Using Linoleic Acid Embedded Cellulose Acetate Membranes to in

Apr 15, 2015 - Using Linoleic Acid Embedded Cellulose Acetate Membranes to in Situ Monitor Polycyclic Aromatic Hydrocarbons in Lakes and Predict Their...
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Using Linoleic Acid Embedded Cellulose Acetate Membranes to in Situ Monitor Polycyclic Aromatic Hydrocarbons in Lakes and Predict Their Bioavailability to Submerged Macrophytes Yuqiang Tao,* Bin Xue,* and Shuchun Yao State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, 210008, China S Supporting Information *

ABSTRACT: To date no passive sampler has been used to predict bioavailability of contaminants to macrophytes. Here a novel passive sampler, linoleic acid embedded cellulose acetate membrane (LAECAM), was developed and used to in situ measure the freely dissolved concentrations of ten polycyclic aromatic hydrocarbons in the sediment porewaters and the water columns of two lakes in both winter and summer and predict their bioavailability to the shoots of resident submerged macrophytes (Potamogeton malainus, Myriophyllum spicata, Najas minor All., and Vallisneria natans (Lour.) Hara). PAH sampling by LAECAMs could reach equilibrium within 21 days. The influence of temperature on LAECAM-water partition coefficients was 0.0008− 0.0116 log units/°C. The method of LAECAM was comparable with the active sampling methods of liquid−liquid extraction combined with f DOC adjustment, centrifugation/solid-phase extraction (SPE), and filtration/SPE but had several advantages. After lipid normalization, concentrations of the PAHs in LAECAMs were not significantly different from those in the macrophytes. In contrast, concentrations of the PAHs in the triolein containing passive sampler (TECAM) deployed simultaneously with LAECAM were much higher. The results suggest that linoleic acid is more suitable than triolein as the model lipid for passive samplers to predict bioavailability of PAHs to submerged macrophytes. the freely dissolved concentrations of POPs.5 Passive samplers such as solid-phase microextraction fibers (SPMEs),6 semipermeable membrane devices (SPMDs),7 triolein embedded cellulose acetate membranes (TECAMs),8 low-density polyethylene membranes (LDPEs), 9 thin polyoxymethylene sheets,10 passive integrative sampling devices,11 and C18 Empore disks12 have been used for in situ sampling of POPs in waters, which offer several advantages over large-volume sampling approaches.13 Among these passive samplers, model lipids containing devices, in particular the triolein containing ones, have been widely used to predict bioavailability of POPs to fish and aquatic invertebrates.7,14 However, to date no passive sampler has been used for in situ prediction of bioavailability of POPs to submerged macrophytes.

1. INTRODUCTION Persistent organic pollutants (POPs) are present in lakes worldwide.1 They can be accumulated by primary producers and then transferred to higher trophic levels via the food webs, posing great risks to ecosystems and human health. Ecological risk and human health assessments are often challenged by a limited amount of site-specific data for contaminant concentrations in resident aquatic biota. In situ monitoring provides a means to fill this knowledge gap. However, in situ monitoring of POPs in lakes particularly at large scale is a difficult task, which largely depends on the development of the sampling techniques. To achieve sufficient sensitivity, some large-volume sampling approaches, such as in situ water pumping,2 on-board filtration/ liquid−liquid extraction,3 and on-board filtration/solid-phase extraction (SPE)4 have been applied in field studies. However, these sampling methods are either labor-intensive or costly to implement and are not feasible for large-scale sampling programs. Additionally, these methods tend to overestimate © XXXX American Chemical Society

Received: September 30, 2014 Revised: April 10, 2015 Accepted: April 15, 2015

A

DOI: 10.1021/acs.est.5b00863 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

sampler was composed of two separable units, namely a semipermeable membrane and a rigid cylindrical protective housing. The semipermeable membrane was prepared as aforementioned. The cylindrical protective housing was made of stainless steel pipe, which was screwed with two stainless steel tips. The tips were perforated with 100-mesh screens through which water and fine-grained sediment particles were able to pass and come in, while larger objects were kept outside. The protective housing was attached to two ropes and a float. Laboratory Verification. To validate the method, LAECAMs were embedded in eight surface sediments collected from Lake Taihu to sample the PAHs in the sediment porewaters at room temperature. They were collected from the sediments after 30 days. The freely dissolved concentrations of the PAHs in the sediment porewaters and the water columns measured by LAECAMs (CW−LAECAM) were calculated as

