Field application of passive sampling techniques for sensing hydrophobic organic contaminants

Trends in Environmental Analytical Chemistry 1 (2014) e19–e24

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Trends in Environmental Analytical Chemistry journal homepage: www.elsevier.com/locate/teac

Review

Field application of passive sampling techniques for sensing hydrophobic organic contaminants Lian-Jun Bao, Eddy Y. Zeng * State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

A R T I C L E I N F O

A B S T R A C T

Keywords: In situ passive sampling Hydrophobic organic contaminants Global monitoring network Site-specific partition coefficient Bioavailability assessment Flux measurement

Progress in the field application of passive sampling techniques for sensing hydrophobic organic contaminates (HOCs) was reviewed. Field applications of in situ passive sampling methods can be categorized as (1) measurement of atmospheric and dissolved HOCs; (2) measurement of site-specific phase partition coefficients; (3) bioavailability assessment; and (4) measurement of inter-compartmental flux. Compared to the comprehensive global monitoring of atmospheric HOCs, in situ measurements of dissolved HOCs in open water and sediment porewater with passive samplers have remained limited. Polymer-coated fibers with small sampling volumes are preferable for determining site-specific phase partition coefficients, which are important parameters in fugacity-based modeling of the geochemical fate of HOCs. Furthermore, field assessment of bioavailability with ex situ or in situ passive samplers needs to be further validated, whereas toxicity assessment can be improved with biodegradation or ingestion of particle-bound HOCs by worms taken into account. A passive sampling device was developed and used to obtain the diffusive fluxes of dichlorodiphenyltrichloroethane and its metabolites across the sediment–water interface, but measurements of fluxes of HOCs across the soil–air and air–water interfaces have been far from success. ß 2013 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurement of atmospheric and freely dissolved HOCs Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Sediment porewater . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Measurement of site-specific partition coefficients . . . . . Bioavailability assessment . . . . . . . . . . . . . . . . . . . . . . . . . Measurement of inter-compartmental fluxes . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Hydrophobic organic contaminants (HOCs), such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and polybrominated diphenyl ethers (PBDEs), in environmental compartments (e.g., air, water

* Corresponding author. Tel.: +86 20 85291421; fax: +86 20 85290706. E-mail address: [email protected] (E.Y. Zeng). 2214-1588/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.teac.2013.11.003

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and sediment) have been paid much attention in recent decades. Monitoring of HOCs in air, water and sediment across the globe has provided updates on worldwide occurrence, global geochemical transport and ecological risk of HOCs [1,2]. Conventional sampling methods for sensing HOCs in environmental compartments follow a general protocol of sample collection with active samplers, i.e., a high-volume air sampler, pump and grab sampler, filtration or extraction (Soxhlet and liquid–liquid extraction), and then purification and instrumental analysis [3]. These active sampling methods require power supply which may be difficult to satisfy in

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remote areas. Thus use of conventional active sampling methods to monitor HOCs in the global environment is nearly impossible. Passive sampling techniques are easy to operate and costeffective, hence create new opportunities for monitoring HOCs in air, water, soil and sediment throughout the world. For instance, the Global Atmospheric Passive Sampling Network initiated in December 2004 conducted a survey of persistent organic pollutants (POPs) and priority chemicals in air with passive air samplers [4]. Until now, this monitoring program has covered more than 55 sites in urban, rural and remote regions of seven continents [5]. Lohmann and Muir [2] also called for establishing a monitoring network of POPs in global aquatic environment using passive sampling devices, especially with polyethylene (PE) as the sorption phase. Passive sampling techniques are mainly based on the phase partitioning of target analytes resulting from the chemical potential gradient between the sampler’s sorbent phase and sampling matrix [6]. Generally, available passive samplers for HOCs, such as semipermeable member device (SPMD) [7], solid phase microextraction (SPME) fiber [8], polyurethane foam (PUF) disk [1], PE device [9] and polyoxymethlyene film (POM) [10], consist of a sorption phase (sorbent) and a protection/ support mechanism used to minimize physical/microbial damage and/or facilitate subsequent instrumental analysis. Early applications of passive sampling techniques mainly involved laboratory measurement by passive samplers of dissolved concentrations of HOCs in water or sediment collected

