© Wuhan University 2022

0 Introduction

Perfluorinated compounds (PFCs) have been synthesized and widely applied in different industrial and commercial fields such as surfactants, coatings, water repellents for leather and textiles, metal plating and fire-fighting foams^{[1, 2]}. The high-energy C—F bonds convert PFCs into non-biodegradable, highly persistent and bio-accumulative compounds. Among them, perfluorooctanoic acid (PFOA) is one of the most abundant PFCs in the environment, and it is also the final decomposition of many PFCs^[3]. At present, there is no special regulation for the disposal of the products containing perfluoro compounds^[4]. When the service life of these products is over, most of them will be discarded into the landfill or incineration plant. The landfill leachate is an important source in PFCs, no matter whether it has been processed or not^[5]. In addition, the annual dry weight of municipal sewage sludge in China is 8.5 million tons, including 18 ng·g^-1 perfluorooctane sulphonate (PFOS) and 8.3 ng·g^-1 PFOA, and nearly 35% of the sludge will be discharged to the landfill^[6]. Leachate from municipal solid waste (MSW) incineration power plant is also susceptible to contain PFOA.

In general, the concentrations of PFOA in the water environment are mostly in level of ng·L^-1 or μg·L^-1. When the PFOA concentration is below the detection limit of some methods, it is necessary to enrich the PFOA in the sample. The solid phase extraction (SPE) is a sample preparation process, and it can separate dissolved or suspended compounds from other compounds in a liquid mixture according to their physical and chemical properties. SPE is the most common approach employed for sample preparation. At present, the analytical methods of trace PFCs in environmental samples have been widely studied, but they are usually based on a single matrix^[7-9]. Leachate has complex composition and high chromaticity. It was reported that the higher chroma of the extract is not conducive to the detection of perfluoroalkyl compound (PFASs) when using SPE^[10]. Even though several studies about PFCs have been involved in the improvement of SPE conditions, they may not be suitable for PFOA or MSW leachate. There is a lack of systematic research on the pretreatment and analysis of perfluorinated compounds in complex matrixes, such as landfill leachate samples^[11].

Therefore, the aim of this study is to optimize the conditions of SPE and obtain relatively pure PFOA in raw and treated leachate samples. The conditions include the types of SPE cartridge, activation reagent, pH value, ionic strength and eluent.

1 Materials and Methods

1.1 Chemicals and Reagents

Reagents in High Performance Liquid Chromatography (HPLC) grade including acetonitrile (MeCN), methanol (MeOH), formic acid (FA), and ammonium acetate were purchased from Sinopharm Chemical Reagent Co., Ltd, China. PFOA (98%) was purchased from Aladdin Bio-Chem Technology Co., Ltd, China. Reagents in AR grade such as ammonia (NH₃·H₂O) and potassium chloride (KCl) were purchased from Cologne Chemical Co., Ltd, Chengdu, China. Weak anion exchange (WAX) (6 cc, 200 mg, 50 mm) solid phase extraction and hydrophilic-lipophilic balance(HLB)(6 cc, 200 mg, 30 mm) SPE cartridges were acquired from Waters (Milford, MA, USA). Bulk C18 SPE sorbent (40-60 μm, 60 A) was purchased from Jinan Bona Biological Technology C o., Ltd, China. Pure water was produced by Water Purifier system.

The HPLC system (LC200, SHIMADZU Co., Ltd, Japan) consisted of a multi-channel pump, an autosampler (set at 4 ℃), a column oven and a system controller coupled with a mass spectrometer consisting of a triple quadrupole, ion trap and ion source of electrospray ionization (ESI), ultrasonic cleaner, water purifier machine, pH-meter (PHS-3C), one over ten-thousand analytical balance (FA2204B) and light microscope.

