Eggshell Membrane-Based Biotemplating of Mixed Hemimicelle

Jul 15, 2014 - ABSTRACT: A new solid-phase extraction (SPE) format was demonstrated, based on eggshell membrane (ESM) templating of the mixed ...
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Eggshell Membrane-Based Biotemplating of Mixed Hemimicelle/ Admicelle as a Solid-Phase Extraction Adsorbent for Carcinogenic Polycyclic Aromatic Hydrocarbons Weidong Wang,†,‡ Bo Chen,† and Yuming Huang*,† †

Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education; College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China ‡ Basic Department of Rongchang Campus, Southwest University, Chongqing 402460, China S Supporting Information *

ABSTRACT: A new solid-phase extraction (SPE) format was demonstrated, based on eggshell membrane (ESM) templating of the mixed hemimicelle/admicelle of linear alkylbenzenesulfonates (LAS) as an adsorbent for the enrichment of carcinogenic polycyclic aromatic hydrocarbons (PAHs) in environmental aqueous samples. The LAS mixed hemimicelle/admicelle formation and SPE of the target PAHs were conducted simultaneously by adding the organic target and LAS through a column filled with 500 mg of ESM. The effect of various factors, including LAS concentration, solution pH, ionic strength, and humic acid concentration on the recoveries of PAHs were investigated and optimized. The results showed that LAS concentration and solution pH had obvious effect on extraction of PAHs, and the recoveries of PAHs compounds decreased in the presence of salt and humic acid. Under the optimized analytical conditions, the present method could respond down to 0.1−8.6 ng/L PAHs with a linear calibration ranging from 0.02 to 10 μg/L, showing a good PAHs enrichment ability with high sensitivity. The developed method was used satisfactorily for the detection of PAHs in environmental water samples. The mixed hemimicelle/admicelle adsorbent exhibited high extraction efficiency to PAHs and good selectivity with respect to natural organic matter and was advantageous over commercial C18 adsorbent, for example, high extraction yield, high breakthrough volume, and easy regeneration. KEYWORDS: biomaterials, eggshell membrane, mixed hemimicelles, solid-phase extraction, polycyclic aromatic hydrocarbons

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INTRODUCTION Because of very low concentrations of the organic pollutants in the environment, it is in urgent need of an extraction procedure to enrich these targets before analysis. Among a great diversity of sample pretreatment methods, solid-phase extraction (SPE) is less time-consuming and solvent-consuming than liquid− liquid extraction. Nowadays, SPE has been widely applied in enrichment of various organic pollutants at trace level.1−3 Although the commercial adsorbents including C8, C18, or Oasis HLB as SPE adsorbents have been widely utilized for sample pretreatment in environmental analysis,4,5 the search for new extraction materials for SPE is still highly needed. This is because the silica-based materials present some shortcomings, such as pH instability and the possibility of discrepant results because of the unrestrained existence of free silanol groups.6 In the past few years, a liquid−solid extraction system, based on hemimicelles (monolayers of the adsorbed surfactant aggregates at the solid−liquid interface) and admicelles (bilayers of the adsorbed surfactant aggregates at the solid−liquid interface), has been developed to extract organic pollutants from various environmental matrices.7−11 The process of extracting analytes in hemimicelles and admicelles is defined as “adsolubilization”, and it is suggested that both hemimicelles and admicelles regions are desirable for the SPE method.7 Unlike the commercial adsorbents such as C8, C18, or Oasis HLB that can be directly used for extraction of analytes, in the SPE method based on hemimicelles and admicelles, the © 2014 American Chemical Society

adsorbents are generated by the adsorption of ionic surfactants on the solid surface of mineral oxides.7−13 Until now, almost all studies concentrated on the adsolubilization behavior of target analytes in the hemimicelle/admicelle aggregates formed on the surface of chemical structures, such as alumina, ferric oxyhydroxides, silica, and titanium dioxide.7−18 There is no work yet on the hemimicelle/admicelle aggregates formation on the surface of biological material. Eggshell membrane (ESM), existing between the egg white and the inner surface of the eggshell, is a worthless biomaterial after the production of egg derivatives in food industry. Its main components are biological molecules and protein fibers. ESM promises very good gas and water permeability and biocompatibility. Recently, ESM has been investigated as an efficient carrier of active components due to its cavity microstructure.19 Importantly, due to the presence of amines, amides, and carboxylic groups on the surface of ESM,20,21 ESM has been widely used in the removal of various pollutants, material preparation, catalysis, and organic synthesis.22−25 On the basis of the principle of hard and soft acids and bases, hardbase groups such as amino groups and carboxylic groups on ESM favor binding to hard acid, such as H+, through strong Received: Revised: Accepted: Published: 8051

January 1, 2014 July 9, 2014 July 15, 2014 July 15, 2014 dx.doi.org/10.1021/jf501877k | J. Agric. Food Chem. 2014, 62, 8051−8059

