Magnetic solid phase extraction of typical polycyclic aromatic hydrocarbons from environmental water samples with metal organic framework MIL-101 (Cr) modified zero valent iron nano-particles

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Magnetic solid phase extraction of typical polycyclic aromatic hydrocarbons from environmental water samples with metal organic framework MIL-101 (Cr) modified zero valent iron nano-particles Qingxiang Zhou ∗ , Man Lei, Yalin Wu, Yongyong Yuan College of Geosciences, China University of Petroleum Beijing, Beijing 102249, China

a r t i c l e

i n f o

Article history: Received 2 September 2016 Received in revised form 15 January 2017 Accepted 19 January 2017 Available online xxx Keywords: Metal-organic framework material Fe@MIL-101 Magnetic solid phase extraction Polycyclic aromatic hydrocarbons

a b s t r a c t Metal-organic framework material has been paid more attention because of its good physical and chemical properties. Nanoscale zero valent iron is also in the center of concern recently. Combination of their merits will give impressive results. Present study firstly synthesized a new magnetic nanomaterial nano-scale zero valent iron-functionalized metal-organic framworks MIL-101 (Fe@MIL-101) by co-precipitation method. The morphology and structure of the as-prepared Fe@MIL-101 were characterized by transmission electron microscopy and X-ray diffraction, etc. The experimental results showed that Fe@MIL-101 earned good adsorption ability to polycyclic aromatic hydrocarbons. The limits of detection of developed magnetic solid phase extraction were all below 0.064 ␮g L−1 and precision can be expressed as relative standard deviation (RSD, %) and which was better than 4.4% (n = 6). The real water analysis indicated that the spiked recoveries were satisfied, and Fe@MIL-101 earned excellent reusability. All these demonstrated that Fe@MIL-101 exhibited excellent adsorption capability to polycyclic aromatic hydrocarbons and would be a good adsorbent for development of new monitoring methods for environmental pollutants. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are a large group of important organic pollutants and produced by incomplete combustion of organic materials, such as coal, oil, gas, wood and other petroleum products during industrial processes and other human activities [1–6]. PAHs are highly hydrophobic, non-polar, cytotoxic, mutagenic, carcinogenic and teratogenic [7–11]. Exposure to PAHs causes a variety of negative health impacts such as various cancers, breaking down hormone systems and depressing immune function [3,7]. The environmental Protection Agency (EPA) of United States has listed 16 PAHs as priority pollutants. Various methods have been developed for the determination of PAHs in different environmental samples including gas chromatography-flame ionization detection (GC-FID) [3], high performance liquid chromatographyfluorescence detection-diode-array detection (HPLC-FLD-DAD) [4], gas chromatography-mass spectrometry (GC–MS) [12–16], high performance liquid chromatography-fluorescence detection (HPLC-FLD) [1,10,17], high performance liquid chromatography-

∗ Corresponding author. E-mail address: [email protected] (Q. Zhou).

ultraviolet detection (HPLC-UV) [18,19], etc. As the PAHs are usually present in environmental samples at trace levels, their direct determination is not possible. Therefore, an efficient preconcentration technique is required prior to their instrumental analysis. Magnetic solid phase extraction (MSPE) has recently attracted much more attention as a promising preconcentration technique based on magnetic nanoparticles (MNPs) [20,21]. Unlike the conventional preconcentration techniques, MSPE dramatically simplifies the procedure and improves the extraction efficiency through magnetic separation and variety of adsorbents. It also shows advantages of high recovery, high enrichment factor, short extraction time and low consumption of organic solvents [22]. Bare MNPs are not directly used due to their high chemical reactivity, easily aggregation and oxidation in air, and so surface coating and functionalization is necessary and important [23,24]. Various materials have been investigated for the coating and functionalization of MNPs, including silica [25], molecular imprinted polymers (MIPs) [26], carbon-based materials [22,27], metal-organic frameworks (MOFs) [23,28,29], etc. MOFs, a class of hybrid inorganic-organic microporous multidimensional frameworks crystalline materials based on selfassembly straightforwardly from metal ions with organic linkers via coordination bonds, are known as porous coordination poly-

