Chemosphere 137 (2015) 9–13
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Characterizing the distribution of selected PBDEs in soil, moss and reindeer dung at Ny-Ålesund of the Arctic Zhen Wang ⇑, Guangshui Na, Xindong Ma, Linke Ge, Zhongsheng Lin, Ziwei Yao National Marine Environmental Monitoring Center, Dalian 116023, China
h i g h l i g h t s Selected PBDEs in soil, moss and reindeer dung are investigated at Ny-Ålesund. Distribution in soil, moss and dung correlates with physicochemical properties. °
Significant linear relationship was observed between logQSM and logp L.
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Article history: Received 20 January 2015 Received in revised form 10 April 2015 Accepted 12 April 2015 Available online 15 May 2015 Keywords: PBDEs Ny-Ålesund Soil Moss Reindeer dung
a b s t r a c t Distribution of 12 selected polybrominated diphenyl ethers (PBDEs) was characterized in soil, moss and reindeer dung samples collected simultaneously at Ny-Ålesund of the Arctic. The average PBDE concentrations were 42 pg/g (dry weight) in soil, 122 pg/g in moss and 72 pg/g in reindeer dung. Significant log/ log-linear relationship was observed between the soil/moss quotients (QSM) and the sub-cooled liquid vapor pressures of PBDEs (r2 = 0.80). Moreover, excellent log/log-linear relationships between QSM and the octanol/air partition coefficients as well as between the moss/dung quotient (QMD) and the octanol/water partition coefficients of PBDEs were also observed, indicating that the physicochemical properties of PBDEs are appropriate parameters for characterizing the distribution of PBDEs in soil, moss and reindeer dung at Ny-Ålesund. Capsule abstract: Significant log-linear correlations were observed between physicochemical properties of PBDEs and their soil/moss (moss/dung) quotients. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Polybrominated diphenyl ethers (PBDEs) have been used extensively as additive flame retardants in commercial and household products for several decades. They can release into the environments during the production, use and the dismantling of the products due to their weak chemical bond binding (Osako et al., 2004; Wang and Jiang, 2008). PBDEs have drawn most scientific and public attentions because of their persistence, semi-volatility and lipophilicity. These characters result in their long-range atmospheric transport (LRAT) potential to remote and polar areas. PBDEs can reach soil via wet/dry deposition during their LRAT, and tend to be strongly absorbed in soil owing to their persistence and lipophilicity. Thus, soil has been regarded as an appropriate pollution ‘‘snapshot’’ of the surrounding atmospheric pollution of persistent organic pollutants (POPs) (Migaszewski et al., 2002; ⇑ Corresponding author. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.chemosphere.2015.04.030 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
Heywood et al., 2006). For example, Meijer et al. (2003a, 2003b) observed excellent relations of the soil/air partition coefficients of organochlorine pesticides with their octanol/air partition coefficients (KOA) and octanol/water partition coefficients (KOW). To date, moss and pine needles have been widely applied for estimating atmospheric POPs as typical natural passive air samplers (Wang et al., 2009b, 2009d). Uptake of POPs via lipid-rich cuticula of plant from the ambient atmosphere is one of key reasons for the contamination of aerial plant parts by POPs (Harmens et al., 2013). Previous studies indicated that the primary mechanism controlling the accumulation of POPs in pine needles is the lipid contents rather than the surface to volume ratio (Yang et al., 2007). For moss, little is known about the mechanism of controlling the accumulation of POPs in mosses. However, it can be believed that moss accumulates POPs mainly from the ambient atmosphere. In general, compounds with lower subcooled liquid vapor pressure (p°L) are prone to bind with particles, whereas those with higher p°L mainly exist in vapor phase. Thus, the different physicochemical properties of POPs have additional effects on the
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characteristics of the deposition to plants and soil. Relationships between the vegetation/air partition coefficients and KOA (or p°L) for typical POPs have been reported recently (Wegmann et al., 2004; Moeckel et al., 2008). According to these findings, the distribution of POPs between soil and air (between moss and air) is assumed to be characterized with their physicochemical properties, such as p°L and KOA. Moreover, bioaccumulation of POPs in biota and trophic transfer in food web are influenced by their physicochemical properties (Kelly et al., 2007; Ma et al., 2013, 2014). Once POPs are ingested into body by animals, they are hard to be degraded owing to their persistence, and one of main elimination pathways from body is believed to be via feces (Yamaguchi et al., 1986). Therefore, the distribution of POPs in animal’s food and dung is expected to be correlated to their physicochemical properties. Findings stated above imply similarities in distribution and accumulation trends of POPs between soil and plant (between plant and dung) in relation to the different p°L, KOW and KOA of POPs. However, there have been few reports so far on the application of these physicochemical parameters for describing the distribution of PBDEs in such soil/plant/animal dung. To clarify this, concentrations of selected PBDEs were analyzed simultaneously in soil, moss and reindeer dung collected at Ny-Ålesund in the Norwegian Arctic, and the relations of distribution coefficients in soil/moss/reindeer dung to the physicochemical properties of PBDEs were discussed in the present study. 