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Figure 1. (A) Scanning electron microscopy image of the ESM. (B) ζ potential of ESM by adsorption of LAS at pH 4.97. (C) Effect of solution pH values on the ζ potential of ESM. Error bars denote standard deviations based on three measurements. LAS Adsorption onto ESM. To obtain the adsorption isotherm of LAS on ESM, a series of 100 mL of 0.05−50 mmol/L LAS aqueous solutions (pH 5.0) was prepared and transferred into conical flasks containing 100 mg of ESM. Then, all flasks were placed vertically on a flat surface in a water bath at 25 °C and shaken at 100 rpm for 1 h to ensure the equilibrium between LAS aqueous solution and ESM. The free LAS concentration in solution was analyzed by HPLC−UV (see the Supporting Information). SPE Procedure. Before adsorption of LAS, ESM was cut into small pieces (about 0.5 mm × 0.5 mm). An ESM-packed cartridge was organized by amending a C18 cartridge (1 g, 6 mL, polypropylene) from SUPELCO Corporation, USA. The 500 mg of the as-prepared ESM was filled into the emptied cartridge. Because ESM is a complex biomaterial probably containing various impurities such as inorganic cations and organic matters,22,31 in order to remove these possible impurities, the homemade ESM column was washed consecutively with CH2Cl2, hexane, CH3OH, CH3CN, and ultrapure water, 5 mL for each. Before sample loading, the ESM-packed column was pretreated by 5 mL of acetone and 5 mL of ultrapure water. Extraction of 10 PAHs was fulfilled by passing 500 mL of the PAH standard solution in water or 100−500 mL of the different water samples (500 mL for the tap water sample, the Jialingjiang water sample and the Changjiang water sample, 100 mL for wastewater from the effluent of a constructed wetland treatment system) containing LAS surfactants (the amount of LAS added was kept 0.5 mmol/g ESM) through the 0.5 g ESM-packed cartridge (for 500 mL sample) or 0.1 g ESM-packed cartridge (for 100 mL sample) at 6 mL/min flow rate. After extraction, 10 mL of ultrapure water was used to wash the homemade ESM column. Then, the column was dried under the negative pressure of 20 mmHg for 15 min, and the enriched 10 PAHs were eluted with 5 mL of acetone. Finally, a volume of 20 μL was used for HPLC−UV analysis of 10 PAHs. The recovery of the target PAHs was used as the criterion for optimizing the parameter for the proposed SPE method, which was obtained based on the ratio of the obtained content of PAHs to the expected content of PAHs, displayed as a percentage. Instrumental Analysis. The HPLC instrument (JASCO Corporation, Japan) consisted of two inert pumps, a column thermostat, and a UV detector. The PAHs were separated on a C18 column (250 mm × 4.6 mm, 5 μm). The mobile phase was a mixture of acetonitrile and water at a ratio of 80−20% (v/v), and its flow rate was set at 1 mL/ min. The column temperature was 25 °C. The chromatographic data were recorded at the wavelength of 254 nm. The HPLC−UV system was controlled by the JASCO Chrompass (JASCO Corporation, Japan). Analytical method for LAS determination was based our previous work,32 and the detailed procedure is provided in the Supporting Information under “LAS Determination by HPLC−UV” (see the Supporting Information). The ζ potential was measured on a Zetasizer Nano ZS90 apparatus (Malvern, U.K.). Analysis of Environmental Water Samples and Comparative Study. In this study, river water, tap water, and wastewater samples

ionic interactions in solution, making ESM surface positively charged under acidic condition. If an anionic surfactant, such as linear alkylbenzenesulfonates (LAS), is added to ESM, the interaction via electrostatic attraction between negatively charged basic sulfonate groups of LAS and the positively charged acidic surface of ESM, results in the formation of hemimicelle/admicelle aggregates on the ESM, which can be used as adsorbents for the enrichment of polycyclic aromatic hydrocarbons (PAHs). On this basis, a new kind of adsorbents, namely, the biotemplating of hemimicelle/admicelle, is obtained. Hence, the objectives of this study were (1) to study the LAS surfactant adsorption onto ESM and mixed hemimicelle/admicelle formation on ESM biomaterial; (2) to investigate the feasibility of using ESM templating of the mixed hemimicelle/admicelle of LAS as new SPE adsorbent for the extraction of 10 PAHs, namely, naphthalene (Nap), acenaphthylene (Acy), fluorene (Flo), phenanthrene (Ph), anthracene (An), fluoranthene (Fl), pyrene (Py), benz[a]anthracene (BaA), benzo[b]fluoranthene (BbF), and benzo[k]fluoranthene (BkF), from environmental water samples. These target PAHs were considered because they are ubiquitous in our environment and are frequently found in rivers and wastewaters.26,27 Thus, they always were chosen as the target PAH compounds in the analysis of environmental water samples;26−30 (3) to optimize the predominant factors affecting the enrichment efficiency of this novel adsorbent for PAH target compounds.

