Environmental Engineering Reference
In-Depth Information
levels. Cornelissen et al. (2001) developed a simple method to measure the
rapidly desorbed recalcitrant compounds, whereby Tenax beads were slur-
ried with sediment, extracted with organic solvent, and analyzed by gas
chromatography. They have shown that this functionally defined rapidly
desorbed fraction was useful for predicting the extent to which microorgan-
isms could remediate sediment (Cornelissen et al., 1998). Mayer et al. (2000)
used microextraction fibers (200-_m polydimethylsiloxane) to measure the
freely dissolved recalcitrant compounds' level in sediment pore water, and
Kraaij (2001) used this analytical method to correlate the freely dissolved
fraction of a number of recalcitrant compounds with bioconcentration factors
for tubificidae. Steady-state accumulation of recalcitrant compounds in
benthic deposit feeders was fully reconciled with equilibrium partitioning
of rapidly desorbing compounds between sediment, pore water, and deposit
feeders. MacRae and Hall (1998) compared results obtained using polyeth-
ylene tube dialysis (PTD) to those obtained using Tenax and a semipermeable
membrane device. In general, the results were similar, but the PTD method
was able to liberate more PAH from the sediment. Johnson and Weber (2001)
used superheated (subcritical) water to measure the slowing desorbing frac-
tion of recalcitrant compounds from geosorbents and used the information
to predict long-term rates of recalcitrant compounds' desorption from soils
and sediments.
2.8 Microbial degradation recalcitrant compounds in
sediment
One implicit assumption in making TBP and EqP predictions is that all of
the biologically available sedimentary recalcitrant compounds' fraction bio-
accumulates and can cause toxicity. That is, recalcitrant compounds are not
degraded by microorganisms, or the portions of the biologically available
fraction of bound recalcitrant compounds that are microbially degraded are
constant from site to site. However, microorganisms are efficient and effec-
tive at recycling chemical elements. They are simple life forms that have
minimal metabolic maintenance energy requirements. This enables them to
thrive on substrates that do not yield much energy (e.g., recalcitrant com-
pounds) and are only available at very low concentrations. Tang et al. (1998)
have shown that bioremediation can reduce the levels of pyrene taken up
by earthworms by a factor of 10. The rapidly desorbing fraction of sedi-
ment-associated PAH is preferentially degraded by microorganisms (Corne-
lissen et al., 1998), and the rates of biodegradation of the slowly desorbing
PAH fraction are limited by the desorption rate (Carmichael et al., 1997). If,
as Kraaij et al. (2002) and others suggest (see above), the bioavailable fraction
of recalcitrant compounds in sediment is generally equivalent to the rapidly
desorbing fraction, then part of the bioavailable (i.e., rapidly desorbing)
fraction will bioaccumulate in benthic biota and part will be degraded by
microorganisms (Figure 2.4). Pore water recalcitrant compound levels will
