Superhydrophobic magnetic nanoparticle-free fatty acid regenerated from waste cooking oil for the enrichment of carcinogenic polycyclic aromatic hydrocarbons in sewage sludges and landfill leachates

Abstract

In this study, Fe₃O₄ nanoparticles grafted with superhydrophobic free fatty acids from waste cooking oil (FFA@MNP) were successfully fabricated as an adsorbent for the magnetic solid phase extraction (MSPE) technique. The synthesized nanomaterial (FFA@MNP) was analyzed using Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, water contact angle analysis and vibrating sample magnetometry, which confirmed the successful functionalization of free fatty acids onto the surface of Fe₃O₄ nanoparticles. The adsorption efficiency of the synthesised FFA@MNP in MSPE was evaluated by the extraction of five selected polycyclic aromatic hydrocarbons (PAHs), namely, fluorene (Flu), fluoranthene (FLT), pyrene (Pyr), chrysene (Cry), and benzo(a)pyrene (BaP), from environmental samples prior to HPLC-DAD analysis. The MSPE method was optimized for various parameters such as adsorbent dosage, type of organic eluent, volume of organic eluent, extraction time, desorption time, pH of the solution and sample volume. The low detection limit for the proposed MSPE was found to be 0.001 to 0.05 ng mL⁻¹. Under optimized conditions, the newly synthesized material was applied as an adsorbent for environmental leachate and sludge samples for enrichment of the selected PAHs from these complex matrices. The stability and reusability studies demonstrated that FFA@MNP could be used for up to five cycles. Due to hydrophobic interactions between the long alkyl chains of FFA@MNP and PAHs, good recoveries (82.8–116.6%) with low relative standard deviations ranging from 5.2% to 11.0% were obtained for the spiked leachate samples. For the spiked sludge samples, the recovery range was found to be from 72.3% to 119.9%, with a satisfactory % RSD (4.8–11.8%).

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1. Introduction

The chemical contamination of our environment over the past few decades as a result of hazardous pollutants, such as polycyclic aromatic hydrocarbons (PAHs), is becoming a serious threat owing to the carcinogenic and mutagenic nature of these pollutants. PAHs belongs to the class of potential pollutants which are listed as priority pollutants by the United States Environmental Protection Agency.^1,2 These PAHs accumulate in our environment due to anthropogenic activities such as combustion of fossil fuel and industrial pollution.³ Due to their unique chemical structures, consisting of many aromatic rings, PAHs are considered to be environmentally persistent organic compounds which may pose dangers to our environment and to living beings.⁴

Polycyclic aromatic hydrocarbons (PAHs) are one of the most common classes of toxic organic pollutants, they are usually detected in landfills. They are released and formed in these areas mainly via petroleum hydrocarbon residues and direct burning of landfills. Therefore, PAHs have been frequently detected in landfill leachates,^5,6 with concentrations of up to 7966 μg L⁻¹ depending on the variety of their composition and the degree of industrial pollution.⁷ Moreover, due to the high lipophilicity and low biodegradability of PAHs in our environment, they are also found in sewage sludge.^8–10 The detection of PAHs in sewage sludge has become an important issue in Europe, as the maximum level of PAHs allowed in sewage sludge is 6000 ng g⁻¹ (calculated as the sum of acenapthene, phenanthrene, fluorene, fluoranthene, pyrene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[g,h,i]perylene and indeno[1,2,3-cd]pyrene) as released by the 3rd draft directive of the European Union. In addition, PAHs in landfills from pyrolytic processes are complex mixtures of non-substituted compounds which can be considered to be ecologically toxic chemicals. Considering these problems, there is a need for a rapid and low-cost analytical method for the detection of PAHs in leachate and sludge samples.

