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J. Sep. Sci. 2015, 00, 1–8

Li Liu Yunhui Hao Yiqian Ren Chun Wang Qiuhua Wu Zhi Wang College of Science, Agricultural University of Hebei, Baoding, China Received December 23, 2014 Revised January 22, 2015 Accepted January 29, 2015

Research Article

Magnetic nanoporous carbon as an adsorbent for the extraction of phthalate esters in environmental water and aloe juice samples In this work, magnetic nanoporous carbon with high surface area and ordered structure was synthesized using cheap commercial silica gel as template and sucrose as the carbon source. The prepared magnetic nanoporous carbon was firstly used as an adsorbent for the extraction of phthalate esters, including diethyl phthalate, diallyl phthalate, and di-n-propylphthalate, from lake water and aloe juice samples. Several parameters that could affect the extraction efficiency were optimized. Under the optimum conditions, the limit of detection of the method (S/N = 3) was 0.10 ng/mL for water sample and 0.20 ng/mL for aloe juice sample. The linearity was observed over the concentration range of 0.50–150.0 and 1.0– 200.0 ng/mL for water and aloe juice samples, respectively. The results showed that the magnetic nanoporous carbon has a high adsorptive capability toward the target phthalate esters in water and aloe juice samples. Keywords: Aloe juice / High-performance liquid chromatography / Magnetic nanoporous carbon / Phthalate esters / Water DOI 10.1002/jssc.201401457

1 Introduction To measure a trace level of contaminants in a sample, sample preconcentration is usually necessary before chromatography analysis. Nowadays, various pretreatment techniques have been employed to preconcentrate target analytes from different sample matrices, such as ultrasonic extraction [1], LLE [2, 3], SPE [4, 5], LPME [6, 7], SPME [8–10], stir bar sorptive extraction [11], and magnetic solid-phase extraction (MSPE) [12,13]. MSPE is an efficient extraction method which uses magnetic or magnetically modified nanoparticles as SPE sorbents. The magnetic adsorbents have more advantages than the traditional adsorbents. For example, magnetic adsorbent can be readily isolated from the sample solution by the only use of a magnet without the need of additional filtrations or centrifugations, which makes the phase separation easier and faster. Therefore, MSPE has been widely used in many fields [14–16]. The magnetic adsorbent plays a key role in MSPE. To improve the extraction efficiency for some analytes, the Correspondence: Dr. Qiuhua Wu, College of Science, Agricultural University of Hebei, Baoding 071001, Hebei, China Fax: +86-312-7528292 E-mail: [email protected]

Abbreviations: DEP, diethyl phthalate; DAP, diallyl phthalate; DPP, di-n-propyl-phthalate; LR, linearity range; MNPC, magnetic nanoporous carbon; MSPE, magnetic solid-phase extraction; NPC, nanoporous carbon materials; SG, silica gel; PAE, phthalate esters

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development of new magnetic adsorbent materials with high selectivity and high adsorption capacities has been a significant research field. Recently, many research groups have explored C18 microspheres [17], carbon nanotubes [18], activated carbon [19], polymers [20, 21], and graphene [22] based Fe3 O4 nanoparticles to extract of various contaminants. Nanoporous carbon materials (NPC), due to its high pore volume, high-specific surface area, high chemical stability, good electrical conductivity, and thermal conductivity, interconnected frameworks as well as tunable pore size [23, 24], have been paid much attention from researchers. Up to date, various reports about the applications of NPC as the adsorbent for the extraction or removal of organic pollutants has been found, such as for the removal of dyes [25, 26] from aqueous solutions; for the adsorption of phenol [27], organic compounds [28] and so on. So far, many techniques including catalytic activation with various metallic species [29], sol–gel techniques [30], template method [31–34], and carbonization of mesoporous polymers [35] have been developed to fabricate NPC. Among them, the most common methods are template methods [23, 36]. Various templates including silica [37], metal–organic frameworks [31] and molecular sieve [32, 38] have been successfully used for the synthesis of NPC. Since silica gel (SG) has many properties such as thermal and mechanical stability, large surface area, rapid adsorption kinetics, and modifiable surface properties [39, 40], various reports about the applications of SG-based materials as the adsorbent for the extraction or removal of organic pollutants have been found. For example, Wang et al. have applied silica gel functionalized with a ditopic zwitterionic