Submerged macrophytes are primary producers in aquatic ecosystems. Many of them have very few roots or rhizoids, but their shoots can form dense beds on sediments. They contain a large amount of C16, C18, and C24 unsaturated fatty acid and esters and are able to accumulate and redistribute POPs in waters.15 Compared with fish and aquatic invertebrates, submerged macrophytes are sessile biomonitors of POPs with lower lipid contents. However, lots of lakes lack submerged macrophytes due to serious eutrophication and contamination. Furthermore, obtaining necessary amounts of submerged macrophytes from lakes is challenging and time-consuming. Additionally, different growth rates may exist for the same submerged macrophyte species living in different lakes even in different areas of the same lake. These factors weaken the ability to use submerged macrophytes to monitor aquatic POPs. Similar to fish and aquatic invertebrates, the nature of the bioconcentration process of POPs by submerged macrophytes is largely the result of lipid−water partitioning.15,16 It is thereby essential to seek a suitable lipid containing passive sampler as a surrogate for submerged macrophytes to monitor aquatic POPs. In this study, linoleic acid (9,12-octadecadienoic acid), a major component of macrophyte lipids,17 was embedded in the matrix of cellulose acetate polymer to develop a novel semipermeable membrane sampler, linoleic acid embedded cellulose acetate membrane (LAECAM). The method validation of LAECAM was conducted with eight surface sediments in the laboratory. LAECAMs were then deployed for in situ measurement of the freely dissolved concentrations of ten polycyclic aromatic hydrocarbons (PAHs) in the sediment porewaters and the water columns in two lakes in Nanjing, China, in both winter and summer. They were finally applied for in situ prediction of bioavailability of the PAHs to the shoots of resident submerged macrophytes (Potamogeton malainus, Myriophyllum spicata, Najas minor All., and Vallisneria natans (Lour.) Hara). The method of LAECAM was also compared with three active sampling techniques (liquid−liquid extraction, filtration/SPE and centrifugation/SPE) and another passive sampler (TECAM) to demonstrate its reliability.

C W−LAECAM =

C LAECAM KLAECAM−W

(1)

where CLAECAM is the concentration of the PAH in LAECAM when sampling equilibrium is reached; KLAECAM−W is the LAECAM−water partition coefficient of the PAHs. The sediment porewaters were collected by centrifugation when LAECAMs were collected, and then filtrated with 0.7-μm GF/F glass fiber filters. Content of dissolved organic carbon ( f DOC) of the filtrates was measured. One hundred milliliters of the filtrate were extracted three times with 20 mL of n-hexane. The extraction solutions of n-hexane were combined and evaporated, solvent exchanged to acetonitrile (0.1 mL) and stored prior to the analysis of the PAHs. Concentrations of the PAHs measured by liquid−liquid extraction were further adjusted for f DOC and DOC−water partition coefficients (KDOC) of the PAHs as C W−DOC =

C W−L 1 + fDOC KDOC

(2)

where CW−DOC is the concentration of the PAH in the porewater measured by liquid−liquid extraction with f DOC adjustment; CW−L is the concentration of the PAH in the porewater measured by liquid−liquid extraction without f DOC adjustment; KDOC of the PAHs was calculated according to the review work of Burkhard20 as

