Environmental Engineering Reference
In-Depth Information
metals/metalloids, where the reduced form is less soluble than the high
oxidation form, as is the case for enzymatic U(VI) reduction to U(IV) (Lovley
1993
), while biomethylation can yield highly volatile derivatives. Both mech-
anisms can result in a decrease in the concentration of soluble metals in
contaminated water. While the microbial enzymatic reduction of radio-
nuclides including U(VI), Tc(VII), Np(V) and Pu(IV) has been demonstrated
(Lovley
1993
; Lloyd & Macaskie
1996
; Lloyd et al.
2000a
; Boukhalfa et al.
2007
),
biomethylation of radionuclides has received little attention.
Lovley and co-workers were the first to demonstrate the dissimilatory reduc-
tion of U(VI) by the Fe(III)-reducing bacteria Geobacter metallireducens GS-15 and
Shewanella oneidensis (formerly Alteromonas putrefacies), a process by which energy
is conserved for anaerobic growth of these organisms (Lovley et al.
1991
; Lovley
1993
). Other bacteria able to reduce U(VI) (but without conserving energy),
include the sulphate-reducing bacteria Desulfovibrio desulfuricans (Lovley &
Phillips
1992
) and Desulfosporosinus sp. (Suzuki et al.
2004
), in addition to
Clostridium sp. ATCC 53464 (Francis et al.
1994
), Salmonella subterranean strain
FRC1 (Shelobolina et al.
2004
) and Anaeromyxobacter dehalogenans strain 2CP-C
(Wu et al.
2006
).
The mechanism of enzymatic reduction of uranium has been characterised
most comprehensively in the sulphate-reducing bacterium Desulfovibrio vulgaris,
which contains a periplasmic cytochrome c3, identified as the terminal reduc-
tase for the reduction of U(VI) (Lovley et al.
1993
). Similar mechanisms may be
important in Geobacter spp. (Lloyd et al.
2002
), but the terminal reductase for
U(VI) in this organism remains to be identified unequivocally. Interestingly,
it was shown recently that Geobacter sulfurreducens cannot reduce NpO
2
þ
, even
though it reduces UO
2
2
þ
efficiently (Renshaw et al.
2005
). The authors suggested
that the enzyme system responsible for uranium reduction is capable of trans-
ferring one electron to an actinyl ion, and the instability of the resulting U(V)
then generates U(IV) via disproportionation. The reduction of Np(V) is not
possible, however, because it appears the enzyme is specific for hexavalent
actinides, and cannot transfer an electron to NpO
2
þ
(Renshaw et al.
2005
).
In contrast, it has been known for some time that S. oneidensis is able to reduce
Np(V) (Lloyd et al.
2000b
), and more recent studies have confirmed that cell
suspensions of S. oneidensis are able to enzymatically reduce unchelated Np(V)
to insoluble Np(IV)(s), but cell suspensions of G. metallireducens are unable to
reduce Np(V) (Icopini et al.
2007
), suggesting the factors controlling enzymatic
reduction of Np(V) are complex.
These two organisms are also able to reduce highly soluble Tc(VII) to insol-
uble lower-valence species enzymatically (Lloyd & Macaskie
1996
). However, in
the subsurface, Fe(III)-reducing bacteria can also indirectly reduce and precipi-
tate Tc(VII) via biogenic Fe(II), and this mechanism is especially efficient when
the Fe(II) is associated with mineral phases, e.g., when biogenic magnetite is
