Comparing black carbon types in sequestering polybrominated diphenyl ethers (PBDEs) in sediments
Introduction
Polybrominated diphenyl ethers (PBDEs) are a group of hydrophobic brominated flame retardants used widely in a variety of consumer products ranging from automobile accessories, plastics, textiles, furnishing foams, to electronic appliances to prevent the development of fire (Eguchi et al., 2011, Rahman et al., 2001). There have been primarily three commercial formulations of PBDEs in use, i.e., penta-, octa-, and deca-BDE, among which penta-BDE has attracted more attention because of its dominant global consumption and relatively higher ecological toxicities as compared to the more highly brominated PBDEs (La Guardia et al., 2006). Many PBDE congeners resulting from the penta-BDE formulations have been frequently found in both environmental compartments (Hale et al., 2003, Nylund et al., 1992) and humans (Meironyte et al., 1999, Toms et al., 2009).
Congeners contained in the penta-BDE products are highly hydrophobic with log Kow > 6 for most congeners, and thus they are mainly associated with the bed sediment in the environment (Lacorte et al., 2003, Mai et al., 2005). As demonstrated previously for many other hydrophobic organic contaminants (HOCs, e.g., PCBs, PAHs), the bulk sediment concentration is often a poor indicator of the potential bioavailability of PBDEs to benthic organisms due to their strong interactions with the organic matter phase (Cui et al., 2013, Jia et al., 2012, Liu et al., 2011). Rather, the freely dissolved concentration (Cfree), which may be measured using passive samplers such as solid phase microextraction (SPME), has been shown to correlate closely with the bioavailability of HOCs (Oleszczuk et al., 2012). On the other hand, a large number of studies have shown that black carbon (BC) may enhance the sorption or sequestration of HOCs in sediment (Accardi-Dey and Gschwend, 2002, Jeong et al., 2008, Jonker and Koelmans, 2002, Millward et al., 2005, Pignatello et al., 2006, Yang et al., 2009a, Yang et al., 2009b). BC has been used as a remediation option at contaminated sites to sequester HOCs in the bed sediment and reduce their flux to the water column (Beckingham and Ghosh, 2011, Chai et al., 2012, Oen et al., 2012, Werner et al., 2010). However, BC materials vary greatly in their origin and physicochemical properties (e.g., surface area, microporosity) (Chai et al., 2012, Choi et al., 2013, Rakowska et al., 2012), and little is known about their differences in sequestering sediment PBDEs (Barring et al., 2002).
In this study, we developed a matrix-SPME method by using disposable polydimethylsiloxane (PDMS) fibers to measure Cfree of common penta-BDE congeners in sediments, and then evaluated three different types of BC, i.e., biochar, charcoal, and activated carbon, in their effects on Cfree in sediments. Findings from this study may be used for optimizing the selection of BC materials in mitigating PBDE contamination in sediments.
Section snippets
Chemicals and PDMS fiber
Standards of 2,2′,4,4′-tetrabromodiphenyl ether (BDE 47), 2,2′,3,4,4′-pentabromodiphenyl ether (BDE 85), 2,2′,4,4′,5-pentabromodiphenyl ether (BDE 99), 2,2′,4,4′,6-pentabromodiphenyl ether (BDE 100), 2,2′,4,4′,5,5′-hexabromodiphenyl ether (BDE 153), and 2,2′,4,4′,5,6′-hexabromodiphenyl ether (BDE 154) in isooctane (50 μg/mL) were purchased from AccuStandard (New Haven, CT). External surrogate decachlorobiphenyl (PCB 209) was purchased from Fisher Scientific (Pittsburgh, PA). All other solvents
PDMS–water partition coefficient (KPDMS)
The KPDMS was measured under controlled conditions by dividing CPDMS over Cfree at equilibrium. The average log KPDMS values for BDE 47, 85, 99, 100, 153, and 154 were 5.89, 6.24, 6.68, 6.45, 6.80, and 6.94, respectively. The large KPDMS values suggested a strong affinity of PBDEs for the PDMS polymer (Table 2). The partition of PBDE congeners to PDMS polymer followed the same trend as their hydrophobicity or Kow (Jia et al., 2012). The log Kow values were found to generally follow a linear
Conclusions
In this study, a matrix-SPME method was developed for measuring Cfree of PBDEs in sediments and the method was further used to compare the efficiency of different BC materials in sequestering PBDEs by decreasing Cfree. Findings from this study showed that although Cfree of PBDEs was reduced by BC amendment, the reduction varied greatly among different types of BC, with activated carbon showing substantially much greater efficiency than biochar or charcoal. In addition, likely due to the
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