Environmental Engineering Reference
In-Depth Information
Biosorption
Bioaccumulation
M
2+
M
2+
M
2+
2L
2+
(out)
M
2L
M
2
(in)
2L
Microbially enhanced
chemisorption of metals
e.g. Hydrogen uranyl phoshate
MICROBIAL
CELL
HPO
4
2
+ M
2+
MHPO
4
e
-
MO
2
2+
Metal
(oxidised soluble)
CO
3
2-
+ M
2+
MCO
3
MO
2
Metal
(reduced insoluble)
H
2
S
+ M
2+
MS
Biotransformation
Biomineralisation
e.g. Bioreduction
Figure 11.1.
Mechanisms of metal/radionuclide-microbe interactions.
studies have investigated sorption of U(VI) (Francis et al.
2004
; Merroun et al.
2005
), with far less known about the sorption of other radionuclides, such as
Pu (Ohnuki et al.
2007
). For a more detailed overview of this research area, the
reader is referred to Lloyd and Macaskie (
2000
).
Bioaccumulation, which can be defined as energy-dependent metal uptake,
has been demonstrated for most physiologically important metal ions and some
radionuclides which can enter the cell as chemical 'surrogates' using these
transport systems. There have been just a few investigations of the intracellular
accumulation of actinides, and almost all of these have concentrated on
uranium (Francis et al.
2004
, Suzuki & Banfield
2004
). It has been suggested that
intracellular uptake of uranium is metabolism-independent, and results from
increased cell-membrane permeability caused, for example, by uranium toxicity
(Suzuki & Banfield
1999
). In other studies, bioaccumulation has been proposed
as a mechanism promoting uranium tolerance in Arthrobacter spp., although
the final fate of the uranium taken up into the cell remains to be confirmed
(Suzuki & Banfield
2004
; Martinez et al.
2006
; Geissler
2007
). In contrast, the
intracellular uptake of Pu(IV) in Microbacterium flavescens was reported to be an
active, metabolism-dependent transport process ( John et al.
2001
).
Micro-organisms can also catalyse the direct transformation of toxic metals
and metalloids to less soluble or more volatile forms via two distinct
enzymatic mechanisms. Bioreduction can result in precipitation of some