1.2 Experimental Water Quality

Raw and treated leachate samples were collected from MSW incineration power plant in Chongqing, China between 2017 and 2018. Raw leachate samples (unfiltered, 2 L) were taken from the leachate lift station before the leachate was pumped for treatment; Treated leachate samples (unfiltered, 2 L) were taken from the terminal Disk-Tube Reverse Osmosis (DTRO) effluent. The water quality of treated leachate is shown in Table 1.

All samples were collected in polyethylene (PE) bottles pre-washed with methanol. Samples were then stored at 4 ℃ and extracted for analysis within two weeks of sampling. Polytetrafluoroethylene (PTFE)-based materials were avoided throughout the sampling and analysis to avoid potential sample contamination.

1.3 Chemical Analysis

1.3.1 Sample preparation

In order to remove large particles, the raw and treated leachate samples should be centrifuged at 12 000 r/min for 15 min. After centrifugation, the supernatants of leachate samples were spiked with external tagging prior to extraction using SPE method. Different subsample aliquots of leachate (50 mL for raw leachate and 500 mL for treated leachate) were used for SPE depending on the sample type, respectively. Extracts were then filtered (0.45 μm) and transferred to a 10 mL PP tube, and the final volume was adjusted to a given volume of methanol:ultrapure water (80:20, V/V) prior to injection.

1.3.2 HPLC-ESI-MS/MS analysis

All samples were analyzed by HPLC-MS/MS system. Chromatographic separation was performed using a Phenomenex Synergi Fusion C-18 (4 mm, 502 mm i.d.) column (Phenomenex, Torrance, CA, USA) equipped with the corresponding guard (42 mm i.d.) column (Phenomenex, Torrance, CA, USA). Aliquots of 5 μL were injected into the column operated at 30 ℃. The flow rate was set to 0.22 mL·min^-1. A mixture of 5 mmol/L ammonium acetate and methanol with a volume ratio of 65:35 (solvent A) and MeOH (solvent B) were applied as mobile phases.

The mass spectrometer was operated in electrospray negative ionization mode (ESIe). For target quantitative analysis, data acquisition was performed in multiple reaction monitoring (MRM) mode at desolvation temperature 340 ℃ and ion voltage 4 000 V. The MS conditions were optimized to provide the highest signal intensity. Quantification of target analytes was done using an external calibration curve of freshly prepared standards with a range 10-10 000 ng·mL^-1 (7 points).

1.4 Experimental Procedure

1.4.1 Single factor experiments

The first part of the study was the influence of single factor experiments such as types of SPE cartridge, pH value and ionic strength on extraction of PFOA in leachate samples, and it was analyzed by HPLC-MS/MS system. The methods were marked as follow:

SPE cartridge: Three types of SPE cartridges such as C18 (octadecylsilane), WAX (weak anion exchange) and HLB (copolymer) were studied. SPE cartridges were activated by 10 mL ammonia water methanol and 4 mL ultrapure water respectively, then 4 mL formic acid (2%) solution was used during leaching, with 5 mL ammonia methanol solution (0.1%) as eluent.

pH value: Before SPE, adjust the sample pH value to 3, 4, 5, 6, 7, respectively, then the extraction is carried out followed by the same steps as SPE cartridge.

Ionic strength: A certain amount of KCl was added to 500 mL PFOA aqueous solution to enhance the ionic strength before SPE. In this research, the dosage of KCl was 0, 50, 100, 200, 500, 2 000 mg. Each combination of parameters was tested in triplicate.

Manual SPE and injection were adopted. Figure 1 shows the full steps of the manual SPE. 50 mL was used as the sample volume when the sample was raw leachate and SPE 500 mL when the sample was DTRO effluent for the SPE in this research. For ion exchange SPE cartridges, the flow rate should not be greater than 2 mL·min^-1, while for others it should not exceed 5 mL·min^-1. At last, 5 mL ammonia water methanol solution was used for smooth elution of PFOA from cartridges.

