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Polybrominated diphenyl ethers (PBDEs) in soils along a rural-urban-rural transect: Sources, concentration gradients, and profiles
Environmental Pollution 159 (2011) 3666e3672
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Polybrominated diphenyl ethers (PBDEs) in soils along a rural-urban-rural transect: Sources, concentration gradients, and profiles Bondi Gevao a, *, Abdul Nabi Ghadban a, Saif Uddin a, Foday M. Jaward b, Majed Bahloul a, Jamal Zafar a a b
Department of Environmental Science, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait Environmental and Occupational Health, College of Public Health, University of South Florida, 13201 Bruce B. Downs Blvd. MDC56, Tampa, FL 33612-3805, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2011 Received in revised form 12 July 2011 Accepted 23 July 2011
This study reports concentrations of PBDEs in surface soil samples collected along a 140 km transect across Kuwait to assess the role of urban centers as sources of persistent organic pollutants to the surrounding environment. The SPBDE concentrations varied by a factor of w250 and ranged from 289 to 80,078 pg g1 d.w. The concentrations of PBDEs in Kuwait City were significantly higher (p < 0.01) than those collected from sites outside the city supporting the hypothesis that urban centers are sources of PBDEs. The congener profiles were dominated by BDE-209, accounting for 93% of the PBDEs in the soil samples. The concentrations of all congeners (except BDE-209) were highly correlated with percent organic carbon (%OC) (p > 0.05) when the data from Kuwait City was omitted from the analysis. These findings suggest that soil concentrations outside the urban centers were close to equilibrium with the atmosphere. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Polybrominated diphenyl ethers Surface soil Persistent organic pollutants Atmospheric deposition, Urban pulse
1. Introduction Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardants in a broad range of consumer products including electrical components, household appliances, furniture, textiles, etc. (Hazrati and Harrad, 2006; Harrad et al., 2008). They were manufactured at three levels of bromination as technical mixtures: the penta-; Octa-; and Deca-formulations. They bear acute similarities in molecular structure, and physico-chemical properties, with other persistent organic pollutants (POPs) like polychlorinated biphenyls (PCBs), polychlorinated dibenzo-pdioxins and furans (PCDD/Fs). PBDEs have a high potential to leach out of the polymers to which they are added since they are not covalently bonded into the fabric of the polymers (Alaee et al., 2003). The ventilation of homes and offices is therefore likely to be a source to ambient (outdoor) air (Wilford et al., 2004). Evidence of high indoor-outdoor gradient of PBDEs was recently reported in different cities (Butt et al., 2004; Wilford et al., 2004), further implicating indoor sources as contributors to ambient (outdoor) concentrations. PBDEs and other semi-volatile organic compounds primarily enter soil via wet and dry atmospheric deposition. Another pathway through which these compounds penetrate soil is the
* Corresponding author. E-mail address: [email protected] (B. Gevao). 0269-7491/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2011.07.021
application of sewage sludge to agricultural land. In Kuwait, sewage sludge from wastewater treatment plants are dumped in the desert and this may constitute an important pathway for these compounds to the terrestrial environment. Plants have been reported to scavenge POPs from air. Therefore, plant litter is another pathway for the entry of POPs into soil. Although vegetation has been reported to constitute an important reservoir for semi-volatile organic compounds (SOCs) (Choi et al., 2008), it is not likely to be an important delivery mechanism to soil in Kuwait because of scant vegetative cover for most of the year. Soils and sediments are important reservoirs of POPs since they act as repositories during periods of maximum input into the environment. When atmospheric concentrations decrease, especially following bans/restrictions on the use of these POPs, such as with PCBs and organochlorine pesticides, soils and sediments once thought of as permanent sinks have been shown to act as important secondary sources supplying these chemicals to the atmosphere. It is therefore important to obtain information on the levels and fate of these compounds in these reservoirs. PBDEs have been reported in various environmental media in Kuwait including indoor air/dust (Gevao et al., 2006a,b), ambient air (Gevao et al., 2006c, 2010), sediments (Gevao et al., 2006d), biota (Gevao et al., 2011), and in sewage sludge (Gevao et al., 2008). To date, however, there is virtually no information on the levels of PBDEs in soils in the Middle East in general or in Kuwait in particular. In 2009, the commercial penta-bromodiphenyl ethers and Octa-bromodiphenyl ethers were included in Annex A (elimination) of the Stockholm
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Convention target list of chemicals (UNEP, 2009). Prior to this date, however, the EU had banned these chemicals in 2004 (Cox and Ethymiou, 2003) and there were voluntary restrictions on their use in the US since 2006 (de Wit et al., 2006). This study presents, for the first time, data on PBDEs in soils collected along a north (upwind)esouth (downwind) transect across Kuwait. A secondary objective of the study was to investigate the “urban pulse” effect postulated by Harrad and Hunter (2006) which suggests that urban areas are sources of PBDEs to remote regions. In the current study, pollutants “outgassing” from Kuwait City are expected to be transported in the southerly direction which is the dominant wind direction. It is expected that soils downwind of Kuwait City will contain higher concentrations of PBDEs compared with those sampled upwind. It is hoped that this study will provide background information on PBDEs in soil against which the effectiveness of the control measures on their continued use in consumer products by the Stockholm Convention can be assessed in the future.
respectively. Samples were collected at approximately 20 km intervals along a 140 km northesouth transect from the KuwaiteIraq border through Kuwait to the Saudi border (Fig. 1). The transect was coincident with the predominant northwestsoutheast wind direction across Kuwait, providing an opportunity to investigate the “urban pulse” theory postulated by Harrad and Hunter (2006). At each sampling site surface (0e5 cm), soil samples were collected using a stainless steel hand held auger. To ensure that samples from each site were representative, three samples were collected in a triangular fashion about 5 m apart (approximately 10 m from the road) and pooled to give a composite sample prior to sub-sampling. The samples were immediately transferred into clean, solvent-rinsed, amber glass jars and stored in a cool box for transport to the laboratory. All utensils used in the collection, pooling and sub-sampling were thoroughly washed and rinsed with acetone and hexane between sampling sites to minimize the likelihood of cross-contamination. Each sampling location was given a unique location reference and coordinates were recorded using a global positioning system (GPS). All samples were given a unique labeling code that identified sample type, time of collection, and other important information. In the laboratory, the samples were sieved onto solvent-rinsed aluminium foil to remove debris and other large particles. The sieve was cleaned and aluminium foil replaced between samples. The samples were immediately transferred to clean; solvent-rinsed amber glass bottles and kept at 20 C until analysis.
2. Materials and methods
Soil samples (w20 g) were extracted in a Soxhlet apparatus using 1:1 v/v mixture of DCM:hexane. Although the soil samples were dry, 10 g of prebaked (450 C for 12 h) sodium sulfate was added to remove any residual water, and spiked with PBDE congeners (BDE 35 and BDE 181) to monitor analytical recovery. The extracts were reduced in volume on a TurbovapÔ, solvent exchanged to hexane and interfering compounds removed by column chromatography using 10 g silica and 5 g alumina (and 0.5 cm anhydrous Na2SO4 at the top of the column to prevent the column from contact with air). The compounds of interest were eluted with 100 ml 1:1 mixture of hexane:DCM. The eluent was then blown down on a TurbovapÔ concentrator, transferred to 2 ml vials and blown down under a gentle stream of nitrogen. 50 ml of dodecane was added during this blow down stage to ensure the samples did not dry out. The samples were then transferred to 100 ml glass inserts, and spiked with Mirex (10 ml of 10 ng/ml) internal standard, used for volume correction and to adjust for variations in instrument response. The sample extracts were analyzed on a Shimadzu GC 2010 (Shimadzu, Tokyo, Japan) gas chromatograph
2.1. Chemicals and reagents All solvents used in this work were of analytical grade and purchased through VWR Scientific (USA). Silica (Baker, 100e200 mesh), Alumina and sodium sulfate (Baker) were also purchased through VWR Scientific (USA). The PBDE analytical standard (EO-5278) was purchased from CIL. The following PBDE congeners were in the standard mix (28, 47, 99, 100, 153, 154, 183, and 209). Individual standards for BDEs 35 (EO-4109) and 181 (EO-4927) were purchased separately from CIL. 2.2. Sample collection Soil samples were collected in November 2010. The mean, minimum and maximum temperatures over sampling period were 26 C, 21 C and 34 C
2.3. Extraction and analyses
Fig. 1. Soil sampling locations along a major road from Iraq to Saudi Arabia. Arrow shows the dominant wind direction across Kuwait.
