Preparation of Micelle Supported Magnetic Hydroxylated Multi-walled Carbon Nanotubes for Determination of PAHs

Transcription

1 2017 Asia-Pacific Engineering and Technology Conference (APETC 2017) ISBN: Preparation of Micelle Supported Magnetic Hydroxylated Multi-walled Carbon Nanotubes for Determination of PAHs Ming-Yu Wang, Shao-Jun Zhang, En-Rui Zhao and Jia-Gan Li ABSTRACT * A nano-scaled micelle supported Fe3O4 magnetic nanoparticles decorated hydroxylated multi-walled carbon nanotubes material was synthesized. The material was facilely synthesized between carbon nanotubes and Fe 2+. The prepared material used in dispersive solid-phase extraction for investigation of polycyclic aromatic hydrocarbons (PAHs) in seawater for the first time. The application showed good response (R2 > ) in the range of µg/l, satisfactory reproducibility and high precision. Limits of detection of PAHs ranged from to µg/l (S/N=3). The spiked recovery of established method ( %) was times higher than that of the conventional method, and the corresponding relative standard deviation (RSD) was in the range of %. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants that formed from both natural and anthropogenic sources that have mutagenic and carcinogenic properties. Because of their hydrophobicity, PAHs and its metabolites tend to be interacted with DNA and are genotoxic, causing malignancies and heritable genetic damage in humans. Therefore, close attention has been paid to the research of PAHs[1]. Directly determination of PAHs in environmental samples using analytical instrument is still challenging because of the extremely low concentrations and complex unknown interferences. Therefore, pre-concentration is required [2]. Modern pretreatment technique dispersive solidphase extraction (DSPE) greatly simplified the pre-concentration and cleaning processes that overcome the limitations of the classical techniques. Many researches have focused either on the synthesis or the modification of nanomaterial adsorbents such as nanofibers [3], graphene [4], glassy carbon [5], multi-walled Ming-Yu Wang 1,2, Shao-Jun Zhang 1,*, En-Rui Zhao 1, Jia-Gan Li 1 1 College of Naval Architecture and Marine Engineering, Shandong Jiaotong University, 1508 Hexing Road, , Weihai, China 2 Ocean School, Yantai University, 32 Qingquan Road, Yantai, China Corresponding author: shaojunyt@163.com 1711

2 carbon nanotubes (MWCNTs) [6]. Among them, MWCNTs is considered as a most widely used adsorbent materials. MWCNTs exhibit good affinity to polycyclic conjugated compounds, which is beneficial for formation of stacking interaction between nanomaterial and analyte [7]. There have no report for determination of PAHs using MWCNTs as adsorbent. In the current study, a surfactant modified magnetic hydroxylated multi-walled carbon nanotubes (MMWCNTs-OH) was prepared and used as an adsorbent for DSPE process. MMWCNTs-OH was utilized as the stationary phase for the micelles and the solution was charged by adjusting the ph of the sample. Extraction of MMWCNTs-OH was achieved by applying an external magnetic field. Results from the proposed method were also compared with those of the classical SPE method. The proposed technique greatly simplifies the extraction process while still maintaining high sensitivity and high recovery rate. EXPERIMENTAL Chemicals and Reagents Methanol, acetonitrile, acetone and n-hexane were purchased from Merck Chemicals Pty. Ltd., Germany. Surfactant DTAB, CTAB, SDBS and SDS were purchased from Aladdin Reagents (Shanghai) Co., Ltd., China. A standard mixture of the USA-EPA 16 priority PAHs (2000 µg/ml, dichloromethane) was purchased from Sigma-Aldrich Pty. Ltd., USA. Synthesis of MMWCNTs-OH MWCNTs-OH was prepared based on the method of Ling [8]. MMWCNTs-OH was prepared under alkaline conditions with oxidation-reduction reaction using MWCNTs and Fe 2+ according the method of Zhang with some modification [9]. Scanning electron microscope (SEM, LEO1530, Zeiss, Germany), X-ray diffraction (XRD, 6000, Shimadzu, Japan), Fourier transform infrared spectroscopy (FTIR, 8400S, Shimadzu, Japan) and vibrating sample magnetometer (VSM, 7410, Lake Shore, USA) were used for characterization of the properties of the prepared material. Sample Collection and Extraction In the sample extraction process, sample solution (100 ml) was adjusted to ph3 by adding HCl to it. MMWCNTs-OH solution 1.0 ml (1.0 mg/ml) containing CTAB (1.0 mg/ml) was added to the sample solution. Extraction was stirred for 20 min. MMWCNTs-OH was collected by a magnet. After desorption with 1.0 ml acetone, the acetone solution was analyzed by using Gas Chromatography-Mass Spectrometer (GC/MS). GC/MS system (GC/MS-QP 2010 Plus, Shimadzu, Japan) equipped with a DB-5MS column was used for quantitative analysis of PAHs. The ion source temperature was 220 C. GC/MS interface temperature was 250 C. The column temperature was maintained at 70 C for 1 min, then raised to 300 C by using a heating rate of 5 C/min and maintained at 300 C for 5 min. Later, the temperature was raised to 310 C by using a heating rate of 5 C/min and maintained at this value for 1 min. 1712

