Spatial and Vertical Distribution of Short Chain Chlorinated Paraffins

Feb 14, 2011 - Furthermore, soil vertical distributions indicated that lower chlorinated (Cl5−6) and shorter chain (C10−12) congeners are more pro...
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Spatial and Vertical Distribution of Short Chain Chlorinated Paraffins in Soils from Wastewater Irrigated Farmlands Lixi Zeng,† Thanh Wang,† Wenya Han,‡ Bo Yuan,† Qian Liu,† Yawei Wang,*,† and Guibin Jiang† †

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡ Foreign Economic Cooperation Office, Ministry of Environmental Protection of the People’s Republic of China, Beijing 100035, China

bS Supporting Information ABSTRACT: Chlorinated paraffins (CPs) are one of the most complex groups of halogenated contaminants in the environment. However, studies of short chain CPs (SCCPs) in China are very scarce. In this study, the concentrations and distribution of SCCPs in farm soils from a wastewater irrigated area in China were investigated. SCCPs were detected in all topsoil samples, with the sum of the concentrations (ΣSCCPs) in the range of 159.9-1450 ng/g (dry weight, dw). A noticeable spatial trend and specific congener distribution were observed in the wastewater irrigated farmland. Soil vertical profiles showed that ΣSCCP concentrations below the plowed layer decreased exponentially and had a significant positive relationship (R2 > 0.83) with total organic carbon in soil cores. Furthermore, soil vertical distributions indicated that lower chlorinated (Cl5-6) and shorter chain (C10-12) congeners are more prone to migrate to deeper soil layers compared to highly chlorinated and longer chain congeners. This work demonstrated that effluents from sewage treatment plants (STPs) could be a significant source of SCCPs to the ambient environment and wastewater irrigation can lead to higher accumulation of SCCPs in farm soils.

’ INTRODUCTION Industrial chlorinated paraffins (CPs) are synthesized by direct free radical chlorination of n-alkane feedstock from petroleum distillation with molecular chlorine.1 They consists of thousands of different isomers, enantiomers, and diastereomers.2 Generally, CPs can be classified according to their chain length into short chain CPs (SCCPs, C10-13), medium chain CPs (MCCPs, C14-17), and long chain CPs (LCCP, C17-30) with chlorine contents usually varying between 40 and 70%.3,4 Among these, SCCPs have attracted most concerns due to their higher propensity for environmental release and higher toxicity to aquatic organisms relative to MCCP and LCCP. During the past decades, SCCPs have been used as lubricants, plasticizers, and additives, as well as flame retardants in a wide range of applications, due to their desirable physical and chemical properties such as thermal stability, variable viscosity, flame resistance, and low vapor pressure. Release into the environment can occur through different pathways during production, transport, usage, etc.5-7 As consequence, SCCPs have been found in various environmental matrixes such as water, sediment, air, soil, and biota,8-11 and they are also considered to be persistent in air and sediments.12 They are considered as one of the most complex groups of halogenated contaminants present in the environment. Detectable levels in sediment and biota from remote Arctic locations suggest the occurrence of long-range r 2011 American Chemical Society

transport of SCCPs via the atmosphere or ocean currents.13 The high log Kow (4.4-8) indicates that SCCPs have the potential to be bioaccumulated and biomagnified in aquatic and terrestrial food webs.14,15 SCCPs have been listed as priority hazardous substances in the European Union because they are considered carcinogenic to rats and may cause chronic toxicity to aquatic organisms.16 Moreover, they are also currently under review as candidate persistent organic pollutants (POPs) in the Stockholm Convention. Currently, information on the environmental behavior and levels of SCCPs are very limited compared to other POPs, mainly due to their complex composition and the lack of suitable analytical methods.17,18 Limited studies showed that effluents, reclaimed wastewater, and sewage sludge from sewage treatment plants (STP) could be a significant contamination source of SCCPs to the ambient environment,19 which is similar to the finding for other POPs.20,21 Stevens et al. 19 reported SCCP concentrations in the range of 6.9-200 μg/g dry weight (dw) in sewage sludge from 14 sewage treatment plans in the United Kingdom, and the highest concentrations were found in sludge Received: November 7, 2010 Accepted: January 19, 2011 Revised: January 11, 2011 Published: February 14, 2011 2100