2. Materials and methods 2.1. Sample collection Total 32 samples of surface soil (upper 5 cm) (12 samples), moss (12 samples) and reindeer dung (8 samples) were collected in July 2013 from 12 sites at Ny-Ålesund (78°550 N, 11°560 E). The detailed information of sampling sites is listed in Table S1. The average temperature during sampling is 5 °C. Soil, moss and reindeer dung were collected simultaneously within an area of 50 50 m at each sites (there are not reindeer dung samples at 4 sites). All samples were collected with stainless steel shovel, kept in aluminum foil pre-cleaned with acetone and transported on ice to the laboratory. Before analysis, soil, moss and reindeer dung samples were freeze-dried, ground, sieved (80 mesh) and stored in sealed glass containers at 20 °C until analysis. 2.2. Reagents, extraction and analysis A standard mixture containing 21 PBDE congeners was purchased from AccuStandards (USA). The 13C-labeled surrogate standards (BDE-3, 15, 28, 47, 99, 138, 153, 154, and 183) and 13 C-labeled internal standard (68A-IS, 13C-PCB-138) were purchased from Wellington Laboratories Inc. (Canada). All solvents used in this study were pesticide grade and obtained from Fisher Scientific (USA). Samples (5 g) were spiked with 13C-labeled surrogate standards (1 ng) and then extracted with a mixture of dichloromethane (DCM) and hexane (v:v, 1:1; 150 mL). Acidic silica (15 g) was added to the extract of moss and reindeer dung samples to remove lipid, and activated copper powder (2 g) was added to soil samples to remove sulfur. Then the extract was concentrated with a rotary evaporator, cleaned with multilayer column filled from the bottom with activated silica gel (1 g), basic silica gel (4 g), activated silica gel (1 g), acidic silica gel (8 g), activated silica gel (1 g), AgNO3 silica gel (2 g) topped with 4 g of anhydrous Na2SO4. Samples were eluted with 70 mL hexane and 100 mL DCM/hexane (1:1). The first fraction was discarded and the second fraction was collected and concentrated to 20 lL. Finally, 1 ng of 13C-PCB138 was added as
internal standard and mixed completely prior to instrumental analysis. PBDEs were analyzed using an Ultra GC coupled with a Trace DSQ II MS (Thermo, USA) in selected ion monitoring (SIM) mode with electron capture negative chemical ionization (ECNCI) mode. Analytes were separated on a DB-5MS (30 m 0.25 mm 0.25 lm). Helium was used as carrier gas at a flow rate of 1.0 mL/min, methane as reagent gas at a flow rate of 2.0 mL/min, and the injection volume was 2.0 lL in the splitless mode. Organic matter contents (OM) of soil and reindeer dung were measured by loss-on-ignition at 450 °C. The lipid contents of moss were determined by extracting moss (2–3 g) in an ultrasonic apparatus for 2 h using a mixture of hexane/acetone (1:1), and calculated by weighting the residues of the evaporated extracts.
2.3. QA/QC Four laboratory blanks (method blanks) were extracted and analyzed in the same way as the real samples. The results showed all targeted PBDEs in blanks were below the detection limits. The recoveries of the surrogate compounds (13C-labeled BDE-47, -99 and -153) are listed in Table S4, and PBDE concentrations were corrected using recoveries of surrogate compounds. The method detection limits (MDL) of individual PBDE congeners were calculated based on ten-fold of the standard deviation of the minimum standard solution divided by an assumed sample weight of 5 g (Ma et al., 2013). The data of MDL are listed in Table 1.
3. Results and discussion 3.1. Concentration patterns of PBDEs The data of PBDE concentrations in soil, moss and reindeer dung are listed in Table 1 and Tables S1–S3. Concentrations (dry weight) of the 12 PBDE congeners (RPBDEs) at Ny-Ålesund were 42 ± 23 (11–89) pg/g in soil, 122 ± 59 (48–259) pg/g in moss and 72 ± 20 (38–102) pg/g in reindeer dung, respectively. The difference of PBDE concentrations was not significant (p > 0.05) among different sampling sites, that is to say, the influence of local (point) sources on PBDE concentrations at Ny-Ålesund was weak. RPBDEs in soil at Ny-Ålesund were compared to the values from the south and east edge of Tibetan Plateau (Wang et al., 2009a; Zheng et al., 2012) and other remote regions, such as Russian Arctic (16–230 pg/g) (de Wit et al., 2006), but much lower than those in Norwegian background soils (median: 970 pg/g) (Hassanin et al., 2004). RPBDEs in moss were in comparison to the PBDE concentrations in Moss samples (H. splendens) collected from Norway (12–339 pg/g) (Mariussen et al., 2008). However, there are few reports on PBDE levels in reindeer dung. In this study, there was no relationship observed between RPBDEs and OM of soil and reindeer dung (and lipid contents of moss) (Figs. S1–S3). Fig. 1 shows the mean proportions of PBDE congeners in the three media. Clearly, the congener specific patterns are different in various media. For low brominated congeners (i.e., BDE-17, -28, -47), the proportions were higher in moss than those in soil, whereas for higher brominated congeners (i.e., BDE-138, -154), the values were lower in moss. The observations can be explained with the physicochemical properties of individual PBDE congeners and their different accumulation routes in soil and moss. As stated above, moss accumulate PBDEs mainly from the ambient atmosphere, and PBDEs in soil mainly from dry/wet deposition of particles, thus soil would accumulate more lower p°L or higher KOA compounds, such as BDE-138 and -154, compared to moss.
