Surface Complexation Modeling of U(VI) Adsorption by Aquifer Sediments from a Former Mill Tailings Site at Rifle, Colorado

Abstract

A study of U(VI) adsorption by aquifer sediment samples from a former uranium mill tailings site at Rifle, Colorado, was conducted under oxic conditions as a function of pH, U(VI), Ca, and dissolved carbonate concentration. Batch adsorption experiments were performed using <2 mm size sediment fractions, a sand-sized fraction, and artificial groundwater solutions prepared to simulate the field groundwater composition. To encompass the geochemical conditions of the alluvial aquifer at the site, the experimental conditions ranged from 6.8 x 10(-8) to 10(-5) M in [U(VI)](tot), 7.2 to 8.0 in pH, 3.0 x 10(-3) to 6.0 x 10(-3) M in [Ca(2+)], and 0.05 to 2.6% in partial pressure of carbon dioxide. Surface area normalized U(VI) adsorption K(d) values for the sand and <2 mm sediment fraction were similar, suggesting a similar reactive surface coating on both fractions. A two-site two-reaction, nonelectrostatic generalized composite surface complexation model was developed and successfully simulated the U(VI) adsorption data. The model successfully predicted U(VI) adsorption observed from a multilevel sampling well installed at the site. A comparison of the model with the one developed previously for a uranium mill tailings site at Naturita, Colorado, indicated that possible calcite nonequilibrium of dissolved calcium concentration should be evaluated. The modeling results also illustrate the importance of the range of data used in deriving the best fit model parameters.

Acetate Availability and its Influence on Sustainable Bioremediation of Uranium-Contaminated Groundwater

“…Calculation of U(VI) distributions between the sediment and aqueous phase were based on the equilibrium surface complexation model of Hyun et al (2009). A porosity of 27% in the aquifer was used to determine the ratio of groundwater to sediments.…”

Geochemical, mineralogical and microbiological characteristics of sediment from a naturally reduced zone in a uranium-contaminated aquifer

“…Although the reduced sediments may be a relatively small fraction of the total mass of the aquifer sediment, their ability to seasonally sequester and/or release U may affect the overall behavior of the contaminant in the aquifer . Estimates of sediment composition, mineralogy, and natural attenuation capacity are often based on representative aquifer sediment samples (e.g., Hyun et al, 2009), but the heterogeneity caused by naturally reduced zones may be an overlooked yet important component in understanding long-term U behavior.…”

Dependence of particle concentration effect on pH and redox for arsenic removal by FeS-coated sand under anoxic conditions

“…The distribution coefficient is sometimes portrayed as a constant (Weber and DiGiano, 1996). However, in this system, as in others (e.g., Fox et al, 2006;Hyun et al, 2009), the distribution coefficient varied as a function of pH and concentration of the solute. At pH 7 and pH 9, the value of K d is essentially independent of the SSR.…”