Summary
In this study, single-walled carbon nanotubes (SWNTs) were used to determine organochlorine pesticides (chlorothalonil and pentachloronitrobenzene) in water using dispersive solid-phase extraction (DSPE), followed by gas chromatography (GC). The optimal adsorption conditions were determined by analyzing the effect of adsorbent dosage, adsorption time, eluent type and volume, and elution time. Under the optimal conditions, a good linearity was obtained at concentrations from 10 to 400 μg L−1 with correlation coefficients ranging from 0.9991 to 0.9986. The limits of detection (LOD) for the two organochlorine pesticides were 0.025 and 0.049 μg L−1, and the limits of quantification (LOQ) were 0.080 and 0.156 μg L−1, respectively. The accuracy of the proposed method was evaluated by measuring the recovery of the spiked samples, which ranged from 82.5% to 110.5% at spiking levels of 0.5–10 μg L−1 with relative standard deviations lower than 5.6% (n = 6). This method was successfully applied to determine the target analytes in canal water, drinking water, and water taken from the inlets and outlets of a wastewater treatment plant. The results demonstrate that the developed method has great potential for determining the two organochlorine pesticides in water samples.
Introduction
Organochlorine pesticides (OCPs) are highly-stable and fat-soluble synthetic insecticides that persist in the environment and pose health risks to humans and other organisms when they enter the food chain via biological enrichment [1]. To ensure environmental safety and health, it necessary to establish a convenient, fast, and accurate detection method to effectively control organic chlorine resides in the environment [2].
Currently, organochlorine pesticide detection methods mainly include gas chromatography (GC) [3, 4], GC-MS [5, 6], and HPLC-MS/MS [7, 8]. Of these, GC has been widely used for pesticide residue analysis due to its rapid analysis. Sample pretreatment is necessary when analyzing environmental samples, and the main pretreatment methods for detecting and analyzing organochlorines in water samples are liquid-liquid extraction [9, 10], solid-phase extraction (SPE) [11], and Soxhlet extraction [12]. SPE is a relatively new sample pretreatment technique that is used to separate, purify, and concentrate samples. Compared with traditional liquid-liquid extraction, SPE effectively separates analytes from interfering components, improves the analyte recovery rate, and simplifies the sample pretreatment process. It is widely used in food, environment, medicine, chemical, and other industries [13–15]. Dispersive solid-phase extraction (DSPE) is a novel sample preparation technique that offers advantages such as simplicity, low solvent use, safety, and automation by adding a sorbent to the extract to remove matrix contaminants. Recently, DSPE has been successfully used to purify liquid samples and extracts [16–19]. Here, single-walled carbon nanotubes (SWNTs) were prepared by an arc discharge method, and their use as a dispersive solid-phase extraction adsorbent for OCPs was studied. This is the first time single-walled carbon nanotubes was used for the dispersive solid-phase extraction; (2) chlorothalonil and pentachloronitrobenzene in water samples could be completely extracted by single-walled carbon nanotubes; (3) single-walled carbon nanotubes as adsorbents for DSPE coupled with GC were developed for determination of chlorothalonil and pentachloronitrobenzene in water sample; and (4) satisfied sensitivity and recoveries were obtained.
Experimental
Instrumentation
Chromatographic analysis was performed using a Shimadzu GC instrument coupled with an electron capture detector (ECD) (Shimadzu Co., Japan). An Extrapid SPE apparatus from Beijing Labtech Instruments Co., Ltd. was used. Scanning electron microscopy (SEM) micrographs were taken using a Hitachi S-4800 500/VP SEM. FT-IR spectra (4000-400 cm−1) were recorded on a 1,600 series Perkin-Elmer FTIR Spectrometer (MA, USA) using KBr pellets. Nitrogen adsorption/desorption isotherms were obtained using an Autosorb-iQ-MP automatic gas adsorption analyzer.