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using splitless injection on a 15 m DB5-ms column (0.25 mm i.d., 0.25 mm film thickness) and helium as carrier gas. The oven program was 150 C for 1 min, ramped at 20 C min1 to 250 C, 4 C min1 to 290 C, and held for 25 min. This gas chromatograph was coupled to a Shimadzu 2010 Mass Selective Detector operated in electron capture negative chemical ionization (ECNCI) using selected ion monitoring (SIM), with methane as reagent gas. The ions m/z 79 and 81 were monitored for PBDEs and 402/404 for Mirex. Operating conditions were as follows: injector temperature was set at 250 C; ion source 230 C; quadrupole 106 C; transfer line 300 C. Identification and quantification was carried out against 5 calibration standards of known concentrations.
2.4. QA/QC An analytical blank was processed for every 5 samples. In the absence of appropriate blanks, anhydrous Na2SO4 (previously baked at 450 C) was spiked with surrogates and taken through the entire analytical process as for actual samples to serve as matrix blanks. A peak was positively identified if it was within 0.05 min of the retention time in the calibration standard and quantified only if the S/N 3 and the ratio of the target ion to its qualifier ion was within 20% of the standard value. The PBDEs present in the appropriate blanks were subtracted from those in the sample extracts. The method detection limits (MDLs) were calculated as the mean blank þ 3 SD. The method detection limits ranged from 110 pg (BDE 28) to 1.1 ng (BDE-209). These were converted to concentrations by dividing by the average weight of soil (20 g) analyzed in the study, resulting in detection limits of 5.5 pg g1 (BDE-28) to 55 pg g1 (BDE-209). Average recoveries (%) for surrogates spiked in samples were between 70 (10 SD) for BDE 35 and 84 (5 SD) for BDE 181. External recoveries for the entire method were carried out by spiking six pre-extracted thimbles containing 10 g anhydrous Na2SO4 that had been previously baked at 450 C for 12 h, with a working standard containing all PBDE congeners. The extraction and work-up procedures were identical to those for actual samples. The recoveries were found to be 90 12% for all congeners. The accuracy and precision of the analytical method was further assessed by replicate analyses (n ¼ 6) of a certified indoor dust reference material (SRM 2585). The results compared very well with the certified values of all the congeners (Fig. 2).
3. Results and discussion The concentrations of PBDEs were calculated by dividing the amounts by the actual weight of soil extracted after adjusting for moisture. A total of 8 PBDE congeners was regularly detected in samples and quantified. These are: BDE 28 (2,4,40 -TriBDE); BDE-47 (2,20 ,4,40 -Tetra-BDE); BDE 99 (2,20 ,4,40 ,5-Penta-BDE); BDE 100 (2,20 ,4,40 ,6-Penta-BDE); BDE-153 (2,20 ,4,40 ,5,50 -Hexa-BDE); BDE-154 (2,20 ,4,40 ,5,60 -Hexa-BDE); BDE-183 (2,20 ,3,4,40 ,50 , 6-Hepta-BDE) and BDE-209 (2,20 ,3,30 4,40 ,5,50 ,6,60 -Deca-BDE). In the discussion that follows, SPBDE refers to the sum of all the congeners measured in
Fig. 2. Comparison between average (n ¼ 6) concentrations of PBDE congeners in SRM 2585 in this study and their certified values. The error bars represent standard deviations.
this study whereas S7PBDEs refers to the concentrations of the penta-congeners (BDEs 28, 47, 99, 100, 153, 154, and 183). The concentrations of PBDEs in the samples are summarized in Fig. 3 and the congener specific concentrations are given in Tables 1 and 2 on a dry weight, and organic carbon normalized concentration basis respectively. The data in Fig. 3 is arranged along the northesouth direction from the KuwaiteIraq border to the KuwaiteSaudi Arabia border which is coincident with the predominant wind direction in the country; the natural gradient for pollutant input from long-range atmospheric transport. The SPBDE concentrations varied by a factor of w250 and ranged from 289 to 80,078 pg g1 d.w., whereas the concentrations of the S7PBDEs (the total contribution of the congeners that are predominantly present in the penta-BDE formulations) ranged from 26 to 6132 pg g1. The concentrations of PBDEs in Kuwait City (shaded portion) were significantly higher (p < 0.01) than those collected from sites outside the city evidenced by a clear pulse in the profile given in Fig. 3. This magnitude of the pulse in concentration within the city, estimated as the ratio of the average concentration in samples collected from the city (sites 5, 6 and 7), to the average concentration at all the other locations outside the city is ca 5 and 24 for SPBDE and S7PBDEs (penta-BDEs) respectively. When organic carbon normalized soil concentrations are used the magnitude of the pulse is 3.3 and 7.6 for SPBDEs and S7PBDEs respectively. This urban pulse for SPBDE is similar to that reported by Harrad and Hunter (2006) for a similar study in Birmingham UK although the magnitude of the pulse for S7PBDEs (penta-congeners) is much higher. It has been hypothesized that urban centers are net sources of pollutants to the surrounding areas because pollutants emitted from urban areas are transported in the atmosphere and deposited in remote locations. Since the predominant wind direction in the study area is from the northwest (upwind) to the southeast (downwind) across Kuwait City, it is plausible to expect higher concentrations of PBDEs in soil downwind of the urban source compared to those sampled upwind. The average concentrations of PBDEs in soil samples collected downwind of Kuwait City (sites 8e11) were 1.5 times the average concentrations collected upwind (sites 1e4) of the City. This finding lends support to the hypothesis that urban centers are sources of pollutants to surrounding areas. Table 3 compares the concentrations measured in this study with those reported in other parts of the world where urban and/or background soils have been analyzed for PBDEs. In order to
Fig. 3. S7PBDE and BDE-209 concentrations in soil samples collected along the transect across Kuwait.
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Table 1 Concentration of PBDEs (pg g1 d.w.) in soil samples along the IraqeKuwaiteSaudi Arabia transect. Site
1 2 3 4 5 6 7 8 9 10 11
Description
100 km Upwind of Kuwait City Center 80 km Upwind of Kuwait City Center 60 km Upwind of Kuwait City Center 40 km Upwind of Kuwait City Center 20 km Upwind of Kuwait City Center Kuwait City Center 20 km Downwind of Kuwait City Center 40 km Downwind of Kuwait City Center 60 km Downwind of Kuwait City Center 80 km Downwind of Kuwait City Center 100 km Downwind of Kuwait City Center
Congeners 28
47
100
99
154
153
183
209
SPBDEs
47:99 ratio
1.1 1.1 2.1 3.7 3.3 63.4 13.8 4.0 1.9 0.5 3.4
12.2 11.4 62.3 35.5 68.7 2099.6 321.6 81.4 21.8 8.0 46.0
1.0 1.5 22.5 2.7 13.9 392.0 78.1 25.0 2.4 1.5 7.8
7.4 9.4 130.3 19.3 107.9 2956.5 584.2 149.2 13.8 14.0 40.9
1.1 2.0 15.9 3.5 8.2 224.8 71.0 14.6 1.5 2.0 7.4
1.9 1.2 23.1 1.7 11.8 294.5 79.6 22.0 2.7 1.7 5.0
1.3 1.7 3.8 0.8 12.7 101.7 31.7 18.0 3.7 2.7 2.4
424.6 2118.6 7340.1 362.7 2894.8 60,464.8 78,898.5 15,280.1 241.7 3449.2 2864.0
450.6 2146.8 7600.0 430.0 3121.3 66,597.2 80,078.5 15,594.2 289.5 3479.6 2977.0
1.65 1.21 0.48 1.84 0.64 0.71 0.55 0.55 1.58 0.58 1.12
meaningfully compare the data between studies, the concentrations of the congeners frequently reported in the literature (defined in this study as S7PBDEs) are given with the total concentrations reported in the respective studies. The concentrations of the S7PBDEs in background soils in this study (24.7e296 pg g1 d.w.) are higher than those reported in Harbin City, China (2.45e55.9 pg g1 d.w.) (Xu et al., 2009); comparable to rural soils in Birmingham UK (73.5e285) (Harrad and Hunter, 2006), floodplain soil of the Saginaw River Watershed, Michigan, USA (20e830 pg g1 d.w.), mean background soils in the UK (400 pg g1 d.w.) (Hassanin et al., 2004) and soils from a botanical garden in Manaus, Brazil (434 pg g1 d.w.) (Thorenz et al., 2010). in soil samples from Kuwait City The S7PBDEs (226e6031 pg g1 d.w.) is also comparable to that reported for urban soil samples from Birmingham, UK (401e3890 pg g1 d.w.) (Harrad and Hunter, 2006), in urban soils from Mainz, Germany (1043 pg g1 d.w.) (Thorenz et al., 2010); Pujalt, Spain (6100 pg g1 d.w.) (Eljarrat et al., 2008), and in Shanghai, China (23.6e3799 pg g1 d.w.). The concentrations reported in this study and those cited above, represent the lower range of PBDE values in soils around the world as much higher values have been recorded in soils from electronic recycling activities (Wang et al., 2005; Cai and Jiang, 2006; Leung et al., 2007; Yang et al., 2008; Ma et al., 2009) and agricultural soils that have been amended with sewage sludge (Matscheko et al., 2002; Eljarrat et al., 2008). 3.1. Technical mixtures in soil samples The congener mixture in the soil samples suggests that two main technical formulations can be identified: deca- and pentamixtures (Fig. 4). The congener profiles were dominated by BDE209, the dominant congener in the technical deca-formulation,