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EXPERIMENTAL PROCEDURES

Reagents and Chemicals. The ESM was obtained from the eggshells of boiled eggs, which were provided by the local supermarket. After the obtained ESM was treated by 0.5% diluted hydrochloric acid20 for 30 min, it was rinsed with proper ultrapure water, dried at 80 °C, and stored at 4 °C until use. Ten PAH standards of Nap, Acy, Flo, Ph, An, Fl, Py, BaA, BbF, and BkF were provided by Sigma−Aldrich (Shanghai, China). The stock PAH standard mixture was prepared in methanol media. The working solution of 10 PAHs was prepared every day by proper dilution of stock PAH solution using ultrapure water. Commercial LAS was supplied by Fluka Corporation, in a mixture containing a proportional composition of the different homologues as follows: C10, 14.56%; C11, 38.00%; C12, 30.19%; and C13, 17.25%. Acetonitrile and methanol (high-performance liquid chromatography (HPLC) grade) were purchased from Scharlau Chemie S.A. (Scharlau Chemie S.A., Spain). All other solvents and reagents were of analytical reagent grade and provided by the Chongqing Taixin Chemical Co. Ltd. (Beibei, Chongqing). The ultrapure water was used throughout. 8052

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−47.6 to −50.9 mV when the amount of LAS added increased from 3.0 to 5.0 mmol/g ESM. This suggests that additional increase in the amount of surfactant did not significantly increase LAS adsorption on ESM. In such a case, the LAS surfactant begins to form micelles in solution, and the aqueous LAS micelles are in equilibrium with LAS admicelles. This results in a distribution of target analytes in two kinds of LAS aggregates, which is unsuitable for SPE application.17 In a word, previous works proved that adsorption of surfactant on inorganic oxides resulted in formation of hemimicelle, hemimicelle/admicelle mixture, as well as admicelle.7−9,11,12,15 Similarly, the adsorption phenomenon of LAS on ESM under acidic conditions also led to formation of hemimicelle, hemimicelle/admicelle mixture, as well as admicelle. Figure S2 (Supporting Information) depicts the adsorption isotherms of LAS homologues on ESM. As can be seen, the C10 to C13 LAS isotherms showed a similar trend (see Figure S2, Supporting Information). The representative fitting parameters by Langmuir37 and Freundlich38 adsorption equations proved that the Langmuir equation ideally described the adsorption data (r2 > 0.99, see Table S1, Supporting Information), suggesting the Langmuir isotherm as a better fit with the experimental data than the Freundlich isotherm. This confirms the monolayer adsorption of LAS onto ESM surface. Optimization of SPE Procedure for PAHs Extraction. Effect of the Amount of LAS Surfactant. The extraction performance of the mixed hemimicell/admicelle adsorbent to PAHs was investigated, and a HPLC−UV instrument was used for the detection of 10 PAHs in aqueous samples after preconentration by the adsorbent. During the extraction process, the amount of LAS is one of the important factors in the present method because the formation of the mixed hemimicelle/admicelle depends on this condition directly.7,11,17,18 Thus, the effect of the LAS amount on extraction of 10 PAH compounds was investigated in detail. As shown in Figure 2, when there was no LAS in aqueous solution, the recoveries of 10 PAHs were rather low, showing that PAHs in aqueous media were hardly adsorbed on ESM in the absence of LAS. With increase in the amount of LAS, the recoveries of PAHs increased. This indicated that the mixed hemimicelle/

were used for the detection of 10 PAHs by the proposed method. The river water samples were collected from the Chongqing sections of the Jialingjiang River (Beibei, Chongqing, China) and the Changjiang River (Jiulongpo, Chongqing, China), respectively. Tap water and wastewater samples were sampled from our lab and an effluent of a wastewater treatment system in Yongchuan (Chongqing, China). Prior to sample pretreatment, all of the aqueous samples were filtered through a 0.45 μm mixed fiber membrane (Shanghai Xinya Purification Device Factory, Shanghai, China); then, the pH value of the resulting filtrate was regulated to 5.0 by dilute HCl. For a comparison, water sample preconcentration by C18 was conducted following the published procedures.33,34 Briefly, the C18 SPE cartridge (0.5 g, 3 mL, polypropylene, SUPELCO Corporation, USA) was preconditioned by 5 mL of methanol and 5 mL of ultrapure water. Then, 100−500 mL of water sample spiked with the target PAHs was pumped through the cartridge at 6 mL/min, followed by a rinsing procedure by 10 mL of ultrapure water. After the column was dried for 15 min, the enriched 10 PAHs were eluted with 5 mL of acetonitrile. Finally, 20 μL of the obtained acetonitrile solution was used for HPLC−UV analysis of 10 PAHs.

