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Abstract

In the present work ordered mesoporous carbon materials soft templating-acidic (STA) were applied as adsorbents in magnetic solid phase extraction technique for isolation of selected phthalates from water samples. STA materials were obtained by soft-templating method in acidic environment. Nickel nanoparticles were added on the STA synthesis stage, thus the obtained material acquired magnetic properties. The analysis of phthalates after the extraction was carried out by gas chromatography-mass spectrometry method. Fast and easy procedure was elaborated, obtaining recoveries close to 90% for some phthalates.

Keywords

Gas chromatography magnetic solid phase extraction mass spectrometry mesoporous carbon materials phthalates water samples

Introduction

The analysis of organic compounds in nature and living organisms, and the evaluation of their influence on the human health, is very important direction of the environment pollutions analysis. Nowadays most of the convenient objects are made from plastics. Based on their production, organic additives, such as antioxidants, stabilizers, and plasticizers, have influence on the processing and durability of plastics. Organic compounds, present in plastics, have influence on many properties of these materials. In some cases they are responsible for the unfavorable influence on living organisms. Freshwater fish (belonging to species Rutilus rutilus) were exposed to treated sewage effluent (containing phthalates and other endocrine disruptors) for 300 days. The reproductive, endocrine, immune, genotoxic, and nephrotoxic effects were observed in fish on exposure (Liney et al., 2006). Long-term exposure of male mice to phthalates can lead to adverse effects in the male reproductive system, specifically on Sertoli cell functions (Narender et al., 2015).

Plasticizers are very important additives to plastic. According to the International Union of Pure and Applied Chemistry (IUPAC), plasticizers are substances added to materials to increase their elasticity, workability or extensibility (Sanches Silva et al., 2006). Plasticizers can be used to restrict inflammability or they can be used as thermal stabilizers. Plasticizers such as phthalic acid esters (phthalates) are often used in food and drinks packing materials. Properties of such materials depend on the chemical character and the content of plasticizers which generally tend to migrate. Phthalates with high molecular masses, e.g. bis(2-ethylhexyl) benzene-1,2-dicarboxylate (DEHP), are applied mainly as the plasticizer in polyvinyl chloride. Phthalates with smaller molecular masses, e.g. dimethyl benzene-1,2-dicarboxylate (DMP), diethyl benzene-1,2-dicarboxylate (DEP), dibutyl benzene-1,2-dicarboxylate (DBP), 2-O-benzyl 1-O-butyl benzene-1,2-dicarboxylate (BBP), are used in cosmetics for smell and as color stabilizers (Sanches Silva et al., 2006). Due to hydrophobicity of phthalates, these compounds migrate into the aqueous solution easily. But when the product contains a low level of free oil or organic solvents (e.g. ethanol), significant amounts of phthalates can migrate from packaging to the product (Ye et al., 2009). The maximum allowable concentrations of studied phthalates in water were not yet found. Environmantal Protection Agency (EPA) has determined the allowable concentration for DEHP as 6 µg/L. However some information was found. The Minnesota Department of Health determined the following concentrations: for DBP 20 µg/L, for BBP 100 µg/L, and for DEHP 7 µg/L (Environmental Health Division, Phthalates and Drinking Water, 2013).

The exposure to high concentrations of some phthalates may lead to hormonal disturbances. A group of scientists from the University in Rochester (USA) did some research and found that these substances may disturb the development of genital organs by stopping the testosterone function in men and gestation shortening in women. It was noticed that these changes could be the reason for gonads cancer in men. Furthermore when persons are exposed to phthalates for longer time some changes were observed in DNA of epithelial cells of the nose and throat mucous membrane (Hauser et al., 2004).