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Figure 1. Sampling locations around the Liangshui River (LSR) and the corresponding level distribution of total SCCPs in topsoils.

samples from industrial catchments. In another study, however, elevated levels of SCCPs plus MCCPs (590 μg/g) were reported in sludge from a rural catchment without noticeable industrial activities, implicating that agricultural soils can be contaminated by high concentrations of CPs due to sewage sludge application.19,22 CPs continue to be produced in China in large amounts. Annual production volumes have increased every year since the 1990s.23 However, research on SCCPs in the environment in China is still very limited, and very little information is available on the environmental behavior and risk assessment.24,25 Due to the permanent shortage of water in North China, wastewater irrigation for crops in peri-urban farmlands has been a common practice since the 1970s and thus might be a potential source of many contaminants including heavy metals, pathogens, and POPs to farm soils. Furthermore, these contaminants in farm soils might be further transferred to crops, vegetables, and finally to humans through food consumption. To date, little attention has been paid to the distribution and environmental fate of SCCPs in agricultural soils irrigated by wastewater. In this study, we evaluated and compared the distribution of SCCPs in farm soils at an urban-rural fringe, in order to discern the potential contamination of SCCPs by wastewater irrigation. Furthermore, the vertical distributions of SCCPs were systematically studied to assess the accumulation and penetration behavior of SCCPs in soils. It is hoped that this work will provide further insight into the source and transport of SCCPs in the environment.

’ MATERIALS AND METHODS Sample Collection. Tongzhou district is located in the southeast suburbs of Beijing. Liangshui River (LSR) flows from west to east through the district, which not only receives wastewater from both treated/untreated industrial and local municipal waste but also receives effluents discharged from a large sewage treatment plant (STP) through a main sewage channel. In this work, soil samples were collected around LSR. In this sampling

area, the predominant soil types are loamy soils, and winter wheat-summer maize is the main crop rotation practice. The sampling map and sites are shown in Figure 1. The sampling sites were categorized as wastewater irrigated only (sites A and B irrigated directly with wastewater from the sewage channel, sites C to H irrigated with wastewater from LSR) and mixed irrigated (sites I to L) which is defined as irrigation with both groundwater from wells and wastewater from a network of small ditches that branched from the LSR. Additional soil samples from a control area were collected from farm plots irrigated solely by groundwater (site M). In all, thirteen composite topsoil samples (A-M sites) and four soil cores (B, C, G, and J sites) were collected. Each topsoil sample (0-20 cm) was obtained by mixing at least five adjacent subsamples from one farm field (four corners and the center; approximately 10  10 m2). Soil core samples were sectioned into 2 or 4 cm layers. Detailed information on sample collection is available in our previous paper,26 and sample preparation can be found in the Supporting Information. Instrumental Analysis. Instrumental analysis was performed on a 7890A gas chromatograph (GC) coupled with a 7000B triple quadrupole mass spectrometer (Agilent, USA). An aliquot of 1 μL of the final extract was injected with a 7683B Series Injector (Agilent, USA) in pulse splitless mode into a DB-5MS (30 m length, 0.25 mm i.d., 0.25 μm film thickness) capillary column at an injector temperature of 275 °C. Helium was used as carrier gas at a constant flow of 1.0 mL/min. The oven temperature program for the chromatographic separation was as follows: 1 min isothermal at 100 °C, increased to 160 at 30 °C/min, held for 5 min, then ramped to 310 at 30 °C/min, and held for 17 min. The low resolution mass spectrometer (LRMS) was coupled with GC in electron capture negative ionization (ECNI) mode using methane as reagent gas. The transfer line temperature and ion source temperature was set to 275 and 200 °C, respectively. The two most abundant isotopes of the [M - Cl]- ions of SCCPs and MCCPs with 5-10 chlorine atoms were recorded in the selected ion monitoring (SIM) mode (60 ms dwell time per ion). The highest and second most abundant isotope ions of [M - Cl]- were used for quantification and confirmation, respectively. The most 2101

