Development of a fate and transport model for biodegradation of PBDE congeners in sediments☆
Graphical abstract
Introduction
Polybrominated diphenyl ethers (PBDEs) are a group of flame retardants used in building materials, electronics, furniture, motor vehicles, airplanes, plastics, polyurethane foams, and textiles (Sutton et al., 2014). They are released into the environment during their manufacture, incineration of municipal waste, deposition to landfills, discharge to municipal wastewater treatment plants, or emitted directly to atmosphere as particulate matter (ATSDR, 2004). PBDEs cause adverse effects on humans and animals due to their toxicity, persistence and bioaccumulative properties (USEPA, 2010). PBDEs were produced as commercial mixtures: PentaBDE, OctaBDE and DecaBDE (Sutton et al., 2014), where the former two were phased out of production in US in 2004, while DecaBDE (primarily 209) was phased out in 2013. Likewise, these commercial mixtures were evaluated and their use was banned at different times by the European Union and the Stockholm Convention. These source control efforts only address new production and use, and do not apply to all countries; imported articles may still contain DecaBDE in particular. Furthermore, certain provisions are present for continued use of DecaBDEs in aviation industry, in electrical wiring and cables, etc. (POPs, 2008). Consequently, for proper management of sites contaminated with PBDEs, development of tools such as fate and transport (F&T) models are necessary.
Physicochemical properties of congeners belonging to a family of hydrophobic organic pollutants differ from one another, affecting their environmental fate. In the literature, models that simulate F&T of a family of hydrophobic organic pollutants (e.g. PCBs, PBDEs) typically focus on total concentrations, either as a single variable (i.e., total congener concentration) (Connolly et al., 2000; Greene et al., 2013; LimnoTech, 2007; Shen et al., 2012) or the sum of individual congeners (i.e., multiple variables) (Davis, 2004; Zhang et al., 2009, 2008). So far, fewer studies exist on F&T modeling of PBDEs (Oram et al., 2008b; Rowe, 2009) when compared to those for PCBs (Connolly et al., 2000; Davis, 2004; Greene et al., 2013; LimnoTech, 2007; Shen et al., 2012; Zhang et al., 2009, 2008). There is only one F&T model in the literature for PCBs and PBDEs (Davis, 2004; Oram et al., 2008b) which considers photolytic, biotic and chemical degradation as loss terms for individual congeners. However, that model does not consider products of biodegradation (i.e., dehalogenation of one congener into another). Field (Bedard and May, 1996; Lombard et al., 2014; Sutton et al., 2015; Venier et al., 2014) and laboratory (Bedard, 2003; Ding et al., 2013; Huang et al., 2014; Robrock et al., 2008; Sowers and May, 2013; Tokarz et al., 2008) studies indicate that biodegradation of hydrophobic organics (i.e., PCBs and PBDEs) can substantially create changes in congener patterns, via conversion of one congener into another, which has been ignored in F&T modeling. Additionally, congeners do not possess the same toxicity (Van den Berg et al., 2006) on organisms or not all congeners are bioaccumulative (Oram et al., 2008b). Therefore, incorporation of congener specific degradation pathways, as well as physicochemical processes that affect congener-specific concentrations, should be considered in F&T modeling. To the best of our knowledge, there is no model in the literature that integrates biodegradation products of PBDEs into simulation of F&T processes.
In this study, a new model, Fate and Transport of Hydrophobic Pollutants – FTHP, was developed to incorporate biodegradation pathways of individual congeners, i.e. anaerobic debromination between congeners, as well as F&T mechanisms in sediments. In the model, biodegradation rate constants for congener specific degradation pathways are estimated by the Anaerobic Dehalogenation Model (ADM) (Karakas and Imamoglu, 2017) or assigned using information from the literature. FTHP was tested for a location representing the Lower South Bay sediments of San Francisco (SF) Bay, which is known to be contaminated by PBDEs (SFEI, 2020). Following calibration and validation of FTHP with almost annually collected 16 years of SF data, potential outcomes of various remedial strategies (i.e. natural attenuation, dredging, biostimulation) under the assumptions of constant and time-dependent water column concentrations were evaluated for a management period of 20 years.
Section snippets
FTHP model
The model is developed to simulate F&T of hydrophobic pollutants in sediments and applied here for PBDEs. Framework and some assumptions of the model are based on the RECOVERY Model (Ruiz et al., 2001). System boundaries are set as the water-sediment interface and deeper sediment (Fig. 1). Similar to Boyer et al. (1994), it is assumed that water and sediment layers are each well-mixed and concentrations in sediment vary with depth where linear equilibrium sorption and first order kinetics
Calibration and validation of FTHP model
For the system studied, an initial sensitivity analysis was performed to identify the parameters FTHP was sensitive to. Accordingly, initial values of nine input parameters (Cs, Kow, vb, vs, focw, focs, Dm, km,i and TSS) were taken from Table C.1 (median of TSS and average of sediment porosity values in Table C.1 as initial values). Relative sensitivities of congener concentrations to selected nine model input parameters were assessed by changing a given input parameter value at arbitrarily
Conclusions
Development of models that integrate biodegradation of individual congeners and their products will enable a focus on and control of congeners with higher toxicity. This may help reduce risks to aquatic organisms and humans and apply better management with the help of a congener-specific concentration prediction tool. In this sense, FTHP is the first attempt (to the best of our knowledge) in inclusion of biodegradation of individual congeners and their products during simulation of F&T of
CRediT authorship contribution statement
Filiz Karakas: Conceptualization, Methodology, Software, Formal analysis, Writing - original draft, Writing - review & editing. Aysegul Aksoy: Conceptualization, Writing - review & editing. Ipek Imamoglu: Supervision, Conceptualization, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Filiz Karakas was financially partly supported by the Scientific and Technological Research Council of Turkey (TUBITAK Project No: 115Y122).
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This paper has been recommended for acceptance by Eddy Y. Zeng.