with active sampling methods and transported to the laboratory. However, factors impacting the partitioning processes of dissolved HOCs among the field environmental compartments, such as temperature and salinity, may not be accurately accounted for in laboratory simulation [11]. On the other hand, substantial losses (as much as 50–70%) of PAHs may occur during transport and storage of water samples, even if the samples are stored in dark for only 48 h in glass vials [12]. In addition, particle-bound HOCs can be released during the freezing and thawing processes of solid samples [13]. Therefore, in situ passive sampling of HOCs may be an advantageous approach over active sampling in many field applications, and it is worthwhile to comprehend the current state of passive sampling techniques suitable for field applications. So far, available literature reviews on passive sampling techniques have mostly focused on calibration methods, assembly of passive samplers, and the principles or limitations of limited applications, e.g., biomedical analysis, in vivo analysis, and bioavailability measurement [14–18]. Conversely, field applications of passive sampling methods have seldom been reviewed. To fill this knowledge gap, the present review compiles and analyzes available information about the new development of typical passive samplers (Table 1 and Fig. 1) for sensing HOCs in air, water and sediments. The compiled information is synthesized to characterize the field application of passive sampling techniques, such as monitoring data, assessment of bioavailability, determination of site-specific partition coefficients and inter-compartmental flux measurement.

Fig. 1. Typical field passive samplers: (a) polyurethane foam (PUF) disks [1]; (b) polydimethylsiloxane (PDMS) in situ sampler [13]; (c) polyethylene (PE) and polyoxymethlyene (POM) mounted on a sediment-penetrating rods [32]; (d) multi-section passive sampler [31], LDPE = low density polyethylene, GFF = glass fiber filter; (e) infinite-sink benthic flux chamber [55], SPMD = semipermeable member device; (f) passive sampling device [56].

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Table 1 Detection limits of hydrophobic organic chemicals with typical passive samplers based on calibration methods in the field application. Sampler

Sorbent phase

Calibration method

Detection limits

Field sites (i.e.,)

Reference

PUF (4.4 g) (0.0213 g/cm3)

Sampling rate calibration (mass transfer is air-controlled)

5 (PAHsg) 0.2–11 (PCNsh) 0.8–10 (PBDEsi) 0.25–5 (PCBsj) 0.4–14 (OCPsk) 0.41–1.2 (PCBs) 0.43–1.1 (neutral PFCsl) 0.0087–0.18 (ionic PFCs)

London

[58] [58] [59] [59] [59] [60] [60] [60]

0.05–0.4 (PBDEs) 73 (p,p0 -DDE) 43 (o,p0 -DDE) 90 (o,p0 -DDD) 0.04–60 (PAHs) 0.6–8.3 (DDTsm)

Scheldt estuaryo South California Bight

Reservoirs (China) Hailing Bay (China)

[7] [21] [22] [21] [26] [26]

0.005–50 (PAHs)

Elbe Riverp

[13]

Hailing Bay (China)

[31]

3

Air (pg/m ) PUFa disk

SIPb disk

PUF impregnated with XAD-4 PUF (4.4 g; 0.0213 g/cm3) XAD-4 (0.5 g; 0.75 mm)

Water (pg/L) SPMDc SPMEd-based sampler

Triolein (0.27 g) PDMSe fiber (100 mm)

Passive water sampler

LDPEf (10 g; 50 mm)

Sediment porewater (pg/L) In situ sampler Muti-section passive sampler

PDMS fibers (length: 95 mm; 30 mm) LDPE (0.13 g; 50 mm)

Equilibrium

Equilibrium Sampling rate-calibrated

n

810–1880 (DDXs )

London

Point Reyes, California

a

PUF = polyurethane foam. b SIP = sorbent-impregnated polyurethane foam. c SPMD = semipermeable member device. d SPME = solid phase microextraction. e PDMS = polydimethylsiloxane. f LDPE = low density polyethylene. g PAHs = polycyclic aromatic hydrocarbons. h PCNs = polychlorinated naphthalenes. i PBDEs = polybrominated diphenyl ethers. j PCBs = polychlorinated biphenyls. k OCPs = organochlorine pesticides. l PFCs = polyfluoroalkyl compounds. m Sum of p,p0 -DDT, p,p0 -DDD, p,p0 -DDE, o,p0 -DDT, o,p0 -DDD and o,p0 -DDE. n Sum of p,p0 -DDT, p,p0 -DDD, p,p0 -DDE, o,p0 -DDT, o,p0 -DDD, o,p0 -DDE, p,p0 -DDMU, p,p0 -DDNU and p,p0 -DBP. o In the Netherland. p Near Geesthacht/Germany.

