Abiotic U(VI) reduction by sorbed Fe(II) on natural sediments

作者：Fox Patricia M^*; Davis James A; Kukkadapu Ravi; Singer David M; Bargar John; Williams Kenneth H

来源：Geochimica et Cosmochimica Acta, 2013, 117: 266-282.

DOI：10.1016/j.gca.2013.05.003

Laboratory experiments were performed as a function of aqueous Fe(II) concentration to determine the uptake and oxidation of Fe(II), and Fe(II)-mediated abiotic reduction of U(VI) by aquifer sediments from the DOE Rifle field research site in Colorado, USA. Mossbauer analysis of the sediments spiked with aqueous Fe-57(II) showed that Fe-57(II) was oxidized on the mineral surfaces to Fe-57(III) and most likely formed a nano-particulate Fe(III)-oxide or ferrihydrite-like phase. The extent of Fe-57 oxidation decreased with increasing Fe-57(II) uptake, such that 98% was oxidized at 7.3 mu mol/g Fe and 41% at 39.6 mu mol/g Fe, indicating that the sediments had a limited capacity for oxidation of Fe(II). Abiotic U(VI) reduction was observed by XANES spectroscopy only when the Fe(II) uptake was greater than approximately 20 mu mol/g and surface-bound Fe(II) was present, possibly as oligomeric Fe(II) surface species. The degree of U(VI) reduction increased with increasing Fe(II)-loading above this level to a maximum of 18% and 36% U(IV) at pH 7.2 (40.7 mu mol/g Fe) and 8.3 (56.1 mu mol/g Fe), respectively in the presence of 400 ppm CO2. Greater U(VI) reduction was observed in CO2-free systems [up to 44% and 54% at pH 7.2 (17.3 mu mol/g Fe) and 8.3 (54.8 mu mol/g Fe), respectively] compared to 400 ppm CO2 systems, presumably due to differences in aqueous U(VI) speciation. While pH affects the amount of Fe(II) uptake onto the solid phase, with greater Fe(II) uptake at higher pH, similar amounts of U(VI) reduction were observed at pH 7.2 and 8.3 for a similar Fe(II) uptake. Thus, it appears that abiotic U(VI) reduction is controlled primarily by sorbed Fe(II) concentration and aqueous U(VI) speciation. The range of Fe(II) loadings tested in this study are within the range observed in biostimulation experiments at the Rifle site, suggesting that Fe(II)-mediated abiotic U(VI) reduction could play a significant role in field settings. Published by Elsevier Ltd.

• 出版日期2013-9-15