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mers. MOFs have attracted remarkable attention because of their unprecedented properties such as designable and tunable composition, structure, porosity and functionality. The large pore volume, high surface area, chemical tenability, thermal stability, framwors flexibility and dynamics, enhanced selectivity and stability and selective penetration make MOFs show great potential in gas storage, luminescence, electrode materials, carriers for nanomaterials, molecular separation, catalysis, sensing, binomedicine, drug delivery, energy storage, adsorption based removal, etc [30–32]. Up to date, many MOFs have been developed and they have different dimensionalities and different topologies which are achieved based on metal ion geometry and the bridging ligands’ bonding mode [33–35]. MOFs can be categorized with or without the guest species based on the structural dimensionality. In one dimensional MOFs, coordination bonds are spread over the polymer in one direction. In two-dimensional MOFs, the single type layers are superimposed through either edge to edge or staggered type of stacking by weak interactions. There are two possibilities for the accommodation of guest molecules in two-dimensional structures: one is the space between grids of the layers and the other is the space between the layers. In three-dimensional MOFs, frameworks are highly porous and stable because of the coordination bonds spread in three directions. The often used synthetic methods included hydrothermal method, microwave and ultrasonic method, electrochemical method, mechanochemical method, and diffusion method [36], etc. MOFs have successfully applied for the removal of sulf- and nitrogen-containing compounds, sample pretreatment including solid phase extraction and solid phase microextraction of pollutants, which have been reviewed in references [37–39], etc. Among them, some types of MOFs such as ZIF-8, MIL-100 (Fe) and MIL-101 (Cr) are water stable [40,41]. As one of the most prominent MOFs, MIL-101 (Cr) has been successfully used as adsorbent due to its advantages of large surface area, high porosity, numerous unsaturated metal sites, excellent chemical stability and cost-effective [42–44]. The often-used MNPs for MSPE were Fe3 O4 , which has been directly used or modified as effective adsorbents. Recently, nanoscale zero valent iron was paid more attention because of its good properties such as increased specific surface area and more reactive sites on the surface, and it is very easy to prepare core-shell Fe@SiO2 by one step method under normal temperature and pressure, and which directly provides the matrice layer SiO2 for coating or functionalization, but such approach is not reported. Present study described a new magnetic material Fe@MIL-101, which will combine the merits of magnetic separation and high adsorption ability of MIL-101. The prepared Fe@MIL-101 was evaluated for the first time as a magnetic solid phase extraction (MSPE) adsorbent for the preconcentration of four typical PAHs such as naphthalene (Nap), anthracene (Ant), fluoranthene (FL) and pyrene (Pyr) from environmental water samples coupled to high performance liquid chromatography-variable wavelength detection (HPLC-VWD).

2. Experimental 2.1. Reagents and materials Ferric chloride hexahydrate (FeCl3 ·6H2 O) was obtained from Sinopharm Chemical Reagent Co. Ltd. Polyvinylpyrrolidone (PVP), tetraethyl orthosilicate (TEOS), terephthalic acid and N,Ndimethylformamide (DMF) were obtained from Aladdin Chemistry Co. Ltd. Sodium borohydride (NaBH4 ) and sodium hydroxide (NaOH) were obtained from Tianjin Guangfu Technology Development Co. Ltd. Triethylamine, hydrochloric acid (HCl), sodium chloride (NaCl), ethanol, acetone and n-hexane were obtained from Beijing Chemical Works. HPLC-grade methanol and acetoni-

