Accumulation and Transformation of Halogenated Biphenyl and Diphenyl Ethers in Urban Sediments

Polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) are common urban contaminants. Very often, PBDE and PCB distributions in sediment cores differ from those in industrial formulations-this is thought to be an artifact of various pre- and post-depositional transport processes. 49 PBDE and 132 PCB congeners in five urban Chicago and six Arkansas (AR) sediment cores were characterized to understand the extent of contamination and to investigate the two main types of post-depositional transport processes: microbial facilitated in-situ transformation and porewater transport through diffusion and advection. Chicago sediments had lower PBDE and much higher PCB levels compared to AR sediments near PBDE manufacturing facilities. The microbial community structure in Chicago and AR surface sediments were characterized using 454-pyrosequencing. Although putative PBDE and PCB degraders were observed in all surface sediments, no correlations were observed between relative abundance of putative degraders and PBDE and PCB concentrations. Dehalococcoides spp. capable of a wide range of PBDE and PCB degradation, were found in small abundance only in three surface sediments. A striking feature was the high abundance of unidentified microorganisms in sediments contaminated with high levels of PBDEs and PCBs. Evidence for microbial dechlorination was observed in only one of the 11 cores. The presence of putative PCB degrading microorganisms coincided with other dehalogenation indicators such as increased in low molecular weight PCB congeners concomitant with decreased in high molecular weight PCB congeners, decreased degree of dechlorination, and decreased abundance of meta-substituted PCB congeners. Taken collectively, these lines of evidence suggest PCB degradation. PBDE and PCB transport mobility through diffusion and advection-diffusion processes were stochastically modeled using a Monte Carlo approach for known transport parameters and measured site characteristics. Four different transport scenarios were considered: diffusion with organic carbon only sorption, diffusion with organic carbon and black carbon co-sorption, diffusion-advection with organic carbon sorption, and diffusion-advection with organic carbon and black carbon co-sorption. The highest transport mobility for both PBDEs and PCBs were achieved under diffusion-advection with organic carbon sorption. Transport mobility decreased for larger PBDE and PCB homologs. Under a forty year maximal transport scenario, predicted mass losses for PBDE and PCB homologs were very small or negligible. Thus we conclude that post-depositional transport and microbial dehalogenation cannot account for large differences between PBDE and PCB distribution patterns in technical products and those observed in the sediment, and thus must occur prior to deposition.