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Schiff base ligand (SG-H2L1) for the sorption of Cu(II) and SO4 2– ions [41]. Sigot et al. have used silica gel for the sorption of octamethylcyclotetrasiloxane [42]. Recently, silica gel also has been successfully employed as the template for the preparation of NPC [34, 43, 44]. Ruhit et al. have used SGtemplated carbons for hydrogen storage [44]. However, the application of SG-based magnetic NPC as the adsorbent for the extraction of trace pollutants from real samples has still remained untouched in the literature. Phthalate esters (PAEs) are widely used in the manufacture of plastics, polyvinyl chloride products, medical devices, building materials, children’s toys, and cosmetics as plasticizers to make polymer materials more flexible and moldable [45]. Since PAEs are not chemically bound to plastic, they can migrate or evaporate into plants, environment water, soil, food [13]. Some studies seem to show that several PAEs are suspected to be human cancer causing agents and endocrine disruptors [46]. Considering the potential hazard of PAEs, it is highly desirable to develop new methods for the determination of trace level of PAEs in different samples. In this work, a new magnetic nanoporous carbon (MNPC) was synthesized using silica gel as the template and sucrose as carbon source. It was explored as an adsorbent for the extraction of three PAEs from lake water and aloe juice samples before HPLC–UV detection. To the best of our knowledge, this is the first report about the application of SG-based MNPC in MSPE.

2 Materials and methods 2.1 Reagents and materials Silica gel was purchased from Qingdao Haiyang Chemical Company (Qingdao, China). FeCl3 ·6H2 O and FeCl2 ·4H2 O were purchased from Chengxin Chemical Reagents Company (Baoding, China). HPLC-grade methanol and acetonitrile were obtained from Sinopharm Chemical Reagent (Beijing, China). H2 SO4 (95%), acetone, NH3 ·H2 O, hydrochloric acid (HCl), sodium hydroxide (NaOH), and all other reagents were purchased from Beijing Chemical Reagents Company (Beijing, China). The water used throughout the work was double-distilled on a SZ-93 automatic double-distiller purchased from Shanghai Yarong Biochemistry Instrumental Factory (Shanghai, China). Certified pesticide standards (99%) of diethyl phthalate (DEP), diallyl phthalate (DAP) and di-n-propyl-phthalate (DPP) were purchased from Aladdin Reagent (Shanghai, China). A mixed stock solution containing DEP, DAP, and DPP each at 20.0 ␮g/mL was prepared in methanol. The standard solutions were stored at 4⬚C and protected from light. Lake water samples were collected from Qiandaohu (Hangzhou, China); bottled aloe juice was purchased from a local supermarket. All the solvents and samples were filtered through a 0.45 ␮m membrane to eliminate particulate matter before analysis.  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2.2 Apparatus HPLC was carried out on a LC–20AT LC (Shimadzu, Japan) that consists of two pumps and a Model SPD-20A UV/vis 104 detector. The separation was performed on a Promosil C18 column (250 mm × 4.6 mm id, 5.0 ␮m) from BonnaAgela technologies (Tianjin, China). The morphology and size of MNPC were observed by TEM using a JEOL model JEM-2011(HR) at an accelerating voltage of 200 kV. The TEM specimens were prepared by dispersing the MNPC on a copper grid.

