, a minimum concentration of 3 mg is needed to achieve the maximum extraction. The decrease observed for higher concentrations can be ascribed to a potential aggregation of MWCNTsFe3O4 MNPs-TMOSPT-EET, which results in a reduced surface available for analyte interaction. Therefore, a dispersion containing the MWCNTs-Fe3O4 MNPs-TMOSPT-EET at a concentration of 3 mg was selected as optimal. Fig.2a The pH of the aqueous solution is another significant variable that controls the adsorption of Hg (II) on MWCNTs-Fe3O4 MNPs-TMOSPT-EET–water interfaces. Hence, the effect of pH on the adsorption of Hg (II) onto the adsorbent investigated in the pH range of 2–11 using dilute HCl or NaOH for adjustment (Fig. 2b). It can be noted that the adsorption of mercury increases with 13

ACCEPTED MANUSCRIPT increasing pH of the solution. The absorption signals remained almost constant within the interval 6–8 and then slightly decreases as the pH becomes more basic. Therefore a pH of 6

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chosen as the best choice.

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Fig.2b

After MDSPME, the adsorbent should be dried under water bath, because chloroform which was

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used for more and easily Hg (II) extraction efficiency, has to be removed and evaporated. The

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drying temperature of the settled investigated in four different temperatures 40, 50, 60, 70 and 80 o

C. Better recoveries were resulted at 50 and 70 oC, therefore drying at 50 oC selected as optimal.

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To study the effect of chloroform volume on the analytical performance, experiments were performed by using 1.8 ml of methanol and the volume of chloroform adjusted in the range of

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10–120 µL. The extraction efficiency increases with increasing the volume of chloroform from 10 to 80 µL. However, the extraction efficiency decreased when the volume of chloroform was

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larger than 80 µL. It seems that 1.8 ml of methanol could not disperse superabundant extraction solvent to form fine droplets well, resulting in a decrease in extraction efficiency. So, 60 µL chosen as the optimal extraction solvent volume. The experimental conditions were fixed and included the use of different volumes of methanol 0.5-5.5 ml. It was found that the extraction efficiency, increased by increasing the volume of methanol up to 1.8 ml and then decreased with further increasing of methanol volume. This phenomenon can be explained as follows: chloroform was not dispersed well when using a smaller volume of methanol and thus the extraction efficiency was lower and in larger amount of methanol, it has an inhibition effect on mercury ion interaction with the adsorbent. Hence, 1.8 ml of methanol selected for further investigation.

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ACCEPTED MANUSCRIPT The other important variable studied was the eluent condition. For this purpose, different solvents: nitric acid, hydrochloric acid, EDTA solution, methanol and ethanol were evaluated as

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eluent, being the volume fixed at 5mL in all instances. Since maximum elution was obtained by

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applying nitric acid solution and it offered the best elution values.

The concentration of nitric acid needed to obtain a quantitative elution of Hg (II) studied

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between 0.5 and 2.5 mol.L-1. As the result, 1 moL.L-1 of nitric acid selected to optimize the

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following variables. 3.2. Adsorption isotherm

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Two different models of adsorption isotherm, Langmuir [27] and Freundlich [28] models were used to describe the relationship between the adsorbed amount of Hg (II) and its equilibrium

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concentration in solution. Langmuir and Freundlich isotherm models are represented in Table 1:

Table1. Adsorption isotherm model equations and parameters for the adsorption of MWCNTsFe3O4 MNPs-TMOSPT-EET towards Hg (II) from aqueous solution.

Isotherm

Formula

Linear form

Parameters

qe=KFCe1/n

lnqe=lnKF+1/n lnCe

KF

8.7

n

1.92

R2

0.92

qmax(mg.g-1)

23.04

models Frundlich

Langmuir

qe=qmaxbCe/1+b

Ce/qe=1/qmaxb+Ce/qmax

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ACCEPTED MANUSCRIPT b(L.mg-1)

0.29

R2

0.99

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Ce

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Where in Langmuir equation, Ce is the equilibrium concentration of the adsorbent (mg.L-1), qe (mg.g-1) is the equilibrium adsorption capacity of adsorbent; qm (mg.g-1) is the maximum

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adsorption capacity; KL (L.mg-1) is the Langmuir adsorption equilibrium constant related to the

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adsorption energy. In Freundlich isotherm, KF ((mg. g-1)/(mg. L-1) 1/n) and n are isotherm constants that indicate the content and strength of the adsorption, respectively. The 1/n factor

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also indicate the heterogeneity factor, adsorption intensity and the type of isotherm to be favourable (0.1 < 1/n < 0.5) or unfavourable (1/n > 2). In terms of R2 values, the applicability of

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the above two models for present experimental data approximately followed the order: Langmuir > Freundlich. It shown that the Langmuir equation had the best fit to the experimental data. The

