|Research in Chemistry Division|
Interactions of Pu with Desferrioxamine Siderophores Can Affect Bioavailability and Mobility
C. E. Ruggiero,
M . P. Neu, J. H. Matonic, S. D. Reilly,
Plutonium is thought to exist mostly as very low-soluble and/or strongly sorbed plutonium(IV) hydroxide and oxide species in the environment and therefore has low risk of becoming mobile or bioavailable. However, compounds that solubilize plutonium or change the oxidation state can significantly increase its bioavailabilty and mobility. We are examining the fundamental inorganic chemistry of actinides with microbial siderophores in order to understand how they could affect actinide biogeochemisty. Siderophores are low molecular weight, strong, metal-chelating agents produced by most microbes to bind and deliver iron into microbial cells via active transport systems.
We have focused on the tri-hydroxamate siderophores desferrioxamine B and E (DFB and DFE, or generically, DFO) because they are well-studied siderophores and are readily available (Figure 1). Hydroxamate siderophores have been estimated to be present in at 0.1-0.01 ÁM concentrations in soils. The equilibrium constant for the Pu(IV)-DFB complex formation reaction has been estimated to be l030.8. This is higher than many organic chelators.
Figure 1: The desferrioxamine siderophores DFB (left) and DFE (right).
We have examined the redox chemistry of plutonium with DFO and have investigated the ability of DFO to solubilize Pu(OH)4 solid. We have found that the Pu(IV)-DFO complex is a thermodynamic sink: no matter what oxidation state of Pu is present initially (III, IV, V, or VI), desferrioxamines eventually cause the Pu(IV)-DFO complex to form.
When DFO is added to a Pu(III) solution, the Pu(III) is oxidized quickly to form Pu(IV)-DFO. We have isolated single crystals from this reaction. X-ray analysis reveals the product to be Al(H2O)6[Pu(DFE)(H2O)3]2(CF3SO3)5·14H2O, the first plutonium-siderophore complex to be structurally characterized. The asymmetric unit contains the Pu(DFE)(H2O)3+ cation (Figure 2), a hexaaquoaluminum(III), four trifluoromethanesulfonates, and seven waters.
Figure 2: Metal-free DFE (left), Fe(III)-DFE Complex (middle), and Pu(IV)-DFE Complex (right). Oxygen atoms are shown in red, oxygen atoms of water molecules are maroon, nitrogen atoms are in blue, carbon atoms are in- black, the iron Fe(III) atom is yellow, and the Pu(IV) atom is green.
The nine-coordinate Pu atom is bound by DFE in approximately one hemisphere and by three waters in the other. The polytopal geometry of the Pu coordination sphere is slightly distorted, tricapped, trigonal prismatic. This is the first discrete molecule containing a nine-coordinate Pu(IV) ion. The Fe(III)-DFE complex and the DFE ligand (without metal) have also been structurally characterized (Figure 2).
When a solution of DFO is added to a solution of Pu(VI) at pH = 2 in an equal molar ratio, the Pu(VI) is instantly reduced to Pu(V). The reduction occurs as rapidly as could be detected, even at concentrations as low as 40 mM Pu. Stoichiometric titration of Pu(VI) into a DFO solution showed that up to twelve molar equivalents of Pu(VI) could be rapidly reduced to Pu(V) per DFO, corresponding to four reducing equivalents per hydroxamate of the DFO molecule. The Pu(V) solution that initially forms slowly reduces to form the Pu(IV)-DFO complex.
The rates of both the initial reduction of Pu(VI) to Pu(V) and the subsequent reduction of Pu(V) to Pu(IV) depend on pH. If the reaction is performed at pH 1 to 5.5, the reduction of Pu(VI) to Pu(V) is instant;, but, the subsequent reduction of the Pu(V) to Pu(IV)-DFO is significantly slower, taking months to fully reduce. However, if the pH is raised above 5.5 after the initial reduction reaction, the Pu(V) is instantly and irreversibly reduced to Pu(IV), and the amount of Pu(IV) formed is proportional to the pH. If the reaction is started above pH=6, the Pu(VI) is instantly and irreversibly reduced directly to Pu(IV)-DFO.