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Table 1 Method detection limits (MDL), average (ave), standard deviation (std), minimum, maximum values of individual and total concentrations of the 12 PBDE congeners in soil, moss and reindeer dung (pg/g dry weight) and their logp°L, logKOA and logKOW values at 5 °C. Congeners
BDE-17 BDE-28 BDE-47 BDE-66 BDE-71 BDE-85 BDE-99 BDE-100 BDE-138 BDE-154 BDE-153 BDE-183 RPBDEs a b c
MDL
0.5 0.3 0.5 0.3 0.5 0.3 0.5 0.3 0.5 0.3 0.3 0.3
Logp°L (Pa)a
5.10 5.18 6.60 6.95 7.32 8.68 8.46 8.35 10.78 10.22 10.13 12.25
LogKOAb
9.83 10.43 11.16 11.77 11.16 12.50 12.50 11.90 13.83 13.23 13.83 14.57
LogKOWc
5.79 5.96 6.76 6.45 6.30 7.32 7.27 7.49 7.59 7.89 7.58 8.35
Soil
Moss
Reindeer dung
ave
std
min
max
ave
std
min
max
ave
std
2.9 2.8 4.3 5.5 5.3 1.8 9.7 2.3 5.3 4.0 3.6 4.1 42
2.3 2.7 5.1 6.9 3.7 1.3 6.1 1.9 4.1 3.2 2.5 2.4 23
0.7 0.4 0.9 0.4 0.9 0.5 3.7 0.4 1.0 0.7 0.3 0.4 11
7.5 8.9 19 22 12 3.7 19 7.2 12 11 8.3 6.7 89
14 27 30 10 6.8 1.1 23 7.6 1.9 1.3 3.1 1.7 122
8.9 17 22 8.8 6.5 1.4 11 10 1.5 0.6 6.8 0.9 59
1.1 4.6 9.0 1.0 1.4 0.4 6.4 0.5 0.5 0.6 0.4 0.9 48
29 54 93 28 22 4.4 41 33 5.4 2.7 22 2.7 259
10 5.3 9.5 7.0 6.6 4.7 15 4.6 7.2 6.1 nd nd 72
3.5 4.4 5.3 3.7 5.3 2.2 10 3.3 4.9 3.4 nd nd 20
min 6.8 1.3 3.0 2.7 0.7 1.5 3.0 1.2 1.0 0.4
38
max 17 13 18 13 13 7.2 29 11 17 10
102
Calculated using the Software SPARC (accessed in July, 2014). Derived based on the method of Li et al. (2006). Calculated based on the method of Li et al. (2008).
with their physiochemical parameters, such as p°L and KOA. To verify the above hypothesis, the dimensionless soil/moss quotient (QSM) was defined for each compound:
Q SM ¼ C S =C M
ð2Þ
where CS and CM are PBDE concentrations in soil and moss (pg/g), respectively. This quotient can effectively reduce the impact of different sampling site loads and organic matter contents both in soil and moss. Due to the concentrations of some PBDE congeners below detection limits in soil and/or moss, the corresponding QSM of these congeners were not calculated. As shown in Fig. 2, QSM displayed an excellent log/log-linear relationship with p°L of the 12 PBDE congeners in soil and moss samples collected at Ny-Ålesund:
log Q SM ¼ 0:25 log pL 2:24 ð5 CÞ
Fig. 1. Percentage of individual PBDE congeners to the total concentration.
Of the 12 congeners, BDE-47 is predominant in moss and BDE99 is the most congener in soil and reindeer dung. Some studies indicated that, except for BDE-209, BED-47 was the most abundant congener in environmental media (de Wit et al., 2006; Wang et al., 2009c), however, some found that BDE-99 was the most one (Mariussen et al., 2008). The concentration discrepancies of BDE47 and -99 in different media can be explained partly by the biotransformation of BDE-99 to -47 (Wang et al., 2009c). Nevertheless, the ratios of (BDE-47 + BDE-99)/RPBDEs did not vary significantly. In the present study, although the proportions of BDE-47 (and BDE-99) to RPBDEs were different (Fig. 1), the percentage of (BDE-47 + BDE-99) to RPBDEs differed insignificantly (32.8%, 41.6% and 30.3% in soil, moss and reindeer dung, respectively).
ð3Þ
Moreover, a significant correlation between logQSM and logKOA was observed as expected (r2 = 0.71, p < 0.01) (Fig. S4). Here logp°L and logKOA (Table 1) were derived at the average ambient temperature (5 °C) according to Software SPARC (accessed in July, 2014) and Li et al. (2006), respectively. Values of logQSM increase significantly with the decrease of logp°L or increase of logKOA, suggesting an increase in accumulation of more hydrophobic PBDEs in soil as compared to moss. The results further confirmed that the physicochemical properties of PBDEs are appropriate for describing their distribution in the terrestrial environments, especially in soil and
3.2. Relations of physicochemical properties with the distribution in soil, moss and reindeer dung POPs with lower p°L and/or higher KOA are favored for accumulation in soil, hence, physicochemical properties could be responsible for the observed different profiles of PBDEs in soil, moss and reindeer dung. p°L is often used to describe the relation with the gas/particle partition coefficients (KP) of PBDEs using the following linear free energy relationship:
log K P ¼ mr log pL þ br
ð1Þ
where mr and br are regression parameters. As expected, the distribution of PBDEs in soil, moss and reindeer dung could be described
Fig. 2. Significant linear correlation between logQSM and logp°L of the 11 PBDE congeners (except for BDE-183).
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correlation between logKOW and logQMD for PAHs in moss and reindeer dung. The trend between logQMD and logKOW can be explained with respect to different absorption and excretion potential for various PBDE congeners. Generally, absorption and excretion of POPs in gastrointestinal tract are two uncoupled processes (Norstrom, 2002). According to the micelle-mediated diffusion model proposed by Kelly et al. (2004), bioaccumulation of POPs via unidirectional micelle transport (and hence dietary uptake) correlated positively with logKOW owing to enhanced absorption rates for higher logKOW compounds. However, the process of getting rid of such chemicals from the organism is the reverse transport process in comparison to absorption. As a result, less PBDEs with higher logKOW occur in dung. 4. Conclusions Fig. 3. Correlation between logQMD and logKOW for individual PBDE congeners except for BDE-153 and -183. Values of logKOW for PBDEs were derived from Li et al. (2008).