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RESULTS AND DISCUSSION LAS Adsorption on ESM. It is essential to obtain the adsorption isotherms of LAS on ESM because this is important to locating the ranges where different aggregates form and to interpreting the SPE behavior of the selected analytes when using the mixed hemimicelle/admicelle-based SPE.13,17 The Fourier transform (FT) IR spectrum of ESM exhibits significant peaks at 3300−3500 cm−1 (ascribed to N−H stretching mode), 1630 cm−1 (ascribed to N−H bending mode), and 1350−1385 cm−1 (ascribed to C−N stretching mode) (see Figure S1, Supporting Information), proving the presence of amines and amides in ESM.21 In addition, ESM contains macroporous network structure (Figure 1A), consisting of highly cross-linked protein fibers (about 1−3 μm in diameter). The presence of macropores benefits mass transfer as an adsorbent. Figure 1B depicts the ζ potential change of ESM by adsorption of LAS at pH 4.97. The isotherm is similar to those in previous works.8,17 Generally, the ionic surfactant adsorption isotherms can be classified into three areas, namely, hemimicelles, mixed hemimicelles, and admicelles.35,36 In the first area, the ζ potential of ESM at pH 4.97 (below the point of zero charge of ESM, about 5.50, see Figure 1C) is positive, and LAS anionic surfactants are sparsely adsorbed onto the positively charged ESM surface. In such a case, adsorption of surfactants occurs primarily via the electrostatic attraction between the positive surface charges of ESM and the negative sulfonate surfactant head groups, resulting in a single-layer coverage (hemimicelles). Hence, the ζ potential decreased from positive to zero. In the area of the mixed hemimicelles, with increase in the amount of LAS, the surface charge of ESM turned to negative due to charge neutralization. For example, the ζ potential gradually decreased from about 1.0 to −41.7 mV when the amount of LAS added increased from 0.5 to 2.0 mmol/g ESM (Figure 1B). This is because the lateral hydrophobic interactions among surfactant tails results in the aggregation of the surfactant molecules at the solid−liquid interface, leading to the formation of bilayers called admicelles. Both hemimicelles and admicelles areas are suitable for the SPE method.7,17 From the ξ potential isotherm (Figure 1B), the range 0.1−0.8 mmol/g ESM is in the mixed hemimicelles regions, which is suitable for SPE. In the third region, further increases in the amount of LAS caused no obvious change of the surface charge; thus, the ζ potential of ESM remained almost constant. For example, the ζ potential shifted from

Figure 2. Effect of the amount of LAS on recoveries of PAHs. Concentration of sample solution: Nap, 2 μg/L; Acy, 4 μg/L; Flo, 0.4 μg/L; Ph, 0.2 μg/L; An, 0.2 μg/L; Fl, 0.4 μg/L; Py, 0.2 μg/L; BaA, 0.2 μg/L; BbF, 0.4 μg/L; BkF, 0.2 μg/L; SPE column packed with 500 mg of ESM; flow rate of sample solution: 4 mL/min; eluent: 5 mL acetone; pH of sample solution: 5.0; volume of sample solution: 500 mL. Error bars denote standard deviations based on three measurements. 8053

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of ESM and the existing state of LAS surfactant in aqueous solution. Finally, pH of 5.0 was adopted in this work. The pH effect can be attributed to the fact that ESM contains some functional groups such as −NH2, −CONH2, and −COOH groups, which can be protonated or deprotonated relying on pH value of the sample solution.35,41 As indicated previously, the pHpzc of ESM is about 5.5 (Figure 1C). Hence, the positively charged sites would form on the ESM surface under acidic media. In the acidic solution (pH ≤ 5.5), the quantitative recoveries of PAHs were obtained (Figure 3), suggesting that the electrostatic attraction may play a key role in hemimicellar/ admicelle formation derived from LAS retained on ESM. At a pH greater than 5.5, the ESM surface became neutral or negatively charged, leading to the decrease of LAS assembled onto ESM and finally leading to the decrease of the target PAH recoveries. The LAS retention decreased at a pH of above 5. This may be caused by the competition effect between the hydroxyl group and the sulfonic group at the identical sorption site of ESM. In fact, it has been found that the adsorption of anions on the positively charged surface decreased by increasing solution pH.42 In addition, the electrostatic repulsion between the negatively charged ESM surface and the negatively charged sulfonic groups will increase at higher pH. Figure 1C shows that the surface potential of ESM was negatively charged at higher pH values. It became more negative at pH from about 5.5 to 9.0, making electrostatic repulsion between the negatively charged basic sulfonic groups of LAS and ESM stronger at higher pH values than that at more acid pH values. Thus, the LAS adsorption on the surface of ESM decreased at higher pH values, leading to low recoveries of PAHs. The result further indicated the importance of the mixed hemimicelle/ admicelle formation arising from LAS for the extraction of PAHs. Another experiment was performed to examine the influence of ionic strength on the extraction of PAHs in the presence of sodium chloride. The experimental results indicated that the addition of NaCl reduced the retention of 10 PAH compounds on ESM templating of hemimicelle/admicelle adsorbents (see Figure S4, Supporting Information). For example, the recoveries of 10 PAHs decreased by about 10% to about 25%, as the NaCl was increased from 0% to 2.0% (w/v). A similar result has been observed in the literatures on the solvent microextraction of nitroaromatic explosives in water samples43 and ion-pair hollow fiber-protected liquid-phase microextraction of acidic herbicides.44 However, it is noteworthy that the quantitative recovery (recovery >95%) of the target PAHs could be obtained when NaCl concentration was ≤1.0% (w/v). This suggested that the fresh water matrix had a minor effect on extraction of 10 PAH compounds in terms of ion strength. In general, the addition can cause a decrease in the solubility of analytes in an aqueous media,45 which benefits the extraction of target analytes. However, in our case, the recovery of 10 PAHs decreased in the presence of salt. One possible reason for this finding is the reduced electrostatic pull between negatively charged LAS and the positively charged surface of ESM. In order to confirm this hypothesis, the effect of NaCl addition on the ζ potential of LAS solution was studied. It is evident that the ζ potential of LAS solution increased with an increasing concentration of NaCl (see Figure S5, Supporting Information). This would lead to the reduced sorption affinity between negatively charged LAS and the positively charged surface of ESM, which would cause a reduced sorption of PAHs and hence a decrease in recovery. On the basis of the above result,

