Environmental Engineering Reference
In-Depth Information
[42]. The effects of U(VI) chemical speciation, electron donors and competing
electron acceptors on U(VI) reduction activity by
Shewanella
have recently
been reported. In addition, the identity and subcellular location of reduction
products have been described, and genetic analysis of U(VI) reduction-deficient
mutant strains is underway.
2.1 U(VI) Chemical Speciation
U(VI) chemical speciation is an important variable controlling microbial
U(VI) reduction activity. In oxidizing aqueous environments at circumneutral
pH (and in the absence of phosphate), U(VI) is found as soluble uranyl ion
(UO
2
2
+
) complexes or as the crystalline solid metaschoepite (UO
3
.
2H
2
O).
U(IV) precipitates in reducing environments as uraninite (UO
2
). The relative
solubility of U(VI) compared with uraninite (only 10
−
8
MatpH
>
5; [74]) forms
the basis of alternate bioremediation strategies.
Uranyl ion is highly soluble and readily complexes with either inorganic
(e.g., hydroxyl, carbonate, phosphate, sulfate and calcium) or organic (e.g.,
acetate, malonate, citrate and oxalate) ligands in aqueous solution. The com-
plexing ligand changes the reduction potential of U(VI), thus affecting micro-
bial reduction activity. In terms of reduction potential, hydroxo complexes are
the most favorable form of complexed U(VI). Complexation by carbonate de-
creases the reduction potential of U(VI). Complexation of U(VI)-carbonate by
calcium (forming Ca-UO
2
-CO
3
complexes) decreases the reduction potential
to such an extent that U(VI) reduction by
S. putrefaciens
CN32 nearly ceases
[7]. The formation of Ca-UO
2
-CO
3
complexes also inhibits U(VI) reduction by
D. sulfuricans
and
G. sulfurreducens
. Neither fumarate nor Tc(VII) reduction
activities are inhibited by Ca
2
+
, indicating that the effects of Ca
2
+
are U(VI)
reduction-specific.
In the absence of carbonate or at pH less than 6 in the presence of carbonate,
organic ligands bound to U(VI) also affect uranium solubility and bioavailabil-
ity. Citrate, for example, binds U(VI) with varying strength as a function of pH
[70]. At pH greater than 6 and at low citrate concentrations, the highly soluble
(UO
2
)
3
Cit
2
species predominates over the (UO
2
)
2
Cit
2
species.
S. alga
reduces
U(VI) bound to citrate and other multidentate aliphatic complexes such as mal-
onate and oxalate more rapidly than U(VI) bound to monodentate aliphatic
complexes such as acetate, while the opposite trend is found with
D. desulfuri-
cans
[25]. U(VI) adsorbs to carboxyl, phosphoryl, and amine functional groups
on the
S. putrefaciens
200 cell surface, and a ligand exchange reaction may take
place between the cell surface or U(VI) terminal reductase(s) and the U(VI)
complexes prior to reduction [26].