accounting for 93% (range, 83e99%) of the PBDEs in the soil samples. This was followed in decreasing order of magnitude by BDE-47 (range, 0.2e8.3%; mean, 2.5%), BDE 99 (range, 0.4e4.8%; mean 2.2%) with the other congeners individually contributing less than 1% to the total concentrations in each sample. The concentrations of BDE-209 ranged from 242 to 78,899 pg g1 with a mean of 15,849 pg g1 d.w. The BDE-209 profile is markedly similar to the S7PBDE profile in that the levels in background soils were generally low with a pulse in Kuwait City (see Fig. 3). If BDE-209 is discarded from the analyses and the congener mixture is normalized to S7PBDEs, it becomes apparent that the penta-formulation is also an important source of PBDEs in Kuwait (Fig. 5). The congener distributions expressed as a percent of S7PBDEs (excluding BDE-209), are given in Fig. 5 for samples collected upwind of Kuwait City, within Kuwait City, and downwind of Kuwait City. Also plotted in Fig. 5 are the percent contributions of these congeners in two penta-formulations (Bromkal 70-5DE and DE-71) for comparison. Although subtle differences exist, the congener profiles in soil samples are remarkably similar to those in both penta-formulations assuming that the congener composition reported by La Guardia et al. (2006) is representative of penta-BDE technical mixtures. Hassanin et al. (2004) argued that this similarity in the soil profiles to that in the technical penta-formulations indicate that the transfer of congeners from source-sink operate with similar efficiencies across the penta-PBDE congeners and that minimal weathering would have occurred during atmospheric transport or within the soil. This indeed may be the case in this study as the distance between the main urban source to the furthermost soil sampling location was less than 100 km which may not be sufficient transport distance for significant changes in composition to occur. The subtle deviations between the observed congener profiles in soil samples and those in the technical penta-formulations may
Table 2 Concentration of PBDEs (pg g1 OC) in soil samples collected along a transect across Kuwait. Site
Description
1 2 3 4 5 6 7 8 9 10 11
100 km Upwind of Kuwait City Center 80 km Upwind of Kuwait City Center 60 km Upwind of Kuwait City Center 40 km Upwind of Kuwait City Center 20 km Upwind of Kuwait City Center Kuwait City Center 20 km Downwind of Kuwait City Center 40 km Downwind of Kuwait City Center 60 km Downwind of Kuwait City Center 80 km Downwind of Kuwait City Center 100 km Downwind of Kuwait City Center
Congener 28
47
100
99
154
153
183
209
SPBDEs
147.5 201.9 161.8 400.6 229.1 1896.2 192.8 222.8 167.6 46.6 472.7
1596.2 2054.8 4767.5 3804.0 4793.9 62,818.0 4489.9 4553.6 1899.5 738.2 6448.7
131.6 264.7 1721.4 292.0 969.8 11,729.1 1090.8 1396.3 209.3 135.6 1096.5
966.6 1691.4 9966.4 2071.3 7533.5 88,456.5 8155.4 8347.7 1199.6 1281.9 5737.9
145.9 360.5 1213.6 374.8 572.0 6725.4 990.5 819.2 130.1 186.9 1041.6
247.1 211.4 1766.6 182.6 822.4 8811.5 1110.6 1230.3 239.2 157.6 699.8
164.1 302.5 288.7 83.9 888.6 3041.9 442.1 1007.2 318.9 246.7 336.5
55,511.3 381,465.9 561,499.6 38,856.9 202,082.8 1,809,084.7 1,101,356.2 855,087.7 210,73.4 316,671.7 401,331.2
58,910.2 386,553.1 581,385.7 460,65.9 217,892.1 1,992,563.3 1,117,828.2 872,664.7 25,237.6 319,465.2 417,164.9
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Table 3 Comparison of PBDE soil concentrations (pg g1 d.w.) between this study and other studies. Location
Congener BDE-28
BDE-47
BDE-154
41.8e410
29.7e357 n/a
4.3e10.8 34.6e69.0
88.1e131
17.1e25.2 13.6e25.6
12e21 7e520 8e200 50e1400 8e49 12e860
78e3200 190e3200 63e1400
8e470 19e600 8e240 11e360 38e1200 14e420 18e230 11e270 13e310
<10e20 <10e20
10e520 <10e270 630 91 71 34
<10e160 <10e90 1080 92
a b c d e
Sum Sum Sum Sum Sum
of of of of of
18
38e392
BDE-153
Birmingham, UK (rural)
10e680 10e420 690 490
117e1710
BDE-100
13e116
UK grassland UK woodland Norwegian woodland Harbin City, China Shiawassee floodplain, USA Saginaw floodplain, USA Saginaw floodplain, USA Pujalt, Spain Mainz (urban), Germany Manaus, Brazil Bratislava (urban), Slovakia Shanghai (urban), China Kuwait-urban (This study) Kuwait-rural (This study)
101e909
BDE-99
Birmingham, UK (urban)
8.0e81.4
7.4e149.2
1.0e25.0
<10e70 <10e20 940 290 330 16
14.1e19.8
<10e60 <10e20 930 43 33 26
BDE-183
BDE-209
SPBDEs
Reference
401e3890b Harrad and Hunter, 2006 73.2e285b Harrad and Hunter, 2006 10e900 n/a 15e5000a Hassanin et al., 2004 a 10e7000 n/a 75e5600 Hassanin et al., 2004 9e130 90e2600a Hassanin et al., 2004
1870 19
n/a
<40e19,220 <40e2160 14,600 760 500
30e1400 20e830 6100d 1043c 434c 161c
Yum et al., 2008 Yum et al., 2008 Eljarrat et al., 2008 Thorenz et al., 2010 Thorenz et al., 2010 Thorenz et al., 2010
1.1e15.9
0.8e18.0
289e15,594 24.7e296b
congeners 47, 99, 100, 153, 154. congeners 28, 47, 99, 100, 153, 154. 17, 28, 47, 66, 100, 99, 154, 153, and 183. 47,100, 99, 154, 153, 183. 28 congeners.
be due to several factors including the range of commercial mixtures used in treating products. Two penta-formulations are commercially available: DE-71, produced by the Great Lakes Chemicals and the European formulation Bromkal 70-5DE. Both formulations have similar congener mixtures except that DE-71 has 10% higher levels of BDE 99 than BDE-47 (La Guardia et al., 2006). Also, the composition of congeners in technical mixtures from the same manufacturer may not be constant between batches. For instance, the percent composition of the two major congeners, BDE-47 and BDE-99 in Bromkal 70-5DE reported by Sjödin et al. (1998) are 37% and 35% respectively while Rayne and Ikonomou (2002) report 51% and 34%, and La Guardia et al. (2006) report 38% and 48% for an identical mixture. The differences in physico-chemical properties between the congeners may be relevant in explaining the subtle differences
Fig. 4. Contribution of the commercial mixtures to the SPBDE concentration measured in soil samples collected along the transect across Kuwait.
observed in the soil profiles and the technical penta-mixtures. For example, the BDE-47:-99 ratio in the soil samples range from 0.63 in Kuwait City to 0.92 in background soils, slightly lower than the ratios in the penta-mixtures (1.05, Bromkal 70-5DE (30); 0.95 La Guardia et al. (29)) but similar to the ratio in DE-71 the pentaformulation manufactured in the United States (0.78, La Guardia et al., 2006). Similar 47:99 ratios have been previously reported in soil samples (e.g. Hassanin et al., 2004; Harrad and Hunter, 2006; Duan et al., 2010). It has been argued that a higher KOA for BDE 99 compared to BDE-47 results in a preferentially higher atmospheric deposition and better soil retention of BDE 99 resulting in a low 47:99 in soils relative to their ratio in the technical penta-mixtures (Hassanin et al., 2004; Harrad and Hunter, 2006; Duan et al., 2010). To further illustrate how differences in the physico-chemical properties of PBDE congeners would have resulted in the observed changes in congener composition in soil samples, we plotted the ratio (“R”) of the sum of BDE-47 þ BDE-99 þ BDE-100 to the sum of BDE-153 þ BDE-154 along the transect (Fig. 6). This ratio was initially proposed by Hites (2004) to examine changes in congener
Fig. 5. Congener mixture expressed as a percent of penta-BDE congeners.
