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Determination of Sudan red contaminants at trace level from water samples by magnetic solid-phase extraction using Fe@NiAl-layered double hydroxide coupled with HPLC
Environmental Sciences Europevolume 31, Article number: 34 (2019)
Sudan red dyes are widely used as color additions in various fields. Due to their potential carcinogenicity and large consumption, they have absorbed much more attention and become a research hot topic. There have been a few of existing methods for their analysis from samples; however, it is of great importance to develop simple, sensitive, and low-cost analytical method for public health and safety. This article described a new method based on magnetic Fe@NiAl-layered double hydroxides for enrichment of Sudan red dyes in combination with high-performance liquid chromatography.
The as-prepared magnetic Fe@NiAl-layered double hydroxides (Fe@NiAl-LDHs) exhibited good adsorption capacity for Sudan red dyes, and could be used as an effective adsorbent for preconcentration of three Sudan Red dyes from environmental water samples. The proposed method provided low limits of detection in the range of 0.002–0.005 μg L−1 with magnetic solid-phase extraction in combination with high-performance liquid chromatography.
The developed method utilized the easy to prepare magnetic materials as adsorbents and conventional analytical instrument and provided high sensitivity, which provided a robust and low consumption of toxic organic solvent alternative method. The results of present study are of great value for interested laboratories to monitor the level of Sudan red dyes in water samples or develop sensitive determination methods to monitor other pollutants at trace level in environmental waters.
Sudan Red dyes are lipophilic orange-red azo dyes [1, 2] and widely applied as color agents in various fields such as food, oils, waxes, shoe polishes, and plastics due to their simple synthetic method, low cost, and favorable availabilities [3,4,5,6]. Nevertheless, Sudan Red dyes are regarded as category 3 carcinogenic by the International Agency for Research on Cancer (IARC) and can cause genetic mutations as well as cancer . Many countries have banned the use of Sudan dyes as food additives [8, 9]. However, Sudan Red dyes are often found in food products as an additive to enhance the appearance of the food products such as chili powder, curry, chili sauces, curcuma, sausage, and so on [10,11,12]. Hence, it has become a hot topic attracting the public attention to detect and evaluate the safety of Sudan dyes.
Until now, plenty of analytical methods have been developed for the determination of Sudan Red dyes such as fluorescence quenching , electrochemical methods [14, 15], surface-enhanced Raman scattering method [16, 17], capillary electrophoresis , high-performance liquid chromatography–mass spectrometry (LC–MS) , ultra-performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) , high-performance liquid chromatography (HPLC) [21, 22], and so on. In general, it is difficult to directly analyze the target analytes because they are present in environment at low concentration and the sample matrix are complex. Hence it is of great value to develop simple and effective sample pretreatment technologies. At present, several sample pretreatment technologies have been established such as supercritical-fluid extraction, liquid–liquid microextraction, size-exclusion chromatography, solid-phase microextraction, solid-phase extraction, dispersive solid-phase extraction, and so on [23,24,25,26,27,28,29,30,31,32,33,34]. Among them, solid-phase extraction (SPE), an effective technology in environment sample pretreatment, has been widely used for the enrichment of environmental pollutants on account of its simplicity, good enrichment efficiency, and less consumption of toxic solvent .
Nowadays, magnetic solid-phase extraction (MSPE), a new mode of SPE, has absorbed more attention on a consequence of its favorable characteristics such as high extraction efficiency, simply separation, and saving time [36, 37]. It has been widely used for the preconcentration and detection of many environmental pollutants such as phenols, polycyclic aromatic hydrocarbons, 2,4,6-trinitrotoluene, and phthalate esters [38,39,40,41]. As an effective and promising pretreatment technique, magnetic nanoparticles (MNPs) are the core factor. In conventional MSPE, Fe3O4 is the often-used magnetic core of the adsorbent material and little attention has been put on nanoscale zero-valent iron (NZVI). Noticeably, NZVI has recently drawn scientific researchers’ attention on account of its small particle size, high reactivity, and large surface area . Nonetheless, the bare NZVI is unstable and easily oxidized, and so it is of great significance to modify surface to improve the stability of NZVI and increase the extraction efficiency.
