Geochemical conditions affecting uranium fate and transport in soil and groundwater in the presence of phosphate.

摘要

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(UO 2)(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)2UO 2 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.