As PAHs are present in leachate and sludge samples at trace levels, a pre-concentration step is required to isolate and enrich this pollutant from matrices before instrumental analysis. Recently, a greener approach for sample pretreatment, known as magnetic solid phase extraction (MSPE), was introduced to enrich different analytes, such as phenol,^11,12 phthalate ester,^13,14 organophosphorus pesticides^15,16 and PAHs.^17,18 MSPE provides high extraction efficiency, rapid extraction kinetics, a high enrichment factor and high breakthrough volume compared to conventional extraction methods.¹⁹ This method is based on the use of a magnetizable adsorbent,^20,21 which is usually based on magnetite (Fe₃O₄) nanoparticles (MNPs). Owing to the superparamagnetic properties of MNPs,^22,23 the magnetic nano-sorbents can be simply collected with an external magnetic field, which enhances the effectiveness of the pre-treatment procedure. Because of their unique features, MNPs have been widely applied in water purification.¹⁹ Additionally, MNPs have high surface areas, lower toxicity,^24,25 and facile synthesis processes. Thus, they are suitable candidates as solid sorbents for the extraction of hazardous environmental pollutants. However, the defects of MNPs, including instability in acidic environments, agglomeration in aqueous media, ready oxidation upon exposure to air and poor extraction efficiency due to their highly hydrophilic nature,^26,27 limit their use as adsorbents for the extraction of organic pollutants from environmental samples. Thus, an alternative solution can be achieved by developing newly modified MNPs with intense hydrophobizing agents based on economic and eco-friendly natural biomaterials.

Herein, we highlight the facile synthesis of a new sorbent based on a green approach, namely, the impregnation of magnetite (Fe₃O₄) nanoparticle MNPs with biomaterials. Here, magnetic nanoparticles functionalized with free fatty acids from waste cooking oil (FFA@MNP) were used as an MSPE adsorbent for the extraction of PAHs from environmental samples. High amounts of free fatty acids can be produced in waste cooking oil through a hydrolytic process that occurs in the presence of oil and water from food. The chemical structure of free fatty acids, which consist of long alkyl chains, makes them an excellent hydrophobizing agent to tune the hydrophilic surface of MNPs for selective extraction of PAHs from complex matrices. Waste cooking oil was selected because it is easily available and economic. In addition, the utilization of a hazardous pollutant, waste cooking oil, for the determination of PAHs (Fig. 1) from the environment is a good solution for waste management. Thus, we introduce a new MSPE adsorbent, FFA@MNP, which combines high selectivity and environmental stability, for the extraction of PAHs from polluted environments.

Fig. 1 Types and structures of PAHs used in this study.

2. Experimental section

2.1 Reagents and chemicals

N,N-dimethylformamide (DMF), (3-aminopropyl)triethoxysilane (APTES), aqueous ammonia, ethanol, choloroform, diethyl ether, methanol, acetonitrile, hexane and ethyl acetate were obtained from Merck. Ferrous chloride tetrahydrate, ferric chloride hexahydrate, potassium iodide, iodine monochloride, starch, potassium hydroxide and sodium thiosulphate were purchased from Sigma Aldrich. Flu, FLT, Pyr, Cry and BaP were purchased from Supelco. The standard stock solutions (100 mg L⁻¹) were prepared in methanol and stored in a dark amber glass at 4 °C to prevent degradation. The working solutions were prepared daily by diluting the stock solution with deionized water. Water was deionized prior to use.

The waste cooking oil used in this study was collected from a restaurant in the city of Bangsar, Kuala Lumpur. The collected waste cooking oil was used in the restaurant for frying purposes at about 180 °C for 2 days. Before use, the waste cooking oil samples were filtered in order to eliminate particulate materials and other impurities. This waste cooking oil was characterized by its acid and iodine values (Table 1). In addition, the fatty acid profile of the oil was analysed, as shown in Table 2. The gas–liquid chromatography reference standard for fatty acid methyl esters was purchased from Supelco.

Table 1 Waste cooking oil characterization

Property	Unit	Value
Acid value	mg KOH g^−1	8.74
Iodine value	g × 10^2 g^−1	7.31

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Table 2 Fatty acid composition of waste cooking oil

Fatty acid		Fatty acid composition (%)
Palmitic	(C16 : 0)	40.56
Stearic	(C18 : 0)	5.46
Oleic	(C18 : 1)	41.73
Linoleic	(C18 : 2)	12.26

