U(VI) adsorption to heterogeneous subsurface media: application of a surface complexation model

Area of Science:

• Geochemistry
• Environmental Science
• Nuclear Chemistry

Background:

• Uranium (VI) contamination in subsurface environments poses environmental risks.
• Understanding uranium adsorption to soil is critical for predicting its transport and remediation.
• Iron and manganese oxides are significant components of subsurface media affecting contaminant behavior.

Purpose of the Study:

• To investigate the pH-dependent adsorption of Uranium (VI) onto heterogeneous subsurface media.
• To evaluate the applicability of a ferrihydrite adsorption model to complex environmental samples.
• To assess the influence of carbonate chemistry on Uranium (VI) adsorption.

Main Methods:

• Adsorption experiments using subsurface media from DOE sites.
• Utilizing a ferrihydrite adsorption model for prediction.
• Varying pH, carbonate concentration, and CO2 partial pressure.

Main Results:

• A ferrihydrite adsorption model successfully predicted Uranium (VI) adsorption trends.
• The model showed predictive capability even under varied conditions with a root mean square error of 0.163-0.408.
• Uranium (VI) adsorption decreased at pH >10, indicating potential mobility in extreme environments.
• Carbonate chemistry significantly impacts Uranium (VI) adsorption, with differences observed between constant CO2 partial pressure and constant total carbonate concentration.

Conclusions:

• The ferrihydrite adsorption model serves as a reliable first approximation for predicting Uranium (VI) adsorption and transport.
• Subsurface Uranium (VI) mobility is influenced by pH and carbonate chemistry, especially in extreme environments.
• Experimental conditions simulating subsurface environments require careful consideration of carbonate system dynamics.