admicelle began to form on the surface of ESM in the presence of LAS, which obviously enhanced the retention of the target PAH compounds. The PAH adsolubilization increased with the amount of LAS surfactants rising up to 0.50 mmol/g ESM, then remained stable until 0.60 mmol/g ESM, and decreased above 0.60 mmol/g ESM. Hence, 0.50 mmol of LAS/g ESM was selected as the optimal addition of surfactant in this investigation, corresponding to 0.5 mmol/L LAS concentration in aqueous solution. This concentration was lower than the critical micelle concentration (CMC) of LAS ranging from 1.8 to 4 mmol/L depending on the solution chemistry and homologues.39,40 This interesting phenomenon suggested that this novel hemimicelle could occupy the highest adsorptive capacity for PAHs when LAS was self-organized on ESM in a specified pattern of saturated monolayer. The formation of anionic hemimicellar/admicelles was conductive to the generation of analyte−adsorbent mixed aggregates. As expected, the adsolubilization of PAHs gradually reduced for free LAS concentrations close to critical micelle concentration. Desorption Condition. Several organic solvents (CH3CN and CH3OH, acetone, CH2Cl2 and hexane) were selected for elution of analytes from ESM templating of hemimicelle/ admicelle adsorbents. The effects of the kind and the volume of eluent on desorption were further investigated from 1 to 5 mL. The obtained results proved that desorption ability of acetone was better than that of CH3CN, CH3OH, hexane and CH2Cl2 (see the Figure S3 of the Supporting Information). Hence, acetone was selected in this work. The effect of volume of acetone as an eluent was investigated in the range of 1−5 mL. The results demonstrated that the recoveries of PAHs increased with volume of acetone as an eluent to 4 mL and then remained stable ranging from 93.8 to 116.9%. Finally, 5 mL of acetone was chosen as an optimized eluent to get the quantitative results. Effect of Sample pH and Ion Strength. The influence of solution pH in extraction of PAHs was examined in the range from 1.0 to 9.0. The retention of all analytes on ESM showed no significant change with solution pH from 1.0 to 5.0 (Figure 3). It is interesting that, for the studied analytes, the adsorption decreased sharply when solution pH was above 5.0. Thus, for practical aqueous samples, pH adjustment is necessary to guarantee quantitative retention of the target PAHs on ESM. This is because solution pH determines both the surface charge

Figure 3. Effect of solution pH on recoveries of PAHs. The amount of LAS: 0.5 mmol/g ESM; other conditions were the same as those in Figure 2. Error bars denote standard deviations based on three measurements. 8054

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(86% and 89%, respectively) at 750 mL of sample volume were a little bit lower than those in Figure 4. This indicated that, if the amount of LAS interacting with the ESM adsorbent was kept constant and large enough (0.5 mmol/g ESM in present study) to form the mixed hemimicelle/admicelle onto ESM, the manner for loading LAS surfactant had no obvious effect on the enrichment ability of the mixed hemimicelles/admicelles-based adsorbent for PAHs. However, direct addition of LAS into the water sample before SPE adopted in our study can simplify the experimental separation procedure as compared to a common way to work with hemimicelles and/or admicelles. The effect of the sample flow rate was studied in the 3−10 mL/min range. The satisfactory recoveries of PAHs (82.4− 116.7%) were obtained in the studied range of flow rate, thus allowing the fast pretreatment of the practical samples. Finally, 6 mL/min was chosen for further experiment. Effect of Humic Acid. The natural organic matter (NOM) in the aqueous samples may cause a matrix interference with extraction of the organic pollutants (e.g., PAHs). In this work, the influence of NOM on the extraction of PAHs was studied by using humic acid (HA) as a model of NOM. As shown in Figure S7 (Supporting Information), the recoveries of PAH compounds reduced in the presence of HA. However, HA did not cause obvious effect on the extraction of 10 PAHs when the concentration of HA changed from 0 to 10 mg/L dissolved total organic carbon (TOC). This can be explained based on the property of HA. HA is a weak acid and contains phenolic and carboxylic groups in its structure.46 The pKa values of humic substances range from 3 to 7.46 At pH 5.0, a portion of −COOH groups is deprotonated via ionization of HA. The degree of HA ionization increased with the increasing of HA concentration, leading to the competitive adsorption between LAS and HA on ESM. This phenomenon could result in the decline of recoveries of 10 PAHs by weakening the formation of hemimicelle and admicelle on the surface of ESM, which was further confirmed by the significant reduction of extraction recoveries of 10 PAH compounds at a concentration of HA above 10 mg/L. The concentration of TOC in river water samples used in this work ranges from 3.44 to 3.50 mg/L. Thus, it can be concluded that HA had minor effect on the detection of PAHs in river water. This suggested that the proposed SPE format showed good selectivity with respect to natural organic matter. Analytical Performance. The above-selected experimental conditions were employed to measure the analytical performance, in order to determine whether the developed method could be used to the analysis of PAHs in aqueous samples. The analytical data, including linear range, correlation coefficients, detection limits, and precision, are shown in Table 1. As can be seen, good linear relationships could be obtained for different analytes in the 0.02−10 μg/L range. Detection limits (S/N = 3) of 10 PAH compounds were in the range 0.1−8.6 ng/L under the above-mentioned extraction conditions, proving that the developed method was highly sensitive for the detection of PAHs. The relative standard deviations (RSDs) were from 1.3% to 3.8% (n = 6) for 10 PAHs at concentration levels of 0.2−2 μg/L on the same column on the same day. In addition, the RSD of intercolumn and intracolumn was below 9% (n = 3, Table 2), showing that the reproducibility among the ESM cartridges was desirable. Analysis of Environmental Water Samples and Comparative Study. The developed method was applied to detecting PAHs in several aqueous samples. Also, a comparison of the