2. Measurement of atmospheric and freely dissolved HOCs

2.2. Water

2.1. Air

Opposite to the frequent and comprehensive programs of monitoring atmospheric HOCs with passive air samplers, passive sampling of aqueous HOCs in open water has been scarce and spotted. Concentration levels of aqueous HOCs in rivers, lakes or estuaries have been determined mainly in association with development and field validation of a novel passive sampler or a new quantitative method with an available sampler. For example, Zeng et al. developed and field-tested an SPME-based sampler using polydimethylsiloxane (PDMS)-coated fiber as the sorbent phase [21], and later used it to monitor the spatial distribution of p,p0 -DDE (<0.073–2.6 ng/L) throughout the Southern California Bight [21,22]. Ouyang et al. [8] tested three types of SPME-based passive sampler (fiber-retracted, PDMS-rod and PDMS membrane) in Hamilton Harbor (Canada) and obtained average concentrations of PAHs with the fiberretracted sampler at 113.1  14.6, 33.1  2.0 and 53.2  7.2 ng/ L at the depth of 1, 11 and 21 m, respectively, of the water column. In other studies, SPMD was deployed at six locations in Scheldt estuary (two sites) and the North Sea (four sites) along the Dutch Coast and ten freshwater sites in Amsterdam, and the timeweighted concentrations of BDE congeners (0.0001–0.005 ng/L) and PCBs (0.04–1.3 ng/L) were obtained [7,23]. In contrast to the high cost with SPME fiber and complex post-sampling processing procedures with SPMD, PE and POM-based samplers are costeffective and relatively simple to handle and therefore have had more application in field monitoring. For example, PE-based

Air passive samplers, especially PUF disk (Fig. 1a), have been used to measure atmospheric HOCs at urban, rural and background sites across the globe. Pozo et al. [1] introduced the Global Atmospheric Passive Sampling study to demonstrate the feasibility of passive sampler (PUF disk) for determining the global spatial distribution of atmospheric HOCs. Results from the sampling period of December 2004 to March 2005 presented the first snapshot of global distribution of gaseous HOCs [1,19]. Analysis of continuous monitoring data from December 2004 to December 2005 identified a peak around mid-latitude (20–808 N) of the northern hemisphere in the atmospheric profile of PCBs, which was consistent with the global PCBs emission pattern [4]. In addition, PUF-derived concentration levels of POPs (i.e., PCBs, dichlorodiphenyltrichloroethane (DDT) and its metabolites, sum of which is designated as DDTs, polychorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) and dioxin-like PCBs) in ambient air of Africa, Pacific & Australia, Asia, Europe, Latin America and North America were assessed by Bogdal et al. [20]. Results showed that concentrations of atmospheric PCDD/Fs (9–678 fg WHO98 TEQ/m3) in Africa and Latin America were even higher than those (i.e., United Kingdom: <50 fg TEQ/m3 (2005–2008)) in Europe, suggesting the need for further monitoring of HOCs in the atmosphere of developing countries.

