Uranium(VI) adsorption and surface complexation modeling onto vadose sediments from the Savannah River Site

Enthalpy of Uranium Adsorption onto Hematite

“…This model is also consistent with spectroscopic studies that indicate that U VI forms an inner-sphere bidentate complex with hematite or other iron (hydr)­oxide surfaces. ,,− Additionally, the hematite site density and protonation constants ( K + , K – ; Table ) used in this SCM were defined based on the hematite crystal structure and from hematite potentiometric titration data pulled from the literature. This SCM and other similar SCMs suggest that, for the pH range examined in this work, there is no need to incorporate uranyl–carbonate surface complexes. ,, Therefore, the equilibrium constant of the U VI –hematite surface complex, (FeO) 2 UO 2 , is the only required fitting parameter. This approach avoids overparameterization of the SCM and allows us to focus our efforts on understanding the thermodynamics of U VI adsorption at multiple temperatures.…”

“…This SCM and other similar SCMs suggest that, for the pH range examined in this work, there is no need to incorporate uranyl− carbonate surface complexes. 23,26,37 Therefore, the equilibrium constant of the U VI −hematite surface complex, (FeO) 2 UO 2 , is the only required fitting parameter. This approach avoids overparameterization of the SCM and allows us to focus our efforts on understanding the thermodynamics of U VI adsorption at multiple temperatures.…”

Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments

“…These U zones did not exhibit the same morphology as the particles with which they were associated (e.g., see Figure j–l), and scatter plots of the U intensity versus the Al intensity exhibited a diffuse cloud of points near the origin (Figures and ). Previous researchers have identified U­(IV) adsorbed to clays in laboratory , and model field systems; also, U­(VI) sorption to clay minerals is known. , It is also entirely possible that U and C may form ternary surface complexes; however, it was not possible from our EXAFS spectroscopic analysis to assess whether U (as either +IV or +VI) was coordinated to organic functional groups or clay minerals surfaces, or both.…”

Response to Comment on “Enthalpy of Uranium Adsorption onto Hematite”

“…Furthermore, although ternary uranyl−carbonate surface complexes have been identified using EXAFS 6,8 or other spectroscopic tools, 17 the contribution of this species to the overall adsorption of uranium remains unclear, given that binary, noncarbonate uranyl surface complexes (e.g., (≡FeO) 2 UO 2 ) have been shown to adequately simulate U VI adsorption across a broad range of pH values, including those where aqueous uranyl− carbonate complexes are prevalent. 18,19 Our work represents a step forward in understanding environmentally relevant uranium adsorption chemistry. Although we may speculate about the influence of carbonate on U VI adsorption at elevated temperatures, we agree with Kersten that the thermodynamic data necessary to model this behavior are not yet available.…”

“…Even if the formation of ternary uranyl–carbonate surface complexes is exothermic, the surface complexation reaction given in our work remains a valid part of the overall thermodynamics describing U VI adsorption, and the formation of (≡FeO) 2 UO 2 on the hematite surface is expected to increase with increasing temperature. Furthermore, although ternary uranyl–carbonate surface complexes have been identified using EXAFS , or other spectroscopic tools, the contribution of this species to the overall adsorption of uranium remains unclear, given that binary, noncarbonate uranyl surface complexes (e.g., (≡FeO) 2 UO 2 ) have been shown to adequately simulate U VI adsorption across a broad range of pH values, including those where aqueous uranyl–carbonate complexes are prevalent. , …”