trile were obtained from Thermo fisher (NJ, USA) and MREDA (USA). Chromium nitrate nonahydrate (Cr(NO3 )2 ·9H2 O), terephthalic acid (H2 BDC), hydrofluoric acid (HF, 48%), naphthalene (Nap), anthracene (Ant), fluoranthene (FL) and pyrene (Pyr) were purchased from Aladdin Chemistry Co. Ltd. Except methanol and acetonitrile, all reagents are analytical grade. The ultrapure water was used in all experiments. 2.2. Instruments An Agilent 1260 HPLC system equipped with an ultraviolet variable wavelength detector (VWD) (Santa Clara, USA) was used for the analysis of all the experiments. An InertSustain C18 column (4.6 × 250 mm, 5 ␮m) was used for separation. The mobile phase was a mixture of methanol-water (90:10, v/v) at a flow rate of 1.0 mL min−1 . The temperature of column was controlled at 25 ◦ C, and the detection wavelength was set at 240 nm. The injection volume was set at 50 ␮L. Transmission electron microscopy (TEM) was observed by a JEM-2100. X-ray diffraction (XRD) measurements were performed on a Bruker D8 Advance (Germany). 2.3. One step synthesis of Fe@MIL-101(Cr) The Fe@SiO2 core-shell nanocomposite was firstly synthesized with a KBH4 reduction method as reported previously combined with a Stöber method [45]. Briefly, 1.45 g ferric trichloride hexahydrate (FeCl3 ·6H2 O) and 50 mg polyvinylpyrrolidone (PVP) were dissolved in 60 mL 70% (v/v) ethanol solution in a 250 mL three necked flask which contained a mechanical stir bar. Then 0.2 mL of tetraethyl orthosilicate (TEOS) was added for the synthesis of SiO2 shell. The mixture was ultrasonic dispersed for 20 min under stirring. Next, 0.7 g sodium borohydride (NaBH4 ) was dissolved in 20 mL deionized water, and this aqueous solution with was dropwise added into the flask with vigorous stirring. The ferric iron was reduced according to the following reaction scheme: 4Fe3+ + 3BH− + 9H2 O → 4Fe0 ↓ + 3H2 BO− 3 + 12H 4

+

+ 6H2 ↑

After 2 h, the black Fe@SiO2 particles were separated by a magnet and then washed repeatedly with ethanol and deionized water. The whole process was carried out in a nitrogen atmosphere. MIL-101 was synthesized via hydrothermal method as reported previously [46] where hydrofluoric acid was used as a mineralizing agent. 1200 mg chromic nitrate, 498 mg terephthalic acid, 0.3 mL HF and 14.4 mL ultrapure water were mixed in a teflon-lined stainless steel reactor and kept at 200 ◦ C for 12 h. After cooling, DMF was added to remove unreacted free terephthalic acid at 80 ◦ C. The MIL101 was obtained by centrifugation. Then ethanol was added to remove DMF or terephthalic acid resided at 80 ◦ C. The product was centrifuged again and finally dried over night at 80 ◦ C. The synthesis of Fe@MIL-101 was as follows: 300 mg MIL-101 was ultrasonically dispersed in 40 mL ethanol for 20 min. Under mechanical agitation, other 40 mL ethanol containing 300 mg Fe@SiO2 was added dropwise into the above solution and continued to react for 2 h. The product was washed with ethanol and water for several times, and then dried in a vacuum oven at 60 ◦ C for 12 h. The overall preparation process is illustrated in Fig. 1. 2.4. MSPE procedure The extraction of four polycyclic aromatic hydrocarbons (PAHs) was studied using the prepared Fe@MIL-101 (Cr) material. First, 50 mg Fe@MIL-101 (Cr) was added into 100 mL sample solution in a 250 mL glass vial. The mixture was shaken on a temperaturecontrolled water bath shaker for 40 min. Next, a magnet was deposited at the bottom of the vial, the Fe@MIL-101 (Cr) nanopar-

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Fig. 1. Schematic illustration of the synthetic process of Fe@MIL-101(Cr).

Fig. 2. (a) TEM and EDS images of Fe@MIL-101(Cr) nanoparticles, (b) XRD patterns of (a)Fe@SiO2 , (b) MIL-101(Cr), (c) Fe@MIL-101(Cr).

ticles were isolated from the aqueous phase and the supernatant was discarded. Subsequently, the target analytes were eluted from the Fe@MIL-101 (Cr) magnetic nanoparticles with 5 mL acetonitrile in 5 min. The desorption was repeated two more times. Finally, the eluate was dried with N2 at 60 ◦ C and dissolved in 200 ␮L methanol, and 50 ␮L was injected into the HPLC system for analysis.