2. MATERIALS AND METHODS Preparation of the Semipermeable Membranes. LAECAMs were prepared according to our patents.18,19 Homogeneous solution of cellulose acetate (17.5 wt %) was prepared in the solvent mixture of acetone (69 wt %) and 1,4dioxane (10 wt %). The additives of anhydrous magnesium perchlorate (2 wt %) and linoleic acid (1.5 wt %) were then added by ultrasonic oscillation for 30 min. The solution was finally kept at 25 °C overnight. To control the thickness of the membranes, this solution was cast on a glass plate by spreading it between thin wires (200 μm diameter) with a glass stick. After solvent evaporation for 60 s at 25 °C, the membranes were immersed into a distilled water coagulation bath (25 °C) for at least 10 min to ensure complete phase separation, and then rinsed with distilled water. TECAMs were prepared using the same method as LAECAMs except for the addition of triolein instead of linoleic acid. All the membranes were kept in distilled water and precleaned by extraction with n-hexane before use. There was no obvious loss or degradation of linoleic acid in LAECAMs and of triolein in TECAMs when they were kept in distilled water and dialyzed in n-hexane. Sampler Design. The illustration of the passive sampler is shown in Figure S1 in the Supporting Information. The

KDOC = 0.08K OW

(3)

CW−LAECAM was compared with CW−DOC and CW−L. Field Deployment in the Water Columns. LAECAMs with protective housings were deployed simultaneously in the water columns at two sites in Lake Yueya (0.17 km2, 1.50 m) and at two sites in Lake Xuanwu (3.68 km2, 1.14 m) during Jan 13th and Feb 13th (winter, 7−6 °C) and May 12th and June 12th 2014 (summer, 27−32 °C), respectively. The sampling sites are shown in Figure S2. LAECAMs were collected after 1, 4, 7, 11, 14, 18, 21, and 30 day deployments for the sampling kinetics study, and 5 L of water were collected simultaneously. One litter of the collected water was filtrated with a 0.7-μm GF/F glass fiber filter, then extracted with a C18 SPE column, and finally eluted with 18 mL of n-hexane/dichloromethane (4:1 v/v). The eluate was evaporated, solvent exchanged to acetonitrile, and stored prior to the analysis of the PAHs. Field Deployment in the Sediment Porewaters. LAECAMs with protective housings were deployed in the sediment porewaters at two sites in Lake Xuanwu during Jan B

DOI: 10.1021/acs.est.5b00863 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Data Analysis and Quality Control. Calibration standards with six concentration levels were prepared. Results were obtained from three replicates. Calibration standards and solvent blanks were run with each set of analysis, and the blank values were subtracted. Blanks of LAECAMs were measured, and the concentrations of the PAHs in the blanks were subtracted. Extraction recoveries for the PAHs in LAECAMs were measured by adding the standard solutions of the PAHs into acetone and 1,4-dioxane when the membranes were prepared, which were 79% for Nap and higher than 90% for the other PAHs. The method quantitation limit (MQL) was determined for each PAH by measuring the coincident instrumental response in blank samples taken through the entire processing and analysis procedure. MQL was defined as the mean plus ten standard deviations.11 For those PAHs of which MQLs were below the levels of the calibration curves used in the HPLC analysis, MQLs were set at the values of the lowest levels of the calibration curves used in quantifying the PAHs. MQLs for the PAHs in the water columns and porewaters were calculated accordingly using eq 1. The means and standard deviations were calculated using Origin 7.0. One-Sample T Test was used to compare the means. Analyses of variance (ANOVA) followed by a Tukey’s pairwise comparison test were used to evaluate the differences in means between groups. Results were considered to be significant if p ≤ 0.05.