vegetation. In addition to the physicochemical properties, some factors would influence the distribution of different PBDE congeners in soil and moss, such as meteorological conditions, different uptake and loss processes in soil and moss. Nevertheless, more than 70% of the total variation of logQSM was explained by differences of PBDEs characterized in terms of logp°L or logKOA. Considering poorly understanding of the effects of factors stated above in the Arctic on the accumulation in soil and moss, and the strong collinearity of p°L, KOA and KOW, it is difficult to regress a polyparameter relation to explain more variation (Weiss, 2000). Moss and pine needle are typical natural passive air samplers as stated above. They can capture gas and particle phase POPs from the atmosphere, and the equilibrium state is expected to be approached for atmospheric POPs between moss (pine needle) and air (Wang et al., 2009b; Harmens et al., 2013). As stated by Wang et al. (2009b), the relation of logQSM with logp°L of selected polycyclic aromatic hydrocarbons (PAHs) was similar to logKP versus logp°L of gas/particle partitioning for PAHs, which carried the information of logKP and would be a ‘‘mirror image’’ of the gas/particle partitioning of PAHs in the atmosphere. Furthermore, previous study suggested that mr value of Eq. (1) would approach -1 under equilibrium condition and mr should inter-relate with intercept br (Pankow and Bidleman, 1992; Li et al., 2014). Similarly, it can be assumed that the slope of the linear regression of logQSM versus logp°L would be expected to be a theoretical value. Interestingly, similar linear correlation between logp°L and the logarithm of soil/vegetation (pine needle and moss) was observed in previous studies, and their slopes were 0.25, 0.161, 0.22 and 0.14, respectively (Weiss, 2000; Wang et al., 2009b, 2009c). These slopes do not differ greatly, implying similar patterns in distribution between soil and vegetation in relation to logp°L of the measured POPs. The results also presented the field-measured data to support the above assumption. However, due to the complexity of sorption process of POPs in vegetation and few knowledge of the effects of factors as discussed above, the quantitative model for describing the distribution processes and ad/absorption characteristics in soil and vegetation has so far not been fully developed and explained.Similar to QSM, the dimensionless moss/dung quotient (QMD) was calculated according to the following equation:
Q MD ¼ C M =C D
ð4Þ
where CD is PBDE concentrations in reindeer dung (pg/g). A significant linear relationship between logQMD and logKOW was observed (Fig. 3). LogQMD of PBDEs increase with the increase of their logKOW, indicating that PBDE congeners with higher logKOW are prone to accumulation in moss compared to reindeer dung. Similarly, Wang et al. (2009c) also observed the positive linear
The ranges of RPBDEs at Ny-Ålesund were 11–89 pg/g in soil, 48–259 pg/g in moss and 38–102 pg/g in reindeer dung, respectively. Significant linear correlation of logQSM versus logp°L indicates that logp°L can be used to describe the distribution of PBDEs in soil and moss. The relationships of logQSM logKOA and logQMD logKOW also confirm that the physicochemical properties of PBDEs, such as logp°L, are appropriate parameters for characterizing the distribution of PBDEs in soil, moss and reindeer dung at Ny-Ålesund of the Arctic. Acknowledgements This study was supported by the National Natural Science Foundation of China (21007012, 21377032), the Chinese Polar Environment Comprehensive Investigation & Assessment Programmes (02-01, 03-04, 04-01 and 04-03) and the polar science key laboratory foundation of SOA (KP201208). We express our sincere gratitude to the Chinese Arctic and Antarctic Administration. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2015.04.030. References de Wit, C.A., Alaee, M., Muir, D.C., 2006. Levels and trends of brominated flame retardants in the Arctic. Chemosphere 64, 209–233. Harmens, H., Foan, L., Simon, V., Mills, G., 2013. Terrestrial mosses as biomonitors of atmospheric POPs pollution: a review. Environ. Pollut. 173, 245–254. Hassanin, A., Breivik, K., Meijer, S.N., Steinnes, E., Thomas, G.O., Jones, K.C., 2004. PBDEs in European background soils: Levels and factors controlling their distribution. Environ. Sci. Technol. 38, 738–745. Heywood, E., Wright, J., Wienburg, C.L., Black, H.I.J., Long, S.M., Osborn, D., 2006. Factors influencing the national distribution of polycyclic aromatic hydrocarbons and polychlorinated biphenyls in British soils. Environ. Sci. Technol. 40, 7629–7635. Kelly, B.C., Gobas, F.A.P.C., McLachlan, M.S., 2004. Intestinal absorption and biomagnification of organic contaminants in fish, wildlife, and humans. Environ. Toxicol. Chem. 23, 2324–2336. Kelly, B.C., Ikonomou, M.G., Blair, J.D., Morin, A.E., Gobas, F.A., 2007. Food webspecific biomagnification of persistent organic pollutants. Science 317, 236– 239. Li, X.H., Chen, J.W., Zhang, L., Qiao, X.L., Huang, L.P., 2006. The fragment constant method for predicting octanol–air partition coefficients of persistent organic pollutants at different temperatures. J. Phys. Chem. Ref. Data 35, 1365–1384. Li, L.N., Xie, S.D., Cai, H., Bai, X.Y., Xue, Z., 2008. Quantitative structure-property relationships for octanol–water partition coefficients of polybrominated diphenyl ethers. Chemosphere 72, 1602–1606. Li, Y.F., Ma, W.L., Yang, M., 2014. Prediction of gas/particle partitioning of polybrominated diphenyl ethers (PBDEs) in global air: a theoretical study. Atmos. Chem. Phys. Dissc. 14, 23415–23451. Ma, X., Zhang, H., Yao, Z., Zhao, X., Wang, L., Wang, Z., 2013. Bioaccumulation and trophic transfer of polybrominated diphenyl ethers (PBDEs) in a marine food web from Liaodong Bay, North China. Mar. Pollut. Bull. 74, 110–115.