3. Results and discussion 3.1. Characterization of Fe2 @MIL-101(Cr) NPs The morphology of Fe@MIL-101(Cr) magnetic nanoparticles were analyzed by the TEM technique. As can be seen from TEM image in Fig. 2A, the bare Fe particles were spherical. After coated

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Fig. 4. Optimization of desorption parameters. A, Selection of desorption solvent; B, Selection of volume of desorption solvent; C, Selection of desorption time.

Fig. 3. Optimization of the enrichment parameters. A, Selection of Fe@MIL-101(Cr) amount; B, Selection of extraction time; C, sample pH; D, Selection of sample volume.

with gray silica shell, the average size of the spherical Fe@SiO2 was approximately about 10–20 nm. The Fe@SiO2 magnetic microspheres were closely encapsulated by the MIL-101 materials. Also, Fe, O, Si, C and Cr could be directly visualized from the EDS pattern in Fig. 2A.

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Table 1 Analytical performance of the proposed method. Analytes

Linearity range (␮g L−1 )

Correlation coefficient (r2 )

RSD (%) (n = 6)

Enrichment factors

Limits of detection (␮g L−1 )

Nap Ant FL Pyr

0.2–500 0.2–500 0.2–500 0.2–500

0.999 0.998 0.999 0.999

4.4 1.3 1.8 1.5

461 482 438 429

0.064 0.059 0.049 0.044

Table 2 Analytical Results in Real Water Samples. Water sample

Spiked (␮g L−1 )

Nap

Ant

FL

Pyr

Jingmi diversion canal

0 5 10

nd 93.0a ± 1.8b 92.2 ± 1.0

ndc 96.5 ± 0.8 96.3 ± 1.0

nd 87.6 ± 2.0 87.5 ± 1.1

nd 85.8 ± 1.4 85.7 ± 0.8

Qintun River water

0 5 10

nd 92.3 ± 1.9 92.2 ± 1.1

nd 97.3 ± 1.6 96.7 ± 1.1

nd 86.6 ± 1.5 85.9 ± 0.7

nd 85.8 ± 1.2 86.2 ± 1.7

Ming Tombs Reservoir

0 5 10

nd 92.5 ± 1.5 93.2 ± 2.2

nd 96.8 ± 1.6 95.5 ± 1.2

nd 87.3 ± 0.8 86.5 ± 2.1

nd 86.6 ± 1.5 86.1 ± 1.8

Shahe River water

0 5 10

nd 93.7 ± 1.7 92.6 ± 1.2

nd 97.3 ± 1.7 96.9 ± 1.1

nd 86.0 ± 1.3 86.3 ± 1.0

nd 86.3 ± 0.7 85.7 ± 2.2

a b c

Recovery(%)

Mean of three determinations. Standard deviation for three determinations. Not detected.

Fig. 5. HPLC chromatogram of Shahe River water sample obtained from MSPE with Fe@MIL-101(Cr). (a) blank; (b) spiked at 5 ␮g L−1 ; (c) spiked at 10 ␮g L−1 .

XRD patterns of Fe@SiO2 , MIL-101 and as-prepared Fe@MIL101(Cr) were shown in Fig. 2B. From the figure, there is a typical diffraction peak of Fe0 in the XRD pattern of Fe@SiO2 , illustrating a thin lay of SiO2 was coated on the surface of Fe nanoparticles. The diffraction peak of Fe@SiO2 was still kept after encapsulated with MIL-101, indicating that Fe@SiO2 could keep its structure during the synthesis procedure. In addition, both Fe@SiO2 and MIL-101 were present in the Fe@MIL-101 because they were simply mixed and not subjected to reaction to ensure chemical bonding. 3.2. Optimization of MSPE parameters In order to select the optimum dosage of the Fe@MIL-101(Cr) nanoparticles for the magnetic solid phase extraction of the 4 PAHs, different amounts of the Fe@MIL-101(Cr) nanoparticles were investigated in the range from 20 to 60 mg. As can be seen in Fig. 3A, for selected PAHs, the recoveries increased with the increase of adsor-