13th and Feb 13th and at two sites in Lake Yueya during May 12th and June 12th 2014 (Figure S2). The devices were sunk by their own weights and immersed in the sediments without additional ballasts. The devices were collected after a 30 day deployment. Surface sediments were collected from the sampling sites when LAECAMs were deployed and collected, respectively. The porewaters of the surface sediments were collected by centrifugation, then filtrated and extracted with C18 SPE columns for the analysis of the PAHs. Prediction of Bioavailability of the PAHs to the Macrophytes. LAECAMs and TECAMs with protective housings were deployed simultaneously in the water column at site 1 in Lake Xuanwu during May 12th and June 12th 2014 (Figure S2). Shoots of the resident submerged macrophytes were collected together with LAECAMs and TECAMs. Because these macrophytes have very few or no roots or rhizoids, roots or rhizoids were not able to be collected. Shoots of the macrophytes were washed, freeze-dried and sliced. An aliquot of 0.1 g freeze-dried shoots was extracted with 70 mL of nhexane/dichloromethane (1:1 v/v) at 60 °C for 24 h using Soxhlet devices. The extracts were evaporated, purged to about 1−2 mL under a gentle nitrogen stream, and then cleaned with a silica (2 g, 6% water)/anhydrous sodium sulfate composite column and eluted with 20 mL of n-hexane/dichloromethane (4:1 v/v). The eluates were evaporated, solvent exchanged to acetonitrile (1 mL) and stored prior to the analysis of the PAHs. An aliquot of 0.2 g freeze-dried shoots was extracted with 70 mL of chloroform and methanol (2:1, v/v) at 75 °C for 24 h using Soxhlet devices to measure the content of lipids. The extract was dried in a rotary evaporator, redissolved in 20 mL of n-hexane and then filtered through a 0.7-μm GF/F glass fiber filter into a preweighed glass tube to remove precipitates. The glass tube with the extract was then dried to a constant weight. The weight of the residue in the glass tube was considered as the weight of the lipids.21 Extraction of the PAHs from LAECAMs and TECAMs. After sampling, LAECAMs and TECAMs were rinsed with distilled water and dipped in 1 mol/L HCl and 1 mol/L NaOH to remove the particles on the outer surface. They were then rinsed with distilled water, wiped up with clean filter paper, and dialyzed in 20 mL of n-hexane for 24 h and rinsed with nhexane (1 mL × 3). The n-hexane solutions were combined, then evaporated under a gentle nitrogen stream, and finally solvent exchanged to acetonitrile (1 mL) and stored prior to the analysis of the PAHs. LAECAMs and TECAMs were stable in n-hexane, with negligible weight loss of lipids and cellulose acetate during dialysis. Chemical Analysis. Naphthalene (Nap), phenanthrene (Phe), anthracene (Ant), fluoranthene (Fla), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), and benzo[a]pyrene (BaP) were analyzed using a HPLC-FLD (Shimadzu LC20AT). Chromatographic separation and resolution were best achieved using a reverse-phase C18 column (4.6 × 250 mm, 5 μm particle size) with the mobile phase of acetonitrile/water (0.75 mL/min, 0−10 min 60:40, 10−50 min from 60:40 to 100:0, and post run 5 min). The C18 column was kept at 25 °C. The excitation wavelength was 260 nm. The emission wavelength was 380 nm for Nap, Phe, Ant, and Fla, and 420 nm for Pyr, BaA, Chr, BbF, BkF, and BaP. Standard samples of the PAHs were used for the quality and quantity analysis.

3. RESULTS AND DISCUSSION Method Validation. Sampling kinetics of the PAHs by LAECAMs is illustrated in Figure S3. The results indicate that sampling of Nap, Phe, and Ant by LAECAMs can reach equilibrium within 14 days, and sampling of Fla, Pyr, BaA, Chr, BbF, BkF, and BaP can reach equilibrium within 21 days. Sampling kinetics of the PAHs by LAECAMs are comparable with those by other passive samplers, such as semipermeable membrane devices (SPMDs)7 and low-density polyethylene membranes (LDPEs).22 The logarithm of LAECAM−water partition coefficients of the PAHs (KLAECAM−W) was measured in the water columns in Lake Xuanwu in both winter (7−6 °C) and summer (27−32 °C), which is shown in Table 1. Significant linear correlations are found between log KLAECAM−W and log Kow of the PAHs, which can be expressed as log KLAECAM − W = 0.722log Kow + 0.549, R2 = 0.976 (in winter 7 − 6 °C)

(4)

Table 1. Logarithm of n-Octanol−Water Partition Coefficients (log Kow) and LAECAM−Water Partition Coefficients of the PAHs (KLAECAM−W)

C

PAHs

log Kow

log KLAECAM−W (7−6 °C)

log KLAECAM−W (27−32 °C)

Nap Phe Ant Fla Pyr BaA Chr BbF BkF BaP

3.45 4.46 4.54 5.30 5.20 5.91 5.61 5.78 6.20 6.35

2.90 3.74 4.00 4.36 4.48 4.75 4.58 4.75 5.01 5.05

3.15 3.92 4.20 4.50 4.49 4.91 4.72 4.78 5.19 5.32 DOI: 10.1021/acs.est.5b00863 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 1. Concentrations of the PAHs in the porewaters of eight surface sediments collected from Lake Taihu measured by the methods of liquid− liquid extraction, LAECAM and liquid−liquid extraction combined with f DOC adjustment, respectively. The label of the x-axis is the number of the sediments. The treatments with significant difference (p ≤ 0.05) are denoted with asterisk.