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Characterizing the distribution of selected PBDEs in soil, moss and reindeer dung at Ny-Ålesund of the Arctic Zhen Wang ⇑, Guangshui Na, Xindong Ma, Linke Ge, Zhongsheng Lin, Ziwei Yao National Marine Environmental Monitoring Center, Dalian 116023, China
h i g h l i g h t s Selected PBDEs in soil, moss and reindeer dung are investigated at Ny-Ålesund. Distribution in soil, moss and dung correlates with physicochemical properties. °
Significant linear relationship was observed between logQSM and logp L.
a r t i c l e
i n f o
Article history: Received 20 January 2015 Received in revised form 10 April 2015 Accepted 12 April 2015 Available online 15 May 2015 Keywords: PBDEs Ny-Ålesund Soil Moss Reindeer dung
a b s t r a c t Distribution of 12 selected polybrominated diphenyl ethers (PBDEs) was characterized in soil, moss and reindeer dung samples collected simultaneously at Ny-Ålesund of the Arctic. The average PBDE concentrations were 42 pg/g (dry weight) in soil, 122 pg/g in moss and 72 pg/g in reindeer dung. Significant log/ log-linear relationship was observed between the soil/moss quotients (QSM) and the sub-cooled liquid vapor pressures of PBDEs (r2 = 0.80). Moreover, excellent log/log-linear relationships between QSM and the octanol/air partition coefficients as well as between the moss/dung quotient (QMD) and the octanol/water partition coefficients of PBDEs were also observed, indicating that the physicochemical properties of PBDEs are appropriate parameters for characterizing the distribution of PBDEs in soil, moss and reindeer dung at Ny-Ålesund. Capsule abstract: Significant log-linear correlations were observed between physicochemical properties of PBDEs and their soil/moss (moss/dung) quotients. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Polybrominated diphenyl ethers (PBDEs) have been used extensively as additive flame retardants in commercial and household products for several decades. They can release into the environments during the production, use and the dismantling of the products due to their weak chemical bond binding (Osako et al., 2004; Wang and Jiang, 2008). PBDEs have drawn most scientific and public attentions because of their persistence, semi-volatility and lipophilicity. These characters result in their long-range atmospheric transport (LRAT) potential to remote and polar areas. PBDEs can reach soil via wet/dry deposition during their LRAT, and tend to be strongly absorbed in soil owing to their persistence and lipophilicity. Thus, soil has been regarded as an appropriate pollution ‘‘snapshot’’ of the surrounding atmospheric pollution of persistent organic pollutants (POPs) (Migaszewski et al., 2002; ⇑ Corresponding author. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.chemosphere.2015.04.030 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
Heywood et al., 2006). For example, Meijer et al. (2003a, 2003b) observed excellent relations of the soil/air partition coefficients of organochlorine pesticides with their octanol/air partition coefficients (KOA) and octanol/water partition coefficients (KOW). To date, moss and pine needles have been widely applied for estimating atmospheric POPs as typical natural passive air samplers (Wang et al., 2009b, 2009d). Uptake of POPs via lipid-rich cuticula of plant from the ambient atmosphere is one of key reasons for the contamination of aerial plant parts by POPs (Harmens et al., 2013). Previous studies indicated that the primary mechanism controlling the accumulation of POPs in pine needles is the lipid contents rather than the surface to volume ratio (Yang et al., 2007). For moss, little is known about the mechanism of controlling the accumulation of POPs in mosses. However, it can be believed that moss accumulates POPs mainly from the ambient atmosphere. In general, compounds with lower subcooled liquid vapor pressure (p°L) are prone to bind with particles, whereas those with higher p°L mainly exist in vapor phase. Thus, the different physicochemical properties of POPs have additional effects on the
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characteristics of the deposition to plants and soil. Relationships between the vegetation/air partition coefficients and KOA (or p°L) for typical POPs have been reported recently (Wegmann et al., 2004; Moeckel et al., 2008). According to these findings, the distribution of POPs between soil and air (between moss and air) is assumed to be characterized with their physicochemical properties, such as p°L and KOA. Moreover, bioaccumulation of POPs in biota and trophic transfer in food web are influenced by their physicochemical properties (Kelly et al., 2007; Ma et al., 2013, 2014). Once POPs are ingested into body by animals, they are hard to be degraded owing to their persistence, and one of main elimination pathways from body is believed to be via feces (Yamaguchi et al., 1986). Therefore, the distribution of POPs in animal’s food and dung is expected to be correlated to their physicochemical properties. Findings stated above imply similarities in distribution and accumulation trends of POPs between soil and plant (between plant and dung) in relation to the different p°L, KOW and KOA of POPs. However, there have been few reports so far on the application of these physicochemical parameters for describing the distribution of PBDEs in such soil/plant/animal dung. To clarify this, concentrations of selected PBDEs were analyzed simultaneously in soil, moss and reindeer dung collected at Ny-Ålesund in the Norwegian Arctic, and the relations of distribution coefficients in soil/moss/reindeer dung to the physicochemical properties of PBDEs were discussed in the present study. 