Chemicals and reagents
SWNTs were purchased from Beijing Deke Daojin Science and Technology Co., Ltd., China, its specific surface area of SWNTs is 96.9 m2 g−1, the pore volume is 0.42 cm3 g−1, and the average pore size is 14.7 nm. SWCNTs was ultrasonic dispersion in 12 mol L−1 HCl for 30 min, the lower layer sample was washed with deionization for several times and pH = 6–7 of the supernatant.The black flake sample was obtained at 60 °C vacuum drying for experimental, 50 mg of dried sample was measured and added with 40 mL of 8 mol L−1 HNO3 and 9.2 mol L−1 HSO4. The above operation was repeated to obtain SWCNTs purified by mixed acid.
Chlorothalonil (CTL) and pentachloronitrobenzene (PCNB) with a 99.5% purity and a concentration of 100 mg L−1 were purchased from the Agro-Environmental Protection Institute, Ministry of Agriculture, China. Methanol, ethanol, acetone, tetrahydrofuran, and acetonitrile were purchased from Fisher Scientific, USA. Microporous membranes (0.22 μm) were obtained from Dikma Technologies (Beijing, China).
Standard curve configuration
Standard solution configuration: 1 mL of CTL standard solution with a concentration of 1,000.0 mg L−1 was diluted to 100 mL with n-hexane and and the concentration was 1 mg L−1. The standard solution of PCNB with a concentration of 1 mg L−1 was prepared using the same method. The CTL and PCNB standard solutions were mixed and used on-site.
Sample preparation
A natural water sample (20 mL) was placed in a 50 mL centrifuge tube. Then, 160 mg of SWNTs were added, and the mixture was vortexed for 5 min, centrifuged for 5 min at 10,000 r min−1, and then the supernatant was discarded. 5 mL ethyl acetate was added into the centrifuge tube, shaken for 5 min, and centrifuged at 10,000 r min−1 for 10 min. 1 mL of the supernatant was extracted and blown dry with nitrogen in a water bath at 20 °C. The resulting residue was re-dissolved in 1 mL n-hexane, then passed through a 0.22 μm microporous membrane filter for GC analysis.
Discussion
Adsorption of OCP pesticides by SWNTs
First, 5 mL of mixed standard solutions of CTL and PCNB with concentrations of 5 μg L−1 were prepared, and various amounts of SWNTs were added. After vortex oscillation and centrifugation, 1 mL of the supernatant was extracted and dried with nitrogen in a water bath at room temperature. Then, 1 mL of n-hexane was used to re-dissolve the residue and was then filtered through a 0.22 μm microporous membrane for GC analysis.
Selection of adsorbent dosage
Adsorbent dosage is one of the main factors that affects DSPE. Different amounts of SWNTs (100, 120, 140, 160, 180, and 200 mg) were added to 5 mL of a mixed standard solution containing 50 μg L−1 organochlorine, then subjected to vortex oscillation for 3 min and centrifugation for 5 min at 10,000 r min−1. The supernatant was extracted and passed through a 0.22 μm microporous membrane filter, and then analyzed by GC. The GC analysis results (Fig. 1) show that when the amount of adsorbent was between 100 and 140 mg, the removal rate of organochlorine in the mixed standard solution gradually increased with an increasing amount of SWNTs, but OCPs were still detected. When the adsorbent dosage was increased to 160 mg, the removal rate of CTL reached 88%, and the removal rate of quintozene reached 73%. The removal rate of the two OCPs remained unchanged when the absorbent dosage was further increased, indicating that 160 mg SWNTs was the optimal adsorbent amount. Therefore, 160 mg SWNTs was used for subsequent DSPE experiments.
Effect of different amounts of SWCNTs on removal of two OCPs
Citation: Acta Chromatographica Acta Chromatographica 33, 2; 10.1556/1326.2020.00840
Selection of pH
CTL and PCNB can slowly hydrolyze in alkaline solution, so the effect of pH (3, 4, 5, 6, 7) on the removal rate of the two OCPs was investigated (Fig. 2). When pH was between 3 and 5, the removal rate of CTL and PCNB in the mixed standard solution gradually increased with the solution pH. When the pH increased to 5, the removal rate of CTL and PCNB reached a maximum and began to slowly decline when it further increased to 7. SWNTs readily accept protons at low pH values that competitively bind to the adsorbent. As the pH increased, the positive charge density on the surface of SWNTs gradually decreased, thus increasing the electrostatic attractions between the adsorbent and CTL, as well as PCNB. The adsorption rapidly increased, showing that the optimal pH was 5.
