Environmental Engineering Reference
In-Depth Information
to live even in radioactively contaminated environments (Selenska-Pobell
2002
;
Lloyd
2003
; Fredrickson et al.
2004
; Ruggiero et al.
2005
; Akob et al.
2007
), and
they can affect radionuclide speciation via a number of mechanisms which are
potentially useful for scalable, cost-effective bioremediation of sediments and
waters impacted by nuclear waste (Keith-Roach & Livens
2002
; Lloyd
2003
; Suzuki
et al.
2003
; Lloyd & Renshaw
2005a
;Brodieet al.
2006
; Fomina et al.
2007
).
The aim of this chapter is to give an overview of known interactions of micro-
organisms with key radionuclides, focusing on potential roles in controlling
radionuclide mobility in the subsurface. We will then discuss the influence of
microbial processes on the immobilisation of radionuclides described in recent
laboratory studies from our groups, focusing on two priority contaminants:
technetium and uranium. The former studies address the impact of bioreduc-
tion strategies on solubility and include work on stimulated bioreduction
(achieved via added organic electron donor) on technetium solubility, while
the latter studies demonstrate the immobilisation of uranium in sediments
from a uranium mining waste pile without the addition of a carbon source.
Finally, due to the widespread use of nitric acid in the nuclear sector, the
multiple influences of nitrate and nitrate-reducing bacteria on the solubility
of radionuclides will be discussed, especially where the activities of these
organisms impact on the biogeochemistry of uranium and technetium.
Microbial interactions with radionuclides
The environmental fate of a radionuclide is governed by the interplay between
the background matrix of the radioactive material, the often complex chemistry
of the radionuclide in question and a broad range of chemical factors associ-
ated with the environment that has been impacted by the radioactive material
in question (Lloyd & Renshaw
2005b
). In addition, microbial activity will have
a profound effect on the solubility of radionuclides via a complex range of
often overlapping mechanisms including biosorption, bioaccumulation,
biotransformation, biomineralisation and microbially enhanced chemisor-
ption of heavy metals (
Fig. 11.1
). For a more extensive discussion of these
microbe radionuclide interactions, the reader is directed to the following
reviews (Lloyd & Lovley
2001
; Keith-Roach & Livens
2002
; Pedersen
2005
;
Renshaw et al.
2007
). A brief synopsis, focusing on interactions with subsurface
micro-organisms, is given below.
Biosorption describes the metabolism-independent sorption of heavy metals
and radionuclides to biomass and encompasses both adsorption and absorp-
tion. Multiple studies over the past three decades have confirmed that a wide
range of micro-organisms are capable of efficient biosorption of radionuclides
(Francis et al.
2004
; Merroun et al.
2005
; Ohnuki et al.
2005
). Both living and dead
biomass is capable of biosorption, with ligands involved in metal binding
including carboxyl, amine, hydroxyl, phosphate and sulphhydryl groups. Most
