Sequential extraction method for determination of Fe(II/III) and U(IV/VI) in suspensions of iron-bearing phyllosilicates and uranium.
Iron-bearing phyllosilicates strongly influence the redox state and mobility of uranium because of their limited hydraulic conductivity, high specific surface area, and redox reactivity. Standard extraction procedures cannot be accurately applied for the determination of clay-Fe(II/III) and U(IV/VI) in clay mineral-U suspensions such that advanced spectroscopic techniques are required. Instead, we developed and validated a sequential extraction method for determination of clay-Fe(II/III) and U(IV/VI) in clay-U suspensions. In our so-called "H(3)PO(4)-HF-H(2)SO(4) sequential extraction" method, H(3)PO(4)-H(2)SO(4) is used first to solubilize and remove U, and the remaining clay pellet is subject to HF-H(2)SO(4) digestion. Physical separation of U and clay eliminates valence cycling between U(IV/VI) and clay-Fe(II/III) that otherwise occurred in the extraction solutions and caused analytical discrepancies. We further developed an "automated anoxic KPA" method to measure soluble U(VI) and total U (calculate U(IV) by difference) and modified the conventional HF-H(2)SO(4) digestion method to eliminate a series of time-consuming weighing steps. We measured the kinetics of uraninite oxidation by nontronite using this sequential extraction method and anoxic KPA method and measured a stoichiometric ratio of 2.19 ± 0.05 mol clay-Fe(II) produced per mol U(VI) produced (theoretical value of 2.0). We found that we were able to recover 98.0-98.5% of the clay Fe and 98.1-98.5% of the U through the sequential extractions. Compared to the theoretical stoichiometric ratio of 2.0, the parallel extractions of 0.5 M HCl for clay-Fe(II) and 1 M NaHCO(3) for U(VI) leached two-times more Fe(II) than U(VI). The parallel extractions of HF-H(2)SO(4) for clay Fe(II) and 1 M NaHCO(3) for U(VI) leached six-times more Fe(II) than U(VI).
Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16801-1408, USA.
Fubo
F
Luan
William D
WD
Burgos
D005591:Chemical Fractionation / Q000379:methods; D005290:Ferric Compounds / Q000032:analysis* / Q000737:chemistry; D005296:Ferrous Compounds / Q000032:analysis* / Q000737:chemistry; D006858:Hydrofluoric Acid / Q000737:chemistry; D017640:Silicates / Q000737:chemistry*; D017693:Sodium Bicarbonate / Q000737:chemistry; D013464:Sulfuric Acids / Q000737:chemistry; D017974:Uranium Compounds / Q000032:analysis* / Q000737:chemistry
D005290:Ferric Compounds; D005296:Ferrous Compounds; D017640:Silicates; D013464:Sulfuric Acids; D017974:Uranium Compounds; D017693:Sodium Bicarbonate; C033158:sulfuric acid; D006858:Hydrofluoric Acid