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2.2 Instrumentation

The particle size and morphology were investigated using a JEOL JEM-2100F transmission electron microscope (TEM). EDX analysis was performed using a scanning electronic microscope (SEM HITACHI SU8220, OXFORD Instruments) equipped with an energy dispersive X-ray spectrometer. FT-IR spectrometry (Spectrum 400 PerkinElmer) was performed using the ATR technique in absorption mode with 4 scans, a resolution of ±4 cm⁻¹, and a wavenumber range of 4000 to 450 cm⁻¹ with a diamond detector. X-ray diffraction (XRD) patterns were recorded using an Empyrean X-ray diffractometer from 2θ = 15° to 75° using Cu Kα radiation (λ = 1.5418 Å) at a scan rate of 0.02 s⁻¹. Contact angle (CA) measurements were conducted using a contact angle instrument (TL 100 and TL 101). The magnetizations of the bare and functionalized magnetic nanoparticles were measured using a vibrating sample magnetometer (VSM LakeShore 7400 series). The acid (AV) and iodine (IV) values of the waste cooking oil were determined according to the standard method in the PORIM Test Methods.²⁸ The fatty acid methyl esters from waste cooking oil were prepared using an acid catalysed (sulphuric acid) method.²⁹ The analysis of the fatty acid composition of the waste cooking oil was performed using a Shimadzu 2010 gas chromatograph (Shimadzu, Kyoto, Japan) equipped with a split/splitless injector and a flame ionization detector (FID). A BPX50 capillary column (SGE Analytical Science, Australia) (30 m × 0.25 mm i.d. 0.25 μm film thickness) was applied for the separation of fatty acids. Helium (with 99.999% purity) was used as the carrier gas at a constant flow rate of 59.2 mL min⁻¹. The chromatographic conditions were controlled as follows: the temperature of the detector and injector were set at 260 and 139 °C, respectively. The injection port was operated in split mode. The oven temperature was initially set at 139 °C for 2 min, then increased by 8 °C min⁻¹ to 165 °C, by 3 °C min⁻¹ to 192 °C, by 13.7 °C min⁻¹ to 240 °C; finally, the temperature was maintained at 240 °C for 11 min.

2.3 HPLC analysis

An HPLC system consisting of a Shimadzu (Tokyo, Japan) LC-20AT pump, an SPD-M20A diode array detector, an SIL-20A HT auto sampler and a CTO-10AS VP column oven was used for the separation and quantification of PAHs. The separation was conducted on a C-18 reverse column (250 mm × 4.6 mm; particle size 5 μm; Hypersil Gold, Thermo Science USA). A mobile phase of acetonitrile–water (80 : 20, v/v) was used at a flow rate of 1.0 mL min⁻¹. The injection volume was 10 μL. The detection of each PAH was set as follows: 5.54 min, 254 nm (for Flu); 6.77 min, 284 nm (for FLT); 7.22 min, 270 nm (for Pyr); 8.03 min, 266 nm (for Cry); 10.82 min, 266 nm (for BaP).

2.4 Synthesis of free fatty acid functionalized Fe₃O₄ nanoparticles (FFA@MNP)

Fig. 2 depicts the procedures for the preparation of FFA@MNP. The magnetite nanoparticles (MNPs) were prepared via the chemical co-precipitation method.³⁰ Based on this method, FeCl₂·4H₂0 (3.1736 g) and FeCl₃·6H₂O (7.5709 g) were dissolved in 320 mL deionized water. The mixed solution was stirred under nitrogen at 80 °C for 1 hour. Then, 40 mL of aqueous ammonia was rapidly added to the reaction mixture; the mixture was stirred under nitrogen for another 1 hour and then cooled to room temperature. The obtained nanoparticles were magnetically collected and washed five times with hot water until the filtrate became neutral.

Fig. 2 Schematic of the synthesis of FFA@MNP. (R = the alkyl chains of the free fatty acids).

MNP-APTES was synthesized according to the reported method.³¹ Briefly, the hydrolysed APTES was added to 100 mL of the magnetite fluids in a 250 mL round bottom flask. The mixture was stirred for 5 hours at 60 °C under nitrogen protection. Later, the solution was cooled at room temperature and the product was collected using filtration, then washed with ethanol and deionized water several times, respectively. The obtained product was dried under vacuum at 70 °C.

To synthesize FFA@MNP, 1.000 g of MNP-APTES was dispersed in 10 mL of DMF, followed by the addition of 2.000 g of waste cooking oil to the solution; the mixture was stirred overnight at 60 °C under nitrogen protection. After the solution was cooled to room temperature, the resulting brownish product was washed with ethanol and deionized water several times, respectively, by magnetic decantation. Then, the final product was dried under vacuum at 70 °C for 24 hours.