B. Gevao et al. / Environmental Pollution 159 (2011) 3666e3672
composition in guillemot eggs collected between 1980 and 2000 from the Great Lakes catchment. It was speculated that the positive correlation between the ratio and year of sample collection may have been due to possible changes in the composition of the technical mixtures used in the USA. This ratio has subsequently been used to assess the relative contributions of the penta- and octa-PBDE technical mixtures in samples (Yum et al., 2008; Jiang et al., 2010). In this study, the ratio (“R”) in soil along the transect may be interpreted as representing the degree of weathering of the technical formulation following its release from their source regions. This ratio (“R”) in the technical penta-mixture lies between 10 (DE-71) and 12 (Bromkal 70-5DE). The ratio is expected to decrease with distance from source regions. Following their release to the atmosphere, the fate of different congeners will be dictated by their properties. In the atmosphere, semi-volatile organic compounds undergo temperature dependent gas-particle partitioning with the heavier congeners preferentially partitioning onto particles. They are consequently more likely to be preferentially scavenged and deposited from a given air mass than lower molecular weight congeners which may preferentially be in the gas phase. As a result of their association with particles, higher molecular weight congeners are more likely deposited closer to source region (urban centers) and also preferentially retained in soils relative to the lower molecular weight congeners. These processes of preferential deposition and/or retention in soil mediated by the physico-chemical properties will ultimately result in the fractionation of the congeners which will result in the alteration of this ratio from that in the penta-formulation. It will be expected that the magnitude of “R” in soil will decrease with distance from potential source regions as heavier congeners are preferentially retained in soil relative to the more volatile congeners. In this study, the ratio (“R”) decreased in the following order: Kuwait city (range, 9.5e11; mean, 10.4) > downwind of the urban source (range, 6.2e7.6; mean, 7.3) > ratio upwind of Kuwait City (range, 3.4e5.5, mean, 5.6). The lowest value of “R” in this study
Fig. 6. Ratio of the sum of the concentrations of BDE-47 þ -99 þ -100 divided by the sum of BDE-153 plus BDE-154 in soil samples collected along the transect across Kuwait.
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(3.4) was from the northernmost site upwind of Kuwait City which mostly receives its pollutant input from long-range transport from other regions. This is followed by a gradual increase in “R” to a maximum in Kuwait City (mean 10.4) and a gentle decrease in samples downwind of the urban source. The inflection point in the profile occurs in the city center with the maximum PBDE soil concentrations. The decrease in the magnitude of “R” in the soil samples collected downwind of urban source (Kuwait City) is gentler because the soil samples downwind will receive fresh inputs of congeners outgassing from the city. 3.2. Soil organic matter It is often suggested that soil organic matter plays a significant role in the distribution of POPs in soils/sediments because they are hydrophobic contaminants. Consequently, the concentrations derived from soil/sediment analyses are often normalized to the organic carbon (OC) content. In this study, the soil organic carbon content of soils ranged from 0.4 to 2.5% except in Kuwait City where the values were between 3 and 7%. The organic carbon content of the soils (especially soil samples collected outside Kuwait City) are generally lower than those reported in other parts of the world (Hassanin et al., 2004; Harrad and Hunter, 2006; Yum et al., 2008; Thorenz et al., 2010). The higher OC content of soils within the City may be due to mulching of soils for greenery operations. Congener specific OC normalized soil concentrations in this study are given in Table 2 and are comparable to those reported in urban and remote soils from Birmingham, UK (Harrad and Hunter, 2006) and Norwegian woodland soils (Hassanin et al., 2004) but lower than those reported for UK grassland and woodland soils (Hassanin et al., 2004). The %OC concentrations in soils tracked the S7PBDE (excluding 209) concentrations remarkably well (Fig. 7). When BDE-209 was included, however, differences in profiles were evident especially in samples collected within Kuwait City where the OC content was significantly higher than in the other samples. Congener specific soil concentrations were highly correlated with % OC (Pearson correlation coefficients, R2, ranging between 0.7 and 0.82, p > 0.05) when the data from Kuwait City was omitted from the analysis. However, when the entire dataset was included in the assessment, none of the congeners was significantly correlated with OC. These findings suggest that the soil concentrations outside the urban centers were close to equilibrium with the atmosphere. It
Fig. 7. Concentrations of S7PBDEs and percent organic carbon content in soils along a transect across Kuwait.
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can be further deduced from the data that the concentrations of PBDEs in soils within Kuwait City were far from a steady state with air and are a function of primary emissions. Acknowledgments We are grateful to the management of Kuwait Institute for Scientific Research for funding this research. References Alaee, M., Arias, P., Sjodin, A., Bergman, A., 2003. An overview of commercially used brominated flame retardants, their applications, their use patterns in different countries/regions and possible modes of release. Environ. Int. 29, 683e689. Butt, C.M., Diamond, M.L., Truong, J., Ikonomou, M.G., ter Schure, A.H., 2004. Spatial distribution of polybrominated diphenyl ethers in Southern Ontario as measured in indoor and outdoor window organic films. Environ. Sci. Technol. 38, 724e731. Cai, Z., Jiang, G., 2006. Determination of polybrominated diphenyl ethers in soil from e-waste recycling site. Talanta 70, 88e90. Choi, S.D., Li, H., Su, Y., Gevao, B., Harner, T., Staebler, R.M., Wania, F., 2008. Depletion of gaseous polycyclic aromatic hydrocarbons by a forest canopy. Atmos. Chem. Phys. 8, 4105e4113. Cox, P.M., Ethymiou, P., 2003. Directive 2003/11/EC of the European parliament and of the council of February 6, 2003 amending for the 24th time Council Directive 76/669/EEC relating to restrictions on the marketting and use of certain dangerous substances and preparations (pentabromodiphenyl ether, octabromodiphenyl ether). Off. J. Eur. Union 42, 45e46. de Wit, C., Alaee, M., Muir, D.C., 2006. Levels and trends of brominated flame retardants in the Arctic. Chemosphere 64, 209e233. Duan, Y.-P., Meng, X.-Z., Yang, C., Pan, Z.-Y., Chen, L., Yu, R., Li F., T., 2010. Polybrominated diphenyl ethers in background surface soils from the Yangtze River Delta (YRD), China: occurrence, sources, and inventory. Environ. Sci. Pollut. Res. 17, 948e956. Eljarrat, E., Marsh, G., Labandeira, A., Barcelo, D., 2008. Effect of sewage sludes contaminated with polybrominated diphenylethers on agricultural soils. Chemosphere 71, 1079e1086. Gevao, B., Al-Bahloul, M., Al-Ghadban, A.N., Al-Omair, A., Ali, L., Zafar, J., Helaleh, M., 2006a. House dust as a source of human exposure to polybrominated diphenyl ethers in Kuwait. Chemosphere 64, 603e608. Gevao, B., Al-Bahloul, M., Al-Ghadban, A.N., Ali, L., Al-Omair, A., Helaleh, M., AlMatrouk, K., Zafar, J., 2006b. Polybrominated diphenyl ethers in indoor air Kuwait: implications for human exposure. Atmos. Environ. 40, 1419e1426. Gevao, B., Al-Omair, A., Sweetman, A., Al-Bahloul, M., Al-Ali, L., Helaleh, M., Zafar, J., 2006c. Passive-sampler derived air concentrations for polybrominated diphenyl ethers and polycyclic aromatic hydrocarbons in Kuwait. Environ. Toxicol. Chem. 25, 1496e1502. Gevao, B., Beg, M.U., Al-Ghadban, A.N., Al-Omair, A., Helaleh, M., Zafar, J., 2006d. Spatial distribution of polybrominated diphenyl ethers in coastal marine sediments receiving industrial and municipal effluents in Kuwait. Chemosphere 62, 1078e1086. Gevao, B., Jaward, F.M., Al-Bahloul, M., Ud din, S., Beg, M.U., Zafar, J., 2011. Polybrominated diphenyl ethers in three commercially important fish from the Northwestern Arabian Gulf: occurrence, concentration, and profiles. Arch. Environ. Contam. Toxicol. 60, 636e642. Gevao, B., Jaward, F.M., MacLeod, M., Jones, K.C., 2010. Diurnal fluctuations in polybrominated diphenyl ether concentrations during and after a severe dust storm episode in Kuwait City, Kuwait. Environ. Sci. Technol. 44, 8114e8120. Gevao, B., Muzaini, S., Helaleh, M., 2008. Occurrence and concentrations of polybrominated diphenyl ethers in sewage sludge from three wastewater treatment plants in Kuwait. Chemosphere 71, 242e247.