2.3 Synthesis of MNPC The synthesis procedure of MNPC is illustrated in Scheme 1. NPC was prepared using silica gel as template and sucrose as a carbon source according to the literature method with some modifications [47]. Firstly, 1.0 g silica gel was mixed homogeneously with an aqueous solution composed of 5 mL of distilled water and 1.5 g of sucrose under stirring for 50 min at room temperature. Then, 0.19 g H2 SO4 (98 wt%) was added to the solution. After being magnetically stirred for 10 min, the mixture was heated for 6 h at 100⬚C and for 6 h at 160⬚C in an oven. Then, the mixture was cooled to room temperature and the resultant black precipitate was ground to a fine powder. After the addition of 0.19 g of H2 SO4 (98 wt%), 1.5 g of sucrose and 5 mL of distilled water, the mixture was heated again for 6 h at 100⬚C and for 6 h at 160⬚C. The obtained SG/sucrose composite was carbonized in a conventional furnace at 900⬚C for 2 h in nitrogen flow. Subsequently, the silica gel template was removed by mixing the composite with 20 mL of HF (25 wt%) for 10 h and the obtained nanoporous carbon was rinsed with ethanol and distilled water, respectively, to neutralize the material surface. Finally, the resultant product was dried in an oven and then SG-based NPC was obtained. MNPC was synthesized by the in situ chemical coprecipitation of Fe2+ and Fe3+ in alkaline solution in the presence NPC. The magnetic composite was prepared by suspending 1.0 g NPC in 500 mL of solution containing 0.85 g (4.33 mmol) FeCl2 ·4H2 O and 2.34 g (8.66 mmol) FeCl3 ·6H2 O at 50⬚C under N2 atmosphere. After the solution was sonicated (200 W, 40 kHz) for 10 min, 40 mL 14% NH3 ·H2 O aqueous solution was added drop-wise to precipitate the iron oxide, and then the reaction was carried out at 50⬚C for 1 h under constant mechanical stirring. The precipitate was separated from the aqueous dispersion by an external magnetic field, washed with double-distilled water until the pH became 7. Finally, the MNPC composite was dried under vacuum to remove absorbed water.

2.4 MSPE procedure Firstly, 10 mg MNPC was accurately weighed and then put into a 50.0 mL sample solution, followed by the adjusting of www.jss-journal.com

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Scheme 1. Synthesis of MNPC.

its pH to 6 with 1 mol/L HCl solution. To completely trap the target PAEs, the mixture was shaken on a slow-moving platform shaker for 15 min. After that, MNPC was separated rapidly from the solution under an external magnetic field. Subsequently, the supernatant was discarded and then the residual mixture was totally transferred to a 10 mL centrifuge tube. Secondly, the MNPC was aggregated again by positioning a strong magnet to the outside of tube wall so that the residual solution could be completely removed by a pipette. Thirdly, 0.2 mL acetone was added to the centrifuge tube and vortex for 30 s to desorb the target PAEs from the MNPC. Three replicate desorption were performed. Finally, the desorption solvent was transferred to a 2 mL microcentrifuge tube and then 20.0 ␮L of the resultant solution was injected into the HPLC system for analysis.

ratios of acetonitrile to water as mobile phase were investigated on a Promosil C18 column (250 mm × 4.6 mm id, 5.0 ␮m) for the separation of PAEs. As a result, the best separation was achieved with acetonitrile/water (75:25 v/v) as mobile phase at a flow rate of 1.0 mL/min. The UV monitoring wavelength was chosen at 225 nm because all the analytes had the maximum absorbance at that wavelength.

3 Results and discussion

3.3.1 Effect of the amount of MNPC

3.1 Characterization of MNPC

In the MSPE process, amount of adsorbent is one of the primary factors that influence the extraction efficiency of the analytes. To choose the optimum amount of the MNPC required for quantitative recoveries for phthalate esters, different quantities of MNPC were tested in the range between 1 and 25 mg. The effect of dosage of the MNPC on the extraction efficiency of the phthalate esters from spiked aqueous samples was presented in Fig. 3A. The extraction recoveries of the three phthalate esters increased with increasing sorbent dosage from 1 to 10 mg, and then stabilized with further increases of sorbent dosage, which means that the adsorption equilibrium has been achieved. According to the above results, 10 mg MNPC was selected as the final amount of the magnetic adsorbents used in the following studies.