2.5 MSPE procedure

Firstly, 10 mg of FFA@MNP were placed in a 15 mL water sample containing trace levels of PAHs. The solution was shaken for 10 minutes to facilitate the adsorption of PAHs onto FFA@MNP (Fig. 3). Subsequently, a Nd–Fe–B magnet was applied to isolate the FFA@MNP from the sample solution. After the solution became clear, the supernatant was removed. The PAHs adsorbed on FFA@MNP were eluted with 1.5 mL of organic eluent (acetonitrile) by shaking for 10 minutes. The collected eluate was dried under a stream of N₂ and then re-dissolved in 0.5 mL of acetonitrile. Finally, a 10 μL portion of the eluate was injected into the HPLC system for analysis.

Fig. 3 Illustration of adsorption of the targeted PAHs on the hydrophobic frameworks of FFA@MNP during the MSPE procedure. (R = the alkyl chains of the free fatty acids).

2.6 Real sample preparation

All the samples, including leachate and sludge, were collected from a landfill at Jeram, Kuala Selangor. The leachate samples were filtered through a 0.22 μm cellulose membrane immediately after sampling and were stored at 4 °C before analysis. Then, 150 mL of filtered leachate sample was used for MSPE by the method mentioned above.

For the sludge sample, the collected sample was dried at room temperature, ground and sieved. The extraction of soil was carried out by ultrasound-assisted extraction (UAE) using the following procedure: 1 g of the sludge sample was exactly weighed and placed in a 50 mL centrifuge tube; 3 mL of methanol was added, and the mixture was sonicated for 10 min. After centrifugation at 3500 rpm for 5 min, the supernatant was filtered using a PTFE syringe filter (13 mm, 0.22 μm pore size) and was transferred into the sample vial.³² Subsequently, the total volume was made up to 150 mL with distilled water, and the samples were analysed by MSPE as described previously.

3. Results and discussion

3.1 Characterizations of the FFA@MNP

3.1.1 Morphological and elemental analysis. The TEM images of MNPs, MNP-APTES and FFA@MNP in Fig. 4A–C clearly indicate that the prepared nanoparticles are spherical in shape and have uniform nano-size distributions. From the diameter distributions (Fig. 4A′–C′), the average diameters of MNPs, MNP-APTES and FFA@MNP were found to be 4.9 nm, 6.90 nm and 7.23 nm, respectively. The increases in the average diameters of MNP-APTES and FFA@MNP suggested the significant encapsulation of (3-aminopropyl)triethoxysilane (APTES) and the free fatty acid layer, respectively, onto the surface of MNPs. Additionally, elemental analysis was performed to confirm the presence of elements such as N, O, Fe, Si and C in the prepared MNPs, MNP-APTES and FFA@MNP. The analysis results exhibited that MNPs contain 74.5% Fe and 24.5% O without any traces of other elements, thus confirming the formation of pure MNPs. After modification with APTES, the EDS of MNP-APTES depicted the additional presence of 0.9% N, 5.0% C and 1.3% Si in addition to Fe and O. Moreover, in the case of FFA@MNP, the EDS result showed the presence of 26.3% C and 33.0% O; the increased percentages of C and O reveal the presence of free fatty acids, which were successfully introduced onto the surface of MNP-APTES.

Fig. 4 TEM images of (A) MNPs; (B) MNP-APTES; (C) FFA@MNP; size distributions of (A') MNPs; (B') MNP-APTES; (C') FFA@MNP.