Effect of pH on removal of two OCPs
Citation: Acta Chromatographica Acta Chromatographica 33, 2; 10.1556/1326.2020.00840
Effect of adsorption time
Using the optimized test conditions, the effects of adsorption time (1, 3, 5, 7, 9 min) on the removal of target compounds was investigated. Fig. 3 shows that the removal rate of CTL and PCNB gradually increased with the shaking time from 1 to 3 min. At 3 min, the removal rate of PCNB reached 89.4%, and continuing to increase the shaking time did not further increase the removal rate of PCNB. When the shaking time was 5 min, the removal rate of CTL reached 95.6%, and further increasing the adsorption time did not increase the CTL adsorption amount. When 160 mg SWNTs was used as the adsorbent, equilibrium adsorption of PCNB was reached in 3 min, while that of CTL was reached at 5 min.
Effect of different adsorption time on removal of two OCPs
Citation: Acta Chromatographica Acta Chromatographica 33, 2; 10.1556/1326.2020.00840
Selection of eluent types
160 mg SWNTs powder was added to four different standard solutions (5 mL, 5 μg L−1) containing a mixture of CTL and PCNB. The solutions were shaken for 5 min, then centrifuged for 5 min at 10,000 r min−1, filtered, and the supernatant was discarded. 3.0 mL acetone, n-hexane, ethyl acetate, and acetone + n-hexane (V/V = 1:1) were used as eluents, shaken for 10 min, and centrifuged. 1 mL of the supernatant was extracted and placed in a water bath at 40 °C to dry. The effect of the four eluents on the recovery of the target compound was investigated, and the results are shown in Fig. 4. Hexane was used to re-dissolve the residue for GC analysis to investigate the effects of the four different eluents on the target recovery. Using ethyl acetate as the eluent showed the highest recovery rate.
Effect of different eluenting sovlent on recovery of two OCPs
Citation: Acta Chromatographica Acta Chromatographica 33, 2; 10.1556/1326.2020.00840
Selection of eluent volume
Using the optimized conditions, the effects of different volumes of ethyl acetate eluent (1, 3, 5, 7, and 9 mL) on the recovery of the two OCPs were investigated. The results show that as the eluent volume increased, the recovery rates of the two OCPs also gradually increased. When the volume increased to 3 mL, the CTL recovery rate reached a maximum of 97.2%, and when the volume continued to increase to 5 mL, the recovery of PCNB reached a maximum. When the volume increased to 9 mL, the recoveries of the two OCPs did not increase further, so 5 mL ethyl acetate was chosen.
Selection of elution time
According to the optimized conditions, the effects of elution time (2, 4, 6, 8, and 10 min) on the recovery of the two pesticides were investigated. The results show that as the elution time increased, the recovery rate gradually increased. When the elution time was 8 min, the recovery rate reached a maximum, and further increasing the elution time did not further increase the recovery rate. Thus, an elution time of 8 min was used to maximize experimental efficiency.
Adsorption mechanism
According to the experimental results, CTL and PCNB were adsorbed on the surface of SWCNTs through π-π interactions. PCNB contain a nitro group, which can easily form positively-charged ions that are adsorbed by SWCNTs. Additionally, intermolecular hydrogen bonds formed between the hydroxyl and carboxyl groups of SWNTs, as well as between CTL and PCNB at the end of SWCNTs. These further enhanced adsorption of the target compounds.
Method validation
Standard curve, quantitative limit, and detection limit
A standard solution (0.1 mL) containing 100 mg L−1 PCNB was placed in a 10 mL volumetric flask. A standard solution with a concentration of 1 mg L−1 CTL was also prepared. The standard solution containing CTL and PCNB was diluted with n-hexane, and a series of standard solutions were prepared with mass concentrations of 10.0, 20.0, 40.0, 80.0, 100.0, 200.0, and 400 μg L−1. The standard solution containing CTL and PCNB was analyzed using GC using the mass concentration of the two OCPs as the x-axis and the instrument response as the y-axis to obtain a linear curve. The limits of detection (LOD) and limits of quantification (LOQ) with 3-times and 10-times signal-to-noise ratios were calculated, respectively, and the results are shown in Table 1.