The morphology of the NPC and MNPC was observed by TEM. Figure 1A and B presents the structural properties of SG-based NPC and SG-based MNPC, revealing that Fe3 O4 nanoparticles are dispersed well on the substrate of NPC and the structure order of silica gel template is well preserved in the carbon replica. The microporous characteristics and macroscopic morphology of the MNPC were analyzed by nitrogen adsorption–desorption isotherms (Fig. 2). It was determined that the Saito–Foley median pore width, total adsorption average pore width, total pore volume, and the experimental multipoint BET surface area of the MNPC were 0.76 nm, 7.59 nm, 0.71 cm3 /g, and 374.9 m2 /g, respectively.

3.3 Optimization of MSPE conditions To achieve a high extraction efficiency of the new MNPC for the three phthalate esters, several parameters, including sample pH, salt concentration, extraction time, and the type and the amount of the adsorbent, were investigated and optimized.

3.3.2 Effect of extraction time 3.2 Optimization of HPLC conditions To obtain the best chromatographic separations (high response, good peak resolution, and short analysis time) for the analytes, several analysis conditions, including mobile phase, flow rate of the mobile phase, UV wavelength, were optimized. For the separation of PAEs, reversed-phase HPLC has been most commonly employed. In this study, different  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Extraction time is usually an important factor that influences the extraction of the analytes because MSPE is a partition process of the target analytes between the adsorbent and sample solution. To obtain the optimum extraction time, the extraction time profiles of the target compounds were studied by increasing the extraction time from 5 to 30 min while other parameters being held at constant values. Results obtained are shown in Fig. 3B. The recovery–time curve revealed that www.jss-journal.com

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Figure 1. (A) TEM of the NPC composite; (B) TEM of the MNPC composite.

Figure 2. The N2 adsorption–desorption isotherms of the MNPC.

the maximum extraction efficiency could be attained at 15 min of the extraction time for all the three phthalate esters. Thus, the extraction time of 15 min was selected. 3.3.3 Effect of sample solution pH The pH of the aqueous sample is of great importance on the extraction efficiency of the MNPC for the phthalate esters because it affects both the charge species and density on the sorbent surface and the existing state of the target analytes. The effect of pH on MSPE extraction efficiency was carried out in the pH range from 2.0 to 12.0 by adding different amounts of either 1 mol/L NaOH or 1 mol/L HCl solution into the sample solution. The results obtained are shown in Fig. 3C. The recovery–pH curve revealed that no obvious extraction efficiency changes were observed for the three phthalate esters when the pH of the sample solution was changed, which could be attributed to the fact that the PAEs exist as neutral compounds under ordinary conditions and the PAEs existing forms are unlikely influenced by the change of the sample solution pH. Generally, the pH of the aloe juice samples was in the range from 3.0 to 4.0. Therefore, the pH of the sample solution was not adjusted.

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3.3.4 Ionic strength Generally, the presence of salt in sample solution has two functions. On the one hand, the addition of salt can decrease the solubility of the analytes in the sample solution, which is favorable for the extraction. However, on the other hand, when the concentrations of salt increases, the viscosity and density of the solution also will increase, which is unfavorable for the extraction [48]. NaCl is the most commonly used inorganic salt for adjusting the salt concentration for an aqueous solution. In this work, the effect of salt addition on the extraction efficiency was evaluated by increasing NaCl concentration in the range from 0–30% to the sample. Results obtained were shown in Fig. 3D. The results showed that the extraction recoveries of the PAEs decreased with increased NaCl concentrations. Therefore, no NaCl was added into the sample solution in further experiments.

3.4 Reusability of the adsorbent To evaluate the reusability of the MNPC, the used adsorbent was washed twice by vortexing for 60 s first with acetone (2 mL × 2) and then with 3 mL water before it was reused

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Figure 3. Effect of optimization extraction conditions on MSPE efficiency. (A) Effect of MNPC dosage on the extraction efficiency of the phthalate esters; (B) Effect of extraction time on the extraction efficiency of the phthalate esters; (C) Effect of sample solution pH on the extraction efficiency of the phthalate esters; (D) Effect of salt addition on the extraction efficiency of the phthalate esters.

for the next extraction of the analytes. After such washing, no sample carryover for the analytes was observed. Further experimental results showed that the MNPC could be reused at least 20 times without a significant loss of the adsorption ability and the magnetism, suggesting that the MNPC has a quite good stability.