Phthalates are estimated by gas chromatography (GC) and liquid chromatography (LC) methods, using different types of detectors. Most of phthalates are semi-volatile and they have a nonpolar character, therefore the GC coupled with mass spectrometry is the most often used for analysis of these compounds. Due to the complexity of the matrix, sample preparation is very important while detecting very small concentrations of phthalates by GC-MS analysis. The most popular methods of samples preparation are: liquid–liquid extraction (LLE) (Das et al., 2014; Net et al., 2014; Sharman et al., 1994), solid phase extraction (SPE) (Cai et al., 2003; Li et al., 2008; Liou et al., 2014; Zheng et al., 2014), and solid phase microextraction (SPME) (Devier et al., 2013; Li et al., 2004; Lirio et al., 2016; Su et al., 2010). Unfortunately SPME has some drawbacks such as high costs of equipment and the memory effect of the fiber. LLE is a time-consuming method and often require large volumes of organic solvents. SPE is also a time-consuming and rather expensive technique (Luo et al., 2012). An ideal sample preparation technique should be simple, quick, and relatively cheap. The new kind of SPE, named magnetic solid phase extraction (MSPE), is easier and faster than the other techniques. Adsorbent need not be packed to the column as in traditional SPE, and the separation of the adsorbent (with isolated analytes) from sample solution is achieved by applied external magnetic field (Chen et al., 2011a, 2011b; Ding et al., 2010; Gao et al., 2010, 2011; Jeddi et al., 2015; Xu et al., 2016).

There are several kinds of materials used as adsorbents in MSPE. One of the most often applied materials are carbon nanotubes, which has strong adsorbent properties in relation to hydrophobic organic substances, which was described for the first time by Iijima (1991). To apply nanotubes to MSPE, they ought to be settled on magnetic particles, so they can interact with the magnetic field. Similar procedure is applied to graphene adsorbents. The simple method of drifting graphene layers on magnetic properties is elaborated by Luo et al. (2011). Except graphene and carbon nanotubes other substances are drifted on the magnetic core, e.g. the silica gel, octadecyl phase or different polymers (Ding et al., 2010; Huang et al., 2014; Li et al., 2011; Meng et al., 2011).

The aim of the present work was to use ordered mesoporous carbon materials containing Ni nanoparticles in MSPE without the necessity of drifting them on magnetic particles.

Experimental

Reagents and equipment

For phthalate esters standard solution EPA Phthalate Ester Mix from Supelco (USA) was used. One milliliter of methanol standard in ampule contains six phthalates: DMP, DEP, DBP, BBP, DEHP, and dioctyl benzene-1,2-dicarboxylate (DNOP). The concentration of each phthalate was 2 mg/mL. Propan-2-one (acetone), methanol, acetonitrile, n-hexane, and ethyl acetate, all GC grade, were purchased from VWR International (USA). NaCl pure for analysis was from POCH (Poland). The water used for the experiment was from Milli-Q system. For GC-MS analysis PerkinElmer’s Clarus 600 gas chromatograph coupled with Clarus 600 T mass spectrometer was used. For structural parameters low-temperature nitrogen adsorption isotherms were determined using ASAP 2020 from Micromeritics.

For STA with Ni nanoparticles (STA-Ni) synthesis resorcinol (1,3-dihydroxybenzene, C₆H₄(OH)₂) was purchased from Sigma–Aldrich, Germany. Formaldehyde, hydrochloric acid (HCl, 35–38 wt%), and ethanol (96%) were purchased from Chempur, Poland. Poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock co-polymer Lutrol F127 ((EO)₁₀₁(PO)₅₆(EO)₁₀₁, Mw = 12,200 Da) was obtained from BASF, Germany. Nickel nanoparticles were purchased from Sigma–Aldrich.