Linear equation, linear range, r, LOD, LOQ of two OCPs
| Analyte | Linear equation (µg L−1) | Linear range (µg L−1) | r | LOD (µg L−1) | LOQ (µg L−1) | RSDP | RSDA |
| CTL | Y = 12,647.613X−83.45 | 10–400 | 0.9991 | 0.025 | 0.080 | 4.6 | 3.9 |
| PCNB | Y = 6,354.512X−36.42 | 20–200 | 0.9986 | 0.049 | 0.156 | 5.1 | 4.8 |
Precision
A mixed standard solution (5 mL) of PCNB and CTL was prepared at a concentration of 1.0 μg L−1. An actual water sample was used to prepare a standard sample with a concentration of 5 μg L−1, and 6 sample solutions were prepared in parallel according to the optimized conditions. The response values of the sample were used to calculate the content, and the RSD was calculated and shown in Table 2.
The average recoveries and RSDs of two OCPs spiked in environmental water samples (n = 6)
| Analyte | Spiked (µg L−1) | Drinking water | Canal water | Wastewater treatment plant | |
| Inlets | Outlets | ||||
| CTL | 0.5 | 93.2(4.6) | 84.6(4.4) | 106.3(5.3) | 86.4(3.4) |
| 5.0 | 88.9(3.6) | 97.5(3.6) | 95.3(2.6) | 105.4(4.8) | |
| 10.0 | 86.3(5.1) | 103.4(5.4) | 92.0(5.6) | 96.5(5.1) | |
| PCNB | 0.5 | 105.3(4.2) | 110.5(3.6) | 83.2(5.0) | 91.8(2.1) |
| 5.0 | 83.2(3.5) | 95.7(3.5) | 82.5(3.6) | 102.1(3.5) | |
| 10.0 | 95.6(4.4) | 82.8(5.2) | 87.62(3.3) | 89.6(5.2) | |
Accuracy
Real water samples were used, and 20 mL of spiked samples with concentrations of 0.5, 5.0, and 10.0 μg L−1 for a total of 15 samples were prepared according to the optimized conditions. The peak area concentrations of the target compounds were calculated, and the RSD of each sample was calculated six times in parallel. The results in Table 2 show that the average recoveries of CTL and PCNB were 82.5–110.5%, and the RSD ranged from 2.1 to 5.6%, respectively.
Analysis of real water samples
The optimized analysis method was used to analyze, drinking water, canal water, inlet water, and outlet water at a sewage treatment plant. Blank tests and water-recovery tests were also conducted. CTL was detected in the water sample from the sewage treatment plant at a concentration of 0.064 μg L−1, which was lower than the quantitative detection limit of the method. The concentration of the added sample was 1.0 μg L−1, and the calculated recovery rate was 96.8%. The results are satisfactory and indicate that the established method was accurate and reliable and can be used to detect and analyze OCPs in environmental water samples. The chromatogram of the mixed standard solution (a), spiked samples (b), and real water samples (c) are shown in Fig. 5.
GC chromatogram of the target in mixed standard solution(a), spiked samples (b) and real water samples(c) CTL (tR = 8.43), PCNB (tR = 8.88)
Citation: Acta Chromatographica Acta Chromatographica 33, 2; 10.1556/1326.2020.00840
Conclusion
This is the first time single-walled carbon nanotubes as adsorbents was used for the dispersive solid-phase extraction coupled with GC were developed for determination of CTL and PCNB in water sample; CTL and PCNB in water samples could be completely extracted by single-walled carbon nanotubes; satisfied sensitivity and recoveries were obtained. These results indicate that the proposed method can be used for the efficient determination of trace organochlorine pesticides in various water samples.
Funding
Heilongjiang Reclamation Area General Bureau Support Program (HKKYZD190717)
Heilongjiang Bayi Agricultural University Support Program for San Heng San Zong (No. ZRCQC201910, No. PTJH201905).
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This work was supported by the Key Laboratory of Functional Inorganic Materials Chemistry.
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