Fig. 1 Manual solid phase extraction schematic diagram

The second part of the study was RSM optimization. The influence of three parameters on the SPE recovery of PFOA was assessed: i) volume of activator; ii) ionic strength; iii) volume of eluent. The Box-Behnken design (BBD) consists of 17 experimental runs for the three variables, the volume of activator ranging from 8 to 12 mL, and ionic strength as potassium chloride dosage ranging from 400 to 600 mg, and volume of eluent ranging from 4 to 6 mL. The Design-Expert 12 software was used for the design, mathematical modeling, statistical analysis, and optimization of the variables.

2 Results and Discussion

2.1 Activation Reagents

Reverse phase C18 SPE cartridges and ion exchange WAX SPE cartridges were used for the extraction of PFOA from leachate samples. Before the extraction, it was necessary to activate and rinse SPE cartridge by polar organic solvent to remove impurities and create a solvent environment. Polar organic solvents such as methanol might react with the non-polar alkyl group on the adsorbent surface, and it would form a thin methanol film on the surface of adsorbent, which could well dissolve each other with water, and after using ultra pure water to balance the pillars, the sample solution could fully contact with adsorption materials to improve the reproducibility and recovery rate of extraction of PFOA from water samples. Figure 2(a) and (b) show the electron microscope of C18 adsorption material that be magnified by 400 times after activation. Usually water-soluble organic solvents such as methanol or acetonitrile are selected as activators. Although the polarity of the two is similar, it was considered that acetonitrile is more toxic than methanol, so methanol was selected as an activation solvent in this research.

Fig. 2 Electron microscope of C18 adsorption material

2.2 SPE Cartridge

The recovery values for all tested SPE cartridges ranged from 45% to 85.38%. The values were presented in Fig. 3. For the treated landfill leachate samples, the obtained results showed that the best recovery rate was 85.38% for PFOA with RSD lower than 5% in WAX cartridges. The lowest recovery rate for PFOA were received in C18, and the extraction effect was improved with the increase of the amount of adsorbed material in the C18 cartridges. When the filler amount was 1 000 mg/mL, the recovery rate reached 76%, and it was gradually approaching to that of HLB and WAX cartridges. It shows the potential of C18 can be developed, however, its cost will substantially increase at the same time. The adsorption selectivity of C18 is not high, and most non-polar compounds or polar compounds can be adsorbed on the surface of C18 adsorbent, thereby enhancing the matrix effect and increasing the measurement interference.

Fig. 3 PFOA recovery rates for different SPE cartridges

There was no effect on the rule when the samples changed from DTRO effluent of leachate to raw leachate, but the flow rates of the samples were generally reduced in both SPE cartridges, and the C18 cartridges were more likely to be blocked. Therefore, considering the accuracy, time and cost of SPE, the WAX cartridge was very suitable for the analysis of most fluorocarbon compounds with different length of carbon chain.

WAX is a mixed-mode, reversed-phase/weak anion-exchange sorbent. Its structure is based on WAX functionalized with piperazine moieties capable to bind anions at acidic pH. Figure 4 shows the chemical structural formula of the mixed-mode, reversed-phase/weak anion-exchange sorbent, and its benzene ring has a strong hydrophobic interaction with a tertiary amine providing weak anion exchange capacity. When the target has negative charge and the adsorbent has positive charge, it will be retained on the SPE cartridges. Owing to the negative functional group carboxylic acid, PFOA can be well preserved by the WAX cartridges.