2. Materials and methods 2.1. Sample collection Total 32 samples of surface soil (upper 5 cm) (12 samples), moss (12 samples) and reindeer dung (8 samples) were collected in July 2013 from 12 sites at Ny-Ålesund (78°550 N, 11°560 E). The detailed information of sampling sites is listed in Table S1. The average temperature during sampling is 5 °C. Soil, moss and reindeer dung were collected simultaneously within an area of 50 50 m at each sites (there are not reindeer dung samples at 4 sites). All samples were collected with stainless steel shovel, kept in aluminum foil pre-cleaned with acetone and transported on ice to the laboratory. Before analysis, soil, moss and reindeer dung samples were freeze-dried, ground, sieved (80 mesh) and stored in sealed glass containers at 20 °C until analysis. 2.2. Reagents, extraction and analysis A standard mixture containing 21 PBDE congeners was purchased from AccuStandards (USA). The 13C-labeled surrogate standards (BDE-3, 15, 28, 47, 99, 138, 153, 154, and 183) and 13 C-labeled internal standard (68A-IS, 13C-PCB-138) were purchased from Wellington Laboratories Inc. (Canada). All solvents used in this study were pesticide grade and obtained from Fisher Scientific (USA). Samples (5 g) were spiked with 13C-labeled surrogate standards (1 ng) and then extracted with a mixture of dichloromethane (DCM) and hexane (v:v, 1:1; 150 mL). Acidic silica (15 g) was added to the extract of moss and reindeer dung samples to remove lipid, and activated copper powder (2 g) was added to soil samples to remove sulfur. Then the extract was concentrated with a rotary evaporator, cleaned with multilayer column filled from the bottom with activated silica gel (1 g), basic silica gel (4 g), activated silica gel (1 g), acidic silica gel (8 g), activated silica gel (1 g), AgNO3 silica gel (2 g) topped with 4 g of anhydrous Na2SO4. Samples were eluted with 70 mL hexane and 100 mL DCM/hexane (1:1). The first fraction was discarded and the second fraction was collected and concentrated to 20 lL. Finally, 1 ng of 13C-PCB138 was added as
internal standard and mixed completely prior to instrumental analysis. PBDEs were analyzed using an Ultra GC coupled with a Trace DSQ II MS (Thermo, USA) in selected ion monitoring (SIM) mode with electron capture negative chemical ionization (ECNCI) mode. Analytes were separated on a DB-5MS (30 m 0.25 mm 0.25 lm). Helium was used as carrier gas at a flow rate of 1.0 mL/min, methane as reagent gas at a flow rate of 2.0 mL/min, and the injection volume was 2.0 lL in the splitless mode. Organic matter contents (OM) of soil and reindeer dung were measured by loss-on-ignition at 450 °C. The lipid contents of moss were determined by extracting moss (2–3 g) in an ultrasonic apparatus for 2 h using a mixture of hexane/acetone (1:1), and calculated by weighting the residues of the evaporated extracts.
2.3. QA/QC Four laboratory blanks (method blanks) were extracted and analyzed in the same way as the real samples. The results showed all targeted PBDEs in blanks were below the detection limits. The recoveries of the surrogate compounds (13C-labeled BDE-47, -99 and -153) are listed in Table S4, and PBDE concentrations were corrected using recoveries of surrogate compounds. The method detection limits (MDL) of individual PBDE congeners were calculated based on ten-fold of the standard deviation of the minimum standard solution divided by an assumed sample weight of 5 g (Ma et al., 2013). The data of MDL are listed in Table 1.
3. Results and discussion 3.1. Concentration patterns of PBDEs The data of PBDE concentrations in soil, moss and reindeer dung are listed in Table 1 and Tables S1–S3. Concentrations (dry weight) of the 12 PBDE congeners (RPBDEs) at Ny-Ålesund were 42 ± 23 (11–89) pg/g in soil, 122 ± 59 (48–259) pg/g in moss and 72 ± 20 (38–102) pg/g in reindeer dung, respectively. The difference of PBDE concentrations was not significant (p > 0.05) among different sampling sites, that is to say, the influence of local (point) sources on PBDE concentrations at Ny-Ålesund was weak. RPBDEs in soil at Ny-Ålesund were compared to the values from the south and east edge of Tibetan Plateau (Wang et al., 2009a; Zheng et al., 2012) and other remote regions, such as Russian Arctic (16–230 pg/g) (de Wit et al., 2006), but much lower than those in Norwegian background soils (median: 970 pg/g) (Hassanin et al., 2004). RPBDEs in moss were in comparison to the PBDE concentrations in Moss samples (H. splendens) collected from Norway (12–339 pg/g) (Mariussen et al., 2008). However, there are few reports on PBDE levels in reindeer dung. In this study, there was no relationship observed between RPBDEs and OM of soil and reindeer dung (and lipid contents of moss) (Figs. S1–S3). Fig. 1 shows the mean proportions of PBDE congeners in the three media. Clearly, the congener specific patterns are different in various media. For low brominated congeners (i.e., BDE-17, -28, -47), the proportions were higher in moss than those in soil, whereas for higher brominated congeners (i.e., BDE-138, -154), the values were lower in moss. The observations can be explained with the physicochemical properties of individual PBDE congeners and their different accumulation routes in soil and moss. As stated above, moss accumulate PBDEs mainly from the ambient atmosphere, and PBDEs in soil mainly from dry/wet deposition of particles, thus soil would accumulate more lower p°L or higher KOA compounds, such as BDE-138 and -154, compared to moss.