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Environmental Science & Technology abundant isotopes of the [M]- (m/z 419.8) and [M - Cl]- (m/z 254.9) ions were, respectively, selected for the surrogate standard 13 C10-trans-chlordane and recovery standard ε-HCH. The analysis of SCCP congeners can interfere with MCCP congeners with similar mass and different molecular formula when low resolution mass spectrometer in ECNI mode was employed. To minimize such interference, we developed an analytical method using GCECNI-LRMS combined with chemical calculation for determination of CPs based on previous reported methods.27-29 In this analytical method, SCCP and MCCP congeners were simultaneously detected at each injection. A detailed description about the modified method is shown in the Supporting Information. In order to improve the sensitivity, all monitored ions of SCCPs and MCCPs were divided into four groups by mutual combination: C10 and C15, C11 and C16, C12 and C17, and C13 and C14. Each group consisted of similar mass congeners of SCCPs and MCCPs. Each sample was subjected to four individual injections in order to determine the molecular composition of the sample. Method of Identification and Quantification. Identification of CP congener groups was performed by comparison of retention time, signal shape, and correct isotope ratio according to the previous works by Reth and Oehme.12,29 The actual relative integrated signals for each congener were obtained by correcting the SIM signals of [M - Cl]- ions from isotopic abundance and response factors. This procedure can be found in the Supporting Information. Congener group abundance profiles were established using the actual relative integrated signals, followed by chemical calculation to determine the relative concentrations of the molecular component in the commercial standards and environmental samples.27 The applied quantification procedure has been described by Reth and Oehme.30 On the basis of this method, a reliable quantification can be achieved even if the degree of chlorination between the samples and the reference standards is different. Quality Assurance and Quality Control (QA/QC). Glassware and sodium sulfate were solvent rinsed and heated at 450 °C prior to use. Each batch of 10 samples included one procedural blank. In general, levels of SCCPs in procedural blanks were close to or under the limit of detection (LOD) for total SCCPs. The LOD was estimated at 100 ng/g. The reported data in this study were, therefore, not blank corrected. SCCP/MCCP concentrations of samples lower than the LOD were treated as half of the LOD. Average recoveries for the surrogate standard, 13C10-trans-chlordane, were between 84 and 102%. Average recoveries for three SCCP references with chlorine content 51.5, 55.5, and 63.0% were between 82.5 and 95% (Figure SI-1, Supporting Information).

’ RESULTS AND DISCUSSION All concentrations were reported on a dry weight (dw) basis. The results indicated that SCCPs were dominant in all soil samples. MCCPs, with C14 as predominant congeners, could be detected in only a few samples at low levels. Therefore, only SCCP were reported and discussed in this paper. Spatial Distribution of SCCPs in Topsoil. In this study region, spatial concentrations and chlorine contents of SCCPs in topsoil were in the range of 159.9-1450 ng/g and 59.2-62% (Table SI-1, Supporting Information), respectively. As shown in Figure 1, farm fields (sites A and B) closest to the sewage channel, which receives treated effluents discharged from the STP, showed higher concentrations of ΣSCCPs than the other sampling sites. The highest concentrations of SCCPs found at sites A