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maximum adsorption capacity calculated by this function was 23.04 mg·g-1. On the other hand, the values of 1/n are all less than 1, indicative of high adsorption intensity. 5. Thermodynamic study

The relationship between the equilibrium constant (K0) and Gibbs free energy change (∆Gº, kj.mol-1) of the adsorption process can be obtained from Eq 1: G0   RT ln K0

(1)

Where, R and T are the universal gas constant (8.314 J.mol-1K-1) and the absolute temperature (K), respectively. Values of K0 can be calculated from Eq 2 [29]: K

qe Ce

(2)

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ACCEPTED MANUSCRIPT The enthalpy (∆Hº, kj.mol-1) and entropy change (∆Sº, Jmol-1K-1) for adsorption were calculated

S H  R RT

(3)

All the thermodynamic parameters are presented in Table 2.

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ln K 0 

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from the slope and intercept of the plot of lnK0 versus 1/T using Eq 3:

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Table 2. Thermodynamic data for the adsorption of MWCNTs-Fe3O4 MNPs-TMOSPT-EET towards Hg (II) from aqueous solution. ∆G(KJ.mol-

(mg.L-1)

1

20

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∆s(J.mol-

1

)

1

)

33.591

140.3

-7.114

2.92

22.921

102.71

-8.129

3.28

33.591

140.3

-7.711

3.11

22.921

102.71

10

-9.11

3.61

33.591

140.3

5

-8.338

3.3

22.921

102.71

10

-9.526

3.72

33.591

140.3

5

-8.610

3.36

22.921

102.71

D TE

10

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∆Hº(KJ.mol-

3.08

5 30

LnK

-7.518

5 25

)

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Concentration

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Temperature

6. Adsorption kinetics Kinetics involves the study of the rates of chemical operations and facilitates an understanding of the factors that find those rates [30]. The study of chemical kinetics involves careful monitoring of the experimental conditions that influence the speed of a chemical reaction in its race toward equilibrium. These studies yield information about the possible mechanisms of adsorption and 17

ACCEPTED MANUSCRIPT the different transition states formed on the way to a final adsorbate– adsorbent complex. The kinetic adsorption data processed to understand the dynamics of the adsorption process in terms

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of the orderliness of the rate constant. Three known kinetic models employed to investigate the

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mechanism of the adsorption. The kinetic equations and parameters represented in Table 3. Table 3. Adsorption kinetic models equations and parameters for the adsorption of MWCNTs-Fe3O4

Liner form

dq  K 1 (q e  q t ) dt

Log (qe  qt )  Log (qe )  ( K1

Parameters

)t 2.303

qe

1.94

K1

3.34

R2

0.204

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order [32]

qt)2

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0.395 0.984

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Pseudo second

1

(mg.L-1)

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order[31]

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Pseudo first

Formula

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Kinetic models

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MNPs@TMOSPT-EET towards Hg (II) from aqueous solution.

0.913 t 1 t   2 qt K 2 q e qe

qe K2

-0.465 R2

0.218 0.995 0.981

qt  k id t  Ci

Intra particle diffusion[33] 0.48

Kid

2.73 R2

0.783 0.924 18

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In the above equations, K1 (min-1), K2 (g. mol-1 min), Kid (mole. g-1. min-0.5), C (mole. g-1), qe and

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qt are the rate constant of the pseudo-first-order adsorption, the rate constant of the pseudo second-order adsorption and the intra particle diffusion rate constant, constant proportional to the

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heaviness of the boundary layer, the amounts of mercury adsorbed on adsorbent (mg.g-1) at

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equilibrium and at time t, respectively.

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The plotting of log (qe–qt) versus time (t) for the pseudo-first order kinetic model and qt versus t for intra particle diffusion model did not converge well and did not produce straight lines at

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the studied temperatures. When the pseudo-second order adsorption equation applied by plotting (t/qt) versus time (t), all the data converged well in a straight line with a high correlation

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coefficient (R2). Based on these results, it is clear that the equilibrium adsorption from the pseudo-second order model is much closer to the experimental. It could be seen that the

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experimental data were fitted better by the pseudo-second order model with R2 of 0.981-0.995 than the other two models (R2 of 0.783-0.984). The high values of R2 implied the good applicability of the pseudo second-order kinetic equations for the Hg (II) adsorption on MWCNTs-Fe3O4 MNPs-TMOSPT-EET. These results suggested that chemical adsorption might be involved in the adsorption process. 7. Analytical performance and method validation In order to demonstrate the validation of the proposed method, the analytical features of the method such as linear range of the calibration curve, limit of detection (LOD), accuracy and precision were obtained. By different concentration of Hg (II) at the optimum conditions, the calibration curve obtained (n=3). The calibration curve was linear in the range of 9-1000 ng mL-

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. The equation of the line was A= 0.7926C-0.0047 where A is the absorbance and C is

concentration of metal ion in µg mL-1, the regression coefficient for the line was 0.9994.