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samplers have been deployed in Boston Harbor (Massachusetts) [9], Auke Lake (Alaska) [24], Narragansett Bay (Rhode Island) [25], four reservoirs and Hailing Bay of Guangdong Province, China [26], and the Palos Verdes Shelf of California, USA [27]. In open Baltic Sea, the median concentrations of PCDD/Fs and PCBs deduced from POM sampler were 2.3 pg toxic equivalents/m3 and 15 pg/L, respectively [10], while the freely dissolved concentrations of PCBs in overlying water of the Grasse River varied between 10 and 30 ng/L from 2006 to 2009 [28]. Overall, the use of passive sampling methods for measuring HOCs in open waters has been gradually extended to wider areas in the world; however, such field application has yet to gain large momentum. Despite the plea by Lohmann and Muir [2] in 2010 for establishing a global network for monitoring HOCs in waters, there has been essentially no progress toward this proposed goal. Because monitoring of HOCs, including POPs and other persistent organic chemicals, in open waters of background or remote regions would provide critical information for evaluating the effectiveness of the Stockholm Convention on POPs, it is significant to initiate and accelerate on building a global monitoring network based on passive sampling methods. 2.3. Sediment porewater Similar to passive sampling of HOCs in open waters, in situ measurement of HOCs in sediment porewater has been conducted only at a limited number of sites. In fact, most data were obtained by passive sampling of field-collected sediment in laboratory [29]. In situ exposure of passive samplers was conducted only in a few sites, i.e., South San Francisco Bay [30], a town canal (Entenfleet, Germany) [13], Elbe River (Near Geesthacht/Germany) [13], and Hailing Bay of South China [31]. Under the field conditions, PE and in situ PDMS samplers (Fig. 1b) were placed within the top layer of sediment to measure the single point porewater concentrations of PCBs and PAHs [13,30]. Different from this one-point measurement, two approaches have been developed, which can measure chemical concentrations in sediment porewater of different depths. Oen et al. [32] used PE or POM-mounted sedimentpenetrating rods (Fig. 1c) to obtain sediment porewater concentrations of PCBs at 0–40 cm depths with 5-cm intervals. In addition, Liu et al. [31] developed a multi-section passive sampler with low density PE (LDPE) as the sorbent phase (Fig. 1d), capable of measuring porewater concentration profiles of HOCs at every 2 cm. Field deployment of this multi-section sampler in Hailing Bay of South China obtained vertical sediment porewater concentration profiles of DDT and its metabolites within 40 cm depth, which were consistent with the metabolism pathways of DDT. Apparently, the capability of measuring vertical profiles of sediment porewater HOCs has a high stake in assessing the mobility of sediment contaminants and the effectiveness of remediation efforts, and subsequently in protecting aquatic environments and ultimately human health. Freely dissolved concentrations of HOCs in sediment porewater are usually greater than those in overlying water and considered as being bioavailable for benthic organisms. Consequently, monitoring HOCs in sediment porewater with passive samplers is expected to draw more attention in the coming years due to its importance for minimizing human health risk through aquatic foodweb transfer. In field application, one cost-effective approach would be to conduct passive sampling of both sediment porewater and overlying HOCs with the same sampler. 3. Measurement of site-specific partition coefficients The partition coefficient of an HOC between organic carbon (OC)/dissolved OC (DOC) and water (Koc or Kdoc) is a critical

parameter for fugacity-based modeling of its geochemical fate. Although Koc or Kdoc can be estimated empirically, accumulated field data suggested that measured and predicted values are often substantially different [33], e.g., the measured log Koc values of phenanthrene (6.07–7.03) in Boston Harbor of Massachusetts, USA were two orders of magnitude larger than the predicted value (4.12) [34]. To account for the impact of organic matter on Kdoc, humic acid (Aldrich) was used in the determination of Kdoc, but laboratory-determined Kdoc values of benzo[a]pyrene were still substantially different from their site-specific values, probably attributed to the difference between the commercial humic acid and naturally occurring DOC [35]. Passive sampling techniques have two advantages over liquid–liquid extraction for determining site-specific Koc or Kdoc. First, only the freely dissolved target compounds would be detected by passive samplers, whereas dissolved and DOM-bound target compounds may be retained by liquid–liquid extraction, thereby underestimating Koc or Kdoc [36]. Second, non-depletive extraction of HOCs with passive sampling does not exhaust target compound in water or porewater other than liquid–liquid extraction. Vaes et al. [37] suggested using <5% of the change in the concentrations of a target compound before and after extraction as the criterion of non-depletive extraction, i.e., the equilibration of the target compound between two interacting phases is not impacted by the extraction. SPME fibers with polymer coatings of 7–100 mm thickness take up small sample volumes and are strong sorbents for HOCs. As a result, they are preferable choices for non-depletive extraction, and thereby have been often used in determination of site-specific Koc or Kdoc [38]. For example, the log Koc values of five PAH congeners (phenanthrene: 4.65  0.03; pyrene: 5.26  0.02; benz[a]anthracene: 6.21  0.01; benzo[b]fluoranthene: 6.72  0.01; and benzo[ghi]perylene: 7.18  0.06) in soil of Denmark were estimated from the aqueous concentrations obtained with negligible SPME fibers and fraction of OC divided by the total concentrations in soil [39]. Similarly, the log Kdoc values of eight BDE congeners (BDE-28, 47, -66, -99, -100, -153, -154 and -183) in sediment collected from four sites (Greasy Creek, Benton County, Oregon; Jordan Lake Reservoir, Chatham County, North Carolina; Leaf Lake, El Dorado County, California; San Diego Creek, Orange County, California) ranged from 5.10 to 8.02 [40]. These data have filled in a large gap in site-specific Koc or Kdoc, and are more preferable than the predicted values in phase-partition modeling. In addition, a previous study has demonstrated that Koc values of phenanthrene varied with different compositions of organic materials in sediment, i.e., lignite, charcoal, activated carbon, and lignite coke [41]. Therefore, site-specific Koc or Kdoc derived with passive sampling are vital to elevating the predicting accuracy of fugacity modeling under different environmental condition. 4. Bioavailability assessment Passive samplers are able to measure freely dissolved concentrations of HOCs, and hence can be used to assess the bioavailability of HOCs in sediment or soil. Numerous studies have suggested that the log-transformed amounts of HOCs sorbed on passive sampling phases, such as PE, SPME fiber and POM, are significantly correlated with those (normalized in lipid) in benthic organisms (Nereis virens, Lumbriculus variegates, Hexagenia sp., and Eisenia fetida) exposed to the same sediment or soil in laboratory experiments [42–44]. However, environmental factors such as temperature, salinity and food variability are expected to have some effects on the internal bioaccumulation of HOCs in organisms under field conditions. Van der Heijden et al. [45] found that in situ SPME fiber was a better method for predicting accumulation of PAHs in L. variegates than ex situ SPME. In addition, the concentrations of petroleum