During recent years, layered double hydroxides (LDHs), also named anionic clays, have drawn tremendous interest owing to its potential applications electrocatalysts, photocatalysts, adsorbents, ion-exchangers [43,44,45,46], etc. LDHs can be defined as the following formula [M(II)1−xM(III)x(OH)2]x+[An−]x/n·mH2O. M(II) represents divalent cations (Ca2+, Mg2+, Zn2+, Mn2+, Ni2+, etc.), M(III) represents trivalent cations (Al3+, Cr3+, Fe3+, etc.), An− represents interlayer anions (Cl−, NO3−, ClO4−, CO32−, etc.), respectively, while x is the molar ratio of M(III)/[M(II) + M(III)] ranging from 0.17 to 0.33 [47, 48]. Structurally, LDHs consist of stacked brucite-like M2+(OH)2 hydroxide layers. In the layers, some of the M2+ ions are replaced by M3+ ions through isomorphous substitutions. Anions are needed between adjacent layers to maintain charge neutrality . The special structure of LDHs results in excellent properties including large surface area, high anionic exchange capability, high porosity, thermal stability, good dispersion , etc.
Up to now, there are few reports on the enrichment of Sudan dyes with LDHs. In this work, a novel composite Fe@NiAl-LDHs was synthesized to combine the merits of magnetic NZVI and NiAl-LDHs by a new two-step method. This composite possessed comprehensive advantages both easy separation of magnetic materials and large adsorption capacity of LDHs. The Fe@NiAl-LDHs were investigated as a MSPE adsorbent for the enrichment of Sudan Red II, Sudan Red III, and Sudan Red IV. The parameters influencing the extraction efficiency including eluent, amount of adsorbents, adsorption time, elution volume, elution time, pH, ionic strength, humic acid concentration, and sample volume were optimized.
Materials and apparatus
Ferric chloride hexahydrate (FeCl3·6H2O) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Hydrazine hydrate (N2H4·H2O, > 98%), Sudan Red II, Sudan Red III, and Sudan Red IV were obtained from Aladdin Chemistry Co., Ltd (Shanghai, China). Sodium hydroxide was obtained from Tianjin Guangfu Technology Development Co., Ltd (Tianjin, China). Sodium carbonate, ethanol, and acetone were obtained from Beijing Chemical Works (Beijing, China). Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and aluminum nitrate nonahydrate (Al(NO3)3·9H2O) were obtained from Beijing Yili Chemical Co., Ltd (Beijing, China). Methanol and acetonitrile (HPLC grade) were obtained from J&K Scientific Co., Ltd (Beijing, China). The deionized water was used in all experiments.
A high-performance liquid chromatography (HPLC) system (Shimadzu, Kyoto, Japan) equipped with a UV detector (SPD-20A) and a pump (LC-20AD) was used for analysis in all experiments. An Agilent InertSustain C18 (150 × 4.6 mm, 5 μm) was used for separation and the mobile phase was acetonitrile with a flow rate of 1.0 mL min−1. The detection wavelength was set at 520 nm and injection volume set at 20 μL. Transmission electron microscopy (TEM) images were obtained with a JEM-2100 Transmission Electron Microscope (Japan). X-ray diffraction (XRD) was measured with a Bruker D8 Advance Diffractometer (Germany).
Synthesis of Fe@NiAl-LDHs
The Fe@NiAl-LDHs were prepared via a simple two-step method. Specifically, The Fe nanocubes were prepared according to a previous report with minor adjustment . The preparation procedure was as follows: 2.4 g FeCl3·6H2O was dissolved in 12 mL ethanol, and 3.0 g NaOH and 4.8 mL N2H4·H2O were added into the reactor under vigorous mechanical stirring. Then, the mixture was sealed in a 50 mL autoclave and heated at 80 °C for 10 h. After that, cooled to room temperature, the particulate materials were collected by a magnet and washed with water and ethanol for several times. Finally, the obtained materials were dried in vacuum at 50 °C overnight.
The Fe@NiAl-LDHs were synthesized by co-precipitation method. 300 mg Fe nanotubes, 812 mg Ni(NO3)2·6H2O, and 525 mg Al(NO3)3·9H2O were dissolved in 100 mL deionized water and the mixture was ultrasonically for 20 min. Then a mixed solution containing NaOH and Na2CO3 was added dropwise into the above-mentioned mixture under vigorous mechanical stirring to adjust the pH at about 10. After that, the solution was stirred for another 2 h at 60 °C. Eventually, the black powders were collected by a magnet and washed with deionized water for several times and dried in vacuum at 50 °C overnight.
First of all, 50 mg Fe@NiAl-LDHs nanoparticles were dispersed into 50 mL deionized water containing target compounds at a concentration of 10 μg L−1. The suspension was stirred for 1 h. Secondly, the adsorbent was separated from liquid phase via putting a magnet under the bottom of the beaker and the supernatant was taken out. Then, the target contaminants were eluted for 9 min from the adsorbent with 6 mL acetone. After that, the eluent was separated by a magnet and blown to near dryness with nitrogen gas flow. Eventually, the residue was redissolved in 200 μL methanol, and 20 μL was injected into HPLC for analysis. The whole experimental procedure is shown in the following Fig. 1.
