Energy, Environmental and Chemical Engineering
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
Soil and groundwater contamination with actinides like uranium is a serious environmental concern. Phosphate addition to uranium-contaminated soil and groundwater can potentially provide long-term in-situ U(VI) immobilization by precipitation of low solubility U(VI)-phosphates. Reactions at the iron: oxy)hydroxide-water interface can control macroscopic transport and long-term stability of uranium. First, the interactions among phosphate, U(VI), and goethite: alpha-FeOOH) were investigated in a year-long batch experimental study. Dissolved U(VI) and phosphate concentrations were interpreted within a reaction-based modeling framework. U(VI) uptake mechanism varied with the aqueous composition. For most initially supersaturated conditions, chernikovite, H3O(UO2)(PO4)*3H2O(s), nucleated homogeneously, but heterogeneous nucleation probably occurred in cases of mild supersaturation. For conditions undersaturated with respect to chernikovite, phosphate-enhanced U(VI) adsorption indicated the formation of a U(VI)-phosphate-Fe(III) oxide ternary surface complex. Second, molecular-scale structures of adsorbed and precipitated U(VI) from batch experiments were probed using X-ray absorption fine-structure: XAFS) spectroscopy for different total U(VI) concentrations over a pH range 4-7 in the absence and presence of phosphate. The structure of precipitated U(VI) fit the meta-autunite group structure. While U(VI) adsorbed as bidentate edge-sharing =Fe(OH)2UO2 and bidentate corner-sharing: =FeOH)2UO2 surface complexes in the absence of phosphate, it formed a ternary surface complex: =FeO)2UO2PO4 in the presence of phosphate. Third, the effect of transport on U(VI) uptake and remobilization mechanisms and rate was examined. Continuous-flow stirred tank reactor: CFSTR) experiments at pH 4 were conducted under conditions supersaturated and undersaturated with respect to chernikovite and analyzed using a combination of measured dissolved concentrations, microscopy, and XAFS spectroscopy. The rates of dominant U(VI) and phosphate uptake and remobilization mechanisms in the absence and presence of goethite were quantified using a flow-through reactor model. Finally, the effects of simultaneous Fe(III) uptake on iron: oxy)(hydr)oxides on U(VI) and phosphate uptake and remobilization were investigated at pH 4. Goethite-coated sand packed columns and goethite-containing CFSTRs were used to simulate environmental conditions favoring the growth of iron: oxy)(hydr)oxide. While the presence of co-influent Fe(III) increased the extent and rate of phosphate uptake its presence not only decreased U(VI) uptake on goethite but also limited the formation of stable phosphate-induced uranium surface species.
Singh, Abhas, "Geochemical Conditions Affecting Uranium(VI) Fate And Transport In Soil And Groundwater In The Presence Of Phosphate" (2010). All Theses and Dissertations (ETDs). 324.