3.1.2 Functional group analysis. In order to confirm the presence of characteristic functional groups in the prepared MNPs, MNP-APTES and FFA@MNP adsorbents, FT-IR spectrometry analysis was carried out, and the results are shown in Fig. 5. In the spectra of MNPs, the presence of the IR bands at approximately 538, 3401 and 1607 cm⁻¹ may be attributed to the Fe–O band, the stretching vibration of the surface adsorbed water and the O–H groups on the surface of MNPs, respectively. The additional IR peaks that appeared in the spectrum of MNP-APTES at ∼3410 and ∼1594 cm⁻¹ may be assigned to the N–H stretching vibration and the NH₂ group of APTES. Moreover, the IR band appearing at ∼944 cm⁻¹ was allocated to the Si–O group, revealing the adsorption of silane polymer onto the surface of MNPs and the formation of covalent bonds with the O–H groups. After modification with free fatty acids, new characteristic absorptions of the C=O group at ∼1712 cm⁻¹ and the NHCO group at ∼1405 cm⁻¹ were observed. Furthermore, the appearance of new peaks at ∼2925 and ∼2860 cm⁻¹ can be related to the asymmetric and symmetric methylene and methyl vibrations, respectively. These peaks confirmed the presence of the free fatty acids, which were functionalized successfully onto the surface of MNP-APTES.

Fig. 5 FTIR spectra of MNPs, MNP-APTES and FFA@MNP.

3.1.3 Wettability analysis. The hydrophobicities of MNPs and FFA@MNP were measured by water contact angle (CA) analysis. Water droplets on the MNPs substrate immediately spread out, indicating that the surface of MNPs is highly hydrophilic (Fig. 6A). In contrast, FFA@MNP displayed superhydrophobic properties, as the contact angle of FFA@MNP was found to be 151.12° (Fig. 6B), revealing that free fatty acid layer was successfully introduced onto the surface of MNPs. Hence, FFA@MNP could extract PAHs selectively from environmental samples, owing to this unique feature. Additionally, the relationship between pH and CA on the surface of FFA@MNP was analysed and is depicted in Fig. 6C. It was found that FFA@MNP displayed repellence behaviour toward aqueous solutions with pH values ranging from 1 to 12, as all water droplets with various pH values still exhibited spherical shapes after being placed on the FFA@MNP bed. These results reveal that FFA@MNP is highly hydrophobic at a wide pH range, suggesting the versatility of FFA@MNP as an MSPE sorbent toward corrosive environmental samples.

Fig. 6 The water contact angles of (A) MNPs and (B) FFA@MNP; and (C) the relationship between the pH of the water droplet and the water CA on the FFA@MNP surface.

3.1.4 Magnetic behaviour. The magnetic behaviour of the as-synthesized nanoparticles was studied using VSM analysis. Fig. 7 demonstrates the VSM spectra of MNPs, MNP-APTES, and FFA@MNP. As is evident from Fig. 7, all the prepared nanosorbents possessed superparamagnetic behaviour, as their remanence and coercivity values are almost negligible. The maximal saturation magnetizations of MNPs, MNP-APTES, and FFA@MNP were found to be 63.30 emu g⁻¹, 58.23 emu g⁻¹, and 53.28 emu g⁻¹, respectively. The decreases in the magnetization values of MNP-APTES and FFA@MNP compared to that of MNPs suggests that the former two samples contain a non-magnetic domain of APTES and a free fatty acid layer surrounding MNPs, respectively. These imply that the prepared magnetic nanoparticles could be dispersed into aqueous solution readily and isolated from the matrix easily by using an external magnet (see photos inset of Fig. 7).

Fig. 7 VSM magnetization curves of MNPs, MNP-APTES and FFA@MNP.

3.1.5 Crystallinity properties. The crystallinity of MNPs, MNP-APTES, and FFA@MNP were analyzed by XRD. As shown in Fig. 8, MNPs showed good crystallinity, with diffraction peaks appearing at the 2θ values of 30.42°, 35.59°, 43.42°, 53.63°, 57.35° and 63.02°, which can be assigned to the (220), (311), (400), (422), (511) and (440) cubic spinel planes of Fe₃O₄, respectively, in accordance with JCPDS 88-0866. Furthermore, the MNP-APTES and FFA@MNP spectra showed approximately identical characteristic diffraction peaks, revealing that the surface modification of MNPs does not alter its phase. Furthermore, decreases in the intensities of the diffraction peaks are observed in the cases of MNP-APTES and FFA@MNP, which may be due to the presence of an amorphous layer of APTES and free fatty acid on MNPs.

Fig. 8 XRD patterns of MNPs, MNP-APTES and FFA@MNP.