bent amount from 20 to 50 mg. Further increasing the adsorbent amount over 50 mg resulted in no obvious increase of the recoveries. Usually more adsorbents means more adsorption active sites for target analytes, however, over a certain amount of absorbent, the used eluting solvent was not enough for the elution and which led to the decrease of the recoveries. Thus, to ensure the sufficient extraction, 50 mg Fe@MIL-101(Cr) nanoparticles were used in the following experiments. The effect of extraction time on the extraction efficiency was investigated by changing extraction time from 20 to 60 min. Fig. 3B shows that the recoveries of target PAHs increased from 20 to 40 min and then changed slightly due to the fact that the extraction equilibrium was achieved. 40 min was sufficient to achieve satisfactory extraction efficiency for target PAHs. The sample pH often plays an important role in the extraction process, since it can influence the stability of the adsorbent, the charge of the adsorbent surface, and the existing state of the ana-

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lytes, further affect the extraction efficiencies of the analytes. Fig. 3C shows the effect of sample pH on the MSPE of PAHs, and the results showed that the maximum recoveries for all 4 PAHs were achieved at about pH 7. The reason for this may be explained as that MIL101 and Fe@SiO2 combined through the electrostatic interaction, the surface charge of MIL-101 was positive at low pH values and the interaction between MIL-101 and Fe@SiO2 would be weakened, thus decreasing the amount of MIL-101 and affecting the extraction efficiency. On the other hand, the interactions between the PAHs and Fe@MIL-101(Cr) may be ␲ ␲ interactions and hydrophobic effects between phenyl rings of MIL-101 and PAHs. Moreover, van der-Waals interactions also exist between the MIL-101 and PAHs. Because of the stability PAHs, the sample pH will have no effect on existing state of PAHs. However, sample pH will plays an important role in the stability of Fe@MIL-101, and then affecting the adsorption capacity of PAHs. MIL-101 is a hybrid inorganic-organic micro-porous crystalline material through self-assembled straightforwardly between Cr(III) and terephthalic acid via coordination bonds. Trivalent chromium is easily hydrolyzed under alkaline conditions which will result in the collapse of the skeleton. Hence, the MIL-101(Cr) is stable in acidic and neutral conditions. Based on these, pH 7.0 was selected in the further experiments. Generally, the addition of salt to the sample is often used method for obtaining higher extraction efficiency in extraction procedure. In this study, the effect of salt addition on the recoveries of target PAHs was investigated in the NaCl concentration range of 0–20% (w/v). The results showed that the recoveries decreased with the addition of NaCl concentration increased to 5% and slightly increased with the addition of NaCl concentration increased to 15%. The addition of NaCl firstly performed salting-in effect which promote the dissolving of analytes, which would account for the results with adding NaCl concentration 0–5%. And then the addition of NaCl would perform decreased salting-out effect, which was favorable for the extraction and accounted for the slight increase of recoveries with NaCl adding up to 15%. However, continuous increase of NaCl concentration will result in the increase of the viscosity of the solution and which affected the extraction recoveries of PAHs. In a word, adding of NaCl does not achieve expected high extraction efficiencies. Humic acid is an often present substance in water system, which will influence the extraction of analytes. Here it was also tested in the range of 0–30 mg L−1 . The recoveries of the analytes decreased with increasing the concentration of the humic acid. It may be explained that the humic acid provided adsorption sites for PAHs and some PAHs would be adsorbed on the humic acid, decreasing the extraction efficiency of adsorbent. That is to say, when water containing high amount of humic acid, simple extraction method is not enough for the accurate concentration of PAHs. The amount of PAHs adsorbed onto humic acid should be considered. Usually, the amount of humic acid is very small in the water samples, and this extraction method could be used. Sample volume is also an important impact factor on extraction efficiency, which is investigated in the range of 20–100 mL. Fig. 3D demonstrates that the recoveries of the target PAHs almost remained constant by varying the sample volume from 20 to 100 mL. Generally larger sample volume resulted in higher enrichment factor. Thus, in order to get a higher enrichment factor, 100 mL sample was adopted for the following experiments. 3.3. Optimization of desorption parameters Desorption solvent plays an important role in the MSPE procedure. The proper selection of desorption solvent can result in a better elution of analytes and a higher recovery. So the following five solvents were considering as desorption solvent including methanol, ethanol, acetone, acetonitrile and dichloromethane. The