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(945.2 ng/g) and B (1450 ng/g), similar to the findings in our previous work on PCBs in this region,26 implied that effluents from STP were likely to be a potential source of SCCPs to the surrounding environment. Sampling site C, which is located near the junction of the sewage channel and LSR, also had a significantly high concentration of ΣSCCPs (846.0 ng/g). Conversely, ΣSCCP levels at sites D to H adjacent to the LSR (with the exception of site G) were all 2 to 3-fold lower relative to those measured at sites A and B. Sampling sites I to L, which are situated farther away from LSR and irrigated with both groundwater and wastewater, and the control site M, showed lower concentrations of ΣSCCPs (range of 159.9-291.5 ng/g, dry weight, dw) relative to those at sites A to H. The change of ΣSCCP concentration with sampling sites is shown in Figure SI-2, Supporting Information. This decreasing trend further suggests that STP and wastewater irrigation from LSR might be responsible for the accumulation of SCCPs in farm soils. Similar distribution trends of PCBs and elevated levels of heavy metals and PAHs at those farms closest to LSR have also been found in the same region, further implicating LSR as a major vector for many contaminants to surrounding areas.26,31,32 Moreover, a significant correlation was found in topsoil samples between SCCPs and PCBs levels in the topsoil samples based on our previous work (R2 = 0.72, p < 0.05), whereas no significant relationship was found between the spatial distribution of total organic carbon (TOC) content and ΣSCCPs (R2 = 0.01, p > 0.05). Variation in Congener Group Abundance Profile with Sampling Sites. The congener group patterns of SCCPs at each sampling site were also studied. Figure 2 shows four representative congener group abundance profiles from sampling sites B, C, G, and J and the others are illustrated in the Supporting Information (Figure SI 3-4). As can be seen, the congener profiles are variable among the different sampling sites. In general, the relative contributions of SCCP homologues and carbon chain lengths can reflect the distribution trend of SCCPs in the local region.33 At the bank of the wastewater channel (site A and B), both high SCCP concentrations and a high variation of congeners with different carbon atoms (C10-13) and chlorine atoms (Cl5-10) were observed. SCCP congeners of longer carbon chain (C12-13) were slightly more predominant than those of shorter carbon chain (C10-11). The distribution pattern of carbon chain groups in site A (ΣC10 = 27.2%, ΣC11 = 19.2%, ΣC12 = 21.8%, ΣC13 = 31.8%) is similar to that of site B (ΣC10 = 23.1%, ΣC11 = 19.9%, ΣC12 = 25.6%, ΣC13 = 31.4%). SCCPs (C10-13 chain) with Cl6-7 were found to be the most predominant among the targeted congeners. This is consistent with the composition of most SCCP industrial products and reference standards. A different homologue pattern was observed at the sampling site C at the confluent of the LSR and the wastewater channel (Figure 2). The profile in this site showed a predominance of shorter carbon chain congeners with C10 and C11 formula groups (64.7% of the total ion abundance). Similarly, lower chlorinated congeners (Cl5-7) were also found to be most abundant in site C. The congener distribution patterns of the sampling sites along downstream of LSR and the more distant areas including the mixed irrigation sites (Figure SI 1-2, Supporting Information) showed similar profiles except for site G, where shorter chain (C10-11) congeners were much more abundant than longer chain (C12-13) congeners. At some sites (E, I, and L), higher chlorinated congeners such as C12 and C13 homologues with 2102

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Figure 2. Representative SCCP congener group abundance profiles for sampling sites B, C, G, and J.

Figure 3. Composition profiles of SCCP congeners with Cl5-10 (a) and C10-13 (b) in different sampling sites.

Cl9-10 could even not be detected. The homologue profile of SCCPs at the control site M (groundwater irrigated only) also showed a similar congener group abundance profile as those sites mentioned above, with C10 and C11 homologue groups accounting for up to 83.1% of the total ion abundance (Figure SI-3, Supporting Information). The comparability of these congener profiles showed that SCCP contamination in these farm soils was likely derived from the same source. Site G showed a distinct congener group abundance profile. Compared to other adjacent sampling sites, where C10 and C11 are the major homologues, the profile in site G showed relatively higher percentages of C12 and C13 (ΣC12-13 = 45%), which were

at least 10% higher than the nearby sites F and H. Moreover, higher chlorinated congeners with Cl8-10 were more abundant (Figure 2) than in other sampling sites. The reason for this is currently unclear, but the characteristic congener group pattern might indicate local source(s) nearby this site. Figure 3 shows a more clear spatial distribution of SCCP congener groups in topsoil categorized by the number of chloride atoms (Cl5-10) and carbon chain length (C10-13) among the thirteen sampling sites. If the sampling sites having the highest concentrations of SCCPs (sites A and B) as reference sampling sites are considered, with the increasing distance along the path of LSR (sites C to H), a noticeable homologue distribution trend 2103

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Environmental Science & Technology can be seen in that the percentage of lower chlorinated (Cl5-6) and shorter chain congeners (C10-11) increased gradually (excluding site G). Moreover, the relative abundance of C10 homologue groups increased on an average of about 10% when the mode of irrigation changed from wastewater (site A-H) to mixed irrigation (site I-L) or groundwater irrigation (site M). A possible reason for the different distribution pattern is that lower chlorinated and shorter chain congeners have relative higher volatility and water solubility34,35 and thus have more facilitated transport abilities than those of higher chlorinated and longer chain congeners. Contamination of these compounds to these regions might be a common result derived from short-range

Figure 4. Vertical concentration profiles of SCCPs from four soil cores: sites B, C, G, and J.