The rates of the reduction steps also depend on the ratio of DFO to Pu. At ratios from one molar equivalent DFB reacting with one molar equivalent plutonium (1 DFB : 1 Pu) to 1 DFB : 4 Pu, the reduction of Pu(VI) to Pu(V) is instant. At ratios from 1 DFB : 6 Pu to 1 DFB : 12 Pu, the rate of reduction is slower, but still very rapid (< 1 hour). In For the reaction at a ratio of 1 DFB: 12 Pu, up to 20% Pu(VI) is still present after 10 minutes, allowing spectroscopic observation of a probable Pu(VI)-DFB species that must form as a first step in the reduction process.
The rate of secondary reduction of the Pu(V) to Pu(IV) is also faster at higher DFB-to-Pu ratios. The presence of excess DFB acts as a thermodynamic driving force for the formation of a stable, soluble Pu(IV)-DFB species. In the reaction performed at a ratio of 1 DFB : 12 Pu, where the DFB is completely oxidized, only Pu(V) is detected in solution, but a precipitate slowly forms. Presumably, any Pu(IV) that forms as a result of the disproportionation of Pu(V) slowly precipitates out of solution as the hydroxide.
Surprisingly, the Pu(IV)-DFO complex is still reactive. When Pu(VI) is added to a solution of the Pu(IV)-DFB or -DFE complex at pH = 2 or pH = 9, the Pu(VI) is rapidly reduced, despite the fact the DFO is already complexed to Pu(IV). The reaction is slower than the reaction without Pu(IV) initially present. Variable temperature NMR (nuclear magnetic resonance) indicates that the Pu(IV)-DFO complexes are highly fluxional and may undergo ligand exchange with free DFO, which would allow for DFO interaction with and reduction of the Pu(VI).
Despite the fact that the Pu(IV)-DFB complex has an exceptional large equilibrium constant for its formation, the desferrioxamine siderophores are poor at solubilizing solid Pu(OH)4. Pu(IV) hydroxide is slowly solubilized by chelates such as EDTA, citrate, and tiron. We have measured rates of approximately 1.13 mM, 0.16 mm, and 0.10 mm per day reaching 310 mM, 41 mM, and 27 mM, respectively, after 253 days. However, the siderophores DFE and DFB are 50 to 500 times slower than EDTA with rates of 0.02 (DFE) and 0.002 mM/day (DFB), although they are faster than controls without a chelator present. These results are unexpected given that the thermodynamic equilibrium constants for the formation of the Pu(IV) tiron, citrate, and EDTA complexes are lower than for the formation of the Pu(IV)-DFO complexes. In fact, EDTA solubilization of Pu(OH)4 was 10 times slower after the plutonium is pretreated with DFB (0.11 mm per day). These surprising results suggest that the desferrioxamine siderophores are actually passivating the surface of the Pu(OH)4 and thereby inhibiting solubilization.
We have shown that siderophores and potentially other naturally produced chelators could play a major role in the environmental behavior of plutonium. (Figure 3.) The basic chemistry is becoming clearer: siderophores have high formation constants for Pu(IV) complexes, which could keep Pu(IV) species solubilized and mobile. Their higher formation constants indicate that siderophores can "steal" plutonium from other chelators, such as EDTA and NTA, present with plutonium wastes. Hydroxamate siderophores have a large reducing capacity for Pu(VI) and Pu(V), leading to the formation of Pu(IV) siderophore complexes. Siderophores could can very slowly solubilize Pu solids, but they could also interfere with solubilization of Pu solids by other chelators.
Figure 3. Microbial interactions with actinides in the environment. All processes could increase or decrease solubility and/or mobility depending on specific bacteria and numerous environmental and chemical factors.
We are now beginning to address more complex questions regarding Pu biogeochemistry: Can bacteria bioaccumulate plutonium by actively transporting siderophore-Pu complexes? (we think so), by surface absorption of Pu onto the bacteria capsule (we know so), or by unchelated passive diffusion? Will bacteria that accumulate plutonium increase the mobility of plutonium (biocolloid formation) or decrease it (biofilm formation)? Can siderophores or other microbial chelators solubilize other forms of Pu in the environment, such as Pu absorbed on mineral, bound by humate, or precipitated by bacteria redox processes? What kinds of ternary complexes can form in the presence of siderophores and other microbial chelators, and will they lead to an increase or decrease in mobility of plutonium? How will local environmental changes caused by bacteria effect plutonium speciation? These processes could play critical roles in plutonium biogeochemistry, which has only begun to be investigated. Understanding how bacteria effect plutonium environmental chemistry is crucial for both the safe long-term storage of plutonium wastes and for proposed bioremediation strategies of plutonium and the organic wastes often present with plutonium.