NaCl was not used in the coming experiments. In addition, in order to clarify the effect of water-structure making or waterstructure breaking characters of the added salts on the extraction of 10 PAHs, the effects of KCl and KSCN as water-structure breaking salts23 on the extraction of PAHs were examined. On the basis of the results shown in Figure S4 (Supporting Information), similar variation trends could be observed for KCl and KSCN, namely, the recoveries of 10 PAHs declined as concentrations of KCl and KSCN increased. Also, they showed a similar trend with NaCl as water-structure making salt23 (see Figure S4, Supporting Information). The result indicated that both water-structure making salt and water-structure breaking salt had similar effect on the adsorption of PAHs. This is probably because PAHs are hydrophobic compounds; the hydrogen bonds in water have little influence on the solvation pattern of 10 PAHs. Hence, the water-structure making or water-structure breaking characters of the added salts had minor effect on the interaction between ESM and PAHs. Breaking through Volume and Flow Rate. In the present study, LAS hemimicelle/admicelle formation and the target extraction occurred simultaneously. Thus, experiments for determining the sample breaking through volume were realized by passing sample solution through a column filled with 500 mg of ESM. The volume of the aqueous sample was varied from 100 to 1000 mL with the spiked concentration of a 0.2−4 μg/L for the PAH aqueous solution, in which the amount of LAS added was kept 0.25 mmol. The experimental results are shown in Figure 4. As can be seen, when the sample solution volume

Figure 4. Effect of sample volume on recoveries of PAHs. The amount of LAS: 0.5 mmol/g ESM; other conditions were the same as those in Figure 2. Error bars denote standard deviations based on three measurements.

increased from 100 to 750 mL, the quantitative recoveries of PAHs were obtained. Whereas, as the volume of sample solution was increased to 1000 mL, the recoveries of the 10 PAHs decreased. Finally, the sample volume of 500 mL was adopted for further study. It should be indicated that additional experiments were also performed to determine the sample breaking through volume by a common way to work with hemimicelles and/or admicelles. In order to do this, first, 0.25 mmol of LAS in 50 mL of water solution was loaded to 0.5 g of ESM-packed cartridge, then 100−1000 mL of sample solutions spiked with a 0.2−4 μg/L PAH standard solution was loaded to the cartridge. The results demonstrated that, when the sample solution volume increased from 100 to 750 mL, the quantitative recoveries of PAHs were obtained (see Figure S6, Supporting Information). However, the recoveries of the BbF and BkF 8055

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Table 1. Linear Range, Correlation Coefficients, Limits of Detection (LOD), and Relative Standard Deviation (RSD) Using ESM Templating of the Mixed Hemimicelle/ Admicelle as SPE analyte

linear range (μg/L)

correlation coefficient

RSD (%) (n = 6)

LOD (ng/L)

Nap Acy Flo Ph An Fl Py BaA BbF BkF

0.2−10 0.2−10 0.05−2.5 0.02−1.0 0.025−1.25 0.02−5 0.02−5 0.02−5 0.05−2.5 0.02−5

0.9983 0.9928 0.9941 0.9947 0.9907 0.9929 0.9726 0.9865 0.9893 0.9836

3.8 3.2 0.6 2.8 3.6 1.3 2.5 2.3 2.0 1.3

8.6 7.1 0.3 0.1 0.6 1.0 1.8 0.9 1.2 1.3

Figure 5. HPLC−UV chromatograms of PAHs from (A) 500 mL of the Changjiang River sample; (B) 500 mL of the Changjiang River sample spiked with 0.40 μg/L Nap, 0.80 μg/L Acy, 0.08 μg/L Flo, 0.04 μg/L Ph, 0.04 μg/L An, 0.04 μg/L Fl, 0.04 μg/L Py, 0.04 μg/L BaA, 0.08 μg/L BbF, and 0.04 μg/L BkF; (C) 500 mL of the ultrapure water spiked with 2 μg/L Nap, 4 μg/L Acy, 0.4 μg/L Flo, 0.2 μg/L Ph, 0.2 μg/L An, 0.4 μg/L Fl, 0.2 μg/L Py, 0.2 μg/L BaA, 0.4 μg/L BbF, 0.2 μg/L BkF after preconcentrated by ESM templating of the mixed hemimicelle/admicelle as SPE adsorbent. The amount of LAS: 0.5 mmol/g ESM; flow rate of sample solution: 6 mL/min; other conditions were the same as those in Figure 2. Peak identifications: (1) Nap; (2) Acy; (3) Flo; (4) Ph; (5) An; (6) Fl; (7) Py; (8) BaA; (9) BbF; (10) BkF.