3.2 Optimization of MSPE conditions

To achieve better extraction efficiency of the tested PAHs using FFA@MNP, several parameters, such as the adsorbent dosage, type of organic eluent, volume of organic eluent, extraction time, desorption time, pH of the solution and sample volume, were optimized. Each experiment was performed in triplicate. The experimental conditions for MSPE using the prepared FFA@MNP were herein optimized using a standard aqueous solution spiked with 100 ng mL⁻¹. Chromatography of the peak area was used to evaluate the influence of these factors on the extraction efficiency of the MSPE of all the tested PAHs.

3.2.1 Adsorbent dosage. To assure adequate extraction of all the tested PAHs, different dosages of FFA@MNP sorbent (5 to 30 mg) were investigated. Fig. 9A shows that the recovery of all the tested PAHs increased with increasing adsorbent dosage to 25 mg, except fluorene. After the amount of adsorbent was increased to 30 mg, there was no significant improvement in the extraction efficiency. Therefore, 25 mg of FFA@MNP was used as the optimised dosage for the following experiments to ensure the complete adsorption of tested PAHs from the sample solution.

Fig. 9 The effects of (A) adsorbent dosage; (B) organic eluent; (C) solution pH; and (D) sample volume on the extraction efficiency of PAHs.

3.2.2 Type of organic eluent. A suitable eluting solvent is an important factor which should be investigated in order to achieve better extraction efficiency. Hexane, acetonitrile, ethyl acetate and toluene were tested as possible eluents for the desorption of PAHs in this experiment. As is evident from Fig. 9B, 2.5 mL of ethyl acetate was the best eluting solvent, as it gave better recoveries for almost all the targeted PAHs. Moreover, the chosen eluent, ethyl acetate, is water immiscible, which facilitates the removal or elimination of water from the final extraction, thereby shortening the time needed to concentrate the extracts.³³ Thus, ethyl acetate was used as the eluent for further studies.

3.2.3 Extraction and desorption time. Extraction time profiles by FFA@MNP were investigated at times ranging from 5 to 30 minutes. As the extraction time increased, the response also increased significantly. All the PAHs gave higher responses at 25 minutes of extraction time, and equilibrium was achieved when the extraction time was prolonged to 30 minutes. Hence, an extraction time of 25 minutes was optimised as the sufficient contact time for the targeted PAHs to attain adsorption equilibrium on the FFA@MNP surface. For desorption, the time was varied from 5 to 30 minutes; it was found that 25 minutes of desorption time was sufficient to desorb all the tested PAHs from FFA@MNP. Therefore, 25 min was selected for both the extraction and desorption times.

3.2.4 pH of the solution. One of the most important features in extraction studies, as stated by various researchers, is the considerable role of pH in adsorption/desorption efficiency. pH plays an important role in the adsorption efficacy of the adsorbent, seemingly owing to its impact on the surface charge of the adsorbent material and the degree of ionization of the PAHs present in the aqueous solution, together with the ionization/dissociation of the functional groups present on the active sites of the adsorbate molecules.^34–36 In this experiment, the effect of the sample solution pH on the extraction efficiency was studied at different pH values (i.e., 2.0, 4.0, 6.0, 6.5, 7.0, 8.0 and 10.0). It can be seen from Fig. 9C that good recoveries for all targeted PAHs were achieved at the natural pH of PAHs (pH 6.5). However, low recoveries for all PAHs were observed under acidic and basic conditions. These results can be explained by the alteration of the amount of protons available in the solution. Under acidic conditions, there are many protons present due to the protonation of residual hydroxyl groups on the surface of MNPs, which may saturate the sorbent sites, making the surface of the FFA@MNP more cationic; meanwhile, under basic conditions, the deprotonation of the remaining hydroxyl groups from MNPs would make the surface of the sorbent more anionic. As a result, the hydrophobicity of the FFA@MNP would be reduced, resulting in less interaction between the targeted PAHs and the alkyl chains of FFA@MNP. Therefore, pH 6.5 was chosen for subsequent experiments.