was not significantly different from the corresponding CW−L and CW−DOC (Figure 1). However, in the case of BaP, CW−LAECAM was much lower than CW−L. It may be ascribed to the extremely high hydrophobicity of BaP (log Kow = 6.35), which results in its high affinity for dissolved organic matter. Because the method of liquid−liquid extraction is unable to separate the freely dissolved fraction and the fraction affiliated to dissolved organic matter, the freely dissolved concentration of BaP in the porewaters was overestimated by the method of liquid−liquid extraction. After adjusted for f DOC and KDOC, CW−LAECAM of BaP was not significantly different from the corresponding CW−DOC (Figure 1). The results suggest that the method of LAECAM is comparable with the method of liquid−liquid extraction combined with f DOC adjustment, which is able to be used to sample the freely dissolved fraction of the PAHs in the presence of high concentration of DOC. Measurement of the PAHs in the Water Columns. CW−LAECAM of the PAHs in the water columns of Lake Yueya was calculated using eq 1. Concentrations of the PAHs in the water columns were also measured by the method of filtration/ SPE when LAECAMs were deployed and collected, respectively. The freely dissolved concentrations of the PAHs in the water columns at both sites changed little during the sampling periods of LAECAMs in both winter and summer (Figure 2), which suggests that the water columns were not depleted in the PAHs during the sampling periods of LAECAMs. CW−LAECAM of the PAHs in the water columns at sites 1 and 2 in Lake Yueya ranged from 5.05 to 181.73 ng/L and from 7.32 to 279.07 ng/L in winter, and varied from 3.29 to 154.86 ng/L and from 3.28 to 152.85 ng/L in summer (Figure 2). Nap and Phe were the dominant PAHs in the water columns of Lake Yueya in winter, with their mean freely dissolved concentrations as high as 246.4

log KLAECAM − W = 0.701 log Kow + 0.813, R2 = 0.980 (in summer 27 − 32 °C)

(5)

Log KLAECAM−W of the PAHs in summer is 0.019−0.266 log units higher than those in winter. The influence of temperature on KLAECAM−W of the PAHs is small, which is 0.0008−0.0116 log units/°C (Table 1). The results are consistent with the studies of Booij et al.22 and Vrana et al.23 which suggested that SPMD−water partition coefficients and C18 Empore disk− water partition coefficients did not significantly change with temperature. The results are also in line with the study of Leo et al. which indicated that the influence of temperature on Kow was small for a number of compounds (0.001−0.01 log units/ °C).24 KLAECAM−W of the PAHs at other temperatures can thereby be estimated. MQLs of Nap, Phe, Ant, Fla, Pyr, BaA, Chr, BbF, BkF, and BaP are 1.44, 0.79, 0.50, 0.59, 0.44, 0.33, 0.38, 0.19, 0.22, and 0.18 ng/L in water, which are one to two orders of magnitude lower than the corresponding concentrations in the water columns of lakes, such as Lake Taihu,25 Lake Chaohu,26 Lake Xuanwu, and Lake Yueya (this study) in China. CW−LAECAM and CW−DOC of the PAHs in the porewaters of the surface sediments collected from Lake Taihu were calculated using eqs 1 and 2. CW−LAECAM was compared with CW−DOC and CW−L (Figure 1). Results of Nap, Phe, Pyr, and BaP are shown as the examples of 2−5 ring PAHs here. CW−LAECAM of Nap, Phe, Pyr, and BaP ranged from 539.4 to 1687.5 ng/L, from 130.0 to 1514.1 ng/L, from 137.8 to 1305.4 ng/L, and from 2.3 to 16.1 ng/L, respectively. Although f DOC in the sediment porewaters was high (from 18.7 to 41.6 mg/L), sampling of the PAHs by LAECAMs was not affected by DOC. Statistic analysis indicated that CW−LAECAM of Nap, Phe, and Pyr D

