Environmental Engineering Reference
In-Depth Information
be repeated a number of times. Although relatively effective, this process is not as
efficient at metal removal as a bio-precipitation process.
Sulfur reducing bacterial systems include the use of Thipaq at the Budelco
zinc refinery in the Netherlands, the Metex anaerobic sludge reactor in Linde,
Germany, and the Bio-Substrat anaerobic micro-carrier reactor, also in Germany. All
of these processes share the requirement for anaerobic conditions needed by sulfate
reducing bacteria to produce insoluble metal sulfides [97, 111, 112]. This bio-
transformation into precipitates detoxifies the metals by making them biologically
unavailable while at the same instance removing them from solution onto bacterial
support matrices. These systems all require anoxic environments thus complicating
bioreactor design.
Two bio-techniques have been implemented in the field for soils and wetland sed-
iment bioremediation. In one system, a combination of leaching sediments with acid
followed by bacterial conversion has been used by Nakamura and colleagues [113].
This was achieved by employing an indigenous Minamata Bay bacterial strain,
Pseudoalteromonas haloplactis
[114]. In the other system, Daly and his group
combined
mer
activity with thiol-containing metallothionein production in bacte-
ria to form Hg(II) that binds to metallothionein proteins in subsurface treatments
[115, 116]. These techniques have the disadvantages of either releasing substan-
tial amounts of Hg(0) into the atmosphere or of not being developed for effluent
treatment purposes.
3.8.3 Potential Aerobic Metal-Sulfide Bioremediation
Bioremediation processes using aerobic precipitation of metals as sulfides may
not have the limitations of the systems discussed above. It should be pointed out
that metal sulfide biotransformation is quite a distinct process from that of metal
ion-exchange mechanisms documented in some algae [25, 117] and cyanobacteria
[118-120]. Potential algal and cyanobacterial bioremediation systems using metal
sulfide production do not require anoxic environments, thereby greatly facilitating
bioreactor design. In addition, the phototrophic capabilities of these organisms pro-
vide the advantage of using light as their energy source and they possess enhanced
aerobic metabolisms over anaerobic bacteria.
Mercury sulfide synthesis occurs in cyanobacteria such as
Limnothrix plancton-
ica
[24], and is particularly efficient in the red alga
Galdieria sulphuraria
[75, 95].
It is not known if other photosynthetic organisms that effectively biosorb metal
ions such as the cyanobacterium spirulina [121, 122] also form metal sulfides.
Galdieria sulphuraria
can rapidly biotransform over 90% of Hg(II) into
β
-HgS
(K
1/2
≈
20 min). This species' extraordinary metabolic adaptability makes it a key
prospective organism to develop for bioremediation purposes as it can adjust to
extreme conditions and it is very tolerant to toxic metal exposure [123]. This may
have particular importance because many contaminated sites contain more than one
metal source and thus the ability for the bioremediating species to be tolerant to a