3 RESULTS AND DISCUSSION Synthesis and Characterization of Materials The synthesis of MMWCNTs-OH procedure was as follows: FeCl3 6H2O (0.060 g) and FeCl2 4H2O (0.020 g) were dissolved in 40 ml of deionized water. The solution was constantly stirred in a 100 ml flask. Subsequently, MWCNTs- OH (0.100 g) was added into the mixed solution followed by the slow addition of 30 ml (5%, v/v) ammonium hydroxide solution at 80 C, and the mixture was stirred vigorously for 60 min. The whole reaction process was performed under nitrogen gas protection. Thereafter, the black precipitates formed (MMWCNTs- OH) were separated from the reaction medium via centrifugation at 2000 r min 1 and then washed with deionized water several times. The MMWCNTs was collected by a magnet and dried under vacuum. The concentration of MMWCNTs- OH was 12.5 mg/ml. The SEM image of MMWCNTs-OH showed that Fe3O4 magnetic nanoparticles (MNPs) were well-deposited on the surface of the MWCNTs surface and no occurrence of aggregates was observed. The average particle diameter of Fe3O4 MNPs was 10 nm. The SEM image of MMWCNTs-OH modified with CTAB confirmed highly porous morphology of the adsorbent along with uniform size distribution of MMWCNTs-OH. After the reaction, MMWCNTs-OH not only maintained the morphology of MWCNTs, but also showed a good ability as a stationary phase carrier. The magnetic properties of MMWCNTs-OH were determined by VSM. As shown in Fig. 1a, MMWCNTs-OH showed super paramagnetic character that made it a good adsorbent for the DSPE process. The maximum value for the saturation magnetization of MMWCNTs-OH was emu/g, which was sufficient to analyze PAHs in the collected samples. MMWCNTs-OH were rapidly dispersed after exposure to the external magnetic field. This phenomenon was caused due to the reason that the MMWCNTs-OH had a large number of polar functional groups which not only provided abundant adsorption sites for micelles but also improved the dispersion of MMWCNTs-OH in water. The magnetic properties of MMWCNTs-OH@CTAB were also determined. The maximum saturation magnetization was slightly decreased to emu/g, which was still sufficient to analyze the PAHs in the samples. Fourier transform infrared (FTIR) spectroscopy has been used to explore the molecular adsorption, transmission and the creation of a molecular character of the prepared material. As shown in Fig. 1b, the vibrational band assignments (cm -1 ) of the MWCNTs-OH were summarized as follows: 1118 cm -1 (C-O), 1260 cm -1 (=C- O-C-OH), 1625 cm -1 (=C-O-H), 1715 cm -1 (C=O), 3439 cm -1 (O-H). The peak comparison between MWCNTs, MWCNTs-OH and MMWCNTs-OH showed that oxygen functional groups were almost entirely removed. 1713