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atmospheric deposition and environmental transport by wastewater irrigation. SCCP Concentrations in Soil Cores. In general, the concentrations of POPs often decrease exponentially under the airsolid interface of undisturbed soils.36 Figure 4 shows the vertical distributions of total SCCP concentrations in soil cores from sites B, C, G, and J. The levels of SCCPs were almost uniform along the typical plowing depth in agricultural soils (∼20-25 cm) and began to decrease exponentially below the tillage layer and thereafter level off at depths below ∼50 cm. A linear correlation between levels of SCCPs and PCBs was found in soil cores,26 and coefficient R2 values were around 0.95 for both soil core B and G and 0.84 in core C (p < 0.05). Moreover, a significant positive relationship was also found between total organic carbon (TOC) content and SCCP concentrations in these soil cores (R2 = 0.32, 0.85, and 0.83 for soil core B, G, and J, p < 0.05), which suggested that TOC might be a major factor governing the vertical transport of SCCPs in these agricultural soils. The decreasing trend in vertical concentration profile for core J (lowest SCCP concentrations among the soil cores) is not as apparent as that of core B (the highest SCCP concentrations). As mentioned above, as TOC is probably an important parameter influencing the vertical transport of SCCPs, higher contents of TOC in soil layers of core B below the plowing depth than those in core J may lead to the difference of vertical concentration profiles. Another possible reason is the different irrigation sources, leading to inputs with different congener composition, between site B (wastewater irrigated) and J (mixed irrigated) (Figure 2). Soil core B has a higher relative abundance of longer chain (C12-13) and higher chlorinated (Cl7-10) homologue groups, that are more restricted in penetrating

Figure 5. Composition profiles of SCCP congeners with Cl5-10 and C10-13 for each layer in soil core B (a, b) and core G (c, d), respectively. 2104

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Environmental Science & Technology deeper into the soil due to their higher Kow and lower water solubility. Variation of Congener Group Patterns with Sampling Depth. The vertical distributions of SCCP congeners based on chlorine and carbon atom numbers were examined in the four soil cores (core B, C, G, and J) to further investigate the migration potential of the various SCCP congeners in soils. Figure 5a,b shows the composition profiles of SCCP congener groups with Cl5-10 and C10-13 in soil core B. Six congener groups (ΣCl5, ΣCl6, ΣCl7, ΣCl8, ΣCl9, ΣCl10) at the top layer accounted for 18.1, 20.9, 32.5, 13.7, 10.4, and 4.4% of ΣSCCPs, respectively, while at the deeper layers (at 46 cm), ΣCl5 and ΣCl6 increased to 23.8 and 48.9% while ΣCl7, ΣCl8, ΣCl9, and ΣCl10 decreased to 20.1, 5.2, 1.6, and 0.4%, respectively. The relative abundance of C10, C11, C12, and C13 homologue groups were 29.7, 26.8, 24.9, and 18.6% at the top layer and then changed to around 48.3, 29.0, 14.4, and 8.3% at lower depths (about 46 cm), respectively. The congener profiles within each homologue group for soil core B (Figure SI-5, Supporting Information) further explicitly demonstrated the differences in downward movement. With the increase of soil depth, the proportions of lower chlorinated congeners (Cl5-6) significantly increased, while for those congeners with 9-10 chlorine atoms, the percentage decreased drastically. In some cases, SCCP congeners with Cl10 were even not detected in the deeper soils. Another three soil cores (core C, G, and J) were also studied. Figure 5c,d shows the composition profiles of SCCP congener groups with Cl5-10 and C10-13 in soil core G. Vertical homologue profiles of individual carbon chain congener groups with Cl5-10 in this core are illustrated in Figure SI-6, Supporting Information. A similar vertical variation of SCCP congener distribution with sampling depth was also found in soil cores C, G, and J (Figure SI-7, Supporting Information). These results strongly indicated that SCCP congeners have different transport abilities depending on their physicochemical properties, which are shown not only on the vertical distributions in soil cores but also on the spatial variations at different sampling sites. Sijm et al. 37 reported the log Kow for all possible SCCP congeners to be at the range of 4.8-7.6 and found a significant positive relationship between log Kow of the SCCP congener and total number of chlorine and carbon atoms. Table SI-2 (Supporting Information) lists the estimated log Kow of some selected SCCP congener groups (C10Cl6-9, C13Cl6-9).37 To further investigate if the vertical movement of SCCP congener groups is related with the estimated log Kow, we also calculated the ratio (r) of the concentration of a lower log Kow congener divided by that of a higher log Kow congener at a same soil layer and analyzed the change of this ratio with soil depth. Figure SI-8 (Supporting Information) depicted the change of r-values along the soil profile with the log Kow of selected congeners. As can be seen, r-values generally showed increasing trends along the soil depth. These results clearly indicated that SCCP congeners with comparatively lower Kow can more easily penetrate deeper into the soils. Thus, shorter chain and lower chlorinated congeners which have comparatively lower Kow and higher water solubility might have greater potential for leaching along the soil column. However, the degradation potential of the different congeners might also be a governing factor for the observations, which should be investigated in detail in future studies. This work demonstrated that irrigating farms with effluents from STP and wastewater might introduce elevated levels of SCCPs into soils and thus might further increase body burden to