3.2.5 Sample volume. The FFA@MNP sorbent has high extraction efficiency and possesses great potential for the pre-treatment of large volumes of water samples. In order to investigate the effect of sample volume on the extraction efficiency of targeted PAHs, sample volumes of solution were studied ranging from 15 to 200 mL (Fig. 9D). As revealed in Fig. 9D, the recoveries for all PAHs increased when the sample volume was increased to 150 mL, and equilibrium was reached as the sample volume was increased further. This may be due to the non-availability of the active sites of FFA@MNP, which were fully occupied with the targeted PAHs, thereby reducing the available sites for further PAH adsorption. Hence, 150 mL of solution volume was selected for optimum sample loading.

3.2.6 Reusability of the adsorbent. The stability and reusability of an adsorbent are important parameters for its practical applications and must be thoroughly investigated. Thus, reusability studies of FFA@MNP were carried out to examine the effects of repeated usage cycles on the adsorption efficiency. In order to study the recycling of the used FFA@MNP, the adsorbent after the first cycle was recovered through magnetic decantation, washed with ethyl acetate, and then dried before the next MSPE application. Fig. 10 shows the reusability results, which depict no obvious decrease in the recoveries of the selected PAHs even after five cycles of reuse, demonstrating the high stability and reusability efficiency of FFA@MNP for the MSPE procedure.

Fig. 10 Reusability of the sorbent for extraction of PAHs.

3.3 Analytical performance

Under the optimized conditions, a series of experiments with regard to the linearity, limit of detection, limit of quantification and precision were evaluated to validate the developed method. The results are listed in Table 3. It can be seen from the table that the present method has a wide linear range and good method precision. In this case, all the tested PAHs exhibited good linearity, with correlation coefficients ranging from 0.9955 to 0.9998. The LOD and LOQ values of the PAHs were found to be in the ranges from 0.001 to 0.05 ng mL⁻¹ and from 0.004 to 0.2 ng mL⁻¹, respectively. Precision, expressed as relative standard deviation (RSD), was assessed in terms of repeatability (from five independent sample preparations, intra-day RSD) and reproducibility (studied during three consecutive days, inter day RSD); the values for intra-day RSD% were between 2.9 and 6.5%, and those of inter-day RSD% were in the range of 1.5–2.5%.

Table 3 Analytical performance data of the proposed method (FFA@MNP-MSPE)

Analyte	Linearity	R^2	LOD (ng mL^−1)	LOQ (ng mL^−1)	Precision
	LDR (ng mL^−1)				Intra-day (RSD%, n = 5)	Inter-day (RSD%, n = 3)
Flu	0.1–100	0.9996	0.05	0.2	6.4	2.2
FLT	0.01–100	0.9955	0.009	0.03	6.5	1.5
Pyr	0.01–100	0.9987	0.008	0.03	3.1	2.5
Cry	0.01–50	0.9998	0.003	0.009	5.3	2.3
BaP	0.01–100	0.9977	0.001	0.004	2.9	2.1

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3.4 Application to real samples

To demonstrate the applicability and reliability of the proposed method, it was applied to some real samples, including leachate and sludge collected from Jeram Landfill, Kuala Selangor. To test the accuracy of the method, the recoveries of the PAHs from leachate and sludge at spiking levels of 0.5 ng mL⁻¹, 5 ng mL⁻¹, and 50 ng mL⁻¹ were measured. The obtained results are shown in Table 4. It can be seen that the relative recoveries for all tested PAHs in leachate were in the range of 82.8–116.6%, with RSDs (n = 5) ranging from 5.2% to 11.0%; this implies good performance of the presented method for the determination of PAHs in real samples.

Table 4 The recoveries and standard deviations of PAHs in real environmental samples with spiked concentrations of 0.5 ng mL⁻¹, 5 ng mL⁻¹, and 50 ng mL⁻¹ for each analyte

Analyte	Spiked (ng mL^−1)	Leachate (n = 5)		Sludge (n = 5)
		Recovery (%)	RSD (%)	Recovery (%)	RSD (%)
Flu	0.5	111.5	7.5	112.7	9.6
	5	94.0	9.7	98.3	9.6
	50	107.1	6.5	87.8	5.5
FLT	0.5	112.1	6.7	112.7	9.6
	5	116.6	11.0	103.6	8.6
	50	105.2	8.2	101.0	8.6
Pyr	0.5	115.3	6.0	117.6	4.8
	5	94.7	10.1	99.1	11.7
	50	82.8	6.1	95.1	9.9
Cry	0.5	100.4	7.6	116.7	11.2
	5	99.2	9.5	119.9	11.8
	50	105.8	6.7	107.2	11.7
BaP	0.5	100.0	9.0	116.4	11.7
	5	97.6	7.6	72.3	8.9
	0.5	110.6	5.2	73.4	7.9