Results and discussion
Characterization of Fe@NiAl-LDHs
Figure 2a shows the TEM image and corresponding EDS spectrum of Fe@NiAl-LDHs. As revealed by the image, Fe nanocubes were about 300 nm in diameter. The surface of Fe nanoparticles was uniformly covered with NiAl-LDHs and its diameter was about 350–400 nm. The element composition of material was analyzed by EDS. As can be seen from Fig. 2a, the material contained N, O, Al, Fe, and Ni elements. Figure 2b displayed the XRD patterns of NiAl-LDHs, Fe0, and Fe@NiAl-LDHs. For NiAl-LDHs, four evident diffraction peaks at about 2θ = 11.3°, 23.2°, 35.5°, and 61.7° were similar to a previous report . As Fe0 was concerned, three obvious diffraction peaks were at about 2θ = 44.8°, 65.2°, and 82.7°, which was quite consistent with a previous study . Diffraction Peaks of iron oxides or hydroxides were undetected. Thus, Fe0 was synthesized as expected. For Fe@NiAl-LDHs nanoparticles, five remarkable diffraction peaks at about 2θ = 11.3°, 35.5°, 44.8°, 65.2°, and 82.7° illustrated that NiAl-LDHs were successfully coated on the surface of Fe nanocubes. Diffraction peaks at about 2θ = 23.2° and 61.7° were not obvious. The possible reason was that the two diffraction peaks were too weak in comparison with the diffraction peaks of Fe0. These data demonstrated that magnetic Fe@NiAl-LDHs nanoparticles were successfully prepared.
Optimization of MSPE
Eluent plays an important role in the procedure of MSPE. It is crucial to choose the best eluent because the physical and chemical properties of different organic solvents will put a profound effect on the elution efficiency. In order to achieve much higher elution efficiency, four eluent solvents including methanol, ethanol, acetone and acetonitrile were investigated. The results were shown in Fig. 3a. It was obviously that four organic solvents resulted in different preconcentration performance, and the largest peak area was obtained with acetone as the eluent. The enrichment performance of ethanol was better than that of methanol. As acetonitrile was concerned, it resulted in better performance for Sudan Red III, similar performance for Sudan Red I and poor performance for Sudan IV than that with ethanol as the eluent. The reason may be that the polarity of acetone is the smallest among the four solvents, and the polarity of the three Sudan Red dyes is very poor and which make them have higher solubility in acetone based the principle of similarity. Therefore, acetone was chosen for use in subsequent experiments.
In general, the dosage of adsorbent is one of the most important factors which influence the enrichment efficiency, and which was investigated in the range of 40–100 mg. Figure 3b shows that the enrichment efficiency increased with the increase of adsorbent dosage, and the largest peak area of all analytes was achieved when 80 mg adsorbent was used. After that, the enrichment efficiency changed very little with continuous increase of the adsorbent dosage. This may contribute to the fact that the increase of adsorbent dosage resulted in the increase of the active adsorption sites, and further led to the increase of the adsorbed amount of analytes. However, it got equilibrium between adsorbent and target contaminants when the adsorbent dosage increased to 80 mg. With the continuous increase of adsorbent dosage, the enrichment performance will keep the constant. Hence, 80 mg adsorbent was used in the following experiments.
Adsorption time is another significant factor affecting the enrichment process. In order to investigate the effect of adsorption time, adsorption time was investigated in the range of 20–100 min. As shown in Fig. 3c, the peak area increased with the increase of adsorption time up to 60 min and kept constant when the adsorption time was over 60 min. In order to achieve best enrichment efficiency as well as saving time, 60 min was used in the following experiments.
In the elution procedure, both volume of eluent and elution time are two important factors, which were optimized in the range of 3–9 mL and 3–15 min, respectively. The results were exhibited in Fig. 3d, e. From the two figures, it was found that the maximum peak area was obtained with 7.5 mL eluent eluting for 9 min. Afterwards, the peak area changed very few with the increase of eluent volume and elution time.
It is known that sample pH is vital in MSPE process, because it can affect the existing states of adsorbent and analytes, and further affect the enrichment performance. Sample pH was optimized in the range of pH 3–11. Figure 3f shows that largest peak area of the analytes was achieved at pH 7. There was a definite possibility that the adsorbent was dissolved under acidic condition and the structure of NiAl-LDHs was broken when the pH was lower than pH 7, while the pH was at a higher value, the target contaminants were easy to be ionized and the solubility of analytes in water increased. Hence pH 7 was used in the followed experiments.