results were shown in Fig. 4A, and it was observed that the desorption ability of acetonitrile was superior to that of other four solvents. Volume of acetonitrile is a crucial factor for desorption process, which will affect desorption efficiency. Therefore, the effect of the volume of acetonitrile was further optimized in the range from 2 to 6 mL. Fig. 4B shows that the minimum volume of acetonitrile required for complete desorption of PAHs was found to be 5 mL. Besides these two factors, elution time was another impact factor, and which was investigated in the range of 2–6 min. Fig. 4C exhibited that the recoveries of the target PAHs increased with the increase of elution time and reached the maximum value when the elution time was 5 min. 3.4. Reusability Reusability is an important parameter to evaluate the new magnetic material Fe@MIL-101. In order to examine the reusability of the Fe@MIL-101 magnetic nanoparticles, the used adsorbent was washed with acetonitrile and water for several times to keep them clean for reuse investigation. The experimental result exhibited that the Fe@MIL-101 could be reused at least 10 times by less than 2.4% loss in the recoveries, which indicated that Fe@MIL-101 earned good reusability. 3.5. Analytical performance The linearity range, correlation coefficients (r2 ), limits of detection (LODs) and the repeatability were studied under the optimum experimental conditions for the analysis of Nap, Ant, FL and Pyr. The results were summarized in Table 1. The linearity for all the analytes existed in the concentration range of 0.2–500 ␮g L−1 with the correlation coefficients (r2 ) above 0.998. The LODs, calculated based on signal-to-noise ratios(S/N = 3), were in the range of 0.044–0.064 ␮g L−1 . The relative standard deviation was better than 4.4%. Du et al. [47] developed MSPE method for the determination of PAHs with magnetic metal-organic framework MIL-100(Fe) microspheres in combination with HPLC with fluorescence detector. The magnetic core was Fe3 O4 . The LODs for Nap, Ant, FL and Pyr were 2.11, 0.057, 0.604, and 0.397 ␮gL−1 , respectively. The lower limit of the linear range for these PAHs were 5, 0.5, 1 and 0.5 ␮gL−1 , respectively. Yan et al. [48] reported a MSPE method with Fe3 O4 @SiO2 -MIL 101 as the adsorbents for the detection of PAHs, which resulted in a better sensitivity. We can see that the LODs of the three PAHs in present study were better than that reported by Du et al. and the result of Ant was similar. The lower limit of linear range was also better. However, these results were a little lower than that reported by Yan et al. From the point of real analysis, present study was a strongly competitive tool for monitoring of such pollutants because of their merits. To further evaluate the applicability of developed method, four water samples were analyzed including Jingmi diversion canal, Qintun River water, Ming Tombs Reservoir and Shahe River. Water samples unspiked and spiked with Nap, Ant, FL and Pyr at two concentration levels (5 and 10 ␮g L−1 ) were analyzed by the proposed method (n = 3). The results were listed in Table 2. Among the four real water samples, no analytes were detected. Recoveries of four analytes at two concentration levels were in the range of 85.7–97.3%, with RSDs within 2.2%. The results revealed that the established method could be successfully applied to the analysis of polycyclic aromatic hydrocarbons at trace level in real samples. The HPLC chromatogram of real water sample was shown in Fig. 5. 4. Conclusion In present study, metal organic framework MIL-101 modified nanoscale zero valent iron core-shell magnetic microspheres Fe@

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MIL-101 was designed and synthesized, which earned magnetism and good adsorption capacity. As a novel efficient adsorbent it has demonstrated good adsorption for PAHs. The developed magnetic solid phase extraction for enrichment of PAHs has exhibited good merits such as good adsorbent material with easiness to synthesize and regenerate of Fe@SiO2 @MIl-101, high surface areas and good adsorption capacity, high extraction efficiency, and simple operation for extraction and elution, low consumption of toxic organic solvent, etc. The sensitivity was very good with LODs at ppt level, and the developed method was an excellent alternative for the determination of polycyclic aromatic hydrocarbons in real water samples.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21377167).

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