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The limit of detection (LOD) and limit of quantification (LOQ) are defined as: LOD = 3 s/m and

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LOQ = 10s/m. (Where s is the standard deviation of blank peak currents in five runs and m is the slope of the calibration curve). The detection limit (LOD) of the presented DMSPE study

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calculated under optimal experimental conditions after application of the preconcentration

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procedure to blank solutions. LOD based on the standard deviation of the blank was 1.5±0.27 ng mL-1 (n=5). The limit of quantification (LOQ) for Hg (II) determination by proposed method was

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5.0 ± 0.32 ng mL-1 (n=5).

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8. Application of proposed method to fresh & spring water, Caspian Sea, canned tuna fish samples and hemodialysis solution

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In order to evaluate the efficiency of synthesized adsorbent in pre-concentration and extraction of Hg (II), from real samples, the developed method applied to determination of mercury ions in real samples. Concentrations of mercury in fresh & spring water, Caspian Sea, canned tuna fish sample and haemodialysis’ solutions were determined using standard addition method. The results are listed in Table 4.

The separate aliquot of the real samples spiked with known concentrations of the analyte. It is carried through the pre-concentration and analytical procedure to determine the matrix interferences. The recoveries of the spiked real sample solutions are evaluated to determine accuracy in a given matrix. The validation of the presented procedure is performed by the analysis of standard reference material (SRM). The method accuracy determined by means of a

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The relative recoveries and matrix effects were determined for spiked samples (n  =  5).

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Relative recovery was calculated by dividing the analytical signal for a sample spiked before extraction by the signal for an equal concentration sample in the same matrix spiked after

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extraction. The relative recovery values close to 100% indicate the lack of matrices effect and

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good accuracy of the procedure.

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Table 4. Median values for Hg (II) determination in different kinds of samples (n=5). Found amounts in

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real sample was subtracted from the amount found in spiked samples. Hg(II) Added (ng mL-1)

Found (ng mL-1)

*

RSD% (n=5)

0 5.0 10 20

30.00 5.12 10.05 21.00

96.6 97.57 103.44

4.29 3.53 3.20 3.31

0 5 10 20

45.00 5.43 10.39 20.06

99.63 99.42 98.09

3.38 2.51 2.67 2.07

0 5 10 20

28.00 5.21 10.26 20.2

98.67 99.8 99.6

4.87 4.57 3.91 3.26

0 5

6.00 5.02

99.2

5.01 4.22

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Sample

Fresh water

Spring water

Caspian Sea

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R. R%

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100.09 100.19

3.65 3.56

0 5

**B.D.L 4.99

99.8

3.48

10 20

10.1 19.96

101 99.8

3.60 2.76

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Hemodialysis solution

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Canned tuna fish

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*R. R%: Relative Recovery %

**B.D.L: below detection limit

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9. Conclusion

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A novel MWCNTs-Fe3O4 MNPs-TMOSPT-EET adsorbent was successfully synthesized and applied in MDMSPE and preconcentration of Hg (II) by CVAAS. The proposed method takes in

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the advantages of being practical, simple, inexpensive, sensitive and quick method for mercury

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determination in some different complex samples. The developed method has a good detection limit and a large dynamic linear range, making it desirable for the trace analysis of Hg (II).

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10. Acknowledgments

We would wish to thank Payame Noor University for supporting of this research.

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Figure captions:

MNP-TMOSPT.

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Fig.1a FT-IR spectra of MWCNTs-Fe3O4 MNPs -TMOSPT-EET composite and MWCNT-Fe3O4

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Fig.1b XRD pattern of MWCNTs-Fe3O4 MNPs -TMOSPT-EET. Fig. 1c Magnetization curves of MWCNTs-Fe3O4 MNPs and MWCNTs-Fe3O4 MNPs TMOSPT-EET.

Fig. 1d TGA curve of MWCNTs-Fe3O4 MNPs -TMOSPT-EET. Fig.1e TEM image of MWCNTs-Fe3O4 MNPs -TMOSPT-EET. Fig.2a. Amount of MWCNTs-Fe3O4 MNPs -TMOSPT-EET in extraction phase (mg). Fig.2b. Influence of pH solution on uptake of Hg (II) from aqueous solution by MWCNTs-Fe3O4 MNPs -TMOSPT-EET.

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ACCEPTED MANUSCRIPT *Highlights (for review) •

A magnetic chelating agent 1-(2-ethoxyphenyl) -3-(4-ethoxyphenyl) triazene

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functionalized multi-walled carbon nanotubes with silica shell is developed as a sorbent. Cold vapor atomic absorption spectroscopy is used for analysis.

•

Hg(II) has been selected as target compounds.

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Target ion is determined in water, hemodialysis solution and fish samples.

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•

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