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Moreover, the developed method was used for determination of the tested PAHs in sludge samples. As is obvious from Table 4, the recoveries for the spiked sludge were found to be in the range of 72.3–119.9%, with RSDs (n = 5) ranging from 4.8% to 11.8%. These results imply that the established method can be applied even for the analysis of PAHs at trace levels in complex matrices. Fig. 11 illustrates typical HPLC-DAD chromatograms of the PAHs extracted from leachate by the proposed procedure, before and after spiking with 50 ng mL⁻¹ of each PAH.

Fig. 11 HPLC-DAD chromatograms of the PAHs after extraction using the proposed MSPE:leachate sample spiked with 50 ng mL⁻¹ of each PAH (A), and non-spiked leachate sample (B). (1) Flu, (2) FLT, (3) Pyr, (4) Cry, and (5) BaP. The inset shows photographs of magnetic nanoparticles dispersed in solution (left) and separated from water solution under an external magnetic field (right).

3.5 Comparison of FFA@MNP with other reported MSPE adsorbents

The extraction efficiencies of FFA@MNP adsorbent for the enrichment of PAHs were compared with other MSPE adsorbents reported in the literature (as shown in Table 5). Comparison of the % Recovery, % RSD and LOD reveals that FFA@MNP is an efficient MSPE adsorbent compared to previously reported MSPE adsorbents.

Table 5 Comparison of the % recovery, % RSD and LOD of the current work with other reported MSPE adsorbents

Matrix	Technique/adsorbent	% recovery	% RSD	LODs (ng mL^−1)	Ref.
Aqueous samples	MSPE-GC/magnetic C18 microspheres	35.0 to 99.0	<10.0	0.8 to 36	37
Lake water	MSPE-HPLC/Fe3O4–SiO2-MIL-101	81.3 to 98.7	3.0 to 10.8	0.0028 to 0.027	38
Lake water	MSPE-GC/Fe3O4–octadecylphosphonic acid	53.5 to 103.5	0.8 to 7.6	0.014 to 0.0644	39
Tap–river–seawater	MSPE-HPLC/Fe3O4–graphene oxide	76.8 to 103.2	1.7 to 11.7	0.09 to 0.19	40
Tap–well–river water	MSPE-GC/Fe3O4–CNFs	93.7 to 100.9	3.2 to 9.8	0.008 to 0.03	41
Leachate–sludge	MSPE-HPLC/Fe3O4–CN/IL	89.50 to 110.2	1.2 to 4.5	0.86 to 1.95	42
Leachate–sludge	MSPE-HPLC/FFA@MNP	72.3 to 119.9	4.8 to 11.8	0.001 to 0.05	This work

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4. Conclusion

In this research, superhydrophobic Fe₃O₄ nanoparticles with free fatty acids from waste cooking oil (FFA@MNP) were successfully synthesized and applied for the enrichment of trace level PAHs from leachate and sludge samples. The obtained detection limits of Flu, FLT, Pyr, Cry and BaP were in the range of 0.001 to 0.05 ng mL⁻¹, and good recoveries were obtained in ranges of 72.3–119.9% and 82.8–116.6% for spiked sludge and leachate samples, respectively. Moreover, the novel FFA@MNP adsorbent exhibited good reproducibility and environmental stability for the detection of trace PAHs from environmental samples. With these remarkable features, FFA@MNP shows great potential as an MSPE adsorbent for the enrichment and analysis of trace hydrophobic organic pollutants in environmental samples.

Acknowledgements

The authors would like to take this chance to express their appreciation to the University of Malaya for a research grant (RP020A-16SUS) and IPPP grant PG042-2015A. The authors also gratefully acknowledge the School of Bioprocess Engineering, University of Malaysia Perlis for providing a fellowship to the one of the authors, Siti Khalijah Binti Mahmad Rozi. The authors thank Dr Cheng Sit Foon and Miss Kok Wai Ming for their help with fatty acid analysis.

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