DOI: 10.1021/acs.est.5b00863 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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method of LAECAM is comparable with the method of filtration/SPE to measure the PAHs in water columns. Measurement of the PAHs in the Sediment Porewaters. CW−LAECAM of the PAHs in the sediment porewaters of Lake Xuanwu and Lake Yueya was calculated using eq 1. The method of centrifugation/SPE was also used to measure the concentrations of the PAHs in the sediment porewaters of the two lakes when LAECAMs were deployed and collected, respectively. Concentrations of the PAHs in the sediment porewaters of the two lakes showed little variation during the sampling periods of LAECAMs (Figure 3), which suggests that the porewaters were not depleted in the PAHs during the sampling periods. CW−LAECAM of the PAHs in the sediment porewaters at the two sites in Lake Xuanwu ranged from 35.2 to 516.1 ng/L and from 18.4 to 588.1 ng/L, respectively (Figure 3). Fla and Pyr were the dominant PAHs in the sediment porewaters of Lake Xuanwu. CW−LAECAM of Phe, Pyr, Fla, Chr, and BbF in the sediment porewaters at these two sites was significantly different (Figure 3). Site specific sediment organic carbon−water partitioning coefficients (Koc) for the PAHs are listed in Table S1. Different CW‑LAECAM of Phe, Pyr, Fla, Chr, and BbF in the sediment porewaters can be ascribed to the different Koc at these two sites. The results suggest that LAECAMs are sensitive to the difference in Koc at different sites. CW−LAECAM of the PAHs in the sediment porewaters at the two sites in Lake Yueya ranged from 6.12 to 222.23 ng/L and from 10.45 to 329.81 ng/L, respectively, which was much lower than the concentrations in Lake Xuanwu. Nap, Phe, Pyr and Fla were the dominant PAHs in the sediment porewaters of Lake Yueya. The freely dissolved concentrations of the PAHs in the sediment porewaters of Lake Xuanwu and Lake Yueya measured by LAECAMs were much higher than those of Delfzijl Harbor measured by LDPEs28 and those of Oslo Harbor measured by SPMDs.29 The freely dissolved concentrations of the PAHs in the sediment porewaters of Lake Yueya were much higher than the corresponding concentrations in the water columns (Figures 2 and 3), indicating fluxes direction of the PAHs from the porewaters to the overlying water in Lake Yueya. CW−LAECAM of the PAHs in the sediment porewaters at both sites in Lake Xuanwu and Lake Yueya was not significantly different from the corresponding concentrations measured by the method of centrifugation/SPE (Figure 3). The results suggest that the method of LAECAM is comparable with the method of centrifugation/SPE to measure the freely dissolved PAHs in field sediment porewaters in both winter and summer. Prediction of Bioavailability of the PAHs to Macrophytes. Although rooted submerged macrophytes are able to absorb POPs from both sediment and overlying water, the roots or rhizoids of submerged macrophytes only constitute 1− 3% of the total plant biomass,30 so the uptake of POPs by roots only constitute a small fraction of the total uptake. Because the studied macrophytes have very few or no roots or rhizoids, roots or rhizoids were not able to be collected in this study. Uptake of the PAHs by roots or rhizoids was thereby not studied here. Previous studies have suggested that acropetal translocation of hydrophobic compounds from roots or rhizoids to shoots in submerged macrophytes is much less than that in terrestrial plants and emergent plants,31−34 even does not occur35 because submerged macrophytes lack transport processes driven by transpiration. Therefore, the uptake of POPs by shoots of submerged macrophytes mainly comes from the overlying water.

Figure 2. Concentrations of the PAHs in the water columns of Lake Yueya in both winter (20140113−20140213) and summer (20140512−20140612) measured by filtration/solid-phase extraction (SPE) at the beginning and the end of the sampling periods of LAECAMs and by in situ LAECAMs.