Fig. 4 Chemical structural formula of WAX cartridge adsorbent

2.3 pH Value

The effect of pH value on PFOA extraction using WAX column was shown in Fig. 5. It could be seen from the diagram that the effect of pH value on SPE was significant. When the sample was the treated landfill leachate, the recovery rate of PFOA was 74.64%-85.09%. When the pH value was 5, the recovery rate of PFOA reached the maximum value of 85.09%. The recovery rate decreased when the pH value was lower or higher than 5. For example, when the pH value was 3, the recovery rate was 74.64%. When pH value was 7, the recovery rate was 81.38%. In addition, it was found that when the pH was in a weak acid condition, such as 5-7, the recovery rate was higher than 80% and stable. When the sample was raw leachate, the recovery rate of PFOA was 67.04%-82.38%. When the pH value was 5, the recovery rate of PFOA reached the maximum value of 82.38%. When the pH value was higher or less than 5, the recovery rate was reduced. For example, when pH=3, the recovery rate was 67.04%, which did not meet the requirements (70%-120%). When pH value was 7, the recovery rate reached 77.32%.

Fig. 5 The effect of pH value on PFOA extraction

In summary, PFOA solid phase extraction of leachate obtained the best extraction effect when pH was 5. It was presumed that when pH was below 5, the dissociation of PFOA in landfill leachate was inhibited, and the competitive adsorption in the extraction process was beneficial to other substances such as humic acid. For weak anion exchange, it was better to adjust the samples pH value to at least 2 pH units higher than the pK_a value of analyte, and to at least 2 pH units lower than the pK_a value of adsorbents. The pK_a of the adsorbent used in this research ranged from 9 to 10 while PFOA is 2.6. In most cases, high or low pH value was unfavorable to the adsorption, for the adsorbent and adsorbate were negatively charged contributing to electrostatic repulsion when the pH value was too high, while adsorbents and adsorbate will carry positive charge when the pH value was too low contributing to electrostatic repulsion. Hence, it could be concluded that the best pH value was 5 in this research.

2.4 Ionic Strength

Figure 6 shows the effect of ionic strength on extraction of target analytes. The results show that the addition of KCl below 100 mg per 500 mL sample has little effect on SPE. When the dosage is between 100 and 500 mg, the recovery rate of PFOA is significantly improved with the increase in quantity of KCl, while more than 500 mg, the variation of the extraction effect gradually decreases with the increase of the dosage and the recovery rate tends to decrease. It is speculated that the adsorption mechanism is hydrophobic in addition to electrostatic action. When the electrostatic repulsion between the adsorbed organic matters on the surface of adsorbents is more obvious to hinder further adsorption, the increasing ionic strength will weaken the electrostatic repulsion at this time, so it is conducive to the adsorption of PFOA, then the hydrophobic interaction between the analyte and adsorbents plays an important role. After the further increase of ionic strength, the newly added electrolyte ions may compete with the ion exchange of adsorbate ions, thus inhibiting the adsorption, or because the adsorbate ions and electrolyte ions form ion pairs, the effective concentration of adsorbate ions decreases, resulting in reduced adsorption capacity^[12]. In addition, the experimental results show that with the increase of ionic strength, the flow velocity of water samples gradually decreases and the extraction time increases.

Fig. 6 Effect of ion intensity on PFOA extraction

In view of the above analysis, for the water samples with low ionic strength, such as treated landfill leachate, it was recommended to pre-add 500 mg KCl in samples, while for samples with high ionic strength, such as raw landfill leachate, there was no need for addition of external electrolytes.

2.5 RSM Optimization

Single factor analysis of SPE conditions showed that ionic strength had great influence on PFOA extraction efficiency. According to BS ISO 25101-2009, for reference, the activation method was optimized with 10 mL ammonia methanol and 4 mL water passing through the column successively, keeping the extraction column wet, and elution method with 5 mL ammonia methanol passing through the column. Based on the results of single factor experiment, the response surface of the extraction recovery was taken as the response value, and the SPE conditions were optimized by the response surface method of three factors and three levels. The design of various factors and levels was shown in Table 2, and the results of the test were shown in Table 3.