Generally, the solubility of analyte is easily influenced by ionic strength of the solution. For this reason, we can change ionic strength to lower the solubility of target contaminant to improve the adsorption efficiency. A series of experiments were designed to investigate the effect of ionic strength on adsorption efficiency at five levels in the range of 0–20% (w/v) with the addition of NaCl to adjust the ionic strength of solution. The results are shown in Fig. 3g. Figure 3g shows that the peak areas of analytes increased with the increase of NaCl concentration from 0 to 15%. The reason may be that the addition of NaCl led to the salting-out effect, which decreased the solubility of Sudan dyes in aqueous solution. The enrichment efficiency decreased with the concentration of NaCl increasing to 20%, which may be due to the fact that the viscosity of solution increased which decreased the migration rate of the analytes onto the surface of adsorbent and less of target analytes were adsorbed onto the adsorbent in the same time interval. Hence, the concentration of NaCl was set at 15% (w/v).
Humic acid is usually present in almost all nature waters. The existence of humic acid may cause a certain influence on the adsorption of analytes by competitive adsorption. A series of concentrations of humic acid in the range of 0–20 μgmL−1 were investigated. Figure 3h illustrates that the peak areas of analytes had no obvious change with different concentrations of humic acid. It was very probable that adsorbent had stronger adsorption capacity to Sudan dyes than humic acid. Therefore, no humic acid was added in order to save time.
Sample volume is also an important factor for achieving higher sensitivity. In this experiment, sample volume was optimized in the range from 30 to 110 mL and the result is shown in Fig. 3i. It was found that the peak areas of analytes had a sight increase as the sample volume increased from 30 to 70 mL. After that, the peak areas decreased as the sample volume increased from 70 to 110 mL, which was due to that the amount of adsorbent per volume decreased and the migration distance to the surface of adsorbent increased, which led to the decrease of peak areas in limited time. Therefore, 70 mL was utilized in later experiments.
Analytical performance of the proposed method
Under the optimal experimental conditions, analytical parameters of this proposed method including linearity, limits of detection, and precisions were investigated. The results are listed in Table 1. Excellent linearity was observed in the range of 0.01–300 μg L−1 with good correlation coefficients (r2 ≥ 0.997) for three Sudan dyes. The limits of detection (LOD, S/N = 3) were 0.004 μg L−1 for Sudan Red II, 0.002 μg L−1 for Sudan Red III, and 0.005 μg L−1 for Sudan Red IV, respectively. The precisions (RSD, n = 6) were 2.81% for Sudan Red II, 1.21% for Sudan Red III, and 5.07% for Sudan Red IV, respectively.
Real sample analysis
The proposed method was evaluated with three different natural water samples including Changping Park water, Ming Tombs Reservoir water, and Binhe Park water. The results are listed in Table 2 and the typical chromatograms of water samples are shown in Fig. 4. Sudan dyes were not detected in all three natural water samples. The mean spiked recoveries were all in the range of 97.6–105.7%. Besides, the proposed method was compared with other previous reported methods for the enrichment and determination of Sudan dyes from the viewpoint of linearity range and LOD. The results are depicted in Table 3. It is obvious that present method possessed wider linearity range and lower LODs. Based on the experimental results, the developed method based on Fe@NiAl-LDHs had good practicability in the enrichment and determination of Sudan Red contaminants in water samples.
In present work, Fe@NiAl-LDHs, a kind of novel magnetic nanoparticles, was firstly synthesized and used in magnetic solid-phase extraction prior to HPLC–UV for the enrichment and determination of Sudan dyes in environmental water samples. The developed method provided an excellent linearity in the range of 0.01–300 μg L−1 and low LODs in the range of 0.002–0.005 μg L−1 for three Sudan dyes. Fe@NiAl-LDHs exhibited excellent enrichment capability to Sudan Red dyes and the developed MSPE-HPLC method possessed advantages such as simplicity, fastness, easy to separate, and high enrichment efficiency compared with other previous methods, which can match the demand of determining trace Sudan dyes in natural water samples, and would be a good alternative tool for the enrichment and detection of Sudan dyes contaminants in environmental water samples.
Availability of data and materials
Fe@NiAl-layered double hydroxides
high-performance liquid chromatography
international agency for research on cancer
high-performance liquid chromatography–mass spectrometry
ultra-performance liquid chromatography–tandem mass spectrometry
magnetic solid-phase extraction
nanoscale zero-valent iron
layered double hydroxides
limits of detection
high-performance liquid chromatography-ultraviolet detector
magnetic solid-phase extraction-high-performance liquid chromatography
dispersive liquid–liquid microextraction
microwave-assisted liquid–liquid microextraction
ultrafast liquid chromatography
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The work was supported by the National Natural Science Foundation of China (21377167, 21677177).
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