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hydrocarbons determined by in situ SPME tended to understate the amounts in worms [46]. Nevertheless, these two studies demonstrated that laboratory-based POM samplers were the same or better in predicting the amounts of PAHs and petroleum hydrocarbon in worms than in situ SPME fiber and may be an alternative choice for in situ assessing the bioavailability of HOCs in sediment. On the other hand, incidents of sediment and soil contamination by petroleum residues from oil leaking are on the rise [47]. Related toxicity tests showed that low content (0.01%) of unresolved complex mixture in sediment contaminated with petroleum residues increased the bioavailability of PAHs in L. variegates, whereas a higher level (5%) posed opposite effects [48]. Apparently, further efforts are needed to determine whether in situ or ex situ passive samplers (i.e., POM) are better predictors of the bioavailability of sediment-associated HOCs and petroleum residues. Another application of passive sampling techniques is the toxicity assessment of sediment HOCs in aquatic organisms. Li et al. [49] found that the coefficient of correlation between the mortality to Hyalella azteca (expressed in probit) and toxic unit of cypermethrin was elevated from 0.26 to 0.52, with the toxic unit estimated from matrix-SPME fiber instead of OC-normalized method. This is probably because freely dissolved HOCs derived with matrix-SPME fiber may be able to penetrate through cell membranes and subsequently cause toxic effect on organisms. Moreover, Tenax-extractable concentrations of permethrin in sediments were correlated to various endpoints of its chronic toxicity to Chironomus dilutus. [50]. However, the median lethal and effects concentrations, estimated from OC-normalized, SPME fiber and Tenax, of bifenthrin to C. dilutus and H. azteca in different sediments were inconsistent, probably resulting from biological degradation or ingestion of sediment particles by the worms [51]. 5. Measurement of inter-compartmental fluxes Molecular diffusion flux of an HOC across the interface of sediment–water, soil–air or air–water not only represents the magnitude of inter-compartmental transfer but also indicates the direction of such transfer; hence it is an important process within the geochemical cycling of the HOC. Since passive sampling can minimize the disturbance of HOCs distributed between two interacting compartments, passive sampler may be a powerful tool for measuring inter-compartmental fluxes of HOCs. However, to our knowledge, there have been few successful measurements of fluxes of HOCs between the air–water and soil– air interfaces using passive samplers. Lohmann et al. [25] determined air–water exchange gradients with PE passive samplers in Narragansett Bay of Rhode Island, USA and observed net volatilization trends for most PAHs, but they did not calculate the air–water fluxes of PAHs, probably due to the lack of wind speed and rainfall data. Wind speed is a key parameter in calculating air–water flux [52], whereas rainfalls occurring during sampling period may increase the dissolved concentration of PAHs in water [25]. In an attempt to measure air–soil fluxes of PAHs, Zhang et al. [53] utilized a modified PUF disk to determine the vertical concentration profiles of gaseous PAHs and determined soil fugacity of PAHs through soil concentrations obtained with Soxhlet extraction. In such an approach, the fluxes from soil to air may have been overestimated because PAHs in both dissolved and bound forms were included in calculating the soil fugacity [54]. Furthermore, two passive samplers, an infinite-sink benthic flux chamber (Fig. 1e) and a passive sampling device (Fig. 1f), have been developed to measure diffusive fluxes of HOCs across the sediment–water interface [55,56]. The benthic chamber with SPMD as the sorbent phase (Fig. 1e) was deployed in Oslo Harbor of Norway, and escaping fluxes from sediment to overlying water of

