Qafoku and Icenhower reported in 2008 that there are interesting developments in the area of U redox reactions in the presence of solid phases. The authors indicated previously mentioned that in the presence of soil mineral catalysts, the redox reaction may occur on the Fe(II) exchanged surface of a solid phase, or on the surface of a Fe(II)-bearing mineral.
However, the explanation for the enhanced reactivity of sorbed Fe(II) remains ambiguous, although recent studies have shed some light on this subject (Boyanov et al. 2007). These authors conducted experiments to gain further insights into the U–Fe redox process at a complexing, non-conducting surface such as carboxyl-functionalized polystyrene.
It was reported that in the Fe ? surface carboxyl system, a transition from monomeric to oligomeric Fe(II) surface species was observed between pH 7.5 and pH 8.4 (Boyanov et al. 2007). In the U ? surface carboxyl system, the U(VI) cation was adsorbed as a mononuclear uranylcarboxyl complex at both pH 7.5 and 8.4.
In the ternary U ? Fe ? surface carboxyl system, U(VI) was not reduced by the solvated or adsorbed Fe(II) at pH 7.5 over a 4-month period, whereas complete and rapid reduction to U(IV) nanoparticles occurred at pH 8.4 (Boyanov et al. 2007).
Boyanov et al. (2007) also reported that the U(IV) product reoxidized rapidly upon exposure to air, but it was stable over a 4-month period under anoxic
conditions. The U(IV)–Fe coordination was consistent with an inner-sphere electron transfer mechanism between the redox centers and involvement of Fe(II) atoms in both steps of the reduction from U(VI) to U(IV). The inability of Fe(II) to reduce U(VI) in solution and at pH 7.5 in the U ? Fe ? carboxyl
system was explained by the formation of a transient, ‘‘dead-end’’ U(V)–Fe(III) complex that blocked the U(V) disproportionation pathway after the first electron transfer.
The authors suggested that the increased reactivity at pH 8.4 relative to pH 7.5 could be attributed to the reaction of U(VI) with an Fe(II) oligomer, whereby the bonds between Fe atoms facilitated the transfer of a second electron to the hypothetical U(V)–Fe(III) intermediate, which may explain the commonly observed higher efficiency of uranyl reduction by adsorbed or structural Fe(II) relative to aqueous Fe(II) (Boyanov et al. 2007).
However, recent results have shown that at low Fe(II) concentrations, sorbed Fe(II) species on hematite are transient and quickly undergo interfacial
electron transfer with structural Fe(III) (LareseCasanova and Scherrer 2007), forming a Fe(III) surface coating layer. The formation of a stable, sorbed Fe(II) phase in hematite was observed only at higher Fe(II) concentrations and coincided with the macroscopically observed change in isotherm slope,
and with the estimated surface site saturation, suggesting that the finite capacity for interfacial electron transfer is influenced by surface properties
(Larese-Casanova and Scherrer 2007).
Definitely, the heterogeneous reduction of U(VI) by sorbed Fe(II) is an important pathway for immobilization of aqueous U(VI) species in subsurface
environments, where the homogeneous redox reaction of the aqueous U(VI):Fe(II) couple is slow. However, many questions remain unanswered:
(i) What is the extent of sorbed U(VI) reduction by sorbed Fe(II) on hematite or other Fe(III) oxides and hydroxide surfaces?
(ii) Is sorbed U(VI) more competitive than structural Fe(III) for the electrons of sorbed Fe(II) on hematite?
(iii) What are the conditions that may control the competition between sorbed U(VI) and structural Fe(III) for sorbed Fe(II) electrons?
(iv) How do other Fe(III) oxides behave?
The authors concluded that U(VI) interactions with the soil minerals is an exciting area of research which is evolving on a daily basis. Studies conducted over the last few years have provided evidence for such interactions and have described U geochemical behavior in complex natural systems under different physical, biogeochemical, hydrological, and reducing or oxidizing conditions. This review covers important aspects of the aqueous U(VI) interactions with the soil minerals in natural systems under oxic and slightly alkaline conditions.
The physical and chemical complexity of the natural systems complicate the development of a mechanistic understanding of the interactions of the aqueous U(VI) species with soil minerals. Nevertheless information from carefully designed and executed experiments conducted with such materials provide helpful information to better understand the nature of aqueous U(VI) interactions with soil minerals, and to predict system’s behavior and U contaminant mobility
and transport. Areas that warrant further attention and study are also proposed in this review paper.
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