The results of BBD design regression analysis show that the relationship between the recovery rate of PFOA extraction and various factors is as follows:

$\begin{array}{l}y=-\mathrm{149.1815}+\mathrm{10.06938}A+\mathrm{0.35343}B+\mathrm{31.19625}C\\ -\mathrm{3.125}\times {\mathrm{10}}^{-\mathrm{4}}AB+\mathrm{0.07}AC+\mathrm{6.6775}\times {\mathrm{10}}^{-\mathrm{3}}BC\\ -\mathrm{0.48388}{A}^{\mathrm{2}}-\mathrm{3.428}\times {\mathrm{10}}^{-\mathrm{4}}{B}^{\mathrm{2}}-\mathrm{3.0555}{C}^{\mathrm{2}}\\ \end{array}$(1)

where y is the recovery rate of PFOA extraction, A is the volume of activator, B is ionic strength, and C is the volume of eluent.

Table 4 shows the results of the two models fitting response surface analysis of variance: linear correlation coefficient between the response value and the variable is R²=444.76/455.42=0.976 6, F-measure is 32.43, and P < 0.05 shows that the regression model is significant. The value of F is 2.32, which shows that the model imitation has a good result.

The 3D surface map and the contour map can reflect the influence of various factors on the response value, and the interaction between the factors on the response value. The steepening of the surface indicates that the effect of this factor on the response value is more significant. When the characteristic value is positive, there is a minimum value; when it is negative, there is a maximum value; when it has both positive and negative, no extreme value exists; The elliptical contour map can be obtained through the projection on the surface of the three-dimensional space point, and the optimal test condition is at the center. From Figs.7-9, we can see from 3D surface maps that there are extreme values. Contour lines are elliptical, and there is a large angle between the axis of the ellipse and the coordinate axis, indicating that there is no significant interaction between the factors. Three factors were set to float within the selected area to get the best extraction recovery rate (Fig. 9). The results of model optimization are as follows: the amount of activated methanol is 10 mL, the amount of potassium chloride is 600 mg, the eluent is 6 mL 1% ammonia methanol solution, and the theoretical recovery rate of PFOA is 95.67%. The recovery rate of the verification result is 93.34%, so the relative error of the prediction is 2.43%, which indicates that the optimization result is reliable.

Fig. 7 Effect of volume of activator and ionic strength on extraction recovery

Fig. 8 Effect of volume of activator and volume of eluent on extraction recovery

Fig. 9 Effect of ionic strength and eluent volume on extraction recovery

3 Conclusion

1) The single factor analysis showed that the best extraction effect could be obtained when using the manual SPE, with methanol as activator of WAX cartridges and 4 mL ultrapure water for transition. It was better to add a 500 mg KCl in the solution to improve the ionic strength, and the best pH in this research was 5.

2) The optimum value of the volume of activator, ionic strength and eluent was 10 mL, 600 mg, 6 mL 1% ammonia methanol solution, respectively, using response surface methodology, and the theoretical maximum of extraction recovery rate was 95.67%.

3) After verification, the recovery rate was 93.34% (RSD=0.28%), so the relative error of the prediction was 2.43%. The measured value was very close to the predicted value, showing that the response surface method was reliable for the optimization of the SPE conditions of PFOA in raw and treated landfill leachate. This study provides some theoretical support for the extraction of PFOA in landfill leachate and the development of related methods.

References

All Tables

All Figures

	Fig. 1 Manual solid phase extraction schematic diagram
In the text

	Fig. 2 Electron microscope of C18 adsorption material
In the text

	Fig. 3 PFOA recovery rates for different SPE cartridges
In the text

	Fig. 4 Chemical structural formula of WAX cartridge adsorbent
In the text

	Fig. 5 The effect of pH value on PFOA extraction
In the text

	Fig. 6 Effect of ion intensity on PFOA extraction
In the text

	Fig. 7 Effect of volume of activator and ionic strength on extraction recovery
In the text

	Fig. 8 Effect of volume of activator and volume of eluent on extraction recovery
In the text

	Fig. 9 Effect of ionic strength and eluent volume on extraction recovery
In the text