± 26.8 and 144.3 ± 13.5 ng/L. The abundance of Pyr and Fla increased in summer. Nap, Phe, Pyr, and Fla became the dominant PAHs in the water columns of Lake Yueya in summer, with their mean freely dissolved concentrations as high as 127.7 ± 8.5, 158.5 ± 8.5, 94.7 ± 5.3, and 49.1 ± 4.7 ng/ L. The freely dissolved concentrations of the PAHs in the water columns of Lake Yueya were much higher than those of a constructed wetland in Columbia measured by a passive integrative sampler11 and those in four reservoirs of Guangdong Province, China, measured by LDPEs.27 In both winter and summer, CW−LAECAM of the PAHs in the water columns at both sites of Lake Yueya was not significantly different from the concentrations measured by the method of filtration/SPE (Figure 2). The results demonstrate that the E

DOI: 10.1021/acs.est.5b00863 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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The dry weight lipid contents of P. malainus, M. spicata, N. minor, and V. natans were 3.71%, 3.34%, 9.79%, and 8.84%, respectively. Although the lipid contents of the submerged macrophytes were different, there was no significant difference in the concentrations of the PAHs in the macrophytes after lipid normalization (Figure 4). The results verify that lipids

Figure 4. Concentrations of the PAHs in the macrophytes, LAECAMs and TECAMs on the basis of the dry weight of the lipid contents. The treatments with significant difference (p ≤ 0.05) are denoted with asterisks.

dominate the uptake of the PAHs by the submerged macrophytes. Our previous studies indicated that the partition contribution by cellulose acetate was not important relative to that by the model lipids for the compounds with log Kow > 3.0.21 Concentrations of the PAHs in LAECAMs and TECAMs were thereby normalized based on the contents of linoleic acid and triolein, respectively. The dry weight lipid contents of both LAECAM and TECAM were 7.14%. After lipid normalization, concentrations of the PAHs in LAECAMs were not significantly different from those in the submerged macrophytes (Figure 4), which demonstrates that the affinity of the PAHs for linoleic acid is comparable with that for the macrophyte lipids. In contrast, concentrations of the PAHs in TECAMs were significantly higher than those in both LAECAMs and the submerged macrophytes after lipid normalization. The logarithm of TECAM−water partition coefficients of the PAHs (KTECAM−W) was calculated according to our previous study38 as log KTECAM − W = 0.98log Kow − 0.28

(R2 = 0.998)

Take Nap, Phe, Pyr, and BaP for the examples. Log KTECAM‑W of Nap, Phe, Pyr, and BaP is 3.10, 4.09, 4.91, and 5.94,38 which is much higher than the values of 2.90, 3.74, 4.36, and 5.05 of log KLAECAM−W (Table 1). The compositions and properties of LAECAMs and TECAMs are almost the same except for the different model lipids. The concentration ratios of the PAHs in TECAMs and LAECAMs (CTECAM/CLAECAM) are consistent with the corresponding ratios of KTECAM−W/KLAECAM−W. The higher concentrations of the PAHs in TECAMs than those in LAECAMs can thereby be ascribed to the higher affinity of the PAHs for triolein than for linoleic acid. Therefore, linoleic acid may be more suitable than triolein to be the model lipid for passive samplers for in situ prediction of bioavailability of PAHs to submerged macrophytes. Method Overview. The results suggest that the method of LAECAM is comparable with the active sampling methods of liquid−liquid extraction combined with f DOC adjustment, centrifugation/SPE and filtration/SPE. However, our method

Figure 3. Concentrations of the PAHs in the sediment porewaters of Lake Xuanwu (winter, 20140113−20140213) and Lake Yueya (summer, 20140512−20140612) measured by centrifugation/solidphase extraction (SPE) at the beginning and the end of the sampling periods of LAECAMs and by in situ LAECAMs.

