| Summary: | Bottom sediments in waters near industrial areas and cities are frequently contaminated
with polycyclic aromatic hydrocarbons (PAH). This observation is of particular concern
because some PAH are proven carcinogens and mutagens, and cancerous lesions in bottom fish
have been correlated with high PAH concentrations in sediment. In the Vancouver, B.C. area, a
number of severely impacted sites have been identified.
This research was carried out to investigate the feasibility of using nitrate to stimulate
the biodegradation of PAH in anoxic marine sediments. It is based on the assumption that the
factor that most often limits microbial activity in sediments that receive a high organic load is
the lack of available electron acceptors. Oxygen, usually the most efficient electron acceptor, is
only sparingly soluble in water and may not be provided to the sediment at a sufficient rate to
meet the demand. Nitrate can be used as an alternative electron acceptor by denitrifying
microorganisms, is very water soluble, and yields relatively innocuous gaseous products which
can leave the system upon reduction.
In laboratory experiments, nitrate addition stimulated the loss of some added PAH in
False Creek sediment incubated under anoxic conditions for up to 20 weeks. The smaller, less
hydrophobic compounds were degraded more quickly than the higher molecular weight
compounds, and benzo(a)pyrene, a 5-ring, highly carcinogenic PAH, was not degraded under
the test conditions. The half lives of the low molecular weight compounds ranged from
approximately 4 to 13 weeks. When they were degraded, the half lives of the high molecular
weight compounds were from 16 to 70 weeks. PAH degradation was inhibited when nitrate
was provided as calcium nitrate, probably as a result of calcium phosphate precipitation leading
to phosphate limitation.
Since PAH degradation seemed to be related to the solubility and probable
bioavailability of the PAH, two degradation experiments were carried out which included a
measure of the "available" fraction using False Creek sediment and one with sediment taken
near the ESSO refinery. In this availability measure, called reverse semipermeable membrane
device (rSPMD) extraction, the sample is sealed in a polyethylene tube and dialyzed against a
solvent. The contaminant must desorb from the solid phase and diffuse through the tubing to
reach the solvent, thus only the available fraction is measured. Degradation of the more readily
available added PAH occurred more rapidly than that of endogenous material. The PAH
degradation rates were higher in the ESSO sediment than in the False creek sediment. The high
molecular weight compounds were also more readily available as measured by rSPMD from the
ESSO sediment.
The likelihood of successful bioremediation at these two sites was examined. In situ
nitrate addition is unlikely to be effective at the False Creek site because degradation of the
larger compounds is too slow. Furthermore, the metal concentrations at the site could exert
toxic effects even after harmful organics are removed. PAH biodegradation at the ESSO site
would likely be accomplished more quickly and with a lower nitrate dose, however
benzo(a)pyrene could also persist there. Bioremediation in a slurry phase reactor under
denitrifying conditions might be more successful since the process can be optimized to favour
desorption of the pollutants.
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