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Table 1 Method detection limits (MDL), average (ave), standard deviation (std), minimum, maximum values of individual and total concentrations of the 12 PBDE congeners in soil, moss and reindeer dung (pg/g dry weight) and their logp°L, logKOA and logKOW values at 5 °C. Congeners
BDE-17 BDE-28 BDE-47 BDE-66 BDE-71 BDE-85 BDE-99 BDE-100 BDE-138 BDE-154 BDE-153 BDE-183 RPBDEs a b c
MDL
0.5 0.3 0.5 0.3 0.5 0.3 0.5 0.3 0.5 0.3 0.3 0.3
Logp°L (Pa)a
5.10 5.18 6.60 6.95 7.32 8.68 8.46 8.35 10.78 10.22 10.13 12.25
LogKOAb
9.83 10.43 11.16 11.77 11.16 12.50 12.50 11.90 13.83 13.23 13.83 14.57
LogKOWc
5.79 5.96 6.76 6.45 6.30 7.32 7.27 7.49 7.59 7.89 7.58 8.35
Soil
Moss
Reindeer dung
ave
std
min
max
ave
std
min
max
ave
std
2.9 2.8 4.3 5.5 5.3 1.8 9.7 2.3 5.3 4.0 3.6 4.1 42
2.3 2.7 5.1 6.9 3.7 1.3 6.1 1.9 4.1 3.2 2.5 2.4 23
0.7 0.4 0.9 0.4 0.9 0.5 3.7 0.4 1.0 0.7 0.3 0.4 11
7.5 8.9 19 22 12 3.7 19 7.2 12 11 8.3 6.7 89
14 27 30 10 6.8 1.1 23 7.6 1.9 1.3 3.1 1.7 122
8.9 17 22 8.8 6.5 1.4 11 10 1.5 0.6 6.8 0.9 59
1.1 4.6 9.0 1.0 1.4 0.4 6.4 0.5 0.5 0.6 0.4 0.9 48
29 54 93 28 22 4.4 41 33 5.4 2.7 22 2.7 259
10 5.3 9.5 7.0 6.6 4.7 15 4.6 7.2 6.1 nd nd 72
3.5 4.4 5.3 3.7 5.3 2.2 10 3.3 4.9 3.4 nd nd 20
min 6.8 1.3 3.0 2.7 0.7 1.5 3.0 1.2 1.0 0.4
38
max 17 13 18 13 13 7.2 29 11 17 10
102
Calculated using the Software SPARC (accessed in July, 2014). Derived based on the method of Li et al. (2006). Calculated based on the method of Li et al. (2008).
with their physiochemical parameters, such as p°L and KOA. To verify the above hypothesis, the dimensionless soil/moss quotient (QSM) was defined for each compound:
Q SM ¼ C S =C M
ð2Þ
where CS and CM are PBDE concentrations in soil and moss (pg/g), respectively. This quotient can effectively reduce the impact of different sampling site loads and organic matter contents both in soil and moss. Due to the concentrations of some PBDE congeners below detection limits in soil and/or moss, the corresponding QSM of these congeners were not calculated. As shown in Fig. 2, QSM displayed an excellent log/log-linear relationship with p°L of the 12 PBDE congeners in soil and moss samples collected at Ny-Ålesund:
log Q SM ¼ 0:25 log pL 2:24 ð5 CÞ
Fig. 1. Percentage of individual PBDE congeners to the total concentration.
Of the 12 congeners, BDE-47 is predominant in moss and BDE99 is the most congener in soil and reindeer dung. Some studies indicated that, except for BDE-209, BED-47 was the most abundant congener in environmental media (de Wit et al., 2006; Wang et al., 2009c), however, some found that BDE-99 was the most one (Mariussen et al., 2008). The concentration discrepancies of BDE47 and -99 in different media can be explained partly by the biotransformation of BDE-99 to -47 (Wang et al., 2009c). Nevertheless, the ratios of (BDE-47 + BDE-99)/RPBDEs did not vary significantly. In the present study, although the proportions of BDE-47 (and BDE-99) to RPBDEs were different (Fig. 1), the percentage of (BDE-47 + BDE-99) to RPBDEs differed insignificantly (32.8%, 41.6% and 30.3% in soil, moss and reindeer dung, respectively).
ð3Þ
Moreover, a significant correlation between logQSM and logKOA was observed as expected (r2 = 0.71, p < 0.01) (Fig. S4). Here logp°L and logKOA (Table 1) were derived at the average ambient temperature (5 °C) according to Software SPARC (accessed in July, 2014) and Li et al. (2006), respectively. Values of logQSM increase significantly with the decrease of logp°L or increase of logKOA, suggesting an increase in accumulation of more hydrophobic PBDEs in soil as compared to moss. The results further confirmed that the physicochemical properties of PBDEs are appropriate for describing their distribution in the terrestrial environments, especially in soil and
3.2. Relations of physicochemical properties with the distribution in soil, moss and reindeer dung POPs with lower p°L and/or higher KOA are favored for accumulation in soil, hence, physicochemical properties could be responsible for the observed different profiles of PBDEs in soil, moss and reindeer dung. p°L is often used to describe the relation with the gas/particle partition coefficients (KP) of PBDEs using the following linear free energy relationship:
log K P ¼ mr log pL þ br
ð1Þ
where mr and br are regression parameters. As expected, the distribution of PBDEs in soil, moss and reindeer dung could be described
Fig. 2. Significant linear correlation between logQSM and logp°L of the 11 PBDE congeners (except for BDE-183).
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correlation between logKOW and logQMD for PAHs in moss and reindeer dung. The trend between logQMD and logKOW can be explained with respect to different absorption and excretion potential for various PBDE congeners. Generally, absorption and excretion of POPs in gastrointestinal tract are two uncoupled processes (Norstrom, 2002). According to the micelle-mediated diffusion model proposed by Kelly et al. (2004), bioaccumulation of POPs via unidirectional micelle transport (and hence dietary uptake) correlated positively with logKOW owing to enhanced absorption rates for higher logKOW compounds. However, the process of getting rid of such chemicals from the organism is the reverse transport process in comparison to absorption. As a result, less PBDEs with higher logKOW occur in dung. 4. Conclusions Fig. 3. Correlation between logQMD and logKOW for individual PBDE congeners except for BDE-153 and -183. Values of logKOW for PBDEs were derived from Li et al. (2008).