Synthesis of ordered mesoporous carbon materials

For the phthalates extraction by MEPS ordered mesoporous carbon materials obtained by soft-templating in acidic environment method were used. In the synthesis 2.5 g of triblock copolymer Lutrol F127 and 2.5 g of resorcinol were dissolved in 11.9 mL of ethanol and 6.6 mL of water. After complete dissolution of copolymer, the Ni nanoparticles were added. The mass of the precursor was adjusted to achieve 10 wt% or 20 wt% of nickel in the final products. After the addition of the precursor, the solution was vigorously stirred for 30 min. Then 2.2 mL of HCl was added and solution was stirred for another 30 min. Next, 2.5 mL of formaldehyde was added and the solution was stirred until it became white and milky; stirring was continued for another 30 min. Subsequently, the magnetic bar was removed and the solution was left for gravitational separation for 3.5–4 h. Following separation, the organic layer was transferred into a Petri dish and dried in open air and room temperature until uniform film was formed. After that, the dish was transferred into an oven and dried at 100℃ for 24 h. Afterward the sample was transferred from the Petri dish to a quartz boat and carbonized in a tube furnace in flowing nitrogen 99.999% (20 L/h). The temperature program was as follows: first, the sample was heated to 180℃ at 2℃/min rate and held for 5 h. Next, temperature was elevated from 180℃ to 400℃ at 2℃/min rate and further from 400℃ to 850℃ at 5℃/min rate. The sample was kept at 850℃ for 2 h. The detailed STA synthesis procedure is described in literature works (Choma et al., 2012, 2008). The simplified diagram of STA synthesis is presented in Figure 1 (Jedynak et al., 2013). Scanning electron microscopy (SEM) images of STA-Ni-20% material are shown in Figure 2. In Figure 2(a) it can be seen that Ni nanoparticles are evenly distributed in carbon material. In Figure 2(b) a single Ni nanoparticle is shown.

Figure 1.

The schema of STA-Ni synthesis by soft-templating method (Jedynak et al., 2013).

Figure 2.

SEM images of STA-Ni 20%, fraction of 0.40–0.63 mm, where (a) SEM of material magnified 25,821x; (b) SEM of material magnified 224,548 x.

In the present work STA materials with 10 wt% and 20 wt% of nickel were achieved. These materials were powdered and sieved to separate two fractions with different grain size of 0.32–0.40 mm and 0.40–0.63 mm. Finally the STA-Ni 20% was used for phthalates extraction and structural properties of this material were analyzed by low-temperature nitrogen adsorption. Obtained results are shown in Table 1 and they are compared with parameters available in the literature (Choma et al., 2012) concerning the same material obtained by the same synthesis procedure.

Table 1.

Structural parameters of mesoporous STA-Ni 20% material obtained on the basis of low-temperature nitrogen adsorption isotherms.

STA-Ni-20%	S_BET (m²/g)	V_t (cm³/g)	V_mi (cm³/g)	V_me (cm³/g)	w_me (nm)	w_mi (nm)	Mesoporosity (%)
Synthesized in present work	660	0.57	0.12	0.44	6.60	1.93	77
Synthesized in work by Choma et al. (2012)	659	0.58	0.14	0.44	6.91	1.92	76

S_BET: specific surface area; V_t: total pore volume; V_mi: volume of micropores; V_me: mesopore volume; w_me: diameter of mesopores, w_mi: diameter of micropores.

The definition of mesopores was based on IUAPC classification stating that the pore size is between 2 and 50 nm (Lu et al., 2004). S_BET values were calculated using Barret-Joyner-Halenda method. V_mi and V_me were calculated using alpha-s method and w_mi and w_me using Kruk-Jaroniec-Sayari method. As it can be seen all determined parameter values are very similar. This fact proves that the synthesis of STA-Ni-20% material is repeatable.

Calibration curves preparation

Working solutions were prepared from standard solution by dilution in methanol and they were used for calibration curves and linearity range determination. Concentration of each phthalate amounted: 0.25; 0.5; 1.0; 2.5; 5.0; 10.0; 25.0 mg/L. After working solution preparation each solution was analyzed for five times, using GC-MS method (described in “GC-MS analysis” section) and on the basis of obtained peak areas, calibration curves were determined. The linearity range for all curves amounted 0.25–25.0 mg/L. Limits of detection (LOD) were obtained on the signal to noise ratio 3:1. Limits of quantification (LOQ) were calculated as LOQ = 3x LOD. Studied phthalates and calibration curves data are shown in Table 2.

