Geology topics

P.M. Fox

Publications and source records attributed to P.M. Fox.

3 recordsLinked to original sources

USGS2009

Surface complexation modeling of U(VI) adsorption by aquifer sediments from a former mill tailings site at Rifle, Colorado

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 <2mm 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 ?? 10 -8 to 10 -5 M in [U(VI)]tot, 7.2 to 8.0 in pH, 3.0 ?? 10 -3 to 6.0 ?? 10 -3 M in [Ca 2+ ], and 0.05 to 2.6% in partial pressure of carbon dioxide. Surface area normalized U(VI) adsorption Kd 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. ?? 2009 American Chemical Society.

Environmental Science & Technology↗

USGS2009

The kinetics of iodide oxidation by the manganese oxide mineral birnessite

The kinetics of iodide (I − ) and molecular iodine (I 2 ) oxidation by the manganese oxide mineral birnessite (δ-MnO 2 ) was investigated over the pH range 4.5–6.25. I − oxidation to iodate ( IO 3 - ) "> (IO3-) proceeded as a two-step reaction through an I 2 intermediate. The rate of the reaction varied with both pH and birnessite concentration, with faster oxidation occurring at lower pH and higher birnessite concentration. The disappearance of I − from solution was first order with respect to I − concentration, pH, and birnessite concentration, such that − d [I − ]/ dt = k [I − ][H + ][MnO 2 ], where k , the third order rate constant, is equal to 1.08 ± 0.06 × 10 7 M −2 h −1 . The data are consistent with the formation of an inner sphere I − surface complex as the first step of the reaction, and the adsorption of I − exhibited significant pH dependence. Both I 2 , and to a lesser extent, IO 3 - "> IO3- sorbed to birnessite. The results indicate that iodine transport in mildly acidic groundwater systems may not be conservative. Because of the higher adsorption of the oxidized I species I 2 and IO 3 - "> IO3- , as well as the biophilic nature of I 2 , redox transformations of iodine must be taken into account when predicting I transport in aquifers and watersheds.

Geochimica et Cosmochimica Acta↗

USGS2006

The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz

Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO 2 (CO 3 ) 3 2− and Ca 2 UO 2 (CO 3 ) 3 0 (aq), on inhibition of microbial reduction and uranium(VI) aqueous speciation in contaminated groundwater. However, to our knowledge, there have been no direct studies of the effects of these species on U(VI) adsorption by mineral phases. The sorption of U(VI) on quartz and ferrihydrite was investigated in NaNO 3 solutions equilibrated with either ambient air (430 ppm CO 2 ) or 2% CO 2 in the presence of 0, 1.8, or 8.9 mM Ca 2+ . Under conditions where the Ca 2 UO 2 (CO 3 ) 3 0 (aq) species predominates U(VI) aqueous speciation, the presence of Ca in solution lowered U(VI) adsorption on quartz from 77% in the absence of Ca to 42% and 10% at Ca concentrations of 1.8 and 8.9 mM, respectively. U(VI) adsorption to ferrihydrite decreased from 83% in the absence of Ca to 57% in the presence of 1.8 mM Ca. Surface complexation model predictions that included the formation constant for aqueous Ca 2 UO 2 (CO 3 ) 3 0 (aq) accurately simulated the effect of Ca 2+ on U(VI) sorption onto quartz and ferrihydrite within the thermodynamic uncertainty of the stability constant value. This study confirms that Ca 2+ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers.

Geochimica et Cosmochimica Acta↗