vegetation. In addition to the physicochemical properties, some factors would influence the distribution of different PBDE congeners in soil and moss, such as meteorological conditions, different uptake and loss processes in soil and moss. Nevertheless, more than 70% of the total variation of logQSM was explained by differences of PBDEs characterized in terms of logp°L or logKOA. Considering poorly understanding of the effects of factors stated above in the Arctic on the accumulation in soil and moss, and the strong collinearity of p°L, KOA and KOW, it is difficult to regress a polyparameter relation to explain more variation (Weiss, 2000). Moss and pine needle are typical natural passive air samplers as stated above. They can capture gas and particle phase POPs from the atmosphere, and the equilibrium state is expected to be approached for atmospheric POPs between moss (pine needle) and air (Wang et al., 2009b; Harmens et al., 2013). As stated by Wang et al. (2009b), the relation of logQSM with logp°L of selected polycyclic aromatic hydrocarbons (PAHs) was similar to logKP versus logp°L of gas/particle partitioning for PAHs, which carried the information of logKP and would be a ‘‘mirror image’’ of the gas/particle partitioning of PAHs in the atmosphere. Furthermore, previous study suggested that mr value of Eq. (1) would approach -1 under equilibrium condition and mr should inter-relate with intercept br (Pankow and Bidleman, 1992; Li et al., 2014). Similarly, it can be assumed that the slope of the linear regression of logQSM versus logp°L would be expected to be a theoretical value. Interestingly, similar linear correlation between logp°L and the logarithm of soil/vegetation (pine needle and moss) was observed in previous studies, and their slopes were 0.25, 0.161, 0.22 and 0.14, respectively (Weiss, 2000; Wang et al., 2009b, 2009c). These slopes do not differ greatly, implying similar patterns in distribution between soil and vegetation in relation to logp°L of the measured POPs. The results also presented the field-measured data to support the above assumption. However, due to the complexity of sorption process of POPs in vegetation and few knowledge of the effects of factors as discussed above, the quantitative model for describing the distribution processes and ad/absorption characteristics in soil and vegetation has so far not been fully developed and explained.Similar to QSM, the dimensionless moss/dung quotient (QMD) was calculated according to the following equation:
Q MD ¼ C M =C D
ð4Þ
where CD is PBDE concentrations in reindeer dung (pg/g). A significant linear relationship between logQMD and logKOW was observed (Fig. 3). LogQMD of PBDEs increase with the increase of their logKOW, indicating that PBDE congeners with higher logKOW are prone to accumulation in moss compared to reindeer dung. Similarly, Wang et al. (2009c) also observed the positive linear
The ranges of RPBDEs at Ny-Ålesund were 11–89 pg/g in soil, 48–259 pg/g in moss and 38–102 pg/g in reindeer dung, respectively. Significant linear correlation of logQSM versus logp°L indicates that logp°L can be used to describe the distribution of PBDEs in soil and moss. The relationships of logQSM logKOA and logQMD logKOW also confirm that the physicochemical properties of PBDEs, such as logp°L, are appropriate parameters for characterizing the distribution of PBDEs in soil, moss and reindeer dung at Ny-Ålesund of the Arctic. Acknowledgements This study was supported by the National Natural Science Foundation of China (21007012, 21377032), the Chinese Polar Environment Comprehensive Investigation & Assessment Programmes (02-01, 03-04, 04-01 and 04-03) and the polar science key laboratory foundation of SOA (KP201208). We express our sincere gratitude to the Chinese Arctic and Antarctic Administration. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2015.04.030. References de Wit, C.A., Alaee, M., Muir, D.C., 2006. Levels and trends of brominated flame retardants in the Arctic. Chemosphere 64, 209–233. Harmens, H., Foan, L., Simon, V., Mills, G., 2013. Terrestrial mosses as biomonitors of atmospheric POPs pollution: a review. Environ. Pollut. 173, 245–254. Hassanin, A., Breivik, K., Meijer, S.N., Steinnes, E., Thomas, G.O., Jones, K.C., 2004. PBDEs in European background soils: Levels and factors controlling their distribution. Environ. Sci. Technol. 38, 738–745. Heywood, E., Wright, J., Wienburg, C.L., Black, H.I.J., Long, S.M., Osborn, D., 2006. Factors influencing the national distribution of polycyclic aromatic hydrocarbons and polychlorinated biphenyls in British soils. Environ. Sci. Technol. 40, 7629–7635. Kelly, B.C., Gobas, F.A.P.C., McLachlan, M.S., 2004. Intestinal absorption and biomagnification of organic contaminants in fish, wildlife, and humans. Environ. Toxicol. Chem. 23, 2324–2336. Kelly, B.C., Ikonomou, M.G., Blair, J.D., Morin, A.E., Gobas, F.A., 2007. Food webspecific biomagnification of persistent organic pollutants. Science 317, 236– 239. Li, X.H., Chen, J.W., Zhang, L., Qiao, X.L., Huang, L.P., 2006. The fragment constant method for predicting octanol–air partition coefficients of persistent organic pollutants at different temperatures. J. Phys. Chem. Ref. Data 35, 1365–1384. Li, L.N., Xie, S.D., Cai, H., Bai, X.Y., Xue, Z., 2008. Quantitative structure-property relationships for octanol–water partition coefficients of polybrominated diphenyl ethers. Chemosphere 72, 1602–1606. Li, Y.F., Ma, W.L., Yang, M., 2014. Prediction of gas/particle partitioning of polybrominated diphenyl ethers (PBDEs) in global air: a theoretical study. Atmos. Chem. Phys. Dissc. 14, 23415–23451. Ma, X., Zhang, H., Yao, Z., Zhao, X., Wang, L., Wang, Z., 2013. Bioaccumulation and trophic transfer of polybrominated diphenyl ethers (PBDEs) in a marine food web from Liaodong Bay, North China. Mar. Pollut. Bull. 74, 110–115.
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