4 15 (a) MMWCNTs-OH (b) MWCNT-OH M (emu g -1 ) MMWCNTs-OH@CTAB MWCNTs-OH Transmitance (A.U.) MMWCNT-OH Magnetic Field (Oe) MMWCNT-OH@CTAB Wavenumver (cm -1 ) Figure 1. (a) Magnetic hysteresis loop of MMWCNTs-OH. (b) FTIR spectroscopy of MWCNTs-OH, MMWCNTs-OH, MMWCNTs-OH@CTAB. Optimization of the Extraction Procedure In order to obtain higher adsorption capacity for PAHs, cationic (DTAB and CTAB) and anionic (SDS and SDBS) surfactants were added to act as modifiers to form micelles. As seen in Fig. 2a, PAHs recoveries of cationic surfactants were higher than the anionic surfactants. CTAB obtained the highest recovery, which indicated that CTAB was the most suitable surfactant for the extraction of PAHs. The formation of CTAB micelles in MMWCNTs-OH was studied through the determination of critical micelle concentration (CMC) (see Fig. 2b). The concentration of the surface active agent increases to the CMC and that is when, the interaction between the opposite surface charges present in the surfactant and the nanomaterial takes place [10]. Hence, formation and deposition of the initial micelle takes place on the surface of the nanomaterial. The tail of the hydrophobic part of the micelle was exposed to aqueous solution. The MMWCNTs-OH surface modified by the surfactant was changed into a hydrophobic entity and thus became favorable to adsorb weakly polar organic compounds. Four organic solvents, acetonitrile, methanol, acetone and dichlorine were tested the effect of elution solvent. Acetone showed a more profound and favorable effect than all the other solvents. Thus, 1.0 ml/mg MMWCNTs-OH acetone was used as the elution solvent in the desorption experiments. Recovery yields (%) (a) CTAB SDS DTAB SDBS NAP ACE ACT FLU PHE ANT FLT PYR BaA CHR BbF BkF BaP IcP DaA BgP k ( s/cm) (b) CMC PAHs Concentration of CTAB (m mol/l) Figure 2 (a) Effect of DTAB, CTAB, SDS and SDBS on adsorption effect. (b) Critical micelle concentration (CMC) of CTAB. 1714

5 Method Validation As shown in Table 1, correlation coefficient (R 2 > ) was observed between the peak area and the concentration of PAHs in the range of µg/l (Table 1). Values for the LOD and LOQ of MMWCNTs-OH@CTAB DSPE method were in the ranges of µg/l (S/N=3) and µg/l. These results showed that the proposed method achieved a high precision. Application of the method to the analysis of real samples. The results were satisfactory with the spiked recoveries in the range of %, and the corresponding RSD was in the range of %. Practical analysis of PAHs in seawater included 16 main PAHs, and the total amount was µg/m 3. Table 1. Method validation of MMWCNTs-OH@CTAB as adsorbent for DSPE. PAHs Abb. R 2 LOD Recovery RSD variability (µg/l) (%) (%) (µg/m 3 ) Naphthalene NAP ±2.4 Acenaphthylene ACE ±1.2 Acenaphthene ACT ±1.8 Fluorene FLU ±2.5 Phenanthrene PHE ±1.5 Anthracene ANT ±0.9 Fluoranthene FLT ±0.8 Pyrene PYR ±0.1 Benz(a)anthracene BaA ±2.1 Chrysene CHR ±0.3 Benzo(b)fluoranthene BbF ±0.2 Benzo(k)fluoranthene BkF ±0.4 Benzo(a)pyrene BaP ±0.4 Indeno(1,2,3-cd)pyrene IcP ±0.5 Dibenz(a,h)anthracene DaA ±0.1 Benzo(g,h,i)perylene BgP ±0.2 Abb: Abbreviation CONCLUSIONS In this work, MMWCNTs-OH was synthesized along with the micelle extraction and was used as a novel adsorbent to develop DSPE analysis. The synthesis procedure was simple and the loading capacity was high. MMWCNTs- OH can be charged and supported as anionic micelle. MMWCNTs-OH@CTAB exhibited good adsorption ability for PAHs. The recovery of proposed method increased about times than that of the conventional SPE method. The method was successfully applied to the analysis of the PAHs in polluted seawater. A traditional method for collection and extraction of PAHs was also performed, and the results showed that in the traditional process lost 27.52% compounds. As the sample collection time was short from sampling to analysis, which cost less than 10% time of the traditional method so as to decrease the PAHs losses to minimum. ACKNOWLEDGMENTS This research was supported by the funds of Shandong Provincical Higher Educational Science and Technology Foundation, China (J14LD04; J16LA61), Shandong Provincial Natural Science Foundation, China (ZR2014JL038), National Natural Science Foundation of China ( ). 1715

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