3.5 Desorption conditions 3.5.1 The type of desorption solvent In this work, to ensure the effective desorption of all the three phthalate esters analytes from the magnetic adsorbent, the most commonly used three organic solvent (acetone, acetonitrile, and methanol) as desorption solvents were investigated. The results (Fig. 4) indicated that under the same extraction and desorption conditions, acetone gave the highest desorption efficiency. Based on this experimental result, acetone was selected to elute the analytes.

3.6 Method validation 3.6.1 Linearity, repeatability, and LODs A series of double-distilled water and PAEs-free aloe juice samples spiked with each of the PAEs at different concentration levels of (0.3, 0.5, 1.0, 2.0, 5.0, 8.0, 15.0, 30.0, 60.0, 150.0, 200.0 ng/mL) were prepared for the establishment of the calibration curve. For concentration each level, five replicate extractions, and determinations were performed under the optimized experimental conditions, and results were presented in Table 1. For water sample, good linearity was observed over the concentration range of 0.5–150.0 ng/mL for the three phthalate esters. For aloe juice samples, the linear response was in the range of 1.0–200.0 ng/mL for the three PAEs. The LODs for the PAEs calculated at S/N = 3 was 0.10 ng/mL for lake water sample and 0.20 ng/mL for aloe juice samples. The correlation coefficients (r) ranged from 0.9966 to 0.9998. 3.6.2 Real sample analysis

3.5.2 Effect of the desorption solvent volume In this work, the effect of desorption solvent volume in the range of 0.1–0.8 mL on the desorption efficiency of phthalate esters analytes was investigated with acetone as desorption solvent. The results indicated that all the three phthalate esters analytes could be desorbed quantitatively from the magnetic adsorbent by rinsing the sorbent with 0.2 mL acetone for three times (0.2 mL × 3).  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The practical applicability of the developed method was examined by determining the three phthalate esters in aloe juice samples and lake water samples under optimized conditions. The analytical results and the recovery for the spiked samples are listed in Table 2. As a result, 4.01 ng/mL of DEP and 0.51 ng/mL of DAP were found in lake water sample. 3.75 ng/mL of DEP was found in aloe juice. To evaluate the accuracy of the method, lake water samples and www.jss-journal.com

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Figure 4. Effect of the desorption solvent on the recoveries of the phthalate esters.

Table 1. Analytical performance data for the three phthalate esters in lake water and aloe juice samples by the MSPE method

Lake water sample

Aloe juice sample

PAEs

LRa) (ng/mL)

r

RSD (%)

LOD (ng/mL)

LRa) (ng/mL)

r

RSD (%)

LOD (ng/mL)

DEP DAP DPP

0.5150.0 0.5150.0 0.5150.0

0.9992 0.9997 0.9998

5.3 4.8 4.7

0.10 0.10 0.10

1.0200.0 1.0200.0 1.0200.0

0.9982 0.9994 0.9966

6.8 5.3 4.8

0.20 0.20 0.20

a) LR, linear range.

Table 2. Recoveries obtained for the determination of DEP, DAP and DPP in lake water and aloe juice samples

Lake water (n = 5) PAEs

Spiked (ng/mL)

Found (ng/mL)

DEP

0 4.0 20.0 0 4.0 20.0 0 4.0 20.0

4.01 7.99 22.71 0.51 4.43 19.17 nda) 3.81 19.61

DAP

DPP

Aloe juice samples (n = 5) Rb) (%)

RSD (%)

99.50 93.50

5.2 4.8

98.00 93.30

4.8 4.2

95.30 98.00

7.0 5.8

Found (ng/mL) 3.75 7.61 23.17 nda) 4.17 20.02 nda) 3.90 19.68

Rb) (%)

RSD (%)

96.50 97.10

4.7 4.6

104.30 100.10

6.8 5.1

97.60 98.40

7.3 6.7

a) nd, not detected. b) R, recovery of the method.

aloe juice samples were spiked with 4.0 and 20.0 ng/mL of each of the three phthalate esters. For each spiked concentration, five replicate analyses were performed. As a result, the recoveries for the three phthalate esters fell in the range from 93.30–104.30% with RSDs between 4.2–7.3%, showing that the method had a good accuracy and the established method is applicable for real sample analysis. Figure 5 shows the typical chromatograms of the three phthalate esters for both spiked and unspiked lake water and aloe juice samples.

