Environ. Sci. Technol. 2005, 39, 9517-9522
Microcosm Experiments to Assess the Effects of Temperature and Microbial Activity on Polychlorinated Biphenyl Transport in Anaerobic Sediment KATHLEEN M. MCDONOUGH AND DAVID A. DZOMBAK* Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
Increased polychlorinated biphenyl (PCB) loadings from sediment to the water column under low-flow conditions during late spring and summer months have been observed in the Grasse River (Massena, NY) and other PCBcontaminated rivers in the United States. Temperature appears to be an important factor affecting this phenomenon, as increased sediment temperature leads to increased microbial and bioturbator activity as well as increased rates of diffusion and desorption. Laboratory-scale sediment microcosms were developed and used to study with finescale resolution the effects of temperature and microbial activity on PCB transport in river sediment. Over the time course of these experiments, with the extraction procedures used, irreversible sorption of each congener to sediment was observed and increased with aging of the sediment. Temperature-dependent transport was observed for PCB congeners 2,4,5-trichlorobiphenyl (BZ29), 2,5dichlorobiphenyl (BZ9), and 2-chlorobiphenyl (BZ1) in Grasse River sediment and a synthetic sediment system. The fastest transport of the congeners occurred in biologically active Grasse River sediment followed by biologically inactive (autoclaved) Grasse River sediment, and synthetic sediment. The increased transport in biologically active sediment demonstrated the importance of microbial activity, in particular gas bubble generation, in PCB transport in nearsurface sediments.
Introduction Increased polychlorinated biphenyl (PCB) loadings from sediment to the water column under low-flow conditions during late spring to summer months have been observed in many PCB-contaminated rivers in the United States (15), including the Grasse River in Massena, NY (6). Sediment temperature appears to have an important role in governing the increased sediment-to-water column loading of PCBs (7). The increased sediment temperature during late spring and summer months can lead to increased rates of physicochemical processes such as desorption, diffusion, and solubilization and increased rates of biological processes. The transport in and release of PCB compounds from sediment to the water column during low-flow conditions in a river system is facilitated through physicochemical pro* Corresponding author phone: (412)268-2946; fax: (412)268-7813; e-mail:
[email protected]. 10.1021/es051249p CCC: $30.25 Published on Web 11/10/2005
2005 American Chemical Society
cesses, microorganism activity, and bioturbation. PCBs desorb from sediment and diffuse through pore water to the sediment-water interface. Under the anoxic conditions prevailing in most sediments, adapted indigenous microorganisms can transform PCBs from highly chlorinated compounds to less chlorinated and therefore more mobile congeners (8-12). Microorganism activity can also generate gas bubbles. Movement of the gas through sediment can aid in the release of hydrophobic organic compounds by carrying particles attached to the gas bubbles out of the sediment (13) and by making the sediment more porous through the creation of preferential pathways within the sediment (14). PCBs can also partition into the vapor phase of the bubble and be transported out of sediment (13). Bioturbation can facilitate PCB transport from the sediment to the water column by mixing sediment, resulting in more contaminated sediment reaching the sediment-water interface (15-17) and spraying particles from the sediment into the sedimentwater boundary layer (18), resulting in greater contact of over-flowing water with PCB-contaminated sediments. The increased sediment temperature during late spring and summer months leads to increased microbial and bioturbation activity (19-21) and to increased rates of physicochemical PCB transport and release processes such as desorption and diffusion. This research aimed to elucidate the effect of temperature and temperature-governed microbial activity on PCB transport in anaerobic near-surface sediments. The effects of temperature on nonadvective PCB transport in a clean, wellcharacterized, synthetic sediment consisting of sand and kaolin, and also in biologically active and inactive Grasse River sediments were studied. The results obtained with synthetic and biologically inactive (autoclaved) sediment microcosms were used to isolate physicochemical effects of temperature on apparent PCB diffusion. PCB transport in biologically inactive Grasse River and synthetic sediment was compared to that in biologically active Grasse River sediment to elucidate the effects of microbial activity on PCB fate and transport at various temperatures.
Materials and Methods Microcosm Setup. An experimental system was designed to study PCB fate and transport in anaerobic near-surface sediment with fine spatial resolution. The design, setup, analysis procedures, and verification of the experimental system are discussed in detail elsewhere (22). Aluminum cylindrical containers (Elemental Container Inc., Union, NJ) 127 mm tall and 27 mm in diameter were used as the microcosm containers. These containers were filled with a spiked sediment layer as the source of PCB contamination to the system, a nonspiked layer of the same sediment used to observe PCB movement away from the spiked layer, water, and a 1 in. × 1 in. semipermeable membrane device (SPMD) (Environmental Sampling Technologies, Inc., St. Joseph, MO) to capture dissolved-phase PCBs (23), and capped (Figure 1). Throughout the 48-week duration of the experiments PCBs did not break through the sediment layer, but SPMDs were included to detect any possible congener break through. Glass microcosms with the same contents as the Grasse River and synthetic sediment microcosms were set up in 100- mL glass serum bottles with rubber septa and aluminum seals, to observe gas generation and sample headspace gas (7). These glass microcosms consisted of duplicates of each sediment system (active, killed, and synthetic) held at each specified temperature. Headspace gases monitored throughout the experimental period included nitrogen, oxygen, VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Molecular Weight, Solubility, and Octanol-Water Partition Coefficient (kow) for PCB Congeners BZ 1, 9, and 29a
BZ 1 BZ 9 BZ 29
FIGURE 1. Schematic diagram of the experimental microcosms. The dotted block in the water column section indicates location of the SPMD. Drawing is not to scale.
TABLE 1. Grasse River and Synthetic Sediment Properties Grasse River sediment synthetic sediment total organic carbon moisture content porosity specific gravity bulk density
3.7%a,c 75%d 62%e 2.60f 0.81 g/cm3 e
0.5%b,c 40%d 49%e 2.65f 1.2 g/cm3 e
a Modified EPA Method 9060 (25), analysis performed by American Interplex Laboratories, Little Rock, AR. b Modified ASTM Method D3178 (26), analysis performed by Microbac Laboratories, Inc., Warrendale, PA. c Different laboratories were used for total organic carbon analysis but both laboratories used fundamentally similar methods (sediment was combusted and CO2 was collected and quantified) and followed proper quality control procedures. d ASTM Method D2216 (27). e The porosity and bulk density were measured by taking a known volume of a saturated sediment sample and weighing it, drying the sediment for 48 h (until there was no change in weight), and weighing the dry sediment sample. f ASTM Method D854 (28).
carbon dioxide, and methane. Information on the analysis of headspace gases can be found in the Supporting Information. Three types of sediment were used in the microcosms: low-PCB concentration (
