Intercomparison and Refinement of Surface Complexation Models for U(VI) Adsorption onto Goethite Based on a Metadata...

Area of Science:

• Environmental Science
• Geochemistry
• Surface Chemistry

Background:

• Uranium adsorption onto goethite influences its mobility in subsurface environments.
• Existing surface complexation models (SCMs) often fail when applied to diverse conditions.
• A need exists for quantitatively evaluating SCM variations and developing more robust models.

Purpose of the Study:

• To intercompare and refine SCMs for uranium adsorption on goethite.
• To develop a more robust model applicable across a wide range of environmental conditions.
• To improve the quantitative understanding of U(VI)-goethite surface reactions.

Main Methods:

• Metadata analysis of a composite dataset covering wide ranges of pH, solid/sorbate ratios, and carbonate concentrations.
• Development of a series of SCMs with varying complexity, systematically evaluated for global best fit.
• Inclusion of goethite-uranyl-carbonate ternary surface complexes in all models.

Main Results:

• Spectroscopically informed models showed best fit with a triple-plane model; double-layer models with bidentate complexes also performed comparably.
• Models neglecting the bidentate nature of uranyl surface complexation performed poorly.
• Models omitting carbonate adsorption or detailed charge distributions did not significantly compromise fitting accuracy.

Conclusions:

• A systematic approach to SCM development using large, varied datasets enhances understanding of U(VI)-goethite surface reactions.
• The refined models offer a path toward a unified set of reactions and equilibrium constants for reactive transport models.
• Improved SCMs will increase confidence in predicting uranium adsorption onto goethite in subsurface environments.