Environmental Engineering Reference
In-Depth Information
congeners. Processes M and Q, for example, are less effective in dechlorinat-
ing Aroclor 1260 than is process N. Thus, the extent of PCB dechlorination
that occurs is dependent on which microorganisms are present and active,
and more extensive dechlorination can be achieved by combining dechlori-
nation processes with complementary activities. This has occurred naturally
in the Upper Hudson River, where PCB dechlorination was first recognized.
In several locations the combined activities of at least the first four processes
listed in Table 6.1 have removed most of the meta- and para-chlorines from
Aroclor 1242, with the result that about 90% of the remaining PCBs are mono-
and dichlorinated congeners substituted only in the ortho-positions. These
congeners constitute less than 8% of Aroclor 1242.
In laboratory experiments, PCB dechlorination by complementary pro-
cesses was achieved using two different approaches. First, the addition of
FeSO 4 fosters simultaneous dechlorination by processes M and Q (Zwiernik
et al., 1998), and second, enhanced dechlorination of Aroclor 1260 was real-
ized using sequential inoculations of process N and process Q microorgan-
isms (Quensen et al., 1990b).
6.3.2 FeSO 4 amendment
Prior experience indicates that meta-dechlorination is more readily achieved
than para-dechlorination and that process Q dechlorination is not reliably
achievable. While investigating the use of ferrous sulfate as a way of pre-
cipitating heavy metals, which can inhibit dechlorination, a means to rescu-
ing process Q activity was discovered. Aroclor 1242 dechlorination was more
extensive in microcosms to which 10 m M FeSO 4 was added, but dechlori-
nation occurred only subsequent to FeSO 4 depletion. Our thinking is that at
least some of the dechlorinating microorganisms are sulfate reducers (i.e.,
they dechlorinate PCBs in the absence of sulfate) and that the addition of a
small amount of sulfate allowed them to increase in numbers. We also believe
that the iron precipitated the sulfide formed and that the para-dechlorinating
microorganisms are sensitive to this sulfide. Evidence for this last point
comes from the fact that precipitation of sulfides with FeCl 2 or PbCl 2 also
enhanced dechlorination but to a lesser extent (Zwiernik et al., 1998). Adding
FeSO 4 to the PCB-dechlorinating microcosms altered the dechlorination pat-
tern achieved. Without the addition of FeSO 4 , pattern M resulting from
meta-dechlorination alone was achieved. With ferrous sulfate, pattern C
resulting from the combined meta- and para-dechlorinating activities of
processes M and Q was achieved. Thus, the addition of FeSO 4 is one way of
achieving enhanced PCB dechlorination through the combined activities of
complementary dechlorination processes.
6.3.3 Sequential inoculations
A second way of obtaining a combination of complementary dechlorination
activities is to use inocula from different sources. The results from such an
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