Table 2.

Phthalates studied and calibration curve data.

Phthalate	R² (regression coefficient)	RSD (%) n = 5	LOD (mg/L)	LOQ (mg/L)
DMP	1.000	6.25	0.05	0.15
DEP	0.999	7.51	0.05	0.15
DBP	0.999	7.62	0.05	0.15
BBP	0.996	8.01	0.10	0.30
DEHP	0.991	8.25	0.10	0.30
DNOP	0.989	9.35	0.10	0.30

In studies related to the optimization method, working solution was prepared by adding 1 mL of methanol standard solution to 9 mL of water. Total volume of the working solution sample was 10 mL and the final concentration of each phthalate amounted 5.0 mg/L. For the experiment water from Milli-Q system was used and it was examined before analysis by MSPE-GC-MS method, described in next sections. No phthalates were found in Milli-Q water. Using received analysis results and calibration curves data, recoveries for six phthalates after various MSPE on STA-Ni materials were calculated.

MSPE procedure

MSPE procedure schema is presented in Figure 3.

Figure 3.

The schema of MSPE procedure using STA-Ni material as sorbent.

For the extraction procedure 15 mL vials with silicon screw caps were used. Hundred milligrams of STA sorbent with various Ni content and different grain diameters were placed into the vials. After weighing adsorbents were put in vials and 5 mL of acetone was added for adsorbent conditioning. Vials were twisted and shaken for 2 min by ultrasonic agitation at room temperature. Then STA carbons were attracted by Nd-Fe-B (40 x 15 x 5 mm) magnet to the vial wall and acetone was effused. After STA conditioning step, acetone was analyzed each time by GC-MS method to eliminate phthalates present in adsorbents and acetone before extraction step. Next adsorbent was released from magnet and 10 mL of working solution was added into the vial and shaken for 2 min by ultrasonic agitation at room temperature. After extraction magnet was added again to the vial wall and solution was effused. Two milliiters of acetone were added into the vial for analyte elution and it was shaken for 1 min by ultrasonic agitation. The extraction parameters are described in “Extraction conditions optimization” section. The last step was transferring the acetone extract into clean glass vial and evaporation to 1 mL volume in synthetic air flow. Sample, prepared as it was described above, was transferred into 1.5 mL chromatographic glass vial, tightly closed, and finally delivered for GC-MS analysis. Extraction of each sample was repeated for three times and each extract was analyzed five times using GC-MS method.

GC-MS analysis

Phthalates were separated on Elite-5MS 30 m x 0.25 mm x 0.25 µm capillary column and the following temperature program was applied: 50℃ held for 1 min, then increased at rate 25℃/min up to 100℃ and then 5℃/min up to 300℃ maintained for 5 min. Volume of 5 µL was injected each time in 1:10 split mode. Injection port temperature was 250℃. Helium carrier gas (≥99.9999% purity) was maintained at a constant flow rate of 2 mL/min. The ion source and transfer line temperature were set at 250℃. Solvent delay was 4 min and the MS analysis was performed in scan mode. The chromatographic analysis parameters were set experimentally, but it is not described in this work.

Results and discussion

The example chromatogram of phthalates separation is presented in Figure 4. The recovery values for phthalates adsorption from water (working solution) for STA-Ni-10% and STA-Ni-20% are shown in Table 3. The error values, given in Table 3, were counted on the basis of relative standard deviation for n = 5.

Figure 4.

Example chromatogram of determination of six phthalates by GC-MS method.

Table 3.

The recovery values of phthalates determined by MSPE-GC-MS method in working solution using STA-Ni-10% and STA-Ni-20%.