Harrad, S., Hunter, S., 2006. Concentrations of polybrominated diphenyl ethers in air and soil on a rural-urban transect across a major UK conurbation. Environ. Sci. Technol. 40, 4548e4553. Harrad, S., Ibarra, C., Diamond, M., Melymuk, L., Robson, M., Douwes, J., Roosens, L., Dirtu, A.C., Covaci, A., 2008. Polybrominated diphenyl ethers in domestic indoor dust from Canada, New Zealand, United Kingdom and United States. Environ. Int. 34, 232e238. 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, 738e745. Hazrati, S., Harrad, S., 2006. Causes of variability in concentrations of polychlorinated biphenyls and polybrominated diphenyl ethers in indoor air. Environ. Sci. Technol. 40, 7584e7589. Hites, R.A., 2004. Polybrominated diphenyl ethers in the environment and in people: a meta-analysis of concentrations. Environ. Sci. Technol. 38, 945e956. Jiang, Y., Wang, X., Zhu, K., Wu, M.S., Sheng, G., Fu, J., 2010. Occurrence, compositional profiles and possible sources of polybrominated diphenyl ethers in urban soils of Shanghai, China. Chemosphere 80, 131e136. La Guardia, L., Hale, R.C., Harvey, E., 2006. Detailed polybrominated diphenyl ether (PBDE) congener composition of the widely used penta-, octa-, and deca-PBDE technical flame-retardant mixtures. Environ. Sci. Technol. 40, 6247e6254. Leung, A.O., Luksemburg, W.J., Wong, A.S., Wong, M.H., 2007. Spatial distribution of polybrominated diphenyl ethers and polychlorinated dibenzo-p-dioxins and dibenzofurans in soil and combusted residue at Guiyu, an electronic waste recycling site in southeast China. Environ. Sci. Technol. 41, 2730e2737. Ma, J., Addink, R., Yun, S., Cheng, J., Wang, W., Kannan, K., 2009. Polybrominated dibenzo-p-dioxins/dibenzofurans and polybrominated diphenyl ethers in soil, vegetation, workshop-floor dust, and electronic shredder residue from an electronic waste recycling facility and in soils from a chemical industrial complex in eastern China. Environ. Sci. Technol. 43, 7350e7356. Matscheko, N., Tysklind, M., de Wit, C., Bergek, S., Andersson, R., Sellstrom, U., 2002. Application of sewage sludge to arable land-soil concentrations of polybrominated diphenyl ethers and polychorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls, and their accumulation in earthworms. Environ. Toxicol. Chem. 21, 2515e2525. Rayne, S., Ikonomou, M.G., 2002. Reconstructing source PBDE congener profiles from SPMDs in the Fraser River, British Columbia, Canada: comparison to commercial mixtures. Environ. Toxicol. Chem. 21, 2292e2300. Sjödin, A., Jakobsson, E., Kierkegaard, A., Marsh, G., Sellstrom, U., 1998. Gas chromatographic identification and quantification of polybrominated diphenyl ethers in a commercial product, Bromkal 70-5DE. J. Chromatogr. A 822, 83e89. Thorenz, U.R., Bandowe, B.A.M., Sobocka, J., Wilcke, W., 2010. Method optimization to measure polybrominated diphenyl etteh (PBDE) concentrations in soils of Bratislava, Slovakia. Environ. Pollut. 158, 2208e2217. UNEP, 2009. The nine new POPs under the Stockholm convention. Retrieved March 12, 2010, from. http://chm.pops.int/Programmes/NewPOPs/The9newPOPs/ tabid/672/language/en-US/Default.aspx. Wang, D., Cai, Z., Jiang, G., Leung, A., Wong, M.H., Wong, W.K., 2005. Determination of polybrominated diphenyl ethers in soil and sediment from an electronic waste recycling facility. Chemosphere 60, 810e816. Wilford, B.H., Harner, T., Zhu, J., Shoeib, M., Jones, K.C., 2004. A passive sampling survey of polybrominated diphenyl ether flame retardants in indoor and outdoor air in Ottawa, Canada: implications for sources and exposure. Environ. Sci. Technol. 38, 5312e5318. Xu, W., Nanqi, R., Hong, Q., Wanli, M., Yifan, L., 2009. Levels and distribution of brominated flame retardants in the soil of Harbin in China. J. Environ. Sci. 21, 1541e1546. Yang, Z.Z., Zhao, X.R., Zhao, Q., Qin, Z.F., Qin, X.F., Xu, X.B., Jin, Z.X., Xu, C.X., 2008. Polybrominated diphenyl ethers in leaves and soil from typical electronic waste polluted area in South China. Bull. Environ. Contam. Toxicol. 80, 340e344. Yum, S.H., Addink, R., McCabe, J.M., Ostaszewski, A., Mackenzie-Taylor, D., Taylor, A.B., Kannan, K., 2008. Polybrominated diphenyl ethers and polybrominated biphenyls in sediment and floodplain soils of the Saginaw river watershed, Michigan, USA. Arch. Environ. Contam. Toxicol. 55, 1e10.
Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Polybrominated diphenyl ethers (PBDEs) in soils along a rural-urban-rural transect: Sources, concentration gradients, and profiles Bondi Gevao a, *, Abdul Nabi Ghadban a, Saif Uddin a, Foday M. Jaward b, Majed Bahloul a, Jamal Zafar a a b
Department of Environmental Science, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait Environmental and Occupational Health, College of Public Health, University of South Florida, 13201 Bruce B. Downs Blvd. MDC56, Tampa, FL 33612-3805, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2011 Received in revised form 12 July 2011 Accepted 23 July 2011
This study reports concentrations of PBDEs in surface soil samples collected along a 140 km transect across Kuwait to assess the role of urban centers as sources of persistent organic pollutants to the surrounding environment. The SPBDE concentrations varied by a factor of w250 and ranged from 289 to 80,078 pg g1 d.w. The concentrations of PBDEs in Kuwait City were significantly higher (p < 0.01) than those collected from sites outside the city supporting the hypothesis that urban centers are sources of PBDEs. The congener profiles were dominated by BDE-209, accounting for 93% of the PBDEs in the soil samples. The concentrations of all congeners (except BDE-209) were highly correlated with percent organic carbon (%OC) (p > 0.05) when the data from Kuwait City was omitted from the analysis. These findings suggest that soil concentrations outside the urban centers were close to equilibrium with the atmosphere. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Polybrominated diphenyl ethers Surface soil Persistent organic pollutants Atmospheric deposition, Urban pulse
1. Introduction Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardants in a broad range of consumer products including electrical components, household appliances, furniture, textiles, etc. (Hazrati and Harrad, 2006; Harrad et al., 2008). They were manufactured at three levels of bromination as technical mixtures: the penta-; Octa-; and Deca-formulations. They bear acute similarities in molecular structure, and physico-chemical properties, with other persistent organic pollutants (POPs) like polychlorinated biphenyls (PCBs), polychlorinated dibenzo-pdioxins and furans (PCDD/Fs). PBDEs have a high potential to leach out of the polymers to which they are added since they are not covalently bonded into the fabric of the polymers (Alaee et al., 2003). The ventilation of homes and offices is therefore likely to be a source to ambient (outdoor) air (Wilford et al., 2004). Evidence of high indoor-outdoor gradient of PBDEs was recently reported in different cities (Butt et al., 2004; Wilford et al., 2004), further implicating indoor sources as contributors to ambient (outdoor) concentrations. PBDEs and other semi-volatile organic compounds primarily enter soil via wet and dry atmospheric deposition. Another pathway through which these compounds penetrate soil is the
* Corresponding author. E-mail address: [email protected] (B. Gevao). 0269-7491/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2011.07.021
application of sewage sludge to agricultural land. In Kuwait, sewage sludge from wastewater treatment plants are dumped in the desert and this may constitute an important pathway for these compounds to the terrestrial environment. Plants have been reported to scavenge POPs from air. Therefore, plant litter is another pathway for the entry of POPs into soil. Although vegetation has been reported to constitute an important reservoir for semi-volatile organic compounds (SOCs) (Choi et al., 2008), it is not likely to be an important delivery mechanism to soil in Kuwait because of scant vegetative cover for most of the year. Soils and sediments are important reservoirs of POPs since they act as repositories during periods of maximum input into the environment. When atmospheric concentrations decrease, especially following bans/restrictions on the use of these POPs, such as with PCBs and organochlorine pesticides, soils and sediments once thought of as permanent sinks have been shown to act as important secondary sources supplying these chemicals to the atmosphere. It is therefore important to obtain information on the levels and fate of these compounds in these reservoirs. PBDEs have been reported in various environmental media in Kuwait including indoor air/dust (Gevao et al., 2006a,b), ambient air (Gevao et al., 2006c, 2010), sediments (Gevao et al., 2006d), biota (Gevao et al., 2011), and in sewage sludge (Gevao et al., 2008). To date, however, there is virtually no information on the levels of PBDEs in soils in the Middle East in general or in Kuwait in particular. In 2009, the commercial penta-bromodiphenyl ethers and Octa-bromodiphenyl ethers were included in Annex A (elimination) of the Stockholm
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Convention target list of chemicals (UNEP, 2009). Prior to this date, however, the EU had banned these chemicals in 2004 (Cox and Ethymiou, 2003) and there were voluntary restrictions on their use in the US since 2006 (de Wit et al., 2006). This study presents, for the first time, data on PBDEs in soils collected along a north (upwind)esouth (downwind) transect across Kuwait. A secondary objective of the study was to investigate the “urban pulse” effect postulated by Harrad and Hunter (2006) which suggests that urban areas are sources of PBDEs to remote regions. In the current study, pollutants “outgassing” from Kuwait City are expected to be transported in the southerly direction which is the dominant wind direction. It is expected that soils downwind of Kuwait City will contain higher concentrations of PBDEs compared with those sampled upwind. It is hoped that this study will provide background information on PBDEs in soil against which the effectiveness of the control measures on their continued use in consumer products by the Stockholm Convention can be assessed in the future.
