title: Reduction of uranium(VI) by soluble iron(II) conforms with thermodynamic predictions.
abstract: Soluble Fe(II) can reduce soluble U(VI) at rapid rates and in accordance with thermodynamic predictions. This was established by initially creating acidic aqueous solutions in which the sole oxidants were soluble U(VI) species and the sole reductants were soluble Fe(II) species. The pH of the solution was then increased by stepwise addition of OH(-), thereby increasing the potential for electron transfer from Fe(II) to U(VI). For each new pH value resulting from addition of base, values of ΔG for the Fe(II)-mediated reduction of U(VI) were calculated using the computed distribution of U and Fe species and possible half reaction combinations. For initial conditions of pH 2.4 and a molar ratio of Fe(II) to U(VI) of 5:1 (1 mM Fe(II) and 0.2 mM U(VI)), ΔG for U(VI) reduction was greater than zero, and U(VI) reduction was not observed. When sufficient OH(-) was added to exceed the computed equilibrium pH of 5.4, ΔG for U(VI) reduction was negative and soluble Fe(II) species reacted with U(VI) in a molar ratio of ∼2:1. X-ray absorption near-edge structure (XANES) spectroscopy confirmed production of U(IV). A decrease in pH confirmed production of acidity as the reaction advanced. As solution pH decreased to the equilibrium value, the rate of reaction declined, stopping completely at the predicted equilibrium pH. Initiation of the reaction at a higher pH resulted in a higher final ratio of U(IV) to U(VI) at equilibrium.
authors.affiliation: Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, USA.
authors.forename: Xin
authors.initials: X
authors.lastname: Du
authors.forename: Benjaporn
authors.initials: B
authors.lastname: Boonchayaanant
authors.forename: Wei-Min
authors.initials: WM
authors.lastname: Wu
authors.forename: Scott
authors.initials: S
authors.lastname: Fendorf
authors.forename: John
authors.initials: J
authors.lastname: Bargar
authors.forename: Craig S
authors.initials: CS
authors.lastname: Criddle
mesh_terms: D002254:Carbonates / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D011848:Radioactive Pollutants / Q000737:chemistry*; D013816:Thermodynamics; D014501:Uranium / Q000737:chemistry*
chemical_list: D002254:Carbonates; D005296:Ferrous Compounds; D011848:Radioactive Pollutants; D014501:Uranium