Phthalate	Recovery (%)
Phthalate	10% Ni; 0.32–0.40 mm	10% Ni; 0.40–0.63 mm	20% Ni; 0.32–0.40 mm	20% Ni; 0.40–0.63 mm
DMP	72 ± 8	62 ± 8	68 ± 4	93 ± 8
DEP	64 ± 4	56 ± 4	62 ± 8	87 ± 7
DBP	41 ± 5	36 ± 6	31 ± 7	77 ± 6
BBP	35 ± 4	35 ± 4	29 ± 5	41 ± 4
DEHP	23 ± 3	22 ± 4	19 ± 2	31 ± 3
DNOP	7 ± 2	9 ± 2	12 ± 2	23 ± 3

The analysis of data, given in Table 3, leads to following conclusions. In case of STA-Ni containing 10% of Ni, the grain size of adsorbent has very small influence on the recovery values. The recoveries are generally small and decrease along with the growth of phthalates molecules’ size. For STA-Ni containing 20% of Ni recoveries of phthalates are clearly greater for 0.40–0.63 mm grain size than for smaller grains. Coinstantaneously greater content of Ni (20%) conduces the higher recoveries than for adsorbent containing 10% of Ni. However, in each case the recovery values decrease with increasing molecular weight of phthalates. For amount of Ni and at each grain dimension the best recoveries were observed for DMP.

The mechanism of the phthalates adsorption on the STA-Ni adsorbent is mainly connected to different interactions of phthalates molecules (with various molecule structures) with hydrophobic surface of mesoporous sorbent. However the influence of paramagnetic properties of nickel on efficiency of phthalates adsorption cannot be excluded (Table 3). The adsorption energy of phthalates containing various alkide chains and ligands is various. In Prokupkova et al.’s work (2002) the values of log K_OW (octanol/water partitioning coefficient) for six phthalates are presented. The log K_OW values are in range from 1.61 to 7.73. Furthermore in Wideł et al.’s work (2015), authors presented distribution coefficients of studied phthalates between methanol and polymeric stationary phase and between water and polymeric stationary phase. All these data indicate different polarity of phthalates.

Optimization of extraction conditions

Extraction time

The MSPE-GC-MS procedure performed for STA materials and various extraction times was used to check the influence of different extraction time for recovery values. The extraction was carried out as it was described in “MSPE procedure” section. For the procedure, 10 mL samples with 5 mg/L of each phthalate concentration were used. Acetone was used for 1 min elution. Elution longer than 1 min did not result in higher recovery of phthalates. The results are shown in Figure 5.

Figure 5.

The dependence of phthalates recoveries on extraction time for 100 mg of STA-Ni-20% used in MSPE; 1 min elution with acetone.

As it can be seen in Figure 5 the highest recoveries are obtained with 2 min extraction time. In next studies, every time 10 mL samples with 5 mg/L of each phthalate concentration were used. Extraction time was 2 min and acetone was used for 1 min elution.

Adsorbent amount

The various amount of adsorbent was also studied. For the extraction 50, 100 and 150 mg of adsorbent were used to evaluate the optimum amount. It was demonstrated in Figure 6 that the 100 mg of adsorbent is the best choice.

Figure 6.

The dependence of phthalate recoveries on adsorbent amount of STA-Ni-20% used in MSPE; 2 min extraction and 1 min elution with acetone.

Salt addition

The analyses with NaCl addition to water sample (working solution) were performed to check what is the influence of ionic strength on extraction efficiency. The NaCl final concentration in the examined samples amounted 5%. The obtained results are presented in Table 4. The recovery increase for STA-Ni was however very small and depended on analyzed phthalates. Therefore salt addition was not applied in optimized analytical procedure.

Table 4.

The influence of salt addition on phthalates extraction efficiency using STA-Ni-20%; grain diameter 0.40–0.63 mm.