Table 2. Reproducibility among Home-Made SPE Columns RSD (%, n = 3) (for single column)

analyte

concentration level (μg/L)

Nap Acy Flo Ph Ant Fl Py BaA BbF BkF

2 2 0.5 0.2 0.25 1 1 1 0.5 1

column 1 column 2 2.4 6.3 1.2 5.2 1.2 0.8 2.3 0.9 2.6 0.7

4.8 1.5 2.0 4.0 2.2 4.4 3.9 1.8 1.3 1.6

column 3

RSD (%, n = 3) (among columns)

3.1 4.2 2.0 1.5 0.4 1.6 2.0 0.8 1.5 3.0

8.4 0.9 7.8 5.7 3.6 4.5 6.7 5.3 2.5 3.9

on water sample preconcentration by C18 were conducted following published procedures.33,34 For C18 SPE adsorbent, the recoveries of 10 PAHs varied from 45.6% to 101.0% for tap water and river water (Table 3 and Table S2, Supporting Information), and the recoveries of 10 PAHs varied from 50.8% to 75.2% for wastewater (see Table S2, Supporting Information). Hence, compared with traditional C18 SPE adsorbents, LAS hemimicelle/admicelle adsorbent exhibited higher extraction recoveries for PAHs in various aqueous samples. In conclusion, this work has well illustrated the potential of ESM templating of the mixed hemimicelle/admicelle as a novel SPE adsorbent for the preconcentration of PAHs in environmental aqueous samples. This method provided highly sensitive analysis of PAHs with a LOD of 0.1−8.6 ng/L, linearity of the response from 0.02 to 10 μg/L, and good selectivity with respect to NOM. The proposed SPE protocol was effective for the concentration of trace PAHs in environmental water sample before HPLC−UV analysis. Compared with commercially available C18 materials, the design of ESM templated hemimicelle/admicelle adsorbent for PAHs extraction has advantages as follows: (1) ESM is a biocompatible material and ESM degrades readily; thus, it can be used as a green biomaterial with little secondary contamination; (2) the peculiar macroporous network of eggshell membrane consisting of cross-linking protein fibers benefits mass transfer as a adsorbent; (3) ESM is low-cost and can be obtained in a massive amount as an waste product of food industry. It is believed that the ESM templating of the mixed hemimicelle/admicelle may be a valuable adsorbent for other micropollutants.

mixed hemimicelle/admicelle-based SPE adsorbent and conventional SPE C18 adsorbent for the extraction of 10 PAH compounds was made. Figure 5 and Figure S8 (Supporting Information) present the chromatograms of the Changjiang River water sample, wastewater sample, PAH-spiked ultrapure water, PAH-spiked Changjiang River water sample, and PAHspiked wastewater sample, enriched using LAS hemimicelle/ admicelle as adsorbent. The satisfactory recoveries of PAHs were obtained (77.8−112.7%) for all the tested water samples (see Table 3 and Table S2, Supporting Information), although the relatively low recoveries of PAHs were obtained for the wastewater sample (77.8−95.7%) (see Table S2, Supporting Information). This suggests that competitive adsorption processes might occur between the wastewater sample matrix and LAS onto ESM. This may be due to the enhancement of the coextraction of humic substances in the present study. In fact, we determined TOC content in the studied water samples in this work, and the concentration levels (mg/L) are as follows: 3.44−3.50 for river water samples, and 12.04 for the constructed wetland water sample. The NOM in the wastewater was 3 times higher than that in the river water sample. Higher TOC content would cause lower recovery, which was consistent with the result in Figure S7 (Supporting Information). The comparative experiments were performed to obtain comprehensive data of C18 and hemimicelle/ admicelle adsorbent, using several practical aqueous samples such as tap water, river water, and wastewater. The experiments 8056

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Table 3. Determination Results and Recoveries of 10 PAHs in Water Samples found (μg/L) analyte

spiked (μg/L)

Tap Water Sample Nap 0.00 0.40 2.00 Acy 0.00 0.80 4.00 Flo 0.00 0.08 0.40 Ph 0.00 0.04 0.20 An 0.00 0.04 0.20 Fl 0.00 0.04 0.20 Py 0.00 0.08 0.40 BaA 0.00 0.04 0.20 BbF 0.00 0.08 0.40 BkF 0.00 0.04 0.20

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proposed adsorbent − 0.40 1.90 − 0.89 3.87 − 0.08 0.38 − 0.04 0.20 − 0.04 0.20 − 0.04 0.19 − 0.08 0.38 − 0.05 0.19 − 0.08 0.39 − 0.04 0.20

