Article pubs.acs.org/est
Reduction of U(VI) Incorporated in the Structure of Hematite Eugene S. Ilton,†,* Juan S. Lezama Pacheco,‡ John R. Bargar,‡ Zhi Shi,† Juan Liu,† Libor Kovarik,§3 Mark H. Engelhard,§3 and Andrew R. Felmy† †
Pacific Northwest National Laboratory, PO Box 999, MSIN: K8-96, Richland, Washington 99352, United States Chemistry and Catalysis Division, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, California 94025, United States §3 Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, Washington, 99352 ‡
S Supporting Information *
ABSTRACT: U(VI) doped hematite was synthesized and exposed to two different organic reductants with E0 of 0.23 and 0.70 V. A combination of HAADF-TEM and EXAFS provided evidence that uranium was incorporated in hematite in uranate, likely octahedral coordination. XPS indicated that structurally incorporated U(VI) was reduced to U(V), whereas non-incorporated U(VI) was reduced to U(IV). Specifically, the experiments indicate that U(V) was the dominant oxidation state of uranium in hematite around Eh −0.24 to −0.28 V and pH 7.7−8.6 for at least up to 5 weeks of reaction time. U(V), but not U(IV), was also detected in hematite at Eh +0.21 V (pH 7.1−7.3). The results support the hypothesis, based on previous experimental and theoretical work, that the stability field of U(V) is widened relative to U(IV) and U(VI) in uranate coordination environments where the coordination number of U is less than 8.
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8, appears to widen the stability of field of U(V) relative to both U(VI) and U(IV). Although these were small cluster calculations and likely missed important contributions from the extended system, they nonetheless serve an important heuristic function. The question then arises whether or not the presence of U(V) is due to a constraint on the coordination environment of U. In this regard, Ilton et al.11 also noted a correlation between U(V) and high coordination with second nearest neighbor Fe (as well as with uranate coordination, see above). Although the U(V) bearing phase(s) was not identified, incorporation of U(V) into an Fe rich mineral was considered. The possibility that U(VI) can be incorporated into Fe(III) (hydr)oxides (under oxidizing conditions) has been hinted at in studies of U(VI) sorption by Fe(III) (hydr)oxides which showed an increase in an unextractable pool of U(VI) over time.23,24 More definitive evidence for structural incorporation of U(VI) in Fe (hydr)oxides was given by the EXAFS study by Duff et al.25 on coprecipitation of U(VI) with hematite. Complementary work on the association of U with goethite formed by reductively induced transformation of ferrihydrite also yielded tantalizing hints that U(VI) and/or U(V) could be incorporated into the structure of the goethite reaction product in uranate coordination.26−29 Classical atomistic simulations of U(IV), U(V), and U(VI) in the structure of various Fe (hydr)oxides, where U was a priori restricted to uranate
INTRODUCTION It has long been thought that the only two uranium oxidation states of environmental importance are U(IV) and U(VI). The likely, and perhaps reasonable, basis for the belief that the intermediate U(V) is not relevant under environmental conditions is the well documented rapid disproportionation of aqueous U(V) outside of a very narrow Eh-pH window.1−8 However, beginning in 2004−2005, laboratory evidence started accumulating for the existence of sorbed pentavalent uranium, well outside the nominal stability of U(V)aq, during the reduction of U(VI) by ferrous minerals9−11 and during the oxidative-dissolution of both SIMFUEL12−18 and particulate UO2.19,20 Evidence for a transitory aqueous U(V) carbonate species has also been found during microbial reduction of U(VI).21 Nonetheless, sorbed U(V) persisted for weeks in the study by Ilton et al.,11 suggesting that it could be more than just a transitory intermediate. A common theme was that U(V) associated with solids was never detected as a simple adsorbed complex, although its presence has not been ruled out; rather, it appeared to be incorporated in polymerized uranium forms or secondary precipitates10,11 during reduction by Fe(II) or in the structure of UO2+x during oxidation of UO2.19 Further, in one study, higher U(V) was correlated to increasing uranate coordinated uranium;11 where the term uranate refers to a relatively symmetric coordination environment that lacks the short dioxo uranyl bonds. These findings spurred theoretical modeling on the role of the coordination environment of uranium in the reduction of U(VI) to U(V) to U(IV). Recent ab initio calculations22 showed that uranate coordination, combined with a first shell coordination number, CN, less than © 2012 American Chemical Society
Received: Revised: Accepted: Published: 9428
April 18, 2012 July 21, 2012 July 27, 2012 July 27, 2012 dx.doi.org/10.1021/es3015502 | Environ. Sci. Technol. 2012, 46, 9428−9436
Environmental Science & Technology
Article
Table 1. Experimental Conditions (Solution Analyses)a Q : H2Qd (mM) sample A1 A2 A3 A4 A5 A6 A7e H1 H2 H3
b
hr
PH 3 24 3 24 24 840 24 3 24 840
7.9 8.6 7.7 7.8 7.8 7.7 7.9 7.1 7.2 7.3
c
Eh (V) −0.24 −0.28 −0.27 −0.25 −0.25 −0.24 −0.24 na 0.21 0.22
initial 0.027 0.027 0.270 0.270 0.270 0.270 0.270 nd nd nd
: : : : : : : : : :
0.143 0.143 1.430 1.430 1.430 1.430 1.430 1.480 1.480 1.480
final 0.089 0.104 0.310 0.346 0.343 0.334 0.322 nd 0.014 0.015
: : : : : : : : : :
0.107 0.082 1.220 1.327 1.337 1.264 1.206 1.477 1.431 1.414
U(M) 8.00 1.70 7.00 1.30 6.76 3.63 2.02 1.80 1.05 9.12
× × × × × × × × × ×
Fe(M) −9
10 10−9 10−9 10−9 10−10 10−9 10−9 10−8 10−8 10−9
1.44 1.06 4.41 5.51 3.98 4.22 2.23