respectively. Samples were collected at approximately 20 km intervals along a 140 km northesouth transect from the KuwaiteIraq border through Kuwait to the Saudi border (Fig. 1). The transect was coincident with the predominant northwestsoutheast wind direction across Kuwait, providing an opportunity to investigate the “urban pulse” theory postulated by Harrad and Hunter (2006). At each sampling site surface (0e5 cm), soil samples were collected using a stainless steel hand held auger. To ensure that samples from each site were representative, three samples were collected in a triangular fashion about 5 m apart (approximately 10 m from the road) and pooled to give a composite sample prior to sub-sampling. The samples were immediately transferred into clean, solvent-rinsed, amber glass jars and stored in a cool box for transport to the laboratory. All utensils used in the collection, pooling and sub-sampling were thoroughly washed and rinsed with acetone and hexane between sampling sites to minimize the likelihood of cross-contamination. Each sampling location was given a unique location reference and coordinates were recorded using a global positioning system (GPS). All samples were given a unique labeling code that identified sample type, time of collection, and other important information. In the laboratory, the samples were sieved onto solvent-rinsed aluminium foil to remove debris and other large particles. The sieve was cleaned and aluminium foil replaced between samples. The samples were immediately transferred to clean; solvent-rinsed amber glass bottles and kept at 20 C until analysis.
2. Materials and methods
Soil samples (w20 g) were extracted in a Soxhlet apparatus using 1:1 v/v mixture of DCM:hexane. Although the soil samples were dry, 10 g of prebaked (450 C for 12 h) sodium sulfate was added to remove any residual water, and spiked with PBDE congeners (BDE 35 and BDE 181) to monitor analytical recovery. The extracts were reduced in volume on a TurbovapÔ, solvent exchanged to hexane and interfering compounds removed by column chromatography using 10 g silica and 5 g alumina (and 0.5 cm anhydrous Na2SO4 at the top of the column to prevent the column from contact with air). The compounds of interest were eluted with 100 ml 1:1 mixture of hexane:DCM. The eluent was then blown down on a TurbovapÔ concentrator, transferred to 2 ml vials and blown down under a gentle stream of nitrogen. 50 ml of dodecane was added during this blow down stage to ensure the samples did not dry out. The samples were then transferred to 100 ml glass inserts, and spiked with Mirex (10 ml of 10 ng/ml) internal standard, used for volume correction and to adjust for variations in instrument response. The sample extracts were analyzed on a Shimadzu GC 2010 (Shimadzu, Tokyo, Japan) gas chromatograph
2.1. Chemicals and reagents All solvents used in this work were of analytical grade and purchased through VWR Scientific (USA). Silica (Baker, 100e200 mesh), Alumina and sodium sulfate (Baker) were also purchased through VWR Scientific (USA). The PBDE analytical standard (EO-5278) was purchased from CIL. The following PBDE congeners were in the standard mix (28, 47, 99, 100, 153, 154, 183, and 209). Individual standards for BDEs 35 (EO-4109) and 181 (EO-4927) were purchased separately from CIL. 2.2. Sample collection Soil samples were collected in November 2010. The mean, minimum and maximum temperatures over sampling period were 26 C, 21 C and 34 C
2.3. Extraction and analyses
Fig. 1. Soil sampling locations along a major road from Iraq to Saudi Arabia. Arrow shows the dominant wind direction across Kuwait.
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using splitless injection on a 15 m DB5-ms column (0.25 mm i.d., 0.25 mm film thickness) and helium as carrier gas. The oven program was 150 C for 1 min, ramped at 20 C min1 to 250 C, 4 C min1 to 290 C, and held for 25 min. This gas chromatograph was coupled to a Shimadzu 2010 Mass Selective Detector operated in electron capture negative chemical ionization (ECNCI) using selected ion monitoring (SIM), with methane as reagent gas. The ions m/z 79 and 81 were monitored for PBDEs and 402/404 for Mirex. Operating conditions were as follows: injector temperature was set at 250 C; ion source 230 C; quadrupole 106 C; transfer line 300 C. Identification and quantification was carried out against 5 calibration standards of known concentrations.
2.4. QA/QC An analytical blank was processed for every 5 samples. In the absence of appropriate blanks, anhydrous Na2SO4 (previously baked at 450 C) was spiked with surrogates and taken through the entire analytical process as for actual samples to serve as matrix blanks. A peak was positively identified if it was within 0.05 min of the retention time in the calibration standard and quantified only if the S/N 3 and the ratio of the target ion to its qualifier ion was within 20% of the standard value. The PBDEs present in the appropriate blanks were subtracted from those in the sample extracts. The method detection limits (MDLs) were calculated as the mean blank þ 3 SD. The method detection limits ranged from 110 pg (BDE 28) to 1.1 ng (BDE-209). These were converted to concentrations by dividing by the average weight of soil (20 g) analyzed in the study, resulting in detection limits of 5.5 pg g1 (BDE-28) to 55 pg g1 (BDE-209). Average recoveries (%) for surrogates spiked in samples were between 70 (10 SD) for BDE 35 and 84 (5 SD) for BDE 181. External recoveries for the entire method were carried out by spiking six pre-extracted thimbles containing 10 g anhydrous Na2SO4 that had been previously baked at 450 C for 12 h, with a working standard containing all PBDE congeners. The extraction and work-up procedures were identical to those for actual samples. The recoveries were found to be 90 12% for all congeners. The accuracy and precision of the analytical method was further assessed by replicate analyses (n ¼ 6) of a certified indoor dust reference material (SRM 2585). The results compared very well with the certified values of all the congeners (Fig. 2).
3. Results and discussion The concentrations of PBDEs were calculated by dividing the amounts by the actual weight of soil extracted after adjusting for moisture. A total of 8 PBDE congeners was regularly detected in samples and quantified. These are: BDE 28 (2,4,40 -TriBDE); BDE-47 (2,20 ,4,40 -Tetra-BDE); BDE 99 (2,20 ,4,40 ,5-Penta-BDE); BDE 100 (2,20 ,4,40 ,6-Penta-BDE); BDE-153 (2,20 ,4,40 ,5,50 -Hexa-BDE); BDE-154 (2,20 ,4,40 ,5,60 -Hexa-BDE); BDE-183 (2,20 ,3,4,40 ,50 , 6-Hepta-BDE) and BDE-209 (2,20 ,3,30 4,40 ,5,50 ,6,60 -Deca-BDE). In the discussion that follows, SPBDE refers to the sum of all the congeners measured in
Fig. 2. Comparison between average (n ¼ 6) concentrations of PBDE congeners in SRM 2585 in this study and their certified values. The error bars represent standard deviations.
this study whereas S7PBDEs refers to the concentrations of the penta-congeners (BDEs 28, 47, 99, 100, 153, 154, and 183). The concentrations of PBDEs in the samples are summarized in Fig. 3 and the congener specific concentrations are given in Tables 1 and 2 on a dry weight, and organic carbon normalized concentration basis respectively. The data in Fig. 3 is arranged along the northesouth direction from the KuwaiteIraq border to the KuwaiteSaudi Arabia border which is coincident with the predominant wind direction in the country; the natural gradient for pollutant input from long-range atmospheric transport. The SPBDE concentrations varied by a factor of w250 and ranged from 289 to 80,078 pg g1 d.w., whereas the concentrations of the S7PBDEs (the total contribution of the congeners that are predominantly present in the penta-BDE formulations) ranged from 26 to 6132 pg g1. The concentrations of PBDEs in Kuwait City (shaded portion) were significantly higher (p < 0.01) than those collected from sites outside the city evidenced by a clear pulse in the profile given in Fig. 3. This magnitude of the pulse in concentration within the city, estimated as the ratio of the average concentration in samples collected from the city (sites 5, 6 and 7), to the average concentration at all the other locations outside the city is ca 5 and 24 for SPBDE and S7PBDEs (penta-BDEs) respectively. When organic carbon normalized soil concentrations are used the magnitude of the pulse is 3.3 and 7.6 for SPBDEs and S7PBDEs respectively. This urban pulse for SPBDE is similar to that reported by Harrad and Hunter (2006) for a similar study in Birmingham UK although the magnitude of the pulse for S7PBDEs (penta-congeners) is much higher. It has been hypothesized that urban centers are net sources of pollutants to the surrounding areas because pollutants emitted from urban areas are transported in the atmosphere and deposited in remote locations. Since the predominant wind direction in the study area is from the northwest (upwind) to the southeast (downwind) across Kuwait City, it is plausible to expect higher concentrations of PBDEs in soil downwind of the urban source compared to those sampled upwind. The average concentrations of PBDEs in soil samples collected downwind of Kuwait City (sites 8e11) were 1.5 times the average concentrations collected upwind (sites 1e4) of the City. This finding lends support to the hypothesis that urban centers are sources of pollutants to surrounding areas. Table 3 compares the concentrations measured in this study with those reported in other parts of the world where urban and/or background soils have been analyzed for PBDEs. In order to
Fig. 3. S7PBDE and BDE-209 concentrations in soil samples collected along the transect across Kuwait.