It is often assumed that metabolism of hydrophobic contaminants in macrophytes is negligible, and that contaminant accumulation is a passive process governed by the compound’s affinity for the lipids of plants.16,17 The partitionlimited model proposed by Chiou et al.36 demonstrated that the partition contribution by carbohydrates became unimportant relative to that by the lipids for compounds with log Kow > 3.0. Jonker37 found that the affinity of PAHs for cellulose was about 400 times lower than that for n-octanol. Thereby, concentrations of the PAHs in the shoots of the submerged macrophytes were normalized based on the lipid contents here. F

DOI: 10.1021/acs.est.5b00863 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

and analysis procedures of LAECAMs are simple and costeffective, they may be applied for large-scale monitoring programs of POPs in lakes at different environmental temperatures, which will be of great help to ecological risk and human health assessments.

has several advantages and the greatest one is its ability of in situ sampling. To achieve sufficient sensitivity, large-volume of water, porewater and sediment have to be collected and transported when the methods of liquid−liquid extraction combined with f DOC adjustment, centrifugation/SPE and filtration/SPE are used. The sediment is often stored by freezing for several days before it arrives at the laboratory. When freezing and thawing the sediment, chemicals that were previously particle-bound and thus not bioavailable might be released. It will alter freely dissolved concentrations and thus lead to overestimation of the effective concentration. In contrast, only a small piece of LAECAM is needed to be deployed in field due to the high affinity of the PAHs for linoleic acid. Furthermore, since KDOC varies greatly with the properties of DOC, the results obtained from the method of liquid−liquid extraction combined with f DOC adjustment will deviate from the true concentrations if the properties of DOC differ greatly from site to site. The deviation is especially magnified if the hydrophobicity of the chemical is extremely high. Changes in water quality, and loss of POPs due to centrifugation and filtration will result in deviations when the methods of centrifugation/SPE and filtration/SPE are used. In contrast, LAECAM is sensitive to the slight differences in KOC and KDOC at different sites or lakes, and is deployed in water columns or immersed in sediment porewaters by its own weight without additional disturbance to the water quality. Additionally, the concentrations obtained by active sampling approaches such as liquid−liquid extraction combined with f DOC adjustment, centrifugation/SPE and filtration/SPE are snapshots of environmental values, whereas those from LAECAMs are time-integrated. This deviation between active sampling approaches and passive approaches will be magnified if concentrations of the POPs in lakes change dramatically during the sampling periods. The field results and MQLs demonstrate that the method of LAECAM is able to in situ monitor PAHs in water columns and sediment porewaters in different lakes in both winter and summer. With the calibration of temperature, LAECAMs may be used in field at different environmental temperatures. After lipid normalization, concentrations of the PAHs in LAECAMs were not significantly different from those in the shoots of resident submerged macrophytes. Because unpurified macrophyte lipids generally contain a range of lipid classes, which often include some constituents of relatively low molecular weight and polarity, minor amounts of macrophyte lipids may diffuse through the membranes during the dialysis process. In order to avoid the leakage and interferences of unpurified macrophyte lipids, high purity linoleic acid may be more suitable than macrophyte lipids to be the model lipid for passive samplers. Triolein is generally the largest mass fraction of neutral lipids in freshwater fish.39 Although triolein has been widely used as the model lipid for passive samplers to predict bioavailability of POPs to aquatic organisms,8,15 the results of this study indicate that linoleic acid is more suitable than triolein to be the model lipid for passive samplers to predict bioavailability of PAHs to shoots of submerged macrophytes. Therefore, this study, for the first time to our best knowledge, successfully provides a linoleic acid containing passive sampler (LAECAM) to in situ monitor PAHs in lakes and predict their bioavailability to shoots of resident submerged macrophytes. The ability of LAECAM to in situ predict bioavailability of POPs to roots or rhizoids of submerged macrophytes will be tested in our future study. Because the preparation, sampling,

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ASSOCIATED CONTENT

S Supporting Information *

Illustrations of linoleic acid embedded cellulose acetate membrane (LAECAM) with the protective housing, illutstration of the sampling sites, sampling kinetics of the PAHs by LAECMs, and Koc of the PAHs at two sites in Lake Xuanwu. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). Tel.: +86-25-86882219. Fax: +86-25-57714759. *E-mail: [email protected] (B.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by Natural Science Foundation of China (21107118 and 41471400), Natural Science Foundation of Jiangsu Province (BK20141514), Foundation of Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (NIGLAS2010KXJ01), and National Basic Technological Research of China (2014FY110400).

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DOI: 10.1021/acs.est.5b00863 Environ. Sci. Technol. XXXX, XXX, XXX−XXX