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3.7 Comparison of the MSPE method with other methods A comparison of linearity range (LR), LOD, RSD, extraction times obtained in the present MSPE method with that obtained by other reported methods such as stirring-assisted dispersive liquid–liquid microextraction [49], dispersive solidphase extraction [50], SPE [51,52], SPME [53], MSPE [54], combined with HPLC, GC–MS, and GC for the determination of phthalate esters is shown in Table 3. As can be seen, the LOD

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Table 3. Comparison of presented method with other sample preparation techniques

Methods

Sample

Linearity (ng/mL)

LOD (ng/mL)

Extraction time (min)

RSD (%)

Refs.

MSA–DLLME–HPLC DSPE–GC–MS SPE–HPLC SPE–HPLC SPME–GC MSPE–GC MSPE–HPLC MSPE–HPLC

Water Water Water Water Plastic film Packaged food Aloe juice Water

1.0–1000.0 20.0–500.0 1.0–200.0 2.0–200.0 0.125–1250.0 25.0–10 000.0 1.0–200.0 0.5–150.0

0.13–0.38 2.0–7.0 0.35–0.43 0.18–0.86 0.003–0.063 26.30 –36.40 0.20 0.10

5 30 s _ _ 40 30 15 15

< 4.0 1.0–10.0 2.0–4.1 _ 0.7–9.3 < 6.0 4.6–7.3 4.2–7.0

[49] [50] [51] [52] [53] [54] This method This method

MSA–DLLME, magnetic stirring-assisted dispersive liquid–liquid microextraction; DSPE, dispersive solid-phase extraction.

this study indicates that the MSPE–HPLC method based on magnetic NPC as the adsorbent is suitable for the extraction of three PAEs from lake water and aloe juice samples with sufficient sensitivity and simplicity.

4 Conclusions In this study, a new magnetic nanoporous carbon was synthesized using cheap, commercial available silica gel as template and used as an efficient adsorbent for the extraction of the PAEs from lake water and aloe juice samples. The results clearly show that the MNPC can be used as an effective adsorbent for the simple and rapid extraction of phthalate esters from water and aloe juice samples. Applying MNPC as adsorbent for the extraction of other organic pollutants from complex samples can be expected.

Figure 5. The typical chromatograms of blank aloe juice sample (A), the blank aloe juice sample spiked with phthalate esters at each concentration of 18.0 ng/mL (B), blank lake water sample (C), and the blank lake water sample spiked with phthalate esters at each concentration of 9.0 ng/mL (D). Peak identification: 1. DEP, 2. DAP, 3. DPP.

Financial supports from the National Natural Science Foundation of China (No. 31171698, 31471643), the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009), the Natural Science Foundations of Hebei (B2012204028) and the Scientific and Technological Research Foundation of Department of Education of Hebei Province (ZD20131033) are gratefully acknowledged. The authors have declared no conflict of interest.

and LR of the established MSPE method is better than that of other reported methods except for that of SPME [53], but the detection techniques of the SPME method [53], GC with flame ionization detection, is more sensitive than HPLC– UV. Moreover, SPME required a much longer extraction time than most of the other methods, and SPME also suffers from some problems such as sample carryover, comparatively expensive, and fiber breakage. It should be pointed out that the sensitivity of developed MSPE method is better than that of the MSPE–GC methods using magnetic multiwalled carbon nanotubes/poly(vinyl alcohol) cryogel as adsorbents, which indicated that magnetic nanoporous carbon is a very efficient adsorbent and has higher adsorption ability. Overall,  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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