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humans via consumption of potential contaminated crops. As this study was only conducted at a typical wastewater irrigation region in north China, further works are recommended to study the contamination and distribution patterns of SCCPs in other wastewater irrigation regions. More attention should be paid to assess the potential environmental risks of SCCPs associated with wastewater irrigation or sludge application.

’ ASSOCIATED CONTENT

bS

Supporting Information. Tables of chlorine content of SCCPs in soil samples and the estimated log Kow of some selected congener groups of SCCPs; total ion chromatograms of the standard SCCPs before and after pretreatment; figure of SCCP concentrations at different sampling sites; SCCP congener group abundance profiles at other sampling sites; vertical SCCP congener profiles of individual carbon group in core B and G; composition profiles of SCCP congeners in core C and J; figure of changes of concentration ratios of selected congeners with depth. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Address: State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China; tel: 8610-6284-9334; fax: 8610-6284-9339; e-mail: ywwang@ rcees.ac.cn.

’ ACKNOWLEDGMENT This work was jointly supported by the National Basic Research Program of China (2009CB421605), the National Natural Science Foundation (20897011, 20921063, 21007078, 21077114), Chinese Academy of Sciences (KZCX2-YW-QN409), and the China Postdoctoral Science Foundation (20090460544, 201003166). ’ REFERENCES (1) Feo, M. L.; Eljarrat, E.; Barcelo, D. Occurrence, fate and analysis of polychlorinated n-alkanes in the environment. TrAC, Trends Anal. Chem. 2009, 28 (6), 778–791. (2) Santos, F. J.; Parera, J.; Galceran, M. T. Analysis of polychlorinated n-alkanes in environmental samples. Anal. Bioanal. Chem. 2006, 386 (4), 837–857. (3) Bayen, S.; Obbard, J. P.; Thomas, G. O. Chlorinated paraffins: A review of analysis and environmental occurrence. Environ. Int. 2006, 32 (7), 915–929. (4) Pellizzato, F.; Ricci, M.; Held, A.; Emons, H. Analysis of shortchain chlorinated paraffins: a discussion paper. J. Environ. Monit. 2007, 9 (9), 924–930. (5) Reth, M.; Ciric, A.; Christensen, G. N.; Heimstad, E. S.; Oehme, M. Short- and medium-chain chlorinated paraffins in biota from the European Arctic - differences in homologue group patterns. Sci. Total Environ. 2006, 367 (1), 252–260. (6) Thomas, G. O.; Farrar, D.; Braekevelt, E.; Stern, G.; Kalantzi, O. I.; Martin, F. L.; Jones, K. C. Short and medium chain length chlorinated paraffins in UK human milk fat. Environ. Int. 2006, 32 (1), 34–40. (7) Castells, P.; Parera, J.; Santos, F. J.; Galceran, M. T. Occurrence of polychlorinated naphthalenes, polychlorinated biphenyls and shortchain chlorinated paraffins in marine sediments from Barcelona (Spain). Chemosphere 2008, 70 (9), 1552–1562. 2105

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