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pyrene and PCB-52 were determined at 300–1600 and 2– 8 ng m2 d1, respectively [55]. However, there are two intrinsic deficits with the benthic chamber. First, the chamber is normally so deployed that its interior space is isolated from overlying water, thus only the escaping fraction of HOCs from sediment is measured by the chamber; therefore the chamber is unable to measure depositing fluxes from overlying water to sediment. Second, flux measurement using the benthic chamber is done assuming the diffusive boundary layers at the chamber’s sorbent phase–water and sediment–water interfaces have the same length. Apparently, large measurement uncertainties may arise if this assumption is not satisfied. To overcome the above-mentioned deficits, a passive sampling device with LDPE as the sorbent phase (Fig. 1f) was developed, which was able to measure concentration profiles of HOCs near the sediment–water interface, from which sediment–water diffusive fluxes were calculated with a self-developed mathematical model [56]. This passive sampler was field-tested in Hailing Bay of South China where sediment was heavily contaminated with DDTs, and diffusive fluxes of individual DDT metabolites (5.9– 150 ng m2 d1) were obtained, comparable to those (5.5– 85 ng m2 d1) obtained by a custom-made benthic chamber with LDPE as the sorbent phase [57]. More significantly, the sampling device was able to acquire deposing diffusive fluxes of o,p0 -DDT and p,p0 -DDT from overlying water to sediment (9.8– 140 ng m2 d1), which was not distinguished with the benthic chamber. 6. Conclusions A few observations can be derived from the above discussions. First, the measurement of atmospheric HOCs with passive samplers has been spread across the globe, whereas in situ passive sampling of HOCs in open water and sediment porewater has remained limited. Taken into account the cost effectiveness, a global monitoring network for measuring dissolved HOCs in open water and porewater may be established with one passive sampling approach. Second, field assessment of the bioavailability of sediment-associated HOCs and petroleum residues with in situ or ex situ passive samplers need to be further tested, while biodegradation or ingestion of particle-bound HOCs by benthic organisms should be accounted for in sediment toxicity assessment using passive sampling techniques. Third, measurements of fluxes of HOCs across both the air–soil and air–water interfaces have been far from success by passive sampling methods. On the other hand, a passive sampling device has been developed and successfully used to measuring the escaping/depositing fluxes of DDTs at the sediment–water interface under field conditions. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (Nos. 21277144 and 41121063), the Natural Science Foundation of Guangdong Province (No. S2012020011076), and the Ministry of Science and Technology of China (No. 2012ZX07503-003-002). This is contribution No. IS-1779 from GIGCAS. References [1] K. Pozo, T. Harner, F. Wania, D.C.G. Muir, K.C. Jones, L.A. Barrie, Environ. Sci. Technol. 40 (2006) 4867. [2] R. Lohmann, D. Muir, Environ. Sci. Technol. 44 (2010) 860. [3] P. Konieczka, L. Wolska, J. Namies´nik, Trends Anal. Chem. 29 (2009) 706. [4] K. Pozo, T. Harner, S.C. Lee, F. Wania, D.C.G. Muir, K.C. Jones, Environ. Sci. Technol. 43 (2009) 796. [5] M. Koblizkova, S. Genualdi, S.C. Lee, T. Harner, Environ. Sci. Technol. 46 (2012) 391.

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