Environmental Engineering Reference
In-Depth Information
present. This was first studied in G. sulfurreducens, which used hydrogen as the
electron donor for enzymatic reduction of Tc(VII) (presumably via a NiFe
hydrogenase identified in other bacteria as the terminal reductase for Tc(VII))
(Lloyd et al.
1997
; Marshall et al.
2008
). When acetate was supplied as the
electron donor, however, enzymatic reduction of Tc(VII) was not possible, but
Tc(VII) reduction was very efficient when Fe(III) oxides were provided with
the acetate and reduced to biogenic Fe(II)-bearing magnetite, which could then
act as an electron shuttle to Tc(VII) and cause reduction. Thus, Fe(II)-bearing
biomagnetite was shown to be an excellent reductant for Tc(VII).
Finally, the mechanism of plutonium reduction by G. metallireducens and
S. oneidensis MR-1 has also been studied recently, and it was demonstrated that
both organisms produce very little Pu(III) enzymatically from Pu(IV)(OH)
4
,
whereas in the presence of ethylenediaminetetraacetic acid (EDTA), most of
the Pu(IV)(OH)
4
(am) was reduced to Pu(III) (Boukhalfa et al.
2007
). Inefficient
enzymatic production of Pu(III) from Pu(IV)(OH)
4
was also identified in
G. sulfurreducens, even though the organism is very adept at reducing a broad
range of extracellular Fe(III) and Mn(IV) minerals (Renshaw et al.
2008
).
Biomineralisation is the process by which metals and radionuclides can be
precipitated with microbially generated ligands, e.g., phosphate, sulphide or
carbonate. In these examples, the microbial ligands accumulate to high con-
centrations around the cell, and the cell surface provides a nucleation site for
precipitation, resulting in efficient removal of the radionuclide from solution
(Renshaw et al.
2007
). The process of biomineralisation can be induced by
secretion of inorganic compounds, such as orthophosphate groups, which can
directly bind U(VI) in insoluble polycrystalline uranyl hydrogen phosphate or
in meta-autunite-like mineral phases (Macaskie et al.
1992
; Merroun et al.
2006
;
Beazley et al.
2007
; Jroundi et al.
2007
). In addition to direct precipitation by
microbially generated ligands, actinide ions can also be removed from solution
by chemisorption to biogenic minerals ('microbially enhanced chemisorption')
(Macaskie et al.
1994
).
Biogeochemistry of technetium reduction in sediments
Technetium is a significant, long-lived (
99
Tc half-life
10
5
years) radio-
active contaminant from nuclear fuel cycle operations. It is highly mobile in its
oxidised form (as Tc(VII)O
4
) but is scavenged to sediments in its reduced forms
(predominantly poorly soluble Tc(IV)). As part of a long-term collaboration
between our groups in Manchester and Leeds, we have been studying the
biogeochemical behaviour of Tc and its potential environmental mobility, to
better inform bioremediation approaches and safety case assessments.
Initial experiments used microcosms constructed from Tc-free Humber
Estuary surface sediments (with their indigenous microbial populations),
which were spiked with low levels (<5
m
M) of TcO
4
, and technetium solubility
ΒΌ
2.15