Phthalate	Recovery (%)
Phthalate	STA-Ni-20%	STA-Ni-20% + 5% NaCl
DMP	93 ± 8	94 ± 8
DEP	87 ± 7	93 ± 8
DBP	77 ± 6	78 ± 6
BBP	41 ± 4	45 ± 5
DEHP	31 ± 3	33 ± 3
DNOP	23 ± 3	20 ± 3

Solvent for elution

Except acetone all other solvents were used in MSPE procedure: methanol, acetonitrile, n-hexane-ethyl acetate (50:50). However extraction with acetone showed the best efficiency results. In case other three solvent extraction efficiencies were noticeably worse.

Mixing

Except sonication magnetic stirrer was also used to check how mixing influence on phthalates recovery. Magnetic stirrer usage was insufficient and the recovery values were about two times lower than using sonication.

Real sample analysis

For real samples analysis 20% STA-Ni (0.40–0.63 mm fraction) was used. The analyses were carried out using optimized MSPE method. The water samples were from: electric kettle made of plastic, after boiling the water; mineral water in plastic bottle from leading Polish mineral water producer (No. 1); spring water in plastic bottle from supermarket (No. 2). Both waters in plastic bottles were exposed to sun rays for three weeks to show that phthalates can migrate from plastic to water under influence of sun rays. In all samples DEHP was found and in kettle sample additionally DBP was found. The water used for kettle experiment was purchased from Milli-Q system and it was phthalate free. It shows that kettle plastic materials are quite dangerous for human health, because phthalates migrate from material to water. The obtained amounts of detected phthalates were calculated using calibration curves. DBP was found only in water from kettle; DEHP was found in all samples studied. Results are presented in Table 5.

Table 5.

The concentration values of phthalates determined by MSPE-GC-MS method in real water samples using STA-Ni-20%; grain diameter 0.40–0.63 mm.

Phthalate	Amount (mg/L)
Phthalate	Water from kettle	water No. 1	water No. 2
DMP	–	–	–
DEP	–	–	–
DBP	0.6 ± 0.07	–	–
BBP	–	–	–
DEHP	0.9 ± 0.07	0.6 ± 0.07	0.4 ± 0.07
DNOP	–	–	–

Conclusion

The MSPE method is quick and easy to use. It does not require any columns or special pressure extraction chambers. The MSPE method is used to analyze small quantities of analytes. A novelty of presented work is the use of mesoporous carbon materials containing Ni nanoparticles. Innovative is that Ni nanoparticles are added to material in the synthesis stage. Obtained material has magnetic properties and it is ready to use, it does not have to be modified. Obtained carbon materials with Ni nanoparticles are good ordered and show high level of mesoporosity.

The main objective of this work was to investigate the applicability of new kind of adsorbent in MSPE. Investigations indicate that phthalates adsorption mechanism on STA-Ni materials is complicated. Therefore the application of these sorbents for simultaneous extraction of various phthalates is ambiguous. The best extraction efficiencies were obtained for phthalates with small molecular masses. MSPE-GC-MS method using STA-Ni can be applied for liquid samples, particularly for beverages and environmental samples.

Footnotes

Acknowledgements

The authors would like to thank Prof Jerzy Choma from Military University of Technology in Warsaw for essential verification of this work and Dr Anna Śrębowata from Polish Academy of Science for help with SEM images’ performance.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This research was funded by National Science Centre (Poland) under Grant DEC-2012/05/N/ST5/00246.

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Investigation of mesoporous carbon materials by magnetic solid phase extraction of selected phthalates from water samples

Abstract

Keywords

Introduction

Experimental

Reagents and equipment

Synthesis of ordered mesoporous carbon materials

Calibration curves preparation

MSPE procedure

GC-MS analysis

Results and discussion

Optimization of extraction conditions

Extraction time

Adsorbent amount

Salt addition

Solvent for elution

Mixing

Real sample analysis

Conclusion

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

References