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Table 1 Concentration of PBDEs (pg g1 d.w.) in soil samples along the IraqeKuwaiteSaudi Arabia transect. Site
1 2 3 4 5 6 7 8 9 10 11
Description
100 km Upwind of Kuwait City Center 80 km Upwind of Kuwait City Center 60 km Upwind of Kuwait City Center 40 km Upwind of Kuwait City Center 20 km Upwind of Kuwait City Center Kuwait City Center 20 km Downwind of Kuwait City Center 40 km Downwind of Kuwait City Center 60 km Downwind of Kuwait City Center 80 km Downwind of Kuwait City Center 100 km Downwind of Kuwait City Center
Congeners 28
47
100
99
154
153
183
209
SPBDEs
47:99 ratio
1.1 1.1 2.1 3.7 3.3 63.4 13.8 4.0 1.9 0.5 3.4
12.2 11.4 62.3 35.5 68.7 2099.6 321.6 81.4 21.8 8.0 46.0
1.0 1.5 22.5 2.7 13.9 392.0 78.1 25.0 2.4 1.5 7.8
7.4 9.4 130.3 19.3 107.9 2956.5 584.2 149.2 13.8 14.0 40.9
1.1 2.0 15.9 3.5 8.2 224.8 71.0 14.6 1.5 2.0 7.4
1.9 1.2 23.1 1.7 11.8 294.5 79.6 22.0 2.7 1.7 5.0
1.3 1.7 3.8 0.8 12.7 101.7 31.7 18.0 3.7 2.7 2.4
424.6 2118.6 7340.1 362.7 2894.8 60,464.8 78,898.5 15,280.1 241.7 3449.2 2864.0
450.6 2146.8 7600.0 430.0 3121.3 66,597.2 80,078.5 15,594.2 289.5 3479.6 2977.0
1.65 1.21 0.48 1.84 0.64 0.71 0.55 0.55 1.58 0.58 1.12
meaningfully compare the data between studies, the concentrations of the congeners frequently reported in the literature (defined in this study as S7PBDEs) are given with the total concentrations reported in the respective studies. The concentrations of the S7PBDEs in background soils in this study (24.7e296 pg g1 d.w.) are higher than those reported in Harbin City, China (2.45e55.9 pg g1 d.w.) (Xu et al., 2009); comparable to rural soils in Birmingham UK (73.5e285) (Harrad and Hunter, 2006), floodplain soil of the Saginaw River Watershed, Michigan, USA (20e830 pg g1 d.w.), mean background soils in the UK (400 pg g1 d.w.) (Hassanin et al., 2004) and soils from a botanical garden in Manaus, Brazil (434 pg g1 d.w.) (Thorenz et al., 2010). in soil samples from Kuwait City The S7PBDEs (226e6031 pg g1 d.w.) is also comparable to that reported for urban soil samples from Birmingham, UK (401e3890 pg g1 d.w.) (Harrad and Hunter, 2006), in urban soils from Mainz, Germany (1043 pg g1 d.w.) (Thorenz et al., 2010); Pujalt, Spain (6100 pg g1 d.w.) (Eljarrat et al., 2008), and in Shanghai, China (23.6e3799 pg g1 d.w.). The concentrations reported in this study and those cited above, represent the lower range of PBDE values in soils around the world as much higher values have been recorded in soils from electronic recycling activities (Wang et al., 2005; Cai and Jiang, 2006; Leung et al., 2007; Yang et al., 2008; Ma et al., 2009) and agricultural soils that have been amended with sewage sludge (Matscheko et al., 2002; Eljarrat et al., 2008). 3.1. Technical mixtures in soil samples The congener mixture in the soil samples suggests that two main technical formulations can be identified: deca- and pentamixtures (Fig. 4). The congener profiles were dominated by BDE209, the dominant congener in the technical deca-formulation,
accounting for 93% (range, 83e99%) of the PBDEs in the soil samples. This was followed in decreasing order of magnitude by BDE-47 (range, 0.2e8.3%; mean, 2.5%), BDE 99 (range, 0.4e4.8%; mean 2.2%) with the other congeners individually contributing less than 1% to the total concentrations in each sample. The concentrations of BDE-209 ranged from 242 to 78,899 pg g1 with a mean of 15,849 pg g1 d.w. The BDE-209 profile is markedly similar to the S7PBDE profile in that the levels in background soils were generally low with a pulse in Kuwait City (see Fig. 3). If BDE-209 is discarded from the analyses and the congener mixture is normalized to S7PBDEs, it becomes apparent that the penta-formulation is also an important source of PBDEs in Kuwait (Fig. 5). The congener distributions expressed as a percent of S7PBDEs (excluding BDE-209), are given in Fig. 5 for samples collected upwind of Kuwait City, within Kuwait City, and downwind of Kuwait City. Also plotted in Fig. 5 are the percent contributions of these congeners in two penta-formulations (Bromkal 70-5DE and DE-71) for comparison. Although subtle differences exist, the congener profiles in soil samples are remarkably similar to those in both penta-formulations assuming that the congener composition reported by La Guardia et al. (2006) is representative of penta-BDE technical mixtures. Hassanin et al. (2004) argued that this similarity in the soil profiles to that in the technical penta-formulations indicate that the transfer of congeners from source-sink operate with similar efficiencies across the penta-PBDE congeners and that minimal weathering would have occurred during atmospheric transport or within the soil. This indeed may be the case in this study as the distance between the main urban source to the furthermost soil sampling location was less than 100 km which may not be sufficient transport distance for significant changes in composition to occur. The subtle deviations between the observed congener profiles in soil samples and those in the technical penta-formulations may
Table 2 Concentration of PBDEs (pg g1 OC) in soil samples collected along a transect across Kuwait. Site
Description
1 2 3 4 5 6 7 8 9 10 11
100 km Upwind of Kuwait City Center 80 km Upwind of Kuwait City Center 60 km Upwind of Kuwait City Center 40 km Upwind of Kuwait City Center 20 km Upwind of Kuwait City Center Kuwait City Center 20 km Downwind of Kuwait City Center 40 km Downwind of Kuwait City Center 60 km Downwind of Kuwait City Center 80 km Downwind of Kuwait City Center 100 km Downwind of Kuwait City Center
Congener 28
47
100
99
154
153
183
209
SPBDEs
147.5 201.9 161.8 400.6 229.1 1896.2 192.8 222.8 167.6 46.6 472.7
1596.2 2054.8 4767.5 3804.0 4793.9 62,818.0 4489.9 4553.6 1899.5 738.2 6448.7
131.6 264.7 1721.4 292.0 969.8 11,729.1 1090.8 1396.3 209.3 135.6 1096.5
966.6 1691.4 9966.4 2071.3 7533.5 88,456.5 8155.4 8347.7 1199.6 1281.9 5737.9
145.9 360.5 1213.6 374.8 572.0 6725.4 990.5 819.2 130.1 186.9 1041.6
247.1 211.4 1766.6 182.6 822.4 8811.5 1110.6 1230.3 239.2 157.6 699.8
164.1 302.5 288.7 83.9 888.6 3041.9 442.1 1007.2 318.9 246.7 336.5
55,511.3 381,465.9 561,499.6 38,856.9 202,082.8 1,809,084.7 1,101,356.2 855,087.7 210,73.4 316,671.7 401,331.2
58,910.2 386,553.1 581,385.7 460,65.9 217,892.1 1,992,563.3 1,117,828.2 872,664.7 25,237.6 319,465.2 417,164.9
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Table 3 Comparison of PBDE soil concentrations (pg g1 d.w.) between this study and other studies. Location
Congener BDE-28
BDE-47
BDE-154
41.8e410
29.7e357 n/a
4.3e10.8 34.6e69.0
88.1e131
17.1e25.2 13.6e25.6
12e21 7e520 8e200 50e1400 8e49 12e860
78e3200 190e3200 63e1400
8e470 19e600 8e240 11e360 38e1200 14e420 18e230 11e270 13e310
<10e20 <10e20
10e520 <10e270 630 91 71 34
<10e160 <10e90 1080 92
a b c d e
Sum Sum Sum Sum Sum
of of of of of
18
38e392
BDE-153
Birmingham, UK (rural)
10e680 10e420 690 490
117e1710
BDE-100
13e116
UK grassland UK woodland Norwegian woodland Harbin City, China Shiawassee floodplain, USA Saginaw floodplain, USA Saginaw floodplain, USA Pujalt, Spain Mainz (urban), Germany Manaus, Brazil Bratislava (urban), Slovakia Shanghai (urban), China Kuwait-urban (This study) Kuwait-rural (This study)
101e909
BDE-99
Birmingham, UK (urban)
8.0e81.4
7.4e149.2
1.0e25.0
<10e70 <10e20 940 290 330 16
14.1e19.8
<10e60 <10e20 930 43 33 26
BDE-183
BDE-209
SPBDEs
Reference
401e3890b Harrad and Hunter, 2006 73.2e285b Harrad and Hunter, 2006 10e900 n/a 15e5000a Hassanin et al., 2004 a 10e7000 n/a 75e5600 Hassanin et al., 2004 9e130 90e2600a Hassanin et al., 2004
1870 19
n/a
<40e19,220 <40e2160 14,600 760 500
30e1400 20e830 6100d 1043c 434c 161c
Yum et al., 2008 Yum et al., 2008 Eljarrat et al., 2008 Thorenz et al., 2010 Thorenz et al., 2010 Thorenz et al., 2010
1.1e15.9
0.8e18.0
289e15,594 24.7e296b
congeners 47, 99, 100, 153, 154. congeners 28, 47, 99, 100, 153, 154. 17, 28, 47, 66, 100, 99, 154, 153, and 183. 47,100, 99, 154, 153, 183. 28 congeners.