C18 − 0.38 1.50 − 0.74 2.77 − 0.08 0.18 − 0.03 0.13 − 0.04 0.14 − 0.04 0.14 − 0.07 0.27 − 0.03 0.14 − 0.07 0.28 − 0.03 0.10

recovery (%) proposed adsorbent

found (μg/L) C18

99.2 ± 1.4 94.9 ± 2.2

93.9 ± 3.0 75.2 ± 4.0

111.1 ± 2.8 96.7 ± 2.7

92.2 ± 2.7 69.3 ± 1.3

98.2 ± 3.1 96.1 ± 3.4

98.0 ± 1.7 45.6 ± 2.8

109.0 ± 1.6 99.6 ± 0.7

71.6 ± 2.1 64.3 ± 4.1

107.9 ± 3.9 97.7 ± 1.6

91.4 ± 3.5 71.0 ± 1.4

97.9 ± 3.6 96.2 ± 3.7

84.1 ± 5.5 68.5 ± 1.6

107.7 ± 7.7 97.4 ± 2.2

93.3 ± 5.8 68.7 ± 1.7

112.7 ± 2.7 94.6 ± 0.7

78.8 ± 2.6 69.1 ± 1.4

100.0 ± 8.1 97.5 ± 0.6

88.9 ± 9.6 68.9 ± 1.2

97.4 ± 8.9 101.1 ± 1.0

70.4 ± 6.4 50.3 ± 0.9

analyte

proposed adsorbent

Changjiang Water Sample Nap 0.00 0.01 0.40 0.38 2.00 1.91 Acy 0.00 − 0.80 0.77 4.00 3.88 Flo 0.00 0.01 0.08 0.09 0.40 0.39 Ph 0.00 − 0.04 0.04 0.20 0.19 An 0.00 0.12 0.04 0.16 0.20 0.30 Fl 0.00 − 0.04 0.04 0.20 0.19 Py 0.00 0.03 0.08 0.11 0.40 0.40 BaA 0.00 − 0.04 0.04 0.20 0.19 BbF 0.00 − 0.08 0.08 0.40 0.40 BkF 0.00 − 0.04 0.04 0.20 0.19

C18 0.01 0.39 1.35 − 0.72 2.75 − 0.08 0.32 − 0.03 0.13 0.01 0.05 0.14 − 0.36 0.13 − 0.07 0.30 − 0.03 0.15 − 0.07 0.30 − 0.03 0.10

proposed adsorbent

C18

93.3 ± 7.6 95.2 ± 0.9

94.7 ± 4.6 67.2 ± 2.2

96.3 ± 1.6 97.1 ± 1.0

89.9 ± 2.7 68.7 ± 2.3

92.7 ± 6.3 94.8 ± 2.3

98.0 ± 1.7 80.0 ± 2.8

98.2 ± 10.9 94.2 ± 1.1

75.3 ± 2.1 62.6 ± 2.0

96.8 ± 0.6 94.7 ± 1.0

84.8 ± 2.6 66.0 ± 0.8

102.1 ± 7.2 96.2 ± 0.9

88.9 ± 5.5 66.7 ± 2.4

97.4 ± 4.4 92.3 ± 3.8

83.3 ± 5.8 74.7 ± 1.7

98.4 ± 2.7 96.7 ± 2.6

80.3 ± 5.2 76.3 ± 1.4

94.6 ± 2.7 99.8 ± 2.7

86.8 ± 5.4 74.2 ± 1.8

102.6 ± 11.8 97.2 ± 3.5

66.7 ± 9.6 49.7 ± 0.9

Funding

ASSOCIATED CONTENT

S Supporting Information *

Financial supports by the National Natural Science Foundation of China (21277111), the Fundamental Research Funds for the Central Universities (XDJK2010C043), and the Doctor Foundation of Southwest University (2010BSr07) are gratefully acknowledged.

The values of the parameters of the Langmuir and Freundlich equations and the values of the linear regression coefficients (Table S1); determination results and recoveries of PAHs in water samples (Table S2); FT−IR spectrum of ESM (Figure S1); adsorption isotherms, Langmuir isotherms, and Freundlich isotherms for the adsorption of four LAS homologues on ESM (Figure S2); effect of eluent solvent on extraction efficiency of PAHs (Figure S3); effects of NaCl content, KCl content, and KSCN content on recoveries of PAHs (Figure S4); effect of NaCl content on the ζ potential of 0.5 mM LAS solution (Figure S5); effect of sample volume on recoveries of PAHs based on a common way to work with hemimicelles and/or admicelles (Figure S6); effect of HA concentration on recoveries of PAHs (Figure S7); and HPLC−UV chromatograms of wastewater sample from the effluent of a constructed wetland treatment system after preconcentration by ESM templating of the mixed hemimicelle/admicelle as SPE adsorbent (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

■

spiked (μg/L)

recovery (%)

Notes

The authors declare no competing financial interest.

■

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AUTHOR INFORMATION

Corresponding Author

*Phone/fax: +86 23 68254843; e-mail: yuminghuang2000@ yahoo.com. 8057

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