Reactive Transport of U and V from Abandoned Uranium Mine Wastes

Abstract

The reactive transport of uranium (U) and vanadium(V) from abandoned mine wastes collected from the Blue Gap/Tachee Claim-28 mine site in Arizona was investigated by integrating flow-through column experiments with reactive transport modeling, and electron microscopy. The mine wastes were sequentially reacted in flow-through columns at pH 7.9 (10 mM HCO₃^-) and pH 3.4 (10 mM CH₃COOH) to evaluate the effect of environmentally relevant conditions encountered at Blue Gap/Tachee on the release of U and V. The reaction rate constants (k_m) for the dissolution of uranyl-vanadate (U-V) minerals predominant at Blue Gap/Tachee were obtained from simulations with the reactive transport software, PFLOTRAN. The estimated reaction rate constants were within 1 order of magnitude for pH 7.9 (k_m = 4.8 × 10^-13 mol cm^-2 s^-1) and pH 3.4 (k_m = 3.2 × 10^-13 mol cm^-2 s^-1). However, the estimated equilibrium constants (K_eq) for U-V bearing minerals were more than 6 orders of magnitude different for reaction at circumneutral pH (K_eq = 10^-38.65) compared to acidic pH (K_eq = 10^-44.81). These results coupled with electron microscopy data suggest that the release of U and V is affected by water pH and the crystalline structure of U-V bearing minerals. The findings from this investigation have important implications for risk exposure assessment, remediation, and resource recovery of U and V in locations where U-V-bearing minerals are abundant.

Figure 1

Measured and simulated effluent concentrations and reactive transport model (PFLOTRAN) of U and V, from mine waste (circle) and background soil (squares) during batch and continuous flow-through column experiments at circumneutral pH (using 10 mM HCO₃⁻). (A) U concentrations from batch experiments versus time; (B) V concentration from batch experiments versus time; (C) U concentrations from column experiments versus pore volumes; (D) V concentration from column experiments versus pore volumes. The curve fitting resulting from the reactive transport model are presented with dashed lines.

Figure 2

Measured and simulated effluent concentrations and reactive transport model (PFLOTRAN) of U and V, from mine waste (circle) and background soil (squares) during batch and continuous flow-through column experiments at acidic pH (using 10 mM C₆H₈O₆ and CH₃COOH). (A) U concentrations from batch experiments versus time; (B) V concentration from batch experiments versus time; (C) U concentrations from column experiments versus pore volumes; (D) V concentration from column experiments versus pore volumes. The curve fittings resulting from the reactive transport model are presented with dashed lines.

Figure 3

Transmission electron microscopy (TEM) images, energy dispersive spectroscopy (EDS) spectra and selected area electron diffraction (SAED) patterns for unreacted mine waste samples indicating the co-occurrence of amorphous and crystalline U–V bearing minerals: (A, D) TEM images of U–V bearing mineral phases; (B, E) EDS spectra identifying the presence of U–V bearing minerals; (C, F) SAED patterns confirming the co-occurrence of amorphous and crystalline U–V bearing mineral phase.