be due to several factors including the range of commercial mixtures used in treating products. Two penta-formulations are commercially available: DE-71, produced by the Great Lakes Chemicals and the European formulation Bromkal 70-5DE. Both formulations have similar congener mixtures except that DE-71 has 10% higher levels of BDE 99 than BDE-47 (La Guardia et al., 2006). Also, the composition of congeners in technical mixtures from the same manufacturer may not be constant between batches. For instance, the percent composition of the two major congeners, BDE-47 and BDE-99 in Bromkal 70-5DE reported by Sjödin et al. (1998) are 37% and 35% respectively while Rayne and Ikonomou (2002) report 51% and 34%, and La Guardia et al. (2006) report 38% and 48% for an identical mixture. The differences in physico-chemical properties between the congeners may be relevant in explaining the subtle differences
Fig. 4. Contribution of the commercial mixtures to the SPBDE concentration measured in soil samples collected along the transect across Kuwait.
observed in the soil profiles and the technical penta-mixtures. For example, the BDE-47:-99 ratio in the soil samples range from 0.63 in Kuwait City to 0.92 in background soils, slightly lower than the ratios in the penta-mixtures (1.05, Bromkal 70-5DE (30); 0.95 La Guardia et al. (29)) but similar to the ratio in DE-71 the pentaformulation manufactured in the United States (0.78, La Guardia et al., 2006). Similar 47:99 ratios have been previously reported in soil samples (e.g. Hassanin et al., 2004; Harrad and Hunter, 2006; Duan et al., 2010). It has been argued that a higher KOA for BDE 99 compared to BDE-47 results in a preferentially higher atmospheric deposition and better soil retention of BDE 99 resulting in a low 47:99 in soils relative to their ratio in the technical penta-mixtures (Hassanin et al., 2004; Harrad and Hunter, 2006; Duan et al., 2010). To further illustrate how differences in the physico-chemical properties of PBDE congeners would have resulted in the observed changes in congener composition in soil samples, we plotted the ratio (“R”) of the sum of BDE-47 þ BDE-99 þ BDE-100 to the sum of BDE-153 þ BDE-154 along the transect (Fig. 6). This ratio was initially proposed by Hites (2004) to examine changes in congener
Fig. 5. Congener mixture expressed as a percent of penta-BDE congeners.
B. Gevao et al. / Environmental Pollution 159 (2011) 3666e3672
composition in guillemot eggs collected between 1980 and 2000 from the Great Lakes catchment. It was speculated that the positive correlation between the ratio and year of sample collection may have been due to possible changes in the composition of the technical mixtures used in the USA. This ratio has subsequently been used to assess the relative contributions of the penta- and octa-PBDE technical mixtures in samples (Yum et al., 2008; Jiang et al., 2010). In this study, the ratio (“R”) in soil along the transect may be interpreted as representing the degree of weathering of the technical formulation following its release from their source regions. This ratio (“R”) in the technical penta-mixture lies between 10 (DE-71) and 12 (Bromkal 70-5DE). The ratio is expected to decrease with distance from source regions. Following their release to the atmosphere, the fate of different congeners will be dictated by their properties. In the atmosphere, semi-volatile organic compounds undergo temperature dependent gas-particle partitioning with the heavier congeners preferentially partitioning onto particles. They are consequently more likely to be preferentially scavenged and deposited from a given air mass than lower molecular weight congeners which may preferentially be in the gas phase. As a result of their association with particles, higher molecular weight congeners are more likely deposited closer to source region (urban centers) and also preferentially retained in soils relative to the lower molecular weight congeners. These processes of preferential deposition and/or retention in soil mediated by the physico-chemical properties will ultimately result in the fractionation of the congeners which will result in the alteration of this ratio from that in the penta-formulation. It will be expected that the magnitude of “R” in soil will decrease with distance from potential source regions as heavier congeners are preferentially retained in soil relative to the more volatile congeners. In this study, the ratio (“R”) decreased in the following order: Kuwait city (range, 9.5e11; mean, 10.4) > downwind of the urban source (range, 6.2e7.6; mean, 7.3) > ratio upwind of Kuwait City (range, 3.4e5.5, mean, 5.6). The lowest value of “R” in this study
Fig. 6. Ratio of the sum of the concentrations of BDE-47 þ -99 þ -100 divided by the sum of BDE-153 plus BDE-154 in soil samples collected along the transect across Kuwait.
3671
(3.4) was from the northernmost site upwind of Kuwait City which mostly receives its pollutant input from long-range transport from other regions. This is followed by a gradual increase in “R” to a maximum in Kuwait City (mean 10.4) and a gentle decrease in samples downwind of the urban source. The inflection point in the profile occurs in the city center with the maximum PBDE soil concentrations. The decrease in the magnitude of “R” in the soil samples collected downwind of urban source (Kuwait City) is gentler because the soil samples downwind will receive fresh inputs of congeners outgassing from the city. 3.2. Soil organic matter It is often suggested that soil organic matter plays a significant role in the distribution of POPs in soils/sediments because they are hydrophobic contaminants. Consequently, the concentrations derived from soil/sediment analyses are often normalized to the organic carbon (OC) content. In this study, the soil organic carbon content of soils ranged from 0.4 to 2.5% except in Kuwait City where the values were between 3 and 7%. The organic carbon content of the soils (especially soil samples collected outside Kuwait City) are generally lower than those reported in other parts of the world (Hassanin et al., 2004; Harrad and Hunter, 2006; Yum et al., 2008; Thorenz et al., 2010). The higher OC content of soils within the City may be due to mulching of soils for greenery operations. Congener specific OC normalized soil concentrations in this study are given in Table 2 and are comparable to those reported in urban and remote soils from Birmingham, UK (Harrad and Hunter, 2006) and Norwegian woodland soils (Hassanin et al., 2004) but lower than those reported for UK grassland and woodland soils (Hassanin et al., 2004). The %OC concentrations in soils tracked the S7PBDE (excluding 209) concentrations remarkably well (Fig. 7). When BDE-209 was included, however, differences in profiles were evident especially in samples collected within Kuwait City where the OC content was significantly higher than in the other samples. Congener specific soil concentrations were highly correlated with % OC (Pearson correlation coefficients, R2, ranging between 0.7 and 0.82, p > 0.05) when the data from Kuwait City was omitted from the analysis. However, when the entire dataset was included in the assessment, none of the congeners was significantly correlated with OC. These findings suggest that the soil concentrations outside the urban centers were close to equilibrium with the atmosphere. It
Fig. 7. Concentrations of S7PBDEs and percent organic carbon content in soils along a transect across Kuwait.
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