Environ. Sci. Technol. 2008, 42, 4831–4836
Modeling the Importance of Biota and Black Carbon As Vectors of Polybrominated Diphenyl Ethers (PBDEs) in the Baltic Sea Ecosystem TUOMAS J. MATTILA* AND MATTI VERTA Finnish Environment Institute, Mechelininkatu 34a, P.O. Box 140 FI-00251, Helsinki, Finland
Received January 28, 2008. Revised manuscript received April 6, 2008. Accepted April 9, 2008.
The POPCYCLING-Baltic model, a nonsteady state spatially resolved mass balance model of chemical transport in the Baltic Sea environment, was modified to include black carbon (BC), dissolved organic carbon (DOC) and food-web bioaccumulation. The importance of these modifications to the transport of PBDE congeners BDE-47, -99, -153, and -209 was assessed by comparing time-series simulated with and without black carbon and biota between 1970 and 2005. Inclusion of black carbon improved the model fit to measurements from air, soil, and biota, and had a major effect on the mass balance. Modeled bulk concentrations of PBDEs in sediments and soils increased by a factor of 3 while concentrations in biota decreased by a factor of 2-5. Black carbon also doubled the recovery time of the system due to the limited availability of PBDEs for degradation. In comparison, the inclusion of biota had only a minor effect on the overall mass balance and recovery times. The modified model is constructed as a flexible matrix and can also be applied to persistent organic pollutants in other ecosystems besides the Baltic Sea.
Introduction Polybrominated Diphenyl Ethers As Environmental Pollutants. Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardants in clothing, electronics, polyurethane insulation, and plastics (1). High environmental concentrations have been found on urban surfaces (2) and also in arctic biota (3). Similar rising concentration trends have been observed in sediments (4) and in fish (5). Environmental levels of PBDEs have grown rapidly since the 1960s when their use became widespread. They have been identified as environmental pollutants because of their persistence, long-range transport potential (6), and large production volumes (1). Although PBDE usage is urban, observed concentrations show that its distribution is global. The production and use of pentabrominated BDEs was banned in the European Union in 2002 and they have been selected as one of the priority pollutants of the Water Framework Directive (Commission proposal COM(2006)397 final). The major emission routes for PBDEs are evaporation from products (7), waste incineration (3), and leaching from landfills (8). The complex environmental behavior of persistent organic pollutants such as the PBDEs can be studied * Corresponding author e-mail:
[email protected]. 10.1021/es800278m CCC: $40.75
Published on Web 05/29/2008
2008 American Chemical Society
quantitatively by using environmental fate models (9). Few such studies have been conducted in the European region: Prevedeuros et al. (10) tested their emission scenarios with the EVnBETR model and Palm et al. (11) used the POPCYCLING Baltic-fugacity model (12) to predict future concentration trends following the partial ban of PBDE usage. Both studies found that although temporal trends were well explained by the model, absolute concentrations in sediments, soil, and biota were underestimated. This study aims to find out if the POPCYCLING Baltic model could be improved with two state-of-the-art additions: aquatic biota and adsorption to black carbon. Black Carbon and Biota As Transporters of PBDES. Black carbon (e.g., soot and char) has a large surface to volume ratio and a strong affinity for nonpolar substances (13) such as polycyclic aromatic hydrocarbons (PAHs), dioxins, and furans (PCDD/Fs), PCBs, and PBDEs (14). In some cases, sorption to black carbon may be the dominant process determining the environmental fate of chemicals. For example, in industrialized regions, up to 80-100% of PAHs can be bound to black carbon. Persistent organic pollutants are bound to black carbon by two complementary mechanisms: occlusion inside pores during soot formation and surface adsorption during and after soot formation (13). Based on sorption experiments (14), the surface adsorption of PBDEs to soot is approximately 1-3 orders of magnitude stronger than to amorphous organic carbon. In a previous modeling study (15), the inclusion of black carbon in the mass balance made PAHs attach more strongly to solid matrices, more persistent, and reduced their availability for biological uptake. Aquatic biota is regularly monitored and can be a significant exposure route in human health risk assessments. In aquatic ecotoxicology there is a long tradition of bioaccumulation studies, and several mechanistic bioaccumulation models have been developed (c.f., overview in ref 16). While the biota is usually treated as a recipient of pollutants, it can also be an important vector between geographical regions, and aquatic and terrestrial ecosystems (17). In the Baltic Sea, it has been estimated that commercial fishing may remove more PCBs from the Baltic Sea than chemical degradation (18). In addition to transportation, biota (blue mussels, Mytilus edulis) have been reported to increase the gross sedimentation rates of PCBs on the Swedish coast by 17-50% (19). Although biota has relatively low volumes, the speed of biological transport processes justifies the inclusion of biological compartments to multimedia transport models (17).
Materials and Methods Extension of the Previous Conceptual Model. The original POPCYCLING Baltic model describes the mass balances of persistent organic pollutants in the Baltic Sea catchment area. In the model, the region is subdivided into 4 atmospheric, 15 water (including sediment and water subsystems), and 10 terrestrial (including forest canopy, soil, fresh water, and sediment subsystems) compartments (12). We translated the original model into the MATLAB modeling environment and restructured the model equations into a linear algebra form. As a result, the transport processes between compartments were described as a single differential equation, which allowed further modification of the amount of environmental compartments and the connectivity between them (eq 1): dM(t) d[V(t)Z(t)f(t)] ) ) D(t) · f(t) + E(t) dt dt VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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where M ) vector of chemical mass in environmental compartments (mol), V ) vector of compartment volumes (m3), Z ) vector of fugacity capacities of chemical in compartments (mol m-3Pa1-), f ) vector of fugacities of chemical in compartments (Pa), D ) matrix of time dependent transfer coefficients (mol h-1 Pa1-), E ) vector of emissions (mol/h), and t ) time (h). The environmental parameters required to calculate the volumes, fugacity capacities, and transport coefficients were obtained from the documentation of POPCYCLING Baltic (12). After model translation, some of the model parameters were updated: subzero sediment and water temperatures used in ice cover calculations were partially replaced with empirical measurements (20) and air-soil interactions were described using the method presented in ref 21. In addition absorption to dissolved organic carbon (DOC) was also included (details given in the following chapter). These changes decreased the annual fluctuation in fugacity capacities (Supporting Information, Figure S2) but did not affect the mass balances significantly. The model translation process was validated by comparing model output with that of the original model (Supporting Information, Figure S1). Details of the transport matrix are given in Supporting Information Figure S4. Including Sorption to Soot and Dissolved Organic Carbon. A Langmuir isotherm can be used to model the adsorption of PBDEs to black carbon (22), but unfortunately the nonlinear isotherm cannot be applied to the linear transport equation (eq 1). In order to apply the isotherm, we had to linearize it by assuming that the pollutant concentrations were small enough (50% of the total net sedimentation rate (sedimentation less resuspension), when black carbon was not taken into account. With black carbon, the biodeposition only accounted for
